increase tcp receive window on linux

Question

Similar to Setting TCP receive window in C and working with tcpdump in Linux and Why changing value of SO_RCVBUF doesn't work?, I'm a unable to increase the initial tcp receive window greater than 5888 on ubuntu linux 2.6.32-45

#!/usr/bin/python
from socket import socket, SOL_SOCKET, SO_RCVBUF, TCP_WINDOW_CLAMP
sock = socket()
sock.setsockopt(SOL_SOCKET, SO_RCVBUF, 65536)
sock.setsockopt(SOL_SOCKET, TCP_WINDOW_CLAMP, 32768)
sock.connect(('google.com', 80))

tcpdump says:

me > google: Flags [S], seq 3758517838, win 5840, options [mss 1460,sackOK,TS val 879735044 ecr 0,nop,wscale 6], length 0
google > me: Flags [S.], seq 597037042, ack 3758517839, win 62392, options [mss 1430,sackOK,TS val 541301157 ecr 879735044,nop,wscale 6], length 0
me > google: Flags [.], ack 1, win 92, options [nop,nop,TS val 879735051 ecr 541301157], length 0

sysctl -a | grep net.*mem says:

net.core.wmem_max = 131071
net.core.rmem_max = 131071
net.core.wmem_default = 112640
net.core.rmem_default = 112640
net.core.optmem_max = 10240
net.ipv4.igmp_max_memberships = 20
net.ipv4.tcp_mem = 77376        103168  154752
net.ipv4.tcp_wmem = 4096        16384   3301376
net.ipv4.tcp_rmem = 4096        87380   3301376
net.ipv4.udp_mem = 77376        103168  154752
net.ipv4.udp_rmem_min = 4096
net.ipv4.udp_wmem_min = 4096

Could there be something else putting a receive window limit on my connection?

Answer 1

That looks like the effects of TCP slow-start. This kernel thread from 2008 offers up a good explanation (I've linked to the last response in the thread):

• SO_RCVBUF doesn't change receiver advertised window

If you keep watching the stream, the window size should increase, up to the maximum you set.