This document describes the packet flows in DHCPv4, DHCPv6 and
DHCPv4-over-DHCPv6 for the client and the server.

DHCPv4 client
-------------

State machine (including startup and timeouts)
 |
send_<message_type>()
 |            |
send_packet() |
              |
             add_timeout() (for retry)

I/O receive handler got_one()
 |
receive_packet()
 |
do_packet() (bootp_packet_handler)
 |              |           |
parse_options() |           |
               bootp()      |
                |          dhcp()
               dhcpoffer()  |
                           dhcp{offer,nack,ack}()
                            |
                           manage lease & state machine

DHCPv6 client
-------------

State machine (including startup and timeouts)
 |
do_xxx()
 |
append IA_NA, IA_TA and/or IA_PD
 |             |
send_packet6() |
               |
              add_timeout() (for retry)

I/O receive handler got_one_v6()
 |
receive_packet6()
 |
do_packet6() (dhcpv6_packet_handler) (relayed or direct message)
 |
parse_option_buffer()
 |
dhcpv6()
 |
xxx_handler() (v6_handler)
 |            |
valid_reply() |
              |
             manage lease (dhc6_leaseify(), ...) & state machine

DHCPv4-over-DHCPv6 client, DHCPv4 side
--------------------------------------

State machine (including startup and timeouts)
 |
send_<message_type>()
 |                  |
send_dhcpv4_query() |
 |   	            |
 V                  |
to DHCPv6 side      |
                   add_timeout() (for retry)

from DHCPv6 side
 |
dhcpv4o6_handler() (including DHCP4o6 IPC state machine)
 |
recv_dhcpv4_response()
 |
parse_options()
 |
bootp()/dhcp() (cf DHCPv4 client)

DHCPv4-over-DHCPv6 client, DHCPv6 side
--------------------------------------

I/O receive handler got_one_v6()
 |
do_packet6() (dhcpv6_packet_handler) (relayed, dhcpv4 or direct message)
 |
parse_option_buffer()
 |
dhcpv6()
 |
forw_dhcpv4_response() (when the message is a DHCPv4-response)
 |
to DHCPv4 side

from DHCPv4 side
 |
dhcpv4o6_handler() (including DHCP4o6 IPC state machine)
 |
forw_dhcpv4_query()
 |
send_packet6() to all DHCP4o6 server addresses or multicast

DHCPv4 server
-------------

I/O receive handler got_one()
 |
receive_packet()
 |
do_packet() (bootp_packet_handler)
 |               |
parse_options()  |
                 |
                bootp()/dhcp()
                 |
                locate_network()
                 |
                manage lease (find_lease*(), ...)
                 |         |
                dhcpxxx()  |
                          send_packet()

DHCPv6 server
-------------

I/O receive handler got_one_v6()
 |
receive_packet6()
 |
do_packet6() (dhcpv6_packet_handler) (relayed or direct message)
 |
parse_option_buffer()
 |
dhcpv6()
 |
build_dhcpv6_reply()
 |              |
send_packet6()  |
               dhcpv6_<message_type>()
                |
               valid_client_{msg,resp}()
                |
               lease_to_client()/start_reply()/...

DHCPv4-over-DHCPv6 server DHCPv4 side
-------------------------------------

from DHCPv6 side
 |
dhcpv4o6_handler()
 |
recv_dhcpv4_query()
 |
inline code from do_packet6(), e.g., parse_option_buffer()
 |
inline code from dhcpv6(), e.g., build_dhcpv6_reply()
 |
to DHCPv6 side

DHCPv4-over-DHCPv6 server DHCPv6 side
-------------------------------------

I/O receive handler got_one_v6()
 |
receive_packet6()
 |
do_packet6() (dhcpv6_packet_handler) (relayed or direct message)
 |
parse_option_buffer()
 |
dhcpv6()
 |
build_dhcpv6_reply() (relayed or direct message)
 |                   |
forw_dhcpv4_query()  |
 |                  dhcp4o6_relay_forw()
to DHCPv4 side       |
                    parse_option_buffer()
                     |
                    build_dhcpv6_reply() (recursive call)

from DHCPv6 side
 |
dhcpv4o6_handler()
 |
send_dhcpv4_response()
 |
send_packet6()