[Nice] Why are TURN allocates necessary on both sides of a call?

[John Selbie]

>From experience, I know that in SIP calls using ICE, both endpoints of a
call will allocate a TURN relay port and use that as an address candidate
to send to the other side. Additionally, the endpoints can use STUN or the
mapped-address in the TURN allocation response to infer the server
reflexive address as a higher priority candidate.

Two questions:

Why is it important that both sides of a call allocate a TURN relay port as
opposed to just letting the inviter side do TURN+STUN and have the callee
do STUN only for gathering address candidates? If there a network topology
in which ICE could not converge unless both sides had a TURN relay as an
address candidate.

If both sides of the call allocate a TURN server address, is there a
network topology that will causes ICE to converge on sending packets
through both TURN servers? Does is obviously the lowest priority candidate
pair, but can it happen?

jrs
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