Re: [dpdk-dev] Where is the padding code in DPDK?

["Morten Brørup" <mb@smartsharesystems.com>]

Anatoly,

This differs from the Linux kernel's behavior, where padding belongs in the NIC driver layer, not in the protocol layer. If you pass a runt frame (too short packet) to a Linux NIC driver's transmission function, the NIC driver (or NIC hardware) will pad the frame to make it valid. E.g. look at the rhine_start_tx() function in the kernel: https://elixir.bootlin.com/linux/v4.9.137/source/drivers/net/ethernet/via/via-rhine.c#L1800

If DPDK does not pad short frames passed to the egress function of the NIC drivers, it should be noted in the documentation - this is not the expected behavior by protocol developers.

Or even better: The NIC hardware (or driver) should ensure padding, possibly considering it a TX Offload feature. Generating packets shorter than 60 bytes data is common - just consider the amount of TCP ACK packets, which are typically only 14 + 20 + 20 = 54 bytes (incl. the 14 byte Ethernet header).


Med venlig hilsen / kind regards
- Morten Brørup

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Burakov, Anatoly
> Sent: Wednesday, November 14, 2018 11:18 AM
> To: Sam
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] Where is the padding code in DPDK?
> 
> On 14-Nov-18 5:45 AM, Sam wrote:
> > OK, then shortly speaking, DPDK will NOT care about padding.
> > NIC will care about padding while send and recv with NIC.
> > kernel will care about while send and recv with vhostuser port.
> >
> > Is that right?
> 
> I cannot speak for virtio/vhost user since i am not terribly familiar
> with them. For regular packets, generally speaking, packets shorter
> than
> 60 bytes are invalid. Whether DPDK does or does not care about padding
> is irrelevant, because *you* are attempting to transmit packets that
> are
> not valid. You shouldn't rely on this behavior.
> 
> >
> >
> > Burakov, Anatoly <anatoly.burakov@intel.com
> > <mailto:anatoly.burakov@intel.com>> 于2018年11月13日周二 下午5:29写道：
> >
> >     On 13-Nov-18 7:16 AM, Sam wrote:
> >      > Hi all,
> >      >
> >      > As we know, ethernet frame must longer then 64B.
> >      >
> >      > So if I create rte_mbuf and fill it with just 60B data, will
> >      > rte_eth_tx_burst add padding data, let the frame longer then
> 64B
> >      >
> >      > If it does, where is the code?
> >      >
> >
> >     Others can correct me if i'm wrong here, but specifically in case
> of
> >     64-byte packets, these are the shortest valid packets that you
> can
> >     send,
> >     and a 64-byte packet will actually carry only 60 bytes' worth of
> packet
> >     data, because there's a 4-byte CRC frame at the end (see Ethernet
> frame
> >     format). If you enabled CRC offload, then your NIC will append
> the 4
> >     bytes at transmit. If you haven't, then it's up to each
> individual
> >     driver/NIC to accept/reject such a packet because it can rightly
> be
> >     considered malformed.
> >
> >     In addition, your NIC may add e.g. VLAN tags or other stuff,
> again
> >     depending on hardware offloads that you have enabled in your TX
> >     configuration, which may push the packet size beyond 64 bytes
> while
> >     having only 60 bytes of actual packet data.
> >
> >     --
> >     Thanks,
> >     Anatoly
> >
> 
> 
> --
> Thanks,
> Anatoly