Data Communication and Computer Networking, Lecture notes of Data Communication Systems and Computer Networks

Download Data Communication and Computer Networking and more Lecture notes Data Communication Systems and Computer Networks in PDF only on Docsity!

Computer Networks

Chapter 2: Protocols and the

TCP/IP Protocol Suite

Protocols

• Cooperative action is necessary — computer networking is not only to exchange bytes — huge system with several utilities and functions. For examples - error detection - Encryption - Routing - etc.
• For proper communication, entities in different systems must speak the same language — there must be mutually acceptable conventions and rules about the content, timing and underlying mechanisms
• Those conventions and associated rules are referred as “PROTOCOLS”

A Real World Example to Protocol Architecture philosopher-translator-secretary architecture Issues:

• peer-to-peer protocols are independent of each other —for example, secretaries may change the comm. medium to email —or the translators may agree on using another common language
• Each layer adds a header

Simplified File Transfer

Architecture

File Transfer Application Layer: Application specific commands, passwords and the actual file(s) – high level data Communications Service Module: reliable transfer of those data – error detection, ordered delivery of data packets, etc. Network Module: actual transfer of data and dealing with the network – if the network changes, only this module is affected, not the whole system

A General Three Layer Model

• Generalize the previous example for a generic application — we can have different applications (e-mail, file transfer, …)
• Network Access Layer
• Transport Layer
• Application Layer

Network Access Layer

• Exchange of data between the computer and the network
• Sending computer provides address of destination — so that network can route
• Different switching and networking techniques — Circuit switching — Packet switching — LANs — etc.
• This layer may need specific drivers and interface equipment depending on type of network used.
• But upper layers do not see these details — independence property

Application Layer

• Support for different user applications

• e.g. e-mail, file transfer

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 number in TCP/IP protocol stack

Protocol Data Units (PDU)

• User data is passed from layer to layer

• Control information is added/removed to/from

user data at each layer

—Header (and sometimes trailer) —each layer has a different header/trailer

• Data + header + trailer = PDU (Protocol Data

Unit)

—This is basically what we call packet

—each layer has a different PDU

Transport PDU

• Transport layer may fragment user data

• Each fragment has a transport header added

—Destination port —Sequence number

• since the transport layer may split application data into smaller packets —Error detection code (generally at trailer)

Operation of a Protocol

Architecture

Transport Header Network Header Network Header Transport Header (Network PDU)

Standard Protocol

Architectures

• Common set of conventions

• Nonstandard vs. standard protocols

—Nonstandard: K sources and L receivers lead to K*L different protocols —If common protocol used, we design only once

• Products from different vendors interoperate

—Customers do not stick to a specific vendor —If a common standard is not implemented in a product, then that product’s market is limited; customers like standard products

OSI Reference Model

• Open Systems Interconnection (OSI)

• Reference model

—provides a general framework for standardization —defines a set of layers and services provided by each layer —one or more protocols can be developed for each layer

• Developed by the International Organization for

Standardization (ISO)

—also published by ITU-T (International Telecommunications Union)

OSI Reference Model

• A layered model

—Seven layers – seven has been presented as the optimal number of layer

• Delivered too late (published in 1984)!

—by that time TCP/IP started to become the de facto standard

• Although no OSI-based protocol survived, the

model is still valid (in the textbooks)

—For Data Link Layer (that we will see later) OSI protocols are still valid