From 4fcc804f9775027485f84884c869d13749594d34 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Philipp=20Kr=C3=BCger?= <philipp.krueger1@gmail.com>
Date: Mon, 1 Dec 2025 13:02:32 +0100
Subject: [PATCH] fix: Ignore PATH_ACKs on abandoned paths

---
 quinn-proto/src/connection/mod.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/quinn-proto/src/connection/mod.rs b/quinn-proto/src/connection/mod.rs
index 9a983b003..bcd7a4845 100644
--- a/quinn-proto/src/connection/mod.rs
+++ b/quinn-proto/src/connection/mod.rs
@@ -2258,6 +2258,12 @@ impl Connection {
         path: PathId,
         ack: frame::Ack,
     ) -> Result<(), TransportError> {
+        if self.abandoned_paths.contains(&path) {
+            // See also https://www.ietf.org/archive/id/draft-ietf-quic-multipath-17.html#section-3.4.3-3
+            // > PATH_ACK frames received with an abandoned path ID are silently ignored, as specified in Section 4.
+            trace!("silently ignoring PATH_ACK on abandoned path");
+            return Ok(());
+        }
         if ack.largest >= self.spaces[space].for_path(path).next_packet_number {
             return Err(TransportError::PROTOCOL_VIOLATION("unsent packet acked"));
         }