NDT-DEV email list created

Status: Accepted
Owner: sekharnv73
Labels: Type-Defect Priority-Medium

New issue 8 by sekharnv73: MSS modified by middlebox message displayed in  
statistics window even the MSS is not modified by middlebox
http://code.google.com/p/ndt/issues/detail?id=8

What steps will reproduce the problem?

1. Connect server and client in a 1GBit Switch
2. Start both servers (fakewww and web100)
3. Set the following tcp paramters in both server and client.
        echo 1 > /proc/sys/net/ipv4/tcp_moderate_rcvbuf
        echo 4194304 > /proc/sys/net/core/wmem_max
        echo 4194304 > /proc/sys/net/core/rmem_max
        echo "4096 87380 4194304" > /proc/sys/net/ipv4/tcp_rmem
        echo "4096 16384 4194304" > /proc/sys/net/ipv4/tcp_wmem
        echo "1" > /proc/sys/net/ipv4/tcp_timestamps
        echo "1" > /proc/sys/net/ipv4/tcp_window_scaling
        echo "1" > /proc/sys/net/ipv4/tcp_sack
        echo "0" > /proc/sys/net/ipv4/tcp_ecn
4. Click the start button to start the test and wait till client complete
the test.
5. Check the statistics window.

What is the expected output? What do you see instead?

It is observed that in statistics window "Information: Network Middlebox is
modifying MSS variable" message displayed. In this setup the MSS is not
modified by middlebox (switch) but the report says MSS is modified by the
middlebox. MSS value is 1488 in this case. Since the TCP option timestamps,
window scaling are enabled MSS is 1488.
If the TCP options timestamps and window scaling are disabled the MSS value
is displayed as 1460 and the messaged "Network Middlebox is modifying MSS
variable" is not displayed.

Please provide any additional information below.

Attachments:
        MSS modified problem.png  186 KB

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