Re: CVS commit: src/sys/net/if_vlan.c

[Matt Thomas <matt%3am-software.com@localhost>]

> On Mar 30, 2015, at 8:26 AM, Christoph Badura <bad%bsd.de@localhost> wrote:
> 
> On Sun, Mar 29, 2015 at 01:30:43PM +0000, Ryota Ozaki wrote:
>> Module Name:	src
>> Committed By:	ozaki-r
>> Date:		Sun Mar 29 13:30:43 UTC 2015
>> 
>> Modified Files:
>> 	src/sys/net: if_vlan.c
>> 
>> Log Message:
>> Correct frame padding length
>> 
>> vlan pads a frame with zeros up to 68 bytes
>> (ETHER_MIN_LEN + ETHER_VLAN_ENCAP_LEN). It expects
>> that even if the frame is untagged, it keeps 64 bytes
>> at least. However, it lacks concern about CRC
>> (4 bytes). So a sending frame can be 72 (68 + 4) bytes.
> 
> I don't get it. ETHER_MIN_LEN includes the 4 bytes for the CRC of a
> minimum sized packet.  Therefore ETHER_MIN_LEN + ETHER_VLAN_ENCAP_LEN is
> the right size, isn't it?

No, it has 4 bytes too many because the 4 bytes for CRC still needs to be added by the hardware.

> BTW, what do you mean by "it lacks concern about CRC"?

It lacks compensating for the fact the CRC has been stripped by the time vlan gets the packet.