[ih] Internet addressing history section

[Noel Chiappa]

    > From: Dave Taht

    > These days I periodically work on the babeld DV routing protocol. Among
    > other things it now has an RTT based metric ... It's generally been my
    > hope, that with the addition of fair queuing ... that the early
    > experiences with DV being problematic were... early bufferbloat.

I don't think so (and in any events IMPs had such a small amount of buffering
that they couldn't do extreme buffer-bloat if they wanted to!) Newer updates
being in the processing queue behind older updates (which is kind of the
key thing that's going on in buffer-bloat, IIRC) were no doubt an issue, but
the problems were more fundamental.

The thing is that a DV routing architecture is fundamentally a distributed
computation - i.e. node A does a tiny bit of the work, hands its intermediate
data output to node B, repeat many times. This is fundamentally different
from LS and its descendants, where everyone gets the basic data at about the
same time, and then does all the computation _locally_, in parallel.

Although both links and processors are _much_ faster than they used to be,
the speed of light (IOW point-point transmission delays) hasn't changed.
So in whatever equation one uses to describe the settling time (i.e. the
amount of time needed for everyone's tables to fully update and settle down),
a major term will not have improved.

Although the two different approaches are probably not _that_ far off, now,
since real-time delay (RTD) in flooding data is also significant. The
difference will be if the DV calculation requires multiple intermediate
computational steps (i.e. a node having to process more than one incoming
routing table update for a given destination), in which the RTD will repeat
and accumulate, so it will probably always have a higher settling time than
Map Distibution approaches such as LS.

Anyway, if one is dealing with a network in which the rate of connectivity
change is faster, in real time, than the settling time, hilarity ensues. The
ARPANET's use of a poorly smoothed delay metric (well, load-sensitive routing
_was_ a goal), which increased the dynamicity of the inputs, made things
considerably worse.

Now, since then work has been done on DV algorithms, to improve
thing like count-to-infinity, etc, which I didn't pay a lot of attention to,
since for other reason (policy routing, Byzantine robustness, etc) I decided
MD was the way to go. I just did not like the 'distributed computation'
aspect (which is fundamental in DV); it just felt less robust.

Anyway, IIRC, that DV work reduced the number of multiple intermediate
computational steps (above) in many (most? all?) cases, so that would be
a big help on that influence on settling time.

Alas, I don't remember any key names/titles to look up on that work; maybe
some early IGRP thing references it? One name that keep coming up in my mind
as associate with it is Zaw-Sing Su. There's someone else who I think did
more, but I just can't rember who it was.


But clearly BGP 'works', so maybe this is a case where it's 'good enough',
and an less capable technology has won out.

Anyway, you should read the 'problem analysis' section of BBN 3803;
available here:

  https://apps.dtic.mil/dtic/tr/fulltext/u2/a053450.pdf

and see to what degree it still applies.

	Noel