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(svn r21365) -Document: the reasoning for some of the network configuration defaults

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rubidium 2010-11-30 20:09:44 +00:00
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docs/multiplayer.txt
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@ -114,6 +114,33 @@ Multiplayer Manual for OpenTTD
about 1.5 kilobytes per second up and down. To decrease this amount, setting
'frame_freq' to 1 will reduce it to roughly 1 kilobyte per second per client.
- OpenTTD's default settings for maximum number of clients, and amount of data
from clients to process are chosen to not influence the normal playing of
people, but to prevent or at least make it less likely that someone can
perform a (distributed) denial-of-service attack on your server by causing
an out-of-memory event by flooding the server with data to send to all
clients. The major factor in this is the maximum number of clients; with
32 clients "only" sending one chat message causes 1024 messages to be
distributed in total, with 64 clients that already quadruples to 4096. Given
that upstream bandwidth is usually the limiting factor, a queue of packets
that need to be sent will be created.
To prevent clients from exploiting this "explosion" of packets to send we
limit the number of incoming data, resulting in effectively limiting the
amount of data that OpenTTD will send to the clients. Even with the default
limits it is possible to generate about 70.000 packets per second, or about
7 megabit per second of traffic.
Given that OpenTTD kicks clients after they have not reacted within about 9
seconds from sending a frame update packet it would be possible that OpenTTD
keeps about 600.000 packets in memory, using about 50 megabytes of memory.
Given that OpenTTD allows short bursts of packets, you can have slightly
more packets in memory in case of a distributed denial of service attack.
When increasing the amount of incoming data, or the maximum number of
clients the amount of memory OpenTTD needs in case of a distributed denial
of service attack is linearly related to the amount of incoming data and
quadratic to the amount of clients. In short, a rule of thumb for, the
maximum memory usage for packets is:
#max_clients * #max_clients * bytes_per_frame * 10 KiB.
6. Some useful things
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