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Re: #158: Proxy-Connection and Keep-alive

From: Amos Jeffries <squid3@treenet.co.nz>
Date: Mon, 28 Nov 2011 23:20:38 +1300
Message-ID: <4ED36076.6080509@treenet.co.nz>
To: ietf-http-wg@w3.org
On 28/11/2011 8:01 p.m., Willy Tarreau wrote:
> Hi Mark,
> On Mon, Nov 28, 2011 at 03:44:40PM +1100, Mark Nottingham wrote:
>> <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/158>
>> Description:
>>> The Keep-Alive header is referred to in a few places, but not defined as a HTTP/1.1 header. This should be addressed (e.g., in an appendix).
>>> Likewise, Proxy-Connection is still (!) in common use, but has been deprecated for at least a decade. A few words in an appendix may encourage implementers to stop this practice.
>> We already have text in p1 about Keep-Alive, so it seemed best to just modify it slightly to also cover Proxy-Connection. I made a proposal on-ticke:
>>> Proposal - replace p1 A.1.2 with:
>>> Most HTTP/1.0 implementations use a separate connection for each request. However, some implement persistent connections using the Keep-Alive negotiation mechanism described in Section 19.7.1 of [RFC2068].
>>> In contrast, persistent connections are the default for HTTP/1.1; they do not need to be explicitly negotiated, as in HTTP/1.0. This is because there were various interoperability problems in the implementations of HTTP/1.0 persistent connections, especially when a proxy is involved.
>>> For example, some existing HTTP/1.0 clients might send Keep-Alive to a proxy server that doesn't understand Connection, which would then erroneously forward it to the next inbound server, which would establish the Keep-Alive connection and result in a hung HTTP/1.0 proxy waiting for the close on the response. The result is that HTTP/1.0 clients must be prevented from using Keep-Alive when talking to proxies.
>>> However, talking to proxies is the most important use of persistent connections, so that prohibition is clearly unacceptable. Some early attempts to correct this was to use the Proxy-Connection request header, but this approach was also unworkable, for similar reasons.
>>> Therefore, some other mechanism for indicating a persistent connection was needed -- one that is safe to use even when talking to an old proxy that ignores Connection. Persistent connections are the default for HTTP/1.1 messages; we introduce a new keyword (Connection: close) for declaring non-persistence. See Section 8.1.
>>> As a result, HTTP/1.1 clients ought not to send the Proxy-Connection or Keep-Alive header; at best, they will waste bytes in requests, and at worst they can cause interoperability problems.
>> We should also probably register Proxy-Connection with a status of "obsoleted".
>> Comments?
> I like this. Shouldn't we add something like this :
>    A proxy receiving an HTTP/1.1 request must ignore Proxy-Connection and
>    Keep-Alive headers and consider only the Connection header.

Which brings up the old problem of "Proxy-Connection: keep-alive" 
passing through middleware which ignores and relays it. End-to-end trouble.

Better to mandate that they are both obsolete and thus dropped. Leading 
to other troubles, but less damaging ones.

For the record Squid drops Keep-Alive ... no problems.

Proxy-Connection is another story. On Marks request we started dropping 
it on relayed requests a few years back. Lots of admin questions about 
where its gone and problems appeared with IIS and a mix of other 
Microsoft software not handling Connection: in responses to requests 
sending Proxy-Connection. Yes, I dare to point the finger at MS server 
software being the sole obvious culprits here, non-MS server software 
seems to have no problems. Client software is a different story altogther.

Reports seem to have declined recently, but there are still signs that 
all this trouble software are still depending on Proxy-Connection acting 
like Connection. Mostly in the form of traffic traces with 
Proxy-Connection: and no Connection header.

> and :
>    A client wishing to use persistent connections with a proxy running an
>    unknown HTTP version may explicitly send "Connection: keep-alive". This
>    statement will be silently ignored by 1.1 proxies and will enable use of
>    persistent connections on 1.0 proxies which understand the Connection header.
> I've encountered a number of issues when deploying haproxy as an intermediary
> between some clients and ancient Squid versions which made me use the
> Proxy-Connection header only in this case because the Connection header was
> not always usable. From what I recall, the issue appeared if haproxy only
> altered the connection header and not the proxy-connection, because both the
> proxy and the client would consider only the later.
> It's easy to notice in a response that it's HTTP/1.0, but in the request,
> the client speaks 1.1 and the intermediary does not know the next hop will
> prefer Proxy-Connection over Connection.
> For instance, if I send this to my local squid 2.6-stable13 :
>    HEAD http://www.yahoo.fr/ HTTP/1.1
>    Host: www.yahoo.fr
>    Proxy-connection: keep-alive
>    Connection: close
> The connection remains alive in the response :
>    HTTP/1.0 301 Moved Permanently
>    Date: Mon, 28 Nov 2011 06:39:55 GMT
>    Cache-Control: max-age=3600
>    Location: http://fr.yahoo.com/
>    Vary: Accept-Encoding
>    Content-Type: text/html; charset=utf-8
>    Age: 165
>    Content-Length: 81
>    Server: YTS/1.19.5
>    X-Cache: MISS from px.home.local
>    Via: HTTP/1.1 rc1.ops.ird.yahoo.com (YahooTrafficServer/1.19.5 [cHs f ]), 1.0 px.home.local:3128 (squid/2.6.STABLE13)
>    Proxy-Connection: keep-alive
> But at least it seems that Squid falls back to Connection if it does not
> see Proxy-Connection. It just defaults to close if neither is present,
> which makes a difference with standard HTTP/1.1, so clients will be tempted

Since Squid versions which do that are 1.0. That is expected yes?

The newer versions advertising 1.1 assume keep-alive unless 
Proxy-Connection, Connection or the config file says otherwise (in that 
order today). There was a bug in POST/PUT/DELETE keep-alive which we 
have now resolved.

> to continue to send "Proxy-Connection: keep-alive" with such proxies. While
> Squid is quite smart and interoperable, I suspect that other proxies are
> harder to get right when it comes to the Connection header. So probably that
> the Proxy-Connection header will still live for a long time because of this
> if we don't suggest an alternative solution as above.

If the agreement here is that it is safe to ignore and drop 
Proxy-Connection Squid can implement that immediately. The only thing 
I'm against is ignoring it completely (implying passing it thru).

Received on Monday, 28 November 2011 10:21:35 UTC

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