sdp-media-rule

An sdp-media-rule groups all of the descriptors that are associated with a specific media-type into single entity, thus allowing the user to perform manipulation operations on a media-specific portion of the SDP. For example, a user can construct an sdp-media-rule to change an attribute of the audio media type.

Like a mime-sdp-rule, all match-value and new-value operations performed at this level are executed against the entire media-group as a complete string. Given the sample SDP below, if a media-level-descriptor is configured to operate against the application group, the match-value and new-values would operate only on designated portion.

        v=0
        o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
        s=SDP Seminar
        i=A Seminar on the session description protocol
        u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
        e=[e-mail] (Mark Handley)
        c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
        a=recvonly
        m=audio 49170 RTP/AVP 0
        m=video 51372 RTP/AVP 31
        m=application 32416 udp wb
        a=orient:portrait

A configuration parameter media-type is used to specify the media group on which to operate. It contains all of the descriptors including the m-line up to the next m-line. This parameter is a string field and must match the media-type exactly as it appears within the SDP. The special media-type media can be used to refer to all media types. This is particularly useful when performing an add operation, when the user wants to add a media section between the first and second medias, but does not know what media type they are. Otherwise, during an add operation, the media section would be added before the specified media-type (if no index parameter was provided).

The types of descriptors used at the sdp-media-rule level are m, i, c, b, k, and a, the descriptors specific to the media description.

This level of granularity affords the user a very simple way to making subtle changes to the media portion of the SDP. For instance, it is very common to have to change the name of an audio format (for example G729 converted to g729b), or to add attributes specific to a certain media-type.

The index operator is supported for the media-type parameter (for example, media-type audio[1]). Like header rules, if no index is supplied, this means operate on all media-types that match the given name. For specifying specific media-types, the non-discrete indices are also supported (for example, ^ - last). Adding a media-type, without any index supplied indicates that the media should be added at the beginning. The special media-type media uses the index as an absolute index to all media sections, while a specific media-type will index relative to that given media type.

For sdp-media-rules set to an action of add where the media-type is set to media, the actual media type is obtained from the new-value, or more specifically, the string after m= and before the first space.

Given the following SDP:

        v=0
        o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
        c=IN IP4 224.2.17.12/127
        t=2873397496 2873404696
        m=audio 49170 RTP/AVP 0
        m=audio 48324 RTP/AVP 8
        m=video 51372 RTP/AVP 31

With the sdp-media-rule:

sdp-media-rule
        name                       smr
        media-type                 audio[1]
        action                     manipulate
        comparison-type            case-sensitive
        match-value
        new-value                  "m=audio 1234 RTP/AVP 8 16"

This rule operates on the 2nd audio line, changing the port and adding another codec, resulting in the SDP:

        v=0
        o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
        c=IN IP4 224.2.17.12/127
        t=2873397496 2873404696
        m=audio 49170 RTP/AVP 0
        m=audio 1234 RTP/AVP 8 16
        m=video 51372 RTP/AVP 31

The following rule, however:

sdp-media-rule
        name                       smr
        media-type                 media[1]
        action                     add
        comparison-type            case-sensitive
        match-value
        new-value                  "m=video 1234 RTP/AVP 45"

adds a new video media-type at the 2nd position of all media-lines, resulting in the SDP:

        v=0
        o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
        c=IN IP4 224.2.17.12/127
        t=2873397496 2873404696
        m=audio 49170 RTP/AVP 0
        m=video 1234 RTP/AVP 45
        m=audio 48324 RTP/AVP 8
        m=video 51372 RTP/AVP 31