[Sip] Outbound issues: slides 9 and 10

[Rohan Mahy <rohan at ekabal.com>]

The issues on slide 9 (how does a registrar verify that an edge proxy 
supports outbound) and slide 10 (what does "Supported: outbound" mean) 
deal with detecting that all the required hops support outbound.  Slide 
9 specifically uncovered a problem with outbound proxies that add a 
Path URI but don't include a flow token.

The proposed solution is as follows.  If a proxy inserts a Path header 
with a flow token consistent with the rules described in outbound, it 
MUST add the ;ob parameter to the proxy URI that has the flow token.

The registrar verifies that any Path header URIs contain the ;ob 
parameter.  If there are any Path URIs without the ob parameter, the 
registrar does not return Supported: outbound.  Furthermore, the 
registrar needs to check that the first hop after the UAC is 
represented in the Path list (unless the registrar recognizes that the 
first hop after the UAC is in the same domain and doesn't need to 
include Path URIs with a flow token).  Note that this implies that 
whatever the proxy puts in the Via (IP address or FQDN), it must put 
the same hostname in the Path header (but there is no need for a 
complete URI comparison).  If the first hop is not correctly 
represented, the registrar ignores the reg-id parameter and does not 
return Supported: outbound.

The implication of this is that an outbound-proxy that supports 
outbound needs to support Path and flow tokens for registrations to all 
foreign domains.  Inside the same domain, the first-hop proxy and the 
registrar can still use Path to communicate with each other, but they 
can use some other private mechanism to disaggregate this 
functionality.

thanks,
-rohan


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