Lecture 4: TCP and TFRC

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TCP"Reno, NewReno, SACK

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TCP Reno

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0

2

4

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8

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14 cwnd ssthresh

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new ack:"if (cwnd < sstresh)" cwnd += 1"else" cwnd += 1/cwnd

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timeout:"retransmit 1st unacked"ssthresh = cwnd/2"cwnd = 1"

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3rd duplicate ACK:"fast retransmission" (ie, retransmit 1st unack) "fast recovery" (details today)"ssthresh = cwnd = cwnd/2

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TCP’s rule"

send more packets if "L + cwnd > H"

[ L .. H-1 ] are outstanding packets 4 September 2009 CS5229, Semester 1, 2009/10

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Data Packets

ACK Packets (received up to this seq number)

Congestion Window

Sequence Number

Time 4 September 2009 CS5229, Semester 1, 2009/10

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3rd dup ack:"retransmit 1st unacked"ssthresh = cwnd/2"cwnd = cwnd/2

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X

Incorrectly clamping the congestion window too soon (without fast recovery)

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fast recovery:"

keep the pipe occupied"

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X

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X

X

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TCP Reno timeout with multiple losses in a window

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TCP NewReno

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X

X

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3rd dup ack:"retransmit 1st unacked"ssthresh = cwnd/2"cwnd = cwnd/2 + 3"remember highest

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“complete” ack:"(all are acked) "cwnd = ssthresh

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“partial” ack:"(acknowledge n packets)"retransmit"cwnd = cwnd - n + 1

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Note: RFC2581/RFC2582 give the accurate/gory details. Simplified version is presented here (eg. cwnd vs FlightSize, update of cwnd upon partial ACK).

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What does a dup ACK tell us?

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“Coarse Feedback”

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TCP SACK

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Use TCP header options to report

received segments.

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SACK Blocks:"

1st block - report most recently received segments"

subsequent blocks - repeat most recent previous blocks

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pipe: num of outstanding packets in the pipe."

send only if pipe < cwnd

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scoreboard: which packets have been received?

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X

X

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3rd dup ack:"pipe = cwnd - 3"retransmit 1st unacked"ssthresh = cwnd/2"cwnd = cwnd/2 + 3

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subsequent dup ack:"cwnd++"pipe--"

if pipe < cwnd" send packet, pipe++

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“partial” ack:"retransmit"cwnd = cwnd - n + 1"pipe -= 2"if pipe < cwnd" send packet, pipe++

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Idea of SACK:"

Which packet has left the network?"Where is the gap?"

Decouple when to send and what to send.

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TCP SACK recovers faster than NewReno with multiple losses in a window.

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Deployment

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77%

15%

8%

NewReno

Reno

Tahoe

Feb 2004

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70% "SACK capable

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TCP BIC/CUBIC"Linux 2.6.x

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Compound TCP "MS Windows Vista

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TFRC"equation-based"

congestion control

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Not everyone uses TCP

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UDP:"

Media streaming"Gaming"

VoIP

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Why not congestion controlled?"

1. UDP has low delay, no need full reliability. UDP "

is not congestion controlled.

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Why not congestion controlled?"

2. No incentive. OTOH, there are incentives NOT to use

congestion control.

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70 Sally Floyd, h+p://www.icir.org/floyd/tcp_unfriendly.html 4 September 2009 CS5229, Semester 1, 2009/10

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“Unresponsive Flows”

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Bad: lead to unfairness and congestion collapse.

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Unfairness: unresponsive flows consume more bandwidth

than congestion controlled flows.

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Congestion Collapse: wasting bandwidth by sending packets that will be dropped

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Today: "a TCP-friendly"

unreliable protocol

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Idea: "send at a rate that a TCP

flow would send

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we can do the AIMD-thing"at the source, or

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equation-based "congestion control

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steady"fair"

responsive

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how to determine"RTT"tRTO"p

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p is not packet loss rate"but "

loss event rate

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RTT can fluctuates

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how to initialize?

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slow start"(until loss occur)"

Tnow = min(2Tprev, 2Trecv)

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no loss history, how?"

solve p given T, RTT

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TFRC is now part of DCCP

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