4. AF_PACKET Poll Mode Driver

The AF_PACKET socket in Linux allows an application to receive and send raw packets. This Linux-specific PMD binds to an AF_PACKET socket and allows a DPDK application to send and receive raw packets through the Kernel.

In order to improve Rx and Tx performance this implementation makes use of PACKET_MMAP, which provides a mmapped ring buffer, shared between user space and kernel, that’s used to send and receive packets. This helps reducing system calls and the copies needed between user space and Kernel.

4.1. Options and inherent limitations

The following options can be provided to set up an af_packet port in DPDK. Some of these, in turn, will be used to configure the PACKET_MMAP settings.

  • iface - name of the Kernel interface to attach to (required);

  • qpairs - number of Rx and Tx queues (optional, default 1);

  • qdisc_bypass - set PACKET_QDISC_BYPASS option in AF_PACKET (optional, disabled by default);

  • fanout_mode - set fanout algorithm. Possible choices: hash, lb, cpu, rollover, rnd, qm (optional, default hash);

  • blocksz - PACKET_MMAP block size (optional, default 4096);

  • framesz - PACKET_MMAP frame size (optional, default 2048B; Note: multiple of 16B);

  • framecnt - PACKET_MMAP frame count (optional, default 512).

For details regarding fanout_mode argument, you can consult the PACKET_FANOUT documentation.

As an example, when fanout_mode=cpu is selected, the PACKET_FANOUT_CPU mode will be set on the sockets, so that on frame reception, the socket will be selected based on the CPU on which the packet arrived.

Only one fanout_mode can be chosen. If left unspecified, the default is to use the PACKET_FANOUT_HASH behavior of AF_PACKET for frame reception.

Because this implementation is based on PACKET_MMAP, and PACKET_MMAP has its own pre-requisites, it should be noted that the inner workings of PACKET_MMAP should be carefully considered before modifying some of these options (namely, blocksz, framesz and framecnt above).

As an example, if one changes framesz to be 1024B, it is expected that blocksz is set to at least 1024B as well (although 2048B in this case would allow two “frames” per “block”).

This restriction happens because PACKET_MMAP expects each single “frame” to fit inside of a “block”. And although multiple “frames” can fit inside of a single “block”, a “frame” may not span across two “blocks”.

For the full details behind PACKET_MMAP’s structures and settings, consider reading the PACKET_MMAP documentation in the Kernel.

4.2. Prerequisites

This is a Linux-specific PMD, thus the following prerequisites apply:

  • A Linux Kernel;

  • A Kernel bound interface to attach to (e.g. a tap interface).

4.3. Set up an af_packet interface

The following example will set up an af_packet interface in DPDK with the default options described above (blocksz=4096B, framesz=2048B and framecnt=512):

--vdev=eth_af_packet0,iface=tap0,blocksz=4096,framesz=2048,framecnt=512,qpairs=1,qdisc_bypass=0,fanout_mode=hash

4.4. Features and Limitations

  • The PMD will re-insert the VLAN tag transparently to the packet if the kernel strips it, as long as the RTE_ETH_RX_OFFLOAD_VLAN_STRIP is not enabled by the application.

  • The PMD will add the kernel packet timestamp with nanoseconds resolution and UNIX origo, i.e. time since 1-JAN-1970 UTC, if RTE_ETH_RX_OFFLOAD_TIMESTAMP is enabled.