Transcript Blue: An Alternative Approach to Active Queue Management Wu-chang Feng June 23, 2001

Blue: An Alternative Approach to
Active Queue Management
Wu-chang Feng
June 23, 2001
Outline
• Motivation
• Congestion control and queue management today (TCP,
Drop-tail, RED)
• Blue
• Conclusion
Motivation
• Exponential increase in network demand
– Rise in packet loss rates
• 17% loss rates reported [Paxson97]
– Decrease in link utilization and goodput
– Potential for congestion collapse [Jacobson88]
• Goal
– Queue management algorithms for maximizing network
efficiency in times of heavy congestion
– 0% packet loss, 100% link utilization, low queuing delay
Congestion control today
• TCP
– Instrumental in preventing congestion collapse
– Limits transmission rate at the source
– Window-based rate control
• Increased and decreased based on implicit signals from the
network (acknowledgments and packet loss)
• Slow-start
• Fast-retransmit, Fast-recovery
• Congestion avoidance
Drop-tail queue management
• Default queue management mechanism
• Packets dropped upon queue overflow
• Problems
– Global synchrony (poor utilization)
– Late congestion notification (packet loss)
• Solution
– Randomize
– Early detection of incipient congestion
RED queue management
• RED (Random Early Detection) [Floyd93]
– Keep EWMA of queue length (Qave) using weight wq
– Increase in EWMA triggers random drops
• Basic algorithm
1
Pdrop
maxp
0
maxth
minth
Qave
Problems with RED
• Hard to tune
• Does not adapt well to traffic
• [Feng99], [Anjum99], [Christiansen2000], [May99],
[Ott99], ….
• Why?
– Wrong control variable being used
– Queue length indicates congestion not severity
Blue
• Class of fundamentally different queue management
algorithms
• Decouple congestion management from queue length
• Rely only on queue and link history
• Example
– Increase aggressiveness when loss rates high
– Decrease aggressiveness when link underutilized
RED example
Sending rate increases above L Mbs
Sinks generate DupACKs/ECN
DupACKs/ECN travel back
Sources
A
L Mbs
B
Sinks
Rate > L Mbs
Rate > L Mbs
Rate > L Mbs
Rate > L Mbs
Sources detect congestion
Sources see sustained CN
Rate < L Mbs
Queue increases
Queue increases, EWMA increases to trigger RED
Queue increases some more
Queue increases some more
Queue overflows, maxth triggered
Queue clears, but under-utilization imminent
Ideal example (Blue)
Sources
A
L Mbs
B
Sinks
Sinks generate
DupACKs/ECN
Rate = L Mbs
Queue drops and/or ECN marks at steady rate
Rate = Exactly what will keep sources at L Mbs
Example Blue algorithm
• Single dropping/marking probability
– Increase upon packet loss
– Decrease when link underutilized
– Freeze value upon changing
Upon packet loss:
if ((now - last_update) > freeze_time) then
Pmark = Pmark + d1
last_update = now
Upon link idle:
if ((now - last_update) > freeze_time) then
Pmark = Pmark - d2
last_update = now
Blue parameter selection
• freeze_time: set to prevent multiple updates on a single
round-trip (~10ms-100ms)
• d1/d2: set to allow pm to range from 0 to 1 on the order
of seconds
• [Paxson97], [Balakrishnan98]
Evaluation
•
•
•
•
1000 and 4000 Pareto on/off TCP sources
Router queue sizes 100KB (17.8ms) to 1000KB (178ms)
RED (wq = 0.0002, 0.002, 0.02, 0.2)
Blue
– freeze_time = 10ms, 100ms
– d1 = 0.02, 0.0025 ; d2 = 0.002, 0.00025
100 Mbs
100 Mbs
1ms, 5ms, 20ms
45 Mbs
45 Mbs
10ms
10ms
1ms, 5ms, 20ms
Blue evaluation
• 1000 sources
Blue evaluation
• 4000 sources
Understanding Blue
• Experiment
– 50 sources added every 10 seconds
• Queue length plots
RED
Blue
Understanding Blue
• Marking behavior
RED
Blue
Understanding Blue w/ECN timeouts
• Lack of ECN timeouts allows packet loss with pm=1
• With ECN timeouts Blue similarly outperforms RED
RED
Blue
Understanding Blue w/ECN timeouts
• Marking behavior
RED
Blue
Blue evaluation w/ECN timeouts
• 1000 sources
Blue evaluation w/ECN timeouts
• 4000 sources
Implementation
• FreeBSD 2.2.7 + ALTQ
IBM PC 365
(200 MHz/64 MB)
Winbook XL
(233 MHz/32 MB)
100 Mbs
100 Mbs
Intellistation Mpro
(400 MHz/128 MB)
IBM PC 360
(150 MHz/64 MB)
10 Mbs
Intellistation Zpro
(200 MHz/64 MB)
Thinkpad 770
(266 MHz/64 MB)
Blue Evaluation
Loss rates
Link utilization
Conclusion