==================
TCP Load-Balancing
==================

A TCP load-balancer is a frontend TCP/IP connection terminator that forwards
inbound connections to any one of a list of configured backends, while it
usually monitors the availability of each backend in order to ensure that any
service associated with the connection remains available and functional.

As such, a variety of aspects are crucial to the sustained operations of a TCP
load-balancer;

#.  Monitor backend service availability on as accurate an application level as
    possible [#]_,

#.  Conform to TCP/IP connection specifications and client- and server-node
    implementation details [#]_,

#.  Provide transparent proxying capabilities, such that the receiving backend
    can determine the original source IP address and port of the connection
    [#]_,

#.  (...)

.. rubric:: Footnotes

.. [#]

    This means that for a web server backend node, an actual `GET`, `HEAD` or
    similar HTTP command needs to be issued against the web server application,
    such that a managed expectation is fullfilled.

.. [#]

    A TCP connection is created with a handshake -- a `SYN` packet is sent out,
    a `SYN,ACK` packet is received in response, and a final `ACK` is required
    to establish the connection.

    This final `ACK` packet may also have the `FIN` flag set, indicating that
    the connection is to be considered terminated. It is supposed to be
    responded to by the other end of the TCP connection, and that response in
    turn is supposed to be acknowledged.

.. [#]

    The need for transparent proxying may not be required for protocols that
    the load-balancer can introspect, apply policy rules to, conditionally
    forward on to a (sub-)set of backend nodes, or respond to the client on.

    Examples of such include the HTTP protocol, for which a backend node's web
    server can interpret the `X-HTTP-Forwarded-For` header as containing the
    original source IP address for the connection.