Re: [v6ops] A common problem with SLAAC in "renumbering" scenarios

[Fernando Gont <[e-mail]>]

Hi, Tim,

On 26/2/19 06:17, Tim Chown wrote:
>> On 25 Feb 2019, at 18:36, Fernando Gont <[e-mail]> wrote:
>>
>> On 25/2/19 07:32, Richard Patterson wrote:
>>> On Sat, 23 Feb 2019 at 05:22, Fernando Gont <[e-mail]> wrote:
>>>
>>>> * As per RFC4862, it turns out that you cannot remove a stale prefix vy
>>>> sending an RA wiht a "Prefix Lifetime" of 0. SO, with current standards,
>>>> not even the CPEs can (even if they wanted to) do something to fix the
>>>> problem.
>>>
>>> The Valid Lifetime cannot be zeroed or shortened below 2 hours, but
>>> the Preferred Lifetime can.  So we can't invalidate the prefix, but we
>>> can deprecate it so it's not used for new outbound sessions.   This is
>>> what we've implemented in our CPEs, after an unavoidable change in
>>> prefix, and it seems to have mitigated (or reduced the impact of) the
>>> issue.
>>
>> Agreed. That said, with the current default lifetime values, things
>> become tricky:
>> At least in theory, you should be sending such RAs for "Valid Lifetime"
>> seconds. With a default of one month, that means that, in theory, you
>> should be sending such RAs for a month. And if there are multiple
>> crash-and-reboot events, you should advertise each of the stale
>> prefixes. (I assume the CPE just records the last one)
> 
> When / where was the default Valid Lifetime chosen to be one month?

Section 6.2.1 of RFC 4861:

      AdvPrefixList
                     A list of prefixes to be placed in Prefix
                     Information options in Router Advertisement
                     messages sent from the interface.

                     Default: all prefixes that the router advertises
                     via routing protocols as being on-link for the
                     interface from which the advertisement is sent.
                     The link-local prefix SHOULD NOT be included in the
                     list of advertised prefixes.

                     Each prefix has an associated:

                        AdvValidLifetime
                             The value to be placed in the Valid
                             Lifetime in the Prefix Information option,
                             in seconds.  The designated value of all
                             1's (0xffffffff) represents infinity.
                             Implementations MAY allow AdvValidLifetime
                             to be specified in two ways:

                               - a time that decrements in real time,
                                 that is, one that will result in a
                                 Lifetime of zero at the specified time
                                 in the future, or

                               - a fixed time that stays the same in
                                 consecutive advertisements.

                             Default: 2592000 seconds (30 days), fixed
                             (i.e., stays the same in consecutive
                             advertisements).


Cheers,
-- 
Fernando Gont
SI6 Networks
e-mail: [e-mail]
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