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Re: HTTP2 question about setting the initial connection window size

From: Martin Thomson <mt@lowentropy.net>
Date: Tue, 10 Jan 2023 08:38:08 +1100
Message-Id: <314c3edf-d003-4841-8e02-f34bf1a1b4b8@betaapp.fastmail.com>
To: ietf-http-wg@w3.org
On Tue, Jan 10, 2023, at 02:14, Kevin Grigorenko wrote:
> Hi, I'm looking into enhancing the Wireshark HTTP2 dissector to track 
> the connection and session window sizes to investigate flow control 
> bottlenecks. I have a question about RFC9113 ยง6.9.2:
> "When an HTTP/2 connection is first established, new streams are 
> created with an initial flow-control window size of 65,535 octets. The 
> connection flow-control window is also 65,535 octets. Both endpoints 
> can adjust the initial window size for new streams by including a value 
> for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame. The connection 
> flow-control window can only be changed using WINDOW_UPDATE frames."
> Does the last sentence mean that the connection flow-control window 
> size can only be changed using a WINDOW_UPDATE frame _after_ the 

Not quite.  SETTINGS_INITIAL_WINDOW_SIZE governs the initial window for streams, not the connection as a whole.

See https://datatracker.ietf.org/doc/html/rfc9113#SETTINGS_INITIAL_WINDOW_SIZE

The text is reiterating that the initial size of the connection-level flow control window cannot be changed with a setting, it can only be changed by sending WINDOW_UPDATE frames.
Received on Monday, 9 January 2023 21:38:44 UTC

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