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2.1. The RTCP Attribute
The RTCP attribute is used to document the RTCP port used for media
stream, when that port is not the next higher (odd) port number
following the RTP port described in the media line. The RTCP
attribute is a "value" attribute, and follows the general syntax
specified page 18 of [RFC2327]: "a=<attribute>:<value>". For the
RTCP attribute:
* the name is the ascii string "rtcp" (lower case),
* the value is the RTCP port number and optional address.
The formal description of the attribute is defined by the following
ABNF [RFC2234] syntax:
rtcp-attribute = "a=rtcp:" port [nettype space addrtype space
connection-address] CRLF
Huitema Standards Track [Page 2]
RFC 3605 RTCP attribute in SDP October 2003
In this description, the "port", "nettype", "addrtype" and
"connection-address" tokens are defined as specified in "Appendix A:
SDP Grammar" of [RFC2327].
Example encodings could be:
m=audio 49170 RTP/AVP 0
a=rtcp:53020
m=audio 49170 RTP/AVP 0
a=rtcp:53020 IN IP4
126.16.64.4 m=audio 49170 RTP/AVP 0
a=rtcp:53020 IN IP6 2001:2345:6789:ABCD:EF01:2345:6789:ABCD
The RTCP attribute MAY be used as a media level attribute; it MUST
NOT be used as a session level attribute. Though the examples below
relate to a method that will return only unicast addresses, both
unicast and multicast values are valid.