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CS 5523 Lecture 4:
Network Overview
] Questions on Laboratory 1 ] Catch up from Lectures 2 and 3 ] Questions from Chapter 2 ] Emphasis of network overview ] Network models ] Overview of routing
Discussion Questions from CDK Chapter 2
[CDK 2.4]
A search engine is a web server that responds to client requests to search in its stored indexes and (concurrently) runs several web crawler tasks to build and update the indexes. What are the requirements for synchronization between these concurrent activities?
Discussion Questions from CDK Chapter 2
[CDK 2.9]
Distinguish between buffering and caching.
Discussion Questions from CDK Chapter 2
[CDK 2.12]
For each of the factors that contribute to the time taken to transmit a message between two processes over a communication channel, state what measures would be needed to set a bound on its contribution to the total time. Why are these measures not provided in current general-purpose distributed systems?
Napster versus Gnutella
] What services do they provide? ] Do they work the same way? ] In what sense are they peer-to-peer? ] What architectural models do they use?
If you are interested in reading more ---
] Peer-to-peer: Harnessing the Power of Disruptive Technologies ed. by Andy Oram, O’Reilly Press, 2001. ] www.napster.com ] www.gnutella.com
Napster and Gnutella are “peer-to-peer” applications.
What are we going to emphasize?
] This is not a networks course ] Quick run-through of CDK Chapter 3 ] CDK Chapter 3 provides some networking terminology ] Emphasize services built on TCP and UDP ] What are consequences to the application?
Basic terminology:
] Communication subsystem
] Host - devices that use a network for communication
] Node - any computer or switching device on a network
] Subnet - (CDK’s definition) set of interconnected nodes that employ same technology to communicate among themselves.
(Not a standard definition of subnet - which usually denotes an organization of IP addresses and a physical location for routing.)
What are the network issues?
Why are these network issues important for operating systems?
] Performance ] Scalability ] Reliability ] Security ] Mobility ] Quality of service ] Multicasting
Implementation details:
] Packet transmission
] Switching schemes
] Protocols
] Routing
] Congestion control
] Internetworking
Switching schemes:
] Broadcast
] Circuit switching
] Packet switching
] Frame relay
What are these schemes and how do they differ?
Protocols:
CDK:
… a well-known set of rules and formats to be used for communication between processes...
Protocols specify both data format and the exchange mechanism.
What is the difference between a protocol layer and a protocol suite?
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 3.2 (CDK) Conceptual layering of protocol software
Layer n
Layer 2 Layer 1
Message sent Message received
Communication medium Sender Recipient
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 3.5 (CDK) OSI protocol summary
Layer Description Examples Application Protocols that are designed to meet the communication requirements of specific applications, often defining the interface to a service. HTTP,CORBA IIOP^ FTP^ , SMTP, Presentation Protocols at this level transmit data in a network representation that is independent of the representations used in individual computers, which maydiffer. Encryption is also performed in this layer, if required. Secure Sockets (SSL),CORBA DataRep. Session At this level reliability and adaptation are performed, such as detection of failures and automatic recovery. Transport This is the lowest level at which messages (rather than packets) are handled. Messages are addressed to communication ports attached to processes,Protocols in this layer may be connection-oriented or connectionless. TCP, UDP Network Transfers data packets between computers in a specific network. In a WAN or an internetwork this involves the generation of a route passing throughrouters. In a single LAN no routing is required. IP, ATM virtual circuits Data link Responsible for transmission of packets between nodes that are directlyconnected by a physical link. In a WAN transmission is between pairs of routers or between routers and hosts. In a LAN it is between any pair of hosts.
Ethernet MAC,ATM cell transfer, PPP Physical The circuits and hardware that drive the network. It transmits sequences ofbinary data by analogue signalling, using amplitude or frequency modulation of electrical signals (on cable circuits), light signals (on fibre optic circuits)or other electromagnetic signals (on radio and microwave circuits).
Ethernet base- bandsignalling, ISDN
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 3.6 (CDK) Internetwork layers
Underlying network
Application
Network interface
Transport
Internetwork Internetwork packets
Network-specific packets
Message Layers
Internetwork protocols
Underlying network protocols
Packet assembly:
] Transport layer usually handles assembly and reassembly ] Packets consist of a header and a data. ] If the data > MTU (maximum transfer unit), must be divided into multiple packets. ] A transport address is network address of host + port number.
Packet delivery:
Two approaches to delivery in the network layer ] Datagram delivery ] Virtual circuit packet delivery Are these the same as connectionless and connection-oriented protocols?
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 3.7 (CDK) Routing in a wide area network
Hosts (^) Links or local networks
A
D E
B
C
1 2
5
3 4
6 Routers
Internetworking terminology:
] router ] bridge ] hub ] switch ] tunnel
How do these differ?
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 3.10 (CDK) Simplified view of the QMW Computer Science network
file
compute
dialup
hammer
henry hotpoint
138.37.88. 138.37.88.
bruno138.37.88.
router/^ sickle
138.37.95.240/29 138.37.95.
138.37.95.
copper138.37.88.
firewall
web
138.37.95.248/
server
desktop computers 138.37.88.xx
subnet
subnet
Eswitch
138.37. server server server
138.37.88.
138.37.94.246^ custard
desktop computers
Eswitch
138.37.
hub hub
Staff subnet Student subnet
otherservers
firewallrouter/ 138.37.94.
☎
Eswitch: Ethernet switch^ 1000 Mbps Ethernet
100 Mbps Ethernet
file server/gateway printers
Campusrouter
Campusrouter
138.37.94.xx
Instructor’s Guide for Coulouris, Dollimore and Kindberg© Addison-Wesley Publishers 2000 Distributed Systems: Concepts and Design Edn. 3
Figure 3. Tunnelling for IPv6 migration
A IPv6^ IPv6 B
IPv6 encapsulated in IPv4 packets
Encapsulators
IPv4 network
