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From: Sofia Baran <sofia.baran@gmx.net>
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Date: Wed, 26 Sep 2018 10:25:07 +0200
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Subject: [dpdk-users] order of 'segmented' UDP packets
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Hi,

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. Could it be some offload mechanisms, influencing 
the sending order? Maybe someone can help.

Thanks and best regards
Sofia Baran