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Chapter 20
Network Layer
Protocols:
ARP, IPv4, ICMPv4,
IPv6, and ICMPv6
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Protocols at Network Layer
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IP is responsible for host-to-host delivery of
datagrams from source to destination.
ARP: Find the MAC (Physical) address of the next
hop. Data link layer encapsulates this address into
the frame
ICMP: Handle unusual situations such as the
occurrence of an error.
IP is meant for unicast. For Multicast, we need IGMP.
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ARP
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Hosts and routers are recognized at the network level
by their IP addresses. IP is unique.
At physical network level, we use MAC. MAC is
unique locally but not necessarily universally.
We need both IP and MAC address because a
physical network, such as Ethernet, can have two
different protocols at the network layer, such as IP
and IPX (Novell), at the same time. Likewise, a
packet at a network layer such as IP may pass
through different physical networks, such as Ethernet
and Token Ring.
IP and MAC address need to be mapped.
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ARP mapping
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Static Mapping
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Having a table of IP and MAC address mapping in all the
machines.
Limitations if MAC address changes due to
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Change of network card
In LocalTalk, MAC address changes when a machine is turned on
A mobile computer can move from one network to another and
so can gain different MAC address.
Dynamic mapping
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Each machine knows one of the two addresses.
Use a protocol to get the other address.
ARP & RARP. RARP is now replaced by DHCP.
ARP associates an IP address with its MAC address.
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ARP Operation
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ARP request has the sender’s IP and MAC & the receiver’s IP
address. It’s a broadcast as the physical address of the receiver
is unknown.
All hosts in the network processes this request but only the
intended recipient responds. Response is unicast.
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ARP Packet Format
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Hardware Type: 16-bits; Defining the type of
network; Ethernet is 1.
Protocol Type: 16-bits; IPv4 is 0800.
Hardware and Protocol length is 8-bits.
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Encapsulation of ARP Packet
Start of Frame Delimiter -SDF
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Delivery of the Datagram
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Sender knows the IP address of the target.
IP asks ARP to create ARP request message. It includes
sender’s and target’s IP and physical addresses. Target
Physical address is all 0s.
Message is passed to data link layer, encapsulated in a
frame using sender’s physical address. The physical
destination address is the broadcast address.
All machines drop the packet except the targeted
machine. Target machine identifies the IP address.
Target machine sends the ARP reply with its physical
address.
Sender receives the reply and knows the physical
address of target.
IP datagram, carries data for target machine, is now
encapsulated in a frame and is unicast to the destination.
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Four cases using ARP
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Four cases using ARP
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Case 1: Map the destination IP address to the
physical address of the destination host (host
MAC).
Case 2: Map the IP address of the router to the
physical address (router MAC).
Case 3: IP of next router is mapped to the
physical address (MAC of next router)
Case 4: Destination IP is mapped to the
destination MAC
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Example 1
A host with IP address 130.23.3.20 and physical address
B23455102210 has a packet to send to another host with IP address
130.23.43.25 and physical address A46EF45983AB. The two hosts
are on the same Ethernet network. Show the ARP request and reply
packets encapsulated in Ethernet frames.
Solution
Figure 20.6 shows the ARP request and reply packets. Note that the ARP
data field in this case is 28 bytes, and that the individual addresses do not fit
in the 4-byte boundary. That is why we do not show the regular 4-byte
boundaries for these addresses. Note that we use hexadecimal for every
field except the IP addresses.
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Figure 20.6
Example 1
Start of Frame Delimiter -SDF
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IP
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Host-to-host network layer delivery protocol for
the Internet.
Unreliable and connectionless datagram protocol
Best-effort: no error control or flow control.
Has error detection mechanism to discard the
packets that are corrupted.
For reliability, use IP with TCP.
Each datagram is delivered independently and via
different routes.
Datagrams: Packets of IP layer.
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Variable-length packet consisting of header [20 to 60
bytes] and data.
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IP datagram
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HLEN must be multiplied by 4 to get the length in bytes
Differentiated services: QoS
Total length = length of data + header length
TTL: number of hops; approx 2 times the maximum number of
routes between any two hosts.
Protocol: Defines the higher level protocol that uses IP layer.
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Protocols: Multiplexing
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Value
Protocol
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1
ICMP
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2
IGMP
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6
TCP
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UDP
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89
OSPF
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Checksum
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Checksum only covers the header and not
data.
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Calculate Checksum
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Data checksum is handled by higher-level
protocols that encapsulate the data in the IP
datagram.
Header changes when packets travels on the
network but data does not change.
Divide the IP header into 16-bit sections.
Value of checksum field is set to zero.
All the sections are added and the sum is
complemented.
The result is inserted in the checksum field.
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Example of checksum calculation
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Fragmentation
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Fragmentation
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Each router on the network decapsulates the IP datagram from the
frame it receives, process it, and then encapsulates it in another frame.
Format and size depends on the incoming and outgoing physical
network.
IP datagram must be divided to make it possible to pass through these
physical networks. This is called fragmentation
Maximum Transfer Unit (MTU)
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To make IP independent of the physical network, the maximum length of
IP is equal to the largest maximum transfer unit (MTU) 65,535 bytes.
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Fragmentation Fields
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Identification: Same for all the fragments. Helps destination in
reassembly of fragments.
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Flags: 3-bit field.
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First bit is reserved.
Second bit is called Don’t fragment bit. If set, don’t fragment the packet.
If set and still needs fragmentation, discard the packet and send an
ICMP message to the source host.
Third field is more fragment bit. If 0 means that this is the last fragment.
Fragmentation offset
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13-bit field shows the relative position of this fragment with respect to
the whole datagram.
It is the offset of the data in the original datagram measured in units of
8 bytes.
Forces hosts or routers that fragment datagrams to choose the size of
each fragment so that the first byte number is divisible by 8.
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Fragmentation Example
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If needed, the packets are fragmented. Each
fragmented datagram has a header of its own. A
fragmented datagram may itself be fragmented if it
encounters a network with an even smaller MTU.
Fragmentation is done at source or at the routers on
the fly. Re-assembly is done only at the destination.
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ICMP: Internet Control Message Protocol
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IP lacks error control.
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Lacks of assistance mechanisms.
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No error-reporting or checking.
What happens if router can not find a final destination?
What if time-to-live filed has zero value?
What will happen if final destination has not received all the
fragments within a pre-determined time limit?
A host sometimes needs to determine if a router or another host is
alive.
And sometimes a network manager needs information from
another host or router.
ICMP (Internet Control Message Protocol) is a network
layer protocol.
ICMP Messages are encapsulated inside IP datagrams
before going to the lower layer.
Protocol field in IP header is 1 for ICMP.
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ICMP Encapsulation
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Types of ICMP Messages
1. Error-Reporting Messages
2. Query Messages
Error-Reporting Messages:
 Chance of error always exists and ICMP
handles error reporting
 Error reporting messages are always sent
to the original source.
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ICMP uses the source IP address to send
the error message to the source
(originator) of the datagram.
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Error-Reporting Messages
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Error-Reporting Messages
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Destination unreachable
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Router can not route a datagram or a host cannot deliver
a datagram, the datagram is discard and message sent
to the source.
Source Quench
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IP is connectionless protocol and so no control of flow or
congestion.
Source has no idea whether the destination host has
been overwhelmed with datagrams.
When a router or host discards a datagram due to
congestion, it sends a source-quench message to the
sender of the datagram.
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To inform sender about the loss of the datagram
To warn the source that there is congestion in the path and that
the source should slow down the sending process.
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Error-Reporting Messages
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Time-exceeded: Generated in two cases
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Parameter Problem
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When a router receives a datagram with TTL as 0, then the router
discards the datagram and sends a message to the source.
When all fragments that make up a message do not arrive at the
destination host within a certain time limit.
If a router or the destination host discovers an ambiguous or
missing value in any field of the datagram, it discards the the
datagram and sends a message back to the source.
Redirection
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Routing decision on routers is made using routing protocols but in
the case of hosts, there is no routing protocol.
A host may send a datagram, which is destined for another
network, to the wrong router. In this case, the router that receives
the datagram will forward the datagram to the correct router.
However, to update the routing table of a host, it sends a
redirection message back to this host.
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Query Messages
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In the case of query message, a node sends a message that is
answered in a specific format by the destination node.
Echo request and reply: for diagnostic purposes
Time-stamp request and reply: to determine the round trip time
and also to synchronize the clocks in two machines.
Address mask request and reply: Request by a host to know its
subnet mask to the router.
Router solicitation and advertisement: Solicitation is request by
host to know the route to send a packet. Reply comes from the
router as advertisement.
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IPv6
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IPv4
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Two level address structure. Inefficient.
For real-time audio and video transmission, there is no
minimum delay strategies and reservation of resources.
No security mechanism [encryption and authentication
of data].
IPv6 [IPng: IP next generation]
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Larger address space: 128 bits long.
Better header format: Options are separated from base
header. This simplifies and speeds up the routing
process because most of the options do not need to be
checked by routers.
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IPv6
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IPv6 [IPng: IP next generation]
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New options: For additional functionalities.
Allowance for extension
Support for resource reservation: Type-of-service has
been removed but a mechanism called flow label has
been added to enable the source to request special
handling of the packet. This mechanism can be used to
support traffic such as real-time audio and video.
Support for more security: encryption and
authentication options in IPv6 provide confidentiality
and integrity of the packet.
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IPv6 Address
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Hexadecimal colon notation
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Abbreviation
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Divided into 8 sections, each 2 bytes in length
Two bytes in hexadecimal requires four hexadecimal digits.
Leading zeros of a section (four digits between two colons)
can be omitted.
Only the leading zeros can be dropped, not the trailing
zeros.
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Abbreviated Address
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Abbreviations are possible if there are consecutive sections
consisting of zeros only.
We can remove the zeros altogether and replace them with
a double semicolon. But only once per address.
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CIDR Address
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IPv6 allows classless addressing and CIDR notation.
Categories of addresses
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Unicast: Single computer.
Anycast: A group of computers with addresses that have the
same prefix. All computers connected to the same physical
network share the same prefix address. A packet sent to an
anycast address must be delivered to exactly one of the
members of the group.
Multicast: Group of computers that may or may not share
the same prefix and may or may not be connected to the
same physical network. A packet sent to a multicast address
must be delivered to each member of the set.
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Format of an IPv6 Datagram
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Format of an IPv6 Datagram
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Base header: 40 bytes; Data + extension header= 65535
bytes
Ver: 4 bit; Priority: 4 bit
Flow label: 24 bit; Payload length: length of IP datagram
excluding the base header.
Next header: 8-bit field defining the header that follows
the base header in the datagram. Sometimes, the pointer
points to the upper-layer protocol’s header.
Hop limit: TTL.
Source / Destination Address: 16-bytes (128 bit). IP Add.
Fragmentation in IPv6 is possible only at the source.
Source must find the MTU using MTU discovery
technique. OR MTU=576 bytes (smallest possible size)
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Comparison of Network Layers in v4 and v6
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ICMPv6 is designed for IPv6
Some protocols that were independent in version 4
are now part of ICMPv6.
ARP and IGMP in version 4 are combined in ICMPv6.
RARP is dropped from the suite because it is seldom
used.
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IP Transition Strategies
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Dual Stack: station should run IPv4 and IPv6 simultaneously
until all the Internet uses IPv6. (Host DNS Query)
Tunneling: A strategy used when two computers using IPv6
want to communicate with each other when the packet must
pass through a region that uses IPv4. To pass this region,
IPv4 address is needed. IPv6 packet is encapsulated in an
IPv4 packet when it enters the region, and the IPv6 packet
leaves its capsule when it exits the region.
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Dual Stack and Tunneling
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Header Translation
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When the majority of the Internet has moved to IPv6
but some systems still use IPv4.
Sender wants to use IPv6, but the receiver does not
understand IPv6.
Tunneling does not work in this situation because the
packet must be in the IPv4 format to be understood by
the receiver. In this case, the header format must be
changed totally through header translation.
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