Computer Networks - Department of Computer Sciencemaccabe/classes/585/FALL02/chap05.pdf · Chapter 5: The Network Layer Computer Networks Maccabe Computer Science Department The University

Chapter 5: The Network LayerComputer Networks

Maccabe

Computer Science DepartmentThe University of New Mexico

September 2002

Topics

End-to-end transmission, routing

Design Issues

Routing Algorithms

Congestion Control Algorithms

Quality of Service

Internetworking

The Network Layer in the Internet

Chapter 5 – p.2/117

Store and Forward

A E FProcess P2

Packet

LAN

Router Carrier's equipment

Process P1

B

H1 H2

D

C

1


Datagram Routing

A E F Process P2

LAN

Router

1

Carrier's equipment

Process P1

B

H1 H2

D

C

Packet

3

4

2

A –

B B

initially

C C

D B

E C

F C

Dest.

A –

B B

later

A's table

C C

D B

E B

F B

A A

B A

C's table

C –

D D

E E

F E

A C

B D

E's table

C C

D D

E –

F F

Line Chapter 5 – p.4/117

Connection Routing

E FProcess P2

LAN

Router

1

Carrier's equipment

Process P1

Process P3

B

H1

H3

H2

D

C

3

42

H1 1

H3 1

C 1

C 2

A's table

A 1

A 2

E 1

E 2

C's table

C 1

C 2

F 1

F 2

E's table

A

In Out


Comparison between Datagram andVirtual-Circuit

Issue Datagram subnet Virtual-circuit subnet

Circuit setup Not needed Required

Addressing Each packet contains the fullsource and destination address

Each packet contains a short VCnumber

State Routers do not hold state informa-tion about connections

Each VC requires router tablespace per connection

Routing Each packet is routed indepen-dently

Route chosen when VC is set up;all packets follow it

Router failures None, except for packets lost dur-ing the crash

All VCs that pass through thefailed router are terminated

Quality of service Difficult Easy if enough resources can beallocated in advance for each VC

Congestion control Difficult Easy if enough resources can beallocated in advance for each VC


Topics


Design Issues

Routing Algorithms


Quality of Service

Internetworking



Routing AlgorithmsIntroduction

Routers

forwarding (moving packets)

updating routing tables

Issues

correctness, simplicity, robustness, stability, fairness,

optimality

Adaptive, nonadaptive (static)


Conflict between Fairness andOptimality

X X′

A B C

A' B' C'

Shut off packets between X and X’ maximizesthroughput

Other measures may be more appropriate


Routing AlgorithmsTopics

Optimality PrincipleShortest Path RoutingFloodingDistance Vector RoutingLink State RoutingHierarchical RoutingBroadcast RoutingMulticast RoutingRouting for Mobile HostsRouting in Ad Hoc NetworksNode Lookup in Peer-to-Peer Networks


Optimality Principle

A path is a sequence of routersIf r0 � r1 � � � � � ri � � � � � rn � 1 � rn is a optimal path from r0to rn, then for any ri on this path, the optimal pathfrom ri to rn follows the same pathRationale: you could replace ri to rn with theoptimal to improve the path from r0 to rnSink tree: optimal paths to the root

B

A

F

D EC

J

N

O

IH

G

L

M

K

(a)

B

A

F

D EC

J

N

O

IH

G

L

M

K

(b) Chapter 5 – p.11/117

Shortest Path Routing

Dijkstra’s shortest path algorithm

A D1

2

6

G

4

(a)

F (∞, −) D (∞,−)

A

B 7 C

2

H

33

2

2 FE

1

22

6

G

4

A

(c)

A

B (2, A) C (9, B)

H (∞, −)

E (4, B)

G (6, A)

F (6, E) D (∞,−)A

(e)

A

B (2, A) C (9, B)

H (9, G)

E (4, B)

G (5, E)

F (6,E) D (∞,−)A

(f)

A

B (2, A) C (9, B)

H (8, F)

E (4, B)

G (5, E)

F (6, E) D (∞,1)A

(d)

A

B (2, A) C (9, B)

H (∞, −)

E (4, B)

G (5, E)

F (∞, −) D (∞, −)A

H

E

G(b)

B (2, A) C (∞, −)

H (∞, −)

E (∞, −)

G (6, A)


Flooding

Duplicate incoming packet on outbound links

Damping

hop count

keep track of messages in the routers

keep track of routers in the messages

Selective flooding: rule out outbound links


Distance Vector Routing

Each router maintains a vector of best distances toeach destination

Other names: Bellman-Ford, Ford-Fulkerson, RIP

Update

each router knows “distance to neighbors”

every T sec, the routers exchange their distance vectors

for a neighbor X the distance to i is Xi

�

m where Xi is the

distance repoted by X and m is the distance to X .


Distance Vector RoutingExample

(a)

A B C D

E

I J K L

F GH

Router

0122540142318172192429

2436182772031200112233

2031198301960147229

2128362422403119221009

820282017301812100615

AA

IH

II

HH

I−

KK

To A I H K Line

New estimated delay from J

ABCDEFGHIJKL

JA JI JH JKdelay delaydelaydelay

is is is is8 10 12 6

New routing table for J

Vectors received from J's four neighbors

(b)


Distance Vector RoutingCount-to-infinity Problem

Good news spreads fastBad news may take a long timea) A comes on lineb) A Drops out

A B C D E

• • • • • • •

• • • 4

1

1

1

1

2

2

2

3

3

InitiallyAfter 1 exchange

After 2 exchanges

After 3 exchanges

After 4 exchanges

A B C D E

1 2 3 4

• • • •

2 3 4

3 4

4

6

3

3

5

5

4

4

6

5

567 6 7

87 8 7

InitiallyAfter 1 exchange

After 2 exchanges

After 3 exchanges

After 4 exchangesAfter 5 exchanges

After 6 exchanges...

(a) (b)


Link State Routing

Discover the neighbors and learn their networkaddresses

Measure the delay of cost to each neighbor

Construct a packet describing what you learned

Send this packet to all other routers

Compute the shortest path to every other router


Link State RoutingLearning about the Neighbors

HELLO, reply with router Id

Handling LANS – model the LAN as a nodeRouter

A

B

C

D E

C

D E

H

I

F

G G H

IF

N

A

B

LAN

(a) (b)


Link State RoutingMeasuring Line Cost

ECHO, measure round-trip time; repeat for betteraccuracymeasure load?

including load makes sense in general

counter example: oscillation

West East

B

A

D

E

C F

G

H

J

I


Link State RoutingBuilding Link State Packets

Easy to build, question is “when to build?”B C

E F

A D61

2

8

5 7

4 3

(a)

A

Seq.

Age

B C D E F

B 4

E 5

Seq.

Age

A 4

C 2

Seq.

Age

B 2

D 3

Seq.

Age

C 3

F 7

Seq.

Age

A 5

C 1

Seq.

Age

B 6

D 7

F 6 E 1 F 8 E 8

Link State Packets

(b)


Link State RoutingDistributing Link State Packets

Flooding with some variationsAdd age field, decrement every sec and discardwhen 0Holding areaACKs

D 21 59 1 0 0 0 1 1

C 20 60 1 0 1 0 1 0

E 21 59 0 1 0 1 0 1

F 21 60 1 1 0 0 0 1

A 21 60 0 1 1 1 0 0

Source Seq. Age A C F A C F Data

Send flags ACK flags


Link State RoutingComputing the Routes

Reconstruct entire graph

Two values for each link (one in each direction)

Run Dijkstra’s shortest path locally


Hierarchical Routing

RegionsMay not get shortest paths (1A to 5C)

Region 1 Region 2

Region 3 Region 5Region 4

1B

1A

1C

2A 2B

2C

5B 5C5A

5E5D

2D

4A

4B 4C

3A

3B

1B 1

1C 1

1B 2

1B 3

1B 3

1B 4

1C 3

1C 2

1C 3

1C 4

1C 4

1C 4

1C 5

1B 5

1C 6

1C 5

– –1A

1C

2A

2B

2C

2D

3A

3B

4A

4B

4C

5A

5B

5C

5D

5E

1B

Line HopsDest.

Full table for 1A

1A

1C

2

3

4

5

1B

Line HopsDest.

Hierarchical table for 1A

1B 1

1C 1

1B 2

1C 2

1C 3

1C 4

– –

(a) (b) (c)


Broadcast Routing

Multicast (send to each destination)

Flooding

Multidestination routing (list of addresses)

Spanning tree (need to construct the tree)

Reverse path forwarding

mimic spanning tree without set-up step

forward packets that arrive on the best path from the source

discard packets arriving on non optimal paths from the

source


Broadcast RoutingReverse Path Forwarding

I

F H J N

A D GKE O M O

GC D N

BH L

L B

A

E

H

B CD

F

J

G

O

MK

LN

I

(a)

AB C

D

G

J

O

FI

E

H

K

L

M

N

(b) (c)

KE

H

a) network

b) sink tree

c) tree built by reverse path forwarding


Multicast Routing

Group management (join, leave, create and destroy)

Spanning trees

Built by routers with complete information

PRUNE messages (don’t send me more messages for this

group) – scalability

Core-based trees, send messages to core node which

relays to the group


Multicast RoutingExample

1, 2

1

1, 2

2 1 12

2

1

2

1, 2

1, 2

2 2

1

1

11

11

1

2

2

2

2 2

(a) (b)

(c) (d) Chapter 5 – p.27/117

Routing for Mobile HostsExample Environment

Foreign agent

Foreign LAN

WANMAN

Home LAN

Home agent

Mobile host

Wireless cell

Stationary hostsMigratory hostsRoaming hosts


Mobile Hosts

Home locationHome agent, Foreign agent

Registration on a remote networkmobile host finds foreign agent (wait or ask)

register with foreign agent: home address, current data link

address, security information

foreign agent contacts home agent: security information is

used to authenticate the mobile host

home agent inspects security information and ACKs

foreign agent makes an entry in its table


Mobile HostsExample

1. Packet is sent to the mobile host's home address

2. Packet is tunneled to the foreign agent

3. Sender is given foreign agent's address

4. Subsequent packets are tunneled to the foreign agent


Ad Hoc NetworksRoute Discovery

Routers are mobile

Examples

military battlefield

fleet of ships at sea

emergency workers at an earthquake

a gathering of nerds with notebook computers and no

802.11 infrastructure


Ad Hoc NetworksRoute Discovery

On demand route generation

CBA

D

E

G

IH

F

(a)

CBA

D

E

G

IH

F

(b)

Range of A's broadcast

CBA

D

E

G

IH

F

(c)

CBA

D

E

G

IH

F

(d)


Ad Hoc NetworksROUTE REQUEST/REPLY packet

REQUESTSource address

Request ID

Destination address

Source sequence #

Dest. sequence #

Hop count

REPLYSource address

Destination address

Destination sequence #

Hop count

Lifetime


Ad Hoc NetworksRoute Maintenance

CBA

D

E

IH

F

(b)(a)

A

Dest.

A

Next hop

1

Distance

F, G

Active neighbors

B B 1 F, G

C B 2 F

E G 2

F F 1 A, B

G G 1 A, B

H F 2 A, B

I G 2 A, B

Other fields


Peer-to-Peer Networks

0

16

2,3

4

4 4

3

2

2,3

0,1

0,1

0,1

31 1

17 15

230

1418

29 3

19 13

428

1220

27

26

25

24 8

7

23 9

6

22 10

5

21 11

2 4

3 4 Node 1's finger table

5 7

9 12

17 20

Start

IP a

ddr

o

f suc

cess

or

5 7


8 12

12 12

20 20

Start

IP a

ddr

o

f suc

cess

or

13 15


16 20

20 20

28 1

Start

IP a

ddr

o

f suc

cess

or

(b)(a)

Node identifier

Actual node


Topics


Design Issues

Routing Algorithms


Quality of Service

Internetworking



Congestion

Maximum carrying

capacity of subnet

Pac

kets

del

iver

edPerfect

Desirable

Congested

Packets sent

Too many packets causes performance degradationChapter 5 – p.37/117

CongestionCauses

Inadequate memory for outgoing flows

Nagle (1997), unbounded router memory make the

problem worse (retransmissions)

Slow processors

Slow outbound lines


CongestionIssues

Solutions

increase capacity

decrease load

Congestion control versus flow control

Congestion control is global to the network – emergent

Flow control is point-to-point


CongestionControl Principles, Open Loop

“Design it right”

decide when to accept new traffic

decide when and which packets to discard

scheduling


CongestionControl Principles, Closed Loop

monitor and determine where congestion occurspercentage of packets discardedqueue lengths

number of packets retransmitted due to timeout

average and standard deviation in packet delaypass information to places where action can betaken

notify traffic source

use a bit field on outgoing packets to notify neighbors

send probe packetsadjust system operation to correct the problem

route around congestion

reactivity is an issueChapter 5 – p.41/117

Congestion Prevention Policies

Layer Policies

Transport Retransmission policyOut-of-order caching policyAcknowledgement policyFlow control policyTimeout determination

Network Virtual circuits versus datagram inside the subnetPacket queueing and service policyPacket discard policyRouting algorithmPacket lifetime management

Data link Retransmission policyOut-of-order caching policyAcknowledgement policyFlow control policy


Congestion ControlVirtual-Circuits

Admission controlonce congestion has been detected no new VCs are

acceptedRouting around congestion

A

Congestion

Virtual circuit

Congestion

B

A

B

(a) (b)

Resource reservations


Congestion ControlDatagram

Link utilization

unew

� auold

�

1 � a�

f

a decayf instantaneous utilization (1 or 0)

Enter warning state when u exceeds a thresholdWarning bits

router sets warning bitdestination sets warning bit in ACK

sender reduces transmission rateChoke Packets: more direct, notify sender that itneeds to slow downHop-by-hop choke: immediate relief, but requiresbuffer space in intermediate routers Chapter 5 – p.44/117

Congestion ControlChoke versus Hop-by-hop

(a) (b)

Choke

Choke

B C

A D

E F

ChokeReduced

flow

Flow is still at maximum rate

Flow is reduced

B C

A D

E FHeavy flow

Choke

Choke

Choke

Reduced flow


Load Shedding

Heavy artilleryWhich packets?

wine – older is better, drop newer packets

file transfer, avoid intermediate transmissions

milk – newer is better, drop older packets

video streams, old frames are useless

needs application informationapps need incentive to request lower levels of service

Random Early Detection (RED) (Floyd andJacobson, 1993)

lost packets mean congestion

router monitors queues, drops random packets


Jitter Control

High jitterLow jitter

Minimum delay

(due to speed of

light)

Delay

(a)

Frac

tion

of p

acke

ts

Frac

tion

of p

acke

ts

Delay

(b)


Topics


Design Issues

Routing Algorithms


Quality of Service

Internetworking



Quality of Service

Requirements

Techniques

Integrated Services

Differentiated Services

Label Switching and MPLS


Quality of Service Requirements

Application Reliability Delay Jitter Bandwidth

E-mail High Low Low Low

File transfer High Low Low Medium

Web access High Medium Low Medium

Remote login High Medium Medium Low

Audio on demand Low Low High Medium

Video on demand Low Low High High

Telephony Low High High Low

Videoconferencing Low High High High


ATM QoS Services

1. Constant bit rate (e.g., telephony)

2. Real-time variable bit rate (e.g., compressedvideoconferencing)

3. Non-real-time variable bit rate (e.g., watching amovie over the Internet)

4. Available bit rate (e.g., file transfer)


QoS TechniquesOverprovisioning

Provide more capacity (buffers, bandwidth, etc) thanyou could need

Typical of real-times systems, estimate how muchyou need and add 30%

Expensive

you may get better at estimating how much is needed

but you purchased the expensive version of the router

Telephone company


QoS TechniquesBuffering

Buffer flows at the receiver before delivery

avoids jitter

common in audio streams (think about CD players in cars)

Example

0 5

1 2 3 4 5 6 7 8

10Time (sec)

Time in buffer

15 20

Gap in playback

Packet removed from buffer

1 2 3 4 5 6 7 8Packet arrives at buffer

1 2 3 4 5 6 7 8Packet departs source


QoS TechniquesTraffic Shaping

Deal with the problem at the sender, rather than thereceiverHere’s my outgoing traffic pattern, can you handleit?

service level agreement

traffic policing

Sliding window protocols introduce burstytransmissionTechniques

leaky bucket

token bucket


QoS Techniques, Traffic ShapingLeaky Bucket

Faucet

Leaky bucket

Water drips out of the hole at a constant rate

Host computer

Packet

The bucket holds packets

Unregulated flow

Regulated flow

Network

Interface containing

a leaky bucket

Water

(b)(a)

Turner (1986)single server with constant service timehandling variable sized packets


QoS Techniques, Traffic ShapingLeaky Bucket Example

(a)

Time (msec)0 500

0 500

(b)

Time (msec)

25 MB/sec for 40 msec


a) Offered loadb) Resulting load


QoS Techniques, Traffic ShapingToken Bucket

Add tokens at a regular rate (token bucket haslimited capacity)Transmit requires data and tokenTokens represent burst capability

Host computer

Networks

Host computer

The bucket holds tokens

Networks

One token is added

to the bucket every ∆T

(a) (b)


QoS Techniques, Traffic ShapingToken Bucket Example

0 500

0 500

0 500

(c)

(d)

(e)

Time (msec)

Time (msec)

Time (msec)








QoS Techniques, Traffic ShapingToken Bucket feeding Leaky Bucket

0 500

(f)

Time (msec)




QoS TechniquesResource Reservation

ResourcesBandwidth

Buffer space

CPU cyclesQueueing theory

T � 1µ

� 11 � λ

�

µ

� 1µ

� 11 � ρ

µ service rateλ arrival rate

ρ � λ�

µ CPU utilization


QoS TechniquesAdmission Control

flow specification – intermediates can reduce theparameters

Example (RFCs 2210 and 2211)

Parameter Unit

Token bucket rate Bytes/sec

Token bucket size Bytes

Peak data rate Bytes/sec

Minimum packet size Bytes

Maximum packet size Bytes


QoS TechniquesProportional Routing

Most routers use “best path” routing

Could also divide traffic to a destination overmultiple outbound legs equally or in proportion tobandwidth


QoS TechniquesPacket Scheduling

Fair queueinga queue per flow per output portqueues are serviced in round robin fashion

higher bandwidth to flows that use large packetsDemers et al. (1990)

simulate byte-by-byte round robin

sort in reverse finishing order

example

O

(a) (b)

1A 6 11 15 19 20

2B 7 12 16

3C 8

4D 9 13 17

5

C

Packet

B

D

E

A

8

Finishing time

16

17

18

20E 10 14 18

Weighted fair queueing Chapter 5 – p.63/117

Integrated Services

Streaming multimedia

RFCs 2205-2210

Flow based algorithms or integrated services

Unicast and multicast services

unicast, e.g., streaming video from new service

multicast, e.g., digital TV broadcast


Integrated ServicesRSVP

A

D

G

J

C

F

I

L

B

K

H

E

1 2

3 4 5

Receivers

Senders

A

D

G

J

C

F

I

L

B

K

H

E

1 2

3 4 5

1 2

3 4 5

A

D

G

J

C

F

I

L

B

K

H

E

(a) (b) (c)Chapter 5 – p.65/117

Integrated ServicesRSVP

A

D

G

J

C

F

Bandwidth reserved for source 1

Bandwidth reserved for source 2

I

L

B

K

H

E

1 2

3 4 5

A

D

G

J

C

F

I

L

B

K

H

E

1 2 2

3 4 5

A

D

G

J

C

F

I

L

B

K

H

E

(b) (c)(a)

3 4 5

1


Differentiates ServicesExpedited Forwarding

Regular packets

Expedited packets


Differentiated ServicesAssured Forwarding

Packets Classifier Marker

Four priority classes

Shaper/ dropper

Output line

Ingress router

Class Queued packet

1

2

3

4


Label Switching and MPLS

PPP MPLS IP

Label QoS S TTL

20Bits

Headers

3 1 8

TCP User data CRC


Topics


Design Issues

Routing Algorithms


Quality of Service

Internetworking



Internetworking

Network Differences

Connecting Networks

Concatenated Virtual Circuits

Connectionless Internetworking

Tunneling

Internetwork Routing

Fragmentation


Multiple Networks/Protocols

Legacy technologyCheaper computers, more localized decisionsNew technologyInterconnected networks

Ethernet

Connection to Internet

Switch

Mainframe

Router

802.11

Notebook computer

FDDI ring

SNA network ATM network


Network Differences

Item Some possibilities

Service offered Connection oriented versus connectionless

Protocols IP, IPX, SNA, ATM, MPLS, AppleTalk, etc.

Addressing Flat (802) versus Hierarchical (IP)

Multicasting Present or absent (also broadcasting)

Packet size Every network has its own maximum

Quality of service Present or absent; many different kinds

Error handling Reliable, ordered, and unordered delivery

Flow control Sliding window, rate control =, other, or none

Congestion control Leaky bucket, token bucket, RED, choke packets, etc.

Security Privacy rules, encryption, etc.

Parameters Different timeouts, flow specifications, etc.

Accounting By connect time, by packet, by byte, or not at all


Ethernets connected by a Switch ora Router

LAN 1

Legend

Packet

Header

Trailer

LAN 2(b)

RouterSwitch

S D

LAN 1 LAN 2(a)

S D

Switch: transports entire frame based on MACaddressRouters: packet is extracted from the frame andaddress in packet is used for routingFundamental difference: routers understand thenetwork protocol layer


Concatenated Virtual Circuits

Initially VC networks were commonDatagram is the current fashion, but fashionschangeConcatenating virtual circuits

X. 25

M

M

OSI

M

M ATM

End-to-end concatenated virtual circuits

Router

Host

Multiprotocol router

SNA

1

2

Terminology: gateway = multiprotocol routerChapter 5 – p.75/117

Connectionless Internetworking

Datagram model

M

M

M

M

2

Host

Router

1

Packets travel individually and can take different routes

Multiprotocol router

Issuesconversion – may be very difficult to supportaddressing – IP to SNA address conversion?

Universal packet format – IP


Comparison

Concatenated virtual circuitsAdvantages (same as VCs in a subnet)

reserved buffers

guaranteed sequencing

short headers

avoid problems due to delayed or duplicated packetsDisadvantages

table space required when connections are idle

not alternate routing for congestion or failure

difficult when a subnet is unreliable datagramDatagram

potential for congestion, ability to adapt congestion

can easily mix connection oriented and datagram subnetsChapter 5 – p.77/117

Tunneling

General internetwork connection is hardTunneling: source and destination network are thesame

IPIP IPIP

1 2

Acts like a serial line

WAN

Multiprotocol routerEthernet in Paris

Ethernet frame Ethernet frame

Header

TunnelEthernet in London

IP packet inside payload field of the WAN packet

AnalogyCar English Channel

Paris London

Railroad track

Railroad carriage


Internetwork Routing

An internetwork and its graph

1

2 3

54

(Multiprotocol) router

Network

A B

E F

C D

(a) (b)

A

B

E

DC

F

Two level routinginterior gateway protocol

exterior gateway protocolIssues

crossing international boundaries

differences in costsChapter 5 – p.79/117

FragmentationReasons

Hardware (e.g., the size of an Ethernet frame).

Operating systems (e.g., all buffers are 512 bytes).

Protocols (e.g., the number of bits in the packetlength field).

Compliance with some (inter)national standard.

Desire to reduce error-induced retransmissions tosome level.

Desire to prevent one packet from occupying thechannel too long.


FragmentationTransparent versus Nontransparent

G1 G2 G3 G4

G1 G2 G3 G4

Packet

Network 1

G1 fragments a large packet

G2 reassembles the fragments

G3 fragments

again

G4 reassembles

again

Network 2

(a)

Packet

G1 fragments a large packet

The fragments are not reassembled until the final destination (a host) is reached

(b)


FragmentationElementary Fragment Size

Number of the first elementary fragment in this packet

Packet number

End of packet bit

27 0 1 A B C D E F G H I J

27 0 0 A B C D E F G H 27 8 1 I J

27 0 0 A B C D E 27 5 0 F G H 27 8 1 I J

Header

1 byte

Header Header

Header Header Header

(a)

(b)

(c)


Topics


Design Issues

Routing Algorithms


Quality of Service

Internetworking



Internet Design Principles

1. Make it work2. Keep it simple (KISS)3. Make clear choices4. Exploit modularity5. Expect heterogeneity6. Avoid static options and parameters7. Look for good design; it need not be perfect8. Be strict when sending and tolerant when receiving9. Think about scalability

10. Consider performance and cost


The InternetTopics

The IP Protocol

IP Addresses

Internet Control Protocols

OSPF–The Interior Gateway Routing Protocol

BGP–The Exterior Gateway Routing Protocol

Internet Multicast

Mobile IP

IPv6


The Internet

Leased lines to Asia A U.S. backbone

Leased transatlantic

line

Regional network

A

IP Ethernet LAN

C

D

SNA network

Tunnel

IP token ring LAN

A European backbone

IP routerNational network

HostB

IP Ethernet LAN

1 2


IPIPv4 Header

Version IHL Type of service Total length

Identification

Time to live Protocol

Fragment offset

Header checksum

Source address

Destination address

Options (0 or more words)

D F

M F

32 Bits

Notes:

big endianoptions: up to 40 bytes (5 32-bit words)header checksum: (1’s comp addition, recompute)


IPSome Options

Option Description

Security Specifies how secret the datagram is

Strict source routing Gives the complete path to be followed

Loose source routing Gives a list of routers not to be missed

Record route Makes each router append its IP address

Timestamp Makes each router append its address andtimestamp

http://www.iana.org/assignments/parameters


IP AddressesClass Based Addressing

32 Bits

Range of host addresses

1.0.0.0 to 127.255.255.255

128.0.0.0 to 191.255.255.255

192.0.0.0 to 223.255.255.255

224.0.0.0 to 239.255.255.255

240.0.0.0 to 255.255.255.255

Class

0 Network Host

10 Network Host

110 Network Host

1110 Multicast address

1111 Reserved for future use

A

B

C

D

E

ICANN: Internet Corporation for Assigned Names andNumbers


IP AddressesSpecial Addresses

This host

A host on this network

Broadcast on the local network

0

Host

Network

127 (Anything)

Broadcast on a distant network

Loopback

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0 0 0 0. . .

. . .1 1 1 1 1 1 1 1


IP AddressesIssues

Networks may be too large: subnetworks

Not enough addresses: CIDR

three bears problem

too big, too small, just right is really too big

sparse address space

too many hosts: NAT

ISP with 65,000 subscribers

large corporate network

home network


IP AddressesSubnets

Bad choice of names

subnet mask

Notation xxx.yyy.zzz.www/nn

PC

Music

French

English

Art

PhysicsEthernet

Math

To ISP

EE

CS

Router

Main router


IP AddressesSubnetting a Class B

32 Bits

Subnet mask

10 Network Subnet Host

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0

Internet routers

network addresses, not machine addresses

can’t simply use most significant 4 bits to identifythe network (needs netmask)


IP AddressingCIDR

Classless InterDomain Routing

If you want 2000 addresses, you get 2048 on a2408 address bounddary

routers use longest (prefix) match

aggregation to reduce prefix length

University First address Last address How many Written as

Cambridge 194.24.0.0 194.24.7.255 2048 194.24.0.0/21

Edinburgh 194.24.8.0 194.21.11.255 1024 194.24.8.0/22

(Available) 194.24.12.0 194.21.15.255 1024 194.24.12.0/22

Oxford 194.24.16.0 194.21.31.255 4096 194.24.16.0/20


IP AddressingNAT

NAT–Network Address TranslationUse un-routable addresseslet firewall replace source addressreturning packets? gack!

1

2

3

4

5

6

7NAT box/firewall

PC Leased line

Packet after translation

Packet before translationCompany

LAN

Company router

Server

ISP's router

10.0.0.1 198.60.42.12

Boundary of company premises


IP AddressingWhy NAT is wrong

1. Violates IP model (each machine has uniqueaddress)

2. Connections on a connectionless network – NATbox crashes

3. Violates layering principles

4. Assumes particular protocols (TCP and UDP)

5. Some apps put their IP address in the messagebody (FTP)

6. Port limitation is 61,440


Control Protocols

ICMP (Internet Control Message Protocol)

ARP (Address Resolution Protocol)

RARP (Reverse ARP), BOOTP (Boot Protocol),DHCP (Dynamic Host Configuration Protocol)


Control ProtocolsICMP

Message type Description

Destination unreachable Packet could not be delivered

Time exceeded Time to live field hit 0

Parameter problem Invalid header field

Source quench Choke packet

Redirect Teach a router about geography

Echo Ask a machine if it is alive

Echo reply Yes, I am alive

Timestamp request Same as Echo with timestamp

Timestamp reply Same as Echo reply with timestamp

http://www.iana.org/assignments/icmp-parameters Chapter 5 – p.98/117

Control ProtocolsARP

RFC 826IP Address to MAC AddressOptimization: bcast IP address on Boot (updateremote ARP caches)Proxy ARP

F2

F1 F3

E1

1 2 3 4

E2 E3 E4 E5 E6

192.31.65.7 192.31.65.5

To

WA

N

CS Router has 2 IP addresses

192.31.60.4192.31.65.1

EE Router has 2 IP addresses

192.31.60.7192.31.63.3

192.31.63.8

Ethernet addresses

Campus FDDI ring

192.31.60.0

CS Ethernet 192.31.65.0

EE Ethernet 192.31.63.0


Control ProtocolsRARP, BOOTP, DHCP

RARP: RFC 903; MAC address to IP address

BOOTP: RFCs 951, 1048, and 1084; uses UDP(forwarded by routers)

DHCP: RFCs 2131 and 2132; dynamic IP

Newly-booted host looking for its IP address

DHCP Discover packet (broadcast)

Unicast packet from DHCP relay to DHCP server

DHCP relay

DHCP server

Other

networks

Router

0

Chapter 5 – p.100/117

OSPF

Generalautonomous system

interior versus exterior

OSPF became standard for interior in 1990Requirements

open – published in open literature

must support a variety of distance metrics

adaptable to changes in the network

must use type of service (later dropped)

load balancing by splitting

hierarchical routing

some security

Chapter 5 – p.101/117

OSPFAutonomous system to graph

Kinds of connectionspoint-to-point (router to router)multiaccess networks with/without broadcasting

(a)

(b)

A B C

LAN 1

D

G

E F

JIH

WAN 1

WAN 3

WAN 2

LAN 2

W1

A B C D

G

W2 F

J

W3L1

L2H

2 3

2

4

10 12

6

17

8

13

1

3

16

4

122

E

I3

Chapter 5 – p.102/117

OSPFAreas

A network or set of contiguous networksno overlap

not necessarily exhaustiveBackbone

area 0

all areas are connected to the backbone

each router connected to two or more areas is in the

backboneWithin an area shortest path

all routers in an area have the same link state database

routers that connect multiple areas have database for all

areas

Chapter 5 – p.103/117

OSPF4 Kinds of Routers

AS 1 AS 2

AS 3 AS 4

Internal router

Backbone

Backbone router

Area

Area border router

AS boundary router

BGP protocol connects the ASes

Chapter 5 – p.104/117

OSPFAdjacent Routers

Routers on the same LAN are neighbors

Inefficient to have every router on a LAN talk toevery other router on the LAN

Designated and backup routers

Designated router is adjacent to other routers on itsLAN

Non adjacent routers do not exchange information

Chapter 5 – p.105/117

OSPFMessage Types

Message type Description

Hello Used to discover who the neighbors are

Link state update Provides the sender’s costs to its neighbors

Link state ack Acknowledge link state update

Database description Announce which updates the sender has

Link state request Requests information from the partner

Chapter 5 – p.106/117

BGP

OSPF recommended within an ASBGP required between ASesOSPF uses any routes

BGP incorporates policiessome routers may refuse to be intermediaries

may want to avoid paths

Kinds of networksstubs – only one BGP connection

multiconnected – refuse to carry transit traffic

transit networks

Chapter 5 – p.107/117

BGP

Like RIP, BGP uses a distance vectorUnlike RIP, BGP exchanges path information

Information F receives from its neighbors about D From B: "I use BCD" From G: "I use GCD" From I: "I use IFGCD" From E: "I use EFGCD"

J

H

D

C

G

I

(a) (b)

F

B

A

E

Chapter 5 – p.108/117

Internet Multicast

Class D addresses, 28 bits

Best effort delivery to all subscribers

Permanent addresses

224.0.0.1: all systems on a LAN

224.0.0.2: all routers on a LAN

224.0.0.5: all OSPF routers on a LAN

224.0.0.6: all designated OSPF routers on a LAN

Temporary addresses must be joined by processes

IGMP (Internet Group Management Protocol)

Chapter 5 – p.109/117

Mobile IP

Problem: IP address includes network address andhost addressHacks:

give the machine a new IP address for each network

use complete IP address for routing (unlikely)

IETF Working Group goalsmobile hosts can use IP address anywhere

cannot change software on fixed hosts

cannot change routers software or tables

most packets from mobile hosts should not take detours

no overhead when mobile host is at home

Chapter 5 – p.110/117

Mobile IP

Solutionmobile host registers with a foreign agent

foreign host contacts home agent with care-of-address

initial packets delivered to home agent (ARP)

home agent forwards packets to mobile host

home agent sends care-of-address to sender

Issuesgratuitous ARP (when leaving or returning)

locating foreign agents – advertisements or arrivals

mobile hosts need to notify foreign agents of departure

authentication

Chapter 5 – p.111/117

IPv6Goals

Support billions of hosts (even if address space isinefficient)

Reduce size of routing tables

Simplify protocol for faster processing in routers

Provide better security

Pay more attention to type of service

Aid multicasting by allowing scopes

Allow for future protocol evolution

Permit old and new protocols to co-exist

Chapter 5 – p.112/117

IPv6Results

Longer addresses

Header simplification (7 versus 13 fields)

Better support for options (easier for routers to skip)

Security (many have been retrofitted to IPv4)

Chapter 5 – p.113/117

IPv6Main Header

Flow label for pseudo connectionsNext header for extension headers

32 Bits

Version Traffic class Flow label

Payload length Next header Hop limit

Source address (16 bytes)

Destination address (16 bytes)

Chapter 5 – p.114/117

IPv6Lost Fields

IHL field gone, because headers are fixed length

Fragmentation fields removed

Minimum length 576 to 1280 (1024 plus headers)

Header checksum

Chapter 5 – p.115/117

IPv6Extension Headers

Extension header Description

Hop-by-hop options Miscellaneous information for routers

Destination options Additional information for the destination

Routing Loose list of routers to visit

Fragmentation Management of datagram fragments

Authentication Verification of the sender’s identity

Encrypted security payload Information about the encrypted contents

Next header

Jumbo payload length

0 194 4

Next headerHeader extension

lengthRouting type Segments left

Type-specific data

Chapter 5 – p.116/117

IPv6Controversies

Addresses8, 16, 20 bytes? 16 was a compromise

how many addresses per square meter?

how many new addresses per second?8-bit hop count

32 hops are currently common, 255 won’t be enough

header bloat

long haul links, hierarchyMaximum packet sizeRemove header checksumMobile hostsSecurity

Chapter 5 – p.117/117

Documents

Computer Networks - Department of Computer Sciencemaccabe/classes/585/FALL02/chap05.pdf · Chapter 5: The Network Layer Computer Networks Maccabe Computer Science Department The University