Subject: [Sip] T4

From: [email-masked]
Date: Mon, 25 Feb 2002 09:33:11 +0200

I included this in my comments in bis-07, but never got an answer.

Why is T4 limited to 5 seconds for timer K in the case of non-INVITE (section 17.1.2.2)?

Regards,
Hisham Khartabil

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From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 27 Feb 2002 03:15:09 -0500

> Why is T4 limited to 5 seconds for timer K in the case of non-INVITE

Sorry, I thought I had answered this somewhere.

The idea is that you've sent a bunch of BYE requests, for example, and
now you've got the final response. You want to hang around to catch any
retransmits of that final response. How long to wait? The interesting
thing about responses to BYE, is that they are not perioidcally sent;
they are sent only in response to a request. So, the delay between
sending a request, and getting a response, is equal to some value (say
T4'), which represents the amount of time it takes for the request to
get to the server, for the server to resend the response, and for the
response to arrive at the client. Let T4 be the maximum value of T4' you
might ever see, which I have set at around 5s. This should be more than
sufficient. So, this means that if you've sent a retransmit of the
request at time t, you would have received the response by t+T4 with
very high probability, or you won't get one at all. So, if a UAC sends
requests at times t1,t2,t3,t4, the responses would have arrived at
t1+T4, t2+T4, t3+T4, t4+T4. The largest of those is t4+T4. Thus, we need
to wait at least T4 seconds after the last request. Instead, we wait T4
after the first response, which is always more time (its simpler that
way). 

Hope that helps.

-Jonathan R.

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