ADR-0005: Protocol surface — memcache over TCP and native RDMA, HTTP object path, no UDP, no auth

Status: Accepted (2026-06-09). Supersedes the earlier memcache-UDP / kernel-bypass exploration.

Amended 2026-07-15: native reliable-connected InfiniBand/RoCE is an additional memcache transport. It is not TCP over IPoIB; ADR-0020 defines its control-ring and registered bulk-window data plane.

Context

The memcache UDP path is semantically wrong for large objects: its framing targets small responses (request-id + datagram seq/count), there is no flow control and no retransmit, and a 1 GB value becomes ~700k datagrams the client must reassemble across packet loss. Decision on 2026-06-09: drop UDP.

Decision

• memcache protocol over reliable streams only: ordinary TCP, plus the optional native RDMA v3 endpoint. There is still no memcache UDP. The TCP endpoint implements classic and meta commands; the initial native RDMA endpoint implements the classic subset: get, gets, set, add, replace, cas, delete, stats, version, quit. Not: incr/decr, CAS-as-coordination, counter mutation, touch/gat, slab/LRU admin. The value block is length-prefixed, so we stream it out for fast time-to-first-byte.
• No ranged GET on the memcache port — the protocol has no partial-get/resume. Range & partial reads belong to HTTP.
• HTTP object-serving path: HTTP/1.1 first (TLS via OpenSSL, keep-alive, Range requests, streaming, cache-control, low-copy send, backpressure-aware). HTTP/2 later. HTTP/3 deferred — it rides QUIC/UDP, but that is a separate, congestion-controlled, integrated-library decision (ngtcp2 + nghttp3), distinct from the raw memcache-UDP path we just dropped. Revisit post-v1.
• No built-in authentication anywhere. TLS is transport only. Auth/policy = trusted network, public-edge use, or a reverse proxy (nginx/Envoy/HAProxy/Caddy) in front.

Consequences

• ➕ The ordinary deployment retains one ubiquitous substrate (TCP); native RDMA is an explicit, separately configured fast path rather than a compatibility requirement.
• ➕ Clear contract: streaming + range on HTTP; simple key ops on memcache/TCP.
• ➖ Clients that require memcache-UDP are unsupported — intended.

Amendment (2026-06-09): SSL is required at build, optional at runtime

OpenSSL is a required build dependency (not compile-time optional). What’s optional is enabling HTTPS when launching the server. The HTTP front-end can run: - HTTP only (plaintext) — default; - HTTPS only — --no-http plus --tls-cert/--tls-key; - both at once — a plaintext listener and a TLS listener on separate ports, simultaneously.

Config: independent enable_http (--http-port, default 8080) and enable_https (--https-port, default 8443; turned on by supplying --tls-cert + --tls-key). Both HTTP and HTTPS may be off — a memcache-only server is valid (the classic cache case; memcache/TCP serves reads too). The only rule: at least one listener overall (memcache, HTTP, or HTTPS). TLS stays transport-only (no auth). The keyless-identity hash (ADR-0014) has vendored scalar and SHA-NI implementations; OpenSSL is used only where the runtime length/architecture policy selects its SHA-256 fallback.