1. Where are we? Chapter 3 and 4 are focused on getting the data from one place to another. Switching and routing Review the next slides First our goal is to have applications on two hosts communicating with each other The data needs to be augmented so it can be sent to the destination, understood and redirected at all points in between and interpreted correctly when it arrives.

2. Host Application Host Application Host Channel

3. Application programs Process-to-process channels Hardware Host-to-host connectivity

4. Encapsulation Notice how the data is encapsulated in a HHP header that has addressing information and the RRP header that tells the destination which application to apply to the data.

5. RRPDataHHP Application program Application program Host 1Host 2 Data RRP Data HHP Data RRP Data HHP

6. Chapter 3: Packet Switching Core job of switches – take packets that arrive on an input port and forward (or switch) them to the correct output port Key problem – finite bandwidth of outputs, packets coming from several sources that all need to be switched to the same destination port can overload the capacity (congestion) of the output port.

7. Chapter Outline Types of Switching Datagrams forwarding tables Virtual circuits Source routing Bridges and LAN switches Learning bridges Spanning tree algorithm Broadcast and multicast

8. Chapter Outline - continued ATM Cells Segmentation and reassembly Switching hardware Throughput Scalability Ports and fabric

10. Switching protocol T3 STS-1

11. Input ports T3 STS-1 T3 STS-1 Switch Output ports

12. Switching and Forwarding Different types of media for inputs and outputs is common Basic idea (figure 3.3) – packet comes in on input port, switch forwards it to the correct output port Basic question: how does the switch decide which output port to place each packet on? Three approaches

13. Datagrams Enough info included with each datagram to allow any switch along the way to decide where to send the datagram. This approach uses forwarding tables Virtual circuits A Virtual Circuit Identifier (VCI) is carried along with each packet, this information tells each switch what path (circuit) to forward it on.

14. continued Source routing Put a list of ports in the path from the source to the destination in each packet. Note: this is different then the VCI approach, which gives each packet a VCI and that along with the input port will tell the next switch in line what to do with the packet

15. Datagrams Study the next figure and the forwarding table on page 175. Also the ICND book has a better, indepth discussion of forwarding tables and learning

16. 0 13 2 0 13 2 0 13 2 Switch 3 Host B Switch 2 Host A Switch 1 Host C Host D Host E Host F Host G Host H

17. Virtual Circuit Look over the next figure 2 nd paragraph on page 177 details how this all works

18. 0 13 2 0 13 2 0 13 2 5 11 4 7 Switch 3 Host B Switch 2 Host A Switch 1

19. Variable Control 8 Address 16 Frame checksum 16 Flag (0x7E) 8 Flag (0x7E) 8 Data

20. Source Routing Look over figure 3.7 (next)

22. Header entering switch Header leaving switch DCBAPtr (a)(b)(c) DCBADCBA DCBAPtrDCBDCBA

23. Workstation as Packet Switch You don’t have to have specialized hardware to do switching (or routing from Chapter 4). A workstation can do it as shown in figure 3.9 Performance is limited however

25. Bridges and LAN Switches Learning bridges See ICND for a more detailed discussion of this (important) concept Changes in hosts and switches must propagate through the network of switches Loops are major problem Spanning tree algorithm Again a good discussion in ICND Know terminology

26. A Bridge BC XYZ Port 1 Port 2

27. B3 A C E D B2 B5 B B7 K F H B4 J B1 B6 G I

30. Limitations of Bridges Scalability Should only connect a few (tens) LANs with bridges No really good way to impose hierarchy on an extended set of LANs with bridges However VLANs can help Bridges forward all broadcasts (routers don’t)

31. WX B1B2 Y Z VLAN 100 VLAN 200

32. ATM Will not spend as much time on this as Ethernet Connection-oriented, packet-switched technology Cells Fixed cell size See page 199 (points 1 and 2) for key advantages Format – see next figure

33. GFCHEC (CRC-8) 41631 8 VPIVCICLPTypePayload 384 (48 bytes)8

34. Segmentation and reassembly 48 byte ATM packets don’t hold much information Larger messages must be fragmented and the put back together at the destination ATM Adaptation Layer (AAL) is a protocol layer added that sits between ATM and a variable-length protocol that might use ATM like IP

35. AAL ATM AAL ATM ……

36. CS-PDU header CS-PDU trailer User data 44 bytes ATM header AAL header Cell payload AAL trailer Padding

38. ATM in the LAN Section 3.3.5 goes over several issues when using ATM in the LAN Switched technology vs. the shared media technology of Ethernet Common protocols like ARP (Address Resolution Protocol) don’t work the same at all.

40. Higher-layer protocols (IP, ARP,...) Signalling + LANE AAL5 ATM PHY ATM PHY Higher-layer protocols (IP, ARP,...) Signalling + LANE AAL5 ATM HostSwitchHost PHY Ethernet-like interface

41. Switching Hardware Throughput Very difficult to define the throughput of a switch Size/scalability How fast does the cost of a size n switch increase as a function of n 2 or n 3 ? cost

42. Hardware Ports Fabric Study the knockout switch structure

43. Input port Input port Input port Input port Output port Output port Output port Output port Fabric

44. D 1234 Outputs Inputs D D D DD D D DD D D D D

45. (c) Shifter Buffers (b) Shifter Buffers (a) Shifter Buffers

46. Shared-Media switch See figure 3.31 Recall the example of a workstation as a switch

47. MuxBuffer memoryDemux Write control Read control InputsOutputs ……