Transport Layer

Transport Layer

1. The transport layer dictates how data is to be sent and received between individual devices.

2. It uses the protocols of TCP and UDP to achieve this.

3. TCP

   • Stands for Transport Control Protocl
   • TCP is a connection-based protocol. This allows it to use various methods to ensure that packets arrive safely, such as delivery acknowledgment and retransmission. However, establishing a connection takes time and the overhead of TCP is higher than UDP.
   • Prior to data transmission using TCP a "three-way handshake" must take place between the communicating devices. The initiating device sends a packet to the recipient device. If the recipient successfully receives this packet it sends back a packet of its own in acknowledgement. The initiating device then sends back another packet acknowledging the acknowledgment and communication can begin.
   • TCP packet headers contain the source and destination ports of the packets, the position of the packet in the sequence of packets, an acknowledgement number for delivery acknowledgment, a checksum and many other fields.
   • TCP uses congestion control to delay the sending of packets until a network has more capacity available. This prevents packet loss but may also result in delays for the client.

4. UDP

   • Stands for User Datagram Protocol
   • UDP is a lightweight connectionless protocol. It has minimal error correction or recognition capability but has less overhead and is faster than TCP due to the lack of handshaking.
   • Due to its connectionless nature UDP does not require the establishment of an End-to-End connection. The host merely sends packets to the client without checking to see if they arrive.
   • UDP packet headers contain the source port of the packets, the destination port, the length of the packet and a checksum for basic error recognition.
     • Due to the connectionless nature of UDP errors cannot be rectified. Corrupted packets are simply discarded.
     • The header does not dictate the position of the packet in the file being sent so files may be recreated in a garbled fashion.
   • UDP does not have congestion control capabilities. It simply sends packets out as fast as possible. This can increase the risk of file loss or corruption on busy networks.