6. Basic Forwarding Sample Application

The Basic Forwarding sample application is a simple skeleton example of a forwarding application.

It is intended as a demonstration of the basic components of a DPDK forwarding application. For more detailed implementations see the L2 and L3 forwarding sample applications.

6.1. Compiling the Application

To compile the application export the path to the DPDK source tree and go to the example directory:

export RTE_SDK=/path/to/rte_sdk

cd ${RTE_SDK}/examples/skeleton

Set the target, for example:

export RTE_TARGET=x86_64-native-linuxapp-gcc

See the DPDK Getting Started Guide for possible RTE_TARGET values.

Build the application as follows:

make

6.2. Running the Application

To run the example in a linuxapp environment:

./build/basicfwd -l 1 -n 4

Refer to DPDK Getting Started Guide for general information on running applications and the Environment Abstraction Layer (EAL) options.

6.3. Explanation

The following sections provide an explanation of the main components of the code.

All DPDK library functions used in the sample code are prefixed with rte_ and are explained in detail in the DPDK API Documentation.

6.3.1. The Main Function

The main() function performs the initialization and calls the execution threads for each lcore.

The first task is to initialize the Environment Abstraction Layer (EAL). The argc and argv arguments are provided to the rte_eal_init() function. The value returned is the number of parsed arguments:

int ret = rte_eal_init(argc, argv);
if (ret < 0)
    rte_exit(EXIT_FAILURE, "Error with EAL initialization\n");

The main() also allocates a mempool to hold the mbufs (Message Buffers) used by the application:

mbuf_pool = rte_mempool_create("MBUF_POOL",
                               NUM_MBUFS * nb_ports,
                               MBUF_SIZE,
                               MBUF_CACHE_SIZE,
                               sizeof(struct rte_pktmbuf_pool_private),
                               rte_pktmbuf_pool_init, NULL,
                               rte_pktmbuf_init,      NULL,
                               rte_socket_id(),
                               0);

Mbufs are the packet buffer structure used by DPDK. They are explained in detail in the “Mbuf Library” section of the DPDK Programmer’s Guide.

The main() function also initializes all the ports using the user defined port_init() function which is explained in the next section:

for (portid = 0; portid < nb_ports; portid++) {
    if (port_init(portid, mbuf_pool) != 0) {
        rte_exit(EXIT_FAILURE,
                 "Cannot init port %" PRIu8 "\n", portid);
    }
}

Once the initialization is complete, the application is ready to launch a function on an lcore. In this example lcore_main() is called on a single lcore.

lcore_main();

The lcore_main() function is explained below.

6.3.2. The Port Initialization Function

The main functional part of the port initialization used in the Basic Forwarding application is shown below:

static inline int
port_init(uint8_t port, struct rte_mempool *mbuf_pool)
{
    struct rte_eth_conf port_conf = port_conf_default;
    const uint16_t rx_rings = 1, tx_rings = 1;
    struct ether_addr addr;
    int retval;
    uint16_t q;

    if (port >= rte_eth_dev_count())
        return -1;

    /* Configure the Ethernet device. */
    retval = rte_eth_dev_configure(port, rx_rings, tx_rings, &port_conf);
    if (retval != 0)
        return retval;

    /* Allocate and set up 1 RX queue per Ethernet port. */
    for (q = 0; q < rx_rings; q++) {
        retval = rte_eth_rx_queue_setup(port, q, RX_RING_SIZE,
                rte_eth_dev_socket_id(port), NULL, mbuf_pool);
        if (retval < 0)
            return retval;
    }

    /* Allocate and set up 1 TX queue per Ethernet port. */
    for (q = 0; q < tx_rings; q++) {
        retval = rte_eth_tx_queue_setup(port, q, TX_RING_SIZE,
                rte_eth_dev_socket_id(port), NULL);
        if (retval < 0)
            return retval;
    }

    /* Start the Ethernet port. */
    retval = rte_eth_dev_start(port);
    if (retval < 0)
        return retval;

    /* Enable RX in promiscuous mode for the Ethernet device. */
    rte_eth_promiscuous_enable(port);

    return 0;
}

The Ethernet ports are configured with default settings using the rte_eth_dev_configure() function and the port_conf_default struct:

static const struct rte_eth_conf port_conf_default = {
    .rxmode = { .max_rx_pkt_len = ETHER_MAX_LEN }
};

For this example the ports are set up with 1 RX and 1 TX queue using the rte_eth_rx_queue_setup() and rte_eth_tx_queue_setup() functions.

The Ethernet port is then started:

retval  = rte_eth_dev_start(port);

Finally the RX port is set in promiscuous mode:

rte_eth_promiscuous_enable(port);

6.3.3. The Lcores Main

As we saw above the main() function calls an application function on the available lcores. For the Basic Forwarding application the lcore function looks like the following:

static __attribute__((noreturn)) void
lcore_main(void)
{
    const uint8_t nb_ports = rte_eth_dev_count();
    uint8_t port;

    /*
     * Check that the port is on the same NUMA node as the polling thread
     * for best performance.
     */
    for (port = 0; port < nb_ports; port++)
        if (rte_eth_dev_socket_id(port) > 0 &&
                rte_eth_dev_socket_id(port) !=
                        (int)rte_socket_id())
            printf("WARNING, port %u is on remote NUMA node to "
                    "polling thread.\n\tPerformance will "
                    "not be optimal.\n", port);

    printf("\nCore %u forwarding packets. [Ctrl+C to quit]\n",
            rte_lcore_id());

    /* Run until the application is quit or killed. */
    for (;;) {
        /*
         * Receive packets on a port and forward them on the paired
         * port. The mapping is 0 -> 1, 1 -> 0, 2 -> 3, 3 -> 2, etc.
         */
        for (port = 0; port < nb_ports; port++) {

            /* Get burst of RX packets, from first port of pair. */
            struct rte_mbuf *bufs[BURST_SIZE];
            const uint16_t nb_rx = rte_eth_rx_burst(port, 0,
                    bufs, BURST_SIZE);

            if (unlikely(nb_rx == 0))
                continue;

            /* Send burst of TX packets, to second port of pair. */
            const uint16_t nb_tx = rte_eth_tx_burst(port ^ 1, 0,
                    bufs, nb_rx);

            /* Free any unsent packets. */
            if (unlikely(nb_tx < nb_rx)) {
                uint16_t buf;
                for (buf = nb_tx; buf < nb_rx; buf++)
                    rte_pktmbuf_free(bufs[buf]);
            }
        }
    }
}

The main work of the application is done within the loop:

for (;;) {
    for (port = 0; port < nb_ports; port++) {

        /* Get burst of RX packets, from first port of pair. */
        struct rte_mbuf *bufs[BURST_SIZE];
        const uint16_t nb_rx = rte_eth_rx_burst(port, 0,
                bufs, BURST_SIZE);

        if (unlikely(nb_rx == 0))
            continue;

        /* Send burst of TX packets, to second port of pair. */
        const uint16_t nb_tx = rte_eth_tx_burst(port ^ 1, 0,
                bufs, nb_rx);

        /* Free any unsent packets. */
        if (unlikely(nb_tx < nb_rx)) {
            uint16_t buf;
            for (buf = nb_tx; buf < nb_rx; buf++)
                rte_pktmbuf_free(bufs[buf]);
        }
    }
}

Packets are received in bursts on the RX ports and transmitted in bursts on the TX ports. The ports are grouped in pairs with a simple mapping scheme using the an XOR on the port number:

0 -> 1
1 -> 0

2 -> 3
3 -> 2

etc.

The rte_eth_tx_burst() function frees the memory buffers of packets that are transmitted. If packets fail to transmit, (nb_tx < nb_rx), then they must be freed explicitly using rte_pktmbuf_free().

The forwarding loop can be interrupted and the application closed using Ctrl-C.