[Bug 2032817] Re: [OVN] packet drops when provider network MTU exceeds tenant network MTU

[Edward Hope-Morley]

** Also affects: cloud-archive
   Importance: Undecided
       Status: New

** Also affects: neutron (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: neutron (Ubuntu Noble)
   Importance: Undecided
       Status: New

** Also affects: neutron (Ubuntu Questing)
   Importance: Undecided
       Status: New

** Also affects: neutron (Ubuntu Resolute)
   Importance: Undecided
       Status: New

** Changed in: neutron (Ubuntu Resolute)
       Status: New => Fix Released

** Also affects: cloud-archive/caracal
   Importance: Undecided
       Status: New

** Also affects: cloud-archive/gazpacho
   Importance: Undecided
       Status: New

** Also affects: cloud-archive/flamingo
   Importance: Undecided
       Status: New

** Also affects: cloud-archive/epoxy
   Importance: Undecided
       Status: New

** Also affects: cloud-archive/dalmatian
   Importance: Undecided
       Status: New

** Changed in: cloud-archive/gazpacho
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/2032817

Title:
  [OVN] packet drops when provider network MTU exceeds tenant network
  MTU

Status in Ubuntu Cloud Archive:
  New
Status in Ubuntu Cloud Archive caracal series:
  New
Status in Ubuntu Cloud Archive dalmatian series:
  New
Status in Ubuntu Cloud Archive epoxy series:
  New
Status in Ubuntu Cloud Archive flamingo series:
  New
Status in Ubuntu Cloud Archive gazpacho series:
  Fix Released
Status in neutron:
  Fix Released
Status in neutron package in Ubuntu:
  Fix Released
Status in neutron source package in Noble:
  New
Status in neutron source package in Questing:
  New
Status in neutron source package in Resolute:
  Fix Released

Bug description:
  At the moment, we have a provider network running 1500 MTU and then
  some tenant networks running 1450 MTU.

  We've noticed that southbound traffic from the provider network to the
  tenant network will result in packets being dropped since the MTU is
  smaller.

  So, the VM would not be able to get any traffic if the packet size >
  1450, since the provider network operates at 1500 and hands over 1500
  packets to the tap interface which gets dropped by the kernel:

  [Wed Aug 23 14:33:19 2023] tapdf9341b0-6d: dropped over-mtu packet: 1456 > 1450
  ...

  It seems that OVN should fragment packets when it notices the MTU of
  the target NAT is smaller than it's own, otherwise traffic goes into a
  nowhere.

  OVN: 23.03.0

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