Computer Networks (EENG 4810)

Computer Networks- Course Objectives & Scope - 1

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Computer Networks (EENG

4810)


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Course Objectives & Scope


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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


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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

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Lesson 1:History of Computer

Networks

History of Computer Networks - 1 6

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- 2 7

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?

Lesson 1: History of Computer Networks - 3

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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 trafficPaul Baran of Rand Institute investigated packet switching for secure voice communication over military networksDonald Davies and Roger Scantlebury were developing ideas on packet switching at the National Physical Lab, England.


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(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.


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Leonard Kleinrock with IMP


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(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.


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.



(Continued)Proprietary Networks such asIBM’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).



(Early Internet Features)Cerf and Kahn’s TCP (quite different from now)It combined reliable in-sequence delivery of data by end-system retransmission (as now) with forwarding (as IP now)Realization of usefulness of separation of unreliable, non-flow 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.


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(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

History of Computer Networks - 11

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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 UniversitiesCSNET (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.


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Proliferation of Networks 1980-90 (Continued)

Simple Message Transfer Protocol (SMTP): E-Mail 1982Deployment 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.


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Proliferation of Networks 1980-90

(The Minitel Project)French Minitel project paralleling ARPAnet

Ambitious projest sponsored by the French GovernmentX.25 protocol suite using virtual circuitsBy mid-90’s, it offered more than 20, 000 services- from home banking to research databaseUsed by more than 20% of the populationGenerated over $1 billion in revenueWas in most French homes 10 years before Americans had ever heard of the Internet.

History of Computer Networks -15 19

Internet Explosion: The1990sEarly 90’s Arpanet decommissioned as Milnet

and Defense Data Net grew enough to carry all defense-related 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.


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.


Internet Explosion: Second half of1990sMicrosoft (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 emailWeb browsing & internet commerceinstant messaging with contact lists pioneered by ICQpeer-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.


Developments of Last DecadeFinancial 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 routers3 Important developments

High Speed Access Internet Access (Cable/DSL/Wireless LANs)Secure applicationsP2P (Point-to-point Networking)


Three Important Recent Developments

I- High Speed Internet AccessIncreased penetration of broadband residential Internet via Cable and DSL with applications such as high-quality Video on Demand and high quality Video ConferencingIncreased 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



II- SecurityIntrusion 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.



III- P2P NetworkingP2P 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 - 22

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Summary and Follow-upIn 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.

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Lesson 2:Overview of Computer

Networks

Overview of Computer Networks - 1

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Preview of the Lesson 2In 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.


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Computer Networks- Definition & PerspectivesReference:

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

http://en.wikipedia.org/wiki/Computer_network

http://en.wikipedia.org/wiki/Computer_network


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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


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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


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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 MansMAN links between LANs and WANs are usually microwave/ infra-red/radio.


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IV - Wide Area Network (WAN)Covers wide geographical areas spanning multiple cities.Works on leased lines and connects multiple LANsUses protocols such as TCP/IP, x.25, Frame Relay and ATMUsually 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)


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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).


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Network Categorization based on the Network Topology

Bus Network

Star Network

Ring Network

Grid Network

Toroidal Networks and Hypercubes

Tree and Hyper-tree Networks


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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.


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Nuts and Bolts view of Computer Network with Internet- Network of

Networks


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Network Building BlocksSwitch- 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


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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.


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“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)– pc’s workstations, servers– PDA (Personal Digital Assistant)’s

phones, toasters

running network apps :Communication links (Network Access)– fiber, coaxial cable, copper, radio,

satellite

Switches, routers, bridges, gateways (Network Core)

local ISP

companynetwork

regional ISP

router workstation

servermobile


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What’s a protocol?Human protocols:

A way of communication between humans

Dictated by local cultureGreeting, response, action takenExamples: “Hey, got time?,” “I have a dumb question,” This is so and so..”

Network protocols: Machines rather than humans involved, but all Internet communication activity is governed by protocolsDictated by standardsProtocols define format, order of messages sent and received among network entities, and actions taken on message transmission and receiptExample: TCP/IP, ISO


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Human and Network Protocol Examples

Hi

Hi

Got thetime?

2:00

TCP connection req.

TCP connectionreply.Get http://www.ee.unt.edu/public/guturu

<file>time


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ProtocolsBuilding 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


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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


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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.


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Steps in Organization of air travel

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing


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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


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Distributed implementation of layer functionality

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Dep

art

ing

air

port

arr

ivin

g

air

port

intermediate air traffic sites

airplane routing airplane routing

Layers: each layer implements a service via its own intra-layer actions relying on services provided by layer below


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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

transport

network

link

physical

•Network: addressing and routing of datagrams from source to destination (e,g. Ip & other routing protocols)•Link: logical organization of data bits transmitted

on a particular medium; framing, addressing, error correction/detection (check sum) e.g. ppp, ethernet

•Physical: bits “on the wire” Defines physical

Properties of various media e.g. Ether-Net cable size

•7-layer OSI protocol (of ISO) has session (reply and response packet pairing) and presentation layers (data syntax, encryption) above transport and below application layer.


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Layering: logical communication

applicationtransportnetwork

linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

Each layer:• distributed• “entities”

implement layer functions at each node

• entities perform actions, exchange messages with peers


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Layering: logical communication (continued)


linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

data

dataE.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

transport

transport

ack


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Layering: physical communication


linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

data

data


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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


linkphysical


linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame


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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 numberError-detection code


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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 UnitICI: Interface Control InformationIDU: Interface Data UnitPDU: Protocol Data Unit

Overview of Computer Networks -29

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Summary of the Lesson 2In 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.

Lesson 3: Application Layer - 1 57

Lesson 3: Preview/Objectives

High level view of network application protocolsclient server paradigmservice models

learn about protocols by examining popular application-level protocols such as

dnssmtppop ftp (Next Lesson)http (Next Lesson)Multimedia (Next Lesson)


Application layer – Some JargonApplications (e.g., email, file

transfer, the Web): communicating, distributed processes

running in network hosts in “user 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

transportnetworkdata linkphysical

application


application



Network applications: some jargon

A process is a program that is running within a host.

Within the same host, two processes communicate with inter-process 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-browserE-mail: mail readerstreaming audio/video: media player


Typical Application has two pieces:Client and Server

Client-server paradigm

application


application


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 cliente.g., Web server sends requested Web page, mail server delivers e-mail

request

reply

Lesson 3:Application Layer - 5 61

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


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”


Transport service requirements of common

appsApplication

file transfere-mail

Web documentsreal-time audio/video

stored audio/videointeractive games

financial apps

Data loss

no lossno lossloss-tolerantloss-tolerant

loss-tolerantloss-tolerantno loss

Bandwidth

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

Time Sensitive

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no


Services provided by Internet transport protocols

TCP service:• connection-oriented: setup

required between client, server

• reliable transport between sending and receiving process

• flow control: sender won’t overwhelm receiver

• congestion control: throttle sender when network overloaded

• does not provide: timing, minimum bandwidth guarantees

UDP service:• unreliable data

transfer between sending and receiving process

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

Q: why bother? Why is there a UDP?


Internet application protocols and corresponding

transport protocolsApplication

e-mailremote terminal access

Web file transfer

streaming multimedia

remote file server

Internet telephony

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

Underlyingtransport protocolTCPTCPTCPTCPTCP or UDP

TCP or UDPtypically UDP


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


DNS: Domain Name SystemApplication providing Mapping between IP addresses and domain namedistributed database implemented in hierarchy of many name serversapplication-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)

note: core Internet function implemented as application-layer protocolcomplexity at network’s “edge”


DNS name servers

Two types Name servers-Local name servers:

each ISP, company has local (default) name serverhost DNS query first goes to local name server

Authoritative name server:for a host: stores that host’s IP address, namecan perform name/address translation for that host’s name

Why not centralize DNS?single point of failuretraffic volumedistant centralized databaseMaintenancedoesn’t scale!

Hence, the distributed organization where server has all name-to-IP address mappings.


DNS: Root name servers

contacted by local name server that can not resolve nameroot name server:

contacts authoritative name server if name mapping not knowngets mappingreturns mapping to local name server

~ dozen root name servers worldwide


Simple DNS ScenarioHost 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.

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

authorititive name serverdns.umass.edu

local name serverdns.eurecom.fr

1

23

45

6


A More Complex DNS Scenario

Root name server:may not know authoratiative name server, butmay know intermediate name server: who to contact to find authoritative name server


gaia.cs.umass.edu

root name server


1

23

4 5

6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8


DNS: iterated queriesrecursive query:

puts burden of name resolution on contacted name serverheavy load?

iterated query:contacted server replies with name of server to contact“I don’t know this name, but ask this server”


gaia.cs.umass.edu

root name server


1

23

4

5 6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated query


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 2136http://www.ietf.org/html.charters/dnsind-charter.html


DNS recordsDNS: distributed db storing resource records (RR)

Type=NSname is domain (e.g. foo.com)value is IP address of authoritative name server for this domain

RR format: (name, value, type,ttl)

Type=Aname is hostnamevalue is IP address

Type=CNAMEname is an alias name for some “cannonical” (the real) namevalue is cannonical nameType=MXvalue is hostname of mail server associated with name


DNS protocol & messagesDNS 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


DNS protocol & messages (Continued)

Name, type fields for a query

RRs in reponseto query

records forauthoritative servers

additional “helpful”info that may be used


Electronic MailThree major components:

user agents mail servers simple mail transfer protocol: smtp

User Agenta.k.a. “mail reader”composing, editing, reading mail messagese.g., Eudora, Outlook, elm, Netscape Messengeroutgoing, incoming messages stored on server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


Electronic Mail: mail servers

Mail Servers mailbox contains incoming messages (yet to be read) for usermessage queue of outgoing (to be sent) mail messagessmtp protocol between mail servers to send email messages

client: sending mail server“server”: receiving mail server

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


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


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)


Sample smtp interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection


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


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!

• body– the “message”, ASCII

characters only

header

body

blankline


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 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

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data


MIME typesContent-Type: type/subtype;

parametersText

example subtypes: plain, html

Imageexample subtypes: jpeg, gif

Audioexampe subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding)

Videoexample subtypes: mpeg, quicktime

Applicationother data that must be processed by reader before “viewable”example subtypes: msword, octet-stream


Multipart TypeFrom: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789Content-Transfer-Encoding: quoted-printableContent-Type: text/plain

Dear Bob, Please find a picture of a crepe.--98766789Content-Transfer-Encoding: base64Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data --98766789--


Mail access protocols

• 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.

useragent

sender’s mail server

useragent

SMTP SMTP POP3 orIMAP

receiver’s mail server


POP3 protocolauthorization 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

C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

S: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK user successfully logged on


How POP3 Works?

Note : DNS name or IP address of ISP server is typically configured when email is set up.


POP3 versus IMAPPOP3 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: Summary and Follow-up

We had a High level view of network application protocols using

client server paradigmservice models

We learned about three of the most common application-level protocols

dnssmtppop

In the next class, we deal with three very popular application protocols

ftp httpMultimedia

92

Lesson 4: More Application Layer

Protocols

Lesson 4: More Application Layer Protocols - 1

93


Learn about the following popular application-level protocols

ftp http Multimedia


94

ftp: The file transfer protocol

transfer file to/from remote hostclient/server model

client: side that initiates transfer (either to/from remote)server: remote host

ftp: RFC 959ftp server: port 21

file transferFTP

server

FTPuser

interface

FTPclient

local filesystem

remote filesystem

user at host


95

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

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20


96

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

control channel• USER username• PASS password• dir/ls return list of files

in current directory

• Put filename retrieves (gets) file

• Get filename stores (puts) a local file on remote host

Sample return codes• 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


97

The Web: some jargonWeb page

consists of “objects”addressed by a URL

Most Web pages consist of:

base HTML page, andseveral referenced objects.

URL has three components: protocol, host name and path name:

User agent for Web is called a browser:

MS Internet ExplorerNetscape Communicator

Server for Web is called Web server:

Apache (public domain)MS Internet Information Server

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


98

The Web: the http protocolhttp: hypertext transfer

protocolWeb’s application layer protocolclient/server model

client: browser that requests, receives, “displays” Web objectsserver: Web server sends objects in response to requests

http1.0: RFC 1945http1.1: RFC 2068

PC runningExplorer

Server running

NCSA Webserver

Mac runningNavigator

http request

http re

quest

http response

http re

sponse

The Internet

DNS Server Ip request

Ip response


99

Navigation through The WebMultiple servers may come into playThe same client/server model

client: browser that requests, receives, “displays” Web objectsserver: Web server sends objects in response to requests

Browser determines URL and asks DNS for IP addressBrowser makes TCP connection on port 80

PC runningExplore

r

abc.com Webserver

http request ( following

hyperlink to abc.com)

htt

p r

equest

http response with a

page having hyperlink to XYZ.com

htt

p r

esp

onse

The Internet

XYZ.com Webserver

DNS Server Ip request

Ip response


100

More about the http protocol

http: TCP transport service:client initiates TCP connection (creates socket) to server, port 80server accepts TCP connection from clienthttp 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

Protocols that maintain “state” are complex!past history (state) must be maintainedif server/client crashes, their views of “state” may be inconsistent, must be reconciled

aside


101

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

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

time

(contains text, references to 10

jpeg images)


102

http example (cont.)

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 objects

4. http server closes TCP connection.

time


103

Non-persistent and persistent connections

Non-persistentHTTP/1.0server parses request, responds, and closes TCP connection2 Request-response messages to fetch each objectEach object transfer suffers from slow start

Persistentdefault for HTTP/1.1on 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.

But most 1.0 browsers useparallel TCP connections.


104

http message format: request

• two types of http messages: request, response

• http request message:– ASCII (human-readable format)

GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr

(extra carriage return, line feed)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message


105

http request message: general format

http Request Example

106Lesson 4: More Application Layer Protocols – 13.1


107

http message format: response

HTTP/1.0 200 OK Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ...

status line(protocol

status codestatus phrase)

header lines

data, e.g., requestedhtml file

http Response Example

108

Lesson 4: More Application Layer Protocols – 14.1


109

http response status codes

200 OK– request succeeded, requested object later in this

message

301 Moved Permanently– requested object moved, new location specified

later in this message (Location:)

400 Bad Request– request message not understood by server

404 Not Found– requested document not found on this server

505 HTTP Version Not Supported

In first line in server->client response message.

A few sample codes:


110

Trying out http (client side) for yourself

1. Telnet to your favorite Web server:

Opens TCP connection to port 80(default http server port) at www.eurecom.fr.Anything typed in sent to port 80 at www.eurecom.fr

telnet www.eurecom.fr 80

2. Type in a GET http request:GET /~ross/index.html HTTP/1.0 By typing this in (hit carriage

return twice), you sendthis minimal (but complete) GET request to http server

3. Look at response message sent by http server!


111

User-server interaction: authenticationAuthentication goal: control

access to server documentsstateless: client must present authorization in each requestauthorization: typically name, password

authorization: header line in requestif no authorization presented, server refuses access, sends

WWW authenticate:

header line in response

client server

usual http request msg401: authorization req.

WWW authenticate:

usual http request msg

+ Authorization:lineusual http response

msg


+ Authorization:lineusual http response

msgtime

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


112

User-server interaction: cookiesserver sends “cookie” to client in response must

Set-cookie: 1678453

client presents cookie in later requests

cookie: 1678453

server matches presented-cookie with server-stored info

authenticationremembering user preferences, previous choices

client server

usual http request msgusual http response

+Set-cookie: #


cookie: #usual http response

msg


cookie: #usual http response msg

cookie-spectificaction

cookie-spectificaction


113

User-server interaction: conditional GET

• Goal: don’t send object if client has up-to-date stored (cached) version

• client: specify date of cached copy in http requestIf-modified-since:

<date>

• server: response contains no object if cached copy up-to-date: HTTP/1.0 304 Not Modified

client server

http request msgIf-modified-since:

<date>

http responseHTTP/1.0

304 Not Modified

object not

modified

http request msgIf-modified-since:

<date>

http responseHTTP/1.1 200 OK

…

<data>

object modified


114

Web Caches (proxy 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

Goal: satisfy client request without involving origin server

client

Proxyserver

client

http request

http re

quest

http response

http re

sponse

http re

quest

http re

sponse

http requesthttp response

origin server

origin server


115

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– link out of

institutional/local ISP network often bottleneck

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache


116

Streaming Audio (Music on Demand)Some cases web-sever provides link to audio server. Media

player gets the file using Real-time Streaming Protocol (RTSP).


117

Media PlayerFunctions1. User Interface Management 2. Transmission error handling 3. Decompression of music 4. Elimination of jitter.

Lesson 4: More Application Layer Protocols -24

118

Media Player Function: Elimination of Jitter

Concept of push and pull media servers


119

Internet Radio


120

Internet Telephony

The ITU


121

H.323 Protocol StackRTP- Real-time Transport Protocol, RTCP- Real-time Transport Control Protocol, RAS- Registration/Admission/Status. H.245 channel is used to negotiate call parameters such as support for video or conference calls, Codecs supported, and so on.

Used for Congestion control

Allows terminals join and leave zones , request and return bandwidths and provide status updates.

G.711,

G.723.1,

etc.


122

Call Flow in H.323


123

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.


124

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.


125

Video on DemandHere 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.


126

Video-servers

TapeDVD

Magnetic Disk

RAM

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= ?


127


Revisiting the client-server paradigm, we dealt with three very popular application protocols

ftp httpMultimedia

Audio-serversH.323SIPVideo-on-Demand

Next we will take up how to program applications using transport layer services (i.e. TCP/UDP sockets)

128

Lesson 5: Writing Applications using Transport Layer

Facilities

Lesson 5: Writing Applications using Transport Layer Facilities-1

129


Learn about the usage of the following transport layer facilities for writing client-server applications

UDP socketsTCP sockets

Learn the difference between connection-oriented and connectionless transport layer services.


130

Socket programming

Socket API• introduced in BSD4.1

UNIX, 1981• explicitly created, used,

released by apps • client/server paradigm • two types of transport

service via socket API: – unreliable datagram – reliable, byte stream-

oriented

a 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

socket


131

Socket-programming using TCPSocket: a door between application process and

end-end-transport protocol (UDP or TCP)TCP service: reliable transfer of bytes from one

process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet


132

Socket programming with TCPClient 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

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint


133

Socket programming with TCPExample 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

client socket

inFromUser

ou

tToS

erv

er

iin

Fro

mS

er v

er


134

Client/server socket interaction: TCP

wait for incomingconnection request

Socket connectionSocket =welcomeSocket.accept()

create socket,port=x, forincoming request:

welcomeSocket =

ServerSocket()

create socket,connect to hostid, port=x

clientSocket = Socket()

connectionSocket.close()

read reply fromclientSocket

clientSocket.close()

Server (running on hostid) Client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

Unix 4.1c BSD: socket()

connect()

Unix 4.1c BSD: socket() bind() listen() accept()

InputStream Socket.getInputStream() OutputStream Socket.getOutputStream()


135

Example: Java TCP clientimport java.io.*; import java.net.*; class TCPClient {

public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence;

BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));

Socket clientSocket = new Socket("hostname", 6789);

DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

Createinput stream

Create client socket,

connect to server

Createoutput stream

attached to socket


136

Example: Java TCP client (cont.)

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine();

outToServer.writeBytes(sentence + '\n');

modifiedSentence = inFromServer.readLine();

System.out.println("FROM SERVER: " + modifiedSentence);

clientSocket.close(); } }

Createinput stream

attached to socket

Send lineto server

Read linefrom server


137

Example: Java server (TCP)import java.io.*; import java.net.*;

class TCPServer {

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()));

Createwelcoming socket

at port 6789

Wait, on welcomingsocket for contact

by client

Create inputstream, attached

to socket

Lesson 5: Writing Applications on Transport Layer Facilities-10

138

Example: Java TCP server (cont.)

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

capitalizedSentence = clientSentence.toUpperCase() + '\n';

outToClient.writeBytes(capitalizedSentence); } } }

Read in linefrom socket

Create outputstream,

attached to socket

Write out lineto socket

End of while loop,loop back and wait foranother client connection


139

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

UDP: transmitted data may be received out of order, or lost

application viewpoint

UDP provides unreliable transfer of groups of bytes (“datagrams”)

between client and server


140

Client/Server socket interaction: UDP

closeclientSocket

Server (running on hostid)

read reply fromclientSocket

create socket,clientSocket = DatagramSocket()

Client

Create, address (hostid, port=x,send datagram request using clientSocket

create socket,port=x, forincoming request:serverSocket = DatagramSocket()

read request fromserverSocket

write reply toserverSocketspecifying clienthost address,port umber

Unix 4.1c BSD: socket() bind() receivefrom()

Unix 4.1c BSD: socket() bind() sendto()


141

Example: Java client (UDP)import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine();

sendData = sentence.getBytes();

Createinput stream

Create client socket

Translate hostname to IP

address using DNS


142

Example: Java UDP client (cont.)

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }

}

Create datagram with data-to-send,

length, IP addr, port

Send datagramto server

Read datagramfrom server

Lesson 5: Writing Applications onTransport Layer Facilities-15

143

Example: Java server (UDP)

import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

serverSocket.receive(receivePacket);

Createdatagram socket

at port 9876

Create space forreceived datagram

Receivedatagra

m


144

Example: Java UDP server (cont)

String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase();

sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }

}

Get IP addrport #, of

sender

Write out datagramto socket

End of while loop,loop back and wait foranother datagram

Create datagramto send to client


145


In this class,Learned about the usage of the following transport layer facilities for writing application

UDP socketsTCP 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.

146

Lesson 6: Transport Layer

Transport Layer - 1 147

Lesson 6: Preview and Objectives

Overview of transport layer services:Multiplexing/de-multiplexingConnectionless 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 services and protocolsProvide logical

communication between app’ processes running on different hostsTransport protocols run in end systems Transport versus network layer services:

network layer: data transfer between end systemstransport layer: data transfer between processes

relies on, enhances, network layer services

application


application


networkdata linkphysical



networkdata linkphysicalnetwork

data linkphysical

logical end-end transport


Transport-layer ServicesInternet transport services:

Unreliable (“best-effort”), unordered unicast or multicast delivery (UDP)

Reliable, in-order unicast delivery (TCP)

congestion controlflow controlconnection setup

Services not available:

real-timebandwidth guaranteesreliable multicast

application


application





networkdata linkphysicalnetwork

data linkphysical

logical end-end transport

Transport Layer -4 150


M P2applicationtransportnetwork

Multiplexing/demultiplexingSegment - unit of data

exchanged between transport layer entities – aka TPDU: transport

protocol data unitreceiver

HtHn

Demultiplexing: delivering received segments to correct app layer processes

segment

segment Mapplicationtransportnetwork

P1M

M MP3 P4

segmentheader

application-layerdata


Multiplexing/Demultiplexing

multiplexing/demultiplexing:Based on sender, receiver port numbers, IP addresses

source, dest port #s in each segmentrecall: well-known port numbers for specific applications

Gathering data from multiple app processes, enveloping data with header (later used for demultiplexing)

source port # dest port #

32 bits

applicationdata

(message)

other header fields

TCP/UDP segment format

Multiplexing:


Multiplexing/Demultiplexing: examples

host A server Bsource port: xdest. port: 23

source port:23dest. port: x

port use: simple telnet app

Web clienthost A

Webserver B

Web clienthost C

Source IP: CDest IP: B

source port: x

dest. port: 80

Source IP: CDest IP: B

source port: y

dest. port: 80

port use: Web server

Source IP: ADest IP: B

source port: x

dest. port: 80


UDP: User Datagram Protocol [RFC 768]

“no frills,” “bare bones” Internet transport protocol“best effort” service, UDP segments may be:

lostdelivered out of order to app

connectionless:no handshaking between UDP sender, receivereach UDP segment handled independently of others

Why is there a UDP?no connection establishment (which can add delay)simple: no connection state at sender, receiversmall segment headerno congestion control: UDP can blast away as fast as desired


More on UDPOften used for streaming multimedia apps

loss tolerantrate sensitive

Other UDP uses (why?):

DNSSNMP

Reliable transfer over UDP: add reliability at application layer

application-specific error recovery!

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header


UDP checksum

Sender:Treat segment contents as sequence of 16-bit integersChecksum: addition (1’s complement sum) of segment contentsSender puts checksum value into UDP checksum field

Receiver:Compute checksum of received segmentCheck if computed checksum equals checksum field value:

NO - error detectedYES - no error detected. But maybe errors nonetheless? More later ….

Goal: detect “errors” (e.g., flipped bits) in transmitted segment


Principles of Reliable data transfer

Important in app., transport, link layersTop-10 list of important networking topics!

Characteristics of unreliable channel will determine complexity of reliable data transfer protocol (RDT)


Reliable data transfer: getting started

sendside

receiveside

rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer

udt_send(): called by rdt,to transfer packet over unreliable channel to

receiver

rdt_rcv(): called when packet arrives on rcv-side of channel

deliver_data(): called by rdt to deliver data to

upper


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

state1

state2

event causing state transitionactions taken on state transition

state: when in this “state”

next state uniquely

determined by next event

eventactions


Rdt1.0: reliable transfer over a reliable channel

underlying channel perfectly reliableno bit errorsno loss of packets

separate FSMs for sender, receiver:sender sends data into underlying channelreceiver read data from underlying channel


Rdt2.0: channel with bit errorsunderlying 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 OKnegative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errorssender retransmits pkt on receipt of NAKhuman scenarios using ACKs, NAKs?

new mechanisms in rdt2.0 (beyond rdt1.0):error detectionreceiver feedback: control msgs (ACK,NAK) rcvr->sender


rdt2.0: FSM specification

sender FSM receiver FSM


rdt2.0: in action (no errors)



rdt2.0: in action (error scenario)



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 pktsender retransmits current pkt if ACK/NAK garbledreceiver discards (doesn’t deliver up) duplicate pkt

Sender sends one packet, then waits for receiver response

stop and wait


rdt2.1: sender, handles garbled ACK/NAKs


rdt2.1: receiver, handles garbled ACK/NAKs


rdt2.1: discussion

Sender:seq # added to pkttwo seq. #’s (0,1) will suffice. Why?must check if received ACK/NAK corrupted twice as many states

state must “remember” whether “current” pkt has 0 or 1 seq. #

Receiver:must check if received packet is duplicate

state indicates whether 0 or 1 is expected pkt seq #

note: receiver can not know if its last ACK/NAK received OK at sender


rdt2.2: a NAK-free protocolsame functionality as rdt2.1, using ACKs onlyinstead of NAK, receiver sends ACK for the last packet received OK

receiver must explicitly include seq # of pkt being ACKed

duplicate ACK at sender results in same action as NAK: retransmit current pkt

senderFSM

!


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 retransmitsyuck: drawbacks?

Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this timeif pkt (or ACK) just delayed (not lost):

retransmission will be duplicate, but use of seq. #’s already handles thisreceiver must specify seq # of pkt being ACKed

requires countdown timer


rdt3.0 sender


rdt3.0 in action


rdt3.0 in action


Performance of rdt3.0

• rdt3.0 works, but performance stinks• example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet:

Ttransmit=8kb/pkt

10**9 b/sec= 8 microsec

Utilization = U = =8 microsec

30.016 msecfraction of time

sender busy sending = 0.00015

– 1KB pkt every 30 msec -> 33kB/sec throughput over 1 Gbps link

– network protocol limits use of physical resources!



We had an overview of transport layer services:Multiplexing/de-multiplexingConnectionless 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.

175

Lesson 7: TCP

Lesson 7: TCP- 1 176

Lesson 7- TCP: Preview/Objectives

TCP Segment (Message) FormatStudy of Connection-oriented data transport (TCP) with facilities for:

Connection ManagementReliable data transfer with one of the two usual methods:

Go back to NSelective Repeat

Flow ControlCongestion Control

Lesson 7: TCP - 2 177

TCP Segment (Message) Structure


TCP Connection Management with 3-way

Handshake


TCP Connection Closing Sequence


TCP Connection Management- Client Side

State Transitions

CLOSING

Receive FIN/ Send ACK

Receive ACK/ Send Nothing

Receive FIN & ACK/ Send ACK

Sharp lines depict unusual states and transitions.


TCP Connection Management- Server Side

State Transitions

SYN_SENT

Send SYN

Receive SYN/ Send SYN &ACK (Simultaneous open)

Receive RST/ Send Nothing

Sharp lines depict unusual states and transitions.


States of The TCP Connection Management

FSM


Pipelined protocolsPipelining: sender allows multiple, “in-

flight”, yet-to-be-acknowledged pktsrange of sequence numbers must be increasedbuffering at sender and/or receiver

Two generic forms of pipelined protocols: go-Back-N, selective repeat


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 sizeWhile 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.


Go-Back-NSender:

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 pkttimeout(n): retransmit pkt n and all higher seq # pkts in window


GBN: sender extended FSM


GBN: receiver extended FSM

receiver simple:ACK-only: always send ACK for correctly-received pkt with highest in-order seq #

may generate duplicate ACKsneed only remember expectedseqnum

out-of-order pkt: discard (don’t buffer) -> no receiver buffering!ACK pkt with highest in-order seq #


GBN inaction


Maximum Window Size

The sequence number dilemmaEach 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 !!


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 0Receiver sends Ack with expected seq.#1Sender sends frames 1, 2, 3, 4, 5, 6, 7, 0Receiver sends Ack with expected seq.#1Sender receives Ack with seq.#1 and cannot decide whether all frames have been received correctly or all are lost in transit.


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 windowN consecutive seq #’sagain limits seq #s of sent, unACKed pkts


Selective repeat: sender, receiver windows


Selective repeat

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 receivedif n smallest unACKed pkt, advance window base to next unACKed seq #

sender pkt n in [rcvbase, rcvbase+N-1]

send ACK(n)out-of-order: bufferin-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt

pkt n in [rcvbase-N,rcvbase-1]

ACK(n)

otherwise: ignore

receiver


Selective repeat in action


Selective repeat:dilemmaExample:

seq #’s: 0, 1, 2, 3window 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?


Complementary ProblemConsider the following example:

Assume a 3-bit sequence numberSender transmits segments 0-6 to the receiverReceiver gets all the segments in good shape and acknowledges with expected Seq.# 7.Now, lightning strikes and all Acks are lostSender times out and retransmits segment 0The 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.


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.


Reliable Data Transfer Protocols- A Comparative

StudyStop-and-Wait Protocol

Simple, but performance leaves much to be desired!

Go-Back-NBetter performance, but more complicated. Possibly wasteful if large blocks of packets need to be retransmitted

Selective RepeatA pain to implement – needs multiple timers, but better performance through individual packet management


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?


Flow Control in TCP

RcvWindow = RcvBuffer – [LastByteRcvd – LastByteRead] LastByteSent – LastByteAcked <= RcvWindow Possible Blocking @ Sender -> TCP Solution?


Silly Window Syndrome

Sender is slow- Sends a byte at a timeNetwork bandwidth badly usedNagle’s algorithm- Wait, bunch and sendAdvisable 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


General Congestion Control Mechanisms

End to End Congestion ControlNetwork-assisted Congestion Control

Direct feedback from router with a choke packetRouter 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.


Congestion Control in TCPThree components of TCP congestion control algorithm

Additive Increase Multiplicative Decrease

Slow start

Reaction to timeout events


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 FSMsReliable data transfer with one of the two usual methods:

Go back to NSelective Repeat

Flow Control with RcvWindow information3 features of TCP Congestion Control Mechanism .

Next class, we proceed on to the Network Layer.

206

Lesson 8: Introduction to Network Layer

Lesson 8: Introduction to Network Layer - 1

207

Lesson 8- Introduction to Network Layer:

Preview/ObjectivesOverview of network layer functionsForwardingRoutingCall setup (sometimes)

Network Models- Virtual Circuits versus Datagram NetworksRouting Algorithms

Desirable CharacteristicsClassificationDifferent known typesOverview of graph theory based algorithms


208

Network layer functions

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.

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.

Call setup: some network architectures require router call setup along path before data flows









application


application


Network layer protocols exist in every switch whether host (end system) or router (intermediate switch).


209

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?

? ??virtual circuit

or datagram?

The most important abstraction provided

by network layer:

serv

ice a

bst

ract

ion


210

Virtual circuits

call setup for each call before data can flow and teardowneach 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.

“source-to-dest path behaves much like telephone circuit”

performance-wisenetwork actions along source-to-destination path


211

Virtual circuits: signaling protocols

used to setup, maintain teardown VCused in ATM, frame-relay, X.25not used in today’s Internet

application


application


1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data


212

Datagram networks: the Internet model

no call setup at network layerrouters: no state about end-to-end connections

no network-level concept of “connection”

packets typically routed using destination host IDpackets between same source-dest pair may take different paths

application


application


1. Send data 2. Receive data


213

Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

Internet model being extented: Intserv, Diffserv


214

Datagram or VC network: why?Internet

data exchange among computers

“elastic” service, no strict timing req.

“smart” end systems (computers)

can adapt, perform control, error recoverysimple inside network, complexity at “edge”

many link types different characteristicsuniform service difficult

ATMevolved from telephonyhuman conversation:

strict timing, reliability requirementsneed for guaranteed service

“dumb” end systemstelephonescomplexity inside network


215

RoutingThe 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:

correctnesssimplicityrobustnessstabilityfairness


216

Routing (Continued)Most important:

optimalityefficiency

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:

delaycostthroughput


217

Routing InformationRouting decisions are generally based on some knowledge of the state of the network.

Delay on certain linksCost through certain nodesPacket lossetc.

This information may have to be dynamically collected. This leads to overhead which in turn reduces the utilization.


218

Routing Algorithms

Graph abstraction for routing algorithms:graph nodes are routersgraph edges are physical links

link cost: delay, $ cost, or congestion level

Goal: determine “good” path

(sequence of routers) thru network from source to

dest.

Routing Algorithm

A

ED

CB

F

2

2

13

1

1

2

53

5

“good” path:typically means minimum cost pathother definitions possible


219

Routing Algorithm classification

Global or decentralized information?

Global:all routers have complete topology, link cost infoExample: “link state” algorithms

Decentralized: router knows physically-connected neighbors, link costs to neighborsiterative process of computation, exchange of info with neighborsExample: “distance vector” algorithms

Static or dynamic?Static:

routes change slowly over time

Dynamic: routes change more quickly

Proactive (periodic update)Reactive (in response to link cost changes)


220

Different Types of RoutingFixed Routing:

Static Routing Tables, Pre-computed Routes

Flooding:Simple but inefficient! WHY?

Hot Potato RoutingSimple, not very efficient, unpredictable

Random RoutingSimple, unpredictable, statistically fair (locally)

Adaptive Routingsophisticated, expensive, efficient, complex...


221

Random RoutingSometimes called probabilistic routing!Here, the probability of a packet being forwarded on a particular link is a function of conditions on this link.

– Pi = Probability of link i being selected

– Ri = Data rate on link i

j j

ii R

RP


222

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 ?


223

Adaptive RoutingAdaptive Routing Techniques are used in almost all packet-switching networks.

ARPANET

Routing decisions change in response to changes in the network.

Network FailureCongestion

Adaptive routing strategies can improve performance.Adaptive routing strategies can aid congestion control.


224

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 measurementsadjacent nodesall nodes in the network


225

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, actualcost, delay, etc.)

D

FE

A

B

C

4

9

12

1

6

3


226

Some of the established shortest-path algorithms in traditional graph theory are:

Dijkstra’s shortest path algorithmBellman-Ford AlgorithmFloyd-Warshall Algorithm

The main difference between the algorithms is the type of augmentation through each iteration.

Dijkstra: nodesBellman-Ford: number of arcs (links) in the pathFloyd-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.


227

Lesson 8- Introduction to Network Layer: Summary and Follow-up

We had an overview of network layer functionsForwardingRoutingCall 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 NetworksWe looked into the following aspects of Routing Algorithms

Desirable CharacteristicsClassificationDifferent known typesOverview of graph theory based algorithms

In the next class, we study in detail some of the shortest path routing algorithms.

228

Lesson 9: Routing Algorithms for Network Layer

Lesson 9: Routing Algorithms for Network Layer - 1

229

Lesson 9: Routing Algorithms for Network Layer- Preview/Objectives

We study two routing algorithmsDikstra’s link State algorithmDistance vector (Bellman Ford) algorithm

We work out examples

We discuss the count-to-infinity problem


230

A Link-State Routing AlgorithmDijkstra’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 nodeiterative: 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


231

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


232

Dijkstra’s Algorithm: An Example

Step012345

start NA

ADADE

ADEBADEBC

ADEBCF

D(B),p(B)2,A2,A2,A

D(C),p(C)5,A4,D3,E3,E

D(D),p(D)1,A

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F

2

2

13

1

1

2

53

5


233

A Discussion on Dijkstra’s algorithmAlgorithm 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

A

D

C

B1 1+e

e0

e

1 1

0 0

A

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

initially … recomputerouting

… recompute … recompute


234

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 destinationAll cycles not containing the destination have nonnegative length (cost).


235

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?


236

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!


237

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 directly-attached 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:

D (Y,Z)X

distance from X toY, via Z as next hop

c(X,Z) + min {D (Y,w)}Z

w

=

=


238

Distance Table: An Example

A

E D

CB7

8

1

2

1

2 D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

Des

tinat

ionD (C,D)

Ec(E,D) + min {D (C,w)}

Dw=

= 2+2 = 4

D (A,D)E

c(E,D) + min {D (A,w)}D

w== 2+3 = 5

D (A,B)E

c(E,B) + min {D (A,w)}B

w== 8+6 = 14

loop!

loop!


239

Distance table gives routing table

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

A

B

C

D

A,1

D,5

D,4

D,2

Outgoing link to use, cost

dest

inat

ion

Distance table Routing table


240

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– neighbors then notify their

neighbors if necessary

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

Each node:


241

Distance Vector Algorithm

1 Initialization:

2 for all adjacent nodes v:

3 D (*,v) = infty /* the * operator means "for all rows" */

4 D (v,v) = c(X,v)

5 for all destinations, y

6 send min D (y,w) to each neighbor /* w over all X's neighbors */

X

X

Xw

At all nodes, X:


242

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 if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D (y,V) = D (y,V) + d 16

17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval 22

23 if we have a new min D (Y,w) for any destination Y 24 send new value of min D (Y,w) to all neighbors 25

26 forever

w

XX

X

X

X

w

w


243

Distance Vector Algorithm: An Example

X Z12

7

Y


244

Distance Vector Algorithm: example (contd.)

X Z12

7

Y

D (Y,Z)X

c(X,Z) + min {D (Y,w)}w=

= 7+1 = 8

Z

D (Z,Y)X

c(X,Y) + min {D (Z,w)}w=

= 2+1 = 3

Y


245

Distance Vector: link cost changesLink 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)

X Z14

50

Y1

algorithmterminates“good

news travelsfast”


246

Distance Vector: link cost changes

Link cost changes:• good news travels

fast • bad news travels

slow - “count to infinity” problem!

X Z14

50

Y60

algorithmcontinues

on!


247

Distance Vector: poisoned reverseIf 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?

X Z14

50

Y60

algorithmterminates


248

Comparison of LS and DV algorithmsMessage 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


249

Lesson 9: Routing Algorithms for Network Layer- Summary

and Follow-upWe studied two routing algorithms

Dikstra’s link State algorithmDistance 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

250

Lesson 10: IP & The Internet

Lesson 10: IP & The Internet - 1 251

Lesson 10: IP & The Internet- Preview/Objectives

We see how the Internet- the network of networks works

Study the IP message and address structures

We study a number of Protocols & AlgorithmsICMPARP & RARP/BOOTP/DHCPRIP /OSPF & BGP

We discuss how the count-to-infinity problem is addressed in the BGP.


The Internet


How Internet Handles Traffic Flow through Different

Networks?


The IP Message FormatHeader Length in 32-bit words

Don’t Fragment (e.g. Memory Image)

More Fragments (All but the last have it !)

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)

Originally had Delay, Throughput and

Reliability flags. Now it has 4 queuing priority classes, 3 discard probabilities and historical service classes.

Tells whether to give the datagram to TCP or UDP or some other process.

Tells to which datagram the newly arrived fragment belongs.


The IP Address Formats


Reserved IP Addresses


The Internet Network layer

routingtable

Host, router network layer functions:

Routing protocols•path selection•ARP, RARP/BOOTP/ DHCP•RIP/OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer


The Internet Control Message Protocol

Each ICMP message is encapsulated in an IP packet

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.


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.


DHCP


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).


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).


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


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- 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 structuresWe studied a number of Protocols & Algorithms

ICMPARP & RARP/BOOTP/DHCPRIP /OSPF & BGP

We discussed how the count-to-infinity problem is addressed in the BGP.

Next class, we proceed on to Data-link layer.

267

Lesson 11: Introduction to Data

Link Layer

Lesson 11: Introduction to Data Link Layer - 1

268

Lesson 11: Introduction to Data Link Layer -Preview/ObjectivesWe study the principles behind various link layer services such as

Error Detection and correctionMultiple 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


269

Link Layer: Setting the Context


270

Link Layer & Data Link Protocol

• two physically connected devices:– host-router, router-router, host-host

• unit of data: frame


linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card


271

Link Layer ServicesFraming and link access:

encapsulate datagram into frame, adding header, trailerimplement 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?


272

More Link Layer ServicesFlow 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


273

Link Layer: Implementationimplemented in “adapter”

e.g., PCMCIA card, Ethernet card typically includes: RAM, DSP chips, host bus interface, and link interface


linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card


274

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 layerTransport layer relies on this and manages with simpler Check Sum.


275

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 standards8-bit G used for protecting 5-byte ATM headers

GCRC-32 = 100000100110000010001110110110111


276

More About CRCCRC is also known as polynomial codeCRC 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.


277

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)


278

Multiple Access protocolssingle 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 transmitcommunication 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


279

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 Accessallow collisions“recover” from collisions

“Taking turns”tightly coordinate shared access to avoid collisionsGoal: efficient, fair, simple, decentralized


280

Random Access protocolsWhen 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 collisionshow to recover from collisions (e.g., via delayed retransmissions)

Examples of random access MAC protocols:slotted ALOHAALOHACSMA and CSMA/CD


281

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.


282

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]


283

Slotted ALOHA

Like Pure-ALOHA with additional requirements:

The channel is slotted in timeEach user is required to synchronize the start of packet transmission to coincide with the slot boundary (only complete collision would occur, avoid partial collision)


284

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


285

Limit on the Slotted Aloha efficiencyQ: 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 ...

= 1/e = .37 as N -> infinity (we will see in the next slide)

At best: channeluse for useful transmissions 37%of time!


286

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)

Find the maximum value of S using the established formula:

Solution: Setting ds/dp = 0, we get,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

eNLim

N

N

111


287

Pure & Slotted Aloha Efficiency LimitsP(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

S =

thro

ughput

=

“goodput”

(

succ

ess

rate

)

G = offered load = Np0.5 1.0 1.5 2.0

0.1

0.2

0.3

0.4

Pure Aloha

Slotted Aloha protocol constrainseffective channelthroughput!


288

Lesson 11: Introduction to Data Link Layer –Summary & Follow-up

We studied the principles behind various link layer services e.g.

Error Detection and correctionMultiple 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 AddressingReliable Data Transfer & Flow control (already done in the TCP class)

We studied and analyzed Pure and Slotted ALOHA Protocols- precursors of CSMA/CD.

Next class, we proceed on to Link layer technologies and study CSMA/CD, Ethernet and other protocols & Technologies.

289

Lesson 12: Link Layer Technologies

Lesson 12: Link Layer Technologies - 1

290

Lesson 12: Link Layer Technologies-Preview/Objectives

We study specific link layer technologies and their implementation

Current Multiple Access MAC (Medium Access Control) Protocols-

CSMA/CDChannel Partitioning“Taking Turns” type – Token Ring

Ethernet Hubs, Bridges and SwitchesPPPATMIEEE 802.11 LANs


291

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.


292

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.


293

CSMA collisions

Collisions can occur:Propagation delay means two nodes may not yethear each other’s transmissionCollision:Entire packet transmission time wasted

spatial layout of nodes along ethernet

Note:Role of distance and propagation delay in determining collision probability.


294

CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMAcollisions detected within short timecolliding transmissions aborted, reducing channel wastage persistent or non-persistent retransmission

Collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signalsdifficult in wireless LANs: receiver shut off while transmitting

Same human analogy of the polite conversationalist


295

IEEE 802.3 CSMA/CDUses 1-persistent CSMA algorithm.Rules:

if the channel is idle then transmitif the channel is busy, then continue to listen until idle then transmit immediatelyif 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


296

CSMA/CD Collision Detection


297

“Taking Turns” MAC protocols

Channel partitioning MAC protocols:share channel efficiently at high loadinefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!

Random access MAC protocolsefficient at low load: single node can fully utilize channelhigh load: collision overhead

“Taking turns” protocolslook for best of both worlds!


298

“Taking Turns” MAC protocolsPolling:

• master node “invites” slave nodes to transmit in turn

• Request to Send, Clear to Send messages

• concerns:– polling overhead – latency– single point of failure

(master)

Token passing:control token passed from one node to next sequentially.token messageconcerns:

token overhead latencysingle point of failure (token)


299

Reservation-based protocolsDistributed Polling:

time divided into slotsbegins with N short reservation slots

reservation slot time equal to channel end-end propagation delay station with message to send posts reservationreservation seen by all stations

after reservation slots, message transmissions

ordered by known priority


300

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/CDcarrier sensing: easy in some technoligies (wire), hard in others (wireless)CSMA/CD used in Ethernet

Taking Turnspolling from a central cite, token passing


301

Ethernet“Dominant” LAN technology:

Cheap $20 for 100Mbs!First wildey used LAN technologySimpler, cheaper than token LANs and ATMKept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Etheretsketch


302

Ethernet Frame StructureSending 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 ratesLast two 11’s of the 8th for alerting about something important to come.


303

Ethernet Frame Structure (Continued)

Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not matchType: 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

46-1500 bytes

6 bytes 6 bytes 4 bytes2 bytes8 bytes


304

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}


305

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}


306

Ethernet Technologies: 10Base2


307

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


308

More on10BaseT and 100BaseT

Max distance from node to Hub is 100 metersHub can disconnect “jabbering adapterHub can gather monitoring information, statistics for display to LAN administrators


309

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


310

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)


311

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?


312

PPP- State Model PPP’s Link Control Protocol (LCP) manages the states.

Configure-request frame (a PPP Frame with protocol set to LCP value- Co21) and configure-ack/configure-nak/ configure-reject responses received.

Physical layer presence indicated by carrier detection or admin action

Terminate request and ACK exchange


313

ATMAAL1- 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.


314

AAL5 PDU


315

ATM Cell Header VCI- Virtual circuit identifier

PT- payload type

CLP- Cell Priority Bit

HEC- Header Error Control


316

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


317

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


318

IEEE 802.11 Standards

802.11b 2.4-2.485 GHZ up to 11 Mbps

Standard Frequency Range Data Range

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 -30

319

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 -31

320

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 transmission and ACK.


321

Lesson 12: Link Layer Technologies-Summary/Follow-

upWe studied the following link layer technologies and their implementation

Current Multiple Access MAC (Medium Access Control) Protocols-

CSMA/CDChannel Partitioning“Taking Turns” type – Token Ring

Ethernet Hubs, Bridges and SwitchesPPPATMIEEE 802.11 LANs

Next class, we take up Physical Layer

322

Lesson 13: Introduction to Physical Layer

Lesson 13: Introduction to Physical Layer -1

323

Lesson 13: Introduction to Physical Layer

-Preview/ObjectivesWe study physical layer functionality and 3 types of transmission

Simplex Half DuplexFull Duplex

Signals and their propertiesRelation between bandwidth and data rate

Lesson 13: Introduction to Physical Layer - 2

324

Physical LayerPhysical layer is concerned with data transmissionData 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.


325

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:


Data Transmission- Some Terminology


326

Transmission in only one direction; one station is the transmitter, the other the receiver. Examples:

One-Way StreetKeyboard-Computer connectionComputer-Monitor connectionTV BroadcastCan you think of other simplex examples?

Simplex Transmission


327

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-TalkiesCB-RadiosTraditional Ethernet (Coax or 10baseT)

Half-Duplex Transmission


328

Transmission in both directions simultaneously. Both stations can send and receive at the same time. Examples:

Regular 2-way streetFull-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.

Full Duplex Transmission


329

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


330

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!

Continuous and Discrete Signals


331

The simplest sort of signal is a periodic signal.

Here, T is said to be the period. T is the smallest value that satisfies the equation.

Definition: a signal s(t) is periodic if and only if

ttsTts )()(

Periodic Signal


332

The sine wave is the fundamental continuous signal. We can represent the sine wave by 3 parameters:

Amplitude (A)Frequency (f)Phase ()

)ftsin(2As(t)

Sinusoid- The Fundamental

Continuous Signal


333

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.

Amplitude, Frequency and Phase


334

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.

Here, v represents the velocity of the signal.

vT

Wavelength of a Signal


335

The Frequency-Domain Concept allows us to represent a signal as the sum of constituent frequencies. For example:

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.

s(t) = sin(2f1t) + 1/3 sin(2(3f1)t)

Frequency Domain Representation of

Signals


336

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.

Frequency Domain Analysis


337

BandwidthPhysical property of the transmission mediumDepends on length, thickness, construction, etc.Range of frequencies transmitted without being strongly attenuatedIn 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.


338

Fourier Analysis- An Overview

Any reasonably behaved periodic signal can be expressed as a sum (possibly infinite) of sines and cosines as follows:

Sine and cosine term pair for a value of n is called nth harmonic.Root Mean Square (RMS) amplitude √an

2+bn2

indicates the significance of the nth harmonic.

s(t)=c/2 + Σn=1 to

∞ansin(2nft)

+ Σn=1 to

∞bncos(2nft)


339

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 -18

340

Lesson 13: Introduction to Physical Layer –Summary and

Follow-upWe studied physical layer functionality and 3 types of transmission


We studied Signals and their properties (particularly Fourier Analysis)Relation between bandwidth and data rateNext class, we study about wireless access technologies.

341

Lesson 14: Physical Layer (Wireless Access)

Lesson 14: Physical Layer (Wireless Access) - 1

342

Lesson 14: Physical Layer (Wireless

Access)-Preview/ObjectivesWe study in this lessonTwo 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, EvolutionaryTechnologies- FDMA (e.g. AMPS), TDMA (e.g. GSM), CDMA (e.g IS-95/CDMA-2000), WCDMAMobility Management


343

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 hand-over/hand-off is used.

A B C D

BSC-X

MSC or PDSN/GGSN

BSC-Y

RNC in UMTS jargon


344

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.


345

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)


346

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.


347

How do you do & Undo?


348

Spreading Example


349

De-spreading (Recovery of Previously Spread Data) for the

same Example


350

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)


351

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


352

Lesson 14: Physical Layer (Wireless

Access)-Summary/Follow-upWe studied in this lessonTwo kinds of wireless access

Fixed (e.g. fixed wireless systems using traditional mobile access technologies, wi-fi)Mobile

Mobile Access:Generations 1-3Technologies- FDMA (e.g. AMPS), TDMA (e.g. GSM), CDMA (e.g IS-95/CDMA-2000), WCDMAMobility Management

353

Lesson 15: Introduction to Network Security

Lesson 15: Introduction to Network Security - 1

354

Lesson 15: Introduction Network Security-Preview/Objectives

We study in this lessonWhat is security? What all it entails?

CryptographyAuthenticationMessage Integrity

Types of Keys for encryption, their distribution and certificationFamous Public Key Algorithm (RSA)


355

Friends and enemies: Alice, Bob, Trudy

Well-known in network security worldBob, Alice (close friends) want to communicate “securely”Trudy, the “intruder” may intercept, delete, add messages

Figure 7.1 goes here


356

What is network security?

Secrecy: only sender, intended receiver should “understand” message contents

sender encrypts messagereceiver 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


357

Internet security threats I- Packet Sniffing

Packet sniffing is possible because the media is broadcast typepromiscuous NIC reads all packets passing byany one can read all unencrypted data (e.g. passwords) e.g.: C sniffs B’s packets

A

B

C

src:B dest:A payload


358

Internet security threats II- IP SpoofingIP 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.

A

B

C

src:B dest:A payload


359

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

A

B

C

SYN

SYNSYNSYN

SYN

SYN

SYN


360

Jargon of cryptography

symmetric key crypto: sender, receiver keys identical

public-key crypto: encrypt key public, decrypt key secret


plaintext plaintext

ciphertext

KA

KB


361

Symmetric key cryptographySubstitution cipher: substituting one thing for

anothermonoalphabetic cipher: substitute one letter for another

plaintext: abcdefghijklmnopqrstuvwxyz

ciphertext: mnbvcxzasdfghjklpoiuytrewq

Plaintext: bob. i love you. aliceciphertext: nkn. s gktc wky. mgsbc

E.g.:

Q: How hard to break this simple cipher?:brute force (how hard?)other?


362

Symmetric key crypto: DESDES: Data Encryption Standard

US encryption standard [NIST 1993]56-bit symmetric key, 64 bit plaintext inputHow secure is DES?

DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 monthsno known “backdoor” decryption approach

making DES more secureuse three keys sequentially (3-DES) on each datumuse cipher-block chaining


363

Symmetric key

crypto: DES

initial permutation 16 identical “rounds” of

function application, each using different 48 bits of key

final permutation

DES operation


364

Public Key Cryptographysymmetric key

cryptorequires sender, receiver know shared secret keyQ: how to agree on key in first place (particularly if never “met”)?

public key cryptographyradically different approach [Diffie-Hellman76, RSA78]sender, receiver do not share secret keyencryption key public (known to all) decryption key private (known only to receiver)


365

Public key cryptography



366

Public key encryption algorithms

need d ( ) and e ( ) such that

d (e (m)) = m BB

B B

need private and public keys

for d ( ) and e ( ), respectively

BB

Two inter-related requirements:

1

2

RSA: Rivest, Shamir, Adelson algorithm


367

RSA: Encryption, decryption0. Given (n,e) and (n,d) as computed above

1. To encrypt bit pattern, m, compute

c = m mod n

e (i.e., remainder when m is divided by n)e

2. To decrypt received bit pattern, c, compute

m = c mod n

d (i.e., remainder when c is divided by n)d

m = (m mod n)

e mod n

dMagichappens!


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RSA: Choosing keys1. 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).


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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.

m me c = m mod ne

12 248832 17

c m = c mod nd

17 481968572106750915091411825223072000 12

cd

encrypt:

decrypt:


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RSA: How does it work?m = (m mod

n)

e mod n

d

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.

To prove: , we use two theorems:

(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 TheoremIn 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).


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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.


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Lesson 15: Introduction to Network

Security-Summary/Follow-upWe studied in this lessonWhat is security? What all it entails?

CryptographyAuthenticationMessage Integrity

Types of Keys for encryption, their distribution and certificationFamous Public Key Algorithm (RSA)

In the next class, we take up other security issues (e.g. authentication) and some applications.

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Lesson 16: Network Security

(Continued)

Lesson 16: Network Security (Continued) - 1

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Lesson 16: Network Security (Continued)-Preview/Objectives

We study in this lessonA more detailed view of the following security features:

AuthenticationMessage IntegrityKey distribution and certification

Security in practice:Application layer: secure e-mailTransport layer: Internet commerce, SSL, SET Network layer: IP security


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Authentication

Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0: Alice says “I am Alice”

Failure scenario??


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Authentication: another try

Protocol ap2.0: Alice says “I am Alice” and sends her IP address along to “prove” it.

Failure scenario??


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Authentication: another try

Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.

Failure scenario?


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Authentication: yet another try

Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.

Failure scenario?

I am Aliceencrypt(password)


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Authentication: yet another tryGoal: avoid playback attack

Failures, drawbacks?


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


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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


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ap5.0: security holeMan (woman) in the middle attack:

Trudy poses as Alice (to Bob) and as Bob (to Alice)

Need “certified” public keys (more later …)


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Digital Signatures

Cryptographic technique analogous to hand-written 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.


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More on Digital Signatures

• Suppose Alice receives msg m, and digital signature dB(m)

• Alice verifies m signed by Bob by applying Bob’s public key eB to dB(m) then checks eB(dB(m) ) = m.

• If eB(dB(m) ) = m, whoever signed m must have used Bob’s private key.

Alice thus verifies that:– Bob signed m.– No one else signed

m.– Bob signed m and

not m’.Non-repudiation:

– Alice can take m, and signature dB(m) to court and prove that Bob signed m.


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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’).


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Digital signature = Signed message digest

Bob sends digitally signed message:

Alice verifies signature and integrity of digitally signed message:


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Hash Function Algorithms• Internet

checksum would make a poor message digest.– Too easy to find

two messages with same checksum.

• MD5 hash function widely used. – Computes 128-bit

message digest in 4-step process.

– arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x.

• SHA-1 is also used.– US standard– 160-bit message digest


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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)


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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 KB-

KDC(A,R1)

• Alice sends Bob KB-KDC(A,R1), Bob extracts R1

• Alice, Bob now share the symmetric key R1.


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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


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Secure e-mail

• 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.

• Alice wants to send secret e-mail message, m, to Bob.


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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.


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Secure e-mail (continued)

• Alice wants to provide secrecy, sender authentication, message integrity.

Note: Alice uses both her private key, Bob’s public key.


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Pretty good privacy (PGP)

• Internet e-mail encryption scheme, a de-facto standard.

• Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.

• Provides secrecy, sender authentication, integrity.

• Inventor, Phil Zimmerman, was target of 3-year federal investigation.

---BEGIN PGP SIGNED MESSAGE---Hash: SHA1

Bob:My husband is out of town tonight.Passionately yours, Alice

---BEGIN PGP SIGNATURE---Version: PGP 5.0Charset: noconvyhHJRHhGJGhgg/

12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2

---END PGP SIGNATURE---

A PGP signed message:


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Secure sockets layer (SSL)

• PGP provides security for a specific network app.

• SSL works at transport layer. Provides security to any TCP-based app using SSL services.

• SSL: used between WWW browsers, servers for I-commerce (shttp).

• SSL security services:– server authentication– data encryption – client authentication

(optional)

• 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.


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SSL (continued)

Encrypted SSL session:• Browser generates

symmetric session key, encrypts it with server’s public key, sends encrypted key to server.

• Using its private key, server decrypts session key.

• Browser, server agree that future messages will be encrypted.

• All data sent into TCP socket (by client or server) is encrypted with session key.

• SSL: basis of IETF Transport Layer Security (TLS).

• SSL can be used for non-Web applications, e.g., IMAP.

• Client authentication can be done with client certificates.


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Secure electronic transactions (SET)• designed for payment-card

transactions over Internet.• provides security services

among 3 players:– customer– merchant– merchant’s bankAll 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


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IPSEC: Network Layer Security• Network-layer secrecy:

– sending host encrypts the data in IP datagram

– TCP and UDP segments; ICMP and SNMP messages.

• Network-layer authentication– destination host can

authenticate source IP address

• Two principle protocols:– authentication header (AH)

protocol– encapsulation security

payload (ESP) protocol

• For both AH and ESP, source, destination handshake:– create network-layer

logical channel called a service agreement (SA)

• Each SA unidirectional.• Uniquely determined by:

– security protocol (AH or ESP)

– source IP address– 32-bit connection ID


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ESP Protocol• Provides secrecy, host

authentication, data integrity.

• Data, ESP trailer encrypted.• Next header field is in ESP

trailer.

• ESP authentication field is similar to AH authentication field.

• Protocol = 50.


399

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.)


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Lesson 16: Network Security (Continued)-Summary/Follow-up

We studied in this lessonA more detailed view of the following security features:

AuthenticationMessage IntegrityKey distribution and certification

Application of those security features in practice:

Application layer: secure e-mailTransport layer: Internet commerce, SSL, SET Network layer: IP security (IPSec)

Documents

Computer Networks (EENG 4810)