Subject: Re: [RRG] draft-fuller-lisp-alt-01.txt

From: Robin Whittle <[email-masked]>
Date: Tue, 29 Jan 2008 10:02:08 +1100

Hi Iljitsch,

I am not sure why each ALT router would need to decapsulate the
traffic packet.  Doesn't it get forwarded to the correct ETR
according to its outer destination address - that of the destination
host, which the authoritative ETR has advertised into the ALT
network?  (I don't understand how this works with multiple ETRs.)

You wrote:

> It seems highly suboptimal to use GRE tunneling for this when
> LISP has its own tunneling mechanism. Why not use regular LISP
> encapsulation/decapsulation? It certainly wouldn't want to
> solve the MTU problem twice.

As far as I know, no LISP ID attempts to handle the PMTU /
fragmentation problem.  Nor do I see any mention of this in GRE's
RFC 2784.

I have already criticised ALT for being too slow, costly and
unreliable.  A global query server network is wholly unsuitable for
the new map-encap architecture.  Local query servers (APT and Ivip)
are the only practical way of supporting caching ITRs.

I have a further criticism of ALT - that the total path along the
LISP ALT routers will often be very long indeed, making the system
still slower and more costly and unreliable than if the query
traversed half the circumference of the Earth via ordinary Internet
routers.  I will discuss this in a separate message.

Also, I can't see how ALT could work with LISP's "Proxy Tunnel
Routers" (the same as Ivip's "anycast ITRs in the core").  This is
only possible if the EIDs are part of prefixes which are advertised
in the conventional BGP system.  However, the ID (page 7) states:

  Unlike in LISP 1.0, LISP 1.5 EIDs are not routed on the public
  Internet; instead, they are only routable over a separate, virtual
  topology referred to as the LISP Alternative Virtual Network.

Yet on page 9, ALT routers are required to operate as a "a proxy-ITR
capacity to support non-LISP sites."  I assume ALT is an instance of
LISP 1.5 - but maybe it is an extension of 1.5.

 - Robin

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