Re: [MMUSIC] ICE PRIORITY with multiple TURN servers

["Stach, Thomas" <[e-mail]>]

Justin,

From: mmusic [mailto:[e-mail]] On Behalf Of Justin Uberti
Sent: Freitag, 6. Juni 2014 03:09
To: [e-mail]; Mallinath Bareddy; Jiayang Liu; Eric Rescorla
Subject: [MMUSIC] ICE PRIORITY with multiple TURN servers

Consider the case where a single endpoint has 2 TURN servers, and allocates a relay candidate from both, from the same local IP. What should their PRIORITY values be?

Quoth 5245, S 4.1.2:
   Each candidate for a media stream MUST have a unique
   priority that MUST be a positive integer between 1 and (2**31 - 1).

OK, so they have to have different priorities, i.e. either their type preference or local preference must be different. But then, looking at S 4.1.2.1<http://4.1.2.1>:
   The type preference MUST be identical for all
   candidates of the same type and MUST be different for candidates of
   different types.

Hmm, this means their local_preference must be different. But later in S 4.1.2.1<http://4.1.2.1>:
   The local preference MUST be an integer from 0 to 65535 inclusive.
   It represents a preference for the particular IP address from which
   the candidate was obtained, in cases where an agent is multihomed.

Obviously, if they come from the same local IP, this cannot be satisfied. What to do?
[TS] Well, that paragraph continues:

  When there is only a single IP address, this value SHOULD be set to
   65535.  More generally, if there are multiple candidates for a
   particular component for a particular media stream that have the same
   type, the local preference MUST be unique for each one.

So the case of two TURN servers is covered.
What you propose below is OK, but if desired you could  also rank A1, A2, B1, B2. It’s your freedom of choice.
Regards
Thomas

It seems like the easiest solution would be to use some of the local preference to represent the ordering, so that both IP preference and server preference and represented, and therefore relay candidates from IPs 1 and 2 and Turn Servers A and B would stack rank as
A1
B1
A2
B2