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

Nick Hilliard nick at foobar.org
Mon Jun 22 13:55:33 UTC 2020

Masataka Ohta wrote on 22/06/2020 13:49:
> But, it should be noted that a single class B routing table entry

"a single class B routing table entry"?  Did 1993 just call and ask for  
its addressing back? :-)

> But, it should be noted that a single class B routing table entry
> often serves for an organization with 10000s of users, which is
> at least our case here at titech.ac.jp.
> It should also be noted that, my concern is scalability in ISP side. 

This entire conversation is puzzling: we already have "hierarchical  
routing" to a large degree, to the extent that the public DFZ only sees  
aggregate routes exported by ASNs.  Inside ASNs, there will be internal  
aggregation of individual routes (e.g. an ISP DHCP pool), and possibly  
multiple levels of aggregation, depending on how this is configured.  
Aggregation is usually continued right down to the end-host edge, e.g. a  
router might have a /26 assigned on an interface, but the hosts will be  
aggregated within this /26.

> If you have 1000 PEs, you should be serving for somewhere around 1000
> customers.
> And, if I understand BGP-MP correctly, all the routing information of
> all the customers is flooded by BGP-MP in the ISP.

Well, maybe.  Or maybe not.  This depend on lots of things.

> Then, it should be a lot better to let customer edges encapsulate
> L2 or L3 over IP, with which, routing information within customers
> is exchanged by customer provided VPN without requiring extra
> overhead of maintaining customer local routing information by the
> ISP. 

If you have 1000 or even 10000s of PEs, injecting simplistic  
non-aggregated routing information is unlikely to be an issue.  If you  
have 1,000,000 PEs, you'll probably need to rethink that position.

If your proposition is that the nature of the internet be changed so  
that route disaggregation is prevented, or that addressing policy be  
changed so that organisations are exclusively handed out IP address  
space by their upstream providers, then this is simple matter of  
misunderstanding of how impractical the proposition is: that horse  
bolted from the barn 30 years ago; no organisation would accept  
exclusive connectivity provided by a single upstream; and today's world  
of dense interconnection would be impossible on the terms you suggest.  
You may not like that there are lots of entries in the DFZ and many  
operators view this as a bit of a drag, but on today's technology, this  
can scale to significantly more than what we foresee in the medium-long  
term future.


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