Notes on Transport Layers

The protocols are the medium that creates a connection between the computers for data transmission. They are a set of rules that are standard and designed for the users to communicate. These transmission protocols are made of layers. One of them is the transport layer that adds error handling information like Cyclic Redundancy Check (CRC).

Delivery of Data Packets

Packets are small bits of information that are broken down into small units of transmission for sending. 

Structure of Packets-

1. Header- The header consists of the information about the packet number, length of the packet, originating address, protocol and destination address.
2. Payload- It is considered the body of the packet. It contains the data that is needed to be delivered.
3. Trailer- It is also called a footer and may use some units of the receiving device and may perform some error checking.

The packets can be delivered by two means- 

• Direct Delivery- In this method, the origin or the last router and destination are in the same physical network.
• Indirect Delivery– Indirect delivery means the packets are transmitted from one entity to another until it reaches the destination in the same physical network.

The packets are delivered at a fast rate, and the maximum length of the information is allowed for each packet. For this purpose, the message received to the transport layer is first divided by transport protocol into packets.

The flow of data, segmentation, desegmentation and error control

The Transmission Control Protocol is the one that controls the flow of data and performs segmentation and desegmentation, and error control.

• The flow of Data- The flow of data is necessary for communication between networks that have different speeds. An example is that a mobile phone should process the data received from the server and regulate the data before it overloads the server. The sender cannot send more data until they get an acknowledgement from the receiver for receiving the data.

• Segmentation – In this process, the data is split into small segments, which are referred to as Data Streams by TCP/IP. To place this data in the TCP data field, the segmentation of data is necessary. After segmentation, the data segments and header of TCP are passed on further to IP. 

• Desegmentation – In this process, the segments are reassembled back into original data.  For example- If data is entered in HTTP format, the server loads the web page and encapsulates the text in the header. It passes the encoded text to TCP. After receiving the data segments, TCP reassembles the coded text into readable form by the web page.
• Error control- TCP gives a sequence number to each byte of a stream. It helps in recognising the errors and retransmitting them back for correction. TCP performs error recovery from end-to-end communication between server and client.

Five classes of Transfer Protocol 

The five classes of Transfer Protocol are-

1. Transmission Control Protocol/ Internet Protocol (TCP/IP)- The TCP/IP is used by the internet and deals with sequential data and handles packet forwarding. When a message or file is sent through a network, the TCP breaks them down into small packets. Each packet has a header with the destination address, and these packets are transferred individually. 

• File Transfer Protocol – The file transfer protocol also involves the Secure File Transfer Protocol (SFTP) and Trivial File Transfer Protocol (TFTP). FTP and SFTP function almost similarly from transferring the data from the server to the client. The TFTP operates a bit differently by setting up small units where the data can be transferred quickly without a traffic load.
• HyperText Transfer Protocol (HTTP)- In HTTP/HTTPs, the data sent from client to server is encrypted. All the application layers use this protocol. The file is transmitted in the form of hypertext which is readable by a web browser. The Transport layer plays a role in identifying the traffic and verifies correct packet arrival.
• E-mail Protocols (POP3, SMTP, IMAP)- Transferring the files in the form of email documents. 

• Simple Mail Transfer Protocol- The email is sent to the SMTP server and forwarded to the server for identifying how the messages need to be sent to the receiver.
• Post Office Protocol- POP3 has 3 versions. Port 995 for encrypted traffic, port110 for unencrypted traffic and for unencrypted traffic in IMAP, it uses port 143, port 993 for encrypted ones.
• User Datagram Protocol- It is a substitution for Transmission Control Protocol.

It does not carry out all the error checking done by the TCP and primarily is employed for creating low latency linking with various networks.

Conclusion–

We come to know that these methods are introduced to reduce the traffic load on the webserver and to minimise the long delays that take place while communicating between client and server. It means that not all packets are encrypted and secured, or the network traffic load is safe, but these protocols and packets can mitigate the high risk from attackers who create malicious network traffic.

Data packets by the transport layer enable multiple connections to occur in the same networking sites.