MMUSIC WG                                               M. Garcia-Martin
Internet-Draft                                                  Ericsson
Intended status: Standards Track                         S. Veikkolainen
Expires: September 27, 2012 February 28, 2013                                         Nokia
                                                               R. Gilman
                                                          March 26,
                                                         August 27, 2012

   Miscellanoues Capabilities Negotiation in the Session Description
                             Protocol (SDP)
              draft-ietf-mmusic-sdp-miscellaneous-caps-00
              draft-ietf-mmusic-sdp-miscellaneous-caps-01

Abstract

   SDP has been extended with a capability negotiation mechanism
   framework that allows the endpoints to negotiate transport protocols
   and attributes.  This framework has been extended with a media
   capabilities negotiation mechanism that allows endpoints to negotiate
   additional media-related capabilities.  This negotiation is embedded
   into the widely-used SDP offer/answer procedures.

   This memo extends the SDP capability negotiation framework to allow
   endpoints to negotiate three additional SDP capabilities.  In
   particular, this memo provides a mechanism to negotiate bandwidth
   ('b=' line), connection data ('c=' line), and titles ('i=' line for
   each session or media).

Status of this Memo

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   This Internet-Draft will expire on September 27, 2012. February 28, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Extensions to SDP  . . . . . . . . . . . . . . . . . . . .  4
       3.1.1.  Bandwidth Capability . . . . . . . . . . . . . . . . .  7
       3.1.2.  Connection Data Capability . . . . . . . . . . . . . .  9
       3.1.3.  Title Capability . . . . . . . . . . . . . . . . . . . 11
     3.2.  Session Level versus Media Level . . . . . . . . . . . . . 14
     3.3.  Offer/Answer model extensions  . . . . . . . . . . . . . . 14
       3.3.1.  Generating the Initial Offer . . . . . . . . . . . . . 15
       3.3.2.  Generating the Answer  . . . . . . . . . . . . . . . . 15
       3.3.3.  Offerer Processing of the Answer . . . . . . . . . . . 15
       3.3.4.  Modifying the Session  . . . . . . . . . . . . . . . . 15
   4.  Field Replacement Rules  . . . . . . . . . . . . . . . . . . . 15
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     5.1.  New SDP Attributes . . . . . . . . . . . . . . . . . . . . 16
     5.2.  New Option Tags  . . . . . . . . . . . . . . . . . . . . . 17
     5.3.  New SDP Capability Negotiation Configuration Parameters  . 17
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

1.  Introduction

   The Session Description Protocol (SDP) [RFC4566] is intended for
   describing multimedia sessions for the purposes of session
   announcement, session invitation, and other forms of multimedia
   session initiation.  SDP has been extended with a capability
   negotiation mechanism framework [RFC5939] which allows the endpoints
   to negotiate capabilities, such as support for Real-time Transport
   Protocol (RTP) [RFC3550] and Secure Real-time Transport Protocol
   (SRTP) [RFC3711].  The SDP media capabilities
   [I-D.ietf-mmusic-sdp-media-capabilities] provides negotiation
   capabilities to media lines as well.

   The capability negotiation is embedded into the widely used SDP
   offer/answer procedure [RFC3264].  This memo provides the means to
   negotiate further capabilities than those specified in the SDP
   capability negotiation mechanism framework [RFC5939] and the SDP
   media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities].  In particular, this memo
   provides a mechanism to negotiate bandwidth ('b='), connection data
   ('c='), and session or media titles ('i=').

   Since the three added capabilities are highly unconnected, it is not
   expected that implementations will support all of them at the same
   time.  Instead, it is expected that applications will choose their
   needed capability for their specific purpose.  Due to this, we are
   writing the normative part pertaining to each capability in a self-
   contained section: Section 3.1.1 describes the bandwidth capability
   extension, Section 3.1.2 describes the connection data capability
   extension, and Section 3.1.3 describes the title capability
   extension.  Separate option tags are defined for each capability.

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 BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant
   implementations.

3.  Protocol Description

3.1.  Extensions to SDP

   The SDP Capability Negotiation Framework [RFC5939] and the SDP media
   capabilities negotiation [I-D.ietf-mmusic-sdp-media-capabilities]
   specify attributes for negotiating SDP capabilities.  These documents
   specify new attributes (e.g., 'acap', 'tcap', 'rmcap', 'omcap') for
   achieving their purpose.  In this document we define three new
   additional capability attributes for SDP lines of the the general
   form:

      type=value

   for types 'b', 'c', and 'i'.  The corresponding capability attributes
   are respectively defined defined as:

   o  'bcap': bandwidth capability

   o  'ccap': connection data capability

   o  'icap': title capability

   From the sub-rules of attribute ('a=') line in SDP [RFC4566], SDP
   attributes are of the form:

         attribute          = (att-field ":" att-value) / att-field
         att-field          = token
         att-value          = byte-string

   Capability attributes use only the 'att-field:att-value' form.

   The new attributes may be referenced in potential configurations
   ('a=pcfg') or in latent configurations ('a=lcfg'), as productions
   conforming to the <extension-config-list> as respectively defined in
   RFC 5939 [RFC5939] and the SDP media capabilities specification
   [I-D.ietf-mmusic-sdp-media-capabilities].

         extension-config-list = ["+"] ext-cap-name "=" ext-cap-list
         ext-cap-name          = 1*(ALPHA / DIGIT)
                                 ; ALPHA and DIGIT defined in RFC 5234
         ext-cap-list          = 1*VCHAR  ; VCHAR defined in RFC 5234

   The optional "+" is used to indicate that the extension is mandatory
   and MUST be supported in order to use that potential configuration.

   The new attributes may also be referenced in actual configurations
   ('a=acfg') as productions conforming to the <sel-extension-config>
   defined in [RFC5939].

         sel-extension-config = ext-cap-name "=" 1*VCHAR

   The specific parameters are defined in the individual description of
   each capability, below.

   The 'bcap', 'ccap', and 'icap' capability attributes can be provided
   either at the session or media level.  According to the SDP
   Capability Negotiation [RFC5939], each extension capability must
   specify the implication of making it part of a configuration at the
   media level.

   According to SDP [RFC4566], 'b=', 'c=', and 'i=' lines may appear
   either at session or media level.  In line with this, the 'bcap',
   'ccap', and 'icap' capability attributes, when declared at session
   level, are to be interpreted as-if that attribute was provided with
   that value at the session level.  The 'bcap', 'ccap' and 'icap'
   capability attributes declared at media level, are to be interpreted
   as-if that capability attribute was declared at the media level.

   For example, extending the example in
   [I-D.ietf-mmusic-sdp-media-capabilities] with 'icap' and 'bcap'
   capability attributes, we get the following SDP:

         v=0
         o=- 25678 753849 IN IP4 192.0.2.1
         s=
         c=IN IP4 192.0.2.1
         t=0 0
         a=bcap:1 CT:200
         a=icap:1 Video conference
         m=audio 54320 RTP/AVP 0
         a=rmcap:1 L16/8000/1
         a=rmcap:2 L16/16000/2
         a=pcfg:1 m=1|2, pt=1:99,2:98
         m=video 66544 RTP/AVP 100
         a=rmcap:3,4 H263-1998/90000
         a=rtpmap:100 H264/90000
         a=pcfg:10 m=3 pt=3:101 b=1 i=1

     Figure 1: Example SDP offer with bcap and icap defined at session
                                   level

   The above SDP defines one PCMU audio stream and one H.264 video
   stream.  It also defines two RTP-based media capabilities ('rmcap'
   numbered 1 and 2), using L16 audio at 8 kbps and 16 kbps,
   respectively, as well as two RTP-based media capabilities for H.263
   video ('rmcap' numbered 3 and 4).  The RTP-based media capabilities
   all appear at the media level.  The example also contains a single
   bandwidth capability ('bcap') and a single title capability ('icap'),
   both defined at session level.  According to the definition above,
   when the capabilities defined in the 'bcap' and 'icap' attributes are
   referenced from the potential configuration, in the resulting SDP
   they are to be interpreted as session level attributes (but the RTP-
   based media capabilities are to be interpreted as media level
   attributes).

3.1.1.  Bandwidth Capability

   According to RFC 4566 [RFC4566] the bandwidth field denotes the
   proposed bandwidth to be used by the session or media.  In this memo
   we specify the bandwidth capability attribute which can also appear
   either at session or media level.  The bandwidth field is specified
   in RFC 4566 [RFC4566] with the following syntax:

      b=<bwtype>:<bandwidth>

   where <bwtype> is an alphanumeric modifier giving the meaning of the
   <bandwidth> figure.

   In this document, we define a new capability attribute: the Bandwidth
   capability attribute 'bcap'.  This attribute lists bandwidth as
   capabilities according to the following definition:

      "a=bcap:" bw-cap-num 1*WSP bwtype ":" bandwidth CRLF

   where <bw-cap-num> is a unique integer between 1 and 2^31-1 (both
   included) user to number the bandwidth capability, and the other
   elements are as defined for the 'b=' field in SDP [RFC4566].

   This format satisfies the general attribute production rules in SDP
   [RFC4566] according to the following Augmented Backus-Naur Form
   (ABNF) [RFC5234] syntax:

         att-field       =/ "bcap"
         att-value       =/ bw-cap-num 1*WSP bwtype ":" bandwidth
         bw-cap-num      = 1*10(DIGIT)   ; DIGIT defined in RFC 5234

                  Figure 2: Syntax of the bcap attribute

   Negotiation of bandwidth per media stream can be useful when
   negotiating media encoding capabilities with different bandwidths.

3.1.1.1.  Configuration Parameters

   The SDP capability negotiation framework [RFC5939] provides for the
   existence of the 'pcfg' and 'acfg' attributes.  The concept is
   extended by the SDP media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] with an 'lcfg' attribute
   that conveys latent configurations.

   Extensions to the 'pcfg' and 'lcfg' attributes are defined through
   <extension-config-list>, and extensions to the 'acfg' attribute are
   defined through the <sel-extension-config> as defined in the SDP
   Capability Negotiation [RFC5939].

   In this document we extend the <extension-config-list> field to be
   able to convey lists of bandwidth capabilities in latent or potential
   configurations, according to the following Augmented Backus-Naur Form
   (ABNF) [RFC5234] syntax:

      extension-config-list  =/ bandwidth-config-list
      bandwidth-config-list  = ["+"] "b=" bw-cap-list *(BAR bw-cap-list)
                                  ; BAR defined in RFC 5939
      bw-cap-list            = bw-cap-num *("," bw-cap-num)
      bw-cap-num             = 1*10(DIGIT)   ; DIGIT defined in RFC 5234

     Figure 3: Syntax of the bandwidth parameter in 'lcfg' and 'pcfg'
                                attributes

   Each bandwidth capability configuration is a comma-separated list of
   bandwidth capability attribute numbers where <bw-cap-num> refers to
   the <bw-cap-num> bandwidth capability numbers defined explicitly
   earlier in this document, and hence must be between 1 and 2^31-1
   (both included).  Alternative bandwidth configurations are separated
   by a vertical bar ("|").

   The above syntax is very flexible, allowing referencing to multiple
   'b=' lines per configuration, even for the same <bwtype>.  While the
   need for such definitions is not seen, we have not restricted this,
   as it is not restricted in SDP [RFC4566] either.

   The bandwidth parameter to the actual configuration attribute
   ('a=acfg') is formulated as a <sel-extension-config> with

      ext-cap-name = "b"

   hence

        sel-extension-config =/ sel-bandwidth-config
        sel-bandwidth-config = "b=" bw-cap-list  ; bw-cap-list as above.

     Figure 4: Syntax of the bandwidth parameter in 'acfg' attributes

3.1.1.2.  Option tag

   The SDP Capability Negotiation Framework [RFC5939] allows for
   capability negotiation extensions to be defined.  Associated with
   each such extension is an option tag that identifies the extension in
   question.  Hereby, we define a new option tag "bcap-v0" that
   identifies support for the bandwidth capability.  The endpoints using
   the 'bcap' capability attribute SHOULD add the option tag to other
   existing option tags present in the 'csup' and 'creq' attributes in
   SDP, according to the procedures defined in the SDP Capability
   Negotiation Framework [RFC5939].

3.1.2.  Connection Data Capability

   According to SDP [RFC4566], the connection data field in SDP contains
   the connection data, and it has the following syntax:

      c=<nettype> <addrtype> <connection-address>

   where <nettype> indicates the network type, <addrtype> indicates the
   address type, and the <connection-address> is the connection address,
   which is dependent on the address type.

   At the moment, network types already defined include "IN", which
   indicates Internet network type, and "ATM" (see RFC 3108 [RFC3108]),
   used for describing Asynchronous Transfer Mode (ATM) bearer
   connections.  The Circuit-Switched (CS) descriptions in SDP document
   [I-D.ietf-mmusic-sdp-cs] adds a "PSTN" network type for expressing a
   Public Switched Telephone Network (PSTN) circuit switch.

   SDP [RFC4566] permits specification of connection data at the session
   or at the media level.  In order to permit negotiation of connection
   data at the media level, we define the connection data capability
   attribute ('a=ccap') in the form:

      "a=ccap:" conn-cap-num 1*WSP nettype SP addrtype SP connection-
      address CRLF

   where <conn-cap-num> is a unique ordinal identifier of the connection
   data capability, and the other elements are as defined in [RFC4566].

   This format corresponds to the [RFC4566] attribute production rules
   according to the following Augmented Backus-Naur Form (ABNF)
   [RFC5234] syntax:

         att-field       =/ "ccap"
         att-value       =/ conn-cap-num 1*WSP nettype SP addrtype
                           SP connection-address
         conn-cap-num    = 1*DIGIT   ; 1 to 2^31-1, inclusive

                  Figure 5: Syntax of the ccap attribute

   The 'ccap' capability attribute allows for expressing alternative
   connection address ('c=') lines in SDP as part of the SDP capability
   negotiation process.  The 'ccap' capability attribute is intended to
   be used only when there is no other mechanism available for
   negotiating alternative connection address information, such as when
   the <nettype> is different among the alternative addresses.  The
   'ccap' attribute MUST NOT be used in situations where an existing
   mechanism (such as Interactive Connectivity Establishment (ICE)
   [RFC5245]) can be used to select between different connection
   addresses.

3.1.2.1.  Configuration Parameters

   The SDP Capability Negotiation Framework [RFC5939] provides for the
   existence of the 'pcfg' and 'acfg' attributes, which can carry one or
   more potential configurations to be negotiated.  The concept is
   extended by the the Media Capabilities Negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] with an 'lcfg' attribute
   that conveys latent configurations.

   In this document we define a <connection-config> parameter to be used
   to specify a connection data capability in a potential or latent
   configuration attribute.  The parameter follows the form of an
   <extension-config-list>, with

      ext-cap-name = "c"

      ext-cap-list = conn-cap-list

   where, according to the following Augmented Backus-Naur Form (ABNF)
   [RFC5234] syntax:

         extension-config-list =/ conn-config-list
         conn-config-list      = "c=" conn-cap-list
         conn-cap-list         = conn-cap-num *(BAR conn-cap-num)
         conn-cap-num          = 1*DIGIT   ; 1 to 2^32-1 inclusive

      Figure 6: Syntax of the connection data parameter in 'lcfg' and
                             'pcfg' attributes

   Each capability configuration alternative contains a single
   connection data capability attribute number and refers to the conn-
   cap-num capability number defined explicitly earlier in this
   document, and hence must be between 1 and 2^31-1 (both included).
   The connection data capability allows the expression of only a single
   capability in each alternative, rather than a list of capabilities,
   since no more than a single connection data field is permitted per
   media block.  Nevertheless, it is still allowed to express
   alternative potential connection configurations separated by a
   vertical bar ("|").

   The connection data parameter to the actual configuration attribute
   ('a=acfg') is formulated as a <sel-extension-config> with

      ext-cap-name = "c"

   hence

         sel-extension-config =/ sel-connection-config
         sel-connection-config = "c=" conn-cap-num  ; as defined above.

        Figure 7: Syntax of the connection data parameter in 'acfg'
                                attributes

3.1.2.2.  Option tag

   The SDP Capability Negotiation Framework [RFC5939] solution allows
   for capability negotiation extensions to be defined.  Associated with
   each such extension is an option tag that identifies the extension in
   question.  Hereby, we define a new option tag of "ccap-v0" that
   identifies support for the connection data capability.  This option
   tag SHOULD be added to other existing option tags present in the
   'csup' and 'creq' attributes in SDP, according to the procedures
   defined in the SDP Capability Negotiation Framework [RFC5939].

3.1.3.  Title Capability

   SDP [RFC4566] provides for the existence of an information field
   expressed in the format of the 'i=' line, which can appear either at
   the session level or at the media level.  An 'i=' line that is
   present at the session level is known as the "session name", and its
   purpose is to convey a human-readable textual information about the
   session.

   The 'i=' line in SDP can also appear at the media level, in which
   case it is used to provide human-readable information about the media
   stream to which it is related, e.g., it may indicate the purpose of
   the media stream.  The 'i=' line is not to be confused with the label
   attribute ('a=label:', [RFC4574]) which provides a machine-readable
   tag.  It is foreseen that applications declaring capabilities related
   to different configurations of a media stream may need to provide
   different identifying information for each of those configurations.
   That is, a party might offer alternative media configurations for a
   stream, each of which represents a different presentation of the same
   or similar information.  For example, an audio stream might offer
   English or Spanish configurations, or a video stream might offer a
   choice of video source such as speaker camera, group camera, or
   document viewer.  The title capability is needed to inform the
   answering user in order to select the proper choice, and the label is
   used to inform the offering machine which choice the answerer has
   selected.  Hence, there is value in defining a mechanism to provide
   titles of media streams as capabilities.

   According to SDP [RFC4566], the session information ('i=') line has
   the following syntax:

      "i=" text

   where "text" represents a human-readable text indicating the purpose
   of the session or media stream.

   In this document we define a new capability attribute: the Title
   capability 'icap'.  This attribute lists session or media titles as
   capabilities, according to the following definition:

      "a=icap:" title-cap-num 1*WSP text

   where <title-cap-num> is a unique integer between 1 and 2^31-1 (both
   included) user to number the unique ordinal identifier of the
   particular title capability and <text> is a human-readable text that
   indicates the purpose of the session or media stream it is supposed
   to characterize.

   As an example, one might use:

      a=icap:1 Document Camera

   to define a title capability number 1 to identify a particular source
   of a media stream.

   The title capability attribute satisfies the general attribute
   production rules in SDP [RFC4566] according to the following
   Augmented Backus-Naur Form (ABNF) [RFC5234] syntax:

         att-field       =/ "icap"
         att-value       =/ title-cap-num 1*WSP text
                                     ; text defined in RFC 4566
         title-cap-num   = 1*10(DIGIT)   ; DIGIT defined in RFC 5234

                  Figure 8: Syntax of the icap attribute

3.1.3.1.  Configuration Parameters

   The SDP Capability Negotiation Framework [RFC5939] provides for the
   existence of the 'pcfg' and 'acfg' attributes.  The concept is
   extended by the SDP media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities] with an 'lcfg' attribute
   that conveys latent configurations.

   In this document, we define an <title-config-list> parameter to be
   used to convey title capabilities in a potential or latent
   configuration.  This parameter is defined as an <extension-config-
   list> with the following associations:

      ext-cap-name = "i"

      ext-cap-list = title-cap-list

   This leads to the following definition for the title capability
   parameter:

         extension-config-list =/ title-config-list
         title-config-list     = ["+"] "i=" title-cap-list
         title-cap-list        = title-cap-num *(BAR title-cap-num)
                                         ; BAR defined in RFC 5939
         title-cap-num         = 1*10(DIGIT) ; DIGIT defined in RFC 5234

     Figure 9: Syntax of the title capability parameter in 'lcfg' and
                             'pcfg' attributes

   Each potential capability configuration contains a single title
   capability attribute number where 'title-cap-num' is the title
   capability number defined explicitly earlier in this document, and
   hence must be between 1 and 2^31-1 (both included).  The title
   capability allows the expression of only a single capability in each
   alternative, since no more than a single title field is permitted per
   block.  Nevertheless, it is still allowed to express alternative
   potential title configurations separated by a vertical bar ("|").

   An endpoint includes a plus sign ("+") in this configuration
   attribute to mandate support for this extension.  An endpoint that
   receives this attribute prefixed with a plus sign and does not
   support this extension MUST treat that potential configuration as not
   valid.

   The title parameter to the actual configuration attribute ('a=acfg')
   is formulated as a <sel-extension-config> with

      ext-cap-name = "i"

   hence

         sel-extension-config =/ sel-title-config
         sel-title-config = "i=" title-cap-num  ; as defined above.

       Figure 10: Syntax of the title parameter in 'acfg' attributes

3.1.3.2.  Option Tag

   At present, it is difficult to envision a scenario in which the
   'icap' attribute must be supported or the offer must be rejected.  In
   most cases, if the icap attribute or its contents were to be ignored,
   an offered configuration could still be chosen based on other
   criteria such as configuration numbering.  However, one might imagine
   an SDP offer that contained English and Spanish potential
   configurations for an audio stream.  The session might be
   unintelligible if the choice is based on configuration numbering,
   rather than informed user selection.  Based on such considerations,
   it may well prove useful to announce the ability to use the icap
   attribute and its contents to select media configurations, or to
   inform the user about the selected configuration(s).  Therefore, we
   define a new option tag of "icap-v0" that identifies support for the
   title capability.  This option tag SHOULD be added to other existing
   option tags present in the 'csup' and/or 'creq' attributes in SDP,
   according to the procedures defined in the SDP Capability Negotiation
   Framework [RFC5939].  The discussion above suggests that "icap-v0"
   will typically appear in a 'csup' attribute, but rarely in a 'creq'
   attribute.

3.2.  Session Level versus Media Level

   The 'bcap', 'ccap' and 'icap' attributes can appear at the session
   level and/or at the media level.  Endpoints MUST interpret
   capabilities declared at session level as part of the session level
   in the resulting SDP for that particular configuration.  Endpoints
   MUST interpret capabilities declared at media level as part of the
   media level in the resulting SDP for that particular configuration.

   If a 'bcap' capability for the same bwtype is declared at both
   session and media level, the media level attribute overrides the
   value of the session level attribute.

   To avoid confusion, the <type-attr-num> for each 'a=bcap', 'a=ccap',
   and 'a=icap' line must be unique across all capability attributes of
   the same type within the entire session description.

3.3.  Offer/Answer model extensions

   In this section, we define extensions to the offer/answer model
   defined in SDP Offer/Answer Model [RFC3264] and extended in the SDP
   Capability Negotiation [RFC5939] to allow for bandwidth and title
   capabilities to be used with the SDP Capability Negotiation
   framework.

3.3.1.  Generating the Initial Offer

   When an endpoint generates an initial offer and wants to use the
   functionality described in the current document, it first defines
   appropriate values for the bandwidth, connection data, and/or title
   capability attributes according to rules defined in [RFC4566] for
   'b=', 'c=' and 'i=' lines.  The endpoint then MUST include the
   respective capability attributes and associated values in the SDP
   offer.  The preferred configurations for each media stream are
   identified following the media line in a 'pcfg' attribute.  Bandwidth
   and title capabilities may also be referenced in latent
   configurations in an 'lcfg' attribute, defined in [RFC5939].

   The offer SHOULD include the level of capability negotiation
   extensions needed to support this functionality in a 'creq'
   attribute.

3.3.2.  Generating the Answer

   When the answering party receives the offer, and if it supports the
   required capability negotiation extensions, it SHOULD select the most
   preferred configuration it can support for each media stream, and
   build the answer accordingly, as defined in Section 3.6.2 of the SDP
   Capability Negotiation [RFC5939].

3.3.3.  Offerer Processing of the Answer

   When the offerer receives the answer, it MUST process the media lines
   according to normal SDP processing rules to identify the media
   stream(s) accepted by the answer, if any.  The 'acfg' attribute, if
   present, may be used to verify the proposed configuration used to
   form the answer, and to infer the lack of acceptability of higher-
   preference configurations that were not chosen.

3.3.4.  Modifying the Session

   If, at a later time, one of the parties wishes to modify the
   operating parameters of a session, e.g. by adding a new media stream,
   or by changing the properties used on an existing stream, it may do
   so via the mechanisms defined for SDP offer/answer [RFC3264].

4.  Field Replacement Rules

   To simplify the construction of SDP records, given the need to
   include fields within the media description in question for endpoints
   that do not support capabilities negotiation, we define some simple
   field-replacement rules for those fields invoked by potential or
   latent configurations.  In particular, any 'i=' or 'c=' line invoked
   by a configuration MUST replace the corresponding line, if present
   within the media description in question.  Any 'b=' line invoked by a
   configuration MUST replace any 'b=' of the same bandwidth type at the
   media level.

5.  IANA Considerations

5.1.  New SDP Attributes

   IANA is hereby requested to register new attributes in the "att-field
   (both session and media level)" of the "Session Description Protocol
   (SDP) Parameteres" registry, according to the following registration
   form:

      Attribute name:      bcap
      Long form name:      Bandwidth Capability
      Type of attribute:   Both media and session level
      Subject to charset:  No
      Purpose:             Negotiate session or media-level bandwidths
      Appropriate values:  See RFC XXXX
           [Note to the RFC Editor: Please replace the above RFC XXXX
           with the RFC number of this specification.
      Contact name:        Miguel A. Garcia,
                           Miguel.A.Garcia@ericsson.com

      Attribute name:      ccap
      Long form name:      Connection Data Capability
      Type of attribute:   Both media and session level
      Subject to charset:  No
      Purpose:             Negotiate media-level connection data
      Appropriate values:  See RFC XXXX
           [Note to the RFC Editor: Please replace the above RFC XXXX
           with the RFC number of this specification.
      Contact name:        Miguel A. Garcia,
                           Miguel.A.Garcia@ericsson.com

      Attribute name:      icap
      Long form name:      Title Capability
      Type of attribute:   Both media and session level
      Subject to charset:  Yes
      Purpose:             Negotiate human-readable information
                           describing the session or media
      Appropriate values:  See RFC XXXX
           [Note to the RFC Editor: Please replace the above RFC XXXX
           with the RFC number of this specification.
      Contact name:        Miguel A. Garcia,
                           Miguel.A.Garcia@ericsson.com

5.2.  New Option Tags

   IANA is hereby requested to add the new option tags "bcap-v0",
   "ccap-v0", and "icap-v0", defined herein, to the "SDP Capability
   Negotiation Option Tag subregistry" of the "Session Description
   Protocol (SDP) Parameters" registry.

5.3.  New SDP Capability Negotiation Configuration Parameters

   IANA is hereby requested to add the new parameter identifiers "b" for
   "bandwidth", "c" for "connection data", and "i" for "title" to the
   "SDP Capability Negotiation Potential Configuration Parameters"
   subregistry of the "Session Description Protocol (SDP) Parameters"
   registry.  These parameters are permitted in 'lcfg', 'acfg', and
   'pcfg' attributes.

6.  Security Considerations

   This document provides an extension on top of RFC 4566 [RFC4566], RFC
   3264 [RFC3264], SDP Capability Negotiation Framework [RFC5939], and
   SDP media capabilities negotiation
   [I-D.ietf-mmusic-sdp-media-capabilities].  As such, the security
   considerations of those documents apply.

   The bandwidth capability attribute may be used for reserving
   resources at endpoints and intermediaries which inspect the SDP.
   Modification of the bandwidth value by an attacker can lead to the
   network being underutilized (too high bandwidth value) or congested
   (too low bandwidth value).  In case it is essential to protect the
   bandwidth value, one of the security mechanisms proposed in [RFC5939]
   should be used.

   The 'i=' line and thus the value carried in the title capability
   attribute is intended for human-readable description only.  It should
   not be parsed programmatically.

7.  Acknowledgments

   Thanks to Christer Holmberg, Alf Heidermark, and Ingemar Johansson
   for arguing for the existence of this document and early reviewing
   it.  Thanks to Flemming Andreasen, Andrew Allen, and Jonathan Lennox
   for a detailed review and many improvement suggestions.

8.  References

8.1.  Normative References

   [I-D.ietf-mmusic-sdp-media-capabilities]
              Gilman, R., Even, R., and F. Andreasen, "SDP Media
              Capabilities Negotiation",
              draft-ietf-mmusic-sdp-media-capabilities-13
              draft-ietf-mmusic-sdp-media-capabilities-14 (work in
              progress), March July 2012.

   [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.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
              Capability Negotiation", RFC 5939, September 2010.

8.2.  Informative References

   [I-D.ietf-mmusic-sdp-cs]
              Garcia, M. and S. Veikkolainen, "Session Description
              Protocol (SDP) Extension For Setting Up Audio and Video
              Media Streams Over Circuit-Switched Bearers In The Public
              Switched Telephone Network (PSTN)",
              draft-ietf-mmusic-sdp-cs-10
              draft-ietf-mmusic-sdp-cs-11 (work in progress),
              March May 2012.

   [RFC3108]  Kumar, R. and M. Mostafa, "Conventions for the use of the
              Session Description Protocol (SDP) for ATM Bearer
              Connections", RFC 3108, May 2001.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.

              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574, August 2006.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

Authors' Addresses

   Miguel A. Garcia-Martin
   Ericsson
   Calle Via de los Poblados 13
   Madrid,   28033
   Spain

   Phone: +34 91 339 1000
   Email: miguel.a.garcia@ericsson.com

   Simo Veikkolainen
   Nokia
   P.O. Box 407
   NOKIA GROUP, FI  00045
   Finland

   Phone: +358 50 486 4463
   Email: simo.veikkolainen@nokia.com

   Robert R. Gilman
   3243 W. 11th Ave. Dr.
   Broomfield, Colorado  80020
   U.S.A.

   Phone: +1 303 898 9780
   Email: bob_gilman@comcast.net