anding the Session Initiation Protocol A tag is a cryptographically random number with at least 32 bits of random- ness, which is added to To and From headers to uniquely identify a dialog. The examples of Chapters 2 and 10 show the use of the tag header parameter. The To header in the initial INVITE will not contain a tag. A caller must include a tag in the From header, although a RFC 2543 user agent generally will not do SO as it was optional in that specification. Excluding 100 Trying, all responses will have a tag added to the To header. The sending or reception of a response containing a From tag creates an early dialog. A tag returned in a 200 OK response is then incorporated as a dialog identifier and used in all future requests for this Call- ID. A tag is never copied across calls. Any response generated by a proxy will have a tag added by the proxy. An ACK gen- erated by either a user agent or a proxy will always copy the From tag of the response in the ACK request. If a UAC receives responses containing different tags, this means that the responses are from different UASs, and hence the INVITE has been forked. It is up to the UAC as to how to deal with this situation. For example, the UAC could establish separate sessions with each of the responding UAS. The dialogs would contain the same From, Call - ID, and CSeq, but would have dif- ferent tags in the To header. The UAC also could BYE certain legs and establish only one session. Note that tags are not part of the To or From URI but are part of the header and always placed outside any "<>". 4.4 Message Bodies Message bodies in SIP may contain various types of information. They may con- tain SDP information, which can be used to convey media information or QoS or even security information. The optional Content-Disposition header is used to indicate the intended use of the message body. If not present, the function is assumed to be session, which means that the body describes a media session. Besides session, the other defined function is render, which m