TCP Friendly Streaming
Ketan Mayer-Patel
CS 294-9 :: Fall 2003
TCP Congestion Control Review
• 3 phases
– Slow-start
• Probing for initial congestion level.
– Congestion Avoidance
• Additive increase, multiplicative decrease
– Fast Retransmission/Recovery
• Optimizations
CS 294-9 :: Fall 2003
Slow-Start
•
•
•
•
At beginning or after an RTO loss.
Window set to 1 segment.
Every new ACK grows window by 1.
If ssthresh not yet set:
– Keep going until RTO loss occurs.
– Set ssthresh to 1/2 window size.
– Start over.
• If ssthresh set, transition to avoidance phase.
CS 294-9 :: Fall 2003
Congestion Avoidance
• Grow window by 1/window for every ACK.
• When RTO loss occurs:
– Set ssthresh to half window size.
– Set window to 1.
– Go back to slow start.
CS 294-9 :: Fall 2003
Fast Retransmit/Recover
• If three duplicate ACK’s detected:
– Retransmit segment identified by duplicate
ACK’s.
– Set ssthresh to 1/2 window size.
– Set window size to ssthresh.
– As long as dup acks keep coming, use the
following window size equation:
• window + # dup acks
– If ack changes, go back to avoidance.
– If RTO occurs, back to slow start.
CS 294-9 :: Fall 2003
AIMD Graphically
Window Size
• TCP congestion control mostly modeled as
being in AIMD phase in steady-state.
ssthresh
Time
CS 294-9 :: Fall 2003
Steady State Control Laws
• Increase:
wt+R = wt + a
• Decrease
wt+R = wt *b
CS 294-9 :: Fall 2003
AIMD Fairness
• AIMD motivated by fairness and stability.
• Claim is that AIMD will push multiple TCP
connections into a fair and stable operating
point.
• Formally unproven, but can provide
intuition graphically.
CS 294-9 :: Fall 2003
Window Size of A
AIMD Fairness Graphically
Window Size of B
CS 294-9 :: Fall 2003
AIMD Fairness Problems
• What assumptions are made by the
graphical “proof”
– Synchronous reaction by both participants.
• What governs when TCP reacts?
– Round-trip time.
– Thus, longer RTT connections might lose to
shorter RTT connections.
CS 294-9 :: Fall 2003
Loss as Congestion
• TCP congestion control couples loss with
congestion.
– Every loss is a congestion event.
• Where does loss come from?
– Overflowing queues in bottleneck routers.
– Rarely from network level congestion.
CS 294-9 :: Fall 2003
RED
• Random Early Drop
• Introduced as a way of providing
congestion feedback before queues
completely full.
• Basic operation:
– Router keeps track of weighted average queue length.
– Below some low threshold, nothing done.
– Between low and high thresholds, probability of
dropping increases linearly
– Above high threshold, everything dropped.
CS 294-9 :: Fall 2003
RED Graphically
Instantaneous queue length
Max
queue length
Forced drop
Max
threshold
Probabilistic
early drop
Min
threshold
No drop
Time
Initial drop probability
100%
maxp
2003
minth CS 294-9 :: Fall
max
th
Weighted
Average
Queue Length
ECN
• Explicit Congestion Notification
– Instead of dropping packet, mark it.
– Receiver echoes the mark back to sender in the
next ACK (and all ACK’s thereafter).
– Sender clears the mark after it makes
adjustment.
• Recent evidence that shows that ECN
essential for any active queue management
scheme.
CS 294-9 :: Fall 2003
MM Streams and TCP
• Congestion control only works if everyone
plays along.
• Unrestricted datagram traffic will clobber
TCP.
• Clearly, need congestion control, but in
what form.
CS 294-9 :: Fall 2003
Why not TCP?
• Could just use TCP without the reliability
part. Why might this be problematic?
– Large, sudden changes in rate.
– Caused by slow start, RTO’s, etc.
CS 294-9 :: Fall 2003
TCP Friendliness
• To avoid being labeled “unreactive”, we
need to “look” like TCP.
• Two basic strategies:
– Use a “TCP-compatible” congestion control
algorithm to determing sending rate.
– Use encoding and streaming strategies that can
deal with rapid rate changes induced by TCP.
– Combine these together.
CS 294-9 :: Fall 2003
TCP-Compatible Cong. Control
• What’s the right metric for compatibility?
– A control algorithm is TCP-compatible (or
friendly) if it achieves the same long-term
throughput as TCP given similar network
conditions.
CS 294-9 :: Fall 2003
Analyzing TCP
Want an expression for
bandwidth of connection in
terms of loss and delay.
B = # packets * S / #RTT * RTT
# packets = 3/2 n2
B = (3/2 n*S) / RTT
Window Size
(# Segments)
Loss probability = L
Expected loss = 1
# packets such that
E(loss) = 1 given L is 1/L
n = sqrt(2/(3L))
2 * ssthresh
ssthresh
n
Time
(# RTT’s)
B = sqrt(3/(2L))*S/RTT
CS 294-9 :: Fall 2003
RAP
• Rate-based version of TCP AIMD
– Periodically probe for greater bandwidth
• No more than once per RTT
– Immediately react to congestion.
• AIMD applied to inter-packet gap
– No longer need to maintain a “window”
– Presumes constant packet sizes
– Appropriate constants provide TCP-friendliness
• Coupled with fine-grained adaptation
– This is key.
– Will see more about this in the papers.
CS 294-9 :: Fall 2003
Binomial Congestion Control
• Bansal and Balakrishnan, 2001
• Non-linear congestion control law.
– Parameterized by k and l
– AIMD is simply one point in parameter space.
• Control laws:
– wt+R = wt + a / wtk
– wt+R = wt - b * wtl
• When k = 0, and l = 1, reduces to AIMD
• Other parameter choices proven to be TCPcompatible but generate smoother rate changes.
CS 294-9 :: Fall 2003
Rate-based Congestion Control
• Previous analysis of TCP congestion control
is naïve.
– Padhye et al. Provide a better analysis.
M  packet size
BTCP 
t RTT
M

2 Dl
3Dl 

l 1  32l 2
 tout min 1,3

3
8



• Basic idea:
l  loss fraction

D  number of packets ACK' ed
by one ACK
– Keep measures of RTT and loss and modulate
rate in accordance to TCP analysis.
– TCP Vegas proposes to do this with TCP itself.
CS 294-9 :: Fall 2003
TFRC
• Motivation
– TCP-friendly congestion control without sudden rate changes.
• Equation-based
– Estimates loss and RTT and plugs them directly into analytical
estimate for TCP throughput
• Estimating loss
– Loss “event” as opposed to pure loss
• Want to avoid counting more than one loss per RTT
• Loss even interval = # of packets between loss events.
• Loss rate is weighted average of last 10 loss event intervals.
CS 294-9 :: Fall 2003