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Routing Protocols Interconnected ASes


Routing technology mainly refers to the routing algorithm. Internet routing protocol characteristics and classification. Among them, the routing algorithm can be divided into static and dynamic routing algorithm routing algorithm. Internet routing protocols are characterized by: the choice of protocol is adaptive (ie, dynamic); is a distributed routing protocols; the use of hierarchical routing protocols, namely, sub-autonomous system of internal and external autonomous system routing protocol. Internet routing protocols are divided into two categories: Interior Gateway Protocol (IGP, specific agreements have RIP and OSPF, etc.) and Exterior Gateway Protocol (EGP, currently the most used is BGP).

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									                                                                               Hierarchical Routing
                                                            Our routing study thus far - idealization
                                                            • all routers identical
                                                            • network “flat”
              Routing Protocols                             … not true in practice
                                                             scale: with 200 million                    administrative autonomy
                                                               destinations:                            •     internet = network of networks
                                                             •    can’t store all dest’s in             •     each network admin may want
                                                                  routing tables!                             to control routing in its own
                                                             •    routing table exchange                      network
                                                                  would swamp links!

              Hierarchical Routing                                        Interconnected ASes
• aggregate routers into
  regions, “autonomous            Gateway router                 3c
                                                                     3a                                          2c
  systems” (AS)                   • Direct link to router   3b                                              2a
                                                                   AS3                                                 2b
                                                                               1c                                AS2
• routers in same AS run            in another AS                         1a              1b
  same routing protocol                                                        1d               AS1
                                                                                                                  • Forwarding table is
   – “intra-AS” routing                                                                                             configured by both
                                                                                                                    intra- and inter-AS
   – routers in different AS                                                        Intra-AS
     can run different intra-AS                                                     Routing
                                                                                                  Routing           routing algorithm
                                                                                    algorithm     algorithm
     routing protocol                                                                                                  – Intra-AS sets entries for
                                                                                         Forwarding                      internal dests
                                                                                                                       – Inter-AS & Intra-As sets
                                                                                                                         entries for external dests

                                                                              Example: Setting forwarding table in router
                Inter-AS tasks                                                                    1d
• Suppose router in AS1                   AS3
  receives datagram for                   1. to propagate this           • Suppose AS1 learns (via inter-AS protocol) that subnet x is
  which dest is outside of                   reachability AS1 needs:       reachable via AS3 (gateway 1c) but not via AS2.
  AS1                                     2. to learn which dests are    • Inter-AS protocol propagates reachability info to all internal
   – Router should forward                                                 routers.
                                             reachable through AS2
     packet towards one of
     the gateway routers, but                and which through info to   • Router 1d determines from intra-AS routing info that its
     which one?                              all routers in AS1            interface I is on the least cost path to 1c.
                                          Job of inter-AS routing!       • Puts in forwarding table entry (x,I).

               3a                                        2c                                     3c
        3b                                          2a                                             3a                                               2c
             AS3                                               2b                          3b                                                 2a
                          1c                             AS2
                                                                                                 AS3                                                     2b
                                                                                                                   1c                              AS2
                    1a              1b
                          1d              AS1                                                                1a              1b
                                                                                                                    1d             AS1

Example: Choosing among multiple ASes                                    Example: Choosing among multiple ASes
 • Now suppose AS1 learns from the inter-AS protocol                      • Now suppose AS1 learns from the inter-AS protocol
   that subnet x is reachable from AS3 and from AS2.                        that subnet x is reachable from AS3 and from AS2.
 • To configure forwarding table, router 1d must                          • To configure forwarding table, router 1d must
   determine towards which gateway it should forward                        determine towards which gateway it should forward
   packets for dest x.                                                      packets for dest x.
 • This is also the job on inter-AS routing protocol!                     • Hot potato routing: send packet towards closest of two

                  3a                                       2c            Learn from inter-AS
                                                                                                        Use routing info
                                                                                                                                  Hot potato routing:
                                                                                                                                                               Determine from
           3b                                        2a                                                  from intra-AS                                      forwarding table the
                AS3                                             2b       protocol that subnet        protocol to determine           Choose the            interface I that leads
                               1c                         AS2
                                                                          x is reachable via          costs of least-cost              gateway            to least-cost gateway.
                         1a                                               multiple gateways              paths to each               that has the               Enter (x,I) in
                                         1b   AS1
                                                                                                        of the gateways           smallest least cost
                               1d                                                                                                                             forwarding table

            Intra-AS Routing                  RIP ( Routing Information Protocol)
                                              • Distance vector algorithm
• Also known as Interior Gateway Protocols    • Included in BSD-UNIX Distribution in 1982
  (IGP)                                       • Distance metric: # of hops (max = 15 hops)
• Most common Intra-AS routing protocols:
                                                                                              From router A to subsets:
  – RIP: Routing Information Protocol                                                              destination hops
                                                         u               v
                                                                                                      u          1
  – OSPF: Open Shortest Path First                           A       B        w                       v          2
                                                                                                      w         2
                                                                                                      x          3
  – IGRP: Interior Gateway Routing Protocol                                      x                    y          3
    (Cisco proprietary)                              z       C       D                                z          2

        RIP advertisements                                       RIP: Example
• Distance vectors: exchanged among           w                  x                                   y
  neighbors every 30 sec via Response                    A                   D                B
  Message (also called advertisement)
• Each advertisement: list of up to 25        Destination Network            Next Router          Num. of hops to dest.
  destination nets within AS                        w                                A                     2
                                                    y                                B                     2
                                                    z                                B                     7
                                                    x                                --                    1
                                                    ….                               ….                    ....
                                                                         Routing table in D

                        RIP: Example
Dest   Next    hops
                                                                                    RIP: Link Failure and Recovery
 w      -      1
 x      -      1           from A to D                                           If no advertisement heard after 180 sec -->
 z      C      4
 ….     …     ...                                                                   neighbor/link declared dead
                                                                                     – routes via neighbor invalidated
  w                    x                                  y
              A                D                  B                                  – new advertisements sent to neighbors
                                                                                     – neighbors in turn send out new
                               C                                                       advertisements (if tables changed)
 Destination Network          Next Router             Num. of hops to dest.
        w                           A                          2
        y                           B                          2
        z                           B A                        7 5
        x                           --                         1
        ….                          ….                         ....
                             Routing table in D

                  RIP Table processing                                            OSPF (Open Shortest Path First)
   • RIP routing tables managed by application-                                  • “open”: publicly available
     level process called route-d (daemon)                                       • Uses Link State algorithm
   • advertisements sent in UDP packets,                                           – LS packet dissemination
     periodically repeated                                                         – Topology map at each node
                                                                                   – Route computation using Dijkstra’s algorithm
                  routed                                 routed

       Transprt                                                Transprt          • OSPF advertisement carries one entry per
        (UDP)                                                     (UDP)            neighbor router
       network        forwarding                  forwarding      network
         (IP)           table                        table            (IP)       • Advertisements disseminated to entire AS (via
       link                                                           link         flooding)
       physical                                                   physical         – Carried in OSPF messages directly over IP (rather
                                                                                     than TCP or UDP

    OSPF “advanced” features (not in
                                                                              Hierarchical OSPF
• Security: all OSPF messages authenticated (to prevent
  malicious intrusion)
• Multiple same-cost paths allowed (only one path in RIP)
• For each link, multiple cost metrics for different TOS
  (e.g., satellite link cost set “low” for best effort; high for
  real time)
• Integrated uni- and multicast support:
   – Multicast OSPF (MOSPF) uses same topology data
     base as OSPF
• Hierarchical OSPF in large domains.

               Hierarchical OSPF                                      Internet inter-AS routing: BGP
• Two-level hierarchy: local area, backbone.
   – Link-state advertisements only in area                        • BGP (Border Gateway Protocol): the de facto
   – each nodes has detailed area topology; only know                standard
     direction (shortest path) to nets in other areas.             • BGP provides each AS a means to:
• Area border routers: “summarize” distances to nets in              1. Obtain subnet reachability information from
  own area, advertise to other Area Border routers.                     neighboring ASs.
• Backbone routers: run OSPF routing limited to                      2. Propagate reachability information to all AS-
  backbone.                                                             internal routers.
• Boundary routers: connect to other AS’s.                           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”

                     BGP basics                                               Distributing reachability info
• Pairs of routers (BGP peers) exchange routing info                  •       With eBGP session between 3a and 1c, AS3 sends prefix
  over semi-permanent TCP connections: BGP sessions                           reachability info to AS1.
   – BGP sessions need not correspond to physical links.              •       1c can then use iBGP do distribute this new prefix reach info to
• When AS2 advertises a prefix to AS1, AS2 is                                 all routers in AS1
  promising it will forward any datagrams destined to                 •       1b can then re-advertise new reachability info to AS2 over 1b-to-
                                                                              2a eBGP session
  that prefix towards the prefix.
                                                                      •       When router learns of new prefix, creates entry for prefix in its
   – AS2 can aggregate prefixes in its advertisement                          forwarding table.

          3c                                                                    3c
              3a                                  2c                                  3a                                  2c
       3b                                   2a                                3b                                   2a
            AS3                                        2b                           AS3                                       2b
                         1c                      AS2                                             1c                     AS2
                    1a            1b                                                        1a         1b
                  AS1     1d                                                              AS1    1d
                                                   eBGP session                                                            eBGP session
                                                       iBGP session                                                           iBGP session

    Path attributes & BGP routes                                                           BGP route selection
• When advertising a prefix, advert includes BGP
                                                                          •        Router may learn about more than 1 route to
    – prefix + attributes = “route”                                                some prefix. Router must select route.
• Two important attributes:                                               •        Elimination rules:
    – AS-PATH: contains ASs through which prefix advertisement has                 1.   Local preference value attribute: policy decision
      passed: AS 67 AS 17
                                                                                   2.   Shortest AS-PATH
    – NEXT-HOP: Indicates specific internal-AS router to next-hop
      AS. (There may be multiple links from current AS to next-hop-                3.   Closest NEXT-HOP router: hot potato routing
      AS.)                                                                         4.   Additional criteria
• When gateway router receives route advertisement,
  uses import policy to accept/decline.

                     BGP messages                                                      BGP routing policy
                                                                                                            legend:   provider
 • BGP messages exchanged using TCP.                                                     B                            network
 • BGP messages:                                                      W         A
   – OPEN: opens TCP connection to peer and                                               C                           network:
     authenticates sender                                                                            Y
   – UPDATE: advertises new path (or withdraws                                     -BGPnew: a simple BGP scenario
                                                                          Figure 4.5
     old)                                                           • A,B,C are provider networks
   – KEEPALIVE keeps connection alive in                            • X,W,Y are customer (of provider networks)
     absence of UPDATES; also ACKs OPEN
     request                                                        • X is dual-homed: attached to two networks
   – NOTIFICATION: reports errors in previous                         – X does not want to route from B via X to C
     msg; also used to close connection                               – .. so X will not advertise to B a route to C

                BGP routing policy (2)                                 Why different Intra- and Inter-AS routing?
                                        legend:      provider
                     B                               network         Policy:
                                 X                                   • Inter-AS: admin wants control over how its traffic routed,
                                                     customer          who routes through its net.
                      C                              network:
                                                                     • Intra-AS: single admin, so no policy decisions needed
               -BGPnew: a simple BGP scenario
      Figure 4.5
• A advertises to B the path AW                                      • hierarchical routing saves table size, reduced update
• B advertises to X the path BAW                                       traffic
• Should B advertise to C the path BAW?                              Performance:
   – No way! B gets no “revenue” for routing CBAW since neither W    • Intra-AS: can focus on performance
     nor C are B’s customers
                                                                     • Inter-AS: policy may dominate over performance
   – B wants to force C to route to w via A
   – B wants to route only to/from its customers!

Placing routing into context


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