[Freeciv-Dev] Re: (PR#6707) Announce server on LAN

["andrearo@xxxxxxxxxxxx" <andrearo@xxxxxxxxxxxx>]

On Mon, 3 Nov 2003, Raimar Falke wrote:
> On Sun, Nov 02, 2003 at 01:51:03PM -0800, andrearo@xxxxxxxxxxxx wrote:
> > > > > Nice idea. I have a problem however with the constants. Why TTL=4? Why
> > > > > not 3 or 5?
> > > >
> > > > Good question.. It depends about how far you want the packet to reach,
> > > > since TTL is decremented every time a packet goes through a router.
> > >
> > > > It could probably also be 3 or 5, depending about how you define
> > > > "LAN".
> > >
> > > And that is the point. If we hardcode it in the code we have to choose
> > > a good value. A not hardcoded solution would be better. Is this
> > > possible?
> >
> > Do you want TTL as a runtime parameter to the client and server? I've
> > never used any software that requires you to do that.
> > I can't think of another way to set the TTL.
> >
> > > > > Why a SERVER_LAN_GROUP of 225.0.0.1? Can't this be
> > > > > autodetected somehow?
> > > >
> > > > The server and client have to be in the same multicast group.
> > >
> > > Uhh I overlooked this. I assumed that the patch uses a broadcast. So
> > > what requirements are needed to use multicast? What setup must be done
> > > to enable it? Where do I have to set 225.0.0.1 to join this multicast
> > > group?
> >
> > As a user you don't have to setup anything. The application layer
> > software in the client and server tells the kernel to give it
> > packets from the multicast group.  I found this useful:
> > http://www.tldp.org/HOWTO/Multicast-HOWTO-2.html
>
> Looking at this and pinging around on some systems I got responses
> when I ping 224.0.0.1 but no answer when I used 225.0.0.1. This is
> logical from the above URL:
>
>    224.0.0.1 is the all-hosts group. If you ping that group, all
>    multicast capable hosts on the network should answer, as every
>    multicast capable host must join that group at start-up on all it's
>    multicast capable interfaces.
>
> This means that none of the other hosts which answered to 224.0.0.1
> are part of 225.0.0.1.
>
> In addition the URL states:
>
>    In any case, range 224.0.0.0 through 224.0.0.255 is reserved for
>    local purposes (as administrative and maintenance tasks) and
>    datagrams destined to them are never forwarded by multicast
>    routers.
>
> So if we choose 224.0.0.1 we know that all hosts are on this group AND
> we know that the TTL is irrelevant since the packets are routed.
>
> The only problem I see is that the server is behind a router. So how
> do you define LAN?
>
> If all this is correct I'm favoring as the default a scan in 224.0.0.1
> with no special TTL and (optionally if you want to code it) "an export
> mode" where you can input a special multicast group and optionally a
> TTL.
>
>       Raimar


I agree, judging from the HOWTO, 224.0.0.1 should be the default group.

 Andreas Røsdal