Information in SIP Messages

Obviously, there is more to SIP signaling than the messages outlined in Figure
8-9. To start with, each SIP request or response contains addresses for
the calling and called parties. Each such address is known as a SIP uniform
resource locator (URL) and has the format “SIP:user@domain.” This is somewhat similar to an e-mail URL, which has the format mailto:user@
domain. A SIP user might well want to have the same values for user and
domain in his or her SIP and e-mail addresses, which would make it very
easy to know how to contact a SIP user—much easier than having to
remember a telephone number.
Several requests and many responses can be sent between SIP entities.
For example, if, in the example of Figure 8-9, the called user were not available,
then the response “Temporarily Unavailable” (status code 480) could
have been returned, rather than the 200 (OK).
Not only are there several requests and many responses, many information
elements can be contained in those requests and responses. In SIP,
these information elements are known as header fields. For example, when
sending an INVITE, the message contains not only a session description
and the to and from addresses (contained in the To and From header fields),
but it can also contain a Subject header field. This field indicates the reason
for the call and can be presented to the called user, who may choose to
accept or reject the call based on the subject in question. One can easily
imagine this capability being used to filter out unwanted telemarketing
calls.
Other header fields include, for example, Call ID, Date, Timestamp, Inreply-
to, Retry-after, and Priority. The Retry-after header could be used, for
example, with the 480 (Temporarily unavailable) response to indicate when
the caller should try the call again (if ever). One of the most important
header fields is Content-type, which indicates the type of additional information
included in the message. For example, when a user issues an
INVITE message, the message includes a session description. The Contenttype
field indicates how that session description is coded so that the
receiver of the message can understand whether or not that type of session
can be supported.