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Network Layer 3
OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
Overview
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Layer 3 Network Layer
• Layer 3 responsibilities
• IP as an example
Network Layer Overview
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Responsible to route data (datagrams) through
an internetwork to destination host.
• Data packets
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User data
• Route update packets
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Information to map and navigate layer 3 networks
Not responsible for error-free communications
Communications are connectionless
Communications are not ordered
Defines broadcast domains
Requires an addressing scheme
Primary device is router
Examples: IP and IPX
TCP/IP Suite Layer 3 protocols
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IP (Internet Protocol)
ARP (Address Resolution Protocol)
ICMP (Internet Control Messaging
Protocol)
IGMP (Internet Group Messaging
Protocol)
The Internet Protocol IP
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The Internet Protocol (IP) is the Network Layer
protocol used in the Internet suite of protocols.
IP provides:
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Datagram Delivery without reliability checking
Addressing through a fixed length IP address
No method of flow control
No ordering of packets
Connectionless service
Fragmentation and reassembly of long packets
The full specification is described in RFC 7911
1All
internet protocols are described by specifications called
Request for comments (RFC)
IP Packet Structure
8 bits
Version
Header Length
Type of Service
Total Length
Identifier
Flags
Fragment Offset
Time to Live
Protocol
Header Checksum
Source Address
Destination Address
Options + Padding
0 10 0
00
0
= 1 bit
16 bits
24 bits
32 bits
Address Resolution Protocol (ARP)
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ARP is used to discover the MAC address
for the destination device or the “Next
Hop” router on the path to the destination.
It does this by broadcasting a message to
all asking for the MAC address that
matches the known IP address.
The device who owns the IP address
answers back with its own MAC address
Some argue that ARP should be at layer 2
ARP Packet Structure
Hardware Type
08 00
Hardware Address Length 06
Protocol Address Length 04
Protocol Type
Operation (Opcode)
Sender's Hardware Address
Sender's IP Address
Target's Hardware Address
Target's IP Address
Internet Control Messaging
Protocol (ICMP)
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Sends control and error messages
throughout the IP network
Examples:
• Echo and Echo reply
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Ping is a utility that sends an ICMP echo to the
destination host. The host replies with an ICMP echo
reply
• Source Quench
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If a device is sending data faster than it can be
received, a source squelch message is sent to the
sender telling it to SLOW DOWN.
• Destination Unreachable
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There is no way to get to the network you have
requested
Internet Group Messaging Protocol
(IGMP)
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Used between IP hosts and their
immediate neighbor multicast agents to
support the allocation of temporary group
addresses and the addition and deletion of
members of a group.
A multicast group is a set of hosts that
wish to receive the same data as others
within their group.
• A multicast transmission is addressed to a
group, not to an individual machine.
Streaming media is a good example of a use
for multicasting
Unicast Datagrams
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A unicast message is sent from one
host to exactly one host.
Host A wants to send a 5 meg
message to hosts B,C and D.
The message is sent 3 times
requiring 15 meg of bandwidth
Multicast Datagrams
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Multicast datagrams are sent once to the
multicast group address and received by
all hosts in the multicast group.
Host A wants to send a 5 meg message to
hosts B,C and D.
IGMP is used to create a multicast group
Host A sends the message to the Multicast
group address
Hosts B,C and D receive the datagram(s)
Total bandwidth used is 5 meg.