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Chapter 2: Application Layer

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주요 내용

❒ 클라이언트/서버❒ 서비스모델

❒ 응용계층 프로토콜

(application layer protocols)

❒ http❒ ftp❒ smtp❒ pop ❒ dns❒ 소켓프로그래밍

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응용계층

❒ 분산응용에서의통신

사용자공간

메시지교환

예) 이메일, 웹, 파일전송❒ 응용계층프로토콜

❍ 응용간교환되는메시지와그에따른행동규정

❍ 트랜스포트계층(TCP,UDP)등하위계층의서비스이용

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

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용어 정의

❒ 프로세스( process): 호스트에서작동중인

프로그램

❒ Interprocesscommunication :같은호스트내의프로세스간통신

❒ 다른호스트에있는프로세스들: 응용계층프로토콜을통해메시지교환

❒ 사용자에이전트: 사용자와네트워크응용간의인터페이스

❍ 웹(응용) –브라우저(에이전트)

❍ E-메일: 메일리더❍ 비디오스트리밍: 미디어플레이어

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클라이언트-서버

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

클라이언트:❒ 서버에접속시도

❒ 서버에게서비스요청

❒ 웹: 브라우저가클라이언트서버:❒ 요청받은서비스제공

❒ 예) 웹서버가요청받은웹페이지제공

요청

응답

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응용계층프로토콜

API: application programming interface

❒ 응용계층-트랜스포트계층인터페이스

❒ 소켓(socket): 인터넷API

❍ 두프로세스가소켓을통해메시지교환

Q: 어떻게다른호스트에있는 프로세스를구분하는가?

❍ IP address❍ port number

… lots more on this later.

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응용이요구하는트랜스포트서비스는?

신뢰성(reliability)❒ 오디오/비디오등은약간의데이터손실감수

❒ 화일전송, E-메일등은신뢰성요구

지연(Delay)❒ 오디오응용등은최저

지연요구대역폭(Bandwidth)❒ 멀티미디어응용은일정대역폭요구

❒ 다른응용은탄력적(elastic)

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주요 응용의 트랜스 포트서비스 요구

응용응용응용응용

file transfere-mail

Web documentsreal-time audio/video

stored audio/videointeractive games

financial apps

신뢰성신뢰성신뢰성신뢰성

no lossno lossloss-tolerantloss-tolerant

loss-tolerantloss-tolerantno loss

대역폭대역폭대역폭대역폭

elasticelasticelasticaudio: 5Kb-1Mbvideo:10Kb-5Mbsame as above few Kbps upelastic

최저최저최저최저지연지연지연지연요구요구요구요구

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no

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트랜스포트프로토콜이제공하는서비스

TCP :❒ 연결형(connection-

oriented) ❒ 신뢰성(reliable

transport)❒ 흐름제어(flow control) ❒ 정체제어(congestion

control)❒ 제공안함:대역폭, 최저지연보장

UDP:❒ 신뢰성보장안함

❒ 비연결형

Q: UDP의존재이유는?다중화(응용간)

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인터넷 응용: 프로토콜및 사용하는 트랜스포트프로토콜

응용응용응용응용

e-mailremote terminal access

Web file transfer

streaming multimedia

remote file serverInternet telephony

응용계층응용계층응용계층응용계층프로토콜프로토콜프로토콜프로토콜

smtp [RFC 821]telnet [RFC 854]http [RFC 2068]ftp [RFC 959]proprietary(e.g. RealNetworks)NSFproprietary(e.g., Vocaltec)

사용하는사용하는사용하는사용하는트랜스포트트랜스포트트랜스포트트랜스포트프로토콜프로토콜프로토콜프로토콜

TCPTCPTCPTCPTCP or UDP

TCP or UDPtypically UDP

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웹관련용어

❒ 웹페이지(Web page):베이스 HTML 페이지 + 오브젝트들

❒ URL로지정하는오브젝트(object)

❒ URL: 호스트이름 /패스

❒ 웹브라우저(사용자에이전트):

❍ MS Internet Explorer❍ Netscape Communicator

❒ 웹서버:❍ 아파치

❍ MS Internet Information Server

www.someSchool.edu/someDept/pic.gif

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웹: http 프로토콜

http: hypertext transfer protocol

❒ client/server model❒ http1.0: RFC 1945❒ http1.1: RFC 2068

PC runningExplorer

Server running

NCSA Webserver

Mac runningNavigator

http request

http request

http response

http response

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http

http: TCP 이용❒ 클라이언트가서버로

TCP (80 포트) 연결시도(소켓을만듦)

❒ 서버가연결요청을수락

❒ http 메시지교환❒ TCP 연결종료

http 는 “stateless”❒ 클라이언트의과거요청을기록하지않음

❒ Cf. 쿠키(cookies)

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http 예사용자입력 URL

www.someSchool.edu/someDepartment/home.index

1a. http client initiates TCP connection to http server (process) at www.someSchool.edu. Port 80 is default for http server.

2. http client sends http request message (containing URL) into TCP connection socket

1b. http server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. http server receives request message, forms response message containing requested object (someDepartment/home.index), sends message into socket

time

(contains text, references to 10

jpeg images)

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http 예(계속)

5. http client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects

6. Steps 1-5 repeated for each of 10 jpeg objects

4. http server closes TCP connection.

time

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HTTP/1.0 과 HTTP/1.1 비교

HTTP/1.0❒ server parses request,

responds, and closes TCP connection

❒ 각오브젝트를가져오는2 RTT 소요

❒ 각 오브젝트마다 slow start O/H

HTTP/1.1❒ 하나의 TCP 연결로모든오브젝트를가져옴

❒ 베이스 HTML을받는즉시참조된모든오브젝트를가져옴(pipelining).

❒ 적은 수의 RTT와 slow start O/H

❒ 패킷수감소(여러개의연결시도등을다중화)

But most 1.0 browsers useparallel TCP connections.

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http 메시지형식: request

❒ two types of http messages: request, response❒ http request message: ASCII

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)

request line(GET, POST,

HEAD commands)

headerlines

Carriage return, line feed

indicates end of message

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http request 메세지: general format

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http message format: respone

HTTP/1.0 200 OK Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html

data data data data data ...

status line(protocol

status codestatus phrase)

headerlines

data, e.g., requestedhtml file

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http response status codes

200 OK❍ request succeeded, requested object later in this message

301 Moved Permanently❍ requested object moved, new location specified later in

this message (Location:)400 Bad Request

❍ request message not understood by server404 Not Found

❍ requested document not found on this server505 HTTP Version Not Supported

In first line in server->client response message.A few sample codes:

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Trying out http (client side) for yourself

1. Telnet to your favorite Web server:Opens TCP connection to port 80(default http server port) at www.eurecom.fr.Anything typed in sent to port 80 at www.eurecom.fr

telnet www.eurecom.fr 80

2. Type in a GET http request:GET /~ross/index.html HTTP/1.0 By typing this in (hit carriage

return twice), you sendthis minimal (but complete) GET request to http server

3. Look at response message sent by http server!

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사용자-서버: 인증(authentication)

❒ stateless: request할때마다인증필요( 브라우저가캐싱가능)

❒ authorization: 대개 name, password

❍ authorization: header line in request

❍ authorization 이없으면, 서버는접근거절하고

WWW authenticate:

을보낸다.

client serverusual http request msg401: authorization req.WWW authenticate:

usual http request msg+ Authorization:line

usual http response msg

usual http request msg+ Authorization:line

usual http response msg time

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사용자-서버: 쿠키(cookies)

❒ 서버가사용자에게쿠키를보냄Set-cookie: 1678453

❒ 클라이언트다음번request할때쿠키제시cookie: 1678453

❒ 서버는저장된쿠키와비교하여

❍ 인증

❍ 사용자기호, 이전선택등을파악하여개인화된서비스제공

client serverusual http request msgusual http response +Set-cookie: #

usual http request msgcookie: #

usual http response msg

usual http request msgcookie: #

usual http response msg

쿠키에따른처리

쿠키에따른처리

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사용자-서버: conditional GET

❒ 사용자가최신내용을캐쉬하고있으면보내지

않음

❒ 클라이언트: 캐쉬한시간표시If-modified-since: <date>

❒ 서버: 변하지않았으면보내지않음: HTTP/1.0 304 Not Modified

클라이언트 서버

http request msgIf-modified-since:

<date>

http responseHTTP/1.0

304 Not Modified

오브젝트불변

http request msgIf-modified-since:

<date>

http responseHTTP/1.1 200 OK

…

<data>

오브젝트변함

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웹캐쉬 (프록시 서버)

❒ 사용자는브라우저설정: 웹캐시를통해웹접속하도록

❒ 클라이언트는모든all http request를웹캐시에보냄

❍ Hit 시에는웹캐쉬가직접

❍ Miss 시에는원래서버에접속후

원래 서버대신에 사용자의 요청을 서비스

클라이언트

프록시서버

클라이언트

http request

http request

http response

http response

http request

http response

http requesthttp response

원래서버

원래서버

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웹캐싱을하는이유?

❒ 응답시간단축: 캐쉬가가까이있을때

❒ 원거리서버로의트래픽

감소❍ 외부로나가는링크가보통병목

원래서버

인터넷

기관네트워크 10 Mbps LAN

1.5 Mbps 억세스 링크

캐쉬

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ftp: 파일전송프로토콜

❒ 원격 서버로의파일송수신

❒ 클라이언트/서버모델❒ ftp: RFC 959❒ ftp server: port 21

파일 전송 FTP서버

FTP사용자인터페이스

FTP클라이언트

지역 파일시스템

원격 파일시스템

사용자

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ftp: 제어, 데이터연결의분리

❒ 클라이언트는서버의 TCP 포트 21번으로접속시도

❒ 두개의TCP 연결을연다:❍ 제어: 명령과응답을교환

“out of band control”

❍ 데이타: 파일송수신❒ ftp 서버는 “state”를유지 : 현재의디렉토리, 이전인증등

FTP클라이언트

FTP서버

TCP 제어 연결port 21

TCP 데이터 연결port 20

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ftp 명령과 응답

명령(예):❒ 제어채널을통해 ASCII로❒ USER username

❒ PASS password

❒ LIST return list of file in current directory

❒ RETR filename retrieves (gets) file

❒ STOR filename stores (puts) file onto remote host

응답코드(예)❒ status code and phrase (as

in http)❒ 331 Username OK,

password required

❒ 125 data connection already open; transfer starting

❒ 425 Can’t open data connection

❒ 452 Error writing file

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E-메일

세가지요소:❒ user agents ❒ mail servers ❒ simple mail transfer

protocol: smtp

사용자에이전트

❒ 메일리더

❒ 메일메시지, 제작, 편집, 읽기❒ 예) Eudora, Outlook, elm,

Netscape Messenger❒ 들어오고나가는메시지는서버에저장

사용자메일박스

나가는메시지 큐

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: mail servers

Mail Servers❒ mailbox contains incoming

messages (yet to be read) for user

❒ message queue of outgoing (to be sent) mail messages

❒ smtp protocol between mail servers to send email messages

❍ client: sending mail server

❍ “server”: receiving mail server

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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Electronic Mail: smtp [RFC 821]

❒ uses tcp to reliably transfer email msg from client to server, port 25

❒ direct transfer: sending server to receiving server❒ three phases of transfer

❍ handshaking (greeting)❍ transfer of messages❍ closure

❒ command/response interaction❍ commands: ASCII text❍ response: status code and phrase

❒ messages must be in 7-bit ASCII

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Sample smtp interactionS: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

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try smtp interaction for yourself:

❒ telnet servername 25

❒ see 220 reply from server❒ enter HELO, MAIL FROM, RCPT TO, DATA, QUIT

commandsabove lets you send email without using email client

(reader)

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smtp: final words

❒ smtp uses persistent connections

❒ smtp requires that message (header & body) be in 7-bit ascii

❒ certain character strings are not permitted in message (e.g., CRLF.CRLF). Thus message has to be encoded (usually into either base-64 or quoted printable)

❒ smtp server uses CRLF.CRLF to determine end of message

Comparison with http❒ http: pull❒ email: push

❒ both have ASCII command/response interaction, status codes

❒ http: each object is encapsulated in its own response message

❒ smtp: multiple objects message sent in a multipart message

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Mail message format

smtp: protocol for exchanging email msgs

RFC 822: standard for text message format:

❒ header lines, e.g.,❍ To:❍ From:❍ Subject:different from smtp

commands!❒ body

❍ the “message”, ASCII characters only

header

body

blankline

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Message format: multimedia extensions

❒ MIME: multimedia mail extension, RFC 2045, 2056❒ additional lines in msg header declare MIME content

type

From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

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MIME typesContent-Type: type/subtype; parameters

Text❒ example subtypes: plain,

html

Image❒ example subtypes: jpeg,

gif

Audio❒ exampe subtypes: basic

(8-bit mu-law encoded), 32kadpcm (32 kbps coding)

Video❒ example subtypes: mpeg,

quicktime

Application❒ other data that must be

processed by reader before “viewable”

❒ example subtypes: msword, octet-stream

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Multipart TypeFrom: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789

--98766789Content-Transfer-Encoding: quoted-printableContent-Type: text/plain

Dear Bob, Please find a picture of a crepe.--98766789Content-Transfer-Encoding: base64Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data --98766789--

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Mail access protocols

❒ SMTP: delivery/storage to receiver’s server❒ Mail access protocol: retrieval from server

❍ POP: Post Office Protocol [RFC 1939]• authorization (agent <-->server) and download

❍ IMAP: Internet Mail Access Protocol [RFC 1730]• more features (more complex)• manipulation of stored msgs on server

❍ HTTP: Hotmail , Yahoo! Mail, etc.

useragent

sender’s mail server

useragent

SMTP SMTP POP3 orIMAP

receiver’s mail server

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POP3 protocol

authorization phase❒ client commands:

❍ user: declare username❍ pass: password

❒ server responses❍ +OK

❍ -ERR

transaction phase, client:❒ list: list message numbers❒ retr: retrieve message by

number❒ dele: delete❒ quit

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents>S: . C: dele 1 C: retr 2 S: <message 1 contents>S: . C: dele 2 C: quit S: +OK POP3 server signing off

S: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK user successfully logged on