The Maximum Segment Size (MSS) used on the LAN connection can affect TCP performance. The MSS used on a standard Ethernet connection is 1460 bytes. You can also configure a line to use Jumbo frames with an MSS of 9000 bytes. The byte increase improves the performance of programs that send a high volume of data, such as a file transfer program, when the application is using a large message size. However, on systems using MCP v3 Networking, this difference is much smaller as much larger buffers are sent to the network hardware.

For example, assume that a program does a WRITE of 5000 bytes. Using standard Ethernet would require four TCP frames (3 x 1460 + 620). Using Jumbo frames, only one TCP frame would need to be written.

Use Jumbo frames for file transfers or with programs that perform WRITEs using large buffers instead of sending data using a smaller buffer.

The value of MSS can also affect how quickly an incoming TCP frame is ACK'd. If the number of bytes received is GEQ to two times the MSS value, then an ACK is sent as soon as the data is READ. If the un-ACK'd data is less than this, the ACK is not sent until the ACK timer expires (200ms).