From: Joel Dodson <[EMAIL PROTECTED]>
UA1 UA2
---------- INVITE ---------->
<---------- 200 OK ----------
At this point UA2 roams to a different subnet and its IP address changes.
---------- ACK ---------->
The ACK is being sent to the IP address specified in the Contact
header in the 200OK, which is no longer valid.
Well, the ACK is likely to be very quick after the 200. OTOH, the 200
might be a minute or more after the INVITE, and the 200 gets to UA1
based on the address specified in the first Via. (Here we have to
assume that UA1 is mobile in order to make this interesting.) In
theory, you can put a DNS name in a Via header, but I don't know of
any phone that supports that.
I suppose if the Contact header never contained an IP address this
wouldn't be an issue, but then there has to be a separate network
component that keeps the mapping (Dynamic DNS I guess) to UA2s
current IP address. What about the SDP though? Would you put an
FQDN in the SDP?
According to RFC 4566, the connection address may be an FQDN. I've
never heard of that being supported, either.
But maybe there's no better choice -- you can't successfully identify
a network location with an address that has to be updated more
frequently than the updating mechanism makes possible. In SIP, the
worst case is a prolonged INVITE transaction, which can last for
minutes. If your IP address becomes invalid with less warning than
that, you've got to use FQDNs.
Dale
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