Hello John,

>> James, Does setting the lower bound lower than the initial retransmission
>>interval accomplish anything?  I was under the impression that the first
>>retransmission of a packet uses the initial interval, and the intervals
>>grow longer after that.  Am I mistaken?

[1  ]     Retransmission Interval Lower Bound (Secs)
[2  ]     Initial Retransmission Interval (Secs)

This is the lowest configurable values for these 2 variables (and yes the 
help documentation for "Retransmission Interval Lower Bound" is wrong... the 
lowest value is [1] seconds).  This is the configuration that I would use in 
all cases with exception to ***weird*** network's like TCP over satellite or 
using 3000's as a router and in these cases I would want a network expert 
involved in devising a timer change as a "workaround".

A feature of the MPE implementation of the "Retransmission Interval Lower 
Bound" is that the lowest value is [1] second.  Note:  On some machines it 
is possible to be less.

**FOR A NEW CONNECTION BEING ESTABLISHED, the "Initial Retransmission 
Interval" time is used.  SYN packets are sent out on the basis of this 
timer.  The first packet is sent out on a connection request and if no 
response, the second SYN at 2 seconds, the third at +4 seconds, the forth at 
+8 seconds, the fifth at +16 seconds, the sixth at +32 seconds, the seventh 
at +64 seconds and the eight at +128 seconds.

I did 8 iterations here because I specified in the configuration the value 
of:

[8  ]     Maximum Retransmissions per Packet

Note:   The number of actual retransmission may actually be bounded by the 
timer:

[360  ]   Maximum Time to Wait For Remote Response (Sec)

If the sum of time for the timers exceeds this configuration value the 
retransmission is not performed and the connection attempt is dropped...  or 
after the 8th retransmission times out, the connection attempt is dropped.

----------

**FOR AN EXISTING CONNECTION the TCP packet transmission starts at the 
"Initial Retransmission Interval" configured value of [2] seconds and then 
is computed (for the details see Douglass E. Comer's book - Internetworking 
with TCP/IP [fourth edition - vol 1] - page 226) to be the smoothed round 
trip time to send a TCP packet and receive an acknowledgement with a minimal 
value of the "Retransmission Interval Lower Bound (Secs)" configured value 
of [1] second.

On local LAN links and extended links where the performance of an 
acknowledged TCP packet is on a smoothed average less than [1] second the 
"Retransmission Interval Lower Bound" [1] second will be the computed 
"Retransmission Timer" value.  For poor quality links, the "Retransmission 
Timer" value will increase to be the smoothed round trip time to send a TCP 
packet and receive an acknowledgement which may be greater than [1] second 
or [2] seconds or [5]  seconds or more...

----------

The BOTTOM LINE answer to your question... is the configured TCP timers are 
being used for (2) purposes... first for the initial connection 
establishment (syn-syn/ack-ack) and secondly for ongoing data transfer.  The 
"Initial Retransmission Interval" comes into play on initial connection 
establishment and is a starting point for initial data transfer.  The 
computed "Retransmission Timer" value is the real timer performing 
retransmissions once a connection is established, but for very fast and 
error free networks this timer has a lower bound of the "Retransmission 
Interval Lower Bound.

I hope this helps...

P.S.  Making the "Retransmission Interval Lower Bound" bigger than the 
"Initial Retransmission Interval" will break MPE's TCP just in case you are 
curious.

Regards,

James Hofmeister
Email: <first>.<last>@hp.com
Hewlett Packard - Global Solutions Engineering (WTEC)
P.S. My Ideals are my own, not necessarily my employers. 

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