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					          Packet Switching

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

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

             T3                                   T3
             T3               Switch              T3
          STS-1                                   STS-1
          Input                                   Output
          ports                                   ports
   – Approaches: datagram/connectionless, virtual
     circuit/connection-oriented, and source routing
• Advantages
   – cover large geographic area (tolerate latency)
   – support large numbers of hosts (scalable bandwidth)
Switching and Forwarding




   A switch provides a star topology.
             Datagram Switching
• No connection setup phase
• Each packet forwarded independently
• Sometimes called connectionless model
                                Host D



• Analogy: postal           3
                                   0 Switch 1
                                       1
                                                    Host E
                                                                                Host F

  system               Host C
                                                       2 Switch 2
                                   2            3          1


• Each switch                                          0

  maintains a                   Host A

  forwarding
  (routing) table                                                     0 Switch 3 Host B
                                                     Host G
                                                               1        3

                                                                      2


                                                                   Host H
               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.
   Datagram Model
    Destination   Port
    A              3
    B              0
    C              3
    D              3
    E              2
    F              1
    G              0
    H              0


Forwarding table for switch 2
             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
           Virtual Circuit Model

 VC        Incoming Incoming Outgoing Outgoing
 Table     Interface VCI     Interface VCI
Switch 1   2        5        1        11

Switch 2   3        11       0        7

Switch 3   0        7        3        4


Virtual circuit table entries for three switches
          Virtual Circuit Switching
• Explicit connection setup (and tear-down) phase
• Subsequence packets follow same circuit
• Sometimes called connection-oriented model
                             0 Switch 1
                      3        1
                                                   2 Switch 2
                             2                 3     1
                      5
                                          11
                                                   0
• Analogy:
  phone call              Host A

                                                           7
• Each switch                                          1
                                                               0 Switch 3
                                                                  3
  maintains a VC                                                            4
                                                                                Host B
                                                               2
  table
             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.
            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)
                 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.
                 Source Routing
         0 Switch 1
                                                                 0
  3          1
                                                            3        1
                                  2 Switch 2
         2                    3        1                         2
3 0 1                 1 3 0
                                  0



      Host A

                                      0 1 3
                                               0 Switch 3
                                           1     3

                                                            Host B
                                               2
 Implementation and Performance

• A general-purpose workstation with a
  number of network interfaces
• A specialized switching device
        Bridges and Extended LANs
• LANs have physical limitations (e.g., 2500m)
• Connect two or more LANs with a bridge
   – accept and forward strategy


                  A    B    C


                                         Port 1
                                Bridge
                                         Port 2

                                         X        Y   Z



• An Ethernet bridge can carry as 10n Mbps, where n is the
  number of port.
               Learning Bridges
• Do not forward when unnecessary
• Maintain forwarding table
                                              Host   Port
       A   B    C
                                                A    1
                             Port 1             B    1
                    Bridge                      C    1
                             Port 2
                                                X    2
                                                Y    2
                             X        Y   Z     Z    2


• Learn table entries based on source address
• Table is an optimization; need not to be complete
• Always forward broadcast frames
         Spanning Tree Algorithm
                                       A

• Problem: loops                           B3
                                                                        B


  in the previous design      C                      B5

                                                D              B7
                                      B2                                    K

                                  E                                 F


                                                    B1

                                  G                                 H


                                           B6             B4
                                       I
                                                                    J


• Bridges run a distributed spanning tree algorithm
   – select which bridges actively forward
   – developed by Radia Perlman
   – now IEEE 802.1 specification
             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         A

  over each LAN for which it            B3
                                                                       B


  is the designated bridge       C           B5

                                                D             B7
                                      B2                                   K
                                  E                                F


                                                    B1

                                  G                                H

                                           B6            B4
                                       I
                                                                   J
               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
            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 messages claiming to be
  the root
        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.
           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.
            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
           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.
           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
           Cell Switching (ATM)
• ATM media - Commonly transmitted over
  SONET
  –   DS-1/T1
  –   NxDS-1
  –   DS-3
  –   Multi-mode fiber (155Mbps)
  –   SONET/SDH
  –   (622 Mbps)
                                               12
                    ATM Network

workstation




LAN Switch


              UNI
                                  ATM Switch



  Router
 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
               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
               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
                       Cell Format
• User-Network Interface (UNI)
   4        8          16        3      1        8         384 (48 bytes)
  GFC      VPI         VCI      Type   CLP   HEC (CRC-8)      Payload


   –    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
             ATM Architecture

                                 Application
   Upper Layer Protocols
                                 Presentation

                                   Session
   ATM Adaptation Layer
   (AAL)                          Transport

1     2     3/4    5      SAAL    Network
(CBR) (VBR) (SMDS) (Data)


         ATM Layer                Data Link

Transmission-convergence
physical medium dependent          Physical
                            ATM Adaptation layer

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

Transmission-convergence
physical medium dependent
              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
AAL Protocols and PDU
          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
         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
                               ATM Layer


                            • Responsible for ATM
                              cell transmissions
                            • Maps network layer
   Upper Layer Protocols
                              address to ATM address


   ATM Adaptation Layer


        ATM Layer

Transmission-convergence
physical medium dependent
                            Physical Layer

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


physical medium dependent
   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



                 AAL                AAL

           …                                  …
                 ATM                ATM
                            AAL 3/4
• Convergence Sublayer Protocol Data Unit (CS-PDU)
       8     8      16          < 64 KB   0– 24   8    8     16

      CPI   Btag   BASize    User data    Pad     0   Etag   Len



  –   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
                     Cell Format
    40         2      4    10    352 (44 by tes)     6       10

ATM header   Ty pe   SEQ   MID   Pay load          Length   CRC-10



– 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
                               AAL5
• CS-PDU Format
     < 64 KB    0– 47 by tes   16      16     32

      Data         Pad     Reserv ed   Len   CRC-32


  – 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

				
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posted:11/22/2011
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