Proposal: remove IPv6-only RA draft bits to adopt DHCP option (RFC 8925)

Hi everyone,

There is an implementation of the DRAFT_IETF_6MAN_IPV6ONLY_FLAG draft in 
the OS. It is the excellent work of Bjoern (@bz), both the 
Internet-Draft and its implementation.

I'm requesting removal of the draft-specific bits (which is not compiled 
by default), but first a short history from an outsider's reading of the 
IETF archives.

The draft's history is unfortunate. @bz had a great idea about making a 
network automatically become IPv6-only by advertising it as a RA flag.
However, the idea had a small flaw: RAs can be trivially forged and 
could be used to maliciously disable v4 networks, so RA was not a safe 
transport for such a flag.
IMHO, the same attack surface could exist for DHCP, but DHCP deployments 
are commonly protected by DHCP snooping in practice.
That led to the conclusion that a DHCP option would be a safer place for 
this signal.
The draft was eventually abandoned (mailing-list archive: 
https://mailarchive.ietf.org/arch/msg/ipv6/7nwZ6BUqbSqEC11eTqVqCOZwGI8/).

Shortly after, someone else (google) submitted the same idea as a DHCP 
option, which became RFC 8925.
Although the original idea came from Bjoern, neither his name nor his 
draft is acknowledged in that RFC.
I have not discussed this with Bjoern (cc'ed), only observed the 
sequence of events.
I appreciated his work, it appears to be his last draft.

We should move forward and align with RFC 8925.
I use the DHCP option at my company and at home, mobiles and most 
devices support it well.
I'd like to make this work on my FreeBSD boxes as well.

In short, I'm asking for willingness to remove or replace the 
EXPERIMENTAL/DRAFT_IETF_6MAN_IPV6ONLY_FLAG bits and adopt the 
DHCP-option-based approach (RFC 8925).
The current code locations referencing the draft are:
Kernel:
sys/netinet6/nd6_rtr.c: lines 107–115, 251–355, 602–604, 782–784
sys/netinet6/nd6.h: lines 77–82
sys/netinet/icmp6.h: #define ND_RA_FLAG_IPV6_ONLY 0x02
sys/net/if_ethersubr.c: lines ~478–497, 544–560

Userland:
grep -r DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/rtadvd/rtadvd.c:#ifdef DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/rtadvd/Makefile:CFLAGS+=     -DDRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/rtadvd/config.c:#ifdef DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/rtadvd/config.c:#ifdef DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/rtadvd/config.c:#ifdef DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/rtadvd/rtadvd.h:#ifdef DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/ndp/Makefile:CFLAGS+=        -DDRAFT_IETF_6MAN_IPV6ONLY_FLAG
./usr.sbin/ndp/ndp.c:#ifdef DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./sbin/ifconfig/af_nd6.c:#ifdef DRAFT_IETF_6MAN_IPV6ONLY_FLAG
./sbin/ifconfig/Makefile:CFLAGS+= -DDRAFT_IETF_6MAN_IPV6ONLY_FLAG

The existing implementation is reusable, but I want to ensure Bjoern and 
others are comfortable with reworking/removing the draft-specific code 
and moving to RFC 8925.
Please reply if you have concerns, objections, or if you're ok with this 
removal of this option.

-- 
Pouria