Download Networks - Faculty - Genesee Community College

Chapter 5
OSI Network Layer
Modified by Profs. Chen and
Cappellino
Objectives



The Network layer encapsulation allows the Transport
layer contents to be passed to the destination within a
network or on another network entirely.
Communication between networks is facilitated by a
process called routing.
Upon completion of this chapter, you will be able to:





Identify the role of the Network layer as it describes
communication from one end device to another end device.
Examine the most common Network layer protocol, Internet
Protocol (IP), and its features for providing connectionless
and best-effort service.
Understand the principles used to guide the division, or
grouping, of devices into networks.
Understand the hierarchical addressing of devices and how
this allows communication between networks.
Understand the fundamentals of routes, next-hop addresses,
and packet forwarding to a destination network.
Network Layer – Communication from Host to Host

Transport layer (OSI Layer 4), which manages
the data transport between the processes
running on each end host
VS

Network layer (OSI Layer 3) protocols ____
_______________ and ___________ used to
____________ from one host to another host.
 Network
layer doesn’t care what data it is
actually caring

To accomplish this end-to-end
transport, the Network layer uses 4
basic processes:
1. ______________________
2. ______________________
3. ______________________
4. ______________________
Communication from Host to Host
1. Addressing
Both

end devices must have _____________________.
device AKA ___________________________
2. Encapsulation
Recall:
Network layer PDU is call what?
As the data travels down the OSI from the source host,
the Network layer (L3) receives the Transport layer (L4)
PDU –AKA…

_______________________________________________
Among


other info, the Packet contains two addresses:
The address of the host _______________________ known as
the _____________________________________________
The address of the _________ host is called _______________.
Communication from Host to Host
3. Routing
Source
and destination hosts are often not connected to the same
network.
Network layer provides services to direct these packets to their
destination host.
_____________________________________________, known
as __________________, en route to a ___________________

Each router (network) that a packet passes through is called a _____.
The
router ____________________________ and direct packets
toward their destination.

Known as _____________________________
4. Decapsulation
Finally,
the packet arrives at the destination host and is processed
at the Network Layer.
The __________________ by the Network layer and __________
________________________________________________
Network Layer Protocols

Protocols implemented at the Network layer
that carry user data include:
Internet
Protocol version 4 (___________)
Internet Protocol version 6 (____________)
Novell Internetwork Packet Exchange (______)
AppleTalk

The Internet Protocol (____ and IPv6) is the
__________________________ data
carrying protocol and will be the focus of this
course.
IP V4 Protocol- most widely used…


Internet Protocol (IP) is a ____________________
IPv4 basic characteristics:
____________________
- No connection is established
before sending data packets.
__________________________________ - No
overhead is used to guarantee packet delivery.
_____________________________ - Operates
independently of the medium carrying the data.
More to come on these…

FYI: IP version 6 (IPv6) is developed and being
implemented in some areas.
IPv6
will operate alongside IPv4 and may replace it in the future.
IP V4 - Connectionless Service


In Connectionless data communications IP
packets are __________________________
____________________ that they are coming.
IP does ________________:

___________________________________ to
establish an end-to-end connection,
________________________________ to maintain the
connection.

With connectionless service, packets __________
at the destination _______________________.
upper
layer services will have to resolve any issues.
IP V4 - Best Effort (unreliable)

Since protocols at other layers can manage
reliability (for example, TCP of the TCP/IP protocol suite), IP is
allowed to function very efficiently at the Network
layer.

makes IP more _______________________________
for _________________________________.

IP packet header does not include fields required
for reliable data delivery and so therefore is an
unreliable protocol
Unreliable
meaning IP does not have the capability to
manage, and recover from, undelivered or corrupt
packets.



____________________________ of packet delivery.
____________________________ for data.
_____________________________________________
IP V4 - Media Independent

Network layer is ___________________________
________________________________
 IP
packet can be communicated electrically over
cable, via light waves over fiber, or wirelessly as
radio signals.


Data Link layer is concerned with transmission over
the communications medium.
One major characteristic of the media that the
_________________________________:
 The
_________________________________
________________________________

Known as: Maximum Transmission Unit (_______).
 The
Data Link layer passes the MTU to the Network
layer.
 The Network layer then determines how large to
create the packets.

A ____________________________________
when forwarding it from one media to another
media if a smaller MTU is required.
 This
process is called ______________________ or
fragmentation.
Encapsulating the Transport layer Segment

IPv4 encapsulates, the Transport
layer segment
The
process of encapsulating data
by layer enables the _________
____________________________
___________________________.
_____________ - remains
______________________ during
the Network layer processes.

Routers can implement different
Network layer protocols
concurrently
_________
only ___________ with
the ________________________
that encapsulates the segment.
IPv4 Packet
Red square indicates the
HEADER portion of the
packet
We will discuss a few
field- refer to the text or
curriculum for details on
all fields
IP V4 Packet Header fields

_____________________ (32 bits)
represents

_____________________ (32 bits)
represents

the source Network layer host address.
the destination Network layer host address.
______________________ (8 bits)
Indicates





the ____________________________
The _____________________________________
Typically- initially set to 123 seconds
Value is ________________ at each hop
When the value becomes zero, the router discards the packet
Assures packets don’t loop around indefinitely between routers
causing a routing loop.
Neat TTL tidbit…
Shows # of hops (TTL) for a
packet of a particular type of
OS running on a particular
processor or platform
IP V4 Packet Header

________________ (8 bits)
Indicates
the data __________________ that the packet
is ___________________________.
Example values are: 01 ICMP; 06 TCP; 17 UDP

_____________________________ (8 bits)
The field is used to ___________________________.
This value enables router to give priority to certain traffic
 Example: Telephony voice data might be considered high priority
packets

_____________________________ (13 bits)
If
a packet needs to be fragmented, the fragment offset
field _______________________________________
___________ in the reconstruction at the destination
Typically set to 0 indicating there is no offset

For additional field see curriculum- we have
discussed the key ones…
Networks – Separating Hosts into Common Groups


As the number of hosts on the network grows,
more planning is required to manage and address
the network.
It is more practical and manageable to ________
________________________________.
These
smaller networks are often called __________ or
______________________.

Networks can be grouped based on three common
factors that include:
1. _________________________________

Grouping hosts at the same location into separate networks can
improve network management and operation.
Continued on next slide…
Separating Hosts into Common Groups cont…
2. ______________________________
Users
who have ______________ typically
use common software, tools, and access
common hardware
We can often reduce the traffic by placing
the ____________ to support the users in
the ________________________________

Users using bandwidth intensive apps should
be separated from other users
3. ________________________
___________________________________
___________ and devices based on the
need for those resources

Ownership of a resource could be based on
a company or part of a company or
department
Why Separate Hosts into Networks (subnets)?
 Improve
Performance
 Increase Security
 Address Management issues
More to come on all of these…
Improve Performance


A ______________________________
__________________________ due to
increased data traffic as well as
increased control and management traffic
An increase in control traffic is often due
to _________________________
A broadcast
is a message _______________
____________________________

Receiving host processing excess broadcast
traffic will cause a degradation in it’s
performance
____________________
generate ________
______________________________
_________________________ to enable data
communication on networks.
Broadcasts __________________, is known
as a _____________________________

_____________________ also _______
____________________________
Increase Security



The Internet has grown as businesses and
organizations have developed their own IP
networks that link to it.
Dividing networks based on ownership means
that _________________________________
______________________________________
_____________________________.
Same security concept applies within a
corporate network
Dividing
a network into subnets based on user
access is a means ___________________________
__________________________________________
__________________________________________
Security between networks is _____________ by
devices such as a router or ____________ appliance
Address Management Issues


_________________________________
_________________________________
Dividing large networks so that hosts who
need to communicate are grouped
together allows host to know or easily
learn the Layer two address of other hosts
in that subnet
Increases

efficiency
For ____________________________,
the hosts only need to ______________
_________________, to which they send
packets for all other destinations
addresses.
This
intermediary device is called a ________
and is generally a __________________________
__________________________________________
How do we Separate Hosts into Networks?
Hierarchical Addressing
More to come …
Hierarchical Addressing

Network layer addressing schemes are hierarchical.
Enables


communication between networks and over the Internet.
What is an example of a hierarchical addressing scheme?
Using hierarchical addressing means that the __________
_______________________________________________
______________________________
The
_______________________ of the address indicates the
________________________________________________

Routers forward packets between networks by referring only to the part
of the Network layer address that is required to direct the packet toward
the destination network.
By
the time the packet arrives at the destination host network, the
whole network layer address of the destination host will have been
used to deliver the packet.

____________________________ can be created if large
networks need be ______________________________
Address
would be: _____________________________________
More to come on next slide…
Hierarchical Addressing cont…

Logical 32-bit IPv4 address
 Are _________________________________
 Each octet is converted to its decimal value
 4 decimal values separated by a dot (period).
 For example - 192.168.18.57

The IPv4 address is hierarchical and is made up of
two parts (three parts when subnetted).
 The ________ part identifies the ____________ and
 the __________ part identifies a _________________.
 In this example, the first three octets, (192.168.18), can
identify the network portion of the address, and the last
octet, (57) identifies the host.
 In a subnetted address:


first part is network, second is subnet, third is host
In subnetted network, the ____________________
_________________________________ of the
address.
 These
borrowed host bits represent the different
subnetworks within the range of the original network.
Hierarchical Addressing cont…

The number of bits of an address used as the
network (and subnet) portion is called the
_______________________
For
example if a network uses 24 bits to express the
network portion of an address the _______________
_____________________________-

In the devices in an IPv4 network, a separate 32bit number called a _______________________
_________________________________________________
the prefix length or subnet mask
enables the ________________________________.
Chapter 6 in this course will cover IPv4 network
addressing and subnetworking in detail.
Support communication Outside our Network


_____________ a local area network or a
_________________, _______________
_________________________________
______________________ (router).
When a host needs to communicate with
a network outside the local area network,
the router connected to our local area
network or subnet, acts as a gateway
The
_________________________________
__________________________________
_______________ is the _________________
___________________ that is connected to
the same network as the host.

The router also needs a _________ that
defines ______________________ next.
This is called the ___________________
Carrying Data End to End via IP Packets


If communication is between hosts on two
different networks
The local network delivers the packet from the
_______________________________
The router ________________________ of
the packet _________ address and forwards
the packet to the appropriate interface.
If the destination network is (NOW) ________
______________ to this router, the packet is
_______________________________
If the destination network is _____________
___________________, the packet is
forwarded on to a second router- __________
The packet forwarding then becomes the
responsibility of this second router.
This continues until the destination is reached
or the max. number of hops is reached
At each hop, the forwarding decisions are based
on the information in the IP packet header.
 ________________________________________
_____________________________________
A Gateway – The way out of Our Network


The gateway, also known as the _________________, is
needed to _____________________________________
This gateway is a _________________ connected to the
local network.
The
gateway interface has a ______________________ that
______________________________________________

Default Gateway- configured on a host
Both
the host IPv4 address and the gateway address must have
the same network (and subnet, if used) portion of their respective
addresses.

Confirming the Gateway
Host
IP address, default gateway etc. can be viewed by issuing
the ipconfig or _______________ command at the command line
of a Windows computer.
The ______________ command used on a Linux or UNIX host
A Gateway – The way out of Our Network

A ___________________
______________________
______________________
for each packet that arrives
at the gateway interface.
This
forwarding process is
referred to as ____________
If a route to a destination
____________________,
the packet cannot be
forwarded and is _________

Router uses a routing table
to determine next hop…
A Route – The Path to a Network

The ___________ stores ________________
__________________________________
 Connected
networks are directly attached to
one of the router interfaces.

These interfaces are the gateways for the hosts on
different local networks.
 ____________________
can be __________
configured by the network admin. or _________
____________________________________

Routes in a routing table have three main
features:
 ___________________________
 _________________________
 _____________________________________
______________________________________
________________________________

The _____________________________ is
used to forward a packet when the destination
network is not represented by any other route
in the routing table.
Note: The routing process and the role of metrics are the subject of a later
course and will be covered in detail there.
Routing Table Entries- a closer look



A _________ in the routing table could refer to a
_______________________________________
__________________________
When forwarding a packet, the ___________
__________________________________
EXAMPLE: As shown in the figure, if a packet
arrives at a router with the destination address of
10.1.1.55, the router forwards the packet to a
next-hop router associated with a route to
network 10.1.1.0.

If a route to 10.1.1.0 is not listed on the routing,
but a route to 10.1.0.0 is available, the packet is
forwarded to the next-hop router for that network.
 Therefore, the precedence of route selection for
the packet going to 10.1.1.55 would be:
1. 10.1.1.0
2. 10.1.0.0
3. 10.0.0.0
4. 0.0.0.0 (Default route if configured)
5. Dropped
See next slide…
Default Route

AKA __________________
A __________________ is a route that
______________________________
All
or any is denoted by the address _______
The default route is used to _____________
for which there is _____________________
for the destination network.
A Route – Host Routing Table

______ also require a _______________ to
ensure that Network layer packets created
by that host, are directed to the correct
destination network.
The
host routing table is _________________ by
the ____________________________________
___________________________



Recall: the routing table in a _______
contains both _______________________
Local routing table info from a computer can
be examined at the command line by
issuing the _______________________
commands.
You can use the following options for the
route command to modify the routing table
contents:
route ADD
route
DELETE
route CHANGE
The Next-Hop – Where the Packet Goes Next

A _____________________ of the device that
will ____________________________
Next-hop
for a host on the network, will be the ______
______________________ (default gateway)
Next-hop for a router is the __________________ that
will process the packet

In the routing table of a router, each route lists a
next-hop for each destination address that is
encompassed by the route.
__________________________________________________
because they are directly cabled to that router

Some _________________________________
indicating there are multiple paths to the same
destination network.
Packet Forwarding – Moving toward its Destination

A router will do one of three things with a packet it
receives:
 ____________________________________
 ____________________________________
 ______________________



if…
No default route is programmed OR
The destination network address doesn’t exist
Packet Examination
 Packets
that arrive at a router's interfaces are
encapsulated as a Data Link layer (Layer 2) PDU.

What s the special name for the PDU at L2?
 The
____________________________ so that the
packet and destination IP address can be examined.

Next Hop Selection
 Destination
(IP) address in a packet is examined and
the routing table is referenced


If the destination network is ______________ to the
router, the packet is first _______________________
protocol then ______________________ to which that
network is connected.
Otherwise, on to the next-hop router or dropped
Packet Forwarding – Using the Default Route

Review: As shown in the figure, if the
routing table does not contain a more
specific route entry for an arriving packet,
the packet is forwarded to the interface
indicated by a default route, if one exists.
This
process may occur a number of times
until the packet reaches its destination
network.

Default routes are important:
It
is not possible for all routers to have a route
to every possible network on the Internet.
_____________________________________
_____________________ at a router that has
a specific route to the destination network.
Sees the default route
(Gateway of last resort)
Routing Protocols – Sharing the Routes

Recall: route information within a routing
table can be manually configured on the
router or learned dynamically from other
routers in the same internetwork.
For
accurate routing decisions, the _____________
__________________________________________
_____________________ that the router can access.


Out-of-date routing information means that packets may not
be forwarded to the most appropriate next-hop,
Two types of routing
________
Routing
_________ Routing
More to come on both…
Static Routing

______________________________________ to remote
networks can be _________________ on the router.
This
is known as static routing.
Upside- ________________________________
A _______________________________________________

If the router is connected to a number of other routers,
knowledge of the internetworking structure is required for
_________________________________
If
the internetwork structure changes or if new networks become
available, these changes ________________________________
______________________________ that is running static routing

What do you think will happen if timely manual updating does not occur?
Must be manually
configured
Dynamic Routing


The administrative maintenance involved
and manual up-dating needed in Static
routing makes it an undesirable routing
option
Routing protocols are the answer:
Set
of rules by which ___________________
____________________________________



When routers become aware of changes to the
network, the information is automatically
passed on to other routers.
When a router _______________________
______________________________________
____________________________ and, in turn,
passes the information to other routers.
Common routing protocols are:
Routing
Information Protocol (_________)
Enhanced Interior Gateway Routing Protocol
(________________)
Open Shortest Path First (___________)
Dynamic Routing cont…

Costs involved in routing protocols
First,
the ________________________________
___________________________ that consumes
network bandwidth.
Second, _______________________________ by
protocols such as EIGRP and OSPF- involves ________
___________________________________

In many internetworks, a _______________ of
static, dynamic, and default routes are used to
provide the necessary routes.
Configuration of routing
protocols covered extensively in
later courses…