
4.2.3.1 Retransmission Timeout Calculation
Connected: An Internet Encyclopedia
4.2.3.1 Retransmission Timeout Calculation
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Connected: An Internet Encyclopedia
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Requests For Comments
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RFC 1122
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4. TRANSPORT PROTOCOLS
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4.2 TRANSMISSION CONTROL PROTOCOL  TCP
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4.2.3 SPECIFIC ISSUES
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Next: 4.2.3.2 When to Send an ACK Segment
4.2.3.1 Retransmission Timeout Calculation
4.2.3.1 Retransmission Timeout Calculation
A host TCP MUST implement Karn's algorithm and Jacobson's
algorithm for computing the retransmission timeout ("RTO").
 Jacobson's algorithm for computing the smoothed round
trip ("RTT") time incorporates a simple measure of the
variance [TCP:7].
 Karn's algorithm for selecting RTT measurements ensures
that ambiguous roundtrip times will not corrupt the
calculation of the smoothed roundtrip time [TCP:6].
This implementation also MUST include "exponential backoff"
for successive RTO values for the same segment.
Retransmission of SYN segments SHOULD use the same algorithm
as data segments.
 DISCUSSION:

There were two known problems with the RTO calculations
specified in RFC793. First, the accurate measurement
of RTTs is difficult when there are retransmissions.
Second, the algorithm to compute the smoothed round
trip time is inadequate [TCP:7], because it incorrectly
assumed that the variance in RTT values would be small
and constant. These problems were solved by Karn's and
Jacobson's algorithm, respectively.
The performance increase resulting from the use of
these improvements varies from noticeable to dramatic.
Jacobson's algorithm for incorporating the measured RTT
variance is especially important on a lowspeed link,
where the natural variation of packet sizes causes a
large variation in RTT. One vendor found link
utilization on a 9.6kb line went from 10% to 90% as a
result of implementing Jacobson's variance algorithm in
TCP.
The following values SHOULD be used to initialize the
estimation parameters for a new connection:
 RTT = 0 seconds.
 RTO = 3 seconds. (The smoothed variance is to be
initialized to the value that will result in this RTO).
The recommended upper and lower bounds on the RTO are known
to be inadequate on large internets. The lower bound SHOULD
be measured in fractions of a second (to accommodate high
speed LANs) and the upper bound should be 2*MSL, i.e., 240
seconds.
 DISCUSSION:

Experience has shown that these initialization values
are reasonable, and that in any case the Karn and
Jacobson algorithms make TCP behavior reasonably
insensitive to the initial parameter choices.
Next: 4.2.3.2 When to Send an ACK Segment
Connected: An Internet Encyclopedia
4.2.3.1 Retransmission Timeout Calculation






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