I'm capturing http traffic with tcpdump and am interested in TCP slow start and how window sizes increase:
$ sudo tcpdump -i eth1 -w wget++.tcpdump tcp and port 80
When I view the dump file with Wireshark the progression of window sizes looks normal, i.e. 5840, 5888, 5888, 8576, 11264, etc...
But when I view the dump file via
$ tcpdump -r wget++.tcpdump -tnN | less
I get what seem to be nonsensical windows sizes ( IP addresses omitted for brevity ):
: S 1069713761:1069713761(0) win 5840 <mss 1460,sackOK,timestamp 24220583 0,nop,wscale 7>
: S 1198053215:1198053215(0) ack 1069713762 win 5672 <mss 1430,sackOK,timestamp 2485833728 24220583,nop,wscale 6>
: . ack 1 win 46 <nop,nop,timestamp 24220604 2485833728>
: . 1:1419(1418) ack 1 win 46 <nop,nop,timestamp 24220604 2485833728>
: P 1419:2002(583) ack 1 win 46 <nop,nop,timestamp 24220604 2485833728>
: . ack 1419 win 133 <nop,nop,timestamp 2485833824 24220604>
: . ack 2002 win 178 <nop,nop,timestamp 2485833830 24220604>
Is there a way to get normal / absolute window sizes on the command line?
The window sizes are correct - they're just unscaled.
The connection initiator has set a wscale (window scaling factor) of 7, so its subsequent win values must be multiplied by 128 to get the window size in bytes. Thus the win 46 indicates a window of 5888 bytes.
The connection recipient has set a wscale of 6, so its win values must be multiplied by 64. Thus win 133 indicates a window of 8512 bytes, and win 178 indicates 11392 bytes.
Also, if the tool (wireshark or tcpdump, it doesn't matter) doesn't see the syn, it has to print the unscaled value, which can fool you
来源：https://stackoverflow.com/questions/3254574/wireshark-and-tcpdump-r-strange-tcp-window-sizes