draft-ietf-mmusic-sdp-capability-negotiation-02.txt   draft-ietf-mmusic-sdp-capability-negotiation-03.txt 
MMUSIC Working Group F. Andreasen MMUSIC Working Group F. Andreasen
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Intended Status: Proposed Standard February 13, 2007 Intended Status: Proposed Standard February 19, 2007
Expires: August 2007 Expires: August 2007
SDP Capability Negotiation SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-02.txt draft-ietf-mmusic-sdp-capability-negotiation-03.txt
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that By submitting this Internet-Draft, each author represents that
any applicable patent or other IPR claims of which he or she is any applicable patent or other IPR claims of which he or she is
aware have been or will be disclosed, and any of which he or she aware have been or will be disclosed, and any of which he or she
becomes aware will be disclosed, in accordance with Section 6 of becomes aware will be disclosed, in accordance with Section 6 of
BCP 79. BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on August 13, 2007. This Internet-Draft will expire on August 19, 2007.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
The Session Description Protocol (SDP) was intended for describing The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. SDP was invitation, and other forms of multimedia session initiation. SDP was
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The solution provided in this document provides a general SDP The solution provided in this document provides a general SDP
capability negotiation framework. It also defines specifically how to capability negotiation framework. It also defines specifically how to
provide attributes and transport protocols as capabilities and provide attributes and transport protocols as capabilities and
negotiate them using the framework. Extensions for other types of negotiate them using the framework. Extensions for other types of
capabilities (e.g. media types and formats) may be provided in other capabilities (e.g. media types and formats) may be provided in other
documents. documents.
Table of Contents Table of Contents
1. Introduction...................................................3 1. Introduction...................................................3
2. Conventions used in this document..............................5 2. Conventions used in this document..............................6
3. SDP Capability Negotiation Solution............................6 3. SDP Capability Negotiation Solution............................6
3.1. Solution Overview.........................................6 3.1. Solution Overview.........................................6
3.2. Version and Extension Indication Attributes...............9 3.2. Version and Extension Indication Attributes...............9
3.2.1. Supported Capability Negotiation Extensions Attribute9 3.2.1. Supported Capability Negotiation Extensions Attribute9
3.2.2. Required Capability Negotiation Extension Attribute.10 3.2.2. Required Capability Negotiation Extension Attribute.10
3.3. Capability Attributes....................................12 3.3. Capability Attributes....................................12
3.3.1. Attribute Capability Attribute......................12 3.3.1. Attribute Capability Attribute......................12
3.3.2. Transport Protocol Capability Attribute.............13 3.3.2. Transport Protocol Capability Attribute.............14
3.3.3. Extension Capability Attributes.....................15
3.4. Configuration Attributes.................................15 3.4. Configuration Attributes.................................15
3.4.1. Potential Configuration Attribute...................15 3.4.1. Potential Configuration Attribute...................15
3.4.2. Actual Configuration Attribute......................18 3.4.2. Actual Configuration Attribute......................19
3.5. Offer/Answer Model Extensions............................20 3.5. Offer/Answer Model Extensions............................20
3.5.1. Generating the Initial Offer........................20 3.5.1. Generating the Initial Offer........................20
3.5.2. Generating the Answer...............................21 3.5.2. Generating the Answer...............................23
3.5.3. Offerer Processing of the Answer....................22 3.5.2.1. Example Views of Potential Configurations......26
3.5.4. Modifying the Session...............................22 3.5.3. Offerer Processing of the Answer....................28
3.6. Interactions with ICE....................................23 3.5.4. Modifying the Session...............................29
3.7. Processing Media before Answer...........................24 3.6. Interactions with ICE....................................29
4. Examples......................................................24 3.7. Processing Media before Answer...........................31
4.1. Best-Effort Secure RTP...................................24 4. Examples......................................................31
4.2. Multiple Transport Protocols.............................27 4.1. Best-Effort Secure RTP...................................31
4.3. Session-Level MIKEY and Media Level Security Descriptions30 4.2. Multiple Transport Protocols.............................34
4.3. Session-Level MIKEY and Media Level Security Descriptions37
4.4. Capability Negotiation with Interactive Connectivity 4.4. Capability Negotiation with Interactive Connectivity
Establishment.................................................30 Establishment.................................................37
5. Security Considerations.......................................30 5. Security Considerations.......................................37
6. IANA Considerations...........................................30 6. IANA Considerations...........................................39
7. To Do and Open Issues.........................................30 6.1. New SDP Attributes.......................................39
8. Acknowledgments...............................................30 6.2. New SDP Capability Negotiation Option Tag Registry.......40
9. Change Log....................................................31 6.3. New SDP Capability Negotiation Potential Configuration
9.1. draft-ietf-mmusic-sdp-capability-negotiation-02..........31 Parameter Registry............................................40
9.2. draft-ietf-mmusic-sdp-capability-negotiation-01..........31 7. To Do and Open Issues.........................................41
9.3. draft-ietf-mmusic-sdp-capability-negotiation-00..........32 8. Acknowledgments...............................................41
10. References...................................................34 Change Log......................................................41
10.1. Normative References....................................34 9................................................................41
10.2. Informative References..................................34 9.1. draft-ietf-mmusic-sdp-capability-negotiation-03..........41
Author's Addresses...............................................36 9.2. draft-ietf-mmusic-sdp-capability-negotiation-02..........41
Intellectual Property Statement..................................36 9.3. draft-ietf-mmusic-sdp-capability-negotiation-01..........42
Full.............................................................37 9.4. draft-ietf-mmusic-sdp-capability-negotiation-00..........43
Copyright Statement..............................................37 10. References...................................................44
Acknowledgment...................................................37 10.1. Normative References....................................44
10.2. Informative References..................................44
Author's Addresses...............................................47
Intellectual Property Statement..................................47
Full Copyright Statement.........................................47
Acknowledgment...................................................48
1. Introduction 1. Introduction
The Session Description Protocol (SDP) was intended for describing The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. The SDP invitation, and other forms of multimedia session initiation. The SDP
contains one or more media stream descriptions with information such contains one or more media stream descriptions with information such
as IP-address and port, type of media stream (e.g. audio or video), as IP-address and port, type of media stream (e.g. audio or video),
transport protocol (possibly including profile information, e.g. transport protocol (possibly including profile information, e.g.
RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other
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Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and SRTP as alternatives. RTP is the
default (actual configuration), but SRTP is the preferred one default (actual configuration), but SRTP is the preferred one
(potential configuration): (potential configuration):
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=creq: v0 a=creq: cap-v0
a=tcap:1 RTP/SAVP a=tcap:1 RTP/SAVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1 a=pcfg:1 t=1 a=1
The "m=" line indicates that Alice is offering to use plain RTP with 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 PCMU or G.729. The required base and extensions are provided by the
"a=creq" attribute, which includes the option tag "v0" to indicate "a=creq" attribute, which includes the option tag "cap-v0" to
that the base framework defined here must be supported. The indicate that the base framework defined here must be supported. The
capabilities are provided by the "a=tcap" and "a=acap" attributes. capabilities are provided by the "a=tcap" and "a=acap" attributes.
The transport capabilities ("a=tcap") indicate that secure RTP under The transport capabilities ("a=tcap") indicate that secure RTP under
the AVP profile ("RTP/SAVP") is supported with an associated the AVP profile ("RTP/SAVP") is supported with an associated
transport capability handle of 1. The "acap" attribute provides an transport capability handle of 1. The "acap" attribute provides an
attribute capability with a handle of 1. The attribute capability is attribute capability with a handle of 1. The attribute capability is
a "crypto" attribute, which provides the keying material for SRTP a "crypto" attribute, which provides the keying material for SRTP
using SDP security descriptions [SDES]. The "a=pcfg" attribute using SDP security descriptions [SDES]. The "a=pcfg" attribute
provides the potential configuration included in the offer by provides the potential configuration included in the offer by
reference to the capability parameters. One alternative is provided; reference to the capability parameters. One alternative is provided;
it has a configuration number of 1 and it consists of transport it has a configuration number of 1 and it consists of transport
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att-value = *WSP option-tag-list att-value = *WSP option-tag-list
option-tag-list = option-tag *(COMMA option-tag) option-tag-list = option-tag *(COMMA option-tag)
option-tag = token ; defined in [SDP] option-tag = token ; defined in [SDP]
COMMA = *WSP "," *WSP ; defined in [RFC4234] COMMA = *WSP "," *WSP ; defined in [RFC4234]
Note that white-space is permitted before the option-tag-list. Also, Note that white-space is permitted before the option-tag-list. Also,
implementers familiar with SIP should note that the above definition implementers familiar with SIP should note that the above definition
of COMMA differs from the one in [RFC3261]. of COMMA differs from the one in [RFC3261].
A special base option tag with a value of "v0" is defined for the A special base option tag with a value of "cap-v0" is defined for the
basic SDP capability negotiation framework. Entities use this option basic SDP capability negotiation framework. Entities use this option
tag with the "a=csup" attribute to indicate support for the SDP tag with the "a=csup" attribute to indicate support for the SDP
capability negotiation framework specified in this document. capability negotiation framework specified in this document.
The following examples illustrates the use of the "a=csup" attribute The following examples illustrates the use of the "a=csup" attribute
with the "v0" option tags and two hypothetical option tags, "foo" and with the "cap-v0" option tags and two hypothetical option tags, "foo"
"bar": and "bar":
a=csup: v0 a=csup: cap-v0
a=csup: foo a=csup: foo
a=csup: bar a=csup: bar
a=csup: v0, foo, bar a=csup: cap-v0, foo, bar
The "a=csup" attribute can be provided at the session and the media- 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 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 SDP. When provided at the media-level, it applies to the media
in question only (option-tags provided at the session level apply as description in question only (option-tags provided at the session
well). There can be one or more "a=csup" attributes at both the level apply as well). There can be one or more "a=csup" attributes at
session and media-level (one or more per media stream in the latter both the session and media-level (one or more per media description
case). in the latter case).
Whenever an entity that supports one or more extensions to the SDP Whenever an entity that supports one or more extensions to the SDP
Capability Negotiation framework generates an SDP, it SHOULD include Capability Negotiation framework generates an SDP, it SHOULD include
the "a=csup" attribute with the option tags for the extensions it the "a=csup" attribute with the option tags for the extensions it
supports at the session and/or media-level, unless those option tags supports at the session and/or media-level, unless those option tags
are already provided in one or more "a=creq" attribute (see Section 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. ) at the relevant levels. The base option tag MAY be included.
3.2.2. Required Capability Negotiation Extension Attribute 3.2.2. Required Capability Negotiation Extension Attribute
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The "creq" attribute adheres to the RFC 4566 "attribute" production, The "creq" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = *WSP option-tag-list att-value = *WSP option-tag-list
where "option-tag-list" is defined in Section 3.2.1. Note that where "option-tag-list" is defined in Section 3.2.1. Note that
white-space is permitted before the option-tag-list. white-space is permitted before the option-tag-list.
The following examples illustrate the use of the "a=creq" attribute The following examples illustrate the use of the "a=creq" attribute
with the "v0" base option tag and two hypothetical option tags, "foo" with the "cap-v0" base option tag and two hypothetical option tags,
and "bar": "foo" and "bar":
a=creq: v0 a=creq: cap-v0
a=creq: foo a=creq: foo
a=creq: bar a=creq: bar
a=creq: v0, foo, bar a=creq: cap-v0, foo, bar
The "a=creq" attribute can be provided at the session and the media- 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 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 SDP. When provided at the media-level, it applies to the media-stream
in question only (required option tags provided at the session level 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 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 the session and media-level (one or more per media stream in the
latter case). latter case).
When an entity generates an SDP and it requires the recipient of that When an entity generates an SDP and it requires the recipient of that
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is already known that the receiving entity supports those option-tags is already known that the receiving entity supports those option-tags
at the relevant levels (in which case their inclusion is OPTIONAL). 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 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 supports the required option-tags from the offer, and the
answerer does not require any additional option-tags beyond what answerer does not require any additional option-tags beyond what
was listed in either the required ("a=creq") or supported was listed in either the required ("a=creq") or supported
("a=csup") attributes from the offer, then the answerer is not ("a=csup") attributes from the offer, then the answerer is not
required to include a required ("a=creq") attribute with any required to include a required ("a=creq") attribute with any
option-tags that may need to be supported (such as the base option option-tags that may need to be supported (such as the base option
tag - "v0"). tag - "cap-v0").
A recipient that receives an SDP and does not support one or more of A recipient that receives an SDP and does not support one or more of
the required extensions listed in a "creq" attribute, MUST NOT the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP capability negotiation defined in this document. For perform the SDP capability negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies non-supported extensions provided at the session-level, this implies
that SDP capability negotiation MUST NOT be performed at all. For that SDP capability negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP non-supported extensions at the media-level, this implies that SDP
capability negotiation MUST NOT be performed for the media stream in capability negotiation MUST NOT be performed for the media stream in
question. question.
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place, i.e. all the capability negotiation attributes should be place, i.e. all the capability negotiation attributes should be
ignored. In that case, the entity SHOULD include a "csup" attribute ignored. In that case, the entity SHOULD include a "csup" attribute
listing the SDP capability negotiation extensions it actually listing the SDP capability negotiation extensions it actually
supports. supports.
This ensures that introduction of the SDP capability negotiation This ensures that introduction of the SDP capability negotiation
mechanism does not introduce any new failure scenarios. mechanism does not introduce any new failure scenarios.
The above rules apply to the base option tag as well. Thus, entities The above rules apply to the base option tag as well. Thus, entities
compliant to this specification MUST include a "creq" attribute (at compliant to this specification MUST include a "creq" attribute (at
least in an offer) that includes the option tag "v0" as illustrated least in an offer) that includes the option tag "cap-v0" as
below: illustrated below:
a=creq: v0 a=creq: cap-v0
3.3. Capability Attributes 3.3. Capability Attributes
In this section, we present the new attributes associated with In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability indicating the capabilities for use by the SDP Capability
negotiation. negotiation.
3.3.1. Attribute Capability Attribute 3.3.1. Attribute Capability Attribute
Attributes can be expressed as negotiable parameters by use of a new Attributes and their associated values can be expressed as
attribute capability attribute ("a=acap"), which is defined as capabilities by use of a new attribute capability attribute
follows: ("a=acap"), which is defined as follows:
a=acap: <att-cap-num> <att-par> a=acap: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^31-1 (both 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 included) used to number the attribute capability and <att-par> is an
attribute ("a=") in its full '<type>=<value>' form (see [SDP]). attribute ("a=") in its full '<type>=<value>' form (see [SDP]).
The "acap" attribute adheres to the RFC 4566 "attribute" production, The "acap" attribute adheres to the RFC 4566 "attribute" production,
with an att-value defined as follows: with an att-value defined as follows:
att-value = *WSP att-cap-num 1*WSP att-par att-value = *WSP att-cap-num 1*WSP att-par
att-cap-num = 1*DIGIT ;defined in [RFC4234] att-cap-num = 1*DIGIT ;defined in [RFC4234]
att-par = attribute ;defined in RFC 4266 att-par = attribute ;defined in RFC 4566
Note that white-space is permitted before the att-cap-num. The "acap" Note that white-space is permitted before the att-cap-num. The "acap"
attribute can be provided at the session level for session-level attribute can be provided at the session level for session-level
attributes and the media level for media-level attributes. The "acap" attributes and the media level for media-level attributes. The "acap"
attribute MUST NOT be used to provide a media-level attribute at the attribute MUST NOT be used to provide a media-level attribute at the
session-level or vice versa. session-level or vice versa.
Each occurrence of the "acap" attribute in the entire session Each occurrence of the "acap" attribute in the entire session
description MUST use a different value of <att-cap-num>. description MUST use a different value of <att-cap-num>.
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capability were used instead (e.g. a reserved transport capability capability were used instead (e.g. a reserved transport capability
number could be used to refer to the transport protocol in the "m=" 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 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 "m=" line (e.g. to translate between plain RTP and secure RTP), then
the potential configuration referencing that implicit transport the potential configuration referencing that implicit transport
capability may no longer be correct. With explicit capabilities, we capability may no longer be correct. With explicit capabilities, we
avoid this pitfall, although the potential configuration preference avoid this pitfall, although the potential configuration preference
(see Section 3.4.1. ) may not reflect that of the middle-box (which (see Section 3.4.1. ) may not reflect that of the middle-box (which
some may view as a feature). some may view as a feature).
3.3.3. Extension Capability Attributes
The SDP Capability Negotiation framework allows for new capabilities
to be defined as extensions and used with the general capability
negotiation framework. The syntax and semantics of such new
capability attributes are not defined here, however in order to be
used with potential configurations, they MUST allow for a numeric
handle to be associated with each capability. This handle will be
used as a reference within the potential and actual configuration
attributes (see Section 3.4.1. and 3.4.2. ). The definition of such
extension capability attributes MUST also state whether they can be
applied at the session-level, media-level, or both.
3.4. Configuration Attributes 3.4. Configuration Attributes
3.4.1. Potential Configuration Attribute 3.4.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows: Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <config-number> <pot-cfg-list> a=pcfg: <config-number> <pot-cfg-list>
where <config-number> is an integer between 1 and 2^31-1 (both where <config-number> is an integer between 1 and 2^31-1 (both
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Below, we provide an example of the "a=pcfg" attribute in a complete Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting media description in order to properly indicate the supporting
attributes: attributes:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVPF 0 18 m=audio 3456 RTP/AVPF 0 18
a=creq: v0 a=creq: cap-v0
a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=tcap: 1 RTP/AVPF RTP/AVP a=tcap: 1 RTP/AVPF RTP/AVP
a=tcap: 3 RTP/SAVP RTP/SAVPF a=tcap: 3 RTP/SAVP RTP/SAVPF
a=pcfg:1 t=4|3 a=1 a=pcfg:1 t=4|3 a=1
a=pcfg:8 t=1|2 a=pcfg:8 t=1|2
We have two potential configurations listed here. The first one (and We have two potential configurations listed here. The first one (and
most preferred, since its configuration number is "1") indicates that most preferred, since its configuration number is "1") indicates that
either of the profiles RTP/SAVPF or RTP/SAVP (specified by the either of the profiles RTP/SAVPF or RTP/SAVP (specified by the
transport protocol capability numbers 4 and 3) can be supported with transport protocol capability numbers 4 and 3) can be supported with
attribute capability 1 (the "crypto" attribute); RTP/SAVPF is attribute capability 1 (the "crypto" attribute); RTP/SAVPF is
preferred over RTP/SAVP since its capability number (4) is listed preferred over RTP/SAVP since its capability number (4) is listed
first in the preferred potential configuration. The second potential first in the preferred potential configuration. The second potential
configuration indicates that the RTP/AVPF of RTP/AVP profile can be configuration indicates that the RTP/AVPF of RTP/AVP profile can be
used, with RTP/AVPF being the preferred one. This non secure RTP used, with RTP/AVPF being the preferred one. This non secure RTP
alternative is the less preferred one since its configuration number alternative is the less preferred one since its configuration number
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attribute). attribute).
3.5. Offer/Answer Model Extensions 3.5. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model In this section, we define extensions to the offer/answer model
defined in [RFC3264] to allow for potential configurations to be defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute offers that may be included in an offer, where they constitute offers that may be
accepted by the answerer instead of the actual configuration(s) accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s). included in the "m=" line(s).
[EDITOR'S NOTE: Multicast considerations have been omitted for The procedures defined in the following subsections apply to both
now.] unicast and multicast streams.
TO DO: Elaborate and firm up offer/answer procedures.
3.5.1. Generating the Initial Offer 3.5.1. Generating the Initial Offer
An offerer that wants to use the SDP capability negotiation An offerer that wants to use the SDP capability negotiation
extensions defined in this document MUST include the following in the extensions defined in this document MUST include the following in the
offer: offer:
o an SDP capability negotiation required extensions attribute ("a- o An SDP capability negotiation required extensions attribute
creq") that contains the option tag "v0". It must either be ("a=creq") as defined in Section 3.2.2. that contains the option
provided at the session-level or for each individual media stream. tag "cap-v0".
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 This attribute SHOULD be provided at the session-level (if there
3.3.1. ) if alternative attribute parameter values are to be is only a single media stream, then it may make sense to include
indicated as offerer capabilities or be negotiated. it at the media-level). If one or more additional option tags are
required to be supported for the entire session description, then
option tags for those extensions MUST be included in the session-
level "creq" attribute. For each media description that requires
one or more capability negotiation extensions not listed at the
session-level, a "creq" attribute containing the required
extensions for that media description MUST be included within the
media description as well (in accordance with Section 3.2.2. ).
o one or more transport protocol capability attributes (as defined o An attribute capability attribute ("a=acap") as defined in Section
in Section 3.3.2. ) if alternative transport protocols are to be 3.3.1. for each attribute name and associated value that needs to
to be indicated as offerer capabilities or be negotiated. be indicated as a capability in the offer.
o one or more potential configuration attributes (as defined in Session-level attributes and associated values MUST be provided in
Section 3.4. ) if alternative potential configurations are to be attribute capabilities at the session-level only, whereas media-
negotiated. level attributes and associated values MUST be provided in
attribute capabilities at the media-level only. Attributes that
can be provided at either the session- or media-level can be
represented as attribute capabilities at either the session- or
media-level. If there is not a need to indicate any attributes as
attribute capabilities, then there will not be any "a=acap"
attributes either.
o one or more required capability negotiation extension attributes o One or more a transport protocol capability attributes ("a=tcap")
(as defined in Section 3.2.2. ), if the answerer is required to as defined in Section 3.3.2. with values for each transport
support one or more SDP capability negotiation extensions. protocol that needs to be indicated as a capability in the offer.
Transport protocol capabilities that apply to multiple media
descriptions SHOULD be provided at the session-level whereas
transport protocol capabilities that apply to a specific media
description ("m=" line) only, SHOULD be provided within that
particular media description. If there is not a need to indicate
any transport protocols as transport protocol capabilities, then
there will not be any "a=tcap" attributes either.
o One or more extension capability attributes (as outlined in
Section 3.3.3. ) for each extension capability that is referenced
by a potential configuration.
o One or more potential configuration attributes ("a=pcfg") as
defined in Section 3.4.1. within each media description where
alternative potential configurations are to be negotiated. Each
potential configuration attribute MUST adhere to the rules
provided in Section 3.4.1. and the additional rules provided
below.
The offerer SHOULD furthermore include the following: The offerer SHOULD furthermore include the following:
o one or more supported capability negotiation extension attributes o One or more supported capability negotiation extension attributes
("a=csup" as defined in Section 3.2.1. ), if the offerer supports ("a=csup") as defined in Section 3.2.2. if the offerer supports
one or more SDP capability negotiation extensions that have not one or more capability negotiation extensions not included in a
been included in one or more "a=creq" attributes at the relevant corresponding "a=creq" attribute (i.e. at the session-level or om
session and media level(s). the same media description). Option tags provided in "a=csup"
attributes at the session-level indicate extensions supported for
the entire session description whereas option tags provided in
"a=csup" attributes in a media description indicate extensions
supported for that particular media description only.
The capabilities provided merely indicate what the offerer is capable Capabilities provided in an offer merely indicate what the offerer is
of doing. They do not constitute a commitment or even an indication capable of doing. They do not constitute a commitment or even an
to actually use them. This applies to potential configurations listed indication to actually use them. Each potential configuration however
at the session level as well. Conversely, each of the potential constitutes an alternative offer that the offerer would like to use.
configurations listed at the media level constitutes an alternative The potential configurations may be used by the answerer to negotiate
offer which may be used to negotiate and establish the session. and establish the session.
The offerer MUST include one or more potential configuration
attributes ("a=pcfg") within each media description where the offerer
wants to provide alternative offers (in the form of potential
configurations). Each potential configuration attribute in a given
media description MUST contain a unique configuration number and one
or more potential configuration parameters, as described in Section
3.4.1. Each potential configuration parameter MUST refer to a
capability that is provided either at the session-level or within
that particular media description; otherwise, the potential
configuration is considered invalid.
The current actual configuration is included in the "m=" line (as The current actual configuration is included in the "m=" line (as
defined by [RFC3264]). Per [RFC3264], once the offerer generates the defined by [RFC3264]). Note that the actual configuration is by
offer, he must be prepared to receive incoming media in accordance definition the least-preferred configuration, and hence the answerer
with that offer. That rule applies here as well, but for the actual will seek to negotiate use of one of the potential configurations
configurations only; media received by the offerer according to one instead. If the offerer wishes a different preference for the actual
of the potential configurations MAY be discarded, until the offerer configuration, the offerer MUST include a corresponding potential
receives an answer indicating what the actual configuration is. Once configuration with the relevant configuration number (which indicates
that answer is received, incoming media MUST be processed in the relative preference between potential configurations); this
corresponding potential configuration should simply duplicate the
actual configuration.
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
provided in the offer 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 accordance with the actual configuration indicated and the answer
received. received (provided the offer/answer exchange completed succesfully).
3.5.2. Generating the Answer 3.5.2. Generating the Answer
When the answerer receives an offer with valid SDP capability When receiving an offer, the answerer MUST check for the presence of
negotiation information in it and in particular with one or more a required capability negotiation extension attribute ("a=creq")
valid potential configuration information attributes present, it may provided at the session level and containing the option tag "cap-v0".
use any of the potential configurations as an alternative offer. A If one is found, then capability negotiation MUST be performed for
potential configuration information attribute is valid if all of the each media description that contains a potential configuration
capabilities (attribute capabilities, transport protocol capabilities attribute ("a=pcfg"). If none is found, then the answerer MUST check
and any extension capabilities) it references are present and valid each offered media description for a required capability negotiation
themselves. extension attribute ("a=creq") containing the option tag "cap-v0" and
one or more potential configuration attributes. Capability
negotiation MUST be performed for each such media description in
accordance with the procedures described below.
The answerer MUST first ensure that it supports any additional
required capability negotiation extensions:
o If a session-level "creq" attribute is provided, and it contains
an option-tag that the answerer does not support, then the
answerer MUST NOT use any of the potential configuration
attributes provided for any of the media descriptions. Instead,
the normal offer/answer procedures MUST continue as per [RFC3264].
Furthermore, the answerer MUST include a session-level supported
capability negotiation extensions attribute ("a=csup") with option
tags for the capability negotiation extensions supported by the
answerer.
o If a media-level "creq" attribute is provided, and it contains an
option tag that the answerer does not support, then the answerer
MUST NOT use any of the potential configuration attributes
provided for that particular media description. Instead, the
offer/answer procedures MUST continue as per [RFC3264].
Furthermore, the answerer MUST include a supported capability
negotiation extensions attribute ("a=csup") in that media
description with option tags for the capability negotiation
extensions supported by the answerer for that media description.
Assuming all required capability negotiation extensions are
supported, the answerer now proceeds as follows.
For each media description where capability negotiation is to be
performed (i.e. all required capability negotiation extensions are
supported and at least one valid potential configuration attribute is
present), the answerer MUST attempt to perform capability negotiation
by using the most preferred potential configuration that is valid. A
potential configuration is valid if:
1. It is in accordance with the syntax and semantics provided in
Section 3.4.1.
2. It contains a configuration number that is unique within that
media description.
3. All attribute capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.3.1. )
and each of them are furthermore provided either at the session-
level or within this particular media description.
4. All transport protocol capabilities referenced by the potential
configuration are valid themselves (as defined in Section 3.3.2. )
and each of them are furthermore provided either at the session-
level or within this particular media description.
5. All extension capabilities referenced by the potential
configuration and supported by the answerer are valid themselves
(as defined by that particular extension) and each of them are
furthermore provided either at the session-level or within this
particular media description. Unknown or unsupported extension
capabilities MUST be ignored.
The most preferred valid potential configuration in a media
description is the valid potential configuration with the lowest
configuration number. The answerer MUST now process the offer for
that media stream based on the most preferred valid potential
configuration. Conceptually, this entails the answerer constructing
an (internal) offer that consists of the offer SDP, with the
following changes:
o If a transport protocol capability is included in the potential
configuration, then it replaces the transport protocol provided in
the "m=" line for that media description.
o If a session-level attribute capability is included, then it is
added to the list of session-level attributes for the session
description.
o If a media-level attribute capability is included, then it is
added to the list of media-level attributes for that particular
media description.
o If a supported extension capability is included, then it is
processed in accordance with the rules provided for that
particular extension capability.
Note that whereas a transport protocol from the potential
configuration replaces the transport protocol in the actual
configuration, an attribute capability from the potential
configuration is instead added to the actual configuration. In some
cases, this may result in having one or more meaningless attributes
from the actual configuration; such meaningless attributes SHOULD
simply be ignored.
For example, if the actual configuration was using Secure RTP and
included an "a=crypto" attribute for the SRTP keying material,
then use of a potential configuration that uses plain RTP would
make the "crypto" attribute meaningless. Rather than requiring
the actual configuration attributes to be present as attribute
capabilities as well (which would increase the message size) and
then have the potential configuration completely replace the
actual configuration, we instead make the use of attribute
capabilities additive to the session description.
Please refer to Section 3.5.2.1. for examples of how the answerer may
conceptuall "see" the resulting offered alternative potential
configurations.
If the answerer is not able to support the most preferred valid
potential configuration for the media description, the answerer MUST
proceed to the second-most preferred valid potential configuration
for the media description, etc. If the answerer is not able to
support any of the valid potential configurations, the answerer MUST
process the offer per normal offer/answer rules, i.e. the actual
configuration provided will be used as the least preferred
alternative.
Once the answerer has selected an offered configuration for the media
stream, the answerer MUST generate a valid answer SDP based on the
selected configuration as "seen" by the answerer. Furthermore, if the
answerer selected one of the potential configurations in a media
description, the answerer MUST include an actual configuration
attribute within that media description that identifies the
configuration number for that potential configuration as well as the
actual parameters that were used from that potential configuration
(if the potential configuration included alternatives, only the
selected alternatives must be included). Only the known and supported
parameters will be included. Unknown or unsupported parameters MUST
NOT be included in the actual configuration attribute.
If the answerer supports one or more capability negotiation
extensions that were not included in a required capability
negotiation extensions attribute in the offer, then the answerer
SHOULD furthermore include a supported capability negotiation
attribute ("a=csup") at the session-level with option tags for the
extensions supported across media streams. Also, if the answerer
supports one or more capability negotiation extensions for particular
media descriptions only, then a supported capability negotiation
attribute with those option-tags SHOULD be included within each
relevant media description.
The actual configuration is contained in the media description's "m=" The actual configuration is contained in the media description's "m="
line. The answerer can send media to the offerer in accordance with line. The answerer can send media to the offerer in accordance with
the actual configuration, however if it chooses to use one of the the actual configuration as soon as it receives the offer, however it
alternative potential configurations, media sent to the offerer may MUST NOT do so if it chooses an alternative potential configuration.
be discarded by the offerer until the answer is received. If the answerer chooses one of the potential configurations, then the
answerer MAY start to send media to the offerer in accordance with
the selected potential configuration, however the offerer MAY discard
such media until the offerer receives the answer.
If the answerer chooses to accept one of the alternative potential 3.5.2.1. Example Views of Potential Configurations
configurations instead of the actual configuration, the answerer MUST
generate an answer as if the offer contained that potential The following examples illustrate how the answerer may conceptually
configuration instead of the actual configuration included. The "see" a potential configuration. Consider the following offered SDP:
answerer MUST also include an actual configuration attribute in the
answer that identifies the potential configuration from the offer v=0
used by the answerer. The actual configuration attribute in the o=alice 2891092738 2891092738 IN IP4 lost.example.com
answer MUST include information about the attribute capabilities, s=Secret discussion
transport protocol parameters, and extension capabilities from the t=0 0
potential configuration that were used to generate the answer. c=IN IP4 lost.example.com
a=creq: cap-v0
a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=tcap:1 RTP/SAVP RTP/AVP
m=audio 39000 RTP/AVP 98
a=rtpmap:98 AMR/8000
a=acap:2 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1|2
m=video 42000 RTP/AVP 31
a=rtpmap:31 H261/90000
a=acap:3 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
a=pcfg:1 t=1 a=1|3
This particular SDP offers an audio stream and a video stream, each
of which can either use plain RTP (actual configuration) or secure
RTP (potential configuration). Furthermore, two different keying
mechanisms are offered, namely session-level Key Management
Extensions using MIKEY (attribute capability 1) and media-level SDP
Security Descriptions (attribute capabilities 2 and 3). There are
several alternative configurations here, however, below we show the
one the answerer "sees" when using potential configuration 1 for both
audio and video, and furthermore using attribute capability 1 (MIKEY)
for both (we have removed all the capability negotiation attributes
for clarity):
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion
t=0 0
c=IN IP4 lost.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 39000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31
a=rtpmap:31 H261/90000
Note that the transport protocol in the media descriptions indicate
use of secure RTP.
Below, we show the offer the answerer "sees" when using potential
configuration 1 for both audio and video and furthermore using
attribute capability 2 and 3 respectively (SDP security descriptions)
for the audio and media stream:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion
t=0 0
c=IN IP4 lost.example.com
m=audio 39000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
m=video 42000 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
Again, note that the transport protocol in the media descriptions
indicate use of secure RTP.
And finally, we show the offer the answerer "sees" when using
potential configuration 1 with attribute capability 1 (MIKEY) for the
audio stream, and potential configuration 1 with attribute capability
3 (SDP security descriptions) for the video stream:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion
t=0 0
c=IN IP4 lost.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
m=audio 39000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
3.5.3. Offerer Processing of the Answer 3.5.3. Offerer Processing of the Answer
When the offerer included potential configurations for a media When the offerer attempted to use SDP Capability Negotiation in the
stream, it MUST examine the answer for the presence of an actual offer, the offerer MUST examine the answer for actual use of
configuration attribute for each such media stream. If the attribute capability negotiation.
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 For each media description where the offerer included a potential
configurations were used by the answerer to generate the answer. This configuration attribute, the offerer MUST first examine the media
includes all the types of capabilities from the potential description for the presence of an actual configuration attribute
("a=acfg"). If an actual configuration attribute is not present in a
media description, then the offerer MUST process the answer SDP for
that media stream per the normal offer/answer rules defined in
[RFC3264]. However, if one is found, then the offerer MUST instead
process the answer as follows:
o The actual configuration attribute specifies which of the
potential configurations were used by the answerer to generate the
answer. This includes all the capabilities from the potential
configuration offered, i.e. the attribute capabilities ("a=acap"), configuration offered, i.e. the attribute capabilities ("a=acap"),
transport protocol capabilities ("a=tcap"), and any extension transport protocol capabilities ("a=tcap"), and any extension
capability parameters included. capability parameters included.
The offerer MUST now process the answer as if the offer had contained o The offerer MUST now process the answer in accordance with the
the potential configuration as the actual configuration in the media rules in [RFC3264], except that it must be done as if the offer
description ("m=" line) and relevant attributes in 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 If the offer/answer exchange was succesful, and if the answerer
offer as the actual configuration, then the offerer SHOULD perform selected one of the potential configurations from the offer as the
another offer/answer exchange, where the offer contains the selected actual configuration, then the offerer SHOULD perform another
offer/answer exchange: The new offer should contain the selected
potential configuration as the actual configuration, i.e. with the potential configuration as the actual configuration, i.e. with the
actual configuration used in the "m=" line and any other relevant actual configuration used in the "m=" line and any other relevant
attributes. This second offer/answer exchange will not modify the attributes. This second offer/answer exchange will not modify the
session anyway, however it will help intermediaries that look at the session in any way, however it will help intermediaries that look at
SDP, but do not understand the capability negotiation extensions, to the SDP, but do not understand or support the capability negotiation
understand the details of the negotiated media streams. extensions, to understand the details of the media stream(s) that
were actually negotiated.
3.5.4. Modifying the Session 3.5.4. Modifying the Session
Potential configurations may be included in subsequent offers as Capabilities and potential configurations may be included in
defined in [RFC3264, Section 8]. The procedure for doing so is subsequent offers as defined in [RFC3264, Section 8]. The procedure
similar to that described above with the answer including an for doing so is similar to that described above with the answer
indication of the actual configuration used by the answerer. including an indication of the actual configuration used by the
answerer.
If the answer indicates use of a potential configuration from the If the answer indicates use of a potential configuration from the
offer, then a second offer/answer exchange using that potential offer, then a second offer/answer exchange using that potential
configuration as the actual configuration SHOULD be performed. configuration as the actual configuration SHOULD be performed.
3.6. Interactions with ICE 3.6. Interactions with ICE
Interactive Connectivity Establishment (ICE) [ICE] provides a Interactive Connectivity Establishment (ICE) [ICE] provides a
mechanism for verifying connectivity between two endpoints by sending mechanism for verifying connectivity between two endpoints by sending
STUN messages directly between the media endpoints. The basic ICE STUN messages directly between the media endpoints. The basic ICE
skipping to change at page 25, line 34 skipping to change at page 32, line 29
Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one: default, but SRTP is the preferred one:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=creq: v0 a=creq: cap-v0
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP FEC_ORDER=FEC_SRTP
a=pcfg:1 t=1 a=1 a=pcfg:1 t=1 a=1
The "m=" line indicates that Alice is offering to use plain RTP with 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 PCMU or G.729. Alice indicates that support for the base protocol
defined here is required by including the "a=creq" attribute defined here is required by including the "a=creq" attribute
containing the value "v0". The capabilities are provided by the containing the value "cap-v0". The capabilities are provided by the
"a=tcap" and "a=acap" attributes. The "tcap" capability indicates "a=tcap" and "a=acap" attributes. The "tcap" capability indicates
that both Secure RTP and normal RTP are supported. The "acap" that both Secure RTP and normal RTP are supported. The "acap"
attribute provides a capability parameter with a handle of 1. The attribute provides a capability parameter with a handle of 1. The
capability parameter is a "crypto" attribute, which provides the capability parameter is a "crypto" attribute, which provides the
keying material for SRTP using SDP security descriptions [SDES]. The keying material for SRTP using SDP security descriptions [SDES]. The
"a=pcfg" attribute provides the potential configurations included in "a=pcfg" attribute provides the potential configurations included in
the offer by reference to the capabilities. A single potential the offer by reference to the capabilities. A single potential
configuration with a configuration number of "1" is provided. It configuration with a configuration number of "1" is provided. It
includes is transport protocol capability 1 (RTP/SAVP, i.e. secure includes is transport protocol capability 1 (RTP/SAVP, i.e. secure
RTP) together with the attribute capability 1, i.e. the crypto RTP) together with the attribute capability 1, i.e. the crypto
skipping to change at page 28, line 15 skipping to change at page 35, line 10
Negotiation extensions, a Secure RTP session will be established. Negotiation extensions, a Secure RTP session will be established.
The best-effort Secure RTP negotiation is illustrated by the The best-effort Secure RTP negotiation is illustrated by the
offer/answer exchange below, where Alice sends an offer to Bob: offer/answer exchange below, where Alice sends an offer to Bob:
Alice Bob Alice Bob
| (1) Offer (SRTP and RTP) | | (1) Offer (SRTP and RTP) |
|--------------------------------->| |--------------------------------->|
| | | |
| (2) Answer (SRTP) |@@ | (2) Answer (SRTP) |
|<---------------------------------| |<---------------------------------|
| | | |
| (3) Offer (SRTP) | | (3) Offer (SRTP) |
|--------------------------------->| |--------------------------------->|
| | | |
| (4) Answer (SRTP) | | (4) Answer (SRTP) |
|<---------------------------------| |<---------------------------------|
Alice's offer includes RTP and SRTP as alternatives. RTP is the Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one: default, but SRTP is the preferred one:
v=0 v=0
o=- 25678 753849 IN IP4 128.96.41.1 o=- 25678 753849 IN IP4 128.96.41.1
s= s=
c=IN IP4 128.96.41.1 c=IN IP4 128.96.41.1
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 18 m=audio 3456 RTP/AVP 0 18
a=creq: v0 a=creq: cap-v0
a=tcap:1 RTP/SAVP RTP/AVP a=tcap:1 RTP/SAVP RTP/AVP
a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP FEC_ORDER=FEC_SRTP
a=pcfg:5 t=1 a=1 a=pcfg:5 t=1 a=1
a=pcfg:10 t=2 a=pcfg:10 t=2
The "m=" line indicates that Alice is offering to use plain RTP with 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 PCMU or G.729. Alice indicates that support for the base protocol
defined here is required by including the "a=creq" attribute defined here is required by including the "a=creq" attribute
containing the value "v0". The capabilities are provided by the containing the value "cap-v0". The capabilities are provided by the
"a=tcap" and "a=acap" attributes. The capabilities indicate that "a=tcap" and "a=acap" attributes. The capabilities indicate that
both Secure RTP and normal RTP are supported. The "acap" attribute both Secure RTP and normal RTP are supported. The "acap" attribute
provides a capability parameter with a handle of 1. The capability provides a capability parameter with a handle of 1. The capability
parameter is a "crypto" attribute in the capability set, which parameter is a "crypto" attribute in the capability set, which
provides the keying material for SRTP using SDP security descriptions provides the keying material for SRTP using SDP security descriptions
[SDES]. The "a=pcfg" attribute provides the potential configurations [SDES]. The "a=pcfg" attribute provides the potential configurations
included in the offer by reference to the capabilities. Two included in the offer by reference to the capabilities. Two
alternatives are provided; the first one with preference "5" (and alternatives are provided; the first one with preference "5" (and
hence the preferred one since the preference on the second one is hence the preferred one since the preference on the second one is
"10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP) "10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP)
skipping to change at page 30, line 16 skipping to change at page 37, line 15
4.3. Session-Level MIKEY and Media Level Security Descriptions 4.3. Session-Level MIKEY and Media Level Security Descriptions
[EDITOR'S NOTE: Example to be added] [EDITOR'S NOTE: Example to be added]
4.4. Capability Negotiation with Interactive Connectivity Establishment 4.4. Capability Negotiation with Interactive Connectivity Establishment
[EDITOR'S NOTE: Example to be added] [EDITOR'S NOTE: Example to be added]
5. Security Considerations 5. Security Considerations
TBD. The SDP Capability Negotiation Framework is defined to be used within
the context of the offer/answer model, and hence all the offer/answer
security considerations apply here as well. Similarly, the Session
Initiation Protocol (SIP) uses SDP and the offer/answer model, and
hence, when used in that context, the SIP security considerations
apply as well.
However, SDP Capability Negotiations introduces additional security
issues. Its use as a mechanism to enable alternative transport
protocol negotiation (secure and non-secure) as well as its ability
to negotiate use of more or less secure keying methods and material
warrant further security considerations. Also, the (continued)
support for receiving media before answer combined with negotiation
of alternative transport protocols (secure and non-secure) warrant
further security considerations. We discuss these issues below.
The SDP capability negotiation framework allows for an offered media
stream to both indicate and support various levels of security for
that media stream. Different levels of security can for example be
negotiated by use of alternative attribute capabilities each
indicating more or less secure keying methods as well as more or less
strong ciphers. Since the offerer indicates support for each of these
alternatives, he will presumably accept the answerer seemingly
selecting any of the offered alternatives. If an attacker can modify
the SDP offer, he can thereby force the negotiation of the weakest
security mechanism that the offerer is willing to accept. This may in
turn enable the attacker to compromise the security of the negotiated
media stream. Similarly, if the offerer wishes to negotiate use of a
secure media stream (e.g. secure RTP), but includes a non-secure
media stream (e.g. plain RTP) as a valid (but less preferred)
alternative, then an attacker that can modify the offered SDP will be
able to force the establishment of an insecure media stream. The
solution to both of these problems involves the use of integrity
protection over the SDP. Ideally, this integrity protection provides
end-to-end integrity protection in order to protect from any man-in-
the-middle attack; secure multiparts such as S/MIME [SMIME] provide
one such solution, however S/MIME requires use and availability of a
Public Key Infrastructure (PKI). A slightly less secure alternative
when using SIP, but generally much easier to deploy in practice
(since it does not require a PKI), is to use SIP Identity [RFC4474];
this requires the existence of an authentication service (see
[RFC4474]). Yet another, and considerably less secure, alternative is
to use hop-by-hop security only, e.g. TLS or IPSec thereby ensuring
the integrity of the offered SDP on a hop-by-hop basis. Note however
that SIP proxies or other intermediaries processing the SIP request
at each hop are able to perform a man-in-the-middle attack by
modifying the offered SDP.
Per the normal offer/answer procedures, as soon as the offerer has
generated an offer, the offerer must be prepared to receive media in
accordance with that offer. The SDP Capability Negotiation preserves
that behavior for the actual configuration in the offer, however the
offerer has no way of knowing which configuration (actual or
potential) configuration was actually selected by the offerer, until
an answer indication is received. This opens up a new security issue
where an attacker may be able to interject media towards the offerer
until the answer is received. For example, the offerer may use plain
RTP as the actual configuration and secure RTP as an alternative
potential configuration. Even though the answerer selects secure RTP,
the offerer will not know that until he receives the answer, and
hence an attacker will be able to send media to the offerer
meanwhile. The easiest protection against such an attack is to not
offer use of the non-secure media stream in the actual configuration,
however that may in itself have undesirable side-effects: If the
answerer does not support the non-secure media stream and also does
not support the capability negotiation framework, then negotiation of
the media stream will fail. Alternatively, SDP security preconditions
[sprecon] can be used. This will ensure that media is not flowing
until session negotiation has completed and hence the selected
configuration is known. Use of preconditions however requires both
side to support them. If they don't, and use of them is required, the
session will fail. As a (limited) work around to this, it is
RECOMMENDED that SIP entities generate an answer SDP and send it to
the offerer as soon as possible, for example in a 183 Session
Progress message. This will limit the time during which an attacker
can send media to the offerer.
Additional security considerations apply to the answer SDP as well.
The actual configuration attribute tells the offerer which potential
configuration the answer was actually based on, and hence an attacker
that can either modify or remove the actual configuration attribute
in the answer can cause session failure as well as extend the time
window during which the offerer will accept incoming media that does
not conform to the actual answer. The solutions to this SDP answer
integrity problem are the same as for the offer, i.e. use of end-to-
end integrity protection, SIP identity, or hop-by-hop protection. The
mechanism to use depends on the mechanisms supported by the offerer
as well as the acceptable security trade-offs.
6. IANA Considerations 6. IANA Considerations
TBD. 6.1. New SDP Attributes
[EDITOR'S NOTE: Need to define registry and procedures for option The IANA is hereby requested to register the following new SDP
tags] attributes as follows:
[EIDTOR'S NOTE: Need to define registry and procedures for extension Attribute name: csup
capabilities] Long form name: Supported capability negotiation extensions
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Option tags for supported SDP capability
negotiation extensions
Appropriate values: See Section 3.2.1.
Attribute name: creq
Long form name: Required capability negotiation extensions
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Option tags for required SDP capability
negotiation extensions
Appropriate values: See Section 3.2.2.
Attribute name: acap
Long form name: Attribute capability
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Attribute capability containing an attribute
name and associated value
Appropriate values: See Section 3.3.1.
Attribute name: tcap
Long form name: Transport Protocol Capability
Type of attribute: Session-level and media-level
Subject to charset: No
Purpose: Transport protocol capability listing one or
more transport protocols
Appropriate values: See Section 3.3.2.
Attribute name: pcfg
Long form name: Potential Configuration
Type of attribute: Media-level
Subject to charset: No
Purpose: Potential configuration for SDP capability
negotiation
Appropriate values: See Section 3.4.1.
Attribute name: acfg
Long form name: Actual configuration
Type of attribute: Media-level
Subject to charset: No
Purpose: Actual configuration for SDP capability
negotiation
Appropriate values: See Section 3.4.2.
6.2. New SDP Capability Negotiation Option Tag Registry
The IANA is hereby requested to create a new SDP Capability
Negotiation Option Tag registry. An IANA SDP capability negotiation
option tag registration MUST be documented in an RFC in accordance
with the [RFC2434] Specification Required policy. The RFC MUST
provide the name of the option tag, a syntax and a semantic
specification of any new SDP attributes and any extensions to the
potential and actual configuration attributes provided in this
document. New SDP attributes that are intended to be capabilities for
use by the capability negotiation framework MUST adhere to the
guidelines provided in Section 3.3.3. Extensions to the potential and
actual configuration attributes MUST adhere to the syntax provided in
Section 3.4.1. and 3.4.2.
The option tag "cap-v0" is defined in this document and the IANA is
hereby requested to register this option tag.
6.3. New SDP Capability Negotiation Potential Configuration Parameter
Registry
The IANA is hereby requested to create a new SDP Capability
Negotiation Potential Configuration Parameter registry. An IANA SDP
Capability Negotiation potential configuration registration MUST be
document in an RFC in accordance with the [RFC2434] Specification
Required policy. The RFC MUST define the syntax and semantics of each
new potential configuration parameter. The syntax MUST adhere to the
syntax provided for extensions in Section 3.4.1. and the semantics
MUST adhere to the semantics provided for extensions in Section
3.4.1. and 3.4.2. Associated with each registration MUST be the
encoding name for the parameter as well as a short descriptive name
for it.
The potential configuration parameters "a" for "attribute" and "t"
for "transport protocol" are defined in this document and the IANA is
hereby requested to register these.
7. To Do and Open Issues 7. To Do and Open Issues
o Look for "EDITOR'S NOTE" throughout the document. o Look for "EDITOR'S NOTE" throughout the document.
8. Acknowledgments 8. Acknowledgments
This document is heavily influenced by the discussions and work done This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined in document itself or the overall direction of the solution defined in
here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg
Ott, Colin Perkins, and Thomas Stach. Ott, Colin Perkins, and Thomas Stach.
Francois Audet and Dan Wing provided useful comments on earlier Francois Audet and Dan Wing provided useful comments on earlier
versions of this document. versions of this document.
9. Change Log 9. Change Log
9.1. draft-ietf-mmusic-sdp-capability-negotiation-02 9.1. draft-ietf-mmusic-sdp-capability-negotiation-03
The following are the major changes compared to version -02:
o Base option tag name changed from "v0" to "cap-v0".
o Added new section on extension capability attributes
o Firmed up offer/answer procedures.
o Added security considerations
o Added IANA considerations
9.2. draft-ietf-mmusic-sdp-capability-negotiation-02
The following are the major changes compared to version -01: The following are the major changes compared to version -01:
o Potential configurations are no longer allowed at the session o Potential configurations are no longer allowed at the session
level level
o Renamed capability attributes ("capar" to "acap" and "ctrpr" to o Renamed capability attributes ("capar" to "acap" and "ctrpr" to
"tcap") "tcap")
o Changed name and semantics of the initial number (now called o Changed name and semantics of the initial number (now called
configuration number) in potential configuration attributes; must configuration number) in potential configuration attributes; must
now be unique and can be used as a handle now be unique and can be used as a handle
o Actual configuration attribute now includes configuration number o Actual configuration attribute now includes configuration number
from the selected potential configuration attribute from the selected potential configuration attribute
skipping to change at page 31, line 43 skipping to change at page 42, line 30
selected a potential configuration selected a potential configuration
o Updated rules (and added restrictions) for referencing media- and o Updated rules (and added restrictions) for referencing media- and
session-level capabilities in potential configurations (at the session-level capabilities in potential configurations (at the
media level) media level)
o Added initial section on ICE interactions o Added initial section on ICE interactions
o Added initial section on receiving media before answer o Added initial section on receiving media before answer
9.2. draft-ietf-mmusic-sdp-capability-negotiation-01 9.3. draft-ietf-mmusic-sdp-capability-negotiation-01
The following are the major changes compared to version -00: The following are the major changes compared to version -00:
o Media capabilities are no longer considered a core capability and o Media capabilities are no longer considered a core capability and
hence have been removed. This leaves transport protocols and hence have been removed. This leaves transport protocols and
attributes as the only capabilities defined by the core. attributes as the only capabilities defined by the core.
o Version attribute has been removed and an option tag to indicate o Version attribute has been removed and an option tag to indicate
the actual version has been defined instead. the actual version has been defined instead.
skipping to change at page 32, line 29 skipping to change at page 43, line 16
configuration attributes. configuration attributes.
o Potential configurations at the session level now limited to o Potential configurations at the session level now limited to
indicate latent capability configurations. Consequently, an actual indicate latent capability configurations. Consequently, an actual
configuration attribute can no longer be provided at the session configuration attribute can no longer be provided at the session
level. level.
o Cleaned up capability and potential configuration terminology - o Cleaned up capability and potential configuration terminology -
they are now two clearly different things. they are now two clearly different things.
9.3. draft-ietf-mmusic-sdp-capability-negotiation-00 9.4. draft-ietf-mmusic-sdp-capability-negotiation-00
Version 00 is the initial version. The solution provided in this Version 00 is the initial version. The solution provided in this
initial version is based on an earlier (individual submission) initial version is based on an earlier (individual submission)
version of [SDPCapNeg]. The following are the major changes compared version of [SDPCapNeg]. The following are the major changes compared
to that document: to that document:
o Solution no longer based on RFC 3407, but defines a set of similar o Solution no longer based on RFC 3407, but defines a set of similar
attributes (with some differences). attributes (with some differences).
o Various minor changes to the previously defined attributes. o Various minor changes to the previously defined attributes.
skipping to change at page 34, line 33 skipping to change at page 44, line 33
[RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in [RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October Session Description Protocol (SDP)", RFC 3605, October
2003. 2003.
[RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
[SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
10.2. Informative References 10.2. Informative References
[RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail [RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996. November 1996.
[RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 2327, April 1998. Description Protocol", RFC 2327, April 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
skipping to change at page 36, line 28 skipping to change at page 46, line 33
Traversal for Offer/Answer Protocols", work in progress, Traversal for Offer/Answer Protocols", work in progress,
January 2007. January 2007.
[ICETCP] J. Rosenberg, "TCP Candidates with Interactive Connectivity [ICETCP] J. Rosenberg, "TCP Candidates with Interactive Connectivity
Establishment (ICE)", work in progress, October 2006. Establishment (ICE)", work in progress, October 2006.
[RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration [RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration
of Resource Management and Session Initiatio Protocol of Resource Management and Session Initiatio Protocol
(SIP)", RFC 3312, October 2002. (SIP)", RFC 3312, October 2002.
[SMIME] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
[RFC4474] J. Peterson, and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session Initiation
Protocol (SIP)", RFC 4474, August 2006.
[sprecon] Andreasen, F. and D. Wing, "Security Preconditions for
Session Description Protocol Media Streams", Work in
Progress, October 2006.
Author's Addresses Author's Addresses
Flemming Andreasen Flemming Andreasen
Cisco Systems Cisco Systems
Edison, NJ Edison, NJ
Email: fandreas@cisco.com Email: fandreas@cisco.com
Intellectual Property Statement Intellectual Property Statement
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