9. IPv4 Multicast Sample Application

9.4.1. Memory Pool Initialization

The IPv4 Multicast sample application uses three memory pools. Two of the pools are for indirect buffers used for packet duplication purposes. Memory pools for indirect buffers are initialized differently from the memory pool for direct buffers:

packet_pool = rte_pktmbuf_pool_create("packet_pool", NB_PKT_MBUF, 32,
                    0, PKT_MBUF_DATA_SIZE, rte_socket_id());
header_pool = rte_pktmbuf_pool_create("header_pool", NB_HDR_MBUF, 32,
                    0, HDR_MBUF_DATA_SIZE, rte_socket_id());
clone_pool = rte_pktmbuf_pool_create("clone_pool", NB_CLONE_MBUF, 32,
                    0, 0, rte_socket_id());

The reason for this is because indirect buffers are not supposed to hold any packet data and therefore can be initialized with lower amount of reserved memory for each buffer.

9.4.2. Hash Initialization

The hash object is created and loaded with the pre-configured entries read from a global array:

static int

init_mcast_hash(void)
{
    uint32_t i;
    mcast_hash_params.socket_id = rte_socket_id();

    mcast_hash = rte_fbk_hash_create(&mcast_hash_params);
    if (mcast_hash == NULL){
        return -1;
    }

    for (i = 0; i < N_MCAST_GROUPS; i ++){
        if (rte_fbk_hash_add_key(mcast_hash, mcast_group_table[i].ip, mcast_group_table[i].port_mask) < 0) {
                    return -1;
        }
    }
    return 0;
}

9.4.3. Forwarding

All forwarding is done inside the mcast_forward() function. Firstly, the Ethernet* header is removed from the packet and the IPv4 address is extracted from the IPv4 header:

/* Remove the Ethernet header from the input packet */

iphdr = (struct ipv4_hdr *)rte_pktmbuf_adj(m, sizeof(struct ether_hdr));
RTE_ASSERT(iphdr != NULL);
dest_addr = rte_be_to_cpu_32(iphdr->dst_addr);

Then, the packet is checked to see if it has a multicast destination address and if the routing table has any ports assigned to the destination address:

if (!IS_IPV4_MCAST(dest_addr) ||
   (hash = rte_fbk_hash_lookup(mcast_hash, dest_addr)) <= 0 ||
   (port_mask = hash & enabled_port_mask) == 0) {
       rte_pktmbuf_free(m);
       return;
}

Then, the number of ports in the destination portmask is calculated with the help of the bitcnt() function:

/* Get number of bits set. */

static inline uint32_t bitcnt(uint32_t v)
{
    uint32_t n;

    for (n = 0; v != 0; v &= v - 1, n++)
       ;
    return n;
}

This is done to determine which forwarding algorithm to use. This is explained in more detail in the next section.

Thereafter, a destination Ethernet address is constructed:

/* construct destination Ethernet address */

dst_eth_addr = ETHER_ADDR_FOR_IPV4_MCAST(dest_addr);

Since Ethernet addresses are also part of the multicast process, each outgoing packet carries the same destination Ethernet address. The destination Ethernet address is constructed from the lower 23 bits of the multicast group OR-ed with the Ethernet address 01:00:5e:00:00:00, as per RFC 1112:

#define ETHER_ADDR_FOR_IPV4_MCAST(x) \
    (rte_cpu_to_be_64(0x01005e000000ULL | ((x) & 0x7fffff)) >> 16)

Then, packets are dispatched to the destination ports according to the portmask associated with a multicast group:

for (port = 0; use_clone != port_mask; port_mask >>= 1, port++) {
    /* Prepare output packet and send it out. */

    if ((port_mask & 1) != 0) {
        if (likely ((mc = mcast_out_pkt(m, use_clone)) != NULL))
            mcast_send_pkt(mc, &dst_eth_addr.as_addr, qconf, port);
        else if (use_clone == 0)
             rte_pktmbuf_free(m);
   }
}

The actual packet transmission is done in the mcast_send_pkt() function:

static inline void mcast_send_pkt(struct rte_mbuf *pkt, struct ether_addr *dest_addr, struct lcore_queue_conf *qconf, uint8_t port)
{
    struct ether_hdr *ethdr;
    uint16_t len;

    /* Construct Ethernet header. */

    ethdr = (struct ether_hdr *)rte_pktmbuf_prepend(pkt, (uint16_t) sizeof(*ethdr));

    RTE_ASSERT(ethdr != NULL);

    ether_addr_copy(dest_addr, &ethdr->d_addr);
    ether_addr_copy(&ports_eth_addr[port], &ethdr->s_addr);
    ethdr->ether_type = rte_be_to_cpu_16(ETHER_TYPE_IPv4);

    /* Put new packet into the output queue */

    len = qconf->tx_mbufs[port].len;
    qconf->tx_mbufs[port].m_table[len] = pkt;
    qconf->tx_mbufs[port].len = ++len;

    /* Transmit packets */

    if (unlikely(MAX_PKT_BURST == len))
        send_burst(qconf, port);
}

9.4.4. Buffer Cloning

This is the most important part of the application since it demonstrates the use of zero- copy buffer cloning. There are two approaches for creating the outgoing packet and although both are based on the data zero-copy idea, there are some differences in the detail.

The first approach creates a clone of the input packet, for example, walk though all segments of the input packet and for each of segment, create a new buffer and attach that new buffer to the segment (refer to rte_pktmbuf_clone() in the rte_mbuf library for more details). A new buffer is then allocated for the packet header and is prepended to the cloned buffer.

The second approach does not make a clone, it just increments the reference counter for all input packet segment, allocates a new buffer for the packet header and prepends it to the input packet.

Basically, the first approach reuses only the input packet’s data, but creates its own copy of packet’s metadata. The second approach reuses both input packet’s data and metadata.

The advantage of first approach is that each outgoing packet has its own copy of the metadata, so we can safely modify the data pointer of the input packet. That allows us to skip creation if the output packet is for the last destination port and instead modify input packet’s header in place. For example, for N destination ports, we need to invoke mcast_out_pkt() (N-1) times.

The advantage of the second approach is that there is less work to be done for each outgoing packet, that is, the “clone” operation is skipped completely. However, there is a price to pay. The input packet’s metadata must remain intact, so for N destination ports, we need to invoke mcast_out_pkt() (N) times.

Therefore, for a small number of outgoing ports (and segments in the input packet), first approach is faster. As the number of outgoing ports (and/or input segments) grows, the second approach becomes more preferable.

Depending on the number of segments or the number of ports in the outgoing portmask, either the first (with cloning) or the second (without cloning) approach is taken:

use_clone = (port_num <= MCAST_CLONE_PORTS && m->pkt.nb_segs <= MCAST_CLONE_SEGS);

It is the mcast_out_pkt() function that performs the packet duplication (either with or without actually cloning the buffers):

static inline struct rte_mbuf *mcast_out_pkt(struct rte_mbuf *pkt, int use_clone)
{
    struct rte_mbuf *hdr;

    /* Create new mbuf for the header. */

    if (unlikely ((hdr = rte_pktmbuf_alloc(header_pool)) == NULL))
        return NULL;

    /* If requested, then make a new clone packet. */

    if (use_clone != 0 && unlikely ((pkt = rte_pktmbuf_clone(pkt, clone_pool)) == NULL)) {
        rte_pktmbuf_free(hdr);
        return NULL;
    }

    /* prepend new header */

    hdr->pkt.next = pkt;

    /* update header's fields */

    hdr->pkt.pkt_len = (uint16_t)(hdr->pkt.data_len + pkt->pkt.pkt_len);
    hdr->pkt.nb_segs = (uint8_t)(pkt->pkt.nb_segs + 1);

    /* copy metadata from source packet */

    hdr->pkt.in_port = pkt->pkt.in_port;
    hdr->pkt.vlan_macip = pkt->pkt.vlan_macip;
    hdr->pkt.hash = pkt->pkt.hash;
    hdr->ol_flags = pkt->ol_flags;
    rte_mbuf_sanity_check(hdr, RTE_MBUF_PKT, 1);

    return hdr;
}