1

Network Layer 4-1

Network Layer: Routing

Computer Networking: ATop Down ApproachFeaturing the Internet,2nd edition.Jim Kurose, Keith RossAddison-Wesley, July2002.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers).They’re in PowerPoint form so you can add, modify, and delete slides(including this one) and slide content to suit your needs. They obviouslyrepresent a lot of work on our part. In return for use, we only ask thefollowing: If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, thatyou note that they are adapted from (or perhaps identical to) our slides, andnote our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2002J.F Kurose and K.W. Ross, All Rights Reserved

Network Layer 4-2

Routing

Graph abstraction forrouting 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

F2

21

3

1

1

2

53

5

❒ “good” path:❍ typically means minimum

cost path❍ other def’s possible

2

Network Layer 4-3

Routing Algorithm classificationGlobal or decentralized

information?Global:❒ all routers have complete

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

connected neighbors, linkcosts to neighbors

❒ iterative process ofcomputation, exchange ofinfo with neighbors

❒ “distance vector” algorithms

Static or dynamic?Static:❒ routes change slowly

over timeDynamic:❒ routes change more

quickly❍ periodic update❍ in response to link

cost changes

Network Layer 4-4

A 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 pathsfrom one node (‘source”) toall other nodes❍ gives routing table for

that node❒ iterative: after k iterations,

know least cost path to kdest.’s

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

j. cost infinite if not directneighbors

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

❒ p(v): predecessor nodealong path from source to v,that is next v

❒ N: set of nodes whose leastcost path definitively known

3

Network Layer 4-5

Dijsktra’s Algorithm

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

Network Layer 4-6

Dijkstra’s algorithm: example

Step012345

start NA

ADADE

ADEBADEBC

ADEBCF

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

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

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

D(E),p(E)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

A

ED

CB

F2

21

3

1

1

2

53

5

4

Network Layer 4-7

Dijkstra’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

AD

CB

1 1+e

e0

e1 1

0 0

Network Layer 4-8

Distance Vector Routing Algorithm

iterative:❒ continues until no nodes

exchange info.❒ self-terminating: no

“signal” to stopasynchronous:❒ nodes need not

exchange info/iterate inlock step!

distributed:❒ each node

communicates only withdirectly-attachedneighbors

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)}Zw

=

=

5

Network Layer 4-9

Distance Table: example

A

E D

CB7

81

2

1

2D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

E cost to destination via

dest

inat

ion

D (C,D)E

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

= 2+2 = 4

D (A,D)E

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

= 2+3 = 5

D (A,B)E

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

= 8+6 = 14

loop!

loop!

Network Layer 4-10

Distance Vector Algorithm: example

X Z12

7

Y

6

Network Layer 4-11

Distance Vector Algorithm: example

X Z12

7

Y

D (Y,Z)X c(X,Z) + min {D (Y,w)}w== 7+1 = 8

Z

D (Z,Y)X c(X,Y) + min {D (Z,w)}w== 2+1 = 3

Y

Network Layer 4-12

Distance Vector: link cost changes

Link cost changes:❒ node detects local link cost change❒ updates distance table (line 15)❒ if cost change in least cost path, notify

neighbors (lines 23,24)

X Z14

50

Y1

algorithmterminates“good

news travelsfast”

7

Network Layer 4-13

Distance Vector: link cost changes

Link cost changes:❒ good news travels fast❒ bad news travels slow -

“count to infinity” problem!X Z

14

50

Y60

algorithmcontinues

on!

Network Layer 4-14

Distance Vector: poisoned reverse

If Z routes through Y to get to X :❒ Z tells Y its (Z’s) distance to X is infinite

(so Y won’t route to X via Z)❒ will this completely solve count to infinity

problem?

X Z14

50

Y60

algorithmterminates

8

Network Layer 4-15

Comparison 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 happensif router malfunctions?

LS:❍ node can advertise

incorrect link cost❍ each node computes only

its own tableDV:

❍ DV node can advertiseincorrect path cost

❍ each node’s table used byothers

• error propagate thrunetwork

Network Layer 4-16

Hierarchical Routing

scale: with 200 milliondestinations:

❒ can’t store all dest’s inrouting tables!

❒ routing table exchangewould swamp links!

administrative autonomy❒ internet = network of networks❒ each network admin may want

to control routing in its ownnetwork

Our routing study thus far - idealization❒ all routers identical❒ network “flat”… not true in practice

9

Network Layer 4-17

Hierarchical Routing

❒ aggregate routers intoregions, “autonomoussystems” (AS)

❒ routers in same AS runsame routing protocol❍ “intra-AS” routing

protocol❍ routers in different AS

can run different intra-ASrouting protocol

❒ special routers in AS❒ run intra-AS routing

protocol with all otherrouters in AS

❒ also responsible forrouting to destinationsoutside AS❍ run inter-AS routing

protocol with othergateway routers

gateway routers

Network Layer 4-18

Intra-AS and Inter-AS routingGateways:

•perform inter-ASrouting amongstthemselves•perform intra-ASrouters with otherrouters in theirAS

inter-AS, intra-ASrouting in

gateway A.c

network layerlink layer

physical layer

a

b

b

aaC

A

Bd

A.aA.c

C.bB.a

cb

c

10

Network Layer 4-19

Intra-AS and Inter-AS routing

Host h2

a

b

b

aaC

A

Bd c

A.aA.c

C.bB.a

cb

Hosth1

Intra-AS routingwithin AS A

Inter-AS routingbetween A and B

Intra-AS routingwithin AS B

❒ We’ll examine specific inter-AS and intra-ASInternet routing protocols shortly

Network Layer 4-20

Routing in the Internet❒ The Global Internet consists of Autonomous

Systems (AS) interconnected with each other:❍ Stub AS: small corporation: one connection to other

AS’s❍ Multihomed AS: large corporation (no transit):

multiple connections to other AS’s❍ Transit AS: provider, hooking many AS’s together

❒ Two-level routing:❍ Intra-AS: administrator responsible for choice of

routing algorithm within network❍ Inter-AS: unique standard for inter-AS routing: BGP

11

Network Layer 4-21

Internet AS HierarchyInter-AS border (exterior gateway) routers

Intra-AS interior (gateway) routers

Network Layer 4-22

Intra-AS Routing

❒ Also known as Interior Gateway Protocols (IGP)❒ Most common Intra-AS routing protocols:

❍ RIP: Routing Information Protocol

❍ OSPF: Open Shortest Path First

❍ IGRP: Interior Gateway Routing Protocol (Ciscoproprietary)

12

Network Layer 4-23

RIP ( Routing Information Protocol)

❒ Distance vector algorithm❒ Included in BSD-UNIX Distribution in 1982❒ Distance metric: # of hops (max = 15 hops)

❍ Can you guess why?

❒ Distance vectors: exchanged among neighbors every 30sec via Response Message (also called advertisement)

❒ Each advertisement: list of up to 25 destination netswithin AS

Network Layer 4-24

RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B 7

x -- 1…. …. ....

w x y

z

A

C

D B

Routing table in D

13

Network Layer 4-25

RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B A 7 5

x -- 1…. …. ....

Routing table in D

w x y

z

A

C

D B

Dest Next hops w - - x - - z C 4 …. … ...

Advertisementfrom A to D

Network Layer 4-26

RIP: Link Failure and RecoveryIf no advertisement heard after 180 sec --> neighbor/link

declared dead❍ routes via neighbor invalidated❍ new advertisements sent to neighbors❍ neighbors in turn send out new advertisements (if

tables changed)❍ link failure info quickly propagates to entire net❍ poison reverse used to prevent ping-pong loops

(infinite distance = 16 hops)

14

Network Layer 4-27

RIP Table processing

❒ RIP routing tables managed by application-levelprocess called route-d (daemon)

❒ advertisements sent in UDP packets, periodicallyrepeated

physicallink

network forwarding (IP) table

Transprt (UDP)

routed

physicallink

network (IP)

Transprt (UDP)

routed

forwardingtable

Network Layer 4-28

RIP Table example (continued)Router: giroflee.eurocom.fr

❒ Three attached class C networks (LANs)❒ Router only knows routes to attached LANs❒ Default router used to “go up”❒ Route multicast address: 224.0.0.0❒ Loopback interface (for debugging)

Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ --------- 127.0.0.1 127.0.0.1 UH 0 26492 lo0 192.168.2. 192.168.2.5 U 2 13 fa0 193.55.114. 193.55.114.6 U 3 58503 le0 192.168.3. 192.168.3.5 U 2 25 qaa0 224.0.0.0 193.55.114.6 U 3 0 le0 default 193.55.114.129 UG 0 143454

15

Network Layer 4-29

OSPF (Open Shortest Path First)

❒ “open”: publicly available❒ Uses Link State algorithm

❍ LS packet dissemination❍ Topology map at each node❍ Route computation using Dijkstra’s algorithm

❒ OSPF advertisement carries one entry per neighborrouter

❒ Advertisements disseminated to entire AS (via flooding)❍ Carried in OSPF messages directly over IP (rather than TCP or

UDP

Network Layer 4-30

OSPF “advanced” features (not in RIP)

❒ Security: all OSPF messages authenticated (to preventmalicious intrusion)

❒ Multiple same-cost paths allowed (only one path in RIP)❒ For each link, multiple cost metrics for different TOS

(e.g., satellite link cost set “low” for best effort; high forreal time)

❒ Integrated uni- and multicast support:❍ Multicast OSPF (MOSPF) uses same topology data

base as OSPF❒ Hierarchical OSPF in large domains.

16

Network Layer 4-31

Hierarchical OSPF

Network Layer 4-32

Hierarchical OSPF

❒ Two-level hierarchy: local area, backbone.❍ Link-state advertisements only in area❍ each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.❒ Area border routers: “summarize” distances to nets in

own area, advertise to other Area Border routers.❒ Backbone routers: run OSPF routing limited to

backbone.❒ Boundary routers: connect to other AS’s.

17

Network Layer 4-33

Inter-AS routing in the Internet: BGP

Figure 4.5.2-new2: BGP use for inter-domain routing

AS2 (OSPF

intra-AS

routing)

AS1 (RIP intra-AS

routing) BGP

AS3 (OSPF intra-AS

routing)

BGP

R1 R2

R3

R4

R5

Network Layer 4-34

Internet inter-AS routing: BGP

❒ BGP (Border Gateway Protocol): the de factostandard

❒ Path Vector protocol:❍ similar to Distance Vector protocol❍ each Border Gateway broadcast to neighbors

(peers) entire path (i.e., sequence of AS’s) todestination

❍ BGP routes to networks (ASs), not individual hosts❍ E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z

18

Network Layer 4-35

Internet inter-AS routing: BGP

Suppose: gateway X send its path to peer gateway W❒ W may or may not select path offered by X

❍ cost, policy (don’t route via competitors AS), loopprevention reasons.

❒ If W selects path advertised by X, then:Path (W,Z) = w, Path (X,Z)

❒ Note: X can control incoming traffic by controlling it routeadvertisements to peers:❍ e.g., don’t want to route traffic to Z -> don’t advertise

any routes to Z

Network Layer 4-36

BGP: controlling who routes to you

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer

network:

provider

network

❒ A,B,C are provider networks❒ X,W,Y are customer (of provider networks)❒ X is dual-homed: attached to two networks

❍ X does not want to route from B via X to C❍ .. so X will not advertise to B a route to C

19

Network Layer 4-37

BGP: controlling who routes to you

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer

network:

provider

network

❒ A advertises to B the path AW❒ B advertises to X the path BAW❒ Should B advertise to C the path BAW?

❍ No way! B gets no “revenue” for routing CBAW since neither Wnor C are B’s customers

❍ B wants to force C to route to w via A❍ B wants to route only to/from its customers!

Network Layer 4-38

BGP operation

Q: What does a BGP router do?❒ Receiving and filtering route advertisements from

directly attached neighbor(s).❒ Route selection.

❍ To route to destination X, which path )of severaladvertised) will be taken?

❒ Sending route advertisements to neighbors.

20

Network Layer 4-39

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-40

NAT: Network Address Translation

❒ Motivation: local network uses just one IP address as far asoutside word is concerned:❍ no need to be allocated range of addresses from ISP: -

just one IP address is used for all devices❍ can change addresses of devices in local network

without notifying outside world❍ can change ISP without changing addresses of devices

in local network❍ devices inside local net not explicitly addressable, visible

by outside world (a security plus).

21

Network Layer 4-41

NAT: Network Address TranslationImplementation: NAT router must:

❍ outgoing datagrams: replace (source IP address, port #) of everyoutgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address,new port #) as destination addr.

❍ remember (in NAT translation table) every (source IP address, port#) to (NAT IP address, new port #) translation pair

❍ incoming datagrams: replace (NAT IP address, new port #) in destfields of every incoming datagram with corresponding (source IPaddress, port #) stored in NAT table

Network Layer 4-42

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

110.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40, 80

NAT translation tableWAN side addr LAN side addr138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80D: 10.0.0.1, 3345 4

S: 138.76.29.7, 5001D: 128.119.40.186, 802

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80D: 138.76.29.7, 5001 3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

22

Network Layer 4-43

NAT: Network Address Translation

❒ 16-bit port-number field:❍ 60,000 simultaneous connections with a single

LAN-side address!❒ NAT is controversial:

❍ routers should only process up to layer 3❍ violates end-to-end argument

• NAT possibility must be taken into account by appdesigners, e.g., P2P applications

❍ address shortage should instead be solved byIPv6

Network Layer 4-44

Mobility: approaches

❒ Let routing handle it: routers advertise permanentaddress of mobile-nodes-in-residence via usual routingtable exchange.❍ routing tables indicate where each mobile located❍ no changes to end-systems

❒ Let end-systems handle it:❍ indirect routing: communication from correspondent

to mobile goes through home agent, then forwardedto remote

❍ direct routing: correspondent gets foreign address ofmobile, sends directly to mobile

23

Network Layer 4-45

Mobility: approaches

❒ Let routing handle it: routers advertise permanentaddress of mobile-nodes-in-residence via usual routingtable exchange.❍ routing tables indicate where each mobile located❍ no changes to end-systems

❒ let end-systems handle it:❍ indirect routing: communication from correspondent

to mobile goes through home agent, then forwardedto remote

❍ direct routing: correspondent gets foreign address ofmobile, sends directly to mobile

notscalable

to millions of mobiles

Network Layer 4-46

Mobility: registration

End result:❒ Foreign agent knows about mobile❒ Home agent knows location of mobile

wide areanetwork

home networkvisited network

1

mobile contactsforeign agent onentering visitednetwork

2

foreign agent contacts homeagent home: “this mobile isresident in my network”

24

Network Layer 4-47

Mobility via Indirect Routing

wide areanetwork

homenetwork

visitednetwork

3

24

1correspondentaddresses packetsusing home addressof mobile

home agent interceptspackets, forwards toforeign agent

foreign agentreceives packets,forwards to mobile

mobile repliesdirectly tocorrespondent

Network Layer 4-48

Indirect Routing: comments❒ Mobile uses two addresses:

❍ permanent address: used by correspondent (hencemobile location is transparent to correspondent)

❍ care-of-address: used by home agent to forwarddatagrams to mobile

❒ foreign agent functions may be done by mobile itself❒ triangle routing: correspondent-home-network-mobile

❍ inefficient whencorrespondent, mobileare in same network

25

Network Layer 4-49

Forwarding datagrams to remote mobile

Permanent address:128.119.40.186

Care-of address:79.129.13.2

dest: 128.119.40.186

packet sent bycorrespondent

dest: 79.129.13.2 dest: 128.119.40.186

packet sent by home agent to foreignagent: a packet within a packet

dest: 128.119.40.186

foreign-agent-to-mobile packet

Network Layer 4-50

Indirect Routing: moving between networks

❒ suppose mobile user moves to another network❍ registers with new foreign agent❍ new foreign agent registers with home agent❍ home agent update care-of-address for mobile❍ packets continue to be forwarded to mobile (but with

new care-of-address)❒ Mobility, changing foreign networks transparent:

on going connections can be maintained!

26

Network Layer 4-51

Mobility via Direct Routing

wide areanetwork

homenetwork

visitednetwork

4

2

41correspondentrequests, receivesforeign address ofmobile

correspondent forwardsto foreign agent

foreign agentreceives packets,forwards to mobile

mobile repliesdirectly tocorrespondent

3

Network Layer 4-52

Mobility via Direct Routing: comments

❒ overcome triangle routing problem❒ non-transparent to correspondent:

correspondent must get care-of-address fromhome agent❍ What happens if mobile changes networks?

27

Network Layer 4-53

Mobile IP

❒ RFC 3220❒ has many features we’ve seen:

❍ home agents, foreign agents, foreign-agentregistration, care-of-addresses, encapsulation(packet-within-a-packet)

❒ three components to standard:❍ agent discovery❍ registration with home agent❍ indirect routing of datagrams

Network Layer 4-54

Mobile IP: agent discovery❒ agent advertisement: foreign/home agents advertise

service by broadcasting ICMP messages (typefield = 9)

RBHFMGV bits

reserved

type = 16

type = 9 code = 0

= 9

checksum

= 9

router address

standard ICMP fields

mobility agent advertisement

extension

length sequence #

registration lifetime

0 or more care-of-addresses

0 8 16 24

R bit: registrationrequired

H,F bits: homeand/or foreign agent

28

Network Layer 4-55

Mobile IP: registration example

visited network: 79.129.13/24 home agent

HA: 128.119.40.7 foreign agent

COA: 79.129.13.2

COA: 79.129.13.2

….

ICMP agent adv. Mobile agent

MA: 128.119.40.186

registration req.

COA: 79.129.13.2 HA: 128.119.40.7

MA: 128.119.40.186 Lifetime: 9999 identification:714 ….

registration req.

COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186

Lifetime: 9999 identification: 714 encapsulation format ….

registration reply

HA: 128.119.40.7

MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format

….

registration reply

HA: 128.119.40.7 MA: 128.119.40.186

Lifetime: 4999 Identification: 714 ….

time

Network Layer 4-56

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from networkserver when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected an “on”Support for mobile users who want to join network (more shortly)

DHCP overview:❍ 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

29

Network Layer 4-57

DHCP client-server scenario

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

DHCP server

arriving DHCP client needsaddress in thisnetwork

Network Layer 4-58

DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offersrc: 223.1.2.5, 67dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACKsrc: 223.1.2.5, 67dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs