It is similar sending data (the announcement ) without any rules and regulations (class decorum).
This is where a protocol comes in - A set of rules that defines what to communicate , when to communicate and how to communicate , ensuring the message reaches the right place in the right way.

In this blog , we will deep dive into TCP(Transmission Control Program Protocol) and uncover how it brings order , reliability and discipline to data communication - opening the black box behind one the most important protocols of the networking.

What is TCP ?

TCP is like a teacher distributing the papers to student by student (data sent in the order) in the exam room .
Each student confirms (Acknowledgement) , if a paper is missing , teacher re-gives the paper to that student (re-transmission).
Teacher waits if students aren’t ready (flow control) and silence in the class ensure no confusion(congestion control).

Teacher ensures that paper is delivered to each student , in order and reliably , not fast and Chaotic .

TCP is a protocol that allows devices to communicate reliably over the network. It ensures that reaches the data at the destination in the right order and in the right way .
It works at Transport Layer (Layer - 4 ) in the OSI model and is essential part of the TCP/IP suite.

TCP Working

TCP works step by steps:-

1. Connection is created

• TCP makes sure before sending the data , both sides are ready . It uses 3-way handshake for the connection establishment.

3-way Handshake

• It is calling someone : ring → Hello → okay , talk

• In similar way, TCP uses syn , syn-ack and ack segment to establish connection.

1. Synchronize(SYN) :-

   Client send the syn segment to receiver for requesting the connection .

2. Synchronize-Acknowledge (SYN-ACK) :-

   Server responds with syn-ack segment , acknowledging the request and agreeing to the connection.

3. Acknowledgement :-

   Client again sends an ack segment to confirm the connection is established.

4. Segmenting:-

   When an application sends the data , it is broken into smaller chunks called segment . Each segment has a header containing the information like sequence numbers , ports and flags

5. Data transmission:-

   Segments are transmitted over the network . They may arrive out-of-order or different paths.

6. Acknowledgement & Reliability

   Receiver sends an ack for successfully received segments . If an ack is not received within a time-limit , segment is re-transmitted.

7. Flow Control

   TCP uses window size to control the amount of the data , ensuring the receiver is not overloaded.

8. Congestion Control

   TCP adjusts its sending rate based on network to avoid traffic congestion

9. Connection Termination

   After data is transmitted , connection is terminated using FIN and ACK messages.

Sequence number and Acknowledgement in TCP

Sequence number

Sequence number tells from which the DATA byte starts . TCP counts bytes , not packets.

From the diagram:

Clients sends SYN , SEQ = 100 which means data will start from 100.

After the handshake, first data:

• DATA (100 bytes), SEQ = 101.

  Next data:

• DATA (200 bytes), SEQ = 201

  SEQ always increases by number of bytes sent.

Acknowledgement Number

It tells the byte number expected. It means ”I received everything before this byte”.

From the diagram:

ACK = 401 means I have received bytes 101 to 400. Sends byte 401 next .

After client sends 50 bytes (SEQ = 401)

Server replies :

• ACK = 451 means Data till byte 450 received.

Packet loss and re-transmission Flow

1. Data Transmission
   The sender transmits data segments, each with a sequence number that identifies the starting byte of the data.

2. Packet Loss Occurs
   Due to network issues, a data packet may get lost before reaching the receiver.

3. No Acknowledgement Received
   Since the receiver does not get the packet, it cannot send an ACK for that data.

4. Sender Waits for Timeout
   The sender waits for a specific time called the retransmission timeout (RTO).

5. Loss Detection
   If no ACK is received before the timeout expires, the sender assumes the packet is lost.

6. Retransmission
   The sender retransmits the same packet using the same sequence number.

7. Acknowledgement After Retransmission
   Once the receiver gets the retransmitted packet, it sends an ACK indicating the next expected byte.

8. Normal Transmission Continues
   Data transfer continues smoothly after successful acknowledgement.

Connection Termination

TCP uses 4-way Handshake for the connection termination.

• 1. FIN:
  The client sends a FIN packet to the server, indicating that it has finished sending data and wants to close its side of the connection.

  2. ACK:
  The server replies with an ACK, confirming that it has received the client’s close request.

  At this point, the client cannot send data, but the server can still send data if required.

  3. FIN:
  When the server finishes sending its data, it sends a FIN packet to the client, indicating that it also wants to close its side of the connection.

  4. ACK:
  The client sends an ACK to the server, confirming that it has received the server’s FIN request.