[Bug 1671951] Re: networkd should allow configuring IPV6 MTU

[Ryan Harper]

This also needs some discussion w.r.t how to handle IPV6-only devices;
namely if you want to set the IPV6 MTU to something higher than the
underlying device's current setting then the device MTU would need to be
raised as well.  That needs to be implemented and added to the patch.

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Title:
  networkd should allow configuring IPV6 MTU

Status in systemd package in Ubuntu:
  New

Bug description:
  1) Zesty
  2) systemd-232-19
  3) I need to configure the IPV6 MTU for tunneling by adding an IPv6MTUBytes=1480 value in the .network file for an interface with an IPV6 static address in the [Network] section
  4) networkd does not parse or read the value and does not apply this configuration to the interface.

  Upstream has discussed this issue here:

  https://github.com/systemd/systemd/pull/1533

  But it's been closed in favor of only setting via RA.

  However, we know of multiple use-case which are currently supported in
  ifdupdown where we want to retain control over IPV6 MTU values outside
  of PMTU Discovery configurations.

  
  Some context from those discussions

  
  >> Client systems that route their ipv6 packets to a 6in4 router also
  >> have to have their ipv6 mtu lowered.  They could lower their link mtu,
  >> so their ipv6 packets are small enough, but that reduces performance
  >> of their ipv4 network.

          Yes.  Anything that creates a PMTUD black hole can result in
  situations where the higher header overhead of IPv6 will cause IPv4 to
  pass but IPv6 traffic to be dropped.

          One example here is egress from an ipsec tunnel wherein the next
  hop MTU is too low for IPv6 datagrams to pass.  Another is VM ->
  whatever -> host bridge -> tunnel ingress.  If the datagram cannot enter
  the tunnel due to size, it is dropped, and an ICMP response uses the
  tunnel address as a source, which may not be routable back to the
  origin.  This one is an issue with IPv4 as well, and is one case where
  manually setting the IPv6 MTU lower than the (also manually set) device
  MTU is of benefit.

          In essence, any of these sort of cases that require an explicit
  setting of the device MTU will likely require a setting of the IPv6 mtu
  as well to account for its larger header overhead.

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