MMUSIC Working Group C. Holmberg
Internet-Draft Ericsson
Updates: 3264 (if approved) H. Alvestrand
Intended status: Standards Track Google
Expires: October 14, 2017 C. Jennings
Cisco
April 12, 2017
Negotiating Media Multiplexing Using the Session Description Protocol
(SDP)
draft-ietf-mmusic-sdp-bundle-negotiation-38.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, specified by 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. The
specification also updates RFC 3264, to allow usage of zero port
values without meaning that media is rejected.
There are multiple ways to correlate the bundled RTP packets with the
appropriate media descriptions. This specification defines a new
Real-time Transport Protocol (RTP) 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-
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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
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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 October 14, 2017.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 7
5. SDP Grouping Framework BUNDLE Extension . . . . . . . . . . . 7
6. SDP 'bundle-only' Attribute . . . . . . . . . . . . . . . . . 8
7. SDP Information Considerations . . . . . . . . . . . . . . . 9
7.1. Connection Data (c=) . . . . . . . . . . . . . . . . . . 9
7.2. Bandwidth (b=) . . . . . . . . . . . . . . . . . . . . . 9
8. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 9
8.1. Mux Category Considerations . . . . . . . . . . . . . . . 10
8.2. Generating the Initial SDP Offer . . . . . . . . . . . . 10
8.2.1. Suggesting the offerer BUNDLE address . . . . . . . . 11
8.2.2. Example: Initial SDP Offer . . . . . . . . . . . . . 11
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8.3. Generating the SDP Answer . . . . . . . . . . . . . . . . 12
8.3.1. Answerer Selection of Offerer Bundle Address . . . . 13
8.3.2. Answerer Selection of Answerer BUNDLE Address . . . . 14
8.3.3. Moving A Media Description Out Of A BUNDLE Group . . 14
8.3.4. Rejecting A Media Description In A BUNDLE Group . . . 15
8.3.5. Example: SDP Answer . . . . . . . . . . . . . . . . . 15
8.4. Offerer Processing of the SDP Answer . . . . . . . . . . 15
8.5. Modifying the Session . . . . . . . . . . . . . . . . . . 16
8.5.1. Suggesting a new offerer BUNDLE address . . . . . . . 16
8.5.2. Adding a media description to a BUNDLE group . . . . 17
8.5.3. Moving A Media Description Out Of A BUNDLE Group . . 17
8.5.4. Disabling A Media Description In A BUNDLE Group . . . 18
9. Protocol Identification . . . . . . . . . . . . . . . . . . . 18
9.1. STUN, DTLS, SRTP . . . . . . . . . . . . . . . . . . . . 19
10. RTP Considerations . . . . . . . . . . . . . . . . . . . . . 19
10.1. Single RTP Session . . . . . . . . . . . . . . . . . . . 19
10.1.1. Payload Type (PT) Value Reuse . . . . . . . . . . . 20
10.2. Associating RTP/RTCP Streams With Correct SDP Media
Description . . . . . . . . . . . . . . . . . . . . . . 20
10.3. RTP/RTCP Multiplexing . . . . . . . . . . . . . . . . . 25
10.3.1. SDP Offer/Answer Procedures . . . . . . . . . . . . 26
11. ICE Considerations . . . . . . . . . . . . . . . . . . . . . 28
11.1. SDP Offer/Answer Procedures . . . . . . . . . . . . . . 29
11.1.1. Generating the Initial SDP Offer . . . . . . . . . . 29
11.1.2. Generating the SDP Answer . . . . . . . . . . . . . 29
11.1.3. Offerer Processing of the SDP Answer . . . . . . . . 30
11.1.4. Modifying the Session . . . . . . . . . . . . . . . 30
12. DTLS Considerations . . . . . . . . . . . . . . . . . . . . . 30
13. RTP Header Extensions Consideration . . . . . . . . . . . . . 31
14. Update to RFC 3264 . . . . . . . . . . . . . . . . . . . . . 31
14.1. Original text of section 5.1 (2nd paragraph) of RFC 3264 31
14.2. New text replacing section 5.1 (2nd paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 31
14.3. Original text of section 8.2 (2nd paragraph) of RFC 3264 32
14.4. New text replacing section 8.2 (2nd paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 32
14.5. Original text of section 8.4 (6th paragraph) of RFC 3264 32
14.6. New text replacing section 8.4 (6th paragraph) of RFC
3264 . . . . . . . . . . . . . . . . . . . . . . . . . . 32
15. RTP/RTCP extensions for identification-tag transport . . . . 33
15.1. RTCP MID SDES Item . . . . . . . . . . . . . . . . . . . 34
15.2. RTP SDES Header Extension For MID . . . . . . . . . . . 34
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
16.1. New SDES item . . . . . . . . . . . . . . . . . . . . . 35
16.2. New RTP SDES Header Extension URI . . . . . . . . . . . 35
16.3. New SDP Attribute . . . . . . . . . . . . . . . . . . . 36
16.4. New SDP Group Semantics . . . . . . . . . . . . . . . . 36
17. Security Considerations . . . . . . . . . . . . . . . . . . . 37
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18. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 38
18.1. Example: Bundle Address Selection . . . . . . . . . . . 38
18.2. Example: BUNDLE Extension Rejected . . . . . . . . . . . 40
18.3. Example: Offerer Adds A Media Description To A BUNDLE
Group . . . . . . . . . . . . . . . . . . . . . . . . . 41
18.4. Example: Offerer Moves A Media Description Out Of A
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 43
18.5. Example: Offerer Disables A Media Description Within A
BUNDLE Group . . . . . . . . . . . . . . . . . . . . . . 45
19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 46
20. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 47
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 55
21.1. Normative References . . . . . . . . . . . . . . . . . . 55
21.2. Informative References . . . . . . . . . . . . . . . . . 57
Appendix A. Design Considerations . . . . . . . . . . . . . . . 58
A.1. UA Interoperability . . . . . . . . . . . . . . . . . . . 59
A.2. Usage of port number value zero . . . . . . . . . . . . . 61
A.3. B2BUA And Proxy Interoperability . . . . . . . . . . . . 61
A.3.1. Traffic Policing . . . . . . . . . . . . . . . . . . 62
A.3.2. Bandwidth Allocation . . . . . . . . . . . . . . . . 62
A.4. Candidate Gathering . . . . . . . . . . . . . . . . . . . 62
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 63
1. Introduction
When multimedia communications are established, each 5-tuple reserved
for an individual media stream consume additional resources
(especially when Interactive Connectivity Establishment (ICE)
[RFC5245] is used). For this reason, it is attractive to use a
5-tuple for multiple media streams.
This specification defines a way to use a single address:port
combination (BUNDLE address) for receiving media specified by
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 specified by 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 specified by a BUNDLE group.
Once the offerer and the answerer have negotiated a BUNDLE group,
they associate their respective BUNDLE address with each "m=" line in
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the BUNDLE group. The BUNDLE addresses are used to receive all media
specified by the BUNDLE group.
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. The specification also updates RFC 3264, to
allow usage of zero port values without meaning that media is
rejected.
As defined in RFC 4566 [RFC4566], the semantics of assigning the same
transport address (IP address and port) 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] source description (SDES) item, 'MID', and a new RTP
SDES header extension that can be used to associate RTP streams with
media descriptions.
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 described by 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
associate a unique address with each "m=" line within an offer and
answer, according to the procedures in [RFC4566] and [RFC3264]
2. Terminology
"m=" line: SDP bodies contain one or more media descriptions. Each
media description is identified by an SDP "m=" line.
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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 associated
with only one "m=" line in an offer or answer.
Shared address: An IP address and port combination that is associated
with multiple "m=" lines within an offer or answer.
Offerer BUNDLE-tag: The first identification-tag in a given SDP
'group:BUNDLE' attribute identification-tag list in an offer.
Answerer BUNDLE-tag: The first identification-tag in a given SDP
'group:BUNDLE' attribute identification-tag list in an answer.
Offerer BUNDLE address: Within a given BUNDLE group, an IP address
and port combination used by an offerer to receive all media
specified by 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
specified by 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 specified by a given BUNDLE group.
Initial offer: The first offer, within an SDP session (e.g. a SIP
dialog when the Session Initiation Protocol (SIP) [RFC3261] is used
to carry SDP), in which the offerer indicates that it wants to create
a given BUNDLE group.
Subsequent offer: An offer which contains a BUNDLE group that has
been created as part of a previous offer/answer exchange.
Identification-tag: A unique token value that is used to identify an
"m=" 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-
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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].
4. Applicability Statement
The mechanism in this specification only applies to the Session
Description Protocol (SDP) [RFC4566], when used together with the SDP
offer/answer mechanism [RFC3264]. Declarative usage of SDP is out of
scope of this document, and is thus undefined.
5. SDP Grouping Framework BUNDLE Extension
This section defines a new SDP Grouping Framework 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 [RFC4961] is used.
All media associated with a BUNDLE group MUST be transport using the
same transport-layer protocol (e.g., UDP or TCP).
The BUNDLE extension is indicated using an SDP 'group' attribute with
a "BUNDLE" semantics value [RFC5888]. An identification-tag is
associated with 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.
NOTE: The order of the "m=" lines listed in the SDP 'group:BUNDLE'
attribute identification-tag list does not have to be the same as the
order in which the "m=" lines occur in the SDP.
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Section 8 defines the detailed SDP Offer/Answer procedures for the
BUNDLE extension.
6. SDP 'bundle-only' Attribute
This section defines a new SDP media-level attribute [RFC4566],
'bundle-only'. 'bundle-only' is a property attribute [RFC4566], and
hence has no value.
Name: bundle-only
Value: N/A
Usage Level: media
Charset Dependent: no
Example:
a=bundle-only
In order to ensure that an answerer that does not support the BUNDLE
extension always rejects a bundled "m=" line, the offerer can assign
a zero port value to the "m=" line. According to [RFC3264] 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 the offerer BUNDLE address has been selected, the offerer
does not need to include the 'bundle-only' attribute in subsequent
offers. By associating the offerer BUNDLE address with an "m=" line
of a subsequent offer, the offerer will ensure that the answerer will
either keep the "m=" line within the BUNDLE group, or the answerer
will have to reject the "m=" line.
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.
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7. SDP Information Considerations
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.
7.1. 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.
7.2. Bandwidth (b=)
An offerer and answerer MUST use the rules and restrictions defined
in [I-D.ietf-mmusic-sdp-mux-attributes] for associating the SDP
bandwidth (b=) line with bundled "m=" lines.
8. SDP Offer/Answer Procedures
This section describes the SDP Offer/Answer [RFC3264] procedures for:
o Negotiating and creating a BUNDLE group; and
o Selecting the BUNDLE addresses (offerer BUNDLE address and
answerer BUNDLE address); and
o Adding an "m=" line to a BUNDLE group; and
o Moving an "m=" line out of a BUNDLE group; and
o Disabling an "m=" line within a BUNDLE group.
The generic rules and procedures defined in [RFC3264] and [RFC5888]
also apply to the BUNDLE extension. For example, if an offer is
rejected by the answerer, the previously negotiated 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.
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The procedures in this section are independent of the media type or
"m=" line proto value 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)
[I-D.ietf-ice-rfc5245bis] mechanism.
SDP offers and answers can contain multiple BUNDLE groups. The
procedures in this section apply independently to a given BUNDLE
group.
8.1. Mux Category Considerations
When an offerer or answerer associates SDP attributes with a bundled
"m=" line (including any bundle-only "m=" line) associated with a
shared address, IDENTICAL and TRANSPORT mux category SDP attributes
[I-D.ietf-mmusic-sdp-mux-attributes] are associated with the "m="
line only if the "m=" line is also associated with the offerer/
answerer BUNDLE-tag. Otherwise the offerer/answerer MUST NOT
associate such SDP attributes with the "m=" line. The rule above
does not apply to a bundled "m=" line associated with a unique
address.
NOTE: As bundled "m=" lines (including any bundle-only "m=" line)
associated with a shared address will share the same IDENTICAL and
TRANSPORT mux category SDP attributes, and attribute values, there is
no need to associate such SDP attributes with each "m=" line. The
attributes and attribute values are implicitly applied to each "m="
line.
The semantics of some SDP attributes only apply to specific types of
media. For example, the semantics of the SDP 'rtcp-mux' and SDP
'rtcp-mux-only' attributes only apply to "m=" lines describing RTP-
based media. However, as described in Section 8.1, there are cases
where IDENTICAL and TRANSPORT mux category SDP attributes are only
associated with the "m=" line associated with the BUNDLE-tag. That
means that media-specific IDENTICAL and TRANSPORT mux category
attributes can be associated with an "m=" line associated with
another type of media.
8.2. Generating the Initial SDP Offer
When an offerer generates an initial offer, in order to create a
BUNDLE group, it MUST:
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o Assign a unique address to each "m=" line within the offer,
following the procedures in [RFC3264], unless the media line is a
'bundle-only' "m=" line (see below); and
o Add an SDP 'group:BUNDLE' attribute to the offer; and
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 8.2.1].
If the offerer wants to request that the answerer accepts a given
bundled "m=" line only if the answerer keeps the "m=" line within the
BUNDLE group, the offerer MUST:
o Associate an SDP 'bundle-only' attribute [Section 8.2.1] 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 8.5.4].
[Section 18.1] shows an example of an initial offer.
8.2.1. Suggesting the offerer BUNDLE address
In the offer, the address associated with the "m=" line associated
with the offerer BUNDLE-tag indicates the address that the offerer
suggests as the offerer BUNDLE address.
The "m=" line associated with the offerer BUNDLE-tag MUST NOT contain
a zero port value or an SDP 'bundle-only' attribute.
8.2.2. Example: Initial SDP Offer
The example shows an initial SDP offer. The offer includes two "m="
lines in the SDP, and suggests that both are included in a BUNDLE
group. The audio "m=" line is associated with the offerer BUNDLE-tag
(placed first in the SDP group:BUNDLE attribute identificatoin-id
list).
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SDP Offer
v=0
o=alice 2890844526 2890844526 IN IP4 atlanta.example.com
s=
c=IN IP4 atlanta.example.com
t=0 0
a=group:BUNDLE foo bar
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
8.3. Generating the SDP Answer
When an answerer generates an answer that 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
corresponding 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 corresponding offer.
If the answer contains a BUNDLE group, the answerer MUST:
o Select an Offerer BUNDLE Address [Section 8.3.1]; and
o Select an Answerer BUNDLE Address [Section 8.3.2];
The answerer is allowed to select a new Answerer BUNDLE address each
time it generates an answer to an offer.
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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 8.3.3]; or
o Reject the "m=" line [Section 8.3.4];
If the answerer keeps a bundle-only "m=" line within the BUNDLE
group, it follows the procedures (associates the answerer BUNDLE
address with 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 8.3.4].
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 8.5.4].
8.3.1. Answerer Selection of Offerer Bundle Address
In an offer, the address (unique or shared) associated with the
bundled "m=" line associated with the offerer BUNDLE-tag indicates
the address that the offerer suggests as the offerer BUNDLE address
[Section 8.2.1]. The answerer MUST check whether that "m=" line
fulfils the following criteria:
o The answerer will not move the "m=" line out of the BUNDLE group
[Section 8.3.3]; and
o The answerer will not reject the "m=" line [Section 8.3.4]; 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
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the identification-tag list, the answerer MUST NOT create the BUNDLE
group. In addition, unless the answerer rejects the whole offer, the
answerer MUST apply the answerer procedures for moving an "m=" line
out of a BUNDLE group [Section 8.3.3] to each bundled "m=" line in
the offer when creating the answer.
[Section 18.1] shows an example of an offerer BUNDLE address
selection.
8.3.2. Answerer Selection of Answerer BUNDLE Address
When the answerer selects a BUNDLE address for itself, referred to as
the answerer BUNDLE address, it MUST associate that address with each
bundled "m=" line within the created BUNDLE group in the answer.
The answerer MUST NOT associate the answerer BUNDLE address with an
"m=" line that is not within the BUNDLE group, or to an "m=" line
that is within another BUNDLE group.
[Section 18.1] shows an example of an answerer BUNDLE address
selection.
8.3.3. Moving A Media Description Out Of A BUNDLE Group
When an answerer wants to move an "m=" line out of a BUNDLE group, it
MUST first check the following criteria:
o In the corresponding offer, the "m=" line is associated with a
shared address (e.g. a previously selected offerer BUNDLE
address); or
o In the corresponding offer, an SDP 'bundle-only' attribute is
associated with the "m=" line, and the "m=" line contains a zero
port value.
If either criteria above is fulfilled, the answerer MUST reject the
"m=" line [Section 8.3.4].
Otherwise, if in the corresponding offer the "m=" line is associated
with a unique address, the answerer MUST associate a unique address
with the "m=" line in the answer (the answerer does not reject the
"m=" line).
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.
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8.3.4. Rejecting A Media Description In A BUNDLE Group
When an answerer rejects an "m=" line, it MUST associate an address
with a zero port value with the "m=" line in the answer, according to
the procedures in [RFC3264].
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.
8.3.5. Example: SDP Answer
The example shows an SDP answer, based on the SDP offer in
[Section 8.2.2]. The answers acceppts both "m=" lines in the BUNDLE
group.
SDP Answer
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=rtcp-mux
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
8.4. Offerer Processing of the SDP Answer
When an offerer receives an answer, if the answer contains a BUNDLE
group, the offerer MUST check that any bundled "m=" line in the
answer was indicated as bundled in the corresponding offer. If there
is no mismatch, the offerer MUST use the offerer BUNDLE address,
selected by the answerer [Section 8.3.1], as the address for each
bundled "m=" line.
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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.
8.5. Modifying the Session
When an offerer generates a subsequent offer, it MUST associate the
previously selected offerer BUNDLE address [Section 8.3.1] with each
bundled "m=" line (including any bundle-only "m=" line), except if:
o The offerer suggests a new offerer BUNDLE address [Section 8.5.1];
or
o The offerer wants to add a bundled "m=" line to the BUNDLE group
[Section 8.5.2]; or
o The offerer wants to move a bundled "m=" line out of the BUNDLE
group [Section 8.5.3]; or
o The offerer wants to disable the bundled "m=" line
[Section 8.5.4].
In addition, the offerer MUST select an offerer BUNDLE-tag
[Section 8.2.1] associated with the previously selected offerer
BUNDLE address, unless the offerer suggests a new offerer BUNDLE
address.
8.5.1. Suggesting a new offerer BUNDLE address
When an offerer generates an offer, in which it suggests a new
offerer BUNDLE address [Section 8.2.1], the offerer MUST:
o Assign the address (shared address) to each "m=" line within the
BUNDLE group; or
o Assign the address (unique address) to one bundled "m=" line.
In addition, the offerer MUST indicate that the address is the new
suggested offerer BUNDLE address [Section 8.2.1].
NOTE: Unless the offerer associates the new suggested offerer BUNDLE
address with each bundled "m=" line, it can associate unique
addresses with any number of bundled "m=" lines (and the previously
selected offerer BUNDLE address to any remaining bundled "m=" line)
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if it wants to suggest multiple alternatives for the new offerer
BUNDLE address.
8.5.2. 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 to the added "m=" line; or
o Assign the previously selected offerer BUNDLE address to the added
"m=" line; or
o If the offerer associates a new (shared address) suggested offerer
BUNDLE address with each bundled "m=" line [Section 8.5.1], also
associate that address with the added "m=" line.
In addition, the offerer MUST add the identification-tag associated
with the added "m=" line to the SDP 'group:BUNDLE' attribute
identification-tag list with the BUNDLE group [Section 8.2.1].
NOTE: Assigning a unique address to the "m=" line allows the answerer
to move the "m=" line out of the BUNDLE group [Section 8.3.3],
without having to reject the "m=" line.
If the offerer associates a unique address with the added "m=" line,
and if the offerer suggests that address as the new offerer BUNDLE
address [Section 8.5.1], the offerer BUNDLE-tag MUST represent the
added "m=" line [Section 8.2.1].
If the offerer associates a new suggested offerer BUNDLE address with
each bundled "m=" line [Section 8.5.1], including the added "m="
line, the offerer BUNDLE-tag MAY represent the added "m=" line
[Section 8.2.1].
[Section 18.3] shows an example where an offerer sends an offer in
order to add a bundled "m=" line to a BUNDLE group.
8.5.3. 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 associate a unique address with the "m=" line; and
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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 the removed "m=" line is associated with the previously
selected BUNDLE-tag, the offerer needs to suggest a new BUNDLE-tag
[Section 8.2.1].
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 8.5.2] to the "m=" line.
[Section 18.4] shows an example of an offer for moving an "m=" line
out of a BUNDLE group.
8.5.4. 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 associate an address with a zero port value with 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 18.5] shows an example of an offer for disabling an "m="
line within a BUNDLE group.
9. Protocol Identification
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 received data with
the correct protocol.
In addition, if received data can be associated with more than one
bundled "m=" line, there MUST exist a publicly available
specification which describes a mechanism for associating the
received data with the correct "m=" line.
This document describes a mechanism to identify the protocol of
received data among the STUN, DTLS and SRTP protocols (in any
combination), when UDP is used as transport-layer protocol, but does
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not describe how to identify different protocols transported on DTLS.
While the mechanism is generally applicable to other protocols and
transport-layer protocols, any such use requires further
specification around how to multiplex multiple protocols on a given
transport-layer protocol, and how to associate received data with the
correct protocols.
9.1. STUN, DTLS, SRTP
Section 5.1.2 of [RFC5764] describes a mechanism to identify the
protocol of a received packet among the STUN, Datagram Transport
Layer Security (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
describes a mechanism for associating the received DTLS packet with
the correct "m=" line.
[Section 10.2] describes how to associate the packets in a received
SRTP stream with the correct "m=" line.
10. RTP Considerations
10.1. Single RTP Session
All RTP-based media within a single BUNDLE group belong to a single
RTP session [RFC3550].
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 only if each codec associated with the payload type number
shares an identical codec configuration [Section 10.1.1].
o The proto value in each bundled RTP-based "m=" line MUST be
identical (e.g. RTP/AVPF).
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o The RTP MID header extension MUST be enabled, by associating an
SDP 'extmap' attribute [RFC5285], with a 'urn:ietf:params:rtp-
hdrext:sdes:mid' URI value, with each bundled RTP-based "m=" line
in every offer and answer.
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 can be reused by another
source (possibly associated with a different bundled "m=" line) after
a delay of 5 RTCP reporting intervals (the delay is to ensure the
SSRC has timed out, in case the RTCP BYE packet was lost [RFC3550]).
10.1.1. Payload Type (PT) Value Reuse
Multiple bundled "m=" lines might represent RTP based media. As all
RTP based media specified by 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.ietf-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 Streams With Correct SDP Media Description
NOTE: The text in this section is copied from Appendix B of JSEP.
The community has not yet agreed on the text.
As described in [RFC3550], RTP packets are associated with RTP
streams [RFC7656]. Each RTP stream is identified by an SSRC value,
and each RTP packet includes an SSRC field that is used to associate
the packet with the correct RTP stream. RTCP packets also use SSRCs
to identify which RTP streams the packet relates to. However, a RTCP
packet can contain multiple SSRC fields, in the course of providing
feedback or reports on different RTP streams, and therefore can be
associated with multiple such streams.
In order to be able to process received RTP/RTCP packets correctly,
it must be possible to associate an RTP stream with the correct "m="
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line, as the "m=" line and SDP attributes associated with the "m="
line contain information needed to process the packets.
As all RTP streams associated with a BUNDLE group use the same
address:port combination for sending and receiving RTP/RTCP packets,
the local address:port combination cannot be used to associate an RTP
stream with the correct "m=" line. In addition, multiple RTP streams
might be associated with the same "m=" line.
An offerer and answerer can inform each other which SSRC values they
will use for an RTP stream by using the SDP 'ssrc' attribute
[RFC5576]. However, an offerer will not know which SSRC values the
answerer will use until the offerer has received the answer providing
that information. Due to this, before the offerer has received the
answer, the offerer will not be able to associate an RTP stream with
the correct "m=" line using the SSRC value associated with the RTP
stream. In addition, the offerer and answerer may start using new
SSRC values mid-session, without informing each other using the SDP
'ssrc' attribute.
In order for an offerer and answerer to always be able to associate
an RTP stream with the correct "m=" line, the offerer and answerer
using the BUNDLE extension MUST support the mechanism defined in
section 14, where the offerer and answerer insert the identification-
tag associated with an "m=" line (provided by the remote peer) into
RTP and RTCP packets associated with a BUNDLE group.
When using this mechanism, the mapping from an SSRC to an
identification-tag is carried in RTP header extensions or RTCP SDES
packets, as specified in section 14. Since a compound RTCP packet
can contain multiple RTCP SDES packets, and each RTCP SDES packet can
contain multiple chunks, a single RTCP packet can contain several
SSRC to identification-tag mappings. The offerer and answerer
maintain tables used for routing that are updated each time an RTP/
RTCP packet contains new information that affects how packets should
be routed.
However, some implementations of may not include this identification-
tag in their RTP and RTCP traffic when using the BUNDLE mechanism,
and instead use a payload type based mechanism for demuxing. In this
situation, each "m=" line MUST use unique payload type values, in
order for the payload type to be a reliable indicator of the relevant
"m=" line for the RTP stream. Note that when using payload type
based demuxing, an SSRC will be mapped to an "m=" line by the first
packet with that SSRC, and the mapping will not be changed even if
the same SSRC is received with a different payload type. In other
words, the SSRC cannot to "move" to a different "m=" line simply by
changing the payload type.
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Applications can implement RTP stacks in many different ways. The
algorithm below details one way that demultiplexing can be
accomplished, but is not meant to be prescriptive about exactly how
an RTP stack needs to be implemented. Applications MAY use any
algorithm that achieves equivalent results to those described in the
algorithm below.
To prepare for demultiplexing RTP/RTCP packets to the correct "m="
line, the following steps MUST be followed for each BUNDLE group.
Construct a table mapping MID to "m=" line for each "m=" line in
this BUNDLE group. Note that an "m=" line may only have one MID.
Construct a table mapping incoming SSRC to "m=" line for each "m="
line in this BUNDLE group and for each SSRC configured for
receiving in that "m=" line.
Construct a table mapping outgoing SSRC to "m=line" for each "m="
line in this BUNDLE group and for each SSRC configured for sending
in that "m=" line.
Construct a table mapping payload type to "m=" line for each "m="
line in the BUNDLE group and for each payload type configured for
receiving in that "m=" line. If any payload type is configured
for receiving in more than one "m=" line in the BUNDLE group, do
not it include it in the table, as it cannot be used to uniquely
identify a "m=" line.
Note that for each of these tables, there can only be one mapping
for any given key (MID, SSRC, or PT). In other words, the tables
are not multimaps.
As "m=" lines are added or removed from the BUNDLE groups, or their
configurations are changed, the tables above MUST also be updated.
For each RTP packet received, the following steps MUST be followed to
route the packet to the correct "m=" section within a BUNDLE group.
Note that the phrase 'deliver a packet to the "m=" line' means to
further process the packet as would normally happen with RTP/RTCP, if
it were received on a transport associated with that "m=" line
outside of a BUNDLE group (i.e., if the "m=" line were not BUNDLEd),
including dropping an RTP packet if the packet's PT does not match
any PT in the "m=" line.
If the packet has a MID, and that MID is not in the table mapping
MID to "m=" line, drop the packet and stop.
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If the packet has a MID, and the packet's extended sequence number
is greater than that of the last MID update, as discussed in
[RFC7941], Section 4.2.6, update the incoming SSRC mapping table
to include an entry that maps the packet's SSRC to the "m=" line
for that MID.
If the packet's SSRC is in the incoming SSRC mapping table, check
that the packet's PT matches a PT included on the associated "m="
line. If so, route the packet to that associated "m=" line and
stop; otherwise drop the packet and stop.
If the packet's payload type is in the payload type table, update
the the incoming SSRC mapping table to include an entry that maps
the packet's SSRC to the "m=" line for that payload type. In
addition, route the packet to the associated "m=" line and stop.
Otherwise, drop the packet.
For each RTCP packet received (including each RTCP packet that is
part of a compound RTCP packet), the packet MUST be routed to the
"m=" line for the RTP streams it contains information about. This
routing is type-dependent, as each kind of RTCP packet has its own
mechanism for associating it with the relevant RTP streams.
Packets for which no appropriate "m=" line can be identified (i.e.,
for unknown RTP streams) are not relevant in the context of this
algorithm and MAY be dropped. This situation may occur with certain
multiparty RTP topologies.
Rules for handling the various types of RTCP packets are explained
below.
If the packet is of type SDES, for each chunk in the packet whose
SSRC is found in the incoming SSRC table, deliver a copy of the
packet to the "m=" line associated with that SSRC. In addition,
for any SDES MID items contained in these chunks, if the MID is
found in the table mapping MID to "m=" line, update the incoming
SSRC table to include an entry that maps the chunk's SSRC to the
"m=" line associated with that MID, unless the packet is older
than the packet that most recently updated the mapping for this
SSRC, as discussed in [RFC7941], Section 4.2.6.
Note that if an SDES packet is received as part of a compound RTCP
packet, the SSRC to "m=" line mapping may not exist until the SDES
packet is handled (e.g., in the case where RTCP for a source is
received before any RTP packets). Therefore, when processing a
compound packet, any contained SDES packet MUST be handled first.
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If the packet is of type BYE, it indicates that the RTP streams
referenced in the packet are ending. Therefore, for each SSRC
indicated in the packet that is found in the incoming SSRC table,
first deliver a copy of the packet to the "m=" line associated
with that SSRC, but then remove the entry for that SSRC from the
incoming SSRC table after an appropriate delay to account for
"straggler packets", as specified in [RFC3550], Section 6.2.1.
If the packet is of type SR or RR, for each report block in the
report whose "SSRC of source" is found in the outgoing SSRC table,
deliver a copy of the RTCP packet to the "m=" line associated with
that SSRC. In addition, if the packet is of type SR, and the
sender SSRC for the packet is found in the incoming SSRC table,
deliver a copy of the packet to the "m=" line associated with that
SSRC.
If the implementation supports RTCP XR and the packet is of type
XR, as defined in [RFC3611], for each report block in the report
whose "SSRC of source" is is found in the outgoing SSRC table,
deliver a copy of the RTCP packet to the "m=" line associated with
that SSRC. In addition, if the sender SSRC for the packet is
found in the incoming SSRC table, deliver a copy of the packet to
the "m=" line associated with that SSRC.
If the packet is a feedback message of type RTPFB or PSFB, as
defined in [RFC4585], it will contain a media source SSRC, and
this SSRC is used for routing certain subtypes of feedback
messages. However, several subtypes of PSFB messages include
target SSRC(s) in a section called Feedback Control Information
(FCI). For these messages, the target SSRC(s) are used for
routing.
If the packet is a feedback message that does not include target
SSRCs in its FCI section, and the media source SSRC is found in
the outgoing SSRC table, deliver the packet to the "m=" line
associated with that SSRC. RTPFB and PSFB types that are handled
in this way include:
Generic NACK: [RFC4585] (PT=RTPFB, FMT=1).
Picture Loss Indication (PLI): [RFC4585] (PT=PSFB, FMT=1).
Slice Loss Indication (SLI): [RFC4585] (PT=PSFB, FMT=2).
Reference Picture Selection Indication (RPSI): [RFC4585]
(PT=PSFB, FMT=3).
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If the packet is a feedback message that does include target
SSRC(s) in its FCI section, it can either be a request or a
notification. Requests reference a RTP stream that is being sent
by the message recipient, whereas notifications are responses to
an earlier request, and therefore reference a RTP stream that is
being received by the message recipient.
If the packet is a feedback request that includes target SSRC(s),
for each target SSRC that is found in the outgoing SSRC table,
deliver a copy of the RTCP packet to the "m=" line associated with
that SSRC. PSFB types that are handled in this way include:
Full Intra Request (FIR): [RFC5104] (PT=PSFB, FMT=4).
Temporal-Spatial Trade-off Request (TSTR): [RFC5104] (PT=PSFB,
FMT=5).
H.271 Video Back Channel Message (VBCM): [RFC5104] (PT=PSFB,
FMT=7).
Layer Refresh Request (LRR): [I-D.ietf-avtext-lrr] (PT=PSFB,
FMT=TBD).
If the packet is a feedback notification that include target
SSRC(s), for each target SSRC that is found in the incoming SSRC
table, deliver a copy of the RTCP packet to the "m=" line
associated with that SSRC. PSFB types that are handled in this
way include:
Temporal-Spatial Trade-off Notification (TSTN): [RFC5104]
(PT=PSFB, FMT=6). This message is a notification in response
to a prior TSTR.
If the packet is of type APP, the only routing information
included is the source of the packet, and therefore the packet
could be related to any existing "m=" line. Accordingly, deliver
a copy of the packet to each "m=" line.
10.3. RTP/RTCP Multiplexing
Within a BUNDLE group, the offerer and answerer MUST enable RTP/RTCP
multiplexing [RFC5761] for the RTP-based media specified by the
BUNDLE group.
When RTP/RTCP multiplexing is enabled, the same address:port
combination will be used for sending all RTP packets and the RTCP
packets associated with the BUNDLE group. Each endpoint will send
the packets towards the BUNDLE address of the other endpoint. The
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same address:port combination MAY be used for receiving RTP packets
and RTCP packets.
10.3.1. SDP Offer/Answer Procedures
This section describes how an offerer and answerer use the SDP 'rtcp-
mux' attribute [RFC5761] and the SDP 'rtcp-mux-only' attribute
[I-D.ietf-mmusic-mux-exclusive] to negotiate usage of RTP/RTCP
multiplexing for RTP-based media associated with a BUNDLE group.
The mux category [I-D.ietf-mmusic-sdp-mux-attributes] of the SDP
'rtcp-mux' and 'rtcp-mux-only' attributes is IDENTICAL. Section 8.1
describes the details regarding which bundled "m=" lines an offerer
and answerer associates the attributes with.
RTP/RTCP multiplexing only applies to RTP-based media. However, as
described in Section 8.1, within a BUNDLE group the SDP 'rtcp-mux'
and SDP 'rtcp-mux-only' attributes might be associated with a non-
RTP-based bundled "m=" line.
10.3.1.1. Generating the Initial SDP Offer
When an offerer generates an initial offer, if the offer contains one
or more RTP-based bundled "m=" lines (or, if there is a chance that
RTP-based "m=" lines will later be added to the BUNDLE group), the
offerer MUST associate an SDP 'rtcp-mux' attribute [RFC5761] with one
or more "m=" lines, following the procedures for IDENTICAL mux
category attributes in Section 8.1. In addition, the offerer MAY
associate an SDP 'rtcp-mux-only' attribute
[I-D.ietf-mmusic-mux-exclusive] with the same "m=" lines.
NOTE: Whether the offerer associates the SDP 'rtcp-mux-only'
attribute depends on whether the offerer supports fallback to usage
of a separate port for RTCP in case the answerer moves one or more
RTP-based "m=" line out of the BUNDLE group in the answer.
NOTE: If the offerer associates an SDP 'rtcp-mux' attribute with one
or more bundled "m=" lines, but does not associate an SDP 'rtcp-mux-
only' attribute, the offerer can also associate an SDP 'rtcp'
attribute [RFC3605] with one or more RTP-based "m=" line in order to
provide a fallback port for RTCP, as described in [RFC5761].
However, the fallback port will only be used for RTP-based "m=" lines
moved out of the BUNDLE group by the answerer.
In the initial offer, the address:port combination for RTCP MUST be
unique in each bundled RTP-based "m=" line (excluding a bundle-only
"m=" line), similar to RTP.
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10.3.1.2. Generating the SDP Answer
When an answerer generates an answer, if the answerer supports RTP-
based media, and if a bundled "m=" line in the offer contained an SDP
'rtcp-mux' attribute, the answerer MUST enable usage of RTP/RTCP
multiplexing, even if there currently are no RTP-based "m=" lines
within the BUNDLE group. The answerer MUST associate an SDP 'rtcp-
mux' attribute with "m=" lines within the BUNDLE group in the answer
following the procedures for IDENTICAL mux category attributes in
Section 8.1. In addition, if the "m=" line in the offer contained an
an SDP "rtcp-mux-only" attribute, the answerer MUST associate an SDP
"rtcp-mux-only" attribute with the "m=" line in the answer.
If the "m=" line associated with the offerer BUNDLE-tag in the offer
contained an SDP 'rtcp-mux-only' attribute, and if the answerer moves
an RTP-based "m=" line out of the BUNDLE group in the answer
Section 8.3.3, the answerer MUST either associate the attribute with
the moved "m=" line (and enable RTP/RTCP multiplexing for the media
associated with the "m=" line), or reject the "m=" line
Section 8.3.4.
The answerer MUST NOT associate an SDP 'rtcp' attribute with any "m="
line within the BUNDLE group in the answer. The answerer will use
the port value of the selected offerer BUNDLE address for sending RTP
and RTCP packets associated with each RTP-based bundled "m=" line
towards the offerer.
If the usage of RTP/RTCP multiplexing within a BUNDLE group has been
negotiated in a previous offer/answer transaction, the answerer MUST
associate an SDP 'rtcp-mux' attribute with the "m=" line associated
with the answerer BUNDLE-tag in the answer. It is not possible to
disable RTP/RTCP multiplexing within a BUNDLE group.
10.3.1.3. Offerer Processing of the SDP Answer
When an offerer receives an answer, if the answerer has accepted the
usage of RTP/RTCP multiplexing (see Section 10.3.1.2), the answerer
follows the procedures for RTP/RTCP multiplexing defined in
[RFC5761]. The offerer will use the port value associated with the
answerer BUNDLE address for sending RTP and RTCP packets associated
with each RTP-based bundled "m=" line towards the answerer.
NOTE: It is considered a protocol error if the answerer has not
accepted the usage of RTP/RTCP multiplexing for RTP-based "m=" lines
that the answerer included in the BUNDLE group.
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10.3.1.4. Modifying the Session
When an offerer generates a subsequent offer, the offerer MUST
associate an SDP 'rtcp-mux' attribute with a bundled "m=" line,
following the procedures for IDENTICAL mux category attributes in
Section 8.1.
If the offerer wants to add a bundled RTP-based "m=" line to the
BUNDLE group, it MAY also associate an SDP 'rtcp-mux-only' attribute
with a bundled "m=", following the procedures for IDENTICAL mux
category attributes in Section 8.1. This allows the offerer to
mandate RTP/RTCP multiplexing for the added "m=" line (or the "m="
line to be rejected by the answerer) even if the answerer does not
accept the "m=" line within the BUNDLE group.
11. ICE Considerations
This section describes how to use the BUNDLE grouping extension
together with the Interactive Connectivity Establishment (ICE)
mechanism [I-D.ietf-ice-rfc5245bis].
The generic procedures for negotiating usage of ICE using SDP,
defined in [I-D.ietf-mmusic-ice-sip-sdp], also apply to usage of ICE
with BUNDLE, with the following exceptions:
o When 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.
o Among bundled "m=" lines (including any bundle-only "m=" line)
with which the offerer has associated a shared address, the
offerer only associates ICE-related media-level SDP attributes
with the "m=" line associated with the offerer BUNDLE-tag,
following the procedures in Section 8.1.
o Among "m=" lines with which the answerer has associated a shared
address within a BUNDLE group, the answerer only associates ICE-
related media-level SDP attributes with the "m=" line associated
with the answerer BUNDLE-tag, following the procedures in
Section 8.1.
Support and usage of ICE mechanism together with the BUNDLE extension
is OPTIONAL.
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11.1. SDP Offer/Answer Procedures
When an offerer associates a unique address with a bundled "m=" line
(excluding any bundle-only "m=" line), the offerer MUST associate SDP
'candidate' attributes (and other applicable ICE-related media-level
SDP attributes), containing unique ICE properties (candidates etc),
with the "m=" line, according to the procedures in
[I-D.ietf-mmusic-ice-sip-sdp].
When an offerer associates a shared address with a bundled "m=" line,
the offerer MUST associate SDP 'candidate' attributes (and other
applicable ICE-related media-level SDP attributes) with the "m=" line
following the procedures in Section 8.1.
When an answerer associates a shared address with an "m=" line within
a BUNDLE group, if the answerer MUST associate SDP 'candidate'
attributes (and other applicable ICE-related media-level SDP
attributes) with the "m=" line following the procedures in
Section 8.1.
NOTE: As most ICE-related media-level SDP attributes belong to the
TRANSPORT mux category [I-D.ietf-mmusic-sdp-mux-attributes], the
offerer and answerer follow the procedures in Section 8.1 when
deciding whether to associate an attribute with a bundled "m=" line.
However, in the case of ICE-related media-level attributes, the rules
apply to all attributes (see note below), even if they belong to a
different mux category.
NOTE: The following ICE-related media-level SDP attributes are
defined in [I-D.ietf-mmusic-ice-sip-sdp]: 'candidiate', 'remote-
candidates', 'ice-mismatch', 'ice-ufrag', 'ice-pwd', and 'ice-
pacing'.
11.1.1. Generating the Initial SDP Offer
When an offerer generates an initial offer, the offerer MUST
associate ICE-related media-level SDP attributes with bundled "m="
lines forllowin the procedures in [Section 11.1].
11.1.2. Generating the SDP Answer
When an answerer generates an answer that contains a BUNDLE group,
the answer MUST associate ICE-related SDP attributes to "m=" lines
within the BUNDLE group according to [Section 11.1].
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11.1.3. 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
associate the ICE properties associated with the offerer BUNDLE
address, selected by the answerer [Section 8.3.1], with each bundled
"m=" line.
11.1.4. Modifying the Session
When an offerer generates a subsequent offer, it MUST associate ICE
properties to bundled "m=" lines following the procedures in
[Section 11.1].
12. DTLS Considerations
One or more media streams within a BUNDLE group might use the
Datagram Transport Layer Security (DTLS) protocol [RFC6347] in order
to encrypt the data, or to negotiate encryption keys if another
encryption mechanism is used to encrypt media.
When DTLS is used within a BUNDLE group, the following rules apply:
o There can only be one DTLS association [RFC6347] associated with
the BUNDLE group; and
o Each usage of the DTLS association within the BUNDLE group MUST
use the same mechanism for determining which endpoints (the
offerer or answerer) become DTLS client and DTLS server; and
o Each usage of the DTLS association within the Bundle group MUST
use the same mechanism for determining whether an offer or answer
will trigger the establishment of a new DTLS association, or
whether an existing DTLS association will be used; and
o If the DTLS client supports DTLS-SRTP [RFC5764] it MUST include
the 'use_srtp' extension [RFC5764] in the DTLS ClientHello message
[RFC5764], The client MUST include the extension even if the usage
of DTLS-SRTP is not negotiated as part of the multimedia session
(e.g., SIP session [RFC3261].
NOTE: The inclusion of the 'use_srtp' extension during the initial
DTLS handshake ensures that a DTLS renegotiation will not be required
in order to include the extension, in case DTLS-SRTP encrypted media
is added to the BUNDLE group later during the multimedia session.
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13. RTP Header Extensions Consideration
When [RFC5285] RTP header extensions are used in the context of this
specification, the identifier used for a given extension MUST
identify the same extension across all the bundled media
descriptions.
14. Update to RFC 3264
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).
14.1. Original text of section 5.1 (2nd paragraph) of RFC 3264
For recvonly and sendrecv streams, the port number and address in the
offer indicate where the offerer would like to receive the media
stream. For sendonly RTP streams, the address and port number
indirectly indicate where the offerer wants to receive RTCP reports.
Unless there is an explicit indication otherwise, reports are sent to
the port number one higher than the number indicated. The IP address
and port present in the offer indicate nothing about the source IP
address and source port of RTP and RTCP packets that will be sent by
the offerer. A port number of zero in the offer indicates that the
stream is offered but MUST NOT be used. This has no useful semantics
in an initial offer, but is allowed for reasons of completeness,
since the answer can contain a zero port indicating a rejected stream
(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.
14.2. New text replacing section 5.1 (2nd paragraph) of RFC 3264
For recvonly and sendrecv streams, the port number and address in the
offer indicate where the offerer would like to receive the media
stream. For sendonly RTP streams, the address and port number
indirectly indicate where the offerer wants to receive RTCP reports.
Unless there is an explicit indication otherwise, reports are sent to
the port number one higher than the number indicated. The IP address
and port present in the offer indicate nothing about the source IP
address and source port of RTP and RTCP packets that will be sent by
the offerer. A port number of zero in the offer by default indicates
that the stream is offered but MUST NOT be used, but an extension
mechanism might specify different semantics for the usage of a zero
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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.
14.3. 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.
14.4. 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."
14.5. 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.
14.6. 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
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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.
15. RTP/RTCP extensions for identification-tag transport
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 SDES header extension [RFC7941], which
is used to carry the 'MID' RTCP SDES item in RTP packets.
The SDES item and RTP SDES header extension make it possible for a
receiver to associate each RTP stream with with a specific "m=" line,
with which the receiver has associated an identification-tag, even if
those "m=" lines are part of the same RTP session. The RTP SDES
header extension also ensures that the media recipient gets the
identification-tag upon receipt of the first decodable media and is
able to associate the media with the correct application.
A media recipient informs the media sender about the identification-
tag associated with an "m=" line through the use of an 'mid'
attribute [RFC5888]. The media sender then inserts the
identification-tag in RTCP and RTP packets sent to the media
recipient.
NOTE: This text above defines how identification-tags are carried in
SDP Offers and Answers. The usage of other 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 after joining the session, and SHOULD be sent
regularly thereafter. The exact number of RTCP packets in which this
SDES item is sent is intentionally not specified here, as it will
depend on the expected packet loss rate, the RTCP reporting interval,
and the allowable overhead.
The RTP SDES header extension for carrying the 'MID' RTCP SDES SHOULD
be included in some RTP packets at the start of the session and
whenever the SSRC changes. It might also be useful to include the
header extension in RTP packets that comprise access points in the
media (e.g., with video I-frames). The exact number of RTP packets
in which this header extension is sent is intentionally not specified
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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
NOT terminate sessions in such cases, as the identification-tag is
likely to arrive soon.
15.1. RTCP MID SDES Item
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MID=TBD | length | identification-tag ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The identification-tag payload is UTF-8 encoded, as in SDP.
The identification-tag is not zero terminated.
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.]
15.2. RTP SDES Header Extension For MID
The payload, containing the identification-tag, of the RTP SDES
header extension element can be encoded using either the one-byte or
two-byte header [RFC7941]. The identification-tag payload is UTF-8
encoded, as in SDP.
The identification-tag is not zero terminated. Note, that the set of
header extensions included in the packet needs to be padded to the
next 32-bit boundary using zero bytes [RFC5285].
As the identification-tag is included in either an RTCP SDES item or
an RTP SDES header extension, or both, there should be some
consideration about the packet expansion caused by the
identification-tag. To avoid Maximum Transmission Unit (MTU) issues
for the RTP packets, the header extension's size needs to be taken
into account when encoding the media.
It is recommended that the identification-tag is kept short. Due to
the properties of the RTP header extension mechanism, when using the
one-byte header, a tag that is 1-3 bytes will result in a minimal
number of 32-bit words used for the RTP SDES header extension, in
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case no other header extensions are included at the same time. Note,
do take into account that some single characters when UTF-8 encoded
will result in multiple octets. The identification-tag MUST NOT
contain any user information, and applications SHALL avoid generating
the identification-tag using a pattern that enables application
identification.
16. IANA Considerations
16.1. New SDES item
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
[RFC EDITOR NOTE: Please replace TBD with the assigned SDES
identifier value.]
This document adds the MID SDES item to the IANA "RTP SDES item
types" registry as follows:
Value: TBD
Abbrev.: MID
Name: Media Identification
Reference: RFCXXXX
16.2. New RTP SDES Header Extension URI
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
This document defines a new extension URI in the RTP SDES Compact
Header Extensions sub-registry of the RTP Compact Header Extensions
registry sub-registry, according to the following data:
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Extension URI: urn:ietf:params:rtp-hdrext:sdes:mid
Description: Media identification
Contact: christer.holmberg@ericsson.com
Reference: RFCXXXX
The SDES item does not reveal privacy information about the users.
It is simply used to associate RTP-based media with the correct SDP
media description (m- line) in the SDP used to negotiate the media.
The purpose of the extension is for the offerer to be able to
associate received multiplexed RTP-based media before the offerer
receives the associated SDP answer.
16.3. New SDP Attribute
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
This document defines a new SDP media-level attribute, 'bundle-only',
according to the following data:
Attribute name: bundle-only
Type of attribute: media
Subject to charset: No
Purpose: Request a media description to be accepted
in the answer only if kept within a BUNDLE
group by the answerer.
Appropriate values: N/A
Contact name: Christer Holmberg
Contact e-mail: christer.holmberg@ericsson.com
Reference: RFCXXXX
Mux category: NORMAL
16.4. New SDP Group Semantics
[RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
document.]
This document registers the following semantics with IANA in the
"Semantics for the "group" SDP Attribute" subregistry (under the
"Session Description Protocol (SDP) Parameters" registry:
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Semantics Token Reference
------------------------------------- ------ ---------
Media bundling BUNDLE [RFCXXXX]
17. Security Considerations
The security considerations defined in [RFC3264] and [RFC5888] apply
to the BUNDLE extension. Bundle does not change which information,
e.g., RTP streams, flows over the network, with the exception of the
usage of the MID SDES item as discussed below. Primarily it changes
which addresses and ports, and thus in which (RTP) sessions that the
information is flowing in. This affects the security contexts being
used and can cause previously separated information flows to share
the same security context. This has very little impact on the
performance of the security mechanism of the RTP sessions. In cases
where one would have applied different security policies on the
different RTP streams being bundled, or where the parties having
access to the security contexts would have differed between the RTP
stream, additional analysis of the implications are needed before
selecting to apply BUNDLE.
The identification-tag, independent of transport, RTCP SDES packet or
RTP header extension, can expose the value to parties beyond the
signaling chain. Therefore, the identification-tag values MUST be
generated in a fashion that does not leak user information, e.g.,
randomly or using a per-bundle group counter, and SHOULD be 3 bytes
or less, to allow them to efficiently fit into the MID RTP header
extension. Note that if implementations use different methods for
generating identification-tags this could enable fingerprinting of
the implementation making it vulnerable to targeted attacks. The
identification-tag is exposed on the RTP stream level when included
in the RTP header extensions, however what it reveals of the RTP
media stream structure of the endpoint and application was already
possible to deduce from the RTP streams without the MID SDES header
extensions. As the identification-tag is also used to route the
media stream to the right application functionality it is also
important that the value received is the one intended by the sender,
thus integrity and the authenticity of the source are important to
prevent denial of service on the application. Existing SRTP
configurations and other security mechanisms protecting the whole
RTP/RTCP packets will provide the necessary protection.
When the BUNDLE extension is used, the set of configurations of the
security mechanism used in all the bundled media descriptions will
need to be compatible so that they can simultaneously used in
parallel, at least per direction or endpoint. When using SRTP this
will be the case, at least for the IETF defined key-management
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solutions due to their SDP attributes (a=crypto, a=fingerprint,
a=mikey) and their classification in
[I-D.ietf-mmusic-sdp-mux-attributes].
The security considerations of "RTP Header Extension for the RTP
Control Protocol (RTCP) Source Description Items" [RFC7941] requires
that when RTCP is confidentiality protected that any SDES RTP header
extension carrying an SDES item, such as the MID RTP header
extension, is also protected using commensurate strength algorithms.
However, assuming the above requirements and recommendations are
followed there are no known significant security risks with leaving
the MID RTP header extension without confidentiality protection.
Thus, the requirements in RFC 7941 MAY be ignored for the MID RTP
header extension. Security mechanisms for RTP/RTCP are discussed in
Options for Securing RTP Sessions [RFC7201], for example SRTP
[RFC3711] can provide the necessary security functions of ensuring
the integrity and source authenticity.
18. Examples
18.1. Example: Bundle Address Selection
The example below shows:
o An offer, in which the offerer associates a unique address with
each bundled "m=" line within the BUNDLE group.
o An answer, in which the answerer selects the offerer BUNDLE
address, and then selects its own BUNDLE address (the answerer
BUNDLE address) and associates it with each 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
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN 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=rtcp-mux
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
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18.2. Example: BUNDLE Extension Rejected
The example below shows:
o An offer, in which the offerer associates a unique address with
each bundled "m=" line within the BUNDLE group.
o An answer, in which the answerer rejects the offered BUNDLE group,
and associates a unique address with 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=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 10002 RTP/AVP 31 32
b=AS:1000
a=mid:bar
a=rtcp-mux
a=rtpmap:31 H261/90000
a=rtpmap:32 MPV/90000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
SDP Answer (2)
v=0
o=bob 2808844564 2808844564 IN IP4 biloxi.example.com
s=
c=IN IP4 biloxi.example.com
t=0 0
m=audio 20000 RTP/AVP 0
b=AS:200
a=rtcp-mux
a=rtpmap:0 PCMU/8000
m=video 30000 RTP/AVP 32
b=AS:1000
a=rtcp-mux
a=rtpmap:32 MPV/90000
18.3. Example: Offerer Adds A Media Description To A BUNDLE Group
The example below shows:
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o A subsequent offer (the BUNDLE group has been created as part of a
previous offer/answer exchange), in which the offerer adds a new
"m=" line, represented by the "zen" identification-tag, to a
previously negotiated BUNDLE group, associates a unique address
with the added "m=" line, and associates the previously selected
offerer BUNDLE address with each of the other bundled "m=" lines
within the BUNDLE group.
o An answer, in which the answerer associates the answerer BUNDLE
address with each bundled "m=" line (including the newly added
"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 zen
m=audio 10000 RTP/AVP 0 8 97
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 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=rtcp-mux
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=
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c=IN IP4 biloxi.example.com
t=0 0
a=group:BUNDLE foo bar zen
m=audio 20000 RTP/AVP 0
b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 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
18.4. Example: Offerer Moves A Media Description Out Of A BUNDLE Group
The example below shows:
o 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, associates a unique
address with the moved "m=" line, and associates the offerer
BUNDLE address with each other bundled "m=" line within the BUNDLE
group.
o An answer, in which the answerer moves the "m=" line out of the
BUNDLE group, associates a unique address with the moved "m="
line, and associates the answerer BUNDLE address with 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
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b=AS:200
a=mid:foo
a=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 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=rtcp-mux
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=rtcp-mux
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=rtcp-mux
a=rtpmap:66 H261/90000
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18.5. Example: Offerer Disables A Media Description Within A BUNDLE
Group
The example below shows:
o 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 a BUNDLE group, assigns a zero port
number to the disabled "m=" line, and associates the offerer
BUNDLE address with each of the other bundled "m=" lines within
the BUNDLE group.
o 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 associates the answerer BUNDLE address with 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=rtcp-mux
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 iLBC/8000
a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
m=video 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
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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=rtcp-mux
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
19. 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.
The SDP examples are also modified versions from the ones in the
Alvestrand proposal.
Thanks to Paul Kyzivat, Martin Thomson, Flemming Andreasen, Thomas
Stach, Ari Keranen, Adam Roach, Christian Groves, Roman Shpount,
Suhas Nandakumar, Nils Ohlmeier, Jens Guballa, Raju Makaraju and
Justin Uberti for reading the text, and providing useful feedback.
Thanks to Bernard Aboba, Cullen Jennings, Peter Thatcher, Justin
Uberti, and Magnus Westerlund for providing the text for the section
on RTP/RTCP stream association.
Thanks to Magnus Westerlund, Colin Perkins and Jonathan Lennox for
providing help and text on the RTP/RTCP procedures.
Thanks to Spotify for providing music for the countless hours of
document editing.
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20. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-37
o The following GitHub pull request was merged:
o https://github.com/cdh4u/draft-sdp-bundle/pull/33
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-36
o The following GitHub pull requests were merged:
o https://github.com/cdh4u/draft-sdp-bundle/pull/32
o - extmap handling in BUNDLE.
o https://github.com/cdh4u/draft-sdp-bundle/pull/31
o - Additional Acknowledgement text added.
o https://github.com/cdh4u/draft-sdp-bundle/pull/30
o - MID SDES item security procedures updated
o https://github.com/cdh4u/draft-sdp-bundle/pull/29
o - Appendix B of JSEP moved into BUNDLE.
o - Associating RTP/RTCP packets with SDP m- lines.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-35
o Editorial changes on RTP streaming mapping section based on
comments from Colin Perkins.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-34
o RTP streams, instead of RTP packets, are associated with m- lines.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-33
o Editorial changes based on comments from Eric Rescorla and Cullen
Jennings:
o - Changes regarding usage of RTP/RTCP multiplexing attributes.
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o - Additional text regarding associating RTP/RTCP packets with SDP
m- lines.
o - Reference correction.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-32
o Editorial changes based on comments from Eric Rescorla and Cullen
Jennings:
o - Justification for mechanism added to Introduction.
o - Clarify that the order of m- lines in the group:BUNDLE attribute
does not have to be the same as the order in which the m- lines
are listed in the SDP.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-31
o Editorial changes based on GitHub Pull requests by Martin Thomson:
o - https://github.com/cdh4u/draft-sdp-bundle/pull/2
o - https://github.com/cdh4u/draft-sdp-bundle/pull/1
o Editorial change based on comment from Diederick Huijbers (9th
July 2016).
o Changes based on comments from Flemming Andreasen (21st June
2016):
o - Mux category for SDP bundle-only attribute added.
o - Mux category considerations editorial clarification.
o - Editorial changes.
o RTP SDES extension according to draft-ietf-avtext-sdes-hdr-ext.
o Note whether Design Considerations appendix is to be kept removed:
o - Appendix is kept within document.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-30
o Indicating in the Abstract and Introduction that the document
updates RFC 3264.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-29
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o Change based on WGLC comment from Colin Perkins.
o - Clarify that SSRC can be reused by another source after a delay
of 5 RTCP reporting intervals.
o Change based on WGLC comment from Alissa Cooper.
o - IANA registry name fix.
o - Additional IANA registration information added.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-28
o - Alignment with exclusive mux procedures.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-27
o - Yet another terminology change.
o - Mux category considerations added.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-26
o - ICE considerations modified: ICE-related SDP attributes only
added to the bundled m- line representing the selected BUNDLE
address.
o - Reference to draft-ietf-mmusic-ice-sip-sdp added.
o - Reference to RFC 5245 replaced with reference to draft-ietf-ice-
rfc5245bis.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-25
o - RTP/RTCP mux procedures updated with exclusive RTP/RTCP mux
considerations.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-24
o - Reference and procedures associated with exclusive RTP/RTCP mux
added
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-23
o - RTCP-MUX mandatory for bundled RTP m- lines
o - Editorial fixes based on comments from Flemming Andreasen
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Changes from draft-ietf-mmusic-sdp-bundle-negotiation-22
o - Correction of Ari's family name
o - Editorial fixes based on comments from Thomas Stach
o - RTP/RTCP correction based on comment from Magnus Westerlund
o -- http://www.ietf.org/mail-archive/web/mmusic/current/
msg14861.html
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-21
o - Correct based on comment from Paul Kyzivat
o -- 'received packets' replaced with 'received data'
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-20
o - Clarification based on comment from James Guballa
o - Clarification based on comment from Flemming Andreasen
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-19
o - DTLS Considerations section added.
o - BUNDLE semantics added to the IANA Considerations
o - Changes based on WGLC comments from Adam Roach
o -- http://www.ietf.org/mail-archive/web/mmusic/current/
msg14673.html
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-18
o - Changes based on agreements at IETF#92
o -- BAS Offer removed, based on agreement at IETF#92.
o -- Procedures regarding usage of SDP "b=" line is replaced with a
reference to to draft-ietf-mmusic-sdp-mux-attributes.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-17
o - Editorial changes based on comments from Magnus Westerlund.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-16
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o - Modification of RTP/RTCP multiplexing section, based on comments
from Magnus Westerlund.
o - Reference updates.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-15
o - Editorial fix.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-14
o - Editorial changes.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-13
o Changes to allow a new suggested offerer BUNDLE address to be
assigned to each bundled m- line.
o Changes based on WGLC comments from Paul Kyzivat
o - Editorial fixes
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-12
o Usage of SDP 'extmap' attribute added
o SDP 'bundle-only' attribute scoped with "m=" lines with a zero
port value
o Changes based on WGLC comments from Thomas Stach
o - ICE candidates not assigned to bundle-only m- lines with a zero
port value
o - Editorial changes
o Changes based on WGLC comments from Colin Perkins
o - Editorial changes:
o -- "RTP SDES item" -> "RTCP SDES item"
o -- "RTP MID SDES item" -> "RTCP MID SDES item"
o - Changes in section 10.1.1:
o -- "SHOULD NOT" -> "MUST NOT"
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o -- Additional text added to the Note
o - Change to section 13.2:
o -- Clarify that mid value is not zero terminated
o - Change to section 13.3:
o -- Clarify that mid value is not zero terminated
o -- Clarify padding
o Changes based on WGLC comments from Paul Kyzivat
o - Editorial changes:
o Changes based on WGLC comments from Jonathan Lennox
o - Editorial changes:
o - Defintion of SDP bundle-only attribute alligned with structure
in 4566bis draft
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-11
o Editorial corrections based on comments from Harald Alvestrand.
o Editorial corrections based on comments from Cullen Jennings.
o Reference update (RFC 7160).
o Clarification about RTCP packet sending when RTP/RTCP multiplexing
is not used (http://www.ietf.org/mail-archive/web/mmusic/current/
msg13765.html).
o Additional text added to the Security Considerations.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-10
o SDP bundle-only attribute added to IANA Considerations.
o SDES item and RTP header extension added to Abstract and
Introduction.
o Modification to text updating section 8.2 of RFC 3264.
o Reference corrections.
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o Editorial corrections.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-09
o Terminology change: "bundle-only attribute assigned to m= line" to
"bundle-only attribute associated with m= line".
o Editorial corrections.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-08
o Editorial corrections.
o - "of"->"if" (8.3.2.5).
o - "optional"->"OPTIONAL" (9.1).
o - Syntax/ABNF for 'bundle-only' attribute added.
o - SDP Offer/Answer sections merged.
o - 'Request new offerer BUNDLE address' section added
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-07
o OPEN ISSUE regarding Receiver-ID closed.
o - RTP MID SDES Item.
o - RTP MID Header Extension.
o OPEN ISSUE regarding insertion of SDP 'rtcp' attribute in answers
closed.
o - Indicating that, when rtcp-mux is used, the answerer MUST NOT
include an 'rtcp' attribute in the answer, based on the procedures
in section 5.1.3 of RFC 5761.
Changes from draft-ietf-mmusic-sdp-bundle-negotiation-06
o Draft title changed.
o Added "SDP" to section names containing "Offer" or "Answer".
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 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).
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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.
21. References
21.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003,
<http://www.rfc-editor.org/info/rfc3605>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
BCP 131, RFC 4961, DOI 10.17487/RFC4961, July 2007,
<http://www.rfc-editor.org/info/rfc4961>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
<http://www.rfc-editor.org/info/rfc5764>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>.
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[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC7941] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP
Header Extension for the RTP Control Protocol (RTCP)
Source Description Items", RFC 7941, DOI 10.17487/RFC7941,
August 2016, <http://www.rfc-editor.org/info/rfc7941>.
[I-D.ietf-ice-rfc5245bis]
Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", draft-ietf-ice-
rfc5245bis-08 (work in progress), December 2016.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
(work in progress), December 2016.
[I-D.ietf-mmusic-mux-exclusive]
Holmberg, C., "Indicating Exclusive Support of RTP/RTCP
Multiplexing using SDP", draft-ietf-mmusic-mux-
exclusive-11 (work in progress), February 2017.
[I-D.ietf-mmusic-ice-sip-sdp]
Petit-Huguenin, M., Keranen, A., and S. Nandakumar, "Using
Interactive Connectivity Establishment (ICE) with Session
Description Protocol (SDP) offer/answer and Session
Initiation Protocol (SIP)", draft-ietf-mmusic-ice-sip-
sdp-12 (work in progress), March 2017.
21.2. Informative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<http://www.rfc-editor.org/info/rfc3611>.
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[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <http://www.rfc-editor.org/info/rfc5104>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
<http://www.rfc-editor.org/info/rfc5576>.
[RFC7160] Petit-Huguenin, M. and G. Zorn, Ed., "Support for Multiple
Clock Rates in an RTP Session", RFC 7160,
DOI 10.17487/RFC7160, April 2014,
<http://www.rfc-editor.org/info/rfc7160>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://www.rfc-editor.org/info/rfc7201>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
[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-02 (work in progress), January 2015.
[I-D.ietf-avtext-lrr]
Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
Message", draft-ietf-avtext-lrr-04 (work in progress),
April 2017.
Appendix A. Design Considerations
One of the main issues regarding the BUNDLE grouping extensions has
been whether, in SDP Offers and SDP Answers, the same port value
should be inserted in "m=" lines associated with a BUNDLE group, as
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the purpose of the extension is to negotiate the usage of a single
address:port combination for media specified by 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.
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.
A.1. 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 knows 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.2. Usage of port number value zero
In an SDP Offer or SDP Answer, the media specified by 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.3. B2BUA And Proxy Interoperability
Some back to back user agents may be configured in a mode where if
the incoming call leg contains an SDP attribute the B2BUA does not
understand, the B2BUA still generates that SDP attribute in the Offer
for the outgoing call leg. Consider a B2BUA that did not understand
the SDP "rtcp" attribute, defined in RFC 3605, yet acted this way.
Further assume that the B2BUA was configured to tear down any call
where it did not see any RTCP for 5 minutes. In this case, if the
B2BUA received an Offer like:
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, a B2BUA 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.3.1. 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.3.2. 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 specified by all "m=" lines try to use that
bandwidth. That may either simply lead to bad user experience, or to
termination of the call.
A.4. Candidate Gathering
When using ICE, a candidate needs to be gathered for each port. This
takes approximately 20 ms extra for each extra "m=" line due to the
NAT pacing requirements. All of this gather can be overlapped with
other things while for exampe a web-page is loading to minimize the
impact. If the client only wants to generate TURN or STUN ICE
candidates for one of the "m=" lines and then use trickle ICE
[I-D.ietf-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
allocations 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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