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Windows Scaling

asked 2020-07-20 19:15:19 +0000

BruceO gravatar image

Do both sides of a TCP communication have to allow windows scaling (tcp.option_kind ==3) for scaling to take affect? Or can one side support it and the side other not?

thank you.

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The Wiki page has been updated with an example.

Chuckc gravatar imageChuckc ( 2020-07-23 02:22:17 +0000 )edit

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answered 2020-07-20 19:40:14 +0000

Chuckc gravatar image

updated 2020-07-20 19:45:23 +0000

Request for Comments: 1323 TCP Extensions for High Performance

 This option is an offer, not a promise; both sides must send
 Window Scale options in their SYN segments to enable window
 scaling in either direction.

Article with graphs here mentions that setting the scale factor to 1 (2^0) allows it to be "supported" without having to allocate extra memory.

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This is why sometimes you will see one side of the connection (usually whoever is sending the bulk of the data) offer the Window Scale option, but use a scale factor of 0. This means "Hey, I am offering this option so that you can use your window scale on your side, but I'm not going to use a multiplier on my end. You can, but I'm just going to let my advertised window be what it is."

But if both sides don't offer, then the option cannot be used as ChuckC mentioned.

ctgreer gravatar imagectgreer ( 2020-07-22 16:20:25 +0000 )edit


Thank you for your response, and the link to the Cloudshark article.

"Window Scaling only works if both sides include it as an option during the initial 3-way handshake (even if their multipliers are different). "


BruceO gravatar imageBruceO ( 2020-07-22 22:47:52 +0000 )edit

@BruceO, if an answer has solved your issue, for the benefit of others who may have the same question, please accept it by clicking the checkmark to the left of the answer.

grahamb gravatar imagegrahamb ( 2020-07-23 08:08:33 +0000 )edit

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Asked: 2020-07-20 19:15:19 +0000

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Last updated: Jul 22 '20