Re: [DNSOP] Comments on draft-ietf-dnsop-svcb-httpssvc-02

Mark Andrews <> Mon, 20 April 2020 05:37 UTC

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From: Mark Andrews <>
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Date: Mon, 20 Apr 2020 15:37:33 +1000
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To: Alessandro Ghedini <>
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Subject: Re: [DNSOP] Comments on draft-ietf-dnsop-svcb-httpssvc-02
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> On 17 Apr 2020, at 20:19, Alessandro Ghedini <> wrote:
> Hello,
> 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.
> 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).
> 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’ve
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’s implementation is available at:

> 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.
> 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).
> 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.
> 5. Section 2.1.1 says
>   The presentation format for SvcFieldValue is a whitespace-separated
>   list of elements representing a key-value pair, with an absent value
>   or "=" indicating an empty value.
> It took me longer than I'd like to admit to understand the "with an absent value
> or "=" indicating an empty value" part. I'd suggest rewording that paragraph to
> something like:
>   The presentation format for SvcFieldValue is a whitespace-separated list of
>   key=value pairs (e.g. "key123=value1 keys456=value2"). When the value, or
>   both the value and the "=" are omitted, the value should be interpreted as
>   being empty.
> Or something better :)
> 6. In Section 2.2 it says (in reference to param field values):
>   o  an octet string of the length defined by the previous field.
> 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.
> 7. Section 4.3 says:
>   All DNS servers SHOULD treat the SvcParam portion of the SVCB RR...
> Should it be SvcFieldValue instead of SvcParam? "SvcParam" is not mentioned
> anywhere else.
> 8. Maybe I'm missing something, but the following sentence in Section 6.4 seems
> wrong:
>   When SvcDomainName is ".", server operators SHOULD NOT include these hints,
>   because they are unlikely to convey any performance benefit.
> My understanding is that ipv4hint and ipv6hint are the way to solve the ESNI
> multi-CDN problem, so let's say I have "" that CNAMEs to both
> "cname.cdn-a.example" and "cname.cdn-b.example". A client queries both A and
> HTTPSVC concurrently for "", and receives two answers (the answer
> to the A query points to CDN A, while the answer to HTTPSSVC points to CDN B):
>      3600 IN CNAME cname.cdn-a.example
>    cname.cdn-a.example 3600 IN A
> and
>      3600 IN CNAME cname.cdn-b.example
>    cname.cdn-b.example 3600 IN HTTPSSVC 1 . alpn=h3 esniconfig="..."
> My understanding is that in this case the client could end up connecting to
> (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).
> 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.
> 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.
> 10. Section B.2: s/pther/other/
> Cheers
> [0]
> _______________________________________________
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Mark Andrews, ISC
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