Internet DRAFT - draft-ietf-mmusic-sdp-bundle-negotiation
draft-ietf-mmusic-sdp-bundle-negotiation
MMUSIC Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 3264,5888,7941 (if approved) H. Alvestrand
Intended status: Standards Track Google
Expires: June 18, 2019 C. Jennings
Cisco
December 15, 2018
Negotiating Media Multiplexing Using the Session Description Protocol
(SDP)
draft-ietf-mmusic-sdp-bundle-negotiation-54.txt
Abstract
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 also allow assigning a zero
port value to a "m=" section in cases where the media described by
the "m=" section is not disabled or rejected.
This specification updates RFC 5888, to also allow an SDP 'group'
attribute to contain an identification-tag that identifies a "m="
section with the port set to zero.
This specification defines a new RTP Control Protocol (RTCP) source
description (SDES) item and a new RTP header extension that can be
used to correlate bundled RTP/RTCP packets with their appropriate
"m=" section.
This specification updates RFC 7941, by adding an exception, for the
MID RTP header extension, to the requirement regarding protection of
an SDES RTP header extension carrying an SDES item for the MID RTP
header extension.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. BUNDLE Mechanism . . . . . . . . . . . . . . . . . . . . 4
1.3. Protocol Extensions . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 8
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5. SDP Grouping Framework BUNDLE Extension . . . . . . . . . . . 8
6. SDP 'bundle-only' Attribute . . . . . . . . . . . . . . . . . 9
7. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 10
7.1. Generic SDP Considerations . . . . . . . . . . . . . . . 10
7.1.1. Connection Data (c=) . . . . . . . . . . . . . . . . 10
7.1.2. Bandwidth (b=) . . . . . . . . . . . . . . . . . . . 11
7.1.3. Attributes (a=) . . . . . . . . . . . . . . . . . . . 11
7.2. Generating the Initial SDP Offer . . . . . . . . . . . . 12
7.2.1. Suggesting the Offerer tagged 'm=' section . . . . . 13
7.2.2. Example: Initial SDP Offer . . . . . . . . . . . . . 13
7.3. Generating the SDP Answer . . . . . . . . . . . . . . . . 14
7.3.1. Answerer Selection of tagged 'm=' sections . . . . . 16
7.3.2. Moving A Media Description Out Of A BUNDLE Group . . 16
7.3.3. Rejecting a Media Description in a BUNDLE Group . . . 17
7.3.4. Example: SDP Answer . . . . . . . . . . . . . . . . . 18
7.4. Offerer Processing of the SDP Answer . . . . . . . . . . 19
7.5. Modifying the Session . . . . . . . . . . . . . . . . . . 19
7.5.1. Adding a Media Description to a BUNDLE group . . . . 20
7.5.2. Moving a Media Description Out of a BUNDLE Group . . 21
7.5.3. Disabling a Media Description in a BUNDLE Group . . . 21
8. Protocol Identification . . . . . . . . . . . . . . . . . . . 22
8.1. STUN, DTLS, SRTP . . . . . . . . . . . . . . . . . . . . 22
9. RTP Considerations . . . . . . . . . . . . . . . . . . . . . 23
9.1. Single RTP Session . . . . . . . . . . . . . . . . . . . 23
9.1.1. Payload Type (PT) Value Reuse . . . . . . . . . . . . 24
9.2. Associating RTP/RTCP Streams with the Correct SDP Media
Description . . . . . . . . . . . . . . . . . . . . . . . 24
9.3. RTP/RTCP Multiplexing . . . . . . . . . . . . . . . . . . 30
9.3.1. SDP Offer/Answer Procedures . . . . . . . . . . . . . 30
10. ICE Considerations . . . . . . . . . . . . . . . . . . . . . 32
11. DTLS Considerations . . . . . . . . . . . . . . . . . . . . . 33
12. RTP Header Extensions Consideration . . . . . . . . . . . . . 34
13. Update to RFC 3264 . . . . . . . . . . . . . . . . . . . . . 34
13.1. Original text of section 5.1 (2nd paragraph) of RFC 3264 34
13.2. New text replacing section 5.1 (2nd paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 35
13.3. Original text of section 8.4 (6th paragraph) of RFC 3264 35
13.4. New text replacing section 8.4 (6th paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 35
14. Update to RFC 5888 . . . . . . . . . . . . . . . . . . . . . 36
14.1. Original text of section 9.2 (3rd paragraph) of RFC 5888 36
14.2. New text replacing section 9.2 (3rd paragraph) of RFC
5888 . . . . . . . . . . . . . . . . . . . . . . . . . . 36
15. RTP/RTCP extensions for identification-tag transport . . . . 36
15.1. RTCP MID SDES Item . . . . . . . . . . . . . . . . . . . 37
15.2. RTP SDES Header Extension For MID . . . . . . . . . . . 38
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
16.1. New SDES item . . . . . . . . . . . . . . . . . . . . . 38
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16.2. New RTP SDES Header Extension URI . . . . . . . . . . . 39
16.3. New SDP Attribute . . . . . . . . . . . . . . . . . . . 39
16.4. New SDP Group Semantics . . . . . . . . . . . . . . . . 40
17. Security Considerations . . . . . . . . . . . . . . . . . . . 40
18. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 41
18.1. Example: Tagged m= Section Selections . . . . . . . . . 41
18.2. Example: BUNDLE Group Rejected . . . . . . . . . . . . . 43
18.3. Example: Offerer Adds a Media Description to a BUNDLE
Group . . . . . . . . . . . . . . . . . . . . . . . . . 45
18.4. Example: Offerer Moves a Media Description Out of a
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 46
18.5. Example: Offerer Disables a Media Description Within a
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 48
19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50
20. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 50
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 61
21.1. Normative References . . . . . . . . . . . . . . . . . . 61
21.2. Informative References . . . . . . . . . . . . . . . . . 64
Appendix A. Design Considerations . . . . . . . . . . . . . . . 65
A.1. UA Interoperability . . . . . . . . . . . . . . . . . . . 65
A.2. Usage of Port Number Value Zero . . . . . . . . . . . . . 67
A.3. B2BUA And Proxy Interoperability . . . . . . . . . . . . 67
A.3.1. Traffic Policing . . . . . . . . . . . . . . . . . . 68
A.3.2. Bandwidth Allocation . . . . . . . . . . . . . . . . 68
A.4. Candidate Gathering . . . . . . . . . . . . . . . . . . . 68
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 69
1. Introduction
1.1. Background
When the SDP offer/answer mechanism [RFC3264] is used to negotiate
the establishment of multimedia communication sessions, if separate
transports (5-tuples) are negotiated for each individual media
stream, each transport consumes additional resources (especially when
Interactive Connectivity Establishment (ICE)
[I-D.ietf-ice-rfc5245bis] is used). For this reason, it is
attractive to use a single transport for multiple media streams.
1.2. BUNDLE Mechanism
This specification defines a way to use a single transport (BUNDLE
transport) for sending and receiving media (bundled media) described
by multiple SDP media descriptions ("m=" sections). The address:port
combination used by an endpoint for sending and receiving bundled
media is referred to as the BUNDLE address:port. The set of SDP
attributes that are applied to each "m=" section within a BUNDLE
group is referred to as BUNDLE attributes. The same BUNDLE transport
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is used for sending and receiving bundled media, which means that the
symmetric Real-time Transport Protocol (RTP) mechanism [RFC4961] is
always used for RTP-based bundled media.
This specification defines a new SDP Grouping Framework [RFC5888]
extension called 'BUNDLE'. The extension can be used with the
Session Description Protocol (SDP) Offer/Answer mechanism [RFC3264]
to negotiate which "m=" sections will become part of a BUNDLE group.
In addition, the offerer and answerer [RFC3264] use the BUNDLE
extension to negotiate the BUNDLE addresses:ports (offerer BUNDLE
address:port and answerer BUNDLE address:port) and the set of BUNDLE
attributes (offerer BUNDLE attributes and answerer BUNDLE attributes)
that will be applied to each "m=" section within the BUNDLE group.
The use of a BUNDLE transport allows the usage of a single set of
Interactive Connectivity Establishment (ICE)
[I-D.ietf-ice-rfc5245bis] candidates for the whole BUNDLE group.
A given BUNDLE address:port MUST only be associated with a single
BUNDLE group. If an SDP offer or answer contains multiple BUNDLE
groups, the procedures in this specification apply to each group
independently. All RTP-based bundled media associated with a given
BUNDLE group belong to a single RTP session [RFC3550].
The BUNDLE extension is backward compatible. Endpoints that do not
support the extension are expected to generate offers and answers
without an SDP 'group:BUNDLE' attribute, and are expected to assign a
unique address:port to each "m=" section within an offer and answer,
according to the procedures in [RFC4566] and [RFC3264].
1.3. Protocol Extensions
In addition to defining the new SDP Grouping Framework extension,
this specification defines the following protocol extensions and RFC
updates:
o The specification defines a new SDP attribute, 'bundle-only',
which can be used to request that a specific "m=" section (and the
associated media) is used only used if kept within a BUNDLE group.
o The specification updates RFC 3264 [RFC3264], to also allow
assigning a zero port value to a "m=" section in cases where the
media described by the "m=" section is not disabled or rejected.
o The specification defines a new RTP Control Protocol (RTCP)
[RFC3550] source description (SDES) item, 'MID', and a new RTP
SDES header extension that can be used to associate RTP streams
with "m=" sections.
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o The specification updates [RFC7941], by adding an exception, for
the MID RTP header extension, to the requirement regarding
protection of an SDES RTP header extension carrying an SDES item
for the MID RTP header extension.
2. Terminology
o "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.
o 5-tuple: A collection of the following values: source address,
source port, destination address, destination port, and transport-
layer protocol.
o Unique address:port: An address:port combination that is assigned
to only one "m=" section in an offer or answer.
o Offerer BUNDLE-tag: The first identification-tag in a given SDP
'group:BUNDLE' attribute identification-tag list in an offer.
o Answerer BUNDLE-tag: The first identification-tag in a given SDP
'group:BUNDLE' attribute identification-tag list in an answer.
o Suggested offerer tagged "m=" section: The bundled "m=" section
identified by the offerer BUNDLE-tag in an initial BUNDLE offer,
before a BUNDLE group has been negotiated.
o Offerer tagged "m=" section: The bundled "m=" section identified
by the offerer BUNDLE-tag in a subsequent offer. The "m=" section
contains characteristics (offerer BUNDLE address:port and offerer
BUNDLE attributes) applied to each "m=" section within the BUNDLE
group.
o Answerer tagged "m=" section: The bundled "m=" section identified
by the answerer BUNDLE-tag in an answer (initial BUNDLE answer or
subsequent). The "m=" section contains characteristics (answerer
BUNDLE address:port and answerer BUNDLE attributes) applied to
each "m=" section within the BUNDLE group.
o BUNDLE address:port: An address:port combination that an endpoint
uses for sending and receiving bundled media.
o Offerer BUNDLE address:port: the address:port combination used by
the offerer for sending and receiving media.
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o Answerer BUNDLE address:port: the address:port combination used by
the answerer for sending and receiving media.
o BUNDLE attributes: IDENTICAL and TRANSPORT multiplexing category
SDP attributes. Once a BUNDLE group has been created, the
attribute values apply to each bundled "m=" section within the
BUNDLE group.
o Offerer BUNDLE attributes: IDENTICAL and TRANSPORT multiplexing
category SDP attributes included in the offerer tagged "m="
section.
o Answerer BUNDLE attributes: IDENTICAL and TRANSPORT multiplexing
category SDP attributes included in the answerer tagged "m="
section.
o BUNDLE transport: The transport (5-tuple) used by all media
described by the "m=" sections within a BUNDLE group.
o BUNDLE group: A set of bundled "m=" sections, created using an SDP
Offer/Answer exchange, which uses a single BUNDLE transport, and a
single set of BUNDLE attributes, for sending and receiving all
media (bundled media) described by the set of "m=" sections. The
same BUNDLE transport is used for sending and receiving bundled
media.
o 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.
o Bundle-only "m=" section: A bundled "m=" section that contains an
SDP 'bundle-only' attribute.
o Bundled media: All media associated with a given BUNDLE group.
o Initial BUNDLE offer: The first offer, within an SDP session (e.g.
a SIP dialog when the Session Initiation Protocol (SIP) [RFC3261]
is used to carry SDP), in which the offerer indicates that it
wants to negotiate a given BUNDLE group.
o Initial BUNDLE answer: The answer to an initial BUNDLE offer in
which the offerer indicates that it wants to negotiate a BUNDLE
group, and where the answerer accepts the creation of the BUNDLE
group. The BUNDLE group is created once the answerer sends the
initial BUNDLE answer.
o Subsequent offer: An offer which contains a BUNDLE group that has
been created as part of a previous offer/answer exchange.
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o Subsequent answer: An answer to a subsequent offer.
o Identification-tag: A unique token value that is used to identify
an "m=" section. The SDP 'mid' attribute [RFC5888] in an "m="
section carries the unique identification-tag assigned to that
"m=" section. The session-level SDP 'group' attribute [RFC5888]
carries a list of identification-tags, identifying the "m="
sections associated with that particular 'group' attribute.
3. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
4. Applicability Statement
The mechanism in this specification only applies to the Session
Description Protocol (SDP) [RFC4566], when used together with the SDP
offer/answer mechanism [RFC3264]. Declarative usage of SDP is out of
scope of this document, and is thus undefined.
5. SDP Grouping Framework BUNDLE Extension
This section defines a new SDP Grouping Framework [RFC5888]
extension, 'BUNDLE'. The BUNDLE extension can be used with the SDP
Offer/Answer mechanism to negotiate a set of "m=" sections that will
become part of a BUNDLE group. Within a BUNDLE group, each "m="
section uses a BUNDLE transport for sending and receiving bundled
media. Each endpoint uses a single address:port combination for
sending and receiving the bundled media.
The BUNDLE extension is indicated using an SDP 'group' attribute with
a semantics value [RFC5888] of "BUNDLE". An identification-tag is
assigned to each bundled "m=" section, and each identification-tag is
listed in the SDP 'group:BUNDLE' attribute identification-tag list.
Each "m=" section whose identification-tag is listed in the
identification-tag list is associated with a given BUNDLE group.
SDP bodies can contain multiple BUNDLE groups. Any given bundled
"m=" section MUST NOT be associated with more than one BUNDLE group
at any given time.
NOTE: The order of the "m=" sections listed in the SDP 'group:BUNDLE'
attribute identification-tag list does not have to be the same as the
order in which the "m=" sections occur in the SDP.
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The multiplexing category [I-D.ietf-mmusic-sdp-mux-attributes] for
the 'group:BUNDLE' attribute is 'NORMAL'.
Section 7 defines the detailed SDP Offer/Answer procedures for the
BUNDLE extension.
6. SDP 'bundle-only' Attribute
This section defines a new SDP media-level attribute [RFC4566],
'bundle-only'. 'bundle-only' is a property attribute [RFC4566], and
hence has no value.
In order to ensure that an answerer that does not support the BUNDLE
extension always rejects a bundled "m=" section in an offer, the
offerer can assign a zero port value to the "m=" section. According
to [RFC3264] an answerer will reject such an "m=" section. By
including an SDP 'bundle-only' attribute in a bundled "m=" section,
the offerer can request that the answerer accepts the "m=" section
only if the answerer supports the BUNDLE extension, and if the
answerer keeps the "m=" section within the associated BUNDLE group.
Name: bundle-only
Value: N/A
Usage Level: media
Charset Dependent: no
Example:
a=bundle-only
Once the offerer tagged "m=" section and the answerer tagged "m="
section have been selected, an offerer and answerer will include an
SDP 'bundle-only' attribute in, and assign a zero port value to,
every other bundled "m=" section.
The usage of the 'bundle-only' attribute is only defined for a
bundled "m=" section with a zero port value. Other usage is
unspecified.
Section 7 defines the detailed SDP Offer/Answer procedures for the
'bundle-only' attribute.
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7. SDP Offer/Answer Procedures
This section describes the SDP Offer/Answer [RFC3264] procedures for:
o Negotiating a BUNDLE group; and
o Suggesting and selecting the tagged "m=" sections (offerer tagged
"m=" section and answerer tagged "m=" section); and
o Adding an "m=" section to a BUNDLE group; and
o Moving an "m=" section out of a BUNDLE group; and
o Disabling an "m=" section within a BUNDLE group.
The generic rules and procedures defined in [RFC3264] and [RFC5888]
also apply to the BUNDLE extension. For example, if an offer is
rejected by the answerer, the previously negotiated addresses:ports,
SDP parameters and characteristics (including those associated with a
BUNDLE group) apply. Hence, if an offerer generates an offer in
order to negotiate a BUNDLE group, and the answerer rejects the
offer, the BUNDLE group is not created.
The procedures in this section are independent of the media type or
"m=" line proto value assigned to a bundled "m=" section. Section 9
defines additional considerations for RTP based media. Section 6
defines additional considerations for the usage of the SDP 'bundle-
only' attribute. Section 10 defines additional considerations for
the usage of Interactive Connectivity Establishment (ICE)
[I-D.ietf-ice-rfc5245bis] mechanism.
Offers and answers can contain multiple BUNDLE groups. The
procedures in this section apply independently to a given BUNDLE
group.
7.1. Generic SDP Considerations
This section describes generic restrictions associated with the usage
of SDP parameters within a BUNDLE group. It also describes how to
calculate a value for the whole BUNDLE group, when parameter and
attribute values have been assigned to each bundled "m=" section.
7.1.1. Connection Data (c=)
The "c=" line nettype value [RFC4566] associated with a bundled "m="
section MUST be 'IN'.
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The "c=" line addrtype value [RFC4566] associated with a bundled "m="
section MUST be 'IP4' or 'IP6'. The same value MUST be associated
with each "m=" section.
NOTE: Extensions to this specification can specify usage of the
BUNDLE mechanism for other nettype and addrtype values than the ones
listed above.
7.1.2. Bandwidth (b=)
An offerer and answerer MUST use the rules and restrictions defined
in [I-D.ietf-mmusic-sdp-mux-attributes] for associating the SDP
bandwidth (b=) line with bundled "m=" sections.
7.1.3. Attributes (a=)
An offerer and answerer MUST include SDP attributes in every bundled
"m=" section where applicable, following the normal offer/answer
procedures for each attribute, with the following exceptions:
o In the initial BUNDLE offer, the offerer MUST NOT include
IDENTICAL and TRANSPORT multiplexing category SDP attributes
(BUNDLE attributes) in bundle-only "m=" sections. The offerer
MUST include such attributes in all other bundled "m=" sections.
In the initial BUNDLE offer each bundled "m=" line can contain a
different set of BUNDLE attributes, and attribute values. Once
the offerer tagged "m=" section has been selected, the BUNDLE
attributes contained in the offerer tagged "m=" section will apply
to each bundled "m=" section within the BUNDLE group.
o In a subsequent offer, or in an answer (initial of subsequent),
the offerer and answerer MUST include IDENTICAL and TRANSPORT
multiplexing category SDP attributes (BUNDLE attributes) only in
the tagged "m=" section (offerer tagged "m=" section or answerer
tagged "m=" section). The offerer and answerer MUST NOT include
such attributes in any other bundled "m=" section. The BUNDLE
attributes contained in the tagged "m=" section will apply to each
bundled "m=" section within the BUNDLE group.
o In an offer (initial BUNDLE offer or subsequent), or in an answer
(initial BUNDLE answer or subsequent), the offerer and answerer
MUST include SDP attributes of other categories than IDENTICAL and
TRANSPORT in each bundled "m=" section that a given attribute
applies to. Each bundled "m=" line can contain a different set of
such attributes, and attribute values, as such attributes only
apply to the given bundled "m=" section in which they are
included.
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NOTE: A consequence of the rules above is that media-specific
IDENTICAL and TRANSPORT multiplexing category SDP attributes which
are applicable only to some of the bundled "m=" sections within the
BUNDLE group might appear in the tagged "m=" section for which they
are not applicable. For instance, the tagged "m=" section might
contain an SDP 'rtcp-mux' attribute even if the tagged "m=" section
does not describe RTP-based media (but another bundled "m=" section
within the BUNDLE group does describe RTP-based media).
7.2. Generating the Initial SDP Offer
The procedures in this section apply to the first offer, within an
SDP session (e.g. a SIP dialog when the Session Initiation Protocol
(SIP) [RFC3261] is used to carry SDP), in which the offerer indicates
that it wants to negotiate a given BUNDLE group. This could occur in
the initial offer, or in a subsequent offer, of the SDP session.
When an offerer generates an initial BUNDLE offer, in order to
negotiate a BUNDLE group, it MUST:
o Assign a unique address:port to each bundled "m=" section,
following the procedures in [RFC3264], excluding any bundle-only
"m=" sections (see below); and
o Pick a bundled "m=" section as the suggested offerer tagged "m="
section [Section 7.2.1]; and
o Include SDP attributes in the bundled "m=" sections following the
rules in [Section 7.1.3]; and
o Include an SDP 'group:BUNDLE' attribute in the offer; and
o Place the identification-tag of each bundled "m=" section in the
SDP 'group:BUNDLE' attribute identification-tag list. The offerer
BUNDLE-tag indicates the suggested offerer tagged "m=" section.
NOTE: When the offerer assigns unique addresses:ports to multiple
bundled "m=" sections, the offerer needs to be prepared to receive
bundled media on each unique address:port, until it receives the
associated answer and finds out which bundled "m=" section (and
associated address:port combination) the answerer has selected as the
offerer tagged "m=" section.
If the offerer wants to request that the answerer accepts a given
bundled "m=" section only if the answerer keeps the "m=" section
within the negotiated BUNDLE group, the offerer MUST:
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o Include an SDP 'bundle-only' attribute [Section 7.2.1] in the "m="
section; and
o Assign a zero port value to the "m=" section.
NOTE: If the offerer assigns a zero port value to a bundled "m="
section, but does not include an SDP 'bundle-only' attribute in the
"m=" section, it is an indication that the offerer wants to disable
the "m=" section [Section 7.5.3].
[Section 7.2.2] and [Section 18.1] show an example of an initial
BUNDLE offer.
7.2.1. Suggesting the Offerer tagged 'm=' section
In the initial BUNDLE offer, the bundled "m=" section indicated by
the offerer BUNDLE-tag is the suggested offerer tagged "m=" section.
The address:port combination associated with the "m=" section will be
used by the offerer for sending and receiving bundled media if the
answerer selects the "m=" section as the offerer tagged "m=" section
[Section 7.3.1]. In addition, if the answerer selects the "m="
section as the offerer tagged "m=" section, the BUNDLE attributes
included in the "m=" section will be applied to each "m=" section
within the negotiated BUNDLE group.
The offerer MUST NOT suggest a bundle-only "m=" section as the
offerer tagged "m=" section.
It is RECOMMENDED that the suggested offerer tagged "m=" section is a
bundled "m=" section that the offerer believes 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.
7.2.2. Example: Initial SDP Offer
The example shows an initial BUNDLE offer. The offer includes two
"m=" sections in the offer, and suggests that both "m=" sections are
included in a BUNDLE group. The audio "m=" section is the suggested
offerer tagged "m=" section, indicated by placing the identification-
tag associated with the "m=" section (offerer BUNDLE-tag) first in
the SDP group:BUNDLE attribute identification-id list.
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SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP6 2001:db8::3
s=
c=IN IP6 2001:db8::3
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
7.3. Generating the SDP Answer
When an answerer generates an answer (initial BUNDLE answer or
subsequent) that contains a BUNDLE group the following general SDP
grouping framework restrictions, defined in [RFC5888], also apply to
the BUNDLE group:
o The answerer is only allowed to include a BUNDLE group in an
initial BUNDLE answer if the offerer requested the BUNDLE group to
be created in the corresponding initial BUNDLE offer; and
o The answerer is only allowed to include a BUNDLE group in a
subsequent answer if the corresponding subsequent offer contains a
previously negotiated BUNDLE group; and
o The answerer is only allowed to include a bundled "m=" section in
an answer if the "m=" section was indicated as bundled in the
corresponding offer; and
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o The answerer is only allowed to include a bundled "m=" section in
the same BUNDLE group as the bundled "m=" line in the
corresponding offer.
In addition, when an answerer generates an answer (initial BUNDLE
answer or subsequent) that contains a BUNDLE group, the answerer
MUST:
o In case of an initial BUNDLE answer, select the offerer tagged
"m=" section using the procedures in Section 7.3.1. In case of a
subsequent answer, the offerer tagged "m=" section is indicated in
the corresponding subsequent offer, and MUST NOT be changed by the
answerer; and
o Select the answerer tagged "m=" section [Section 7.3.1]; and
o Assign the answerer BUNDLE address:port to the answerer tagged
"m=" section; and
o Include an SDP 'bundle-only' attribute in, and assign a zero port
value to, every other bundled "m=" section within the BUNDLE
group; and
o Include SDP attributes in the bundled "m=" sections following the
rules in [Section 7.1.3]; and
o Include an SDP 'group:BUNDLE' attribute in the answer; and
o Place the identification-tag of each bundled "m=" section in the
SDP 'group:BUNDLE' attribute identification-tag list. The
answerer BUNDLE-tag indicates the answerer tagged "m=" section
[Section 7.3.1].
If the answerer does not want to keep an "m=" section within a BUNDLE
group, it MUST:
o Move the "m=" section out of the BUNDLE group [Section 7.3.2]; or
o Reject the "m=" section [Section 7.3.3].
The answerer can modify the answerer BUNDLE address:port, add and
remove SDP attributes, or modify SDP attribute values, in a
subsequent answer. Changes to the answerer BUNDLE address:port and
the answerer BUNDLE attributes will be applied to each bundled "m="
section within the BUNDLE group.
NOTE: If a bundled "m=" section in an offer contains a zero port
value, but the "m=" section does not contain an SDP 'bundle-only'
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attribute, it is an indication that the offerer wants to disable the
"m=" section [Section 7.5.3].
7.3.1. Answerer Selection of tagged 'm=' sections
When the answerer selects the offerer tagged "m=" section, it first
checks the suggested offerer tagged "m=" section [Section 7.2.1].
The answerer MUST check whether the "m=" section fulfils the
following criteria:
o The answerer will not move the "m=" section out of the BUNDLE
group [Section 7.3.2]; and
o The answerer will not reject the "m=" section [Section 7.3.3]; and
o The "m=" section does not contain a zero port value.
If all of the criteria above are fulfilled, the answerer MUST select
the "m=" section as the offerer tagged "m=" section, and MUST also
mark the corresponding "m=" section in the answer as the answerer
tagged "m=" section. In the answer the answerer BUNDLE-tag indicates
the answerer tagged "m=" section.
If one or more of the criteria are not fulfilled, the answerer MUST
pick the next identification-tag in the identification-tag list in
the offer, and perform the same criteria check for the "m=" section
indicated by that identification-tag. If there are no more
identification-tags in the identification-tag list, the answerer MUST
NOT create the BUNDLE group. Unless the answerer rejects the whole
offer, the answerer MUST apply the answerer procedures for moving an
"m=" section out of a BUNDLE group [Section 7.3.2] or rejecting an
"m=" section within a BUNDLE group [Section 7.3.3] to every bundled
"m=" section in the offer when creating the answer.
[Section 18.1] shows an example of an offerer BUNDLE address:port
selection.
[Section 7.3.4] and [Section 18.1] show an example of an answerer
tagged "m=" section selection.
7.3.2. Moving A Media Description Out Of A BUNDLE Group
When an answerer generates the answer, if the answerer wants to move
a bundled "m=" section out of the negotiated BUNDLE group, the
answerer MUST first check the following criteria:
o In the corresponding offer, the "m=" section is within a
previously negotiated BUNDLE group; and
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o In the corresponding offer, the "m=" section contains an SDP
'bundle-only' attribute.
If either criterium 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 [Section 7.3.3], or keep the "m=" section
within the BUNDLE group in the answer and later create an offer where
the "m=" section is moved out of the BUNDLE group [Section 7.5.2].
NOTE: One consequence of the rules above is that, once a BUNDLE group
has been negotiated, a bundled "m=" section can not be moved out of
the BUNDLE group in an answer. Instead an offer is needed.
When the answerer generates an answer, in which it moves a bundled
"m=" section out of a BUNDLE group, the answerer:
o MUST assign a unique address:port to the "m=" section; and
o MUST include any applicable SDP attribute in the "m=" section,
using the normal offer/answer procedures for the each Attributes;
and
o 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.
o MUST NOT include an SDP 'bundle-only' attribute to the "m="
section; and
Because an answerer is not allowed to move an "m=" section from one
BUNDLE group to another within an answer [Section 7.3], if the
answerer wants to move an "m=" section from one BUNDLE group to
another it MUST first move the "m=" section out of the current BUNDLE
group, and then generate an offer where the "m=" section is added to
another BUNDLE group [Section 7.5.1].
7.3.3. Rejecting a Media Description in a BUNDLE Group
When an answerer wants to reject a bundled "m=" section in an answer,
it MUST first check the following criterion:
o In the corresponding offer, the "m=" section is the offerer tagged
"m=" section.
If the criterium above is fulfilled the answerer can not reject the
"m=" section in the answer. The answerer can either reject the whole
offer, reject each bundled "m=" section within the BUNDLE group, or
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keep the "m=" section within the BUNDLE group in the answer and later
create an offer where the "m=" section is disabled within the BUNDLE
group [Section 7.5.3].
When an answerer generates an answer, in which it rejects a bundled
"m=" section, the answerer:
o MUST assign a zero port value to the "m=" section, according to
the procedures in [RFC3264]; and
o 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
o MUST NOT include an SDP 'bundle-only' attribute in the "m="
section.
7.3.4. Example: SDP Answer
The example below shows an answer, based on the corresponding offer
in [Section 7.2.2]. The answerer accepts both bundled "m=" sections
within the created BUNDLE group. The audio "m=" section is the
answerer tagged "m=" section, indicated by placing the
identification-tag associated with the "m=" section (answerer BUNDLE-
tag) first in the SDP group;BUNDLE attribute identification-id list.
The answerer includes an SDP 'bundle-only' attribute in, and assigns
a zero port value to, the video "m=" section.
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SDP Answer
v=0
o=bob 2808844564 2808844564 IN IP6 2001:db8::1
s=
c=IN IP6 2001:db8::1
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 32
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
7.4. Offerer Processing of the SDP Answer
When an offerer receives an answer, if the answer contains a BUNDLE
group, the offerer MUST check that any bundled "m=" section in the
answer was indicated as bundled in the corresponding offer. If there
is no mismatch, the offerer MUST apply the properties (BUNDLE
address:port, BUNDLE attributes etc) of the offerer tagged "m="
section (selected by the answerer [Section 7.3.1]) to each bundled
"m=" section within the BUNDLE group.
NOTE: As the answerer might reject one or more bundled "m=" sections
in an initial BUNDLE offer, or move a bundled "m=" section out of a
BUNDLE group, a given bundled "m=" section in the offer might not be
indicated as bundled in the corresponding answer.
If the answer does not contain a BUNDLE group, the offerer MUST
process the answer as a normal answer.
7.5. Modifying the Session
When a BUNDLE group has previously been negotiated, and an offerer
generates a subsequent offer, the offerer MUST:
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o Pick one bundled "m=" section as the offerer tagged "m=" section.
The offerer can either pick the "m=" section that was previously
selected by the answerer as the offerer tagged "m=" section, or
pick another bundled "m=" section within the BUNDLE group; and
o Assign a BUNDLE address:port (previously negotiated or newly
suggest) to the offerer tagged "m=" section; and
o Include an SDP 'bundle-only' attribute in, and assign a zero port
value to, every other bundled "m=" section within the BUNDLE
group; and
o Include SDP attributes in the bundled "m=" sections following the
rules in [Section 7.1.3]; and
o Include an SDP 'group:BUNDLE' attribute in the offer; and
o Place the identification-tag of each bundled "m=" section in the
SDP 'group:BUNDLE' attribute identification-tag list. The offerer
BUNDLE-tag indicates the offerer tagged "m=" section.
The offerer MUST NOT pick a given bundled "m=" section as the offerer
tagged "m=" section if:
o The offerer wants to move the "m=" section out of the BUNDLE group
[Section 7.5.2]; or
o The offerer wants to disable the "m=" section [Section 7.5.3].
The offerer can modify the offerer BUNDLE address:port, add and
remove SDP attributes, or modify SDP attribute values, in the
subsequent offer. Changes to the offerer BUNDLE address:port and the
offerer BUNDLE attributes will (if the offer is accepted by the
answerer) be applied to each bundled "m=" section within the BUNDLE
group.
7.5.1. Adding a Media Description to a BUNDLE group
When an offerer generates a subsequent offer, in which it wants to
add a bundled "m=" section to a previously negotiated BUNDLE group,
the offerer follows the procedures in Section 7.5. The offerer
either picks the added "m=" section, or an "m=" section previously
added to the BUNDLE group, as the offerer tagged "m=" section.
NOTE: As described in Section 7.3.2, the answerer can not move the
added "m=" section out of the BUNDLE group in its answer. If the
answer wants to move the "m=" section out of the BUNDLE group, it
will have to first accept it into the BUNDLE group in the answer, and
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then send a subsequent offer where the "m=" section is moved out of
the BUNDLE group [Section 7.5.2].
7.5.2. Moving a Media Description Out of a BUNDLE Group
When an offerer generates a subsequent offer, in which it want to
remove a bundled "m=" section from a BUNDLE group, the offerer:
o MUST assign a unique address:port to the "m=" section; and
o MUST include SDP attributes in the "m=" section following the
normal offer/answer rules for each attribute; and
o 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
o MUST NOT assign an SDP 'bundle-only' attribute to the "m="
section.
For the other bundled "m=" sections within the BUNDLE group, the
offerer follows the procedures in [Section 7.5].
An offerer MUST NOT move an "m=" section from one BUNDLE group to
another within a single offer. If the offerer wants to move an "m="
section from one BUNDLE group to another it MUST first move the
BUNDLE group out of the current BUNDLE group, and then generate a
second offer where the "m=" section is added to another BUNDLE group
[Section 7.5.1].
[Section 18.4] shows an example of an offer for moving an "m="
section out of a BUNDLE group.
7.5.3. Disabling a Media Description in a BUNDLE Group
When an offerer generates a subsequent offer, in which it want to
disable a bundled "m=" section from a BUNDLE group, the offerer:
o MUST assign a zero port value to the "m=" section, following the
procedures in [RFC4566]; and
o 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
o MUST NOT assign an SDP 'bundle-only' attribute to the "m="
section.
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For the other bundled "m=" sections within the BUNDLE group, the
offerer follows the procedures in [Section 7.5].
[Section 18.5] shows an example of an offer and answer for disabling
an "m=" section within a BUNDLE group.
8. Protocol Identification
Each "m=" section within a BUNDLE group MUST use the same transport-
layer protocol. If bundled "m=" sections use different upper-layer
protocols on top of the transport-layer protocol, there MUST exist a
publicly available specification 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.
8.1. STUN, DTLS, SRTP
Section 5.1.2 of [RFC5764] describes a mechanism to identify the
protocol of a received packet among the STUN, DTLS and SRTP protocols
(in any combination). If an offer or answer includes a bundled "m="
section that represents these protocols, the offerer or answerer MUST
support the mechanism described in [RFC5764], and no explicit
negotiation is required in order to indicate support and usage of the
mechanism.
[RFC5764] does not describe how to identify different protocols
transported on DTLS, only how to identify the DTLS protocol itself.
If multiple protocols are transported on DTLS, there MUST exist a
specification describing a mechanism for identifying each individual
protocol. In addition, if a received DTLS packet can be associated
with more than one "m=" section, there MUST exist a specification
which describes a mechanism for associating the received DTLS packets
with the correct "m=" section.
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[Section 9.2] describes how to associate the packets in a received
SRTP stream with the correct "m=" section.
9. RTP Considerations
9.1. Single RTP Session
All RTP-based media within a single BUNDLE group belong to a single
RTP session [RFC3550].
Since a single BUNDLE transport is used for sending and receiving
bundled media, the symmetric RTP mechanism [RFC4961] MUST be used for
RTP-based bundled media.
Since a single RTP session is used for each BUNDLE group, all "m="
sections representing RTP-based media within a BUNDLE group will
share a single SSRC numbering space [RFC3550].
The following rules and restrictions apply for a single RTP session:
o A specific payload type value can be used in multiple bundled "m="
sections only if each codec associated with the payload type
number shares an identical codec configuration [Section 9.1.1].
o The proto value in each bundled RTP-based "m=" section MUST be
identical (e.g., RTP/AVPF).
o The RTP MID header extension MUST be enabled, by including an SDP
'extmap' attribute [RFC8285], with a 'urn:ietf:params:rtp-
hdrext:sdes:mid' URI value, in each bundled RTP-based "m=" section
in every offer and answer.
o A given SSRC MUST NOT transmit RTP packets using payload types
that originate from different bundled "m=" sections.
NOTE: The last bullet above is to avoid sending multiple media types
from the same SSRC. If transmission of multiple media types are done
with time overlap, RTP and RTCP fail to function. Even if done in
proper sequence this causes RTP Timestamp rate switching issues
[RFC7160]. However, once an SSRC has left the RTP session (by
sending an RTCP BYE packet), that SSRC can be reused by another
source (possibly associated with a different bundled "m=" section)
after a delay of 5 RTCP reporting intervals (the delay is to ensure
the SSRC has timed out, in case the RTCP BYE packet was lost
[RFC3550]).
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[RFC7657] defines Differentiated Services (Diffserv) considerations
for RTP-based bundled media sent using a mixture of Diffserv
Codepoints.
9.1.1. Payload Type (PT) Value Reuse
Multiple bundled "m=" sections might describe RTP based media. As
all RTP based media associated with a BUNDLE group belong to the same
RTP session, in order for a given payload type value to be used
inside more than one bundled "m=" section, all codecs associated with
the payload type number MUST share an identical codec configuration.
This means that the codecs MUST share the same media type, encoding
name, clock rate and any parameter that can affect the codec
configuration and packetization.
[I-D.ietf-mmusic-sdp-mux-attributes] lists SDP attributes, whose
attribute values are required to be identical for all codecs that use
the same payload type value.
9.2. Associating RTP/RTCP Streams with the Correct SDP Media
Description
As described in [RFC3550], RTP packets are associated with RTP
streams [RFC7656]. Each RTP stream is identified by an SSRC value,
and each RTP packet includes an SSRC field that is used to associate
the packet with the correct RTP stream. RTCP packets also use SSRCs
to identify which RTP streams the packet relates to. However, a RTCP
packet can contain multiple SSRC fields, in the course of providing
feedback or reports on different RTP streams, and therefore can be
associated with multiple such streams.
In order to be able to process received RTP/RTCP packets correctly,
it MUST be possible to associate an RTP stream with the correct "m="
section, as the "m=" section and SDP attributes associated with the
"m=" section contains information needed to process the packets.
As all RTP streams associated with a BUNDLE group use the same
transport for sending and receiving RTP/RTCP packets, the local
address:port combination part of the transport cannot be used to
associate an RTP stream with the correct "m=" section. In addition,
multiple RTP streams might be associated with the same "m=" section.
An offerer and answerer can inform each other which SSRC values they
will use for an RTP stream by using the SDP 'ssrc' attribute
[RFC5576]. However, an offerer will not know which SSRC values the
answerer will use until the offerer has received the answer providing
that information. Due to this, before the offerer has received the
answer, the offerer will not be able to associate an RTP stream with
the correct "m=" section using the SSRC value associated with the RTP
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stream. In addition, the offerer and answerer may start using new
SSRC values mid-session, without informing each other using the SDP
'ssrc' attribute.
In order for an offerer and answerer to always be able to associate
an RTP stream with the correct "m=" section, the offerer and answerer
using the BUNDLE extension MUST support the mechanism defined in
Section 15, where the offerer and answerer insert the identification-
tag associated with an "m=" section (provided by the remote peer)
into RTP and RTCP packets associated with a BUNDLE group.
When using this mechanism, the mapping from an SSRC to an
identification-tag is carried in RTP header extensions or RTCP SDES
packets, as specified in Section 15. Since a compound RTCP packet
can contain multiple RTCP SDES packets, and each RTCP SDES packet can
contain multiple chunks, a single RTCP packet can contain several
SSRC to identification-tag mappings. The offerer and answerer
maintain tables used for routing that are updated each time an RTP/
RTCP packet contains new information that affects how packets 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:
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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
[RFC7941], 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.
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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
[RFC7941], Section 4.2.6.
Note that if an SDES packet is received as part of a compound RTCP
packet, the SSRC to "m=" section mapping might not exist until the
SDES packet is handled (e.g., in the case where RTCP for a source
is received before any RTP packets). Therefore, it can be
beneficial for an implementation to delay RTCP packet routing,
such that it either prioritizes processing of the SDES item to
generate or update the mapping, or buffers the RTCP information
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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 [RFC3550], 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 [RFC3611], for each report block in the report
whose "SSRC of source" is found in the outgoing SSRC table,
deliver a copy of the XR packet to the "m=" section associated
with that SSRC. In addition, if the sender SSRC for the packet is
found in the incoming SSRC table, deliver a copy of the XR packet
to the "m=" section associated with that SSRC.
If the RTCP packet is a feedback message of type RTPFB or PSFB, as
defined in [RFC4585], it will contain a media source SSRC, and
this SSRC is used for routing certain subtypes of feedback
messages. However, several subtypes of PSFB and RTPFB messages
include 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:
Generic NACK: [RFC4585] (PT=RTPFB, FMT=1).
Picture Loss Indication (PLI): [RFC4585] (PT=PSFB, FMT=1).
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Slice Loss Indication (SLI): [RFC4585] (PT=PSFB, FMT=2).
Reference Picture Selection Indication (RPSI): [RFC4585]
(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:
Full Intra Request (FIR): [RFC5104] (PT=PSFB, FMT=4).
Temporal-Spatial Trade-off Request (TSTR): [RFC5104] (PT=PSFB,
FMT=5).
H.271 Video Back Channel Message (VBCM): [RFC5104] (PT=PSFB,
FMT=7).
Temporary Maximum Media Bit Rate Request (TMMBR): [RFC5104]
(PT=RTPFB, FMT=3).
Layer Refresh Request (LRR): [I-D.ietf-avtext-lrr] (PT=PSFB,
FMT=10).
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:
Temporal-Spatial Trade-off Notification (TSTN): [RFC5104]
(PT=PSFB, FMT=6). This message is a notification in response
to a prior TSTR.
Temporary Maximum Media Bit Rate Notification (TMMBN): [RFC5104]
(PT=RTPFB, FMT=4). This message is a notification in response
to a prior TMMBR, but can also be sent unsolicited.
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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.
9.3. RTP/RTCP Multiplexing
Within a BUNDLE group, the offerer and answerer MUST enable RTP/RTCP
multiplexing [RFC5761] for the RTP-based bundled media (i.e., the
same transport will be used for both RTP packets and RTCP packets).
In addition, the offerer and answerer MUST support the SDP 'rtcp-mux-
only' attribute [I-D.ietf-mmusic-mux-exclusive].
9.3.1. SDP Offer/Answer Procedures
This section describes how an offerer and answerer use the SDP 'rtcp-
mux' attribute [RFC5761] and the SDP 'rtcp-mux-only' attribute
[I-D.ietf-mmusic-mux-exclusive] to negotiate usage of RTP/RTCP
multiplexing for RTP-based bundled media.
RTP/RTCP multiplexing only applies to RTP-based media. However, as
described in Section 7.1.3, within an offer or answer the SDP 'rtcp-
mux' and SDP 'rtcp-mux-only' attributes might be included in a
bundled "m=" section for non-RTP-based media (if such "m=" section is
the offerer tagged "m=" section or answerer tagged "m=" section).
9.3.1.1. Generating the Initial SDP BUNDLE Offer
When an offerer generates an initial BUNDLE offer, if the offer
contains one or more bundled "m=" sections for RTP-based media (or,
if there is a chance that "m=" sections for RTP-based media will
later be added to the BUNDLE group), the offerer MUST include an SDP
'rtcp-mux' attribute [RFC5761] in each bundled "m=" section
(excluding any bundle-only "m=" sections). In addition, the offerer
MAY include an SDP 'rtcp-mux-only' attribute
[I-D.ietf-mmusic-mux-exclusive] in one or more bundled "m=" sections
for RTP-based media.
NOTE: Whether the offerer includes the SDP 'rtcp-mux-only' attribute
depends on whether the offerer supports fallback to usage of a
separate port for RTCP in case the answerer moves one or more "m="
sections for RTP-based media out of the BUNDLE group in the answer.
NOTE: If the offerer includes an SDP 'rtcp-mux' attribute in the
bundled "m=" sections, but does not include an SDP 'rtcp-mux-only'
attribute, the offerer can also include an SDP 'rtcp' attribute
[RFC3605] in one or more RTP-based bundled "m=" sections in order to
provide a fallback port for RTCP, as described in [RFC5761].
However, the fallback port will only be applied to "m=" sections for
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RTP-based media that are moved out of the BUNDLE group by the
answerer.
In the initial BUNDLE offer, the address:port combination for RTCP
MUST be unique in each bundled "m=" section for RTP-based media
(excluding a bundle-only "m=" section), similar to RTP.
9.3.1.2. Generating the SDP Answer
When an answerer generates an answer, if the answerer supports RTP-
based media, and if a bundled "m=" section in the corresponding offer
contained an SDP 'rtcp-mux' attribute, the answerer MUST enable usage
of RTP/RTCP multiplexing, even if there currently are no bundled "m="
sections for RTP-based media within the BUNDLE group. The answerer
MUST include an SDP 'rtcp-mux' attribute in the answerer tagged "m="
section, following the procedures for BUNDLE attributes
[Section 7.1.3]. In addition, if the "m=" section that is selected
as the offerer tagged "m=" section contained an SDP "rtcp-mux-only"
attribute, the answerer MUST include an SDP "rtcp-mux-only" attribute
in the answerer tagged "m=" section.
In an initial BUNDLE offer, if the suggested offerer tagged "m="
section contained an SDP 'rtcp-mux-only' attribute, the "m=" section
was for RTP-based media, and the answerer does not accept the "m="
section in the created BUNDLE group, the answerer MUST either move
the "m=" section out of the BUNDLE group [Section 7.3.2], include the
attribute in the moved "m=" section and enable RTP/RTCP multiplexing
for the media associated with the "m=" section, or reject the "m="
section [Section 7.3.3].
The answerer MUST NOT include an SDP 'rtcp' attribute in any bundled
"m=" section in the answer. The answerer will use the port value of
the tagged offerer "m=" section sending RTP and RTCP packets
associated with RTP-based bundled media towards the offerer.
If the usage of RTP/RTCP multiplexing within a BUNDLE group has been
negotiated in a previous offer/answer exchange, the answerer MUST
include an SDP 'rtcp-mux' attribute in the answerer tagged "m="
section . It is not possible to disable RTP/RTCP multiplexing within
a BUNDLE group.
9.3.1.3. Offerer Processing of the SDP Answer
When an offerer receives an answer, if the answerer has accepted the
usage of RTP/RTCP multiplexing [Section 9.3.1.2], the answerer
follows the procedures for RTP/RTCP multiplexing defined in
[RFC5761]. The offerer will use the port value of the answerer
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tagged "m=" section for sending RTP and RTCP packets associated with
RTP-based bundled media towards the answerer.
NOTE: It is considered a protocol error if the answerer has not
accepted the usage of RTP/RTCP multiplexing for RTP-based "m="
sections that the answerer included in the BUNDLE group.
9.3.1.4. Modifying the Session
When an offerer generates a subsequent offer, the offerer MUST
include an SDP 'rtcp-mux' attribute in the offerer tagged "m="
section, following the procedures for IDENTICAL multiplexing category
attributes in Section 7.1.3.
10. ICE Considerations
This section describes how to use the BUNDLE grouping extension
together with the Interactive Connectivity Establishment (ICE)
mechanism [I-D.ietf-ice-rfc5245bis].
The generic procedures for negotiating usage of ICE using SDP,
defined in [I-D.ietf-mmusic-ice-sip-sdp], also apply to usage of ICE
with BUNDLE, with the following exceptions:
o 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 bundled "m=" section within
the BUNDLE group.
o The generic SDP attribute offer/answer considerations
[Section 7.1.3] also apply to ICE-related attributes. Therefore,
when an offer sends an initial BUNDLE offer (in order to negotiate
a BUNDLE group) the offerer include ICE-related media-level
attributes in each bundled "m=" section (excluding any bundle-only
"m=" section), and each "m=" section MUST contain unique ICE
properties. When an answerer generates an answer (initial BUNDLE
answer or subsequent) that contains a BUNDLE group, and when an
offerer sends a subsequent offer that contains a BUNDLE group,
ICE-related media-level attributes are only included in the tagged
"m=" section (suggested offerer tagged "m=" section or answerer
tagged "m=" section), and the ICE properties are applied to each
bundled "m=" section within the BUNDLE group.
NOTE: Most ICE-related media-level SDP attributes belong to the
TRANSPORT multiplexing category [I-D.ietf-mmusic-sdp-mux-attributes],
and the generic SDP attribute offer/answer considerations for
TRANSPORT multiplexing category apply to the attributes. However, in
the case of ICE-related attributes, the same considerations also
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apply to ICE-related media-level attributes that belong to other
multiplexing categories.
NOTE: The following ICE-related media-level SDP attributes are
defined in [I-D.ietf-mmusic-ice-sip-sdp]: 'candidate', 'remote-
candidates', 'ice-mismatch', 'ice-ufrag', 'ice-pwd', and 'ice-
pacing'.
Initially, before ICE has produced selected candidate pairs that will
be used for media, there might be multiple transports established (if
multiple candidate pairs are tested). Once ICE has selected
candidate pairs, they form the BUNDLE transport.
Support and usage of ICE mechanism together with the BUNDLE extension
is OPTIONAL, and the procedures in this section only apply when the
ICE mechanism is used. Note that applications might mandate usage of
the ICE mechanism even if the BUNDLE extension is not used.
NOTE: If the trickle ICE mechanism [I-D.ietf-mmusic-trickle-ice-sip]
is used, an offerer and answerer might assign a port value of '9',
and an IPv4 address of '0.0.0.0' (or, the IPv6 equivalent '::') to
multiple bundled "m=" sections in the initial BUNDLE offer. The
offerer and answerer will follow the normal procedures for generating
the offers and answers, including picking a bundled "m=" section as
the suggested offerer tagged "m=" section, selecting the tagged "m="
sections etc. The only difference is that media can not be sent
until one or more candidates have been provided. Once a BUNDLE group
has been negotiated, trickled candidates associated with a bundled
"m=" section will be applied to all bundled "m=" sections within the
BUNDLE group.
11. DTLS Considerations
One or more media streams within a BUNDLE group might use the
Datagram Transport Layer Security (DTLS) protocol [RFC6347] in order
to encrypt the data, or 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:
o There can only be one DTLS association [RFC6347] associated with
the BUNDLE group; and
o 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
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o 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
o If the DTLS client supports DTLS-SRTP [RFC5764] it MUST include
the 'use_srtp' extension [RFC5764] in the DTLS ClientHello message
[RFC5764]. The client MUST include the extension even if the
usage of DTLS-SRTP is not negotiated as part of the multimedia
session (e.g., SIP session [RFC3261]).
NOTE: The inclusion of the 'use_srtp' extension during the initial
DTLS handshake ensures that a DTLS renegotiation will not be required
in order to include the extension, in case DTLS-SRTP encrypted media
is added to the BUNDLE group later during the multimedia session.
12. RTP Header Extensions Consideration
When [RFC8285] RTP header extensions are used in the context of this
specification, the identifier used for a given extension MUST
identify the same extension across all the bundled media
descriptions.
13. Update to RFC 3264
This section updates RFC 3264, in order to allow extensions to define
the usage of a zero port value in offers and answers for other
purposes than removing or disabling media streams. The following
sections of RFC 3264 are updated:
o Section 5.1 (Unicast Streams).
o Section 8.4 (Putting a Unicast Media Stream on Hold).
13.1. Original text of section 5.1 (2nd paragraph) 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
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(Section 6). Furthermore, existing streams can be terminated by
setting the port to zero (Section 8). In general, a port number of
zero indicates that the media stream is not wanted.
13.2. New text replacing section 5.1 (2nd paragraph) 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 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.
13.3. Original text of section 8.4 (6th paragraph) of RFC 3264
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.
13.4. New text replacing section 8.4 (6th paragraph) of RFC 3264
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
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semantics of the zero port number usage. An agent MUST be capable of
receiving SDP with a connection address of 0.0.0.0, in which case it
means that neither RTP nor RTCP is to be sent to the peer.
14. Update to RFC 5888
This section updates RFC 5888 [RFC5888]), in order to allow
extensions to allow an SDP 'group' attribute containing an
identification-tag that identifies a "m=" section with the port set
to zero Section 9.2 (Group Value in Answers) of RFC 5888 is updated.
14.1. Original text of section 9.2 (3rd paragraph) of RFC 5888
SIP entities refuse media streams by setting the port to zero in the
corresponding "m" line. "a=group" lines MUST NOT contain
identification-tags that correspond to "m" lines with the port set to
zero.
14.2. New text replacing section 9.2 (3rd paragraph) of RFC 5888
SIP entities refuse media streams by setting the port to zero in the
corresponding "m" line. "a=group" lines MUST NOT contain
identification-tags that correspond to "m" lines with the port set to
zero, but an extension mechanism might specify different semantics
for including identification-tags that correspond to such "m=" lines.
15. RTP/RTCP extensions for identification-tag transport
SDP Offerers and Answerers [RFC3264] can associate identification-
tags with "m=" sections within SDP Offers and Answers, using the
procedures in [RFC5888]. Each identification-tag uniquely represents
an "m=" section.
This section defines a new RTCP SDES item [RFC3550], 'MID', which is
used to carry identification-tags within RTCP SDES packets. This
section also defines a new RTP SDES header extension [RFC7941], which
is used to carry the 'MID' RTCP SDES item in RTP packets.
The SDES item and RTP SDES header extension make it possible for a
receiver to associate each RTP stream with 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'
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attribute [RFC5888]. The media sender then inserts the
identification-tag in RTCP and RTP packets sent to the media
recipient.
NOTE: This text above defines how identification-tags are carried in
SDP Offers and Answers. The usage of other signaling protocols for
carrying identification-tags is not prevented, but the usage of such
protocols is outside the scope of this document.
[RFC3550] defines general procedures regarding the RTCP transmission
interval. The RTCP MID SDES item SHOULD be sent in the first few
RTCP packets 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.
15.1. RTCP MID SDES Item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MID=TBD | length | identification-tag ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The identification-tag payload is UTF-8 encoded [RFC3629], as in SDP.
The identification-tag is not zero terminated.
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.]
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15.2. RTP SDES Header Extension For MID
The payload, containing the identification-tag, of the RTP SDES
header extension element can be encoded using either the one-byte or
two-byte header [RFC7941]. The identification-tag payload is UTF-8
encoded, as in SDP.
The identification-tag is not zero terminated. Note, that the set of
header extensions included in the packet needs to be padded to the
next 32-bit boundary using zero bytes [RFC8285].
As the identification-tag is included in either an RTCP SDES item or
an RTP SDES header extension, or both, there 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.
16. IANA Considerations
16.1. New SDES item
[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
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16.2. New RTP SDES Header Extension URI
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
This document defines a new extension URI in the RTP SDES Compact
Header Extensions sub-registry of the RTP Compact Header Extensions
registry sub-registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:sdes:mid
Description: Media identification
Contact: IESG (iesg@ietf.org)
Reference: RFCXXXX
The SDES item does not reveal privacy information about the users.
It is simply used to associate RTP-based media with the correct SDP
media description ("m=" section) in the SDP used to negotiate the
media.
The purpose of the extension is for the offerer to be able to
associate received multiplexed RTP-based media before the offerer
receives the associated SDP answer.
16.3. New SDP Attribute
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
This document defines a new SDP media-level attribute, 'bundle-only',
according to the following data:
Attribute name: bundle-only
Type of attribute: media
Subject to charset: No
Purpose: Request a media description to be accepted
in the answer only if kept within a BUNDLE
group by the answerer.
Appropriate values: N/A
Contact name: IESG
Contact e-mail: iesg@ietf.org
Reference: RFCXXXX
Mux category: NORMAL
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16.4. New SDP Group Semantics
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
This document registers the following semantics with IANA in the
"Semantics for the "group" SDP Attribute" subregistry (under the
"Session Description Protocol (SDP) Parameters" registry:
Semantics Token Reference
------------------------------------- ------ ---------
Media bundling BUNDLE [RFCXXXX]
Mux category: NORMAL
17. Security Considerations
The security considerations defined in [RFC3264] and [RFC5888] apply
to the BUNDLE extension. Bundle does not change which information,
e.g., RTP streams, flows over the network, with the exception of the
usage of the MID SDES item as discussed below. Primarily it changes
which addresses and ports, and thus in which (RTP) sessions the
information 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
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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 [I-D.ietf-mmusic-sdp-mux-attributes].
The security considerations of "RTP Header Extension for the RTP
Control Protocol (RTCP) Source Description Items" [RFC7941] requires
that when RTCP is confidentiality protected, then any SDES RTP header
extension carrying an SDES item, such as the MID RTP header
extension, is also protected using commensurate strength algorithms.
However, assuming the above requirements and recommendations are
followed, there are no known significant security risks with leaving
the MID RTP header extension without confidentiality protection.
Therefore, this specification updates RFC 7941 by adding the
exception that this requirement MAY be ignored for the MID RTP header
extension. Security mechanisms for RTP/RTCP are discussed in Options
for Securing RTP Sessions [RFC7201], for example SRTP [RFC3711] can
provide the necessary security functions of ensuring the integrity
and source authenticity.
18. Examples
18.1. Example: Tagged m= Section Selections
The example below shows:
o An initial BUNDLE offer, in which the offerer wants to negotiate a
BUNDLE group, and indicates the audio m= section as the suggested
offerer tagged "m=" section.
o An initial BUNDLE answer, in which the answerer accepts the
creation of the BUNDLE group, selects the audio m= section in the
offer as the offerer tagged "m=" section, selects the audio "m="
section in the answer as the answerer tagged "m=" section and
assigns the answerer BUNDLE address:port to that "m=" section.
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SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP6 2001:db8::3
s=
c=IN IP6 2001:db8::3
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP6 2001:db8::1
s=
c=IN IP6 2001:db8::1
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 32
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:32 MPV/90000
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a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
18.2. Example: BUNDLE Group Rejected
The example below shows:
o An initial BUNDLE offer, in which the offerer wants to negotiate a
BUNDLE group, and indicates the audio m= section as the suggested
offerer tagged "m=" section.
o An initial BUNDLE answer, in which the answerer rejects the
creation of the BUNDLE group, generates a normal answer and
assigns a unique address:port to each "m=" section in the answer.
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SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP6 2001:db8::3
s=
c=IN IP6 2001:db8::3
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP6 2001:db8::1
s=
c=IN IP6 2001:db8::1
t=0 0
m=audio 20000 RTP/AVP 0
b=AS:200
a=rtcp-mux
a=rtpmap:0 PCMU/8000
m=video 30000 RTP/AVP 32
b=AS:1000
a=rtcp-mux
a=rtpmap:32 MPV/90000
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18.3. Example: Offerer Adds a Media Description to a BUNDLE Group
The example below shows:
o A subsequent offer, in which the offerer adds a new bundled "m="
section (video), indicated by the "zen" identification-tag, to a
previously negotiated BUNDLE group, indicates the new "m=" section
as the offerer tagged "m=" section and assigns the offerer BUNDLE
address:port to that "m=" section.
o A subsequent answer, in which the answerer indicates the new video
"m=" section in the answer as the answerer tagged "m=" section and
assigns the answerer BUNDLE address:port to that "m=" section.
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP6 2001:db8::3
s=
c=IN IP6 2001:db8::3
t=0 0
a=group:BUNDLE zen foo bar
m=audio 0 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=bundle-only
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtcp-mux
a=rtpmap:66 H261/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
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SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP6 2001:db8::1
s=
c=IN IP6 2001:db8::1
t=0 0
a=group:BUNDLE zen foo bar
m=audio 0 RTP/AVP 0
b=AS:200
a=mid:foo
a=bundle-only
a=rtpmap:0 PCMU/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 32
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 20000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtcp-mux
a=rtpmap:66 H261/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
18.4. Example: Offerer Moves a Media Description Out of a BUNDLE Group
The example below shows:
o A subsequent offer, in which the offerer removes a "m=" section
(video), indicated by the "zen" identification-tag, from a
previously negotiated BUNDLE group, indicates one of the bundled
"m=" sections (audio) remaining in the BUNDLE group as the offerer
tagged "m=" section and assigns the offerer BUNDLE address:port to
that "m=" section.
o A subsequent answer, in which the answerer removes the "m="
section from the BUNDLE group, indicates the audio "m=" section in
the answer as the answerer tagged "m=" section and assigns the
answerer BUNDLE address:port to that "m=" section.
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SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP6 2001:db8::3
s=
c=IN IP6 2001:db8::3
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 50000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtcp-mux
a=rtpmap:66 H261/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP6 2001:db8::1
s=
c=IN IP6 2001:db8::1
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
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m=video 0 RTP/AVP 32
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 60000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtcp-mux
a=rtpmap:66 H261/90000
18.5. Example: Offerer Disables a Media Description Within a BUNDLE
Group
The example below shows:
o A subsequent offer, in which the offerer disables (by assigning a
zero port value) a "m=" section (video), indicated by the "zen"
identification-tag, from a previously negotiated BUNDLE group,
indicates one of the bundled "m=" sections (audio) remaining
active in the BUNDLE group as the offerer tagged "m=" section and
assigns the offerer BUNDLE address:port to that "m=" section.
o A subsequent answer, in which the answerer disables the "m="
section, indicates the audio "m=" section in the answer as the
answerer tagged "m=" section and assigns the answerer BUNDLE
address:port to that "m=" section.
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP6 2001:db8::3
s=
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
c=IN IP6 2001:db8::3
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
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a=rtpmap:97 iLBC/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 31 32
c=IN IP6 2001:db8::3
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 66
a=mid:zen
a=rtpmap:66 H261/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP6 2001:db8::1
s=
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
c=IN IP6 2001:db8::1
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 32
c=IN IP6 2001:db8::1
b=AS:1000
a=mid:bar
a=bundle-only
a=rtpmap:32 MPV/90000
a=extmap:1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 66
a=mid:zen
a=rtpmap:66 H261/90000
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19. Acknowledgements
The usage of the SDP grouping extension for negotiating bundled media
is based on similar alternatives proposed by Harald Alvestrand and
Cullen Jennings. The BUNDLE extension described in this document is
based on the different alternative proposals, and text (e.g., SDP
examples) 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 Charlie Kaufman for performing the Sec-Dir review.
Thanks to Linda Dunbar for performing the Gen-ART review.
Thanks to Spotify for providing music for the countless hours of
document editing.
20. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-51
o Changes based on IESG reviews.
o - Clarification of 'initial offer' terminology.
o - Merging of tagged m- section selection sections.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-50
o Changes based on IESG reviews.
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o - Adding of tagged m- section concept.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-49
o Changes based on IESG reviews.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-48
o Changes based on Sec-Dir review by Charlie Kaufman.
o - s/unique address/unique address:port
o Changes based on Gen-ART review by Linda Dunbar.
o Mux category for group:BUNDLE attribute added.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-47
o Changes based on AD review by Ben Campbell.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-46
o Pre-RFC5378 disclaimer removed put back.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-45
o Mux category for SDP 'group:BUNDLE' attribute added.
o https://github.com/cdh4u/draft-sdp-bundle/pull/54
o Pre-RFC5378 disclaimer removed.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-44
o Minor editorial nits based on pull request by Colin P.
o https://github.com/cdh4u/draft-sdp-bundle/pull/53
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-43
o Changes based on WG chairs review.
o Text added in order to close GitHub issues by Taylor B.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-42
o Changes based on final WG review.
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Changes from draft-ietf-mmusic-sdp-bundle-negotiation-41
o Update to section 6 o RFC 3264:
o https://github.com/cdh4u/draft-sdp-bundle/pull/47
o Editorial clarification on BUNDLE address selection:
o https://github.com/cdh4u/draft-sdp-bundle/pull/46
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-40
o Editorial changes and technical restrictions in order to make the
specification more understandable:
o https://github.com/cdh4u/draft-sdp-bundle/pull/45
o - BUNDLE address is only assigned to m- section indicated by
BUNDLE-tag.
o - bundle-only attribute also used in answers and subsequent
offers.
o - Answerer cannot reject, or remove, the bundled m- section that
contains the BUNDLE address.
o - ICE Offer/Answer sections removed, due to duplicated
information.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-39
o Editorial terminology changes.
o RFC 5285 reference replaced by reference to RFC 8285.
o https://github.com/cdh4u/draft-sdp-bundle/pull/44
o - Clarify that an m- section can not be moved between BUNDLE
groups without first moving the m- section out of a BUNDLE group.
o https://github.com/cdh4u/draft-sdp-bundle/pull/41
o - Addition of BUNDLE transport concept.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-38
o Changes to RTP streaming mapping section based on text from Colin
Perkins.
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o The following GitHub pull requests were merged:
o https://github.com/cdh4u/draft-sdp-bundle/pull/34
o - Proposed updates to RTP processing
o https://github.com/cdh4u/draft-sdp-bundle/pull/35
o - fixed reference to receiver-id section
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-37
o The following GitHub pull request was merged:
o https://github.com/cdh4u/draft-sdp-bundle/pull/33
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-36
o The following GitHub pull requests were merged:
o https://github.com/cdh4u/draft-sdp-bundle/pull/32
o - extmap handling in BUNDLE.
o https://github.com/cdh4u/draft-sdp-bundle/pull/31
o - Additional Acknowledgement text added.
o https://github.com/cdh4u/draft-sdp-bundle/pull/30
o - MID SDES item security procedures updated
o https://github.com/cdh4u/draft-sdp-bundle/pull/29
o - Appendix B of JSEP moved into BUNDLE.
o - Associating RTP/RTCP packets with SDP m- lines.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-35
o Editorial changes on RTP streaming mapping section based on
comments from Colin Perkins.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-34
o RTP streams, instead of RTP packets, are associated with m- lines.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-33
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o Editorial changes based on comments from Eric Rescorla and Cullen
Jennings:
o - Changes regarding usage of RTP/RTCP multiplexing attributes.
o - Additional text regarding associating RTP/RTCP packets with SDP
m- lines.
o - Reference correction.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-32
o Editorial changes based on comments from Eric Rescorla and Cullen
Jennings:
o - Justification for mechanism added to Introduction.
o - 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
o Editorial changes based on GitHub Pull requests by Martin Thomson:
o - https://github.com/cdh4u/draft-sdp-bundle/pull/2
o - https://github.com/cdh4u/draft-sdp-bundle/pull/1
o Editorial change based on comment from Diederick Huijbers (9th
July 2016).
o Changes based on comments from Flemming Andreasen (21st June
2016):
o - Mux category for SDP bundle-only attribute added.
o - Mux category considerations editorial clarification.
o - Editorial changes.
o RTP SDES extension according to draft-ietf-avtext-sdes-hdr-ext.
o Note whether Design Considerations appendix is to be kept removed:
o - Appendix is kept within document.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-30
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o Indicating in the Abstract and Introduction that the document
updates RFC 3264.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-29
o Change based on WGLC comment from Colin Perkins.
o - Clarify that SSRC can be reused by another source after a delay
of 5 RTCP reporting intervals.
o Change based on WGLC comment from Alissa Cooper.
o - IANA registry name fix.
o - Additional IANA registration information added.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-28
o - Alignment with exclusive mux procedures.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-27
o - Yet another terminology change.
o - Mux category considerations added.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-26
o - ICE considerations modified: ICE-related SDP attributes only
added to the bundled m- line representing the selected BUNDLE
address.
o - Reference to draft-ietf-mmusic-ice-sip-sdp added.
o - Reference to RFC 5245 replaced with reference to draft-ietf-ice-
rfc5245bis.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-25
o - RTP/RTCP mux procedures updated with exclusive RTP/RTCP mux
considerations.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-24
o - Reference and procedures associated with exclusive RTP/RTCP mux
added
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-23
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o - RTCP-MUX mandatory for bundled RTP m- lines
o - Editorial fixes based on comments from Flemming Andreasen
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-22
o - Correction of Ari's family name
o - Editorial fixes based on comments from Thomas Stach
o - RTP/RTCP correction based on comment from Magnus Westerlund
o -- http://www.ietf.org/mail-archive/web/mmusic/current/
msg14861.html
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-21
o - Correct based on comment from Paul Kyzivat
o -- 'received packets' replaced with 'received data'
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-20
o - Clarification based on comment from James Guballa
o - Clarification based on comment from Flemming Andreasen
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-19
o - DTLS Considerations section added.
o - BUNDLE semantics added to the IANA Considerations
o - Changes based on WGLC comments from Adam Roach
o -- http://www.ietf.org/mail-archive/web/mmusic/current/
msg14673.html
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-18
o - Changes based on agreements at IETF#92
o -- BAS Offer removed, based on agreement at IETF#92.
o -- 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
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o - Editorial changes based on comments from Magnus Westerlund.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-16
o - Modification of RTP/RTCP multiplexing section, based on comments
from Magnus Westerlund.
o - Reference updates.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-15
o - Editorial fix.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-14
o - Editorial changes.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-13
o Changes to allow a newly suggested offerer BUNDLE address to be
assigned to each bundled m- line.
o Changes based on WGLC comments from Paul Kyzivat
o - Editorial fixes
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-12
o Usage of SDP 'extmap' attribute added
o SDP 'bundle-only' attribute scoped with "m=" lines with a zero
port value
o Changes based on WGLC comments from Thomas Stach
o - ICE candidates not assigned to bundle-only m- lines with a zero
port value
o - Editorial changes
o Changes based on WGLC comments from Colin Perkins
o - Editorial changes:
o -- "RTP SDES item" -> "RTCP SDES item"
o -- "RTP MID SDES item" -> "RTCP MID SDES item"
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o - Changes in section 10.1.1:
o -- "SHOULD NOT" -> "MUST NOT"
o -- Additional text added to the Note
o - Change to section 13.2:
o -- Clarify that mid value is not zero terminated
o - Change to section 13.3:
o -- Clarify that mid value is not zero terminated
o -- Clarify padding
o Changes based on WGLC comments from Paul Kyzivat
o - Editorial changes:
o Changes based on WGLC comments from Jonathan Lennox
o - Editorial changes:
o - Defintion of SDP bundle-only attribute alligned with structure
in 4566bis draft
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-11
o Editorial corrections based on comments from Harald Alvestrand.
o Editorial corrections based on comments from Cullen Jennings.
o Reference update (RFC 7160).
o Clarification about RTCP packet sending when RTP/RTCP multiplexing
is not used (http://www.ietf.org/mail-archive/web/mmusic/current/
msg13765.html).
o Additional text added to the Security Considerations.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-10
o SDP bundle-only attribute added to IANA Considerations.
o SDES item and RTP header extension added to Abstract and
Introduction.
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o Modification to text updating section 8.2 of RFC 3264.
o Reference corrections.
o Editorial corrections.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-09
o Terminology change: "bundle-only attribute assigned to m= line" to
"bundle-only attribute associated with m= line".
o Editorial corrections.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-08
o Editorial corrections.
o - "of"->"if" (8.3.2.5).
o - "optional"->"OPTIONAL" (9.1).
o - Syntax/ABNF for 'bundle-only' attribute added.
o - SDP Offer/Answer sections merged.
o - 'Request new offerer BUNDLE address' section added
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-07
o OPEN ISSUE regarding Receiver-ID closed.
o - RTP MID SDES Item.
o - RTP MID Header Extension.
o OPEN ISSUE regarding insertion of SDP 'rtcp' attribute in answers
closed.
o - 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
o Draft title changed.
o Added "SDP" to section names containing "Offer" or "Answer".
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o Editorial fixes based on comments from Paul Kyzivat
(http://www.ietf.org/mail-archive/web/mmusic/current/
msg13314.html).
o Editorial fixed based on comments from Colin Perkins
(http://www.ietf.org/mail-archive/web/mmusic/current/
msg13318.html).
o - Removed text about extending BUNDLE to allow multiple RTP
sessions within a BUNDLE group.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-05
o Major re-structure of SDP Offer/Answer sections, to align with RFC
3264 structure.
o Additional definitions added.
o - Shared address.
o - Bundled "m=" line.
o - Bundle-only "m=" line.
o - Offerer suggested BUNDLE mid.
o - Answerer selected BUNDLE mid.
o 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.
o 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.
o SDP 'bundle-only' attribute section added.
o Connection data nettype/addrtype restrictions added.
o RFC 3264 update section added.
o 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
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o Updated Offerer procedures (http://www.ietf.org/mail-
archive/web/mmusic/current/msg12293.html).
o Updated Answerer procedures (http://www.ietf.org/mail-
archive/web/mmusic/current/msg12333.html).
o Usage of SDP 'bundle-only' attribute added.
o Reference to Trickle ICE document added.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-02
o 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.
o Cullen Jennings added as co-author.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-01
o No changes. New version due to expiration.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-00
o No changes. New version due to expiration.
Changes from draft-holmberg-mmusic-sdp-multiplex-negotiation-00
o Draft name changed.
o Harald Alvestrand added as co-author.
o "Multiplex" terminology changed to "bundle".
o Added text about single versus multiple RTP Sessions.
o Added reference to RFC 3550.
21. References
21.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
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[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002, <https://www.rfc-
editor.org/info/rfc3264>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003, <https://www.rfc-
editor.org/info/rfc3605>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
BCP 131, RFC 4961, DOI 10.17487/RFC4961, July 2007,
<https://www.rfc-editor.org/info/rfc4961>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010, <https://www.rfc-
editor.org/info/rfc5761>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010, <https://www.rfc-
editor.org/info/rfc5764>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010, <https://www.rfc-
editor.org/info/rfc5888>.
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[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7941] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP
Header Extension for the RTP Control Protocol (RTCP)
Source Description Items", RFC 7941, DOI 10.17487/RFC7941,
August 2016, <https://www.rfc-editor.org/info/rfc7941>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General
Mechanism for RTP Header Extensions", RFC 8285,
DOI 10.17487/RFC8285, October 2017, <https://www.rfc-
editor.org/info/rfc8285>.
[I-D.ietf-ice-rfc5245bis]
Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", draft-ietf-ice-
rfc5245bis-20 (work in progress), March 2018.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
(work in progress), December 2016.
[I-D.ietf-mmusic-mux-exclusive]
Holmberg, C., "Indicating Exclusive Support of RTP/RTCP
Multiplexing using SDP", draft-ietf-mmusic-mux-
exclusive-12 (work in progress), May 2017.
[I-D.ietf-mmusic-ice-sip-sdp]
Petit-Huguenin, M., Nandakumar, S., and A. Keranen,
"Session Description Protocol (SDP) Offer/Answer
procedures for Interactive Connectivity Establishment
(ICE)", draft-ietf-mmusic-ice-sip-sdp-20 (work in
progress), April 2018.
[I-D.ietf-mmusic-trickle-ice-sip]
Ivov, E., Stach, T., Marocco, E., and C. Holmberg, "A
Session Initiation Protocol (SIP) Usage for Trickle ICE",
draft-ietf-mmusic-trickle-ice-sip-14 (work in progress),
February 2018.
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21.2. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002, <https://www.rfc-
editor.org/info/rfc3261>.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<https://www.rfc-editor.org/info/rfc3611>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <https://www.rfc-editor.org/info/rfc5104>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006, <https://www.rfc-
editor.org/info/rfc4585>.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
<https://www.rfc-editor.org/info/rfc5576>.
[RFC7160] Petit-Huguenin, M. and G. Zorn, Ed., "Support for Multiple
Clock Rates in an RTP Session", RFC 7160,
DOI 10.17487/RFC7160, April 2014, <https://www.rfc-
editor.org/info/rfc7160>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<https://www.rfc-editor.org/info/rfc7201>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015, <https://www.rfc-
editor.org/info/rfc7656>.
[RFC7657] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015, <https://www.rfc-
editor.org/info/rfc7657>.
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[I-D.ietf-ice-trickle]
Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
"Trickle ICE: Incremental Provisioning of Candidates for
the Interactive Connectivity Establishment (ICE)
Protocol", draft-ietf-ice-trickle-21 (work in progress),
April 2018.
[I-D.ietf-avtext-lrr]
Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
Message", draft-ietf-avtext-lrr-07 (work in progress),
July 2017.
Appendix A. Design Considerations
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:
o 1) Interoperability with existing UAs.
o 2) Interoperability with intermediary Back to Back User Agent
(B2BUA) and proxy entities.
o 3) Time to gather, and the number of, ICE candidates.
o 4) Different error scenarios, and when they occur.
o 5) SDP Offer/Answer impacts, including usage of port number value
zero.
A.1. UA Interoperability
Consider the following SDP Offer/Answer exchange, where Alice sends
an SDP Offer to Bob:
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SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
m=audio 10000 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 10002 RTP/AVP 97
a=rtpmap:97 H261/90000
SDP Answer
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
m=audio 20000 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 20002 RTP/AVP 97
a=rtpmap:97 H261/90000
RFC 4961 specifies a way of doing symmetric RTP but that is a later
extension to RTP and Bob 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:
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SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
m=audio 10000 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 98
a=rtpmap:98 H261/90000
will result in the second "m=" section being considered an SDP error
because it has the same port as the first line.
A.2. Usage of Port Number Value Zero
In an 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.
A.3. B2BUA And Proxy Interoperability
Some back to back user agents may be configured in a mode where if
the incoming call leg contains an SDP attribute the B2BUA does not
understand, the B2BUA still generates that SDP attribute in the Offer
for the outgoing call leg. Consider a B2BUA that did not understand
the SDP "rtcp" attribute, defined in RFC 3605, yet acted this way.
Further assume that the B2BUA was configured to tear down any call
where it did not see any RTCP for 5 minutes. In this case, if the
B2BUA received an Offer like:
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SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=rtcp:53020
It would be looking for RTCP on port 49171 but would not see any
because the RTCP would be on port 53020 and after five minutes, it
would tear down the call. Similarly, a B2BUA that did not understand
BUNDLE yet put 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.
A.3.1. Traffic Policing
Sometimes intermediaries do not act as B2BUAs, in the sense that they
don't modify SDP bodies, nor do they terminate SIP dialogs. 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.
A.3.2. Bandwidth Allocation
Sometimes intermediaries do not act as B2BUAs, in the sense that they
don't modify SDP bodies, nor do they terminate SIP dialogs. 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.
A.4. Candidate Gathering
When using ICE, a candidate needs to be gathered for each port. This
takes approximately 20 ms extra for each extra "m=" section due to
the NAT pacing requirements. All of this gathering can be overlapped
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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
[I-D.ietf-ice-trickle] to get the non host ICE candidates for the
rest of the "m=" sections, it MAY do that and will not need any
additional gathering time.
Some people have suggested a TURN extension to get a bunch of TURN
allocations at once. This would only provide a single STUN result so
in cases where the other end did not support BUNDLE, 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.
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Harald Tveit Alvestrand
Google
Kungsbron 2
Stockholm 11122
Sweden
Email: harald@alvestrand.no
Cullen Jennings
Cisco
400 3rd Avenue SW, Suite 350
Calgary, AB T2P 4H2
Canada
Email: fluffy@iii.ca
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