netdev - Re: [PATCH] tcp: allow the initial receive window to be greater than 64KiB

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Date:   Sat, 12 Feb 2022 20:23:19 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Tian Lan <tilan7663@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, Andrew.Chester@...sigma.com,
        Tian Lan <Tian.Lan@...sigma.com>
Subject: Re: [PATCH] tcp: allow the initial receive window to be greater than 64KiB

On Sat, Feb 12, 2022 at 8:06 PM Tian Lan <tilan7663@...il.com> wrote:
>
> From: Tian Lan <Tian.Lan@...sigma.com>
>
> Commit 13d3b1ebe287 ("bpf: Support for setting initial receive window")
> introduced a BPF_SOCK_OPS option which allows setting a larger value
> for the initial advertised receive window up to the receive buffer space
> for both active and passive TCP connections.
>
> However, the commit a337531b942b ("tcp: up initial rmem to 128KB and SYN
> rwin to around 64KB") would limit the initial receive window to be at most
> 64KiB which partially negates the change made previously.
>
> With this patch, the initial receive window will be set to the
> min(64KiB, space) if there is no init_rcv_wnd provided. Else set the
> initial receive window to be the min(init_rcv_wnd * mss, space).


I do not see how pretending to have a large rcvwin is going to help
for passive connections,
given the WIN in SYN and SYNACK packet is not scaled.

So this patch I think is misleading. Get over it, TCP has not been
designed to announce more than 64KB
in the 3WHS.

The only way a sender could use your bigger window would be to violate
TCP specs and send more than 64KB in the first RTT,
assuming the receiver has in fact a RWIN bigger than 64K ????