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CSC 600 Internetworking with TCP/IP Unit 6a: IP Routing and Exterior Routing Protocols (Ch. 14, 15) Dr. Cheer-Sun Yang Spring 2001.

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Presentation on theme: "CSC 600 Internetworking with TCP/IP Unit 6a: IP Routing and Exterior Routing Protocols (Ch. 14, 15) Dr. Cheer-Sun Yang Spring 2001."— Presentation transcript:

1 CSC 600 Internetworking with TCP/IP Unit 6a: IP Routing and Exterior Routing Protocols (Ch. 14, 15) Dr. Cheer-Sun Yang Spring 2001

2 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).

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

4 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”.

5 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.


7 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.



10 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.


12 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?


14 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.

15 Automatic Propagation of Routing Information The Internet is not static!

16 Distance Vector (Bellman-Ford) Routing



19 Gateway-to-Gateway Protocol (GGP)  Sometimes known as exterior routing protocols.  It is a true distance-vector protocol.  It measures distance in router hops.

20 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.

21 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.  R 1 and R 2 are in one AS, while R 3 is not.  If R 3 sends datagrams via R 1 for sending datagrams to R 2, it is not optimal.


23 Hidden Networks

24 Architectural Approach: Autonomous 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.


26 Architectural Approach: Autonomous 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.

27 Interior Routing Protocols Routing Information Protocol (RIP): a distance vector (Bellman-Ford) Open Shortest Path First Protocol (OSPF): a link state algorithm (Dijkstra’s algorithm)

28 Exterior Routing Protocol Border Gateway Protocol (BGP)

29 Application of IRP and ERP

30 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

31 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



34 BGP Messages


36 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






42 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)

43 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


45 The Key Restriction of EGP An exterior gateway protocol does not communicate or interpret distance metrices, even if metrics are available.

46 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.

47 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

48 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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