TCP: An Overview

TCP is a widely used network protocol. It's a "reliable" protocol that runs on top of an unreliable protocol: IP, short for Internet Protocol.

Let's dive deeper and understand how TCP works under the hood.

We'll start by looking at IP (short for Internet Protocol), the protocol that TCP builds on top of.

IP - The Internet's postal service

When a program sends data over the network using IP, the data is broken up and sent as multiple "packets".

Each packet contains:

• a header section
• a data section

The header contains a source and destination address, much like an envelope that you send through your local postal service.

The important similarity between IP and a postal service is that packets are not guaranteed to arrive at the destination. Although every effort is made to get it there, sometimes packets get lost in transit.

Furthermore, if you send 5 packets at once, there's no guarantee that they'll arrive at their destination at the same time or in the same order.

TCP was created to address these limitations of IP.

TCP's Guarantees

Primarily, TCP offers two guarantees: (a) Reliable delivery of packets and (b) In-order delivery of packets.

Guarantee #1 - Reliable Delivery

TCP ensures that no packets are lost in transit. It does this by asking the receiver to acknowledge all sent packets, and re-transmitting any packets if an acknowledgement isn't received.

Guarantee #2 - Ordered Delivery

In addition to guaranteeing packets reach their destination, TCP also guarantees that the packets are delivered in order. It does this by labelling each packet with a sequence number. The receiver tracks these numbers and reorders out-of-sequence packets. If a packet is missing, the receiver waits for it to be re-transmitted.

TCP Connections

TCP is a connection-oriented protocol, which means that to interact over TCP a program must first "establish a connection". To do this, one program takes the role of a "server", and the other program takes the role of a "client".

The server waits for connections, and the client initiates a connection. Once a connection is established, the client & server can both receive and send data (it's a two-way channel).

A TCP connection is identified using a unique combination of four values:

• destination IP address
• destination port number
• source IP address
• source port number

For example, let's say you're connecting to google.com. The values for your TCP connection will look like this:

• destination IP address: x.x.x.x (Google's web server)
• destination port number: 443 (Default port used by HTTPS)
• source IP address: y.y.y.y (your computer)
• source port number: 26789 (a random number, unused by any other connection on your computer)

If your browser opens multiple connections to Google's server, only the "source port number" will change, the rest will remain the same.

TCP Handshake

The TCP handshake is how clients establish connections with servers. This is a 3-step process.

Step 1: SYN

First, the client initiates the connection by sending a SYN (synchronize) packet to the server, indicating a request to establish a connection. This packet also contains a sequence number to maintain the order of the packets being sent.

Step 2: SYN-ACK

The server, upon receiving this SYN packet, sends back a SYN-ACK (synchronize-acknowledge) packet.

Step 3: ACK

In the final step of this three-way handshake, the client acknowledges the server's SYN-ACK packet by sending an ACK (acknowledge) packet. The connection is considered established once this last packet is received by the server.

The three-way handshake ensures a reliable connection is created, verifies both devices are ready for data transmission, and sets initial sequence numbers for proper ordering of data packets.