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RFC 6871

Session Description Protocol (SDP) Media Capabilities Negotiation

Pages: 55
Proposed Standard
Updates:  5939
Part 2 of 4 – Pages 13 to 34
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Top   ToC   RFC6871 - Page 13   prevText

3.3. New Capability Attributes

In this section, we present the new attributes associated with indicating the media capabilities for use by the SDP capability negotiation. The approach taken is to keep things similar to the existing media capabilities defined by the existing media descriptions ("m=" lines) and the associated "rtpmap" and "fmtp" attributes. We use media subtypes and "media capability numbers" to link the relevant media capability parameters. This permits the capabilities to be defined at the session level and be used for multiple streams, if desired. For RTP-based media formats, payload types are then specified at the media level (see Section A media capability merely indicates possible support for the media type and media format(s) and parameters in question. In order to actually use a media capability in an offer/answer exchange, it MUST be referenced in a potential configuration. Media capabilities, i.e., the attributes associated with expressing media capability formats, parameters, etc., can be provided at the session level and/or the media level. Media capabilities provided at the session level may be referenced in any "pcfg" or "lcfg" attribute at the media level (consistent with the media type), whereas media capabilities provided at the media level may be referenced only by the "pcfg" or "lcfg" attribute within that media stream. In either case, the scope of the <med-cap-num> is the entire session description. This enables each media capability to be uniquely referenced across the entire session description (e.g., in a potential configuration).

3.3.1. The Media Format Capability Attributes

Media subtypes can be expressed as media format capabilities by use of the "a=rmcap" and "a=omcap" attributes. The "a=rmcap" attribute MUST be used for RTP-based media, whereas the "a=omcap" attribute MUST be used for non-RTP-based (other) media formats. The two attributes are defined as follows:
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   a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate>

   a=omcap:<media-cap-num-list> <format-name>

   where <media-cap-num-list> is a (list of) media capability number(s)
   used to number a media format capability, the <encoding name> or
   <format-name> is the media subtype, e.g., H263-1998, PCMU, or T38,
   <clock rate> is the encoding rate, and <encoding parms> are the media
   encoding parameters for the media subtype.  All media format
   capabilities in the list are assigned to the same media type/subtype.
   Each occurrence of the "rmcap" and "omcap" attribute MUST use unique
   values in their <media-cap-num-list>; the media capability numbers
   are shared between the two attributes and the numbers MUST be unique
   across the entire SDP session.  In short, the "rmcap" and "omcap"
   attributes define media format capabilities and associate them with a
   media capability number in the same manner as the "rtpmap" attribute
   defines them and associates them with a payload type number.
   Additionally, the attributes allow multiple capability numbers to be
   defined for the media format in question by specifying a range of
   media capability numbers.  This permits the media format to be
   associated with different media parameters in different
   configurations.  When a range of capability numbers is specified, the
   first (leftmost) capability number MUST be strictly smaller than the
   second (rightmost), i.e., the range increases and covers at least two

   In ABNF [RFC5234], we have:

   media-capability-line = rtp-mcap / non-rtp-mcap

   rtp-mcap           = "a=rmcap:" media-cap-num-list
                           1*WSP encoding-name "/" clock-rate
                           ["/" encoding-parms]
   non-rtp-mcap       = "a=omcap:" media-cap-num-list 1*WSP format-name
   media-cap-num-list = media-cap-num-element
                        *("," media-cap-num-element)
   media-cap-num-element = media-cap-num
                                / media-cap-num-range
   media-cap-num-range = media-cap-num "-" media-cap-num
   media-cap-num      = NonZeroDigit *9(DIGIT)
   encoding-name      = token ;defined in RFC 4566
   clock-rate         = NonZeroDigit *9(DIGIT)
   encoding-parms     = token
   format-name        = token ;defined in RFC 4566
   NonZeroDigit       = %x31-39    ; 1-9
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   The encoding-name, clock-rate, and encoding-params are as defined to
   appear in an "rtpmap" attribute for each media type/subtype.  Thus,
   it is easy to convert an "rmcap" attribute line into one or more
   "rtpmap" attribute lines, once a payload type number is assigned to a
   media-cap-num (see Section 3.3.5).

   The format-name is a media format description for non-RTP-based media
   as defined for the <fmt> part of the media description ("m=" line) in
   SDP [RFC4566].  In simple terms, it is the name of the media format,
   e.g., "t38".  This form can also be used in cases such as Binary
   Floor Control Protocol (BFCP) [RFC4585] where the fmt list in the
   "m=" line is effectively ignored (BFCP uses "*").

   The "rmcap" and "omcap" attributes can be provided at the session
   level and/or the media level.  There can be more than one "rmcap" and
   more than one "omcap" attribute at both the session and media levels
   (i.e., more than one of each at the session level and more than one
   of each in each media description).  Media capability numbers cannot
   include leading zeroes, and each media-cap-num MUST be unique within
   the entire SDP record; it is used to identify that media capability
   in potential, latent, and actual configurations, and in other
   attribute lines as explained below.  Note that the media-cap-num
   values are shared between the "rmcap" and "omcap" attributes; hence,
   the uniqueness requirement applies to the union of them.  When the
   media capabilities are used in a potential, latent, or actual
   configuration, the media formats referred by those configurations
   apply at the media level, irrespective of whether the media
   capabilities themselves were specified at the session or media level.
   In other words, the media capability applies to the specific media
   description associated with the configuration that invokes it.
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   For example:

      o=- 24351 621814 IN IP4
      c=IN IP4
      t=0 0
      a=rmcap:1 L16/8000/1
      a=rmcap:2 L16/16000/2
      a=rmcap:3 H263-1998/90000
      a=omcap:4 example
      m=audio 54320 RTP/AVP 0
      a=pcfg:1 m=1|2, pt=1:99,2:98
      m=video 66544 RTP/AVP 100
      a=rtpmap:100 H264/90000
      a=pcfg:10 m=3 pt=3:101
      a=tcap:1 TCP
      a=pcfg:11 m=4 t=1

3.3.2. The Media Format Parameter Capability Attribute

This attribute is used to associate media format specific parameters with one or more media format capabilities. The form of the attribute is a=mfcap:<media-caps> <list of parameters> where <media-caps> permits the list of parameters to be associated with one or more media format capabilities and the format parameters are specific to the type of media format. The mfcap lines map to a single traditional SDP "fmtp" attribute line (one for each entry in <media-caps>) of the form a=fmtp:<fmt> <list of parameters> where <fmt> is the media format parameter defined in RFC 4566 [RFC4566], as appropriate for the particular media stream. The "mfcap" attribute MUST be used to encode attributes for media capabilities, which would conventionally appear in an "fmtp" attribute. The existing "acap" attribute MUST NOT be used to encode "fmtp" attributes. The "mfcap" attribute adheres to SDP [RFC4566] attribute production rules with media-format-parameter-capability = "a=mfcap:" media-cap-num-list 1*WSP fmt-specific-param-list fmt-specific-param-list = text ; defined in RFC 4566
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   Note that media format parameters can be used with RTP-based and non-
   RTP-based media formats. Media Format Parameter Concatenation Rule
The appearance of media subtypes with a large number of formatting options (e.g., AMR-WB [RFC4867]), coupled with the restriction that only a single "fmtp" attribute can appear per media format, suggests that it is useful to create a combining rule for "mfcap" parameters that are associated with the same media capability number. Therefore, different mfcap lines MAY include the same media-cap-num in their media-cap-num-list. When a particular media capability is selected for processing, the parameters from each mfcap line that references the particular capability number in its media-cap-num-list are concatenated together via ";", in the order the "mfcap" attributes appear in the SDP record, to form the equivalent of a single "fmtp" attribute line. This permits one to define a separate mfcap line for a single parameter and value that is to be applied to each media capability designated in the media-cap-num-list. This provides a compact method to specify multiple combinations of format parameters when using codecs with multiple format options. Note that order-dependent parameters SHOULD be placed in a single mfcap line to avoid possible problems with line rearrangement by a middlebox. Format parameters are not parsed by SDP; their content is specific to the media type/subtype. When format parameters for a specific media capability are combined from multiple "a=mfcap" lines that reference that media capability, the format-specific parameters are concatenated together and separated by ";" for construction of the corresponding format attribute ("a=fmtp"). The resulting format attribute will look something like the following (without line breaks): a=fmtp:<fmt> <fmt-specific-param-list1>; <fmt-specific-param-list2>; ... where <fmt> depends on the transport protocol in the manner defined in RFC 4566 [RFC4566]. SDP cannot assess the legality of the resulting parameter list in the "a=fmtp" line; the user must take care to ensure that legal parameter lists are generated. The "mfcap" attribute can be provided at the session level and the media level. There can be more than one "mfcap" attribute at the session or media level. The unique media-cap-num is used to associate the parameters with a media capability.
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   As a simple example, a G.729 capability is, by default, considered to
   support comfort noise as defined by Annex B.  Capabilities for G.729
   with and without comfort noise support may thus be defined by:

      a=rmcap:1,2 G729/8000
      a=mfcap:2 annexb:no

   Media capability 1 supports G.729 with Annex B, whereas media
   capability 2 supports G.729 without Annex B.

   Example for H.263 video:

      a=rmcap:1 H263-1998/90000
      a=rmcap:2 H263-2000/90000
      a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
      a=mfcap:2 profile=2;level=2.2

   Finally, for six format combinations of the Adaptive Multi-Rate

      a=rmcap:1-3 AMR/8000/1
      a=rmcap:4-6 AMR-WB/16000/1
      a=mfcap:1,2,3,4 mode-change-capability=1
      a=mfcap:5,6 mode-change-capability=2
      a=mfcap:1,2,3,5 max-red=220
      a=mfcap:3,4,5,6 octet-align=1
      a=mfcap:1,3,5 mode-set=0,2,4,7
      a=mfcap:2,4,6 mode-set=0,3,5,6

   So that AMR codec #1, when specified in a "pcfg" attribute within an
   audio stream block (and assigned payload type number 98) as in:

      a=pcfg:1 m=1 pt=1:98

   is essentially equivalent to the following:

      m=audio 49170 RTP/AVP 98
      a=rtpmap:98 AMR/8000/1
      a=fmtp:98 mode-change-capability=1; \
      max-red=220; mode-set=0,2,4,7

   and AMR codec #4 with payload type number 99, depicted by the
   potential configuration:

      a=pcfg:4 m=4, pt=4:99
Top   ToC   RFC6871 - Page 19
   is equivalent to the following:

      m=audio 49170 RTP/AVP 99
      a=rtpmap:99 AMR-WB/16000/1
      a=fmtp:99 mode-change-capability=1; octet-align=1; \

   and so on for the other four combinations.  SDP could thus convert
   the media capabilities specifications into one or more alternative
   media stream specifications, one of which can be chosen for the

3.3.3. The Media-Specific Capability Attribute

Attributes and parameters associated with a media format are typically specified using the "rtpmap" and "fmtp" attributes in SDP, and the similar "rmcap" and "mfcap" attributes in SDP media capabilities. Some SDP extensions define other attributes that need to be associated with media formats, for example, the "rtcp-fb" attribute defined in RFC 4585 [RFC4585]. Such media-specific attributes, beyond the "rtpmap" and "fmtp" attributes, may be associated with media capability numbers via a new media-specific attribute, "mscap", of the following form: a=mscap:<media caps star> <att field> <att value> where <media caps star> is a (list of) media capability number(s), <att field> is the attribute name, and <att value> is the value field for the named attribute. Note that the media capability numbers refer to media format capabilities specified elsewhere in the SDP ("rmcap" and/or "omcap"). If a range of capability numbers is specified, the first (leftmost) capability number MUST be strictly smaller than the second (rightmost). The media capability numbers may include a wildcard ("*"), which will be used instead of any payload type mappings in the resulting SDP (see, e.g., RFC 4585 [RFC4585] and the example below). In ABNF, we have: media-specific-capability = "a=mscap:" media-caps-star 1*WSP att-field ; from RFC 4566 1*WSP att-value ; from RFC 4566 media-caps-star = media-cap-star-element *("," media-cap-star-element) media-cap-star-element = (media-cap-num [wildcard]) / (media-cap-num-range [wildcard]) wildcard = "*"
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   Given an association between a media capability and a payload type
   number as specified by the "pt=" parameters in a "pcfg" attribute
   line, a mscap line may be translated easily into a conventional SDP
   attribute line of the form:

      a=<att field>":"<fmt> <att value> ; <fmt> defined in SDP [RFC4566]

   A resulting attribute that is not a legal SDP attribute, as specified
   by RFC 4566, MUST be ignored by the receiver.

   If a media capability number (or range) contains a wildcard character
   at the end, any payload type mapping specified for that media-
   specific capability (or range of capabilities) will use the wildcard
   character in the resulting SDP instead of the payload type specified
   in the payload type mapping ("pt" parameter) in the configuration

   A single mscap line may refer to multiple media capabilities by use
   of a capability number range; this is equivalent to multiple mscap
   lines, each with the same attribute values (but different media
   capability numbers), one line per media capability.

   Multiple mscap lines may refer to the same media capability, but,
   unlike the "mfcap" attribute, no concatenation operation is defined.
   Hence, multiple mscap lines applied to the same media capability are
   equivalent to multiple lines of the specified attribute in a
   conventional media record.

   Here is an example with the "rtcp-fb" attribute, modified from an
   example in RFC 5104 [RFC5104] (with the session level and audio media
   omitted).  If the offer contains a media block like the following
   (note the wildcard character),

      m=video 51372 RTP/AVP 98
      a=rtpmap:98 H263-1998/90000
      a=tcap:1 RTP/AVPF
      a=rmcap:1 H263-1998/90000
      a=mscap:1 rtcp-fb ccm tstr
      a=mscap:1 rtcp-fb ccm fir
      a=mscap:1* rtcp-fb ccm tmmbr smaxpr=120
      a=pcfg:1 t=1 m=1 pt=1:98

   and if the proposed configuration is chosen, then the equivalent
   media block would look like the following
Top   ToC   RFC6871 - Page 21
      m=video 51372 RTP/AVPF 98
      a=rtpmap:98 H263-1998/90000
      a=rtcp-fb:98 ccm tstr
      a=rtcp-fb:98 ccm fir
      a=rtcp-fb:* ccm tmmbr smaxpr=120

3.3.4. New Configuration Parameters

Along with the new attributes for media capabilities, new extension parameters are defined for use in the potential configuration, the actual configuration, and/or the new latent configuration defined in Section 3.3.5. The Media Configuration Parameter (m=)
The media configuration parameter is used to specify the media format(s) and related parameters for a potential, actual, or latent configuration. Adhering to the ABNF for extension-config-list in RFC 5939 [RFC5939] with ext-cap-name = "m" ext-cap-list = media-cap-num-list [*(BAR media-cap-num-list)] we have media-config-list = ["+"] "m=" media-cap-num-list *(BAR media-cap-num-list) ;BAR is defined in RFC 5939 ;media-cap-num-list is defined above Alternative media configurations are separated by a vertical bar ("|"). The alternatives are ordered by preference, most-preferred first. When media capabilities are not included in a potential configuration at the media level, the media type and media format from the associated "m=" line will be used. The use of the plus sign ("+") is described in RFC 5939. The Payload Type Number Mapping Parameter (pt=)
The payload type number mapping parameter is used to specify the payload type number to be associated with each RTP-based media format in a potential, actual, or latent configuration. We define the payload type number mapping parameter, payload-number-config-list, in accordance with the extension-config-list format defined in RFC 5939 [RFC5939]. In ABNF:
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   payload-number-config-list = ["+"] "pt=" media-map-list
   media-map-list      = media-map *("," media-map)
   media-map           = media-cap-num ":" payload-type-number
                            ; media-cap-num is defined in Section 3.3.1
   payload-type-number = NonZeroDigit *2(DIGIT) ; RTP payload
                                                ; type number

   The example in Section 3.3.7 shows how the parameters from the rmcap
   line are mapped to payload type numbers from the "pcfg" "pt"
   parameter.  The use of the plus sign ("+") is described in RFC 5939

   A latent configuration represents a future capability; hence, the
   "pt=" parameter is not directly meaningful in the "lcfg" attribute
   because no actual media session is being offered or accepted.  It is
   permitted in order to tie any payload type number parameters within
   attributes to the proper media format.  A primary example is the case
   of format parameters for the Redundant Audio Data (RED) [RFC2198]
   payload, which are payload type numbers.  Specific payload type
   numbers used in a latent configuration MAY be interpreted as
   suggestions to be used in any future offer based on the latent
   configuration, but they are not binding; the offerer and/or answerer
   may use any payload type numbers each deems appropriate.  The use of
   explicit payload type numbers for latent configurations can be
   avoided by use of the parameter substitution rule of Section 3.3.7.
   Future extensions are also permitted.  Note that leading zeroes are
   not permitted. The Media Type Parameter
When a latent configuration is specified (always at the media level), indicating the ability to support an additional media stream, it is necessary to specify the media type (audio, video, etc.) as well as the format and transport type. The media type parameter is defined in ABNF as media-type = ["+"] "mt=" media; media defined in RFC 4566 At present, the media-type parameter is used only in the latent configuration attribute, and the use of the "+" prefix to specify that the entire attribute line is to be ignored if the mt= parameter is not understood is unnecessary. However, if the media-type parameter is later added to an existing capability attribute such as "pcfg", then the "+" would be useful. The media format(s) and transport type(s) are specified using the media configuration parameter ("+m=") defined above, and the transport parameter ("t=") defined in RFC 5939 [RFC5939], respectively.
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3.3.5. The Latent Configuration Attribute

One of the goals of this work is to permit the exchange of supportable media configurations in addition to those offered or accepted for immediate use. Such configurations are referred to as "latent configurations". For example, a party may offer to establish a session with an audio stream, and, at the same time, announce its ability to support a video stream as part of the same session. The offerer can supply its video capabilities by offering one or more latent video configurations along with the media stream for audio; the responding party may indicate its ability and willingness to support such a video session by returning a corresponding latent configuration. Latent configurations returned in SDP answers MUST match offered latent configurations (or parameter subsets thereof). Therefore, it is appropriate for the offering party to announce most, if not all, of its capabilities in the initial offer. This choice has been made in order to keep the size of the answer more compact by not requiring acap, rmcap, tcap, etc. lines in the answer. Latent configurations may be announced by use of the latent configuration attribute, which is defined in a manner very similar to the potential configuration attribute. The latent configuration attribute combines the properties of a media line and a potential configuration. A latent configuration MUST include a media type (mt=) and a transport protocol configuration parameter since the latent configuration is independent of any media line present. In most cases, the media configuration (m=) parameter needs to be present as well (see Section 4 for examples). The "lcfg" attribute is a media-level attribute. The "lcfg" attribute is defined as a media-level attribute since it specifies a possible future media stream. However, the "lcfg" attribute is not necessarily related to the media description within which it is provided. Session capability attributes ("a=sescap") may be used to indicate supported media stream configurations. Each media line in an SDP description represents an offered simultaneous media stream, whereas each latent configuration represents an additional stream that may be negotiated in a future offer/answer exchange. Session capability attributes may be used to determine whether a latent configuration may be used to form an offer for an additional simultaneous stream or to reconfigure an existing stream in a subsequent offer/answer exchange.
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   The latent configuration attribute is of the form:

        a=lcfg:<config-number> <latent-cfg-list>

   which adheres to the SDP [RFC4566] "attribute" production with
   att-field and att-value defined as:

      att-field  = "lcfg"
      att-value  = config-number 1*WSP lcfg-cfg-list
      config-number = NonZeroDigit *9(DIGIT)  ;DIGIT defined in RFC 5234
      lcfg-cfg-list = media-type 1*WSP pot-cfg-list
                                  ; as defined in RFC 5939
                                  ; and extended herein

   The media-type (mt=) parameter identifies the media type (audio,
   video, etc.)  to be associated with the latent media stream, and it
   MUST be present.  The pot-cfg-list MUST contain a transport-protocol-
   config-list (t=) parameter and a media-config-list (m=) parameter.
   The pot-cfg-list MUST NOT contain more than one instance of each type
   of parameter list.  As specified in RFC 5939 [RFC5939], the use of
   the "+" prefix with a parameter indicates that the entire
   configuration MUST be ignored if the parameter is not understood;
   otherwise, the parameter itself may be ignored.

   Media stream payload numbers are not assigned by a latent
   configuration.  Assignment will take place if and when the
   corresponding stream is actually offered via an "m=" line in a later
   exchange.  The payload-number-config-list is included as a parameter
   to the "lcfg" attribute in case it is necessary to tie payload
   numbers in attribute capabilities to specific media capabilities.

   If an "lcfg" attribute invokes an "acap" attribute that appears at
   the session level, then that attribute will be expected to appear at
   the session level of a subsequent offer when and if a corresponding
   media stream is offered.  Otherwise, "acap" attributes that appear at
   the media level represent media-level attributes.  Note, however,
   that "rmcap", omcap, "mfcap", "mscap", and "tcap" attributes may
   appear at the session level because they always result in media-level
   attributes or "m=" line parameters.

   The configuration numbers for latent configurations do not imply a
   preference; the offerer will imply a preference when actually
   offering potential configurations derived from latent configurations
   negotiated earlier.  Note, however, that the offerer of latent
   configurations MAY specify preferences for combinations of potential
   and latent configurations by use of the "sescap" attribute defined in
   Section 3.3.8.  For example, if an SDP offer contains, say, an audio
Top   ToC   RFC6871 - Page 25
   stream with "pcfg:1", and two latent video configurations, "lcfg:2"
   and "lcfg:3", then a session with one audio stream and one video
   stream could be specified by including "a=sescap:1 1,2|3".  One audio
   stream and two video streams could be specified by including
   "a=sescap:2 1,2,3" in the offer.  In order to permit combinations of
   latent and potential configurations in session capabilities, latent
   configuration numbers MUST be different from those used for potential
   configurations.  This restriction is especially important if the
   offerer does not require cmed-v0 capability and the recipient of the
   offer doesn't support it.  If the "lcfg" attribute is not recognized,
   the capability attributes intended to be associated with it may be
   confused with those associated with a potential configuration of some
   other media stream.  Note also that leading zeroes are not permitted
   in configuration numbers.

   If a cryptographic attribute, such as the SDES "a=crypto:" attribute
   [RFC4568], is referenced by a latent configuration through an "acap"
   attribute, any keying material required in the conventional
   attribute, such as the SDES key/salt string, MUST be included in
   order to satisfy formatting rules for the attribute.  Since the
   keying material will be visible but not actually used at this stage
   (since it's a latent configuration), the value(s) of the keying
   material MUST NOT be a real value used for real exchange of media,
   and the receiver of the "lcfg" attribute MUST ignore the value(s).

3.3.6. Enhanced Potential Configuration Attribute

The present work requires new extensions (parameters) for the "pcfg" attribute defined in the SDP capability negotiation base protocol [RFC5939]. The parameters and their definitions are "borrowed" from the definitions provided for the latent configuration attribute in Section 3.3.5. The expanded ABNF definition of the "pcfg" attribute is a=pcfg: <config-number> [<pot-cfg-list>] where config-number = 1*DIGIT ;defined in [RFC5234] pot-cfg-list = pot-config *(1*WSP pot-config) pot-config = attribute-config-list / ;def in [RFC5939] transport-protocol-config-list / ;defined in [RFC5939] extension-config-list / ;[RFC5939] media-config-list / ; Section payload-number-config-list ; Section Except for the extension-config-list, the pot-cfg-list MUST NOT contain more than one instance of each parameter list.
Top   ToC   RFC6871 - Page 26 Returning Capabilities in the Answer
Potential and/or latent configuration attributes may be returned within an answer SDP to indicate the ability of the answerer to support alternative configurations of the corresponding stream(s). For example, an offer may include multiple potential configurations for a media stream and/or latent configurations for additional streams. The corresponding answer will indicate (via an "acfg" attribute) the configuration accepted and used to construct the base configuration for each active media stream in the reply, but the reply MAY also contain potential and/or latent configuration attributes, with parameters, to indicate which other offered configurations would be acceptable. This information is useful if it becomes desirable to reconfigure a media stream, e.g., to reduce resource consumption. When potential and/or latent configurations are returned in an answer, all numbering MUST refer to the configuration and capability attribute numbering of the offer. The offered capability attributes need not be returned in the answer. The answer MAY include additional capability attributes and/or configurations (with distinct numbering). The parameter values of any returned "pcfg" or "lcfg" attributes MUST be a subset of those included in the offered configurations and/or those added by the answerer; values MAY be omitted only if they were indicated as alternative sets, or optional, in the original offer. The parameter set indicated in the returned "acfg" attribute need not be repeated in a returned "pcfg" attribute. The answerer MAY return more than one "pcfg" attribute with the same configuration number if it is necessary to describe selected combinations of optional or alternative parameters. Similarly, one or more session capability attributes ("a=sescap") MAY be returned to indicate which of the offered session capabilities is/are supportable by the answerer (see Section 3.3.8). Note that, although the answerer MAY return capabilities beyond those included by the offerer, these capabilities MUST NOT be used to form any base level media description in the answer. For this reason, it is advisable for the offerer to include most, if not all, potential and latent configurations it can support in the initial offer, unless the size of the resulting SDP is a concern. Either party MAY later announce additional capabilities by renegotiating the session in a second offer/answer exchange.
Top   ToC   RFC6871 - Page 27 Payload Type Number Mapping
When media format capabilities defined in "rmcap" attributes are used in potential configuration lines, the transport protocol uses RTP and it is necessary to assign payload type numbers. In some cases, it is desirable to assign different payload type numbers to the same media format capability when used in different potential configurations. One example is when configurations for AVP and SAVP are offered: the offerer would like the answerer to use different payload type numbers for encrypted and unencrypted media, so the offerer can decide whether or not to render early media that arrives before the answer is received. For example, if use of AVP was selected by the answerer, then media received by the offerer is not encrypted; hence, it can be played out prior to receiving the answer. Conversely, if SAVP was selected, cryptographic parameters and keying material present in the answer may be needed to decrypt received media. If the offer configuration indicated that AVP media uses one set of payload types and SAVP a different set, then the offerer will know whether media received prior to the answer is encrypted or not by simply looking at the RTP payload type number in the received packet. This association of distinct payload type number(s) with different transport protocols requires a separate pcfg line for each protocol. Clearly, this technique cannot be used if the number of potential configurations exceeds the number of possible payload type numbers. Processing of Media-Format-Related Conventional Attributes for Potential Configurations
When media capabilities negotiation is employed, SDP records are likely to contain conventional attributes such as "rtpmap", "fmtp", and other media-format-related lines, as well as capability attributes such as "rmcap", omcap, "mfcap", and "mscap" that map into those conventional attributes when invoked by a potential configuration. In such cases, it MAY be appropriate to employ the delete-attributes option [RFC5939] in the attribute configuration list parameter in order to avoid the generation of conflicting "fmtp" attributes for a particular configuration. Any media-specific attributes in the media block that refer to media formats not used by the potential configuration MUST be ignored.
Top   ToC   RFC6871 - Page 28
   For example:

      o=- 25678 753849 IN IP4
      c=IN IP4
      t=0 0
      m=audio 3456 RTP/AVP 0 18 100
      a=rtpmap:100 telephone-event
      a=fmtp:100 0-11
      a=rmcap:1 PCMU/8000
      a=rmcap:2 G729/8000
      a=rmcap:3 telephone-event/8000
      a=mfcap:3 0-15
      a=pcfg:1 m=2,3|1,3 a=-m pt=1:0,2:18,3:100

   In this example, PCMU is media capability 1, G729 is media capability
   2, and telephone-event is media capability 3.  The a=pcfg:1 line
   specifies that the preferred configuration is G.729 with extended
   DTMF events, second is G.711 mu-law with extended DTMF events, and
   the base media-level attributes are to be deleted.  Intermixing of
   G.729, G.711, and "commercial" DTMF events is least preferred (the
   base configuration provided by the "m=" line, which is, by default,
   the least preferred configuration).  The "rtpmap" and "fmtp"
   attributes of the base configuration are replaced by the "rmcap" and
   "mfcap" attributes when invoked by the proposed configuration.

   If the preferred configuration is selected, the SDP answer will look
   like the following

      o=- 25678 753849 IN IP4
      c=IN IP4
      t=0 0
      m=audio 3456 RTP/AVP 18 100
      a=rtpmap:100 telephone-event/8000
      a=fmtp:100 0-15
      a=acfg:1 m=2,3 pt=1:0,2:18,3:100
Top   ToC   RFC6871 - Page 29

3.3.7. Substitution of Media Payload Type Numbers in Capability Attribute Parameters

In some cases, for example, when an RFC 2198 [RFC2198] redundancy audio subtype (RED) capability is defined in an "mfcap" attribute, the parameters to an attribute may contain payload type numbers. Two options are available for specifying such payload type numbers. They may be expressed explicitly, in which case they are bound to actual payload types by means of the payload type number parameter (pt=) in the appropriate potential or latent configuration. For example, the following SDP fragment defines a potential configuration with redundant G.711 mu-law m=audio 45678 RTP/AVP 0 a=rtpmap:0 PCMU/8000 a=rmcap:1 PCMU/8000 a=rmcap:2 RED/8000 a=mfcap:2 0/0 a=pcfg:1 m=2,1 pt=2:98,1:0 The potential configuration is then equivalent to m=audio 45678 RTP/AVP 98 0 a=rtpmap:0 PCMU/8000 a=rtpmap:98 RED/8000 a=fmtp:98 0/0 A more general mechanism is provided via the parameter substitution rule. When an "mfcap", "mscap", or "acap" attribute is processed, its arguments will be scanned for a payload type number escape sequence of the following form (in ABNF): ptn-esc = "%m=" media-cap-num "%" ; defined in Section 3.3.1 If the sequence is found, the sequence is replaced by the payload type number assigned to the media capability number, as specified by the "pt=" parameter in the selected potential configuration; only actual payload type numbers are supported -- wildcards are excluded. The sequence "%%" (null digit string) is replaced by a single percent sign and processing continues with the next character, if any.
Top   ToC   RFC6871 - Page 30
   For example, the above offer sequence could have been written as

      m=audio 45678 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      a=rmcap:1 PCMU/8000
      a=rmcap:2 RED/8000
      a=mfcap:2 %m=1%/%m=1%
      a=pcfg:1 m=2,1 pt=2:98,1:0

   and the equivalent SDP is the same as above.

3.3.8. The Session Capability Attribute

Potential and latent configurations enable offerers and answerers to express a wide range of alternative configurations for current and future negotiation. However, in practice, it may not be possible to support all combinations of these configurations. The session capability attribute provides a means for the offerer and/or the answerer to specify combinations of specific media stream configurations that it is willing and able to support. Each session capability in an offer or answer MAY be expressed as a list of required potential configurations, and MAY include a list of optional potential and/or latent configurations. The choices of session capabilities may be based on processing load, total bandwidth, or any other criteria of importance to the communicating parties. If the answerer supports media capabilities negotiation, and session configurations are offered, it MUST accept one of the offered configurations, or it MUST refuse the session. Therefore, if the offer includes any session capabilities, it SHOULD include all the session capabilities the offerer is willing to support. The session capability attribute is a session-level attribute described by "a=sescap:" <session num> <list of configs>
Top   ToC   RFC6871 - Page 31
   which corresponds to the standard value attribute definition with

           att-field        = "sescap"
           att-value        = session-num 1*WSP list-of-configs
                              [1*WSP optional-configs]
           session-num      = NonZeroDigit *9(DIGIT)  ; DIGIT defined
                                                      ; in RFC 5234
           list-of-configs  = alt-config *("," alt-config)
           optional-configs = "[" list-of-configs "]"
           alt-config       = config-number *("|" config-number)

   The session-num identifies the session: a lower-number session is
   preferred over a higher-number session, and leading zeroes are not
   permitted.  Each alt-config list specifies alternative media
   configurations within the session; preference is based on config-num
   as specified in RFC 5939 [RFC5939].  Note that the session preference
   order, when present, takes precedence over the individual media
   stream configuration preference order.

   Use of session capability attributes requires that configuration
   numbers assigned to potential and latent configurations MUST be
   unique across the entire session; RFC 5939 [RFC5939] requires only
   that "pcfg" configuration numbers be unique within a media
   description.  Also, leading zeroes are not permitted.

   As an example, consider an endpoint that is capable of supporting an
   audio stream with either one H.264 video stream or two H.263 video
   streams with a floor control stream.  In the latter case, the second
   video stream is optional.  The SDP offer might look like the
   following (offering audio, an H.263 video streams, BFCP and another
   optional H.263 video stream) -- the empty lines are added for
   readability only (not part of valid SDP):

      o=- 25678 753849 IN IP4
      c=IN IP4
      t=0 0
      a=sescap:2 1,2,5,[3]
      a=sescap:1 1,4

      m=audio 54322 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
Top   ToC   RFC6871 - Page 32
      m=video 22344 RTP/AVP 102
      a=rtpmap:102 H263-1998/90000
      a=fmtp:102 CIF=4;QCIF=2;F=1;K=1
      i=main video stream
      a=rmcap:1 H264/90000
      a=mfcap:1 profile-level-id=42A01E; packetization-mode=2
      a=acap:1 label:13
      a=pcfg:4 m=1 a=1 pt=1:104

      m=video 33444 RTP/AVP 103
      a=rtpmap:103 H263-1998/90000
      a=fmtp:103 CIF=4;QCIF=2;F=1;K=1
      i=secondary video (slides)

      m=application 33002 TCP/BFCP *
      a=floorid:1 m-stream:11 12

   If the answerer understands MediaCapNeg, but cannot support the
   Binary Floor Control Protocol, then it would respond with (invalid
   empty lines in SDP included again for readability):

      o=- 25678 753849 IN IP4
      c=IN IP4
      t=0 0
      a=sescap:1 1,4

      m=audio 23456 RTP/AVP 0
      a=rtpmap:0 PCMU/8000

      m=video 41234 RTP/AVP 104
      a=rtpmap:104 H264/90000
      a=fmtp:104 profile-level-id=42A01E; packetization-mode=2
      a=acfg:4 m=1 a=1 pt=1:104
Top   ToC   RFC6871 - Page 33
      m=video 0 RTP/AVP 103

      m=application 0 TCP/BFCP *

   An endpoint that doesn't support media capabilities negotiation, but
   does support H.263 video, would respond with one or two H.263 video
   streams.  In the latter case, the answerer may issue a second offer
   to reconfigure the session to one audio and one video channel using
   H.264 or H.263.

   Session capabilities can include latent capabilities as well.  Here's
   a similar example in which the offerer wishes to initially establish
   an audio stream, and prefers to later establish two video streams
   with chair control.  If the answerer doesn't understand Media CapNeg,
   or cannot support the dual video streams or flow control, then it may
   support a single H.264 video stream.  Note that establishment of the
   most favored configuration will require two offer/answer exchanges.

      o=- 25678 753849 IN IP4
      c=IN IP4
      t=0 0
      a=sescap:1 1,3,4,5
      a=sescap:2 1,2
      a=sescap:3 1

      a=rmcap:1 H263-1998/90000
      a=mfcap:1 CIF=4;QCIF=2;F=1;K=1
      a=tcap:1 RTP/AVP TCP/BFCP
      m=audio 54322 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 22344 RTP/AVP 102
      a=rtpmap:102 H264/90000
      a=fmtp:102 profile-level-id=42A01E; packetization-mode=2
      a=lcfg:3 mt=video t=1 m=1 a=31,32
      a=acap:31 label:12
      a=acap:32 content:main
      a=lcfg:4 mt=video t=1 m=1 a=41,42
      a=acap:41 label:13
      a=acap:42 content:slides
Top   ToC   RFC6871 - Page 34
      a=lcfg:5 mt=application m=51 t=51
      a=tcap:51 TCP/BFCP
      a=omcap:51 *
      a=acap:51 setup:passive
      a=acap:52 connection:new
      a=acap:53 floorid:1 m-stream:12 13
      a=acap:54 floor-control:s-only
      a=acap:55 confid:4321
      a=acap:56 userid:1234

   In this example, the default offer, as seen by endpoints that do not
   understand capabilities negotiation, proposes a PCMU audio stream and
   an H.264 video stream.  Note that the offered lcfg lines for the
   video streams don't carry "pt=" parameters because they're not needed
   (payload type numbers will be assigned in the offer/answer exchange
   that establishes the streams).  Note also that the three "rmcap",
   "mfcap", and "tcap" attributes used by "lcfg:3" and "lcfg:4" are
   included at the session level so they may be referenced by both
   latent configurations.  As per Section 3.3, the media attributes
   generated from the "rmcap", "mfcap", and "tcap" attributes are always
   media-level attributes.  If the answerer supports Media CapNeg, and
   supports the most desired configuration, it would return the
   following SDP:

      o=- 25678 753849 IN IP4
      c=IN IP4
      t=0 0
      a=sescap:1 1,3,4,5
      a=sescap:2 1,2
      a=sescap:3 1
      m=audio 23456 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 0 RTP/AVP 102
      a=lcfg:3 mt=video t=1 m=1 a=31,32
      a=lcfg:4 mt=video t=1 m=1 a=41,42
      a=lcfg:5 mt=application t=2

   This exchange supports immediate establishment of an audio stream for
   preliminary conversation.  This exchange would presumably be followed
   at the appropriate time with a "reconfiguration" offer/answer
   exchange to add the video and chair control streams.

(next page on part 3)

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