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From: Michael Barker <mikeb01@gmail.com>
Date: Sat, 29 Sep 2018 22:19:18 +1200
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Subject: [dpdk-users] Calculating Packet Length
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Hi,

I've new to DPDK and have been started by sending ARP packets.  I have a
question around how to set the mbuf data_len and pkt_size.  I Initially did
the following:

    struct rte_mbuf* arp_pkt = rte_pktmbuf_alloc(mbuf_pool);
    const size_t pkt_size = sizeof(struct ether_addr) + sizeof(struct
arp_hdr);

    arp_pkt->data_len = pkt_size;
    arp_pkt->pkt_len = pkt_size;

Which is based on ptpclient.c sample code.  However after setting all of
the fields, the packet either doesn't get sent or has some of the data
truncated from the end of the packet when viewed in Wireshark.  If I modify
the size to be the following:

    const size_t pkt_size = sizeof(struct ether_addr) + sizeof(struct
arp_hdr) + 8;

It works as expected.  I'm wondering where the extra 8 bytes come from?  Is
there a better way to calculate the packet length?

Using dpdk 18.08, Linux - kernel 4.15.0-33.

Mike.