Help Broadcasting a UDP packet on the LAN:URGENT

[Barney Wolff]

On Thu, Oct 23, 2003 at 02:23:57PM -0400, Charles Swiger wrote:
> >What are you going to do when IPv6 comes into more general use, since
> >it has no broadcast address?
> 
> Are you asking what a IPv4-to-IPv6 translator (like gif?) should do, or 
> are you worried about the case of a machine configured for IPv6 only 
> and not for IPv4?  I expect that most people will be using IPv4 for 
> quite some time; we don't have to do something for the IPv6-only case 
> to still have this be useful.

My expectation is the same as yours, but I strongly believe that
anyone doing a new design that deliberately ignores IPv6 is being very
shortsighted.  "Quite some time" is now only years, not decades.

> >>Interactions with VLANs, for instance.  If you send an
> >>all-ones broadcast on an interface that has one or more VLANs 
> >>configured,
> >>do you repeat them "on" each VLAN as well?  Ugh.  What about
> >>point-to-point links?  Are those always considered gateways to a 
> >>foreign
> >>network, or just another form of locally attached network?
> >
> >The multicast notion would suggest that this be handled as a special
> >case of multicast, with a pseudo group that can't occur naturally.
> >That way you get "for free" to control which interfaces should send
> >the broadcast, based on group membership.
> 
> Multicast and broadcast addressing are working at layer-3, but the 
> point of using VLAN tags is to create logically 'seperate' networks 
> where the flow of traffic is being handled/segregated at layer-2 rather 
> than layer-3.

VLANs are meant to allow segregating a physical switch into multiple
logical switches.  VLAN tagging is used on inter-switch trunks, so
that multiple logical switches can be connected by a single physical
circuit.  In either case, whether the switch *itself* (that is, its
management interface) is on a particular VLAN depends on whether it
has a level-3 address on that VLAN, at least in the normal case that
a VLAN corresponds to an IP subnet.  Since we're talking here of
sending IP packets, that reasoning would seem to apply.  (For level 2
purposes such as spanning tree, of course the switch is "on" every
configured VLAN.)

> >The whole VLAN thing is nasty, but I'd say that the general issue is
> >does the box itself have a virtual interface on the VLAN, or is it
> >merely switching on it.  If the former, you send packets and process
> >received packets up the stack.  If the latter, you just do what any
> >switch/bridge would do, because "you" (ie, higher layers) are not 
> >really
> >on that layer-3 network.
> 
> The all-ones broadcast is supposed to go to all physically connected 
> network segments, regardless of whether a particular interface is 
> ifconfig'ured with an IP that is part of a particular layer-3 subnet.  
> You should be able to send the broadcast packet out from an interface 
> which is up but does not have an IPv4 address assigned, right?

In what sense are you using "supposed" - required by some standard,
or simply what you'd like to have happen?  If the former, please point
to the standard.  Sending a packet out from an interface with no IP
address assigned leads immediately to the question of what source IP
address to use, and how a responder knows where to send its response.
Perhaps functions like this would be better accomplished at the link
layer, not using IP at all, like ARP.  If that were done,
an interface using VLAN tagging should send the frame on each of its
configured VLANs.  "Physical" doesn't mean much when using VLANs.
Among other things, the thing at the other end of the cable from a
port using VLAN tagging may not approve of a frame sent
with no tag (although cisco's can be configured with a default VLAN
for that case).

-- 
Barney Wolff         http://www.databus.com/bwresume.pdf
I'm available by contract or FT, in the NYC metro area or via the 'Net.