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From Stefan Eissing <>
Subject Re: mod_http2 / H2WindowSize default
Date Mon, 16 Nov 2015 11:41:18 GMT

> Am 14.11.2015 um 11:14 schrieb Bert Huijben <>:
>                 Hi,
> I was wondering why mod_http2 currently uses a default window size of 65536.

Pure mistake on my part. I put this in almost at the start of development and never reviewed

Thanks for catching it. I am just about to commit a change that
a) set the default to 65535
b) will not send a setting for it to the client unless it has been configured to have another

I also removed the sending of the MAX_HEADER_LIST_SIZE, since it is not enforced in any way

Hope this change works for you. 


> The http2 protocol defines a default window size of 65535, just 1 byte less than the
current default. Changing the default requires transferring this setting to every client in
the initial settings frame, while sticking to 65535 would allow not doing that.
> I can imagine that a completely different default makes sense (and allowing to configure
this certainly does), but this 1 byte of the protocol default doesn’t make much sense to
> This is just 6 bytes wasted in the handshake for no obvious benefit.
> I would recommend changing the default to 65535 or perhaps to something completely different.
(1MB… Don’t know). But sending a setting that is less than 0.002% off the protocol default
doesn’t make much sense to me.
> Note that this size is not really used by httpd itself. This size is used by clients
that need to transfer request bodies to httpd. This number of bytes specifies the maximum
number of bytes the client is allowed to send before it needs to receive a WINDOW_UPDATE from
the server, allowing it to transfer more data.
> The client determines the default in the other direction.
>                 Bert

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