Hi, please note that this is for discussing a specific task: "Extend neighbhor discovery to support 6LoWPAN", this discussion should be on a separate thread on lwip-devel list.

Short answer: as always we'll do our best to support RFCs, but since this is a lightweight stack we may limit some functionality.

If you have any comments/suggestions/questions regarding 6LowPAN, please send them to the lwip-devel list, I'd be very happy to discuss more there.

Netif goes below layer 3, and can have many functions. Adaptation and Layer 2 definitely fit there. You could split it more, but in a Light-weight system it fits well in a single layer.

I am posting a patch with the current 6LowPAN code, until it is ready for merging to head.

6LoWPAN compresses/decompresses the IPv6 header and adds new features for ND. I don't know why it is placed below netif. In fact, it should be the adaption layer between layer 3 and layer 2, I think.

I don't think strict layer isolation is a must-have in a tiny embedded stack like lwIP where the smallest code size and memory requirements is the main goal (after basic security about input of course). We know what we are doing everywhere and reading the whole source code to understand how it works is not out of the question.

FYI I'm starting work on adding 6LowPAN support.

At the moment it is implemented as a netif.

I don't know if ND is a layer 3 protocol that is layer-2-aware, or viceversa. In practice, at least for LwIP, it is a mixed layer protocol.

I think that we need to follow a similar approach with 6LowPAN.

So far I've been able to run it with ND module as is, although I agree that some API to work with the router list would be beneficial. Integration of prefixes might require some hacking initially, especially as AFAIK there is no standard to assign prefixes for header compression. It seems to be ad hoc.

My best guess is that the info gathered by nd6/mld6 should be available, probably (desirable) by a callback function, so link-level protocols could use it. Some way to detect changes is also desirable.
(IMHO,) I don't really think the netif but the 6LoWPAN implementation should care about this. Point is: is there an open source 6LoWPAN implementation to provide hooks/callbacks to ?
Should this task turn to "write a 6LoWPAN module" ? (perhaps followed by a ROLL module ?)

For use cases that implement a 6LoWPAN inerface, the context identifiers and prefixes distributed as a part of neighbor discovery procedures need to be provide to the 6LoWPAN layer to enable efficient compression and decompression of the IP header. This interface today does not exist in lwIP as lWIP does not accound for the 6LoWPAN. It will be extremly useful to enable notifying new prefixes and context identifier to the netif which can set these for the 6LoWPAN layer. Below is a short diagram to depict position of 6LoWPAN when using lwIP stack.

          +----------------+------------------+
          |      ND6       |       MLD6       |   
          +----------------+------------------+
          |            IPv6/ICMPv6            |
          +-----------------------------------+
          |              netif                |
          +-----------------------------------+
          |             6LoWPAN               |
          +-----------------------------------+