Chapter 2 Applications and Layered Architectures Protocols, - PowerPoint PPT Presentation

Chapter 2 Applications and Layered Architectures Protocols, Services & Layering

Layers, Services & Protocols l The overall communications process between two or more machines connected across one or more networks is very complex l Layering partitions related communications functions into groups that are manageable l Each layer provides a service to the layer above l Each layer operates according to a protocol

Layer l A set of related communication functions that can be managed and grouped together Examples of Layers l Application Layer: communications functions that are used by application programs l HTTP (Browsing), DNS (URL to IP address), SMTP (email) l Transport Layer: end-to-end communications between two processes in two machines l TCP Reliable Stream Service, UDP Datagram Service l Network Layer: communications between machines l Internet Protocol (IP) best effort service

Protocols l A protocol is a set of rules that governs how two or more communicating entities in a layer are to interact l Messages that can be sent and received l Actions that are to be taken when a certain event occurs, e.g. sending or receiving messages, expiry of timers l The purpose of a protocol is to provide a service to the layer above

Example: HTTP l HTTP is an application layer protocol l Retrieves documents on behalf of a browser application program l HTTP specifies fields in request messages and response messages l Request types; Response codes l Content type, options, cookies, … l HTTP specifies actions to be taken upon receipt of certain messages

HTTP Protocol GET HTTP HTTP Client Server Response l HTTP provides service to a browsing application l HTTP messages exchanged between HTTP client and HTTP server l HTTP client and server are processes running in two different machines across the Internet l HTTP uses the reliable stream transfer service provided by TCP (Transmission Control Protocol)

HTTP uses service of TCP HTTP HTTP server client Response GET Port 80 Port 1127 TCP TCP GET GET Response TCP 80, 1127 bytes Response 1127, 80

Example: TCP l TCP is a transport layer protocol l Provides reliable byte stream service between two processes in two computers across the Internet l Sequence numbers keep track of the bytes that have been transmitted and received l Error detection and retransmission used to recover from transmission errors, losses, duplications l TCP is connection-oriented : the sender and receiver must first establish an association and set initial sequence numbers before data is transferred l Connection ID is specified uniquely by (send port #, send IP address, receive port #, receiver IP address) l HTTP server has well-known port # 80

Example: DNS Protocol l DNS (Domain Name System) is an application layer protocol l DNS is a distributed database that resides in multiple machines in the Internet l DNS protocol allows queries of different types l Name-to-address or Address-to-name l Mail exchange l DNS usually involves short messages and so uses service provided by UDP l Well-known port 53

Local Authoritative Name Name Server Server 1 5 4 2 3 6 Root Name Server l Local Name Server: resolve frequently-used names l University department, ISP l Contacts Root Name server if it cannot resolve query l Root Name Servers: 13 globally (actually 100’s) l Resolves query or refers query to Authoritative Name Server l Authoritative Name Server: last resort l Every machine must register its address with at least two authoritative name servers

Web Browsing Application l World Wide Web allows users to access resources (i.e. documents) located in computers connected to the Internet l Documents prepared using HyperText Markup Language (HTML) l A browser application program is used to access the web l The browser displays HTML documents that include links to other documents l Each link references a Uniform Resource Locator (URL) that gives the name of the machine and the location of the given document l Let � s see what happens when a user clicks on a link

1. DNS A. 64.15.247.200 Q. www.nytimes.com? l User clicks on http://www.nytimes.com/ l URL contains Internet name of machine (www.nytimes.com), but not Internet address l Internet needs Internet address to send packets to a machine l Browser software uses DNS protocol to send query for Internet address l DNS system responds with Internet address

2. TCP ACK ACK, TCP Connection Request From: 64.15.247.200 Port 80 To:128.100.11.13 Port 1127 TCP Connection Request From: 128.100.11.13 Port 1127 To: 64.15.247.200 Port 80 l Browser uses HTTP to send request for document l HTTP server waits for requests by listening to a well-known port number (80 for HTTP) l HTTP client sends request messages through an � ephemeral port number, � e.g. 1127 l HTTP needs a TCP connection between the HTTP client and HTTP server to transfer messages reliably

3. HTTP Content 200 OK GET / HTTP/1.1 l HTTP client sends its request message: � GET … � l HTTP server sends a status response: � 200 OK � l HTTP server sends requested file l Browser displays document l Clicking a link sets off a chain of events across the Internet! l DNS, TCP, HTTP, Browser display

Example: UDP l UDP is a transport layer protocol l Provides best-effort datagram service between two processes in two computers across the Internet l Port numbers distinguish various processes in the same machine l UDP is connectionless l Datagram is sent immediately l Quick, simple, but not reliable l Used in DNS, real-time voice,…

Summary l Layers: related communications functions l Application Layer: HTTP, DNS l Transport Layer: TCP, UDP l Network Layer: IP l Services: a protocol provides a communications service to the layer above l TCP provides connection-oriented reliable stream service l UDP provides connectionless datagram service l Each layer builds on services of lower layers l HTTP builds on top of TCP l DNS builds on top of UDP l TCP and UDP build on top of IP

Chapter 2 Applications and Layered Architectures OSI Reference Model

OSI Reference Model l A seven-layer abstract reference model for a network architecture l Provides a framework for the development of protocols l Unified view of layers, protocols, and services which is used in the development of new protocols l Detailed standards were developed for each layer, but most of these are not in use l TCP/IP protocols preempted deployment of OSI protocols

7-Layer OSI Reference Model Application Application End-to-End Protocols Application Application Layer Layer Presentation Presentation Layer Layer Session Session Layer Layer Transport Transport Layer Layer Network Network Network Network Layer Layer Layer Layer Data Link Data Link Data Link Data Link Layer Layer Layer Layer Physical Physical Physical Physical Layer Layer Layer Layer Communicating End Systems One or More Network Nodes

Physical Layer l Transfers bits across link l Definition & specification of the physical aspects of a communications link l Mechanical: cable, plugs, pins... l Electrical/optical: modulation, signal strength, voltage levels, bit times, … l functional/procedural: how to activate, maintain, and deactivate physical links… l Ethernet, DSL, cable modem, cell phone,… l Twisted-pair cable, coaxial cable optical fiber, radio, infrared, …

Data Link Layer l Transfers frames between directly connected machines l Groups bits into frames l Detection of bit errors; Retransmission of frames l Activation, maintenance, & deactivation of data link connections l Access control for local area networks (WIFI, Ethernet) l Flow control between machines of different speeds frames Data Link Data Link Layer Layer bits Physical Physical Layer Layer

Network Layer l Transfers packets across multiple links and/or multiple networks l Addressing must scale to large networks l Nodes jointly execute routing algorithm to determine paths across the network l Forwarding transfers packet along hops in path l Congestion control to deal with traffic surges l Connection setup, maintenance, and teardown when connection-based

Internetworking Ethernet LAN l Internetworking is part of network layer and provides l l l l l l transfer of packets across multiple possibly dissimilar ATM networks Network ATM l Gateways (routers) direct packets across networks Switch ATM ATM H Switch Switch ATM H Switch Net 3 Net 3 G Net 1 Net 1 G G G Net 5 Net 5 Net 4 G Net 2 G H Net 2 H G = gateway H = host

Transport Layer l Transfers data end-to-end from process in a machine to process in another machine l Reliable stream transfer or quick-and-simple single- block transfer l Port numbers enable multiplexing l Message segmentation and reassembly l Connection setup, maintenance, and release Transport Transport Layer Layer Network Network Network Network Layer Layer Layer Layer Communication Network

Application & Upper Layers l Application Layer: Provides services that are frequently Application required by applications: DNS, Application web acess, file transfer, email… Application Layer l Presentation Layer: machine- Application Layer Presentation independent representation of Transport Layer data… Layer Session l Session Layer: dialog Layer management, recovery from Transport Layer errors, … Incorporated into Application Layer