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Transport MMUSIC Working Group RTP SDP Bundle Multiplexing RTCWEB CLUE RTCWEB MMUSIC AVT WEB Browser 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. This specification 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. This specification defines a new RTP Control Protocol (RTCP) source description (SDES) item and a new RTP header extension that van be used to correlate bundled RTP/RTCP packets with their appropriate "m=" section.

When the SDP offer/answer mechanism 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) 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 Real-time Transport Protocol (RTP) mechanism is always used for RTP-based bundled media. This specification defines a new SDP Grouping Framework extension called 'BUNDLE'. The extension can be used with the Session Description Protocol (SDP) Offer/Answer mechanism 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 the BUNDLE address:port. In addition to negotiating the BUNDLE group, the offerer and answerer use the BUNDLE extension to negotiate the BUNDLE addresses:ports, one for the offerer (offerer BUNDLE address) and one for the answerer (answerer BUNDLE address:port). Once the offerer and the answerer have negotiated the BUNDLE addresses:ports, and a BUNDLE group has been formed, they assign their respective BUNDLE address:port to each "m=" section within the BUNDLE group. The endpoints then use the BUNDLE addresses:ports for sending and receiving the bundled media associated with the BUNDLE group. The use of a BUNDLE transport allows the usage of a single set of Interactive Connectivity Establishment (ICE) candidates for the whole BUNDLE group. This specification 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. This specification 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. As defined in RFC 4566 , 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 defines a new RTP Control Protocol (RTCP) source description (SDES) item, 'MID', and a new RTP SDES header extension that can be used to associate RTP streams with "m=" sections. A given BUNDLE address:port MUST only be associated with a single BUNDLE group. If an SDP offer or answer contains multiple BUNDLE groups, the procedures in this specification apply to each group independently. All RTP based media flows described by a single BUNDLE group belong to a single RTP session . 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 and .

"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: 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. 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. Suggested Offerer BUNDLE address:port: before an offerer BUNDLE address:port has been selected by the answerer, or when the offerer wants to change a previously selected offerer BUNDLE address:port, the address:port combination that the offerer wants to use for sending and receiving media. While suggested by the offerer, the selection of the offerer BUNDLE address:port is done by the answerer. Answerer BUNDLE address:port: the address:port combination used by the answerer for sending and receiving media. 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 (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 offer: The first offer, within an SDP session (e.g. a SIP dialog when the Session Initiation Protocol (SIP) 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 in an "m=" section carries the unique identification-tag assigned to that "m=" section. The session-level SDP 'group' attribute 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 .

The mechanism in this specification only applies to the Session Description Protocol (SDP) , when used together with the SDP offer/answer mechanism . Declarative usage of SDP is out of scope of this document, and is thus undefined.

This section defines a new SDP Grouping Framework 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 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 mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'group:BUNDLE' attribute is 'NORMAL'. defines the detailed SDP Offer/Answer procedures for the BUNDLE extension.

This section defines a new SDP media-level attribute , 'bundle-only'. 'bundle-only' is a property attribute , 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, the offerer can assign a zero port value to the "m=" section. According to an answerer will reject such an "m=" section. By including an SDP 'bundle-only' attribute in such an "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. Once the offerer and answerer BUNDLE addresses:ports have been selected, an offerer and answerer only assign the BUNDLE address:port to one bundled "m=" section. The offerer and answerer assign a zero port value and includes an SDP 'bundle-only' attribute 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. 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 how to calculate a value for the whole BUNDLE group, when parameter and attribute values have been assigned to each bundled "m=" section.

The "c=" line nettype value associated with a bundled "m=" section MUST be 'IN'. The "c=" line addrtype value 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 for associating the SDP bandwidth (b=) line with bundled "m=" sections.

This section describes the SDP Offer/Answer procedures for: Negotiating a BUNDLE group; and Selecting the BUNDLE addresses:ports (offerer BUNDLE address:port and answerer BUNDLE address:port); and Adding an "m=" section to a BUNDLE group; and 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 and 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. defines additional considerations for RTP based media. defines additional considerations for the usage of the SDP 'bundle-only' attribute. defines additional considerations for the usage of Interactive Connectivity Establishment (ICE) mechanism. SDP offers and answers can contain multiple BUNDLE groups. The procedures in this section apply independently to a given BUNDLE group.

When a BUNDLE group is initially negotiated, and a unique address is assigned to each bundled "m=" section (excluding any bundle-only "m=" section) in the initial offer [], any IDENTICAL and TRANSPORT mux category SDP attributes included in the BUNDLE group MUST explicitly be included in each bundled "m=“ section (excluding any bundle-only "m=" sections). When an offerer or answerer includes SDP attributes in bundled "m=" sections within a BUNDLE group for which the offerer and answerer BUNDLE addresses:ports have been selected, IDENTICAL and TRANSPORT mux category SDP attributes are only included in the "m=" section indicated by the BUNDLE-tag in the offer or answer. The SDP attribute values are implicitly applied to each bundled "m=" section (including any bundle-only "m=" section). The offerer and answerer MUST NOT include such SDP attributes in any other bundled "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 , there are cases where IDENTICAL and TRANSPORT mux category SDP attributes are only included in the "m=" sections indicated by 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, to negotiate a BUNDLE group, it MUST: Assign a unique address to each "m=" section within the offer, following the procedures in , excluding any bundle-only "m=" sections (see below); and 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; and Indicate which unique address the offerer suggests as the offerer BUNDLE address:port []. 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: Include an SDP 'bundle-only' attribute [] in the "m=" section; and Assign a zero port value to the "m=" section. NOTE: If the offerer assigns a zero port value to an "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 []. NOTE: If the offerer assigns unique addresses to multiple bundled "m=" sections, the offerer needs to be prepared to receive bundled media on each unique address, until it receives the associated answer and finds out which address:port combination has been selected as the offerer BUNDLE-address. [] and [] show an example of an initial offer.

In the offer, the address:port combination assigned to the "m=" section indicated by the offerer BUNDLE-tag indicates the offerer BUNDLE address:port, i.e., the address:port combination that the offerer suggests for sending and receiving bundled media. The offerer BUNDLE-tag MUST NOT represent a bundle-only "m=" section. Hence, the offer MUST contain at least one bundled "m=" section with a unique address (and a non-zero port value). It is RECOMMENDED that the offerer assigns the suggested offerer BUNDLE address:port to a bundled "m=" section that the offerer assumes it 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.

The example shows an initial SDP offer. The offer includes two "m=" sections in the SDP, and suggests that both are included in a BUNDLE group. The audio "m=" section is indicated by the offerer BUNDLE-tag (placed first in the SDP group:BUNDLE attribute identification-id list).

When an answerer generates an answer that contains a BUNDLE group, the following general SDP grouping framework restrictions, defined in , also apply to the BUNDLE group: The answerer MUST NOT include a BUNDLE group in the answer, unless the offerer requested the BUNDLE group to be negotiated in the corresponding offer; and The answerer MUST NOT include an "m=" section within a BUNDLE group, unless the offerer requested the "m=" section to be within that BUNDLE group in the corresponding offer. If the answer contains multiple BUNDLE groups, the answerer MUST NOT move an "m=" section from one BUNDLE group to another. If the answer contains a BUNDLE group, the answerer MUST: Select an offerer BUNDLE address:port []; and Select an answerer BUNDLE address:port []. The answerer is allowed to select a new answerer BUNDLE address:port 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: Move the "m=" section out of the BUNDLE group []; or Reject the "m=" section []. When the answerer creates the answer, it selects the offerer BUNDLE address:port [] and the answerer BUNDLE address:port []. The answerer then assigns the answerer BUNDLE address:port to the bundled "m=" section indicated by the answerer BUNDLE-tag. In every other bundled "m=" section the answerer includes an SDP 'bundle-only' attribute and assigns a zero port value to the "m=" section. If the answerer does not want to keep a bundle-only "m=" section within the BUNDLE group, it MUST reject the "m=" section []. 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 [].

In an offer, the bundled "m=" section indicated by the offerer BUNDLE-tag contains the suggested offerer BUNDLE address:port, i.e, the address:port combination that the offerer wants to use for sending and receiving bundled media []. The answerer MUST check whether that "m=" section fulfils the following criteria: The answerer will not move the "m=" section out of the BUNDLE group []; and The answerer will not reject the "m=" section []; and The "m=" section does not contain a zero port value. If all of the criteria above are fulfilled, the answerer MUST select the suggested offerer BUNDLE address:port. 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 [] or rejecting an "m=" section within a BUNDLE group [] to every bundled "m=" section in the offer when creating the answer. [] shows an example of an offerer BUNDLE address:port selection.

When the answerer selects a BUNDLE address:port for itself (answerer BUNDLE address:port), the answerer MUST assign the answerer BUNDLE address:port to the "m=" section that contains the selected offerer BUNDLE address:port in the corresponding offer. The answerer BUNDLE-tag represents that "m=" section in the answer. To every other bundled "m=" section the answerer MUST assign a zero port value and include an SDP 'bundle-only' attribute. The answerer MUST NOT assign an answerer BUNDLE address:port to an "m=" section that is not within the BUNDLE group, or to an "m=" section that is within another BUNDLE group. [] and [] show an example of an answerer BUNDLE address:port selection.

When an answerer wants to move a bundled "m=" section out of a BUNDLE group in an answer, it MUST first check the following criteria: In the corresponding offer, an offerer BUNDLE address:port (previously selected [] or newly suggested []) has been assigned to the "m=" section by the offerer; or In the corresponding offer, the "m=" section contains an SDP 'bundle-only' attribute and a zero port value. If either criteria above is fulfilled, the answerer can not move the "m=" section out of the BUNDLE group in the answer. The answerer can either 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 moved out of the BUNDLE group []. 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 to the "m=" section; and 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. 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 "m=" section out of the current BUNDLE group, and then generate an offer where the "m=" section is added to another BUNDLE group [].

When an answerer wants to reject a bundled "m=" section in an answer, it MUST first check the following criteria: In the corresponding offer, an offerer BUNDLE address:port (previously selected [] or newly suggested []) has been assigned to the "m=" section by the offerer. If the criteria above is fulfilled, the answerer can not reject the "m=" section in the answer (unless the answerer rejects each bundled "m=" section within the BUNDLE group). The answerer can either 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 []. 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 ; and 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.

The example below shows an SDP answer, based on the SDP offer in []. The answerer accepts both "m=" sections within the BUNDLE group. The answerer assigns the answerer BUNDLE address:port to the "m=" section indicated by the answerer BUNDLE-tag. The answerer assigns a zero port value and an SDP 'bundle-only' attribute to the other bundled "m=" section.

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:port, selected by the answerer [], 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 (i.e., a BUNDLE group has previously been negotiated), it MUST assign the previously selected offerer BUNDLE address:port [], or a newly suggested offerer BUNDLE address:port [], to exactly one "m=" section within the BUNDLE group. The offerer MUST NOT assign an offerer BUNDLE address:port (previously selected [] or newly suggested []) to a bundled "m=" section if: The offerer wants to move the bundled "m=" section out of the BUNDLE group []; or The offerer wants to disable the bundled "m=" section []. To every other "m=" section within the BUNDLE group, the offerer MUST assign a zero port value and an SDP 'bundle-only' attribute. When the offerer generates a subsequent offer, the offerer BUNDLE-tag MUST represent the bundled "m=" section to which the offerer BUNDLE address:port (previously negotiated or newly suggested) has been assigned.

When an offerer generates an offer, in which it suggests a new offerer BUNDLE address:port [], the offerer MUST: Assign the newly suggested offerer BUNDLE address:port to exactly one "m=" section within the BUNDLE group; and Assign a zero port value and an SDP 'bundle-only' attribute to every other "m=" section within the BUNDLE group.

When an offerer generates an offer, in which it wants to add a new bundled "m=" section, the offerer MUST: Assign the offerer BUNDLE address:port (previously selected [] or newly suggested []) to the added "m=" section; or Assign a zero port value and an SDP 'bundle-only' attribute to the added "m=" section (in this case the offerer BUNDLE address:port is assigned to another "m=" section within the BUNDLE group). 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 []. NOTE: If the offerer also wants to suggest a new offerer BUNDLE address:port to the BUNDLE group, the offerer can assign the newly suggested offerer BUNDLE address:port either to the added "m=" section, or to some other "m=" section within the BUNDLE group []. [] 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, the offerer: MUST assign a unique address to the "m=" section; and 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. 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 []. [] 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, the offerer: MUST assign a zero port value to the "m=" section, following the procedures in ; and 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. [] shows an example of an offer and answer 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 upper-layer protocols on top of the transport-layer protocol, there MUST exist a publicly available specification which describes a mechanism how to associate 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 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 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 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 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 , and no explicit negotiation is required in order to indicate support and usage of the mechanism. 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 packets with the correct "m=" section. [] 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 . Since a single BUNDLE transport is used for sending and receiving bundled media, the symmetric RTP mechanism 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 . 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 []. 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 , 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 . 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 ).

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. lists SDP attributes, whose attribute values are required to be identical for all codecs that use the same payload type value.

As described in , RTP packets are associated with RTP streams . 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 . 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 , 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 . 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 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 payload type based mechanism 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 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 MID to "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 "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 "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 payload type to "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 MID to "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 , Section 4.2.6, update the MID associated with the RTP stream to match the MID carried in the RTP packet, 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, updating 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 MID to "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 , 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, then remove the entry for that SSRC from the incoming SSRC table after an appropriate delay to account for "straggler packets", as specified in , Section 6.2.1. If the RTCP packet is of type SR or 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 RTCP XR and the packet is of type XR, as defined in , 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 or PSFB, as defined in , 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 target SSRC(s) in a section called Feedback Control Information (FCI). For these messages, the target SSRC(s) are 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: (PT=RTPFB, FMT=1). (PT=PSFB, FMT=1). (PT=PSFB, FMT=2). (PT=PSFB, FMT=3). If the RTCP packet is a feedback message that does include target SSRC(s) in its FCI section, it can either be a request or a notification. Requests reference a RTP stream that is being sent by the message recipient, whereas notifications are responses to an earlier request, and therefore reference a RTP stream that is being received by the message recipient. If the RTCP packet is a feedback request that includes 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: (PT=PSFB, FMT=4). (PT=PSFB, FMT=5). (PT=PSFB, FMT=7). (PT=RTPFB, FMT=3). (PT=PSFB, FMT=TBD). If the RTCP packet is a feedback notification that includes 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 matching SSRC. PSFB and RTPFB types that are handled in this way include: (PT=PSFB, FMT=6). This message is a notification in response to a prior TSTR. (PT=RTPFB, FMT=4). This message is a notification in response to a prior TMMBR, but 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 recognise the APP packet, then it MUST be discarded.

Within a BUNDLE group, the offerer and answerer MUST enable RTP/RTCP multiplexing 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 and the SDP 'rtcp-mux-only' attribute to negotiate usage of RTP/RTCP multiplexing for RTP-based media associated with a BUNDLE group. The mux category of the SDP 'rtcp-mux' and 'rtcp-mux-only' attributes is IDENTICAL. 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 , 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 (if such "m=" line is indicated by a BUNDLE-tag). NOTE: The offer in which the offerer indicates that it wants to create a BUNDLE group can be the initial offer of the session, or a subsequent offer within the session. As defined in , within the scope of this document the initial offer always refers to the offer in which the offerer indicates that it wants to create a BUNDLE group, no matter whether that offer is the initial offer, or a subsequent offer, within the session.

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 in each bundled "m=" section (excluding any bundle-only "m=" sections), following the procedures for IDENTICAL mux category attributes in . In addition, the offerer MAY include an SDP 'rtcp-mux-only' attribute in a RTP-based bundled "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 the bundled "m=" sections, but does not include an SDP 'rtcp-mux-only' attribute, the offerer can also include an SDP 'rtcp' attribute in one or more RTP-based bundled "m=" sections in order to provide a fallback port for RTCP, as described in . 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 the bundled "m=" section indicated by the answerer BUNDLE-tag, following the procedures for IDENTICAL mux category attributes in . In addition, if the "m=" section in the offer contained an SDP "rtcp-mux-only" attribute, the answerer MUST include an SDP "rtcp-mux-only" attribute in the bundled "m=" section indicated by the answerer BUNDLE-tag in the answer. If the "m=" section indicated by 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 [], the answerer MUST either 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 []. 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:port 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 exchange, 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 ), the answerer follows the procedures for RTP/RTCP multiplexing defined in . The offerer will use the port value associated with the answerer BUNDLE address:port 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 the bundled "m=" section indicated by the offerer BUNDLE-tag, following the procedures for IDENTICAL mux category attributes in .

This section describes how to use the BUNDLE grouping extension together with the Interactive Connectivity Establishment (ICE) mechanism . The generic procedures for negotiating usage of ICE using SDP, defined in , also apply to usage of ICE with BUNDLE, with the following exceptions: When the BUNDLE transport has been established, ICE connectivity checks and keep-alives only need to be performed for the BUNDLE transport, instead of per individual "m=" section within the BUNDLE group. In an offer, if the offer assigns a unique address to one or more bundled "m=" sections (excluding any bundle-only "m=" sections), the offerer MUST include ICE-related media-level attributes in each of those "m=" sections. If the offerer assigns an offerer BUNDLE address:port (previously selected [] or newly suggested []) to a bundled "m=" section (the "m=" section indicated by the offerer BUNDLE-tag), the offerer only includes ICE-related media-level SDP attributes in that "m=" section, following the procedures in . In an answer, the answerer only includes ICE-related media-level SDP attributes in the bundled "m=" section to which the answerer has assigned the answerer BUNDLE address:port (the "m=" section indicated by the answerer BUNDLE-tag), following the procedures in . Initially, before ICE has produced a candidate pair that will be used for media, there might be 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, 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.

When an offerer assigns a unique address to one or more bundled "m=" sections (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 each of those "m=" sections, following the procedures in . When an offerer assigns a BUNDLE address:port (previously selected or newly suggested) to a bundled "m=" section, (the "m=" section indicated by the offerer BUNDLE-tag) the offerer MUST only include SDP 'candidate' attributes (and other applicable ICE-related media-level SDP attributes) in that "m=" section, following the procedures in . When an answerer assigns a BUNDLE address to an "m=" section within a BUNDLE group (the "m=" section represented by the answerer BUNDLE-tag), the answerer onlys include SDP 'candidate' attributes (and other applicable ICE-related media-level SDP attributes) in that "m=" section, following the procedures in . The answerer MUST NOT include ICE-related media-level SDP attributes in any other "m=" sections. NOTE: As most ICE-related media-level SDP attributes belong to the TRANSPORT mux category , the offerer and answerer follow the procedures in 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 : 'candidate', 'remote-candidates', 'ice-mismatch', 'ice-ufrag', 'ice-pwd', and 'ice-pacing'.

One or more media streams within a BUNDLE group might use the Datagram Transport Layer Security (DTLS) protocol 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: There can only be one DTLS association associated with the BUNDLE group; and 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; and 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 whether 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 . The client MUST include the extension even if the usage of DTLS-SRTP is not negotiated as part of the multimedia session (e.g., SIP session . 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 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 updates RFC 3264, in order to allow extensions to define the usage of a zero port value in offers and answers for other purposes than removing or disabling media streams. The following sections of RFC 3264 are updated: Section 5.1 (Unicast Streams). Section 6 (Generating the Answer). Section 8.4 (Putting a Unicast Media Stream on Hold).

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.

An offered stream MAY be rejected in the answer, for any reason. If a stream is rejected, the offerer and answerer MUST NOT generate media (or RTCP packets) for that stream. To reject an offered stream, the port number in the corresponding stream in the answer MUST be set to zero. Any media formats listed are ignored. At least one MUST be present, as specified by SDP.

An offered stream MAY be rejected in the answer, for any reason. If a stream is rejected, the offerer and answerer MUST NOT generate media (or RTCP packets) for that stream. A port number of zero in the answer by default indicates that the offered stream is rejected, but an extension mechanism might specify different semantics for the usage of a zero port value. If a stream is rejected, any media formats listed are ignored. At least one MUST be present, as specified by SDP.

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 is to 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 is to be sent to the peer.

SDP Offerers and Answerers can associate identification-tags with "m=" sections within SDP Offers and Answers, using the procedures in . Each identification-tag uniquely represents an "m=" section. This section defines a new RTCP SDES item , 'MID', which is used to carry identification-tags within RTCP SDES packets. This section also defines a new RTP SDES header extension , 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 an 'mid' attribute . 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 signaling protocols for carrying identification-tags is not prevented, but the usage of such protocols is outside the scope of this document. 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, 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.

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 . 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 . As the identification-tag is included in either an RTCP SDES item or 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 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 user- or 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:

[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:

[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:

[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:

The security considerations defined in and 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 is flowing. 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 . The security considerations of "RTP Header Extension for the RTP Control Protocol (RTCP) Source Description Items" requires that when RTCP is confidentiality protected, and 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. 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 , for example SRTP can provide the necessary security functions of ensuring the integrity and source authenticity.

The example below shows: An offer, in which the offerer assigns a unique address to each bundled "m=" section within the BUNDLE group. An answer, in which the answerer selects the offerer BUNDLE address:port, and then selects its own BUNDLE address (the answerer BUNDLE address:port) and assigns it to the bundled "m=" section indicated by the answerer BUNDLE-tag.

The example below shows: An offer, in which the offerer assigns a unique address to each bundled "m=" section within the BUNDLE group. An answer, in which the answerer rejects the offered BUNDLE group, and assigns a unique address to each "m=" section (following normal RFC 3264 procedures).

The example below shows: A subsequent offer (the BUNDLE group has been created as part of a previous offer/answer exchange), in which the offerer adds a new "m=" section, indicated by the "zen" identification-tag, to a previously negotiated BUNDLE group, assigns the previously selected offerer BUNDLE address:port to the added "m=" section, indicated by the offerer BUNDLE-tag. To every other bundled "m=" section the offerer assigns a zero port value and includes an SDP 'bundle-only' attribute. An answer, in which the answerer assigns the answerer BUNDLE address:port to the bundled "m=" section indicated by the answerer BUNDLE-tag. To every other bundled "m=" section the answerer assigns a zero port value and includes an SDP 'bundle-only' attribute.

The example below shows: A subsequent offer (the BUNDLE group has been created as part of a previous offer/answer transaction), in which the offerer moves a bundled "m=" section, indicated by the "zen" identification-tag, out of a BUNDLE group, assigns a unique address to the moved "m=" section, and assigns the previously selected offerer BUNDLE address:port to another bundled "m=" section, indicated by the offerer BUNDLE-tag. To every other bundled "m=" section the offerer assigns a zero port value and includes an SDP 'bundle-only' attribute. An answer, in which the answerer moves the "m=" section out of the BUNDLE group, assigns a unique address to the moved "m=" section, and assigns the answerer BUNDLE address:port to the bundled "m=" section indicated by the answerer BUNDLE-tag. To every other bundled "m=" section the answerer assigns a zero port value and includes an SDP 'bundle-only' attribute.

The example below shows: A subsequent offer (the BUNDLE group has been created as part of a previous offer/answer transaction), in which the offerer disables a bundled "m=" section indicated by the "zen" identification-tag, within a BUNDLE group, assigns a zero port number to the disabled "m=" section, and assigns the offerer BUNDLE address:port to another bundled "m=" section, indicated by the offerer BUNDLE-tag. To every other bundled "m=" section the offerer assigns a zero port value and includes an SDP 'bundle-only' attribute. An answer, in which the answerer moves the disabled "m=" sections out of the BUNDLE group, assigns a zero port value to the disabled "m=" section, and assigns the answerer BUNDLE address:port to the bundled "m=" section indicated by the answerer BUNDLE-tag. To every other bundled "m=" section the answerer assigns a zero port value and includes an SDP 'bundle-only' attribute.

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) 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, 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] Changes from draft-ietf-mmusic-sdp-bundle-negotiation-47 Changes based on AD review by Ben Campbell. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-46 Pre-RFC5378 disclaimer removed put back. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-45 Mux category for SDP 'group:BUNDLE' attribute added. https://github.com/cdh4u/draft-sdp-bundle/pull/54 Pre-RFC5378 disclaimer removed. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-44 Minor editorial nits based on pull request by Colin P. https://github.com/cdh4u/draft-sdp-bundle/pull/53 Changes from draft-ietf-mmusic-sdp-bundle-negotiation-43 Changes based on WG chairs review. Text added in order to close GitHub issues by Taylor B. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-42 Changes based on final WG review. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-41 Update to section 6 o RFC 3264: https://github.com/cdh4u/draft-sdp-bundle/pull/47 Editorial clarification on BUNDLE address selection: https://github.com/cdh4u/draft-sdp-bundle/pull/46 Changes from draft-ietf-mmusic-sdp-bundle-negotiation-40 Editorial changes and technical restrictions in order to make the specification more understandable: https://github.com/cdh4u/draft-sdp-bundle/pull/45 - BUNDLE address is only assigned to m- section indicated by BUNDLE-tag. - bundle-only attribute also used in answers and subsequent offers. - Answerer cannot reject, or remove, the bundled m- section that contains the BUNDLE address. - ICE Offer/Answer sections removed, due to duplicated information. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-39 Editorial terminology changes. RFC 5285 reference replaced by reference to RFC 8285. https://github.com/cdh4u/draft-sdp-bundle/pull/44 - Clarify that an m- section can not be moved between BUNDLE groups without first moving the m- section out of a BUNDLE group. https://github.com/cdh4u/draft-sdp-bundle/pull/41 - Addition of BUNDLE transport concept. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-38 Changes to RTP streaming mapping section based on text from Colin Perkins. The following GitHub pull requests were merged: https://github.com/cdh4u/draft-sdp-bundle/pull/34 - Proposed updates to RTP processing https://github.com/cdh4u/draft-sdp-bundle/pull/35 - fixed reference to receiver-id section Changes from draft-ietf-mmusic-sdp-bundle-negotiation-37 The following GitHub pull request was merged: https://github.com/cdh4u/draft-sdp-bundle/pull/33 Changes from draft-ietf-mmusic-sdp-bundle-negotiation-36 The following GitHub pull requests were merged: https://github.com/cdh4u/draft-sdp-bundle/pull/32 - extmap handling in BUNDLE. https://github.com/cdh4u/draft-sdp-bundle/pull/31 - Additional Acknowledgement text added. https://github.com/cdh4u/draft-sdp-bundle/pull/30 - MID SDES item security procedures updated https://github.com/cdh4u/draft-sdp-bundle/pull/29 - Appendix B of JSEP moved into BUNDLE. - Associating RTP/RTCP packets with SDP m- lines. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-35 Editorial changes on RTP streaming mapping section based on comments from Colin Perkins. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-34 RTP streams, instead of RTP packets, are associated with m- lines. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-33 Editorial changes based on comments from Eric Rescorla and Cullen Jennings: - Changes regarding usage of RTP/RTCP multiplexing attributes. - Additional text regarding associating RTP/RTCP packets with SDP m- lines. - Reference correction. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-32 Editorial changes based on comments from Eric Rescorla and Cullen Jennings: - Justification for mechanism added to Introduction. - Clarify that the order of m- lines in the group:BUNDLE attribute does not have to be the same as the order in which the m- lines are listed in the SDP. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-31 Editorial changes based on GitHub Pull requests by Martin Thomson: - https://github.com/cdh4u/draft-sdp-bundle/pull/2 - https://github.com/cdh4u/draft-sdp-bundle/pull/1 Editorial change based on comment from Diederick Huijbers (9th July 2016). Changes based on comments from Flemming Andreasen (21st June 2016): - Mux category for SDP bundle-only attribute added. - Mux category considerations editorial clarification. - Editorial changes. RTP SDES extension according to draft-ietf-avtext-sdes-hdr-ext. Note whether Design Considerations appendix is to be kept removed: - Appendix is kept within document. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-30 Indicating in the Abstract and Introduction that the document updates RFC 3264. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-29 Change based on WGLC comment from Colin Perkins. - Clarify that SSRC can be reused by another source after a delay of 5 RTCP reporting intervals. Change based on WGLC comment from Alissa Cooper. - IANA registry name fix. - Additional IANA registration information added. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-28 - Alignment with exclusive mux procedures. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-27 - Yet another terminology change. - Mux category considerations added. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-26 - ICE considerations modified: ICE-related SDP attributes only added to the bundled m- line representing the selected BUNDLE address. - Reference to draft-ietf-mmusic-ice-sip-sdp added. - Reference to RFC 5245 replaced with reference to draft-ietf-ice-rfc5245bis. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-25 - RTP/RTCP mux procedures updated with exclusive RTP/RTCP mux considerations. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-24 - Reference and procedures associated with exclusive RTP/RTCP mux added Changes from draft-ietf-mmusic-sdp-bundle-negotiation-23 - RTCP-MUX mandatory for bundled RTP m- lines - Editorial fixes based on comments from Flemming Andreasen Changes from draft-ietf-mmusic-sdp-bundle-negotiation-22 - Correction of Ari's family name - Editorial fixes based on comments from Thomas Stach - RTP/RTCP correction based on comment from Magnus Westerlund -- http://www.ietf.org/mail-archive/web/mmusic/current/msg14861.html Changes from draft-ietf-mmusic-sdp-bundle-negotiation-21 - Correct based on comment from Paul Kyzivat -- 'received packets' replaced with 'received data' Changes from draft-ietf-mmusic-sdp-bundle-negotiation-20 - Clarification based on comment from James Guballa - Clarification based on comment from Flemming Andreasen Changes from draft-ietf-mmusic-sdp-bundle-negotiation-19 - DTLS Considerations section added. - BUNDLE semantics added to the IANA Considerations - Changes based on WGLC comments from Adam Roach -- http://www.ietf.org/mail-archive/web/mmusic/current/msg14673.html Changes from draft-ietf-mmusic-sdp-bundle-negotiation-18 - Changes based on agreements at IETF#92 -- BAS Offer removed, based on agreement at IETF#92. -- Procedures regarding usage of SDP "b=" line is replaced with a reference to to draft-ietf-mmusic-sdp-mux-attributes. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-17 - Editorial changes based on comments from Magnus Westerlund. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-16 - Modification of RTP/RTCP multiplexing section, based on comments from Magnus Westerlund. - Reference updates. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-15 - Editorial fix. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-14 - Editorial changes. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-13 Changes to allow a newly suggested offerer BUNDLE address to be assigned to each bundled m- line. Changes based on WGLC comments from Paul Kyzivat - Editorial fixes Changes from draft-ietf-mmusic-sdp-bundle-negotiation-12 Usage of SDP 'extmap' attribute added SDP 'bundle-only' attribute scoped with "m=" lines with a zero port value Changes based on WGLC comments from Thomas Stach - ICE candidates not assigned to bundle-only m- lines with a zero port value - Editorial changes Changes based on WGLC comments from Colin Perkins - Editorial changes: -- "RTP SDES item" -> "RTCP SDES item" -- "RTP MID SDES item" -> "RTCP MID SDES item" - Changes in section 10.1.1: -- "SHOULD NOT" -> "MUST NOT" -- Additional text added to the Note - Change to section 13.2: -- Clarify that mid value is not zero terminated - Change to section 13.3: -- Clarify that mid value is not zero terminated -- Clarify padding Changes based on WGLC comments from Paul Kyzivat - Editorial changes: Changes based on WGLC comments from Jonathan Lennox - Editorial changes: - Defintion of SDP bundle-only attribute alligned with structure in 4566bis draft Changes from draft-ietf-mmusic-sdp-bundle-negotiation-11 Editorial corrections based on comments from Harald Alvestrand. Editorial corrections based on comments from Cullen Jennings. Reference update (RFC 7160). Clarification about RTCP packet sending when RTP/RTCP multiplexing is not used (http://www.ietf.org/mail-archive/web/mmusic/current/msg13765.html). Additional text added to the Security Considerations. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-10 SDP bundle-only attribute added to IANA Considerations. SDES item and RTP header extension added to Abstract and Introduction. Modification to text updating section 8.2 of RFC 3264. Reference corrections. Editorial corrections. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-09 Terminology change: "bundle-only attribute assigned to m= line" to "bundle-only attribute associated with m= line". Editorial corrections. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-08 Editorial corrections. - "of"->"if" (8.3.2.5). - "optional"->"OPTIONAL" (9.1). - Syntax/ABNF for 'bundle-only' attribute added. - SDP Offer/Answer sections merged. - 'Request new offerer BUNDLE address' section added Changes from draft-ietf-mmusic-sdp-bundle-negotiation-07 OPEN ISSUE regarding Receiver-ID closed. - RTP MID SDES Item. - RTP MID Header Extension. OPEN ISSUE regarding insertion of SDP 'rtcp' attribute in answers closed. - Indicating that, when rtcp-mux is used, the answerer MUST NOT include an 'rtcp' attribute in the answer, based on the procedures in section 5.1.3 of RFC 5761. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-06 Draft title changed. Added "SDP" to section names containing "Offer" or "Answer". Editorial fixes based on comments from Paul Kyzivat (http://www.ietf.org/mail-archive/web/mmusic/current/msg13314.html). Editorial fixed based on comments from Colin Perkins (http://www.ietf.org/mail-archive/web/mmusic/current/msg13318.html). - Removed text about extending BUNDLE to allow multiple RTP sessions within a BUNDLE group. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-05 Major re-structure of SDP Offer/Answer sections, to align with RFC 3264 structure. Additional definitions added. - Shared address. - Bundled "m=" line. - Bundle-only "m=" line. - Offerer suggested BUNDLE mid. - Answerer selected BUNDLE mid. Q6 Closed (IETF#88): An Offerer MUST NOT assign a shared address to multiple "m=" lines until it has received an SDP Answer indicating support of the BUNDLE extension. Q8 Closed (IETF#88): An Offerer can, before it knows whether the Answerer supports the BUNDLE extension, assign a zero port value to a 'bundle-only' "m=" line. SDP 'bundle-only' attribute section added. Connection data nettype/addrtype restrictions added. RFC 3264 update section added. Indicating that a specific payload type value can be used in multiple "m=" lines, if the value represents the same codec configuration in each "m=" line. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-04 Updated Offerer procedures (http://www.ietf.org/mail-archive/web/mmusic/current/msg12293.html). Updated Answerer procedures (http://www.ietf.org/mail-archive/web/mmusic/current/msg12333.html). Usage of SDP 'bundle-only' attribute added. Reference to Trickle ICE document added. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-02 Mechanism modified, to be based on usage of SDP Offers with both different and identical port number values, depending on whether it is known if the remote endpoint supports the extension. Cullen Jennings added as co-author. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-01 No changes. New version due to expiration. Changes from draft-ietf-mmusic-sdp-bundle-negotiation-00 No changes. New version due to expiration. Changes from draft-holmberg-mmusic-sdp-multiplex-negotiation-00 Draft name changed. Harald Alvestrand added as co-author. "Multiplex" terminology changed to "bundle". Added text about single versus multiple RTP Sessions. Added reference to RFC 3550.

One of the main issues regarding the BUNDLE grouping extensions has been whether, in SDP Offers and SDP 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 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: 1) Interoperability with existing 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.

Consider the following SDP Offer/Answer exchange, where Alice sends an SDP Offer to Bob:

RFC 4961 specifies a way of doing symmetric RTP but that is a later extension 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 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 led 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 an SDP data structure where SDP with "m=" sections with the same port such as:

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: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 which 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.

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:

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 BUNDLE 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.

Sometimes intermediaries do not act as B2BUAs, 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 B2BUAs, 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 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 TURN or STUN ICE candidates for one of the "m=" lines and then use trickle ICE 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 would be quick in the cases where both sides supported BUNDLE and would fall back to a successful call in the other cases.