Chapter 8 LAN and Switch Flow Control

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Chapter 8LAN and Switch Flow Control
Presenter Leica Yeh
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Agenda

• The need for flow control
• Flow control in Half duplex network
• Flow control in the Full duplex network
• PAUSE Function
• IEEE 802.3x flow control
• Flow control symmetry

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The Need for Flow control

• Packet loss
• Due to the Bit Error
• Due to the Buffer unavailability
• Due to the Buffer congestion
• Flow control (prevent the packet loss)
• End-to-end flow control gt High layer
• Transport Control Protocol (TCP)
• Positive Acknowledgment and Retransmission(PAR)
• Data link layer flow control gt Low layer
• Discussing in this chapter

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Switch congestion

• Fig 8.1

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Flow control in Half duplex network

• Backpressure
• Force Collision with the incoming frames
• MAC layer 16 successive collision
• Management view excessive collision errors
• Necessarily Backpressure
• Collision on the congested port
• Doesnt collision on the un-congested port
• To check the DA of the incoming frames

Congested
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Forced carries sense

• Fig 8.3

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Flow control in the Full duplex network

• Definition the MAC control layer
• Between the MAC control client(IP) and MAC layer
• Specification of the flow control mechanisms
• Function of the MAC control layer
• Generate Operation the MAC Control frames

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Client and control frames

• Fig 8.6

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MAC control frame format

• Fig 8.7

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PAUSE Function

• To implement flow control on the Full duplex
• Only for using across a single link
• Cant be used on a half duplex
• Can be used between
• A pair of end station
• A switch and an end station
• A switch-to-switch link

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PAUSE Function (cont.)

• The PAUSE function does not
• Solve the steady-state network congestion
• Cure a sustained overload
• Provide end-to-end flow control
• only across a single link
• Provide complexity beyond a simple stop-start
  mechanism
• Dont provide for credit-based schemes
• Dont provide on rate-based flow control

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Overview of PAUSE function

• A very simple stop-start mechanism
• non-zero zero timer
• Destination Address
• DA01-80-C2-00-00-01
• Why Multicast address ?
• Sent on a shared LAN
• Having not to know the unicast address
• specific multicast address, reserved for standard
  bridge (Blocked)

switch
PAUSE
hub
Multicast PAUSE
understand
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Overview of PAUSE function (cont.)

• Source Address
• contain the unicast address
• Type Field
• reserved value 0x8808
• MAC control Opcode and Parameters
• Opcode 0x0001
• pause_time 2-bytes
• 512 bit-time incremental counter
• PAUSE time pause_time x 512 bit-time

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PAUSE Timer ranges

• Table 8.1

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Configuration of Flow control capability

• Optional feature Pause function
• Manual configuration
• Auto-Negotiation configuration
• table 8.2

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IEEE 802.3x Flow control

• Inserting PAUSE frames in the Tx. Queue
• In time transmission
• High priority in Tx queue
• simple keep hard-code in logic

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IEEE 802.3x Flow control (cont.)

• Parsing Received PAUSE Frames
• Parsing Destination Address
• well-know DA 01-80-C2-00-00-01
• Parsing Type field
• Reserved for MAC control 0x8808
• Parsing MAC Control Opcode
• for PAUSE function 0x0001
• Frame Check Sequence(FCS)
• Must be valid FCS
• ignoring the already-received-and decode PAUSE

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IEEE 802.3x Flow control (cont.)

• PAUSE Timing
• No Extra frame allowed
• One Extra frame allowed

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IEEE 802.3x Flow control (cont.)

• Buffering Requirements
• Non-preempted
• One maximum-length frame on Tx
• 1542bytesMax.lengthVLANPreambleStart-of-frame
  IFG
• One PAUSE frame time
• 84bytesPausePreambleStart-of-frameIFG
• The PAUSE frame decode time allowance
• High number for Giga speed
• One Maximum-length frame on Rx
• 1522bytesMax.lengthVLAN
• The round-trip propagation delay of the link
• Table 8.3

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IEEE 802.3x Flow control (cont.)

• Buffer Thresholds
• High-water mark Issue PAUSE command
• Low-water mark Cancel PAUSE command
• fig 8.10

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IEEE 802.3x Flow control (cont.)

• Selection of PAUSE times
• OxFFFF Crossing the High-water mark
• Ox0000 Crossing the Low-water mark
• Longer PAUSE time
• Effect on the Higher layer protocol
• Time-out the connections , Break virtual circuits
• Dealing with Unreliable delivery
• PAUSE frames Not guarantee
• In the event of the PAUSE loss
• Due to the transient error
• restart the flow (pause_time 0x0000)

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Flow Control Symmetry

• Symmetric Flow Control
• Switch-to-Switch full duplex link
• Both devices will deal with High frames
• Both devices have similar buffer memory
  constraints
• Asymmetric Flow Control

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PAUSE Priority Resolution
Reference by IEEE 802.3z Auto-Negotiation