"Scope of Applicability This document describes the use of the MESSAGE method for sending instant messages using a metaphor similar to that of a two-way pager or SMS enabled handset. That is@ there are no explicit association between messages. Each IM stands alone--any sense of a ""conversation"" only exists in the client user interface@ or perhaps in the user's own imagination. We contrast this with a ""session"" model@ where there is an explicit conversation with a clear beginning and end. In the SIP environment@ an IM session would be a media session initiated with an INVITE transaction and terminated with a BYE transaction. There is value in each model. Most modern IM clients offer both user experiences. The user can choose to send an IM to a contact@ or he can choose to invite one or more contacts to join a conversation. The pager model makes sense when the user wishes to send a small number of short IMs to a single (or small number of) recipients. The session model makes sense for extended conversations@ joining chat groups@ if there is a need to associate a conversation with some other SIP initiated session@ etc. There may be a temptation to simulate a session of IMs by initiating a dialog@ then sending MESSAGE requests in the context of that dialog. This is not an adequate solution for IM sessions@ in that this approach forces the MESSAGE requests to follow the same network path as any other SIP requests@ even though the MESSAGE requests arguably carry media rather than signaling. IM applications are typically high volume@ and we expect the IM volume in sessions to be even higher. This will likely cause congestion problems if sent over a transport without congestion control@ and there is no clear mechanism in SIP to prevent some hop from forwarding a MESSAGE request over UDP. Additionally@ MESSAGE requests sent over an existing dialog must@ by the nature of SIP@ go to the same destination as any other request sent in that dialog. This prevents any separation between the IM endpoint and the signaling endpoint. This is not an acceptable limitation for the session-model of instant messaging. The authors recognize that there may be valid reasons to send MESSAGE requests in the context of a dialog. For example@ one participant in a voice session may wish to send an IM to another participant@ and associate that IM with the session. But implementations SHOULD NOT create dialogs for the primary purpose of associating MESSAGE requests with one another. Note that this statement does not prohibit using SIP to initiate a media session made up of IMs@ just like any other session. Indeed@ we expect the solution for IM sessions to use that metaphor. The reader should avoid confusing the concepts of a SIP dialog and a media session."