For years, we have been keeping non-upstream patch

  patches.suse/netfilter-ip_conntrack_slp.patch

in our kernels. The patch adds conntrack helper for SLP protocol. However, the
patch has been rejected by upstream

  https://www.spinics.net/lists/netfilter-devel/msg26254.html

because since kernel 3.6, there is a netfilter infrastructure for userspace
conntrack helpers and in kernel helpers are deprecated.

The SLP helper is trivial so that it won't be difficult to write a userspace
helper for conntrack-tools package (or even use e.g. existing one for mdns)
and use it.