35. Libpcap and Ring Based Poll Mode Drivers

35.1.1.1. Device Streams

Multiple ways of stream definitions can be assessed and combined as long as the following two rules are respected:

• A device is provided with two different streams - reception and transmission.
• A device is provided with one network interface name used for reading and writing packets.

The different stream types are:

• rx_pcap: Defines a reception stream based on a pcap file. The driver reads each packet within the given pcap file as if it was receiving it from the wire. The value is a path to a valid pcap file.

  rx_pcap=/path/to/file.pcap

• tx_pcap: Defines a transmission stream based on a pcap file. The driver writes each received packet to the given pcap file. The value is a path to a pcap file. The file is overwritten if it already exists and it is created if it does not.

  tx_pcap=/path/to/file.pcap

• rx_iface: Defines a reception stream based on a network interface name. The driver reads packets coming from the given interface using the Linux kernel driver for that interface. The value is an interface name.

  rx_iface=eth0

• tx_iface: Defines a transmission stream based on a network interface name. The driver sends packets to the given interface using the Linux kernel driver for that interface. The value is an interface name.

  tx_iface=eth0

• iface: Defines a device mapping a network interface. The driver both reads and writes packets from and to the given interface. The value is an interface name.

  iface=eth0

35.1.1.2. Examples of Usage

Read packets from one pcap file and write them to another:

$RTE_TARGET/app/testpmd -l 0-3 -n 4 \
    --vdev 'net_pcap0,rx_pcap=file_rx.pcap,tx_pcap=file_tx.pcap' \
    -- --port-topology=chained

Read packets from a network interface and write them to a pcap file:

$RTE_TARGET/app/testpmd -l 0-3 -n 4 \
    --vdev 'net_pcap0,rx_iface=eth0,tx_pcap=file_tx.pcap' \
    -- --port-topology=chained

Read packets from a pcap file and write them to a network interface:

$RTE_TARGET/app/testpmd -l 0-3 -n 4 \
    --vdev 'net_pcap0,rx_pcap=file_rx.pcap,tx_iface=eth1' \
    -- --port-topology=chained

Forward packets through two network interfaces:

$RTE_TARGET/app/testpmd -l 0-3 -n 4 \
    --vdev 'net_pcap0,iface=eth0' --vdev='net_pcap1;iface=eth1'

35.1.1.3. Using libpcap-based PMD with the testpmd Application

One of the first things that testpmd does before starting to forward packets is to flush the RX streams by reading the first 512 packets on every RX stream and discarding them. When using a libpcap-based PMD this behavior can be turned off using the following command line option:

--no-flush-rx

It is also available in the runtime command line:

set flush_rx on/off

It is useful for the case where the rx_pcap is being used and no packets are meant to be discarded. Otherwise, the first 512 packets from the input pcap file will be discarded by the RX flushing operation.

$RTE_TARGET/app/testpmd -l 0-3 -n 4 \
    --vdev 'net_pcap0,rx_pcap=file_rx.pcap,tx_pcap=file_tx.pcap' \
    -- --port-topology=chained --no-flush-rx

35.1.3.1. Usage Examples

To create two pseudo-Ethernet ports where all traffic sent to a port is looped back for reception on the same port (error handling omitted for clarity):

#define RING_SIZE 256
#define NUM_RINGS 2
#define SOCKET0 0

struct rte_ring *ring[NUM_RINGS];
int port0, port1;

ring[0] = rte_ring_create("R0", RING_SIZE, SOCKET0, RING_F_SP_ENQ|RING_F_SC_DEQ);
ring[1] = rte_ring_create("R1", RING_SIZE, SOCKET0, RING_F_SP_ENQ|RING_F_SC_DEQ);

/* create two ethdev's */

port0 = rte_eth_from_rings("net_ring0", ring, NUM_RINGS, ring, NUM_RINGS, SOCKET0);
port1 = rte_eth_from_rings("net_ring1", ring, NUM_RINGS, ring, NUM_RINGS, SOCKET0);

To create two pseudo-Ethernet ports where the traffic is switched between them, that is, traffic sent to port 0 is read back from port 1 and vice-versa, the final two lines could be changed as below:

port0 = rte_eth_from_rings("net_ring0", &ring[0], 1, &ring[1], 1, SOCKET0);
port1 = rte_eth_from_rings("net_ring1", &ring[1], 1, &ring[0], 1, SOCKET0);

This type of configuration could be useful in a pipeline model, for example, where one may want to have inter-core communication using pseudo Ethernet devices rather than raw rings, for reasons of API consistency.

Enqueuing and dequeuing items from an rte_ring using the rings-based PMD may be slower than using the native rings API. This is because DPDK Ethernet drivers make use of function pointers to call the appropriate enqueue or dequeue functions, while the rte_ring specific functions are direct function calls in the code and are often inlined by the compiler.

Once an ethdev has been created, for either a ring or a pcap-based PMD, it should be configured and started in the same way as a regular Ethernet device, that is, by calling rte_eth_dev_configure() to set the number of receive and transmit queues, then calling rte_eth_rx_queue_setup() / tx_queue_setup() for each of those queues and finally calling rte_eth_dev_start() to allow transmission and reception of packets to begin.