Re: [v6ops] Combining IPv6 ND and DHCPv6 into a single, unified function

Brian E Carpenter <brian.e.carpenter@gmail.com> Mon, 27 November 2017 23:07 UTC

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Subject: Re: [v6ops] Combining IPv6 ND and DHCPv6 into a single, unified function
To: "STARK, BARBARA H" <bs7652@att.com>, Fred Baker <fredbaker.ietf@gmail.com>, Michael Richardson <mcr+ietf@sandelman.ca>
Cc: "dhcwg@ietf.org" <dhcwg@ietf.org>, "v6ops@ietf.org" <v6ops@ietf.org>, "6man@ietf.org" <6man@ietf.org>
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From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Organization: University of Auckland
Message-ID: <8a5feb3a-d18e-1773-0539-3da22e582c70@gmail.com>
Date: Tue, 28 Nov 2017 12:06:53 +1300
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On 28/11/2017 02:19, STARK, BARBARA H wrote:
>>> My understanding is that Fred Templin essentially wants to carry DHCP
>>> (configuration) options in ND/RA.
>>
>> On thing that I have never understood is the lack of an RA option that carries
>> any DHCPv6 option. This entire silly debate would end if RAs encompassed
>> DHCP options.
> 
> Thus forcing either routers or hosts (or both) to support two distinct mechanisms to do the same thing if there is to be any hope of interoperability. 

Yes and no. Let's consider a host that needs a particular piece of
information. As an example, consider the address of the next-hop router
for a given prefix. There are three separate parts to this:

1. Receiving some sort of message containing an appropriate option.
2. Decoding the option
3. Storing the result.

The important one is #3. The mechanism for storing the result
is unavoidable, and stateful, whatever the delivery protocol
and encoding.

In the real world, most hosts today have several versions of
#1 in their code: RA, DHCP, RADIUS come to mind. (What they
enable is another matter.)

So where's the duplication? It seems to be in the decoding
step. With a little software engineering, there could be
a single decoder that handles all the possible formats, and
doesn't care which protocol delivered it.

> And if the host supports both, then it also needs to implement code for which wins when both are present and the options provided have conflicting info.

I suspect that is needed anyway, in the general case.

> If you really want DHCPv6 options in RAs for closed ecosystem use cases, include a huge caveat that routers are not required to support both mechanisms -- routers are only required to do DHCPv6.

Yes, if an option is primarily defined for DHCP, that makes sense.

> Any host that only implements receipt of DHCPv6 options via RA must not expect to operate in an open ecosystem. Put the burden on the hosts.

Probably. Unless this thing becomes popular.

   Brian