Week14 lec1


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Week14 lec1

  1. 1. Chapter 4 Network Layer Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007.
  2. 2. Hierarchical Routing  All routers run the same routing Algorithm? … not true in practice Scale: With millions of destinations:  Can’t store all destinations in routing tables  Exchange of Routing updates would swamp links! Administrative Autonomy  Internet = network of networks  Each network administrator may want to control routing in its own network
  3. 3. Hierarchical Routing Gateway Routers  One or more of the into “Autonomous routers in an AS will Systems” (ASs). have the added task of  Routers in same administrative domain forwarding packets to destinations outside  Routers in same AS run the AS. same routing protocol  Routers are organized   The routing algorithm within an autonomous system is called an “intra-AS” routing protocol Routers in different AS can run different intraAS routing protocol 1c 1a 1d 1b AS1
  4. 4. Interconnected ASs 3c 3b 3a 2a AS3 1c 1a 1d 2c 2b AS2 1b AS1 1b, 1c, 2a and 3a are Gateway Routers
  5. 5. Inter-AS tasks AS1 must: 1. Learn which destinations are reachable through AS2, which through AS3 2. Propagate this reachability information to all routers in AS1.  Configure Forwarding table Job of inter-AS routing protocol!  Suppose router in AS1 receives datagram destined outside of AS1:  Router should forward packet to gateway router, but which one? 3c 3a 3b AS3 1a 2a 1c 1d 1b Intra-AS Routing algorithm AS1 Inter-AS Routing algorithm Forwarding table 2c AS2 2b
  6. 6. Example: Setting forwarding table in router 1d  Suppose AS1 learns (via inter-AS protocol) that network x reachable via AS3 (gateway 1c) but not via AS2.  Inter-AS protocol propagates reachability info to all internal routers.  Router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c.  installs forwarding table entry (x,I) x 3c 3a 3b AS3 1a 2a 1c 1d 1b AS1 2c 2b AS2
  7. 7. Example: Choosing among multiple ASs  Now suppose AS1 learns from inter-AS protocol that a network x is reachable from AS3 and from AS2.  To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x.  Hot Potato Routing: Send packet towards closest of two routers. x 3c 3a 3b AS3 1a 2a 1c 1d 1b 2c AS2 AS1 2b
  8. 8. Example: Choosing among multiple ASs Learn from inter-AS protocol that network x is reachable via multiple gateways Use routing info from intra-AS protocol to determine costs of least-cost paths to each of the gateways Hot potato routing: Choose the gateway that has the smallest least cost Determine the interface I that leads to least-cost gateway. Enter (x,I) in forwarding table Steps in adding an outside AS destination in a routers forwarding Table
  9. 9. Routing Protocols
  10. 10. Path Vector Routing
  11. 11. Path Vector Routing Loop Prevention If a router receives a message, checks to see if its autonomous system is in the path list to the destination. If it is, looping is involved and the message is ignored. Policy Routing If one of the path in the path vector is against the policy it can ignore that path and does not update its routing table.
  12. 12. Inter-AS Routing in Internet: BGP  BGP (Border Gateway Protocol)  BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all ASinternal routers. 3. Determine “good” routes to subnets based on reachability information and policy.  Allows subnet to advertise its existence to rest of Internet: “I am here”
  13. 13. BGP Basics  Pairs of routers (BGP peers) exchange routing info over TCP connections using port 179.  For each TCP connection, the two routers at the end of the connection are called BGP Peers  The TCP connection along with all the BGP messages sent over the connection is called a BGP Session  Internal (IBGP) neighbours  A pair of BGP speakers within the same AS  External (EBGP) neighbours  Two BGP speakers from two different AS eBGP session 3c 3a 3b AS3 1a AS1 iBGP session 2a 1c 1d 1b 2c AS2 2b
  14. 14. BGP Basics  BGP allows each AS to learn which destinations are reachable via its neighboring ASs.  Destinations are not hosts but instead are CIDRized prefixes.  E.g. AS1 has four subnets attached to it It will aggregate the prefixes of these four and will advertise the single prefix  AS1 and AS2 send reachability information trough their gateway routers 1b and 2a.  When a gateway router receives e-BGP learned prefixes, the gateway router uses iBGP sessions to distribute the prefixes to other routers in the AS.
  15. 15. Distributing Reachability info  Using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.  1c can then use iBGP do distribute new prefix info to all routers in AS1  1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session  When router learns of new prefix, it creates entry for prefix in its forwarding table. eBGP session 3c 3a 3b AS3 1a AS1 iBGP session 2a 1c 1d 1b 2c AS2 2b
  16. 16. Path Attributes & BGP Routes  In BGP an AS is recognized by its globally unique Autonomous System Number (ASN).  Assigned by ICANN Regional Registries.  Advertised prefix includes BGP attributes.  prefix + attributes = “route”  Two important attributes:  AS-PATH and NEXT-HOP
  17. 17. Path Attributes & BGP Routes  AS-PATH  Contains ASs through which prefix advertisement has passed.  When a prefix is passed into an AS, the As adds its ASN to the AS-PATH attribute. E.g suppose that the prefix is first advertised from AS2 to AS1. If AS1 then advertises the prefix to AS3, AS-PATH would be AS2 AS1.  Detect and prevent looping advertisements
  18. 18. Routing Loop Prevention AS2 [ Net1, (AS3,AS4) ] [ Net1, (AS2,AS3,AS4)] AS3 AS1 [Net1, (AS4)] AS4 Net1 [ Net1, (AS1,AS2,AS3,AS4)] AS3 won’t forward this further