Re: [RRG] ALT's strong aggregation often leads to *very* long paths

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Robin Whittle <rw@firstpr.com.au>

    > Thanks very much for this beautiful illustration ....
    > For those without PowerPoint, I have a PNG of it here

Thanks for that.

Looking at the picture, though, I am completely confused by this one too (in
the sense of 'it has no relationship to how I thought things worked').

ITRs (and ETRs) are not routers (although they are colocated with routers in
a single box), and they are not connected in a mesh. In other words, I would
never expect to see a packet sent from one ITR to another. Ever.

ITRs are mapping/wrapping devices: they take a packet, map the destination ID
to an RLOC (of the ETR), wrap the packet up in a wrapper with that RLOC as
the destination, and send that wrapped packet off through the *router*
network to the ETR. When it gets there, the ETR takes the wrapper off, and
forwards the packet onto the destination.

In other words, the path for the average inter-site packet looks like:

Step A - 0 or more local routers
Step B - ITR
Step C - 1 or more backbone routers
Step D - ETR
Step E - 0 or more local routers



    >> it shows a sequence of LISP ITRs that a packets traverses from source
    >> host to the destination host

A packet would normally only ever transit a single ITR.

    >> while passing through several levels of aggregation hierarchy.

The aggregation hierarchy is only there for the *resolution* phase (like
DNS); there is no hierarchy associated with the handling of user data packets.

    >> Referring to the diagram, once the EID space x/24 has been mapped to
    >> locator ETR1 and this mapping information is known, then why can't
    >> ITR4 have a route entry that points to ITR8 (pink arrow) as the next
    >> hop towards destination x/24 (locator ETR1).

Mapping entries are not routing entries; i.e. one mapping entry never points
to another mapping entry. A mapping entry only ever points to the destination
ETR.

	Noel

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