Data Communication: Network and Transport Layers
In data communication, effective communication is only achieved when data, is successfully moved from the one destination to the other. The transport and the network layers are responsible for moving data from one end of the network to the other. The two layers mainly overlap which is a major reason they are analyzed and discussed together. The paper addresses the functions of the transport layer and the network layer. There are three functions of the transport layer that include; segmenting, session management and linking the application layer to the network layer. On the other hand, the network layer has two primary functionalities; routing and addressing. The transport and network layer protocols specify certain protocols that determine how the packets are to be organized. However, the transmission control protocol (TCP) also known as the internet protocol (IP) takes prevalence where the network and transport layers are concerned.
The transport layer provides a link between the application layer and the network layer. In addition to this, the layer is also responsible for the end-end delivery of messages. On one end, the transport layer accepts application layer outgoing messages. Some of the messages that the transport layer accepts in this regard are emailing and web data. Once the messages are received, they are segmented for transmission. The protocol data unit in this layer responsible for the segmentation is called a segment. Once the messages are segmented, they are handled over to the network layer. Here the network layer routes the messages through the network by determining the best possible route from the source to the required destination. There are a number of variables that the network layer considers prior to choosing a route. The chosen route happens to be the best approach to be adopted by the packets. An IP is appended to the message before being routed, and the packet is passed to the data link link. Here, the layers add a frame known as the Ethernet frame and instructs the physical layer on when to transmit the data.
Both the network and transport layers accept incoming messages from the data link layers and organize them coherently before passing them to the application layer. The process of message transfer is a coordination where the transport layer breaks the message into segments, passes the segments to the Network layer for routing, which in turn passes the data to the data link layer for transmission. As previously stated, the transport layer is also responsible for addressing and routing by the network layer. However, there exists a coordination between the two layers as will be evaluated in this paper. To achieve these functionalities, there are a number of protocols prevalent in the two layers. Of all the protocols, one emerges as dominant among them; the TCP/IP.
The transmission control protocol or the internet protocol (TCP/IP) was originally developed for the department of defense in the United States. However, the layer is currently the most popular protocol in the world and as such it is used on the Internet. There is an efficient transmission of data using this protocol as it enhances error-free transmissions, performs error checking and handles large files for sending across unreliable networks with a high guarantee of a low rate of data corruption. Another popularity of the link emanates from the compatibility of the protocol with data link protocols. The layer has two parts, and one part is on the transport (TCP) that links the application layer to the network layer. The layer is responsible for performing segmentation where the data is broken into segments. The segments are then numbered and transmitted to the destination which a high rate of reliability. Once the segments reach the destination, they are properly ordered to provide a coherent meaning that corresponds with the initial meaning of the transmitted message. The other part is the IP that is the network layer protocol responsible for addressing and routing functions. For all the computers through which the message passes, an IP is used. The IP is responsible for routing the message to the desired destination. TCP only plays a role when the data enters or leaves the application layer. For this reason, the software only needs to be active at the sender and receiver side.
A TCP comprises of 24 bytes of information control. The source and destination ports are defined in the header information. The defined destination port is responsible for specifying to the TCP, which application layer the packet should be sent. The source port, on the other hand, is responsible for specifying the origin of the packet. There is a sequence number that is associated with the TCP that enables the destination to reassemble the packets in the correct order and ensure that there are no lost segments. Another form of transport layer protocols present is the User Datagram Protocol (UDP) that sends message segments inform of datagrams. The protocol is commonly used where there is the need to send a single small packet. The protocol eliminates the need of the TCP to segment the packet and reassemble them on receipt. There are four fields present in a UDP and additional source and destination ports and the application layer packet. As opposed to the TCP, UDP does not check for lost datagrams, and it is for this reason that the protocol is rarely used. However, the decision regarding what protocol to use is left to the application programming engineer.
The IP is present at the network layer, and the PDUs present at the network layer are known as packets. There are two version of IP in use; the IP version 4 and the IP version 6. The former is an older version that is characterized by the presence of 24 bytes. It contains the source and destination, the packet number, and the packet length. Due to the increase in the communication devices, the IPv4 is being dropped in favor of the most robust IPv6. The latter is characterized by 40 bytes and offers a more increased address size from the initial 32 bits to 128 bits. In addition to this, IPv6 offers a simpler addressing and routing approach that enhances a variety of new, more stable approaches to routing and addressing.
The transport layer has a variety of functions among them linking to the application layer, segmenting and management of sessions. With every computer having a unique port address, it becomes easy for the transport layer to identify the source and destination of the packets as provided by their respective computers. In segmenting, TCP ensures that large volumes of messages are transmitted in the form of segments that are broken down and relied on different routes and later reassembled at the destination. The protocol oversees that all the segments are received on the receiving end. However, a major challenge facing this protocol is determining the size of the segments. It is commonly to have messages being broken down into varying segment sizes. Session management involves establishing a link and retaining the link during the communication of the networked devices. There are two approaches to managing sessions that are; connection-oriented and connectionless messaging approach.
In conclusion, there are three different types of addresses present in computers. The application layer address, the data link layer address and the network layer address. Data link layer address are part of the hardware while the network layer and application layer addresses are set by the software. The process of translating n application layer address to a network address or a network address to a data link layer address is known as address resolution. The domain name server is responsible for address resolution on receiving a special message.