computer networks dclab-ghcho 2000 fall 1 course syllabus (i) lecturer : gihwan cho office : room...

Computer Networks DCLAB-ghcho 2000 Fall

1

Course Syllabus (I) Lecturer : Gihwan Cho

office : room 607 (voice 3437) [email protected]

This material is based on: Andrew S. Tannenbaum, Computer Networks, Third Edition,

Prentice Hall, ISBN 0-13-349945-6 D. Comer, Internetworking with TCP/IP, Third Edition, Prenti

ce Hall, ISBN 0-13-227836-7 Course objectives

understanding of the basic principles of computer networks, Internet and its protocols

understanding in detail of the main subjects of computer networks, Internet and its protocols


2

Courses Syllabus (II) Course outline

lecturer will present the basic principles of computer networks, Internet and its protocols

the lecture will suggest some of main subjects in computer network (some of these may be proposed by yourself)

then, you are planed to choose an identified subject, and work elaborately for it, and finally be taken a presentation for the identified subject

Expected works 1 presentation for 1 subject 2 examinations (mid, final)

Lecture information cs.chonbuk.ac.kr/~ghcho/courses/netsem.html


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Courses Syllabus (III) Policy on the presentation

is given I hour presentation time per person is recommended to use about 30 slides in ppt format is checked mainly how much you are understand for the sele

cted subject the presentation material should be send to lecture at least 1

2 hour before you take it (if not, …) if you can not have a presentation at an arranged time, you

must consult with the other to exchange the time, and inform me at least 24 hour before the arranged time (if not, …)

for the poor presentation (mainly, in the understanding viewpoint), another chance would be given

the presentation will be tested with two examinations Grading

presentation 40 : material 10, understanding 20, Q/A 10 exam. 60 : mid. 30, final 30


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Proposed Presentation Subjects1. Modems

2. Fiber optics

3. B-ISDN (ATM)

4. XDSL

5. Switching system

6. Sliding window

7. CSMA/CD

8. Distance vector routing

9. Link state routing

10. IGMP (Internet group mgmt protocol)

11. Multicast protocols

12. TCP flow control (nagle’s algorithm)

13. RTT in TCP (Kern’s algorithm)

14. TCP congestion control (slow start)

15. ICMP (Internet Control Message Protocol)

16. BOOTP (Boot Protocol)

17. DHCP (Dynamic Host Config. Protocol)

18. SNMP (Simple Network Mgm’t Protocol)

19. DNS (Domain Name System)

20. B2B (Business to Business)

21. P2P (Point to Point)

22. DoS (Denial of Service)

23. Network security

24. Wireless network

25. UMTS vs. IMT2000

26. Satellite network

27. Client-server programming

28. VoD framework (DSM-CC)


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Lecture Topic 1

Overall Introduction of

Computer Networks

What good is a computer network Network technologies Network software ISO/OSI 7 layer model TCP/IP model Network standardization


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Introduction (I) Old model was the Mainframe based computer center

current model of computing environment is an interconnected collection of autonomous computers

We call such a system a Computer Network (CN) a computer network is a set of nodes that are interconnected to

permit the exchange of information An CN is necessary for a Distributed System but is not the same

in a distributed system, the existence of multiple autonomous systems is transparent

What good is a CN economic and technical Issues: resource sharing; reliability;

cost savings (client/server); scalability; communications (email) consumer issues: telecommuting; interactive entertainment and

video on demand; socio-political interaction; selling stuff yucky issues: hate speech; snooping/employee monitoring;

misinformation; ownership & copyright; theft & hacking


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Network Technology A network is a carrier of information between 2 or more entities Generally two types of transmission technology: broadcast

single channel is shared by all the machines on the network messages sent by one node are received by all use special address field in message to specify target of

comm.(broadcast/multicast) usually small (geographically) networks

point-to-point connections between individual pairs of machines message may pass through many pairs of point-to-point

connections to get from source to destination often machines may have multiple point-to-point connections large (geographically) networks..


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Classes of Networks : LAN LAN (Local Area Network)

maximum distance not more than a few kms ownership by a single organization transmission speed of at least several Mbps (tens to hundre

ds are economical) often broadcast, shared media based some widely used standards include:

IEEE 802.3 - Ethernet IEEE 803.5 - Token ring FDDI ATM

an important issue in broadcast LANs is the allocation of the shared channel (media access control)

control may be static (time division multiplexing) or dynamic (contention or arbitration)


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LAN : Two Broadcast Networks


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Classes of Networks : MAN MAN (Metropolitan Area Network)

distances between 5 and 50 kms data rate above 1 Mbps standards: IEEE 802.6 DQDB, FDDI, and ATM


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Classes of Networks : WAN WAN (Wide Area Network)

spans entire states or countries data rate of 1.544, and 45 Mbps common higher data rates are available with the wide deployment of

ATM backbone networks often owned by multiple organizations usually separate communications functions from application

functions transmission lines: circuits, channels or trunks switching elements : Intermediate Systems, Packet

Switching Node, Data Switching Exchange, Router, etc. Intermediate systems store a complete packet before

forwarding it : store-and-forward; packet switched; point-to-point network


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WAN : Host and the Subnet


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WAN : Network Topologies


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Classes of Networks : Wireless Networks Wireless Network

wireless comm. has been used from 1901, by G. Marconi, but its technology did not known, why?

channel multiplexing : AMPS, TDMA, CDMA two up-coming information technologies invent a new compu

ting paradigm, so mobile computing mobile hosts : PDA, palm-top, notebook mobile packet data : IMT 2000, UMTS, GPRS

permit user to access information anytime, anywhere one-line mobility vs. off-line mobility must provide seamless connectivity actually it would be based a fixed network, possibly Internet,

to be a general information network why wireless ? - timely news, and way too much of it

- information where people want to be


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Classes of Networks : Internetworks Internetworks : an interconnected network of networks

direct links : a full-mesh of point-to-point links => n(n-1)/2 links indirect links : bus, star, ring, tree …

unlike a single WAN, internetworks often interconnect different, incompatible networks, so an abbreviated word, Internet

use special types of intermediate systems called Gateways cf) repeater, bridge, router, gateway

InternetInternet


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Network Software Network software is highly structured This technique has been immensely successful The key is Layered design

each layer provides a service to the layer above each layer hides details of how the service is provided to the

layer above the Nth layer on one machine talks to or interacts with the Nth

layer on another machine Conventions and rules governing this interaction are specified

by the Layer N Protocol a protocol is an agreement about how communications are

to proceed, without a protocol, communication can be difficult or even impossible

e.g. telephone conversation, postal addresses


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Network Hierarchy : Protocols


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Network Software : Protocols (Cont.) Information is not actually transferred directly between peer laye

r N entities peer layer N entities carry on a virtual communication using t

he services of the layers below layer N passes data and control information down to (or rece

ives data and control from) Layer N-1 until the physical medium is reached

Interfaces exist between each layer Interface defines which primitive functions and services layer N-

1 provides to layer N Want layers to:

perform a well defined, logically related set of functions minimize the amount of infor. need to pass between layers keep interfaces “clean” to allow easy and transparent replac

ement of layers


19

Network Software : Layering The set of protocols and layers together make up the Network

Architecture a network architecture Specification must provide enough

information to allow implementation in hardware/software implementation specific details are not part of the

architecture and should be irrelevant for inter-operation with one protocol per layer we have a Protocol Stack

Layering is used in other software, e.g. UNIX OS For network software the important difference is that we are not

allowed to violate layering (layer 5 cannot directly access layer 1)

For network software it’s important layers don’t peek into headers of other layers and rely on protocol data of other layers


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Network Software : Layering (Cont.)


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The Benefit of Layered Protocols The network architectures, protocols and protocol stacks are the

fundamentals of computer networks multilayer communications protocols allow:

ready adaptation of successful protocols to new technology (prevent obsolescence)

migration of protocols from software implementation (slow) to hardware (fast) as they evolve

separate data and control information support differing levels of abstraction (message, packet, fram

e) with different sizes allow segmentation of large messages peer process abstraction facilitates reduction of difficult desig

n task (a network architecture) into smaller manageable tasks (protocol layer architecture)

typically lower layer protocols of “network software” are implemented in silicon (hardware)


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Understanding Services and Protocols The protocol is a set of rules about the format and meaning of

data units exchanged by the peer entities within a layer The service is a set of primitives (operations) that a layer

provider to the layer above it The interface tells the processes above it how to access it, that

is, it specifies what parameters are and what results to expect protocol is used by entity to implement services protocol and/or it’s implementation can change and as long

as the service (interface) remains unchanged higher layers are happy and continue to work

like in abstract data types or object orientation we decouple interface and implementation


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Network Software : Design Issues Addressing and Routing Data transfer : simplex, half duplex, full duplex Connection management : # of logical channels per connection Error recovery : error detection, correction, retransmission Message ordering : full, partial, causal Flow control / Rate control Assembly / Disassembly Multiplexing

TDM (Time Division Multiplexing) FDM (Frequency Division Multiplexing) CDM (Code Division Multiplexing)


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Network Software : QoS Quality of Service


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The ISO/OSI Reference Model Developed by the International Standards Organization (ISO) to f

acilitate the intern’l standardization of communications protocols ISO basic reference model for Open Systems Interconnect (henc

e: ISO/OSI), started in the mid-1970’s the reference model itself is not a network architecture (doesn

’t specify any protocols or services) ISO also developed network architecture standards

No assumptions are made regarding: programming language bindings operating system bindings applications programming interfaces

Biggest problems very long time to complete the model and protocol standards very hard to understand the detailed standards difficult (expensive) to get the standards documents


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ISO/OSI 7-Layer Reference Model


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OSI Layer 1 : Physical Layer Primary function is transmitting raw bits over a physical

communications channel Primary design issues include: mechanical, electrical, coding,

physical characteristics how many pins in the connector what voltage represents a “1” versus a “0” etc.

By “raw bits” we mean there is no interpretation of the bits - stream of bits in and bits out


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OSI Layer 2 : Data Link Layer Primary function is to make Layer 1 into what appears to be a

channel free of undetected errors Deals with data in chunks (typically 100s-1000s of bytes)

generally called Frames This layer must create/recognize frame boundaries

remember - physical layer does not care often requires special bit patterns to signal boundaries may have to deal with possibility of pattern appearing in data

Among the key issues dealt with are: error handling (e.g. corrupted frame) flow control providing various qualities of service

For Broadcast networks a key issue is controlling access to the channel: use a sub-layer called the Media Access Control (MAC)


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OSI Layer 3 : Network Layer Primary function is control the operation of the subnet (layers

below) Among the key issues dealt with are:

how routing packets from source to destination through the network (or multiple networks) using static or dynamic routing algorithms

controlling congestion in the subnet accounting functions (for billing) translating between protocols across heterogeneous

networks (address, packet size, …) concerned with addressing


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OSI Layer 4 : Transport Layer First end-to-end layer Uses the network to (most often) provide higher layers with a

connection oriented, reliable, error free channel that delivers messages (or byte stream) in order

May provide other types of transport services Generally requires address (or naming) May also perform flow control Often performs multiplexing of multiple transport connections

over one or more network connections


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OSI Layer 5, 6 : Session, Presentation Session layer

sort of an unwanted layer, this layer is usually very thin and little more than a pass through for most protocols

manages dialog control (e.g. may manage who’s turn to talk in a high-level half-duplex protocol)

manages synchronization of transactions which may need to be able to roll back in case of a crash

Presentation layer rather than being concerned with moving information the

presentation layer is concerned with the interpretation of information representation

ensures that the syntax and meaning is the same for each participant in a communication

provides for standard representation and may provide capabilities for conversion of data


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OSI Layer 7 : Application Layer, and The layer where end-user applications live All the rest of the layers exist to support these applications Layering exists so we can move these around to different

machines, and so they can communicate across any platforms - Open Systems Interconnect

Review: functions of the OSI layers layer 1 (physical): transmission of bits layer 2 (data link): transmission of frames on one given link layer 3 (network): routing of packets through the network layer 4 (transport): end-to-end delivery of messages layer 5 (session): end-to-end conversation, synchronization layer 6 (presentation): formatting, encryption, and

compression layer 7 (application): user applications


33

TCP/IP Protocol Suite Advanced Research Project Agency (ARPA) of DoD sponsored

the development of ARPANET in 1970s. TCP/IP has been adopted as the ARPANET protocol suite TCP/IP became popular by the inclusion of this protocol in BSD

Unix system Transport layer-TCP

provides fully reliable, connection-oriented service byte-stream transmission

Another transport layer-UDP provides unreliable, connectionless service User datagram (message) transmission

Network layer- IP IP provides datagram service it is connectionless unreliable service IP handles routing


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TCP/IP Suite and OSI 7-Layer Model


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Comparison of the Two Reference Models


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A Critique of the OSI Model & Protocols (I)


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A Critique of the OSI Model & Protocols Bad technology

session, presentation (small) vs. data link, network (big) IBM SNA (System Network Architecture) 7 layers complexity of model (services, protocol spec.) : difficult to imple

ment, inefficient in operation addressing, flow control, error control are repeated in each layer inappropriate features in particular layers: eg. virtual terminal ha

ndling -> application layer ignore the importance of connectionless services & protocols telecommunications approach ; eg, indication primitive

Bad implementation huge & slow implementations due to complexity of the model an

d protocols => bad impression good first impl. of TCP/IP in Berkeley UNIX => good impression

Bad politics


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A Critique of the TCP/IP Model Bad software engineering

spec. and implementation go hand-in-hand not distinguish service, interface, and protocol

Not at all general model Host-to-network layer

not a layer but an interface between the network and data link layer

Not distinguish the physical and data link layers Ad hoc protocols

OSI -> model TCP/IP -> protocol


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Network Standardization (I)


40

Network Standardization (II)


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Network Standardization (III)


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Network Standardization (IV)


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Lecture Topic 2

Physical Layer

Physical layer functions Support for framing of information Analog and digital transmission Transmission media Switching techniques Integrated Services Digital Network (ISDN) xDSL: x Digital Subscriber Line Technologies Asynchronous Transfer Mode (ATM) Networks


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Hardware Building Blocks in Computer Network


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Physical/Data Link Layer Interface

NL

DLL

PL

Frame

HDR

ACK

HDR

Sender Receiver

Transmitted Bits


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Synchronization and Framing The simplest way to communicate the bit stream is to use unipo

lar modulation:

Issues are: how to keep the correct pace when reading the bits how to find the start time and the end time

Use some timing mechanism at the receiver so that it reads the bits every T seconds starting from T1 + T/2

Problem: receiver clock cannot tick exactly every T seconds; timing may slowly drift, or fall in the wrong

bits 1 0 1 1 0 0 1

receivedsignal

time

T

T1 T2


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Framing with Start-Stop Bits

STARTbit=0

STOPbit=1

T

lineidle

N data bits = 0 or 1

START bit indicates the beginning of a character

STOP bit concludes the transmission of a character, equivalent to a return to an idle state

Solution: specify a short maximum length for the bit sequences (this is called asynchronous transmission)


48

Line Coding

Synchronization can be achieved via self-synchronizingcodes; Manchester encoding is a widely used self-synchronizing code

1 0 0 1 1 0 1

time

Bits

Bit 1 is indicated by an upward transition in the middle of the bit time,Bit 0 is indicated by a downward transition

+V

-V


49

Analog and Digital Transmission

Definition analog signal : represents information with a continuously var

ying electromagnetic wave, e.g. telephone, TV digital signal : represents information with a sequence of volta

ge pulses, e.g. computer carrier signal : an analog electromagnetic wave that carries in

formation modulation : the process of encoding onto a carrier signal

A/D conversion converts an analog signal into a digital signal, required 3 steps

• sampling• quantization• coding


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Sending Digital Signal using an Analog Carrier

AM, FM, PM


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Sampling


52

Quantization


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Coding


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Transmission Media : Twisted Pair Copper

Very common media is twisted pair usually 2 copper wires (~ 1mm diameter) shielded (STP)

or unshielded (UTP) twisting reduces tendency to become an antenna ubiquitous because of the telephone system can be used for analog or digital transmission usually require amplifiers (for analog) or repeaters

(digital) every few kilometers bandwidth related to distance and thickness but Mbps are

possible


55

Transmission Media : Baseband Coaxial

Coaxial cable (Baseband) “Baseband” => single digital channel 50 Ohm cable usually used for digital transmission shielding provides high bandwidth and good noise immuni

ty 1 ~ 2 Gbps on a 1 kilometer cable now largely replaced by fiber optics on long-haul route used for cable television and some original 10 base Ether

net and for 10 base T (Thinwire)


56

Transmission Media : Broadband Coaxial Coaxial cable (Broadband)

75 Ohm used for analog transmission can transmit 300 MHz for long distances (100 km) digital signal requires analog transceivers divides bandwidth into multiple channels channels can transmit TV, audio or digital and mix digital and an

alog transmissions (6Mhz for a TV channel) inferior to baseband but lots of cable in place due to historical de

velopment of broadcasting (so analog amplifiers are required) directional transmission => 2 cables (transmit and receive) in a tr

ee structure, or a single cable with two frequency bands : different ways to alloca

te frequency bands- subsplit 5-30 MHz for inbound and 40-300 MHz for outbound- midsplit 5-116 MHz for inbound and 168-300 MHz for outbound


57

Transmission Media : Fiber Optics (I)

Fiber Optic cable properties of refraction allow light to be trapped inside a sl

ender glass strand and propagate for very long distances with little loss

may use LED(Light Emitting Diode)s (cheaper - usually with multi-mode) or Lasers (expensive - usually with single mode)

depending on diameter of fiber and wavelength of light there may be multiple paths for a given light ray depending on incident angle of refraction (multi-mode fiber - usually 62.5 micron core) or there may be just a single path (single-mode fiber - usually 8 micron core)

multi-mode is cheaper and more common and gives up to 500 Mbps at 2-4 kilometers


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Transmission Media : Fiber Optics (II)

single-mode is more expensive and gives up to 2 Gbps to about 30 kilometers

attenuation depends on wavelength of light and there are 3 nice windows (.85, 1.3 and 1.55 micron - most devices use first or second)

light disperses (signal smears out) especially in multi-mode since individual rays take paths of different lengths

fiber requires electro-optics for conversion of electrical signals to/from optical signals

often use Active Repeaters and ring topology or passive star topology to distribute signals (divides signal power among the arms)

fiber is immune to thermal noise


59

Modems Due to attenuation, delay distortion, and noise, it is undesirab

le to send wide range frequencies (square waves) - baseband signaling (known as DC signaling)

Modulation is used to solve DC signaling problem and uses a 1000-2000 Hz as a carrier signal Amplitude Modulation, Frequency Modulation, (frequency

shift keying), phase modulation, etc.


60

RS-232 and RS-449

It is a physical protocol to interface computer with modems specify mechanical, electrical, functional, and procedural

interface

Computeror

TerminalModem

Protective Ground (1)

Transmit (2)

Receive (3)

Request to Send (4)

Clear to Send (5)

Data Set Ready (6)

Common Return (7)

Carrier Detect (8)

Date Terminal Ready (20)


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Switching Basis (I)


62

Switching Basis (II)


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Circuit Switching (I) Circuit Switching

communications is done via a dedicated path between end stations

this path is a sequence of links between intermediate nodes

three phases are required: Circuit Establishment (Call Setup) : end station A

requests connection to end station B results in sequence of connection establishments

Data Transfer : information is transmitted in analog or binary format

Circuit Disconnection : sequence of disconnection of individual links


64

Circuit Switching (II) Need circuit setup end-to-end before any data transfer can tak

e place disadvantage: setup may introduce appreciable delay advantage: after set up there is usually low delay and little

variance in delay - effectively a wire Characteristics good for voice so wide spread because of voic

e and the PSTN Not optimal for many digital applications but can be expected t

o continue to be widely used in WANs As you might suspect a key device is the Switch!

generally digital in modern switches several different technologies for switching carrier charges based on distance and call time


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Switches

Function of the switch is to provide atransparent signal path betweenattached devices.

Control unit: 1) establishesconnections (usually at request ofconnected devices) 2) maintains theconnection (may require continuousmanipulation of switching elementsdepending on multiplexing used) and3) handle disconnection

Control Unit

Digital

Switch

NETWORKINTERFACE

FULLDUPLEX

LINES

SWITCH


66

Space Division Switches (I)

Example of Space-divisionSwtich - signal paths arephsyically divided

Disadvantages: number ofcrosspoints grows with N2

costly for large switches

Failure of crosspoint elementeliminates possibility ofconnecting associated nodes

Inefficient use of crosspointseven when all lines are used

1 2 3 4 5 6 7 8

1

2

3

4

5

6

7

8

8 x 8 Crossbar switchFull Duplex Inputchannels

Full Duplex Outputchannels


67

Space Division Switches (II)

Use Multi-stage switches toovercome inefficiencies ofsimple crossbar

Advantages over single-stage switching matrixinclude:

Number of crosspointelements reduced -improving utilization

Multiple paths to connecttwo given end points -improves reliability

Disadvantage: Requiresmore complex control

May introduce blocking

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

1

2

3

4

5

5 x 2

5 x 2

2 x 2

2 x 2

2 x 5

2 x 5

Three stage Space-division switch


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Packet Switches (I) Packet Switching

unlike voice traffic, computer interactions usually involve long periods of idle time - making circuit switching inefficient

circuit switching requires the two end stations to talk at the same data rate

a solution to these problems is to break up the communication into chunks with a relatively small maximum size - will store temporarily at intermediate system

now we must add some control information to each packet so it can be routed through the network

send the packet through sequence to intermediate systems to get from source to destination


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Packet Switches (II) advantages over circuit switching include:

dynamic sharing of circuits rather than dedication of circuit to single connection increases efficiency of utilization

eliminates idle time slots by queuing up available packets

other advantages over circuit switching include: end stations can operate with connections of different

data rates circuit switched network will likely block under heavy

load but packet switching we can still stuff packets into the network but delay increases

we can prioritize packets for lower delay charge per packet, byte or bit independent of distance


70

Packet Switches Techniques Datagram

each packet is independent of others each may take a different path through the network so

may arrive out of order some may be lost and possibly some may be duplicated intermediate system make a routing decision on each

packet call setup time avoided

Virtual Circuit a call setup process establishes a preplanned route

through the network all packets follow the same route (hence the name) intermediate systems make routing decision only on call

setup packets better support for sequencing and error control


71

Comparison of Switching Techniques

Circuit Swtiching

A x y B

Datagram Pkt Switch

A x y B A x y B

Virtual Circuit Pkt Switch


72

Narrowband ISDN Mid 1980’s the telecomm’s decided to invent the replacement

for the analog phone system in anticipation of customer demand for end-to-end digital service

Since it was to provide integrated voice and non-voice service they called it Integrated Services Digital Network - ISDN

Use on a limited set of standardized facilities Support for both circuit switching and packet switching Based on 64 kbps connections - fundamental building block of

ISDN (hence “narrowband”) Provide intelligent services Layered architecture which can be mapped to OSI Variety of physical configurations


73

Conceptual view of ISDN

PacketSwitchedNetwork

CircuitSwitchedNetwork

OtherNetwork

Databases

OtherServices

ISDN Central OfficeCustomer ISDN Equipement

Desktop System

Telephone

PBX

Router

IBM Compatible

Mac II

Laser printer

Digital "Pipe"


74

ADSL

POTS Upstream Channel Downstream Channel

3.4 30 138 1104(KHz)

It provides up to 7Mbps downstreamUp to 500 Kbps downstream traffice


75

ADSL Technologies ADSL Applications

VOD, home shopping, internet access, remote LAN access, multimedia access

ADSL speed v.s. distance

18,000 feet 1.544 Mbps (T1)

16,000 feet 2.048 Mbps (E1)

12,000 feet 6.312 Mbps (DS2)

9,000 feet 8.448 Mbps.


76

B-ISDN A single physical network integrates variety of services The Broadband Integrated Services Digital Network (BISDN)

was defined by CCITT to meet this objective Problems

QoS requirements for services widely different

- voice : Real time (low delay jitter), tolerates occasional losses

- data : Usually no real-time requirements, error-free, guaranteed delivery

- video : High bandwidth, low delay and jitter traffic characteristics also widely different certain applications require synchronization among multipl

e traffic streams


77

Services and Protocol Based ATM and AAL

CO AALCS sublayer

CL AALCS sublayer

CBR AALCS sublayer

VBR AALCS sublayer

Segmentation and Reassembly AAL Sublayer

ATM Layer

Physical Layer (SONET / SDH)

CO data applications(AAL type 3, 5)

CL data applications(AAL type 4)

CBR applications(AAL type 1)

VBR applications(AAL type 2)


78

ATMATM Layer cell multipexing and demultiplexing extracts / attaching headers Generic Flow Control (GFC) Virtual Path Identifier (VPI) Virtual Circuit Identifier (VCI) Payload Type Identifier (PTI) Cell Loss Priority (CLP) Header Error Check (HEC)

Header (5 bytes)

Payload (48 bytes)

8 bits

53 bytes

GFC VPI

VPI VCI

VCI

VCI PTI CLP

HEC

8 7 6 5 4 3 2 1

VPI

VPI VCI

VCI

VCI PTI CLP

HEC

8 7 6 5 4 3 2 1

(a) ATM cell format

(b) ATM cell header format across UNI (c) ATM cell header format across NNI


79

Lecture Topic 3

Data Link Layer

Data Link Layer Functions Data Link Layer Design Issues Framing Techniques Error Detecting and Corrections Error control and flow control Media Access Control Sublayer Carrier Sense Multiple Access Protocols Standard LAN Protocols Bridges


80

Data Link Layer Primary service is transferring data from the network layer on

the source to the network layer on the destination DL converts the bit pipe provided by the physical layer into a

frame link DL layer may be designed to offer a variety of services

generalized as:• unacknowledged connectionless service• acknowledged connectionless service• acknowledged connection-oriented service

Most commonly the DL implements reliable and ordered frame links

Frames received incorrectly are retransmitted, known as Automatic Repeat Request (ARQ)

Sender is informed of transmission errors by timers and acknowledgements


81

Data Link Layer Design Issues


82

Data Link Layer Design Issues


83

Data Link Layer Operation

NL

DLL

PL

Retransmitif timeout

ACK ifcorrectFrame

CRC

ACK

CRC

Sender Receiver

correct and ordered


84

Framing in the DL Recall DL uses services provided by physical layer (raw bit

stream, not necessarily error free) Usually DL organizes the bit stream into discrete frames to

perform error detection on frames (perhaps by Checksum) How can we organize this bit stream when we may not have any

guarantees regarding timing (i.e. we can’t just insert gaps like the spaces between words - would waste bandwidth)?

Several methods are commonly used and we can use a combination of methods

Recall we usually add some protocol control information at each layer as a header (and maybe a trailer)

Many simple techniques developed for early ASCII character transmission character counting start/end characters


85

Character Counting


86

01111110 001111110100101011100111111100 01111110

01111110 00111110101001010111001111101100 01111110

01111110 001111110100101011100111111100 01111110

inserted 0 inserted 0

deleted 0 deleted 0

Stuffing

Destuffing

Start and end of a frame is delimited using flags, typically the bit pattern 01111110

To make sure the flag does not occur in the middle of a packet, a bit stuffing is used: insert a zero after a five consecutive ones appear

Bit destuffing is performed at the receiver to recover the original Example of bit (de)stuffing

Start and End Characters


87

Error Control Noise can introduce transmission errors Optical communication channels typically have a bit error rate

(BER) on the order of 10e-9

Transmission lines typically have a larger BER: 10e-7 is typical If a packet is N bits long, then the packet error rate (PER) is given

by PER = 1 - (1 - BER) N

Error Control

Error Correction Error Detection

Parity checks CRC codes

Provide enough redundant data with each block of data in the frame to allow receiver to reconstruct the data in event of error

Just enough redundancy to allow receiver to detect than an error has occured

(so, retransmission required)


88

Error Detection and Correction (I) Hamming distance : the number of bit positions in which two code

words differ


89

Error Detection and Correction (II) The error detecting and error correcting properties of a code dep

end on its Hamming distance Main idea : choose 2n codewords such that the Hamming distanc

e of the complete code is maximized! let A be a complete code, then to detect “d” error, HD(A) d + 1, and to correct “d” error, HD(A) 2d + 1

Single bit error correction : in case of each of 2m legal messages n+1 bits required: so, (n+1) 2m 2n, where n = m + r (m+r+1) 2r ; given m, this puts a low limit on r ex) m=4, r=3

Error detection : parity bit, CRC Error correction : Hamming code


90

Parity Bits (I) Add a single bit (parity bit) to each character so that the total num

ber of ones is even (even parity) or odd (odd parity). If di = i-th data bit,

then, parity bit f = d1 d2 … dn

An example : Parity checks only detect odd number of errors

d1 d2 d3 d4 d5 d6 d7

1 0 0 1 1 1 0 0 (even parity)

Transmitted: Received:

1 0 1 1 0 1 0 0

d1 d7 . . .

1 0 0 1 0 1 0 0

1 0 0 0 0 1 0 0

1 0 0 0 1 1 0 0

1 error

2 errors

3 errors

Status:

detected

not detected

detected


91

Parity Bits (II)


92

Cyclic Redundancy Code (CRC) Powerful method : used in most computer networks Small amount of hardware required Consider a message m = 11011001 (where the left-most bit repr

esents the most significant bit : big endian) the corresponding polynomial representation of the message

is: M(X) = X7 + X6 + X4 + X3 + 1• G(x) : degree r (r+1 bits) generator polynomial• R(x) : CRC polynomial, r bits• T(x) : text transmitted

Addition, subtraction, multiplication and division of polynomials are done with modulo two arithmetic


93

CRC Encoding/Decoding Process Encoding

• step 1: add r zero bits to the low-order end of the frame (this corresponds to Xr M(X) )

• step 2: divide Xr M(X) by G(X), giving a quotient Q(X) and remainder R(X) so that

Xr M(X) = Q(X) G(X) R(X), or

Xr M(X) R(X) = Q(X) G(X)• step 3: transmit

T(X) = Xr M(X) R(X) Decoding

• receive C(X) = T(X) E(X),

where E(X) is the polynomial representing errors• step 1: divide C(X) by G(X)• step 2: if remainder = 0, no error; else, errors detected


94

CRC Generators in Standards


95

CRC Example


96

Hamming Code Parity bit and CRC catch errors, but can we correct them without

retransmitting information? =: Hamming code Hamming codes, unlike CRC, contain the information necess

ary to locate a single bit error Procedure

place message bits in their non-power-of-two Hamming positions

build a table listing the binary representation each of the message bit positions

calculate the check bit Hamming code

check bits : b1, b2, b4, b8, b16, ….

data bits : b3, b5, b6, b7, b9, ….


97

Hamming Code Example (I)


98

Hamming Code Example (II)


99

Hamming Code Example (III)


100

Hamming Code Example (IV)

Now, sent message is 1011011 how do we check for a single bit error in the sent message? as an example, if you received 1011001


101

Error Control and Flow Control (I) Error control

feedback mechanisms • positive acknowledge

cumulate ack.

selective ack.• negative acknowledge

timer

sequence number


102

Error Control and Flow Control (II) Flow control

feedback mechanisms • window-based protocols

A sample frame structure for our protocols

StartFrame

Delimiter

FrameControl

Data EndFrame

Delimiter

FrameType

Seq.# Ack


103

Unrestricted Simplex Protocol Transmitter and receiver are always ready, processing time is

negligible and buffer space is not a problem No error control, no flow control Seemingly useless and unrealistic but can provide simple model

for communication between software layers of protocol stack within a host


104

Simplex Stop-and-Wait Protocol Drop assumption that network layer is already ready to receive

data (that is, receiver has finite buffer, finite processing time Still assume channel is error free and simplex Now we need to prevent the sender from flooding the receiver

with frames


105

Simplex Stop-and-Wait Protocol for a Noisy Channel (I)

Drop assumption that channel is error free Now frames might be corrupted or even lost completely - but

assume we can detect errors (say via checksum) Receiver must inform sender of damaged or lost frame and

sender will retransmit, it has to distinguish between multiple copies of the same frame

Use a Sequence number to distinguish frames


106

Simplex Stop-and-Wait Protocol for a Noisy Channel (II)


107

Simplex Stop-and-Wait Protocol for a Noisy Channel (III)

ARQ(Automatic Repeat reQuest) or PAR(Positive Acknowledge Retransmission) protocols early time or delayed ack. problem worse-case engineering of the time interval


108

Simplex Stop-and-Wait Protocol for a Noisy Channel (IV)

Full duplex data transmission considerations two physical circuits, each one for simplex data traffic

• bandwidth waste (one circuit) one physical circuit, interleaving data and ack. frames

• the type field in the header of the frames piggybacking (ack. field in the frame header)

• reduce the # of frame types as small as possible• advantages

– bandwidth improvement– processing time improvement– buffer space improvement

• disadvantages– heavy dependent of the availability of frames to be tra

nsmitted by the receive


109

Sliding Window Protocols (I) Stop & wait protocol is not efficient in high speed networks Need to allow multiple frames in transmit at one time

this requires more error control and flow control Very robust protocols addressing these issues are known as Slidi

ng Window Protocols Key idea is that the sender maintains a Sending Window

set of sequence numbers corresponding to frames it is permitted to send (unacked frames)

The receiver also maintains a Receiving Window the set of sequence numbers of frames it is permitted to recei

ve (may be out of order) Full duplex, acknowledgement may piggyback on data packet ABP(Alternating Bit Protocol) is sliding window with window size

= 1 (one bit sliding window)


110

One Bit Sliding Window Protocols (I)


111

One Bit Sliding Window Protocols (II)


112

Sliding Window Protocols (II) Two windows can be different sizes, may change size Sequence numbers represent frames sent for which an ack. has

not been received sender buffers the frames whose sequence # are still in the w

indow for possible retransmission New packets handed down by the NL are given the next highest

sequence # and the upper edge of the window advanced when an ack. is received from the physical layer the lower ed

ge of the window is advanced If the window reaches maximum size N the DL refuses to accept

packets from the NL until an ack. arrives and a buffer is freed if a frame is received with the sequence # equal to the lower

bound of the window it is processed, the data passed to the NL, an ack. is generated and the window is rotated by one


113

Sliding Window with Pipelining: Go-Back-N Protocol


114

The sender can have up to W unacknowledged frames The receiver has 1 window, and acknowledges a correct frame

with an ACK that has the same number as the frame

Sliding Window with Pipelining: Go-Back-N Protocol


115

Permits more than a single unacknowledged frame to eliminate “dead time” waiting for ACKs, so cumulative ack.

Now, the receiver has n windows,and may get the frames in the wrong order from the PL, but it delivers the frames to the NL in order

Selective Repeat Protocol (SRP)


116

How to set the timer?

Error control is not confined to the data link layer, but applied to higher layer protocols

Timer


117

A simulation project assignment will be posted on the class web

Sliding Window : Summary


118

Our previous discussion of DL protocols concerned point-to-point communications (one sender/one receiver)

For a local computer network a fully interconnected topology is not economical and would requires adding interfaces to every system whenever a new computer was added to the network

A more economical approach is the use of a broadcast network with shared media : multiaccess, or random access channels

Medium Access Control (MAC) Sublayer (I)


119

Static channel allocation FDM (Frequency Division Multiplexing)

• only a small and fixed number of users• problem with fixed engineering, assume n channel

users n blocking even if bandwidth is wastedusers n bandwidth waste inherently

• computer comm. traffic is burst peak/mean = 1000/1 TDM (Time Division Multiplexing)

Dynamic channel allocation the network has some number N of independent stations

generating frames at a rate (usually assumed to Poisson) probability a station generates a frame in interval t is given by

t a single channel is shared by all stations (physical peers) stations detect a collision if more than one frame transmission

overlaps

Medium Access Control (MAC) Sublayer (II)


120

Goal : let users transmit whenever they have something to send

Procedure

1. transmit whenever you have data to send

2. listen to the broadcast : the sending host can always find out if its packet was destroyed just by listening to the downward broadcast one round-trip time later

3. if the packet was destroyed, wait a random amount of time and send it again

Note that if the first bit of a new packet overlaps with the last bit of a packet almost finished, both packets are totally destroyed

Due to the collisions and idle periods, pure ALOHA is limited to approximately 18% throughput in the nest case

Can we improve this

Pure ALOHA


121

1-persistent CSMA when a station has data to send, it first listens to the

channel to see if it is busy (carrier sense) the station transmits with probability of 1 whenever it finds

the channel idle the propagation delay has an important effect on the

performance of this protocol

Carrier Sense Multiple Access (CSMA)


122

Persistent CSMA (I)


123

Non-persistent CSMA the station begins transmitting when the channel is idle if the channel is busy, the channel waits a random period

time and senses the channel again

Persistent CSMA (II)


124

P-persistent CSMA this protocol applies to slotted channels when a station becomes ready to send, it senses the channel

Persistent CSMA (III)


125

CSMA


126

At the beginning of the contention period, stations can transmit After a station detects a collision, it aborts the transmission,

waits a random period of time, and then tries again In the worst case a station cannot seize the channel until it has

transmitted for 2 without hearing a collision, where is the propagation delay from end to end

CSMA with Collision Detection (CSMA/CD)


127

IEEE 802.3 (Ethernet) uses CSMA/CD protocol cable length maximum is specified for each media type so t

hat we can guarantee that collisions will be detected now most dominant media is twisted pair (10baseT) minimum frame size is 64 bytes to ensure collision detectio

n maximum data in frame is 1500 bytes a simple, easy to implement contention protocol - utilization

decreases as offered load increases - low delay characteristics => exponential backoff

Ethernet (I)


128

Ethernet (II)


129

IEEE 802 LAN standards (I)


130

LAN has a physical limitations (500 meter Ethernet) Connect 2 or more LANs with a bridge

Bridges (I)


131

Transparent bridges do not forward when unnecessary maintain forwarding table (hash table)

Bridges (II)


132

Spanning tree bridges Extend LANs, sometimes have loops

Bridges (III)


133

Limitations of bridges do not scale

• broadcasting does not scale• spanning tree algorithm does not scale

do not accommodate heterogeneity• same format for address in network’s frame header

beware of transparency• bridge congestion• latency is higher and variable• frame ordering

Bridges (IV)


134

Lecture Topic 4

Network Layer

Routing algorithms Congestion control algorithms InterNetworking issues IP (Internet Protocol) Internet routing IPv6 protocol


135

The Network layer (OSI layer 3)

The Network layer is responsible for communication all the way from source to destination, that is addressing and routing three host identifiers

names : what object is (a location independent characteristic of a network entity)

addresses : where it is (a function of the location of the destination)

routes : how to get there (something that depends on both the source and destination)

Routing may include multiple hops through intermediate systems and include crossing several different networks

It also may be concerned with congestion and optimization of the subnet


136

Classes of Routing Algorithms

Adaptive Algorithms collect information dynamically from other routers (IS’s) either lo

cally or globally adjust to changes in network topology adjust to changes in network traffic load differ in metrics used and frequency of information gathering

Non-Adaptive Algorithms often called static routing predetermined routing policies programmed into router (IS) does not adjust to changes in network topology does not adjust to changes in network traffic


137

Optimality Principle (I)

An important consideration in routing algorithms irrespective of traffic or topology: suppose that router R2 lies on the optimal path from router R1 to

router R3 then the same path contains the optimal router for R2 to R3 otherwise, if there was a different optimal path from R2 to R3 the

optimal path for R1 to R3 would include that path

R1R3

R2


138

C

Optimality Principle (II)

B

A

D E

I J

K

N

OM

L

H

GF

C

B

A

D E

I J

K

N

OM

L

H

GF

SUBNETWORK SINK TREE for B


139

Static Routing Algorithms : Shortest Path Shortest path

simple and widely used have to determine the metric used to define “shortest” - may be n

umber of hops, or could be queuing delay or some cost generally “path length” will be a function of a variety of measures -

cost, bandwidth, traffic, delay, etc. with appropriate weighting of each component

Dijkstra’s algorithm each node is labeled with its best path from source initially all labels are tentative and assigned large values (infinity) algorithm proceeds to find best paths node labels are changed to reflect better paths and made perma

nent when the shortest path from the source is determined permanent labels can not change once assigned


140

A

B

D

22

1

2

7

33

22

H

46

G

E F

C

A

B

D (inf, -)

22

1

2

7

33

22

H (inf, -)

46

G (6, A)

E (inf, -) F (inf, -)

C (inf,-)(2,A)


141

A

B

D (inf, -)

22

1

2

7

33

22

H (inf, -)4

6

G (6, A)

E (4,B) F (inf, -)

C (9, B)(2,A)

A

B

D (inf, -)

22

1

2

7

33

224

6

G (5,E)

E (4,B) F (6,E)

C (9,B)(2,A)

H (inf, -)


142

A

B

D (inf, -)

22

1

2

7

33

224

6

G (5,E)

E (4,B) F (6,E)

C (9,B)(2,A)

H (9,G)

A

B

D (inf, -)

22

1

2

7

33

224

6

G (5,E)

E (4,B) F (6,E)

C (9,B)(2,A)

H (8,F)


143

Static Routing Algorithms : Flooding Flooding

send every packet from each input line to every output line use hop counter and limit to damp the growth of packets or mark packets to keep from resending a packet already

flooded or use selective flooding - flood only on output lines making

progress toward the destination flooding always chooses shortest path because it tries every

path simultaneously! used as benchmark or in military applications


144

Static Routing Algorithms : Flow Based

Flow based routing considers traffic instead of just topology depends on relatively stable traffic and determination of

average delay for each line - so for whole subnet try to find routes that minimize average delay for the whole

subnet need to know topology, average traffic and capacity of each

path in advance


145

Flow-based Routing (I)


146

Flow-based Routing (II)

20

20

20

50

Ci (kbps) C (pkts/sec) Ti (ms) Weight

25

25

12.5

12.5

91

77

154

20

0.171

0.146

0.134

0.159

i

1

234

Line (pkts/sec)

5

76

8

AB

BC

CD

AE

EF

FD

BF

EC

14

12

6

11

13

8

10

8

10

10

20

20

62.5

25

2525

71

222

67

59

0.073

0.098

0.122

0.098

mean delay for the whole subnet is 91 msec, thus 1/ = 800 bits mean delay with weight is 86 msec,

thus the weight = the fraction of the total traffic using that line


147

Dynamic Routing Algorithms : Distance Vector

Distance vector routing (Bellman-Ford, Ford-Fulkerson) one of the most commonly used types of algorithms (original AR

PANET, Internet RIP, BGP) each router maintains a vector giving the best know distance to

a destination and which output line to use vectors are kept in tables updated by exchanging information wi

th neighboring routers table gives preferred outgoing line and estimate of distance for

each destination router the distance metric might be number of hops, time delay in millis

econds, total number of packet queued along the path … a router assumed to know the metric of its neighbors can suffer from slow propagation of information about routes tha

t have gone down


148

Distance Vector Routing (I)


149

Distance Vector Routing (II)


150

Distance Vector Routing (III)


151

Distance Vector Routing (IV)

A B C D

E F GH

IJ K L

A 0 24 20 21 8 AB 12 36 31 28 20 AC 25 18 19 36 28 ID 40 27 8 24 20 HE 14 7 30 22 17 IF 23 20 19 40 30 IG 18 31 6 31 18 HH 17 20 0 19 12 H I 21 0 14 22 10 IJ 9 11 7 10 0 -K 24 22 22 0 6 KL 29 33 9 9 15 k JA JI JH JK new routing

8 10 12 6

A I H K

Router uses Echo packetsto estimate delays to neighbors

New for J Line


152

Distance Vector Routing (V) : Count-to-infinity problem


153

Dynamic Routing Algorithms : Link State Link State Routing

distance vector routing • queue length as the delay metrics, not considered link bandwi

dth• long convergence time

1) each router must discover neighbors• sends a HELLO packet on each line

2) measure delay or cost to each neighbor• send ECHO packet and measure time to get response

3) build an information packet periodically or at specific events (neighbor router goes down or comes up)

4) send the information packet to all other routers• use modified flooding

5) compute the shortest path to every other router• run Dijkstra’s algorithm at the local router


154

Building Link State Package

A

B

D

42

75E

C

F

31

6

8

ASeq.AgeB 4E 5

BSeq.AgeA 4C 2F 6

CSeq.AgeB 2D 3E 1

Link State Packets for all six routersThe most difficult issue is when to build the linkstate information packet and how to distribute it


155

Distributing the Link State Packets

Use flooding to distribute link state packets Each packet contains a sequence number that is incremented for ea

ch new packet generated by the router Routers keep track of all pairs (source, seq.) they see It checks the incoming link state packets

if new, it is forwarded on all lines except the one it came on if not new, (duplicate), it is discarded

Once a router has a full set of link state packets, the entire subnet graph can be constructed

Dijkstra’s Algorithm can be run locally to construct the shortest path for n routers and each with k neighbors

• we need memory proportional to kn


156

Hierarchical Routing As network size grows, routing tables and CPU time increase

exponentially Hierarchical routing is used to reduce these size, but increase

number of hops to reach a remote destination


157

MobileHost

ForeignHost

HomeAgent

WirelessCell

Routing in Mobile Networks


158

Routing for Mobile Hosts

All users assumed to have home location that never changes (Home Agent)

Foreign agents broadcast information packets mobile host registers with foreign agent foreign agent contacts home agent that replies with ack. After pro

per authentication, security Once ack is received, the mobile user is registered Mobile hosts de-register once they leave the area


159

Broadcast Routing

Broadcast by repetitive sending the message for each destination flooding might be an interesting approach to perform

broadcasting multi-destination routing

• each packet has a list of destinations or a bit map for desired destinations

• router generates a new copy for each output line to be used to reach the desired destinations

– partition the destinations among the output lines and eventually the node will receive only one destination

sink tree or spanning tree Broadcast incoming packets only on the spanning tree lines

makes excellent use of bandwidth problem is that each router must have knowledge of some

spanning tree


160

C

B

AD E

I J

K

N

OM

L

H

GF

SUBNETWORK(5 hops, 24 packets)

C

B

A

Reverse Path Forwarding(4 hops, 14 packets)

C

BA

D EI J

K

N

OM

LH

GF

SINK TREE for B

F

HG NK

I JD

H D

L M O

I

L F E O

M G L H

E


161

C

B

A

D E

I J

K

N

OM

L

H

GF

SINK TREE for B

1,2

1

1

2

2 1

C

B

A

I

SINK TREE for B

1

C

B

A

J

K

F

1

11

22

2

Multicast Routing


162

Congestion Control (I)

If packet traffic becomes too heavy routers can begin to loose packets

It an arise because of sudden burst of incoming traffic on several lines all destined for same output line

Processors in some routers might be slow Some lines may have lower bandwidth Congestion tends to generate more congestion as packet time

-outs expire and packets are retransmitted, buffers become locked waiting for ack’s, etc.

Congestion control is a global issue in the network (vs. flow control which is a point-to-point issue between a sender and receiver)


163

Congestion Control (II)


164

Congestion Control (III)


165

General Principles of Congestion Control

We can take closed loop or open loop viewpoint for open loop control

• decide when to accept new traffic• decide when to discard packets• decide which packets to discard• make decisions independent of current state

for closed loop control• based on feedback loop concept• actively monitor network to detect congestion• pass information to systems where action is needed• adjust network operation to correct the congestion


166

Open Loop Systems

Layer PoliciesTransport

Network

Data Link

Retransmission PolicyOut-of-order Caching PolicyAcknowledgement PolicyFlow Control PolicyTimeout Determination

Virtual Circuits vs. DatagramPacket Queuing and Servicing PolicyPacket Discard PolicyRouting AlgorithmPacket Lifetime Management

Retransmission PolicyOut-of-order Caching PolicyAcknowledgement PolicyFlow Control Policy


167

Closed Loop Control

We need metrics to monitor for congestion percentage of packets discarded due to buffer space queue lengths number of retransmissions packet delay measures

When congestion is detected may send control packets to traffic sources

• may add to the congestion May use bits in existing packet headers of control packets

Dynamically query other routers about potential congestion before it occurs

Problem is to get time scale correct to keep from oscillating


168

DL Policies Impact on Congestion

Retransmission Policy affects how fast a times out waiting for an Ack and which packets are retransmitted on timeout short timeout and Go Back N => more load long timeout and Selective Repeat => less load

Caching Policy affects how receiver deals with packets received out of order caching out of order packets lightens network load => nee

d for buffer memory at receiver Ack Policy can affect congestion

piggybacking could result in extra timeouts and retransmissions

may also save on traffic Flow Control Policy (e.g. Window size) impacts offered load

large window size => higher load


169

NL Policies Impact on Congestion Choice of service strategy

virtual circuits may lead to congested links congestion control algorithms may not work with Datagram service

Packet queuing and service policy affects where congestion may occur maintain an input Q per line; maintain an output Q per line; or both? how are the queues serviced? are there priorities?

Packet discard policy affects how we determine which packets to drop in the router when there is congestion age, # of hops, some priority

Routing algorithm choice may evenly distribute or concentrate traffic possibly leading to congestion

Packet Lifetime concerns how long the packet can bounce around the net before being discarded too long => congestion as packets bounce around too short => senders time-out and retransmit


170

TL Policies Impact on Congestion

Recall TL is first end-to-end layer Peer transport entities talk across the internetwork So issues are basically the same as the DL where stations ar

e logically adjacent (point-to-point) Primary difference is that it is much more difficult to determine

timeout values across an internetworked environment than across a LAN

Short timeouts contribute to congestion by generating more packets while long timeouts impact response time when packets are lost


171

The Leaky Bucket Algorithm

Outflow is either zero or a constant rate Packets spilling over the edge of the

bucket are lost Single-server queue with constant

service time One packet (drop) output per “clock tick” Easy to implement in NIC Particularly easy when packets are all

fixed length (e.g. ATM cells) For packets of varying size we can

modify to allow fixed number of bytes per clock tick instead of fixed number of packets

Enforces fixed maximum output rate


172

The Token Bucket Algorithm

We may want output rate to be able to speed up when a burst arrives

Instead of packets the bucket holds Tokens which are generated into the bucket at a fixed rate

One packet may be transmitted for each Token in the bucket

If no Token a packet may not be transmitted until a new Token is generated

Effectively lets the idle host save Tokens for a burst up to the limit of Tokens the bucket can hold

If the bucket is full new Tokens are discarded (but not packets)


173

Internetworking Issues

Expect that there will continue to be a large variety of protocols at each layer

Interconnecting heterogeneous networks will introduce many conflicts

To provide services we want Network layer to accommodate: different addressing schemes different maximum packet sizes different network access mechanisms …

Network layer may have to accommodate: different timeout values error recovery status reporting routing & congestion control user access control


174

Internetwork Approaches

G G

G

H1 R

H2

R

Connectionless with datagrams

S

S

H1

S

S

Connection oriented with virtual circuits

MH2

SS

S


175

Connection Oriented Approach

Build virtual circuit pathway through the internetwork between the source and the destination

Switches maintain information about the virtual circuits The connection oriented approach is often more appropriate when the

internetwork is homogeneous Benefits of virtual circuit based internetworking include:

resource allocation at circuit setup sequencing is guaranteed low header overhead no duplicate packets

Drawbacks of internetworking based Virtual Circuit include: switch resources needed for each circuit switch failure brings down the whole connection certain paths may be susceptible to congestion difficult to incorporate non-VC based network into the internetwork


176

Connectionless Approach

For connectionless we route the packets through the network with routers performing a role similar to the switches but packets do not need to all follow the same route useful for heterogeneous networks

Gateways interconnect networks and are given differing names depending on the layer repeaters - physical layer bridges - DL/MAC layer routers (Gateways, Multiprotocol Routers) - Network Layer transport Gateways - Transport Layer application Gateways (e.g. email gateway) - Application Layer probably not useful at Presentation or Session layer


177

Gateways In this case the gateway performs a routing and translation function

between network A and network B

Network A Network B

HOST HOST


178

Routing

In this case the gateway performs a routing function between network A and network B

IP Network A IP Network B

HOST HOST


179

Tunneling In this case the gateway does not translate to the WAN protocol

between network A and network B but wraps the IP packet in a WAN packet and sends it transparently (tunnels) across the WAN. A & B seem to have a direct serial link.

Network A Network B

HOST HOST

WAN


180

Fragmentation If our data has to traverse many diverse networks it’s likely that they wi

ll have different maximum data “payload” sizes This may be determined by OS (device driver) parameters, physical or

data link layer hardware or optimization efforts Usually the size of PDU payload increases in higher layers (higher leve

ls of abstraction) Internetwork has to deal with differences - usually means we have to fr

agment larger packets Easy part - Gateway is allowed to break up a packet into fragments an

d send each fragment as a separate piece Hard part - Gateway has to put pieces back together to reconstruct the

original packet So the question is - do we need to put them back together again? As usual there are two competing viewpoints

transparent fragmentation non-transparent fragmentation


181

Transparent Fragmentation

Fragments recombined at each Gateway and original sized packet delivered at destination

Requires all packets to leave network via same Gateway so some performance loss

Gateway needs to know when all fragments have been received Fragmenting, recombining, fragmenting, recombining… as

packet traverses internet introduces overhead and reduces performance

G2G1 R3H1 R1R2R1 H1

G7 G8


182

Non-Transparent Fragmentation

Do not recombine fragments at each intermediate Gateway so each fragment becomes an independent packet

Allows fragments to take separate paths Recombination takes place at the destination host

G4G2 G5H1 G3 H1G1

G7

G6

G8


183

The Internet Protocol (IP)

Internet is not a physical network, but it is a method of internetworking physical networks and a set of conventions for using networks that allow the computers they reach to Internet

The collection of networks and gateways that use the TCP/IP protocol suite and that function as a single, cooperative virtual network

A collection of autonomous systems (in other word ‘domain’) interconnected by one or more backbones

Loose, collaborative structure with AS’s organized into Regional Networks interconnected into the larger Internet

Developed from the DARPAnet, NSFnet and grew from the original TCP/IP protocol suite and was designed for internetworking from the start

Provides best effort datagram service to transport Layer


184

Internetworking


185

IP (Internet Protocol) (I)


186

IP (Internet Protocol) (II)


187

IP (Internet Protocol) (III)


188

IP Addressing


189

IP Addressing - Special Addresses


190

Subnetting

The notion of splitting an internet address into a network and host portion didn't work well in practice

It required the central authority to handle all requests for address for networks, of which there were many more than anticipated

A better approach, soon adopted, is to divide the internet address into three pieces: an institution #, a network #, and a host #

The institution is given a range of addresses The address bits following the institution # (still officially the network

#) are divided into subnet (or real network) number and host # This division is specified, for each institution, by a bit mask known

as the subnet mask


191

Internet Control Message Protocol (ICMP) IP Standards specify that compliant implementations must also

implement ICMP (RFC 792) ICMP provides a mechanism to provide feedback about problems

in the network ICMP packets may be sent by routers or hosts and are generated

“at the NL” ICMP exists at the NL but is a user of NL services - I.e. uses IP

datagram service ICMP packets are usually generated by a host or router in

response to a previous datagram ICMP packets have a 64 bit header which includes:

type (8 bits) - type of ICMP packet code (8 bits) - specifies parameters of the packet checksum (16 bits) - checksum for entire ICMP packet parameters (32 bits) - specifies parameters to large for Code


192

Types of ICMP Packets The header is usually followed by additional information depending

on packet type When the packet refers to a previous datagram the additional info. i

ncludes the IP header and first 64 bits of the original datagram Inclusion of first 64 bits of data after the IP header is to allow IP entit

y to determine which IP user was associated with the datagram Types of packets include:

destination unreachable - e.g. router can’t reach dst network time exceeded - TTL of datagram expires parameter error - semantic error in IP header Src Quench - simple flow control redirect - advise host of better route echo (reply) - test communications timestamp (reply) - allow determination of delay address mask req (reply) - inform host of LAN’s subnet mask


193

ICMP Examples : ping Use ICMP echo request/reply Source can calculate round trip time (RTT) of packets


194

ICMP Examples : traceroute Records the route that packets take To determine the route, progressively increase TTL


195

Some ICMP Packet formats (I)

Type Code Checksum

Unused

Type Code Checksum

Identifier Sequence #

Originate timestamp

Type Code Checksum

Ptr Unused

IP Header + 64 bits original dg

Type Code Checksum


Originate timestamp

Receive timestamp

Transmit timestamp

Dst. unreachable, time exceeded, src quench Timestamp

Parameter error

Timestamp reply


196

Some ICMP Packet formats (II)

Type Code Checksum


Address mask request

Echo, Echo Reply

Redirect

Type Code Checksum

Gateway IP Address


Type Code Checksum



Type Code Checksum


Address Mask

Address mask reply


197

Mapping IP addresses to the DL

Consider an 802.3 LAN running IP Recall DL has it’s own 48-bit addresses used to identify LLC ent

ities on the LAN NL superimposes an internetwork on top of the LAN and provid

es it’s own 32-bit IP address space DL knows nothing about IP addresses How do these two sets of addresses get mapped to each other?

A B C D

Who is 1.2.3.4?

That’s me!

Ethernet


198

Address Resolution Protocol (ARP) (I) Another control protocol which resides at the NL is ARP ARP builds a DL broadcast frame with a packet “what’s the DL addre

ss for IP address w.x.y.z?” and sends it Broadcast frame is received by all hosts and one says “that’s me!” or

another says “I know” Host recognizing the IP address builds a response giving the DL addr

ess to IP address mapping and sends it to the sender This is a simple and effective protocol which eliminates need for main

taining static tables A sender broadcasts the ARP request packet with it’s destination xfffff

fffffff address field But, the broadcasting is too expensive to use repeatedly whenever a

host wants to send a packet How can it be solved? When a host receives an ARP reply, it saves the sender’s IP address

and corresponding physical address in its cache for successive lookups


199

Address Resolution Protocol (ARP) (II) Is it be possible more refinement? The sender’s IP-to-physical address binding is included in every ARP

broadcast; receivers update the binding information in their cache ARP is a low-level protocol that hides the underlying network physical

addressing, permitting one to assign an arbitrary IP address to every machine

ARP is a part of the physical network system, and is not a part of the internet protocols

Reverse address resolutoin protocol (RARP) ARP finds out Ethernet address that corresponds to a given IP RARP finds the IP address of the host using an Ethernet address

associated with the Ethernet card• when the machine is booted, it broadcasts its 48-bit Ethernet a

ddress and ask for its IP address• RARP server that is available at each network responds with t

he IP address


200

The Internet Routing Architecture Internet = a core system + a set of autonomous systems The core system is the glue, as which

is controlled by the INOC(Internet Network Operations Center) provides reliable and consistent routers for all possible dest. does not use the default route has complete infor. about optimal routes to all possible dest.

The autonomous system is an ever-growing component of core system, as which is a collection of networks and gateways managed by one admin

istrative authority are hierarchically grouped into an autonomous system (nesting) allows gateways to advertise only the reachability of those netwo

rks within the gateway’s autonomous system restricts the Internet’s topology to a tree structure in which a core

system forms the root - only one path from the core system


201

Routing Protocols in IP

Core system : GGP (Gateway-to-Gateway Protocol) Core and autonomous system(s) : EGP (Exterior Gateway Protocol) Autonomous system : IGP (Interior Gateway Protocol) Initial DARPA Internet protocol for GGP was Routing Information

Protocol (RIP) - same Distance Vector routing As the Internet has grown very large, RIP is being replaced by Open

Shortest Path First (OSPF) - Link State protocol Widely used IP routing protocol for Exterior gateways is Border

Gateway Protocol (BGP)

Core System

Gateway 1 Gateway 3Gateway 2

Autonomous System 1

Autonomous System 3

Autonomous System 2


202

Table Driven IP Routing The IP routing algorithm employs an Internet routing table on each

machine (host and router), which contains information about the possible destinations and how to reach them

It consults the table to decide where to send the datagram Then what information should be kept in routing tables?

minimal information principle : keep network prefix only- makes routing efficient and keeps routing table small

information hiding principle : the details of specific hosts confined to the local environment : next- hop routing

- the routing table in a router only specifies one step along the path from the router to a destination

Default routing : If no route appears in the table, the routing routines send the datagram to a default router it makes their routing decisions efficiently to possible distant

destinations


203

Table Driven IP Routing (An Example)

Network10.0.0.0

Q Network20.0.0.0

R Network30.0.0.0

S Network40.0.0.0

40.0.0.720.0.0.5

20.0.0.6

30.0.0.6

30.0.0.710.0.0.5

To reach hostson network

30.0.0.0

Route tothis address

10.0.0.0

40.0.0.0

20.0.0.0 Deliver Directly

Deliver Directly

20.0.0.5

30.0.0.7


204

IP Routing Algorithm

Route_IP_Datagram(datagram, routing_table)

Extract destination IP address, ID, from datagram

Compute IP address of destination network, IN

if IN matches any directly connected network address

send datagram to destination over that network;

else if ID appears as a host-specific route

route datagram as specified in the table;

else if IN appears in routing table

route datagram as specified in the table;

else if a default route has been specified

route datagram to the default gateway;

else declare a routing error;


205

Routing Protocols in IP IP routing is based on the destination network ID alone, what?

all IP traffic for a given network tales the same path regardless to the delay or throughput of physical network

only the final router can determine if the destination exists or is operational, the router only can report the delivery to the sender

each router routes traffic independently - someone should find out if two-way communication is always possible

IP routing selects the next hop to be sent the datagram, what? where does IP store the next hop address? not IP itself! IP simply passes the datagram and the next hop address to the

network interface software (so-called network driver) the driver software responsible for the physical network over

which the datagram must be sent - binds the next hop IP address to a physical address, forms a frame, and sends it


206

IPv6 Protocol

IPv6 is the formal name of the protocol recommended by the IETF’ IPng group, its objectives are: support large global internetwork support new low-end Internet devices (PDAs, mobile comput

ers, consumers, devices) support the networked multimedia services

The Challenges from IPv4 plenty of addresses reduced administrative overhead opportunity for better routing support for address renumbering improved header processing reasonable security support for host mobility QoS control capability


207

IPv6 HeaderPriority to distinguish packets whose sourcescan (can not) be flow controlledValues 8 through 15 used for real-time traffic

Vers Prior Flow Level

Payload Length Next Header Hop Limit

Source Address (128)

Destination Address (128)

15 310

Other option headers …

IP payload : TCP header (variable)

Next HeaderHeader Length

Hop-by-hop option (variable)

Next HeaderHeader Length

10 X 32 bit= 40 octets


208

IPv6 Addresses

Two-level structure of the IPv4 address, what? Space are 340,282,366,920,938,463,463,374,607,431,768,211,456

(2^^96 times that of IPv4) An address is represented as x:x:x:x:x:x:x:x (x is 16 bit long)

(ex, fedc:ba45:00d4:4354:f345:ad23:546d:232c) Compression 0’s (ex, ff01:0:0:0:0:0:0:43 => ff01::43) Combination between the IPv4 address and IPv6’s one

- IPv4 compatible address => ::IPv4 address (eg. x:x:x:x:x:x:d.d.d.d)

- IPv4 mapped address => ::ffff:IPv4 address IPv6 addresses are identifiers for interfaces, not nodes A single interface may be assigned multiple IPv6 addresses of any t

ype, that is, unicast, anycast, multicast

010 REGISTRY INTERFACESUBNETSUBSCRIBERPROVIDER


209

Lecture Topic 5

Transport Layer

Transport layer concepts Protocol layering Port number TCP protocol TCP flow control TCP congestion control TCP protocol congestion UDP protocol Network programming interfaces


210

Transport Layer (I)

The transport layer must provide higher layers with the illusion of an end-to-end connection, especially in connectionless networks

Its protocol responsible for providing support for end-to-end exchange of data between two processes, it may concerned with: optimizing the use of the network service providing a requested quality of service to the TL service user

When an application in one host wants to communicate with an application in another host, it must set up a transport layer connection to that application naïve approach


211

Transport Layer (II)

Problems with naïve approach duplicate connection request or accept connection paskets

solution: introduces sequence numbers and a 3 way handshake


212

Transport Layer (III)

3 way handshake

The transport layer, like the data link layer, must provide a flow-control and error-controlled link the DLL is hop-by-hop (node-to-node), while the TL is end-to-end

The same flow and error control protocols used in the data link layer may be used with the transport layer one additional concern is packet resequencing


213

Transport Layer (IV)

Sliding window with out of order arrivals sender side window is unaffected by out of order reception of

packets at the receiver receiver side window, however, behaves differently when packets

are ale to arrive out of order


214

Protocol Layering in Internet

IP Layer

N.I

Others...

Sender

Net 1

IP Layer

N.I

IP Layer

N.I

Others...

Receiver

IP Layer

N.I

Net 2 Net 3

High Level Layer

IP Layer

NI Layer

Conceptual Layers

Software Organization

Protocol 1 Protocol 3Protocol 2

IP Module

NI 1 NI 2 NI 3


215

Transport Layer Responsible for providing support for end-to-end exchange of data b

etween two processes TL may be concerned with optimizing the use of the network service TL may be concerned with providing a requested quality of service t

o the TL service user Two TL protocols - Transport Control Protocol (TCP) and User Data

gram Protocol (UDP) TCP is connection oriented UDP is connectionless (minimal service on top of IP)

TCP provides reliable byte-stream communications between a pair of TCP user processes across an unreliable network

Functionally equivalent to Class 4 ISO Transport but TCP is stream oriented

TCP was designed to dynamically adapt to properties of the internetwork and being robust to many kinds of failures


216

TCP A TCP entity runs on each host supporting the TCP/IP protocol suite The entity may be a kernel process which interacts with the IP entity TCP users exchange streams of data bytes but TCP entity breaks these

up into segments of 64KB or less for transmission TCP service is obtained by having both the sender and receiver create

end points (sockets) each socket has a socket number (address) consisting of

IP address of the host 16-bit number local to the host (port)

connections are identified by the socket identifiers at both ends (socket1, socket2)

Current operating system support multiprogramming multiple applications would be executed simultaneously; multitask


217

Port Number (I) A process is the ultimate destination for a message, but IP delivers a

datagram to only the destination host, and processes are created and destroyed dynamically process identifier would be changed in times much reasonable to identify destinations from the functions

Instead of thinking of a process as the ultimate destination, Internet provides a set of abstract destination points called protocol port, which is possible for more than one user process at a time to be using either

TCP or UDP consist of 16-bit integer

When a client process wants to contact a server, the client must have a way of identifying the server that it wants assuming that the client knows the server’s IP address, how does

the client identify the particular server process To solve this problem, a group of well-known ports are defined

the port 1 - 255 (1 - 1023 for BSD UNIX) are reserved


218

Port Number (II) Now, the hierarchical addressing scheme is:

IP datagram contains the two 32-bit IP addresses also IP header contains a protocol identifier UDP or TCP header contains the two 16-bit port # for identifying a

user process (TCP ports are independent of UDP port)

Network IP

06TCP

21 25

FTP SMTP

Protocol “06” is the TCP protocol

Port determineswhich applicationgets incomingdata

17UDP

7

ECHO

69

TFTP

203.234.18.72

IP address identifies this machine


219

# Network services, Internet styleFTP 21TELNET 23echo 7/tcpecho 7/udpftp 21/tcptelnet 23/tcpsmtp 25/tcp #mailtime 37/udp #timserverfinger 79/tcppop 109/tcp #postofficenntp 119/tcp # USENET News Transfer Protocolntp 123/udp # network time protocolsnmp 161/udp # SNMP Network Management

Some Well Known PortsPort numbers less than 1024 are reserved for standard services:


220

TCP Service Model

TCP connection is a byte stream and not a message stream if a sending process writes 4 512-byte chunks to a TCP stream

it might be delivered as 4 512 byte chunks two 1024 byte chunks one 2048 byte chunk

the data can be sent immediately or buffered it in order to collect a larger amount to send at once

TCP provides a push mechanism to send data immediately without any delay user can require TCP to transmit all outstanding data by setting

the PUSH flag receiver TCP delivers this pushed data immediately

TCP provides an urgent mechanism TCP allows user to mark data as urgent (CTRL-C) TCP user at receiving end decides what to do in response


221

TCP Protocol IP datagrams with TCP specified in protocol field are presented to

the TCP entity to reconstruct data streams TCP entity uses timers to deal with lost packets, retransmission a

nd out of order Use sliding window protocol - receiver ack refers to the next expe

cted segment TCP has only one type of TPDU - segment Minimum header size is 20 octets

segment must fit in maximum IP packet 65535 B each network has a maximum transfer unit(MTU) and each se

gment must fit in that MTU


222

TCP Segment Header

After the options, up to 65,535 -20 -20 = 65,495 data bytes may followsegments without data used for ack and control messages

Source Port

0 8 16 24 31

Sequence number

Option (if any)

Destination Port

Checksum

Acknowledgement number

...

Data

Urgent Pinter

WindowHlen Reserved Flags

Padding


223

TCP TPDU Header Fields (I) Source/destination Ports - defines the local end points of the co

nnection Sequence Number - Sequence # of first data octet in the segme

nt except if SYN flag is set, then it’s initial sequence number N and first data octet is N+1

Ack number refers to the next byte expected seq and ack are both 32 bits long because every byte in TCP str

eam is numbered TCP Header Length - Number of 32-bit words in the TCP heade

r Reserved - 6 bits reserved for future use


224

TCP TPDU Header Fields (II) Flags

URG - Urgent pointer field is used to indicate a byte offset from the current urgent data

ACK - Acknowledgement field is valid/ignored PSH - to indicate that data is PUSHED RST - Reset the connection due to crashes, failures SYN - is used to establish connections FIN - release connection No more data sent

Flow control is handled using variable-size sliding window window size 0 is legal

Checksum - add all 16-bit words in One’s complement and then take the 1’s complement of the sum

options used to add new capabilities use large segment to reduce header overhead increase window size from 16 bits to 30 bits


225

Service Request Primitive Types (I)

Unspecified Passive Open - listen for open from any remote destination

Fully Specified Passive Open - listen for open from specific remote destination

Active Open - request connection to specific remote destination

Active Open with Data - request connection to specific remote destination and send data with the open request

Send - transfer data across an established connection Allocate - issue incremental allocation for receive data Close - close a connection gracefully Abort - close a connection ungracefully Status - check on connection status


226

Service Response Primitive Types (II)

Open ID - inform TCP user of connection name assigned to pending connection requested by Open primitive

Open Failure - report failure of Active Open Open Success - report completion of Active Open Deliver - report arrival of data Closing - report that remote TCP user issued a close and all d

ata sent by remote user has been delivered Terminate - reports that the connection has been terminated Status Response - returns status of current connection Error - reports serivce request or internal error


227

TCP Connection Setup

Uses 3-way handshake One side uses Listen and Accept primitives to passively wait for an

open request (usually a server) Client side issues a Connect (active open) request to establish a

connection (SYN bit on, ACK bit off)


228

TCP Flow Control (I)

TCP uses a modified version of the sliding window In ack., TCP uses the “window size” field to tell the sender how ma

ny bytes it may transmit TCP uses bytes, not packets, as sequence numbers


229

TCP Flow Control (II)


230

TCP Flow Control Problems (I)

The small packet problem occurs when the source sends many small packets

The silly window syndrome occurs when the destination reads a small number of bytes at a

time from its buffer Consider an interactive application where the source host sends e

ach keystroke one at a time to the destination host each keystroke is 1 byte, after adding TCP/IP overhead, a 41 b

yte packet is generated when the destination receives the packet, it returns a 40 byte a

ck. packet when the destination removes the byte from its buffer, a 40 byt

e window update packet is sent some applications echo the types character back to the source,

creating another 41-byte packet The small packet problem seriously degrades throughputs


231

TCP Flow Control Problems (II)

The small packet problem (SPP)


232

How TCP Solves the SPP Nagle’s algorithm

when data is sent one byte at a time, send only the first byte buffer all remaining bytes until the first one is acknowledged after receiving the ack., send all the buffered bytes in one packet

This algorithm reduces the amount of bandwidth required to support interactive applications


233

TCP Flow Control Problems (III)

The silly window syndrome (SWS) Consider an application where the source sends in large blocks of

data but the destination reads bytes from its buffer 1 byte at a time each time the destination reads a byte from its buffer, it returns a

window update to the source the source sees that it is only free to send 1 more byte so it sends

a single byte this process repeats itself all the data has been sent, 1 byte at a

time Clark’s solution

prevent the receiver application from reading only 1 byte from its TCP buffer

the receiver should only read from the TCP buffer when it has sufficient application buffer space to handle a large chunk of data

the sender may also help by refusing to send small data packets


234

TCP Flow Control Problems (IV)

The silly window syndrome (SWS)


235

TCP Retransmission

When a packet remains unacknowledged for a period of time, TCP assumes it is lost and retransmits it

TCP tries to calculate the round trip time (RTT) for a packet and its acknowledgement

From the RTT, TCP can guess how long it should wait before timing out


236

RTT Calculation


237

Smoothing the RTT Measurement

First, we must smooth the RTT due to variations in delay within the network, as

is typically equal to 0.875 The timeout value is then calculated by multiplying the smoothed

RTT by some factor (greater than 1) called , as

Timeout = X SRTT

This coefficient of is included to callow for some variation in the round trip times


238

Problem with RTT Calculation

Karn’s algorithm never update RTT measurements based on acknowledgement

s from retransmitted packets


239

Another Problem with RTT Calculation

RTT measurements can sometimes fluctuate severely Smoothed RTT is not a good reflection of RTT in these cases

Solution : use Jacobson/Karels algorithm:


240

TCP Congestion Control (I)

Congestion control is based on the principle that no new packets can be allowed until the old one is going (law of conservation of packets) TCP achieve this by dynamically manipulating the window size

How do we detect congestion? Detect congestion is difficult

lost packets could be due noisy transmission line congested routers or switches

in WANs, frequent timeouts are typically caused by the existence of congestion

noisy channels (wireless) lead to timeouts congested routers, or lack of buffer space at receivers lead

to discarding incoming packets


241

TCP Congestion Control (II)

During connection establishment, you setup window size based on buffer size

Potential congestion causes are due to network capacity and receiver capacity

TCP maintain two windows Receiver Window(RW) - determined by buffer size Congestion Window(CW) - proportion to network capacity

Number of bytes that can be transmitted is the minimum of the receiver window and the congestion window

Examples if the RW says the sender can transmit 8K, but the CW is

only 4K, the the sender may only transmit 4K The TCP congestion control algorithm makes use of :

slow start congestion avoidance (linear increase thresholds)


242

TCP Congestion Algorithm (Slow Start) Sender initializes the congestion window to maximum segment size

and the receiver window to maximum buffer space TCP slow start

congestion window starts small, at 1 segment size each time a transmitted segment is acknowledged, the

congestion window is increased by one maximum segment size Congestion window keeps growing exponential until it becomes

equal to receiver window Packet losses indicate congestion

these are determined by using timers at the sender When a timeout occurs, the congestion window is reduced to one

maximum segment size and everything starts over this leads to low throughput


243

Slow Start


244

TCP Linear Increase Threshold (I) Establish a threshold at which the rate increase is linear instead of

exponential to improve efficiency Algorithm

start the threshold at 64K start the congestion window size at 1 segment size increase the congestion window size exponentially using slow

start until the threshold is reached once the threshold is passed only increase the congestion

window size by 1 segment size for each congestion window of data transmitted

if a timeout occurs, reset the congestion window size to 1 segment and set threshold to ½ of MIN (sliding window, congestion window)


245

TCP Linear Increase Threshold (II)


246

UDP

UDP is an unreliable transport protocol UDP does not provide:

flow or error control connection management guaranteed in-order packet delivery

UDP is almost a “null” transport layer Why UDP?

no connection needs to be set up throughput may be higher because UDP packets are

easier to process, especially at the source the user doesn’t care if the data is transmitted reliably the user wants to implement his or her own transport

protocol


247

UDP Protocol

Specified in RFC 768 Low overhead since there’s not much for it to do: header is 8

octets length field specifies length of the entire UDP segment (header

+ data) checksum applies to segment plus pseudo-header and is same

as with TCP on error segment is just discarded

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