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Re: [hybi] Reliable message delivery (was Re: Technical feedback.)

Maciej Stachowiak <mjs@apple.com> Tue, 02 February 2010 03:55 UTC

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From: Maciej Stachowiak <mjs@apple.com>
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Date: Mon, 01 Feb 2010 19:56:06 -0800
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To: "Thomson, Martin" <Martin.Thomson@andrew.com>
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Subject: Re: [hybi] Reliable message delivery (was Re: Technical feedback.)
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On Feb 1, 2010, at 6:42 PM, Thomson, Martin wrote:

>> That is fine for TCP; the 0xff 0x00 is redundant.
> 
> What I get from this is that TCP connections haemorrhaging data can be solved without support in the higher layer - all that it requires is that the implementation of the next layer be aware of the limitations of TCP and that it uses shutdown(...) or whatever is necessary.
> 
> That's a pretty strong argument against putting specific close semantics in WS.  I don't buy the "let's make our protocol transport-layer-agnostic argument"; that only leads to the protocol picking up unnecessary baggage.

I'm not overly concerned about being transport-layer-agnostic on this feature - after all, other transports may not need the special close semantics. For example, SCTP likely does not. But I am concerned about whether a close handshake using only half-closes is as widely implementable as we would like. Two concerns here: (1) it's been reported that some TCP stacks do not actually expose the ability to do a half-close - it would be a shame if you could never write a WebSocket client or server for such an OS; (2) When using SSL as the transport, does the same issue arise, and can SSL implementations be convinced to do a half-close on demand?

If either of these concerns makes half-close-only impractical, then I think Jamie's suggestion is the best: allow any of half-close, explicit close frame, or both. 

 - Maciej

> 
> If an explicit close notification is required for an alternative transport, then let those who are porting the protocol to that environment add a shim for that purpose.
> 
> Having text that explains the assumptions of the current solution and maybe even suggests an implementation option (preferably by reference) seems most appropriate to me.
> 
> --Martin
> 
> 
>> Please note: The close-frame is *not* for acknowledgement of prior
>> messages.  It is a mechanism to prevent closes from turning into
>> "connection reset" TCP errors and the data-discarding behaviour.  It
>> would be *most unwise* of an endpoint to refuse to send the
>> close-frame (or shutdown) because it was keeping it to acknowledge an
>> expected message - because that would force the other end to keep a
>> connection open for a long timeout, when receiving close and lack of
>> an expected message is precisely a situation when connections should
>> be closed quickly because an unrecoverable error has occured.
> 
> This too is useful information - but this could be applied to the FIN, rather than the close frame.  That is, if you receive a FIN, that doesn't mean that the other peer received everything that you've already sent.  When you put it like that, it doesn't seem like much of a stretch ;)

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