Re: [dpdk-dev] Packet crafting....

[sabu kurian <sabu2kurian@gmail.com>]

Thanks on the reply Richardson. You did mention something like "Don't
forget that when you get the mbuf from rte_pktmbuf_alloc, you also need to
set the length value to the appropriate size." .

 Like I'm allocating the mbuf using rte_mbuf . The rte_mempool_create has
already set the maximum packet size for a mbuf and the number of mbuf's and
all. So do I really need to use rte_pktmbuf_alloc ?


On Fri, Mar 14, 2014 at 4:38 PM, Richardson, Bruce <
bruce.richardson@intel.com> wrote:

>
> > From: sabu kurian [mailto:sabu2kurian@gmail.com]
> > Sent: Friday, March 14, 2014 10:09 AM
> > To: Richardson, Bruce
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] Packet crafting....
> >
> > Thank you very much Richardson for your valuable reply. But is there
> another way of doing it using the rte_pktmbuf_append / rte_pktmbuf_prepend
> ? Can you please tell me on how to do that ?
>
> You could use pktmbuf_append and pktmbuf_prepend, but they are primarily
> designed to add headers/footers on to an existing packet that already has
> headers, so that you don't need to move the existing data. In the case of
> having to craft a packet from scratch, there is no existing data in the
> mbuf, so the whole packet can just be filled in sequentially. [Don't forget
> that when you get the mbuf from rte_pktmbuf_alloc, you also need to set the
> length value to the appropriate size.]
>
> However, if you first write the data to the mbuf and then want to add the
> headers in order, like a packet moving down through layers of a stack,  you
> can use prepend to add udp, then ip, then your ethernet header, but this
> will be no faster than just writing the data directly to the mbuf space,
> and may be slightly slower as each prepend call has to just the length
> values and data pointer values in the mbuf. Each header prepended also uses
> up headroom in the mbuf, so you can only add 128-bytes of headers by
> default to each packet.
>
> In short, if you are creating a single packet, I'd recommend just getting
> the data pointer from the returned mbuf and writing directly to that.
>
> Regards,
> /Bruce
>