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Page 7 of 16
20.16 CSeq
A CSeq header field in a request contains a single decimal sequence
number and the request method. The sequence number MUST be
expressible as a 32-bit unsigned integer. The method part of CSeq is
case-sensitive. The CSeq header field serves to order transactions
within a dialog, to provide a means to uniquely identify
transactions, and to differentiate between new requests and request
retransmissions. Two CSeq header fields are considered equal if the
sequence number and the request method are identical. Example:
CSeq: 4711 INVITE
20.17 Date
The Date header field contains the date and time. Unlike HTTP/1.1,
SIP only supports the most recent RFC 1123 [20] format for dates. As
in [H3.3], SIP restricts the time zone in SIP-date to "GMT", while
RFC 1123 allows any time zone. An RFC 1123 date is case-sensitive.
The Date header field reflects the time when the request or response
is first sent.
Rosenberg, et. al. Standards Track [Page 170]
RFC 3261 SIP: Session Initiation Protocol June 2002
The Date header field can be used by simple end systems without a
battery-backed clock to acquire a notion of current time.
However, in its GMT form, it requires clients to know their offset
from GMT.
Example:
Date: Sat, 13 Nov 2010 23:29:00 GMT
20.18 Error-Info
The Error-Info header field provides a pointer to additional
information about the error status response.
SIP UACs have user interface capabilities ranging from pop-up
windows and audio on PC softclients to audio-only on "black"
phones or endpoints connected via gateways. Rather than forcing a
server generating an error to choose between sending an error
status code with a detailed reason phrase and playing an audio
recording, the Error-Info header field allows both to be sent.
The UAC then has the choice of which error indicator to render to
the caller.
A UAC MAY treat a SIP or SIPS URI in an Error-Info header field as if
it were a Contact in a redirect and generate a new INVITE, resulting
in a recorded announcement session being established. A non-SIP URI
MAY be rendered to the user.
Examples:
SIP/2.0 404 The number you have dialed is not in service
Error-Info: <sip: This e-mail address is being protected from spambots. You need JavaScript enabled to view it >
20.19 Expires
The Expires header field gives the relative time after which the
message (or content) expires.
The precise meaning of this is method dependent.
The expiration time in an INVITE does not affect the duration of the
actual session that may result from the invitation. Session
description protocols may offer the ability to express time limits on
the session duration, however.
The value of this field is an integral number of seconds (in decimal)
between 0 and (2**32)-1, measured from the receipt of the request.
Example:
Expires: 5
