// Copyright (C) 2016 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

/**
 @page dhcpv4o6Dhcp6 DHCPv4-over-DHCPv6 DHCPv6 Server Side

Kea supports DHCPv4-over-DHCPv6 using cooperating DHCPv6 and DHCPv4
servers. This section describes the DHCPv6 server side. For its
DHCPv6 counter-part, see @ref dhcpv4o6Dhcp4.

@section dhcp6to4Ipc DHCPv6-to-DHCPv4 Inter Process Communication

The @c Dhcp6to4Ipc class is derived from the base @c Dhcp4o6IpcBase as
a singleton class (by the static @ref isc::dhcp::Dhcp6to4Ipc::instance
function).  @ref isc::dhcp::Dhcp6to4Ipc::open is called to open IPC
UDP sockets and to register @ref isc::dhcp::Dhcp6to4Ipc::handler on
external sockets on the @c IfaceMgr.

@section dhcp6to4Process DHCPv6-to-DHCPv4 Packet Processing

Following the DHCPv4-over-DHCPv6 packet flow:

- a DHCPv6 DHCPv4-QUERY is received from a DHCPv4-over-DHCPv6 client
  (standard incoming DHCPv6 message processing is done until
   the message type specific part)

- the isc::dhcp::Dhcpv6Srv::processDhcp4Query method which verifies
  a DHCPv4 message option is present and calls the (inherited) @ref
  isc::dhcp::Dhcp4o6IpcBase::send method to transmit the DHCPv4-QUERY
  with the interface name and remote IPv6 address to the DHCPv4 server

- the DHCPv4 server processes the DHCPv4-QUERY and returns a
  DHCPv4-RESPONSE using the IPC

- the interface manager (@c IfaceMgr) using the external socket
  mechanism invokes @ref isc::dhcp::Dhcp6to4Ipc::handler

- @ref isc::dhcp::Dhcp6to4Ipc::handler receives the packet by
  (inherited) @ref isc::dhcp::Dhcp4o6IpcBase::receive which
  decodes and strips the ISC Vendor option. From this point
  the code mimics the sending part of standard DHCPv6 packet
  processing.

- the callouts registered for "buffer6_send" hook point are called
  (@ref dhcpv6HooksBuffer6Send). Note the "pkt6_send" hook is not used
  because the matching query packet is not available

- the DHCPv6 DHCPv4-RESPONSE packet is sent back to the IPv6 remote address

*/