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Scalability Challenges in Interdomain Routing and Address Management

This document discusses the intricacies of interdomain routing and the efficiency of hierarchical addressing schemes in network design. It highlights issues related to classful addressing, particularly the inefficiencies of Class B and Class C addresses, and presents subnetting strategies to enhance routing scalability. The significance of Autonomous Systems (AS) is explored, along with the role of Classless Inter-Domain Routing (CIDR) in managing routing tables. Additionally, it addresses the complexities of route propagation and the need for flexibility in routing decisions amongst different networks.

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Scalability Challenges in Interdomain Routing and Address Management

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  1. Interdomain Routing CS 332

  2. How to Make Routing Scale • Flat versus Hierarchical Addresses • Inefficient use of Hierarchical Address Space • class C with 2 hosts (2/255 = 0.78% efficient) • class B with 256 hosts (256/65535 = 0.39% efficient) • Demand for Class B the problem. So why not just assign 2 class C’s for a 50% efficiency rate? • Still Too Many Networks • routing tables do not scale • route propagation protocols do not scale CS 332

  3. NSFNET backbone Stanford ISU BARRNET MidNet … regional regional Westnet regional Berkeley P ARC UNL KU UNM NCAR UA Internet Structure Recent Past CS 332

  4. Internet Structure • Autonomous system (AS) • Administered Independently of other ASs • Want to be able to control various ways in which network is configured, used, etc. • Select their own intranetwork routing protocol • Perhaps select own link metrics, etc. • Advantageous because it provides finer hierarchy • Good for scalability CS 332

  5. Network number Host number Class B address 111111111111111111111111 00000000 Subnet mask (255.255.255.0) Network number Subnet ID Host ID Subnetted address Subnetting • Add another level to address/routing hierarchy: subnet • Subnet masks define variable partition of host part • Subnets visible only within site CS 332

  6. Subnet Mask • Written in dotted quad notation (like IP addresses) • Exactly one mask per subnet (all hosts on given subnet have same subnet mask) • Subnet number of host (or of subnet) = bitwise AND of subnet mask and IP address 11111111 11111111 11111111 10000000 10000000 01100000 00100010 00001111 10000000 01100000 00100010 00000000 CS 332

  7. Subnetting (cont) • To send IP packet: • Host performs bitwise AND of its subnet mask with destination IP address • If result is same subnet number as sending host, then destination is on same subnet, so forward directly (Note: Arp unaffected) • Else send packet to a router to be forwarded to another subnet • New routing table entries: <SubnetNumber, SubnetMask, NextHop> replaces <NetworkNumber, NextHop> CS 332

  8. Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 128.96.34.15 128.96.34.1 H1 R1 Subnet mask: 255.255.255.128 128.96.34.130 Subnet number: 128.96.34.128 128.96.34.139 128.96.34.129 H2 R2 H3 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 Subnet Example Forwarding table at router R1 Subnet Number Subnet Mask Next Hop 128.96.34.0 255.255.255.128 interface 0 128.96.34.128 255.255.255.128 interface 1 128.96.33.0 255.255.255.0 R2 CS 332

  9. Forwarding Algorithm D = destination IP address for each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D if D1 = SubnetNum if NextHop is an interface deliver datagram directly to D else deliver datagram to NextHop • Use a default router if nothing matches • Not necessary for all 1s in subnet mask to be contiguous • Can put multiple subnets on one physical network(?!) • Subnets not visible from the rest of the Internet CS 332

  10. The Key • It’s important to remember that both subnetting and supernetting are attempts to help make routing scale • Even for an AS like U of R, subnetting can help shrink routing tables, though this isn’t really a serious issue here • Supernetting is really intended to make Internet routing scale – it benefits primarily the internet service providers and backbone routers, where the real scale problems exist. CS 332

  11. 3185* 319* 52* 31* 51* 5* backbone 317* 3172* 534* 7* 3174* 73* 76* 748* 317483* 317482* Supernetting (CIDR) • What we’re shooting for: CS 332

  12. Supernetting (CIDR) • Called CIDR: Classless Inter-Domain Routing • Assign block of contiguous network numbers to nearby networks (in same AS or using same ISP) • Aggregates routes: single entry for many networks • E.g. Class B addresses 192.4.16-192.4.31 have same top 20 bits, so a single 20 bit network address gets packets to correct AS. • Restrict block sizes to powers of 2 • Represent network numbers with(length, value)pair • All routers must understand CIDR addressing CS 332

  13. Supernetting (CIDR) • Assign block of contiguous network numbers to nearby networks • Called CIDR: Classless Inter-Domain Routing • Represent blocks with a single pair (first_network_address, count) • Restrict block sizes to powers of 2 CS 332

  14. Interdomain Routing • Much more difficult than intradomain routing • Scale: Internet backbone router has 50,000+ prefixes • Impossible to calculate path costs: Different ASs mean different link-state metrics which may not be comparable. • Focus is on reachability, not optimality, and this is plenty difficult all by itself • Trust: If you trust another AS, you trust their routing advertisements and their network system configuration info. • Need for flexibility: “Use provider A only for these addresses”, “Use AS X in preference to AS Y”, etc. CS 332

  15. Route Propagation • Know a smarter router • hosts know local router (on same physical network) • local routers know how to get to border router (and to each other) • Regional ISP routers know how to get to its customers, and also to a border (gateway) router to a backbone provider • Backbone (core) routers know everything (or at least how to get what they need) CS 332

  16. Route Propagation • Two-level route propagation hierarchy • interior gateway protocol (each AS selects its own) • Also called intradomain routing • exterior gateway protocol (Internet-wide standard) • Also called interdomain routing • Note again efficiency of default routes (AS need only know inside AS and how to get out of AS) CS 332

  17. EGP: Exterior Gateway Protocol • Overview • designed for tree-structured Internet • This and other limitations caused it to be replaced by BGP • Protocol messages • neighbor acquisition: one router requests that another be its peer; peers exchange reachability information • neighbor reachability: one router periodically tests if the another is still reachable; exchange HELLO/ACK messages; uses a k-out-of-n rule • routing updates: peers periodically exchange their routing tables (distance-vector) CS 332

  18. Large corporation “ ” Consumer ISP Peering point Backbone service provider Peering point Consumer ” ISP “ “ Consumer ISP ” Large corporation Small corporation Internet Structure Today multihomed AS transit ASs stub AS CS 332

  19. BGP-4: Border Gateway Protocol • Concept of AS Types • stub AS: has a single connection to one other AS • carries local traffic only • multihomed AS: has connections to more than one AS • refuses to carry transit traffic • transit AS: has connections to more than one AS • carries both transit and local traffic CS 332

  20. BGP-4: Border Gateway Protocol • Each AS has (aside from possibly 16 bit ID): • one or more border routers (need not be same as the BGP speaker) • one BGP speaker that advertises (to other BGP speakers): • local networks • other reachable networks (transit AS only) • gives complete path information (neither DVR nor link-state, though closer to DVR) • Avoids loops CS 332

  21. BGP-4: Border Gateway Protocol Border routers CS 332

  22. 128.96 Customer P 192.4.153 (AS 4) Regional provider A (AS 2) Customer Q 192.4.32 (AS 5) 192.4.3 Backbone network (AS 1) Customer R 192.12.69 (AS 6) Regional provider B (AS 3) Customer S 192.4.54 (AS 7) 192.4.23 BGP Example • Speaker for AS2 advertises reachability to P and Q • network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2 • Speaker for backbone advertises • networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path (AS1, AS2). • Speaker can cancel previously advertised paths stubs Transit networks CS 332

  23. Avoiding Loops • Because of full path info, this scenario can be avoided: • AS 1 learns it can reach • Network 10.0.1 through • AS 2, it advertises this to • AS 3, who in turn advertises • it back to AS 2. If AS 2 • decides that it should send • packets for 10.0.1 through • AS 3, we’ve got a loop. AS 1 AS 3 AS 2 CS 332

  24. Final BGP Notes • BGP was designed to work with CIDR, so the “network” numbers that are passed around are really variable length prefixes, as used in CIDR • Typically written 144.166.206/19 and the like • Number of nodes participating in BGP is on order of number of Ass (much smaller than number of networks) • Finding good interdomain route amounts to finding path to the right border router, and there are only a few of these per AS • Complexity of intradomain routing is on order of number of networks in the particular AS CS 332

  25. Integrating Intra and Inter • Stub AS (very common): border router “injects” default route into intradomain protocol • Non-stub, but non backbone: Border routers inject learned (either through BGP or static config) info into intradomain protocol • Backbone: IBGP (interior BGP): Too much info to inject into traditional intradomain protocol (10,000 prefixes = > big LSP + complex shortest path info). Traditional intradomain + protocols for querying border routers. CS 332

  26. Scalability (again) • Nodes using BGP = O(number of ASs) • Finding good interdomain route = finding path to correct border router (few per AS) • Complexity of intradomain = O(number physical networks in AS) • Tradeoff between scalability and optimality • Hierarchy hides info, hinders optimality • Hiding info key to scaling, since nodes don’t need global info • In large networks, scalability more important CS 332

  27. IP Version 6 • Features • 128-bit addresses (classless) • multicast • real-time service • authentication and security • autoconfiguration • end-to-end fragmentation • protocol extensions • Header • 40-byte “base” header • extension headers (fixed order, mostly fixed length) • fragmentation • source routing • authentication and security • other options CS 332

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