Download power-point presentation

Computer Networks
(EENG 4810)
Computer Networks- Course
Objectives & Scope - 1
1
Course Objectives & Scope
Computer Networks- Course
Objectives & Scope - 2
2
In this class, you are expected to learn A brief History of Computer Networks
Categorization of Computer Networks
Network Services and Internet Perspective
Network Components- Nuts and Bolts View
General Concepts of Network Design
Protocols and Layered Communication Architecture
Network Programming
Computer Networks- Course
Objectives & Scope - 3
3
This class, however, does not deal with Network Hardware Design
Comparative analyses of different protocol standards
Special purpose networks such as ad hoc sensor nets
Applications of Queuing Theory to Network traffic
control
Computer Networks- Course
Objectives & Scope - 4
4
Lesson 1:
History of Computer Networks
5
Preview of the Lesson 1
In this lesson, we cover History of Computer
Networks organized into approximately 5 decades.
In passing, we get a hang of what all a computer
network can do
History of Computer Networks - 1
6
History of Computer Networks
Development of Packet Switching: 1961-72
Proprietary Networks and Internetworking: 1972-80
Proliferation of Networks: 1980-90
Internet Explosion: 1990-2000
Developments of Last Decade: Bubble burst?
Social Networks?
History of Computer Networks- 2
7
Development of Packet Switching: 1961-72
Telephone network - World’s dominant communication
network , uses circuit switching. (Early 1960s)
Three research groups around the world independently
invented packet switching (1964 – 1967)
Leonard Kleinrock at MIT used queuing theory to demonstrate
effectiveness of packet switching for bursty traffic
Paul Baran of Rand Institute investigated packet switching
for secure voice communication over military networks
Donald Davies and Roger Scantlebury were developing ideas
on packet switching at the National Physical Lab, England.
Lesson 1: History of Computer
Networks - 3
8
Development of Packet Switching: 1961-72
(continued)
J.C.R. Licklider and Laurence Roberts led the CS
program at ARPA (Advanced Projects Research
Agency) and published a plan for ARPAnet in 1967.
Arpanet was the ancestor of today’s Internet.
Early Packet switches were known as Interface
Message Processors (IMPs). BBN got the contract.
First IMP was installed at UCLA on Labor Day 1969
under Kleinrock’s supervision. Later 3 more at SRI,
UCSB and University of Utah.
Lesson 1: History of Computer
Networks - 4
9
Leonard Kleinrock with IMP
Lesson 1: History of Computer
Networks - 5
10
Development of Packet Switching: 1961-72
(continued)
First use of the net of 4 nodes was remote login from
UCLA to SRI; it resulted in system crash.
Robert Kahn demonstrated 15-node ARPAnet in 1972
ICCN.
First host to host protocol was Network Control
Protocol (NCP).
Ray Tomlinson at BBN wrote the first e-mail program
in 1972.
Lesson 1: History of Computer
Networks - 6
11
Proprietary Networks and Internet 1972-80
ALOHAnet- microwave satellite net linking universities
on Hawaii islands (Norman Abramson 1970).
Telenet- a BBN commercial packet network and
Cyclades- a French Packet Net by Louis Pouzin.
Time-sharing networks such as Tymnet and GE
Information Services Net (late 60s and early 70s).
Metcalfe’s PhD thesis proposing Ethernet.
History of Computer Networks - 7
12
Proprietary Networks and Internet 1972-80
(Continued)
Proprietary Networks such as
IBM’s (1969-74) System Network Architecture (SNA)
paralleling the ARPAnet (Schwartz 1977).
DEC’s DECnet and Xerox corporation’s XNA.
Vincent Cerf and Robert Kahn (Cerf 1974)Architecture for interconnecting Networks (They
coined the word Internet for network of networks).
DARPA’s packet satellite and packet-radio networks
(Kahn 1978).
History of Computer Networks - 8
13
Proprietary Networks and Internet 1972-80
(Early Internet Features)
Cerf and Kahn’s TCP (quite different from now)
It combined reliable in-sequence delivery of data by endsystem retransmission (as now) with forwarding (as IP now)
Realization of usefulness of separation of unreliable, nonflow controlled end-to end transport service for applications
such as packetized voice led to separation of IP.
Three internet protocols TCP, IP and UDP - conceptually in
place by the end of 1970’s.
Main features of their InterNet- Minimalism, autonomy
(no internal changes required for interconnection), Best
effort delivery, stateless routers and decentalized control.
History of Computer Networks - 9
14
Proprietary Networks and Internet 1972-80
(Early Ethernet Features)
Abramson’s ALOHA protocol- a multiple-access protocol for
communication among geographically distributed users by a single
shared broadcast medium.
Metcalfe and Bogg’s EtherNet protocol for wire-based shared networks
was originally motivated by the need to connect multiple PCprinters
Lesson 1: History of Computer
Networks - 10
15
Proliferation of Networks 1980-90
100 nodes by late 70’s
New national networks (100,000 by the end of 80’s)
BITNET for email and FTP services among many North East
Universities
CSNET (computer Science Network) for researchers with no
access to APRPAnet.
NSF-net for access to NSF-sponsored super-computing
centers
Starting with a backbone of 56 kbps, NSF net was
running at 1.5 Mbps by the end of the decade.
History of Computer Networks - 11
16
Proliferation of Networks 1980-90
(Continued)
Simple Message Transfer Protocol (SMTP): E-Mail
1982
Deployment of TCP /IP replacing NCP (Jan. 1, 1983)
FTP- The File Transfer Protocol defined (1983).
Host-based TCP Congestion Control (Jacobson 1988).
Domain Name System (DNS)- mapping between
human readable Internet computer name and 32-bit
IP address.
Lesson 1: History of Computer
Networks - 12
17
Proliferation of Networks 1980-90
(The Minitel Project)
French Minitel project paralleling ARPAnet
Ambitious projest sponsored by the French Government
X.25 protocol suite using virtual circuits
By mid-90’s, it offered more than 20, 000 services- from
home banking to research database
Used by more than 20% of the population
Generated over $1 billion in revenue
Was in most French homes 10 years before Americans had
ever heard of the Internet.
Lesson 1: History of Computer
Networks - 14
18
Internet Explosion: The1990s
Early 90’s Arpanet decommissioned as Milnet and
Defense Data Net grew enough to carry all defenserelated traffic.
NSF lifted restrictions on commercial use of NSFnet
(1991). NSFnet began to serve as a backbone and was
later decommissioned it in 1995.
Web invented at CERN by Tim Berners-Lee (89-91)
Developed intial versions of HTML, HTTP, a web server and
a web browser - Based on the original work on Hypertext in
1940s by Bush (1945) and in 1960s by Ted Nelson
Marc Andreesen developed Mosaic- Popular GUI
browser.
History of Computer Networks -15
19
Internet Explosion: First half of1990s
Marc Andreesen and Jim Clark formed Mosaic
Communications in 1994(it later became Netscape).
By 1995, University students were able surf web.
Big and small companies started transacting on the web
and transact commerce over the web.
History of Computer Networks -16
20
Internet Explosion: Second half of1990s
Microsoft (MS) started making browsers (1996) and this
started the war with NetScape which MS won later.
E-mail evolved with address books, attachments, hot
links, multimedia support.
4 Killer applications
Web accessible email
Web browsing & internet commerce
instant messaging with contact lists pioneered by ICQ
peer-to-peer file sharing of MP3s , pioneered by Napster .
By late 90’s, 50 million computers with 100+ million
users on the web. 1 GBs Back bone link speeds achieved.
History of Computer Networks -17
21
Developments of Last Decade
Financial turmoil, many start-ups collapsed. Still many
companies like eBay, Yahoo, Amazon and Cisco emerged
as winners despite setbacks in their stock prices.
Advances in content distribution, internet telephony,
high speed LANs and fast routers
3 Important developments
High Speed Access Internet Access (Cable/DSL/Wireless
LANs)
Secure applications
P2P (Point-to-point Networking)
History of Computer Networks -18
22
Three Important Recent Developments
I- High Speed Internet Access
Increased penetration of broadband residential Internet
via Cable and DSL with applications such as highquality Video on Demand and high quality Video
Conferencing
Increased ubiquity of public Wi-Fi nets (with 11 Mbps
and higher speeds)
Internet access via mobile phones of 3rd Generation &
Beyond; proliferation of social networks
History of Computer Networks -19
23
Three Important Recent Developments
II- Security
Intrusion detection methods for early warning of denial
of service attacks through worms (e.g. Blaster worm)
that infect systems and clog networks.
Use of Firewalls to filter unwanted traffic before it
enters the network.
Use of IP-traceback to pinpoint the origin of attacks.
History of Computer Networks -20
24
Three Important Recent Developments
III- P2P Networking
P2P application exploits resources (memory, disk-space,
content and CPU cycles) in user’s computers.
It gives significant autonomy from central servers.
KaZaA is the most popular p2P-file sharing system.
Currently, this network has 4 million connected systems
and its traffic constitutes 20-50% of Internet traffic.
History of Computer Networks -21
25
Summary and Follow-up
In this lesson, we covered History of Computer Networks
organized into approximately 5 decades.
In passing, we found what all a computer networks can do.
This will help you to write the first chapter of your project
report i.e. to prepare a table of requirements for your own
network!
You got used to some terminology e.g. circuit switching,
packet switching, firewalls, etc. If any of those concepts are
not clear, you may search the web, discuss with me or wait on
till we take them up in a greater detail later.
Explore the concepts- Circuit/Virtual Circuit/Packet
switching on the web.
History of Computer Networks 22
26
Lesson 2:
Overview of Computer Networks
27
Preview of the Lesson 2
In this lesson, we try to answer the question- What is a
Computer Network?
We try to view computer networks from different
perspectives. In other words, we try to answer the question:
what are all the different types computer networks?
We will have an overview of different components of a
computer network (Internet).
We also study a little bit of how the interconnected
computers communicate with one another, that is, we will
have cursory glance at protocol stacks.
Overview of Computer Networks 1
28
Computer Networks- Definition &
Perspectives
Reference:
http://en.wikipedia.org/wiki/Computer_network
What is a Computer Network?
A system for communication among two or more computers.
What are all the different types computer networks?
Different ways of categorization of Computer networks are:
– Range or extent of the network
– Inter-nodal functional relationship
– Network Topology
– Specialized functions of the nodes
Overview of Computer Networks 2
29
Network Categorization based on the Range
I- Personal Area Network (PAN)
With a reach of a few meters, connects home/small office
devices/computers or higher level net/Internet (in the latter case
called an uplink)
could be wired (using Universal Serial Bus, shortly USB, or
Fire-wire) or wireless (using blue-tooth or IrDA, that is,
Infrared Data Association)
Blue Tooth PAN is also called Piconet
IEEE 802.15.1 adapts Physical and MAC layers from
Bluetooth 1.1
Zigbeee is a proprietary technology for low power radios based
on IEEE 802.15.4
Overview of Computer Networks 3
30
Network Categorization based on the Range
II - Local Area Network (LAN)
Range is less than 1000 m2
Could be used in home, small office or university.
Earlier popular LAN was proprietary - DataPoint’s ArcNet
IEEE later produced two LAN standards- Ether Net (IEEE
802.3) and Token Ring (IEEE 802.5)
LAN speeds could be 10/100 Mbps (Ether Net) and 4/16/100
mbps/1 Gbps (Token Ring)
Wireless LANs- IEEE 802.11 (Wi-Fi)- speeds up to 56 Mbps
Overview of Computer Networks 4
31
Network Categorization based on the Range
III - Metropolitan Area Network (MAN)
Spans a city or a big campus with range up to 200 km (125 miles)
Earlier technologies used for MANs were:
Fiber Distributed Data Interface (FDDI)
Switched Megabit Data Service (as defined by IEEE 802.6 MAN
standard) using either B-ISDN or Distributed Dual-Queue Dual Bus
(DQDB) with speeds 1.5/45 Mbs.
Asynchronous Transfer Mode (ATM)
Above technologies are being displaced by 1GB Ether Net based
Mans
MAN links between LANs and WANs are usually microwave/
infra-red/radio.
Overview of Computer Networks 5
32
Network Categorization based on the Range
IV - Wide Area Network (WAN)
Covers wide geographical areas spanning multiple cities.
Works on leased lines and connects multiple LANs
Uses protocols such as TCP/IP, x.25, Frame Relay and ATM
Usually used to connect different sites of an organization or
service provider. For this reason, it is being replaced by Virtual
Private Networks (VPNs).
VPNs are of two types- i) Secure (they use leased lines and use
protocols like IPSEC ii) Trusted (They rely on security of single
provider’s network and use protocols such as Multi-protocol label
switching (MPLS) and Layer 2 Tunneling Protocol (L2TP)
Overview of Computer Networks 6
33
Network Categorization based on the
Functional Relationship of the Nodes
Client- Server Network
Multi-tier architecture (GUI, business logic and DB
could be in 3 separate tiers)
Peer-to-Peer Network (each node acts as both a
client and server, e.g. in case of e-mail).
Overview of Computer Networks 7
34
Network Categorization based on the
Network Topology
Bus Network
Star Network
Ring Network
Grid Network
Toroidal Networks and Hypercubes
Tree and Hyper-tree Networks
Overview of Computer Networks 8
35
Network Categorization based on Specialized
Function
Storage Area Network (SAN)- used for connecting multiple storage
devices such as disk controllers and tape libraries to a server.
Server Farms (Network of servers maintained by an enterprise)
Process Control Network- transmits data between measurement
and control units.
Value Added Network (VAN)- a third party network put up to
add value (e.g. maintenance & admin) to an enterprise network
SOHO (small office home office) Network- use ethernet/Wi-Fi
Wireless Community Networks- meant for hobbyists and use
wireless LANs- outgrowths of amateur radio clubs.
Overview of Computer Networks 9
36
Nuts and Bolts view of Computer Network with
Internet- Network of Networks
Overview of Computer Networks 10
37
Network Building Blocks
Switch
- connects computing devices to host computers, allowing a
large number of devices to share a limited number of ports
Router
- a Protocol-dependent device that connects sub-networks
together
Bridge
- a device that interconnects local or remote networks
Gateway
- a device that can interconnect networks with different,
incompatible communications
Overview of Computer Networks 11
38
Network Building Blocks (Continued)
Network hosts, workstations, etc.
- they generally represent the source and sink (destination) of
data traffic (packets)
Multiplexer
- telecommunications device that funnels multiple signals onto a
single channel
Transceiver
- (short for transmitter-receiver), is a device that both transmits
and receives analog or digital signals.
Firewall
- a system or group of systems that enforces an access control
policy between an organization's network and the Internet for
purposes of security.
Overview of Computer Networks 12
39
“Nuts and bolts” view of the Internet
It is a loosely hierarchical network of
networks (some private intranets) with
millions of connected computing devices:
Hosts, end-systems (Network Edge)
router
server
workstation
mobile
local ISP
– pc’s workstations, servers
– PDA (Personal Digital Assistant)’s
phones, toasters
regional ISP
running network apps :
Communication links (Network Access)
– fiber, coaxial cable, copper, radio, satellite
company
network
Switches, routers, bridges, gateways
(Network Core)
Overview of Computer Networks 13
40
What’s a protocol?
Human protocols:
A way of communication
between humans
Network protocols:
Machines rather than humans
involved, but all Internet
communication activity is
governed by protocols
Dictated by local culture
Dictated by standards
Greeting, response, action
Protocols define format, order
taken
of messages sent and received
among network entities, and
Examples: “Hey, got time?,”
actions taken on message
“I have a dumb question,”
transmission and receipt
This is so and so..”
Example: TCP/IP, ISO
Overview of Computer Networks 14
41
Human and Network Protocol Examples
Hi
TCP connection
req.
Hi
TCP connection
reply.
Got the
time?
Get http://www.ee.unt.edu/public/guturu
2:00
<file>
time
Overview of Computer Networks 15
42
Protocols
Building blocks of a network architecture
Each protocol object has two different interfaces
– service interface: defines operations on this protocol
– peer-to-peer interface: defines messages exchanged
with peer
Term “protocol” is overloaded
– specification of peer-to-peer interface
– module that implements this interface
Overview of Computer Networks 16
43
Why Protocol “Layers?”
Networks are complex;
they have many
heterogeneous “pieces”:
– Hosts, routers, links
of various media,
Application entities,
protocols, hardware,
software …
Question:
How to achieve effective
communication in this
mess?
Simple Answer:
Divide & Conquer
Overview of Computer Networks 17
44
Why layering?
Divide & Conquer Policy to handle Complex systems:
Explicit structure allows identification of complex system’s
pieces and their inter-relationships.
– Following slides present an example of a layered real-life
protocol.
Modularization eases maintenance and updating of system
– change of implementation of layer’s service transparent to
rest of system e.g., change in gate procedure doesn’t affect
rest of system
Cost: Layering may affect efficiency, but is inevitable.
Overview of Computer Networks 18
45
Steps in Organization of air travel
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
Overview of Computer Networks 19
46
Layered services in air travel
Counter-to-counter delivery of person+bags
baggage-claim-to-baggage-claim delivery
people transfer: loading gate to arrival gate
runway-to-runway delivery of plane
airplane routing from source to destination
Overview of Computer Networks 20
47
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
arriving airport
Departing airport
Distributed implementation of layer functionality
intermediate air traffic sites
airplane routing
airplane routing
Layers: each layer implements
airplane routing
a service via its own intra-layer
actions relying on services
Overview of Computer Networks provided by layer below
21
48
Internet protocol stack
• Application: supporting network applications (e.g. ftp, smtp, http)
• Transport: host-host data transfer, defines quality and nature of data
delivery (e.g. tcp, udp)
application
•Network: addressing and routing of datagrams from
source to destination (e,g. Ip & other routing protocols)
transport
•Link: logical organization of data bits transmitted
on a particular medium; framing, addressing, error
network
correction/detection (check sum) e.g. ppp, ethernet
•Physical: bits “on the wire” Defines physical
link
Properties of various media e.g. Ether-Net cable size
•7-layer OSI protocol (of ISO) has session (reply and
physical
response packet pairing) and presentation layers (data
syntax, encryption) above transport and below
application layer.
Overview of Computer Networks 22
49
Layering: logical communication
Each layer:
• distributed
• “entities”
implement layer
functions at each
node
• entities perform
actions, exchange
messages with
peers
application
transport
network
link
physical
application
transport
network
link
physical
Overview of Computer Networks 23
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
50
Layering: logical communication (continued)
E.g.: transport
• Take data from app
• Add addressing,
reliability check info
to form “datagram”
• Send datagram to
peer
• Wait for peer to ack
receipt
• Analogy: post office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
Overview of Computer Networks 24
application
transport
network
link
physical
network
link
physical
data
application
transport
transport
network
link
physical
51
Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
Overview of Computer Networks 25
data
application
transport
network
link
physical
52
Protocol layering and data
Each layer takes data from above, adds header
information to create new data unit and passes
new data unit to layer below
source
M
Ht
M
Hn Ht M
Hl Hn Ht M
destination
application
transport
network
link
physical
application
transport
network
link
physical
Overview of Computer Networks 26
Ht
M
message
M
segment
Hn Ht M
Hl Hn Ht M
datagram
frame
53
Protocol Data Units
The combination of data from the next higher layer
and control information is referred to as PDU.
Control Information in the Transport Layer may include:
Destination Service Access Point (DSAP)
Sequence number
Error-detection code
Overview of Computer Networks 27
54
Service Access Point
A Service Access Point (SAP) is the location where a
layer (N-1) entity provides service for a layer (N)
entity.
SDU: Service Data Unit
ICI: Interface Control Information
IDU: Interface Data Unit
PDU: Protocol Data Unit
Overview of Computer Networks 28
55
Summary of the Lesson 2
In this lesson, we addressed the question- What is a
Computer Network?
We studied the classification of computer networks
from different perspectives i.e. had a taxonomic view.
We had a components view of the computer network.
We have also studied a little bit of how the
interconnected computers communicate with one
another, that is, we had cursory glance at protocol
layers/stacks.
Overview of Computer Networks 29
56
Lesson 3: Preview/Objectives
High level view of network application protocols
client server paradigm
service models
learn about protocols by examining popular application-level
protocols such as
dns
smtp
pop
ftp (Next Lesson)
http (Next Lesson)
Multimedia (Next Lesson)
Lesson 3: Application Layer - 1
57
Application layer – Some Jargon
Applications (e.g., email, file transfer, the
Web): communicating, distributed
application
transport
processes
network
data link
running in network hosts in “user physical
space”
exchange messages to implement app
Application-layer protocols
one “piece” of an app
define messages exchanged by apps
and actions taken
Depend on user services provided by
lower layer protocols
application
transport
network
data link
physical
Lesson 3: Application Layer - 2
application
transport
network
data link
physical
58
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.
Web-browser
E-mail: mail reader
streaming audio/video: media player
Lesson 3: Application Layer - 3
59
Client-server paradigm
Typical Application 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 e-mail
application
transport
network
data link
physical
Lesson 3: Application Layer - 4
request
reply
application
transport
network
data link
physical
60
Client-Server Communication
Client and Sever, as a matter of fact, any two applications on
different hosts, communicate using what is called an API:
application programming interface that
defines interface between application and transport layer e.g.
socket: the Internet API
two processes communicate by writing data into socket and reading data out
of socket
How does a process “identify” the other process with which it
wants to communicate?
IP address of host running other process
“Port number” - allows receiving host to determine to which local
process the message should be delivered
Lesson 3:Application Layer - 5
61
Services Provided by the Transport Layer to
Applications
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
Timing
• some apps (e.g., Internet telephony, interactive games) require
low delay to be “effective”
Lesson 3:Application Layer - 6
62
Transport service requirements of common
apps
Application
file transfer
e-mail
Web documents
real-time audio/video
stored audio/video
interactive games
financial apps
Data loss
Bandwidth
no loss
no loss
loss-tolerant
loss-tolerant
elastic
elastic
elastic
audio: 5Kb-1Mb
video:10Kb-5Mb
loss-tolerant same as above
loss-tolerant few Kbps up
no loss
elastic
Lesson 3:Application Layer - 7
Time Sensitive
no
no
no
yes, 100’s msec
yes, few secs
yes, 100’s msec
yes and no
63
Services provided by Internet transport protocols
TCP service:
UDP service:
• connection-oriented: setup required • unreliable data transfer
between sending and
between client, server
receiving process
• reliable transport between sending
• does not provide: connection
and receiving process
setup, reliability, flow
• flow control: sender won’t
control, congestion control,
overwhelm receiver
timing, or bandwidth
• congestion control: throttle sender
guarantee
when network overloaded
Q: why bother? Why is there a
• does not provide: timing, minimum
UDP?
bandwidth guarantees
Lesson 3:Application Layer - 8
64
Internet application protocols and
corresponding transport protocols
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)
NFS
proprietary
(e.g., Vocaltec)
Lesson 3:Application Layer - 9
Underlying
transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
TCP or UDP
typically UDP
65
DNS: Domain Name System
People: many identifiers:
SSN, Passport #
Name
Internet hosts, routers:
IP address (32 bit) - used for
addressing datagrams
“Name”, e.g., gaia.cs.umass.edu used by humans
Lesson 3: Application Layer - 10
66
DNS: Domain Name System
Application providing Mapping between IP addresses and
domain name
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”
Lesson 3: Application Layer - 11
67
DNS name servers
Two types Name serversWhy not centralize DNS?
Local name servers:
single point of failure
each ISP, company has local
traffic volume
(default) name server
distant centralized database
host DNS query first goes to
Maintenance
local name server
doesn’t scale!
Authoritative name server:
Hence, the distributed organization
where server has all name-to-IP
address mappings.
for a host: stores that host’s IP
address, name
can perform name/address
translation for that host’s name
Lesson 3: Application Layer - 12
68
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
~ dozen root name servers
worldwide
Lesson 3: Application Layer - 13
69
Simple DNS Scenario
Host surf.eurecom.fr wants IP
address of
gaia.cs.umass.edu
1. Contacts its local DNS
server, dns.eurecom.fr
2. dns.eurecom.fr contacts root
name server, if necessary
3. root name server contacts
authoritative name server,
dns.umass.edu, if necessary
4, 5 & 6 are responses in
reverse order.
root name server
2
4
5
local name server
dns.eurecom.fr
1
3
authorititive name server
dns.umass.edu
6
requesting host
gaia.cs.umass.edu
surf.eurecom.fr
Lesson 3: Application Layer - 14
70
A More Complex DNS Scenario
root name server
Root name server:
may not know
authoratiative name
server, but
may know intermediate
name server: who to
contact to find
authoritative name
server
6
2
7
local name server
dns.eurecom.fr
1
8
requesting host
3
intermediate name server
dns.umass.edu
4
5
authoritative name server
dns.cs.umass.edu
surf.eurecom.fr
gaia.cs.umass.edu
Lesson 3: Application Layer - 15
71
DNS: iterated queries
root name server
recursive query:
puts burden of name
resolution on contacted
name server
heavy load?
iterated query:
2
3
4
7
local name server
contacted server replies
with name of server to
contact
“I don’t know this name,
but ask this server”
iterated query
dns.eurecom.fr
1
8
intermediate name server
dns.umass.edu
5
6
authoritative name server
dns.cs.umass.edu
requesting host
surf.eurecom.fr
Lesson 3: Application Layer - 16
gaia.cs.umass.edu
72
DNS: caching and updating records
once (any) name server learns mapping, it caches
mapping
– cache entries timeout (disappear) after some time
update/notify mechanisms under design by IETF
RFC 2136
http://www.ietf.org/html.charters/dnsind-charter.html
Lesson 3: Application Layer - 17
73
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type,ttl)
Type=A
Type=CNAME
name is hostname
value is IP address
Type=NS
name is domain (e.g. foo.com)
value is IP address of
authoritative name server for
this domain
name is an alias name for
some “cannonical” (the
real) name
value is cannonical name
Type=MX
value is hostname of mail server
associated with name
Lesson 3: Application Layer - 18
74
DNS protocol & messages
DNS protocol : query and repy messages, both with same
message format
msg header
• identification: 16 bit # for
query, repy to query uses same
#
• flags:
– query or reply
– recursion desired
– recursion available
– reply is authoritative
Lesson 3: Application Layer - 19
75
DNS protocol & messages (Continued)
Name, type fields
for a query
RRs in reponse
to query
records for
authoritative servers
additional “helpful”
info that may be used
Lesson 3: Application Layer - 20
76
Electronic Mail
Three major components:
user agents
mail servers
simple mail transfer protocol: smtp
outgoing
message queue
user mailbox
user
agent
mail
server
User Agent
SMTP
a.k.a. “mail reader”
composing, editing, reading mail
mail
messages
server
e.g., Eudora, Outlook, elm, Netscape
Messenger
user
outgoing, incoming messages stored
agent
on server
Lesson 3: Application Layer - 21
user
agent
SMTP
SMTP
mail
server
user
agent
user
agent
user
agent
77
Electronic Mail: mail servers
user
agent
Mail Servers
mailbox contains incoming
messages (yet to be read) for user
message queue of outgoing (to be
sent) mail messages
smtp protocol between mail
servers to send email messages
client: sending mail server
“server”: receiving mail server
mail
server
user
agent
SMTP
SMTP
SMTP
mail
server
mail
server
user
agent
user
agent
user
agent
user
agent
Lesson 3: Application Layer - 22
78
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
Lesson 3: Application Layer - 23
79
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)
Lesson 3: Application Layer - 24
80
Sample smtp interaction
S:
C:
S:
C:
S:
C:
S:
C:
S:
itself
C:
C:
C:
S:
C:
S:
220 hamburger.edu
HELO crepes.fr
250 Hello crepes.fr, pleased to meet you
MAIL FROM: <[email protected]>
250 [email protected]... Sender ok
RCPT TO: <[email protected]>
250 [email protected] ... Recipient ok
DATA
354 Enter mail, end with "." on a line by
Do you like ketchup?
How about pickles?
.
250 Message accepted for delivery
QUIT
221 hamburger.edu closing connection
Lesson 3: Application Layer - 25
81
smtp: Some Observations
• smtp uses persistent connections
• smtp requires that message
(header & body) be in 7-bit ascii
• certain character strings are not
permitted in message (e.g.,
CRLF.CRLF). Thus message
has to be encoded (usually into
either base-64 or quoted
printable)
• smtp server uses CRLF.CRLF
to determine end of message
Comparison with http
• http: pull
• email: push
• both have ASCII
command/response interaction,
status codes
• http: each object is encapsulated
in its own response message
• smtp: multiple objects message
sent in a multipart message
Lesson 3: Application Layer - 26
82
Mail message format
smtp: protocol for exchanging email
msgs
RFC 822: standard for text
message format:
• header lines, e.g.,
– To:
– From:
– Subject:
different from smtp commands!
header
blank
line
body
• body
– the “message”, ASCII characters
only
Lesson 3: Application Layer - 27
83
Message format: multimedia extensions
• MIME (Multipurpose Internet Mail extension): Contains
multimedia mail extensions, RFC 2045, 2056
• additional lines in msg header declare MIME content type
MIME version
method used
to encode data
multimedia data
type, subtype,
parameter declaration
encoded data
From: [email protected]
To: [email protected]
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
.........................
......base64 encoded data
Lesson 3: Application Layer - 28
84
MIME types
Content-Type: type/subtype; parameters
Text
Video
example subtypes: plain, html
example subtypes: mpeg,
quicktime
Image
example subtypes: jpeg, gif
Audio
exampe subtypes: basic (8-bit
mu-law encoded), 32kadpcm (32
kbps coding)
Application
other data that must be
processed by reader before
“viewable”
example subtypes: msword,
octet-stream
Lesson 3: Application Layer - 29
85
Multipart Type
From: [email protected]
To: [email protected]
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=98766789
--98766789
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain
Dear Bob,
Please find a picture of a crepe.
--98766789
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
.........................
......base64 encoded data
--98766789--
Lesson 3: Application Layer - 30
86
Mail access protocols
user
agent
SMTP
POP3 or
IMAP
SMTP
sender’s mail
server
user
agent
receiver’s mail
server
• SMTP: delivery/storage to receiver’s server
• Mail access protocol: retrieval from server
– POP3: Post Office Protocol version 3 [RFC 1939]
• authorization (agent <-->server) and download
– IMAP: Internet Mail Access Protocol [RFC 2060]
• more features (more complex)
• manipulation of stored msgs on server
– Webmail/HTTP: Hotmail , Yahoo! Mail, etc.
Lesson 3: Application Layer - 31
87
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
S:
C:
S:
C:
S:
C:
S:
S:
S:
C:
S:
S:
C:
C:
S:
S:
C:
C:
S:
+OK POP3 server ready
user alice
+OK
pass hungry
+OK user successfully logged
list
1 498
2 912
.
retr 1
<message 1 contents>
.
dele 1
retr 2
<message 1 contents>
.
dele 2
quit
+OK POP3 server signing off
Lesson 3: Application Layer - 32
88
on
How POP3 Works?
Note : DNS name or IP address of ISP server is typically configured when email is set up.
Lesson 3: Application Layer - 33
89
POP3 versus IMAP
POP3 is widely used because of simplicity and
robustness.
Both allow downloads from different places, but
POP3 assumes user will clear out all messages from
server on every contact and works offline after that.
This makes email spread on different machines.
IMAP (Internet Message Access Protocol) assumes
messages remain indefinitely on the server.
IMAP provides facilities to manipulate messages/
mailboxes on the server
Lesson 3: Application Layer - 34
90
Lesson 3: Summary and Follow-up
We had a High level view of network application protocols using
client server paradigm
service models
We learned about three of the most common application-level protocols
dns
smtp
pop
In the next class, we deal with three very popular application protocols
ftp
http
Multimedia
Lesson 3: Application Layer - 35
91
Lesson 4: More Application
Layer Protocols
92
Lesson 4: Preview/Objectives
Learn about the following popular application-level
protocols
ftp
http
Multimedia
Lesson 4: More Application Layer
Protocols - 1
93
ftp: The file transfer protocol
FTP
user
interface
user
at host
FTP
client
file transfer
local file
system
FTP
server
remote file
system
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
Lesson 4: More Application Layer
Protocols - 2
94
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
TCP control connection
port 21
FTP
client
Lesson 4: More Application Layer
Protocols - 3
TCP data connection
port 20
FTP
server
95
ftp commands, responses
Sample commands:
Sample return codes
• sent as ASCII text over control
channel
• USER username
• PASS password
• dir/ls return list of files in
current directory
• 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
• Put filename retrieves (gets) file
• Get filename stores (puts) a local
file on remote host
Lesson 4: More Application Layer
Protocols - 4
96
The Web: some jargon
Web page
consists of “objects”
addressed by a URL
Most Web pages consist of:
base HTML page, and
several referenced objects.
URL has three components:
protocol, host name and path
name:
http://www.someSchool.edu/someDept/pic.gif
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
Lesson 4: More Application Layer
Protocols - 5
97
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
DNS Server
PC running
Explorer
Server
running
NCSA Web
server
The Internet
Mac running
Navigator
Lesson 4: More Application Layer
Protocols - 6
98
Navigation through The Web
Multiple servers may come into
play
The same client/server model
PC running
client: browser that requests, Explorer
receives, “displays” Web
objects
server: Web server sends
objects in response to
requests
Browser determines URL and
XYZ.com Web
asks DNS for IP address
server
Browser makes TCP connection
on port 80
Lesson 4: More Application Layer
Protocols - 7
DNS Server
abc.com Web
server
The Internet
99
More about the http protocol
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
aside
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
Lesson 4: More Application Layer
Protocols - 8
100
Further Details for the http example
Suppose user enters URL
www.someSchool.edu/someDepartment/home.index
1a. http client initiates TCP connection to
http 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
time
(contains text,
references to 10
jpeg images)
1b. http server at host
www.someSchool.edu waiting for
TCP connection at port 80.
“accepts” connection, notifying
client
3. http server receives request message,
forms response message containing
requested object
(someDepartment/home.index),
sends message into socket
Lesson 4: More Application Layer
Protocols - 9
101
http example (cont.)
4. http server closes TCP connection.
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
time
objects
Lesson 4: More Application Layer
Protocols - 10
102
Non-persistent and persistent connections
Non-persistent
HTTP/1.0
server parses request,
responds, and closes TCP
connection
2 Request-response
messages to fetch each
object
Each object transfer
suffers from slow start
But most 1.0 browsers use
parallel TCP connections.
Persistent
default for HTTP/1.1
on same TCP connection: server
parses request, responds, parses
new request,..
Client sends requests for all
referenced objects as soon as it
receives base HTML.
Fewer Request-response
messages and less slow start.
Lesson 4: More Application Layer
Protocols - 11
103
http message format: request
• two types of http messages: request, response
• http request message:
– ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
Carriage return,
line feed
indicates end
of message
GET /somedir/page.html HTTP/1.0
User-agent: Mozilla/4.0
Accept: text/html, image/gif,image/jpeg
header Accept-language:fr
lines
(extra carriage return, line feed)
Lesson 4: More Application Layer
Protocols - 12
104
http request message: general format
Lesson 4: More Application Layer
Protocols - 13
105
http Request Example
Lesson 4: More Application Layer
Protocols – 13.1
106
http message format: response
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
html file
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 ...
Lesson 4: More Application Layer
Protocols - 14
107
http Response Example
Lesson 4: More Application Layer
Protocols – 14.1
108
http response status codes
In first line in server->client response message.
A few sample 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
Lesson 4: More Application Layer
Protocols - 15
109
Trying out http (client side) for yourself
1. Telnet to your favorite Web server:
Opens TCP connection to port 80
telnet www.eurecom.fr 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:
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
3. Look at response message sent by http server!
Lesson 4: More Application Layer
Protocols - 16
110
User-server interaction: authentication
Authentication goal: control access to
server documents
client
stateless: client must present
authorization in each request
authorization: typically name,
password
authorization: header line in
request
if no authorization presented,
server refuses access, sends
server
usual http request msg
401: authorization req.
WWW authenticate:
usual http request msg
+ Authorization:line
usual http response msg
WWW authenticate:
usual http request msg
+ Authorization:line
header line in response
usual http response msg
Browser caches name & password so
that user does not have to repeatedly enter it.
Lesson 4: More Application Layer
Protocols - 17
time
111
User-server interaction: cookies
server sends “cookie” to
client in response must
Set-cookie: 1678453
client presents cookie in
later requests
cookie: 1678453
server matches presentedcookie with server-stored
info
authentication
remembering user
preferences, previous
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
Lesson 4: More Application Layer
Protocols - 18
cookiespectific
action
cookiespectific
action
112
User-server interaction: conditional GET
client
• Goal: don’t send object if client
http request msg
has up-to-date stored (cached)
If-modified-since:
<date>
version
http response
• client: specify date of cached
HTTP/1.0
304 Not Modified
copy in http request
server
object
not
modified
If-modified-since: <date>
• server: response contains no
object if cached copy up-todate:
HTTP/1.0 304 Not
Modified
http request msg
If-modified-since:
<date>
http response
object
modified
HTTP/1.1 200 OK
…
<data>
Lesson 4: More Application Layer
Protocols - 19
113
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
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
origin
server
Proxy
server
client
client
Lesson 4: More Application Layer
Protocols - 20
origin
server
114
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
origin
servers
public
Internet
1.5 Mbps
access link
institutional
network
– link out of
institutional/local ISP
network often bottleneck
Lesson 4: More Application Layer
Protocols - 21
10 Mbps LAN
institutional
cache
115
Streaming Audio (Music on Demand)
Some cases web-sever provides link to audio server. Media player gets the file using Realtime Streaming Protocol (RTSP).
Lesson 4: More Application Layer
Protocols - 22
116
Media Player
Functions
1. User Interface Management 2. Transmission error handling 3. Decompression of music 4.
Elimination of jitter.
Lesson 4: More Application Layer
Protocols - 23
117
Media Player Function: Elimination of Jitter
Concept of push and pull media servers
Lesson 4: More Application Layer
Protocols -24
118
Internet Radio
Lesson 4: More Application Layer
Protocols - 25
119
Internet Telephony
The ITU
Lesson 4: More Application Layer
Protocols - 26
120
H.323 Protocol Stack
RTP- Real-time Transport Protocol, RTCP- Real-time Transport Control Protocol, RASRegistration/Admission/Status. H.245 channel is used to negotiate call parameters such
as support for video or conference calls, Codecs supported, and so on.
G.711,
G.723.1,
etc.
Used for
Congestion
control
Allows terminals
join and leave
zones , request and
return bandwidths
and provide status
updates.
Lesson 4: More Application Layer
Protocols - 27
121
Call Flow in H.323
Lesson 4: More Application Layer
Protocols - 28
122
Session Initiation Protocol (SIP)
•A light-weight protocol designed to inter-work with existing
internet applications. You can click and initiate telephone call
•A text based protocol modeled on HTTP.
•Interoperability could be a problem in the future.
Lesson 4: More Application Layer
Protocols -29
123
Video- Still and Moving Images
MPEG-1 output consists of 4 kinds of frames;
• I (Intra-coded) frames: Self-contained JPEG-encoded still pictures
•P (Predictive) frames: Block-by-block difference with last frame
•B (Bidirectional) frames: Differences between last and next frames
•D (DC-coded): Block averages used for last forward.
Lesson 4: More Application Layer
Protocols - 30
124
Video on Demand
Here MPEG-2 is more applicable. It is similar to MPEG-1,
but uses 10x10 blocks on place of 8x8. It also supports both
progressive and interlaced images.
Lesson 4: More Application Layer
Protocols - 31
125
Video-servers
Zipf’s Law: Most popular movie is seven times as
popular as the 7th popular movie. kth popular
movie will have C/k of total requests where C= ?
Lesson 4: More Application Layer
Protocols - 32
RAM
Magnetic Disk
DVD
Tape
126
Lesson 4: Summary and Follow-up
Revisiting the client-server paradigm, we dealt with three
very popular application protocols
ftp
http
Multimedia
Audio-servers
H.323
SIP
Video-on-Demand
Next we will take up how to program applications using
transport layer services (i.e. TCP/UDP sockets)
Lesson 4: More Application Layer
Protocols -33
127
Lesson 5: Writing Applications
using Transport Layer Facilities
128
Lesson 5: Preview/Objectives
Learn about the usage of the following transport layer
facilities for writing client-server applications
UDP sockets
TCP sockets
Learn the difference between connection-oriented and
connectionless transport layer services.
Lesson 5: Writing Applications
using Transport Layer Facilities-1
129
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
socket
a local-host created/owned
application,
OS-controlled interface (a “door”)
into which
application process can both send
and
receive messages to/from another
(remote or
local) application process
Lesson 5: Writing Applications
using Transport Layer Facilities-2
130
Socket-programming using TCP
Socket: a door between application process and end-endtransport protocol (UDP or TCP)
TCP service: reliable transfer of bytes from one process to
another
controlled by
application
developer
controlled by
operating
system
process
process
socket
TCP with
buffers,
variables
socket
TCP with
buffers,
variables
internet
controlled by
application
developer
controlled by
operating
system
host or
server
host or
server
Lesson 5: Writing Applications
using Transport Layer Facilities-3
131
Socket programming with TCP
Client must contact server
• server process must first be
running
• server must have created
socket (door) that welcomes
client’s contact
Client contacts server by:
• creating client-local TCP
socket
• specifying IP address, port
number of server process
• When client creates socket: client
TCP establishes connection to
server TCP
• When contacted by client, server
TCP creates new socket for server
process to communicate with client
– allows server to talk with
multiple clients
application viewpoint
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
Lesson 5: Writing Applications
using Transport Layer Facilities-4
132
Socket programming with TCP
Example client-server app:
• client reads line from standard
input (inFromUser stream) ,
sends to server via socket
(outToServer stream)
• server reads line from socket
• server converts line to uppercase,
sends back to client
• client reads, prints modified line
from socket (inFromServer
stream)
Input stream: sequence of bytes
into process
Output stream: sequence of
bytes out of process
Lesson 5: Writing Applications
using Transport Layer Facilities-5
client socket
133
Client/server socket interaction: TCP
Server (running on hostid)
Unix 4.1c BSD:
socket()
bind()
listen() accept()
Client
create socket,
port=x, for
incoming request:
Unix 4.1c BSD:
socket()
welcomeSocket =
ServerSocket()
TCP
connection setup
wait for incoming
connection request
Socket connectionSocket =
welcomeSocket.accept()
read request from
connectionSocket
connect()
create socket,
connect to hostid, port=x
clientSocket =
Socket()
send request using
clientSocket
InputStream Socket.getInputStream()
OutputStream Socket.getOutputStream()
write reply to
connectionSocket
read reply from
clientSocket
connectionSocket.close()
clientSocket.close()
Lesson 5: Writing Applications
using Transport Layer Facilities-6
134
Example: Java TCP client
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
Lesson 5: Writing Applications
using Transport Layer Facilities-7
135
Example: Java TCP client (cont.)
Create
input stream
attached to socket
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
Send line
to server
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
Read line
from server
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
}
}
Lesson 5: Writing Applications
using Transport Layer Facilities-8
136
Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
Create
welcoming socket
at port 6789
Wait, on welcoming
socket for contact
by client
Create input
stream, attached
to socket
public static void main(String argv[]) throws Exception
{
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));
Lesson 5: Writing Applications
using Transport Layer Facilities-9
137
Example: Java TCP server (cont.)
Create output
stream, attached
to socket
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
Read in line
from socket
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
Write out line
to socket
outToClient.writeBytes(capitalizedSentence);
}
}
}
End of while loop,
loop back and wait for
another client connection
Lesson 5: Writing Applications on
Transport Layer Facilities-10
138
Socket programming with UDP
UDP: no “connection” between
client and server
• no handshaking
• sender explicitly attaches IP
address and port of destination
• server must extract IP address,
port of sender from received
datagram
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
UDP: transmitted data may be
received out of order, or lost
Lesson 5: Writing Applications on
Transport Layer Facilities-11
139
Client/Server socket interaction: UDP
Server (running on hostid)
Unix 4.1c BSD:
socket()
bind()
receivefrom()
Client
create socket,
port=x, for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port umber
Lesson 5: Writing Applications on
Transport Layer Facilities-12
create socket,
clientSocket =
DatagramSocket()
Unix 4.1c BSD:
socket()
bind()
sendto()
Create, address (hostid, port=x,
send datagram request
using clientSocket
read reply from
clientSocket
close
clientSocket
140
Example: Java client (UDP)
import java.io.*;
import java.net.*;
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS
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();
Lesson 5: Writing Applications on
Transport Layer Facilities-13
141
Example: Java UDP client (cont.)
Create datagram with
data-to-send,
length, IP addr, port
Send datagram
to server
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
Read datagram
from server
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
Lesson 5: Writing Applications on
Transport Layer Facilities-14
142
Example: Java server (UDP)
import java.io.*;
import java.net.*;
Create
datagram socket
at port 9876
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)
{
Create space for
received datagram
Receive
datagram
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Lesson 5: Writing Applications
onTransport Layer Facilities-15
143
Example: Java UDP server (cont)
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
Get IP addr
port #, of
sender
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
Create datagram
to send to client
Write out
datagram
to socket
serverSocket.send(sendPacket);
}
}
}
End of while loop,
loop back and wait for
another datagram
Lesson 5: Writing Applications on
Transport Layer Facilities-16
144
Lesson 5: Summary and Follow-up
In this class,
Learned about the usage of the following transport layer
facilities for writing application
UDP sockets
TCP sockets
Learned the difference between connection-oriented and
connectionless transport layer services.
In the following classes, we study the transport layer itself.
In other words, we find the ways of implementing transport
layer functionalities.
Lesson 5: Writing Applications on
Transport Layer Facilities-17
145
Lesson 6: Transport Layer
146
Lesson 6: Preview and Objectives
Overview of transport layer services:
Multiplexing/de-multiplexing
Connectionless and unreliable data transport (UDP)
Connection-oriented and reliable data transport (TCP)
Study an Incremental Approach to the Design of Reliable
Data Transfer Mechanisms in order to:
Get an insight into how industrial products are usually evolved
starting with simpler user-models/assumptions and proceeding on
with more and more complex ones (big-bangs are rather rare!)
Get a perspective on the TCP ‘s reliable data transfer mechanisms
Transport Layer - 1
147
Transport services and protocols
Provide logical communication
between app’ processes running
on different hosts
Transport protocols run in end
systems
Transport versus network layer
services:
application
transport
network
data link
physical
network layer: data transfer
between end systems
transport layer: data transfer
between processes
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
relies on, enhances, network layer
services
Transport Layer - 2
148
Transport-layer Services
Internet transport services:
Unreliable (“best-effort”),
unordered unicast or
multicast delivery (UDP)
Reliable, in-order unicast
delivery (TCP)
application
transport
network
data link
physical
congestion control
flow control
connection setup
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
Services not available:
application
transport
network
data link
physical
real-time
bandwidth guarantees
reliable multicast
Transport Layer - 3
149
Multiplexing/demultiplexing
Segment - unit of data
exchanged between
transport layer entities
– aka TPDU: transport
protocol data unit
application-layer
data
segment
header
segment
M
M
H n segment
receiver
P3
M
P1
Ht
Demultiplexing: delivering
received segments to
correct app layer processes
M
application
transport
network
application
transport
network
Transport Layer -4
P4
M
P2
application
transport
network
150
Multiplexing/Demultiplexing
Multiplexing:
Gathering data from multiple
app processes, enveloping
data with header (later used
for demultiplexing)
multiplexing/demultiplexing:
Based on sender, receiver
port numbers, IP addresses
source, dest port #s in each
segment
recall: well-known port
numbers for specific
applications
Transport Layer -5
32 bits
source port #
dest port #
other header fields
application
data
(message)
TCP/UDP segment format
151
Multiplexing/Demultiplexing: examples
host A
source port: x
dest. port: 23
Web client
host C
server B
source port:23
dest. port: x
Source IP: C
Dest IP: B
source port: y
dest. port: 80
port use: simple telnet app
Web client
host A
Source IP: A
Dest IP: B
source port: x
dest. port: 80
Source IP: C
Dest IP: B
source port: x
dest. port: 80
Web
server B
port use: Web server
Transport Layer - 6
152
UDP: User Datagram Protocol [RFC 768]
“no frills,” “bare bones”
Internet transport protocol
“best effort” service, UDP
segments may be:
lost
delivered out of order to
app
connectionless:
no handshaking between
UDP sender, receiver
each UDP segment
handled independently of
others
Why is there a UDP?
no connection establishment
(which can add delay)
simple: no connection state at
sender, receiver
small segment header
no congestion control: UDP
can blast away as fast as
desired
Transport Layer - 7
153
More on UDP
Often used for streaming
multimedia apps
loss tolerant
rate sensitive
Other UDP uses
32 bits
Length, in
bytes of UDP
segment,
(why?): including
header
DNS
SNMP
Reliable transfer over UDP:
add reliability at application
layer
application-specific error
recovery!
Transport Layer - 8
source port #
dest port #
length
checksum
Application
data
(message)
UDP segment format
154
UDP checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
Sender:
Receiver:
Treat segment contents as
sequence of 16-bit integers
Checksum: addition (1’s
complement sum) of segment
contents
Sender puts checksum value
into UDP checksum field
Compute checksum of received
segment
Check if computed checksum
equals checksum field value:
NO - error detected
YES - no error detected. But
maybe errors nonetheless?
More later ….
Transport Layer - 9
155
Principles of Reliable data transfer
Important in app., transport, link layers
Top-10 list of important networking topics!
Characteristics of unreliable channel will determine complexity of
reliable data transfer protocol (RDT)
Transport Layer - 10
156
Reliable data transfer: getting started
rdt_send(): called from above,
(e.g., by app.). Passed data to
deliver to receiver upper layer
deliver_data(): called by
rdt to deliver data to upper
send
side
udt_send(): called by rdt,
to transfer packet over
unreliable channel to receiver
receive
side
rdt_rcv(): called when packet
arrives on rcv-side of channel
Transport Layer - 11
157
Reliable data transfer: getting started
We’ll:
incrementally develop sender, receiver sides of reliable data
transfer protocol (rdt)
consider only unidirectional data transfer
but control info will flow on both directions!
use finite state machines (FSM) to specify sender, receiver
event causing state transition
actions taken on state transition
state: when in this
“state” next state
uniquely determined
by next event
state
1
event
actions
Transport Layer - 12
state
2
158
Rdt1.0: reliable transfer over a reliable channel
underlying channel perfectly reliable
no bit errors
no loss of packets
separate FSMs for sender, receiver:
sender sends data into underlying channel
receiver read data from underlying channel
Transport Layer - 13
159
Rdt2.0: channel with bit errors
underlying channel may flip bits in packet
recall: UDP checksum to detect bit errors
the question: how to recover from errors:
acknowledgements (ACKs): receiver explicitly tells sender that
pkt received OK
negative acknowledgements (NAKs): receiver explicitly tells
sender that pkt had errors
sender retransmits pkt on receipt of NAK
human scenarios using ACKs, NAKs?
new mechanisms in rdt2.0 (beyond rdt1.0):
error detection
receiver feedback: control msgs (ACK,NAK) rcvr->sender
Transport Layer - 14
160
rdt2.0: FSM specification
sender FSM
receiver FSM
Transport Layer - 15
161
rdt2.0: in action (no errors)
sender FSM
receiver FSM
Transport Layer - 16
162
rdt2.0: in action (error scenario)
sender FSM
receiver FSM
Transport Layer - 17
163
rdt2.0 has a fatal flaw!
What happens if ACK/NAK
corrupted?
sender doesn’t know what
happened at receiver!
can’t just retransmit: possible
duplicate
What to do?
sender ACKs/NAKs receiver’s
ACK/NAK? What if sender
ACK/NAK lost?
retransmit, but this might cause
retransmission of correctly
received pkt!
Handling duplicates:
sender adds sequence number to
each pkt
sender retransmits current pkt if
ACK/NAK garbled
receiver discards (doesn’t deliver
up) duplicate pkt
stop and wait
Sender sends one packet,
then waits for receiver
response
Transport Layer - 18
164
rdt2.1: sender, handles garbled ACK/NAKs
Transport Layer - 19
165
rdt2.1: receiver, handles garbled ACK/NAKs
Transport Layer - 20
166
rdt2.1: discussion
Sender:
seq # added to pkt
two seq. #’s (0,1) will
suffice. Why?
must check if received
ACK/NAK corrupted
twice as many states
Receiver:
must check if received
packet is duplicate
state indicates whether 0 or
1 is expected pkt seq #
state must “remember”
whether “current” pkt has 0
or 1 seq. #
note: receiver can not know
if its last ACK/NAK
received OK at sender
Transport Layer - 21
167
rdt2.2: a NAK-free protocol
same functionality as
rdt2.1, using ACKs only
instead of NAK, receiver
sends ACK for the last
packet received OK
receiver must explicitly
include seq # of pkt being
ACKed
sender
FSM
!
duplicate ACK at sender
results in same action as
NAK: retransmit current
pkt
Transport Layer - 22
168
rdt3.0: channels with errors and loss
New assumption: underlying
channel can also lose
packets (data or ACKs)
checksum, seq. #, ACKs,
retransmissions will be of
help, but not enough
Q: how to deal with loss?
sender waits until certain
data or ACK lost, then
retransmits
yuck: drawbacks?
Approach: sender waits
“reasonable” amount of time
for ACK
retransmits if no ACK received
in this time
if pkt (or ACK) just delayed (not
lost):
retransmission will be
duplicate, but use of seq. #’s
already handles this
receiver must specify seq # of
pkt being ACKed
requires countdown timer
Transport Layer -23
169
rdt3.0 sender
Transport Layer - 24
170
rdt3.0 in action
Transport Layer - 25
171
rdt3.0 in action
Transport Layer - 26
172
Performance of rdt3.0
• rdt3.0 works, but performance stinks
• example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet:
T
transmit
Utilization = U =
=
8kb/pkt
10**9 b/sec
= 8 microsec
fraction of time
sender busy sending
=
8 microsec
30.016 msec
= 0.00015
– 1KB pkt every 30 msec -> 33kB/sec throughput over 1 Gbps link
– network protocol limits use of physical resources!
Transport Layer - 27
173
Lesson 6: Summary and Follow-up
We had an overview of transport layer services:
Multiplexing/de-multiplexing
Connectionless and unreliable data transport (UDP)
Connection-oriented and reliable data transport (TCP)
We studied an Incremental Approach to the Design of Reliable Data
Transfer Mechanisms (i.e. increasingly complex versions of RDT
protocol) in order to:
Get an insight into how industrial products are usually evolved starting with
simpler user-models/assumptions and proceeding on with more and more
complex ones (big-bangs are rather rare!)
Get a perspective on the TCP ‘s reliable data transfer mechanisms
Next class, we study TCP protocol with all the facilities it provides.
Transport Layer - 28
174
Lesson 7: TCP
175
Lesson 7- TCP: Preview/Objectives
TCP Segment (Message) Format
Study of Connection-oriented data transport (TCP) with
facilities for:
Connection Management
Reliable data transfer with one of the two usual methods:
Go back to N
Selective Repeat
Flow Control
Congestion Control
Lesson 7: TCP- 1
176
TCP Segment (Message) Structure
Lesson 7: TCP - 2
177
TCP Connection Management with 3way Handshake
Lesson 7: TCP - 3
178
TCP Connection Closing Sequence
Lesson 7: TCP - 4
179
TCP Connection Management- Client
Side State Transitions
Receive ACK/
Send Nothing
CLOSING
Receive FIN
& ACK/ Send
ACK
Receive FIN/
Send ACK
Sharp lines depict unusual states and transitions.
Lesson 7: TCP - 5
180
TCP Connection Management- Server
Side State Transitions
Sharp lines depict unusual states and transitions.
Send
SYN
Receive RST/
Send Nothing
SYN_SENT
Receive SYN/
Send SYN &ACK
(Simultaneous open)
Lesson 7: TCP - 6
181
States of The TCP Connection
Management FSM
Lesson 7: TCP - 7
182
Pipelined protocols
Pipelining: sender allows multiple, “in-flight”, yet-to-beacknowledged pkts
range of sequence numbers must be increased
buffering at sender and/or receiver
Two generic forms of pipelined protocols: go-Back-N,
selective repeat
Lesson 7: TCP - 8
183
Go-back-N ARQ
It is the most commonly used sliding window protocol!
Here, the sender may send a series of frames.
The number of unacknowledged frames is determined by the
window size
While no errors occur, the receiver will acknowledge the
receipt of frames with RR# (receiver ready).
A frame in error will be rejected with REJ# and discarded by
the receiver.
Upon receiving a REJ#, the sender must retransmit the
frame in error and all frames that were sent thereafter.
Lesson 7: TCP - 9
184
Go-Back-N
Sender:
k-bit seq # in pkt header
“window” of up to N, consecutive unack’ed pkts allowed
ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK”
may receive duplicate ACKs (see receiver)
timer for each in-flight pkt
timeout(n): retransmit pkt n and all higher seq # pkts in window
Lesson 7: TCP - 10
185
GBN: sender extended FSM
Lesson 7: TCP - 11
186
GBN: receiver extended FSM
receiver simple:
ACK-only: always send ACK for correctly-received pkt
with highest in-order seq #
may generate duplicate ACKs
need only remember expectedseqnum
out-of-order pkt:
discard (don’t buffer) -> no receiver buffering!
ACK pkt with highest in-order seq #
Lesson 7: TCP - 12
187
GBN in
action
Lesson 7: TCP - 13
188
Maximum Window Size
The sequence number dilemma
Each frame has a k-bit field to represent its
corresponding sequence number (0..2k-1)
What is the maximum window size we can allow for
Go-Back-N?
Answer: 2k-1
Why not 2k ?? DISCUSS !!
Lesson 7: TCP - 20
189
A Problem Similar To Circular-Q Problem
Example: Let’s say we use a 3-bit sequence number.
Consider the following sequence of events
Sender sends frame 0
Receiver sends Ack with expected seq.#1
Sender sends frames 1, 2, 3, 4, 5, 6, 7, 0
Receiver sends Ack with expected seq.#1
Sender receives Ack with seq.#1 and cannot decide
whether all frames have been received correctly or all are
lost in transit.
Lesson 7: TCP - 21
190
Selective Repeat
receiver individually acknowledges all correctly received
pkts
buffers pkts, as needed, for eventual in-order delivery to upper
layer
sender only resends pkts for which ACK not received
sender timer for each unACKed pkt
sender window
N consecutive seq #’s
again limits seq #s of sent, unACKed pkts
Lesson 7: TCP - 14
191
Selective repeat: sender, receiver windows
Lesson 7: TCP - 15
192
Selective repeat
receiver
sender
pkt n in [rcvbase, rcvbase+N-1]
data from above :
if next available seq # in
window, send pkt
timeout(n):
Send pkt n again, restart timer
ACK(n) in [sendbase,sendbase+N]:
mark pkt n as received
if n smallest unACKed pkt,
advance window base to next
unACKed seq #
send ACK(n)
out-of-order: buffer
in-order: deliver (also deliver
buffered, in-order pkts),
advance window to next notyet-received pkt
pkt n in [rcvbase-N,rcvbase-1]
ACK(n)
otherwise:
ignore
Lesson 7: TCP - 16
193
Selective repeat in action
Lesson 7: TCP - 17
194
Selective repeat:dilemma
Example:
seq #’s: 0, 1, 2, 3
window size=3
receiver sees no difference in two
scenarios!
incorrectly passes duplicate data as
new in (a)
Q: what relationship between seq # size
and window size?
Lesson 7: TCP - 18
195
Complementary Problem
Consider the following example:
Assume a 3-bit sequence number
Sender transmits segments 0-6 to the receiver
Receiver gets all the segments in good shape and
acknowledges with expected Seq.# 7.
Now, lightning strikes and all Acks are lost
Sender times out and retransmits segment 0
The receiver has advanced its window to accept segments
7, 0-5 and since frame 0 is one that is within that range,
it is accepted.
Lesson 7: TCP - 23
196
Actual Window Size
The problem shown in the example is that there is an overlap
between the sending and receiving windows.
Hence, the solution to the window-size problem is to limit
the maximum window size to half the range of the sequence
number range
That is, for a k-bit sequence number field: 2k-1,
Show that: (MaxSeqNum + 1)/2 = 2k-1.
Lesson 7: TCP - 24
197
Reliable Data Transfer Protocols- A
Comparative Study
Stop-and-Wait Protocol
Simple, but performance leaves much to be desired!
Go-Back-N
Better performance, but more complicated. Possibly
wasteful if large blocks of packets need to be
retransmitted
Selective Repeat
A pain to implement – needs multiple timers, but better
performance through individual packet management
Lesson 7: TCP - 19
198
Selective-Reject ARQ
In this ARQ mechanism the sender only retransmits
those frames for which a negative ACK (SREJ) has
been received or for that timed out.
The receiver does not discard frames which are
delivered out of order.
Question: What about the permissible window size?
Lesson 7: TCP - 22
199
Flow Control in TCP
RcvWindow = RcvBuffer – [LastByteRcvd – LastByteRead]
LastByteSent – LastByteAcked <= RcvWindow
Possible Blocking @ Sender -> TCP Solution?
Lesson 7: TCP - 25
200
Silly Window Syndrome
Sender is slow- Sends a byte at a time
Network bandwidth badly used
Nagle’s algorithm- Wait, bunch and send
Advisable to disable in interactive applications- cursor movement
may look erratic and make user unhappy
Receiver is slow- Takes a byte at a time for an interactive
application
Clarke’s solution- wait till a decent amount of space is available
and advertise the receiver window size,
Complementary to Nagle’s and both can work together
Lesson 7: TCP - 26
201
General Congestion Control Mechanisms
End to End Congestion Control
Network-assisted Congestion Control
Direct feedback from router with a choke packet
Router marks a field in packet. Upon receipt of the packet,
receiver sends a notification to the sender. (Full RTT required!)
Network-assisted Congestion not possible in TCP as there
is no support from IP.
Lesson 7: TCP - 27
202
Congestion Control in TCP
Three components
of TCP congestion
control algorithm
Additive Increase
Multiplicative
Decrease
Slow start
Reaction to
timeout events
Lesson 7: TCP - 28
203
Lesson 7- TCP: Summary & Follow up
We have studied TCP Segment (Message) Format and what
each field of the message is meant for.
Study of Connection-oriented data transport (TCP) with
facilities for:
Connection Management FSMs
Reliable data transfer with one of the two usual methods:
Go back to N
Selective Repeat
Flow Control with RcvWindow information
3 features of TCP Congestion Control Mechanism .
Next class, we proceed on to the Network Layer.
Lesson 7: TCP - 30
205
Lesson 8: Introduction to
Network Layer
206
Lesson 8- Introduction to Network Layer:
Preview/Objectives
Overview of network layer functions
Forwarding
Routing
Call setup (sometimes)
Network Models- Virtual Circuits versus Datagram
Networks
Routing Algorithms
Desirable Characteristics
Classification
Different known types
Overview of graph theory based algorithms
Lesson 8: Introduction to Network
Layer - 1
207
Network layer functions
Network layer protocols exist in every switch whether host (end system) or
router (intermediate switch).
application
Three important functions:
Switching- Moving packets (frames) that come
into a switch interface and forward them on the
interface that leads to the destination. Switching
implies forwarding- ability to determine the
interface to which a frame should be directed.
Switching has more of hardware connotation and
forwarding refers to software aspect.
transport
network
data link
physical
Routing: Determination of path or route taken by
packets from source to destination. There exist
many routing algorithms for doing this. As
against forwarding which refers to transfer of
packets from an incoming link to an outgoing
link, routing refers collective interaction via
routing protocols for path determination.
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
Call setup: some network architectures require
router call setup along path before data flows
Lesson 8: Introduction to Network
Layer - 2
208
Network service model
Q: What service model for
“channel” transporting
packets from sender to
receiver?
guaranteed bandwidth?
preservation of inter-packet
timing (no jitter)?
loss-free delivery?
in-order delivery?
congestion feedback to sender?
Lesson 8: Introduction to Network
Layer - 3
The most important
abstraction provided
by network layer:
? ?
?
virtual circuit
or
datagram?
209
Virtual circuits
“source-to-dest path behaves much like telephone circuit”
performance-wise
network actions along source-to-destination path
call setup for each call before data can flow and teardown
each packet carries VC identifier (not destination host OD)
every router on source-dest path s maintain “state” for each passing
connection
transport-layer connection only involved two end systems
link, router resources (bandwidth, buffers) may be allocated to VC
to get circuit-like performance.
Lesson 8: Introduction to Network
Layer - 4
210
Virtual circuits: signaling protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.25
not used in today’s Internet
application
transport
network
data link
physical
5. Data flow begins
4. Call connected
1. Initiate call
Lesson 8: Introduction to Network
Layer - 5
6. Receive data
3. Accept call
2. incoming call
application
transport
network
data link
physical
211
Datagram networks: the Internet model
no call setup at network layer
routers: no state about end-to-end connections
no network-level concept of “connection”
packets typically routed using destination host ID
packets between same source-dest pair may take different paths
application
transport
network
data link
physical
1. Send data
2. Receive data
Lesson 8: Introduction to Network
Layer - 6
application
transport
network
data link
physical
212
Network layer service models:
Network
Architecture
Guarantees ?
Service
Model
Bandwidth
Loss
Order
Timing
best effort
none
no
no
no
ATM
CBR
yes
yes
yes
ATM
VBR
yes
yes
yes
ATM
ABR
no
yes
no
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
Internet
Congestion
feedback
Internet model being extented: Intserv, Diffserv
Lesson 8: Introduction to Network
Layer - 7
213
Datagram or VC network: why?
Internet
ATM
data exchange among computers
“elastic” service, no strict
timing req.
“smart” end systems (computers)
can adapt, perform control,
error recovery
simple inside network,
complexity at “edge”
many link types
different characteristics
uniform service difficult
evolved from telephony
human conversation:
strict timing, reliability
requirements
need for guaranteed service
“dumb” end systems
telephones
complexity inside network
Lesson 8: Introduction to Network
Layer - 8
214
Routing
The primary function of a packet network is to accept
packets from a source and deliver them to a destination node.
The process of forwarding the packets through the network is
referred to a routing (routing has more of a global concept as
against forwarding).
Routing mechanisms have a set of requirements:
correctness
simplicity
robustness
stability
fairness
Lesson 8: Introduction to Network
Layer - 9
215
Routing (Continued)
Most important:
optimality
efficiency
Routing directly impacts the performance of the network!
WHY?
In order to route packets on optimal routes through the
network to their destinations, we must first decide what is to
be optimized:
delay
cost
throughput
Lesson 8: Introduction to Network
Layer - 10
216
Routing Information
Routing decisions are generally based on some
knowledge of the state of the network.
Delay on certain links
Cost through certain nodes
Packet loss
etc.
This information may have to be dynamically
collected. This leads to overhead which in turn
reduces the utilization.
Lesson 8: Introduction to Network
Layer - 11
217
Routing Algorithms
Routing Algorithm
Goal: determine “good” path
(sequence of routers) thru
network from source to dest.
5
B
link cost: delay, $ cost, or
congestion level
C
2
A
Graph abstraction for
routing algorithms:
graph nodes are routers
graph edges are physical
links
3
2
1
D
3
1
5
F
1
E
2
“good” path:
typically means minimum
cost path
other definitions possible
Lesson 8: Introduction to Network
Layer - 12
218
Routing Algorithm classification
Global or decentralized
information?
Static or dynamic?
Static:
routes change slowly over time
Dynamic:
routes change more quickly
Proactive (periodic update)
Reactive (in response to
link cost changes)
Global:
all routers have complete topology,
link cost info
Example: “link state” algorithms
Decentralized:
router knows physically-connected
neighbors, link costs to neighbors
iterative process of computation,
exchange of info with neighbors
Example: “distance vector”
algorithms
Lesson 8: Introduction to Network
Layer - 13
219
Different Types of Routing
Fixed Routing:
Static Routing Tables, Pre-computed Routes
Flooding:
Simple but inefficient! WHY?
Hot Potato Routing
Simple, not very efficient, unpredictable
Random Routing
Simple, unpredictable, statistically fair (locally)
Adaptive Routing
sophisticated, expensive, efficient, complex...
Lesson 8: Introduction to Network
Layer - 14
220
Random Routing
Sometimes called probabilistic routing!
Here, the probability of a packet being forwarded on
a particular link is a function of conditions on this
link.
Pi 
R
R
i
j
j
– Pi = Probability of link i being selected
– Ri = Data rate on link i
Lesson 8: Introduction to Network
Layer - 15
221
Random Routing (Continued)
Note: Random Routing is probabilistic, i.e., the link
with the largest capacity may not be the one chosen
for every transmission.
We can formulate a static and dynamic (adaptive)
version of the routing algorithm.
Can you think of other measurements (metrics) to
compute Pi ?
Lesson 8: Introduction to Network
Layer - 16
222
Adaptive Routing
Adaptive Routing Techniques are used in almost all
packet-switching networks.
ARPANET
Routing decisions change in response to changes in the
network.
Network Failure
Congestion
Adaptive routing strategies can improve performance.
Adaptive routing strategies can aid congestion control.
Lesson 8: Introduction to Network
Layer - 17
223
Shortest Path Routing Algorithms
Shortest-path routing mechanisms are based on graph theoretic
concepts.
The challenge is to reformulate centralized forms of these
algorithms to work in a distributed setting, such as a
communication network.
The information upon routing decisions are based may come
from
local measurements
adjacent nodes
all nodes in the network
Lesson 8: Introduction to Network
Layer - 18
224
Graph-Theoretic Formulation
Problem:
Find a least cost path between any two nodes of a graph.
Network viewed as a graph:
Vertices (switches)
Edges (links)
Cost on each edge
(congestion, actual
cost, delay, etc.)
A
3
9
B
2
1
F
E
6
4
C
1
Lesson 8: Introduction to Network
Layer - 19
D
225
Some of the established shortest-path algorithms in
traditional graph theory are:
Dijkstra’s shortest path algorithm
Bellman-Ford Algorithm
Floyd-Warshall Algorithm
The main difference between the algorithms is the type of
augmentation through each iteration.
Dijkstra: nodes
Bellman-Ford: number of arcs (links) in the path
Floyd-Warshall: set of nodes in the path (all s-d pairs)
These algorithms have been formulated in a centralized
manner and must be mapped into a distributed
environment.
Lesson 8: Introduction to Network
Layer - 20
226
Lesson 8- Introduction to Network Layer:
Summary and Follow-up
We had an overview of network layer functions
Forwarding
Routing
Call setup (sometimes)
In passing studied the subtle differences between switching, forwarding
and routing.
We made a comparative study of Network Models- Virtual Circuits versus
Datagram Networks
We looked into the following aspects of Routing Algorithms
Desirable Characteristics
Classification
Different known types
Overview of graph theory based algorithms
In the next class, we study in detail some of the shortest path routing
algorithms.
Lesson 8: Introduction to Network
Layer - 21
227
Lesson 9: Routing Algorithms
for Network Layer
228
Lesson 9: Routing Algorithms for Network
Layer- Preview/Objectives
We study two routing algorithms
Dikstra’s link State algorithm
Distance vector (Bellman Ford) algorithm
We work out examples
We discuss the count-to-infinity problem
Lesson 9: Routing Algorithms for
Network Layer - 1
229
A Link-State Routing Algorithm
Dijkstra’s algorithm
net topology, link costs known
to all nodes
accomplished via “link state
broadcast”
all nodes have same info
computes least cost paths from
one node (‘source”) to all other
nodes
gives routing table for that
node
iterative: after k iterations,
know least cost path to k dest.’s
Notation:
c(i,j): link cost from node i to j.
cost infinite if not direct neighbors
D(v): current value of cost of
path from source to destination V
p(v): predecessor node along path
from source to v, that is next v
N: set of nodes whose least cost
path definitively known
Lesson 9: Routing Algorithms for
Network Layer - 2
230
Dijsktra’s Algorithm
1 Initialization:
2 N = {A}
3 for all nodes v
4
if v adjacent to A
5
then D(v) = c(A,v)
6
else D(v) = infinity
7
8 Loop
9 find w not in N such that D(w) is a minimum
10 add w to N
11 update D(v) for all v adjacent to w and not in N:
12
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
14 shortest path cost to w plus cost from w to v */
15 until all nodes in N
Lesson 9: Routing Algorithms for
Network Layer - 3
231
Dijkstra’s Algorithm: An Example
Step
0
1
2
3
4
5
start N
A
AD
ADE
ADEB
ADEBC
ADEBCF
D(B),p(B)
2,A
2,A
2,A
D(D),p(D)
1,A
D(C),p(C)
5,A
4,D
3,E
3,E
D(E),p(E)
infinity
2,D
D(F),p(F)
infinity
infinity
4,E
4,E
4,E
5
B
3
C
2
A
2
1
D
3
1
5
F
1
E
2
Lesson 9: Routing Algorithms for
Network Layer - 4
232
A Discussion on Dijkstra’s algorithm
Algorithm complexity: n nodes
• each iteration: need to check all nodes, w, not in N (the set)
• n*(n+1)/2 comparisons: O(n**2)
• more efficient implementations possible: O(nlogn)
Oscillations possible:
• e.g., link cost = amount of carried traffic
D
A
1
0
0
0
C
1
1+e
B
e
initially
D
0
1
e
2+e
A
1+e 1
C
0
B
0
… recompute
routing
0
D
1
A
2+e
0 0
C 1+e
… recompute
Lesson 9: Routing Algorithms for
Network Layer - 5
B
2+e
D
0
A
1+e 1
C
0
B
e
… recompute
233
Bellman-Ford (Distance Vector)
The algorithm iterates on # of arcs in a path.
The original algorithm is a single destination shortest
path algorithm.
Let D(h)i be the shortest ( h) path length from node i to
node 1 (the destination).
By definition, D(h)1= 0 h.
Assumptions:
There exists at least one path from every node to the
destination
All cycles not containing the destination have nonnegative
length (cost).
Lesson 9: Routing Algorithms for
Network Layer - 6
234
Bellman Ford Algorithm- Preliminaries
• NOTE: Let SD(i,j) be the shortest distance from node
i to node j. In an undirected graph, we clearly have:
SD(i,j) = SD(j,i).
• This may not be true for a Digraph.
• Why is the assumption of cycles with nonnegative
cost important?
• Length (hops) is just one of many possible routing
metrics. Can you think of others?
Lesson 9: Routing Algorithms for
Network Layer - 7
235
Bellman-Ford Algorithm
• The Bellman-Ford Algorithm:
– Step 1: Set D(0)i =  i
– Step 2: For each h  0 compute D(h+1)i as
D(h+1)i = minj[D(h)j + dj,i] i  1
– where dj,i is the cost (length) of link lj,i
• We say that the algorithm has terminated when D(h)i
= D(h-1)i i
• In a network with N nodes, the algorithm terminates
after at most N iterations!
Lesson 9: Routing Algorithms for
Network Layer - 8
236
Distance Vector Routing Algorithm
Iterative:
• continues until no nodes
exchange info.
• self-terminating: no
“signal” to stop
Asynchronous:
• nodes need not exchange
info/iterate in lock step!
Distributed:
• each node communicates
only with directlyattached neighbors
Distance Table data structure
• each node has its own
• row for each possible destination
• column for each directly-attached
neighbor to node
• example: in node X, for destination Y
via neighbor Z:
X
D (Y,Z)
distance from X to
= Y, via Z as next hop
= c(X,Z) + min {DZ(Y,w)}
Lesson 9: Routing Algorithms for
Network Layer - 9
w
237
Distance Table: An Example
7
A
B
1
C
2
8
1
E
E
2
D ()
A
B
D
A
1
14
5
B
7
8
5
C
6
9
4
D
4
11
2
D
E
D (C,D) = c(E,D) + min {DD(C,w)}
w
= 2+2 = 4
E
D (A,D) = c(E,D) + min {DD(A,w)}
= 2+3 = 5
E
D (A,B)
w
loop!
= c(E,B) + min {D B(A,w)}
w
= 8+6 = 14
cost to destination via
loop!
Lesson 9: Routing Algorithms for
Network Layer - 10
238
Distance table gives routing table
E
cost to destination via
Outgoing link
D ()
A
B
D
A
1
14
5
A
A,1
B
7
8
5
B
D,5
C
6
9
4
C
D,4
D
4
11
2
D
D,2
Distance table
to use, cost
Routing table
Lesson 9: Routing Algorithms for
Network Layer - 11
239
Distance Vector Routing: An Overview
Iterative, asynchronous: each
local iteration caused by:
• local link cost change
• message from neighbor: its
least cost path change from
neighbor
Distributed:
• each node notifies neighbors
only when its least cost path
to any destination changes
Each node:
wait for (change in local link cost of
msg from neighbor)
recompute distance table
if least cost path to any dest has
changed, notify neighbors
– neighbors then notify their
neighbors if necessary
Lesson 9: Routing Algorithms for
Network Layer - 12
240
Distance Vector Algorithm
At all nodes, X:
1 Initialization:
2 for all adjacent nodes v:
3 DX(*,v) = infty
/* the * operator means "for all rows" */
X
4 D (v,v) = c(X,v)
5 for all destinations, y
6 send min
w
DX(y,w) to
each neighbor
/* w over all X's neighbors */
Lesson 9: Routing Algorithms for
Network Layer - 13
241
Distance Vector Algorithm (cont.)
8 loop
9 wait (until I see a link cost change to neighbor V
10
or until I receive update from neighbor V)
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
if (c(X,V) changes by d)
/* change cost to all dest's via neighbor v by d */
/* note: d could be positive or negative */
for all destinations y: DX(y,V) = D X(y,V) + d
else if (update received from V wrt destination Y)
/* shortest path from V to some Y has changed */
/* V has sent a new value for its min DV(Y,w) */
w
/* call this received new value is "newval" */
for the single destination y: DX(Y,V) = c(X,V) + newval
if we have a new min w D X(Y,w) for any destination Y
send new value of min D X(Y,w) to all neighbors
w
forever
Lesson 9: Routing Algorithms for
Network Layer - 14
242
Distance Vector Algorithm: An Example
X
2
Y
7
1
Z
Lesson 9: Routing Algorithms for
Network Layer - 15
243
Distance Vector Algorithm: example (contd.)
X
2
Y
7
1
Z
X
Z
X
Y
D (Y,Z) = c(X,Z) + minw{D (Y,w)}
= 7+1 = 8
D (Z,Y) = c(X,Y) + minw {D (Z,w)}
= 2+1 = 3
Lesson 9: Routing Algorithms for
Network Layer - 16
244
Distance Vector: link cost changes
Link cost changes:
• node detects local link cost change
• updates distance table (line 15)
• if cost change in least cost path, notify
neighbors (lines 23,24)
1
X
4
Y
50
1
Z
algorithm
terminates
“good
news
travels
fast”
Lesson 9: Routing Algorithms for
Network Layer - 17
245
Distance Vector: link cost changes
Link cost changes:
• good news travels fast
• bad news travels slow “count to infinity” problem!
60
X
4
Y
50
1
Z
algorithm
continues
on!
Lesson 9: Routing Algorithms for
Network Layer - 18
246
Distance Vector: poisoned reverse
If Z routes through Y to get to X :
• Z tells Y its (Z’s) distance to X is infinite
(so Y won’t route to X via Z)
• will this completely solve count to infinity
problem?
60
X
4
Y
50
1
Z
algorithm
terminates
Lesson 9: Routing Algorithms for
Network Layer - 19
247
Comparison of LS and DV algorithms
Message complexity
• LS: with n nodes, E links, O(nE)
msgs sent each
• DV: exchange between neighbors
only
– convergence time varies
Speed of Convergence
• LS: O(n**2) algorithm requires
O(nE) msgs
– may have oscillations
• DV: convergence time varies
– may be routing loops
– count-to-infinity problem
Robustness: what happens if
router malfunctions?
LS:
– node can advertise incorrect
link cost
– each node computes only its
own table
DV:
– DV node can advertise
incorrect path cost
– each node’s table used by
others
• error propagate thru network
Lesson 9: Routing Algorithms for
Network Layer - 20
248
Lesson 9: Routing Algorithms for Network
Layer- Summary and Follow-up
We studied two routing algorithms
Dikstra’s link State algorithm
Distance vector (Bellman-Ford) algorithm
• We work ed out examples
• We discussed the count-to-infinity problem
• Next class, we continue with more on Internet & IP
Lesson 9: Routing Algorithms for
Network Layer - 21
249
Lesson 10: IP & The Internet
250
Lesson 10: IP & The InternetPreview/Objectives
We see how the Internet- the network of networks works
Study the IP message and address structures
We study a number of Protocols & Algorithms
ICMP
ARP & RARP/BOOTP/DHCP
RIP /OSPF & BGP
We discuss how the count-to-infinity problem is addressed in the
BGP.
Lesson 10: IP & The Internet - 1
251
The Internet
Lesson 10: IP & The Internet - 2
252
How Internet Handles Traffic Flow through
Different Networks?
Lesson 10: IP & The Internet - 3
253
The IP Message Format
Header Length
in 32-bit words
Originally had Delay, Throughput and
Reliability flags. Now it has 4 queuing
priority classes, 3 discard probabilities
and historical service classes.
Don’t Fragment (e.g.
Memory Image)
More Fragments (All
but the last have it !)
Tells to which
datagram the
newly arrived
fragment
belongs.
Tells whether to give the datagram to
TCP or UDP or some other process.
Variable Length field (in multiples of
32-bits) meant for inclusion by
subsequent versions new Info.
Original Options: Security, strict source
routing, loose source coding (gives list of
routers not to be missed), Timestamp
(enforces each router to append its address &
Timestamp- useful for debugging)
Lesson 10: IP & The Internet - 4
254
The IP Address Formats
Lesson 10: IP & The Internet - 5
255
Reserved IP Addresses
Lesson 10: IP & The Internet - 6
256
The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Network
layer
Routing protocols
•path selection
•ARP, RARP/BOOTP/
DHCP
•RIP/OSPF, BGP
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
routing
table
ICMP protocol
•error reporting
•router “signaling”
Link layer
physical layer
Lesson 10: IP & The Internet - 7
257
The Internet Control Message Protocol
Each ICMP message is encapsulated in an IP packet
Lesson 10: IP & The Internet - 8
258
ARP (Address Resolution
Protocol)
• Used in IPV4 (over Ethernet) to get the
hardware/link/MAC address of the machine
with IP address
• ARP message of the form “I am
X1.X2.X3.X4, tell me who is Y1.Y2,Y3,Y4 is
sent using LAN (say, ETHERNET) broadcast
address (all 1’s) in an ethernet packet.
• Only the concerned system sends ARP
response; others discard.
Lesson 1: History of Computer
Networks - 2
259
RARP, BOOTP and DHCP
RARP- Reverse Address Resolution Protocol
Useful for diskless workstations getting binary image of O/S from remote file server.
BOOTP (Bootstrap Protocol)
Invented because destination address of all 1’s in RARP is not portable to RARP
server across network
Uses UDP.
DHCP (Dynamic Host Configuration Protocol)
has largely replaced RARP & BOOTP
DHCP relay agents, in the network of the source, intercept all DHCP discover packets
and unicast them to the DHCP server across the network. DHCP.
Lesson 10: IP & The Internet - 10
260
DHCP
Lesson 10: IP & The Internet - 11
261
OSPF (Open Shortest Path First)
Interior Gateway Protocol for
routing within Autonomous Systems (ASes).
It Supports
point-to-point routing between two routers
multi-access networks with Broadcasting (e.g. LANs)
and
multi-access networks without broad casting (e.g.
WANs).
Lesson 10: IP & The Internet - 12
262
OSPF (Open Shortest Path First)
Interior Gateway Protocol (routing within Autonomous Systems (ASes).
Supports- point-to-point routing between two routers, multi-access networks with Broadcasting (e.g. LANs) and
multi-access networks without broad casting (e.g. WANs).
Lesson 10: IP & The Internet - 13
263
OSPF (Continued)
Original Interior gateway protocol was RIP (Routing Information Protocol) based on the Bellman-Ford
algorithm in ARPANET. Now replaced by an extension of the LS algorithm. It is open, dynamic
(adaptable to changes), supports other metrics e.g. delay, routing based on types of service, hierarchical
systems, security, tunneling, and does load balancing
Lesson 10: IP & The Internet - 14
264
BGP (Boarder Gateway Protocol)
Exterior Gateway Protocol used between ASes
Uses Distance Vector (DV) routing, but solves the count to infinity problem by keeping
track paths , not just the costs to destination.
Policies based on political, security or economic considerations configured into BGP
routers by Scripts.
Lesson 10: IP & The Internet - 15
265
Lesson 10: IP & The Internet- Summary
and Follow-up
We have seen how the Internet- the network of networks
works (particularly, the tunneling concept)
We Studied the IP message and address structures
We studied a number of Protocols & Algorithms
ICMP
ARP & RARP/BOOTP/DHCP
RIP /OSPF & BGP
We discussed how the count-to-infinity problem is
addressed in the BGP.
Next class, we proceed on to Data-link layer.
Lesson 10: IP & The Internet - 16
266
Lesson 11: Introduction to
Data Link Layer
267
Lesson 11: Introduction to Data Link Layer Preview/Objectives
We study the principles behind various link layer services
such as
Error Detection and correction
Multiple access (sharing the broadcast channel)
Point-to-point (Single wire e.g. SLIP/PPP)
Broadcast (Shared wire e.g. Ethernet, WaveLan etc.
Switched (e.g. Switched Ethernet, ATM, etc.)
Link layer Addressing (ARP- already done!)
Reliable Data Transfer & Flow control (already done in the
context of TCP)
We study Pure and Slotted Protocols- precursors of
CSMA/CD
Lesson 11: Introduction to Data
Link Layer - 1
268
Link Layer: Setting the Context
Lesson 11: Introduction to Data
Link Layer - 2
269
Link Layer & Data Link Protocol
• two physically connected devices:
– host-router, router-router, host-host
• unit of data: frame
M
Ht
M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
network
link
physical
Hl Hn Ht M
frame
adapter card
Lesson 11: Introduction to Data
Link Layer - 3
270
Link Layer Services
Framing and link access:
encapsulate datagram into frame, adding header, trailer
implement channel access if shared medium,
‘physical addresses’ used in frame headers to identify source and
destination
different from IP address!
Reliable delivery between two physically connected
devices:
we learned how to do this already (in the context of TCP)!
seldom used on low bit error link (fiber, some twisted pair)
wireless links: high error rates
Q: why both link-level and end-end reliability?
Lesson 11: Introduction to Data
Link Layer - 4
271
More Link Layer Services
Flow Control:
pacing between sender and receivers
Error Detection:
errors caused by signal attenuation, noise.
receiver detects presence of errors and
signals sender for retransmission or drops frame
Error Correction:
receiver identifies and corrects bit error(s) without
resorting to retransmission
Lesson 11: Introduction to Data
Link Layer - 5
272
Link Layer: Implementation
implemented in “adapter”
e.g., PCMCIA card, Ethernet card
typically includes: RAM, DSP chips, host bus
interface, and link interface
M
Ht
M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
network
link
physical
Hl Hn Ht M
frame
adapter card
Lesson 11: Introduction to Data
Link Layer - 6
273
Error Detection in Link Layer
Error Detection:
Parity bit (single bit indication, but even number of flips
can’t be detected)
Check Sum is simple, but not enough (even number of
flips in the opposite direction give the same value)
Cyclic Redundancy Check is more rigorous and hence used
in link layer
Transport layer relies on this and manages with simpler
Check Sum.
Lesson 11: Introduction to Data
Link Layer - 7
274
Cyclic Redundancy Check Code
For r-bit CRC code, (r+1)-bit
Generator (G) is required.
Most Significant Bit of G = 1
8-, 12-, 16-, 32-bit G’s defined
by International standards
8-bit G used for protecting 5byte ATM headers
GCRC-32 =
100000100110000010001110
110110111
Lesson 11: Introduction to Data
Link Layer - 8
275
More About CRC
CRC is also known as polynomial code
CRC Formula Derivation:
D.2r XOR R = n G
D.2r = n G XOR R
R = remainder (D.2r/G ) when subtraction in the division is done by
XOR.
CRC can detect
Burst errors (consecutive bit errors) of size < r+1
Under some assumptions, bust errors of size > r+1 can be
detected with probability 1 – 0.5 r
Each CRC standard can detect any odd number of bit errors.
Lesson 11: Introduction to Data
Link Layer - 9
276
Multiple Access Links and Protocols
Three types of “links”:
point-to-point (single wire, e.g. PPP, SLIP)
broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.)
switched (e.g., switched Ethernet, ATM etc)
Lesson 11: Introduction to Data
Link Layer - 10
277
Multiple Access protocols
single shared communication channel
two or more simultaneous transmissions by nodes: interference
only one node can send successfully at a time
multiple access protocol:
distributed algorithm that determines how stations share channel, i.e.,
determine when station can transmit
communication about channel sharing must use channel itself!
what to look for in multiple access protocols:
synchronous or asynchronous
information needed about other stations
robustness (e.g., to channel errors)
performance
Lesson 11: Introduction to Data
Link Layer - 11
278
MAC Protocols: A Taxonomy
Three broad classes:
Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency)
allocate piece to node for exclusive use
Random Access
allow collisions
“recover” from collisions
“Taking turns”
tightly coordinate shared access to avoid collisions
Goal: efficient, fair, simple, decentralized
Lesson 11: Introduction to Data
Link Layer - 12
279
Random Access protocols
When node has packet to send
transmit at full channel data rate R.
no a priori coordination among nodes
two or more transmitting nodes -> “collision”,
random access MAC protocol specifies:
how to detect collisions
how to recover from collisions (e.g., via delayed retransmissions)
Examples of random access MAC protocols:
slotted ALOHA
ALOHA
CSMA and CSMA/CD
Lesson 11: Introduction to Data
Link Layer - 13
280
Pure (Unslotted) ALOHA
Users are not synchronized.
Each user transmits a data packet when ready.
In the event of two or more packets collide
(overlap in time), each user involved realized this
and retransmit the packet after a randomized
delay.
Lesson 11: Introduction to Data
Link Layer - 14
281
Pure ALOHA (Continued)
• unslotted Aloha: simpler, no synchronization
• packet needs transmission:
– send without awaiting for beginning of slot
• collision probability includes two overlapping intervals:
– packet sent at t0 collide with other packets sent in [t0-1, t0+1]
Lesson 11: Introduction to Data
Link Layer - 15
282
Slotted ALOHA
Like Pure-ALOHA with additional
requirements:
The channel is slotted in time
Each user is required to synchronize the start of
packet transmission to coincide with the slot
boundary (only complete collision would occur,
avoid partial collision)
Lesson 11: Introduction to Data
Link Layer - 16
283
Slotted Aloha - Further Details
• time is divided into equal size slots (= packet trans. time)
• node with new arriving packets: transmit at beginning of
next slot
• if collision: retransmit packet in future slots with probability
p, until successful.
Success (S), Collision (C), Empty (E) slots
Lesson 11: Introduction to Data
Link Layer - 17
284
Limit on the Slotted Aloha efficiency
Q: what is max fraction slots successful?
A: Suppose N stations have packets to send
– each transmits in slot with probability p
– prob. successful transmission S is:
by single node: S= p (1-p)(N-1)
by any of N nodes
S = Probability (only one transmits)
= N p (1-p)(N-1)
… choosing optimum p as N -> infinity ...
At best: channel
use for useful
transmissions 37%
of time!
= 1/e = .37 as N -> infinity (we will see in the next slide)
Lesson 11: Introduction to Data
Link Layer - 18
285
Derivation of Slotted Aloha efficiency Limit
S = Probability of success of any of the N nodes (i.e. only one transmits)
= N p (1-p)(N-1)
N
 1
Find the maximum value of S using the established formula: Lim 1 
 N
Solution: Setting ds/dp = 0, we get,
N 

1
e
N. (1-p)(N-1) _ N p (N-1) (1-p)(N-2) = 0
 p = 1/N
Putting this value “p” in S and taking limits we get,
S = 1/e
Lesson 11: Introduction to Data
Link Layer - 19
286
Pure & Slotted Aloha Efficiency Limits
P(success by given node) = P(node transmits) .
P(no other node transmits in [p0-1,p0] .
P(no other node transmits in [p0,p0+1]
= p . (1-p)(N-1) .(1-p)(N-1)
P(success by any of N nodes) = N p . (1-p)(N-1). (1-p)(N-1)
… choosing optimum p as N -> infty ...
= 1/(2e) using similar derivation = .18
0.4
0.3
Slotted Aloha
0.2
0.1
protocol constrains
effective channel
throughput!
Pure Aloha
0.5
1.0
1.5
2.0
G = offered load = Np
Lesson 11: Introduction to Data
Link Layer - 20
287
Lesson 11: Introduction to Data Link Layer –
Summary & Follow-up
We studied the principles behind various link layer services e.g.
Error Detection and correction
Multiple access (sharing the broadcast channel)
Point-to-point (Single wire e.g. SLIP/PP)
Broadcast (Shared wire e.g. Ethernet, WaveLan etc.
Switched (e.g. Switched Ethernet, ATH, etc.)
Link layer Addressing
Reliable Data Transfer & Flow control (already done in the TCP class)
We studied and analyzed Pure and Slotted ALOHA Protocolsprecursors of CSMA/CD.
Next class, we proceed on to Link layer technologies and study
CSMA/CD, Ethernet and other protocols & Technologies.
Lesson 11: Introduction to Data
Link Layer - 21
288
Lesson 12: Link Layer
Technologies
289
Lesson 12: Link Layer TechnologiesPreview/Objectives
We study specific link layer technologies and their
implementation
Current Multiple Access MAC (Medium Access Control)
ProtocolsCSMA/CD
Channel Partitioning
“Taking Turns” type – Token Ring
Ethernet Hubs, Bridges and Switches
PPP
ATM
IEEE 802.11 LANs
Lesson 12: Link Layer
Technologies - 1
290
Carrier Sense Multiple Access (CSMA)
Used in radio network.
Propagation delay is small compared to packet
transmission time.
Avoid collision by listening to the carrier before
transmission.
Lesson 12: Link Layer
Technologies - 2
291
CSMA: Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire packet
If channel sensed busy, defer transmission
Persistent CSMA: retry immediately with
probability p when channel becomes idle (may cause
instability)
Non-persistent CSMA: retry after random interval
human analogy: don’t interrupt others! Good
Manners protocol.
Lesson 12: Link Layer
Technologies - 3
292
CSMA collisions
spatial layout of nodes along ethernet
Collisions can occur:
Propagation delay means
two nodes may not yet
hear each other’s transmission
Collision:
Entire packet transmission time wasted
Note:
Role of distance and propagation delay in
determining collision probability.
Lesson 12: Link Layer
Technologies - 4
293
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
colliding transmissions aborted, reducing channel wastage
persistent or non-persistent retransmission
Collision detection:
easy in wired LANs: measure signal strengths, compare
transmitted, received signals
difficult in wireless LANs: receiver shut off while
transmitting
Same human analogy of the polite conversationalist
Lesson 12: Link Layer
Technologies - 5
294
IEEE 802.3 CSMA/CD
Uses 1-persistent CSMA algorithm.
Rules:
if the channel is idle then transmit
if the channel is busy, then continue to listen until idle then
transmit immediately
if a collision is detected during the transmission, immediately
cease transmitting the frame and transmit a jamming signal to
ensure everyone knows the collision, hence the name collision
detection (CD)
After transmitting the jamming signal, then wait a random
time and attempt to transmit again
Lesson 12: Link Layer
Technologies - 6
295
CSMA/CD Collision Detection
Lesson 12: Link Layer
Technologies - 7
296
“Taking Turns” MAC protocols
Channel partitioning MAC protocols:
share channel efficiently at high load
inefficient at low load: delay in channel access, 1/N
bandwidth allocated even if only 1 active node!
Random access MAC protocols
efficient at low load: single node can fully utilize channel
high load: collision overhead
“Taking turns” protocols
look for best of both worlds!
Lesson 12: Link Layer
Technologies - 8
297
“Taking Turns” MAC protocols
Polling:
• master node “invites”
slave nodes to transmit
in turn
• Request to Send, Clear
to Send messages
• concerns:
Token passing:
control token passed from one node
to next sequentially.
token message
concerns:
token overhead
latency
single point of failure (token)
– polling overhead
– latency
– single point of failure
(master)
Lesson 12: Link Layer
Technologies - 9
298
Reservation-based protocols
Distributed Polling:
time divided into slots
begins with N short reservation slots
reservation slot time equal to channel end-end propagation delay
station with message to send posts reservation
reservation seen by all stations
after reservation slots, message transmissions ordered by known
priority
Lesson 12: Link Layer
Technologies - 10
299
Summary of MAC protocols
What can we do with a shared media?
Channel Partitioning, by time, frequency or code
Time Division,Code Division, Frequency Division
Random partitioning (dynamic),
ALOHA, S-ALOHA, CSMA, CSMA/CD
carrier sensing: easy in some technoligies (wire), hard in others
(wireless)
CSMA/CD used in Ethernet
Taking Turns
polling from a central cite, token passing
Lesson 12: Link Layer
Technologies - 11
300
Ethernet
“Dominant” LAN technology:
Cheap $20 for 100Mbs!
First wildey used LAN technology
Simpler, cheaper than token LANs and ATM
Kept up with speed race: 10, 100, 1000 Mbps
Metcalfe’s Etheret
sketch
Lesson 12: Link Layer
Technologies - 12
301
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble:
7 bytes with pattern 10101010 followed by one byte with
pattern 10101011
Used to synchronize receiver, sender clock rates
Last two 11’s of the 8th for alerting about something
important to come.
Lesson 12: Link Layer
Technologies - 13
302
Ethernet Frame Structure (Continued)
Addresses: 6 bytes, frame is received by all adapters on a
LAN and dropped if address does not match
Type: indicates the higher layer protocol, mostly IP but others
may be supported such as Novell IPX and AppleTalk)
CRC: checked at receiver, if error is detected, the frame is
simply dropped
8 bytes
6 bytes
6 bytes 2 bytes
46-1500 bytes
Lesson 12: Link Layer
Technologies - 14
4 bytes
303
Ethernet: CSMA/CD Algorithm
A: sense channel, if idle
then {
transmit and monitor the channel;
If detect another transmission
then {
abort and send jam signal;
update # collisions;
delay as required by exponential backoff algorithm;
goto A
}
else {done with the frame; set collisions to zero}
}
else {wait until ongoing transmission is over and goto A}
Lesson 12: Link Layer
Technologies - 15
304
Ethernet’s CSMA/CD- Finer Details
Jam Signal: make sure all other transmitters are aware of
collision; 48 bits;
Exponential Backoff:
• Goal: adapt retransmission attempts to estimated current
load
– heavy load: random wait will be longer
• first collision: choose K from {0,1}; delay is K x 512 bit
transmission times
• after second collision: choose K from {0,1,2,3}…
• after ten or more collisions, choose K from
{0,1,2,3,4,…,1023}
Lesson 12: Link Layer
Technologies - 16
305
Ethernet Technologies: 10Base2
10: 10Mbps; 2: under 200 meters max cable length
thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments
repeater repeats bits it hears on one interface to its other
interfaces: physical layer device only!
Lesson 12: Link Layer
Technologies - 17
306
10BaseT and 100BaseT
• 10/100 Mbps rate; latter called “fast ethernet”
• T stands for Twisted Pair
• Hub to which nodes are connected by twisted pair, thus “star
topology”
• CSMA/CD implemented at hub
Lesson 12: Link Layer
Technologies - 18
307
More on10BaseT and 100BaseT
Max distance from node to Hub is 100 meters
Hub can disconnect “jabbering adapter
Hub can gather monitoring information, statistics for
display to LAN administrators
Lesson 12: Link Layer
Technologies - 19
308
Gbit Ethernet
use standard Ethernet frame format
allows for point-to-point links and shared broadcast channels
in shared mode, CSMA/CD is used; short distances between
nodes to be efficient
uses hubs, called here “Buffered Distributors”
Full-Duplex at 1 Gbps for point-to-point links
Lesson 12: Link Layer
Technologies - 20
309
PPP- Format
Flag field mark the beginning and end of the PPP frame
What is the use of the same address and control fields?
Protocol- values depend on the upper layer (network) protocol
receiving the data: AppleTalk (29), IPCP (8021)
Lesson 12: Link Layer
Technologies - 21
310
PPP- Format- How differentiate Data and
Control Info in the Header?
Answer: A technique called byte stuffing. An escape byte 01111101
precedes the flags byte appearing as data. What about escape byte itself?
Lesson 12: Link Layer
Technologies - 22
311
PPP- State Model
PPP’s Link Control Protocol (LCP) manages the states.
Physical layer
presence indicated by
carrier detection or
admin action
Terminate request
and ACK exchange
Configure-request frame (a PPP Frame
with protocol set to LCP value- Co21)
and configure-ack/configure-nak/
configure-reject responses received.
Lesson 12: Link Layer
Technologies - 23
312
ATM
AAL1- Constant bit rate services AAL2- Variable bit rate (e.g. video) services AAL5- IP Services
AAL (ATM Adaptation Layer)- Performs error detection; Equivalent to Transport layer as it is responsible for
segmentation & Reassembly.
Lesson 12: Link Layer
Technologies - 24
313
AAL5 PDU
Lesson 12: Link Layer
Technologies - 25
314
ATM Cell Header
VCI- Virtual circuit identifier
PT- payload type
CLP- Cell Priority Bit
HEC- Header Error Control
Lesson 12: Link Layer
Technologies - 26
315
ATM Physical Layer
At the bottom of the ATM protocol stack
Uses T1/T3, SONET/SDH (synchronous Optical Network/Synchronous
Digital Hierarchy) over a single-mode fiber.
T1/T3 frames over fiber, microwave and copper
Like T1/T3, SONET/SDH have frame structures to establish sync between
transmitters and receivers.
Cell based with no frames (clock at receiver is derived from a transmitted signal)
Standardized rates for SONET
OC-1: 51.84 Mbps
OC-3: 155.52 Mbps
OC-12: 622.08 Mbps
OC-48: 2.5 Gbps
Lesson 12: Link Layer
Technologies - 27
316
Wi-Fi: 802.11 Wireless LANs
Building Block of Wi-Fi LAN
architecture is Basic Service Set
(BSS) containing
a base station, known as access
point (AP)
One or more wireless stations
WI-FI Uses CSMA/CA
LANs that deploy APs are called
Infrastructure Wireless LANs
Lesson 12: Link Layer
Technologies - 28
317
IEEE 802.11 Standards
Standard
Frequency Range
Data Range
802.11b
2.4-2.485 GHZ
up to 11 Mbps
802.11a
5.1- 5.8 GHZ
up to 54 Mbps
802.11g
2.4-2.485 GHZ
up to 54 Mbps
• 802.11b mostly sufficient for home networks with DSL or broadband
Cable. 802a have higher bit rates, but have lesser transmission distance
for the same power. 802g’s have both high speed and low power
advantages.
Lesson 12: Link Layer
Technologies - 29
318
How Wi-Fi works
Once AP is installed, it is given 1 or 2 word Service Set
Identifier (SSID). It is also given channel numbers- 85 MHz
in 802.11b, for example, divided into 11 channels.
As per wifi standard, AP periodically transmits beacon
frames with its SSID and MAC Address
Wireless station tries to access an AP using 802.11
association protocol.
When channel is sensed idle, a station (AP or other
station) transmits frame after a time called Distributed
Inter-Frame Space (DIFS)
Lesson 12: Link Layer
Technologies -30
319
How Wi-Fi works (continued)
When channel is busy, it takes a random back off value and
freezes it. Only when it is idle, it starts counting down and
transmits when count is zero. This is for collision avoidance.
Once the frame is transmitted, waits for ACK.
If ACK is received and another frame is required and starts
again with a random back off value.
If ACK is not received, same process is repeated with a larger
back-off value.
Collision is avoided for large frames by Request to Send (RTS)
and Clear To Send (CTS) protocol message exchanges before data
12: Link Layer
320
transmission and ACK. Lesson
Technologies -31
Lesson 12: Link Layer TechnologiesSummary/Follow-up
We studied the following link layer technologies and their
implementation
Current Multiple Access MAC (Medium Access Control)
ProtocolsCSMA/CD
Channel Partitioning
“Taking Turns” type – Token Ring
Ethernet Hubs, Bridges and Switches
PPP
ATM
IEEE 802.11 LANs
Next class, we take up Physical Layer
Lesson 12: Link Layer
Technologies - 32
321
Lesson 13: Introduction to
Physical Layer
322
Lesson 13: Introduction to Physical Layer Preview/Objectives
We study physical layer functionality and 3 types of
transmission
Simplex
Half Duplex
Full Duplex
Signals and their properties
Relation between bandwidth and data rate
Lesson 13: Introduction to
Physical Layer -1
323
Physical Layer
Physical layer is concerned with data transmission
Data transmission occurs between a transmitter and a
receiver.
The media may be guided or unguided:
guided: twisted pair, coaxial cable, and fiber.
unguided: through air, water, or vacuum.
Either type of transmission is based on electromagnetic waves.
A direct link is the signal transmission path between two
devices with no intermediate device other than repeaters
and amplifiers.
Lesson 13: Introduction to
Physical Layer - 2
324
Data Transmission- Some Terminology
A guided medium is point-to-point if
it provides a direct link between two devices;
the medium is shared by only those two devices;
In a multi-point configuration, more than two devices
share the transmission medium.
We distinguish 3 forms of transmission:
Simplex
Half Duplex
Full Duplex
Lesson 13: Introduction to
Physical Layer - 3
325
Simplex Transmission
Transmission in only one direction; one station is the
transmitter, the other the receiver.
Examples:
One-Way Street
Keyboard-Computer connection
Computer-Monitor connection
TV Broadcast
Can you think of other simplex examples?
Lesson 13: Introduction to
Physical Layer - 4
326
Half-Duplex Transmission
Half Duplex: Transmission in both directions
possible, but NOT at the same time. Here, the
attached stations are both, sender and receiver.
Examples:
One-Lane Road with access control lights. While
cars go in one directions, cars going the opposite
way must wait.
Walkie-Talkies
CB-Radios
Traditional Ethernet (Coax or 10baseT)
Lesson 13: Introduction to
Physical Layer - 5
327
Full Duplex Transmission
Transmission in both directions simultaneously. Both
stations can send and receive at the same time.
Examples:
Regular 2-way street
Full-Duplex repeated Ethernet (Gbit Ethernet)
Full Duplex transmission can be accomplished in two
ways:
Separated physical transmission media
Divided channel capacity and separation of signals in
different directions.
Lesson 13: Introduction to
Physical Layer - 6
328
What is transmitted?
Signals are transmitted; could be electrical, optical ,
etc.
Signals can be expressed in two ways:
in the Time-Domain, the signal intensity varies over time;
i.e., as a function of time, f(t)
in the Frequency-Domain, the signal is expressed as a
function of the constituent frequencies, the set of sinusoid
signals which make up the signal.
We need to distinguish between 2 types of signals:
Continuous;
Discrete
Lesson 13: Introduction to
Physical Layer - 7
329
Continuous and Discrete Signals
A continuous signal is one in which the signal
intensity varies in a smooth fashion over time. There
are no breaks (poles) or discontinuities.
A discrete signal is one in which the signal intensity
maintains a constant level for some period of time and
then changes to another constant level.
Note: A discrete signal may consist of more than just 2
constant levels; i.e., discrete does not mean binary!
Lesson 13: Introduction to
Physical Layer - 8
330
Periodic Signal
The simplest sort of signal is a periodic signal.
Definition: a signal s(t) is periodic if and only if
s(t  T )  s(t )    t  
Here, T is said to be the period. T is the smallest value
that satisfies the equation.
Lesson 13: Introduction to
Physical Layer - 9
331
Sinusoid- The Fundamental
Continuous Signal
The sine wave is the fundamental continuous signal.
We can represent the sine wave by 3 parameters:
Amplitude (A)
Frequency (f)
Phase ()
s(t)  A sin(2  ft   )
Lesson 13: Introduction to
Physical Layer - 10
332
Amplitude, Frequency and Phase
Amplitude (A): is the peak value or strength of the
signal over time. (in Volts, Watts, etc.)
Frequency (f): is the rate (in cycles per second, or
Hertz (Hz)) at which the signal repeats.
The period T can be computed as T=1/f. T is the amount
of time taken for one repetition.
Phase (): is the measure of the relative position in
time within a single period of the signal.
Lesson 13: Introduction to
Physical Layer - 11
333
Wavelength of a Signal
The Wavelength () of a signal is the distance
occupied by a single cycle (or period). In other words,
it is the distance between to points of corresponding
phase of two consecutive cycles.
  vT
Here, v represents the velocity of the signal.
Lesson 13: Introduction to
Physical Layer - 12
334
Frequency Domain
Representation of Signals
The Frequency-Domain Concept allows us to
represent a signal as the sum of constituent
frequencies. For example:
s(t) = sin(2f1t) + 1/3 sin(2(3f1)t)
The components of s(t) are sine waves of
frequencies f1 and 3f1.
Fourier analysis is the method of decomposing
signals into the constituent sinusoids.
Lesson 13: Introduction to
Physical Layer - 13
335
Frequency Domain Analysis
When all of the frequency components are integer
multiples of one frequency f1, f1 is called the
fundamental frequency.
The period of the total signal is equal to the period of
the fundamental frequency.
The spectrum of a signal is the range of frequencies
that it contains. In our example, the spectrum extends
from f1 to 3 f1.
Lesson 13: Introduction to
Physical Layer - 14
336
Bandwidth
Physical property of the transmission medium
Depends on length, thickness, construction, etc.
Range of frequencies transmitted without being strongly
attenuated
In our example, the bandwidth required to send the signal
without distortion is 3f1- f1 = 2f1.
Note that most of the energy in the signal is contained in a
relative narrow band of frequencies. This is referred to as the
effective bandwidth required. In this case, a medium with lower
bandwidth can transmit this signal with tolerable distortion.
Lesson 13: Introduction to
Physical Layer - 15
337
Fourier Analysis- An Overview
Any reasonably behaved periodic
signal can be expressed as a sum
(possibly infinite) of sines and
cosines as follows:
s(t)=c/2 + Σn=1 to ∞ansin(2nft)
+ Σn=1 to ∞bncos(2nft)
Sine and cosine term pair for a
value of n is called nth harmonic.
Root Mean Square (RMS)
amplitude √an2+bn2 indicates
the significance of the nth
harmonic.
Lesson 13: Introduction to
Physical Layer - 16
338
Relation between Data Rate and
Bandwidth
At b bits/sec, time required
to send 8-bits = 8/b sec.
Freq. of 1st harmonic will
be b/8 Hz. How many
harmonica pass through a
voice grade line with 3000
Hz cut-off?
Lesson 13: Introduction to
Physical Layer - 17
339
Lesson 13: Introduction to Physical Layer –
Summary and Follow-up
We studied physical layer functionality and 3 types of
transmission
Simplex
Half Duplex
Full Duplex
We studied Signals and their properties (particularly
Fourier Analysis)
Relation between bandwidth and data rate
Next class, we study about wireless access technologies.
Lesson 13: Introduction to
Physical Layer -18
340
Lesson 14: Physical
Layer (Wireless Access)
341
Lesson 14: Physical Layer (Wireless Access)Preview/Objectives
We study in this lesson
Two kinds of wireless access
Fixed (e.g. fixed wireless systems using traditional mobile access
technologies, wi-fi)
Mobile
Mobile Access:
Generations 1-3, 2.5, Evolutionary
Technologies- FDMA (e.g. AMPS), TDMA (e.g. GSM), CDMA
(e.g IS-95/CDMA-2000), WCDMA
Mobility Management
Lesson 14: Physical Layer
(Wireless Access) - 1
342
How Wireless Systems Work?
• Depending upon in which cell
mobile is, it will be able to access
a particular base station.
• Call will be se up via a Base
Station controller (BSC) and a
Mobile Switching Center (MSC)
after a lot of call processing
(control or signaling messages)
back and forth.
• Phone could be stationary (fixed)
or mobile- but in case of mobile
phones a technique called handover/hand-off is used.
RNC in UMTS
MSC or PDSN/GGSN
jargon
BSC-X
A
Lesson 14: Physical Layer
(Wireless Access) - 2
B
C
BSC-Y
D
343
Multiple Access
Each pair of users
enjoy a dedicated,
private circuit
through the
transmission
medium (air in
case of wireless
systems), unaware
of the existence of
other users.
Lesson 14: Physical Layer
(Wireless Access) - 3
344
Generations of Wireless Technologies
• 1st Generation Mobile Phones (Analog Voice)
– Push to Talk Systems (e.g. CB radios, police radios) in late 1950s
– IMTS (Improved Mobile Telephone Systems) 1960s
– AMPS (Advanced Mobile Phone Systems) 1982 by Bell Labs
• 2nd Generation (Digital Voice)
– D-AMPS, GSM and CDMA (IS-95)
• 3rd Generation
– 1XRTT, CDMA-200 and UMTS (Universal Mobile
Telecommunications System) based on W-CDMA.
• Beyond 3g (B3g)- Evolutionary (1xEVDV, 1xEVDO, etc.)
• 2.5 G
– Enhanced Data Rates for GSM (Edge) and GPRS (General Packet
Radio Services)
Lesson 14: Physical Layer
(Wireless Access) - 4
345
CDMA-Spread Spectrum
• Slow varying (low frequency) data signal is spread
over a large spectrum using a fast (high frequency
signal
• CDMA spreading principle- Anything we can do , we
can undo.
Lesson 14: Physical Layer
(Wireless Access) - 5
346
How do you do & Undo?
Lesson 14: Physical Layer
(Wireless Access) - 6
347
Spreading Example
Lesson 14: Physical Layer
(Wireless Access) - 7
348
De-spreading (Recovery of Previously Spread
Data) for the same Example
Lesson 14: Physical Layer
(Wireless Access) - 8
349
How do you handle mixture of signals from
multiple users?
• Use orthogonal signals (e.g. Walsh codes) for spreading.
• Two signals are orthogonal if their XOR sum has equal
number of 1’s and 0’s (e.g. 111111 and 101010)
Lesson 14: Physical Layer
(Wireless Access) - 9
350
Mobility Management
• Hand-off/Hand-over
• Two types
– Soft-handoff (Continuous connection with two base
stations and seamless transfer)
– Hard-handoff (mobile stops transmitting, adjusts its
parameters and restarts)
• Intersystem (control is passed to a new MSC)
• Intra-system
Lesson 14: Physical Layer
(Wireless Access) - 10
351
Lesson 14: Physical Layer (Wireless Access)Summary/Follow-up
We studied in this lesson
Two kinds of wireless access
Fixed (e.g. fixed wireless systems using traditional mobile access
technologies, wi-fi)
Mobile
Mobile Access:
Generations 1-3
Technologies- FDMA (e.g. AMPS), TDMA (e.g. GSM), CDMA
(e.g IS-95/CDMA-2000), WCDMA
Mobility Management
Lesson 14: Physical Layer
(Wireless Access) - 11
352
Lesson 15: Introduction to
Network Security
353
Lesson 15: Introduction Network SecurityPreview/Objectives
We study in this lesson
What is security? What all it entails?
Cryptography
Authentication
Message Integrity
Types of Keys for encryption, their distribution and
certification
Famous Public Key Algorithm (RSA)
Lesson 15: Introduction to
Network Security - 1
354
Friends and enemies: Alice, Bob, Trudy
Figure 7.1 goes here
Well-known in network security world
Bob, Alice (close friends) want to communicate “securely”
Trudy, the “intruder” may intercept, delete, add messages
Lesson 15: Introduction to
Network Security - 2
355
What is network security?
Secrecy: only sender, intended receiver should
“understand” message contents
sender encrypts message
receiver decrypts message
Authentication: sender, receiver want to confirm
identity of each other
Message Integrity: sender, receiver want to ensure
message not altered (in transit, or afterwards)
without detection
Lesson 15: Introduction to
Network Security - 3
356
Internet security threats I- Packet Sniffing
Packet sniffing is possible because
the media is broadcast type
promiscuous NIC reads all packets passing by
any one can read all unencrypted data (e.g. passwords)
e.g.: C sniffs B’s packets
C
A
src:B dest:A
payload
B
Lesson 15: Introduction to
Network Security - 4
357
Internet security threats II- IP Spoofing
IP Spoofing (e.g. C pretending to be B) is done by:
Generation of “raw” IP packets directly from application,
putting any value into IP source address field such that
receiver can’t tell if source is spoofed
More generic name for this kind of attack- Sybil attack where
even bogus messages can be introduced in the network.
C
A
src:B dest:A
payload
B
Lesson 15: Introduction to
Network Security - 5
358
Internet security threats III: Denial of
Service Attack
This attack is done by
– A flood of maliciously generated packets that “swamp”
receiver
– Distributed DOS (DDOS): multiple coordinated sources
that swamp receiver e.g. C and remote host SYN-attack A
C
A
SYN
SYN
SYN
SYN
SYN
B
SYN
SYN
Lesson 15: Introduction to
Network Security - 6
359
Jargon of cryptography
plaintext
K
K
B
A
plaintext
ciphertext
Figure 7.3 goes here
symmetric key crypto: sender, receiver keys identical
public-key crypto: encrypt key public, decrypt key secret
Lesson 15: Introduction to
Network Security - 7
360
Symmetric key cryptography
Substitution cipher: substituting one thing for another
monoalphabetic cipher: substitute one letter for another
plaintext: abcdefghijklmnopqrstuvwxyz
ciphertext: mnbvcxzasdfghjklpoiuytrewq
E.g.:
Plaintext: bob. i love you. alice
ciphertext: nkn. s gktc wky. mgsbc
Q: How hard to break this simple cipher?:
brute force (how hard?)
other?
Lesson 15: Introduction to
Network Security - 8
361
Symmetric key crypto: DES
DES: Data Encryption Standard
US encryption standard [NIST 1993]
56-bit symmetric key, 64 bit plaintext input
How secure is DES?
DES Challenge: 56-bit-key-encrypted phrase (“Strong
cryptography makes the world a safer place”) decrypted (brute
force) in 4 months
no known “backdoor” decryption approach
making DES more secure
use three keys sequentially (3-DES) on each datum
use cipher-block chaining
Lesson 15: Introduction to
Network Security - 9
362
Symmetric key
crypto: DES
DES operation
initial permutation
16 identical “rounds” of
function application,
each using different 48
bits of key
final permutation
Lesson 15: Introduction to
Network Security - 10
363
Public Key Cryptography
symmetric key crypto
requires sender, receiver
know shared secret key
Q: how to agree on key
in first place
(particularly if never
“met”)?
public key cryptography
radically different
approach [DiffieHellman76, RSA78]
sender, receiver do not
share secret key
encryption key public
(known to all)
decryption key private
(known only to receiver)
Lesson 15: Introduction to
Network Security - 11
364
Public key cryptography
Figure 7.7 goes
here
Lesson 15: Introduction to
Network Security - 12
365
Public key encryption algorithms
Two inter-related requirements:
1 need dB( ) and eB( ) such that
d (e (m)) = m
B B
2
need private and public keys
for dB( ) and eB( ), respectively
RSA: Rivest, Shamir, Adelson algorithm
Lesson 15: Introduction to
Network Security - 13
366
RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt bit pattern, m, compute
c = me mod n (i.e., remainder when m e is divided by n)
2. To decrypt received bit pattern, c, compute
m = cd mod n (i.e., remainder when c d is divided by n)
Magic
happens!
m = (me mod n)
Lesson 15: Introduction to
Network Security - 14
d mod n
367
RSA: Choosing keys
1. Choose two large prime numbers p, q.
(e.g., 1024 bits each)
2. Compute n = pq, z = (p-1)(q-1)
3. Choose e (with e<n) that has no common factors
with z. (e, z are “relatively prime”).
4. Choose d such that ed-1 is exactly divisible by z.
(in other words: ed mod z = 1 ).
5. Public key is (n,e). Private key is (n,d).
Lesson 15: Introduction to
Network Security - 15
368
RSA example:
Bob chooses p=5, q=7. Then n=35, z=24.
e=5 (so e, z relatively prime).
d=29 (so ed-1 exactly divisible by z.
encrypt:
decrypt:
c
17
m
me
12
248832
d
c
481968572106750915091411825223072000
Lesson 15: Introduction to
Network Security - 16
c = me mod n
17
m = cd mod n
12
369
RSA: How does it work?
To prove: m = (me mod n) d mod n , we use two theorems:
1. Fermat’s little theorem :(x p-1 mod p = 1), when p is prime and x is
prime to p.
2. Chinese Reminder Theorem : If a = b mod p and a=b mod q where p
and q are relatively prime, a=b mod pq.
(me)d =med= med-1.m= mh(p-1)(q-1).m = 1h(q-1).m (mod p) = m (mod p)
(me)d =med= med-1.m= mh(p-1)(q-1).m = 1h(p-1).m (mod q) = m (mod q)
Hence, (me)d = m (mod pq) by Chinese Reminder Theorem
In the above, h is an integer . Since ed-1 is divisible by z=(p-1)(q-1),
ed-1 = hz =h(p-1)(q-1).
Lesson 15: Introduction to
Network Security - 17
370
RSA: Is it the end of Public Key
Cryptography?
No. Recently, another algorithm called Elliptic Curve
Cryptography is getting popular as it is even more difficult to
break.
Lesson 15: Introduction to
Network Security - 18
371
Lesson 15: Introduction to Network
Security-Summary/Follow-up
We studied in this lesson
What is security? What all it entails?
Cryptography
Authentication
Message Integrity
Types of Keys for encryption, their distribution and
certification
Famous Public Key Algorithm (RSA)
In the next class, we take up other security issues (e.g.
authentication) and some applications.
Lesson 15: Introduction to
Network Security - 19
372
Lesson 16: Network Security
(Continued)
373
Lesson 16: Network Security (Continued)Preview/Objectives
We study in this lesson
A more detailed view of the following security features:
Authentication
Message Integrity
Key distribution and certification
Security in practice:
Application layer: secure e-mail
Transport layer: Internet commerce, SSL, SET
Network layer: IP security
Lesson 16: Network Security
(Continued) - 1
374
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
Failure scenario??
Lesson 16: Network Security
(Continued) - 2
375
Authentication: another try
Protocol ap2.0: Alice says “I am Alice” and sends her IP
address along to “prove” it.
Failure scenario??
Lesson 16: Network Security
(Continued) - 3
376
Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Failure scenario?
Lesson 16: Network Security
(Continued) - 4
377
Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
I am Alice
encrypt(password)
Failure scenario?
Lesson 16: Network Security
(Continued) - 5
378
Authentication: yet another try
Goal: avoid playback attack
Nonce: number (R) used only once in a lifetime
ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice
must return R, encrypted with shared secret key
Figure 7.11 goes here
Failures, drawbacks?
Lesson 16: Network Security
(Continued) - 6
379
Authentication: ap5.0
ap4.0 requires shared symmetric key
– problem: how do Bob, Alice agree on key
– can we authenticate using public key techniques?
ap5.0: use nonce, public key cryptography
Figure 7.12 goes here
Lesson 16: Network Security
(Continued) - 7
380
ap5.0: security hole
Man (woman) in the middle attack: Trudy poses as
Alice (to Bob) and as Bob (to Alice)
Figure 7.14 goes here
Need “certified” public
keys (more later …)
Lesson 16: Network Security
(Continued) - 8
381
Digital Signatures
Cryptographic technique
analogous to handwritten signatures.
• Sender (Bob) digitally signs
document, establishing he is
document owner/creator.
• Verifiable, nonforgeable:
recipient (Alice) can verify
that Bob, and no one else,
signed document.
Simple digital signature for
message m:
• Bob encrypts m with his
private key dB, creating signed
message, dB(m).
• Bob sends m and dB(m) to
Alice.
Lesson 16: Network Security
(Continued) - 9
382
More on Digital Signatures
• Suppose Alice receives
Alice thus verifies that:
msg m, and digital
– Bob signed m.
signature dB(m)
– No one else signed m.
• Alice verifies m signed by
– Bob signed m and not m’.
Bob by applying Bob’s
Non-repudiation:
public key eB to dB(m)
– Alice can take m, and
then checks eB(dB(m) ) =
signature dB(m) to court
m.
and prove that Bob signed
• If eB(dB(m) ) = m,
m.
whoever signed m must
have used Bob’s private
key.
Lesson 16: Network Security
383
(Continued) - 10
Message Digests
Computationally expensive to
public-key-encrypt long
messages
Goal: fixed-length,easy to
compute digital signature,
“fingerprint”
• apply hash function H to m,
get fixed size message digest,
H(m).
Hash function properties:
• Many-to-1
• Produces fixed-size msg digest
(fingerprint)
• Given message digest x,
computationally infeasible to find m
such that x = H(m)
• computationally infeasible to find
any two messages m and m’ such
that H(m) = H(m’).
Lesson 16: Network Security
384
(Continued) - 11
Digital signature = Signed message digest
Bob sends digitally signed
message:
Alice verifies signature and
integrity of digitally signed
message:
Lesson 16: Network Security
(Continued) - 12
385
Hash Function Algorithms
• Internet checksum would • MD5 hash function widely
make a poor message
used.
digest.
– Computes 128-bit message
– Too easy to find two
digest in 4-step process.
messages with same
– arbitrary 128-bit string x,
checksum.
appears difficult to construct
msg m whose MD5 hash is
equal to x.
• SHA-1 is also used.
– US standard
– 160-bit message digest
Lesson 16: Network Security
(Continued) - 13
386
Trusted Intermediaries
Problem:
– How do two entities
establish shared secret
key over network?
Solution:
– trusted key distribution
center (KDC) acting as
intermediary between
entities
Problem:
– When Alice obtains Bob’s
public key (from web site,
e-mail, diskette), how
does she know it is Bob’s
public key, not Trudy’s?
Solution:
– trusted certification
authority (CA)
Lesson 16: Network Security
(Continued) - 14
387
Key Distribution Center (KDC)
• Alice,Bob need shared
symmetric key.
• KDC: server shares
different secret key with
each registered user.
• Alice, Bob know own
symmetric keys, KA-KDC
KB-KDC , for
communicating with
KDC.
• Alice communicates with KDC,
gets session key R1, and KBKDC(A,R1)
• Alice sends Bob
KB-KDC(A,R1), Bob extracts R1
• Alice, Bob now share the
symmetric key R1.
Lesson 16: Network Security
(Continued) - 15
388
Certification Authorities
• Certification authority (CA)
binds public key to particular
entity.
• Entity (person, router, etc.)
can register its public key with
CA.
– Entity provides “proof of
identity” to CA.
– CA creates certificate
binding entity to public
key.
– Certificate digitally signed
by CA.
• When Alice wants Bob’s public
key:
• gets Bob’s certificate (Bob or
elsewhere).
• Apply CA’s public key to Bob’s
certificate, get Bob’s public key
Lesson 16: Network Security
(Continued) - 16
389
Secure e-mail
• Alice wants to send secret e-mail message, m, to Bob.
• generates random symmetric private key, KS.
• encrypts message with KS
• also encrypts KS with Bob’s public key.
• sends both KS(m) and eB(KS) to Bob.
Lesson 16: Network Security
(Continued) - 17
390
Secure e-mail (continued)
• Alice wants to provide sender authentication message integrity.
• Alice digitally signs message.
• sends both message (in the clear) and digital signature.
Lesson 16: Network Security
(Continued) - 18
391
Secure e-mail (continued)
• Alice wants to provide secrecy, sender authentication,
message integrity.
Note: Alice uses both her private key, Bob’s public key.
Lesson 16: Network Security
(Continued) - 19
392
Pretty good privacy (PGP)
• Internet e-mail encryption
A PGP signed message:
scheme, a de-facto standard.
---BEGIN PGP SIGNED MESSAGE--Hash: SHA1
• Uses symmetric key
cryptography, public key
Bob:My husband is out of town
cryptography, hash function,
tonight.Passionately yours, Alice
and digital signature as
---BEGIN PGP SIGNATURE--described.
Version: PGP 5.0
Charset: noconv
• Provides secrecy, sender
yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJhFEv
authentication, integrity.
ZP9t6n7G6m5Gw2
• Inventor, Phil Zimmerman, was ---END PGP SIGNATURE--target of 3-year federal
investigation.
Lesson 16: Network Security
(Continued) - 20
393
Secure sockets layer (SSL)
• PGP provides security for a
specific network app.
• SSL works at transport layer.
Provides security to any TCPbased app using SSL services.
• SSL: used between WWW
browsers, servers for Icommerce (shttp).
• SSL security services:
• Server authentication:
– SSL-enabled browser includes
public keys for trusted CAs.
– Browser requests server certificate,
issued by trusted CA.
– Browser uses CA’s public key to
extract server’s public key from
certificate.
• Visit your browser’s security
menu to see its trusted CAs.
– server authentication
– data encryption
– client authentication (optional)
Lesson 16: Network Security
(Continued) - 21
394
SSL (continued)
Encrypted SSL session:
•
•
•
•
• SSL: basis of IETF
Browser generates symmetric
Transport Layer Security
session key, encrypts it with
(TLS).
server’s public key, sends
• SSL can be used for nonencrypted key to server.
Web applications, e.g.,
Using its private key, server
IMAP.
decrypts session key.
Browser, server agree that future • Client authentication can
messages will be encrypted.
be done with client
All data sent into TCP socket (by
certificates.
client or server) is encrypted with
session key.
Lesson 16: Network Security
(Continued) - 22
395
Secure electronic transactions (SET)
• designed for payment-card
transactions over Internet.
• provides security services
among 3 players:
– customer
– merchant
– merchant’s bank
All must have certificates.
• SET specifies legal meanings
of certificates.
– apportionment of
liabilities for transactions
• Customer’s card number passed
to merchant’s bank without
merchant ever seeing number in
plain text.
– Prevents merchants from
stealing, leaking payment
card numbers.
• Three software components:
– Browser wallet
– Merchant server
– Acquirer gateway
Lesson 16: Network Security
(Continued) - 23
396
IPSEC: Network Layer Security
• Network-layer secrecy:
– sending host encrypts the data in • For both AH and ESP, source,
destination handshake:
IP datagram
– create network-layer logical
– TCP and UDP segments; ICMP
channel called a service
and SNMP messages.
agreement (SA)
• Network-layer authentication
• Each SA unidirectional.
– destination host can authenticate
• Uniquely determined by:
source IP address
– security protocol (AH or
• Two principle protocols:
ESP)
– authentication header (AH)
– source IP address
protocol
– 32-bit connection ID
– encapsulation security payload
(ESP) protocol
Lesson 16: Network Security
(Continued) - 24
397
ESP Protocol
• Provides secrecy, host
• ESP authentication field
authentication, data integrity.
is similar to AH
authentication field.
• Data, ESP trailer encrypted.
• Protocol = 50.
• Next header field is in ESP
trailer.
Lesson 16: Network Security
(Continued) - 25
398
Authentication Header (AH) Protocol
• Provides source host
authentication, data integrity,
but not secrecy.
• AH header inserted between
IP header and IP data field.
• Protocol field = 51.
• Intermediate routers process
datagrams as usual.
AH header includes:
• connection identifier
• authentication data: signed
message digest, calculated over
original IP datagram, providing
source authentication, data
integrity.
• Next header field: specifies type
of data (TCP, UDP, ICMP, etc.)
Lesson 16: Network Security
(Continued) - 26
399
Lesson 16: Network Security (Continued)Summary/Follow-up
We studied in this lesson
A more detailed view of the following security features:
Authentication
Message Integrity
Key distribution and certification
Application of those security features in practice:
Application layer: secure e-mail
Transport layer: Internet commerce, SSL, SET
Network layer: IP security (IPSec)
Lesson 16: Network Security
(Continued) - 27
400