Controlling TCP Packet Sizes

7. How do I control packet size in TCP tests?

There are three ways to control packet size in TCP tests. First, on systems that support it, the MSS option (#9) is

the best way to do this. The next best option is to force the MTU size as described in #10 and #11. Finally,

the packet size can be controlled by setting the send_buffer_size to the desired payload size and disabling

Nagle’s algorithm for that pair. Nagle’s algorithm is enabled by default on all systems to prevent small packets

from flooding the network; by disabling Nagle you can force small packets to be sent. You may see erratic or

low-performing behavior on some systems from using this method.

8. What is the TCP MSS?

MSS stands for Maximum Segment Size. The default MSS is based on the MTU size of the network interface

i.e. MSS = MTU – 40 (20 bytes IP header + 20 bytes TCP header). Some systems, like Linux, allow this to be

adjusted at the socket level. On these systems, you can insert a ‘Maximum Segment Size’ option at the top of a

Chariot script to control the size of frames

9. How do I control the MSS from a Chariot script?

The MSS can be controlled by the endpoint on a per-connection basis when running on Linux or IBM AIX platforms.

To enable this option, first select Insert->Maximum segment size option at Endpoint X…

Next, set the MSS to your desired value:

10. How do I change the packet size/MSS/MTU on a Windows system?

Microsoft Windows does not allow per-connection modification of the MSS; it can only be modified by

changing the actual MTU size of the interface. This can be done by modifying the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{xxxx-xxx-xxx- xxx}\MTU. The best way to find the correct interface key is to look at the IP address field in each one

How do I set the Maximum Transmission Unit (MTU) size for Microsoft operating systems?

Solution

To configure MTU size for Windows, you have to set a registry variable.

For Windows NT 4.0 or Windows 2000, open the registry (use REGEDT32) to: HKEY_LOCAL_MACHINE\
SYSTEM\CurrentControlSet\Services\"Adapter Name"\Parameters\Tcpip 
Where "Adapter Name" is a hexadecimal string in the format XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX that corresponds to the MAC address of the network adapter.

Create a new variable of type REG_DWORD called MTU and enter the number of bytes up to the MTU of the underlying network. For example, Ethernet is usually 1500.

This information was obtained from the Windows NT Resource Kit, which is shipped as a part of the developer's assistance program.

For Windows 95 and Windows 98, open the Registry (use REGEDIT) to: HKEY_LOCAL_MACHINE\
System\CurrentControlSet\Services\Class\NetTrans\

Figure out which key is the TCP/IP protocol for your connection by looking at the other values, then open up that key. The keys in this directory use 000n as their naming convention, where n is a number from 0 to 9. The TCP/IP key will contain a parameter named DriverDesc set to "TCP/IP". Inside that 000n key, create a new string variable called "MaxMTU" and enter your value. 1500 is the default. This information was obtained fromWindows 95 Networking FAQ maintained by Richard Graves.

Max frame Size (Bytes)	MTU = Max frame Size – (Preamble + IPG)
64	46 = 64 -18
512	494 = 512 - 18
1024	1006 = 1024 - 18
1518	1500 = 1518 - 18