MMUSIC Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 3264 (if approved) H. Alvestrand
Intended status: Standards Track Google
Expires: February 23, 2015 C. Jennings
Cisco
August 22, 2014
Negotiating Media Multiplexing Using the Session Description Protocol
(SDP)
draft-ietf-mmusic-sdp-bundle-negotiation-08.txt
Abstract
This specification defines a new SDP Grouping Framework extension,
"BUNDLE", that can be used with the Session Description Protocol
(SDP) Offer/Answer mechanism to negotiate the usage of bundled media,
which refers to the usage of a single 5-tuple for sending and
receiving media associated with multiple SDP media descriptions ("m="
lines).
This specification also updates sections 5.1, 8.1 and 8.2 of RFC
3264, in order to allow an answerer to in an SDP answer assign a non-
zero port value to an "m=" line, even if the offerer in the
associated SDP offer had assigned a zero port value to the "m=" line.
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/.
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 February 23, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 6
5. SDP Grouping Framework BUNDLE Extension Semantics . . . . . . 6
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . 6
5.2.1. General . . . . . . . . . . . . . . . . . . . . . . . 6
5.2.2. SDP Information Considerations . . . . . . . . . . . 7
5.2.3. Generating the Initial SDP Offer . . . . . . . . . . 8
5.2.4. Generating the SDP Answer . . . . . . . . . . . . . . 8
5.2.5. Offerer Processing of the SDP Answer . . . . . . . . 10
5.2.6. Modifying the Session . . . . . . . . . . . . . . . . 11
6. SDP 'bundle-only' Attribute . . . . . . . . . . . . . . . . . 13
6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . 13
6.2.1. Generating the Initial SDP Offer . . . . . . . . . . 13
6.2.2. Generating the SDP Answer . . . . . . . . . . . . . . 13
6.2.3. Offerer Processing of the SDP Answer . . . . . . . . 14
6.2.4. Modifying the Session . . . . . . . . . . . . . . . . 14
7. Protocol Identification . . . . . . . . . . . . . . . . . . . 15
7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.2. STUN, DTLS, SRTP . . . . . . . . . . . . . . . . . . . . 15
8. RTP Considerations . . . . . . . . . . . . . . . . . . . . . 15
8.1. Single RTP Session . . . . . . . . . . . . . . . . . . . 15
8.1.1. General . . . . . . . . . . . . . . . . . . . . . . . 16
8.1.2. Payload Type (PT) Value Re-usage . . . . . . . . . . 16
8.2. Associating RTP/RTCP Packets With Correct SDP Media
Description . . . . . . . . . . . . . . . . . . . . . . . 16
8.3. RTP/RTCP Multiplexing . . . . . . . . . . . . . . . . . . 17
8.3.1. General . . . . . . . . . . . . . . . . . . . . . . . 17
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8.3.2. SDP Offer/Answer Procedures . . . . . . . . . . . . . 18
9. ICE Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.2. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . 20
9.2.1. Generating the Initial SDP Offer . . . . . . . . . . 20
9.2.2. Generating the SDP Answer . . . . . . . . . . . . . . 20
9.2.3. Offerer Processing of the SDP Answer . . . . . . . . 20
9.2.4. Modifying the Session . . . . . . . . . . . . . . . . 20
9.2.5. Keep-alives . . . . . . . . . . . . . . . . . . . . . 20
10. Update to RFC 3264 . . . . . . . . . . . . . . . . . . . . . 20
10.1. General . . . . . . . . . . . . . . . . . . . . . . . . 20
10.2. Original text of section 5.1 (2nd paragraph) of RFC 3264 21
10.3. New text replacing section 5.1 (2nd paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.4. Original text of section 8.2 (2nd paragraph) of RFC 3264 21
10.5. New text replacing section 8.2 (2nd paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.6. Original text of section 8.4 (6th paragraph) of RFC 3264 22
10.7. New text replacing section 8.4 (6th paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11. RTP/RTCP extensions for mid value transport . . . . . . . . . 22
11.1. General . . . . . . . . . . . . . . . . . . . . . . . . 22
11.2. RTP MID SDES Item . . . . . . . . . . . . . . . . . . . 23
11.3. RTP MID Header Extension . . . . . . . . . . . . . . . . 24
11.4. IANA Considerations . . . . . . . . . . . . . . . . . . 24
12. Security Considerations . . . . . . . . . . . . . . . . . . . 24
13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 25
13.1. Example: Bundle Address Selection . . . . . . . . . . . 25
13.2. Example: Bundle Mechanism Rejected . . . . . . . . . . . 26
13.3. Example: Offerer Adds A Media Description To A BUNDLE
Group . . . . . . . . . . . . . . . . . . . . . . . . . 27
13.4. Example: Offerer Moves A Media Description Out Of A
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 29
13.5. Example: Offerer Disables A Media Description Within A
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 31
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 33
16. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 33
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
17.1. Normative References . . . . . . . . . . . . . . . . . . 35
17.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Design Considerations . . . . . . . . . . . . . . . 36
A.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 37
A.2. UA Interoperability . . . . . . . . . . . . . . . . . . . 37
A.3. Usage of port number value zero . . . . . . . . . . . . . 39
A.4. B2BUA And Proxy Interoperability . . . . . . . . . . . . 39
A.4.1. Traffic Policing . . . . . . . . . . . . . . . . . . 40
A.4.2. Bandwidth Allocation . . . . . . . . . . . . . . . . 40
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A.5. Candidate Gathering . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
In the IETF RTCWEB WG, a need to use a single 5-tuple for sending and
receiving media associated with multiple SDP media descriptions ("m="
lines) has been identified. This would e.g. allow the usage of a
single set of Interactive Connectivity Establishment (ICE) [RFC5245]
candidates for multiple media descriptions. Normally different media
types (audio, video etc) will be described using different media
descriptions.
This specification defines a new SDP Grouping Framework [RFC5888]
extension, "BUNDLE", that can be used with the Session Description
Protocol (SDP) Offer/Answer mechanism [RFC3264] to negotiate the
usage of bundled media, which refers to the usage of a single 5-tuple
for sending and receiving media associated with multiple SDP media
descriptions ("m=" lines).
The offerer and answerer [RFC3264] use the BUNDLE mechanism to
negotiate BUNDLE addresses, one for the offerer (offerer BUNDLE
address) and one for the answerer (answerer BUNDLE address) to be
used for the bundled media associated with a BUNDLE group.
Once the offerer and the answerer have negotiated a BUNDLE group, and
the associated BUNDLE addresses, each endpoint can assign its BUNDLE
address to each "m=" line within, and use the address to send and
receive all media associated with, the BUNDLE group.
NOTE: As defined in RFC 4566 [RFC4566], the semantics of assigning
the same port value to multiple "m=" lines are undefined, and there
is no grouping defined by such means. Instead, an explicit grouping
mechanism needs to be used to express the intended semantics. This
specification provides such an extension.
SDP bodies can contain multiple BUNDLE groups. Each BUNDLE group
MUST use a unique 5-tuple. Any given "m=" line can only be
associated with a single BUNDLE group.
The procedures in this specification apply independently to a given
BUNDLE group.
All Real-time Transport Protocol (RTP) [RFC3550] based media flows
associated with a single BUNDLE group belong to a single RTP session
[RFC3550].
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The BUNDLE mechanism is backward compatible. Endpoints that do not
support the BUNDLE mechanism are expected to generate SDP offers and
SDP answers without an SDP 'group:BUNDLE' attribute, and are expected
to assign a unique address to each "m=" line within an SDP offer and
SDP answer, according to the procedures in [RFC4566] and [RFC3264]
This specification also updates sections 5.1, 8.1 and 8.2 of
[RFC3264], in order to allow an answerer to assign a non-zero port
value to an "m=" line in an SDP answer, even if the offerer in the
associated SDP offer had assigned a zero port value to the "m=" line.
2. Terminology
5-tuple: A collection of the following values: source address, source
port, destination address, destination port and protocol.
Unique address: An IP address and IP port combination that is
assigned to a single "m=" line in an SDP offer or SDP answer.
Shared address: An IP address and IP port combination that is
assigned to multiple "m=" lines in an SDP offer or SDP answer.
Offerer suggested BUNDLE mid: The first mid value in a given SDP
'group:BUNDLE' attribute mid list in an SDP offer.
Answerer selected BUNDLE mid: The first mid value in a given SDP
'group:BUNDLE' attribute mid list in an SDP answer.
Offerer BUNDLE address: Within a given BUNDLE group, an IP address
and IP port combination used by an offerer to receive all media
associated with each "m=" line within the BUNDLE group.
Answerer BUNDLE address: Within a given BUNDLE group, an IP address
and IP port combination used by an answerer to receive all media
associated with each "m=" line within the BUNDLE group.
BUNDLE group: A set of "m=" lines, created using an SDP offer/answer
exchange, for which a single 5-tuple is used to send and receive
media. Each endpoint uses its BUNDLE address, associated with the
BUNDLE group, to send and receive the media.
Bundled "m=" line: An "m=" line, in an SDP offer or SDP answer,
associated with a BUNDLE group.
Bundle-only "m=" line: An "m=" line, to which an SDP 'bundle-only'
attribute has been assigned.
Bundled media: All media associated with a BUNDLE group.
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Initial SDP offer: The first SDP offer, within an SDP session, in
which the offerer indicates that it wants to create a given BUNDLE
group.
Subsequent SDP offer: An SDP offer which contains a BUNDLE group that
has been created as part of a previous SDP offer/answer exchange.
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].
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 Semantics
5.1. General
This section defines a new SDP Grouping Framework extension, BUNDLE.
The BUNDLE extension can be indicated using an SDP session-level
'group' attribute. Each SDP Media Description ("m=" line) that is
grouped together, using SDP media-level mid attributes, belongs to a
given BUNDLE group.
5.2. SDP Offer/Answer Procedures
5.2.1. General
This section describes usage of the SDP offer/answer mechanism
[RFC3264] for negotiating usage of the BUNDLE mechanism, for creating
a BUNDLE group, for selecting the BUNDLE addresses (offerer BUNDLE
address and answerer BUNDLE address), for adding an "m=" line to a
BUNDLE group, for moving an "m=" line out of a BUNDLE group, and for
disabling an "m=" line within a BUNDLE group.
The generic rules and procedures defined in [RFC3264] and [RFC5888]
also apply to the BUNDLE mechanism. For example, if an SDP 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 SDP offer in which the
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offerer wants to create a BUNDLE group, and the answerer rejects the
SDP 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 8]
defines additional considerations for RTP based media. [Section 6]
defines additional considerations for the usage of the SDP 'bundle-
only' attribute. [Section 9] defines additional considerations for
the usage of Interactive Connectivity Establishment (ICE) mechanism
[RFC5245].
5.2.2. SDP Information Considerations
5.2.2.1. 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 assigned to each bundled "m=" line,
how to calculate a value for the whole BUNDLE group.
5.2.2.2. Connection Data (c=)
The "c=" line nettype value [RFC4566] assigned to a bundled "m=" line
MUST be 'IN'.
The "c=" line addrtype value [RFC4566] assigned to a bundled "m="
line MUST be 'IP4' or 'IP6'. The same value MUST be assigned to each
"m=" line.
NOTE: Extensions to this specification can specify usage of the
BUNDLE mechanism for other nettype and addrtype values than the ones
listed above.
5.2.2.3. Bandwidth (b=)
The total proposed bandwidth is the sum of the proposed bandwidth for
each bundled "m=" line.
5.2.2.4. Attributes (a=)
[I-D.nandakumar-mmusic-sdp-mux-attributes] defines rules and
restrictions for assigning different types of SDP attributes to a
bundled "m=" line.
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5.2.3. Generating the Initial SDP Offer
5.2.3.1. General
When an offerer generates an initial SDP offer, in order to create a
BUNDLE group, the offerer MUST in the SDP offer assign a unique
address to each "m=" line with a non-zero port value, following the
procedures in [RFC3264].
The offerer MUST in the SDP offer insert an SDP session level
'group:BUNDLE' attribute, associated with the BUNDLE group, and
assign an SDP 'mid' attribute [RFC5888] to each "m=" line that the
offerer wants to be within the BUNDLE group, and place the 'mid'
attribute value in the 'group:BUNDLE' attribute mid list.
[Section 13.1] shows an example of an initial SDP offer.
5.2.3.2. Request offerer BUNDLE address selection
When an offerer generates an initial SDP offer, in order to create a
BUNDLE group, the offerer MUST in the SDP offer indicate which unique
address, associated with one of the "m=" lines that the offerer wants
to be within the BUNDLE group, that the offerer wants the answerer to
select as the offerer BUNDLE address [Section 5.2.4.2]. In the SDP
offer, the offerer BUNDLE mid value represents that address.
5.2.4. Generating the SDP Answer
5.2.4.1. RFC 5888 restrictions
When an answerer generates an SDP answer, the following restrictions,
defined in [RFC5888], also apply a BUNDLE group:
o 1) The answerer MUST NOT in the SDP answer include a BUNDLE group,
unless the offerer in the associated SDP offer requested the
BUNDLE group to be created; and
o 2) The answerer MUST NOT in the SDP answer include an "m=" line
within a BUNDLE group, unless the offerer in the associated SDP
offer requested the "m=" line to be within the BUNDLE group.
5.2.4.2. Answerer Selection of Offerer Bundle Address
When an answerer generates an SDP answer, it MUST select a BUNDLE
address for the offerer, referred to as the offerer BUNDLE address.
The answerer MUST select an address which the offerer in the
associated SDP offer requested to be within the BUNDLE group.
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In the SDP offer, the offerer suggested BUNDLE mid represents the
"m=" line to which the offerer in the SDP offer has assigned the
address that it wants the answerer to select as the offerer BUNDLE
address [Section 5.2.3.2]. The answerer MUST first select the "m="
line associated with the offerer suggested BUNDLE mid, and check
whether it fulfils the following criteria:
o The answerer will in the SDP answer create the BUNDLE group;
o The answerer will not in the SDP answer move the "m=" line out of
the BUNDLE group [Section 5.2.4.4];
o The answerer will not in the SDP answer reject the "m=" line
[Section 5.2.4.5]; and
o The offerer did not in the associated SDP offer assign a zero port
value to the "m=" line.
If all of the criteria above is fulfilled, the answerer MUST select
the address associated with the "m=" line as the offerer BUNDLE
address.
If all of the criteria is not fulfilled, the answerer MUST select the
next mid value in the mid list, and perform the same criteria check
for the "m=" line associated with the mid value.
In the SDP answer, the answerer selected BUNDLE mid value represents
the "m=" line which address (in the associated SDP offer) the
answerer has selected as the offerer BUNDLE address.
[Section 13.1] shows an example of an offerer BUNDLE address
selection.
5.2.4.3. Answerer Selection of Answerer BUNDLE Address
When an answerer generates an SDP answer, the answerer MUST select a
BUNDLE address for itself, referred to as the answerer BUNDLE
address, and in the SDP answer assign the answerer BUNDLE address to
each "m=" line within the created BUNDLE group.
The answerer MUST NOT in the SDP answer assign the answerer BUNDLE
address to an "m=" line that is not associated with the BUNDLE group,
or to an "m=" line that is associated with another BUNDLE group.
The answerer is allowed to select a new answerer BUNDLE address in
every SDP answer that the answerer generates.
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[Section 13.1] shows an example of an answerer BUNDLE address
selection.
5.2.4.4. Moving A Media Description Out Of A BUNDLE Group
When an answerer generates an SDP answer, in which the answerer moves
a bundled "m=" line out a BUNDLE group, the answerer assigns an
address to the moved "m=" line based on the type of address that the
offerer in the associated SDP offer assigned to the "m=" line.
o If the offerer in the SDP offer has assigned a shared address
(e.g. a previously selected offerer BUNDLE address) to the "m="
line, the answerer MUST in the SDP answer reject the moved "m="
line, according to the procedures in [Section 5.2.4.5].
o If the offerer in the SDP offer assigned an SDP 'bundle-only'
attribute to the "m=" line, the answerer MUST in the SDP answer
reject the moved "m=" line, according to the procedures in
[Section 5.2.4.5].
o If the offerer in the SDP offer assigned a unique address to the
"m=" line, the answerer MUST in the SDP answer assign a unique
address to the moved "m=" line.
In addition, in either case above, the answerer MUST NOT in the SDP
answer include a mid value, associated with the moved "m=" line, in
the SDP 'group:BUNDLE' attribute mid list associated with the BUNDLE
group.
5.2.4.5. Rejecting A Media Description In A BUNDLE Group
When an answerer generates an SDP answer, in which the answerer
rejects an "m=" line, the answerer MUST in the SDP answer assign an
address with a zero port value to the rejected "m=" line, according
to the procedures in [RFC4566].
In addition, the answerer MUST NOT in the SDP answer include a mid
value, associated with the rejected "m=" line, in the SDP
'group:BUNDLE' attribute mid list associated with the BUNDLE group.
5.2.5. Offerer Processing of the SDP Answer
5.2.5.1. General
When an offerer receives an SDP answer, the offerer MUST apply the
selected offerer BUNDLE address to each bundled "m=" line. If the
offerer generates a subsequent SDP offer, the offerer MUST in the SDP
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offer assign the offerer BUNDLE address to each bundled "m=" line
(including any 'bundle-only' "m=" line) [Section 5.2.6].
If the SDP answer does not contain a BUNDLE group, the offerer MUST
cease to use any procedure associated with the BUNDLE mechanism.
5.2.5.2. Bundle Address Synchronization (BAS)
If the selected offerer BUNDLE address is different than the address
that the offerer in the associated SDP offer assigned to a bundled
"m=" line (including an "m=" line that the offerer in the SDP offer
added to an existing BUNDLE group [Section 5.2.6.2]), and the bundled
"m=" line was not rejected [Section 5.2.4.5], or moved out of the
BUNDLE group [Section 5.2.4.4] by the answerer, the offerer SHOULD as
soon as possible generate a subsequent SDP offer, in which the
offerer assigns the offerer BUNDLE address to each bundled "m=" line.
This procedure is referred to as Bundle Address Synchronization
(BAS), and the SDP offer is referred to as a BAS Offer.
The offerer MAY in the BAS offer modify any SDP parameter.
NOTE: It is important that the BAS offer gets accepted by the
answerer. For that reason the offerer needs to consider the
necessity to in the BAS offer modify SDP parameters that could get
the answerer to reject the BAS offer. Disabling "m=" lines, or
reducing the number of codecs, in a BAS offer is considered to have a
low risk of being rejected.
NOTE: The main purpose of the BAS offer is to ensure that
intermediaries, that might not support the BUNDLE mechanism, have
correct information regarding the address is going to be used to
transport the bundled media.
[Section 13.1] shows an example where an offerer sends a BAS offer.
5.2.6. Modifying the Session
5.2.6.1. General
When an offerer generates a subsequent SDP offer, the offerer MUST in
the SDP offer assign the previously selected offerer BUNDLE address
[Section 5.2.4.2] to each bundled "m=" line (including any bundle-
only "m=" line), unless the offerer in the SDP offer moves the "m="
line out of the BUNDLE group [Section 5.2.6.3], or disables the "m="
line [Section 5.2.6.4].
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If the SDP offerer in the SDP offer adds an "m=" line to the BUNDLE
group [Section 5.2.6.2], the offerer MAY assign the previously
selected offerer BUNDLE address to the added "m=" line.
In addition, the offerer MUST in the SDP offer indicate which address
(unique or previously selected offerer BUNDLE address) it wants the
answerer to select as the offerer BUNDLE address, following the
procedures in [Section 5.2.3.2]. The offerer MUST do this even if
the offerer in the SDP offer assigns a previously selected offerer
BUNDLE address to each bundled "m=" line.
5.2.6.2. Adding a media description to a BUNDLE group
When an offerer generates an SDP offer, in which the offerer wants to
add an "m=" line to a BUNDLE group, the offerer assigns in the SDP
offer an address (unique or previously selected offerer BUNDLE
address) to the "m=" line, assigns an SDP 'mid' attribute to the "m="
line, and places the mid value in the SDP 'group:BUNDLE' attribute
mid list associated with the BUNDLE group [Section 5.2.3.2].
NOTE: If the offerer wants the answerer to select the address
associated with the added "m=" as the offerer BUNDLE address, the
offerer suggested BUNDLE mid MUST represent the added "m=" line
[Section 5.2.3.2].
[Section 13.3] shows an example where an offerer sends an SDP offer
in order to add an "m=" line to a BUNDLE group.
5.2.6.3. Moving A Media Description Out Of A BUNDLE Group
When an offerer generates an SDP offer, in which the offerer wants to
move a bundled "m=" line out of a BUNDLE group, the offerer MUST
assign a unique address to the "m=" line. In addition, the offerer
MUST NOT place a mid value associated with the "m=" line in the SDP
'group:BUNDLE' attribute mid list associated with the BUNDLE group.
NOTE: The offerer MAY keep a previously assigned SDP 'mid' attribute
in an "m=" line that it wants to move out of a BUNDLE group, e.g. if
the mid value is used for some other SDP grouping extension than
BUNDLE.
[Section 13.4] shows an example where an offerer sends an SDP offer
in order to move an "m=" line out of a BUNDLE group.
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5.2.6.4. Disabling A Media Description In A BUNDLE Group
When an offerer generates an SDP offer, in which the offerer wants to
disable a bundled "m=" line, the offerer MUST assign an address with
a zero port alue to the "m=" line, following the procedures in
[RFC4566]. In addition, the offerer MUST NOT place a mid value
associated with the "m=" line in the SDP 'group:BUNDLE' attribute mid
list associated with the BUNDLE group.
NOTE: The offerer MAY assign an SDP 'mid' attribute to an "m=" line
that it wants to disable, e.g. if the mid value is used for some
other SDP grouping extension than BUNDLE.
[Section 13.5] shows an example where an offerer sends an SDP offer
in order to disable an "m=" line within a BUNDLE group.
6. SDP 'bundle-only' Attribute
6.1. General
This section defines a new SDP media-level attribute [RFC4566],
'bundle-only'. An offerer can in an SDP offer assign a 'bundle-only'
"m=" line to a bundled "m=" line (including an "m=" line that the
offerer wants to add to the BUNDLE group [Section 5.2.6.2]), in order
to ensure that the answerer only accepts the "m=" line if the
answerer supports the BUNDLE mechanism, and if the answerer in the
SDP answer keeps the "m=" line within the BUNDLE group.
6.2. SDP Offer/Answer Procedures
6.2.1. Generating the Initial SDP Offer
When an offerer generates an initial SDP offer, in order to create a
BUNDLE group, the offerer can in the SDP offer assign an SDP 'bundle-
only' attribute to an "m=" line that the offerer wants to be within
the BUNDLE group.
The offerer MUST in the SDP offer assign a zero port value the
bundle-only "m=" line.
6.2.2. Generating the SDP Answer
When the answerer selects the offerer BUNDLE address
[Section 5.2.4.2], the answerer MUST also take a bundle-only "m="
line with a non-zero port value into consideration.
If the offerer in the SDP offer has assigned a zero port value to a
bundle-only "m=" line, and if the answerer accepts the "m=" line, the
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answerer will treat the "m=" line as any other bundle "m=" line when
the answerer generates the SDP answer [Section 5.2.4].
NOTE: If the offerer in the SDP offer has assigned a zero port value
to a bundled "m=" line, but the offerer has not assigned a 'bundle-
only' SDP attribute to the "m=" line, it is an indication that the
offerer wants to disable the "m=" line [Section 5.2.6.4].
If the answerer in the SDP answer does not keep the bundle-only "m="
line within the BUNDLE group, the answerer MUST in the SDP answer
reject the "m=" line [Section 5.2.4.5].
The answerer MUST NOT in the SDP answer assign an SDP 'bundle-only'
attribute to an "m=" line (even if the offerer in the associated SDP
offer has assigned a 'bundle-only' attribute to the "m="line).
6.2.3. Offerer Processing of the SDP Answer
When the offerer receives an SDP answer, the offerer follows the
procedures in [Section 5.2.5]. If the offerer in the associated SDP
offer assigned an SDP 'bundle-only' attribute to an "m=" line, and
the "m=" line was accepted (and was kept within the BUNDLE group) by
the answerer, the selected offerer BUNDLE address also applies to the
"m=" line.
6.2.4. Modifying the Session
When an offerer creates a subsequent SDP offer, the offerer follows
the procedures in [Section 5.2.6]. If the offerer in the SDP offer
assigns an SDP 'bundle-only' attribute to a bundled "m=" line, in
order to ensure that the answerer accepts the "m=" line only if the
answerer keeps the "m=" line within the BUNDLE group, the offerer
MUST NOT assign a zero port value to the "m=" line. Instead, the
offerer MUST in the SDP offer assign the offerer BUNDLE address or,
if the "m=" line is added to the BUNDLE group [Section 5.2.6.2],
either the offerer BUNDLE address or a unique address, to the "m="
line.
NOTE: The offerer can in a subsequent SDP offer assign an SDP
'bundle-only' attribute to a bundled "m=" line even if the offerer
did not assign a 'bundle-only' attribute to the "m=" line in a
previous SDP offer.
If the offerer in the SDP offer wants to move a bundled "m=" line out
of a BUNDLE group [Section 5.2.6.3], the offerer MUST NOT in the SDP
offer assign a 'bundle-only' attribute to the "m=" line.
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If the offerer in the SDP offer wants to disable a bundled "m=" line
[Section 5.2.6.4], the offerer MUST NOT in the SDP offer assign a
'bundle-only' attribute to the "m=" line.
7. Protocol Identification
7.1. General
If bundled "m=" lines represent different transport protocols, there
MUST exist a specification which describes a mechanism, for this
specific transport protocol combination, how to associate a received
packet with the correct transport protocol.
In addition, if a received packet can be associated with more than
one bundled "m=" line, there MUST exist a specification which
describes a mechanism how to associated the received packet with the
correct "m=" line.
7.2. STUN, DTLS, SRTP
Section 5.1.2 of [RFC5764] describes a mechanism how to identify the
protocol among the STUN, DTLS and SRTP protocols (in any
combination). If an offer or answerer in SDP offers or answers
include bundled "m=" lines that represent these protocols, the
offerer or answerer MUST support the mechanism described in
[RFC5764], and no explicit negotiation is required in order to
indicate support and usage of the mechanism.
[RFC5764] does not describe how to identify different protocols
transported on DTLS, only how to identify the DTLS protocol itself.
If multiple protocols are transported on DTLS, there MUST exist a
specification describing a mechanism how to identify each individual
protocol. In addition, if a received DTLS packet can be associated
with more than one "m=" line, there MUST exist a specification which
describes a mechanism how to associate the received DTLS packet with
the correct "m=" line.
[Section 8.2] describes how to associate a received (S)RTP packet
with the correct "m=" line.
8. RTP Considerations
8.1. Single RTP Session
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8.1.1. 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 8.1.2].
o The "proto" value in each bundled "m=" line MUST be identical
(e.g. RTP/AVPF).
o A given SSRC SHOULD NOT transmit RTP packets using payload types
that originates 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 fails to function. Even if done in
proper sequence this causes RTP Timestamp rate switching issues [ref
to draft-ietf-avtext-multiple-clock-rates].
8.1.2. Payload Type (PT) Value Re-usage
Multiple bundled "m=" lines might represent RTP based media. As all
RTP based media associated with a BUNDLE group belong to the same RTP
session, in order for a given payload type value to used inside more
than one bundled "m=" line, all codecs associated with the payload
type numbers MUST share an identical codec configuration. This means
that the codecs MUST share the same media type, encoding name, clock
rate and any parameter that can affect the codec configuration and
packetization. [I-D.nandakumar-mmusic-sdp-mux-attributes] lists SDP
attributes, which attribute values must be identical for all codecs
that use the same payload type value.
8.2. Associating RTP/RTCP Packets With Correct SDP Media Description
In general, there are multiple mechanisms that can be used by an
endpoint in order to associate received RTP/RTCP packets with the
bundled "m=" line representing the RTP packets. Such mechanisms
include using the local address:port combination on which the RTP
packets are received, the payload type value carried inside the RTP
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packets, the SSRC values carried inside the RTP packets, and other
"m=" line specific information carried inside the RTP packets.
As all RTP/RTCP packets associated with a BUNDLE group are sent and
received using the same 5-tuple, the local address:port combination
cannot be used to associate received RTP packets with the correct
"m=" line.
As described in [Section 8.1.2], 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 SDP offer and answer inform each
other which SSRC values they will use inside sent RTP/RTCP packets
by, by assigning an SDP 'ssrc' attribute [RFC5576] to each bundled
"m=" line which contains a payload type value that is also used
inside another bundled "m=" line. As the SSRC values will be carried
inside the RTP/RTCP packets, the offerer and answerer can then use
that information to associate received RTP packets with the correct
"m=" line. However, an offerer will not know which SSRC values the
answerer will use until it has received the SDP answer providing that
information. Due to this, before the offerer has received the SDP
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 mechanism MUST use the mechanism
defined in Section 11, where the remote endpoint inserts the SDP
'mid' attribute value of an "m=" line in RTP and RTCP packets
associated with that "m=" line.
8.3. RTP/RTCP Multiplexing
8.3.1. 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 5-tuples will be
used for sending and receiving the RTP packets and the RTCP packets.
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8.3.2. SDP Offer/Answer Procedures
8.3.2.1. 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 associated
with a BUNDLE group.
8.3.2.2. Generating the Initial SDP Offer
When an offerer generates an initial SDP offer, if the offerer wants
to negotiate usage of RTP/RTCP multiplexing within a BUNDLE group,
the offerer MUST in the SDP offer assign an SDP 'rtcp-mux' attribute
[RFC5761] to each bundled "m=" line (including any bundle-only "m="
line). In addition, the offerer MUST in the SDP offer assign an SDP
'rtcp' attribute [RFC3605] to each bundled "m=" line (including any
bundle-only "m=" line), with an attribute value that is identical to
the port value assigned to the "m=" line itself.
If the offerer does not want to negotiate usage of RTP/RTCP
multiplexing, the offerer MUST NOT assign the SDP attributes above to
any bundled "m=" line.
8.3.2.3. Generating the SDP Answer
8.3.2.3.1. Generating the SDP Answer to an Initial SDP Offer
When the answerer generates an SDP answer to an initial SDP offer, if
the offerer in the associated SDP offer indicated support of RTP/RTCP
multiplexing [RFC5761] within a BUNDLE group, the answerer MUST in
the SDP answer either accept or reject usage of RTP/RTCP
multiplexing.
If the answerer accepts usage of RTP/RTCP multiplexing within the
BUNDLE group, the answerer MUST in the SDP answer assign an SDP
'rtcp-mux' attribute to each bundled "m=" line. The answerer MUST
NOT in the SDP answer assign an SDP 'rtcp' attribute to any bundled
"m=" line.
If the answerer rejects usage of RTP/RTCP multiplexing within the
BUNDLE group, the answerer MUST NOT in the SDP answer assign an SDP
'rtcp-mux' or SDP 'rtcp' attribute to any bundled "m=" line.
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8.3.2.3.2. Generating the SDP Answer to a Subsequent SDP Offer
When the answerer generates an SDP answer to a subsequent SDP offer,
if the offerer in the associated SDP offer indicated support of RTP/
RTCP multiplexing [RFC5761] within a BUNDLE group, the answerer MUST
in the SDP answer assign an SDP 'rtcp-mux' attribute and SDP 'rtcp'
attribute to each bundled "m=" line.
NOTE: The BUNDLE mechanism does not allow the answerer to, in a
subsequent SDP answer, disable usage of RTP/RTCP multiplexing, if the
offerer in the associated SDP offer indicates that it wants to
continue using RTP/RTCP multiplexing.
8.3.2.4. Offerer Processing of the SDP Answer
When the offerer receives an SDP answer, it follows the procedures
defined in [RFC5245].
8.3.2.5. Modifying the Session
When an offerer generates a subsequent SDP offer, if the offerer
wants to negotiate usage of RTP/RTCP multiplexing within a BUNDLE
group, or if the offerer wants to continue usage of previously
negotiated RTP/RTCP multiplexing within the BUNDLE group, the offerer
MUST in the SDP offer assign 'rtcp-mux' and 'rtcp' attributes to each
bundled "m=" line (including bundle-only "m=" lines), unless the "m="
line is disabled or removed from the BUNDLE group.
If the offerer does not want to negotiate usage of RTP/RTCP
multiplexing within the BUNDLE group, or if the offerer wants to
disable previously negotiated usage of RTP/RTCP multiplexing within a
BUNDLE group, the offerer MUST NOT in the SDP offer assign 'rtcp-mux'
and 'rtcp' attributes to any bundled "m=" line.
NOTE: It is RECOMMENDED that, once usage of RTP/RTCP multiplexing has
been negotiated within a BUNDLE group, that the usage of not
disabled. Disabling RTP/RTCP multiplexing means that the offerer and
answerer need to reserve new IP ports, to be used for sending and
receiving RTCP packets.
9. ICE Considerations
9.1. General
This section describes how to use the BUNDLE grouping extension
together with the Interactive Connectivity Establishment (ICE)
mechanism [RFC5245].
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Support and usage of ICE mechanism together with the BUNDLE mechanism
is optional.
9.2. SDP Offer/Answer Procedures
9.2.1. Generating the Initial SDP Offer
When an offerer generates an initial SDP offer, which contains a
BUNDLE group, the offerer MUST assign ICE candidates [RFC5245] to
each bundled "m=" line, except to an "m=" line to which the offerer
assigns a zero port value (e.g. a bundle-only "m=" line). The
offerer MUST assign unique ICE candidate values to each "m=" line.
9.2.2. Generating the SDP Answer
When an answerer generates and SDP Answer, which contains a BUNDLE
group, the answerer MUST assign ICE candidates to each bundled "m="
line. The answerer MUST assign identical ICE candidate values to
each bundled "m=" line.
9.2.3. Offerer Processing of the SDP Answer
When the offerer receives an SDP answer, it follows the procedures
defined in [RFC5245].
9.2.4. Modifying the Session
When an offerer generates a subsequent SDP offer, for each bundled
"m=" line to which the offerer assigns its BUNDLE address, the
offerer MUST assign identical ICE candidate values. The offerer MUST
assign the ICE candidate values associated with the "m=" line that
was used by the answerer to select the offerer BUNDLE address [ref-
to-be-added].
9.2.5. Keep-alives
Once it is known that both endpoints support, and accept to use, the
BUNDLE grouping extension, ICE connectivity checks and keep-alives
only needs to be performed for the whole BUNDLE group, instead of for
each bundled "m=" line.
10. Update to RFC 3264
10.1. General
This section replaces the text of the following sections of RFC 3264:
o Section 5.1 (Unicast Streams).
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o Section 8.2 (Removing a Media Stream).
o Section 8.4 (Putting a Unicast Media Stream on Hold).
10.2. 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.
10.3. 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.
10.4. 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.
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10.5. 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.
10.6. 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.
10.7. 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.
11. RTP/RTCP extensions for mid value transport
11.1. General
SDP Offerers and Answerers [RFC3264] can assign values, mid values,
to SDP Media Descriptions (m= lines) within SDP Offers and Answers,
using the procedures in [RFC5888]. Each mid value uniquely
references an m= line.
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This section defines a new RTP SDES item [RFC3550], 'MID', which is
used to carry mid values within RTCP SDES packets. This section also
defines a new RTP header extension [RFC5285], which can be used to
carry the mid value in RTP packets.
The SDES item and RTP header extension makes is possible for a
receiver to associate received RTCP- and RTP packets with a specific
m= line, to which the receiver has assigned a mid value, even if
those m= lines are part of the same RTP session. The endpoint
informs the remote endpoint about the mid values using the procedures
in [RFC5888], and the remote endpoint then inserts the mid values in
RTCP- and RTP packets sent towards the other endpoint.
NOTE: This text above defines how the mid value is carried in SDP
Offers and Answers. The usage of other signalling protocols for
carrying the mid value is not prevented, but the usage of such
protocols is outside the scope of this document.
The RTP MID SDES item SHOULD be sent in the first few RTCP packets
sent on joining the session, and SHOULD be sent regularly thereafter.
The exact number of RTCP packets in which this SDES item is sent is
intentionally not specified here, as it will depend on the expected
packet loss rate, the RTCP reporting interval, and the allowable
overhead.
The RTP MID header extension SHOULD be included in some RTP packets
at the start of the session and whenever the SSRC changes. It might
also be useful to include the header extension in RTP packets that
comprise random access points in the media (e.g., with video
I-frames). The exact number of RTP packets in which this header
extension is sent is intentionally not specified here, as it will
depend on expected packet loss rate and loss patterns, the overhead
the application can tolerate, and the importance of immediate receipt
of the mid value.
For robustness purpose, endpoints need to be prepared for situations
where the mid value is delayed, and SHOULD NOT terminate sessions in
such cases, as the mid value is likely to arrive soon.
11.2. RTP 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 | mid value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The mid value payload is UTF-8 encoded, as in SDP.
11.3. 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 mid value payload is UTF-8 encoded, as in
SDP.
11.4. IANA Considerations
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.]
This document adds the MID SDES item to the IANA "RTP SDES item
types" registry as follows:
Value: TBD
Abbrev.: MID
Name: Media Identification
Reference: RFCXXXX
This document defines a new extension URI in the RTP Compact Header
Extensions subregistry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:sdes:mid
Description: Media identification
Contact: christer.holmberg@ericsson.com
Reference: RFCXXXX
12. Security Considerations
This specification does not significantly change the security
considerations of SDP which can be found in Section X of TBD.
TODO: Think carefully about security analysis of reuse of same SDES
key on multiple "m=" lines when the far end does not use BUNDLE and
warn developers of any risks.
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13. Examples
13.1. Example: Bundle Address Selection
The example below shows:
o 1. An SDP offer, in which the offerer assigns a unique address to
each bundled "m=" line within the BUNDLE group.
o 2. An SDP 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 SDP 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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10002 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
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a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
a=rtpmap:32 MPV/90000
SDP Offer (3)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
13.2. Example: Bundle Mechanism Rejected
The example below shows:
o 1. An SDP offer, in which the offerer assigns a unique address to
each bundled "m=" line within the BUNDLE group.
o 2. An SDP 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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10002 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
m=audio 20000 RTP/AVP 0
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 30000 RTP/AVP 32
b=AS:1000
a=rtpmap:32 MPV/90000
13.3. Example: Offerer Adds A Media Description To A BUNDLE Group
The example below shows:
o 1. An SDP offer, in which the offerer adds a new "m=" line,
represented by the "zen" mid value, to a previously negotiated
BUNDLE group, assigns a unique address to the added "m=" line, and
assigns the previously selected offerer BUNDLE address to each of
the other bundled "m=" lines within the BUNDLE group.
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o 2. An SDP 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 SDP 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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 20000 RTP/AVP 66
a=mid:zen
b=AS:1000
a=rtpmap:66 H261/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar zen
m=audio 20000 RTP/AVP 0
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
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a=rtpmap:32 MPV/90000
m=video 20000 RTP/AVP 66
a=mid:zen
b=AS:1000
a=rtpmap:66 H261/90000
SDP Offer (3)
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar zen
m=audio 10000 RTP/AVP 0 8 97
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 10000 RTP/AVP 66
a=mid:zen
b=AS:1000
a=rtpmap:66 H261/90000
13.4. Example: Offerer Moves A Media Description Out Of A BUNDLE Group
The example below shows:
o 1. An SDP offer, 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 SDP 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 other
bundled "m=" line within the BUNDLE group.
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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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 50000 RTP/AVP 66
b=AS:1000
a=rtpmap:66 H261/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
a=rtpmap:32 MPV/90000
m=video 60000 RTP/AVP 66
b=AS:1000
a=rtpmap:66 H261/90000
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13.5. Example: Offerer Disables A Media Description Within A BUNDLE
Group
The example below shows:
o 1. An SDP offer, in which the offerer disables a bundled "m="
line within BUNDLE group, assigns a zero port number 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 SDP 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 other bundled "m=" lines within the BUNDLE group.
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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
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
m=video 10000 RTP/AVP 31 32
a=mid:bar
b=AS:1000
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
m=video 0 RTP/AVP 66
a=rtpmap:66 H261/90000
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 20000 RTP/AVP 0
a=mid:foo
b=AS:200
a=rtpmap:0 PCMU/8000
m=video 20000 RTP/AVP 32
a=mid:bar
b=AS:1000
a=rtpmap:32 MPV/90000
m=video 0 RTP/AVP 66
a=rtpmap:66 H261/90000
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14. IANA Considerations
This document requests IANA to register the new SDP Grouping semantic
extension called BUNDLE.
15. 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 mechanism 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 and Martin Thompson for taking the the time to
read the text along the way, and providing useful feedback.
16. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
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 SDP
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).
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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 mechanism.
o Q8 Closed (IETF#88): An Offerer can, before it knows whether the
Answerer supports the BUNDLE mechanism, 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).
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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.
17. References
17.1. 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.
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[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.nandakumar-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-01
(work in progress), February 2014.
17.2. 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.
[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.
[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
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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 number
value should be inserted in "m=" lines associated with a BUNDLE
group, as the purpose of the extension is to negotiate the usage of a
single 5-tuple for media associated with the "m=" lines. Issues with
both approaches, discussed in the Appendix have been raised. The
outcome was to specify a mechanism which uses SDP Offers with both
different and identical port number values.
Below are the primary issues that have been considered when defining
the "BUNDLE" grouping extension:
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 number values, and one of those port would be used for
the 5-tuple, problems would occur if an endpoint wants to disable/
reject the "m=" line associated with that port, by setting the port
number value to zero. After that, no "m=" line would contain the
port number value which is used for the 5-tuple. In addition, it is
unclear what would happen to the ICE candidates associated with the
"m=" line, as they are also used for the 5-tuple.
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
that generated an Offer with capabilities it does not understand is
not compliant with the specifications.
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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.
Authors' Addresses
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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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