[SIPForum-discussion] What is Session timer ?

[Pranav Damele]

Hello Khoa.

SIP Session Timers allows a periodic refreshing of a SIP session using the
RE-INVITE message. The refreshing allows both the user agent and proxy to
determine if the SIP session is still active. The SIP Session Timer is a
keep alive mechanism for SIP sessions that allow User Agents (UA) or
proxies to determine the status of a session and to release it if it is not
active, even if a BYE has not been received.
When a UAC (User Agent Client) sends an INVITE, it includes a supported
header field with the option tag 'timer'. This indicates support for this
extension. The request passes through proxies, any one of which may have an
interest in establishing a session timer. Each proxy can insert a
Session-Expires header field and a Min-SE header field into the request (if
none is already there) or alter the value of existing Session-Expires and
Min-SE header fields.
If the Session-Expires interval is too low for a proxy (lower than the
value of Min-SE that the proxy would wish to assert), the proxy rejects the
request with a 422 response. That response contains a Min-SE header field
identifying the minimum session interval it is willing to support. The UAC
will try again, this time including the Min-SE header field in the request.
The header field contains the largest Min-SE header field it observed in
all 422 responses previously received. This way, the minimum timer meets
the constraints of all proxies along the path.
After several INVITE/422 iterations, the request eventually arrives at the
UAS. The UAS can adjust the value of the session interval as if it were a
proxy; when done, it places the final session interval into the
Session-Expires header field in a 2xx response. The Session-Expires header
field also contains a 'refresher' parameter, which indicates who is doing
the refreshing -- the UA that is currently the UAC, or the UA that is
currently the UAS. As the 2xx response travels back through the proxy
chain, each proxy can observe the final session interval but can't change
it.
>From the Session-Expires header field in the response, both UA's know that
a session timer is active, when it will expire, and who is refreshing. At
some point before the expiration, the currently active refresher generates
a session refresh request, which is a re-INVITE or UPDATE request. If the
refresher never gets a response to that session refresh request, it sends a
BYE to terminate the session. Similarly, if the other side never gets the
session refresh request before the session expires, it sends a BYE.
The refresh requests sent once the session is established are processed
identically to the initial requests, as described above. This means that a
successful session refresh request will extend the session, as desired.

*Pranav Damele*
Senior Engineer - Testing
 *Mobile:* +91-9818-92-4466
*Email:* pranavdamele at gmail.com
*IM:* pranavdamele (Skype)
 *http://in.linkedin.com/in/pranavdamele*


On 27 February 2013 12:45, Khoa Pham <onmyway133 at gmail.com> wrote:

> Session timer is defined in RFC 4028, but what is it used for ?
> Is that the max duration a call can be ?
>
> --
> Khoa Pham
> HCMC University of Science
> Faculty of Information Technology
>
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