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Re: httpbis-p2 : 1xx messages may not always be treated as 100

From: Willy Tarreau <w@1wt.eu>
Date: Fri, 9 Jul 2010 20:13:05 +0200
To: Yves Lafon <ylafon@w3.org>
Cc: ietf-http-wg@w3.org
Message-ID: <20100709181305.GA7380@1wt.eu>
Hello Yves,

On Fri, Jul 09, 2010 at 12:21:23PM -0400, Yves Lafon wrote:
(...)
> In fact, it's not that 100 is a special state, it is more because 101
> is special.
> <<
>       - 1xx: Informational - Request received, continuing process
> >>
> <<
>    A client MUST be prepared to accept one or more 1xx status responses
>    prior to a regular response, even if the client does not expect a 100
>    (Continue) status message. Unexpected 1xx status responses MAY be
>    ignored by a user agent.
> 
>    Proxies MUST forward 1xx responses, unless the connection between the
>    proxy and its client has been closed, or unless the proxy itself
>    requested the generation of the 1xx response. (For example, if a
>    proxy adds a "Expect: 100-continue" field when it forwards a request,
>    then it need not forward the corresponding 100 (Continue)
>    response(s).)
> >>
> The "prior to a regular response" show that 101 is the culprit, and
> not 100.

Indeed, I agree with your analysis. The 101 could have been a 2xx, just
like a 200 in response to a CONNECT means a tunnel is established.

> However, 101 definition says:
> <<
>    The server understands and is willing to comply with the client's
>    request, via the Upgrade message header field (section 14.42), for a
>    change in the application protocol being used on this connection.
> >>
> but Upgrade is a hop-by-hop header
> <<
>    The Upgrade header field only applies to the immediate connection.
>    Therefore, the upgrade keyword MUST be supplied within a Connection
>    header field (section 14.10) whenever Upgrade is present in an
>    HTTP/1.1 message.
> >>
> Meaning that Upgrade request should not be transmitted by a proxy,
> unless the proxy knows how to deal with 101 and re-add the Upgrade
> header (and act accordingly afterwards).
> 
> If the client is sending a Upgrade: header to a proxy not knowing how
> to cope with it, it should be removed, and no 101 can flow back, so
> the general rules of 1xx are still correct.

I now see your point, except that the 101 will flow back, without the
Upgrade header, which results in the client being able to identify the
failure. This is what I've observed on my (quite old) squid here, as
well as on another commercial proxy (bluecoat I believe though not
certain).

> >So by considering that 1xx could all be treated as 100, I broke support
> >for 101 because when a server replied with 101, I would wait for a
> >second message to come.
> 
> In your case, you are just not supporting 101 and it is fine, unless
> you forward the Upgrade headers.

Yes, indeed, that seems OK to me now. Anyway, I'm wondering if we would
not need a clear statement for this 101 case. I'm concerned about reverse
proxies and load balancers which generally don't modify the stream that
passes through them, and which will not remove the Upgrade header but
will still forward 101, processing it as 1xx. As you say this is the only
corner case, maybe we should explain the proper failure method to adopt
when 101 is not implemented (which could very well be something like
"if an intermediate receives a 101 which it cannot handle, and it is not
able to remove the Upgrade header, it must ensure by any means that the
server's response will not be forwarded back intact to the client, so
that the client does not believe the connection is upgraded. Closing the
connection is one possible solution".

Thanks for your explanations.
Willy
Received on Friday, 9 July 2010 18:13:35 GMT

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