sending order of 'segmented' UDP packets


when creating an UDP packet, I need to use two mbufs - one containing the UDP header (hdr) and another holding the payload (pay):

struct rte_mbuf* hdr = rte_pktmbuf_alloc(hdrmp);

struct rte_mbuf* pay = rte_pktmbuf_alloc(paymp);

// filling ether, IP, UDP header

ip_hdr->version_ihl = 0x40 | 0x05; // (*) without 0x05 order it ok

// setting sizes and linkage

hdr->data_len = sizeof(struct ether_hdr) + sizeof(struct ipv4_hdr) + sizeof(struct udp_hdr);

pay->data_len = payloadSize;

hdr->pkt_len = hdr->data_len + pay->data_len;

pay->pkt_len = hdr->pkt_len;

hdr->next = pay;

hdr->nb_segs = 2;

When sending plenty of such UDP packets using rte_eth_tx_burst(), all of them were sent correctly, but the sending order seems to be random. When using just a single mbuf for an UDP packet, the sending order is always the order of the packets in the tx array - which is what I expect. Using the header-payload-separation approach and omitting the IP header size info in the ip_hdr field - resulting in a wrong IP packet - the sending order gets ok.

I’m using the mlx5 PMD, NIC is a ConnectX-5. Could it be some offload mechanisms, influencing the sending order? Maybe someone can help.

Thanks and best regards

Sofia Baran