Siddhi Mehta 94

Pg-1
ACN-QB-2022-23-PT-1....
ACN Questions of 2, 4, 6 marks:

1. Give IP address representation notations in IPv4

ANSWERS:
 An IP address is a 32-bit address.
 The IP addresses are unique.
Address space rule:
 The address space in a protocol That uses N-bits to define an Address is = 2N
 The address space of IPv4 is 2^32 or4,294,967,296.
Address Space Notations:
 Binary Notation: 01110101 10010101 00011101 11101010
 Dotted-decimal notation

 Hexadecimal Notation

2 Give IP address representation notations in IPv6.

ANSWERS:
IPv6 Address Representation Examples:
 2031:0000:130F:0000:0000:09C0:876A:130B
 2031:0:130f::9c0:876a:130b
 FF01:0:0:0:0:0:0:1 >>> FF01::1
 0:0:0:0:0:0:0:1 >>> ::1
 0:0:0:0:0:0:0:0 >>> ::
Notations in 128 bit:
 Dotted decimal 123.145.20.34
 hexadecimal notation. 23BA:1234:00B1:0000:BF30:3456:000A:FFFF
 Mixed representation 23BA:1234:123:56:BF30:3456:000A:FFFF
 CIDR notation. FDC1:AB23:0:FFFF/27
 Siddhi Mehta 94
Pg-2
3. Draw flowchart of deciding class of IP address
ANSWERS:

4. Describe static, dynamic translation and load balancing

ANSWERS:
Static Translation:
 Map a range of external address to the same size block of internal addresses
- Firewall just does a simple translation of each address
 Port forwarding - map a specific port to come through the Firewall rather than all ports;
 useful to expose a specific service on the internal network to the public network
Dynamic Translation (IP Masquerading):
 Also called Network Address and Port Translation (NAPT)
 Individual hosts inside the Firewall are identified based on of each connection flowing
through the firewall.
Since a connection doesn’t exist until an internal host requests a connection through the firewall to an
external host, and most Firewalls only open ports only for the addressed host only that host can route
back into the internal network
 IP Source routing could route back in; but most Firewalls block incoming source route
packets
 NAT only prevents external hosts from making connections to internal hosts.
 Some protocols won’t work; protocols that rely on separate connections back into the local
network
 Theoretical max of 216 connections, actual is much less
Load Balancing:
A firewall that will dynamically map a request to a pool of identical clone machines
– often done for really busy web sites
– each clone must have a way to notify the Firewall of its current load so the Fire
wall can choose a target machine
– or the firewall just uses a dispatching algorithm like round robin
 Siddhi Mehta 94
Pg-3
5. Draw IPV4 header explains its blocks and its sub blocks in short
ANSWERS:

Version
 Which version of the protocol the datagram belongs to.
 The current version number is 4.
 Next version: 6
IHL (Header length)
 The number of 32-bit words in the header
 Because this is 4 bits, the max header length is 15 words (i.e. 60 bytes)
 The header is at least 20 bytes, but options may make it bigger
Type of Service
 Contains a 3-bit precedence field (that is ignored today), 4 service bits, and 1 unused bit.
 The four service bits can be:
o 1000 - minimize delay
o 0100 - maximize throughput
o 0010 - maximize reliability
o 0001 - minimize monetary cost
 This is a "hint" of what characteristics of the physical layer to use
 The Type of Service is not supported in most. Implementations. However, some
implementations have extra fields in the routing table to indicate delay, throughput, reliability,
and monitory cost.
Total Length
 Siddhi Mehta 94
Pg-4
 total length of the datagram in bytes.
 we know where the data starts by the header length
 we know the size of the data by computing "total length - header length"
Identification
 Uniquely identifies the datagram.
 Usually incremented by 1 each time a datagram is sent.
 All fragments of a datagram contain the same identification value.
 This allows the destination host to determine which fragment belongs to which datagram.
Flags and Fragmentation Offset
 Used for fragmentation
 DF means do not fragment. It is a request to routers not to fragment the datagram since the
destination is incapable of putting the pieces back together.
 MF means more fragments to follow. All fragments except the last one have this bit set. It is
needed to know if all fragments of a datagram have arrived.
Fragment offset
Number of fragments
Time to Live
 Upper limit of routers
 usually set to 32 or 64.
 decremented by each router that processes the datagram,
 router discards the datagram when TTL reaches 0.
Protocol
 Tells IP where to send the datagram up to.
 6 means TCP
 17 means UDP
Header checksum
Only covers the header, not the data

 Used to verify the header, and is recomputed at each router hop.

 This field is left out of IPv6 which relies on the transport layer for verification.
Source IP address
The sender
Destination IP address
the final destination
Options
 Optional data.
 Siddhi Mehta 94
Pg-5
 Some examples include having the router put in a IP address of router and a time stamp so the
final destination knows how long it took to get to each hop.
The source and destination in the IP header are the original source and the final destination! The
physical layer addresses pass the datagram from router to router. So, while the physical layer
addresses change from router to router, the source and destination IP addresses in the IP datagram
remain constant!
The checksum
 How to compute a checksum?
o Put a 0 in the checksum field.
o Add each 16-bit value together.
o Add in any carry
o Inverse the bits and put that in the checksum field.
 To check the checksum:
o Add each 16-bit value together (including the checksum).
o Add in carry.
o Inverse the bits.
o The result must be 0.
 Remember, only the bits in the header are calculated in the IP checksum.
 Note: all other fields of the IP header are identical to the first packet (except the checksum)
 Siddhi Mehta 94
Pg-6
6. Draw IPV6 header explains its blocks and its sub blocks in short
ANSWERS:
IPv6 Packet Header Format
The IPv6 protocol defines a set of headers, including the basic IPv6 header and the IPv6 extension
headers. The following figure shows the fields that appear in the IPv6 header and the order in which
the fields appear.

The following list describes the function of each header field.

 Version – 4-bit version number of Internet Protocol = 6.
 Traffic class – 8-bit traffic class field.
 Flow label – 20-bit field. designed to provide special handling for a particular flow of data.
 Payload length – 16-bit unsigned integer, which is the rest of the packet that follows the IPv6
header, in octets.
 Next header – 8-bit selector. Identifies the type of header that immediately follows the IPv6
header. Uses the same values as the IPv4 protocol field.
 Hop limit – 8-bit unsigned integer. Decremented by one by each node that forwards the
packet. The packet is discarded if the hop limit is decremented to zero.
 Source address – 128 bits. The address of the initial sender of the packet.
 Destination address – 128 bits. The address of the intended recipient of the packet. The
intended recipient is not necessarily the recipient if an optional routing header is present.
 Siddhi Mehta 94
Pg-7
7. Enlist blocks of IPV4 header explain any one in short
ANSWERS:

Version
 Which version of the protocol the datagram belongs to.
 The current version number is 4.
 Next version: 6
IHL (Header length)
 The number of 32-bit words in the header
 Because this is 4 bits, the max header length is 15 words (i.e. 60 bytes)
 The header is at least 20 bytes, but options may make it bigger
Type of Service
 Contains a 3-bit precedence field (that is ignored today), 4 service bits, and 1 unused bit.
 The four service bits can be:
o 1000 - minimize delay
o 0100 - maximize throughput
o 0010 - maximize reliability
o 0001 - minimize monetary cost
 Siddhi Mehta 94
Pg-8
 This is a "hint" of what characteristics of the physical layer to use
 The Type of Service is not supported in most. Implementations. However, some
implementations have extra fields in the routing table to indicate delay, throughput, reliability,
and monitory cost.
Total Length
 total length of the datagram in bytes.
 we know where the data starts by the header length
 we know the size of the data by computing "total length - header length"
Identification
 Uniquely identifies the datagram.
 Usually incremented by 1 each time a datagram is sent.
 All fragments of a datagram contain the same identification value.
 This allows the destination host to determine which fragment belongs to which datagram.
Flags and Fragmentation Offset
 Used for fragmentation
 DF means do not fragment. It is a request to routers not to fragment the datagram since the
destination is incapable of putting the pieces back together.
 MF means more fragments to follow. All fragments except the last one have this bit set. It is
needed to know if all fragments of a datagram have arrived.
Fragment offset
Number of fragments
Time to Live
 Upper limit of routers
 usually set to 32 or 64.
 decremented by each router that processes the datagram,
 router discards the datagram when TTL reaches 0.
Protocol
 Tells IP where to send the datagram up to.
 6 means TCP
 17 means UDP
Header checksum
Only covers the header, not the data

 Used to verify the header, and is recomputed at each router hop.

 This field is left out of IPv6 which relies on the transport layer for verification.
 Siddhi Mehta 94
Pg-9
Source IP address
The sender
Destination IP address
the final destination
Options
 Optional data.
 Some examples include having the router put in a IP address of router and a time stamp so the
final destination knows how long it took to get to each hop.
The source and destination in the IP header are the original source and the final destination! The
physical layer addresses pass the datagram from router to router. So, while the physical layer
addresses change from router to router, the source and destination IP addresses in the IP datagram
remain constant!
The checksum
 How to compute a checksum?
o Put a 0 in the checksum field.
o Add each 16-bit value together.
o Add in any carry
o Inverse the bits and put that in the checksum field.
 To check the checksum:
o Add each 16-bit value together (including the checksum).
o Add in carry.
o Inverse the bits.
o The result must be 0.
 Remember, only the bits in the header are calculated in the IP checksum.
 Note: all other fields of the IP header are identical to the first packet (except the checksum)
 Contains a 3-bit precedence field (that is ignored today), 4 service bits, and 1 unused bit.
 The four service bits can be:
o 1000 - minimize delay
o 0100 - maximize throughput
 Siddhi Mehta 94
Pg-10
8. Enlist OSI Layer-wise protocols
ANSWERS:

9. Explain fields and sub fields of OPTION format

ANSWERS:
OPTIONS:
The header of the IP datagram is made of two parts:

✔ a fixed part- The fixed part is 20 bytes long and was discussed in the previous section. ✔ a
variable part-The variable part comprises the options, which can be a maximum of 40 bytes.
Options,

✔ as the name implies, are not required for a datagram.

✔ They can be used for network testing and debugging.

✔ Although options are not a required part of the IP header, option processing is required of the IP
software.

✔ This means that all implementations must be able to handle options if they are present in
the header.
 Siddhi Mehta 94
Pg-11
Format
The format of an option is composed of:
∙ A 1-byte type field,
∙ A 1-byte length field, and
∙ A variable-sized value field.
The three fields are often referred to as Type-Length-Value or TLV.

Type
The type field is 8 bits long and contains three subfields: copy, class, and number.
 Copy. This 1-bit subfield controls the presence of the option in fragmentation When its value is 0, it
means that the option must be copied only to the first fragment. If its value is 1, it means the option
must be copied to all fragments.
 Class. This 2-bit subfield defines the general purpose of the option. When its value is 00, it means
that the option is used for datagram control. When its value is 10, it means that the option is used for
debugging and management. The other two possible values (01 and 11) have not yet been defined.
 Number. This 5-bit subfield defines the type of option. Although 5 bits can define up to 32 different
types, currently only 6 types are in use. These will be discussed in a later section.
Length
The length field defines the total length of the option including the type field and the length field
itself. This field is not present in all of the option types.
Value
The value field contains the data that specific options require. Like the length field, this field is also
not present in all option types.
 Option Types
 Siddhi Mehta 94
Pg-12
There are only six options are currently being used. Two of these are1-byte options, and they do not
require the length or the data fields. Four of them are multiple-byte options; they require the length
and the data fields.

10. Enlist types/categories of OPTION.

ANSWERS:
There are only six options are currently being used. Two of these are1-byte options, and they do not
require the length or the data fields. Four of them are multiple-byte options; they require the length
and the data fields.

11 Explain Record Route Strict Source-route Record, Loose Source-route Record

ANSWERS:
Record-Route Option
 is used to record the Internet routers that handle the datagram.
 It can list up to nine router IP addresses since the maximum size of the header is 60 bytes,
 which must include 20 bytes for the base header.
 This implies that only 40 bytes are leftover for the option part.
 The source creates placeholder fields in the option to be filled by the visited routers.
 Siddhi Mehta 94
Pg-13
 The pointer field is an offset integer field containing the byte number of the first empty entry/
it points to the first available entry.
 The source creates empty fields for the IP addresses in the data field of the option. When the
datagram leaves the source, all of the fields are empty.
 The pointer field has a value of 4, pointing to the first empty field.
 When the datagram is traveling, each router that processes the datagram compares the value
of the pointer with the value of the length.
 If the value of the pointer is greater than the value of the length, the option is full and no
changes are made.
 However, if the value of the pointer is not greater than the value of the length, the router
inserts its outgoing IP address in the next empty field
 (Remember that a router has more than one IP address).
In this the router adds the IP address of its interface from which the datagram is leaving.
The router then increments the value of the pointer by 4.

A strict-source-route option is
 Used by the source to predetermine a route for the datagram as it travels through the Internet.
Dictation of a route by the source can be useful for several purposes.
 The sender can choose a route with a specific type of service, such as minimum delay or
throughput.
 Alternatively, it may choose a route that is safer or more reliable for the sender’s purpose. For
example, a sender can choose a route so that its datagram does not travel through a
competitor’s network.
 If a datagram specifies a strict source route, all of the routers defined in the option must be
visited by the datagram. A router must not be visited if its IP address is not listed in the
datagram.
 If the datagram visits a router that is not on the list, the datagram is discarded and an error
message is issued.
 If the datagram arrives at the destination and some of the entries were not visited, it will also
be discarded and an error message issued.
 Regular users of the Internet, however, are not usually aware of the physical topology of the
Internet. Consequently, strict source routing is not the choice of most users.
 Siddhi Mehta 94
Pg-14

 It is similar to the record route option with the exception that all of the IP addresses are
entered by the sender.
 When the datagram is traveling, each router that processes the datagram compares the value
of the pointer with the value of the length.
 If the value of the pointer is greater than the value of the length, the datagram has visited all
of the predefined routers.
 The datagram cannot travel anymore; it is discarded and an error message is created.
 If the value of the pointer is not greater than the value of the length, the router compares the
destination IP address with its incoming IP address:
 If they are equal, it processes the datagram, swaps the IP address pointed by the pointer with
the destination address, increments the pointer value by 4, and forwards the datagram.
 If they are not equal, it discards the datagram and issues an error message. Figure 7.17 shows
the actions taken by each router as a datagram-travels from source to destination.

Loose-Source-Route Option
 A loose-source-route option is similar to the strict source route, but it is more relaxed.
 Each router in the list must be visited, but the datagram can visit other routers as well.
 Siddhi Mehta 94
Pg-15
12. Draw architecture of Mobile IP and explain its functioning
ANSWERS:

Mobile Hosts
 When a host moves from one network to another, the IP addressing structure needs to be
modified.
 Several solutions have been proposed.

Changing the Address:

One simple solution is to let the mobile host change its address as it goes to the new network. The
host can use DHCP to obtain a new address to associate it with the new network.
This approach has several drawbacks.
 First, the configuration files would need to be changed.
 Second, each time the computer moves from one network to another, it must be rebooted.
 Third, the DNS tables need to be revised so that every other host in the Internet is aware
of the change.
 Fourth, if the host roams from one network to another during a transmission, the data
exchange will be interrupted.
 This is because the ports and IP addresses of the client and the server must remain
constant for the duration of the connection
Two Addresses:
 The approach that is more feasible is the use of two addresses.
 The host has its original address, called the Home address, and A temporary address,
called the care-of address.
 The home address is permanent; it associates the host to its home network, the network
that is the permanent home of the host.
 The care-of address is temporary. When a host moves from one network to another, the
care-of address changes; it is associated with the foreign network, the network to which
the host moves.
When a mobile host visits a foreign network, it receives its care-of address during the agent discovery
and registration phase
AGENTS
 Siddhi Mehta 94
Pg-16
To make the change of address transparent to the rest of the Internet requires a home Agent and a
Foreign Agent.
Figure shows the position of a home agent relative to the home network and a foreign agent relative to
the foreign network.

We have shown the home and the foreign agents as routers, but we need to emphasize that their
specific function as an agent is performed in the application layer. In other words, they are both
routers and hosts.
Home Agent
 The home agent is usually a router attached to the home network of the mobile host.
 The home agent acts on behalf of the mobile host when a remote host sends a packet to the
mobile host.
 The home agent receives the packet and sends it to the foreign agent.
Foreign Agent
 The foreign agent is usually a router attached to the foreign network.
 The foreign agent receives and delivers packets sent by the home agent to the mobile host.
 The mobile host can also act as a foreign agent.
 In other words, the mobile host and the foreign agent can be the same.
 However, to do this, a mobile host must be able to receive a care-of address by itself, which
can be done through the use of DHCP.
 In addition, the mobile host needs the necessary software to allow it to communicate with the
home agent and to have two addresses:
1. its home address and
2. its care-of address.
 This dual addressing must be transparent to the application programs.
 When the mobile host acts as a foreign agent, the care-of address is called a co- located care-
of address.
 Siddhi Mehta 94
Pg-17
13. State different Agents involved in Mobile IP with their roles
ANSWERS:
AGENTS
To make the change of address transparent to the rest of the Internet requires a home Agent and a
Foreign Agent.
Figure shows the position of a home agent relative to the home network and a foreign agent relative to
the foreign network.

We have shown the home and the foreign agents as routers, but we need to emphasize that their
specific function as an agent is performed in the application layer. In other words, they are both
routers and hosts.
Home Agent
 The home agent is usually a router attached to the home network of the mobile host.
 The home agent acts on behalf of the mobile host when a remote host sends a packet to the
mobile host.
 The home agent receives the packet and sends it to the foreign agent.
Foreign Agent
 The foreign agent is usually a router attached to the foreign network.
 The foreign agent receives and delivers packets sent by the home agent to the mobile host.
 The mobile host can also act as a foreign agent.
 In other words, the mobile host and the foreign agent can be the same.
 However, to do this, a mobile host must be able to receive a care-of address by itself, which
can be done through the use of DHCP.
 In addition, the mobile host needs the necessary software to allow it to communicate with the
home agent and to have two addresses:
3. its home address and
4. its care-of address.
 This dual addressing must be transparent to the application programs.
 When the mobile host acts as a foreign agent, the care-of address is called a co- located care-
of address.
 Siddhi Mehta 94
Pg-18
14. States three phases of Mobile IP. Explain any one in short
ANSWERS:
THREE PHASES
To communicate with a remote host, a mobile host goes through three phases:
Agent Discovery:
During the agent discovery phase, the Home Agent and Foreign Agent advertise their services on the
network by using the ICMP.
Router Discovery Protocol (IRDP). The Mobile Node listens to these advertisements to determine if it
is connected to its home network or foreign network.
 If a Mobile Node determines that it is connected to a foreign network, it acquires a care-of
address.
Two types of care-of addresses exist:
 Care-of address acquired from a Foreign Agent Co-located care-of address
 When the Mobile Node hears a Foreign Agent advertisement and detects that it has moved
outside of its home network, it begins registration.
Registration
The Mobile Node is configured with the IP address and mobility security association (which includes
the shared key) of its Home Agent. In addition, the Mobile Node is configured with either its home IP
address, or another user identifier, such as a Network Access Identifier.

Data Transfer. associated request is in its pending list as well as proper authentication of the Home
Agent. If the registration reply is not valid, the Mobile Node discards the reply. If a valid registration
reply specifies that the registration is accepted, the Mobile Node is confirmed that the mobility agents
are aware of its roaming. In the co-located care-of address case, it adds a tunnel to the Home Agent.
Subsequently, it sends all packets to the Foreign Agent.
 a successful Mobile IP registration sets up the routing mechanism for transporting packets to and
from the Mobile Node as it roams.
 Siddhi Mehta 94
Pg-19
15. State flags used in Fragmentation, explain fragmentation with simple example
ANSWERS:
Flags: This is a three-bit field.
 The first bit is reserved (not used).
 The second bit is called the do not fragment bit.
If its value is 1, the machine must not fragment the datagram. If it cannot pass the datagram through
any available physical network, it discards the datagram and sends an ICMP error message to the
source host.If its value is 0, the datagram can be fragmented if necessary.
 The third bit is called the more fragment bit.
If its value is 1, it means the datagram is not the last fragment; there are more fragments after this one.

If its value is 0, it means thesis the last or only fragment.

Fragmentation example:

 Remember that the value of the offset is measured in units of 8 bytes. This is done because the
length of the offset field is only 13 bits long and cannot represent a sequence of bytes greater than
8191.
 This forces hosts or routers that fragment datagrams to choose the size of each fragment so that
the first byte number is divisible by 8.
 The value of the identification field is the same in all fragments.
 Notice the value of the flags field with the more bit set for all fragments except the last.

Also, the value of the offset field for each fragment is shown.
 Siddhi Mehta 94
Pg-20

In this case the value of the offset field is always relative to the original datagram.
For example, in the figure, the second fragment is itself fragmented later to two fragments of 800
bytes and 600 bytes, but the offset shows the relative position of the fragments to the original data. It
is obvious that even if each fragment follows a different path and arrives out of order, the final
destination host can reassemble the original datagram from the fragments received (if none of them is
lost) using the following strategy:
a. The first fragment has an offset field value of zero.
b. Divide the length of the first fragment by 8. The second fragment has an offset value equal to that
result.
c. Divide the total length of the first and second fragment by 8. The third fragment has an offset
value equal to that result.
d. Continue the process. The last fragment has a more bit value of 0.
 Siddhi Mehta 94
Pg-21
16. Give significance of ICMP Checksum. Give example of checksum calculation.
ANSWERS:
ICMP Checksum
Checksum Calculation
The sender follows these steps using one’s complement arithmetic:
1. The checksum field is set to zero.
2. The sum of all the 16-bit words (header and data) is calculated.
3. The sum is complemented to get the checksum.
4. The checksum is stored in the checksum field.
Checksum Testing
The receiver follows these steps using one’s complement arithmetic:
1. The sum of all words (header and data) is calculated.
2. The sum is complemented.
3. If the result obtained in step 2 is 16 0s, the message is accepted; otherwise, it is rejected.
17. Draw IPV6 Header format in detail.
ANSWERS:

The following list describes the function of each header field.

 Version – 4-bit version number of Internet Protocol = 6.
 Traffic class – 8-bit traffic class field.
 Siddhi Mehta 94
Pg-22
 Flow label – 20-bit field.
 Payload length – 16-bit unsigned integer, which is the rest of the packet that follows the IPv6
header, in octets.
 Next header – 8-bit selector. Identifies the type of header that immediately follows the IPv6 header.
Uses the same values as the IPv4 protocol field.
 Hop limit – 8-bit unsigned integer. Decremented by one by each node that forwards the packet. The
packet is discarded if the hop limit is decremented to zero.
 Source address – 128 bits. The address of the initial sender of the packet.
 Destination address – 128 bits. The address of the intended recipient of the packet. The intended
recipient is not necessarily the recipient if an optional routing header is present.

18. Draw formats of IPV6 Extension Header types

ANSWERS:
1) hop-by-hop option,

It is used when the source needs to pass information to all routers visited by the datagram.
2) routing (source routing) header,

The source routing extension header combines the concepts of the strict source route and the loose
source route options of IPv4.
3) Destination options header,
The destination option is used when the source needs to pass information to the destination only.
Intermediate routers are not permitted access to this information. The format of the destination option
is the same as the hop-by-hop option (refer back to Figure 27.5). So far, only the Pad1 and PadN
options have been defined.
 Siddhi Mehta 94
Pg-23

4) TCP header

5) fragmentation,

6) Authentication And Encryption security Payload Header (AH & ESP)

 Siddhi Mehta 94
Pg-24

19. Differentiate static and dynamic routing any four points.

ANSWERS:
Routing Algorithms are of two types:
1. Static / Non-Adaptive routing Algorithms:
 The network topology determines the final path. All the possible paths which are already
calculated are loaded into routing table.
 Suitable for small networks
 Advantages:
  Minimal CPU / memory overhead.
  Granular control on how traffic is routed.
  Simple to configure and maintain.
  Secure as only defined routes can be accessed.
  Bandwidth is not used for sending routing updates.
Disadvantage is : inability to respond quickly in case of network failure.
2. Dynamic / Adaptive routing Algorithms:
 Can change their routing decision on the basis of some changes made in topology
 Each router can check the network status by communicating with the neighbors so the
changes in the topology are reflected to all routers.
 Router can calculate the suitable path to the final destination.
Advantages:
  Simple to configure on large networks.
  Will dynamically choose a different /better route if a link goes down.
  Ability to load balance between multiple links.
Disadvantages:
 It has complexity
 Updates are shared between routers, thus consuming bandwidth.
 Routing protocols put additional load on router CPU/RAM.
 Siddhi Mehta 94
Pg-25
20. Draw IPv6 Address Representation EUI 64 - Global and stateless Autoconfiguration.
ANSWERS:
 Siddhi Mehta 94
Pg-26
21 Explain Virtual Private Network with proper diagram
ANSWERS:

22. Describe features of VPN.

ANSWERS:
• Security
• Reliability
• Scalability
• Network Management
• Policy Management

23. State and explain types of VPN Protocols

ANSWERS:
Types of VPN protocols
 Two VPN types are based on different VPN security protocols.
 Siddhi Mehta 94
Pg-27
 Each of these VPN protocols offer different features and levels of security,
1. Internet Protocol Security or IPSec:
 IPSec is used to secure Internet communication across an IP network.
 Secures Internet Protocol communication by authenticating the session and encrypts each data
packet during the connection.
 IPSec operates in two modes, Transport mode and Tunneling mode, to protect data transfer
between two different networks.
 The transport mode encrypts the message in the data packet and the tunneling mode encrypts
the entire data packet.
 Also used with other security protocols to enhance the security system.
2. Layer 2 Tunneling Protocol (L2TP):
 is a tunneling protocol that is usually combined with another VPN security protocol like
IPSec to create a highly secure VPN connection.
 creates a tunnel between two L2TP connection points and
 IPSec protocol encrypts the data and handles secure communication between the tunnel.
3. Point – to – Point Tunneling Protocol (PPTP):
 creates a tunnel and encapsulates the data packet.
 It uses a Point-to-Point Protocol (PPP) to encrypt the data between the connection.
 is one of the most widely used VPN protocol and has been in use since the time of Windows
95. Apart from Windows, PPTP is also supported on Mac and Linux.
4. Secure Sockets Layer (SSL) and Transport Layer Security (TLS):
SSL (Secure Sockets Layer) and TLS (Transport Layer Security)
 create a VPN connection where the web browser acts as the client and user access is restricted
to specific applications instead of entire network.
 is most commonly used by online shopping websites and service providers.
 Web browsers switch to SSL with ease and with almost no action required from the user,
since web browsers come integrated with SSL and TLS. SSL connections have https in the
beginning of the URL instead of http.
5. OpenVPN:
 is an open source VPN that is useful for creating Point-to-Point and Site-to-Site connections.
 uses a custom security protocol based on SSL and TLS protocol.
6. Secure Shell (SSH):
 Secure Shell or SSH creates the VPN tunnel through which the data transfer happens and also
ensures that the tunnel is encrypted.
 SSH connections are created by a SSH client and data is transferred from a local port on to
the remote server through the encrypted tunnel.
 Siddhi Mehta 94
Pg-28
24. Explain given types of VPN in short: 1) Remote Access VPN 2) Site-to-Site VPN
ANSWERS:
Types of VPN
VPN is a Virtual Private Network that allows a user to connect to a private network over the Internet
securely and privately. VPN creates an encrypted connection, known as VPN tunnel, and all Internet
traffic and communication is passed through this secure tunnel. Thus, keeping the user data secure and
private.
There are two basic VPN types which are explained below.
1. Remote Access VPN
 allows a user to connect to a private network and access its services and resources remotely.
 The connection between the user and the private network happens through the Internet and
the connection is secure and private.
 useful for business users as well as home users.
 A corporate employee, while traveling, uses a VPN to connect to his/her company’s private
network and remotely access files and resources on the private network.
 Home users, or private users of VPN, use VPN services to bypass regional restrictions on the
Internet and access blocked websites.
 Users conscious of Internet security also use VPN services to enhance their Internet security
and privacy.
2. Site – to – Site VPN
 also called as Router-to-Router VPN
 Mostly used in the corporates. Companies, with offices in different geographical locations,
use Site-to-site VPN to connect the network of one office location to the network at another
office location.
 When multiple offices of the same company are connected using Site-to-Site VPN type, it is
called as Intranet based VPN.
 When companies use Site-to-site VPN type to connect to the office of another company, it is
called as Extranet based VPN.
 Basically, Site-to-site VPN create a virtual bridge between the networks at geographically
distant offices and connect them through the Internet and maintain a secure and private
communication between the networks.
 Since Site-to-site VPN is based on Router-to-Router communication, in this VPN type one
router acts as a VPN Client and another router as a VPN Server. The communication between
the two routers starts only after an authentication is validated between the two.
 Siddhi Mehta 94
Pg-29
25. Explain Dual Stack IP implementation
ANSWERS:

 Dual-stack IP implementations provide complete IPv4 and IPv6 protocol stacks in the
operating system of a computer or network device on top of the common physical layer
implementation, such as Ethernet.
 This permits dual-stack hosts to participate in IPv6 and IPv4 networks simultaneously.
 The method is defined in RFC 4213.
 A device with dual-stack implementation in the operating system has an IPv4 and IPv6
address, and can communicate with other nodes in the LAN or the Internet using either IPv4
or IPv6.
 The Domain Name System (DNS) protocol is used by both IP protocols to resolve fully
qualified domain names (FQDN) and IP addresses, but dual stack requires that the resolving
DNS server can resolve both types of addresses.
 Such a dual stack DNS server would hold IPv4 addresses in the A records, and IPv6 addresses
in the AAAA records.
 Depending on the destination that is to be resolved, a DNS name server may return an IPv4 or
IPv6 IP address, or both.
 A default address selection mechanism, or preferred protocol, needs to be configured either
on hosts or the DNS server.
 The IETF(Internet Engineering Task Force) has published Happy Eyeballs called Fast
Fallback) is an algorithm published by the IETF which can make dual- stack applications
more responsive to users )to assist dual stack applications, so that they can connect using both
IPv4 and IPv6, but prefer an IPv6 connection if it is available.
 dual-stack also needs to be implemented on all routers between the host and the service for
which the DNS server has returned a IPv6 address.
 Dual-stack clients should only be configured to prefer IPv6, if the network is able to forward
IPv6 packets using the IPv6 versions of routing protocols.
 When dual stack networks protocols are in place the application layer can be migrated to
IPv6.
 While dual-stack is supported by major operating system and network device vendors, legacy
networking hardware and servers don't support IPv6.
 Siddhi Mehta 94
Pg-30
26. Describe transition from IPV4 to IPV6
ANSWERS:
Complete transition from IPv4 to IPv6 might not be possible because IPv6 is not backward
compatible. This results in a situation where either a site is on IPv6 or it is not. It is unlike
implementation of other new technologies where the newer one is backward compatible so the older
system can still work with the newer version without any additional changes. To overcome this short-
coming, we have a few technologies that can be used to ensure slow and smooth transition from IPv4
to IPv6.
Three Transition from IPv4 to IPv6 strategies are
1. Dual Stack
2. Tunnelling
3. Header Translation
1. DUAL STACK
In this kind of strategy, a station has a dual stack of protocols run IPv4 and IPv6 simultaneously. To
determine which version to use when sending a packet to a destination, the source host queries the
DNS. If the DNS returns an IPv4 address, the source host sends an IPv4 packet. If the DNS returns an
IPv6 address, the source host sends an IPv6 packet.

2. Tunnelling
Tunnelling is a strategy used when two computers using IPv6 want to communicate with each other
and the packet must pass through a region that uses IPv4.
 To pass through this region, the packet must have an IPv4 address. So the IPv6 packet is
encapsulated in an IPv4 packet when it enters the region.
 To make it clear that the IPv4 packet is carrying an IPv6 packet as data the protocol value is set to
41.
 Siddhi Mehta 94
Pg-31
3. Header Translation
In this case, the header format must be totally changed through header translation. The header of the
IPv6 packet is converted to an IPv4 header see figure.

27. State benefits/comparison of IPV6 over IPV4.

ANSWERS:
The header length field is eliminated in IPv6 because the length of the header is fixed in this version.
 The service type field is eliminated in IPv6.
 The traffic class and flow label fields together take over the function of the service type field.
 The total length field is eliminated in IPv6 and replaced by the payload length field.
 The identification, flag, and offset fields are eliminated from the base header in IPv6.
 They are included in the fragmentation extension header.
 The TTL field is called hop limit in IPv6.
 The protocol field is replaced by the next header field.
 The header checksum is eliminated because the checksum is provided by upper layer
protocols; hence not needed at this level.
 The option fields in IPv4 are implemented as extension headers in IPv6.

28. Explain following strategies of transition with diagram 1)Dual Stack, 2) Tunnelling 3) Header
Translation
ANSWERS:
TRANSITION FROM IPv4 TO IPv6
Because of the huge number of systems on the Internet, the transition from IPv4 to IPv6 cannot
happen suddenly. It will take a considerable amount of time before every system in the Internet can
move from IPv4 to IPv6. The transition must be smooth to prevent any problems between IPv4 and
IPv6 systems.
Three strategies have been devised by the IETF to help the transition shown in fig:
 Siddhi Mehta 94
Pg-32
Dual Stack
 It is recommended that all hosts, before migrating completely to version 6, have a dual stack
of protocols.
 In other words, a station must run IPv4 and IPv6 simultaneously until all the Internet uses
IPv6. The layout of a dual-stack configuration is :

 To determine which version to use when sending a packet to a destination, the source host
queries the DNS.
 If the DNS returns an IPv4 address, the source host sends an IPv4 packet.
 If the DNS returns an IPv6 address, the source host sends an IPv6 packet.
Tunneling:
 Is a strategy used when two computers using IPv6 want to communicate with each other and
the packet must pass through a region that uses IPv4.
 To pass through this region, the packet must have an IPv4 address.
 So the IPv6 packet is encapsulated in an IPv4 packet when it enters the region, and it leaves
its capsule when it exits the region.
 It seems as if the IPv6 packet passes goes through a tunnel at one end and emerges at the
other end.
 To make it clear that the IPv4 packet is carrying an IPv6 packet as data, the protocol value is
set to 41.

Header translation from ipv4 to ipv6:

 is necessary when the majority of the Internet has moved to IPv6 but some systems still use
IPv4.
 The sender wants to use IPv6, but the receiver does not understand IPv6.
 Tunnelling does not work in this situation because the packet must be in the IPv4 format to be
understood by the receiver.
 In this case, the header format must be totally changed through header translation.
 The header of the IPv6 packet is converted to an IPv4 header
 Siddhi Mehta 94
Pg-33

Header translation uses the mapped address to translate an IPv6 address to an IPv4 address.
The following lists some rules used in transforming an IPv6 packet header to an IPv4 packet
header.
 The IPv6 mapped address is changed to an IPv4 address by extracting the rightmost 32 bits.
 The value of the IPv6 priority field is discarded.
 The type of service field in IPv4 is set to zero.
 The checksum for IPv4 is calculated and inserted in the corresponding field.
 The IPv6 flow label is ignored.
 Compatible extension headers are converted to options and inserted in the IPv4 header. Some
may have to be dropped.
 The length of IPv4 header is calculated and inserted into the corresponding field.
 The total length of the IPv4 packet is calculated and inserted in the corresponding field.
29. Give significance of Autoconfiguration and Re-numbering concepts and explain
ANSWERS:
AUTOCONFIGURATION
  One of the interesting features of IPv6 addressing is the auto-configuration of hosts.
  In IPv4, the host and routers are originally configured manually by the network manager.
  Dynamic Host Configuration Protocol, DHCP, can be used to allocate an IPv4 address to a
host that joins the network.
  In IPv6, DHCP protocol can still be used to allocate an IPv6 address to host, but a host can
also configure itself.
When a host in IPv6 joins a network, it can configure itself using the following process:
1. The host first creates a link local address for itself. This is by taking the 10-bit link Local prefix
(1111 1110 10), adding 54 zeros, and adding the 64-bit interface identifier, which any host knows
how to generate it from its interface card. The result is a 128-bit link local address.
2. The host then tests to see if this link local address is unique and not used by Other hosts. Since the
64-bit interface identifier is supposed to be unique, the link local address generated is unique with a
high probability. However, to be sure, the host sends a neighbor solicitation message and waits for
neighbor advertisement message. If any host in the subnet is using this link local address, the process
fails and the host cannot auto-configure itself; it needs to use other means such as DHCP protocol for
this purpose.
3. If the uniqueness of the link local address is passed, the host stores this address as its link-local
address (for private communication), but it still needs a global unicast address. The host then sends a
router solicitation message to a local router. If there is a router running on the network, the host
receives a router advertisement message that includes the global unicast prefix and the subnet prefix
that the host needs to add to its interface identifier to generate its global unicast address. If the router
 Siddhi Mehta 94
Pg-34
cannot help the host with the configuration, it informs the host in the router advertisement message
(by setting a flag). The host then needs to use other means for configuration.
Example:
Assume a host with Ethernet address ( F5-A9-23-11-9B-E2) has joined the network. What would be
its global unicast address if the global unicast prefix of the organization is 3A21:1216:2165 and the
subnet identifier is A245:1232.
Solution
The host first creates its interface identifier as F7A9:23FF:FE11:9BE2
using the Ethernet address read from its card. The host then creates its link-local address as
FE80::F7A9:23FF:FE11:9BE2
Assuming that this address is unique, the host sends a router solicitation message and receives the
router advertisement message that announces the combination of global unicast prefix and the subnet
identifier as 3A21:1216:2165:A245:1232.
The host then appends its interface identifier to this prefix to find and store its global unicast address
as: 3A21:1216:2165:A245:1232:F7A9:23FF:FE11:9BE2
RENUMBERING:
 To allow sites to change the service provider, renumbering of the address prefix ( ) was built
into IPv6 addressing.
 Each site is given a prefix by the service provider to which it is connected.
 If the site changes the provider, the address prefix needs to be changed.
 A router to which the site is connected can advertise a new prefix and
 let the site use the old prefix for a short time before disabling it.
 In other words, during the transition period, a site has two prefixes.
 The main problem in using the renumbering mechanism is the support of the DNS, which
needs to propagate the new addressing associated with a domain name.
 A new protocol for DNS, called Next Generation DNS, is under study to provide support for
this mechanism.


30. Explain types of OSPF Packets in detail

ANSWERS:

Hello Message
OSPF uses the hello message to create neighbourhood relationships and to test the reach ability of
neighbours. This is the first step in link state routing. Before a router can flood all of the other routers
 Siddhi Mehta 94
Pg-35
with information about its neighbours, it must first greet its neighbours. It must know if they are alive,
and it must know if they are reachable (see Figure 11.46).

Database Description Message

When a router is connected to the system for the first time or after a failure, it needs the complete link
state database immediately. It cannot wait for all link state update packets to come from every other
router before making its own database and calculating its routing table. Therefore, after a router is
connected to the system, it sends hello packets to greet its neighbours. If this is the first time that the
neighbours hear from the router, they send a database description message. The database description
packet does not contain complete database information; it only gives an outline, the title of each line
in the database. The newly connected router examines the outline and finds out which lines of
information it does not have. It then sends one or more link state request packets to get full
information about that particular link. When two routers want to exchange database description
packets, one of them takes the role of master and the other the role of slave. Because the message can
be very long, the contents of the data- base can be divided into several messages. The format of the
database description packet is shown in Figure 11.47. The fields are as follows:

Link State Request Packet

The format of the link state request packet is shown in Figure 11.48. This is a packet that is sent by a
router that needs information about a specific route or routes. It is answered with a link state update
packet. It can be used by a newly connected router to request more information about some routes
after receiving the database description packet. The three fields here are part of the LSA header,
which has already been discussed. Each set of the three fields is a request for one single LSA. The set
is repeated if more than one advertisement is desired.
 Siddhi Mehta 94
Pg-36

Link State Acknowledgment Packet

OSPF makes routing more reliable by forcing every router to acknowledge the receipt of every link
state update packet. The format of the link state acknowledgment packet is shown in Figure 11.49. It
has the common OSPF header and the general LSA header. These two sections are sufficient to
acknowledge a packet.

31 Describe given types of BGP Messages: 1) Open 2) Update 3) Keepalive 4) Notification

ANSWERS:
- OPEN: opens TCP connection to peer and authenticates sender
- UPDATE: advertises new path (or withdraws old)
- KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request
- NOTIFICATION: reports errors in previous msg; also used to close connection

32. Draw and explain diagram of Double-crossing inefficiency in Mobile IP

ANSWERS:
Double crossing occurs when a remote host communicates with a mobile host that has moved to the
same network (or site) as the remote host.
 Siddhi Mehta 94
Pg-37
  When the mobile host sends a packet to the remote host, there is no inefficiency; the
communication is local.
  However, when the remote host sends a packet to the mobile host, the packet crosses the
Internet twice.
  Since a computer usually communicates with other local computers (principle of locality),
the inefficiency from double crossing is significant.

33. Explain ICMP messages categories Query. Error reporting with is sub-categories in short
ANSWERS:
Error reporting
One of the main responsibilities of ICMP is to report errors. Although technology has produced
increasingly reliable transmission media, errors still exist and must be handled. IP is an unreliable
protocol. This means that error checking and error control are not a concern of IP. ICMP was
designed, in part, to compensate for this shortcoming. However, ICMP does not correct errors, it
simply reports them. Error correction is left to the higher-level protocols. Error messages are always
sent to the original source because the only information available in the datagram about the route is
the source and destination IP addresses. ICMP uses the source IP address to send the error message to
the source (originator) of the datagram.

Destination Unreachable
When a router cannot route a datagram or a host cannot deliver a datagram, the datagram is discarded
and the router or the host sends a destination-unreachable message back to the source host that
initiated the datagram.
Source Quench
The source-quench message in ICMP was designed to add a kind of flow control and congestion
control to the IP. When a router or host discards a datagram due to congestion, it sends a source-
quench message to the sender of the datagram. This message has two purposes. First, it informs the
source that the datagram has been discarded. Second, it warns the source that there is congestion
somewhere in the path and that the source should slow down (quench) the sending process.
Time Exceeded
The time-exceeded message is generated in two cases:
1. Whenever a router decrements a datagram with a time-to-live value to zero, it discards the
datagram and sends a time-exceeded message to the original source.
2. When the final destination does not receive all of the fragments in a set time, it discards the
received fragments and sends a time-exceeded message to the original source.
Parameter Problem
 Siddhi Mehta 94
Pg-38
Any ambiguity in the header part of a datagram can create serious problems as the datagram travels
through the Internet. If a router or the destination host discovers an ambiguous or missing value in any
field of the datagram, it discards the datagram and sends a parameter-problem message back to the
source.
Redirection
Host A wants to send a datagram to host B. Router R2 is obviously the most efficient routing choice,
but host A did not choose router R2. The datagram goes to R1 instead. R1, after consulting its table,
finds that the packet should have gone to R2. It sends the packet to R2 and, at the same time, sends a
redirection message to host A. Host A’s routing table can now be updated.
Query Messages
In addition to error reporting, ICMP can also diagnose some network problems. This is accomplished
through the query messages. A group of five different pairs of messages have been designed for this
purpose, but three of these pairs are deprecated today, as we discuss later in the section. Only two
pairs are used today: echo request and replay and timestamp request and replay. In this type of ICMP
message, a node sends a message that is answered in a specific format by the destination node.

echo-request message
 An echo-request message can be sent by a host or router. An echo-reply message is sent by the
host or router that receives an echo-request message.
 Echo-request and echo-reply messages can be used by network managers to check the operation
of the IP protocol.
 Echo-request and echo-reply messages can test the reachability of a host. This is usually done by
invoking the ping command.
Timestamp Request and Reply
Timestamp-request and timestamp-reply messages can be used to calculate the round-trip time
between a source and a destination machine even if their clocks are not synchronized.
Deprecated Messages
Three pairs of messages are declared obsolete by IETF:
1. Information request and replay messages are not used today because their duties are done by
Address Resolution Protocol (ARP)
2. Address mask request and reply messages are not used today because their duties are done by
Dynamic Host Configuration Protocol (DHCP)
3. Router solicitation and advertisement messages are not used today because them duties are done by
Dynamic Host Configuration Protocol (DHCP),
34. Describe any on in detail 1) Triangular Routing 2) Double crossing.
ANSWERS:
Double crossing occurs when a remote host communicates with a mobile host that has moved to the
same network (or site) as the remote host.
 Siddhi Mehta 94
Pg-39

 When the mobile host sends a packet to the remote host, there is no inefficiency; the
communication is local.
 However, when the remote host sends a packet to the mobile host, the packet crosses the
Internet twice.
 Since a computer usually communicates with other local computers (principle of locality), the
inefficiency from double crossing is significant.
Triangle routing:
 the less severe case, occurs when the remote host communicates with a mobile host that is not
attached to the same network (or site) as the mobile host.
 When the mobile host sends a packet to the remote host, there is no inefficiency.
 However, when the remote host sends a packet to the mobile host, the packet goes from the
remote host to the home agent and then to the mobile host.
 The packet travels the two sides of a triangle, instead of just one side

35. Enlist any 8 features of Open Shortest Path First (OSPF) routing
ANSWERS:
 OSPF is an interior gateway protocol (IGP).
 It runs within a single routing domain, such as an autonomous system (AS).
 It uses a concept called areas, to optimize network traffic and simplify administration.
 It uses Dijkstra's algorithm to compute the shortest route to each destination.
 It runs over IP protocol but does not use a transport protocol (such as TCP or UDP) to
encapsulate its data.
 It encapsulates its data directly in IP packets with protocol number 89.
 It uses its own error detection and correction mechanism.

36. Explain Border Gateway Protocol (BGP) and state it's any two characteristics.
ANSWERS:
BGP (Border Gateway Protocol): BGP basics:
 the Internet standard External Gateway Protocol (EGP).
 BGP detects modifications to routing tables and selectively communicates those changes to other
routers over TCP/IP.
 Siddhi Mehta 94
Pg-40
 Internet providers commonly use BGP to join their networks together.
 Larger business sometimes also use BGP to connect multiple internal networks.
 Professionals consider BGP the most challenging of all routing protocols to master due to its
configuration complexity.
 More recent than Distance vector and Link state routing.
 Not only exchanges of info. About the existence of destination networks but also exchanges the
path on how to reach the destination.
 Path info. Is used to determine the best paths and to prevent routing loops. Path vector table is
BGP-Border Gateway protocol
BGP provides each AS a means to:
1. Obtain subnet reachability information from neighboring ASs.
2. Propagate the reachability information to all routers internal to the AS.
3. Determine “good” routes to subnets based on reachability information and policy.
4. Allows a subnet to advertise its existence to rest of the Internet: “I am here”
5. Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP
sessions
6. Note that BGP sessions do not correspond to physical links.
7. When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to
that prefix towards the prefix.
 AS2 can aggregate prefixes in its advertisement
Distributing reachability info:
  With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.
  1c can then use iBGP do distribute this new prefix reach info to all routers in AS1
  1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session
  When router learns about a new prefix, it creates an entry for the prefix in its forwarding table.

Path attributes & BGP routes

  When advertising a prefix, advert includes BGP attributes. - prefix + attributes = “route”
  Two important attributes:
 Siddhi Mehta 94
Pg-41
- AS-PATH: contains the ASs through which the advert for the prefix passed: AS 67 AS 17
- NEXT-HOP: Indicates the specific internal-AS router to next-hop AS. (There may be multiple links
from current AS to next-hop-AS.)
  When gateway router receives route advert, uses import policy to accept/decline. BGP route
selection:
  Router may learn about more than 1 route to some prefix. Router must select route.
  Elimination rules:

1. Local preference value attribute: policy decision

2. Shortest AS-PATH
3. Closest NEXT-HOP router: hot potato routing
4. Additional criteria
BGP messages:
  BGP messages exchanged using TCP.
  BGP messages:

37. Explain routing example with graph a routing tables for each node
ANSWERS:

38. Explain 1) Distance Vector protocol 2) Link state Protocol

ANSWERS:
Prerequisite – Classification of Routing Algorithms
Distance Vector Routing –
 It is a dynamic routing algorithm in which each router computes distance between itself and each
possible destination i.e., its immediate neighbours.
 The router shares its knowledge about the whole network to its neighbours and accordingly updates
table based on its neighbours.
 The sharing of information with the neighbour’s takes place at regular intervals.
 It makes use of Bellman Ford Algorithm for making routing tables.
 Problems – Count to infinity problem which can be solved by splitting horizon.
 Siddhi Mehta 94
Pg-42
– Good news spread fast and bad news spread slowly.
– Persistent looping problem i.e., loop will be there forever.
Link State Routing –
 It is a dynamic routing algorithm in which each router shares knowledge of its neighbours with
every other router in the network.
 A router sends its information about its neighbours only to all the routers through flooding.
 Information sharing takes place only whenever there is a change.
 It makes use of Dijkstra’s Algorithm for making routing tables.
 Problems – Heavy traffic due to flooding of packets.
– Flooding can result in infinite looping which can be solved by using Time to leave (TTL) field.

39. Draw and explain RIP message format in detail/Explain types of Links used in OSPF
ANSWERS:

❑ Command. This 8-bit field specifies the type of message: request (1) or response (2).

❑ Version. This 8-bit field defines the version. In this book we use version 1, but at the end of this
section, we give some new features of version 2.

❑ Family. This 16-bit field defines the family of the protocol used. For TCP/IP the value is 2.

❑ Network address. The address field defines the address of the destination network. RIP has
allocated 14 bytes for this field to be applicable to any protocol. However, IP currently uses only 4
bytes. The rest of the address is filled with 0s.

❑ Distance. This 32-bit field defines the hop count (cost) from the advertising router to the
destination network.
Note that part of the message is repeated for each destination network. We refer to this as an entry.
 Siddhi Mehta 94
Pg-43
40 Describe router solicitation and router advertisement
ANSWERS:
 Siddhi Mehta 94
Pg-44
41. Explain Link Local steps using stateless autoconfiguration. 1) generation 2) uniqueness test 3)
Address Assignment 4) Router Contact 5) Router direction 6) Global address Configuration
ANSWERS:
1. Link-Local Address Generation:
The device generates a link-local address.
Recall that this is one of the two types of local-use IPv6 addresses.
Link-local addresses have ―1111 1110 10 for the first ten bits.
 The generated address uses those ten bits followed by 54 zeroes and then the 64-bit interface
identifier.
 This will be derived from the data link layer (MAC) address or it may be a ―token generated in
some other manner.
2. Link-Local Address Uniqueness Test:
 The node tests to ensure that the address it generated isn't for some reason already in use on
the local network.
 if the link-local address came from a MAC address, if it was based on a generated token.
 It sends a Neighbour Solicitation message using the
 Neighbour Discovery (ND) protocol. It listens for a Neighbour Advertisement in response, it
indicates that another device is already using its link-local address; if so, either a new address
must be generated, or auto-configuration fails and another method must be employed.
3. Link-Local Address Assignment:
 Assuming the uniqueness test passes, the device assigns the link-local address to its IP
interface.
 This address can be used for communication on the local network, but not on the wider
Internet (since link-local addresses are not routed).
4. Router Contact: The node next attempts to contact a local router for more information on
continuing the configuration.
 This is done either by listening for Router Advertisement messages sent periodically by
routers, or by sending a specific Router Solicitation to ask a router for information on what to
do next.
 This is in reference with IPv6 Neighbour Discovery protocol.
5. Router Direction:
 The router provides direction to the node on how to proceed with the auto-configuration.
 It may tell the node that on this network “stateful” auto-configuration is in use, and tell it the
address of a DHCP server to use.
 Means it will tell the host how to determine its global Internet address.
6. Global Address Configuration:
 Assuming that stateless auto-configuration is in use on the network,
 the host will configure itself with its globally-unique Internet address.
 This address is generally formed from a network prefix provided to the host by the router,
combined with the device's identifier as generated in the first step.
 Siddhi Mehta 94
Pg-45
 Numerous advantages over both manual and server-based configuration helpful in supporting
mobility of IP devices, as they can move to new networks and
 Get a valid address without any knowledge of local servers or network prefixes.
 It still allows management of IP addresses using the (IPv6-compatible) version of DHCP if that is
desired.
 Routers on the local network will typically tell hosts which type of auto-configuration is
supported using special flags in ICMPv6 Router Advertisement messages.
 IPv6 includes an interesting feature called stateless address auto-configuration, which allows a
host to actually determine its own IPv6 address from its layer two address by following a special
procedure.

42. Explain Dijkstra’s shortest path algorithm

ANSWERS:
Dijkstra’s Algorithm:
 Step-1: Source node is initialized and can be indicated as a filled circle filled.
 Step-2: Initialize path cost to neighbouring nodes (adjacent node) or link cost is computed and
these nodes are relabelled considering source node.
 Step-3: Examine all the adjacent nodes and find the smallest label and make it permanent.
 Step-4: The smallest label node is now working node the step-2 and step-3 are repeated till
the destination node reaches.
Siddhi Mehta 94
Pg-46
 Siddhi Mehta 94
Pg-47
43. Explain Bellman Ford algorithm.
ANSWERS:

44. Difference between Classful routing and classless routing

ANSWERS:

S.NO Classful Routing Classless Routing

In classful routing, VLSM (Variable Length While in classless routing, VLSM (Variable
1. Subnet Mask) is not supported. Length Subnet Mask) is supported.

2. Classful routing requires more bandwidth. While it requires less bandwidth.

In classful routing, hello messages are not While in classless routing, hello messages are
3. used. used.

4. Classful routing does not import subnet mask. Whereas it imports subnet mask.

In classful routing, address is divided into

three parts which are: Network, Subnet and While in classless routing, address is divided
5. Host. into two parts which are: Subnet and Host.

In classful routing, regular or periodic

6. updates are used. Whereas in this, triggered updates are used.

In classful routing, CIDR (Classless Inter- While in classless routing, CIDR (Classless
7. Domain Routing) is not supported. Inter-Domain Routing) is supported.

In classful routing, subnets are not displayed While in classless routing, subnets are
8. in another major subnet. displayed in another major subnet.

In classful routing, fault can be detected While in classless routing, fault detection is
9. easily. little tough.

45. Advantages of RIP-V1/V2.OSPF BGP.

ANSWERS:
Advantages:
 AS can includes several hundred routers
 Compatible upgrade includes subnet routing, authentication, CIDR, aggregation, route tags
and multicast transmission,
 Subnet support Uses partitioning using variable-length subnets.
 Low requirement in memory and processing at node.
 RIP and RIP-v2 are used for IPv4 while RIPng is designed for IPv6.
 Siddhi Mehta 94
Pg-48
Advantages of BGP:
 Very robust and scalable routing protocol.
 CIDR is used to reduce the size of internet routing Table.
 Easily solves count-to-infinity problem. It’s an Open standard.
 Its hierarchical routing protocol with AS area-0 at top of the hierarchy.
 Use link state algorithm, and OSPF diameter of network can be much large than that of RIP.
 Supports variable Length subnet mask, results in efficient use of networking resources.
 Uses multicasting within areas.
 After initialization OSPF only sends updates on routing table sections those have changed,
not to entire routing table hence it uses less network bandwidth.
 Using areas, OSPF networks can be logically segmented to improve administration and
decreases size of routing tables.
Advantages of BGP:
 Very robust and scalable routing protocol.
 CIDR is used to reduce the size of internet routing Table.
 Easily solves count-to-infinity problem.

46. List and Explain RIP Times. 1)Periodic 2) Expiration 3) Garbage Collection
ANSWERS:
Different Timers in RIP are:
 Periodic Timer: Randomly set to each router (25-35 sec)
 Expiration Time: for validity of a route (180 sec)
 Garbage collection: If a route becomes invalid and route is removed/ eliminated from
table(120 sec)