Table 6.3 (continued)
LINE DESCRIPTION
From: User A <sip:[email protected]> The Fromheader contains a display
;tag=34kd92kfs name (User A) followed by the URI
of the request recipient enclosed in
<>(sip:[email protected]). The
tagparameter is a pseudo random
string generated uniquely for each
dialog.
Call-ID: 4r59899D8g10c3413 The Call-IDheader contains a
unique identifier for this call
(session). It is usually made up of a
locally unique pseudorandom string.
All requests and responses during
the call will contain this same
Call-ID.
Max-Forwards: 70 The Max-Forwardsheader field is a
hop count that is decremented by
each proxy server that forwards a
request. When the count goes to
zero, a 483 Too Many Hops
response is returned.
CSeq: 1 INVITE CSeqis the Command Sequence
number, which contains an integer
( 1 ) a space, then the request
method (INVITE). Each successive
request (command) during the call
will have a higher CSeqnumber. The
caller and called parties each main-
tain their own separate CSeqcounts.
Contact: sip:[email protected] Contactcontains one or more SIP
URIs that provide information for the
other party in the session to contact
User A.
Content-Length: 126 Content-Lengthis the octet (byte)
count of the message body ( 126 )
that follows the list of SIP headers
and is separated from the headers
by a single CRLF. A Content-
Lengthof 0 indicates no message
body.The details of SIP headers will be discussed as needed in the explanations
that follow. For a full description and examples of all SIP headers, see [7].
SIP Overview 105