Subject: [lisp] ETR selection?

From: "Flinck, Hannu (NSN - FI/Espoo)" <[email-masked]>
Date: Thu, 30 Dec 2010 14:17:52 +0200

Hello Vince and others

I read LISP Map server draft (version 6) and was wondering the
following:

In section 4.3 Map-Server Processing it is mentioned that "...the
Map-Server verifies that the destination EID matches an EID-prefix for
which it has one or more registered ETRs, then re-encapsulates and
forwards the now-Encapsulated Map-Request to a matching ETR."

So, there is a set of ETR matching the EID-prefix out of which a one ETR
is chosen. What would be the basis for selecting the target ETR out of
the set of matching ETRs? 

Best regards
Hannu

---

From: [email-masked] (Noel Chiappa)
Date: Thu, 30 Dec 2010 12:10:08 -0500 (EST)

> From: "Flinck, Hannu (NSN - FI/Espoo)" <[email-masked]>

    > Map-Server verifies that the destination EID matches an EID-prefix
    > for which it has one or more registered ETRs, then re-encapsulates
    > and forwards the now-Encapsulated Map-Request to a matching ETR."

    > So, there is a set of ETR matching the EID-prefix out of which a one
    > ETR is chosen. What would be the basis for selecting the target ETR
    > out of the set of matching ETRs?

Off the top of my head, and as a personal opinion, I would assume it would
be a random selection (although it need not be); remember, the only
packets that should be going through the path you are asking about are
Map-Requests, and all ETRs for a given EID block should return the same
data in the Map-Reply, so it should not matter which ETR the Map-Request
is sent to.

The only reason I can think of to prefer the algorithm of 'pick one at
pseudo-random' is that if an ETR fails, it may take a minute or two for
the MS to notice that the ETR is down, and drop it; during that time
period, if the Map-Request is retransmitted, if it's sent to the same
(down) ETR that's not much good. A circulating sweep through all the ETRs
for a given EID block would be just as good (and cheaper to implement), so
maybe that's the way to go, actually.

I may have missed something, though - anyone else have a different view?

	Noel