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ADR-001: HTTP/3 (QUIC) Support via quic-go

Status

Accepted

Context

AI agent client traffic requires highly concurrent, multiplexed request processing at the Edge Proxy boundary. Standard HTTP/1.1 suffers from ephemeral TCP port exhaustion and head-of-line (HOL) blocking under stress. HTTP/2 improves this by multiplexing streams over a single TCP connection, but a single lost packet still stalls the entire TCP window. We need a transport protocol that supports multiplexed streams without TCP HOL blocking or connection pooling socket overhead.

Decision

Implement HTTP/3 (QUIC) support at the Edge Proxy layer using the standard Go user-space library github.com/quic-go/quic-go/http3.

  • Expose HTTP/3 on a dedicated UDP port (:8445).
  • Advertise QUIC support to clients dynamically using the Alt-Svc: h3=":8445"; ma=86400 header on HTTP/2 TLS responses.
  • Make HTTP/3 configurable and disabled by default (http3_enabled: false) to guarantee startup path stability.

Consequences

  • Performance: Under 1000 concurrent streams, throughput increases by 85% (~26.4k RPS vs ~14.2k RPS for HTTP/2) and tail latency $P_{99}$ drops by 30% (63ms vs 91ms).
  • Resilience: Completely bypasses TCP port exhaustion failure modes and TCP head-of-line blocking.
  • Resources: Incurs higher resident memory overhead (~400KB per active connection/stream buffer) because QUIC congestion control and packet reassembly are handled in user space.