Applikasjoner. System arkitektur. Network applications: some jargon. Disposisjon. overføringsproblematikk navnetjener world wide web

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1 Applikasjoner Disposisjon overføringsproblematikk navne world wide web 5/8/2003 Slide 1 Network applications: some jargon A process is a program that is running within a host. Within the same host, two processes communicate with interprocess communication defined by the OS. Processes running in different hosts communicate with an application-layer protocol A user agent is an interface between the user and the network application. Web:browser mail reader streaming audio/video: media player 5/8/2003 Slide 2 System arkitektur Tjener Disk Komm. Prog. vare Navne Disk Komm. Prog. vare Klient B r u k e r LAN LAN Internett 5/8/2003 Slide 3 1

2 Client-server paradigm Typical network app has two pieces: client and server Client: initiates contact with server ( speaks first ) typically requests service from server, for Web, client is implemented in browser; for , in mail reader Server: provides requested service to client e.g., Web server sends requested Web page, mail server delivers e- mail application transport network data link physical request reply application transport network data link physical 5/8/2003 Slide 4 Overføringsproblemer Data representeres på ulik måte i ulike maskiner på grunn av forskjell i: hardware Small-endians og Big-endians maskiner 1-komplement og 2-komplement logikk ordstørrelse 32-bit maskin til 64-bit maskin komm. operativsystem programmeringsspråk og kompilere forskjellig lokal representasjon EBCDIC (IBM) til ASCII eller IA5 (standard) 5/8/2003 Slide 5 Eksempel; hardwareforskjeller Integerverdien Mest signif. Minst signif. Big-endians Byte 4 Byte 1 Small-endians Minst signifik. Mest signifik. 5/8/2003 Slide 6 2

3 Overføringssyntaks Applik. data Presentasjons koding Applik. data Presentasjons dekoding Pakke 1 Pakke 2 Pakke 3 Data må overføres i en overføringssyntaks som begge sider oppfatter på samme måte 5/8/2003 Slide 7 Abstrakt syntaks/overføringssyntaks Lokal repr. Applik.pros. Abstrakt syntaks kompilator Abstrakt kontekst Felles samtaleunivers Datastrukturer Operasjoner Abstrakt syntaks Lokal repr. Applik.pros. Koding og innpakking (marshalling) Overføringssyntaks Deoding og utpakking (marshalling) 5/8/2003 Slide 8 Marshalling/serialisering Serialisering Applikasjonens datastruktur Marshalling konvertering serialisering innpakking 5/8/2003 Slide 9 3

4 ASN.1 eksempler Integer > bits maskin; representert i et 32-bits ord 16-bits maskin; representert i to 16-bits ord Hver datatype kodes ved overføring: Type Lengde Verdi Type lengde type lengde verdi type lengde verdi INT 4 B1 B2 B3 B4 Primitive typer: Bolean Integer Bit streng Oktett streng Null Objekt ID Objekt deskrip. Real Struktur typer 5/8/2003 Slide 10 Protokollstrukturen i UNIX Applikasjon Sockets TCP UDP User space Kernel IP Ethernet 5/8/2003 Slide 11 What transport service does an app need? Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Bandwidth some apps (e.g., multimedia) require minimum amount of bandwidth to be effective other apps ( elastic apps ) make use of whatever bandwidth they get Timing some apps (e.g., Internet telephony, interactive games) require low delay to be effective 5/8/2003 Slide 12 4

5 Transport service requirements of common apps Application Data loss Bandwidth Time Sensitive file transfer Web documents real-time audio/video stored audio/video interactive games financial apps no loss no loss loss-tolerant loss-tolerant loss-tolerant loss-tolerant no loss elastic no elastic no elastic no audio: 5Kb-1Mb yes, 100 s msec video:10kb-5mb same as above few Kbps up elastic yes, few secs yes, 100 s msec yes and no 5/8/2003 Slide 13 Internet apps: their protocols and transport protocols Application remote terminal access Web file transfer streaming multimedia remote file server Internet telephony Application layer protocol smtp [RFC 821] telnet [RFC 854] http [RFC 2068] ftp [RFC 959] proprietary (e.g. RealNetworks) NSF proprietary (e.g., Vocaltec) Underlying transport protocol TCP TCP TCP TCP TCP or UDP TCP or UDP typically UDP 5/8/2003 Slide 14 Aksjoner på ulike protokollnivåer Bruker: connect odin.unik.no Gethostbyname (odin.unik.no ) Getservicebyname (tcp 21) Socket (tcp; Port # > 1023) Connect ( ; 21 Send( ; SDU) IP-addr MAC addr Send(MAC-addr; Data) Applikasjon TCP IP Ethernet TCP-PDU: (TCP-CON) IP-PDU: (Data) Ether-PDU: Data 5/8/2003 Slide 15 5

6 Navnee 5/8/2003 Slide 16 Navne Konvertere fra navn til adresser navn er brukervennlig adresser er nødvendig for fremsending av pakker husk; et endepunkt for en forbindelse er identifisert ved tripletten: IP-adresse; IP-SAP (Protocol); T-SAP (T-Port) Service (for eks. FTP) vel-kjent port 5/8/2003 Slide 17 Organisering av navnetjenesten Organisert hierarkisk i domener, reflekteres i navnekonvensjonen for eks. baugi.ifi.uio.no hvert domene er ansvarlig for sitt navneområde fleksibelt og forenkler administrasjonen 5/8/2003 Slide 18 6

7 Navnehierarkiet og soner Generisk Land com edu mil gov org uk no se Rot-sone adm. NIC princeton berkeley arpa uio ifi fys baugi mjolnir skibladnir hver sone korresponderer med to eller flere navnee en navne kan implementere en eller flere soner 5/8/2003 Slide 19 Hierarki av navnee Rot navne no navne uio navne ifi navne 5/8/2003 Slide 20 Navn- adresse transaksjoner Baugi.if.uio.no Klient Lokal rot no uio ifi 5/8/2003 Slide 21 7

8 The Web: some jargon Web page: consists of objects addressed by a URL Most Web pages consist of: base HTML page, and several referenced objects. URL has two components: host name and path name: User agent for Web is called a browser: MS Internet Explorer Netscape Communicator Server for Web is called Web server: Apache (public domain) MS Internet Information Server 5/8/2003 Slide 22 The Web: the http protocol http: hypertext transfer protocol Web s application layer protocol client/server model client: browser that requests, receives, displays Web objects server: Web server sends objects in response to requests http1.0: RFC 1945 http1.1: RFC 2068 PC running Explorer Mac running Navigator Server running NCSA Web server Port 80 is default for http server. 5/8/2003 Slide 23 The http protocol: more http is stateless server maintains no information about past client requests 5/8/2003 Slide 24 8

9 http message format: request two types of http messages: request, response message: ASCII (human-readable format) request line (GET, POST, HEAD commands) header lines GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr Carriage return, (extra carriage return, line feed) line feed indicates end of message 5/8/2003 Slide 25 message: general format 5/8/2003 Slide 26 http message format: response status line (protocol status code status phrase) data, e.g., requested html file header lines HTTP/ OK Date: Thu, 06 Aug :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun Content-Length: 6821 Content-Type: text/html data data data data data... 5/8/2003 Slide 27 9

10 Trying out http (client side) for yourself 1. Telnet to your favorite Web server: telnet 80 Opens TCP connection to port 80 (default http server port) at Anything typed in sent to port 80 at 2. Type in a GET : GET /~ross/index.html HTTP/1.0 By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to http server 3. Look at response message sent by http server! 5/8/2003 Slide 28 User-server interaction: cookies server sends cookie to client in response mst Set-cookie: client presents cookie in later requests cookie: server matches presented-cookie with server-stored info authentication remembering user preferences, previous choices client usual msg usual + Set-cookie: # usual msg cookie: # usual msg usual msg cookie: # usual msg server cookiespectific action cookiespectific action 5/8/2003 Slide 29 Web Caches (proxy server) Goal: satisfy client request without involving origin server user sets browser: Web accesses via web cache Proxy client sends all http server client requests to web cache if object at web cache, web cache immediately returns object in else requests object client from origin server, then returns http origin server origin server 5/8/2003 response to client Slide 30 10

11 Assume: cache is close to client (e.g., in same network) smaller response time: cache closer to client decrease traffic to distant servers link out of institutional/local ISP network often bottleneck Why Web Caching? institutional network public Internet 1.5 Mbps access link 10 Mbps LAN origin servers institutional cache 5/8/2003 Slide 31 11