How to determine TCP initial window size and scaling option? Which factors affect the determination?

Resolution

Unless problems are encountered with a particular system, or network path, the system defaults will provide good performance and should be left alone.

Determining Initial TCP window size value

There are several factors which affect the initial TCP window size:

• The size of socket's receive buffer
• TCP maximum segment size (MSS)
• The window scale extension
• TCP Timestamps

• tcp_workaround_signed_windows parameter

• Here is the relevant code in kernel 2.6 of Red Hat Enterprise Linux:

if (mss > (1<<*rcv_wscale)) {
     int init_cwnd = 4;
     if (mss > 1460*3)
          init_cwnd = 2;
     else if (mss > 1460)
          init_cwnd = 3;
     if (*rcv_wnd > init_cwnd*mss)
          *rcv_wnd = init_cwnd*mss;
}

• Generally, if the MSS is 1460 bytes, the initial window size is going to be 5840 (1460 * 4), although there are still other factors that could change the size. For example, it can be 5792 when TCP Timestamp is set because there ACK timestamp overhead is 12 bytes.

net.ipv4.tcp_timestamps = 1

• Note1 :

MSS = MTU - size of the TCP header (40 bytes)

• Note2 :
  It is possible to set a larger initial window size, up to 65535 bytes. The value can be 65160 if the maximum size of receive buffer is set to 67108864 (64MB) or larger and other values are kept unchanged.

• For more information about the initial window size, refer to RFC 3390 and When tcp_window_scaling is set, what is the relationship among MTU, MSS and Window size

Determining TCP window scale

• The base window size can not exceed 65535 bytes due to limitations of the TCP header:

The TCP header uses a 16 bit field to report the receive
window size to the sender.  Therefore, the largest window
that can be used is 2**16 = 65K bytes.
                                               - RFC 1323

• The Window Scale TCP option allows windows larger than 65K bytes by using a scale factor to multiply the window size value. This factor is set based on maximum size of receive buffer used by TCP sockets:

net.ipv4.tcp_rmem = 4096    87380    4194304

• The code below shows how the kernel defines the scale factor:

if (wscale_ok) {
       space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
       while (space > 65535 && (*rcv_wscale) < 14) {
               space >>= 1;
               (*rcv_wscale)++;
       }
}

So, if the maximum size of the receive buffer, tcp_rmem, is set to 4194304, the scale factor will be 7.

Note : the scale factor can be up to 14.

For more information, refer to RFC 1323.

Relevant kernel parameters

These parameters can affect the TCP window size determination:

net.ipv4.tcp_timestamps
net.core.rmem_max
net.ipv4.tcp_rmem
net.ipv4.tcp_adv_win_scale
net.ipv4.tcp_window_scaling
net.ipv4.tcp_workaround_signed_windows