src/core/net/nd6.hpp

/*
 *  Copyright (c) 2020, The OpenThread Authors.
 *  All rights reserved.
 *
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions are met:
 *  1. Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *  2. Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *  3. Neither the name of the copyright holder nor the
 *     names of its contributors may be used to endorse or promote products
 *     derived from this software without specific prior written permission.
 *
 *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
 *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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 *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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 */

/**
 * @file
 *   This file includes definitions for IPv6 Neighbor Discovery (ND).
 *
 * See RFC 4861 (https://tools.ietf.org/html/rfc4861) and RFC 4191 (https://tools.ietf.org/html/rfc4191).
 */

#ifndef OT_CORE_NET_ND6_HPP_
#define OT_CORE_NET_ND6_HPP_

#include "openthread-core-config.h"

#if OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE

#include <stdint.h>

#include <openthread/netdata.h>
#include <openthread/platform/infra_if.h>
#include <openthread/platform/toolchain.h>

#include "border_router/infra_if.hpp"
#include "common/const_cast.hpp"
#include "common/encoding.hpp"
#include "common/equatable.hpp"
#include "common/heap_array.hpp"
#include "net/icmp6.hpp"
#include "net/ip6.hpp"
#include "net/ip6_headers.hpp"
#include "thread/network_data_types.hpp"

namespace ot {
namespace Ip6 {
namespace Nd {

typedef NetworkData::RoutePreference  RoutePreference;  ///< Route Preference
typedef Data<kWithUint16Length>       Icmp6Packet;      ///< A data buffer for an ICMPv6 packet.
typedef otPlatInfraIfLinkLayerAddress LinkLayerAddress; ///< An infra-if link-layer address.

/**
 * Represents the variable length options in Neighbor Discovery messages.
 *
 * @sa PrefixInfoOption
 * @sa RouteInfoOption
 */
OT_TOOL_PACKED_BEGIN
class Option
{
    friend class RouterAdvert;

public:
    enum Type : uint8_t
    {
        kSourceLinkLayerAddr    = 1,  ///< Source Link Layer Address Option.
        kTargetLinkLayerAddr    = 2,  ///< Target Link Layer Address Option.
        kTypePrefixInfo         = 3,  ///< Prefix Information Option.
        kTypeRouteInfo          = 24, ///< Route Information Option.
        kTypeRecursiveDnsServer = 25, ///< Recursive DNS Server (RDNSS) Option.
        kTypeRaFlagsExtension   = 26, ///< RA Flags Extension Option.
        kTypeNat64Prefix        = 38, ///< NAT64 Prefix Option (aka PREF64 Option).
    };

    static constexpr uint16_t kLengthUnit = 8; ///< The unit of length in octets.

    /**
     * Gets the option type.
     *
     * @returns  The option type.
     */
    uint8_t GetType(void) const { return mType; }

    /**
     * Sets the option type.
     *
     * @param[in] aType  The option type.
     */
    void SetType(Type aType) { mType = aType; }

    /**
     * Sets the length based on a given total option size in bytes.
     *
     * Th option must end on a 64-bit boundary, so the length is derived as `(aSize + 7) / 8 * 8`.
     *
     * @param[in]  aSize  The size of option in bytes.
     */
    void SetSize(uint16_t aSize) { mLength = static_cast<uint8_t>(DivideAndRoundUp(aSize, kLengthUnit)); }

    /**
     * Returns the size of the option in bytes.
     *
     * @returns  The size of the option in bytes.
     */
    uint16_t GetSize(void) const { return mLength * kLengthUnit; }

    /**
     * Sets the length of the option (in unit of 8 bytes).
     *
     * @param[in]  aLength  The length of the option in unit of 8 bytes.
     */
    void SetLength(uint8_t aLength) { mLength = aLength; }

    /**
     * Returns the length of the option (in unit of 8 bytes).
     *
     * @returns  The length of the option in unit of 8 bytes.
     */
    uint16_t GetLength(void) const { return mLength; }

    /**
     * Indicates whether or not this option is valid.
     *
     * @retval TRUE   The option is valid.
     * @retval FALSE  The option is not valid.
     */
    bool IsValid(void) const { return mLength > 0; }

private:
    class Iterator : public Unequatable<Iterator>
    {
    public:
        Iterator(void);
        Iterator(const void *aStart, const void *aEnd);

        const Option &operator*(void) { return *mOption; }
        void          operator++(void) { Advance(); }
        void          operator++(int) { Advance(); }
        bool          operator==(const Iterator &aOther) const { return mOption == aOther.mOption; }

    private:
        static const Option *Next(const Option *aOption);
        void                 Advance(void);
        const Option        *Validate(const Option *aOption) const;

        const Option *mOption;
        const Option *mEnd;
    };

    uint8_t mType;   // Type of the option.
    uint8_t mLength; // Length of the option in unit of 8 octets, including the `mType` and `mLength` fields.
} OT_TOOL_PACKED_END;

/**
 * Represents the Prefix Information Option.
 *
 * See section 4.6.2 of RFC 4861 for definition of this option [https://tools.ietf.org/html/rfc4861#section-4.6.2]
 */
OT_TOOL_PACKED_BEGIN
class PrefixInfoOption : public Option, private Clearable<PrefixInfoOption>
{
    friend class Clearable<PrefixInfoOption>;

public:
    static constexpr Type kType = kTypePrefixInfo; ///< Prefix Information Option Type.

    static constexpr uint8_t kOnLinkFlag           = 0x80; ///< On-link - L flag.
    static constexpr uint8_t kAutoConfigFlag       = 0x40; ///< Autonomous address-configuration - A flag.
    static constexpr uint8_t kDhcp6PdPreferredFlag = 0x10; ///< DHCPv6-PD preferred - P flag.

    typedef uint8_t Flags; ///< Represents the flags as a bitmap.

    /**
     * Initializes the Prefix Info option with proper type and length and sets all other fields to zero.
     */
    void Init(void);

    /**
     * Gets the raw flag bitmap.
     *
     * @return The flags bitmap.
     */
    Flags GetFlags(void) const { return mFlags; }

    /**
     * Sets the raw flag bitmap.
     *
     * @param[in] aFlags  The flags bitmap.
     */
    void SetFlags(Flags aFlags) { mFlags = aFlags; }

    /**
     * Indicates whether or not the on-link flag is set.
     *
     * @retval TRUE  The on-link flag is set.
     * @retval FALSE The on-link flag is not set.
     */
    bool IsOnLinkFlagSet(void) const { return (mFlags & kOnLinkFlag) != 0; }

    /**
     * Sets the on-link (L) flag.
     */
    void SetOnLinkFlag(void) { mFlags |= kOnLinkFlag; }

    /**
     * Clears the on-link (L) flag.
     */
    void ClearOnLinkFlag(void) { mFlags &= ~kOnLinkFlag; }

    /**
     * Indicates whether or not the autonomous address-configuration (A) flag is set.
     *
     * @retval TRUE  The auto address-config flag is set.
     * @retval FALSE The auto address-config flag is not set.
     */
    bool IsAutoAddrConfigFlagSet(void) const { return (mFlags & kAutoConfigFlag) != 0; }

    /**
     * Sets the autonomous address-configuration (A) flag.
     */
    void SetAutoAddrConfigFlag(void) { mFlags |= kAutoConfigFlag; }

    /**
     * Clears the autonomous address-configuration (A) flag.
     */
    void ClearAutoAddrConfigFlag(void) { mFlags &= ~kAutoConfigFlag; }

    /**
     * Indicates whether or not the DhCPv6-PD Preferred (P) flag is set.
     *
     * @retval TRUE  The DHCPv6-PD Preferred (P) flag is set.
     * @retval FALSE The DHCPv6-PD Preferred (P) flag is not set.
     */
    bool IsDhcp6PdPreferredFlagSet(void) const { return (mFlags & kDhcp6PdPreferredFlag) != 0; }

    /**
     * Sets the valid lifetime of the prefix in seconds.
     *
     * @param[in]  aValidLifetime  The valid lifetime in seconds.
     */
    void SetValidLifetime(uint32_t aValidLifetime) { mValidLifetime = BigEndian::HostSwap32(aValidLifetime); }

    /**
     * THis method gets the valid lifetime of the prefix in seconds.
     *
     * @returns  The valid lifetime in seconds.
     */
    uint32_t GetValidLifetime(void) const { return BigEndian::HostSwap32(mValidLifetime); }

    /**
     * Sets the preferred lifetime of the prefix in seconds.
     *
     * @param[in]  aPreferredLifetime  The preferred lifetime in seconds.
     */
    void SetPreferredLifetime(uint32_t aPreferredLifetime)
    {
        mPreferredLifetime = BigEndian::HostSwap32(aPreferredLifetime);
    }

    /**
     * THis method returns the preferred lifetime of the prefix in seconds.
     *
     * @returns  The preferred lifetime in seconds.
     */
    uint32_t GetPreferredLifetime(void) const { return BigEndian::HostSwap32(mPreferredLifetime); }

    /**
     * Returns the prefix length (in bits).
     *
     * @returns The prefix length (in bits).
     */
    uint8_t GetPrefixLength(void) const { return mPrefixLength; }

    /**
     * Sets the prefix length (in bits).
     *
     * @param[in] aPrefixLength  The prefix length (in bits).
     */
    void SetPrefixLength(uint8_t aPrefixLength) { mPrefixLength = aPrefixLength; }

    /**
     * Sets the prefix.
     *
     * @param[in]  aPrefix  The prefix contained in this option.
     */
    void SetPrefix(const Prefix &aPrefix);

    /**
     * Gets the prefix in this option.
     *
     * @param[out] aPrefix   Reference to a `Prefix` to return the prefix.
     */
    void GetPrefix(Prefix &aPrefix) const;

    /**
     * Indicates whether or not the option is valid.
     *
     * @retval TRUE  The option is valid
     * @retval FALSE The option is not valid.
     */
    bool IsValid(void) const;

    PrefixInfoOption(void) = delete;

private:
    // Prefix Information Option
    //
    //   0                   1                   2                   3
    //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |     Type      |    Length     | Prefix Length |L|A|R|P| Rsvd1 |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                         Valid Lifetime                        |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                       Preferred Lifetime                      |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                           Reserved2                           |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                                                               |
    //  +                                                               +
    //  |                                                               |
    //  +                            Prefix                             +
    //  |                                                               |
    //  +                                                               +
    //  |                                                               |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //
    //  Reference for P Flag (DHCPv6-PD preferred flag):
    //  https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-6man-pio-pflag.xml

    uint8_t  mPrefixLength;      // The prefix length in bits.
    uint8_t  mFlags;             // The flags field.
    uint32_t mValidLifetime;     // The valid lifetime of the prefix.
    uint32_t mPreferredLifetime; // The preferred lifetime of the prefix.
    uint32_t mReserved2;         // The reserved field.
    Address  mPrefix;            // The prefix.
} OT_TOOL_PACKED_END;

static_assert(sizeof(PrefixInfoOption) == 32, "invalid PrefixInfoOption structure");

/**
 * Represents the Route Information Option.
 *
 * See section 2.3 of RFC 4191 for definition of this option. [https://tools.ietf.org/html/rfc4191#section-2.3]
 */
OT_TOOL_PACKED_BEGIN
class RouteInfoOption : public Option, private Clearable<RouteInfoOption>
{
    friend class Clearable<RouteInfoOption>;

public:
    static constexpr uint16_t kMinSize = kLengthUnit;    ///< Minimum size (in bytes) of a Route Info Option
    static constexpr Type     kType    = kTypeRouteInfo; ///< Route Information Option Type.

    /**
     * Initializes the option setting the type and clearing (setting to zero) all other fields.
     */
    void Init(void);

    /**
     * Sets the route preference.
     *
     * @param[in]  aPreference  The route preference.
     */
    void SetPreference(RoutePreference aPreference);

    /**
     * Gets the route preference.
     *
     * @returns  The route preference.
     */
    RoutePreference GetPreference(void) const;

    /**
     * Sets the lifetime of the route in seconds.
     *
     * @param[in]  aLifetime  The lifetime of the route in seconds.
     */
    void SetRouteLifetime(uint32_t aLifetime) { mRouteLifetime = BigEndian::HostSwap32(aLifetime); }

    /**
     * Gets Route Lifetime in seconds.
     *
     * @returns  The Route Lifetime in seconds.
     */
    uint32_t GetRouteLifetime(void) const { return BigEndian::HostSwap32(mRouteLifetime); }

    /**
     * Sets the prefix and adjusts the option length based on the prefix length.
     *
     * @param[in]  aPrefix  The prefix contained in this option.
     */
    void SetPrefix(const Prefix &aPrefix);

    /**
     * Gets the prefix in this option.
     *
     * @param[out] aPrefix   Reference to a `Prefix` to return the prefix.
     */
    void GetPrefix(Prefix &aPrefix) const;

    /**
     * Tells whether this option is valid.
     *
     * @returns  A boolean indicates whether this option is valid.
     */
    bool IsValid(void) const;

    /**
     * Calculates the minimum option length for a given prefix length.
     *
     * The option length (which is in unit of 8 octets) can be 1, 2, or 3 depending on the prefix length. It can be 1
     * for a zero prefix length, 2 if the prefix length is not greater than 64, and 3 otherwise.
     *
     * @param[in] aPrefixLength   The prefix length (in bits).
     *
     * @returns The option length (in unit of 8 octet) for @p aPrefixLength.
     */
    static uint8_t OptionLengthForPrefix(uint8_t aPrefixLength);

    /**
     * Calculates the minimum option size (in bytes) for a given prefix length.
     *
     * @param[in] aPrefixLength   The prefix length (in bits).
     *
     * @returns The option size (in bytes) for @p aPrefixLength.
     */
    static uint16_t OptionSizeForPrefix(uint8_t aPrefixLength)
    {
        return kLengthUnit * OptionLengthForPrefix(aPrefixLength);
    }

    RouteInfoOption(void) = delete;

private:
    // Route Information Option
    //
    //   0                   1                   2                   3
    //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |     Type      |    Length     | Prefix Length |Resvd|Prf|Resvd|
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                        Route Lifetime                         |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                   Prefix (Variable Length)                    |
    //  .                                                               .
    //  .                                                               .
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    static constexpr uint8_t kPreferenceOffset = 3;
    static constexpr uint8_t kPreferenceMask   = 3 << kPreferenceOffset;

    uint8_t       *GetPrefixBytes(void) { return AsNonConst(AsConst(this)->GetPrefixBytes()); }
    const uint8_t *GetPrefixBytes(void) const { return reinterpret_cast<const uint8_t *>(this) + sizeof(*this); }

    uint8_t  mPrefixLength;  // The prefix length in bits.
    uint8_t  mResvdPrf;      // The preference.
    uint32_t mRouteLifetime; // The lifetime in seconds.
    // Followed by prefix bytes (variable length).

} OT_TOOL_PACKED_END;

static_assert(sizeof(RouteInfoOption) == 8, "invalid RouteInfoOption structure");

/**
 * Represents an RA Flags Extension Option.
 *
 * See RFC-5175 [https://tools.ietf.org/html/rfc5175]
 */
OT_TOOL_PACKED_BEGIN
class RaFlagsExtOption : public Option, private Clearable<RaFlagsExtOption>
{
    friend class Clearable<RaFlagsExtOption>;

public:
    static constexpr Type kType = kTypeRaFlagsExtension; ///< RA Flags Extension Option type.

    /**
     * Initializes the RA Flags Extension option with proper type and length and sets all flags to zero.
     */
    void Init(void);

    /**
     * Tells whether this option is valid.
     *
     * @returns  A boolean indicates whether this option is valid.
     */
    bool IsValid(void) const { return GetSize() >= sizeof(*this); }

    RaFlagsExtOption(void) = delete;

private:
    // RA Flags Extension Option
    //
    //   0                   1                   2                   3
    //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |     Type      |    Length     |         Bit fields available ..
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  ... for assignment                                              |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                .

    uint8_t mFlags[6];
} OT_TOOL_PACKED_END;

static_assert(sizeof(RaFlagsExtOption) == 8, "invalid RaFlagsExtOption structure");

/**
 * Represents the NAT64 Prefix Option (aka PREF64 Option).
 *
 * See section 4 of RFC 8781 for definition of this option [https://tools.ietf.org/html/rfc8781#section-4]
 */
OT_TOOL_PACKED_BEGIN
class Nat64PrefixOption : public Option, private Clearable<Nat64PrefixOption>
{
    friend class Clearable<Nat64PrefixOption>;

public:
    static constexpr Type kType = kTypeNat64Prefix; ///< NAT64 Prefix Option Type.

    /**
     * Initializes the NAT64 Prefix option with proper type and length and sets all other fields to zero.
     */
    void Init(void);

    /**
     * Sets the NAT64 prefix lifetime.
     *
     * @param[in] aLifetime   The prefix lifetime in seconds.
     */
    void SetLifetime(uint32_t aLifetime);

    /**
     * Returns the NAT64 prefix lifetime in seconds.
     *
     * The NAT64 prefix lifetime is encoded by scaled lifetime in units of 8 seconds.
     *
     * @returns The prefix lifetime in seconds.
     */
    uint32_t GetLifetime(void) const
    {
        return (BigEndian::HostSwap16(mPrefixAttr) >> kScaledLifetimeOffset) * kLifetimeScalingUnit;
    }

    /**
     * Sets the prefix.
     *
     * @param[in]  aPrefix  The prefix contained in this option.
     *
     * @retval kErrorNone         Successfully set the prefix.
     * @retval kErrorInvalidArgs  The prefix length in @p aPrefix is not a valid NAT64 prefix length (must be one of
     *                            32, 40, 48, 56, 64, or 96).
     */
    Error SetPrefix(const Prefix &aPrefix);

    /**
     * Gets the prefix in this option.
     *
     * @param[out] aPrefix   Reference to a `Prefix` to return the prefix.
     *
     * @retval kErrorNone   Successfully retrieved the prefix.
     * @retval kErrorParse  The Prefix Length Code is not valid.
     */
    Error GetPrefix(Prefix &aPrefix) const;

    /**
     * Indicates whether or not the option is valid.
     *
     * @retval TRUE  The option is valid
     * @retval FALSE The option is not valid.
     */
    bool IsValid(void) const;

    Nat64PrefixOption(void) = delete;

private:
    // NAT64 Prefix Option
    //
    //   0                   1                   2                   3
    //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |     Type      |    Length     |     Scaled Lifetime     | PLC |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                                                               |
    //  +                                                               +
    //  |              Highest 96 bits of the Prefix                    |
    //  +                                                               +
    //  |                                                               |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    static constexpr uint32_t kLifetimeScalingUnit  = 8;    // Lifetime is scaled in units of 8 seconds.
    static constexpr uint16_t kScaledLifetimeOffset = 3;    // Scaled Lifetime offset in `mPrefixAttr`.
    static constexpr uint16_t kPrefixLengthCodeMask = 0x07; // Prefix Length Code mask in `mPrefixAttr`.

    static const uint8_t kPrefixLengths[]; // Map from prefix length code to prefix length in bits.

    /**
     * Returns the NAT64 prefix length code.
     *
     * The prefix length code values 0, 1, 2, 3, 4, and 5 indicate the NAT64 prefix length of 96, 64, 56, 48, 40, and 32
     * bits, respectively.
     *
     * @returns The prefix length code.
     */
    uint8_t GetPrefixLengthCode(void) const { return BigEndian::HostSwap16(mPrefixAttr) & kPrefixLengthCodeMask; }

    /**
     * Sets the NAT64 prefix length code.
     *
     * @param[in] aPrefixLengthCode   The prefix length code.
     */
    void SetPrefixLengthCode(const uint8_t aPrefixLengthCode);

    uint16_t mPrefixAttr; // The prefix attributes (Scaled Lifetime and Prefix Length Code).
    uint8_t  mPrefixMsb[12];

} OT_TOOL_PACKED_END;

static_assert(sizeof(Nat64PrefixOption) == 16, "invalid Nat64PrefixOption structure");

/**
 * Represents the Recursive DNS Server (RDNSS) Option.
 *
 * See section 5.1 of RFC 8106 [https://datatracker.ietf.org/doc/html/rfc8106#section-5.1].
 */
OT_TOOL_PACKED_BEGIN
class RecursiveDnsServerOption : public Option, private Clearable<RecursiveDnsServerOption>
{
    friend class Clearable<RecursiveDnsServerOption>;

public:
    static constexpr uint16_t kMinSize = kLengthUnit;             ///< Minimum size (in bytes) of a RDNSS Option.
    static constexpr Type     kType    = kTypeRecursiveDnsServer; ///< Route Information Option Type.

    /**
     * Initializes the option setting the type and clearing (setting to zero) all other fields.
     */
    void Init(void);

    /**
     * Tells whether this option is valid.
     *
     * @returns  A boolean indicates whether this option is valid.
     */
    bool IsValid(void) const { return (GetLength() > 0); }

    /**
     * Sets the Lifetime field.
     *
     * @param[in]  aLifetime  The lifetime in seconds.
     */
    void SetLifetime(uint32_t aLifetime) { mLifetime = BigEndian::HostSwap32(aLifetime); }

    /**
     * Gets the Lifetime fields
     *
     * @returns  The Lifetime in seconds.
     */
    uint32_t GetLifetime(void) const { return BigEndian::HostSwap32(mLifetime); }

    /**
     * Gets the numbers of IPv6 addresses.
     *
     * @returns Number of IPv6 addresses.
     */
    uint16_t GetNumAddresses(void) const { return IsValid() ? (GetLength() - 1) / 2 : 0; }

    /**
     * Returns a pointer to array of IPv6 addresses of DNS server.
     *
     * @returns A pointer to the array of IPv6 addresses.
     */
    const Address *GetAddresses(void) const
    {
        return reinterpret_cast<const Address *>(reinterpret_cast<const uint8_t *>(this) + sizeof(*this));
    }

    /**
     * Returns a pointer to array of IPv6 addresses of DNS server.
     *
     * @returns A pointer to the array of IPv6 addresses.
     */
    Address *GetAddresses(void) { return AsNonConst(AsConst(this)->GetAddresses()); }

    /**
     * Returns the IPv6 address at a given index.
     *
     * Caller MUST ensure that @p aIndex is valid and smaller than `GetNumberOfAddresses()`. Otherwise the behavior
     * of this method is undefined.
     *
     * @param[in] aIndex   The index.
     *
     * @returns The IPv6 address at @p aIndex.
     */
    const Address &GetAddressAt(uint16_t aIndex) const { return GetAddresses()[aIndex]; }

    /**
     * Calculates the option length for a given number of IPv6 addresses.
     *
     * @param[in] aNumAddresses   Number of IPv6 addresses
     *
     * @returns The option length (in unit of 8 octets) for @p aNumAddresses.
     */
    static uint8_t OptionLengthFor(uint8_t aNumAddresses);

    RecursiveDnsServerOption(void) = delete;

private:
    // RDNSS Option
    //
    //   0                   1                   2                   3
    //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |     Type      |     Length    |           Reserved            |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                           Lifetime                            |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                                                               |
    //  :            Addresses of IPv6 Recursive DNS Servers            :
    //  |                                                               |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    uint16_t mReserved;
    uint32_t mLifetime;
    // Followed by IPv6 Addresses of DNS servers (variable length).

} OT_TOOL_PACKED_END;

static_assert(sizeof(RecursiveDnsServerOption) == 8, "invalid RecursiveDnsServerOption structure");

/**
 * Defines the ND6 Tx Message.
 */
class TxMessage
{
public:
    /**
     * Gets the prepared ND6 message as an `Icmp6Packet`.
     *
     * @param[out] aPacket   A reference to an `Icmp6Packet`.
     */
    void GetAsPacket(Icmp6Packet &aPacket) const { aPacket.Init(mArray.AsCArray(), mArray.GetLength()); }

    /**
     * Appends bytes from a given buffer to the ND6 message.
     *
     * @param[in] aBytes     A pointer to the buffer containing the bytes to append.
     * @param[in] aLength    The buffer length.
     *
     * @retval kErrorNone    Bytes are appended successfully.
     * @retval kErrorNoBufs  Insufficient available buffers to grow the message.
     */
    Error AppendBytes(const uint8_t *aBytes, uint16_t aLength);

    /**
     * Appends a Source/Target Link Layer Address Option to the ND6 message.
     *
     * @param[in] aLinkLayerAddress    The AIL Layer Address.
     * @param[in] aType                The type of Link Layer Address Option, Source or Target
     *
     * @retval kErrorNone    Option is appended successfully.
     * @retval kErrorNoBufs  Insufficient available buffers to grow the message.
     */
    Error AppendLinkLayerOption(LinkLayerAddress &aLinkLayerAddress, Option::Type aType);

    /**
     * Appends an object to the ND6 message.
     *
     * @tparam    ObjectType   The object type to append to the message.
     *
     * @param[in] aObject      A reference to the object to append to the message.
     *
     * @retval kErrorNone      Successfully appended the object.
     * @retval kErrorNoBufs    Insufficient available buffers to grow the message.
     */
    template <typename ObjectType> Error Append(const ObjectType &aObject)
    {
        static_assert(!TypeTraits::IsPointer<ObjectType>::kValue, "ObjectType must not be a pointer");

        return AppendBytes(reinterpret_cast<const uint8_t *>(&aObject), sizeof(ObjectType));
    }

protected:
    static constexpr uint16_t kCapacityIncrement = 256;

    Option *AppendOption(uint16_t aOptionSize);

    Heap::Array<uint8_t, kCapacityIncrement> mArray;
};

/**
 * Defines Router Advertisement components.
 */
class RouterAdvert
{
public:
    /**
     * Represent an RA message header.
     *
     * See section 2.2 of RFC 4191 [https://datatracker.ietf.org/doc/html/rfc4191]
     */
    OT_TOOL_PACKED_BEGIN
    class Header : public Equatable<Header>, public Clearable<Header>
    {
        friend class Clearable<Header>;

    public:
        /**
         * Initializes the Router Advertisement message with
         * zero router lifetime, reachable time and retransmission timer.
         */
        Header(void) { SetToDefault(); }

        /**
         * Indicates whether the header is valid by checking the type field to match Router Advertisement ICMPv6 type.
         *
         * @retval TRUE  The header is valid.
         * @retval FALSE The header is not valid.
         */
        bool IsValid(void) const { return GetType() == Icmp::Header::kTypeRouterAdvert; }

        /**
         * Sets the RA message to default values.
         */
        void SetToDefault(void);

        /**
         * Sets the checksum value.
         *
         * @param[in]  aChecksum  The checksum value.
         */
        void SetChecksum(uint16_t aChecksum) { mChecksum = BigEndian::HostSwap16(aChecksum); }

        /**
         * Sets the Router Lifetime in seconds.
         *
         * @param[in]  aRouterLifetime  The router lifetime in seconds.
         */
        void SetRouterLifetime(uint16_t aRouterLifetime) { mRouterLifetime = BigEndian::HostSwap16(aRouterLifetime); }

        /**
         * Gets the Router Lifetime (in seconds).
         *
         * Router Lifetime set to zero indicates that the sender is not a default router.
         *
         * @returns  The router lifetime in seconds.
         */
        uint16_t GetRouterLifetime(void) const { return BigEndian::HostSwap16(mRouterLifetime); }

        /**
         * Sets the default router preference.
         *
         * @param[in]  aPreference  The router preference.
         */
        void SetDefaultRouterPreference(RoutePreference aPreference);

        /**
         * Gets the default router preference.
         *
         * @returns  The router preference.
         */
        RoutePreference GetDefaultRouterPreference(void) const;

        /**
         * Indicates whether or not the Managed Address Config Flag is set in the RA message header.
         *
         * @retval TRUE   The Managed Address Config Flag is set.
         * @retval FALSE  The Managed Address Config Flag is not set.
         */
        bool IsManagedAddressConfigFlagSet(void) const { return (mFlags & kManagedAddressConfigFlag) != 0; }

        /**
         * Sets the Managed Address Config Flag in the RA message.
         */
        void SetManagedAddressConfigFlag(void) { mFlags |= kManagedAddressConfigFlag; }

        /**
         * Indicates whether or not the Other Config Flag is set in the RA message header.
         *
         * @retval TRUE   The Other Config Flag is set.
         * @retval FALSE  The Other Config Flag is not set.
         */
        bool IsOtherConfigFlagSet(void) const { return (mFlags & kOtherConfigFlag) != 0; }

        /**
         * Sets the Other Config Flag in the RA message.
         */
        void SetOtherConfigFlag(void) { mFlags |= kOtherConfigFlag; }

        /**
         * Indicates whether or not the SNAC Router Flag is set in the RA message header.
         *
         * @retval TRUE   The SNAC Router Flag is set.
         * @retval FALSE  The SNAC Router Flag is not set.
         */
        bool IsSnacRouterFlagSet(void) const { return (mFlags & kSnacRouterFlag) != 0; }

        /**
         * Sets the SNAC Router Flag in the RA message header.
         */
        void SetSnacRouterFlag(void) { mFlags |= kSnacRouterFlag; }

        /**
         * This method returns the ICMPv6 message type.
         *
         * @returns The ICMPv6 message type.
         */
        Icmp::Header::Type GetType(void) const { return static_cast<Icmp::Header::Type>(mType); }

    private:
        // Router Advertisement Message
        //
        //   0                   1                   2                   3
        //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        //  |     Type      |     Code      |          Checksum             |
        //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        //  | Cur Hop Limit |M|O| |Prf| |S| |       Router Lifetime         |
        //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        //  |                         Reachable Time                        |
        //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        //  |                          Retrans Timer                        |
        //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        //  |   Options ...
        //  +-+-+-+-+-+-+-+-+-+-+-+-

        static constexpr uint8_t kManagedAddressConfigFlag = 1 << 7;
        static constexpr uint8_t kOtherConfigFlag          = 1 << 6;
        static constexpr uint8_t kSnacRouterFlag           = 1 << 1;
        static constexpr uint8_t kPreferenceOffset         = 3;
        static constexpr uint8_t kPreferenceMask           = 3 << kPreferenceOffset;

        uint8_t  mType;
        uint8_t  mCode;
        uint16_t mChecksum;
        uint8_t  mCurHopLimit;
        uint8_t  mFlags;
        uint16_t mRouterLifetime;
        uint32_t mReachableTime;
        uint32_t mRetransTimer;
    } OT_TOOL_PACKED_END;

    static_assert(sizeof(Header) == 16, "Invalid RA `Header`");

    /**
     * Represents a received RA message.
     */
    class RxMessage
    {
    public:
        /**
         * Initializes the RA message from a received packet data buffer.
         *
         * @param[in] aPacket   A received packet data.
         */
        explicit RxMessage(const Icmp6Packet &aPacket)
            : mData(aPacket)
        {
        }

        /**
         * Gets the RA message as an `Icmp6Packet`.
         *
         * @returns The RA message as an `Icmp6Packet`.
         */
        const Icmp6Packet &GetAsPacket(void) const { return mData; }

        /**
         * Indicates whether or not the received RA message is valid.
         *
         * @retval TRUE   If the RA message is valid.
         * @retval FALSE  If the RA message is not valid.
         */
        bool IsValid(void) const
        {
            return (mData.GetBytes() != nullptr) && (mData.GetLength() >= sizeof(Header)) &&
                   (GetHeader().GetType() == Icmp::Header::kTypeRouterAdvert);
        }

        /**
         * Gets the RA message's header.
         *
         * @returns The RA message's header.
         */
        const Header &GetHeader(void) const { return *reinterpret_cast<const Header *>(mData.GetBytes()); }

        /**
         * Indicates whether or not the received RA message contains any options.
         *
         * @retval TRUE   If the RA message contains at least one option.
         * @retval FALSE  If the RA message contains no options.
         */
        bool ContainsAnyOptions(void) const { return (mData.GetLength() > sizeof(Header)); }

        /**
         * Returns pointer to the start of option bytes (after header).
         *
         * @returns Pointer to start of options.
         */
        const uint8_t *GetOptionStart(void) const { return (mData.GetBytes() + sizeof(Header)); }

        /**
         * Gets the length (number of bytes) of options.
         *
         * @returns Number of bytes after header specifying RA options.
         */
        uint16_t GetOptionLength(void) const { return ContainsAnyOptions() ? mData.GetLength() - sizeof(Header) : 0; }

        // The following methods are intended to support range-based `for`
        // loop iteration over `Option`s in the RA message.

        Option::Iterator begin(void) const { return Option::Iterator(GetOptionStart(), GetDataEnd()); }
        Option::Iterator end(void) const { return Option::Iterator(); }

    private:
        const uint8_t *GetDataEnd(void) const { return mData.GetBytes() + mData.GetLength(); }

        Data<kWithUint16Length> mData;
    };

    /**
     * Represents an RA message to be sent.
     */
    class TxMessage : public ot::Ip6::Nd::TxMessage
    {
    public:
        /**
         * Appends a Prefix Info Option to the RA message.
         *
         * @param[in] aPrefix             The prefix.
         * @param[in] aValidLifetime      The valid lifetime in seconds.
         * @param[in] aPreferredLifetime  The preferred lifetime in seconds.
         * @param[in] aFlags              The PIO flags to use.
         *
         * @retval kErrorNone    Option is appended successfully.
         * @retval kErrorNoBufs  Insufficient available buffers to grow the message.
         */
        Error AppendPrefixInfoOption(const Prefix           &aPrefix,
                                     uint32_t                aValidLifetime,
                                     uint32_t                aPreferredLifetime,
                                     PrefixInfoOption::Flags aFlags);

        /**
         * Appends a Route Info Option to the RA message.
         *
         * @param[in] aPrefix             The prefix.
         * @param[in] aRouteLifetime      The route lifetime in seconds.
         * @param[in] aPreference         The route preference.
         *
         * @retval kErrorNone    Option is appended successfully.
         * @retval kErrorNoBufs  Insufficient available buffers to grow the message.
         */
        Error AppendRouteInfoOption(const Prefix &aPrefix, uint32_t aRouteLifetime, RoutePreference aPreference);

        /**
         * Appends a NAT64 Prefix Option to the RA message.
         *
         * @note This is intended for testing only. An OTBR should not advertise a NAT64 prefix option; it should only
         * process received ones.
         *
         * @param[in] aPrefix         The prefix.
         * @param[in] aLifetime       The lifetime in seconds.
         *
         * @retval kErrorNone         Option is appended successfully.
         * @retval kErrorInvalidArgs  Unsupported length for NAT64 prefix.
         * @retval kErrorNoBufs       Insufficient available buffers to grow the message.
         */
        Error AppendNat64PrefixOption(const Prefix &aPrefix, uint32_t aLifetime);

        /**
         * Append a Recursive DNS Server Option to the RA message.
         *
         * @note This is intended for testing only. An OTBR should not advertise an RDNSS option; it should only
         * process received ones.
         *
         * @param[in] aAddresses     A pointer to an array of IPv6 addresses.
         * @param[in] aNumAddresses  Number of addresses in @p aAddresses array.
         * @param[in] aLifetime      The lifetime in seconds.
         *
         * @retval kErrorNone    Option is appended successfully.
         * @retval kErrorNoBufs  Insufficient available buffers to grow the message.
         */
        Error AppendRecursiveDnsServerOption(const Address *aAddresses, uint8_t aNumAddresses, uint32_t aLifetime);

        /**
         * Indicates whether or not the received RA message contains any options.
         *
         * @retval TRUE   If the RA message contains at least one option.
         * @retval FALSE  If the RA message contains no options.
         */
        bool ContainsAnyOptions(void) const { return (mArray.GetLength() > sizeof(Header)); }
    };

    RouterAdvert(void) = delete;
};

/**
 * Implements the Router Solicitation message.
 *
 * See section 4.1 of RFC 4861 for definition of this message.
 * https://tools.ietf.org/html/rfc4861#section-4.1
 */
OT_TOOL_PACKED_BEGIN
class RouterSolicitHeader
{
public:
    /**
     * Initializes the Router Solicitation message.
     */
    RouterSolicitHeader(void);

private:
    Icmp::Header mHeader; // The common ICMPv6 header.
} OT_TOOL_PACKED_END;

static_assert(sizeof(RouterSolicitHeader) == 8, "invalid RouterSolicitHeader structure");
/**
 * Represents a Neighbor Solicitation (NS) message.
 */
OT_TOOL_PACKED_BEGIN
class NeighborSolicitHeader : public Clearable<NeighborSolicitHeader>
{
public:
    /**
     * Initializes the Neighbor Solicitation message header.
     */
    NeighborSolicitHeader(void);

    /**
     * Indicates whether the Neighbor Solicitation message is valid (proper Type and Code).
     *
     * @retval TRUE  If the message header is valid.
     * @retval FALSE If the message header is not valid.
     */
    bool IsValid(void) const { return (mType == Icmp::Header::kTypeNeighborSolicit) && (mCode == 0); }

    /**
     * Gets the Target Address field.
     *
     * @returns The Target Address.
     */
    const Address &GetTargetAddress(void) const { return mTargetAddress; }

    /**
     * Sets the Target Address field.
     *
     * @param[in] aTargetAddress  The Target Address.
     */
    void SetTargetAddress(const Address &aTargetAddress) { mTargetAddress = aTargetAddress; }

private:
    // Neighbor Solicitation Message (RFC 4861)
    //
    //   0                   1                   2                   3
    //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |     Type      |     Code      |          Checksum             |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                           Reserved                            |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                                                               |
    //  +                                                               +
    //  |                                                               |
    //  +                       Target Address                          +
    //  |                                                               |
    //  +                                                               +
    //  |                                                               |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |   Options ...
    //  +-+-+-+-+-+-+-+-+-+-+-+-

    uint8_t  mType;
    uint8_t  mCode;
    uint16_t mChecksum;
    uint32_t mReserved;
    Address  mTargetAddress;
} OT_TOOL_PACKED_END;

static_assert(sizeof(NeighborSolicitHeader) == 24, "Invalid NeighborSolicitHeader definition");

/**
 * Represents a Neighbor Advertisement (NA) message.
 */
OT_TOOL_PACKED_BEGIN
class NeighborAdvertMessage : public Clearable<NeighborAdvertMessage>
{
public:
    NeighborAdvertMessage(void);

    /**
     * Indicates whether the Neighbor Advertisement message is valid (proper Type and Code).
     *
     * @retval TRUE  If the message is valid.
     * @retval FALSE If the message is not valid.
     */
    bool IsValid(void) const { return (mType == Icmp::Header::kTypeNeighborAdvert) && (mCode == 0); }

    /**
     * Indicates whether or not the Router Flag is set in the NA message.
     *
     * @retval TRUE   The Router Flag is set.
     * @retval FALSE  The Router Flag is not set.
     */
    bool IsRouterFlagSet(void) const { return (mFlags & kRouterFlag) != 0; }

    /**
     * Sets the Router Flag in the NA message.
     */
    void SetRouterFlag(void) { mFlags |= kRouterFlag; }

    /**
     * Indicates whether or not the Solicited Flag is set in the NA message.
     *
     * @retval TRUE   The Solicited Flag is set.
     * @retval FALSE  The Solicited Flag is not set.
     */
    bool IsSolicitedFlagSet(void) const { return (mFlags & kSolicitedFlag) != 0; }

    /**
     * Sets the Solicited Flag in the NA message.
     */
    void SetSolicitedFlag(void) { mFlags |= kSolicitedFlag; }

    /**
     * Indicates whether or not the Override Flag is set in the NA message.
     *
     * @retval TRUE   The Override Flag is set.
     * @retval FALSE  The Override Flag is not set.
     */
    bool IsOverrideFlagSet(void) const { return (mFlags & kOverrideFlag) != 0; }

    /**
     * Sets the Override Flag in the NA message.
     */
    void SetOverrideFlag(void) { mFlags |= kOverrideFlag; }

    /**
     * Gets the Target Address field.
     *
     * @returns The Target Address.
     */
    const Address &GetTargetAddress(void) const { return mTargetAddress; }

    /**
     * Sets the Target Address field.
     *
     * @param[in] aTargetAddress  The Target Address.
     */
    void SetTargetAddress(const Address &aTargetAddress) { mTargetAddress = aTargetAddress; }

private:
    // Neighbor Advertisement Message (RFC 4861)
    //
    //   0                   1                   2                   3
    //   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |     Type      |     Code      |          Checksum             |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |R|S|O|                     Reserved                            |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |                                                               |
    //  +                                                               +
    //  |                                                               |
    //  +                       Target Address                          +
    //  |                                                               |
    //  +                                                               +
    //  |                                                               |
    //  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //  |   Options ...
    //  +-+-+-+-+-+-+-+-+-+-+-+-

    static constexpr uint8_t kRouterFlag    = (1 << 7);
    static constexpr uint8_t kSolicitedFlag = (1 << 6);
    static constexpr uint8_t kOverrideFlag  = (1 << 5);

    uint8_t  mType;
    uint8_t  mCode;
    uint16_t mChecksum;
    uint8_t  mFlags;
    uint8_t  mReserved[3];
    Address  mTargetAddress;
} OT_TOOL_PACKED_END;

static_assert(sizeof(NeighborAdvertMessage) == 24, "Invalid NeighborAdvertMessage definition");

} // namespace Nd
} // namespace Ip6
} // namespace ot

#endif // OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE

#endif // OT_CORE_NET_ND6_HPP_