[dcc2] SIDs and connections to self.

[myndzi]

I don't like the renaming the index bit every time, but I see how it
would make sense (allow you to store data you initiated or they
initiated in the same table)...


On Sun, 27 Jun 2004 15:29:59 +0200, Szymon Stefanek <pragma at kvirc.net> wrote:
> 
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> On Sunday 27 June 2004 09:17, myndzi wrote:
> 
> > The other option is simply to ensure that all the required info
> > remains in the message. That way you don't have to "remember"
> > anything, merely react to incoming data. That may be the 'way to go'.
> > Sorry for the mass posting ;P
> 
> I guess that the applications WILL always have some private data to remember.
> 
> A "mental" schema for implementing the data structure lookup might be the
> following:
> 
> You keep a hashtable keyed to the triple (<SID>,<NICK>,<MID>) where <SID> is
> the identifier of the session, <NICK> is the REMOTE nickname and <MID> is the
> lasts message counter SENT.
> 
> To initiate a DCC2 handshake to <NICK>:
>         - generate a <SID>
>         - memorize your data keyed to (<SID>,<NICK>,1) in the hash
>         - send the first handshake message by using <SID> and <MID>=1
> 
> When you receive a message (<SID>,<MID>) from <NICK> you proceed as follows:
> 
>         - If <MID>=1 then this is a new session to handle.
>                 It may have been initiated by a remote party or yourself, but you shouldn't
>                 care about that. Memorize your data structure keyed to (<SID>,<NICK>,2)
>                 and continue the handshake by sending a message with that triple.
>         - Search the triple <SID>,<MID> - 1 ,<nick> in the hash.
>                 If you have a match then this is a valid session initiated earlier. If <MID>
>                 is ODD then the session has been initiated by yourself , otherwise it has
>                 been initiated by a remote party. Remove the data structure from the hash
>                 and rememorize it again keyed to (<SID>,<NICK>,<MID>+1). Continue
>                 the handshake by sending a message with <MID>+1.
>         - Error: discard the message.
> 
> The bad news is that a remote party can mess up in several ways.
> For example, if it receives a message with MID=1 and sends the reply without
> increasing it (still with MID 1) then you may end up in an infinite loop.
> 
> For this reason when you receive a message with (<SID>,<MID> = 1) from <NICK>
> you should perform this additional check:
> 
>         - If there exists a data structure keyed to (<SID>,<N>,<NICK>) with <N>
>                         GREATER OR EQUAL to <MID> then:
>                 - If <NICK> is NOT you, then the message is an error.
>                 - If <NICK> is you, then:
>                         - If <N> is EQUAL to <MID> then it is a "dcc-to-self" session.
>                         - If <N> is GREATER than <MID> you have a bug in your program.
> 
> The rules above can be also implemented cleanly using two hashes: one for the
> locally generated sessions and one for the remotely generated. In this case
> the number of lookups increases.
> 
> - --
> 
> Szymon Stefanek
> 
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