EEL 6786 Advanced Networking Hardware Design Lecture 4 Buffer Management in Routers

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EEL 6786Advanced Networking Hardware
DesignLecture 4 Buffer Management in Routers
Prof. Taskin Koçak School of EECS,
UCF tkocak_at_cs.ucf.edu Course URL
http//www.cs.ucf.edu/tkocak/eel6786.html
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Introduction

• All routers contain buffers to hold packets
  during times of congestion
• The performance of a high-speed router is
  often dictated by the speed of its packet
  buffers.
• Buffers are implemented using commercial DRAMs.
• DRAMs are low cost, however they are not
  optimized for speed but size.

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Packet Buffering (Queuing)

• General paradigm is store-and-forward
• Incoming packet placed in queue
• Outgoing packet placed in queue
• When queue is full, choose packet to discard

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Buffer Placement

• Input queuing

Output queuing
Combined input/output queuing
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Head-of-line Blocking

• Input queueing holds packets until the switch is
  able to direct them to the appropriate output.
• The problem with this is that with FIFO queueing
  discipline, packets destined for other output
  ports that are free may be blocked this is known
  as head-of-line blocking
• This can be solved by replacing FIFO discipline
  with virtual output queues.

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Queueing Priorities

• Multiple queues used to enforce priority among
  packets
• Incoming packet
• Assigned priority as function of contents
• Placed in appropriate priority queue
• Queueing discipline
• Examines priority queues
• Chooses which packet to send

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Examples Of Queueing Disciplines

• Priority Queueing
• Assign unique priority number to each queue
• Choose packet from highest priority queue that
  is
• nonempty
• Known as strict priority queueing
• Can lead to starvation

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Examples Of Queueing Disciplines (cont.)

• Weighted Round Robin (WRR)
• Assign unique priority number to each queue
• Process all queues round-robin
• Compute N, max number of packets to select
  from a queue proportional to priority
• Take up to N packets before moving to next
  queue
• Works well if all packets equal size

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Examples Of Queueing Disciplines (cont.)

• Weighted Fair Queueing (WFQ)
• Make selection from queue proportional to
  priority
• Use packet size rather than number of packets
• Allocates priority to amount of data from a
  queue rather than number of packets

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Example Implementation

• IDT FIFO
• Part (72T20128)
• Programmable
• 256Kx20b 512Kx10b
• Ability to read and write on both edges of the
  clock (DDR)

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Multi-queue example

• Again from IDT
• Part 72T51268
• up to 256 prioritization queues can be
  configured
• 7.2 Gbps throughput