Subject: Re: [rrg] Please respond: Questions from the IESG as to whether a WG forming BOF is necessary for LISP

From: [email-masked] (Noel Chiappa)
Date: Wed, 28 Jan 2009 23:05:41 -0500 (EST)

> From: Christian Vogt <[email-masked]>

    > Portability of EID prefixes requires that the ALT topology follows
    > EID-addressing. This leads to potentially longer-than-optimal paths
    > along the ALT topology. Vice versa, if path stretch is to be limited,
    > then portability of EID prefixes must be restricted.

Yes, but this is only for the first packet(s), until the mapping gets back to
the ITR. Do you really think a few extra hops there are going to be a
problem? (Especially given that due to EGP/IGP information flow barriers,
paths probably aren't completely optimal anyway....)

Also, what percentage of DNS resolutions take more than one round-trip
(because of the use of multi-level DNS names)? Surely that's an even worse
delay (several round-trips, as opposed to simply a few extra hops), but I
don't think I've heard complaints about that?

        Noel
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From: Christian Vogt <[email-masked]>
Date: Sat, 31 Jan 2009 11:55:10 -0800

[ Forked from thread:  "Please respond: Questions from the IESG as to
whether a WG forming BOF is necessary for LISP". ]

On Jan 28, 2009, Noel Chiappa wrote:

> (because of the use of multi-level DNS names)? Surely that's an even
> worse delay (several round-trips, as opposed to simply a few extra
> hops), but I don't think I've heard complaints about that?

Noel -

There are two significant differences between the delays introduced by
DNS during hostname resolution, and the delays introduced by a pull-
based mapping system:

- DNS-related delays do not affect transport protocols, whereas pull-
   based mapping systems do.

- DNS does not create off-path dependencies for routing, whereas a
   pull-based mapping system does.  In pull-based mapping systems, at
   least the mapping requests must travel via an overlay because the
   direct routing path is not yet known at the request time.  This
   creates an off-path dependency for routing.

The first bullet is a performance issue.  Experimentation could help in
finding out how much of a problem this is.  The second bullet is a
robustness issue.  More analysis and discussion would help to better
understand the consequences of this.

- Christian

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From: Yakov Rekhter <[email-masked]>
Date: Sat, 31 Jan 2009 15:47:11 -0800

Christian,

> understand the consequences of this.

There is yet another difference - when an ITR goes down, all the
flows that used to go through that ITR would need to be re-routed
through some other ITR(s). But in order for that other ITR(s) to correctly
forward these flows, that other ITR(s) needs to acquire mapping for
all these flows. What would these other ITR(s) do with the data
arriving on these flows, as these ITRs are acquiring the mapping ?
What impact would that have on the overall system performance ?

There is no analogy to this in the context of name resolution (DNS).

Yakov.