3rd Edition: Chapter 4

1
Chapter 4Network LayerPart A
The majority of these slides are adapted from Jim
Kurose, Keith Ross, Addison-Wesley, July
2007.Slides from other sources (CISCO CCNA) and
from Vasos Vassiliou are also included in this
presentation.
2
Chapter 4 Network Layer

• Chapter goals
• understand principles behind network layer
  services
• routing (path selection)
• dealing with scale
• how a router works
• advanced topics IPv6, mobility

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Chapter 4 Network Layer

• 4. 1 Introduction
• 4.2 Virtual circuit and datagram networks
• 4.3 Whats inside a router
• 4.4 IP Internet Protocol
• Datagram format
• IPv4 addressing
• Subnetting
• DHCP
• ICMP
• IPv6

• 4.5 Routing algorithms
• Link state
• Distance Vector
• Hierarchical routing
• 4.6 Routing in the Internet
• RIP
• OSPF
• BGP
• 4.7 Broadcast and multicast routing

4
Network layer

• transport segment from sending to receiving host
• on sending side encapsulates segments into
  datagrams
• on receiving side, delivers segments to transport
  layer
• network layer protocols in every host, router
• Router examines header fields in all IP datagrams
  passing through it

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The Internet Network layer

• Host, router network layer functions

Transport layer TCP, UDP
Network layer
Link layer
physical layer
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Key Network-Layer Functions

• analogy
• routing process of planning trip from source to
  dest
• forwarding process of getting through single
  interchange

• forwarding move packets from routers input to
  appropriate router output
• routing determine route taken by packets from
  source to dest.
• Routing algorithms

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Path Determination
8
Routing and Forwarding Tables
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Network layer connection and connection-less
service

• Datagram network provides network-layer
  connectionless service
• VC network provides network-layer connection
  service
• Analogous to the transport-layer services, but
• Service host-to-host
• No choice network provides one or the other
• Implementation in the core

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Datagram networks

• no call setup at network layer
• routers no state about end-to-end connections
• no network-level concept of connection
• packets forwarded using destination host address
• packets between same source-dest pair may take
  different paths

1. Send data
2. Receive data
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IP Header Format
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IP Addressing introduction
223.1.1.1

• IP address 32-bit identifier for host, router
  interface
• They uniquely identify each device on an IP
  network.
• Every host (computer, networking device,
  peripheral) must have a unique address.
• interface connection between host/router and
  physical link

223.1.2.9
223.1.1.4
223.1.1.3
223.1.1.1 11011111 00000001 00000001 00000001
223
1
1
1
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IP Address Format Dotted Decimal Notation
The binary-to-decimal and decimal-to-binary
conversion will be detailed later in this course.
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IP Address Classes The First Octet
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IP Address Ranges
127 (01111111) is a Class A address reserved
for loopback testing and cannot be assigned to a
network.
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Reserved Address
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Public IP Addresses
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Private IP Addresses
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IP addressing CIDR

• CIDR Classless InterDomain Routing
• subnet portion of address of arbitrary length
• address format a.b.c.d/x, where x is bits in
  subnet portion of address

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Subnets

• IP address
• subnet part (high order bits)
• host part (low order bits)
• Whats a subnet ?
• device interfaces with same subnet part of IP
  address
• can physically reach each other without
  intervening router
• Why subnet ?
• Smaller networks are easier to manage.
• Overall traffic is reduced.
• You can more easily apply network security
  policies.

223.1.1.1
223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
LAN
223.1.3.2
223.1.3.1
network consisting of 3 subnets
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Subnets

• Recipe
• To determine the subnets, detach each interface
  from its host or router, creating islands of
  isolated networks. Each isolated network is
  called a subnet.

Subnet mask /24
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Subnets
223.1.1.2

• How many?

223.1.1.1
223.1.1.4
223.1.1.3
223.1.7.0
223.1.9.2
223.1.9.1
223.1.7.1
223.1.8.0
223.1.8.1
223.1.2.6
223.1.3.27
223.1.2.1
223.1.2.2
223.1.3.2
223.1.3.1
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What a Subnet Mask Does

• Tells the router the number of bits to look at
  when routing
• Defines the number of bits that are significant
• Used as a measuring tool, not to hide anything

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Possible Subnets and Hosts for a Class C Network
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Possible Subnets and Hosts for a Class B Network
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Possible Subnets and Hosts for a Class A Network
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End System Subnet Mask Operation
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Octet Values of a Subnet Mask
Subnet masks, like IP addresses, are represented
in the dotted decimal format like 255.255.255.0
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Default Subnet Masks
Example Class A address (decimal) 10.0.0.0 Exampl
e Class A address (binary) 00001010.00000000.0000
0000.00000000 Default Class A mask
(binary) 11111111.00000000.00000000.00000000 Defa
ult Class A mask (decimal) 255.0.0.0 Default
classful prefix length /8
Example Class B address (decimal) 172.16.0.0 Exam
ple Class B address (binary) 10010001.10101000.00
000000.00000000 Default Class B mask
(binary) 11111111.11111111.00000000.00000000 Defa
ult Class B mask (decimal) 255.255.0.0 Default
classful prefix length /16
Example Class C address (decimal) 192.168.42.0 Ex
ample Class C address (binary) 11000000.10101000.
00101010.00000000 Default Class C mask
(binary) 11111111.11111111.11111111.00000000 Defa
ult Class C mask (decimal) 255.255.255.0 Default
classful prefix length /24
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Procedure for Implementing Subnets

• Determine the IP address assigned by the registry
  authority.
• Based on the organizational and administrative
  structure, determine the number of subnets
  required.
• Based on the address class and required number of
  subnets, determine the number of bits you need to
  borrow from the host ID.
• Determine the binary and decimal value of the
  subnet mask.
• Apply the subnet mask to the network IP address
  to determine the subnet and host addresses.
• Assign subnet addresses to specific interfaces.

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Eight Easy Steps for Determining Subnet Addresses
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Eight Easy Steps for Determining Subnet Addresses
(Cont.)
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Example Applying a Subnet Mask for a Class C
Address
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Hierarchical addressing route aggregation
Hierarchical addressing allows efficient
advertisement of routing information
Organization 0
Organization 1
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16
ISPs-R-Us
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Hierarchical addressing more specific routes
ISPs-R-Us has a more specific route to
Organization 1
Organization 0
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16 or 200.23.18.0/23
ISPs-R-Us
Organization 1
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IP addresses how to get one?

• Q How does a host get IP address?
• hard-coded by system admin in a file
• Windows control-panel-gtnetwork-gtconfiguration-gttc
  p/ip-gtproperties
• UNIX /etc/rc.config
• DHCP Dynamic Host Configuration Protocol
  dynamically get address from as server
• plug-and-play

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Network Connection
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DHCP Dynamic Host Configuration Protocol

• Goal allow host to dynamically obtain its IP
  address from network server when it joins network
• Can renew its lease on address in use
• Allows reuse of addresses (only hold address
  while connected an on)
• Support for mobile users who want to join network
  (more shortly)
• DHCP overview
• host broadcasts DHCP discover msg
• DHCP server responds with DHCP offer msg
• host requests IP address DHCP request msg
• DHCP server sends address DHCP ack msg

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DHCP client-server scenario
223.1.2.1
DHCP

223.1.1.1
server

223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
arriving DHCP client needs address in
this network
223.1.1.3
223.1.3.27

223.1.3.2
223.1.3.1

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DHCP client-server scenario
arriving client
DHCP server 223.1.2.5
DHCP offer
src 223.1.2.5, 67 dest 255.255.255.255,
68 yiaddrr 223.1.2.4 transaction ID
654 Lifetime 3600 secs
DHCP request
src 0.0.0.0, 68 dest 255.255.255.255,
67 yiaddrr 223.1.2.4 transaction ID
655 Lifetime 3600 secs
time
DHCP ACK
src 223.1.2.5, 67 dest 255.255.255.255,
68 yiaddrr 223.1.2.4 transaction ID
655 Lifetime 3600 secs
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IP addressing the last word...

• Q How does an ISP get block of addresses?
• A ICANN Internet Corporation for Assigned
• Names and Numbers
• allocates addresses
• manages DNS
• assigns domain names, resolves disputes

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Getting a datagram from source to dest.
forwarding table in A

• IP datagram

• datagram remains unchanged, as it travels source
  to destination
• addr fields of interest here

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Getting a datagram from source to dest.
forwarding table in A
misc fields
data
223.1.1.1
223.1.1.3

• Starting at A, send IP datagram addressed to B
• look up net. address of B in forwarding table
• find B is on same net. as A
• link layer will send datagram directly to B
  inside link-layer frame
• B and A are directly connected

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Getting a datagram from source to dest.
forwarding table in A
misc fields
data
223.1.1.1
223.1.2.3

• Starting at A, dest. E
• look up network address of E in forwarding table
• E on different network
• A, E not directly attached
• routing table next hop router to E is 223.1.1.4
• link layer sends datagram to router 223.1.1.4
  inside link-layer frame
• datagram arrives at 223.1.1.4
• continued..

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Getting a datagram from source to dest.
forwarding table in router
misc fields
data
223.1.1.1
223.1.2.3

• Arriving at 223.1.4, destined for 223.1.2.2
• look up network address of E in routers
  forwarding table
• E on same network as routers interface 223.1.2.9
  
• router, E directly attached
• link layer sends datagram to 223.1.2.2 inside
  link-layer frame via interface 223.1.2.9
• datagram arrives at 223.1.2.2!!! (hooray!)

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ARP
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Host-to-Host Packet Delivery (1 of 22)
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Host-to-Host Packet Delivery (2 of 22)
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Host-to-Host Packet Delivery (3 of 22)
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Host-to-Host Packet Delivery (4 of 22)
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Host-to-Host Packet Delivery (5 of 22)
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Host-to-Host Packet Delivery (6 of 22)
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Host-to-Host Packet Delivery (7 of 22)
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Host-to-Host Packet Delivery (8 of 22)
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Host-to-Host Packet Delivery (9 of 22)
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Host-to-Host Packet Delivery (10 of 22)
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Host-to-Host Packet Delivery (11 of 22)
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Host-to-Host Packet Delivery (12 of 22)
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Host-to-Host Packet Delivery (13 of 22)
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Host-to-Host Packet Delivery (14 of 22)
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Host-to-Host Packet Delivery (15 of 22)
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Host-to-Host Packet Delivery (16 of 22)
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Host-to-Host Packet Delivery (17 of 22)
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Host-to-Host Packet Delivery (18 of 22)
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Host-to-Host Packet Delivery (19 of 22)
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Host-to-Host Packet Delivery (20 of 22)
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Host-to-Host Packet Delivery (21 of 22)
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Host-to-Host Packet Delivery (22 of 22)
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Default Gateway