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

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Message-ID: <101663cb4d7d43e9b6bfa946f6b8036b@exmbdft6.ad.twosigma.com>
Date:   Sun, 13 Feb 2022 17:37:35 +0000
From:   Tian Lan <Tian.Lan@...sigma.com>
To:     Eric Dumazet <edumazet@...gle.com>, Tian Lan <tilan7663@...il.com>
CC:     netdev <netdev@...r.kernel.org>,
        Andrew Chester <Andrew.Chester@...sigma.com>
Subject: RE: [PATCH] tcp: allow the initial receive window to be greater than
 64KiB

> 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 ????

Just want to clarify, the intent of this patch is not trying to violate the TCP protocol. The goal is to allow the receiver advertise a much "larger" rcv_wnd once the connection is established. So instead of having the receiver grow the rcv_wnd starting with 64KiB, the receiver could advertise the new rcv_wnd based on a much larger initial window. 

Sorry for the confusion, happy to chat more if you have questions or concerns. 

-----Original Message-----
From: Eric Dumazet <edumazet@...gle.com> 
Sent: Saturday, February 12, 2022 11:23 PM
To: Tian Lan <tilan7663@...il.com>
Cc: netdev <netdev@...r.kernel.org>; Andrew Chester <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 ????