Lecture 10 - Routing Part II

8/2/2019 Lecture 10 - Routing Part II

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Routing StrategiesRouting StrategiesRouting StrategiesRouting Strategies

Fixed

Flooding

Random

Adaptive

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Properties of Fixed RoutingProperties of Fixed RoutingProperties of Fixed RoutingProperties of Fixed Routing

Single permanent route for each source to destination pair

Determine routes using a least cost algorithm

Route fixed, at least until a change in network topology

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Fixed Routing ExampleFixed Routing ExampleFixed Routing ExampleFixed Routing Example

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FixedFixedFixedFixed

RoutingRoutingRoutingRouting

TablesTablesTablesTables

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Properties of FloodingProperties of FloodingProperties of FloodingProperties of Flooding

No network info required

Packet sent by node to every neighbor

Incoming packets retransmitted on every link except incoming link

Eventually a number of copies will arrive at destination

Each packet is uniquely numbered so duplicates can be discarded

Nodes can remember packets already forwarded to keep network load in bounds

Can include a hop count in packets

All possible routes are tried

Very robust

At least one packet will have taken minimum hop count route Can be used to set up vir tual circuit

All nodes are visited

Useful to distribute information (e.g. routing)

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FloodingFloodingFloodingFlooding

ExampleExampleExampleExample

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Properties of Random RoutingProperties of Random RoutingProperties of Random RoutingProperties of Random Routing

Node selects one outgoing path for retransmission of incoming

packet

Selection can be random or round robin

Can select outgoing path based on probability calculation

No network info needed

Route is typically not least cost nor minimum hop

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Properties of Adaptive RoutingProperties of Adaptive RoutingProperties of Adaptive RoutingProperties of Adaptive Routing

Used by almost all packet switching networks

Routing decisions change as conditions on the network change

Failure

Congestion

Requires info about network

Decisions more complexTradeoff between quality of network info and overhead

Reacting too quickly can cause oscillation

Too slowly to be relevant

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Dynamic Routing Example Distance Vector RoutingModern computer networks tend to use dynamic routing

algorithms.

Distance vector routing algorithms work by having each routermaintain a table giving the best known distance to each destinationand which line to use when they get there.

These tables are updated regularly.

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Dynamic Routing Example Distance Vector RoutingRouter A computes the delay to its neighbors B and C as follows:

B: 2msec, C: 3msec.

Each of Routers A neighbors, in this case B and C, send delay vectorsthat show the millisecond delay from each of their machines to allrouters, as follows.Router B> A: 3, B:0, C: 7, D: 6, E: 4, F: 10

Router C> A: 3, B:6, C: 0, D: 3, E: 12, F: 7.

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A

B

C

D

F

E

Nodes

Communication Link

Dynamic Routing Example Distance Vector Routing Routing Table for Router A is then calculated

Router A sends updates to all its neighbours.

The measure could be anything. In this case it is delay time in milliseconds.

Updates ripple through the network as each router does this calculation.

Destination Link Delay

Router A -- 0

Router B AB 2

Router C AC 3

Router D AC 6

Router E AB 6

Router F AC 10

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Key issues in this technique :

Knowledge about the whole network

Routing only to neighbors

Information sharing at regular intervals

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Example Distance Vector Routing

At start up a routers knowledge of the internetwork is sparse.Prepare a routing table with following fields:

Network id

Cost

Next hop

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Sharing of information in distance vector routing

Routing table distribution in distance vector routing

Updating routing table of router A

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Final routing tablesRouter applies the following rules for updating

process1. If the advertised station is not in the routing table, the

router should add the advertised information to the

table.

2. If the advertised destination is in the routing table.

a) If the next hop field is the same, the router should replace

the entry in the table with the advertised one. Note that even

if the advertised hop count is larger the advertised entry

should replace the entry in the table because the new

information invalidates the old.

b) If the next hop field is not the same.i. If the advertised hop count is smaller than the one in the

table, the router should replace the entry in the table

with the new one.

ii. If the advertised hop count is not smaller(same or

larger), the router should do nothing.32

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Updating process

(Rule 2.a)(Rule 2.a)

(Rule 1)(Rule 1)

(Rule 2.b.i)(Rule 2.b.i)

(Rule 2.b.ii)(Rule 2.b.ii)

(Rule 2.b.ii)(Rule 2.b.ii)

Adaptive RoutingAdaptive RoutingAdaptive RoutingAdaptive Routing ---- AdvantagesAdvantagesAdvantagesAdvantages

Improved performance

Aid congestion control

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Two-node instability

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Both A and B know

where X is.

Link between A and Xfails. A updates its

table immediately.

But before A can tell B,

B sends its info to A!

A, using Bs info, up-dates its table (wrongly!).

Then A send its tableto B and B updates its

table (again wrongly).

Both routers keep up-dating tables, event-

ually hitting infinity. Inthe meantime, chaos!

Solutions to two-node instability Possible Solutions to two-node instability:

Define infinity to be a much smaller value, such as 100. Then itdoesnt take too long to become stable. But now you cant usedistance vector routing in large networks.

Split Horizon instead of flooding entire table to each node,only part of its table is sent. More precisely, if node B thinksthat the optimum router to reach X is via A, then B does notneed to advertise this piece of info to A the info has alreadycome from A.

Split Horizon and Poison Reverse Normally, the distance

vector protocol uses a timer. If there is no news about a route,the node deletes the route from its table. So when A neverhears from B about the route to X, it deletes it. Instead, Node Bstill advertises the value for X, but if the source of info is A, itreplaces the distance with infinity, saying Do not use thisvalue; what I know about this route comes from you.

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Three-node instability - no solution here!?

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Lecture 10 - Routing Part II