Subject: Re: [Status] Summary of STATUS BOF and the next steps

From: [email-masked] (Noel Chiappa)
Date: Tue, 3 Sep 2013 10:19:12 -0400 (EDT)

> From: John E Drake <[email-masked]>

    > What is this 'segment routing' silliness? The correct term is 'source
    > routing'.

After a quick review of the I-D, I'm not so sure. To me, "source routing"
means... the source picks the path (specified either at a high level - 'loose
source route' - or at a low level - 'strict source route').

Which is why in Nimrod (RFC-1992) we did not use the term "source route",
because it tended to confuse people - newcomers saw that phrase "source
route" and applied their existing mental models - and went way wrong.

The I-D isn't as clear as I'd like (it seems to jumble together architectural
description and engineering detail - in the early sections, at least), but to
me, the first question to answer is 'who makes the path selection decisions'.

We have 'hop by hop' as a clearly understood term for one end of that
spectrum, and 'source routing' (classic version) for another end, but for
things in the middle (e.g. use of MPLS paths to carry datagram traffic along
an MPLS flow for _part_ of the packet's entire end-end path) we don't really
have clear terminology.

I tried, but could never come up with a good one: I tried "unitary routing"
for Nimrod's model (emphasizing that _one_ entity was in charge of selecting
the path, although perhaps in terms of high-level entities, with the path
selection process then recursing down to actual hardware, all using the same
basic model), but I was never happy with that. I later started using
"explicit routing", but I'm not sure that was any better.

"Segment routing" might in fact be a pretty good term to describe what this
design does (at least, as best I understand it).

But I'd suggest to the I-D writers that they focus on 'who makes the path
selection decision(s)' first, and only later talk about 'how that decisision
is carried out' (be it by a sequence of flow labels, or whatever - those are
secondary questions).

    > From: AshwoodsmithPeter <[email-masked]>

    > The term "source routing" has been around for a very long time .. Its
    > also the term used in the literature etc

Which means that it already has a meaning in people's minds - and, worse yet,
since it has been used to cover a lot of ground, those meanings may be
_different_ for different people.

(We really need better taxonomy of how paths are selected - as Saltzer
memorably said in RFC-1498, "trying to discuss the issues with too few
well-defined [terms] at hand" leads to confusion. There are a lot more
options for how paths are selected than just 'hop-by-hop' and 'classic source
route' - e.g. the existing MPLS stuff I alluded to above.)

Anyway, if the concept in this I-D is in some ways significantly different
from 'classic' source routing, use of that term will be confusing, so we
should not re-use that term.

	Noel