[nginx] QUIC: do not shrink congestion window after losing an MTU probe.

[noreply at nginx.com]

details:   https://github.com/nginx/nginx/commit/6bf13e9d57bbc664ac055cdb58c738b09a0f0189
branches:  master
commit:    6bf13e9d57bbc664ac055cdb58c738b09a0f0189
user:      Roman Arutyunyan <arut at nginx.com>
date:      Mon, 6 Jan 2025 16:27:03 +0400
description:
QUIC: do not shrink congestion window after losing an MTU probe.

As per RFC 9000, Section 14.4:

    Loss of a QUIC packet that is carried in a PMTU probe is therefore
    not a reliable indication of congestion and SHOULD NOT trigger a
    congestion control reaction.

---
 src/event/quic/ngx_event_quic_ack.c       | 8 ++++++++
 src/event/quic/ngx_event_quic_migration.c | 1 +
 src/event/quic/ngx_event_quic_transport.h | 1 +
 3 files changed, 10 insertions(+)

diff --git a/src/event/quic/ngx_event_quic_ack.c b/src/event/quic/ngx_event_quic_ack.c
index bc99947bd..d16545a1d 100644
--- a/src/event/quic/ngx_event_quic_ack.c
+++ b/src/event/quic/ngx_event_quic_ack.c
@@ -755,6 +755,14 @@ ngx_quic_congestion_lost(ngx_connection_t *c, ngx_quic_frame_t *f)
         goto done;
     }
 
+    if (f->ignore_loss) {
+        ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                       "quic congestion lost ignore t:%M win:%uz if:%uz",
+                       now, cg->window, cg->in_flight);
+
+        goto done;
+    }
+
     cg->recovery_start = now;
     cg->window /= 2;
 
diff --git a/src/event/quic/ngx_event_quic_migration.c b/src/event/quic/ngx_event_quic_migration.c
index 3caae88e5..463eeb503 100644
--- a/src/event/quic/ngx_event_quic_migration.c
+++ b/src/event/quic/ngx_event_quic_migration.c
@@ -923,6 +923,7 @@ ngx_quic_send_path_mtu_probe(ngx_connection_t *c, ngx_quic_path_t *path)
 
     frame->level = ssl_encryption_application;
     frame->type = NGX_QUIC_FT_PING;
+    frame->ignore_loss = 1;
 
     qc = ngx_quic_get_connection(c);
     ctx = ngx_quic_get_send_ctx(qc, ssl_encryption_application);
diff --git a/src/event/quic/ngx_event_quic_transport.h b/src/event/quic/ngx_event_quic_transport.h
index 3e320391a..dcd763df1 100644
--- a/src/event/quic/ngx_event_quic_transport.h
+++ b/src/event/quic/ngx_event_quic_transport.h
@@ -271,6 +271,7 @@ struct ngx_quic_frame_s {
     unsigned                                    need_ack:1;
     unsigned                                    pkt_need_ack:1;
     unsigned                                    ignore_congestion:1;
+    unsigned                                    ignore_loss:1;
 
     ngx_chain_t                                *data;
     union {