Re: alt-svc and proxies

Martin Thomson <> Mon, 04 January 2016 03:54 UTC

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Date: Mon, 04 Jan 2016 14:50:30 +1100
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From: Martin Thomson <>
To: Piotr Galecki <>
Cc: HTTP Working Group <>
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Subject: Re: alt-svc and proxies
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We've discussed intermediation a lot on this list and concluded that
the current text was adequate.

A proxy in the sense you are concerned about is a client, as you can
see from

A proxy can forward Alt-Svc because clients that are configured to use
a proxy for a request will do so regardless of what they learn about
alternative services:

> A client configured to use a proxy for a given request SHOULD NOT directly connect to an alternative service for this request, but instead route it through that proxy.

On 4 January 2016 at 14:32, Piotr Galecki
<> wrote:
> When reviewing the alt-svc draft it is not entirely clear to me from the draft how proxies should process Alt-Svc headers and
> how they should behave when a response with the Alt-Svc header is received.
> This should be defined.
> IMO Alt-Svc header support should be required from proxies going forward.
> Proxies should behave in the same/similar way as clients in respect to Alt-Svc headers.
> They should cache information about alternative service available
> and use the alt service for subsequent requests to origin.
> Forward proxies supporting alternative services should also remove Alt-Svc headers
> when forwarding a response to client.
> This is because the protocol used for the client to proxy connection is defined (by user or WPAD) in proxy configuration settings
> and it should not be modified by the alternative service header.
> I'd like to propose the following changes:
> - change "client" to "client (or intermediary)" in the text throughout the draft wherever appropriate
> - possibly to section 2.4 or other add text:
>   "Intermediaries when receiving a response with Alt-Svc header SHOULD cache the availability of the alternative service
>    and use the alt service when forwarding subsequent requests to the origin,  provided the alternative service information is fresh.
>    In order to continue using the user (or WPAD) configured protocol for the client to proxy connection
>    forward proxies supporting alternative services SHOULD remove Alt-Svc header when forwarding a response to client."
> Thanks,
> Piotr