컴퓨터 네트워크

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컴퓨터 네트워크

PART 02 프로토콜

(chapter 04 응용계층 )

임효택

E-mail : [email protected]

Home page : http://kowon.dongseo.ac.kr/~htlim

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chapter 04 응용계층

 OSI 참조 모델 응용

4.1.1 세션 , 표현 계층

 세션 계층의 개관

 OSI 참조 모델 5 계층에 해당

 전송 제어 기능을 상위 계층에 제공

 서비스 측면의 기능들을 제공하는 데 목적이 있다 .

 제공 서비스

 기본적으로 연결형 서비스를 제공한다 .

 전송되는 데이터의 순서는 중요한 의미를 가진다 .

 지원되는 서비스의 종류가 많다 .

 기능단위와 사용자가 필요한 기능단위를 선택하여 사용할 수 있다 .

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chapter 04 응용계층

 연결형 서비스 과정

– 연결 설정 단계 , 데이터 전송 단계 , 연결 해제 단계로 나눈다 .

 세션 연결의 특성

– (a) 트랜스포트와 세션 연결이 일대 일로 대응한다 . ( 가장 일반적 ) – (b) 다시 새로운 세션 연결을 시작할 때 트랜스포트 연결을 그대로 사용

할수 있다 .

– (c) 하나의 세션 연결에 대하여 트랜스포트 연결이 중간에 해제되었다가

다시 연결되어 사용된다 .

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chapter 04 응용계층

 대화 관리 (Dialogue management)

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chapter 04 응용계층

 동기 (Synchronization)

– 두 세션 사용자 간의 데이터를 교환하다가 문제가 발생하였을때

미리 정해 놓은 동기점으로 되돌아가 다시 시작하도록 하는 것 .

– 대동기점 , 소동기점

[ 그림 4.3] 세션 계층에서의 동기점

[ 그림 4.4] 대동기점과 소동기점

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chapter 04 응용계층

 액티비티 (Activity)

– 세션 사용자 간에 어떤 일의 논리적인 단위를 표시하기 위한 것 . – 독립적인 특성을 가지며 영향을 받지 않는다 .

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chapter 04 응용계층

 토큰 (Token)

– 어떠한 서비스를 수행할 수 있는 권리를 나타내는 것

» 데이터토큰 , 해제 토큰 , 소동기 토큰 , 대동기 / 액티비티 토큰

 서비스 프리미티브와 SPDU

 서비스 프리미티브는 ISO 8326 또는 X.215 에 정의됨 .

 SPDU 는 ISO8327, X.255 에 정의 됨 .

 에러검출 및 복구의 기능이 없다 .

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chapter 04 응용계층

4.1.2 응용계층

 응용 서비스 요소

 응용 프로그램이 필요로 하는 최소의 단위를 응용 서비스 요소라 한다 .

 엔티티 : 특정한 하나의 응용 프로그램에 필요한 통신 서비스 요소들의 결합

형태

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chapter 04 응용계층

 SASE

– FTAM (file access & management) – VT (virtual terminal)

– MOTIS (message oriented text interchange system) – JTM (job transfer & manipulation)

– RDA (remote database access) – 트랜젝션 처리

– OSI 관리

 CASE

– ACSE (association control service element) – CCR (commitment concurrency & recovery) – ROS (remote operation service)

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chapter 04 응용계층

 메시지 처리 시스템 (MHS) 서비스

 대표적인 예 : 전자우편

 파일 전송 , 접근과 관리 (FTAM)

 디렉토리 서비스

 각 자원의 위치에 관계하는 명칭을 알고 필요한 어드레스를 요구하는 기능

 가상 터미널 서비스

 가상 터미널이 응용 프로그램 또는 터미널 이용자에게 제공하는 서비스

 OSI 관리

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Application 이 필요한 트랜스포트 서비스는

?

Data loss

 some apps (e.g., audio) can tolerate some loss

 other apps (e.g., file transfer, telnet) require 100% reliable

data transfer

Timing

 some apps (e.g., Internet

telephony, interactive games) require low delay to be

“effective”

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

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common apps

Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games financial apps

Data loss no loss no loss

loss-tolerant loss-tolerant loss-tolerant loss-tolerant no loss

Bandwidth elastic

elastic elastic

audio: 5Kb-1Mb video:10Kb-5Mb same as above few Kbps up elastic

Time Sensitive no

no no

yes, 100’s msec

yes, few secs

yes, 100’s msec

yes and no

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Internet 응용 프로토콜과 트랜스포트 프로토콜

Application e-mail 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

typically UDP

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Web 상식

 Web page:

 consists of “objects”

 addressed by a URL

 Web 페이지의 구성 :

 base HTML page, and

 several referenced objects.

 URL 의 두가지 구성요소 : 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

www.someSchool.edu/someDept/pic.gif

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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

Server running NCSA Web

server Mac running

Navigator

http request

http request http re

sponse

http response

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The http protocol: more

http: TCP transport service:

 client initiates TCP connection (creates socket) to server, port 80

 server accepts TCP connection from client

 http messages (application-layer protocol messages) exchanged between browser (http client) and Web server (http server)

 TCP connection closed

http is “stateless”

 server maintains no information about past client requests

Protocols that maintain “state” are complex!

 past history (state) must be maintained

 if server/client crashes, their views of “state” may be

inconsistent, must be reconciled

aside

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http example

Suppose user enters URL

www.someSchool.edu/someDepartment/home.index

1a. http client initiates TCP connection to h ttp 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.ed u waiting for TCP connection at port 8 0. “accepts” connection, notifying clien t

3. http server receives request message, for ms response message containing reque sted object (someDepartment/home.inde x), sends message into socket

time

(contains text, references to 10

jpeg images)

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http example (cont.)

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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Non-persistent and persistent connections

Non-persistent

 HTTP/1.0

 server parses request, respond s, and closes TCP connection

 2 RTTs to fetch each object

 Each object transfer suffers fro m slow start

Persistent

 default for HTTP/1.1

 on same TCP connection: serve r, parses request, responds, pa rses new request,..

 Client sends requests for all ref erenced objects as soon as it re ceives base HTML.

 Fewer RTTs and less slow start.

But most 1.0 browsers use

parallel TCP connections.

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Non-Persistent HTTP: Response time

Definition of RTT: time to send a small packet to travel from client to server and back.

Response time:

 one RTT to initiate TCP connection

 one RTT for HTTP request and first few bytes of HTTP

response to return

 file transmission time total = 2RTT+transmit time

time to transmit file

initiate TCP connection

RTT request file

RTT file received

time time

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Persistent HTTP

Nonpersistent HTTP issues:

 requires 2 RTTs per object

 OS overhead for each TCP connection

 browsers often open parallel TCP connections to fetch referenced objects

Persistent HTTP

 server leaves connection open after sending response

 subsequent HTTP messages between same client/server sent over open connection

Persistent without pipelining:

 client issues new request only when previous response has been received

 one RTT for each referenced object

Persistent with pipelining:

 default in HTTP/1.1

 client sends requests as soon as it encounters a referenced

object

 as little as one RTT for all the referenced objects

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request

 two types of http messages: request, response

 http request message:

 ASCII (human-readable format)

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)

header

lines

Carriage return,

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http request message: 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 code status phrase)

header lines

data, e.g.,

requested

html 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 server

404 Not Found

 requested document not found on this server

505 HTTP Version Not Supported

In first line in server->client response message.

A few sample codes:

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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 send

this minimal (but complete)

GET request to http server

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Web Caches (proxy server)

 user sets browser: Web accesses via web cache

 client sends all http requests to web cache

 if object at web cache, web cache immediately returns object in http response

 else requests object from origin server, then returns http response to client

Goal:

satisfy client request without involving origin server

client

Proxy server

client

http request http request http re

sponse http response

http request http response http request http re

sponse

origin server origin server

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Why Web Caching?

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

origin servers

public Internet

institutional

network 10 Mbps LAN 1.5 Mbps

access link

institutional

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Web caches (proxy server)

 user sets browser: Web accesses via cache

 browser sends all HTTP requests to cache

 object in cache: cache returns object

 else cache requests object from origin server, then returns object to client

Goal:

satisfy client request without involving origin server

client

Proxy server

client

HTTP r

equest HTTP request HTTP r

esponse HTTP response

HTTP request HTTP response

origin server origin server

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More about Web caching

 Cache acts as both client and server

 Typically cache is installed by ISP (university, company, residential ISP)

Why Web caching?

 Reduce response time for client request.

 Reduce traffic on an institution’s access link.

 Internet dense with caches:

enables “poor” content

providers to effectively deliver content (but so does P2P file sharing)

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Caching example

Assumptions

 average object size = 100,000 bits

 avg. request rate from institution’s browsers to origin servers = 15/sec

 delay from institutional router to any origin server and back to router = 2 sec

Consequences

 utilization on LAN = 15%

 utilization on access link = 100%

 total delay = Internet delay + access delay + LAN delay

= 2 sec + minutes + milliseconds

origin servers

public Internet

institutional

access link

institutional

cache

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Caching example (cont)

Possible solution

 increase bandwidth of access link to, say, 10 Mbps

Consequences

 utilization on LAN = 15%

 utilization on access link = 15%

 Total delay = Internet delay + access delay + LAN delay

= 2 sec + msecs + msecs

 often a costly upgrade

origin servers

public Internet

institutional

network 10 Mbps LAN 10 Mbps

access link

institutional

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Caching example (cont)

Install cache

 suppose hit rate is .4

Consequence

 40% requests will be satisfied almost immediately

 60% requests satisfied by origin server

 utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)

 total avg delay = Internet delay + access delay + LAN delay = . 6*(2.01) secs + .4*milliseconds

< 1.4 secs

origin servers

public Internet

institutional

access link

institutional

cache

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ftp: the file transfer protocol

 transfer file to/from remote host

 client/server model

 client: side that initiates transfer (either to/from remote)

 server: remote host

 ftp: RFC 959

 ftp server: port 21

file transfer FTP server userFTP

interface

clientFTP

local file system

remote file system user

at host

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ftp: separate control, data connections

 ftp client contacts ftp server at port 21, specifying TCP as

transport protocol

 two parallel TCP connections opened:

 control: exchange commands, responses between client, server.

“out of band control”

 data: file data to/from server

 ftp server maintains “state”:

current directory, earlier authentication

client FTP FTP

server

TCP control connection port 21

TCP data connection port 20

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ftp commands, responses

Sample commands:

 sent as ASCII text over control channel

 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

Sample return codes

 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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TCP Connection Establishment

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Control Connection

 Control functions (commands) and reply codes are transferred over the control connection.

 Example:-

^Commands Reply Codes

USER 331

PASS 230

CWD 250

PWD 257

TYPE I 200

PASV 227

STOR 125

226

-For more commands and Reply codes -Check on RFC 959

http://www.faqs.org/rfcs/rfc959.html

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Ethereal Captured Screen For FTP

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-Control connection

Works Done here

1. Send username seongyee

2. Response with 331 (PasswordRequired)

3. Send password password

4. Response with 230 (Successful Login)

5. Change Directory with CWD

6. Response with 250 (CWD command successed)

7. List Current Directory with PWD

8. Response with 257 (List Current Directory)

9. Set the data type to I

10. Response with 200 (Type Set to I)

11. Set to Passive Mode

12. Response with 277(Entering Passive Mode)

13. Download Chap2.ppt by sending STOR Command

14. Response with 125(Data Connection Opened) 15. Response with 226(Transfer complete)

Data

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Packet Capture For Data Connection

Another TCP connection where 192.168.112.94:3287 to 203.241.187.71:1163

Established For data connection

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chapter 04 응용계층

E-mail(

전자메일 )

 컴퓨터 사용자가 동일한 컴퓨터 또는 네트워크에 연결된 다른 컴퓨터 사용자와 보통의 메일 서비스를 전산망 상에서 온라인으로 사용할 수 있는 서비스 .

 TCP/IP는 전자메일을 주고받기 위한 SMTP 를 정의한다 .

 [그림 4.13] 은 TCP/IP 를 사용한 전자메일의 전체적인 구성도를 보여준다 .

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Electronic Mail

Three major components:

 user agents

 mail servers

 simple mail transfer protocol: smtp User Agent

 a.k.a. “mail reader”

 composing, editing, reading mail m essages

 e.g., Eudora, Outlook, elm, Netscap e Messenger

 outgoing, incoming messages store d on server

user mailbox outgoing message queue

servermail

agentuser

agentuser servermail

agentuser agentuser

servermail

agentuser

SMTP

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Electronic Mail: mail servers

Mail Servers

 mailbox contains incoming message s (yet to be read) for user

 message queue of outgoing (to be s ent) mail messages

 smtp protocol between mail servers to send email messages

 client: sending mail server

 “server”: receiving mail server

servermail

agentuser

agentuser servermail

agentuser

SMTP

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chapter 04 응용계층

 SMTP 동작

 SMTP 동작의 세 가지 단계

1. SMTP

클라이언트와 서버 간의 연결이 확립된다 .

2.

전자메일이 연결을 따라 전송된다 .

3.

연결이 해제된다 .

 SMTP Command , Reply

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Command From Client To Server

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Reply From Server to Client

-Quite similar with FTP

-For more command and reply , refer to RFC 2821

http://rfc.net/rfc2821.html

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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 interaction

S: 220 hamburger.edu C: HELO crepes.fr

S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]>

S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]>

S: 250 [email protected] ... 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 commands

above lets you send email without using email client (reader)

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Ethereal Captured Screen For SMTP

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Works Flow in SMTP

Steps involved

1. Open TCP connection to port 25 of the server 2. Server Response with code 220 (Service Ready) 3. Client send command EHLO (Client authentication) 4. Server Response with code 250 (OK)

5. Client send command AUTH PLAIN

6. Server Response with code 235 (Authentication successes) 7. Sender send command MAIL ( Enter sender name)

8. Server Response with code 250 (OK)

9. Sender send command RCPT TO ( Enter recipient name) 10. Server Response with code 250 (OK)

11. Sender send command DATA (Beginning Transmission) 12. Server Response with code 354 (Start Email Input)

13. Sender send command Message Body (The body of the msg)

14. Server Response with code 250 (OK)

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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.

agentuser

sender’s mail server agentuser

SMTP SMTP POP3 or IMAP

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: .

S: +OK POP3 server ready C: user alice

S: +OK

C: pass hungry

S: +OK

user successfully logged on

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chapter 04 응용계층

4.2.5 DNS

 DNS (Domain Name Server)

 IP 주소는 사람이 기억하기에 어려워 기억하기 쉽게 바꾸어 놓은 것이

도메인 이름이다 .

 컴퓨터가 속해 있는 기관이나 국가에 따라서 계층적으로 형성됨

 Domain Name 을 IP 주소로 또는 그 반대로 바꿔 주는 것을

DNS(Domain Name System) 이라 한다 .

 도메인 네임

– 도메인 네임의 구조 : 호스트 이름 . 소속단체 . 단체성격 . 소속국가

 Nslookup 명령

– 가끔씩 컴퓨터가 도메인 주소를 이해 못하는 경우가 발생할 수 있으며 ,

사용자가 특별히 특정 호스트 이름에 해당하는 IP 주소를 알고 싶을 때

사용하는 명령어

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DNS: Domain Name System

People: many identifiers:

 SSN, name, passport #

Internet hosts, routers:

 IP address (32 bit) - used for addressing datagrams

 “name”, e.g.,

ww.yahoo.com - used by humans

Q: map between IP addresses and name ?

Domain Name System:

 distributed database implemented in hierarchy of many name servers

 application-layer protocol host, routers, name servers to

communicate to resolve names (address/name translation)

 note: core Internet function, implemented as application- layer protocol

 complexity at network’s “edge”

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DNS

Why not centralize DNS?

 single point of failure

 traffic volume

 distant centralized database

 maintenance doesn’t scale!

DNS services

 Hostname to IP address translation

 Host aliasing

 Canonical and alias names

 Mail server aliasing

 Load distribution

 Replicated Web servers: set of IP addresses for one

canonical name

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Root DNS Servers

com DNS servers org DNS servers edu DNS servers poly.edu

DNS servers

umass.edu DNS servers yahoo.com

DNS servers amazon.com DNS servers

pbs.org

DNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1

^st

approx:

 Client queries a root server to find com DNS server

 Client queries com DNS server to get amazon.com DNS server

 Client queries amazon.com DNS server to get IP address for

www.amazon.com

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DNS: Root name servers

 contacted by local name server that can not resolve name

 root name server:

 contacts authoritative name server if name mapping not known

 gets mapping

 returns mapping to local name server

13 root name servers worldwide

b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA

f Internet Software C. Palo Alto, CA (and 17 other locations)

i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt)

m WIDE Tokyo a Verisign, Dulles, VA

c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD

g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations)

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TLD and Authoritative Servers

 Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.

 Network solutions maintains servers for com TLD

 Educause for edu TLD

 Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).