bgp convergence

Reaction: BGP convergence, divergence & the ‘net

Let’s have a little talk about BGP convergence.

We tend to make a number of assumptions about the Internet, and sometimes these assumptions don’t always stand up to critical analysis. . . . On the Internet anyone can communicate with anyone else – right? -via APNIC

Geoff Huston’s recent article on the reality of Internet connectivity—no, everyone cannot connect to everyone—prompted a range of reactions from various folks I know.

For instance, BGP is broken! After all, any routing protocol that can’t provide basic reachability to every attached destination must be broken, right? The problem with this statement is it assumes BGP is, at core, a routing protocol. To set the record straight, BGP is not, at heart, a routing protocol in the traditional sense of the term. BGP is a system used to describe bilateral peering arrangements between independent parties in a way that provides loop free reachability information. The primary focus of BGP is not loop free reachability, but policy.

After all, BGP convergence is a big deal, right? Part of the problem here is that we use BGP as a routing protocol in some situations (for instance, on data center fabrics), so we have a hard time adjusting our thinking to the original peering policy based focus it was designed for. In the larger ‘net, it’s not a bug that some destinations are unreachable from some sources. It’s an expression of policy, and hence it’s a feature. There are certainly times when such policies are unintentional, but unintentional/unplanned policy is policy just the same as intentional/planned policy is.

We shouldn’t declare BGP broken for doing something it’s supposed to do.

There’s another point here, as well: Some networks never converge. And that’s okay. This is, perhaps, even harder for network engineers to get their heads around. I’ve spent twenty years making sure networks converge quickly, as loop free as possible, with as little chance for failure as possible, and using the least number of resources possible. But every network in the world doesn’t always have to converge to a single view of the topology and reachability. Really!

The problem here is the micro and macro views of the world. The ‘net doesn’t converge for two reason.

First, there’s that pesky policy problem again. Policy, in the real world, never converges. There are always contradictory policies, and policies will often form bistable states. This is maddening, of course, to the mind of an engineer, but it’s just reality intruding on our little bubble. Bubbles are, after all, meant to be burst.

Second, there’s that whole CAP theorem thing in there someplace. Not many people understand the application of CAP to routing, so I’m stuffing a post or two on this on my todo list, but just remember: you can choose to a Consistent database, a database that is Accessible by every reader/user all the time, or a database that can be Partitioned. If you think about it, routing protocols are readable by every network device all the time, and they are partitioned among all the routers/intermediate systems in the network. Which means… They aren’t going to be consistent.

As in, if you feed a routing protocol enough changes often enough, it won’t ever converge—because it’s eventual consistency will always be catching up with reality. This is just the way the world is built—piling all the SDN unicorn magic in the world into routing isn’t going to solve this one, folks. On a network the size of the Internet, someone, somewhere, is always going to be changing something. This cripples BGP convergence; the ‘net never converges.

In the history of ideas, perhaps BGP shouldn’t float to the top as one of the most brilliant (and Tony and Yaakov would probably even agree with you)—but it has, on the other hand, been one of the most successful. It’s just tight enough to work often enough to rely on the connectivity as described, and it’s just loose enough to allow policies to be injected where they need to be. No such system is ever going to be “perfect.”

We could beat our heads against a wall trying, of course, but even virtual reality has physical limitations.