Hi, Conventional OpenStack Networking service (neutron) drivers such as Open vSwitch and Linux bridge have a long history of MTU issues involving overlay networks. For example, consider the following typical virtual network architecture:
1) A provider network using native Ethernet or 802.1q tagging connects the physical and virtual network infrastructure. Typically uses a 1500 or 9000 MTU. 2) A private network using an overlay such as VXLAN or GRE. Overlay protocol overhead reduces the effective MTU available to a VM. 3) A VM attaching to the private network. 4) A virtual router connecting the provider and private networks. For a packet outbound from a VM, we can simply set the MTU of the network interface in the VM to account for overlay protocol overhead. For example, if the underlying network supports a 1500 MTU, a VM attaching to a VXLAN overlay network should use a 1450 MTU. We can provide this MTU to the VM via DHCP, RA, or manual configuration of the interface. For a packet inbound to a VM, the situation becomes more interesting. The host sending the packet usually resides outside of the overlay network and attempts to use the MTU available to it, typically 1500 or greater. The provider network also uses a 1500 MTU. The virtual router contains interfaces on the provider network using a 1500 MTU and the private network using a 1450 MTU. As the packet attempts to traverse the router, it hits the MTU disparity between 1500 and 1450 which causes the provider interface to originate an ICMP message for PMTU discovery containing the 1450 MTU. The host receives this message and sends the packet again using the 1450 MTU. Currently, both the Open vSwitch and Linux bridge drivers incorrectly place MTU disparities in layer-2 components which cannot originate ICMP messages and drop packets that exceed the MTU. Using the native layer-3 implementation, how does OVN handle MTU disparities between provider networks and private networks using an overlay protocol? Thanks, Matt
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