Re: problem statement [was Re: New Version Notification for draft-hinden-ipv4flag-00.txt]

Brian E Carpenter <> Wed, 22 November 2017 02:32 UTC

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Subject: Re: problem statement [was Re: New Version Notification for draft-hinden-ipv4flag-00.txt]
To: David Farmer <>
Cc: Nick Hilliard <>, Lorenzo Colitti <>, IETF IPv6 Mailing List <>
References: <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <>
From: Brian E Carpenter <>
Organization: University of Auckland
Message-ID: <>
Date: Wed, 22 Nov 2017 15:32:27 +1300
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On 22/11/2017 13:42, David Farmer wrote:
> On Tue, Nov 21, 2017 at 6:15 PM, Brian E Carpenter <
>> wrote:
>> On 22/11/2017 11:33, Nick Hilliard wrote:
>>> Lorenzo Colitti wrote:
>>>> On Wed, Nov 22, 2017 at 12:47 AM, Nick Hilliard <>
>> wrote:
>>>>> There would also be a requirement
>>>>> for the network edge to be able to filter out RAs with this option,
>>>> Why is it not sufficient to configure the routers that emit the RAs?
>>> because you're assuming congruity between ipv4 and ipv6 connectivity on
>>> a network, which is not codified in any ietf document that I'm aware of.
>>>  If there is a incongruity between management of the two protocols, then
>>> the l2 network must have a mechanism to stop an ipv6 gateway from
>>> attempting shutting down ipv4 services.
>> This is a very interesting point. If every IPv6 router on a link believes
>> there is no IPv4 router/DHCP service, they would all send flag==1 in the
>> proposed solution. So the proposal only works if at least one IPv6 router
>> is colocated with an IPv4 router, and therefore knows to send flag==0.
>> Oops.
> This is a problem only if the flag is set automatically and their is no
> mechanism to override the automatic setting.  Most routers with an IPv6
> address configured on an interface by default will send an RA unless you
> configure them not to.  So, an IPv6 router without IPv4 configured on the
> interface should set this flag, unless configured otherwise.  In other
> words you need to be able to tell an IPv6 router there is IPv4 on the
> interface even if router is not providing it.
> It is quite possible that the IPv6 topology is incongruous from the IPv4
> topology, however it is doubtful than the management is also incongruous.

Yes, but do you think we should push a mechanism that assumes correct
configuration by humans in order to work reliably? 
(Arguing against my own proposal, not for the first time ;-).