Transcript Chapter 2 Protocols and Architecture

Term Paper
• Phase-I is Due on Tuesday, February 19 in
class timing
—Submit a Hard Copy of your paper (MS word
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• GA for CPEG-471 Office hours (Faraz Arain)
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—Thursday (12:00 to 2:30 Pm)
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William Stallings
Data and Computer
Communications
Chapter 2
Protocols and Architecture
Need For Protocol Architecture
• E.g. File transfer
—Source must activate comms. Path or inform network
of destination
— Source must check destination is prepared to receive
— File transfer application on source must check
destination file management system will accept and
store file for his user
— May need file format translation
• Each problem needs to address separately
• Task broken into subtasks
• Implemented separately in layers in stack
Key Elements of a Protocol
• Syntax
—Data formats
—Signal levels
• Semantics
—Control information
—Error handling
• Timing
—Speed matching (e.g., indicates flow control)
—Sequencing (e.g., avoid packet duplication)
Protocol Architecture
• Implementation of File Transfer
—File transfer example can have 4 problems
—Implementation of “File Transfer” could use three
modules
• File transfer application (Top most layer)
• Communication service module (Middle Layer)
• Network access module (Lower Layer)
Implementation of File Transfer
Architecture
Module-1 Determine weather file transfer app at sys Y is ready to
receive file?
Layer1  Determine weather file translation is required or not?
Module-2 Determine weather Computer Y is ready to receive the Data?
Module-3 Determine weather a direct link or an indirect link exists between X & Y
A Three Layer Model
1. Network Access Layer
2. Transport Layer
3. Application Layer
A Three Layer Model
• Network Access Layer
—Exchange of data between the computer and the
network
—Sending computer provides address of destination
—May invoke levels of service (i.e., QoS parameters)
—Dependent on type of network used (packet or CKT
switched Network etc.)
• Transport Layer
—Ensures reliable data exchange between 2 hosts
• Application Layer
—Support for different user applications
—e.g. e-mail, file transfer
Protocol Architectures and
Networks
 Port
 IP Address
A Three Layer Architecture
Destination SAP
SAP-1
SAP-2
Y+ SAP-2
Y+ SAP-2
Y
Y
Protocol Data Units
Message-1
Protocol data unit (PDU)
Message-2
Operation of a Protocol
Architecture
Open Systems
Interconnection
(OSI)
OSI  Definition
• Open Systems Interconnection
• Developed by the International Organization for
Standardization (ISO)
• It has seven layers
• TCP/IP is the refinement of OSI
• Questions
— Why TCP/IP doesn’t have presentation & session layers?
— How many layers a reference model should have ?
OSI – Advantage of 7 Layers Model
• Each layer performs a subset of the required
communication functions
— Sub tasks are assigned to one of the layers depending on the nature
• Each layer relies on the next lower layer to perform more
primitive functions
— such as application layer relies on transport layer to send the data and
commands..
• Each layer provides services to the next higher layer
— Such as Network layer offer services to Transport layer that can be
used at N/W-Layer SAP
• Changes in one layer should not require changes in other
layers
— Layers are entirely independent and transparent to other layers
The OSI 7-Layers functioning
OSI as Framework for
Standardization
Services are well defined:
(Service Definition)
Protocols are well defined:
(Protocol Specification)
Service Primitives and
Parameters
• Services need to be well defined by means of
service definition
• Services between adjacent layers expressed in
terms of primitives and parameters
• Primitives specify function to be performed
—Just like Op-code in Assembly
• Parameters pass data and control info
—Just like Op-rand in Assembly
TCP/IP Protocol Architecture
•
TCP/IP was originally developed by the US
Defense Advanced Research Project Agency
(DARPA) for its packet switched network
(ARPANET)
•
5 Layers of TCP/IP.
1.
2.
3.
4.
5.
Application Layer
Transport Layer or Host to host
Internet Layer or Network Layer
Network Access Layer or Data Link Layer
Physical Layer
1- Physical Layer
• Responsible to deal with interface between
source & destination
• Physical interface between data transmission
device (e.g. computers) and transmission
medium or network
• Characteristics of transmission medium
• Signal levels
• Data rates
• etc.
2- Network Access Layer or
Data Link Layer
• Exchange of data between end system and the network
• Responsible to deliver frames reliably from hop to hop (hop could be
DLL device such as bridges or switches)
• If within the LAN, upper layers can leave reliability issues (error/flow
control) on the DLL
• Guarantee Error Free Delivery of Data from one hop to the
other (not N/W to N/W OR not end-to-end)
• DLL is divided into 2 sub layers
LAN-1
LAN-2
Host A
Host C
DLL
DLL
Host B
Router
Network
Router
DLL
Host D
3- Internet Layer (IP) or
Network Layer
• Systems may be attached to different networks (such as Host-A at
LAN-1 & Host-D at LAN-2)
• Lowest layer that deals with end-to-end transmission
• Implemented in end systems and router
• Guarantee Error Free Delivery of Packets from one N/W to
the other N/W (not end-to-end)
LAN-1
LAN-2
Network
Host A
Host C
R
R
DLL
DLL
Router-1
Router-2
R
DLL
R
Host B
N/W Layer
N/W Layer
Host D
Layer 4 and Layer 5
• Layer 4 Transport Layer
— Guarantee Error Free Delivery of Message from sourcehost to the destination-host (End-to-End reliability)
— Offers connection oriented and connection less services
— Reliability includes: Error detection and correction, flow control,
packet duplication etc…
— Runs only on host not on the network
• Layer 5  Application Layer
—Provide interface between end-user & applications
—Support several users applications
• For example: FTP, Telnet, SSH, SMTP
OSI v TCP/IP
TCP and UDP
• TCP (connection oriented Protocol)
— Establish a reliable logical connection between the source & the
destination hosts
— Need to know the source & destination port addresses
— Logical connection is monitored by TCP
— TCP segments (TCP PDU) can be transmitted through the secure
connection
— TCP is reliable but allow relatively slower communication
• UDP (Connection Less Protocol)
— Is a connection less protocol
— Does not guarantee the reliable transmission of UDP segments
— UDP header has limited control information
— Faster but not reliable at all
• Which protocol should I use then (TCP or UDP ???)
TCP/IP Concepts
PDUs in TCP/IP
Some Protocols in TCP/IP Suite
Required Reading
• Stallings chapter 2
• Comer,D. Internetworking with TCP/IP volume I
• Comer,D. and Stevens,D. Internetworking with
TCP/IP volume II and volume III, Prentice Hall
• Halsall, F. Data Communications, Computer
Networks and Open Systems, Addison Wesley
• RFCs