enet/docs/design.dox

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@page Features Features and Architecture

ENet evolved specifically as a UDP networking layer for the
multiplayer first person shooter Cube. Cube necessitated low latency
communication 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
guarantees 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).

*/