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

[Wojciech Dec <[e-mail]>]

Hi Ian,

Just to be clear: I'm ok with lw46 defining a specific functional mode as I
believe it does in this draft, 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).
- Clarification of WAN selection or assumptions

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...

continued inline...


On 17 February 2014 17:15, Ian Farrer <[e-mail]> wrote:

> Hi Woj,
>
> Please see inline.
>
> Cheers,
> Ian
>
> On 16 Feb 2014, at 17:32, Wojciech Dec <[e-mail]> wrote:
>
> Hi Ian,
>
> you haven't replied on my high level comment - would appreciate if you
> introduced changes to that effect in the draft.
>
> 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.
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. 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"


> Either way, I don't really agree with your justification points for a
> change:
> - The PSID and the port range are algorithmically derived & related as per
> the MAP algorithm (covered in Section 5.1)
>  [ian] The PSID usage in lw4o6 is written so that you don't have to
> implement the MAP algorithm to use it (hence the should requirement for
> a=0). This makes it functionally identical to the PSID originally described
> in draft-bajko-pripaddrassign. So lw4o6 does not require you to implement
> the MAP algorithm, but it doesn't prevent you from using MAP to implement
> it (which was the reason for the agreement on using the MAP PSID DHCP
> format).
>
I'm actually fine with the current lw46 text in section 5.1, but your
response seems to conflict with that. I'll start another thread on this
topic.



>  - The assigned IPv4 address and port-set form part of the IPv6 address
> of a lwB4, and are included in the interface-identifier in the same place
> and format as MAP (Section 5.1)
> [ian] This is noted and already referenced in the lw4o6 draft.
> - The lw46 CPE should be provisioned using the MAP DHCP (also Section 5.1).
> [ian] The draft which is called map-dhcp has been re-written extensively
> with input from people across the WG to be a DHCP option for provisioning
> map & lw4o6. It is now map-dhcp by filename alone and once it progresses,
> won't even be called that anymore. How does this make lw4o6
> a 'specialisation' of MAP?
> As it's potentially a significant change that you are requesting here, I'd
> appreciate the opinions of other members of the WG on this.
>


>
>> 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] This is entirely implementation specific and something that hasn't
> stopped millions of CPEs being built and deployed that support both RFC6333
> and RFC7084 (né RFC6204). What's the problem that you are trying to solve?
>

Woj> None of those texts attempt to create a deterministic ip interface,
and in case of 6333 that is a known operational issue. 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.

Coincidentally, all of the latter are options allowed for by RFC7084.


>
>  * Processing of IPv4 fragments Section 5.2. Text " The lwB4 MAY require
>> that the UDP, TCP, or ICMP header be completely contained within the
>> fragment that contains fragment offset equal to zero."
>>
>> What does that mean? Is it trying to say that the IPv4 MTU > 40?
>>
>>
>>
>> [ian] The text is taken directly from RFC6146. I guess that doesn't
>> specify is as a defined MTU size of > 40 bytes (or anything else) as the
>> IPv4 header could feasibly be longer than 20 bytes if options were included
>> in it, so the fragmented L4 header problem could still exist.
>>
>
> Here we're talking about tunneling, and I cannot see a case whereby the
> IPv4 TCP/UDP header would get fragmented, unless the MTU of the IPv6 tunnel
> was <40. rfc6146 is about translation where other effects come to apply.
>
>
> [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?

>
>>
>> Section 5.2: "
>>
>> For incoming packets carrying TCP or UDP IPv4 fragments with a non-
>>
>>    zero checksum, after de-capsulation, the lwB4 MAY elect to queue the
>>
>>    fragments as they arrive and perform NAPT44 on all fragments at the
>>
>>    same time"
>>
>> ?? Queue based on what? Presumably the fragment identifier field, and if so then that is no different to 2473.
>>
>>
>>
>> [ian] Updated the text to note that.
>>
>
> Ok, but in essence, it would be actually better removing the text since it
> adds nothing above 2473.
>
>
> [ian] The text in the current version reads:
>
> For incoming packets carrying TCP or UDP IPv4 fragments with a non-
>    zero checksum, after de-capsulation, the lwB4 MAY elect to queue the
>    fragments as they arrive and perform NAPT44 on all fragments at the
>    same time.  In this case, the incoming 5-tuple is determined by
>    extracting the appropriate fields from the received packet, as
>    described in [RFC2473].  Alternatively, a lwB4 MAY translate the de-
>    capsulated fragments as they arrive, by storing information that
>    allows it to compute the 5-tuple for fragments other than the first.
>
> In the context of the above, is this a problem?
>

Woj> Well, what strikes me is that the above text does not bring in
anything other than what's already in 2473 for IPinIP fragementation and
[RFC5508], and  [RFC5382] for NAT. The text should refer to those
documents, or file errata. In other words, IPv4inIPv6 fragments and
handling of IPv4 fragments with NAT44 is not a lwB4 specific function, and
is already well documented.




>
>
>>
>> * 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.

Cheers,
Wojciech.