Protocol Evolution and Compatibility

Once a protocol is deployed, it is hard to change—TCP’s fixed header caused extensions to take ten years, and TLS middleboxes’ buggy behavior toward unknown extensions led to ossification. QUIC has built-in version negotiation and GREASE (random insertion of meaningless extensions) to combat middlebox ossification from the start—this is a required course for all new protocols.

Overview

Protocols are not immutable—once deployed, they are hard to change. The textual flexibility of HTTP/1.1 headers allowed new features (Cache-Control, CORS) to be added painlessly, but TCP’s fixed header made extensions difficult (TCP Fast Open took 10 years to become widespread). QUIC has built-in version negotiation and GREASE (random insertion of meaningless extensions) from the start—to combat protocol ossification caused by middleboxes’ buggy behavior toward unknown fields. All newly designed protocols should implement GREASE and explicit extension points.

Forward vs Backward

Forward compatible (new client, old server):
  Unknown fields: skip (protobuf, TLV)
  Unknown frame type: ignore (HTTP/2)
  → Implementation: "be liberal in what you accept"

Backward compatible (old client, new server):
  Default value filling
  Version negotiation
  Capability bitmaps (TLS cipher suites)

Negotiation and Fallback

HTTP Upgrade GET / HTTP/1.1 + Upgrade: h2c 101 Switching or ignored

TLS ALPN ClientHello ALPN=[h2, http/1.1] ServerHello ALPN="h2"

HTTP Alt-Svc Alt-Svc: h3=":443" Use HTTP/3 for the next connection (Does not upgrade the current connection)

Problem: Middleboxes (firewalls, NATs) only recognize byte patterns they know
  → New protocol extension → Dropped (unrecognized) → Protocol cannot evolve

GREASE: Client randomly inserts meaningless extensions/values:
  TLS: Random cipher suites in ClientHello
  HTTP/2: Random SETTINGS parameters
  QUIC: Random transport parameters

Purpose: Middleboxes must correctly handle unknown values (ignore, not reject)
  → Future real extensions will not be blocked by middleboxes