Subject: [MMUSIC] ICE-16: some questions

From: "Zingerle, Meinrad" <[email-masked]>
Date: Mon, 9 Jul 2007 12:56:31 +0200

#1) resolving role conflicts 
  "5.2. Determining Role
   ...
    Both Lite: The agent which generated the offer which started the ICE
	processing MUST take the controlling role, and the other MUST
take
	the controlled role. In this case, no connectivity checks are
	ever sent. Rather, once the offer/answer exchange completes,
each
	agent performs the processing described in Section 8 without
	connectivity checks. It is possible that both agents will
believe
	they are controlled or controlling. In the latter case, the
	conflict is resolved through glare detection capabilities in the
	signaling protocol carrying the offer/answer exchange.
    .."
Why the "glare detection capabilities in the signaling protocol" can
resolve the role conflict in a Lite-Lite scenario but not in Full-Full
one?

#2) candidates pairs for the same media stream and same foundation
    "7.1.2.2.3. Updating Pair States
	The agent sets the state of the pair that generated the check to
	Succeeded. The success of this check might also cause the state
of
	other checks to change as well. The agent MUST perform the
following
	two steps:
	   1. The agent changes the states for all other Frozen pairs
for the
		same media stream and same foundation to Waiting.
Typically
		these other pairs will have different component IDs but
not
		always.
      ..."
In which cases different candidate pairs with same fondation and
belonging to the same media may have the same component ID? 
And in case there are two candidate pairs with same fondation and
component ID belonging to the same media: how does an ICE agent handle
it? As alternatives, or ..?

#3) Peer reflexive candidates
In my understanding peer reflexive candidates will be discovered in the
following two scenarios only:
a) the agent is behind a smmetric NAT 
b) the agent belongs to private network that has more than one NAT exits
to the public network
         
    +---+           +-----+          +----+
    | L | ---+------|NAT-1|----------|STUN|---- .......
    +---+    |      +-----+          +----+           .
             |                                        |
             |      +-----+                           |        +---+
             +------|NAT-2|---------------------------+--------| R |
                    +-----+                                    +---+ 

Is that incorrect or are there other scenarios where peer reflexive
candidates will be learned?
And am I right, that in scenario a) STUN checks with the server
reflexive candidate of agent L wouldn't succeed?
In case b) the selection of the path through the NAT-2 box is a routing
decision and I'm not sure whether using the server reflexive candidate
of agent L would work. Would it?

Thanks,
  Meinrad

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 09 Jul 2007 15:19:13 -0400

Responses inline.

> resolve the role conflict in a Lite-Lite scenario but not in Full-Full

THey are far less efficient in the signaling case. Also in the signaling 
case you may have a transient period during which you send and receive 
media from different candidates. When both sides are ice-lite this is OK 
since there are no NAT. For full ICE this would fail.

> belonging to the same media may have the same component ID? 

If for some reason you have allocated two host candidates from the same 
interface but different ports. Largely useless but possible.

> it? As alternatives, or ..?

Yes, alternatives as always.

> b) the agent belongs to private network that has more than one NAT exits

(a) is the most common case.
(b) is another case, yes.

There is also additional cases. Philip Matthews some time back had 
uncovered some really nasty topologies where this could happen. He had a 
writeup on a website somewhere but I cannot find the URL.

> reflexive candidate of agent L wouldn't succeed?

Right.

> of agent L would work. Would it?

Yes it should.

Thanks,
Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                   600 Lanidex Plaza
Cisco Fellow                                   Parsippany, NJ 07054-2711
Cisco Systems
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