I disagree with this approach. Being very very specific in what you accept and very very specific in what you emit seems better to me.
A BGP Update message is mostly just a container of Type-Length-Value attributes. As long as the TLV structure is intact, you should be able to just pass on those TLVs without problems to any peers that the route is destined for.
The problem fundamentally is three things:
1. The original BGP RFC suggests tearing down the connection upon receiving an erroneous message. This is a terrible idea, especially for transitive attributes: you'll just reconnect and your peer will resend you the same message, flapping over and over, and the attribute is likely to not even be your peer's fault. The modern recommendation is Treat As Withdraw, i.e. remove any matching routes from the same peer from your routing table.
2. A lack of fuzz testing and similar by BGP implementers (Arista in this case)
3. Even for vendors which have done such testing, a number of have decided (IMO stupidly) to require you to turn on these robustness features explicitly.
In practice there are many ways to allow a protocol to evolve, and being liberal in what you accept is just about the worst way to achieve that. The most obvious alternative is to version the protocol, and have each node support multiple versions.
Old nodes will simply not receive messages for a version of the protocol they do not speak. The subset of nodes supporting a new version can translate messages into older versions of the protocol where it makes sense, and they can do this because they speak the new protocol, so can make an intelligent decision. This allows the network to function as a single entity even when only a subset is able to communicate on the newer protocol.
With strict versioning and compliance to specification, reference validators can be built and fitted as barriers between subnetworks so that problems in one are less likely to spread to others. It becomes trivial for anyone to quickly detect problems in the network.
You're imagining a world where things get specified and implemented completely correctly. Which does not exist and probably can't!
There's a difference between unknown extensions following a known format, and data that's simply broken (e.g. offset pointer past end of data).
And generally I think critics of Postel are lacking the context in which they were made. You and probably others would actually make similar decisions than Postel for many particular issues.
https://en.wikipedia.org/wiki/Adaptive_chosen-ciphertext_att...
One, if everyone is doing something different from the spec it is hard to figure out what they are really doing and what they mean. Long term you have confidence things will continue to work even when someone else writes their own version which otherwise might also deviate from the spec.
Two, it is easier to modify the spec as more features are dreamed up if you have confidence that the spec is boss meaning someone else didn't already use that field for something different (which you may not have heard about yet).
Three, if you agree to a spec you can audit it (think security), if nobody even knows what the spec is that is much harder.
Following the spec is harder in the early days. You have to put more effort into the spec because you can't discover a problem and just patch it in code. However the internet is far past those days. We need a spec that is the rule that everyone follows exactly.
The internet is ossified because middleboxes stick their noses where they shouldn't. If they just route IP packets, we could have had nice things like SCTP...
It's not a concession you want to make unless you really have to.
And that's because the end-user is at the mercy of, but not party to, an over the air interface between the producer and consumer that you can't verify ahead of time.
So if you're consuming a stream of supposed xhtml `<p>foo<p>bar</p>`, you have to decide if you want to screw the user for the producer's mistake for a single fuck up in the website's footer.