Leino, Tammy wrote:
Iljitsch, thank you for your comprehensive remarks. I think my
mistake was in believing an IPv6 router does not have to be
configured to send RAs, but the DHCPv6 server could serve the same
purpose as the RAs. It appears DHCPv6 was meant to supplement RAs.
As an embedded developer, there is a lot of overhead in terms of code
space and memory requirements in the DHCPv6 protocol, particularly
for customers wishing to just distribute DNS servers. It would be
nice if all the configuration options of DHCPv6 had been added to
RAs. This way, we would save on memory.
Errr... the RAs would become larger and bandwidth would be wasted sometimes.
I'd rather extend the RA to include the DHCP Server address such that
the mobile does not need to discover the DHCP Server address, thus
gaining time by eliminating Solicit and Advertise DHCP messages.
But these things are relatively widely implemented and modifying
anything in the existing operation risks taking a lot of time...
Alex
Thank you again for your time and advice.
Best Regards, Tammy
-----Original Message----- From: Iljitsch van Beijnum
[mailto:[EMAIL PROTECTED] Sent: Monday, August 13, 2007 6:18 AM To:
Leino, Tammy Cc: John Jason Brzozowski (JJMB); Hemant Singh
(shemant); [email protected] Subject: Re: prefix length determination for
DHCPv6
On 11-aug-2007, at 1:09, Leino, Tammy wrote:
The reason I am not assuming there is a router on link configured
to send RAs with prefix options is because I don't see the point of
DHCPv6 configuring addresses if a router is configured to do the
same job.
:-)
Since the prefix length is carried in a prefix option of an RA, I
have to assume these will be absent on the link; otherwise, the
node would not be using DHCPv6 to obtain an address.
But then, why do you need addresses when you have no external
connectivity? A lot of stuff will work just fine over link local
addresses, although the need to include a scope identifier gets in
the way of some applications.
(There is of course the consideration that at this time, very
few
IPv6 implementations can configure an address through DHCPv6.)
Is this because of some shortcoming(s) in the specification or do
network managers find DHCPv6 unnecessary?
First of all, the DHCPv6 specification was finished relatively late,
in 2003 if I'm not mistaken. Most major vendors had already
implemented IPv6 by then and I'm guessing that deployment levels so
far haven't exactly given them much reason to drastically update
their IPv6 implementations.
Then there is the confusion about stateful and stateless DHCPv6 and
the whole discussion about the meaning of the M and O bits in router
advertisements. I think this created uncertainty in the market place
for some time.
Since address configuration has always happened and continues to
happen through stateless autoconfiguration, DHCPv6 address assignment
wasn't implemented widely, and you can only use this mechanism when
you can be sure that both the server and the clients support it. It
looks like now that IPv6 is gaining more widespread attention that
more people want this because it's the same as in IPv4, but "old
school" IPv6 users are generally happy with stateless
autoconfiguration.
That leaves:
If they are not using DHCPv6, how do they distribute DNS servers
and other configuration information?
In practice? They don't. As long as you have IPv4, you can use IPv4
for this.
Other than that, you mostly manually configure an address. Although
DHCPv6 allows you to do this automatically, the trouble is that you
can't be sure that there will be a DHCPv6 server everywhere you go
so in practice, the advantage of DHCPv6 over manual configuration is
limited. There was talk about using well-known anycasted addresses
for this but that never came off the ground.
In the near future we'll have draft-jeong-dnsop-ipv6-dns-
discovery-12.txt which is now in the RFC Editor queue. However, this
will have the same problem as DHCPv6: you can't be sure that someone
is making the info available until it's extremely widely
implemented.
I am interested in learning how well received DHCPv6 has been. We
have been slow to implement it in our OS because of low demand.
My opinion: DHCPv6 is a complex protocol that uses a relatively high
number of messages. In the past, UDP-based protocols have been
especially susceptible to security problems in their implementations.
As such, I'm very happy to do without it when I don't need it and I
feel stronly that DHCPv6 should NOT be initiated by clients by
default but only if RA flags indicate that they should do this, and
DNS configuration should be possible without DHCPv6.
Having said that, there can be situations where it's useful to assign
an address to an IPv6 host, so there is certainly a place for
DHCPv6. And prefix delegation is brilliant, this is a great way to
provision IPv6 CPEs with an address block.
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