[dcc2] SIDs and connections to self.

[Szymon Stefanek]

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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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