Troubleshooting
Problem
This document describes the TCP R2 retransmission function within the IBM i Change TCP Attributes (CHGTCPA) command.
Resolving The Problem
TCP retransmissions are based on TCP/IP communications, where packets are sent to a host, and those packets failing to be acknowledged by the remote host. When the system is forced to send that data again, it is known as a TCP retransmit or TCP retransmission packet. The IBM i controls TCP/IP retransmissions using the Change TCP Attributes (CHGTCPA) command, with the parameter TCP R2 retransmission count (TCPR2CNT)
By default, this value is set to 16. This indicates the System i will retransmit a packet up to 16 times before the connection will expire and ultimately terminate. The underlying timing mechanism of a TCP retransmit is based on a calculation of the round trip time between the client and the host. The common timing referred to is based on the initial retransmit occurring 250ms (.250 seconds) after the initial packet goes unacknowledged. The timer is then exponentially backed off. For example, using the 250ms base timer, this is how the corresponding retransmits are timed:
| R2# | Interval | Total Time |
| 1 | 0.25 | 0.25 |
| 2 | 0.50 | 0.75 |
| 3 | 1.0 | 1.75 |
| 4 | 2.0 | 3.75 |
| 5 | 4.0 | 7.75 |
| 6 | 8.0 | 15.75 |
| 7 | 16 | 31.75 |
| 8 | 32 | 63.75 |
| 9 | 64 | 128 |
| 10 | 128 | 256 |
| 11 | 128 | 384 |
| 12 | 128 | 512 |
| 13 | 128 | 640 |
| 14 | 128 | 768 |
| 15 | 128 | 896 |
| 16 | 128 | 1024 = ~17 minutes |
Notice at retransmit (R2#) 11, we maintain the 128-second interval versus backing off further to 256 seconds. This is an intentional design to keep the retransmits to a maximum delay of approximately 2 minutes. The total duration of a connection going through full retransmissions would be roughly 17 minutes in the example above. The numbers above are a general example of the timing mechanism; however, these are very close to real world times used in client environments.
Historical Number
564994179
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Document Information
Modified date:
18 December 2019
UID
nas8N1012183