Telecommunication Protocols Laboratory Course

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March 4, 2004 http://www.abo.fi/~lpetre/teleprot/teleprot.html 1

Telecommunication Protocols Laboratory Course

Lecture 1

G721 (KTF)

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Behind the name

• Telecommunication: the science and technology of

transmitting information (words, sounds, images) over great distances, in the form of electromagnetic signals (telegraph, telephone, radio, TV)

• In short: communication at a distance

• Protocol: a set of conventions governing the treatment and especially the formatting of data in an electronic

communications system

=> underlying framework : COMPUTER NETWORKS

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Preliminaries

• 2 credits, 20h–course, weeks 10-19, Thu from 8:30 to 10 AM

• Goal of the course: to understand the

specifics of telecommunication protocols

• How to get there

– Simulate protocols

– Teams formed, of 2-4 persons

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Preliminaries 2

• How to get to the goal

– Each team chooses their protocols and simulate them in whatever framework they agree upon – The result has to work

– User interface not essential but not neglected

either

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Other details

• Lectures 1-6: teaching, forming groups, choosing protocols, start implementing

• Remaining time: implementation and feedback

• Individuals are graded based on the log

• There is no exam

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Networks

• Definition

– autonomous computing systems (NODES) are interconnected

• Result

– resources and information are accessible, independently of their physical location

• Advantages

– resource sharing and redundancy – parallel processing

⇒ improved reliability, availability, and performance

• Tradeoff

− increased complexity

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Network taxonomy

• Criterion: transmission technology

– Broadcast links – Point-to-point links

• Criterion: scale (geographical extent)

– Very local area networks (personal area network) – Local area networks LAN

– Metropolitan area networks MAN – Wide area networks WAN

– Internetworks

• Criterion: organization

– Private networks – Public networks

• Other criterions

– Wireless networks – Home networks

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Networks for Communication

• Basic form of communications

– shared memory – message exchange

• Networks are loosely coupled, they are links w/o memory

⇒ there is no shared memory

⇒ applications residing on different nodes cooperate ONLY by passing messages to each other

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Communication

Application Application

Network

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Network Layers

• Networks organized as stacks of layers

• Each layer

– is built upon the one below it

– offers certain services to the higher layers

– shields higher layers from the implementation of the services

• The number, contents, functions and names of layers depends on the network

• Layer n on one machine communicates with layer n on another machine

– Rules and conventions used in this: layer n protocol

– Entities comprising the corresponding layers on different machines:

peers (processes, hardware devices, humans)

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Interfaces

• Between each pair of adjacent layers

• Define which primitive operations and services the lower layer makes available to the upper one

• Clean interfaces between layers -> an important issue for network designers

⇒each layer performs a specific collection of well-understood functions

• Clear-cut interfaces

• Make it simpler to replace the implementation of one layer with a completely different implementation

• Minimize the amount of information that must be passed between layers

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Layers, protocols, and interfaces

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Network architecture

• Is a set of layers and protocols

• The spec of the architecture should contain enough

information for building layers that obbey the protocols

• Interfaces and details of implementation are not part of the architecture

• Protocol stack Æ list of protocols, one per layer, used by a certain system

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Protocol Stack 1

I like rabbits Location A

3

2

1

3

2

1 Location B

Message Philosopher

Translator

Secretary Information

for the remote translator

Information for the remote secretary L: Dutch

Ik vind konijnen leuk

Fax #--- L: Dutch Ik vind konijnen leuk

J'aime bien les lapins

L: Dutch Ik vind konijnen leuk

Fax #--- L: Dutch Ik vind konijnen leuk

The philosopher-translator-secretary architecture.

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Protocol Stack 2

Example information flow supporting virtual communication in layer 5.

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Design Issues for the Layers

• Addressing

– Layers need to identify senders and receivers

• Data Transfer Rules

– Directions of data travel, logical channels

• Error Control

– Many error-detecting and error-controlling codes exist – The peers have to agree on which ones are used

– The receiver has to inform the sender about correctly received messages

• Flow Control

– Keep a faster sender from swamping a slow receiver

• Arbitrarily long messages

• Multiplexing and demultiplexing

– Using the same connection for multiple, unrelated communications

• Routing

– If multiple paths between source and destination exist, a route must be chosen

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Types of services

• Layers offer 2 types of services to the layers above them

– Connection-oriented – Connectionless

• Connection-oriented network service

– Service user establishes connection, uses it, then releases it – Connection Æ tube

– Negotiation between sender, receiver, subnet on parameters such as: maximum message size, quality of service, etc

• Connectionless network service

– Each message carries the full destination address and is routed through the system independently of all the others

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Types of services 2

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Quality of service

• Usually

– refers to reliability of services (not losing data)

– implemented by having the receiver acknowledge the receipt of each message so the sender is sure it arrived

– acknowledgement process introduces overhead and delays

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Service primitives

• A service is formally specified by a set of primitives (operations) available to a user process to access the service

(Primitives for implementing a simple connection-oriented service)

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Service primitives 2

Packets sent in a simple client-server interaction on a connection-oriented network.

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Services and protocols

• Service

– Set of primitives that a layer provides to the layer above it

– Defines what operations the layer is prepared to perform on behalf of its users

– Relates to an interface between 2 layers: service provider and service user

• Protocol

– Set of rules governing the format and meaning of packets (messages) that are exchanged by peer entities within a layer – Implements service definitions

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A layer with services and protocols

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Reference models

• Network architectures

– OSI reference model – TCP/IP reference model

• OSI

– The associated protocols are not really used – The model is quite general and still valid

– Features discussed at each layer are still important

• TCP/IP

– The model is not of much use – The protocols are widely used

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OSI/ISO reference model

• Based on a proposal of ISO (International Standards Organization)

• Intended as a first step toward international standardization of protocols used in the various layers (1983, revised 1995)

• Called ISO/OSI (Open Systems Interconnection)

– It deals with connecting open systems (systems open for communication with other systems)

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OSI model

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OSI model principles

• A layer should be created where a different abstraction is needed

• Each layer should perform a well-defined function

• The function of each layer should be chosen so that

internationally standardized protocols can be then defined

• The layer boundaries should be chosen to minimize the information flow across interfaces

• The number of layers should be large enough that distinct functions are not thrown together in the same layer and small enough so that architecture does not become unhandy

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Physical layer

• Concerned with transmitting raw bits over a communication channel

• When a 1-bit is sent, a 1-bit has to be received, not a 0-bit

• Typical questions

– How many volts used to represent a 1, how many for a 0 – How many ns a bit lasts

– Whether transmission can proceed simultaneously in both directions – How the initial connection is established and how is it then torn down – How many pins the network connector has and what are their uses

• Design issues deal with

– Mechanical, electrical and timing interfaces – Physical transmission medium

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Data Link Layer

• Transforms a raw transmission facility into a line that

appears free of undetected transmission errors to the above network layer

• Obliges the sender to break up input data into data frames (few hundred or thousands bytes each) and transmit frames sequentially

• If service reliable => receiver confirms correct receipt of each frame by sending back an acknowledgement frame

• Flow control regulations and error handling are integrated

• Medium access control sublayer

– Controls access to a shared channel (for broadcast networks)

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Network layer

• Controls the operation of a subnet

• Key design issue: how are packets routed from source to destination

• Routes: static / determined at intialization / highly dynamic

• Controls the bottlenecks in the subnet

• Handles quality of service issues: delay, transit time, jitter, etc

• Compatibility among different networks (addressing, max message sizes, different protocols)

• Broadcast networks: this layer is very thin

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Transport layer

• Accepts data from the above layers and manipulates it, ensuring that it arrives correctly at the destination

• Efficiency is important

• Hiding the hardware from the above layers also important

• When a connection is established, the type of service is determined, e.g.

– Error-free, point-to-point channel – Transporting isolated messages – Broadcasting

• True end-to-end layer

– a program on the source machine carries on a conversation with a similar program on the destination machine using message headers and control messages

– In contrast, layers 1-3 are chained

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Session layer

• Allows users on different machines to establish sessions between them

• Session services

– Dialog control: keeping track of whose turn it is to transmit

– Token management: preventing 2 parties from attempting the same critical operation at the same time

– Synchronization: check-pointing long transmissions to allow them to continue from where they were after a crash

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Presentation layer

• Concerned with the syntax and semantics of the transmitted information

• Data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used on the wire

– Useful for computers with different data representation

• Manages these abstract data structures and allows higher-level data structures to be defined and exchanged

– E.g. banking records

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Application layer

• Contains a variety of protocols commonly needed by users

• E.g.

– HTTP – basis for WWW – File transfer

– Electronic mail – Network news

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TCP/IP reference model

• US Department of Defense sponsored a research network called ARPANET (1960s) that needed ultimately to

interconnect with satellites and radio networks Î a reference architecture was needed

• Major goal: the ability to connect multiple networks in a seamless way

• Another major goal: the ability to survive loss of subnet hardware with existing conversations not being broken off

• Also: flexible architecture

⇒a packet-switching network, based on a connectionless internetwork layer was conceived

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TCP/IP model

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Internet layer

• Connectionless internetwork layer that holds TCP/IP architecture together

• It permits hosts to inject packets into any network and have them travel independently to the destination (in possibly another

network)

• Defines an official packet format and protocol called IP (internet protocol)

• It delivers IP packets to their destination

• Major issues: packet routing and avoiding congestion

• Similar in functionality to OSI network layer

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Transport layer

• Allows peer entities on the source and destination hosts to carry on a conversation

• Similar to the OSI transport layer

• 2 end-to-end transport protocols are defined here:

– TCP (transmission control protocol) – UDP (user datagram protocol)

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TCP

• Reliable connection oriented protocol

– A byte stream originating on one machine is delivered w/o error on any other machine in the internet

– It fragments the byte stream into discrete messages and passes each one to the internet layer

– At destination the receiving TCP process reassembles the received messages into the output stream

– Also handles flow control

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UDP

• Unreliable, connectionless protocol

• Used for applications not desiring TCP’s sequencing or flow control but wishing to provide their own

• Widely used also for one-shot, client-server type request- reply queries and applications

– Prompt delivery more important than accurate delivery

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TCP/IP Other Layers

• Application layer - contains all the higher level protocols

– TELNET, FTP, SMTP, DNS, NNTP, HTTP

• Host-to-network layer

– Not really specified

– The host should connect to the network (with some protocol) so that it could send IP packets to it

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TCP/IP Protocols and Networks

(initially)

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Similarities between OSI and TCP/IP reference models

• Both are based on the concept of a stack of independent protocols

• Functionality of layers is roughly similar

– Layers up through and including the transport layer provide an end- to-end, network-independent transport service to processes wishing to communicate

– These layers are application-oriented users of the transport service

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Central concepts to OSI

• Service: what the layer does; it defines the layer’s semantics

• Interface: tells the processes above it how to access it

– It specifies the parameters and expected results

• Protocol: implements the services

– Peer protocols used in a layer are the layer’s own business

– The layer can use any protocols as long as it gets the job done (provides the offered services)

– The layer can also change the protocols at will without affecting software in higher layers

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Protocols and models

• OSI model was devised before the corresponding protocols

– The model was not biased toward one set of protocols

– Designers did not have experience and did not know well what to put in which layer

• TCP/IP model was devised just as a description of the existing protocols

– The protocols hence fit perfectly into the layers

– The model does not fit any other protocol stack => not useful for describing other, non-TCP/IP networks

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Other differences

• Except for the (inter)network, transport and application layers, other layers are different in OSI and TCP/IP

• Connection vs connectionless services

– OSI supports both in the network layer but only connection- oriented in the transport layer

– TCP/IP has only connectionless services in the internet layer but supports both in the transport layer

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A Critique of the OSI Model and Protocols

Why OSI did not take over the world

– Bad timing

– Bad technology

– Bad implementations – Bad politics

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Bad Timing

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A Critique of the TCP/IP Reference Model

Problems:

• Service, interface, and protocol not distinguished

• Not a general model

• Host-to-network “layer” not really a layer

• No mention of physical and data link layers

• Minor protocols deeply entrenched, hard to replace

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Telecommunication Protocols Laboratory Course