[EME] Re: transport recovery at the APP layer ?

[Joe Touch <[e-mail]>]

Rémi Després wrote:
> Joe Touch wrote:
>> Rémi Després wrote:
>>   
>>> ... A transport layer acknowledgement (TCP ACK) doesn't mean that the
>>> destination application has seen the data.
>>>     
>> It never did. But receiving an ACK to a FIN does.
>>   
> However regrettable it may be :'( , it's no true in all cases either.
>  If a FIN is received by a TCP stack, it MUST be acked within 0.5 s to
> conform to RFC1122  -  4.2.3.2.
>  It MAY be acked long before that. .
>  If the application hasn't absorbed all buffered data before the ACK is
> sent, remaining data MAY never be received by the app.

An ACK is generated - but if there is outstanding data, it will not ACK
the seq num of the FIN.

The ACK _of the FIN_ means that all subsequent data has been received
correctly (as determined by the TCP checksum, which is not strong) by
the TCP at the other end.

As per below, there are other steps I should have noted to ensure that
the app has _read_ the data. As you and Paul observe, that may be
semantically insufficient - even if the app reads the data, if you want
to know if the app acted on it (e.g., a bank transaction), knowing that
the item was delivered or even received by the app is insufficient.

>  Since changing all TCP stacks would not be realistic, this fact of life
> has to be faced.

>>> I guess what is meant by "E2E" should be clarified: transport stack
>>> to transport stack, or app to app?
>>>  TCP provides only the former.
>>>  What TCP does guarantee is that all received data have been sent, in
>>> the same order and with no intermediate omission.
>>>     
>> when the connection closes, the app HAS received the data.
>>   
>  See above.

When your side registers a CLOSED state, it means that the other side
has issued a CLOSE, which means the app has received all the data. It
may not have acted on it (which would require the app to ACK), but the
app has it.

>> ... if the TCP
>> connection closes it knows that the other application has received the
>> data. That's no longer true in spliced connections.
>>   
> It's NEVER true (see above).

I've tried to show how this is ensured above. Please indicate where that
is incorrect. I admit I'm not absolutely sure of the mechanistic
details, but a successful close means the application has _read_ the
data you sent (which again may be insufficient).

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