Operating Systems Lecture Notes

16 April 2012 • Network Layer

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Outline

• Network layer.
• IP and naming.
• Subnetworks and forwarding.
• Routing algorithms.

Network Services

Interconnections

• Interconnecting lots of machines.
  • Naming (addressing).
  • Message forwarding and routing.

Network Layer

• Computer-to-computer message transfer.
• Network services.
  • Forwarding and routing.
  • Guaranteed delivery, bandwith, other qualties.
  • Security.
• Network layer protocols.
  • Virtual circuits or datagram networks.
• Routers.

Virtual Circuits

• A three-phase protocol: connect, transfer, disconnect.
  • Path discovery and resource reservation.
  • Move the packets through the circuit.
  • Release resrouces.

Datagram Packets

• Independent, per-packet routing.
• Routing is hop-by-hop based on local collected information.

IP

• The network-layer protocol for the Internet.
• Datagram networks.
• IPv4 and IPv6.
• Datagram structure.

Naming

• IP addresses identify interfaces.
• An IPv4 address is a 32-bit number divded into four 8-bit bytes (dotted quad).
  • An address has a network/host structure.
  • There are four network classes.
• IP addresses are assigned statically or dynamically.

Subnetworking

Forwarding

• Each router has a routing table.
• Each entry is a (subnet, subnet mask, gateway) triple.
  • The mask removes the host part of the IP address.
  • Matching networks get sent to the associated gateway.

Routing Algorithms

• Routing tables direct traffic on to the next node.
  • Subject to constraints like least cost or quickest delivery.
• It's a graph problem.

Algorithm Types

• All-pairs shortest path problems.
• Global (synchronized) or local (asynchronous) routing algorithms.
  • Global vs non-global knowledge.
  • Distance vector algorithms.

Summary

• Networks present a heterogenous address space.

References

• Internet Protocol, RFC 791, Internet Engineering Task Force, September, 1981.

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This page last modified on 2011 October 9.