How does TCP work?

TCP provides reliable, connection-oriented delivery of data in IP networks.

Key stages and mechanisms:

1. Connection establishment (Three-way Handshake):

   • The client sends a SYN packet with an initial sequence number (ISN).
   • The server responds with a SYN-ACK packet, confirming receipt of SYN and sending its ISN.
   • The client sends an ACK packet, confirming receipt of SYN-ACK.

2. Data transfer:

   • Data is segmented.
   • Each segment has a header with a sequence number and checksum.
   • The receiver sends an acknowledgment (ACK) upon receiving a segment, indicating the next expected segment number.

3. Flow control:

   • The receiver informs the sender of the available buffer size (window size).
   • The sender does not send more data than the receiver's window size, preventing buffer overflow.

4. Error control:

   • Checksums are used to verify segment integrity.
   • If a segment is lost or corrupted, the receiver does not send an ACK for it. The sender retransmits the segment after a timeout.

5. Congestion control:

   • Mechanisms to prevent network congestion (e.g., Slow Start, Congestion Avoidance, Fast Retransmit, Fast Recovery).
   • The window size dynamically adjusts based on network conditions (packet loss, RTT).

6. Connection termination (Four-way Handshake):

   • One side sends a FIN packet indicating it will not send more data.
   • The other side acknowledges the FIN (ACK) and may continue sending its data.
   • When the other side is ready, it sends its FIN packet.
   • The first side acknowledges the FIN (ACK).

Differences from UDP:

Characteristic	TCP	UDP
Orientation	Connection establishment	Connectionless
Reliability	High (guaranteed delivery, order)	Low (no guarantees)
Error control	Yes	No
Flow control	Yes	No
Congestion control	Yes	No
Speed	Lower	Higher
Header size	Larger (20-60 bytes)	Smaller (8 bytes)