User's Guide

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3.4.6.2 Network Maximum Roundtrip Time
ID: 65
Maximum network roundtrip time, or network roundtrip for short, is the time it takes a
packet to get to a destination and the destination to return an ACK / NAK to originator.
The roundtrip also accounts for queue delays, and local retries.
Network roundtrip time is highly dependent on network topology, and may change for
networks with the same n number of nodes but different topologies by a factor of 10 or
even 100 or more for large n.
As a rule of thunb, the max. network roundtrip time should be greater than about 20%
of the average roundtrip time from a given topology.
Roundtrip time resolution is 125 milli seconds and defined by a 2 byte value. Valid
values are [32..992] seconds, parameter setting [256..7.936].
Internally, originator application data is time stamped with a retry or resend timestamp,
which is used to control retries. After each successful (locally acknowledged) retry, the
resend timestamp is reset.
A packet will be resent when the time since last transmission is in the interval of
[network roundtrip] sec. to [network roundtrip]+64 sec.
A packet will only be resent when the packet age is less than TTL - [network roundtrip]
If for example the network roundtrip is 32 seconds (network roundtrip value is 1), and
TTL is 96 seconds (TTL value is 3), the packet will perhaps be retried once, if this retry
happens to fall between 32 to 64 seconds.
The retry period is between 32 and 96 seconds, so to make sure retry happens, the TTL
must have a value at least 3 units (96 seconds) higher than network roundtrip.
See section 3.4.6.3 for more details on the Payload Data Time To Live – TTL parameter.
3.4.6.3 Payload Data Time to Live – TTL for Acknowledged Payload
ID: 66
TTL is the maximum age a globally retried acknowledged packet can have. When a
packet reaches this age, it is removed.
Other data types, such as ACK and NAK are removed when their age reaches [Network
MaxÌmum Roundtrip Rime] / 2.
In general; TTL = [max. network roundtrip time] * R + 3, where R is the number of
desired global retries.