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Re: HTTP and half close (WAS: HTTP client abort detection)

From: Roy T. Fielding <fielding@ebuilt.com>
Date: Sat, 17 Mar 2001 23:03:28 -0800
To: Miles Sabin <MSabin@interx.com>
Cc: http-wg@cuckoo.hpl.hp.com
Message-ID: <20010317230328.A914@waka.ebuilt.net>
> That's all very well, but it's only one of the possible choices 
> for the mapping of TCP/socket API behaviour on to HTTP behaviour.
> It'd be nice to have some justification for why the choice should 
> be made this way rather than some other, and some reassurance 
> that this is the way the choice has been made in real 
> implementations. 

There are literally millions of theoretical possibilties that are
not mentioned in a protocol spec.  If they were all mentioned, then
nobody would have time to read the spec.

The reasoning in this case is very simple: be conservative in what you
send and liberal in what you accept.  If you follow that principle, there
is only one choice, and you will find it in every robust implementation of
HTTP over TCP.  Go ahead and check the source code at apache.org.

> It also leaves me puzzled about the question of delimiting
> request entities. If a client TCP half close *isn't* considered 
> as an HTTP connection close, then why can't we use that that as 
> an alternative to Content-Length or chunking? Given what you've
> said above, the parenthesis in,
> 
>    5.By the server closing the connection. (Closing the 
>      connection cannot be used to indicate the end of a request 
>      body, since that would leave no possibility for the server 
>      to send back a response.)
> 
> simply doesn't apply. Why not allow EOF as a request entity
> delimiter?

Because there is no value obtained by allowing it, and the cost would be
to add an HTTP requirement that is specific to those TCP API that provide
half-close as an option.  If there exists a platform for which the
application-layer cannot discern a half-close condition from an error
condition, then the server on that platform cannot know if it has
received the entire request or only a partial request, and since this
can impact the fidelity of carrying out that request, you have defined
a protocol which is less robust than what is defined in RFC 2616.
Therefore, it is not an option allowed by HTTP/1.1.

> > Note that the above applies to the socket API.  There may be a 
> > completely different algorithm for determining when the 
> > connection is closed when using other API, particularly event-
> > based ones.
> 
> Could you elaborate on this? It seems to imply that we might
> have client and server HTTP over TCP implementations, one built 
> on the socket API, one built on something else, each with
> different mappings of TCP behaviour to HTTP behaviour, hence that 
> the two might not be fully interoperable. I'm not sufficiently 
> familiar with non-socket APIs to say whether this would be likely 
> to be a problem in practice, but it certainly seems like a 
> worrying prospect.

They are interoperable right now precisely because HTTP does not
define things in terms of TCP.  There are many platforms that have
HTTP servers without any TCP -- the Web was created in a world where
TCP was not the universal standard for transport (not that it is today).
The intermediary boxes that translate from non-TCP server network to
TCP client network do not know anything about HTTP, and thus an HTTP
message can only be sent through that gateway if neither side makes
implementation assumptions about the other beyond those carried within
the HTTP message.

> > The reason this isn't defined in the HTTP spec is because it is 
> > not an interoperability issue for the application protocol -- 
> > it is an implementation detail that is entirely dependent on 
> > the nature of the lower-layer API used by the application.  It 
> > is the stuff for a good book on network programming.
> 
> Perhaps it's not an issue for the abstract protocol, but it is
> an issue for it's most common concrete realization. Isn't this
> sort of interaction between the transport layer and the
> application layer precisely the sort of thing which prompted
> Jim Gettys and Alan Freiers "HTTP Connection Management" ID?
> 
> On the face of it, it'd be useful to have one or more transport
> layer specific profiles as adjuncts to the abstract protocol
> specification.

There is absolutely no question that there is value in documenting
implementation profiles for protocols.  I am particularly fond of the
ones written by W. Richard Stevens.  But there is also a damn good
reason why those profiles are better written by individual authors
rather than within a standards organization, and that is because they
deal with how a standard should be implemented rather than what the
standard should be.

Cheers,

Roy T. Fielding, Chief Scientist, eBuilt, Inc.
                 2652 McGaw Avenue
                 Irvine, CA 92614-5840  fax:+1.949.609.0001
                 (fielding@ebuilt.com)  <http://www.eBuilt.com>
Received on Sunday, 18 March 2001 07:19:33 EST

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