Subject: [Sip] Maintaining transport between servers (was: Transport parm in Rec ord-Route)

From: [email-masked]
Date: Wed, 27 Jun 2001 08:18:59 -0600

I had another thought on this prompting me to rename the thread.  The goal
is to use the same transport between two servers for subsequent
transactions.  Simply putting the transport in a record-route has an
undesirable side affect.  That route could be used by the hop on either side
of the server that added it.  In the original example, if UAS sent a
subsequent request it might try to do it over TLS when all that is needed
there is UDP.  We need some mechanism in SIP to communicate a desired
transport between two specific servers in the Route list that will work for
subsequent requests in either direction.

John

> We have encountered a problem with the requirement in 16.3 where
>Record-Route MUST NOT contain the transport parameter.  I realize this was
>designed to handle the possibility of requiring a UA that does not support
a
>particular transport to use said transport, resulting in a failure.
>However, this also causes problems when the proxy must use only one
>transport for all transactions.  Take for example a proxy (B) in a private
>network that has a TLS connection with a trusted peer proxy (A) on the open
>Internet.  The flow for an Invite might be something like:
>
>UAC -> proxy (A) -> proxy (B) -> UAS
>
> Now if both proxies are record-routed without the transport parm, the
>initial transaction could be setup on TLS OK based on proxy (A) routing
>decisions, but if a subsequent transaction such as BYE came from the UAC,
>proxy (A) would see a Route header with no transport parm, use it as the
>req-URI which, lacking a transport parm results in using default UDP (see
>reference below).  Since proxy (B) only trusts and listens for requests
over
>a TLS connection, the request fails.
>
>From section 2, descrition of URL parameters:
>
>UDP is to be assumed when no explicit transport parameter is included.
>
> Perhaps some new verbage is needed describing proxy behaviour and how to
>select the transport protocol when not specified, i.e. look for open TLS or
>TCP sockets to the destination first.  This still might not help if for
>example the transport was TCP and the socket timed out due to inactivity.
>There may be other ways around this problem I have not thought through yet.
>Comments?
>
>
>John Hearty
>Level3
>

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