chapter 2: application layer › ~jsteach › cosc4377 › 2001fall › notes › chapter2a.pdf2:...

COSC 4377, Spring 2001 - Chapter 2, Part A 9/18/01

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2: Application Layer 1

chapter 2: Application LayerChapter goals:r conceptual +

implementation aspectsof network applicationprotocolsm client server

paradigmm service models

r learn about protocols byexamining popularapplication-levelprotocols

More chapter goalsr specific protocols:

m httpm ftpm smtpm popm dns

r programming networkapplicationsm socket programming


Applications and application-layer protocolsApplication: communicating,

distributed processesm running in network hosts

and in “user space”m exchange messages to

implement appm e.g., email, file transfer,

the WebApplication-layer protocols

m one (big) “piece” of anetwork application

m define messages exchangedby apps and actions taken

m use services provided bylower layer protocols , e.g.,TCP, UDP

applicationtransportnetworkdata linkphysical




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Network applications: some jargon

r A process is a programthat is running within ahost.

r Within the same host, twoprocesses communicatewith interprocesscommunication defined bythe OS.

r Processes running indifferent hostscommunicate with anapplication-layer protocol

r A user agent is aninterface between theuser and the networkapplication.m Web:browserm E-mail: mail readerm streaming audio/video:

media player


Client-server paradigmTypical network app has two

pieces: client and server applicationtransportnetworkdata linkphysical


Client:r initiates contact with server

(“speaks first”)r typically requests service

from server,r for Web, client is implemented

in browser; for e-mail, in mailreader

Server:r active in listening moder responds and provides

requested service to clientr e.g., Web server sends

requested Web page

request

reply


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Application-layer protocols (cont).

API: applicationprogramming interface

r defines interfacebetween applicationand transport layer

r socket: Internet APIm two processes

communicate by sendingdata into socket,reading data out ofsocket

Q: how does a process“identify” the otherprocess with which itwants to communicate?m IP address of host

running other processm “port number” - allows

receiving host todetermine to whichlocal process themessage should bedelivered

… lots more on this later.


What transport service does an app need?Data lossr some apps (e.g., audio) can

tolerate some lossr other apps (e.g., file

transfer, telnet) require100% reliable data transfer Timing

r some apps (e.g., Internettelephony, interactivegames) require low delay tobe “effective”

Bandwidthr some apps (e.g., multimedia)

require minimum amount ofbandwidth to be “effective”

r other apps (“elastic apps”)make use of whateverbandwidth they get


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Transport service requirements of common apps

Application

file transfere-mail

Web documentsreal-time audio/video

stored audio/videointeractive games

financial apps

Data loss

no lossno lossloss-tolerantloss-tolerant

loss-tolerantloss-tolerantno loss

Bandwidth

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

Time Sensitive

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no


Services provided by Internettransport protocols

TCP service:r connection-oriented: setup

required between client,server

r reliable transport betweensending and receiving process

r flow control: sender won’toverwhelm receiver

r congestion control: throttlesender when networkoverloaded

r does not provide: timing,minimum bandwidthguarantees

UDP service:r unreliable data transfer

between sending andreceiving process

r does not provide:connection setup,reliability, flow control,congestion control, timing,or bandwidth guarantee

Q: why bother? Why isthere a UDP?


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Internet apps: their protocols and transportprotocols

Application

e-mailremote terminal access

Web file transfer

streaming multimedia

remote file serverInternet telephony

Applicationlayer protocol

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

Underlyingtransport protocol

TCPTCPTCPTCPTCP or UDP

TCP or UDPtypically UDP


The Web: some jargonr Web page:

m consists of “objects”m addressed by a URL

r Most Web pages consistof:m base HTML file, andm several referenced

objects.r URL (Uniform Resource

Locator) has three parts:protocol, host name(w/port), and path name:

r User agent for Web iscalled a browser:m MS Internet Explorerm Netscape Communicator

r Server for Web iscalled Web server:m Apache (public domain)m MS Internet

Information Server

http://www.someSchool.edu:port/someDept/pic.gif


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The Web: the http protocolhttp: hypertext transfer

protocolr Web’s application layer

protocolr client/server model

m client: browser thatrequests, receives,“displays” Web objects

m server: Web serversends objects inresponse to requests

r http1.0: RFC 1945, May 1996r http1.1: RFC 2068, Jan. 1997

PC runningExplorer

Server running

NCSA Webserver

Mac runningNavigator

http request

http reque

st

http response

http respo

nse


The http protocol: morehttp: TCP transport

service:r client initiates TCP

connection (creates socket)to server, port 80

r server accepts TCPconnection from client

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

r TCP connection closed

http is “stateless”r server maintains no

information aboutpast client requests

Protocols that maintain“state” are complex!

r past history (state) mustbe maintained

r if server/client crashes,their views of “state” maybe inconsistent, must bereconciled

aside


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http exampleSuppose user enters URL

http://www.someSchool.edu/someDepartment/home.index

1a. http client initiates TCPconnection to http server(process) atwww.someSchool.edu. Port 80is default for http server.

2. http client sends http requestmessage (containing URL) intoTCP connection socket

1b. http server at hostwww.someSchool.edu waitingfor TCP connection at port 80.“accepts” connection, notifyingclient

3. http server receives requestmessage, forms responsemessage containing requestedobject(someDepartment/home.index),sends message into sockettime

(contains text,references to

10 jpeg images)


http example (cont.)

5. http client receives responsemessage containing html file,displays html. Parsing htmlfile, finds 10 referenced jpegobjects

6. Steps 1-5 repeated for eachof 10 jpeg objects

4. http server closes TCPconnection.

time


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Non-persistent and persistent connectionsNon-persistentr HTTP/1.0r server parses request,

responds, and closesTCP connection

r 2 RTTs (round-trip time)to fetch each object

r Each object transfersuffers from slowstart

Persistentr default for HTTP/1.1r on same TCP

connection: server,parses request,responds, parses newrequest,..

r Client sends requestsfor all referencedobjects as soon as itreceives base HTML.(pipelined)

r Fewer RTTs and lessslow start.

But most 1.0 browsers useparallel TCP connections.


http message format: requestr two types of http messages: request, responser http request message:

m 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, line feed

indicates end of message


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http request message: general format


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)

header lines

data, e.g., requestedhtml file


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http response status codes

200 OKm request succeeded, requested object later in this message

301 Moved Permanentlym requested object moved, new location specified later in

this message (Location:)400 Bad Request

m request message not understood by server404 Not Found

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

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


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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User-server interaction: authentication

Authentication goal: controlaccess to server documents

r stateless: client must presentauthorization in each request

r authorization: typically name,passwordm authorization: header

line in requestm if no authorization

presented, server refusesaccess, sendsWWW authenticate:

header line in response

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 timeBrowser caches name & password sothat user does not have to repeatedly enter it.


User-server interaction: cookies

r server sends “cookie” toclient in response mstSet-cookie: 1678453

r client presents cookie inlater requestscookie: 1678453

r server matchespresented-cookie withserver-stored infom authenticationm remembering user

preferences, previouschoices

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

usual http request msgcookie: #


usual http request msgcookie: #


cookie-specificaction

cookie-specificaction


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User-server interaction: conditional GET

r Goal: don’t send object ifclient has up-to-date stored(cached) version

r client: specify date ofcached copy in http requestIf-modified-since:

<date>

r server: response containsno object if cached copy up-to-date:HTTP/1.0 304 Not

Modified

client serverhttp request msg

If-modified-since:<date>

http responseHTTP/1.0

304 Not Modified

object not

modified

http request msgIf-modified-since:

<date>

http responseHTTP/1.1 200 OK

…<data>

object modified


Web Caches (proxy server)

r user sets browser:Web accesses via webcache

r client sends all httprequests to web cachem if object at web

cache, web cacheimmediately returnsobject in httpresponse

m else requests objectfrom origin server,then returns httpresponse to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

http request

http reque

st

http response

http respo

nse

http reque

st

http respo

nse

http requesthttp response

origin server

origin server


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Why Web Caching?Assume: cache is “close”

to client (e.g., in samenetwork)

r smaller response time:cache “closer” toclient

r decrease traffic todistant serversm link out of

institutional/local ISPnetwork oftenbottleneck

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache


ftp: the file transfer protocol

r transfer file to/from remote hostr client/server model

m client: side that initiates transfer (either to/fromremote)

m server: remote hostr ftp: RFC 959r ftp server: port 21

file transfer FTPserver

FTPuser

interfaceFTP

client

local filesystem

remote filesystem

userat host


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ftp: separate control, data connections

r ftp client contacts ftp serverat port 21, specifying TCP astransport protocol

r two parallel TCP connectionsopened:m control: exchange

commands, responsesbetween client, server.

“out of band control”m data: file data to/from

serverr ftp server maintains “state”:

current directory, earlierauthentication

FTPclient

FTPserver

TCP control connectionport 21

TCP data connectionport 20


ftp commands, responsesSample commands:r sent as ASCII text over

control channelr USER usernamer PASS password

r LIST return list of file incurrent directory

r RETR filename retrieves(gets) file

r STOR filename stores(puts) file onto remotehost

Sample return codesr status code and phrase (as

in http)r 331 Username OK,

password requiredr 125 data connection

already open;transfer starting

r 425 Can’t open dataconnection

r 452 Error writingfile


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Electronic MailMajor Components:r mail user agents (MUA) and

mailer process (e.g. sendmail)r mail serversr simple mail transfer protocol:

SMTP / ESMTPr mail access protocols (e.g.

POP, IMAP)User Agentr a.k.a. “mail reader”r composing, editing, reading

mail messagesr e.g., Eudora, Outlook, elm,

pine, Netscape Messenger

sendingMUA

types calls sendingmail

server

receivingmail

server

Internet

on the sending network

on the receiving network

Bob’smailbox

Alice

delivers to

Bob’sMUA

Bob’snotifier Bob


Electronic Mail: mail serversMail Serversr mailbox contains incoming

messages (yet to be read)for user

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

r smtp protocol used betweenmail servers to send emailmessages (routing messages)m client: sending mail serverm “server”: receiving mail

server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragentmail

server

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


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Electronic Mail: smtp [RFC 821]r uses tcp to reliably transfer email msg from client to server,

port 25r direct transfer: sending server to receiving serverr three phases of transfer

m handshaking (greeting)m transfer of messagesm closure

r command/response interactionm commands: ASCII textm response: status code and phrase

r messages must be in 7-bit ASCIIr ESMTP [RFC 1869] - SMTP Service Extension: 8-bit data

transfer


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:

r telnet servername 25r see 220 reply from serverr enter HELO, MAIL FROM, RCPT TO, DATA, QUIT

commandsabove lets you send email without using email client

(reader)


smtp: final wordsr smtp uses persistent

connectionsr smtp requires that message

(header & body) be in 7-bitascii

r certain character strings arenot permitted in message (e.g.,CRLF.CRLF). Thus message hasto be encoded (usually intoeither base-64 or quotedprintable)

r smtp server uses CRLF.CRLFto determine end of message

r esmtp can take 8-bit data

Comparison with httpr http: pullr email: pushr both have ASCII

command/responseinteraction, status codes

r http: each object isencapsulated in its ownresponse message

r smtp: multiple objectsmessage sent in a multipartmessage


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Mail message formatsmtp: protocol for exchanging

email msgsRFC 822: standard for text

message format:r header lines

(keyword: values), e.g.,m To:m From:m Subject:different from smtp

commands!r body

m the “message”, ASCIIcharacters only

header

body

blankline


Message format: multimedia extensionsr MIME: Multipurpose Internet Mail Extension, RFCs 2045-2049,

especially RFC 2045 and 2046r additional lines in msg header declare MIME content type

From: [email protected]: [email protected]: Picture of yummy crepe.MIME-Version: 1.0Content-Transfer-Encoding: base64Content-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

Textr example subtypes: plain,

html

Imager example subtypes: jpeg,

gif

Audior example subtypes: basic

(8-bit mu-law encoded),32kadpcm (32 kbpscoding)

Videor example subtypes: mpeg,

quicktime

Applicationr other data that must be

processed by readerbefore “viewable”

r example subtypes:msword, octet-stream


Multipart TypeFrom: [email protected]: [email protected]: Picture of yummy crepe.MIME-Version: 1.0Content-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

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

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

m IMAP: Internet Mail Access Protocol [RFC 1730]m HTTP or Webmail

useragent

sender’s mail server

useragent

SMTP SMTP POP3 orIMAP

receiver’s mail server


POP3 protocolauthorization phaser client commands:

m user: declare usernamem pass: password

r server responsesm +OKm -ERR

transaction phase, client:r list: list message numbersr retr: retrieve message by

numberr dele: deleter 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


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Mail access protocolsr Problems with POP

m viewed mail resides on a specific user machine and cannotbe accessed from other machines.

r IMAP• maintains folder hierarchy on the server• can receive/download only components of a message,

e.g., header or certain attachmentsr HTTP

m Hotmail , Yahoo! Mail, Novell MyRealBox.com, etc.m Similar in concept to IMAP but with a web interface

chapter 2: application layer › ~jsteach › cosc4377 › 2001fall › notes › chapter2a.pdf2:...

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