[sr-dev] rtpengine ICE droppings

[Richard Fuchs]

On 07/06/14 08:32, Peter Villeneuve wrote:
> Thanks Richard,
> 
> That does clear up some confusion on my end.
> 
> /This is caused by the incorrect priorities of the other candidates.
> Rtpengine added itself as lowest possible priority "host" candidate,
> which however is still a higher priority than the other candidates,
> because they have an incorrect(?) priority value./
> 
> Hmm, if that were so, then shouldn't the rtpengine "host" priority be
> smaller than the "true" host priority (let's leave the srflx aside for
> the sake of clarity)?
> You can see that 2130706432 is still higher priority than 1694498815,
> which seems to suggest something even weirder is going on.
> 
> The clients I'm using are the latest nightly CSipSimple, and AFAIK
> pjsip's ICE implementation follows the RFC, including the algo for
> attributing priority.
> Very strange indeed. I'm going to try and investigate this further, but
> any hints or suggestions are very welcome.


The recommended formula from the RFC is:

   priority = (2^24)*(type preference) +
              (2^8)*(local preference) +
              (2^0)*(256 - component ID)

The type preference for host candidates is 126 and 100 for server
reflexive. Local preference is 65535 for highest preference, 0 for
lowest. So the primary host candidate should have a priority of
(2^24)*126 + (2^8)*65535 + 256 = 2130706432, and the lowest priority
host candidate would have (2^24)*126 + (2^8)*0 + 256 = 2113929472. So my
last reply actually described things backwards, but the problem still
remains: if you go with the recommended formula, even the lowest
priority host candidate comes out with a higher priority than whatever
CSipSimple is using. I have no idea how it comes up with 1694498815.

As for the srflx one, highest priority would be (2^24)*100 + (2^8)*65535
+ 256 = 1694498816, which is actually close to the priority of the host
candidate. With a bit of playing around, it looks like CSipSimple uses a
type priority of 110 for srflx and 100 for host candidates, which is not
what the RFC recommends.

In this case, the only way for rtpengine to insert itself with a lower
priority is to deviate from the recommended formula. In itself that
wouldn't be a problem, but I can't tell if and which side effects that
would have.

cheers