IPv6 prefixes longer then /64: are they possible in DOCSIS networks?

Dmitry Cherkasov doctorchd at gmail.com
Thu Dec 1 10:11:12 UTC 2011


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:

* 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;
* If the RA contains a prefix advertisement with the A bit set to 0,
the CM MUST NOT perform SLAAC on that prefix.

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
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.

You are completely right that prefix delegation enforce DHCPv6 so
SLAAC mentioned above can be used only for CMs, not for CPE.

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

Dmitry Cherkasov

2011/11/30 Brzozowski, John <John_Brzozowski at cable.comcast.com>:
> Technically this is not true.  SLAAC is not prohibited, it does come with
> 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
> =========================================
> John Jason Brzozowski
> Comcast Cable
> e) mailto:john_brzozowski at 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 at 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 at 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. There's
>>>> 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 is,
>>> 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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