Subject: [Ecrit] Location by Reference considered harmful for Emergency Calls

From: "Brian Rosen" <[email-masked]>
Date: Fri, 21 Oct 2005 09:03:22 -0400

I'd like to challenge the entire notion of location by reference for
emergency calls.  

What you want for emergency calls is for it to work when things go wrong. 

If you don't have location, you can't route, and because it's the Internet,
that means you don't know what continent you are in, let alone which PSAP
you need to go to.

Location by reference means that we can't figure out where you are unless we
can dereference the location.  When things go wrong, you can lose
connectivity.  This is related to Henning's requirement about if there is
connectivity between the PSAP and the UA, it ought to work.

Loss of connectivity to the LIS has two bad characteristics:

1. You may not be able to get the location at the time of the call to route.
That affects both the first mapping and any subsequent routing due to, for
example, an ESRP.
2. The PSAP might not be able to get the location, because it can't get to
the LIS.

The first characteristic can be migrated somewhat by mapping early, before
the call.  This may fix the first route, but not anything the ESRP does.  If
the first route is fixed, we probably have a gross idea of where you are
(say, a state level), but nothing that is really helpful to route.

The second characteristic can't be mitigated at all.

I also don't like adding entities that have to be reliable at call time.  We
need some, but fewer is better.

It seems to me that, for emergency calls, we have to have a really big
overriding reason that trumps this issue.  I don't see it; location by
reference is just a nice to have because it bits the signaling is carrying
smaller, and it gives the LIS operator more control over the location.  In
the emergency calling domain, those are much less important than reliability
of route and location delivery to the PSAP.

And as was pointed out, you can't use the first dereference, and then send
location so that downstream entities can use it directly; proxies can't add
bodies.

Location by reference may have more value in other use cases, where the
extreme reliability is not much of a concern.

If location by reference had a requirement that the endpoint that has a
location must be able to dereference itself (i.e., you can always ask where
you are), then the endpoint could do the dereference at, say, boot time, and
then use location by value for emergency calls.

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From: Henning Schulzrinne <[email-masked]>
Date: Fri, 21 Oct 2005 09:34:01 -0400

There is an additional, smaller problem, namely reference validity. Once 
you have a reference, all parties better understand how long the 
reference is valid and retrievable. If the reference times out and the 
call gets transferred to a first responder, there's a problem. I'm sure 
this can be solved, but it introduces yet another failure case.

There's also a privacy issue: As soon as the PSAP requests location by 
reference, the location provider now knows that I placed an emergency 
call. This is not information that I'd necessarily want to share with 
parties that I barely know.

Henning