Re: alternative service probability parameter

On Fri, May 8, 2015 at 12:49 PM, Amos Jeffries <> wrote:

> On 9/05/2015 1:37 a.m., Bence Béky wrote:
> > Hello httpbis,
> >
> > This is a heads up about a parameter that we have been using in
> > Alternate-Protocol headers, and are planning to use in Alt-Svc headers
> > and ALTSVC frames.  It is "p=" (probability), that Google servers emit
> > and Chrome observes.  This takes a numerical value between 0.0 and 1.0
> > inclusive, and tells the client to only observe the alternative
> > service with that given probability (and ignore it otherwise).  This
> > parameter can be used for finer grade load balancing, for gradual
> > rollout of a new protocol, and for performance testing.
> Why not reusing the standard qvalue / ranking, like other standard
> documents do?
>   <>

​Interesting idea! I can definitely see reusing the syntax, but I suspect
it needs some added semantics. In particular, the way we use p= at Google
is to ​make sure that if p=.5 then 50% of users will use the alternative
and 50% will not. But more strongly than that, we need to have all users in
the "use it" group to use it for all servers that advertise p=.5. Because
page loads often pull in resources from multiple domains/server it is
critical that we be able to do realistic A/B comparisons. From reading the
qvalue section it doesn't sound like it fits exactly. But I could imagine a
small change to the Alt-Svc draft to clarify this in the Alt-Svc context.

The other question I have is if we want to do 50% use the alternative 50%
don't, I assume we would need to emit 2 alternatives both with the same q=
value. The first is the "real" alternative, the second is the default
protocol. Is that how you would imagine this working?



Received on Friday, 8 May 2015 21:05:40 UTC