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IP Router Design and Optics Tutorial

A quick tutorial on IP router design, optics, and routing with a focus on the internals of best-effort and QoS routers. Exploring the potential for optics in router design.

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IP Router Design and Optics Tutorial

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  1. A quick tutorial on IP Router design Optics and Routing Seminar October 10th, 2000 Nick McKeown nickm@stanford.edu http://www.stanford.edu/~nickm 1

  2. Outline • Where IP routers sit in the network • What IP routers look like • What do IP routers do? • Some details: • The internals of a “best-effort” router • Lookup, buffering and switching • The internals of a “QoS” router • Can optics help? 2

  3. Outline (next time) • The way routers are really built. • Evolution of their internal workings. • What limits their performance. • The effect that DWDM is having on switch/router design. • The way the network is built today. • Discussion: The scope for optics 3

  4. Outline • Where IP routers sit in the network • What IP routers look like • What do IP routers do? • Some details: • The internals of a “best-effort” router • Lookup, buffering and switching • The internals of a “QoS” router • Can optics help? 4

  5. The Internet is a mesh of routers (in theory) IP Core router The Internet Core IP Edge Router 5

  6. What do they look like? Access routers e.g. ISDN, ADSL Core ATM switch Core router e.g. OC48c POS 6

  7. Basic Architectural Componentsof an IP Router Routing Protocols Routing Table Control Plane Datapath” per-packet processing Forwarding Table Switching 7

  8. Per-packet processing in an IP Router 1. Accept packet arriving on an incoming link. 2. Lookup packet destination address in the forwarding table, to identify outgoing port(s). 3. Manipulate packet header: e.g., decrement TTL, update header checksum. 4. Send packet to the outgoing port(s). 5. Buffer packet in the queue. 6. Transmit packet onto outgoing link. 8

  9. Outline • Where IP routers sit in the network • What IP routers look like • What do IP routers do? • Some details: • The internals of a “best-effort” router • Lookup, buffering and switching • The internals of a “QoS” router • Can optics help? 9

  10. Basic Architectural ComponentsDatapath: per-packet processing 1. Ingress 2. Interconnect 3. Egress Forwarding Table Forwarding Decision Forwarding Table Forwarding Decision Forwarding Table Forwarding Decision 10

  11. Forwarding Engine Packet header payload Router Routing Lookup Data Structure Destination Address Outgoing Port Forwarding Table Dest-network Port 65.0.0.0/8 3 128.9.0.0/16 1 149.12.0.0/19 7 11

  12. The Search Operation is not a Direct Lookup (Outgoing port, label) (Incoming port, label) Memory Address Data IP addresses: 32 bits long  4G entries 12

  13. The Search Operation is also not an Exact Match Search • Hashing • Balanced binary search trees Exact match search:search for a key in a collection of keys of the same length. Relatively well studied data structures: 13

  14. 128.9.16.14 Example Forwarding Table Prefix length IP prefix: 0-32 bits 142.12.0.0/19 128.9.0.0/16 65.0.0.0/8 0 232-1 224 65.0.0.0 65.255.255.255 14

  15. 128.9.16.14 Prefixes can Overlap Longest matching prefix 128.9.176.0/24 128.9.16.0/21 128.9.172.0/21 142.12.0.0/19 65.0.0.0/8 128.9.0.0/16 0 232-1 Routing lookup:Find the longest matching prefix (aka the most specific route) among all prefixes that match the destination address. 15

  16. 32 24 Prefix Length 8 128.9.16.14 Prefixes Difficulty of Longest Prefix Match 128.9.176.0/24 128.9.16.0/21 128.9.172.0/21 142.12.0.0/19 128.9.0.0/16 65.0.0.0/8 16

  17. 31.25 Mpps  33 ns DRAM: 50-80 ns, SRAM: 5-10 ns Lookup Rate Required Year Line Line-rate (Gbps) 40B packets (Mpps) 1998-99 OC12c 0.622 1.94 1999-00 OC48c 2.5 7.81 2000-01 OC192c 10.0 31.25 2002-03 OC768c 40.0 125 17

  18. Source: http://www.telstra.net/ops/bgptable.html 10,000/year Size of the Forwarding Table Number of Prefixes 95 96 97 98 99 00 Year 18

  19. Basic Architectural ComponentsDatapath: per-packet processing 1. Ingress 2. Interconnect 3. Egress Forwarding Table Forwarding Decision Forwarding Table Forwarding Decision Forwarding Table Forwarding Decision 19

  20. InterconnectsTwo basic techniques Input Queueing Output Queueing Usually a non-blocking switch fabric (e.g. crossbar) Usually a fast bus 20

  21. Memory b/w = 2N.R Memory b/w = (N+1).R InterconnectsOutput Queueing Individual Output Queues Centralized Shared Memory 1 2 N 1 2 N 21

  22. InterconnectsCentralized Shared Memory Numerous work has proven and made possible: • Fairness • Delay Guarantees • Delay Variation Control • Loss Guarantees • Statistical Guarantees Output 1 Input 1 Input 2 Output 2 Large, single dynamically allocated memory buffer: N writes per “cell” time N reads per “cell” time. Limited by memory bandwidth. Input N Output N 22

  23. Output QueueingHow fast can we make centralized shared memory? 5ns SRAM Shared Memory • 5ns per memory operation • Two memory operations per packet • Therefore, up to 160Gb/s • In practice, closer to 80Gb/s 1 2 N 200 byte bus 23

  24. Memory b/w = 2R InterconnectsInput Queueing with Crossbar Arbiter Data In Data Out configuration 24

  25. 58.6% Input QueueingHead of Line Blocking Delay Load 100% 25

  26. Head of Line Blocking 26

  27. Input QueueingVirtual output queues 27

  28. Input QueueingVirtual Output Queues Delay Load 100% 28

  29. Complex! Memory b/w = 2R Input QueueingVirtual output queues Arbiter 29

  30. Other Non-Blocking FabricsClos Network 30

  31. Expansion factor required = 2-1/N Other Non-Blocking FabricsClos Network 31

  32. Other Non-Blocking FabricsSelf-Routing Networks 000 000 001 001 010 010 011 011 100 100 101 101 110 110 111 111 32

  33. Other Non-Blocking FabricsSelf-Routing Networks The Non-blocking Batcher Banyan Network Bitonic Sorter Self-Routing Network 3 7 7 7 7 7 7 000 7 2 5 0 4 6 6 001 5 3 2 5 5 4 5 010 2 5 3 1 6 5 4 011 6 6 1 3 0 3 3 100 0 1 0 4 3 2 2 101 1 0 6 2 1 0 1 110 4 4 4 6 2 2 0 111 • Fabric can be used as scheduler. • Batcher-Banyan network is blocking for multicast. 33

  34. Outline • Where IP routers sit in the network • What IP routers look like • What do IP routers do? • Some details: • The internals of a “best-effort” router • Lookup, buffering and switching • The internals of a “QoS” router • Can optics help? 34

  35. Admission Control Reservation Packet Classification Output Scheduling Policing & Access Control Ingress Interconnect Egress 1. 2. 3. Basic Architectural Components Routing Protocols Routing Table Control Plane Datapath” per-packet processing Forwarding Table Switching 35

  36. Limitation: Memory b/w Limitation: Interconnect b/w Power & Arbitration Limitation: Memory b/w Basic Architectural ComponentsDatapath: per-packet processing 1. Ingress 2. Interconnect 3. Egress Classifier Table Forwarding Table Policing & Access Control Forwarding Decision Classifier Table Forwarding Table Policing & Access Control Forwarding Decision Classifier Table Forwarding Table Policing & Access Control Forwarding Decision 36

  37. Outline • Where IP routers sit in the network • What IP routers look like • What do IP routers do? • Some details: • The internals of a “best-effort” router • Lookup, buffering and switching • The internals of a “QoS” router • Can optics help? 37

  38. Can optics help? Cynical view: • A packet switch (e.g. an IP router) must have buffering. • Optical buffering is not feasible. • Therefore, optical routers are not feasible. • Hence, “optical switches” are circuit switches (e.g. TDM, space or Lambda switches). 38

  39. Can optics help? Open-minded view: • Optics seem ill-suited to processing intensive functions, or where random access memory is required. • Optics seems well-suited to bufferless, reconfigurable datapaths. 39

  40. Can optics help? Typical IP Router Linecard Lookup Tables Buffer & State Memory Buffered or Bufferless Fabric Packet Processing Buffer Mgmt & Scheduling Physical Layer Framing & Maintenance Buffer Mgmt & Scheduling Buffer & State Memory Arbitration OC192c linecard: ~10-30M gates ~2Gbits of memory ~2 square feet >$10k cost Backplane 40

  41. Can optics help? • Linecard? • The linecard is processing & memory intensive. • Interconnect? • Arbitration is very processing intensive. • The fabric can be a bufferless datapath… • How fast can an optical datapath be reconfigured? 41

  42. Outline for next time… • The way IP routers are really built. • Evolution of their internal workings. • What limits their performance. • The effect that DWDM is having on switch/router design. • The way the network is built today. • Discussion: The scope for optics 42

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