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15-441 Computer Networking. Lecture 18 – More TCP & Congestion Control. Outline. TCP reliability TCP congestion control. Reliability Challenges. Congestion related losses Variable packet delays What should the timeout be? Reordering of packets

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15 441 computer networking

15-441 Computer Networking

Lecture 18 – More TCP & Congestion Control

outline
Outline
  • TCP reliability
  • TCP congestion control

Lecture 18: 03-22-2005

reliability challenges
Reliability Challenges
  • Congestion related losses
  • Variable packet delays
    • What should the timeout be?
  • Reordering of packets
    • How to tell the difference between a delayed packet and a lost one?

Lecture 18: 03-22-2005

tcp go back n variant
TCP = Go-Back-N Variant
  • Sliding window with cumulative acks
    • Receiver can only return a single “ack” sequence number to the sender.
    • Acknowledges all bytes with a lower sequence number
    • Starting point for retransmission
    • Duplicate acks sent when out-of-order packet received
  • But: sender only retransmits a single packet.
    • Reason???
      • Only one that it knows is lost
      • Network is congested  shouldn’t overload it
  • Error control is based on byte sequences, not packets.
    • Retransmitted packet can be different from the original lost packet – Why?

Lecture 18: 03-22-2005

round trip time estimation
Round-trip Time Estimation
  • Wait at least one RTT before retransmitting
  • Importance of accurate RTT estimators:
    • Low RTT estimate
      • unneeded retransmissions
    • High RTT estimate
      • poor throughput
  • RTT estimator must adapt to change in RTT
    • But not too fast, or too slow!
  • Spurious timeouts
    • “Conservation of packets” principle – never more than a window worth of packets in flight

Lecture 18: 03-22-2005

original tcp round trip estimator
Original TCP Round-trip Estimator
  • Round trip times exponentially averaged:
    • New RTT = a (old RTT) + (1 - a) (new sample)
    • Recommended value for a: 0.8 - 0.9
      • 0.875 for most TCP’s
  • Retransmit timer set to (b * RTT), where b = 2
    • Every time timer expires, RTO exponentially backed-off
  • Not good at preventing spurious timeouts
    • Why?

Lecture 18: 03-22-2005

jacobson s retransmission timeout
Jacobson’s Retransmission Timeout
  • Key observation:
    • At high loads round trip variance is high
  • Solution:
    • Base RTO on RTT and standard deviation
      • RTO = RTT + 4 * rttvar
    • new_rttvar = b * dev + (1- b) old_rttvar
      • Dev = linear deviation
      • Inappropriately named – actually smoothed linear deviation

Lecture 18: 03-22-2005

rtt sample ambiguity

A

B

Original transmission

RTO

ACK

Sample

RTT

retransmission

RTT Sample Ambiguity

A

B

  • Karn’s RTT Estimator
    • If a segment has been retransmitted:
      • Don’t count RTT sample on ACKs for this segment
      • Keep backed off time-out for next packet
      • Reuse RTT estimate only after one successful transmission

Original transmission

X

RTO

Sample

RTT

retransmission

ACK

Lecture 18: 03-22-2005

timestamp extension
Timestamp Extension
  • Used to improve timeout mechanism by more accurate measurement of RTT
  • When sending a packet, insert current timestamp into option
    • 4 bytes for timestamp, 4 bytes for echo
  • Receiver echoes timestamp in ACK
    • Actually will echo whatever is in timestamp
  • Removes retransmission ambiguity
    • Can get RTT sample on any packet

Lecture 18: 03-22-2005

timer granularity
Timer Granularity
  • Many TCP implementations set RTO in multiples of 200,500,1000ms
  • Why?
    • Avoid spurious timeouts – RTTs can vary quickly due to cross traffic
    • Make timers interrupts efficient
  • What happens for the first couple of packets?
    • Pick a very conservative value (seconds)

Lecture 18: 03-22-2005

fast retransmit
Fast Retransmit
  • What are duplicate acks (dupacks)?
    • Repeated acks for the same sequence
  • When can duplicate acks occur?
    • Loss
    • Packet re-ordering
    • Window update – advertisement of new flow control window
  • Assume re-ordering is infrequent and not of large magnitude
    • Use receipt of 3 or more duplicate acks as indication of loss
    • Don’t wait for timeout to retransmit packet

Lecture 18: 03-22-2005

fast retransmit12

Packets

Acks

Fast Retransmit

Retransmission

X

Duplicate Acks

Sequence No

Time

Lecture 18: 03-22-2005

outline13
Outline
  • TCP reliability
  • TCP congestion control

Lecture 18: 03-22-2005

tcp congestion control
TCP Congestion Control
  • Changes to TCP motivated by ARPANET congestion collapse
  • Basic principles
    • AIMD
    • Packet conservation
    • Reaching steady state quickly
    • ACK clocking

Lecture 18: 03-22-2005

slide15
AIMD
  • Distributed, fair and efficient
  • Packet loss is seen as sign of congestion and results in a multiplicative rate decrease
    • Factor of 2
  • TCP periodically probes for available bandwidth by increasing its rate

Rate

Time

Lecture 18: 03-22-2005

implementation issue
Implementation Issue
  • Operating system timers are very coarse – how to pace packets out smoothly?
  • Implemented using a congestion window that limits how much data can be in the network.
    • TCP also keeps track of how much data is in transit
  • Data can only be sent when the amount of outstanding data is less than the congestion window.
    • The amount of outstanding data is increased on a “send” and decreased on “ack”
    • (last sent – last acked) < congestion window
  • Window limited by both congestion and buffering
    • Sender’s maximum window = Min (advertised window, cwnd)

Lecture 18: 03-22-2005

congestion avoidance
Congestion Avoidance
  • If loss occurs when cwnd = W
    • Network can handle 0.5W ~ W segments
    • Set cwnd to 0.5W (multiplicative decrease)
  • Upon receiving ACK
    • Increase cwnd by (1 packet)/cwnd
      • What is 1 packet?  1 MSS worth of bytes
      • After cwnd packets have passed by  approximately increase of 1 MSS
  • Implements AIMD

Lecture 18: 03-22-2005

congestion avoidance sequence plot

Packets

Acks

Congestion Avoidance Sequence Plot

Sequence No

Time

Lecture 18: 03-22-2005

congestion avoidance behavior
Congestion Avoidance Behavior

Congestion

Window

Time

Cut

Congestion

Window

and Rate

Grabbing

back

Bandwidth

Packet loss

+ Timeout

Lecture 18: 03-22-2005

packet conservation
Packet Conservation
  • At equilibrium, inject packet into network only when one is removed
    • Sliding window and not rate controlled
    • But still need to avoid sending burst of packets  would overflow links
      • Need to carefully pace out packets
      • Helps provide stability
  • Need to eliminate spurious retransmissions
    • Accurate RTO estimation
    • Better loss recovery techniques (e.g. fast retransmit)

Lecture 18: 03-22-2005

tcp packet pacing
TCP Packet Pacing
  • Congestion window helps to “pace” the transmission of data packets
  • In steady state, a packet is sent when an ack is received
    • Data transmission remains smooth, once it is smooth
    • Self-clocking behavior

Pb

Pr

Sender

Receiver

As

Ar

Ab

Lecture 18: 03-22-2005

reaching steady state
Reaching Steady State
  • Doing AIMD is fine in steady state but slow…
  • How does TCP know what is a good initial rate to start with?
    • Should work both for a CDPD (10s of Kbps or less) and for supercomputer links (10 Gbps and growing)
  • Quick initial phase to help get up to speed (slow start)

Lecture 18: 03-22-2005

slow start packet pacing
Slow Start Packet Pacing
  • How do we get this clocking behavior to start?
    • Initialize cwnd = 1
    • Upon receipt of every ack, cwnd = cwnd + 1
  • Implications
    • Window actually increases to W in RTT * log2(W)
    • Can overshoot window and cause packet loss

Lecture 18: 03-22-2005

slow start example

One RTT

0R

1

One pkt time

1R

1

2

3

2R

2

3

4

6

5

7

4

5

6

7

3R

8

10

12

14

9

11

13

15

Slow Start Example

Lecture 18: 03-22-2005

slow start sequence plot

Packets

Acks

Slow Start Sequence Plot

.

.

.

Sequence No

Time

Lecture 18: 03-22-2005

return to slow start
Return to Slow Start
  • If packet is lost we lose our self clocking as well
    • Need to implement slow-start and congestion avoidance together
  • When timeout occurs set ssthresh to 0.5w
    • If cwnd < ssthresh, use slow start
    • Else use congestion avoidance

Lecture 18: 03-22-2005

tcp saw tooth behavior
TCP Saw Tooth Behavior

Congestion

Window

Timeouts

may still

occur

Time

Slowstart

to pace

packets

Fast

Retransmit

and Recovery

Initial

Slowstart

Lecture 18: 03-22-2005

important lessons
Important Lessons
  • TCP timeout calculation  how is RTT estimated
  • Modern TCP loss recovery
    • Why are timeouts bad?
    • How to avoid them?  e.g. fast retransmit
  • How does TCP implement AIMD?
    • Sliding window, slow start & ack clocking
    • How to maintain ack clocking during loss recovery  fast recovery

Lecture 18: 03-22-2005