Re: [Softwires] I-D Action: draft-ietf-softwire-lw4over6-06.txt

[Suresh Krishnan <[e-mail]>]

Hi Woj,

On 02/25/2014 05:12 AM, Wojciech Dec wrote:
> Hi Qi,
>
> your answers didn't answer the majority of the concerns raised. And some
> of those raised by Ole still also stand:
> - Clean-up text that does not belong to lw46 (eg 2473 fixes, NAT44 best
> practice).
> - Clarification of WAN selection or assumptions
> - Clarify questionable use of ICMPv6 error code (to report lack of v4
> binding)
>
> I still see no reason why they cannot be addressed by edits to the draft
> that don't change the matter that lw46 is a standalone draft.

In all fairness, your comments came in a few hours before the draft 
submission deadline for this IETF. It is unreasonable to expect the 
issues to get addressed by draft changes, at least until the submission 
reopens. I do think the above points merit a response from the document 
editors.

 > - Text clarifying relation to MAP-E (both use MAP PSID algo, address
 > embedding, IPv4inIPv6 and NAT44 w/ address sharing. MAP allows state to
 > be ranged from N to 1 per CPE. lw46 focuses on the 1:1 case)

Why do you think this is required here? Isn't this the goal of the 
unified CPE work (to clarify the commonalities between MAP and lw4over6)?

Thanks
Suresh

>
>
> Snipped text Inline... Woj>
>
>
> On 21 February 2014 10:25, Qi Sun <[e-mail]
> <mailto:[e-mail]>> wrote:
>
>
>     Hi Woj,
>
>
>     On 2014-2-19, at 下午4:34, Wojciech Dec wrote:
>>
>>     Just to be clear: I'm ok with lw46 defining a specific functional
>>     mode as I believe it does in this draft,
>
>     [Qi] The outcome of ietf88 was lw4o6 and map are two mechanisms, but
>     both use the Softwire DHCP for provisioning (at least according to
>     my memory). That is why the WG decided to remove the statement of
>     "lw4o6 is a subset of MAP-E" is from the Softwire DHCP Option draft.
>
>     And lw4o6 can use DHCPv4 over DHCPv6 and PCP for provisioning. This
>     is documented in the current lw4o6 draft, and some teams are doing
>     the documentation work. (We have demoed in IETF85 that lw4o6 can
>     work with DHCPv4 over IPv6 and PCP). The point is, lw4o6
>     architecture is quite flexible.
>
>
>
> Woj>I didn't quite comment on the DHCP draft, nor on how flexible and
> clear *you think* the lw46 draft is, etc. I commented on the fact that
> that lw46 shares a lot with MAP-E and the fact that it is built on the
> same technology blocks as MAP-E, but realizes solely the 1:1 part should
> be clarified.
>
> Irrespective of this, there is overall no reason for having text in the
> lw46 draft that fixes 2473 or NAT44 best practices, etc.
>
>
>>     also leaving "as-is" the DHCP part of it (i.e. it's a capability
>>     that can be signalled using the lw46 container, etc).
>
>>     General items remain open (as commented):
>>     - Cleanup text that does not belong to lw46 (eg 2473 fixes, or
>>     NAT44 best practice).
>
>     [Qi] IMO, that text guarantees the interoperation between lwAFTRs
>     and lwB4s from different vendors, which bothered us a little when
>     carrying out the interop test. I think it doesn't hurt to make it
>     specific.
>
>
> Woj>A couple of points:  Firstly: Given that the lwB4 CPE uses the MAP
> algorithm for the PSID and embeds the IPv4+PSID in its IPv6 address, the
> text is now in shape that a lwB4 would be interoperable with a MAP-E BR.
> This is actually good convergence/progress. What remains open in that
> happening is actually weeding out the possible source of differences,
> which appear to be in the nuances of ICMP handling.
>
> Secondly: It claims to be a standards track document (currently), and it
> carries text that selectively duplicates text in other standard rfcs
> without deferring to those RFCs. If there are any reasons why this
> "cherry picking" was made, while other recommendations (say re NAT44)
> were not adopted, they should be clarified. In general for rfc2473,
> there I see so far no zero reason to have some other text here, as
> opposed to errata. Same for NAT44 really.
>
> Then again, perhaps the draft should be interpreted as an RFP of some
> kind, which it actually reads a lot like. In which case it should not be
> a standard track document.
>
>
>>     - Clarification of WAN selection or assumptions
>
>     [Qi] I think Ian has expressed cleared in previous mails.
>
>
>
> Woj>Please see my reply to Ian - looking forward to an answer in the draft:
> --snip--
> So, in terms of the lw46 draft specifying how a CPE is to get to the IP
> prefix of  "the WAN interface" is an important aspect, or at least
> stating the assumptions about this WAN interface. Some obvious questions
> apply here:
> is the WAN the interface which has the default route? Is it the
> interface that has a DHCPv6 client? What if there are multiple such
> "WAN" interfaces? What if a single "WAN" has multiple IPv6 prefixes?
> What if it has NO global IPv6 prefix? What if it doesn't have a /64
> prefix. etc.
> --snip--
> One doesn't see the draft answering any of these at the moment.
>
>
>>
>>     That said, abundant technical evidence (summarized in previous
>>     mail) indicates that this mode is in near total overlap with MAP
>>     1:1, certainly much more than at the outset of this work. Given
>>     this, why not align the text and actually make MAP 1:1 and lw46 be
>>     the same? On all of the data plane IPv6, IPv4, NAT44, tunneling
>>     and ICMP handling parts I see there no reason for there being a
>>     difference...
>
>     [Qi] I think lw4o6 is clean and simple, and flexible to work with
>     different provisioning mechanisms (signaling through the Softwire
>     DHCP as is agreed by the WG).
>
>
>>
>>     continued inline...
>>
>>
>>
>>
>>         [ian] The draft already contains the following text (and has
>>         for some time):
>>
>>             Lightweight 4over6 provides a solution for a hub-and-spoke softwire
>>             architecture only.  It does not offer direct, meshed IPv4
>>             connectivity between subscribers without packets traversing the AFTR.
>>             If this type of meshed interconnectivity is required,
>>             [I-D.ietf-softwire-map] provides a suitable solution.
>>
>>         So, are you not happy with the wording of the current
>>         reference, or are you requesting that the draft is rewritten
>>         to be cast as a ‘specialised' case of MAP?
>>
>>     What I pointed out is that the Lw46 draft has major if near total
>>     overlap with MAP-E 1:1 mode. Stating that lw46 realizes this mode,
>>     in a standalone way from the N:1 mesh mode, is what I propose.
>>     This would greatly help in implementing it and "digesting" the
>>     array of solutions.
>
>     [Qi] I don't see any "help" here...
>
>
> Woj>To an operator and implementer, it would be very advantageous that
> all of IPv4 in IPv6 and NAT44 data plane aspects, as specified by
> Softwire drafts are the same, unless it is made clear as to why they
> should be different, and what does it bring. This applies especially to
> lw46 and MAP, since there is so much overlap between the solutions.
>
>
>>     Additional comment Now that you mention it, the stated
>>     characterisation of map isn't quite right; MAP does not force the
>>     use of mesh mode, nor is mesh mode its defining characteristic.
>
>     [Qi] From my reading, the text doesn't characterize what MAP is. It
>     just says "if you want to use meshed IPv4 connectivity, then use
>     MAP". It doesn't prevent you from using MAP to do anything else. So
>     I think the text is OK.
>
>
> Woj> Well, that is "characterisation". From dictionary: "The act of
> describing the character or qualities of someone or something"
> And as stated, it is not quite correct, i.e. mesh-mode is not the
> defining feature of MAP. The state optimization (stateless) is, of which
> mesh mode is a by product (optional actually in the spec). The text
> needs to be changed.
>
>
>>     The characterisation should be restated as " lw46 does not seek to
>>     offer the optimization of AFTR subscriber state and the AFTR is
>>     expected to have configuration state for each lwB4 node. If
>>     optimization of this state is required, [id-ietf-softwire-map]
>>     provides a suitable solution ".
>>     Alternatively, state that "lw46 describes a
>>     deployment/implementation of MAP 1:1"
>>>
>>>
>>>>
>>>>             Detailed comments:
>>>>
>>>>             * Section 5.1 WAN prefix selection and address forming -
>>>>             as per Ole's comment. This is indeed hand wavy in the
>>>>             current draft (how to select the WAN?)
>>>>
>>>>             [ian] I’ve updated with the wording that I proposed to Ole.
>>>>
>>>>             The CPE WAN interface is a fairly well used term in a
>>>>             number of other RFCs. Both RFC6333 and RFC7084 use the
>>>>             term (and have implementations based on them) without
>>>>             defining how the CPE has identified it. Why does that
>>>>             need to be specifically defined here?
>>>>
>>>>
>>>>         Woj>  What is the WAN interface is, and how should the CPE
>>>>         "find it" not said. That other drafts fail to clarify that
>>>>         doesn't make it quite right…
>>>
>>>
>>>         [ian] No, this is related to fragmentation in respect to the
>>>         RFC1858 'Tiny Fragment attack' which affects  the IPv4
>>>         payload. The same problems exist for the lwB4s NAT44 function
>>>         as exist for a NAT64 in this respect, which is why it was
>>>         included.
>>>
>>>
>>>     Woj> Ok, but what does "MAY require" mean, and how should a
>>>     device go about requiring it?
>>
>     [Qi] Please see page 18 of RFC6146:
>
>            *  The NAT64 MAY require that the UDP, TCP, or ICMP header be
>               completely contained within the fragment that contains
>     fragment
>               offset equal to zero.
>
>
> Woj> Well, that text as you point comes from NAT64, and concerns IPv6
> header chain to IPv4 header "fitting". In lwB4, unless I misunderstand,
> the only fragmentation there may be is that of IPv4 that would be driven
> by an IPv4 MTU < 40 (violation of IPv4 min MTU 68). So what exactly the
> draft is requiring here isn't clear.
> In any case the fragmentation behaviour is not something specific to
> lw46, so it would be best if you pointed the draft at the source.
>
>
>>>
>>>
>>>             * Section 5.2: Text:
>>>
>>>             For incoming de-capsulated IPv4 packets carrying UDP packets with a
>>>
>>>                 zero checksum, if the lwB4 has enough resources, the lwB4 MUST
>>>
>>>                 reassemble the packets and MUST calculate the checksum.  If the lwB4
>>>
>>>                 does not have enough resources, then it MUST silently discard the
>>>
>>>                 packets.
>>>
>>>             Wouldn't it be easier to say that the lwb4 MAY reassemble
>>>             the packet and recalculate the checksum? It's clearly not
>>>             a MUST...
>>>
>>>             [ian] Why is it not a MUST? If one condition is met you
>>>             MUST do this, if a different condition is met, you MUST
>>>             do that. A MAY would make re-assembly of the packets
>>>             optional, meaning that an implementation could not
>>>             implement support for fragment reassembly at all and
>>>             still say that it is compliant with the spec.
>>>
>>>
>>>         If it were a MUST then something would horribly break if it
>>>         were not done. The above clearly says that this reassembly
>>>         operation is at the discretion of the device,  subject to an
>>>         arbitrary "enough resources" condition. If you'd like it to
>>>         be a MUST then the condition should be removed.
>>
>>         [ian] Then surely re-assembly is a SHOULD requirement, with a
>>         a MUST to discard if cannot be re-assembled. As I said, if it
>>         was a MAY requirement, then implementation would be completely
>>         optional.
>>
>>
>>     Woj> Yes, and that's what the above text reads from a test
>>     perspective, i.e. since one has no way of determining whether
>>     there are "enough resources", a lwB4 that discards all such
>>     packets would comply. As I said, if one would like to tighten the
>>     spec; a) the un-testable "enough resources" condition should be
>>     removed b) with that either a MUST used, if this is something
>>     super important, or a SHOULD (as it seems to me). Alternatively a
>>     MAY as "nice to have" may also do.
>
>     [Qi] I prefer to use a SHOULD for re-asembly and a MUST for
>     discarding the packet if it cannot be reassembled.
>
>
> Woj> Well, so it's a SHOULD then. Please note: the "enough resources"
> condition as is, is un-testable across implementations, so adding such
> "dead" text is rather pointless.
>
> Cheers,
> Wojciech.
>
>
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