Computer Networking Lecture Notes

6 September 2012 • Computer Networking

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Outline

• Networks, computer and otherwise.
• Network architecture and protocol software.
• Network security.

Networks

• What is a network?
• A network is a collection of interesting places interconnected by transmission media.
  • A place is interesting for what information it has or actions it can take.
  • Media transmits objects suitable to the interesting places.

Examples

• The Interstate system.
  • Cites are interesting places.
  • Roads are the transmission medium.
• Your body.
  • Your skin and brain are the interesting places
  • Nerves are the transmission medium.

Computer Networks

• The nodes are computers, dedicated routers, special purpose computers, …
• The transmission lines are coax, twisted pair, fiber, radio
  • Point-to-point interconnections.
  • Multiple access interconnections.
• Media transmits information.

Why Bother?

• The ability to transmit among interesting places allows new capabilities.
  • Resources can be shared; information can be disseminated.
• General-purpose computers are programmable.
• Programmable hosts leads to new network uses.
• Compare to the phone system.
  • Relative to VoIP or Asterisk.

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.

Recursive Networks

• With enough hosts, a single routed network becomes impractical (doesn’t scale).
  • When horizontal growth is exhausted, try vertical growth.
• Build a network of networks by considering each routed network as an interesting place.
• This approach can be repeated to outrun growth.

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 Protocols

• A protocol defines the rules of communication.
  • The syntax and semantics of communication.
• On the outside, network programming is mainly developing protocols.
  • Also inside the network, but there’s lots of other aspects to cover too.

Protocol Layering

• Network encapsulation can be viewed as a stack.
• A network packet from A to D is put in link-layer packets A to B, B to C, and C to D.
  • Each layer produces packets that become the payload of the lower-layer’s packets.
  • This is almost correct, but TCP/UDP cheats to detect errors in IP-level information addresses.

Conceptual Architectures

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

The ISO OSI Stack

• The ISO OSI stack has seven layers:
  • 7: Application (top, most abstract)
  • 6: Presentation
  • 5: Session
  • 4: Transport
  • 3: Network
  • 2: Data-link
  • 1: Physical (bottom, most concrete)

OSI Layers

• The ISO OSI stack has seven layers:
  • 7: Application, network coding (e-mail).
  • 6: Presentation, encoding (XML, AES).
  • 5: Session, connections (RPC, multicast).
  • 4: Transport, end-to-end (TCP/IP, TP4)
  • 3: Network, edge-to-edge (AppleTalk)
  • 2: Data-link, framing and access (Ethernet).
  • 1: Physical, data transmission (wireless).

Addressing

• Each node has a unique address.
  • Or at least an effectively unique address.
• Each layer can have its own addressing.
  • Link layer has 48-bit Ethernet addresses.
  • Network layer has 32-bit IP addresses.
  • Transport layer has 16-bit port addresses.
• Routing delivers data across multiple hops.
• Broad- or multicasting have special address ranges.

Internet Protocol

• The Internet Protocol (IP) is networking’s 800 lb gorilla.
  • Supported almost everywhere (resource-constrained environments).
• Each host-link interface has a unique 4-byte (32-bit) IP address.
  • For example: 192.100.64.153 (www.monmouth.edu).
  • IPv6 uses 16-byte (128-bit) addresses

The IP Reference Model

• The Internet Protocol (IP) model parallels the OSI model.
  • The IP model is specific to the IP effort.
  • The OSI model is general to networking.
• The IP model collapses OSI layers.

  TCP/IP		OSI
  Application		Application, Presentation, Session
  Transport		Transport
  IP		Network
  Access		Data Link, Physical

The Hourglass Architecture

• Many application protocols and many transmission media.
  • Quickly leading to a scaling problem.
• Application protocols in TCP and UDP; IP over many network types.
• The Hourglass architecture.
  • IP is a common, simple protocol.
  • All networks can support it.
  • Compare to compiler architectures.

Inter-Process Communication

• Applications reside on hosts; many applications on a host.
• Networking transmits among hosts.
• A transmission arrives at a host; then what?
• Provide both inter-host and inter-process communication abstractions.
• The solution is to encapsulate an application protocol in IP.

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

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This page last modified on 2012 September 6.