Conclusions of Last Call for draft-ietf-spfbis-4408bis

[Pete Resnick <[e-mail]>]

Below is the list of issues brought up during Last Call of 
draft-ietf-spfbis-4408bis. I have tried to collect together the common 
issues and tease out the ones that are slightly different. Below each 
issue, I've given what I take to be the answer to the issue (either the 
change that needs to be made, or the explanation of why no change is 
necessary). I've not put names to the objections or the answers, simply 
because multiple people gave different versions of the same objections 
or the same answers, and sorting that out seemed useless.

Issues:

1. Overloading of the TXT RR for this use is bad.
     - As far as I can tell, there is nobody that disagrees with this 
statement. However, it is also not in-and-of-itself an objection to the 
document: Nobody seems to have argued that this document should forbid 
use of the TXT RR completely in SPF. The only question is whether the 
document should provide a transition mechanism to the SPF RR or whether 
it is reasonable to go forward with this protocol using only the TXT RR. 
There is the "precedent"objection I will discuss in 2 below, but the 
specific technical objections seem mostly *not* to be about forbidding 
TXT RR use from the get-go. Some of the specific objections could be 
taken as arguments against the document going forward if it has no 
transition mechanism, but I believe all of those have either been 
addressed or can be addressed with clarifying language in the document:
     - The only complete solution to many of the problems that fall 
under this category is if *all* misuse of TXT RR went away. There has 
been no convincing argument put forward that this is plausible in the 
foreseeable future.

     1a. TXT RR can cause large RDATA.
         - In theory, that's certainly true. But I have not seen an 
argument that SPF is causing a major problem to date, nor that there is 
an expectation that it will in the future.
         - There were some suggested text clarifications for section 3.4 
to make it clear which size limitations relate to DNS response size, UDP 
payload size, or MTU size. They seem reasonable and were not objected 
to. I'll work with the editor and others to clarify that text.

     1b. Use of TXT RR can cause collisions with other applications.
         - Again, there is no indication that this has caused a problem 
for SPF (or others) to date, and SPF requires rejection of TXT RR data 
that does not conform to the spec, so I see no evidence of the existing 
harm, nor of a solid reason to believe there will be future harm. 
(Again, I'm leaving aside the "precedent" arguments until 2.)
         - There appears to be an effort underway to document (via an 
IANA registry) such uses to minimize the potential for these sorts of 
collision problems in the future.

     1c. Use of TXT RR for multiple purposes makes it impossible to do 
access control based on type of data (i.e., to allow delegation of the 
management for TXT RRs that are solely for SPF use).
         - Many organizations have been managing these records already; 
no reasoning was given that this fine-grained management is necessary.
         - Delegation is possible by pointing a TXT RR of (e.g.) 
example.com to _spf.example.com, delegating the latter.

     - All of the above was discussed extensively in the WG and taken 
into consideration. Given the charter limitations, a reasonable choice 
was made.

2. Use of the TXT RR sets a bad precedent for future use.
     - Several people responded that some additional text in 3.1 or 
elsewhere (either by way of some sort of applicability guidance or an 
overt IESG Statement) would address this issue. I think an IESG 
Statement is unnecessary since I have not heard significant objection to 
including some guidance, and I think something reasonable along these 
lines can be crafted. I'll work with the editor and others to get such 
text in the document.
     - The impediments that caused SPF to use TXT RR in the first place 
are mostly gone. New protocols are unlikely to face the same challenges.

3. Removing SPF RR support is a charter violation.
     - Because the original spec has a non-interoperable mechanism for 
use, this constituted an "error" in the spec that was to be corrected.

4. A new transition mechanism from TXT RR to SPF RR should be put into 
the spec.
     - This was extensively considered by the WG.
     - Backward compatibility would require support of TXT RR for the 
forseeable future anyway.
     - The proposed transition mechanisms have technical issues: 
Doubling request traffic (e.g., doing queries in parallel), introducing 
delays (e.g., querying SPF RR first and running into firewalls, etc).
     - This would be a new, unchartered requirement for the WG.
     - The widely held conclusion was that such a transition plan would 
not be undertaken by implementers. (See also 5.)

5. That there will be a lack of adoption of SPF RR was based on RFC 
6686, which does not support the conclusion.

     5a. There is some current use of the SPF RR; it will increase if we 
put in a new transition mechanism.
         - 6686 showed only minimal use, and reports of those in the 
industry shows the use dropping (i.e., the momentum is in the other 
direction).
         - Bigger sites are conservative and will not transition for 
fear of breaking current usage.
         - No solid case was made for why to believe that the transition 
would occur.

     5b. Use of SPF RR is on the increase now.
         - Claims that use is increasing are only anecdotal, and 
disagree with the experience of those in the industry.

     5c. Things may have changed since 6686. We should do more data 
collection.
         - There's no reason to believe that the small amounts of 
recently presented data are representative.
         - Nobody presented any basis to doubt the folks working in the 
industry.
         - There has been ample opportunity (and motivation) for folks 
outside of the WG to do more data collection; none has been presented.
         - It is an unreasonable burden to place on the WG at this point.

There were a few smaller issues:
     6. The term "SPF records" is confusing because it could refer to 
SPF RR.
         - Because the document no longer uses SPF RR, this shouldn't be 
a problem in practice.
     7. Clarifications are needed regarding the number of lookups to do 
in 4.6.4.
         - This will be reviewed prior to publication.
     8. There should be a limit on PTR lookups.
         - This was already considered by the WG and rejected.

The only looming issue large issue is the architectural one:

9. Using TXT RR for this purpose violates the architecture of the DNS.

I list this separately because this is not about the immediate technical 
implications (like objection 1) or a claim about precedence setting 
(like objection 2), but appears to be a claim that violating the 
architecture is in and of itself a reason to not put something on the 
standards track. The problem I have with this is, short of actual 
technical harm, I'm not sure how to judge this. Our processes judge 
whether standards should be adopted on the basis of interoperability, 
deployment, and solving a useful problem. We have many examples of 
protocols that violate architectural principles, but standardize them 
nonetheless. (We can all name our most hated.) We have found it more 
useful to document how to interoperate with protocols that may not be 
ideal but are widely deployed rather than reject them on principle. I 
can't see how to treat this protocol differently.

[There were a number of "straw men": There were responses to objections 
that never got brought up during Last Call, and a number of followups to 
responses to objections where the response was not one offered. Examples 
of these included:
     - A response to the objection that we haven't let the Experiment 
run long enough. (Nobody ever argued that during Last Call.)
     - A followup denying the contention that there is limited server 
support for SPF RRs. (Nobody ever said that the reason to use TXT RR now 
was because of limited server support.)
I have ignored these threads.]

So, my conclusions in summary are:

- The document needs to make a statement in the document clarifying why 
the SPF RR is no longer used in the spec and making it clear that no 
precedent should be created by this protocol's continued use of TXT RR.

- A few clarifications are required in the text (size limitations, 
perhaps number of lookup limitations).

- The remainder of the objections were fully considered and understood 
by the WG, and were addressed to a reasonable extent, and therefore that 
there is rough consensus to go forward with this document.

pr

-- 
Pete Resnick<http://www.qualcomm.com/~presnick/>
Qualcomm Technologies, Inc. - [phone]