< draft-ietf-avt-rtp-new-11.txt   draft-ietf-avt-rtp-new-12.txt >
A new Request for Comments is now available in online RFC libraries.
Internet Engineering Task Force Audio/Video Transport Working Group RFC 3550
Internet Draft Schulzrinne/Casner/Frederick/Jacobson
draft-ietf-avt-rtp-new-11.txt Columbia U./Packet Design/
Cacheflow/Packet Design
November 20, 2001
Expires: May 2002
RTP: A Transport Protocol for Real-Time Applications
STATUS OF THIS MEMO
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Abstract
This memorandum is a revision of RFC 1889 in preparation for
advancement from Proposed Standard to Draft Standard status.
This memorandum describes RTP, the real-time transport protocol. RTP
provides end-to-end network transport functions suitable for
applications transmitting real-time data, such as audio, video or
simulation data, over multicast or unicast network services. RTP does
not address resource reservation and does not guarantee quality-of-
service for real-time services. The data transport is augmented by a
control protocol (RTCP) to allow monitoring of the data delivery in a
manner scalable to large multicast networks, and to provide minimal
control and identification functionality. RTP and RTCP are designed
to be independent of the underlying transport and network layers. The
protocol supports the use of RTP-level translators and mixers.
This specification is a product of the Audio/Video Transport working
group within the Internet Engineering Task Force. Comments are
solicited and should be addressed to the working group's mailing list
at avt@ietf.org and/or the authors.
Contents
1 Introduction ........................................ 3
1.1 Terminology ......................................... 5
2 RTP Use Scenarios ................................... 5
2.1 Simple Multicast Audio Conference ................... 6
2.2 Audio and Video Conference .......................... 6
2.3 Mixers and Translators .............................. 7
2.4 Layered Encodings ................................... 8
3 Definitions ......................................... 8
4 Byte Order, Alignment, and Time Format .............. 11
5 RTP Data Transfer Protocol .......................... 12
5.1 RTP Fixed Header Fields ............................. 12
5.2 Multiplexing RTP Sessions ........................... 15
5.3 Profile-Specific Modifications to the RTP Header
................................................................ 16
5.3.1 RTP Header Extension ................................ 17
6 RTP Control Protocol -- RTCP ........................ 17
6.1 RTCP Packet Format .................................. 19
6.2 RTCP Transmission Interval .......................... 21
6.2.1 Maintaining the number of session members ........... 26
6.3 RTCP Packet Send and Receive Rules .................. 26
6.3.1 Computing the RTCP transmission interval ............ 27
6.3.2 Initialization ...................................... 28
6.3.3 Receiving an RTP or non-BYE RTCP packet ............. 29
6.3.4 Receiving an RTCP BYE packet ........................ 29
6.3.5 Timing Out an SSRC .................................. 30
6.3.6 Expiration of transmission timer .................... 30
6.3.7 Transmitting a BYE packet ........................... 31
6.3.8 Updating we_sent .................................... 32
6.3.9 Allocation of source description bandwidth .......... 32
6.4 Sender and Receiver Reports ......................... 33
6.4.1 SR: Sender report RTCP packet ....................... 33
6.4.2 RR: Receiver report RTCP packet ..................... 39
6.4.3 Extending the sender and receiver reports ........... 40
6.4.4 Analyzing sender and receiver reports ............... 41
6.5 SDES: Source description RTCP packet ................ 43
6.5.1 CNAME: Canonical end-point identifier SDES item ..... 44
6.5.2 NAME: User name SDES item ........................... 46
6.5.3 EMAIL: Electronic mail address SDES item ............ 46
6.5.4 PHONE: Phone number SDES item ....................... 46
6.5.5 LOC: Geographic user location SDES item ............. 47
6.5.6 TOOL: Application or tool name SDES item ............ 47
6.5.7 NOTE: Notice/status SDES item ....................... 47
6.5.8 PRIV: Private extensions SDES item .................. 48
6.6 BYE: Goodbye RTCP packet ............................ 49
6.7 APP: Application-defined RTCP packet ................ 50
7 RTP Translators and Mixers .......................... 51
7.1 General Description ................................. 51
7.2 RTCP Processing in Translators ...................... 54
7.3 RTCP Processing in Mixers ........................... 55
7.4 Cascaded Mixers ..................................... 56
8 SSRC Identifier Allocation and Use .................. 57
8.1 Probability of Collision ............................ 57
8.2 Collision Resolution and Loop Detection ............. 58
8.3 Use with Layered Encodings .......................... 62
9 Security ............................................ 63
9.1 Confidentiality ..................................... 63
9.2 Authentication and Message Integrity ................ 65
10 Congestion Control .................................. 65
11 RTP over Network and Transport Protocols ............ 65
12 Summary of Protocol Constants ....................... 67
12.1 RTCP packet types ................................... 67
12.2 SDES types .......................................... 68
13 RTP Profiles and Payload Format Specifications ...... 68
14 IANA Considerations ................................. 71
A Algorithms .......................................... 71
A.1 RTP Data Header Validity Checks ..................... 75
A.2 RTCP Header Validity Checks ......................... 80
A.3 Determining the Number of RTP Packets Expected and
Lost ........................................................... 80
A.4 Generating SDES RTCP Packets ........................ 81
A.5 Parsing RTCP SDES Packets ........................... 82
A.6 Generating a Random 32-bit Identifier ............... 83
A.7 Computing the RTCP Transmission Interval ............ 86
A.8 Estimating the Interarrival Jitter .................. 93
B Changes from RFC 1889 ............................... 94
C Security Considerations ............................. 98
D Full Copyright Statement ............................ 98
E Addresses of Authors ................................ 99
1 Introduction
[Note to the RFC Editor: This paragraph and the first paragraph of
the Abstract are to be deleted when this draft is published as an
RFC. All RFC XXXX should be filled in with the RFC number of the RTP
Profile for Audio and Video Conferences as it is submitted for Draft
Standard status. Readers are directed to Appendix B, Changes from RFC
1889, for a listing of the changes that have been made in this
draft.]
This memorandum specifies the real-time transport protocol (RTP),
which provides end-to-end delivery services for data with real-time
characteristics, such as interactive audio and video. Those services
include payload type identification, sequence numbering, timestamping
and delivery monitoring. Applications typically run RTP on top of UDP
to make use of its multiplexing and checksum services; both protocols
contribute parts of the transport protocol functionality. However,
RTP may be used with other suitable underlying network or transport
protocols (see Section 11). RTP supports data transfer to multiple
destinations using multicast distribution if provided by the
underlying network.
Note that RTP itself does not provide any mechanism to ensure timely
delivery or provide other quality-of-service guarantees, but relies
on lower-layer services to do so. It does not guarantee delivery or
prevent out-of-order delivery, nor does it assume that the underlying
network is reliable and delivers packets in sequence. The sequence
numbers included in RTP allow the receiver to reconstruct the
sender's packet sequence, but sequence numbers might also be used to
determine the proper location of a packet, for example in video
decoding, without necessarily decoding packets in sequence.
While RTP is primarily designed to satisfy the needs of multi-
participant multimedia conferences, it is not limited to that
particular application. Storage of continuous data, interactive
distributed simulation, active badge, and control and measurement
applications may also find RTP applicable.
This document defines RTP, consisting of two closely-linked parts:
o the real-time transport protocol (RTP), to carry data that has
real-time properties.
o the RTP control protocol (RTCP), to monitor the quality of
service and to convey information about the participants in an
on-going session. The latter aspect of RTCP may be sufficient
for "loosely controlled" sessions, i.e., where there is no
explicit membership control and set-up, but it is not
necessarily intended to support all of an application's
control communication requirements. This functionality may be
fully or partially subsumed by a separate session control
protocol, which is beyond the scope of this document.
RTP represents a new style of protocol following the principles of
application level framing and integrated layer processing proposed by
Clark and Tennenhouse [10]. That is, RTP is intended to be malleable
to provide the information required by a particular application and
will often be integrated into the application processing rather than
being implemented as a separate layer. RTP is a protocol framework
that is deliberately not complete. This document specifies those
functions expected to be common across all the applications for which
RTP would be appropriate. Unlike conventional protocols in which
additional functions might be accommodated by making the protocol
more general or by adding an option mechanism that would require
parsing, RTP is intended to be tailored through modifications and/or
additions to the headers as needed. Examples are given in Sections
5.3 and 6.4.3.
Therefore, in addition to this document, a complete specification of
RTP for a particular application will require one or more companion
documents (see Section 13):
o a profile specification document, which defines a set of
payload type codes and their mapping to payload formats (e.g.,
media encodings). A profile may also define extensions or
modifications to RTP that are specific to a particular class
of applications. Typically an application will operate under
only one profile. A profile for audio and video data may be
found in the companion RFC XXXX [1].
o payload format specification documents, which define how a
particular payload, such as an audio or video encoding, is to
be carried in RTP.
A discussion of real-time services and algorithms for their
implementation as well as background discussion on some of the RTP
design decisions can be found in [11].
1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant RTP implementations.
2 RTP Use Scenarios
The following sections describe some aspects of the use of RTP. The
examples were chosen to illustrate the basic operation of
applications using RTP, not to limit what RTP may be used for. In
these examples, RTP is carried on top of IP and UDP, and follows the
conventions established by the profile for audio and video specified
in the companion RFC XXXX.
2.1 Simple Multicast Audio Conference
A working group of the IETF meets to discuss the latest protocol
draft, using the IP multicast services of the Internet for voice
communications. Through some allocation mechanism the working group
chair obtains a multicast group address and pair of ports. One port
is used for audio data, and the other is used for control (RTCP)
packets. This address and port information is distributed to the
intended participants. If privacy is desired, the data and control
packets may be encrypted as specified in Section 9.1, in which case
an encryption key must also be generated and distributed. The exact
details of these allocation and distribution mechanisms are beyond
the scope of RTP.
The audio conferencing application used by each conference
participant sends audio data in small chunks of, say, 20 ms duration.
Each chunk of audio data is preceded by an RTP header; RTP header and
data are in turn contained in a UDP packet. The RTP header indicates
what type of audio encoding (such as PCM, ADPCM or LPC) is contained
in each packet so that senders can change the encoding during a
conference, for example, to accommodate a new participant that is
connected through a low-bandwidth link or react to indications of
network congestion.
The Internet, like other packet networks, occasionally loses and
reorders packets and delays them by variable amounts of time. To cope
with these impairments, the RTP header contains timing information
and a sequence number that allow the receivers to reconstruct the
timing produced by the source, so that in this example, chunks of
audio are contiguously played out the speaker every 20 ms. This
timing reconstruction is performed separately for each source of RTP
packets in the conference. The sequence number can also be used by
the receiver to estimate how many packets are being lost.
Since members of the working group join and leave during the
conference, it is useful to know who is participating at any moment
and how well they are receiving the audio data. For that purpose,
each instance of the audio application in the conference periodically
multicasts a reception report plus the name of its user on the RTCP
(control) port. The reception report indicates how well the current
speaker is being received and may be used to control adaptive
encodings. In addition to the user name, other identifying
information may also be included subject to control bandwidth limits.
A site sends the RTCP BYE packet (Section 6.6) when it leaves the
conference.
2.2 Audio and Video Conference
If both audio and video media are used in a conference, they are
transmitted as separate RTP sessions RTCP packets are transmitted for
each medium using two different UDP port pairs and/or multicast
addresses. There is no direct coupling at the RTP level between the
audio and video sessions, except that a user participating in both
sessions should use the same distinguished (canonical) name in the
RTCP packets for both so that the sessions can be associated.
One motivation for this separation is to allow some participants in
the conference to receive only one medium if they choose. Further
explanation is given in Section 5.2. Despite the separation,
synchronized playback of a source's audio and video can be achieved
using timing information carried in the RTCP packets for both
sessions.
2.3 Mixers and Translators
So far, we have assumed that all sites want to receive media data in
the same format. However, this may not always be appropriate.
Consider the case where participants in one area are connected
through a low-speed link to the majority of the conference
participants who enjoy high-speed network access. Instead of forcing
everyone to use a lower-bandwidth, reduced-quality audio encoding, an
RTP-level relay called a mixer may be placed near the low-bandwidth
area. This mixer resynchronizes incoming audio packets to reconstruct
the constant 20 ms spacing generated by the sender, mixes these
reconstructed audio streams into a single stream, translates the
audio encoding to a lower-bandwidth one and forwards the lower-
bandwidth packet stream across the low-speed link. These packets
might be unicast to a single recipient or multicast on a different
address to multiple recipients. The RTP header includes a means for
mixers to identify the sources that contributed to a mixed packet so
that correct talker indication can be provided at the receivers.
Some of the intended participants in the audio conference may be
connected with high bandwidth links but might not be directly
reachable via IP multicast. For example, they might be behind an
application-level firewall that will not let any IP packets pass. For
these sites, mixing may not be necessary, in which case another type
of RTP-level relay called a translator may be used. Two translators
are installed, one on either side of the firewall, with the outside
one funneling all multicast packets received through a secure
connection to the translator inside the firewall. The translator
inside the firewall sends them again as multicast packets to a
multicast group restricted to the site's internal network.
Mixers and translators may be designed for a variety of purposes. An
example is a video mixer that scales the images of individual people
in separate video streams and composites them into one video stream
to simulate a group scene. Other examples of translation include the
connection of a group of hosts speaking only IP/UDP to a group of
hosts that understand only ST-II, or the packet-by-packet encoding
translation of video streams from individual sources without
resynchronization or mixing. Details of the operation of mixers and
translators are given in Section 7.
2.4 Layered Encodings
Multimedia applications should be able to adjust the transmission
rate to match the capacity of the receiver or to adapt to network
congestion. Many implementations place the responsibility of rate-
adaptivity at the source. This does not work well with multicast
transmission because of the conflicting bandwidth requirements of
heterogeneous receivers. The result is often a least-common
denominator scenario, where the smallest pipe in the network mesh
dictates the quality and fidelity of the overall live multimedia
"broadcast".
Instead, responsibility for rate-adaptation can be placed at the
receivers by combining a layered encoding with a layered transmission
system. In the context of RTP over IP multicast, the source can
stripe the progressive layers of a hierarchically represented signal
across multiple RTP sessions each carried on its own multicast group.
Receivers can then adapt to network heterogeneity and control their
reception bandwidth by joining only the appropriate subset of the
multicast groups.
Details of the use of RTP with layered encodings are given in
Sections 6.3.9, 8.3 and 11.
3 Definitions
RTP payload: The data transported by RTP in a packet, for
example audio samples or compressed video data. The payload
format and interpretation are beyond the scope of this
document.
RTP packet: A data packet consisting of the fixed RTP header, a
possibly empty list of contributing sources (see below),
and the payload data. Some underlying protocols may require
an encapsulation of the RTP packet to be defined. Typically
one packet of the underlying protocol contains a single RTP
packet, but several RTP packets MAY be contained if
permitted by the encapsulation method (see Section 11).
RTCP packet: A control packet consisting of a fixed header part
similar to that of RTP data packets, followed by structured
elements that vary depending upon the RTCP packet type. The
formats are defined in Section 6. Typically, multiple RTCP
packets are sent together as a compound RTCP packet in a
single packet of the underlying protocol; this is enabled
by the length field in the fixed header of each RTCP
packet.
Port: The "abstraction that transport protocols use to
distinguish among multiple destinations within a given host
computer. TCP/IP protocols identify ports using small
positive integers." [12] The transport selectors (TSEL)
used by the OSI transport layer are equivalent to ports.
RTP depends upon the lower-layer protocol to provide some
mechanism such as ports to multiplex the RTP and RTCP
packets of a session.
Transport address: The combination of a network address and port
that identifies a transport-level endpoint, for example an
IP address and a UDP port. Packets are transmitted from a
source transport address to a destination transport
address.
RTP media type: An RTP media type is the collection of payload
types which can be carried within a single RTP session. The
RTP Profile assigns RTP media types to RTP payload types.
RTP session: The association among a set of participants
communicating with RTP. For each participant, the session
is defined by a particular pair of destination transport
addresses (one network address plus a port pair for RTP and
RTCP). The destination transport address pair may be common
for all participants, as in the case of IP multicast, or
may be different for each, as in the case of individual
unicast network addresses and port pairs. In a multimedia
session, each medium is carried in a separate RTP session
with its own RTCP packets. The multiple RTP sessions are
distinguished by different port number pairs and/or
different multicast addresses.
Synchronization source (SSRC): The source of a stream of RTP
packets, identified by a 32-bit numeric SSRC identifier
carried in the RTP header so as not to be dependent upon
the network address. All packets from a synchronization
source form part of the same timing and sequence number
space, so a receiver groups packets by synchronization
source for playback. Examples of synchronization sources
include the sender of a stream of packets derived from a
signal source such as a microphone or a camera, or an RTP
mixer (see below). A synchronization source may change its
data format, e.g., audio encoding, over time. The SSRC
identifier is a randomly chosen value meant to be globally
unique within a particular RTP session (see Section 8). A
participant need not use the same SSRC identifier for all
the RTP sessions in a multimedia session; the binding of
the SSRC identifiers is provided through RTCP (see Section
6.5.1). If a participant generates multiple streams in one
RTP session, for example from separate video cameras, each
MUST be identified as a different SSRC.
Contributing source (CSRC): A source of a stream of RTP packets
that has contributed to the combined stream produced by an
RTP mixer (see below). The mixer inserts a list of the SSRC
identifiers of the sources that contributed to the
generation of a particular packet into the RTP header of
that packet. This list is called the CSRC list. An example
application is audio conferencing where a mixer indicates
all the talkers whose speech was combined to produce the
outgoing packet, allowing the receiver to indicate the
current talker, even though all the audio packets contain
the same SSRC identifier (that of the mixer).
End system: An application that generates the content to be sent
in RTP packets and/or consumes the content of received RTP
packets. An end system can act as one or more
synchronization sources in a particular RTP session, but
typically only one.
Mixer: An intermediate system that receives RTP packets from one
or more sources, possibly changes the data format, combines
the packets in some manner and then forwards a new RTP
packet. Since the timing among multiple input sources will
not generally be synchronized, the mixer will make timing
adjustments among the streams and generate its own timing
for the combined stream. Thus, all data packets originating
from a mixer will be identified as having the mixer as
their synchronization source.
Translator: An intermediate system that forwards RTP packets
with their synchronization source identifier intact.
Examples of translators include devices that convert
encodings without mixing, replicators from multicast to
unicast, and application-level filters in firewalls.
Monitor: An application that receives RTCP packets sent by
participants in an RTP session, in particular the reception
reports, and estimates the current quality of service for
distribution monitoring, fault diagnosis and long-term
statistics. The monitor function is likely to be built into
the application(s) participating in the session, but may
also be a separate application that does not otherwise
participate and does not send or receive the RTP data
packets (since they are on a separate port). These are
called third-party monitors. It is also acceptable for a
third-party monitor to receive the RTP data packets but not
send RTCP packets or otherwise be counted in the session.
Non-RTP means: Protocols and mechanisms that may be needed in
addition to RTP to provide a usable service. In particular,
for multimedia conferences, a control protocol may
distribute multicast addresses and keys for encryption,
negotiate the encryption algorithm to be used, and define
dynamic mappings between RTP payload type values and the
payload formats they represent for formats that do not have
a predefined payload type value. Examples of such protocols
include the Session Initiation Protocol (SIP) (RFC 2543
[13]), H.323 [14] and applications using SDP (RFC 2327
[15]), such as RTSP (RFC 2326 [16]). For simple
applications, electronic mail or a conference database may
also be used. The specification of such protocols and
mechanisms is outside the scope of this document.
4 Byte Order, Alignment, and Time Format
All integer fields are carried in network byte order, that is, most
significant byte (octet) first. This byte order is commonly known as
big-endian. The transmission order is described in detail in [3].
Unless otherwise noted, numeric constants are in decimal (base 10).
All header data is aligned to its natural length, i.e., 16-bit fields
are aligned on even offsets, 32-bit fields are aligned at offsets
divisible by four, etc. Octets designated as padding have the value
zero.
Wallclock time (absolute date and time) is represented using the
timestamp format of the Network Time Protocol (NTP), which is in
seconds relative to 0h UTC on 1 January 1900 [4]. The full resolution
NTP timestamp is a 64-bit unsigned fixed-point number with the
integer part in the first 32 bits and the fractional part in the last
32 bits. In some fields where a more compact representation is
appropriate, only the middle 32 bits are used; that is, the low 16
bits of the integer part and the high 16 bits of the fractional part.
The high 16 bits of the integer part must be determined
independently.
An implementation is not required to run the Network Time Protocol in
order to use RTP. Other time sources, or none at all, may be used
(see the description of the NTP timestamp field in Section 6.4.1).
However, running NTP may be useful for synchronizing streams
transmitted from separate hosts.
The NTP timestamp will wrap around to zero some time in the year
2036, but for RTP purposes, only differences between pairs of NTP
timestamps are used. So long as the pairs of timestamps can be
assumed to be within 68 years of each other, using modulo arithmetic
for subtractions and comparisons makes the wraparound irrelevant.
5 RTP Data Transfer Protocol
5.1 RTP Fixed Header Fields
The RTP header has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first twelve octets are present in every RTP packet, while the
list of CSRC identifiers is present only when inserted by a mixer.
The fields have the following meaning:
version (V): 2 bits
This field identifies the version of RTP. The version
defined by this specification is two (2). (The value 1 is
used by the first draft version of RTP and the value 0 is
used by the protocol initially implemented in the "vat"
audio tool.)
padding (P): 1 bit
If the padding bit is set, the packet contains one or more
additional padding octets at the end which are not part of
the payload. The last octet of the padding contains a count
of how many padding octets should be ignored, including
itself. Padding may be needed by some encryption
algorithms with fixed block sizes or for carrying several
RTP packets in a lower-layer protocol data unit.
extension (X): 1 bit
If the extension bit is set, the fixed header MUST be
followed by exactly one header extension, with a format
defined in Section 5.3.1.
CSRC count (CC): 4 bits
The CSRC count contains the number of CSRC identifiers that
follow the fixed header.
marker (M): 1 bit
The interpretation of the marker is defined by a profile.
It is intended to allow significant events such as frame
boundaries to be marked in the packet stream. A profile MAY
define additional marker bits or specify that there is no
marker bit by changing the number of bits in the payload
type field (see Section 5.3).
payload type (PT): 7 bits
This field identifies the format of the RTP payload and
determines its interpretation by the application. A profile
MAY specify a default static mapping of payload type codes
to payload formats. Additional payload type codes MAY be
defined dynamically through non-RTP means (see Section 3).
A set of default mappings for audio and video is specified
in the companion RFC XXXX [1]. An RTP source MAY change
the payload type during a session, but this field SHOULD
NOT be used for multiplexing separate media streams (see
Section 5.2).
A receiver MUST ignore packets with payload types that it
does not understand.
sequence number: 16 bits
The sequence number increments by one for each RTP data
packet sent, and may be used by the receiver to detect
packet loss and to restore packet sequence. The initial
value of the sequence number SHOULD be random
(unpredictable) to make known-plaintext attacks on
encryption more difficult, even if the source itself does
not encrypt according to the method in Section 9.1, because
the packets may flow through a translator that does.
Techniques for choosing unpredictable numbers are discussed
in [17].
timestamp: 32 bits
The timestamp reflects the sampling instant of the first
octet in the RTP data packet. The sampling instant MUST be
derived from a clock that increments monotonically and
linearly in time to allow synchronization and jitter
calculations (see Section 6.4.1). The resolution of the
clock MUST be sufficient for the desired synchronization
accuracy and for measuring packet arrival jitter (one tick
per video frame is typically not sufficient). The clock
frequency is dependent on the format of data carried as
payload and is specified statically in the profile or
payload format specification that defines the format, or
MAY be specified dynamically for payload formats defined
through non-RTP means. If RTP packets are generated
periodically, the nominal sampling instant as determined
from the sampling clock is to be used, not a reading of the
system clock. As an example, for fixed-rate audio the
timestamp clock would likely increment by one for each
sampling period. If an audio application reads blocks
covering 160 sampling periods from the input device, the
timestamp would be increased by 160 for each such block,
regardless of whether the block is transmitted in a packet
or dropped as silent.
The initial value of the timestamp SHOULD be random, as for
the sequence number. Several consecutive RTP packets will
have equal timestamps if they are (logically) generated at
once, e.g., belong to the same video frame. Consecutive RTP
packets MAY contain timestamps that are not monotonic if
the data is not transmitted in the order it was sampled, as
in the case of MPEG interpolated video frames. (The
sequence numbers of the packets as transmitted will still
be monotonic.)
SSRC: 32 bits
The SSRC field identifies the synchronization source. This
identifier SHOULD be chosen randomly, with the intent that
no two synchronization sources within the same RTP session
will have the same SSRC identifier. An example algorithm
for generating a random identifier is presented in Appendix
A.6. Although the probability of multiple sources choosing
the same identifier is low, all RTP implementations must be
prepared to detect and resolve collisions. Section 8
describes the probability of collision along with a
mechanism for resolving collisions and detecting RTP-level
forwarding loops based on the uniqueness of the SSRC
identifier. If a source changes its source transport
address, it must also choose a new SSRC identifier to avoid
being interpreted as a looped source (see Section 8.2).
CSRC list: 0 to 15 items, 32 bits each
The CSRC list identifies the contributing sources for the
payload contained in this packet. The number of identifiers
is given by the CC field. If there are more than 15
contributing sources, only 15 can be identified. CSRC
identifiers are inserted by mixers (see Section 7.1), using
the SSRC identifiers of contributing sources. For example,
for audio packets the SSRC identifiers of all sources that
were mixed together to create a packet are listed, allowing
correct talker indication at the receiver.
5.2 Multiplexing RTP Sessions
For efficient protocol processing, the number of multiplexing points
should be minimized, as described in the integrated layer processing
design principle [10]. In RTP, multiplexing is provided by the
destination transport address (network address and port number) which
define an RTP session. For example, in a teleconference composed of
audio and video media encoded separately, each medium SHOULD be
carried in a separate RTP session with its own destination transport
address.
Separate audio and video streams SHOULD NOT be carried in a single
RTP session and demultiplexed based on the payload type or SSRC
fields. Interleaving packets with different RTP media types but using
the same SSRC would introduce several problems:
1. If, say, two audio streams shared the same RTP session and
the same SSRC value, and one were to change encodings and
thus acquire a different RTP payload type, there would be
no general way of identifying which stream had changed
encodings.
2. An SSRC is defined to identify a single timing and sequence
number space. Interleaving multiple payload types would
require different timing spaces if the media clock rates
differ and would require different sequence number spaces
to tell which payload type suffered packet loss.
3. The RTCP sender and receiver reports (see Section 6.4) can
only describe one timing and sequence number space per SSRC
and do not carry a payload type field.
4. An RTP mixer would not be able to combine interleaved
streams of incompatible media into one stream.
5. Carrying multiple media in one RTP session precludes: the
use of different network paths or network resource
allocations if appropriate; reception of a subset of the
media if desired, for example just audio if video would
exceed the available bandwidth; and receiver
implementations that use separate processes for the
different media, whereas using separate RTP sessions
permits either single- or multiple-process implementations.
Using a different SSRC for each medium but sending them in the same
RTP session would avoid the first three problems but not the last
two.
5.3 Profile-Specific Modifications to the RTP Header
The existing RTP data packet header is believed to be complete for
the set of functions required in common across all the application
classes that RTP might support. However, in keeping with the ALF
design principle, the header MAY be tailored through modifications or
additions defined in a profile specification while still allowing
profile-independent monitoring and recording tools to function.
o The marker bit and payload type field carry profile-specific
information, but they are allocated in the fixed header since
many applications are expected to need them and might
otherwise have to add another 32-bit word just to hold them.
The octet containing these fields MAY be redefined by a
profile to suit different requirements, for example with a
more or fewer marker bits. If there are any marker bits, one
SHOULD be located in the most significant bit of the octet
since profile-independent monitors may be able to observe a
correlation between packet loss patterns and the marker bit.
o Additional information that is required for a particular
payload format, such as a video encoding, SHOULD be carried in
the payload section of the packet. This might be in a header
that is always present at the start of the payload section, or
might be indicated by a reserved value in the data pattern.
o If a particular class of applications needs additional
functionality independent of payload format, the profile under
which those applications operate SHOULD define additional
fixed fields to follow immediately after the SSRC field of the
existing fixed header. Those applications will be able to
quickly and directly access the additional fields while
profile-independent monitors or recorders can still process
the RTP packets by interpreting only the first twelve octets.
If it turns out that additional functionality is needed in common
across all profiles, then a new version of RTP should be defined to
make a permanent change to the fixed header.
5.3.1 RTP Header Extension
An extension mechanism is provided to allow individual
implementations to experiment with new payload-format-independent
functions that require additional information to be carried in the
RTP data packet header. This mechanism is designed so that the header
extension may be ignored by other interoperating implementations that
have not been extended.
Note that this header extension is intended only for limited use.
Most potential uses of this mechanism would be better done another
way, using the methods described in the previous section. For
example, a profile-specific extension to the fixed header is less
expensive to process because it is not conditional nor in a variable
location. Additional information required for a particular payload
format SHOULD NOT use this header extension, but SHOULD be carried in
the payload section of the packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| defined by profile | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extension |
| .... |
If the X bit in the RTP header is one, a variable-length header
extension MUST be appended to the RTP header, following the CSRC list
if present. The header extension contains a 16-bit length field that
counts the number of 32-bit words in the extension, excluding the
four-octet extension header (therefore zero is a valid length). Only
a single extension can be appended to the RTP data header. To allow
multiple interoperating implementations to each experiment
independently with different header extensions, or to allow a
particular implementation to experiment with more than one type of
header extension, the first 16 bits of the header extension are left
open for distinguishing identifiers or parameters. The format of
these 16 bits is to be defined by the profile specification under
which the implementations are operating. This RTP specification does
not define any header extensions itself.
6 RTP Control Protocol -- RTCP
The RTP control protocol (RTCP) is based on the periodic transmission
of control packets to all participants in the session, using the same
distribution mechanism as the data packets. The underlying protocol
MUST provide multiplexing of the data and control packets, for
example using separate port numbers with UDP. RTCP performs four
functions:
1. The primary function is to provide feedback on the quality
of the data distribution. This is an integral part of the
RTP's role as a transport protocol and is related to the
flow and congestion control functions of other transport
protocols (see Section 10 on the requirement for congestion
control). The feedback may be directly useful for control
of adaptive encodings [18,19], but experiments with IP
multicasting have shown that it is also critical to get
feedback from the receivers to diagnose faults in the
distribution. Sending reception feedback reports to all
participants allows one who is observing problems to
evaluate whether those problems are local or global. With a
distribution mechanism like IP multicast, it is also
possible for an entity such as a network service provider
who is not otherwise involved in the session to receive the
feedback information and act as a third-party monitor to
diagnose network problems. This feedback function is
performed by the RTCP sender and receiver reports,
described below in Section 6.4.
2. RTCP carries a persistent transport-level identifier for an
RTP source called the canonical name or CNAME, Section
6.5.1. Since the SSRC identifier may change if a conflict
is discovered or a program is restarted, receivers require
the CNAME to keep track of each participant. Receivers may
also require the CNAME to associate multiple data streams
from a given participant in a set of related RTP sessions,
for example to synchronize audio and video. Inter-media
synchronization also requires the NTP and RTP timestamps
included in RTCP packets by data senders.
3. The first two functions require that all participants send
RTCP packets, therefore the rate must be controlled in
order for RTP to scale up to a large number of
participants. By having each participant send its control
packets to all the others, each can independently observe
the number of participants. This number is used to
calculate the rate at which the packets are sent, as
explained in Section 6.2.
4. A fourth, OPTIONAL function is to convey minimal session
control information, for example participant identification
to be displayed in the user interface. This is most likely
to be useful in "loosely controlled" sessions where
participants enter and leave without membership control or
parameter negotiation. RTCP serves as a convenient channel
to reach all the participants, but it is not necessarily
expected to support all the control communication
requirements of an application. A higher-level session
control protocol, which is beyond the scope of this
document, may be needed.
Functions 1-3 SHOULD be used in all environments, but particularly in
the IP multicast environment. RTP application designers SHOULD avoid
mechanisms that can only work in unicast mode and will not scale to
larger numbers. Transmission of RTCP MAY be controlled separately for
senders and receivers, as described in Section 6.2, for cases such as
unidirectional links where feedback from receivers is not possible.
6.1 RTCP Packet Format
This specification defines several RTCP packet types to carry a
variety of control information:
SR: Sender report, for transmission and reception statistics
from participants that are active senders
RR: Receiver report, for reception statistics from participants
that are not active senders and in combination with SR for
active senders reporting on more than 31 sources
SDES: Source description items, including CNAME
BYE: Indicates end of participation
APP: Application specific functions
Each RTCP packet begins with a fixed part similar to that of RTP data
packets, followed by structured elements that MAY be of variable
length according to the packet type but MUST end on a 32-bit
boundary. The alignment requirement and a length field in the fixed
part of each packet are included to make RTCP packets "stackable".
Multiple RTCP packets can be concatenated without any intervening
separators to form a compound RTCP packet that is sent in a single
packet of the lower layer protocol, for example UDP. There is no
explicit count of individual RTCP packets in the compound packet
since the lower layer protocols are expected to provide an overall
length to determine the end of the compound packet.
Each individual RTCP packet in the compound packet may be processed
independently with no requirements upon the order or combination of
packets. However, in order to perform the functions of the protocol,
the following constraints are imposed:
o Reception statistics (in SR or RR) should be sent as often as
bandwidth constraints will allow to maximize the resolution of
the statistics, therefore each periodically transmitted
compound RTCP packet MUST include a report packet.
o New receivers need to receive the CNAME for a source as soon
as possible to identify the source and to begin associating
media for purposes such as lip-sync, so each compound RTCP
packet MUST also include the SDES CNAME except when the
compound RTCP packet is split for partial encryption as
described in Section 9.1.
o The number of packet types that may appear first in the
compound packet needs to be limited to increase the number of
constant bits in the first word and the probability of
successfully validating RTCP packets against misaddressed RTP
data packets or other unrelated packets.
Thus, all RTCP packets MUST be sent in a compound packet of at least
two individual packets, with the following format:
Encryption prefix: If and only if the compound packet is to be
encrypted according to the method in Section 9.1, it MUST
be prefixed by a random 32-bit quantity redrawn for every
compound packet transmitted. If padding is required for
the encryption, it MUST be added to the last packet of the
compound packet.
SR or RR: The first RTCP packet in the compound packet MUST
always be a report packet to facilitate header validation
as described in Appendix A.2. This is true even if no data
has been sent or received, in which case an empty RR MUST
be sent, and even if the only other RTCP packet in the
compound packet is a BYE.
Additional RRs: If the number of sources for which reception
statistics are being reported exceeds 31, the number that
will fit into one SR or RR packet, then additional RR
packets SHOULD follow the initial report packet.
SDES: An SDES packet containing a CNAME item MUST be included
in each compound RTCP packet, except as noted in Section
9.1. Other source description items MAY optionally be
included if required by a particular application, subject
to bandwidth constraints (see Section 6.3.9).
BYE or APP: Other RTCP packet types, including those yet to be
defined, MAY follow in any order, except that BYE SHOULD be
the last packet sent with a given SSRC/CSRC. Packet types
MAY appear more than once.
An individual RTP participant SHOULD send only one compound RTCP
packet per report interval in order for the RTCP bandwidth per
participant to be estimated correctly (see Section 6.2), except when
the compound RTCP packet is split for partial encryption as described
in Section 9.1. If there are too many sources to fit all the
necessary RR packets into one compound RTCP packet without exceeding
the maximum transmission unit (MTU) of the network path, then only
the subset that will fit into one MTU SHOULD be included in each
interval. The subsets SHOULD be selected round-robin across multiple
intervals so that all sources are reported.
It is RECOMMENDED that translators and mixers combine individual RTCP
packets from the multiple sources they are forwarding into one
compound packet whenever feasible in order to amortize the packet
overhead (see Section 7). An example RTCP compound packet as might be
produced by a mixer is shown in Fig. 1. If the overall length of a
compound packet would exceed the MTU of the network path, it SHOULD
be segmented into multiple shorter compound packets to be transmitted
in separate packets of the underlying protocol. This does not impair
the RTCP bandwidth estimation because each compound packet represents
at least one distinct participant. Note that each of the compound
packets MUST begin with an SR or RR packet.
An implementation SHOULD ignore incoming RTCP packets with types
unknown to it. Additional RTCP packet types may be registered with
the Internet Assigned Numbers Authority (IANA) as described in
Section 14.
6.2 RTCP Transmission Interval
RTP is designed to allow an application to scale automatically over
session sizes ranging from a few participants to thousands. For
example, in an audio conference the data traffic is inherently self-
limiting because only one or two people will speak at a time, so with
multicast distribution the data rate on any given link remains
relatively constant independent of the number of participants.
However, the control traffic is not self-limiting. If the reception
reports from each participant were sent at a constant rate, the
control traffic would grow linearly with the number of participants.
Therefore, the rate must be scaled down by dynamically calculating
if encrypted: random 32-bit integer
|
|[--------- packet --------][---------- packet ----------][-packet-]
|
| receiver chunk chunk
V reports item item item item
--------------------------------------------------------------------
R[SR #sendinfo #site1#site2][SDES #CNAME PHONE #CNAME LOC][BYE##why]
--------------------------------------------------------------------
| |
|<----------------------- compound packet ----------------------->|
|<-------------------------- UDP packet ------------------------->|
#: SSRC/CSRC identifier
Figure 1: Example of an RTCP compound packet
the interval between RTCP packet transmissions.
For each session, it is assumed that the data traffic is subject to
an aggregate limit called the "session bandwidth" to be divided among
the participants. This bandwidth might be reserved and the limit
enforced by the network. If there is no reservation, there may be
other constraints, depending on the environment, that establish the
"reasonable" maximum for the session to use, and that would be the
session bandwidth. The session bandwidth may be chosen based on some
cost or a priori knowledge of the available network bandwidth for the
session. It is somewhat independent of the media encoding, but the
encoding choice may be limited by the session bandwidth. Often, the
session bandwidth is the sum of the nominal bandwidths of the senders
expected to be concurrently active. For teleconference audio, this
number would typically be one sender's bandwidth. For layered
encodings, each layer is a separate RTP session with its own session
bandwidth parameter.
The session bandwidth parameter is expected to be supplied by a
session management application when it invokes a media application,
but media applications MAY set a default based on the single-sender
data bandwidth for the encoding selected for the session. The
application MAY also enforce bandwidth limits based on multicast
scope rules or other criteria. All participants MUST use the same
value for the session bandwidth so that the same RTCP interval will
be calculated.
Bandwidth calculations for control and data traffic include lower-
layer transport and network protocols (e.g., UDP and IP) since that
is what the resource reservation system would need to know. The
application can also be expected to know which of these protocols are
in use. Link level headers are not included in the calculation since
the packet will be encapsulated with different link level headers as
it travels.
The control traffic should be limited to a small and known fraction
of the session bandwidth: small so that the primary function of the
transport protocol to carry data is not impaired; known so that the
control traffic can be included in the bandwidth specification given
to a resource reservation protocol, and so that each participant can
independently calculate its share. It is RECOMMENDED that the
fraction of the session bandwidth allocated to RTCP be fixed at 5%.
It is also RECOMMENDED that 1/4 of the RTCP bandwidth be dedicated to
participants that are sending data so that in sessions with a large
number of receivers but a small number of senders, newly joining
participants will more quickly receive the CNAME for the sending
sites. When the proportion of senders is greater than 1/4 of the
participants, the senders get their proportion of the full RTCP
bandwidth. While the values of these and other constants in the
interval calculation are not critical, all participants in the
session MUST use the same values so the same interval will be
calculated. Therefore, these constants SHOULD be fixed for a
particular profile.
A profile MAY specify that the control traffic bandwidth may be a
separate parameter of the session rather than a strict percentage of
the session bandwidth. Using a separate parameter allows rate-
adaptive applications to set an RTCP bandwidth consistent with a
"typical" data bandwidth that is lower than the maximum bandwidth
specified by the session bandwidth parameter.
The profile MAY further specify that the control traffic bandwidth
may be divided into two separate session parameters for those
participants which are active data senders and those which are not.
Following the recommendation that 1/4 of the RTCP bandwidth be
dedicated to data senders, the RECOMMENDED default values for these
two parameters would be 1.25% and 3.75%, respectively. When the
proportion of senders is greater than 1/4 of the participants, the
senders get their proportion of the sum of these parameters. Using
two parameters allows RTCP reception reports to be turned off
entirely for a particular session by setting the RTCP bandwidth for
non-data-senders to zero while keeping the RTCP bandwidth for data
senders non-zero so that sender reports can still be sent for inter-
media synchronization. This may be appropriate for systems operating
on unidirectional links or for sessions that don't require feedback
on the quality of reception.
The calculated interval between transmissions of compound RTCP
packets SHOULD also have a lower bound to avoid having bursts of
packets exceed the allowed bandwidth when the number of participants
is small and the traffic isn't smoothed according to the law of large
numbers. It also keeps the report interval from becoming too small
during transient outages like a network partition such that
adaptation is delayed when the partition heals. At application
startup, a delay SHOULD be imposed before the first compound RTCP
packet is sent to allow time for RTCP packets to be received from
other participants so the report interval will converge to the
correct value more quickly. This delay MAY be set to half the
minimum interval to allow quicker notification that the new
participant is present. The RECOMMENDED value for a fixed minimum
interval is 5 seconds.
An implementation MAY scale the minimum RTCP interval to a smaller
value inversely proportional to the session bandwidth parameter with
the following limitations:
o For multicast sessions, only active data senders MAY use the
reduced minimum value to calculate the interval for
transmission of compound RTCP packets.
o For unicast sessions, the reduced value MAY be used by
participants that are not active data senders as well, and the
delay before sending the initial compound RTCP packet MAY be
zero.
o For all sessions, the fixed minimum SHOULD be used when
calculating the participant timeout interval (see Section
6.3.5) so that implementations which do not use the reduced
value for transmitting RTCP packets are not timed out by other
participants prematurely.
o The RECOMMENDED value for the reduced minimum in seconds is
360 divided by the session bandwidth in kilobits/second. This
minimum is smaller than 5 seconds for bandwidths greater than
72 kb/s.
The algorithm described in Section 6.3 and Appendix A.7 was designed
to meet the goals outlined in this section. It calculates the
interval between sending compound RTCP packets to divide the allowed
control traffic bandwidth among the participants. This allows an
application to provide fast response for small sessions where, for
example, identification of all participants is important, yet
automatically adapt to large sessions. The algorithm incorporates the
following characteristics:
o The calculated interval between RTCP packets scales linearly
with the number of members in the group. It is this linear
factor which allows for a constant amount of control traffic
when summed across all members.
o The interval between RTCP packets is varied randomly over the
range [0.5,1.5] times the calculated interval to avoid
unintended synchronization of all participants [20]. The
first RTCP packet sent after joining a session is also delayed
by a random variation of half the minimum RTCP interval.
o A dynamic estimate of the average compound RTCP packet size is
calculated, including all those received and sent, to
automatically adapt to changes in the amount of control
information carried.
o Since the calculated interval is dependent on the number of
observed group members, there may be undesirable startup
effects when a new user joins an existing session, or many
users simultaneously join a new session. These new users will
initially have incorrect estimates of the group membership,
and thus their RTCP transmission interval will be too short.
This problem can be significant if many users join the session
simultaneously. To deal with this, an algorithm called "timer
reconsideration" is employed. This algorithm implements a
simple back-off mechanism which causes users to hold back RTCP
packet transmission if the group sizes are increasing.
o When users leave a session, either with a BYE or by timeout,
the group membership decreases, and thus the calculated
interval should decrease. A "reverse reconsideration"
algorithm is used to allow members to more quickly reduce
their intervals in response to group membership decreases.
o BYE packets are given different treatment than other RTCP
packets. When a user leaves a group, and wishes to send a BYE
packet, it may do so before its next scheduled RTCP packet.
However, transmission of BYE's follows a back-off algorithm
which avoids floods of BYE packets should a large number of
members simultaneously leave the session.
This algorithm may be used for sessions in which all participants are
allowed to send. In that case, the session bandwidth parameter is the
product of the individual sender's bandwidth times the number of
participants, and the RTCP bandwidth is 5% of that.
Details of the algorithm's operation are given in the sections that
follow. Appendix A.7 gives an example implementation.
6.2.1 Maintaining the number of session members
Calculation of the RTCP packet interval depends upon an estimate of
the number of sites participating in the session. New sites are added
to the count when they are heard, and an entry for each SHOULD be
created in a table indexed by the SSRC or CSRC identifier (see
Section 8.2) to keep track of them. New entries MAY be considered not
valid until multiple packets carrying the new SSRC have been received
(see Appendix A.1), or until an SDES RTCP packet containing a CNAME
for that SSRC has been received. Entries MAY be deleted from the
table when an RTCP BYE packet with the corresponding SSRC identifier
is received, except that some straggler data packets might arrive
after the BYE and cause the entry to be recreated. Instead, the entry
SHOULD be marked as having received a BYE and then deleted after an
appropriate delay.
A participant MAY mark another site inactive, or delete it if not yet
valid, if no RTP or RTCP packet has been received for a small number
of RTCP report intervals (5 is RECOMMENDED). This provides some
robustness against packet loss. All sites must have the same value
for this multiplier and must calculate roughly the same value for the
RTCP report interval in order for this timeout to work properly.
Therefore, this multiplier SHOULD be fixed for a particular profile.
For sessions with a very large number of participants, it may be
impractical to maintain a table to store the SSRC identifier and
state information for all of them. An implementation MAY use SSRC
sampling, as described in [21], to reduce the storage requirements.
An implementation MAY use any other algorithm with similar
performance. A key requirement is that any algorithm considered
SHOULD NOT substantially underestimate the group size, although it
MAY overestimate.
6.3 RTCP Packet Send and Receive Rules
The rules for how to send, and what to do when receiving an RTCP
packet are outlined here. An implementation that allows operation in
a multicast environment or a multipoint unicast environment MUST meet
the requirements in Section 6.2. Such an implementation MAY use the
algorithm defined in this section to meet those requirements, or MAY
use some other algorithm so long as it provides equivalent or better
performance. An implementation which is constrained to two-party
unicast operation SHOULD still use randomization of the RTCP
transmission interval to avoid unintended synchronization of multiple
instances operating in the same environment, but MAY omit the "timer
reconsideration" and "reverse reconsideration" algorithms in Sections
6.3.3, 6.3.6 and 6.3.7.
To execute these rules, a session participant must maintain several
pieces of state:
tp: the last time an RTCP packet was transmitted;
tc: the current time;
tn: the next scheduled transmission time of an RTCP packet;
pmembers: the estimated number of session members at the time tn
was last recomputed;
members: the most current estimate for the number of session
members;
senders: the most current estimate for the number of senders in
the session;
rtcp_bw: The target RTCP bandwidth, i.e., the total bandwidth
that will be used for RTCP packets by all members of this
session, in octets per second. This will be a specified
fraction of the "session bandwidth" parameter supplied to
the application at startup.
we_sent: Flag that is true if the application has sent data
since the 2nd previous RTCP report was transmitted.
avg_rtcp_size: The average compound RTCP packet size, in octets,
over all RTCP packets sent and received by this
participant. The size includes lower-layer transport and
network protocol headers (e.g., UDP and IP) as explained in
Section 6.2.
initial: Flag that is true if the application has not yet sent
an RTCP packet.
Many of these rules make use of the "calculated interval" between
packet transmissions. This interval is described in the following
section.
6.3.1 Computing the RTCP transmission interval
To maintain scalability, the average interval between packets from a
session participant should scale with the group size. This interval
is called the calculated interval. It is obtained by combining a
number of the pieces of state described above. The calculated
interval T is then determined as follows:
1. If there are any senders (senders > 0) in the session, but
the number of senders is less than 25% of the membership
(members), the interval depends on whether the participant
is a sender or not (based on the value of we_sent). If the
participant is a sender (we_sent true), the constant C is
set to the average RTCP packet size (avg_rtcp_size) divided
by 25% of the RTCP bandwidth (rtcp_bw), and the constant n
is set to the number of senders. If we_sent is not true,
the constant C is set to the average RTCP packet size
divided by 75% of the RTCP bandwidth. The constant n is set
to the number of receivers (members - senders). If the
number of senders is greater than 25%, senders and
receivers are treated together. The constant C is set to
the average RTCP packet size divided by the total RTCP
bandwidth and n is set to the total number of members.
2. If the participant has not yet sent an RTCP packet (the
variable initial is true), the constant Tmin is set to 2.5
seconds, else it is set to 5 seconds.
3. The deterministic calculated interval Td is set to
max(Tmin, n*C).
4. The calculated interval T is set to a number uniformly
distributed between 0.5 and 1.5 times the deterministic
calculated interval.
5. The resulting value of T is divided by e-3/2=1.21828 to
compensate for the fact that the timer reconsideration
algorithm converges to a value of the RTCP bandwidth below
the intended average.
This procedure results in an interval which is random, but which, on
average, gives at least 25% of the RTCP bandwidth to senders and the
rest to receivers. If the senders constitute more than one quarter of
the membership, this procedure splits the bandwidth equally among all
participants, on average.
6.3.2 Initialization
Upon joining the session, the participant initializes tp to 0, tc to
0, senders to 0, pmembers to 1, members to 1, we_sent to false,
rtcp_bw to the specified fraction of the session bandwidth, initial
to true, and avg_rtcp_size to the probable size of the first RTCP
packet that the application will later construct. The calculated
interval T is then computed, and the first packet is scheduled for
time tn = T. This means that a transmission timer is set which
expires at time T. Note that an application MAY use any desired
approach for implementing this timer.
The participant adds its own SSRC to the member table.
6.3.3 Receiving an RTP or non-BYE RTCP packet
When an RTP or RTCP packet is received from a participant whose SSRC
is not in the member table, the SSRC is added to the table, and the
value for members is updated once the participant has been validated
as described in Section 6.2.1. The same processing occurs for each
CSRC in a validated RTP packet.
When an RTP packet is received from a participant whose SSRC is not
in the sender table, the SSRC is added to the table, and the value
for senders is updated.
For each compound RTCP packet received, the value of avg_rtcp_size is
updated: avg_rtcp_size = (1/16)*packet_size + (15/16)* avg_rtcp_size,
where packet_size is the size of the RTCP packet just received.
6.3.4 Receiving an RTCP BYE packet
Except as described in Section 6.3.7 for the case when an RTCP BYE is
to be transmitted, if the received packet is an RTCP BYE packet, the
SSRC is checked against the member table. If present, the entry is
removed from the table, and the value for members is updated. The
SSRC is then checked against the sender table. If present, the entry
is removed from the table, and the value for senders is updated.
Furthermore, to make the transmission rate of RTCP packets more
adaptive to changes in group membership, the following "reverse
reconsideration" algorithm SHOULD be executed when a BYE packet is
received that reduces members to a value less than pmembers:
o The value for tn is updated according to the following
formula: tn = tc + (members/pmembers)(tn - tc).
o The value for tp is updated according the following formula:
tp = tc - (members/pmembers)(tc - tp).
o The next RTCP packet is rescheduled for transmission at time
tn, which is now earlier.
o The value of pmembers is set equal to members.
This algorithm does not prevent the group size estimate from
incorrectly dropping to zero for a short time due to premature
timeouts when most participants of a large session leave at once but
some remain. The algorithm does make the estimate return to the
correct value more rapidly. This situation is unusual enough and the
consequences are sufficiently harmless that this problem is deemed
only a secondary concern.
6.3.5 Timing Out an SSRC
At occasional intervals, the participant MUST check to see if any of
the other participants time out. To do this, the participant computes
the deterministic (without the randomization factor) calculated
interval Td for a receiver, that is, with we_sent false. Any other
session member who has not sent an RTP or RTCP packet since time tc -
MTd (M is the timeout multiplier, and defaults to 5) is timed out.
This means that its SSRC is removed from the member list, and members
is updated. A similar check is performed on the sender list. Any
member on the sender list who has not sent an RTP packet since time
tc - 2T (within the last two RTCP report intervals) is removed from
the sender list, and senders is updated.
If any members time out, the reverse reconsideration algorithm
described in Section 6.3.4 SHOULD be performed.
The participant MUST perform this check at least once per RTCP
transmission interval.
6.3.6 Expiration of transmission timer
When the packet transmission timer expires, the participant performs
the following operations:
o The transmission interval T is computed as described in
Section 6.3.1, including the randomization factor.
o If tp + T is less than or equal to tc, an RTCP packet is
transmitted. tp is set to tc, then another value for T is
calculated as in the previous step and tn is set to tc + T.
The transmission timer is set to expire again at time tn. If
tp + T is greater than tc, tn is set to tp + T. No RTCP packet
is transmitted. The transmission timer is set to expire at
time tn.
o pmembers is set to members.
If an RTCP packet is transmitted, the value of initial is set to
FALSE. Furthermore, the value of avg_rtcp_size is updated:
avg_rtcp_size = (1/16)*packet_size + (15/16)* avg_rtcp_size, where
packet_size is the size of the RTCP packet just transmitted.
6.3.7 Transmitting a BYE packet
When a participant wishes to leave a session, a BYE packet is
transmitted to inform the other participants of the event. In order
to avoid a flood of BYE packets when many participants leave the
system, a participant MUST execute the following algorithm if the
number of members is more than 50 when the participant chooses to
leave. This algorithm usurps the normal role of the members variable
to count BYE packets instead:
o When the participant decides to leave the system, tp is reset
to tc, the current time, members and pmembers are initialized
to 1, initial is set to 1, we_sent is set to false, senders is
set to 0, and avg_rtcp_size is set to the size of the compound
BYE packet. The calculated interval T is computed. The BYE
packet is then scheduled for time tn = tc + T.
o Every time a BYE packet from another participant is received,
members is incremented by 1 regardless of whether that
participant exists in the member table or not, and when SSRC
sampling is in use, regardless of whether or not the BYE SSRC
would be included in the sample. members is NOT incremented
when other RTCP packets or RTP packets are received, but only
for BYE packets. Similarly, avg_rtcp_size is updated only for
received BYE packets. senders is NOT updated when RTP packets
arrive; it remains 0.
o Transmission of the BYE packet then follows the rules for
transmitting a regular RTCP packet, as above.
This allows BYE packets to be sent right away, yet controls their
total bandwidth usage. In the worst case, this could cause RTCP
control packets to use twice the bandwidth as normal (10%) -- 5% for
non BYE RTCP packets and 5% for BYE.
A participant that does not want to wait for the above mechanism to
allow transmission of a BYE packet MAY leave the group without
sending a BYE at all. That participant will eventually be timed out
by the other group members.
If the group size estimate members is less than 50 when the
participant decides to leave, the participant MAY send a BYE packet
immediately. Alternatively, the participant MAY choose to execute
the above BYE backoff algorithm.
In either case, a participant which never sent an RTP or RTCP packet
MUST NOT send a BYE packet when they leave the group.
6.3.8 Updating we_sent
The variable we_sent contains true if the participant has sent an RTP
packet recently, false otherwise. This determination is made by using
the same mechanisms as for managing the set of other participants
listed in the senders table. If the participant sends an RTP packet
when we_sent is false, it adds itself to the sender table and sets
we_sent to true. The reverse reconsideration algorithm described in
Section 6.3.4 SHOULD be performed to possibly reduce the delay before
sending an SR packet. Every time another RTP packet is sent, the
time of transmission of that packet is maintained in the table. The
normal sender timeout algorithm is then applied to the participant --
if an RTP packet has not been transmitted since time tc - 2T, the
participant removes itself from the sender table, decrements the
sender count, and sets we_sent to false.
6.3.9 Allocation of source description bandwidth
This specification defines several source description (SDES) items in
addition to the mandatory CNAME item, such as NAME (personal name)
and EMAIL (email address). It also provides a means to define new
application-specific RTCP packet types. Applications should exercise
caution in allocating control bandwidth to this additional
information because it will slow down the rate at which reception
reports and CNAME are sent, thus impairing the performance of the
protocol. It is RECOMMENDED that no more than 20% of the RTCP
bandwidth allocated to a single participant be used to carry the
additional information. Furthermore, it is not intended that all
SDES items will be included in every application. Those that are
included SHOULD be assigned a fraction of the bandwidth according to
their utility. Rather than estimate these fractions dynamically, it
is recommended that the percentages be translated statically into
report interval counts based on the typical length of an item.
For example, an application may be designed to send only CNAME, NAME
and EMAIL and not any others. NAME might be given much higher
priority than EMAIL because the NAME would be displayed continuously
in the application's user interface, whereas EMAIL would be displayed
only when requested. At every RTCP interval, an RR packet and an SDES
packet with the CNAME item would be sent. For a small session
operating at the minimum interval, that would be every 5 seconds on
the average. Every third interval (15 seconds), one extra item would
be included in the SDES packet. Seven out of eight times this would
be the NAME item, and every eighth time (2 minutes) it would be the
EMAIL item.
When multiple applications operate in concert using cross-application
binding through a common CNAME for each participant, for example in a
multimedia conference composed of an RTP session for each medium, the
additional SDES information MAY be sent in only one RTP session. The
other sessions would carry only the CNAME item. In particular, this
approach should be applied to the multiple sessions of a layered
encoding scheme (see Section 2.4).
6.4 Sender and Receiver Reports
RTP receivers provide reception quality feedback using RTCP report
packets which may take one of two forms depending upon whether or not
the receiver is also a sender. The only difference between the sender
report (SR) and receiver report (RR) forms, besides the packet type
code, is that the sender report includes a 20-byte sender information
section for use by active senders. The SR is issued if a site has
sent any data packets during the interval since issuing the last
report or the previous one, otherwise the RR is issued.
Both the SR and RR forms include zero or more reception report
blocks, one for each of the synchronization sources from which this
receiver has received RTP data packets since the last report. Reports
are not issued for contributing sources listed in the CSRC list. Each
reception report block provides statistics about the data received
from the particular source indicated in that block. Since a maximum
of 31 reception report blocks will fit in an SR or RR packet,
additional RR packets SHOULD be stacked after the initial SR or RR
packet as needed to contain the reception reports for all sources
heard during the interval since the last report. If there are too
many sources to fit all the necessary RR packets into one compound
RTCP packet without exceeding the MTU of the network path, then only
the subset that will fit into one MTU SHOULD be included in each
interval. The subsets SHOULD be selected round-robin across multiple
intervals so that all sources are reported.
The next sections define the formats of the two reports, how they may
be extended in a profile-specific manner if an application requires
additional feedback information, and how the reports may be used.
Details of reception reporting by translators and mixers is given in
Section 7.
6.4.1 SR: Sender report RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| RC | PT=SR=200 | length | header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| NTP timestamp, most significant word | sender
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ info
| NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sender's packet count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sender's octet count |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
| fraction lost | cumulative number of packets lost | 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_2 (SSRC of second source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
: ... : 2
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| profile-specific extensions |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The sender report packet consists of three sections, possibly
followed by a fourth profile-specific extension section if defined.
The first section, the header, is 8 octets long. The fields have the
following meaning:
version (V): 2 bits
Identifies the version of RTP, which is the same in RTCP
packets as in RTP data packets. The version defined by this
specification is two (2).
padding (P): 1 bit
If the padding bit is set, this individual RTCP packet
contains some additional padding octets at the end which
are not part of the control information but are included in
the length field. The last octet of the padding is a count
of how many padding octets should be ignored, including
itself (it will be a multiple of four). Padding may be
needed by some encryption algorithms with fixed block
sizes. In a compound RTCP packet, padding is only required
on one individual packet because the compound packet is
encrypted as a whole for the method in Section 9.1. Thus,
padding MUST only be added to the last individual packet,
and if padding is added to that packet, the padding bit
MUST be set only on that packet. This convention aids the
header validity checks described in Appendix A.2 and allows
detection of packets from some early implementations that
incorrectly set the padding bit on the first individual
packet and add padding to the last individual packet.
reception report count (RC): 5 bits
The number of reception report blocks contained in this
packet. A value of zero is valid.
packet type (PT): 8 bits
Contains the constant 200 to identify this as an RTCP SR
packet.
length: 16 bits
The length of this RTCP packet in 32-bit words minus one,
including the header and any padding. (The offset of one
makes zero a valid length and avoids a possible infinite
loop in scanning a compound RTCP packet, while counting
32-bit words avoids a validity check for a multiple of 4.)
SSRC: 32 bits
The synchronization source identifier for the originator of
this SR packet.
The second section, the sender information, is 20 octets long and is
present in every sender report packet. It summarizes the data
transmissions from this sender. The fields have the following
meaning:
NTP timestamp: 64 bits
Indicates the wallclock time (see Section 4) when this
report was sent so that it may be used in combination with
timestamps returned in reception reports from other
receivers to measure round-trip propagation to those
receivers. Receivers should expect that the measurement
accuracy of the timestamp may be limited to far less than
the resolution of the NTP timestamp. The measurement
uncertainty of the timestamp is not indicated as it may not
be known. On a system that has no notion of wallclock time
but does have some system-specific clock such as "system
uptime", a sender MAY use that clock as a reference to
calculate relative NTP timestamps. It is important to
choose a commonly used clock so that if separate
implementations are used to produce the individual streams
of a multimedia session, all implementations will use the
same clock. Until the year 2036, relative and absolute
timestamps will differ in the high bit so (invalid)
comparisons will show a large difference; by then one hopes
relative timestamps will no longer be needed. A sender
that has no notion of wallclock or elapsed time MAY set the
NTP timestamp to zero.
RTP timestamp: 32 bits
Corresponds to the same time as the NTP timestamp (above),
but in the same units and with the same random offset as
the RTP timestamps in data packets. This correspondence may
be used for intra- and inter-media synchronization for
sources whose NTP timestamps are synchronized, and may be
used by media-independent receivers to estimate the nominal
RTP clock frequency. Note that in most cases this timestamp
will not be equal to the RTP timestamp in any adjacent data
packet. Rather, it MUST be calculated from the
corresponding NTP timestamp using the relationship between
the RTP timestamp counter and real time as maintained by
periodically checking the wallclock time at a sampling
instant.
sender's packet count: 32 bits
The total number of RTP data packets transmitted by the
sender since starting transmission up until the time this
SR packet was generated. The count SHOULD be reset if the
sender changes its SSRC identifier.
sender's octet count: 32 bits
The total number of payload octets (i.e., not including
header or padding) transmitted in RTP data packets by the
sender since starting transmission up until the time this
SR packet was generated. The count SHOULD be reset if the
sender changes its SSRC identifier. This field can be used
to estimate the average payload data rate.
The third section contains zero or more reception report blocks
depending on the number of other sources heard by this sender since
the last report. Each reception report block conveys statistics on
the reception of RTP packets from a single synchronization source.
Receivers SHOULD NOT carry over statistics when a source changes its
SSRC identifier due to a collision. These statistics are:
SSRC_n (source identifier): 32 bits
The SSRC identifier of the source to which the information
in this reception report block pertains.
fraction lost: 8 bits
The fraction of RTP data packets from source SSRC_n lost
since the previous SR or RR packet was sent, expressed as a
fixed point number with the binary point at the left edge
of the field. (That is equivalent to taking the integer
part after multiplying the loss fraction by 256.) This
fraction is defined to be the number of packets lost
divided by the number of packets expected, as defined in
the next paragraph. An implementation is shown in Appendix
A.3. If the loss is negative due to duplicates, the
fraction lost is set to zero. Note that a receiver cannot
tell whether any packets were lost after the last one
received, and that there will be no reception report block
issued for a source if all packets from that source sent
during the last reporting interval have been lost.
cumulative number of packets lost: 24 bits
The total number of RTP data packets from source SSRC_n
that have been lost since the beginning of reception. This
number is defined to be the number of packets expected less
the number of packets actually received, where the number
of packets received includes any which are late or
duplicates. Thus packets that arrive late are not counted
as lost, and the loss may be negative if there are
duplicates. The number of packets expected is defined to
be the extended last sequence number received, as defined
next, less the initial sequence number received. This may
be calculated as shown in Appendix A.3.
extended highest sequence number received: 32 bits
The low 16 bits contain the highest sequence number
received in an RTP data packet from source SSRC_n, and the
most significant 16 bits extend that sequence number with
the corresponding count of sequence number cycles, which
may be maintained according to the algorithm in Appendix
A.1. Note that different receivers within the same session
will generate different extensions to the sequence number
if their start times differ significantly.
interarrival jitter: 32 bits
An estimate of the statistical variance of the RTP data
packet interarrival time, measured in timestamp units and
expressed as an unsigned integer. The interarrival jitter J
is defined to be the mean deviation (smoothed absolute
value) of the difference D in packet spacing at the
receiver compared to the sender for a pair of packets. As
shown in the equation below, this is equivalent to the
difference in the "relative transit time" for the two
packets; the relative transit time is the difference
between a packet's RTP timestamp and the receiver's clock
at the time of arrival, measured in the same units.
If Si is the RTP timestamp from packet i, and Ri is the
time of arrival in RTP timestamp units for packet i, then
for two packets i and j, D may be expressed as D(i,j) =
(R_j - R_i) - (S_j - S_i) = (R_j - S_j) - (R_i - S_i)
The interarrival jitter SHOULD be calculated continuously
as each data packet i is received from source SSRC_n, using
this difference D for that packet and the previous packet
i-1 in order of arrival (not necessarily in sequence),
according to the formula J_i = J_i-1 + (|D(i-1,i)| - J_i-
1)/16
Whenever a reception report is issued, the current value of
J is sampled.
The jitter calculation MUST conform to the formula
specified here in order to allow profile-independent
monitors to make valid interpretations of reports coming
from different implementations. This algorithm is the
optimal first-order estimator and the gain parameter 1/16
gives a good noise reduction ratio while maintaining a
reasonable rate of convergence [22]. A sample
implementation is shown in Appendix A.8. See Section 6.4.4
for a discussion of the effects of varying packet duration
and delay before transmission.
last SR timestamp (LSR): 32 bits
The middle 32 bits out of 64 in the NTP timestamp (as
explained in Section 4) received as part of the most recent
RTCP sender report (SR) packet from source SSRC_n. If no SR
has been received yet, the field is set to zero.
delay since last SR (DLSR): 32 bits
The delay, expressed in units of 1/65536 seconds, between
receiving the last SR packet from source SSRC_n and sending
this reception report block. If no SR packet has been
received yet from SSRC_n, the DLSR field is set to zero.
Let SSRC_r denote the receiver issuing this receiver
report. Source SSRC_n can compute the round-trip
propagation delay to SSRC_r by recording the time A when
this reception report block is received. It calculates the
total round-trip time A-LSR using the last SR timestamp
(LSR) field, and then subtracting this field to leave the
round-trip propagation delay as (A- LSR - DLSR). This is
illustrated in Fig. 2. Times are shown in both a
hexadecimal representation of the 32-bit fields and the
equivalent floating-point decimal representation. Colons
indicate a 32-bit field divided into a 16-bit integer part
and 16-bit fraction part.
This may be used as an approximate measure of distance to
cluster receivers, although some links have very asymmetric
delays.
[10 Nov 1995 11:33:25.125 UTC] [10 Nov 1995 11:33:36.5 UTC]
n SR(n) A=b710:8000 (46864.500 s)
---------------------------------------------------------------->
v ^
ntp_sec =0xb44db705 v ^ dlsr=0x0005:4000 ( 5.250s)
ntp_frac=0x20000000 v ^ lsr =0xb705:2000 (46853.125s)
(3024992005.125 s) v ^
r v ^ RR(n)
---------------------------------------------------------------->
|<-DLSR->|
(5.250 s)
A 0xb710:8000 (46864.500 s)
DLSR -0x0005:4000 ( 5.250 s)
LSR -0xb705:2000 (46853.125 s)
-------------------------------
delay 0x 6:2000 ( 6.125 s)
Figure 2: Example for round-trip time computation
6.4.2 RR: Receiver report RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| RC | PT=RR=201 | length | header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
| fraction lost | cumulative number of packets lost | 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_2 (SSRC of second source) | report
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
: ... : 2
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| profile-specific extensions |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the receiver report (RR) packet is the same as that of
the SR packet except that the packet type field contains the constant
201 and the five words of sender information are omitted (these are
the NTP and RTP timestamps and sender's packet and octet counts). The
remaining fields have the same meaning as for the SR packet.
An empty RR packet (RC = 0) MUST be put at the head of a compound
RTCP packet when there is no data transmission or reception to
report.
6.4.3 Extending the sender and receiver reports
A profile SHOULD define profile-specific extensions to the sender
report and receiver report if there is additional information that
needs to be reported regularly about the sender or receivers. This
method SHOULD be used in preference to defining another RTCP packet
type because it requires less overhead:
o fewer octets in the packet (no RTCP header or SSRC field);
o simpler and faster parsing because applications running under
that profile would be programmed to always expect the
extension fields in the directly accessible location after the
reception reports.
The extension is a fourth section in the sender- or receiver-report
packet which comes at the end after the reception report blocks, if
any. If additional sender information is required, then for sender
reports it would be included first in the extension section, but for
receiver reports it would not be present. If information about
receivers is to be included, that data SHOULD be structured as an
array of blocks parallel to the existing array of reception report
blocks; that is, the number of blocks would be indicated by the RC
field.
6.4.4 Analyzing sender and receiver reports
It is expected that reception quality feedback will be useful not
only for the sender but also for other receivers and third-party
monitors. The sender may modify its transmissions based on the
feedback; receivers can determine whether problems are local,
regional or global; network managers may use profile-independent
monitors that receive only the RTCP packets and not the corresponding
RTP data packets to evaluate the performance of their networks for
multicast distribution.
Cumulative counts are used in both the sender information and
receiver report blocks so that differences may be calculated between
any two reports to make measurements over both short and long time
periods, and to provide resilience against the loss of a report. The
difference between the last two reports received can be used to
estimate the recent quality of the distribution. The NTP timestamp is
included so that rates may be calculated from these differences over
the interval between two reports. Since that timestamp is independent
of the clock rate for the data encoding, it is possible to implement
encoding- and profile-independent quality monitors.
An example calculation is the packet loss rate over the interval
between two reception reports. The difference in the cumulative
number of packets lost gives the number lost during that interval.
The difference in the extended last sequence numbers received gives
the number of packets expected during the interval. The ratio of
these two is the packet loss fraction over the interval. This ratio
should equal the fraction lost field if the two reports are
consecutive, but otherwise it may not. The loss rate per second can
be obtained by dividing the loss fraction by the difference in NTP
timestamps, expressed in seconds. The number of packets received is
the number of packets expected minus the number lost. The number of
packets expected may also be used to judge the statistical validity
of any loss estimates. For example, 1 out of 5 packets lost has a
lower significance than 200 out of 1000.
From the sender information, a third-party monitor can calculate the
average payload data rate and the average packet rate over an
interval without receiving the data. Taking the ratio of the two
gives the average payload size. If it can be assumed that packet loss
is independent of packet size, then the number of packets received by
a particular receiver times the average payload size (or the
corresponding packet size) gives the apparent throughput available to
that receiver.
In addition to the cumulative counts which allow long-term packet
loss measurements using differences between reports, the fraction
lost field provides a short-term measurement from a single report.
This becomes more important as the size of a session scales up enough
that reception state information might not be kept for all receivers
or the interval between reports becomes long enough that only one
report might have been received from a particular receiver.
The interarrival jitter field provides a second short-term measure of
network congestion. Packet loss tracks persistent congestion while
the jitter measure tracks transient congestion. The jitter measure
may indicate congestion before it leads to packet loss. The
interarrival jitter field is only a snapshot of the jitter at the
time of a report and is not intended to be taken quantitatively.
Rather, it is intended for comparison across a number of reports from
one receiver over time or from multiple receivers, e.g., within a
single network, at the same time. To allow comparison across
receivers, it is important the the jitter be calculated according to
the same formula by all receivers.
Because the jitter calculation is based on the RTP timestamp which
represents the instant when the first data in the packet was sampled,
any variation in the delay between that sampling instant and the time
the packet is transmitted will affect the resulting jitter that is
calculated. Such a variation in delay would occur for audio packets
of varying duration. It will also occur for video encodings because
the timestamp is the same for all the packets of one frame but those
packets are not all transmitted at the same time. The variation in
delay until transmission does reduce the accuracy of the jitter
calculation as a measure of the behavior of the network by itself,
but it is appropriate to include considering that the receiver buffer
must accommodate it. When the jitter calculation is used as a
comparative measure, the (constant) component due to variation in
delay until transmission subtracts out so that a change in the
network jitter component can then be observed unless it is relatively
small. If the change is small then it is likely to be
inconsequential.
6.5 SDES: Source description RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| SC | PT=SDES=202 | length | header
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC/CSRC_1 | chunk
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1
| SDES items |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC/CSRC_2 | chunk
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2
| SDES items |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
The SDES packet is a three-level structure composed of a header and
zero or more chunks, each of which is composed of items describing
the source identified in that chunk. The items are described
individually in subsequent sections.
version (V), padding (P), length:
As described for the SR packet (see Section 6.4.1).
packet type (PT): 8 bits
Contains the constant 202 to identify this as an RTCP SDES
packet.
source count (SC): 5 bits
The number of SSRC/CSRC chunks contained in this SDES
packet. A value of zero is valid but useless.
Each chunk consists of an SSRC/CSRC identifier followed by a list of
zero or more items, which carry information about the SSRC/CSRC. Each
chunk starts on a 32-bit boundary. Each item consists of an 8-bit
type field, an 8-bit octet count describing the length of the text
(thus, not including this two-octet header), and the text itself.
Note that the text can be no longer than 255 octets, but this is
consistent with the need to limit RTCP bandwidth consumption.
The text is encoded according to the UTF-8 encoding specified in RFC
2279 [5]. US-ASCII is a subset of this encoding and requires no
additional encoding. The presence of multi-octet encodings is
indicated by setting the most significant bit of a character to a
value of one.
Items are contiguous, i.e., items are not individually padded to a
32-bit boundary. Text is not null terminated because some multi-octet
encodings include null octets. The list of items in each chunk MUST
be terminated by one or more null octets, the first of which is
interpreted as an item type of zero to denote the end of the list.
No length octet follows the null item type octet, but additional null
octets MUST be included if needed to pad until the next 32-bit
boundary. Note that this padding is separate from that indicated by
the P bit in the RTCP header. A chunk with zero items (four null
octets) is valid but useless.
End systems send one SDES packet containing their own source
identifier (the same as the SSRC in the fixed RTP header). A mixer
sends one SDES packet containing a chunk for each contributing source
from which it is receiving SDES information, or multiple complete
SDES packets in the format above if there are more than 31 such
sources (see Section 7).
The SDES items currently defined are described in the next sections.
Only the CNAME item is mandatory. Some items shown here may be useful
only for particular profiles, but the item types are all assigned
from one common space to promote shared use and to simplify profile-
independent applications. Additional items may be defined in a
profile by registering the type numbers with IANA as described in
Section 14.
6.5.1 CNAME: Canonical end-point identifier SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CNAME=1 | length | user and domain name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The CNAME identifier has the following properties:
o Because the randomly allocated SSRC identifier may change if a
conflict is discovered or if a program is restarted, the CNAME
item MUST be included to provide the binding from the SSRC
identifier to an identifier for the source that remains
constant.
o Like the SSRC identifier, the CNAME identifier SHOULD also be
unique among all participants within one RTP session.
o To provide a binding across multiple media tools used by one
participant in a set of related RTP sessions, the CNAME SHOULD
be fixed for that participant.
o To facilitate third-party monitoring, the CNAME SHOULD be
suitable for either a program or a person to locate the
source.
Therefore, the CNAME SHOULD be derived algorithmically and not
entered manually, when possible. To meet these requirements, the
following format SHOULD be used unless a profile specifies an
alternate syntax or semantics. The CNAME item SHOULD have the format
"user@host", or "host" if a user name is not available as on single-
user systems. For both formats, "host" is either the fully qualified
domain name of the host from which the real-time data originates,
formatted according to the rules specified in RFC 1034 [6], RFC 1035
[7] and Section 2.1 of RFC 1123 [8]; or the standard ASCII
representation of the host's numeric address on the interface used
for the RTP communication. For example, the standard ASCII
representation of an IP Version 4 address is "dotted decimal", also
known as dotted quad. Other address types are expected to have ASCII
representations that are mutually unique. The fully qualified domain
name is more convenient for a human observer and may avoid the need
to send a NAME item in addition, but it may be difficult or
impossible to obtain reliably in some operating environments.
Applications that may be run in such environments SHOULD use the
ASCII representation of the address instead.
Examples are "doe@sleepy.megacorp.com" or "doe@192.0.2.89" for a
multi-user system. On a system with no user name, examples would be
"sleepy.megacorp.com" or "192.0.2.89".
The user name SHOULD be in a form that a program such as "finger" or
"talk" could use, i.e., it typically is the login name rather than
the personal name. The host name is not necessarily identical to the
one in the participant's electronic mail address.
This syntax will not provide unique identifiers for each source if an
application permits a user to generate multiple sources from one
host. Such an application would have to rely on the SSRC to further
identify the source, or the profile for that application would have
to specify additional syntax for the CNAME identifier.
If each application creates its CNAME independently, the resulting
CNAMEs may not be identical as would be required to provide a binding
across multiple media tools belonging to one participant in a set of
related RTP sessions. If cross-media binding is required, it may be
necessary for the CNAME of each tool to be externally configured with
the same value by a coordination tool.
Application writers should be aware that private network address
assignments such as the Net-10 assignment proposed in RFC 1597 [23]
may create network addresses that are not globally unique. This would
lead to non-unique CNAMEs if hosts with private addresses and no
direct IP connectivity to the public Internet have their RTP packets
forwarded to the public Internet through an RTP-level translator.
(See also RFC 1627 [24].) To handle this case, applications MAY
provide a means to configure a unique CNAME, but the burden is on the
translator to translate CNAMEs from private addresses to public
addresses if necessary to keep private addresses from being exposed.
6.5.2 NAME: User name SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAME=2 | length | common name of source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is the real name used to describe the source, e.g., "John Doe,
Bit Recycler, Megacorp". It may be in any form desired by the user.
For applications such as conferencing, this form of name may be the
most desirable for display in participant lists, and therefore might
be sent most frequently of those items other than CNAME. Profiles MAY
establish such priorities. The NAME value is expected to remain
constant at least for the duration of a session. It SHOULD NOT be
relied upon to be unique among all participants in the session.
6.5.3 EMAIL: Electronic mail address SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EMAIL=3 | length | email address of source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The email address is formatted according to RFC 822 [9], for example,
"John.Doe@megacorp.com". The EMAIL value is expected to remain
constant for the duration of a session.
6.5.4 PHONE: Phone number SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PHONE=4 | length | phone number of source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The phone number SHOULD be formatted with the plus sign replacing the
international access code. For example, "+1 908 555 1212" for a
number in the United States.
6.5.5 LOC: Geographic user location SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LOC=5 | length | geographic location of site ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Depending on the application, different degrees of detail are
appropriate for this item. For conference applications, a string
like "Murray Hill, New Jersey" may be sufficient, while, for an
active badge system, strings like "Room 2A244, AT&T BL MH" might be
appropriate. The degree of detail is left to the implementation
and/or user, but format and content MAY be prescribed by a profile.
The LOC value is expected to remain constant for the duration of a
session, except for mobile hosts.
6.5.6 TOOL: Application or tool name SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOOL=6 | length | name/version of source appl. ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A string giving the name and possibly version of the application
generating the stream, e.g., "videotool 1.2". This information may be
useful for debugging purposes and is similar to the Mailer or Mail-
System-Version SMTP headers. The TOOL value is expected to remain
constant for the duration of the session.
6.5.7 NOTE: Notice/status SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NOTE=7 | length | note about the source ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following semantics are suggested for this item, but these or
other semantics MAY be explicitly defined by a profile. The NOTE item
is intended for transient messages describing the current state of
the source, e.g., "on the phone, can't talk". Or, during a seminar,
this item might be used to convey the title of the talk. It should be
used only to carry exceptional information and SHOULD NOT be included
routinely by all participants because this would slow down the rate
at which reception reports and CNAME are sent, thus impairing the
performance of the protocol. In particular, it SHOULD NOT be included
as an item in a user's configuration file nor automatically generated
as in a quote-of-the-day.
Since the NOTE item may be important to display while it is active,
the rate at which other non-CNAME items such as NAME are transmitted
might be reduced so that the NOTE item can take that part of the RTCP
bandwidth. When the transient message becomes inactive, the NOTE item
SHOULD continue to be transmitted a few times at the same repetition
rate but with a string of length zero to signal the receivers.
However, receivers SHOULD also consider the NOTE item inactive if it
is not received for a small multiple of the repetition rate, or
perhaps 20-30 RTCP intervals.
6.5.8 PRIV: Private extensions SDES item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PRIV=8 | length | prefix length | prefix string...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | value string ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This item is used to define experimental or application-specific SDES
extensions. The item contains a prefix consisting of a length-string
pair, followed by the value string filling the remainder of the item
and carrying the desired information. The prefix length field is 8
bits long. The prefix string is a name chosen by the person defining
the PRIV item to be unique with respect to other PRIV items this
application might receive. The application creator might choose to
use the application name plus an additional subtype identification if
needed. Alternatively, it is RECOMMENDED that others choose a name
based on the entity they represent, then coordinate the use of the
name within that entity.
Note that the prefix consumes some space within the item's total
length of 255 octets, so the prefix should be kept as short as
possible. This facility and the constrained RTCP bandwidth SHOULD NOT
be overloaded; it is not intended to satisfy all the control
communication requirements of all applications.
SDES PRIV prefixes will not be registered by IANA. If some form of
the PRIV item proves to be of general utility, it SHOULD instead be
assigned a regular SDES item type registered with IANA so that no
prefix is required. This simplifies use and increases transmission
efficiency.
6.6 BYE: Goodbye RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| SC | PT=BYE=203 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| length | reason for leaving ... (opt)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The BYE packet indicates that one or more sources are no longer
active.
version (V), padding (P), length:
As described for the SR packet (see Section 6.4.1).
packet type (PT): 8 bits
Contains the constant 203 to identify this as an RTCP BYE
packet.
source count (SC): 5 bits
The number of SSRC/CSRC identifiers included in this BYE
packet. A count value of zero is valid, but useless.
The rules for when a BYE packet should be sent are specified in
Sections 6.3.7 and 8.2.
If a BYE packet is received by a mixer, the mixer SHOULD forward the
BYE packet with the SSRC/CSRC identifier(s) unchanged. If a mixer
shuts down, it SHOULD send a BYE packet listing all contributing
sources it handles, as well as its own SSRC identifier. Optionally,
the BYE packet MAY include an 8-bit octet count followed by that many
octets of text indicating the reason for leaving, e.g., "camera
malfunction" or "RTP loop detected". The string has the same encoding
as that described for SDES. If the string fills the packet to the
next 32-bit boundary, the string is not null terminated. If not, the
BYE packet MUST be padded with null octets to the next 32-bit
boundary. This padding is separate from that indicated by the P bit
in the RTCP header.
6.7 APP: Application-defined RTCP packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| subtype | PT=APP=204 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name (ASCII) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| application-dependent data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The APP packet is intended for experimental use as new applications
and new features are developed, without requiring packet type value
registration. APP packets with unrecognized names SHOULD be ignored.
After testing and if wider use is justified, it is RECOMMENDED that
each APP packet be redefined without the subtype and name fields and
registered with IANA using an RTCP packet type.
version (V), padding (P), length:
As described for the SR packet (see Section 6.4.1).
subtype: 5 bits
May be used as a subtype to allow a set of APP packets to
be defined under one unique name, or for any application-
dependent data.
packet type (PT): 8 bits
Contains the constant 204 to identify this as an RTCP APP
packet.
name: 4 octets
A name chosen by the person defining the set of APP packets
to be unique with respect to other APP packets this
application might receive. The application creator might
choose to use the application name, and then coordinate the
allocation of subtype values to others who want to define
new packet types for the application. Alternatively, it is
RECOMMENDED that others choose a name based on the entity
they represent, then coordinate the use of the name within
that entity. The name is interpreted as a sequence of four
ASCII characters, with uppercase and lowercase characters
treated as distinct.
application-dependent data: variable length
Application-dependent data may or may not appear in an APP
packet. It is interpreted by the application and not RTP
itself. It MUST be a multiple of 32 bits long.
7 RTP Translators and Mixers
In addition to end systems, RTP supports the notion of "translators"
and "mixers", which could be considered as "intermediate systems" at
the RTP level. Although this support adds some complexity to the
protocol, the need for these functions has been clearly established
by experiments with multicast audio and video applications in the
Internet. Example uses of translators and mixers given in Section 2.3
stem from the presence of firewalls and low bandwidth connections,
both of which are likely to remain.
7.1 General Description
An RTP translator/mixer connects two or more transport-level
"clouds". Typically, each cloud is defined by a common network and
transport protocol (e.g., IP/UDP) plus a multicast address and
transport level destination port or a pair of unicast addresses and
ports. (Network-level protocol translators, such as IP version 4 to
IP version 6, may be present within a cloud invisibly to RTP.) One
system may serve as a translator or mixer for a number of RTP
sessions, but each is considered a logically separate entity.
In order to avoid creating a loop when a translator or mixer is
installed, the following rules MUST be observed:
o Each of the clouds connected by translators and mixers
participating in one RTP session either MUST be distinct from
all the others in at least one of these parameters (protocol,
address, port), or MUST be isolated at the network level from
the others.
o A derivative of the first rule is that there MUST NOT be
multiple translators or mixers connected in parallel unless by
some arrangement they partition the set of sources to be
forwarded.
Similarly, all RTP end systems that can communicate through one or
more RTP translators or mixers share the same SSRC space, that is,
the SSRC identifiers MUST be unique among all these end systems.
Section 8.2 describes the collision resolution algorithm by which
SSRC identifiers are kept unique and loops are detected.
There may be many varieties of translators and mixers designed for
different purposes and applications. Some examples are to add or
remove encryption, change the encoding of the data or the underlying
protocols, or replicate between a multicast address and one or more
unicast addresses. The distinction between translators and mixers is
that a translator passes through the data streams from different
sources separately, whereas a mixer combines them to form one new
stream:
Translator: Forwards RTP packets with their SSRC identifier
intact; this makes it possible for receivers to identify
individual sources even though packets from all the sources
pass through the same translator and carry the translator's
network source address. Some kinds of translators will pass
through the data untouched, but others MAY change the
encoding of the data and thus the RTP data payload type and
timestamp. If multiple data packets are re-encoded into
one, or vice versa, a translator MUST assign new sequence
numbers to the outgoing packets. Losses in the incoming
packet stream may induce corresponding gaps in the outgoing
sequence numbers. Receivers cannot detect the presence of a
translator unless they know by some other means what
payload type or transport address was used by the original
source.
Mixer: Receives streams of RTP data packets from one or more
sources, possibly changes the data format, combines the
streams in some manner and then forwards the combined
stream. Since the timing among multiple input sources will
not generally be synchronized, the mixer will make timing
adjustments among the streams and generate its own timing
for the combined stream, so it is the synchronization
source. Thus, all data packets forwarded by a mixer MUST be
marked with the mixer's own SSRC identifier. In order to
preserve the identity of the original sources contributing
to the mixed packet, the mixer SHOULD insert their SSRC
identifiers into the CSRC identifier list following the
fixed RTP header of the packet. A mixer that is also itself
a contributing source for some packet SHOULD explicitly
include its own SSRC identifier in the CSRC list for that
packet.
For some applications, it MAY be acceptable for a mixer not
to identify sources in the CSRC list. However, this
introduces the danger that loops involving those sources
could not be detected.
The advantage of a mixer over a translator for applications like
audio is that the output bandwidth is limited to that of one source
even when multiple sources are active on the input side. This may be
important for low-bandwidth links. The disadvantage is that receivers
on the output side don't have any control over which sources are
passed through or muted, unless some mechanism is implemented for
remote control of the mixer. The regeneration of synchronization
information by mixers also means that receivers can't do inter-media
synchronization of the original streams. A multi-media mixer could do
it.
[E1] [E6]
| |
E1:17 | E6:15 |
| | E6:15
V M1:48 (1,17) M1:48 (1,17) V M1:48 (1,17)
(M1)-------------><T1>-----------------><T2>-------------->[E7]
^ ^ E4:47 ^ E4:47
E2:1 | E4:47 | | M3:89 (64,45)
| | |
[E2] [E4] M3:89 (64,45) |
| legend:
[E3] --------->(M2)----------->(M3)------------| [End system]
E3:64 M2:12 (64) ^ (Mixer)
| E5:45 <Translator>
|
[E5] source: SSRC (CSRCs)
------------------->
Figure 3: Sample RTP network with end systems, mixers and translators
A collection of mixers and translators is shown in Figure 3 to
illustrate their effect on SSRC and CSRC identifiers. In the figure,
end systems are shown as rectangles (named E), translators as
triangles (named T) and mixers as ovals (named M). The notation "M1:
48(1,17)" designates a packet originating a mixer M1, identified with
M1's (random) SSRC value of 48 and two CSRC identifiers, 1 and 17,
copied from the SSRC identifiers of packets from E1 and E2.
7.2 RTCP Processing in Translators
In addition to forwarding data packets, perhaps modified, translators
and mixers MUST also process RTCP packets. In many cases, they will
take apart the compound RTCP packets received from end systems to
aggregate SDES information and to modify the SR or RR packets.
Retransmission of this information may be triggered by the packet
arrival or by the RTCP interval timer of the translator or mixer
itself.
A translator that does not modify the data packets, for example one
that just replicates between a multicast address and a unicast
address, MAY simply forward RTCP packets unmodified as well. A
translator that transforms the payload in some way MUST make
corresponding transformations in the SR and RR information so that it
still reflects the characteristics of the data and the reception
quality. These translators MUST NOT simply forward RTCP packets. In
general, a translator SHOULD NOT aggregate SR and RR packets from
different sources into one packet since that would reduce the
accuracy of the propagation delay measurements based on the LSR and
DLSR fields.
SR sender information: A translator does not generate its own
sender information, but forwards the SR packets received
from one cloud to the others. The SSRC is left intact but
the sender information MUST be modified if required by the
translation. If a translator changes the data encoding, it
MUST change the "sender's byte count" field. If it also
combines several data packets into one output packet, it
MUST change the "sender's packet count" field. If it
changes the timestamp frequency, it MUST change the "RTP
timestamp" field in the SR packet.
SR/RR reception report blocks: A translator forwards reception
reports received from one cloud to the others. Note that
these flow in the direction opposite to the data. The SSRC
is left intact. If a translator combines several data
packets into one output packet, and therefore changes the
sequence numbers, it MUST make the inverse manipulation for
the packet loss fields and the "extended last sequence
number" field. This may be complex. In the extreme case,
there may be no meaningful way to translate the reception
reports, so the translator MAY pass on no reception report
at all or a synthetic report based on its own reception.
The general rule is to do what makes sense for a particular
translation.
A translator does not require an SSRC identifier of its
own, but MAY choose to allocate one for the purpose of
sending reports about what it has received. These would be
sent to all the connected clouds, each corresponding to the
translation of the data stream as sent to that cloud, since
reception reports are normally multicast to all
participants.
SDES: Translators typically forward without change the SDES
information they receive from one cloud to the others, but
MAY, for example, decide to filter non-CNAME SDES
information if bandwidth is limited. The CNAMEs MUST be
forwarded to allow SSRC identifier collision detection to
work. A translator that generates its own RR packets MUST
send SDES CNAME information about itself to the same clouds
that it sends those RR packets.
BYE: Translators forward BYE packets unchanged. A translator
that is about to cease forwarding packets SHOULD send a BYE
packet to each connected cloud containing all the SSRC
identifiers that were previously being forwarded to that
cloud, including the translator's own SSRC identifier if it
sent reports of its own.
APP: Translators forward APP packets unchanged.
7.3 RTCP Processing in Mixers
Since a mixer generates a new data stream of its own, it does not
pass through SR or RR packets at all and instead generates new
information for both sides.
SR sender information: A mixer does not pass through sender
information from the sources it mixes because the
characteristics of the source streams are lost in the mix.
As a synchronization source, the mixer SHOULD generate its
own SR packets with sender information about the mixed data
stream and send them in the same direction as the mixed
stream.
SR/RR reception report blocks: A mixer generates its own
reception reports for sources in each cloud and sends them
out only to the same cloud. It MUST NOT send these
reception reports to the other clouds and MUST NOT forward
reception reports from one cloud to the others because the
sources would not be SSRCs there (only CSRCs).
SDES: Mixers typically forward without change the SDES
information they receive from one cloud to the others, but
MAY, for example, decide to filter non-CNAME SDES
information if bandwidth is limited. The CNAMEs MUST be
forwarded to allow SSRC identifier collision detection to
work. (An identifier in a CSRC list generated by a mixer
might collide with an SSRC identifier generated by an end
system.) A mixer MUST send SDES CNAME information about
itself to the same clouds that it sends SR or RR packets.
Since mixers do not forward SR or RR packets, they will
typically be extracting SDES packets from a compound RTCP
packet. To minimize overhead, chunks from the SDES packets
MAY be aggregated into a single SDES packet which is then
stacked on an SR or RR packet originating from the mixer.
A mixer which aggregates SDES packets will use more RTCP
bandwidth than an individual source because the compound
packets will be longer, but that is appropriate since the
mixer represents multiple sources. Similarly, a mixer
which passes through SDES packets as they are received will
be transmitting RTCP packets at higher than the single
source rate, but again that is correct since the packets
come from multiple sources. The RTCP packet rate may be
different on each side of the mixer.
A mixer that does not insert CSRC identifiers MAY also
refrain from forwarding SDES CNAMEs. In this case, the SSRC
identifier spaces in the two clouds are independent. As
mentioned earlier, this mode of operation creates a danger
that loops can't be detected.
BYE: Mixers MUST forward BYE packets. A mixer that is about to
cease forwarding packets SHOULD send a BYE packet to each
connected cloud containing all the SSRC identifiers that
were previously being forwarded to that cloud, including
the mixer's own SSRC identifier if it sent reports of its
own.
APP: The treatment of APP packets by mixers is application-
specific.
7.4 Cascaded Mixers
An RTP session may involve a collection of mixers and translators as
shown in Figure 3. If two mixers are cascaded, such as M2 and M3 in
the figure, packets received by a mixer may already have been mixed
and may include a CSRC list with multiple identifiers. The second
mixer SHOULD build the CSRC list for the outgoing packet using the
CSRC identifiers from already-mixed input packets and the SSRC
identifiers from unmixed input packets. This is shown in the output
arc from mixer M3 labeled M3:89(64,45) in the figure. As in the case
of mixers that are not cascaded, if the resulting CSRC list has more
than 15 identifiers, the remainder cannot be included.
8 SSRC Identifier Allocation and Use
The SSRC identifier carried in the RTP header and in various fields
of RTCP packets is a random 32-bit number that is required to be
globally unique within an RTP session. It is crucial that the number
be chosen with care in order that participants on the same network or
starting at the same time are not likely to choose the same number.
It is not sufficient to use the local network address (such as an
IPv4 address) for the identifier because the address may not be
unique. Since RTP translators and mixers enable interoperation among
multiple networks with different address spaces, the allocation
patterns for addresses within two spaces might result in a much
higher rate of collision than would occur with random allocation.
Multiple sources running on one host would also conflict.
It is also not sufficient to obtain an SSRC identifier simply by
calling random() without carefully initializing the state. An example
of how to generate a random identifier is presented in Appendix A.6.
8.1 Probability of Collision
Since the identifiers are chosen randomly, it is possible that two or
more sources will choose the same number. Collision occurs with the
highest probability when all sources are started simultaneously, for
example when triggered automatically by some session management
event. If N is the number of sources and L the length of the
identifier (here, 32 bits), the probability that two sources
independently pick the same value can be approximated for large N
[25] as 1 - exp(-N**2 / 2**(L+1)). For N=1000, the probability is
roughly 10**-4.
The typical collision probability is much lower than the worst-case
above. When one new source joins an RTP session in which all the
other sources already have unique identifiers, the probability of
collision is just the fraction of numbers used out of the space.
Again, if N is the number of sources and L the length of the
identifier, the probability of collision is N / 2**L. For N=1000, the
probability is roughly 2*10**-7.
The probability of collision is further reduced by the opportunity
for a new source to receive packets from other participants before
sending its first packet (either data or control). If the new source
keeps track of the other participants (by SSRC identifier), then
before transmitting its first packet the new source can verify that
its identifier does not conflict with any that have been received, or
else choose again.
8.2 Collision Resolution and Loop Detection
Although the probability of SSRC identifier collision is low, all RTP
implementations MUST be prepared to detect collisions and take the
appropriate actions to resolve them. If a source discovers at any
time that another source is using the same SSRC identifier as its
own, it MUST send an RTCP BYE packet for the old identifier and
choose another random one. (As explained below, this step is taken
only once in case of a loop.) If a receiver discovers that two other
sources are colliding, it MAY keep the packets from one and discard
the packets from the other when this can be detected by different
source transport addresses or CNAMEs. The two sources are expected
to resolve the collision so that the situation doesn't last.
Because the random SSRC identifiers are kept globally unique for each
RTP session, they can also be used to detect loops that may be
introduced by mixers or translators. A loop causes duplication of
data and control information, either unmodified or possibly mixed, as
in the following examples:
o A translator may incorrectly forward a packet to the same
multicast group from which it has received the packet, either
directly or through a chain of translators. In that case, the
same packet appears several times, originating from different
network sources.
o Two translators incorrectly set up in parallel, i.e., with the
same multicast groups on both sides, would both forward
packets from one multicast group to the other. Unidirectional
translators would produce two copies; bidirectional
translators would form a loop.
o A mixer can close a loop by sending to the same transport
destination upon which it receives packets, either directly or
through another mixer or translator. In this case a source
might show up both as an SSRC on a data packet and a CSRC in a
mixed data packet.
A source may discover that its own packets are being looped, or that
packets from another source are being looped (a third-party loop).
Both loops and collisions in the random selection of a source
identifier result in packets arriving with the same SSRC identifier
but a different source transport address, which may be that of the
end system originating the packet or an intermediate system.
Therefore, if a source changes its source transport address, it MAY
also choose a new SSRC identifier to avoid being interpreted as a
looped source. (This is not MUST because in some applications of RTP
sources may be expected to change addresses during a session.) Note
that if a translator restarts and consequently changes the source
transport address (e.g., changes the UDP source port number) on which
it forwards packets, then all those packets will appear to receivers
to be looped because the SSRC identifiers are applied by the original
source and will not change. This problem can be avoided by keeping
the source transport addressed fixed across restarts, but in any case
will be resolved after a timeout at the receivers.
Loops or collisions occurring on the far side of a translator or
mixer cannot be detected using the source transport address if all
copies of the packets go through the translator or mixer, however
collisions may still be detected when chunks from two RTCP SDES
packets contain the same SSRC identifier but different CNAMEs.
To detect and resolve these conflicts, an RTP implementation MUST
include an algorithm similar to the one described below, though the
implementation MAY choose a different policy for which packets from
colliding third-party sources are kept. The algorithm described below
ignores packets from a new source or loop that collide with an
established source. It resolves collisions with the participant's own
SSRC identifier by sending an RTCP BYE for the old identifier and
choosing a new one. However, when the collision was induced by a loop
of the participant's own packets, the algorithm will choose a new
identifier only once and thereafter ignore packets from the looping
source transport address. This is required to avoid a flood of BYE
packets.
This algorithm requires keeping a table indexed by the source
identifier and containing the source transport addresses from the
first RTP packet and first RTCP packet received with that identifier,
along with other state for that source. Two source transport
addresses are required since, for example, the UDP source port
numbers may be different on RTP and RTCP packets. However, it may be
assumed that the network address is the same in both source transport
addresses.
Each SSRC or CSRC identifier received in an RTP or RTCP packet is
looked up in the source identifier table in order to process that
data or control information. The source transport address from the
packet is compared to the corresponding source transport address in
the table to detect a loop or collision if they don't match. For
control packets, each element with its own SSRC id, for example an
SDES chunk, requires a separate lookup. (The SSRC id in a reception
report block is an exception because it identifies a source heard by
the reporter, and that SSRC id is unrelated to the source transport
address of the RTCP packet sent by the reporter.) If the SSRC or CSRC
is not found, a new entry is created. These table entries are removed
when an RTCP BYE packet is received with the corresponding SSRC id
and validated by a matching source transport address, or after no
packets have arrived for a relatively long time (see Section 6.2.1).
Note that if two sources on the same host are transmitting with the
same source identifier at the time a receiver begins operation, it
would be possible that the first RTP packet received came from one of
the sources while the first RTCP packet received came from the other.
This would cause the wrong RTCP information to be associated with the
RTP data, but this situation should be sufficiently rare and harmless
that it may be disregarded.
In order to track loops of the participant's own data packets, the
implementation MUST also keep a separate list of source transport
addresses (not identifiers) that have been found to be conflicting.
As in the source identifier table, two source transport addresses
MUST be kept to separately track conflicting RTP and RTCP packets.
Note that the conflicting address list should be short, usually
empty. Each element in this list stores the source addresses plus
the time when the most recent conflicting packet was received. An
element MAY be removed from the list when no conflicting packet has
arrived from that source for a time on the order of 10 RTCP report
intervals (see Section 6.2).
For the algorithm as shown, it is assumed that the participant's own
source identifier and state are included in the source identifier
table. The algorithm could be restructured to first make a separate
comparison against the participant's own source identifier.
if (SSRC or CSRC identifier is not found in the source
identifier table) {
create a new entry storing the data or control source
transport address, the SSRC or CSRC id and other state;
}
/* Identifier is found in the table */
else if (table entry was created on receipt of a control packet
and this is the first data packet or vice versa) {
store the source transport address from this packet;
}
else if (source transport address from the packet does not match
the one saved in the table entry for this identifier) {
/* An identifier collision or a loop is indicated */
if (source identifier is not the participant's own) {
/* OPTIONAL error counter step */
if (source identifier is from an RTCP SDES chunk
containing a CNAME item that differs from the CNAME
in the table entry) {
count a third-party collision;
} else {
count a third-party loop;
}
abort processing of data packet or control element;
/* MAY choose a different policy to keep new source */
}
/* A collision or loop of the participant's own packets */
else if (source transport address is found in the list of
conflicting data or control source transport
addresses) {
/* OPTIONAL error counter step */
if (source identifier is not from an RTCP SDES chunk
containing a CNAME item or CNAME is the
participant's own) {
count occurrence of own traffic looped;
}
mark current time in conflicting address list entry;
abort processing of data packet or control element;
}
/* New collision, change SSRC identifier */
else {
log occurrence of a collision;
create a new entry in the conflicting data or control
source transport address list and mark current time;
send an RTCP BYE packet with the old SSRC identifier;
choose a new SSRC identifier;
create a new entry in the source identifier table with
the old SSRC plus the source transport address from
the data or control packet being processed;
}
}
In this algorithm, packets from a newly conflicting source address
will be ignored and packets from the original source address will be
kept. If no packets arrive from the original source for an extended
period, the table entry will be timed out and the new source will be
able to take over. This might occur if the original source detects
the collision and moves to a new source identifier, but in the usual
case an RTCP BYE packet will be received from the original source to
delete the state without having to wait for a timeout.
If the original source address was through a mixer (i.e., learned as
a CSRC) and later the same source is received directly, the receiver
may be well advised to switch to the new source address unless other
sources in the mix would be lost. Furthermore, for applications such
as telephony in which some sources such as mobile entities may change
addresses during the course of an RTP session, the RTP implementation
SHOULD modify the collision detection algorithm to accept packets
from the new source transport address. To guard against flip-flopping
between addresses if a genuine collision does occur, the algorithm
SHOULD include some means to detect this case and avoid switching.
When a new SSRC identifier is chosen due to a collision, the
candidate identifier SHOULD first be looked up in the source
identifier table to see if it was already in use by some other
source. If so, another candidate MUST be generated and the process
repeated.
A loop of data packets to a multicast destination can cause severe
network flooding. All mixers and translators MUST implement a loop
detection algorithm like the one here so that they can break loops.
This should limit the excess traffic to no more than one duplicate
copy of the original traffic, which may allow the session to continue
so that the cause of the loop can be found and fixed. However, in
extreme cases where a mixer or translator does not properly break the
loop and high traffic levels result, it may be necessary for end
systems to cease transmitting data or control packets entirely. This
decision may depend upon the application. An error condition SHOULD
be indicated as appropriate. Transmission MAY be attempted again
periodically after a long, random time (on the order of minutes).
8.3 Use with Layered Encodings
For layered encodings transmitted on separate RTP sessions (see
Section 2.4), a single SSRC identifier space SHOULD be used across
the sessions of all layers and the core (base) layer SHOULD be used
for SSRC identifier allocation and collision resolution. When a
source discovers that it has collided, it transmits an RTCP BYE
packet on only the base layer but changes the SSRC identifier to the
new value in all layers.
9 Security
Lower layer protocols may eventually provide all the security
services that may be desired for applications of RTP, including
authentication, integrity, and confidentiality. These services have
been specified for IP in [26]. Since the initial audio and video
applications using RTP needed a confidentiality service before such
services were available for the IP layer, the confidentiality service
described in the next section was defined for use with RTP and RTCP.
That description is included here to codify existing practice. New
applications of RTP MAY implement this RTP-specific confidentiality
service for backward compatibility, and/or they MAY implement IP
layer security services. The overhead on the RTP protocol for this
confidentiality service is low, so the penalty will be minimal if
this service is obsoleted by lower layer services in the future.
Alternatively, other services, other implementations of services and
other algorithms may be defined for RTP in the future if warranted.
The selection presented here is meant to simplify implementation of
interoperable, secure applications and provide guidance to
implementors. No claim is made that the methods presented here are
appropriate for a particular security need. A profile may specify
which services and algorithms should be offered by applications, and
may provide guidance as to their appropriate use.
Key distribution and certificates are outside the scope of this
document.
9.1 Confidentiality
Confidentiality means that only the intended receiver(s) can decode
the received packets; for others, the packet contains no useful
information. Confidentiality of the content is achieved by
encryption.
When encryption of RTP or RTCP is desired, all the octets that will
be encapsulated for transmission in a single lower-layer packet are
encrypted as a unit. For RTCP, a 32-bit random number MUST be
prepended to the unit before encryption to deter known plaintext
attacks. For RTP, no prefix is required because the sequence number
and timestamp fields are initialized with random offsets.
For RTCP, an implementation MAY segregate the individual RTCP packets
in a compound RTCP packet into two separate compound RTCP packets,
one to be encrypted and one to be sent in the clear. For example,
SDES information might be encrypted while reception reports were sent
in the clear to accommodate third-party monitors that are not privy
to the encryption key. In this example, depicted in Fig. 4, the SDES
information MUST be appended to an RR packet with no reports (and the
random number) to satisfy the requirement that all compound RTCP
packets begin with an SR or RR packet. The SDES CNAME item is
required in either the encrypted or unencrypted packet, but not both.
The same SDES information SHOULD NOT be carried in both packets as
this may compromise the encryption.
UDP packet UDP packet
----------------------------- ------------------------------
[random][RR][SDES #CNAME ...] [SR #senderinfo #site1 #site2]
----------------------------- ------------------------------
encrypted not encrypted
#: SSRC identifier
Figure 4: Encrypted and non-encrypted RTCP packets
The presence of encryption and the use of the correct key are
confirmed by the receiver through header or payload validity checks.
Examples of such validity checks for RTP and RTCP headers are given
in Appendices A.1 and A.2.
To be consistent with existing practice, the default encryption
algorithm is the Data Encryption Standard (DES) algorithm in cipher
block chaining (CBC) mode, as described in Section 1.1 of RFC 1423
[27], except that padding to a multiple of 8 octets is indicated as
described for the P bit in Section 5.1. The initialization vector is
zero because random values are supplied in the RTP header or by the
random prefix for compound RTCP packets. For details on the use of
CBC initialization vectors, see [28]. Implementations that support
encryption SHOULD always support the DES algorithm in CBC mode as the
default to maximize interoperability. This method was chosen because
it has been demonstrated to be easy and practical to use in
experimental audio and video tools in operation on the Internet.
Other encryption algorithms MAY be specified dynamically for a
session by non-RTP means. It is RECOMMENDED that stronger encryption
algorithms such as Triple-DES be used in place of the default
algorithm.
As an alternative to encryption at the IP level or at the RTP level
as described above, profiles MAY define additional payload types for
encrypted encodings. Those encodings MUST specify how padding and
other aspects of the encryption are to be handled. This method allows
encrypting only the data while leaving the headers in the clear for
applications where that is desired. It may be particularly useful for
hardware devices that will handle both decryption and decoding. It
is also valuable for applications where link-level compression of RTP
and lower-layer headers is desired and confidentiality of the payload
(but not addresses) is sufficient since encryption of the headers
precludes compression.
9.2 Authentication and Message Integrity
Authentication and message integrity services are not defined at the
RTP level since these services would not be directly feasible without
a key management infrastructure. It is expected that authentication
and integrity services will be provided by lower layer protocols.
10 Congestion Control
All transport protocols used on the Internet need to address
congestion control in some way [29]. RTP is not an exception, but
because the data transported over RTP is often inelastic (generated
at a fixed or controlled rate), the means to control congestion in
RTP may be quite different from those for other transport protocols
such as TCP. In one sense, inelasticity reduces the risk of
congestion because the RTP stream will not expand to consume all
available bandwidth as a TCP stream can. However, inelasticity also
means that the RTP stream cannot arbitrarily reduce its load on the
network to eliminate congestion when it occurs.
Since RTP may be used for a wide variety of applications in many
different contexts, there is no single congestion control mechanism
that will work for all. Therefore, congestion control SHOULD be
defined in each RTP profile as appropriate. For some profiles, it may
be sufficient to include an applicability statement restricting the
use of that profile to environments where congestion is avoided by
engineering. For other profiles, specific methods such as data rate
adaptation based on RTCP feedback may be required.
11 RTP over Network and Transport Protocols
This section describes issues specific to carrying RTP packets within
particular network and transport protocols. The following rules apply
unless superseded by protocol-specific definitions outside this
specification.
RTP relies on the underlying protocol(s) to provide demultiplexing of
RTP data and RTCP control streams. For UDP and similar protocols, RTP
SHOULD use an even destination port number and the corresponding RTCP
stream SHOULD use the next higher (odd) destination port number. For
applications that take a single port number as a parameter and derive
the RTP and RTCP port pair from that number, if an odd number is
supplied then the application SHOULD replace that number with the
next lower (even) number to use as the base of the port pair. For
applications in which the RTP and RTCP destination port numbers are
specified via explicit, separate parameters (using a signaling
protocol or other means), the application MAY disregard the
restrictions that the port numbers be even/odd and consecutive
although the use of an even/odd port pair is still encouraged. The
RTP and RTCP port numbers MUST NOT be the same since RTP relies on
the port numbers to demultiplex the RTP data and RTCP control
streams.
In a unicast session, both participants need to identify a port pair
for receiving RTP and RTCP packets. Both participants MAY use the
same port pair. A participant MUST NOT assume that the source port of
the incoming RTP or RTCP packet can be used as the destination port
for outgoing RTP or RTCP packets. When RTP data packets are being
sent in both directions, each participant's RTCP SR packets MUST be
sent to the port that the other participant has specified for
reception of RTCP. The RTCP SR packets combine sender information for
the outgoing data plus reception report information for the incoming
data. If a side is not actively sending data (see Section 6.4), an
RTCP RR packet is sent instead.
It is RECOMMENDED that layered encoding applications (see Section
2.4) use a set of contiguous port numbers. The port numbers MUST be
distinct because of a widespread deficiency in existing operating
systems that prevents use of the same port with multiple multicast
addresses, and for unicast, there is only one permissible address.
Thus for layer n, the data port is P + 2n, and the control port is P
+ 2n + 1. When IP multicast is used, the addresses MUST also be
distinct because multicast routing and group membership are managed
on an address granularity. However, allocation of contiguous IP
multicast addresses cannot be assumed because some groups may require
different scopes and may therefore be allocated from different
address ranges.
The previous paragraph conflicts with the SDP specification, RFC 2327
[15], which says that it is illegal for both multiple addresses and
multiple ports to be specified in the same session description
because the association of addresses with ports could be ambiguous.
It is intended that this restriction will be relaxed in a revision of
RFC 2327 to allow an equal number of addresses and ports to be
specified with a one-to-one mapping implied.
RTP data packets contain no length field or other delineation,
therefore RTP relies on the underlying protocol(s) to provide a
length indication. The maximum length of RTP packets is limited only
by the underlying protocols.
If RTP packets are to be carried in an underlying protocol that
provides the abstraction of a continuous octet stream rather than
messages (packets), an encapsulation of the RTP packets MUST be
defined to provide a framing mechanism. Framing is also needed if the
underlying protocol may contain padding so that the extent of the RTP
payload cannot be determined. The framing mechanism is not defined
here.
A profile MAY specify a framing method to be used even when RTP is
carried in protocols that do provide framing in order to allow
carrying several RTP packets in one lower-layer protocol data unit,
such as a UDP packet. Carrying several RTP packets in one network or
transport packet reduces header overhead and may simplify
synchronization between different streams.
12 Summary of Protocol Constants
This section contains a summary listing of the constants defined in
this specification.
The RTP payload type (PT) constants are defined in profiles rather
than this document. However, the octet of the RTP header which
contains the marker bit(s) and payload type MUST avoid the reserved
values 200 and 201 (decimal) to distinguish RTP packets from the RTCP
SR and RR packet types for the header validation procedure described
in Appendix A.1. For the standard definition of one marker bit and a
7-bit payload type field as shown in this specification, this
restriction means that payload types 72 and 73 are reserved.
12.1 RTCP packet types
abbrev. name value
SR sender report 200
RR receiver report 201
SDES source description 202
BYE goodbye 203
APP application-defined 204
These type values were chosen in the range 200-204 for improved
header validity checking of RTCP packets compared to RTP packets or
other unrelated packets. When the RTCP packet type field is compared
to the corresponding octet of the RTP header, this range corresponds
to the marker bit being 1 (which it usually is not in data packets)
and to the high bit of the standard payload type field being 1 (since
the static payload types are typically defined in the low half). This
range was also chosen to be some distance numerically from 0 and 255
since all-zeros and all-ones are common data patterns.
Since all compound RTCP packets MUST begin with SR or RR, these codes
were chosen as an even/odd pair to allow the RTCP validity check to
test the maximum number of bits with mask and value.
Additional RTCP packet types may be registered through IANA (see
Section 14).
12.2 SDES types
abbrev. name value
END end of SDES list 0
CNAME canonical name 1
NAME user name 2
EMAIL user's electronic mail address 3
PHONE user's phone number 4
LOC geographic user location 5
TOOL name of application or tool 6
NOTE notice about the source 7
PRIV private extensions 8
Additional SDES types may be registered through IANA (see Section
14).
13 RTP Profiles and Payload Format Specifications
A complete specification of RTP for a particular application will
require one or more companion documents of two types described here:
profiles, and payload format specifications.
RTP may be used for a variety of applications with somewhat differing
requirements. The flexibility to adapt to those requirements is
provided by allowing multiple choices in the main protocol
specification, then selecting the appropriate choices or defining
extensions for a particular environment and class of applications in
a separate profile document. Typically an application will operate
under only one profile in a particular RTP session, so there is no
explicit indication within the RTP protocol itself as to which
profile is in use. A profile for audio and video applications may be
found in the companion RFC XXXX. Profiles are typically titled "RTP
Profile for ...".
The second type of companion document is a payload format
specification, which defines how a particular kind of payload data,
such as H.261 encoded video, should be carried in RTP. These
documents are typically titled "RTP Payload Format for XYZ
Audio/Video Encoding". Payload formats may be useful under multiple
profiles and may therefore be defined independently of any particular
profile. The profile documents are then responsible for assigning a
default mapping of that format to a payload type value if needed.
Within this specification, the following items have been identified
for possible definition within a profile, but this list is not meant
to be exhaustive:
RTP data header: The octet in the RTP data header that contains
the marker bit and payload type field MAY be redefined by a
profile to suit different requirements, for example with
more or fewer marker bits (Section 5.3, p. 14).
Payload types: Assuming that a payload type field is included,
the profile will usually define a set of payload formats
(e.g., media encodings) and a default static mapping of
those formats to payload type values. Some of the payload
formats may be defined by reference to separate payload
format specifications. For each payload type defined, the
profile MUST specify the RTP timestamp clock rate to be
used (Section 5.1, p. 12).
RTP data header additions: Additional fields MAY be appended to
the fixed RTP data header if some additional functionality
is required across the profile's class of applications
independent of payload type (Section 5.3, p. 14).
RTP data header extensions: The contents of the first 16 bits of
the RTP data header extension structure MUST be defined if
use of that mechanism is to be allowed under the profile
for implementation-specific extensions (Section 5.3.1, p.
14).
RTCP packet types: New application-class-specific RTCP packet
types MAY be defined and registered with IANA.
RTCP report interval: A profile SHOULD specify that the values
suggested in Section 6.2 for the constants employed in the
calculation of the RTCP report interval will be used. Those
are the RTCP fraction of session bandwidth, the minimum
report interval, and the bandwidth split between senders
and receivers. A profile MAY specify alternate values if
they have been demonstrated to work in a scalable manner.
SR/RR extension: An extension section MAY be defined for the
RTCP SR and RR packets if there is additional information
that should be reported regularly about the sender or
receivers (Section 6.4.3, p. 32).
SDES use: The profile MAY specify the relative priorities for
RTCP SDES items to be transmitted or excluded entirely
(Section 6.3.9); an alternate syntax or semantics for the
CNAME item (Section 6.5.1); the format of the LOC item
(Section 6.5.5); the semantics and use of the NOTE item
(Section 6.5.7); or new SDES item types to be registered
with IANA.
Security: A profile MAY specify which security services and
algorithms should be offered by applications, and MAY
provide guidance as to their appropriate use (Section 9, p.
50).
String-to-key mapping: A profile MAY specify how a user-provided
password or pass phrase is mapped into an encryption key.
Congestion: A profile SHOULD specify the congestion control
behavior appropriate for that profile.
Underlying protocol: Use of a particular underlying network or
transport layer protocol to carry RTP packets MAY be
required.
Transport mapping: A mapping of RTP and RTCP to transport-level
addresses, e.g., UDP ports, other than the standard mapping
defined in Section 11, p. 52 may be specified.
Encapsulation: An encapsulation of RTP packets may be defined to
allow multiple RTP data packets to be carried in one
lower-layer packet or to provide framing over underlying
protocols that do not already do so (Section 11, p. 52).
It is not expected that a new profile will be required for every
application. Within one application class, it would be better to
extend an existing profile rather than make a new one in order to
facilitate interoperation among the applications since each will
typically run under only one profile. Simple extensions such as the
definition of additional payload type values or RTCP packet types may
be accomplished by registering them through IANA and publishing their
descriptions in an addendum to the profile or in a payload format
specification.
14 IANA Considerations
Additional RTCP packet types and SDES item types may be registered
through the Internet Assigned Numbers Authority (IANA). Since these
number spaces are small, allowing unconstrained registration of new
values would not be prudent. To facilitate review of requests and to
promote shared use of new types among multiple applications, requests
for registration of new values must be documented in an RFC or other
permanent and readily available reference such as the product of
another cooperative standards body (e.g., ITU-T). Other requests may
also be accepted, under the advice of a "designated expert." (Contact
the IANA for the contact information of the current expert.)
RTP profile specifications SHOULD register with IANA a name for the
profile in the form "RTP/xxx", where xxx is a short abbreviation of
the profile title. These names are for use by higher-level control
protocols, such as the Session Description Protocol (SDP), RFC 2327
[15], to refer to transport methods.
A Algorithms
We provide examples of C code for aspects of RTP sender and receiver
algorithms. There may be other implementation methods that are faster
in particular operating environments or have other advantages. These
implementation notes are for informational purposes only and are
meant to clarify the RTP specification.
The following definitions are used for all examples; for clarity and
brevity, the structure definitions are only valid for 32-bit big-
endian (most significant octet first) architectures. Bit fields are
assumed to be packed tightly in big-endian bit order, with no
additional padding. Modifications would be required to construct a
portable implementation.
/*
* rtp.h -- RTP header file
*/
#include <sys/types.h>
/*
* The type definitions below are valid for 32-bit architectures and
* may have to be adjusted for 16- or 64-bit architectures.
*/
typedef unsigned char u_int8;
typedef unsigned short u_int16;
typedef unsigned int u_int32;
typedef short int16;
/*
* Current protocol version.
*/
#define RTP_VERSION 2
#define RTP_SEQ_MOD (1<<16)
#define RTP_MAX_SDES 255 /* maximum text length for SDES */
typedef enum {
RTCP_SR = 200,
RTCP_RR = 201,
RTCP_SDES = 202,
RTCP_BYE = 203,
RTCP_APP = 204
} rtcp_type_t;
typedef enum {
RTCP_SDES_END = 0,
RTCP_SDES_CNAME = 1,
RTCP_SDES_NAME = 2,
RTCP_SDES_EMAIL = 3,
RTCP_SDES_PHONE = 4,
RTCP_SDES_LOC = 5,
RTCP_SDES_TOOL = 6,
RTCP_SDES_NOTE = 7,
RTCP_SDES_PRIV = 8
} rtcp_sdes_type_t;
/*
* RTP data header
*/
typedef struct {
unsigned int version:2; /* protocol version */
unsigned int p:1; /* padding flag */
unsigned int x:1; /* header extension flag */
unsigned int cc:4; /* CSRC count */
unsigned int m:1; /* marker bit */
unsigned int pt:7; /* payload type */
unsigned int seq:16; /* sequence number */
u_int32 ts; /* timestamp */
u_int32 ssrc; /* synchronization source */
u_int32 csrc[1]; /* optional CSRC list */
} rtp_hdr_t;
/*
* RTCP common header word
*/
typedef struct {
unsigned int version:2; /* protocol version */
unsigned int p:1; /* padding flag */
unsigned int count:5; /* varies by packet type */
unsigned int pt:8; /* RTCP packet type */
u_int16 length; /* pkt len in words, w/o this word */
} rtcp_common_t;
/*
* Big-endian mask for version, padding bit and packet type pair
*/
#define RTCP_VALID_MASK (0xc000 | 0x2000 | 0xfe)
#define RTCP_VALID_VALUE ((RTP_VERSION << 14) | RTCP_SR)
/*
* Reception report block
*/
typedef struct {
u_int32 ssrc; /* data source being reported */
unsigned int fraction:8; /* fraction lost since last SR/RR */
int lost:24; /* cumul. no. pkts lost (signed!) */
u_int32 last_seq; /* extended last seq. no. received */
u_int32 jitter; /* interarrival jitter */
u_int32 lsr; /* last SR packet from this source */
u_int32 dlsr; /* delay since last SR packet */
} rtcp_rr_t;
/*
* SDES item
*/
typedef struct {
u_int8 type; /* type of item (rtcp_sdes_type_t) */
u_int8 length; /* length of item (in octets) */
char data[1]; /* text, not null-terminated */
} rtcp_sdes_item_t;
/*
* One RTCP packet
*/
typedef struct {
rtcp_common_t common; /* common header */
union {
/* sender report (SR) */
struct {
u_int32 ssrc; /* sender generating this report */
u_int32 ntp_sec; /* NTP timestamp */
u_int32 ntp_frac;
u_int32 rtp_ts; /* RTP timestamp */
u_int32 psent; /* packets sent */
u_int32 osent; /* octets sent */
rtcp_rr_t rr[1]; /* variable-length list */
} sr;
/* reception report (RR) */
struct {
u_int32 ssrc; /* receiver generating this report */
rtcp_rr_t rr[1]; /* variable-length list */
} rr;
/* source description (SDES) */
struct rtcp_sdes {
u_int32 src; /* first SSRC/CSRC */
rtcp_sdes_item_t item[1]; /* list of SDES items */
} sdes;
/* BYE */
struct {
u_int32 src[1]; /* list of sources */
/* can't express trailing text for reason */
} bye;
} r;
} rtcp_t;
typedef struct rtcp_sdes rtcp_sdes_t;
/*
* Per-source state information
*/
typedef struct {
u_int16 max_seq; /* highest seq. number seen */
u_int32 cycles; /* shifted count of seq. number cycles */
u_int32 base_seq; /* base seq number */
u_int32 bad_seq; /* last 'bad' seq number + 1 */
u_int32 probation; /* sequ. packets till source is valid */
u_int32 received; /* packets received */
u_int32 expected_prior; /* packet expected at last interval */
u_int32 received_prior; /* packet received at last interval */
u_int32 transit; /* relative trans time for prev pkt */
u_int32 jitter; /* estimated jitter */
/* ... */
} source;
A.1 RTP Data Header Validity Checks
An RTP receiver SHOULD check the validity of the RTP header on
incoming packets since they might be encrypted or might be from a
different application that happens to be misaddressed. Similarly, if
encryption according to the method described in Section 9 is enabled,
the header validity check is needed to verify that incoming packets
have been correctly decrypted, although a failure of the header
validity check (e.g., unknown payload type) may not necessarily
indicate decryption failure.
Only weak validity checks are possible on an RTP data packet from a
source that has not been heard before:
o RTP version field must equal 2.
o The payload type must be known, and in particular it must not
be equal to SR or RR.
o If the P bit is set, then the last octet of the packet must
contain a valid octet count, in particular, less than the
total packet length minus the header size.
o The X bit must be zero if the profile does not specify that
the header extension mechanism may be used. Otherwise, the
extension length field must be less than the total packet size
minus the fixed header length and padding.
o The length of the packet must be consistent with CC and
payload type (if payloads have a known length).
The last three checks are somewhat complex and not always possible,
leaving only the first two which total just a few bits. If the SSRC
identifier in the packet is one that has been received before, then
the packet is probably valid and checking if the sequence number is
in the expected range provides further validation. If the SSRC
identifier has not been seen before, then data packets carrying that
identifier may be considered invalid until a small number of them
arrive with consecutive sequence numbers. Those invalid packets MAY
be discarded or they MAY be stored and delivered once validation has
been achieved if the resulting delay is acceptable.
The routine update_seq shown below ensures that a source is declared
valid only after MIN_SEQUENTIAL packets have been received in
sequence. It also validates the sequence number seq of a newly
received packet and updates the sequence state for the packet's
source in the structure to which s points.
When a new source is heard for the first time, that is, its SSRC
identifier is not in the table (see Section 8.2), and the per-source
state is allocated for it, s->probation should be set to the number
of sequential packets required before declaring a source valid
(parameter MIN_SEQUENTIAL ) and s->max_seq initialized to seq-1 s-
>probation marks the source as not yet valid so the state may be
discarded after a short timeout rather than a long one, as discussed
in Section 6.2.1.
After a source is considered valid, the sequence number is considered
valid if it is no more than MAX_DROPOUT ahead of s->max_seq nor more
than MAX_MISORDER behind. If the new sequence number is ahead of
max_seq modulo the RTP sequence number range (16 bits), but is
smaller than max_seq , it has wrapped around and the (shifted) count
of sequence number cycles is incremented. A value of one is returned
to indicate a valid sequence number.
Otherwise, the value zero is returned to indicate that the validation
failed, and the bad sequence number plus 1 is stored. If the next
packet received carries the next higher sequence number, it is
considered the valid start of a new packet sequence presumably caused
by an extended dropout or a source restart. Since multiple complete
sequence number cycles may have been missed, the packet loss
statistics are reset.
Typical values for the parameters are shown, based on a maximum
misordering time of 2 seconds at 50 packets/second and a maximum
dropout of 1 minute. The dropout parameter MAX_DROPOUT SHOULD be a
small fraction of the 16-bit sequence number space to give a
reasonable probability that new sequence numbers after a restart will
not fall in the acceptable range for sequence numbers from before the
restart.
void init_seq(source *s, u_int16 seq)
{
s->base_seq = seq;
s->max_seq = seq;
s->bad_seq = RTP_SEQ_MOD + 1; /* so seq == bad_seq is false */
s->cycles = 0;
s->received = 0;
s->received_prior = 0;
s->expected_prior = 0;
/* other initialization */
}
int update_seq(source *s, u_int16 seq)
{
u_int16 udelta = seq - s->max_seq;
const int MAX_DROPOUT = 3000;
const int MAX_MISORDER = 100;
const int MIN_SEQUENTIAL = 2;
/*
* Source is not valid until MIN_SEQUENTIAL packets with
* sequential sequence numbers have been received.
*/
if (s->probation) {
/* packet is in sequence */
if (seq == s->max_seq + 1) {
s->probation--;
s->max_seq = seq;
if (s->probation == 0) {
init_seq(s, seq);
s->received++;
return 1;
}
} else {
s->probation = MIN_SEQUENTIAL - 1;
s->max_seq = seq;
}
return 0;
} else if (udelta < MAX_DROPOUT) {
/* in order, with permissible gap */
if (seq < s->max_seq) {
/*
* Sequence number wrapped - count another 64K cycle.
*/
s->cycles += RTP_SEQ_MOD;
}
s->max_seq = seq;
} else if (udelta <= RTP_SEQ_MOD - MAX_MISORDER) {
/* the sequence number made a very large jump */
if (seq == s->bad_seq) {
/*
* Two sequential packets -- assume that the other side
* restarted without telling us so just re-sync
* (i.e., pretend this was the first packet).
*/
init_seq(s, seq);
}
else {
s->bad_seq = (seq + 1) & (RTP_SEQ_MOD-1);
return 0;
}
} else {
/* duplicate or reordered packet */
}
s->received++;
return 1;
}
The validity check can be made stronger requiring more than two
packets in sequence. The disadvantages are that a larger number of
initial packets will be discarded (or delayed in a queue) and that
high packet loss rates could prevent validation. However, because the
RTCP header validation is relatively strong, if an RTCP packet is
received from a source before the data packets, the count could be
adjusted so that only two packets are required in sequence. If
initial data loss for a few seconds can be tolerated, an application
MAY choose to discard all data packets from a source until a valid
RTCP packet has been received from that source.
Depending on the application and encoding, algorithms may exploit
additional knowledge about the payload format for further validation.
For payload types where the timestamp increment is the same for all
packets, the timestamp values can be predicted from the previous
packet received from the same source using the sequence number
difference (assuming no change in payload type).
A strong "fast-path" check is possible since with high probability
the first four octets in the header of a newly received RTP data
packet will be just the same as that of the previous packet from the
same SSRC except that the sequence number will have increased by one.
Similarly, a single-entry cache may be used for faster SSRC lookups
in applications where data is typically received from one source at a
time.
A.2 RTCP Header Validity Checks
The following checks SHOULD be applied to RTCP packets.
o RTP version field must equal 2.
o The payload type field of the first RTCP packet in a compound
packet must be equal to SR or RR.
o The padding bit (P) should be zero for the first packet of a
compound RTCP packet because padding should only be applied,
if it is needed, to the last packet.
o The length fields of the individual RTCP packets must total to
the overall length of the compound RTCP packet as received.
This is a fairly strong check.
The code fragment below performs all of these checks. The packet type
is not checked for subsequent packets since unknown packet types may
be present and should be ignored.
u_int32 len; /* length of compound RTCP packet in words */
rtcp_t *r; /* RTCP header */
rtcp_t *end; /* end of compound RTCP packet */
if ((*(u_int16 *)r & RTCP_VALID_MASK) != RTCP_VALID_VALUE) {
/* something wrong with packet format */
}
end = (rtcp_t *)((u_int32 *)r + len);
do r = (rtcp_t *)((u_int32 *)r + r->common.length + 1);
while (r < end && r->common.version == 2);
if (r != end) {
/* something wrong with packet format */
}
A.3 Determining the Number of RTP Packets Expected and Lost
In order to compute packet loss rates, the number of packets expected
and actually received from each source needs to be known, using per-
source state information defined in struct source referenced via
pointer s in the code below. The number of packets received is simply
the count of packets as they arrive, including any late or duplicate
packets. The number of packets expected can be computed by the
receiver as the difference between the highest sequence number
received ( s->max_seq ) and the first sequence number received ( s-
>base_seq ). Since the sequence number is only 16 bits and will wrap
around, it is necessary to extend the highest sequence number with
the (shifted) count of sequence number wraparounds ( s->cycles ).
Both the received packet count and the count of cycles are maintained
the RTP header validity check routine in Appendix A.1.
extended_max = s->cycles + s->max_seq;
expected = extended_max - s->base_seq + 1;
The number of packets lost is defined to be the number of packets
expected less the number of packets actually received:
lost = expected - s->received;
Since this signed number is carried in 24 bits, it SHOULD be clamped
at 0x7fffff for positive loss or 0x800000 for negative loss rather
than wrapping around.
The fraction of packets lost during the last reporting interval
(since the previous SR or RR packet was sent) is calculated from
differences in the expected and received packet counts across the
interval, where expected_prior and received_prior are the values
saved when the previous reception report was generated:
expected_interval = expected - s->expected_prior;
s->expected_prior = expected;
received_interval = s->received - s->received_prior;
s->received_prior = s->received;
lost_interval = expected_interval - received_interval;
if (expected_interval == 0 || lost_interval <= 0) fraction = 0;
else fraction = (lost_interval << 8) / expected_interval;
The resulting fraction is an 8-bit fixed point number with the binary
point at the left edge.
A.4 Generating SDES RTCP Packets
This function builds one SDES chunk into buffer b composed of argc
items supplied in arrays type , value and length b
char *rtp_write_sdes(char *b, u_int32 src, int argc,
rtcp_sdes_type_t type[], char *value[],
int length[])
{
rtcp_sdes_t *s = (rtcp_sdes_t *)b;
rtcp_sdes_item_t *rsp;
int i;
int len;
int pad;
/* SSRC header */
s->src = src;
rsp = &s->item[0];
/* SDES items */
for (i = 0; i < argc; i++) {
rsp->type = type[i];
len = length[i];
if (len > RTP_MAX_SDES) {
/* invalid length, may want to take other action */
len = RTP_MAX_SDES;
}
rsp->length = len;
memcpy(rsp->data, value[i], len);
rsp = (rtcp_sdes_item_t *)&rsp->data[len];
}
/* terminate with end marker and pad to next 4-octet boundary */
len = ((char *) rsp) - b;
pad = 4 - (len & 0x3);
b = (char *) rsp;
while (pad--) *b++ = RTCP_SDES_END;
return b;
}
A.5 Parsing RTCP SDES Packets
This function parses an SDES packet, calling functions find_member()
to find a pointer to the information for a session member given the
SSRC identifier and member_sdes() to store the new SDES information
for that member. This function expects a pointer to the header of the
RTCP packet.
void rtp_read_sdes(rtcp_t *r)
{
int count = r->common.count;
rtcp_sdes_t *sd = &r->r.sdes;
rtcp_sdes_item_t *rsp, *rspn;
rtcp_sdes_item_t *end = (rtcp_sdes_item_t *)
((u_int32 *)r + r->common.length + 1);
source *s;
while (--count >= 0) {
rsp = &sd->item[0];
if (rsp >= end) break;
s = find_member(sd->src);
for (; rsp->type; rsp = rspn ) {
rspn = (rtcp_sdes_item_t *)((char*)rsp+rsp->length+2);
if (rspn >= end) {
rsp = rspn;
break;
}
member_sdes(s, rsp->type, rsp->data, rsp->length);
}
sd = (rtcp_sdes_t *)
((u_int32 *)sd + (((char *)rsp - (char *)sd) >> 2)+1);
}
if (count >= 0) {
/* invalid packet format */
}
}
A.6 Generating a Random 32-bit Identifier
The following subroutine generates a random 32-bit identifier using
the MD5 routines published in RFC 1321 [30]. The system routines may
not be present on all operating systems, but they should serve as
hints as to what kinds of information may be used. Other system calls
that may be appropriate include
o getdomainname() ,
o getwd() , or
o getrusage()
"Live" video or audio samples are also a good source of random
numbers, but care must be taken to avoid using a turned-off
microphone or blinded camera as a source [17].
Use of this or similar routine is RECOMMENDED to generate the initial
seed for the random number generator producing the RTCP period (as
shown in Appendix A.7), to generate the initial values for the
sequence number and timestamp, and to generate SSRC values. Since
this routine is likely to be CPU-intensive, its direct use to
generate RTCP periods is inappropriate because predictability is not
an issue. Note that this routine produces the same result on repeated
calls until the value of the system clock changes unless different
values are supplied for the type argument.
/*
* Generate a random 32-bit quantity.
*/
#include <sys/types.h> /* u_long */
#include <sys/time.h> /* gettimeofday() */
#include <unistd.h> /* get..() */
#include <stdio.h> /* printf() */
#include <time.h> /* clock() */
#include <sys/utsname.h> /* uname() */
#include "global.h" /* from RFC 1321 */
#include "md5.h" /* from RFC 1321 */
#define MD_CTX MD5_CTX
#define MDInit MD5Init
#define MDUpdate MD5Update
#define MDFinal MD5Final
static u_long md_32(char *string, int length)
{
MD_CTX context;
union {
char c[16];
u_long x[4];
} digest;
u_long r;
int i;
MDInit (&context);
MDUpdate (&context, string, length);
MDFinal ((unsigned char *)&digest, &context);
r = 0;
for (i = 0; i < 3; i++) {
r ^= digest.x[i];
}
return r;
} /* md_32 */
/*
* Return random unsigned 32-bit quantity. Use 'type' argument if you
* need to generate several different values in close succession.
*/
u_int32 random32(int type)
{
struct {
int type;
struct timeval tv;
clock_t cpu;
pid_t pid;
u_long hid;
uid_t uid;
gid_t gid;
struct utsname name;
} s;
gettimeofday(&s.tv, 0);
uname(&s.name);
s.type = type;
s.cpu = clock();
s.pid = getpid();
s.hid = gethostid();
s.uid = getuid();
s.gid = getgid();
/* also: system uptime */
return md_32((char *)&s, sizeof(s));
} /* random32 */
A.7 Computing the RTCP Transmission Interval
The following functions implement the RTCP transmission and reception
rules described in Section 6.2. These rules are coded in several
functions:
o rtcp_interval() computes the deterministic calculated
interval, measured in seconds. The parameters are defined in
Section 6.3.
o OnExpire() is called when the RTCP transmission timer expires.
o OnReceive() is called whenever an RTCP packet is received.
Both OnExpire() and OnReceive() have event e as an argument. This is
the next scheduled event for that participant, either an RTCP report
or a BYE packet. It is assumed that the following functions are
available:
o Schedule(time t, event e) schedules an event e to occur at
time t. When time t arrives, the function OnExpire is called
with e as an argument.
o Reschedule(time t, event e) reschedules a previously scheduled
event e for time t.
o SendRTCPReport(event e) sends an RTCP report.
o SendBYEPacket(event e) sends a BYE packet.
o TypeOfEvent(event e) returns EVENT_BYE if the event being
processed is for a BYE packet to be sent, else it returns
EVENT_REPORT.
o PacketType(p) returns PACKET_RTCP_REPORT if packet p is an
RTCP report (not BYE), PACKET_BYE if its a BYE RTCP packet,
and PACKET_RTP if its a regular RTP data packet.
o ReceivedPacketSize() and SentPacketSize() return the size of
the referenced packet in octets.
o NewMember(p) returns a 1 if the participant who sent packet p
is not currently in the member list, 0 otherwise. Note this
function is not sufficient for a complete implementation
because each CSRC identifier in an RTP packet and each SSRC in
a BYE packet should be processed.
o NewSender(p) returns a 1 if the participant who sent packet p
is not currently in the sender sublist of the member list, 0
otherwise.
o AddMember() and RemoveMember() to add and remove participants
from the member list.
o AddSender() and RemoveSender() to add and remove participants
from the sender sublist of the member list.
double rtcp_interval(int members,
int senders,
double rtcp_bw,
int we_sent,
double avg_rtcp_size,
int initial)
{
/*
* Minimum average time between RTCP packets from this site (in
* seconds). This time prevents the reports from `clumping' when
* sessions are small and the law of large numbers isn't helping
* to smooth out the traffic. It also keeps the report interval
* from becoming ridiculously small during transient outages like
* a network partition.
*/
double const RTCP_MIN_TIME = 5.;
/*
* Fraction of the RTCP bandwidth to be shared among active
* senders. (This fraction was chosen so that in a typical
* session with one or two active senders, the computed report
* time would be roughly equal to the minimum report time so that
* we don't unnecessarily slow down receiver reports.) The
* receiver fraction must be 1 - the sender fraction.
*/
double const RTCP_SENDER_BW_FRACTION = 0.25;
double const RTCP_RCVR_BW_FRACTION = (1-RTCP_SENDER_BW_FRACTION);
/*
/* To compensate for "unconditional reconsideration" converging to a
* value below the intended average.
*/
double const COMPENSATION = 2.71828 - 1.5;
double t; /* interval */
double rtcp_min_time = RTCP_MIN_TIME;
int n; /* no. of members for computation */
/*
* Very first call at application start-up uses half the min
* delay for quicker notification while still allowing some time
* before reporting for randomization and to learn about other
* sources so the report interval will converge to the correct
* interval more quickly.
*/
if (initial) {
rtcp_min_time /= 2;
}
/*
* If there were active senders, give them at least a minimum
* share of the RTCP bandwidth. Otherwise all participants share
* the RTCP bandwidth equally.
*/
n = members;
if (senders > 0 && senders < members * RTCP_SENDER_BW_FRACTION) {
if (we_sent) {
rtcp_bw *= RTCP_SENDER_BW_FRACTION;
n = senders;
} else {
rtcp_bw *= RTCP_RCVR_BW_FRACTION;
n -= senders;
}
}
/*
* The effective number of sites times the average packet size is
* the total number of octets sent when each site sends a report.
* Dividing this by the effective bandwidth gives the time
* interval over which those packets must be sent in order to
* meet the bandwidth target, with a minimum enforced. In that
* time interval we send one report so this time is also our
* average time between reports.
*/
t = avg_rtcp_size * n / rtcp_bw;
if (t < rtcp_min_time) t = rtcp_min_time;
/*
* To avoid traffic bursts from unintended synchronization with
* other sites, we then pick our actual next report interval as a
* random number uniformly distributed between 0.5*t and 1.5*t.
*/
t = t * (drand48() + 0.5);
t = t / COMPENSATION;
return t;
}
void OnExpire(event e,
int members,
int senders,
double rtcp_bw,
int we_sent,
double *avg_rtcp_size,
int *initial,
time_tp tc,
time_tp *tp,
int *pmembers)
{
/* This function is responsible for deciding whether to send
* an RTCP report or BYE packet now, or to reschedule transmission.
* It is also responsible for updating the pmembers, initial, tp,
* and avg_rtcp_size state variables. This function should be called
* upon expiration of the event timer used by Schedule(). */
double t; /* Interval */
double tn; /* Next transmit time */
/* In the case of a BYE, we use "unconditional reconsideration" to
* reschedule the transmission of the BYE if necessary */
if (TypeOfEvent(e) == EVENT_BYE) {
t = rtcp_interval(members,
senders,
rtcp_bw,
we_sent,
*avg_rtcp_size,
*initial);
tn = *tp + t;
if (tn <= tc) {
SendBYEPacket(e);
exit(1);
} else {
Schedule(tn, e);
}
} else if (TypeOfEvent(e) == EVENT_REPORT) {
t = rtcp_interval(members,
senders,
rtcp_bw,
we_sent,
*avg_rtcp_size,
*initial);
tn = *tp + t;
if (tn <= tc) {
SendRTCPReport(e);
*avg_rtcp_size = (1./16.)*SentPacketSize(e) +
(15./16.)*(*avg_rtcp_size);
*tp = tc;
/* We must redraw the interval. Don't reuse the
one computed above, since its not actually
distributed the same, as we are conditioned
on it being small enough to cause a packet to
be sent */
t = rtcp_interval(members,
senders,
rtcp_bw,
we_sent,
*avg_rtcp_size,
*initial);
Schedule(t+tc,e);
*initial = 0;
} else {
Schedule(tn, e);
}
*pmembers = members;
}
}
void OnReceive(packet p,
event e,
int *members,
int *pmembers,
int *senders,
double *avg_rtcp_size,
double *tp,
double tc,
double tn)
{
/* What we do depends on whether we have left the group, and
* are waiting to send a BYE (TypeOfEvent(e) == EVENT_BYE) or
* an RTCP report. p represents the packet that was just received. */
if (PacketType(p) == PACKET_RTCP_REPORT) {
if (NewMember(p) && (TypeOfEvent(e) == EVENT_REPORT)) {
AddMember(p);
*members += 1;
}
*avg_rtcp_size = (1./16.)*ReceivedPacketSize(p) +
(15./16.)*(*avg_rtcp_size);
} else if (PacketType(p) == PACKET_RTP) {
if (NewMember(p) && (TypeOfEvent(e) == EVENT_REPORT)) {
AddMember(p);
*members += 1;
}
if (NewSender(p) && (TypeOfEvent(e) == EVENT_REPORT)) {
AddSender(p);
*senders += 1;
}
} else if (PacketType(p) == PACKET_BYE) {
*avg_rtcp_size = (1./16.)*ReceivedPacketSize(p) +
(15./16.)*(*avg_rtcp_size);
if (TypeOfEvent(e) == EVENT_REPORT) {
if (NewSender(p) == FALSE) {
RemoveSender(p);
*senders -= 1;
}
if (NewMember(p) == FALSE) {
RemoveMember(p);
*members -= 1;
}
if(*members < *pmembers) {
tn = tc + (((double) *members)/(*pmembers))*(tn - tc);
*tp = tc - (((double) *members)/(*pmembers))*(tc - *tp);
/* Reschedule the next report for time tn */
Reschedule(tn, e);
*pmembers = *members;
}
} else if (TypeOfEvent(e) == EVENT_BYE) {
*members += 1;
}
}
}
A.8 Estimating the Interarrival Jitter
The code fragments below implement the algorithm given in Section
6.4.1 for calculating an estimate of the statistical variance of the
RTP data interarrival time to be inserted in the interarrival jitter
field of reception reports. The inputs are r->ts , the timestamp from
the incoming packet, and arrival , the current time in the same
units. Here s points to state for the source; s->transit holds the
relative transit time for the previous packet, and s->jitter holds
the estimated jitter. The jitter field of the reception report is
measured in timestamp units and expressed as an unsigned integer, but
the jitter estimate is kept in a floating point. As each data packet
arrives, the jitter estimate is updated:
int transit = arrival - r->ts;
int d = transit - s->transit;
s->transit = transit;
if (d < 0) d = -d;
s->jitter += (1./16.) * ((double)d - s->jitter);
When a reception report block (to which rr points) is generated for
this member, the current jitter estimate is returned:
rr->jitter = (u_int32) s->jitter;
Alternatively, the jitter estimate can be kept as an integer, but
scaled to reduce round-off error. The calculation is the same except
for the last line:
s->jitter += d - ((s->jitter + 8) >> 4);
In this case, the estimate is sampled for the reception report as:
rr->jitter = s->jitter >> 4;
B Changes from RFC 1889
Most of this RFC is identical to RFC 1889. There are no changes in
the packet formats on the wire, only changes to the rules and
algorithms governing how the protocol is used. The biggest change was
an enhancement to the scalable timer algorithm for calculating when
to send RTCP packets:
o The algorithm for calculating the RTCP transmission interval
specified in Sections 6.2 and 6.3 and illustrated in Appendix
A.7 is augmented to include "reconsideration" to minimize
transmission in excess of the intended rate when many
participants join a session simultaneously, and "reverse
reconsideration" to reduce the incidence and duration of false
participant timeouts when the number of participants drops
rapidly. Reverse reconsideration is also used to possibly
shorten the delay before sending RTCP SR when transitioning
from passive receiver to active sender mode.
o Section 6.3.7 specifies new rules controlling when an RTCP BYE
packet should be sent in order to avoid a flood of packets
when many participants leave a session simultaneously.
o The requirement to retain state for inactive participants for
a period long enough to span typical network partitions was
removed from Section 6.2.1. In a session where many
participants join for a brief time and fail to send BYE, this
requirement would cause a significant overestimate of the
number of participants. The reconsideration algorithm added in
this revision compensates for the large number of new
participants joining simultaneously when a partition heals.
It should be noted that these enhancements only have a significant
effect when the number of session participants is large (thousands)
and most of the participants join or leave at the same time. This
makes testing in a live network difficult. However, the algorithm was
subjected to a thorough analysis and simulation to verify its
performance. Furthermore, the enhanced algorithm was designed to
interoperate with the algorithm in RFC 1889 such that the degree of
reduction in excess RTCP bandwidth during a step join is proportional
to the fraction of participants that implement the enhanced
algorithm. Interoperation of the two algorithms has been verified
experimentally on live networks.
Other functional changes were:
o Section 6.2.1 specifies that implementations may store only a
sampling of the participants' SSRC identifiers to allow
scaling to very large sessions. Algorithms are specified in
RFC 2762 [21].
o In Section 6.2 it is specified that RTCP sender and receiver
bandwidths to be set as separate parameters of the session
rather than a strict percentage of the session bandwidth, and
may be set to zero. The requirement that RTCP was mandatory
for RTP sessions using IP multicast was relaxed.
o Also in Section 6.2 it is specified that the minimum RTCP
interval may be scaled to smaller values for high bandwidth
sessions, and that the initial RTCP delay may be set to zero
for unicast sessions.
o Timing out a participant is to be based on inactivity for a
number of RTCP report intervals calculated using the receiver
RTCP bandwidth fraction even for active senders.
o Sections 7.2 and 7.3 specify that translators and mixers
should send BYE packets for the sources they are no longer
forwarding.
o Rule changes for layered encodings are defined in Sections
2.4, 6.3.9, 8.3 and 11. In the last of these, it is noted that
the address and port assignment rule conflicts with the SDP
specification, RFC 2327 [15], but it is intended that this
restriction will be relaxed in a revision of RFC 2327.
o The convention for using even/odd port pairs for RTP and RTCP
in Section 11 was clarified to refer to destination ports.
The requirement to use an even/odd port pair was removed if
the two ports are specified explicitly. For unicast RTP
sessions, distinct port pairs may be used for the two ends
(Sections 3, 7.1 and 11).
o A new Section 10 was added to explain the requirement for
congestion control in applications using RTP.
o In Section 8.2, the requirement that a new SSRC identifier
MUST be chosen whenever the source transport address is
changed has been relaxed to say that a new SSRC identifier MAY
be chosen. Correspondingly, it was clarified that an
implementation MAY choose to keep packets from the new source
address rather than the existing source address when a
collision occurs, and SHOULD do so for applications such as
telephony in which some sources such as mobile entities may
change addresses during the course of an RTP session.
o An indentation bug in the RFC 1889 printing of the pseudo-code
for the collision detection and resolution algorithm in
Section 8.2 has been corrected by translating the syntax to
pseudo C language, and the algorithm has been modified to
remove the restriction that both RTP and RTCP must be sent
from the same source port number.
o The description of the padding mechanism for RTCP packets was
clarified and it is specified that padding MUST only be
applied to the last packet of a compound RTCP packet.
o In Section A.1, initialization of base_seq was corrected to be
seq rather than seq - 1, and the text was corrected to say the
bad sequence number plus 1 is stored.
o Clamping of number of packets lost in Section A.3 was
corrected to use both positive and negative limits.
o The specification of "relative" NTP timestamp in the RTCP SR
section now defines these timestamps to be based on the most
common system-specific clock, such as system uptime, rather
than on session elapsed time which would not be the same for
multiple applications started on the same machine at different
times.
Non-functional changes:
o It is specified that a receiver MUST ignore packets with
payload types it does not understand.
o In Fig. 2, the floating point NTP timestamp value was
corrected and the UTC timezone was specified.
o The inconsequence of NTP timestamps wrapping around in the
year 2036 is explained.
o The policy for registration of RTCP packet types and SDES
types was clarified in a new Section 14, IANA Considerations.
The suggestion that experimenters register the numbers they
need and then unregister those which prove to be unneeded has
been removed in favor of using APP and PRIV. Registration of
profile names was also specified.
o The reference for the UTF-8 character set was changed from an
X/Open Preliminary Specification to be RFC 2279.
o The last paragraph of the introduction in RFC 1889, which
cautioned implementers to limit deployment in the Internet,
was removed because it was deemed no longer relevant.
o Small clarifications of the text have been made in several
places, some in response to questions from readers. In
particular:
- A definition for "RTP media type" is given in Section 3 to
allow the explanation of multiplexing RTP sessions in
Section 5.2 to be more clear regarding the multiplexing of
multiple media.
- The definition for "non-RTP means" was expanded to include
examples of other protocols constituting non-RTP means.
- The description of the session bandwidth parameter is
expanded in Section 6.2.
- The effect of varying packet duration on the jitter
calculation was explained in Section 6.4.4.
- The method for terminating and padding a sequence of SDES
items was clarified in Section 6.5.
- The Security section adds a formal reference to IPSEC now
that it is available, and says that the confidentiality
method defined in this specification is primarily to codify
existing practice. It is RECOMMENDED that stronger
encryption algorithms such as Triple-DES be used in place of
the default algorithm. It is also noted that payload-only
encryption is necessary to allow for header compression.
- The method for partial encryption of RTCP was clarified; in
particular, SDES CNAME is carried in only one part when the
compound RTCP packet is split.
- It is clarified that only one compound RTCP packet should be
sent per reporting interval and that if there are too many
active sources for the reports to fit in the MTU, then a
subset of the sources should be selected round-robin over
multiple intervals.
- A note was added in Appendix A.1 that packets may be saved
during RTP header validation and delivered upon success.
- Section 7.3 now explains that a mixer aggregating SDES
packets uses more RTCP bandwidth due to longer packets, and
a mixer passing through RTCP naturally sends packets at
higher than the single source rate, but both behaviors are
valid.
- Section 13 clarifies that an RTP application may use
multiple profiles but typically only one in a given session.
- The terms MUST, SHOULD, MAY, etc. are used as defined in RFC
2119.
- The bibliography was divided into normative and non-
normative references.
C Security Considerations
RTP suffers from the same security liabilities as the underlying
protocols. For example, an impostor can fake source or destination
network addresses, or change the header or payload. Within RTCP, the
CNAME and NAME information may be used to impersonate another
participant. In addition, RTP may be sent via IP multicast, which
provides no direct means for a sender to know all the receivers of
the data sent and therefore no measure of privacy. Rightly or not,
users may be more sensitive to privacy concerns with audio and video
communication than they have been with more traditional forms of
network communication [31]. Therefore, the use of security mechanisms
with RTP is important. These mechanisms are discussed in Section 9.
RTP-level translators or mixers may be used to allow RTP traffic to
reach hosts behind firewalls. Appropriate firewall security
principles and practices, which are beyond the scope of this
document, should be followed in the design and installation of these
devices and in the admission of RTP applications for use behind the
firewall.
D Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
E Addresses of Authors
Henning Schulzrinne
Department of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
Stephen L. Casner
Packet Design
2465 Latham Street
Mountain View, CA 94040
United States
electronic mail: casner@acm.org
Ron Frederick
Cacheflow Inc.
650 Almanor Avenue
Sunnyvale, CA 94085
United States
electronic mail: ronf@cacheflow.com
Van Jacobson
Packet Design
2465 Latham Street
Mountain View, CA 94040
United States
electronic mail: van@packetdesign.com
Acknowledgments
This memorandum is based on discussions within the IETF Audio/Video
Transport working group chaired by Stephen Casner and Colin Perkins.
The current protocol has its origins in the Network Voice Protocol
and the Packet Video Protocol (Danny Cohen and Randy Cole) and the
protocol implemented by the vat application (Van Jacobson and Steve
McCanne). Christian Huitema provided ideas for the random identifier
generator. Extensive analysis and simulation of the timer
reconsideration algorithm was done by Jonathan Rosenberg.
References
Normative References
[1] H. Schulzrinne and S. Casner, "RTP profile for audio and video
conferences with minimal control," Request for Comments XXXX,
Internet Engineering Task Force. Work in progress.
[2] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," Request for Comments (Best Current Practice) 2119, Internet
Engineering Task Force, Mar. 1997.
[3] J. Postel, "Internet protocol," Request for Comments (Standard)
791, Internet Engineering Task Force, Sept. 1981.
[4] D. L. Mills, "Network time protocol (version 3) specification,
implementation," Request for Comments (Draft Standard) 1305, Internet
Engineering Task Force, Mar. 1992.
[5] F. Yergeau, "UTF-8, a transformation format of ISO 10646,"
Request for Comments (Proposed Standard) 2279, Internet Engineering
Task Force, Jan. 1998.
[6] P. V. Mockapetris, "Domain names - concepts and facilities,"
Request for Comments (Standard) 1034, Internet Engineering Task
Force, Nov. 1987.
[7] P. V. Mockapetris, "Domain names - implementation and
specification," Request for Comments (Standard) 1035, Internet
Engineering Task Force, Nov. 1987.
[8] R. T. Braden, "Requirements for internet hosts - application and
support," Request for Comments (Standard) 1123, Internet Engineering
Task Force, Oct. 1989.
[9] D. Crocker, "Standard for the format of ARPA internet text
messages," Request for Comments (Standard) 822, Internet Engineering
Task Force, Aug. 1982.
Non-Normative References
[10] D. D. Clark and D. L. Tennenhouse, "Architectural considerations
for a new generation of protocols," in SIGCOMM Symposium on
Communications Architectures and Protocols , (Philadelphia,
Pennsylvania), pp. 200--208, IEEE, Sept. 1990. Computer
Communications Review, Vol. 20(4), Sept. 1990.
[11] H. Schulzrinne, "Issues in designing a transport protocol for
audio and video conferences and other multiparticipant real-time
applications." expired Internet draft, Oct. 1993.
[12] D. E. Comer, Internetworking with TCP/IP , vol. 1. Englewood
Cliffs, New Jersey: Prentice Hall, 1991.
[13] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments (Proposed
Standard) 2543, Internet Engineering Task Force, Mar. 1999.
[14] International Telecommunication Union, "Visual telephone systems
and equipment for local area networks which provide a non-guaranteed
quality of service," Recommendation H.323, Telecommunication
Standardization Sector of ITU, Geneva, Switzerland, May 1996.
[15] M. Handley and V. Jacobson, "SDP: session description protocol,"
Request for Comments (Proposed Standard) 2327, Internet Engineering
Task Force, Apr. 1998.
[16] H. Schulzrinne, A. Rao, and R. Lanphier, "Real time streaming
protocol (RTSP)," Request for Comments (Proposed Standard) 2326,
Internet Engineering Task Force, Apr. 1998.
[17] D. Eastlake, 3rd, S. Crocker, and J. Schiller, "Randomness
recommendations for security," Request for Comments (Informational)
1750, Internet Engineering Task Force, Dec. 1994.
[18] J.-C. Bolot, T. Turletti, and I. Wakeman, "Scalable feedback
control for multicast video distribution in the internet," in SIGCOMM
Symposium on Communications Architectures and Protocols , (London,
England), pp. 58--67, ACM, Aug. 1994.
[19] I. Busse, B. Deffner, and H. Schulzrinne, "Dynamic QoS control
of multimedia applications based on RTP," Computer Communications ,
vol. 19, pp. 49--58, Jan. 1996.
[20] S. Floyd and V. Jacobson, "The synchronization of periodic
routing messages," in SIGCOMM Symposium on Communications
Architectures and Protocols (D. P. Sidhu, ed.), (San Francisco,
California), pp. 33--44, ACM, Sept. 1993. also in [32].
[21] J. Rosenberg and H. Schulzrinne, "Sampling of the group Title: RTP: A Transport Protocol for Real-Time
membership in RTP," Request for Comments (Experimental) 2762, Applications
Internet Engineering Task Force, May 1999. Author(s): H. Schulzrinne, S. Casner, R. Frederick,
V. Jacobson
Status: Standards Track
Date: July 2003
Mailbox: schulzrinne@cs.columbia.edu, casner@acm.org,
ronf@bluecoat.com, van@packetdesign.com
Pages: 104
Characters: 259985
Obsoletes: 1889
[22] J. A. Cadzow, Foundations of digital signal processing and data I-D Tag: draft-ietf-avt-rtp-new-12.txt
analysis New York, New York: Macmillan, 1987.
[23] Y. Rekhter, B. Moskowitz, D. Karrenberg, and G. de Groot, URL: ftp://ftp.rfc-editor.org/in-notes/rfc3550.txt
"Address allocation for private internets," Request for Comments ftp://ftp.rfc-editor.org/in-notes/rfc3550.ps
(Informational) 1597, Internet Engineering Task Force, Mar. 1994.
[24] E. Lear, E. Fair, D. Crocker, and T. Kessler, "Network 10 This memorandum describes RTP, the real-time transport protocol. RTP
considered harmful (some practices shouldn't be codified)," Request provides end-to-end network transport functions suitable for
for Comments (Informational) 1627, Internet Engineering Task Force, applications transmitting real-time data, such as audio, video or
June 1994. simulation data, over multicast or unicast network services. RTP
does not address resource reservation and does not guarantee
quality-of-service for real-time services. The data transport is
augmented by a control protocol (RTCP) to allow monitoring of the
data delivery in a manner scalable to large multicast networks, and
to provide minimal control and identification functionality. RTP and
RTCP are designed to be independent of the underlying transport and
network layers. The protocol supports the use of RTP-level
translators and mixers.
[25] W. Feller, An Introduction to Probability Theory and its Most of the text in this memorandum is identical to RFC 1889 which it
Applications, Volume 1 , vol. 1. New York, New York: John Wiley and obsoletes. There are no changes in the packet formats on the wire,
Sons, third ed., 1968. only changes to the rules and algorithms governing how the protocol
is used. The biggest change is an enhancement to the scalable timer
algorithm for calculating when to send RTCP packets in order to
minimize transmission in excess of the intended rate when many
participants join a session simultaneously.
[26] S. Kent and R. Atkinson, "Security architecture for the internet This document is a product of the Audio/Video Tranposrt Working Group
protocol," Request for Comments (Proposed Standard) 2401, Internet of the IETF.
Engineering Task Force, Nov. 1998.
[27] D. Balenson, "Privacy enhancement for internet electronic mail: This is now a Draft Standard Protocol.
Part III: algorithms, modes, and identifiers," Request for Comments
(Proposed Standard) 1423, Internet Engineering Task Force, Feb. 1993.
[28] V. L. Voydock and S. T. Kent, "Security mechanisms in high-level This document specifies an Internet standards track protocol for
network protocols," ACM Computing Surveys , vol. 15, pp. 135--171, the Internet community, and requests discussion and suggestions
June 1983. for improvements. Please refer to the current edition of the
"Internet Official Protocol Standards" (STD 1) for the
standardization state and status of this protocol. Distribution
of this memo is unlimited.
[29] S. Floyd, "Congestion Control Principles," Request for Comments This announcement is sent to the IETF list and the RFC-DIST list.
(Best Current Practice) 2914, Internet Engineering Task Force, Sep. Requests to be added to or deleted from the IETF distribution list
should be sent to IETF-REQUEST@IETF.ORG. Requests to be
added to or deleted from the RFC-DIST distribution list should
be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
2000. Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
help: ways_to_get_rfcs. For example:
[30] R. Rivest, "The MD5 message-digest algorithm," Request for To: rfc-info@RFC-EDITOR.ORG
Comments (Informational) 1321, Internet Engineering Task Force, Apr. Subject: getting rfcs
1992.
[31] S. Stubblebine, "Security services for multimedia conferencing," help: ways_to_get_rfcs
in 16th National Computer Security Conference , (Baltimore,
Maryland), pp. 391--395, Sept. 1993.
[32] S. Floyd and V. Jacobson, "The synchronization of periodic Requests for special distribution should be addressed to either the
routing messages," IEEE/ACM Transactions on Networking , vol. 2, pp. author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
122--136, Apr. 1994. specifically noted otherwise on the RFC itself, all RFCs are for
unlimited distribution.echo
Submissions for Requests for Comments should be sent to
RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
Authors, for further information.
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