Internet Engineering Task Force (IETF)                     H. Alvestrand
Request for Comments: 8830                                        Google
Category: Standards Track                                   January 2021
ISSN: 2070-1721

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.

このメカニズムは、SDPシグナリングを使用して、「メディア記述」のSDPコンセプトとMediaStream / MediaStreamTrackのWebリアルタイム通信(WebRTC)コンセプトの間の関連付けをシグナリングするために使用されます。

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.

このドキュメントは、インターネット技術特別調査委員会(IETF)の製品です。これは、IETFコミュニティのコンセンサスを表しています。パブリックレビューを受け、Internet Engineering Steering Group(IESG)による公開が承認されました。インターネット標準の詳細については、RFC7841のセクション2をご覧ください。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at


Copyright Notice


Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.

Copyright(c)2021 IETFTrustおよびドキュメントの作成者として識別された人物。全著作権所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

このドキュメントは、このドキュメントの発行日に有効なBCP 78およびIETFドキュメントに関連するIETFトラストの法的規定(の対象となります。これらのドキュメントは、このドキュメントに関するお客様の権利と制限について説明しているため、注意深く確認してください。このドキュメントから抽出されたコードコンポーネントには、Trust LegalProvisionsのセクション4.eで説明されているSimplifiedBSD Licenseテキストが含まれている必要があり、Simplified BSDLicenseで説明されているように保証なしで提供されます。

Table of Contents


   1.  Introduction
     1.1.  Terminology
     1.2.  Structure of This Document
     1.3.  Why a New Mechanism Is Needed
     1.4.  The WebRTC MediaStream
   2.  The MSID Mechanism
   3.  Procedures
     3.1.  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.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Design Considerations, Rejected Alternatives
   Author's Address
1. Introduction
1. はじめに
1.1. Terminology
1.1. 用語

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.

MediaStream:MediaStreamTracks [W3C.CR-mediacapture-streams]のアセンブリ。1つのMediaStreamには、同じタイプまたは異なるタイプの複数のMediaStreamTracksを含めることができます。

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 MediaStreams.


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.

メディアの説明:[RFC4566]で、「m =」フィールドで始まり、次の「m =」フィールドまたはセッションの説明の終わりで終わるフィールドのセットとして定義されています。

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

キーワード「MUST」、「MUST NOT」、「REQUIRED」、「SHALL」、「SHALL NOT」、「SHOULD」、「SHOULD NOT」、「RECOMMENDED」、「NOT RECOMMENDED」、「MAY」、「OPTIONAL」「このドキュメントでは、BCP 14 [RFC2119] [RFC8174]で説明されているように、ここに示すように、すべて大文字で表示される場合にのみ解釈されます。

1.2. Structure of This Document
1.2. このドキュメントの構造

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 WebRTC [RFC8825].

このドキュメントは、RTPストリームに識別子を添付し、それらが形成するグループに識別子を添付できる新しいセッション記述プロトコル(SDP)[RFC4566]メカニズムを追加します。WebRTC [RFC8825]で使用するように設計されています。

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 [W3C-WebRTC].

セクション3では、「msid」属性を使用して、WebRTC API [W3C-WebRTC]をサポートするMediaStreamTracksとMediaStreamsの関連付けを通知するために必要なセマンティック情報と手順を示します。

1.3. Why a New Mechanism Is Needed
1.3. 新しいメカニズムが必要な理由

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] is used).

メディアがRTP [RFC3550]によって伝送される場合、各RTPストリームは、同期ソース(SSRC)によってRTPセッション内で区別されます。各RTPセッションは、異なるトランスポートアソシエーション(厳密に言えば、RTP / RTCP多重化[RFC5761]がない限り、1つはRTPに使用され、もう1つはRTP制御プロトコル(RTCP)に使用される2つのトランスポートアソシエーション)上にあることによって他のすべてのRTPセッションと区別されます。中古)。

SDP [RFC4566] gives a format for describing an SDP session that can contain multiple media descriptions. According to the model used in [RFC8829], 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.

SDP [RFC4566]は、複数のメディア記述を含むことができるSDPセッションを記述するためのフォーマットを提供します。[RFC8829]で使用されているモデルによると、各メディアの説明は正確に1つのメディアソースを記述しています。複数のメディアソースがRTPセッションで伝送される場合、これはBUNDLE [RFC8843]を使用して通知されます。BUNDLEが使用されていない場合、各メディアソースは独自のRTPセッションで伝送されます。

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
1.4. 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.

W3C WebRTC API仕様[W3C-WebRTC]は、WebRTCエンティティ間の通信がMediaStreamTracksを含むMediaStreamsを介して行われることを指定しています。MediaStreamTrackは通常、RTPセッションで単一のSSRCを使用して伝送され、RTPストリームを形成します。用語の衝突は残念です。前方誤り訂正やサイマルキャストなどの機能をサポートするために、おそらく追加のRTPセッション内に追加のSSRCが存在する可能性があります。これらの追加のSSRCは、この仕様の影響を受けません。

MediaStreamTracks are unidirectional; they carry media in one direction only.


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 attribute.


This usage is described in Section 3.


2. The MSID Mechanism
2. MSIDメカニズム

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 "appdata" field.

このドキュメントでは、新しいSDP [RFC4566]メディアレベルの「msid」属性を定義しています。この新しい属性により、エンドポイントは、[W3C-WebRTC]で定義されているように、個別のメディア記述で記述されているRTPストリームを適切なMediaStreamsに関連付けることができます。また、エンドポイントが「appdata」フィールドに各MediaStreamTrackの識別子を保持できるようにします。

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 [RFC5234] grammar:

属性の値は、次のABNF [RFC5234]文法によって指定されます。

     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:


msid:examplefoo examplebar

msid:examplefoo examplebar

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.

識別子( "msid-id")は、SDP記述のスコープ内のグループを一意に識別します。

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 "msid-appdata" MUST 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.


3. Procedures
3. 手順

This section describes the procedures for associating media descriptions representing MediaStreamTracks within MediaStreams, as defined in [W3C-WebRTC].


In the Javascript API described in that specification, each MediaStream and MediaStreamTrack has an "id" attribute, which is a DOMString.


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 MediaStream".


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 be ignored.


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.

* セッション記述に新しいMSID「識別子」値があり、それが「-」でない場合、受信者は、新しいMediaStreamが追加されたことをアプリケーションに通知できます。

* 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 corresponding MediaStream.

* セッション記述が更新されて、MSID「識別子」値と1つ以上の異なる「appdata」値を持つメディア記述を持つ場合、受信者は、新しいMediaStreamTracksが追加されたことをアプリケーションに通知し、それらが追加されたMediaStreamを記録できます。。これは、MediaStreamTrackが追加され、対応するMediaStreamがないことを示す、特別な値「-」を含む、異なるMSID「識別子」値ごとに実行されます。

* 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.

* 「appdata」値のないMSID「識別子」値が表示される場合は、送信者が受信者にMediaStreamTrackの目的の識別子を通知しなかったことを意味し、受信者は作成されたMediaStreamTrackの「id」値を独自に割り当てます。。「appdata」値を持たないメディアセクション内のすべてのMSIDは、同じMediaStreamTrackを参照していると見なされます。

* 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 has ended.

* セッションの説明が更新され、特定のメディアの説明に「msid」属性がリストされなくなると、受信者は、対応するMediaStreamTrackが終了したことをアプリケーションに通知できます。

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 of [RFC3550], 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 [RFC8829].


3.1. Handling of Nonsignaled Tracks
3.1. 信号のないトラックの処理

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 an issue.

MSIDメカニズムを使用するエンティティと通信する場合、MediaStream ID値が関連付けられていない着信RTPパケットを受信できるのは、最初のネゴシエーションの後、応答者がすでに確立されている接続にMediaStreamTrackを追加して開始するネゴシエーションが実行されるときだけです。提供者が回答を受け取る前にデータを送信します。最初のネゴシエーションでは、Interactive Connectivity Establishment(ICE)の候補とフィンガープリントが交換されるまでパケットは流れないため、これは問題ではありません。

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 [RFC8843] (if present) to associate the RTP packets with a specific media section.

* RTPパケットが最初に受信されると、メディアのタイプ(PayloadTypeで伝送)に基づいて適切なMediaStreamTrackが作成され、MID RTPヘッダー拡張[RFC8843](存在する場合)を使用してRTPパケットが特定のメディアセクションに関連付けられます。

* If the connection is not in the RTCSignalingState "stable", it will wait at this point.

* 接続がRTCSignalingStateの「安定」状態にない場合、この時点で待機します。

* When the connection is in the RTCSignalingState "stable", it will assign ID values.

* 接続がRTCSignalingStateの「安定」状態にある場合、ID値が割り当てられます。

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.

* 「msid」属性がある場合、上記のように、その属性を使用してMediaStreamTrackおよび関連するMediaStreamsの「id」フィールドにデータを入力します。

* 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".

* 「msid」属性がない場合、MediaStreamTrackの識別子はランダムに生成された文字列に設定され、WebIDLの「label」属性が「Non-WebRTCstream」に設定されたMediaStreamの一部として通知されます。

* After deciding on the "id" field to be applied to the MediaStreamTrack, the track will be signaled to the user.

* MediaStreamTrackに適用する「id」フィールドを決定した後、トラックはユーザーに通知されます。

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 been discarded.


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 6.3.4 and 6.3.5 of [RFC3550] or by disabling the media description by setting its port to zero.


3.2. Detailed Offer/Answer Procedures
3.2. 詳細なオファー/回答手順

These procedures are given in terms of sections recommended by [RFC3264]. 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 JavaScript Session Establishment Protocol (JSEP) [RFC8829].


3.2.1. Generating the Initial Offer
3.2.1. 最初のオファーの生成

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.

オファー内のメディアの説明ごとに、関連付けられた発信MediaStreamTrackがある場合、オファー提供者は、MediaStreamTrackが関連付けられている各MediaStreamのセクションに1つの「a = msid」属性を追加します。属性の「識別子」フィールドは、MediaStreamのWebIDL「id」属性に設定されます。送信者がMediaStreamTracksの識別子を通知する場合、「appdata」フィールドはMediaStreamTrackのWebIDL「id」属性に設定されます。それ以外の場合は省略されます。

3.2.2. Answerer Processing of the Offer
3.2.2. オファーの回答者処理

For each media description in the offer and each "a=msid" attribute in the media description, the receiver of the offer will perform the following steps:

オファーの各メディアの説明およびメディアの説明の各「a = msid」属性について、オファーの受信者は次の手順を実行します。

* Extract the "appdata" field of the "a=msid" attribute, if present.

* 「a = msid」属性の「appdata」フィールドが存在する場合は、それを抽出します。

* 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.

* 「appdata」フィールドが存在する場合:「appdata」フィールドと同じWebIDL「id」属性を持つMediaStreamTrackがすでに存在し、「終了」状態にないかどうかを確認します。そのようなMediaStreamTrackが見つからない場合は、作成します。

* 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.

* 「appdata」フィールドが存在せず、MediaStreamTrackがこのメディアセクションに関連付けられていない場合は、MediaStreamTrackを作成し、将来使用するためにこのメディアセクションに関連付けます。

* Extract the "identifier" field of the "a=msid" attribute.

* 「a = msid」属性の「識別子」フィールドを抽出します。

* Check if a MediaStream with the same WebIDL "id" attribute already exists. If not, create it.

* 同じWebIDL「id」属性を持つMediaStreamがすでに存在するかどうかを確認します。そうでない場合は、作成します。

* Add the MediaStreamTrack to the MediaStream.

* MediaStreamTrackをMediaStreamに追加します。

* Signal to the user that a new MediaStreamTrack is available.

* 新しいMediaStreamTrackが利用可能であることをユーザーに通知します。

3.2.3. Generating the Answer
3.2.3. 答えを生成する

The answer is generated in exactly the same manner as the offer. "a=msid" values in the offer do not influence the answer.

回答は、オファーとまったく同じ方法で生成されます。オファーの「a = msid」値は回答に影響しません。

3.2.4. Offerer Processing of the Answer
3.2.4. 回答の提供者処理

The answer is processed in exactly the same manner as the offer.


3.2.5. Modifying the Session
3.2.5. セッションの変更

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.

* 以前のオファー/アンサー交換の結果として作成され、「終了」状態ではない各MediaStreamTrackについて、「appdata」フィールドを持つ現在のSDPに「a = msid」属性がまだあるかどうかを確認します。トラックのWebIDL「id」属性と同じです。

* If no such attribute is found, stop the MediaStreamTrack. This will set its state to "ended".

* そのような属性が見つからない場合は、MediaStreamTrackを停止します。これにより、状態が「終了」に設定されます。

3.3. Example SDP Description
3.3. SDPの説明の例

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. IANAの考慮事項
4.1. Attribute Registration in Existing Registries
4.1. 既存のレジストリでの属性登録

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].

IANAは、[RFC4566]の手順に従って、「Session Description Protocol(SDP)Parameters」レジストリ内の「att-field」(メディアレベルのみ)レジストリに「msid」属性を登録しました。

The "msid" registration information is as follows:


Contact name, email: IETF, contacted via, or a successor address designated by IESG

連絡先名、電子メール:IETF、mmusic @ ietf.org経由で連絡、またはIESGによって指定された後継アドレス

Attribute name: msid


Attribute syntax:


                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

O / A手順:RFC8830で説明されています

Appropriate values: The details of appropriate values are given in RFC 8830 (this document).

適切な値:適切な値の詳細は、RFC 8830(このドキュメント)に記載されています。

Mux Category: NORMAL


The Mux Category is defined in [RFC8859].


5. Security Considerations
5. セキュリティに関する考慮事項

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 application.


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.

識別子を不注意に生成すると、プライバシーに配慮した情報が漏洩する可能性があります。[W3C.CR-mediacapture-streams]は、このようなリークを回避するために、Universally Unique IDentifier(UUID)クラス3または4をベースとして識別子を生成することを推奨しています。

No other attacks have been identified that depend on this mechanism.


6. References
6. 参考文献
6.1. Normative References
6.1. 引用文献

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <>.

[RFC2119] Bradner、S。、「要件レベルを示すためにRFCで使用するキーワード」、BCP 14、RFC 2119、DOI 10.17487 / RFC2119、1997年3月、<>。

[RFC3550] 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, <>.

[RFC3550] Schulzrinne、H.、Casner、S.、Frederick、R。、およびV. Jacobson、「RTP:A Transport Protocol for Real-Time Applications」、STD 64、RFC 3550、DOI 10.17487 / RFC3550、2003年7月、<>。

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, <>.

[RFC4566] Handley、M.、Jacobson、V。、およびC. Perkins、「SDP:Session Description Protocol」、RFC 4566、DOI 10.17487 / RFC4566、2006年7月、< / rfc4566>。

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <>.

[RFC5234] Crocker、D.、Ed。およびP.Overell、「構文仕様の拡張BNF:ABNF」、STD 68、RFC 5234、DOI 10.17487 / RFC5234、2008年1月、<>。

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <>.

[RFC8174] Leiba、B。、「RFC 2119キーワードにおける大文字と小文字のあいまいさ」、BCP 14、RFC 8174、DOI 10.17487 / RFC8174、2017年5月、<>。

[RFC8829] Uberti, J., Jennings, C., and E. Rescorla, Ed., "JavaScript Session Establishment Protocol (JSEP)", RFC 8829, DOI 10.17487/RFC8829, January 2021, <>.

[RFC8829] Uberti、J.、Jennings、C。、およびE. Rescorla、Ed。、「JavaScript Session Establishment Protocol(JSEP)」、RFC 8829、DOI 10.17487 / RFC8829、2021年1月、<>。

[RFC8859] Nandakumar, S., "A Framework for Session Description Protocol (SDP) Attributes When Multiplexing", RFC 8859, DOI 10.17487/RFC8859, January 2021, <>.

[RFC8859] Nandakumar、S。、「多重化時のセッション記述プロトコル(SDP)属性のフレームワーク」、RFC 8859、DOI 10.17487 / RFC8859、2021年1月、<>。

[W3C-WebRTC] Jennings, C., Boström, H., and J-I. Bruaroey, "WebRTC 1.0: Real-time Communication Between Browsers", W3C Proposed Recommendation, <>.

[W3C-WebRTC] Jennings、C.、Boström、H。、およびJ-I。Bruaroey、「WebRTC 1.0:ブラウザ間のリアルタイム通信」、W3C提案の推奨事項、<>。

[W3C.CR-mediacapture-streams] Jennings, C., Aboba, B., Bruaroey, J.-I., and H. Boström, "Media Capture and Streams", W3C Candidate Recommendation, <>.

[W3C.CR-mediacapture-streams] Jennings、C.、Aboba、B.、Bruaroey、J.-I。、およびH.Boström、「Media Capture and Streams」、W3C Candidate Recommendation、<https:// www。>。

6.2. Informative References
6.2. 参考引用

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002, <>.

[RFC3264] Rosenberg、J。およびH. Schulzrinne、「Session Description Protocol(SDP)を使用したオファー/アンサーモデル」、RFC 3264、DOI 10.17487 / RFC3264、2002年6月、< info / rfc3264>。

[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, DOI 10.17487/RFC5761, April 2010, <>.

[RFC5761] Perkins、C。およびM. Westerlund、「単一ポートでのRTPデータと制御パケットの多重化」、RFC 5761、DOI 10.17487 / RFC5761、2010年4月、< rfc5761>。

[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, DOI 10.17487/RFC5888, June 2010, <>.

[RFC5888] Camarillo、G。およびH. Schulzrinne、「The Session Description Protocol(SDP)Grouping Framework」、RFC 5888、DOI 10.17487 / RFC5888、2010年6月、<>。

[RFC7656] 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, DOI 10.17487/RFC7656, November 2015, <>.

[RFC7656] 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、DOI 10.17487 / RFC7656、2015年11月、<>。

[RFC8825] Alvestrand, H., "Overview: Real-Time Protocols for Browser-Based Applications", RFC 8825, DOI 10.17487/RFC8825, January 2021, <>.

[RFC8825] Alvestrand、H。、「Overview:Real-Time Protocols for Browser-Based Applications」、RFC 8825、DOI 10.17487 / RFC8825、2021年1月、<>。

[RFC8843] Holmberg, C., Alvestrand, H., and C. Jennings, "Negotiating Media Multiplexing Using the Session Description Protocol (SDP)", RFC 8843, DOI 10.17487/RFC8843, January 2021, <>.

[RFC8843] Holmberg、C.、Alvestrand、H。、およびC. Jennings、「Session Description Protocol(SDP)を使用したメディア多重化のネゴシエーション」、RFC 8843、DOI 10.17487 / RFC8843、2021年1月<https:// www。>。

[WEBIDL] Chen, E. and T. Gu, "Web IDL", W3C Editor's Draft, August 2020, <>.

[WEBIDL] Chen、E。およびT. Gu、「Web IDL」、W3C編集者ドラフト、2020年8月、<>。

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.

もう1つの提案は、「msid」値をRTCP SR(送信者レポート)パケットの属性内に配置することです。これは、MediaStream用に現在構成されているすべてのトラックを見たことを知る機能を提供しません。

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.

このドキュメントの多くのドラフトで生き残った提案は、MSIDを汎用メカニズムとして定義することでした。このメカニズムの使用法の特定のセマンティクスは、「a = wms-semantic」属性によって定義されます。これは2015年4月に削除されました。



This note is based on sketches from, among others, Justin Uberti and Cullen Jennings.


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.

Flemming Andreasen、Ben Campbell、Miguel Garcia、Martin Thomson、Ted Hardie、Adam Roach、Magnus Westerlund、Alissa Cooper、Sue Hares、Paul Kyzivatが、このドキュメントのレビューに尽力してくれたことに特に感謝します。

Author's Address


Harald Alvestrand Google Kungsbron 2 SE-11122 Stockholm Sweden

Harald Alvestrand Google Kungsbron 2SE-11122ストックホルムスウェーデン