Post on 21-Dec-2015
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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 – 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