Subject: Re: [lisp] Fwd: Re: Re: A mapping system design-- specificationof LISP+ALT

From: [email-masked] (Noel Chiappa)
Date: Thu, 3 Dec 2009 14:09:25 -0500 (EST)

> From: "Joel M. Halpern" <[email-masked]>

    > if the default conceptually represents all unmapped ID space, then if
    > it is responding to a request, it probably needs to respond with
    > a /32 (or /128 for v6 based IDs) answer.

Duhhhh - I didn't think of that! I'm so used to the 'responds with the same
mapping that's in the mapping database', it never crossed my mind that the
contents of the reply could be different! Hmmm, I wonder if that trick can be
used elsewhere...

Anyway, in the ALT at least, there might be an issue, because the PETR is
going to be advertizing that it has mappings to the entire namespace. I guess
the fix there is that it advertizes the availability of that mapping with an
artifically high metric, so it only gets requests if nobody is advertizing a
_real_ mapping for that chunk of the namespace. It all PETRs start with the
same 'base' high number in the metric, the request will be routed to whichever
PETR is closest to the ITR.

    > One could argue that it can respond with a nice short prefix. But, the
    > ITR would need to explicitly check any other EID anyway. So responding
    > with the short prefix actually does not do any good.

Let me make sure I followed that. You're saying that responding with a
short-prefix mapping would be bad, because in some sense the ITR should check
for mappings for other EIDS anyway (i.e. to avoid using the PETR match, and
sending traffic for another site to the PETR, and then getting a 'that box is
better' message from the PETR, perhaps from Sam's mechanism); it would thus
be better not to respond with a short-prefix mapping anyway.

	Noel

---

From: "Darrel Lewis (darlewis)" <[email-masked]>
Date: Thu, 3 Dec 2009 11:11:42 -0800

> going to be advertizing that it has mappings to the entire 
> namespace.

Noel,  It dosen't follow that the Proxy-ETR needs to participate into
the ALT.

An ITR can locally choose to point to an ETR for non-LISP sites.  There
is no mapping requirements for decapsulation (which is all a Proxy-ETR
does).

-Darrel