What is a TCP segment?

A TCP segment is a unit of data at the transport layer (L4) used by the TCP protocol to transfer information between applications.

In short: a TCP segment is a piece of application data, wrapped by TCP with control information for reliable transmission.

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#Why a TCP segment is needed

Applications transfer data in large blocks. TCP:

• splits the data stream into segments
• sends them over the network
• reassembles them in the correct order on the receiver's side

This lets TCP:

• control delivery
• detect loss
• retransmit data

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#What a TCP segment consists of

A TCP segment consists of two parts:

#1. The TCP header

Contains the control information needed for reliability:

• source port
• destination port
• sequence number
• acknowledgment number
• flags (SYN, ACK, FIN, and others)
• window size (for flow control)
• checksum

It is the header that makes TCP a reliable protocol.

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#2. The payload (data)

This is:

• the application's data
• a part of the overall data stream

The payload size depends on:

• the MTU
• TCP settings
• the state of the network

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#What happens to a TCP segment next

When transmitted over the network:

1. TCP forms a segment
2. It is passed to the network layer
3. The TCP segment is encapsulated in an IP packet
4. The IP packet is transmitted over the network

Routers:

• do not see TCP segments
• work only with IP packets

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#Important to remember

• a TCP segment belongs to L4
• it exists only between the end nodes
• it provides:
  • reliability
  • order
  • no duplicates
• UDP uses a similar unit, but without these guarantees

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#In short, for the interview

A TCP segment is a unit of data at the transport layer containing the TCP header and application data, used for reliable transmission of information between applications over an end-to-end connection.