[lwip-devel] [bug #49125] Need a way to iterate multicast groups for MAC filtering

[Daniel Elstner]

URL:
  <http://savannah.nongnu.org/bugs/?49125>

                 Summary: Need a way to iterate multicast groups for MAC
filtering
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: danielk
            Submitted on: Mon 19 Sep 2016 04:42:14 AM CEST
                Category: None
                Severity: 3 - Normal
              Item Group: Feature Request
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head

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Details:

In my Ethernet interface code, I am implementing the MLD MAC filter callback
in order to reconfigure the hardware address matching of the MAC controller.

Although I got it working in principle, the implementation is unnecessarily
complicated because the API does not provide a way to iterate over all
multicast groups of a netif. My hardware supports multicast address filtering
by means of matching a hash, and the hash is a function of *all* multicast
addresses I want to match. This means that using the current API, I need to
keep track of added/removed IPv6 multicast addresses, essentially duplicating
information that the MLD implementation already keeps track of. This wastes
memory and complicates the code.

Actually even with table-based address matching hardware, it would be
necessary to keep track of added groups just to know which table indices they
occupy. So this is not limited to hashing.

Simply making the "mld_group_list" variable public would already be enough to
fix this, since it is easy enough to skip over other netifs while iterating.
If that's considered too internal to expose, the MLD code could perhaps be
changed to store the groups directly in the netif struct.

The same considerations would also apply to IGMP. Interestingly, there is this
comment in the IGMP code:

 * igmp group structure - there is
 * a list of groups for each interface
 * these should really be linked from the interface, but
 * if we keep them separate we will not affect the lwip original code
 * too much

I'd say linking the groups from the interface would make life a lot easier for
anyone implementing the MAC filter callbacks.




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