Introduction to The Transport Layer of The Osi Model

Introduction

The transport layer is the 4thlayer of the OSI model. Transport layer offers peer-to-peer and end-to-end connection between two processes on remote hosts. Transport layer offers reliable connection-oriented and connectionless depending on the application. Transport layer takes data from Application layer and then breaks it into smaller size segments, numbers each byte, and hands over to Network Layer for delivery. The most common transport layer protocols are TCP Transmission Control Protocol (TCP) and the connection less UDP User Datagram Protocol (UDP) both protocols are connection-oriented. Other protocols of transport layers are

• ATP: AppleTalk Transaction Protocol
• CUDP: Cyclic UDP
• DCCP: Datagram Congestion Control Protocol
• FCP: Fibre Channel Protocol
• IL: IL Protocol
• MPTCP: Multipath TCP
• RDP: Reliable Data Protocol
• RUDP: Reliable User Datagram Protocol
• SCTP: Stream Control Transmission Protocol
• SPX: Sequenced Packet Exchange
• SST, Structured Stream Transport
• TCP: Transmission Control Protocol
• µTP: Micro Transport Protocol

Any host may have multipleapplications that are communicating across the network. Each of theseapplications will be communicating with one or more applications on remotehosts. It is the responsibility of the Transport layer to maintain the multiple communication streams between these applications.

Topic analysis

As each application creates a stream data to be sent to a remote application, this data must be prepared to be sent across the media in manageable pieces. The Transport layer protocolsdescribe services that segment this data from the Application layer. This includes the encapsulation required on each piece of data. Each piece of application data requires headers to be added at the Transport layer to indicate to which communication it is associated.

At the receiving host, each pieceof data may be directed to the appropriate application. Additionally, these individualpieces of data must also be reconstructed into a complete data stream that isuseful to the Application layer. The protocols at the Transport layer describethe how the Transport layer header information is used to reassemble the datapieces into streams to be passed to the Application layer.

In order to pass data streams to the proper applications, the Transport layer must identify the target application. To accomplish this, the Transport layer assigns an application anidentifier. The TCP/IP protocols call this identifier a port number. Each software process that needs to access the network is assigned a port number unique in that host. This port number is used in the transport layer header to indicate to which application that piece of data is associated.

The Transportlayer is the link between the Application layer and the lower layer that are responsible for network transmission. This layer accepts data from different conversations and passes it down to the lower layers as manageable pieces thatcan be eventually multiplexed over the media. Applications do not need to knowthe operational details of the network in use.

The applications generate data that is sent from one application to another, without regard to the destination host type, the type of media over which the data must travel, the path taken by the data, the congestion on a link, or the size of the network. Additionally,the lower layers are not aware that there are multiple applications sending data on the network. Their responsibility is to deliver data to the appropriate device. The Transport layer then sorts these pieces before delivering them tothe appropriate application.

Controlling the Conversations:

• Segmentation and Reassembly – Most networks have alimitation on the amount of data that can be included in a single PDU. TheTransport layer divides application data into blocks of data that are anappropriate size. At the destination, the Transport layer reassembles the databefore sending it to the destination application or service.
• Conversation Multiplexing – There may be manyapplications or services running on each host in the network. Each of theseapplications or services is assigned an address known as a port so that theTransport layer can determine with which application or service the data isidentified. In addition to using the information contained in the headers, for the basic functions of data segmentation and reassembly, some protocols at the Transport layer provide: Connection-oriented conversation, Reliable delivery, Ordered data reconstruction, Flow control transport layer services. Establishing a Session.

The Transport layer can provide this connection orientation by creating a session between the applications. These connections prepare the applications to communicate witheach other before any data is transmitted. Within these sessions, the data fora communication between the two applications can be closely managed.

• Reliable Delivery. For many reasons, it is possible for a piece of data to become corrupted, or lost completely, as it is transmitted over the network. The Transport layer can ensure that all pieces reach their destination by having the source device to retransmit any data that is lost.
• Same Order Delivery. Because networks may provide multiple routes that can have different transmission times, data can arrive in the wrong order. By numbering and sequencing the segments, the Transport layer can ensure that these segments are reassembled into the proper order.
• Flow Control. Network hosts have limited resources, such as memory or bandwidth. When Transport layer is aware that these resources are overtaxed, some protocols can request that the sending application reduce the rate of data flow. This is done at the Transport layer by regulating the amount of data the source transmits as a group. Flow control can prevent the loss of segments on the network and avoid the need for retransmission.

Error Control : Error Control is performed end to end in this layer to ensure that the complete message arrives at the receiving transport layer without any error. Error Correction is done through retransmission.

The two most common Transport layer protocols of TCP/IP protocol suite are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Both protocols manage the communication of multiple applications. The differences between the two are the specific functions that each protocol implements.

1. User Datagram Protocol (UDP). UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for lowoverhead data delivery. The pieces of communication in UDP are called datagrams. These datagrams are sent as "best effort" by this Transport layer protocol. Applications that use UDP include: Domain Name System (DNS), Video Streaming, Voice over IP (VoIP)
2. Transmission Control Protocol (TCP). TCP is a connection-oriented protocol, described in RFC 793. TCP incurs additional overhead to gain functions. Additional functions specified by TCP are the same order delivery, reliable delivery, and flow control. Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data, whereas each UDP segment only has 8 bytes of overhead. See the figure for a comparison.