Information and Communications
D.M.Chathuranga Dimuthu Dissanayaka
Department of Information Technology
2nd year (2011-2012)
THE 7 LAYERS OF OSI
1. Identify and name the device(s) used in Layer 3 of OSI model and
describe their functionality With respect to OSI model.
The third layer of the OSI Model is network layer. Network layer is most commonly
known as the layer where routing takes place. A router's main job is to get packets
from one network to another.Routers is that each interface on a router has its own
IP address, because each of those interfaces is on a different networks
The Network layer knows the address of the neighboring nodes in the network,
packages output with the correct network address information, selects routes and
quality of service and recognizes and forwards to the Transport layer incoming
messages for local host domains.
Among existing protocol that generally map to the OSI network layer are the
Internet Protocol (IP) part of TCP/IP and NetWare IPX/SPX. Both IP Version 4 and
IP Version 6 (IPv6) map to the OSI network layer.
As mentioned above, the Internet Protocol works on this layer. This means that
when we see an IP address, for example 192.168.0.1, this IP address maps to the
Network layer in the OSI model, in other words only the Network layer deals with
or cares about IP addresses in the OSI model. To keep things simple, IP is analyzed
under the "Protocols" section.
Devices in the network layer
Router: a specialized network device that determines the next network point
to which it can forward a data packet towards the ultimate destination of the
packet. Unlike a gateway, it cannot interface different protocols
Bridge router (brouter): a device that combines router and bridge
functionality and therefore works on (OSI layers 2 and 3.)
The network layer moves data from one end point to another by implementing
the following functions:
Layer 3 address is purely a logical address which is independent of any particular
hardware; a MAC address is associated with particular hardware and hardware
An example of layer 3 addressing is the Internet Protocol (IP) addressing. An
illustration of an IP address can be seen.
It is the job of the network layer to move data from one point to its destination. To
accomplish this, the network layer must be able to plan a route for the data to
traverse. A combination of hardware and software routines accomplishes this task
known as routing. When a router receives a packet from a source it first needs to
determine the destination address. It does this by removing the headers previously
added by the data link layer and reading the address from the predetermined
location within the packet as defined by the standard in use.
When a router sends a packet down to the data link layer which then adds headers
before transmitting the packet to its next point, this is an example of encapsulation
for the data link layer.
Like the data link layer, the network layer is also responsible for encapsulating
data it receives from the layer above it. In this case it would be from the data
received from layer 4, the transport layer. Actually, every layer is responsible for
encapsulating data it receives from the layer above it. Even the seventh and last
layer, the application layer, because an application encapsulates data it receives
When the network layer sends data down to the data link layer it can sometimes
run into trouble. That is, depending on what type of data link layer technology is in
use the data may be too large. This requires the network layer have the ability to
split the data up into smaller chunks which can each be sent to the data link layer
in turn. This process is known as fragmentation.
Error handling is an important aspect of the network layer. one source of errors is
when routers do not find the destination address in their routing table. In that
case, the router needs to generate a destination unreachable error. Another
possible source of errors is the TTL (time to live) value of the packet. If the network
layer determines that the TTL has reached a zero value, a time exceeded error is
generated. Both the destination unreachable error and the time exceeded error
messages conform to specific standards as defined in the Internet Control Message
Another responsibility of the network layer is congestion control. Any given
network device has an upper limit as to the amount of throughput the device can
handle. This upper limit is always creeping upward but there are still times when
there is just too much data for the device to handle. This is the motivation for
2. Identify and name the devices used in Layer 2 of OSI model and
describe their functionality with Respect to OSI model.
Devicesin the Data link layer
Switch: a device that allocates traffic from one network segment to certain
lines (intended destination(s)) which connect the segment to another network
Bridge: a device that connects multiple network segments along the data
Multilayer switch: a switch which, in addition to switching on OSI layer 2,
provides functionality at higher protocol layers.
Bridge router (brouter): a device that combines router and bridge
functionality and therefore works on ( OSI layers 2 and 3.
The data link layer provides functional and procedural methods of transferring
data between two points. There are five general functions which the Data Link layer
is responsible for. These functions are:
Logical Link Control
Media Access Control
Logical Link Control
The Logical Link Control (LLC) is usually considered a sublayer of the Data Link
layer (DLL), as opposed to a function of the Data Link layer. This Logical Link
Control sublayer is primarily concerned with multiplexing protocols to be sent over
Media Access Control (MAC) sublayer. The LLC does this by splitting up the data to
be sent into smaller frames and adding descriptive information to these frames,
Media Access Control
Like Logical Link Control, the Media Access Control (MAC) is considered a sublayer
of the Data Link layer, as opposed to a function of the Data Link layer. Included in
this sublayer is what is known as the MAC address. The MAC address provides
this sublayer with a unique identifier so that each network access point can
communicate with the network. The MAC sublayer is also responsible for the
actual access to the network cable, or communication medium.
If one were to simply send data out onto the network medium not much would
happen. The receiver has to know how, and when, to read the data. This can
happen in a number of ways and is the sole purpose of framing. In general terms,
framing organizes the data to be transferred and surrounds this data with
descriptive information, called headers. What, and how much, information these
headers contain is determined by the protocol used on the network, like Ethernet.
The structure of a frame adhering to the Ethernet protocol is shown below in
Addressing in layer 2 happens, with the MAC address of the MAC sublayer. It is
very important not to confuse this with network or IP addressing. It can be helpful
to associate the MAC address with a specific network access point and the network
or IP address associated with an entire device Speaking of routers that routers
operate in layer 3 not layer 2. Switches and hubs do operate in layer two, and
therefore direct data based on layer 2 addressing (MAC addresses) and are
unaware of IP or network addressing.
Error Detection and Handling
Whenever data is sent over any kind of transmission medium, there exists a
chance that the data will not be received exactly as it was sent. This can be due to
many factors including interference and, in the case of long transmissions,
signal attenuation. So, how can a receiver know if the data received is error free?
There are several methods that can be implemented to accomplish this. Some of
these methods are simple and somewhat effective – others are complicated and
3. Describe how a Network hub and a Network switch differ when
they operate. (You must identifytheir difference when they are
operating. You have to explain in detail from the level of IP
A switch is effectively a higher-performance alternative to a hub. People tend to
benefit from a switch over a hub if their home network has four or more
computers, or if they want to use their home network for applications that generate
significant amounts of network traffic, like multiplayer games or heavy music file
sharing. In most other cases, home networkers will not notice an appreciable
difference between hubs and switch
Technically speaking, hubs operate using a broadcast model and switches operate
using a virtual circuit model. When four computers are connected to a hub, for
example, and two of those computers communicate with each other, hubs simply
pass through all network traffic to each of the four computers. Switches, on the
other hand, are capable of determining the destination of each individual traffic
element and selectively forwarding data to the one computer that actually needs it.
By generating less network traffic in delivering messages, a switch performs better
than a hub on busy networks.
Differencebetween hub and switch.
Hubs and switches are different types of network equipment that connect devices.
They differ in the way that they pass on the network traffic that they receive.
The term „hub‟ is sometimes used to refer to
any piece of network equipment that connects
PCs together, but it actually refers to a multiport repeater. This type of device simply
passes on all the information it receives, so
that all devices connected to its ports receive
Hubs repeat everything they receive and can be used to extend the network.
However, this can result in a lot of unnecessary traffic being sent to all devices on
the network. Hubs pass on traffic to the network regardless of the intended
destination; the PCs to which the packets are sent use the address information in
each packet to work out which packets are meant for them. In a small network
repeating is not a problem but for a larger, more heavily used network, another
piece of networking equipment (such as a switch) may be required to help reduce
the amount of unnecessary traffic being generated.
Switches control the flow of network
information in each packet. A switch
learns which devices are connected to
its ports and then forwards on packets
to the appropriate port only. This
allows simultaneous communication
across the switch, improving bandwidth.
This switching operation reduces the amount of unnecessary traffic that would
have occurred if the same information had been sent from every port (as with a
Switches and hubs are often used in the same network; the hubs extend the
network by providing more ports, and the switches divide the network into smaller,
less congested sections.
Use a Hub or Switch
In a small network (less than 30 users), a hub (or collection of hubs) can easily
cope with the network traffic generated and is the ideal piece of equipment to use
for connecting the users.
When the network gets larger (about 50 users) may need to use a switch to divide
the groups of hubs, to cut down the amount of unnecessary traffic being generated.
If there is a hub or switch with Network Utilization LEDs, use the LEDs to view the
amount of traffic on the network. If the traffic is constantly high, you may need to
divide up the network using a switch.
Network with a hub
Network with a Switch
4. Explain CSMA/CD and CSMA/CA protocols and identify the OSI
layer that they belong.
CSMA/CD: - Carrier sense multiple access with collision detection
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a media
access control method used most notably in local area networking using
early Ethernet technology. It uses a carrier sensing scheme in which a
transmitting data station detects other signals while transmitting a frame, and
stops transmitting that frame, transmits a jam signal, and then waits for a random
time interval before trying to resend the frame.
CSMA/CD is a modification of pure carrier sense multiple accesses (CSMA).
CSMA/CD is used to improve CSMA performance by terminating transmission as
soon as a collision is detected, thus shortening the time required before a retry can
A collision occurs when two or more devices on a network attempt to transmit over
a single data channel (e.g., a twisted pair copper wire cable or an optical fiber
cable) simultaneously. It is detected by all participating devices, and, after a brief,
random, and different interval of time (called a back off delay) has elapsed for each
device, the devices attempt to transmit again. If another collision occurs, the time
intervals from which the random waiting times are selected are increased step-bystep in a process referred to as exponential back off.
CSMA/CD operates at the physical layer is the bottom level in the OSI (open
systems interconnection) seven layer model, which is used to standardize and
simplify definitions with regard to computer networks
CSMA/CA: -Carrier sense multiple access with collision avoidance
Carrier sense multiple access with collision avoidance (CSMA/CA) in computer
networking, is a network multiple access method in which carrier sensing is used,
but nodes attempt to avoid collisions by transmitting only when the channel is
sensed to be "idle". When they do transmit, nodes transmit their packet data in its
It is particularly important for wireless networks, where the collision detection of
the alternative CSMA/CD is unreliable due to the hidden node problem.
CSMA/CA is a protocol that operates in the Data Link Layer of the OSI model.
1. Describe about the IPv4 header by identifying its different fields
Internet Protocol version 4 (IPv4) is the fourth version in the development of
the Internet Protocol the Internet, and routes most traffic on the Internet. However,
a successor protocol.
IPv4 is a connectionless protocol for use on packet-switched networks. It operates
on a best effort delivery model; in that it does not guarantee delivery, nor does it
assure proper sequencing or avoidance of duplicate delivery. These aspects,
including data integrity, are addressed by an upper layer transport protocol, such
as the Transmission Control Protocol (TCP).
Of the approximately four billion addresses allowed in IPv4, three ranges of address
are reserved for use in private networks. These ranges are not routable outside of
private networks, and private machines cannot directly communicate with public
networks. They can, however, do so through network address translation.
The class “A” network 127.0.0.0 is reserved for loopback. IP packets whose source
addresses belong to this network should never appear outside a host. Themodus
operandi of this network expands upon that of a loopback interface:
IP packets whose source and destination addresses belong to the network of the
same loopback interface are returned to that interface;
IP packets whose source and destination addresses belong to networks of different
interfaces of the same host, one of them being a loopback interface, are forwarded
The IPv4 packet header consists of 14 fields, of which 13 are required. The 14th
field is optional (red background in table) and aptly named: options. The fields in
the header are packed with the most significant byte first (big endian), and for the
diagram and discussion, the most significant bits are considered to come first
(MSB 0 bit numbering). The most significant bit is numbered 0, so the version field
is actually found in the four most significant bits of the first byte, for example.
IPv4 Header Format
0 1 2 3 4 5 6 7 8 9
1 1 1 1 1
0 1 2 3 4
1 1 1
7 8 9
2 2 2 2 2
5 6 7 8 9
Destination IP Address
Source IP Address
Options (if IHL > 5)
Time To Live
Internet Header Length (IHL)
The second field (4 bits) is the Internet Header Length (IHL), which is the number
of 32-bit words in the header. Since an IPv4 header may contain a variable number
of options, this field specifies the size of the header the minimum value for this
field is 5, which is a length of 5×32 = 160 bits = 20 bytes. Being a 4-bit value, the
maximum length is 15 words (15×32 bits) or 480 bits = 60 bytes.
2. Describe about the IPv6 header by identifying its different fields
An IPv6 packet is the smallest message entity exchanged via the Internet Protocol
across an Internet Protocol version 6 (IPv6) network.
Packets consist of control information for addressing and routing, and
a payload consisting of user data. The control information in IPv6 packets is
subdivided into a mandatory fixed header and optional extension headers. The
payload of an IPv6 packet is typically a datagram or segment of the higherlevel Transport Layer protocol, but may be data for an Internet Layer orLink
IPv6 packets are typically transmitted over a Link Layer protocol, such
as Ethernet which encapsulates each packet in a frame, but this may also be a
higher layer tunneling protocol, such asIPv4 when using 6to4 or Teredo transition
Routers do not fragment IPv6 packets, as they do for IPv4. Hosts are "strongly
of MTUs greater than the smallest MTU of 1280 octets. Hosts may
use fragmentation to send packets larger than the observed path MTU.
Offsets Octet 0
Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
32 Payload Length
64 Source Address
192 Destination Address
Version Traffic Class Flow Label
Version (4 bits)
3. Find how IPv6 packets are routed over IPv4 networks
6to4 is an Internet transition mechanism for migrating from IPv4 to IPv6, a system
that allows IPv6 packets to be transmitted over an IPv4 network without the need
to configure explicit tunnels. Special relay servers are also in place that allows 6to4
networks to communicate with native IPv6 networks.
6to4 is especially relevant during the initial phases of deployment to full, native
IPv6 connectivity, since IPv6 is not required on nodes between the host and the
destination. However, it is intended only as a transition mechanism and is not
meant to be used permanently.
6to4 may be used by an individual host, or by a local IPv6 network. When used by
a host, it must have a global IPv4 address connected, and the host is responsible
for encapsulation of outgoing IPv6 packets and de capsulation of incoming 6to4
packets. If the host is configured to forward packets for other clients, often a local
network, it is then a router.
Most IPv6 networks use auto configuration, which requires the last 64 bits for the
host. The first 64 bits are the IPv6 prefix. The first 16 bits of the prefix are always
2002: the next 32 bits are the IPv4 address, and the last 16 bits of the prefix are
available for addressing multiple IPv6 subnets behind the same 6to4 router. Since
the IPv6 hosts using auto configuration already have determined the unique 64 bit
host portion of their address, they must simply wait for a Router Advertisement
indicating the first 64 bits of prefix to have a complete IPv6 address. A 6to4 router
will know to send an encapsulated packet directly over IPv4 if the first 16 bits are
2002, using the next 32 as the destination, or otherwise send the packet to a wellknown relay server, which has access to native IPv6.
6to4 does not facilitate interoperation between IPv4-only hosts and IPv6-only hosts.
6to4 is simply a transparent mechanism used as a transport layer between IPv6
Due to the high levels of misconfigured hosts and poor performance observed, an
advisory about how 6to4 should be deployed was published in August 2011.
Address block allocation
For any 32-bit global IPv4 address that is assigned to a host, a 48-bit 6to4 IPv6
prefix can be constructed for use by that host (and if applicable the network behind
it) by appending the IPv4 address to 2002: /16.
For example the global IPv4 address 192.0.2.4 has the corresponding 6to4
prefix 2002:c000:0204: /48. This gives a prefix length of 48 bits, which leaves
room for a 16-bit subnet field and 64 bit host addresses within the subnets.
Any IPv6 address that begins with the 2002:/16 prefix (in other words, any address
with the first two octets of 2002 hexadecimal) is known as a 6to4 address, as
opposed to a native IPv6 address which does not use transition technologies.
Note that using a reserved IPv4 address, such as those provided by RFC 1918, is
undefined, since these networks are disallowed from being routed on the public
Internet. For example, using 192.168.1.1 as the router's WAN address would be
invalid since a return packet would not be able to determine the destination IPv4
address of the actual send
Routing between 6to4 and native IPv6
To allow hosts and networks using 6to4 addresses to exchange traffic with hosts
using "native" IPv6 addresses, "relay routers" have been established. A relay router
connects to an IPv4 network and an IPv6 network. 6to4 packets arriving on an IPv4
interface will have their IPv6 payloads routed to the IPv6 network, while packets
arriving on the IPv6 interface with a destination address prefix of 2002:/16 will be
encapsulated and forwarded over the IPv4 network.
There is a difference between a "relay router" and a "border router" (also known as
a "6to4 border router"). A 6to4 border router is an IPv6 router supporting a 6to4
pseudo-interface. It is normally the border router between anIPv6 site and a widearea IPv4 network, where the IPv6 site uses 2002:/16 co-related to the IPv4
address used later on. On the other hand, a "relay router" is a 6to4 router
Configured to support transit routing between 6to4 addresses and pure native IPv6
To allow a 6to4 host to communicate with the native IPv6 Internet, it must have its
IPv6 default gateway set to a 6to4 address which contains the IPv4 address of a
6to4 relay router. To avoid the need for users to set this up manually,
the anycast address of 18.104.22.168 has been allocated for the purpose of sending
packets to a 6to4 relay router. Note that when wrapped in 6to4 with the subnet
and hosts fields set to zero this IPv4 address (22.214.171.124) becomes the IPv6
address 2002:c058:6301::. To ensure BGP routing propagation, a short prefix
of 126.96.36.199/24 has been allocated for routes pointed at 6to4 relay routers that
use this anycast IP address. Providers willing to provide 6to4 service to their clients
or peers should advertise the anycast prefix like any other IP prefix, and route the
prefix to their 6to4 relay.
Packets from the IPv6 Internet to 6to4 systems must be sent to a 6to4 relay router
by normal IPv6 routing methods. The specification states that such relay routers
must only advertise 2002: /16 and not subdivisions of it to prevent IPv4 routes
pollute the routing tables of IPv6 routers. From here they can then be sent over the
IPv4 Internet to the destination.
For a 6to4 host to have fast and reliable connectivity with a host natively using the
IPv6 Internet, both the 6to4 host and the native IPv6 host must have a route to a
fast, reliable and correctly configured relay server. The 6to4 host's ISP can ensure
that outgoing packets go to such a relay, but they have no control over the relay
used for the responses from the native IPv6 host. A variant called IPv6 rapid
deployment ("6rd") uses the same basic principles as 6to4 but uses a relay
operated by the 6rd user's ISP for traffic in both directions. To achieve this address
block allocated by the user's ISP is used instead of 2002:/16.