Date: Fri, 5 Jun 2009 12:15:27 -0700 (PDT)
From: david@lang.hm
To: Alexander Clouter <alex@digriz.org.uk>
cc: linux-kernel@vger.kernel.org, sefi@s-e-f-i.de
Subject: Re: When does Linux drop UDP packets?
In-Reply-To: <20090605191025.GG2014@woodchuck>
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References: <20090604145347.GA27692@miyuki> <buslf6-j24.ln1@woodchuck.wormnet.eu> <alpine.DEB.1.10.0906041618200.7953@asgard> <20090605191025.GG2014@woodchuck>
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On Fri, 5 Jun 2009, Alexander Clouter wrote:

> Hi,
>
> * david@lang.hm <david@lang.hm> [2009-06-04 16:19:56-0700]:
>>
>> On Thu, 4 Jun 2009, Alexander Clouter wrote:
>>
>>> Philipp Reh <sefi@s-e-f-i.de> wrote:
>>>>
>>>> I have the following setting in which a client that resides on the same
>>>> physical network as a server wants to receive any UDP packet that
>>>> arrives on any of its interfaces sent by that server.
>>>>
>>> Read up about multicasting, it will do what you want, does not depend on
>>> the IP address of the destination workstation and will also cross
>>> subnets if you want it to.
>>>
>>> It's dead easy to transmit and receive multicast traffic, broadcasting
>>> network traffic is so 1980's :)
>>
>> there is only a difference between multicast and broadcast traffic if you
>> are spanning subnets.
>>
> Well yes and no.  Broadcast traffic is *always* handled by the kernel as
> only the kernel can tell if it is interested in it or not.  With
> multicast the NIC is configured to only pass particular
> Ethernet multicast packets up to the kernel.
>
> By using broadcast traffic the load (okay, hardly a big problem
> now-a-days) hits *all* the workstations on the subnet, with multicast,
> only those interested in the traffic receive it.

true, but only for some NICs, and even those tend to have a fairly small 
number of slots for the filters. past these limits the OS handles it all 
just like broadcasts.

David Lang
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