[quote]
8.1.3.3 Vias

If more than one Via header field value is present in a response, the
UAC SHOULD discard the message.

The presence of additional Via header field values that precede
the originator of the request suggests that the message was
misrouted or possibly corrupted.
[/quote]

That probably led to the confusion - at least it confused me. It only deals with *responses*, not with *requests*, and says that only the *UAC* (user agent client, the originator of the request that caused the response, which is not neccessarily a user agent) should drop the message, not any proxies in between.

UAC adds Via1
Proxy adds Via2
Proxy adds Via3
SIP arrives at destination, sends a response the same way back
Proxy removes Via3 from response
Proxy removes Via2 from response
UAC should see exactly one Via: Via1, pointing to itself

If a response with more than one Via arrives back at the UAC, something is amiss, and the message should be dropped.

Maybe, if you explain it to them in this way they accept that they indeed have to proxy/route SIP with more than 1 Via-Header, since it’s up to the originator of the request (you) to validate that rule.

Really sad if a carrier tries to justify its failures with this argument.

However, we have been using this one carrier for many months now, without even so much as considering changing anything on our end - so all along they have been dealing with the two Via headers without any issue, until today……..