Subject: RE: ICE's reliable 18x [was RE: [MMUSIC] Remaining ICE open issues]

From: "Kevin Johns" <[email-masked]>
Date: Mon, 23 Oct 2006 17:04:05 -0600

Philip,

Yep. I had pretty much convinced myself things were ok, but am glad to
get affirmation from others!

Thanks for taking the time.

Kevin 

-----Original Message-----
From: Philip Matthews [mailto:[email-masked]] 
Sent: Monday, October 23, 2006 2:26 PM
To: Kevin Johns
Cc: Dan Wing; IETF MMUSIC WG
Subject: Re: ICE's reliable 18x [was RE: [MMUSIC] Remaining ICE open
issues]

On 19-Oct-06, at 11:39 , Kevin Johns wrote:

> Dan,
>
> I had the call flow backward in my head, so I don't think this is an 
> issue as I originally thought. Here is what I was thinking:
>
> UE A -> INVITE -> UE B; The INVITE contains a set of candidate 
> addresses for UE A Once UE B gathers its candidates, it can being 
> sending connectivity checks in parallel with sending the 18X
>
> The First set of STUN Requests will fail since the NAT for UE A will 
> block them as UE A has yet to transmit any STUN requests (it is still 
> waiting for the 18x or has received it and is still building its list)

> and establish a binding. Depending on how long it takes UE A to build 
> its list and send its checks, UE B may retransmit the 18x having not 
> received any STUN requests.
>
> While the first set of STUN checks will fail for UE B, I do not see 
> that this will result in a re-transmission of the 18x.
>

Kevin:

If I understand your scenario correctly, then I think the following
answers your question.

If the NAT in front of UE A has address-dependent mapping, then you are
correct in saying that the first set of STUN checks from UE B are likely
to be dropped by A's NAT.

Once the 18x arrives at UE A, then UE A will start its own set of STUN
checks.
These checks will install permission entries in A's NAT. Assuming that
the NAT in front of UA B is reasonably nice (= not address-dependent
mapping), then these STUN packets will make it to UA B, and a reply will
come back.

In addition, they will "trigger" UA B to resend its checks (see section
7.8 of the ICE-11 draft). These triggered checks are in addition to the
normal resending mechanism of STUN.

So assuming the 18x gets from UE B to UE A, no resending of this 18x is
required.

Does this help?

- Philip

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