Message Session Relay Protocol - Usage in SIP

Usage in SIP

MSRP can be used within a SIP session:

• to do instant messaging in a one-to-one or one-to-many mode
• to do an attachment file transfer
• to do some photo sharing (e.g., Image Share) based on prior exchange of capabilities between the user endpoints

MSRP session is set up through SIP's offer-answer model. The SDP m-line media type is message and the proto is either TCP/MSRP for MSRP over TCP and TCP/TLS/MSRP for MSRP over secure TLS. Furthermore, the MSRP URI is specified in a path attribute.

A full SDP example, as provided by the RFC:

v=0 o=alice 2890844526 2890844527 IN IP4 alice.example.com s= - c=IN IP4 alice.example.com t=0 0 m=message 7394 TCP/MSRP * a=accept-types:text/plain a=path:msrp://alice.example.com:7394/2s93i9ek2a;tcp

The fact that MSRP contains the address and port and at the same time the c-line holds the address and m-line contain the port is a cause for ambiguity. Generally, other media types use the c-line and m-line to describe the address, but the MSRP RFC 4975 states this information is actually specified by the MSRP path. This may cause some devices to improperly set up the session, in particular, a B2BUA may require to alter the path per each MSRP message between the different devices. To overcome this, RFC 6714 "CEMA for MSRP" changes the way CEMA-aware devices use SDP, which makes CEMA-aware MSRP B2BUA implementations much simpler and more efficient.