congestion control algorithms : open questions

http://www.nlnetlabs.nl/

Congestion Control Algorithms: Open Questions

Benno Overeinder

NLnet Labs

http://www.nlnetlabs.nl/ NLnetLabs

What This Talk is Not About• Details of TCP congestion avoidance and control

algorithms• Research on improvements of TCP congestion

avoidance algorithms• Measurements of TCP congestion avoidance

algorithm performance• None of this, but

– highlight current open question and future research


Congestion Control Over the Years

• Global congestion collapse (1986)• TCP Tahoe (1988) and TCP Reno (1990)• TCP New Reno (1998)• …• TCP over long fat networks (2002–2003)• TCP and bufferbloat (2011)


Common Congestion Control Algorithms

• FreeBSD/Solaris– TCP New Reno– Reno: “classic” congestion avoidance – improves retransmission during the fast-recovery phase

• Linux– TCP CUBIC– BIC: optimized congestion control algorithm for LFN– CUBIC: less aggressive and more systematic derivative

• Windows– Compound TCP– achieve good performance for LFNs, while not harm fairness


Fairness

One mechanism at a time Mixed mechanisms


SHAPE OF CONGESTION WINDOW INCREASE FUNCTION


Shape of Congestion Window Increase Function


Convex vs. Concave-Convex: H-TCP vs. CUBIC


Distribution of cwnd for Convex and Concave Increase

Distribution of cwnd at back-off for convex and concave updatesversus loss probability. Key: + convex, o concave-convex


BUFFERING AND BUFFERBLOAT


Impact of Buffering

Throughput and cwnd for Reno, buffer size 1 x BPD, 10 MBps link, 100 ms RTT.

With buffering, flow throughput and cwnd are fundamentally different quantities, and only weakly related.


Bufferbloat• Trend to provide large buffers to network equipment

– rule of thumb buffer to accommodate 250 ms traffic– e.g., 1 Gb/s interface requires 32 MB buffer

• Flow of packets slows down traveling from fast to slow network– buffer absorbs, temporary delay packets – packets queued in network only drops if buffer is full

• TCP congestion algorithm– relies on packet drops to determine available bandwidth– keeps speeding up and slowing down the transmission rate to find

equilibrium– packet drops must occur in a timely manner– …


Active Queue Management (AQM)

• Random Early Detection (RED)– random and early notification of congestion– variants FRED, SRED, with notion of flows– no synchronisation à la drop-tail

• CHOKe– penalize misbehaving flows– similar to SRED, but less complex

• CoDel– improve overall performance of RED– easier to manage, does not require manual configuration


Congestion Control, Latency, and AQM


CONCLUDING


Remy Computer-Generated Congestion Control

• Specify– prior knowledge and

assumptions of network– objective to achieve (e.g.,

throughput and delay)

• Outperforms existing (w/ ns2 simulations)– TCP New Reno, TCP Cubic,

Compound (at end-points)– in many cases outperforms

Cubic/FQ-CoDeL (requires network changes) Results for dumbbell topology, n=12,

15 Mbps.


Summary• Congestion control problem has changed

– from: there is congestion, what do we do?– via: networks are empty, what do we do?– to: how do we get all this stuff deployed and let it interoperate?

• After 20+ years still interesting and important problem• One size does not fit all?

– FreeBSD modCC dynamic load/unload CC algorithm– For discussion: Internet at large might agree on model to prepare a

“one-size-fits-all” Remy?

• IETF/IRTF– IETF: AQM, CONEX, RMCAT– IRTF: ICCRG

congestion control algorithms : open questions

Documents

tcp congestion algorithms

tcp reno

yearsglobal congestion

tcp tahoe

classic congestion avoidance

buffer size

control algorithmsresearch

cubic http