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List:       keepalived-devel
Subject:    Re: [Keepalived-devel] about multicast
From:       Alexandre Cassen <Alexandre.Cassen () wanadoo ! fr>
Date:       2004-04-11 21:17:18
Message-ID: 6.0.1.1.2.20040411230331.01b89e08 () mail
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Hi Joe,

>Am wondering how multicast works when you have an IP sending/receiving
>multicast packets, but the IP is not configured on a NIC.
>Ganglia does this, copying data between nodes using multicast from/to
>239.2.11.71:8649. I see the IP with `netstat -an` but this IP
>is not seen with `ip addr show`.
>
>I seem to remember that keepalived uses multicast and I seem to
>remember a socket being involved, but I can't remember whether
>an IP was involved.
>
>I looked at the Linux Multicast HOWTO, but this only talks
>about multicast between machines which have class D addresses
>configured on NICs.

In mcast fashion, Class D address is not configured to NIC. you just join 
or leave the Class D, so called the mcast group. For mcast you can consider 
2 differents design, most of common applications using multicast are done 
over UDP, but you can also create your own mcast protocol as VRRP or HSRP 
does, that way you are using mcast at same layer as UDP without adding UDP 
overhead. Since mcast is not connection oriented the both design UDP or 
pure RAW protocol are allowed. This contrast with the new SCTP protocol 
which add retransmission and connection oriented design in a one-to-many 
design (called associations).

So in mcast you must distinguish the sending and the receiving source. if 
using the UDP transport then you can bind sending/receiving points to 
special IP. On RAW fashion, you bind directly to device. Keepalived/VRRP 
operate at RAW implementing its own protocol, use a pair of 
sending/receiving socket on each interface VRRP instances run.

Best regards,
Alexandre 




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