otocols.

    @ivar _unprocessed: bytes received, but not yet broken up into messages /
        sent to stringReceived.  _compatibilityOffset must be updated when this
        value is updated so that the C{recvd} attribute can be generated
        correctly.
    @type _unprocessed: C{bytes}

    @ivar structFormat: format used for struct packing/unpacking. Define it in
        subclass.
    @type structFormat: C{str}

    @ivar prefixLength: length of the prefix, in bytes. Define it in subclass,
        using C{struct.calcsize(structFormat)}
    @type prefixLength: C{int}

    @ivar _compatibilityOffset: the offset within C{_unprocessed} to the next
        message to be parsed. (used to generate the recvd attribute)
    @type _compatibilityOffset: C{int}
    r#