Re: [DNSOP] [Ext] Questions / concerns with draft-ietf-dnsop-svcb-https (in RFC Editor queue)

[Brian Dickson <[e-mail]>]

Here are some proposed text changes, per Warren's invitation to send text:

In section 1.2, change:

2.  TargetName: The domain name of either the alias target (for
       AliasMode) or the alternative endpoint (for ServiceMode).

to:

2.  TargetName: Either the domain name of the alias target (for
       AliasMode) or the host name of the alternative endpoint (for
ServiceMode).

In section 2.4.2, change:

   As legacy clients will not know to use this record, service operators
   will likely need to retain fallback AAAA and A records alongside this
   SVCB record, although in a common case the target of the SVCB record
   might offer better performance, and therefore would be preferable for
   clients implementing this specification to use.

to:

   As legacy clients will not know to use this record, service operators
   will likely need to retain fallback AAAA and A records at the service
name,
   although in a common case the target of the SVCB record
   might offer better performance, and therefore would be preferable for
   clients implementing this specification to use.


In section 2.4.3, change:

   In ServiceMode, the TargetName and SvcParams within each resource
   record associate an alternative endpoint for the service with its
   connection parameters.

to:

   In ServiceMode, the TargetName and SvcParams within each resource
   record associate an alternative endpoint for the service with its
   connection parameters. The TargetName MUST be a host name
   (as defined in [DNSTerm].)

In section 3, the following changes are proposed; they introduce a new term
LASTNAME to be used to disambiguate the $QNAME reference so as to remove
ATTRLEAF prefixes from the appended target:


   1.  Let $QNAME be the service name plus appropriate prefixes for the
       scheme (see Section 2.3).

becomes:

   1.  Let $QNAME be the service name plus appropriate prefixes for the
       scheme (see Section 2.3). Let $LASTNAME be the service name without
any prefixes.



   3.  If an AliasMode SVCB record is returned for $QNAME (after
       following CNAMEs as normal), set $QNAME to its TargetName
       (without additional prefixes) and loop back to step 2, subject to
       chain length limits and loop detection heuristics (see
       Section 3.1).

becomes:

   3.  If an AliasMode SVCB record is returned for $QNAME (after
       following CNAMEs as normal), set $QNAME to its TargetName
       (without additional prefixes), set $LASTNAME to this new $QNAME and
loop back to step 2, subject to
       chain length limits and loop detection heuristics (see
       Section 3.1).


   Once SVCB resolution has concluded, whether successful or not, SVCB-
   optional clients SHALL append to the priority list an endpoint
   consisting of the final value of $QNAME, the authority endpoint's
   port number, and no SvcParams.  (This endpoint will be attempted
   before falling back to non-SVCB connection modes.  This ensures that
   SVCB-optional clients will make use of an AliasMode record whose
   TargetName has A and/or AAAA records but no SVCB records.)

becomes:

   Once SVCB resolution has concluded, whether successful or not, SVCB-
   optional clients SHALL append to the priority list an endpoint
   consisting of the final value of $LASTNAME, the authority endpoint's
   port number, and no SvcParams.  (This endpoint will be attempted
   before falling back to non-SVCB connection modes.  This ensures that
   SVCB-optional clients will make use of an AliasMode record whose
   TargetName has A and/or AAAA records but no SVCB records.)

   If the client is SVCB-optional, and connecting using this list of
   endpoints has failed, the client now attempts to use non-SVCB
   connection modes.

becomes:

   If the client is SVCB-optional, and connecting using this list of
   endpoints has failed, the client MAY attempt to use non-SVCB
   connection modes, using the origin name (without prefixes),

   the authority endpoint's port number, and no SvcParams.


One additional suggested addition to the end of section 3.1 is:

   If DNS responses are cryptographically protected, and at least
   one HTTPS AliasMode record has been received successfully,
   clients MAY apply Section 9.5 (HSTS equivalent) restrictions
   even when reverting to non-SVCB connection modes. Clients

   also MAY treat resolution or connection failures subsequent

   to the initial cryptographically protected AliasMode record

   as fatal.

[Brian's note: this last would provide some guidance to implementers of
clients: a signed HTTPS AliasMode record is a strong signal that the DNS
operator is discouraging fallback, albeit at a "MAY" level.]

NB: The 2.4.3 change could be removed as it is mostly independent, as could
the last addition to 3.1.
The 1.2 change is very minor, is not too important but presents a succinct
clarification on the hostname vs domain name thing.
The 2.4.2 change and section 3 changes together are fixes for the
prefix/no-prefix issue (which was basically a scrivener's error, and does
not change the semantics at all.) They should stay or go as one unit.

Brian

On Tue, Aug 30, 2022 at 12:08 AM Brian Dickson <
[e-mail]> wrote:

>
>
> On Sat, Aug 27, 2022 at 3:00 PM Ben Schwartz <[e-mail]> wrote:
>
>>
>>
>> On Fri, Aug 26, 2022 at 10:49 PM Brian Dickson <
>> [e-mail]> wrote:
>>
>>>
>>>  Fail fast may not be appealing, but in some (probably the majority of)
>>> cases, it may be the most correct option.
>>>
>>> It may also be the case that the zone owner knows whether this is the
>>> case.
>>> I think it is much more likely that explicitly declaring the situation
>>> (if known) is more useful than having several billion clients independently
>>> attempting to infer whether the first option will even work, let alone
>>> provide a useful alternative to the second or third.
>>>
>>
>> In fact, there is one way for the zone owner to disable fallback: enable
>> ECH.  Fallback is not compatible with ECH, so ECH-aware clients will
>> disable fallback when the ServiceMode records contain ECH.
>>
>>
> Wait, what?
>
> This whole discussion was raised from the perspective of zone owners
> publishing AliasMode apex records.
> Those owners would not be operating the CDN, which is the whole point of
> using a CNAME or AliasMode.
> I.e., the zone owner would be the one wanting to disable fallback, but
> would not be in a position to do what you suggest.
>
> The domain's contents are served via a CDN, where the CDN requires
> delegation of control, most often with CNAME (or AliasMode at the apex).
> The ServiceMode records are placed on the CDN operated zone (in order to
> avoid the first connection to establish the AltSvc stuff).
>
> The AliasMode record cannot be combined with ECH, since no SvcParams are
> allowed. The zone owner is not using ServiceMode, that is the declared
> assumption.
>
> If that (ECH) is the only way to disable fallback, that's what the focused
> discussion needed to elicit, and I think some slight adjustments are needed
> to at least facilitate zone owners preventing fallback. The mechanism
> doesn't need to be added to the draft, but likely would get put into a
> separate draft or a -bis document. However, there needs to be some daylight
> between the fallback method and the mandatory SVCB/HTTPS components, in
> order to allow for that development.
>
> BTW, the concern is less about singleton zone owners than it is about
> large scale integrated DNS management of zones in order to accommodate CDN
> usage.
>
> Note also, this issue is not strictly limited to vertical integration
> among products/services of the DNS operator; there are large scale
> inter-provider (DNS and other services) open partnerships (controlled by
> their mutual customers) that have need for the programmatic ability to
> assign CDNs and enable/disable fallback (if fallback is part of the
> specification).
> (For those interested, the not-yet-an-IETF standard for interoperability
> between DNS and service providers is Domain Connect. The intent is to
> revive the draft for that, which previously lived in the REGEXT WG.)
>
> I think converting the fallback in the draft into MAY, and having active
> discussions, dev, test, and deployment on a voluntary basis outside of the
> scope of the current draft, is the fastest path to solving the "no
> fallback" signaling issue, and to getting the draft published (with a few
> minor tweaks).
>
> I'll review the other comments, as well as Warren and Viktor's recent
> messages, and see if I can come up with some proposed text to make very
> limited changes to the draft.
>
> Brian
>