Routing Protocols CT

7/27/2019 Routing Protocols CT

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Understands and Utilizes Layer 3 Information Forward “packets” towards their destination

networks on a hop-by-hop basis The forwarding router does not know if a next hop

router is the destination router just another hop

Maintains Tables to aid in decision

Act as addressing center for network Each Port on a Router is a Network or Subnet

Interconnects LANs and WANs

Translation of Protocols - LAN Based (IP, IPX) to WANBased (Frame Relay)



Multiprotocol Routing Provides Like Services for All Routed Protocols like IP,

IPX, DEC, Novell etc.

Best Path Determination

With dynamic routing enabled, it providesalternate route for a failed route



That part of the NW layer software responsiblefor deciding which output line an incomingpacket should be transmitted on.

If subnet uses datagrams, decision is made afresh

for every arriving pkt since best route may havechanged since last time.

If subnet uses VCs, routing decisions are madeonly when a new VC is being established;

thereafter the pkts follow previously establishedroute (Session Routing).



Correctness Simplicity

Robustness

Ability to cope with the changes in the topology andtraffic without requiring all the jobs in all the hosts tobe aborted and the NW to be rebooted everytimesome router crashes.

Stability

Continued efficient performance giving the desiredoutput over a period of time.

Fairness

Giving an equal chance to everybody



Rapid Convergence

Convergence is the process of agreement, by allrouters, on optimal routes.

When NW topology changes, routers distribute

routing update messages that permeate NWs,stimulating recalculation of optimal routes andeventually causing all routers to agree on theseroutes. Routing algorithms that converge slowly cancause routing loops or network outages.

Optimality

Reducing the number of hops to reduce delay andbandwidth consumed thereby improving thethroughput.



Thus, set of optimal routes from all sources to agiven destination form a tree rooted at thedestination. Such a tree is called a sink tree.

Optimality Principle

It states that if a router J is on the optimal path

from router I to router K, then the optimal path

from J to K also falls along the same route.



A sink tree does not contain any loops, so eachpacket will be delivered within a finite andbounded number of hops. In practice, life is notquite this easy. Links and routers can go down

and come back up during operation, so differentrouters may have different ideas about thecurrent topology.

Also, we have quietly finessed the issue ofwhether each router has to individually acquirethe information on which to base its sink treecomputation, or whether this information iscollected by some other means.



A routing protocol sets up routing table in routers.

Router makes a local choice depending on global topology

To make correct local decisions, each router mustknow something about global state

Global state is inherently large, dynamic and hardto collect

A routing protocol must intelligently summarizerelevant information



Non-Adaptive Algorithms Do not base their routing decisions on estimates of

current traffic and topology.

The choice of routes to get from I to J, is computedoffline, in advance and downloaded to the router.

The procedure is also called Static Routing.



Adaptive Algorithms Change their routing decisions to reflect changes in

traffic and topology.

Adaptive algorithms differ in Where they get their info from?

Locally From adjacent routers, or

From all routers

When do they change their routes? Every delta T secs

When the load changes When the topology changes

What metric is used for optimization? Distance

No of hops

Estimated transit time.



Uses a route that a network

administrator enters into the router

Static Route

Uses a route that a network protocoladjusts automatically for topology or

traffic changes

Dynamic Route



Route table acquires information in two ways.The information may be entered manually, bymeans of a static route entry, or automatically byone of several systems of automatic informationdiscovery and sharing known as dynamic routing protocols.

The procedure for statically routing aninternetwork has three steps: For each data link within the internetwork, identify all

addresses (subnet or network). For each router, identify all data links not directly

connected to that router. For each router, write a route statement for each data

link not directly connected to it.



(config)# ip route 192.168.1.0 255.255.255.0

192.168.1.193

(config)# ip route 10.4.6.0 255.255.255.0

192.168.1.193 (config)# ip route 10.4.7.0 255.255.255.0

192.168.1.193



Dynamic routing protocols are built around analgorithm Routing algorithm must, at a minimum, specify the

following: A procedure for passing reachability information

about networks to other routers. A procedure for receiving reachability information

from other routers A procedure for determining optimal routes based on

the reachability information it has and for recordingthis information in a route table

A procedure for reacting to, compensating for, andadvertising topology changes in an internetwork



All networks within an internetwork must beconnected to a router, and wherever a router hasan interface on a network that interface musthave an address on the network. This address isthe originating point for reachability information.

Routing path selection parameters:- metric is a variable assigned to routes as a means

of ranking them from best to worst or from mostpreferred to least preferred but it simply countsrouter hops

bandwidth plays a role in how efficiently traffictravels through the networks. Reliability:- The path with highest reliability would

be selected as best.



Shortest Path Routing Build a graph of the subnet

Each node of the graph represents a router and each arcof the graph represents a link.

To choose a route between a pair of routers, the

algorithm finds the shortest path between them on thegraph.

Metrics for determination of shortest path

No of hops

Geographical distance

Mean queuing and transmission delay Bandwidth utilization

Communication cost

Average traffic



Shortest Path Routing - General Case Labels on the arc computed as functions of

distance, BW, average traffic, comn cost, meanqueuing length, measured delay etc.

By changing the weighting function thealgorithm would then compute the “shortest”path measured according to:

any one criteria, or

combination of criteria.



A

B (2,A) C (&,-)

D (&,-)E(&,-)

F(&,-)

G (6,A) H (&,-)(b)

A

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

D (&,-)E(4,B)

F(&,-)

G (6,A) H (&,-)(c)

D (&,-) A

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

E(4,B)F(6,E)

G (5,E) H (&,-)(d)

A

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

D (&,-)E(4,B)

F(6,E)

G (5,E) H (9,G)(e)

A

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

D (&,-)E(4,B)

F(6,E)

G (5,E) H (8,F)(f)

A

B C

DE F

G H

2

73

6

4

2

3

22

(a)

1

2



Every incoming pkt is sent out on every outgoingline except the one it arrived on

Generates a vast number of duplicate pkts.

Flooding is not practical in most applications.

Can be used in mil applications ,due to its tremendousrobustness, it can handle losses of a part of thesubnet.

Can be used in distributed database applications toupdate data concurrently



Optimisation - dampen the process of flooding. Hop counter

Contained in the header of each pkt

Decremented at each hop

Pkt discarded when the counter reaches zero

Hop counter initialized to the length of the path fromsource to destn

If length not known the counter can be initialized to theworst case, i.e. the full diameter of the subnet.

Selective flooding

Routers do not send every incoming pkt on everyoutgoing line, but only on those lines that are goingapprox in the right direction.



Developed by Cisco in the mid-1980’s as a better

alternative solution to RIP (Routing InformationProtocol).

A distance vector protocol

Main goal was to be a robust protocol for routing

within an autonomous system IGRP was initially decided to run in any network

environments, but ported to run in OSIConnectionless-Network Protocol (CLNP).

Many organizations with large internetworks

replaced RIP with IGRP, because it is superior than RIP in:

More scalability with supported hop-count up to 255

More flexibility with sophisticated metric, a composite

metric of 5 variables; and Multi-path support acrossunequal path. 3



Routing Protocols

Interior Exterior

Dist-Vector Link-State Hybrid

RIP IGRP OSPF EIGRP

EGPBGP

4



5

Interior Routing protocols operated within anAutonomous System.

Exterior Routing protocols operated betweenAutonomous Systems.



Definition:

Collection of routers and networks under the same

administration or routing policy.

Usually under single ownership, trust and

administrative control.

When using AS:

Multi-home networks to different providers.

Routing policy is different to external peers.

6



Characteristics:

• Internal connectivity: All parts of an AS must remainconnected, meaning that its all routers mustexchange routing information in order to maintain theconnectivity.

• A single routing protocol required to run in an AS,between all routers.

• In 1982, terminology, routers inside an AS were called“interior gateways” and the protocol was an “Interior Gateway Protocol” (IGP).

• Each AS is identified by a 16-bit “AS number”. EGP is us ed to

exchange informat ion among A ss .

7



Mathematically compare routes using somemeasurement of distance (distance vector).

Routers send all or a portion of their routing table in

a routing-update message at regular intervals toeach of their neighboring routers.

As routing information proliferates, routers:

• Identify new destinations

• Learn failure through the network

• Calculate distance to all known destinations.

Distance Vector Routing

8



Distance Vector Routing (2/3)

Rout ing table E

9



Destination Outgoing Link Distance

A Local 0

B

C

1 2

A

CB

Routin g table A

Routin g table B

Destination Outgoing Link Distance A

B Local 0

Destination Outgoing Link Distance A

C Local 0

Routing table C

• Initially, each router initializing its

local knowledge: remember its own

address and be able to identify the

links attached to it.

• It then broadcasts its routing table toall its neighbors.

• It updates its routing table when

noticing a change.

• The process continues until the

network converges


A Local 0

B 1 1

C

Update

broadca

st

Update

Destination Outgoing Link Distance A 2 1

B 2 2

C Local 0

Broadcast


A Local 0

B 1 1

C 2 1

Update

Destination Outgoing Link Distance A 1 1

B Local 0

C 1 2

Broadcast

10



Count To Infinity


A Local 0

B 1 1

C 2 11 2

A

CB


A 1 1

B Local 0

C 1 2

Routin g table A

Routing table B

A capturesthe

change


A Local 0

B 1 1

C 2 Inf


A Local 0

B 1 1

C 2 3

Update routing table

U p d a t i n g


A 1 1

B Local 0

C 1 4

11



Solution: Split Horizon• Split horizon is based on very simple precaution: it is

never useful to send information about a route back in

the direction from which it came.

Split Horizon


A Local 0

B 1 1

C 2 inf

1 2

A

CB


A 1 1

B Local 0

C 1 2

Routin g table A

Routin g table B

U p d a t e


A 1 1

B Local 0

C 1 inf

The Network converges !

12



Solution: Poison-reverse update• Is a way in which a router tells its neighboring router that

one of the routers is no longer connected; or disqualifies

a route back a long the interface on which it learned the

route by setting the unconnected router to a defined

number.

Poison-reverse update


A Local 0

B 1 1

C 2 1

1 2

A

CB


A 1 1

B Local 0

C 1 2

Routin g table A

Routin g table B


A 1 1

B Local 0

C 1 inf


A

captures

and

updates


A Local 0

B 1 1

C 2 inf B modifies

routing table

before

sending

13



Solution: Triggered Update• Is an attempt to increase the responsiveness of the

protocol by requesting nodes to send message as soon

as they notice a change in their routing table with out

having to wait for the end of the period.

Triggered Update


A Local 0

B 1 1

C 2 1

1 2

A

CB


A 1 1

B Local 0

C 1 2

Routin g table A

Routin g table B

U p d a t e


A 1 1

B Local 0

C 1 inf


A captures

the change

and

updates


A Local 0

B 1 1

C 2 inf

A send

update

immediately

to B

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Original ARPANET routing algorithm.

Each router maintains a table containing oneentry for each router in the subnet.

Each entry has two parts: preferred outgoing line to use for that destination,

and

an estimate of the time or distance to thatdestination.

These tables are updated by exchanging infowith the neighbors.

RIP and IGRP are distance-vector routingprotocols.



Distance Vector Routing has a serious drawbackin practice known as Count-To-Infinity problem.

It reacts rapidly to good news, but slowly to badnews.

Example: Consider a five node linear subnet withthe delay metric being the no of hops



Many ad hoc solutions to the count-to-infinity problem have been proposed in the literature, each

one more complicated and less useful than the one

before it.

The split horizon algorithm works the same way asdistance vector routing, except that the distance to

X is not reported on the line that packets for X are

sent on (actually, it is reported as infinity).



1 2 3 4

inf 2 3 4inf inf 3 4

inf inf inf 4

inf inf inf inf

inf=infinity



When CD line goesdown. A thinks it has apath to D through B and

B thinks it has a path toD through A.

A and B will count toinfinity.



Distance Vector Routing replaced by Link StateRouting due to following problems:-

Delay metric did not take line bandwidth into account.

Algorithm often took too long to converge.

In Link State Routing each router must:- Discover its neighbors and learn their NW address.

Measure delay to each of its neighbors.

Construct a packet telling all it has just learned.

Send this pkt to all other routers.

Compute the shortest path to every other router.



Learning about neighbours

On booting router sends special hello packet oneach point-to-point line.

Routers on other end reply by sending their

globally unique names.



Measuring Line Delay/ Cost Each router must have a reasonable estimate

of delay to each of its neighbors.

Determined by sending a special ECHO

Packet over the line that the other side isreqd to send back imdtly.

To take current load in account, round triptimer is started when Echo Packet is queued.

Otherwise timer is started when it reacheshead of queue.

Should we take load into account?



Including queuing cost: can use

the best line, but may lead torouting table oscillating.

Same bandwidth

on the two links



Building Link State Packets

Once the info is collected, pkts containing all thedata are built.

Pkt contains ID of sender, sequence No, age andlist of neighbors.

Delay to each neighbor is also given. These pkts may be built after regular intervals or

on some significant event.



Distributing Link State Pkts Flooding is used to distribute pkts.

Each pkt contains a sequence No

which is incremented for each new pktsent.

Sequence No of new pkt is checkedagainst that already seen. New pkt is

fwd on all lines except the one it cameon else it is discarded.



Distributing Link State Pkts (Contd…) Problems:

Wrapping around of sequence No. will causeconfusion – thus 32 bit sequence No. is used.

If a router crashes it will start again with seq nozero which will be rejected as duplicate. Sequence No. may get corrupted leading to

discarding of subsequent pkts.

Solution - Include the age of each pkt anddecrement it once per second/hop.A pkt is

discarded when the age hits zero. All link state pkts are acknowledged on point

to point lines.



Distributing Link State Pkts Data structure used to buffer pkts at a router



Computing New Routes After accumulating full set of link state pkts, a router

constructs the entire subnet graph.

Then, Shortest Path Algorithm is used to determinethe routing tables and normal operation resumed.

Disadvantages: For large subnets, large memory reqd to store input

data.

Computational time is large.

Example Protocols: OSPF, IS-IS(Intermediate system,

Intermediate System)



Link-state algorithm floods routing information toall nodes in the internetwork. Each router,however, sends only the portion of the routingtable that describes the state of its own links.Thus, each router builds a picture of the entirenetwork in its routing tables.

In Distance vector algorithm, each router sendsall or some portion of its routing table to itsneighbors only. Distance vector algorithms know

only about their neighbors.

In essence, link-state algorithms send smallupdates everywhere, while distance vector algorithms send larger updates only to

neighboring routers.



Link-state algorithms are less prone to routingloops and converge faster than distance vector algorithms.

Link-state algorithms require more CPU power

and memory than distance vector algorithms -more expensive to implement and support.

Link-state protocols are generally more scalablethan distance vector protocols.



As the size of the NW increases: Routing table grow proportionally.

More router memory consumed.

More CPU time needed to scan the tables.

More bandwidth required to send the status reports.

NW may grow to a point where it is no longer feasible for every router to have an entry for every other router.



In Hierarchical routing the routers are dividedinto regions.

Each router knows how to route pkts todestinations within its own region.

It knows nothing about the internal structure ofother regions.

For very large networks, these regions may begrouped into clusters, the clusters into zones, the

zones into groups, and so on



Advantages?



Unfortunately, the gain in routing table space arenot free.

There is a penalty to be paid in the form ofincreased path length.

For example, the best route from 1A to 5C is viaregion 2, but with hierarchical routing all traffic toregion 5 goes via region 3, because that is better for most destinations in region 5.

When a single network becomes very large, howmany levels should the hierarchy have?

Answer: The optimal number of levels for an N

router subnet is ln N.



When a new user enters an area, either by connecting to it, or

just wandering into the cell, his computer must register itself

with the foreign agent there.



Find a foreign agent ? The mobile host registers with the foreign agent,

giving its:

home address,

current data link layer address, and some security information.

The foreign agent contacts the mobile host’s home agent.

The message from the foreign agent to the home

agent contains the foreign agent’s network address.

It also includes the security information, to convincethe home agent that the mobile host is really there.



The home agent examines the security

information, which contains a time stamp, toprove that it was generated within the past fewseconds.

On being satisfied, it tells the foreign agent to

proceed. When the foreign agent gets the

acknowledgement from the home agent, itmakes an entry in its tables and informs the

mobile host that it is now registered. The event of user leaving an area should also be

ideally announced to allow deregistration.

But many users abruptly turn off their

computers when done. So, what do you do?



Used in appln where msg has to be send to allhosts.

One method could be sending a distinct packetto each destination.

Requires no special feature from subnet Wastes bandwidth and require the source to have a

complete list of all destinations.

Least desired method.

Second method is Flooding .

Generates too many packets and consumes too muchbandwidth.

Third method is multi-destination routing.



Each packet contains either a list of destinationsor a bit map indicating the desired destinations.

When a packet arrives at a router, the router checks all the destinations to determine the setof output lines that will be needed.

The router generates a new copy of the packetfor each output line to be used and includes ineach packet only those destinations that are touse the line.

In effect, the destination set is partitioned amongthe output lines.

After some hops each pkt will carry only one destand can be treated as normal pkt



A fourth broadcast algorithm makes explicit useof the sink tree for the router initiating thebroadcast, or any other convenient spanning tree.

This method makes excellent use of bandwidth,

generating the absolute minimum number ofpackets necessary to do the job.

The only problem is that each router must haveknowledge of some spanning tree for it to beapplicable.



dynamic routing is always better than staticrouting. It's important to keep in mind that theprimary duty of a dynamic routing protocol is toautomatically detect and adapt to topologicalchanges in the internetwork. The price of this

"automation" is paid in bandwidth and maybequeue space, in memory, and in processing time.

A frequent objection to static routing is that it ishard to administer. This criticism may be true ofmedium to large topologies with many

alternative routes, but it is certainly not true ofsmall internetworks with few or no alternativeroutes



Protocol Examples Characteristics

Distance

Vector

RIP v1 and RIP v2

Interior Gateway

Routing Protocol

(IGRP)

• Copies routing tables to neighbors

• Updates frequently

• RIP v1 / v2 use hop count as metric

• Views the network from the perspective of the neighbors

• Slow to converge

• Susceptible to routing loops

• Easy to configure and administer

• Consumes a lot of bandwidth

Link-state Open Shortest Path

First (OSPF)

Intermediate-System

to Intermediate-

System (IS-IS)

• Uses shortest path

• Updates are event triggered

• Sends link-state packets to all network routers

• Has common view of network

• Fast to converge

• Not as susceptible to routing loops

• Harder to configure

• Requires more memory and processing power than distance

vector

• Consumes less bandwidth than distance vector

Routing Protocols CT

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