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

[Brian Haley]

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

** Changed in: neutron (Ubuntu Questing)
       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:
  Fix Released
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:
  Fix Released
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:
  Fix Released
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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