RFC4028 Session Timers in the SIP

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RFC4028 Session Timers in Session Initiation Protocol
http://www.faqs.org/rfcs/rfc4028.html


Abstract:
This document defines an extension to the Session Initiation Protocol
   (SIP).  This extension allows for a periodic refresh of SIP sessions
   through a re-INVITE or UPDATE request.  The refresh allows both user
   agents and proxies to determine whether the SIP session is still
   active.  The extension defines two new header fields:
   Session-Expires, which conveys the lifetime of the session, and
   Min-SE, which conveys the minimum allowed value for the session
   timer.

note:
SIP里面的session timer是很重要的，在IMS网络中，网元之间必须要有heart-beat
机制，用来保证session的有效性。如果session不再有效，应该及时回收资源。