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Re: IPv6 prefixes longer then /64: are they possible in DOCSIS networks?
From: "Brzozowski, John" <John_Brzozowski () Cable Comcast com>
Date: Fri, 2 Dec 2011 02:33:43 +0000

See below.

On 12/1/11 5:11 AM, "Dmitry Cherkasov" <doctorchd () gmail com> wrote:


Due to your note I carefully read again Cable Labs specs and found
that really SLAAC is not prohibited. According to CM-SP-MULPIv3.0:
[jjmb] I was part of the team that wrote IPv6 for DOCSIS, so I know the
history well.  ;)

* If the M bit in the RA is set to 1, the CM (cable modem) MUST use
DHCPv6 ...;
* If there are no prefix information options in the RA, the CM MUST
NOT perform SLAAC;
[jjmb] even if there are PIOs and the A bit is set to 0, the CM will
not/must not perform SLAAC.

* If the RA contains a prefix advertisement with the A bit set to 0,
the CM MUST NOT perform SLAAC on that prefix.
[jjmb] yes, see above.

That means that if M bit in the RA is set to 0 and RA contains a
prefix advertisement with the A bit set to 1 nothing prevents CM from
[jjmb] correct.

And if so we probably better reserve /64 per network just in case we
may use SLAAC in it in the future. While we do not use SLAAC we can
shorten the range of actually used IPv6 addresses by using longer then
/64 prefix.
[jjmb] I suppose, again not sure why you would want to take this route.
This also assumes no PIOs in the RA.  Please note there are other
operational reason why SLAAC is not a truly deployable alternative.  We
can discuss off list if you are interested.

You are completely right that prefix delegation enforce DHCPv6 so
SLAAC mentioned above can be used only for CMs, not for CPE.
[jjmb] similar to cable modems, CPEs that only request or require IA_NA
could conceivably use SLAAC.  Same caveat and comments as above.

Just a note: as far as I can see available DOCSIS 3.0 CMTSes do not
support the ability of SLAAC for CMs currently (checked Casa and Cisco
[jjmb] I am sure you make it work on at least one of the above. :)

Dmitry Cherkasov

2011/11/30 Brzozowski, John <John_Brzozowski () cable comcast com>:
Technically this is not true.  SLAAC is not prohibited, it does come
side affects that complicate the deployment of IPv6.  It is technically
feasible to use SLAAC, it is just not practical in most cases.

Stateful DHCPv6 is the preferred mechanism for address and configuration
assignment.  Prefix delegation requires the use of stateful DHCPv6 in
DOCSIS networks.

John Jason Brzozowski
Comcast Cable
e) mailto:john_brzozowski () cable comcast com
o) 609-377-6594
m) 484-962-0060
w) http://www.comcast6.net

On 11/29/11 7:09 AM, "Dmitry Cherkasov" <doctorchd () gmail com> wrote:


SLAAC is prohibited for using in DOCSIS networks, router
advertisements that allow SLAAC must be ignored by end-devices,
therefore DHCPv6 is the only way of configuring (if not talking about
statical assignment). I have seen at least Windows7 handling this
properly in its default configuration: it starts DHCPv6 negotiation
instead of auto-configuration.

Dmitry Cherkasov

2011/11/29 Steven Bellovin <smb () cs columbia edu>:

On Nov 28, 2011, at 4:51 52PM, Owen DeLong wrote:

On Nov 28, 2011, at 7:29 AM, Ray Soucy wrote:

It's a good practice to reserve a 64-bit prefix for each network.
That's a good general rule.  For point to point or link networks you
can use something as small as a 126-bit prefix (we do).

Technically, absent buggy {firm,soft}ware, you can use a /127.
actual benefit to doing anything longer than a /64 unless you have
buggy *ware (ping pong attacks only work against buggy *ware),
and there can be some advantages to choosing addresses other than
::1 and ::2 in some cases. If you're letting outside packets target
point-to-point links, you have bigger problems than neighbor table
attacks. If not, then the neighbor table attack is a bit of a

The context is DOCSIS, i.e., primarily residential cable modem users,
the cable company ISPs do not want to spend time on customer care and
hand-holding.  How are most v6 machines configured by default?  That
what did Microsoft do for Windows Vista and Windows 7?  If they're set
stateless autoconfig, I strongly suspect that most ISPs will want to
with that and hand out /64s to each network.  (That's apart from the
question of why they should want to do anything else...)

               --Steve Bellovin, https://www.cs.columbia.edu/~smb

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