[dpdk-users] Calculating Packet Length

Wiles, Keith keith.wiles at intel.com
Sat Sep 29 16:39:39 CEST 2018

> On Sep 29, 2018, at 5:19 AM, Michael Barker <mikeb01 at gmail.com> wrote:
> 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);

This does seem to be wrong and sizeof(struct ether_hdr) should have used. As the L2 header is normally 14 bytes in size.

A packet in DPDK must be at least 60 bytes in length is the hardware appends the Frame checksum (4 bytes) because all ethernet frames must be at least 64 bytes in the wire. Some hardware will pad the frame out the correct length then add the FCS (Frame Checksum) bytes. Some hardware will discard the frame and count it as a runt or fragment. Just to be safe I always make sure the length of the frame is 60 bytes at least using a software check.

Hope that helps you. 

Also it looks like the3 ptpclient.c file may need to be fixed.

>    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.


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