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?

  • What is not a use case about testing? Wouldn't you want to test a multicast application as you develop it? This allows you to test it on a single machine. – Ron Maupin Oct 28 '14 at 14:24
  • 1
    I don't think it isn't a valid use case. I'm just hoping it isn't the only use case. That's why I'm waiting before selecting an answer. – Eddie Oct 28 '14 at 14:33
  • You asked the question; you just don't like the answer. Testing is absolutely a valid use case. Suppose you want to develop a multicast application. If you can't test it locally, you risk flooding a network that others may use. Once the application works, you just turn off that bit and put it into production. – Ron Maupin Oct 28 '14 at 14:39
  • @RonMaupin Its not that I don't like your answer, I just wanted to keep the question open to see if there are other use cases. I knew already of the 'testing' use case, and it is absolutely a valid answer. But if you take a look, I did ask for some use cases, and not just a use case. – Eddie Oct 28 '14 at 15:00
  • OK, I understand. Just think what the unicast loopback is used for. The multicast loopback is used for the same sorts of things. – Ron Maupin Oct 28 '14 at 15:25
5

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

  • 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 – Sander Steffann 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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