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Re: [tcpm] tcp window size

Yuchung Cheng <ycheng@google.com> Mon, 20 May 2019 16:54 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Mon, 20 May 2019 09:53:40 -0700
Message-ID: <CAK6E8=dhqEGLOyTwW522b2nNua=KdQdv-MgyM_ioj05i3+ARKA@mail.gmail.com>
To: Marc <gaardiolor@gmail.com>
Cc: "tcpm@ietf.org Extensions" <tcpm@ietf.org>
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Subject: Re: [tcpm] tcp window size
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On Mon, May 20, 2019 at 9:23 AM Marc <gaardiolor@gmail.com> wrote:
>
> Hello experts,
>
> I've been trying to find definitive information about tcp window size
> negotiation, and if it exists at all.
>
> Take a browser that downloads a file (receiver) and a webserver (sender). If
> - both sender and receiver support window scaling
> - the receiver advertises a TCP winsize of 256KB
> - the sender advertises a window size of 32KB
> - the sender has enough space in its TCP send buffer
> (/proc/sys/net/ipv4/tcp_wmem on linux)
>
> How much 'outstanding unacked data' is the sender allowed to have,
> 32KB or 256KB ?
>
> I can't see any logic for it being limited to the window size of the
> sender (32KB), I think it depends on both the receiver winsize and the
> sender TCP Send Buffer Size, but I need to be sure.
>
> Reading https://tools.ietf.org/html/rfc793 also suggests there is no
> such thing like window size 'negotiation' and each end of a TCP
> connection just advertises its own to indicate how much data it can
> receive, independently.
>
> Is that correct ?
That's correct. It's FYI not negotiated.
>
> Thanks!
>
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