[Ns-developers] Learning Bridge, NetDevice API changes

[Gustavo Carneiro]

2008/7/5 Mathieu Lacage <mathieu.lacage at sophia.inria.fr>:

> On Sat, 2008-07-05 at 13:13 +0100, Gustavo Carneiro wrote:
>
> >         I would expect to be able to set the device promiscuous bit on
> >         globally
> >         as root on linux. i.e., "ifconfig ethx promisc" ? So, if we do
> >         the same
> >         thing, we probably should be fine.
> >
> > If we're going down linux road, at least we should add a packet type
> > parameter to make it easy for protocol handlers to ignore certain
> > packets:
> >
> > unsigned char pkt_type;
> >
> >         Packet classification used in delivering it. The driver is
> >         responsible for setting it to PACKET_HOST (this packet is for
> >         me), PACKET_BROADCAST, PACKET_MULTICAST, or PACKET_OTHERHOST
> >         (no, this packet is not for me). Ethernet drivers don't modify
> >         pkt_type explicitly because eth_type_trans does it for them.
> >
> > (from http://www.xml.com/ldd/chapter/book/ch14.html)
>
> What is the use-case for that ? I don't have anything against this idea
> but, I really don't see what problem this feature will solve.


I don't know, but I guess it makes it simple and efficient to decide what to
do with a packet.  For example I see linux kernel uses it in ip_forward:

[...]
 if (skb <http://lxr.linux.no/linux/+code=skb>->pkt_type
<http://lxr.linux.no/linux/+code=pkt_type> != PACKET_HOST
<http://lxr.linux.no/linux/+code=PACKET_HOST>)
 <http://lxr.linux.no/linux/net/ipv4/ip_forward.c#L68>       goto drop
<http://lxr.linux.no/linux/+code=drop>;


http://lxr.linux.no/linux/net/ipv4/ip_forward.c#L67

Of course, personally I would prefer completely separate flow paths for
packets received promiscuously and normal packets.  But if you insist in
mixing it all together and don't provide a way to distinguish (IPv4 does not
count, not all world is IP based), someone will be requesting another API
change in a couple of months.  If not sooner.

-- 
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert