[https://tools.ietf.org/html/draft-ietf-mmusic-msid-08](https://tools.ietf.org/html/draft-ietf-mmusic-msid-08) - NB: Removed in subsequent drafts, most likely will not be used eventually. Used here for compatibility with Google Chrome and Mozilla Firefox. A session-level attribute is defined for signalling the semantics associated with an msid grouping.This allows msid groupings with different semantics to coexist. This OPTIONAL attribute gives the group identifier and its group semantic; it carries the same meaning as the ssrc-group-attr of RFC 5576 section 4.2, but uses the identifier of the group rather than a list of SSRC values. This attribute MUST be present if "a=msid" is used. An empty list of identifiers is an indication that the sender supports the indicated semantic, but has no msid groupings of the given type in the present SDP. An identifier of "*" is an indication that all "a=msid" lines in the SDP have this specific semantic. If "*" is not used, each msid-id in the SDP MUST appear in one and only one "msid-semantic" line. The name of the attribute is "msid-semantic". The value of the attribute is given by the following ABNF: msid-semantic-value = msid - semantic msid-list msid-semantic = token ; see RFC 4566 msid-list = *(" " msid-id) / " *" The semantic field holds values from the IANA registriy "Semantics for the msid-semantic SDP attribute" (which is defined in Section 6 of [https://tools.ietf.org/html/draft-ietf-mmusic-msid-08](https://tools.ietf.org/html/draft-ietf-mmusic-msid-08) ). An example msid-semantic might look like this, if a semantic LS was registered by IANA for the same purpose as the existing LS grouping semantic: a= msid-semantic:LS xyzzy forolow This means that the SDP description has two lip sync groups, with the group identifiers xyzzy and forolow, respectively. The msid-semantic attribute can occur more than once, but MUST NOT occur more than once with the same msid-semantic value.