Re: [DNSOP] HTTP dns-alt-svc draft

[Ben Schwartz <[e-mail]>]

On Sat, Jun 23, 2018 at 6:51 AM Shumon Huque <[e-mail]> wrote:

> On Sat, Jun 23, 2018 at 12:00 AM Shumon Huque <[e-mail]> wrote:
>
>> In other threads, Erik Nygren suggested that we review the proposed
>> DNS record for HTTP Alternative Services draft:
>>
>>     https://tools.ietf.org/html/draft-schwartz-httpbis-dns-alt-svc-02
>>     (You might also want to read RFC7838 for background).
>>
>
> Another comment on this draft:
>
> I noticed that RFC7838 says:
>
>    The Alt-Svc field value can have multiple values:
>
>    Alt-Svc: h2="alt.example.com:8000", h2=":443"
>
> So, presumably my example in the last message was not quite correct
> for representing multiple target hosts for the service:
>
> Instead of:
>
>  _443._https.example.com. 900 IN ALTSVC "h2=\"cdn1.example.org:443\""
>  _443._https.example.com. 900 IN ALTSVC "h2=\"cdn2.example.org:8443\""
>
> It probably is:
>
>  _443._https.example.com. 900 IN ALTSVC "h2=\"cdn1.example.org:443\",
> h2=\"cdn2.example.org:443\""
>
>
> It also says:
>
>    When multiple values are present, the order of the values reflects
>    the server's preference (with the first value being the most
>    preferred alternative).
>
> The preference order of the values does not permit load balancing.
> So, if a site wants to do load balancing, as many do today, I assume
> they would have to employ only one target hostname, with multiple address
> records,
>

It seems to me that a site could publish multiple ALTSVC RRs, each of which
could contain multiple targets.


> and still rely on random/shuffle ordered return of the address
> record set from name resolution functions.
>

At the stage of processing ALTSVC responses, we are talking about new
client logic, so the significance of ordering is up to us.  The draft is a
bit hazy on this point, but we could add an explicit requirement that
clients shuffle the ALTSVC RRSET before processing it.  Would that help?


> In this sense, SRV is more
> flexible since it supports both priority and proportional load balancing.
>

Yes, that's true.  So far, no one has asked us for this kind of explicit
percentage load-balancing.  However, because the format is extensible, we
can add it later as a new Alt-Svc parameter.


>
> Shumon.
>
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