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QUIC: do not block ACKs by congestion control.

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Previously, it was not possible to send acknowledgments if the
congestion window was limited or temporarily exceeded, such as
after sending a large response or MTU probe.  If ACKs were not
received from the peer for some reason to update the in-flight
bytes counter below the congestion window, this might result in
a stalled connection.

The fix is to send ACKs regardless of congestion control.  This
meets RFC 9002, Section 7:
: Similar to TCP, packets containing only ACK frames do not count
: toward bytes in flight and are not congestion controlled.

This is a simplified implementation to send ACK frames from the
head of the queue.  This was made possible after 6f5f17358.

Reported in trac ticket #2621 and subsequently by Vladimir Homutov:
https://mailman.nginx.org/pipermail/nginx-devel/2025-April/ZKBAWRJVQXSZ2ISG3YJAF3EWMDRDHCMO.html
This commit is contained in:
Sergey Kandaurov 2025-04-25 23:32:24 +04:00
parent adda704158
commit d6bd356e19
1 changed files with 17 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
src/event/quic/ngx_event_quic_output.c
View File

@ -55,7 +55,8 @@ static ssize_t ngx_quic_send_segments(ngx_connection_t *c, u_char *buf,
size_t len, struct sockaddr *sockaddr, socklen_t socklen, size_t segment);
#endif
static ssize_t ngx_quic_output_packet(ngx_connection_t *c,
ngx_quic_send_ctx_t *ctx, u_char *data, size_t max, size_t min);
ngx_quic_send_ctx_t *ctx, u_char *data, size_t max, size_t min,
ngx_uint_t ack_only);
static void ngx_quic_init_packet(ngx_connection_t *c, ngx_quic_send_ctx_t *ctx,
ngx_quic_header_t *pkt, ngx_quic_path_t *path);
static ngx_uint_t ngx_quic_get_padding_level(ngx_connection_t *c);
@ -131,8 +132,7 @@ ngx_quic_create_datagrams(ngx_connection_t *c)
ngx_memzero(preserved_pnum, sizeof(preserved_pnum));
#endif
while (cg->in_flight < cg->window) {
do {
p = dst;
len = ngx_quic_path_limit(c, path, path->mtu);
@ -158,7 +158,8 @@ ngx_quic_create_datagrams(ngx_connection_t *c)
return NGX_OK;
}
n = ngx_quic_output_packet(c, ctx, p, len, min);
n = ngx_quic_output_packet(c, ctx, p, len, min,
cg->in_flight >= cg->window);
if (n == NGX_ERROR) {
return NGX_ERROR;
}
@ -187,7 +188,8 @@ ngx_quic_create_datagrams(ngx_connection_t *c)
ngx_quic_commit_send(c);
path->sent += len;
}
} while (cg->in_flight < cg->window);
return NGX_OK;
}
@ -315,6 +317,10 @@ ngx_quic_allow_segmentation(ngx_connection_t *c)
bytes += f->len;
if (qc->congestion.in_flight + bytes >= qc->congestion.window) {
return 0;
}
if (bytes > len * 3) {
/* require at least ~3 full packets to batch */
return 1;
@ -364,7 +370,7 @@ ngx_quic_create_segments(ngx_connection_t *c)
if (len && cg->in_flight + (p - dst) < cg->window) {
n = ngx_quic_output_packet(c, ctx, p, len, len);
n = ngx_quic_output_packet(c, ctx, p, len, len, 0);
if (n == NGX_ERROR) {
return NGX_ERROR;
}
@ -521,7 +527,7 @@ ngx_quic_get_padding_level(ngx_connection_t *c)
static ssize_t
ngx_quic_output_packet(ngx_connection_t *c, ngx_quic_send_ctx_t *ctx,
u_char *data, size_t max, size_t min)
u_char *data, size_t max, size_t min, ngx_uint_t ack_only)
{
size_t len, pad, min_payload, max_payload;
u_char *p;
@ -585,6 +591,10 @@ ngx_quic_output_packet(ngx_connection_t *c, ngx_quic_send_ctx_t *ctx,
{
f = ngx_queue_data(q, ngx_quic_frame_t, queue);
if (ack_only && f->type != NGX_QUIC_FT_ACK) {
break;
}
if (len >= max_payload) {
break;
}