UDPデータグラムのパース/構築と、疑似ヘッダによるチェックサム

// ip-icmp-udp/include/ipicmpudp/udp.hpp
struct UdpDatagram
{
    std::uint16_t source_port;
    std::uint16_t destination_port;
    std::span<const std::byte> payload;
};

enum class UdpParseError
{
    DatagramTooShort,
    LengthMismatch,
    ChecksumMismatch,
};

std::expected<UdpDatagram, UdpParseError> parse_udp_datagram(
    std::span<const std::byte> datagram,
    Ipv4Address source_ip,
    Ipv4Address destination_ip
);

std::vector<std::byte> build_udp_datagram(
    std::uint16_t source_port,
    std::uint16_t destination_port,
    std::span<const std::byte> payload,
    Ipv4Address source_ip,
    Ipv4Address destination_ip
);

// ip-icmp-udp/src/udp.cpp（抜粋）
std::vector<std::byte> build_pseudo_header(
    Ipv4Address source_ip,
    Ipv4Address destination_ip,
    std::uint16_t udp_length
)
{
    std::vector<std::byte> pseudo(PseudoHeaderSize);
    std::span<std::byte> bytes(pseudo);

    std::ranges::copy(source_ip, bytes.begin());
    std::ranges::copy(destination_ip, bytes.begin() + 4);
    bytes[8] = std::byte{0};
    bytes[9] = static_cast<std::byte>(IpProtocol::Udp);
    detail::write_uint16(bytes.subspan<10, 2>(), udp_length);

    return pseudo;
}

// ip-icmp-udp/src/udp.cpp（抜粋）
std::uint16_t compute_udp_checksum(
    std::span<const std::byte> udp_message,
    Ipv4Address source_ip,
    Ipv4Address destination_ip
)
{
    auto pseudo = build_pseudo_header(
        source_ip, destination_ip, static_cast<std::uint16_t>(udp_message.size())
    );

    std::vector<std::byte> combined(pseudo.size() + udp_message.size());
    std::ranges::copy(pseudo, combined.begin());
    std::ranges::copy(udp_message, combined.begin() + pseudo.size());

    return compute_checksum(combined);
}

// ip-icmp-udp/src/udp.cpp（抜粋）
const auto checksum = compute_udp_checksum(bytes, source_ip, destination_ip);
detail::write_uint16(bytes.subspan<6, 2>(), checksum == 0 ? std::uint16_t{0xFFFF} : checksum);

// ip-icmp-udp/src/ipv4.cpp（抜粋）
std::vector<std::byte> build_ipv4_packet(
    IpProtocol protocol,
    Ipv4Address source,
    Ipv4Address destination,
    std::uint16_t identification,
    std::span<const std::byte> payload
)
{
    constexpr std::size_t HeaderSize = 20;
    constexpr std::uint8_t Ttl = 64;

    std::vector<std::byte> packet(HeaderSize + payload.size());
    std::span<std::byte> bytes(packet);

    bytes[0] = std::byte{0x45}; // version 4, IHL 5
    bytes[1] = std::byte{0x00};
    detail::write_uint16(bytes.subspan<2, 2>(), static_cast<std::uint16_t>(packet.size()));
    detail::write_uint16(bytes.subspan<4, 2>(), identification);
    detail::write_uint16(bytes.subspan<6, 2>(), 0); // フラグメンテーションは扱わない
    bytes[8] = static_cast<std::byte>(Ttl);
    bytes[9] = static_cast<std::byte>(protocol);
    detail::write_uint16(bytes.subspan<10, 2>(), 0); // checksum 仮置き
    std::ranges::copy(source, bytes.begin() + 12);
    std::ranges::copy(destination, bytes.begin() + 16);
    std::ranges::copy(payload, bytes.begin() + HeaderSize);

    const auto checksum = compute_checksum(bytes.subspan(0, HeaderSize));
    detail::write_uint16(bytes.subspan<10, 2>(), checksum);

    return packet;
}

// ip-icmp-udp/apps/udp_echo_client.cpp（抜粋）
auto udp_payload = ipicmpudp::build_udp_datagram(
    LocalPort, target_port, std::as_bytes(std::span(message)), *sender_ip, *target_ip
);
auto ip_packet = ipicmpudp::build_ipv4_packet(
    ipicmpudp::IpProtocol::Udp, *sender_ip, *target_ip, 1, udp_payload
);

std::vector<std::byte> frame(14 + ip_packet.size());
std::span<std::byte> frame_bytes(frame);
std::ranges::copy(*target_mac, frame_bytes.begin());
std::ranges::copy(*sender_mac, frame_bytes.begin() + 6);
frame_bytes[12] = std::byte{0x08};
frame_bytes[13] = std::byte{0x00}; // EtherType = IPv4(0x0800)
std::ranges::copy(ip_packet, frame_bytes.begin() + 14);