//! ICMPv6 message handling for FIPS.
//!
//! Implements ICMPv6 error message generation per RFC 4443.
//! Currently supports Destination Unreachable (Type 1) for
//! packets that cannot be routed.

use std::net::Ipv6Addr;

/// ICMPv6 message types.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum Icmpv6Type {
    /// Destination Unreachable (error).
    DestinationUnreachable = 1,
    /// Packet Too Big (error).
    PacketTooBig = 2,
    /// Time Exceeded (error).
    TimeExceeded = 3,
    /// Parameter Problem (error).
    ParameterProblem = 4,
    /// Echo Request.
    EchoRequest = 128,
    /// Echo Reply.
    EchoReply = 129,
}

/// ICMPv6 Destination Unreachable codes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum DestUnreachableCode {
    /// No route to destination.
    NoRoute = 0,
    /// Communication administratively prohibited.
    AdminProhibited = 1,
    /// Beyond scope of source address.
    BeyondScope = 2,
    /// Address unreachable.
    AddressUnreachable = 3,
    /// Port unreachable.
    PortUnreachable = 4,
    /// Source address failed policy.
    SourcePolicy = 5,
    /// Reject route to destination.
    RejectRoute = 6,
}

/// IPv6 header next-header value for ICMPv6.
pub const IPPROTO_ICMPV6: u8 = 58;

/// Minimum IPv6 MTU - ICMPv6 responses must not exceed this.
const MIN_IPV6_MTU: usize = 1280;

/// IPv6 header length.
const IPV6_HEADER_LEN: usize = 40;

/// ICMPv6 header length (type + code + checksum + unused/data).
const ICMPV6_HEADER_LEN: usize = 8;

/// Maximum original packet bytes to include in ICMPv6 error.
const MAX_ORIGINAL_PACKET: usize = MIN_IPV6_MTU - IPV6_HEADER_LEN - ICMPV6_HEADER_LEN;

/// FIPS base encapsulation overhead for DataPacket (excluding port payload).
///
/// This is the fixed overhead for a SessionDatagram carrying an FSP DataPacket,
/// used by the send path's CP-flag guard to check whether piggybacked coords
/// fit within the transport MTU. For IPv6 effective MTU calculations, use
/// [`FIPS_IPV6_OVERHEAD`] which accounts for port multiplexing and header
/// compression.
///
/// Breakdown (traced through the actual send path):
///
/// ```text
/// FMP outer header (cleartext AAD)              16
///   common prefix (4) + receiver_idx (4) + counter (8)
/// FMP AEAD ciphertext:
///   timestamp (4) + msg_type (1)                 5   [FMP inner header]
///   ttl (1) + path_mtu (2) + src (16) + dst (16) 35  [SessionDatagram body]
///   FSP header (4 prefix + 8 counter)            12   [cleartext AAD]
///   FSP AEAD ciphertext:
///     timestamp (4) + msg_type (1) + flags (1)    6   [FSP inner header]
///     <application data>
///     Poly1305 tag                               16   [FSP AEAD]
/// FMP Poly1305 tag                              16   [FMP AEAD]
///                                              ────
///                                               106
/// ```
///
/// Note: the FMP inner header msg_type byte IS the SessionDatagram msg_type
/// byte (shared, not double-counted). The "35 bytes" is the SessionDatagram
/// body after msg_type is consumed by the dispatch layer.
pub const FIPS_OVERHEAD: u16 = 16 + 16 + 5 + 35 + 12 + 6 + 16; // 106 bytes

/// FIPS encapsulation overhead for compressed IPv6 shim traffic (port 256).
///
/// With port multiplexing (4 bytes) and IPv6 header compression (format byte +
/// 6 residual bytes, stripping 34 bytes of addresses/version/payload length),
/// the net overhead for IPv6 packets is 77 bytes.
///
/// ```text
/// Wire size = FIPS_OVERHEAD(106) + port_header(4) + format(1) + residual(6) + upper_payload
///           = 117 + (ipv6_len - 40)
///           = ipv6_len + 77
/// ```
pub const FIPS_IPV6_OVERHEAD: u16 = 77;

/// Calculate the effective IPv6 MTU for FIPS-encapsulated traffic.
///
/// Given a transport MTU (e.g., UDP payload size), returns the maximum
/// IPv6 packet size (including IPv6 header) that can be transmitted
/// through the FIPS mesh after IPv6 header compression.
pub fn effective_ipv6_mtu(transport_mtu: u16) -> u16 {
    transport_mtu.saturating_sub(FIPS_IPV6_OVERHEAD)
}

/// Check if we should send an ICMPv6 error for this packet.
///
/// Returns false if the packet is:
/// - Too short to be valid IPv6
/// - Not IPv6
/// - An ICMPv6 error message itself
/// - Has a multicast source address
/// - Has a multicast destination address
/// - Has an unspecified source address (::)
pub fn should_send_icmp_error(packet: &[u8]) -> bool {
    // Must have at least an IPv6 header
    if packet.len() < IPV6_HEADER_LEN {
        return false;
    }

    // Must be IPv6
    let version = packet[0] >> 4;
    if version != 6 {
        return false;
    }

    // Extract source address
    let src = Ipv6Addr::from(<[u8; 16]>::try_from(&packet[8..24]).unwrap());

    // Don't send errors for unspecified source
    if src.is_unspecified() {
        return false;
    }

    // Don't send errors for multicast source (first byte 0xff)
    if src.octets()[0] == 0xff {
        return false;
    }

    // Extract destination address
    let dst = Ipv6Addr::from(<[u8; 16]>::try_from(&packet[24..40]).unwrap());

    // Don't send errors for multicast destination (first byte 0xff)
    // e.g., ff02::2 (all-routers) from Router Solicitation
    if dst.octets()[0] == 0xff {
        return false;
    }

    // Don't send errors for ICMPv6 error messages (types 0-127)
    let next_header = packet[6];
    if next_header == IPPROTO_ICMPV6 && packet.len() > IPV6_HEADER_LEN {
        let icmp_type = packet[IPV6_HEADER_LEN];
        // ICMPv6 error messages are types 0-127
        if icmp_type < 128 {
            return false;
        }
    }

    true
}

/// Build an ICMPv6 Destination Unreachable response.
///
/// Takes the original packet that couldn't be delivered and returns
/// a complete IPv6 packet containing the ICMPv6 error response.
///
/// Arguments:
/// - `original_packet`: The packet that couldn't be routed
/// - `code`: The specific unreachable reason
/// - `our_addr`: Our FIPS address (source of the error)
///
/// Returns None if the original packet is invalid.
pub fn build_dest_unreachable(
    original_packet: &[u8],
    code: DestUnreachableCode,
    our_addr: Ipv6Addr,
) -> Option<Vec<u8>> {
    // Validate original packet
    if original_packet.len() < IPV6_HEADER_LEN {
        return None;
    }

    // Extract destination from original packet (becomes our destination)
    let dest_addr = Ipv6Addr::from(<[u8; 16]>::try_from(&original_packet[8..24]).unwrap());

    // Calculate how much of the original packet to include
    let original_len = original_packet.len().min(MAX_ORIGINAL_PACKET);
    let icmpv6_len = ICMPV6_HEADER_LEN + original_len;
    let total_len = IPV6_HEADER_LEN + icmpv6_len;

    let mut response = vec![0u8; total_len];

    // === IPv6 Header ===
    // Version (4) + Traffic Class (8) + Flow Label (20)
    response[0] = 0x60; // Version 6, TC high bits = 0
    // response[1..4] = 0 (TC low bits + flow label)

    // Payload length (ICMPv6 header + body)
    let payload_len = icmpv6_len as u16;
    response[4..6].copy_from_slice(&payload_len.to_be_bytes());

    // Next header = ICMPv6
    response[6] = IPPROTO_ICMPV6;

    // Hop limit
    response[7] = 64;

    // Source = our address
    response[8..24].copy_from_slice(&our_addr.octets());

    // Destination = original source
    response[24..40].copy_from_slice(&dest_addr.octets());

    // === ICMPv6 Header ===
    let icmp_start = IPV6_HEADER_LEN;

    // Type = Destination Unreachable
    response[icmp_start] = Icmpv6Type::DestinationUnreachable as u8;

    // Code
    response[icmp_start + 1] = code as u8;

    // Checksum placeholder (calculated below)
    // response[icmp_start + 2..icmp_start + 4] = 0

    // Unused (4 bytes of zeros for Dest Unreachable)
    // response[icmp_start + 4..icmp_start + 8] = 0

    // === ICMPv6 Body ===
    // As much of original packet as fits
    response[icmp_start + ICMPV6_HEADER_LEN..].copy_from_slice(&original_packet[..original_len]);

    // Calculate checksum
    let checksum = icmpv6_checksum(&response[icmp_start..], &our_addr, &dest_addr);
    response[icmp_start + 2..icmp_start + 4].copy_from_slice(&checksum.to_be_bytes());

    Some(response)
}

/// Build an ICMPv6 Packet Too Big response.
///
/// RFC 4443 Section 3.2: Packet Too Big Message
///
/// ## Wire Format
/// ```text
/// 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=2    |     Code=0    |          Checksum             |
/// +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/// |                             MTU                               |
/// +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/// |                    As much of invoking packet                 |
/// +               as possible without exceeding 1280              +
/// |                                                               |
/// ```
///
/// ## Parameters
/// - `original_packet`: The oversized IPv6 packet that triggered this error
/// - `mtu`: The MTU value to report (effective IPv6 MTU after FIPS overhead)
/// - `our_addr`: Our FIPS IPv6 address (source of ICMP message)
///
/// ## Returns
/// Complete IPv6 packet containing the ICMP Packet Too Big message,
/// ready to be written to the TUN interface.
pub fn build_packet_too_big(
    original_packet: &[u8],
    mtu: u32,
    our_addr: Ipv6Addr,
) -> Option<Vec<u8>> {
    // Validate original packet
    if original_packet.len() < IPV6_HEADER_LEN {
        return None;
    }

    // Must be IPv6
    let version = original_packet[0] >> 4;
    if version != 6 {
        return None;
    }

    // Extract source address from original packet (becomes ICMP destination)
    let src_addr = Ipv6Addr::from(<[u8; 16]>::try_from(&original_packet[8..24]).unwrap());

    // Don't send ICMP in response to:
    // - Multicast sources (ff00::/8)
    // - Unspecified source (::)
    if src_addr.is_unspecified() || src_addr.octets()[0] == 0xff {
        return None;
    }

    // Don't send ICMP in response to ICMP errors (avoid loops)
    let next_header = original_packet[6];
    if next_header == IPPROTO_ICMPV6 && original_packet.len() > IPV6_HEADER_LEN {
        let icmp_type = original_packet[IPV6_HEADER_LEN];
        // ICMPv6 error messages are types 0-127
        if icmp_type < 128 {
            return None;
        }
    }

    // Calculate how much of the original packet to include
    // RFC 4443: "as much of invoking packet as possible without exceeding 1280"
    let original_len = original_packet.len().min(MAX_ORIGINAL_PACKET);
    let icmpv6_len = ICMPV6_HEADER_LEN + original_len;
    let total_len = IPV6_HEADER_LEN + icmpv6_len;

    let mut response = vec![0u8; total_len];

    // === IPv6 Header ===
    // Version (4) + Traffic Class (8) + Flow Label (20)
    response[0] = 0x60; // Version 6, TC high bits = 0
    // response[1..4] = 0 (TC low bits + flow label)

    // Payload length (ICMPv6 header + body)
    let payload_len = icmpv6_len as u16;
    response[4..6].copy_from_slice(&payload_len.to_be_bytes());

    // Next header = ICMPv6
    response[6] = IPPROTO_ICMPV6;

    // Hop limit
    response[7] = 64;

    // Source = our address
    response[8..24].copy_from_slice(&our_addr.octets());

    // Destination = original source
    response[24..40].copy_from_slice(&src_addr.octets());

    // === ICMPv6 Header ===
    let icmp_start = IPV6_HEADER_LEN;

    // Type = Packet Too Big
    response[icmp_start] = Icmpv6Type::PacketTooBig as u8;

    // Code = 0 (always 0 for Packet Too Big)
    response[icmp_start + 1] = 0;

    // Checksum placeholder (calculated below)
    // response[icmp_start + 2..icmp_start + 4] = 0

    // MTU (4 bytes, network byte order per RFC 4443 §3.2)
    response[icmp_start + 4..icmp_start + 8].copy_from_slice(&mtu.to_be_bytes());

    // === ICMPv6 Body ===
    // As much of original packet as fits
    response[icmp_start + ICMPV6_HEADER_LEN..].copy_from_slice(&original_packet[..original_len]);

    // Calculate checksum
    let checksum = icmpv6_checksum(&response[icmp_start..], &our_addr, &src_addr);
    response[icmp_start + 2..icmp_start + 4].copy_from_slice(&checksum.to_be_bytes());

    Some(response)
}

/// Calculate ICMPv6 checksum per RFC 4443.
///
/// The checksum is calculated over a pseudo-header plus the ICMPv6 message.
fn icmpv6_checksum(icmpv6_message: &[u8], src: &Ipv6Addr, dst: &Ipv6Addr) -> u16 {
    let mut sum: u32 = 0;

    // Pseudo-header: source address (16 bytes)
    for chunk in src.octets().chunks(2) {
        sum += u16::from_be_bytes([chunk[0], chunk[1]]) as u32;
    }

    // Pseudo-header: destination address (16 bytes)
    for chunk in dst.octets().chunks(2) {
        sum += u16::from_be_bytes([chunk[0], chunk[1]]) as u32;
    }

    // Pseudo-header: upper-layer packet length (4 bytes, as u32)
    let len = icmpv6_message.len() as u32;
    sum += len >> 16;
    sum += len & 0xffff;

    // Pseudo-header: next header (padded to 4 bytes)
    sum += IPPROTO_ICMPV6 as u32;

    // ICMPv6 message (with checksum field = 0)
    let mut i = 0;
    while i + 1 < icmpv6_message.len() {
        // Skip the checksum field (bytes 2-3)
        if i == 2 {
            i += 2;
            continue;
        }
        sum += u16::from_be_bytes([icmpv6_message[i], icmpv6_message[i + 1]]) as u32;
        i += 2;
    }

    // Handle odd byte
    if i < icmpv6_message.len() {
        sum += (icmpv6_message[i] as u32) << 8;
    }

    // Fold 32-bit sum to 16 bits
    while sum >> 16 != 0 {
        sum = (sum & 0xffff) + (sum >> 16);
    }

    // One's complement
    !(sum as u16)
}

#[cfg(test)]
mod tests {
    use super::*;

    fn make_ipv6_packet(src: Ipv6Addr, dst: Ipv6Addr, next_header: u8, payload: &[u8]) -> Vec<u8> {
        let mut packet = vec![0u8; IPV6_HEADER_LEN + payload.len()];

        // Version + TC + Flow Label
        packet[0] = 0x60;

        // Payload length
        let len = payload.len() as u16;
        packet[4..6].copy_from_slice(&len.to_be_bytes());

        // Next header
        packet[6] = next_header;

        // Hop limit
        packet[7] = 64;

        // Source
        packet[8..24].copy_from_slice(&src.octets());

        // Destination
        packet[24..40].copy_from_slice(&dst.octets());

        // Payload
        packet[IPV6_HEADER_LEN..].copy_from_slice(payload);

        packet
    }

    #[test]
    fn test_should_send_error_valid_packet() {
        let src = "fd00::1".parse().unwrap();
        let dst = "fd00::2".parse().unwrap();
        let packet = make_ipv6_packet(src, dst, 17, &[0u8; 8]); // UDP

        assert!(should_send_icmp_error(&packet));
    }

    #[test]
    fn test_should_not_send_error_unspecified_source() {
        let src = Ipv6Addr::UNSPECIFIED;
        let dst = "fd00::2".parse().unwrap();
        let packet = make_ipv6_packet(src, dst, 17, &[0u8; 8]);

        assert!(!should_send_icmp_error(&packet));
    }

    #[test]
    fn test_should_not_send_error_multicast_source() {
        let src = "ff02::1".parse().unwrap();
        let dst = "fd00::2".parse().unwrap();
        let packet = make_ipv6_packet(src, dst, 17, &[0u8; 8]);

        assert!(!should_send_icmp_error(&packet));
    }

    #[test]
    fn test_should_not_send_error_multicast_destination() {
        let src = "fe80::1".parse().unwrap();
        let dst = "ff02::2".parse().unwrap(); // all-routers multicast
        let packet = make_ipv6_packet(src, dst, 17, &[0u8; 8]);

        assert!(!should_send_icmp_error(&packet));
    }

    #[test]
    fn test_should_not_send_error_for_icmp_error() {
        let src = "fd00::1".parse().unwrap();
        let dst = "fd00::2".parse().unwrap();
        // ICMPv6 Destination Unreachable (type 1)
        let icmp_payload = [1u8, 0, 0, 0, 0, 0, 0, 0];
        let packet = make_ipv6_packet(src, dst, IPPROTO_ICMPV6, &icmp_payload);

        assert!(!should_send_icmp_error(&packet));
    }

    #[test]
    fn test_should_send_error_for_icmp_echo() {
        let src = "fd00::1".parse().unwrap();
        let dst = "fd00::2".parse().unwrap();
        // ICMPv6 Echo Request (type 128) - informational, not error
        let icmp_payload = [128u8, 0, 0, 0, 0, 0, 0, 0];
        let packet = make_ipv6_packet(src, dst, IPPROTO_ICMPV6, &icmp_payload);

        assert!(should_send_icmp_error(&packet));
    }

    #[test]
    fn test_should_not_send_error_short_packet() {
        let packet = vec![0u8; 20]; // Too short for IPv6
        assert!(!should_send_icmp_error(&packet));
    }

    #[test]
    fn test_build_dest_unreachable() {
        let src: Ipv6Addr = "fd00::1".parse().unwrap();
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        let original = make_ipv6_packet(src, dst, 17, &[0u8; 8]);

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let response = build_dest_unreachable(&original, DestUnreachableCode::NoRoute, our_addr);

        assert!(response.is_some());
        let response = response.unwrap();

        // Check IPv6 header
        assert_eq!(response[0] >> 4, 6); // Version
        assert_eq!(response[6], IPPROTO_ICMPV6); // Next header

        // Check source is our address
        let resp_src = Ipv6Addr::from(<[u8; 16]>::try_from(&response[8..24]).unwrap());
        assert_eq!(resp_src, our_addr);

        // Check destination is original source
        let resp_dst = Ipv6Addr::from(<[u8; 16]>::try_from(&response[24..40]).unwrap());
        assert_eq!(resp_dst, src);

        // Check ICMPv6 type and code
        assert_eq!(response[IPV6_HEADER_LEN], 1); // Type = Dest Unreachable
        assert_eq!(response[IPV6_HEADER_LEN + 1], 0); // Code = No Route
    }

    #[test]
    fn test_build_dest_unreachable_invalid_input() {
        let short_packet = vec![0u8; 20];
        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();

        let response =
            build_dest_unreachable(&short_packet, DestUnreachableCode::NoRoute, our_addr);
        assert!(response.is_none());
    }

    #[test]
    fn test_build_dest_unreachable_truncates_large_packet() {
        let src: Ipv6Addr = "fd00::1".parse().unwrap();
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        // Large payload
        let original = make_ipv6_packet(src, dst, 17, &[0u8; 2000]);

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let response = build_dest_unreachable(&original, DestUnreachableCode::NoRoute, our_addr);

        assert!(response.is_some());
        let response = response.unwrap();

        // Response must not exceed minimum MTU
        assert!(response.len() <= MIN_IPV6_MTU);
    }

    #[test]
    fn test_build_packet_too_big() {
        let src: Ipv6Addr = "fd00::1".parse().unwrap();
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        let original = make_ipv6_packet(src, dst, 17, &[0u8; 1200]); // Large UDP packet

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let mtu = 1070u32;
        let response = build_packet_too_big(&original, mtu, our_addr);

        assert!(response.is_some());
        let response = response.unwrap();

        // Check IPv6 header
        assert_eq!(response[0] >> 4, 6); // Version
        assert_eq!(response[6], IPPROTO_ICMPV6); // Next header

        // Check source is our address
        let resp_src = Ipv6Addr::from(<[u8; 16]>::try_from(&response[8..24]).unwrap());
        assert_eq!(resp_src, our_addr);

        // Check destination is original source
        let resp_dst = Ipv6Addr::from(<[u8; 16]>::try_from(&response[24..40]).unwrap());
        assert_eq!(resp_dst, src);

        // Check ICMPv6 type and code
        assert_eq!(response[IPV6_HEADER_LEN], 2); // Type = Packet Too Big
        assert_eq!(response[IPV6_HEADER_LEN + 1], 0); // Code = 0

        // Check MTU value (32-bit field per RFC 4443 §3.2)
        let reported_mtu = u32::from_be_bytes([
            response[IPV6_HEADER_LEN + 4],
            response[IPV6_HEADER_LEN + 5],
            response[IPV6_HEADER_LEN + 6],
            response[IPV6_HEADER_LEN + 7],
        ]);
        assert_eq!(reported_mtu, mtu);

        // Response must not exceed minimum MTU
        assert!(response.len() <= MIN_IPV6_MTU);
    }

    #[test]
    fn test_build_packet_too_big_invalid_input() {
        let short_packet = vec![0u8; 20];
        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();

        let response = build_packet_too_big(&short_packet, 1280, our_addr);
        assert!(response.is_none());
    }

    #[test]
    fn test_build_packet_too_big_multicast_source() {
        let src: Ipv6Addr = "ff02::1".parse().unwrap(); // Multicast
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        let original = make_ipv6_packet(src, dst, 17, &[0u8; 1200]);

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let response = build_packet_too_big(&original, 1280, our_addr);

        // Should not send ICMP for multicast source
        assert!(response.is_none());
    }

    #[test]
    fn test_build_packet_too_big_unspecified_source() {
        let src = Ipv6Addr::UNSPECIFIED;
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        let original = make_ipv6_packet(src, dst, 17, &[0u8; 1200]);

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let response = build_packet_too_big(&original, 1280, our_addr);

        // Should not send ICMP for unspecified source
        assert!(response.is_none());
    }

    #[test]
    fn test_build_packet_too_big_for_icmp_error() {
        let src: Ipv6Addr = "fd00::1".parse().unwrap();
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        // ICMPv6 Destination Unreachable (type 1) - an error message
        let icmp_payload = [1u8, 0, 0, 0, 0, 0, 0, 0];
        let original = make_ipv6_packet(src, dst, IPPROTO_ICMPV6, &icmp_payload);

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let response = build_packet_too_big(&original, 1280, our_addr);

        // Should not send ICMP in response to ICMP error
        assert!(response.is_none());
    }

    #[test]
    fn test_build_packet_too_big_for_icmp_echo() {
        let src: Ipv6Addr = "fd00::1".parse().unwrap();
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        // ICMPv6 Echo Request (type 128) - informational, not error
        let icmp_payload = [128u8, 0, 0, 0, 0, 0, 0, 0];
        let original = make_ipv6_packet(src, dst, IPPROTO_ICMPV6, &icmp_payload);

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let response = build_packet_too_big(&original, 1280, our_addr);

        // Should send ICMP for informational messages
        assert!(response.is_some());
    }

    #[test]
    fn test_build_packet_too_big_truncates_large_packet() {
        let src: Ipv6Addr = "fd00::1".parse().unwrap();
        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
        // Very large payload
        let original = make_ipv6_packet(src, dst, 17, &[0u8; 2000]);

        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
        let response = build_packet_too_big(&original, 1070, our_addr);

        assert!(response.is_some());
        let response = response.unwrap();

        // Response must not exceed minimum MTU
        assert!(response.len() <= MIN_IPV6_MTU);
    }

    /// Verify that when the ICMP source is set to the original packet's
    /// destination (the remote peer), the PTB is correctly formed.
    ///
    /// This is the critical fix for the PMTUD blackhole: Linux ignores
    /// ICMPv6 PTBs whose source matches a local address. By using the
    /// remote peer's address as the ICMP source, the kernel sees the PTB
    /// as coming from a "remote router" and honors it.
    #[test]
    fn test_build_packet_too_big_remote_source_for_pmtud() {
        let local_addr: Ipv6Addr = "fd41::1".parse().unwrap();
        let remote_addr: Ipv6Addr = "fddf::2".parse().unwrap();
        let original = make_ipv6_packet(local_addr, remote_addr, 6, &[0u8; 1200]); // TCP

        // Pass remote_addr as our_addr — this is what send_icmpv6_packet_too_big
        // does after the fix (original packet's dst = remote peer).
        let response = build_packet_too_big(&original, 1203, remote_addr);
        assert!(response.is_some());
        let response = response.unwrap();

        // PTB source must be the remote peer (not local)
        let ptb_src = Ipv6Addr::from(<[u8; 16]>::try_from(&response[8..24]).unwrap());
        assert_eq!(
            ptb_src, remote_addr,
            "PTB source must be remote peer address"
        );

        // PTB destination must be the local sender (original src)
        let ptb_dst = Ipv6Addr::from(<[u8; 16]>::try_from(&response[24..40]).unwrap());
        assert_eq!(
            ptb_dst, local_addr,
            "PTB destination must be original sender"
        );

        // Verify ICMPv6 type/code
        assert_eq!(response[IPV6_HEADER_LEN], 2); // Type = Packet Too Big
        assert_eq!(response[IPV6_HEADER_LEN + 1], 0); // Code = 0

        // Verify reported MTU
        let reported_mtu = u32::from_be_bytes([
            response[IPV6_HEADER_LEN + 4],
            response[IPV6_HEADER_LEN + 5],
            response[IPV6_HEADER_LEN + 6],
            response[IPV6_HEADER_LEN + 7],
        ]);
        assert_eq!(reported_mtu, 1203);

        // Verify checksum is valid (recalculate and compare)
        let stored_checksum =
            u16::from_be_bytes([response[IPV6_HEADER_LEN + 2], response[IPV6_HEADER_LEN + 3]]);
        let recomputed = icmpv6_checksum(&response[IPV6_HEADER_LEN..], &remote_addr, &local_addr);
        assert_eq!(stored_checksum, recomputed, "ICMPv6 checksum must be valid");
    }
}