Subject: RE: [Sipping] stateful and stateless 503 within overload slides

From: "Brett Tate" <[email-masked]>
Date: Wed, 5 Dec 2007 13:30:43 -0500

> With stateless 503, the server sends 503s in 
> response to incoming INVITEs during overload. It does not 
> lookup up transactions and does not create one. The idea is 
> that a server can do this very quickly and respond to lots of 
> INVITEs (which is much better than dropping them). Stateful 
> 503 is regular processing of 503s.

If using UDP (or rfc2543 device), wouldn't there be a compliance or
processing issue concerning "stateless" 503 when dealing with request
retries?

Example A: Received INVITE; processing it without sending 503; now
overloaded and receive retry; 503 sent.  Isn't subsequently sending
anything other than 200 or 503 to the INVITE now basically considered an
abnormal situation?

Example B: Same as above; however 503 sent to original INVITE and the
retry is processed without returning a 503.  Isn't subsequently sending
anything other than 200 or 503 to the INVITE now basically considered an
abnormal situation?

Thanks for the response,
brett

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From: Jeroen van Bemmel <[email-masked]>
Date: Wed, 05 Dec 2007 19:41:42 +0100

Indeed, we have seen this very issue in a real deployment. Stateless 503 
responses can cause protocol violations (multiple, different final 
response for the same INVITE) at the transition from overload to 
non-overload (eg first 503 and then 200 OK for a retransmission)

There is no easy way to avoid this, except sending 503 statefully (the 
other option is to wait for a period of at least 32 seconds with no 
INVITEs, after which there shouldn't be any more retransmissions. But 
this may take forever)

Regards,
Jeroen

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From: Volker Hilt <[email-masked]>
Date: Wed, 05 Dec 2007 11:05:31 -0800

Brett,

> processing issue concerning "stateless" 503 when dealing with request
Yes, this may occur for stateless 503. Basically it is possible that two 
different responses are returned for an INVITE, first a 200 and then a 
503 or vice versa.

The main idea for looking into stateless 503 was to make sending 503 as 
quick as possible for a proxy under overload. The drawbacks are, as 
you've noted, the inconsistencies that can be created by this. The graph 
Jonathan has presented yesterday showed that if we compare the 
performance of statefull and stateless 503 at the *same* rate, the 
overall performance of statefull 503s is better since it avoids these 
inconsistencies. Thus, if the difference in processing time for 
statefull and stateless is small, statefull is perferrable, even from a 
performance point of view (not to mention the avoidance of inconsistencies).

Thanks,

Volker

-- 
Volker Hilt                      Bell Labs / Alcatel-Lucent
phone: +1 732 332 6432           101 Crawfords Corner Rd
email: [email-masked]     Holmdel, NJ 07733