Re: Unusual use of IP Broadcast Address

[Steve Deering <[e-mail]>]

Wendy,

It is legal for a host or router to send packets addressed to the Limited
Broadcast Address (255.255.255.255) on any of its attached subnets; in fact,
that is the preferred address to use for IP broadcasting on any subnet
(see Host Requirements RFC 1122, section 3.3.6).  Your two serial lines
are simply two additional subnets attached to your Sun.

One issue that arises is the following: if an application on a multihomed
host or a router originates a packet destined to 255.255.255.255, which
interface should it be sent over?  In my opinion, the service interface to
the IP module should provide a means for the application to specify the
outgoing interface (it might be implicit in the application's choice of IP
source address or explicit as, say, a separate parameter to the Send()
operation or a control operation performed before the Send() operation).
Unfortunately, BSD-derived Unixes do not provide any such means, but
rather limit the use of 255.255.255.255 to one interface only
(configurable through the ifconfig command).  If you are stuck with
such an implementation, the best you can do is configure one of the
serial lines to use the 255.255.25.255 broadcast address, and configure
the Ethernet and the other serial line to use Directed Broadcast addresses,
i.e., addresses of the form <net><subnet><-1>.

Steve Deering

P.S.  I suggest that you consider using IP multicast rather than IP broadcast;
      the service interface to IP multicast does include the ability to
      choose the outgoing interface.