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

[Felipe Reyes]

Public bug reported:

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

** Affects: ovn (Ubuntu)
     Importance: Undecided
         Status: New

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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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