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Re: two-phase send concerns

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 07 Dec 95 15:50:42 PST
Message-Id: <9512072350.AA02967@acetes.pa.dec.com>
To: rg@server.net
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
    Jeff> 	If the server doesn't want to receive the large body, it
    Jeff> immediately replies with its 4xx or 5xx response, and
    Jeff> immediately closes (not resets) the connection.
    
    Jeff> 	If the client manages to read the 4xx or 5xx response, it must
    Jeff> honor it and should reflect it to the user.
    
    I really don't like the "If".  401 (not to mention 3xx) -require- the
    client to read the response, because they aren't fatal, and they
    contain critical information to making the transaction succeed!
    If there is any doubt that the client might not read the response,
    then we are doing something wrong.
    
Good point, although this is actually the result of an ambiguity
in what I wrote.

When I wrote "4xx or 5xx response", I was essentially qouting from
Roy's draft.  It occurred to me while I was writing that message
that the two-phase mechanism really needed an explicit status
code for use here, which is why I ended up including this
bit:
    We might also want to do this:
        Servers rejecting an entity-body for reasons of size
        must respond with
                413 Request too large
So I would restate the phrases you object to as:

    	If the server doesn't want to receive the large body, it
    immediately replies with a 413 response, and
    immediately closes (not resets) the connection.
    
    	If the client manages to read the 413 response, it must
    honor it and should reflect it to the user.

In other words, the server should only prematurely close the
connection for this specific purposes (request too large)
and SHOULD NOT prematurely close the connection for other
4xx and 5xx statuses (although the server is always allowed
to prematurely close the connection if it has to ... for example,
if it crashes and reboots!)

    The protocol should be deterministic in any case where the physical
    connection hasn't been screwed up.  I'm all for fault tolerance and
    fallback mechanisms, but they should not be the "standard" way of
    moving data around.
    
I think my proposal is fully deterministic if no networking
failures occur, although it does require at least one retry
in some rare cases.

    Anyway, everyone keeps talking about the 4xx and 5xx -fatal- cases.
    These are the easy ones.  In any scenario, please think in terms of a
    harder case, such as the client wants to PUT a giant chunk of data to
    a location that requires authentication, or tries to POST to a
    resource that has moved, etc.

The real question here is "if there are two or more reasons why
a request would fail, which one is reported in the status code?"
(this is because each response carries exactly one status code).

One answer would be "it doesn't matter" (more precisely, "it's
up to the server implementer").  Take your first example: if the
client wants to PUT a zillion bytes to a location that requires
authentication, then does it really matter why it fails?  Either
way, it can't be done.

We could take that approach, but I think it might be too crude.
For example, suppose that a server will accept a 1 KB chunk
from anyone, but wants authentication before it will accept
a 20MB chunk.  If a 20 MB Put arrives without authentication,
it would be more useful to respond with 401 Unauthorized than
413 Request too large, since if the client sees the 401 it
could presumably retry after authentication, whereas if it
simply sees the 413, it has no clue.

Even in this scenario, it may be the case that the server
implementor might have some reason for sending the 413
response in preference to the 401 response (e.g., an organizational
security policy that tries to obscure the reasons for
a failure), so I think the HTTP spec could recommend a
set of status-code precedences, but not require it.

-Jeff
Received on Thursday, 7 December 1995 16:01:50 EST

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