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tcp.analysis.bytes_in_flight times lower than communicated window size

asked 2022-09-26 05:53:18 +0000

Hi. I noticed that when I'm copying a file over SSH (not SFTP) the communicated TCP window size is ~ 250k, but the actual bytes_in_flight never raise above 50k. What could be the reason? Does Windows ignore this packet field? Is there a tweak for that? The sender is Windows 10 and the receiver is Windows 7.

$ ssh -oCompression=no [email protected] 'dd bs=1M if=/dev/zero' | pv -f >/dev/null
4.53MiB 0:00:27 [ 297KiB/s]

When multiplexing, it's possible to reach the theoretical throughput:

$ ( for ((i=0;i<20;i++)); do ssh -oCompression=no [email protected] 'dd bs=1M if=/dev/zero' & sleep 1; done ) | pv -f >/dev/null
 102MiB 0:01:08 [1.86MiB/s]

The ping is 129ms:

$ ping 192.168.149.55
Pinging 192.168.149.55 with 32 bytes of data:
Reply from 192.168.149.55: bytes=32 time=129ms TTL=126

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answered 2022-09-26 08:48:44 +0000

hugo.vanderkooij gravatar image

The Windows size is what a sender may transmit. It is not a requirement that a client must push each connection to the limit of the allowed windows size.

There might be limit on the server or client preventing it from eating up all CPU resources. Our team still curses the backup process that started at 01:10 at one particular customer and caused alarms to go of on about 15 to 30 minutes after that because our management connections started to fail in the night.

So I suggest you check resource usage on both client and server. as I suspect that is the limititation you need to look into. (Or accept as a wise precaution.)

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Asked: 2022-09-26 05:53:18 +0000

Seen: 47 times

Last updated: Sep 26