55. Tun|Tap Poll Mode Driver

The rte_eth_tap.c PMD creates a device using TAP interfaces on the local host. The PMD allows for DPDK and the host to communicate using a raw device interface on the host and in the DPDK application.

The device created is a TAP device, which sends/receives packet in a raw format with a L2 header. The usage for a TAP PMD is for connectivity to the local host using a TAP interface. When the TAP PMD is initialized it will create a number of tap devices in the host accessed via ifconfig -a or ip command. The commands can be used to assign and query the virtual like device.

These TAP interfaces can be used with Wireshark or tcpdump or Pktgen-DPDK along with being able to be used as a network connection to the DPDK application. The method enable one or more interfaces is to use the --vdev=net_tap0 option on the DPDK application command line. Each --vdev=net_tap1 option given will create an interface named dtap0, dtap1, and so on.

The interface name can be changed 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, but when testing or with a static configuration the developer may need a fixed MAC address style. Using the option mac=fixed you can create a fixed known MAC address:

--vdev=net_tap0,mac=fixed

The MAC address will have a fixed value with the last octet incrementing by one for each interface string containing mac=fixed. The MAC address is formatted as 02:’d’:’t’:’a’:’p’:[00-FF]. Convert the characters to hex and you get the actual MAC address: 02:64:74:61:70:[00-FF].

–vdev=net_tap0,mac=”02:64:74:61:70:11”

The MAC address will have a user value passed as string. The MAC address is in format with delimiter :. The string is byte converted to hex and you get the actual 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).

After the DPDK application is started you can send and receive packets on the interface using the standard rx_burst/tx_burst APIs in DPDK. From the host point of view you can use any host tool like tcpdump, Wireshark, ping, Pktgen and others to communicate with the DPDK application. The DPDK application may not understand network protocols like IPv4/6, UDP or TCP unless the application has been written to understand these protocols.

If you need the interface as a real network interface meaning running and has a valid IP address then you can do this with the following commands:

sudo ip link set dtap0 up; sudo ip addr add 192.168.0.250/24 dev dtap0
sudo ip link set dtap1 up; sudo ip addr add 192.168.1.250/24 dev dtap1

Please change the IP addresses as you see fit.

If routing is enabled on the host you can also communicate with the DPDK App over the internet via a standard socket layer application as long as you account for the protocol handling in the application.

If you have a Network Stack in your DPDK application or something like it you can utilize that stack to handle the network protocols. Plus you would be able to address the interface using an IP address assigned to the internal interface.

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

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

The TUN PMD allows user to create a TUN device on host. The PMD allows user to transmit and receive packets via DPDK API calls with L3 header and payload. The devices in host can be accessed via ifconfig or ip command. TUN interfaces are passed to DPDK rte_eal_init arguments as --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.

55.1. Flow API support

The tap PMD supports major flow API pattern items and actions, when running on linux kernels above 4.2 (“Flower” classifier required). The kernel support can be checked with this command:

zcat /proc/config.gz | ( grep 'CLS_FLOWER=' || echo 'not supported' ) |
tee -a /dev/stderr | grep -q '=m' &&
lsmod | ( grep cls_flower || echo 'try modprobe cls_flower' )

Supported 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 (requires kernel 4.9)

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 tap1 parent 1:

55.1.1. 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

55.2. Multi-process sharing

It is possible to attach an existing TAP device in a secondary process, by declaring it as a vdev with the same name as in the primary process, and without any parameter.

The port attached in a secondary process will give access to the statistics and the queues. Therefore it can be used for monitoring or Rx/Tx processing.

The IPC synchronization of Rx/Tx queues is currently limited:

  • Maximum 8 queues shared

  • Synchronized on probing, but not on later port update

55.3. Example

The following is a simple example of using the TAP PMD with the Pktgen packet generator. It requires that the socat utility is installed on the test system.

Build DPDK, then pull down Pktgen and build pktgen using the DPDK SDK/Target used to build the dpdk you pulled down.

Run pktgen from the pktgen directory in a terminal with a commandline like the following:

sudo ./app/app/x86_64-native-linux-gcc/app/pktgen -l 1-5 -n 4        \
 --proc-type auto --log-level debug --socket-mem 512,512 --file-prefix pg   \
 --vdev=net_tap0 --vdev=net_tap1 -b 05:00.0 -b 05:00.1                  \
 -b 04:00.0 -b 04:00.1 -b 04:00.2 -b 04:00.3                            \
 -b 81:00.0 -b 81:00.1 -b 81:00.2 -b 81:00.3                            \
 -b 82:00.0 -b 83:00.0 -- -T -P -m [2:3].0 -m [4:5].1                   \
 -f themes/black-yellow.theme

Verify with ifconfig -a command in a different xterm window, should have a dtap0 and dtap1 interfaces created.

Next set the links for the two interfaces to up via the commands below:

sudo ip link set dtap0 up; sudo ip addr add 192.168.0.250/24 dev dtap0
sudo ip link set dtap1 up; sudo ip addr add 192.168.1.250/24 dev dtap1

Then use socat to create a loopback for the two interfaces:

sudo socat interface:dtap0 interface:dtap1

Then on the Pktgen command line interface you can start sending packets using the commands start 0 and start 1 or you can start both at the same time with start all. The command str is an alias for start all and stp is an alias for stop all.

While running you should see the 64 byte counters increasing to verify the traffic is being looped back. You can use set all size XXX to change the size of the packets after you stop the traffic. Use pktgen help command to see a list of all commands. You can also use the -f option to load commands at startup in command line or Lua script in pktgen.

55.4. RSS specifics

Packet distribution in TAP is done by the kernel which has a default distribution. This feature is adding RSS distribution based on eBPF code. The default eBPF code calculates RSS hash based on Toeplitz algorithm for a fixed RSS key. It is calculated on fixed packet offsets. For IPv4 and IPv6 it is calculated over src/dst addresses (8 or 32 bytes for IPv4 or IPv6 respectively) and src/dst TCP/UDP ports (4 bytes).

The RSS algorithm is written in file tap_bpf_program.c which does not take part in TAP PMD compilation. Instead this file is compiled in advance to eBPF object file. The eBPF object file is then parsed and translated into eBPF byte code in the format of C arrays of eBPF instructions. The C array of eBPF instructions is part of TAP PMD tree and is taking part in TAP PMD compilation. At run time the C arrays are uploaded to the kernel via BPF system calls and the RSS hash is calculated by the kernel.

It is possible to support different RSS hash algorithms by updating file tap_bpf_program.c In order to add a new RSS hash algorithm follow these steps:

  1. Write the new RSS implementation in file tap_bpf_program.c

BPF programs which are uploaded to the kernel correspond to C functions under different ELF sections.

  1. Install LLVM library and clang compiler versions 3.7 and above

  2. Compile tap_bpf_program.c via LLVM into an object file:

    clang -O2 -emit-llvm -c tap_bpf_program.c -o - | llc -march=bpf \
-filetype=obj -o <tap_bpf_program.o>

4. Use a tool that receives two parameters: an eBPF object file and a section name, and prints out the section as a C array of eBPF instructions. Embed the C array in your TAP PMD tree.

The C arrays are uploaded to the kernel using BPF system calls.

tc (traffic control) is a well known user space utility program used to configure the Linux kernel packet scheduler. It is usually packaged as part of the iproute2 package. Since commit 11c39b5e9 (“tc: add eBPF support to f_bpf”) tc can be used to uploads eBPF code to the kernel and can be patched in order to print the C arrays of eBPF instructions just before calling the BPF system call. Please refer to iproute2 package file lib/bpf.c function bpf_prog_load().

An example utility for eBPF instruction generation in the format of C arrays will be added in next releases

TAP reports on supported RSS functions as part of dev_infos_get callback: RTE_ETH_RSS_IP, RTE_ETH_RSS_UDP and RTE_ETH_RSS_TCP. Known limitation: TAP supports all of the above hash functions together and not in partial combinations.

55.5. Systems supporting flow API

  • “tc flower” classifier requires linux kernel above 4.2

  • eBPF/RSS requires linux kernel above 4.9

RH7.3

No flow rule support

RH7.4

No RSS action support

RH7.5

No RSS action support

SLES 15, kernel 4.12

No limitation

Azure Ubuntu 16.04, kernel 4.13

No limitation

55.6. Limitations

  • Rx/Tx must have the same number of queues.