[dpdk-dev] Where is the padding code in DPDK?
Morten Brørup
mb at smartsharesystems.com
Wed Nov 14 11:51:36 CET 2018
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 at dpdk.org] On Behalf Of Burakov, Anatoly
> Sent: Wednesday, November 14, 2018 11:18 AM
> To: Sam
> Cc: dev at 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 at intel.com
> > <mailto:anatoly.burakov at 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
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