Re: [dhcwg] Follow up from IETF-95 - draft-ietf-dhc-dhcpv6-prefix-length-hint-issue

["Bernie Volz (volz)" <[e-mail]>]

Some additional comments below (BV>).


-          Bernie

From: tianxiang li [mailto:[e-mail]]
Sent: Thursday, May 19, 2016 10:43 AM
To: H. Peter Anvin <[e-mail]>
Cc: Bernie Volz (volz) <[e-mail]>; 神明達哉 <[e-mail]>; dhcwg <[e-mail]>
Subject: Re: [dhcwg] Follow up from IETF-95 - draft-ietf-dhc-dhcpv6-prefix-length-hint-issue

Hi,

Thank you for providing your valuable comments regarding the prefix-length hint issues draft. Please see my response inline below(TX>).  Please correct me where I'm wrong, and whether I left out anything.

Best regards,
Tianxiang

2016-05-19 16:28 GMT+08:00 H. Peter Anvin <[e-mail]<mailto:[e-mail]>>:
On 05/02/16 10:57, Bernie Volz (volz) wrote:
> Jinmei:
>
> Here's I think a simple example ...
>
> Requesting router solicits for a /56.  Delegating router has none to
> offer (perhaps temporary issue or the configuration doesn't currently
> allow). So, delegating router returns a /60. The requesting router
> uses this.
>
> Now, it does periodic renewals. If it just sends the delegated-prefix
> (/60) in the IA_PD, the server will keep renewing this. Since the
> delegating router has no other information.
>
> The current RFC3633 never says that the requesting router should
> continue to include a ::/56 as a hint in the renewal (though it also
> doesn't prohibit it).
>
> Thus, even if the delegating router is reconfigured (or the temporary
> issue is resolved), the requesting router would continue to use the
> /60 (perhaps not being able to operate fully). Yet, if the requesting
> router included the ::/56 as a hint (in addition to the /60
> delegated-prefix), the delegating router COULD assign it a /56 when
> it was able to.
>
> There's also a separate issue of if the requesting router where to
> Solicit (and say for the /56 hint), should the server abandon the /60
> that it might still have leased or prefer a /56 (or something shorter
> than a /60 it has)? That might be less of an issue for this case as
> that would generally just be up to the server's policy (as to how
> strong the hint it treated).
>
> To handle the case I mentioned above, we did resort in our server to
> remembering the hint the client provided during the Solicit so that
> we can re-evaluate as to whether to keep renewing the current
> delegated prefix or re-evaluate whether we had something "better" to
> offer the client.
>
> Note that this works both ways - a delegating router might only have
> had a /56 to delegated when the client requested a /60, but that
> generally is a less interesting case because the requesting router
> can just use the longer prefix.
>
Funny enough, I just went to this list with the explicit intent to ask
some of the questions that I believe this document ought to answer, but
quite frankly it doesn't look from my reading that it does.
TX>The draft mainly covers issues discussed in the previous IETF meetings and the mailing list, it might not be complete, so we appreciate any suggestions on what problems or solutions we haven't covered regarding the prefix-length hint.

I am concerned about client behavior personally, so this is from this
perspective.  This comes from my having written a patchset (which I need
to update) for ISC dhclient to do prefix requests.

My concern is when the ::/n prefix hint should be included.  My
assumption has always been that a single IA_PD always correspond to
prefixes that are to be configured together, whereas multiple IA_PD
delegations would mean prefixes for separate entities.  If this is NOT
the intent it probably needs to be clarified.
TX>Currently in the draft, we recommend the requesting router to use one IA_PD option, to contain two IA_PREFIX options. One IA_PREFIX option containing the current delegated prefix, the other containing the prefix-length hint. So even if the delegating router is able to assign a new prefix to the client, it would be in the same IA_PD option with the same IAID.

It is also worth noting that a server which receives requests for
multiple IA_PD delegations is free to aggregate those if appropriate,
although I have no idea if actual servers in the field do.  It is still
unclear to me if it would be better for the client to push multiple
IA_PD options upstream -- giving the server the option to delegate or
not delegate -- or if it should forcibly aggregate at the client level.
TX>Aggregation is indeed a problem regarding multiple IA_PDs, although the draft right now does not cover the that scenario. This is one of the complexities we tried to avoid.

Bernie raises several of the same issues that I tripped over, not
surprising.


Now, what I think IS clear from the client point of view:

a) When creating a SOLICIT message, and there currently is no lease for
an existing prefix of the required length, the SOLICIT message SHOULD
include the ::/n hint for the desired prefix length.

TX>This is one of the intended uses for the prefix-length hint in the Solicit message, we will add some text to explain it clearer in the draft.

b) When creating a SOLICIT message and there are prefixes which have
been used in the past, they SHOULD be included even if they are too narrow.

Thus, an IA_PD request might look like:

        2001:db8:85a3:2345::/64
        ::/60

meaning "please give me a /60 if you can, otherwise please give me
2001:db8:85a3:2345::/64 [again]".
TX>I think case b) is a bit more complicated as the client's preference for the prefix-length could have changed since its last prefix delegation, so the previous prefix might not be suitable anymore. We could add a sentence saying "If the requesting router's configuration did not change during this period..."

BV> My personal opinion is what value is there here in the client including the /64. Sure, it could, but why should it if it really isn’t what it wants? Again, if the client does want to include /64, it should send both. But in this case, I see little reason for it to send the /64.

BV> I also think not including the /64 is better because it is more clear that client wants a /60.


What I think is NOT clear from the client point of view:

a) When creating a SOLICIT message, and we have had at least one prefix
of sufficient length, SHOULD ::/n be included?  That is, should an IA_PD
request look like:

        2001:db8:85a3:7890::/60
        ::/60
TX>I think the latter is implied by the former.  Currently in the draft, if the client requests for a specific prefix (e.g. 2001:db8:85a3:7890::/60), the server will first try to return the requested prefix, if it is not available,  the server will try to assign a prefix with the same length (/60). So I don't think the ::/n prefix-length hint still needs to be included in this case.

BV> That is a very good question. It really depends on what each type of hint means. I think we’re all clear that ::/60 means “Client wants a /60”. But does “2001::/60” means “give client that prefix or, if server can’t, /60 is the hint” (assuming no other explicit hint is included). I think we could definitely decide that if a client sends <prefix>/<length> and not ::/<length>, then length should be treated as the hint if the explicitly requested prefix isn’t available. But I don’t think the current 3633 (or the bis document) reads like that.

... or is the latter implied by the former, even if that specific prefix
is unavailable?  What is the request looks like:

        2001:db8:85a3:7890::/60
        2001:db8:85a3:2345::/64
TX>We don't yet cover the use case of the client requesting more than one specific prefix, does there exist such a use case?

BV> This one does make the previous issue more complex as is the /60 or /64 the hint if the server can’t grant the requested value. That is why the ::/<length> might be best to be explicit always as to the hint.

b) Are there messages *other* than SOLICIT where the hint SHOULD get
included?
TX>The client could also include the prefix-length hint in the Renew/Rebind message. For example, if the client's hint was not honored during Solicit, the client could include the prefix-length hint in the Renew/Rebind message, to see if the server has the prefix available at that time. This is covered in section 3.4 and 3.5 of the draft.

BV> Correct, This all applies to Renew and Rebind!!!

c) Whether or not the client SHOULD be encouraged to transmit a hint if
it has a *wider* prefix assigned than it needs?

[My feeling is that we want to encourage the client to send the hint for
the preferred prefix length, longer or shorer, in SOLICIT, RENEW and
REBIND messages unless servers in the field are known to malfuction that
way.]
TX>The ideal case would be that the client gets exactly what it needs. Thus, saving the wider prefix for clients that could not function without it. However, this would cause a lot of complexities in the protocol, as both clients with a "wider" and "shorter" prefix would continue to request for the preferred prefix-length, which would also cause a lot of burden on the server. I don't know whether it is worth the cost. Especially, if this is going to be a Standards Track document, then it has to be the desired behavior for all clients.

BV> I don’t think there is any harm. It may cost the server some processing to ‘see’ if there is a better prefix available.

BV> One issue here that is important to consider is that a wider prefix (shorter prefix length) might be best to retain as it avoids renumbering (either if server was able to grant less wide (longer length) or if something changes on the requesting router – more interfaces added). I could see a client doing this on a Solicit but less likely to do it on a Renew/Rebind to avoid renumbering.


d) If the client MUST/SHOULD to forcibly do aggregation or not, in the
case that it is OK for the client side to deal with multiple disjoint
prefixes.

[I suspect that current servers will not deal well with client
nonaggregation.]

e) If the client SHOULD or SHOULD NOT to respond to a narrower prefix
than requested by issuing a new request for multiple prefixes (via
multiple IA_PD options).
TX>Right now in the draft, if the client received a narrower prefix than it requested, it is encouraged to include the prefix-length hint in the Renew/Rebind messages(in one IA_PD option, using two IA_PREFIX options), and do a graceful switch over if the server is able to grant the desired prefix (section 3.4 and 3.5 of the draft).

BV> This is what we want.

f) If there is ever a reason to NOT send a prefix hint?  Would that
implicitly mean ::/64, and if so, should that be documented?
TX>If the client does not send a prefix-length hint, then how the server act would depend on the server default policy. The servers might return what the client got the last time, or it could return a prefix of default length (Which could be a /64).

BV> That is why I think sending the hint is good – if the server must grant a new prefix (old cannot be extended for whatever reason), it knows what prefix length to use in providing a new one. (Note: Of course, if we make it clear that a 2001::/<length> IAPREFIX means the hint is <length> unless another explicit hint (::/<length>) is provided, then we can relax the need to send an explicit hint when the lengths are equal.)

        -hpa