Packet Switching

1. Packet Switching Outline Switching and Forwarding Bridges and LAN Switches Cell Switching (ATM) Switching Hardware

2. Problem: Not all networks are directly connected • Limitations of the directly connected networks: • How many hosts can be attached. • How large of geographic area a single network can serve. • A switch is used to enable packets (a limit-size block of data) to travel from one host to another. • The jobs of a switch are: • Forward packets • Handle contention • Solve the congestion (Chapter 6) • Two technologies are focused in this chapter: • LAN switching • Asynchronous transfer mode (ATM)

3. Switching and Forwarding Outline Store-and-Forward Switches Bridges and Extended LANs Cell Switching Segmentation and Reassembly

4. Switching and Forwarding • A switch is a multi-input, multi-output device, which transfers packets from an input to one or more outputs. • A switch establishes the star topology: • Large networks can be built by interconnecting a number of switches. • We can build networks of large geographic scope. • Adding a new host to the network does not necessarily mean the hosts will get worse performance. Switched network is considered more scalable.

5. T3 T3 Switch T3 T3 STS-1 STS-1 Input Output ports ports Scalable Networks • Switch is the main function of the network layer. • forwards packets from input port to output port • port selected based on address in packet header • Approaches: datagram/connectionless, virtual circuit/connection-oriented, and source routing • Advantages • cover large geographic area (tolerate latency) • support large numbers of hosts (scalable bandwidth)

6. Switching and Forwarding A switch provides a star topology.

7. Host D Host E 0 Switch 1 Host F 3 1 Switch 2 2 Host C 2 3 1 0 Host A 0 Switch 3 Host B Host G 1 3 2 Host H Datagram Switching • No connection setup phase • Each packet forwarded independently • Sometimes called connectionless model • Analogy: postal system • Each switch maintains a forwarding (routing) table

8. Datagram Model • There is no round trip time delay waiting for connection setup; a host can send data as soon as it is ready. • Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up or running. • Each packet is forwarded independently. • A switch or link failure might not have any serious effect on communication if it is possible to route around link and node failures. • Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.

9. Datagram Model Forwarding table for switch 2

10. Virtual Circuit Model • The virtual circuit model requires a virtual connection from the source host to the destination to be set up before the connection. • It is a two-stage process: connection setup and data transfer. • Two approaches to establish connection state: permanent virtual circuit (PVC) by a network administrator and switched virtual circuit (SVC) by signalling. • A entry in PVC contains: • An incoming interface for the incoming packets • A virtual circuit identifier (VCI) • An outgoing interface • A VCI for the outgoing packets

11. Virtual Circuit Model Virtual circuit table entries for three switches

12. 0 Switch 1 3 1 2 Switch 2 2 3 1 5 11 0 Host A 7 0 Switch 3 1 3 4 Host B 2 Virtual Circuit Switching • Explicit connection setup (and tear-down) phase • Subsequence packets follow same circuit • Sometimes called connection-oriented model • Analogy: phone call • Each switch maintains a VC table

13. Virtual Circuit Model • Typically wait full RTT for connection setup before sending first data packet. • While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small. • If a switch or a link in a connection fails, the connection is broken and a new one needs to be established. • Connection setup provides an opportunity to reserve resources.

14. Virtual Circuit Model • In a datagram network, each packet competes with other packet. In the virtual model, different quality of service (QoS) can be provided. QoS means some performance-related guarantee. • Examples of virtual circuit technologies: • X.25 - packet-switching technology which was designed for transmitting analog data such as voice conversations. • Frame Relay – construct virtual private network (VPNs). • asynchronous transfer mode (ATM)

15. Source Routing • All the information about network topology for switching is provided by the source host. • Possible ways to implement source routing: • Place a number to each output of each switch in the header. • Put an ordered list of switch ports in the header and rotate this list as Figure 3.7. • Source routing can be used in both datagram and virtual networks. The Internet Protocol includes a source route option. • Source routing suffers from a scaling problem.

16. Source Routing

17. Implementation and Performance • A general-purpose workstation with a number of network interfaces • A specialized switching device

18. A B C Port 1 Bridge Port 2 Z X Y Bridges and Extended LANs • LANs have physical limitations (e.g., 2500m) • Connect two or more LANs with a bridge • accept and forward strategy • An Ethernet bridge can carry as 10n Mbps, where n is the number of port.

19. A B C Port 1 Bridge Port 2 Z X Y Learning Bridges • Do not forward when unnecessary • Maintain forwarding table Host Port A 1 B 1 C 1 X 2 Y 2 Z 2 • Learn table entries based on source address • Table is an optimization; need not to be complete • Always forward broadcast frames

20. A B B3 C B5 D B7 K B2 E F B1 G H B6 B4 I J Spanning Tree Algorithm • Problem: loops in the previous design • Bridges run a distributed spanning tree algorithm • select which bridges actively forward • developed by Radia Perlman • now IEEE 802.1 specification

21. A B B3 C B5 D B7 K B2 E F B1 G H B6 B4 I J Algorithm Overview • Each bridge has unique id (e.g., B1, B2, B3) • Select bridge with smallest id as root • Select bridge on each LAN closest to root as designated bridge (use id to break ties) • Each bridge forwards frames over each LAN for which it is the designated bridge

22. Algorithm Details • Bridges exchange configuration messages • id for bridge sending the message • id for what the sending bridge believes to be root bridge • distance (hops) from sending bridge to root bridge • Each bridge records current best configuration message for each port • Initially, each bridge believes it is the root

23. Algorithm Detail (cont) • When learn not root, stop generating config messages • in steady state, only root generates configuration messages • When learn not designated bridge, stop forwarding config messages • in steady state, only designated bridges forward config messages • Root continues to periodically send config messages • If any bridge does not receive config message after a period of time, it starts generating config messagesclaiming to be the root

24. Broadcast and Multicast • Forward all broadcast/multicast frames • current practice • Each host in a multicast group must periodically send a frame with the address for the group in the source field of the frame header.

25. Limitations of Bridges • Do not scale • The spanning tree algorithm does not scale • Broadcast does not scale. It is not necessary to broadcast messages to all hosts in a large environment. • Do not accommodate heterogeneity • Caution: beware of transparency. Bridges might drop frames.

26. Cell Switching (ATM) • Architecture Features • Similarities between ATM and packet switching • Transfer of data in discrete chunks • Multiple logical connections over single physical interface • In ATM flow on each logical connection is in fixed sized packets called cells • Minimal error and flow control • Reduced overhead • Data rates (physical layer) 25.6Mbps to 622.08Mbps

27. Cell Switching (ATM) • Connection-oriented packet-switched network • Used in both WAN and LAN settings • Signaling (connection setup) Protocol: Q.2931 • An ITU-T specification defining user-to-network interface signaling for Broadband ISDN. • Discover a suitable route • Responsible for allocating resources at the switches • The QoS capabilities of ATM are one of its greatest strengths.

28. Cell Switching (ATM) • Two Addressing schemes • Public ATM networks use 8-octet format (E.164 standard) • Computers attached to private ATM network use 20-octet Network Service Access Point (NSAP) address (ATM Forum) • Packets are called cells – Fixed length 53 bytes • 5-byte header + 48-byte payload • Commonly transmitted over SONET • other physical layers possible

29. Cell Switching (ATM) • ATM media - Commonly transmitted over SONET • DS-1/T1 • NxDS-1 • DS-3 • Multi-mode fiber (155Mbps) • SONET/SDH • (622 Mbps)

30. 12 ATM Network workstation LAN Switch UNI ATM Switch Router

31. Variable vs. Fixed-Length Packets • No Optimal Length • if small: high header-to-data overhead • if large: low utilization for small messages • Fixed-Length Easier to Switch in Hardware • simpler • enables parallelism

32. Big vs Small Packets • Small Improves Queue behavior • finer-grained pre-emption point for scheduling link • maximum packet = 4KB • link speed = 100Mbps • transmission time = 4096 x 8/100 = 327.68us • high priority packet may sit in the queue 327.68us • in contrast, 53 x 8/100 = 4.24us for ATM • near cut-through behavior • two 4KB packets arrive at same time • link idle for 327.68us while both arrive • at end of 327.68us, still have 8KB to transmit • in contrast, ATM can transmit first cell after 4.24us • at end of 327.68us, just over 4KB left in queue

33. Big vs. Small (cont) • Small Improves Latency (for voice) • voice digitally encoded at 64KBps (8-bit samples at 8KHz) • need full cell’s worth of samples before sending cell • example: 1000-byte cells implies 125ms per cell (too long) • smaller latency implies no need for echo cancellers • ATM Compromise: 48 bytes = (32+64)/2

34. 4 16 3 1 8 8 384 (48 bytes) GFC VPI VCI Type CLP HEC (CRC-8) Payload Cell Format • User-Network Interface (UNI) • host-to-switch format (telephone companies and customers) • GFC: Generic Flow Control (still being defined) • VCI: Virtual Circuit Identifier • VPI: Virtual Path Identifier • Type: management, congestion control, AAL5 (later) • CLPL Cell Loss Priority • HEC: Header Error Check (CRC-8) • Network-Network Interface (NNI) • switch-to-switch format (phone companies) • GFC becomes part of VPI field

35. ATM Architecture Application Upper Layer Protocols Presentation Session ATM Adaptation Layer (AAL) Transport Network 1 (CBR) 2 (VBR) 3/4 (SMDS) 5 (Data) SAAL Data Link ATM Layer Transmission-convergence physical medium dependent Physical

36. ATM Adaptation layer Upper Layer Protocols • Supports multiple-application operations • Type of user payload is identified • Maps higher layer information into ATM cell payload. • Handle transmission errors • Segmentation and re-assembly • Handle lost and misinserted cells • Flow control and timing CS 1 (CBR) 2 (VBR) 3/4 (SMDS) 5 (Data) SAAL SAR ATM Layer Transmission-convergence physical medium dependent

37. ATM Adaptation Sub Layers • Convergence Sublayer (CS) • Functions needed to support specific applications using AAL • AAL user attaches at SAP • Segmentation and Reassembly(SAR) • Responsible for creating 48 byte payload for ATM cells. • Also unpacks cell payload data received from ATM layer for delivery up to CS sublayer

38. AAL Protocols and PDU

39. AAL Applications • Support for information transfer protocol not based on ATM • PCM (voice) • Assemble bits into cells • Re-assemble into constant flow • IP • Map IP packets onto ATM cells • Fragment IP packets • Use LAPF over ATM to retain all IP infrastructure

40. Supported Application Types • Circuit emulation • VBR voice and video • General data service • IP over ATM • Multiprotocol encapsulation over ATM (MPOA) • IPX, AppleTalk, DECNET) • LAN emulation

41. ATM Layer • Responsible for ATM cell transmissions • Maps network layer address to ATM address Upper Layer Protocols ATM Adaptation Layer ATM Layer Transmission-convergence physical medium dependent

42. Physical Layer Divided into two sublayers: • Transmission Convergence • Synchronization of transmission & reception • Cell delineation • Error control • Physical Medium Dependent (PMD) • Specifies physical medium used Upper Layer Protocols ATM Adaptation Layer ATM Layer Transmission-convergence physical medium dependent

43. AAL AAL … … A TM A TM Segmentation and Reassembly • ATM Adaptation Layer (AAL) • AAL 1 and 2 designed for applications that need guaranteed rate (e.g., voice, video) • AAL 3/4 designed for packet data • AAL 5 is an alternative standard for packet data

44. AAL 3/4 • Convergence Sublayer Protocol Data Unit (CS-PDU) • CPI: commerce part indicator (version field) • Btag/Etag:beginning and ending tag • BAsize: hint on amount of buffer space to allocate • Length: size of whole PDU

45. Cell Format • Type • BOM: beginning of message • COM: continuation of message • EOM end of message • SEQ: sequence of number • MID: message id • Length: number of bytes of PDU in this cell

46. AAL5 • CS-PDU Format • pad so trailer always falls at end of ATM cell • Length: size of PDU (data only) • CRC-32 (detects missing or misordered cells) • Cell Format • end-of-PDU bit in Type field of ATM header