[SIPForum-discussion] What is Session timer ?

[Khoa Pham]

@Sasikanth: Thanks. I used SIP workbench to log SIP traffic. And found that
it is the CLIENT who is sending the UPDATE request, not the server

@Pranav Damele: Thanks. I see the UA sending UPDATE, not re-INIVITE. Says
the Session timer is 100s, I see the UA
sending UPDATE every 50s (half the Session timer)

And, if the other UA is not responding. We must wait 50s. It is a very long
long time. How to overcome this ?



On Thu, Feb 28, 2013 at 2:14 AM, Pranav Damele <pranavdamele at gmail.com>wrote:

> 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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>


-- 
Khoa Pham
HCMC University of Science
Faculty of Information Technology
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