Linux Control Plane Integration
This plugin allows VPP to integrate with the Linux kernel. The general model is that Linux is the network stack, i.e. it has the control plane protocols, like ARP, IPv6 ND/MLD, ping, etc, and VPP provides a SW based ASIC for forwarding.
VPP owns the interfaces in the system; physical (.e.g PCI), quasi physical (e.g. vhost), or virtual (e.g. tunnel). However, for the Linux networking stack to function it needs a representation of these interfaces; it needs a mirror image in the kernel. For this mirror we use a TAP interface, if the VPP interface is multi-point, a TUN if it’s point-to-point. A physical and its mirror form an interface ‘pair’.
The host interface has two identities; the sw_if_index of the TAP and the virtual interface index in the kernel. It may be in a Linux network namespace.
The creation of the interface pairs is required from the control plane. It can be statically configured in the VPP startup configuration file. The intent here is to make the pair creation explicit, rather than have VPP guess which of the interfaces it owns require a mirror.
Linux will send and receive packets on the mirrored tap/tun interfaces. Any configuration that is made on these Linux interfaces, also needs to be applied on the corresponding physical interface in VPP.
This is functionality is provided by the “linux_nl” plugin.
Linux will own the [ARP/ND] neighbor tables (which will be copied via netlink to VPP also). This means that Linux will send packets with the peer’s MAC address in the rewrite to VPP. The receiving TAP interface must therefore be in promiscuous mode.
The basic principle is to x-connect traffic from a Linux host interface (received on the tap/tun) to its paired the physical, and vice-versa.
Host to Physical
All packets sent by the host, and received by VPP on a tap/tun should be sent to its paired physical interface. However, they should be sent with the same consequences as if they had originated from VPP, i.e. they should be subject to all output features on the physical interface. To achieve this there is a per-IP-address-family (AF) node inserted in the per-AF input feature arc. The node must be per-AF, since it must be a sibling of a start node for the ipX-output feature arc. This node uses the packet’s L2 rewrite to search for the adjacency that VPP would have used to send this packet; this adjacency is stored in the buffer’s meta data so that it is available to all output features. Then the packet is sent through the physical interface’s IP output feature arc.
All ARP packets are x-connected from the tap to the physical.
Physical to Host
All ARP packets received on the physical are sent to the paired tap. This allows the Linux network stack to build the neighbor table.
IP packets that are punted are sent to the host. They are sent on the tap that is paired with the physical on which they were originally received. The packet is sent on the tap/tun ‘exactly’ as it was received (i.e. with the L2 rewrite) but post any translations that input features may have made.
When using this plugin disable the ARP, ND, IGMP plugins; this is the task for Linux. Disable ping plugin, since Linux will now respond.