The Application Layer - Computer Networks - Lecture Slides, Slides of Computer Networks

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Computer Networking

Technology I

The Application Layer

Application Layer

The Application Layer is the reason the rest of the network exists – to serve applications Most of the software familiar to end users are applications Email, FTP, newsgroups, chat, the Web, streaming video, video conferencing, IPTV, etc. We focus first on key concepts related to the Application Layer, then discuss some specific applications in detail

Client-server Architecture

In client-server architecture, the server Handles requests from many clients, and Is generally always available Often has a fixed IP address Clients generally don’t communicate with each other, and may be on or off independently of each other and the server Client-server applications include email, FTP,the Web, remote login

Client-server Architecture

Complex infrastructure intensive apps might require several types of servers – database, web, etc. Multiple servers may be needed to keep up with the volume of client requests, hence the need for a server farm or data center

P2P Architecture

P2P architecture is inherently self-scalable Millions of computers may participate, because each computer adds capacity at the same time it adds possible workload Managing contents of a P2P application can be difficult Only one computer may have a particular file, and there’s no control over when that computer is available

P2P Architecture

Key challenges in a good P2P app include ISP friendly, since most residential connections are designed for far more bandwidth down than up, and P2P doesn’t follow this Security, danger of over-sharing Incentives for people to participate

Process Communication

Any network application (no matter which architecture) needs to communicate between hosts using processes In this sense, a process is a program running on a client, server, or peer host Processes may communicate with other processes on the same host; thisis controlled by the host’s operating system (OS) We are interested in processes that communicate between hosts

Process Communication

Processes exchange messages The sending or client process creates a message and sends it into the network The receiving or server process gets the message from the network and might reply Notice that client and server process only relate to their relative roles in sending a message, not the client-server or other architectures mentioned earlier

processSockets

TCP with buffers, variables

socket

host or server

process

TCP with buffers, variables

socket

host or server

Internet

controlled by OS

controlled by app developer process

TCP with buffers, variables

socket

host or server

processprocess

TCP with buffers, variables

TCP with buffers, variables

socket

host or server

process

TCP with buffers, variables

socket

host or server

processprocess

TCP with buffers, variables

TCP with buffers, variables

socket

host or server

Internet

controlled by OS

controlled by app developer

Could be UDP on both ends

Sockets

A socket is the Application

Programming Interface (API)

between application and

the network

The API is all the developer sees of the

network connection Docsity.com

Addressing Processes

Client process Server processIP address

Socket (^) Port

TCP or UDP^ Internet

and lower Layers

TCP or UDP and lower Layers

Sockets send packets Ports listen for them

Port Number

Port numbers follow default values, set by

the IANA, unless specified otherwise

21 = FTP

23 = Telnet

25 = SMTP

53 = DNS

Application vs its protocols

A single application often needs to

use several application-layer

protocols

A web browser might use HTTP, but

also FTP, telnet, gopher, etc.

An email application might use POP3, Docsity.com

RFC Summary

For an RFC which lists the current

RFC standards, look in the RFC Index

for

“Internet Official Protocol Standards”

The current one is RFC 5000, dated May