Instant Messaging (IM) refers to the transfer of messages between users in near real-time. These messages are usually, but not required to be, short. IMs are often used in a conversational mode, that is, the transfer of messages back and forth is fast enough for participants to maintain an interactive conversation.

This document proposes the MESSAGE method, an extension to the Session Initiation Protocol (SIP) that allows the transfer of Instant Messages. Since the MESSAGE request is an extension to SIP, it inherits all the request routing and security features of that protocol. MESSAGE requests carry the content in the form of MIME body parts. MESSAGE requests do not themselves initiate a SIP dialog; under normal usage each Instant Message stands alone, much like pager messages. MESSAGE requests may be sent in the context of a dialog initiated by some other SIP request.

The size of MESSAGE requests outside of a media session MUST NOT exceed 1300 bytes, unless the UAC has positive knowledge that the message will not traverse a congestion-unsafe link at any hop, or that the message size is at least 200 bytes less than the lowest MTU value found en route to the UAS. Larger payloads may be sent as part of a media session, or using some type of content-indirection.

To prevent the replay of old SIP requests, all signed MESSAGE requests and responses MUST contain a Date header field covered by the message signature. Any message with a date older than several minutes in the past, or which is more than several minutes in the future, SHOULD be answered with a 400 (Incorrect Date or Time) message, unless such messages arrive repeatedly from the same source, in which case they MAY be discarded without sending a response.