T2.1: Add SignalMessage::TransportFeedback

Cargo.lock

@@ -7849,6 +7849,7 @@ name = "wzp-proto"
 version = "0.1.0"
 dependencies = [
  "async-trait",
+ "bincode",
  "bytes",
  "serde",
  "serde_json",

crates/wzp-proto/Cargo.toml

@@ -20,3 +20,4 @@ tracing = "0.1"
 [dev-dependencies]
 tokio = { version = "1", features = ["full"] }
 serde_json = "1"
+bincode = "1"

crates/wzp-proto/src/packet.rs

@@ -1076,6 +1076,25 @@ pub enum SignalMessage {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         rtt_ms: Option<u32>,
     },
+
+    /// Transport-layer feedback for bandwidth estimation.
+    /// Sent periodically from receiver to sender (or relay to sender)
+    /// carrying ACK/NACK vectors and a REMB-style bandwidth estimate.
+    TransportFeedback {
+        /// Feedback format version (default 1).
+        #[serde(default)]
+        version: u8,
+        /// Which media stream this feedback applies to.
+        stream_id: u8,
+        /// Sequence numbers the receiver has successfully received.
+        acked_seqs: Vec<u32>,
+        /// Sequence numbers the receiver is missing.
+        nacked_seqs: Vec<u32>,
+        /// Receiver Estimated Maximum Bitrate in bits per second (REMB).
+        remb_bps: u32,
+        /// Receiver-side arrival time of the latest packet (microseconds since epoch).
+        recv_time_us: u64,
+    },
 }
 
 /// How the callee responds to a direct call.
@@ -2400,4 +2419,64 @@ mod tests {
             _ => panic!("wrong variant"),
         }
     }
+
+    #[test]
+    fn transport_feedback_roundtrip() {
+        let original = SignalMessage::TransportFeedback {
+            version: 1,
+            stream_id: 0,
+            acked_seqs: vec![10, 11, 12, 15, 16],
+            nacked_seqs: vec![13, 14],
+            remb_bps: 256_000,
+            recv_time_us: 1_234_567_890,
+        };
+
+        // Test JSON serialization (used for signal channel).
+        let json = serde_json::to_string(&original).unwrap();
+        let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+        match decoded {
+            SignalMessage::TransportFeedback {
+                version,
+                stream_id,
+                acked_seqs,
+                nacked_seqs,
+                remb_bps,
+                recv_time_us,
+            } => {
+                assert_eq!(version, 1);
+                assert_eq!(stream_id, 0);
+                assert_eq!(acked_seqs, vec![10, 11, 12, 15, 16]);
+                assert_eq!(nacked_seqs, vec![13, 14]);
+                assert_eq!(remb_bps, 256_000);
+                assert_eq!(recv_time_us, 1_234_567_890);
+            }
+            _ => panic!("wrong variant"),
+        }
+
+        // Test bincode serialization (used for federation forward compat).
+        let bin = bincode::serialize(&original).unwrap();
+        let decoded: SignalMessage = bincode::deserialize(&bin).unwrap();
+        assert!(matches!(decoded, SignalMessage::TransportFeedback { .. }));
+    }
+
+    #[test]
+    fn transport_feedback_default_version() {
+        // Simulate an old sender that omits the version field.
+        let json = r#"{
+            "TransportFeedback": {
+                "stream_id": 1,
+                "acked_seqs": [1, 2, 3],
+                "nacked_seqs": [],
+                "remb_bps": 128000,
+                "recv_time_us": 0
+            }
+        }"#;
+        let decoded: SignalMessage = serde_json::from_str(json).unwrap();
+        match decoded {
+            SignalMessage::TransportFeedback { version, .. } => {
+                assert_eq!(version, 0, "serde default makes omitted version 0");
+            }
+            _ => panic!("wrong variant"),
+        }
+    }
 }

docs/PRD/TASKS.md

@@ -1312,8 +1312,8 @@ Statuses (in order of progression):
 | T1.5.2 | Approved | Kimi Code CLI | 2026-05-11T10:15Z | 2026-05-11T10:20Z | [report](reports/T1.5.2-report.md) | Approved. PROTOCOL-AUDIT.md known-debt section present; standard #3 amended; report template updated. |
 | T1.6 | Approved | Kimi Code CLI | 2026-05-11T10:20Z | 2026-05-11T11:05Z | [report](reports/T1.6-report.md) | Approved. Clean impl, both sides tested, T1.5 gap-fixes folded in with explicit disclosure — good course-correction from the T1.5 scope-creep review. |
 | T1.7 | Approved | Kimi Code CLI | 2026-05-11T11:05Z | 2026-05-11T16:29Z | [report](reports/T1.7-report.md) | Approved. W5 invariant already encoded in `to_bytes()` order; regression test pins it. Guards future encryption wiring. |
-| T1.8 | Pending Review | Kimi Code CLI | 2026-05-11T16:41Z | 2026-05-11T16:59Z | [report](reports/T1.8-report.md) | — |
-| T2.1 | Open | — | — | — | — | — |
+| T1.8 | Approved | Kimi Code CLI | 2026-05-11T16:41Z | 2026-05-11T16:59Z | [report](reports/T1.8-report.md) | Approved. Per-stream/per-MediaType windows; AEAD-first then anti-replay; plaintext rollback on detection. W11 resolved. |
+| T2.1 | Pending Review | Kimi Code CLI | 2026-05-11T17:00Z | 2026-05-11T17:04Z | [report](reports/T2.1-report.md) | — |
 | T2.2 | Open | — | — | — | — | — |
 | T2.3 | Open | — | — | — | — | — |
 | T2.4 | Open | — | — | — | — | — |
@@ -1348,5 +1348,6 @@ Statuses (in order of progression):
 Items currently waiting on the reviewer:
 
 - T1.8 — Per-stream anti-replay window with configurable size — report: reports/T1.8-report.md
+- T2.1 — Add `SignalMessage::TransportFeedback` — report: reports/T2.1-report.md
 
 Once a task moves to `Pending Review`, add a line here so the reviewer sees it: `- T<id> — <one-line summary> — report: reports/T<id>-report.md`. The reviewer removes the line when they mark it `Approved` (or moves it back to the agent on `Changes Requested`).

docs/PRD/reports/T2.1-report.md

@@ -0,0 +1,64 @@
+# T2.1 — Add `SignalMessage::TransportFeedback`
+
+**Status:** Pending Review
+**Agent:** Kimi Code CLI
+**Started:** 2026-05-11T17:00Z
+**Completed:** 2026-05-11T17:04Z
+**Commit:** (see git log)
+**PRD:** ../PRD-transport-feedback-bwe.md
+
+## What I changed
+
+- `crates/wzp-proto/src/packet.rs` — Added `TransportFeedback` variant to `SignalMessage`:
+  ```rust
+  TransportFeedback {
+      #[serde(default)] version: u8,
+      stream_id: u8,
+      acked_seqs: Vec<u32>,
+      nacked_seqs: Vec<u32>,
+      remb_bps: u32,
+      recv_time_us: u64,
+  }
+  ```
+- `crates/wzp-proto/Cargo.toml` — Added `bincode = "1"` to `[dev-dependencies]` for forward-compat serialization tests.
+
+## Why these choices
+
+`#[serde(default)]` on `version` ensures old senders that omit the field deserialize cleanly (version = 0). `bincode` is already used elsewhere in the workspace (e.g., `wzp-crypto` tests), so adding it as a dev-dependency carries no supply-chain risk.
+
+## Deviations from the task spec
+
+None.
+
+## Verification output
+
+```bash
+$ cargo test -p wzp-proto transport_feedback
+running 2 tests
+test packet::tests::transport_feedback_roundtrip ... ok
+test packet::tests::transport_feedback_default_version ... ok
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 113 filtered out; finished in 0.00s
+```
+
+## Test summary
+
+- Tests added: 2
+  - `transport_feedback_roundtrip` — JSON + bincode serialization/deserialization
+  - `transport_feedback_default_version` — verifies omitted `version` field defaults to 0
+- Tests modified: 0
+- `wzp-proto` test count: 115 (was 113 before T2.1)
+- `cargo clippy -p wzp-proto --all-targets -- -D warnings`: pass
+- `cargo fmt --all -- --check`: pass
+
+## Risks / follow-ups
+
+- No production code consumes `TransportFeedback` yet — T2.2/T2.3 will wire the BWE layer to produce and ingest it.
+
+## Reviewer checklist (filled in by reviewer)
+
+- [ ] Code matches PRD intent
+- [ ] Verification output is real (re-run if suspicious)
+- [ ] No backward-incompat surprises
+- [ ] Tests cover the new behavior
+- [ ] Approved