[#1460] options doc update

- created new table with supported RAI suboptions
 - added RFCs 3011, 3993
 - removed outdated reference do 1497 from limitations section

doc/sphinx/arm/dhcp4-srv.rst

@@ -1751,8 +1751,6 @@ currently has no means to validate it.
    +----------------------------------------+------+---------------------------+-------------+-------------+
    | name-service-search                    | 117  | uint16                    | true        | false       |
    +----------------------------------------+------+---------------------------+-------------+-------------+
-   | subnet-selection                       | 118  | ipv4-address              | false       | false       |
-   +----------------------------------------+------+---------------------------+-------------+-------------+
    | domain-search                          | 119  | fqdn                      | true        | false       |
    +----------------------------------------+------+---------------------------+-------------+-------------+
    | vivco-suboptions                       | 124  | binary                    | false       | false       |
@@ -1793,7 +1791,8 @@ configured explicitly by sysadmin. The `dhcp-parameter-request-list` is sent by
 never be sent by the server. The `dhcp-renewal-time` and `dhcp-rebinding-time` are governed by
 `renew-timer` and `rebind-timer` parameters. The `dhcp-client-identifier` is echoed back with the
 value sent by the client. The `fqdn` option is part of the DDNS and D2 configuration. The `dhcp-agent-options`
-is being sent by the relay agent.
+is being sent by the relay agent. The `subnet-selection` option, if present in client's messages, will
+be used in the subnet selection process.
 
 .. table:: List of standard DHCPv4 options managed by Kea on its own and not directly configurable by an administrator
 
@@ -1824,6 +1823,8 @@ is being sent by the relay agent.
    +----------------------------------------+------+---------------------------+-------------+-------------+
    | dhcp-agent-options                     | 82   | empty                     | false       | true        |
    +----------------------------------------+------+---------------------------+-------------+-------------+
+   | subnet-selection                       | 118  | ipv4-address              | false       | false       |
+   +----------------------------------------+------+---------------------------+-------------+-------------+
 
 The following table lists all option types used in the previous two tables with a description of
 what values are accepted for them.
@@ -1917,6 +1918,31 @@ what values are accepted for them.
    |                 | 2147483647.                                           |
    +-----------------+-------------------------------------------------------+
 
+Kea also supports Relay Agent Information (RAI) option, sometimes referred to as relay option, agent
+option or simply option 82. The option itself is just a container and doesn't convey any information
+on its own. The following table contains a list of RAI sub-options that Kea can understand. The RAI
+and its sub-options are inserted by the relay agent and received by Kea. There is no need for Kea
+to be configured with those options as Kea only receives them.
+
+.. table:: List of RAI sub-options that Kea can understand.
+
+   +--------------------+------+----------------------------------------------------------------------+
+   | Name               | Code | Comment                                                              |
+   +====================+======+======================================================================+
+   | circuit-id         | 1    | Used when host-reservation-identifiers is set to `circuit-id`.       |
+   +--------------------+------+----------------------------------------------------------------------+
+   | remote-id          | 2    | Can be used with flex-id to identify hosts.                          |
+   +--------------------+------+----------------------------------------------------------------------+
+   | link selection     | 5    | If present, is used to select appropriate subnet.                    |
+   +--------------------+------+----------------------------------------------------------------------+
+   | subscriber-id      | 6    | Can be used with flex-id to identify hosts.                          |
+   +--------------------+------+----------------------------------------------------------------------+
+   | relay-source-port  | 19   | If sent by the relay, Kea will send back its responses to this port. |
+   +--------------------+------+----------------------------------------------------------------------+
+
+All other RAI sub-options can be used in client classification to classify incoming packets to specific classes
+and/or by flex-id to construct unique device identifier.
+
 .. _dhcp4-custom-options:
 
 Custom DHCPv4 Options
@@ -6345,7 +6371,7 @@ The following standards are currently supported:
 
 -  *BOOTP Vendor Information Extensions*, `RFC
    1497 <https://tools.ietf.org/html/rfc1497>`__: This requires the open
-   source BOOTP hook to be loaded.
+   source BOOTP hook to be loaded. See :ref:`hooks-bootp` for details.
 
 -  *Dynamic Host Configuration Protocol*, `RFC
    2131 <https://tools.ietf.org/html/rfc2131>`__: Supported messages are
@@ -6358,9 +6384,18 @@ The following standards are currently supported:
    Domain Name (15), DNS Servers (6), IP Address Lease Time (51), Subnet
    mask (1), and Routers (3).
 
+-  *The IPv4 Subnet Selection Option for DHCP*, `RFC
+   3011 <https://tools.ietf.org/html/rfc3011>`__: The subnet selection option
+   is supported. If received in a packet, it will be used in the subnet selection
+   process.
+
 -  *DHCP Relay Agent Information Option*, `RFC
-   3046 <https://tools.ietf.org/html/rfc3046>`__: Relay Agent Information
+   3046 <https://tools.ietf.org/html/rfc3046>`__: Relay Agent Information,
-   option is supported.
+   Circuit-id, and Remote-id options are supported.
+
+-  *Link Selection sub-option for the Relay Agent Option*, `RFC 3527
+   <https://tools.ietf.org/html/rfc3527>`__: The link selection sub-option
+   is supported.
 
 -  *Vendor-Identifying Vendor Options for Dynamic Host Configuration
    Protocol version 4*, `RFC
@@ -6368,6 +6403,10 @@ The following standards are currently supported:
    Vendor Class and Vendor-Identifying Vendor-Specific Information
    options are supported.
 
+-  *Subscriber-ID Suboption for the DHCP Relay Agent Option*, `RFC
+   3993 <//https://tools.ietf.org/html/rfc3993>`__: The subscriber-id
+   option is supported.
+
 -  *The Dynamic Host Configuration Protocol (DHCP) Client Fully
    Qualified Domain Name (FQDN) Option*, `RFC 4702
    <https://tools.ietf.org/html/rfc4702>`__: The Kea server is able to
@@ -6409,10 +6448,6 @@ are clearly marked as such.
 -  BOOTP (`RFC 951 <https://tools.ietf.org/html/rfc951>`__) is not
    supported. This is a design choice; historic BOOTP support is not planned.
 
--  BOOTP with vendor information extensions
-   (`RFC 1497 <https://tools.ietf.org/html/rfc1497>`__) is supported
-   by the BOOTP hooks library; see :ref:`hooks-bootp` for details.
-
 -  On Linux and BSD system families the DHCP messages are sent and
    received over the raw sockets (using LPF and BPF) and all packet
    headers (including data link layer, IP, and UDP headers) are created