Subject: [Ecrit] PSAP callbacks

From: Paul Kyzivat <[email-masked]>
Date: Wed, 28 Mar 2012 14:31:03 +0200

Having just had the discussion of this in the meeting, I just realized 
what must be the attack that Christer, Andrew, etc. have in mind:

If a "carrier" has a user/device who has been "barred" - prevented from 
initiating or receiving calls, except for emergency calls, then it wants 
to ensure that it only delivers "legitimate" emergency callbacks to that 
user.

In this case that carrier won't find it sufficient to trust caller 
assertions that this is an emergency callback. Complicit callers could 
reach this barred phone by suitably marking their calls.

But how much *standard* machinery is needed to protect against that? 
Could this not be mitigated to a sufficient degree via private means? 
Presumably this doesn't need to be 100% reliable - if some small number 
of calls can leak through to barred phones that could be an acceptable 
tradeoff.

Aside from this case I think it could be sufficient to honor the 
caller's intent until the call reaches something that is in a position 
to remember when the last emergency call occurred. That thing can then 
apply policy to decide whether to accept the call, and whether to 
prioritize it or not.

So maybe the barring case can be considered separately.

	Thanks,
	Paul