Transcript Chapter 1: Foundation

Chapter 1
NETWORKS OVERVIEW
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Chapter 1
OSI Architecture
The OSI 7-layer Model
OSI – Open Systems Interconnection
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Physical Layer
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Handles the transmission of raw bits over a communication link
Data Link Layer
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Chapter 1
Description of Layers
Collects a stream of bits into a larger aggregate called a frame
Network adaptor along with device driver in OS implement the
protocol in this layer
Frames are actually delivered to hosts
Network Layer
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Handles routing among nodes within a packet-switched network
Unit of data exchanged between nodes in this layer is called a
packet
The lower three layers are implemented on all network nodes
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Transport Layer
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Chapter 1
Description of Layers
Implements a process-to-process channel
Unit of data exchanges in this layer is called a
message
There is a disagreement on the top three
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Session, Presentation, and Application
Mainly because the are not always present
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Chapter 1
Description of Layers
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Session Layer
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Presentation Layer
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Provides a name space that is used to tie together the potentially
different transport streams that are part of a single application
Ex., tie the audio and video together in videoconference
Concerned about the format of data exchanged between peers
Ex., integer formats, audio/video format, most/least significant
Application Layer
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Include things like the Hypertext Transfer Protocol
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Basis the world wide web
Used by web browsers
The transport layer and the higher layers typically run only on endhosts and not on the intermediate switches and routers
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Internet Architecture
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Sometimes called TCP/IP
Evolved from an earlier packet-switched
network called ARPANET
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Internet and ARPANET were funded by ARPA
(Advanced Research Projects Agency)
Both existed before the OSI architecture
Both affected the OSI model
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Internet Protocol Graph
Chapter 1
Internet Architecture
Alternative view of the
Internet architecture. The
“Network” layer shown here
is sometimes referred to as
the “sub-network” or “link”
layer.
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Internetworking
What is internetwork
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An arbitrary collection of networks interconnected to provide
some sort of host-to-host packet delivery service
A simple internetwork where H represents hosts and R represents routers
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Internetworking
What is IP
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IP stands for Internet Protocol
Key tool used today to build scalable, heterogeneous
internetworks
It runs on all the nodes in a collection of networks and defines
the infrastructure that allows these nodes and networks to
function as a single logical internetwork
A simple internetwork showing the protocol layers
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IP Service Model
Two parts
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Global Addressing Scheme
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Provides a way to identify all hosts in the network
Datagram (Connectionless) model for data delivery
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Best-effort delivery (unreliable service)
packets are lost
packets are delivered out of order
duplicate copies of a packet are delivered
packets can be delayed for a long time
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Packet Format
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Global Addresses
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IP addresses Properties
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globally unique
hierarchical: network + host
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Network part: identifies the network the host is attached to
Host: identifies a unique host on that network
Ethernet addresses, even globally unique, are flat (no structure and thus no
meaning) and can not be use for routing
Note that a router is attached to at least two networks, so it must
have an IP address on each port/interface
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Thus it is more precise to think of IP addresses as belonging to interfaces
rather than to hosts
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Global Addresses
Approximately, 4 Billion IP address, half are A type, ¼ is
B type, and 1/8 is C type
(a) Class A (b) Class B (c) Class C
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Global Addresses
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Class A was intended for Wide Area Networks
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Thus there should a very few of them
Class B was intended for a modest size networks (like a
campus)
Class C is for the large number of LANs
However, these classifications are not flexible and
today’s IP addresses are normally “classless” as we will
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Format
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4 bytes, each byte is represented by a decimal number
Dot notation
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10.3.2.4
128.96.33.81
192.12.69.77
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Strategy
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IP Datagram Forwarding
every datagram contains destination's address
if directly connected to destination network, then forward to
host
if not directly connected to destination network, then forward to
some router
forwarding table maps network number into next hop
each host has a default router
each router maintains a forwarding table
Example (router R2)
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Subnetting
The network number part was designed to uniquely
identify exactly one physical network
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However, this approach has some problems
A network with only 2 hosts has to have at least a class C
network!!
A network with only 256 hosts has to have at least a class B
network!!
Thus, we will waste our valuable IP address space
Solution
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Subnetting
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Subnetting
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Key Idea
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Allocate a single network number and use it for several physical
networks
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called subnets
Several things need to be done
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Subnets need to be physically close to each other
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Configure all nodes on each subnet with a subnet mask
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From the Internet point of view, they all look ONE network
A perfect situation to use subnetting is for large campus or corporation
It masks the network part
Introduces the subnet number
All nodes on the same subnet have the same subnet number and the same mask
The IP address of a nodes ANDed with the subnet mask
give the subnet number
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IP AND subnet mask  subnet number
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Subnetting
Increases the number
of networks and
reduces the number of
hosts
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Classless Addressing
Subnetting has a counterpart, sometimes called
supernetting, but often called Classless Interdomain
Routing, CIDR (pronounced cider)
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Classless Addressing
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Requires to hand out blocks of class C addresses that
share a common prefix
The convention is to place a /X after the prefix where X is
the prefix length in bits
For example, the 20-bit prefix for all the networks
192.4.16 through 192.4.31 is represented as 192.4.16/20
By contrast, if we wanted to represent a single class C
network number, which is 24 bits long, we would write it
192.4.16/24
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Chapter 1
Classless Addressing
Route aggregation with CIDR
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Different Protocols
ARP (Address Resolution Protocol)
DHCP (Dynamic Host Configuration Protocol)
ICMP (Internet Control Message Protocol)
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Simple Demultiplexer (UDP)
Format for UDP header (Note: length and checksum fields
should be switched)
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Simple Demultiplexer (UDP)
UDP Message Queue
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TCP Header
TCP Header Format
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Connection Establishment/Termination in TCP
Timeline for three-way handshake algorithm
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