Answer sheet of switching & routing

1. How can you define contention network?

Contention network describe the situation where two or more nodes attempt to transmit a message

across the same wire at the same time.

In contention networks, any node that has a packet to send, merely sends the packet

It is clear that this type of network frequently experiences collisions

The more nodes trying to communicate, the higher the chance of collisions

Thus, contention networks are severely limited in the number of hosts possible

2. What is carrier sense multiple access? Define carrier sensing hardware with diagram?

Carrier Sense Multiple Access (CSMA) is a network protocol that listens to or senses network signals on

the carrier/medium before transmitting any data. CSMA is implemented in Ethernet network and is a

part of the Media Access Control (MAC) protocol.

To reduce the number of collisions, the medium is tested for a signal before each transmission

If a signal exists, the node waits

Signal testing can be anything from detection of an electrical signal, to testing for photons

Collisions can still occur (although less often)

If a node tests for a signal before a transmission from another node, and transmits after, a

collision occurs

3. Define the components of a router with diagram?

Hardware components of a router:

Network interfaces

Interconnection network

Processor with a memory and CPU

Commercial routers:

Interconnection network and interface cards are sophisticated

Processor is only responsible for control functions (route processor)

Almost all forwarding is done on interface cards

http://www.webopedia.com/TERM/N/network.html

http://www.webopedia.com/TERM/N/node.html

Functional Components:

4. “Router has modularized network interfaces” Explain?

Often, routers have modularized network interfaces

a. One can add/remove/replace network interfaces as needs change

b. Some routers can accept network interface modules of different types (e.g. Ethernet, Token Ring)

c. Each network interface would have its own:

i. Input buffer

ii. Output buffer

iii. Routing processor (in high-end routers)

5. What are the differences between hub, layer2 and layer3 switches? Explain with necessary

diagram?

Interface Card

Interconnection Network

Interface Card Interface Card

Processor

CPUMemory

Hub:

Transmission from a station received by central hub and retransmitted on all outgoing lines

Only one transmission at a time

Layer 2 Switch:

Incoming frame switched to one outgoing line

Many transmissions at same time

Flat address space

Broadcast storm

Only one path between any 2 devices

Solution 1: sub networks connected by routers

Solution 2: layer 3 switching, packet-forwarding logic in hardware

Layer 3 Switch:

Uses layer 3 routing to determine a path for packets

Once a path is found, subsequent packets are switched

This switching typically occurs on layer 2

6. Define packet tunneling in Internet with necessary diagram?

Tunneling is the transmission of data in such a way that the routing nodes in the network are unaware

that the transmission is from a different network.

Working principle:

Source sends packets to an intermediate gateway

Intermediate gateways put the whole packet into the payload field (don't interpret it).

The destination will understand the packet

Tunneling a packet from Paris to London

7. What is shortest path routing? Find the shortest path from a source node to destination node?

Shortest Path Routing is a static routing algorithm that just finds the shortest path.

A graph is used to represent the network.

a. Each node of the graph represents a router.

b. Each arc of the graph represents a communication link.

c. To choose the route between a given pair of routers, the algorithm just finds the shortest

path between them on the graph.

Metric used in the shortest path.

a. Number of hops

b. Geographic distance in miles/kilometers

c. Transmission delay fastest path

The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working

node.

8. Explain different types of packet fragmentation with diagram? Suppose a 19 byte packet to be

transmitted along with 3 byte maximum segment length network. Show the segment numbering

systems of the packet. The packet number is 15.

Transparent Fragmentation

With transparent fragmentation, end hosts (sender and receiver) are unaware that fragmentation has

taken place.

A router fragments a packet, and the next-hop router on the same network reassembles the fragments

back into the original packet.

Non-Transparent Fragmentation:

As before, routers fragment packets when needed. Routers along the path do not reassemble.

Destination hosts perform re-assembly (if needed).

(a) Transparent fragmentation. (b) Nontransparent fragmentation.

9. Explain briefly the functions of routing processor.

• Routing processors have two functions:

1. Maintain and exchange routing data with other routers in the network

Often this involves computing the forwarding table from data received by other routers

2. Use the forwarding table data to configure the switching fabric to forward the packet to the

correct output port

10. Describe briefly the construction of routers of different generation with necessary diagram.

1st Generation

• This architecture is still used in low end routers

• Arriving packets are copied to main memory via direct memory access (DMA)

• Interconnection network is shared bus

• All IP forwarding functions are performed in the central processor.

• Routing cache at processor can accelerate the routing table lookup.

2nd Generation

• Keeps shared bus architecture, but offloads most IP forwarding to interface cards

• Interface cards have local route cache and processing elements

Fast path: If routing entry is found in local cache, forward packet directly to outgoing interface

Slow path: If routing table entry is not in cache, packet must be handled by central CPU

3rd Generation

• Interconnection network is a switch fabric (e.g., a crossbar switch)

• Distributed architecture:

Memory

Shared Bus

DMA

MAC

DMA

MAC

Interface

Card

DMA

MAC

Route Processor

Interface

Card

Interface

Card

CacheCPU

Shared Bus

Interface

Cards

DMA

MAC

DMA

MAC

DMA

MAC

Route Cache

Memory

Route Cache

Memory

Route Cache

Memory

Route Processor

slow path

fast path

MemoryCacheCPU

slow path

fast path

– Interface cards operate independently of each other

– No centralized processing for IP forwarding

• These routers can be scaled to many hundred interface cards and to aggregate capacity of > 1

Terabit per second

11. Find the minimum spanning tree for the weighted graph in figure.

https://www.youtube.com/watch?v=3isltkMwxEs

12. Describe the routing protocol for mobile host with necessary diagram.

All hosts are assumed to have a permanent home location (home address) that never changes.

Each area has one or more foreign agents (FA), keeping track of all mobile hosts (MH) visiting the area.

Each area has a home agent (HA), which keep track of hosts whose home is in the area but are

currently visiting another area.

A WAN to which LANs, MANs, and wireless cells are attached

• When a new host enters an area, it registers with the FA.

– Each FA periodically announces its existence and address. The newly-arrived mobile host

(MH) waits for one of these messages. If no message is received, it broadcasts a message

and asks for FAs.

– The MH sends its home address, link layer address, and some security info to the FA.

Interface

Cards CPU

Memory

Route

Processor

Memory

Route

Processing

MAC

Switch

Fabric

Interface

Switch

Fabric

Memory

Route

Processing

MAC

Switch

Fabric

Interface

https://www.youtube.com/watch?v=3isltkMwxEs

– The FA contracts the HA.

– The HA examines the security info and records the temporary location of the MH.

– The FA gets ACK from HA, and informs MH that it has been registered.

Packet routing for mobile users

13. How can you define TCP/IP tunneling over ATM? Explain the categories of services that ATM

provides.

TCP/IP tunneling allows transmission of TCP/IP packets over ATM (and other non-TCP/IP) networks

The use of TCP/IP over these networks allows applications which normally only run on TCP/IP networks to operate on all networks

ATM provides 4 service categories: o CBR (Constant Bit Rate): A constant bandwidth is reserved and guaranteed by the

network o UBR (Unspecified Bit Rate): Data is transmitted when bandwidth is available, and not

when bandwidth is not available o ABR (Available Bit Rate): The network will provide feedback about network

congestion, under the assumption the node will adjust its transmission to meet the current availability of network bandwidth

o VBR (Variable Bit Rate): A minimum bandwidth is reserved and guaranteed by the network, although available bandwidth may increase/peak above this value

RT (Real Time): Fluctuation of bandwidth is minimized

Used for non-guaranteed streaming audio/video NRT (Non Real Time): Any bandwidth available is used

Used for downloads

14. What is distance vector routing? Explain with example.

Distance Vector Routing algorithm – Dynamic routing

a. Each router maintains a table (vector), giving the best known distance to each destination

and the outgoing line to get there.

b. These tables are updated by exchanging information with the neighbors.

c. The metric used might be the number of hops, the time delay, or the number of queued

packets.

d. The router is assumed to know the “distance” to each of its neighbors.

(a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.

15. Explain the link state routing construction routing packets for a subnet.

Link State Routing is a dynamic routing.

Each router must do the following:

a. Discover its neighbors, learn their network address.

b. Measure the delay or cost to each of its neighbors.

c. Construct a packet telling all it has just learned.

d. Send this packet to all other routers.

e. Compute the shortest path to every other router.

(a) A subnet. (b) The link state packets for this subnet

16. Explain the hierarchical routing with necessary diagram.

With the increase of network/routers, it is infeasible to have an entry for each router. The

hierarchical routing is required.

a. Divide the routers into regions.

b. The router only knows details to route packets to the destination within the same region.

c. But may not be optimal (e.g., the best route from 1A to 5C is via region 2, but since the

route via region 3 is better for most nodes in region 5.

Hierarchical routing

17. Name and define the hardware components of a router. Show the routers functional components

with diagram.

Check number 3 answer.

18. What is ATM network? Define the logical connections used in ATM networks.

ATM networks are packet-switched, but still create a (virtual) circuit through the network

a. Before transfer can occur, the network must create a path (called a virtual circuit) between the two machines

b. Once the virtual circuit (VC) has been established, packets can be transferred between the machines

Use optical fiber similar to that used for FDDI networks

- ATM runs on network hardware called SONET

ATM cells (packets) are 53 octets long

- 5 bytes of header information - 48 bytes of data

Logical Connections:

- VCC (Virtual Channel Connection): a logical connection analogous to virtual circuit in X.25

- VPC (Virtual Path Connection): a bundle of VCCs with same endpoints

19. Explain ATM protocol architecture. Define ATM cell format.

Fixed-size packets called cells

Streamlined: minimal error and flow control

2 protocol layers relate to ATM functions:

a. Common layer providing packet transfers

b. Service dependent ATM adaptation layer (AAL)

AAL maps other protocols to ATM

Protocol model has three planes:

• User

• Control

• management

ATM Cell Format: Data Cells

• Generic flow control: Control traffic flow.

• Virtual path identifier: An identifier for the virtual path/circuit

• Virtual channel identifier: An identifier to identify which channel within the specified virtual

path/circuit

• Payload Type: 3 flag bits

• Cell loss priority: Should the cell be discarded in the event of a congested switch?

• Header error check: Cyclical redundancy check for the cell header

20. Explain the way of creating ATM virtual circuits

Creating a virtual circuit has been compared to making a telephone call

A network node sends a request to the ATM switch specifying the destination

The switch interacts with any other switches necessary to align themselves to form a complete path

When communication is complete, the node sends a disconnect message to the switch

The switch will then notify all switches involved to release the connection

21. What do you mean by segmentation & reassembly in ATM network? Define ATM switching with

necessary diagram.

Segmentation:

• Segmentation is the process of turning a chunk of data into a group of ATM cells

– For example, the chunk of data might be a packet from another type of network

• e.g. An Ethernet frame

• Since ATM cells travel on the same virtual circuit, they do not arrive out of order

– Thus sequencing information is not necessary

• Each sequence of 48 octets is sent in its own cell

Reassembly:

• Reassembly is the process of recombining ATM cells into the original data chunk

• As the destination node receives cells, it removes the 48 octets and appends it to the end of a

buffer

• One of the configuration parameters, called ‘Payload Type’ is used to indicate the final cell

ATM Switching:

• As can be seen in the diagram, ATM switches exist entirely in the hardware layer

• As a result, they are much faster than routers, which require software execution

– Routers must read packets from electronic signals into a memory buffer (which is slow)

– Routers then convert packets back into electronic signals onto a new network connection

22. Write the drawbacks of ATM network.

Small, finite sized cells provide faster transmission speeds

- However, 53 octet cells are incompatible with other technologies which are in widespread use

ATM addressing also differs significantly from other forms of addressing

- For example the TCP/IP protocol suite is the most common network protocol system

- Most Internet applications are based on TCP/IP

ATM networks are not broadcast networks

- Each cell only arrives at its intended destination

- Broadcasting & multicasting are not directly supported

23. Show the internetworking using concatenated virtual circuits and connectionless internetworking.

Concatenated Virtual Circuits:

a. A connection to a remote host is set up by concatenating virtual circuits in all networks it

passes by.

b. Gateways response for converting packet format and maintaining VC.

c. Work best when all network have the same properties.

i. All reliable or all unreliable.

Can also be done on transport layer

Internetworking using concatenated virtual circuits

Connectionless internetworking:

– inject datagram’s into subnets and hope for the best

– packets may not follow the same route

– Also works on VC subnet.

A connectionless internet

24. Name and define the network properties.

Scope: A network should provide services to several applications

Scalability: A network should operate efficiently when deployed on a small-scale as well as on a large-

scale

Robustness: A network should operate in spite of failures or lost data

Self-Stabilization: A network, after a failure or other problem, should return to normal (or near

normal) without human intervention

Auto configurability: A network should optimize its own parameters in order to achieve better

performance

Safety: A network should prevent failures as well as prevent failures from affecting other areas of the

network

Configurability: A network’s parameters should be configurable to improve performance

Determinism: Two networks with identical conditions should yield identical results

Migration: It should be possible to add new features to a network without disruption of network

service

25. Explain the TCP/IP protocol in action using a simplified network route.

The TCP/IP Protocol in Action

Consider the following simplified network

route

The source (S) and destination (D) are

separated by two routers (R1, R2)

S DR1 R2

51


Let’s consider a web browser, using HTTP

The web browser on S sends a packet to the web server on D

The application layer (i.e. the browser) provides the logical (IP) addresses for S (IPS) and D (IPD)

The application layer also provides the port numbers for the source (PortS) and destination (PortD)

S DR1 R2

HTTP Req

52


The Transport layer (TCP) uses the port

numbers (e.g. 2765 and 80) to create a TCP

packet (sometimes called a segment):

S DR1 R2Source Port: 2765

Destination Port: 80

HTTP Req 53

Source IP: 137.207.140.71

Dest IP: 24.87.204.16


The Internet (i.e. IP) layer uses the IP

addresses specified by the application layer

to create an IP datagram

e.g. 137.207.140.71, 24.87.204.16

Next, a route is determined for the packet,

using S’s routing table

S only needs one router’s address (R1)

S DR1 R2

TCP Segment

HTTP Req 54

Source MAC: MACS

Dest MAC: MACR1

IP Datagram


The MAC addresses of S and R1 (MACS and

MACR1) are used to create a network frame

If the MAC address of R1 is not known, ARP

(address resolution protocol) is used

S DR1 R2

TCP Segment

HTTP Req55

Source MAC: MACS

Dest MAC: MACR1

IP Datagram


Let’s simplify the picture (for clarity)

In subsequent steps the IP datagram and its

contents will not change very much

S DR1 R2

56

Source MAC: MACS

Dest MAC: MACR1

IP Datagram


The network frame is transmitted on the

network to R1

This is possible since S and R1 are both

members of the same network

S DR1 R2

57

IP Datagram


R1 will extract the IP datagram from the payload of the network frame

R1 looks up the destination IP address (IPD) in it’s routing table, to determine which router should get the datagram next (R2)

S DR1 R2

58

Source MAC: MACR1

Dest MAC: MACR2

IP Datagram


R1 uses its own MAC address (MACR1) and

R2’s MAC address (MACR2) to create another

network frame

S DR1 R2

59

Source MAC: MACR1

Dest MAC: MACR2

IP Datagram


The network frame is received by R2, and the

IP datagram is extracted from it’s payload

R2 uses its routing table to lookup IPD

In this case, R2 is directly connected to D

This is called direct routing

S DR1 R2

60

ARP Request

IP: 24.87.204.16

MAC: ?IP Datagram


Most likely, R2 does not have the MAC

address of D (MACD)

The address resolution protocol (ARP) is used

to determine the MAC address:

S DR1 R2

61

ARP Response

IP: 24.87.204.16

MAC: 08-7F-3C-90-0C-DFIP Datagram


D recognizes it’s IP address and responds

with its MAC address (MACD)

e.g. 08-7F-3C-90-0C-DF

S DR1 R2

62

Source MAC: MACR2

Dest MAC: MACD

IP Datagram


A network frame is created by R2 now that

the MAC address is known

The frame is sent directly to D

S DR1 R2

63

Source MAC: MACR2

Dest MAC: MACD

IP Datagram


D extracts the IP datagram from the network

frame (which is discarded)

The IP datagram’s payload is passed to the

transport layer

S DR1 R2

64


The Transport layer (within D’s operating

system), will use the port numbers specified

in the TCP segment to determine to which

application it should send the segment

In this case, to the application bound to port

80 (the web server)

S DR1 R2Source Port: 2765

Destination Port: 80

HTTP Req65


Now, the web server on D has the HTTP

request, and it processes it

An HTTP response is sent back using the

same process

The web server uses the same IP addresses

and logical addresses as the last message

S DR1 R2HTTP Req

66

26. Name & define different types of network bridges.

There are four types of network bridges:

1. Transparent basic bridge

2. Source routing bridge

3. Transparent learning bridge

4. Transparent spanning bridge

The Transparent Basic Bridge

The simplest type of bridge is the transparent basic bridge. It stores the traffic until it can transmit it to

the next network. The amount of time the data is stored is very brief. Traffic is sent to all ports except the

port from which the bridge received the data. No conversion of traffic is performed by a bridge. In this

regard, the bridge is similar to a repeater.

Source Routing Bridge

The route through the LAN internet is determined by the source (originator) of the traffic hence this bridge

is called as source routing bridge. The routing information field (RIF) in the LAN frame header, contains the

information of route followed by the LAN network.

The Transparent Learning Bridge

The transparent bridge finds the location of user using the source and destination address. When the

frame is received at the bridge it checks its source address and the destination address. The destination

address is stored if it was not found in a routing table. Then the frame sent to all LAN excluding the LAN

from which it came. The source address is also stored in the routing table. If another frame is arrived in

which the previous source address is now its destination address then it is forwarded to that port.

The Transparent Spanning Tree Bridge

These bridges use a subnet of the full topology to create a loop free operation.

27. Short notes on routers input buffer, output buffer, processor, switching fabric.

Input Buffer:

The incoming packets of a network interface are placed in input buffers

These are banks of very high speed memory for packet queuing prior to processing

The packet is stored here until the routing processor is available

The network interface may have a routing processor, which would:

a. have a copy of the forwarding table (to prevent concurrent access)

b. lookup the destination address in this forwarding table, to determine the correct output port

c. configure the switching fabric to forward the packet to the correct output buffer

Low-end routers would share one routing processor

Output Buffer:

The switching fabric gets the packet to the right output port

However, that port’s network may not be immediately available

The packets are stored in the output buffer until the network is available

Routing Processor:

• A routing processor is software which executes on a CPU:

– Off-the-shelf CPU

• These are very inexpensive

• However, the performance of these CPUs is low since they are not optimized for

the types of operations a router typically needs to perform

– Application-Specific Integrated Circuit (ASIC)

• These are expensive to design (time and money)

• They are optimized for typical routing operations

• High-end routers use these to achieve higher performance levels

Switching Fabric:

• Switching fabric’s job is to move packets from the input buffer into the correct output buffer

– The routing processor determines the correct output port, using the forwarding table

• Switching fabric comes in 3 major types:

– In-memory switching fabric:

• The packets are input into the routing processor’s memory, and output into the

correct output buffer

– Bus-based switching fabric:

• The packets move along a shared bus (similar to a network bus) to the correct

output buffer

– Crossbar switching fabric:

• The packets move along a grid of redundant buses

• If any bus fails, alternate paths exist so that forwarding can continue

28. What is switching? Write the reasons for switching in communication.

The establishing, on-demand, of an individual connection from a desired inlet to a desired outlet within a set of inlets and outlets for as long as is required for the transfer of information.

Switching implies directing of information flows in communications networks based on known rules

Switching takes place in specialized network nodes

Data switched on bit, octet, frame or packet level

Size of a switched data unit is variable or fixed Reasons for switching in communication:

Switches allow reduction in overall network cost by reducing number and/or cost of transmission links required to enable a given user population to communicate

Limited number of physical connections implies need for sharing of transport resources, which means

better utilization of transport capacity

use of switching

Switching systems are central components in communications networks

29. Describe the main building block of a switch. Define basic type of switching networks.

30. Explain different types of switching modes. Define label switching.

Types of switching modes:

• Circuit switching • Cell switching • Packet switching

– Routing – Layer 3 - 7 switching

– Label switching

Circuit Switching:

End-to-end circuit established for a connection

Signaling used to set-up, maintain and release circuits

Circuit offers constant bit rate and constant transport delay

Equal quality offered to all connections

Transport capacity of a circuit cannot be shared

Applied in conventional telecommunications networks (e.g. PDH/PCM and N-ISDN)

Cell switching: • Virtual circuit (VC) established for a connection • Data transported in fixed length frames (cells), which carry information needed for routing cells along established VCs • Forwarding tables in network nodes • Signaling used to set-up, maintain and release VCs as well as update forwarding tables • VCs offer constant or variable bit rates and transport delay • Transport capacity of links shared by a number of connections (statistical multiplexing) • Different quality classes supported • Applied, e.g. in ATM networks

Packet switching:

No special transport path established for a connection

Variable length data packets carry information used by network nodes in making forwarding decisions

No signaling needed for connection setup

Forwarding tables in network nodes are updated by routing protocols

No guarantees for bit rate or transport delay

Best effort service for all connections in conventional packet switched networks

Transport capacity of links shared effectively

Applied in IP (Internet Protocol) based networks

Label Switching:

Evolved from the need to speed up connectionless packet switching and utilize L2-switching in packet forwarding

A label switched path (LSP) established for a connection

Forwarding tables in network nodes

Signaling used to set-up, maintain and release LSPs

A label is inserted in front of a L3 packet (behind L2 frame header)

Packets forwarded along established LSPs by using labels in L2 frames

Quality of service supported

Applied, e.g. in ATM, Ethernet and PPP

Generalized label switching scheme (GMPLS) extends MPLS to be applied also in optical networks, i.e., enables light waves to be used as LSPs

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Answer sheet of switching & routing