n a NOTIFY request. 4.1.10 MESSAGE The MESSAGE method [13] is used to transport instant messages (IM) using SIP. IM usually consists of short message exchanged in near-real time by SIP Request Messages Table 4.10 Mandatory Header Fields for a NOTIFY Call-ID Max-Forwards Contact Event Subscription-State Allow-Events participants engaged in a "conversation." MESSAGES may be sent within a dia- log or outside a dialog, but they do not establish a dialog by themselves. The actual message content is carried in the message body as a MIME attachment. All UAs that support the MESSAGE method must support plain / text format, they may support other formats such as message/cpim [14] or text/html, or many others. A MESSAGE request normally receives a 200 OK response to indicate that the message has been delivered to the final destination. An Instant Message response should not be sent in the message body of a 200 OK, but rather a separate MESSAGE request sent to the original sender. A 202 Accepted response indicates that the request has reached a store-and-forward device and will likely eventually be delivered to the final destination. In neither case does the 2xx response confirm that the message content has been rendered to the user. A MESSAGE request may use the im (Instant Message) URI scheme [14] in a Request-URI, although a client should try to resolve to a sip or sips. An example MESSAGE call flow is shown in Figure 4.9. Note that the MESSAGE method is not the only application of instant messaging with SIP. It is also possible to use SIP to establish an instant message session in a completely analogous way that SIP is commonly used to establish a media session. An INVITE could be used to establish the session with a SDP body that describes the instant message protocol to be used directly between the two users. IM sessions have been proposed using SIP MESSAGE, Common Presence and Instant Messaging (CPIM) [14], and even Jabber [15]. An exam- ple call flow is shown in Figure 4.10. SIP: Understanding the Session Initiation