An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 91
500
50
450
45
400
40
Cwnd (packets)
350
300
250
200
150
Goodput (mbit/sec)
35
30
25
20
15
100
Normal Acking
50
Delayed Acking
Delayed Acking with ABC
0
0 50 100 150 200 250 300
Time (seconds)
10
Normal Acking
5
Delayed Acking
Delayed Acking with ABC
0
0 50 100 150 200 250 300
Time (seconds)
(a) CWND
(b) Goodput
Figure 5.6: Effect of delayed acks upon cwnd dynamics.
delayed acks is less aggressive, and increases cwnd at half the rate compared
to a TCP connection without delayed acknowledgments.
This results in
reduced throughput over short timescales after each congestion event for any
TCP connection where the receiver is using delayed acknowledgments. This
is shown in Figure 5.6 for machines on a private dummynet network (See
Appendix A.5).
This is especially noticeable in the slow start phase, because in slow start
each arriving ack increases the congestion window by one segment, thus
effectively doubling the rate at which packets are sent.
The Linux kernel implements delayed acking by default [Bra89], but imposes
a less conservative delayed acking time of up-to 200ms. Linux introduces
a feature called quick acks [SK02] which helps to reduce the problems
of reduced responsiveness when using delayed acking during slow start. The
idea is to disable delayed acks at the receiver for the first n packets of the
connection, where n is a configurable parameter, acknowledging every packet
at the beginning of the connection and thus achieving the equilibrium state
(congestion avoidance) in shorter time.
On the other hand, such a policy would increase the probability of net-