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From: Mark Andrews <marka@isc.org>
In-Reply-To: <20200417101932.GA2035@wakko.flat11.house>
Date: Mon, 20 Apr 2020 15:37:33 +1000
Cc: dnsop@ietf.org
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Message-Id: <225371AF-E4C4-48B3-B7A5-7022DD49B819@isc.org>
References: <20200417101932.GA2035@wakko.flat11.house>
To: Alessandro Ghedini <alessandro@ghedini.me>
Archived-At: <https://mailarchive.ietf.org/arch/msg/dnsop/m_jqkfcq0OI-Sj6YCkZJcGHDuVo>
Subject: Re: [DNSOP] Comments on draft-ietf-dnsop-svcb-httpssvc-02
Precedence: list



> On 17 Apr 2020, at 20:19, Alessandro Ghedini <alessandro@ghedini.me> =
wrote:
>=20
> Hello,
>=20
> First off, I have started implementing support for SVCB and HTTPSSVC =
as part of
> the dnspython library [0] and I'd be interested in doing some interop =
testing
> with other people's implementations.
>=20
> I also have a few comments/questions about the draft, apologies if =
they have
> already been discussed in the past (haven't been following the draft =
from the
> start).
>=20
> 1. For interop testing purposes it would be very helpful if the draft =
listed
> commonly agreed upon code-points for the new RR types. Ideally in the =
form of an
> official early assignment from IANA,

Lets wait until we are certain that the format will not change.  Every =
time I=E2=80=99ve
updated the branch there has been a non backwards compatible change.

> but if that's not possible picking a couple
> of codepoints at random from the "private use" range would also be =
helpful. In
> my implementation I'm currently using "65481" for SVCB and "65482" for =
HTTPSSVC.

BIND=E2=80=99s implementation is available at:

https://gitlab.isc.org/isc-projects/bind9/-/merge_requests/2135

> 2. The structure of the draft is a bit odd, as it lists a bunch of =
examples
> before introducing any of the records. This was a bit confusing to me, =
so I'd
> suggest moving sections 1.5 and 1.6 _before_ the examples (that is, =
immediately
> after the introduction). It might also be a good idea to just move the =
examples
> to an Appendix instead.
>=20
> 3. Would it make sense to move the ESNI/ECHO config paramenter to the =
ESNI/ECHO
> draft instead? This way the DNS draft wouldn't need to depend on the =
ESNI draft
> (so e.g. if ESNI ends up taking longer, this draft could be published =
without
> having to wait for it).
>=20
> 4. What is the point of differentiating between AliasForm and =
ServiceForm? Like,
> couldn't the draft just say that the SvcFieldValue is an optional =
field and be
> done with that? It seems like not having to explicitly differentiate =
the two
> cases would simplify the draft a bit without sacrificing much, though =
I might
> be missing something.
>=20
> 5. Section 2.1.1 says
>=20
>   The presentation format for SvcFieldValue is a whitespace-separated
>   list of elements representing a key-value pair, with an absent value
>   or "=3D" indicating an empty value.
>=20
> It took me longer than I'd like to admit to understand the "with an =
absent value
> or "=3D" indicating an empty value" part. I'd suggest rewording that =
paragraph to
> something like:
>=20
>   The presentation format for SvcFieldValue is a whitespace-separated =
list of
>   key=3Dvalue pairs (e.g. "key123=3Dvalue1 keys456=3Dvalue2"). When =
the value, or
>   both the value and the "=3D" are omitted, the value should be =
interpreted as
>   being empty.
>=20
> Or something better :)
>=20
> 6. In Section 2.2 it says (in reference to param field values):
>=20
>   o  an octet string of the length defined by the previous field.
>=20
> It might be good to say here that the format of this octet string is =
defined
> according to the corresponding SvcParamKey, and then reference section =
6 for
> ths currently defined keys. The same applies for section 2.1.1 for the
> presentation format.
>=20
> 7. Section 4.3 says:
>=20
>   All DNS servers SHOULD treat the SvcParam portion of the SVCB RR...
>=20
> Should it be SvcFieldValue instead of SvcParam? "SvcParam" is not =
mentioned
> anywhere else.
>=20
> 8. Maybe I'm missing something, but the following sentence in Section =
6.4 seems
> wrong:
>=20
>   When SvcDomainName is ".", server operators SHOULD NOT include these =
hints,
>   because they are unlikely to convey any performance benefit.
>=20
> My understanding is that ipv4hint and ipv6hint are the way to solve =
the ESNI
> multi-CDN problem, so let's say I have "www.example.net" that CNAMEs =
to both
> "cname.cdn-a.example" and "cname.cdn-b.example". A client queries both =
A and
> HTTPSVC concurrently for "www.example.net", and receives two answers =
(the answer
> to the A query points to CDN A, while the answer to HTTPSSVC points to =
CDN B):
>=20
>    www.xample.net      3600 IN CNAME cname.cdn-a.example
>    cname.cdn-a.example 3600 IN A 192.0.2.1
>=20
> and
>=20
>    www.xample.net      3600 IN CNAME cname.cdn-b.example
>    cname.cdn-b.example 3600 IN HTTPSSVC 1 . alpn=3Dh3 esniconfig=3D"..."=

>=20
> My understanding is that in this case the client could end up =
connecting to
> 192.0.2.1 (CDN A) with CDN B's ESNI config (or e.g. with the wrong =
ALPN). So if
> the server doesn't provide IP hints there would indeed be impact on =
performance
> because the client would just straight up fail to connect initially =
(e.g. maybe
> CDN A doesn't support HTTP/3, but CDN B's HTTPSSVC says the client can =
use it,
> or just because of the wrong ESNI config).
>=20
> Long story short, I don't think the text should discourage setting =
ipv4hint and
> ipv6hint here. I get that it's SHOULD NOT and not MUST NOT, but it's =
pretty
> confusing nevertheless.
>=20
> 9. Speaking of multi-CDN, AFAICT the problem is mentioned only once in =
the whole
> draft and only in relation to ESNI. However this is not ESNI-specific =
and also
> affects e.g. HTTP/3 as per the example above. So I think it would be =
useful to
> go into a little more detail on this.
>=20
> 10. Section B.2: s/pther/other/
>=20
> Cheers
>=20
> [0] https://github.com/rthalley/dnspython/pull/452
>=20
> _______________________________________________
> DNSOP mailing list
> DNSOP@ietf.org
> https://www.ietf.org/mailman/listinfo/dnsop

--=20
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka@isc.org