Section 188.8.131.52). 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).
a=rmcap:<media-cap-num-list> <encoding-name>/<clock-rate> [/<encoding-parms>] 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 numbers. 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
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.
For example: v=0 o=- 24351 621814 IN IP4 192.0.2.2 s= c=IN IP4 192.0.2.2 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 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
Note that media format parameters can be used with RTP-based and non- RTP-based media formats. 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.
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 codec: 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
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; \ mode-set=0,3,5,6 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 answer. 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 = "*"
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 attribute. 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
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 Section 3.3.5. 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. RFC5939]. In ABNF:
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 [RFC5939]. 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. 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.
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.
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
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). 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 184.108.40.206 payload-number-config-list ; Section 220.127.116.11 Except for the extension-config-list, the pot-cfg-list MUST NOT contain more than one instance of each parameter list.
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.
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.
For example: v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=creq:med-v0 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 a=pcfg:2 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 v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=csup:med-v0 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
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.
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.
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): v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=creq:med-v0 a=sescap:2 1,2,5, a=sescap:1 1,4 m=audio 54322 RTP/AVP 0 a=rtpmap:0 PCMU/8000 a=pcfg:1
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=label:11 a=pcfg:2 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) a=label:12 a=pcfg:3 m=application 33002 TCP/BFCP * a=setup:passive a=connection:new a=floorid:1 m-stream:11 12 a=floor-control:s-only a=confid:4321 a=userid:1234 a=pcfg:5 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): v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.22 t=0 0 a=csup:med-v0 a=sescap:1 1,4 m=audio 23456 RTP/AVP 0 a=rtpmap:0 PCMU/8000 a=acfg:1 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
m=video 0 RTP/AVP 103 a=acfg:3 m=application 0 TCP/BFCP * a=acfg:5 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. v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=creq:med-v0 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 a=pcfg:1 m=video 22344 RTP/AVP 102 a=rtpmap:102 H264/90000 a=fmtp:102 profile-level-id=42A01E; packetization-mode=2 a=label:11 a=content:main a=pcfg: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
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: v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.22 t=0 0 a=csup:med-v0 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 a=acfg:1 m=video 0 RTP/AVP 102 a=pcfg:2 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.