Computer Networking Lecture Notes

2014 September 24 • Internetworking

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Outline

• Function
• Hierarchy
• Architecture
  • Piece parts.
  • Integration
• Protocols
  • Border gateway protocols.

Where Are We?

• How do we go from the left to the right?
  • The Internet, how does it work?

Internetworking

• Internetworking combines independent networks into a common network.
  • An internetwork or internet.
  • Note: an internet, the Internet.

Intenetworking Functions

An internet should provide
• Uniform network services.
  • Homogeneous behavior on heterogeneous systems.
• Transport-layer (at least) services.
  • There is no middle of an internetwork.
• The independent networks maintain their separate identifies.

IP-Address Hierarchy

• IP addresses provide a simple, two-level hierarchy.

  

• Outside a network, only the network exists.

  

More Hierarchy

• IP (v4) addresses don't scale well to 100,000s of networks.
  • Expensive to maintain and use.
• Large scale needs intermediate structure between networks and an internet.

  

• Extra-technical motivations also intrude.
  • Organizations and policies.

More Structure

• Networks group themselves into autonomous systems (ASs, rfc 1930).
  • An AS is also a (relatively) small internet.
• An AS is controlled by a single entity (a corporation, an agency, a government).
• Each AS has a 32-bit autonomous system number (ASN) assigned by the Internet Assigned Number Authority (IANA).
  • Around 74,000 AS numbers have been allocated.

AS Traffic

Local traffic is traffic into or out of an autonomous system. Transit traffic is traffic through an autonomous system.

AS Types

• A stub AS is singly-connected autonomous system.
• A multi-homed AS is a multiply-connected AS that doesn't carry transit traffic.
• A transit AS is a multi-homed AS that carries transit traffic.

  

AS Hierarchy

• An internet of autonomous systems has (roughly) a three-level structure.

  1. Backbone
  2. Service providers.
  3. Customers

• Backbone and provider levels are sets of peer ASs (roughly).
• Customers are individual ASs or smaller networks (roughly).

Recursion

• An autonomous system may contain smaller versions of itself.
  • A non-AS group of networks may organize in an AS hierarchy.
• Example: dear old MU.
  • Backbone: campus-wide ring.
  • Provider: each building wired into the ring.
  • Customers: each department wired into the building.

What About Scaling?

• IP addresses aggregate hosts as a network address.
• Autonomous systems aggregate networks as ASs.
  • But AS numbers are not addresses.
• Each AS associates host addresses with ASs.
  • Unaccessed networks can be ignored.
  • Backbone ASs are still on the hook for everything.

Connecting

• What is a transport-layer connection?
• It is a path from one host through autonomous systems to the other host.

  

• Finding that path is the routing problem.
  • Or just routing.
  • The path is the route.

AS Routing Policy

• An AS has a set of rules governing traffic through the AS.
  • Multi-homed ASs have a no transit traffic rule.
  • Microsoft won't route through Apple's AS, and vice versa.
• An AS's rule set is known as its routing policy.
• AS policies can get complicated quickly.
  • And AS organizations tend to be competitive rather than cooperative.

AS Routing

• Routing through autonomous systems in an internet should
  1. Produce efficient and effective routes between hosts
  2. Respect the constituent ASs routing policies.
• Respecting policy takes precedence over effective routes.

AS Routing Protocols

• Autonomous systems use several routing protocols to route through an internet.
• The border gateway protocol (BGP, rfc 4271) is the main protocol used among ASs.

Border Routers

• A border router is a host in an autonomous system through which traffic moves.

  

  • Sometimes known as an edge router or a gateway.
• ASs are connected via border routers.
  • An AS has at least one border router.

BGP Routers

• A BGP router is a host in an autonomous system that communicates using BGP.
  • Border routers are usually BGP routers, but don't have to be.
• An AS has to have at least one BGP router.
• BGP routers
  1. collect and distribute routing information with connected ASs.
  2. construct new routes based on collected routing information.

Internet Routes

• A path through an internet is a sequence of autonomous systems ending at a network.
  • For example: AS₁, AS₂, …, AS_k, a.b.c.d/n.
• An AS's BGP routers maintain the path list.
  • Potentially one path for every network.
• The BGP routers periodically advertise a subset of their path list to connected ASs.
  • The AS policy determines the list advertised.

Path Finding

• A BGP router determines new paths from path information advertised by connected ASs.
  • Add paths to previously unknown networks.
  • Replace old paths with newer, better paths.
  • Drop old paths that now violate policy.
• Internet path finding can be a difficult task.
  • Local policies constrain and complicate choice.
  • Connected ASs may advertise poor-quality information.

Summary

• An internet combines many networks into a single network.
  • This is a recursive definition.
• A tree-shaped hierarchy scales up to global-sized internets.
  • For the most part.
• The constituent networks still maintain their autonomy.
  • And that's what makes it interesting.

References

• The Global Internet (Section 4.1) in Computer Networking: A Systems Approach by Larry Peterson and Bruce Davie, Morgan-Kaufman, 2011.
• Internetworking (Section 5.5) in Computer Networks by Andrew Tanenbaum and David Wetherall, Prentice Hall, 2010.

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This page last modified on 2014 September 22.