stk-code/lib/enet/design.txt

* 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).