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Question

Pardon my limited knowledge in networks.

I am trying to setup two small programs as client and server, which join to a particular multicast group. The servers sends some arbitrary data to the group and the client receives the data as it listens to the group.

This of course is possible with UDP programs which I have already done. However I need to work with MAC addresses instead of IP s. And I have a few questions regarding this:

1. Is it possible to fix a range of MAC addresses for my multicast application ? As I understand, the least significant bit of the first octet in the MAC address signifies if it is multicast or not. So the multicast MAC should look like 01:*:*:*:*:* . So I guess, there should be some way to use a wide range of MAC addresses (except for those reserved).

2. Many places it is written as All multicast MAC addresses begin with: 0100.5e . Is it always true? AFAIK this prefix is fixed in the MAC address which leaves space for the rest of the IP address bits to be mapped here. But what if my application doesn't care about IP addresses and listens to multicast groups based on MAC addresses (if it is possible) ?. Can't I keep the first octet as 01 and use a wide range for my multicast addressing ?

3. How to send packets to a multicast group and listen to a multicast group only based on MAC addresses and irrespective of IP ? ie the multicast group addresses are defined based on MACs.

I hope my questions made sense.

Answer 1

If you are not using IP (or UDP on top of that) then the default way of mapping IP addresses to mac addresses have probably no added value for you. These mappings are specified for example in:

• http://en.wikipedia.org/wiki/IP_multicast#Layer_2_delivery
• rfc1112 http://tools.ietf.org/html/rfc1112 paragraph 6.4

If you insist on using layer2 and multicasting it will probably be sufficient to send out frames with the LSB of the first octet set (the multicast bit, ref: http://en.wikipedia.org/wiki/File:MAC-48_Address.svg ). And put your own mapping on top of that. But you should take into account that:

• you probably need to force your NIC to pass the data, often NIC's filter out frames they are interested in hardware. And these filters are typically set when joining a group or eg when setting the ALLMULTI flag).
• There is probably networking equipment in the middle and some pseudo-intelligent switches may refuse to forward data unless they have seen IGMP joins (ref igmp snooping).
• And you code will obviously need to deal with raw layer 2 sockets to read out the data.

Considering the ease of just using UDP this may be a lot of work.