From 5bd653e9d2fce4220009baa85edb103f722061ca Mon Sep 17 00:00:00 2001
From: ospab <ospab@ospab.host>
Date: Sun, 17 May 2026 02:56:16 +0300
Subject: [PATCH] fix: immediately ACK duplicate packets instead of silently
 dropping them to unblock client retries when ACKs are lost

---
 Cargo.lock                | 10 +++++-----
 ostp-core/src/protocol.rs | 19 ++++++++++++++++++-
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 07a345e..df1b0a3 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -745,7 +745,7 @@ checksum = "c08d65885ee38876c4f86fa503fb49d7b507c2b62552df7c70b2fce627e06381"
 
 [[package]]
 name = "ostp"
-version = "0.1.57"
+version = "0.1.58"
 dependencies = [
  "anyhow",
  "base64",
@@ -762,7 +762,7 @@ dependencies = [
 
 [[package]]
 name = "ostp-client"
-version = "0.1.57"
+version = "0.1.58"
 dependencies = [
  "anyhow",
  "bytes",
@@ -780,7 +780,7 @@ dependencies = [
 
 [[package]]
 name = "ostp-core"
-version = "0.1.57"
+version = "0.1.58"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -813,7 +813,7 @@ dependencies = [
 
 [[package]]
 name = "ostp-server"
-version = "0.1.57"
+version = "0.1.58"
 dependencies = [
  "anyhow",
  "bytes",
@@ -828,7 +828,7 @@ dependencies = [
 
 [[package]]
 name = "ostp-tun-helper"
-version = "0.1.57"
+version = "0.1.58"
 dependencies = [
  "anyhow",
  "chrono",
diff --git a/ostp-core/src/protocol.rs b/ostp-core/src/protocol.rs
index e2e982d..f3b6d48 100644
--- a/ostp-core/src/protocol.rs
+++ b/ostp-core/src/protocol.rs
@@ -229,7 +229,11 @@ impl ProtocolMachine {
             let nonce = u64::from_be_bytes(raw_vec[4..12].try_into().unwrap());
             
             if nonce < self.expected_recv_nonce {
-                // Duplicate or delayed packet already processed, drop silently
+                // Duplicate packet! The ACK we sent was likely lost or delayed.
+                // We MUST trigger an immediate ACK to unblock the sender's congestion window.
+                if let Some(ack_frame) = self.force_build_ack()? {
+                    return Ok(ProtocolAction::SendDatagram(ack_frame));
+                }
                 return Ok(ProtocolAction::Noop);
             }
 
@@ -469,6 +473,19 @@ impl ProtocolMachine {
         Ok(Some(frame))
     }
 
+    fn force_build_ack(&mut self) -> Result<Option<Bytes>, ProtocolError> {
+        let payload = self.build_ack_payload();
+        if payload.is_empty() {
+            self.ack_pending = false;
+            return Ok(None);
+        }
+
+        let frame = self.build_control_datagram(0, FrameKind::Ack, payload)?;
+        self.ack_pending = false;
+        self.last_ack_sent = Instant::now();
+        Ok(Some(frame))
+    }
+
     fn build_ack_payload(&self) -> Bytes {
         const MAX_RANGES: usize = 8;
         let mut ranges = Vec::new();