Routing In the Internet

• An autonomous system is a system where all the routers are controlled by a single entity.
  • A collection of IP networks.
  • Employing a common routing policy
  • Usually the AS has full information about all routers and links.
  • Examples:
    • ISP
    • Campus or corporate network.
  • Stallings states that there must be a path from every node to every other node. (But not necessarily direct).
• Autonomous systems run interior routing protocols
• Routing protocols between different AS are called exterior routing protocols
• Kurose discusses intra-AS and inter-AS protocols.
• Interior routing protocols
  • RIP: Routing Information Protocol
    • R.I.P. RIP R.I.P.
    • Uses Bellman-Ford
    • This records the next hop to each destination, as well as a distance to the ultimate destination.
    • routed was part of BSD
    • Uses hop count for path weight.
    • Maximum path weight was 15
    • Sent update every 30 seconds.
    • This became a problem as networks grew.
    • Even worse when clocks were in sync.
    • The kernel kept the routing table.
      • Routed was a process which was allowed to manipulate this table.
      • The ip portion of the stack used the table when finding the address to send the packet to.
      • 
  • OSPF: Open Shortest Path First
    • Uses Dijkstra's
    • Along with IS-IS this seems to be the current standard.
    • Employs flooding.
    • Every 30 min, broadcast link state.
    • Or when the link state changes.
    • OSPF Provides
      • Authentication, enabling security
      • Multiple shortest cost paths.
      • Support for unicast and multicast
      • Hierarchy within an AS
    • Hierarchy within an AS
      • Subsections of the graph can be labeled as an area
      • Routers within an area exchange information.
      • But have a default route to the outside world.
      • area border routers are routers with a connection to the AS backbone
      • These routers participate in the Global configuration
      • Border routers talk to other AS routers.
      • 
  • IS-IS: Intermediate System to Intermediate System.
    • Much link OSPF
    • But built for the ISO architecture, not IP
    • Wikipidea says it differers in details, but the operation is similar.
• Exterior Protocols
  • BGP: Border Gateway Protocol
    • Kurose states that this is extremely complex
      • Many books
      • RFC's
      • And MANY months practice to understand
    • Permanent TCP connections are maintained between routers in different AS
    • These BGP Peers exchange information
      • The Identity of each AS they are connected to
      • List of all networks within the AS
    • They also exchange information about other AS they are connected to.
  • CSPF: Constrained Shortest Path First.