IP: Network Layeree122/sp04/network layer.pdfGoals and Tasks Goals of Network Layer Guide packets from source to destination Use network links “efficiently” (e.g., prefer shorter

IP: Network Layer

OverviewAddressingRouting

TOC – IP

OverviewGoals and Tasks RoutingSwitchingIssuesBasic ideas

TOC – IP – Overview

Goals and TasksGoals of Network Layer

Guide packets from source to destinationUse network links “efficiently” (e.g., prefer shorter and fasterroutes)

AddressingAgree on addressing scheme to identify nodesIP addresses are location-based (similar to telephone numbers)This structure reduces the information routers must keepDifferent types of addresses

RoutingRouters exchange information to “learn” network topology Routers then calculate good routes to the different destinationsRouters store the results of these calculations in “routing tables” Different routing algorithms

TOC – IP – Overview – Goals and Tasks

RoutingDefinition

Finding path from source to destinationTypes: Path based on

Flow“Type or Traffic”Source/DestinationDestination Internet

A (S D): 1, 2, 3B (S D): 1, 2, 3C (S D): 1, 4, 5, 3

A (S D): 1, 2, 3B (S D): 1, 2, 3C (S D): 1, 4, 5, 3

Voice (S D): 1, 2, 3Data (S D): 1, 4, 5, 3

Voice (S D): 1, 2, 3Data (S D): 1, 4, 5, 3

11 22

44 55

33SS DD

S’S’ D’D’

(S D): 1, 2, 3(S’ D’): 1, 4, 5, 3

(S D): 1, 2, 3(S’ D’): 1, 4, 5, 3

(S D): 1, 2, 3(S’ D): 1, 2, 3

(S D): 1, 2, 3(S’ D): 1, 2, 3

TOC – IP – Overview – Routing

SwitchingDefinition

Sending the bits along the pathApproaches

Circuit (Telephone; Lightwave)Packet

Virtual Circuit (ATM)Datagram (Ethernet, IP)

NotesA circuit or VC can be a link in an IP networkAn Ethernet LAN can be a link in an IP network

TOC – IP – Overview – Switching

Switching (cont.)Datagram v/s Virtual Circuit

Datagram routingEach packet to be forwarded independently

Virtual CircuitEach packet from same “flow” uses same routeMore state (pick the “right” granularity)

QoS sensitive networks use VC’s and signalingFind a route that has the resources available for the connection. “Reserve” the resources before sending data packets

TOC – IP – Overview – Switching

IssuesScalability [great in IP]

Millions of nodesRouting tables should remain “small”Updates should be manageable

Topology Changes [good in IP]Routers compute new routes as topology changesChanges should not affect most tables

Performance [poor in IP]Link utilization should be well-balanced [not in practice]Updates should be fast [not always]Ideally, some flows would have a guaranteed rate [no]Network should detect configuration errors or other errors [no]Network should protect itself against attacks [no]

TOC – IP – Overview – Issues

Basic IdeasAddressing

Layer 2: Local scheme, typically flat not scalableLayer 3: Location based and hierarchical scalableTemporary addresses for mobile nodesNetwork Address Translation to reuse addresses

Routing Route is based on destination only (roughly: shortest path)Network decomposed into domainsInterdomain routing: Uses a path-vector algorithmIntradomain routing: Uses a link state or a distance vector algorithm

VariationsMulticast; P2P; Ad Hoc; Sensors; Content Distribution Networks

AddressingExamplesClass-Based AddressingCIDR: Classless Interdomain RoutingAssigning AddressesDHCPNetwork Address Translation

TOC – IP – Addressing

ExamplesFlat AddressingHierarchical AddressingInternetworkingLayers 2 and 3

TOC – IP – Addressing – Examples

Flat Addressing

3311

22

5544

66

a

b

a

b

c a

b

c a

b

baba

2: b3: a4: a5: a6: a

2: b3: a4: a5: a6: a

1: a3: b4: b5: b6: b

1: a3: b4: b5: b6: b

1: a2: b4: c5: c6: c

1: a2: b4: c5: c6: c

1: a2: a3: a5: c6: b

1: a2: a3: a5: c6: b 1: a

2: a3: a4: a5: b

1: a2: a3: a4: a5: b

Address Ports 1: a2: a3: a4: a6: b

1: a2: a3: a4: a6: b

Routing Table: One per nodeDestination Exit Port

Addresses are arbitrary; not based on topology (e.g., Ethernet)N nodes N -1 entries in every routing table; not scalable

TOC – IP – Addressing – Examples – Flat

Hierachical Addresses

1.11.11.21.2

1.31.3

2.22.22.12.1

2.32.3

a

b

a

b

c a

b

c a

b

baba

1.3: bDefault: a

1.3: bDefault: a

1.2: a1.3: b

Default: c

1.2: a1.3: b

Default: c

2.2: c2.3: b

Default: a

2.2: c2.3: b

Default: a2.3: b

Default: a2.3: b

Default: a

1.2: aDefault: b

1.2: aDefault: b

2.2: bDefault: a

2.2: bDefault: a

Addresses are arranged based on topology (e.g., IP)Few entries in each routing table; scalable

TOC – IP – Addressing – Examples – Hierarchical

InternetworkingRecall the basic internetworking scheme of IP:

1.4x

1.7y

2.5z

2.4u

3.6v 3.8

w

1.2t

1.*: localDefault: y1.*: local

Default: y

1.2: t1.7: y

1.2: t1.7: y

IP

Local

x y | 1.4 3.8 | data

z u | 1.4 3.8 | data

v w | 1.4 3.8 | data

a

b

d

a

1.*: local4.*: bDefault: a

1.*: local4.*: bDefault: a

1.4: x1.7: y

1.4: x1.7: y

TOC – IP – Addressing – Examples – Internetworking

Layers 2 and 3EthernetSwitch

EthernetSwitch

Router

p

Phy PhyPhy PhyPhy

Transport

Application

Phy

Transport

Application

Phy Phy

Destination Address B Local to port pLocal address B Layer 2 address w

Phy Phy

Linky

NetworkC D

LinkvLinkLink

x

NetworkA

Linkw

NetworkB

Link

Destination Address B Next Hop CLocal address C Layer 2 address y

TOC – IP – Addressing – Examples – Layers 2/3

Class-Based AddressesAddressesScalability Problem

TOC – IP – Addressing – Class

Addresses

Addressing reflects internet hierarchy

32 bits divided into 2 parts:

Class A

Class B

Class C

network host 00

network host 1160

network host 1240

~2 million nets256 hosts

8

0

1 0

TOC – IP – Addressing – Class - Addresses

Scalability ProblemExample: an organization initially needs 100 addresses

Allocate it a class C addressOrganization grows to need 300 addressesClass B address is allocated. (~64K hosts) That’s overkill -a huge wasteOnly about 8200 class B addresses!Artificial Address crises

TOC – IP – Addressing – Class - Scalability

Classless Internet Domain Routing (CIDR)

CIDR allows networks to be assigned on arbitrary bit boundaries.

Address ranges can be assigned in chunks of 2k k=1…32 Idea - use aggregation - provide routing for a large number of customers by advertising one common prefix.

This is possible because nature of addressing is hierarchicalSummarization reduces the size of routing tables, but maintains connectivity. Aggregation

Scalability and survivability of the Internet

TOC – IP – Addressing – CIDR

CIDR (cont.)Suppose fifty computers in a network are assigned IP addresses 128.23.9.0 - 128.23.9.49

They share the prefix 128.23.9Is this the longest prefix?

Range is 01111111 00001111 00001001 00000000 to01111111 00001111 00001001 00110001

How to write 01111111 00001111 00001001 00X?Convention: 128.23.9.0/26There are 32-27=6 bits for the 50 computers

26 = 64 addresses


CIDR (cont.)

Specify a range of addresses by a prefix: X/YThe prefix common to the entire range is the first Y bits of X.X: The first address in the range has prefix XY: 232-Y addresses in the range

Example 128.5.10/23Common prefix is 23 bits: 01000000 00000101 0000101Number of addresses: 29 = 512

Prefix aggregationCombine two address ranges128.5.10/24 and 128.5.11/24 gives 128.5.10/23

Routers match to longest prefix

TOC IP Addressing – – – CIDR

CIDR Longest prefix match routing

1100, 1101, 1111


1110

1001, 1011, 1010

0100, 0001

111

1101111

0

10

b

da

c

Dest. a b c d1100 3 2 1 01001 1 1 2 01111 4 3 1 0

Length of longest prefixmatch for given port

CIDR (cont.)Example

128.32.134.12 128.32.134.27

128.32.112.15

128.32.12.54

R1

R4R3

128.32.112128.32.134

128.32Default

Default

128.32.134

128.32.32128.32.12128.32.112Default

128.32.12


CIDR - Subnets

e1:

H1

R1

H2

H3e1

e2

e3

e4 e5

IP1

IP3

H1: IP1 Mask: 255.255.255.0

H2: IP2 Mask: 255.255.255.0

H3: IP3 Mask: 255.255.255.0

H1: Is H3 on same subnet as I am?

Yes if IP3/24 = IP1/24

R2IP2


e2:

e1:

CIDR (cont.)Direct Delivery

H1

R1

H2

H3e1

e2

e3

e4 e5

IP1

IP2

IP3

R2

IP1|IP2|X

Who is IP2?all|e1

I am IP2e2:e1|e2

e2|e1

IP1 IP2 on same subnetIP1 IP2 on same subnet

Address Resolution Protocol = Layer 3 Address Layer 2 AddressAddress Resolution Protocol = Layer 3 Address Layer 2 Address


CIDR (cont.)Indirect Delivery IP1 IP3 not on same subnetIP1 IP3 not on same subnet

H1

R1

H2

H3e1

e2

e3

e4 e5

IP1

IP3

R2

IP1|IP3|Xe4|e1

IP1|IP3|XSH

Who is IP3?all|e5

I am IP3e5|e3

IP1|IP3|Xe3|e5

IP2

Note: Fragmentation may be required at R1TOC – IP – Addressing – CIDR

Assigning IP address (Ideally)

A host gets its IP address from the IP address block of its organizationAn organization gets an IP address block from its ISP’s address blockAn ISP gets its address block from its own provider OR from one of the 3 routing registries:

ARIN: American Registry for Internet NumbersRIPE: Reseaux IP EuropeensAPNIC: Asia Pacific Network Information Center

Each Autonomous System (AS) is assigned a 16-bit number (65536 total)

Currently 14,000 AS’s in use

TOC – IP – Addressing – Assigning Addresses

DHCP – Dynamic Host Configuration Protocol

IdeaTemporary addresses assigned “on demand”

AdvantagesEnables to reuse addresses

You come to a classroom with a laptopDial-up users

Automates the assignment of addressesDisadvantage

Cannot be a server (how to find address?)

TOC – IP – Addressing – DHCP

DHCP (cont.)

OperationsDHCP server maintains list of available addressesClient requests an address

Client sends “DHCP discover message”(“me all” = [0…0 | 1…1])Server replies with “DHCP offer”Client asks for address; server provides one…

Client can extend/release the leaseServer and client can test address

TOC – IP – Addressing – DHCP

NAT

OverviewExampleHow NAT works

TOC – IP – Addressing – NAT

OverviewShortage of IP AddressesCIDR may not be enoughIPv6 may take a long time until deployedNAT enables reuse of addressesPrivate Addresses:

10.0.0.0 - 10.255.255.255172.16.0.0 - 172.31.255.255196.168.0.0 - 196.168.255.255

See IETF RFC 1631 (1994)

TOC – IP – Addressing – NAT – Overview

ExampleHome Network

One IP address (IPa) is visible outside

IPa (typically DHCP)

IPb(DHCP with NAT)

IPc(DHCP with NAT)

NAT

Note: Can be extended to a set of addresses instead of only one (IPa)In that case, some “static” addresses can be reserved for servers …

TOC – IP – Addressing – NAT – Example

How it worksTrick: Use TCP port to distinguish computersThere are 64k port numbers, the first 1k are reserved

IPa

IPc

NAT

IPx[IPb | IPx | TCPm | TCPn | …]

[IPa | IPx | TCPb | TCPn | …]

[TCPb IPb, TCPm]

[IPx | IPa | TCPn | TCPb | …]

[IPx | IPb | TCPn | TCPm | …]

IPbTOC IP Addressing NAT – – – – How

RoutingRouting Sub-FunctionsHierarchicalTypes of Protocol

TOC – IP – Routing

Routing Sub-Functions

Topology Update: Characterize and maintain connectivity

Discover neighborsMeasure “distance” (one or more metric)Disseminate

Route Computation:Kind of path: Multicast, UnicastCentralized or Distributed AlgorithmPolicyHierarchy

Switching: Forward the packets at each node

TOC – IP – Routing – Sub-Functions

Hierarchical Routing The internet has many Administrative Domains

A

B

C

31

2

12

10

13

11

6

7

8

5

4

TOC – IP – Routing – Hierachical

Hierarchical Routing Border Routers

6

4

3

2

13

A

B

C

2

4

3

6

13

7

8

5

1 12

1011

OSPF

RIP

IGRP

BGP


Hierarchical Routing Interdomain & Intradomain

A

B

C

6

7

8

5

4

31

2

12

10

13

11

6

4

3

2

13

B

2

4

3

6

13

OSPF

RIP

IGRP

BGP

InterDomainInterDomain

IntraDomain

IntraDomain

IntraDomain


Types of Routing ProtocolOverviewLink StateDistance VectorLink State vs. Distance VectorPath Vector: Interdomain Routing

TOC – IP – Routing – Types

Overview

Topology changes can be detected by nearby nodesThese changes must be reflected in the routes

Mechanisms for disseminating informationLink State: Communicate the names and costs of neighbors. Each node maintains the entire topology. E.g. used in OSPFDistance Vector: Communicate current distance estimates of node to every other node. E.g. used in RIPPath Vector: Communicate current estimates of preferred paths from node to every other node. E.g. used in BGP

TOC – IP – Routing – Types – Overview

Overview

AABB

CCDD

2

1

1

3

A: [B, 2], [C, 1]B: [A, 2], [D, 1]C: [A, 1], [D, 3]D: [B, 1], [C, 3]

1) Exchange Link States 2) Each node computesthe shortest paths tothe others

LINK STATE

AABB

CCDD

2

1

1

3

DISTANCE VECTOR

0

0AA

BB

CCDD

2

1

1

3

1

03

AABB

CCDD

2

1

1

3

AABB

CCDD

2

1

1

3

PATH VECTOR

D

DAA

BB

CCDD

2

1

1

3

B,D

C,D

AABB

CCDD

2

1

1

3

“Don’t like B”

TOC IP Routing Types – – – – Overview

Link State Protocols

OverviewLink State Advertisements Shortest Path Algorithm: Dijkstra

TOC – IP – Routing – Types – Link State

Overview

1. Every node learns the topology of the network

Flooding of Link State Packets (LSP)2. An efficient shortest path algorithm

computes routes to every other node3. Node updates Forwarding Table

TOC – IP – Routing – Types – Link State - Overview

Link State Advertisements

Link State PacketsFlooding ExampleSome Issues

TOC – IP – Routing – Types – Link State - LSA

Link State Packets

SourceSequence Number

AgeList of Neighbors

Every router sends Link State Packets (LSPs) to all of its neighborsLSPs arrive and wait in buffers to be “accepted”If node j receives a LSP from node k it compares the sequence numbers. If this is the most recent one from k, send to N(j)-{k}.

This way each router can send its LSP to all other routersAge starts out at 7. At any router, value is decremented every 8seconds. At 0 discard.As long as sequence don’t wrap this works

Otherwise things can get ugly

TOC – IP – Routing – Types – Link State – LSA – LSP

LSP - Example

6

7

8

5

4

31

2

12

10

13

11

TOC – IP – Routing – Types – Link State – LSA – Example

LSP - Example

6

7

8

5

4

31

2

12

10

13

11


LSP - Example

6

7

8

5

4

31

2

12

10

13

11


LSP - Example

6

7

8

5

4

31

2

12

10

13

11


Some Issues

What happens if some routers are much faster at transmitting LSPs?What happens if sequence numbers wrap?What happens when a partitioned network is reconstituted?What about security?Etc., etc.Many lines of code

TOC – IP – Routing – Types – Link State – LSA – Issues

Dijkstra

Every node knows the graphAll link weights are >= 0

Goal at node 1: Find the shortest paths from 1 to all the other nodes.Each node computes the same shortest paths so they all agree on the routesStrategy at node 1: Find the shortest paths in order of increasing path length

1

3

4

6

2

5

1

4

11

41

3

2

1

TOC – IP – Routing – Types – Link State - Dijkstra

DijkstraIDEA: Given P(k) we can find P(k+1) efficiently:To get P(k+1), observe that

1. This node cannot be in P(k)2. It must be one hop away from some node

in P(k)Suppose 2 were false. We picked i

Node i has no edge into P(k)There must be a node x, not in P(k) such thatx is one hop away from P(k) andD(1,i)=D(1,x)+D(x,i)

But then, D(1,x) < D(1,i) and we would have picked x instead.

Pick node(s) that is one hop away from P(k) that is closest to 1.Keep iterating until all nodes are in P

Notationc(i,j) >=0 :cost of link from (I,j)D(1,i): Shortest path from 1 to i.D(1,x,i): Shortest path from 1 to i via xLet P(k) be the set of nodes k-closest to 1

P(2)={1,2}

1

3

4

6

2

5

1

4

11

41

3D(1,5)=2D(1,6,5)=5 2

1


Dijkstra

13

4

6

2

5

1

41 1

41

2

1

13

6

2

51

1 4

4 2

P(2)={1,2}D(1,2)=1

13

4

6

2

51

1

3 2

3

5

P(4)={1,2,3,5,6}D(1,3)=3D(1,6)=3

13

4

6

2

51

1

3 2

3

6

P(3)={1,2,5}D(1,5)=2


Dijkstra - Forwarding Table

At node 5

Outgoing Cost

1 2 2

2 2 1

3 3 1

4 3 3

6 6 1

1

3

4

6

2

5

1

4

11

41

3

2

1


Distance Vector Protocol

Bellman – FordWhy does it work?Counting to InfinityBad News Travel SlowlyAsynchronous Bellman – FordOscillations

TOC – IP – Routing – Types – DV

Bellman-Ford

1

3

4

6

2

5

1

4

11

41

2

3

1

i Di

1 (0,1,∞,∞,∞,4)2 (1,0,3,∞, 1,∞)3 (∞,3,0,2, 1,∞)4 (∞,∞,2,0, 4,∞)5 (∞,1,1,4, 0,1)6 (4,∞,∞,∞,1,0)

C(3,4) = 2

Initially

Communicate current distance estimates of node to every other node

This is called its distance vector: Di = (D(i,1),D(i,2),…,D(i,n))Initially, assume that D(i,j) = c(i,j) if there is a link ij

= ∞ otherwiseThe nodes do not need to learn the entire topology

Just the distance estimates (vectors) of their neighbors

Periodically each node sends its distance vector to all of its neighbors

TOC – IP – Routing – Types – DV – Bellman-Ford

1

3

4

6

2

5

1

4

11

41

2

3

1

Bellman-FordUpdate: when receive estimatesD(i,d) = minjεN(i) {c(i,j) + D(j,d)}

3 gets updates from 2 and 5

i Di

1 (0,1,∞,∞,∞,4)


2 (1,0,3,∞, 1,∞)3 (∞,3,0,2, 1,∞)4 (∞,∞,2,0, 4,∞)5 (∞,1,1,4, 0,1)6 (4,∞,∞,∞,1,0)

D(3,1) = min{c(3,2) + D(2,1), c(3,5) + D(5,1)}

= min{ 3 + 1 , 1 + ∞ }= 4

1

3

4

6

2

5

1

4

11

41

2

3

1

Bellman-FordFocus on destination 1Here are the values of D(i,1):

i 1 2 3 4 5 6 71 0 0 0 0 0 0 02 ∞ 1 1 1 1 1 13 ∞ ∞ 4 3 3 3 3

4 ∞ ∞ ∞ 6 5 5 55 ∞ ∞ 2 2 2 2 26 ∞ 4 4 3 3 3 3

step


Why does this compute shortest paths?

Suppose in every tick each node sends its distance vector.

Assume that initial distances are ∞At time h, node i has as an estimate of the shortest path to node j that has <= h+1 hops!Dh+1(i,j) = minkεN(i) {Dh(k,j) + c(i,k)}

13

4

6

2

5

1

3 2

3

513

6

2

5

1 4

13

4

6

2

5

1

41 1

41

23

1

13

4

6

2

5

1

1

6

21 3

6

4 2 3 24

TOC – IP – Routing – Types – DV – Why

Counting to Infinity

A B C

012

A B C

0

All links cost 1

4 3

A B C

06 5

Ping-Pong to Eternity

TOC – IP – Routing – Types – DV – Counting to Infinity

Bad News Travels Slowly…4 3

2

1

1

11

M

1

D(2,1)=2, D(3,1)=1, D(4,1)=2

TOC – IP – Routing – Types – DV – Bad News

Bad News Travels Slowly…4 3

2

1

1

11

M

1Node 2 takes about M Iterations to figure out thatD(2,1)=M

Fundamental Cause: After a network change, think of the networkprotocol running from time 0. The initial conditions are arbitrary…

•Tricks exist to get around these problems but not fool proof

TOC – IP – Routing – Types – DV – Bad News

Asynchronous Bellman Ford

In general, nodes are using different and possibly inconsistent estimatesIf no link changes after some time t, the algorithm will eventually converge to the shortest pathNo synchronization required at all…

TOC – IP – Routing – Types – DV – Asynchronous

Oscillations

Link costs must reflect link speed AND congestionUnder both LSP and DV routing occurs over a tree

The costs of the links of this tree will increaseThe other links will not be congested

Their costs will dropRouting protocol will shift traffic and create a new treeThis process of shifting and reshifting can be severeWay out: Change congestion costs slowly (exponential averaging) – Route dampening

TOC – IP – Routing – Types – DV – Oscillations

Oscillations - ExampleHeavy Load High Delay

1

2

3

4

5 5

1 1

Traffic

Light Load Low Delay

1

2

3

4

1 5

5 1

Traffic

Light Load Low Delay

Heavy Load High Delay

TOC – IP – Routing – Types – DV – Oscillations

Link State vs. Distance VectorNo clear winner

LS is robust since it each node computes its own routes independently

Suffers from the weaknesses of the topology update protocol. Inconsistency etc.Excellent choice for a well engineered network within one administrative domainE. g. OSPF

DV works well when the network is large since it requires no synchronization and has a trivial topology update algorithm

Suffers from convergence delaysVery simple to implement at each nodeExcellent choice for large networksE.g. RIP

TOC – IP – Routing – Types – LS vs. DV

Documents

IP: Network Layeree122/sp04/network layer.pdfGoals and Tasks Goals of Network Layer Guide packets from source to destination Use network links “efficiently” (e.g., prefer shorter