[PATCH 2 of 4] QUIC: always add ACK frame to the queue head

[Sergey Kandaurov]

> On 27 Jul 2023, at 16:42, Roman Arutyunyan <arut at nginx.com> wrote:
> 
> # HG changeset patch
> # User Roman Arutyunyan <arut at nginx.com>
> # Date 1690461509 -14400
> #      Thu Jul 27 16:38:29 2023 +0400
> # Node ID 0d12ada84c168c62e9bae847af2725641da583d0
> # Parent  2fd16fc76920ef0b8ea2fa64858934e38c4477c5
> QUIC: always add ACK frame to the queue head.
> 
> Previously it was added to the tail as all other frames.  However, if the
> amount of queued data is large, it could delay the delivery of ACK, which
> could trigger frames retransmissions and slow down the connection.
> 
> diff --git a/src/event/quic/ngx_event_quic_output.c b/src/event/quic/ngx_event_quic_output.c
> --- a/src/event/quic/ngx_event_quic_output.c
> +++ b/src/event/quic/ngx_event_quic_output.c
> @@ -1175,7 +1175,9 @@ ngx_quic_send_ack(ngx_connection_t *c, n
>     frame->u.ack.range_count = ctx->nranges;
>     frame->u.ack.first_range = ctx->first_range;
> 
> -    ngx_quic_queue_frame(qc, frame);
> +    ngx_queue_insert_head(&ctx->frames, &frame->queue);
> +
> +    frame->len = ngx_quic_create_frame(NULL, frame);
> 
>     return NGX_OK;
> }

place frame->len first, to other frame assignments?

Otherwise, looks good.

-- 
Sergey Kandaurov