TCP Protocol

network Network MOC

---

The Transmission Control Protocol focuses on reliability and is common for most tasks. Setup requires a handshake and can retransmit data. The downside of TCP is its slow transmission compared to the UDP Protocol.

Handshake

1. Computer A sends packet with the synchronize flag (SYN) to computer B with a randomly generated sequence number.
2. Computer B replies with synchronize flag (SYN) and acknowledge flag (ACK) containing another random number and the acknowledgement number, which is the sequence number of Computer A incremented by 1.
3. Computer A responds with just the acknowledge flag (ACK) and their acknowledgment number.

sequenceDiagram
	participant A as Computer A
	participant B as Computer B
	A->>B: [SYN] Seq=5 Len=0
	B-->>A: [SYN ACK] Seq=3 Ack=6 Len=0
	A->>B: [ACK] Seq=6 Ack=4 Len=0

Transmission

Unlike the handshake and teardown, data is sent with the Len showing length of data and the next acknowledge number is the previous Len + 1.

sequenceDiagram
	participant A as Computer A
	participant B as Computer B
	A->>B: [PSH, ACK] Seq=5 Ack=2 Len=12 [Data = Hello World]
	B-->>A: [ACK] Seq=2 Ack=13 Len=0

Teardown

1. Computer A sends a finish packet (FIN) with current sequence number. The ACK is to acknowledge the previous package.
2. Computer B responds with ACK with acknowledgment number as Computer A’s sequence number incremented by 1.
3. Computer B sends a FIN + ACK with same acknowledgment number.
4. Computer A sends a ACK packet with acknowledgment number as Computer B’s sequence number incremented by 1.

sequenceDiagram
	participant A as Computer A
	participant B as Computer B
	A->>B: [FIN, ACK] Seq=13 Ack=1 Len=0
	B-->>A: [ACK] Seq=1 Ack=14 Len=0
	B-->>A: [FIN, ACK] Seq=1 Ack=14 Len=0
	A->>B: [ACK] Seq=14 Ack=2 Len=0