Computer networks-3

This is continuation of part 2, we are going to
discuss

ISO OSI reference model
Reference model
formally defines what is meant by a layer, a service etc.
Service architecture
describes the services provided by each layer and the service
access point
Protocol architecture
set of protocols that implement the service architecture
compliant service architectures may still use non-compliant
protocol architectures

The seven Layers
Presentation
Application
Session
Transport
Network
Data Link
Physical
Presentation
Application
Session
Transport
Network
Data Link
Physical
Network
Data Link
Physical
End system End systemIntermediate
system

The seven Layers - protocol stack
Presentation
Application
Session
Transport
Network
Data Link
Physical
Presentation
Application
Session
Transport
Network
Data Link
Physical
data
DH+data+DT
bits
data
data
data
data
AH
PH
SH
TH
Network
Data Link
Physical
dataNH
 Session and presentation layers are not so important, and are often ignoredSession and presentation layers are not so important, and are often ignored

Physical layer
Moves bits between physically connected end-systems
Standard prescribes
coding scheme to represent a bit
shapes and sizes of connectors
bit-level synchronization
Internet
technology to move bits on a wire, wireless link, satellite channel etc.

Datalink layer
(Reliable) communication over a single link.
Introduces the notion of a frame
set of bits that belong together
Idle markers tell us that a link is not carrying a frame
Begin and end markers delimit a frame
Internet
a variety of data link layer protocols
most common is Ethernet
others are FDDI, SONET, HDLC

Datalink layer (contd.)
Datalink layer protocols are the first layer of software
Very dependent on underlying physical link properties
Usually bundle both physical and datalink in hardware.

Ethernet (broadcast link)
 end-system must receive only bits meant for it
 need data link-layer address
 also need to decide who gets to speak next
 these functions are provided by Medium Access sub layer (MAC)

Network layer
Carries data from source to destination.
Logically concatenates a set of links to form the abstraction of an
end-to-end link
Allows an end-system to communicate with any other end-system
by computing a route between them
Hides idiosyncrasies of data link layer
Provides unique network-wide addresses
Found both in end-systems and in intermediate systems

Network layer types
 In datagram networks
 provides both routing and data forwarding
 In connection-oriented network
 separate data plane and control plane
 data plane only forwards and schedules data (touches every
byte)
 control plane responsible for routing, call-establishment, call-
teardown (doesn’t touch data bytes)

 Internet
 network layer is provided by Internet Protocol (IP)
 found in all end-systems and intermediate systems
 provides abstraction of end-to-end link
 segmentation and reassembly
 packet-forwarding, routing, scheduling
 unique IP addresses
 can be layered over anything, but only best-effort service
Network layer (contd.)

Network layer (contd.)
At intermediate systems
participates in routing protocol to create routing tables
responsible for forwarding packets
schedules the transmission order of packets
chooses which packets to drop

At end-systems
 primarily hides details of data link layer
 segments and reassemble
 detects errors

Transport layer
Reliable end-to-end communication.
creates the abstraction of an error-controlled, flow-controlled and
multiplexed end-to-end link
 (Network layer provides only a ‘raw’ end-to-end service)
Some transport layers provide fewer services
e.g. simple error detection, no flow control, and no retransmission
 Internet
 TCP provides error control, flow control, multiplexing
 UDP provides only multiplexing

Transport layer (contd.)
Error control
GOAL: message will reach destination despite packet loss, corruption and
duplication
ACTIONS: retransmit lost packets; detect, discard, and retransmit corrupted
packets; detect and discard duplicated packets
Flow control
match transmission rate to rate currently sustainable on the path to
destination, and at the destination itself
Multiplexes multiple applications to the same end-to-end
connection
adds an application-specific identifier (port number) so that receiving end-
system can hand in incoming packet to the correct application

Session layer
Not common
Provides full-duplex service, expedited data delivery, and session
synchronization
Internet
doesn’t have a standard session layer

Session layer (cont.)
Duplex
if transport layer is simplex, concatenates two transport endpoints
together
Expedited data delivery
allows some messages to skip ahead in end-system queues, by using a
separate low-delay transport layer endpoint
Synchronization
allows users to place marks in data stream and to roll back to a pre
specified mark

Presentation layer
Usually ad hoc
Touches the application data
(Unlike other layers which deal with headers)
Hides data representation differences between applications
characters (ASCII, Unicode, EBCDIC.)
Can also encrypt data
Internet
no standard presentation layer
only defines network byte order for 2- and 4-byte integers

Application layer
The set of applications that use the network
Doesn’t provide services to any other layer

Computer networks-3

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