january 22, 2002ce80n -- lecture #61 ce80n introduction to networks & the internet dr. chane l....

January 22, 2002 CE80N -- Lecture #6 1

CE80NCE80NIntroduction to NetworksIntroduction to Networks

&&The InternetThe Internet

Dr. Chane L. Fullmer

UCSC

Winter 2002


ReadingReading

Chapter 15 – – IP: Software To Create A Virtual Network


Communication ProtocolCommunication Protocol

A common language computers use to exchange messages.– Specifying exact format and

meaning of each message– Sending and receiving


What’s a protocol?What’s a protocol?

human protocols: “what’s the time?” “I have a question” Introductions

… specific msgs sent

… specific actions taken when msgs received, or other events

network protocols: machines rather than

humans all communication activity

in Internet governed by protocols


What’s a protocol?What’s a protocol?

Q: Other human protocol?

Hi

Hi

Got thetime?

2:00

TCP connection req.

TCP connectionreply.Get http://gaia.cs.umass.edu/index.htm

<file>time

Human Protocol Computer Protocol


Protocol “Layers”Protocol “Layers”

Networks are complex! many “pieces”:

– hosts– routers– links of various

media– applications– protocols– hardware,

software

Question: Is there any hope of organizing structure of

network?

Or at least our discussion of networks?


Organization of air travelOrganization of air travel

a series of steps

Arrival at airport

baggage (check)

gates (load)

runway takeoff

airplane routing

Departure from airport

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing


Organization of air travelOrganization of air travel: a different view: a different view

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

Airport arrival

baggage (check)

gates (load)

runway takeoff

airplane routing

Airport departure

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing


Layered air travel: servicesLayered air travel: services

Airport-to-Airport delivery of person+bags

baggage-claim-to-baggage-claim delivery

people transfer: loading gate to arrival gate

runway-to-runway delivery of plane

airplane routing from source to destination

January 22, 2002 10

DistributedDistributed implementation of layer functionality implementation of layer functionality

Arrival

baggage (check)

gates (load)

runway takeoff

airplane routingDep

art

ing

air

port

Departure

baggage (claim)

gates (unload)

runway landing

airplane routing arr

ivin

g

air

port

airplane routing

intermediate air traffic sites

airplane routing airplane routing

January 22, 2002 11

DistributedDistributed implementation of layer functionality implementation of layer functionality

gates (load)

runway takeoff

airplane routingDep

art

inggates (unload)

runway landing

airplane routing arr

ivin

g

intermediate air traffic sites


Layering ModelLayering Model

Purpose is to divide and conquer complex software and hardware needed to implement services

Partition services and functions needed in system into layers Each layer of service is provided by peer protocol entities Extensibility (new protocols and services easily added) Communication can be point-to-point or multipoint

Layer-N Protocol Entity

Layer-N Protocol Entity

Layer-(N - 1)Protocol Entity

Layer-(N - 1) Protocol Entity

(virtual communication)

Layer N packets

interface

NODE A NODE B

protocol


Why layering?Why layering?

Dealing with complex systems: explicit structure allows identification,

relationship of complex system’s pieces– layered reference model for discussion

modularization eases maintenance, updating of system– change of implementation of layer’s service

transparent to rest of system– e.g., change in gate procedure doesn’t affect

rest of system Is layering considered harmful?


Internet protocol stackInternet protocol stack application: supporting network

applications– ftp, smtp, http

transport: host-host data transfer– tcp, udp

network: routing of datagrams from source to destination– ip, routing protocols

link: data transfer between neighboring network elements– ppp, ethernet

physical: bits “on the wire”

application

transport

network

link

physical


Layering: logical communication Layering: logical communication

applicationtransportnetwork

linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

Each layer: distributed “entities”

implement layer functions at each node

entities perform actions, exchange messages with peers


Layering: Layering: logical logical communication communication


linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

data

data

E.g.: transport take data from

application add addressing,

reliability check info to form “datagram”

send datagram to peer

wait for peer to ack receipt

analogy: post office

data

transport

transport

ack


Layering: physical communication Layering: physical communication


linkphysical


linkphysical


linkphysical


linkphysical

networklink

physical

data

data


Protocol layering and dataProtocol layering and data

Each layer takes data from above adds header information to create new data unit passes new data unit to layer below


linkphysical


linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame


PHYSICAL

Open Systems Interconnect (OSI) StackOpen Systems Interconnect (OSI) Stack

LINK

End-user services (mail, file transfer)

Formatting, encryption, compression of data

Setup and management of end-to-end dialogue

End-to-end delivery of messages to processes

End-to-end transmission of packets in net

Transmission of packets over a link

Transmission of bit over physical media

Web access

TCP

IP

PPP, CSMA/CD

SONET

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATION

Proposed by the International Standards Organization

Specifies the functions at each layer, not the protocols that implement them


Open Systems InterconnectOpen Systems Interconnect

Source: Link 2 Semiconductorhttp://www.link2semiconductor.com/articles/dob010911_p.htm


Description of OSI layersDescription of OSI layers– Application layer: The application layer provides services

to a calling computer program. An application layer might take care of all the downloads involved in transferring a web page (that is, the text, graphics, and other files) to a browser (the client program or application).

– Presentation layer: Takes care of any data format translations that might be needed to take the particular bits representing, for example, a number in the client computer and convert them to a "universal" number representation recognized by the communications system.

– Session layer: This layer is responsible for controlling exchange of information, for example by having the client and server take turns transmitting data.


OSI Layers (continued)OSI Layers (continued)

— Transport layer: The transport layer is responsible for getting messages from one computer to another. Network layer: The network layer is responsible for getting data across a communications network from one host computer to another. Data Link layer: The data link layer (often abbreviated to "link" layer or DLL) gets data from one network node (e.g. a computer or router) to another. Physical layer: The physical layer is the set of specifications that describe the actual medium of transmission. For example, a physical layer specification might include the type of connectors and wire to be used in a cable linking two machines, as well as the function of each wire and voltage levels that specify a "1" or "0", allowed rate of transmission, and so on.


Basic Functionality: Basic Functionality: Internet Protocol (IP)Internet Protocol (IP)

IP defines computer communication details.– Specifying how packets are formed– Specifying how routers forward each packet

• IP Forwarding

Computers connecting to the Internet must follow the IP rules.


IP Software On Every MachineIP Software On Every Machine

Computer hardware does not understand IP.– Connecting a computer to the Internet

does not mean it can use the InternetComputers need IP software before

using the Internet.– Windows– Unix– Mac OS


Internet Packets Are Called Internet Packets Are Called DatagramsDatagrams

IP Datagrams are packets that follow the IP specifications.– Traveling across the

Internet independent of sender


The Illusion Of A Giant The Illusion Of A Giant NetworkNetwork

Any computer can send IP datagrams to any other computer providing they have IP software installed (Universal Service)

The Internet operates like a virtual network.

The Virtual NetworkThe Virtual Network

Figure 15.1 The view of the Internet that IP software provides. Users and application programs treat the Internet like a single large network that allow arbitrary numbers of computers to communicate.


The Reality Of Internal The Reality Of Internal StructureStructure

The Internet contains a complex physical structure users never see– Interconnecting networks with routers

Internet Internal StructureInternet Internal Structure

Figure 15.2 A small example of the physical structure that remains hidden inside the Internet. Each computer attaches to a single network; routers interconnect the networks.


Datagrams Travel In PacketsDatagrams Travel In Packets

IP datagram defines a standard for all Internet packets. Routers:– Encloses the data before sending out the

packet• “Data Encapsulation”


IP datagram formatIP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.


Every Computer Is Assigned A Every Computer Is Assigned A Unique AddressUnique Address

Each computer attached to the Internet must be assigned a unique address.– One computer must know the address

of another before it can communicate


Internet AddressesInternet Addresses

The unique number assigned to a computer is its Internet (IP) address.

Each computer (including routers) need to have an IP address.


An Odd IP Address SyntaxAn Odd IP Address Syntax

Computer stores IP address in four binary units called bytes.

“one twenty-eight dot one fourteen dot one dot zero”


IP Addresses Are Not RandomIP Addresses Are Not Random

IP addresses are not random.Computers on the same network

have the same prefix.


IP Addressing: introductionIP Addressing: introduction IP address: 32-bit

identifier for host, router interface

interface: connection between host, router and physical link– router’s typically have

multiple interfaces– host may have

multiple interfaces– IP addresses

associated with interface, not host, router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11


IP AddressingIP Addressing IP address:

– network part (high order bits)

– host part (low order bits)

What’s a network ? (from IP address perspective)– device interfaces with

same network part of IP address

– can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 IP networks(for IP addresses starting with 223, first 24 bits are network address)

LAN


IP AddressingIP Addressing

How to find the networks?

Detach each interface from router, host

create “islands of isolated networks

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Interconnected system consisting

of six networks


IP AddressesIP Addresses

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

given notion of “network”, let’s re-examine IP addresses:

“class-full” addressing:


IP addressing: CIDRIP addressing: CIDR

Classful addressing: – inefficient use of address space, address space exhaustion– e.g., class B net allocated enough addresses for 65K hosts,

even if only 2K hosts in that network

CIDR: Classless InterDomain Routing– network portion of address of arbitrary length– address format: a.b.c.d/x, where x is # bits in network

portion of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23


IP addresses: how to get one?IP addresses: how to get one?

Q: How does host get IP address? hard-coded by system admin in a file

– Wintel: control-panel->network->config->tcp/ip->properties (reboot)

– UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: – dynamically get address: “plug-and-play”– host broadcasts “DHCP discover” msg– DHCP server responds with “DHCP offer” msg– host requests IP address: “DHCP request” msg– DHCP server sends address: “DHCP ack” msg


IP addresses: how to get one?IP addresses: how to get one?

Network (network portion):get allocated portion of ISP’s address

space:ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23


IP addressing: the last word...IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned Names and Numbers

– allocates addresses– manages DNS– assigns domain names, resolves

disputes


Hierarchical addressing: route aggregationHierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

•Hierarchical addressing allows efficient advertisement of routing information:


Hierarchical addressing: more specific Hierarchical addressing: more specific routesroutes

•ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...


A Trip Through The InternetA Trip Through The Internet

A router must choose between two paths that both lead to the destination.– Choosing the shortest path


Getting a datagram from source to dest.Getting a datagram from source to dest.

IP datagram:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

miscfields

sourceIP addr

destIP addr data

datagram remains unchanged, as it travels source to destination

addr fields of interest here

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

routing table in A



223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Starting at A, given IP datagram addressed to B:

look up net. address of B find B is on same net. as A link layer will send datagram

directly to B inside link-layer frame– B and A are directly

connected


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

miscfields223.1.1.1223.1.1.3data



223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Starting at A, dest. E: look up network address of E E on different network

– A, E not directly attached routing table: next hop router

to E is 223.1.1.4 link layer sends datagram to

router 223.1.1.4 inside link-layer frame

datagram arrives at 223.1.1.4 continued…..

miscfields223.1.1.1223.1.2.3 data



223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Arriving at 223.1.4, destined for 223.1.2.2

look up network address of E E on same network as

router’s interface 223.1.2.9 – router, E directly attached

link layer sends datagram to 223.1.2.2 inside link-layer

frame via interface 223.1.2.9 datagram arrives at

223.1.2.2!!! (hooray!)

miscfields223.1.1.1223.1.2.3 data network router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

Dest. next


RoutingRouting

Graph abstraction for routing algorithms:

graph nodes are routers

graph edges are physical links– link cost: delay, $ cost,

or congestion level

Goal: determine “good” path

(sequence of routers) thru network from source to

dest.

Routing protocol

A

ED

CB

F

2

2

13

1

1

2

53

5

“good” path:– typically means

minimum cost path– other def’s possible


Animation of Routing AlgorithmsAnimation of Routing Algorithms

Dijkstra’s algorithm:http://ciips.ee.uwa.edu.au/~morris/Year2/PLDS210/

dijkstra.html

http://www-b2.is.tokushima-u.ac.jp/~ikeda/suuri/dijkstra/DijkstraApp.shtml?demo3

Distance Vector algorithm:http://www-mm.informatik.uni-mannheim.de/

veranstaltungen/animation/routing/ripdvmrp/


Routing Algorithm classificationRouting Algorithm classification

Global or decentralized information?

Global: all routers have complete

topology, link cost info “link state” algorithmsDecentralized: router knows physically-

connected neighbors, link costs to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms

Static or dynamic?Static: routes change slowly

over time

Dynamic: routes change more

quickly– periodic update– in response to link

cost changes


A Link-State Routing AlgorithmA Link-State Routing Algorithm

Dijkstra’s algorithm net topology, link costs

known to all nodes– accomplished via “link

state broadcast” – all nodes have same info

computes least cost paths from one node (‘source”) to all other nodes– gives routing table for

that node iterative: after k iterations,

know least cost path to k dest.’s

Notation: c(i,j): link cost from node

i to j. cost infinite if not direct neighbors

D(v): current value of cost of path from source to dest. V

p(v): predecessor node along path from source to v, that is next v

N: set of nodes whose least cost path definitively known


Dijkstra’s AlgorithmDijkstra’s Algorithm1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infty 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N


Dijkstra’s algorithm: exampleDijkstra’s algorithm: exampleStep

012345

start NA

ADADE

ADEBADEBC

ADEBCF

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

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

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

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F

2

2

13

1

1

2

53

5


Dijkstra’s algorithm, discussionDijkstra’s algorithm, discussionAlgorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n*(n+1)/2 comparisons: O(n**2) more efficient implementations possible: O(nlogn)

Oscillations possible: e.g., link cost = amount of carried traffic

A

D

C

B1 1+e

e0

e

1 1

0 0

A

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

initially… recompute

routing… recompute … recompute


Distance Vector Routing AlgorithmDistance Vector Routing Algorithm

iterative: continues until no

nodes exchange info. self-terminating: no

“signal” to stop

asynchronous: nodes need not

exchange info/iterate in lock step!

distributed: each node

communicates only with directly-attached neighbors

Distance Table data structure each node has its own row for each possible

destination column for each directly-

attached neighbor to node example: in node X, for dest. Y

via neighbor Z:

D (Y,Z)X

distance from X toY, via Z as next hop

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

w

=

=


Distance Table: exampleDistance Table: example

A

E D

CB7

8

1

2

1

2

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

D (C,D)E

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

w== 2+2 = 4

D (A,D)E

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

w== 2+3 = 5

D (A,B)E

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

w== 8+6 = 14

loop!

loop!


Distance table gives routing tableDistance table gives routing table

D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

dest

inat

ion

A

B

C

D

A,1

D,5

D,4

D,4

Outgoing link to use, cost

dest

inat

ion

Distance table Routing table


Distance Vector Routing: overviewDistance Vector Routing: overview

Iterative, asynchronous: Each local iteration caused

by: – local link cost change

– message from neighbor: its least cost path change from neighbor

Distributed: each node notifies neighbors

only when its least cost path to any destination changes– neighbors then notify their

neighbors if necessary

wait for (change in local link cost of msg from neighbor)

recompute distance table

if least cost path to any dest

has changed, notify neighbors

Each node:


Distance Vector Algorithm:Distance Vector Algorithm:

1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infty /* the * operator means "for all rows" */ 4 D (v,v) = c(X,v) 5 for all destinations, y 6 send min D (y,w) to each neighbor /* w over all X's neighbors */

XX

Xw

At all nodes, X:

CE80N -- Lecture #6 63

Distance Vector Algorithm (cont.):Distance Vector Algorithm (cont.):8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) 11 12 if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: DX(y,V) = DX(y,V) + d 16 17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its minw DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: DX(Y,V) = c(X,V) + newval 22 23 if we have a new minw DX(Y,w)for any destination Y 24 send new value of minw DX (Y,w) to all neighbors 25 26 forever


Comparison of LS and DV algorithmsComparison of LS and DV algorithms

Message complexity LS: with n nodes, E links,

O(nE) msgs sent each DV: exchange between

neighbors only– convergence time varies

Speed of Convergence LS: O(n2) algorithm requires

O(nE) msgs– may have oscillations

DV: convergence time varies– may be routing loops– count-to-infinity problem

Robustness: what happens if router malfunctions?

LS: – node can advertise

incorrect link cost

– each node computes only its own table

DV:– DV node can advertise

incorrect path cost

– each node’s table used by others

• error propagate thru network


ICMP: Internet Control Message ProtocolICMP: Internet Control Message Protocol

used by hosts, routers, gateways to communication network-level information– error reporting:

unreachable host, network, port, protocol

– echo request/reply (used by ping)

network-layer “above” IP:– ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header


SummarySummary

Protocols and protocol stack– Internet stack, OSI stack

IP addressing IP forwarding Routing

– Link State– Distance Vector


GlossaryGlossary Protocol

– The rules two or more computers must follow to exchange messages

Internet Protocol – (IP) Specification for the format of packets

computers use when communicating across the Internet

IP Datagram – A packet of data sent across the Internet


GlossaryGlossary

Virtual Network– Appearance of a single, seamless network

system Internet Address

– (IP Address) An unique number assigned to a computer attached to the Internet

january 22, 2002ce80n -- lecture #61 ce80n introduction to networks & the internet dr. chane l....

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