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diff --git a/docs/design.dox b/docs/design.dox new file mode 100755 index 0000000..daed221 --- /dev/null +++ b/docs/design.dox @@ -0,0 +1,126 @@ +/** +@page Features Features and Architecture + +ENet evolved specifically as a UDP networking layer for the +multiplayer first person shooter Cube. Cube necessitated low latency +communcation with data sent out very frequently, so TCP was an +unsuitable choice due to its high latency and stream orientation. UDP, +however, lacks many sometimes necessary features from TCP such as +reliability, sequencing, unrestricted packet sizes, and connection +management. So UDP by itself was not suitable as a network protocol +either. No suitable freely available networking libraries existed at +the time of ENet's creation to fill this niche. + +UDP and TCP could have been used together in Cube to benefit somewhat +from both of their features, however, the resulting combinations of +protocols still leaves much to be desired. TCP lacks multiple streams +of communication without resorting to opening many sockets and +complicates delineation of packets due to its buffering behavior. UDP +lacks sequencing, connection management, management of bandwidth +resources, and imposes limitations on the size of packets. A +significant investment is required to integrate these two protocols, +and the end result is worse off in features and performance than the +uniform protocol presented by ENet. + +ENet thus attempts to address these issues and provide a single, +uniform protocol layered over UDP to the developer with the best +features of UDP and TCP as well as some useful features neither +provide, with a much cleaner integration than any resulting from a +mixture of UDP and TCP. + +@section CM Connection Management + +ENet provides a simple connection interface over which to communicate +with a foreign host. The liveness of the connection is actively +monitored by pinging the foreign host at frequent intervals, and also +monitors the network conditions from the local host to the foreign +host such as the mean round trip time and packet loss in this fashion. + +@section Sequencing Sequencing + +Rather than a single byte stream that complicates the delineation of +packets, ENet presents connections as multiple, properly sequenced +packet streams that simplify the transfer of various types of data. + +ENet provides sequencing for all packets by assigning to each sent +packet a sequence number that is incremented as packets are sent. ENet +guarentees that no packet with a higher sequence number will be +delivered before a packet with a lower sequence number, thus ensuring +packets are delivered exactly in the order they are sent. + +For unreliable packets, ENet will simply discard the lower sequence +number packet if a packet with a higher sequence number has already +been delivered. This allows the packets to be dispatched immediately +as they arrive, and reduce latency of unreliable packets to an +absolute minimum. For reliable packets, if a higher sequence number +packet arrives, but the preceding packets in the sequence have not yet +arrived, ENet will stall delivery of the higher sequence number +packets until its predecessors have arrived. + +@section Channels Channels + +Since ENet will stall delivery of reliable packets to ensure proper +sequencing, and consequently any packets of higher sequence number +whether reliable or unreliable, in the event the reliable packet's +predecessors have not yet arrived, this can introduce latency into the +delivery of other packets which may not need to be as strictly ordered +with respect to the packet that stalled their delivery. + +To combat this latency and reduce the ordering restrictions on +packets, ENet provides multiple channels of communication over a given +connection. Each channel is independently sequenced, and so the +delivery status of a packet in one channel will not stall the delivery +of other packets in another channel. + +@section Reliability Reliability + +ENet provides optional reliability of packet delivery by ensuring the +foreign host acknowledges receipt of all reliable packets. ENet will +attempt to resend the packet up to a reasonable amount of times, if no +acknowledgement of the packet's receipt happens within a specified +timeout. Retry timeouts are progressive and become more lenient with +every failed attempt to allow for temporary turbulence in network +conditions. + +@section FaR Fragmentation and Reassembly + +ENet will send and deliver packets regardless of size. Large packets +are fragmented into many smaller packets of suitable size, and +reassembled on the foreign host to recover the original packet for +delivery. The process is entirely transparent to the developer. + +@section Aggregation Aggregation + +ENet aggregates all protocol commands, including acknowledgements and +packet transfer, into larger protocol packets to ensure the proper +utilization of the connection and to limit the opportunities for +packet loss that might otherwise result in further delivery latency. + +@section Adaptability Adaptability + +ENet provides an in-flight data window for reliable packets to ensure +connections are not overwhelmed by volumes of packets. It also +provides a static bandwidth allocation mechanism to ensure the total +volume of packets sent and received to a host don't exceed the host's +capabilities. Further, ENet also provides a dynamic throttle that +responds to deviations from normal network connections to rectify +various types of network congestion by further limiting the volume of +packets sent. + +@section Portability Portability + +ENet works on Windows and any other Unix or Unix-like platform +providing a BSD sockets interface. The library has a small and stable +code base that can easily be extended to support other platforms and +integrates easily. ENet makes no assumptions about the underlying +platform's endianess or word size. + +@section Freedom Freedom + +ENet demands no royalties and doesn't carry a viral license that would +restrict you in how you might use it in your programs. ENet is +licensed under a short-and-sweet MIT-style license, which gives you +the freedom to do anything you want with it (well, almost anything). + +*/ + |