6

On an IPv6 multicast packet, the last nibble of the first hextet represents the Scope of the packet. This defines effectively how far you want your multicast packet to travel.

The address makeup is as follows:

|   8    |  4 |  4 |              112 bits               |
+--------+----+----+-------------------------------------+
|11111111|flgs|scop|              group ID               |
+--------+----+----+-------------------------------------+

The "scop" bits can have any of the following values:

1  Interface-Local scope
2  Link-Local scope
4  Admin-Local scope
5  Site-Local scope
8  Organization-Local scope
E  Global scope

(all others are reserved / not in use)

My question is specifically surrendering the "Interface-Local" scope. I understand academically what it means, the multicast packet travels only so far as the local interface (aka, doesn't actually hit the wire). But practically, what are some use cases for the Interface-Local scope?

0
5

Node-local scope is mostly useful for inter-process communication. The sender and receiver can be different processes on the same node.

2
  • Can you provide any more detail or information about this? In what instances would I need one process to multicast a packet to multiple other processes? I understand process to process over unicast, but I'm still not fully understanding the multicast aspect of it.
    – Eddie
    Nov 3 '14 at 15:11
  • If you have one process providing information and multiple processes that want to hear it then multicast is much nicer than multiple-unicast sessions. Multicast is a nice publish/subscribe system Nov 3 '14 at 19:30
3

Just like the unicast loopback address (::1/128) it can be used for testing.

See RFC3513: "interface-local scope spans only a single interface on a node, and is useful only for loopback transmission of multicast."

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