Feature/bbr v3

.dockerignore

@@ -0,0 +1,7 @@
+.zig-cache
+zig-cache
+zig-out
+.git
+.github
+.claude
+.serena

README.md

@@ -8,30 +8,46 @@ A QUIC protocol library for Zig. Sans-I/O — you own the socket; the library ow
 - TLS 1.3 server handshake with AES-128-GCM and ChaCha20-Poly1305 (RFC 9001)
 - Session resumption and 0-RTT
 - Loss recovery, RTT estimation, PTO (RFC 9002)
-- CUBIC congestion control (RFC 9438)
+- CUBIC and BBR v3 congestion control (RFC 9438)
 - Stream multiplexing and flow control
 - Path migration and NAT rebinding
+- Pacing with wire-time accounting
+- Packet coalescing (RFC 9000 §12.2)
 - PMTUD, retry tokens, key rotation, ECN
 - Ed25519 and P-256 certificates
 - Zero external dependencies
 
 ## Build
 
 ```sh
-zig build test    # run tests
-zig build         # build server binary
+zig build test                  # run tests (default: BBR)
+zig build test -Dcongestion=cubic  # run tests with CUBIC
+zig build                       # build server binary
+zig build -Dcongestion=cubic    # build with CUBIC
 ```
 
 Requires Zig 0.16.0-dev or later.
 
 ## Interop Results
 
 <!-- INTEROP_START -->
-Tested against ngtcp2 client — 22/22 passing, goodput 9394 kbps on 10 Mbps link:
-
-| Result | Test cases |
-| :---: | --- |
-| ✅ Pass (22/22) | handshake, transfer, longrtt, chacha20, multiplexing, retry, resumption, zerortt, http3, blackhole, keyupdate, ecn, amplificationlimit, handshakeloss, transferloss, handshakecorruption, transfercorruption, v2, ipv6, rebind-port, rebind-addr, connectionmigration |
+Tested against 11 QUIC clients via [quic-interop-runner](https://github.com/quic-interop/quic-interop-runner) on a 10 Mbps / 30 ms RTT link:
+
+| Client | Tests | Goodput |
+| --- | --- | --- |
+| ngtcp2 | 22/22 | 9432 kbps |
+| quic-go | 20/20 | 9507 kbps |
+| quiche | 18/18 | — |
+| neqo | 19/22 | — |
+| kwik | 19/21 | 7849 kbps |
+| picoquic | 16/22 | — |
+| mvfst | 12/16 | 9496 kbps |
+| aioquic | 13/21 | 9190 kbps |
+| lsquic | — | 9454 kbps |
+| msquic | — | 7937 kbps |
+| quinn | — | 9462 kbps |
+
+Test cases: handshake, transfer, longrtt, chacha20, multiplexing, retry, resumption, zerortt, http3, blackhole, keyupdate, ecn, amplificationlimit, handshakeloss, transferloss, handshakecorruption, transfercorruption, v2, ipv6, rebind-port, rebind-addr, connectionmigration
 <!-- INTEROP_END -->
 
 ## Limitations

TODO.md

@@ -0,0 +1,20 @@
+# Performance TODO
+
+## High-bandwidth scaling (target: 100 Gbps)
+
+### Ring buffer sizing
+- [ ] Make `SEND_QUEUE_DEPTH` runtime-configurable (currently 256, overflows at ~3 Gbps/30ms)
+- [ ] Make `MAX_SENT` runtime-configurable (currently 256, same limit — evictions break loss detection)
+- [ ] Scale `SEND_BUF_SIZE` per-stream based on negotiated BDP (currently 64 KB, tight at 10 Mbps)
+
+### Syscall reduction
+- [ ] GSO (`UDP_SEGMENT`) for Linux — batch N QUIC packets into 1 sendmsg (60× fewer send syscalls at 1 Gbps)
+- [ ] recvmmsg for Linux — batch receive multiple datagrams per syscall
+- [ ] Increase `SEND_BATCH` and `BATCH_SIZE` for higher packet rates (currently 32/16)
+
+### Zero-copy send path
+- [ ] Encrypt directly into send queue slot (currently: pkt_scratch → enc_scratch → sq[].buf = 2 copies per packet)
+
+### Pacing at high rates
+- [ ] Sub-millisecond pacing for >1 Gbps (current 1ms timer tick limits pacing granularity)
+- [ ] Consider io_uring or busy-poll for microsecond-level pacing

build.zig

@@ -4,21 +4,35 @@ pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
+    // Congestion control algorithm selection: bbr (default) or cubic.
+    const Algorithm = enum { bbr, cubic };
+    const congestion = b.option(Algorithm, "congestion", "Congestion control algorithm: bbr (default) or cubic") orelse .bbr;
+    const congestion_cubic = congestion == .cubic;
+
+    const build_options = b.addOptions();
+    build_options.addOption(bool, "congestion_cubic", congestion_cubic);
+    const build_options_mod = build_options.createModule();
+
     // Public module: consumers import this as @import("zquic")
     const zquic_mod = b.addModule("zquic", .{
         .root_source_file = b.path("src/root.zig"),
         .target = target,
         .optimize = optimize,
+        .imports = &.{
+            .{ .name = "build_options", .module = build_options_mod },
+        },
     });
 
     // Static library artifact
+    const lib_mod = b.createModule(.{
+        .root_source_file = b.path("src/root.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+    lib_mod.addImport("build_options", build_options_mod);
     const lib = b.addLibrary(.{
         .name = "zquic",
-        .root_module = b.createModule(.{
-            .root_source_file = b.path("src/root.zig"),
-            .target = target,
-            .optimize = optimize,
-        }),
+        .root_module = lib_mod,
     });
     b.installArtifact(lib);
 
@@ -83,6 +97,8 @@ pub fn build(b: *std.Build) void {
         "src/quic/stream.zig",
         "src/quic/flow_control.zig",
         "src/quic/congestion/cubic.zig",
+        "src/quic/congestion/bbr.zig",
+        "src/quic/congestion/common.zig",
         "src/quic/transport_params.zig",
         "src/quic/loss_recovery.zig",
         "src/quic/tls.zig",
@@ -104,7 +120,11 @@ pub fn build(b: *std.Build) void {
             .target = target,
             .optimize = optimize,
         });
+        mod.addImport("build_options", build_options_mod);
         const t = b.addTest(.{ .root_module = mod });
+        // Connection(16) is ~2.2 MB; Debug mode disables copy elision, creating
+        // ~16 MB of stack frames in accept() + test.  64 MB gives enough headroom.
+        t.stack_size = 64 * 1024 * 1024;
         const run = b.addRunArtifact(t);
         test_step.dependOn(&run.step);
     }
@@ -119,5 +139,6 @@ pub fn build(b: *std.Build) void {
     server_test_mod.addImport("http3", http3_mod);
     server_test_mod.addImport("qpack", qpack_mod);
     const server_test = b.addTest(.{ .root_module = server_test_mod });
+    server_test.stack_size = 64 * 1024 * 1024;
     test_step.dependOn(&b.addRunArtifact(server_test).step);
 }

interop-test.sh

@@ -362,13 +362,14 @@ phase_verify_setup() {
     echo -e "${GREEN}✓${NC} zquic Docker image ready"
 
     # Verify implementations.json includes zquic
-    if grep -q '"zquic"' "$INTEROP_DIR/implementations.json"; then
-        echo -e "${GREEN}✓${NC} zquic registered in implementations.json"
+    local impl_file="$INTEROP_DIR/implementations_quic.json"
+    if grep -q '"zquic"' "$impl_file" 2>/dev/null; then
+        echo -e "${GREEN}✓${NC} zquic registered in implementations_quic.json"
     else
-        echo -e "${YELLOW}⚠${NC} zquic not in implementations.json, adding it..."
-        python3 << 'PYTHON_SCRIPT'
-import json
-config_file = '$INTEROP_DIR/implementations.json'
+        echo -e "${YELLOW}⚠${NC} zquic not in implementations_quic.json, adding it..."
+        python3 - "$impl_file" << 'PYTHON_SCRIPT'
+import json, sys
+config_file = sys.argv[1]
 with open(config_file, 'r') as f:
     config = json.load(f)
 if 'zquic' not in config:
@@ -380,7 +381,7 @@ if 'zquic' not in config:
     with open(config_file, 'w') as f:
         json.dump(config, f, indent=2)
 PYTHON_SCRIPT
-        echo -e "${GREEN}✓${NC} zquic added to implementations.json"
+        echo -e "${GREEN}✓${NC} zquic added to implementations_quic.json"
     fi
 
     echo ""

interop/Dockerfile

@@ -39,7 +39,7 @@ COPY . .
 
 RUN set -e; \
     . /build_env.sh; \
-    zig build -Doptimize=ReleaseSafe -Dtarget="${TARGET}"
+    zig build -Doptimize=ReleaseSafe -Dtarget="${TARGET}" -Dcongestion=bbr
 
 # Stage 2: Runtime image with network simulator support.
 FROM martenseemann/quic-network-simulator-endpoint:latest