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Rate limits is a way to control client->server data flow. This is done
by calculating rate level on every client snac. If client rate goes above alert
server send warning, if it goes above limit server send warning and drop
snacs from client, if it goes above disconnect level client disconnected from
server and can't connect again for some time.
At some point in the logon sequence the client should send
SNAC(01,06) which is the "rate request" packet.
In reply, the server will send the "rate response"
SNAC(01,07) which contain rate limit parameters
formatted as follows:
First comes a word value telling you how many rate classes there are.
Then for each class you get a structure like this:
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| xx xx xx xx |
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dword |
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Window size |
| xx xx xx xx |
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dword |
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Clear level |
| xx xx xx xx |
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dword |
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Alert level |
| xx xx xx xx |
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dword |
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Limit level |
| xx xx xx xx |
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dword |
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Disconnect level |
| xx xx xx xx |
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dword |
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Current level |
| xx xx xx xx |
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dword |
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Max level |
| xx xx xx xx |
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dword |
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Last time; |
| xx |
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byte |
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Current state; |
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And after those you get another set of structures (one for each class)
like this:
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| xx xx |
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word |
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rate group id |
| xx xx |
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word |
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count of pairs in group |
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| xx xx xx xx |
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dword |
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family/subtype pair #1 |
| .... |
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.... |
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.... |
| xx xx xx xx |
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dword |
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family/subtype pair #n |
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The rest of the structure is just count word pairs, a SNAC family and SNAC
subtype for each SNAC that will use the rate information from this class.
If the number of classes received is zero you should not reply. If it is
greater than zero, then you should reply with
SNAC(01,08) - "rate acknowledge" which is just a list of words with
the Class ID of each class you received.
Now for some more explanations about the protocol. For each rate class, ICQ
can be in one of three states: "limited" (state 1), in
which no data is sent; "alert" (state 2), which is when
you're sending too fast, but you aren't yet limited; and "clear"
(state 3), when everything is cool. Every time a packet is sent (assuming
you aren't being "limited"), ICQ updates a "Last Time" value to keep track of
the last sent time (as with the state value, there is a separate time for each
rate class).
It also looks up the time since the previous packet was sent and uses that
to keep a running average of time between packets (I refer to this as the
rate level). This level is calculated using a window size which specifies
how many of the previous times to take into account, as follows:
NewLevel = (Window - 1)/Window * OldLevel + 1/Window * CurrentTimeDiff
There is also a Maximum Level at which this value will be capped. This formula
is for both server and client because they both should calculate current level.
You can check if your calculations is ok - just send
SNAC(01,06) in the middle of a session and compare server value from
SNAC(01,07) with yours.
Once the new level has been calculated, ICQ updates your state as follows:
if your level is less than the Limit Level your state will be set to 1
("limited"); if it's greater than the Limit Level, but less than the Alert
Level, your state will be set to 2 ("alert"); if it's greater than the Alert
Level your state will be 3 ("clear"). If your state was already set to 1,
the calculation is slightly different. It only compares your level against
the Clear Level value - if it's greater than that, your state becomes 3
("clear"), otherwise it remains at 1 ("limited"). And one more thing - icq
often receive SNAC(01,07) with state=114 - this is ok, this mean that you
not limited.
Incidentally, this calculation happens before your packet is actually sent, so
if your state changes to "limited" at this point, the packet won't be sent.
If your state is not "clear", ICQ will also start a timer for some time in
the future (the actual duration is rather complicated so I won't go into
that now) so that your state can be recalculated, giving you a chance to get
back to "clear".
Now that you know the basic protocol, some of the SNAC parameters will make
more sense. The Window Size is the Window mentioned in calculating the
running average time between packets (the level). The Clear, Alert and Limit
Levels are used in calculating the rate state. Disconnect level is the level
at which the ICQ server will disconnect you so it doesn't used by the client.
The Current level is what the level is initially set to you when SNAC 1/07 is
received. The Max Level is what the level is capped at when calculating the
running average.
The Last Time is a duration in milliseconds which is used to set the initial
value of the last sent time (the time is set to the specified duration into
the past, i.e. 1000 milliseconds means 1 second ago). The Start State
specifies the initial rate state, and although the state is immediately
recalculated, the current state does effect that calculation (as explained
above).
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