[Ecrit] Later call-back

[Henning Schulzrinne <hgs at cs.columbia.edu>]

I'll raise an issue that Hannes has mentioned to me: callback by the  
PSAP later. This is actually a general problem, as it would be nice  
for my home proxy to recognize a callback, as I may have special  
policies for handling such calls (automatic whitelist, bypass call  
treatments such as forwarding to voice mail, etc.)

Since we don't assume that the original emergency call traverses the  
home proxy, this has to be stateless, i.e., the PSAP needs to get all  
the information to convince the home agent that this is indeed an  
(emergency) callback. Thus, in general, the caller needs to provide  
the PSAP with some kind of encrypted token that the home proxy can  
recognize. This does not require setting up a new secret, since the  
Digest secret can be-reused for that.

There seem to be several options:

(1) In-Reply-To: If the call-ID is recognizable as an emergency call,  
the home proxy can perform appropriate call treatments. (For example,  
some encrypted version of this information.) This seems like a kludge.

(2) GRUU: Some special GRUU, with the same basic properties and  
problem as (1).

(3) marked call to Contact or AOR: some kind of "I'm a PSAP"  
certificate, with all the usual deployment problems.

(4) New header or Contact parameter: Basically, the caller provides  
the callee with a token that they can present later to prove that the  
call is "special". This seems closely related to the REFER problem  
and maybe we can re-use some of the same techniques.

Any other suggestions?

Henning

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