ONT Notes – Congestion Avoidance, Policing, Shaping, and Link Efficiency

  • Tail drop drawbacks
    • TCP synchronization – Dropping TCP packets from different flows can cause them all to window down and back up again at the same time in cycles.
    • TCP starvation – Non-TCP or aggressive flows can starve everyone else out when TCP throttles back.
    • No differentiated drop – Tail drop doesn’t care who you are, so you get dropped if the queue is full.
  • RED – Random Early Detection
    • Avoids tail drop by randomly dropping packets from the queue before it gets full
    • Only dropped TCP flows slow down instead of everyone who has sent a packet since the queue filled
    • Queues are smaller.
    • Link utilization is more efficient
    • Configured with
      • Minimum threshold – start dropping when the queue is this size
      • Maximum threshold – if the queue is this big, start tail dropping
      • Mark probability denominator (MPD) – 1/MPD is the ratio of packets to drop when between the thresholds
  • WRED – Weighted RED
    • Based on IP precedence or DSCP values
    • Less-important packets are dropped more aggressively than important packets
    • Applied to an interface, VC or a class within a policy map
  • CBWRED – Class based WRED
    • Configured with CBWFQ
  • Policing
    • Limits subrate bandwidth (give you 100kbps on a T1)
    • Limits traffic of certain applications
    • Any traffic that exceeds police is dropped or re-classified; it’s a hard limit
    • Inbound or outbound
  • Shaping
    • Sets a limit but buffers any in excess
    • Requires memory to store the buffer
    • Buffers = delay and/or jitter
    • Outbound only
    • Can respond to network signals like BECNs and FECNs
  • Token and bucket
    • The queue is a bucket; if a byte of data needs to be sent, it needs a token.
    • If there are enough tokens, the traffic is considered conforming.
    • If there aren’t enough tokens, the traffic is considered exceeding, which triggers the drop (policing), re-classify (policing), or buffer (shaping).
  • Frame relay traffic shaping (FRTS)
    • Only controls frame relay traffic
    • Applied on subif or DLCI
    • Support fragmentation and interleaving
    • Reacts to FECNs and BECNs
  • Compression
    • Removed redundancy and patterns in data
    • Less data = less latency
    • Hardware compression or hardware-assisted compression does not involve the main CPU
    • Software compression does
    • Payload compression
    • Header compression
  • Link fragmentation and interleaving
    • Small data might be waiting for larger data pieces to finish sending
    • Chunks data into smaller fragments so they don’t have to wait
    • Interleaving shuffles flows in the Tx queue

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages. If you have any questions, the best way to contact me is through Twitter at @aconaway.

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