How do I alter values for TCP timeouts (SRTT & RTO)

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Hi

Can anyone please help?! : )

I am working with a Win2000 IP network over a HF radio system with high
latencies (ping = 35 secs). Putting the jokes aside of 'why bother'!, we
occasionally get a TCP timeout occurring. I know this is because the reply
takes longer than the TcpTimeout setting. The MS info on this is at

http://www.microsoft.com/resources/...s/2000/server/reskit/en-us/regentry/58802.asp

I have changed the TcpInitialRtt value in the registry but this is an
initial value to start the Smoothed RTT (SRTT) which is then dynamically
calculated
using a given formula discussed in

http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

SRTT is calculated by values such as [alp], [beta]. Retransmit TimeOut (RTO)
is then calculated from SRTT and also has values such as upper and lower
bounds (UBOUND, LBOUND). I have done a lot of searching and can find lots of
information on what I have discussed above, but nothing about where I can set
the values above. The articles indicate that these values may be altered. I
assume these are set in the registry, like TcpInitialRTT, but I cant find out
where.

cheers David
sunny England
 
Not positive, but I believe you'll discover that lbound & ubound are set
in the source code of whatever TCP you're using, and resetting would
involve a recompile of that module. Not highlevel stuff, likely not in
registry.

Further I believe the max (ubound) setting is dynamically adjusted
according to ongoing retransmit durations, and in most implementations
can exceed your 35 second ping by close to a minute and a half.

Is it possible your radio link is occasionally completely blocked for
periods exceedingf a few hundred seconds? Jammed? Hdw/sftw glitch?

As I say, cum grano salis etc.
Hi

Can anyone please help?! : )

I am working with a Win2000 IP network over a HF radio system with high
latencies (ping = 35 secs). Putting the jokes aside of 'why bother'!, we
occasionally get a TCP timeout occurring. I know this is because the reply
takes longer than the TcpTimeout setting. The MS info on this is at

http://www.microsoft.com/resources/...s/2000/server/reskit/en-us/regentry/58802.asp

I have changed the TcpInitialRtt value in the registry but this is an
initial value to start the Smoothed RTT (SRTT) which is then dynamically
calculated
using a given formula discussed in

http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

SRTT is calculated by values such as [alp], [beta]. Retransmit TimeOut (RTO)
is then calculated from SRTT and also has values such as upper and lower
bounds (UBOUND, LBOUND). I have done a lot of searching and can find lots of
information on what I have discussed above, but nothing about where I can set
the values above. The articles indicate that these values may be altered. I
assume these are set in the registry, like TcpInitialRTT, but I cant find out
where.

cheers David
sunny England


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Thanks for your reply Dan

Well recompiling is a bit above my level if that is what is required and
drifts out of scope of our task to get 'out of the box' IP apps to work over
HF albeit with a few tweaks. I consider altering registry settings to fall
within the scope.

I would be disappointed if these values cannot be set somewhere as settings
such as TcpInitialRtt , and TcpMaxDataRetransmissions can be.

These values all refer to RFC 793 and all these values are talked about in
suggested ranges - leading me to assume that they can be altered. Possibly MS
has not implemented the RFC with non-alterable default values while say,
Linux has.

Your right in saying there may be times when the radio cannot simply make a
link hence why some maximum timeout is required. What I'm finding is one node
will have a quick TCP response and dynamically alter its RTT to say 25 secs.
The next time this is not long enough and it will retransmit (half duplex)
over the incoming response. The system generally works 90% but occasionally
gets its 'knickers in a knot'. The data wont get through until a TCP protocol
attempts to retransmit (eg SMTP failed mail retry).

There are 2 ways I can think of to overcome this, set LBOUND to a min, or
give less weight of SRTT to the last measured RTT. This algorithm is given
from one of the links below;

Windows 2000 TCP Retransmission Behavior
TCP starts a retransmission timer when each TCP segment is sent and then
waits for an acknowledgment to come back before the retransmission timer
expires. This technique is the Positive Acknowledgment Retransmission (PAR)
scheme.

In Windows 2000 every new TCP connection request uses an initial
retransmission timer value that is set to three seconds. This value can be
changed using the TcpInitialRtt Registry parameter. If a TCP connection
cannot be established, it is retried TcpMaxDataRetransmissions times (the
default is two times).

For established TCP connections, the number of retransmissions is controlled
by the TcpMaxDataRetransmissions Registry parameter (set to 5 by default).
The retransmission timeout is adjusted dynamically as per RFC 793,"TCP DARPA
Internet Program Protocol Specification" to adjust for the delay
characteristics of the connection. A Smoothed Round Trip Time (SRTT)
calculation is used as described in the following sidebar.

In some situations TCP retransmits before the retransmission timer expires.
This situation commonly occurs when TCP implements a fast retransmit feature,
described in RFC 2581, "TCP Congestion Control" and explained in the
following sidebar.


--------------------------------------------------------------------------------

Calculating Smoothed Round Trip Time (SRTT)

Measure the elapsed time between sending a data octet with a particular
sequence number and receiving an acknowledgment that covers that sequence
number. This measured elapsed time is the Round Trip Time (RTT). Based on the
RTT, compute a Smoothed Round Trip Time (SRTT) as

SRTT = ( [alp] * SRTT ) + ((1-[alp]) * RTT)

[alp] is a weighting factor whose value is between 0 and 1 (0 <= [alp] < 1).
The SRTT is an estimated timeout. The SRTT computation, therefore, takes into
account its old SRTT value and the new RTT value and takes a weighted average
of these two. When [alp] = 0.5, SRTT is just the simple average of old SRTT
and the RTT values:

SRTT = (SRTT + RTT)/2 when [alp] = 0.5

When [alp] is less than 0.5, a greater weight is given to the actual Round
Trip Time (RTT). When [alp] is greater than 0.5, a greater weight is given to
the previous estimated Round Trip Time (SRTT).

When [alp] = 0 or almost 1, you have the two extremes. When [alp] = 0, SRTT
is always set to RTT; and when [alp] is almost 1, a fixed SRTT is used with
little deference to the actual RTT that is implied. Needless to say, the two
extremes are seldom used in actual practice but are useful for understanding
the nature of the weighted average.

The SRTT is then used to compute the retransmission timeout (RTO) using the
following:

RTO = min[UBOUND,max[LBOUND,([beta]*SRTT)]]

UBOUND is an upper bound on the timeout (for example, one minute), and
LBOUND is a lower bound on the timeout (for example, one second). These
values ensure that the RTO will fall between LBOUND and UBOUND. [beta] is a
delay variance factor to change the sensitivity to the delay. If [beta] is
set to 1, RTO will be set to SRTT unless the SRTT value is below LBOUND or
above UBOUND. This value ([beta] = 1) makes TCP sensitive to packet loss and
does not cause TCP to wait for a long time before retransmitting. However,
small delays might cause unnecessary retransmissions. To make TCP less
susceptible to small delays, a larger [beta] value might be used. RFC 793
suggests using a value from 1.3 to 2.0.

above extract from
http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

Dan Seur said:
Not positive, but I believe you'll discover that lbound & ubound are set
in the source code of whatever TCP you're using, and resetting would
involve a recompile of that module. Not highlevel stuff, likely not in
registry.

Further I believe the max (ubound) setting is dynamically adjusted
according to ongoing retransmit durations, and in most implementations
can exceed your 35 second ping by close to a minute and a half.

Is it possible your radio link is occasionally completely blocked for
periods exceedingf a few hundred seconds? Jammed? Hdw/sftw glitch?

As I say, cum grano salis etc.
Hi

Can anyone please help?! : )

I am working with a Win2000 IP network over a HF radio system with high
latencies (ping = 35 secs). Putting the jokes aside of 'why bother'!, we
occasionally get a TCP timeout occurring. I know this is because the reply
takes longer than the TcpTimeout setting. The MS info on this is at

http://www.microsoft.com/resources/...s/2000/server/reskit/en-us/regentry/58802.asp

I have changed the TcpInitialRtt value in the registry but this is an
initial value to start the Smoothed RTT (SRTT) which is then dynamically
calculated
using a given formula discussed in

http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

SRTT is calculated by values such as [alp], [beta]. Retransmit TimeOut (RTO)
is then calculated from SRTT and also has values such as upper and lower
bounds (UBOUND, LBOUND). I have done a lot of searching and can find lots of
information on what I have discussed above, but nothing about where I can set
the values above. The articles indicate that these values may be altered. I
assume these are set in the registry, like TcpInitialRTT, but I cant find out
where.

cheers David
sunny England


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You're very welcome, Dave. I looked at 793 before responding earlier
(1981? that's 10 years after my 1st involvement with
RAND/ISI/DARPA/Arpanet) and came away with the sense that these min/max
settings may or may not be subnet-settable - up to the coders, which
would mean sometimes they're hardcoded. I think some flavors of *NIX
have them settable externally. No idea about MS products. NT3.1 had a
situation where it ate itself if the settings weren't just right in a
high speed (I mean really high speed) net, according to one old post
from Argonne or somewhere.

I hope some other poster with the real scoop on what you can and can't
do with these parameters will come in with good info instead of
speculation like mine. Good luck.

PS: have you gone down all the registry TCP Parameters lists item by
item? Things sometimes have names that are....well....oddly chosen. :-)
Thanks for your reply Dan

Well recompiling is a bit above my level if that is what is required and
drifts out of scope of our task to get 'out of the box' IP apps to work over
HF albeit with a few tweaks. I consider altering registry settings to fall
within the scope.

I would be disappointed if these values cannot be set somewhere as settings
such as TcpInitialRtt , and TcpMaxDataRetransmissions can be.

These values all refer to RFC 793 and all these values are talked about in
suggested ranges - leading me to assume that they can be altered. Possibly MS
has not implemented the RFC with non-alterable default values while say,
Linux has.

Your right in saying there may be times when the radio cannot simply make a
link hence why some maximum timeout is required. What I'm finding is one node
will have a quick TCP response and dynamically alter its RTT to say 25 secs.
The next time this is not long enough and it will retransmit (half duplex)
over the incoming response. The system generally works 90% but occasionally
gets its 'knickers in a knot'. The data wont get through until a TCP protocol
attempts to retransmit (eg SMTP failed mail retry).

There are 2 ways I can think of to overcome this, set LBOUND to a min, or
give less weight of SRTT to the last measured RTT. This algorithm is given
from one of the links below;

Windows 2000 TCP Retransmission Behavior
TCP starts a retransmission timer when each TCP segment is sent and then
waits for an acknowledgment to come back before the retransmission timer
expires. This technique is the Positive Acknowledgment Retransmission (PAR)
scheme.

In Windows 2000 every new TCP connection request uses an initial
retransmission timer value that is set to three seconds. This value can be
changed using the TcpInitialRtt Registry parameter. If a TCP connection
cannot be established, it is retried TcpMaxDataRetransmissions times (the
default is two times).

For established TCP connections, the number of retransmissions is controlled
by the TcpMaxDataRetransmissions Registry parameter (set to 5 by default).
The retransmission timeout is adjusted dynamically as per RFC 793,"TCP DARPA
Internet Program Protocol Specification" to adjust for the delay
characteristics of the connection. A Smoothed Round Trip Time (SRTT)
calculation is used as described in the following sidebar.

In some situations TCP retransmits before the retransmission timer expires.
This situation commonly occurs when TCP implements a fast retransmit feature,
described in RFC 2581, "TCP Congestion Control" and explained in the
following sidebar.


--------------------------------------------------------------------------------

Calculating Smoothed Round Trip Time (SRTT)

Measure the elapsed time between sending a data octet with a particular
sequence number and receiving an acknowledgment that covers that sequence
number. This measured elapsed time is the Round Trip Time (RTT). Based on the
RTT, compute a Smoothed Round Trip Time (SRTT) as

SRTT = ( [alp] * SRTT ) + ((1-[alp]) * RTT)

[alp] is a weighting factor whose value is between 0 and 1 (0 <= [alp] < 1).
The SRTT is an estimated timeout. The SRTT computation, therefore, takes into
account its old SRTT value and the new RTT value and takes a weighted average
of these two. When [alp] = 0.5, SRTT is just the simple average of old SRTT
and the RTT values:

SRTT = (SRTT + RTT)/2 when [alp] = 0.5

When [alp] is less than 0.5, a greater weight is given to the actual Round
Trip Time (RTT). When [alp] is greater than 0.5, a greater weight is given to
the previous estimated Round Trip Time (SRTT).

When [alp] = 0 or almost 1, you have the two extremes. When [alp] = 0, SRTT
is always set to RTT; and when [alp] is almost 1, a fixed SRTT is used with
little deference to the actual RTT that is implied. Needless to say, the two
extremes are seldom used in actual practice but are useful for understanding
the nature of the weighted average.

The SRTT is then used to compute the retransmission timeout (RTO) using the
following:

RTO = min[UBOUND,max[LBOUND,([beta]*SRTT)]]

UBOUND is an upper bound on the timeout (for example, one minute), and
LBOUND is a lower bound on the timeout (for example, one second). These
values ensure that the RTO will fall between LBOUND and UBOUND. [beta] is a
delay variance factor to change the sensitivity to the delay. If [beta] is
set to 1, RTO will be set to SRTT unless the SRTT value is below LBOUND or
above UBOUND. This value ([beta] = 1) makes TCP sensitive to packet loss and
does not cause TCP to wait for a long time before retransmitting. However,
small delays might cause unnecessary retransmissions. To make TCP less
susceptible to small delays, a larger [beta] value might be used. RFC 793
suggests using a value from 1.3 to 2.0.

above extract from
http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

:

Not positive, but I believe you'll discover that lbound & ubound are set
in the source code of whatever TCP you're using, and resetting would
involve a recompile of that module. Not highlevel stuff, likely not in
registry.

Further I believe the max (ubound) setting is dynamically adjusted
according to ongoing retransmit durations, and in most implementations
can exceed your 35 second ping by close to a minute and a half.

Is it possible your radio link is occasionally completely blocked for
periods exceedingf a few hundred seconds? Jammed? Hdw/sftw glitch?

As I say, cum grano salis etc.
Hi

Can anyone please help?! : )

I am working with a Win2000 IP network over a HF radio system with high
latencies (ping = 35 secs). Putting the jokes aside of 'why bother'!, we
occasionally get a TCP timeout occurring. I know this is because the reply
takes longer than the TcpTimeout setting. The MS info on this is at

http://www.microsoft.com/resources/...s/2000/server/reskit/en-us/regentry/58802.asp

I have changed the TcpInitialRtt value in the registry but this is an
initial value to start the Smoothed RTT (SRTT) which is then dynamically
calculated
using a given formula discussed in

http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

SRTT is calculated by values such as [alp], [beta]. Retransmit TimeOut (RTO)
is then calculated from SRTT and also has values such as upper and lower
bounds (UBOUND, LBOUND). I have done a lot of searching and can find lots of
information on what I have discussed above, but nothing about where I can set
the values above. The articles indicate that these values may be altered. I
assume these are set in the registry, like TcpInitialRTT, but I cant find out
where.

cheers David
sunny England


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Thanks again for your reply. Well with your experience and you think they
cant be set I'll think I'll put this venture on hold.

Yes I've looked throught the registry settings but nothing looked remotely
like it is to alter the settings. Some registry settings dont exist and are
default values unless they are actually created (eg TcpInitialRtt).

Dave Patrick from the Registry forum sent me the following link which seems
to show all there is to alter in the MS realm. It is included for info of
anyone interested.

http://support.microsoft.com/?id=120642

Here's just hoping some boffin happens upon my post with the holy grail of
answers! cheers David

Dan Seur said:
You're very welcome, Dave. I looked at 793 before responding earlier
(1981? that's 10 years after my 1st involvement with
RAND/ISI/DARPA/Arpanet) and came away with the sense that these min/max
settings may or may not be subnet-settable - up to the coders, which
would mean sometimes they're hardcoded. I think some flavors of *NIX
have them settable externally. No idea about MS products. NT3.1 had a
situation where it ate itself if the settings weren't just right in a
high speed (I mean really high speed) net, according to one old post
from Argonne or somewhere.

I hope some other poster with the real scoop on what you can and can't
do with these parameters will come in with good info instead of
speculation like mine. Good luck.

PS: have you gone down all the registry TCP Parameters lists item by
item? Things sometimes have names that are....well....oddly chosen. :-)
Thanks for your reply Dan

Well recompiling is a bit above my level if that is what is required and
drifts out of scope of our task to get 'out of the box' IP apps to work over
HF albeit with a few tweaks. I consider altering registry settings to fall
within the scope.

I would be disappointed if these values cannot be set somewhere as settings
such as TcpInitialRtt , and TcpMaxDataRetransmissions can be.

These values all refer to RFC 793 and all these values are talked about in
suggested ranges - leading me to assume that they can be altered. Possibly MS
has not implemented the RFC with non-alterable default values while say,
Linux has.

Your right in saying there may be times when the radio cannot simply make a
link hence why some maximum timeout is required. What I'm finding is one node
will have a quick TCP response and dynamically alter its RTT to say 25 secs.
The next time this is not long enough and it will retransmit (half duplex)
over the incoming response. The system generally works 90% but occasionally
gets its 'knickers in a knot'. The data wont get through until a TCP protocol
attempts to retransmit (eg SMTP failed mail retry).

There are 2 ways I can think of to overcome this, set LBOUND to a min, or
give less weight of SRTT to the last measured RTT. This algorithm is given
from one of the links below;

Windows 2000 TCP Retransmission Behavior
TCP starts a retransmission timer when each TCP segment is sent and then
waits for an acknowledgment to come back before the retransmission timer
expires. This technique is the Positive Acknowledgment Retransmission (PAR)
scheme.

In Windows 2000 every new TCP connection request uses an initial
retransmission timer value that is set to three seconds. This value can be
changed using the TcpInitialRtt Registry parameter. If a TCP connection
cannot be established, it is retried TcpMaxDataRetransmissions times (the
default is two times).

For established TCP connections, the number of retransmissions is controlled
by the TcpMaxDataRetransmissions Registry parameter (set to 5 by default).
The retransmission timeout is adjusted dynamically as per RFC 793,"TCP DARPA
Internet Program Protocol Specification" to adjust for the delay
characteristics of the connection. A Smoothed Round Trip Time (SRTT)
calculation is used as described in the following sidebar.

In some situations TCP retransmits before the retransmission timer expires.
This situation commonly occurs when TCP implements a fast retransmit feature,
described in RFC 2581, "TCP Congestion Control" and explained in the
following sidebar.


--------------------------------------------------------------------------------

Calculating Smoothed Round Trip Time (SRTT)

Measure the elapsed time between sending a data octet with a particular
sequence number and receiving an acknowledgment that covers that sequence
number. This measured elapsed time is the Round Trip Time (RTT). Based on the
RTT, compute a Smoothed Round Trip Time (SRTT) as

SRTT = ( [alp] * SRTT ) + ((1-[alp]) * RTT)

[alp] is a weighting factor whose value is between 0 and 1 (0 <= [alp] < 1).
The SRTT is an estimated timeout. The SRTT computation, therefore, takes into
account its old SRTT value and the new RTT value and takes a weighted average
of these two. When [alp] = 0.5, SRTT is just the simple average of old SRTT
and the RTT values:

SRTT = (SRTT + RTT)/2 when [alp] = 0.5

When [alp] is less than 0.5, a greater weight is given to the actual Round
Trip Time (RTT). When [alp] is greater than 0.5, a greater weight is given to
the previous estimated Round Trip Time (SRTT).

When [alp] = 0 or almost 1, you have the two extremes. When [alp] = 0, SRTT
is always set to RTT; and when [alp] is almost 1, a fixed SRTT is used with
little deference to the actual RTT that is implied. Needless to say, the two
extremes are seldom used in actual practice but are useful for understanding
the nature of the weighted average.

The SRTT is then used to compute the retransmission timeout (RTO) using the
following:

RTO = min[UBOUND,max[LBOUND,([beta]*SRTT)]]

UBOUND is an upper bound on the timeout (for example, one minute), and
LBOUND is a lower bound on the timeout (for example, one second). These
values ensure that the RTO will fall between LBOUND and UBOUND. [beta] is a
delay variance factor to change the sensitivity to the delay. If [beta] is
set to 1, RTO will be set to SRTT unless the SRTT value is below LBOUND or
above UBOUND. This value ([beta] = 1) makes TCP sensitive to packet loss and
does not cause TCP to wait for a long time before retransmitting. However,
small delays might cause unnecessary retransmissions. To make TCP less
susceptible to small delays, a larger [beta] value might be used. RFC 793
suggests using a value from 1.3 to 2.0.

above extract from
http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

:

Not positive, but I believe you'll discover that lbound & ubound are set
in the source code of whatever TCP you're using, and resetting would
involve a recompile of that module. Not highlevel stuff, likely not in
registry.

Further I believe the max (ubound) setting is dynamically adjusted
according to ongoing retransmit durations, and in most implementations
can exceed your 35 second ping by close to a minute and a half.

Is it possible your radio link is occasionally completely blocked for
periods exceedingf a few hundred seconds? Jammed? Hdw/sftw glitch?

As I say, cum grano salis etc.

hotdogdave wrote:

Hi

Can anyone please help?! : )

I am working with a Win2000 IP network over a HF radio system with high
latencies (ping = 35 secs). Putting the jokes aside of 'why bother'!, we
occasionally get a TCP timeout occurring. I know this is because the reply
takes longer than the TcpTimeout setting. The MS info on this is at

http://www.microsoft.com/resources/...s/2000/server/reskit/en-us/regentry/58802.asp

I have changed the TcpInitialRtt value in the registry but this is an
initial value to start the Smoothed RTT (SRTT) which is then dynamically
calculated
using a given formula discussed in

http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

SRTT is calculated by values such as [alp], [beta]. Retransmit TimeOut (RTO)
is then calculated from SRTT and also has values such as upper and lower
bounds (UBOUND, LBOUND). I have done a lot of searching and can find lots of
information on what I have discussed above, but nothing about where I can set
the values above. The articles indicate that these values may be altered. I
assume these are set in the registry, like TcpInitialRTT, but I cant find out
where.

cheers David
sunny England


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Dave, about that boffin -

- There are some very good tech fora other than these MS Winx's. Came
across one or two googling for your lbound/ubound problem.

- You might try posting to a couple of these. You might also try one or
two *nix boards in hopes that some wirehead there has been there done
that to Win2k...those *nix guys like to get down and dirty with system
code in ways that the MS community just cannot.

- And lastly, you might somehow post a query (phone call??) to one of
the big university comp centers involved in very hispeed networking.
With the right attitude/question you might find a guy who knows all
about your problem and has a solution. There are similarities in
ACK/NACK requirements and consequent code mods between ultrafast
interhost TCP and reeeeallllly slllooooowww TCP down stringand can-based
links... Just a thought.

- And REALLY lastly - that radio drop thing - hmmm. ;_0
Thanks again for your reply. Well with your experience and you think they
cant be set I'll think I'll put this venture on hold.

Yes I've looked throught the registry settings but nothing looked remotely
like it is to alter the settings. Some registry settings dont exist and are
default values unless they are actually created (eg TcpInitialRtt).

Dave Patrick from the Registry forum sent me the following link which seems
to show all there is to alter in the MS realm. It is included for info of
anyone interested.

http://support.microsoft.com/?id=120642

Here's just hoping some boffin happens upon my post with the holy grail of
answers! cheers David

:

You're very welcome, Dave. I looked at 793 before responding earlier
(1981? that's 10 years after my 1st involvement with
RAND/ISI/DARPA/Arpanet) and came away with the sense that these min/max
settings may or may not be subnet-settable - up to the coders, which
would mean sometimes they're hardcoded. I think some flavors of *NIX
have them settable externally. No idea about MS products. NT3.1 had a
situation where it ate itself if the settings weren't just right in a
high speed (I mean really high speed) net, according to one old post
from Argonne or somewhere.

I hope some other poster with the real scoop on what you can and can't
do with these parameters will come in with good info instead of
speculation like mine. Good luck.

PS: have you gone down all the registry TCP Parameters lists item by
item? Things sometimes have names that are....well....oddly chosen. :-)

hotdogdave wrote:

Thanks for your reply Dan

Well recompiling is a bit above my level if that is what is required and
drifts out of scope of our task to get 'out of the box' IP apps to work over
HF albeit with a few tweaks. I consider altering registry settings to fall
within the scope.

I would be disappointed if these values cannot be set somewhere as settings
such as TcpInitialRtt , and TcpMaxDataRetransmissions can be.

These values all refer to RFC 793 and all these values are talked about in
suggested ranges - leading me to assume that they can be altered. Possibly MS
has not implemented the RFC with non-alterable default values while say,
Linux has.

Your right in saying there may be times when the radio cannot simply make a
link hence why some maximum timeout is required. What I'm finding is one node
will have a quick TCP response and dynamically alter its RTT to say 25 secs.
The next time this is not long enough and it will retransmit (half duplex)
over the incoming response. The system generally works 90% but occasionally
gets its 'knickers in a knot'. The data wont get through until a TCP protocol
attempts to retransmit (eg SMTP failed mail retry).

There are 2 ways I can think of to overcome this, set LBOUND to a min, or
give less weight of SRTT to the last measured RTT. This algorithm is given
from one of the links below;

Windows 2000 TCP Retransmission Behavior
TCP starts a retransmission timer when each TCP segment is sent and then
waits for an acknowledgment to come back before the retransmission timer
expires. This technique is the Positive Acknowledgment Retransmission (PAR)
scheme.

In Windows 2000 every new TCP connection request uses an initial
retransmission timer value that is set to three seconds. This value can be
changed using the TcpInitialRtt Registry parameter. If a TCP connection
cannot be established, it is retried TcpMaxDataRetransmissions times (the
default is two times).

For established TCP connections, the number of retransmissions is controlled
by the TcpMaxDataRetransmissions Registry parameter (set to 5 by default).
The retransmission timeout is adjusted dynamically as per RFC 793,"TCP DARPA
Internet Program Protocol Specification" to adjust for the delay
characteristics of the connection. A Smoothed Round Trip Time (SRTT)
calculation is used as described in the following sidebar.

In some situations TCP retransmits before the retransmission timer expires.
This situation commonly occurs when TCP implements a fast retransmit feature,
described in RFC 2581, "TCP Congestion Control" and explained in the
following sidebar.


--------------------------------------------------------------------------------

Calculating Smoothed Round Trip Time (SRTT)

Measure the elapsed time between sending a data octet with a particular
sequence number and receiving an acknowledgment that covers that sequence
number. This measured elapsed time is the Round Trip Time (RTT). Based on the
RTT, compute a Smoothed Round Trip Time (SRTT) as

SRTT = ( [alp] * SRTT ) + ((1-[alp]) * RTT)

[alp] is a weighting factor whose value is between 0 and 1 (0 <= [alp] < 1).
The SRTT is an estimated timeout. The SRTT computation, therefore, takes into
account its old SRTT value and the new RTT value and takes a weighted average
of these two. When [alp] = 0.5, SRTT is just the simple average of old SRTT
and the RTT values:

SRTT = (SRTT + RTT)/2 when [alp] = 0.5

When [alp] is less than 0.5, a greater weight is given to the actual Round
Trip Time (RTT). When [alp] is greater than 0.5, a greater weight is given to
the previous estimated Round Trip Time (SRTT).

When [alp] = 0 or almost 1, you have the two extremes. When [alp] = 0, SRTT
is always set to RTT; and when [alp] is almost 1, a fixed SRTT is used with
little deference to the actual RTT that is implied. Needless to say, the two
extremes are seldom used in actual practice but are useful for understanding
the nature of the weighted average.

The SRTT is then used to compute the retransmission timeout (RTO) using the
following:

RTO = min[UBOUND,max[LBOUND,([beta]*SRTT)]]

UBOUND is an upper bound on the timeout (for example, one minute), and
LBOUND is a lower bound on the timeout (for example, one second). These
values ensure that the RTO will fall between LBOUND and UBOUND. [beta] is a
delay variance factor to change the sensitivity to the delay. If [beta] is
set to 1, RTO will be set to SRTT unless the SRTT value is below LBOUND or
above UBOUND. This value ([beta] = 1) makes TCP sensitive to packet loss and
does not cause TCP to wait for a long time before retransmitting. However,
small delays might cause unnecessary retransmissions. To make TCP less
susceptible to small delays, a larger [beta] value might be used. RFC 793
suggests using a value from 1.3 to 2.0.

above extract from
http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

:



Not positive, but I believe you'll discover that lbound & ubound are set
in the source code of whatever TCP you're using, and resetting would
involve a recompile of that module. Not highlevel stuff, likely not in
registry.

Further I believe the max (ubound) setting is dynamically adjusted
according to ongoing retransmit durations, and in most implementations
can exceed your 35 second ping by close to a minute and a half.

Is it possible your radio link is occasionally completely blocked for
periods exceedingf a few hundred seconds? Jammed? Hdw/sftw glitch?

As I say, cum grano salis etc.

hotdogdave wrote:


Hi

Can anyone please help?! : )

I am working with a Win2000 IP network over a HF radio system with high
latencies (ping = 35 secs). Putting the jokes aside of 'why bother'!, we
occasionally get a TCP timeout occurring. I know this is because the reply
takes longer than the TcpTimeout setting. The MS info on this is at

http://www.microsoft.com/resources/...s/2000/server/reskit/en-us/regentry/58802.asp

I have changed the TcpInitialRtt value in the registry but this is an
initial value to start the Smoothed RTT (SRTT) which is then dynamically
calculated
using a given formula discussed in

http://www.informit.com/articles/article.asp?p=130716&seqNum=4&rl=1

SRTT is calculated by values such as [alp], [beta]. Retransmit TimeOut (RTO)
is then calculated from SRTT and also has values such as upper and lower
bounds (UBOUND, LBOUND). I have done a lot of searching and can find lots of
information on what I have discussed above, but nothing about where I can set
the values above. The articles indicate that these values may be altered. I
assume these are set in the registry, like TcpInitialRTT, but I cant find out
where.

cheers David
sunny England


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