TCP Congestion Control and Common AQM Schemes: Quick Revision

1
TCP Congestion Control and Common AQM Schemes
Quick Revision

• Shivkumar Kalyanaraman
• Rensselaer Polytechnic Institute
• shivkuma_at_ecse.rpi.edu
• http//www.ecse.rpi.edu/Homepages/shivkuma
• Based in part upon slides of Prof. Raj Jain
  (OSU), Srini Seshan (CMU), J. Kurose (U Mass),
  I.Stoica (UCB)

2
Overview

• TCP Congestion Control Model and Mechnisms
• TCP Versions Tahoe, Reno, NewReno, SACK, Vegas
  etc
• AQM schemes common goals, RED,

3
TCP Congestion Control

• Maintains three variables
• cwnd congestion window
• rcv_win receiver advertised window
• ssthresh threshold size (used to update cwnd)
• Rough estimate of knee point
• For sending use win min(rcv_win, cwnd)

4
Packet Conservation Self-clocking

• Implications of ack-clocking
• More batching of acks gt bursty traffic
• Less batching leads to a large fraction of
  Internet traffic being just acks (overhead)

5
TCP Slow Start

• Whenever starting traffic on a new connection, or
  whenever increasing traffic after congestion was
  experienced
• Set cwnd 1
• Each time a segment is acknowledged increment
  cwnd by one (cwnd).
• Does Slow Start increment slowly? Not really. In
  fact, the increase of cwnd is exponential!!
• Window increases to W in RTT log2(W)

6
Slow Start Example

• The congestion window size grows very rapidly
• TCP slows down the increase of cwnd when cwnd gt
  ssthresh

cwnd 2
cwnd 4
cwnd 8
7
Slow Start Sequence Plot
. . .
Sequence No
Window doubles every round
Time
8
Congestion Avoidance

• Goal maintain operating point at the left of the
  cliff
• How?
• additive increase starting from the rough
  estimate (ssthresh), slowly increase cwnd to
  probe for additional available bandwidth
• multiplicative decrease cut congestion window
  size aggressively if a loss is detected.
• If cwnd gt ssthresh then each time a segment is
  acknowledged increment cwnd by 1/cwnd
• i.e. (cwnd 1/cwnd).

9
Additive Increase/Multiplicative Decrease (AIMD)
Policy

• Assumption decrease policy must (at minimum)
  reverse the load increase over-and-above
  efficiency line
• Implication decrease factor should be
  conservatively set to account for any congestion
  detection lags etc

10
Congestion Avoidance Sequence Plot
Sequence No
Window grows by 1 every round
Time
11
Slow Start/Congestion Avoidance Eg.

• Assume that ssthresh 8

ssthresh
Cwnd (in segments)
Roundtrip times
12
Putting Everything TogetherTCP Pseudo-code

• Initially
• cwnd 1
• ssthresh infinite
• New ack received
• if (cwnd lt ssthresh)
• / Slow Start/
• cwnd cwnd 1
• else
• / Congestion Avoidance /
• cwnd cwnd 1/cwnd
• Timeout (loss detection)
• / Multiplicative decrease /
• ssthresh win/2
• cwnd 1

while (next lt unack win) transmit next
packet where win min(cwnd, flow_win)
unack
next
seq
win
13
The big picture
cwnd
Timeout
Congestion Avoidance
Slow Start
Time
14
Packet Loss Detection Timeout Avoidance

• Wait for Retransmission Time Out (RTO)
• Whats the problem with this?
• Because RTO is a performance killer
• In BSD TCP implementation, RTO is usually more
  than 1 second
• the granularity of RTT estimate is 500 ms
• retransmission timeout is at least two times of
  RTT
• Solution Dont wait for RTO to expire
• Use fast retransmission/recovery for loss
  detection
• Fall back to RTO only if these mechanisms fail.
• TCP Versions Tahoe, Reno, NewReno, SACK

15
TCP Congestion Control Summary

• Sliding window limited by receiver window.
• Dynamic windows slow start (exponential rise),
  congestion avoidance (additive rise),
  multiplicative decrease.
• Ack clocking
• Adaptive timeout need mean RTT deviation
• Timer backoff and Karns algo during
  retransmission
• Go-back-N or Selective retransmission
• Cumulative and Selective acknowledgements
• Timeout avoidance Fast Retransmit

16
Queuing Disciplines

• Each router must implement some queuing
  discipline
• Queuing allocates bandwidth and buffer space
• Bandwidth which packet to serve next
  (scheduling)
• Buffer space which packet to drop next (buff
  mgmt)
• Queuing also affects latency

Traffic Sources
Traffic Classes
Class A
Class B
Class C
Drop
Scheduling
Buffer Management
17
Typical Internet Queuing

• FIFO drop-tail
• Simplest choice
• Used widely in the Internet
• FIFO (first-in-first-out)
• Implies single class of traffic
• Drop-tail
• Arriving packets get dropped when queue is full
  regardless of flow or importance
• Important distinction
• FIFO scheduling discipline
• Drop-tail drop (buffer management) policy

18
FIFO Drop-tail Problems

• FIFO Issues In a FIFO discipline, the service
  seen by a flow is convoluted with the arrivals of
  packets from all other flows!
• No isolation between flows full burden on e2e
  control
• No policing send more packets ? get more service
• Drop-tail issues
• Routers are forced to have have large queues to
  maintain high utilizations
• Larger buffers gt larger steady state
  queues/delays
• Synchronization end hosts react to same events
  because packets tend to be lost in bursts
• Lock-out a side effect of burstiness and
  synchronization is that a few flows can
  monopolize queue space

19
Queue Management Ideas

• Synchronization, lock-out
• Random drop drop a randomly chosen packet
• Drop front drop packet from head of queue
• High steady-state queuing vs burstiness
• Early drop Drop packets before queue full
• Do not drop packets too early because queue may
  reflect only burstiness and not true overload
• Misbehaving vs Fragile flows
• Drop packets proportional to queue occupancy of
  flow
• Try to protect fragile flows from packet loss
  (eg color them or classify them on the fly)
• Drop packets vs Mark packets
• Dropping packets interacts w/ reliability
  mechanisms
• Mark packets need to trust end-systems to
  respond!

20
Packet Drop Dimensions
Aggregation
Single class
Per-connection state
Class-based queuing
Drop position
Tail
Head
Random location
Early drop
Overflow drop
21
Random Early Detection (RED)
Min thresh
Max thresh
Average Queue Length
P(drop)
1.0
maxP
minth
maxth
Avg queue length
22
Random Early Detection (RED)

• Maintain running average of queue length
• Low pass filtering
• If avg Q lt minth do nothing
• Low queuing, send packets through
• If avg Q gt maxth, drop packet
• Protection from misbehaving sources
• Else mark (or drop) packet in a manner
  proportional to queue length bias to protect
  against synchronization
• Pb maxp(avg - minth) / (maxth - minth)
• Further, bias Pb by history of unmarked packets
• Pa Pb/(1 - countPb)

23
RED Issues

• Issues
• Breaks synchronization well
• Extremely sensitive to parameter settings
• Wild queue oscillations upon load changes
• Fail to prevent buffer overflow as sources
  increases
• Does not help fragile flows (eg small window
  flows or retransmitted packets)
• Does not adequately isolate cooperative flows
  from non-cooperative flows
• Isolation
• Fair queuing achieves isolation using per-flow
  state
• RED penalty box Monitor history for packet
  drops, identify flows that use disproportionate
  bandwidth

24
REM Athuraliya Low 2000

• Main ideas
• Decouple congestion performance measure
• Price adjusted to match rate and clear buffer
• Marking probability exponential in price

REM
RED
1
Avg queue
25
Comparison of AQM Performance
26
What is TCP Throughput?
window
t

• Each cycle delivers 2w2/3 packets
• Assume each cycle delivers 1/p packets 2w2/3
• Delivers 1/p packets followed by a drop
• gt Loss probability p/(1p) p if p is small.
• Hence

27
Law

• Equilibrium window size
• Equilibrium rate
• Empirically constant a 1
• Verified extensively through simulations and on
  Internet
• References
• T.J.Ott, J.H.B. Kemperman and M.Mathis (1996)
• M.Mathis, J.Semke, J.Mahdavi, T.Ott (1997)
• T.V.Lakshman and U.Mahdow (1997)
• J.Padhye, V.Firoiu, D.Towsley, J.Kurose (1998)