Devil's Advocate - Segment Routing, Why?

Masataka Ohta mohta at
Fri Jun 19 15:40:09 UTC 2020

Mark Tinka wrote:

> I wouldn't agree.
> MPLS is a purely forwarding paradigm, as is hop-by-hop IP.

As the first person to have proposed the forwarding paradigm of
label switching, I have been fully aware from the beginning that:

    Conventional Communication over ATM in a Internetwork Layer

    The conventional communication, that is communication that does not
    assume connectivity, is no different from that of the existing IP, of

special, prioritized forwarding should be done only by special
request by end users (by properly designed signaling mechanism, for
which RSVP failed to be) or administration does not scale.

> Even with
> hop-by-hop IP, you need the edge to be routing-aware.

The edge to be routing-aware around itself does scale.

The edge to be routing-aware at the destinations of all the flows
over it does not scale, which is the problem of MPLS.

Though the lack of equipment scalability was unnoticed by many,
thanks to Moore' law, inscalable administration costs a lot.

As a result, administration of MPLS has been costing a lot.

> I wasn't at the table when the MPLS spec. was being dreamed up,

I was there before poor MPLS was dreamed up.

> If you can
> tell me how NOT running MPLS affords you a "hierarchical, scalable"
> routing table, I'm all ears.

Are you saying inter-domain routing table is not "hierarchical,
scalable" except for the reason of multihoming?

As for multihoming problem, see, for example:

> Whether you forward in IP or in MPLS, scaling routing is an ever clear &
> present concern.

Not. Even without MPLS, fine tuning of BGP does not scale.

However, just as using plain IP router costs less than using
MPLS capable IP routers, BGP-only administration costs less than
BGP and MPLS administration.

For better networking infrastructure, extra cost should be spent
for L1, not MPLS or very complicated technologies around it.

							Masataka Ohta

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