From: Mike N. 


With news.barkto.com ping turned back on, I tried to measure the MTU. Using
the command

   ping -f -l xxxx news.barkto.com
(That's lower case L, not capital i)   , varying the payload length, I get
a maximum ping payload of 1472 bytes - and the maximum packet size with
headers of 1514 bytes.   That's also the maximum size I can send on my LAN.

   So it still comes down to testing at the news.barkto.com server to see
what is happening with MTU discovery.   A workaround is to set

SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

\EnablePMTUDiscovery  to zero - Completely enables or disables the PMTU
discovery   mechanism. When PMTU discovery is disabled, an MTU of 576 bytes
is used   for all non-local destination addresses. PMTU discovery is
enabled by default.

  Or, if you really want to spend time tinkering, set
 \EnablePMTUBHDetect  to 1 - Description: Setting this parameter to 1
(True) causes TCP to try to detect Black Hole routers while doing Path MTU
Discovery. A Black Hole router does not return ICMP Destination Unreachable
messages when it needs to fragment a TCP packet with the Don’t Fragment bit
set. TCP depends on receiving these messages to perform Path MTU Discovery.
With this feature enabled, TCP will try to send segments without the Don’t
Fragment bit set if several retransmissions of a segment go unacknowledged.
If the segment is acknowledged as a result, the MSS will be decreased and
the Don’t Fragment bit will be set in future packets on the connection.
Enabling black hole detection increases the maximum number of
retransmissions performed for a given segment.

--- BBBS/NT v4.01 Flag-5