#[cfg(feature = "std")]
use crate::err::ip::HeadersWriteError;
use crate::err::{Layer, LenError, ValueTooBigError};
use crate::*;

/// Internet protocol headers version 4 & 6.
#[derive(Clone, Debug, Eq, PartialEq)]
#[allow(clippy::large_enum_variant)]
pub enum IpHeaders {
    /// IPv4 header & extension headers.
    Ipv4(Ipv4Header, Ipv4Extensions),
    /// IPv6 header & extension headers.
    Ipv6(Ipv6Header, Ipv6Extensions),
}

impl IpHeaders {
    /// Maximum summed up length of all extension headers in bytes/octets.
    pub const MAX_LEN: usize = Ipv6Header::LEN + Ipv6Extensions::MAX_LEN;

    /// Returns references to the IPv4 header & extensions if the header contains IPv4 values.
    pub fn ipv4(&self) -> Option<(&Ipv4Header, &Ipv4Extensions)> {
        if let IpHeaders::Ipv4(header, exts) = self {
            Some((header, exts))
        } else {
            None
        }
    }

    /// Returns references to the IPv6 header & extensions if the header contains IPv6 values.
    pub fn ipv6(&self) -> Option<(&Ipv6Header, &Ipv6Extensions)> {
        if let IpHeaders::Ipv6(header, exts) = self {
            Some((header, exts))
        } else {
            None
        }
    }

    /// Renamed to [`IpHeaders::from_slice`]
    #[deprecated(since = "0.10.1", note = "Renamed to `IpHeaders::from_slice`")]
    #[inline]
    pub fn read_from_slice(
        slice: &[u8],
    ) -> Result<(IpHeaders, IpNumber, &[u8]), err::ip::HeadersSliceError> {
        let (header, payload) = IpHeaders::from_slice(slice)?;
        Ok((header, payload.ip_number, payload.payload))
    }

    /// Read an [`IpHeaders`] from a slice and return the headers & payload of
    /// the IP packet (determined based on the length fields in the IP header).
    ///
    /// Note that his function returns an [`crate::err::LenError`] if the given slice
    /// contains less data then the length fields in the IP header indicate should
    /// be present.
    ///
    /// If you want to ignore these kind of length errors based on the length
    /// fields in the IP headers use [`IpHeaders::from_slice_lax`] instead.
    pub fn from_slice(
        slice: &[u8],
    ) -> Result<(IpHeaders, IpPayloadSlice<'_>), err::ip::HeadersSliceError> {
        use err::ip::{HeaderError::*, HeadersError::*, HeadersSliceError::*};

        if slice.is_empty() {
            Err(Len(err::LenError {
                required_len: 1,
                len: slice.len(),
                len_source: LenSource::Slice,
                layer: err::Layer::IpHeader,
                layer_start_offset: 0,
            }))
        } else {
            match slice[0] >> 4 {
                4 => {
                    // check length
                    if slice.len() < Ipv4Header::MIN_LEN {
                        return Err(Len(err::LenError {
                            required_len: Ipv4Header::MIN_LEN,
                            len: slice.len(),
                            len_source: LenSource::Slice,
                            layer: err::Layer::Ipv4Header,
                            layer_start_offset: 0,
                        }));
                    }

                    // read ihl
                    //
                    // SAFETY:
                    // Safe as the slice length is checked to be at least
                    // Ipv4Header::MIN_LEN (20) at the start.
                    let ihl = unsafe { slice.get_unchecked(0) } & 0xf;

                    //check that the ihl is correct
                    if ihl < 5 {
                        return Err(Content(Ip(Ipv4HeaderLengthSmallerThanHeader { ihl })));
                    }

                    // check that the slice contains enough data for the entire header + options
                    let header_len = usize::from(ihl) * 4;
                    if slice.len() < header_len {
                        return Err(Len(LenError {
                            required_len: header_len,
                            len: slice.len(),
                            len_source: LenSource::Slice,
                            layer: Layer::Ipv4Header,
                            layer_start_offset: 0,
                        }));
                    }

                    let header = unsafe {
                        // SAFETY: Safe as the IHL & slice len has been validated
                        Ipv4HeaderSlice::from_slice_unchecked(core::slice::from_raw_parts(
                            slice.as_ptr(),
                            header_len,
                        ))
                        .to_header()
                    };

                    // check that the total len is at least containing the header len
                    let total_len: usize = header.total_len.into();
                    if total_len < header_len {
                        return Err(Len(LenError {
                            required_len: header_len,
                            len: total_len,
                            len_source: LenSource::Ipv4HeaderTotalLen,
                            layer: Layer::Ipv4Packet,
                            layer_start_offset: 0,
                        }));
                    }

                    // restrict the rest of the slice based on the total len
                    let rest = if slice.len() < total_len {
                        return Err(Len(LenError {
                            required_len: total_len,
                            len: slice.len(),
                            len_source: LenSource::Slice,
                            layer: Layer::Ipv4Packet,
                            layer_start_offset: 0,
                        }));
                    } else {
                        unsafe {
                            core::slice::from_raw_parts(
                                // SAFETY: Safe as the slice length was validated to be at least header_length
                                slice.as_ptr().add(header_len),
                                // SAFETY: Safe as slice length has been validated to be at least total_length_usize long
                                total_len - header_len,
                            )
                        }
                    };

                    let (exts, next_protocol, rest) =
                        Ipv4Extensions::from_slice(header.protocol, rest).map_err(|err| {
                            use err::ip_auth::HeaderSliceError as I;
                            match err {
                                I::Len(mut err) => {
                                    err.layer_start_offset += header_len;
                                    err.len_source = LenSource::Ipv4HeaderTotalLen;
                                    Len(err)
                                }
                                I::Content(err) => Content(Ipv4Ext(err)),
                            }
                        })?;

                    let fragmented = header.is_fragmenting_payload();
                    Ok((
                        IpHeaders::Ipv4(header, exts),
                        IpPayloadSlice {
                            ip_number: next_protocol,
                            fragmented,
                            len_source: LenSource::Ipv4HeaderTotalLen,
                            payload: rest,
                        },
                    ))
                }
                6 => {
                    if slice.len() < Ipv6Header::LEN {
                        return Err(Len(err::LenError {
                            required_len: Ipv6Header::LEN,
                            len: slice.len(),
                            len_source: LenSource::Slice,
                            layer: err::Layer::Ipv6Header,
                            layer_start_offset: 0,
                        }));
                    }
                    let header = {
                        // SAFETY:
                        // This is safe as the slice length is checked to be
                        // at least Ipv6Header::LEN (40) before this code block.
                        unsafe {
                            Ipv6HeaderSlice::from_slice_unchecked(core::slice::from_raw_parts(
                                slice.as_ptr(),
                                Ipv6Header::LEN,
                            ))
                            .to_header()
                        }
                    };

                    // restrict slice by the length specified in the header
                    let (header_payload, len_source) =
                        if 0 == header.payload_length && slice.len() > Ipv6Header::LEN {
                            // In case the payload_length is 0 assume that the entire
                            // rest of the slice is part of the packet until the jumbogram
                            // parameters can be parsed.

                            // TODO: Add payload length parsing from the jumbogram
                            unsafe {
                                (
                                    core::slice::from_raw_parts(
                                        slice.as_ptr().add(Ipv6Header::LEN),
                                        slice.len() - Ipv6Header::LEN,
                                    ),
                                    LenSource::Slice,
                                )
                            }
                        } else {
                            let payload_len: usize = header.payload_length.into();
                            let expected_len = Ipv6Header::LEN + payload_len;
                            if slice.len() < expected_len {
                                return Err(Len(LenError {
                                    required_len: expected_len,
                                    len: slice.len(),
                                    len_source: LenSource::Slice,
                                    layer: Layer::Ipv6Packet,
                                    layer_start_offset: 0,
                                }));
                            } else {
                                unsafe {
                                    (
                                        core::slice::from_raw_parts(
                                            slice.as_ptr().add(Ipv6Header::LEN),
                                            payload_len,
                                        ),
                                        LenSource::Ipv6HeaderPayloadLen,
                                    )
                                }
                            }
                        };

                    let (exts, next_header, rest) =
                        Ipv6Extensions::from_slice(header.next_header, header_payload).map_err(
                            |err| {
                                use err::ipv6_exts::HeaderSliceError as I;
                                match err {
                                    I::Len(mut err) => {
                                        err.layer_start_offset += Ipv6Header::LEN;
                                        err.len_source = len_source;
                                        Len(err)
                                    }
                                    I::Content(err) => Content(Ipv6Ext(err)),
                                }
                            },
                        )?;

                    let fragmented = exts.is_fragmenting_payload();
                    Ok((
                        IpHeaders::Ipv6(header, exts),
                        IpPayloadSlice {
                            ip_number: next_header,
                            fragmented,
                            len_source,
                            payload: rest,
                        },
                    ))
                }
                version_number => Err(Content(Ip(UnsupportedIpVersion { version_number }))),
            }
        }
    }

    /// Reads an [`IpHeaders`] as far as possible without encountering an error &
    /// separates the payload from the given slice with less strict length checks.
    /// This function is usefull for cut off packet or for packets with unset length
    /// fields).
    ///
    /// If you want to only receive correct IpPayloads use [`IpHeaders::from_slice`]
    /// instead.
    ///
    /// The main usecases for this functions are:
    ///
    /// * Parsing packets that have been cut off. This is, for example, useful to
    ///   parse packets returned via ICMP as these usually only contain the start.
    /// * Parsing packets where the `total_len` (for IPv4) or `payload_length` (for IPv6)
    ///   have not yet been set. This can be useful when parsing packets which have been
    ///   recorded in a layer before the length field was set (e.g. before the operating
    ///   system set the length fields).
    ///
    /// # Differences to `from_slice`:
    ///
    /// There are two main differences:
    ///
    /// * Errors in the expansion headers will only stop the parsing and return an `Ok`
    ///   with the successfully parsed parts and the error as optional. Only if an
    ///   unrecoverable error is encountered in the IP header itself an `Err` is returned.
    ///   In the normal `from_slice` function an `Err` is returned if an error is
    ///   encountered in an exteions header.
    /// * `from_slice_lax` ignores inconsistent `total_len` (in IPv4 headers) and
    ///   inconsistent `payload_length` (in IPv6 headers) values. When these length
    ///   values in the IP header are inconsistant the length of the given slice is
    ///   used as a substitute.
    ///
    /// You can check if the slice length was used as a substitude by checking
    /// if the `len_source` value in the returned [`LaxIpPayloadSlice`] is set to
    /// [`LenSource::Slice`]. If a substitution was not needed `len_source`
    /// is set to [`LenSource::Ipv4HeaderTotalLen`] or
    /// [`LenSource::Ipv6HeaderPayloadLen`].
    ///
    /// # When is the slice length used as a fallback?
    ///
    /// For IPv4 packets the slice length is used as a fallback/substitude
    /// if the `total_length` field in the IPv4 header is:
    ///
    ///  * Bigger then the given slice (payload cannot fully be seperated).
    ///  * Too small to contain at least the IPv4 header.
    ///
    /// For IPv6 packet the slice length is used as a fallback/substitude
    /// if the `payload_length` is
    ///
    /// * Bigger then the given slice (payload cannot fully be seperated).
    /// * The value `0`.
    pub fn from_slice_lax(
        slice: &[u8],
    ) -> Result<
        (
            IpHeaders,
            LaxIpPayloadSlice<'_>,
            Option<(err::ip_exts::HeadersSliceError, Layer)>,
        ),
        err::ip::LaxHeaderSliceError,
    > {
        use err::ip::{HeaderError::*, LaxHeaderSliceError::*};

        if slice.is_empty() {
            Err(Len(err::LenError {
                required_len: 1,
                len: slice.len(),
                len_source: LenSource::Slice,
                layer: err::Layer::IpHeader,
                layer_start_offset: 0,
            }))
        } else {
            match slice[0] >> 4 {
                4 => {
                    // check length
                    if slice.len() < Ipv4Header::MIN_LEN {
                        return Err(Len(err::LenError {
                            required_len: Ipv4Header::MIN_LEN,
                            len: slice.len(),
                            len_source: LenSource::Slice,
                            layer: err::Layer::Ipv4Header,
                            layer_start_offset: 0,
                        }));
                    }

                    // read ihl
                    //
                    // SAFETY:
                    // Safe as the slice length is checked to be at least
                    // Ipv4Header::MIN_LEN (20) at the start.
                    let ihl = unsafe { slice.get_unchecked(0) } & 0xf;

                    //check that the ihl is correct
                    if ihl < 5 {
                        return Err(Content(Ipv4HeaderLengthSmallerThanHeader { ihl }));
                    }

                    // check that the slice contains enough data for the entire header + options
                    let header_len = usize::from(ihl) * 4;
                    if slice.len() < header_len {
                        return Err(Len(LenError {
                            required_len: header_len,
                            len: slice.len(),
                            len_source: LenSource::Slice,
                            layer: Layer::Ipv4Header,
                            layer_start_offset: 0,
                        }));
                    }

                    let header = unsafe {
                        // SAFETY: Safe as the IHL & slice len has been validated
                        Ipv4HeaderSlice::from_slice_unchecked(core::slice::from_raw_parts(
                            slice.as_ptr(),
                            header_len,
                        ))
                        .to_header()
                    };

                    // check that the total len is at least containing the header len
                    let total_len: usize = header.total_len.into();

                    // restrict the rest of the slice based on the total len (if the total_len is not conflicting)
                    let (len_source, rest, incomplete) = if total_len < header_len {
                        // fallback to slice len
                        (
                            LenSource::Slice,
                            unsafe {
                                core::slice::from_raw_parts(
                                    // SAFETY: Safe as the slice length was validated to be at least header_length
                                    slice.as_ptr().add(header_len),
                                    // SAFETY: Safe as slice length has been validated to be at least header_len long
                                    slice.len() - header_len,
                                )
                            },
                            false,
                        )
                    } else if slice.len() < total_len {
                        (
                            LenSource::Slice,
                            unsafe {
                                core::slice::from_raw_parts(
                                    // SAFETY: Safe as the slice length was validated to be at least header_length
                                    slice.as_ptr().add(header_len),
                                    // SAFETY: Safe as slice length has been validated to be at least header_len long
                                    slice.len() - header_len,
                                )
                            },
                            true,
                        )
                    } else {
                        (
                            LenSource::Ipv4HeaderTotalLen,
                            unsafe {
                                core::slice::from_raw_parts(
                                    // SAFETY: Safe as the slice length was validated to be at least header_length
                                    slice.as_ptr().add(header_len),
                                    // SAFETY: Safe as slice length has been validated to be at least total_length_usize long
                                    total_len - header_len,
                                )
                            },
                            false,
                        )
                    };

                    let (exts, next_protocol, rest, stop_err) =
                        Ipv4Extensions::from_slice_lax(header.protocol, rest);

                    let stop_err = stop_err.map(|err| {
                        use err::ip_auth::HeaderSliceError as I;
                        use err::ip_exts::HeaderError as OC;
                        use err::ip_exts::HeadersSliceError as O;
                        match err {
                            I::Len(mut l) => O::Len({
                                l.layer_start_offset += header_len;
                                l.len_source = len_source;
                                l
                            }),
                            I::Content(c) => O::Content(OC::Ipv4Ext(c)),
                        }
                    });

                    let fragmented = header.is_fragmenting_payload();
                    Ok((
                        IpHeaders::Ipv4(header, exts),
                        LaxIpPayloadSlice {
                            incomplete,
                            ip_number: next_protocol,
                            fragmented,
                            len_source,
                            payload: rest,
                        },
                        stop_err.map(|v| (v, Layer::IpAuthHeader)),
                    ))
                }
                6 => {
                    if slice.len() < Ipv6Header::LEN {
                        return Err(Len(err::LenError {
                            required_len: Ipv6Header::LEN,
                            len: slice.len(),
                            len_source: LenSource::Slice,
                            layer: err::Layer::Ipv6Header,
                            layer_start_offset: 0,
                        }));
                    }
                    let header = {
                        // SAFETY:
                        // This is safe as the slice length is checked to be
                        // at least Ipv6Header::LEN (40) befpre this code block.
                        unsafe {
                            Ipv6HeaderSlice::from_slice_unchecked(core::slice::from_raw_parts(
                                slice.as_ptr(),
                                Ipv6Header::LEN,
                            ))
                            .to_header()
                        }
                    };

                    // restrict slice by the length specified in the header
                    let payload_len = usize::from(header.payload_length);
                    let (header_payload, len_source, incomplete) =
                        if (header.payload_length == 0) && (Ipv6Header::LEN < slice.len()) {
                            // TODO: Add payload length parsing from the jumbogram
                            unsafe {
                                (
                                    core::slice::from_raw_parts(
                                        // SAFTEY: Safe as we verify what `slice.len() >= Ipv6Header::LEN` above.
                                        slice.as_ptr().add(Ipv6Header::LEN),
                                        // SAFTEY: Safe as we verify what `slice.len() >= Ipv6Header::LEN` above.
                                        slice.len() - Ipv6Header::LEN,
                                    ),
                                    LenSource::Slice,
                                    false,
                                )
                            }
                        } else if (slice.len() - Ipv6Header::LEN) < payload_len {
                            unsafe {
                                (
                                    core::slice::from_raw_parts(
                                        // SAFTEY: Safe as we verify what `slice.len() >= Ipv6Header::LEN` above.
                                        slice.as_ptr().add(Ipv6Header::LEN),
                                        // SAFTEY: Safe as we verify what `slice.len() >= Ipv6Header::LEN` above.
                                        slice.len() - Ipv6Header::LEN,
                                    ),
                                    LenSource::Slice,
                                    true,
                                )
                            }
                        } else {
                            unsafe {
                                (
                                    core::slice::from_raw_parts(
                                        // SAFTEY: Safe as we verify what `slice.len() >= Ipv6Header::LEN` above.
                                        slice.as_ptr().add(Ipv6Header::LEN),
                                        // SAFTEY: Safe as we verify that `(slice.len() - Ipv6Header::LEN) >= payload_len` above.
                                        payload_len,
                                    ),
                                    LenSource::Ipv6HeaderPayloadLen,
                                    false,
                                )
                            }
                        };

                    let (exts, next_header, rest, stop_err) =
                        Ipv6Extensions::from_slice_lax(header.next_header, header_payload);

                    let stop_err = stop_err.map(|(err, layer)| {
                        use err::ip_exts::HeaderError::Ipv6Ext;
                        use err::ip_exts::HeadersSliceError as O;
                        use err::ipv6_exts::HeaderSliceError as I;
                        (
                            match err {
                                I::Len(mut l) => {
                                    l.layer_start_offset += Ipv6Header::LEN;
                                    l.len_source = len_source;
                                    O::Len(l)
                                }
                                I::Content(c) => O::Content(Ipv6Ext(c)),
                            },
                            layer,
                        )
                    });

                    let fragmented = exts.is_fragmenting_payload();
                    Ok((
                        IpHeaders::Ipv6(header, exts),
                        LaxIpPayloadSlice {
                            incomplete,
                            ip_number: next_header,
                            fragmented,
                            len_source,
                            payload: rest,
                        },
                        stop_err,
                    ))
                }
                version_number => Err(Content(UnsupportedIpVersion { version_number })),
            }
        }
    }

    /// Read an IPv4 header & extension headers from a slice and return the slice containing the payload
    /// according to the total_length field in the IPv4 header.
    ///
    /// Note that his function returns an [`err::LenError`] if the given slice
    /// contains less data then the `total_len` field in the IPv4 header indicates
    /// should be present.
    ///
    /// If you want to ignore these kind of length errors based on the length
    /// fields in the IP headers use [`IpHeaders::from_ipv4_slice_lax`] instead.
    pub fn from_ipv4_slice(
        slice: &[u8],
    ) -> Result<(IpHeaders, IpPayloadSlice<'_>), err::ipv4::SliceError> {
        use err::ipv4::SliceError::*;

        // read the header
        let (header, header_rest) = Ipv4Header::from_slice(slice).map_err(|err| {
            use err::ipv4::HeaderSliceError as I;
            match err {
                I::Len(err) => Len(err),
                I::Content(err) => Header(err),
            }
        })?;

        // check that the total length at least contains the header
        let total_len: usize = header.total_len.into();
        let header_len = header.header_len();
        let payload_len = if total_len >= header_len {
            total_len - header_len
        } else {
            return Err(Len(LenError {
                required_len: header_len,
                len: total_len,
                len_source: LenSource::Ipv4HeaderTotalLen,
                layer: Layer::Ipv4Packet,
                layer_start_offset: 0,
            }));
        };

        // limit rest based on ipv4 total length
        let header_rest = if payload_len > header_rest.len() {
            return Err(Len(err::LenError {
                required_len: total_len,
                len: slice.len(),
                len_source: LenSource::Slice,
                layer: Layer::Ipv4Packet,
                layer_start_offset: 0,
            }));
        } else {
            unsafe {
                // Safe as the payload_len <= header_rest.len is verified above
                core::slice::from_raw_parts(header_rest.as_ptr(), payload_len)
            }
        };

        // read the extension header
        let (exts, next_header, exts_rest) =
            Ipv4Extensions::from_slice(header.protocol, header_rest).map_err(|err| {
                use err::ip_auth::HeaderSliceError as I;
                match err {
                    I::Len(mut err) => {
                        err.layer_start_offset += header.header_len();
                        err.len_source = LenSource::Ipv4HeaderTotalLen;
                        Len(err)
                    }
                    I::Content(err) => Exts(err),
                }
            })?;

        let fragmented = header.is_fragmenting_payload();
        Ok((
            IpHeaders::Ipv4(header, exts),
            IpPayloadSlice {
                ip_number: next_header,
                fragmented,
                len_source: LenSource::Ipv4HeaderTotalLen,
                payload: exts_rest,
            },
        ))
    }

    /// Reads an IPv4 header & its extensions headers as far as is possible without encountering an
    /// error & separates the payload from the given slice with less strict length checks.
    /// This function is usefull for cut off packet or for packets with unset length fields).
    ///
    /// If you want to only receive correct IpPayloads use [`IpHeaders::from_ipv4_slice`]
    /// instead.
    ///
    /// The main usecases for this functions are:
    ///
    /// * Parsing packets that have been cut off. This is, for example, useful to
    ///   parse packets returned via ICMP as these usually only contain the start.
    /// * Parsing packets where the `total_len` (for IPv4) have not yet been set.
    ///   This can be useful when parsing packets which have been recorded in a
    ///   layer before the length field was set (e.g. before the operating
    ///   system set the length fields).
    ///
    /// # Differences to `from_ipv4_slice`:
    ///
    /// There are two main differences:
    ///
    /// * Errors in the expansion headers will only stop the parsing and return an `Ok`
    ///   with the successfully parsed parts and the error as optional. Only if an
    ///   unrecoverable error is encountered in the IP header itself an `Err` is returned.
    ///   In the normal `from_slice` function an `Err` is returned if an error is
    ///   encountered in an exteions header.
    /// * `from_ipv4_slice_lax` ignores inconsistent `total_len` values. When the `total_len`
    ///   value in the IPv4 header are inconsistant the length of the given slice is
    ///   used as a substitute.
    ///
    /// You can check if the slice length was used as a substitude by checking
    /// if the `len_source` value in the returned [`crate::LaxIpPayloadSlice`] is set to
    /// [`LenSource::Slice`]. If a substitution was not needed `len_source`
    /// is set to [`LenSource::Ipv4HeaderTotalLen`].
    ///
    /// # When is the slice length used as a fallback?
    ///
    /// For IPv4 packets the slice length is used as a fallback/substitude
    /// if the `total_length` field in the IPv4 header is:
    ///
    ///  * Bigger then the given slice (payload cannot fully be seperated).
    ///  * Too small to contain at least the IPv4 header.
    pub fn from_ipv4_slice_lax(
        slice: &[u8],
    ) -> Result<
        (
            IpHeaders,
            LaxIpPayloadSlice<'_>,
            Option<err::ip_auth::HeaderSliceError>,
        ),
        err::ip::LaxHeaderSliceError,
    > {
        use err::ip::LaxHeaderSliceError::*;

        // read the header
        let (header, header_rest) = Ipv4Header::from_slice(slice).map_err(|err| {
            use err::ip::HeaderError as O;
            use err::ipv4::HeaderError as I1;
            use err::ipv4::HeaderSliceError as I0;
            match err {
                I0::Len(err) => Len(err),
                I0::Content(err) => Content(match err {
                    I1::UnexpectedVersion { version_number } => {
                        O::UnsupportedIpVersion { version_number }
                    }
                    I1::HeaderLengthSmallerThanHeader { ihl } => {
                        O::Ipv4HeaderLengthSmallerThanHeader { ihl }
                    }
                }),
            }
        })?;

        // check that the total len is at least containing the header len
        let total_len: usize = header.total_len.into();

        // restrict the rest of the slice based on the total len (if the total_len is not conflicting)
        let header_len = header.header_len();
        let (len_source, header_rest, incomplete) = if total_len < header_len {
            // fallback to the rest of the slice
            (LenSource::Slice, header_rest, false)
        } else if header_rest.len() < total_len - header_len {
            // fallback to the rest of the slice
            (LenSource::Slice, header_rest, true)
        } else {
            (
                LenSource::Ipv4HeaderTotalLen,
                unsafe {
                    core::slice::from_raw_parts(
                        header_rest.as_ptr(),
                        // SAFETY: Safe as slice length has been validated to be at least total_length_usize long
                        total_len - header_len,
                    )
                },
                false,
            )
        };

        let (exts, next_protocol, payload, mut stop_err) =
            Ipv4Extensions::from_slice_lax(header.protocol, header_rest);

        use err::ip_auth::HeaderSliceError as I;
        if let Some(I::Len(err)) = stop_err.as_mut() {
            err.layer_start_offset += header.header_len();
            err.len_source = len_source;
        }

        let fragmented = header.is_fragmenting_payload();
        Ok((
            IpHeaders::Ipv4(header, exts),
            LaxIpPayloadSlice {
                incomplete,
                ip_number: next_protocol,
                fragmented,
                len_source,
                payload,
            },
            stop_err,
        ))
    }

    /// Read an IPv6 header & extension headers from a slice and return the slice
    /// containing the payload (e.g. TCP, UDP etc.) length limited by payload_length
    /// field in the IPv6 header.
    ///
    /// Note that slice length is used as a fallback value in case the
    /// payload_length in the IPv6 is set to zero. This is a temporary workaround
    /// to partially support jumbograms.
    pub fn from_ipv6_slice(
        slice: &[u8],
    ) -> Result<(IpHeaders, IpPayloadSlice<'_>), err::ipv6::SliceError> {
        use err::ipv6::SliceError::*;

        // read ipv6 header
        let (header, header_rest) = Ipv6Header::from_slice(slice).map_err(|err| {
            use err::ipv6::HeaderSliceError as I;
            match err {
                I::Len(err) => Len(err),
                I::Content(err) => Header(err),
            }
        })?;

        // restrict slice by the length specified in the header
        let (header_payload, len_source) =
            if 0 == header.payload_length && slice.len() > Ipv6Header::LEN {
                // In case the payload_length is 0 assume that the entire
                // rest of the slice is part of the packet until the jumbogram
                // parameters can be parsed.

                // TODO: Add payload length parsing from the jumbogram
                (header_rest, LenSource::Slice)
            } else {
                let payload_len: usize = header.payload_length.into();
                if header_rest.len() < payload_len {
                    return Err(Len(LenError {
                        required_len: payload_len + Ipv6Header::LEN,
                        len: slice.len(),
                        len_source: LenSource::Slice,
                        layer: Layer::Ipv6Packet,
                        layer_start_offset: 0,
                    }));
                } else {
                    unsafe {
                        (
                            core::slice::from_raw_parts(header_rest.as_ptr(), payload_len),
                            LenSource::Ipv6HeaderPayloadLen,
                        )
                    }
                }
            };

        // read ipv6 extensions headers
        let (exts, next_header, exts_rest) =
            Ipv6Extensions::from_slice(header.next_header, header_payload).map_err(|err| {
                use err::ipv6_exts::HeaderSliceError as I;
                match err {
                    I::Len(mut err) => {
                        err.layer_start_offset += Ipv6Header::LEN;
                        err.len_source = len_source;
                        Len(err)
                    }
                    I::Content(err) => Exts(err),
                }
            })?;

        let fragmented = exts.is_fragmenting_payload();
        Ok((
            IpHeaders::Ipv6(header, exts),
            IpPayloadSlice {
                ip_number: next_header,
                fragmented,
                len_source,
                payload: exts_rest,
            },
        ))
    }

    /// Reads an IPv6 header & its extensions headers as far as is possible without encountering an
    /// error & separates the payload from the given slice with less strict length checks.
    /// This function is usefull for cut off packet or for packets with unset length fields).
    ///
    /// If you want to only receive correct IpPayloads use [`IpHeaders::from_ipv6_slice`]
    /// instead.
    ///
    /// The main usecases for this functions are:
    ///
    /// * Parsing packets that have been cut off. This is, for example, useful to
    ///   parse packets returned via ICMP as these usually only contain the start.
    /// * Parsing packets where the `payload_length` (in the IPv6 header) has not
    ///   yet been set. This can be useful when parsing packets which have been
    ///  recorded in a layer before the length field was set (e.g. before the operating
    ///   system set the length fields).
    ///
    /// # Differences to `from_slice`:
    ///
    /// There are two main differences:
    ///
    /// * Errors in the expansion headers will only stop the parsing and return an `Ok`
    ///   with the successfully parsed parts and the error as optional. Only if an
    ///   unrecoverable error is encountered in the IP header itself an `Err` is returned.
    ///   In the normal `from_slice` function an `Err` is returned if an error is
    ///   encountered in an exteions header.
    /// * `from_slice_lax` ignores inconsistent `payload_length` values. When the
    ///   `payload_length` value in the IPv6 header is inconsistant the length of
    ///   the given slice is used as a substitute.
    ///
    /// You can check if the slice length was used as a substitude by checking
    /// if the `len_source` value in the returned [`LaxIpPayloadSlice`] is set to
    /// [`LenSource::Slice`]. If a substitution was not needed `len_source`
    /// is set to [`LenSource::Ipv6HeaderPayloadLen`].
    ///
    /// # When is the slice length used as a fallback?
    ///
    /// The slice length is used as a fallback/substitude if the `payload_length`
    /// field in the IPv6 header is
    ///
    /// * Bigger then the given slice (payload cannot fully be seperated).
    /// * The value `0`.
    pub fn from_ipv6_slice_lax(
        slice: &[u8],
    ) -> Result<
        (
            IpHeaders,
            LaxIpPayloadSlice<'_>,
            Option<(err::ipv6_exts::HeaderSliceError, Layer)>,
        ),
        err::ipv6::HeaderSliceError,
    > {
        // read ipv6 header
        let (header, header_rest) = Ipv6Header::from_slice(slice)?;

        // restrict slice by the length specified in the header
        let payload_len: usize = header.payload_length.into();
        let (header_payload, len_source, incomplete) =
            if payload_len == 0 && (false == header_rest.is_empty()) {
                (header_rest, LenSource::Slice, false)
            } else if payload_len > header_rest.len() {
                (header_rest, LenSource::Slice, true)
            } else {
                unsafe {
                    (
                        core::slice::from_raw_parts(header_rest.as_ptr(), payload_len),
                        LenSource::Ipv6HeaderPayloadLen,
                        false,
                    )
                }
            };

        // read ipv6 extensions headers
        let (exts, next_header, exts_rest, mut stop_err) =
            Ipv6Extensions::from_slice_lax(header.next_header, header_payload);

        use err::ipv6_exts::HeaderSliceError as I;
        if let Some((I::Len(err), _)) = stop_err.as_mut() {
            err.layer_start_offset += Ipv6Header::LEN;
            err.len_source = len_source;
        };

        let fragmented = exts.is_fragmenting_payload();
        Ok((
            IpHeaders::Ipv6(header, exts),
            LaxIpPayloadSlice {
                incomplete,
                ip_number: next_header,
                fragmented,
                len_source,
                payload: exts_rest,
            },
            stop_err,
        ))
    }

    /// Reads an IP (v4 or v6) header from the current position (requires
    /// crate feature `std`).
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    pub fn read<T: std::io::Read + std::io::Seek + Sized>(
        reader: &mut T,
    ) -> Result<(IpHeaders, IpNumber), err::ip::HeaderReadError> {
        use crate::io::LimitedReader;
        use err::ip::{HeaderError::*, HeaderReadError::*, HeadersError::*};

        let value = {
            let mut buf = [0; 1];
            reader.read_exact(&mut buf).map_err(Io)?;
            buf[0]
        };
        match value >> 4 {
            4 => {
                // get internet header length
                let ihl = value & 0xf;

                // check that the ihl is correct
                if ihl < 5 {
                    return Err(Content(Ip(Ipv4HeaderLengthSmallerThanHeader { ihl })));
                }

                // read the rest of the header
                let header_len_u16 = u16::from(ihl) * 4;
                let header_len = usize::from(header_len_u16);
                let mut buffer = [0u8; Ipv4Header::MAX_LEN];
                buffer[0] = value;
                reader.read_exact(&mut buffer[1..header_len]).map_err(Io)?;

                let header = unsafe {
                    // SAFETY: Safe as both the IHL and slice len have been verified
                    Ipv4HeaderSlice::from_slice_unchecked(&buffer[..header_len])
                }
                .to_header();

                // check that the total len is long enough to contain the header
                let total_len = usize::from(header.total_len);
                let mut reader = if total_len < header_len {
                    return Err(Len(LenError {
                        required_len: header_len,
                        len: total_len,
                        len_source: LenSource::Ipv4HeaderTotalLen,
                        layer: Layer::Ipv4Packet,
                        layer_start_offset: 0,
                    }));
                } else {
                    // create a new reader that is limited by the total_len value length
                    LimitedReader::new(
                        reader,
                        total_len - header_len,
                        LenSource::Ipv4HeaderTotalLen,
                        header_len,
                        Layer::Ipv4Header,
                    )
                };

                // read the extension headers if present
                Ipv4Extensions::read_limited(&mut reader, header.protocol)
                    .map(|(ext, next)| (IpHeaders::Ipv4(header, ext), next))
                    .map_err(|err| {
                        use err::ip_auth::HeaderLimitedReadError as I;
                        match err {
                            I::Io(err) => Io(err),
                            I::Len(err) => Len(err),
                            I::Content(err) => Content(Ipv4Ext(err)),
                        }
                    })
            }
            6 => {
                let header = Ipv6Header::read_without_version(reader, value & 0xf).map_err(Io)?;

                // create a new reader that is limited by the payload_len value length
                let mut reader = LimitedReader::new(
                    reader,
                    header.payload_length.into(),
                    LenSource::Ipv6HeaderPayloadLen,
                    header.header_len(),
                    Layer::Ipv6Header,
                );

                Ipv6Extensions::read_limited(&mut reader, header.next_header)
                    .map(|(ext, next)| (IpHeaders::Ipv6(header, ext), next))
                    .map_err(|err| {
                        use err::ipv6_exts::HeaderLimitedReadError as I;
                        match err {
                            I::Io(err) => Io(err),
                            I::Len(err) => Len(err),
                            I::Content(err) => Content(Ipv6Ext(err)),
                        }
                    })
            }
            version_number => Err(Content(Ip(UnsupportedIpVersion { version_number }))),
        }
    }

    /// Writes an IP (v4 or v6) header to the current position (requires
    /// crate feature `std`).
    #[cfg(feature = "std")]
    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
    pub fn write<T: std::io::Write + Sized>(
        &self,
        writer: &mut T,
    ) -> Result<(), HeadersWriteError> {
        use crate::IpHeaders::*;
        use HeadersWriteError::*;
        match *self {
            Ipv4(ref header, ref extensions) => {
                header.write(writer).map_err(Io)?;
                extensions.write(writer, header.protocol).map_err(|err| {
                    use err::ipv4_exts::HeaderWriteError as I;
                    match err {
                        I::Io(err) => Io(err),
                        I::Content(err) => Ipv4Exts(err),
                    }
                })
            }
            Ipv6(ref header, ref extensions) => {
                header.write(writer).map_err(Io)?;
                extensions.write(writer, header.next_header).map_err(|err| {
                    use err::ipv6_exts::HeaderWriteError as I;
                    match err {
                        I::Io(err) => Io(err),
                        I::Content(err) => Ipv6Exts(err),
                    }
                })
            }
        }
    }

    /// Returns the size when the ip header & extensions are serialized
    pub fn header_len(&self) -> usize {
        use crate::IpHeaders::*;
        match *self {
            Ipv4(ref header, ref extensions) => header.header_len() + extensions.header_len(),
            Ipv6(_, ref extensions) => Ipv6Header::LEN + extensions.header_len(),
        }
    }

    /// Returns the last next header number following the ip header
    /// and header extensions.
    pub fn next_header(&self) -> Result<IpNumber, err::ip_exts::ExtsWalkError> {
        use crate::err::ip_exts::ExtsWalkError::*;
        use crate::IpHeaders::*;
        match *self {
            Ipv4(ref header, ref extensions) => {
                extensions.next_header(header.protocol).map_err(Ipv4Exts)
            }
            Ipv6(ref header, ref extensions) => {
                extensions.next_header(header.next_header).map_err(Ipv6Exts)
            }
        }
    }

    /// Sets all the next_header fields in the ipv4 & ipv6 header
    /// as well as in all extension headers and returns the ether
    /// type number.
    ///
    /// The given number will be set as the last "next_header" or
    /// protocol number.
    pub fn set_next_headers(&mut self, last_next_header: IpNumber) -> u16 {
        use IpHeaders::*;
        match self {
            Ipv4(ref mut header, ref mut extensions) => {
                header.protocol = extensions.set_next_headers(last_next_header);
                EtherType::IPV4.0
            }
            Ipv6(ref mut header, ref mut extensions) => {
                header.next_header = extensions.set_next_headers(last_next_header);
                EtherType::IPV4.0
            }
        }
    }

    /// Tries to set the length field in the ip header given the length of data
    /// after the ip header and extension header(s).
    ///
    /// If the payload length is too large to be stored in the length fields
    /// of the ip header an error is returned.
    ///
    /// Note that this function will automatically add the length of the extension
    /// headers is they are present.
    pub fn set_payload_len(&mut self, len: usize) -> Result<(), ValueTooBigError<usize>> {
        use crate::err::ValueType;
        match self {
            IpHeaders::Ipv4(ipv4_hdr, exts) => {
                if let Some(complete_len) = len.checked_add(exts.header_len()) {
                    ipv4_hdr.set_payload_len(complete_len)
                } else {
                    Err(ValueTooBigError {
                        actual: len,
                        max_allowed: usize::from(u16::MAX)
                            - ipv4_hdr.header_len()
                            - exts.header_len(),
                        value_type: ValueType::Ipv4PayloadLength,
                    })
                }
            }
            IpHeaders::Ipv6(ipv6_hdr, exts) => {
                if let Some(complete_len) = len.checked_add(exts.header_len()) {
                    ipv6_hdr.set_payload_length(complete_len)
                } else {
                    Err(ValueTooBigError {
                        actual: len,
                        max_allowed: usize::from(u16::MAX) - exts.header_len(),
                        value_type: ValueType::Ipv4PayloadLength,
                    })
                }
            }
        }
    }

    /// Returns true if the payload is fragmented based on the IPv4 header
    /// or the IPv6 fragment header.
    pub fn is_fragmenting_payload(&self) -> bool {
        match self {
            IpHeaders::Ipv4(ipv4, _) => ipv4.is_fragmenting_payload(),
            IpHeaders::Ipv6(_, exts) => exts.is_fragmenting_payload(),
        }
    }
}

#[cfg(test)]
mod test {
    use crate::{
        err::{
            ip::{HeadersError, HeadersSliceError},
            Layer, LenError,
        },
        ip_number::*,
        test_gens::*,
        *,
    };
    use alloc::{borrow::ToOwned, format, vec::Vec};
    use proptest::prelude::*;
    use std::io::Cursor;

    const EXTENSION_KNOWN_IP_NUMBERS: [IpNumber; 5] = [
        AUTH,
        IPV6_DEST_OPTIONS,
        IPV6_HOP_BY_HOP,
        IPV6_FRAG,
        IPV6_ROUTE,
    ];

    fn combine_v4(v4: &Ipv4Header, ext: &Ipv4Extensions, payload: &[u8]) -> IpHeaders {
        IpHeaders::Ipv4(
            {
                let mut v4 = v4.clone();
                v4.protocol = if ext.auth.is_some() { AUTH } else { UDP };
                v4.total_len = (v4.header_len() + ext.header_len() + payload.len()) as u16;
                v4.header_checksum = v4.calc_header_checksum();
                v4
            },
            ext.clone(),
        )
    }

    fn combine_v6(v6: &Ipv6Header, ext: &Ipv6Extensions, payload: &[u8]) -> IpHeaders {
        let (ext, next_header) = {
            let mut ext = ext.clone();
            let next_header = ext.set_next_headers(UDP);
            (ext, next_header)
        };
        IpHeaders::Ipv6(
            {
                let mut v6 = v6.clone();
                v6.next_header = next_header;
                v6.payload_length = (ext.header_len() + payload.len()) as u16;
                v6
            },
            ext,
        )
    }

    proptest! {
        #[test]
        fn debug(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            assert_eq!(
                format!(
                    "Ipv4({:?}, {:?})",
                    v4,
                    v4_exts
                ),
                format!("{:?}", IpHeaders::Ipv4(v4, v4_exts))
            );
            assert_eq!(
                format!(
                    "Ipv6({:?}, {:?})",
                    v6,
                    v6_exts
                ),
                format!("{:?}", IpHeaders::Ipv6(v6, v6_exts))
            );
        }
    }

    proptest! {
        #[test]
        fn clone_eq(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            {
                let v4 = IpHeaders::Ipv4(v4, v4_exts);
                assert_eq!(v4, v4.clone());
            }
            {
                let v6 = IpHeaders::Ipv6(v6, v6_exts);
                assert_eq!(v6, v6.clone());
            }
        }
    }

    proptest! {
        #[test]
        fn ipv4(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            assert_eq!(
                IpHeaders::Ipv4(v4.clone(), v4_exts.clone()).ipv4(),
                Some((&v4, &v4_exts))
            );
            assert_eq!(
                IpHeaders::Ipv6(v6.clone(), v6_exts.clone()).ipv4(),
                None
            );
        }
    }

    proptest! {
        #[test]
        fn ipv6(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            assert_eq!(
                IpHeaders::Ipv4(v4.clone(), v4_exts.clone()).ipv6(),
                None
            );
            assert_eq!(
                IpHeaders::Ipv6(v6.clone(), v6_exts.clone()).ipv6(),
                Some((&v6, &v6_exts))
            );
        }
    }

    proptest! {
        #[test]
        #[allow(deprecated)]
        fn read_from_slice(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
        ) {
            let header = combine_v4(&v4, &v4_exts, &[]);
            let mut buffer = Vec::with_capacity(header.header_len());
            header.write(&mut buffer).unwrap();

            let actual = IpHeaders::read_from_slice(&buffer).unwrap();
            assert_eq!(actual.0, header);
            assert_eq!(actual.1, header.next_header().unwrap());
            assert_eq!(actual.2, &buffer[buffer.len()..]);
        }
    }

    proptest! {
        #[test]
        fn ip_from_slice(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            use err::ip::{HeadersError::*, HeaderError::*, HeadersSliceError::*};

            // empty error
            assert_eq!(
                IpHeaders::from_slice(&[]),
                Err(Len(err::LenError {
                    required_len: 1,
                    len: 0,
                    len_source: LenSource::Slice,
                    layer: err::Layer::IpHeader,
                    layer_start_offset: 0,
                }))
            );

            // unknown version
            for version_number in 0..=0xfu8 {
                if version_number != 4 && version_number != 6 {
                    assert_eq!(
                        IpHeaders::from_slice(&[version_number << 4]),
                        Err(Content(Ip(UnsupportedIpVersion { version_number })))
                    );
                }
            }

            let payload = [1,2,3,4];

            // v4
            {
                let header = combine_v4(&v4, &v4_exts, &payload);
                let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
                header.write(&mut buffer).unwrap();
                buffer.extend_from_slice(&payload);
                buffer.push(1); // add some value to check the return slice

                // read
                {
                    let actual = IpHeaders::from_slice(&buffer).unwrap();
                    assert_eq!(&actual.0, &header);
                    assert_eq!(
                        actual.1,
                        IpPayloadSlice{
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Ipv4HeaderTotalLen,
                            payload: &payload
                        }
                    );
                }

                // read error ipv4 header
                IpHeaders::from_slice(&buffer[..1]).unwrap_err();

                // read error ipv4 extensions
                if v4_exts.header_len() > 0 {
                    IpHeaders::from_slice(&buffer[..v4.header_len() + 1]).unwrap_err();
                }

                // total length smaller the header
                {
                    let bad_total_len = (v4.header_len() - 1) as u16;

                    let mut buffer = buffer.clone();
                    // inject bad total_len
                    let bad_total_len_be = bad_total_len.to_be_bytes();
                    buffer[2] = bad_total_len_be[0];
                    buffer[3] = bad_total_len_be[1];
                    assert_eq!(
                        IpHeaders::from_slice(&buffer[..]).unwrap_err(),
                        HeadersSliceError::Len(LenError{
                            required_len: v4.header_len(),
                            len: bad_total_len as usize,
                            len_source: LenSource::Ipv4HeaderTotalLen,
                            layer: Layer::Ipv4Packet,
                            layer_start_offset: 0,
                        })
                    );
                }
            }

            // v6
            {
                let header = combine_v6(&v6, &v6_exts, &payload);
                let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
                header.write(&mut buffer).unwrap();
                buffer.extend_from_slice(&payload);
                buffer.push(1); // add some value to check the return slice

                // len error
                {
                    let actual = IpHeaders::from_slice(&buffer).unwrap();
                    assert_eq!(&actual.0, &header);
                    assert_eq!(
                        actual.1,
                        IpPayloadSlice{
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Ipv6HeaderPayloadLen,
                            payload: &payload
                        }
                    );
                }

                // read error header
                IpHeaders::from_slice(&buffer[..1]).unwrap_err();

                // read error ipv4 extensions
                if v6_exts.header_len() > 0 {
                    IpHeaders::from_slice(&buffer[..Ipv6Header::LEN + 1]).unwrap_err();
                }

                // len error (with payload len zero)
                if v6_exts.header_len() > 0 {
                    let mut buffer = buffer.clone();

                    // inject zero as payload len
                    buffer[4] = 0;
                    buffer[5] = 0;

                    assert!(
                        IpHeaders::from_slice(
                            &buffer[..buffer.len() - payload.len() - 2]
                        ).is_err()
                    );
                }
            }
        }
    }

    proptest! {
        #[test]
        fn from_slice_lax(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            use err::ip::{HeaderError::*, LaxHeaderSliceError::*};

            let payload = [1,2,3,4];

            // empty error
            assert_eq!(
                IpHeaders::from_slice_lax(&[]),
                Err(Len(err::LenError {
                    required_len: 1,
                    len: 0,
                    len_source: LenSource::Slice,
                    layer: err::Layer::IpHeader,
                    layer_start_offset: 0,
                }))
            );

            // unknown version
            for version_number in 0..=0xfu8 {
                if version_number != 4 && version_number != 6 {
                    assert_eq!(
                        IpHeaders::from_slice_lax(&[version_number << 4]),
                        Err(Content(UnsupportedIpVersion { version_number }))
                    );
                }
            }

            // v4
            {
                let header = combine_v4(&v4, &v4_exts, &payload);
                let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
                header.write(&mut buffer).unwrap();
                buffer.extend_from_slice(&payload);
                buffer.push(1); // add some value to check the return slice

                // normal read
                {
                    let actual = IpHeaders::from_slice_lax(&buffer).unwrap();
                    assert_eq!(&actual.0, &header);
                    assert_eq!(
                        actual.1,
                        LaxIpPayloadSlice{
                            incomplete: false,
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Ipv4HeaderTotalLen,
                            payload: &payload
                        }
                    );
                }

                // error len smaller then min header len
                for len in 1..Ipv4Header::MIN_LEN {
                    assert_eq!(
                        IpHeaders::from_slice_lax(&buffer[..len]),
                        Err(Len(err::LenError {
                            required_len: Ipv4Header::MIN_LEN,
                            len,
                            len_source: LenSource::Slice,
                            layer: err::Layer::Ipv4Header,
                            layer_start_offset: 0,
                        }))
                    );
                }

                // ihl value error
                {
                    let mut bad_ihl_buffer = buffer.clone();
                    for bad_ihl in 0..5 {
                        bad_ihl_buffer[0] = (bad_ihl_buffer[0] & 0xf0) | bad_ihl;
                        assert_eq!(
                            IpHeaders::from_slice_lax(&bad_ihl_buffer),
                            Err(Content(Ipv4HeaderLengthSmallerThanHeader { ihl: bad_ihl }))
                        );
                    }
                }

                // ihl len error
                for short_ihl in 5..usize::from(v4.ihl()) {
                    assert_eq!(
                        IpHeaders::from_slice_lax(&buffer[..4*short_ihl]),
                        Err(Len(err::LenError {
                            required_len: usize::from(v4.ihl())*4,
                            len: 4*short_ihl,
                            len_source: LenSource::Slice,
                            layer: err::Layer::Ipv4Header,
                            layer_start_offset: 0,
                        }))
                    );
                }

                // total_len bigger then slice len (fallback to slice len)
                for payload_len in 0..payload.len(){
                    let actual = IpHeaders::from_slice_lax(&buffer[..v4.header_len() + v4_exts.header_len() + payload_len]).unwrap();
                    assert_eq!(&actual.0, &header);
                    assert_eq!(
                        actual.1,
                        LaxIpPayloadSlice{
                            incomplete: true,
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Slice,
                            payload: &payload[..payload_len]
                        }
                    );
                }

                // len error ipv4 extensions
                if v4_exts.header_len() > 0 {
                    let (_, _, stop_err) = IpHeaders::from_slice_lax(&buffer[..v4.header_len() + 1]).unwrap();
                    assert!(stop_err.is_some());
                }

                // content error ipv4 extensions
                if v4_exts.auth.is_some() {
                    use err::ip_auth::HeaderError::ZeroPayloadLen;
                    use err::ip_exts::HeadersSliceError::Content;
                    use err::ip_exts::HeaderError::Ipv4Ext;

                    // introduce a auth header zero payload error
                    let mut errored_buffer = buffer.clone();
                    // inject length zero into auth header (not valid, will
                    // trigger a content error)
                    errored_buffer[v4.header_len() + 1] = 0;

                    let (_, _, stop_err) = IpHeaders::from_slice_lax(&errored_buffer).unwrap();

                    assert_eq!(stop_err, Some((Content(Ipv4Ext(ZeroPayloadLen)), Layer::IpAuthHeader)));
                }

                // total length smaller the header (fallback to slice len)
                {
                    let bad_total_len = (v4.header_len() - 1) as u16;

                    let mut buffer = buffer.clone();
                    // inject bad total_len
                    let bad_total_len_be = bad_total_len.to_be_bytes();
                    buffer[2] = bad_total_len_be[0];
                    buffer[3] = bad_total_len_be[1];

                    let actual = IpHeaders::from_slice_lax(&buffer[..]).unwrap();

                    let (v4_header, v4_exts) = header.ipv4().unwrap();
                    let expected_headers = IpHeaders::Ipv4(
                        {
                            let mut expected_v4 = v4_header.clone();
                            expected_v4.total_len = bad_total_len;
                            expected_v4
                        },
                        v4_exts.clone()
                    );
                    assert_eq!(&expected_headers, &actual.0);
                    assert_eq!(
                        actual.1,
                        LaxIpPayloadSlice{
                            incomplete: false,
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Slice,
                            payload: &buffer[v4_header.header_len() + v4_exts.header_len()..],
                        }
                    );
                }
            }

            // v6
            {
                let header = combine_v6(&v6, &v6_exts, &payload);
                let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
                header.write(&mut buffer).unwrap();
                buffer.extend_from_slice(&payload);
                buffer.push(1); // add some value to check the return slice

                // normal read
                {
                    let actual = IpHeaders::from_slice_lax(&buffer).unwrap();
                    assert_eq!(&actual.0, &header);
                    assert_eq!(
                        actual.1,
                        LaxIpPayloadSlice{
                            incomplete: false,
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Ipv6HeaderPayloadLen,
                            payload: &payload
                        }
                    );
                }

                // smaller then header
                for len in 1..Ipv6Header::LEN {
                    assert_eq!(
                        IpHeaders::from_slice_lax(&buffer[..len]),
                        Err(Len(err::LenError {
                            required_len: Ipv6Header::LEN,
                            len,
                            len_source: LenSource::Slice,
                            layer: err::Layer::Ipv6Header,
                            layer_start_offset: 0,
                        }))
                    );
                }

                // extension len error
                if v6_exts.header_len() > 0 {
                    let (actual, _, stop_err) = IpHeaders::from_slice_lax(&buffer[..v6.header_len() + 1]).unwrap();
                    assert_eq!(&actual.ipv6().as_ref().unwrap().0, &header.ipv6().as_ref().unwrap().0);
                    assert!(stop_err.is_some());
                }

                // extension content error
                if v6_exts.auth.is_some() {
                    use err::ip_auth::HeaderError::ZeroPayloadLen;
                    use err::ip_exts::HeadersSliceError::Content;
                    use err::ip_exts::HeaderError::Ipv6Ext;
                    use err::ipv6_exts::HeaderError::IpAuth;

                    // introduce a auth header zero payload error
                    let mut errored_buffer = buffer.clone();
                    let auth_offset = v6.header_len() +
                        v6_exts.hop_by_hop_options.as_ref().map(|h| h.header_len()).unwrap_or(0) +
                        v6_exts.destination_options.as_ref().map(|h| h.header_len()).unwrap_or(0) +
                        v6_exts.routing.as_ref().map(|h| h.routing.header_len()).unwrap_or(0) +
                        // routing.final_destination_options skiped, as after auth
                        v6_exts.fragment.as_ref().map(|h| h.header_len()).unwrap_or(0);

                    // inject length zero into auth header (not valid, will
                    // trigger a content error)
                    errored_buffer[auth_offset + 1] = 0;

                    let (_, _, stop_err) = IpHeaders::from_slice_lax(&errored_buffer).unwrap();
                    assert_eq!(
                        stop_err,
                        Some((Content(Ipv6Ext(IpAuth(ZeroPayloadLen))), Layer::IpAuthHeader))
                    );
                }

                // slice smaller then payload len
                for len in (v6.header_len()+v6_exts.header_len())..buffer.len() - 1 {
                    let actual = IpHeaders::from_slice_lax(&buffer[..len]).unwrap();
                    assert_eq!(&actual.0, &header);
                    assert_eq!(
                        actual.1,
                        LaxIpPayloadSlice{
                            incomplete: true,
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Slice,
                            payload: &payload[..len - v6.header_len() - v6_exts.header_len()]
                        }
                    );
                }

                // payload len zero (fallback to slice len)
                {
                    let mut buffer = buffer.clone();
                    // inject zero as payload len
                    buffer[4] = 0;
                    buffer[5] = 0;

                    let actual = IpHeaders::from_slice_lax(&buffer[..]).unwrap();

                    let (v6_header, v6_exts) = header.ipv6().unwrap();
                    let expected_headers = IpHeaders::Ipv6(
                        {
                            let mut expected_v6 = v6_header.clone();
                            expected_v6.payload_length = 0;
                            expected_v6
                        },
                        v6_exts.clone()
                    );
                    assert_eq!(&expected_headers, &actual.0);
                    assert_eq!(
                        actual.1,
                        LaxIpPayloadSlice{
                            incomplete: false,
                            ip_number: header.next_header().unwrap(),
                            fragmented: header.is_fragmenting_payload(),
                            len_source: LenSource::Slice,
                            payload: &buffer[v6_header.header_len() + v6_exts.header_len()..],
                        }
                    );
                }

            }
        }
    }

    proptest! {
        #[test]
        fn from_ipv4_slice(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
        ) {
            let payload = [1,2,3,4];

            let header = combine_v4(&v4, &v4_exts, &payload);
            let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
            header.write(&mut buffer).unwrap();
            buffer.extend_from_slice(&payload);
            buffer.push(1); // add some value to check the return slice

            // read
            {
                let actual = IpHeaders::from_ipv4_slice(&buffer).unwrap();
                assert_eq!(&actual.0, &header);
                assert_eq!(
                    actual.1,
                    IpPayloadSlice{
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Ipv4HeaderTotalLen,
                        payload: &payload
                    }
                );
            }

            // read error ipv4 header
            IpHeaders::from_ipv4_slice(&buffer[..1]).unwrap_err();

            // read error ipv4 extensions
            if v4_exts.header_len() > 0 {
                IpHeaders::from_ipv4_slice(&buffer[..v4.header_len() + 1]).unwrap_err();
            }

            // total length smaller the header
            {
                let bad_total_len = (v4.header_len() - 1) as u16;

                let mut buffer = buffer.clone();
                // inject bad total_len
                let bad_total_len_be = bad_total_len.to_be_bytes();
                buffer[2] = bad_total_len_be[0];
                buffer[3] = bad_total_len_be[1];
                assert_eq!(
                    IpHeaders::from_ipv4_slice(&buffer[..]).unwrap_err(),
                    err::ipv4::SliceError::Len(LenError{
                        required_len: v4.header_len(),
                        len: bad_total_len as usize,
                        len_source: LenSource::Ipv4HeaderTotalLen,
                        layer: Layer::Ipv4Packet,
                        layer_start_offset: 0,
                    })
                );
            }
        }
    }

    proptest! {
        #[test]
        fn from_ipv4_slice_lax(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any()
        ) {
            use err::ip::{LaxHeaderSliceError::*, HeaderError::*};

            let payload = [1,2,3,4];

            // empty error
            assert_eq!(
                IpHeaders::from_ipv4_slice_lax(&[]),
                Err(Len(err::LenError {
                    required_len: 20,
                    len: 0,
                    len_source: LenSource::Slice,
                    layer: err::Layer::Ipv4Header,
                    layer_start_offset: 0,
                }))
            );

            // build a buffer with a valid packet
            let header = combine_v4(&v4, &v4_exts, &payload);
            let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
            header.write(&mut buffer).unwrap();
            buffer.extend_from_slice(&payload);
            buffer.push(1); // add some value to check the return slice

            // normal read
            {
                let actual = IpHeaders::from_ipv4_slice_lax(&buffer).unwrap();
                assert_eq!(&actual.0, &header);
                assert_eq!(
                    actual.1,
                    LaxIpPayloadSlice{
                        incomplete: false,
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Ipv4HeaderTotalLen,
                        payload: &payload
                    }
                );
            }

            // error len smaller then min header len
            for len in 1..Ipv4Header::MIN_LEN {
                assert_eq!(
                    IpHeaders::from_ipv4_slice_lax(&buffer[..len]),
                    Err(Len(err::LenError {
                        required_len: Ipv4Header::MIN_LEN,
                        len,
                        len_source: LenSource::Slice,
                        layer: err::Layer::Ipv4Header,
                        layer_start_offset: 0,
                    }))
                );
            }

            // ihl value error
            {
                let mut bad_ihl_buffer = buffer.clone();
                for bad_ihl in 0..5 {
                    bad_ihl_buffer[0] = (bad_ihl_buffer[0] & 0xf0) | bad_ihl;
                    assert_eq!(
                        IpHeaders::from_ipv4_slice_lax(&bad_ihl_buffer),
                        Err(Content(Ipv4HeaderLengthSmallerThanHeader { ihl: bad_ihl }))
                    );
                }
            }

            // ihl len error
            for short_ihl in 5..usize::from(v4.ihl()) {
                assert_eq!(
                    IpHeaders::from_ipv4_slice_lax(&buffer[..4*short_ihl]),
                    Err(Len(err::LenError {
                        required_len: usize::from(v4.ihl())*4,
                        len: 4*short_ihl,
                        len_source: LenSource::Slice,
                        layer: err::Layer::Ipv4Header,
                        layer_start_offset: 0,
                    }))
                );
            }

            // total_len bigger then slice len (fallback to slice len)
            for payload_len in 0..payload.len(){
                let actual = IpHeaders::from_ipv4_slice_lax(&buffer[..v4.header_len() + v4_exts.header_len() + payload_len]).unwrap();
                assert_eq!(&actual.0, &header);
                assert_eq!(
                    actual.1,
                    LaxIpPayloadSlice{
                        incomplete: true,
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Slice,
                        payload: &payload[..payload_len]
                    }
                );
            }

            // len error ipv4 extensions
            if v4_exts.header_len() > 0 {
                let (actual, _, stop_err) = IpHeaders::from_ipv4_slice_lax(&buffer[..v4.header_len() + 1]).unwrap();
                assert_eq!(actual.ipv4().unwrap().0, header.ipv4().unwrap().0);
                assert!(stop_err.is_some());
            }

            // content error ipv4 extensions
            if v4_exts.auth.is_some() {
                use err::ip_auth::HeaderSliceError::Content;
                use err::ip_auth::HeaderError::ZeroPayloadLen;

                // introduce a auth header zero payload error
                let mut errored_buffer = buffer.clone();
                // inject length zero into auth header (not valid, will
                // trigger a content error)
                errored_buffer[v4.header_len() + 1] = 0;

                let (_, _, stop_err) = IpHeaders::from_ipv4_slice_lax(&errored_buffer).unwrap();
                assert_eq!(stop_err, Some(Content(ZeroPayloadLen)));
            }

            // total length smaller the header (fallback to slice len)
            {
                let bad_total_len = (v4.header_len() - 1) as u16;

                let mut buffer = buffer.clone();
                // inject bad total_len
                let bad_total_len_be = bad_total_len.to_be_bytes();
                buffer[2] = bad_total_len_be[0];
                buffer[3] = bad_total_len_be[1];

                let actual = IpHeaders::from_ipv4_slice_lax(&buffer[..]).unwrap();

                let (v4_header, v4_exts) = header.ipv4().unwrap();
                let expected_headers = IpHeaders::Ipv4(
                    {
                        let mut expected_v4 = v4_header.clone();
                        expected_v4.total_len = bad_total_len;
                        expected_v4
                    },
                    v4_exts.clone()
                );
                assert_eq!(&expected_headers, &actual.0);
                assert_eq!(
                    actual.1,
                    LaxIpPayloadSlice{
                        incomplete: false,
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Slice,
                        payload: &buffer[v4_header.header_len() + v4_exts.header_len()..],
                    }
                );
            }
        }
    }

    proptest! {
        #[test]
        fn from_ipv6_slice(
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            let payload = [1,2,3,4];
            let header = combine_v6(&v6, &v6_exts, &payload);
            let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
            header.write(&mut buffer).unwrap();
            buffer.extend_from_slice(&payload);
            buffer.push(1); // add some value to check the return slice

            // len error
            {
                let actual = IpHeaders::from_ipv6_slice(&buffer).unwrap();
                assert_eq!(&actual.0, &header);
                assert_eq!(
                    actual.1,
                    IpPayloadSlice{
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Ipv6HeaderPayloadLen,
                        payload: &payload
                    }
                );
            }

            // read error header
            IpHeaders::from_ipv6_slice(&buffer[..1]).unwrap_err();

            // read error ipv4 extensions
            if v6_exts.header_len() > 0 {
                IpHeaders::from_ipv6_slice(&buffer[..Ipv6Header::LEN + 1]).unwrap_err();
            }

            // len error (with payload len zero)
            if v6_exts.header_len() > 0 {
                let mut buffer = buffer.clone();

                // inject zero as payload len
                buffer[4] = 0;
                buffer[5] = 0;

                assert!(
                    IpHeaders::from_ipv6_slice(
                        &buffer[..buffer.len() - payload.len() - 2]
                    ).is_err()
                );
            }
        }
    }

    proptest! {
        #[test]
        fn from_ipv6_slice_lax(
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
            bad_version in 0..0xfu8
        ) {
            use err::ipv6::{HeaderError::*, HeaderSliceError::*};

            let payload = [1,2,3,4];

            // empty error
            assert_eq!(
                IpHeaders::from_ipv6_slice_lax(&[]),
                Err(Len(err::LenError {
                    required_len: Ipv6Header::LEN,
                    len: 0,
                    len_source: LenSource::Slice,
                    layer: err::Layer::Ipv6Header,
                    layer_start_offset: 0,
                }))
            );

            // setup buffer with a valid packet
            let header = combine_v6(&v6, &v6_exts, &payload);
            let mut buffer = Vec::with_capacity(header.header_len() + payload.len() + 1);
            header.write(&mut buffer).unwrap();
            buffer.extend_from_slice(&payload);
            buffer.push(1); // add some value to check the return slice

            // unknown version
            if bad_version != 6 {
                let mut bad_vers_buffer = buffer.clone();
                bad_vers_buffer[0] = (bad_vers_buffer[0] & 0xf) | (bad_version << 4);
                assert_eq!(
                    IpHeaders::from_ipv6_slice_lax(&bad_vers_buffer),
                    Err(Content(UnexpectedVersion { version_number: bad_version }))
                );
            }

            // normal read
            {
                let actual = IpHeaders::from_ipv6_slice_lax(&buffer).unwrap();
                assert_eq!(&actual.0, &header);
                assert_eq!(
                    actual.1,
                    LaxIpPayloadSlice{
                        incomplete: false,
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Ipv6HeaderPayloadLen,
                        payload: &payload
                    }
                );
            }

            // smaller then header
            for len in 1..Ipv6Header::LEN {
                assert_eq!(
                    IpHeaders::from_ipv6_slice_lax(&buffer[..len]),
                    Err(Len(err::LenError {
                        required_len: Ipv6Header::LEN,
                        len,
                        len_source: LenSource::Slice,
                        layer: err::Layer::Ipv6Header,
                        layer_start_offset: 0,
                    }))
                );
            }

            // extension len error
            if v6_exts.header_len() > 0 {
                let (_, _, err) = IpHeaders::from_ipv6_slice_lax(&buffer[..v6.header_len() + 1]).unwrap();
                assert!(err.is_some());
            }

            // extension content error
            if v6_exts.auth.is_some() {
                use err::ip_auth::HeaderError::ZeroPayloadLen;
                use err::ipv6_exts::{HeaderSliceError::Content, HeaderError::IpAuth};

                // introduce a auth header zero payload error
                let mut errored_buffer = buffer.clone();
                let auth_offset = v6.header_len() +
                    v6_exts.hop_by_hop_options.as_ref().map(|h| h.header_len()).unwrap_or(0) +
                    v6_exts.destination_options.as_ref().map(|h| h.header_len()).unwrap_or(0) +
                    v6_exts.routing.as_ref().map(|h| h.routing.header_len()).unwrap_or(0) +
                    // routing.final_destination_options skiped, as after auth
                    v6_exts.fragment.as_ref().map(|h| h.header_len()).unwrap_or(0);

                // inject length zero into auth header (not valid, will
                // trigger a content error)
                errored_buffer[auth_offset + 1] = 0;
                let (_, _, err) = IpHeaders::from_ipv6_slice_lax(&errored_buffer).unwrap();
                assert_eq!(err, Some((Content(IpAuth(ZeroPayloadLen)), Layer::IpAuthHeader)));
            }

            // slice smaller then payload len
            for len in (v6.header_len()+v6_exts.header_len())..buffer.len() - 1 {
                let actual = IpHeaders::from_ipv6_slice_lax(&buffer[..len]).unwrap();
                assert_eq!(&actual.0, &header);
                assert_eq!(
                    actual.1,
                    LaxIpPayloadSlice{
                        incomplete: true,
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Slice,
                        payload: &payload[..len - v6.header_len() - v6_exts.header_len()]
                    }
                );
            }

            // payload len zero (fallback to slice len)
            {
                let mut buffer = buffer.clone();
                // inject zero as payload len
                buffer[4] = 0;
                buffer[5] = 0;

                let actual = IpHeaders::from_ipv6_slice_lax(&buffer[..]).unwrap();

                let (v6_header, v6_exts) = header.ipv6().unwrap();
                let expected_headers = IpHeaders::Ipv6(
                    {
                        let mut expected_v6 = v6_header.clone();
                        expected_v6.payload_length = 0;
                        expected_v6
                    },
                    v6_exts.clone()
                );
                assert_eq!(&expected_headers, &actual.0);
                assert_eq!(
                    actual.1,
                    LaxIpPayloadSlice{
                        incomplete: false,
                        ip_number: header.next_header().unwrap(),
                        fragmented: header.is_fragmenting_payload(),
                        len_source: LenSource::Slice,
                        payload: &buffer[v6_header.header_len() + v6_exts.header_len()..],
                    }
                );
            }
        }
    }

    proptest! {
        #[test]
        fn read(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            bad_ihl in 0u8..5u8,
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            use err::ip::{HeadersError::*, HeaderError::*};

            // no data error
            {
                let mut cursor = Cursor::new(&[]);
                assert!(
                    IpHeaders::read(&mut cursor)
                    .unwrap_err()
                    .io()
                    .is_some()
                );
            }
            // version error
            {
                let mut cursor = Cursor::new(&[0xf << 4]);
                assert_eq!(
                    IpHeaders::read(&mut cursor).unwrap_err().content().unwrap(),
                    Ip(UnsupportedIpVersion {
                        version_number: 0xf
                    })
                );
            }
            // v4
            {
                let header = combine_v4(&v4, &v4_exts, &[]);
                let mut buffer = Vec::with_capacity(header.header_len());
                header.write(&mut buffer).unwrap();

                // read
                {
                    let mut cursor = Cursor::new(&buffer[..]);
                    let actual = IpHeaders::read(&mut cursor).unwrap();
                    assert_eq!(actual.0, header);
                    assert_eq!(actual.1, header.next_header().unwrap());
                }

                // read error ihl smaller then header
                {
                    let mut buffer = buffer.clone();
                    // inject bad ihl
                    buffer[0] = (buffer[0] & 0b1111_0000) | bad_ihl;
                    let mut cursor = Cursor::new(&buffer[..]);
                    assert_eq!(
                        IpHeaders::read(&mut cursor)
                        .unwrap_err()
                        .content()
                        .unwrap(),
                        Ip(Ipv4HeaderLengthSmallerThanHeader{
                            ihl: bad_ihl
                        })
                    );
                }

                // total length smaller the header
                {
                    let bad_total_len = (v4.header_len() - 1) as u16;

                    let mut buffer = buffer.clone();
                    // inject bad total_len
                    let bad_total_len_be = bad_total_len.to_be_bytes();
                    buffer[2] = bad_total_len_be[0];
                    buffer[3] = bad_total_len_be[1];
                    let mut cursor = Cursor::new(&buffer[..]);
                    assert_eq!(
                        IpHeaders::read(&mut cursor)
                        .unwrap_err()
                        .len()
                        .unwrap(),
                        LenError{
                            required_len: v4.header_len(),
                            len: bad_total_len as usize,
                            len_source: LenSource::Ipv4HeaderTotalLen,
                            layer: Layer::Ipv4Packet,
                            layer_start_offset: 0,
                        }
                    );
                }

                // read len error ipv4
                {
                    let mut cursor = Cursor::new(&buffer[..1]);
                    assert!(
                        IpHeaders::read(&mut cursor)
                        .unwrap_err()
                        .io()
                        .is_some()
                    );
                }

                // read error ipv4 extensions
                if v4_exts.header_len() > 0 {
                    let mut cursor = Cursor::new(&buffer[..v4.header_len() + 1]);
                    IpHeaders::read(&mut cursor).unwrap_err();
                }

                // len error in extensions
                if v4_exts.auth.is_some() {
                    let bad_total_len = (buffer.len() - 1) as u16;

                    let mut buffer = buffer.clone();
                    // inject bad total_len
                    let bad_total_len_be = bad_total_len.to_be_bytes();
                    buffer[2] = bad_total_len_be[0];
                    buffer[3] = bad_total_len_be[1];
                    let mut cursor = Cursor::new(&buffer[..]);
                    assert_eq!(
                        IpHeaders::read(&mut cursor)
                        .unwrap_err()
                        .len()
                        .unwrap(),
                        LenError{
                            required_len: buffer.len() - v4.header_len(),
                            len: bad_total_len as usize - v4.header_len(),
                            len_source: LenSource::Ipv4HeaderTotalLen,
                            layer: Layer::IpAuthHeader,
                            layer_start_offset: v4.header_len(),
                        }
                    );
                }

                // extension content error
                if v4_exts.auth.is_some() {
                    let mut buffer = buffer.clone();
                    // inject zero as header len
                    buffer[v4.header_len() + 1] = 0;
                    let mut cursor = Cursor::new(&buffer[..]);
                    assert_eq!(
                        IpHeaders::read(&mut cursor)
                        .unwrap_err()
                        .content()
                        .unwrap(),
                        HeadersError::Ipv4Ext(
                            err::ip_auth::HeaderError::ZeroPayloadLen
                        )
                    );
                }
            }

            // v6
            {
                let header = combine_v6(&v6, &v6_exts, &[]);
                let mut buffer = Vec::with_capacity(header.header_len());
                header.write(&mut buffer).unwrap();

                // ok case
                {
                    let mut cursor = Cursor::new(&buffer[..]);
                    let actual = IpHeaders::read(&mut cursor).unwrap();
                    assert_eq!(actual.0, header);
                    assert_eq!(actual.1, header.next_header().unwrap());
                }

                // io error in v6 header section
                {
                    let mut cursor = Cursor::new(&buffer[..1]);
                    assert!(
                        IpHeaders::read(&mut cursor).unwrap_err().io().is_some()
                    );
                }

                // io error ipv6 extensions
                if v6_exts.header_len() > 0 {
                    let mut cursor = Cursor::new(&buffer[..Ipv6Header::LEN + 1]);
                    assert!(
                        IpHeaders::read(&mut cursor).unwrap_err().io().is_some()
                    );
                }

                // len error in ipv6 extensions
                if v6_exts.header_len() > 0 {
                    // inject an invalid length
                    let mut buffer = buffer.clone();
                    let bad_payload_len = (buffer.len() - header.header_len()) as u16;
                    let bad_payload_len_be = bad_payload_len.to_be_bytes();
                    buffer[4] = bad_payload_len_be[0];
                    buffer[5] = bad_payload_len_be[1];
                    // expect a length error
                    let mut cursor = Cursor::new(&buffer[..]);
                    assert!(
                        IpHeaders::read(&mut cursor).unwrap_err().len().is_some()
                    );
                }

                // extension content error
                if let Some(auth) = v6_exts.auth.as_ref() {
                    // only do it if auth is the last header
                    if v6_exts.routing.is_none() {
                        // inject zero as header len
                        let mut buffer = buffer.clone();
                        let auth_offset = buffer.len() - auth.header_len();
                        buffer[auth_offset + 1] = 0;
                        let mut cursor = Cursor::new(&buffer[..]);
                        assert_eq!(
                            IpHeaders::read(&mut cursor)
                            .unwrap_err()
                            .content()
                            .unwrap(),
                            HeadersError::Ipv6Ext(err::ipv6_exts::HeaderError::IpAuth(
                                err::ip_auth::HeaderError::ZeroPayloadLen
                            ))
                        );
                    }
                }
            }
        }
    }

    proptest! {
        #[test]
        fn write(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            // v4
            {
                let header = combine_v4(&v4, &v4_exts, &[]);
                let mut buffer = Vec::with_capacity(header.header_len());
                header.write(&mut buffer).unwrap();

                let actual = IpHeaders::from_slice(&buffer).unwrap().0;
                assert_eq!(header, actual);

                // write error v4 header
                {
                    let mut buffer = [0u8;1];
                    let mut cursor = Cursor::new(&mut buffer[..]);
                    assert!(
                        header.write(&mut cursor)
                        .unwrap_err()
                        .io()
                        .is_some()
                    );
                }

                // write io error v4 extension headers
                if v4_exts.header_len() > 0 {
                    let mut buffer = [0u8;Ipv4Header::MAX_LEN + 1];
                    let mut cursor = Cursor::new(&mut buffer[..v4.header_len() + 1]);
                    assert!(
                        header.write(&mut cursor)
                        .unwrap_err()
                        .io()
                        .is_some()
                    );
                }

                // write content error v4 extension headers
                if v4_exts.header_len() > 0 {
                    // cause a missing reference error
                    let header = IpHeaders::Ipv4(
                        {
                            let mut v4 = v4.clone();
                            // skips extension header
                            v4.protocol = ip_number::UDP;
                            v4.total_len = (v4.header_len() + v4_exts.header_len()) as u16;
                            v4.header_checksum = v4.calc_header_checksum();
                            v4
                        },
                        v4_exts.clone(),
                    );
                    let mut buffer = [0u8;Ipv4Header::MAX_LEN + IpAuthHeader::MAX_LEN];
                    let mut cursor = Cursor::new(&mut buffer[..]);
                    assert!(header.write(&mut cursor).is_err());
                }
            }

            // v6
            {
                let header = combine_v6(&v6, &v6_exts, &[]);

                // normal write
                let mut buffer = Vec::with_capacity(header.header_len());
                header.write(&mut buffer).unwrap();

                let actual = IpHeaders::from_slice(&buffer).unwrap().0;
                assert_eq!(header, actual);

                // write error v6 header
                {
                    let mut buffer = [0u8;1];
                    let mut cursor = Cursor::new(&mut buffer[..]);
                    assert!(
                        header.write(&mut cursor)
                        .unwrap_err()
                        .io()
                        .is_some()
                    );
                }

                // write error v6 extension headers
                if v6_exts.header_len() > 0 {
                    let mut buffer = [0u8;Ipv6Header::LEN + 1];
                    let mut cursor = Cursor::new(&mut buffer[..]);
                    assert!(
                        header.write(&mut cursor)
                        .unwrap_err()
                        .io()
                        .is_some()
                    );
                }
                // write content error v4 extension headers
                if v6_exts.header_len() > 0 {
                    // cause a missing reference error
                    let header = IpHeaders::Ipv6(
                        {
                            let mut v6 = v6.clone();
                            // skips extension header
                            v6.next_header = ip_number::UDP;
                            v6.payload_length = v6_exts.header_len() as u16;
                            v6
                        },
                        v6_exts.clone(),
                    );
                    let mut buffer = [0u8;Ipv4Header::MAX_LEN + IpAuthHeader::MAX_LEN];
                    let mut cursor = Cursor::new(&mut buffer[..]);
                    assert!(header.write(&mut cursor).is_err());
                }
            }
        }
    }

    proptest! {
        #[test]
        fn header_len(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
        ) {
            assert_eq!(
                v4.header_len() + v4_exts.header_len(),
                IpHeaders::Ipv4(v4, v4_exts).header_len()
            );
            assert_eq!(
                Ipv6Header::LEN + v6_exts.header_len(),
                IpHeaders::Ipv6(v6, v6_exts).header_len()
            );
        }
    }

    proptest! {
        #[test]
        fn next_header(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
            post_header in ip_number_any()
                .prop_filter("Must be a non ipv6 header relevant ip number".to_owned(),
                    |v| !EXTENSION_KNOWN_IP_NUMBERS.iter().any(|&x| v == &x)
                )
        ) {
            {
                let mut header = v4.clone();
                let mut exts = v4_exts.clone();
                header.protocol = exts.set_next_headers(post_header);
                assert_eq!(
                    Ok(post_header),
                    IpHeaders::Ipv4(header, exts).next_header()
                );
            }
            {
                let mut header = v6.clone();
                let mut exts = v6_exts.clone();
                header.next_header = exts.set_next_headers(post_header);
                assert_eq!(
                    Ok(post_header),
                    IpHeaders::Ipv6(header, exts).next_header()
                );
            }
        }
    }

    // TODO set_next_headers

    proptest! {
        #[test]
        fn set_payload_len(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any(),
            payload_len in 0usize..10
        ) {
            // ipv4 (with valid payload length)
            {
                let mut actual = IpHeaders::Ipv4(
                    v4.clone(),
                    v4_exts.clone()
                );
                actual.set_payload_len(payload_len).unwrap();

                assert_eq!(
                    actual,
                    IpHeaders::Ipv4(
                        {
                            let mut re = v4.clone();
                            re.set_payload_len(v4_exts.header_len() + payload_len).unwrap();
                            re
                        },
                        v4_exts.clone()
                    )
                );
            }
            // ipv6 (with valid payload length)
            {
                let mut actual = IpHeaders::Ipv6(
                    v6.clone(),
                    v6_exts.clone()
                );
                actual.set_payload_len(payload_len).unwrap();

                assert_eq!(
                    actual,
                    IpHeaders::Ipv6(
                        {
                            let mut re = v6.clone();
                            re.set_payload_length(v6_exts.header_len() + payload_len).unwrap();
                            re
                        },
                        v6_exts.clone()
                    )
                );
            }

            // v4 (with invalid size)
            {
                let mut actual = IpHeaders::Ipv4(
                    v4.clone(),
                    v4_exts.clone()
                );
                assert!(actual.set_payload_len(usize::MAX).is_err());
            }

            // v6 (with invalid size)
            {
                let mut actual = IpHeaders::Ipv6(
                    v6.clone(),
                    v6_exts.clone()
                );
                assert!(actual.set_payload_len(usize::MAX).is_err());
            }
        }
    }

    proptest! {
        #[test]
        fn is_fragmenting_payload(
            v4 in ipv4_any(),
            v4_exts in ipv4_extensions_any(),
            v6 in ipv6_any(),
            v6_exts in ipv6_extensions_any()
        ) {
            // ipv4
            assert_eq!(
                v4.is_fragmenting_payload(),
                IpHeaders::Ipv4(v4.clone(), v4_exts.clone()).is_fragmenting_payload()
            );

            // ipv6
            assert_eq!(
                v6_exts.is_fragmenting_payload(),
                IpHeaders::Ipv6(v6.clone(), v6_exts.clone()).is_fragmenting_payload()
            );
        }
    }
}