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QUIC: prevent spurious congestion control recovery mode.

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Since recovery_start field was initialized with ngx_current_msec, all
congestion events that happened within the same millisecond or cycle
iteration, were treated as in recovery mode.

Also, when handling persistent congestion, initializing recovery_start
with ngx_current_msec resulted in treating all sent packets as in recovery
mode, which violates RFC 9002, see example in Appendix B.8.

While here, also fixed recovery_start wrap protection.  Previously it used
2 * max_idle_timeout time frame for all sent frames, which is not a
reliable protection since max_idle_timeout is unrelated to congestion
control.  Now recovery_start <= now condition is enforced.  Note that
recovery_start wrap is highly unlikely and can only occur on a
32-bit system if there are no congestion events for 24 days.
This commit is contained in:
Roman Arutyunyan 2025-01-03 13:01:06 +04:00 committed by Roman Arutyunyan
parent 53e7e9eb54
commit 38236bf74f
3 changed files with 44 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
src/event/quic/ngx_event_quic.c
View File

@ -312,7 +312,7 @@ ngx_quic_new_connection(ngx_connection_t *c, ngx_quic_conf_t *conf,
ngx_max(2 * NGX_QUIC_MIN_INITIAL_SIZE,
14720));
qc->congestion.ssthresh = (size_t) -1;
qc->congestion.recovery_start = ngx_current_msec;
qc->congestion.recovery_start = ngx_current_msec - 1;
if (pkt->validated && pkt->retried) {
qc->tp.retry_scid.len = pkt->dcid.len;

54
src/event/quic/ngx_event_quic_ack.c
View File

@ -41,6 +41,7 @@ static ngx_int_t ngx_quic_detect_lost(ngx_connection_t *c,
ngx_quic_ack_stat_t *st);
static ngx_msec_t ngx_quic_pcg_duration(ngx_connection_t *c);
static void ngx_quic_persistent_congestion(ngx_connection_t *c);
static ngx_msec_t ngx_quic_oldest_sent_packet(ngx_connection_t *c);
static void ngx_quic_congestion_lost(ngx_connection_t *c,
ngx_quic_frame_t *frame);
static void ngx_quic_lost_handler(ngx_event_t *ev);
@ -335,6 +336,14 @@ ngx_quic_congestion_ack(ngx_connection_t *c, ngx_quic_frame_t *f)
cg->in_flight -= f->plen;
/* prevent recovery_start from wrapping */
timer = now - cg->recovery_start;
if ((ngx_msec_int_t) timer < 0) {
cg->recovery_start = ngx_quic_oldest_sent_packet(c) - 1;
}
timer = f->send_time - cg->recovery_start;
if ((ngx_msec_int_t) timer <= 0) {
@ -360,14 +369,6 @@ ngx_quic_congestion_ack(ngx_connection_t *c, ngx_quic_frame_t *f)
now, cg->window, cg->in_flight);
}
/* prevent recovery_start from wrapping */
timer = cg->recovery_start - now + qc->tp.max_idle_timeout * 2;
if ((ngx_msec_int_t) timer < 0) {
cg->recovery_start = now - qc->tp.max_idle_timeout * 2;
}
done:
if (blocked && cg->in_flight < cg->window) {
@ -543,19 +544,48 @@ ngx_quic_pcg_duration(ngx_connection_t *c)
static void
ngx_quic_persistent_congestion(ngx_connection_t *c)
{
ngx_msec_t now;
ngx_quic_congestion_t *cg;
ngx_quic_connection_t *qc;
qc = ngx_quic_get_connection(c);
cg = &qc->congestion;
now = ngx_current_msec;
cg->recovery_start = now;
cg->recovery_start = ngx_quic_oldest_sent_packet(c) - 1;
cg->window = qc->path->mtu * 2;
ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
"quic congestion persistent t:%M win:%uz", now, cg->window);
"quic congestion persistent t:%M win:%uz",
ngx_current_msec, cg->window);
}
static ngx_msec_t
ngx_quic_oldest_sent_packet(ngx_connection_t *c)
{
ngx_msec_t oldest;
ngx_uint_t i;
ngx_queue_t *q;
ngx_quic_frame_t *start;
ngx_quic_send_ctx_t *ctx;
ngx_quic_connection_t *qc;
qc = ngx_quic_get_connection(c);
oldest = ngx_current_msec;
for (i = 0; i < NGX_QUIC_SEND_CTX_LAST; i++) {
ctx = &qc->send_ctx[i];
if (!ngx_queue_empty(&ctx->sent)) {
q = ngx_queue_head(&ctx->sent);
start = ngx_queue_data(q, ngx_quic_frame_t, queue);
if ((ngx_msec_int_t) (start->send_time - oldest) < 0) {
oldest = start->send_time;
}
}
}
return oldest;
}

2
src/event/quic/ngx_event_quic_migration.c
View File

@ -186,7 +186,7 @@ valid:
ngx_max(2 * NGX_QUIC_MIN_INITIAL_SIZE,
14720));
qc->congestion.ssthresh = (size_t) -1;
qc->congestion.recovery_start = ngx_current_msec;
qc->congestion.recovery_start = ngx_current_msec - 1;
ngx_quic_init_rtt(qc);
}