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Understanding BGP and Address Allocation: Hierarchies, CIDR, and Routing Policies

This documentation delves into the intricacies of the Border Gateway Protocol (BGP), hierarchical addressing, and the significance of Classless Interdomain Routing (CIDR) in managing address space and aggregation. It outlines how IP address assignments are orchestrated, the problems inherent in classful addressing, and the evolution towards classless approaches to mitigate route table growth and address depletion. Additionally, it explores key concepts of routing policies, multihoming, and the role of Autonomous System Numbers (ASNs) in facilitating efficient internet connectivity.

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Understanding BGP and Address Allocation: Hierarchies, CIDR, and Routing Policies

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  1. 6.829 BGP Recitation Rob Beverly <rbeverly@mit> September 29, 2006

  2. Addressing and Assignment

  3. Area-Routing • Review… • Why does Internet Scale? • Hierarchical Addressing • How are addresses assigned? • Classfull Addressing: • e.g. class A -> first bit 0, 7 bits network, 24 bits host • What’s wrong with classes? • Classless Interdomain Routing (CIDR)

  4. CIDR • Stop-gap measure to prevent: • Address depletion • Route table growth • Arbitrary network boundaries (not byte) • Allows for proper sizing (not just 2^{8,16,24}) • Allows for aggregation • Stroke Format: prefix/mask • e.g. 18.0.0.0/8

  5. CIDR • Example: • 198.61.4.0/24 (class C) • 198.61.5.0/24 (class C) • Aggregate as: 198.61.4.0/23 • What about: • 198.61.3.0/24 (class C) • 198.61.4.0/24 (class C) • Can this be aggregated as: 198.61.3.0/23? No! • 3 = (binary) 00000011 • 4 = (binary) 00000100 • Differ in first 7 bits, so cannot aggregate

  6. Routing Nomenclature • We use lots of acronyms, keep them straight: • IGP: interior gateway protocol, • e.g. OSPF, ISIS, RIP • Optimized for: Shortest Path, loop-free • EGP: exterior gateway protocol, • e.g. BGP • Optimized for: scalability, policy • BGP types: • iBGP: internal BGP • eBGP: external BGP • iBGP != IGP

  7. Brief Tangent: AS & IP Assignment • Useful information for pset and debugging • Who assigns IP addresses? • ARIN to regional registries (RIRs) who subdelegate • Lookup: athena$ whois –h whois.arin.net 18.26.0.25 • Who assigns AS numbers? • ARIN • Range? 2^16 • Lookup: athena$ whois –h whois.arin.net “AS3” • Who maintains IP->AS (or AS->IP) mapping? • Not centralized • Lookup: routing table

  8. BGP Policy

  9. BGP • Autonomous System Numbers (ASNs) • Routing preference: • Customers: advertise all routes to customers, import their routes • Peers: advertise my customers to my peers, import their routes • Providers: advertise my customers, import their routes

  10. BGP Policy • Motivation for peering rules • Org peers with Sprint • Sprint peers with UUNET • Org gets transit from UUNET UUNET dst1 Org Sprint src dst2 Why would Sprint not advertise UUNET routes to Org?

  11. BGP by Example: Hot-Potato ISP A src dst ISP B ISP A and B peer on both east and west coasts

  12. BGP by Example: Hot-Potato ISP A src dst ISP B Consider src to dst conversation

  13. BGP by Example: Hot-Potato ISP A src dst ISP B Note Asymmetric path! – Very Common

  14. BGP by Example: Multihoming • Most customers don’t run BGP: • Simply default route to ISP • ISP injects customer route into BGP (or customer’s address space is from ISP) • Why BGP for multihoming? • Scenarios: • Customer has own address space • Customer has provider address space • Customer multihomes with single ISP • Customer multihomes with two ISPs

  15. Multihoming to Single Provider ISP A Consider two default routes here. What is the outcome? Cust ISP and changes within ISP affect customer’s inbound traffic! Load share outbound traffic: reordering!

  16. Multihoming to Single Provider ISP A Advertise: 18.0.0.0/8 128.30.0.0/16 + MEDs Advertise: 18.0.0.0/8 128.30.0.0/16 + MEDs Cust 18.0.0.0/8 128.30.0.0/16

  17. Multihoming to Single Provider ISP sets localprefs differently on inbound customer routes perhaps based on community ISP A Advertise: 18.0.0.0/8 128.30.0.0/16 Advertise: 18.0.0.0/8 128.30.0.0/16 Cust 18.0.0.0/8 128.30.0.0/16

  18. Multihoming: Own Address Space ISP A ISP B Advertise: 18.0.0.0/8 18.128.0.0/9 Advertise: 18.0.0.0/8 18.0.0.0/9 Cust 18.0.0.0/8

  19. Multihoming: Own Address Space ISP A ISP B backup Advertise: 18.0.0.0/8 Advertise: 18.0.0.0/8 with ASPATH Pre-pending Cust 18.0.0.0/8

  20. Multihoming: Provider Address Space 204.0.0.0/8 204.128.207.0/24 204.0.0.0/8 ISP A ISP B LPM Cust Punches holes in the aggregated routing tables 204.128.207.0/24

  21. Route Reflection

  22. iBGP • Need to distribute routes within the AS • Why not inject into IGP? • How many BGP routes? Lots, ~100-200k • Scalability of link-state database • Too much control traffic flooding • Use iBGP full mesh internally • Never redistribute a route heard via iBGP to other iBGP neighbors

  23. iBGP • Physical and logical topology may be very different • Must have an IGP running first to establish TCP-based BGP sessions Physical Topology Logical Topology

  24. Route Reflection • Full-mesh of iBGP sessions • Requires (n(n-1)/2) iBGP sessions • Not scalable: e.g. 50 routers = 1225 sessions • Solution: hierarchy plus minor tweak to BGP protocol • New types: • Route Reflector (RR) • Route Reflector Client (RC) (no change)

  25. Route Reflection by Example • RRs redistribute routes from RCs to all iBGP neighbors (other RRs) • RRs redistribute routes from all iBGP neighbors to their RCs RR2 RC1 RC3 RR1 RR3 RC2 RC4

  26. Route Reflection by Example • RR1 redistributes to RR2 and RR3 • RR1 redistributes to her client RC1 • RR3 redistributes to RC3 and RC4 RR2 RC1 RC3 RR1 RR3 RC2 RC4 18.0.0.0/8 AS3

  27. BGP Badness

  28. SIGCOMM06: Wang et. al • “A Measurement Study on the Impact of Routing Events on E2E Internet Path Performance” • Experimental Methodology: • BGP Beacon multihomed to 2 AS • Advertises and withdraws on predetermined schedule • Planetlab Active Measurement 37-to-1 • UDP, Ping, Traceroute

  29. Experimental Methodology plab1 plab2 plab3 plab4 Internet BGP Beacon withdraws r1 and r2 on predetermined schedule. Restores r1 And r2 on schedule. r2 r1 BGP Beacon

  30. Routing Events + Path Perf • Loss correlated to both withdrawals and restores • Observe two periods of loss on a withdrawal • Observe loss even when second path is restored – non-intuitive • High-level reasons: BGP policy limiting advertisements, MRAI timer

  31. E2E Traffic Probe Paper attempts to explain these two loss events Seq no Received packet order

  32. Route Withdrawal == loss ISP C = customer Plab Box = peer = traffic flow ISP A ISP B Beacon

  33. Route Withdrawal == loss ISP C Plab Box withdraw withdraw ISP A ISP B withdraw Beacon

  34. Route Withdrawal == loss = traffic flow ISP C Plab Box announce ISP A ISP B Beacon

  35. Route Withdrawal == loss ISP C Plab Box withdraw Z ISP A ISP B Beacon Z sends withdrawal because of policy: never send peer route to another peer

  36. Route Restoration == loss?!? = customer Plab Box = peer = traffic flow Beacon

  37. Route Restoration == loss?!? Plab Box A B Z advertise C advertise C advertises beacon route to B, but waits (due to MRAI) before sending to A Beacon

  38. Route Restoration == loss?!? Plab Box withdraw A B Z ?? No route C B can’t advertise route from C to A because of iBGP rules. Since B has new best route via C, B must poison previous route via Z. Sends withdrawal to A. A has no route, traffic dropped. Beacon

  39. Route Restoration == loss?!? Plab Box withdraw A B Z MRAI timer fires. Announce route, traffic restored. C Beacon

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