Part 11 : Interdomain routing with BGP
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Slides supporting the "Computer Networking: Principles, Protocols and Practice" ebook. The slides can be freely reused to teach an undergraduate computer networking class using the open-source ebook.
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Part 11 : Interdomain routing with BGP
- 1. Week 11 Interdomain routing with BGP
- 2. Agenda • Interdomain routing • BGP basics • BGP convergence • Local Area Networks
- 3. Limitations of local-pref • In theory • Each domain is free to define its order of preference for the routes learned from external peers • How to reach 2001:db8:1/48 from AS3 and AS1 AS3 AS4 Preferred paths for AS4 1. AS3:AS1 2. AS1 Preferred paths for AS3 1. AS4:AS1 2. AS1 2001:db8:1/48
- 4. • AS1 sends its UPDATE messages ... AS1 AS3 AS4 2001:db8:1/48 Preferred paths for AS3 1. AS4:AS1 2. AS1 UPDATE lP: 2001:db8:1/48 lASPath: AS1 Routing table for AS3 2001:db8:1/48 ASPath: AS1 (best) Preferred paths for AS4 1. AS3:AS1 2. AS1 UPDATE lP: 2001:db8:1/48 lASPath: AS1 Routing table for AS4 2001:db8:1/48 ASPath: AS1 (best) Limitations of local-pref
- 5. • First possibility • AS3 sends its UPDATE first... AS1 AS3 AS4 Preferred paths for AS3 1. AS4:AS1 2. AS1 2001:db8:1/48 Routing table for AS3 2001:db8:1/48 ASPath: AS1 (best) UPDATE lP: 2001:db8:1/48 lASPath: AS3:AS1 Preferred paths for AS4 1. AS3:AS1 2. AS1 Routing table for AS4 2001:db8:1/48 ASPath: AS1 2001:db8:1/48 ASPath:AS3:AS1 (best) • Stable route assignment Limitations of local-pref
- 6. • AS4 sends its UPDATE first... AS1 AS3 AS4 l2001:db8:1/48 Preferred paths for AS4 1. AS3:AS1 2. AS1 Routing table for AS4 2001:db8:1/48 ASPath: AS1 (bes Preferred paths for AS3 1. AS4:AS1 2. AS1 UPDATE lPrefix: 2001:db8:1/48 lASPath: AS4:AS1 Routing table for AS3 2001:db8:1/48 ASPath: AS1 2001:db8:1/48 ASPath: AS4:AS1 (best) • Another (but different) stable route assignment Limitations of local-pref
- 7. • AS3 and AS4 send their UPDATE together... AS1 AS3 AS4 Preferred paths for AS3 1. AS4:AS1 2. AS1 2001:db8:1/48 UPDATE lP: 2001:db8:1/48 lASPath: AS3:AS1 Preferred paths for AS4 1. AS3:AS1 2. AS1 UPDATE lP: 2001:db8:1/48 ASPath: AS4:AS1 • AS3 prefers indirect path -> withdraw • AS4 prefers indirect path -> withdraw Limitations of local-pref
- 8. • AS3 and AS4 send their UPDATE together... AS1 AS3 AS4 Preferred paths for AS3 1. AS4:AS1 2. AS1 2001:db8:1/48 Preferred paths for AS4 1. AS3:AS1 2. AS1 WITHDRAW lP: 2001:db8:1/48 • AS3 : indirect route is not available anymore • AS3 will reannounce its direct route... WITHDRAW lP: 2001:db8:1/48 • AS4 : indirect route is not available anymore • AS4 will reannounce its direct route... Limitations of local-pref
- 9. More limitations local pref • Unfortunately, interdomain routing may not converge at all in some cases... • How to reach a destination inside AS0 in this case ? AS1 AS3 AS4 Preferred paths for AS3 1. AS4:AS0 2. other paths AS0 Preferred paths for AS4 1. AS1:AS0 2. other paths Preferred paths for AS1 1. AS3:AS0 2. other paths
- 10. Simple example AS1 AS2 AS3 $ Customer-provider Shared-cost $ • AS1 announces one prefix, p • What is the BGP routing table on AS2 ? • What is the BGP routing table on AS3 ?
- 11. Simple example • AS1 announces one prefix, p AS1 AS2 AS3 AS5 $ Customer-provider Shared-cost $ $ AS4 $ AS4 BGP p via AS1 p via AS1 AS4 BGP p via AS1 p via AS2:AS1 p via AS3:AS2:AS1 AS4 BGP p via AS1 p via AS3:AS2:AS
- 12. Simple example • AS1 announces one prefix, p AS1 AS2 AS3 AS5 $ Customer-provider Shared-cost $ $ AS4 $ $ $ p via AS1 p via AS2:AS1 p via AS3:AS2:AS1 p via AS2:AS1 p via AS1 p via AS4:AS3:AS2:AS1
- 13. Simple example • AS1 announces one prefix, p • link AS1-AS2 fails AS1 AS2 AS3 AS5 $ Customer-provider Shared-cost $ $ AS4 $ $ $ AS2 BGP p via AS1 AS3 BGP p via AS2:AS1 AS4 BGP p via AS1 p via AS2:AS1 p via AS3:AS2:AS Withdraw(p) Withdraw(p) Withdraw(p) Withdraw(p)
- 14. Simple example • AS1 announces one prefix, p • AS2-AS4 fails AS1 AS2 AS3 AS5 $ Customer-provider Shared-cost $ $ AS4 $ $ AS2 BGP p via AS1 AS3 BGP p via AS2:AS1 AS4 BGP p via AS1 p via AS2:AS1 p via AS3:AS2:AS
- 15. Simple example • AS1 announces one prefix, p • AS3-AS4 is initially down and then comes up AS1 AS2 AS3 $ Customer-provider Shared-cost $ $ AS4 $ AS5 $ AS4 BGP p via AS1 p via AS2:AS1 p via AS3:AS2:AS1
- 16. local-pref and economical relationships • In practice, local-pref is often combined with filters to enforce economical relationships AS1 Prov1 Prov2 Peer1 Peer2 Peer3 Peer4 Cust1 Cust2 $ Customer-provider $ Shared-cost $ $ $ Local-pref values used by AS1 > 1000 for the routes received from a Customer 500 – 999 for the routes learned from a Peer < 500 for the routes learned from a Provider
- 17. local-pref • Which path will be used by AS1 to reach AS5 ? • and how will AS5 reach AS1 ? AS1 AS4 AS2 AS3 AS5 $ Customer-provider Shared-cost $ $ $ $ $ AS8 $ AS6 AS7 $ $ Internet paths are often asymmetrical
- 18. Internet 1990s • NSFNet • American backbone • AUP : no commercial traffic • Some regional networks • US regions • national networks in Europe • Universities/research labs • connected to regional networks or directly to
- 19. Internet early 2000s • Tier-1 ISPs • Dozen transit ISPs shared- cost • Uunet, Level3, OTIP, ... • Tier-2 ISPs • Regional/ National ISPs • Tier-3 ISPs • Smaller ISPs, Entreprise Networks, Content providers • Customers of T2 or T1 ISPs • shared-cost with other T3
- 20. Today’s Internet • Hyper Giants • google, microsoft, yahoo, amazon, ... • google peers 70% ISPs • Tier-1 ISPs • Tier-2 ISPs • Tier-3 ISPs • Many peerings at IXPs Craig Labovitz), Scott Iekel-Johnson, Danny McPherson, Jon Oberheide, Farnam Jahanian, Internet Inter-Domain Traffic, SIGCOMM 2010
- 21. Internet size (#AS) Source: http://bgp.potaroo.net
- 22. BGP IPv6 routing tables Source: http://bgp.potaroo.net
- 23. BGP : IPv4 routing tables Source: http://bgp.potaroo.net
- 24. AS7007 incident
- 27. Agenda • Interdomain routing • Local Area Networks • Ethernet • WiFi
Editor's Notes
- In practice, the exchange of BGP UPDATE messages will cease due to the utilization of timers by BGP routers and the routing will stabilize on one of the two stable route assignments.
- Due to the utilization of the local-pref attribute, some paths on the Internet are longer than their optimum length, see : Lixin Gao and Feng Wang , The Extent of AS Path Inflation by Routing Policies, GlobalInternet 2002
- Due to the utilization of the local-pref attribute, some paths on the Internet are longer than their optimum length, see : Lixin Gao and Feng Wang , The Extent of AS Path Inflation by Routing Policies, GlobalInternet 2002
- Due to the utilization of the local-pref attribute, some paths on the Internet are longer than their optimum length, see : Lixin Gao and Feng Wang , The Extent of AS Path Inflation by Routing Policies, GlobalInternet 2002
- Due to the utilization of the local-pref attribute, some paths on the Internet are longer than their optimum length, see : Lixin Gao and Feng Wang , The Extent of AS Path Inflation by Routing Policies, GlobalInternet 2002
- Due to the utilization of the local-pref attribute, some paths on the Internet are longer than their optimum length, see : Lixin Gao and Feng Wang , The Extent of AS Path Inflation by Routing Policies, GlobalInternet 2002
- Due to the utilization of the local-pref attribute, some paths on the Internet are longer than their optimum length, see : Lixin Gao and Feng Wang , The Extent of AS Path Inflation by Routing Policies, GlobalInternet 2002
- This local-pref settings corresponds to the economical relationships between the various ASes. Since AS1 is paid to carry packets towards Cust1 and Cust2, it will select a route towards those networks whenever possible. Since AS1 does not need to pay to carry packets towards Peer1-4, AS1 will select a route towards those networks whenever possible. AS1 will only utilize the routes receive from its providers when there is no other choice. It is shown in the following papers that this way of utilizing the local-pref attribute leads to stable BGP routes : Lixin Gao, Timothy G. Griffin, and Jennifer Rexford, "Inherently safe backup routing with BGP," Proc. IEEE INFOCOM, April 2001 Lixin Gao and Jennifer Rexford, "Stable Internet routing without global coordination," IEEE/ACM Transactions on Networking, December 2001, pp. 681-692 The RPSL policy of AS1 could be as follows : RPSL policy for AS1 aut-num: AS1 import: from Cust1 action set localpref=200; accept Cust1 from Cust2 action set localpref=200; accept Cust2 from Peer1 action set localpref=150; accept Peer1 from Peer2 action set localpref=160; accept Peer2 from Peer3 action set localpref=170; accept Peer3 from Peer4 action set localpref=180; accept Peer4 from Prov1 action set localpref=100; accept ANY from Prov2 action set localpref=100; accept ANY
- Due to the utilization of the local-pref attribute, some paths on the Internet are longer than their optimum length, see : Lixin Gao and Feng Wang , The Extent of AS Path Inflation by Routing Policies, GlobalInternet 2002
- See : L. Subramanian, S. Agarwal, J. Rexford, and RH Katz. Characterizing the Internet hierarchy from multiple vantage points. In IEEE INFOCOM, 2002