[dpdk-users] Too much data being sent with small packet and IXGBE

["terry.montague.1980@btinternet.com" <terry.montague.1980@btinternet.com>]

Hi there,
I'm trying to send a single IGMP control packet out of an Intel X550 to its connected router.
I've looked at the packet in wireshark and its absolutely fine, the ethernet header, the ip header, the ipchecksum offload and the IGMP payload & checksum - there's nothing wrong with it.
It is 46 bytes formatted and the pkt and data lengths of the allocated buffer are all set to 46 bytes. Its a 46 byte packet - end of!
However....  what comes out of the interface is 60 bytes of data - the 46 bytes I want to send + 14 bytes of zeros.
The 14 bytes is curiously equivalent to the size of an ethernet header, but I just cannot see how it is just being added on to the end of the packet data.
Has anyone else observed this effect , or indeed know what's causing it ?
Many thanks
Terry.
From keith.wiles@intel.com  Mon Sep 10 22:39:20 2018
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From: "Wiles, Keith" <keith.wiles@intel.com>
To: "terry.montague.1980@btinternet.com" <terry.montague.1980@btinternet.com>
CC: "users@dpdk.org" <users@dpdk.org>
Thread-Topic: [dpdk-users] Too much data being sent with small packet and IXGBE
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Date: Mon, 10 Sep 2018 20:39:16 +0000
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Subject: Re: [dpdk-users] Too much data being sent with small packet and
 IXGBE
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> On Sep 10, 2018, at 1:51 PM, terry.montague.1980@btinternet.com wrote:
>=20
> Hi there,
> I'm trying to send a single IGMP control packet out of an Intel X550 to i=
ts connected router.
> I've looked at the packet in wireshark and its absolutely fine, the ether=
net header, the ip header, the ipchecksum offload and the IGMP payload & ch=
ecksum - there's nothing wrong with it.
> It is 46 bytes formatted and the pkt and data lengths of the allocated bu=
ffer are all set to 46 bytes. Its a 46 byte packet - end of!
> However....  what comes out of the interface is 60 bytes of data - the 46=
 bytes I want to send + 14 bytes of zeros.
> The 14 bytes is curiously equivalent to the size of an ethernet header, b=
ut I just cannot see how it is just being added on to the end of the packet=
 data.
> Has anyone else observed this effect , or indeed know what's causing it ?

Not sure what you are doing, but the smallest packet on ethernet is 60 byte=
s + 4 bytes CRC for 64 bytes total. Normally anything less then 60 bytes + =
CRC is called a runt packet and are discarded. The NIC is padding the data =
to 60 bytes then adds the 4 byte CRC.

Hope that helps.

> Many thanks
> Terry.

Regards,
Keith