CHAPTER 7PACKET-SWITCHING NETWORKSDept of computer science and engg

6th sem cse

SSE

VIRTUAL CIRCUIT PACKET SWITCHING

•Virtual circuit packet switching establishes a fixed path called VIRTUAL CIRCUITS

VIRTUAL CIRCUIT SETUP PROCEDURE will take place before any packets flow through the network.

Virtual circuit resides in network layer.Figure 2 Delay is incurred when a message is broken into

3 packets and transmitter over the circuit. Same delay as datagram packet switching execpt

for additional delay used to set up the circuit.

Path is determined parameters are set in swiches by exchanging Connect request and Connect confirm messages

If the switch does not have enough resources then it responds with a connect reject message and the setup procedure fails.

Figure 3

•At the input of every switch ,the virtual circuit is identified by VIRTUAL CIRCUIT IDENTIFIER(VCI)

When packet arrives at the input port the VCI in the header is used to access the table.

The table lookup provides the output port to which the packet is fowarded and the VCI that is used at the input port of the next switch.

Call setup procedure sets up chain of pointers across the network that directs the flow of packets

Figure 4

STRUCTURE OF PACKET SWITCH

A packet switch will perform two main functions

ROUTING FORWARDING

Routing function uses algorithm to find path to destination

Stores result in routing table

Forwarding function processes incoming packet from input port

Forwards packet to appropriate output port based on routing table information

GENERIC PACKET SWITCH

Input port, output port, interconnection fabric, switch controller

Input ports and output ports are paired

Line card Contain several input/output port. Capacity of link connecting the fabric is high speed,

fully utilized Implements physical, data link and network layer

functions Symbolic timing, line coding, framing, physical layer

addressing ,error checking

ORGANIZATION OF LINE CARDMADE UP OF VARIOUS CHIPSET

Line card supports medium access control protocol to handle broadcast network.

Implemented by special purpose chipset Network layer routing table in line card Need to perform fast table lookup to find output

port Contain buffers and associated scheduling

algorithms Network processor performs table lookup and

packet scheduling

Figure 6->

CONTROLLER

Contain general purpose processor This manages control and management

functions If controller is connectionless executes

routing protocol If connection oriented handles signally

messages. Controller communicates with line card and

fabric to configure internal parameters.

INTER CONNECTION FABRIC

Transfers packet between line card Crossbar interconnection fabric transfer

packets parallel between input and output ports

Buffers are added to crossbar to accommodate packet contention.

Buffer located at input or output port.

OUTPUT BUFFERING Crossbar with output buffering should run N times

faster than port speed as N packets simultaneously arrive at a particular output

If output is idle, one packet is transmitted rest are buffered.

Only one packet is allowed to proceed to particular output

INPUT BUFFERING 2 packets at input 2,first packet like to go to

output 3,second to output 8 Packet from input buffer 1 also want to go to

output 3 Frist packet from input buffer should wait ,2nd

should wait behind 1st even though output 8 is idle

Results in performance degradation . Problem called HEAD of Line Blocking

BANYAN SWITCH

Solution is building a large switch Composed of 2*2 switching elements

interconnected in certain fashion` Exactly 1 path will exist from input to each output Routing is done in distributed manner Ie appending binary the binary address of the

output number to each packet Switching element at stage I steers a packet based

on ith bit of the address. Bit is 0 the element steers packet to upper output.

Input 1 likes to send it to the output destines to packet 5

The switching element at stage 1 looks at first bit Of the address and steers the packet to its lower

output.At stage 2 it steers the packet to the upper output

since the second bit is 0.at stage 3 steers the packet to lower output and sends to output 5

ROUTING IN PACKET NETWORKS Routing is concerned with determining feasible

paths for packet to follow from source to destination

1)Rapid and accurate delivery of packetsRouting algorithm should operate correctly

Find path to correct destination

Should not take long time to find path

2)Adaptablilty to changes in network topologyNetwork equipment and transmission line will fail

Routing algorithm should adapt and reconsider the path.

3)Adaptability to various source destination traffic loads

Traffic loads change dynamically, hence should adjust path based on the current traffic loads.

4) Ability to route packets away from congested links

Routing algorithm should avoid congested links and balance the load.

5) Ability to determine connectivity of networkRouting system should know connectivity to find

optimal paths

6)Ablility to avoid loops

Inconsistent information will lead to routing loops. Routing system should avoid the loops in presence of distributed routing system.

7)Low OverheadShould obtain connectivity information by exchanging

control messages with routing system. These messages create overhead on bandwidth usage and hence should be minimized.

ROUTING ALGORITHM CLASSIFICATION Routing algorithm is classifies as 1) Static routing 2)Dynamic routing

Static routingPaths are precomputed based on the network

topology, link capacities and other informationComputation is completed the paths are loaded

in the routing table of each node.This remains fixed for a long time.Works when network size is small, and the

traffic doesn’t change in timeIt becomes cumbersome, as network size

increases.Disadvantage is inability react to network

failures

DYNAMIC ROUTING Each node will continuously learn the state of

the network by communicating with the neighbors

Change in network topology is eventually propagated trough nodes

Based on the information the node computes the best path to the destination

Disadvantage is the added complexity with the nodes.

They can also be distinguished as Centralized routing Distributed routing

CENTRALIZED AND DISTRIBUTED ROUTING

In Centralized routing the network control center computes all the path and then uploads the information in the network

In Distributed routing nodes cooperate by the means of message exchanges and perform their own routing computations.

Distributed routing is better than centralized routing but generate inconsistent results ,loop wil develop

If A thinks B is the best path to Z and If B thinks A is the best path to B

Then desired packet to Z have the misfortune of arriving at A or B and stuck in a loop between A and B

ROUTING TABLES Once the routing algorithm defines the set of

path the path information is stored in the routing table.

In virtual circuit packet switching the routing table translates each incoming VCI to outgoing VCI an identifies the output port to which the packet is to be forwarded.

Datagram packet switching the routing table identifies the next hop to forward the packet based on destination address of the packet

Virtual circuits are bidirectional.

A packet sent by node A with VCI 1 will eventually reach node B and from node A with VCI 5 will reach node D

VCI 1 from node A will be translated to 2 and to 7 and finally to 8 and reaches B

When node 1 will receive the header with VCI 1 that node replaces the incoming VCI with 2 ande forwards the packet with node 3

If a packet with VCI 5 arrives at node 1 from node A,the packets is forwarded to node 3 after the VCI is replaced with 3

After arriving at node 3 the packet receives the outgoing VCI 4 and is forwarded to node 4

Node 4 translates the VCI to 5 and forwards the packet to node 5.

Node 5 translates to VCI 2 and delivers the packet to destination node D

The routing table uses node numbers to identify where the packet comes from and where the packet is forwarded.

ROUTING TABLE FOR PACKET SWITCHING NETWORK

Shows routing table for network topology Minimum hoping routing objective Packet destines to node 6 arrives at node 1, the

packet is fowarded to node 3 based on the corresponding routing table entry

Node 3 forwards the packet to node 6

HIERARCHICAL ROUTING The size of routing table is reduced Hosts that are near to each other should

have addresses that have common perfixes Routers will examine the part of address and

decide to route the packet Example of hierarchal address assignment

and flat address assignment

FLOODING Forwards an incoming packet to all the ports

except the one received from Each switch performs the flooding process

and eventually reach the destination Effective routing approach when the routing

information is not available Also effective when source need to send

packet to all the nodes Flooding may easily swamp network

To reduce resource consumption in the network Time to live field (TTL) in each packet Source sends the packet TTL is set to some

number Each node decrements the TTL by 1 before

flooding the packet If value reaches 0 the node discards the packet Avoid wastage of bandwidth, minimum hop

number between two further nodes 2nd ,the node adds identifier to header of packet When node receives the packet that contain the

identifier it discards it as it knows packet already visited the node(prevents looping)

Identified with unique sequence number,node will record the sequence number and source address

If already visited it will discard the packet

DEFLECTION ROUTING Also called hot potato routing. eg: Manhatten street

network

They provide multiple path for each source –destination

Each path tries to forward the packet to the preffered port

If preferred port is busy or congested the packet is deflected to another port

Advantage is node are buffer less, packets do not have to wait for a specific port to become available

If unavailable it will deflect the packet to another port, reaches destination

Since they take alternate path they cannot guarantee in-sequence delivery of packets

Used in optical network where optical buffers are difficult to find

Used to implement high speed switches where topology is regular and buffers are expensive

SHORTEST PATH ROUTING

Routing algorithm is based onShortest path algorithmo Each link represents the cost of using the linko Shortest path between node 2 to node 6 is trough2-4-3-6 and the cost path is 4Metrics used to assign the cost1. Cost ~ 1/capacity : Higher cost to lower capacity

links2. Cost ~ packet delay: It includes queuing delay in

switch buffer and propagation delay in link3 Cost ~ Congestion : Congestion measure is traffic

loading. Avoids Congested link

THE BELLMAN – FORD ALGORITHM

If Neighbor of node A knows the shortest path to node Z ,then node A can determine the shortest path to Z

Ie Node A will calculate the cost/Distance to node Z trough its neighbors by picking the minimum

Dj – Current estimate to minimum cost to destination node

Cij – link cost from node I to node j (c13=c31=2)

(c11=0) (c15=c23= ∞)

Minimum cost from node 2 to node 6 trough node 1 ,node 4 and node 5:

D2= min{C21 + D1 , C24 + D4 ,C23+ D5} = min{3+3,1+3,4+2}

=41.Initialization: Di= ∞ for all i ≠ d

Dd=0

2. Updating: for each I ≠d

Di= Minj {Cij + Dj} for all j ≠ i

Initally all the nodes other than the destination node are at infinite cost to node 6.

(Iteration 1)Node 3 finds that it is connected to node 6 with cost of 1.Node 5 finds it is connected to node 6 with cost 2.Node 3 and 5 update their entries.

(Iteration 2)Node 1 finds it can reach node 6 via node 3 with cost 3.Node 2 finds it can reach node 6 via node 5 with cost 6. Node 4 finds it has path via 3 and 5 with costs 3 and 7 respectively.Node 4 selects the path via node 3.Node 1,2 and 4 update their entries

(Iteration 3)Node 2 finds it can reach node 6 via node 4 with distance 4.Node 2 changes the entry to (4,4) and informs the neighbours

Node 1,4,5 processes the new entry from node 2 but donot find new shortest paths.The algorithm is conservged

Link connecting node 3 and node 6 breaks. Compute minimum cost for each node to destination node.

(update 1)As soon as node 3 detects that link(3,6) breaks, node 3 recomputed the minimum cost to node 6.Node 3 looks for path to 6 trough neighbours, Node 1 and Node 4 and its calculation indicates that the new shortest path is trough node 4 at a cost of 5.Node 3 then sends the new routing updates that is node 1 and node 4

(update 2)Node 1 and 4 recomputes their minimum costs; node 1 findsits shortest path is still trough node 3 but the cost has increased to 7;node 4 finds the shortest path is trough node 2 or node 5 with cost 5.node 4 chooses node 2.Node 1 transmits its routing updates to node 2,node 3 and node 4

Node 4 transmits its routing updates to node 1,2,3,5

(Update 3)Node 1 finds its shortest path is still trough node 3.Node 2 finds its shortest path is still trough node 4 but the cost has increased to 6.Node 3 finds its shortest path is still through node 4 but the cost has increased to 7.Node 4 and Node 5 finds the shortest path is not changed. Node 2 transmits its updates to node1,node4 and node 5 and transmits its updates to node 1 and 4.

(update 4) Node 1 finds its shortest path is through either node 2 or node 3 with cost 9.Suppose that node 1 chooses node 2.Node 4 now finds the shortest path through node 5 with cost 5 since the cost through 2 has incresed to 7.Node 5 does not change its shortest path.Node 1 transmits its updates to node 2,node 3,node 4 and node 4 transmit its updates to node 1,2,3 and 5

(update 5) None of the nodes find short paths

Update node1 node2 node3Before break (2,3) (3,2) (4,1)After break (2,3) (3,2) (2,3)1 (2,3) (3,4)

(2,3) 2 (2,5) (3,4) (2,5)3 (2,5) (3,6) (2,5)4 (2,7) (3,6) (2,7)5 (2,7) (3,8) (2,7)

SPLIT HORIZON WITH POISON REVERSE

Update Node 1 Node 2 Node3Before break (2,3) (3,2) (4,1)After Break (2,3) (3,2) (-1, )1 (2,3) (-1, ) (-1, ) 2 (-1, ) (-1, ) (-1, )

After the link breaks node 3 sets the cost of the destination to infinity,since the minimum cost node 3 has received from node 2 is also infinity.When node 2 received the update message it also sets to infinity.Node 1 learns the destination is unreachable

DIJKSTRA ‘S ALGORTIHM

Alternate algorithm for finding the shortest paths to source to all other nodes.

To identify the closest nodes from the source node in the order of their increasing path cost.

1 InitialalizationN={s}Dj=Csj,for all j ≠ s

2.Finding the next closest nodeDi=min DjAdd i to N3.Update minimum cost from node I to node NDj=min{Dj,Di+Cij}

SOURCE ROUTING VERSUS HOP BY HOP ROUTING

Datagram network each node determines the next hop along the shotest path ,packet travels from source as it follows HOP BY HOP routing to destination

SOURCE ROUTING the path to the destination is determined by source.Explicit routing allow a perticular packet node to determine the path.

Source includes path information in the packet header that contain sequence of nodes to traverse and sufficient information to nodes till packet is fowarded to destination.

Each node examines the header,removes address identifying the node,forwards packet to next node

If we reserve the path information we can traverse the node back to the source by reversing the path.

LINK STATE ROUTING VERSUS DISTANCE VECTOR ROUTING Distance Vector routingNeighboring routes exchange routing tables that sets

the vector of known distance to destination. Bellman ford algorithm is used to find the best path though information from the neighbors. With new path the router wil send new vector to neighbors.

This adapts to change in network topologyLink State RoutingEach router will flood information to the state of the

links that connects to neighborsAllows routers to construct map of entire network and

then uses Djikstra s algorithm. If state is changed the router detects the change and floods the new information to network

ATM NETWORKS Asynchronous transfer mode (ATM) is a method for

multiplexing and switching that supports broad range of services

Connection oriented packet switching technique that provides quality of service(QoS)

Variable bit rate delay bursty traffic processing

TDM Multirate only low,fixed Inefficient Minimal ,high speed

Packet Easily handled variable efficient header and packet

processing

ATM is packet based,can easily handle services and generate information in bursty fashion on variable bit rates.

The abbreviated header of ATM facilitates implementations that result in low delay and high speed

Information flow generated from various users is converted into cell and sent to ATM multiplexer

It arranges the cell into one or more queues and implements some scheduling strategy to determine which order it is transmitted