Applikasjoner
• Disposisjon
Î overføringsproblematikk
Î navnetjener
Î 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.
ÎWeb:browser
ÎE-mail: mail reader
Îstreaming audio/video:
media player
5/8/2003
Slide 2
System arkitektur
Tjener
Klient
Disk
Komm.
Prog.
vare
Disk
Navne
tjener
Komm.
Prog.
vare
B
r
u
k
e
r
LAN
LAN
Internett
5/8/2003
Slide 3
1
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 e-mail, in mail reader
Server:
• provides requested service to client
• e.g., Web server sends requested
Web page, mail server delivers email
application
transport
network
data link
physical
request
reply
application
transport
network
data link
physical
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Slide 4
Overføringsproblemer
• Data representeres på ulik måte i ulike
maskiner på grunn av forskjell i:
Î hardware
– ”Small-endians” og ”Big-endians” maskiner
– 1-komplement og 2-komplement logikk
– ordstørrelse Æ 32-bit maskin til 64-bit maskin komm.
Î operativsystem
Î programmeringsspråk og kompilere
– forskjellig lokal representasjon
– EBCDIC (IBM) til ASCII eller IA5 (standard)
5/8/2003
Slide 5
Eksempel; hardwareforskjeller
Integerverdien 34.677.374
Mest signif.
Big-endians
00000010 00010001 00100010 01111110
Byte 4
Small-endians
Byte 1
01111110 00100010 00010001 00000010
Minst signifik.
5/8/2003
Minst signif.
Mest signifik.
Slide 6
2
Overføringssyntaks
Applik. data
Applik. data
Presentasjons
koding
Pakke 1
Presentasjons
dekoding
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
Abstrakt syntaks
kompilator
Lokal
repr.
Applik.pros.
Koding og
innpakking
(marshalling)
Abstrakt kontekst
Felles samtaleunivers
Datastrukturer
Operasjoner
Abstrakt syntaks
Overføringssyntaks
Lokal
repr.
Applik.pros.
Deoding og
utpakking
(marshalling)
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Slide 8
Marshalling/serialisering
Applikasjonens
datastruktur
Serialisering
5/8/2003
Marshalling
• konvertering
• serialisering
• innpakking
Slide 9
3
ASN.1 eksempler
Integer > 65 535
32-bits maskin; representert i et 32-bits ord
16-bits maskin; representert i to 16-bits ord
Primitive typer:
Hver datatype kodes ved overføring:
Type Lengde Verdi
Type lengde type lengde verdi type lengde verdi
Bolean
Integer
Bit streng
Oktett streng
Null
Objekt ID
Objekt deskrip.
Real
Struktur typer
INT 4
B1 B2 B3 B4
5/8/2003
Slide 10
Protokollstrukturen i UNIX
Applikasjon
User space
Sockets
Kernel
TCP UDP
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
5/8/2003
Timing
• some apps (e.g., Internet
telephony, interactive games)
require low delay to be
“effective”
Slide 12
4
Transport service requirements of common apps
Data loss
Bandwidth
Time Sensitive
file transfer
e-mail
Web documents
real-time audio/video
no loss
no loss
loss-tolerant
loss-tolerant
no
no
no
yes, 100’s msec
stored audio/video
interactive games
financial apps
loss-tolerant
loss-tolerant
no loss
elastic
elastic
elastic
audio: 5Kb-1Mb
video:10Kb-5Mb
same as above
few Kbps up
elastic
Application
yes, few secs
yes, 100’s msec
yes and no
5/8/2003
Slide 13
Internet apps: their protocols and transport
protocols
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
remote file server
Internet telephony
Application
layer protocol
Underlying
transport protocol
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
5/8/2003
Slide 14
Aksjoner på ulike protokollnivåer
Bruker: connect odin.unik.no
Gethostbyname (odin.unik.no Æ 193.156.96.215)
Applikasjon
Getservicebyname (tcp Æ 21)
Socket (tcp; Port # > 1023)
Connect (193.156.96.215; 21
TCP
TCP-PDU:
(TCP-CON)
Send(193.156.96.215; SDU)
IP-addr Æ MAC addr
IP
IP-PDU:
(Data)
Send(MAC-addr; Data)
Ethernet
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Ether-PDU:
Data
Slide 15
5
Navnetjenere
5/8/2003
Slide 16
Navnetjener
• Konvertere fra navn til adresser
Î navn er brukervennlig
Î adresser er nødvendig for fremsending av
pakker
Î husk; et endepunkt for en forbindelse er
identifisert ved tripletten:
• IP-adresse; IP-SAP (Protocol); T-SAP (T-Port)
• Service (for eks. FTP) Æ vel-kjent port
5/8/2003
Slide 17
Organisering av navnetjenesten
• Organisert hierarkisk i domener,
Î reflekteres i navnekonvensjonen
• for eks. ”baugi.ifi.uio.no”
Î hvert domene er ansvarlig for sitt navneområde
Î fleksibelt og forenkler administrasjonen
5/8/2003
Slide 18
6
Navnehierarkiet og soner
Generisk
Land
Rot-sone
adm. NIC
com
edu
mil
gov
org
uk
no se
uio
princeton berkeley arpa
ifi
fys
baugi mjolnir skibladnir
• hver sone korresponderer med to eller flere navnetjenere
• en navnetjener kan implementere en eller flere soner
5/8/2003
Slide 19
Hierarki av navnetjenere
Rot
navnetjener
no
navnetjener
uio
navnetjener
ifi
navnetjener
5/8/2003
Slide 20
Navn- adresse transaksjoner
1
rot
tjener
Baugi.if.uio.no
2
Klient
Lokal
tjener
129.240.64.36
no
tjener
3
4
uio
tjener
ifi
tjener
5/8/2003
Slide 21
7
The Web: some jargon
• Web page:
Îconsists of “objects”
Îaddressed by a URL
• Most Web pages consist
of:
Îbase HTML page, and
Îseveral referenced
objects.
• URL has two components:
host name and path name:
• User agent for Web is
called a browser:
ÎMS Internet Explorer
ÎNetscape
Communicator
• Server for Web is called
Web server:
ÎApache (public
domain)
ÎMS Internet
Information Server
www.someSchool.edu/someDept/pic.gif
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Slide 22
The Web: the http protocol
http: hypertext transfer protocol
• Web’s application layer
protocol
• client/server model
Îclient: browser that
requests, receives,
“displays” Web objects
Îserver: Web server sends
objects in response to
requests
• http1.0: RFC 1945
• http1.1: RFC 2068
htt
PC running
Explorer
htt
pr
equ
est
pr
esp
ons
e
est
equ
Server
pr
nse
htt
running
spo
re
NCSA Web
tp
ht
server
Mac running
Navigator
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
http message format: request
• two types of http messages: request, response
• http request message:
ÎASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.0
User-agent: Mozilla/4.0
Accept: text/html, image/gif,image/jpeg
header Accept-language:fr
lines
Carriage return,
line feed
indicates end
of message
(extra carriage return, line feed)
5/8/2003
Slide 25
http request message: general format
5/8/2003
Slide 26
http message format: response
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
html file
5/8/2003
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 ...
Slide 27
9
Trying out http (client side) for yourself
1. Telnet to your favorite Web server:
telnet www.eurecom.fr 80 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
2. Type in a GET http request:
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to http server
GET /~ross/index.html HTTP/1.0
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: 1678453
• client presents cookie in
later requests
cookie: 1678453
• server matches
presented-cookie with
server-stored info
Îauthentication
Îremembering user
preferences, previous
5/8/2003
choices
server
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
cookiespectific
action
cookiespectific
action
Slide 29
Web Caches (proxy server)
Goal: satisfy client request without involving origin
• server
user sets browser:
Web accesses via web
cache
• client sends all http
requests to web cache
Îif object at web cache,
web cache
immediately returns
object in http response
Îelse requests object
from origin server,
then returns http
response to client
5/8/2003
origin
server
htt
Proxy
t
pr
server reques e
equ
est
p
ns
htt
res
spo
e
r
pon
tp
se
ht
est
htt
pr
equ
equ
pr
nse
htt
est
htt
spo
pr
e
r
esp
p
t
ons
ht
e
client http
client
origin
server
Slide 30
10
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
Îlink out of
institutional/local ISP
network often
5/8/2003
bottleneck
origin
servers
public
Internet
1.5 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
Slide 31
11