One document matched: draft-ietf-mmusic-sdp-capability-negotiation-02.txt
Differences from draft-ietf-mmusic-sdp-capability-negotiation-01.txt
MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems
Intended Status: Proposed Standard February 13, 2007
Expires: August 2007
SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-02.txt
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Copyright (C) The IETF Trust (2007).
Abstract
The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. SDP was
not intended to provide capability indication or capability
negotiation, however over the years, SDP has seen widespread adoption
and as a result it has been gradually extended to provide limited
support for these. SDP and its current extensions however do not have
the ability to negotiate one or more alternative transport protocols
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(e.g. RTP profiles) which makes it particularly difficult to deploy
new RTP profiles such as secure RTP or RTP with RTCP-based feedback.
The purpose of this document is to address that and other real-life
limitations by extending SDP with capability negotiation parameters
and associated offer/answer procedures to use those parameters in a
backwards compatible manner.
The solution provided in this document provides a general SDP
capability negotiation framework. It also defines specifically how to
provide attributes and transport protocols as capabilities and
negotiate them using the framework. Extensions for other types of
capabilities (e.g. media types and formats) may be provided in other
documents.
Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................5
3. SDP Capability Negotiation Solution............................6
3.1. Solution Overview.........................................6
3.2. Version and Extension Indication Attributes...............9
3.2.1. Supported Capability Negotiation Extensions Attribute9
3.2.2. Required Capability Negotiation Extension Attribute.10
3.3. Capability Attributes....................................12
3.3.1. Attribute Capability Attribute......................12
3.3.2. Transport Protocol Capability Attribute.............13
3.4. Configuration Attributes.................................15
3.4.1. Potential Configuration Attribute...................15
3.4.2. Actual Configuration Attribute......................18
3.5. Offer/Answer Model Extensions............................20
3.5.1. Generating the Initial Offer........................20
3.5.2. Generating the Answer...............................21
3.5.3. Offerer Processing of the Answer....................22
3.5.4. Modifying the Session...............................22
3.6. Interactions with ICE....................................23
3.7. Processing Media before Answer...........................24
4. Examples......................................................24
4.1. Best-Effort Secure RTP...................................24
4.2. Multiple Transport Protocols.............................27
4.3. Session-Level MIKEY and Media Level Security Descriptions30
4.4. Capability Negotiation with Interactive Connectivity
Establishment.................................................30
5. Security Considerations.......................................30
6. IANA Considerations...........................................30
7. To Do and Open Issues.........................................30
8. Acknowledgments...............................................30
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9. Change Log....................................................31
9.1. draft-ietf-mmusic-sdp-capability-negotiation-02..........31
9.2. draft-ietf-mmusic-sdp-capability-negotiation-01..........31
9.3. draft-ietf-mmusic-sdp-capability-negotiation-00..........32
10. References...................................................34
10.1. Normative References....................................34
10.2. Informative References..................................34
Author's Addresses...............................................36
Intellectual Property Statement..................................36
Full.............................................................37
Copyright Statement..............................................37
Acknowledgment...................................................37
1. Introduction
The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. The SDP
contains one or more media stream descriptions with information such
as IP-address and port, type of media stream (e.g. audio or video),
transport protocol (possibly including profile information, e.g.
RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other
session and media stream parameters that define the session.
Simply providing media stream descriptions is sufficient for session
announcements for a broadcast application, where the media stream
parameters are fixed for all participants. When a participant wants
to join the session, he obtains the session announcement and uses the
media descriptions provided, e.g., joins a multicast group and
receives media packets in the encoding format specified. If the
media stream description is not supported by the participant, he is
unable to receive the media.
Such restrictions are not generally acceptable to multimedia session
invitations, where two or more entities attempt to establish a media
session that uses a set of media stream parameters acceptable to all
participants. First of all, each entity must inform the other of its
receive address, and secondly, the entities need to agree on the
media stream parameters to use for the session, e.g. transport
protocols and codecs. We here make a distinction between the
capabilities supported by each participant, the way in which those
capabilities can be supported and the parameters that can actually be
used for the session. More generally, we can say that we have the
following:
o A set of capabilities for the session and its associated media
stream components, supported by each side.
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o A set of potential configurations indicating which of those
capabilities can be used for the session and its associated media
stream components.
o A set of actual configurations for the session and its associated
media stream components, which specifies which combinations of
session parameters and media stream components to use and with
what parameters.
o A negotiation process that takes the set of potential
configurations (combinations of capabilities) as input and
provides the actual configurations as output.
SDP by itself was designed to provide only one of these, namely the
actual configurations, however over the years, use of SDP has been
extended beyond its original scope. Session negotiation semantics
were defined by the offer/answer model in RFC 3264. It defines how
two entities, an offerer and an answerer, exchange session
descriptions to negotiate a session. The offerer can include one or
more media formats (codecs) per media stream, and the answerer then
selects one or more of those offered and returns them in an answer.
Both the offer and the answer contain actual configurations;
capabilities and potential configurations are not supported. The
answer however may reduce the set of actual configurations from the
offer as well as extend the set of actual configurations that can be
used to receive media by the answerer.
Other relevant extensions have been defined. Simple capability
declarations, which define how to provide a simple and limited set of
capability descriptions in SDP was defined in RFC 3407. Grouping of
media lines, which defines how media lines in SDP can have other
semantics than the traditional "simultaneous media streams"
semantics, was defined in RFC 3388, etc.
Each of these extensions was designed to solve a specific limitation
of SDP. Since SDP had already been stretched beyond its original
intent, a more comprehensive capability declaration and negotiation
process was intentionally not defined. Instead, work on a "next
generation" of a protocol to provide session description and
capability negotiation was initiated [SDPng]. SDPng however has not
gained traction and has remained as work in progress for an extended
period of time. Existing real-time multimedia communication
protocols such as SIP, RTSP, Megaco, and MGCP continue to use SDP.
SDP and its current extensions however do not address an increasingly
important problem: the ability to negotiate one or more alternative
transport protocols (e.g., RTP profiles). This makes it difficult to
deploy new RTP profiles such as secure RTP (SRTP) [SRTP], RTP with
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RTCP-Based Feedback [AVPF], etc. This particular problem is
exacerbated by the fact that RTP profiles are defined independently.
When a new profile is defined and N other profiles already exist,
there is a potential need for defining N additional profiles, since
profiles cannot be combined automatically. For example, in order to
support the plain and secure RTP version of RTP with and without
RTCP-based feedback, four separate profiles (and hence profile
definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP [SRTP], RTP/AVPF
[AVPF], and RTP/SAVPF [SAVPF]. In addition to the pressing profile
negotiation problem, other important real-life limitations have been
found as well.
The purpose of this document is to define a mechanism that enables
SDP to provide limited support for indicating capabilities and their
associated potential configurations, and negotiate the use of those
potential configurations as actual configurations. It is not the
intent to provide a full-fledged capability indication and
negotiation mechanism along the lines of SDPng or ITU-T H.245.
Instead, the focus is on addressing a set of well-known real-life
limitations. More specifically, the solution provided in this
document provides a general SDP capability negotiation framework. It
also defines specifically how to provide attributes and transport
protocols as capabilities and negotiate them using the framework.
Extensions for other types of capabilities (e.g. media types and
formats) may be provided in other documents.
As mentioned above, SDP is used by several protocols, and hence the
mechanism should be usable by all of these. One particularly
important protocol for this problem is the Session Initiation
Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is
not specific to SIP) to negotiate sessions and hence the mechanism
defined here defines the offer/answer procedures to use for the
capability negotiation framework.
The rest of the document is structured as follows. In Section 3. we
present our SDP capability negotiation solution, which consists of
new SDP attributes and associated offer/answer procedures. In Section
4. we provide examples illustrating its use and in Section 5. we
provide the security considerations.
2. Conventions used in this document
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 [RFC2119].
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3. SDP Capability Negotiation Solution
In this section we first provide an overview of the SDP Capability
negotiation solution. This is followed by definitions of new SDP
attributes for the solution and its associated updated offer/answer
procedures.
3.1. Solution Overview
The solution consists of the following:
o Two new attributes to support versioning and extensions to the
framework itself as follows:
o A new attribute ("a=csup") that lists the supported base and
extension options to the framework.
o A new attribute ("a=creq") that lists the base and or
extensions to the framework that are required to be supported
by the entity receiving the SDP in order to do capability
negotiation.
o Two new attributes used to express capabilities as follows
(additional attributes can be defined as extensions):
o A new attribute ("a=acap") that defines how to list attribute
parameter values ("a=" values) as capabilities.
o A new attribute ("a=tcap") that defines how to list transport
protocols (e.g. "RTP/AVP") as capabilities.
o Two new attributes to negotiate configurations as follows:
o A new attribute ("a=pcfg") that lists the potential
configurations supported. This is done by reference to the
capabilities from the SDP in question. Multiple potential
configurations have an explicitly indicated ordering
associated with them. Extension capabilities can be defined
and referenced in the potential configurations.
o A new attribute ("a=acfg") to be used in an answer SDP. The
attribute identifies which of the potential configurations
from an offer SDP were used as actual configurations to form
the answer SDP. Extension capabilities can be included as
well.
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o Extensions to the offer/answer model that allow for capabilities
and potential configurations to be included in an offer.
Capabilities can be provided at the session level or the media
level. Potential configurations can be included at the media level
only, where they constitute alternative offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s). The answerer indicates which (if
any) of the potential configurations it used to form the answer by
including the actual configuration attribute ("a=acfg") in the
answer. Capabilities may be included in answers as well, where
they can aid in guiding a subsequent new offer.
The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (SRTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default (actual configuration), but SRTP is the preferred one
(potential configuration):
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=creq: v0
a=tcap:1 RTP/SAVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The required base and extensions are provided by the
"a=creq" attribute, which includes the option tag "v0" to indicate
that the base framework defined here must be supported. The
capabilities are provided by the "a=tcap" and "a=acap" attributes.
The transport capabilities ("a=tcap") indicate that secure RTP under
the AVP profile ("RTP/SAVP") is supported with an associated
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transport capability handle of 1. The "acap" attribute provides an
attribute capability with a handle of 1. The attribute capability is
a "crypto" attribute, which provides the keying material for SRTP
using SDP security descriptions [SDES]. The "a=pcfg" attribute
provides the potential configuration included in the offer by
reference to the capability parameters. One alternative is provided;
it has a configuration number of 1 and it consists of transport
protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and
the attribute capability 1, i.e. the crypto attribute provided.
Potential configurations are always preferred over actual
configurations, and hence Alice is expressing a preference for using
secure RTP.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation framework, and hence he accepts the
(preferred) potential configuration for Secure RTP provided by Alice:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a crypto attribute. If Bob supported one or more extensions to the
capability negotiation framework, he would have included those in the
answer as well (in an "a=csup" attribute).
Note that in this particular example, the answerer supported the
capability negotiation extensions defined here, however had he not,
the answerer would simply have ignored the new attributes and
accepted the (actual configuration) offer to use normal RTP. In that
case, the following answer would have been generated instead:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
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t=0 0
m=audio 4567 RTP/AVP 0 18
3.2. Version and Extension Indication Attributes
In this section, we present the new attributes associated with
indicating the SDP capability negotiation extensions supported and
required.
3.2.1. Supported Capability Negotiation Extensions Attribute
The SDP Capability negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined
in Section 6.
The Supported Capability Negotiation Extensions attribute ("a=csup")
contains a comma-separated list of option tags identifying the SDP
Capability negotiation extensions supported by the entity that
generated the SDP. The attribute is defined as follows:
a=csup: <option-tag-list>
RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup"
attribute adheres to the RFC 4566 "attribute" production, with an
att-value defined as follows:
att-value = *WSP option-tag-list
option-tag-list = option-tag *(COMMA option-tag)
option-tag = token ; defined in [SDP]
COMMA = *WSP "," *WSP ; defined in [RFC4234]
Note that white-space is permitted before the option-tag-list. Also,
implementers familiar with SIP should note that the above definition
of COMMA differs from the one in [RFC3261].
A special base option tag with a value of "v0" is defined for the
basic SDP capability negotiation framework. Entities use this option
tag with the "a=csup" attribute to indicate support for the SDP
capability negotiation framework specified in this document.
The following examples illustrates the use of the "a=csup" attribute
with the "v0" option tags and two hypothetical option tags, "foo" and
"bar":
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a=csup: v0
a=csup: foo
a=csup: bar
a=csup: v0, foo, bar
The "a=csup" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media-stream
in question only (option-tags provided at the session level apply as
well). There can be one or more "a=csup" attributes at both the
session and media-level (one or more per media stream in the latter
case).
Whenever an entity that supports one or more extensions to the SDP
Capability Negotiation framework generates an SDP, it SHOULD include
the "a=csup" attribute with the option tags for the extensions it
supports at the session and/or media-level, unless those option tags
are already provided in one or more "a=creq" attribute (see Section
3.2.2. ) at the relevant levels. The base option tag MAY be included.
3.2.2. Required Capability Negotiation Extension Attribute
The SDP Capability negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined
in Section 6.
The Required Capability Negotiation Extensions attribute ("a=creq")
contains a comma-separated list of option tags identifying the SDP
Capability negotiation extensions that MUST be supported by the
entity receiving the SDP in order for that entity to properly process
the SDP Capability negotiation. The attribute is defined as follows:
a=creq: <option-tag-list>
The "creq" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = *WSP option-tag-list
where "option-tag-list" is defined in Section 3.2.1. Note that
white-space is permitted before the option-tag-list.
The following examples illustrate the use of the "a=creq" attribute
with the "v0" base option tag and two hypothetical option tags, "foo"
and "bar":
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a=creq: v0
a=creq: foo
a=creq: bar
a=creq: v0, foo, bar
The "a=creq" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media-stream
in question only (required option tags provided at the session level
apply as well). There can be one or more "a=creq" attributes at both
the session and media-level (one or more per media stream in the
latter case).
When an entity generates an SDP and it requires the recipient of that
SDP to support one or more SDP capability negotiation extensions in
order to properly process the SDP Capability negotiation, the
"a=creq" attribute MUST be included with option-tags that identify
the required extensions at the session and/or media level, unless it
is already known that the receiving entity supports those option-tags
at the relevant levels (in which case their inclusion is OPTIONAL).
An example of this is when generating an answer to an offer. If the
answerer supports the required option-tags from the offer, and the
answerer does not require any additional option-tags beyond what
was listed in either the required ("a=creq") or supported
("a=csup") attributes from the offer, then the answerer is not
required to include a required ("a=creq") attribute with any
option-tags that may need to be supported (such as the base option
tag - "v0").
A recipient that receives an SDP and does not support one or more of
the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP capability negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies
that SDP capability negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP
capability negotiation MUST NOT be performed for the media stream in
question.
When an entity does not support one or more required SDP capability
negotiation extensions, the entity SHOULD proceed as if the SDP
capability negotiation attributes were not included in the first
place, i.e. all the capability negotiation attributes should be
ignored. In that case, the entity SHOULD include a "csup" attribute
listing the SDP capability negotiation extensions it actually
supports.
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This ensures that introduction of the SDP capability negotiation
mechanism does not introduce any new failure scenarios.
The above rules apply to the base option tag as well. Thus, entities
compliant to this specification MUST include a "creq" attribute (at
least in an offer) that includes the option tag "v0" as illustrated
below:
a=creq: v0
3.3. Capability Attributes
In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability
negotiation.
3.3.1. Attribute Capability Attribute
Attributes can be expressed as negotiable parameters by use of a new
attribute capability attribute ("a=acap"), which is defined as
follows:
a=acap: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the attribute capability and <att-par> is an
attribute ("a=") in its full '<type>=<value>' form (see [SDP]).
The "acap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = *WSP att-cap-num 1*WSP att-par
att-cap-num = 1*DIGIT ;defined in [RFC4234]
att-par = attribute ;defined in RFC 4266
Note that white-space is permitted before the att-cap-num. The "acap"
attribute can be provided at the session level for session-level
attributes and the media level for media-level attributes. The "acap"
attribute MUST NOT be used to provide a media-level attribute at the
session-level or vice versa.
Each occurrence of the "acap" attribute in the entire session
description MUST use a different value of <att-cap-num>.
There is a need to be able to reference both session-level and
media-level attributes in potential configurations at the media
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level, and this provides for a simple solution to avoiding overlap
between the references (handles) to each attribute capability.
The <att-cap-num> values provided are independent of similar <cap-
num> values provided for other capability attributes, i.e., they form
a separate name-space for attribute capabilities.
The following examples illustrate use of the "acap" attribute:
a=acap: 1 a=ptime:20
a=acap: 2 a=ptime:30
a=acap: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
SrzKTAv9zV
a=acap: 4 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
The first two provide attribute values for the ptime attribute. The
third provides SRTP parameters by using MIKEY with the key-mgmt
attribute [KMGMT]. The fourth provides SRTP parameters by use of
security descriptions with the crypto attribute [SDES]. Note that the
line-wrapping and new-lines in example three and four are provided
for formatting reasons only - they are not permitted in actual SDP.
Readers familiar with RFC 3407 may notice the similarity between
the RFC 3407 "cpar" attribute and the above. There are however a
couple of important differences, most notably that the "acap"
attribute contains a handle that enables referencing it and it
furthermore supports attributes only (the "cpar" attribute defined
in RFC 3407 supports bandwidth information as well). The "acap"
attribute also is not automatically associated with any particular
capabilities.
3.3.2. Transport Protocol Capability Attribute
Transport Protocols can be expressed as capabilities by use of a new
Transport Protocol Capability attribute ("a=tcap") defined as
follows:
a=tcap: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 2^31-1 (both
included) used to number the transport address capability for later
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reference, and <proto-list> is one or more <proto>, separated by
white space, as defined in the SDP "m=" line.
The "tcap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = *WSP trpr-cap-num 1*WSP proto-list
trpr-cap-num = 1*DIGIT ;defined in [RFC4234]
proto-list = proto *(1*WSP proto) ; defined in RFC 4566
Note that white-space is permitted before the trpr-cap-num. The
"tcap" attribute can be provided at the session- and media-level.
Each occurrence of the "tcap" attribute in the entire session
description MUST use a different value of <trpr-cap-num>. When
multiple <proto> values are provided, the first one is associated
with the value <trpr-cap-num>, the second one with the value one
higher, etc. The <trpr-cap-num> values provided are independent of
similar <cap-num> values provided for other capability attributes,
i.e., they form a separate name-space for transport protocol
capabilities.
Below, we provide examples of the "a=tcap" attribute:
a=tcap: 1 RTP/AVP
a=tcap: 2 RTP/AVPF
a=tcap: 3 RTP/SAVP RTP/SAVPF
The first one provides a capability for the "RTP/AVP" profile defined
in [RFC3551] and the second one provides a capability for the RTP
with RTCP-Based Feedback profile defined in [AVPF]. The third one
provides capabilities for the "RTP/SAVP" and "RTP/SAVPF" profiles.
Transport capabilities are inherently included in the "m=" line,
however they still need to be specified explicitly in a "tcap"
attribute, if they are to be used as a capability. This may seem
redundant (and indeed it is from the offerer's point of view),
however it is done to protect against middle-boxes that may modify
"m=" lines while passing unknown attributes through. If an implicit
capability were used instead (e.g. a reserved transport capability
number could be used to refer to the transport protocol in the "m="
line), and a middle-box were to modify the transport protocol in the
"m=" line (e.g. to translate between plain RTP and secure RTP), then
the potential configuration referencing that implicit transport
capability may no longer be correct. With explicit capabilities, we
avoid this pitfall, although the potential configuration preference
(see Section 3.4.1. ) may not reflect that of the middle-box (which
some may view as a feature).
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3.4. Configuration Attributes
3.4.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <config-number> <pot-cfg-list>
where <config-number> is an integer between 1 and 2^31-1 (both
included).
The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = *WSP config-number 1*WSP pot-cfg-list
config-number = 1*DIGIT ;defined in [RFC4234]
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = pot-attribute-parameter-config /
pot-transport-protocol-config /
pot-extension-config
The missing productions are defined below. Note that white-space is
permitted before the config-number.
The potential configuration attribute can be provided at the media-
level only. The attribute includes a configuration number, which is
an integer between 1 and 2^31-1 (both included). The configuration
number MUST be unique within the media stream. The configuration
number also indicates the relative preference of potential
configurations; lower numbers are preferred over higher numbers.
After the configuration number, one or more potential configuration
parameters MUST be provided. This document defines potential
attribute parameter configurations and potential transport protocol
configurations. Each of these MUST NOT be present more than once in
a particular potential configuration attribute. Potential extension
configurations can be included as well; unknown potential extension
configurations MUST be ignored (if support is required, then the
"a=creq" with a suitable option tag should be used). There can be
more than one potential extension configuration, however each
particular potential extension configuration MUST NOT be present more
than once in a given potential configuration attribute. Together,
these values define a potential configuration.
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There can be multiple potential configurations provided within a
media description. Each of these indicates not only a willingness,
but in fact a desire to use the potential configuration.
Attribute capabilities are included in a potential configuration by
use of the pot-attribute-parameter-config parameter, which is defined
by the following ABNF:
pot-attribute-parameter-config
= "a=" acap-cap-list *(BAR acap-cap-list)
acap-cap-list = att-cap-num *(COMMA att-cap-num)
att-cap-num = 1*DIGIT ;defined in [RFC4234]
BAR = *WSP "|" *WSP ; defined in [RFC4234]
Each potential attribute parameter configuration list is a comma-
separated list of attribute capability numbers where att-cap-num
refers to attribute capability numbers defined above and hence MUST
be between 1 and 2^31-1 (both included). Alternative potential
attribute parameter configurations are separated by a vertical bar
("|"), the scope of which extends to the next alternative (i.e. ","
has higher precedence than "|"). The alternatives are ordered by
preference with the most preferred listed first.
Transport protocol capabilities are included in a potential
configuration by use of the pot-transport-protocol-config parameter,
which is defined by the following ABNF:
pot-transport-protocol-config =
"t=" trpr-cap-num *(BAR trpr-cap-num)
trpr-cap-num = 1*DIGIT ; defined in [RFC4234]
The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 2^31-1 (both included).
Alternative potential transport protocol configurations are separated
by a vertical bar ("|"). The alternatives are ordered by preference
with the most preferred listed first. When transport protocol
capabilities are not included in a potential configuration at the
media level, the transport protocol information from the associated
"m=" line will be used.
In the presence of middle-boxes (the existence of which may not be
known), care should be taken with assuming that the transport
protocol in the "m=" line will not be modified by a middle-box. Use
of an explicit capability will guard against the capability
indications of that.
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Extension capabilities can be included in a potential configuration
as well. Such extensions MUST adhere to the following ABNF:
pot-extension-config = ext-cap-name "="
ext-cap-list *(BAR ext-cap-list)
ext-cap-name = token ; defined in [SDP]
ext-cap-list = ext-cap-num *(COMMA ext-cap-num)
ext-cap-num = 1*DIGIT ; defined in [RFC4234]
The ext-cap-name refers to the type of extension capability and the
ext-cap-num refers to a capability number associated with that
particular type of extension capability. The number MUST be between
1 and 2^31-1 (both included). Alternative potential extension
configurations for a particular extension are separated by a vertical
bar ("|"),the scope of which extends to the next alternative (i.e.
"," has higher precedence than "|"). Unsupported or unknown
potential extension configs MUST be ignored.
The "creq" attribute and its associated rules can be used to ensure
that required extensions are supported in the first place.
Potential configurations can be provided at the media level only,
however it is possible to reference capabilities provided at either
the session or media level. There are certain semantic rules and
restrictions associated with this:
A (media level) potential configuration in a given media description
MUST NOT reference a media-level capability provided in a different
media description; doing so invalidates that potential configuration.
A potential configuration can however reference a session-level
capability. The semantics of doing so (should that potential
configuration be chosen), depends on the type of capability. In the
case of transport capabilities, this has no particular implication.
In the case of attribute capabilities however, it does. More
specifically, the corresponding attribute value (provided within that
attribute capability) will be considered part of the active
configuration at the *session* level. In other words, it will be as-
if that attribute was simply provided with that value at the session-
level in the first place. Note that individual media streams perform
capability negotiation individually, and hence it is possible that
another media stream (where the attribute was part of a potential
configuration) chose a configuration without that session level
attribute. The session-level attribute however remains "active" and
hence applies to the entire session. It is up to the entity that
generates the SDP to ensure that in such cases, the resulting active
configuration SDP is still meaningful.
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The session-level operation of extension capabilities is undefined:
Consequently, if session-level extension capabilities are defined,
they MUST specify the implication of making them part of an active
configuration at the media level.
Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting
attributes:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVPF 0 18
a=creq: v0
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=tcap: 1 RTP/AVPF RTP/AVP
a=tcap: 3 RTP/SAVP RTP/SAVPF
a=pcfg:1 t=4|3 a=1
a=pcfg:8 t=1|2
We have two potential configurations listed here. The first one (and
most preferred, since its configuration number is "1") indicates that
either of the profiles RTP/SAVPF or RTP/SAVP (specified by the
transport protocol capability numbers 4 and 3) can be supported with
attribute capability 1 (the "crypto" attribute); RTP/SAVPF is
preferred over RTP/SAVP since its capability number (4) is listed
first in the preferred potential configuration. The second potential
configuration indicates that the RTP/AVPF of RTP/AVP profile can be
used, with RTP/AVPF being the preferred one. This non secure RTP
alternative is the less preferred one since its configuration number
is "8".
3.4.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential
configurations from an offer SDP were used as actual configurations
in an answer SDP. This is done by reference to the configuration
number and the attribute capabilities and transport protocol
capabilities from the offer that were actually used by the answerer
in his offer/answer procedure. If extension capabilities were used,
those will be included by reference as well. Note that the
configuration number and all capability numbers used are those from
the offer; not the answer.
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The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <act-cfg-list>
The "acfg" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows:
att-value = *WSP config-number 1*WSP act-cfg-list
;config-number defined in Section 3.4.1.
act-cfg-list = capability *(1*WSP capability)
capability = act-attribute-parameter-config /
act-transport-protocol-config /
act-extension-config
act-attribute-parameter-config =
"a=" acap-cap-list ; defined in Section 3.4.1.
act-transport-protocol-config =
"t=" trpr-cap-num ; defined in Section 3.4.1.
act-extension-config =
ext-cap-name "=" ext-cap-list ; defined in Section 3.4.1.
Note that white-space is permitted before the config-number. The
actual configuration ("a=acfg") attribute can be provided at the
media-level only. There MUST NOT be more than one occurrence of an
actual configuration attribute within a given media description.
Below, we provide an example of the "a=acfg" attribute (building on
the previous example with the potential configuration attribute):
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVPF 0
a=creq: 0
a=acfg:1 t=4 a=1
It indicates that the answerer used an offer consisting of potential
configuration number 1 with transport protocol capability 4 from the
offer (RTP/SAVPF) and attribute capability 1 (the "crypto"
attribute).
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3.5. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s).
[EDITOR'S NOTE: Multicast considerations have been omitted for
now.]
TO DO: Elaborate and firm up offer/answer procedures.
3.5.1. Generating the Initial Offer
An offerer that wants to use the SDP capability negotiation
extensions defined in this document MUST include the following in the
offer:
o an SDP capability negotiation required extensions attribute ("a-
creq") that contains the option tag "v0". It must either be
provided at the session-level or for each individual media stream.
Option tags for any other required extensions MUST be included as
well (in accordance with Section 3.2.2. )
o one or more attribute capability attributes (as defined in Section
3.3.1. ) if alternative attribute parameter values are to be
indicated as offerer capabilities or be negotiated.
o one or more transport protocol capability attributes (as defined
in Section 3.3.2. ) if alternative transport protocols are to be
to be indicated as offerer capabilities or be negotiated.
o one or more potential configuration attributes (as defined in
Section 3.4. ) if alternative potential configurations are to be
negotiated.
o one or more required capability negotiation extension attributes
(as defined in Section 3.2.2. ), if the answerer is required to
support one or more SDP capability negotiation extensions.
The offerer SHOULD furthermore include the following:
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o one or more supported capability negotiation extension attributes
("a=csup" as defined in Section 3.2.1. ), if the offerer supports
one or more SDP capability negotiation extensions that have not
been included in one or more "a=creq" attributes at the relevant
session and media level(s).
The capabilities provided merely indicate what the offerer is capable
of doing. They do not constitute a commitment or even an indication
to actually use them. This applies to potential configurations listed
at the session level as well. Conversely, each of the potential
configurations listed at the media level constitutes an alternative
offer which may be used to negotiate and establish the session.
The current actual configuration is included in the "m=" line (as
defined by [RFC3264]). Per [RFC3264], once the offerer generates the
offer, he must be prepared to receive incoming media in accordance
with that offer. That rule applies here as well, but for the actual
configurations only; media received by the offerer according to one
of the potential configurations MAY be discarded, until the offerer
receives an answer indicating what the actual configuration is. Once
that answer is received, incoming media MUST be processed in
accordance with the actual configuration indicated and the answer
received.
3.5.2. Generating the Answer
When the answerer receives an offer with valid SDP capability
negotiation information in it and in particular with one or more
valid potential configuration information attributes present, it may
use any of the potential configurations as an alternative offer. A
potential configuration information attribute is valid if all of the
capabilities (attribute capabilities, transport protocol capabilities
and any extension capabilities) it references are present and valid
themselves.
The actual configuration is contained in the media description's "m="
line. The answerer can send media to the offerer in accordance with
the actual configuration, however if it chooses to use one of the
alternative potential configurations, media sent to the offerer may
be discarded by the offerer until the answer is received.
If the answerer chooses to accept one of the alternative potential
configurations instead of the actual configuration, the answerer MUST
generate an answer as if the offer contained that potential
configuration instead of the actual configuration included. The
answerer MUST also include an actual configuration attribute in the
answer that identifies the potential configuration from the offer
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used by the answerer. The actual configuration attribute in the
answer MUST include information about the attribute capabilities,
transport protocol parameters, and extension capabilities from the
potential configuration that were used to generate the answer.
3.5.3. Offerer Processing of the Answer
When the offerer included potential configurations for a media
stream, it MUST examine the answer for the presence of an actual
configuration attribute for each such media stream. If the attribute
is missing, offerer processing of the answer MUST proceed as defined
by [RFC3264]. If the attribute is present, processing continues as
follows:
The actual configuration attribute specifies which of the potential
configurations were used by the answerer to generate the answer. This
includes all the types of capabilities from the potential
configuration offered, i.e. the attribute capabilities ("a=acap"),
transport protocol capabilities ("a=tcap"), and any extension
capability parameters included.
The offerer MUST now process the answer as if the offer had contained
the potential configuration as the actual configuration in the media
description ("m=" line) and relevant attributes in the offer.
If the answerer selected one of the potential configurations from the
offer as the actual configuration, then the offerer SHOULD perform
another offer/answer exchange, where the offer contains the selected
potential configuration as the actual configuration, i.e. with the
actual configuration used in the "m=" line and any other relevant
attributes. This second offer/answer exchange will not modify the
session anyway, however it will help intermediaries that look at the
SDP, but do not understand the capability negotiation extensions, to
understand the details of the negotiated media streams.
3.5.4. Modifying the Session
Potential configurations may be included in subsequent offers as
defined in [RFC3264, Section 8]. The procedure for doing so is
similar to that described above with the answer including an
indication of the actual configuration used by the answerer.
If the answer indicates use of a potential configuration from the
offer, then a second offer/answer exchange using that potential
configuration as the actual configuration SHOULD be performed.
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3.6. Interactions with ICE
Interactive Connectivity Establishment (ICE) [ICE] provides a
mechanism for verifying connectivity between two endpoints by sending
STUN messages directly between the media endpoints. The basic ICE
specification [ICE] is defined to support UDP-based connectivity
only, however it allows for extensions to support other transport
protocols, such as TCP, which is being specified in [ICETCP]. ICE
defines a new "a=candidate" attribute, which, among other things,
indicates the possible transport protocol(s) to use and then
associates a priority with each of them. The most preferred transport
protocol that *successfully* verifies connectivity will end up being
used.
When using ICE, it is thus possible that the transport protocol that
will be used differs from what is specified in the "m=" line.
Furthermore, since both ICE and SDP Capability Negotiation may now
specify alternative transport protocols, there is a potentially
unintended interaction when using these together.
We provide the following guidelines for addressing that.
[EDITOR'S NOTE: This requires more work]
There are two basic scenarios to consider here:
1) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to
use the one that works (in the preference order specified).
2) A particular media stream can run over different transport
protocols (e.g. UDP, TCP, or TCP/TLS) and the intent is to have the
negotiation process decide which one to use (e.g. T.38 over TCP or
UDP).
In scenario 1, there should be ICE "a=candidate" attributes for UDP,
TCP, etc. but otherwise nothing special in the potential
configuration attributes to indicate the desire to use different
transport protocols (e.g. UDP, or TCP). The ICE procedures
essentially cover the capability negotiation required (by having the
answerer select something it supports and then use of trial and
error).
Scenario 2 does not require a need to support or use ICE. Instead, we
simply use transport protocol capabilities and potential
configuration attributes to indicate the desired outcome.
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The scenarios may be combined, e.g. by offering potential
configuration alternatives where some of them can support one
transport protocol only (e.g. UDP), whereas others can support
multiple transport protocols (e.g. UDP or TCP). In that case, the ICE
candidate attributes should be defined as attribute capabilities and
the relevant ones should then be included in the proper potential
configurations (for example candidate attributes for UDP only for
potential configurations that are restricted to UDP, whereas there
could be candidate attributes for UDP, TCP, and TCP/TLS for potential
configurations that can use all three).
3.7. Processing Media before Answer
The offer/answer model requires an offerer to be able to receive
media in accordance with the offer prior to receiving the answer.
This property is retained with the SDP capability negotiation
extensions defined here, but only when the actual configuration is
selected by the answerer. If a potential configuration is chosen, it
is permissible for the offerer to not process any media received
before the answer is received. This however may lead to clipping.
In the case of SIP, this issue could be solved easily by defining a
precondition [RFC3312] for capability negotiation, however
preconditions are viewed as complicated to implement and they add to
overall session establishment delay by requiring an extra
offer/answer exchange. An alternative is therefore desirable.
The SDP capability negotiation framework does not define such an
alternative, however extensions may do so. For example, one technique
proposed for best-effort SRTP in [BESRTP] is to provide different RTP
payload type mappings for different transport protocols used. The
basic SDP capability negotiation framework defined here does not
include the ability to do so, however extensions that enable that may
be defined.
4. Examples
In this section, we provide examples showing how to use the SDP
Capability Negotiation.
4.1. Best-Effort Secure RTP
The following example illustrates how to use the SDP Capability
negotiation extensions to support so-called Best-Effort Secure RTP.
In that scenario, the offerer supports both RTP and Secure RTP. If
the answerer does not support secure RTP (or the SDP capability
negotiation extensions), an RTP session will be established. However,
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if the answerer supports Secure RTP and the SDP Capability
Negotiation extensions, a Secure RTP session will be established.
The best-effort Secure RTP negotiation is illustrated by the
offer/answer exchange below, where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (SRTP) |
|<---------------------------------|
| |
| (3) Offer (SRTP) |
|--------------------------------->|
| |
| (4) Answer (SRTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=creq: v0
a=tcap:1 RTP/SAVP RTP/AVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=pcfg:1 t=1 a=1
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. Alice indicates that support for the base protocol
defined here is required by including the "a=creq" attribute
containing the value "v0". The capabilities are provided by the
"a=tcap" and "a=acap" attributes. The "tcap" capability indicates
that both Secure RTP and normal RTP are supported. The "acap"
attribute provides a capability parameter with a handle of 1. The
capability parameter is a "crypto" attribute, which provides the
keying material for SRTP using SDP security descriptions [SDES]. The
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"a=pcfg" attribute provides the potential configurations included in
the offer by reference to the capabilities. A single potential
configuration with a configuration number of "1" is provided. It
includes is transport protocol capability 1 (RTP/SAVP, i.e. secure
RTP) together with the attribute capability 1, i.e. the crypto
attribute provided.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation extensions, and hence he accepts the potential
configuration for Secure RTP provided by Alice:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a crypto attribute.
When Alice receives Bob's answer, session negotiation has completed,
however Alice nevertheless generates a new offer using the actual
configuration. This is done purely to assist any middle-boxes that
may reside between Alice and Bob but do not support the capability
negotiation extensions (and hence may not understand the negotiation
that just took place):
Alice's updated offer includes only SRTP, and it is not using the SDP
capability negotiation extensions (Alice could have included the
capabilities as well is she wanted to):
v=0
o=- 25678 753850 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
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inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
The "m=" line now indicates that Alice is offering to use secure RTP
with PCMU or G.729. The "crypto" attribute, which provides the SRTP
keying material, is included with the same value again.
Bob receives the SDP offer from Alice, which he accepts, and then
generates an answer to Alice:
v=0
o=- 24351 621815 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
Bob includes the same crypto attribute as before, and the session
proceeds without change. Although Bob did not include any
capabilities in his answer, he could of course have done so if he
wanted to.
Note that in this particular example, the answerer supported the
capability extensions defined here, however had he not, the answerer
would simply have ignored the new attributes received in step 1 and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated in step 2 instead:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/AVP 0 18
4.2. Multiple Transport Protocols
[EDITOR'S NOTE: Example to be updated - old copy below]
The following example illustrates how to use the SDP Capability
negotiation extensions to support so-called Best-Effort Secure RTP.
In that scenario, the offerer supports both RTP and Secure RTP. If
the answerer does not support secure RTP (or the SDP capability
negotiation extensions), an RTP session will be established. However,
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if the answerer supports Secure RTP and the SDP Capability
Negotiation extensions, a Secure RTP session will be established.
The best-effort Secure RTP negotiation is illustrated by the
offer/answer exchange below, where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (SRTP) |@@
|<---------------------------------|
| |
| (3) Offer (SRTP) |
|--------------------------------->|
| |
| (4) Answer (SRTP) |
|<---------------------------------|
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=creq: v0
a=tcap:1 RTP/SAVP RTP/AVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=pcfg:5 t=1 a=1
a=pcfg:10 t=2
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. Alice indicates that support for the base protocol
defined here is required by including the "a=creq" attribute
containing the value "v0". The capabilities are provided by the
"a=tcap" and "a=acap" attributes. The capabilities indicate that
both Secure RTP and normal RTP are supported. The "acap" attribute
provides a capability parameter with a handle of 1. The capability
parameter is a "crypto" attribute in the capability set, which
provides the keying material for SRTP using SDP security descriptions
[SDES]. The "a=pcfg" attribute provides the potential configurations
included in the offer by reference to the capabilities. Two
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alternatives are provided; the first one with preference "5" (and
hence the preferred one since the preference on the second one is
"10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP)
together with the attribute capability 1, i.e. the crypto attribute
provided. The second one is using transport protocol capability 2.
Note that we could have omitted the second potential configuration
since it equals the actual configuration (which is always the least
preferred configuration).
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation extensions, and hence he accepts the potential
configuration for Secure RTP provided by Alice:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=csup: foo
a=acfg:1 t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a crypto attribute. Finally, Bob supports an SDP capability
negotiation extension with the option tag "foo" and hence he includes
the "a=csup" parameter containing value "foo" in the answer.
Note that in this particular example, the answerer supported the
capability extensions defined here, however had he not, the answerer
would simply have ignored the new attributes and accepted the offer
to use normal RTP. In that case, the following answer would have been
generated instead:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
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m=audio 4567 RTP/AVP 0 18
4.3. Session-Level MIKEY and Media Level Security Descriptions
[EDITOR'S NOTE: Example to be added]
4.4. Capability Negotiation with Interactive Connectivity Establishment
[EDITOR'S NOTE: Example to be added]
5. Security Considerations
TBD.
6. IANA Considerations
TBD.
[EDITOR'S NOTE: Need to define registry and procedures for option
tags]
[EIDTOR'S NOTE: Need to define registry and procedures for extension
capabilities]
7. To Do and Open Issues
o Look for "EDITOR'S NOTE" throughout the document.
8. Acknowledgments
This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined in
here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg
Ott, Colin Perkins, and Thomas Stach.
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Francois Audet and Dan Wing provided useful comments on earlier
versions of this document.
9. Change Log
9.1. draft-ietf-mmusic-sdp-capability-negotiation-02
The following are the major changes compared to version -01:
o Potential configurations are no longer allowed at the session
level
o Renamed capability attributes ("capar" to "acap" and "ctrpr" to
"tcap")
o Changed name and semantics of the initial number (now called
configuration number) in potential configuration attributes; must
now be unique and can be used as a handle
o Actual configuration attribute now includes configuration number
from the selected potential configuration attribute
o Added ABNF throughout
o Specified that answerer should include "a=csup" in case of
unsupported required extensions in offer.
o Specified use of second offer/answer exchange when answerer
selected a potential configuration
o Updated rules (and added restrictions) for referencing media- and
session-level capabilities in potential configurations (at the
media level)
o Added initial section on ICE interactions
o Added initial section on receiving media before answer
9.2. draft-ietf-mmusic-sdp-capability-negotiation-01
The following are the major changes compared to version -00:
o Media capabilities are no longer considered a core capability and
hence have been removed. This leaves transport protocols and
attributes as the only capabilities defined by the core.
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o Version attribute has been removed and an option tag to indicate
the actual version has been defined instead.
o Clarified rules for session-level and media level attributes
provided at either level as well how they can be used in potential
configurations.
o Potential configuration parameters no longer have implicit
ordering; an explicit preference indicator is now included.
o The parameter name for transport protocols in the potential and
actual configuration attributes have been changed "p" to "t".
o Clarified operator precedence within potential and actual
configuration attributes.
o Potential configurations at the session level now limited to
indicate latent capability configurations. Consequently, an actual
configuration attribute can no longer be provided at the session
level.
o Cleaned up capability and potential configuration terminology -
they are now two clearly different things.
9.3. draft-ietf-mmusic-sdp-capability-negotiation-00
Version 00 is the initial version. The solution provided in this
initial version is based on an earlier (individual submission)
version of [SDPCapNeg]. The following are the major changes compared
to that document:
o Solution no longer based on RFC 3407, but defines a set of similar
attributes (with some differences).
o Various minor changes to the previously defined attributes.
o Multiple transport capabilities can be included in a single "tcap"
attribute
o A version attribute is now included.
o Extensions to the framework are formally supported.
o Option tags and the ability to list supported and required
extensions are supported.
o A best-effort SRTP example use case has been added.
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o Some terminology change throughout to more clearly indicate what
constitutes capabilities and what constitutes configurations.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, Internet Mail
Consortium and Demon Internet Ltd., November 1997.
[RFC3264] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002.
[RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October
2003.
[RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
10.2. Informative References
[RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 2327, April 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
"SIP: Session Initiation Protocol", RFC 3261, June 2002.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
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[RFC3551] Schulzrinne, H., and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3551, July
2003.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3851] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
[RFC4091] Camarillo, G., and J. Rosenberg, The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework, RFC 4091, June 2005.
[AVPF] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF)",
Work in Progress, August 2004.
[I-D.jennings-sipping-multipart] Wing, D., and C. Jennings, "Session
Initiation Protocol (SIP) Offer/Answer with Multipart
Alternative", Work in Progress, March 2006.
[SAVPF] Ott, J., and E Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)", Work in Progress,
December 2005.
[SDES] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol Security Descriptions for Media
Streams", RFC 4568, July 2006.
[SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session Description
and Capability Negotiation", Work in Progress, February
2005.
[BESRTP] Kaplan, H., and F. Audet, "Session Description Protocol
(SDP) Offer/Answer Negotiation for Best-Effort Secure Real-
Time Transport Protocol, Work in progress, August 2006.
[KMGMT] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
RFC 4567, July 2006.
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[SDPCapNegRqts] Andreasen, F. "SDP Capability Negotiation:
Requirementes and Review of Existing Work", work in
progress, December 2006.
[SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
progress, December 2006.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[ICE] J. Rosenberg, "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", work in progress,
January 2007.
[ICETCP] J. Rosenberg, "TCP Candidates with Interactive Connectivity
Establishment (ICE)", work in progress, October 2006.
[RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration
of Resource Management and Session Initiatio Protocol
(SIP)", RFC 3312, October 2002.
Author's Addresses
Flemming Andreasen
Cisco Systems
Edison, NJ
Email: fandreas@cisco.com
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Andreasen Expires August 13, 2007 [Page 37]
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