Transcript Ch3_OSI_1
Protocols
Used for communications between entities in a system
Must speak the same language
Entities
User applications
E-mail facilities
Terminals
Systems
Computer
Terminal
Remote sensor
Protocol Cont
Protocol:
A set of rules and conventions
Governs how computers exchange information over a
shared/network medium.
Used for communications between entities in a system
Defines :
What is communicated
How it is communicated
When it is communicated
Varieties of communication protocols exist.
LAN protocols, WAN protocols, Network protocols,
Routing protocols etc.
Key Elements of Protocol
Syntax:
Semantics:
Refers to structure or format of the data (the order they
are presented)
Example - data format, coding
Refers to the meaning of each section of the bits
Example - data information, control information
Timing:
Refer to synchronous
When data should be sent
How fast they can be sent
Example sequencing, speed/delay matching
Protocol Architecture
Protocols in networking follows ‘layered’ architecture
Purpose of this layered architecture:
functions necessary for transfer of data are grouped into
layers.
a set of/each task(s) is separated from others.
several protocols at each layer, each suitable for a
different situation.
work that must be done in networking is broken down
into manageable units.
Protocol Architecture
Task of communication broken up into modules, for
example file transfer could use three modules
File transfer application
Communication service module
Network access module
A simplified architecture for file
transfer
Three-layered Model
Network Access Layer
Exchange of data between the computer and the
network
Sending computer provides address of destination
computer.
May invoke levels of service
Dependent on type of network used (LAN, packet
switched etc.)
Cont…
Transport Layer
Reliable data exchange
Independent of network being used
Independent of application
Application Layer
Support for different user applications
E.g. e-mail, file transfer
Protocols in simplified
architecture
Addressing Requirements
Two levels of addressing required
Each computer needs unique network address
Each application on a (multi-tasking) computer
needs a unique address within the computer
The service access point or SAP
The port on TCP/IP stacks
Protocol Data Unit
At each layer, protocols are used to communicate
Control information is added to user data at each
layer
Transport layer may fragment user data
Each fragment has a transport header added
Destination SAP
Sequence number
Error detection code
This gives a transport protocol data unit
Protocol Data Units
Network PDU
Adds network header
Network address for destination computer
Facilities requests
Standardized Protocol
Architectures
Required for devices to communicate
Vendors have more marketable products
Customers can insist on standards based equipment
Two standards:
OSI Reference model
TCP/IP protocol suite
Never lived up to early promises
Most widely used
Also: IBM Systems Network Architecture (SNA)
OSI Model
The OSI Model
A layered framework for network systems design
allows for communication across all types of
computer systems
consists of 7 layers
each layer defines a segment of process of moving
information
Layered Architecture
Application
Presentation
Session
Transport
Network
Data Link
Physical
Please Do Not Touch Steve’s Pet Alligator
Peer-to-Peer Processes
Application
Presentation
<---peer-to-peer process ---> Application
Presentation
provides services Session
for
Transport
Session
uses services
provided by
Network
Network
Data Link
Data Link
Physical
Physical
machine A
Transport
machine B
The OSI Environment
Functions of the Layers
Physical Layer
Transmits a bit stream over a physical link
Deals with the mechanical and electrical
specifications of the interface and transmission
medium
Specification Responsibilities : defines
interface the devices and the transmission medium
data rates : # of bits sent each second
synchronisation of bits : synchronises both side clocks
Physical Layer (Cont.)
Specification Responsibilities : defines (cont.)
Line configuaration :
concerns with the connection of devices to the medium
point-to-point configuration
multipoint configuaration
2 devices are connected together through a link
a link is shared between several devices
physical topology, defines
how devices are connected (mesh, star, etc.)
Physical Layer (Cont.)
Specification Responsibilities : defines (cont.)
Transmission mode, defines
direction of transmission between 2 devices
simplex, half-duplex, full-duplex
Data Link Layer
node-to-node delivery
fix the errors for the next layer
Specification Responsibility
framing
physical addressing
add headers (souce address, destination address)
flow control
devides the stream of bits into frames
prevent overflow transmission
error control
add mechanism for detecting damages
Data Link Layer
Specification Responsibility (cont.)
access control
for multipoint configuaration
determines which device has control over the link at any given
time
Responsible the delivery of packets between 2
systems on the same network link
Network Layer
Responsible for the source-to-destination
delivery of packet across mulitple network links
ensures that a packet gets from it origin to
destination
Specification Responsibilities
logicall addressing
add source/destination addresses
routing
Network Layer (cont.)
Responsible for end-to-end delivery of individual
packet
each packet is treated independently
gets the packet to the correct computer
Transport Layer
Responsible for source-to-destination (end-toend) delivery of entire message
ensure that the whole message arrives intact and in
order
Specification Responsibilities
service-point addressing
port address
gets the entire message to the correct program on the
computer
Transport Layer (cont.)
Specification Responsibilities (cont.)
segmentation and reassembly
devides message into segment (packet)
addes a sequence number to each segment
flow control
similar to the data link flow control
perform end to end (machine) rather than across a link
error control
makes sure that the entire message arrives at the receiver
without error (damage, loss, etc.)
retransmission mechanism
Session Layer
The network dialog controller
Establishes, maintains, synchronise the
interaction between communication systems
Specification Responsibilities
dialog control
agree to take either simplex, half-duplex, full-duplex mode
synchronisation
allows a process to add checkpoints (synchronisation
points in to a data stream)
Session Layer (Cont.)
synchronisation
a message of 1320 bytes
100
100
checkpoint
100
100
…...
Presentation Layer
Concerns with the syntax and semantics of
information exchanged between 2 systems
Specification Responsibilities
translation
character string
numbers, etc.
different computers use different encoding systems
Presentation Layer (Cont.)
Specification Responsibilities (cont.)
responsible for interoperability between these
different encoding methods
encryption
for sensitive information
encode/decode information
compression
reduce the number of bits to be transmitted
Application Layer
Enables the user (human/software) to access the
network
Specification Responsibilities
Applications : examples
network virtual terminal
to remote login to a remote machine
file transfer
mail services
TCP/IP Protocol Suite
Used in the Internet
Developed before the OSI Model
do not match exactly with OSI Model
5 layers
Application
Transport
Internetworking
Network
Physical
TCP/IP Protocol Suite (Cont.)
Two main protocols
Transmission Control Protocol
Internetworking Protocol
Many other protocols
Hierarchical protocol architecture made up for
interactive modules
each upper-level protocol is supported by one or
more lower-level protocols
TCP/IP Protocol Suite (Cont.)
At the Transport layer, TCP/IP defines two
protocols
TCP
UDP (User Datagram Protocol)
At the Network layer
IP
OSI vs TCP/IP
TCP/IP and the OSI Model
Application
Presentation
Session
Transport
Application
SMTP
FTP
DNS
SNMP NFS
TCP
ICMP IGMP
Network
TELNET
UDP
IP
ARP RARP
Data Link Protocols defined by the underlying
Physical networks
Addressing level
Level in architecture at which entity is named
Unique address for each end system (computer)
and router
Network level address
IP or internet address (TCP/IP)
Network service access point or NSAP (OSI)
Process within the system
Port number (TCP/IP)
Service access point or SAP (OSI)
Trace of Simple Operation
Process associated with port 1 in host A sends
message to port 2 in host B
Process at A hands down message to TCP to
send to port 2
TCP hands down to IP to send to host B
IP hands down to network layer (e.g. Ethernet)
to send to router J
Generates a set of encapsulated PDUs
PDUs in TCP/IP
Standard Organizations
ISO
ITU
ANSI
IEEE
State Department
EIA
Carriers
Other org.