[En-Nut-Discussion] Extra packet in ethernet packet

[Coleman Brumley]

The code should be modified as follows:

sz += nb->nb_dl.sz;
//if (sz & 1) {
//	sz++;
//}

Since the AT91 EMAC isn't limited to only sending odd or even bytes, there
is no need to check. 

Coleman

> -----Original Message-----
> From: en-nut-discussion-bounces at egnite.de [mailto:en-nut-discussion-
> bounces at egnite.de] On Behalf Of Pawel
> Sent: Friday, June 12, 2015 3:42 AM
> To: Ethernut User Chat (English)
> Subject: Re: [En-Nut-Discussion] Extra packet in ethernet packet
> 
>     Hello
> 
> > Hi,
> >
> > On 05.12.2014 20:51, Philipp Burch wrote:
> >> On 05.12.2014 20:42, Mingshu Wang wrote:
> >>> I found an issue in one of our products that is using Nut/OS 4.8.7 and
> AT91SAM7X256. If I send a packet with odd number of characters to my
> computer, an extra byte is padded at the end. Wireshark interprets it as
> "VSS-Monitoring Ethernet trailer".  If the packet size is an even number,
> everything is fine.  Most applications can handle this packet correctly.
But
> some  applications don't read the length in the header. They just process
all
> characters in the packet and think this is a wrong message.
> >> In the EmacPutPacket() function in
> >> nut/arch/arm/dev/atmel/at91_emac.c,
> >> there is this code:
> >>
> >>     sz += nb->nb_dl.sz;
> >>     if (sz & 1) {
> >>         sz++;
> >>     }
> > Applications must always honor the lengths provided in the headers.
> > The minimum size of an Ethernet payload is 46 octets
> >
> > http://wiki.wireshark.org/Ethernet
> > http://tools.ietf.org/html/rfc894
> >
> > So, padding may always occur. Most Ethernet controllers add padding
> > bytes automatically.
> >
> > Some Ethernet controllers cannot transmit odd number of bytes. As far
> > as I can see, the AT91 EMAC doesn't have this limitation. I guess,
> > that the code snippet above had been inherited from another driver.
>     Does anyone resolved this problem?
> Removing "sz++" is not enough to send packet with odd bytes number.
> 
> best regards
> Pawel
> 
> 
> _______________________________________________
> http://lists.egnite.de/mailman/listinfo/en-nut-discussion