Re: [PATCH 4/4] tcp: On partial send (incomplete sendmsg()), request a retransmission right away

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Oct 02, 2025 at 02:06:46AM +0200, Stefano Brivio wrote:
> ...possibly with an updated window value. As we're discarding the
> remaining data, we'll need to receive it again. If we don't request
> a retransmission immediately, we'll see an apparent gap in the
> sequence, and request a retransmission on the next data batch or
> segment, but we're just wasting time like that. In packets:
> 
> 28686   0.000007 88.198.0.164 → 93.235.151.95 16118 TCP 55414 → 47080 [ACK] Seq=80501 Ack=141 Win=65536 Len=16064 [TCP segment of a reassembled PDU]
> 28687   0.000012 88.198.0.164 → 93.235.151.95 16118 TCP [TCP Window Full] 55414 → 47080 [ACK] Seq=96565 Ack=141 Win=65536 Len=16064 [TCP segment of a reassembled PDU]
> 
> on this second data segment, we have a short sendmsg(), and
> 
> 28688   0.000026 93.235.151.95 → 88.198.0.164 54 TCP 47080 → 55414 [ACK] Seq=141 Ack=90721 Win=32128 Len=0
> 
> consequently acknowledge it, without requesting a retransmission,
> 
> 28689   0.000006 88.198.0.164 → 93.235.151.95 8866 HTTP HTTP/1.1 200 ok  (text/css)
> 
> so the server proceeds sending a bit more, but
> 
> 28690   0.000016 93.235.151.95 → 88.198.0.164 54 TCP [TCP Dup ACK 28688#1] 47080 → 55414 [ACK] Seq=141 Ack=90721 Win=32128 Len=0
> 28691   0.000000 93.235.151.95 → 88.198.0.164 54 TCP [TCP Dup ACK 28688#2] 47080 → 55414 [ACK] Seq=141 Ack=90721 Win=32128 Len=0
> 
> we'll throw that data (from frame #28689) away, and finally request
> a retransmission as we spotted the gap now.
> 
> Request a retransmission as soon as we know we'll need it, instead.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tcp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 5a7a607..ae5e7a1 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1876,7 +1876,7 @@ eintr:
>  	}
>  
>  out:
> -	if (keep != -1) {
> +	if (keep != -1 || partial_send) {
>  		/* We use an 8-bit approximation here: the associated risk is
>  		 * that we skip a duplicate ACK on 8-bit sequence number
>  		 * collision. Fast retransmit is a SHOULD in RFC 5681, 3.2.
> -- 
> 2.43.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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