Validation State Doesn't Act By Itself

Originally published at https://blog.pathvector.dev/protocol-in-code-bgp-05/ — part of the free Protocol Lab series. This post is part of Protocol in Code , a free series that reads network protocols not as configuration examples but as logic with inputs, state, and branches — actual code you can read and run. The whole series lives here: github.com/pathvector-studio/protocol-in-code . If you're newer to this material and want a more hands-on, guided on-ramp first, start with the companion Protocol Lab series and come back. Today's module is from the BGP track, Session 05. The source file is src/protocol_in_code/bgp/policy.py , and it builds directly on the origin-validation logic from Session 04. The question to keep in your head Here's the one thing to turn over as you read: What happens after origin validation returns valid , invalid , or not_found — and why does the result still need routing policy before anything happens to the route? There's a piece of folk knowledge that says "RPKI invalid means the router rejects the route." It's the kind of statement that sounds like a rule of the protocol. It isn't. It's one possible policy decision built on top of a validation result . The whole point of this session is to separate those two things in your head, and the code makes the seam impossible to miss. Two layers, not one Validation answers a factual question: does this route's origin AS match what the ROAs say it should be? That's Session 04's job, and its output is a ValidationState . Policy answers a completely different question: given that fact, what do we do ? Drop the route? Keep it but make it less preferred? Accept it normally? That's a local decision — different operators configure it differently, and the same validation result can lead to different actions on different routers. The file models the second layer with three small pieces. First, the set of actions the router can take: class PolicyAction ( str , Enum ): ACCEPT = " accept " DEPRIORITIZE = " de