Subject: [Sipping] Opening a discussion on Multiple Path Retry

From: [email-masked]
Date: Sun, 18 Nov 2007 22:34:02 -0500

In my employer's PBX system, we're starting to consider problems that
show up when there are multiple ways of getting a call to the
destination, and we want the PBX to try alternatives only if the first
attempt failed.  The problem is, SIP doesn't make it particularly
clear whether a call failed due to some difficulty with transport
vs. the call reached the UA and was actively or passively rejected.

I've written an I-D to start the discussion of solutions to this
problem:

http://www.ietf.org/internet-drafts/draft-worley-redundancy-response-02.txt

A new version of I-D, draft-worley-redundancy-response-02.txt has been
successfuly submitted by Dale Worley and posted to the IETF
repository.

Filename:        draft-worley-redundancy-response
Revision:        02
Title:           Supporting Multiple Path Routing in the Session
Initiation Protocol (SIP)
Creation_date:   2007-11-18
WG ID:           Independent Submission
Number_of_pages: 9

Abstract:
An increasing number of SIP architectures implement multiple path
routing (MPR), which is the providing of more than one path for a
call to reach a destination user agent (UA).  To implement MPR well,
the proxy which forks a request onto the redundant paths needs to be
able to determine if a fork that failed reached the destination UA
and was rejected by the UA (and so an alternate path should not be
tried), or if the fork failed before reaching the UA (and so an
alternate path should be attempted).  This document is to begin a
discussion of strategies for making this determination.

Dale

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---

From: "Christer Holmberg" <[email-masked]>
Date: Mon, 19 Nov 2007 07:31:53 +0100

Hi Dale,

Chapter 1 says:

"If the forked request failed and did not reach the destination UA, then
in order implement MPR, the proxy must attempt the next fork.
But if the request did reach the destination UA, and the UA returneda
failure response, sending the request to the same UA via a
different path is unlikely to yield success, and may even degrade the
user experience."

How do you define "same UA"? Based on the IP address in the Contact?

Regarding the problem with proxies forwarding only the "best" final
response (chapter 2), have you considered whether the "199 mechanism"
could be useful?

Regards,

Christer

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]] 
> Sent: 19. marraskuuta 2007 5:34
> To: [email-masked]
> Subject: [Sipping] Opening a discussion on Multiple Path Retry
> 
> In my employer's PBX system, we're starting to consider 
> problems that show up when there are multiple ways of getting 
> a call to the destination, and we want the PBX to try 
> alternatives only if the first attempt failed.  The problem 
> is, SIP doesn't make it particularly clear whether a call 
> failed due to some difficulty with transport vs. the call 
> reached the UA and was actively or passively rejected.
> 
> A new version of I-D, draft-worley-redundancy-response-02.txt 
> has been successfuly submitted by Dale Worley and posted to 
> the IETF repository.
> 
> An increasing number of SIP architectures implement multiple 
> path routing (MPR), which is the providing of more than one 
> path for a call to reach a destination user agent (UA).  To 
> implement MPR well, the proxy which forks a request onto the 
> redundant paths needs to be able to determine if a fork that 
> failed reached the destination UA and was rejected by the UA 
> (and so an alternate path should not be tried), or if the 
> fork failed before reaching the UA (and so an alternate path 
> should be attempted).  This document is to begin a discussion 
> of strategies for making this determination.
> 
> Dale
> 
> 
> Use [email-masked] for questions on current 
> sip Use [email-masked] for new developments of core SIP
>

---

From: [email-masked]
Date: Mon, 19 Nov 2007 18:53:57 -0500

From: "Christer Holmberg" <[email-masked]>

Naively, I mean "the same physical UA".  How one *determines* that is
a question.  But even "the same UA" may a can of worms.  And I note:
'[T]he question of when two destinations are to be considered to be
"the same", and so constitute an MPR set.'

I haven't *considered* it, but it does sound like it would be a useful
approach.

Dale

---

From: Cullen Jennings <[email-masked]>
Date: Thu, 29 Nov 2007 19:37:09 -0800

Could you call out the problem a bit more specifically here to help  
me understand. What error codes are getting returned on things an MPR  
would fix?  I'm not saying there is not problem here, I just am not  
seeing exactly what the problem is. I hope that understanding this  
would make it clearer if we needed a few new error codes, separate  
classes, or just clarification of the existing error codes.

Cullen <with my individual contributor hat on>