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Re: [reSIProcate] DNS question


On Fri, 2006-01-13 at 12:05 +0000, Robert Mansfield wrote:

> Should the application use DNS procedures to ascertain the transport
> to reach the next hop prior to sending the message to the sip stack?
> I've run a test whereby I've submitted a INVITE request using "UDP" in
> the construct of the Via header. The DNS lookup for the target returns
> with a TCP record so the message is sent out via TCP but the text of
> the message still indicates UDP. Is this valid? 

I assume what you mean is that your INVITE has a header that looks like:

Via: SIP/2.0/UDP host.example.com;branch=...

But that the DNS SRV records for host.example.com only provide a TCP
contact.

I would say that the SIP agent sending the INVITE is erroneous, much as
if host.example.com had no DNS records but it was using that name in the
Via.  I say this because the sender of the message is asserting that
responses should be sent to "host.example.com" via UDP, but DNS is
configured such that there *no* UDP contacts for "host.example.com".

Whatever strategy the recipient uses should be considered to be attempts
to recover from erroneous input.  It would probably be better for the
recipient to just drop the transaction, as that will tend to force the
users to diagnose the situation and correct the problem with the sender.

Dale