The main role of the MSRPRelay is to help NAT traversal of media sessions between endpoints located behind NAT. It can also be used for lawful interception or enterprise policy for IM sessions. This extension is defined in RFC 4976.
Message Session Relay Protocol (MSRP) is a protocol for transmitting a series of related instant messages in the context of a session. Message sessions are treated like any other media stream when set up via a rendezvous or session creation protocol such as the Session Initiation Protocol (SIP). MSRP Sessions are defined in RFC 4975.
For more information surf to http://msrprelay.org