Re: [Etherboot-developers] [RFC] Multicast reception....

[<ebi...@ln...>]

Donald J Christensen <dj...@ci...> writes:

> "Eric W. Biederman" wrote:
> ...
> > My understanding of the guts of NIC is limited, so please check me.  I
> > believe NICs have a hardware filter that allows them to receive just
> > broadcast packets, and packets to their own mac address.  So to
> > receive multicast packets I need open up or disable the filter all
> > together.  The simplest solution appears to be disabling the hardware
> > filter and go into promiscous mode, and then replace the hardware
> > filter with a software filter in await_reply.
> 
> I thought at least some NICs have support for monitoring some number
> of multicast addresses, in addition to broadcast and the local MAC.
> I'm not an expert on NICs, but I think it might be worthwhile to
> check the Linux drivers for some of the newer NICs and see if there
> is anything obvious there as far as multicast support.

I have seen some support but unless there is a compelling reason to
enable it, the complexity and the few percetage points performance
gains are not worth it.
 
> > Does anyone know if there is any communication between NICs and
> > switches about what a NIC is listening for that would make promiscous
> > mode a bad state to put NICs into?
> 
> Other than IGMP, I am not aware of any communication along these lines.
> I think the main problem with promiscuous mode is the additional CPU
> overhead if there is a lot of unwanted traffic.  Since you are doing
> this at boot time, it probably is not a concern, especially if all of
> the clients are booting at the same time, in which case there will be
> very little "unwanted" traffic.

Right.  My research seems to back this up.  And this probably explains
why layer 2 switches have such a hard time with multicast traffic.
All they have enough information to do is broadcast it.  So a switch
has to climb up to layer 3 and start interacting with IGMP to even see
where the multicast traffic should go.  Which make switches a very
interesting tradeoff.

When you are running through a switch generally you don't have much
extraneous network traffic except broadcast traffic, so I don't see a
lot of problems.

The real question is for people with ISA NIC's where the bandwidth to
memory is less than the network bandwidth if loosing a little extra
performance is a problem.  Because I'd like to enable promiscous mode
unconditionally.

Working at Cisco do you know why some switches loose 90% of their
multicast traffic.  I haven't had a chance to investigate this one
personally but I have heard it reported of the Cisco gige switches
amoung others.  With enough clients (11+) multicast is still a win
even at 90% packet loss but it would be nice to not need so many
retransmissions.

Eric