Operating Systems Lecture Notes

4 April 2012 • Networking Background

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Outline

• Networking
• Network structure, LANs and WANs.
• Why we network.
• Networking issues and concepts.
• ISO & IETF networking models.
• Networks, OSs, wires, and middleware.

Networking

• A network is an undirected graph (more likely: a hypergraph).
  • Nodes are hosts.
  • Edges are links.
• Adjacent hosts exchange data over the links.
• Connected hosts exchange data via a path through intermediate hosts.
• Networks are dynamic in hosts and links, some more than others.

Networking Illustrated

Network Structure

• Network structure can be classified along many dimensions.
• One important dimension is maximal distance.
  • Local-area networks: buildings and smaller.
  • Wide-area networks: cross country or countries.
  • Global networks: the Internet.
• There are lots of other divisions.

LAN Structure

• LANs typically have a simple linear, tree, or ring structure.

  

WAN Structure

• WANs have a much more fragmented structure.

  

Why Bother?

• Resource sharing.
  • I’ve got a file, you’ve got a printer.
• Computational speed-ups; load sharing.
  • Split a complex, expensive problem into pieces.
• Reliability; redundant systems.
• Communication and coordination.

Why We Network

Networking Issues

• Important issues in networking:
  • Naming and name resolution: what are hosts called, and where are they?
  • Routing: how do messages get through the network?
  • Switching: Circuit, message, or packet switching?
  • Contention: managing the shared network efficiently.

Where’s the OS?

• Naming and name resolution: mostly external libraries, including DNS servers.
• Routing: not at the edge (hosts), mostly in the middle (network processors).
• Switching: A decision made for the OS and abstracted away early.
• Contention: low-level in the hardware; high-level in the software (protocols).

Networking Concepts

• The International Standards Organization (ISO) defined a seven-level conceptual architecture for networking.

  The Open System Interconnection (OSI) model. Physical level 1 is closest to the wire. Application level 7 is furthest from the wire.

Level Examples

7	Application — useful operations (HTTP).
	Presentation — represent data (XML).
5	Session — end-point connections (RPC).
	Transport — group the networks (TCP/IP).
3	Network — group the hosts (AppleTalk).
	Link — read & write the wire (HDLC).
1	Physical — drive the wire (802.11n).

ISO Networking Model

ISO v IETF

Networks in OSs

• Adding a network to an operating system produces (in decreasing complexity):
  • Distributed operating systems: location transparency, rare.
    • Is this even possible?
  • Networked operating systems: location aware, most of what there is.
• In either case the operating system acts as a network host.

Networks as Wires

• Replace intra-system connections between components with networks.
  • NAS and SAN or disks and file systems.
  • X Window System for display-keyboard-mouse.
  • BOOTP for system boot.
  • DHCP for system configuration.
  • CORBA for object-oriented programs.
• Replacing wires with networks is fun.

Middleware

• Middleware is software implementing distributed functions.
  • Middleware provides DNS, peer-to-peer networks, and cloud services, for example.
• Middleware is not the operating system.
  • It uses the operating system as an abstraction, or virtual machine.
  • Middleware pieces may operate in the operating system.

Network Devices

• A network device is an asynchronous information source and sink.

  

Network Packets

An ATM Card

An Ethernet Card

Summary

• Networks are graphs with hosts as nodes and links as edges.
• There are many network configurations, but for systems at the edge abstraction hides it all.
• Networks interpose within a system as well as among systems.
• Among systems, middleware raises the abstraction level.

Credits

• Lucent Technologies TN2238 ATM Network Interface Card by John Sloan under a Creative Commons BY NC SA license.
• TN2238: Front Panel Edge by John Sloan under a Creative Commons BY NC SA license.
• Ethernet NIC 100Mbit PCI by afrank99 under a Creative Commons BY SA license.

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This page last modified on 2012 April 9.