CPS 356: Introduction to Computer Networks Lecture 7: Switching technologies Ch 2.8.2, 3.1, 3.4

1. CPS 356: Introduction to Computer NetworksLecture 7: Switching technologiesCh 2.8.2, 3.1, 3.4 Xiaowei Yang xwy@cs.duke.edu

2. Review: link layer functions Encoding NRZ, NRZI, Manchester, 4B/5B Framing: bit, byte, time-synchronization Error detection Parity bits, Internet checksum, CRC Reliable transmission Stop-and-wait Sliding window Concurrent logical channels Media Access Control Ethernet, Token Ring, Wifi (today)

3. Ethernet: Physical properties Sensing the line; if idle, sends signals CSMA/CD: Carrier Sense Multiple access with Collision Detection 10Base5

4. Collision Domain • Any host hears any other host • A single segment • Multiple segments connected by repeaters • Multiple segments connected by a hub

5. Transmitter Algorithm Begin: Wait until the line is idle and has data to send, the adaptor sends it, and listens to collision If no, go back to Begin else exponentially backoff randomly selects a k between [0,2n-1], waits for k x 51.2 μs to try Begin again Gives up after n reaches 16

6. Collision detection An adaptor senses the signals on the line and compares it with its own If same, no collision; otherwise, collision Sends 32-bit jamming sequence after collision In the worst case, a sender needs to send 512 bits (46+14+4 = 64B) to detection collision Why?

7. One way delay is d A needs to send for 2d duration to detect collision 2d = 512 μs. On a 10Mps Ethernet, corresponds to 512 bits

8. The IEEE 802.3 Baseband 5-4-3 • Five physical segments between any two nodes • Four repeaters between the nodes. • Three of these physical segments can have connected node • Each segment < 500m  Total < 2500m

9. Propagation delay for this maximum-extent Ethernet network is 25.6us • 2*d = 512us (tolerating errors) • Minimum Ethernet packet size is 512 bits (64B) • Header 14B, payload 46B, CRC 4B

10. Today • Types of switching • Datagram • Virtual circuit • Source routing • Switching hardware design

11. Packet switching • Problem: single link networks have limited scale • Ethernet < 1024 hosts, 2500 meters • Wireless limited by radio ranges • Point-to-point links connect only two nodes • Packet switches enable packets to travel from one host to another without being directly connected • A packet switch is a device with several inputs and outputs leading to and from the nodes that the switch interconnects

12. A star topology • A switch has a limited number of inputs and outputs ports • Switches can be connected to each other to build larger networks • Adding a new host may not reduce the performance for other hosts • Not true for shared media networks • Why?

13. Switching • A switch connects links • Each link can be of different types • A switch runs an appropriate link layer protocol • Switching (or forwarding): receive incoming packets and send to different outgoing links

14. Switching technologies • Problem: how does a switch decide on which output port to place each packet? • Solution: look at the packet header and makes a decision • Connectionless: datagram • Connection oriented: virtual circuit • Source routing

15. Challenges • Contention • Input rate exceeds output rate • Multiple input ports may send to the same output port • Switches queue packets until contention disappears • Congestion • When a switch runs out of buffer, it discards packets. • Too frequent packet loss is said to be congested

16. Datagram • Every packet contains the destination address • A global unique identifier • Ethernet has 48-bit addresses • A switch maintains a forwarding table that maps a packet to an output port

17. Switch 2’s forwarding table Q: how does a switch compute the table?

18. Features of datagram switching • Connectionless: hosts can send anytime. No need to wait for connection to set up • Unknown network state: not sure whether a packet can reach the destination • Independent forwarding: packets may take different paths • Robust to failures: a failure of a switch may not disrupt communications • Switches can re-compute forwarding tables

19. Virtual circuit switching • Connection oriented • Set up a virtual circuit • Data transfer • Connection setup phase • Set up connection state • A virtual circuit identifier, an incoming interface, an outgoing interface, and an outgoing virtual circuit identifier

20. Virtual circuit table (switch1) 11 5

21. Virtual circuit switching • Algorithm: • If a packet arrives on the matching incoming port with the matching incoming VCI, it will be sent to the corresponding outgoing port with the corresponding VCI • VCIs are link-local

22. How to setup connection state • Administrator configured • Permanent virtual circuit (PVC) • Admin manually sets up VC tables • Does not suit large networks • Signaling • A host sends messages to dynamically setup or tear down a VC

23. VC setup • A host A sends a setup message to first hop switch, including the final destination address • Similar to a datagram packet • The switch picks an unused VCI to identify the incoming connection, and fills part of the VC table • Why not let the host pick it? • Every switch repeats the process until the packet reaches the destination B • The destination B sends an ack to inform its upstream switch the VCI for the connection

24. VCI VCI IF OF VCI IF OF VCI 3 11 2 2 5 1 Setup B Setup B VCI IF OF VCI 0 7 1 Setup B VCI Setup B VCI IF OF VCI 4 0 7 1

25. VCI VCI IF OF VCI IF OF VCI 3 11 2 2 5 1 ACK, ACK, 4 VCI VCI IF OF VCI 4 4 0 7 1

26. Characteristics of VC switching • Connection setup wait • Data packets contain a small VCI, rather than the full destination addresses • One switch failure tears down the entire connection • Connection sets up require routing algorithms • Setup packet is forwarded using a datagram algorithm

27. VC allows resource reservation • Buffers can be allocated during the setup phase to avoid congestion • An example (X.25) • Buffers allocated during connection setup • Sliding window is run between pairs of nodes (hop-by-hop flow control) • Circuit is rejected if no more buffer

28. Quality of service (QoS) • Connectionless network is difficult to allocate resources • Switches send packets independently • How to associate one packet with other packets? • Virtual circuit can be used to provide different QoS • Allocate a fraction of link bandwidth to each circuit

29. Switching technologies • Connectionless: datagram • Connection oriented: virtual circuit • Source routing

30. Source routing • Source host provides all the information for packets to travel across the network • Packets carry output port numbers • Packets carry switch addresses • Variable header length

31. Rotation Stripping No return path! Pointer Handling source routing headers

32. Loose or strict source routing • Strict • Must visit every node on the path • Loose • Waypoints rather than the complete route

33. Overview • Types of switching • Datagram • Virtual circuit • Source routing • Switching hardware design • How to build switches?

34. Software switch • Packets cross the bus twice • Half of the memory bus speed • 133Mhz, 64-bit wide I/O bus  4Gpbs • Short packets reduce throughput • 1Mpps, 64 bytes packet • Throughput = 512 Mbps • Shared by 10 ports: 51.2Mbps

35. Hardware switches • Ports communicate with the outside world • Eg, maintains VC tables • Switching fabric is simple and fast

36. Performance bottlenecks • Input port • Line speed: 2.48 Gbps • 2.48x109/(64x8) = 4.83 Mpps • Buffering • Head of line blocking • May limit throughput to only 59% • Use output buffers or sophisticated buffer management algorithms to improve performance

37. Fabrics • Shared bus • The workstation switch • Shared memory • Input ports read packets to shared memory • Output ports read them out to links

38. Fabrics • Cross bar • Each output ports need to accept from all input ports

39. Fabrics • Self routing • a self-routing header added by the input port • Most scalable • Often built from 2x2 switching units

40. An example of self-routing • 3-bit numbers are self-routing headers • Multiple 2x2 switching elements • 0: upper output; 1: lower output

41. Summary • Types of switching • Datagram • Forwarding table • Simple, difficult to have QoS • Virtual circuit • Datagram for connection setup • Smaller VC identifiers • Source routing • Packets carry forwarding information • Variable header length • Switching hardware design • Shared bus, shared memory, cross-bar, self-routing • Next lecture • How is forwarding table computed? • ATM network