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-boun...@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.bura...@intel.com > > <mailto:anatoly.bura...@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
