| MMUSIC Working Group | C. Holmberg |
| Internet-Draft | Ericsson |
| Updates: 3264 (if approved) | H. Alvestrand |
| Intended status: Standards Track | |
| Expires: March 26, 2015 | C. Jennings |
| Cisco | |
| September 22, 2014 |
Negotiating Media Multiplexing Using the Session Description Protocol (SDP)
draft-ietf-mmusic-sdp-bundle-negotiation-11.txt
This specification defines a new SDP Grouping Framework extension, 'BUNDLE'. The extension can be used with the Session Description Protocol (SDP) Offer/Answer mechanism to negotiate the usage of a single 5-tuple for sending and receiving media associated with multiple SDP media descriptions ("m="). This is referred to as bundled media. This specification also defines a new SDP attribute, 'bundle-only', which can be used to request that specific media is only used if bundled.
This specification also updates sections 5.1, 8.1 and 8.2 of RFC 3264. The update allows an answerer to assign a non-zero port value to an "m=" line in an SDP answer, even if the "m=" line in the associated SDP offer contained a zero port value.
This specification also defines a new RTP SDES item, and a new RTP header extension, that can be used to carry a value that associates RTP/RTCP packets with a specific media description.
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 http://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 March 26, 2015.
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
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In the IETF RTCWEB WG, a need to use a single 5-tuple for sending and receiving media associated with multiple SDP media descriptions ("m=" lines) has been identified. This would e.g. allow the usage of a single set of Interactive Connectivity Establishment (ICE) [RFC5245] candidates for multiple media descriptions.
This specification defines a new SDP Grouping Framework [RFC5888] extension , 'BUNDLE'. The extension can be used with the Session Description Protocol (SDP) Offer/Answer mechanism [RFC3264] to negotiate the usage of a single 5-tuple for sending and receiving media associated with multiple SDP media descriptions ("m="). This is referred to as bundled media. This specification also defines a new SDP attribute, 'bundle-only', which can be used to request that specific media is only used if bundled.
The offerer and answerer [RFC3264] use the BUNDLE extension to negotiate the 5-tuples (BUNDLE addresses), one for the offerer (offerer BUNDLE address) and one for the answerer (answerer BUNDLE address) to be used for the bundled media associated with a BUNDLE group.
Once the offerer and the answerer have negotiated a BUNDLE group, they assign their respective BUNDLE address to each "m=" line in the BUNDLE group. The BUNDLE address is used to send and receive all media associated with the BUNDLE group.
NOTE: As defined in RFC 4566 [RFC4566], the semantics of assigning the same port value to multiple "m=" lines 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=" line in an SDP answer, even if the "m=" line in the associated SDP offer contained a zero port value.
This specification also defines a new RTP SDES item, and a new RTP header extension, that can be used to carry a value that associates RTP/RTCP packets with a specific media description.
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 Real-time Transport Protocol (RTP) [RFC3550] based media flows associated with 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=" line within an offer and answer, according to the procedures in [RFC4566] and [RFC3264]
5-tuple: A collection of the following values: source address, source port, destination address, destination port and protocol.
Unique address: An IP address and IP port combination that is assigned to only one "m=" line in an offer or answer.
Shared address: An IP address and IP port combination that is assigned to multiple "m=" lines within an offer or answer.
Offerer suggested BUNDLE mid: The first mid value in a given SDP 'group:BUNDLE' attribute mid list in an offer.
Answerer selected BUNDLE mid: The first mid value in a given SDP 'group:BUNDLE' attribute mid list in an answer.
Offerer BUNDLE address: Within a given BUNDLE group, an IP address and IP port combination used by an offerer to receive all media associated with each "m=" line within the BUNDLE group.
Answerer BUNDLE address: Within a given BUNDLE group, an IP address and IP port combination used by an answerer to receive all media associated with each "m=" line within the BUNDLE group.
BUNDLE group: A set of "m=" lines, created using an SDP Offer/Answer exchange, for which each endpoint uses a single 5-tuple to send and receive media. Each endpoint uses its BUNDLE address, associated with the BUNDLE group, to send and receive the media.
Bundled "m=" line: An "m=" line, whose SDP 'mid' attribute value is placed in a SDP 'group:BUNDLE' attribute mid value list in an offer or answer.
Bundle-only "m=" line: A bundled "m=" line with an associated SDP 'bundle-only' attribute.
Bundled media: All media associated with a given BUNDLE group.
Initial offer: The first offer, within an SDP session, 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 SDP Offer/Answer exchange.
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].
This section defines a new SDP Grouping Framework extension [RFC5888], 'BUNDLE'. The BUNDLE extension can be used with the Session Description Protocol (SDP) Offer/Answer mechanism to negotiate the usage of a single 5-tuple for sending and receiving media, referred to as bundled media, associated with multiple SDP media descriptions ("m=" lines). Within a successfully created BUNDLE group, media described with "m=" lines associated with the BUNDLE group will be sent and received using a single 5-tuple.
The BUNDLE extension is indicated using an SDP 'group' attribute with a "BUNDLE" semantics value [RFC5888]. An SDP "mid" attribute is assigned to each bundled "m=" line, and the "mid" attribute value is listed in the 'group:BUNDLE' attribute mid value list. Each "m=" line, whose mid value is listed in the mid value list, is associated with a given BUNDLE group.
SDP bodies can contain multiple BUNDLE groups. Any given bundled "m=" line MUST NOT be associated with more than one BUNDLE group.
[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'.
The 'bundle-only' attribute can be associated with a bundled "m=" line in an offer, to request that the answerer only accepts the "m=" line if the answerer keeps the "m=" line within the associated BUNDLE group.
In order to ensure that an answerer that does not supports the BUNDLE extension always rejects a 'bundle-only' "m=" line, the offerer can assign a zero port value to the "m=" line. According to [RFC4566] an answerer will reject such "m=" line.
The usage of the 'bundle-only' attribute is only defined for a bundled "m=" line within an offer. Other usage is unspecified.
[Section 8] defines the detailed SDP Offer/Answer procedures for the 'bundle-only' attribute.
This section defines the Augmented Backus-Naur Form (ABNF) [RFC5234] for the SDP 'bundle-only' attribute, based on the SDP [RFC4566] grammar.
attribute =/ bundle-only-attribute
bundle-only-attribute = "bundle-only"
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=" line, how to calculate a value for the whole BUNDLE group.
The "c=" line nettype value [RFC4566] assigned to a bundled "m=" line MUST be 'IN'.
The "c=" line addrtype value [RFC4566] assigned to a bundled "m=" line MUST be 'IP4' or 'IP6'. The same value MUST be assigned to each "m=" line.
NOTE: Extensions to this specification can specify usage of the BUNDLE mechanism for other nettype and addrtype values than the ones listed above.
The proposed bandwidth for a bundled "m=" line SHOULD be calculated in the same way as for a non-bundled "m=" line.
The total proposed bandwidth for a BUNDLE group is the sum of the proposed bandwidth for each bundled "m=" line.
The total proposed bandwidth for an offer or answer is the sum of the proposed bandwidth for each "m=" line (bundled and non-bundled) within the offer or answer.
An offerer and answerer MUST use the rules and restrictions defined in [I-D.mmusic-sdp-mux-attributes] for when associating SDP attributes with bundled "m=" lines.
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 transport protocol represented by a bundled "m=" line. [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) mechanism [RFC5245].
The offerer and answerer MUST follow the rules and restrictions defined in [Section 7] when creating offers and answers.
SDP bodies can contain multiple BUNDLE groups. The procedures in this section apply independently to a given BUNDLE group.
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 "m=" line only if the answerer keeps the "m=" line within the BUNDLE group, the offerer MUST:
NOTE: If the offerer assigns a zero port value to an "m=" line, but does not also associate an SDP 'bundle-only' attribute with the "m=" line, it is an indication that the offerer wants to disable the "m=" line [Section 8.5.5].
[Section 16.1] shows an example of an initial offer.
In the offer, the address assigned to the "m=" line associated with the offerer suggested BUNDLE mid indicates the address that the offerer wants the answer to select as the offerer BUNDLE address [Section 8.3.2].
When an answerer generates an answer, which 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=" line within a BUNDLE group, it MUST:
If the answerer keeps a bundle-only "m=" line within the BUNDLE group, it follows the procedures (assigns the answerer BUNDLE address to the "m=" line etc) for any other "m=" line kept within the BUNDLE group.
If the answerer does not want to keep a bundle-only "m=" line within the BUNDLE group, it MUST reject the "m=" line [Section 8.3.5].
The answerer MUST NOT include a 'bundle-only' attribute in an answer."
NOTE: If a bundled "m=" line in an offer contains a port zero value, but the "m=" line does not contain an SDP 'bundle-only' attribute, it is an indication that the offerer wants to disable the "m=" line [Section 8.5.5].
In an offer, the address (unique or shared) assigned to the bundled "m=" line associated with the offerer suggested BUNDLE mid indicates the address that the offerer wants the answer to select as the offerer BUNDLE address [Section 8.2.2]. The answerer MUST check whether the "m=" line fulfils the following criteria:
If all of the criteria above is fulfilled, the answerer MUST select the address associated with the "m=" line as the offerer BUNDLE address. In the answer, the answerer selected BUNDLE mid represents the "m=" line, and the address associated with the "m=" line in the offer becomes the offerer BUNDLE address.
If all of the criteria is not fulfilled, the answerer MUST select the next mid value in the mid list, and perform the same criteria check for the "m=" line associated with that mid value. If there are no more mid values in the mid list, the answerer MUST NOT create the BUNDLE group.
[Section 16.1] 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 the address to each bundled "m=" line within the created BUNDLE group in the answer.
The answerer MUST NOT assign the answerer BUNDLE address to an "m=" line that is not within the BUNDLE group, or to an "m=" line that is within another BUNDLE group.
[Section 16.1] shows an example of an answerer BUNDLE address selection.
When an answerer moves a "m=" line out of a BUNDLE group, it assigns an address to the "m=" line in the answer based on the following rules:
In addition, in either case above, the answerer MUST NOT include a mid value, associated with the moved "m=" line, in the SDP 'group:BUNDLE' attribute mid list associated with the BUNDLE group.
When an answerer rejects an "m=" line, it MUST assign an address with a zero port value to the "m=" line in the answer, according to the procedures in [RFC4566].
In addition, the answerer MUST NOT include a mid value, associated with the rejected "m=" line, in the SDP 'group:BUNDLE' attribute mid list associated with the BUNDLE group.
When an offerer receives an answer, if the answer contains a BUNDLE group, the offerer MUST check that any bundled "m=" line in the answer was indicated as bundled in the associated offer. If there is no mismatch, the offerer MUST apply the offerer BUNDLE address, selected by the answerer [Section 8.3.2], to each bundled "m=" line.
NOTE: As the answerer might reject one or more bundled "m=" lines, or move a bundled "m=" line out of a BUNDLE group, each bundled "m=" line 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 receives an answer, if the answer contains a BUNDLE group, the offerer MUST check whether the offerer BUNDLE address, selected by the answerer [Section 8.3.2], matches what was assigned to each bundled "m=" line (excluding any bundled "m=" line that was rejected, or moved out of the BUNDLE group, by the answer) in the associated offer. If there is a mismatch, the offerer SHOULD as soon as possible generate a subsequent offer, in which it assigns the offerer BUNDLE address to each bundled "m=" line. Such offer is referred to as a Bundle Address Synchronization (BAS) offer.
A BAS offer is typically sent in the following scenarios:
The offerer is allowed to modify any SDP parameter in the BAS offer.
NOTE: It is important that the BAS offer gets accepted by the answerer. For that reason the offerer needs to consider the necessity to modify SDP parameters in the BAS offer, in such a way that could trigger the answerer to reject the BAS offer. Disabling "m=" lines, or reducing the number of codecs, in a BAS offer is considered to have a low risk of being rejected.
NOTE: The main purpose of the BAS offer is to ensure that intermediaries, that might not support the BUNDLE extension, have correct information regarding the address is going to be used to transport the bundled media.
[Section 16.1] shows an example of a BAS offer.
When an offerer generates a subsequent offer, it MUST assign the previously selected offerer BUNDLE address [Section 8.3.2], to each bundled "m=" line (including any bundle-only "m=" line), with the following exceptions:
In addition, the offerer MUST select an offerer suggested BUNDLE mid [Section 8.2.2], even if the offerer does not want the answerer to select a new offerer BUNDLE address.
If the offerer associates an SDP 'bundle-only' attribute with a bundled "m=" line in the subsequent offer, if MUST assign the offerer BUNDLE address to the "m=" line. The offerer MUST NOT assign a unique address, or a zero port value, to a bundle-only "m=" line in a subsequent offer.
NOTE: The offerer can associate an SDP 'bundle-only' attribute with a bundled "m=" line in a subsequent offer, even if the offerer did not associate a 'bundle-only' attribute with the same "m=" line in a previous offer.
When an offerer generates an offer, in which it wants the answerer to select a new offerer BUNDLE address [Section 8.2.2], the offerer MUST:
NOTE: The offerer can assign a unique address to each bundled "m=" line in the offer, or it can assign the previously negotiated offerer BUNDLE address to each "m=" line (except the "m=" line to which it assigns the unique address that it wants the answerer to select as the new offerer BUNDLE address).
When an offerer generates an offer, in which it wants to add a bundled "m=" line to BUNDLE group, the offerer MUST:
NOTE: Adding a unique address to the "m=" line allows the answerer to move the "m=" line out of the BUNDLE group [Section 8.3.4], without having to reject the "m=" line.
If the offerer wants the answerer to select the address associated with the added "m=" line as the new offerer BUNDLE address, the offerer suggested BUNDLE mid MUST represent the added "m=" line [Section 8.2.2].
If the offerer associates an SDP 'bundle-only' attribute with the added "m=" line, the offerer MUST assign the offerer BUNDLE address (selected by the answerer in a previous offer/answer transaction) to the "m=" line.
[Section 16.3] shows an example where an offerer sends an offer in order to add a bundled "m=" line to a BUNDLE group.
When an offerer generates an offer, in which it wants to move a bundled "m=" line out of a BUNDLE group it was added to in a previous offer/answer transaction, the offerer:
NOTE: If an "m=" line, when being moved out of a BUNDLE group, is added to another BUNDLE group, the offerer applies the procedures in [Section 8.5.3] to the "m=" line.
[Section 16.4] shows an example of an offer for moving an "m=" line out of a BUNDLE group.
When an offerer generates an offer, in which it wants to disable a bundled "m=" line (added to the BUNDLE group in a previous offer/answer transaction), the offerer:
[Section 16.5] shows an example of an offer for disabling an "m=" line within a BUNDLE group.
If bundled "m=" lines represent different transport protocols, there MUST exist a publically available specification which describes a mechanism, for this specific transport protocol combination, how to associate a received packet with the correct transport protocol.
In addition, if a received packet can be associated with more than one bundled "m=" line, there MUST exist a publically available specification which describes a mechanism how to associate the received packet with the correct "m=" line.
Section 5.1.2 of [RFC5764] describes a mechanism how to identify the protocol among the STUN, DTLS and SRTP protocols (in any combination). If an offer or answerer in offers or answers include bundled "m=" lines 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 how to identify each individual protocol. In addition, if a received DTLS packet can be associated with more than one "m=" line, there MUST exist a specification which describes a mechanism how to associate the received DTLS packet with the correct "m=" line.
[Section 10.2] describes how to associate a received (S)RTP packet with the correct "m=" line.
All RTP-based media within a single BUNDLE group belong to a single RTP session [RFC3550]. Disjoint BUNDLE groups will form multiple RTP sessions, one per BUNDLE group.
Since a single RTP session is used for each bundle group, all "m=" lines representing RTP-based media in 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 [ref to draft-ietf-avtext-multiple-clock-rates].
Multiple bundled "m=" lines might represent 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 used inside more than one bundled "m=" line, all codecs associated with the payload type numbers 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.mmusic-sdp-mux-attributes] lists SDP attributes, whose attribute values must be identical for all codecs that use the same payload type value.
In general, there are multiple mechanisms that can be used by an endpoint in order to associate received RTP/RTCP packets with the bundled "m=" line representing the RTP packets. Such mechanisms include using the local address:port combination on which the RTP packets are received, the payload type value carried inside the RTP packets, the SSRC values carried inside the RTP packets, and other "m=" line specific information carried inside the RTP packets.
As all RTP/RTCP packets associated with a BUNDLE group are sent and received using the same 5-tuple, the local address:port combination cannot be used to associate received RTP packets with the correct "m=" line.
As described in [Section 10.1.2], the same payload type value might be used inside RTP packets described by multiple "m=" lines. In such cases, the payload type value cannot be used to associate received RTP packets with the correct "m=" line.
An offerer and answerer can in an offer and answer inform each other which SSRC values they will use inside sent RTP/RTCP packets, by assigning an SDP 'ssrc' attribute [RFC5576] to each bundled "m=" line which contains a payload type value that is also used inside another bundled "m=" line. As the SSRC values will be carried inside the RTP/RTCP packets, the offerer and answerer can then use that information to associate received RTP packets with the correct "m=" line. However, an offerer will not know which SSRC values the answerer will use until it 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 received RTP/RTCP packets with the correct "m=" line using the SSRC values.
In order for an offerer and answerer to always be able to associate received RTP and RTCP packets with the correct "m=" line, an offerer and answerer using the BUNDLE extension MUST use the mechanism defined in [Section 13], where the remote endpoint inserts the SDP 'mid' attribute value of an "m=" line in RTP and RTCP packets associated with that "m=" line.
When a BUNDLE group, which contains RTP based media, is created, the offerer and answerer MUST negotiate whether to enable RTP/RTCP multiplexing for the RTP based media associated with the BUNDLE group [RFC5761].
If RTP/RTCP multiplexing is not enabled, separate 5-tuples will be used for sending and receiving the RTP packets and the RTCP packets.
This section describes how an offerer and answerer can use the SDP 'rtcp-mux' attribute [RFC5761] and the SDP 'rtcp' attribute [RFC3605] to negotiate usage of RTP/RTCP multiplexing for RTP based media associated with a BUNDLE group.
When an offerer generates an initial offer, if the offerer wants to negotiate usage of RTP/RTCP multiplexing within a BUNDLE group, the offerer MUST assign an SDP 'rtcp-mux' attribute [RFC5761] to each bundled "m=" line (including any bundle-only "m=" line) in the offer. In addition, the offerer MUST assign an SDP 'rtcp' attribute [RFC3605] to each bundled "m=" line (including any bundle-only "m=" line), with an attribute value that is identical to the port value assigned to the "m=" line itself, in the offer.
If the offerer does not want to negotiate usage of RTP/RTCP multiplexing, it MUST NOT assign the SDP attributes above to any bundled "m=" line.
When an answerer generates an answer, if the offerer indicated support of RTP/RTCP multiplexing [RFC5761] within a BUNDLE group in the associated offer, the answerer MUST either accept or reject the usage of RTP/RTCP multiplexing in the answer.
If the answerer accepts usage of RTP/RTCP multiplexing within the BUNDLE group, it MUST assign an SDP 'rtcp-mux' attribute to each bundled "m=" line in the answer. The answerer MUST NOT assign an SDP 'rtcp' attribute to any bundled "m=" line in the answer.
If the answerer rejects usage of RTP/RTCP multiplexing within the BUNDLE group, it MUST NOT assign an SDP 'rtcp-mux' or SDP 'rtcp' attribute to any bundled "m=" line in the answer.
If the usage of RTP/RTCP multiplexing has been negotiated in a previous offer/answer transaction, and the offerer indicates that it wants to continue using RTP/RTCP multiplexing in a subsequent offer, the answerer MUST assign an SDP 'rtcp-mux' attribute to each bundled "m=" line in the answer. I.e. the answerer MUST NOT disable the usage of RTP/RTCP multiplexing.
When the offerer receives an answer, if the answerer accepts the usage of RTP/RTCP multiplexing, by including an SDP 'rtcp-mux' attribute to each bundled "m=" line in the answer [Section 10.3.2.3], the answerer follows the procedures for RTP/RTCP multiplexing defined in [RFC5761].
If the answerer does not accept the usage of RTP/RTCP multiplexing [Section 10.3.2.3], the offerer MUST use separate 5-tuples for RTP and RTCP.
When an offerer generates a subsequent offer, if it wants to negotiate usage of RTP/RTCP multiplexing within a BUNDLE group, or if it wants to continue usage of RTP/RTCP multiplexing (negotiated in a previous offer/answer transaction), it MUST assign SDP 'rtcp-mux' and 'rtcp' attributes to each bundled "m=" line (including bundle-only "m=" lines, and a bundled "m=" line that the offerer wants to add to the BUNDLE group), unless the offerer wants to disable or remove the "m=" line from the BUNDLE group.
If the offerer does not want to negotiate usage of RTP/RTCP multiplexing within the BUNDLE group, or if it wants to disable usage of RTP/RTCP multiplexing (negotiated in a previous offer/answer transaction), the offerer MUST NOT assign SDP 'rtcp-mux' and 'rtcp' attributes to any bundled "m=" line in the subsequent offer.
NOTE: It is RECOMMENDED that, once usage of RTP/RTCP multiplexing has been negotiated within a BUNDLE group, that the usage is not disabled. Disabling RTP/RTCP multiplexing means that the offerer and answerer need to reserve new IP ports, to be used for sending and receiving RTCP packets.
This section describes how to use the BUNDLE grouping extension together with the Interactive Connectivity Establishment (ICE) mechanism [RFC5245].
The procedures defined in [RFC5245] also apply to usage of ICE with BUNDLE, with the following exception:
Support and usage of ICE mechanism together with the BUNDLE extension is OPTIONAL.
When an offerer or answerer assigns a unique address to a bundled "m=" line (excluding bundle-only "m=" lines), it MUST also assign unique ICE candidates [RFC5245] to the "m=" line.
When an offerer or answerer assigns a shared address (i.e. a previously selected BUNDLE address) to one or more bundled "m=" line (including bundle-only "m=" lines), and when it assigns an address with a zero port value to one or more bundle-only "m=" lines, it MUST assign identical ICE candidates (referred to as shared ICE candidates) to each of those "m=" lines.
When an offerer generates an initial offer, it assigns unique or shared ICE candidates to the bundled "m=" lines, according to [Section 11.1].
When an answerer generates an answer, which contains a BUNDLE group, the answerer MUST assign shared ICE candidates to each bundled "m=" line (including "m=" lines that were indicated as bundle-only in the associated offer) in the answer.
When an offerer receives an answer, if the answerer supports and uses the ICE mechanism and the BUNDLE extension, the offerer MUST assign the ICE candidates, associated with the "m=" line representing the offerer BUNDLE address (selected by the answerer) to each bundled "m=" line.
When an offerer generates a subsequent offer, it assigns unique or shared ICE candidates to the bundled "m=" lines, according to [Section 11.1].
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 (like the offer) 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 assign mid values to SDP Media Descriptions (m= lines) within SDP Offers and Answers, using the procedures in [RFC5888]. Each mid value uniquely identifies an m= line.
This section defines a new RTP SDES item [RFC3550], 'MID', which is used to carry mid values within RTCP SDES packets. This section also defines a new RTP header extension [RFC5285], which can be used to carry the mid value in RTP packets.
The SDES item and RTP header extension makes is possible for a receiver to associate received RTCP- and RTP packets with a specific m= line, to which the receiver has assigned a mid value, even if those m= lines are part of the same RTP session. The endpoint informs the remote endpoint about the mid values using the procedures in [RFC5888], and the remote endpoint then inserts the mid values in RTCP- and RTP packets sent towards the other endpoint.
NOTE: This text above defines how the mid value is carried in SDP Offers and Answers. The usage of other signalling protocols for carrying the mid value 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 RTP MID SDES item SHOULD be sent in the first few RTCP packets sent on 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 MID header extension 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 random 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 mid value.
For robustness purpose, endpoints need to be prepared for situations where the mid value is delayed, and SHOULD NOT terminate sessions in such cases, as the mid value 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 | mid value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The mid value payload is UTF-8 encoded, as in SDP.
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES identifier value.]
The payload, containing the mid value, of the RTP MID header extension element can be encoded using either the one-byte or two-byte header [RFC5285]. The mid value payload is UTF-8 encoded, as in SDP.
[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
This document defines a new extension URI in the RTP Compact Header Extensions subregistry of the Real-Time Transport Protocol (RTP) Parameters 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
This document defines a new SDP media-level attribute, according to the following data:
Attribute name: bundle-only Type of attribute: Media-level 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
The security considerations defined in [RFC3264] and [RFC5888] apply to the BUNDLE extension.
When the BUNDLE extension is used a single set of security credentials might be used for all media streams associated with a BUNDLE group. If the security credentials are compromised, an attacker will have access to all media content.
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10002 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
a=rtpmap:32 MPV/90000
SDP Offer (3)
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
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
m=audio 10000 RTP/AVP 0 8 97
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10002 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/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
m=audio 20000 RTP/AVP 0
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 30000 RTP/AVP 32
b=AS:1000
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 20000 RTP/AVP 66
a=mid:zen
b=AS:1000
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 zen
m=audio 20000 RTP/AVP 0
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
a=rtpmap:32 MPV/90000
m=video 20000 RTP/AVP 66
a=mid:zen
b=AS:1000
a=rtpmap:66 H261/90000
SDP Offer (3)
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 10000 RTP/AVP 66
a=mid:zen
b=AS:1000
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 50000 RTP/AVP 66
b=AS:1000
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
a=rtpmap:32 MPV/90000
m=video 60000 RTP/AVP 66
b=AS:1000
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 0 RTP/AVP 66
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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
a=rtpmap:32 MPV/90000
m=video 0 RTP/AVP 66
a=rtpmap:66 H261/90000
This document requests IANA to register the new SDP Grouping semantic extension called BUNDLE.
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 and Flemming Andreasen for taking the time to read the text along the way, and providing useful feedback.
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-10
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-09
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-08
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-07
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-06
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-05
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-04
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-02
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-01
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-00
Changes from draft-holmberg-mmusic-sdp-multiplex-negotiation-00
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC3264] | Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. |
| [RFC4566] | Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. |
| [RFC5285] | Singer, D. and H. Desineni, "A General Mechanism for RTP Header Extensions", RFC 5285, July 2008. |
| [RFC5761] | Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, April 2010. |
| [RFC5888] | Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, June 2010. |
| [I-D.mmusic-sdp-mux-attributes] | Nandakumar, S., "A Framework for SDP Attributes when Multiplexing", Internet-Draft draft-ietf-mmusic-sdp-mux-attributes-02, July 2014. |
One of the main issues regarding the BUNDLE grouping extensions has been whether, in SDP Offers and SDP Answers, the same port number 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 5-tuple for media associated with the "m=" lines. 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 number values.
Below are the primary issues that have been considered when defining the "BUNDLE" grouping extension:
NOTE: Before this document is published as an RFC, this Appendix might be removed.
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 know 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=" line 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=" lines 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=" line being considered an SDP error because it has the same port as the first line.
In an SDP Offer or SDP Answer, the media associated with an "m=" line 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=" line.
If each "m=" line associated with a BUNDLE group would contain different port number values, and one of those port would be used for the 5-tuple, problems would occur if an endpoint wants to disable/reject the "m=" line associated with that port, by setting the port number value to zero. After that, no "m=" line would contain the port number value which is used for the 5-tuple. In addition, it is unclear what would happen to the ICE candidates associated with the "m=" line, as they are also used for the 5-tuple.
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 B2BUS still generates that SDP attribute in the Offer for the outgoing call leg. Consider an 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 cases, 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, an SBC 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=" line, which means that it might not be enough if media associated with all "m=" lines try to use that bandwidth. That may either simply lead to bad user experience, or to termination of the call.
When using ICE, an candidate needs to be gathered for each port. This takes approximately 20 ms extra for each extra "m=" line due to the NAT pacing requirements. All of this gather can be overlapped with other things while the 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-mmusic-trickle-ice] to get the non host ICE candidates for the rest of the "m=" lines, 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 allocation 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.