RFC 2780 is one of those quiet, load-bearing documents that most engineers never read but whose consequences they encounter every day. It doesn't define a protocol. It defines who gets to hand out numbers in the fixed-width fields of IP, TCP, UDP, and the various IP-in-IP encapsulations — and, crucially, what process someone must go through to obtain one.

By 2000, the IETF had a problem. Fields like the IPv4 Protocol byte, the IPv6 Next Header byte, the Differentiated Services codepoint, and TCP/UDP port numbers were being requested by everyone from serious protocol designers to random vendors who wanted "their" number burned into a wire format. IANA had no consistent rulebook. Some fields were handed out first-come-first-served; others required an RFC; others were governed by informal tradition. Bradner (of "rough consensus and running code" fame) and Paxson wrote RFC 2780 to formalize this.

The document leans on RFC 2434's allocation-policy vocabulary — Standards Action, IESG Approval, Expert Review, Specification Required, First Come First Served — and assigns one to each field of the core internet headers:

• IPv4 Version field (4 bits): Standards Action. Only 16 values exist ever; you don't hand these out lightly. This is why IPv5 was never really "a thing" (it was reserved for ST, the experimental Stream Protocol) and why IPv6 got version 6.
• IPv4 Protocol / IPv6 Next Header (8 bits): IESG Approval or Standards Action. One byte, 256 values, shared across TCP (6), UDP (17), ICMP (1), GRE (47), ESP (50), SCTP (132), and so on. Precious real estate.
• IPv4 TOS / DiffServ field: Standards Action for the codepoint pool, with a chunk reserved for experimental and local use. This is the plumbing behind every QoS deployment.
• TCP/UDP ports: divided into well-known (0–1023, IESG Approval), registered (1024–49151, Expert Review), and dynamic (49152–65535, no registration). This is the framework that governs whether you can "claim" port 5432 for your database protocol.
• TCP option numbers: Standards Action, because a bad TCP option can subtly break middleboxes worldwide.

The genius of RFC 2780 is that it made scarcity legible. Before it, arguing about whether IANA should give your startup a protocol number was a political fight. After it, you could point at a table and say "this field requires Standards Action; write a draft and get it through the IESG." It converted a social process into a procedural one.

It matters today in ways most engineers don't realize. Every time you see a QUIC deployment negotiating over UDP (protocol 17, allocated long before this RFC but grandfathered under its rules), every time a new IPv6 extension header proposal stalls in the IETF, every time someone laments that we can't just "add a new IP protocol number for X" — you are watching RFC 2780 do its job. It's also why RFC 6335 later had to update the port-number rules: the registered range was filling up, and Expert Review needed teeth.

There's a subtle philosophical point buried here too. Bradner and Paxson explicitly write that these fields belong to the Internet community, not to IANA or the IETF. IANA is a custodian, not an owner. That framing — number space as commons — has quietly shaped every subsequent registry debate, from ASN allocation to the DNS root zone.