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diff --git a/docs/multiplayer.txt b/docs/multiplayer.txt index eaa87576e..b85724d46 100644 --- a/docs/multiplayer.txt +++ b/docs/multiplayer.txt @@ -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 ---------------------- |