Layer 2 Multicast

[Fábio Oliveira]

Hi,

I'm working in some Applications and I'm using PacketSocket.

Using only layer 2 is it possible to use Multicast to send frames? And is it possible to define the Multicast groups in run time or do I have to hard-coded in pre-configuration?

Regards,
Fábio Oliveira

[Tommaso Pecorella]

Hi Fábio,

usually L2 only defines unicast and link-local broadcast addresses. However, some technologies may support multicast (e.g., LTE).

It all depends on the MAC and PHY.

Cheers,

T.

[Fábio Oliveira]

Hi Tommaso,

I'm using CSMA and I wan't to know if it is possible to send frames in multicast without L3 configured? All examples that I found use L3, but I'm not interested in that.

And if multicast only with L2 is possible, can I do it in run time or have I to hard-coded in pre-configuration of simulation?

Thanks. Regards,

Fábio

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[Tommaso Pecorella]

Just a question. Why should I dig the code and the standard for you ?

Is my time so less valuable than your ?

Obviously not. It's just that I am less lazy, more curious and definitely more stupid (otherwise I'd simply reply to go and check the standard).

Multicast at L2 in Ethernet.

- http://en.wikipedia.org/wiki/MAC_address - The "b1" bit gives an hint: unicast / multicast. Ethernet does have multicast addresses ("FF:FF:FF:FF:FF:FF" is the broadcast for the records).

- mac48address.h has more hints:

  /**
   * \param address base IPv4 address
   * \returns a multicast address
   */
  static Mac48Address GetMulticast (Ipv4Address address);


  /**
   * \brief Get multicast address from IPv6 address.
   * \param address base IPv6 address
   * \returns a multicast address
   */
  static Mac48Address GetMulticast (Ipv6Address address);


  /**
   * \returns the multicast prefix (01:00:5e:00:00:00).
   */
  static Mac48Address GetMulticastPrefix (void);


  /**
   * \brief Get the multicast prefix for IPv6 (33:33:00:00:00:00).
   * \returns a multicast address.
   */
  static Mac48Address GetMulticast6Prefix (void);

- However, a fast check in CsmaNetDevice::Receive can tell you that any "multicast" packet will be received by the NIC, no matter what the address is.

In other terms, there is no L2 filtering of the incoming "multicast" packets. Indeed, the standard does not state if and how the L2 filtering must be implemented, or if the multicast filtering must be done at L3.

As you can see, if you plan to manage the multicast packets at L2, you'll have to modify the actual csma NetDevice code. How you modify it will tell you if you can modify the groups at run time or not.

T.

[Fábio Oliveira]

Thanks for your reply.

Fabio