Random Early Detection (RED)

1
Random Early Detection (RED)

• Router notifies source before congestion happens
• - just drop the packet (TCP will timeout and
  adjust its window)
• - could make explicit by marking the packet
• Early random drop
• - rather than wait for queue to become full,
  drop each arriving packet with some drop
  probability whenever the average queue length
  exceeds some drop level
• RED details (normal, congestion avoidance,
  congestion control)
• - compute average queue length as weighted
  average of queue lengths each time a packet
  arrives
• - If average is less than minimum threshold,
  enqueue the packet
• - If average exceeds maximum threshold, drop
  arriving packet
• - Otherwise, drop arriving packet with
  probability P

2
RED

• P is a function of average queue length
• - small bursts pass through unharmed
• - only affects sustained overloads
• - P also depends on how long since last drop
  RED counts new packets that have been queued
  while average length has been between the two
  thresholds
• - TempP MaxP (AvgLen - MinThreshold)/(MaxT
  hreshold - MinThreshold)
• P TempP / (1 - count TempP)
• Cooperative sources reduce their rate and get
  lower overall delays, uncooperative sources get
  severe packet loss
• Random drops help avoid global synchronization,
  which occurs when hundreds or thousands of flows
  back off and go into slow start at roughly the
  same time

3
More on RED

• RED is a congestion avoidance mechanism (rather
  than congestion control), i.e. it predicts when
  congestion is about to happen, and reduce the
  rate at which hosts send data just before queue
  saturation
• Probability of dropping a particular flows
  packet(s) is roughly proportional to the share of
  the bandwidth that flow is currently getting
• Max P is typically set to 0.02
• If traffic is bursty, then min threshold should
  be sufficiently large to allow link utilization
  to be maintained at an acceptably high level
• Difference between two thresholds should be
  larger than the typical increase in calculated
  average queue length in one RTT setting max
  threshold to twice min threshold seems reasonable
• RED is fair cant differentiate services with
  fairness
• Unfair or weighted RED lower drop probability
  for higher priority traffic. Priorities set at
  edge routers or hosts

4
A Controlled-Load Scheme

• A connection has to specify its Tspec in terms of
  token bucket rate and depth
• If there arent enough tokens present at the time
  of transmission, the packet is treated as
  non-conformant
• Non-conformant and best-effort traffic injected
  into network unmarked
• Conformant traffic is marked
• At routers, an enhanced RED (ERED) FIFO is used
• Marked packets have a lower drop probability
• Admission control ensures sum of rates does not
  exceed link capacity
• Designed for TCP based applications (assumes
  TCP-reno)

5
Effect of Service Rate

• Compliant throughput observed to be less than the
  reserved rate of service
• Not all tokens generated are used to mark packets
  at the source, or token buffer overflows
• TCPs congestion control is overly conservative
  and does not exploit the reservation the
  connection has
• TCP ceases to transmit due to ack compression (or
  gaps between successive acknowledgments)
• In fast recovery, TCP cuts its window in half by
  halting additional transmissions until half the
  original window clears the network
• Gaps in ack stream also form due to normal
  dynamics of network traffic

6
Effect of Token Bucket Depth

• Solution increase token bucket depth
• But, burst losses can occur
• Or, more buffers in the network are needed
• Large token buckets do not help
• Unless we have better admission control!

7
TCP Adaptations

• Adapt acknowledgment-based transmit triggers to
  rate-based reservation paradigm
• Delayed transmissions a segment is held back for
  a random amount of time when there arent enough
  tokens, thus reducing drop probability of packets
• Not very effective in the presence of reverse
  path congestion
• Timed transmissions whenever a periodic timer
  expires, if there are sufficient tokens, sender
  sends packet as marked, ignoring congestion
  window (still honor receiver advertised window)
• Non-compliant packets are sent only if there is
  room under the congestion window
• A connection receives its reserved rate
• But, does not share excess bandwidth equally with
  best-effort
• Connections with larger reservations are
  penalized more

8
Rate Adaptive Windowing

• Congestion window consists of reserved part
  (RWND) and variable part (CWND)
• RWND equals reserved rate times estimated RTT
• CWND tries to estimate residual capacity and
  share it with other active connections
• In fast recovery, set CWND to RWND
  (CWND-RWND)/2
• In slow start, set CWND to RWND 1, set SSTHRESH
  to minimum of RWND (CWND-RWND)/2 and AWND
• Scale window increase by (CWND-RWND)/CWND
• Sender still sends minimum of CWND and AWND
• Size of receivers buffer must be at least equal
  RWND to sustain reserved rate using TCP

9
Comments

• Token bucket depth should be at least
  TimerInterval times TokenBucketRate
• As timer interrupt interval increases, token
  bucket size (burst size) increases
• Burst losses cause throughput degradation
• Using fine-grained timers allow applications to
  request smaller token buckets
• ERED can be embedded as a class in CBQ to provide
  weighted bandwidth sharing among connections
• Many multimedia applications do not require
  reliability of TCP, but results can be extended
  to TCP-friendly RTP-based applications
• Admission control needs to consider bucket sizes,
  router buffer sizes, ERED parameters

10
Class Based Queueing

• Hierarchical link sharing
• - a priori hierarchical partitioning of a
  links bandwidth among organizations, traffic
  classes, or protocol families
• A class can continue unregulated if
• - the class is not overlimit (i.e. using more
  than its share)
• OR
• - the class has a not-overlimit ancestor at
  level i, and there are no unsatisfied classes at
  levels lower than i
• a class is unsatisfied if underlimit and has
  a persistent backlog