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VFd Overview

Much in the way that a guest operating system requires a hypervisor to provide accessibility to the underlying real operating system and hardware, as well as to ensure policies are enforced, there is a similar need for a NIC hypervisor when virtual functions (VFs) are directly available through SR-IOV[1]. While a bit more lightweight than a hypervisor, VFd can be thought of as the NIC hypervisor inasmuch as it provides the policy enforcement through both configuration and real-time validation of guest[2] requests.

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Figure 1: General relationship of NIC, VFs, "guests" and VFd

Figure 1 illustrates the relationship between the NIC (PF), the configured VFs, the guests, and VFd. VFd acts as the policy implementor by configuring each VF to stipulate which VLAN(s) a VF will receive traffic from, and be allowed to send traffic to. In addition, VFd will configure the PF and VFs to:

  • Strip and insert the outer VLAN tag.
  • Enable NIC bridging allowing guest to guest communication.
  • Allow unknown unicast traffic to be sent to one or more guests.
  • Allow broadcast and multicast traffic to be received/transmitted by a guest.
  • Enforce MAC and/or VLAN spoofing (drop spoofed packets).
  • Configure multiple MAC addresses for the VF.
  • Enable and configure DCB quality of service.

In addition to the NIC configurations listed above, VFd provides the means to access the packet and byte counters allowing for them to be available to the user; something that is not possible when using a kernel driver to manage the VFs. Statistics, both at the PF and VF level include:

  • Packet counts for inbound (Rx) and outbound (Tx)
  • Byte counts, Tx and Rx
  • Error packet count
  • Count of spoofed packets

The Guests

While the illustration shows only one VF allocated to each guest, it is possible to allocate multiple VFs, from different PFs, to a single guest. Of the various guests shown in figure 1, two are illustrations of how guest operating systems, virtual machines[3], can have one, or more, VF allocated as an SR-IOV device. Bare metal applications (e.g. DPDK applications) are also able to make use of the VFs offered up by the NIC and generally use the NIC's kernel VF interface/driver to do so.

Inside of a VM

When a guest operating system is created, and one or more VFs is attached to it, the VF appears as a virtual network interface and standard kernel drivers can be used to treat the interface as an ordinary ethernet device, configurable with well known tools like ifconfig and ip, or a DPDK driver may be used to allow DPDK applications to run in the virtual machine.

What VFd Is Not

While VFd is implemented as a DPDK application, primarily to easily access the network devices, VFd is not a packet processor, and does not insert itself into the path of any packet.


A valid, and often asked, question is why exactly is VFd needed? It would seem that if the hardware offers all of the sophistication that they do, that the kernel drivers would supply the means to configure the hardware such that all of the features could be used. However, at least at the present time, this isn't the case, and there are several configuration options which unfortunately cannot be achieved through the kernel drivers. In order to take full advantage of the hardware, an independent configuration tool, VFd, is required.

Real-time Authorisation

In addition to managing the configurations of all VFs, VFd also provides real-time policy enforcement when a guest makes a request to configure it's interface. The following are examples of guest actions which require the approval of VFd before the requested changes are actually applied o the VF:

  • Add or modify VLAN ID(s)
  • Change default MAC address
  • Add/delete alternate (white list) MAC addresses
  • Set the MTU value

VFd will authorise (ACK) any request provided that the change is within the policy as defined to VFd via the PF and VF configuration files. For example, if a guests requests to add/modify a VLAN ID, it will be permitted only if the requested ID has been defined in the VF's configuration file. MTU changes are limited by the overall maximum MTU defined for each device in the main VFd configuration file. Looking from a policy assurance point of view, VFd provides a service that would otherwise not be available even if all NIC features could be exercised through the kernel driver interface as the drivers provide no mechanism to call out to an independent authority to make these kinds of decisions.

VF Configuration Features

The following sections provide a high level explanation of NIC features which may be managed using VFd. The purpose here is to outline what can be done; for information on how to actually implement these, please refer to the VFd User's Guide.


Spoofing is used in reference to a machine on a network pretending to be another machine, usually by using the other machine's MAC address, or using an unauthorised VLAN ID in packets, or possibly both. VFd forces anti-spoofing settings to be enabled for VLAN IDs[4] under the assumption that a guest assigned to a VLAN must always send packets with the assigned ID, but may send with any source MAC address.

VLAN Considerations

The list of approved VLAN IDs that are given in a VF's configuration file limit the packets that the NIC will forward to the VF. Only packets containing one of the VLAN IDs in the list are allowed to pass. VFd does not permit a VF to see all packets regardless of VLAN ID, nor does it permit untagged packets from being written onto a VF's queue.

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Figure 2: On transmit, insert VLAN tag into untagged packet (top to middle), and into a single tagged packet (middle to bottom). On receipt, the reverse.

Stripping and Inserting VLAN Tags

The VLAN ID, or tag, may be automatically removed, stripped, as a packet is received, and automatically inserted prior to transmission; both of these actions are performed by the NIC and thus are as efficient as is possible. Whether or not one or multiple VLAN IDs are configured for a VF, the stripping of the tag is straight forward: the tag is removed from the packet before the packet is presented to the VF. For the case where a VF receives packets with a single VLAN tag, the process of insertion is also straight forward: the NIC inserts the tag into the packet before transmission. When a VF is permitted send and receive packets using one of several VLAN IDs, the insertion of the correct ID is the responsibility of the sending process, and cannot be added to the packet by the NIC.

Q in Q Stripping

Packets arriving with multiple embedded VLAN IDs (figure 2, bottom), also known as Q in Q, are subject to the same stripping practice as described above. Only the first, or _ outer_ tag, is removed from a received packet, and only the outer tag will be inserted on transmit. Using the illustration as an example, the middle packet would be delivered to the guest after stripping, and the bottom packet would be transmitted. It is the responsibility of the guest software (kernel driver, or DPDK application) to properly distinguish packets, if necessary, based on the VLAN ID which remains in the packet.

MAC Addresses

By default the NIC generates a random MAC address for each VF that is created. This random address is known as the default MAC, and is visible to the guest (e.g. it is what would be displayed in the output of an ifconfig command). If it is necessary, the default MAC address can be supplied in a VF's configuration file which will replace the random address generated by the NIC.

Guest Generated MAC Addresses

It is sometimes desirable for a guest to generate the default MAC, and possibly other white-list MAC addresses. VFd will approve these guest requests provided that:

  • The address is not in use by another VF on the same PF.
  • The number of addresses currently allocated on the VF is not at maximum
  • THe number of addresses currently allocated on the PF is not at maximum.

Guest allocated MAC addresses remain in place until the VF configuration is deleted. When the configuration is deleted any guest allocated addresses are replaced by a single, randomly generated, default address.

Broadcast, Unknown Unicast, Multicast Traffic

VFd allows each VF to be configured to cause the NIC to allow or reject each of these types of traffic. The configuration of a VF provides for three independent settings allowing each to be managed separately. All packets falling within each traffic type are subject to VLAN filtering; it is not possible for a VF to see all *-cast traffic.

PF Configuration Features

There are several configuration options that VFd provides which apply to all VFs on a given NIC, or to all PFs on the NIC, and thus are supplied in the main VFd configuration file. These configuration options are described in the following sections.


By default the NIC bridge is disabled which forces a packet to be transmitted to the switch, and then back, should two guests using VFs on the same PF need to communicate. Enabling the loopback option for a VF will enable the NIC bridge and allow packets to be exchanged between VFs on the PF without having to travel onto the real network.

Flow Control

While evidence suggests that using flow control can significantly impact performance (max throughput), VFd does allow flow control on the NIC to be enabled.

Quality of Service

VFd is able to configure all PFs to enable Datacenter Bridging (DCB) quality of service. This defines a set of traffic classes and allows the assignment of bandwidth shares (percentages) for each VF with respect to each traffic class.

MTU Limit

The overall maximum MTU limit can be set on each PF. When set, an attempt by a guest to increase the MTU value over this limit will be rejected.

Traffic Mirroring

Traffic mirroring is a specialised form of loopback. VFd provides for real-time mirror configuration, independent of VF configuration files, allowing for inbound[5], outbound, or all of a VF's traffic to be duplicated and passed to another VF on the same PF.

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Figure 3: Inbound mirroring showting traffic also being delivered to Guest B.

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Figure 4: Outbound mirroring showing all Tx traffic from Guest A delivered to Guest B.

Mirroring traffic locally has benefits with respect to debugging connectivity issues as well as application issues. However, there are potential impacts, some serious, to the overall throughput of the NIC when traffic is mirrored over the NIC bridge. For instance, a badly behaved application used as the receiver for mirrored traffic has the potential to block all traffic to the source VF.


[1] A basic primer on SR-IOV can be found via the following URL:

[2] For the purposes of this document, we use the term _ guest_ to mean either a guest operating system or any application which has been given direct access to an SR-IOV device.

[3] To avoid confusion, we tend not to use the acronym VM when refering to a guest operating system (virtual machine) as some vendors refer to the virtual functions on the NIC as VMs which could lead to confusion.

[4] For some NICs in order to enable VLAN anti-spoofing, MAC anti-spoofing must also be enabled.

[5] The directional notation of mirrored traffic is relative to the guest, and not to the NIC or switch. Thus _ inbound_ means traffic received; inbound to the guest.

Source: vfd/doc/overview/overview.xfm
Original: 19 February 2018
Revised: 2 March 2018
Formatter: tfm V2.2/0a266