[Xorp-hackers] Multicast routing

[Suresh kannan]

Hi Pavlin,

Sounds great. Thanks for the info. It clears my doubt which i had for long
time.

Thanks,
Regards,
Suresh kannan.

On 3/2/07, Pavlin Radoslavov <pavlin at icir.org> wrote:
>
> > > > How does the XORP multicast router sniff IGMP/MLD messages from the
> > > network?
> > > >
> > > > I checked the some routers' code. Most of them just open a RAW
> socket
> > > with
> > > > protocol set to IPPROTO_IGMP. But using this the router gets the
> > > > packets addressed to 224.0.0.1 only.
> > > >
> > > > How does it receive IGMPv2 and IGMPv3 membership reports sent by
> other
> > > hosts?
> > >
> > > The kernel gets all multicast packets because the interface is set
> > > in ALLMULTI mode (i.e., snoop all multicast packets). This happens
> > > after the multicast routing socket has been opened with
> > > setsockopt(..., MRT_INIT, ...) and then each interface is added with
> > > setsockopt(..., MRT_ADD_VIF, ...).
> >
> >
> > %Suresh : In FreeBSD, IGMP packets are handled in from ipintr() in
> > ip_input.c where it directly goes to igmp_input() (igmp.c). From here,
> it
> > puts packet to applications via rip_input call. So, I guess to receive
> IGMP
> > packets we no need to setsockopt though IGMP ( XORP,etc) adds to VIF;
> thats
> > to enable Mrouting on that interface. Not particularly to receive IGMP
> > control packets. Please correct me if I'm wrong here.
>
> You need MRT_ADD_VIF if you want to guarantee you will receive all
> IGMP packets.
>
> The IP stack will get a multicast packet (including the IGMP packets
> that are always sent to a multicast destination) only if:
>
> (a) The host has explicitly joined the particular multicast group
> on the particular network interface.
>
> OR
>
> (b) The network interface is in IFF_ALLMULTI mode (i.e., to snoop
> and pass all multicast packets to the IP stack).
>
> Otherwise a multicast packet will be ignored by the network
> interface and won't even reach ip_input().
>
> Obviously, you cannot do (a) for each of the multicast groups in the
> whole multicast address space.
> Hence, to guarantee that you will get all IGMP packets you need to
> do (b).
> I believe the only API that allows the user to set the IFF_ALLMULTI
> flag on an interface is by using setsockopt(MRT_ADD_VIF).
> At least, this is the API that is guaranteed to work on all UNIX
> systems that support multicast forwarding.
>
> However, if you care only about the IGMP packets sent to a small
> number of multicast groups, then you could explicitly join each of
> those groups and you don't need setsockopt(MRT_ADD_VIF).
>
> Regards,
> Pavlin
>
> > Then the userland program can receive the IGMP packets with opening
> > > a raw IPPROTO_IGMP socket.
> > >
> > > BTW, note that there could be only one multicast routing socket open
> > > at a time (across all processes).
> > >
> > > Regards,
> > > Pavlin
> > >
> > >
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