Subject: Re: Handling looped back packets

From: Bill Fenner <[email-masked]>
Date: Thu, 4 Jan 2001 15:34:15 -0800

>Should loop back be enabled on Multicast routers.

What does this mean?

>If so, when we send
>out a multicast packet, should it be looped back via the loop-back
>interface. If it is looped back, then looping back forwarded datagrams
>can cause problems, so at what level should the looped back packet be
>discarded or how else can it be handled.

The UNIX multicast router implementation treats packets sent by
the system (as opposed to packets forwarded by the system) as though
they arrived on the interface on which they were sent.  This causes
the fact that the system is a router to be transparent to applications
being run on the system, i.e. the applications see a multi-homed host
with the same interfaces as the router has, and the router sees packets
coming in from external systems.

There's also the question of whether a datagram sent on a socket
gets looped back to other sockets (the IP_MULTICAST_LOOP setsockopt)
but that happens completely independent of routing, so I assume that's
not the question.

  Bill