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enet/design.txt

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+* Why ENet?
+
+    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.
+
+* 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.
+
+* 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.
+
+* 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.
+
+* 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.
+
+* 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.
+
+* 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.
+
+* 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.
+
+* 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.
+
+* 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).
+
+