Linux : TCP Window scaling

How to check whether or not your OS supports TCP Window scaling.

# cat /proc/sys/net/ipv4/tcp_window_scaling 1

1 : enable window scale

0 : disable window scale

[ when disabling TCP window scale ]

Client ( CentOS 6.2 ) ------ web sites

OS informs Window scaling factor when sending syn packet.

# echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

capture and then access to an web site.

# tshark -i eth0 port 80 -w ws_disable.pcap

# wget http://www.google.com

Here’s a syn packet sent from the client

Client -> google

the client does not add window scale option in TCP options.

Flags: 0x02 (SYN)        0... .... = Congestion Window Reduced (CWR): Not set        .0.. .... = ECN-Echo: Not set        ..0. .... = Urgent: Not set        ...0 .... = Acknowledgement: Not set        .... 0... = Push: Not set        .... .0.. = Reset: Not set        .... ..1. = Syn: Set    Options: (16 bytes)        Maximum segment size: 1460 bytes        SACK permitted

[ when enabling Window scale ]

enable window scale

# echo 1 > /proc/sys/net/ipv4/tcp_window_scaling

# tshark -i eth0 port 80 -w ws_enable.pcap # wget http://www.google.com

syn packet

The client adds window scaling factor in TCP options.

Flags: 0x02 (SYN)        0... .... = Congestion Window Reduced (CWR): Not set        .0.. .... = ECN-Echo: Not set        ..0. .... = Urgent: Not set        ...0 .... = Acknowledgement: Not set        .... 0... = Push: Not set        .... .0.. = Reset: Not set        .... ..1. = Syn: Set    Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 807482, TSecr 0        NOP        Window scale: 6 (multiply by 64)  <-  2**6  =64

In this case , Window size will be 1460 * 2**6 = 93440

[ how to change window scaling factor ]

You may change scaling factor by editing the following three files. ( receive buffer )

/proc/sys/net/ipv4/tcp_rmem

/proc/sys/net/core/rmem_default

/proc/sys/net/core/rmem_max

The scaling factor range is from 0 to 14.

- scaling factor : 0

# echo "4096 65535 65535" > /proc/sys/net/ipv4/tcp_rmem # echo 65535 > /proc/sys/net/core/rmem_default # echo 65535 > /proc/sys/net/core/rmem_max

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 1516864, TSecr 0        NOP        Window scale: 0 (multiply by 1)

window size : 1460 * 2**0 = 1460

- scaling factor : 1

# echo 131070 > /proc/sys/net/core/rmem_max # echo 131070 > /proc/sys/net/core/rmem_default # echo "4096 131070 131070" > /proc/sys/net/ipv4/tcp_rmem

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 1665986, TSecr 0        NOP        Window scale: 1 (multiply by 2)

window size : 1460 * 2**1 = 2920

- scaling factor : 2

# echo "4096 196605 196605" > /proc/sys/net/ipv4/tcp_rmem # echo 196605 > /proc/sys/net/core/rmem_default # echo 196605 > /proc/sys/net/core/rmem_max

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 1892175, TSecr 0        NOP        Window scale: 2 (multiply by 4)

- scaling factor : 3

# echo "4096 300000 300000" > /proc/sys/net/core/rmem_default # echo 300000 > /proc/sys/net/core/rmem_max # echo 300000 > /proc/sys/net/core/rmem_default

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 3386446, TSecr 0        NOP        Window scale: 3 (multiply by 8)

- scaling factor : 4

# echo 1048560 > /proc/sys/net/core/rmem_default # echo 1048560 > /proc/sys/net/core/rmem_max # echo "4096 1048560 1048560" > /proc/sys/net/ipv4/tcp_rmem

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 3644467, TSecr 0        NOP        Window scale: 4 (multiply by 16)

- scaling factor : 5

>>> 65535*2**5

2097120

scaling factor becomes 5 ???

# echo 2097120 > /proc/sys/net/core/rmem_default # echo 2097120 > /proc/sys/net/core/rmem_max # echo "4096 2097120 2097120" > /proc/sys/net/ipv4/tcp_rmem

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 3817108, TSecr 0        NOP        Window scale: 5 (multiply by 32)

- scaling factor : 6

>>> 65535*2**6

4194240

# echo 4194240 > /proc/sys/net/core/rmem_default # echo 4194240 > /proc/sys/net/core/rmem_max # echo "4096 4194240 4194240" > /proc/sys/net/ipv4/tcp_rmem

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 4000584, TSecr 0        NOP        Window scale: 6 (multiply by 64)

- scaling factor : 7

>>> 65535*2**7

8388480

# echo 8388480 > /proc/sys/net/core/rmem_default # echo 8388480 > /proc/sys/net/core/rmem_max # echo "4096 8388480 8388480" > /proc/sys/net/ipv4/tcp_rmem

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 4338817, TSecr 0        NOP        Window scale: 7 (multiply by 128)

- scaling factor : 8

>>> 65535*2**8

16776960

# echo "4096 16776960 16776960" > /proc/sys/net/ipv4/tcp_rmem # echo 16776960 > /proc/sys/net/core/rmem_default # echo 16776960 > /proc/sys/net/core/rmem_max

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 4465434, TSecr 0        NOP        Window scale: 8 (multiply by 256)

- scaling factor : 9

>>> 65535*2**9

33553920

# echo 33553920 > /proc/sys/net/core/rmem_default # echo 33553920 > /proc/sys/net/core/rmem_max # echo "4096 33553920 33553920" > /proc/sys/net/ipv4/tcp_rmem

Options: (20 bytes)        Maximum segment size: 1460 bytes        SACK permitted        Timestamps: TSval 4596689, TSecr 0        NOP        Window scale: 9 (multiply by 512)

scale factor 10 – 14 ….. I’m tired of checking … :(