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Introduction to Routers and Switches

Introduction to Routers and Switches. What do they look like?. Access routers e.g. ISDN, ADSL. Core ATM switch. Core router e.g. OC48c POS. Routing Protocols. Routing Table. Forwarding Table. Switching. Basic Architectural Components of an IP Router. Control Plane. Datapath”

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Introduction to Routers and Switches

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  1. Introduction to Routers and Switches

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

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

  4. 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.

  5. General Switch Model Interconnect

  6. IP Switch Model 1. Ingress 2. Interconnect 3. Egress Forwarding Table Forwarding Decision Forwarding Table Forwarding Decision Forwarding Table Forwarding Decision

  7. Forwarding Engine Packet header payload Router Destination Address Routing Lookup Data Structure 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

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

  9. 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:

  10. 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

  11. 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 (the most specific route) among all prefixes that match the destination address.

  12. 32 Prefix Length 24 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

  13. 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

  14. 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

  15. Internal Interconnects 1. Multiplexers 2. Tri-State Devices 3. Shared Memory

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

  17. Shared Memory Bandwidth 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

  18. Input buffered swtich • Independent routing logic per input • FSM • Scheduler logic arbitrates each output • priority, FIFO, random • Head-of-line blocking problem Internconnect

  19. 58.6% Input QueueingHead of Line Blocking Delay Load 100%

  20. Head of Line Blocking

  21. (Virtual) Output Buffered Switch • How would you build a shared pool? N buffers per input

  22. Solving HOL with Input QueueingVirtual output queues

  23. Input QueueingVirtual Output Queues Delay Load 100%

  24. Output scheduling • n independent arbitration problems? • static priority, random, round-robin • simplifications due to routing algorithm? • general case is max bipartite matching

  25. A 1 2 B 3 C 4 D 5 E 6 F Finding a maximum size match Requests • How do we find the maximum size (weight) match? Inputs Outputs

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