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Routing Techniques

Dr. Indranil Sen Gupta Packet Switching Networks Slide 1

Routing in Circuit Switching Networks


Routing in Circuit Switched Networks

• In large circuit-switched networks, connections often require a path

through more than one switch.

• Routing function in public telecommunication networks has been

traditionally quite simple.

– Switches are organized as a tree structure.

– To add some resilience to the network, additional high-usage trunks are

added that cut across the tree structure to connect exchanges with high

Dr. Indranil Sen Gupta Circuit Switching Slide 3

added that cut across the tree structure to connect exchanges with high

volumes of traffic between them.

– Usually a static approach.

• To cope with growing demands, all providers today use a dynamic

approach.

– Routing decisions are influenced by current traffic conditions.

– Switching nodes have a peer relationship with each other rather than a

hierarchical one.

– Routing is more complex and more flexible.

Method 1: Alternate Routing

• The possible routes to be used between two end offices are

predetermined.

– It is the responsibility of the originating switch to select the

appropriate route for each call.


• In practice, usually a different set of pre-planned routes is

used for different time periods.

– Takes advantage of different traffic patterns in different time zones

and different times of day.

Alternate Routing Example


Method 2: Adaptive Routing

• This is designed to enable switches to react to changing

traffic patterns on the network.

– Greater management overhead (switches must exchange

information).


– Has the potential for more effectively optimizing the use of

network resources.

Routing in Packet Switching Networks


Introduction

• One of the most complex and crucial aspect of packet-

switching network design is routing.

– In most subnets, packets will require multiple hops to make the

journey.


• Routing Algorithm:

– That part of the network layer software responsible for deciding

which output line an incoming packet should be transmitted on.

– For datagrams, this decision has to be taken for every arriving

packet.

– For virtual circuits, routing decisions are made only when a new

virtual circuit is being set up.

Desirable properties in a routing algorithm

• Correctness and simplicity: self-explanatory.

• Robustness: ability of the network to deliver packets via some route

even in the face of failures.

• Stability: the algorithm should converge to equilibrium fast in the

.


face of changing conditions in the network.

• Fairness and optimality: obvious requirements, but conflicting.

X

FDB

CA E

Y

X -> Y

A -> B

C -> D

E -> F

Performance Criteria

• The simplest criterion is to choose the minimum-hop route

through the network.

– Easily measured criterion.

• A generalization is least-cost routing.


• A generalization is least-cost routing.

– A cost is associated with each link.

– For any pair of attached stations, the least cost route through the

network is looked for.

• For either case, several well-known algorithms exists for

finding out the optimum path.

– Dijkstra’s algorithm

– Bellman-Ford algorithm

Routing Strategies

• A large number of routing strategies have evolved over the

years.

• Four key strategies shall be discussed:

– Fixed (Static) routing


– Fixed (Static) routing

– Flooding

– Random routing

– Adaptive routing

Fixed Routing

• A route is selected for each source-destination pair of nodes in the

network.

– Any standard least-cost routing algorithm can be used.

– The routes are fixed; they may only change if there is a change in the

topology of the network.

• How fixed routing may be implemented?


• How fixed routing may be implemented?

– A central routing matrix is created, which is stored at a network control

center.

– The matrix shows, for each source-destination pair of nodes, the identity

of the next node on the route.

– From the main routing matrix, routing tables to be used by each individual

node can be developed.

• Main advantage is simplicity, and works well in a reliable network

with stable load. Disadvantage is its lack of flexibility.

Fixed Routing:

example


Flooding

• Every incoming packet is sent out on every outgoing line

except the one it arrived on.

• Flooding generates vast number of duplicate packets, and

suitable damping mechanism must be used.


suitable damping mechanism must be used.

– A hop counter may be contained in the packet header, which is

decremented at each hop, with the packet being discarded when the

counter reaches zero.

• The sender initializes the hop counter. If no estimate is known, it is

set to the full diameter of the subnet.

– Keep track of which packets have been flooded, to avoid sending

them out a second time.

Flooding (contd.)

• A variation which is slightly more practical is

selective flooding.

– The routers do not send every incoming packet out on every line,

only on those lines that go in approximately the same direction.

• Utilities of flooding:


• Utilities of flooding:

– Flooding is highly robust, and could be used to send emergency

messages (e.g. military applications).

– May be used to initially set up the route in a virtual circuit.

• Flooding always chooses the shortest path, since it explores every

possible path in parallel.

– Can be useful for the dissemination of important information to all

nodes (e.g. routing information).

Random Routing

• This has the simplicity and robustness of flooding with far

less traffic load.

– A node selects only one outgoing path for retransmission of an

incoming packet.


– The outgoing link is chosen at random, excluding the link on

which the packet arrived.

• A refinement is to assign a probability to each outgoing

link and to select the link based on that probability.

• The actually route will typically not be the least-cost route.

– Network must carry a higher than optimum traffic load.

Adaptive Routing

• Routing decisions change as conditions on the network

change.

• Two principle conditions affecting routing decisions:

– Failure: When a node or trunk fails, it can no longer be used as part


– Failure: When a node or trunk fails, it can no longer be used as part

of the route.

– Congestion: When a particular portion of the network become

heavily congested, it is desirable to route packets around the area of

congestion.

• For adaptive routing to be possible, network state

information must be exchanged among the nodes.

– More information exchange ==> better routing ==> more overhead.

Adaptive Routing (contd.)

• Several drawbacks of the approach:

– Routing decision is more complex ==> more processing burden on

the switching nodes.

– Depends on status information that is collected at one place but


used at another ==> traffic overhead increases.

– It may react too quickly to changing network state, thereby

producing congestion-producing oscillation.

• Despite the drawbacks, adaptive routing is widely used.

– Improves performance, as seen by the network user.

– Can aid in congestion control.

Adaptive Routing Algorithms

• Several algorithms shall be discussed.

– Isolated adaptive routing.

– ARPANET: first generation.

– ARPANET: second generation.


– ARPANET: second generation.

– ARPANET: third generation.

Isolated Adaptive Routing

• A node routes an incoming packet to the outgoing link

with the shortest queue length Q.

– Balances load on outgoing links.

– The chosen outgoing link may not be heading in the right direction.


– The chosen outgoing link may not be heading in the right direction.

• To take direction into account, each link emanating from

the node has a bias Bi, for each destination i.

– For each arriving packet heading for node i, the node would

choose the outgoing link that minimizes Q+Bi.

– A node would tend to send packets in the right direction, with a

concession made to current traffic delays.

Isolated Adaptive Routing: an example

Node 4’s Bias Table

for Destination 6

To 2


Next node Bias

1 9

2 6

3 3

5 0

To 1To 3

To 5

Packet arrives from node 1 ==> route through node 5

ARPANET: first generation

• Developed in 1969.

• Distributed adaptive algorithm using delay as the

performance criterion.

• Each node maintains two vectors:

– Di = [ di1 di2 …… din]


i i1 i2 in

– Si = [ si1 si2 ……. sin ]

– Here,

• Di is the delay vector for node i

• dij is the current estimate of minimum delay from node i to node j

• n is the number of nodes

• Si is the successor node vector for node i

• sij is the next node in the current minimum-delay route from i to j

Contd.

• Periodically (every 128 ms), each node exchanges its delay

vector with all of its neighbors.

• Using the incoming delay vectors, a node k updates its

vectors as follows:


vectors as follows:

– dkj = Min [dij + lki] over all i in A

– skj = i, using i that minimizes the expression above

– Here,

• A is the set of neighboring nodes for k

• lki is the current estimate of delay from k to i

Contd.

• The estimated link delay is simply the queue length for that

link.

– In building the new routing table, a node will tend to favor

outgoing links with shorter queues.


– Since queue lengths vary rapidly with time, a thrashing situation

may result.

• A packet continues to seek out areas of low congestion rather than

aiming at the destination.

ARPANET: first generation

Illustrative Example


Figures 12.6, 12.7 from the book by Stallings

ARPANET: second generation

• The previous algorithms had several shortcomings:

– Only queue lengths were considered, and not line speed.

• Higher capacity links were not given the favored status they deserved.

– Queue length is only an artificial measure of delay.


– Queue length is only an artificial measure of delay.

• Processing time of a packet before placing it in the queue may itself

be variable.

– The algorithm responds slowly to congestion and delay increases.

• New algorithm proposed in 1979.

– A distributed adaptive algorithm, using delay as the performance

criterion.

– The delay is measured directly, by timestamping the packets.

ARPANET: third generation

• Problem with the previous approaches was that every node

was trying to obtain the best route for all destinations, and

that efforts conflicted.

• It was concluded that under heavy loads, the goal of

routing should be to give the average route a good path


routing should be to give the average route a good path

instead of attempting to give all routes the best path.

– The designers decided that it was unnecessary to change the

overall routing algorithm.

– It was sufficient to change the function that calculates link costs.

• Done in a way to damp routing oscillations and reduce routing

overheads.

• Uses simple concepts from queuing theory.

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