Data Communication and Networks

Lecture 11

Internet RoutingAlgorithms and Protocols

December 5, 2002

Joseph Conron

Computer Science Department

New York University

conron@cs.nyu.edu

Some Perspective on Routing …..

• When we wish to take a long trip by car, we consult a road map.

• The road map shows the possible routes to our destination.

• It might show us the shortest distance, but, it can’t always tell us what we really want to know:– What is the fastest route!

– Why is this not always obvious?

• Question: What’s the difference between you and an IP Packet?

Packets are Dumb, Students are Smart!

• We adapt to traffic conditions as we go.• Packets depend on routers to choose how they get

their destination.• Routers have maps just like we do. These are

called routing tables.• What we want to know is:

– How to these tables get constructed/updated?

– How are routes chosen using these tables?

Static Vs. Dynamic Routing

• Routes are static if they do not change.– Route table is loaded once at startup and all changes are

manual

– Computers at the network edge use static routing.

• Routes are dynamic if the routing table information can change over time (without human intervention.– Internet routers use dynamic routing.

Routing Table Example

Dynamic Routing and Routers

• To insure that routers know how to reach all possible destinations, routers exchange information using a routing protocol.

• But, we cannot expect every router to know about every other router.– Too much Internet traffic would be generated.– Tables would be huge (106 routers)– Algorithms to choose “best” path would never

terminate.

• How to handle this?

Autonomous Systems (AS)

• Routers are divided into groups known as an autonomous systems (AS).

• ASs communicate using an Exterior Routing Protocol (Intra-AS Routing)

• Routers within an AS communicate using an Interior Routing Protocol (Inter-AS Routing)

Interior and Exterior Routing

Why different Intra and Inter-AS routing ?

Policy:

• Inter-AS: admin wants control over how its traffic routed, who routes through its net.

• Intra-AS: single admin, so no policy decisions needed

Scale:

• hierarchical routing saves table size, reduced update traffic

Performance:

• Intra-AS: can focus on performance

• Inter-AS: policy may dominate over performance

Routing Algorithms

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

Routing Algorithm classification

Static or dynamic?

• Static: – routes change slowly over time

• Dynamic: – routes change more quickly

• periodic update

• in response to link cost changes

Routing Algorithm classificationGlobal or decentralized?

• Global:• all routers have complete topology, link cost info• “link state” algorithms

• Decentralized: • router knows physically-connected neighbors, link costs

to neighbors• iterative process of computation, exchange of info with

neighbors• “distance vector” algorithms

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 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

A Link-State Routing Algorithm

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

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) = 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: 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

F

2

2

13

1

1

2

53

5

Dijkstra’s algorithm, discussion

Algorithm 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 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: 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 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,2

Outgoing link to use, cost

dest

inat

ion

Distance table Routing table

Distance 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:

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:

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: D (y,V) = D (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 min DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval 22 23 if we have a new min D (Y,w)for any destination Y 24 send new value of min D (Y,w) to all neighbors 25 26 forever

w

XX

XX

X

w

w

Distance Vector Algorithm: example

X Z12

7

Y

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

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”

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!

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 Z

14

50

Y60

algorithmterminates

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(n**2) algorithm requires O(nE) msgs

– may have oscillations• DV: convergence time varies

– may be routing loops– count-to-infinity problem

Comparison of LS and DV algorithms

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 propagates thru network

Interior and Exterior Routing

Interior Router Protocol (IRP)

• Passes routing information between routers within AS

• May be more than one AS in internet• Routing algorithms and tables may differ between

different AS• Routers need some info about networks outside

their AS• Use exterior router protocol (ERP)• IRP needs detailed model• ERP supports summary information on

reachability

Interior Routing

• Also known as Interior Gateway Protocols (IGP)• Passes routing information between routers within

AS• Most common IGPs:

– RIP: Routing Information Protocol

– OSPF: Open Shortest Path First

• Routing algorithms and tables may differ between different AS

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 every 30 sec via Response Message (also called advertisement)

• Each advertisement: route to up to 25 destination nets

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 neighbor router

• Advertisements disseminated to entire AS (via flooding)

Sample AS

Directed Graph of AS

Exterior Routing

Border Gateway Protocol (BGP)

• For use with TCP/IP internets

• Preferred EGP of the Internet

• Procedures– Neighbor acquisition– Neighbor reachability– Network reachability

Internet Exterior Routing: BGP

• BGP messages exchanged using TCP.

• BGP messages:

– OPEN: opens TCP connection to peer and authenticates sender

– UPDATE: advertises new path (or withdraws old)

– KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request

– NOTIFICATION: reports errors in previous msg; also used to close connection

BGP Procedure

• Open TCP connection

• Send Open message– Includes proposed hold time

• Receiver selects minimum of its hold time and that sent– Max time between Keep alive and/or update

messages

BGP Message Types

• Keep Alive– To tell other routers that this router is still here

• Update– Info about single routes through internet– List of routes being withdrawn– Includes path info

• Origin (IGP or EGP)• AS_Path (list of AS traversed)• Next_hop (IP address of border router)• Multi_Exit_Disc (Info about routers internal to AS)• Local_pref (Inform other routers within AS)• Atomic_Aggregate, Aggregator (Uses address tree structure to

reduce amount of info needed)

Uses of AS_Path and Next_Hop

• AS_Path– Enables routing policy

• Avoid a particular AS• Security• Performance• Quality• Number of AS crossed

• Next_Hop– Only a few routers implement BGP

• Responsible for informing outside routers of routes to other networks in AS

BGP Routing Information Exchange

• Within AS, router builds topology picture using IGP

• Router issues Update message to other routers outside AS using BGP

• These routers exchange info with other routers in other AS

• Routers must then decide best routes