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Chapter 3: Networking and Internetworking

From Coulouris, Dollimore and Kindberg

Distributed Systems: Concepts and Design

Edition 3, © Addison-Wesley 2001

Presentation based on slides by Coulouris et al;modified by Jens B Jørgensen and Jonas Thomsen, University of Aarhus

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Networks – basics

A network consists of: Transmission media (wire, cable, …). Hardware devices (routers, switches, …). Software components (protocol stacks, drivers, …). All together defines a communication subsystem.

Terminology: Host: Computers and other devices that use a network. Node: Any computer or switching device attached to a network. Subnet: Set of interconnected nodes

Unit of routing Collection of nodes that can be reached on the same physical network

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Networks – Design issues (1/2)

Performance (Network) Latency – time for one bit to traverse the network.

Software overheads, routing delays, load, physical distance

Data Transfer Rate – how many bits pr. sec. can traverse the network.

Physical characteristics of the network.

Overload lead to degrading performance. Scalability

Address range, routing tables. Reliability

The physical media is generally reliable (except for wireless) Low level vs. application level reliability (mostly application)

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Networks – Design issues (2/2)

Security A need to produce a secure network environment

Firewalls (protect resources inside, control the use of external resources, runs on a gateway)

Mobility Movement of mobile devices require addressing and routing

schemes to adapt. Quality of service

Meet deadlines, guaranteed bandwidth, bounded latencies Dynamically: Specify minimum acceptable qos. and desired

optimum. Multicasting

Typical communication in between pairs of processes Need for one-to-many communication (N x one-to-one is not

enough)

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Networks – types

LAN: Local Area Network No routing, shared bandwidth, low latency, speeds like disc access

Ethernet, token ring

WAN: Wide Area Network Different organization, long distance, routing, high latency

Telephone networks, dedicated links, internet backbone

MAN: Metropolitan Area Network Limited distance

xDSL, cable modem, fibre ADSL, StofaNet, Bolignet Aarhus, Djurslandsnet

Wireless Network WLAN (IEEE 802.11x) WPAN (infrared, Bluetooth) Mobile networks (GSM, GPRS, UMTS)

Internetworks Linking of different networks to provide common data communication

facilities, independent of involved components, routers / gateways

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Networks – Comparison of types

Range Bandwidth (Mbps) Latency (ms)

LAN 1-2 kms 10 – 1000 1 – 10WAN worldwide 0.010 – 10000 [1] 100 – 500MAN 2-50 kms 1 – 150 10Wireless LAN 0.15-1.5 km 2 – 54 [2] 5 – 20Wireless WAN worldwide 0.010 – 2 100 – 500Internet worldwide 0.010 – 2 100 – 500

[1]: OC-192 over ATM: http://newsroom.cisco.com/dlls/innovators/switching/eugene_wang_profile.html(OC-x (Optical Carier level x). OC-1 = 51,84 Mbps).

[2]: IEEE 803.11a:http://www.wi-fiplanet.com/tutorials/article.php/2109881

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Networks – packet transmission

Message: Sequence of data items of arbitrary length. Messages subdivided into packets of restricted length

Allocate sufficient buffer space Avoid occupying communication channel for long time during

large transfers. Switching schemes:

Broadcast Everything is transmitted to every node (non-switched Ethernet)

Circuit switching Plain Old Telephone System

Packet switching Store and forward

ATM / Frame relay Avoid switching delays, small packets (frames), switched after reception of a

few bytes (5 bytes in ATM)

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Protocols – basics

Protocol: Set of rules and formats to be used for communication between processes in order to perform a given task.

Should include specification of: Sequence of messages that must be exchanged. Format of the data in the messages.

Implemented by a pair of software modules in the sending and receiving computers.

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Protocols – layers

Layer n

Layer 2

Layer 1

Message sent Message received

Communicationmedium

Sender Recipient

Provides a service to the layer aboveExtends the service of the layer below

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Protocols – encapsulation and headers

Presentation header

Application-layer message

Session header

Transport header

Network header

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Protocols – the ISO Open Systems Interconnection (OSI) model

Application

Presentation

Session

Transport

Network

Data link

Physical

Message sent Message received

Sender Recipient

Layers

Communicationmedium

A framework for definition of protocols – not a definition for a specific protocol!

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Protocols – internetwork layers

Underlying network

Application

Network interface

Transport

Internetwork

Internetwork packets

Network-specific packets

MessageLayers

Internetworkprotocols

Underlyingnetworkprotocols

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Protocols – network layer routing (WAN)

Packet delivery: Datagram

One shot, different routes

Virtual Circuit ‘Call’ setup, same route during ‘call’

Don’t confuse with Connection oriented / less!!!

Routing algorithms Routers know next hop Adaptive routing (change route if links break)

Congestion Queues fill up, longer delays, dropped packets Control: Informing nodes of congestion, choke packet,

transmission control (TCP)

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Internet protocols – Internetworking

Internetwork: Network which integrates a number of different subnets.

Needs: Unified internetwork addressing scheme (Internet: IP

addresses) Protocol defining format of internetwork packets and

specifying rules for handling (Internet: IP protocol). Interconnecting components that route packets to

their destinations (Internet: Internet routers).

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Internet protocols – the TCP/IP protocol suite

Messages (UDP) or Streams (TCP)

Application

Transport

Internet

UDP or TCP packets

IP datagrams

Network-specific frames

MessageLayers

Underlying network

Network interface

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Internet protocols – encapsulation and headers

Application message

TCP header

IP header

Ethernet header

Ethernet frame

port

TCP

IP

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Internet protocols – IP

Internet Protocol.Transmits datagrams from one host to another,

if necessary via intermediate routers.Unreliable, best-effort delivery semantics.Address resolution: Conversion of Internet

addresses to network addresses (for a given network, e.g. ARP for Ethernet).

Routing: Each router in the Internet implements IP-layer software to provide a routing algorithm.

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Internet protocols – IP packet layout and addressing

7 24

Class A: 0 Network ID Host ID

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Class B: 1 0 Network ID Host ID

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Class C: 1 1 0 Network ID Host ID

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Class D (multicast): 1 1 1 0 Multicast address

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Class E (reserved): 1 1 1 1 unused0

dataIP address of destinationIP address of source

header

up to 64 kilobytes

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Internet protocols – TCP and UDP

Both: Process to process communication UDP features:

Transport-level replica of IP. No guarantee of delivery. No setup cost, no acknowledgement messages. Message size up to 64 Kbytes (8 Kbytes in praxis).

TCP features: Reliable delivery. Arbitrarily long sequences of bytes (stream). Connection-oriented. Mechanisms: Sequencing, flow control, retransmission,

buffering, checksum.

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Ethernet (IEEE 802.3) – Basics

Carriers Sense Multiple Access with Collision Detection (CSMA/CD)

Xerox – EthernetRandom Access

Stations access medium randomly

Contention Stations contend for time on medium

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Ethernet – ALOHA

Sender Go ahead and send! Retransmit if no ACK

Problems Collisions Low utilization (18%)

Slotted ALOHA is an improvement (max utilization 37%)

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Ethernet – CSMA

Carrier Sense Multiple Access Observations

Propagation time is much less than transmission time All stations know that a transmission has started almost

immediately Sender

First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Wait reasonable time Retransmit if no ACK

Max utilization depends on propagation time (medium length) and frame length

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Ethernet – CSMA/CD

Carrier Sense Multiple Access – Collision Detection Observation: With CSMA, collision occupies medium for

duration of transmission Sender

If medium idle, transmit If busy, listen for idle, then transmit Station listens whilst transmitting If collision detected, transmit jam signal, then cease

transmission

After jam, wait random time then start again Binary exponential back off

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Ethernet –CSMA/CDOperation

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Ethernet – Collision Detection

Bus Collision produces much higher signal voltage than

signal Collision detected if cable signal greater than single

station signal

Star Activity on more than one input is collision Special collision presence signal

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Summary

Networks.Protocols. Internet protocols (TCP/IP).Ethernet