Implementing IGMP protocol and multicast support

[podhrmic]

Hello, for our application I need smoltcp to handle multicast packets (the application communicates over multicast). I am happy to do that and implement this functionality to smoltcp, but I would like to get some pointers and guidance first.

1. IGMP protocol will have to be implemented - I would add another branch on the match expression here https://github.com/m-labs/smoltcp/blob/master/src/iface/ethernet.rs#L249
2. Implement EthernetProtocol::IGMP with underlying logic
3. when IGMP (and multicast) is enabled we check dst_addr() against the list of multicast MAC addresses we are subscribed to (here: https://github.com/m-labs/smoltcp/blob/master/src/iface/ethernet.rs#L244 )
4. appropriately handle timeouts (see https://en.wikipedia.org/wiki/Internet_Group_Management_Protocol ) - not sure if we need a separate timer or we should use the time supplied to iface.poll()

Does it sound like a good approach to you?

[whitequark]

Sounds good! I suggest working on it in small pieces and seeking review early, as I try to ensure the quality of code in smoltcp stays very high.

[whitequark]

@podhrmic This is done, right?

[podhrmic]

@whitequark there is a pending PR #236 but I got to a halt with the ManagedMap(see the comments there). I got the fragmentation working properly in my fork using ManagedSlice so it might be used for reference.

[whitequark]

@podhrmic That PR is for fragmentation but this issue is for IGMP; I thought IGMP fully works now

[podhrmic]

Yes, I believe IGMP support was added in #178