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

[<[e-mail]>]

Hi Woj,

I’ve been out of the office for a couple of days, so sorry for the be late reply.

Please see inline.

Cheers,
Ian

From: Wojciech Dec <[e-mail]<mailto:[e-mail]>>
Date: Wednesday, 19 February 2014 09:34
To: Ian Farrer <[e-mail]<mailto:[e-mail]>>
Cc: Softwires-wg <[e-mail]<mailto:[e-mail]>>
Subject: Re: [Softwires] I-D Action: draft-ietf-softwire-lw4over6-06.txt

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).

[ian] It would help if you could propose text for what you would like to see. The inline discussion has become quite protracted.

General items remain open (as commented):
- Cleanup text that does not belong to lw46 (eg 2473 fixes, or NAT44 best practice).

[ian] The text relating to this was specifically requested, discussed and agreed during the WGLC. It would be better to pick up the discussion on that thread and then document the outcome (It starts with: http://www.ietf.org/mail-archive/web/softwires/current/msg05786.html)

- Clarification of WAN selection or assumptions

[ian] Again, the text was previously discussed and agreed as part of the WGLC. Can you pick up the discussion at http://www.ietf.org/mail-archive/web/softwires/current/msg05792.html

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...

[ian] Let’s discuss this based on your proposed text changes.


continued inline...




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.

[ian] Discussed above.



* 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?

[ian] 'MAY require' is a suggested possible solution which the CPE could implement: If there is not a complete L4 header in the first fragment, then it’s discarded. Implement it if you want to, don’t if you don’t (I.e. a MAY requirement).  I’m really not sure what’s not clear here.


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.

[ian] What’s being added here is the translation of packets as they arrive, instead of queuing, re-assembing, then NATing as-per RFC2743. This is not documented anywhere for NAT44 (to my knowledge), only for NAT64.

This is not saying that RFC2473 is incorrect, it’s saying that there could be another alternative.





* 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.

[ian] How is ‘enough resources’ untestable? Earlier in the section, it already specifically states that the amount of resources allocated for fragmented packets must be limited. The amour of a fixed fragment buffer in use or number of fragments in a fixed sized queue are perfectly testable.

>From a customers perspective, if I tested an implementation that discarded all such fragmented packets in all conditions, then the implementation is not compliant.

Regardless, if we can agree that the lwB4 SHOULD re-assemble if it can and discard if it can’t, then I’ll modify the paragraph to that effect.


Cheers,
Wojciech.