diff --git a/src/cfdp/pdu/mod.rs b/src/cfdp/pdu/mod.rs
index 3d6c611..3f7d344 100644
--- a/src/cfdp/pdu/mod.rs
+++ b/src/cfdp/pdu/mod.rs
@@ -55,6 +55,8 @@ pub enum PduError {
         found: u8,
         expected: Option<FileDirectiveType>,
     },
+    InvalidSegmentRequestFormat,
+    InvalidStartOrEndOfScopeValue,
     /// Invalid condition code. Contains the raw detected value.
     InvalidConditionCode(u8),
     /// Invalid checksum type which is not part of the checksums listed in the
@@ -75,9 +77,15 @@ impl Display for PduError {
             PduError::InvalidEntityLen(raw_id) => {
                 write!(
                     f,
-                    "Invalid PDU entity ID length {raw_id}, only [1, 2, 4, 8] are allowed"
+                    "invalid PDU entity ID length {raw_id}, only [1, 2, 4, 8] are allowed"
                 )
             }
+            PduError::InvalidSegmentRequestFormat => {
+                write!(f, "invalid segment request format for NAK PDU")
+            }
+            PduError::InvalidStartOrEndOfScopeValue => {
+                write!(f, "invalid start or end of scope for NAK PDU")
+            }
             PduError::InvalidTransactionSeqNumLen(raw_id) => {
                 write!(
                     f,
diff --git a/src/cfdp/pdu/nak.rs b/src/cfdp/pdu/nak.rs
index 8b13789..9619c34 100644
--- a/src/cfdp/pdu/nak.rs
+++ b/src/cfdp/pdu/nak.rs
@@ -1 +1,285 @@
+use core::marker::PhantomData;
 
+use crate::{
+    cfdp::{CrcFlag, Direction, LargeFileFlag},
+    ByteConversionError,
+};
+
+use super::{
+    add_pdu_crc, generic_length_checks_pdu_deserialization, CfdpPdu, FileDirectiveType, PduError,
+    PduHeader, WritablePduPacket,
+};
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// Special iterator type for the NAK PDU which allows to iterator over both normal and large file
+/// segment requests.
+pub struct SegmentRequestIter<'a, T> {
+    seq_req_start: &'a [u8],
+    current_idx: usize,
+    phantom: std::marker::PhantomData<T>,
+}
+
+trait FromBytes {
+    fn from_bytes(bytes: &[u8]) -> Self;
+}
+
+impl FromBytes for u32 {
+    fn from_bytes(bytes: &[u8]) -> u32 {
+        u32::from_be_bytes(bytes.try_into().unwrap())
+    }
+}
+
+impl FromBytes for u64 {
+    fn from_bytes(bytes: &[u8]) -> u64 {
+        u64::from_be_bytes(bytes.try_into().unwrap())
+    }
+}
+
+impl<'a, T> Iterator for SegmentRequestIter<'a, T>
+where
+    T: FromBytes,
+{
+    type Item = (T, T);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.current_idx + std::mem::size_of::<T>() * 2 > self.seq_req_start.len() {
+            return None;
+        }
+
+        let start_offset = T::from_bytes(
+            &self.seq_req_start[self.current_idx..self.current_idx + std::mem::size_of::<T>()],
+        );
+        self.current_idx += std::mem::size_of::<T>();
+
+        let end_offset = T::from_bytes(
+            &self.seq_req_start[self.current_idx..self.current_idx + std::mem::size_of::<T>()],
+        );
+        self.current_idx += std::mem::size_of::<T>();
+
+        Some((start_offset, end_offset))
+    }
+}
+
+/// NAK PDU abstraction.
+///
+/// For more information, refer to CFDP chapter 5.2.6.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub struct NakPdu<'seg_reqs> {
+    pdu_header: PduHeader,
+    start_of_scope: u64,
+    end_of_scope: u64,
+    seg_reqs_raw: &'seg_reqs [u8],
+}
+
+impl<'seg_reqs> NakPdu<'seg_reqs> {
+    pub fn new(
+        mut pdu_header: PduHeader,
+        start_of_scope: u64,
+        end_of_scope: u64,
+        seg_reqs_raw: &'seg_reqs [u8],
+    ) -> Result<NakPdu, PduError> {
+        // Force correct direction flag.
+        pdu_header.pdu_conf.direction = Direction::TowardsSender;
+        if pdu_header.common_pdu_conf().file_flag == LargeFileFlag::Large {
+            if seg_reqs_raw.len() % 16 != 0 {
+                return Err(PduError::InvalidSegmentRequestFormat);
+            }
+        } else {
+            if seg_reqs_raw.len() % 8 != 0 {
+                return Err(PduError::InvalidSegmentRequestFormat);
+            }
+            if start_of_scope > u32::MAX as u64 || end_of_scope > u32::MAX as u64 {
+                return Err(PduError::InvalidStartOrEndOfScopeValue);
+            }
+        }
+        let mut nak_pdu = Self {
+            pdu_header,
+            start_of_scope,
+            end_of_scope,
+            seg_reqs_raw,
+        };
+        nak_pdu.pdu_header.pdu_datafield_len = nak_pdu.calc_pdu_datafield_len() as u16;
+        Ok(nak_pdu)
+    }
+
+    pub fn start_of_scope(&self) -> u64 {
+        self.start_of_scope
+    }
+
+    pub fn end_of_scope(&self) -> u64 {
+        self.end_of_scope
+    }
+
+    pub fn get_normal_segment_requests_iterator(&self) -> Option<SegmentRequestIter<'_, u32>> {
+        if self.file_flag() == LargeFileFlag::Large {
+            return None;
+        }
+        Some(SegmentRequestIter {
+            seq_req_start: self.seg_reqs_raw,
+            current_idx: 0,
+            phantom: PhantomData,
+        })
+    }
+
+    pub fn get_large_segment_requests_iterator(&self) -> Option<SegmentRequestIter<'_, u64>> {
+        if self.file_flag() == LargeFileFlag::Normal {
+            return None;
+        }
+        Some(SegmentRequestIter {
+            seq_req_start: self.seg_reqs_raw,
+            current_idx: 0,
+            phantom: PhantomData,
+        })
+    }
+
+    pub fn num_segment_reqs(&self) -> usize {
+        if self.file_flag() == LargeFileFlag::Large {
+            self.seg_reqs_raw.len() / 16
+        } else {
+            self.seg_reqs_raw.len() / 8
+        }
+    }
+
+    pub fn pdu_header(&self) -> &PduHeader {
+        &self.pdu_header
+    }
+
+    fn calc_pdu_datafield_len(&self) -> usize {
+        let mut datafield_len = 1;
+        if self.file_flag() == LargeFileFlag::Normal {
+            datafield_len += 8;
+            datafield_len += self.num_segment_reqs() * 8;
+        } else {
+            datafield_len += 16;
+            datafield_len += self.num_segment_reqs() * 16;
+        }
+        if self.crc_flag() == CrcFlag::WithCrc {
+            datafield_len += 2;
+        }
+        datafield_len
+    }
+
+    pub fn from_bytes(buf: &'seg_reqs [u8]) -> Result<NakPdu, PduError> {
+        let (pdu_header, mut current_idx) = PduHeader::from_bytes(buf)?;
+        let full_len_without_crc = pdu_header.verify_length_and_checksum(buf)?;
+        // Minimum length of 9: 1 byte directive field and start and end of scope for normal file
+        // size.
+        generic_length_checks_pdu_deserialization(buf, 9, full_len_without_crc)?;
+        let directive_type = FileDirectiveType::try_from(buf[current_idx]).map_err(|_| {
+            PduError::InvalidDirectiveType {
+                found: buf[current_idx],
+                expected: Some(FileDirectiveType::NakPdu),
+            }
+        })?;
+        if directive_type != FileDirectiveType::NakPdu {
+            return Err(PduError::WrongDirectiveType {
+                found: directive_type,
+                expected: FileDirectiveType::AckPdu,
+            });
+        }
+        current_idx += 1;
+        let start_of_scope;
+        let end_of_scope;
+        if pdu_header.common_pdu_conf().file_flag == LargeFileFlag::Large {
+            if current_idx + 16 > buf.len() {
+                return Err(PduError::ByteConversionError(
+                    ByteConversionError::FromSliceTooSmall {
+                        found: buf.len(),
+                        expected: current_idx + 16,
+                    },
+                ));
+            }
+            start_of_scope =
+                u64::from_be_bytes(buf[current_idx..current_idx + 8].try_into().unwrap());
+            current_idx += 8;
+            end_of_scope =
+                u64::from_be_bytes(buf[current_idx..current_idx + 8].try_into().unwrap());
+            current_idx += 8;
+        } else {
+            start_of_scope =
+                u64::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+            current_idx += 4;
+            end_of_scope =
+                u64::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+            current_idx += 4;
+        }
+        Self::new(
+            pdu_header,
+            start_of_scope,
+            end_of_scope,
+            &buf[current_idx..full_len_without_crc],
+        )
+    }
+}
+
+impl CfdpPdu for NakPdu<'_> {
+    fn pdu_header(&self) -> &PduHeader {
+        &self.pdu_header
+    }
+
+    fn file_directive_type(&self) -> Option<FileDirectiveType> {
+        Some(FileDirectiveType::NakPdu)
+    }
+}
+
+impl WritablePduPacket for NakPdu<'_> {
+    fn write_to_bytes(&self, buf: &mut [u8]) -> Result<usize, PduError> {
+        let expected_len = self.len_written();
+        if buf.len() < expected_len {
+            return Err(ByteConversionError::ToSliceTooSmall {
+                found: buf.len(),
+                expected: expected_len,
+            }
+            .into());
+        }
+        let mut current_idx = self.pdu_header.write_to_bytes(buf)?;
+        buf[current_idx] = FileDirectiveType::NakPdu as u8;
+        current_idx += 1;
+
+        if self.file_flag() == LargeFileFlag::Large {
+            buf[current_idx..current_idx + 8].copy_from_slice(&self.start_of_scope.to_be_bytes());
+            current_idx += 8;
+            buf[current_idx..current_idx + 8].copy_from_slice(&self.end_of_scope.to_be_bytes());
+            current_idx += 8;
+            // Unwrap is okay here, we checked the file flag.
+            let segments_iter = self.get_large_segment_requests_iterator().unwrap();
+            for (next_start_offset, next_end_offset) in segments_iter {
+                buf[current_idx..current_idx + 8].copy_from_slice(&next_start_offset.to_be_bytes());
+                current_idx += 8;
+                buf[current_idx..current_idx + 8].copy_from_slice(&next_end_offset.to_be_bytes());
+                current_idx += 8;
+            }
+        } else {
+            // Unwrap is okay here, the API should prevent invalid values which would trigger a
+            // panic here.
+            let start_of_scope = u32::try_from(self.start_of_scope).unwrap();
+            let end_of_scope = u32::try_from(self.end_of_scope).unwrap();
+            buf[current_idx..current_idx + 4].copy_from_slice(&start_of_scope.to_be_bytes());
+            current_idx += 4;
+            buf[current_idx..current_idx + 4].copy_from_slice(&end_of_scope.to_be_bytes());
+            current_idx += 4;
+            // Unwrap is okay here, we checked the file flag.
+            let segments_iter = self.get_normal_segment_requests_iterator().unwrap();
+            for (next_start_offset, next_end_offset) in segments_iter {
+                buf[current_idx..current_idx + 4].copy_from_slice(&next_start_offset.to_be_bytes());
+                current_idx += 4;
+                buf[current_idx..current_idx + 4].copy_from_slice(&next_end_offset.to_be_bytes());
+                current_idx += 4;
+            }
+        }
+
+        if self.crc_flag() == CrcFlag::WithCrc {
+            current_idx = add_pdu_crc(buf, current_idx);
+        }
+        Ok(current_idx)
+    }
+
+    fn len_written(&self) -> usize {
+        self.pdu_header.header_len() + self.calc_pdu_datafield_len()
+    }
+}
+
+#[cfg(test)]
+mod tests {}