DPDK：修改捕获的数据包头的最有效方法

Question

i'm trying to modify headers of some GTP packets using the mbuf and mempool libraries, specifically i want to cut out all of ETH, IP, UDP ,GTP layers and obtain a (deep) copy of the packet's payload .

Here's the piece of code that should do the work:

void (const unsigned char* packet, size_t size)
{
    auto outer_header_len = sizeof(ether_header) + sizeof(ip) + sizeof(udphdr) + sizeof(gtp); //length to cut
    uint8_t byte_size = static_cast<uint8_t>(size);
    struct rte_mempool* mbuf_pool;

    struct rte_mbuf *mbuf_pkt = rte_pktmbuf_alloc(mbuf_pool);
    mbuf_pkt->data_len = byte_size;
    mbuf_pkt->pkt_len = byte_size;

    rte_pktmbuf_append(mbuf_pkt, packet[byte_size]);    
    auto payload = rte_pktmbuf_adj(mbuf_pkt, outer_header_len);
}

This function is called from a loop which parses the stream of packets and passes packet and size at each iteration. Since there will be lots of calls, how can i make my code memory efficient and better? Any tips?

Answer 1

There are few issues with the code.

byte_size

uint8_t byte_size = static_cast<uint8_t>(size); I am not sure that is correct, since it basically limiting all packets to 256 bytes, which might not be always the case. So I would just deleted this line, since the size itself is a perfectly fine.

rte_pktmbuf_append()

rte_pktmbuf_append(mbuf_pkt, packet[byte_size]); this function increases the length of mbuf_pkt , but do not change the mbuf bytes themselves. So we have to use the pointer it returns to actually do the copy, ie

ptr = rte_pktmbuf_append(mbuf_pkt, size);
if (ptr != NULL)
    rte_memcpy(ptr, packet + outer_header_len, size -outer_header_len);

Here are also the links to the rte_pktmbuf_append() and rte_memcpy() documentation.

Other Mbuf Length Manipulations

Since the rte_pktmbuf_append() increases the length of the packet, the following lines are redundant:

mbuf_pkt->data_len = byte_size;
mbuf_pkt->pkt_len = byte_size;
auto payload = rte_pktmbuf_adj(mbuf_pkt, outer_header_len);

Performance

We can copy data without the outer header by taking into account outer_header_len during the data copy as in the example before:

ptr = rte_pktmbuf_append(mbuf_pkt, size);
if (ptr != NULL)
    rte_memcpy(ptr, packet + outer_header_len, size -outer_header_len);

A check if the outer_header_len is less than the packet length might be also required, unless there is such a check made earlier in the code.