The transport layer responsible for establishing a temporary communication session between two applications and delivering data between them

Manages end-to-end connections between hosts through sockets
Segments data for manageability and reassembles segmented data at the destination (splitting data into packets and back into data)
It manages data integrity, error correction and delivery guarantee (TCP)

UDP

User Datagram Packet is the basic transport layer protocol, providing an unreliable datagram service
- Online gaming and live video can use it as lost data is OK. Even VPN can use UDP as the tunnelling protocol takes care of lost packets

TCP

Transmission Control Protocol provides flow-control, connection establishment, and reliable transmission of data
- Anything where all transmitted data must be received uses TCP, but there’s a processing overhead for using it and it can be slow
TCP vs UDP
- Think of TCP and UDP as using Royal Mail
- If you are sending money, you will use recorded delivery and would only consider sending more money, once you have acknowledged that the first batch arrived safely
  - This is TCP. It will only send more packets, once receipt has been acknowledged. If not, it will resend.
- If you are just sending a standard letter, you put it in the post and hope it gets there. You would not receive an acknowledgement
  - UDP tries but doesn’t care if it fails. Packets are sent without worrying if previous ones got there OK.
  - It is deliberately light-weight and fast
- TCP’s job is to establish a connection to the remote device before any data is sent
  - It is therefore a connection oriented protocol
  - Once a connection is established, data can flow in both directions (bidirectional)
  - Packets are sent out and numbered (for resequencing once received) and it will wait for acknowledgement before sending more data
- UDP’s job is to efficiently send out data without the overhead of TCP
  - No connection is established before sending data
  - UDP does not insert sequence numbers
    - The packets are expected to arrive as a continuous stream, or they are dropped

While the Internet Layer uses IP Addressing, the Transport Layer uses ports
- The term socket is the combination of IP Address (Extended) and port #

A port is a way to address specific application and have a source and destination port address. Each address is 16 bits.
- It works like an IP Address (Extended), but for applicaiton protocols rather than machines. A bit like having individual names on letter to the same house
There are well known port numbers such as SMTP using port 25, HTTP using port 80, etc.
- However, these can be changed in software