Chapter 25 Internet Routing

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Chapter 25 Internet Routing
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Static Routing

• manually configured routes that do not change
• Used by hosts whose routing table contains one
  static route describing the network to with the
  host is attached
• a default route directs all other traffic to a
  specific router (fig 25.1)

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Dynamic Routing

• routing tables can change automatically over time
  via information learned via routing messages
  from other routers.
• Each router learns about destinations other
  routers can reach, and informs other routers
  about destinations that it can reached (fig 25.2)

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Routing in the Global Internet

• the Internet uses a 2-level routing hierarchy
• Routers and networks are divided into groups
  where each group is known as an autonomous system
  (a contiguous set of networks and routers all
  under control of one administrative authority).
• All routers within a group(autonomous system)
  exchange routing information while one router in
  each group summarizes the information before
  passing it to other groups.

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Internet Routing Protocols

• Interior Gateway Protocol (IGP)
• eg. RIP, OSPF, IGRP used by routers within an
  autonomous system
• Exterior Gateway Protocols(EGP)
• eg.BGP used by a router in one autonomous system
  to exchange routing information with a router in
  another autonomous system (fig 25.3)

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Routing Metrics

• eg. hop count, administrative cost, throughput,
  delay.
• Used by interior gateway protocols but not by
  exterior gateway protocols due to existence of
  different metrics.
• Within an autonomous system, IGP software uses a
  routing metric to choose an optimal path to each
  destination.
• EGP software finds a path to each destination,
  but cannot find an optimal path because it cannot
  compare routing metrics from multiple autonomous
  systems.

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Border Gateway Protocol (BGP)

• routing messages among autonomous systems contain
  routes, each of which is described as a path
  of autonomous systems ( eg. route to autonomous
  system 34 is achieve via autonomous systems 17,
  2, 56, and 12.
• Provision for policies manager can configure
  BGP to restrict routes advertised to outsiders
• Each autonomous system is classified as a transit
  system if it agrees to pass traffic through to
  another autonomous system or as a stub system if
  it does not
• Uses TCP for reliable transport of routing
  messages
• Used by all ISPs to exchange routing information
  with each other and from an authoritative route
  server (which has a copy distributed database of
  all possible destinations in the Internet with
  information about the ISP that owns each
  destination)

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Routing Information Protocol (RIP)

• implemented by a program called routed
• hop count metric with origin-one counting ( ie. A
  directly connected network is 1 hop away, not 0)
• uses unreliable transport (UDP)
• uses broadcast(RIP version 1) or multicast(RIP
  version2) for message delivery
• support for default route propagation

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Routing Information Protocol (cont.)

• uses distance-vector algorithm
• RIP advertises the destinations it can reach
  along with a distance to each destination
• adjacent routers receive the information and
  update their routing tables
• allows hosts to be operate in passive listen-only
  mode ( ie. no advertising)
• RIP packet format (fig 25.5)
• drawbacks of RIP include large routing messages,
  slow propagation of route changes (one router at
  a time), and limited scalability.

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Open Shortest Path First Protocol (OSPF)

• scales well to large organizations
• designed as an interior gateway protocol
• full CIDR and subnet support
• authenticated message exchange
• can import routes from BGP
• Uses hierarchical routing by dividing routers and
  networks in an autonomous system into subnets
  known as areas.
• Each router is within a given area exchange
  link-status messages via broadcasts.
• Summarized routing information are exchanged by
  one router in each area with routers in other
  areas.

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Open Shortest Path First Protocol (cont)

• uses link-state routing algorithm.
• Each router must periodically probe adjacent
  routers and then broadcast a link-status message
• routers that receive the message use Dijkstras
  SPF algorithm to compute the shortest paths using
  it local copy of network graph(fig 25.6) .

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Multicasting

• an application running on any computer can join a
  multicast group at any time and begin receiving a
  copy of all packets sent to the group.
• To join or leave a group, the computer informs a
  nearby router via IGMP (Internet Group Multicast
  Protocol).
• To leave a group, the computer informs the local
  router that it is no longer participating in the
  group.

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Multicasting (cont)

• An IP multicast group is anonymous
• sender and receiver do not know the identity or
  the number of group members
• routers do not know which applications will send
  a datagram to a group since any application on
  any computer can send a datagram to any multicast
  group at any time.
• Membership in a multicast group only defines a
  set of receivers
• sender does not need to join a multicast group
  before sending a message to the group.
• Multicast packets are forwarded using techniques
  such as flood-and-prune, configuration-and-tunneli
  ng, or core-base discovery. 

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Multicast routing protocols

• DVMRP (distance vector multicast routing
  protocol)
• used by Unix program mrouted and Internet
  Multicast backBONE (MBONE)
• CBT (Core Base Trees)
• PIM-SM (protocol independent multicastsparse
  mode)
• PIM-DM (protocol independent multicastdense
  mode)
• MOSPF(multicast extensions to the open shortest
  path first protocol)