Link State Routing - Engineering and Advanced Technology - Massey

159.334 Computer Networks159.334 Computer Networks

Link State Routing

Professor Richard Harris

School of Engineering and AdvancedTechnology (SEAT)

Computer NetworksComputer Networks -- 1/1/22


Presentation Outline

Link State Routing Protocols



Learning Objectives

You will be able to:

Describe the operation of OSPF

Discuss the differences between link state routing andother forms of routing



References

Tanenbaum, “Computer Networks”, 4th Edition

Forouzan, “Data Communications and Networking”, 4th

Edition

Cisco CCNA1 Module 10 - part 1

Stallings, William 2000 ‘Data and ComputerCommunications’, Prentice Hall, Sixth Edition

Russell, Travis 1997 ‘Telecommunications Protocols’,McGraw Hill



Introduction to Link State RoutingProtocols - 2

Link state protocols are based upon theprinciple of a “distributed map”.

All of the nodes in the network have a copy ofthe map and it is regularly updated. The map isrepresented as a database.

We shall examine this database in the followingslides and explain how it is updated using theflooding protocol.

The Link State protocols are known as“Shortest Path First” algorithms as they arebased upon a well-known shortest pathalgorithm developed by Dijkstra.



An Example Network

AABB

CC

EEDD

Link 1 Link 2

Link 5

Link 4

Link 6

Link 3

To illustrate the concept of a link state protocol and itsmode of operation, we shall consider a simple 5 nodeand 6 link network with unit link costs:



The Database - 1

The idea behind Link StateRouting is to have all the nodesmaintain a complete copy of thenetwork map and perform acomputation of the best routesbased on this local copy of themap.

The database oppositerepresents the network shownon the previous slide.

From To Link Distance NumberA B 1 1 2

A D 3 1 1

B A 1 1 2

B C 2 1 2

B E 4 1 1

C B 2 1 2

C E 5 1 1

D A 3 1 1

D E 6 1 1

E B 4 1 1

E C 5 1 1

E D 6 1 2



The Database - 2

Each record in the database is inserted by the onestation that is responsible for it.

It containsan interface identifier (the link number in our case)

a state of the link indicator (not shown in the table)

a distance metric

With the above information, each node can work out theshortest path from itself to all other nodes. This is easilydone with modern computers and the Dijkstra algorithmis very fast - even in larger networks.

As all nodes have the same database, routes arecoherent and there are no loops.



The Flooding Protocol - 1

The main objective of a routing protocol is to generateroutes on the basis of the prevailing conditions in anetwork.

To achieve this objective, Link State protocols mustsend out update information whenever there is a changein the link state.

For example, if Link 1 fails in our network, then amessage must be generated by nodes A and B to informthe other nodes of this change in condition.



The Flooding Protocol - 2

AABB

CC

EEDD

Link 1 Link 2

Link 5

Link 4

Link 6

Link 3

From A to B, Link 1, distance = inf

From B to A, Link 1, distance = inf

From A to B, Link 1, distance = inf

From B to A, Link 1, distance = inf



Flooding Algorithm - 1

The previous slide showed the messages circulating after the update.

Note, however, that the messages should be time-stamped otherwise oldmessages could pollute the database.

The algorithm for flooding can therefore be summarised as:

Receive message. Look for record in database.

If the record is not yet included, add it to the database, broadcast the message.

Else if the number in the base is lower than the number in the message, replacethe value with the new one and broadcast the message.

Else if the number in the base is higher, transmit the current database valuethrough the incoming interface.

Else if both numbers were the same, do nothing.



Flooding Algorithm - 2

Note that the broadcast operation will cause the transmission ofthe message on all interfaces except the incoming one.

If we assume that the message identifier for the first record in thedatabase at A was “1”, then the message sent by A to D would be:

From A to B: Link 1, distance=inf; message #=2

D will update its database and transmit the message to E. E willalso update its database and transmit the message to B and C.

When they receive the second message, B and C will notice thatthe number is the same as in their current databases so that theflooding operation will terminate according to the algorithm.



Bringing up Adjacencies - 1

Consider the case where both links 1 and 6 have failed.

You will see that nodes {A,D} and {B,C,E} have becomeisolated, and each will be aware that the other group hasbecome unreachable.

AABB

CC

EEDD

Link 1 Link 2

Link 5

Link 4

Link 6

Link 3




There will be two distinctly different versions of thedatabase:

From To Link Distance NumberA B 1 inf 2

A D 3 1 1

B A 1 inf 2

B C 2 1 2

B E 4 1 1

C B 2 1 2

C E 5 1 1

D A 3 1 1

D E 6 inf 1

E B 4 1 1

E C 5 1 1

E D 6 1 2

Routing seen from {A,D}


A D 3 1 1

B A 1 inf 2

B C 2 1 2

B E 4 1 1

C B 2 1 2

C E 5 1 1

D A 3 1 1

D E 6 1 1

E B 4 1 1

E C 5 1 1

E D 6 inf 2

Routing seen from {B,C,E}




Notice that in this situation,there is no possibility of a“count to infinity” as indistance vector protocols.

Consider the case whereLink 2 also fails.

An interesting problemoccurs when restoration isundertaken for link 1 in thesecircumstances.


A D 3 1 1

B A 1 inf 2

B C 2 inf 2

B E 4 1 1

C B 2 inf 2

C E 5 1 1

D A 3 1 1

D E 6 1 1

E B 4 1 1

E C 5 1 1

E D 6 inf 2

Routing seen from {B,C,E}




Consider the resetting of link 1 so that it has become operational.

It can be shown that merely distributing this new link information isinsufficient.

We have to guarantee that the two groups end up with the samedatabases. This alignment process is referred to as “Bringing upAdjacencies” in OSPF.

The idea is that the groups must synchronise their databases bykeeping only the most up to date version of each record.

OSPF does this by defining database description packets.

During the first phase of the synchronisation procedure, bothrouters send a complete description of their database records in asequence of description packets.




Each router looks for “interesting records” in their database - forexample those records with remote numbers greater than localnumbers (identifiers).

In the second phase of the synchronisation procedure, each routerwill poll its neighbour for a full copy of these interesting recordsthrough link state requests.

Node ANode A Node DNode D

Once the synchronisation has occurred, the normalflooding procedure will propagate the updated records.



Map Updates

To prevent accidental corruption of the databases in thenetwork, a scheme developed by Radia Perlman hasbeen introduced:

The flooding procedure includes hop-by-hop acknowledgment.

Database description packets are sent in a secure fashion.

Each link state record is protected by a timer is is removed fromthe system if a refreshing packet is not sent in time.

All records are protected by a checksum.

The messages can be authenticated by a password.



“Open Shortest Path First” - OSPF

We have seen that the distance vector protocols are based upon theBellman-Ford procedure. The link state protocols are based uponthe method of Dijkstra.

It turns out that Dijkstra’s algorithm is generally faster thanBellman-Ford’s approach. In fact, if there are N nodes and M links,Bellman-Ford requires o(NM) iterations, whilst Dijkstra requires o(Mlog M) iterations. For large networks, this difference is quitesignificant.

Dijkstra called his method “Shortest Path First”.

The IETF coined the phrase “Open Shortest Path First” because itwas developed in an “open” fashion.



Advantages of Link State Protocols

It appears that most network specialists favour the link stateprotocols.

The reasons are:

Fast, loopless convergence.

Two phases are used: (a) Rapid transmission via flooding and, (b) Localcomputation of the routes. Cf Distance vector protocols.

Support of precise metrics and multiple metrics - if desired.

Avoids the problem of counting to infinity, finer grain metrics are nowpossible.

Support of multiple paths to a destination.

If there are some almost equivalent routes in terms of cost, these may beused in link state protocol systems.

Separate representation of external routes.



Congestion

If the users send a large number of packets via a given subnet, it may getcongested

This can be caused by the following reasons

Poor routing decisions

Nodes may take a long time to keep records (queue lengths, info. from other nodes, etc)or update routing tables

If the incoming traffic from all directions exceeds the outgoing link buffer capacity,queues will grow very large.

If a node has no buffer space, all new packets may have to be discarded. Then thesending node will time-out and retransmit again and again.

Now, since the sending node cannot release its buffer until an acknowledgement comesback from the receiver, it will run out of buffer space too and will start losing packets. Inthis way, the congestion propagates through the subnet

Generally 80% utilisation is critical – queueing section for reasons!



Congestion Control

Congestion control aims to keep number of packets below the levelat which performance falls off dramatically

It is a global issue and involves the behaviour of all users, all nodes,routing algorithms and other processing required for the operationof the subnet including the process at data link layer, network layerand transport layer.

Each process needs to take congestion control into considerationwhile being designed and needs to be optimized globally.

Note: Flow control is an issue in a point-to-point link. It controls afast transmitter from overwhelming a slow receiver, often usingfeedback acknowledgements


Some Current Research Activity

Weight Setting with Linear Programming(Jointly between: R Harris, J Murphy, R Surayasaputra, S Eum)



What Does a Router Do?

0DirectNet2

0DirectNet1

4R3Net4

2R2Net3

CostNext HopDestination

R1

R2

R3Net1 Net2

Net3

Net4

Routing Table for R1

(4)

(2) (4)

Metric values



Why is Traffic Engineering needed?

1

3

2(20, 4)

(0, 4)(0, 2)

distance metriclink load

each link capacity = 15

20 units of trafficfrom node 1 tonode 2

CONGESTION!!!

1 → 2 : 41 → 3 → 2 : 2+4=6

The Shortest Path Routing Problem



Solution

1

3

2

(10, 4+2 = 6)

(10, 4)(10, 2)

Updated link metric

each link capacity = 15

20 units oftraffic from node1 to node 2

1 → 2 : 61 → 3 → 2 : 2+4=6

Equal Cost Multi-Path Solution

…….……..…………….

6R3Net4

6R2Net4

2R2Net3

CostNext HopDestination



Weight Setting using LinearProgramming (LP)

Is a Novel TechniqueCan use existing LP packages (eg CPLEX)

Thus

More robust - won’t break down

It is a very FAST algorithm - up to 100 times faster than searchtechniques

Still get large reduction in max. link utilisation

– AT &T Tabu search technique 15 min – 1.5 hrs ( 90 nodes)

– Our LP technique:10 seconds!!!

• Can easily use existing features to develop new methods (eg linkfailure)

Link State Routing - Engineering and Advanced Technology - Massey

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