Internet Engineering Task Force (IETF) H. Alvestrand
Request for Comments: 8830
Category: Standards Track January 2021
WebRTC MediaStream Identification in the Session Description Protocol
This document specifies a Session Description Protocol (SDP) grouping
mechanism for RTP media streams that can be used to specify relations
between media streams.
This mechanism is used to signal the association between the SDP
concept of "media description" and the Web Real-Time Communication
(WebRTC) concept of MediaStream/MediaStreamTrack using SDP signaling.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8830
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Table of Contents 1.
Structure of This Document 1.3.
Why a New Mechanism Is Needed 1.4.
The WebRTC MediaStream 2.
The MSID Mechanism 3.
Handling of Nonsignaled Tracks 3.2.
Detailed Offer/Answer Procedures 3.2.1.
Generating the Initial Offer 3.2.2.
Answerer Processing of the Offer 3.2.3.
Generating the Answer 3.2.4.
Offerer Processing of the Answer 3.2.5.
Modifying the Session 3.3.
Example SDP Description 4.
IANA Considerations 4.1.
Attribute Registration in Existing Registries 5.
Security Considerations 6.
Normative References 6.2.
Informative References Appendix A
. Design Considerations, Rejected Alternatives
This document uses terminology from [RFC8825
]. In addition, the
following terms are used as described below:
RTP stream: A stream of RTP packets containing media data [RFC7656
MediaStream: An assembly of MediaStreamTracks
[W3C.CR-mediacapture-streams]. One MediaStream can contain
multiple MediaStreamTracks, of the same or different types.
MediaStreamTrack: Defined in [W3C.CR-mediacapture-streams] as a
unidirectional flow of media data (either audio or video, but not
both). Corresponds to the [RFC7656
] term "source stream". One
MediaStreamTrack can be present in zero, one, or multiple
Media description: Defined in [RFC4566
] as a set of fields starting
with an "m=" field and terminated by either the next "m=" field or
the end of the session description.
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in BCP
] when, and only when, they appear in all
capitals, as shown here.
1.2. Structure of This Document
This document adds a new Session Description Protocol (SDP) [RFC4566
mechanism that can attach identifiers to the RTP streams and attach
identifiers to the groupings they form. It is designed for use with
]. Section 1.3
gives the background on why a new mechanism is needed. Section 2
gives the definition of the new mechanism. Section 3
gives the necessary semantic information and procedures for
using the "msid" attribute to signal the association of
MediaStreamTracks to MediaStreams in support of the WebRTC API
1.3. Why a New Mechanism Is Needed
When media is carried by RTP [RFC3550
], each RTP stream is
distinguished inside an RTP session by its Synchronization Source
(SSRC); each RTP session is distinguished from all other RTP sessions
by being on a different transport association (strictly speaking, two
transport associations, one used for RTP and one used for the RTP
Control Protocol (RTCP), unless RTP/RTCP multiplexing [RFC5761
] gives a format for describing an SDP session that can
contain multiple media descriptions. According to the model used in
], each media description describes exactly one media source.
If multiple media sources are carried in an RTP session, this is
signaled using BUNDLE [RFC8843
]; if BUNDLE is not used, each media
source is carried in its own RTP session.
The SDP Grouping Framework [RFC5888
] can be used to group media
descriptions. However, for the use case of WebRTC, there is the need
for an application to specify some application-level information
about the association between the media description and the group.
This is not possible using the SDP Grouping Framework.
1.4. The WebRTC MediaStream
The W3C WebRTC API specification [W3C-WebRTC] specifies that
communication between WebRTC entities is done via MediaStreams, which
contain MediaStreamTracks. A MediaStreamTrack is generally carried
using a single SSRC in an RTP session, forming an RTP stream. The
collision of terminology is unfortunate. There might possibly be
additional SSRCs, possibly within additional RTP sessions, in order
to support functionality like forward error correction or simulcast.
These additional SSRCs are not affected by this specification.
MediaStreamTracks are unidirectional; they carry media in one
In the RTP specification, RTP streams are identified using the SSRC
field. Streams are grouped into RTP sessions and also carry a CNAME.
Neither CNAME nor RTP session corresponds to a MediaStream.
Therefore, the association of an RTP stream to MediaStreams need to
be explicitly signaled.
WebRTC defines a mapping (documented in [RFC8829
]) where one SDP
media description is used to describe each MediaStreamTrack, and the
BUNDLE mechanism [RFC8843
] is used to group MediaStreamTracks into
RTP sessions. Therefore, the need is to specify the identifier (ID)
of the MediaStreamTrack and its associated MediaStream for each media
description, which can be accomplished with a media-level SDP
This usage is described in Section 3
2. The MSID Mechanism
This document defines a new SDP [RFC4566
] media-level "msid"
attribute. This new attribute allows endpoints to associate RTP
streams that are described in separate media descriptions with the
right MediaStreams, as defined in [W3C-WebRTC]. It also allows
endpoints to carry an identifier for each MediaStreamTrack in its
The value of the "msid" attribute consists of an identifier and an
optional "appdata" field.
The name of the attribute is "msid".
The value of the attribute is specified by the following ABNF
msid-value = msid-id [ SP msid-appdata ]
msid-id = 1*64token-char ; see RFC 4566
msid-appdata = 1*64token-char ; see RFC 4566
An example "msid" value for a group with the identifier "examplefoo"
and application data "examplebar" might look like this:
The identifier is a string of ASCII characters that are legal in a
"token", consisting of between 1 and 64 characters.
Application data (msid-appdata) is carried on the same line as the
identifier, separated from the identifier by a space.
The identifier ("msid-id") uniquely identifies a group within the
scope of an SDP description.
There may be multiple "msid" attributes in a single media
description. This represents the case where a single
MediaStreamTrack is present in multiple MediaStreams; the value of
be identical for all occurrences.
Multiple media descriptions with the same value for "msid-id" and
"msid-appdata" are not permitted.
Endpoints can update the associations between RTP streams as
expressed by "msid" attributes at any time.
The "msid" attributes depend on the association of RTP streams with
media descriptions but do not depend on the association of RTP
streams with RTP transports. Therefore, their Mux Category (as
defined in [RFC8859
]) is NORMAL; the process of deciding on "msid"
attributes doesn't have to take into consideration whether or not the
RTP streams are bundled.
This section describes the procedures for associating media
descriptions representing MediaStreamTracks within MediaStreams, as
defined in [W3C-WebRTC].
MediaStream and MediaStreamTrack has an "id" attribute, which is a
The value of the "msid-id" field in the MSID consists of the "id"
attribute of a MediaStream, as defined in the MediaStream's WebIDL
specification [WEBIDL]. The special value "-" indicates "no
The value of the "msid-appdata" field in the MSID, if present,
consists of the "id" attribute of a MediaStreamTrack, as defined in
the MediaStreamTrack's WebIDL specification.
When an SDP session description is updated, a specific "msid-id"
value continues to refer to the same MediaStream, and a specific
"msid-appdata" to the same MediaStreamTrack. There is no memory
apart from the currently valid SDP descriptions; if an MSID
"identifier" value disappears from the SDP and appears in a later
negotiation, it will be taken to refer to a new MediaStream.
If the "msid" attribute does not conform to the ABNF given here, it SHOULD
The following is a high-level description of the rules for handling
SDP updates. Detailed procedures are located in Section 3.2
* When a new MSID "identifier" value occurs in a session
description, and it is not "-", the recipient can signal to its
application that a new MediaStream has been added.
* When a session description is updated to have media descriptions
with an MSID "identifier" value, with one or more different
"appdata" values, the recipient can signal to its application that
new MediaStreamTracks have been added and note to which
MediaStream they have been added. This is done for each different
MSID "identifier" value, including the special value "-", which
indicates that a MediaStreamTrack has been added with no
* If an MSID "identifier" value with no "appdata" value appears, it
means that the sender did not inform the recipient of the desired
identifier of the MediaStreamTrack, and the recipient will assign
the "id" value of the created MediaStreamTrack on its own. All
MSIDs in a media section that do not have an "appdata" value are
assumed to refer to the same MediaStreamTrack.
* When a session description is updated to no longer list any "msid"
attribute on a specific media description, the recipient can
signal to its application that the corresponding MediaStreamTrack
In addition to signaling that the track is ended when its "msid"
attribute disappears from the SDP, the track will also be signaled as
being ended when all associated SSRCs have disappeared by the rules
], Sections 6.3.4
(BYE packet received) and 6.3.5
(timeout), or when the corresponding media description is disabled by
setting the port number to zero. Changing the direction of the media
description (by setting "sendonly", "recvonly", or "inactive"
attributes) will not end the MediaStreamTrack.
The association between SSRCs and media descriptions is specified in
3.1. Handling of Nonsignaled Tracks
Entities that do not use the mechanism described in this document
will not send the "msid" attribute and thus will not send information
allowing the mapping of RTP packets to MediaStreams. This means that
there will be some incoming RTP packets for which the recipient has
no predefined MediaStream ID value.
Note that the handling described below is triggered by incoming RTP
packets, not SDP negotiation.
When communicating with entities that use the MSID mechanism, the
only time incoming RTP packets can be received without an associated
MediaStream ID value is when, after the initial negotiation, a
negotiation is performed where the answerer adds a MediaStreamTrack
to an already established connection and starts sending data before
the answer is received by the offerer. For initial negotiation,
packets won't flow until the Interactive Connectivity Establishment
(ICE) candidates and fingerprints have been exchanged, so this is not
The recipient of those packets will perform the following steps:
* When RTP packets are initially received, it will create an
appropriate MediaStreamTrack based on the type of the media
(carried in PayloadType) and use the MID RTP header extension
] (if present) to associate the RTP packets with a
specific media section.
* If the connection is not in the RTCSignalingState "stable", it
will wait at this point.
* When the connection is in the RTCSignalingState "stable", it will
assign ID values.
The following steps are performed to assign ID values:
* If there is an "msid" attribute, it will use that attribute to
populate the "id" field of the MediaStreamTrack and associated
MediaStreams, as described above.
* If there is no "msid" attribute, the identifier of the
MediaStreamTrack will be set to a randomly generated string, and
it will be signaled as being part of a MediaStream with the WebIDL
"label" attribute set to "Non-WebRTC stream".
* After deciding on the "id" field to be applied to the
MediaStreamTrack, the track will be signaled to the user.
The process above may involve a considerable amount of buffering
before the "stable" state is entered. If the implementation wishes
to limit this buffering, it MUST
signal to the user that media has
It follows from the above that MediaStreamTracks in the "default"
MediaStream cannot be closed by removing the "msid" attribute; the
application must instead signal these as closed when the SSRC
disappears, either according to the rules of Sections 6
] or by disabling the media description by setting its
port to zero.
3.2. Detailed Offer/Answer Procedures
These procedures are given in terms of sections recommended by
]. They describe the actions to be taken in terms of
MediaStreams and MediaStreamTracks; they do not include event
signaling inside the application, which is described in the
3.2.1. Generating the Initial Offer
For each media description in the offer, if there is an associated
outgoing MediaStreamTrack, the offerer adds one "a=msid" attribute to
the section for each MediaStream with which the MediaStreamTrack is
associated. The "identifier" field of the attribute is set to the
WebIDL "id" attribute of the MediaStream. If the sender wishes to
signal identifiers for the MediaStreamTracks, the "appdata" field is
set to the WebIDL "id" attribute of the MediaStreamTrack; otherwise,
it is omitted.
3.2.2. Answerer Processing of the Offer
For each media description in the offer and each "a=msid" attribute
in the media description, the receiver of the offer will perform the
* Extract the "appdata" field of the "a=msid" attribute, if present.
* If the "appdata" field exists: Check if a MediaStreamTrack with
the same WebIDL "id" attribute as the "appdata" field already
exists and is not in the "ended" state. If such a
MediaStreamTrack is not found, create it.
* If the "appdata" field does not exist, and a MediaStreamTrack is
not associated with this media section, create a MediaStreamTrack
and associate it with this media section for future use.
* Extract the "identifier" field of the "a=msid" attribute.
* Check if a MediaStream with the same WebIDL "id" attribute already
exists. If not, create it.
* Add the MediaStreamTrack to the MediaStream.
* Signal to the user that a new MediaStreamTrack is available.
3.2.3. Generating the Answer
The answer is generated in exactly the same manner as the offer.
"a=msid" values in the offer do not influence the answer.
3.2.4. Offerer Processing of the Answer
The answer is processed in exactly the same manner as the offer.
3.2.5. Modifying the Session
On subsequent exchanges, precisely the same procedure as for the
initial offer/answer is followed, but with one additional step in the
parsing of the offer and answer:
* For each MediaStreamTrack that has been created as a result of
previous offer/answer exchanges, and is not in the "ended" state,
check to see if there is still an "a=msid" attribute in the
present SDP whose "appdata" field is the same as the WebIDL "id"
attribute of the track.
* If no such attribute is found, stop the MediaStreamTrack. This
will set its state to "ended".
3.3. Example SDP Description
The following SDP description shows the representation of a WebRTC
PeerConnection with two MediaStreams, each of which has one audio and
one video track. Only the parts relevant to the MSID are shown.
Line wrapping, empty lines, and comments are added for clarity. They
are not part of the SDP.
# First MediaStream - id is 4701...
m=audio 56500 UDP/TLS/RTP/SAVPF 96 0 8 97 98
m=video 56502 UDP/TLS/RTP/SAVPF 100 101
# Second MediaStream - id is 6131....
m=audio 56503 UDP/TLS/RTP/SAVPF 96 0 8 97 98
m=video 56504 UDP/TLS/RTP/SAVPF 100 101
4. IANA Considerations
4.1. Attribute Registration in Existing Registries
IANA has registered the "msid" attribute in the "att-field" (media
level only) registry within the "Session Description Protocol (SDP)
Parameters" registry, according to the procedures of [RFC4566
The "msid" registration information is as follows:
Contact name, email: IETF, contacted via firstname.lastname@example.org, or a
successor address designated by IESG
Attribute name: msid
msid-value = msid-id [ SP msid-appdata ]
msid-id = 1*64token-char ; see RFC 4566
msid-appdata = 1*64token-char ; see RFC 4566
Attribute semantics: Described in RFC 8830
Attribute value: msid-value
Long-form attribute name: MediaStream Identifier
Usage level: media
Subject to charset: The attribute value contains only ASCII
characters and is therefore not subject to the charset attribute.
Purpose: The attribute can be used to signal the relationship
between a WebRTC MediaStream and a set of media descriptions.
O/A Procedures: Described in RFC 8830
Appropriate values: The details of appropriate values are given in RFC 8830
Mux Category: NORMAL
The Mux Category is defined in [RFC8859
5. Security Considerations
An adversary with the ability to modify SDP descriptions has the
ability to switch around tracks between MediaStreams. This is a
special case of the general security consideration that modification
of SDP descriptions needs to be confined to entities trusted by the
If implementing buffering as mentioned in Section 3.1
, the amount of
buffering should be limited to avoid memory exhaustion attacks.
Careless generation of identifiers can leak privacy-sensitive
information. [W3C.CR-mediacapture-streams] recommends that
identifiers be generated using a Universally Unique IDentifier (UUID)
class 3 or 4 as a basis, which avoids such leakage.
No other attacks have been identified that depend on this mechanism.
6.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550
, DOI 10.17487/RFC3550
July 2003, <https://www.rfc-editor.org/info/rfc3550
] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566
, DOI 10.17487/RFC4566
July 2006, <https://www.rfc-editor.org/info/rfc4566
] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234
, January 2008,
] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", BCP 14, RFC 8174
, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174
] Uberti, J., Jennings, C., and E. Rescorla, Ed.,
, DOI 10.17487/RFC8829
, January 2021,
] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859
, January 2021,
Jennings, C., Boström, H., and J-I. Bruaroey, "WebRTC 1.0:
Real-time Communication Between Browsers", W3C Proposed
Jennings, C., Aboba, B., Bruaroey, J.-I., and H. Boström,
"Media Capture and Streams", W3C Candidate Recommendation,
6.2. Informative References
] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264
, June 2002,
] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761
, April 2010,
] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888
, June 2010,
] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656
, November 2015,
] Alvestrand, H., "Overview: Real-Time Protocols for
Browser-Based Applications", RFC 8825
, January 2021,
] Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", RFC 8843
, January 2021,
[WEBIDL] Chen, E. and T. Gu, "Web IDL", W3C Editor's Draft, August
Appendix A. Design Considerations, Rejected Alternatives
One suggested mechanism has been to use CNAME instead of a new
attribute. This was abandoned because CNAME identifies a
synchronization context; one can imagine both wanting to have tracks
from the same synchronization context in multiple MediaStreams and
wanting to have tracks from multiple synchronization contexts within
one MediaStream (but the latter is impossible, since a MediaStream is
defined to impose synchronization on its members).
Another suggestion has been to put the "msid" value within an
attribute of RTCP SR (sender report) packets. This doesn't offer the
ability to know that you have seen all the tracks currently
configured for a MediaStream.
A suggestion that survived for a number of drafts of this document
was to define MSID as a generic mechanism, where the particular
semantics of this usage of the mechanism would be defined by an
"a=wms-semantic" attribute. This was removed in April 2015.
This note is based on sketches from, among others, Justin Uberti and
Special thanks to Flemming Andreasen, Ben Campbell, Miguel Garcia,
Martin Thomson, Ted Hardie, Adam Roach, Magnus Westerlund, Alissa
Cooper, Sue Hares, and Paul Kyzivat for their work in reviewing this
document, with many specific language suggestions.