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RFC 5939

 
 
 

Session Description Protocol (SDP) Capability Negotiation

Part 3 of 4, p. 32 to 54
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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.

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   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.

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   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

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   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

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      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

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      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.

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   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.

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      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.

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   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

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   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).

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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

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   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

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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].

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   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.

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   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.

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   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.

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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

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   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).

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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.

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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).

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   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.

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   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.

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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.



(page 54 continued on part 4)

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