CSC 600 Internetworking with TCP/IP

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CSC 600Internetworking withTCP/IP

• Unit 6a IP Routing and Exterior Routing
  Protocols
• (Ch. 14, 15)
• Dr. Cheer-Sun Yang
• Spring 2001

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Routing Protocols

• Cores, Peers, and Algorithms Distance
  Vector(Bellman-Ford), Link State(Dijkstra),
  Gateway-to-Gateway Protocol(GGP),
• Interior within an autonomous system
• Exterior between two autonomous systems
• Exterior Routing Protocols Border Gateway
  Protocol(BGP)
• Interior Routing Protocols RIP(distance vector),
  OSPF(link state).

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Routing Protocols

• Routing Information
• About topology and delays in the internet
• Routing Algorithm
• Used to make routing decisions based on
  information

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The Evolution of Internet Architecture

• Core system many non-core routers are conneced
  to a set of core routers.
• Peer-to-peer many routers are connected to a
  backbone.
• Architectural many autonomous systems are
  connected to their own gateways and gateways are
  connected as peers.

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Original Internet Architecture and Cores

• A small number of routers kept complete
  information about all possible destinations and a
  large set of routers only kept partial
  information.
• The routing table in a given router contains
  partial information about possible destinations.
• Routing that uses partial information allows
  sites autonomy in making local routing changes.

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Core vs. Noncore

• Core routers are controlled by the Internet
  Network Operations Center (INOC).
• Noncore routers are controlled by individual
  groups.
• This architecture can introduce the possibility
  of inconsistencies that may make some
  destinations unreachable from some sources unless
  the chain of all default routers (core) reaches
  every router in a giant cycle as shown in next
  slide.

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Core System is Impractical

• When the NSFNET became the major part of the
  Internet, the core architecture became
  impractical for the following reasons
• The Internet outgrew a single, centrally managed
  long-haul backbone.
• Not every site could have a core router connected
  to the backbone.
• Because core routers all interacted to ensure
  consistent routing information, the core
  architecture did not scale to arbitrary size.
• The peer-to-peer architecture is formed.

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Routing Becomes Complicated

• For example, how can a datagram be routed from
  host 3 to host 2?
• Which path should be taken?
• How can routing be optimized?
• How can loops be eliminated?

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Summary of Core System Architecture

• A core routing architecture assumes a centralized
  set of routers which serves as the repository of
  information about all possible destinations in an
  internet.
• Core systems work best for internets that have a
  single, centrally managed backbone.
• Expanding the topology to multiple backbones
  makes routing complex attempting to partition
  the core architecture so that all routers use
  default routers introduces potential routing
  loops.

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Automatic Propagation of Routing Information
The Internet is not static!
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Distance Vector (Bellman-Ford) Routing
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Gateway-to-Gateway Protocol (GGP)

• Sometimes known as exterior routing
• protocols.
• It is a true distance-vector protocol.
• It measures distance in router hops.

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Autonomous Systems

• Although it is desirable for routers to exchange
  routing information, it is impractical for all
  routers on an arbitrarily large internet to
  participate in a single routing update protocol.
• The number of routers that participate in a
  single routing protocol must be limited.

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Autonomous Systems

• This idea works fine. However, it implies that
  some routers will be outside the group.
• If a router outside of an AS uses a member of
  the group as the default route, routing will be
  suboptimal.
• R1 and R2 are in one AS, while R3 is not.
• If R3 sends datagrams via R1 for sending
  datagrams to R2, it is not optimal.

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Hidden Networks
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Architectural ApproachAutonomous Systems (AS)

• Group of routers
• Exchange information
• Common routing protocol
• Set of routers and networks managed by single
  organization - an autonomous system
• The Internet is organized into a collection of
  Ass, each of which is normally administered by a
  single entity. A corporation or university campus
  often defines an AS. The NSF backbone forms an AS.

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Architectural ApproachAutonomous Systems (AS)

• Each Autonomous system can select its own routing
  protocol to communicate between the routers in
  that AS. This is called an interior gateway
  protocol (IGP) or intradomain routing protocol.
• Separate routing protocols called exterior
  gateway protocol (EGS) or interdomain routing
  protocol are used between the routers in
  different autonomous systems.

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Interior Routing Protocols

• Routing Information Protocol (RIP) a distance
  vector (Bellman-Ford)
• Open Shortest Path First Protocol (OSPF) a link
  state algorithm (Dijkstras algorithm)

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Exterior Routing Protocol

• Border Gateway Protocol (BGP)

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Application of IRP and ERP
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Border Gateway Protocol (BGP)

• Inter-autonomous system communication
• Coordination among multiple BGP gateways
• Propagation of reachability information
• Next-hop paradigm
• Policy support
• Reliable transport
• Incremental updates
• Support for classless addressing
• Route aggregation
• Authentication

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Border Gateway Protocol (BGP)

• For use with TCP/IP internets
• Preferred EGP of the Internet
• Messages types sent over TCP connections
• Open
• Update advertise or withdraw routes
• Keep alive actively test peer connectivity
• Notification response to an incorrect message
• Procedures
• Neighbor acquisition
• Neighbor reachability
• Network reachability

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BGP Messages
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BGP Procedure

• Open TCP connection
• Send Open message
• Includes proposed hold time
• Receiver selects minimum of its hold time and
  that sent
• Max time between Keep alive and/or update
  messages

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Other Message Types

• Keep Alive
• To tell other routers that this router is still
  here
• Update
• Info about single routes through internet
• List of routes being withdrawn
• Includes path info
• Origin (IGP or EGP)
• AS_Path (list of AS traversed)
• Next_hop (IP address of boarder router)
• Multi_Exit_Disc (Info about routers internal to
  AS)
• Local_pref (Inform other routers within AS)
• Atomic_Aggregate, Aggregator (Uses address tree
  structure to reduce amount of info needed)

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Uses of AS_Path and Next_Hop

• AS_Path
• Enables routing policy
• Avoid a particular AS
• Security
• Performance
• Quality
• Number of AS crossed
• Next_Hop
• Only a few routers implement BGP
• Responsible for informing outside routers of
  routes to other networks in AS

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The Key Restriction of EGP

• An exterior gateway protocol does not communicate
  or interpret distance metrices, even if metrics
  are available.

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The Routing Arbiter System

• For an internet to operate correctly, routing
  information must be globally consistent.
• Individual protocols such as BGP does not
  guarantee global consistency.
• The RA system consists of a replicated
  authenticated database of reachability
  information.Each ISP designates one of the
  routers near a Network Access Point (NAP) to be a
  BGP border router.
• The designated router maintains a connection to
  the route server over which it uses BGP. BGP
  notification messages are exchanged.

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BGP Routing Information Exchange

• Within AS, router builds topology picture using
  IGP
• Router issues Update message to other routers
  outside AS using BGP
• These routers exchange info with other routers in
  other AS
• Routers must then decide best routes

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Notification Message

• Message header error
• Authentication and syntax
• Open message error
• Syntax and option not recognized
• Unacceptable hold time
• Update message error
• Syntax and validity errors
• Hold time expired
• Connection is closed
• Finite state machine error
• Cease
• Used to close a connection when there is no error

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