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Interdomain Routing

Learn about the efficiency of hierarchical address space, subnetting, CIDR, and challenges of interdomain routing to enhance network scalability.

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Interdomain Routing

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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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