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CS 408

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

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)