why am i in this handbasket? (was Devil's Advocate - Segment Routing, Why?)

Masataka Ohta mohta at necom830.hpcl.titech.ac.jp
Sun Jun 21 12:36:38 UTC 2020


Mark Tinka wrote:

>> If information to create labels at or near sources to all the
>> possible destinations is distributed in advance, may be.
> 
> But this is what happens today.

That is a tragedy.

> Whether you do it manually or use a label distribution protocol, FEC's
> are pre-computed ahead of time.
> 
> What am I missing?

If all the link-wise (or, worse, host-wise) information of possible
destinations is distributed in advance to all the possible sources,
it is not hierarchical but flat (host) routing, which scales poorly.

Right?

>> But
>> it is effectively flat routing, or, in extreme cases, flat host
>> routing.
> 
> I still don't get it.

Why, do you think, flat routing does not but hierarchical
routing does scale?

It is because detailed information to reach destinations
below certain level is advertised not globally but only for
small part of the network around the destinations.

That is, with hierarchical routing, detailed information
around destinations is actively hidden from sources.

So, with hierarchical routing, routing protocols can
carry only rough information around destinations, from
which, source side can not construct detailed (often
purposelessly nested) labels required for MPLS.

 > So why create labels on-demand if
 > a box to handle the traffic is already in place and actively working,
 > day-in, day-out?

According to your theory to ignore routing traffic, we can be happy
with global *host* routing table with 4G entries for IPv4 and a lot
lot lot more than that for IPv6. CIDR should be unnecessary
complication to the Internet

With nested labels, you don't need so much labels at certain nesting
level, which was the point of Yakov, which does not mean you don't
need so much information to create entire nested labels at or near
the sources.

The problem is that we can't afford traffic (and associated processing
by all the related routers or things like those) and storage (at or
near source) for routing (or MPLS, SR* or whatever) with such detailed
routing at the destinations.


						Masataka Ohta


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