Overview of Maximum Segment Size

Pre-requisites : TCP/IP model, and OSI Model

In the establishment of TCP connection, Maximum Segment Size (MSS) is vital. MSS refers to field in TCP header, that denotes largest amount of data, in Bytes, that device can receive in TCP segment.

Consider structure of frame in data link layer.

Structure of frame format in Data link layer

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

Example – Ethernet.

1. In Ethernet, the MTU = 1500 Bytes. This is denoted in data link layer.
2. Payload of 1500 B, is received by Network layer and is denoted as 1480 B of datagram/payload and 20 B of header.
3. 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.

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