On Wed, May 17, 2017 at 1:14 PM, Dimitri John Ledkov <[email protected] > wrote:
> I'm very naive, so please bear with me if this is a silly question. As > far as I can tell the existing MTU setting in networkd will apply to > both ipv4 and ipv6 simultaniously. Are you arguing that you want > specific control of MTU and use different values for ipv4 and ipv6? > Yes AFAICT, the MTUBytes field which is a link property, applies to the device itself. IPv6 has a separate *protocol* level MTU where it is used to fragment IPv6 packets so they can be tunneled (among other things) over IPv4. There's also a specific IPv6 MTU setting to prevent IPv6 packets from being to small (I think 1280 is the minimum MTU allowable for IPv6 packets. Subsequently, the kernel has *two* MTUs, Device (ethernet, ppp, etc) and IPv6 protocal device: /sys/class/net/<interface>/mtu ipv6: /proc/sys/net/ipv6/conf/<interface>/mtu Would it not be sufficient to set the appropriatly-lowest/highest common > denominator value for MTU? > It is not. In the case that you want to use 9000 MTU on an ipv6 address, one needs to ensure that the MTU of the underlying device is raised from the default to 9000 otherwise the V6 MTU means nothing. In some cases you want to raise the MTU of the underlying device, but *not* raise the MTU of the IPV6 this is a tunneling case: eth0 mtu 9000 ipv6 mtu 1480 So, yes, we want independent control over v4 and v6 MTU. Note, the upstream discussions (IMHO) punt the problem to Router Advertisements; however what does remain is that we currently have this control with ifupdown (with some help of pre/post scripts). I had a start at getting this working in networkd, however I wasn't able to achieve the independent control > > -- > You received this bug notification because you are subscribed to the bug > report. > https://bugs.launchpad.net/bugs/1671951 > > Title: > networkd should allow configuring IPV6 MTU > > To manage notifications about this bug go to: > https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/ > 1671951/+subscriptions > -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to systemd in Ubuntu. https://bugs.launchpad.net/bugs/1671951 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. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1671951/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : [email protected] Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
