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rfc:rfc5939

Internet Engineering Task Force (IETF) F. Andreasen Request for Comments: 5939 Cisco Systems Category: Standards Track September 2010 ISSN: 2070-1721

     Session Description Protocol (SDP) Capability Negotiation

Abstract

 The Session Description Protocol (SDP) was intended to describe
 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, notably in the form of the offer/answer
 model defined in RFC 3264.  SDP does not define how to negotiate one
 or more alternative transport protocols (e.g., RTP profiles) or
 attributes.  This makes it difficult to deploy new RTP profiles such
 as Secure RTP or RTP with RTCP-based feedback, negotiate use of
 different security keying mechanisms, etc.  It also presents problems
 for some forms of media negotiation.
 The purpose of this document is to address these shortcomings by
 extending SDP with capability negotiation parameters and associated
 offer/answer procedures to use those parameters in a backwards
 compatible manner.
 The document defines a general SDP Capability Negotiation framework.
 It also specifies 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 media formats)
 may be provided in other documents.

Andreasen Standards Track [Page 1] RFC 5939 SDP Capability Negotiation September 2010

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5939.

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Andreasen Standards Track [Page 2] RFC 5939 SDP Capability Negotiation September 2010

Table of Contents

 1. Introduction ....................................................4
 2. Conventions Used in This Document ...............................7
 3. SDP Capability Negotiation Solution .............................7
    3.1. SDP Capability Negotiation Model ...........................7
    3.2. Solution Overview .........................................10
    3.3. Version and Extension Indication Attributes ...............14
    3.4. Capability Attributes .....................................17
    3.5. Configuration Attributes ..................................22
    3.6. Offer/Answer Model Extensions .............................32
    3.7. Interactions with ICE .....................................45
    3.8. Interactions with SIP Option Tags .........................47
    3.9. Processing Media before Answer ............................48
    3.10. Indicating Bandwidth Usage ...............................49
    3.11. Dealing with Large Number of Potential Configurations ....50
    3.12. SDP Capability Negotiation and Intermediaries ............51
    3.13. Considerations for Specific Attribute Capabilities .......52
    3.14. Relationship to RFC 3407 .................................54
 4. Examples .......................................................54
    4.1. Multiple Transport Protocols ..............................54
    4.2. DTLS-SRTP or SRTP with Media-Level Security Descriptions...58
    4.3. Best-Effort SRTP with Session-Level MIKEY and Media-Level
         Security Descriptions .....................................61
    4.4. SRTP with Session-Level MIKEY and Media-Level Security
         Descriptions as Alternatives ..............................66
 5. Security Considerations ........................................69
 6. IANA Considerations ............................................72
    6.1. New SDP Attributes ........................................72
    6.2. New SDP Capability Negotiation Option Tag Registry ........73
    6.3. New SDP Capability Negotiation Potential
         Configuration Parameter Registry ..........................74
 7. Acknowledgments ................................................74
 8. References .....................................................75
    8.1. Normative References ......................................75
    8.2. Informative References ....................................75

Andreasen Standards Track [Page 3] RFC 5939 SDP Capability Negotiation September 2010

1. Introduction

 The Session Description Protocol (SDP) was intended to describe
 multimedia sessions for the purposes of session announcement, session
 invitation, and other forms of multimedia session initiation.  An SDP
 session description 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.  To solve this, RFC 3264 [RFC3264] defined the
 offer/answer model, whereby an offerer constructs an offer SDP
 session description that lists the media streams, codecs, and other
 SDP parameters that the offerer is willing to use.  This offer
 session description is sent to the answerer, which chooses from among
 the media streams, codecs and other session description parameters
 provided, and generates an answer session description with his
 parameters, based on that choice.  The answer session description is
 sent back to the offerer thereby completing the session negotiation
 and enabling the establishment of the negotiated media streams.
 Taking a step back, we can 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.  The capability
    indications by themselves do not imply a commitment to use the
    capabilities in the session.

Andreasen Standards Track [Page 4] RFC 5939 SDP Capability Negotiation September 2010

    Capabilities can, for example, be that the "RTP/SAVP" profile is
    supported, that the "PCMU" (Pulse Code Modulation mu-law) codec is
    supported, or that the "crypto" attribute is supported with a
    particular value.
 o  A set of potential configurations indicating which combinations of
    those capabilities can be used for the session and its associated
    media stream components.  Potential configurations are not ready
    for use.  Instead, they provide an alternative that may be used,
    subject to further negotiation.
    A potential configuration can, for example, indicate that the
    "PCMU" codec and the "RTP/SAVP" transport protocol are not only
    supported (i.e., listed as capabilities), but they are offered for
    potential use in the session.
 o  An actual configuration for the session and its associated media
    stream components, that specifies which combinations of session
    parameters and media stream components can be used currently and
    with what parameters.  Use of an actual configuration does not
    require any further negotiation.
    An actual configuration can, for example, be that the "PCMU" codec
    and the "RTP/SAVP" transport protocol are offered for use
    currently.
 o  A negotiation process that takes the set of actual and potential
    configurations (combinations of capabilities) as input and
    provides the negotiated actual configurations as output.
 SDP by itself was designed to provide only one of these, namely
 listing of the actual configurations; however, over the years, use of
 SDP has been extended beyond its original scope.  Of particular
 importance are the session negotiation semantics that were defined by
 the offer/answer model in RFC 3264.  In this model, both the offer
 and the answer contain actual configurations; separate capabilities
 and potential configurations are not supported.
 Other relevant extensions have been defined as well.  RFC 3407
 [RFC3407] defined simple capability declarations, which extends SDP
 with a simple and limited set of capability descriptions.  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 5888 [RFC5888], 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

Andreasen Standards Track [Page 5] RFC 5939 SDP Capability Negotiation September 2010

 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 defined a
 comprehensive capability negotiation framework and protocol that was
 not bound by existing SDP constraints.  SDPng was not designed to be
 backwards compatible with existing SDP and hence required both sides
 to support it, with a graceful fallback to legacy operation when
 needed.  This, combined with lack of ubiquitous multipart MIME
 support in the protocols that would carry SDP or SDPng, made it
 challenging to migrate towards SDPng.  In practice, SDPng has not
 gained traction and, as of the time of publication of this document,
 work on SDPng has stopped.  Existing real-time multimedia
 communication protocols such as SIP, Real Time Streaming Protocol
 (RTSP), Megaco, and Media Gateway Control Protocol (MGCP) continue to
 use SDP.  However, SDP does not address an increasingly important
 problem: the ability to negotiate one or more alternative transport
 protocols (e.g., RTP profiles) and associated parameters (e.g., SDP
 attributes).  This makes it difficult to deploy new RTP profiles such
 as Secure RTP (SRTP) [RFC3711], RTP with RTCP-based feedback
 [RFC4585], etc.  The 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 [RFC3711], RTP/AVPF [RFC4585], and RTP/SAVPF
 [RFC5124].  In addition to the pressing profile negotiation problem,
 other important real-life limitations have been found as well.
 Keying material and other parameters, for example, need to be
 negotiated with some of the transport protocols, but not others.
 Similarly, some media formats and types of media streams need to
 negotiate a variety of different parameters.
 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 that
 is backwards compatible with existing SDP.  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.

Andreasen Standards Track [Page 6] RFC 5939 SDP Capability Negotiation September 2010

 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 [RFC3264]
 (which is not specific to SIP) to negotiate sessions and hence the
 mechanism defined here provides 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 the SDP Capability Negotiation solution, which consists of
 new SDP attributes and associated offer/answer procedures.  In
 Section 4, we provide examples illustrating its use.  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].

3. SDP Capability Negotiation Solution

 In this section, we first present the conceptual model behind the SDP
 Capability Negotiation framework followed by an overview of the SDP
 Capability Negotiation solution.  We then define new SDP attributes
 for the solution and provide its associated updated offer/answer
 procedures.

3.1. SDP Capability Negotiation Model

 Our model uses the concepts of
 o  Capabilities
 o  Potential Configurations
 o  Actual Configurations
 o  Negotiation Process
 as defined in Section 1.  Conceptually, we want to offer not just the
 actual configuration SDP session description (which is done with the
 offer/answer model defined in [RFC3264]), but the actual
 configuration SDP session description as well as one or more
 alternative SDP session descriptions, i.e., potential configurations.
 The answerer must choose either the actual configuration or one of
 the potential configurations, and generate an answer SDP session
 description based on that.  The offerer may need to perform

Andreasen Standards Track [Page 7] RFC 5939 SDP Capability Negotiation September 2010

 processing on the answer, which depends on the offer that was chosen
 (actual or potential configuration).  The answerer therefore informs
 the offerer which configuration the answerer chose.  The process can
 be viewed *conceptually* as follows:
      Offerer                           Answerer
      =======                           ========
 1) Generate offer with actual
    configuration and alternative
    potential configurations
 2) Send offer with all configurations
 +------------+
 | SDP o1     |
 | (actual    |
 |  config    |
 |            |-+      Offer
 +------------+ |      ----->   3) Process offered configurations
   | SDP o2     |                  in order of preference indicated
   | (potential |               4) Generate answer based on chosen
   |  config 1) |-+                configuration (e.g., o2), and
   +------------+ |                inform offerer which one was
     | SDP o3     |                chosen
     | (potential |
     |  config 2) |-+
     +------------+ |
       | SDP ...    |
       :            :
                                    +------------+
                                    | SDP a1     |
                      Answer        | (actual    |
                      <-----        |  config,o2)|
                                    |            |
 5) Process answer based on         +------------+
    the configuration that was
    chosen (o2), as indicated in
    the answer
 The above illustrates the conceptual model: the actual solution uses
 a single SDP session description, which contains the actual
 configuration (as with existing SDP session descriptions and the
 offer/answer model defined in [RFC3264]) and several new attributes
 and associated procedures, that encode the capabilities and potential
 configurations.  A more accurate depiction of the actual offer SDP
 session description is therefore as follows:

Andreasen Standards Track [Page 8] RFC 5939 SDP Capability Negotiation September 2010

        +--------------------+
        | SDP o1             |
        | (actual            |
        |  config            |
        |                    |
        | +-------------+    |
        | | capability 1|    |
        | | capability 2|    |
        | | ...         |    |
        | +-------------+    |   Offer
        |                    |   ----->
        | +-------------+    |
        | | potential   |    |
        | |   config 1  |    |
        | | potential   |    |
        | |   config 2  |    |
        | | ...         |    |
        | +-------------+    |
        |                    |
        +--------------------+
 The above structure is used for two reasons:
 o  Backwards compatibility:   As noted above, support for multipart
    MIME is not ubiquitous.  By encoding both capabilities and
    potential configurations in SDP attributes, we can represent
    everything in a single SDP session description thereby avoiding
    any multipart MIME support issues.  Furthermore, since unknown SDP
    attributes are ignored by the SDP recipient, we ensure that
    entities that do not support the framework simply perform the
    regular RFC 3264 offer/answer procedures.  This provides us with
    seamless backwards compatibility.
 o  Message size efficiency:   When we have multiple media streams,
    each of which may potentially use two or more different transport
    protocols with a variety of different associated parameters, the
    number of potential configurations can be large.  If each possible
    alternative is represented as a complete SDP session description
    in an offer, we can easily end up with large messages.  By
    providing a more compact encoding, we get more efficient message
    sizes.
 In the next section, we describe the exact structure and specific SDP
 parameters used to represent this.

Andreasen Standards Track [Page 9] RFC 5939 SDP Capability Negotiation September 2010

3.2. Solution Overview

 The solution consists of the following:
 o  Two new SDP attributes to support extensions to the framework
    itself as follows:
    o  A new attribute ("a=csup") that lists the supported base
       (optionally) and any supported extension options to the
       framework.
    o  A new attribute ("a=creq") that lists the extensions to the
       framework that are required to be supported by the entity
       receiving the SDP session description in order to do capability
       negotiation.
 o  Two new SDP 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 an
       attribute name and its associated value (if any) as a
       capability.
    o  A new attribute ("a=tcap") that defines how to list transport
       protocols (e.g., "RTP/AVP") as capabilities.
 o  Two new SDP attributes to negotiate configurations as follows:
    o  A new attribute ("a=pcfg") that lists potential configurations
       supported.  This is done by reference to the capabilities from
       the SDP session description in question.  Extension
       capabilities can be defined and referenced in the potential
       configurations.  Alternative potential configurations have an
       explicit ordering associated with them.  Also, potential
       configurations are by default preferred over the actual
       configuration included in the "m=" line and its associated
       parameters.
       This preference order was chosen to provide maximum backwards
       compatibility for the capability negotiation framework and the
       possible values offered for a session.  For example, an entity
       that wants to establish a Secure RTP media stream but is
       willing to accept a plain RTP media stream (assumed to be the
       least common denominator for most endpoints), can offer plain
       RTP in the actual configuration and use the capability
       negotiation extensions to indicate the preference for Secure
       RTP.  Entities that do not support the capability negotiation
       extensions or Secure RTP will then default to plain RTP.

Andreasen Standards Track [Page 10] RFC 5939 SDP Capability Negotiation September 2010

    o  A new attribute ("a=acfg") to be used in an answer SDP session
       description.  The attribute identifies a potential
       configuration from an offer SDP session description that was
       used as an actual configuration to form the answer SDP session
       description.  Extension capabilities can be included as well.
 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 and the media
    level.  Potential configurations can be included only at the media
    level, where they constitute alternative offers that may be
    accepted by the answerer instead of the actual configuration(s)
    included in the "m=" line(s) and associated parameters.  The
    mechanisms defined in this document enable potential
    configurations to change the transport protocol, add new
    attributes, as well as remove all existing attributes from the
    actual configuration.  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)                |
                |<---------------------------------|
                |                                  |
                | (3) Offer (SRTP)                 |
                |--------------------------------->|
                |                                  |
                | (4) Answer (SRTP)                |
                |<---------------------------------|
                |                                  |

Andreasen Standards Track [Page 11] RFC 5939 SDP Capability Negotiation September 2010

 Alice's offer includes RTP and SRTP as alternatives, where RTP is the
 default (actual configuration), but SRTP is the preferred one
 (potential configuration):
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    m=audio 53456 RTP/AVP 0 18
    a=tcap:1 RTP/SAVP
    a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
    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 capabilities are provided by the "a=tcap" and
 "a=acap" attributes.  The transport capability attribute ("a=tcap")
 indicates that Secure RTP under the AVP profile ("RTP/SAVP") is
 supported with an associated 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 [RFC4568].
 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 preferred
 over the actual configuration included in the offer SDP session
 description, and hence Alice is expressing a preference for using
 Secure RTP.
 Bob receives the SDP session description 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 and generates the following answer SDP session
 description:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    m=audio 54568 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

Andreasen Standards Track [Page 12] RFC 5939 SDP Capability Negotiation September 2010

 Bob includes the "a=acfg" attribute in the answer to inform Alice
 that he based his answer on an offer using potential configuration 1
 with transport protocol capability 1 and attribute capability 1 from
 the offer SDP session description (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 option
 tags for those in the answer as well (in an "a=csup" attribute).
 When Alice receives Bob's answer, session negotiation has completed;
 however, Alice nevertheless generates a new offer using the
 negotiated configuration as the actual configuration.  This is done
 purely to assist any intermediaries that may reside between Alice and
 Bob but do not support the SDP Capability Negotiation framework, 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 framework (Alice could have included the
 capabilities as well if she wanted):
    v=0
    o=- 25678 753850 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    m=audio 53456 RTP/SAVP 0 18
    a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
 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 session description offer from Alice, which he
 accepts, and then generates an answer to Alice:
    v=0
    o=- 24351 621815 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    m=audio 54568 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 have done so if he wanted.

Andreasen Standards Track [Page 13] RFC 5939 SDP Capability Negotiation September 2010

 Note that in this particular example, the answerer supported the
 capability negotiation extensions defined here.  Had he not, he 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 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    m=audio 54568 RTP/AVP 0 18

3.3. Version and Extension Indication Attributes

 In this section, we present the new attributes associated with
 indicating the SDP Capability Negotiation extensions supported and
 required.

3.3.1. 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 6.2.
 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 session description.  The attribute can be provided
 at the session level and the media level, and it is defined as
 follows:
    a=csup: <option-tag-list>
 RFC 4566, Section 9, provides the ABNF [RFC5234] for SDP attributes.
 The "csup" attribute adheres to the RFC 4566 "attribute" production,
 with an att-value defined as follows:
    att-value         = option-tag-list
    option-tag-list   = option-tag *("," option-tag)
    option-tag        = token    ; defined in [RFC4566]
 A special base option tag with a value of "cap-v0" is defined for the
 basic SDP Capability Negotiation framework defined in this document.
 Entities can use this option tag with the "a=csup" attribute to

Andreasen Standards Track [Page 14] RFC 5939 SDP Capability Negotiation September 2010

 indicate support for the SDP Capability Negotiation framework
 specified in this document.  Please note that white space is not
 allowed in this rule.
 The following examples illustrate use of the "a=csup" attribute with
 the "cap-v0" option tag and two hypothetical option tags, "foo" and
 "bar" (note the lack of white space):
    a=csup:cap-v0
    a=csup:foo
    a=csup:bar
    a=csup:cap-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 session description.  When provided at the media level, it
 applies only to the media description in question (option tags
 provided at the session level apply as well).  There MUST NOT be more
 than one "a=csup" attribute at the session level and one at the media
 level (one per media description in the latter case).
 Whenever an entity that supports one or more extensions to the SDP
 Capability Negotiation framework generates an SDP session
 description, 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 3.3.2) at the relevant levels.
 Inclusion of the base option tag is OPTIONAL; support for the base
 framework can be inferred from presence of the "a=pcfg" attribute
 defined in Section 3.5.1.
 Use of the base option tag may still be useful in some scenarios,
 e.g., when using SIP OPTIONS [RFC3261] or generating an answer to an
 offer that did not use the SDP Capability Negotiation framework.

3.3.2. Required Capability Negotiation Extensions Attribute

 The Required Capability Negotiation Extensions attribute ("a=creq")
 contains a comma-separated list of option tags (see Section 3.3.1)
 specifying the SDP Capability Negotiation extensions that MUST be
 supported by the entity receiving the SDP session description, in
 order for that entity to properly process the SDP Capability
 Negotiation attributes and associated procedures.  There is no need
 to include the base option tag ("cap-v0") with the "creq" attribute,

Andreasen Standards Track [Page 15] RFC 5939 SDP Capability Negotiation September 2010

 since any entity that supports the "creq" attribute in the first
 place also supports the base option tag.  Still, it is permissible to
 do so.
    Such functionality may be important if a future version of the
    Capability Negotiation framework were not backwards compatible.
 The attribute can be provided at the session level and the media
 level, and it 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   = option-tag-list
 The following examples illustrate use of the "a=creq" attribute with
 the "cap-v0" base option tag and two hypothetical option tags, "foo"
 and "bar" (note the lack of white space):
    a=creq:cap-v0
    a=creq:foo
    a=creq:bar
    a=creq:cap-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 session description.  When provided at the media level, it
 applies only to the media description in question (required option
 tags provided at the session level apply as well).  There MUST NOT be
 more than one "a=creq" attribute at the session level and one
 "a=creq" attribute at the media level (one per media description in
 the latter case).
 When an entity generates an SDP session description and it requires
 the recipient of that SDP session description to support one or more
 SDP Capability Negotiation extensions (except for the base) at the
 session or media level 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.  If support for an extension is needed only in
 one or more specific potential configurations, the potential
 configuration provides a way to indicate that instead (see Section
 3.5.1).  Support for the basic negotiation framework is implied by

Andreasen Standards Track [Page 16] RFC 5939 SDP Capability Negotiation September 2010

 the presence of an "a=pcfg" attribute (see Section 3.5.1) and hence
 it is not required to include the "a=creq" attribute with the base
 option tag ("cap-v0").
 A recipient that receives an SDP session description 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; instead the recipient MUST proceed as if the SDP
 Capability Negotiation attributes were not included in the first
 place, i.e., the capability negotiation attributes are ignored.  In
 that case, if the SDP session description recipient is an SDP
 answerer [RFC3264], the recipient SHOULD include a "csup" attribute
 in the resulting SDP session description answer listing the SDP
 Capability Negotiation extensions it actually supports.
    This ensures that introduction of the SDP Capability Negotiation
    mechanism by itself does not lead to session failures
 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.
    An entity that does not support the SDP Capability Negotiation
    framework at all, will ignore these attributes (as well as the
    other SDP Capability Negotiation attributes) and not perform any
    SDP Capability Negotiation in the first place.

3.4. Capability Attributes

 In this section, we present the new attributes associated with
 indicating the capabilities for use by the SDP Capability
 Negotiation.

3.4.1. Attribute Capability Attribute

 Attributes and their associated values can be expressed as
 capabilities 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 "<attribute>" or "<attribute>:<value>" form,
 i.e., excluding the "a=" part (see [RFC4566]).  The attribute can be
 provided at the session level and the media level.

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 The "acap" attribute adheres to the RFC 4566 "attribute" production,
 with an att-value defined as follows:
    att-value   = att-cap-num 1*WSP att-par
    att-cap-num = 1*10(DIGIT)  ;defined in [RFC5234]
    att-par     = attribute    ;defined in [RFC4566]
 Note that white space is not permitted before the att-cap-num.
 When the attribute capability contains a session-level attribute,
 that "acap" attribute can only be provided at the session level.
 Conversely, media-level attributes can be provided in attribute
 capabilities at either the media level or session level.  The base
 SDP Capability Negotiation framework however only defines procedures
 for use of media-level attribute capabilities at the media level.
 Implementations that conform only to the base framework MUST NOT
 generate media-level attribute capabilities at the session level;
 however, extensions may change this (see, e.g., [SDPMedCap] for one
 such extension) and hence all implementations MUST still be prepared
 to receive such capabilities (see Section 3.6.2 for processing
 rules).
 Each occurrence of the "acap" attribute in the entire session
 description MUST use a different value of <att-cap-num>.  Consecutive
 numbering of the <att-cap-num> values is not required.
    There is a need to be able to reference both session-level and
    media-level attributes in potential configurations at the media
    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 types of capabilities, i.e., they
 form a separate name-space for attribute capabilities.
 The following examples illustrate use of the "acap" attribute:
    a=acap:1 ptime:20
    a=acap:2 ptime:30
    a=acap:3 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
    AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
    JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
    SrzKTAv9zV
    a=acap:4 crypto:1 AES_CM_128_HMAC_SHA1_32
          inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32

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 The first two attribute capabilities provide attribute values for the
 ptime attribute.  The third provides SRTP parameters by using
 Multimedia Internet KEYing (MIKEY) [RFC3830] with the "key-mgmt"
 attribute [RFC4567].  The fourth provides SRTP parameters by use of
 security descriptions with the "crypto" attribute [RFC4568].  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 session descriptions.
    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, notably that the "acap" attribute
    contains a handle that enables referencing it and it furthermore
    supports only attributes (the "cpar" attribute defined in RFC 3407
    supports bandwidth information as well).  The "acap" attribute
    also is not automatically associated with any particular
    capabilities.  See Section 3.14 for the relationship to RFC 3407.
 Attribute capabilities MUST NOT embed any capability negotiation
 parameters.  This restriction applies to all the capability
 negotiation parameters defined in this document ("csup", "creq",
 "acap", "tcap", "pcfg", and "acfg") as well as any capability
 negotiation extensions defined.  The following examples are thus
 invalid attribute capabilities and MUST NOT be used:
   a=acap:1 acap:2 foo:a       ;Not allowed to embed "acap"
   a=acap:2 a=pcfg:1 t=1 a=1   ;Not allowed to embed "pcfg"
 The reason for this restriction is to avoid overly complex processing
 rules resulting from the expansion of such capabilities into
 potential configurations (see Section 3.6.2 for further details).

3.4.2. 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
 reference, and <proto-list> is one or more <proto>, separated by
 white space, as defined in the SDP "m=" line.  The attribute can be
 provided at the session level and the media level.

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 The "tcap" attribute adheres to the RFC 4566 "attribute" production,
 with an att-value defined as follows:
    att-value      = trpr-cap-num 1*WSP proto-list
    trpr-cap-num   = 1*10(DIGIT)           ;defined in [RFC5234]
    proto-list     = proto *(1*WSP proto)  ;defined in [RFC4566]
 Note that white space is not permitted before the trpr-cap-num.
 The "tcap" attribute can be provided at the session level and the
 media level.  There MUST NOT be more than one "a=tcap" attribute at
 the session level and one at the media level (one per media
 description in the latter case).  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.  There MUST NOT be any capability
 number overlap between different "tcap" attributes in the entire SDP
 session description.  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.  Consecutive numbering of the <trpr-cap-num>
 values in different "tcap" attributes is not required.
 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
    a=tcap:5 UDP/TLS/RTP/SAVP
 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 [RFC4585].  The third one
 provides capabilities for the "RTP/SAVP" (transport capability number
 3) and "RTP/SAVPF" profiles (transport protocol capability number 4).
 The last one provides capabilities for "UDP/TLS/RTP/SAVP", i.e.,
 DTLS-SRTP [RFC5764] (transport capability number 5).
 The "tcap" attribute by itself can only specify transport protocols
 as defined by <proto> in [RFC4566]; however, full specification of a
 media stream requires further qualification of the transport protocol
 by one or more media format descriptions, which themselves often
 depend on the transport protocol.  As an example, [RFC3551] defines
 the "RTP/AVP" transport for use with audio and video codecs (media

Andreasen Standards Track [Page 20] RFC 5939 SDP Capability Negotiation September 2010

 formats), whereas [RFC4145] defines the "TCP" transport, which, for
 example, may be used to negotiate T.38 fax ("image/t38"), etc.  In a
 non-SDP context, some media formats could be viewed as transports
 themselves (e.g., T.38); however, in the context of SDP and SDP
 Capability Negotiation, they are not.  If capability negotiation is
 required for such media formats, they MUST all either be valid under
 the transport protocol indicated in the "m=" line included for the
 media stream description, or a suitable extension must be used, e.g.,
 SDP Media Capabilities [SDPMedCap].
 The ability to use a particular transport protocol is inherently
 implied by including it in the "m=" line, regardless of whether or
 not it is provided in a "tcap" attribute.  However, if a potential
 configuration needs to reference that transport protocol as a
 capability, the transport protocol MUST be included explicitly in a
 "tcap" attribute.
    This may seem redundant (and indeed it is from the offerer's point
    of view), however it is done to protect against intermediaries
    (e.g., middleboxes) that may modify "m=" lines while passing
    unknown attributes through.  If an implicit transport 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 an intermediary 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; however, the potential
    configuration preference (see Section 3.5.1) may not reflect that
    of the intermediary (which some may view as a feature).
 Note that a transport protocol capability may be provided,
 irrespective of whether or not it is referenced in a potential
 configuration (just like any other capability).

3.4.3. Extension Capability Attributes

 The SDP Capability Negotiation framework allows for new types of
 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 SHOULD allow for a
 numeric handle to be associated with each capability.  This handle
 can be used as a reference within the potential and actual
 configuration attributes (see Sections 3.5.1 and 3.5.2).  The
 definition of such extension capability attributes MUST also state
 whether they can be applied at the session level, media level, or

Andreasen Standards Track [Page 21] RFC 5939 SDP Capability Negotiation September 2010

 both.  Note that extensions can have option tags defined for them,
 and option tags MUST be registered with the IANA in accordance with
 the procedures specified in Section 6.2.
 Extension capabilities SHOULD NOT embed any capability negotiation
 parameters.  This applies to all the capability negotiation
 parameters defined in this document as well as any extensions
 defined.  The reason for this restriction is to avoid overly complex
 processing rules resulting from the expansion of such capabilities
 into potential configurations (see Section 3.6.2 for further
 details).  If an extension does not follow the above "SHOULD NOT"
 recommendation, the extension MUST provide a careful analysis of why
 such behavior is both necessary and safe.

3.5. Configuration Attributes

3.5.1. 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 attribute can be provided only at the media level.
 The "pcfg" attribute adheres to the RFC 4566 "attribute" production,
 with an att-value defined as follows:
    att-value      = config-number [1*WSP pot-cfg-list]
    config-number  = 1*10(DIGIT)  ;defined in [RFC5234]
    pot-cfg-list   = pot-config *(1*WSP pot-config)
    pot-config     = attribute-config-list /
                     transport-protocol-config-list /
                     extension-config-list
 The missing productions are defined below.  Note that white space is
 not permitted before the config-number.
 The potential configuration attribute can be provided only at the
 media level and there can be multiple instances of it within a given
 media description.  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 description
 (i.e., it has only media-level scope).  The configuration number also
 indicates the relative preference of potential configurations; lower

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 numbers are preferred over higher numbers.  Consecutive numbering of
 the configuration numbers in different "pcfg" attributes in a media
 description is not required.
 A potential configuration list is normally provided after the
 configuration number.  When the potential configuration list is
 omitted, the potential configuration equals the actual configuration.
 The potential configuration list contains one or more of attribute,
 transport, and extension configuration lists.  A potential
 configuration may for example include attribute capabilities and
 transport capabilities, transport capabilities only, or some other
 combination of capabilities.  If transport capabilities are not
 included in a potential configuration, the default transport for that
 media stream is used.
 The potential configuration lists generally reference one or more
 capabilities (extension configuration lists MAY use a different
 format).  Those capabilities are (conceptually) used to construct a
 new internal version of the SDP session description by use of purely
 syntactic add and (possibly) delete operations on the original SDP
 session description (actual configuration).  This provides an
 alternative potential configuration SDP session description that can
 be used by conventional SDP and offer/answer procedures if selected.
 This document defines attribute configuration lists and transport
 protocol configuration lists.  Each of these MUST NOT be present more
 than once in a particular potential configuration attribute.
 Attribute capabilities referenced by the attribute configuration list
 (if included) are added to the actual configuration, whereas a
 transport capability referenced by the transport protocol
 configuration list (if included) replaces the default transport
 protocol from the actual configuration.  Extension configuration
 lists can be included as well.  There can be more than one extension
 configuration list; however, each particular extension MUST NOT be
 present more than once in a given "a=pcfg" attribute.  Together, the
 various configuration lists define a potential configuration.
 There can be multiple potential configurations in a media
 description.  Each of these indicates not only a willingness, but in
 fact a desire to use the potential configuration.

Andreasen Standards Track [Page 23] RFC 5939 SDP Capability Negotiation September 2010

 The example SDP session description below contains two potential
 configurations:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    m=audio 53456 RTP/AVP 0 18
    a=tcap:1 RTP/SAVP RTP/SAVPF
    a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
    a=pcfg:1 t=1 a=1
    a=pcfg:2 t=2 a=1
 Potential configuration 1 contains a transport protocol configuration
 list that references transport capability 1 ("RTP/SAVP") and an
 attribute configuration list that references attribute capability 1
 ("a=crypto:...").  Potential configuration 2 contains a transport
 protocol configuration list that references transport capability 2
 ("RTP/SAVPF") and an attribute configuration list that references
 attribute capability 1 ("a=crypto:...").
 Attribute capabilities are used in a potential configuration by use
 of the attribute-config-list parameter, which is defined by the
 following ABNF:
    attribute-config-list =  "a=" delete-attributes
    attribute-config-list =/ "a=" [delete-attributes ":"]
                      mo-att-cap-list *(BAR mo-att-cap-list)
    delete-attributes = DELETE ( "m" ; media attributes
                            / "s"    ; session attributes
                            / "ms" ) ; media and session attributes
    mo-att-cap-list   = mandatory-optional-att-cap-list /
                                  mandatory-att-cap-list /
                                     optional-att-cap-list
    mandatory-optional-att-cap-list  = mandatory-att-cap-list
                                           "," optional-att-cap-list
    mandatory-att-cap-list           = att-cap-list
    optional-att-cap-list            = "[" att-cap-list "]"
    att-cap-list      = att-cap-num *("," att-cap-num)
    att-cap-num       = 1*10(DIGIT)     ;defined in [RFC5234]
    BAR               = "|"
    DELETE            = "-"

Andreasen Standards Track [Page 24] RFC 5939 SDP Capability Negotiation September 2010

 Note that white space is not permitted within the attribute-config-
 list rule.
 Each attribute configuration list can optionally begin with
 instructions for how to handle attributes that are part of the actual
 configuration SDP session description (i.e., the "a=" lines present
 in the original SDP session description).  By default, such
 attributes will remain as part of the potential configuration in
 question.  However, if delete-attributes indicates "-m", then all
 attribute lines within the media description in question will be
 deleted in the resulting potential configuration SDP session
 description (i.e., all "a=" lines under the "m=" line in question).
 If delete-attributes indicates "-s", then all attribute lines at the
 session level will be deleted (i.e., all "a=" lines before the first
 "m=" line).  If delete-attributes indicates "-ms", then all attribute
 lines within this media description ("m=" line) and all attribute
 lines at the session level will be deleted.
 The attribute capability list comes next (if included).  It contains
 one or more alternative lists of attribute capabilities.  The
 alternative attribute capability lists are separated by a vertical
 bar ("|"), and each list contains one or more attribute capabilities
 separated by commas (",").  The attribute capabilities are either
 mandatory or optional.  Mandatory attribute capabilities MUST be
 supported in order to use the potential configuration, whereas
 optional attribute capabilities MAY be supported in order to use the
 potential configuration.
 Within each attribute capability list, all the mandatory attribute
 capabilities (if any) are listed first, and all the optional
 attribute capabilities (if any) are listed last.  The optional
 attribute capabilities are contained within a pair of square brackets
 ("[" and "]").  Each attribute capability is merely an attribute
 capability number (att-cap-num) that identifies a particular
 attribute capability by referring to attribute capability numbers
 defined above and hence MUST be between 1 and 2^31-1 (both included).
 The following example illustrates the above:
    a=pcfg:1 a=-m:1,2,[3,4]|1,7,[5]
 where
 o  "a=-m:1,2,[3,4]|1,7,[5]" is the attribute configuration list
 o  "-m" indicates to delete all attributes from the media description
    of the actual configuration

Andreasen Standards Track [Page 25] RFC 5939 SDP Capability Negotiation September 2010

 o  "1,2,[3,4]" and "1,7,[5]" are both attribute capability lists.
    The two lists are alternatives, since they are separated by a
    vertical bar above
 o  "1", "2", and "7" are mandatory attribute capabilities
 o  "3", "4", and "5" are optional attribute capabilities
 Note that in the example above, we have a single handle ("1") for the
 potential configuration(s), but there are actually two different
 potential configurations (separated by a vertical bar).  This is done
 for message size efficiency reasons, which is especially important
 when we add other types of capabilities to the potential
 configuration.  If there is a need to provide a unique handle for
 each, then separate "a=pcfg" attributes with different handles MUST
 be used instead.
 Each referenced attribute capability in the potential configuration
 will result in the corresponding attribute name and its associated
 value (contained inside the attribute capability) being added to the
 resulting potential configuration SDP session description.
 Alternative attribute capability lists 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.  In order for a
 recipient of the SDP session description (e.g., an answerer receiving
 this in an offer) to use this potential configuration, exactly one of
 the alternative lists MUST be selected in its entirety.  This
 requires that all mandatory attribute capabilities referenced by the
 potential configuration are supported with the attribute values
 provided.
 Transport protocol configuration lists are included in a potential
 configuration by use of the transport-protocol-config-list parameter,
 which is defined by the following ABNF:
    transport-protocol-config-list =
                         "t=" trpr-cap-num *(BAR trpr-cap-num)
    trpr-cap-num        = 1*10(DIGIT)   ; defined in [RFC5234]
 Note that white space is not permitted within this rule.
 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 transport protocol capabilities are separated by a
 vertical bar ("|").  The alternatives are ordered by preference with
 the most preferred listed first.  If there are no transport protocol

Andreasen Standards Track [Page 26] RFC 5939 SDP Capability Negotiation September 2010

 capabilities included in a potential configuration at the media
 level, the transport protocol information from the associated "m="
 line MUST be used.  In order for a recipient of the SDP session
 description (e.g., an answerer receiving this in an offer) to use
 this potential configuration, exactly one of the alternatives MUST be
 selected.  This requires that the transport protocol in question is
 supported.
    In the presence of intermediaries (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 an intermediary.
    Use of an explicit transport protocol capability will guard
    against capability negotiation implications of that.
 Extension capabilities can be included in a potential configuration
 as well by use of extension configuration lists.  Extension
 configuration lists MUST adhere to the following ABNF:
    extension-config-list   = ["+"] ext-cap-name "=" ext-cap-list
    ext-cap-name            = 1*(ALPHA / DIGIT)
    ext-cap-list            = 1*VCHAR   ; defined in [RFC5234]
 Note that white space is not permitted within this rule.
 The ext-cap-name refers to the name of the extension capability and
 the ext-cap-list is here merely defined as a sequence of visible
 characters.  The actual extension supported MUST refine both of these
 further.  For extension capabilities that merely need to be
 referenced by a capability number, it is RECOMMENDED to follow a
 structure similar to what has been specified above.  Unsupported or
 unknown potential extension configuration lists in a potential
 configuration attribute MUST be ignored, unless they are prefixed
 with the plus ("+") sign, which indicates that the extension is
 mandatory and MUST be supported in order to use that potential
 configuration.
    The "creq" attribute and its associated rules can be used to
    ensure that required extensions are supported in the first place.
 Extension configuration lists define new potential configuration
 parameters and hence they MUST be registered with IANA per the
 procedures defined in Section 6.3.
 Potential configuration attributes can be provided only at the media
 level; 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:

Andreasen Standards Track [Page 27] RFC 5939 SDP Capability Negotiation September 2010

 A (media-level) potential configuration attribute in a given media
 description MUST NOT reference a media-level capability provided in a
 different media description; doing so invalidates that potential
 configuration (note that a potential configuration attribute can
 contain more than one potential configuration by use of
 alternatives).  A potential configuration attribute can however
 reference a session-level capability.  The semantics of doing so
 depends on the type of capability.  In the case of transport protocol
 capabilities, it has no particular implication.  In the case of
 attribute capabilities, however, it does.  More specifically, the
 attribute name and value (provided within that attribute capability)
 will be considered part of the resulting SDP for that particular
 configuration at the *session* level.  In other words, it will be
 as-if that attribute was provided with that value at the session
 level in the first place.  As a result, the base SDP Capability
 Negotiation framework REQUIRES that potential configurations do not
 reference any session-level attribute capabilities that contain
 media-level attributes (since that would place a media-level
 attribute at the session level).  Extensions may modify this
 behavior, as long as it is fully backwards compatible with the base
 specification.
 Individual media streams perform capability negotiation individually,
 and hence it is possible that one media stream (where the attribute
 was part of a potential configuration) chose a configuration without
 a session-level attribute that was chosen by another media stream.
 The session-level attribute however remains "active" and applies to
 the entire resulting potential configuration SDP session description.
 In theory, this is problematic if one or more session-level
 attributes either conflicts with or potentially interacts with
 another session-level or media-level attribute in an undefined
 manner.  In practice, such examples seem to be rare (at least with
 the SDP attributes that had been defined at time of publication of
 this document).
    A related set of problems can occur if we need coordination
    between session-level attributes from multiple media streams in
    order for a particular functionality to work.  The grouping
    framework [RFC5888] is an example of this.  If we use the SDP
    Capability Negotiation framework to select a session-level group
    attribute (provided as an attribute capability), and we require
    two media descriptions to do this consistently, we could have a
    problem.  The Forward Error Correction (FEC) grouping semantics
    [RFC4756] is one example where this in theory could cause
    problems, however in practice, it is unclear that there is a
    significant problem with the grouping semantics that had been
    defined at time of publication of this document.

Andreasen Standards Track [Page 28] RFC 5939 SDP Capability Negotiation September 2010

 Resolving the above issues in general requires inter-media stream
 constraints and synchronized potential configuration processing; this
 would add considerable complexity to the overall solution.  In
 practice, with the SDP attributes defined at time of publication of
 this document, it does not seem to be a significant problem, and
 hence the base SDP Capability Negotiation solution does not provide a
 solution to this issue.  Instead, it is RECOMMENDED that use of
 session-level attributes in a potential configuration is avoided when
 possible, and when not, that such use is examined closely for any
 potential interaction issues.  If interaction is possible, the entity
 generating the SDP session description SHOULD NOT assume that well-
 defined operation will occur at the receiving entity.  This implies
 that mechanisms that might have such interactions cannot be used in
 security critical contexts.
 The session-level operation of extension capabilities is undefined.
 Consequently, each new session-level extension capability defined
 MUST specify the implication of making it part of a 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 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    m=audio 53456 RTP/AVPF 0 18
    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 RTP/SAVP RTP/SAVPF
    a=pcfg:1 t=4|3 a=1
    a=pcfg:8 t=1|2
 We have two potential configuration attributes 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.  Note that
 although we have a single potential configuration attribute and
 associated handle, we have two potential configurations.

Andreasen Standards Track [Page 29] RFC 5939 SDP Capability Negotiation September 2010

 The second potential configuration attribute indicates that the
 RTP/AVPF or RTP/AVP profiles 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".  Again, note that we have
 two potential configurations here and hence a total of four potential
 configurations in the SDP session description above.

3.5.2. Actual Configuration Attribute

 The actual configuration attribute identifies which of the potential
 configurations from an offer SDP session description was selected and
 used as the actual configuration to generate an answer SDP session
 description.  This is done by including the configuration number and
 the configuration lists (if any) from the offer that were selected
 and used by the answerer in his offer/answer procedure as follows:
 o  A selected attribute configuration MUST include the delete-
    attributes and the known and supported parameters from the
    selected alternative mo-att-cap-list (i.e., containing all
    mandatory and all known and supported optional capability numbers
    from the potential configuration).  If delete-attributes were not
    included in the potential configuration, they will of course not
    be present here either.
 o  A selected transport protocol configuration MUST include the
    selected transport protocol capability number.
 o  A selected potential extension configuration MUST include the
    selected extension configuration parameters as specified for that
    particular extension.
 o  When a configuration list contains alternatives (separated by
    "|"), the selected configuration only MUST be provided.
 Note that the selected configuration number and all selected
 capability numbers used in the actual configuration attribute refer
 to those from the offer: not the answer.
    The answer may for example include capabilities as well to inform
    the offerer of the answerers capabilities above and beyond the
    negotiated configuration.  The actual configuration attribute does
    not refer to any of those answer capabilities though.
 The Actual Configuration Attribute ("a=acfg") is defined as follows:
    a=acfg: <config-number> [<sel-cfg-list>]

Andreasen Standards Track [Page 30] RFC 5939 SDP Capability Negotiation September 2010

 where <config-number> is an integer between 1 and 2^31-1 (both
 included) that refers to the selected potential configuration.  The
 attribute can be provided only at the media level.
 The "acfg" attribute adheres to the RFC 4566 "attribute" production,
 with an att-value defined as follows:
    att-value      = config-number [1*WSP sel-cfg-list]
                      ;config-number defined in Section 3.5.1.
    sel-cfg-list   = sel-cfg *(1*WSP sel-cfg)
    sel-cfg        = sel-attribute-config /
                         sel-transport-protocol-config /
                         sel-extension-config
    sel-attribute-config =
             "a=" [delete-attributes ":"] mo-att-cap-list
                                  ; defined in Section 3.5.1.
    sel-transport-protocol-config =
             "t=" trpr-cap-num    ; defined in Section 3.5.1.
    sel-extension-config =
             ext-cap-name "=" 1*VCHAR   ; defined in Section 3.5.1.
 Note that white space is not permitted before the config-number.
 The actual configuration ("a=acfg") attribute can be provided only at
 the media level.  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 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    m=audio 54568 RTP/SAVPF 0
    a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
    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).  The answerer includes his own "crypto" attribute as
 well.

Andreasen Standards Track [Page 31] RFC 5939 SDP Capability Negotiation September 2010

3.6. 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 alternative offers that
 may be accepted by the answerer instead of the actual
 configuration(s) included in the "m=" line(s).
 The procedures defined in the following subsections apply to both
 unicast and multicast streams.

3.6.1. Generating the Initial Offer

 An offerer that wants to use the SDP Capability Negotiation defined
 in this document MUST include the following in the offer:
 o  Zero or more attribute capability attributes.  There MUST be an
    attribute capability attribute ("a=acap") as defined in Section
    3.4.1 for each attribute name and associated value (if any) that
    needs to be indicated as a capability in the offer.  Attribute
    capabilities may be included irrespective of whether or not they
    are referenced by a potential configuration.
    Session-level attributes and associated values MUST be provided in
    attribute capabilities only at the session level, whereas media-
    level attributes and associated values can be provided in
    attribute capabilities at either the media level or session level.
    Attributes that are allowed at either the session or media level
    can be provided in attribute capabilities at either level.
 o  Zero or more transport protocol capability attributes.  There MUST
    be transport protocol capabilities as defined in Section 3.4.2
    with values for each transport protocol that needs to be indicated
    as a capability in the offer.
    Transport protocol capabilities may be included irrespective of
    whether or not they are referenced by a potential configuration.
    Transport protocols that apply to multiple media descriptions
    SHOULD be provided as transport protocol capabilities at the
    session level whereas transport protocols that apply only to a
    specific media description ("m=" line), SHOULD be provided as
    transport protocol capabilities within that particular media
    description.  In either case, there MUST NOT be more than a single
    "a=tcap" attribute at the session level and a single "a=tcap"
    attribute in each media description.

Andreasen Standards Track [Page 32] RFC 5939 SDP Capability Negotiation September 2010

 o  Zero or more extension capability attributes.  There MUST be one
    or more extension capability attributes (as outlined in Section
    3.4.3) for each extension capability that is referenced by a
    potential configuration.  Extension capability attributes that are
    not referenced by a potential configuration can be provided as
    well.
 o  Zero or more potential configuration attributes.  There MUST be
    one or more potential configuration attributes ("a=pcfg"), as
    defined in Section 3.5.1, in each media description where
    alternative potential configurations are to be negotiated.  Each
    potential configuration attribute MUST adhere to the rules
    provided in Section 3.5.1 and the additional rules provided below.
 If the offerer requires support for one or more extensions (besides
 the base protocol defined here), then the offerer MUST include one or
 more "a=creq" attributes as follows:
 o  If support for one or more capability negotiation extensions is
    required for the entire session description, then option tags for
    those extensions MUST be included in a single session-level "creq"
    attribute.
 o  For each media description that requires support for one or more
    capability negotiation extensions not listed at the session level,
    a single "creq" attribute containing all the required extensions
    for that media description MUST be included within the media
    description (in accordance with Section 3.3.2).
 Note that extensions that only need to be supported by a particular
 potential configuration can use the "mandatory" extension prefix
 ("+") within the potential configuration (see Section 3.5.1).
 The offerer SHOULD furthermore include the following:
 o  A supported capability negotiation extension attribute ("a=csup")
    at the session level and/or media level as defined in Section
    3.3.2 for each capability negotiation extension supported by the
    offerer and not included in a corresponding "a=creq" attribute
    (i.e., at the session level or in the same media description).
    Option tags provided in a "a=csup" attribute at the session level
    indicate extensions supported for the entire session description,
    whereas option tags provided in a "a=csup" attribute in a media
    description indicate extensions supported for only that particular
    media description.

Andreasen Standards Track [Page 33] RFC 5939 SDP Capability Negotiation September 2010

 Capabilities provided in an offer merely indicate what the offerer is
 capable of doing.  They do not constitute a commitment or even an
 indication to use them.  In contrast, each potential configuration
 constitutes an alternative offer that the offerer would like to use.
 The potential configurations MUST be used by the answerer to
 negotiate and establish the session.
 The offerer MUST include one or more potential configuration
 attributes ("a=pcfg") in 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
 zero, one or more potential configuration lists, as described in
 Section 3.5.1.  Each potential configuration list MUST refer to
 capabilities that are provided at the session level or within that
 particular media description; otherwise, the potential configuration
 is considered invalid.  The base SDP Capability Negotiation framework
 REQUIRES that potential configurations not reference any session-
 level attribute capabilities that contain media-level-only
 attributes; however, extensions may modify this behavior, as long as
 it is fully backwards compatible with the base specification.
 Furthermore, it is RECOMMENDED that potential configurations avoid
 use of session-level capabilities whenever possible; refer to Section
 3.5.1.
 The current actual configuration is included in the "m=" line (as
 defined by [RFC3264]) and any associated parameters for the media
 description (e.g., attribute ("a=") and bandwidth ("b=") lines).
 Note that the actual configuration is by default the least-preferred
 configuration, and hence the answerer will seek to negotiate use of
 one of the potential configurations instead.  If the offerer wishes a
 different preference for the actual configuration, the offerer MUST
 include a corresponding potential configuration with the relevant
 configuration number (which indicates the relative preference between
 potential configurations); this corresponding potential configuration
 should simply duplicate the actual configuration.
    This can either be done implicitly (by not referencing any
    capabilities), or explicitly (by providing and using capabilities
    for the transport protocol and all the attributes that are part of
    the actual configuration).  The latter may help detect
    intermediaries that modify the actual configuration but are not
    SDP Capability Negotiation aware.
 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 only for the actual
 configurations provided in the offer: Media received by the offerer

Andreasen Standards Track [Page 34] RFC 5939 SDP Capability Negotiation September 2010

 according to one of the potential configurations MAY be discarded,
 until the offerer receives an answer indicating what the actual
 selected configuration is.  Once that answer is received, incoming
 media MUST be processed in accordance with the actual selected
 configuration indicated and the answer received (provided the
 offer/answer exchange completed successfully).
 The above rule assumes that the offerer can determine whether
 incoming media adheres to the actual configuration offered or one of
 the potential configurations instead; this may not always be the
 case.  If the offerer wants to ensure he does not play out any
 garbage, the offerer SHOULD discard all media received before the
 answer SDP session description is received.  Conversely, if the
 offerer wants to avoid clipping, he SHOULD attempt to play any
 incoming media as soon as it is received (at the risk of playing out
 garbage).  In either case, please note that this document does not
 place any requirements on the offerer to process and play media
 before answer.  For further details, please refer to Section 3.9.

3.6.2. Generating the Answer

 When receiving an offer, the answerer MUST check for the presence of
 a required capability negotiation extension attribute ("a=creq")
 provided at the session level.  If one is found, then capability
 negotiation MUST be performed.  If none is found, then the answerer
 MUST check each offered media description for the presence of a
 required capability negotiation extension attribute ("a=creq") and
 one or more potential configuration attributes ("a=pcfg").
 Capability negotiation MUST be performed for each media description
 where either of those is present in accordance with the procedures
 described below.
 The answerer MUST first ensure that it supports any 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

Andreasen Standards Track [Page 35] RFC 5939 SDP Capability Negotiation September 2010

    provided for that particular media description.  Instead, the
    offer/answer procedures for that media description MUST continue
    as per [RFC3264] (SDP Capability Negotiation is still performed
    for other media descriptions in the SDP session description).
    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 perform capability negotiation by using
 the most preferred potential configuration that is valid to the
 answerer, subject to any local policies.  A potential configuration
 is valid to the answerer if:
 1. It is in accordance with the syntax and semantics provided in
    Section 3.5.1.
 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 3.4.1)
    and each of them is provided either at the session level or within
    this particular media description.
    For session-level attribute capabilities referenced, the
    attributes contained inside them MUST NOT be media-level-only
    attributes.  Note that the answerer can only determine this for
    attributes supported by the answerer.  If an attribute is not
    supported, it will simply be ignored by the answerer and hence
    will not trigger an "invalid" potential configuration.
 4. All transport protocol capabilities referenced by the potential
    configuration are valid themselves (as defined in Section 3.4.2)
    and each of them is 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

Andreasen Standards Track [Page 36] RFC 5939 SDP Capability Negotiation September 2010

    capabilities MUST be ignored, unless they are prefixed with the
    plus ("+") sign, which indicates that the extension MUST be
    supported in order to use that potential configuration.  If the
    extension is not supported, that potential configuration is not
    valid to the answerer.
 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 as follows.  First, all capability negotiation
 parameters from the offer SDP session description are removed,
 thereby yielding an offer SDP session description with the actual
 configuration as if SDP Capability Negotiation was not done in the
 first place.  Secondly, this actual configuration SDP session
 description is modified as follows for each media stream offered,
 based on the capability negotiation parameters included originally:
 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 attribute capabilities are present with a delete-attributes
    session indication ("-s") or media and session indication ("-ms"),
    then all session-level attributes from the actual configuration
    SDP session description MUST be deleted in the resulting potential
    configuration SDP session description in accordance with the
    procedures in Section 3.5.1.  If attribute capabilities are
    present with a delete-attributes media indication ("-m") or media
    and session indication ("-ms"), then all attributes from the
    actual configuration SDP session description inside this media
    description MUST be deleted.
 o  If a session-level attribute capability is included, the attribute
    (and its associated value, if any) contained in it MUST be added
    to the resulting SDP session description.  All such added session-
    level attributes MUST be listed before the session-level
    attributes that were initially present in the SDP session
    description.  Furthermore, the added session-level attributes MUST
    be added in the order they were provided in the potential
    configuration (see also Section 3.5.1).
       This allows for attributes with implicit preference ordering to
       be added in the desired order; the "crypto" attribute [RFC4568]
       is one such example.

Andreasen Standards Track [Page 37] RFC 5939 SDP Capability Negotiation September 2010

 o  If a media-level attribute capability is included, then the
    attribute (and its associated value, if any) MUST be added to the
    resulting SDP session description within the media description in
    question.  All such added media-level attributes MUST be listed
    before the media-level attributes that were initially present in
    the media description in question.  Furthermore, the added media-
    level attributes MUST be added in the order they were provided in
    the potential configuration (see also Section 3.5.1).
 o  If a supported extension capability is included, then it MUST be
    processed in accordance with the rules provided for that
    particular extension capability.
 The above steps MUST be performed exactly once per potential
 configuration, i.e., there MUST NOT be any recursive processing of
 any additional capability negotiation parameters that may (illegally)
 have been nested inside capabilities themselves.
 As an example of this, consider the (illegal) attribute capability
  a=acap:1 acap:2 foo:a
 The resulting potential configuration SDP session description will,
 after the above processing has been done, contain the attribute
 capability
  a=acap:2 foo:a
 However, since we do not perform any recursive processing of
 capability negotiation parameters, this second attribute capability
 parameter will not be processed by the offer/answer procedure.
 Instead, it will simply appear as a (useless) attribute in the SDP
 session description that will be ignored by further processing.
 Note that a transport protocol from the potential configuration
 replaces the transport protocol in the actual configuration, but an
 attribute capability from the potential configuration is simply added
 to the actual configuration.  In some cases, this can result in
 having one or more meaningless attributes in the resulting potential
 configuration SDP session description, or worse, ambiguous or
 potentially even illegal attributes.  Use of delete-attributes for
 the session- and/or media-level attributes MUST be done to avoid such
 scenarios.  Nevertheless, it is RECOMMENDED that implementations
 ignore meaningless attributes that may result from potential
 configurations.

Andreasen Standards Track [Page 38] RFC 5939 SDP Capability Negotiation September 2010

    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.  The answerer may or may
    not ignore such a meaningless attribute.  The offerer can here
    ensure correct operation by using delete-attributes to remove the
    "crypto" attribute (but will then need to provide attribute
    capabilities to reconstruct the SDP session description with the
    necessary attributes deleted, e.g., rtpmaps).
 Also note, that while it is permissible to include media-level
 attribute capabilities at the session level, the base SDP Capability
 Negotiation framework defined here does not define any procedures for
 use of them, i.e., the answerer effectively ignores them.
 Please refer to Section 3.6.2.1 for examples of how the answerer may
 conceptually "see" the resulting offered alternative potential
 configurations.
 The answerer MUST check that he supports all mandatory attribute
 capabilities from the potential configuration (if any), the transport
 protocol capability (if any) from the potential configuration, and
 all mandatory extension capabilities from the potential configuration
 (if any).  If he does not, the answerer MUST proceed to the second
 most preferred valid potential configuration for the media
 description, etc.
 o  In the case of attribute capabilities, support implies that the
    attribute name contained in the capability is supported and it can
    (and will) be negotiated successfully in the offer/answer exchange
    with the value provided.  This does not necessarily imply that the
    value provided is supported in its entirety.  For example, the
    "a=fmtp" parameter is often provided with one or more values in a
    list, where the offerer and answerer negotiate use of some subset
    of the values provided.  Other attributes may include mandatory
    and optional parts to their values; support for the mandatory part
    is all that is required here.
       A side effect of the above rule is that whenever an "fmtp" or
       "rtpmap" parameter is provided as a mandatory attribute
       capability, the corresponding media format (codec) must be
       supported and use of it negotiated successfully.  If this is
       not the offerer's intent, the corresponding attribute
       capabilities must be listed as optional instead.

Andreasen Standards Track [Page 39] RFC 5939 SDP Capability Negotiation September 2010

 o  In the case of transport protocol capabilities, support implies
    that the transport protocol contained in the capability is
    supported and the transport protocol can (and will) be negotiated
    successfully in the offer/answer exchange.
 o  In the case of extension capabilities, the extension MUST define
    the rules for when the extension capability is considered
    supported and those rules MUST be satisfied.
 If the answerer has exhausted all potential configurations for the
 media description, without finding a valid one that is also
 supported, then the answerer MUST process the offered media stream
 based on the actual configuration plus any session-level attributes
 added by a valid and supported potential configuration from another
 media description in the offered SDP session description.
 The above process describes potential configuration selection as a
 per-media-stream process.  Inter-media stream coordination of
 selected potential configurations however is required in some cases.
 First of all, session-level attributes added by a potential
 configuration for one media description MUST NOT cause any problems
 for potential configurations selected by other media descriptions in
 the offer SDP session description.  If the session-level attributes
 are mandatory, then those session-level attributes MUST furthermore
 be supported by the session as a whole (i.e., all the media
 descriptions if relevant).  As mentioned earlier, this adds
 additional complexity to the overall processing and hence it is
 RECOMMENDED not to use session-level attribute capabilities in
 potential configurations, unless absolutely necessary.
 Once the answerer has selected a valid and supported offered
 potential configuration for all of the media streams (or has fallen
 back to the actual configuration plus any added session attributes),
 the answerer MUST generate a valid virtual answer SDP session
 description based on the selected potential configuration SDP session
 description, as "seen" by the answerer using normal offer/answer
 rules (see Section 3.6.2.1 for examples).  The actual answer SDP
 session description is formed from the virtual answer SDP session
 description as follows: if the answerer selected one of the potential
 configurations in a media description, the answerer MUST include an
 actual configuration attribute ("a=acfg") within that media
 description.  The "a=acfg" attribute MUST identify the configuration
 number for the selected potential configuration as well as the actual
 parameters that were used from that potential configuration; if the
 potential configuration included alternatives, the selected
 alternatives only 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.  In the case

Andreasen Standards Track [Page 40] RFC 5939 SDP Capability Negotiation September 2010

 of attribute capabilities, only the known and supported capabilities
 are included; unknown or unsupported attribute capabilities MUST NOT
 be included.
 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 only
 particular media descriptions, then a supported capability
 negotiation attribute with those option tags SHOULD be included
 within each relevant media description.  The required capability
 negotiation attribute ("a=creq") MUST NOT be used in an answer.
 The offerer's originally provided actual configuration is contained
 in the offer media description's "m=" line (and associated
 parameters).  The answerer MAY send media to the offerer in
 accordance with that actual configuration as soon as it receives the
 offer; however, it MUST NOT send media based on that actual
 configuration if it selects an alternative potential configuration.
 If the answerer selects one of the potential configurations, then the
 answerer MAY immediately start to send media to the offerer in
 accordance with the selected potential configuration; however, the
 offerer MAY discard such media or play out garbage until the offerer
 receives the answer.  Please refer to Section 3.9.  for additional
 considerations and possible alternative solutions outside the base
 SDP Capability Negotiation framework.
 If the answerer selected a potential configuration instead of the
 actual configuration, then it is RECOMMENDED that the answerer send
 back an answer SDP session description as soon as possible.  This
 minimizes the risk of having media discarded or played out as garbage
 by the offerer.  In the case of SIP [RFC3261] without any extensions,
 this implies that if the offer was received in an INVITE message,
 then the answer SDP session description should be provided in the
 first non-100 provisional response sent back (per RFC 3261, the
 answer would need to be repeated in the 200 response as well, unless
 a relevant extension such as [RFC3262] is being used).

Andreasen Standards Track [Page 41] RFC 5939 SDP Capability Negotiation September 2010

3.6.2.1. Example Views of Potential Configurations

 The following examples illustrate how the answerer may conceptually
 "see" a potential configuration.  Consider the following offered SDP
 session description:
    v=0
    o=alice 2891092738 2891092738 IN IP4 lost.example.com
    s=
    t=0 0
    c=IN IP4 lost.example.com
    a=tool:foo
    a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
    a=tcap:1 RTP/SAVP RTP/AVP
    m=audio 59000 RTP/AVP 98
    a=rtpmap:98 AMR/8000
    a=acap:2 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 52000 RTP/AVP 31
    a=rtpmap:31 H261/90000
    a=acap:3 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 session description 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 potential 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=
    t=0 0
    c=IN IP4 lost.example.com
    a=tool:foo
    a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
    m=audio 59000 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    m=video 52000 RTP/SAVP 31
    a=rtpmap:31 H261/90000

Andreasen Standards Track [Page 42] RFC 5939 SDP Capability Negotiation September 2010

 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 video stream -- note the order in
 which the resulting attributes are provided:
    v=0
    o=alice 2891092738 2891092738 IN IP4 lost.example.com
    s=
    t=0 0
    c=IN IP4 lost.example.com
    a=tool:foo
    m=audio 59000 RTP/SAVP 98
    a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
    a=rtpmap:98 AMR/8000
    m=video 52000 RTP/SAVP 31
    a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
       a=rtpmap:31 H261/90000
 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=
    t=0 0
    c=IN IP4 lost.example.com
    a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
    a=tool:foo
    m=audio 59000 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    m=video 52000 RTP/SAVP 31
    a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
    a=rtpmap:31 H261/90000

Andreasen Standards Track [Page 43] RFC 5939 SDP Capability Negotiation September 2010

3.6.3. Offerer Processing of the Answer

 When the offerer attempted to use SDP Capability Negotiation in the
 offer, the offerer MUST examine the answer for actual use of SDP
 Capability Negotiation.
 For each media description where the offerer included a potential
 configuration attribute ("a=pcfg"), the offerer MUST first examine
 that media description for the presence of a valid actual
 configuration attribute ("a=acfg").  An actual configuration
 attribute is valid if:
 o  it refers to a potential configuration that was present in the
    corresponding offer, and
 o  it contains the actual parameters that were used from that
    potential configuration; if the potential configuration included
    alternatives, the selected alternatives only MUST be included.
    Note that the answer will include only parameters and attribute
    capabilities that are known and supported by the answerer, as
    described in Section 3.6.2.
 If a valid actual configuration attribute is not present in a media
 description, then the offerer MUST process the answer SDP session
 description for that media stream per the normal offer/answer rules
 defined in [RFC3264].  However, if a valid one is found, the offerer
 MUST instead process the answer as follows:
 o  The actual configuration attribute specifies which of the
    potential configurations was used by the answerer to generate the
    answer for this media stream.  This includes all the supported
    attribute capabilities and the transport capabilities referenced
    by the potential configuration selected, where the attribute
    capabilities have any associated delete-attributes included.
    Extension capabilities supported by the answerer are included as
    well.
 o  The offerer MUST now process the answer in accordance with the
    rules in [RFC3264], except that it must be done as if the offer
    consisted of the selected potential configuration instead of the
    original actual configuration, including any transport protocol
    changes in the media ("m=") line(s), attributes added and deleted
    by the potential configuration at the media and session level, and
    any extensions used.  If this derived answer is not a valid answer
    to the potential configuration offer selected by the answerer, the
    offerer MUST instead continue further processing as it would have
    for a regular offer/answer exchange, where the answer received
    does not adhere to the rules of [RFC3264].

Andreasen Standards Track [Page 44] RFC 5939 SDP Capability Negotiation September 2010

 If the offer/answer exchange was successful, and if the answerer
 selected one of the potential configurations from the offer as the
 actual configuration, and the selected potential configuration
 differs from the actual configuration in the offer (the "m=", "a=",
 etc., lines), then the offerer SHOULD initiate another offer/answer
 exchange.  This second offer/answer exchange will not modify the
 session in any way; however, it will help intermediaries (e.g.,
 middleboxes), which look at the SDP session description but do not
 support the capability negotiation extensions, understand the details
 of the media stream(s) that were actually negotiated.  This new offer
 MUST contain the selected potential configuration as the actual
 configuration, i.e., with the actual configuration used in the "m="
 line and any other relevant attributes, bandwidth parameters, etc.
 Note that, per normal offer/answer rules, the second offer/answer
 exchange still needs to update the version number in the "o=" line
 (<sess-version> in [RFC4566]).  Attribute lines carrying keying
 material SHOULD repeat the keys from the previous offer, unless
 re-keying is necessary, e.g., due to a previously forked SIP INVITE
 request.  Please refer to Section 3.12 for additional considerations
 related to intermediaries.

3.6.4. Modifying the Session

 Capabilities and 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 selected configuration used by
 the answerer.
 If the answer indicates use of a potential configuration from the
 offer, then the guidelines provided in Section 3.6.3 for doing a
 second offer/answer exchange using that potential configuration as
 the actual configuration apply.

3.7. Interactions with ICE

 Interactive Connectivity Establishment (ICE) [RFC5245] provides a
 mechanism for verifying connectivity between two endpoints by sending
 Session Traversal Utilities for NAT (STUN) messages directly between
 the media endpoints.  The basic ICE specification [RFC5245] is only
 defined to support UDP-based connectivity; 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.

Andreasen Standards Track [Page 45] RFC 5939 SDP Capability Negotiation September 2010

 When using ICE, it is thus possible that the transport protocol that
 will be used differs from what is specified in the "m=" line.  Since
 both ICE and SDP Capability Negotiation may specify alternative
 transport protocols, there is a potentially unintended interaction
 when using these together.
 We provide the following guidelines for addressing that.
 There are two basic scenarios to consider:
 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
 connectivity checks).
 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.
 The scenarios may be combined, e.g., by offering potential
 configuration alternatives where some of them can support only one
 transport protocol (e.g., UDP), whereas others can support multiple
 transport protocols (e.g., UDP or TCP).  In that case, there is a
 need for tight control over the ICE candidates that will be used for
 a particular configuration, yet the actual configuration may want to
 use all of the ICE candidates.  In that case, the ICE candidate
 attributes can 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).  Furthermore, use of the
 delete-attributes in a potential configuration can be used to ensure
 that ICE will not end up using a transport protocol that is not
 desired for a particular configuration.

Andreasen Standards Track [Page 46] RFC 5939 SDP Capability Negotiation September 2010

 SDP Capability Negotiation recommends use of a second offer/answer
 exchange when the negotiated actual configuration was one of the
 potential configurations from the offer (see Section 3.6.3).
 Similarly, ICE requires use of a second offer/answer exchange if the
 chosen candidate is not the same as the one in the m/c-line from the
 offer.  When ICE and capability negotiation are used at the same
 time, the two secondary offer/answer exchanges SHOULD be combined to
 a single one.

3.8. Interactions with SIP Option Tags

 SIP [RFC3261] allows for SIP extensions to define a SIP option tag
 that identifies the SIP extension.  Support for one or more such
 extensions can be indicated by use of the SIP Supported header, and
 required support for one or more such extensions can be indicated by
 use of the SIP Require header.  The "a=csup" and "a=creq" attributes
 defined by the SDP Capability Negotiation framework are similar,
 except that support for these two attributes by themselves cannot be
 guaranteed (since they are specified as extensions to the SDP
 specification [RFC4566] itself).
 SIP extensions with associated option tags can introduce enhancements
 to not only SIP, but also SDP.  This is for example the case for SIP
 preconditions defined in [RFC3312].  When using SDP Capability
 Negotiation, some potential configurations may include certain SDP
 extensions, whereas others may not.  Since the purpose of the SDP
 Capability Negotiation is to negotiate a session based on the
 features supported by both sides, use of the SIP Require header for
 such extensions may not produce the desired result.  For example, if
 one potential configuration requires SIP preconditions support,
 another does not, and the answerer does not support preconditions,
 then use of the SIP Require header for preconditions would result in
 a session failure, in spite of the fact that a valid and supported
 potential configuration was included in the offer.
 In general, this can be alleviated by use of mandatory and optional
 attribute capabilities in a potential configuration.  There are
 however cases where permissible SDP values are tied to the use of the
 SIP Require header.  SIP preconditions [RFC3312] is one such example,
 where preconditions with a "mandatory" strength-tag can only be used
 when a SIP Require header with the SIP option tag "precondition" is
 included.  Future SIP extensions that may want to use the SDP
 Capability Negotiation framework should avoid such coupling.

Andreasen Standards Track [Page 47] RFC 5939 SDP Capability Negotiation September 2010

3.9. Processing Media before Answer

 The offer/answer model [RFC3264] 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, the offerer may decide not to process any
 media received before the answer is received.  This may lead to
 clipping.  Consequently, the SDP Capability Negotiation framework
 recommends sending back an answer SDP session description as soon as
 possible.
 The issue can be resolved by introducing a three-way handshake.  In
 the case of SIP, this can, for example, be done by defining a
 precondition [RFC3312] for capability negotiation (or by using an
 existing precondition that is known to generate a second offer/answer
 exchange before proceeding with the session).  However, preconditions
 are often viewed as complicated to implement and they may add to
 overall session establishment delay by requiring an extra
 offer/answer exchange.
 An alternative three-way handshake can be performed by use of ICE
 [RFC5245].  When ICE is being used, and the answerer receives a STUN
 Binding Request for any one of the accepted media streams from the
 offerer, the answerer knows the offer has received his answer.  At
 that point, the answerer knows that the offerer will be able to
 process incoming media according to the negotiated configuration and
 hence he can start sending media without the risk of the offerer
 either discarding it or playing garbage.
 Please note that, the above considerations notwithstanding, this
 document does not place any requirements on the offerer to process
 and play media before answer; it merely provides recommendations for
 how to ensure that media sent by the answerer and received by the
 offerer prior to receiving the answer can in fact be rendered by the
 offerer.
 In some use cases, a three-way handshake is not needed.  An example
 is when the offerer does not need information from the answer, such
 as keying material in the SDP session description, in order to
 process incoming media.  The SDP Capability Negotiation framework
 does not define any such solutions; 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, outside of the actual configuration, while
 still allowing them to be used by the answerer (exchange of keying

Andreasen Standards Track [Page 48] RFC 5939 SDP Capability Negotiation September 2010

 material is still needed, e.g., inband).  The basic SDP Capability
 Negotiation framework defined here does not include the ability to do
 so; however, extensions that enable that may be defined.

3.10. Indicating Bandwidth Usage

 The amount of bandwidth used for a particular media stream depends on
 the negotiated codecs, transport protocol and other parameters.  For
 example the use of Secure RTP [RFC3711] with integrity protection
 requires more bandwidth than plain RTP [RFC3551].  SDP defines the
 bandwidth ("b=") parameter to indicate the proposed bandwidth for the
 session or media stream.
 In SDP, as defined by [RFC4566], each media description contains one
 transport protocol and one or more codecs.  When specifying the
 proposed bandwidth, the worst case scenario must be taken into
 account, i.e., use of the highest bandwidth codec provided, the
 transport protocol indicated, and the worst case (bandwidth-wise)
 parameters that can be negotiated (e.g., a 32-bit Hashed Message
 Authentication Code (HMAC) or an 80-bit HMAC).
 The base SDP Capability Negotiation framework does not provide a way
 to negotiate bandwidth parameters.  The issue thus remains; however,
 it is potentially worse than with SDP per [RFC4566], since it is
 easier to negotiate additional codecs, and furthermore possible to
 negotiate different transport protocols.  The recommended approach
 for addressing this is the same as for plain SDP; the worst case (now
 including potential configurations) needs to be taken into account
 when specifying the bandwidth parameters in the actual configuration.
 This can make the bandwidth value less accurate than in SDP per
 [RFC4566] (due to potential greater variability in the potential
 configuration bandwidth use).  Extensions can be defined to address
 this shortcoming.
 Note, that when using RTP retransmission [RFC4588] with the RTCP-
 based feedback profile [RFC4585] (RTP/AVPF), the retransmitted
 packets are part of the media stream bandwidth when using
 synchronization source (SSRC) multiplexing.  If a feedback-based
 protocol is offered as the actual configuration transport protocol, a
 non-feedback-based protocol is offered as a potential configuration
 transport protocol and ends up being used, the actual bandwidth usage
 may be lower than the indicated bandwidth value in the offer (and
 vice versa).

Andreasen Standards Track [Page 49] RFC 5939 SDP Capability Negotiation September 2010

3.11. Dealing with Large Number of Potential Configurations

 When using the SDP Capability Negotiation, it is easy to generate
 offers that contain a large number of potential configurations.  For
 example, in the offer:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    m=audio 53456 RTP/AVP 0 18
    a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
    a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
       FEC_ORDER=FEC_SRTP
    a=acap:2 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
    a=acap:3 rtcp-fb:0 nack
    a=pcfg:1 t=1 a=1,3|2,3
    a=pcfg:2 t=2 a=1|2
    a=pcfg:3 t=3 a=3
 we have 5 potential configurations on top of the actual configuration
 for a single media stream.  Adding an extension capability with just
 two alternatives for each would double that number (to 10), and doing
 the equivalent with two media streams would again double that number
 (to 20).  While it is easy (and inexpensive) for the offerer to
 generate such offers, processing them at the answering side may not
 be.  Consequently, it is RECOMMENDED that offerers do not create
 offers with unnecessarily large number of potential configurations in
 them.
 On the answering side, implementers MUST take care to avoid excessive
 memory and CPU consumption.  For example, a naive implementation that
 first generates all the valid potential configuration SDP session
 descriptions internally, could find itself being memory exhausted,
 especially if it supports a large number of endpoints.  Similarly, a
 naive implementation that simply performs iterative trial-and-error
 processing on each possible potential configuration SDP session
 description (in the preference order specified) could find itself
 being CPU constrained.  An alternative strategy is to prune the
 search space first by discarding the set of offered potential
 configurations where the transport protocol indicated (if any) is not
 supported, and/or one or more mandatory attribute capabilities (if
 any) are either not supported or not valid.  Potential configurations
 with unsupported mandatory extension configurations in them can be
 discarded as well.

Andreasen Standards Track [Page 50] RFC 5939 SDP Capability Negotiation September 2010

3.12. SDP Capability Negotiation and Intermediaries

 An intermediary is here defined as an entity between a SIP user agent
 A and a SIP user agent B, that needs to perform some kind of
 processing on the SDP session descriptions exchanged between A and B,
 in order for the session establishment to operate as intended.
 Examples of such intermediaries include Session Border Controllers
 (SBCs) that may perform media relaying, Proxy Call Session Control
 Functions (P-CSCFs) that may authorize use of a certain amount of
 network resources (bandwidth), etc.  The presence and design of such
 intermediaries may not follow the "Internet" model or the SIP
 requirements for proxies (which are not supposed to look in message
 bodies such as SDP session descriptions); however, they are a fact of
 life in some deployment scenarios and hence deserve consideration.
 If the intermediary needs to understand the characteristics of the
 media sessions being negotiated, e.g., the amount of bandwidth used
 or the transport protocol negotiated, then use of the SDP Capability
 Negotiation framework may impact them.  For example, some
 intermediaries are known to disallow answers where the transport
 protocol differs from the one in the offer.  Use of the SDP
 Capability Negotiation framework in the presence of such
 intermediaries could lead to session failures.  Intermediaries that
 need to authorize use of network resources based on the negotiated
 media stream parameters are affected as well.  If they inspect only
 the offer, then they may authorize parameters assuming a different
 transport protocol, codecs, etc., than what is actually being
 negotiated.  For these, and other, reasons it is RECOMMENDED that
 implementers of intermediaries add support for the SDP Capability
 Negotiation framework.
 The SDP Capability Negotiation framework itself attempts to help out
 these intermediaries as well, by recommending a second offer/answer
 exchange when use of a potential configuration has been negotiated
 (see Section 3.6.3).  However, there are several limitations with
 this approach.  First of all, although the second offer/answer
 exchange is RECOMMENDED, it is not required and hence may not be
 performed.  Secondly, the intermediary may refuse the initial answer,
 e.g., due to perceived transport protocol mismatch.  Thirdly, the
 strategy is not foolproof since the offer/answer procedures [RFC3264]
 leave the original offer/answer exchange in effect when a subsequent
 one fails.  Consider the following example:
 1. Offerer generates an SDP session description offer with the actual
    configuration specifying a low-bandwidth configuration (e.g.,
    plain RTP) and a potential configuration specifying a high(er)
    bandwidth configuration (e.g., Secure RTP with integrity).

Andreasen Standards Track [Page 51] RFC 5939 SDP Capability Negotiation September 2010

 2. An intermediary (e.g., an SBC or P-CSCF), that does not support
    SDP Capability Negotiation, authorizes the session based on the
    actual configuration it sees in the SDP session description.
 3. The answerer chooses the high(er) bandwidth potential
    configuration and generates an answer SDP session description
    based on that.
 4. The intermediary passes through the answer SDP session
    description.
 5. The offerer sees the accepted answer, and generates an updated
    offer that contains the selected potential configuration as the
    actual configuration.  In other words, the high(er) bandwidth
    configuration (which has already been negotiated successfully) is
    now the actual configuration in the offer SDP session description.
 6. The intermediary sees the new offer; however, it does not
    authorize the use of the high(er) bandwidth configuration, and
    consequently generates a rejection message to the offerer.
 7. The offerer receives the rejected offer.
 After step 7, per RFC 3264, the offer/answer exchange that completed
 in step 5 remains in effect; however, the intermediary may not have
 authorized the necessary network resources and hence the media stream
 may experience quality issues.  The solution to this problem is to
 upgrade the intermediary to support the SDP Capability Negotiation
 framework.

3.13. Considerations for Specific Attribute Capabilities

3.13.1. The "rtpmap" and "fmtp" Attributes

 The base SDP Capability Negotiation framework defines transport
 capabilities and attribute capabilities.  Media capabilities, which
 can be used to describe media formats and their associated
 parameters, are not defined in this document; however, the "rtpmap"
 and "fmtp" attributes can nevertheless be used as attribute
 capabilities.  Using such attribute capabilities in a potential
 configuration requires a bit of care though.
 The rtpmap parameter binds an RTP payload type to a media format
 (e.g., codec).  While it is possible to provide rtpmaps for payload
 types not found in the corresponding "m=" line, such rtpmaps provide
 no value in normal offer/answer exchanges, since only the payload
 types found in the "m=" line are part of the offer (or answer).  This
 applies to the base SDP Capability Negotiation framework as well.

Andreasen Standards Track [Page 52] RFC 5939 SDP Capability Negotiation September 2010

 Only the media formats (e.g., RTP payload types) provided in the "m="
 line are actually offered; inclusion of "rtpmap" attributes with
 other RTP payload types in a potential configuration does not change
 this fact and hence they do not provide any useful information there.
 They may still be useful as pure capabilities though (outside a
 potential configuration) in order to inform a peer of additional
 codecs supported.
 It is possible to provide an "rtpmap" attribute capability with a
 payload type mapping to a different codec than a corresponding actual
 configuration "rtpmap" attribute for the media description has.  Such
 practice is permissible as a way of indicating a capability.  If that
 capability is included in a potential configuration, then delete-
 attributes (see Section 3.5.1) MUST be used to ensure that there is
 not multiple "rtpmap" attributes for the same payload type in a given
 media description (which would not be allowed by SDP [RFC4566]).
 Similar considerations and rules apply to the "fmtp" attribute.  An
 "fmtp" attribute capability for a media format not included in the
 "m=" line is useless in a potential configuration (but may be useful
 as a capability by itself).  An "fmtp" attribute capability in a
 potential configuration for a media format that already has an "fmtp"
 attribute in the actual configuration may lead to multiple fmtp
 format parameters for that media format and that is not allowed by
 SDP [RFC4566].  The delete-attributes MUST be used to ensure that
 there are not multiple "fmtp" attributes for a given media format in
 a media description.
 Extensions to the base SDP Capability Negotiation framework may
 change the above behavior.

3.13.2. Direction Attributes

 SDP defines the "inactive", "sendonly", "recvonly", and "sendrecv"
 direction attributes.  The direction attributes can be applied at
 either the session level or the media level.  In either case, it is
 possible to define attribute capabilities for these direction
 capabilities; if used by a potential configuration, the normal
 offer/answer procedures still apply.  For example, if an offered
 potential configuration includes the "sendonly" direction attribute,
 and it is selected as the actual configuration, then the answer MUST
 include a corresponding "recvonly" (or "inactive") attribute.

Andreasen Standards Track [Page 53] RFC 5939 SDP Capability Negotiation September 2010

3.14. Relationship to RFC 3407

 RFC 3407 defines capability descriptions with limited abilities to
 describe attributes, bandwidth parameters, transport protocols and
 media formats.  RFC 3407 does not define any negotiation procedures
 for actually using those capability descriptions.
 This document defines new attributes for describing attribute
 capabilities and transport capabilities.  It also defines procedures
 for using those capabilities as part of an offer/answer exchange.  In
 contrast to RFC 3407, this document does not define bandwidth
 parameters, and it also does not define how to express ranges of
 values.  Extensions to this document may be defined in order to fully
 cover all the capabilities provided by RFC 3407 (for example, more
 general media capabilities).
 It is RECOMMENDED that implementations use the attributes and
 procedures defined in this document instead of those defined in
 [RFC3407].  If capability description interoperability with legacy
 RFC 3407 implementations is desired, implementations MAY include both
 RFC 3407 capability descriptions and capabilities defined by this
 document.  The offer/answer negotiation procedures defined in this
 document will not use the RFC 3407 capability descriptions.

4. Examples

 In this section, we provide examples showing how to use the SDP
 Capability Negotiation.

4.1. Multiple Transport Protocols

 The following example illustrates how to use the SDP Capability
 Negotiation extensions to negotiate use of one out of several
 possible transport protocols.  The offerer uses the expected least-
 common-denominator (plain RTP) as the actual configuration, and the
 alternative transport protocols as the potential configurations.

Andreasen Standards Track [Page 54] RFC 5939 SDP Capability Negotiation September 2010

 The example is illustrated by the offer/answer exchange below, where
 Alice sends an offer to Bob:
              Alice                               Bob
                | (1) Offer (RTP/[S]AVP[F])        |
                |--------------------------------->|
                |                                  |
                | (2) Answer (RTP/AVPF)            |
                |<---------------------------------|
                |                                  |
                | (3) Offer (RTP/AVPF)             |
                |--------------------------------->|
                |                                  |
                | (4) Answer (RTP/AVPF)            |
                |<---------------------------------|
                |                                  |
 Alice's offer includes plain RTP (RTP/AVP), RTP with RTCP-based
 feedback (RTP/AVPF), Secure RTP (RTP/SAVP), and Secure RTP with RTCP-
 based feedback (RTP/SAVPF) as alternatives.  RTP is the default, with
 RTP/SAVPF, RTP/SAVP, and RTP/AVPF as the alternatives and preferred
 in the order listed:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    m=audio 53456 RTP/AVP 0 18
    a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
    a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
       FEC_ORDER=FEC_SRTP
    a=acap:2 rtcp-fb:0 nack
    a=pcfg:1 t=1 a=1,[2]
    a=pcfg:2 t=2 a=1
    a=pcfg:3 t=3 a=[2]
 The "m=" line indicates that Alice is offering to use plain RTP with
 PCMU or G.729.  The capabilities are provided by the "a=tcap" and
 "a=acap" attributes.  The "tcap" capability indicates that Secure RTP
 with RTCP-based feedback (RTP/SAVPF), Secure RTP (RTP/SAVP), and RTP
 with RTCP-based feedback are supported.  The first "acap" attribute
 provides an attribute capability with a handle of 1.  The capability
 is a "crypto" attribute, which provides the keying material for SRTP
 using SDP security descriptions [RFC4568].  The second "acap"
 attribute provides an attribute capability with a handle of 2.  The

Andreasen Standards Track [Page 55] RFC 5939 SDP Capability Negotiation September 2010

 capability is an "rtcp-fb" attribute, which is used by the RTCP-based
 feedback profiles to indicate that payload type 0 (PCMU) supports
 feedback type "nack".  The "a=pcfg" attributes provide the potential
 configurations included in the offer by reference to the
 capabilities.  There are three potential configurations:
 o  Potential configuration 1, which is the most preferred potential
    configuration specifies use of transport protocol capability 1
    (RTP/SAVPF) and attribute capabilities 1 (the "crypto" attribute)
    and 2 (the "rtcp-fb" attribute).  Support for the first one is
    mandatory whereas support for the second one is optional.
 o  Potential configuration 2, which is the second most preferred
    potential configuration specifies use of transport protocol
    capability 2 (RTP/SAVP) and mandatory attribute capability 1 (the
    "crypto" attribute).
 o  Potential configuration 3, which is the least preferred potential
    configuration (but the second least preferred configuration
    overall, since the actual configuration provided by the "m=" line
    is always the least preferred configuration), specifies use of
    transport protocol capability 3 (RTP/AVPF) and optional attribute
    capability 2 (the "rtcp-fb" attribute).
 Bob receives the SDP session description offer from Alice.  Bob does
 not support any Secure RTP profiles; however, he supports plain RTP
 and RTP with RTCP-based feedback, as well as the SDP Capability
 Negotiation extensions, and hence he accepts the potential
 configuration for RTP with RTCP-based feedback provided by Alice:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    m=audio 54568 RTP/AVPF 0 18
    a=rtcp-fb:0 nack
    a=acfg:1 t=3 a=[2]
 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 3 and optional
 attribute capability 2 from the offer SDP session description (i.e.,
 the RTP/AVPF profile using the "rtcp-fb" value provided).  Bob also
 includes an "rtcp-fb" attribute with the value "nack" value for RTP
 payload type 0.

Andreasen Standards Track [Page 56] RFC 5939 SDP Capability Negotiation September 2010

 When Alice receives Bob's answer, session negotiation has completed,
 however Alice nevertheless chooses to generate a new offer using the
 actual configuration.  This is done purely to assist any
 intermediaries that may reside between Alice and Bob but do not
 support the SDP Capability Negotiation framework (and hence may not
 understand the negotiation that just took place):
 Alice's updated offer includes only RTP/AVPF, and it is not using the
 SDP Capability Negotiation framework (Alice could have included the
 capabilities as well if she wanted):
    v=0
    o=- 25678 753850 IN IP4 192.0.2.1
    s=
    c=IN IP4 192.0.2.1
    t=0 0
    m=audio 53456 RTP/AVPF 0 18
    a=rtcp-fb:0 nack
 The "m=" line now indicates that Alice is offering to use RTP with
 RTCP-based feedback and using PCMU or G.729.  The "rtcp-fb" attribute
 provides the feedback type "nack" for payload type 0 again (but as
 part of the actual configuration).
 Bob receives the SDP session description offer from Alice, which he
 accepts, and then generates an answer to Alice:
    v=0
    o=- 24351 621815 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    m=audio 54568 RTP/AVPF 0 18
    a=rtcp-fb:0 nack
 Bob includes the same "rtcp-fb" attribute as before, and the session
 proceeds without change.  Although Bob did not include any
 capabilities in his answer, he could have done so if he wanted.
 Note that in this particular example, the answerer supported the SDP
 Capability Negotiation framework and hence the attributes and
 procedures 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:

Andreasen Standards Track [Page 57] RFC 5939 SDP Capability Negotiation September 2010

    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    c=IN IP4 192.0.2.2
    t=0 0
    m=audio 54568 RTP/AVP 0 18

4.2. DTLS-SRTP or SRTP with Media-Level Security Descriptions

 The following example illustrates how to use the SDP Capability
 Negotiation framework to negotiate use of SRTP using either SDP
 security descriptions or DTLS-SRTP.  The offerer (Alice) wants to
 establish a Secure RTP audio stream but is willing to use plain RTP.
 Alice prefers to use DTLS-SRTP as the key management protocol, but
 supports SDP security descriptions as well (note that [RFC5763]
 contains additional DTLS-SRTP examples).
 The example is illustrated by the offer/answer exchange below, where
 Alice sends an offer to Bob:
           Alice                                     Bob
             | (1) Offer (RTP/[S]AVP,SDES | DTLS-SRTP)|
             |--------------------------------------->|
             |                                        |
             |<--------- DTLS-SRTP handshake -------->|
             |                                        |
             | (2) Answer (DTLS-SRTP)                 |
             |<---------------------------------------|
             |                                        |
             | (3) Offer (DTLS-SRTP)                  |
             |--------------------------------------->|
             |                                        |
             | (4) Answer (DTLS-SRTP)                 |
             |<---------------------------------------|
             |                                        |
 Alice's offer includes an audio stream that offers use of plain RTP
 and Secure RTP as alternatives.  For the Secure RTP stream, it can be
 established using either DTLS-SRTP or SDP security descriptions:

Andreasen Standards Track [Page 58] RFC 5939 SDP Capability Negotiation September 2010

    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    t=0 0
    c=IN IP4 192.0.2.1
    a=acap:1 setup:actpass
    a=acap:2 fingerprint: SHA-1 \
          4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
    a=tcap:1 UDP/TLS/RTP/SAVP RTP/SAVP
    m=audio 59000 RTP/AVP 98
    a=rtpmap:98 AMR/8000
    a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
    a=pcfg:1 t=1 a=1,2
    a=pcfg:2 t=2 a=3
 The first (and preferred) potential configuration for the audio
 stream specifies use of transport capability 1 (UDP/TLS/RTP/SAVP),
 i.e., DTLS-SRTP, and attribute capabilities 1 and 2 (active/passive
 mode and certificate fingerprint), both of which must be supported to
 choose this potential configuration.  The second (and less preferred)
 potential configuration specifies use of transport capability 2
 (RTP/SAVP) and mandatory attribute capability 3, i.e., the SDP
 security description.
 Bob receives the SDP session description offer from Alice.  Bob
 supports DTLS-SRTP as preferred by Alice and Bob now initiates the
 DTLS-SRTP handshake to establish the DTLS-SRTP session (see [RFC5764]
 for details).
 Bob also sends back an answer to Alice as follows:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    a=setup:active
    a=fingerprint: SHA-1 \
      FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 UDP/TLS/RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=acfg:1 t=1 a=1,2
 For the audio stream, Bob accepted the use of DTLS-SRTP, and hence
 the profile in the "m=" line is "UDP/TLS/RTP/SAVP".  Bob also
 includes a "setup:active" attribute to indicate he is the active

Andreasen Standards Track [Page 59] RFC 5939 SDP Capability Negotiation September 2010

 endpoint for the DTLS-SRTP session as well as the fingerprint for
 Bob's certificate.  Bob's "acfg" attribute indicates that he chose
 potential configuration 1 from Alice's offer.
 When Alice receives Bob's answer, session negotiation has completed
 (and Alice can verify the DTLS handshake using Bob's certificate
 fingerprint in the answer); however, Alice nevertheless chooses to
 generate a new offer using the actual configuration.  This is done
 purely to assist any intermediaries 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 DTLS-SRTP for the audio stream,
 and it is not using the SDP Capability Negotiation framework (Alice
 could have included the capabilities as well if she wanted):
    v=0
    o=- 25678 753850 IN IP4 192.0.2.1
    s=
    t=0 0
    c=IN IP4 192.0.2.1
    a=setup:actpass
    a=fingerprint: SHA-1 \
          4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
    m=audio 59000 UDP/TLS/RTP/AVP 98
    a=rtpmap:98 AMR/8000
 The "m=" line for the audio stream now indicates that Alice is
 offering to use DTLS-SRTP in active/passive mode using her
 certificate fingerprint provided.
 Bob receives the SDP session description offer from Alice, which he
 accepts, and then generates an answer to Alice:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    a=setup:active
    a=fingerprint: SHA-1 \
      FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 UDP/TLS/RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=acfg:1 t=1 a=1,2

Andreasen Standards Track [Page 60] RFC 5939 SDP Capability Negotiation September 2010

 Bob includes the same "setup:active" and fingerprint attributes as
 before, and the session proceeds without change.  Although Bob did
 not include any capabilities in his answer, he could have done so if
 he wanted.
 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 192.0.2.2
    s=
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 RTP/AVP 98
    a=rtpmap:98 AMR/8000
 Finally, if Bob had chosen to use SDP security descriptions instead
 of DTLS-SRTP, the following answer would have been generated:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
    a=acfg:2 t=2 a=3

4.3. Best-Effort SRTP with Session-Level MIKEY and Media-Level Security

    Descriptions
 The following example illustrates how to use the SDP Capability
 Negotiation extensions to support so-called Best-Effort Secure RTP as
 well as alternative keying mechanisms, more specifically MIKEY
 [RFC3830] and SDP security descriptions.  The offerer (Alice) wants
 to establish an audio and video session.  Alice prefers to use
 session-level MIKEY as the key management protocol, but supports SDP
 security descriptions as well.
 The example is illustrated by the offer/answer exchange below, where
 Alice sends an offer to Bob:

Andreasen Standards Track [Page 61] RFC 5939 SDP Capability Negotiation September 2010

           Alice                                     Bob
             | (1) Offer (RTP/[S]AVP[F], SDES|MIKEY)  |
             |--------------------------------------->|
             |                                        |
             | (2) Answer (RTP/SAVP, SDES)            |
             |<---------------------------------------|
             |                                        |
             | (3) Offer (RTP/SAVP, SDES)             |
             |--------------------------------------->|
             |                                        |
             | (4) Answer (RTP/SAVP, SDES)            |
             |<---------------------------------------|
             |                                        |
 Alice's offer includes an audio and a video stream.  The audio stream
 offers use of plain RTP and Secure RTP as alternatives, whereas the
 video stream offers use of plain RTP, RTP with RTCP-based feedback,
 Secure RTP, and Secure RTP with RTCP-based feedback as alternatives:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    t=0 0
    c=IN IP4 192.0.2.1
    a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
    a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF
    m=audio 59000 RTP/AVP 98
    a=rtpmap:98 AMR/8000
    a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
    a=pcfg:1 t=2 a=1|2
    m=video 52000 RTP/AVP 31
    a=rtpmap:31 H261/90000
    a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
    a=acap:4 rtcp-fb:* nack
    a=pcfg:1 t=1 a=1,4|3,4
    a=pcfg:2 t=2 a=1|3
    a=pcfg:3 t=3 a=4
 The potential configuration for the audio stream specifies use of
 transport capability 2 (RTP/SAVP) and either attribute capability 1
 (session-level MIKEY as the keying mechanism) or 2 (SDP security
 descriptions as the keying mechanism).  Support for either of these
 attribute capabilities is mandatory.  There are three potential
 configurations for the video stream.

Andreasen Standards Track [Page 62] RFC 5939 SDP Capability Negotiation September 2010

 o  The first configuration with configuration number 1 uses transport
    capability 1 (RTP/SAVPF) with either attribute capabilities 1 and
    4 (session-level MIKEY and the "rtcp-fb" attribute) or attribute
    capabilities 3 and 4 (SDP security descriptions and the "rtcp-fb"
    attribute).  In this example, the offerer insists on not only the
    keying mechanism being supported, but also that the "rtcp-fb"
    attribute is supported with the value indicated.  Consequently,
    all the attribute capabilities are marked as mandatory in this
    potential configuration.
 o  The second configuration with configuration number 2 uses
    transport capability 2 (RTP/SAVP) and either attribute capability
    1 (session-level MIKEY) or attribute capability 3 (SDP security
    descriptions).  Both attribute capabilities are mandatory in this
    configuration.
 o  The third configuration with configuration number 3 uses transport
    capability 3 (RTP/AVPF) and mandatory attribute capability 4 (the
    "rtcp-fb" attribute).
 Bob receives the SDP session description offer from Alice.  Bob
 supports Secure RTP, Secure RTP with RTCP-based feedback and the SDP
 Capability Negotiation extensions.  Bob also supports SDP security
 descriptions, but not MIKEY, and hence he generates the following
 answer:
    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
    a=acfg:1 t=2 a=2
    m=video 55468 RTP/SAVPF 31
    a=rtpmap:31 H261/90000
    a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
    a=rtcp-fb:* nack
    a=acfg:1 t=1 a=3,4
 For the audio stream, Bob accepted the use of Secure RTP, and hence
 the profile in the "m=" line is "RTP/SAVP".  Bob also includes a
 "crypto" attribute with his own keying material, and an "acfg"
 attribute identifying actual configuration 1 for the audio media
 stream from the offer, using transport capability 2 (RTP/SAVP) and

Andreasen Standards Track [Page 63] RFC 5939 SDP Capability Negotiation September 2010

 attribute capability 2 (the "crypto" attribute from the offer).  For
 the video stream, Bob accepted the use of Secure RTP with RTCP-based
 feedback, and hence the profile in the "m=" line is "RTP/SAVPF".  Bob
 also includes a "crypto" attribute with his own keying material, and
 an "acfg" attribute identifying actual configuration 1 for the video
 stream from the offer, using transport capability 1 (RTP/SAVPF) and
 attribute capabilities 3 (the "crypto" attribute from the offer) and
 4 (the "rtcp-fb" attribute from the offer).
 When Alice receives Bob's answer, session negotiation has completed;
 however, Alice nevertheless chooses to generate a new offer using the
 actual configuration.  This is done purely to assist any
 intermediaries 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 for the audio stream SRTP
 with RTCP-based feedback for the video stream, and it is not using
 the SDP Capability Negotiation framework (Alice could have included
 the capabilities as well is she wanted):
    v=0
    o=- 25678 753850 IN IP4 192.0.2.1
    s=
    t=0 0
    c=IN IP4 192.0.2.1
    m=audio 59000 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 52000 RTP/SAVPF 31
    a=rtpmap:31 H261/90000
    a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
    a=rtcp-fb:* nack
 The "m=" line for the audio stream now indicates that Alice is
 offering to use Secure RTP with PCMU or G.729, whereas the "m=" line
 for the video stream indicates that Alice is offering to use Secure
 RTP with RTCP-based feedback and H.261.  Each media stream includes a
 "crypto" attribute, which provides the SRTP keying material, with the
 same value again.

Andreasen Standards Track [Page 64] RFC 5939 SDP Capability Negotiation September 2010

 Bob receives the SDP session description offer from Alice, which he
 accepts, and then generates an answer to Alice:
    v=0
    o=- 24351 621815 IN IP4 192.0.2.2
    s=
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
    m=video 55468 RTP/SAVPF 31
    a=rtpmap:31 H261/90000
    a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
    a=rtcp-fb:* nack
 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 have done so if he wanted.
 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 192.0.2.2
    s=
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 RTP/AVP 98
    a=rtpmap:98 AMR/8000
    m=video 55468 RTP/AVP 31
    a=rtpmap:31 H261/90000
    a=rtcp-fb:* nack
 Finally, if Bob had chosen to use session-level MIKEY instead of SDP
 security descriptions, the following answer would have been
 generated:

Andreasen Standards Track [Page 65] RFC 5939 SDP Capability Negotiation September 2010

    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    t=0 0
    c=IN IP4 192.0.2.2
    a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyO...
    m=audio 54568 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=acfg:1 t=2 a=1
    m=video 55468 RTP/SAVPF 31
    a=rtpmap:31 H261/90000
    a=rtcp-fb:* nack
    a=acfg:1 t=1 a=1,4
 It should be noted, that although Bob could have chosen session-level
 MIKEY for one media stream, and SDP security descriptions for another
 media stream, there are no well-defined offerer processing rules of
 the resulting answer for this, and hence the offerer may incorrectly
 assume use of MIKEY for both streams.  To avoid this, if the answerer
 chooses session-level MIKEY, then all Secure RTP-based media streams
 SHOULD use MIKEY (this applies irrespective of whether or not SDP
 Capability Negotiation is being used).  Use of media-level MIKEY does
 not have a similar constraint.

4.4. SRTP with Session-Level MIKEY and Media-Level Security

    Descriptions as Alternatives
 The following example illustrates how to use the SDP Capability
 Negotiation framework to negotiate use of either MIKEY or SDP
 security descriptions, when one of them is included as part of the
 actual configuration, and the other one is being selected.  The
 offerer (Alice) wants to establish an audio and video session.  Alice
 prefers to use session-level MIKEY as the key management protocol,
 but supports SDP security descriptions as well.
 The example is illustrated by the offer/answer exchange below, where
 Alice sends an offer to Bob:
           Alice                                     Bob
             | (1) Offer (RTP/[S]AVP[F], SDES|MIKEY)  |
             |--------------------------------------->|
             |                                        |
             | (2) Answer (RTP/SAVP, SDES)            |
             |<---------------------------------------|
             |                                        |

Andreasen Standards Track [Page 66] RFC 5939 SDP Capability Negotiation September 2010

 Alice's offer includes an audio and a video stream.  Both the audio
 and the video stream offer use of Secure RTP:
    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    t=0 0
    c=IN IP4 192.0.2.1
    a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
    m=audio 59000 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
    a=pcfg:1 a=-s:1
    m=video 52000 RTP/SAVP 31
    a=rtpmap:31 H261/90000
    a=acap:2 crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
    a=pcfg:1 a=-s:2
 Alice does not know whether Bob supports MIKEY or SDP security
 descriptions.  She could include attributes for both; however, the
 resulting procedures and potential interactions are not well-
 defined.  Instead, she places a session-level "key-mgmt" attribute
 for MIKEY in the actual configuration with SDP security descriptions
 as an alternative in the potential configuration.  The potential
 configuration for the audio stream specifies that all session-level
 attributes are to be deleted (i.e., the session-level "a=key-mgmt"
 attribute) and that mandatory attribute capability 2 is to be used
 (i.e., the "crypto" attribute).  The potential configuration for the
 video stream is similar, except it uses its own mandatory "crypto"
 attribute capability (2).  Note how the deletion of the session-level
 attributes does not affect the media-level attributes.
 Bob receives the SDP session description offer from Alice.  Bob
 supports Secure RTP and the SDP Capability Negotiation framework.
 Bob also supports both SDP security descriptions and MIKEY.  Since
 the potential configuration is more preferred than the actual
 configuration, Bob (conceptually) generates an internal potential
 configuration SDP session description that contains the "crypto"
 attributes for the audio and video stream, but not the "key-mgmt"
 attribute for MIKEY, thereby avoiding any ambiguity between the two
 keying mechanisms.  As a result, he generates the following answer:

Andreasen Standards Track [Page 67] RFC 5939 SDP Capability Negotiation September 2010

    v=0
    o=- 24351 621814 IN IP4 192.0.2.2
    s=
    t=0 0
    c=IN IP4 192.0.2.2
    m=audio 54568 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:WSJ+PSdFcGdUJShpX1ZjNzB4d1BINUAvLEw6UzF3|2^20|1:32
    a=acfg:1 a=-s:1
    m=video 55468 RTP/SAVP 31
    a=rtpmap:31 H261/90000
    a=crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:AwWpVLFJhQX1cfHJSojd0RmdmcmVCspeEc3QGZiN|2^20|1:32
    a=acfg:1 a=-s:2
 For the audio stream, Bob accepted the use of Secure RTP using SDP
 security descriptions.  Bob therefore includes a "crypto" attribute
 with his own keying material, and an "acfg" attribute identifying the
 actual configuration 1 for the audio media stream from the offer,
 with the delete-attributes ("-s") and attribute capability 1 (the
 "crypto" attribute from the offer).  For the video stream, Bob also
 accepted the use of Secure RTP using SDP security descriptions.  Bob
 therefore includes a "crypto" attribute with his own keying material,
 and an "acfg" attribute identifying actual configuration 1 for the
 video stream from the offer, with the delete-attributes ("-s") and
 attribute capability 2.
 Below, we illustrate the offer SDP session description, when Bob
 instead offers the "crypto" attribute as the actual configuration
 keying mechanism and "key-mgmt" as the potential configuration:

Andreasen Standards Track [Page 68] RFC 5939 SDP Capability Negotiation September 2010

    v=0
    o=- 25678 753849 IN IP4 192.0.2.1
    s=
    t=0 0
    c=IN IP4 192.0.2.1
    a=acap:1 key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
    m=audio 59000 RTP/SAVP 98
    a=rtpmap:98 AMR/8000
    a=crypto:1 AES_CM_128_HMAC_SHA1_32
       inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
    a=acap:2 rtpmap:98 AMR/8000
    a=pcfg:1 a=-m:1,2
    m=video 52000 RTP/SAVP 31
    a=rtpmap:31 H261/90000
    a=acap:3 crypto:1 AES_CM_128_HMAC_SHA1_80
       inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32
    a=acap:4 rtpmap:31 H261/90000
    a=pcfg:1 a=-m:1,4
 Note how we this time need to perform delete-attributes at the media
 level instead of the session level.  When doing that, all attributes
 from the actual configuration SDP session description, including the
 rtpmaps provided, are removed.  Consequently, we had to include these
 rtpmaps as capabilities as well, and then include them in the
 potential configuration, thereby effectively recreating the original
 "rtpmap" attributes in the resulting potential configuration SDP
 session description.

5. Security Considerations

 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 [RFC3264].  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 [RFC3261].
 However, SDP Capability Negotiation 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) warrants
 further security considerations.  We discuss these issues below.

Andreasen Standards Track [Page 69] RFC 5939 SDP Capability Negotiation September 2010

 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 session description offer, he can thereby force the
 negotiation of the weakest security mechanism that the offerer is
 willing to accept.  This may 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 session description 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
 session description.  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 Secure/Multipurpose
 Internet Mail Extensions (S/MIME) [RFC5751] 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, is to use
 SIP Identity [RFC4474]; this requires the existence of an
 authentication service (see [RFC4474]).  Although this mechanism
 still requires a PKI, it only requires that servers (as opposed to
 end-users) have third-party validatable certificates, which
 significantly reduces the barrier to entry by ordinary users.  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 session description on a hop-by-hop basis.  This
 is less secure because SIP allows partially trusted intermediaries on
 the signaling path, and such intermediaries processing the SIP
 request at each hop would be able to perform a man-in-the-middle
 attack by modifying the offered SDP session description.  In simple
 architectures where the two UA's proxies communicate directly, the
 security provided by this method is roughly comparable to that
 provided by the previously discussed signature-based mechanisms.
 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) was selected by the answerer, until an answer indication
 is received.  This opens up a new security issue where an attacker

Andreasen Standards Track [Page 70] RFC 5939 SDP Capability Negotiation September 2010

 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 secure media stream and also does not support the
 capability negotiation framework, then negotiation of the media
 stream will fail.  Alternatively, SDP security preconditions
 [RFC5027] 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
 sides 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 session
 description 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.  Section 3.9
 presents other alternatives as well.
 Additional security considerations apply to the answer SDP session
 description as well.  The actual configuration attribute tells the
 offerer on which potential configuration the answer was based, 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 session description 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.
 As described in Sections 3.1 and 3.11, SDP Capability Negotiation
 conceptually allows an offerer to include many different offers in a
 single SDP session description.  This can cause the answerer to
 process a large number of alternative potential offers, which can
 consume significant memory and CPU resources.  An attacker can use
 this amplification feature to launch a denial-of-service attack
 against the answerer.  The answerer must protect itself from such
 attacks.  As explained in Section 3.11, the answerer can help reduce
 the effects of such an attack by first discarding all potential
 configurations that contain unsupported transport protocols,
 unsupported or invalid mandatory attribute capabilities, or
 unsupported mandatory extension configurations.  The answerer should

Andreasen Standards Track [Page 71] RFC 5939 SDP Capability Negotiation September 2010

 also look out for potential configurations that are designed to pass
 the above test, but nevertheless produce a large number of potential
 configuration SDP session descriptions that cannot be supported.
    A possible way of achieving that is for an attacker to find a
    valid session-level attribute that causes conflicts or otherwise
    interferes with individual media description configurations.  At
    the time of publication of this document, we do not know of such
    an SDP attribute; however, this does not mean it does not exist,
    or that it will not exist in the future.  If such attributes are
    found to exist, implementers should explicitly protect against
    them.
 A significant number of valid and supported potential configurations
 may remain.  However, since all of those contain only valid and
 supported transport protocols and attributes, it is expected that
 only a few of them will need to be processed on average.  Still, the
 answerer must ensure that it does not needlessly consume large
 amounts of memory or CPU resources when processing those as well as
 be prepared to handle the case where a large number of potential
 configurations still need to be processed.

6. IANA Considerations

6.1. New SDP Attributes

 The IANA has registered the following new SDP attributes:
 Attribute name:      csup
 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 3.3.1 of RFC 5939
 Contact name:        Flemming Andreasen, fandreas@cisco.com
 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 3.3.2 of RFC 5939
 Contact name:        Flemming Andreasen, fandreas@cisco.com

Andreasen Standards Track [Page 72] RFC 5939 SDP Capability Negotiation September 2010

 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 3.4.1 of RFC 5939
 Contact name:        Flemming Andreasen, fandreas@cisco.com
 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 3.4.2 of RFC 5939
 Contact name:        Flemming Andreasen, fandreas@cisco.com
 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 3.5.1 of RFC 5939
 Contact name:        Flemming Andreasen, fandreas@cisco.com
 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 3.5.2 of RFC 5939
 Contact name:        Flemming Andreasen, fandreas@cisco.com

6.2. New SDP Capability Negotiation Option Tag Registry

 The IANA has created 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 [RFC5226] IETF
 Review 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 configuration ("a=pcfg") and actual
 configuration ("a=acfg") attributes provided in this document.  If
 the extension defines any new SDP attributes that are intended to be
 capabilities for use by the capability negotiation framework (e.g.,
 similar to "a=acap"), those capabilities MUST adhere to the

Andreasen Standards Track [Page 73] RFC 5939 SDP Capability Negotiation September 2010

 guidelines provided in Section 3.4.3.  Extensions to the potential
 and actual configuration attributes MUST adhere to the syntax
 provided in Sections 3.5.1 and 3.5.2.
 The option tag "cap-v0" is defined in this document, and the IANA has
 registered this option tag.

6.3. New SDP Capability Negotiation Potential Configuration Parameter

    Registry
 The IANA has created a new SDP Capability Negotiation Potential
 Configuration Parameter registry.  An IANA SDP Capability Negotiation
 Potential Configuration registration MUST be documented in an RFC in
 accordance with the [RFC5226] IETF Review 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 3.5.1 and the semantics MUST adhere to the
 semantics provided for extensions in Section 3.5.1 and 3.5.2.
 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
 has registered them.

7. Acknowledgments

 The SDP Capability Negotiation solution defined in this document
 draws on the overall capability negotiation framework that was
 defined by [SDPng].  Also, the SDP Capability Negotiation solution 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
 here: Francois Audet, John Elwell, Roni Even, Miguel Garcia, Robert
 Gilman, Cullen Jennings, Jonathan Lennox, Matt Lepinski, Jean-
 Francois Mule, Joerg Ott, Colin Perkins, Jonathan Rosenberg, Thomas
 Stach, and Dan Wing.
 General Area review comments were provided by Christian Vogt, and
 Stephen Kent provided Security Directorate review comments.  Eric
 Rescorla provided textual input to the Security Considerations.
 Alexey Melnikov, Robert Sparks, and Magnus Westerlund provided
 several review comments as well.

Andreasen Standards Track [Page 74] RFC 5939 SDP Capability Negotiation September 2010

8. References

8.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264, June
            2002.
 [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
            Description Protocol", RFC 4566, July 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
            Syntax Specifications: ABNF", STD 68, RFC 5234, January
            2008.
 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245, April
            2010.

8.2. Informative References

 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            June 2002.
 [RFC3312]  Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
            "Integration of Resource Management and Session Initiation
            Protocol (SIP)", RFC 3312, October 2002.
 [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
            Provisional Responses in Session Initiation Protocol
            (SIP)", RFC 3262, June 2002.
 [RFC3407]  Andreasen, F., "Session Description Protocol (SDP) Simple
            Capability Declaration", RFC 3407, October 2002.
 [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
            Video Conferences with Minimal Control", STD 65, RFC 3551,
            July 2003.

Andreasen Standards Track [Page 75] RFC 5939 SDP Capability Negotiation September 2010

 [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
            Norrman, "The Secure Real-time Transport Protocol (SRTP)",
            RFC 3711, March 2004.
 [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
            Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
            August 2004.
 [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
            the Session Description Protocol (SDP)", RFC 4145,
            September 2005.
 [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
            Authenticated Identity Management in the Session
            Initiation Protocol (SIP)", RFC 4474, August 2006.
 [RFC4567]  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.
 [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
            Description Protocol (SDP) Security Descriptions for Media
            Streams", RFC 4568, July 2006.
 [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
            "Extended RTP Profile for Real-time Transport Control
            Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
            2006.
 [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
            Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
            July 2006.
 [RFC4756]  Li, A., "Forward Error Correction Grouping Semantics in
            Session Description Protocol", RFC 4756, November 2006.
 [RFC5027]  Andreasen, F. and D. Wing, "Security Preconditions for
            Session Description Protocol (SDP) Media Streams", RFC
            5027, October 2007.
 [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
            Real-time Transport Control Protocol (RTCP)-Based Feedback
            (RTP/SAVPF)", RFC 5124, February 2008.
 [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
            Mail Extensions (S/MIME) Version 3.2 Message
            Specification", RFC 5751, January 2010.

Andreasen Standards Track [Page 76] RFC 5939 SDP Capability Negotiation September 2010

 [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
            for Establishing a Secure Real-time Transport Protocol
            (SRTP) Security Context Using Datagram Transport Layer
            Security (DTLS)", RFC 5763, May 2010.
 [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
            Security (DTLS) Extension to Establish Keys for the Secure
            Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
 [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
            Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
 [BESRTP]   Kaplan, H. and F. Audet, "Session Description Protocol
            (SDP) Offer/Answer Negotiation For Best-Effort Secure
            Real-Time Transport Protocol", Work in Progress, October
            2006.
 [ICETCP]   Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
            "TCP Candidates with Interactive Connectivity
            Establishment (ICE)", Work in Progress, September 2010.
 [SDPMedCap]
            Gilman, R., Even, R., and F. Andreasen, "SDP media
            capabilities Negotiation", Work in Progress, July 2010.
 [SDPng]    Kutscher, D., Ott, J., and C. Bormann, "Session
            Description and Capability Negotiation", Work in Progress,
            February 2005.

Author's Address

 Flemming Andreasen
 Cisco Systems
 Iselin, NJ 08830
 USA
 EMail: fandreas@cisco.com

Andreasen Standards Track [Page 77]

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