56. TAP Poll Mode Driver

The TAP Poll Mode Driver (PMD) is a virtual device for injecting packets to be processed by the Linux kernel. This PMD is useful when writing DPDK application for offloading network functionality (such as tunneling) from the kernel.

From the kernel point of view, the TAP device looks like a regular network interface. The network device can be managed by standard tools such as ip and ethtool commands. It is also possible to use existing packet tools such as wireshark or tcpdump.

From the DPDK application, the TAP device looks like a DPDK ethdev. Packets are sent and received in L2 (Ethernet) format. The standard DPDK API’s to query for information, statistics and send/receive packets work as expected.

56.1. Requirements

The TAP PMD requires kernel support for multiple queues in TAP device as well as the multi-queue multiq and incoming ingress queue disciplines. These are standard kernel features in most Linux distributions.

56.2. Arguments

TAP devices are created with the command line --vdev=net_tap0 option. This option may be specified more than once by repeating with a different net_tapX device.

By default, the Linux interfaces are named dtap0, dtap1, etc. The interface name can be specified by adding the iface=foo0, for example:

--vdev=net_tap0,iface=foo0 --vdev=net_tap1,iface=foo1, ...

Normally the PMD will generate a random MAC address. If a static address is desired instead, the mac=fixed can be used:

--vdev=net_tap0,mac=fixed

With the fixed option, the MAC address will have the first octets: as 02:’d’:’t’:’a’:’p’:[00-FF] and the last octets are the interface number.

To specify a specific MAC address use the conventional representation. The string is byte converted to hex, the result is MAC address: 02:64:74:61:70:11.

It is possible to specify a remote netdevice to capture packets from by adding remote=foo1, for example:

--vdev=net_tap,iface=tap0,remote=foo1

If a remote is set, the tap MAC address will be set to match the remote one just after netdevice creation. Using TC rules, traffic from the remote netdevice will be redirected to the tap. If the tap is in promiscuous mode, then all packets will be redirected. In allmulti mode, all multicast packets will be redirected.

Using the remote feature is especially useful for capturing traffic from a netdevice that has no support in the DPDK. It is possible to add explicit rte_flow rules on the tap PMD to capture specific traffic (see next section for examples).

Normally, when the DPDK application exits, the TAP device is marked down and is removed. But this behavior can be overridden by the use of the persist flag, example:

--vdev=net_tap0,iface=tap0,persist ...

56.3. TUN devices

The TAP device can be used as an L3 tunnel only device (TUN). This type of device does not include the Ethernet (L2) header; all packets are sent and received as IP packets.

TUN devices are created with the command line arguments --vdev=net_tunX, where X stands for unique id, example:

--vdev=net_tun0 --vdev=net_tun1,iface=foo1, ...

Unlike TAP PMD, TUN PMD does not support user arguments as MAC or remote user options. Default interface name is dtunX, where X stands for unique id.

56.4. Flow API support

The TAP PMD supports major flow API pattern items and actions.

56.4.1. Requirements

Flow support in TAP driver requires the Linux kernel support of flow based traffic control filter flower. This was added in Linux 4.3 kernel.

The implementation of RSS action uses an eBPF module that requires additional libraries and tools. Building the RSS support requires the clang compiler to compile the C code to BPF target; bpftool to convert the compiled BPF object to a header file; and libbpf to load the eBPF action into the kernel.

Supported match items:

eth: src and dst (with variable masks), and eth_type (0xffff mask).

vlan: vid, pcp, but not eid. (requires kernel 4.9)

ipv4/6: src and dst (with variable masks), and ip_proto (0xffff mask).

udp/tcp: src and dst port (0xffff) mask.

Supported actions:

DROP
QUEUE
PASSTHRU
RSS

It is generally not possible to provide a “last” item. However, if the “last” item, once masked, is identical to the masked spec, then it is supported.

Only IPv4/6 and MAC addresses can use a variable mask. All other items need a full mask (exact match).

As rules are translated to TC, it is possible to show them with something like:

tc -s filter show dev dtap1 parent 1:

56.4.2. Examples of testpmd flow rules

Drop packets for destination IP 192.0.2.1:

testpmd> flow create 0 priority 1 ingress pattern eth / ipv4 dst is 192.0.2.1 \
         / end actions drop / end

Ensure packets from a given MAC address are received on a queue 2:

testpmd> flow create 0 priority 2 ingress pattern eth src is 06:05:04:03:02:01 \
         / end actions queue index 2 / end

Drop UDP packets in vlan 3:

testpmd> flow create 0 priority 3 ingress pattern eth / vlan vid is 3 / \
         ipv4 proto is 17 / end actions drop / end

Distribute IPv4 TCP packets using RSS to a given MAC address over queues 0-3:

testpmd> flow create 0 priority 4 ingress pattern eth dst is 0a:0b:0c:0d:0e:0f \
         / ipv4 / tcp / end actions rss queues 0 1 2 3 end / end

56.6. RSS specifics

The default packet distribution in TAP without flow rules is done by the kernel which has a default flow based distribution. When flow rules are used to distribute packets across a set of queues, an eBPF program is used to calculate the RSS based on Toeplitz algorithm with the given key.

The hash is calculated for IPv4 and IPv6, over src/dst addresses (8 or 32 bytes for IPv4 or IPv6 respectively) and optionally the src/dst TCP/UDP ports (4 bytes).

56.7. Limitations

Since TAP device uses a file descriptor to talk to the kernel, the same number of queues must be specified for receive and transmit.
The RSS algorithm only support L3 or L4 functions. It does not support finer grain selections (for example: only IPV6 packets with extension headers).