Introduction transport layer in computer networking

 Introduction transport layer in computer networking

The transport layer is the fourth layer of the OSI Model. It provides host to host communication services for the application. It receives data from the application, and breaks it into chunks called segments, and sends them to the network router for delivery. The transport layer is operated by Operating system and also referred to Heart of OSI Model. Today, in this post we will explore various protocol of the transport layer.

The most important and common protocols of the transport layer include TCP(Transmission Control Protocol), UDP(User Datagram Protocol) and SCTP(Stream Control Transmission Protocol).

The transport layer is a 4th layer from the top.

The main role of the transport layer is to provide the communication services directly to the application processes running on different hosts.

The transport layer provides a logical communication between application processes running on different hosts. Although the application processes on different hosts are not physically connected, application processes use the logical communication provided by the transport layer to send the messages to each other.

The transport layer protocols are implemented in the end systems but not in the network routers.

A computer network provides more than one protocol to the network applications. For example, TCP and UDP are two transport layer protocols that provide a different set of services to the network layer.

All transport layer protocols provide multiplexing/demultiplexing service. It also provides other services such as reliable data transfer, bandwidth guarantees, and delay guarantees.

Each of the applications in the application layer has the ability to send a message by using TCP or UDP. The application communicates by using either of these two protocols. Both TCP and UDP will then communicate with the internet protocol in the internet layer. The applications can read and write to the transport layer. Therefore, we can say that communication is a two-way process.

Properties of UDP

They provide connectionless service.

The UDP is an unreliable transport protocol with no sessions or flow control and optional error checking.

They don’t provide inherent congestion control or reliability.

UDP is an unreliable service that provides no guarantees for delivery.

They provide unreliable and message-passing transport to applications.

UDP just sends packets as soon as requested and forgets about them.

Standard application using UDP includes:

Trivial File Transfer Protocol (TFTP)

Domain Name System (DNS) name server

Remote Procedure Call (RPC), used by the Network File System (NFS)

Simple Network Management Protocol (SNMP)

Lightweight Directory Access Protocol (LDAP)

Transmission layer protocol

The Transmission Control Protocol (TCP) is a connection-oriented reliable protocol and standardized by rfc793.

Properties of TCP

Connection: TCP provides connections between clients and servers. A TCP client establishes a connection with a server, exchanges data across the connection, and then terminates the connection.

Round-trip time (RTT): TCP estimates RTT between a client and server dynamically so that it knows how long to wait for an acknowledgment.

Sequencing: TCP associates a sequence number with every byte (segment, unit of data that TCP passes to IP.) it sends. TCP reorders out-of-order segments and discards duplicate segments.

Flow control: TCP provides a means for the receiver to govern the amount of data sent by the sender. This is achieved by returning a “window” with every ACK indicating a range of acceptable sequence numbers beyond the last segment successfully received.

Multiplexing: Multiplexing is the process of combining two or more data streams into a single physical connection. TCP provides multiplexing facilities by using source and destination port numbers. These port numbers allow TCP to set up a number of virtual connections over a physical connection and multiplex the data stream through that connection.

Precedence and security: The users of TCP may indicate the security and precedence of their communication.

Full-duplex: an application can send and receive data in both directions on a given connection at any time.

Reliability: TCP requires acknowledgment when sending data. If an acknowledgment is not received, TCP automatically retransmits the data and waits a longer amount of time.

Application: TCP is used in applications such as telnet, World Wide Web (WWW), FTP and electronic mail.

Services provided by the Transport Layer

The services provided by the transport layer are similar to those of the data link layer. The data link layer provides the services within a single network while the transport layer provides the services across an internetwork made up of many networks. The data link layer controls the physical layer while the transport layer controls all the lower layers.

The services provided by the transport layer protocols can be divided into five categories:

End-to-end delivery

Addressing

Reliable delivery

Flow control

Multiplexing