MMUSIC Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 3264 (if approved) H. Alvestrand
Intended status: Standards Track Google
Expires: June 11, 2015 C. Jennings
Cisco
December 8, 2014
Negotiating Media Multiplexing Using the Session Description Protocol
(SDP)
draft-ietf-mmusic-sdp-bundle-negotiation-13.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 address:port combination (BUNDLE address) for receiving media,
referred to as bundled media, associated with multiple SDP media
descriptions ("m=" lines).
To assist endpoints in negotiating the use of bundle this
specification defines a new SDP attribute, 'bundle-only', which can
be used to request that specific media is only used if bundled. This
specification also updates sections 5.1, 8.1 and 8.2 of RFC 3264 to
allow an answerer to assign a non-zero port value to an "m=" line in
an SDP answer, even if the "m=" line in the associated SDP offer
contained a zero port value.
There are multiple ways to correlate the bundled RTP packets with the
appropriate media descriptions. This specification defines a new
RTCP source description (SDES) item and a new RTP header extension
that provides an additional way to do this correlation by using them
to carry a value that associates the RTP/RTCP packets with a specific
media description.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 11, 2015.
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 7
5. SDP Grouping Framework BUNDLE Extension . . . . . . . . . . . 7
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. SDP 'bundle-only' Attribute . . . . . . . . . . . . . . . . . 7
6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. bundle-only . . . . . . . . . . . . . . . . . . . . . . . 8
7. SDP Information Considerations . . . . . . . . . . . . . . . 8
7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.2. Connection Data (c=) . . . . . . . . . . . . . . . . . . 9
7.3. Bandwidth (b=) . . . . . . . . . . . . . . . . . . . . . 9
7.4. Attributes (a=) . . . . . . . . . . . . . . . . . . . . . 9
8. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 9
8.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.2. Generating the Initial SDP Offer . . . . . . . . . . . . 10
8.2.1. General . . . . . . . . . . . . . . . . . . . . . . . 10
8.2.2. Suggesting the offerer BUNDLE address . . . . . . . . 11
8.3. Generating the SDP Answer . . . . . . . . . . . . . . . . 11
8.3.1. General . . . . . . . . . . . . . . . . . . . . . . . 11
8.3.2. Answerer Selection of Offerer Bundle Address . . . . 12
8.3.3. Answerer Selection of Answerer BUNDLE Address . . . . 13
8.3.4. Moving A Media Description Out Of A BUNDLE Group . . 13
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8.3.5. Rejecting A Media Description In A BUNDLE Group . . . 13
8.4. Offerer Processing of the SDP Answer . . . . . . . . . . 14
8.4.1. General . . . . . . . . . . . . . . . . . . . . . . . 14
8.4.2. Bundle Address Synchronization (BAS) . . . . . . . . 14
8.5. Modifying the Session . . . . . . . . . . . . . . . . . . 15
8.5.1. General . . . . . . . . . . . . . . . . . . . . . . . 15
8.5.2. Request a new offerer BUNDLE address . . . . . . . . 15
8.5.3. Adding a media description to a BUNDLE group . . . . 16
8.5.4. Moving A Media Description Out Of A BUNDLE Group . . 16
8.5.5. Disabling A Media Description In A BUNDLE Group . . . 17
9. Protocol Identification . . . . . . . . . . . . . . . . . . . 17
9.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.2. STUN, DTLS, SRTP . . . . . . . . . . . . . . . . . . . . 17
10. RTP Considerations . . . . . . . . . . . . . . . . . . . . . 18
10.1. Single RTP Session . . . . . . . . . . . . . . . . . . . 18
10.1.1. General . . . . . . . . . . . . . . . . . . . . . . 18
10.1.2. Payload Type (PT) Value Reuse . . . . . . . . . . . 19
10.2. Associating RTP/RTCP Packets With Correct SDP Media
Description . . . . . . . . . . . . . . . . . . . . . . 19
10.3. RTP/RTCP Multiplexing . . . . . . . . . . . . . . . . . 20
10.3.1. General . . . . . . . . . . . . . . . . . . . . . . 20
10.3.2. SDP Offer/Answer Procedures . . . . . . . . . . . . 20
11. ICE Considerations . . . . . . . . . . . . . . . . . . . . . 22
11.1. General . . . . . . . . . . . . . . . . . . . . . . . . 22
11.2. SDP Offer/Answer Procedures . . . . . . . . . . . . . . 22
11.2.1. General . . . . . . . . . . . . . . . . . . . . . . 23
11.2.2. Generating the Initial SDP Offer . . . . . . . . . . 23
11.2.3. Generating the SDP Answer . . . . . . . . . . . . . 23
11.2.4. Offerer Processing of the SDP Answer . . . . . . . . 23
11.2.5. Modifying the Session . . . . . . . . . . . . . . . 23
12. Update to RFC 3264 . . . . . . . . . . . . . . . . . . . . . 24
12.1. General . . . . . . . . . . . . . . . . . . . . . . . . 24
12.2. Original text of section 5.1 (2nd paragraph) of RFC 3264 24
12.3. New text replacing section 5.1 (2nd paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.4. Original text of section 8.2 (2nd paragraph) of RFC 3264 25
12.5. New text replacing section 8.2 (2nd paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.6. Original text of section 8.4 (6th paragraph) of RFC 3264 25
12.7. New text replacing section 8.4 (6th paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 25
13. RTP/RTCP extensions for identification-tag transport . . . . 26
13.1. General . . . . . . . . . . . . . . . . . . . . . . . . 26
13.2. RTCP MID SDES Item . . . . . . . . . . . . . . . . . . . 27
13.3. RTP MID Header Extension . . . . . . . . . . . . . . . . 27
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
14.1. New SDES item . . . . . . . . . . . . . . . . . . . . . 27
14.2. New RTP Header Extension URI . . . . . . . . . . . . . . 28
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14.3. New SDP Attribute . . . . . . . . . . . . . . . . . . . 28
15. Security Considerations . . . . . . . . . . . . . . . . . . . 28
16. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 29
16.1. Example: Bundle Address Selection . . . . . . . . . . . 29
16.2. Example: BUNDLE Extension Rejected . . . . . . . . . . . 31
16.3. Example: Offerer Adds A Media Description To A BUNDLE
Group . . . . . . . . . . . . . . . . . . . . . . . . . 32
16.4. Example: Offerer Moves A Media Description Out Of A
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 35
16.5. Example: Offerer Disables A Media Description Within A
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 36
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
18. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 38
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
19.1. Normative References . . . . . . . . . . . . . . . . . . 42
19.2. Informative References . . . . . . . . . . . . . . . . . 43
Appendix A. Design Considerations . . . . . . . . . . . . . . . 43
A.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 43
A.2. UA Interoperability . . . . . . . . . . . . . . . . . . . 44
A.3. Usage of port number value zero . . . . . . . . . . . . . 46
A.4. B2BUA And Proxy Interoperability . . . . . . . . . . . . 46
A.4.1. Traffic Policing . . . . . . . . . . . . . . . . . . 47
A.4.2. Bandwidth Allocation . . . . . . . . . . . . . . . . 47
A.5. Candidate Gathering . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction
This specification defines a way to use a single address:port
combination (BUNDLE address) for receiving media associated with
multiple SDP media descriptions ("m=" lines).
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 the usage of a BUNDLE group. Within the BUNDLE group, a
BUNDLE address is used for receiving media associated with multiple
"m=" lines. This is referred to as bundled media.
The offerer and answerer [RFC3264] use the BUNDLE extension to
negotiate the BUNDLE addresses, one for the offerer (offerer BUNDLE
address) and one for the answerer (answerer BUNDLE address), to be
used for receiving the bundled media associated with a BUNDLE group.
Once the offerer and the answerer have negotiated a BUNDLE group,
they assign their respective BUNDLE address to each "m=" line in the
BUNDLE group. The BUNDLE addresses are used to receive all media
associated with the BUNDLE group.
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The use of a BUNDLE group and a BUNDLE address also allows the usage
of a single set of Interactive Connectivity Establishment (ICE)
[RFC5245] candidates for multiple "m=" lines.
This specification also defines a new SDP attribute, 'bundle-only',
which can be used to request that specific media is only used if kept
within a BUNDLE group.
As defined in RFC 4566 [RFC4566], the semantics of assigning the same
port value to multiple "m=" lines are undefined, and there is no
grouping defined by such means. Instead, an explicit grouping
mechanism needs to be used to express the intended semantics. This
specification provides such an extension.
This specification also updates sections 5.1, 8.1 and 8.2 of RFC 3264
[RFC3264]. The update allows an answerer to assign a non-zero port
value to an "m=" line in an SDP answer, even if the "m=" line in the
associated SDP offer contained a zero port value.
This specification also defines a new Real-time Transport Protocol
(RTP) [RFC3550] SDES item and a new RTP header extension that can be
used to carry a value that associates RTP/RTCP packets with a
specific media description. This can be used to correlate a RTP
packet with the correct media.
SDP bodies can contain multiple BUNDLE groups. A given BUNDLE
address MUST only be associated with a single BUNDLE group. The
procedures in this specification apply independently to a given
BUNDLE group. All RTP based media flows associated with a single
BUNDLE group belong to a single RTP session [RFC3550].
The BUNDLE extension is backward compatible. Endpoints that do not
support the extension are expected to generate offers and answers
without an SDP 'group:BUNDLE' attribute, and are expected to assign a
unique address to each "m=" line within an offer and answer,
according to the procedures in [RFC4566] and [RFC3264]
2. Terminology
5-tuple: A collection of the following values: source address, source
port, destination address, destination port, and transport-layer
protocol.
Unique address: An IP address and port combination that is assigned
to only one "m=" line in an offer or answer.
Shared address: An IP address and port combination that is assigned
to multiple "m=" lines within an offer or answer.
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Offerer BUNDLE-tag: The first identification-tag in a given SDP
'group:BUNDLE' attribute identification-tag list in an offer.
Answerer BUNDLE-tag: The first identification-tag in a given SDP
'group:BUNDLE' attribute identification-tag list in an answer.
Offerer BUNDLE address: Within a given BUNDLE group, an IP address
and port combination used by an offerer to receive all media
associated with each "m=" line within the BUNDLE group.
Answerer BUNDLE address: Within a given BUNDLE group, an IP address
and port combination used by an answerer to receive all media
associated with each "m=" line within the BUNDLE group.
BUNDLE group: A set of "m=" lines, created using an SDP Offer/Answer
exchange, which uses the same BUNDLE address for receiving media.
Bundled "m=" line: An "m=" line, whose identification-tag is placed
in an SDP 'group:BUNDLE' attribute identification-tag list in an
offer or answer.
Bundle-only "m=" line: A bundled "m=" line with an associated SDP
'bundle-only' attribute.
Bundled media: All media associated with a given BUNDLE group.
Initial offer: The first offer, within an SDP session, in which the
offerer indicates that it wants to create a given BUNDLE group.
Subsequent offer: An offer which contains a BUNDLE group that has
been created as part of a previous SDP Offer/Answer exchange.
Identification-tag: A unique token value that is used to identify an
"m=" line. The SDP 'mid' attribute [RFC5888], associated with an
"m=" line, carries an unique identification-tag. The session-level
SDP 'group' attribute [RFC5888] carries a list of identification-
tags, identifying the "m=" lines associated with that particular
'group' attribute.
3. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
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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].
5. SDP Grouping Framework BUNDLE Extension
5.1. General
This section defines a new SDP Grouping Framework extension
[RFC5888], 'BUNDLE'. The BUNDLE extension can be used with the SDP
Offer/Answer mechanism to negotiate the usage of a single
address:port combination (BUNDLE address) for receiving bundled
media.
A single address:port combination is also used for sending bundled
media. The address:port combination used for sending bundled media
MAY be the same as the BUNDLE address, used to receive bundled media,
depending on whether symmetric RTP is used. A given address:port
combination MUST NOT be used for sending media associated with
multiple BUNDLE groups.
All media associated with a BUNDLE group share a single 5-tuple, i.e.
in addition to using a single address:port combination all bundled
media MUST be transported using the same transport-layer protocol.
The BUNDLE extension is indicated using an SDP 'group' attribute with
a "BUNDLE" semantics value [RFC5888]. An identification-tag is
assigned to each bundled "m=" line, and each identification-tag is
listed in the SDP 'group:BUNDLE' attribute identification-tag list.
Each "m=" line, 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=" line MUST NOT be associated with more than one BUNDLE group.
Section 8 defines the detailed SDP Offer/Answer procedures for the
BUNDLE extension.
6. SDP 'bundle-only' Attribute
6.1. General
This section defines a new SDP media-level attribute [RFC4566],
'bundle-only'.
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6.2. bundle-only
Name: bundle-only
Value:
Usage Level: media
Charset Dependent: no
Example:
a=bundle-only
In order to ensure that an answerer that does not supports the BUNDLE
extension always rejects a bundled "m=" line, the offerer can assign
a zero port value to the "m=" line. According to [RFC4566] an
answerer will reject such "m=" line. By associating an SDP 'bundle-
only' attribute with such "m=" line, the offerer can request that the
answerer accepts the "m=" line if the answerer supports the Bundle
extension, and if the answerer keeps the "m=" line within the
associated BUNDLE group.
NOTE: Once an offerer BUNDLE address has been selected, the offerer
can ensure that an bundled "m=" line is accepted by the answerer only
if the answerer keeps the "m=" line within the associated BUNDLE
group by assigning the offerer BUNDLE address to the "m=" line. If
the answerer does not keep that "m=" line within the BUNDLE group,
the answerer will reject it. Therefore, the SDP 'bundle-only'
attribute is not needed in such cases
The usage of the 'bundle-only' attribute is only defined for a
bundled "m=" line with a zero port value, within an offer. Other
usage is unspecified.
Section 8 defines the detailed SDP Offer/Answer procedures for the
'bundle-only' attribute.
7. SDP Information Considerations
7.1. General
This section describes restrictions associated with the usage of SDP
parameters within a BUNDLE group. It also describes, when parameter
and attribute values have been associated with each bundled "m="
line, how to calculate a value for the whole BUNDLE group.
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7.2. Connection Data (c=)
The "c=" line nettype value [RFC4566] associated with a bundled "m="
line MUST be 'IN'.
The "c=" line addrtype value [RFC4566] associated with a bundled "m="
line MUST be 'IP4' or 'IP6'. The same value MUST be associated with
each "m=" line.
NOTE: Extensions to this specification can specify usage of the
BUNDLE mechanism for other nettype and addrtype values than the ones
listed above.
7.3. Bandwidth (b=)
The proposed bandwidth for a bundled "m=" line SHOULD be calculated
in the same way as for a non-bundled "m=" line.
The total proposed bandwidth for a BUNDLE group is the sum of the
proposed bandwidth for each bundled "m=" line.
The total proposed bandwidth for an offer or answer is the sum of the
proposed bandwidth for each "m=" line (bundled and non-bundled)
within the offer or answer.
7.4. Attributes (a=)
An offerer and answerer MUST use the rules and restrictions defined
in [I-D.mmusic-sdp-mux-attributes] for when associating SDP
attributes with bundled "m=" lines.
8. SDP Offer/Answer Procedures
8.1. General
This section describes the SDP Offer/Answer [RFC3264] procedures for:
o Negotiating and creating of a BUNDLE group;
o Selecting the BUNDLE addresses (offerer BUNDLE address and
answerer BUNDLE address);
o Adding an "m=" line to a BUNDLE group;
o Moving an "m=" line out of a BUNDLE group; and
o Disabling an "m=" line within a BUNDLE group.
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The generic rules and procedures defined in [RFC3264] and [RFC5888]
also apply to the BUNDLE extension. For example, if an offer is
rejected by the answerer, the previously negotiated SDP parameters
and characteristics (including those associated with a BUNDLE group)
apply. Hence, if an offerer generates an offer in which the offerer
wants to create a BUNDLE group, and the answerer rejects the offer,
the BUNDLE group is not created.
The procedures in this section are independent of the media type or
transport protocol represented by a bundled "m=" line. Section 10
defines additional considerations for RTP based media. Section 6
defines additional considerations for the usage of the SDP 'bundle-
only' attribute. Section 11 defines additional considerations for
the usage of Interactive Connectivity Establishment (ICE) [RFC5245]
mechanism .
The offerer and answerer MUST follow the rules and restrictions
defined in Section 7 when creating offers and answers.
SDP bodies can contain multiple BUNDLE groups. The procedures in
this section apply independently to a given BUNDLE group.
8.2. Generating the Initial SDP Offer
8.2.1. General
When an offerer generates an initial offer, in order to create a
BUNDLE group, it MUST:
o Assign a unique address to each "m=" line within the offer,
following the procedures in [RFC3264];
o Assign a unique address to each "m=" line within the offer,
following the procedures in [RFC3264];
o Add an SDP 'group:BUNDLE' attribute to the offer;
o Place the identification-tag of each bundled "m=" line in the SDP
'group:BUNDLE' attribute identification-tag list; and
o Indicate which unique address the offerer suggests as the offerer
BUNDLE address [Section 8.2.2].
If the offerer wants to request that the answerer accepts a given
"m=" line only if the answerer keeps the "m=" line within the BUNDLE
group, the offerer MUST:
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o Associate an SDP 'bundle-only' attribute [Section 8.2.2] with the
"m=" line; and
o Assign a zero port value to the "m=" line.
NOTE: If the offerer assigns a zero port value to an "m=" line, but
does not also associate an SDP 'bundle-only' attribute with the "m="
line, it is an indication that the offerer wants to disable the "m="
line [Section 8.5.5].
[Section 16.1] shows an example of an initial offer.
8.2.2. Suggesting the offerer BUNDLE address
In the offer, the address assigned to the "m=" line associated with
the offerer BUNDLE-tag indicates the address that the offerer
suggests as the offerer BUNDLE address.
8.3. Generating the SDP Answer
8.3.1. General
When an answerer generates an answer, which contains a BUNDLE group,
the following general SDP grouping framework restrictions, defined in
[RFC5888], also apply to the BUNDLE group:
o The answerer MUST NOT include a BUNDLE group in the answer, unless
the offerer requested the BUNDLE group to be created in the
associated offer; and
o The answerer MUST NOT include an "m=" line within a BUNDLE group,
unless the offerer requested the "m=" line to be within that
BUNDLE group in the associated offer.
If the answer contains a BUNDLE group, the answerer MUST:
o Select an Offerer BUNDLE Address [Section 8.3.2]; and
o Select an Answerer BUNDLE Address [Section 8.3.3];
The answerer is allowed to select a new Answerer BUNDLE address each
time it generates an answer to an offer.
If the answerer does not want to keep an "m=" line within a BUNDLE
group, it MUST:
o Move the "m=" line out of the BUNDLE group [Section 8.3.4]; or
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o Reject the "m=" line [Section 8.3.5];
If the answerer keeps a bundle-only "m=" line within the BUNDLE
group, it follows the procedures (assigns the answerer BUNDLE address
to the "m=" line etc) for any other "m=" line kept within the BUNDLE
group.
If the answerer does not want to keep a bundle-only "m=" line within
the BUNDLE group, it MUST reject the "m=" line [Section 8.3.5].
The answerer MUST NOT associate an SDP 'bundle-only' attribute with
any "m=" line in an answer.
NOTE: If a bundled "m=" line in an offer contains a zero port value,
but the "m=" line does not contain an SDP 'bundle-only' attribute, it
is an indication that the offerer wants to disable the "m=" line
[Section 8.5.5].
8.3.2. Answerer Selection of Offerer Bundle Address
In an offer, the address (unique or shared) assigned to the bundled
"m=" line associated with the offerer BUNDLE-tag indicates the
address that the offerer suggests as the offerer BUNDLE address
[Section 8.2.2]. The answerer MUST check whether that "m=" line
fulfills the following criteria:
o The answerer will not move the "m=" line out of the BUNDLE group
[Section 8.3.4];
o The answerer will not reject the "m=" line [Section 8.3.5]; and
o The "m=" line does not contain a zero port value.
If all of the criteria above are fulfilled, the answerer MUST select
the address associated with the "m=" line as the offerer BUNDLE
address. In the answer, the answerer BUNDLE-tag represents the "m="
line, and the address associated with the "m=" line in the offer
becomes the offerer BUNDLE address.
If one or more of the criteria are not fulfilled, the answerer MUST
select the next identification-tag in the identification-tag list,
and perform the same criteria check for the "m=" line associated with
that identification-tag. If there are no more identification-tags in
the identification-tag list, the answerer MUST NOT create the BUNDLE
group.
[Section 16.1] shows an example of an offerer BUNDLE address
selection.
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8.3.3. Answerer Selection of Answerer BUNDLE Address
When the answerer selects a BUNDLE address for itself, referred to as
the answerer BUNDLE address, it MUST assign that address to each
bundled "m=" line within the created BUNDLE group in the answer.
The answerer MUST NOT assign the answerer BUNDLE address to an "m="
line that is not within the BUNDLE group, or to an "m=" line that is
within another BUNDLE group.
[Section 16.1] shows an example of an answerer BUNDLE address
selection.
8.3.4. Moving A Media Description Out Of A BUNDLE Group
When an answerer moves a "m=" line out of a BUNDLE group, it assigns
an address to the "m=" line in the answer based on the following
rules:
o In the associated offer, if the "m=" line contains a shared
address (e.g. a previously selected offerer BUNDLE address), the
answerer MUST reject the moved "m=" line [Section 8.3.5];
o In the associated offer, if the "m=" line contains a unique
address, the answerer MUST assign a unique address also to the
"m=" line in the answer; or
o In the associated offer, if an SDP 'bundle-only' attribute is
associated with the "m=" line, and if the "m=" line contains a
zero port value, the answerer MUST reject the "m=" line
[Section 8.3.5].
In addition, in either case above, the answerer MUST NOT place the
identification-tag, associated with the moved "m=" line, in the SDP
'group' attribute identification-tag list associated with the BUNDLE
group.
8.3.5. Rejecting A Media Description In A BUNDLE Group
When an answerer rejects an "m=" line, it MUST assign an address with
a zero port value to the "m=" line in the answer, according to the
procedures in [RFC4566].
In addition, the answerer MUST NOT place the identification-tag,
associated with the rejected "m=" line, in the SDP 'group' attribute
identification-tag list associated with the BUNDLE group.
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8.4. Offerer Processing of the SDP Answer
8.4.1. General
When an offerer receives an answer, if the answer contains a BUNDLE
group, the offerer MUST check that any bundled "m=" line in the
answer was indicated as bundled in the associated offer. If there is
no mismatch, the offerer MUST use the offerer BUNDLE address,
selected by the answerer [Section 8.3.2], as the address for each
bundled "m=" line.
NOTE: As the answerer might reject one or more bundled "m=" lines, or
move a bundled "m=" line out of a BUNDLE group, each bundled "m="
line in the offer might not be indicated as bundled in the answer.
If the answer does not contain a BUNDLE group, the offerer MUST
process the answer as a normal answer.
8.4.2. Bundle Address Synchronization (BAS)
When an offerer receives an answer, if the answer contains a BUNDLE
group, the offerer MUST check whether the offerer BUNDLE address,
selected by the answerer [Section 8.3.2], matches what was assigned
to each bundled "m=" line (excluding any bundled "m=" line that was
rejected, or moved out of the BUNDLE group, by the answer) in the
associated offer. If there is a mismatch, the offerer SHOULD as soon
as possible generate a subsequent offer, in which it assigns the
offerer BUNDLE address to each bundled "m=" line. Such offer is
referred to as a Bundle Address Synchronization (BAS) offer.
A BAS offer is typically sent in the following scenarios:
o The offerer receives an answer to an initial offer, as the bundled
"m=" lines in the initial offer always contain unique addresses
[Section 8.2]; or
o The offerer receives an answer to an offer, in which a new bundled
"m=" line has been added to the BUNDLE group [Section 8.5.3], and
the offerer assigned a unique address to the bundled "m=" line in
the offer.
The offerer is allowed to modify any SDP parameter in the BAS offer.
NOTE: It is important that the BAS offer gets accepted by the
answerer. For that reason the offerer needs to consider the
necessity to modify SDP parameters in the BAS offer, in such a way
that could trigger the answerer to reject the BAS offer. Disabling
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"m=" lines, or reducing the number of codecs, in a BAS offer is
considered to have a low risk of being rejected.
NOTE: The main purpose of the BAS offer is to ensure that
intermediaries, that might not support the BUNDLE extension, have
correct information regarding the address that is going to be used to
transport the bundled media.
[Section 16.1] shows an example of a BAS offer.
8.5. Modifying the Session
8.5.1. General
When an offerer generates a subsequent offer, it MUST assign the
previously selected offerer BUNDLE address [Section 8.3.2], to each
bundled "m=" line (including any bundle-only "m=" line), except if:
o The offerer suggests a new offerer BUNDLE address [Section 8.5.2];
o The offerer wants to add a bundled "m=" line to the BUNDLE group
[Section 8.5.3];
o The offerer wants to move a bundled "m=" line out of the BUNDLE
group [Section 8.5.4]; or
o The offerer wants to disable the bundled "m=" line
[Section 8.5.5].
In addition, the offerer MUST select an offerer BUNDLE-tag
[Section 8.2.2], even if the offerer does not suggest a new offerer
BUNDLE address.
The offerer MUST NOT associate an SDP 'bundle-only' attribute with a
bundled "m=" line in a subsequent offer, unless the offerer also
assigns a zero port value to the "m=" line.
8.5.2. Request a new offerer BUNDLE address
When an offerer generates an offer, in which it suggests a new
offerer BUNDLE address [Section 8.2.2], the offerer MUST:
o Assign a unique address, which the offerer suggests as the new
offerer BUNDLE address, to a bundled "m=" line; and
o Indicate that the offerer suggests the unique address as the new
offerer BUNDLE address [Section 8.2.2]
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NOTE: The offerer can assign a unique address to each bundled "m="
line in the offer, or it can assign the previously selected offerer
BUNDLE address to each "m=" line (except to the "m=" line to which it
assigns the unique address that it suggests as the new offerer BUNDLE
address).
8.5.3. Adding a media description to a BUNDLE group
When an offerer generates an offer, in which it wants to add a
bundled "m=" line to a BUNDLE group, the offerer MUST:
o Assign a unique address, or the previously selected offerer BUNDLE
address, to the "m=" line; and
o Extend the SDP 'group:BUNDLE' attribute identification-tag list
with the BUNDLE group [Section 8.2.2] by adding the
identification-tag associated with the added "m=" line to the
list.
NOTE: Assigning a unique address to the "m=" line allows the answerer
to move the "m=" line out of the BUNDLE group [Section 8.3.4],
without having to reject the "m=" line.
If the offerer suggests the address associated with the added "m="
line as the new offerer BUNDLE address, the offerer BUNDLE-tag MUST
represent the added "m=" line [Section 8.2.2].
[Section 16.3] shows an example where an offerer sends an offer in
order to add a bundled "m=" line to a BUNDLE group.
8.5.4. Moving A Media Description Out Of A BUNDLE Group
When an offerer generates an offer, in which it wants to move a
bundled "m=" line out of a BUNDLE group it was added to in a previous
offer/answer transaction, the offerer:
o MUST assign a unique address to the "m=" line; and
o MUST NOT place the identification-tag associated with the "m="
line in the SDP 'group:BUNDLE' attribute identification-tag list
associated with the BUNDLE group.
NOTE: If an "m=" line, when being moved out of a BUNDLE group, is
added to another BUNDLE group, the offerer applies the procedures in
[Section 8.5.3] to the "m=" line.
[Section 16.4] shows an example of an offer for moving an "m=" line
out of a BUNDLE group.
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8.5.5. Disabling A Media Description In A BUNDLE Group
When an offerer generates an offer, in which it wants to disable a
bundled "m=" line (added to the BUNDLE group in a previous offer/
answer transaction), the offerer:
o MUST assign an address with a zero port value to the "m=" line,
following the procedures in [RFC4566]; and
o MUST NOT place the identification-tag associated with the "m="
line in the SDP 'group:BUNDLE' attribute identification-tag list
associated with the BUNDLE group.
[Section 16.5] shows an example of an offer for disabling an "m="
line within a BUNDLE group.
9. Protocol Identification
9.1. General
Each "m=" line within a BUNDLE group MUST use the same transport-
layer protocol. If bundled "m=" lines use different protocols on top
of the transport-layer protocol, there MUST exist a publicly
available specification which describes a mechanism, for this
particular protocol combination, how to associate a received packet
with the correct protocol.
In addition, if a received packet can be associated with more than
one bundled "m=" line, there MUST exist a publically available
specification which describes a mechanism for associating the
received packet with the correct "m=" line.
9.2. 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 bundled "m="
lines that represent these protocols, the offerer or answerer MUST
support the mechanism described in [RFC5764], and no explicit
negotiation is required in order to indicate support and usage of the
mechanism.
[RFC5764] does not describe how to identify different protocols
transported on DTLS, only how to identify the DTLS protocol itself.
If multiple protocols are transported on DTLS, there MUST exist a
specification describing a mechanism for identifying each individual
protocol. In addition, if a received DTLS packet can be associated
with more than one "m=" line, there MUST exist a specification which
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describes a mechanism for associating the received DTLS packet with
the correct "m=" line.
[Section 10.2] describes how to associate a received (S)RTP packet
with the correct "m=" line.
10. RTP Considerations
10.1. Single RTP Session
10.1.1. General
All RTP-based media within a single BUNDLE group belong to a single
RTP session [RFC3550]. Disjoint BUNDLE groups will form multiple RTP
sessions, one per BUNDLE group.
Since a single RTP session is used for each bundle group, all "m="
lines representing RTP-based media in a bundle group will share a
single SSRC numbering space [RFC3550].
The following rules and restrictions apply for a single RTP session:
o A specific payload type value can be used in multiple bundled "m="
lines if each codec associated with the payload type number shares
an identical codec configuration [Section 10.1.2].
o The "proto" value in each bundled RTP-based "m=" line MUST be
identical (e.g. RTP/AVPF).
o An SDP 'extmap' attribute [RFC5285], with a 'urn:ietf:params:rtp-
hdrext:sdes:mid' URI value, MUST, in every offer and answer, be
associated with each bundled "m=" line representing RTP-based
media.
o A given SSRC MUST NOT transmit RTP packets using payload types
that originate from different bundled "m=" lines.
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 value can later be reused by
another source(possible associated with a different bundled "m="
line.
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10.1.2. Payload Type (PT) Value Reuse
Multiple bundled "m=" lines might represent RTP based media. As all
RTP based media associated with a BUNDLE group belong to the same RTP
session, in order for a given payload type value to be used inside
more than one bundled "m=" line, 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.mmusic-sdp-mux-attributes]
lists SDP attributes, whose attribute values must be identical for
all codecs that use the same payload type value.
10.2. Associating RTP/RTCP Packets With Correct SDP Media Description
There are multiple mechanisms that can be used by an endpoint in
order to associate received RTP/RTCP packets with a bundled "m="
line. Such mechanisms include using the payload type value carried
inside the RTP packets, the SSRC values carried inside the RTP
packets, and other "m=" line specific information carried inside the
RTP packets.
As all RTP/RTCP packets associated with a BUNDLE group are received
(and sent) using single address:port combinations, the local
address:port combination cannot be used to associate received RTP
packets with the correct "m=" line.
As described in [Section 10.1.2], the same payload type value might
be used inside RTP packets described by multiple "m=" lines. In such
cases, the payload type value cannot be used to associate received
RTP packets with the correct "m=" line.
An offerer and answerer can in an offer and answer inform each other
which SSRC values they will use inside sent RTP/RTCP packets, by
associating an SDP 'ssrc' attribute [RFC5576] with each bundled "m="
line which contains a payload type value that is also used inside
another bundled "m=" line. As the SSRC values will be carried inside
the RTP/RTCP packets, the offerer and answerer can then use that
information to associate received RTP packets with the correct "m="
line. However, an offerer will not know which SSRC values the
answerer will use until it has received the answer providing that
information. Due to this, before the offerer has received the
answer, the offerer will not be able to associate received RTP/RTCP
packets with the correct "m=" line using the SSRC values.
In order for an offerer and answerer to always be able to associate
received RTP and RTCP packets with the correct "m=" line, an offerer
and answerer using the BUNDLE extension MUST support the mechanism
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defined in Section 13, where the remote endpoint inserts the
identification-tag associated with an "m=" line in RTP and RTCP
packets associated with that "m=" line.
10.3. RTP/RTCP Multiplexing
10.3.1. General
When a BUNDLE group, which contains RTP based media, is created, the
offerer and answerer MUST negotiate whether to enable RTP/RTCP
multiplexing for the RTP based media associated with the BUNDLE group
[RFC5761].
If RTP/RTCP multiplexing is not enabled, separate address:port
combinations will be used for receiving (and sending) the RTP packets
and the RTCP packets.
10.3.2. SDP Offer/Answer Procedures
10.3.2.1. General
This section describes how an offerer and answerer can use the SDP
'rtcp-mux' attribute [RFC5761] and the SDP 'rtcp' attribute [RFC3605]
to negotiate usage of RTP/RTCP multiplexing for RTP based media
associated with a BUNDLE group.
10.3.2.2. Generating the Initial SDP Offer
When an offerer generates an initial offer, if the offerer wants to
negotiate usage of RTP/RTCP multiplexing within a BUNDLE group, the
offerer MUST associate an SDP 'rtcp-mux' attribute [RFC5761] with
each bundled RTP-based "m=" line (including any bundle-only "m="
line) in the offer. In addition, the offerer MUST associate an SDP
'rtcp' attribute [RFC3605] with each bundled RTP-based "m=" line
(including any bundle-only "m=" line), with an attribute value that
is identical to the port value assigned to the "m=" line itself, in
the offer.
If the offerer does not want to negotiate usage of RTP/RTCP
multiplexing, it MUST NOT associate the SDP attributes above with any
bundled "m=" line.
10.3.2.3. Generating the SDP Answer
When an answerer generates an answer, if the offerer indicated
support of RTP/RTCP multiplexing [RFC5761] within a BUNDLE group in
the associated offer, the answerer MUST either accept or reject the
usage of RTP/RTCP multiplexing in the answer.
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If the answerer accepts usage of RTP/RTCP multiplexing within the
BUNDLE group, it MUST associate an SDP 'rtcp-mux' attribute with each
bundled RTP-based "m=" line in the answer. The answerer MUST NOT
associate an SDP 'rtcp' attribute with any bundled "m=" line in the
answer. The answerer will use the port number of the selected
offerer BUNDLE address for sending RTP and RTCP packets associated
with each bundled RTP-based "m=" line towards the offerer.
If the answerer rejects usage of RTP/RTCP multiplexing within the
BUNDLE group, it MUST NOT associate an SDP 'rtcp-mux' or SDP 'rtcp'
attribute with any bundled "m=" line in the answer. The answerer
MUST, based on the port number of the selected offerer BUNDLE
address, use the next higher (odd) destination port number [RFC3550]
for sending RTCP packets associated with each bundled RTP-based "m="
line towards the offerer.
NOTE: When the answerer rejects usage of RTP/RTCP multiplexing, the
reason for mandating usage of the next higher (odd) destination port
number for RTCP is to allign the procedures for the corresponding
offer.
If the usage of RTP/RTCP multiplexing has been negotiated in a
previous offer/answer transaction, and the offerer indicates that it
wants to continue using RTP/RTCP multiplexing in a subsequent offer,
the answerer MUST associate an SDP 'rtcp-mux' attribute with each
bundled "m=" line in the answer. I.e. the answerer MUST NOT disable
the usage of RTP/RTCP multiplexing.
10.3.2.4. Offerer Processing of the SDP Answer
When the offerer receives an answer, if the answerer accepts the
usage of RTP/RTCP multiplexing, by including an SDP 'rtcp-mux'
attribute to each bundled "m=" line in the answer [Section 10.3.2.3],
the answerer follows the procedures for RTP/RTCP multiplexing defined
in [RFC5761]. The offerer will use the port number of the answerer
BUNDLE address for sending RTP and RTCP packets associated with each
bundled "m=" line towards the answerer.
If the answerer does not accept the usage of RTP/RTCP multiplexing
[Section 10.3.2.3], the offerer MUST use separate address:port
combinations for RTP and RTCP. The offerer will, based on the port
number of the answerer BUNDLE address, use the next higher (odd)
destination port number [RFC3550] for sending RTCP packets associated
with a bundled "m=" line towards the answerer.
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10.3.2.5. Modifying the Session
When an offerer generates a subsequent offer, if it wants to
negotiate usage of RTP/RTCP multiplexing within a BUNDLE group, or if
it wants to continue usage of RTP/RTCP multiplexing (negotiated in a
previous offer/answer transaction), it MUST associate SDP 'rtcp-mux'
and 'rtcp' attributes with each bundled "m=" line (including any
bundled "m=" line that the offerer wants to add to the BUNDLE group),
unless the offerer wants to disable or remove the "m=" line from the
BUNDLE group.
If the offerer does not want to negotiate usage of RTP/RTCP
multiplexing within the BUNDLE group, or if it wants to disable usage
of RTP/RTCP multiplexing (negotiated in a previous offer/answer
transaction), the offerer MUST NOT associate SDP 'rtcp-mux' and
'rtcp' attributes with any bundled "m=" line in the subsequent offer.
NOTE: It is RECOMMENDED that, once usage of RTP/RTCP multiplexing has
been negotiated within a BUNDLE group, that the usage is not
disabled. Disabling RTP/RTCP multiplexing means that the offerer and
answerer need to reserve new ports, to be used for sending and
receiving RTCP packets.
11. ICE Considerations
11.1. General
This section describes how to use the BUNDLE grouping extension
together with the Interactive Connectivity Establishment (ICE)
mechanism [RFC5245].
The procedures defined in [RFC5245] also apply to usage of ICE with
BUNDLE, with the following exception:
o When BUNDLE addresses for a BUNDLE group have been selected for
both endpoints, ICE connectivity checks and keep-alives only need
to be performed for the whole BUNDLE group, instead of per bundled
"m=" line.
Support and usage of ICE mechanism together with the BUNDLE extension
is OPTIONAL.
11.2. SDP Offer/Answer Procedures
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11.2.1. General
When an offerer assigns a unique address to a bundled "m=" line
(excluding and bundle-only "m=" line), it MUST also associate unique
ICE candidates [RFC5245] to the "m=" line.
An offerer MUST NOT assign ICE candidates to a bundle-only "m=" line
with a zero port value.
NOTE: The bundle-only "m=" line, if accepted by the answerer, will
inherit the candidates associated with the selected offerer BUNDLE
address. An answerer that does not support BUNDLE would not accept a
bundle-only "m=" line.
When an offerer or answerer assigns a shared address (i.e. a
previously selected BUNDLE address) to one or more bundled "m="
lines, it MUST associate identical ICE candidates (referred to as
shared ICE candidates) to each of those "m=" lines.
11.2.2. Generating the Initial SDP Offer
When an offerer generates an initial offer, it assigns unique or
shared ICE candidates to the bundled "m=" lines, according to
Section 11.1.
11.2.3. Generating the SDP Answer
When an answerer generates an answer, which contains a BUNDLE group,
the answerer MUST assign shared ICE candidates to each bundled "m="
line (including "m=" lines that were indicated as bundle-only in the
associated offer) in the answer.
11.2.4. Offerer Processing of the SDP Answer
When an offerer receives an answer, if the answerer supports and uses
the ICE mechanism and the BUNDLE extension, the offerer MUST assign
the same ICE candidates, associated with the "m=" line representing
the offerer BUNDLE address (selected by the answerer), to each
bundled "m=" line.
11.2.5. Modifying the Session
When an offerer generates a subsequent offer, it assigns unique or
shared ICE candidates to the bundled "m=" lines, according to
(Section 11.1).
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12. Update to RFC 3264
12.1. General
This section replaces the text of the following sections of RFC 3264:
o Section 5.1 (Unicast Streams).
o Section 8.2 (Removing a Media Stream).
o Section 8.4 (Putting a Unicast Media Stream on Hold).
12.2. 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
(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.
12.3. 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.
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12.4. Original text of section 8.2 (2nd paragraph) of RFC 3264
A stream that is offered with a port of zero MUST be marked with port
zero in the answer. Like the offer, the answer MAY omit all
attributes present previously, and MAY list just a single media
format from amongst those in the offer.
12.5. New text replacing section 8.2 (2nd paragraph) of RFC 3264
A stream that is offered with a port of zero MUST by default be
marked with port zero in the answer, unless an extension mechanism,
which specifies semantics for the usage of a non-zero port value, is
used. If the stream is marked with port zero in the answer, the
answer MAY omit all attributes present previously, and MAY list just
a single media format from amongst those in the offer."
12.6. 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 should be sent to the peer.
12.7. 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
semantics of the zero port number usage. An agent MUST be capable of
receiving SDP with a connection address of 0.0.0.0, in which case it
means that neither RTP nor RTCP should be sent to the peer.
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13. RTP/RTCP extensions for identification-tag transport
13.1. General
SDP Offerers and Answerers [RFC3264] can associate identification-
tags with "m=" lines within SDP Offers and Answers, using the
procedures in [RFC5888]. Each identification-tag uniquely represents
an "m=" line.
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 header extension [RFC5285], which is
used to carry identification-tags in RTP packets.
The SDES item and RTP header extension make it possible for a
receiver to associate received RTCP- and RTP packets with a specific
"m=" line, to which the receiver has assigned an identification-tag,
even if those "m=" lines are part of the same RTP session. The
endpoint informs the remote endpoint about the identification-tag
using the procedures in [RFC5888], and the remote endpoint then
inserts the identification-tag in RTCP- and RTP packets sent towards
the other endpoint.
NOTE: This text above defines how identification-tags are carried in
SDP Offers and Answers. The usage of other signalling protocols for
carrying identification-tags is not prevented, but the usage of such
protocols is outside the scope of this document.
[RFC3550] defines general procedures regarding the RTCP transmission
interval. The RTCP MID SDES item SHOULD be sent in the first few
RTCP packets sent on joining the session, and SHOULD be sent
regularly thereafter. The exact number of RTCP packets in which this
SDES item is sent is intentionally not specified here, as it will
depend on the expected packet loss rate, the RTCP reporting interval,
and the allowable overhead.
The RTP MID header extension SHOULD be included in some RTP packets
at the start of the session and whenever the SSRC changes. It might
also be useful to include the header extension in RTP packets that
comprise random access points in the media (e.g., with video
I-frames). The exact number of RTP packets in which this header
extension is sent is intentionally not specified here, as it will
depend on expected packet loss rate and loss patterns, the overhead
the application can tolerate, and the importance of immediate receipt
of the identification-tag.
For robustness purpose, endpoints need to be prepared for situations
where the reception of the identification-tag is delayed, and SHOULD
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NOT terminate sessions in such cases, as the identification-tag is
likely to arrive soon.
13.2. 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, as in SDP.
The identification-tag is not zero terminated.
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.]
13.3. RTP MID Header Extension
The payload, containing the mid value, of the RTP MID header
extension element can be encoded using either the one-byte or two-
byte header [RFC5285]. The identification-tag payload is UTF-8
encoded, as in SDP.
The identification-tag is not zero terminated. Note, however, that
RTP header extensions that are not a multiple of 32 bits in length
MUST be padded to the next 32-bit boundary using zero bytes; these
padding bytes are not included in the header length field [RFC3550].
14. IANA Considerations
14.1. 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 "RTCP SDES item
types" registry as follows:
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Value: TBD
Abbrev.: MID
Name: Media Identification
Reference: RFCXXXX
14.2. New RTP 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 Compact Header
Extensions subregistry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:sdes:mid
Description: Media identification
Contact: christer.holmberg@ericsson.com
Reference: RFCXXXX
14.3. 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: Christer Holmberg
Contact e-mail: christer.holmberg@ericsson.com
Reference: RFCXXXX
15. Security Considerations
The security considerations defined in [RFC3264] and [RFC5888] apply
to the BUNDLE extension. Bundle does not change which information
flows over the network but only changes which ports that information
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is flowing on and thus has very little impact on the security of the
RTP sessions.
When the BUNDLE extension is used, a single set of security
credentials might be used for all media streams associated with a
BUNDLE group.
When the BUNDLE extension is used, the number of SSRC values within a
single RTP session increases, which increases the risk of SSRC
collision. [RFC4568] describes how SSRC collision may weaken SRTP
and SRTCP encryption in certain situations.
16. Examples
16.1. Example: Bundle Address Selection
The example below shows:
o 1. An offer, in which the offerer assigns a unique address to
each bundled "m=" line within the BUNDLE group.
o 2. An answer, in which the answerer selects the offerer BUNDLE
address, and in which selects its own BUNDLE address (the answerer
BUNDLE address) and assigns it each bundled "m=" line within the
BUNDLE group.
o 3. A subsequent offer (BAS offer), which is used to perform a
Bundle Address Synchronization (BAS).
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=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
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a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Offer (3)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
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16.2. Example: BUNDLE Extension Rejected
The example below shows:
o 1. An offer, in which the offerer assigns a unique address to
each bundled "m=" line within the BUNDLE group.
o 2. An answer, in which the answerer rejects the offered BUNDLE
group, and assigns a unique addresses to each "m=" line (following
normal RFC 3264 procedures).
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SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
m=audio 20000 RTP/AVP 0
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 30000 RTP/AVP 32
b=AS:1000
a=rtpmap:32 MPV/90000
16.3. Example: Offerer Adds A Media Description To A BUNDLE Group
The example below shows:
o 1. A subsequent offer (the BUNDLE group has been created as part
of a previous offer/answer transaction), in which the offerer adds
a new "m=" line, represented by the "zen" identification-tag, to a
previously negotiated BUNDLE group, assigns a unique address to
the added "m=" line, and assigns the previously selected offerer
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BUNDLE address to each of the other bundled "m=" lines within the
BUNDLE group.
o 2. An answer, in which the answerer assigns the answerer BUNDLE
address to each bundled "m=" line (including the newly added "m="
line) within the BUNDLE group.
o 3. A subsequent offer (BAS offer), which is used to perform a
Bundle Address Synchronization (BAS).
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar zen
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 20000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtpmap:66 H261/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar zen
m=audio 20000 RTP/AVP 0
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b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 20000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtpmap:66 H261/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Offer (3)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar zen
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtpmap:66 H261/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
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16.4. Example: Offerer Moves A Media Description Out Of A BUNDLE Group
The example below shows:
o 1. A subsequent offer (the BUNDLE group has been created as part
of a previous offer/answer transaction), in which the offerer
moves a bundled "m=" line out of a BUNDLE group, assigns a unique
address to the moved "m=" line, and assigns the offerer BUNDLE
address to each other bundled "m=" line within the BUNDLE group.
o 2. An answer, in which the answerer moves the "m=" line out of
the BUNDLE group, assigns unique address to the moved "m=" line,
and assigns the answerer BUNDLE address to each of the remaining
bundled "m=" line within the BUNDLE group.
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 50000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtpmap:66 H261/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
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c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 60000 RTP/AVP 66
b=AS:1000
a=mid:zen
a=rtpmap:66 H261/90000
16.5. Example: Offerer Disables A Media Description Within A BUNDLE
Group
The example below shows:
o 1. A subsequent offer (the BUNDLE group has been created as part
of a previous offer/answer transaction), in which the offerer
disables a bundled "m=" line within BUNDLE group, assigns a zero
port number to the disabled "m=" line, and assigns the offerer
BUNDLE address to each of the other bundled "m=" lines within the
BUNDLE group.
o 2. An answer, in which the answerer moves the disabled "m=" line
out of the BUNDLE group, assigns a zero port value to the disabled
"m=" line, and assigns the answerer BUNDLE address to each of the
remaining bundled "m=" line within the BUNDLE group.
SDP Offer (1)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
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a=mid:foo
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10000 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 66
a=mid:zen
a=rtpmap:66 H261/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
b=AS:200
a=mid:foo
a=rtpmap:0 PCMU/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 20000 RTP/AVP 32
b=AS:1000
a=mid:bar
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 0 RTP/AVP 66
a=mid:zen
a=rtpmap:66 H261/90000
17. Acknowledgements
The usage of the SDP grouping extension for negotiating bundled media
is based on a similar alternatives proposed by Harald Alvestrand and
Cullen Jennings. The BUNDLE extension described in this document is
based on the different alternative proposals, and text (e.g. SDP
examples) have been borrowed (and, in some cases, modified) from
those alternative proposals.
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The SDP examples are also modified versions from the ones in the
Alvestrand proposal.
Thanks to Paul Kyzivat, Martin Thomson, Flemming Andreasen, Thomas
Stach and Ari Keraenen for taking the time to read the text along the
way, and providing useful feedback.
18. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
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"
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
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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.
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.
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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".
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.
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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
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.
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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.
19. References
19.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
[I-D.mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-05
(work in progress), November 2014.
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19.2. Informative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605, October
2003.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[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, May 2010.
[RFC7160] Petit-Huguenin, M. and G. Zorn, "Support for Multiple
Clock Rates in an RTP Session", RFC 7160, April 2014.
[I-D.ietf-mmusic-trickle-ice]
Ivov, E., Rescorla, E., and J. Uberti, "Trickle ICE:
Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol", draft-ietf-
mmusic-trickle-ice-01 (work in progress), February 2014.
Appendix A. Design Considerations
A.1. General
One of the main issues regarding the BUNDLE grouping extensions has
been whether, in SDP Offers and SDP Answers, the same port value
should be inserted in "m=" lines associated with a BUNDLE group, as
the purpose of the extension is to negotiate the usage of a single
address:port combination for media associated with the "m=" lines.
Issues with both approaches, discussed in the Appendix have been
raised. The outcome was to specify a mechanism which uses SDP Offers
with both different and identical port values.
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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 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.
NOTE: Before this document is published as an RFC, this
Appendix might be removed.
A.2. UA Interoperability
Consider the following SDP Offer/Answer exchange, where Alice sends
an SDP Offer to Bob:
SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
m=audio 10000 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 10002 RTP/AVP 97
a=rtpmap:97 H261/90000
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SDP Answer
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
m=audio 20000 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 20002 RTP/AVP 97
a=rtpmap:97 H261/90000
RFC 4961 specifies a way of doing symmetric RTP but that is an a
later invention to RTP and Bob can not assume that Alice supports RFC
4961. This means that Alice may be sending RTP from a different port
than 10000 or 10002 - some implementation simply send the RTP from an
ephemeral port. When Bob's endpoint receives an RTP packet, the only
way that Bob know if it should be passed to the video or audio codec
is by looking at the port it was received on. This lead some SDP
implementations to use the fact that each "m=" line had a different
port number to use that port number as an index to find the correct m
line in the SDP. As a result, some implementations that do support
symmetric RTP and ICE still use a SDP data structure where SDP with
"m=" lines with the same port such as:
SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
m=audio 10000 RTP/AVP 97
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 98
a=rtpmap:98 H261/90000
will result in the second "m=" line being considered an SDP error
because it has the same port as the first line.
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A.3. Usage of port number value zero
In an SDP Offer or SDP Answer, the media associated with an "m=" line
can be disabled/rejected by setting the port number value to zero.
This is different from e.g. using the SDP direction attributes, where
RTCP traffic will continue even if the SDP "inactive" attribute is
indicated for the associated "m=" line.
If each "m=" line associated with a BUNDLE group would contain
different port values, and one of those port values would be used for
a BUNDLE address associated with the BUNDLE group, problems would
occur if an endpoint wants to disable/reject the "m=" line associated
with that port, by setting the port value to zero. After that, no
"m=" line would contain the port value which is used for the BUNDLE
address. In addition, it is unclear what would happen to the ICE
candidates associated with the "m=" line, as they are also used for
the BUNDLE address.
A.4. 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 B2BUS still generates that SDP attribute in the Offer
for the outgoing call leg. Consider an B2BUA that did not understand
the SDP "rtcp" attribute, defined in RFC 3605, yet acted this way.
Further assume that the B2BUA was configured to tear down any call
where it did not see any RTCP for 5 minutes. In this cases, if the
B2BUA received an Offer like:
SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=rtcp:53020
It would be looking for RTCP on port 49172 but would not see any
because the RTCP would be on port 53020 and after five minutes, it
would tear down the call. Similarly, an SBC that did not understand
BUNDLE yet put BUNDLE in it's offer may be looking for media on the
wrong port and tear down the call. It is worth noting that a B2BUA
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that generated an Offer with capabilities it does not understand is
not compliant with the specifications.
A.4.1. Traffic Policing
Sometimes intermediaries do not act as B2BUA, in the sense that they
don't modify SDP bodies, nor do they terminate SIP dialogs. Still,
however, they may use SDP information (e.g. IP address and port) in
order to control traffic gating functions, and to set traffic
policing rules. There might be rules which will trigger a session to
be terminated in case media is not sent or received on the ports
retrieved from the SDP. This typically occurs once the session is
already established and ongoing.
A.4.2. Bandwidth Allocation
Sometimes intermediaries do not act as B2BUA, in the sense that they
don't modify SDP bodies, nor do they terminate SIP dialogs. Still,
however, they may use SDP information (e.g. codecs and media types)
in order to control bandwidth allocation functions. The bandwidth
allocation is done per "m=" line, which means that it might not be
enough if media associated with all "m=" lines try to use that
bandwidth. That may either simply lead to bad user experience, or to
termination of the call.
A.5. Candidate Gathering
When using ICE, an candidate needs to be gathered for each port.
This takes approximately 20 ms extra for each extra "m=" line due to
the NAT pacing requirements. All of this gather can be overlapped
with other things while the page is loading to minimize the impact.
If the client only wants to generate TURN or STUN ICE candidates for
one of the "m=" lines and then use trickle ICE
[I-D.ietf-mmusic-trickle-ice] to get the non host ICE candidates for
the rest of the "m=" lines, it MAY do that and will not need any
additional gathering time.
Some people have suggested a TURN extension to get a bunch of TURN
allocation at once. This would only provide a single STUN result so
in cases where the other end did not support BUNDLE, may cause more
use of the TURN server but would be quick in the cases where both
sides supported BUNDLE and would fall back to a successful call in
the other cases.
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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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