Subject: [Sip] OUTBOUND: Direction of keep-alive requests

From: "Christer Holmberg \(JO/LMF\)" <[email-masked]>
Date: Fri, 3 Mar 2006 15:14:49 +0100

Hi,

When the different NAT keep-alive mechanisms has been discussed, it
seems to be assumed that whatever message is sent, it is originated by
the UA (or some other node behind the NAT, on the "private side"). At
some point messages originated from the network has also been mentioned,
but then the comment has been that many NATs require the the keep-alive
messages to be sent from the UA (or some other node behind the NAT).

However, in a requst/response based protocols, there WILL be messages
sent also in the UA-to-NAT direction; the response messages. So, while
the requests from the network would not keep the NAT binding open, the
responses would.

Of course, whatever proxy that would send these requests would have some
kind of B2BUA behavior, and then we get into religious ground again...

The advantage, as I see it, would be that in the easiest form no new
functionality would be needed in the SIP UAs. They would reply to SIP
requests even if they don't understand them, and the binding would be
kept open.

The disadvantage, of course, is that the UA would not detect when the
NAT binding is gone.

But, if we would define some extension similar to the session-timer, the
UA would (assuming it supports the extension) EXPECT these requests.
And, if the requests don't come, the UA could e.g. perform a
re-registration.

This extension could even be combined with some sending of double CRLFs
by the UA, so that the keep-alive requests don't have to be sent that
often (still often enough, though).

This mechanism of course assmes that the responses are sent to the same
IP port/address as where requests would be sent, in order to keep the
binding open.

I am not saying that this solution would be better or worse than
anything else, but at least it is the easiest way to force a client to
send a message towards the NAT :)

Regards,

Christer

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---

From: Juha Heinanen <[email-masked]>
Date: Fri, 3 Mar 2006 23:34:49 +0200

Christer Holmberg (JO/LMF) writes:

 > At some point messages originated from the network has also been mentioned,

there may indeed be such NATs, but looks like none of users of my proxy
have managed to buy one, because nobody has complained.  so are you sure
the problem is worth solving?

-- juha