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CS610 – Detail Questions (Final)
What is the main difference between TCP and UDP? (2)
TCP(Transmission Control Protocol).
• TCP is a connection-oriented protocol.
• A connection can be made from client to server, and from then on any data can be sent
along that connection.
• TCP Provide a Reliability due to connection oriented features.
UDP(User Datagram Protocol).
• A simpler message-based connectionless protocol.
• Messages (packets) can be sent across the network in chunks.
• UDP is Unreliable – as it could get lost on the way.
Why TCP called end to end Protocol.......(2)
TCP established connection between nodes before transmission or receiving the data, due to this
feature, it is called end-to-end protocol.
Why more preference given to TCP as comparative to UDP
TCP provide reliability as it follows connection oriented protocol paradigm that is why TCP is
given preference over UDP.
How is TCP a reliable protocol? (Marks=2)
• TCP is a connection-oriented protocol.
• A connection can be made from client to server, and from then on any data can be sent
along that connection.
• TCP Provide a Reliability due to connection oriented features.
The maximum payload of a TCP segment is 65,495 bytes. Why was such a strange number
chosen?(3)
A TCP segment must fit inside an IP packet. The TCP header is a fixed-format of 20- bytes, and
the IP header is also 20 bytes long. Since the maximum length of an IPv4 packet is 65,535 bytes,
this leaves only 65,535-20-20 = 65,495 bytes left for TCP payload.
Why does UDP exist? Would it not have been enough to just let user processes send raw IP
packets?(3)
UDP instead of raw IP is used because the UDP header contains the port information for the
source and destination machines. This allows the machines to know which processes the packet
came from and should be sent to. This information is not available anywhere in the header for the
raw IP packet. It would not have been enough to just let user processes send raw IP packets
Characteristics of UDP (5)
UDP is connectionless protocol
• The sender isn't aware of the other party's status and send the data anytime it wants to.
• Real-time communication is possible.
The port number is marked on the data and the data is encapsulated to IP packet and sent.
• Reliability of communication is not guaranteed.
• Data can be lost.
Differentiate between UNICAST, MULTICAST AND BROADCAST (6)
Three methods can be used to transmit packets over a network: unicast, multicast, and
broadcast.
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Unicast involves communication between a single sender and a single receiver. This is a type of
point-to-point transmission; since the packet is transmitted to one destination at a time.
Multicast is used to send packets to a group of addresses, represented by a "group address." In
this case, packets are transmitted from a single sender to multiple receivers. Since the same data
packet can be sent to multiple nodes by sending just one copy of the data, the load of the sender
and the overall load of the network are both reduced.
Broadcast involves sending packets to all nodes on a network simultaneously. This type of
transmission is used to establish communication with another host, and for DHCP type methods
of assigning IP addresses. In the first case, it's needed because your packets must know the
proper MAC address to send packets to a machine on your local network. Suppose you know the
IP address (obtained, for example, by querying a DNS server), but have not yet determined the
MAC address that belongs to the node. If you broadcast a packet that asks requests the identity
of the node with that particular IP address, every machine on the network will receive the
message, but only the one with that IP address will respond.
Q5: Differentiate between connectionless and connection-oriented protocol? (6)
Connection-oriented
• Requires a session connection (analogous to a phone call) be established before any
data can be sent.
• This method is often called a "reliable" network service.
• It can guarantee that data will arrive in the same order.
• Connection-oriented services set up virtual links between end systems through a
network.
Connectionless
• Does not require a session connection between sender and receiver.
• The sender simply starts sending packets (called data-grams) to the destination.
• This service does not have the reliability of the connection-oriented method, but it is
useful for periodic burst transfers.
• Neither system must maintain state information for the systems that they send
transmission to or receive transmission from.
• A connectionless network provides minimal services.
Define special IP..(5)
Special IP’s are those IP’s which are not assigned to any machine; these are reserve IP’s, used
for special purpose, for example:
Routing information Protocol define briefly (10)
The Routing Information Protocol (RIP) is a dynamic routing protocol used in local and wide area
networks. As such it is classified as an interior gateway protocol (IGP). It uses the distance-vector
routing algorithm. It was first defined in RFC 1058 (1988). The protocol has since been extended
several times, resulting in RIP Version 2 (RFC 2453). Both versions are still in use today,
however, they are considered to have been made technically obsolete by more advanced
techniques such as Open Shortest Path First (OSPF) and the OSI protocol IS-IS. RIP has also
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been adapted for use in IPv6 networks, a standard known as RIPng (RIP next generation),
published in RFC 2080 (1997).
Features of RIP
 Routing within an autonomous system (IGP)
 Hop count metric
 Unreliable transport (uses UDP)
 Broadcast or multicast delivery
 Default Route Propagation
 Distance vector algorithm
 Passive version for host
What is OSPF.....(2)
Open Shortest Path First (OSPF) is a dynamic routing protocol for use in Internet Protocol (IP)
networks. Specifically, it is a link-state routing protocol and falls into the group of interior gateway
protocols, operating within a single autonomous system (AS).
What is mean by address resolution with centralization message exchange?
A network includes one or more servers that are assigned the task of answering address
resolution requests. It has an advantage that resolution is easier to configure, manage and
control.
To solve the problem the routers and networks in the internet are divided into groups, how large
do you think are these groups? (Marks=2)
There is no limit to determine the size of group, it can be large or small depends upon net work
administrator.
How does a packet pass across an internet? (Marks=3)
• A source creates the packet and places the destination address in a packet header
• The packet is sent to the nearest router
• The router uses the destination address to select the next router on the path to the
destination, and then transmits the packet.
• The packet reaches the router that can deliver the packet to its final destination.
What are some of the metrics used by routing protocols? (Marks=3)
Path length, reliability, delay, bandwidth, load, and communication cost
What are the problem that IP does not guarantee and how to handle these problems? (Marks=3)
IP does not guarantee the following problems:
• Delivery of packets
• Delivery of packets in sequence.
How can Universal Services be achieved? (Marks=5)
The goal of internetworking is universal service across heterogeneous networks. To provide this
service all computers and routers must agree to forward information from a source on one
network to a specified destination. The task is complicated as frame formats and addressing
schemes may differ. The key of achieving universal service is universal protocol software
(TCP/IP).
What are the main advantages and disadvantages of Routing Information Protocol (RIP)?
(Marks=5)
Advantages:
* Simple to implement
* Low requirement in processing and memory at the nodes
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* Suitable for small networks
Disadvantages:
* Slow convergence
* Bouncing effect
* Counting to infinity problem from Routing Basics, RIP
* Poor metrics. RIP only supports a hop count metric, with a maximum value of 15 hops.
Can a router in the global internet use static routing the same way a host does? If so, how large is
the routing table in a router? (Marks=10)
The answers of these questions are complex. Although cases exit where a router uses static
routing, most routers are using dynamic routing.
How can we prove that we have 2,147,483,648 addresses in class A? (2)
In class A, only 1 bit defines the class. The remaining 31 bits are available for the address. With
31 bits, we can have 231 or 2,147,483,648 addresses.
Why is internet multicast routing difficult? (2)
Internet multicast routing is difficult because internet multicast allows arbitrary computer to join
multicast group at any time. It allows arbitrary member to leave multicast group at any time. It
also allows arbitrary computer to send message to a group (even if not a member).
Define what is the Extension Headers in IPv6. (2)
In IPv6, optional internet-layer information is encoded in separate headers that may be placed
between the IPv6 header and the upper- layer header in a packet. There are a small number of
such extension headers, each identified by a distinct Next Header value. As illustrated in these
examples, an IPv6 packet may carry zero, one, or more extension headers, each identified by the
Next Header field of the preceding header:
If IPv4 is so successful, then why change IPv6? (5 marks)
Internet Protocol Version 6 (IPv6) is designed to increase Internet global address space to
accommodate the rapidly increasing numbers of users and applications that require unique global
IP addresses and help enable a global environment where the addressing rules of the network
are again transparent to applications.
Write a note on IPv6 (marks 5)
Why does IPv6 use separate Extension Headers? (3)
There are two reasons:
1. Economy
2. Extensibility
What are IPv6 ADDRESS NOTATION? (5)
The standard notation for IPv6 addresses is to represent the address as eight 16-bit hexadecimal
words separated by ``:'' (colons). For example:
FEDC:BA98:0332:0000:CF8A:000C:2154:7313
Write three new features of IPV6. (3)
• Expanded Addressing
IP address size increases from 32 bits in IPv4 to 128 bits in IPv6, to support more levels
of addressing hierarchy. In addition, IPv6 provides many more addressable IPv6
systems.
• Address Auto-configuration and Neighbor Discovery
• Header Format Simplification
• Improved Support for IP Header Options
• Application Support for IPv6 Addressing
• Additional IPv6 Resources
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How does IP software reassemble fragments that arrive out of order? (3)
• The sender places a unique ID number in the Identification field of each outgoing
datagram
• When a router fragments the datagram, the router copies the identification number into
each fragment
• A receiver uses the identification number and the IP source address in the incoming
fragment to determine the datagram to which the fragment belongs
• The Fragment Offset field tells a receiver how to order fragments within a datagram
A unique ID number of each outgoing datagram
When a router fragments the datagram, the router copies the ID number into each fragment.
The fragment OFFSET field tells a receiver how to order fragments within a given datagram
The process of creating a copy of the original datagram from fragments is called reassembly.
What is the first address in the block if one of the addresses is 167.199.170.82/27? (3)
The prefix length is 27, which means that we must keep the first 27 bits as is and change the
remaining bits (5) to 0s. The following shows the process:
Address in binary: 10100111 11000111 10101010 01010010
Keep the left 27 bits: 10100111 11000111 10101010 01000000
Result in CIDR notation: 167.199.170.64/27
What is the first address in the block if one of the addresses is 140.120.84.24/20? (3)
The prefix length is 20, which means that we must keep the first 20 bits as is and change the
remaining bits (12) to 0s. The following shows the process:
Address in binary: 10001100 1111000 01010100 00011000
Keep the left 20 bits: 10001100 1111000 01010000 00000000
Result in CIDR notation: 140.120.80.0/27
In internet routing how does a host join or leave a group? (3)
A standard protocol exists that allows a host to inform a nearby router whenever the host needs
to join or leave a particular multicast group known as Internet Group Multicast Protocol (IGMP).
The computer uses IGMP to inform the local router about the last application when it leaves.
What is the role of area in open shortest path first (OSPF)? (3)
OSPF allows subdivision of Autonomous System into areas. The link-status information is
propagated within an area. The routes are summarized before being propagated to another area.
It reduces overhead (less broadcast traffic). Because it allows a manager to partition the routers
and networks in an autonomous system into multiple areas, OSPF can scale to handle a larger
number of routers than other IGPs.
Write a note on Address Resolution. (5)
Mapping between a protocol address and a hardware address is called Address Resolution. A
host or router uses address resolution when it needs to send a packet to another computer on the
same physical network. A computer never resolves the address of a computer that attaches to a
remote network.
What is the concept of classes for IP Addresses? Briefly describe each class (10)
The original IP address scheme divides host addresses into three primary classes. The class of
an address determines the boundary between the network prefix and suffix.
The original classes of IP addresses are shown in the figure below.
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What is address resolution? (2 marks)
Mapping between a protocol address and a hardware address is called Address Resolution. A
host or router uses address resolution when it needs to send a packet to another computer on the
same physical network. A computer never resolves the address of a computer that attaches to a
remote network.
In IP datagram format, what does the field SERVICE TYPE do? (2 marks)
SERVICE TYPE shows sender’s preference for low latency, high reliability that is rarely used.
What is stored in routing table? (3 marks)
A routing table is used by TCP/IP network routers to calculate the destinations of messages it is
responsible for forwarding. The table is a small in-memory database managed by the router's
built-in hardware and software.
Routing Table Entries and Sizes
Routing tables contain a list of IP addresses. Each IP address identifies a remote router (or other
network gateway) that the local router is configured to recognize. For each IP address, the routing
table additionally stores a network mask and other data that specifies the destination IP address
ranges that remote device will accept.
What are implementations of NAT? (3 marks)
The figure below shows the implementation of NAT. We can see that the old and new values of
IP source field and destination field are shown with their directions.
NAT device stores state information in table. The value is entered in the table when NAT box
receives outgoing datagram from new
Why organizations don't use a single router to connect its entire network? (3)
Organization seldom uses a single router to connect its entire network for two reasons.
• Because the router must forward each packet, the processor in a given router is
insufficient to handle the traffic.
• Redundancy improved Internet reliability.
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What are the characteristic of border gateway protocol? (5 marks)
The characteristics of BGP follow:
• BGP is an exterior gateway protocol (EGP) used in routing in the Internet. It is an interdomain
routing protocol.
• BGP is a path vector routing protocol suited for strategic routing policies.
• BGP uses TCP Port 179 to establish connections with neighbors.
• BGPv4 implements CIDR.
• eBGP is for external neighbors. It's used between separate autonomous systems.
• iBGP is for internal neighbors. It's used within an AS.
• BGP uses several attributes in the routing-decision algorithm.
• BGP uses confederations and route reflectors to reduce BGP peering overhead.
• The MED (metric) attribute is used between autonomous systems to influence inbound
traffic.
• Weight is used to influence the path of outbound traffic from a single router, configured
locally.
As the Internet grew, the original Classful addressing scheme became a limitation. What were the
new design goals to overcome this limitation? (5 marks)
As the Internet grew, the original Classful addressing scheme became a limitation.
The IP address space was being exhausted because all networks had to choose one of three
possible sizes. Many addresses were unused.
Two new mechanisms were invented to overcome the limitations, which are as follows:
• Subnet addressing
• Classless addressing
Instead of having three distinct address classes, allow the division between prefix and suffix to
occur on an arbitrary boundary. The classless addressing scheme solves the problem by allowing
an ISP to assign a prefix that is, 28 bits long (allowing the host to have up to 14 hosts).
An ISP has class C classful address 193.214.11.0. If this ISP has 3 customers each with 6
computers. Then what addresses will be assigned to those three customers by using classless
addressing? (10 marks)
Is there a comparison between TCP/IP reference model and ISO reference model? (2)
TCP/IP reference model have internet layer whereas ISO reference do not have internet layer.
Does OSPF only share information within an area or does it allow communication between
areas? (2)
OSPF allows subdivision of Autonomous System into areas. The link-status information is
propagated within an area. The routes are summarized before being propagated to another area.
It reduces overhead (less broadcast traffic). Because it allows a manager to partition the routers
and networks in an autonomous system into multiple areas, OSPF can scale to handle a larger
number of routers than other IGPs.
What are the implementations of Network Address Translation? (2)
Implementations of NAT are available e.g.
• Stand-alone hardware device
• IP router with NAT functionality embedded
The figure below shows the implementation of NAT. We can see that the old and new values of
IP source field and destination field are shown with their directions.
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NAT device stores state information in table. The value is entered in the table when NAT box
receives outgoing datagram from new
Describe the difference between static and dynamic routing? (3)
• Static routing is configured by the network administrator. It is not capable of adjusting to
changes in the network without network administrator intervention.
• Dynamic routing adjusts to changing network circumstances by analyzing incoming
routing update messages without administrator intervention.
What is the difference between an interior gateway protocol and an exterior gateway protocol?
Name an example of each. (5)
INTERIOR GATEWAY PROTOCOLS (IGPs):
It is used among routers within autonomous system. The destinations lie within IGP.
Examples are: RIP, IGRP, EIGRP, OSPF.
EXTERIOR GATEWAY PROTOCOLS (EGPs):
It is used among autonomous systems. The destinations lie throughout Internet
Example of EGP is Broader Gateway Protocol (BGP) . BGP is the recommended backbone
peering protocol because it is:
• Industrial Standard.
• Scalable
• Best practice
• Non-Proprietary
• Flexible
As the Internet grew, the original Classful addressing scheme became a limitation, what is was
the designed solution. (5)
As the Internet grew, the original Classful addressing scheme became a limitation. The IP
address space was being exhausted because all networks had to choose one of three possible
sizes. Many addresses were unused. Two new mechanisms were invented to overcome the
limitations, which are as follows:
• Subnet addressing
• Classless addressing
Instead of having three distinct address classes, allow the division between prefix and suffix to
occur on an arbitrary boundary. The classless addressing scheme solves the problem by allowing
an ISP to assign a prefix that is, 28 bits long (allowing the host to have up to 14 hosts).
LIST SOME CHARACTERISTICS OF A CLIENT. (10)
The characteristics of a client are explained below:
• "Client is an arbitrary application program.
• "It becomes client temporarily.
• "It can also perform other computations.
• "It is invoked directly by the user.
• "It runs locally on the user’s computer.
• "It actively initiates contact with a server.
• "It contacts one server at a time.
CHARACTERISTICS OF A SERVER:
The characteristics of a server are explained below:
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• "It is a special-purpose, privileged program.
• "It is dedicated to provide one service.
• "It can handle multiple remote clients simultaneously.
• "It invoked automatically when system boots.
• "It executes forever.
• "It needs powerful computer and operating system.
• "It waits for client contact.
• "It accepts requests from arbitrary clients.
With help of the diagram below, explain TCP Segment Format: (10)
• Source Port: The 16-bit port number of the process that originated the TCP segment on
the source device.
• Destination Port: The 16-bit port number of the process that is the ultimate intended
recipient of the message on the destination device.
• Sequence Number: For normal transmissions, the sequence number of the first byte of
data in this segment. In a connection request (SYN) message, this carries the initial
sequence number (ISN) of the source TCP.
• Acknowledgment Number: When the ACK bit is set, this segment is serving as an
acknowledgment (in addition to other possible duties) and this field contains the
sequence number the source is next expecting the destination to send
• Data Offset: Specifies the number of 32-bit words of data in the TCP header. In other
words, this value times four equals the number of bytes in the header, which must always
be a multiple of four. It is called a “data offset” since it indicates by how many 32-bit
words the start of the data is offset from the beginning of the TCP segment.
• Reserved: 6 bits reserved for future use; sent as zero.
• Control Bits: TCP does not use a separate format for control messages instead certain
bits are set for controlling communication.
• Window: Indicates the number of octets of data the sender of this segment is willing to
accept from the receiver at one time.
• Checksum: A 16-bit checksum for data integrity protection, computed over the entire
TCP datagram plus a special “pseudo header” of fields. It is used to protect the entire
TCP segment against not just errors in transmission, but also errors in delivery. Optional
alternate checksum methods are also supported.
• Urgent Pointer: Used in conjunction with the URG control bit for priority data transfer.
This field contains the sequence number of the last byte of urgent data.
• Options: Set for other options
• Padding: If the Options field is not a multiple of 32 bits in length, enough zeroes are
added to pad the header so it is a multiple of 32 bits.
• Data: The bytes of data being sent in the segment.
How can a datagram be transmitted across a physical network that does not understand the
datagram format? (2)
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The answer lies in a technique known as encapsulation. When an IP datagram is encapsulated in
a frame, the entire datagram is placed in the data area of a frame.
• The network interface layer encapsulates an entire datagram in the data area of a
hardware frame.
• The network hardware ignores the datagram format.
• The hardware treats a frame containing a datagram like any other frame.
• The sender places a frame type in the frame header indicating that the frame contains a
datagram.
• The receiver reads the frame type and knows the frame contains a datagram.
What is the basic concept of Twice NAT (Network Address Translation)? (2)
What format is used for an internet packet? (3)
“To achieve a hierarchy, OSPF allows an autonomous system to be partitioned for routing
purposes”. Does this feature make OSPF more complex or powerful? (3)
Consider the IP addresses: 178.200.127.5 and the corresponding subnet masks 255.255.255.0,
then find out the following: (5)
a. The number of bits used for subnetting. (8-Bits)
b. Total number of host in the subnet (254)
c. The network address of the subnet. (178.200.127.0)
d. The subnet address of the IP address.
Here is the online calculator for subnetting
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http://ping.fm/9gcsS
Write down the comparison of Distance- vector and Link – state algorithm? (5)
DISTANCE-VECTOR ROUTING:
• • It is very simple to implement.
• • Packet switch updates its own routing table first.
• • It is used in RIP.
LINK-STATE ALGORITHM:
• • It is much more complex.
• • Switches perform independent computations.
• • It is used in OSPF.
Descibe in detail what is the purpose of the following table? What sort of information can be
extracted? (10)
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List down and describe atleast five charachteristics of Routing Information Protocol. (10)
It has the following characteristics:
1. "It is used for routing within an autonomous system (IGP).
2. "Hop Count Metric: RIP measures distance in network hops, where each network
between the source and destination counts as a single hop.
3. "It uses UDP for all message transmissions.
4. "RIP is used over LAN. Version 1 of RIP uses hardware broadcast and version 2 allows
delivery via multicast.
5. "It can be used to advertise default route propagation. An organization can use RIP to
install a default route in each router.
6. "It uses distance vector algorithm.
7. "RIP allows hosts to listen passively and update its routing table
What make TCP reliable? Give one technique which makes it reliable (marks 10)
Why class A, B and C are called primary classes (marks 2)
Importance of intelligent network
What will be the shortest possible path for secure communication.
If physical layer not understand the datagram format, how data transmit.......(2)
Describe the process of routing packets.
Routing is the act of moving information across an internet work from a source to a destination.
What are some routing algorithm types?
Static, dynamic, flat, hierarchical, host-intelligent, router-intelligent, intra-domain, inter-domain,
link-state, and distance vector.
Why Three- way handshake is necessary for TCP communication and how three-way handshake
occurs? 10
What are the characteristics of UDP? 5
UDP has the following characteristics.
• It is an end-to-end protocol. It provides application-to-application communication.
• It provides connectionless service.
• It is a Message-Oriented protocol.
• It uses best-effort delivery service.
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• It follows arbitrary interaction.
• It is operating system independent.
In client -server interaction, if no signal is given, how can an application know when
communication has arrived? (3)
One application actively begins execution first and another application waits passively at
prearranged location. This process is called client-server interaction.
What is the concept of IP Address Hierarchy? 3
The IP address hierarchy guarantees two important principles:
1. Each computer is assigned a unique address.
2. Although network number assignment must be coordinated globally, suffixes can be
assigned locally.
How ICMP can be used to trace a route? 3
List of all routers on path from A to B is called the route from A to B. The intermediate routers
send ICMP time exceeded message to the source and destination sends an ICMP destination
unreachable message to the source.
Tracert (Windows version) sends ICMP echo messages with increasing TTL. Router that
decrements TTL to 0 sends ICMP time exceeded message, with router’s address as source
address. First, with TTL 1, gets to first router, which discards and sends time exceeded message.
Next, with TTL 2 gets through first router to second router. It continues until an ICMP echo reply
message from destination is received.
What is meant by the Client-Server Paradigm? (2)
It is used by all network applications. The passive program is called a server and the active
program is called a client.
Find the class of each address:
a. 00000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
c. 10100111 11011011 10001011 01101111
d. 11110011 10011011 11111011 00001111
Solution
a. The first bit is 0. This is a class A address.
b. The first 2 bits are 1; the third bit is 0. This is a class C address.
c. The first bit is 0; the second bit is 1. This is a class B address.
d. The first 4 bits are 1s. This is a class E address.
0 ~ 127  Class A
128 ~ 191  Class B
192 ~ 223  Class C
224 ~ 239  Class D
240 ~ 255  Class E
Find the class of each address:
a. 227.12.14.87 b.193.14.56.22 c.14.23.120.8 d. 252.5.15.111 e.134.11.78.56
Solution
a. The first byte is 227 (between 224 and 239); the class is D.
b. The first byte is 193 (between 192 and 223); the class is C.
c. The first byte is 14 (between 0 and 127); the class is A.
d. The first byte is 252 (between 240 and 255); the class is E.
e. The first byte is 134 (between 128 and 191); the class is B.
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Change the following IP addresses from binary notation to dotted-decimal notation.
a. 10000001 00001011 00001011 11101111
b. 11000001 10000011 00011011 11111111
c. 11100111 11011011 10001011 01101111
d. 11111001 10011011 11111011 00001111
Solution
We replace each group of 8 bits with its equivalent decimal number (see Appendix B) and add
dots for separation:
a. 129.11.11.239
b. 193.131.27.255
c. 231.219.139.111
d. 249.155.251.15
Find the error, if any, in the following IP addresses:
a. 111.56.045.78
b. 221.34.7.8.20
c. 75.45.301.14
d. 11100010.23.14.67
Solution
a. There are no leading zeroes in dotted-decimal notation (045).
b. We may not have more than four numbers in an IP address.
c. In dotted-decimal notation, each number is less than or equal to 255; 301 is outside this range.
d. A mixture of binary notation and dotted-decimal notation is not allowed.
Is there a comparison between TCP/IP reference model and OSI reference model.
Both the TCP/IP and OSI are international standards for data communication. Both use the
concept of protocol layering. These are the conceptual tools that is often used to show how
various protocols and technologies fit together to implement networks. The functions that the
model represents are much the same. This means that the TCP/IP and the OSI models are really
quite similar in nature even if they don't carve up the network functionality pie in precisely the
same way. There is a fairly natural correspondence between the TCP/IP and OSI layers; it just
isn't always a “one-to-one” relationship. Since the OSI model is used so widely, it is common to
explain the TCP/IP architecture both in terms of the TCP/IP layers and the corresponding OSI
layers
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