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 spam bots, 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

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