Re: [straw] AD review of draft-ietf-straw-sip-traceroute-02

[Richard Barnes <[e-mail]>]

On Fri, Apr 11, 2014 at 4:22 PM, Hadriel Kaplan
<[e-mail]>wrote:

>
> On Apr 11, 2014, at 3:29 PM, Richard Barnes <[e-mail]> wrote:
>
> > If B2BUAs implement this spec as-is, then they'll see Max-Forwards: 0
> and answer the call -- they don't know whether this is part of a traceroute
> or not.  So if a legacy UAC sends out a normal, non-traceroute, INVITE with
> loopback, and it happens to time out at one of these B2BUAs, and the UAC
> doesn't know to check for the Reason header for this value (because it's
> legacy), then it ends up talking to the B2BUA instead of the UAS.
>
> Good point. So that's another benefit then. B2BUAs solve call failure
> problems they didn't even know existed.
> ;)
>
> Actually, getting back to serious mode, that could happen even without
> this draft. Max-Forward handling in B2BUAs has always been undefined
> behavior until the loopback draft, afaik.


Good point.  I did not have this in mind.


> It seems like in order to resolve this, you need to either:
> > 1. Require UACs that use loopback to check for Reason (and thus update
> RFC 6849)
>
> I'm fine with that.  Another option is to re-title this draft to be
> "Media-Loopback Usage in SIP" and add some text about that in general too.
>
>
> > 2. Add some consent flag to the INVITE so that the B2BUA can tell
> traceroutes from other INVITEs
>
> It won't help solve the problem you raised fully - it will help if all
> B2BUAs along the path implement this draft, but not if they just implement
> RFC 6849. Because they can still answer calls based on policy, and
> Max-Forwards=0 technically doesn't apply to them as a UAS unless they also
> implement the loopback draft.
>

How about this in 3.2?

"""
The mechanism defined in this document could cause B2BUAs to send a 200
response to an INVITE that is not part of a traceroute.  In such cases, the
UAC would believe that it was exchanging media with the end UAS, when in
reality it is talking to an intermediate B2BUA.  The UAC can recognize this
situation by examining the Reason header.  It is RECOMMENDED that UACs
implementing RFC 6849 perform this check.  (Note that this problem is not
introduced by this document, since the behavior of B2BUAs with regard to
Max-Forwards has been undefined until the publication of
[I-D.ietf-straw-b2bua-loop-detection].)
"""



>
> -hadriel
>
>