Applikasjoner. System arkitektur. Network applications: some jargon. Client-server paradigm

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1 Applikasjoner Network applications: some jargon Disposisjon overføringsproblematikk navnetjener world wide web epost 4/5/2011 Slide 1 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 4/5/2011 Slide 2 Tjener LAN Disk Komm. Prog. vare System arkitektur Navne tjener Internett Disk Komm. Prog. vare Klient LAN 4/5/2011 Slide 3 B r u k e r Typical network app has two pieces: client and server Client-server paradigm 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 4/5/2011 Slide 4 1

2 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) 4/5/2011 Slide 5 Eksempel; hardwareforskjeller Integerverdien Mest signif. Minst signif. Big-endians Byte 4 Byte 1 Small-endians Minst signifik. Mest signifik. 4/5/2011 Slide 6 Overføringssyntaks Abstrakt syntaks/overføringssyntaks Applik. data Presentasjons koding Applik. data Presentasjons dekoding Lokal repr. Applik.pros. Abstrakt syntaks kompilator Abstrakt kontekst Felles samtaleunivers Datastrukturer Operasjoner Abstrakt syntaks Lokal repr. Applik.pros. Pakke 1 Pakke 2 Pakke 3 Data må overføres i en overføringssyntaks som begge sider oppfatter på samme måte Koding og innpakking (marshalling) Overføringssyntaks Dekoding og utpakking (marshalling) 4/5/2011 Slide 7 4/5/2011 Slide 8 2

3 Samtaleunivers Eksempel på samtaleunivers felles oppfatning av hva de to prosessene kan samarbeide om en mer formell beskrivelse: et sett av datastrukturer (resident del) operasjoner som kan utføres på disse strukturene (kommunisert over nettet) <Fligth> :: <Carrier>>Flight-no> <Origin><Departure-time> <Destination><Arrival-time> <Available-seats-class-1> <Available-seats-class-2> <Operations>:: <Type> <Carrier><Fligth-no> <Class><No-of-seats> <Type>:: Inquire Book-seats Cancel-seats resident del kommuniseres over nettet 4/5/2011 Slide 9 4/5/2011 Slide 10 Marshalling/serialisering ASN.1 eksempler Serialisering Applikasjonens datastruktur Marshalling konvertering serialisering innpakking 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 Primitive typer: Bolean Integer Bit streng Oktett streng Null Objekt ID Objekt deskrip. Real Struktur typer INT 4 B1 B2 B3 B4 4/5/2011 Slide 11 4/5/2011 Slide 12 3

4 Protokollstrukturen i UNIX What transport service does an app need? Applikasjon Sockets TCP UDP IP Ethernet User space Kernel 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 4/5/2011 Slide 13 4/5/2011 Slide 14 Transport service requirements of common apps Internet apps: their protocols and transport protocols Application Data loss Bandwidth Time Sensitive Application Application layer protocol Underlying transport protocol 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 elastic elastic audio: 5Kb-1Mb video:10kb-5mb same as above few Kbps up elastic no no no yes, 100 s msec yes, few secs yes, 100 s msec yes and no remote terminal access Web file transfer streaming multimedia remote file server Internet telephony smtp [RFC 821] telnet [RFC 854] http [RFC 2068] ftp [RFC 959] proprietary (e.g. RealNetworks) NSF proprietary (e.g., Vocaltec) TCP TCP TCP TCP TCP or UDP TCP or UDP typically UDP 4/5/2011 Slide 15 4/5/2011 Slide 16 4

5 Aksjoner på ulike protokollnivåer Navnetjener Bruker: connect odin.unik.no Gethostbyname (odin.unik.no ) Getservicebyname (ftp 21) Socket (tcp; Port # > 1023) Connect ( ; 21 Applikasjon TCP TCP-PDU: (TCP-CON) Konvertere fra navn til adresser navn er brukervennlig adresser er nødvendig for fremsending av pakker Send( ; SDU) IP-addr MAC addr IP IP-PDU: (Data) husk; et endepunkt for en forbindelse er Send(MAC-addr; Data) Ethernet Ether-PDU: Data identifisert ved tripletten: IP-adresse; IP-SAP (Protocol); T-SAP (T-Port) Service (for eks. mail) velkjent port 4/5/2011 Slide 17 4/5/2011 Slide 18 Organisering av navnetjenesten Organisert hierarkisk i domener (soner), reflekteres i navnekonvensjonen for eks. smaragd.ifi.uio.no hvert domene er ansvarlig for sitt navneområde fleksibelt og forenkler administrasjonen Navnehierarkiet og soner com edu mil gov org uk no se princeton berkeley arpa Generisk Land uio Rot-sone adm. NIC ifi fys baugi mjolnir skibladnir hver sone korresponderer med to eller flere navnetjenere en navnetjener kan implementere en eller flere soner 4/5/2011 Slide 19 4/5/2011 Slide 20 5

6 Hierarki av navnetjenere Navn-adresse transaksjoner Rot navnetjener 1 rot tjener no navnetjener uio navnetjener smaragd.ifi.uio. no Klient Lokal tjener no tjener uio tjener ifi navnetjener ifi tjener 4/5/2011 Slide 21 4/5/2011 Slide 22 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 4/5/2011 Slide 23 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 2616 (erstatter RFC2068) PC running Explorer Mac running Navigator Server running NCSA Web server Port 80 is default for http server. 4/5/2011 Slide 24 6

7 The http protocol: more http is stateless server maintains no information about past client requests 4/5/2011 Slide 25 http message format: request two types of http messages: request, response http request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) header lines Carriage return, line feed indicates end of message GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr (extra carriage return, line feed) 4/5/2011 Slide 26 http request message: general format 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... 4/5/2011 Slide 27 4/5/2011 Slide 28 7

8 Trying out http (client side) for yourself 1. Telnet to your favorite Web server: telnet Type in a GET http request: GET /~ross/index.html HTTP/1.0 Opens TCP connection to port 80 (default http server port) at Anything typed in sent to port 80 at 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! 4/5/2011 Slide 29 User-server interaction: cookies server sends cookie to client in response msg 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 http request msg usual http response + Set-cookie: # usual http request msg cookie: # usual http response msg usual http request msg cookie: # usual http response msg server cookiespectific action cookiespecific action 4/5/2011 Slide 30 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 requests to web cache client if object at web cache, web cache immediately returns object in http response else requests object from origin server, client then returns http response to client origin server origin server 4/5/2011 Slide 31 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 4/5/2011 Slide 32 8

9 Electronic Mail Format of an Sending/Receiving Mail Addresses User Agent MIME Mail Transfer Agent Mail Access Protocols 4/5/2011 Slide 33 4/5/2011 Slide 34 address User agent 4/5/2011 Slide 35 4/5/2011 Slide 36 9

10 Some examples: More examples: Note: Some examples of command-driven user agents are mail, pine, and elm. Note: Some examples of GUI-based user agents are Eudora, Outlook, Thunderbird and Netscape. 4/5/2011 Slide 37 4/5/2011 Slide 38 MIME MIME header 4/5/2011 Slide 39 4/5/2011 Slide 40 10

11 Data types and subtypes in MIME Type Subtype Description Text Plain Unformatted text Multiport Message Mixed Parallel Digest Alternative RFC822 Partial Ext. Body Body contains ordered parts of different data types Same as above, but no order Similar to mixed, but the default is message/rfc822 Parts are different versions of the same message Body is an encapsulated message Body is a fragment of a bigger message Body is a reference to another message Image JPEG Image is in JPEG GIF Image is in GIF format Video MPEG Video is in MPEG format Audio Basic Single-channel encoding of voice at 8 KHz PostScript Adobe PostScript Application 4/5/2011 Slide 41 Octet-Stream General binary data (8-bit bytes) Category Type 7bit 8bit Binary Base64 Content-transfer transfer encoding Description ASCII characters and short lines Non-ASCII characters and short lines Non-ASCII characters with unlimited-length lines 6-bit blocks of data are encoded into 8-bit ASCII characters Non-ASCII characters are encoded as an equal sign followed by an ASCII code 4/5/2011 Slide 42 MTA client and server Commands and responses 4/5/2011 Slide 43 4/5/2011 Slide 44 11

12 delivery 4/5/2011 Slide 45 12