[Standards-JIG] Re: WHACK

Maciek Niedzielski machekku at uaznia.net
Tue Apr 25 21:48:15 UTC 2006

Hash: SHA1

Peter Saint-Andre wrote:
> Maciek Niedzielski wrote:
>>> Peter Saint-Andre wrote:
>>>>> Kevin mentioned to me an alternative approach, which he swears is also
>>>>> Justin's idea -- whitespace acks (since I like fun names for things, I
>>>>> dub these "whacks"). Whenever an entity receives a stanza, it sends one
>>>>> whitespace character ("whack") to the immediate sender.
>>> Isn't this a bit abusing whitespace-something?
> One person's abuse is another person's elegant solution.

I meant assigning new meaning (whack) to something already used for sth
else (whing).

>>> What if your client would send a whitespace ping just before receiving
>>> my message, and then the connection would break, without delivering the
>>> message? Your server could assume that your ping was receiving confirmation.
> That's an edge case, see below.

My friends already know that I like to invent such edge cases ;)

>>> The problem - in my opinion - that some clients/servers already send
>>> whitespace meaning something else than receiving confirmation.
> Whitespace pings are currently sent by servers to test whether a TCP
> connection is still alive. I don't know of any clients that send
> whitespace pings, but it's possible there are some.

Psi has an option called "prevent NAT timeouts" which (as far as I know)
works by sending whitespace from time to time to keep the connection busy.

> That still seems like a real edge case (given that clients right now are
> not sending whings as far as I know), but yes if that scenario happens
> then the message is lost. But I think that is quite unlikely -- if our
> only problem is total edge cases, then we're better off than we are today.

I'm not an expert on low level TCP, so I couldn't really say how much
probable my example was, but just wanted to write about it, so some
smarter people can think about it ;)

> One possible solution: no client-generated whitespace pings allowed.

It may be too late to enforce this.
And at the same time, if server-generated pings are allowed, the same
situation (in reversed version) is still probable.

- --
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 xmpp:machekku at uaznia.net   Q: Why is that?
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