Transcript nawaporn29255212307

ON DEMAND LECTURE PART II
By Dr. Nawaporn Wisitpongphan
ON DEMAND OUTLINE

Voice Over IP (VoIP)

Hyper Text Transfer Protocol (HTTP)

Domain Name Server (DNS)
INTRODUCTION TO VOIP

Broadband Internet offers high-speed connection

Many applications become viable e.g., multimedia
streaming, VoIP, etc.
The technology for transmitting voice
conversation over the Internet has been available
since early 1980s
 VoIP allows two-way voice transmission over
broadband connections
 VoIP also carries telephony signals as digital
audio

Reduce data rate by using speech data compression
 Encapsulating in a packet stream sent over IP
network

VOIP
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
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Not yet a big player with less than 5% of market
Cost savings, enhanced voice services and new
applications major advantages
VoIP gateways bridge circuit-switched PSTN and
packet-switched Internet

Gateways packetize, and compress voice, route
packets, authenticate users, and manage network of
gateways
VOIP HARDWARE
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Enterprise gateway
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VoIP routers
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Voice cards perform packetization and compression
functions in a router
IP PBX
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Deployed between PBX and WAN device (router) for call
set-up,routing, and conversion
Distributed telephony servers that operat ein packtswitched mode
ISP VoIP gateways

Aggregate incoming traffic and routing
VOIP INFRASTRUCTURE
VOIP ARCHITECTURE
IMPLEMENTING VOIP
VOIP STANDARDS
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H.323
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SIP
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Based on ISDN and limited to point-to-point
applications
Application layer (signaling) protocol
Establishes temp sessions for multimedia conferences,
telephony, mobile phone-to-instant messaging
LDAP



Standard directory server technology for Internet
Enables retrieval of information from multi-vendor
directories
Used for free phone and Internet phone number hosting
THE PHONE WORKS – WHY BOTHER
WITH VOIP
user perspective
carrier perspective
variable compression: tin can to broadcast
quality  no need for dedicated lines
better codecs + silence suppression – packet
header overhead = maybe reduced bandwidth
security through encryption
shared facilities simplify management,
redundancy
caller & talker identification
advanced services
better user interface (more than 12 keys, visual
feedback, semantic rather than stimulus)
cheaper bit switching
no local access fees (but dropping to 1c/min
for PSTN)
transmit as data rather than voiceband data
(14.4 kb/s)
adding video, application sharing is easy
VOIP VS PSTN (PUBLIC SWITCH
TELEPHONE NETWORK)

VoIP can transmit more than one phone call over the same
broadband connection

Conference call, call forwarding, auto redial, caller ID are
free of charge
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Secure calls using protocols such as (Secure Real-Time
Transport Protocol)
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Connection Independence. Can use VoIP anywhere there’s
an Internet
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Integration with other services available
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Mobility
VOIP:
DRAWBACK AND CHALLENGES
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Available Bandwidth
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Network Latency
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Packet Loss
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Jitter

Echo
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Security
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Realiability
THE FIVE-PHASE
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"Technology Trigger" — The first phase of a hype cycle is the "technology
trigger" or breakthrough, product launch or other event that generates
significant press and interest.
"Peak of Inflated Expectations" — In the next phase, a frenzy of publicity
typically generates over-enthusiasm and unrealistic expectations. There may
be some successful applications of a technology, but there are typically more
failures.
"Trough of Disillusionment" — Technologies enter the "trough of
disillusionment" because they fail to meet expectations and quickly become
unfashionable. Consequently, the press usually abandons the topic and the
technology.
"Slope of Enlightenment" — Although the press may have stopped covering
the technology, some businesses continue through the "slope of enlightenment"
and experiment to understand the benefits and practical application of the
technology.
"Plateau of Productivity" — A technology reaches the "plateau of
productivity" as the benefits of it become widely demonstrated and accepted.
The technology becomes increasingly stable and evolves in second and third
generations. The final height of the plateau varies according to whether the
technology is broadly applicable or benefits only a niche market
HYPE CYCLE 2005
HYPE CYCLE 2006
HYPE CYCLE 2007
HYPE CYCLE 2008
HYPE CYCLE 2009
HTTP
ARCHITECTURAL OVERVIEW
Client with Web browser program
 Server with Web Server and pages (html)
 Other servers with Web Servers and pages
 Links between pages
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BROWSER OPERATION
WHEN
USER
CLICKS ON A LINK
B picks the URL from the clicked link
 B gets IP address of Web server from DNS
 B open TCP connection to the (IP, port 80)
 B sends a request for page (HTTP packet)
 W.S. sends the linked page (HTTP packet)


Page is in html language
B. closes TCP connection
 B. interpret html, displays page to user
 B fetches & presents images linked to the file

THE CLIENT SIDE
 non
html in page: PDF, GIF, JPEG, MP3, MPEG, ...
 Plug-ins:
Code installed as an extension to the
browser
 Helper
Applications, invoked by B as a separate
process
Plug-in
Helper Application
SERVER SIDE
Accepts TCP connection
 Gets name of requested file (HTTP packet)
 Gets the file (local disk)
 Sends back the file (HTTP packets)
 Release TCP connection


To improve performance
Maintain cache of files
 Multithreading
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MULTI-THREADED WEB SERVER
Front-end thread accept request, build record
 Pass record to a Working Thread
 All threads share memory , including the cache
 If page not in cache, WT initiates disk read

TASKS OF A WORKING THREAD
Resolving name of the file
 Authenticating client (another lecture)
 Perform access control on client
 Check the cache
 Fetch file from disk
 Determine MIME type of file
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This will be sent to the client
Send reply to client
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Construct HTTP packet(s)
Write in the Web Server log
 What if the CPU can’t handle the load?
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SERVER FARM ON A LAN
Problems
 Each Processing Node has its own cache


P.N. “specialize” with certain files
Both requests and replies via the Front-end
 Solution?
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TCP HANDOFF
Front-end passes the TCP endpoint (IP, port) to the
Processing Node
 Processing Node send page to Client

Normal
TCP Handoff
URLS – UNIFORM RESOURCE LOCATERS
URL provides answers to what?
 What is the name of the page?
 What is the location of the page?
 How to access the page (which protocol)?

?
?
STATELESSNESS AND COOKIES
HTTP is request/reply; stateless
 But, server needs:
 to recognize users (registered?, adapt home page)
 to keep track of visited items (shopping cart)
 Cookies (small text files) keep that info.
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Stored at Client
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C:\Documents and Settings\aviv\Cookies
Identified by domain name of the sending server
COOKIES: STRUCTURE
domain:
where the cookie came from
Path: root of the file tree related to cookie
Content: variableName=value pairs. Anything
Expires – if set it is kept (persistent cookie)
Secure: If set cookie is sent only to secure server
Usages?
USING COOKIES
Casino
server chooses which gambling option it
presents
Store Server puts “items in cart” in the cookie
Web Portal server presents stock prices and Sport
results
sneaky.com records visits of UserID in certain pages
 pages include adds/banners/small pictures
 User not aware its browser visited sneaky.com
 User profile is built, maybe with name/password
USER-SERVER
STATE: COOKIES
Many major Web sites
use cookies
Four components:
1) cookie header line in
the HTTP response
message
2) cookie header line in
HTTP request message
3) cookie file kept on
user’s host and
managed by user’s
browser
4) back-end database at
Web site
Example:
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Susan access Internet
always from same PC
She visits a specific ecommerce site for first
time
When initial HTTP
requests arrives at site,
site creates a unique ID
and creates an entry in
backend database for
ID
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COOKIES: KEEPING “STATE” (CONT.)
client
Cookie file
server
usual http request msg
usual http response +
ebay: 8734
Cookie file
amazon: 1678
ebay: 8734
one week later:
Cookie file
amazon: 1678
ebay: 8734
Set-cookie: 1678
usual http request msg
cookie: 1678
usual http response
msg
usual http request msg
cookie: 1678
usual http response
msg
server
creates ID
1678 for user
cookiespecific
action
cookiespectific
action
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WEB CACHES (PROXY SERVER)
Goal: satisfy client request without involving origin server

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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

origin
server
client
client
Proxy
server
origin
server
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MORE ABOUT WEB CACHING
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Cache acts as both client
and server
Typically cache is installed
by ISP (university,
company, residential ISP)
Why Web caching?
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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
origin
Assumptions
servers
 average object size = 100,000
bits
public
Internet
 avg. request rate from
institution’s browsers to origin
servers = 15/sec
 delay from institutional router to
1.5 Mbps
any origin server and back to
access link
router = 2 sec
institutional
Consequences
utilization on LAN = 15%
 utilization on access link = 100%
 total delay = Internet delay +
access delay + LAN delay
= 2 sec + minutes + milliseconds

network
10 Mbps LAN
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
10 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
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CACHING EXAMPLE (CONT)
origin
servers
Install cache

suppose hit rate is .4
Consequence

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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 +
milliseconds < 1.4 secs
public
Internet
1.5 Mbps
access link
institutional
network
10 Mbps LAN
institutional
cache
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DNS
DNS: DOMAIN NAME SYSTEM
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 applicationlayer protocol
 complexity at network’s
“edge”
Why not centralize DNS?
 single point of failure
 traffic volume
 distant centralized database
 maintenance
doesn’t scale!
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DISTRIBUTED, HIERARCHICAL DATABASE
Root DNS Servers
com DNS servers
yahoo.com
amazon.com
DNS servers DNS servers
org DNS servers
pbs.org
DNS servers
edu DNS servers
poly.edu
umass.edu
DNS serversDNS servers
Client wants IP for www.amazon.com; 1st 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
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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
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also Los Angeles)
d U Maryland College Park, MD
k RIPE London (also Amsterdam,
g US DoD Vienna, VA
i Frankfurt)
Autonomica, Stockholm (plus
h ARL Aberdeen, MD
3 other locations)
j Verisign, ( 11 locations)
m WIDE Tokyo
e NASA Mt View, CA
f Internet Software C. Palo
Alto, CA (and 17 other locations)
13 root name
servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
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TLD AND AUTHORITATIVE SERVERS
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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
 Education for edu TLD
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Authoritative DNS servers: organization’s DNS
servers, providing authoritative hostname to IP
mappings for organization’s servers (e.g., Web and
mail).
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Can be maintained by organization or service provider
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LOCAL NAME SERVER
Does not strictly belong to hierarchy
 Each ISP (residential ISP, company, university)
has one.
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Also called “default name server”
When a host makes a DNS query, query is sent to
its local DNS server

Acts as a proxy, forwards query into hierarchy.
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root DNS server
ITERATED QUERIES
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2
Host at cis.poly.edu
wants IP address for
gaia.cs.umass.edu
3
4
TLD DNS server
5
local DNS server
iterated query:


contacted server replies
with name of server to
contact
“I don’t know this name,
but ask this server”
dns.poly.edu
1
8
requesting host
7
6
authoritative DNS server
dns.cs.umass.edu
cis.poly.edu
gaia.cs.umass.edu
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root DNS server
RECURSIVE QUERIES
recursive query:
 puts burden of name
resolution on
contacted name
server
 heavy load?
2
3
7
6
TLD DNS server
local DNS server
dns.poly.edu
1
5
4
8
requesting host
authoritative DNS server
dns.cs.umass.edu
cis.poly.edu
gaia.cs.umass.edu
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