Stream Control Transmission Protocol (SCTP)-Based Media
Transport in the Session Description Protocol (SDP)
EricssonHirsalantie 1102420JorvasFinlandchrister.holmberg@ericsson.comEricssonHirsalantie 1102420JorvasFinlandSalvatore.Loreto@ericsson.comEricssonHirsalantie 1102420JorvasFinlandGonzalo.Camarillo@ericsson.com
RAI
MMUSICSCTP, SDP, DTLS
The Stream Control Transmission Protocol (SCTP) is a transport protocol
used to establish associations between two endpoints.
This specification describes how to describe SCTP associations using the Session
Description Protocol (SDP), and defines the following new SDP Media Description
protocol identifiers (proto values):'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'.
The specification also describes how to use the new proto values together
with the SDP Offer/Answer mechanism in order to negotiate and establish
SCTP associations, and how to indicate the SCTP application usage.
SDP (Session Description Protocol) provides a
general-purpose format for describing multimedia sessions in announcements
or invitations. TCP-Based Media Transport in the Session Description Protocol
(SDP) specifies a general mechanism for describing and
establishing TCP streams.
Connection-Oriented Media Transport over the Transport Layer Security (TLS)
Protocol in SDP extends RFC4145 for
describing TCP-based media streams that are protected using TLS.
The Stream Control Transmission Protocol (SCTP) is a
transport protocol used to establish associations between two endpoints.
This specification defines how to describe SCTP associations using the
Session Description Protocol (SDP) , and defines
the following new SDP Media Description protocol
identifiers (proto values):'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'.
The specification also describes how to use the new proto values together
with the SDP Offer/Answer mechanism in order to
negotiate and establish SCTP associations, and how to indicate the SCTP
application usage.
NOTE: TLS is designed to run on top of a byte-stream oriented
transport protocol providing a reliable, in-sequence delivery like
TCP. presents serious limitations with transporting TLS
on top of SCTP. Therefore, defining a mechanism to negotiate media streams
transported using TLS on top of SCTP, i.e. 'SCTP/TLS', is outside the scope
of this specification.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described
in BCP 14, RFC 2119 and indicate
requirement levels for compliant implementations.
SCTP Association: A protocol relationship between SCTP endpoints,
composed of the two SCTP endpoints and protocol state information
including Verification Tags and the currently active set of
Transmission Sequence Numbers (TSNs), etc. An association can be
uniquely identified by the transport addresses used by the
endpoints in the association.
SCTP Stream: A unidirectional logical channel established from one to
another associated SCTP endpoint, within which all user messages
are delivered in sequence except for those submitted to the
unordered delivery service.
SCTP Transport address: A transport address is traditionally defined by
a network-layer address, a transport-layer protocol, and a
transport-layer port number. In the case of SCTP running over IP,
a transport address is defined by the combination of an IP address
and an SCTP port number (where SCTP is the transport protocol).
This section defines the following new SDP Media Description (m- line)
protocol identifiers (proto values) for describing an SCTP association:
'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'. The section
also describes how an m- line, associated with the proto values, is created.
The following is the format for an 'm' line, as specified in RFC4566
:
The 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto
values are similar to both the 'UDP' and 'TCP' proto values in that they
only describe the transport-layer protocol and not the upper-layer
protocol.
NOTE: When the 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto values are used,
the underlying transport protocol is respectively UDP and TCP; SCTP is
carried on top of DTLS which is on top of those transport-layer protocols.
The m- line fmt value, identifying the application-layer protocol,
MUST be registered by IANA.
The new proto values are defined as below:
The 'SCTP' proto value describes an SCTP association, as
defined in .
The 'SCTP/DTLS' proto value describes a Datagram Transport Layer
Security (DTLS)
connection on top of an SCTP association, as defined in .
The 'UDP/DTLS/SCTP' proto value describes an SCTP association on top of
a DTLS connection on top of UDP, as defined in
.
The 'TCP/DTLS/SCTP' proto value describes an SCTP association on top of
a DTLS connection on top of TCP, as defined in
.
defines that specifications defining new proto values must
define the rules by which their media format (fmt) namespace is managed.
Use of an existing MIME subtype for the format is encouraged. If no MIME subtype
exists, it is recommended that a suitable one is registered through the
IETF process by production of, or
reference to, a standards-track RFC that defines the transport protocol for the format.
An m- line with a proto value of 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP'
always describe a single SCTP association.
In addition, such m- line MUST further indicate the application-layer protocol
using an 'fmt' identifier. There MUST be exactly one 'fmt' value per m- line associated
with the proto values defined in this specification. The "fmt" namespace associated
with those proto values describes the generic application usage of the entire SCTP
association, including the associated SCTP streams.
NOTE: A mechanism on how to describe, and manage, individual SCTP streams within an
SCTP association, is outside the scope of this specification.
This section defines the ABNF for the SDP media description when associated
with any of the proto values defined in this document.
This specification creates an IANA registry for 'association-usage' values.
This section defines a new SDP media-level attribute, 'sctp-port'.
The attribute can be associated with an SDP media description (m- line)
with a 'UDP/DTLS/SCTP' or a 'TCP/DTLS/SCTP' proto value. In that case
the m- line port value indicates the port of the underlying transport layer
protocol (UDP or TCP), and the 'sctp-port' value indicates the SCTP port.
No default value is defined for the SDP sctp-port attribute. Therefore, if
the attribute is not present, the associated m- line MUST be considered invalid.
Usage of the SDP sctp-port attribute with other proto values
is not specified, and MUST be discarded if received.
The ABNF for the SDP 'sctp-port' attribute is:
The mux category for
the SDP sctp-port' attribute is SPECIAL. Usage of the attribute
is only applicable when associated with 'UDP/DTLS/SCTP' and
'TCP/DTLS/SCTP' proto value m- lines.
As the usage of multiple SCTP associations on top of a single
DTLS connection is outside the scope of this specification,
no mux rules are specified for the 'UDP/DTLS/SCTP' and
'TCP/DTLS/SCTP' proto values. Future extensions, that define
how to negotiate multiplexing of multiple SCTP associations of top of
a single DTLS connection, need to also define the mux rules for the
attribute.
This section defines a new SDP media-level attribute, 'max-message-size'.
The attribute can be associated with an m- line to indicate the maximum
message size (indicated in bytes) that an SCTP endpoint is willing to
receive on the SCTP association associated with the m- line. Different
attribute values can be used in each direction.
The remote peer MUST assume that larger messages will be rejected by the
SCTP endpoint. SCTP endpoints need to decide on appropriate behavior in
case a message that exceeds the maximum size needs to be sent.
If the SDP 'max-message-size' attribute contains a maximum message size
value of zero, it indicates the SCTP endpoint will handle messages of any size,
subject to memory capacity etc.
If the SDP 'max-message-size' attribute is not present, the default value is
64K.
NOTE: This specification only defines the usage of the SDP 'max-message-size'
attribute when associated with an m- line containing one of the following proto
values: 'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP'.
Usage of the attribute with other proto values needs to be defined
in a separate specification.
The ABNF for the SDP 'max-message-size' attribute is:
The mux category for the SDP 'max-message-size' attribute
is SPECIAL. The mux rules depends on the proto value of the
associated m- line. If the proto value is 'SCTP' or 'SCTP/DTLS'
the rules are identical to the rules associated with the TRANSPORT
mux category.
As the usage of multiple SCTP associations on top of a single
DTLS connection is outside the scope of this specification,
no mux rules are specified for the 'UDP/DTLS/SCTP' and
'TCP/DTLS/SCTP' proto values.
The UDP/DTLS/SCTP transport is realized as described below:
SCTP on top of DTLS is realized according to the procedures
defined in ; and
DTLS on top of UDP is realized according to the procedures in
defined in .
NOTE: While allows
multiple SCTP associations on top of a single DTLS connection,
the procedures in this specification only supports the negotiation of a single
SCTP association on top of any given DTLS connection.
The TCP/DTLS/SCTP transport is realized as described below:
SCTP on top of DTLS is realized according to the procedures
defined in ; and
DTLS on top of TCP is realized using the framing method defined in
, with DTLS packets being sent instead of
RTP/RTCP packets, and SDP signaling according to the procedures
defined in this specification.
NOTE: DTLS on top of TCP, without using the framing method defined in
is outside the scope of this specification.
A separate proto value would need to be registered for such
transport realization.
The management of an SCTP association is identical to the management
of a TCP connection. An SCTP endpoint MUST follow the rules in
Section 6 of to manage SCTP associations.
Whether to use the SCTP ordered or unordered delivery service is up
to the applications using the SCTP association, and this specification
does not define a mechanism to indicate the type of delivery service
using SDP.
This specification does not define semantics for the SDP direction
attributes . Unless semantics of these
attributes for an SCTP association usage have been defined, SDP direction
attributes MUST be discarded if present.
The SDP setup attribute is used to determine the 'active/passive'
status of the endpoints, following the procedures for TCP in
.
Both the 'active' and 'passive' endpoint MUST initiate the SCTP association,
and MUST use the same SCTP port as client port and server port
(in order to prevent two separate SCTP associations from being established).
NOTE: The procedure above is different from TCP, where only the 'active'
endpoint initiates the TCP connection .
NOTE: If the underlying transport protocol is UDP or TCP (e.g. if the m- line
proto value is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP'), when the SCTP association
is established it is assumed that any NAT traversal procedures for the underlying
transport protocol has successfully been performed.
If the m- line proto value is 'TCP/DTLS/SCTP', the 'active' endpoint
only MUST initiate the TCP connection, following the procedures in
. Both endpoints MUST still initiate the
SCTP association on top of the TCP connection.
If the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP'
or 'TCP/DTLS/SCTP', the 'active/passive' status is used to determine
the (D)TLS roles of the endpoints. Following the procedures in
, the 'active' endpoint will take
the (D)TLS client role.
Once the DTLS connection has been established, the endpoints MUST NOT
modify (as result of an offer/answer exchange) the TLS roles, or the
'active/passive' status, of the endpoints, unless the underlying transport
protocol is also modified (e.g. if an IP address- or port value associated
with the transport protocol is modified).
If the underlying transport protocol is modified, the endpoints MUST
establish a new DTLS connection. In such case the 'active/passive' status
of the endpoints will again be determined following the procedures in
, and the new status will be used to determine
the (D)TLS roles of the endpoints associated with the new DTLS connection.
NOTE: The procedure above is identical to the one defined for SRTP-DTLS in
.
NOTE: As described in [add-reference], if the Interactive Connectivity
Establishment (ICE) mechanism is used, all ICE
candidates associated with an SCTP association on top of a DTLS connection
as part of the same DTLS connection. Thus, a switch from one candidate pair to
another candidate pair will not trigger the establishment of a new DTLS
connection.
The SDP connection attribute is used following the procedures in
, with the additional SCTP specific
considerations described in this section.
If the m- line proto value is 'TCP/DTLS/SCTP', an SDP connection
attribute associated with that m- line applies to both the
SCTP association and the TCP connection. Therefore, an attribute
'new' value indicates that both a new SCTP association and new TCP
connection have to be established, following the procedures in
.
NOTE: This specification does not define a mechanism which allows
re-establishing of a new SCTP association, while maintaining the
underlying TCP connection.
The SDP connection attribute value does not automatically impact
an existing DTLS connection.
describes in which cases a new DTLS connections will have to be
re-established.
This section defines the SDP Offer/Answer
procedures for negotiating and establishing an SCTP association. Unless
explicitly stated, the procedures apply to all m- line proto values
('SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP') defined
in this specification.
If the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP',
each endpoint MUST provide a certificate fingerprint, using the SDP 'fingerprint'
attribute , if the endpoint supports, and is willing to use,
a cipher suite with an associated certificate.
The authentication certificates are interpreted and validated as
defined in . Self-signed certificates can
be used securely, provided that the integrity of the SDP description
is assured as defined in .
NOTE: The procedures apply to a specific m- line describing an SCTP association.
If an offer or answer contains multiple m- lines describing SCTP associations,
the procedures are applied separately to each m- line.
When the offerer creates an initial offer, the offerer:
MUST, if the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP',
associate an SDP setup attribute, with an 'actpass' value, with the m- line
(see );
MUST, if the m- line proto is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', associate an
SDP 'sctp-port' attribute with the m- line (see );
MUST associate an SDP 'connection' attribute, with a 'new' value, with
the m- line (see ); and
MAY associate an SDP 'max-message-size' attribute with the m- line
(see ).
When the answerer receives an offer, which contains an m- line describing
an SCTP association, if the answerer accepts the m- line it:
MUST insert a corresponding m- line in the answer, with an identical
m- line proto value ;
MUST, if the m- line proto value is 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP',
associate an SDP 'setup' attribute, with an 'active' or 'passive' value, with
the m- line (see );
MUST, if the m- line proto is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', associate an
SDP 'sctp-port' attribute with the m- line (see ); and
MAY associate an SDP 'max-message-size' attribute with the m- line
(see ). The attribute value in
the answer is independent from the value (if present) in the
corresponding m- line of the offer.
Once the answerer has sent the answer, the answerer:
MUST, if an SCTP association associated with the m- line has yet not
been established, or if an existing SCTP association is to be re-established,
initiate the establishing of the SCTP association; and
MUST, if the answerer is the 'active' endpoint, and if an DTLS connection associated
with the m- line is to be established (or re-established), initiate the establishing
of the DTLS connection (by sending a ClientHello message).
If the answerer does not accept the m- line in the offer, it MUST assign a zero port value
to the corresponding m- line in the answer. In addition, the answerer MUST NOT establish an
SCTP association, or a DTLS connection, associated with the m- line.
When the offerer receives an answer, which contains an m- line with a non-zero
port value, describing an SCTP association, the offerer:
MUST, if the offerer is the 'active' endpoint, if the m- line proto value
is 'TCP/DTLS/SCTP', and if a TCP connection used to carry the SCTP association
has not yet been established (or if an existing TCP connection is to be re-established),
initiate the establishing of the TCP connection;
MUST, if an SCTP association associated with the m- line has not yet
been established (or if an existing SCTP association is to be re-established),
initiate the establishing of the SCTP association; and
MUST, if the offerer is the 'active' endpoint, and if a DTLS connection associated
with the m- line is to be established (or if an existing DTLS connection is to be
re-established), initiate the establishing of the DTLS connection (by sending a
ClientHello message).
NOTE: If the m- line proto value is 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP', the
underlying DTLS connection needs to be established before the SCTP association
can be established.
If the m- line in the answer contains a zero port value, the offerer MUST NOT
establish a TCP connection, an SCTP association, or a DTLS connection, associated with
the m- line.
When an offerer sends an updated offer, in order to modify a previously established
SCTP association, it follows the procedures in ,
with the following exceptions:
Unless the offerer wants to re-establish an existing SCTP association, the offerer
MUST associate an SDP connection attribute, with an 'existing' value, with the
m- line; and
If the offerer wants to disable a previously established SCTP association, it MUST
assign a zero port value to the m- line associated with the SCTP association, following
the procedures in .
SCTP supports multihoming. An SCTP endpoint is considered
multihomed if it has more than one IP address on which SCTP
can be used. An SCTP endpoint inform the remote peer about
its IP addresses using the address parameters in the INIT/INIT-ACK
chunk. Therefore, when SDP is used to describe an SCTP association,
while the "c=" line contains the address which was used to negotiate
the SCTP association, multihomed SCTP endpoints might end up using
other IP addresses.
If an endpoint removes the IP address that
it offered in the SDP "c=" line associated with the SCTP association,
it MUST send a new Offer, in which the "c=" line contains an IP address
which is valid within the SCTP association.
NOTE: In some network environments, intermediaries performing gate-
and firewall control using the address information in the SDP "c=" and
"m=" lines to authorize media, and will not pass media sent using
other addresses. In such network environments, if an SCTP endpoints
wants to change the address information on which media is sent and
received, it needs to send an updated Offer, in which the SDP "c="
and "m=" lines contain the new address information.
Multihoming is not supported when sending SCTP on top of DTLS,
as DTLS does not expose address management of the underlying
transport protocols (UDP or TCP) to its upper layer.
SCTP features not present in UDP or TCP, including the checksum
(CRC32c) value calculated on the whole packet (rather than
just the header), and multihoming, introduce new challenges
for NAT traversal.
defines an SCTP specific variant of NAT, which provides similar
features of Network Address and Port Translation (NAPT).
Current NATs typically do not support SCTP.
defines a mechanism for sending SCTP on top of UDP, which makes it
possible to use SCTP with NATs and firewalls that do not support SCTP.
At the time of writing this specification, no procedures have been
defined for using ICE together with SCTP as
transport layer protocol. Such procedures, including the associated
SDP Offer/Answer procedures, are outside the scope of this specification,
and might be defined in a future specification.
When the transport layer protocol is UDP (in case of an SCTP association on top
of a DTLS connection on top of UDP), if ICE is used, the ICE procedures
defined in are used.
When the transport layer protocol is TCP (in case of an SCTP association on top
of a DTLS connection on top of TCP), if ICE is used, the ICE procedures
defined in are used.
Implementations MUST treat all ICE candidate pairs associated with a
an SCTP association on top of a DTLS connection as part of the same
DTLS connection. Thus, there will only be one DTLS handshake even if
there are multiple valid candidate pairs, and shifting from one candidate
pair to another will not impact the DTLS connection. If new candidates
are added, they will also be part of the same DTLS connection.
defines general SDP security considerations, while
, and
define security considerations when using the SDP offer/answer mechanism
to negotiate media streams.
defines general SCTP security considerations, while
defines security considerations when using DTLS on top of
SCTP, and defines security
considerations when using SCTP on top of DTLS.
This specification does not introduce new security considerations in addition
to those defined in the specifications listed above.
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number
of this document.]
This document updates the "Session Description Protocol (SDP) Parameters" registry,
following the procedures in ,
by adding the following values to the table in the SDP "proto" field registry:
TypeSDP NameReferenceprotoSCTP[RFCXXXX]protoSCTP/DTLS[RFCXXXX]protoUDP/DTLS/SCTP[RFCXXXX]protoTCP/DTLS/SCTP[RFCXXXX]
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number
of this document.]
This document defines a new SDP media-level attribute,'sctp-port', as follows:
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number
of this document.]
This document defines a new SDP media-level attribute,'max-message-size', as follows:
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number
of this document.]
This specification creates a new IANA registry, following the procedures in
, for the "fmt" namespace associated with the
'SCTP', 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' protocol identifiers.
Each "fmt" value describes the usage of an entire SCTP association, including
all SCTP streams associated with the SCTP association.
NOTE: Usage indication of individual SCTP streams is outside the scope of this
specification.
The "fmt" value, "association-usage", used with these "proto" is required.
It is defined in [].
As part of this registry, IANA maintains the following information:
The identifier of the
subprotocol, as will be used as the "fmt" value.A reference to the
document in which the association-usage is defined.
association-usage names are to be subject to the "First Come First Served"
IANA registration policy [RFC5226].
IANA is asked to add initial values to the registry.
The authors wish to thank Harald Alvestrand, Randell Jesup, Paul Kyzivat,
Michael Tuexen, Juergen Stoetzer-Bradler, Flemming Andreasen and Ari Keranen for
their comments and useful feedback.
[RFC EDITOR NOTE: Please remove this section when publishing]Changes from draft-ietf-mmusic-sctp-sdp-15
Re-submission due to expiration of previous version.Reference updates.Changes from draft-ietf-mmusic-sctp-sdp-14
Changes based on WGLC comments from Magnus Westerlund.- ABNF clarification that token and port are defined in RFC4566.- Specify 40 as maximum digit character length for the SDP max-message-size value.- Editorial clarification.Changes based on discussions at IETF#92.- Specify that all ICE candidate pairs belong to the same DTLS connection.Changes from draft-ietf-mmusic-sctp-sdp-13
Changes based on comments from Paul Kyzivat.- Text preventing usage of well-known ports removed.- Editorial clarification.Changes from draft-ietf-mmusic-sctp-sdp-12
Mux category rules added for new SDP attributes.Reference to draft-ietf-mmusic-sdp-mux-attributes added.Changes based on comments from Roman Shpount:- Specify that fingerprint or setup roles must not be modified,
unless underlying transport protocol is also modified.Changes based on comments from Ari Keranen:- Editorial corrections.Changes based on comments from Flemming Andreasen:- Clarify that, if UDP/DTLS/SCTP or TCP/DTLS/SCTP is used,
the DTLS connection is established before the SCTP
association.- Clarify that max-message-size value is given in bytes, and
that different values can be used per direction.- Section on fmtp attribute removed.- Editorial corrections.Changes from draft-ietf-mmusic-sctp-sdp-11
Example added.Changes from draft-ietf-mmusic-sctp-sdp-10
SDP max-message-size attribute added to IANA considerations.Changes based on comments from Paul Kyzivat:- Text about max message size removed from fmtp attribute section.Changes from draft-ietf-mmusic-sctp-sdp-09
'DTLS/SCTP' split into 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'Procedures for realizing UDP/DTLS/SCTP- and TCP/DTLS/SCTP transports added.Changes from draft-ietf-mmusic-sctp-sdp-08
Default SCTP port removed:- Usage of SDP sctp-port attribute mandatory.SDP max-message-size attribute defined:- Attribute definition.- SDP Offer/Answer procedures.Text about SDP direction attributes added.Text about TLS role determination added.