[nginx] QUIC: do not increase underutilized congestion window.

[noreply at nginx.com]

details:   https://github.com/nginx/nginx/commit/cd5e4fa1446dff86fafc3b6ffcc11afd527a024f
branches:  master
commit:    cd5e4fa1446dff86fafc3b6ffcc11afd527a024f
user:      Roman Arutyunyan <arut at nginx.com>
date:      Sat, 4 Jan 2025 18:03:46 +0400
description:
QUIC: do not increase underutilized congestion window.

As per RFC 9002, Section 7.8, congestion window should not be increased
when it's underutilized.

---
 src/event/quic/ngx_event_quic_ack.c        | 24 ++++++++++++++++++++++++
 src/event/quic/ngx_event_quic_ack.h        |  1 +
 src/event/quic/ngx_event_quic_connection.h |  1 +
 src/event/quic/ngx_event_quic_output.c     | 12 +++++++++++-
 4 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/src/event/quic/ngx_event_quic_ack.c b/src/event/quic/ngx_event_quic_ack.c
index a6f34348b..bc99947bd 100644
--- a/src/event/quic/ngx_event_quic_ack.c
+++ b/src/event/quic/ngx_event_quic_ack.c
@@ -354,6 +354,14 @@ ngx_quic_congestion_ack(ngx_connection_t *c, ngx_quic_frame_t *f)
         goto done;
     }
 
+    if (cg->idle) {
+        ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                       "quic congestion ack idle t:%M win:%uz if:%uz",
+                       now, cg->window, cg->in_flight);
+
+        goto done;
+    }
+
     if (cg->window < cg->ssthresh) {
         cg->window += f->plen;
 
@@ -377,6 +385,22 @@ done:
 }
 
 
+void
+ngx_quic_congestion_idle(ngx_connection_t *c, ngx_uint_t idle)
+{
+    ngx_quic_congestion_t  *cg;
+    ngx_quic_connection_t  *qc;
+
+    qc = ngx_quic_get_connection(c);
+    cg = &qc->congestion;
+
+    ngx_log_debug1(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                   "quic congestion idle:%ui", idle);
+
+    cg->idle = idle;
+}
+
+
 static void
 ngx_quic_drop_ack_ranges(ngx_connection_t *c, ngx_quic_send_ctx_t *ctx,
     uint64_t pn)
diff --git a/src/event/quic/ngx_event_quic_ack.h b/src/event/quic/ngx_event_quic_ack.h
index 56920c2a5..4ad59660f 100644
--- a/src/event/quic/ngx_event_quic_ack.h
+++ b/src/event/quic/ngx_event_quic_ack.h
@@ -17,6 +17,7 @@ ngx_int_t ngx_quic_handle_ack_frame(ngx_connection_t *c,
 
 void ngx_quic_congestion_ack(ngx_connection_t *c,
     ngx_quic_frame_t *frame);
+void ngx_quic_congestion_idle(ngx_connection_t *c, ngx_uint_t idle);
 void ngx_quic_resend_frames(ngx_connection_t *c, ngx_quic_send_ctx_t *ctx);
 void ngx_quic_set_lost_timer(ngx_connection_t *c);
 void ngx_quic_pto_handler(ngx_event_t *ev);
diff --git a/src/event/quic/ngx_event_quic_connection.h b/src/event/quic/ngx_event_quic_connection.h
index 824c92b57..acc09c142 100644
--- a/src/event/quic/ngx_event_quic_connection.h
+++ b/src/event/quic/ngx_event_quic_connection.h
@@ -169,6 +169,7 @@ typedef struct {
     size_t                            window;
     size_t                            ssthresh;
     ngx_msec_t                        recovery_start;
+    ngx_uint_t                        idle; /* unsigned  idle:1; */
 } ngx_quic_congestion_t;
 
 
diff --git a/src/event/quic/ngx_event_quic_output.c b/src/event/quic/ngx_event_quic_output.c
index 9aa7f37ba..a92a539f3 100644
--- a/src/event/quic/ngx_event_quic_output.c
+++ b/src/event/quic/ngx_event_quic_output.c
@@ -196,7 +196,7 @@ ngx_quic_create_datagrams(ngx_connection_t *c)
 static void
 ngx_quic_commit_send(ngx_connection_t *c)
 {
-    ngx_uint_t              i;
+    ngx_uint_t              i, idle;
     ngx_queue_t            *q;
     ngx_quic_frame_t       *f;
     ngx_quic_send_ctx_t    *ctx;
@@ -206,9 +206,15 @@ ngx_quic_commit_send(ngx_connection_t *c)
     qc = ngx_quic_get_connection(c);
     cg = &qc->congestion;
 
+    idle = 1;
+
     for (i = 0; i < NGX_QUIC_SEND_CTX_LAST; i++) {
         ctx = &qc->send_ctx[i];
 
+        if (!ngx_queue_empty(&ctx->frames)) {
+            idle = 0;
+        }
+
         while (!ngx_queue_empty(&ctx->sending)) {
 
             q = ngx_queue_head(&ctx->sending);
@@ -229,6 +235,8 @@ ngx_quic_commit_send(ngx_connection_t *c)
 
     ngx_log_debug1(NGX_LOG_DEBUG_EVENT, c->log, 0,
                    "quic congestion send if:%uz", cg->in_flight);
+
+    ngx_quic_congestion_idle(c, idle);
 }
 
 
@@ -257,6 +265,8 @@ ngx_quic_revert_send(ngx_connection_t *c, uint64_t pnum[NGX_QUIC_SEND_CTX_LAST])
 
         ctx->pnum = pnum[i];
     }
+
+    ngx_quic_congestion_idle(c, 1);
 }