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Re: IPv6 RA vs DHCPv6 - The chosen one?
From: Tomas Podermanski <tpoder () cis vutbr cz>
Date: Thu, 22 Dec 2011 21:04:42 +0100


On 12/21/11 9:40 PM, Ray Soucy wrote:
I'm afraid you're about 10 years too late for this opinion to make
much difference. ;-)

My opinion is that there is never too late to make thinks easier to
implement and operate, specially in situation when things are
unnecessary complicated. I do not hide that my opinion about SLAAC might
look extreme but I have wrote my reasons for that. I do not expect that
anything will be changed but the fact that I can see discussion about
that topic approximately 2 or 3 times per month  (v6ops, dhcwg, ipv6,
...) and that shows that this problem is pain for many people/ISP. Have
you ever seen similar discussion related to DHCP(v4). I have not,
because there nothing to discuss about. It just works. It works in
simple and logical way.

We have been running IPv6 in production for several years (2008) as
well (answering this email over IPv6 now, actually) yet I have
completely different conclusions about the role of RA and DHCPv6.

Well, then how many devices do you have in the network that uses IPv6?
Do you have implemented first hop  security? What will you do when some
user runs RA flood attack
(http://samsclass.info/ipv6/proj/flood-router6a.htm). What will you do
when some user runs NDP Table Exhaustion Attack
(http://inconcepts.biz/~jsw/IPv6_NDP_Exhaustion.pdf) ? It is quite easy
to bring IPv6 into a server subnet or a small office network. Providing
stable and secure connectivity into the network with thousands of access
port for the paying customers/users is really a serious issue today.

I am very interested how the sites with similar number of access ports
(users/customers) solve that problems. I can see that many ISPs prefer
to solve that by blocking whole IPv6 traffic instead. But it does not
look as a good strategy for deploying IPv6 :-(.  


On Wed, Dec 21, 2011 at 2:16 PM, Tomas Podermanski <tpoder () cis vutbr cz> wrote:

from my perspective the short answer for this never-ending story is:

- SLAAC/RA is totally useless, does not bring any benefit at all
 and should be removed from IPv6 specs
- DHCPv6 should be extended by route options as proposed in
- DHCPv6 should be extended by layer 2 address to identify
 client's L2 address (something that we can see in RFC 6221)
- DHCPv6 should be the common way to autoconfigure an address
 in a IPv6 environment

The long answer is:

I completely disagree with opinion that both DHCPv6 and RA (SLAAC)
should be supported. It is easy to say that both have place but it has
some consequences. I and my colleagues have been working on deploying
IPv6 for a few years and from the operation perspective we conclude into
a quite clear opinion in that area. Both SLAAC and DHCPv6 uses a
opposite principles although the goal is just one. DHCPv6 is based on a
central DHCPv6 server that assigns addresses. SLAAC does opposite -
leaves the decision about the address on a client side. However we have
to run both of them in a network to provide all necessary pieces of
information to the clients (default route and DNS). This brings many
implementation and operational complications.

- Clients have to support both SLAAC and DHCPv6 to be able to work in
 both environments
- There must be solved a situation what to do when SLAAC and DHCPv6
 provides some conflict information (quite long thread with various
 can be found at
- The first hop security have to be solved twice - for SLAAC and for
DHCPv6. Both
 of then uses a differed communication way. SLAAC is part of ICMPv6,
but DHCPv6
 uses "own" UDP communication what does not make things easier.
- SLAAC is usually processed in a kernel, DHCPv6 is usually run as a
 process in the user space. Diagnostic and troubleshooting is more
- DHCPv6 is currently tied with SLAAC (M,O flags), what means that
 a DHCPv6 client have to wait until some RA message arrives to start DHCPv6
 discovery. That unnecessary prolongs whole autoconfiguration process.

Some other issues are also described in [1].

I personally prefer to bury SLAAC once forever for several reasons:
- In fact SLAAC does nothing more what DHCPv6 can do.
- SLAAC is quite difficult to secure. One (really only ONE)  RA packet
can destroy
 the IPv6 connection for all clients in a local network just in a few
 It also happens accidentally by "connection sharing" in Vista, Win7

- The full protection against that behavior it's impossible today.
RA-Guard or
 PACL can be bypassed using extension headers or fragmentation
- With SLAAC is difficult to implement security features like ARP/ND
 protection/inspection, IP source guard/Dynamic lock down, because
 all this techniques are based on a MAC-IP database created during
 a communication with a DHCP server. There are some attempts (ND
protection, SAVI)
 but it does not provide the same level of security.
- Just the same technique was introduced in IPv4 as Router Discovery
(RFC 1256).
 Nobody uses it today. Do we really need go through same death way again?
 (Oh right, we are already going :-( )

Comparing to SLAAC, DHCPv6 have several advantages:
- DHCPv6 is very similar to DHCP(v4) and many people are used to using it.
- DHCPv6 can potentially do much more - for example handle an information
 for PXE, options for IP phones, prefix delegation.
- DHCPv6 allows handle an information only for some hosts or group of
 hosts (differed lease time, search list, DNS atc.). With SLAAC it is
 impossible and all host on a subnet have to share the same set of
 the configuration information.
- Frankly said, I have not found any significant benefit that SLAAC brings.

Unfortunately there is another issue with DUIDs in DHCPv6. But it is a
little bit differed tale.

At the beginning the autoconfiguration was meant as easy to use and easy
to configure but the result turned out into kind of nightmare. For those
who do not know what I am talking about I prepared two images. The first
one shows necessary communication before first regular packet can be
send over IPv4 (http://hawk.cis.vutbr.cz/~tpoder/tmp/autoconf/IPv4.png)
and just the same thing in IPv6
(http://hawk.cis.vutbr.cz/~tpoder/tmp/autoconf/IPv4.png). In IPv4 we
have very simple answer if somebody asks for autoconfiguration  = use
DHCP. In IPv6 the description how things work have to be written into
more than 10 pages [1]. I believe that is not what we really wanted.

For those who are interested in that area there are several
articles/presentations where we mentioned that topic.

[1] IPv6 Autoconfiguration - Best Practice Document

[2] IPv6 - security threads

[3] Deploying IPv6 in University Campus Network - Practical Problems

   Tomas Podermanski

On 12/20/11 8:31 AM, Owen DeLong wrote:
Different operators will have different preferences in different environments.

Ideally, the IETF should provide complete solutions based on DHCPv6 and
on RA and let the operators decide what they want to use in their environments.


On Dec 19, 2011, at 10:27 PM, Ravi Duggal wrote:


IPv6 devices (routers and hosts) can obtain configuration information
about default routers, on-link prefixes and addresses from Router
Advertisements as defined in   Neighbor Discovery.  I have been told
that in some deployments, there is a strong desire not to use Router
Advertisements at all and to perform all configuration via DHCPv6.
There are thus similar IETF standards to get everything that you can
get from RAs, by using DHCPv6 instead.

As a result of this we see new proposals in IETF that try to do
similar things by either extending RA mechanisms or by introducing new
options in DHCPv6.

We thus have draft-droms-dhc-dhcpv6-default-router-00 that extends
DHCPv6 to do what RA does. And now, we have
draft-bcd-6man-ntp-server-ra-opt-00.txt that extends RA to advertise
the NTP information that is currently done via DHCPv6.

My question is, that which then is the more preferred option for the
operators? Do they prefer extending RA so that the new information
loaded on top of the RA messages gets known in the single shot when
routers do neighbor discovery. Or do they prefer all the extra
information to be learnt via DHCPv6? What are the pros and cons in
each approach and when would people favor one over the other?

I can see some advantages with the loading information to RA since
then one is not dependent on the DHCPv6 server. However, the latter
provides its own benefits.

Ravi D.

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