Chapter2 4th Ed Oct 1 2007
Transcript of Chapter2 4th Ed Oct 1 2007
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2: Application Layer 1
Chapter 2
Application Layer
Computer Networking:A Top Down Approach,4thedition.Jim Kurose, Keith RossAddison-Wesley, July2007.
A note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers).
Theyre in PowerPoint form so you can add, modify, and delete slides
(including this one) and slide content to suit your needs. They obviously
represent a lotof work on our part. In return for use, we only ask the
following:
If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, wed like people to use our book!)
If you post any slides in substantially unaltered form on a www site, that
you note that they are adapted from (or perhaps identical to) our slides, and
note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2007
J.F Kurose and K.W. Ross, All Rights Reserved
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2: Application Layer 2
Chapter 2: Application layer
2.1 Principles ofnetwork applications
2.2 Web and HTTP
2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 Socket programmingwith TCP
2.8 Socket programmingwith UDP
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2: Application Layer 3
Chapter 2: Application Layer
Our goals: conceptual,
implementationaspects of network
application protocols transport-layer
service models
client-server
paradigm peer-to-peer
paradigm
learn about protocolsby examining popularapplication-levelprotocols
HTTP FTP
SMTP / POP3 / IMAP
DNS
programming networkapplications
socket API
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2: Application Layer 4
Some network apps
e-mail
web
instant messaging
remote login P2P file sharing
multi-user networkgames
streaming stored videoclips
voice over IP
real-time videoconferencing
grid computing
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2: Application Layer 5
Creating a network app
write programs that run on (different) end
systems
communicate over network
e.g., web server softwarecommunicates with browsersoftware
No need to write softwarefor network-core devices Network-core devices do
not run user applications
applications on end systemsallows for rapid appdevelopment, propagation
applicationtransportnetworkdata link
physical
applicationtransportnetwork
data linkphysical
applicationtransportnetworkdata linkphysical
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2: Application Layer 6
Chapter 2: Application layer
2.1 Principles ofnetwork applications
2.2 Web and HTTP
2.3 FTP 2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 Socket programmingwith TCP
2.8 Socket programmingwith UDP
2.9 Building a Webserver
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2: Application Layer 7
Application architectures
Client-server
Peer-to-peer (P2P)
Hybrid of client-server and P2P
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2: Application Layer 8
Client-server architecture
server: always-on host
permanent IP address
server farms forscaling
clients: communicate with server
may be intermittently
connected may have dynamic IP
addresses
do not communicatedirectly with each other
client/server
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2: Application Layer 9
Pure P2P architecture
noalways-on server
arbitrary end systemsdirectly communicate
peers are intermittentlyconnected and change IPaddresses
Highly scalable butdifficult to manage
peer-peer
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2: Application Layer 10
Hybrid of client-server and P2P
Skype voice-over-IP P2P application centralized server: finding address of remote
party: client-client connection: direct (not through
server)Instant messaging
chatting between two users is P2P centralized service: client presence
detection/location user registers its IP address with central
server when it comes online user contacts central server to find IP
addresses of buddies
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2: Application Layer 12
Sockets
process sends/receivesmessages to/from itssocket
socket analogous to door sending process shoves
message out door
sending process relies ontransport infrastructureon other side of door whichbrings message to socketat receiving process
process
TCP with
buffers,
variables
socket
host orserver
process
TCP with
buffers,
variables
socket
host orserver
Internet
controlledby OS
controlled by
app developer
API: (1) choice of transport protocol; (2) ability to fixa few parameters (lots more on this later)
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2: Application Layer 13
Addressing processes
to receive messages,process must haveidentifier
host device has unique32-bit IP address
Q:does IP address ofhost suffice foridentifying the process?
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2: Application Layer 14
Addressing processes
to receive messages,process must haveidentifier
host device has unique32-bit IP address
Q:does IP address ofhost on which processruns suffice foridentifying the
process? A:No, many
processes can berunning on same host
identifierincludes bothIP addressand portnumbersassociated withprocess on host.
Example port numbers: HTTP server: 80
Mail server: 25
to send HTTP messageto gaia.cs.umass.edu web
server: IP address:128.119.245.12
Port number:80
more shortly
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2: Application Layer 15
App-layer protocol defines
Types of messagesexchanged, e.g., request, response
Message syntax: what fields in messages &
how fields are delineated
Message semantics meaning of information in
fields Rules for when and how
processes send &respond to messages
Public-domain protocols:
defined in RFCs
allows for
interoperability e.g., HTTP, SMTP
Proprietary protocols:
e.g., Skype
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2: Application Layer 16
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 datatransferTiming some apps (e.g.,
Internet telephony,interactive games)require low delay to beeffective
Throughput
some apps (e.g.,multimedia) requireminimum amount ofthroughput to be
effective other apps (elastic apps)
make use of whateverthroughput they get
Security Encryption, data
integrity,
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2: Application Layer 17
Transport service requirements of common apps
Application
file transfer
e-mail
Web documentsreal-time audio/video
stored audio/video
interactive games
instant messaging
Data loss
no loss
no loss
no lossloss-tolerant
loss-tolerant
loss-tolerant
no loss
Throughput
elastic
elastic
elasticaudio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
Time Sensitive
no
no
noyes, 100s msec
yes, few secs
yes, 100s msec
yes and no
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2: Application Layer 18
Internet transport protocols services
TCP service: connection-oriented:setup
required between client andserver processes
reliable transport betweensending and receiving process
flow control:sender wontoverwhelm receiver
congestion control:throttle
sender when networkoverloaded
does not provide:timing,minimum throughputguarantees, security
UDP service: unreliable data transfer
between sending andreceiving process
does not provide:connection setup,reliability, flow control,congestion control, timing,throughput guarantee, or
security
Q:why bother? Why isthere a UDP?
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2: Application Layer 19
Internet apps: application, transport protocols
Application
e-mail
remote terminal access
Webfile transfer
streaming multimedia
Internet telephony
Applicationlayer protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]FTP [RFC 959]
HTTP (eg Youtube),
RTP [RFC 1889]
SIP, RTP, proprietary
(e.g., Skype)
Underlyingtransport protocol
TCP
TCP
TCPTCP
TCP or UDP
typically UDP
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2: Application Layer 22
HTTP overview
HTTP: hypertexttransfer protocol
Webs application layerprotocol
client/server model client:browser that
requests, receives,displays Web objects
server:Web server
sends objects inresponse to requests
PC runningExplorer
Serverrunning
Apache Web
server
Mac runningNavigator
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2: Application Layer 23
HTTP overview (continued)
Uses TCP: client initiates TCP
connection (creates socket)to server, port 80
server accepts TCPconnection from client
HTTP messages (application-layer protocol messages)exchanged between browser
(HTTP client) and Webserver (HTTP server)
TCP connection closed
HTTP is stateless server maintains no
information aboutpast client requests
Protocols that maintainstate are complex!
past history (state) mustbe maintained
if server/client crashes,their views of state maybe inconsistent, must bereconciled
aside
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2: Application Layer 24
HTTP connections
Nonpersistent HTTP
At most one object issent over a TCPconnection.
Persistent HTTP
Multiple objects canbe sent over singleTCP connectionbetween client andserver.
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2: Application Layer 25
Nonpersistent HTTPSuppose user enters URL
www.someSchool.edu/someDepartment/home.index
1a.HTTP client initiates TCPconnection to HTTP server(process) at
www.someSchool.edu on port 80
2.HTTPclient sends HTTPrequest message(containingURL) into TCP connection
socket. Message indicatesthat client wants objectsomeDepartment/home.index
1b.HTTPserver at hostwww.someSchool.edu waiting
for TCP connection at port 80.accepts connection, notifyingclient
3.HTTPserver receives request
message, forms responsemessagecontaining requestedobject, and sends messageinto its socket
time
(contains text,
references to 10
jpeg images)
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2: Application Layer 26
Nonpersistent HTTP (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.HTTPserver closes TCPconnection.
time
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2: Application Layer 28
Persistent HTTP
Nonpersistent HTTP issues: requires 2 RTTs per object
OS overhead for eachTCPconnection
browsers often open parallel
TCP connections to fetchreferenced objects
Persistent HTTP server leaves connection
open after sendingresponse
subsequent HTTP messages
between sameclient/server sent overopen connection
client sends requests assoon as it encounters a
referenced object as little as one RTT for all
the referenced objects
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2: Application Layer 29
HTTP request message
two types of HTTP messages: request, response
HTTP request message: ASCII (human-readable format)
GET /somedir/page.html HTTP/1.1
Host: www.someschool.edu
User-agent: Mozilla/4.0
Connection: close
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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2: Application Layer 30
HTTP request message: general format
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2: Application Layer 31
Uploading form input
Post method:
Web page oftenincludes form input
Input is uploaded toserver in entity body
URL method:
Uses GET method
Input is uploaded inURL field of requestline:
www.somesite.com/animalsearch?monkeys&banana
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2: Application Layer 32
Method types
HTTP/1.0
GET
POST
HEAD asks server to leave
requested object out ofresponse
HTTP/1.1
GET, POST, HEAD
PUT uploads file in entity
body to path specifiedin URL field
DELETE deletes file specified in
the URL field
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2: Application Layer 34
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 server
505 HTTP Version Not Supported
In first line in server->client response message.A few sample codes:
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2: Application Layer 35
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 cis.poly.edu.Anything typed in sent
to port 80 at cis.poly.edu
telnet cis.poly.edu 80
2. Type in a GET HTTP request:
GET /~ross/ HTTP/1.1
Host: cis.poly.edu
By typing this in (hit carriagereturn twice), you send
this minimal (but complete)GET request to HTTP server
3. Look at response message sent by HTTP server!
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2: Application Layer 37
Cookies: keeping state (cont.)
client
server
usual http response msg
usual http response msg
cookie file
one week later:
usual http request msgcookie: 1678 cookie-
specificaction
access
ebay 8734usual http request msg Amazon server
creates ID1678 for usercreate
entry
usual http responseSet-cookie: 1678
ebay 8734amazon 1678
usual http request msgcookie: 1678 cookie-
spectificaction
accessebay 8734
amazon 1678
backenddatabase
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2: Application Layer 38
Cookies (continued)
What cookies can bring: authorization
shopping carts
recommendations
user session state(Web e-mail)
Cookies and privacy: cookies permit sites to
learn a lot about you
you may supply name
and e-mail to sites
aside
How to keep state:
protocol endpoints: maintain stateat sender/receiver over multipletransactions
cookies: http messages carry state
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2: Application Layer 39
Web caches (proxy server)
user sets browser:Web accesses viacache
browser sends allHTTP requests tocache object in cache: cache
returns object else cache requests
object from originserver, then returnsobject to client
Goal:satisfy client request without involving origin server
client
Proxy
server
clientoriginserver
originserver
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2: Application Layer 40
More about Web caching
cache acts as bothclient and server
typically cache isinstalled by ISP
(university, company,residential ISP)
Why Web caching?
reduce response timefor client request
reduce traffic on aninstitutions accesslink.
Internet dense withcaches: enables poor
content providers toeffectively delivercontent (but so doesP2P file sharing)
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2: Application Layer 42
Caching example (cont)
possible solution increase bandwidth of access
link to, say, 10 Mbps
consequence 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
publicInternet
institutionalnetwork 10 Mbps LAN
10 Mbpsaccess link
institutionalcache
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2: Application Layer 46
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 transferFTP
server
FTPuser
interface
FTPclient
local filesystem
remote file
system
userat host
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2: Application Layer 47
FTP: separate control, data connections
FTP client contacts FTP serverat port 21, TCP is transportprotocol
client authorized over controlconnection
client browses remotedirectory by sending commandsover control connection.
when server receives file
transfer command, serveropens 2ndTCP connection (forfile) to client
after transferring one file,server closes data connection.
FTPclient
FTPserver
TCP control connectionport 21
TCP data connectionport 20
server opens another TCPdata connection to transferanother file.
control connection: out of
band FTP server maintains state:
current directory, earlierauthentication
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2: Application Layer 48
FTP commands, responses
Sample commands: sent as ASCII text over
control channel
USER username
PASS password LISTreturn list of file in
current directory
RETR filenameretrieves
(gets) file STOR filenamestores
(puts) file onto remotehost
Sample return codes status code and phrase (as
in HTTP) 331 Username OK,
password required 125 data connection
already open;
transfer starting
425 Cant open data
connection 452 Error writing
file
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2: Application Layer 49
Chapter 2: Application layer
2.1 Principles ofnetwork applications
2.2 Web and HTTP
2.3 FTP
2.4 Electronic Mail SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 Socket programmingwith TCP
2.8 Socket programmingwith UDP
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2: Application Layer 50
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 messages e.g., Eudora, Outlook, elm,
Mozilla Thunderbird
outgoing, incoming messagesstored on server
user mailbox
outgoingmessage queue
mail
server
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
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2: Application Layer 51
Electronic Mail: mail servers
Mail Servers mailboxcontains incoming
messages for user
messagequeueof outgoing
(to be sent) mail messages SMTP protocolbetween mail
servers to send emailmessages
client: sending mail
server server: receiving mail
server
mailserver
useragent
useragent
useragent
mail
server
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
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2: Application Layer 52
Electronic Mail: SMTP [RFC 2821]
uses TCP to reliably transfer email message from clientto 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
S i Ali d t B b
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2: Application Layer 53
Scenario: Alice sends message to Bob
1) Alice uses UA to compose
message and [email protected]
2) Alices UA sends messageto her mail server; messageplaced in message queue
3) Client side of SMTP opensTCP connection with Bobsmail server
4) SMTP client sends Alices
message over the TCPconnection
5) Bobs mail server places themessage in Bobs mailbox
6) Bob invokes his user agent
to read message
useragent
mailserver
mailserver user
agent
1
2 3 4 56
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2: Application Layer 54
Sample SMTP interactionS: 220 hamburger.edu
C: HELO crepes.frS: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM:
S: 250 [email protected]... Sender ok
C: RCPT TO:
S: 250 [email protected] ... Recipient okC: 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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2: Application Layer 55
Try SMTP interaction for yourself:
telnet servername 25 see 220 reply from server
enter HELO, MAIL FROM, RCPT TO, DATA, QUITcommands
above lets you send email without using email client(reader)
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2: Application Layer 56
SMTP: final words
SMTP uses persistentconnections
SMTP requires message(header & body) to be in 7-bit ASCII
SMTP server usesCRLF.CRLFto determineend of message
Comparison with HTTP:
HTTP: pull
SMTP: push
both have ASCIIcommand/responseinteraction, status codes
HTTP: each objectencapsulated in its own
response msg SMTP: multiple objects
sent in multipart msg
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2: Application Layer 57
Mail message format
SMTP: protocol forexchanging email msgs
RFC 822: standard for textmessage format:
header lines, e.g., To:
From:
Subject:
differentfrom SMTPcommands!
body the message, ASCII
characters only
header
body
blankline
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2: Application Layer 58
Message format: multimedia extensions
MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content
type
From: [email protected]
To: [email protected]: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data ..............................
......base64 encoded data
multimedia data
type, subtype,parameter declaration
method usedto encode data
MIME version
encoded data
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2: Application Layer 59
Mail access protocols
SMTP: delivery/storage to receivers 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: gmail, Hotmail, Yahoo! Mail, etc.
useragent
senders mailserver
useragent
SMTP SMTP accessprotocol
receivers mailserver
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2: Application Layer 60
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 bynumber
dele:delete
quit
C: list
S: 1 498
S: 2 912
S: .C: retr 1
S:
S: .
C: dele 1
C: retr 2
S:
S: .
C: dele 2
C: quit
S: +OK POP3 server signing off
S: +OK POP3 server ready
C: user bob
S: +OK
C: pass hungry
S: +OKuser successfully logged on
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2: Application Layer 61
POP3 (more) and IMAP
More about POP3 Previous example uses
download and deletemode.
Bob cannot re-read e-mail if he changesclient
Download-and-keep:copies of messages ondifferent clients
POP3 is statelessacross sessions
IMAP Keep all messages in
one place: the server
Allows user to
organize messages infolders
IMAP keeps user stateacross sessions:
names of folders andmappings betweenmessage IDs and foldername
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2: Application Layer 62
Chapter 2: Application layer
2.1 Principles ofnetwork applications
2.2 Web and HTTP
2.3 FTP
2.4 Electronic Mail SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications 2.7 Socket programming
with TCP
2.8 Socket programmingwith UDP
2.9 Building a Webserver
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2: Application Layer 63
DNS: Domain Name System
People:many identifiers: SSN, name, passport #
Internet hosts, routers: IP address (32 bit) -
used for addressingdatagrams
name, e.g.,ww.yahoo.com - used byhumans
Q:map between IPaddresses and name ?
Domain Name System: distributed database
implemented in hierarchy ofmany name servers
application-layer protocolhost, routers, name servers tocommunicate to resolvenames(address/name translation)
note: core Internet
function, implemented asapplication-layer protocol
complexity at networksedge
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2: Application Layer 64
DNS
Why not centralize DNS? single point of failure
traffic volume
distant centralized
database maintenance
doesnt scale!
DNS services hostname to IP
address translation
host aliasing Canonical, alias names
mail server aliasing
load distribution replicated Web
servers: set of IPaddresses for onecanonical name
Distributed Hierarchical Database
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2: Application Layer 65
Root DNS Servers
com DNS servers org DNS servers edu DNS servers
poly.edu
DNS servers
umass.edu
DNS serversyahoo.com
DNS servers
amazon.com
DNS servers
pbs.org
DNS servers
Distributed, Hierarchical Database
Client wants IP for www.amazon.com; 1stapprox:
client queries a root server to find com DNS server
client queries com DNS server to get amazon.comDNS server
client queries amazon.com DNS server to get IPaddress for www.amazon.com
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2: Application Layer 66
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 nameservers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
e NASA Mt View, CA
f Internet Software C. PaloAlto,CA (and 36 other locations)
i Autonomica, Stockholm (plus
28 other locations)
k RIPE London (also 16 other locations)
m WIDE Tokyo (also Seoul,
Paris, SF)
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also LA)
d U Maryland College Park, MD
g US DoD Vienna, VA
h ARL Aberdeen, MDj Verisign, ( 21 locations)
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2: Application Layer 67
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 TLDAuthoritative DNS servers:
organizations DNS servers, providingauthoritative hostname to IP mappings for
organizations servers (e.g., Web, mail). can be maintained by organization or service
provider
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2: Application Layer 68
Local Name Server
does not strictly belong to hierarchy each ISP (residential ISP, company,
university) has one.
also called default name serverwhen host makes DNS query, query is sent
to its local DNS server acts as proxy, forwards query into hierarchy
DNDNS name
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2: Application Layer 69
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
2 3
4
5
6
authoritative DNS server
dns.cs.umass.edu
78
TLD DNS server
DNS nameresolution example
Host at cis.poly.eduwants IP address forgaia.cs.umass.edu
iterated query: contacted server
replies with name ofserver to contact
I dont know this
name, but ask thisserver
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2: Application Layer 71
DNS: caching and updating records
once (any) name server learns mapping, it cachesmapping
cache entries timeout (disappear) after sometime
TLD servers typically cached in local nameservers Thus root name servers not often visited
update/notify mechanisms under design by IETF
RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html
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2: Application Layer 72
DNS records
DNS:distributed db storing resource records (RR)
Type=NS nameis domain (e.g.
foo.com) valueis hostname of
authoritative nameserver for this domain
RR format: (name, value, type, ttl)
Type=A
nameis hostname valueis IP address
Type=CNAME
nameis alias name for somecanonical (the real) name
www.ibm.com is reallyservereast.backup2.ibm.com
valueis canonical name
Type=MX valueis name of mailserver
associated with name
l
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2: Application Layer 73
DNS protocol, messages
DNS protocol :queryand replymessages, both withsame message format
msg header identification:16 bit #
for query, reply to queryuses same #
flags:
query or reply
recursion desired
recursion available reply is authoritative
DN l
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2: Application Layer 74
DNS protocol, messages
Name, type fieldsfor a query
RRs in response
to query
records forauthoritative servers
additional helpfulinfo that may be used
I i d i DNS
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2: Application Layer 75
Inserting records into DNS
example: new startup Network Utopia register name networkuptopia.com at DNS registrar
(e.g., Network Solutions) provide names, IP addresses of authoritative name server
(primary and secondary)
registrar inserts two RRs into com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
create authoritative server Type A record forwww.networkuptopia.com; Type MX record fornetworkutopia.com
How do people get IP address of your Web site?
Ch t 2 A li ti l
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2: Application Layer 76
Chapter 2: Application layer
2.1 Principles ofnetwork applications app architectures
app requirements
2.2 Web and HTTP 2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications 2.7 Socket programming
with TCP
2.8 Socket programming
with UDP
P P2P hit t
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2: Application Layer 77
Pure P2P architecture
noalways-on server arbitrary end systems
directly communicate
peers are intermittently
connected and change IPaddresses
Three topics: File distribution
Searching for information
Case Study: Skype
peer-peer
Fil Di t ib ti S Cli t P2P
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2: Application Layer 78
File Distribution: Server-Client vs P2P
Question: How much time to distribute filefrom one server to N peers?
us
u2d1d2
u1
uN
dN
Server
Network (withabundant bandwidth)
File, size F
us:server upload
bandwidth
ui
:
peer i uploadbandwidth
di:peer i download
bandwidth
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Fil di t ib ti ti P2P
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2: Application Layer 80
File distribution time: P2P
us
u2d1 d2u1
uN
dN
Server
Network (withabundant bandwidth)
F server must send one
copy: F/ustime
client i takes F/di timeto download
NF bits must bedownloaded (aggregate) fastest possible upload rate: us+ Sui
dP2P= max {F/us, F/min(di), NF/(us+ Sui)}i
S li t P2P l
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2: Application Layer 81
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
N
MinimumD
istributionTime P2P
Client-Server
Server-client vs. P2P: example
Client upload rate = u, F/u = 1 hour, us= 10u, dmin us
Fil di t ib ti BitT t
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2: Application Layer 82
File distribution: BitTorrent
tracker:tracks peersparticipating in torrent
torrent:group ofpeers exchangingchunks of a file
obtain listof peers
trading
chunks
peer
P2P file distribution
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2: Application Layer 83
BitTorrent (1)
file divided into 256KB chunks.
peer joining torrent:
has no chunks, but will accumulate them over time registers with tracker to get list of peers,
connects to subset of peers (neighbors)
while downloading, peer uploads chunks to other
peers. peers may come and go
once peer has entire file, it may (selfishly) leave or(altruistically) remain
BitT t (2)
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2: Application Layer 84
BitTorrent (2)
Pulling Chunks at any given time,
different peers havedifferent subsets of
file chunks periodically, a peer
(Alice) asks eachneighbor for list ofchunks that they have.
Alice sends requestsfor her missing chunks
rarest first
Sending Chunks: tit-for-tat
Alice sends chunks to fourneighbors currentlysending her chunks at thehighest rate
re-evaluate top 4 every
10 secs every 30 secs: randomly
select another peer,starts sending chunks
newly chosen peer mayjoin top 4
optimistically unchoke
BitT t Tit f t t
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2: Application Layer 85
BitTorrent: Tit-for-tat(1) Alice optimistically unchokes Bob
(2) Alice becomes one of Bobs top-four providers; Bob reciprocates(3) Bob becomes one of Alices top-four providers
With higher upload rate,can find better tradingpartners & get file faster!
P2P: searching for information
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2: Application Layer 86
P2P searching for information
File sharing (eg e-mule)
Index dynamicallytracks the locations of
files that peers share. Peers need to tell
index what they have.
Peers search index to
determine where filescan be found
Instant messaging
Index maps usernames to locations.
When user starts IMapplication, it needs toinform index of itslocation
Peers search index todetermine IP addressof user.
Index in P2P system: maps information to peer location
(location = IP address & port number).
P2P: centralized index
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2: Application Layer 87
P2P: centralized index
original Napster design1) when peer connects, it
informs central server: IP address
content2) Alice queries for Hey
Jude
3) Alice requests file from
Bob
centralizeddirectory server
peers
Alice
Bob
1
1
1
12
3
P2P: problems with centralized directory
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2: Application Layer 88
P2P: problems with centralized directory
single point of failure performance bottleneck
copyright infringement:target of lawsuit is
obvious
file transfer isdecentralized, butlocating content ishighly centralized
Query flooding
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2: Application Layer 89
Query flooding
fully distributed no central server
used by Gnutella Each peer indexes the
files it makes availablefor sharing (and noother files)
overlay network: graph edge between peer X
and Y if theres a TCPconnection
all active peers andedges form overlay net
edge: virtual (notphysical) link
given peer typicallyconnected with < 10overlay neighbors
Query flooding
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2: Application Layer 90
Query flood ng
Query
QueryHit
Query
QueryHit
File transfer:
HTTPQuery message
sent over existing TCPconnections
peers forwardQuery message
QueryHitsent overreversepath
Scalability:
limited scopeflooding
Gnutella: Peer joining
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2: Application Layer 91
Gnutella: Peer joining
1. joining peer Alice must find another peer inGnutella network: use list of candidate peers
2. Alice sequentially attempts TCP connections withcandidate peers until connection setup with Bob
3. Flooding:Alice sends Ping message to Bob; Bobforwards Ping message to his overlay neighbors(who then forward to their neighbors.) peers receiving Ping message respond to Alice
with Pong message
4. Alice receives many Pong messages, and can thensetup additional TCP connections
Peer leaving: see homework problem!
Hierarchical Overlay
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2: Application Layer 92
Hierarchical Overlay
between centralizedindex, query floodingapproaches
each peer is either a
super nodeor assigned toa super node TCP connection between
peer and its super node.
TCP connections betweensome pairs of super nodes.
Super node tracks contentin its children
ordinary peer
group-leader peer
neighoring relationships
in overlay network
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Chapter 2: Application layer
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2: Application Layer 95
Chapter 2: Application layer
2.1 Principles ofnetwork applications
2.2 Web and HTTP
2.3 FTP
2.4 Electronic Mail SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications 2.7 Socket programming
with TCP
2.8 Socket programming
with UDP
Socket programming
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2: Application Layer 96
Socket programming
Socket API introduced in BSD4.1 UNIX,
1981
explicitly created, used,released by apps
client/server paradigm
two types of transport
service via socket API: unreliable datagram
reliable, byte stream-oriented
a host-local,
application-created,OS-controlledinterface(a door) into which
application process canboth send and
receivemessages to/fromanother applicationprocess
socket
Goal:learn how to build client/server application that
communicate using sockets
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Socket programming with TCP
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2: Application Layer 98
Socket programming with TCP
Client must contact server
server process must firstbe running
server must have createdsocket (door) thatwelcomes clients contact
Client contacts server by:
creating client-local TCPsocket
specifying IP address, port
number of server process When client creates
socket: client TCPestablishes connection toserver TCP
When contacted by client,server TCP creates newsocketfor server process tocommunicate with client
allows server to talk withmultiple clients
source port numbersused to distinguishclients (more in Chap 3)
TCP provides reliable, in-ordertransfer of bytes (pipe)between client and server
application viewpoint
Client/server socket interaction: TCP
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2: Application Layer 99
Client/server socket interaction: TCP
wait for incomingconnection requestconnectionSocket =
welcomeSocket.accept()
create socket,port=x, for
incoming request:welcomeSocket =
ServerSocket()
create socket,connect to hostid, port=xclientSocket =
Socket()
close
connectionSocket
read reply from
clientSocket
close
clientSocket
Server (running on hostid) Client
send request using
clientSocketread request from
connectionSocket
write reply to
connectionSocket
TCPconnection setup
Stream jargon
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2: Application Layer 100
outToServer
to network from network
inFromServer
inFromUser
keyboard monitor
Process
clientSocket
input
stream
input
stream
output
stream
TCP
socket
Client
process
client TCPsocket
Stream jargon
A streamis a sequence ofcharacters that flow intoor out of a process.
An input streamisattached to some input
source for the process,e.g., keyboard or socket.
An output streamisattached to an outputsource, e.g., monitor or
socket.
Socket programming with TCP
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2: Application Layer 101
Socket programming with TCP
Example client-server app:1) client reads line fromstandard input (inFromUserstream) , sends to server viasocket (outToServer
stream)2) server reads line from socket
3) server converts line touppercase, sends back toclient
4) client reads, prints modifiedline from socket(inFromServerstream)
Example: Java client (TCP)
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2: Application Layer 102
Example: Java client (TCP)
import java.io.*;import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;String modifiedSentence;
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
Createinput stream
Create
client socket,connect to server
Createoutput stream
attached to socket
Example: Java client (TCP) cont
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2: Application Layer 103
Example: Java client (TCP), cont.
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
}
}
Createinput stream
attached to socket
Send lineto server
Read linefrom server
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Example: Java server (TCP) cont
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2: Application Layer 105
Example: Java server (TCP), cont
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence);
}
}
}
Read in linefrom socket
Create outputstream, attached
to socket
Write out lineto socket
End of while loop,loop back and wait foranother client connection
Chapter 2: Application layer
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2: Application Layer 106
Chapter 2: Application layer
2.1 Principles ofnetwork applications
2.2 Web and HTTP
2.3 FTP
2.4 Electronic Mail SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications 2.7 Socket programming
with TCP
2.8 Socket programming
with UDP
Socket programming with UDP
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2: Application Layer 107
Socket programming with UDP
UDP: no connection betweenclient and server
no handshaking
sender explicitly attachesIP address and port of
destination to each packet server must extract IP
address, port of senderfrom received packet
UDP: transmitted data may bereceived out of order, orlost
application viewpoint
UDP provides unreliable transferof groups of bytes (datagrams)between client and server
Client/server socket interaction: UDP
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2: Application Layer 108
Client/server socket interaction: UDP
Server (running on hostid)
close
clientSocket
read datagram from
clientSocket
create socket,
clientSocket =
DatagramSocket()
Client
Create datagram with server IP andport=x; send datagram via
clientSocket
create socket,
port= x.
serverSocket =
DatagramSocket()
read datagram from
serverSocket
write reply to
serverSocket
specifyingclient address,
port number
Example: Java client (UDP)
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2: Application Layer 109
Example: Java client (UDP)
sendPacket
to network from network
receivePacket
inFromUser
keyboard monito r
Process
clientSocket
UDPpacket
inputstream
UDPpacket
UDPsocket
Output: sendspacket (recall
that TCP sentbyte stream)
Input: receivespacket (recall
thatTCP receivedbyte stream)
Client
process
client UDPsocket
Example: Java client (UDP)
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2: Application Layer 110
Example: Java client (UDP)
import java.io.*;import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception
{
BufferedReader inFromUser =new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Create
input streamCreate
client socket
Translatehostname to IP
address using DNS
Example: Java client (UDP), cont.
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2: Application Layer 111
Example Java client (UDP), cont.
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);clientSocket.close();
}
}
Create datagramwith data-to-send,
length, IP addr, port
Send datagramto server
Read datagramfrom server
Example: Java server (UDP)
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2: Application Layer 112
Example Java server (UDP)
import java.io.*;import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Createdatagram socket
at port 9876
Create space forreceived datagram
Receivedatagram
Example: Java server (UDP), cont
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2: Application Layer 113
Example Java server (UDP), cont
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
serverSocket.send(sendPacket);}
}
}
Get IP addrport #, of
sender
Write out
datagramto socket
End of while loop,loop back and wait foranother datagram
Create datagramto send to client
Chapter 2: Summary
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2: Application Layer 114
application architectures client-server
P2P
hybrid
application servicerequirements: reliability, bandwidth,
delay
Internet transportservice model connection-oriented,
reliable: TCP
unreliable, datagrams: UDP
our study of network apps now complete!
specific protocols: HTTP
FTP
SMTP, POP, IMAP
DNS P2P: BitTorrent, Skype
socket programming
Chapter 2: Summary
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Chapter 2 Summary
typical request/replymessage exchange: client requests info or
service server responds with
data, status code
message formats:
headers: fields givinginfo about data
data: info beingmm ni t d
Most importantly: learned aboutprotocolsImportant themes:
control vs. data msgs
in-band, out-of-band centralized vs.
decentralized
stateless vs. stateful
reliable vs. unreliablemsg transfer
complexity at network