| MMUSIC Working Group | C. Holmberg |
| Internet-Draft | Ericsson |
| Updates: 3264 (if approved) | H. Alvestrand |
| Intended status: Standards Track | |
| Expires: May 24, 2018 | C. Jennings |
| Cisco | |
| November 20, 2017 |
Negotiating Media Multiplexing Using the Session Description Protocol (SDP)
draft-ietf-mmusic-sdp-bundle-negotiation-40.txt
This specification defines a new Session Description Protocol (SDP) Grouping Framework extension, '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.
To assist endpoints in negotiating the use of bundle this specification defines a new SDP attribute, 'bundle-only', which can be used to request that specific media is only used if bundled. The specification also updates RFC 3264, to allow assigning a zero port value to a "m= section without meaning that the media described by the "m=" section is disabled or rejected.
When RTP-based media is used, there are multiple ways to correlate bundled RTP packets with the appropriate "m=" section. This specification defines a new Real-time Transport Protocol (RTP) source description (SDES) item and a new RTP header extension that provides an additional way to do this correlation by using them to carry a value that associates the RTP/RTCP packets with a specific "m=" section.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 24, 2018.
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When multimedia communications are established, each transport (5-tuple) reserved for an individual media stream consume additional resources (especially when Interactive Connectivity Establishment (ICE) [I-D.ietf-ice-rfc5245bis] is used). For this reason, it is attractive to use a single transport for multiple media streams.
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 same BUNDLE transport is used for sending and receiving bundled media, which means that the symmetric 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. Within a BUNDLE group, each "m=" section will use a BUNDLE transport for sending and receiving bundled media.
Within a BUNDLE group, each endpoint uses a single address:port combination for sending and receiving bundled media. The address:port combination is referred to as BUNDLE address. In addition to negotiating the BUNDLE group, the offerer and answerer [RFC3264] use the BUNDLE extension to negotiate the BUNDLE addresses, one for the offerer (offerer BUNDLE address) and one for the answerer (answerer BUNDLE address). Once the offerer and the answerer have negotiated the BUNDLE addresses, and a BUNDLE group has been formed, they assign their respective BUNDLE address to each "m=" section within the BUNDLE group. The endpoints then use the BUNDLE addresses for sending and receiving the bundled media associated with the BUNDLE group.
The use of a BUNDLE transport also allows the usage of a single set of Interactive Connectivity Establishment (ICE) [I-D.ietf-ice-rfc5245bis] candidates for the whole BUNDLE group.
This specification also defines a new SDP attribute, 'bundle-only', which can be used to request that specific media is only used if the "m=" section describing the media is kept within a BUNDLE group. The specification also updates RFC 3264, to allow usage of zero port values without meaning that media is rejected.
As defined in RFC 4566 [RFC4566], the semantics of assigning the same transport address (IP address and port) to multiple "m=" sections are undefined, and there is no grouping defined by such means. Instead, an explicit grouping mechanism needs to be used to express the intended semantics. This specification provides such an extension.
This specification also updates sections 5.1, 8.1 and 8.2 of RFC 3264 [RFC3264]. The update allows an answerer to assign a non-zero port value to an "m=" section in an SDP answer, even if the "m=" section in the associated SDP offer contained a zero port value.
This specification also defines a new Real-time Transport Protocol (RTP) [RFC3550] source description (SDES) item, 'MID', and a new RTP SDES header extension that can be used to associate RTP streams with "m=" sections.
SDP bodies can contain multiple BUNDLE groups. A given BUNDLE address MUST only be associated with a single BUNDLE group. The procedures in this specification apply independently to a given BUNDLE group. All RTP based media flows described by a single 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 are expected to assign a unique address to each "m=" section within an offer and answer, according to the procedures in [RFC4566] and [RFC3264]
"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. An 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: An address:port combination that is assigned to only one "m=" section in an offer or answer.
Shared address: An address:port combination that is assigned to multiple "m=" sections within 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.
Offerer BUNDLE address: Within a given BUNDLE group, an address:port combination used by an offerer to receive all media described by each "m=" section within the BUNDLE group.
Answerer BUNDLE address: Within a given BUNDLE group, an address:port combination used by an answerer to receive all media described by each "m=" section within the BUNDLE group.
BUNDLE transport: The transport (5-tuple) used by all media described by the "m=" sections within a BUNDLE group.
BUNDLE group: A set of "m=" sections, created using an SDP Offer/Answer exchange, which uses a single BUNDLE transport for sending and receiving all media described by the set of "m=" sections. The same BUNDLE transport is used for sending and receiving 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 specified by a given BUNDLE group.
Initial offer: The first offer, within an SDP session (e.g. a SIP dialog when the Session Initiation Protocol (SIP) [RFC3261] is used to carry SDP), in which the offerer indicates that it wants to create a given BUNDLE group.
Subsequent offer: An offer which contains a BUNDLE group that has been created as part of a previous offer/answer exchange.
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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [RFC2119].
The mechanism in this specification only applies to the Session Description Protocol (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.
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 will use a BUNDLE transport for sending and receiving bundled media. Each endpoint use a single address:port combination for sending receiving the bundled media.
The BUNDLE extension is indicated using an SDP 'group' attribute with a "BUNDLE" semantics value [RFC5888]. 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.
Section 8 defines the detailed SDP Offer/Answer procedures for the BUNDLE extension.
This section defines a new SDP media-level attribute [RFC4566], 'bundle-only'. 'bundle-only' is a property attribute [RFC4566], and hence has no value.
Name: bundle-only
Value: N/A
Usage Level: media
Charset Dependent: no
Example:
a=bundle-only
In order to ensure that an answerer that does not support the BUNDLE extension always rejects a bundled "m=" section, the offerer can assign a zero port value to the "m=" section. According to [RFC3264] an answerer will reject such "m=" section. By including an SDP 'bundle-only' attribute in such "m=" section, the offerer can request that the answerer accepts the "m=" section if the answerer supports the Bundle extension, and if the answerer keeps the "m=" section within the associated BUNDLE group.
NOTE: Once the offerer BUNDLE address has been selected, the offerer does not need to include the 'bundle-only' attribute in subsequent offers. By assigning the offerer BUNDLE address to an "m=" section of a subsequent offer, the offerer will ensure that the answerer will either keep the "m=" section within the BUNDLE group, or the answerer will have to reject the "m=" section.
The usage of the 'bundle-only' attribute is only defined for a bundled "m=" section with a zero port value, within an offer. Other usage is unspecified.
Section 8 defines the detailed SDP Offer/Answer procedures for the 'bundle-only' attribute.
This section describes restrictions associated with the usage of SDP parameters within a BUNDLE group. It also describes, when parameter and attribute values have been assigned to each bundled "m=" section, how to calculate a value for the whole BUNDLE group.
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.
An offerer and answerer MUST use the rules and restrictions defined in [I-D.ietf-mmusic-sdp-mux-attributes] for associating the SDP bandwidth (b=) line with bundled "m=" section.
This section describes the SDP Offer/Answer [RFC3264] procedures for:
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 SDP parameters and characteristics (including those associated with a BUNDLE group) apply. Hence, if an offerer generates an offer in which the offerer wants to create 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 10 defines additional considerations for RTP based media. Section 6 defines additional considerations for the usage of the SDP 'bundle-only' attribute. Section 11 defines additional considerations for the usage of Interactive Connectivity Establishment (ICE) [I-D.ietf-ice-rfc5245bis] mechanism.
SDP offers and answers can contain multiple BUNDLE groups. The procedures in this section apply independently to a given BUNDLE group.
When an offerer or answerer includes SDP attributes in a bundled "m=" section (including any bundle-only "m=" section) to which a shared address has been assigned, IDENTICAL and TRANSPORT mux category SDP attributes [I-D.ietf-mmusic-sdp-mux-attributes] are included in the "m=" section only if the "m=" section is also associated with the offerer/answerer BUNDLE-tag. Otherwise the offerer/answerer MUST NOT include such SDP attributes in the "m=" section. The rule above does not apply to a bundled "m=" section to which a unique address has been assigned.
NOTE: As bundled "m=" section (including any bundle-only "m=" section) to which a shared address has been assigned will share the same IDENTICAL and TRANSPORT mux category SDP attributes, and attribute values, there is no need to include such SDP attributes in each "m=" section. The attributes and attribute values are implicitly included and applied to each "m=" section.
The semantics of some SDP attributes only apply to specific types of media. For example, the semantics of the SDP 'rtcp-mux' and SDP 'rtcp-mux-only' attributes only apply to "m=" sections describing RTP-based media. However, as described in Section 8.1, there are cases where IDENTICAL and TRANSPORT mux category SDP attributes are only included in the "m=" sections associated with the BUNDLE-tag. That means that media-specific IDENTICAL and TRANSPORT mux category attributes can be included in an "m=" section associated with another type of media.
When an offerer generates an initial offer, in order to create a BUNDLE group, it MUST:
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 BUNDLE group, the offerer MUST:
NOTE: If the offerer assigns a zero port value to an "m=" section, but does not also include an SDP 'bundle-only' attribute in the "m=" section, it is an indication that the offerer wants to disable the "m=" section [Section 8.5.4].
[Section 18.1] shows an example of an initial offer.
In the offer, the address:port combination assigned to the "m=" section associated with the offerer BUNDLE-tag indicates the address:port combination that the offerer suggests as the offerer BUNDLE address.
The offerer MUST NOT assign a zero port value, or an SDP 'bundle-only' attribute, to the "m=" section associated with the offerer BUNDLE-tag.
The example shows an initial SDP offer. The offer includes two "m=" section in the SDP, and suggests that both are included in a BUNDLE group. The audio "m=" section is associated with the offerer BUNDLE-tag (placed first in the SDP group:BUNDLE attribute identification-id list).
SDP Offer v=0 o=alice 2890844526 2890844526 IN IP4 atlanta.example.com s= c=IN IP4 atlanta.example.com 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=rtpmap:31 H261/90000 a=rtpmap:32 MPV/90000 a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
When an answerer generates an answer that contains a BUNDLE group, the following general SDP grouping framework restrictions, defined in [RFC5888], also apply to the BUNDLE group:
If the answer contains a BUNDLE group, the answerer MUST:
The answerer is allowed to select a new Answerer BUNDLE address each time it generates an answer to an offer.
If the answerer does not want to keep an "m=" section within a BUNDLE group, it MUST:
If the answerer keeps a bundle-only "m=" section within the BUNDLE group, it follows the procedures (assigns the answerer BUNDLE address to the "m=" section etc) for any other "m=" section kept within the BUNDLE group.
If the answerer does not want to keep a bundle-only "m=" section within the BUNDLE group, it MUST reject the "m=" section [Section 8.3.4].
The answerer MUST NOT include an SDP 'bundle-only' attribute in any "m=" section in an answer.
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 8.5.4].
In an offer, the address (unique or shared) assigned to the bundled "m=" section associated with the offerer BUNDLE-tag indicates the address that the offerer suggests as the offerer BUNDLE address [Section 8.2.1]. The answerer MUST check whether that "m=" section fulfils the following criteria:
If all of the criteria above are fulfilled, the answerer MUST select the address assigned to the "m=" section as the offerer BUNDLE address. In the answer, the answerer BUNDLE-tag represents the "m=" section, and the address assigned to the "m=" section in the offer becomes the offerer BUNDLE address.
If one or more of the criteria are not fulfilled, the answerer MUST select the next identification-tag in the identification-tag list, and perform the same criteria check for the "m=" section associated with that identification-tag. If there are no more identification-tags in the identification-tag list, the answerer MUST NOT create the BUNDLE group. In addition, 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 8.3.3] to each bundled "m=" section in the offer when creating the answer.
[Section 18.1] shows an example of an offerer BUNDLE address selection.
When the answerer selects a BUNDLE address for itself, referred to as the answerer BUNDLE address, it MUST assign that address to each bundled "m=" section within the created BUNDLE group in the answer.
The answerer MUST NOT assign the answerer BUNDLE address to an "m=" section that is not within the BUNDLE group, or to an "m=" section that is within another BUNDLE group.
[Section 18.1] shows an example of an answerer BUNDLE address selection.
When an answerer wants to move an "m=" section out of a BUNDLE group, it MUST first check the following criteria:
An answerer MUST NOT move an "m=" section from one BUNDLE group to another within an answer. If the answerer 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 an offer where the "m=" section is added to another BUNDLE group [Section 8.5.2].
If either criteria above is fulfilled, the answerer MUST reject the "m=" section [Section 8.3.4].
Otherwise, if a unique address has been assigned to the "m=" section in the corresponding offer, the answerer MUST assign a unique address to the "m=" section in the answer (the answerer does not reject the "m=" section).
In addition, in either case above, the answerer MUST NOT place the identification-tag, associated with the moved "m=" section, in the SDP 'group' attribute identification-tag list associated with the BUNDLE group.
When an answerer rejects an "m=" section, it MUST assign a zero port value to "m=" section in the answer, according to the procedures in [RFC3264].
In addition, the answerer MUST NOT place the identification-tag associated with the rejected "m=" section in the SDP 'group' attribute identification-tag list associated with the BUNDLE group.
The example shows an SDP answer, based on the SDP offer in [Section 8.2.2]. The answers accepts both "m=" sections within the BUNDLE group.
SDP Answer v=0 o=bob 2808844564 2808844564 IN IP4 biloxi.example.com s= c=IN IP4 biloxi.example.com 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 20000 RTP/AVP 32 b=AS:1000 a=mid:bar a=rtpmap:32 MPV/90000 a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
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 use the offerer BUNDLE address, selected by the answerer [Section 8.3.1], as the address for each bundled "m=" section.
NOTE: As the answerer might reject one or more bundled "m=" sections, or move a bundled "m=" section out of a BUNDLE group, each bundled "m=" section in the offer might not be indicated as bundled in the answer.
If the answer does not contain a BUNDLE group, the offerer MUST process the answer as a normal answer.
When an offerer generates a subsequent offer, it MUST assign the previously selected offerer BUNDLE address [Section 8.3.1] to each bundled "m=" section (including any bundle-only "m=" section), except if:
In addition, the offerer MUST select an offerer BUNDLE-tag [Section 8.2.1] associated with the previously selected offerer BUNDLE address, unless the offerer suggests a new offerer BUNDLE address.
When an offerer generates an offer, in which it suggests a new offerer BUNDLE address [Section 8.2.1], the offerer MUST:
In addition, the offerer MUST indicate that the address is the new suggested offerer BUNDLE address [Section 8.2.1].
NOTE: Unless the offerer assigns the new suggested offerer BUNDLE address to each bundled "m=" section, it can assign unique addresses to any number of bundled "m=" sections (and the previously selected offerer BUNDLE address to any remaining bundled "m=" section) if it wants to suggest multiple alternatives for the new offerer BUNDLE address.
When an offerer generates an offer, in which it wants to add a bundled "m=" section to a BUNDLE group, the offerer MUST:
In addition, the offerer MUST place the identification-tag associated with the added "m=" section in the SDP 'group:BUNDLE' attribute identification-tag list associated with the BUNDLE group [Section 8.2.1].
NOTE: Assigning a unique address to the "m=" section allows the answerer to move the "m=" section out of the BUNDLE group [Section 8.3.3], without having to reject the "m=" section.
If the offerer assigns a unique address to the added "m=" section, and if the offerer suggests that address as the new offerer BUNDLE address [Section 8.5.1], the offerer BUNDLE-tag MUST represent the added "m=" section [Section 8.2.1].
If the offerer associates a new suggested offerer BUNDLE address with each bundled "m=" section [Section 8.5.1], including the added "m=" section, the offerer BUNDLE-tag MAY represent the added "m=" section [Section 8.2.1].
[Section 18.3] shows an example where an offerer sends an offer in order to add a bundled "m=" section to a BUNDLE group.
When an offerer generates an offer, in which it wants to move a bundled "m=" section out of a BUNDLE group it was added to in a previous offer/answer transaction, the offerer:
NOTE: If the removed "m=" section is associated with the previously selected BUNDLE-tag, the offerer needs to suggest a new BUNDLE-tag [Section 8.2.1].
NOTE: If an "m=" section, when being moved out of a BUNDLE group, is added to another BUNDLE group, the offerer applies the procedures in [Section 8.5.2] to the "m=" section.
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 8.5.2].
[Section 18.4] shows an example of an offer for moving an "m=" section out of a BUNDLE group.
When an offerer generates an offer, in which it wants to disable a bundled "m=" section (added to the BUNDLE group in a previous offer/answer transaction), the offerer:
[Section 18.5] shows an example of an offer for disabling an "m=" section within a BUNDLE group.
Each "m=" section within a BUNDLE group MUST use the same transport- layer protocol. If bundled "m=" sections use different protocols on top of the transport-layer protocol, there MUST exist a publicly available specification which describes a mechanism, for this particular protocol combination, how to associate received data with the correct protocol.
In addition, if received data can be associated with more than one bundled "m=" section, there MUST exist a publicly available specification which 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 STUN, DTLS and SRTP protocols (in any combination), when UDP is used as transport-layer protocol, but 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 around how to multiplex multiple protocols on a given transport-layer protocol, and how to associate received data with the correct protocols.
Section 5.1.2 of [RFC5764] describes a mechanism to identify the protocol of a received packet among the STUN, Datagram Transport Layer Security (DTLS) and SRTP protocols (in any combination). If an offer or answer includes bundled "m=" section that represent 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 which describes a mechanism for associating the received DTLS packet with the correct "m=" section.
[Section 10.2] describes how to associate the packets in a received SRTP stream with the correct "m=" section.
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 SSRC numbering space [RFC3550].
The following rules and restrictions apply for a single RTP session:
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]).
Multiple bundled "m=" section 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. [I-D.ietf-mmusic-sdp-mux-attributes] lists SDP attributes, whose attribute values must be identical for all codecs that use the same payload type value.
NOTE: The text in this section is copied from Appendix B of JSEP. The community has not yet agreed on the text.
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, a 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 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 SSRC to identification-tag mappings. 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 should be routed.
However, some implementations of may not include this identification-tag in their RTP and RTCP traffic when using the BUNDLE mechanism, and instead use a payload type based mechanism to associate RTP streams with SDP "m=" sections. In this situation, each "m=" section MUST 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. Note that when using the payload type to associate RTP streams with "m=" sections an RTP stream, identified by 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 to "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 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.
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 RTP packet contains one of 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 is passed to the "m=" sections corresponding to the RTP streams it contains information about for additional processing. 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 for which no appropriate "m=" section can be identified MUST be processed as usual by the RTP layer, updating the metadata associated with the corresponding RTP streams, but are not passed to any "m=" section. This situation can occur with certain multiparty RTP topologies, or when RTCP packets are sent containing a subset of the SDES information.
Rules for additional processing of the various types of RTCP packets are explained below.
Within a BUNDLE group, the offerer and answerer MUST enable RTP/RTCP multiplexing [RFC5761] for the RTP-based media specified by the BUNDLE group.
When RTP/RTCP multiplexing is enabled, the same transport will be used for both RTP packets and RTCP packets associated with the BUNDLE group.
This section describes how an offerer and answerer use the SDP 'rtcp-mux' attribute [RFC5761] and the SDP 'rtcp-mux-only' attribute [I-D.ietf-mmusic-mux-exclusive] to negotiate usage of RTP/RTCP multiplexing for RTP-based media associated with a BUNDLE group.
The mux category [I-D.ietf-mmusic-sdp-mux-attributes] of the SDP 'rtcp-mux' and 'rtcp-mux-only' attributes is IDENTICAL. Section 8.1 describes the details regarding which bundled "m=" sections an offerer and answerer associates the attributes with.
RTP/RTCP multiplexing only applies to RTP-based media. However, as described in Section 8.1, within a BUNDLE group the SDP 'rtcp-mux' and SDP 'rtcp-mux-only' attributes might be included in a non-RTP-based bundled "m=" section.
When an offerer generates an initial offer, if the offer contains one or more RTP-based bundled "m=" sections (or, if there is a chance that RTP-based "m=" sections will later be added to the BUNDLE group), the offerer MUST include an SDP 'rtcp-mux' attribute [RFC5761] in one or more "m=" sections, following the procedures for IDENTICAL mux category attributes in Section 8.1. In addition, the offerer MAY include an SDP 'rtcp-mux-only' attribute [I-D.ietf-mmusic-mux-exclusive] in the same "m=" section.
NOTE: Whether the offerer associates 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 RTP-based "m=" section out of the BUNDLE group in the answer.
NOTE: If the offerer includes an SDP 'rtcp-mux' attribute in one or more 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 "m=" sections in order to provide a fallback port for RTCP, as described in [RFC5761]. However, the fallback port will only be used for RTP-based "m=" sections moved out of the BUNDLE group by the answerer.
In the initial offer, the address:port combination for RTCP MUST be unique in each bundled RTP-based "m=" section (excluding a bundle-only "m=" section), similar to RTP.
When an answerer generates an answer, if the answerer supports RTP-based media, and if a bundled "m=" section in the offer contained an SDP 'rtcp-mux' attribute, the answerer MUST enable usage of RTP/RTCP multiplexing, even if there currently are no RTP-based "m=" sections within the BUNDLE group. The answerer MUST include an SDP 'rtcp-mux' attribute in "m=" sections within the BUNDLE group in the answer following the procedures for IDENTICAL mux category attributes in Section 8.1. In addition, if the "m=" section in the offer contained an an SDP "rtcp-mux-only" attribute, the answerer MUST include an SDP "rtcp-mux-only" attribute to the "m=" section in the answer.
If the "m=" section associated with the offerer BUNDLE-tag in the offer contained an SDP 'rtcp-mux-only' attribute, and if the answerer moves an RTP-based "m=" section out of the BUNDLE group in the answer Section 8.3.3, the answerer MUST either include the attribute with in moved "m=" section (and enable RTP/RTCP multiplexing for the media associated with the "m=" section), or reject the "m=" section Section 8.3.4.
The answerer MUST NOT include an SDP 'rtcp' attribute in any "m=" section within the BUNDLE group in the answer. The answerer will use the port value of the selected offerer BUNDLE address for sending RTP and RTCP packets associated with each RTP-based bundled "m=" section towards the offerer.
If the usage of RTP/RTCP multiplexing within a BUNDLE group has been negotiated in a previous offer/answer transaction, the answerer MUST include an SDP 'rtcp-mux' attribute in the "m=" section associated with the answerer BUNDLE-tag in the answer. It is not possible to disable RTP/RTCP multiplexing within a BUNDLE group.
When an offerer receives an answer, if the answerer has accepted the usage of RTP/RTCP multiplexing (see Section 10.3.1.2), the answerer follows the procedures for RTP/RTCP multiplexing defined in [RFC5761]. The offerer will use the port value associated with the answerer BUNDLE address for sending RTP and RTCP packets associated with each RTP-based bundled "m=" section 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.
When an offerer generates a subsequent offer, the offerer MUST include an SDP 'rtcp-mux' attribute in a bundled "m=" section, following the procedures for IDENTICAL mux category attributes in Section 8.1.
If the offerer wants to add a bundled RTP-based "m=" section to the BUNDLE group, it MAY also include an SDP 'rtcp-mux-only' attribute in a bundled "m=" section, following the procedures for IDENTICAL mux category attributes in Section 8.1. This allows the offerer to mandate RTP/RTCP multiplexing for the added "m=" section (or the "m=" section to be rejected by the answerer) even if the answerer does not accept the "m=" section within the BUNDLE group.
This section describes how to use the BUNDLE grouping extension together with the Interactive Connectivity Establishment (ICE) mechanism [I-D.ietf-ice-rfc5245bis].
The generic procedures for negotiating usage of ICE using SDP, defined in [I-D.ietf-mmusic-ice-sip-sdp], also apply to usage of ICE with BUNDLE, with the following exceptions:
Initially, before ICE has produced a candidate pair that will be used for media, there might by multiple transports established (if multiple candidate pairs are tested). Once ICE has produced a transport that will be used for media, that becomes the BUNDLE transport.
Support and usage of ICE mechanism together with the BUNDLE extension is OPTIONAL.
When an offerer assigns a unique address to a bundled "m=" section (excluding any bundle-only "m=" section), the offerer MUST include SDP 'candidate' attributes (and other applicable ICE-related media-level SDP attributes), containing unique ICE properties (candidates etc), in the "m=" section according to the procedures in [I-D.ietf-mmusic-ice-sip-sdp].
When an offerer assigns a shared address to a bundled "m=" section, the offerer MUST include SDP 'candidate' attributes (and other applicable ICE-related media-level SDP attributes) in the "m=" section following the procedures in Section 8.1.
When an answerer assigns a shared address to an "m=" section within a BUNDLE group, the answerer MUST include SDP 'candidate' attributes (and other applicable ICE-related media-level SDP attributes) in the "m=" section following the procedures in Section 8.1.
NOTE: As most ICE-related media-level SDP attributes belong to the TRANSPORT mux category [I-D.ietf-mmusic-sdp-mux-attributes], the offerer and answerer follow the procedures in Section 8.1 when deciding whether to include an attribute in a bundled "m=" section. However, in the case of ICE-related media-level attributes, the rules apply to all attributes (see note below), even if they belong to a different mux category.
NOTE: The following ICE-related media-level SDP attributes are defined in [I-D.ietf-mmusic-ice-sip-sdp]: 'candidiate', 'remote-candidates', 'ice-mismatch', 'ice-ufrag', 'ice-pwd', and 'ice-pacing'.
When an offerer generates an initial offer, the offerer MUST include ICE-related media-level SDP attributes in bundled "m=" sections following the procedures in [Section 11.1].
When an answerer generates an answer that contains a BUNDLE group, the answer MUST include ICE-related SDP attributes in "m=" sections within the BUNDLE group according to [Section 11.1].
When an offerer receives an answer, if the answerer supports and uses the ICE mechanism and the BUNDLE extension, the offerer MUST apply the ICE properties associated with the offerer BUNDLE address, selected by the answerer [Section 8.3.1], to each bundled "m=" section.
When an offerer generates a subsequent offer, it MUST include ICE-related SDP attributes in a bundled "m=" section following the procedures in [Section 11.1].
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 to negotiate encryption keys if another encryption mechanism is used to encrypt media.
When DTLS is used within a BUNDLE group, the following rules apply:
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.
When [RFC8285] RTP header extensions 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.
This section replaces the text of the following sections of RFC 3264:
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.
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 by default 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.
A stream that is offered with a port of zero MUST be marked with port zero in the answer. Like the offer, the answer MAY omit all attributes present previously, and MAY list just a single media format from amongst those in the offer.
A stream that is offered with a port of zero MUST by default be marked with port zero in the answer, unless an extension mechanism, which specifies semantics for the usage of a non-zero port value, is used. If the stream is marked with port zero in the answer, the answer MAY omit all attributes present previously, and MAY list just a single media format from amongst those in the offer."
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.
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, 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 should be sent to the peer.
SDP Offerers and Answerers [RFC3264] can associate identification-tags with "m=" sections within SDP 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 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 an 'mid' attribute [RFC5888]. The media sender then inserts the identification-tag in RTCP and RTP packets sent to the media recipient.
NOTE: This text above defines how identification-tags are carried in SDP Offers and Answers. The usage of other signalling 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 sent 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 purpose, 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.
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=TBD | length | identification-tag ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The identification-tag payload is UTF-8 encoded, as in SDP.
The identification-tag is not zero terminated.
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES identifier value.]
The payload, containing the identification-tag, of the RTP SDES header extension element can be encoded using either the one-byte or two-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 either an RTCP SDES item or an RTP SDES header extension, or both, there should 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 is kept short. Due to the properties of the RTP header extension mechanism, when using the one-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 application identification.
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.]
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES identifier value.]
This document adds the MID SDES item to the IANA "RTP SDES item types" registry as follows:
Value: TBD
Abbrev.: MID
Name: Media Identification
Reference: RFCXXXX
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.]
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: christer.holmberg@ericsson.com
Reference: RFCXXXX
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 SDP answer.
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.]
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: Christer Holmberg
Contact e-mail: christer.holmberg@ericsson.com
Reference: RFCXXXX
Mux category: NORMAL
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document.]
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 Reference
------------------------------------- ------ ---------
Media bundling BUNDLE [RFCXXXX]
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 that the information is flowing in. 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 stream, 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 also 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 simultaneously used in parallel, 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 [I-D.ietf-mmusic-sdp-mux-attributes].
The security considerations of "RTP Header Extension for the RTP Control Protocol (RTCP) Source Description Items" requires that when RTCP is confidentiality protected that 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. Thus, the requirements in RFC 7941 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.
The example below shows:
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
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 IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
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 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
The example below shows:
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
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 IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
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
The example below shows:
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar zen
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 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
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
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar zen
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 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
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=rtpmap:66 H261/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
The example below shows:
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
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 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
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 IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
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 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
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
The example below shows:
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
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 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
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 IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
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 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
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
The usage of the SDP grouping extension for negotiating bundled media is based on a 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) have 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 Keranen, 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, Cullen Jennings, 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 Spotify for providing music for the countless hours of document editing.
[RFC EDITOR NOTE: Please remove this section when publishing]
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One of the main issues regarding the BUNDLE grouping extensions has been whether, in SDP Offers and SDP Answers, the same port value should 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 which uses SDP Offers with both different and identical port values.
Below are the primary issues that have been considered when defining the "BUNDLE" grouping extension:
Consider the following SDP Offer/Answer exchange, where Alice sends an SDP 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 an a later invention to RTP and Bob can not assume that Alice supports RFC 4961. This means that Alice may be sending RTP from a different port than 10000 or 10002 - some implementation simply send the RTP from an ephemeral port. When Bob's endpoint receives an RTP packet, the only way that Bob knows if it should be passed to the video or audio codec is by looking at the port it was received on. This lead some SDP implementations to use the fact that each "m=" section had a different port number to use that port number as an index to find the correct m line in the SDP. As a result, some implementations that do support symmetric RTP and ICE still use a 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.
In an SDP Offer or SDP 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 associated with the BUNDLE group, problems would occur if an endpoint wants to disable/reject the "m=" sectcion associated with that port, by setting the port value to zero. After that, no "m=" section would contain the port value which is used for the BUNDLE address. 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.
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 49172 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 BUNDLE in it's 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.
Sometimes intermediaries do not act as B2BUA, in the sense that they don't modify SDP bodies, nor do they terminate SIP dialogs. Still, however, they may 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 which 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.
Sometimes intermediaries do not act as B2BUA, in the sense that they don't modify SDP bodies, nor do they terminate SIP dialogs. Still, however, they may 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 either simply lead to bad user experience, or to termination of the call.
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 gather can be overlapped with other things while for exampe a web-page is loading to minimize the impact. If the client only wants to generate TURN or STUN ICE candidates for one of the "m=" lines and then use trickle ICE [I-D.ietf-ice-trickle] 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, may cause more use of the TURN server but would be quick in the cases where both sides supported BUNDLE and would fall back to a successful call in the other cases.