1. CPS 356: Introduction to Computer Networks Lecture 7: Switching technologies Ch 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 10Base5 CSMA/CD: Carrier Sense Multiple access with Collision Detection

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 [0,2 n -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 A3 B0 C3 D E F G H 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) Incoming interface Incoming VCI Outgoing interface Outgoing VCI 25111 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. Setup B IF VCI OFVCI 251 Setup B IF VCI OFVCI 3112 Setup B IF VCI OFVCI 071 Setup B IF VCI OFVCI 071 4

25. IF VCI OFVCI 251 IF VCI OFVCI 3112 IF VCI OFVCI 071 4 ACK, 4 4 ACK,

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

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 2.48x10 9 /(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