Transcript IP File

Computer Networks
Computer Networks
Dr. Hussein Al-Bahadili
([email protected])
Department of Computer Information Systems
Faculty of Information Systems and Technology
The Arab Academy for Banking and financial Sciences
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Computer Networks
Chapter 5
The Network Layer
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Network Layer Design Issues
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Store-and-Forward Packet Switching
Services Provided to the Transport Layer
Implementation of Connectionless Service
Implementation of Connection-Oriented Service
Comparison of Virtual-Circuit and Datagram Subnets
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Store-and Forward Packet Switching
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In a store-and-forward packet switching system, a host
with a packet to send transmits it to the nearest router,
either on its own LAN or over a point-to-point link to the
carrier.
The packet is stored there until it has fully arrived so the
checksum can be verified.
Then it is forwarded to the next router along the path until
it reaches the destination host, where it is delivered.
The major components of the system are the carrier's
equipment (routers connected by transmission lines).
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Store-and-Forward Packet Switching
The environment of the network layer protocols.
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Services Provided to the Transport Layer
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The network layer provides services to the transport layer
at the network layer/transport layer interface, these
services are of two types:
1. Connectionless Services (Internet Community).
2. Connection-Oriented Services (ATM Community).
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Services Provided to the Transport Layer
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The network layer services have been designed with the
following goals in mind:
1. The services should be independent of the router
technology.
2. The transport layer should be shielded from the
number, type, and topology of the routers.
3. The network addresses made available to the
transport layer should use a uniform numbering plan,
even across LANs and WANs.
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Connectionless Services
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In connectionless service, packets are injected into
the subnet individually and routed independently of
each other.
No advance setup is needed.
In this context, the packets are frequently called
datagrams (in analogy with telegrams) and the
subnet is called a datagram subnet.
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Connection-Oriented Services
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In connection-oriented service, the path from the
source router to the destination router must be
established before any data packets can be sent.
This connection is called a virtual circuit (VC), in
analogy with the physical circuits set up by the
telephone system, and the subnet is called a virtualcircuit subnet.
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Implementation of Connectionless Service
Routing within a diagram subnet.
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Implementation of Connection-Oriented Service
Routing within a virtual-circuit subnet.
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Comparison of Virtual-Circuit and Datagram Subnets
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Routing Algorithms
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The Optimality Principle
Shortest Path Routing
Flooding
Distance Vector Routing
Link State Routing
Hierarchical Routing
Broadcast Routing
Multicast Routing
Routing for Mobile Hosts
Routing in Ad Hoc Networks
Node Lookup in Peer-to-Peer Networks
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Internetworking
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How Networks Differ
How Networks Can Be Connected
Concatenated Virtual Circuits
Connectionless Internetworking
Tunneling
Internetwork Routing
Fragmentation
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Internetworking
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Internetworking involves connecting two or more
computer networks with some sort of routing device to
exchange traffic back and forth, and to guide traffic on
the correct path across the complete network to their
destination.
Internetworking uses devices called routers.
Connecting together two or more networks with
bridges sometimes inaccurately refer to as
internetworking, but the resulting system is a single
subnetwork, and users require no internetworking
protocol, such as IP, to traverse it.
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Internetworking
A collection of interconnected networks.
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Problems with Internetworking
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Connection-oriented to connectionless
Reordering of packets
Protocol conversions
Address conversions
Multicast packets into a network that
Doesn’t support multicasting
Different maximum packet and payload sizes
Error, flow and congestion controls differ
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How Networks Differ
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Networks can differ in many ways.
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They may differ in their physical layers by having
different modulation techniques.
They may differ in their data link layer by having
different frame formats.
They may differ in their network layer by having
different internetworking protocols.
In internetworking we concern with the difference in
the network layer.
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How Networks Differ
Some of the many ways network can differ.
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How Networks Cab Be Connected
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Networks can be connected by different devices
according to the level of connection.
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Internetworking Devices
Layer
Device
Description
Physical
Layer
Repeaters They just move the bit from one
and hubs network to an identical network.
These are mostly analog devices
and do not understand any thing
about digital protocols (they just
regenerate signals).
Data Link Bridges
They accept frames, examine MAC
Layer
and
addresses and forward the frames
switches to different networks doing minor
protocol translations, e.g., from
Ethernet to FDDI or to 802.11.
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Internetworking Devices
Layer
Network
Layer
Device
Routers
Description
They can connect two networks and
do packet translation, but translations
is increasingly rare. A router that can
handle multiple protocols is called a
multiprotocol router.
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Internetworking Devices
Layer
Transport
Layer
Device
Transport
gateways
Description
They can interface between two
transport connections. For
example, allow packets to flow
between a TCP network and an
SNA network, which have
different transport protocols.
Application Application They translate message
Layer
gateways
semantics for the data being
moved, such as parsing and
changing header fields between
an Internet e-mail and X.400 email.
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Internetworking Devices
(a) Two Ethernets connected by a switch.
(b) Two Ethernets connected by routers.
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Switches vs. Routers
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With a switch, the entire frame is routed based on the
MAC address.
With a router, the packet is extracted from the frame,
the IP address is used to decide where to route the
packet.
Switches do not have to understand the network layer
protocol being used to switch the packets. Routers do.
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Concatenated Virtual Circuits
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A series of virtual circuits can be concatenated to
create a connection-oriented connection.
The virtual circuits are connected by a series of
gateways.
Each gateway converts the packet formats and
virtual-circuit numbers as needed.
This system works best with similar networks.
Concatenated virtual circuits are also common in
the transport layer (SNA to TCP).
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Concatenated Virtual Circuits
Internetworking using concatenated virtual circuits.
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Connectionless Internetworking
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Connectionless internetworking is another internetwork
model, it is also call datagram model.
In this model, the only service the network layer offers
to the transport layer is the ability to inject datagrams
into the subnet and hope for the best.
There is no notion of virtual circuit at all in the network
layer.
This model does not require all packets belonging to
one connection to traverse the same sequence of
gateways, but the may use different routers through the
internetwork.
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Connectionless Internetworking
Connectionless Internet.
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Tunnelling
• Tunnelling is a technique used for passing information
between two networks of the same type of network, but
there is a different network in between.
• This involves sending a packet from one LAN to another
similar LAN with a different WAN in between.
• When the router gets the frame, it pulls the IP packet out
of the frame and drops it into the payload field of
another frame to send across the WAN. This is undone
at the other end, allowing the data to be received on the
other end.
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Tunnelling
Tunnelling a packet from Paris to London
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Tunnelling
Tunnelling a car from France to England.
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Internetwork Routing
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Routing through an internetwork involves constructing a
graph representing the route taken by the packet to
move from one network to another, or in other word find
the routing algorithm, such as the distance vector and
link state algorithm.
This gives a two-level routing algorithm, these are
1. Interior gateway protocol within each network
2. Exterior gateway protocol between the networks
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In an autonomous system (AS), each network is
independent of all others.
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Internetwork Routing
(a) An internetwork. (b) A graph of the internetwork.
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Fragmentation
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Each network imposes some maximum size on its
packets. These limits have various causes, such as
1. Hardware (e.g., the size of an Ethernet frame).
2. Operating system (e.g., all buffers are 514 bytes).
3. Protocols (number of bits in the packet length field).
4. Compliance with some (inter)national standards.
5. Desire to reduce error-induced retransmissions to
some level.
6. Desire to prevent one packet from occupying the
channel too long.
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Fragmentation
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One problem with differing networks is the size limits
placed on packets.
If one network allows only 48 byte packets while
another network allows 65515 byte packets, then it is
difficult to get the large packet over the network that
only allows smaller packets.
How can this be done?
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Fragmentation
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Fragmentation is the process of breaking up a packet into
several smaller packets to send over a network.
The problem isn’t breaking up the packet to send it, but
putting the packet back together on the other end.
There are two different types of fragmentation:
1. Transparent
2. Nontransparent
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Types of Fragmentation
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Transparent fragmentation tries to make the
fragmentation invisible to any other network on
the route by reconstruction the packet each time it
leaves a network.
Non-transparent fragmentation results in all of the
fragmented packets travelling through multiple
networks to get to the destination, leaving the
destination to put them back together.
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Types of Fragmentation
(a) Transparent fragmentation.
(b) Nontransparent fragmentation.
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Numbering Fragments
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When a packet is fragmented, the fragments must be
numbered in such a way that the original data stream
can be reconstructed.
There are two ways of numbering the fragments, these
are:
1.
Use a tree
2.
Define an elementary fragment size small enough
that can pass through every network.
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Tree Numbering Technique
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One way of numbering the fragments is to use a tree.
If packet 0 must be split up, the pieces are called
0.0, 0.1, 0.2, etc.
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If any of these fragments themselves must be
fragmented later on, the pieces are numbered
0.0.0, 0.0.1, 0.0.2, …., 0.1.0, 0.1.1, 0.1.2, etc.``
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Elementary Fragment Size Numbering Technique
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The other way of numbering the fragments is to define
an elementary fragment size small enough that can
pass through every network.
This approach requires two sequence fields in the
internet header:
1. The original packet number
2. The fragment number
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Elementary Fragment Size Numbering Technique
Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.
(b) Fragments after passing through a network with maximum packet size of 8 payload
bytes plus header.
(c) Fragments after passing through a size 5 gateway.
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The Network Layer in the Internet
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The IP Protocol
IP Addresses
Internet Control Protocols
OSPF – The Interior Gateway Routing Protocol
BGP – The Exterior Gateway Routing Protocol
Internet Multicasting
Mobile IP
IPv6
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The Network Layer in the Internet
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Principles that drove the design of the NL in the Internet:
1. Make sure it works
2. Keep it simple
3. Make clear choices
4. Exploit modularity
5. Expect heterogeneity
6. Avoid static options and parameters
7. Look for a good design; it need not be perfect
8. Be strict when sending and tolerant when receiving
9. Think about scalability
10.Consider performance and cost
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Internet Protocol (IP)
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IP is a NL protocol that is used as the glue that
holds the whole Internet together.
It was designed with internetworking in mind.
The job of the NL is to provide a best-effort way to
transport datagrams across the network.
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Communication in the Internet
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The transport layer breaks the data stream and
breaks it up into datagrams.
Datagrams can be up to 64 Kbytes, but are
usually limited to not more than 1500 bytes so
that they can fit into an Ethernet frame.
If frames get fragmented during the trip, that is not
an issue – they will be reconstructed at the
destination machine.
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Communication in the Internet
The Internet is an interconnected collection of many networks.
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The Internet Protocol (IPv4)
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An IP datagram consists of a header part and a text
part.
The header has a 20-byte fixed part and a variable
length optional part.
It is transmitted in big endian order: from left to right,
with the high-order bit of the version field going first.
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The Internet Protocol (IPv4)
The Internet protocol IPv4 header (1/3)
Field
Length
Description
Version
4-bit
Specify the version of the protocol the datagram
belongs to.
IHL
4-bit
Specify the length of the header.
Type of
service
6-bit
Specify the classes of services. These classes
include the four queuing priorities, three discard
probabilities, and historical classes.
Not used
2-bit
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Total length
16-bit
Include everything in the datagram-both the header
and data. The maximum length is 65,535 bytes.
Identification
16-bit
Allow the destination host to determine which
datagram a newly arrived fragment belongs to. All the
fragments of a datagram contain the same
identification value.
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The Internet Protocol (IPv4)
The Internet protocol IPv4 header (2/3)
Field
Length
Description
Not used
1-bit
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DF
1-bit
Don’t fragment
MF
1-bit
More fragments.
Fragment
offset
13-bit
Tells where in the current datagram this fragment
belongs. Since 13-bits are provides, there is a
maximum of 8192 fragments per datagram, giving a
maximum datagram length of 65,536 bytes, one more
than the total length field.
Time of live
8-bit
A counter used to limit the packet lifetimes. It is
supposed to count time in seconds, allowing a
maximum lifetime of 255 seconds.
Protocol
8-bit
Tells the network layer which transport process to give
the datagram to (e.g., TCP, UDP, etc.)
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The Internet Protocol (IPv4)
The Internet protocol IPv4 header (3/3)
Field
Length
Description
Header
checksum
16-bit
Verifies the header only.
Source
address
32-bit
Indicate the network number.
Destination
address
32-bit
Indicate the host number
Options
>=0
Design to provide an escape to allow subsequent
versions of the protocol to include information not
present in the original design, to permit experimenters
to try out new ideas, and to avoid allocating header
bits to information that is rarely needed.
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The Internet Protocol (IPv4)
The IPv4 header.
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Some of the IP Options
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The options are variable length. Each option begins with
a 1-byte code identifying the option. Some options are
followed by a 1-byte option length field, and then one or
more data bytes.
The options field is padded out to multiple of four bytes.
Some of the IP options are:
1. Security
2. Strict source routing
3. Loose source routing
4. Record route
5. Timestamp
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Some of the IP Options
Some of the IP options.
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IP Addresses
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Every host and router on the Internet has an IP address
that encodes: (i) Network number, and (ii) Host number.
The IP address, which is a combination of the network
number and the host number, is unique, and, in
principle, no two machines on the internet have the
same IP address.
All IP addresses are 32 bits long.
The IP address refers to a network interface, so if a host
on two networks, it must have two IP addresses.
However, in practice, most hosts are on one network and
thus have one IP address.
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Classful Addressing
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IP addresses, which are written in dotted decimal
notation, are divided into five classes, namely Class
A, B,C, D, and E.
Range of Host Addresses
Class
Number of
From
To
Network
Host
A
1.0.0.0
127.255.255.255
128
16,777,216
B
128.0.0.0
191.255.255.255
16,384
65,536
C
192.0.0.0
223.255.255.255
2,097,152
256
D
224.0.0.0
239.255.255.255
Multicast address
E
240.0.0.0
255.255.255.255
Reserved for future use
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Classful Addressing
Class A IP address
A unicast IP address that ranges from 1.0.0.1 to
126.255.255.254. The first octet indicates the network,
and the last three octets indicate the host on the
network.
Class B IP address
A unicast IP address that ranges from 128.0.0.1 to
191.255.255.254. The first two octets indicate the
network, and the last two octets indicate the host on
the network.
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Classful Addressing
Class C IP address
A unicast IP address that ranges from 192.0.0.1 to
223.255.255.254. The first three octets indicate the
network, and the last octet indicates the host on the
network. Network Load Balancing provides optional
session support for Class C IP addresses (in addition
to support for single IP addresses) to accommodate
clients that make use of multiple proxy servers at the
client site.
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Classful Addressing
IP address formats.
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Special IP Addresses
Special IP addresses.
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Subnetting
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According to the addressing approached addressed
earlier, all the hosts in the network must have the
same network number.
A LAN may grow to be too large to handle and must
be split into subnets.
The subnets work like small LANs inside a larger LAN,
but allow the entire LAN to look like a single network to
the outside world.
This allows different subnets to be connected within an
organization.
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Subnetting
A campus network consisting of LANs
for various departments.
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Subnet Masks
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To implement subnetting, the router needs a subnet
mask that indicates the split between network +
subnet number and host.
A subnet mask is used by the router to determine
which subnet the packet should travel to.
The mask can be specified in dotted decimal notation
(255.255.252.0) or simply by indicating the size of the
mask (/22)
The subnet mask 255.255.252.0/22 is a 22 bit mask
allowing 64 subnets on a class B network.
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Subnet Masks
A class B network subnetted into 64 subnets.
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Subnet Masks - Example
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The 16-bit for the host number is divided between
subnet number of 6-bit and host number of 10-bit.
This allows up to 64 subnets each with 1022 hosts
(where all 0s and 1s are not used)
The subnets are counting by four as shown below:
Subnet 1: 10000010 00110010 000001|00 00000001
130.
24.
4.
1
Subnet 2: 10000010 00110010 000010|00 00000001
130.
24.
8.
1
Subnet 3: 10000010 00110010 000011|00 00000001
130.
24.
12.
1
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Subnet Masks
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Routers will AND the destination address with the
subnet mask in order to get the address of the router
where the packet should go.
Using this method reduces the number of individual
addresses that each router must store, resulting in
smaller router tables.
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The Three Bears Problem
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We can only have 128 Class A networks with 16 million
hosts each – too big.
We can have 16,384 Class B networks with 64,000 hosts
each – still too big.
We can have 2 million Class C networks with 256 hosts
each – much too small.
There are too many people under-utilizing Class B
networks, resulting in a shortage in IP addresses.
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Classless InterDomain Routing
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As opposed to allocating IP addresses based on class,
the basic idea behind CIDR is to allocate the
remaining IP addresses in variable-sized blocks.
If a site needs (N=732) addresses, we would allocate
the next highest number of addresses based on
boundaries ( b = Int(ln(N) / ln(2)) ).
Number of addresses should be allocated = 2b+1
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This makes routing more difficult as subnet masks do
not work so well.
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Classful Addressing Forwarding
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In a classful system, forwarding works like this:
1. When a packet arrives at a router, a copy of the IP address is
shifted right 28 bits to yield a 4-bit class number.
2. A 16-way branch then sorts packets into A, B, C, and D, with
8 of the cases for class A, 4 of the cases for class B, 2 of the
cases for class C, and 1 each for class D and class E.
3. The code for each class then masked off the 8-, 16-, or 24-bit
network number and right aligned it in a 32-bit word.
4. The network number was then looked up in the A, B, or C
table.
5. Once the entry was found, the outgoing line could be looked
up and the packet forwarded.
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CIDR Forwarding
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CIDR forwarding works like this:
1. Each routing table is extended by giving it a 32-bit mask.
There will be a single routing table for all networks consisting
of an array of (IP address, subnet mask, outgoing line).
2. When a packet comes in, its destination IP address is first
extracted.
3. Then the routing table is scanned entry by entry, masking the
destination address and comparing it to the table entry
looking for a match.
4. It is possible that multiple entries (with different subnet mask
lengths) match, in this case the longest mask is used.
Complex algorithms have been devised to speed up the address
matching process.
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CIDR Forwarding - Example
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Consider an example in which a million of addresses are available
starting at 194.24.0.0.
Find out the starting address and the last address to be assigned for
each university to meet their requirements
Cambridge
2048 addresses
Oxford
4096 addresses
Edinburgh
1024 addresses
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CIDR Forwarding - Example
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The routing tables all over the world are now updated with the three
assigned entries, each entry contains a base address and a subnet
mask. The entries (in binary) are:
C: 11000010 00011000 00000000 00000000
11111111 11111111 11111000 00000000
E: 11000010 00011000 00010000 00000000
11111111 11111111 11111100 00000000
O: 11000010 00011000 00001000 00000000
11111111 11111111 11110000 00000000
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Message Forwarding
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Forwarding a message to the following address:
194.24.17.4
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IP routers forward packets based on the network ID
Single entry in the forwarding table (network ID) for
all the hosts connected to that network – network
aggregation.
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CIDR Forwarding - Example
A set of IP address assignments.
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Network Address Translation (NAT)
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•
•
NAT can be used by companies when they run out of IP
addresses to assign machines.
The NAT process involves using private internal
IP
addresses and then translating those IP addresses to a
valid IP address when leaving the LAN.
This translation is done by a NAT box.
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Placement and Operation of a NAT Box.
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Network Address Translation (NAT)
• To make the NAT scheme possible, three ranges of
IP
addresses have been declared as private addresses.
• Companies may used them internally as they wish. The
only rule is that no packet containing these addresses
may appear on the Internet itself.
• The three reserved ranges are:
10.0.0.0
- 10.255.255.255/8
172.16.0.0 - 172.31.255.255/12
192.168.0.0 - 192.168.255.255/16
(16,777,216 hosts)
(1,048,576 hosts)
(65,536 hosts)
• The first range is the usual choice of most companies.
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Network Address Translation Box (NAT Box)
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•
•
The NAT box is able to translate and keep track of
addresses by using a large translation table.
As incoming packets arrive at the NAT box, it looks
up the source port field of the TCP or the UDP
transport layer protocols which was used as an
index to the internal IP address in the NAT table.
NAT tables are widely used.
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Network Address Translation Issues
1. Violates the architectural model of IP – that each IP
address is for one machine on the Internet.
2. It changes the Internet into a “connection-oriented”
network. The NAT box maintains the state of the
connection, and if it crashes, so does the link.
3. Protocol layer k assumes protocol layer k+1 has put
in the payload, violating layer independence.
4. NAT fails if protocols other than TCP or UDP are
used.
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Network Address Translation Issues
5. If IP addresses are inserted into the payload data (i.e.
text of the message), then the NAT table will not
translate that information and trouble could occur.
6. The limit of a NAT machine is 61,440 machines.
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Internet Control Protocols
•
In addition to the IP, which is used for data transfer,
the Internet has several control protocols used in the
network layer, these include
ICMP
- The Internet Control Message Protocol
ARP
- The Address Resolution Protocol
RARP
- Reverse Address Resolution Protocol
BOOTP - The Bootstrap Protocol
DHCP
- The Dynamic Host configuration Protocol
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The Internet Control Message Protocol (ICMP)
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•
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•
The operation of the Internet is monitored closely by
the routers.
When something unexpected occurs, the event is
reported by the ICMP (Internet Control Message
Protocol), which used also to test the Internet.
About a dozen types of ICMP messages are defined.
Each ICMP message type is encapsulated in an IP
packet.
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The Internet Control Message Protocol (ICMP)
The principal ICMP message types.
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The Address Resolution Protocol (ARP)
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•
Every machine on the Internet has a 32-bit IP
addresses, which differs from the 48-bit Ethernet
address.
In a LAN, the Ethernet boards send and receive
frames based on 48-bit Ethernet address without any
consideration to the 32-bit IP address.
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The Address Resolution Protocol (ARP)
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The question now arises: How do IP addresses get
mapped onto DLL addresses, such as Ethernet?
A better solution is to output a broadcast packet onto
the Ethernet asking: Who owns the requested IP
address?
The reply will come from the machine that has the
requested IP address to tell the Ethernet address of
the machine that has the requested IP address.
The protocol used for asking this question and getting
reply is called ARP (Address Resolution Protocol).
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The Address Resolution Protocol (ARP)
Three interconnected /24 networks: two Ethernets and an FDDI ring.
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The Reverse Address Resolution Protocol (RARP)
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•
ARP solves the problem of finding out which Ethernet
address corresponds to a given IP address.
How can we find the IP address for a particular Ethernet
address?
The first solution revised was to use the Reverse
Address Resolution Protocol (RARP).
This protocol allows a newly-booted workstation to
broadcast its Ethernet address and ask: Does anyone
out there know my IP address?
The RARP server sees this request, looks up the
Ethernet address in its configuration files, and sends
back the corresponding IP address.
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Advantages and Disadvantages of RARP
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•
Advantages
RARP does not require to embed the IP address in the
memory image so that it allows the same image to be
used on all machines. If the IP address were buries
inside the image, each workstation would need its own
image.
Disadvantages
RARP uses a destination address of all 1s (limited
broadcasting) to reach the RARP server. Such
broadcast are not forwarded by routers, so a RARP
server is needed on each network.
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Bootstrap Protocol (BOOTP)
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In order to avoid using an RARP server on each
network, an alternative bootstrap protocol (BOOTP) was
invented.
BOOTP uses UDP messages, which are forwarded over
routers. It also provides a diskless workstation with
additional information, including
1. The IP address of the file server holding the memory image.
2. The IP address of the default router, and subnet mask to use.
•
A serious problem with BOOTP is that it requires manual
configuration of tables mapping IP address to Ethernet
address.
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Dynamic Host Configuration Protocol (DHCP)
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•
To eliminate the error-prune step that may be occurred
due to the manual setup of the IP and Ethernet
addresses, BOOTP was extended and given a new
name: Dynamic Host Configuration Protocol (DHCP).
DHCP allows both manual IP address assignment and
automatic assignment. It is based on the idea of a special
server that assigns IP address to hosts asking for them.
This server needs not be on the same LAN as requesting
host. The DHCP server may not be reachable by
broadcasting, therefore a DHCP relay agent is needed on
each LAN.
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Dynamic Host Configuration Protocol (DHCP)
Operation of DHCP.
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Dynamic Host Configuration Protocol (DHCP)
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To find its IP address, a newly-booted machine
broadcasts a DHCP DISCOVER packet.
The DHCP relay agent on its LAN intercepts all DHCP
broadcasts. When it finds a DHCP DISCOVER packet, it
sends the packet as a unicast packet to the DHCP
server, possibly on a distant network.
The only piece of information the relay agent needs is the
IP address of the DHCP server.
An issue arises: How long an IP address should be
allocated? Addresses are usually leased for a specific
period of time.
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Routing Protocols
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The Internet is made up of a number of autonomous
systems (AS).
Each AS is operated by a different organization and
can use its own routing algorithm inside.
There are two types of routing algorithms:
1. Interior gateway protocol: A routing algorithm within
an AS.
2. Exterior gateway protocol: An algorithm for routing
between ASes.
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Interior Gateway Routing Protocol
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•
The original Internet interior gateway protocol was a
distance vector protocol, namely, the routing
information protocol (RIP) based on the Bellman-Ford
algorithm inherited from the ARPANET.
The RIP has the following disadvantages:
1. It works well in small systems, but less well as ASes get larger.
•
•
2. It suffers from the count-to-infinity problem and generally slow
convergence.
It replaced in May 1979 by a link state protocol.
In 1988, the IETF began work on a new protocol called
OSPF (Open Shortest Path First), which became a
standard in 1990.
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Routing Protocol Requirements
•
The group designing the OSPF protocol had a long list
of requirements that had to be met, these include:
1. The algorithm had to be published in the literature.
2. It had to support a variety of distance matrics,
including physical distance, delay, and so on.
3. It had to be dynamic algorithm, one that adapted to
changes in the topology automatically and quickly.
4. It had to support routing based on type of service.
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Routing Protocol Requirements
5. It had to do load balancing, splitting and load over
multiple lines.
6. It should provide support for hierarchical system.
7. It required some modicum of security to prevent
fun-loving users from spoofing routers by sending
them false routing information.
8. It needed a provision for dealing with routers that
were connected to the Internet via a tunnel.
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OSPF Connections and Networks
•
OSPF supports three kinds of connections and
networks:
1. Point-to-point lines between exactly two routers.
2. Multiaccess networks with broadcasting (most
LANs).
3. Multiaccess networks without broadcasting (most
packet-switched WANs).
•
A multiaccess network is one that can have multiple
routers on it, each of which can directly communicate
with all others (All LANs and WANs have this property).
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OSPF Operation
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•
•
OPSF operates by abstracting the collection of actual
networks, routers, and lines into a directed graph in
which each arc is assigned a cost (distance, delay, etc.).
It then computes the shortest path based on the weights
on the arcs.
A serial connection between two routers is represented
by a pair of arcs, may be of different weight, one in each
direction.
A multiaccess network is represented by a node for the
network itself plus a node for each router.
The arcs from the network node to the routers have
weight 0, and normally not shown on the graph.
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OSPF Operation
(a) An autonomous system. (b) A graph representation of (a).
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OSPF Operation
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•
Many of the ASes in the Internet are themselves large
and nontrivial to manage.
OSPF allows them to be divided into numbered areas,
where an area is a network or a set of non overlapped
contiguous networks.
Every AS has a backbone, called area 0. All area are
connected to the backbone, possibly by tunnels, so it is
possible to go from one area to another area in the AS
via the backbone.
A tunnel is represented as an arc and has a cost.
Each router that is connected to two or more area is part
of the backbone.
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OSPF Normal Operation Routes
•
During normal operation, three kinds of routes may be
needed:
1. Intra-area routing which is the easiest, since the
source router already knows the shortest path to the
destination router.
2. Inter-area routing which always proceeds in three
steps:
 Go from the source to the backbone.
 Go across the backbone to the destination area.
 Go to destination.
3. Inter-AS
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Types of OSPF Routers
•
OPSF distinguishes four classes of routers:
1. Internet routers are wholly within one area.
2. Area border routers connect two or more areas.
3. Backbone routers are on the backbone
4. AS boundary routers talks to routers in other ASes.
•
Routers may also be classified as
1. Adjacent routers which can exchange information
between them.
2. Neighboring routers which don’t exchange
information been them.
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Types of OSPF Routers
The relation between ASes, backbones, and areas in OSPF.
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Types of OSPF Messages
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•
There are a number of standard messages that
adjacent routers exchange at the startup or
periodically.
All these messages are sent as raw IP packets.
The five types of OSPF messages.
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BGP – The Exterior Gateway Routing Protocol
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•
The OPSF protocol is used within a single AS to move
packets as efficiently as possible from the source to the
destination. It doesn’t have to worry about politics.
The Border Gateway Protocol (BGP) is used to
exchange information between ASes, and has to
carefully consider politics.
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BGP – The Exterior Gateway Routing Protocol
•
Typical policies involve political, security, or economic
considerations. A few example of routing constraints are:
1. No transit traffic through certain ASes.
2. Never put Iraq on a route starting at the Pentagon
3. Do not use US to get from British Columbia to
Ontario.
4. Only transit Albania if there is no alternative to the
destination.
5. Traffic staring or ending at IBM should not transit
Microsoft.
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BGP Network Groups
•
Given BGP’s special interest in transit traffic, networks
are grouped into one of three categories:
1. Stub networks: which have only one connection to the
BGP graph. It cannot be used for transit traffic because
there is no one on the other side.
2. Multiconnected networks: which could be used for transit
traffic, except that they refused.
3. Transit networks: such as backbone which are willing to
handle third-party packets, possibly with some restrictions,
and usually for pay.
•
Pairs of BGP routers communicate with each other by
establishing TCP connections.
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Features of BGP
(a) A set of BGP routers. (b) Information sent to F.
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Internet Multicasting
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•
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Normal IP communication is between one sender and
one receiver.
However, for some applications it is useful for a
process to be able to send to a large number of
receivers simultaneously (multicast).
IP supports multicasting, using a class D addresses.
Each class D address identify a group of hosts.
There are 28-bit are available for identifying groups, so
over 250 million groups can exist at the same time.
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Internet Multicasting
•
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•
There are two kinds of group addresses, these are
1. Permanent addresses
2. Temporary addresses
A permanent group is always there and does not have
to be setup, while a temporary group must be created
before it can be used.
Each permanent group has a permanent group
address.
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Internet Multicasting
Examples of permanent group addresses.
224.0.0.1
All systems on a LAN.
224.0.0.2
All routers on a LAN.
224.0.0.5
224.0.0.6
All OSPF routers on a LAN.
All designated OSPF routers on a LAN.
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Internet Group Management Protocol (IGMP)
• Multicasting is implemented by special multicast routers,
which may be collocated with the standard routers.
• A bout once a minute, each multicast router sends a
hardware (DLL) multicast to the hosts on its LAN
(address 224.0.0.1) asking them to report back on the
groups their processes currently belong to.
• Each host sends back a responses for all the class D
addresses it is interested in.
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Internet Group Management Protocol (IGMP)
• These query and response packets use IGMP, which has
two kinds of packets: query and response.
• Each of the above packets are with a simple, fixed format
containing control information in the first word of the
payload field and a class D address in the second word.
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Mobile IP (The Problem)
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•
Every user of the Internet has his own IP address which
consists of two parts: the network number and the host
number.
The message for a particular user is delivered to its
network which is responsible to deliver it to the host.
What will happened when a host wants to move away (be
mobile) to a new site (i.e., network)?
•
To be mobile is a requirement which has to be
considered, because many users of the Internet have
portable computers and want to stay connected to the
Internet when they visit a distant Internet site.
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Mobile IP (Nonpractical Solutions)
•
In order to enable mobile users stay connected to the
Internet, some solutions could be:
1. Giving the machine a new IP address corresponding
to its new locations is unattractive because large
numbers of people, programs, and databases would
have to be informed of the change.
2. Force the routers to use complete IP addresses for
routing, instead of just the network number. However,
this strategy would require each router to have millions
of table entries, at astronomical cost to the Internet.
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Mobile IP (IETF Requirements for the Solution)
•
•
Due to the huge demands by the internet users to have
the ability to connect their notebook computers to the
Internet where ever they were, the IETF set up a
Working Group to find a solution.
The Working Group quickly formulated a number of
requirements. The major ones are
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Mobile IP (IETF Requirements for the Solution)
•
The Working Group quickly formulated a number of
requirements. The major ones are
1. Each mobile host must use its home IP address
anywhere.
2. Software changes to the fixed hosts were not
permitted.
3. Changes to the router software and tables were not
permitted.
4. Most packets shouldn’t detour on the way.
5. No overhead incurred when a mobile host is at
home.
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Mobile IP (The Solution)
•
In what follow a revision for the solution is given
1. Every site that wants to allow its users to roam has
to create a home agent.
2. Every site that wants to allow visitors to use its
network has to create a foreign agent.
3. When a mobile host shows up at a foreign site, it
contacts the foreign agent host there and
registered.
4. The foreign host then contacts the user’s home
and gives it a care-of address, normally the foreign
agent’s own IP address.
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Mobile IP (The Solution)
5. When a packet arrives at the host’s network router.
The router then tries to locate the host by
broadcasting an ARP packet asking about the
Ethernet address of the host.
6. The home agent responds to this query by giving its
own Ethernet address.
7. The router then sends the packet to the home agent.
8. The home agents, in turn, tunnels the packet to the
care-of address by encapsulating them in the payload
field of an IP packet addressed to the foreign agent.
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Mobile IP (The Solution)
9. The foreign agent then de-encapsulates and delivers
the packet to data link address of the mobile host.
10. In addition, the home agent gives the care-of
address to the sender, so future packets can be
tunneled directly to the foreign agent.
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Mobile IP (Gratuitous ARP)
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•
•
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•
At the time the mobile host moves, the router probably
has its Ethernet address cached which is soon-to-beinvalid (i.e., after the host leave the network).
According to the new solution, the mobile host Ethernet
address is replaced by the home agent Ethernet address.
Replacing the mobile host Ethernet address with the
home agent Ethernet address is done by a trick called
gratuitous ARP.
Gratuitous ARP is a specific cache entry of the mobile
host about to leave.
When a mobile host returns later, the same trick is used to
update the router’s cache again.
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Mobile IP (Solution of other Problems)
•
The IETF solution for mobile hosts solves a number of
other problems, such as:
1. How are agents located? For each agent to periodically
broadcast its address and the type of services it is
willing to provide (home, foreign, or both).
When a mobile host arrives somewhere, it can just
listen for these broadcasts (advertisements), and
broadcast a packet announcing its arrival and hope
that the local foreign agent responds to it.
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Mobile IP (Solution of other Problems)
2. What to do about impolite mobile hosts that leave
without saying goodbye? Make the registration valid
only for a fixed time interval. If it is not refreshed
periodically, it times out, so foreign host can clear
tables.
3. What to do about security? Use cryptographic
authentication protocols.
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Mobile IP (Recursive Tunneling)
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•
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Levels of mobility is an important issue to be addressed
by the Mobile IP Working Group.
This problem arises in an airplane with an on-board
Ethernet used by the navigation, avionics, and
passengers computers.
In this setup there are two levels of mobility:
1. The aircraft’s own computers, which are stationary with respect
to the Ethernet, and the passengers’ computers which are
mobile with respect to it
2. The on-board router which is mobile with respect to the routers
on the ground.
Being mobile with respect to a system that is itself
mobile can be handled using recursive tunneling.
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The Internet Protocol (IPv6)
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•
CIDR and NAT increased the number of IPv4
addresses slightly, which were not enough with the
astronomical expansion in the number of users for
the Internet.
In 1990, IETF started work on a new version, one
which would never run out of addresses, would
solve a variety of related problems, and be more
flexible and efficient as well.
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IPv6 Goals
•
The major IPv6 goals were:
1. Support billions of hosts (even with inefficient address space
allocation).
2. Reduce routing table size.
3. Simplify the protocol to speed up packet processing at routers.
4. Provide better security.
5. Pay more attention to type of service to aid in QoS concerns for
real-time data.
6. Aid multicasting by allowing scopes to be specified.
7. Make it possible for a host to roam without changing addresses.
8. Allow space for the protocol to evolve.
9. Permit the old and new protocols to coexist for a number of
years until IPv6 was used exclusively.
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IPv6
• IPv6 was known as SIPP (Simple Internet Protocol Plus).
• Addressing is done with 16 bytes (128 bits) greatly
increasing the address space.
• Reduces the number of fields from 13 to 7.
• Authentication and privacy are parts of the protocol.
• The issue of QoS was addressed.
• IPv6 is not compatible with IPv4, but it is compatible with
other auxiliary Internet protocols, including TCP, UDP,
ICMP, IGMP, OSPF, BGP, and DNS.
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IPv6 Addressing
•
The address space is written as eight groups of four
hex digits, such as:
8000:0000:0000:0000:0123:4567:89AB:CDEF
•
Optimizations:
– Leading zeros can be dropped
– One or more groups of zeros can be replaced with
two semicolons “::”
– IPv4 addresses can be accesses as:
::130.15.1.100
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IPv6 Address Space
•
There are:
– 2128 possible addresses. Or:
– 3 x 1038 possible address. Or:
– If the entire surface of the earth (land and water) were
covered with computers, IPv6 would allow 7 x 1023 IP
addresses per square meter. That’s almost one IP
address for each molecule.
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IPv6 Implementation
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•
•
It was expected that the shift to IPv6 would take about
a decade.
IPv6 groups will begin, tunnelling information between
them.
It is expected that these groups will grow until they
become the majority on the Internet.
Once this happens, it is just time until everyone else
follows suit.
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IPv6 Format
The IPv6 fixed header (required).
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IPv6 Extension Headers
IPv6 extension headers.
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IPv6 Extension Headers
The hop-by-hop extension header for large datagrams (jumbograms).
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IPv6 Extension Header for Routing
The extension header for routing.
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Internet Engineering Task Force (IETF)
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•
•
An open community of network designers, operators,
vendors, and researchers concerned with the evolution of
Internet architecture and the smooth operation of the
Internet.
Technical work is performed by working groups organized
by topic areas (such as routing, transport, and security)
and through mailing lists.
Internet standards are developed in IETF Requests for
Comments (RFCs), which are a series of notes that
discuss many aspects of computing and computer
communication, focusing on networking protocols,
programs, and concepts.
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