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the actual tcp send window is not increased further

asked 2019-05-16 10:17:22 +0000

SteveZhou gravatar image

updated 2019-05-16 12:45:16 +0000

grahamb gravatar image


I am handling an issue that the smb file transfer over tcp does not fill the 1Gbit pipe between the Windows file server and a macOS 10.14.5 client. They are on different subnet so it can be any intermediate devices that is blocking something.

The weird thing I am seeing is that the sender was stuck at sending 8 * SMSS and no more. According to the Ack, the receiver window is still large enough to accept more incoming data, but the actual send window didn't increase to more than 8 * SMSS followed by each of the RTT (Ack). There is no window full reported either.

I am wondering what can be the possible reason that the sending speed was stuck at 8 * SMSS? I am no saying 8 is low, but it was just not as good as, i don't know, 14 * SMSS?


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sorry that I cannot share the pcap here since it's from the customer.

SteveZhou gravatar imageSteveZhou ( 2019-05-16 10:17:49 +0000 )edit

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answered 2019-05-16 11:03:24 +0000

updated 2019-05-16 11:04:02 +0000

It's hard to tell without seeing an actual PCAP, but usually getting stuck on constant (not slowly increasing) Bytes In Flight value points to sender application behavior (send buffer size etc).

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Asked: 2019-05-16 10:17:22 +0000

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Last updated: May 16 '19