Finding out TCP Initial Window segment size in linux

Question

I run Centos

[user ~]$ cat /etc/redhat-release
CentOS release 6.2 (Final)

With kernel

3.0.4-x86_64-linode21

How do I check the TCP initial window size in my OS? I need to verify that the IW is 10.

Answer 1

You can do it with tcpdump:

# show http tcp SYN packets on any interface
tcpdump -n -i any 'tcp[tcpflags] & (tcp-syn|tcp-ack) == tcp-syn and port 80'

Result is like:

12:46:42.632366 IP 10.0.0.1.38758 > 93.184.221.133.80: Flags [S], seq 3142522345, win 14600, options [mss 1460,sackOK,TS val 1588391 ecr 0,nop,wscale 6], length 0
#or
12:43:37.310810 IP 10.0.0.0.48324 > 213.180.146.27.80: S 2019031227:2019031227(0) win 5840 <mss 1460,nop,nop,timestamp 2617840050 0,nop,wscale 6>

Win and mss values are important to you. Your initial window is WIN/MSS, 10 in first case, 4 in second.