netdev - Wrong padding of short packets send by a tagged-vlan interface?

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Date:	Mon, 7 Jul 2008 23:01:29 +0200 (CEST)
From:	Krzysztof Oledzki <olel@....pl>
To:	netdev@...r.kernel.org
cc:	kaber@...sh.net, greearb@...delatech.com
Subject: Wrong padding of short packets send by a tagged-vlan interface?

Hello,

I have a problem that I'm not able to connect from selected linux hosts to 
a newly installed Netware 6.5 server. After two days of debugging I 
discovered that the problem seems to be caused by an insufficient padding 
of a .1q tagged packets.

Currently, if a packet is too short, it is extended to 60(+4) octets for 
both untagged and tagged ones. Unfortunately, when a .1Q tag is removed on 
a receive side, such packet is likely to be dropped as it may become too 
short (56 bytes):

This is a normal (working) ping with 84B (IP) + 14B (eth hdr) = 98B octets:
# ping -c 2 192.168.0.194
PING 192.168.0.194 (192.168.0.194) 56(84) bytes of data.
64 bytes from 192.168.0.194: icmp_seq=1 ttl=128 time=0.137 ms
64 bytes from 192.168.0.194: icmp_seq=2 ttl=128 time=0.141 ms

This is a shorter (working) ping with 46 (IP) + 14B (eth hdr) = 60B octets:
# ping -c 2 192.168.0.194 -s 18
PING 192.168.0.194 (192.168.0.194) 18(46) bytes of data.
26 bytes from 192.168.0.194: icmp_seq=1 ttl=128 time=0.137 ms
26 bytes from 192.168.0.194: icmp_seq=2 ttl=128 time=0.144 ms

This one does not work (45+14 = 59): 
# ping -c 2 192.168.0.194 -s 17
PING 192.168.0.194 (192.168.0.194) 17(45) bytes of data.

--- 192.168.0.194 ping statistics ---
2 packets transmitted, 0 received, 100% packet loss, time 999ms

I believe that my problem can be solved if we start padding .1Q packets 
with 4 more octets but I'm not able to find a proper place where to fix 
it.

I also found a very similar report:
  http://mail.opensolaris.org/pipermail/networking-discuss/2007-March/014585.html
  http://mail.opensolaris.org/pipermail/networking-discuss/2007-March/014588.html

Best regards,

 				Krzysztof Olędzki