linux-kernel - Re: [PATCH net v2 1/2] tcp: process the 3rd ACK with sk

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Message-ID: <CADVnQy=Aky08HJGnozv-Nd97kRHBnxhw+caks+42FUyn+9GbPQ@mail.gmail.com>
Date: Tue, 23 Jul 2024 18:01:19 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Matthieu Baerts <matttbe@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Kuniyuki Iwashima <kuniyu@...zon.com>, netdev@...r.kernel.org, mptcp@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] tcp: process the 3rd ACK with sk_socket for TFO/MPTCP

On Tue, Jul 23, 2024 at 3:09 PM Matthieu Baerts <matttbe@...nel.org> wrote:
>
> Hi Eric,
>
> On 23/07/2024 18:42, Eric Dumazet wrote:
> > On Tue, Jul 23, 2024 at 6:08 PM Matthieu Baerts <matttbe@...nel.org> wrote:
> >>
> >> Hi Eric,
> >>
> >> On 23/07/2024 17:38, Eric Dumazet wrote:
> >>> On Tue, Jul 23, 2024 at 4:58 PM Matthieu Baerts <matttbe@...nel.org> wrote:
> >>>>
> >>>> Hi Eric,
> >>>>
> >>>> +cc Neal
> >>>> -cc Jerry (NoSuchUser)
> >>>>
> >>>> On 23/07/2024 16:37, Eric Dumazet wrote:
> >>>>> On Thu, Jul 18, 2024 at 12:34 PM Matthieu Baerts (NGI0)
> >>>>> <matttbe@...nel.org> wrote:
> >>>>>>
...
> >>> +.045 < S. 1234:1234(0) ack 1001 win 14600 <mss
> >>> 940,nop,nop,sackOK,nop,wscale 6,FO 12345678,nop,nop>
> >>>    +0 > . 1001:1001(0) ack 1 <nop,nop,sack 0:1>  // See here
> >>
> >> I'm sorry, but is it normal to have 'ack 1' with 'sack 0:1' here?
> >
> > It is normal, because the SYN was already received/processed.
> >
> > sack 0:1 represents this SYN sequence.
>
> Thank you for your reply!
>
> Maybe it is just me, but does it not look strange to have the SACK
> covering a segment (0:1) that is before the ACK (1)?
>
> 'ack 1' and 'sack 0:1' seem to cover the same block, no?
> Before Kuniyuki's patch, this 'sack 0:1' was not present.

A SACK that covers a sequence range that was already cumulatively or
selectively acknowledged is legal and useful, and is called a
Duplicate Selective Acknowledgement (DSACK or D-SACK).

A DSACK indicates that a receiver received duplicate data. That can be
very useful in allowing a data sender to determine that a
retransmission was not needed (spurious). If a data sender receives
DSACKs for all retransmitted data in a loss detection episode then it
knows all retransmissions were spurious, and thus it can "undo" its
congestion control reaction to that estimated loss, since the
congestion control algorithm was responding to an incorrect loss
signal. This can be very helpful for performance in the presence of
varying delays or reordering, which can cause spurious loss detection
episodes..

See:

https://datatracker.ietf.org/doc/html/rfc2883
An Extension to the Selective Acknowledgement (SACK) Option for TCP

https://www.rfc-editor.org/rfc/rfc3708.html
"Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream
Control Transmission Protocol (SCTP) Duplicate  Transmission Sequence
Numbers (TSNs) to Detect Spurious Retransmissions"

neal