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Re: [tram] Happy Turnballs

Jonathan Lennox <jonathan@vidyo.com> Mon, 17 April 2017 16:05 UTC

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From: Jonathan Lennox <jonathan@vidyo.com>
To: "Tirumaleswar Reddy (tireddy)" <tireddy@cisco.com>
CC: "tram@ietf.org" <tram@ietf.org>
Thread-Topic: Happy Turnballs
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Date: Mon, 17 Apr 2017 16:05:43 +0000
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Subject: Re: [tram] Happy Turnballs
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> On Apr 17, 2017, at 6:25 AM, Tirumaleswar Reddy (tireddy) <tireddy@cisco.com> wrote:
> 
>> -----Original Message-----
>> From: tram [mailto:tram-bounces@ietf.org] On Behalf Of Jonathan Lennox
>> Sent: Wednesday, March 29, 2017 9:34 AM
>> To: tram@ietf.org
>> Subject: [tram] Happy Turnballs
>> 
>> So I was asked to write up my comments on Happy Eyeballs for TURN.  Here’re
>> my thoughts.
>> 
>> As always with Happy Eyeballs, when connecting to a TURN server specified by
>> a DNS name, do a DNS query for both IPv6 and IPv4, and then (with
>> appropriate timing based on expected success), attempt to connect to both
>> the returned IPv6 and IPv4 addresses, in parallel.
> 
> As per https://tools.ietf.org/html/rfc6555, by default IPv6 should have slight preference (or head-start) over IPv4 unless the client address preference policy is modified giving preference to IPv4 over IPv6.

Right — I was eliding that part, but my intention was that all the timing and policy would follow the recommendations in 6555.

>> If the server is not using stateless cookies, it will instead send back a
>> ServerHello message (and associated messages in that flight).  In this case, the
>> client should gracefully tear down the dispreffered association to avoid
>> lingering server state: it should send a user_canceled alert, followed by a
>> close_notify alert.
> 
> user_cancelled alert is typically triggered when initiated by a user, close_notify alert is sufficient to inform the recipient that the sender will not send 
> any more messages on this connection.

As I said, this section needs a DTLS expert.

Is there usually any server behavioral difference between the two alert types?

>> 3. For UDP, send TURN Allocate requests to both addresses, without
>> authentication information.  If the TURN server is requiring authentication (as
>> it normally will), it will send back 401 Unauthenticated responses.  This does
>> not create server state, so the dispreferred allocation attempt can be silently
>> discarded.
> 
> TURN server would have to remember the Nonce value !

It shouldn’t — normally, Nonce values can be encoded in a way that’s stateless for the server (e.g., timestamp plus an HMAC), and this is usually considered good practice so as to avoid state-exhaustion attacks on the server.

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