10. IP Reassembly Sample Application

10.4.1. IPv4 Fragment Table Initialization

This application uses the rte_ip_frag library. Please refer to Programmer’s Guide for more detailed explanation of how to use this library. Fragment table maintains information about already received fragments of the packet. Each IP packet is uniquely identified by triple <Source IP address>, <Destination IP address>, <ID>. To avoid lock contention, each RX queue has its own Fragment Table, e.g. the application can’t handle the situation when different fragments of the same packet arrive through different RX queues. Each table entry can hold information about packet consisting of up to RTE_LIBRTE_IP_FRAG_MAX_FRAGS fragments.

frag_cycles = (rte_get_tsc_hz() + MS_PER_S - 1) / MS_PER_S * max_flow_ttl;

if ((qconf->frag_tbl[queue] = rte_ip_frag_tbl_create(max_flow_num, IPV4_FRAG_TBL_BUCKET_ENTRIES, max_flow_num, frag_cycles, socket)) == NULL)
{
    RTE_LOG(ERR, IP_RSMBL, "ip_frag_tbl_create(%u) on " "lcore: %u for queue: %u failed\n",  max_flow_num, lcore, queue);
    return -1;
}

10.4.2. Mempools Initialization

The reassembly application demands a lot of mbuf’s to be allocated. At any given time up to (2 * max_flow_num * RTE_LIBRTE_IP_FRAG_MAX_FRAGS * <maximum number of mbufs per packet>) can be stored inside Fragment Table waiting for remaining fragments. To keep mempool size under reasonable limits and to avoid situation when one RX queue can starve other queues, each RX queue uses its own mempool.

nb_mbuf = RTE_MAX(max_flow_num, 2UL * MAX_PKT_BURST) * RTE_LIBRTE_IP_FRAG_MAX_FRAGS;
nb_mbuf *= (port_conf.rxmode.max_rx_pkt_len + BUF_SIZE - 1) / BUF_SIZE;
nb_mbuf *= 2; /* ipv4 and ipv6 */
nb_mbuf += RTE_TEST_RX_DESC_DEFAULT + RTE_TEST_TX_DESC_DEFAULT;
nb_mbuf = RTE_MAX(nb_mbuf, (uint32_t)NB_MBUF);

snprintf(buf, sizeof(buf), "mbuf_pool_%u_%u", lcore, queue);

if ((rxq->pool = rte_mempool_create(buf, nb_mbuf, MBUF_SIZE, 0, sizeof(struct rte_pktmbuf_pool_private), rte_pktmbuf_pool_init, NULL,
    rte_pktmbuf_init, NULL, socket, MEMPOOL_F_SP_PUT | MEMPOOL_F_SC_GET)) == NULL) {

        RTE_LOG(ERR, IP_RSMBL, "mempool_create(%s) failed", buf);
        return -1;
}

10.4.3. Packet Reassembly and Forwarding

For each input packet, the packet forwarding operation is done by the l3fwd_simple_forward() function. If the packet is an IPv4 or IPv6 fragment, then it calls rte_ipv4_reassemble_packet() for IPv4 packets, or rte_ipv6_reassemble_packet() for IPv6 packets. These functions either return a pointer to valid mbuf that contains reassembled packet, or NULL (if the packet can’t be reassembled for some reason). Then l3fwd_simple_forward() continues with the code for the packet forwarding decision (that is, the identification of the output interface for the packet) and actual transmit of the packet.

The rte_ipv4_reassemble_packet() or rte_ipv6_reassemble_packet() are responsible for:

1. Searching the Fragment Table for entry with packet’s <IP Source Address, IP Destination Address, Packet ID>
2. If the entry is found, then check if that entry already timed-out. If yes, then free all previously received fragments, and remove information about them from the entry.
3. If no entry with such key is found, then try to create a new one by one of two ways:
   1. Use as empty entry
   2. Delete a timed-out entry, free mbufs associated with it mbufs and store a new entry with specified key in it.
4. Update the entry with new fragment information and check if a packet can be reassembled (the packet’s entry contains all fragments).
   1. If yes, then, reassemble the packet, mark table’s entry as empty and return the reassembled mbuf to the caller.
   2. If no, then just return a NULL to the caller.

If at any stage of packet processing a reassembly function encounters an error (can’t insert new entry into the Fragment table, or invalid/timed-out fragment), then it will free all associated with the packet fragments, mark the table entry as invalid and return NULL to the caller.

10.4.4. Debug logging and Statistics Collection

The RTE_LIBRTE_IP_FRAG_TBL_STAT controls statistics collection for the IP Fragment Table. This macro is disabled by default. To make ip_reassembly print the statistics to the standard output, the user must send either an USR1, INT or TERM signal to the process. For all of these signals, the ip_reassembly process prints Fragment table statistics for each RX queue, plus the INT and TERM will cause process termination as usual.