Re: [RRG] EXPLISP BOF at the Dublin IETF

[Robin Whittle <rw@firstpr.com.au>]

Hi Dino,

Sorry, I didn't look closely enough at what you wrote or the
diagram.  I planned to look at it properly later, and have now done so.

I added the image to http://www.firstpr.com.au/ip/ivip/lisp-links/
with a link to this discussion.

> The top side shows the packet flow when the mapping doesn't exist
> and the bottom side shows the packet flow when the  mapping does
> exist.

OK. It is hard to show everything on a diagram, but a key point is
the distinction between packets travelling natively on the Net and
those travelling on the ALT network (LAT).

The map request packet, which is perhaps an encapsulated initial
data packet, travels along the GRE tunnel to the nearest ALT router.
 There, over the GRE links to other ALT routers, each ALT router
forwards the packet according to its destination address 240.1.1.1.
 The structure of this LAT network results in the packet arriving at
the correct ETR for this destination host.  The map reply message
goes back via the ordinary Internet (no GRE tunnels or ALT routers)
to the ITR.  Subsequent encapsulated data packets go from the ITR to
the ETR via the ordinary Internet too.

Its not clear to me how the 12.0.0.1 ITR has a tunnel to an ALT
router, unless the dotted line "Low Opex" indicates a tunnel,
without the need for a physical link to this ALT router.

The diagram makes it look easy, with a single GRE hop between the
first and the final ALT router.  However, in reality, it would be
common for the packet to have to ascend the highly aggregated ALT
hierarchy before it could descend the hierarchy towards the ALT
router with a direct link to the correct ETR.  Also, each GRE tunnel
may involve traversing multiple physical routers.  This long path
problem was discussed:

  http://psg.com/lists/rrg/2008/msg00229.html

with a diagram:

  http://www.antd.nist.gov/~ksriram/strong_aggregation.png

and a round-up of the debate:

  http://www.firstpr.com.au/ip/ivip/lisp-links/#long_paths


  - Robin


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