Subject: [Sip] mid-dialog failover: need URI parameter for reg-id to avoid triangular routing

From: "Jeroen van Bemmel" <[email-masked]>
Date: Sat, 28 Oct 2006 17:54:55 +0200

Jonathan,

Regarding your outbound-discovery-mid-dialog draft: I believe it can work, 
but the example given in section 15 has some glitches and may result in 
sub-optimal routing.

The backup flow from the UA x should have a different reg-id, say reg-id=*2* 
(instead of 1 as per the example in the draft). Assume there is also a user 
y registered, via proxy 1 and proxy 3. Then the connection table at the home 
proxy would look like

TLS connection to 1:
    sip:outbound.example.com
    sip:[email-masked];keepalive=stun

TLS connection to 2:
    sip:outbound.example.com
    sip:[email-masked];keepalive=stun

TLS connection to 3:
    sip:outbound.example.com
    sip:[email-masked];keepalive=stun
    sip:[email-masked];keepalive=stun

Consequently, when proxy 2 fails, there will be no "most specific" match for 
message 35, only least specific ones. The request should go to proxy 3, but 
the home proxy could equally likely select proxy 1 (which would then perform 
triangular routing via proxy 2).

To have the home proxy favor proxy 3 in this scenario, I believe we need a 
URI parameter to hold the reg-id, instead of putting it in the user part. If 
we would then define that flow selection should be performed based on the 
URI from which the reg-id parameter is stripped, there would be a partly 
specific match for proxy 3 and failover would work more smoothly

Regards,
Jeroen 
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