1

ITEC801Distributed Systems

Protocols

Coulouris chapter 3

ITEC 801 Protocols 2

Important Things

Things to get from this set of slides What is a protocol? What are protocols for? Why we have many protocols What is the difference between connection-

oriented and connectionless

ITEC 801 Protocols 3

Protocols

Inter-process communication is at the heart of distributed systemsIPC depends on protocolsa protocol is simply the rules by which the communication is carried outa network communications protocol defines all the messages (and their ordering) for a communication

ITEC 801 Protocols 4

Protocol

Protocol definitions include Error detection Message length Message structure etc

Protocol implementations may be in hardware or software

ITEC 801 Protocols 5

How Do Protocols Work?

Transmission of data over network broken down into discrete systematic steps

At each step, certain actions take place

Each step has its own rules and procedures which cannot take place at any other step

ITEC 801 Protocols 6

Protocol Functioning

Steps must be carried out in a consistent order on every computer in the network Sending Computer: Steps carried out-Top

down. Receiving Computer: Steps carried out

bottom-up.

ITEC 801 Protocols 7

Protocols

ITEC 801 Protocols 8

Protocol

There are many protocols

Each protocol has a different purpose and accomplishes different task

Several protocols work together in what is known as a Protocol Stack

Levels of the protocol stack map on to layers of the OSI model

ITEC 801 Protocols 9

Layered Protocols

Computer network protocols are layered

each layer has its own function

for example: Network level: IP Transport level: TCP. Application level

RPC Remote method Invocation (RMI) Message oriented Middleware (MOM)

ITEC 801 Protocols 10

Internet Protocols (some)

IP ICMP

TCP UDP

Network

Transport

Teln

et

FTP

HTTP

Use

r app

s

SN

MP

TFT

P

Use

r app

s

UpperLayers

ITEC 801 Protocols 11

Need for a Protocol Stack

Require a high degree of cooperation between two computer systems that communicate

Activities include: Path determination/conveying control information

to network Ascertaining destination status Ascertaining destination application status Determining format

ITEC 801 Protocols 12

Need for a Protocol Stack

Single module versus multiple modules.

We could implement all communication in a single piece of software

too unwieldy and hard to maintain

ITEC 801 Protocols 13

Internet Protocol Stack: TCP/IP

Standard routable enterprise networking protocol

Technology for connecting dissimilar systems

ITEC 801 Protocols 14

TCP/IP layers:

Application: user interface to Internet.Transport: Transmission Control Protocol (TCP): Connection

oriented reliable communications for applications. User Datagram Protocol (UDP): connectionless

Network: Internet Protocol: For routing and addressing. (RFC 791)Network Interface layer: Inserting/removing frames from network.

ITEC 801 Protocols 15

OSI Reference Model

Defines seven layersPhysical: Concerned with transmission of unstructured bit stream over physical medium.Data link: Reliable transmission of information across physical link.Network: Provides upper layer independence from data transmission/switching technologies.

ITEC 801 Protocols 16

OSI Reference Model

Transport: reliable transfer of data between end points.Session: Control structure for communication between application.Presentation: Independence to application process from data representation.Application: Access to environment.

ITEC 801 Protocols 17

OSI Reference Model

ITEC 801 Protocols 18

Connection-Oriented versus Connectionless

Protocols can be connection–oriented OR Connectionless

Connection oriented: A logical connection is set up between sender and receiver before they actually communicate

Connectionless: Sender and receiver communicate without establishing a logical connection

ITEC 801 Protocols 19

Connection-Oriented

Setup phase when the connection is created

Communication phase during which the communications happens

Pull-down phase when the connection is terminated

ITEC 801 Protocols 20

Protocol Terminology

Entity: active elements in each layer

Peer Entities: Entities in same layer but on different machines

Entities in layer N implement a service used by layer N+1 N layer: Service provider N+1 layer: service user

ITEC 801 Protocols 21

Protocols

Each layer provides service at an access point : SAP (Service Access Point) Each SAP has a unique address that

identifies it.

Agreed set of rules at Interface between any two layers

ITEC 801 Protocols 22

Protocols

Information block passed by layer N+1 to layer N through SAP: Interface Data Unit (IDU)IDU Block= SDU (Service Data Unit) + Control (ICI)ICI: Interface Control InformationSDU information passed along network to peer entity up to layer N+1

ITEC 801 Protocols 23

Protocols

Control: some guidelines for layer N to do its Job.

Processing on SDU by layer N results in PDU (Protocol Data Unit)

PDU = SDU+ Header

PDUs generated at different layers given a different name

ITEC 801 Protocols 24

ICI SDU

SAP

ICI SDU

SDUHeader N-PDU

Layer N+1

Interface

Layer N

Relation Between Layers at an Interface