Related specifications
RFC 4566, SDP: Session Description Protocol, available at http://www.ietf.org/
RFC 3264, An Offer/Answer Model with the Session Description Protocol (SDP), available at http://www.ietf.org/
RFC 4855, Media Type Registration of RTP Payload Formats, available at http://www.ietf.org/
RFC 4856, Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences, available at http://www.ietf.org/
RFC 3960, Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP), available at http://www.ietf.org/
RFC 3556 Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth, available at http://www.ietf.org/
3GPP 26.114 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; IP Multimedia Subsystem (IMS); Multimedia Telephony; Media handling and interaction (Release 8) v8.4, available at http://www.3gpp.org
Implementation notes
Early Media is supported. Reference to RFC 3960 is made in order to identify a specification describing the generation of a local ringing tone, should Early Media not be available.
Bandwidth modifiers are supported. Implementation of the bandwidth modifiers (b=AS:, b=RR: and b=RS:) is according to RFC 3556 and 3GPP 26.114.
The implementation is in accordance with RFC 3264 with the following exceptions:
A port number of zero is used to reject offered media for MT sessions. (Section 5.1, RFC 3264.)
Multicast streams are not supported.
Streams are marked as sendonly when generating an offer for HOLD inside a session and recvonly when generating an ANSWER to a HOLD offer. Streams are marked as recvonly in ANSWER when inactive is received in an offer. Attribute sendrecv is used when resuming from HOLD (in offer and ANSWER) only. (Section 6.1, RFC 3264).
Every ANSWER to any offer contains the most preferred supported codec only. (Section 6.1, RFC 3264.)
Packetisation interval is supported with ptime and maxptime attributes. (Section 6.1, RFC 3264.)
One media stream (audio) only is supported. (Chapter 8, RFC 3264.)
Changing the port number during a session is not supported in the MO (Mobile Originated) direction, but is supported in the MT (Mobile Terminated) direction. In other words, a new offer with a different port number is not supported, but an arrived offer from another source with a changed port number is supported. (Section 8.3.1, RFC 3264.)
Changing the transport of a stream is not supported. (Section 8.3.1, RFC 3264.)
Changing the media type during the session is not supported. (Section 8.3.3, RFC 3264.)
Receiving audio with every codec presented in sent offer is supported.