[Bug 194238] [tcp] Ping attempted with MTU 9000 transmits fragmented packets of size 1500

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194238

--- Comment #4 from commit-hook at freebsd.org ---
A commit references this bug:

Author: melifaro
Date: Mon Nov 17 01:05:32 UTC 2014
New revision: 274611
URL: https://svnweb.freebsd.org/changeset/base/274611

Log:
  Finish r274175: do control plane MTU tracking.

  Update route MTU in case of ifnet MTU change.
  Add new RTF_FIXEDMTU to track explicitly specified MTU.

  Old behavior:
  ifconfig em0 mtu 1500->9000 -> all routes traversing em0 do not change MTU.
  User has to manually update all routes.
  ifconfig em0 mtu 9000->1500 -> all routes traversing em0 do not change MTU.
  However, if ip[6]_output finds route with rt_mtu > interface mtu, rt_mtu
  gets updated.

  New behavior:
  ifconfig em0 mtu 1500->9000 -> all interface routes in all fibs gets updated
  with new MTU unless RTF_FIXEDMTU flag set on them.
  ifconfig em0 mtu 9000->1500 -> all routes in all fibs gets updated with new
  MTU unless RTF_FIXEDMTU flag set on them AND rt_mtu is less than ifp mtu.

  route add ... -mtu XXX automatically sets RTF_FIXEDMTU flag.
  route change .. -mtu 0 automatically removes RTF_FIXEDMTU flag.

  PR:        194238
  MFC after:    1 month
  CR:        D1125

Changes:
  head/sbin/route/route.c
  head/sys/net/if.c
  head/sys/net/route.c
  head/sys/net/route.h
  head/sys/netinet/ip_output.c
  head/sys/netinet6/ip6_output.c

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