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diff --git a/doc/rfc/rfc8843.txt b/doc/rfc/rfc8843.txt new file mode 100644 index 0000000..80d4e31 --- /dev/null +++ b/doc/rfc/rfc8843.txt @@ -0,0 +1,2672 @@ + + + + +Internet Engineering Task Force (IETF) C. Holmberg +Request for Comments: 8843 Ericsson +Updates: 3264, 5888, 7941 H. Alvestrand +Category: Standards Track Google +ISSN: 2070-1721 C. Jennings + Cisco + January 2021 + + + Negotiating Media Multiplexing Using the Session Description Protocol + (SDP) + +Abstract + + This specification defines a new Session Description Protocol (SDP) + Grouping Framework extension called 'BUNDLE'. The extension can be + used with the SDP offer/answer mechanism to negotiate the usage of a + single transport (5-tuple) for sending and receiving media described + by multiple SDP media descriptions ("m=" sections). Such transport + is referred to as a BUNDLE transport, and the media is referred to as + bundled media. The "m=" sections that use the BUNDLE transport form + a BUNDLE group. + + This specification defines a new RTP Control Protocol (RTCP) Source + Description (SDES) item and a new RTP header extension. + + This specification updates RFCs 3264, 5888, and 7941. + +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/rfc8843. + +Copyright Notice + + Copyright (c) 2021 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (https://trustee.ietf.org/license-info) 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. + + This document may contain material from IETF Documents or IETF + Contributions published or made publicly available before November + 10, 2008. The person(s) controlling the copyright in some of this + material may not have granted the IETF Trust the right to allow + modifications of such material outside the IETF Standards Process. + Without obtaining an adequate license from the person(s) controlling + the copyright in such materials, this document may not be modified + outside the IETF Standards Process, and derivative works of it may + not be created outside the IETF Standards Process, except to format + it for publication as an RFC or to translate it into languages other + than English. + +Table of Contents + + 1. Introduction + 1.1. Background + 1.2. BUNDLE Mechanism + 1.3. Protocol Extensions + 1.4. Contradiction regarding bundle-only "m=" sections + 2. Terminology + 3. Conventions + 4. Applicability Statement + 5. SDP Grouping Framework BUNDLE Extension + 6. SDP 'bundle-only' Attribute + 7. SDP Offer/Answer Procedures + 7.1. Generic SDP Considerations + 7.1.1. Connection Data ("c=") + 7.1.2. Bandwidth ("b=") + 7.1.3. Attributes ("a=") + 7.2. Generating the Initial SDP Offer + 7.2.1. Suggesting the Offerer-Tagged 'm=' Section + 7.2.2. Example: Initial SDP Offer + 7.3. Generating the SDP Answer + 7.3.1. Answerer Selection of Tagged 'm=' Sections + 7.3.2. Moving a Media Description Out of a BUNDLE Group + 7.3.3. Rejecting a Media Description in a BUNDLE Group + 7.3.4. Example: SDP Answer + 7.4. Offerer Processing of the SDP Answer + 7.5. Modifying the Session + 7.5.1. Adding a Media Description to a BUNDLE Group + 7.5.2. Moving a Media Description Out of a BUNDLE Group + 7.5.3. Disabling a Media Description in a BUNDLE Group + 8. Protocol Identification + 8.1. STUN, DTLS, and SRTP + 9. RTP Considerations + 9.1. Single RTP Session + 9.1.1. Payload Type (PT) Value Reuse + 9.2. Associating RTP/RTCP Streams with the Correct SDP Media + Description + 9.3. RTP/RTCP Multiplexing + 9.3.1. SDP Offer/Answer Procedures + 10. ICE Considerations + 11. DTLS Considerations + 12. RTP Header Extensions Consideration + 13. Update to RFC 3264 + 13.1. Original Text from RFC 3264, Section 5.1, 2nd Paragraph + 13.2. New Text Replacing RFC 3264, Section 5.1, 2nd Paragraph + 13.3. Original Text from RFC 3264, Section 8.4, 6th Paragraph + 13.4. New Text Replacing RFC 3264, Section 8.4, 6th Paragraph + 14. Update to RFC 5888 + 14.1. Original Text from RFC 5888, Section 9.2, 3rd Paragraph + 14.2. New Text Replacing RFC 5888, Section 9.2, 3rd Paragraph + 15. RTP/RTCP Extensions for identification-tag Transport + 15.1. RTCP MID SDES Item + 15.2. RTP SDES Header Extension For MID + 16. IANA Considerations + 16.1. New SDES Item + 16.2. New RTP SDES Header Extension URI + 16.3. New SDP Attribute + 16.4. New SDP Group Semantics + 17. Security Considerations + 18. Examples + 18.1. Example: Tagged "m=" Section Selections + 18.2. Example: BUNDLE Group Rejected + 18.3. Example: Offerer Adds a Media Description to a BUNDLE + Group + 18.4. Example: Offerer Moves a Media Description Out of a BUNDLE + Group + 18.5. Example: Offerer Disables a Media Description within a + BUNDLE Group + 19. References + 19.1. Normative References + 19.2. Informative References + Appendix A. Design Considerations + A.1. UA Interoperability + A.2. Usage of Port Number Value Zero + A.3. B2BUA and Proxy Interoperability + A.3.1. Traffic Policing + A.3.2. Bandwidth Allocation + A.4. Candidate Gathering + Acknowledgements + Authors' Addresses + +1. Introduction + +1.1. Background + + When the SDP offer/answer mechanism [RFC3264] is used to negotiate + the establishment of multimedia communication sessions, if separate + transports (5-tuples) are negotiated for each individual media + stream, each transport consumes additional resources (especially when + Interactive Connectivity Establishment (ICE) [RFC8445] is used). For + this reason, it is attractive to use a single transport for multiple + media streams. + +1.2. BUNDLE Mechanism + + This specification defines a way to use a single transport (BUNDLE + transport) for sending and receiving media (bundled media) described + by multiple SDP media descriptions ("m=" sections). The address:port + combination used by an endpoint for sending and receiving bundled + media is referred to as the BUNDLE address:port. The set of SDP + attributes that are applied to each "m=" section within a BUNDLE + group is referred to as BUNDLE attributes. The same BUNDLE transport + is used for sending and receiving bundled media, which means that the + symmetric Real-time Transport Protocol (RTP) mechanism [RFC4961] is + always used for RTP-based bundled media. + + This specification defines a new SDP Grouping Framework [RFC5888] + extension called 'BUNDLE'. The extension can be used with the + Session Description Protocol (SDP) Offer/Answer mechanism [RFC3264] + to negotiate which "m=" sections will become part of a BUNDLE group. + In addition, the offerer and answerer [RFC3264] use the BUNDLE + extension to negotiate the BUNDLE addresses:ports (offerer BUNDLE + address:port and answerer BUNDLE address:port) and the set of BUNDLE + attributes (offerer BUNDLE attributes and answerer BUNDLE attributes) + that will be applied to each "m=" section within the BUNDLE group. + + The use of a BUNDLE transport allows the usage of a single set of ICE + [RFC8445] candidates for the whole BUNDLE group. + + A given BUNDLE address:port MUST only be associated with a single + BUNDLE group. If an SDP offer or SDP answer (hereafter referred to + as "offer" and "answer") contains multiple BUNDLE groups, the + procedures in this specification apply to each group independently. + All RTP-based bundled media associated with a given BUNDLE group + belong to a single RTP session [RFC3550]. + + The BUNDLE extension is backward compatible. Endpoints that do not + support the extension are expected to generate offers and answers + without an SDP 'group:BUNDLE' attribute and assign a unique + address:port to each "m=" section within an offer and answer, + according to the procedures in [RFC3264] and [RFC4566]. + +1.3. Protocol Extensions + + In addition to defining the new SDP Grouping Framework extension, + this specification defines the following protocol extensions and RFC + updates. This specification: + + * defines a new SDP attribute, 'bundle-only', which can be used to + request that a specific "m=" section (and the associated media) be + used only if kept within a BUNDLE group. + + * updates RFC 3264 [RFC3264], to also allow assigning a zero port + value to an "m=" section in cases where the media described by the + "m=" section is not disabled or rejected. + + * defines a new RTCP [RFC3550] SDES item, 'MID', and a new RTP SDES + header extension that can be used to associate RTP streams with + "m=" sections. + + * updates [RFC7941] by adding an exception, for the MID RTP header + extension, to the requirement regarding protection of an SDES RTP + header extension carrying an SDES item for the MID RTP header + extension. + +1.4. Contradiction regarding bundle-only "m=" sections + + Since the approval of the WebRTC specification documents, the IETF + has become aware of an inconsistency between the document specifying + JSEP and the document specifying BUNDLE (RFC 8829 and this RFC, + respectively). Rather than delaying publication further to come to a + resolution, the documents are being published as they were originally + approved. The IETF intends to restart work on these technologies, + and revised versions of these documents will be published as soon as + a resolution becomes available. + + The specific issue involves the handling of "m=" sections that are + designated as bundle-only, as discussed in [RFC8829], Section 4.1.1. + Currently, there is divergence between JSEP and BUNDLE, as well as + between these specifications and existing browser implementations: + + * JSEP prescribes that said "m=" sections should use port zero and + add an "a=bundle-only" attribute in initial offers, but not in + answers or subsequent offers. + + * BUNDLE prescribes that these "m=" sections should be marked as + described in the previous point, but in all offers and answers. + + * Most current browsers do not mark any "m=" sections with port zero + and instead use the same port for all bundled "m=" sections; some + others follow the JSEP behavior. + +2. Terminology + + "m=" section: SDP bodies contain one or more media descriptions, + referred to as "m=" sections. Each "m=" section is represented + by an SDP "m=" line and zero or more SDP attributes associated + with the "m=" line. A local address:port combination is + assigned to each "m=" section. + + 5-tuple: A collection of the following values: source address, + source port, destination address, destination port, and + transport-layer protocol. + + Unique address:port: An address:port combination that is assigned + to only one "m=" section in an offer or answer. + + Offerer BUNDLE-tag: The first identification-tag in a given SDP + 'group:BUNDLE' attribute identification-tag list in an offer. + + Answerer BUNDLE-tag: The first identification-tag in a given SDP + 'group:BUNDLE' attribute identification-tag list in an answer. + + Suggested offerer-tagged "m=" section: The bundled "m=" section + identified by the offerer BUNDLE-tag in an initial BUNDLE + offer, before a BUNDLE group has been negotiated. + + Offerer-tagged "m=" section: The bundled "m=" section identified + by the offerer BUNDLE-tag in a subsequent offer. The "m=" + section contains characteristics (offerer BUNDLE address:port + and offerer BUNDLE attributes) that are applied to each "m=" + section within the BUNDLE group. + + Answerer-tagged "m=" section: The bundled "m=" section identified + by the answerer BUNDLE-tag in an answer (initial BUNDLE answer + or subsequent). The "m=" section contains characteristics + (answerer BUNDLE address:port and answerer BUNDLE attributes) + that are applied to each "m=" section within the BUNDLE group. + + BUNDLE address:port: An address:port combination that an endpoint + uses for sending and receiving bundled media. + + Offerer BUNDLE address:port: The address:port combination used by + the offerer for sending and receiving media. + + Answerer BUNDLE address:port: The address:port combination used + by the answerer for sending and receiving media. + + BUNDLE attributes: IDENTICAL and TRANSPORT multiplexing category + SDP attributes. Once a BUNDLE group has been created, the + attribute values apply to each bundled "m=" section within the + BUNDLE group. + + Offerer BUNDLE attributes: IDENTICAL and TRANSPORT multiplexing + category SDP attributes included in the offerer-tagged "m=" + section. + + Answerer BUNDLE attributes: IDENTICAL and TRANSPORT multiplexing + category SDP attributes included in the answerer-tagged "m=" + section. + + BUNDLE transport: The transport (5-tuple) used by all media + described by the "m=" sections within a BUNDLE group. + + BUNDLE group: A set of bundled "m=" sections, created using an + SDP offer/answer exchange, that uses a single BUNDLE transport, + and a single set of BUNDLE attributes, for sending and + receiving all media (bundled media) described by the set of + "m=" sections. The same BUNDLE transport is used for sending + and receiving bundled media. + + Bundled "m=" section: An "m=" section, whose identification-tag + is placed in an SDP 'group:BUNDLE' attribute identification-tag + list in an offer or answer. + + Bundle-only "m=" section: A bundled "m=" section that contains an + SDP 'bundle-only' attribute. + + Bundled media: All media associated with a given BUNDLE group. + + Initial BUNDLE offer: The first offer, within an SDP session + (e.g., a SIP dialog when SIP [RFC3261] is used to carry SDP), + in which the offerer indicates that it wants to negotiate a + given BUNDLE group. + + Initial BUNDLE answer: The answer to an initial BUNDLE offer in + which the offerer indicates that it wants to negotiate a BUNDLE + group, and the answerer accepts the creation of the BUNDLE + group. The BUNDLE group is created once the answerer sends the + initial BUNDLE answer. + + Subsequent offer: An offer that contains a BUNDLE group that has + been created as part of a previous offer/answer exchange. + + Subsequent answer: An answer to a subsequent offer. + + Identification-tag: A unique token value that is used to identify + an "m=" section. The SDP 'mid' attribute [RFC5888] in an "m=" + section carries the unique identification-tag assigned to that + "m=" section. The session-level SDP 'group' attribute + [RFC5888] carries a list of identification-tags, identifying + the "m=" sections associated with that particular 'group' + attribute. + +3. Conventions + + 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. + +4. Applicability Statement + + The mechanism in this specification only applies to SDP [RFC4566], + when used together with the SDP offer/answer mechanism [RFC3264]. + Declarative usage of SDP is out of scope of this document and is thus + undefined. + +5. SDP Grouping Framework BUNDLE Extension + + This section defines a new SDP Grouping Framework [RFC5888] + extension, 'BUNDLE'. The BUNDLE extension can be used with the SDP + offer/answer mechanism to negotiate a set of "m=" sections that will + become part of a BUNDLE group. Within a BUNDLE group, each "m=" + section uses a BUNDLE transport for sending and receiving bundled + media. Each endpoint uses a single address:port combination for + sending and receiving the bundled media. + + The BUNDLE extension is indicated using an SDP 'group' attribute with + a semantics value [RFC5888] of "BUNDLE". An identification-tag is + assigned to each bundled "m=" section, and each identification-tag is + listed in the SDP 'group:BUNDLE' attribute identification-tag list. + Each "m=" section whose identification-tag is listed in the + identification-tag list is associated with a given BUNDLE group. + + SDP bodies can contain multiple BUNDLE groups. Any given bundled + "m=" section MUST NOT be associated with more than one BUNDLE group + at any given time. + + NOTE: The order of the "m=" sections listed in the SDP 'group:BUNDLE' + attribute identification-tag list does not have to be the same as the + order in which the "m=" sections occur in the SDP. + + The multiplexing category [RFC8859] for the 'group:BUNDLE' attribute + is 'NORMAL'. + + Section 7 defines the detailed SDP offer/answer procedures for the + BUNDLE extension. + +6. SDP 'bundle-only' Attribute + + This section defines a new SDP media-level attribute [RFC4566], + 'bundle-only'. 'bundle-only' is a property attribute [RFC4566] and + hence has no value. + + In order to ensure that an answerer that does not support the BUNDLE + extension always rejects a bundled "m=" section in an offer, the + offerer can assign a zero port value to the "m=" section. According + to [RFC3264], an answerer will reject such an "m=" section. By + including an SDP 'bundle-only' attribute in a bundled "m=" section, + the offerer can request that the answerer accepts the "m=" section + only if the answerer supports the BUNDLE extension and if the + answerer keeps the "m=" section within the associated BUNDLE group. + + Name: bundle-only + + Value: N/A + + Usage Level: media + + Charset Dependent: no + + Example: a=bundle-only + + Once the offerer-tagged "m=" section and the answerer-tagged "m=" + section have been selected, an offerer and answerer will include an + SDP 'bundle-only' attribute in, and assign a zero port value to, + every other bundled "m=" section. + + The usage of the 'bundle-only' attribute is only defined for a + bundled "m=" section with a zero port value. Other usage is + unspecified. + + Section 7 defines the detailed SDP offer/answer procedures for the + 'bundle-only' attribute. + +7. SDP Offer/Answer Procedures + + This section describes the SDP offer/answer [RFC3264] procedures for: + + * Negotiating a BUNDLE group; + + * Suggesting and selecting the tagged "m=" sections (offerer-tagged + "m=" section and answerer-tagged "m=" section); + + * Adding an "m=" section to a BUNDLE group; + + * Moving an "m=" section out of a BUNDLE group; and + + * Disabling an "m=" section within a BUNDLE group. + + The generic rules and procedures defined in [RFC3264] and [RFC5888] + also apply to the BUNDLE extension. For example, if an offer is + rejected by the answerer, the previously negotiated addresses:ports, + SDP parameters, and characteristics (including those associated with + a BUNDLE group) apply. Hence, if an offerer generates an offer in + order to negotiate a BUNDLE group, and the answerer rejects the + offer, the BUNDLE group is not created. + + The procedures in this section are independent of the media type or + "m=" line proto value assigned to a bundled "m=" section. Section 6 + defines additional considerations for the usage of the SDP 'bundle- + only' attribute. Section 9 defines additional considerations for + RTP-based media. Section 10 defines additional considerations for + the usage of the ICE [RFC8445] mechanism. + + Offers and answers can contain multiple BUNDLE groups. The + procedures in this section apply independently to a given BUNDLE + group. + +7.1. Generic SDP Considerations + + This section describes generic restrictions associated with the usage + of SDP parameters within a BUNDLE group. It also describes how to + calculate a value for the whole BUNDLE group, when parameter and + attribute values have been assigned to each bundled "m=" section. + +7.1.1. Connection Data ("c=") + + The "c=" line nettype value [RFC4566] associated with a bundled "m=" + section MUST be 'IN'. + + The "c=" line addrtype value [RFC4566] associated with a bundled "m=" + section MUST be 'IP4' or 'IP6'. The same value MUST be associated + with each "m=" section. + + NOTE: Extensions to this specification can specify usage of the + BUNDLE mechanism for other nettype and addrtype values than the ones + listed above. + +7.1.2. Bandwidth ("b=") + + An offerer and answerer MUST use the rules and restrictions defined + in [RFC8859] for associating the SDP bandwidth ("b=") line with + bundled "m=" sections. + +7.1.3. Attributes ("a=") + + An offerer and answerer MUST include SDP attributes in every bundled + "m=" section where applicable, following the normal offer/answer + procedures for each attribute, with the following exceptions: + + * In the initial BUNDLE offer, the offerer MUST NOT include + IDENTICAL and TRANSPORT multiplexing category SDP attributes + (BUNDLE attributes) in bundle-only "m=" sections. The offerer + MUST include such attributes in all other bundled "m=" sections. + In the initial BUNDLE offer, each bundled "m=" line can contain a + different set of BUNDLE attributes and attribute values. Once the + offerer-tagged "m=" section has been selected, the BUNDLE + attributes contained in the offerer-tagged "m=" section will apply + to each bundled "m=" section within the BUNDLE group. + + * In a subsequent offer, or in an answer (initial of subsequent), + the offerer and answerer MUST include IDENTICAL and TRANSPORT + multiplexing category SDP attributes (BUNDLE attributes) only in + the tagged "m=" section (offerer-tagged "m=" section or answerer- + tagged "m=" section). The offerer and answerer MUST NOT include + such attributes in any other bundled "m=" section. The BUNDLE + attributes contained in the tagged "m=" section will apply to each + bundled "m=" section within the BUNDLE group. + + * In an offer (initial BUNDLE offer or subsequent), or in an answer + (initial BUNDLE answer or subsequent), the offerer and answerer + MUST include SDP attributes from categories other than IDENTICAL + and TRANSPORT in each bundled "m=" section that a given attribute + applies to. Each bundled "m=" line can contain a different set of + such attributes, and attribute values, as such attributes only + apply to the given bundled "m=" section in which they are + included. + + NOTE: A consequence of the rules above is that media-specific + IDENTICAL and TRANSPORT multiplexing category SDP attributes that are + applicable only to some of the bundled "m=" sections within the + BUNDLE group might appear in the tagged "m=" section for which they + are not applicable. For instance, the tagged "m=" section might + contain an SDP 'rtcp-mux' attribute even if the tagged "m=" section + does not describe RTP-based media (but another bundled "m=" section + within the BUNDLE group does describe RTP-based media). + +7.2. Generating the Initial SDP Offer + + The procedures in this section apply to the first offer, within an + SDP session (e.g., a SIP dialog when SIP [RFC3261] is used to carry + SDP), in which the offerer indicates that it wants to negotiate a + given BUNDLE group. This could occur in the initial offer, or in a + subsequent offer, of the SDP session. + + When an offerer generates an initial BUNDLE offer, in order to + negotiate a BUNDLE group, it MUST: + + * Assign a unique address:port to each bundled "m=" section, + following the procedures in [RFC3264], excluding any bundle-only + "m=" sections (see below); + + * Pick a bundled "m=" section as the suggested offerer-tagged "m=" + (Section 7.2.1); + + * Include SDP attributes in the bundled "m=" sections following the + rules in Section 7.1.3; + + * Include an SDP 'group:BUNDLE' attribute in the offer; and + + * Place the identification-tag of each bundled "m=" section in the + SDP 'group:BUNDLE' attribute identification-tag list. The offerer + BUNDLE-tag indicates the suggested offerer-tagged "m=" section. + + NOTE: When the offerer assigns unique addresses:ports to multiple + bundled "m=" sections, the offerer needs to be prepared to receive + bundled media on each unique address:port, until it receives the + associated answer and finds out which bundled "m=" section (and + associated address:port combination) the answerer has selected as the + offerer-tagged "m=" section. + + If the offerer wants to request that the answerer accepts a given + bundled "m=" section only if the answerer keeps the "m=" section + within the negotiated BUNDLE group, the offerer MUST: + + * Include an SDP 'bundle-only' attribute (Section 7.2.1) in the "m=" + section, and + + * Assign a zero port value to the "m=" section. + + NOTE: If the offerer assigns a zero port value to a bundled "m=" + section, but does not include an SDP 'bundle-only' attribute in the + "m=" section, it is an indication that the offerer wants to disable + the "m=" section (Section 7.5.3). + + Sections 7.2.2 and 18.1 show an example of an initial BUNDLE offer. + +7.2.1. Suggesting the Offerer-Tagged 'm=' Section + + In the initial BUNDLE offer, the bundled "m=" section indicated by + the offerer BUNDLE-tag is the suggested offerer-tagged "m=" section. + The address:port combination associated with the "m=" section will be + used by the offerer for sending and receiving bundled media if the + answerer selects the "m=" section as the offerer-tagged "m=" section + (Section 7.3.1). In addition, if the answerer selects the "m=" + section as the offerer-tagged "m=" section, the BUNDLE attributes + included in the "m=" section will be applied to each "m=" section + within the negotiated BUNDLE group. + + The offerer MUST NOT suggest a bundle-only "m=" section as the + offerer-tagged "m=" section. + + It is RECOMMENDED that the suggested offerer-tagged "m=" section be a + bundled "m=" section that the offerer believes is unlikely that the + answerer will reject or move out of the BUNDLE group. How such + assumption is made is outside the scope of this document. + +7.2.2. Example: Initial SDP Offer + + The example shows an initial BUNDLE offer. The offer includes two + "m=" sections in the offer and suggests that both "m=" sections are + included in a BUNDLE group. The audio "m=" section is the suggested + offerer-tagged "m=" section, indicated by placing the identification- + tag associated with the "m=" section (offerer BUNDLE-tag) first in + the SDP group:BUNDLE attribute identification-id list. + + SDP Offer + + v=0 + o=alice 2890844526 2890844526 IN IP6 2001:db8::3 + s= + c=IN IP6 2001:db8::3 + t=0 0 + a=group:BUNDLE foo bar + + m=audio 10000 RTP/AVP 0 8 97 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=rtpmap:8 PCMA/8000 + a=rtpmap:97 iLBC/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 10002 RTP/AVP 31 32 + b=AS:1000 + a=mid:bar + a=rtcp-mux + a=rtpmap:31 H261/90000 + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + +7.3. Generating the SDP Answer + + When an answerer generates an answer (initial BUNDLE answer or + subsequent) that contains a BUNDLE group, the following general SDP + Grouping Framework restrictions, defined in [RFC5888], also apply to + the BUNDLE group: + + * The answerer is only allowed to include a BUNDLE group in an + initial BUNDLE answer if the offerer requested the BUNDLE group to + be created in the corresponding initial BUNDLE offer; + + * The answerer is only allowed to include a BUNDLE group in a + subsequent answer if the corresponding subsequent offer contains a + previously negotiated BUNDLE group; + + * The answerer is only allowed to include a bundled "m=" section in + an answer if the "m=" section was indicated as bundled in the + corresponding offer; and + + * The answerer is only allowed to include a bundled "m=" section in + the same BUNDLE group as the bundled "m=" line in the + corresponding offer. + + In addition, when an answerer generates an answer (initial BUNDLE + answer or subsequent) that contains a BUNDLE group, the answerer + MUST: + + * In case of an initial BUNDLE answer, select the offerer-tagged + "m=" section using the procedures in Section 7.3.1. In case of a + subsequent answer, the offerer-tagged "m=" section is indicated in + the corresponding subsequent offer and MUST NOT be changed by the + answerer; + + * Select the answerer-tagged "m=" section (Section 7.3.1); + + * Assign the answerer BUNDLE address:port to the answerer-tagged + "m=" section; + + * Include an SDP 'bundle-only' attribute in, and assign a zero port + value to, every other bundled "m=" section within the BUNDLE + group; + + * Include SDP attributes in the bundled "m=" sections following the + rules in Section 7.1.3; + + * Include an SDP 'group:BUNDLE' attribute in the answer; and + + * Place the identification-tag of each bundled "m=" section in the + SDP 'group:BUNDLE' attribute identification-tag list. The + answerer BUNDLE-tag indicates the answerer-tagged "m=" section + (Section 7.3.1). + + If the answerer does not want to keep an "m=" section within a BUNDLE + group, it MUST: + + * Move the "m=" section out of the BUNDLE group (Section 7.3.2); or + + * Reject the "m=" section (Section 7.3.3). + + The answerer can modify the answerer BUNDLE address:port, add and + remove SDP attributes, or modify SDP attribute values in a subsequent + answer. Changes to the answerer BUNDLE address:port and the answerer + BUNDLE attributes will be applied to each bundled "m=" section within + the BUNDLE group. + + NOTE: If a bundled "m=" section in an offer contains a zero port + value, but the "m=" section does not contain an SDP 'bundle-only' + attribute, it is an indication that the offerer wants to disable the + "m=" section (Section 7.5.3). + +7.3.1. Answerer Selection of Tagged 'm=' Sections + + When selecting the offerer-tagged "m=" section, the answerer MUST + first check whether the "m=" section fulfills the following criteria + Section 7.2.1: + + * The answerer will not move the "m=" section out of the BUNDLE + group (Section 7.3.2); + + * The answerer will not reject the "m=" section (Section 7.3.3); and + + * The "m=" section does not contain a zero port value. + + If all of the criteria above are fulfilled, the answerer MUST select + the "m=" section as the offerer-tagged "m=" section and MUST also + mark the corresponding "m=" section in the answer as the answerer- + tagged "m=" section. In the answer, the answerer BUNDLE-tag + indicates the answerer-tagged "m=" section. + + If one or more of the criteria are not fulfilled, the answerer MUST + pick the next identification-tag in the identification-tag list in + the offer and perform the same criteria check for the "m=" section + indicated by that identification-tag. If there are no more + identification-tags in the identification-tag list, the answerer MUST + NOT create the BUNDLE group. Unless the answerer rejects the whole + offer, the answerer MUST apply the answerer procedures for moving an + "m=" section out of a BUNDLE group (Section 7.3.2) or rejecting an + "m=" section within a BUNDLE group (Section 7.3.3) to every bundled + "m=" section in the offer when creating the answer. + + Section 18.1 shows an example of an offerer BUNDLE address:port + selection. + + Sections 7.3.4 and 18.1 show an example of an answerer-tagged "m=" + section selection. + +7.3.2. Moving a Media Description Out of a BUNDLE Group + + When an answerer generates the answer, if the answerer wants to move + a bundled "m=" section out of the negotiated BUNDLE group, the + answerer MUST first check the following criteria: + + * In the corresponding offer, the "m=" section is within a + previously negotiated BUNDLE group, and + + * In the corresponding offer, the "m=" section contains an SDP + 'bundle-only' attribute. + + If either criterion above is fulfilled, the answerer cannot move the + "m=" section out of the BUNDLE group in the answer. The answerer can + reject the whole offer, reject each bundled "m=" section within the + BUNDLE group (Section 7.3.3), or keep the "m=" section within the + BUNDLE group in the answer and later create an offer where the "m=" + section is moved out of the BUNDLE group (Section 7.5.2). + + NOTE: One consequence of the rules above is that, once a BUNDLE group + has been negotiated, a bundled "m=" section cannot be moved out of + the BUNDLE group in an answer. Instead, an offer is needed. + + When the answerer generates an answer, in which it moves a bundled + "m=" section out of a BUNDLE group, the answerer: + + * MUST assign a unique address:port to the "m=" section; + + * MUST include any applicable SDP attribute in the "m=" section, + using the normal offer/answer procedures for each attribute; + + * MUST NOT place the identification-tag associated with the "m=" + section in the SDP 'group:BUNDLE' attribute identification-tag + list associated with the BUNDLE group; and + + * MUST NOT include an SDP 'bundle-only' attribute to the "m=" + section. + + Because an answerer is not allowed to move an "m=" section from one + BUNDLE group to another within an answer (Section 7.3), if the + answerer wants to move an "m=" section from one BUNDLE group to + another, it MUST first move the "m=" section out of the current + BUNDLE group and then generate an offer where the "m=" section is + added to another BUNDLE group (Section 7.5.1). + +7.3.3. Rejecting a Media Description in a BUNDLE Group + + When an answerer wants to reject a bundled "m=" section in an answer, + it MUST first check the following criterion: + + * In the corresponding offer, the "m=" section is the offerer-tagged + "m=" section. + + If the criterion above is fulfilled, the answerer cannot reject the + "m=" section in the answer. The answerer can reject the whole offer, + reject each bundled "m=" section within the BUNDLE group, or keep the + "m=" section within the BUNDLE group in the answer and later create + an offer where the "m=" section is disabled within the BUNDLE group + (Section 7.5.3). + + When an answerer generates an answer, in which it rejects a bundled + "m=" section, the answerer: + + * MUST assign a zero port value to the "m=" section, according to + the procedures in [RFC3264]; + + * MUST NOT place the identification-tag associated with the "m=" + section in the SDP 'group:BUNDLE' attribute identification-tag + list associated with the BUNDLE group; and + + * MUST NOT include an SDP 'bundle-only' attribute in the "m=" + section. + +7.3.4. Example: SDP Answer + + The example below shows an answer, based on the corresponding offer + in Section 7.2.2. The answerer accepts both bundled "m=" sections + within the created BUNDLE group. The audio "m=" section is the + answerer-tagged "m=" section, indicated by placing the + identification-tag associated with the "m=" section (answerer BUNDLE- + tag) first in the SDP group;BUNDLE attribute identification-id list. + The answerer includes an SDP 'bundle-only' attribute in, and assigns + a zero port value to, the video "m=" section. + + SDP Answer + + v=0 + o=bob 2808844564 2808844564 IN IP6 2001:db8::1 + s= + c=IN IP6 2001:db8::1 + t=0 0 + a=group:BUNDLE foo bar + + m=audio 20000 RTP/AVP 0 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 32 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + +7.4. Offerer Processing of the SDP Answer + + When an offerer receives an answer, if the answer contains a BUNDLE + group, the offerer MUST check that any bundled "m=" section in the + answer was indicated as bundled in the corresponding offer. If there + is no mismatch, the offerer MUST apply the properties (BUNDLE + address:port, BUNDLE attributes, etc.) of the offerer-tagged "m=" + section (selected by the answerer; see Section 7.3.1) to each bundled + "m=" section within the BUNDLE group. + + NOTE: As the answerer might reject one or more bundled "m=" sections + in an initial BUNDLE offer, or move a bundled "m=" section out of a + BUNDLE group, a given bundled "m=" section in the offer might not be + indicated as bundled in the corresponding answer. + + If the answer does not contain a BUNDLE group, the offerer MUST + process the answer as a normal answer. + +7.5. Modifying the Session + + When a BUNDLE group has been previously negotiated, and an offerer + generates a subsequent offer, the offerer MUST: + + * Pick one bundled "m=" section as the offerer-tagged "m=" section. + The offerer can pick either the "m=" section that was previously + selected by the answerer as the offerer-tagged "m=" section or + another bundled "m=" section within the BUNDLE group; + + * Assign a BUNDLE address:port (previously negotiated or newly + suggested) to the offerer-tagged "m=" section; + + * Include an SDP 'bundle-only' attribute in, and assign a zero port + value to, every other bundled "m=" section within the BUNDLE + group; + + * Include SDP attributes in the bundled "m=" sections following the + rules in Section 7.1.3; + + * Include an SDP 'group:BUNDLE' attribute in the offer; and + + * Place the identification-tag of each bundled "m=" section in the + SDP 'group:BUNDLE' attribute identification-tag list. The offerer + BUNDLE-tag indicates the offerer-tagged "m=" section. + + The offerer MUST NOT pick a given bundled "m=" section as the + offerer-tagged "m=" section if: + + * The offerer wants to move the "m=" section out of the BUNDLE group + (Section 7.5.2), or + + * The offerer wants to disable the "m=" section (Section 7.5.3). + + The offerer can modify the offerer BUNDLE address:port, add and + remove SDP attributes, or modify SDP attribute values in the + subsequent offer. Changes to the offerer BUNDLE address:port and the + offerer BUNDLE attributes will (if the offer is accepted by the + answerer) be applied to each bundled "m=" section within the BUNDLE + group. + +7.5.1. Adding a Media Description to a BUNDLE Group + + When an offerer generates a subsequent offer, in which it wants to + add a bundled "m=" section to a previously negotiated BUNDLE group, + the offerer follows the procedures in Section 7.5. The offerer picks + either the added "m=" section or an "m=" section previously added to + the BUNDLE group as the offerer-tagged "m=" section. + + NOTE: As described in Section 7.3.2, the answerer cannot move the + added "m=" section out of the BUNDLE group in its answer. If the + answer wants to move the "m=" section out of the BUNDLE group, it + will have to first accept it into the BUNDLE group in the answer, and + then send a subsequent offer where the "m=" section is moved out of + the BUNDLE group (Section 7.5.2). + +7.5.2. Moving a Media Description Out of a BUNDLE Group + + When an offerer generates a subsequent offer, in which it wants to + remove a bundled "m=" section from a BUNDLE group, the offerer: + + * MUST assign a unique address:port to the "m=" section; + + * MUST include SDP attributes in the "m=" section following the + normal offer/answer rules for each attribute; + + * MUST NOT place the identification-tag associated with the "m=" + section in the SDP 'group:BUNDLE' attribute identification-tag + list associated with the BUNDLE group; and + + * MUST NOT assign an SDP 'bundle-only' attribute to the "m=" + section. + + For the other bundled "m=" sections within the BUNDLE group, the + offerer follows the procedures in Section 7.5. + + An offerer MUST NOT move an "m=" section from one BUNDLE group to + another within a single offer. If the offerer wants to move an "m=" + section from one BUNDLE group to another, it MUST first move the + BUNDLE group out of the current BUNDLE group, and then generate a + second offer where the "m=" section is added to another BUNDLE group + (Section 7.5.1). + + Section 18.4 shows an example of an offer for moving an "m=" section + out of a BUNDLE group. + +7.5.3. Disabling a Media Description in a BUNDLE Group + + When an offerer generates a subsequent offer, in which it wants to + disable a bundled "m=" section from a BUNDLE group, the offerer: + + * MUST assign a zero port value to the "m=" section, following the + procedures in [RFC4566]; + + * MUST NOT place the identification-tag associated with the "m=" + section in the SDP 'group:BUNDLE' attribute identification-tag + list associated with the BUNDLE group; and + + * MUST NOT assign an SDP 'bundle-only' attribute to the "m=" + section. + + For the other bundled "m=" sections within the BUNDLE group, the + offerer follows the procedures in Section 7.5. + + Section 18.5 shows an example of an offer and answer for disabling an + "m=" section within a BUNDLE group. + +8. Protocol Identification + + Each "m=" section within a BUNDLE group MUST use the same transport- + layer protocol. If bundled "m=" sections use different upper-layer + protocols on top of the transport-layer protocol, there MUST exist a + publicly available specification that describes how a mechanism + associates received data with the correct protocol for this + particular protocol combination. + + In addition, if received data can be associated with more than one + bundled "m=" section, there MUST exist a publicly available + specification that describes a mechanism for associating the received + data with the correct "m=" section. + + This document describes a mechanism to identify the protocol of + received data among the Session Traversal Utilities for NAT (STUN), + DTLS, and the Secure Real-time Transport Protocol (SRTP) (in any + combination) when UDP is used as a transport-layer protocol, but it + does not describe how to identify different protocols transported on + DTLS. While the mechanism is generally applicable to other protocols + and transport-layer protocols, any such use requires further + specification that encompasses how to multiplex multiple protocols on + a given transport-layer protocol and how to associate received data + with the correct protocols. + +8.1. STUN, DTLS, and SRTP + + Section 5.1.2 of [RFC5764] describes a mechanism to identify the + protocol of a received packet among the STUN, DTLS, and SRTP + protocols (in any combination). If an offer or answer includes a + bundled "m=" section that represents these protocols, the offerer or + answerer MUST support the mechanism described in [RFC5764], and no + explicit negotiation is required in order to indicate support and + usage of the mechanism. + + [RFC5764] does not describe how to identify different protocols + transported on DTLS, only how to identify the DTLS protocol itself. + If multiple protocols are transported on DTLS, there MUST exist a + specification describing a mechanism for identifying each individual + protocol. In addition, if a received DTLS packet can be associated + with more than one "m=" section, there MUST exist a specification + that describes a mechanism for associating the received DTLS packets + with the correct "m=" section. + + Section 9.2 describes how to associate the packets in a received SRTP + stream with the correct "m=" section. + +9. RTP Considerations + +9.1. Single RTP Session + + All RTP-based media within a single BUNDLE group belong to a single + RTP session [RFC3550]. + + Since a single BUNDLE transport is used for sending and receiving + bundled media, the symmetric RTP mechanism [RFC4961] MUST be used for + RTP-based bundled media. + + Since a single RTP session is used for each BUNDLE group, all "m=" + sections representing RTP-based media within a BUNDLE group will + share a single Synchronization Source (SSRC) numbering space + [RFC3550]. + + The following rules and restrictions apply for a single RTP session: + + * A specific payload type value can be used in multiple bundled "m=" + sections only if each codec associated with the payload type + number shares an identical codec configuration (Section 9.1.1). + + * The proto value in each bundled RTP-based "m=" section MUST be + identical (e.g., RTP/AVPF). + + * The RTP MID header extension MUST be enabled, by including an SDP + 'extmap' attribute [RFC8285], with a 'urn:ietf:params:rtp- + hdrext:sdes:mid' URI value, in each bundled RTP-based "m=" section + in every offer and answer. + + * A given SSRC MUST NOT transmit RTP packets using payload types + that originate from different bundled "m=" sections. + + NOTE: The last bullet above is to avoid sending multiple media types + from the same SSRC. If transmission of multiple media types are done + with time overlap, RTP and RTCP fail to function. Even if done in + proper sequence, this causes RTP timestamp rate switching issues + [RFC7160]. However, once an SSRC has left the RTP session (by + sending an RTCP BYE packet), that SSRC can be reused by another + source (possibly associated with a different bundled "m=" section) + after a delay of 5 RTCP reporting intervals (the delay is to ensure + the SSRC has timed out, in case the RTCP BYE packet was lost + [RFC3550]). + + [RFC7657] defines Differentiated Services (Diffserv) considerations + for RTP-based bundled media sent using a mixture of Diffserv + Codepoints. + +9.1.1. Payload Type (PT) Value Reuse + + Multiple bundled "m=" sections might describe RTP-based media. As + all RTP-based media associated with a BUNDLE group belong to the same + RTP session, in order for a given payload type value to be used + inside more than one bundled "m=" section, all codecs associated with + the payload type number MUST share an identical codec configuration. + This means that the codecs MUST share the same media type, encoding + name, clock rate, and any parameter that can affect the codec + configuration and packetization. [RFC8859] lists SDP attributes, + whose attribute values are required to be identical for all codecs + that use the same payload type value. + +9.2. Associating RTP/RTCP Streams with the Correct SDP Media + Description + + As described in [RFC3550], RTP packets are associated with RTP + streams [RFC7656]. Each RTP stream is identified by an SSRC value, + and each RTP packet includes an SSRC field that is used to associate + the packet with the correct RTP stream. RTCP packets also use SSRCs + to identify which RTP streams the packet relates to. However, an + RTCP packet can contain multiple SSRC fields, in the course of + providing feedback or reports on different RTP streams, and therefore + can be associated with multiple such streams. + + In order to be able to process received RTP/RTCP packets correctly, + it MUST be possible to associate an RTP stream with the correct "m=" + section, as the "m=" section and SDP attributes associated with the + "m=" section contains information needed to process the packets. + + As all RTP streams associated with a BUNDLE group use the same + transport for sending and receiving RTP/RTCP packets, the local + address:port combination part of the transport cannot be used to + associate an RTP stream with the correct "m=" section. In addition, + multiple RTP streams might be associated with the same "m=" section. + + An offerer and answerer can inform each other which SSRC values they + will use for an RTP stream by using the SDP 'ssrc' attribute + [RFC5576]. However, an offerer will not know which SSRC values the + answerer will use until the offerer has received the answer providing + that information. Due to this, before the offerer has received the + answer, the offerer will not be able to associate an RTP stream with + the correct "m=" section using the SSRC value associated with the RTP + stream. In addition, the offerer and answerer may start using new + SSRC values mid-session, without informing each other about using the + SDP 'ssrc' attribute. + + In order for an offerer and answerer to always be able to associate + an RTP stream with the correct "m=" section, the offerer and answerer + using the BUNDLE extension MUST support the mechanism defined in + Section 15, where the offerer and answerer insert the identification- + tag associated with an "m=" section (provided by the remote peer) + into RTP and RTCP packets associated with a BUNDLE group. + + When using this mechanism, the mapping from an SSRC to an + identification-tag is carried in RTP header extensions or RTCP SDES + packets, as specified in Section 15. Since a compound RTCP packet + can contain multiple RTCP SDES packets, and each RTCP SDES packet can + contain multiple chunks, a single RTCP packet can contain several + mappings of SSRC to identification-tag. The offerer and answerer + maintain tables used for routing that are updated each time an RTP/ + RTCP packet contains new information that affects how packets are to + be routed. + + However, some legacy implementations may not include this + identification-tag in their RTP and RTCP traffic when using the + BUNDLE mechanism and instead use a mechanism based on the payload + type to associate RTP streams with SDP "m=" sections. In this + situation, each "m=" section needs to use unique payload type values, + in order for the payload type to be a reliable indicator of the + relevant "m=" section for the RTP stream. If an implementation fails + to ensure unique payload type values, it will be impossible to + associate the RTP stream using that payload type value to a + particular "m=" section. Note that when using the payload type to + associate RTP streams with "m=" sections, an RTP stream, identified + by its SSRC, will be mapped to an "m=" section when the first packet + of that RTP stream is received, and the mapping will not be changed + even if the payload type used by that RTP stream changes. In other + words, the SSRC cannot "move" to a different "m=" section simply by + changing the payload type. + + Applications can implement RTP stacks in many different ways. The + algorithm below details one way that RTP streams can be associated + with "m=" sections, but it is not meant to be prescriptive about + exactly how an RTP stack needs to be implemented. Applications MAY + use any algorithm that achieves equivalent results to those described + in the algorithm below. + + To prepare to associate RTP streams with the correct "m=" section, + the following steps MUST be followed for each BUNDLE group: + + * Construct a table mapping a MID to an "m=" section for each "m=" + section in this BUNDLE group. Note that an "m=" section may only + have one MID. + + * Construct a table mapping SSRCs of incoming RTP streams to an "m=" + section for each "m=" section in this BUNDLE group and for each + SSRC configured for receiving in that "m=" section. + + * Construct a table mapping the SSRC of each outgoing RTP stream to + an "m=" section for each "m=" section in this BUNDLE group and for + each SSRC configured for sending in that "m=" section. + + * Construct a table mapping a payload type to an "m=" section for + each "m=" section in the BUNDLE group and for each payload type + configured for receiving in that "m=" section. If any payload + type is configured for receiving in more than one "m=" section in + the BUNDLE group, do not include it in the table, as it cannot be + used to uniquely identify an "m=" section. + + * Note that for each of these tables, there can only be one mapping + for any given key (MID, SSRC, or PT). In other words, the tables + are not multimaps. + + As "m=" sections are added or removed from the BUNDLE groups, or + their configurations are changed, the tables above MUST also be + updated. + + When an RTP packet is received, it MUST be delivered to the RTP + stream corresponding to its SSRC. That RTP stream MUST then be + associated with the correct "m=" section within a BUNDLE group, for + additional processing, according to the following steps: + + * If the MID associated with the RTP stream is not in the table + mapping a MID to an "m=" section, then the RTP stream is not + decoded and the payload data is discarded. + + * If the packet has a MID, and the packet's extended sequence number + is greater than that of the last MID update, as discussed in + [RFC7941], Section 4.2.2, update the MID associated with the RTP + stream to match the MID carried in the RTP packet, and then update + the mapping tables to include an entry that maps the SSRC of that + RTP stream to the "m=" section for that MID. + + * If the SSRC of the RTP stream is in the incoming SSRC mapping + table, check that the payload type used by the RTP stream matches + a payload type included on the matching "m=" section. If so, + associate the RTP stream with that "m=" section. Otherwise, the + RTP stream is not decoded and the payload data is discarded. + + * If the payload type used by the RTP stream is in the payload type + table, update the incoming SSRC mapping table to include an entry + that maps the RTP stream's SSRC to the "m=" section for that + payload type. Associate the RTP stream with the corresponding + "m=" section. + + * Otherwise, mark the RTP stream as "not for decoding" and discard + the payload. + + If the RTP packet contains one or more Contributing Source (CSRC) + identifiers, then each CSRC is looked up in the incoming SSRC table, + and a copy of the RTP packet is associated with the corresponding + "m=" section for additional processing. + + For each RTCP packet received (including each RTCP packet that is + part of a compound RTCP packet), the packet is processed as usual by + the RTP layer, then associated with the appropriate "m=" sections, + and processed for the RTP streams represented by those "m=" sections. + This routing is type dependent, as each kind of RTCP packet has its + own mechanism for associating it with the relevant RTP streams. + + RTCP packets that cannot be associated with an appropriate "m=" + section MUST still be processed as usual by the RTP layer, which + updates the metadata associated with the corresponding RTP streams. + This situation can occur with certain multiparty RTP topologies or + when RTCP packets are sent containing a subset of the SDES + information. + + Additional rules for processing various types of RTCP packets are + explained below. + + * If the RTCP packet is of type SDES, for each chunk in the packet + whose SSRC is found in the incoming SSRC table, deliver a copy of + the SDES packet to the "m=" section associated with that SSRC. In + addition, for any SDES MID items contained in these chunks, if the + MID is found in the table mapping a MID to an "m=" section, update + the incoming SSRC table to include an entry that maps the RTP + stream associated with the chunk's SSRC to the "m=" section + associated with that MID, unless the packet is older than the + packet that most recently updated the mapping for this SSRC, as + discussed in [RFC7941], Section 4.2.6. + + * Note that if an SDES packet is received as part of a compound RTCP + packet, the SSRC to "m=" section mapping might not exist until the + SDES packet is handled (e.g., in the case where RTCP for a source + is received before any RTP packets). Therefore, it can be + beneficial for an implementation to delay RTCP packet routing, + such that it either prioritizes processing of the SDES item to + generate or update the mapping or buffers the RTCP information + that needs to be routed until the SDES item(s) has been processed. + If the implementation is unable to follow this recommendation, the + consequence could be that some RTCP information from this + particular RTCP compound packet is not provided to higher layers. + The impact from this is likely minor, when this information + relates to a future incoming RTP stream. + + * If the RTCP packet is of type BYE, it indicates that the RTP + streams referenced in the packet are ending. Therefore, for each + SSRC indicated in the packet that is found in the incoming SSRC + table, first deliver a copy of the BYE packet to the "m=" section + associated with that SSRC, and then remove the entry for that SSRC + from the incoming SSRC table after an appropriate delay to account + for "straggler packets", as specified in [RFC3550], Section 6.2.1. + + * If the RTCP packet is of type Sender Report (SR) or Receiver + Report (RR), for each report block in the report whose "SSRC of + source" is found in the outgoing SSRC table, deliver a copy of the + SR or RR packet to the "m=" section associated with that SSRC. In + addition, if the packet is of type SR, and the sender SSRC for the + packet is found in the incoming SSRC table, deliver a copy of the + SR packet to the "m=" section associated with that SSRC. + + * If the implementation supports the RTCP Extended Report (XR) and + the packet is of type XR, as defined in [RFC3611], for each report + block in the report whose "SSRC of source" is found in the + outgoing SSRC table, deliver a copy of the XR packet to the "m=" + section associated with that SSRC. In addition, if the sender + SSRC for the packet is found in the incoming SSRC table, deliver a + copy of the XR packet to the "m=" section associated with that + SSRC. + + * If the RTCP packet is a feedback message of type RTPFB (transport- + layer FB message) or PSFB (payload-specific FB message), as + defined in [RFC4585], it will contain a media source SSRC, and + this SSRC is used for routing certain subtypes of feedback + messages. However, several subtypes of PSFB and RTPFB messages + include a target SSRC(s) in a section called Feedback Control + Information (FCI). For these messages, the target SSRC(s) is used + for routing. + + * If the RTCP packet is a feedback packet that does not include + target SSRCs in its FCI section, and the media source SSRC is + found in the outgoing SSRC table, deliver the feedback packet to + the "m=" section associated with that SSRC. RTPFB and PSFB types + that are handled in this way include: + + Generic NACK: (PT=RTPFB, FMT=1) [RFC4585]. + + Picture Loss Indication (PLI): (PT=PSFB, FMT=1) [RFC4585]. + + Slice Loss Indication (SLI): (PT=PSFB, FMT=2) [RFC4585]. + + Reference Picture Selection Indication (RPSI): (PT=PSFB, + FMT=3) [RFC4585]. + + * If the RTCP packet is a feedback message that does include a + target SSRC(s) in its FCI section, it can either be a request or a + notification. Requests reference an RTP stream that is being sent + by the message recipient, whereas notifications are responses to + an earlier request and therefore reference an RTP stream that is + being received by the message recipient. + + * If the RTCP packet is a feedback request that includes a target + SSRC(s), for each target SSRC that is found in the outgoing SSRC + table, deliver a copy of the RTCP packet to the "m=" section + associated with that SSRC. PSFB and RTPFB types that are handled + in this way include: + + Full Intra Request (FIR): (PT=PSFB, FMT=4) [RFC5104]. + + Temporal-Spatial Trade-off Request (TSTR): (PT=PSFB, FMT=5) + [RFC5104]. + + H.271 Video Back Channel Message (VBCM): (PT=PSFB, FMT=7) + [RFC5104]. + + Temporary Maximum Media Stream Bit Rate Request (TMMBR): (PT=R + TPFB, FMT=3) [RFC5104]. + + Layer Refresh Request (LRR): (PT=PSFB, FMT=10) [LLR-RTCP]. + + * If the RTCP packet is a feedback notification that includes a + target SSRC(s), for each target SSRC that is found in the incoming + SSRC table, deliver a copy of the RTCP packet to the "m=" section + associated with the RTP stream with a matching SSRC. PSFB and + RTPFB types that are handled in this way include: + + Temporal-Spatial Trade-off Notification (TSTN): (PT=PSFB, + FMT=6) [RFC5104]. This message is a notification in + response to a prior TSTR. + + Temporary Maximum Media Stream Bit Rate Notification + (TMMBN): (PT=RTPFB, FMT=4) [RFC5104]. This message is a + notification in response to a prior TMMBR, but it can also + be sent unsolicited. + + If the RTCP packet is of type APP, then it is handled in an + application-specific manner. If the application does not + recognize the APP packet, then it MUST be discarded. + +9.3. RTP/RTCP Multiplexing + + Within a BUNDLE group, the offerer and answerer MUST enable RTP/RTCP + multiplexing [RFC5761] for the RTP-based bundled media (i.e., the + same transport will be used for both RTP packets and RTCP packets). + In addition, the offerer and answerer MUST support the SDP 'rtcp-mux- + only' attribute [RFC8858]. + +9.3.1. SDP Offer/Answer Procedures + + This section describes how an offerer and answerer use the SDP 'rtcp- + mux' [RFC5761] and SDP 'rtcp-mux-only' attributes [RFC8858] to + negotiate usage of RTP/RTCP multiplexing for RTP-based bundled media. + + RTP/RTCP multiplexing only applies to RTP-based media. However, as + described in Section 7.1.3, within an offer or answer the SDP 'rtcp- + mux' and SDP 'rtcp-mux-only' attributes might be included in a + bundled "m=" section for non-RTP-based media (if such "m=" section is + the offerer-tagged "m=" section or answerer-tagged "m=" section). + +9.3.1.1. Generating the Initial SDP BUNDLE Offer + + When an offerer generates an initial BUNDLE offer, if the offer + contains one or more bundled "m=" sections for RTP-based media (or, + if there is a chance that "m=" sections for RTP-based media will + later be added to the BUNDLE group), the offerer MUST include an SDP + 'rtcp-mux' attribute [RFC5761] in each bundled "m=" section + (excluding any bundle-only "m=" sections). In addition, the offerer + MAY include an SDP 'rtcp-mux-only' attribute [RFC8858] in one or more + bundled "m=" sections for RTP-based media. + + NOTE: Whether the offerer includes the SDP 'rtcp-mux-only' attribute + depends on whether the offerer supports fallback to usage of a + separate port for RTCP in case the answerer moves one or more "m=" + sections for RTP-based media out of the BUNDLE group in the answer. + + NOTE: If the offerer includes an SDP 'rtcp-mux' attribute in the + bundled "m=" sections, but does not include an SDP 'rtcp-mux-only' + attribute, the offerer can also include an SDP 'rtcp' attribute + [RFC3605] in one or more RTP-based bundled "m=" sections in order to + provide a fallback port for RTCP, as described in [RFC5761]. + However, the fallback port will only be applied to "m=" sections for + RTP-based media that are moved out of the BUNDLE group by the + answerer. + + In the initial BUNDLE offer, the address:port combination for RTCP + MUST be unique in each bundled "m=" section for RTP-based media + (excluding a bundle-only "m=" section), similar to RTP. + +9.3.1.2. Generating the SDP Answer + + When an answerer generates an answer, if the answerer supports RTP- + based media, and if a bundled "m=" section in the corresponding offer + contained an SDP 'rtcp-mux' attribute, the answerer MUST enable usage + of RTP/RTCP multiplexing, even if there currently are no bundled "m=" + sections for RTP-based media within the BUNDLE group. The answerer + MUST include an SDP 'rtcp-mux' attribute in the answerer-tagged "m=" + section, following the procedures for BUNDLE attributes + (Section 7.1.3). In addition, if the "m=" section that is selected + as the offerer-tagged "m=" section contained an SDP 'rtcp-mux-only' + attribute, the answerer MUST include an SDP 'rtcp-mux-only' attribute + in the answerer-tagged "m=" section. + + In an initial BUNDLE offer, if the suggested offerer-tagged "m=" + section contained an SDP 'rtcp-mux-only' attribute, the "m=" section + was for RTP-based media; thus, the answerer does not accept the "m=" + section in the created BUNDLE group, and the answerer MUST move the + "m=" section out of the BUNDLE group (Section 7.3.2); include the + attribute in the moved "m=" section and enable RTP/RTCP multiplexing + for the media associated with the "m=" section; or reject the "m=" + section (Section 7.3.3). + + The answerer MUST NOT include an SDP 'rtcp' attribute in any bundled + "m=" section in the answer. The answerer will use the port value of + the offerer-tagged "m=" section sending RTP and RTCP packets + associated with RTP-based bundled media towards the offerer. + + If the usage of RTP/RTCP multiplexing within a BUNDLE group has been + negotiated in a previous offer/answer exchange, the answerer MUST + include an SDP 'rtcp-mux' attribute in the answerer-tagged "m=" + section. It is not possible to disable RTP/RTCP multiplexing within + a BUNDLE group. + +9.3.1.3. Offerer Processing of the SDP Answer + + When an offerer receives an answer, if the answerer has accepted the + usage of RTP/RTCP multiplexing (Section 9.3.1.2), the answerer + follows the procedures for RTP/RTCP multiplexing defined in + [RFC5761]. The offerer will use the port value of the answerer- + tagged "m=" section for sending RTP and RTCP packets associated with + RTP-based bundled media towards the answerer. + + NOTE: It is considered a protocol error if the answerer has not + accepted the usage of RTP/RTCP multiplexing for RTP-based "m=" + sections that the answerer included in the BUNDLE group. + +9.3.1.4. Modifying the Session + + When an offerer generates a subsequent offer, the offerer MUST + include an SDP 'rtcp-mux' attribute in the offerer-tagged "m=" + section, following the procedures for IDENTICAL multiplexing category + attributes in Section 7.1.3. + +10. ICE Considerations + + This section describes how to use the BUNDLE grouping extension + together with the ICE mechanism [RFC8445]. + + The generic procedures for negotiating the usage of ICE using SDP, + defined in [RFC8839], also apply to the usage of ICE with BUNDLE, + with the following exceptions: + + * When the BUNDLE transport has been established, ICE connectivity + checks and keepalives only need to be performed for the BUNDLE + transport, instead of per individual bundled "m=" section within + the BUNDLE group. + + * The generic SDP attribute offer/answer considerations + (Section 7.1.3) also apply to ICE-related attributes. Therefore, + when an offerer sends an initial BUNDLE offer (in order to + negotiate a BUNDLE group), the offerer includes ICE-related media- + level attributes in each bundled "m=" section (excluding any + bundle-only "m=" sections), and each "m=" section MUST contain + unique ICE properties. When an answerer generates an answer + (initial BUNDLE answer or subsequent) that contains a BUNDLE + group, and when an offerer sends a subsequent offer that contains + a BUNDLE group, ICE-related media-level attributes are only + included in the tagged "m=" section (suggested offerer-tagged "m=" + section or answerer-tagged "m=" section), and the ICE properties + are applied to each bundled "m=" section within the BUNDLE group. + + NOTE: Most ICE-related media-level SDP attributes belong to the + TRANSPORT multiplexing category [RFC8859], and the generic SDP + attribute offer/answer considerations for the TRANSPORT multiplexing + category apply to the attributes. However, in the case of ICE- + related attributes, the same considerations also apply to ICE-related + media-level attributes that belong to other multiplexing categories. + + NOTE: The following ICE-related media-level SDP attributes are + defined in [RFC8839]: 'candidate', 'remote-candidates', 'ice- + mismatch', 'ice-ufrag', 'ice-pwd', and 'ice-pacing'. + + Initially, before ICE has produced selected candidate pairs that will + be used for media, there might be multiple transports established (if + multiple candidate pairs are tested). Once ICE has selected + candidate pairs, they form the BUNDLE transport. + + Support and usage of the ICE mechanism together with the BUNDLE + extension is OPTIONAL, and the procedures in this section only apply + when the ICE mechanism is used. Note that applications might mandate + usage of the ICE mechanism even if the BUNDLE extension is not used. + + NOTE: If the Trickle ICE mechanism [RFC8840] is used, an offerer and + answerer might assign a port value of '9' and an IPv4 address of + '0.0.0.0' (or, the IPv6 equivalent '::') to multiple bundled "m=" + sections in the initial BUNDLE offer. The offerer and answerer will + follow the normal procedures for generating the offers and answers, + including picking a bundled "m=" section as the suggested offerer- + tagged "m=" section, selecting the tagged "m=" sections, etc. The + only difference is that media cannot be sent until one or more + candidates have been provided. Once a BUNDLE group has been + negotiated, trickled candidates associated with a bundled "m=" + section will be applied to all bundled "m=" sections within the + BUNDLE group. + +11. DTLS Considerations + + One or more media streams within a BUNDLE group might use the + Datagram Transport Layer Security (DTLS) protocol [RFC6347] in order + to encrypt the data or negotiate encryption keys if another + encryption mechanism is used to encrypt media. + + When DTLS is used within a BUNDLE group, the following rules apply: + + * There can only be one DTLS association [RFC6347] associated with + the BUNDLE group; + + * Each usage of the DTLS association within the BUNDLE group MUST + use the same mechanism for determining which endpoints (the + offerer or answerer) become DTLS client and DTLS server; + + * Each usage of the DTLS association within the BUNDLE group MUST + use the same mechanism for determining whether an offer or answer + will trigger the establishment of a new DTLS association or an + existing DTLS association will be used; and + + * If the DTLS client supports DTLS-SRTP, it MUST include the + 'use_srtp' extension in the DTLS ClientHello message [RFC5764]. + The client MUST include the extension even if the usage of DTLS- + SRTP is not negotiated as part of the multimedia session (e.g., + the SIP session [RFC3261]). + + NOTE: The inclusion of the 'use_srtp' extension during the initial + DTLS handshake ensures that a DTLS renegotiation will not be required + in order to include the extension, in case DTLS-SRTP encrypted media + is added to the BUNDLE group later during the multimedia session. + +12. RTP Header Extensions Consideration + + When RTP header extensions [RFC8285] are used in the context of this + specification, the identifier used for a given extension MUST + identify the same extension across all the bundled media + descriptions. + +13. Update to RFC 3264 + + This section updates RFC 3264, in order to allow extensions to define + the usage of a zero port value in offers and answers for purposes + other than removing or disabling media streams. The following + sections are being updated: + + * "Unicast Streams"; see Section 5.1 of [RFC3264] + + * "Putting a Unicast Media Stream on Hold"; see Section 8.4 of + [RFC3264]. + +13.1. Original Text from RFC 3264, Section 5.1, 2nd Paragraph + + | For recvonly and sendrecv streams, the port number and address in + | the offer indicate where the offerer would like to receive the + | media stream. For sendonly RTP streams, the address and port + | number indirectly indicate where the offerer wants to receive RTCP + | reports. Unless there is an explicit indication otherwise, + | reports are sent to the port number one higher than the number + | indicated. The IP address and port present in the offer indicate + | nothing about the source IP address and source port of RTP and + | RTCP packets that will be sent by the offerer. A port number of + | zero in the offer indicates that the stream is offered but MUST + | NOT be used. This has no useful semantics in an initial offer, + | but is allowed for reasons of completeness, since the answer can + | contain a zero port indicating a rejected stream (Section 6). + | Furthermore, existing streams can be terminated by setting the + | port to zero (Section 8). In general, a port number of zero + | indicates that the media stream is not wanted. + +13.2. New Text Replacing RFC 3264, Section 5.1, 2nd Paragraph + + For recvonly and sendrecv streams, the port number and address in the + offer indicate where the offerer would like to receive the media + stream. For sendonly RTP streams, the address and port number + indirectly indicate where the offerer wants to receive RTCP reports. + Unless there is an explicit indication otherwise, reports are sent to + the port number one higher than the number indicated. The IP address + and port present in the offer indicate nothing about the source IP + address and source port of the RTP and RTCP packets that will be sent + by the offerer. By default, a port number of zero in the offer + indicates that the stream is offered but MUST NOT be used, but an + extension mechanism might specify different semantics for the usage + of a zero port value. Furthermore, existing streams can be + terminated by setting the port to zero (Section 8). In general, a + port number of zero by default indicates that the media stream is not + wanted. + +13.3. Original Text from RFC 3264, Section 8.4, 6th Paragraph + + | RFC 2543 [10] specified that placing a user on hold was + | accomplished by setting the connection address to 0.0.0.0. Its + | usage for putting a call on hold is no longer recommended, since + | it doesn't allow for RTCP to be used with held streams, doesn't + | work with IPv6, and breaks with connection oriented media. + | However, it can be useful in an initial offer when the offerer + | knows it wants to use a particular set of media streams and + | formats, but doesn't know the addresses and ports at the time of + | the offer. Of course, when used, the port number MUST NOT be + | zero, which would specify that the stream has been disabled. An + | agent MUST be capable of receiving SDP with a connection address + | of 0.0.0.0, in which case it means that neither RTP nor RTCP + | should be sent to the peer. + +13.4. New Text Replacing RFC 3264, Section 8.4, 6th Paragraph + + RFC 2543 [RFC2543] specifies that placing a user on hold was + accomplished by setting the connection address to 0.0.0.0. Its usage + for putting a call on hold is no longer recommended, since it doesn't + allow for RTCP to be used with held streams, doesn't work with IPv6, + and breaks with connection oriented media. However, it can be useful + in an initial offer when the offerer knows it wants to use a + particular set of media streams and formats, but doesn't know the + addresses and ports at the time of the offer. Of course, when used, + the port number MUST NOT be zero, if it would specify that the stream + has been disabled. However, an extension mechanism might specify + different semantics of the zero port number usage. An agent MUST be + capable of receiving SDP with a connection address of 0.0.0.0, in + which case it means that neither RTP nor RTCP is to be sent to the + peer. + +14. Update to RFC 5888 + + This section updates RFC 5888 [RFC5888], in order for extensions to + allow an SDP 'group' attribute containing an identification-tag that + identifies an "m=" section with the port set to zero. "Group Value + in Answers" (Section 9.2 of [RFC5888]) is updated. + +14.1. Original Text from RFC 5888, Section 9.2, 3rd Paragraph + + | SIP entities refuse media streams by setting the port to zero in + | the corresponding "m" line. "a=group" lines MUST NOT contain + | identification-tags that correspond to "m" lines with the port set + | to zero. + +14.2. New Text Replacing RFC 5888, Section 9.2, 3rd Paragraph + + SIP entities refuse media streams by setting the port to zero in the + corresponding "m" line. "a=group" lines MUST NOT contain + identification-tags that correspond to "m" lines with the port set to + zero, but an extension mechanism might specify different semantics + for including identification-tags that correspond to such "m=" lines. + +15. RTP/RTCP Extensions for identification-tag Transport + + Offerers and answerers [RFC3264] can associate identification-tags + with "m=" sections within offers and answers, using the procedures in + [RFC5888]. Each identification-tag uniquely represents an "m=" + section. + + This section defines a new RTCP SDES item [RFC3550], 'MID', which is + used to carry identification-tags within RTCP SDES packets. This + section also defines a new RTP SDES header extension [RFC7941], which + is used to carry the 'MID' RTCP SDES item in RTP packets. + + The SDES item and RTP SDES header extension make it possible for a + receiver to associate each RTP stream with a specific "m=" section, + with which the receiver has associated an identification-tag, even if + those "m=" sections are part of the same RTP session. The RTP SDES + header extension also ensures that the media recipient gets the + identification-tag upon receipt of the first decodable media and is + able to associate the media with the correct application. + + A media recipient informs the media sender about the identification- + tag associated with an "m=" section through the use of a 'id' + attribute [RFC5888]. The media sender then inserts the + identification-tag in RTCP and RTP packets sent to the media + recipient. + + NOTE: The text above defines how identification-tags are carried in + offers and answers. The usage of other signaling protocols for + carrying identification-tags is not prevented, but the usage of such + protocols is outside the scope of this document. + + [RFC3550] defines general procedures regarding the RTCP transmission + interval. The RTCP MID SDES item SHOULD be sent in the first few + RTCP packets after joining the session and SHOULD be sent regularly + thereafter. The exact number of RTCP packets in which this SDES item + is sent is intentionally not specified here, as it will depend on the + expected packet-loss rate, the RTCP reporting interval, and the + allowable overhead. + + The RTP SDES header extension for carrying the 'MID' RTCP SDES SHOULD + be included in some RTP packets at the start of the session and + whenever the SSRC changes. It might also be useful to include the + header extension in RTP packets that comprise access points in the + media (e.g., with video I-frames). The exact number of RTP packets + in which this header extension is sent is intentionally not specified + here, as it will depend on expected packet-loss rate and loss + patterns, the overhead the application can tolerate, and the + importance of immediate receipt of the identification-tag. + + For robustness, endpoints need to be prepared for situations where + the reception of the identification-tag is delayed and SHOULD NOT + terminate sessions in such cases, as the identification-tag is likely + to arrive soon. + +15.1. RTCP MID SDES Item + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | MID=15 | length | identification-tag ... + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + The identification-tag payload is UTF-8 encoded [RFC3629], as in SDP. + + The identification-tag is not zero terminated. + +15.2. RTP SDES Header Extension For MID + + The payload, containing the identification-tag, of the RTP SDES + header extension element can be encoded using either the 1-byte or + the 2-byte header [RFC7941]. The identification-tag payload is UTF-8 + encoded, as in SDP. + + The identification-tag is not zero terminated. Note that the set of + header extensions included in the packet needs to be padded to the + next 32-bit boundary using zero bytes [RFC8285]. + + As the identification-tag is included in an RTCP SDES item, an RTP + SDES header extension, or both, there needs to be some consideration + about the packet expansion caused by the identification-tag. To + avoid Maximum Transmission Unit (MTU) issues for the RTP packets, the + header extension's size needs to be taken into account when encoding + the media. + + It is recommended that the identification-tag be kept short. Due to + the properties of the RTP header extension mechanism, when using the + 1-byte header, a tag that is 1-3 bytes will result in a minimal + number of 32-bit words used for the RTP SDES header extension, in + case no other header extensions are included at the same time. Note, + do take into account that some single characters when UTF-8 encoded + will result in multiple octets. The identification-tag MUST NOT + contain any user information, and applications SHALL avoid generating + the identification-tag using a pattern that enables user or + application identification. + +16. IANA Considerations + +16.1. New SDES Item + + This document adds the MID SDES item to the IANA "RTP SDES Item + Types" registry as follows: + + Value: 15 + + Abbrev.: MID + + Name: Media Identification + + Reference: RFC 8843 + +16.2. New RTP SDES Header Extension URI + + This document defines a new extension URI in the RTP SDES Compact + Header Extensions sub-registry of the RTP Compact Header Extensions + registry sub-registry, according to the following data: + + Extension URI: urn:ietf:params:rtp-hdrext:sdes:mid + + Description: Media identification + + Contact: IESG (iesg@ietf.org) + + Reference: RFC 8843 + + The SDES item does not reveal privacy information about the users. + It is simply used to associate RTP-based media with the correct SDP + media description ("m=" section) in the SDP used to negotiate the + media. + + The purpose of the extension is for the offerer to be able to + associate received multiplexed RTP-based media before the offerer + receives the associated answer. + +16.3. New SDP Attribute + + This document defines a new SDP media-level attribute, 'bundle-only', + according to the following data: + + Attribute name: bundle-only + + Type of attribute: media + + Subject to charset: No + + Purpose: Request a media description to be accepted in the answer + only if kept within a BUNDLE group by the answerer. + + Appropriate values: N/A + + Contact name: IESG + + Contact e-mail: iesg@ietf.org + + Reference: RFC 8843 + + Mux category: NORMAL + +16.4. New SDP Group Semantics + + This document registers the following semantics with IANA in the + "Semantics for the 'group' SDP Attribute" subregistry (under the + "Session Description Protocol (SDP) Parameters" registry): + + +================+========+==============+===========+ + | Semantics | Token | Mux Category | Reference | + +================+========+==============+===========+ + | Media bundling | BUNDLE | NORMAL | RFC 8843 | + +----------------+--------+--------------+-----------+ + + Table 1 + +17. Security Considerations + + The security considerations defined in [RFC3264] and [RFC5888] apply + to the BUNDLE extension. BUNDLE does not change which information, + e.g., RTP streams, flows over the network, with the exception of the + usage of the MID SDES item as discussed below. Primarily, it changes + which addresses and ports, and thus in which (RTP) sessions, the + information flows to. This affects the security contexts being used + and can cause previously separated information flows to share the + same security context. This has very little impact on the + performance of the security mechanism of the RTP sessions. In cases + where one would have applied different security policies on the + different RTP streams being bundled, or where the parties having + access to the security contexts would have differed between the RTP + streams, additional analysis of the implications are needed before + selecting to apply BUNDLE. + + The identification-tag, independent of transport, RTCP SDES packet, + or RTP header extension, can expose the value to parties beyond the + signaling chain. Therefore, the identification-tag values MUST be + generated in a fashion that does not leak user information, e.g., + randomly or using a per-bundle group counter, and SHOULD be 3 bytes + or less to allow them to efficiently fit into the MID RTP header + extension. Note that if implementations use different methods for + generating identification-tags, this could enable fingerprinting of + the implementation making it vulnerable to targeted attacks. The + identification-tag is exposed on the RTP stream level when included + in the RTP header extensions; however, what it reveals of the RTP + media stream structure of the endpoint and application was already + possible to deduce from the RTP streams without the MID SDES header + extensions. As the identification-tag is also used to route the + media stream to the right application functionality, it is important + that the value received is the one intended by the sender; thus, + integrity and the authenticity of the source are important to prevent + denial of service on the application. Existing SRTP configurations + and other security mechanisms protecting the whole RTP/RTCP packets + will provide the necessary protection. + + When the BUNDLE extension is used, the set of configurations of the + security mechanism used in all the bundled media descriptions will + need to be compatible so that they can be used simultaneously, at + least per direction or endpoint. When using SRTP, this will be the + case, at least for the IETF-defined key-management solutions due to + their SDP attributes ("a=crypto", "a=fingerprint", "a=mikey") and + their classification in [RFC8859]. + + The security considerations of "RTP Header Extension for the RTP + Control Protocol (RTCP) Source Description Items" [RFC7941] require + that when RTCP is confidentiality protected, any SDES RTP header + extension carrying an SDES item, such as the MID RTP header + extension, is also protected using commensurate strength algorithms. + However, assuming the above requirements and recommendations are + followed, there are no known significant security risks with leaving + the MID RTP header extension without confidentiality protection. + Therefore, this specification updates RFC 7941 by adding the + exception that this requirement MAY be ignored for the MID RTP header + extension. Security mechanisms for RTP/RTCP are discussed in + "Options for Securing RTP Sessions" [RFC7201], for example, SRTP + [RFC3711] can provide the necessary security functions of ensuring + the integrity and source authenticity. + +18. Examples + +18.1. Example: Tagged "m=" Section Selections + + The example below shows: + + * An initial BUNDLE offer, in which the offerer wants to negotiate a + BUNDLE group and indicates the audio "m=" section as the suggested + offerer-tagged "m=" section. + + * An initial BUNDLE answer, in which the answerer accepts the + creation of the BUNDLE group, selects the audio "m=" section in + the offer as the offerer-tagged "m=" section, selects the audio + "m=" section in the answer as the answerer-tagged "m=" section, + and assigns the answerer BUNDLE address:port to that "m=" section. + + SDP Offer (1) + + v=0 + o=alice 2890844526 2890844526 IN IP6 2001:db8::3 + s= + c=IN IP6 2001:db8::3 + t=0 0 + a=group:BUNDLE foo bar + + m=audio 10000 RTP/AVP 0 8 97 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=rtpmap:8 PCMA/8000 + a=rtpmap:97 iLBC/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 10002 RTP/AVP 31 32 + b=AS:1000 + a=mid:bar + a=rtcp-mux + a=rtpmap:31 H261/90000 + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + SDP Answer (2) + + v=0 + o=bob 2808844564 2808844564 IN IP6 2001:db8::1 + s= + c=IN IP6 2001:db8::1 + t=0 0 + a=group:BUNDLE foo bar + + m=audio 20000 RTP/AVP 0 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 32 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + +18.2. Example: BUNDLE Group Rejected + + The example below shows: + + * An initial BUNDLE offer, in which the offerer wants to negotiate a + BUNDLE group and indicates the audio "m=" section as the suggested + offerer-tagged "m=" section. + + * An initial BUNDLE answer, in which the answerer rejects the + creation of the BUNDLE group, generates a normal answer, and + assigns a unique address:port to each "m=" section in the answer. + + SDP Offer (1) + + v=0 + o=alice 2890844526 2890844526 IN IP6 2001:db8::3 + s= + c=IN IP6 2001:db8::3 + t=0 0 + a=group:BUNDLE foo bar + + m=audio 10000 RTP/AVP 0 8 97 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=rtpmap:8 PCMA/8000 + a=rtpmap:97 iLBC/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 10002 RTP/AVP 31 32 + b=AS:1000 + a=mid:bar + a=rtcp-mux + a=rtpmap:31 H261/90000 + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + SDP Answer (2) + + v=0 + o=bob 2808844564 2808844564 IN IP6 2001:db8::1 + s= + c=IN IP6 2001:db8::1 + t=0 0 + + m=audio 20000 RTP/AVP 0 + b=AS:200 + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + + m=video 30000 RTP/AVP 32 + b=AS:1000 + a=rtcp-mux + a=rtpmap:32 MPV/90000 + +18.3. Example: Offerer Adds a Media Description to a BUNDLE Group + + The example below shows: + + * A subsequent offer, in which the offerer adds a new bundled "m=" + section (video), indicated by the "zen" identification-tag, to a + previously negotiated BUNDLE group; indicates the new "m=" section + as the offerer-tagged "m=" section; and assigns the offerer BUNDLE + address:port to that "m=" section. + + * A subsequent answer, in which the answerer indicates the new video + "m=" section in the answer as the answerer-tagged "m=" section and + assigns the answerer BUNDLE address:port to that "m=" section. + + SDP Offer (1) + + v=0 + o=alice 2890844526 2890844526 IN IP6 2001:db8::3 + s= + c=IN IP6 2001:db8::3 + t=0 0 + a=group:BUNDLE zen foo bar + + m=audio 0 RTP/AVP 0 8 97 + b=AS:200 + a=mid:foo + a=bundle-only + a=rtpmap:0 PCMU/8000 + a=rtpmap:8 PCMA/8000 + a=rtpmap:97 iLBC/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 31 32 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:31 H261/90000 + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 10000 RTP/AVP 66 + b=AS:1000 + a=mid:zen + a=rtcp-mux + a=rtpmap:66 H261/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + SDP Answer (2) + + v=0 + o=bob 2808844564 2808844564 IN IP6 2001:db8::1 + s= + c=IN IP6 2001:db8::1 + t=0 0 + a=group:BUNDLE zen foo bar + + m=audio 0 RTP/AVP 0 + b=AS:200 + a=mid:foo + a=bundle-only + a=rtpmap:0 PCMU/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 32 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 20000 RTP/AVP 66 + b=AS:1000 + a=mid:zen + a=rtcp-mux + a=rtpmap:66 H261/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + +18.4. Example: Offerer Moves a Media Description Out of a BUNDLE Group + + The example below shows: + + * A subsequent offer, in which the offerer removes an "m=" section + (video), indicated by the "zen" identification-tag, from a + previously negotiated BUNDLE group; indicates one of the bundled + "m=" sections (audio) remaining in the BUNDLE group as the + offerer-tagged "m=" section; and assigns the offerer BUNDLE + address:port to that "m=" section. + + * A subsequent answer, in which the answerer removes the "m=" + section from the BUNDLE group, indicates the audio "m=" section in + the answer as the answerer-tagged "m=" section, and assigns the + answerer BUNDLE address:port to that "m=" section. + + SDP Offer (1) + + v=0 + o=alice 2890844526 2890844526 IN IP6 2001:db8::3 + s= + c=IN IP6 2001:db8::3 + t=0 0 + a=group:BUNDLE foo bar + + m=audio 10000 RTP/AVP 0 8 97 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=rtpmap:8 PCMA/8000 + a=rtpmap:97 iLBC/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 31 32 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:31 H261/90000 + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 50000 RTP/AVP 66 + b=AS:1000 + a=mid:zen + a=rtcp-mux + a=rtpmap:66 H261/90000 + + SDP Answer (2) + + v=0 + o=bob 2808844564 2808844564 IN IP6 2001:db8::1 + s= + c=IN IP6 2001:db8::1 + t=0 0 + a=group:BUNDLE foo bar + + m=audio 20000 RTP/AVP 0 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 32 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 60000 RTP/AVP 66 + b=AS:1000 + a=mid:zen + a=rtcp-mux + a=rtpmap:66 H261/90000 + +18.5. Example: Offerer Disables a Media Description within a BUNDLE + Group + + The example below shows: + + * A subsequent offer, in which the offerer disables (by assigning a + zero port value) an "m=" section (video), indicated by the "zen" + identification-tag, from a previously negotiated BUNDLE group; + indicates one of the bundled "m=" sections (audio) remaining + active in the BUNDLE group as the offerer-tagged "m=" section; and + assigns the offerer BUNDLE address:port to that "m=" section. + + * A subsequent answer, in which the answerer disables the "m=" + section, indicates the audio "m=" section in the answer as the + answerer-tagged "m=" section, and assigns the answerer BUNDLE + address:port to that "m=" section. + + SDP Offer (1) + + v=0 + o=alice 2890844526 2890844526 IN IP6 2001:db8::3 + s= + t=0 0 + a=group:BUNDLE foo bar + + m=audio 10000 RTP/AVP 0 8 97 + c=IN IP6 2001:db8::3 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=rtpmap:8 PCMA/8000 + a=rtpmap:97 iLBC/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 31 32 + c=IN IP6 2001:db8::3 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:31 H261/90000 + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 66 + a=mid:zen + a=rtpmap:66 H261/90000 + + SDP Answer (2) + + v=0 + o=bob 2808844564 2808844564 IN IP6 2001:db8::1 + s= + t=0 0 + a=group:BUNDLE foo bar + + m=audio 20000 RTP/AVP 0 + c=IN IP6 2001:db8::1 + b=AS:200 + a=mid:foo + a=rtcp-mux + a=rtpmap:0 PCMU/8000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 32 + c=IN IP6 2001:db8::1 + b=AS:1000 + a=mid:bar + a=bundle-only + a=rtpmap:32 MPV/90000 + a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid + + m=video 0 RTP/AVP 66 + a=mid:zen + a=rtpmap:66 H261/90000 + +19. References + +19.1. Normative References + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, + DOI 10.17487/RFC2119, March 1997, + <https://www.rfc-editor.org/info/rfc2119>. + + [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model + with Session Description Protocol (SDP)", RFC 3264, + DOI 10.17487/RFC3264, June 2002, + <https://www.rfc-editor.org/info/rfc3264>. + + [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, <https://www.rfc-editor.org/info/rfc3550>. + + [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute + in Session Description Protocol (SDP)", RFC 3605, + DOI 10.17487/RFC3605, October 2003, + <https://www.rfc-editor.org/info/rfc3605>. + + [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November + 2003, <https://www.rfc-editor.org/info/rfc3629>. + + [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. + Norrman, "The Secure Real-time Transport Protocol (SRTP)", + RFC 3711, DOI 10.17487/RFC3711, March 2004, + <https://www.rfc-editor.org/info/rfc3711>. + + [RFC4566] 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>. + + [RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)", + BCP 131, RFC 4961, DOI 10.17487/RFC4961, July 2007, + <https://www.rfc-editor.org/info/rfc4961>. + + [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and + Control Packets on a Single Port", RFC 5761, + DOI 10.17487/RFC5761, April 2010, + <https://www.rfc-editor.org/info/rfc5761>. + + [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer + Security (DTLS) Extension to Establish Keys for the Secure + Real-time Transport Protocol (SRTP)", RFC 5764, + DOI 10.17487/RFC5764, May 2010, + <https://www.rfc-editor.org/info/rfc5764>. + + [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description + Protocol (SDP) Grouping Framework", RFC 5888, + DOI 10.17487/RFC5888, June 2010, + <https://www.rfc-editor.org/info/rfc5888>. + + [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer + Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, + January 2012, <https://www.rfc-editor.org/info/rfc6347>. + + [RFC7941] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP + Header Extension for the RTP Control Protocol (RTCP) + Source Description Items", RFC 7941, DOI 10.17487/RFC7941, + August 2016, <https://www.rfc-editor.org/info/rfc7941>. + + [RFC8174] 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>. + + [RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General + Mechanism for RTP Header Extensions", RFC 8285, + DOI 10.17487/RFC8285, October 2017, + <https://www.rfc-editor.org/info/rfc8285>. + + [RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive + Connectivity Establishment (ICE): A Protocol for Network + Address Translator (NAT) Traversal", RFC 8445, + DOI 10.17487/RFC8445, July 2018, + <https://www.rfc-editor.org/info/rfc8445>. + + [RFC8839] Petit-Huguenin, M., Nandakumar, S., Holmberg, C., Keränen, + A., and R. Shpount, "Session Description Protocol (SDP) + Offer/Answer Procedures for Interactive Connectivity + Establishment (ICE)", RFC 8839, DOI 10.17487/RFC8839, + January 2021, <https://www.rfc-editor.org/info/rfc8839>. + + [RFC8840] Ivov, E., Stach, T., Marocco, E., and C. Holmberg, "A + Session Initiation Protocol (SIP) Usage for Incremental + Provisioning of Candidates for the Interactive + Connectivity Establishment (Trickle ICE)", RFC 8840, + DOI 10.17487/RFC8840, January 2021, + <https://www.rfc-editor.org/info/rfc8840>. + + [RFC8858] Holmberg, C., "Indicating Exclusive Support of RTP and RTP + Control Protocol (RTCP) Multiplexing Using the Session + Description Protocol (SDP)", RFC 8858, + DOI 10.17487/RFC8858, January 2021, + <https://www.rfc-editor.org/info/rfc8858>. + + [RFC8859] Nandakumar, S., "A Framework for Session Description + Protocol (SDP) Attributes When Multiplexing", RFC 8859, + DOI 10.17487/RFC8859, January 2021, + <https://www.rfc-editor.org/info/rfc8859>. + +19.2. Informative References + + [LLR-RTCP] Lennox, J., Hong, D., Uberti, J., Holmer, S., and M. + Flodman, "The Layer Refresh Request (LRR) RTCP Feedback + Message", Work in Progress, Internet-Draft, draft-ietf- + avtext-lrr-07, 2 July 2017, + <https://tools.ietf.org/html/draft-ietf-avtext-lrr-07>. + + [RFC2543] Handley, M., Schulzrinne, H., Schooler, E., and J. + Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, + DOI 10.17487/RFC2543, March 1999, + <https://www.rfc-editor.org/info/rfc2543>. + + [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, + A., Peterson, J., Sparks, R., Handley, M., and E. + Schooler, "SIP: Session Initiation Protocol", RFC 3261, + DOI 10.17487/RFC3261, June 2002, + <https://www.rfc-editor.org/info/rfc3261>. + + [RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., + "RTP Control Protocol Extended Reports (RTCP XR)", + RFC 3611, DOI 10.17487/RFC3611, November 2003, + <https://www.rfc-editor.org/info/rfc3611>. + + [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, + "Extended RTP Profile for Real-time Transport Control + Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, + DOI 10.17487/RFC4585, July 2006, + <https://www.rfc-editor.org/info/rfc4585>. + + [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, + "Codec Control Messages in the RTP Audio-Visual Profile + with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104, + February 2008, <https://www.rfc-editor.org/info/rfc5104>. + + [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific + Media Attributes in the Session Description Protocol + (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009, + <https://www.rfc-editor.org/info/rfc5576>. + + [RFC7160] Petit-Huguenin, M. and G. Zorn, Ed., "Support for Multiple + Clock Rates in an RTP Session", RFC 7160, + DOI 10.17487/RFC7160, April 2014, + <https://www.rfc-editor.org/info/rfc7160>. + + [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP + Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014, + <https://www.rfc-editor.org/info/rfc7201>. + + [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, + <https://www.rfc-editor.org/info/rfc7656>. + + [RFC7657] Black, D., Ed. and P. Jones, "Differentiated Services + (Diffserv) and Real-Time Communication", RFC 7657, + DOI 10.17487/RFC7657, November 2015, + <https://www.rfc-editor.org/info/rfc7657>. + + [RFC8829] Uberti, J., Jennings, C., and E. Rescorla, Ed., + "JavaScript Session Establishment Protocol (JSEP)", + RFC 8829, DOI 10.17487/RFC8829, January 2021, + <https://www.rfc-editor.org/info/rfc8829>. + + [RFC8838] Ivov, E., Uberti, J., and P. Saint-Andre, "Trickle ICE: + Incremental Provisioning of Candidates for the Interactive + Connectivity Establishment (ICE) Protocol", RFC 8838, + DOI 10.17487/RFC8838, January 2021, + <https://www.rfc-editor.org/info/rfc8838>. + +Appendix A. Design Considerations + + One of the main issues regarding the BUNDLE grouping extensions has + been whether, in offers and answers, the same port value can be + inserted in "m=" lines associated with a BUNDLE group, as the purpose + of the extension is to negotiate the usage of a single transport for + media specified by the "m=" sections. Issues with both approaches, + discussed in the Appendix, have been raised. The outcome was to + specify a mechanism that uses offers with both different and + identical port values. + + Below are the primary issues that have been considered when defining + the "BUNDLE" grouping extension: + + 1) Interoperability with existing User Agents (UAs). + + 2) Interoperability with intermediary Back-to-Back User Agent + (B2BUA) and proxy entities. + + 3) Time to gather, and the number of, ICE candidates. + + 4) Different error scenarios and when they occur. + + 5) SDP offer/answer impacts, including usage of port number value + zero. + +A.1. UA Interoperability + + Consider the following SDP offer/answer exchange, where Alice sends + an offer to Bob: + + SDP Offer + + v=0 + o=alice 2890844526 2890844526 IN IP4 atlanta.example.com + s= + c=IN IP4 atlanta.example.com + t=0 0 + + m=audio 10000 RTP/AVP 97 + a=rtpmap:97 iLBC/8000 + m=video 10002 RTP/AVP 97 + a=rtpmap:97 H261/90000 + + SDP Answer + + v=0 + o=bob 2808844564 2808844564 IN IP4 biloxi.example.com + s= + c=IN IP4 biloxi.example.com + t=0 0 + + m=audio 20000 RTP/AVP 97 + a=rtpmap:97 iLBC/8000 + m=video 20002 RTP/AVP 97 + a=rtpmap:97 H261/90000 + + RFC 4961 specifies a way of doing symmetric RTP, but that is a later + extension to RTP, and Bob cannot assume that Alice supports RFC 4961. + This means that Alice may be sending RTP from a different port than + 10000 or 10002 -- some implementations simply send the RTP from an + ephemeral port. When Bob's endpoint receives an RTP packet, the only + way that Bob knows if the packet is to be passed to the video or + audio codec is by looking at the port it was received on. This + prompted some SDP implementations to use a port number as an index to + find the correct "m=" line in the SDP, since each "m"= section + contains a different port number. As a result, some implementations + that do support symmetric RTP and ICE still use an SDP data structure + where SDP with "m=" sections with the same port such as: + + SDP Offer + + v=0 + o=alice 2890844526 2890844526 IN IP4 atlanta.example.com + s= + c=IN IP4 atlanta.example.com + t=0 0 + + m=audio 10000 RTP/AVP 97 + a=rtpmap:97 iLBC/8000 + m=video 10000 RTP/AVP 98 + a=rtpmap:98 H261/90000 + + will result in the second "m=" section being considered an SDP error + because it has the same port as the first line. + +A.2. Usage of Port Number Value Zero + + In an offer or answer, the media specified by an "m=" section can be + disabled/rejected by setting the port number value to zero. This is + different from, e.g., using the SDP direction attributes, where RTCP + traffic will continue even if the SDP 'inactive' attribute is + indicated for the associated "m=" section. + + If each "m=" section associated with a BUNDLE group would contain + different port values, and one of those port values would be used for + a BUNDLE address:port associated with the BUNDLE group, problems + would occur if an endpoint wants to disable/reject the "m=" section + associated with that port, by setting the port value to zero. After + that, no "m=" section would contain the port value that is used for + the BUNDLE address:port. In addition, it is unclear what would + happen to the ICE candidates associated with the "m=" section, as + they are also used for the BUNDLE address:port. + +A.3. B2BUA and Proxy Interoperability + + Some back-to-back user agents may be configured in a mode where if + the incoming call leg contains an SDP attribute the B2BUA does not + understand, the B2BUA still generates that SDP attribute in the Offer + for the outgoing call leg. Consider a B2BUA that did not understand + the SDP 'rtcp' attribute, defined in RFC 3605, yet acted this way. + Further assume that the B2BUA was configured to tear down any call + where it did not see any RTCP for 5 minutes. In this case, if the + B2BUA received an Offer like: + + SDP Offer + + v=0 + o=alice 2890844526 2890844526 IN IP4 atlanta.example.com + s= + c=IN IP4 atlanta.example.com + t=0 0 + + m=audio 49170 RTP/AVP 0 + a=rtcp:53020 + + It would be looking for RTCP on port 49171 but would not see any + because the RTCP would be on port 53020, and after five minutes, it + would tear down the call. Similarly, a B2BUA that did not understand + BUNDLE yet put it in its offer may be looking for media on the wrong + port and tear down the call. It is worth noting that a B2BUA that + generated an Offer with capabilities it does not understand is not + compliant with the specifications. + +A.3.1. Traffic Policing + + Sometimes intermediaries do not act as B2BUAs, in the sense that they + don't modify SDP bodies nor do they terminate SIP dialogs. However, + they may still use SDP information (e.g., IP address and port) in + order to control traffic gating functions and to set traffic policing + rules. There might be rules that will trigger a session to be + terminated in case media is not sent or received on the ports + retrieved from the SDP. This typically occurs once the session is + already established and ongoing. + +A.3.2. Bandwidth Allocation + + Sometimes, intermediaries do not act as B2BUAs, in the sense that + they don't modify SDP bodies nor do they terminate SIP dialogs. + However, they may still use SDP information (e.g., codecs and media + types) in order to control bandwidth allocation functions. The + bandwidth allocation is done per "m=" section, which means that it + might not be enough if media specified by all "m=" sections try to + use that bandwidth. That may simply lead to either bad user + experience or termination of the call. + +A.4. Candidate Gathering + + When using ICE, a candidate needs to be gathered for each port. This + takes approximately 20 ms extra for each extra "m=" section due to + the NAT pacing requirements. All of this gathering can be overlapped + with other things while, e.g., a web page is loading to minimize the + impact. If the client only wants to generate Traversal Using Relays + around NAT (TURN) or STUN ICE candidates for one of the "m=" lines + and then use Trickle ICE [RFC8838] to get the non-host ICE candidates + for the rest of the "m=" sections, it MAY do that and will not need + any additional gathering time. + + Some people have suggested a TURN extension to get a bunch of TURN + allocations at once. This would only provide a single STUN result, + so in cases where the other end did not support BUNDLE, it may cause + more use of the TURN server, but it would be quick in the cases where + both sides supported BUNDLE and would fall back to a successful call + in the other cases. + +Acknowledgements + + The usage of the SDP grouping extension for negotiating bundled media + is based on similar alternatives proposed by Harald Alvestrand and + Cullen Jennings. The BUNDLE extension described in this document is + based on the different alternative proposals, and text (e.g., SDP + examples) has been borrowed (and, in some cases, modified) from those + alternative proposals. + + The SDP examples are also modified versions from the ones in the + Alvestrand proposal. + + Thanks to Paul Kyzivat, Martin Thomson, Flemming Andreasen, Thomas + Stach, Ari Keränen, Adam Roach, Christian Groves, Roman Shpount, + Suhas Nandakumar, Nils Ohlmeier, Jens Guballa, Raju Makaraju, Justin + Uberti, Taylor Brandstetter, Byron Campen, and Eric Rescorla for + reading the text and providing useful feedback. + + Thanks to Bernard Aboba, Peter Thatcher, Justin Uberti, and Magnus + Westerlund for providing the text for the section on RTP/RTCP stream + association. + + Thanks to Magnus Westerlund, Colin Perkins, and Jonathan Lennox for + providing help and text on the RTP/RTCP procedures. + + Thanks to Charlie Kaufman for performing the Sec-Dir review. + + Thanks to Linda Dunbar for performing the Gen-ART review. + + Thanks to Spotify for providing music for the countless hours of + document editing. + +Authors' Addresses + + Christer Holmberg + Ericsson + Hirsalantie 11 + FI-02420 Jorvas + Finland + + Email: christer.holmberg@ericsson.com + + + Harald Tveit Alvestrand + Google + Kungsbron 2 + SE-11122 Stockholm + Sweden + + Email: harald@alvestrand.no + + + Cullen Jennings + Cisco + 400 3rd Avenue SW, Suite 350 + Calgary AB T2P 4H2 + Canada + + Email: fluffy@iii.ca |