Re: [PATCH] tcp: Set PSH flag for last incoming packets in a batch

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 21 Jan 2025 00:49:58 +0100
Stefano Brivio <sbrivio@redhat.com> wrote:

> So far we omitted setting PSH flags for inbound traffic altogether: as
> we ignore the nature of the data we're sending, we can't conclude that
> some data is more or less urgent. This works fine with Linux guests,
> as the Linux kernel doesn't do much with it, on input: it will
> generally deliver data to the application layer without delay.
> 
> However, with Windows, things change: if we don't set the PSH flag on
> interactive inbound traffic, we can expect long delays before the data
> is delivered to the application.
> 
> This is very visible with RDP, where packets we send on behalf of the
> RDP client are delivered with delays exceeding one second:
> 
>   $ tshark -r rdp.pcap -td -Y 'frame.number in { 33170 .. 33173 }' --disable-protocol tls
>   33170   0.030296 93.235.154.248 → 88.198.0.164 54 TCP 49012 → 3389 [ACK] Seq=13820 Ack=285229 Win=387968 Len=0
>   33171   0.985412 88.198.0.164 → 93.235.154.248 105 TCP 3389 → 49012 [PSH, ACK] Seq=285229 Ack=13820 Win=63198 Len=51
>   33172   0.030373 93.235.154.248 → 88.198.0.164 54 TCP 49012 → 3389 [ACK] Seq=13820 Ack=285280 Win=387968 Len=0
>   33173   1.383776 88.198.0.164 → 93.235.154.248 424 TCP 3389 → 49012 [PSH, ACK] Seq=285280 Ack=13820 Win=63198 Len=370
> 
> in this example (packet capture taken by passt), frame #33172 is a
> mouse event sent by the RDP client, and frame #33173 is the first
> event (display reacting to click) sent back by the server. This
> appears as a 1.4 s delay before we get frame #33173.
> 
> If we set PSH, instead:
> 
>   $ tshark -r rdp_psh.pcap -td -Y 'frame.number in { 314 .. 317 }' --disable-protocol tls
>   314   0.002503 93.235.154.248 → 88.198.0.164 170 TCP 51066 → 3389 [PSH, ACK] Seq=7779 Ack=74047 Win=31872 Len=116
>   315   0.000557 88.198.0.164 → 93.235.154.248 54 TCP 3389 → 51066 [ACK] Seq=79162 Ack=7895 Win=62872 Len=0
>   316   0.012752 93.235.154.248 → 88.198.0.164 170 TCP 51066 → 3389 [PSH, ACK] Seq=7895 Ack=79162 Win=31872 Len=116
>   317   0.011927 88.198.0.164 → 93.235.154.248 107 TCP 3389 → 51066 [PSH, ACK] Seq=79162 Ack=8011 Win=62756 Len=53
> 
> here, in frame #116, our mouse event is delivered without a delay and
                  ^^^ 316

-- 
Stefano