Re: [Fwd: I-D ACTION:draft-jennings-midcom-stun-results-00.txt]

Yutaka Takeda wrote:

>>With ICE, a UAS will first check connectivity to UAC's STUN address.
>>The ICE draft suggests doing so on the initial application request
>>(SIP INVITE e.g.) to keep call set up time low. At this 
>>moment however, 
>>UAC's NAT is not primed yet and the STUN check reports no 
>>connectivity.
> 
> 
> My understanding is that the connectivity check is done at the same time
> at both ends. The UAS (responder) may initiate the connectivity check
> first, the UAS will send 183 Session Progress with SDP (accept message)
> back to the UAC (initiator) without waiting for completion of the connectivity
> check.

This is allowed, yes.

> UAC will start the connectivity check immediately after it receives the
> accept message.

Right.

> Timeout duration of STUN transaction is 9.5 seconds
> which is long enought for UAS to initiate the connectivity check before UAS
> side connectivity check is complete.

I think you mean "long enough for the UAC to initiate the connectivity 
check before the UAS side connectivity check is complete".

I believe that will work, yes, though it does REQUIRE the UAS to issue a 
183 immediately, as opposed to waiting as long as it wants.

In the current draft, the original approach for dealing with port 
restricted was to do another ice cycle, i.e., a second o/a exchange 
which would re-do the checks. However, doing it with a single cycle 
would be highly preferred, so I like Yutaka's suggestion on this.

> 
> BTW, 9.5 seconds seems to me too long. I would like to know if there is
> a specific reason for that.

Typical value for transaction timeouts on public Internet. SIP uses an 
even larger number, 32s. STUN is quite aggressive by comparison.

Also, note that with ICE, the clients will use the "default" media path 
- generally through a relay - until the connectivity checks yield 
something better. So you'll still get media flowing even if the checks 
take much longer.

Thanks,
Jonathan R.

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