Re: hardware loop and frame length increase

[Martin T <m4rtntns () gmail com>]

Fulko, Guy thanks for explaining this!


regards,
martin


2011/6/7 Guy Harris <guy () alum mit edu>:
> On Jun 6, 2011, at 3:02 PM, Martin T wrote:
>
> > As you can see, every second I sent and received one frame. The
> > question is, why is the frame, which I receive, 18 bytes longer than
> > the one I sent? I mean what are those 144 0-bits at the end of the
> > each frame back from the hardware loop?
>
> Padding.  Ethernet requires that a frame be a minimum of 64 octets if you don't include the CRC at the end (64 octets 
> if you include the CRC).  If you send a 42-byte frame, and you "receive" it by the networking stack on your machine 
> "looping back" the packet as capture input, you will receive a 42-byte frame; however, on the wire, that frame will 
> have an additional 18 bytes of padding added to it, and if you receive it from the Ethernet adapter, you will see the 
> additional padding bytes.
>
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