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Fundamentals of Computer Networks ECE 478/578

Fundamentals of Computer Networks ECE 478/578. Lecture #18: Policy-Based Routing Instructor: Loukas Lazos Dept of Electrical and Computer Engineering University of Arizona. Routing on the Internet. Scale Issues Network of IP Prefixes indicating different subnets: 150,000-200,000 so far

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Fundamentals of Computer Networks ECE 478/578

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  1. Fundamentals of Computer NetworksECE 478/578 Lecture #18: Policy-Based Routing Instructor: LoukasLazos Dept of Electrical and Computer Engineering University of Arizona

  2. Routing on the Internet • Scale Issues • Network of IP Prefixes indicating different subnets: 150,000-200,000 so far • More than 20,000 organizations owning different “subnets” • Millions of different routes between and within subnets • Privacy Issues • Organizations may not want to reveal internal topologies • Policy Issues • No unique policy for determining the costs of links • Organizations may have preferred routes based on policies/relationships

  3. Autonomous Systems • Definition: A set of routers that has a single routing policy and runs under a single technicaladministration. • Internet Gateway Protocol: An internal routing protocol run within the AS • Routing between ASes: External Gateway Protocol at the early stages of internet • Replaced by the Border Gateway Protocol (BGP)

  4. Types of ASes • Stub AS: an AS with only a single connection to one other Ases (carries local traffic) • Multihomed AS: An AS that has connections to more than one ASes but refuses to carry transit traffic • Transit AS: An AS that has connections to more than one AS and carries both transit and local traffic

  5. Drawbacks of Link-State, DV Routing • Traffic is restricted to shortest path routes • Not all nodes may assign the same metrics to links • Difficult to obtain the same global view of the network • Conflicting information may lead to routing oscillations • Topology information flooded in the entire network • High processing overhead at each node to determine appropriate routes

  6. Path-Vector Routing • Key Idea: Advertise the entire path to a destination (AS) rather than the cost of the path • Based on the Distance Vector algorithm • Send cost metrics to the destination along with the entire routing path to different ASes. AS 2 AS 3 d : (AS 2, AS 3 AS 4) AS 4 AS 1 d AS 5 d: (AS 5 AS 4)

  7. Advantages of Path-Vector Routing • Fast loop detection • AS can locate its own identifier in the path to a destination • Node can simply discard paths with loops • E.g. AS 5 discards route update from AS 1 AS 2 AS 3 AS 4 AS 1 AS 5 d: (AS 1, AS 5, AS 4)

  8. Flexible Policy Implementation • Each node can apply local policies • Path selection: Which path to use? • Path export: Which paths to advertise? • Examples • Node 2 may prefer the path “2, 3, 1” over “2, 1” • Node 1 may not let node 3 hear the path “1, 2” AS 2 AS 3 AS 1

  9. Border Gateway Protocol (BGP) • Interdomain routing protocol for the Internet • Prefix-based path-vector protocol • Policy-based routing based on AS paths • Evolved during the past 15 years • Establish a TCP connection on port 179 • Exchange information on all known active routes • Exchange incremental updates AS 1 AS 2 BGP session

  10. Storing and Learning Routes • BGP router stores multiple routes toward a destination • Policies are applied for the selection of the “best” route • …and which routes shall be advertised • Incremental updates • Announcement • New active routes toward destinations are updated by sending BGP updates • Withdrawal • If a route is no longer active, a withdrawal notification is sent to all neighbors

  11. Routing in BGP • Destination prefix based • Includes route attributes such as next hop and AS path 192.168.0.5 AS 1 AS 3 AS 2 128.122.56.0/16 116.128.1.1 101.108.2.2 12.127.2.1 D: 128.122.56.0/16 AS_path = AS 1 Next _Hop = 192.168.0.5 D: 128.122.56.0/16 AS_path = AS 2 AS 1 Next _Hop = 12.127.2.1

  12. BGP Attributes • AS Path • Sequence of ASes that a route has traversed • Used for detecting loops and applying policies • Next Hop • Next hop to reach a network • Local Preference • Used to influence selection of paths • Path with highest local preference is selected • Multi-Exit Discriminator (MED) • Use to influence exit (entry) point when multiple BGP routers exist in an AS

  13. BGP Path Selection • Follow a minimum hop approach • Shortest AS path • Arbitrary tie break • Example • AS 2 routes to AS 6 through AS 3 • BGP is mainly policy-based routing 192.168.0.5 AS 1 AS 3 AS 2 116.128.1.1 101.108.2.2 12.127.2.1 D: 128.122.56.0/16 AS_path = AS 1 AS 4 AS 6 Next _Hop = 192.168.0.5 D: 128.122.56.0/16 AS_path = AS 3 AS 6 Next _Hop = 101.108.2.2

  14. BGP Policy • Import policy • Filter unwanted routes from neighbor • E.g. prefix that your customer doesn’t own • Manipulate attributes to influence path selection • E.g., assign local preference to favored routes • Export policy • Filter routes you don’t want to tell your neighbor • E.g., don’t tell a peer a route learned from other peer • Manipulate attributes to control what they see • E.g., make a path look artificially longer than it is

  15. Import Policy – Local Preference • Favor one path over another • Used to override shortest AS path principle • Example: prefer a customer AS AS 2 AS 3 Local pref =100 T-mobile AS 4 AS 1 AS 5 Sprint Local pref = 200

  16. Filtering • Import Policy • Discard certain advertised paths • Recognize network or AS based on prefix • Discard routes that contain large ISPs (may induce longer delays) • Discard pre-fixes if not owned • Export Policy • Limit propagation of routing information • Don’t announce routes from one peer to another • Don’t advertise routes containing untrusted ASes • Manipulate attributes to influence the way that other ASes perceive routes

  17. BGP Policy Configuration • Routing policy languages are vendor-specific • Not part of the BGP protocol specification • Different languages for Cisco, Juniper, etc. • Still, all languages have some key features • Policy as a list of clauses • Each clause matches on route attributes • … and either discards or modifies the matching routes • Configuration done by human operators • Implementing the policies of their AS • Business relationships, traffic engineering, security, …

  18. Other Considerations • AS_path can be misleading in terms of delay and # of hops AS 1 AS 4 AS 2 AS 3

  19. Treating Multiple Routers within an AS • BGP routing info is disseminated within an AS using iBGP eBGP AS 1 iBGP

  20. Combination of eBGP with iBGP E A B C D

  21. Hot-potato (deflection) Routing • Incentive • Minimize required storage, probability of buffer overflow • Key idea: Get rid of packets as soon as possible • Packets may not necessarily follow optimal paths (deflect packets wherever possible) • E.g., if two packets must go out on the same link, deflect one of them to another at random Local pref = 90 AS 1 AS 2 AS 3 Local pref = 200 AS 4 AS 5

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