Computer Networking Lecture Notes

5 September 2013 • Computer Networking

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Outline

• Networks.
• Network architecture and protocol software.
• Measuring networks.
• Network security.

Networks

• Why networks?
  • To move bits from one system to another.
• For what purpose?
  • To communicate, mostly.
• Communicate what?
  • It doesn't matter (to a first approximation).
  • But it does matter, somewhat.
    • The more you know, the better you can do.

Networked Systems

• A network is a bit pipe connecting systems A and B.

  More generally, a network connects any two systems with a bit pipe. A complete network.

Network Structure

• For various reasons, a complete network is a useful model but a terrible implementation.

• Abstract the useful behavior, hiding the unhelpful structure. Our goal is to design and build this network cloud.

Communicating Bits

• Two end-points require common knowledge to communicate.
  • Recognize the words used, structure or syntax.
  • Understand the words used, meaning or semantics.
• A pre-arranged (pre-communicated) understanding between end-points.
  • Otherwise communication is difficult and ineffectual.

Protocols

• A protocol is the pre-arranged understanding between end-points.
• A protocol defines the syntax and semantics of communication.
  • The form and meaning of communication.
• Protocols define and instruct network behavior.
  • Necessary for the network user and the network designer-implementer.

Complexity

• Networks are complex, difficult environments.
• Protocols describe complex, difficult behavior.
• The combination of environment and behavior can be overwhelming.
• Impose structure on the network and protocols to control complexity and difficulty.
  • Hardware network structure.
  • Software protocol structure.

Layering

• Layering is a way to controlling complexity.
• A complex system is broken up into simpler layers.

• Each layer implements a slightly more complicated system using the slightly simpler system implemented by the next lower layer. The highest layer implements the desired system.

Layered Services

Example Protocol Stack

The ISO OSI Protocol Stack

• The International Standards Organization (ISO) standardizes lots of things, including networks.
• The ISO defined the Open Systems Interconnection (OSI) model to clarify network function.
  • The seven-layer model.
  • The ISO OSI stack is a conceptual model, not a reference implementation.

Another Example

• This is the Internet Protocol (IP) model.

Stack Relations

Network Performance

• There are many measures of network performance:
  • Throughput - Sustained bits/time to the receiver.
  • Latency - Time to receive the first bit.
  • Delay - Time to receive the last bit.
  • Goodput - TransferSize/Latency (scaled by overhead).
  • Jitter - Variation in latency.

Network Ranges

• Computer networks can be classified by the span they cover:
  • Personal-area networks (1-2 meters, Bluetooth).
  • Local-area networks (1 km, WiFi).
  • Metropolitan-area networks (10 km, Cable).
  • Wide-area network (1000 km, NSFNet).
  • The Internet (10,000 km).

Scaling

• Scaling characterizes how a system changes when size changes.
  • How an ecosystem changes with increasing animal population.
  • How a mutual fund changes with increasing investor dollars.
  • How a network changes with increasing hosts.
• Scale-related change is described with asymptotic (big O) notation.

Scaling Networks

• Scaling fully-connected point-to-point networks to n hosts requires O(n²) links.
• Dedicated switching fabrics can scale subquadratically.
  • The nodes in the fabric are called switches or routers.

Network Security

• Three concerns:
  1. Attack — what happens to the network?
  2. Defense — what are the counter measures?
  3. Protocol security — do protocols hinder or help security?
• There are other security concerns.
  • Physical security; social engineering.

Assumptions

• The assumptions underlying security definitions are crucial.
• Assume Eve (trad., short for eavesdropper)
  • sees everything on the network.
  • can modify and interfere with network traffic at will.
• Make sure you understand the assumptions behind security claims.
  • For example, Microsoft NT and C2 certification.

Consequences

• Ordering from an on-line store serving plain-text information:
  • Eve sees the order, gathers customer information, steals identity.
• Using telnet (plain-text communication):
  • Eve injects her commands into the stream.
• Updating software.
  • Eve changes the vendor’s updates to hers.

Threat Types

• An adversary can violate
  • secrecy assumptions (steal passwords, credit-card numbers),
  • traffic integrity (modify shipping addresses, site spoofing),
  • service availability (denial of service attacks).
• The collected set of danger points is called the attack surface.

Possible Solutions

• Security is based on obscuring information.
  • Not on obscuring security techniques.
• Cryptography (encryption-decryption) is the principle obscuring mechanism.
  • There are others (needle-in-haystack protections).
• What are the assumptions under which cryptography is a suitable solution to security problems?

Solution Examples

• Cryptographic algorithms are the basis for other approaches to protection.
  • Authentication protocols (Kerberos).
  • Message Integrity Protocols (SHA1 message digests).
  • Key distribution (SPKI, a hard problem).

Summary

• Networks link interesting places to allow interaction.
• Protocols determine the rules of interaction.
• Protocol layering combats network complexity through structuring.
• Security maintains desirable characteristics for interactions.

References

• The Structure of the “THE”-Multiprogramming System by Edsger Dikjstra in Communications of the ACM, May, 1968.

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This page last modified on 2012 September 6.