RTP Stream Pause and ResumeEricssonKistavagen 25SE - 164 80 KistaSweden+46107141311bo.burman@ericsson.comwww.ericsson.comEricssonFarogatan 6SE - 164 80 KistaSweden+46107142658muhammad.azam.akram@ericsson.comwww.ericsson.comHuawei TechnologiesTel AvivIsraelroni.even@mail01.huawei.comEricssonFarogatan 6SE- Kista 164 80Sweden+46107148287magnus.westerlund@ericsson.comWith the increased popularity of real-time multimedia applications,
it is desirable to provide good control of resource usage, and users
also demand more control over communication sessions. This document
describes how a receiver in a multimedia conversation can pause and
resume incoming data from a sender by sending real-time feedback
messages when using Real-time Transport Protocol (RTP) for real time
data transport. This document extends the Codec Control Messages (CCM)
RTCP feedback package by explicitly allowing and describing specific use
of existing CCM messages and adding a group of new real-time feedback
messages used to pause and resume RTP data streams. This document
updates RFC 5104.As real-time communication attracts more people, more applications
are created; multimedia conversation applications being one example.
Multimedia conversation further exists in many forms, for example,
peer-to-peer chat application and multiparty video conferencing
controlled by central media nodes, such as RTP Mixers.Multimedia conferencing may involve many participants; each has its
own preferences for the communication session, not only at the start but
also during the session. This document describes several scenarios in
multimedia communication where a conferencing node or participant
chooses to temporarily pause an incoming RTP stream and later resume it when needed. The
receiver does not need to terminate or inactivate the RTP session and
start all over again by negotiating the session parameters, for example
using SIP with SDP
Offer/Answer.Centralized nodes, like RTP Mixers or MCUs, which either uses logic
based on voice activity, other measurements, or user input could reduce
the resources consumed in both the sender and the network by temporarily
pausing the RTP streams that aren't required by the RTP Mixer. If the
number of conference participants are greater than what the conference
logic has chosen to present simultaneously to receiving participants,
some participant RTP streams sent to the RTP Mixer may not need to be
forwarded to any other participant. Those RTP streams could then be
temporarily paused. This becomes especially useful when the media
sources are provided in multiple encoding versions
(Simulcast) or with Multi-Session Transmission (MST) of scalable
encoding such as SVC. There may be some of
the defined encodings or combination of scalable layers that are not
used all of the time.As the RTP streams required at any given point in time is highly
dynamic in such scenarios, using the out-of-band signaling channel for
pausing, and even more importantly resuming, an RTP stream is difficult
due to the performance requirements. Instead, the pause and resume
signaling should be in the media plane and go directly between the
affected nodes. When using RTP for media
transport, using Extended RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/AVPF) appears appropriate. No currently existing RTCP
feedback message explicitly supports pausing and resuming an incoming
RTP stream. As this affects the generation of packets and may even allow
the encoding process to be paused, the functionality appears to match
Codec Control Messages in the RTP Audio-Visual
Profile with Feedback (AVPF) and it is proposed to define the
solution as a Codec Control Message (CCM) extension.The Temporary Maximum Media Bitrate Request (TMMBR) message of CCM is
used by video conferencing systems for flow control. It is desirable to
be able to use that method with a bitrate value of zero for pause and
resume, whenever possible.3rd Generation Partnership ProjectAudio-Visual Profile with Feedback (RFC
4585)Border GatewayCodec Control Messages (RFC 5104)Canonical Name (RTCP SDES)Contributing Source (RTP)Feedback (AVPF)Feedback Control Information (AVPF)Full Intra Refresh (CCM)Feedback Message Type (AVPF)Long-Term Evolution (3GPP)Multipoint Control UnitMaximum Transfer UnitPayload Type (RTP)Real-time Transport Protocol (RFC 3550)RTP Control Protocol (RFC 3550)RTCP Receiver ReportSession Description Protocol (RFC 4566)Signaling GatewaySession Initiation Protocol (RFC 3261)Synchronization Source (RTP)Scalable Video CodingTransmission Control Protocol (RFC 793)Temporary Maximum Media Bitrate Request
(CCM)Temporary Maximum Media Bitrate Notification
(CCM)User Agent (SIP)User Datagram Protocol (RFC 768)In addition to the following, the definitions from RTP, AVPF, CCM, and RTP Taxonomy
also apply in this document.CCM
categorized different RTCP feedback messages into four types,
Request, Command, Indication and Notification. This document
places the PAUSE and RESUME messages into Request category, PAUSED
as Indication and REFUSE as Notification.Request from an RTP stream receiver to
pause a streamRequest from an RTP stream receiver to
resume a paused streamIndication from an RTP stream sender that
a stream is pausedNotification from an RTP stream sender
that a PAUSE or RESUME request will not be honoredThe intermediate RTP node which receives an
RTP stream from different end points, combines them to make one
RTP stream and forwards to destinations, in the sense described in
Topo-Mixer of RTP
Topologies.A member which is part of an RTP
session, acting as receiver, sender or both.An RTP stream sender that has stopped
its transmission, i.e. no other participant receives its RTP
transmission, either based on having received a PAUSE request,
defined in this specification, or based on a local decision.An RTP stream receiver which sends
a PAUSE request, defined in this specification, to other
participant(s).Used as a short term for RTP stream, unless
otherwise noted.Short for RTP stream receiver; the
RTP entity responsible for receiving an RTP stream, usually a
Media Depacketizer.Short for RTP stream sender; the RTP
entity responsible for creating an RTP stream, usually a Media
Packetizer.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.This section discusses the main use cases for RTP stream pause and
resume.This is the most basic use case with an RTP session containing two
End Points. Each End Point sends one or more streams.The usage of RTP stream pause in this use case is to temporarily
halt delivery of streams that the sender provides but the receiver
does not currently use. This can for example be due to minimized
applications where the video stream is not actually shown on any
display, and neither is it used in any other way, such as being
recorded.In this case, since there is only a single receiver of the stream,
pausing or resuming a stream does not impact anyone else than the
sender and the single receiver of that stream.RTCWEB WG's use case and
requirements document defines the following API requirements in
Appendix A, used also by W3C WebRTC WG:The Web API must provide means for the web
application to mute/unmute a stream or stream component(s). When a
stream is sent to a peer mute status must be preserved in the
stream received by the peer.The Web API must provide means for the web
application to cease the sending of a stream to a peer.This memo provides means to optimize transport usage by stop
sending muted streams and start sending again when unmuting.One of the most commonly used topologies in centralized
conferencing is based on the RTP Mixer. The
main reason for this is that it provides a very consistent view of the
RTP session towards each participant. That is accomplished through the
Mixer originating its´ own streams, identified by SSRC, and any
RTP streams sent to the participants will be sent using those SSRCs.
If the Mixer wants to identify the underlying media sources for
its´ conceptual streams, it can identify them using CSRC. The
stream the Mixer provides can be an actual mix of multiple media
sources, but it might also be switching received streams as described
in Sections 3.6-3.8 of .Which streams that are delivered to a given receiver, A, can depend
on several things. It can either be the RTP Mixer´s own logic
and measurements such as voice activity on the incoming audio streams.
It can be that the number of sent media sources exceed what is
reasonable to present simultaneously at any given receiver. It can
also be a human controlling the conference that determines how the
media should be mixed; this would be more common in lecture or similar
applications where regular listeners may be prevented from breaking
into the session unless approved by the moderator. The streams may
also be part of a
Simulcast or scalable encoded (for
Multi-Stream Transmission), thus providing multiple versions
that can be delivered by the RTP stream sender. These examples
indicate that there are numerous reasons why a particular stream would
not currently be in use, but must be available for use at very short
notice if any dynamic event occurs that causes a different stream
selection to be done in the Mixer.Because of this, it would be highly beneficial if the Mixer could
request to pause a particular stream from being delivered to it. It
also needs to be able to resume delivery with minimal delay.Just as for point-to-point, there is only a
single receiver of the stream, the RTP Mixer, and pausing or resuming
a stream does not affect anyone else than the sender and single
receiver of that stream.This use case is similar to the previous section, however the RTP
Mixer is involved in three domains that need to be separated; the
Multicast Network (including participants A and C), participant B, and
participant D. The difference from above is that A and C share a
multicast domain, which is depicted below.If the RTP Mixer pauses a stream from A, it will not only pause the
stream towards itself, but will also stop the stream from arriving to
C, which C is heavily impacted by, might not approve of, and should
thus have a say on.If the Mixer resumes a paused stream from A, it will be resumed
also towards C. In this case, if C is not interested it can simply
ignore the stream and is not impacted as much as above.In this use case there are several receivers of a stream and
special care must be taken as not to pause a stream that is still
wanted by some receivers.An End Point in could potentially
request to pause the delivery of a given stream. Possible reasons
include the ones in the point to
point case above.When the RTP Mixer is only connected to individual unicast paths,
the use case and any considerations are identical to the point to
point use case.However, when the End Point requesting stream pause is connected to
the RTP Mixer through a multicast network, such as A or C in , the use case instead becomes
identical to the one in , only with reverse
direction of the streams and pause/resume requests.An End Point, like A in , could
potentially request to pause the delivery of a given stream, like one
of B's, over any of the SSRCs used by the Mixer by sending a pause
request for the CSRC identifying the stream. However, the authors are
of the opinion that this is not a suitable solution, for several
reasons:The Mixer might not include CSRC in it´s stream
indications.An End Point cannot rely on the CSRC to correctly identify the
stream to be paused when the delivered media is some type of mix.
A more elaborate stream identification solution is needed to
support this in the general case.The End Point cannot determine if a given stream is still
needed by the RTP Mixer to deliver to another session
participant.Due to the above reasons, we exclude this use case from further
consideration.This section describes the requirements that this specification needs
to meet.The first section of this
specification describes some possible reasons why a receiver may pause
an RTP sender. Pausing and resuming is time-dependent, i.e. a receiver
may choose to pause an RTP stream for a certain duration, after which
the receiver may want the sender to resume. This time dependency means
that the messages related to pause and resume must be transmitted to
the sender in real-time in order for them to be purposeful. The pause
operation is arguably not very time critical since it mainly provides
a reduction of resource usage. Timely handling of the resume operation
is however likely to directly impact the end-user's perceived quality
experience, since it affects the availability of media that the user
expects to receive more or less instantly.It is the responsibility of an RTP stream receiver, who wants to
pause or resume a stream from the sender(s), to transmit PAUSE and
RESUME messages. An RTP stream sender who likes to pause itself, can
simply do it. Any indication that an RTP stream is paused is the
responsibility of the RTP stream sender and may in some cases not even
be needed by the stream receiver.The PAUSE and RESUME messages apply to single RTP streams
identified by their SSRC, which means the receiver targets the
sender's SSRC in the PAUSE and RESUME requests. If a paused sender
starts sending with a new SSRC, the receivers will need to send a new
PAUSE request in order to pause it. PAUSED indications refer to a
single one of the sender's own, paused SSRC.An RTP stream sender should not pause an SSRC that some receiver
still wishes to receive. The reason is that in RTP topologies where
the stream is shared between multiple receivers, a single receiver on
that shared network, independent of it being multicast, a mesh with
joint RTP session or a transport Translator based, must not
single-handedly cause the stream to be paused without letting all
other receivers to voice their opinions on whether or not the stream
should be paused. A consequence of this is that a newly joining
receiver, for example indicated by an RTCP Receiver Report containing
both a new SSRC and a CNAME that does not already occur in the
session, firstly needs to learn the existence of paused streams, and
secondly should be able to resume any paused stream. Any single
receiver wanting to resume a stream should also cause it to be
resumed.RTP and RTCP does not guarantee reliable data transmission. It uses
whatever assurance the lower layer transport protocol can provide.
However, this is commonly UDP that provides no reliability guarantees.
Thus it is possible that a PAUSE and/or RESUME message transmitted
from an RTP End Point does not reach its destination, i.e. the
targeted RTP stream sender. When PAUSE or RESUME reaches the RTP
stream sender and are effective, i.e., an active RTP stream sender
pauses, or a resuming RTP stream sender have media data to transmit,
it is immediately seen from the arrival or non-arrival of RTP packets
for that RTP stream. Thus, no explicit acknowledgments are required in
this case.In some cases when a PAUSE or RESUME message reaches the RTP stream
sender, it will not be able to pause or resume the stream due to some
local consideration, for example lack of data to transmit. This error
condition, a negative acknowledgment, may be needed to avoid
unnecessary retransmission of
requests.When the stream is not affected as expected by a PAUSE or RESUME
request, the request may have been lost and the sender of the request
will need to retransmit it. The retransmission should take the round
trip time into account, and will also need to take the normal RTCP
bandwidth and timing rules applicable to the RTP session into account,
when scheduling retransmission of feedback.When it comes to resume requests that are more time critical, the
best resume performance may be achieved by repeating the request as
often as possible until a sufficient number have been sent to reach a
high probability of request delivery, or the stream gets
delivered.A PAUSE request message will need to have a sequence number to
separate retransmissions from new requests. A retransmission keeps the
sequence number unchanged, while it is incremented every time a new
PAUSE request is transmitted that is not a retransmission of a
previous request.Since RESUME always takes precedence over PAUSE and are even
allowed to avoid pausing a stream, there is a need to keep strict
ordering of PAUSE and RESUME. Thus, RESUME needs to share sequence
number space with PAUSE and implicitly references which PAUSE it
refers to. For the same reasons, the explicit PAUSED indication also
needs to share sequence number space with PAUSE and RESUME.A performance comparison between SIP/SDP and RTCP signaling
technologies was made and included in draft versions of this
specification. Using SIP and SDP to
carry pause and resume information means that it will need to traverse
the entire signaling path to reach the signaling destination (either
the remote End Point or the entity controlling the RTP Mixer), across
any signaling proxies that potentially also has to process the SDP
content to determine if they are expected to act on it. The amount of
bandwidth required for a SIP/SDP-based signaling solution is in the
order of at least 10 times more than an RTCP-based solution.
Especially for UA sitting on mobile wireless access, this will risk
introducing delays that are too
long to provide a good user experience, and the bandwidth cost
may also be considered infeasible compared to an RTCP-based solution.
RTCP data is sent through the media path, which is likely shorter
(contains fewer intermediate nodes) than the signaling path, may
anyway have to traverse a few intermediate nodes. The amount of
processing and buffering required in intermediate nodes to forward
those RTCP messages is however believed to be significantly less than
for intermediate nodes in the signaling path. Based on those
considerations, RTCP is chosen as signaling protocol for the pause and
resume functionality.The proposed solution implements PAUSE and RESUME functionality based
on sending AVPF RTCP feedback messages from any RTP session participant
that wants to pause or resume a stream targeted at the stream sender, as
identified by the sender SSRC.It is proposed to re-use CCM TMMBR and
TMMBN to the extent possible, and to define a small set of new
RTCP feedback messages where new semantics is needed. Considerations
that apply when using TMMBR/TMMBN for pause and resume purposes are also
described.A single Feedback message specification is used to implement the new
messages. The message consists of a number of Feedback Control
Information (FCI) blocks, where each block can be a PAUSE request, a
RESUME request, PAUSED indication, a REFUSE response, or an extension to
this specification. This structure allows a single feedback message to
handle pause functionality on a number of streams.The PAUSED functionality is also defined in such a way that it can be
used standalone by the RTP stream sender to indicate a local decision to
pause, and inform any receiver of the fact that halting media delivery
is deliberate and which RTP packet was the last transmitted.This section is intended to be explanatory and therefore
intentionally contains no mandatory statements. Such statements can
instead be found in other parts of this specification.An End Point can use an extension to CCM SDP signaling to declare
capability to understand the messages defined in this specification.
Capability to understand PAUSED indication is defined separately from
the others to support partial implementation, which is specifically
believed to be feasible for the RTP Mixer to Media Sender use
case.For the case when TMMBR/TMMBN are used for pause and resume
purposes, it is possible to explicitly express joint support for TMMBR
and TMMBN, but not for TMMBN only.An RTP stream receiver can choose to request PAUSE at any time,
subject to AVPF timing rules. This also applies when using TMMBR 0 in
the point-to-point case.The PAUSE request contains a PauseID, which is incremented by one
(in modulo arithmetic) with each PAUSE request that is not a
re-transmission. The PauseID is scoped by and thus a property of the
targeted RTP stream (SSRC).When a non-paused RTP stream sender receives the PAUSE request, it
continues to send the RTP stream while waiting for some time to allow
other RTP stream receivers in the same RTP session that saw this PAUSE
request to disapprove by sending a RESUME for the same stream and
with the same PauseID as in the disapproved PAUSE. If such
disapproving RESUME arrives at the RTP stream sender during the wait
period before the stream is paused, the pause is not performed. In
point-to-point configurations, the wait period may be set to zero.
Using a wait period of zero is also appropriate when using TMMBR 0 and
in line with the semantics for that message.If the RTP stream sender receives further PAUSE requests with the
available PauseID while waiting as described above, those additional
requests are ignored.If the PAUSE request or TMMBR 0 is lost before it reaches the RTP
stream sender, it will be discovered by the RTP stream receiver
because it continues to receive the RTP stream. It will also not see
any PAUSED indication or
TMMBN 0 for the stream. The same condition can be caused by the RTP
stream sender having received a disapproving RESUME from a stream
receiver A for a PAUSE request sent by a stream sender B, but that the
PAUSE sender (B) did not receive the RESUME (from A) and may instead
think that the PAUSE was lost. In both cases, a PAUSE request can be
re-transmitted using the same PauseID. If using TMMBR 0 the request
MAY be re-transmitted when the requester fails to receive a TMMBN 0
confirmation.If the pending stream pause is aborted due to a disapproving
RESUME, the PauseID from the disapproved PAUSE is invalidated by the
RESUME and any new PAUSE must use an incremented PauseID (in modulo
arithmetic) to be effective.An RTP stream sender receiving a PAUSE not using the available
PauseID informs the RTP stream receiver sending the ineffective PAUSE
of this condition by sending a REFUSE response that contains the next
available PauseID value. This REFUSE also informs the RTP stream
receiver that it is probably not feasible to send another PAUSE for
some time, not even with the available PauseID, since there are other
RTP stream receivers that wish to receive the stream.A similar situation where an ineffective PauseID is chosen can
appear when a new RTP stream receiver joins a session and wants to
PAUSE a stream, but does not yet know the available PauseID to use.
The REFUSE response will then provide sufficient information to create
a valid PAUSE. The required extra signaling round-trip is not
considered harmful, since it is assumed that pausing a stream is not
time-critical.There may be local considerations making it impossible or
infeasible to pause the stream, and the RTP stream sender can then
respond with a REFUSE. In this case, if the used PauseID would
otherwise have been effective, the REFUSE contains the same PauseID as
in the PAUSE request, and the PauseID is kept as available. Note that
when using TMMBR 0 as PAUSE, that request cannot be refused (TMMBN
> 0) due to the existing restriction in section 4.2.2.2 of that TMMBN SHALL contain the current bounding set,
and the fact that a TMMBR 0 will always be the most restrictive point
in any bounding set.If the RTP stream sender receives several identical PAUSE for an
RTP stream that was already at least once responded with REFUSE and
the condition causing REFUSE remains, those additional REFUSE should
be sent with regular RTCP timing. A single REFUSE can respond to
several identical PAUSE requests.An RTP stream sender can choose to pause the stream at any time.
This can either be as a result of receiving a PAUSE, or be based on
some local sender consideration. When it does, it sends a PAUSED
indication, containing the available PauseID. If the stream was paused
by a TMMBR 0, TMMBN 0 is used as PAUSED indication. What is said on
PAUSED in the rest of this paragraph apply also to the use of TMMBN 0,
except for PAUSED message parameters. Note that PauseID is incremented
when pausing locally (without having received a PAUSE). It also sends
the PAUSED indication in the next two regular RTCP reports, given that
the pause condition is then still effective.The RTP stream sender may want to apply some local consideration to
exactly when the stream is paused, for example completing some media
unit or a forward error correction block, before pausing the
stream.The PAUSED indication also contains information about the RTP
extended highest sequence number when the pause became effective. This
provides RTP stream receivers with first hand information allowing
them to know whether they lost any packets just before the stream
paused or when the stream is resumed again. This allows RTP stream
receivers to quickly and safely take into account that the stream is
paused, in for example retransmission or congestion control
algorithms.If the RTP stream sender receives PAUSE requests with the available
PauseID while the stream is already paused, those requests are
ignored.As long as the stream is being paused, the PAUSED indication MAY be
sent together with any regular RTCP SR or RR. Including PAUSED in this
way allows RTP stream receivers joining while the stream is paused to
quickly know that there is a paused stream, what the last sent
extended RTP sequence number was, and what the next available PauseID
is to be able to construct valid PAUSE and RESUME requests at a later
stage.When the RTP stream sender learns that a new End Point has joined
the RTP session, for example by a new SSRC and a CNAME that was not
previously seen in the RTP session, it should send PAUSED indications
for all its paused streams at its earliest opportunity. It should in
addition continue to include PAUSED indications in at least two
regular RTCP reports.An RTP stream receiver can request to resume a stream with a RESUME
request at any time, subject to AVPF timing rules. If the stream was
paused with TMMBR 0, resuming the stream is made with TMMBR containing
a bitrate value larger than 0. The bitrate value used when resuming
after a PAUSE with TMMBR 0 is either according to known limitations,
or the configured maximum for the stream or session. What is said on
RESUME in the rest of this paragraph apply also to the use of TMMBR
with a bitrate value larger than 0, except for RESUME message
parameters.The RTP stream receiver must include the available PauseID in the
RESUME request for it to be effective.A pausing RTP stream sender that receives a RESUME including the
correct available PauseID resumes the stream at the earliest
opportunity. Receiving RESUME requests for a stream that is not paused
does not require any action and can be ignored.There may be local considerations, for example that the media
device is not ready, making it temporarily impossible to resume the
stream at that point in time, and the RTP stream sender MAY then
respond with a REFUSE containing the same PauseID as in the RESUME.
When receiving such REFUSE with a PauseID identical to the one in the
sent RESUME, RTP stream receivers SHOULD then avoid sending further
RESUME requests for some reasonable amount of time, to allow the
condition to clear.If the RTP stream sender receives several identical RESUME for an
RTP stream that was already at least once responded with REFUSE and
the condition causing REFUSE remains, those additional REFUSE should
be sent with regular RTCP timing. A single REFUSE can respond to
several identical RESUME requests.When resuming a paused stream, especially for media that makes use
of temporal redundancy between samples such as video, the temporal
dependency between samples taken before the pause and at the time
instant the stream is resumed may not be appropriate to use in the
encoding. Should such temporal dependency between before and after the
media was paused be used by the RTP stream sender, it requires the RTP
stream receiver to have saved the sample from before the pause for
successful continued decoding when resuming. The use of this temporal
dependency is left up to the RTP stream sender. If temporal dependency
is not used when the RTP stream is resumed, the first encoded sample
after the pause will not contain any temporal dependency to samples
before the pause (for video it may be a so-called intra picture). If
temporal dependency to before the pause is used by the RTP stream
sender when resuming, and if the RTP stream receiver did not save any
sample from before the pause, the RTP stream receiver can use a FIR request to explicitly ask for a sample
without temporal dependency (for video a so-called intra picture),
even at the same time as sending the RESUME.As stated, TMMBR/TMMBN may be used to provide pause and resume
functionality for the point-to-point case. If the topology is not
point-to-point, TMMBR/TMMBN cannot safely be used for pause or
resume.This is a brief summary of what functionality is provided when
using TMMBR/TMMBN:Corresponds to PAUSE, without the
requirement for any hold-off period to wait for RESUME before
pausing the stream.Corresponds to RESUME when the stream
was previously paused with TMMBR 0. Since there is only a single
RTP stream receiver, there is no need for the RTP stream sender to
delay resuming the stream until after sending TMMBN >0, or to
apply the hold-off period specified in
before increasing the bitrate from zero.Corresponds to PAUSED. Also corresponds to
a REFUSE indication when a stream is requested to be resumed with
TMMBR >0.Cannot be used as REFUSE indication
when a stream is requested to be paused with TMMBR 0, for reasons
stated in .This document introduces three new states for a stream in an RTP
sender, according to the figure and sub-sections below. Any references
to PAUSE, PAUSED, RESUME and REFUSE in this section SHALL be taken to
apply to the extent possible also when TMMBR/TMMBN are used for this
functionality.This state is not new, but is the normal media sending state from
. When entering the state, the PauseID MUST be
incremented by one in modulo arithmetic. The RTP sequence number for
the first packet sent after a pause SHALL be incremented by one
compared to the highest RTP sequence number sent before the pause. The
first RTP Time Stamp for the first packet sent after a pause SHOULD be
set according to capture times at the source.In this state, the RTP stream sender has received at least one
PAUSE message for the stream in question. The RTP stream sender SHALL
wait during a hold-off period for the possible reception of RESUME
messages for the RTP stream being paused before actually pausing RTP
stream transmission. The period to wait SHALL be long enough to allow
another RTP stream receiver to respond to the PAUSE with a RESUME, if
it determines that it would not like to see the stream paused. This
delay period (denoted by 'Hold-off period' in the figure) is
determined by the formula:2 * RTT + T_dither_max,where RTT is the longest round trip known to the RTP stream sender
and T_dither_max is defined in section 3.4 of . The hold-off period MAY be set to 0 by some signaling means when it can be
determined that there is only a single receiver, for example in
point-to-point or some unicast situations.If the RTP stream sender has set the hold-off period to 0 and
receives information that it was an incorrect decision and that there
are in fact several receivers of the stream, for example by RTCP RR,
it MUST change the hold-off to instead be based on the above
formula.An RTP stream is in paused state when the sender pauses its
transmission after receiving at least one PAUSE message and the
hold-off period has passed without receiving any RESUME message for
that stream.When entering the state, the RTP stream sender SHALL send a PAUSED
indication to all known RTP stream receivers, and SHALL also repeat
PAUSED in the next two regular RTCP reports.Following sub-sections discusses some potential issues when an RTP
sender goes into paused state. These conditions are also valid if an
RTP Translator is used in the communication. When an RTP Mixer
implementing this specification is involved between the participants
(which forwards the stream by marking the RTP data with its own SSRC),
it SHALL be a responsibility of the Mixer to control sending PAUSE and
RESUME requests to the sender. The below conditions also apply to the
sender and receiver parts of the RTP Mixer, respectively.When a participant leaves the RTP session, it sends an RTCP BYE
message. In addition to the semantics described in section 6.3.4 and
6.3.7 of RTP, following two conditions
MUST also be considered when an RTP participant sends an RTCP BYE
message,If a paused sender sends an RTCP BYE message, receivers
observing this SHALL NOT send further PAUSE or RESUME requests
to it.Since a sender pauses its transmission on receiving the PAUSE
requests from any receiver in a session, the sender MUST keep
record of which receiver that caused the RTP stream to pause. If
that receiver sends an RTCP BYE message observed by the sender,
the sender SHALL resume the RTP stream.Section 6.3.5 in RTP describes the
SSRC time-out of an RTP participant. Every RTP participant maintains
a sender and receiver list in a session. If a participant does not
get any RTP or RTCP packets from some other participant for the last
five RTCP reporting intervals it removes that participant from the
receiver list. Any streams that were paused by that removed
participant SHALL be resumed.This state can be entered at any time, based on local decision from
the RTP stream sender. As for Paused
State, the RTP stream sender SHALL send a PAUSED indication to
all known RTP stream receivers, when entering the state, and repeat it
in the next two regular RTCP reports.When leaving the state, the stream state SHALL become Playing,
regardless whether or not there were any RTP stream receivers that
sent PAUSE for that stream, effectively clearing the RTP stream
sender's memory for that stream.Section 6 of AVPF defines three types
of low-delay RTCP feedback messages, i.e. Transport layer,
Payload-specific, and Application layer feedback messages. This document
defines a new Transport layer feedback message, this message is either a
PAUSE request, a RESUME request, or one of four different types of
acknowledgments in response to either PAUSE or RESUME requests.The Transport layer feedback messages are identified by having the
RTCP payload type be RTPFB (205) as defined by AVPF. The PAUSE and RESUME messages are
identified by Feedback Message Type (FMT) value in common packet header
for feedback message defined in section 6.1 of AVPF. The PAUSE and RESUME transport feedback
message is identified by the FMT value = TBA1.The Common Packet Format for Feedback Messages defined by AVPF is:For the PAUSE and RESUME messages, the following interpretation of
the packet fields will be:The FMT value identifying the PAUSE and RESUME
message: TBA1Payload Type = 205 (RTPFB)As defined by AVPF, i.e. the length of this
packet in 32-bit words minus one, including the header and any
padding.The SSRC of the RTP session
participant sending the messages in the FCI. Note, for End Points
that have multiple SSRCs in an RTP session, any of its SSRCs MAY be
used to send any of the pause message types.Not used, SHALL be set to 0. The
FCI identifies the SSRC the message is targeted for.The Feedback Control Information (FCI) field consist of one or
more PAUSE, RESUME, PAUSED, REFUSE, or any future extension. These
messages have the following FCI format:The FCI fields have the following definitions:For a PAUSE and RESUME
messages, this value is the SSRC that the request is intended for.
For PAUSED, it MUST be the SSRC being paused. If pausing is the
result of a PAUSE request, the value in PAUSED is effectively the
same as Target SSRC in a related PAUSE request. For REFUSE, it MUST
be the Target SSRC of the PAUSE or RESUME request that cannot change
state. A CSRC MUST NOT be used as a target as the interpretation of
such a request is unclear.The pause feedback type. The values
defined in this specification are as follows,PAUSE request messageRESUME request messagePAUSED indication messageREFUSE indication messageReserved for future useType specific reserved. SHALL be
ignored by receivers implementing this specification and MUST be set
to 0 by senders implementing this specification.Length of the Type Specific
field in 32-bit words. MAY be 0.Message sequence identification.
SHALL be incremented by one modulo 2^16 for each new PAUSE message,
unless the message is re-transmitted. The initial value SHOULD be 0.
The PauseID is scoped by the Target SSRC, meaning that PAUSE,
RESUME, and PAUSED messages therefore share the same PauseID space
for a specific Target SSRC.Defined per pause feedback
Type. MAY be empty.This section contains detailed explanations of each message defined
in this specification. All transmissions of request and indications are
governed by the transmission rules as defined by .Any references to PAUSE, PAUSED, RESUME and REFUSE in this section
SHALL be taken to apply to the extent possible also when TMMBR/TMMBN are used for this
functionality. TMMBR/TMMBN MAY be used instead of the messages defined
in this specification when the effective topology is point-to-point. If
either sender or receiver learns that the topology is not
point-to-point, TMMBR/TMMBN MUST NOT be used for pause/resume
functionality. If the messages defined in this specification are
supported in addition to TMMBR/TMMBN, pause/resume signaling MUST revert
to use those instead. If the topology is not point-to-point and the
messages defined in this specification are not supported, pause/resume
functionality with TMMBR/TMMBN MUST NOT be used.An RTP stream receiver MAY schedule PAUSE for transmission at any
time.PAUSE has no defined Type Specific parameters and Parameter Len
MUST be set to 0.PauseID SHOULD be the available PauseID, as indicated by PAUSED or implicitly determined by
previously received PAUSE or RESUME
requests. A randomly chosen PauseID MAY be used if it was not possible
to retrieve PauseID information, in which case the PAUSE will either
succeed, or the correct PauseID can be found in the returned REFUSE. A PauseID that is matching the
available PauseID is henceforth also called a valid PauseID.PauseID needs to be incremented by one, in modulo arithmetic, for
each PAUSE request that is not a retransmission, compared to what was
used in the last PAUSED indication sent by the media sender. This is
to ensure that the PauseID matches what is the current available
PauseID at the RTP stream sender. The RTP stream sender increments
what it considers to be the available PauseID when entering Playing State.For the scope of this specification, a PauseID larger than the
current one is defined as having a value between and including
(PauseID + 1) MOD 2^16 and (PauseID + 2^14) MOD 2^16, where "MOD" is
the modulo operator. Similarly, a PauseID smaller than the current one
is defined as having a value between and including (PauseID - 2^15)
MOD 2^16 and (PauseID - 1) MOD 2^16.If an RTP stream receiver that sent a PAUSE with a certain PauseID
receives a RESUME with the same PauseID, it is RECOMMENDED that it
refrains from sending further PAUSE requests for some appropriate time
since the RESUME indicates that there are other receivers that still
wishes to receive the stream.If the targeted RTP stream does not pause, if no PAUSED indication
with a larger PauseID than the one used in PAUSE, and if no REFUSE is
received within 2 * RTT + T_dither_max, the PAUSE MAY be scheduled for
retransmission, using the same PauseID. RTT is the observed round-trip
to the RTP stream sender and T_dither_max is defined in section 3.4 of
.When an RTP stream sender in Playing State receives a valid
PAUSE, and unless local considerations currently makes it impossible
to pause the stream, it SHALL enter Pausing State when reaching an
appropriate place to pause in the stream, and act accordingly.If an RTP stream sender receives a valid PAUSE while in Pausing,
Paused or Local Paused States, the
received PAUSE SHALL be ignored.The PAUSED indication MAY be sent either as a result of a valid
PAUSE request, when entering Paused State, or based on a RTP
stream sender local decision, when entering Local Paused State.PauseID MUST contain the available, valid value to be included in a
subsequent RESUME.PAUSED SHALL contain a 32 bit parameter with the RTP extended
highest sequence number valid when the RTP stream was paused.
Parameter Len MUST be set to 1.After having entered Paused or Local Paused State and thus having
sent PAUSED once, PAUSED MUST also be included in the next two regular
RTCP reports, given that the pause condition is then still
effective.While remaining in Paused or Local Paused States, PAUSED MAY be
included in all regular RTCP reports.When in Paused or Local Paused States, It is RECOMMENDED to send
PAUSED at the earliest opportunity and also to include it in the next
two regular RTCP reports, whenever the RTP stream sender learns that
there are End Points that did not previously receive the stream, for
example by RTCP reports with an SSRC and a CNAME that was not
previously seen in the RTP session.An RTP stream receiver MAY schedule RESUME for transmission
whenever it wishes to resume a paused stream, or to disapprove a
stream from being paused.PauseID SHOULD be the valid PauseID, as indicated by PAUSED or implicitly determined by
previously received PAUSE or RESUME
requests. A randomly chosen PauseID MAY be used if it was not possible
to retrieve PauseID information, in which case the RESUME will either
succeed, or the correct PauseID can be found in a returned REFUSE.RESUME has no defined Type Specific parameters and Parameter Len
MUST be set to 0.When an RTP stream sender in Pausing, Paused or Local Paused State receives a
valid RESUME, and unless local considerations currently makes it
impossible to resume the stream, it SHALL enter Playing State and act accordingly.
If the RTP stream sender is incapable of honoring the RESUME request
with a valid PauseID, or receives a RESUME request with an invalid
PauseID while in Paused or Pausing state, the RTP stream sender sends
a REFUSE message as specified below.If an RTP stream sender in Playing State receives a RESUME
containing either a valid PauseID or a PauseID that is less than the
valid PauseID, the received RESUME SHALL be ignored.REFUSE has no defined Type Specific parameters and Parameter Len
MUST be set to 0.If an RTP stream sender receives a valid PAUSE or RESUME request that cannot be fulfilled by
the sender due to some local consideration, it SHALL schedule
transmission of a REFUSE indication containing the valid PauseID from
the rejected request.If an RTP stream sender receives PAUSE or RESUME requests with a
non-valid PauseID it SHALL schedule a REFUSE response containing the
available, valid PauseID, except if the RTP stream sender is in
Playing State and receives a RESUME with a PauseID less than the valid
one, in which case the RESUME SHALL be ignored.If several PAUSE or RESUME that would render identical REFUSE
responses are received before the scheduled REFUSE is sent, duplicate
REFUSE MUST NOT be scheduled for transmission. This effectively lets a
single REFUSE respond to several invalid PAUSE or RESUME requests.If REFUSE containing a certain PauseID was already sent and yet
more PAUSE or RESUME messages are received that require additional
REFUSE with that specific PauseID to be scheduled, and unless the
PauseID number space has wrapped since REFUSE was last sent with that
PauseID, further REFUSE messages with that PauseID SHOULD be sent in
regular RTCP reports.An RTP stream receiver that sent a PAUSE or RESUME request and
receives a REFUSE containing the same PauseID as in the request SHOULD
refrain from sending an identical request for some appropriate time to
allow the condition that caused REFUSE to clear.An RTP stream receiver that sent a PAUSE or RESUME request and
receives a REFUSE containing a PauseID different from the request MAY
schedule another request using the PauseID from the REFUSE
indication.The transmission of any RTCP feedback messages defined in this
specification MUST follow the normal AVPF defined timing rules and
depends on the session's mode of operation.All messages defined in this specification MAY use either Regular,
Early or Immediate timings, taking the following into
consideration:PAUSE SHOULD use Early or Immediate timing, except for
retransmissions that SHOULD use Regular timing.The first transmission of PAUSED for each (non-wrapped) PauseID
SHOULD be sent with Immediate or Early timing, while subsequent
PAUSED for that PauseID SHOULD use Regular timing.RESUME SHOULD always use Immediate or Early timing.The first transmission of REFUSE for each (non-wrapped) PauseID
SHOULD be sent with Immediate or Early timing, while subsequent
REFUSE for that PauseID SHOULD use Regular timing.The capability of handling messages defined in this specification MAY
be exchanged at a higher layer such as SDP. This document extends the
rtcp-fb attribute defined in section 4 of AVPF to include the request for pause and
resume. Like AVPF and CCM , it is RECOMMENDED to use the rtcp-fb
attribute at media level and it MUST NOT be used at session level. This
specification follows all the rules defined in AVPF for rtcp-fb
attribute relating to payload type in a session description.Note: When TMMBR 0 / TMMBN 0 are used to implement pause and
resume functionality (with the restrictions described in this memo),
signaling rtcp-fb attribute with ccm tmmbr parameter is sufficient
and no further signaling is necessary.This specification defines two new parameters to the "ccm" feedback
value defined in CCM, "pause" and
"paused"."pause" represents the capability to understand the RTCP feedback
message and all of the defined FCIs of PAUSE, RESUME, PAUSED and
REFUSE. A direction sub-parameter is used to determine if a given
node desires to issue PAUSE or RESUME requests, can respond to PAUSE
or RESUME requests, or both."paused" represents the functionality of supporting the playing
and local paused states and generate PAUSED FCI when a stream
delivery is paused. A direction sub-parameter is used to determine
if a given node desires to receive these indications, intends to
send them, or both.The reason for this separation is to make it possible for partial
implementation of this specification, according to the different roles
in the use cases section.A sub-parameter named "nowait", indicating that the hold-off time
defined in can be set to 0, reducing
the latency before the stream can paused after receiving a PAUSE
request. This condition occurs when there will be only a single receiver
per direction in the RTP session, for example in point-to-point
sessions. It is also possible to use in scenarios using unidirectional
media. The conditions that allow "nowait" to be set also indicate that
it would be possible to use CCM TMMBR/TMMBN as pause/resume
signaling.A sub-parameter named "dir" is used to indicate in which directions a
given node will use the pause or paused functionality. The node being
configured or issuing an offer or an answer uses the directionality in
the following way. Note that pause and paused have separate and
different definitions.Direction ("dir") values for "pause" is defined as follows:The node intends to send PAUSE and RESUME
requests for other nodes' streams and is thus also capable of
receiving PAUSED and REFUSE. It will not support receiving PAUSE and
RESUME requests.The node supports receiving PAUSE and RESUME
requests targeted for streams sent by the node. It will send PAUSED
and REFUSE as needed. The node will not send any PAUSE and RESUME
requests.The node supports receiving PAUSE and RESUME
requests targeted for streams sent by the node. The node intends to
send PAUSE and RESUME requests for other nodes' streams. Thus the
node is capable of sending and receiving all types of pause
messages. This is the default value. If the "dir" parameter is
omitted, it MUST be interpreted to represent this value.Direction values for "paused" is defined as follows:The node intends to send PAUSED indications
whenever it pauses RTP stream delivery in any of its streams. It has
no need to receive PAUSED indications itself.The node desires to receive PAUSED
indications whenever any stream sent by another node is paused. It
does not intend to send any PAUSED indications.The nodes desires to receive PAUSED
indications and intends to send PAUSED indications whenever any
stream is paused. This is the default value. If the "dir" parameter
is omitted, it MUST be interpreted to represent this value.This is the resulting ABNF, extending
existing ABNF in section 7.1 of CCM:An endpoint implementing this specification and using SDP to signal
capability SHOULD indicate both of the new "pause" and "paused"
parameters with ccm signaling. When negotiating usage, it is possible
select either of them, noting that "pause" contain the full "paused"
functionality. A sender or receiver SHOULD NOT use the messages from
this specification towards receivers that did not declare capability for
it.There MUST NOT be more than one "a=rtcp-fb" line with "pause" and one
with "paused" applicable to a single payload type in the SDP, unless the
additional line uses "*" as payload type, in which case "*" SHALL be
interpreted as applicable to all listed payload types that does not have
an explicit "pause" or "paused" specification.There MUST NOT be more than a single direction sub-parameter per
"pause" and "paused" parameter. There MUST NOT be more than a single
"nowait" sub-parameter per "pause" parameter.An offerer implementing this specification needs to include "pause"
and/or "paused" CCM parameters with suitable directionality parameter
("dir") in the SDP, according to what messages it intends to send and
desires or is capable to receive in the session. It is RECOMMENDED to
include both "pause" and "paused" if "pause" is supported, to enable
at least the "paused" functionality if the answer only supports
"paused" or different directionality for the two functionalities. The
"pause" and "paused" functionalities are negotiated independently,
although the "paused" functionality is part of the "pause"
functionality. As a result, an answerer MAY remove "pause" or "paused"
lines from the SDP depending on the agreed mode of functionality.In offer/answer, the "dir" parameter is interpreted based on the
agent providing the SDP. The node described in the offer is the
offerer, and the answerer is described in an answer. In other words,
an offer for "paused dir=sendonly" means that the offerer intends to
send PAUSED indications whenever it pauses RTP stream delivery in any
of its streams.An answerer receiving an offer with a "pause" parameter with
dir=sendrecv MAY remove the pause line in its answer, respond with
pause keeping sendrecv for full bi-directionality, or it may change
dir value to either sendonly or recvonly based on its capabilities and
desired functionality. An offer with a "pause" parameter with
dir=sendonly or dir=recvonly is either completely removed or accepted
with reverse directionality, i.e. sendonly becomes recvonly or
recvonly becomes sendonly.An answer receiving an offer with "paused" has the same choices as
for "pause" above. It should be noted that the directionality of pause
is the inverse of RTP stream direction, while the directionality of
paused is the same as the RTP stream direction.If the offerer believes that itself and the intended answerer are
likely the only End Points in the RTP session, it MAY include the
"nowait" sub-parameter on the "pause" line in the offer. If an
answerer receives the "nowait" sub-parameter on the "pause" line in
the SDP, and if it has information that the offerer and itself are not
the only End Points in the RTP session, it MUST NOT include any
"nowait" sub-parameter on its "pause" line in the SDP answer. The
answerer MUST NOT add "nowait" on the "pause" line in the answer
unless it is present on the "paused" line in the offer. If both offer
and answer contained a "nowait" parameter, then the hold-off time is
configured to 0 at both offerer and answerer.In declarative use, the SDP is used to configure the node receiving
the SDP. This has implications on the interpretation of the SDP
signaling extensions defined in this draft. First, it is normally only
necessary to include either "pause" or "paused" parameter to indicate
the level of functionality the node should use in this RTP session.
Including both is only necessary if some implementations only
understands "paused" and some other can understand both. Thus
indicating both means use pause if you understand it, and if you only
understand paused, use that.The "dir" directionality parameter indicates how the configured
node should behave. For example "pause" with sendonly:The node intends to send PAUSE and RESUME
requests for other nodes' streams and is thus also capable of
receiving PAUSED and REFUSE. It will not support receiving PAUSE
and RESUME requests.In this example, the configured node should send PAUSE and RESUME
requests if has reason for it. It does not need to respond to any
PAUSE or RESUME requests as that is not supported.The "nowait" parameter, if included, is followed as specified. It
is the responsibility of the declarative SDP sender to determine if a
configured node will participate in a session that will be point to
point, based on the usage. For example, a conference client being
configured for an any source multicast session using SAP will not be in a point to point session,
thus "nowait" cannot be included. An RTSP client receiving a declarative SDP may
very well be in a point to point session, although it is highly
doubtful that an RTSP client would need to support this specification,
considering the inherent PAUSE support in RTSP.The following examples shows use of PAUSE and RESUME messages,
including use of offer-answer:Offer-AnswerPoint-to-Point sessionPoint-to-Multipoint using MixerPoint-to-Multipoint using TranslatorThe below figures contains an example how to show support for
pausing and resuming the streams, as well as indicating whether or not
the hold-off period can be set to 0.The offerer supports all of the messages defined in this
specification and offers a sendrecv stream. The offerer also believes
that it will be the sole receiver of the answerer's stream as well as
that the answerer will be the sole receiver of the offerer's stream
and thus includes the "nowait" sub-parameter for both "pause" and
"paused" parameters.This is the SDP answer:The answerer will not allow its sent streams to be paused or
resumed and thus support pause only in sendonly mode. It does support
paused and intends to send it, and also desires to receive PAUSED
indications. Thus paused in sendrecv mode is included in the answer.
The answerer somehow knows that it will not be a point-to-point RTP
session and has therefore removed "nowait" from the "pause" line,
meaning that the offerer must use a non-zero hold-off time when being
requested to pause the stream.When using TMMBR 0 / TMMBN 0 to achieve pause and resume
functionality, there are no differences in SDP compared to CCM and therefore no such examples are
included here.This is the most basic scenario, which involves two participants,
each acting as a sender and/or receiver. Any RTP data receiver sends
PAUSE or RESUME messages to the sender, which pauses or resumes
transmission accordingly. The hold-off time before pausing a stream is
0. shows the basic pause and resume
operation in Point-to-Point scenario. At time t1, an RTP sender sends
data to a receiver. At time t2, the RTP receiver requests the sender
to pause the stream, using PauseID 3 (which it knew since before in
this example). The sender pauses the data and replies with a PAUSED
containing the same PauseID. Some time later (at time t4) the receiver
requests the sender to resume, which resumes its transmission. The
next PAUSE, sent at time t6, contains an updated PauseID (4). describes the same
point-to-point scenario as above, but using TMMBR/TMMBN signaling. describes what happens if a PAUSE
message from an RTP stream receiver does not reach the RTP stream
sender. After sending a PAUSE message, the RTP stream receiver waits
for a time-out to detect if the RTP stream sender has paused the data
transmission or has sent PAUSED indication according to the rules
discussed in . As the PAUSE message
is lost on the way (at time t2), RTP data continues to reach to the
RTP stream receiver. When the timer expires, the RTP stream receiver
schedules a retransmission of the PAUSE message, which is sent at time
t4. If the PAUSE message now reaches the RTP stream sender, it pauses
the RTP stream and replies with PAUSED.At time t6, the RTP stream receiver wishes to resume the stream
again and sends a RESUME, which is lost. This does not cause any
severe effect, since there is no requirement to wait until further
RESUME are sent and another RESUME are sent already at time t7, which
now reaches the RTP stream sender that consequently resumes the stream
at time t8. The time interval between t6 and t7 can vary, but may for
example be one RTCP feedback transmission interval as determined by
the AVPF rules.The RTP stream receiver did not realize that the RTP stream was
resumed in time to stop yet another scheduled RESUME from being sent
at time t9. This is however harmless since the RESUME PauseID is less
than the valid one and will be ignored by the RTP stream sender. It
will also not cause any unwanted resume even if the stream was paused
based on a PAUSE from some other receiver before receiving the RESUME,
since the valid PauseID is now larger than the one in the stray RESUME
and will only cause a REFUSE containing the new valid PauseID from the
RTP stream sender.In , the receiver requests to
pause the sender, which refuses to pause due to some consideration
local to the sender and responds with a REFUSE message.An RTP Mixer is an intermediate node connecting different
transport-level clouds. The Mixer receives streams from different RTP
sources, selects or combines them based on the application´s
needs and forwards the generated stream(s) to the destination. The
Mixer typically puts its´ own SSRC(s) in RTP data packets
instead of the original source(s).The Mixer keeps track of all the streams delivered to the Mixer and
how they are currently used. In this example, it selects the video
stream to deliver to the receiver R based on the voice activity of the
RTP stream senders. The video stream will be delivered to R using M's
SSRC and with an CSRC indicating the original source.Note that PauseID is not of any significance for the example and is
therefore omitted in the description.The session starts at t1 with S1 being the most active speaker and
thus being selected as the single video stream to be delivered to R
(t2) using the Mixer SSRC but with S1 as CSRC (indicated after the
colon in the figure). Then S2 joins the session at t3 and starts
delivering an RTP stream to the Mixer. As S2 has less voice activity
then S1, the Mixer decides to pause S2 at t4 by sending S2 a PAUSE
request. At t5, S2 acknowledges with a PAUSED and at the same instant
stops delivering RTP to the Mixer. At t6, the user at S2 starts
speaking and becomes the most active speaker and the Mixer decides to
switch the video stream to S2, and therefore quickly sends a RESUME
request to S2. At t7, S2 has received the RESUME request and acts on
it by resuming RTP stream delivery to M. When the RTP stream from t7
arrives at the Mixer, it switches this RTP stream into its SSRC (M) at
t8 and changes the CSRC to S2. As S1 now becomes unused, the Mixer
issues a PAUSE request to S1 at t9, which is acknowledged at t10 with
a PAUSED and the RTP stream from S1 stops being delivered.A transport Translator in an RTP session forwards the message from
one peer to all the others. Unlike Mixer, the Translator does not mix
the streams or change the SSRC of the messages or RTP media. These
examples are to show that the messages defined in this specification
can be safely used also in a transport Translator case. The
parentheses in the figures contains (Target SSRC, PauseID) information
for the messages defined in this specification. describes how a Translator can help
the receiver in pausing and resuming the sender. The sender S sends
RTP data to the receiver R through Translator, which just forwards the
data without modifying the SSRCs. The receiver sends a PAUSE request
to the sender, which in this example knows that there may be more
receivers of the stream and waits a non-zero hold-off time to see if
there is any other receiver that wants to receive the data, does not
receive any disapproving RESUME, hence pauses itself and replies with
PAUSED. Similarly the receiver resumes the sender by sending RESUME
request through Translator. Since this describes only a single pause
operation for a single RTP stream sender, all messages uses a single
PauseID, in this example 3. explains the pause
and resume operations when a transport Translator is involved between
a sender and two receivers in an RTP session. Each message exchange is
represented by the time it happens. At time t1, Sender (S) starts
sending an RTP stream to the Translator, which is forwarded to R1 and
R2 through the Translator, T. R1 and R2 receives RTP data from
Translator at t2. At this point, both R1 and R2 will send RTCP
Receiver Reports to S informing that they receive S's stream.After some time (at t3), R1 chooses to pause the stream. On
receiving the PAUSE request from R1 at t4, S knows that there are at
least one receiver that may still want to receive the data and uses a
non-zero hold-off period to wait for possible RESUME messages. R2 did
also receive the PAUSE request at time t4 and since it still wants to
receive the stream, it sends a RESUME for it at time t5, which is
forwarded to the sender S by the translator T. The sender S sees the
RESUME at time t6 and continues to send data to T which forwards to
both R1 and R2. At t7, the receiver R2 chooses to pause the stream by
sending a PAUSE request with an updated PauseID. The sender S still
knows that there are more than one receiver (R1 and R2) that may want
the stream and again waits a non-zero hold-off time, after which and
not having received any disapproving RESUME, it concludes that the
stream must be paused. S now stops sending the stream and replies with
PAUSED to R1 and R2. When any of the receivers (R1 or R2) chooses to
resume the stream from S, in this example R1, it sends a RESUME
request to the sender. The RTP sender immediately resumes the
stream.Consider also an RTP session which includes one or more receivers,
paused sender(s), and a Translator. Further assume that a new
participant joins the session, which is not aware of the paused
sender(s). On receiving knowledge about the newly joined participant,
e.g. any RTP traffic or RTCP report (i.e. either SR or RR) from the
newly joined participant, the paused sender(s) immediately sends
PAUSED indications for the paused streams since there is now a
receiver in the session that did not pause the sender(s) and may want
to receive the streams. Having this information, the newly joined
participant has the same possibility as any other participant to
resume the paused streams.As outlined in , this specification
requests IANA to allocateThe FMT number TBA1 to be allocated to the PAUSE and RESUME
functionality from this specification.The 'pause' and 'paused' tags to be used with ccm under rtcp-fb
AVPF attribute in SDP.The 'nowait' parameter to be used with the 'pause' and 'paused'
tags in SDP.A registry listing registered values for 'pause' Types.PAUSE, RESUME, PAUSED, and REFUSE with the listed numbers in the
pause Type registry.This document extends the CCM and
defines new messages, i.e. PAUSE and RESUME. The exchange of these new
messages MAY have some security implications, which need to be addressed
by the user. Following are some important implications,Identity spoofing - An attacker can spoof him/herself as an
authenticated user and can falsely pause or resume any source
transmission. In order to prevent this type of attack, a strong
authentication and integrity protection mechanism is needed.Denial of Service (DoS) - An attacker can falsely pause all
source streams which MAY result in Denial of Service (DoS). An
Authentication protocol may prevent this attack.Man-in-Middle Attack (MiMT) - The pausing and resuming of an RTP
source is prone to a Man-in-Middle attack. Public key authentication
may be used to prevent MiMT.Daniel Grondal contributed in the creation and writing of early
versions of this specification.Daniel Grondal made valuable contributions during the initial
versions of this draft.NOTE TO RFC EDITOR: Please remove this section prior to
publication.Replaced most text on relation with other signaling
technologies in previous section 5 with a single, summarizing
paragraph, as discussed at IETF 90 in Toronto, and placed it as
the last sub-section of section 4 (design considerations).Removed unused references.Corrected text in section 6.5 and 6.2 to indicate that a PAUSE
signaled via TMMBR 0 cannot be REFUSEd using TMMBN > 0Improved alignment with RTP Taxonomy draft, including the
change of Packet Stream to RTP StreamEditorial improvements