Reducing dummynet pipe size hangs system

[Michael R. Wayne]

Summary:
   On an old (4.8p10) machine, substantially reducing the pipe size
   on a loaded connection causes the system to freeze.

   Is this a supported feature?

   Did it usd to be broken?

Detail:
   There's a 4.8p10 machine acting as a firewall (runs only ssh and
   ipfw rules, hence the lack of updates).  While trying to copy a
   huge file from a local machine to a remote, the following ipfw
   rules are in place:

      ipfw add  10990 pipe 1 tcp from ${local_host} to ${remote_host}
      ipfw pipe 1     config bw 256kbit/s queue 40KB
      ipfw add  10991 pipe 2 tcp from ${remote_host} to ${local_host}
      ipfw pipe 2     config bw 256Kbit/s queue 40KB

   Since the flie is large enough to require several days to transfer, I
   decided to tweak the pipe sizes off hours to permit it to go faster.
   Crontab:
      0       18      *       *       *       root    ipfw pipe 1     config bw  512kbit/s queue 40KB
      1       18      *       *       *       root    ipfw pipe 2     config bw  512kbit/s queue 40KB
      30      0       *       *       *       root    ipfw pipe 1     config bw 1024kbit/s queue 40KB
      31      0       *       *       *       root    ipfw pipe 2     config bw 1024kbit/s queue 40KB
      30      3       *       *       *       root    ipfw pipe 1     config bw 1200kbit/s queue 40KB
      31      3       *       *       *       root    ipfw pipe 2     config bw 1200kbit/s queue 40KB
      30      6       *       *       *       root    ipfw pipe 1     config bw  256kbit/s queue 40KB
      31      6       *       *       *       root    ipfw pipe 2     config bw  256kbit/s queue 40KB

   Two days in a row now, just after 6:30, the box freezes, refusing
   keyboard input and requiring a reboot.

Queries:
   Is this considered an unsupported use of dummynet?

   If it should work, was it broken back in 4.8 and, if so, when
   was it corrected?  I'd rather not take the time up upgrade the
   box to 4.11 if I'll then be forced to move to 5.4.  

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