Network Layer Fundamentals

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© Cisco и/или Партнеры, 2016 г. Все права защищены.
Конфиденциальная информация Cisco
Network layer

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PLAN
1. Network Layer characteristics
2. IPv4 package
3. IPv6 package
4. Routing methods on hosts
5. Routing tables on routers

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8.1 Network Layer
characteristics

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Network layer
• Provides services that allow end devices to
exchange data
• IP version 4 (IPv4) and IP version 6 (IPv6) are the
communication protocols of the main network layer.
• The network layer performs four basic operations:
• Addressing end devices
• Encapsulation
• Routing
• Deencapsulation

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IP encapsulation
• The IP protocol encapsulates a transport
layer segment.
• The IP can use an IPv4 or IPv6 packet
and does not affect the Layer 4
segment.
• The IP packet will be checked by all
Layer 3 devices as the network
progresses.
• IP addressing does not change from
source to destination.

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IP Protocol Characteristics
IP is designed to have low overhead, and can be described as:
 Without establishing a connection
 Non-guaranteed delivery
 Independence from the environment

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Without establishing a connection
IP protocol. Without establishing a connection.
 The IP does not establish a connection to the destination until the packet is sent.
 Control information is not required (synchronization, confirmations, etc.).
 The destination will receive the packet when it arrives, but no advance IP notifications are
sent.
 If there is a need for connection-oriented traffic, then another protocol will handle this (usually
TCP at the transport layer).

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Non-guaranteed delivery
• IP does not guarantee package
delivery.
Thus, IP reduces overhead because
there is no mechanism to resend data
that is not received.
• The IP is not waiting for
confirmation.
• The IP does not know if another
device is working or if another
device has received a packet.

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Independence from the environment
IP is unreliable:
 It cannot manage or fix undelivered or corrupted
packages.
 IP cannot retransmit information after an error.
 IP cannot restore the sequence of packets.
 IP must rely on other protocols for these functions.
IP is independent of the environment.
 IP has nothing to do with the type of frame required at
the link layer or the type of media at the physical layer.
 IP can be transmitted over any type of media: copper,
fiber optic or wireless channels.

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The network layer sets the maximum data
transfer unit (MTU).
 The network layer gets this from the
control information sent by the link
layer.
 The network then sets the MTU size.
Fragmentation is when layer 3 splits an
IPv4 packet into smaller units.
 Fragmentation causes a delay.
 IPv6 does not fragment packets.
 Example. The router is transitioning
from Ethernet to a slow WAN with a
smaller MTU

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8.2 IPv4 package

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IPv4 packet header
IPv4 is one of the main network layer communication protocols.
The network header has many purposes:
 It ensures that the packet is sent in the right direction (to the destination).
 It contains information for network layer processing in various fields.
 The information in the header is used by all Layer 3 devices that process the packet

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IPv4 packet header
IPv4 Network Header Characteristics:
 It is shown in binary format.
 Contains several fields of information
 The diagram is read from left to right, 4 bytes
per line
 The two most important fields are source and
destination.
Protocols can have one or more functions.

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IPv4 packet header
Among the most important fields in the IPv4 header are the following.
Function Description
Version This is for v4, unlike v6, 4-bit field = 0100
Differentiated services Used for QoS: DiffServ — DS field or old IntServ — ToS or service type
Header Checksum Detecting corruption in the IPv4 header
Lifetime (TTL) The number of jumps of the 3rd level. When it becomes null, the router drops
the packet.
Protocol I.D.s next-level protocol: ICMP, TCP, UDP, etc.
IPv4 source address 32-bit source address
Destination IP address 32-bit destination address

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8.3 IPv6 package

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IPv4 Packet Restrictions
IPv4 has three main limitations:
 Depletion of IPv4 addresses — at the present time, IPv4 addressing capabilities have
been exhausted.
 Lack of end-to-end connectivity - in order for IPv4 to exist for so long, private addressing
and NAT were created. This put an end to direct communication with public addressing.I
 ncreased network complexity — NAT was conceived as a temporary solution, it creates
such problems in the network as a side effect of manipulating the addressing of network
headers. NAT causes latency issues and troubleshooting.

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IPv6 Review
• The Working Group of Internet Technology
Engineers (Internet Engineering Task Force)
• IPv6 overcomes the limitations of IPv4.
• Advantages that IPv6 provides:
• The address space has been increased —
based on a 128-bit address, not a 32-bit
oneImproved packet handling.
• The IPv6 header structure has been simplified
by reducing the number of fields.
• Eliminates the need for NAT — since there is a
huge amount of addressing, there is no need to
use private addressing internally and map to a
shared public address

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IPv6 IPv4 Packet header fields in the IPv6 packet header
• The IPv6 header is simplified, but
not smaller in size.
• The header is fixed with a length of
40 bytes or octets.
• Several IPv4 fields have been
removed to improve performance.
• Some IPv4 fields have been
removed to improve performance:
• Flag
• Fragment Offset
• Header Checksum

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IPv6 packet header
Among the most important fields in the IPv4 header are the following.
Функция Описание
Version This is for IPv6, unlike v4, 4-bit field = 0110
Traffic class Used for QoS: equivalent to the DiffServ — DS field
Flow label Tells the device to process identical stream labels in the same way, a 20-bit
field
Payload length This 16-bit field indicates the length of the data block or payload of the IPv6
packet.
Next heading I.D.s next-level protocol: ICMP, TCP, UDP, etc.
Transition limit Replaces the TTL Level 3 Transition Counter field
IPv4 source address 128-bit source address
Destination IP address 128-bit destination address

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An IPv6 packet can also contain extension headers (EH).
EH header Characteristics:
 provide additional information about the network layer
 are optional
 placed between the IPv6 header and the payload
 used for fragmentation, security, mobility support and more.
Note: Unlike IPv4, routers do not divide IPv6-directed packets into parts.

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8.4 Routing methods on hosts

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Decision on packet forwarding by node
• Packages are always created in the source.
• Each host device creates its own routing table.
• The host can send packets as follows:
• Sam — 127.0.0.1 (IPv4),: :1 (IPv6)
• Local nodes — the destination is on the same local network
• Remote hosts — devices are not on the same LAN

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Decision on packet forwarding by node (Continued)
• The source device determines whether the destination is local or remote
• Detection method:
• The IPv4 Source uses its own IP address and subnet mask, as well as the destination IP address
• The IPv6 Source uses the network address and prefix declared by the local router
• Local traffic is dumped from the host interface for processing by an intermediate device.
• Remote traffic is redirected directly to the default gateway on the local network.

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Default Gateway
The router or layer 3 switch can be the default gateway.
Features of the Default Gateway (HCV):
• It must have an IP address in the same range as the rest of the LAN.
• It can receive data from the local network and is able to redirect traffic from the
local network.
• It can route to other networks.
If a device does not have a default gateway or a bad default gateway, its traffic
will not be able to leave the local network.

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Using the default Gateway
• The host will know the default gateway
(DGW) either statically or via DHCP in
IPv4.
• IPv6 sends DGW via the Request
Router (RS) or can be configured
manually.
• DGW is a static route that will be the
route of last resort in the routing table.
• All devices on the LAN will need a DGW
router if they intend to send traffic
remotely.

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Host Routing Tables
In Windows, print the route or
netstat -r to display the PC routing
table
Three sections displayed by these
two commands:
• Interface List — all potential
interfaces and MAC addressing
• IPv4 Routing Table
• IPv6 Routing Table

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8.5 Introduction to Routing

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The decision to forward router packets
What happens when the router receives a frame from the host device?

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Введение в маршрутизацию
IP Router Routing Table
There are three types of routes in the router's routing table:
 Direct connection — these routes are automatically added by the router, provided that the interface is
active and has addresses.
 Remote routes — these routes do not have a direct connection, they can be explored:
• Manually — with a static route
• Dynamically — using a routing protocol so that routers share information with each other
 Default route — this redirects all traffic to a certain direction if there is no match in the routing table

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Static routing
Static route characteristics:
 Manually configured.
 It is necessary to configure manually by
the administrator when changing the
topology
 Suitable for small networks without
redundancy
 It is often used in combination with the
dynamic routing protocol to configure the
default route

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Dynamic routing
Dynamic routes automatically provide:
 Remote network detection
 Updating routing data
 Choosing the optimal path to
destination networks.
 Finding new best paths when topology
changes
Dynamic routing can also share static
default routes with other routers.

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Introduction to the IPv4 Routing Table
The show ip route command shows the
following route sources:
 the IP address of the local interface with a
direct connection
 C – Directly connected network
 S — Static route was manually configured
by the administrator
 O – OSPF
 D – EIGRP
This command shows the types of routes:
 Directly connected network – C and L
 Remotemarts to remote networks – O, D,
etc.

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New terms and commands
• Encapsulation
• Routing
• De-encapsulation
• Data payload
• Packet
• Internet Protocol Version
4 (IPv4)
• Internet Protocol Version
6 (IPv6)
• Network Layer PDU = IP
Packet
• IP Header
• Best effort delivery
• Media independent
• Connectionless
• Unreliable
• Maximum Transmission
Unit (MTU)
• Version
• Differentiated Services
(DS)
• Time-to-Live (TTL)
• Internet Control Message
Protocol (ICMP)
• Identification, Flags,
Fragment Offset fields
• Network Address
Translation (NAT)
• Traffic Class
• Flow Label
• Payload Length
• Next Header
• Hop Limit
• Extension Headers
• Local host
• Remote host
• Default Gateway

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New terms and commands
• netstat –r
• route print
• interface list
• IPv4 Route Table
• IPv6 Route Table
• directly-connected routes
• remote routes
• default route
• show ip route
• route source
• destination network
• outgoing interface
• administrative distance
• metric
• next-hop
• route timestamp

Network Layer Fundamentals

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