[Bug 1949120] Re: Missing PPMTUD for ICMP and UDP when there are dedicated gateways

[Frode Nordahl]

Thank you for raising this issue, so just to confirm, OVN 20.03 already
does send ICMP type 3 (Destination Unreachable) code 4 (Fragmentation
needed and DF was set) [4] to instances in the cloud, informing the
client of the gateway MTU when the `gateway_mtu` flag is set on the LRP.

So the issue you are describing would only come into play if there are
machines connected directly to the provider network using a MTU > the
OVN LR MTU and/or a external to openstack router has MTU > the OVN LR
MTU on its L3 interface facing the provider network?

4: https://github.com/ovn-
org/ovn/commit/ba11e16288888be643b26c9cbe239eb5c1f4eb41

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https://bugs.launchpad.net/bugs/1949120

Title:
  Missing PPMTUD for ICMP and UDP when there are dedicated gateways

Status in ovn package in Ubuntu:
  New

Bug description:
  According to this bug[0] and these patches[1][2][3] the support to
  emit a ICMP "need to fragment" packet was added in ovn-21.09, Wallaby
  UCA carries ovn-20.12

  This limitation becomes a problem when the overlay network is
  configured to use jumbo frames and the external network uses a MTU of
  1500 .

  [Environment]

  Focal Wallaby with a dedicated ovn-chassis application to act as
  gateway

    ovn-chassis-gw:
      bindings:
        ? ''
        : oam-space
        data: overlay-space
      charm: cs:ovn-dedicated-chassis
      options:
        source: cloud:focal-wallaby/proposed
        bridge-interface-mappings: br-data:bond0.3811
        ovn-bridge-mappings: physnet1:br-data
        prefer-chassis-as-gw: true
      num_units: 2
      to:
      - 1001
      - 1002

  [0] https://bugzilla.redhat.com/show_bug.cgi?id=1547074#c5
  [1] https://github.com/ovn-org/ovn/commit/2c2f1802dcfc6f7d3e3a25a24e0b8f4c7c7f39d8
  [2] https://github.com/ovn-org/ovn/commit/1c9e46ab5c05043a8cd6c47b5fec2e1ac4c962db
  [3] https://github.com/ovn-org/ovn/commit/947e8d450ebaa8ce4ab81cb480a419618f1508c7

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