Subject: This won't work, was: RE: [SIP] T2 and 182 Queued

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 26 Jan 2001 13:34:24 -0500

> -----Original Message-----
> From: Rick Dean [mailto:[email-masked]]
> Sent: Wednesday, January 24, 2001 2:45 PM
> To: [email-masked]
> Subject: Re: [SIP] T2 and 182 Queued
> 
> 
> 
> An algorithm I am familiar with and is allowable
> by the standard, is the UAC retransmits
> an INVITE (same cSeq) ~30 seconds after a 
> provisional response is received to see if the 
> far end is still alive.  Because non-response to 
> a single INVITE is not conclusive,  the UAC
> restarts the 0.5, 1, 2, 4, 4,... sequence at
> this time.  If no response after N INVITES
> (where N <= 7 allowable in the spec) plus some
> time for the far end to do a sequence, the 
> call is considered dead.  This algorithm allows 
> the UAC to decide how frequently he wants to 
> check on the far end.  In the normal case
> each check costs two packets.

This doesn't work.

It doesn't work because stateful proxies between the caller and callee will
absorb these retransmissions. Your scheme will generate lots of traffic just
to verify that your high availability proxy is, in fact, still available,
and tell you absolutely zero about the availability of the far end UAS. 

-Jonathan R.
---
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (973) 952-5000
http://www.dynamicsoft.com

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