What is Maximum Segment Size?

The maximum segment size, or MSS, sets a restriction on the size of data packets that can be sent over a network like the Internet. Every bit of data that moves across a network is divided up into packets. Multiple headers, each containing information about the contents and destination, are appended to packets. The payload, or non-header portion of a packet, is measured by MSS.

MSS is like a scale that measures only the trailer if a data packet is like a transport vehicle, where the payload is the trailer and freight and the header is the truck itself. The truck is not permitted to go to its destination if the trailer weighs too much.

Structure of frame format in Data link layer

The data link layer helps move frames from one node to the next. It consists of a head (which contains information on the number of packets), a tail (which denotes the end of packet flow), and a payload (a portion of the data packet, transmitted. This consists of actual information).
The payload is referred to as the ‘Maximum Transmission Unit’ (MTU). It is the biggest packet or size of frame, specified in Bytes, that can be sent over the network, like the Internet.

Example – Ethernet

In Ethernet, the MTU = 1500 Bytes. This is denoted in data link layer.
Payload of 1500 B, is received by Network layer and is denoted as 1480 B of datagram/payload and 20 B of header.
TPayload of 1480 B, when received by Transport layer, payload size is observed to be 1460 B and header size is seen to be 20 B. Payload here is referred to as ‘Maximum segment size’. For good communications, the number of bytes in data segment and header has to add up to lower than number of bytes in maximum transmission unit.

Note: Thus, Maximum Segment Size refers to highest payload size, an end device is ready to accept within single packet. The maximum Transmission Unit is highest size of packet that can flow through network without being divided.