IPv6 segment routing

Posted Dec 18, 2020 23:04 UTC (Fri) by Cyberax (✭ supporter ✭, #52523)
In reply to: IPv6 segment routing by paulj
Parent article: IPv6 segment routing

> How do you build a distributed control-plane for it that can allow the 'core' to be dumb and at least scale well (i.e., the state grows at a less-than-linear rate, relative to growth in the number of nodes), if not stateless (which to my thinking, implies constant state at each node, regardless of the size of the node).
Individual data-plane routers just need a handful of rules (in the extreme, equal to the number of ports).

> OSPF/IS-IS don't scale up arbitrarily. They need O(N . LogN + M) entries in each flooding domain, for N nodes with M links, and a further O(N.logN) amount of state for vectored destinations outside of flooding domains. RSVP-TE scales much worse again.
You need OSPF/ISIS only for the control plane nodes, and you don't need many of them. Perhaps tens of thousands even for the extremely large networks (e.g. an Amazon AWS region). This amount of state can be easily managed through OSPF/ISIS flooding over gigabit-range links.

You also don't need to do that in hardware (so no worries about TCAM capacity), purely software routing is fine.

> If you know how to make source-routing scale well, you should write a Ph.D. on it - or start a company.
I did look into it, but it's hard. You might have maybe several dozens of very large potential customers and it's a very hard market to get into. Every customer will require a lot of custom integration with their system, and for a startup it's just not feasible.