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
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