Application Layer / Transport Layer

Application Layer / Transport Layer

Application Layer


The application layer is where network applications and their application-layer protocols reside. The internet's application layer contains many protocols, such as the HTTP protocol (which provides for Web document request and transfer), SMTP (which provides for the transfer of e-mail messages), and FTP (which provides for the transfer of files between two end systems). We'll see that certain network functions, such as the translation of human-friendly names for Internet end systems like www.ietf.org to a 32-bit network address, are also done with the help of a particular application-layer protocol, namely, the domain name system (DNS). We'll see in "Application Layer" that it is very easy to create and deploy our own new application-layer protocols.

An application-layer protocol is distributed over numerous end systems, with the application in one end system using the protocol to exchange packets of information with the application in another end system. We'll refer to this packet of information at the application layer as a message.


Transport Layer


The Internet's transport layer transports application-layer messages between application endpoints. In the Internet there are two transport protocols, TCP and UDP, either of which can transport application-layer messages. TCP provides a connection-oriented service to its applications. This service includes guaranteed delivery of application-layer messages to the destination and flow control (that is, sender/receiver speed matching). TCP also breaks long messages into shorter segments and provides a congestion-control mechanism, so that a source throttles its transmission rate when the network is congested. The UDP protocol provides a connectionless service to its applications. This is a no-frills service that provides no reliability, no flow control, and no congestion control. !n this blog, we'll refer to a transport-layer packet as a segment.



Tags

application layer, transport layer, network application

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