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

[Wojciech Dec <[e-mail]>]

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)
- 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)

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.


Snipped text Inline... Woj>


On 21 February 2014 10:25, Qi Sun <[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.