Merge pull request 'Add ACK and NAK PDU abstractions' (#41) from add-ack-nak-pdus into main

Reviewed-on: #41

README.md

@@ -47,3 +47,15 @@ deserializing them with an appropriate `serde` provider like
 
 You can check the [documentation](https://docs.rs/spacepackets) of individual modules for various
 usage examples.
+
+# Coverage
+
+Coverage was generated using [`grcov`](https://github.com/mozilla/grcov). If you have not done so
+already, install the `llvm-tools-preview`:
+
+```sh
+rustup component add llvm-tools-preview
+```
+
+After that, you can simply run `coverage.py` to test the project with coverage. You can optionally
+supply the `--open` flag to open the coverage report in your webbrowser.

coverage.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+import os
+import argparse
+import webbrowser
+
+
+def generate_cov_report(open_report: bool):
+    os.environ["RUSTFLAGS"] = "-Cinstrument-coverage"
+    os.environ["LLVM_PROFILE_FILE"] = "target/coverage/%p-%m.profraw"
+    os.system("cargo test")
+    os.system(
+        "grcov . -s . --binary-path ./target/debug/ -t html --branch --ignore-not-existing "
+        "-o ./target/debug/coverage/"
+    )
+    if open_report:
+        coverage_report_path = os.path.abspath("./target/debug/coverage/index.html")
+        webbrowser.open_new_tab(coverage_report_path)
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Generate coverage report and optionally open it in a browser")
+    parser.add_argument("--open", action="store_true", help="Open the coverage report in a browser")
+    args = parser.parse_args()
+    generate_cov_report(args.open)
+
+
+if __name__ == "__main__":
+    main()

src/cfdp/mod.rs

@@ -48,6 +48,24 @@ pub enum CrcFlag {
     WithCrc = 1,
 }
 
+impl From<bool> for CrcFlag {
+    fn from(value: bool) -> Self {
+        if value {
+            return CrcFlag::WithCrc;
+        }
+        CrcFlag::NoCrc
+    }
+}
+
+impl From<CrcFlag> for bool {
+    fn from(value: CrcFlag) -> Self {
+        if value == CrcFlag::WithCrc {
+            return true;
+        }
+        false
+    }
+}
+
 /// Always 0 and ignored for File Directive PDUs (CCSDS 727.0-B-5 P.75)
 #[derive(Debug, Copy, Clone, PartialEq, Eq, TryFromPrimitive, IntoPrimitive)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
@@ -119,6 +137,22 @@ pub enum LargeFileFlag {
     Large = 1,
 }
 
+/// Transaction status for the ACK PDU field according to chapter 5.2.4 of the CFDP standard.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, TryFromPrimitive, IntoPrimitive)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[repr(u8)]
+pub enum TransactionStatus {
+    /// Transaction is not currently active and the CFDP implementation does not retain a
+    /// transaction history.
+    Undefined = 0b00,
+    Active = 0b01,
+    /// Transaction was active in the past and was terminated.
+    Terminated = 0b10,
+    /// The CFDP implementation does retain a tranaction history, and the transaction is not and
+    /// never was active at this entity.
+    Unrecognized = 0b11,
+}
+
 /// Checksum types according to the
 /// [SANA Checksum Types registry](https://sanaregistry.org/r/checksum_identifiers/)
 #[derive(Debug, Copy, Clone, PartialEq, Eq, TryFromPrimitive, IntoPrimitive)]

src/cfdp/pdu/ack.rs

@@ -0,0 +1,299 @@
+use crate::{
+    cfdp::{ConditionCode, CrcFlag, Direction, TransactionStatus},
+    ByteConversionError,
+};
+
+use super::{
+    add_pdu_crc, generic_length_checks_pdu_deserialization, CfdpPdu, FileDirectiveType, PduError,
+    PduHeader, WritablePduPacket,
+};
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// ACK PDU abstraction.
+///
+/// For more information, refer to CFDP chapter 5.2.4.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub struct AckPdu {
+    pdu_header: PduHeader,
+    directive_code_of_acked_pdu: FileDirectiveType,
+    condition_code: ConditionCode,
+    transaction_status: TransactionStatus,
+}
+
+impl AckPdu {
+    pub fn new(
+        mut pdu_header: PduHeader,
+        directive_code_of_acked_pdu: FileDirectiveType,
+        condition_code: ConditionCode,
+        transaction_status: TransactionStatus,
+    ) -> Result<Self, PduError> {
+        if directive_code_of_acked_pdu == FileDirectiveType::EofPdu {
+            pdu_header.pdu_conf.direction = Direction::TowardsSender;
+        } else if directive_code_of_acked_pdu == FileDirectiveType::FinishedPdu {
+            pdu_header.pdu_conf.direction = Direction::TowardsReceiver;
+        } else {
+            return Err(PduError::InvalidDirectiveType {
+                found: directive_code_of_acked_pdu as u8,
+                expected: None,
+            });
+        }
+        // Force correct direction flag.
+        let mut ack_pdu = Self {
+            pdu_header,
+            directive_code_of_acked_pdu,
+            condition_code,
+            transaction_status,
+        };
+        ack_pdu.pdu_header.pdu_datafield_len = ack_pdu.calc_pdu_datafield_len() as u16;
+        Ok(ack_pdu)
+    }
+
+    pub fn new_for_eof_pdu(
+        pdu_header: PduHeader,
+        condition_code: ConditionCode,
+        transaction_status: TransactionStatus,
+    ) -> Self {
+        // Unwrap okay here, [new] can only fail on invalid directive codes.
+        Self::new(
+            pdu_header,
+            FileDirectiveType::EofPdu,
+            condition_code,
+            transaction_status,
+        )
+        .unwrap()
+    }
+
+    pub fn new_for_finished_pdu(
+        pdu_header: PduHeader,
+        condition_code: ConditionCode,
+        transaction_status: TransactionStatus,
+    ) -> Self {
+        // Unwrap okay here, [new] can only fail on invalid directive codes.
+        Self::new(
+            pdu_header,
+            FileDirectiveType::FinishedPdu,
+            condition_code,
+            transaction_status,
+        )
+        .unwrap()
+    }
+
+    pub fn pdu_header(&self) -> &PduHeader {
+        &self.pdu_header
+    }
+
+    pub fn directive_code_of_acked_pdu(&self) -> FileDirectiveType {
+        self.directive_code_of_acked_pdu
+    }
+
+    pub fn condition_code(&self) -> ConditionCode {
+        self.condition_code
+    }
+
+    pub fn transaction_status(&self) -> TransactionStatus {
+        self.transaction_status
+    }
+
+    fn calc_pdu_datafield_len(&self) -> usize {
+        if self.crc_flag() == CrcFlag::WithCrc {
+            return 5;
+        }
+        3
+    }
+
+    pub fn from_bytes(buf: &[u8]) -> Result<AckPdu, PduError> {
+        let (pdu_header, mut current_idx) = PduHeader::from_bytes(buf)?;
+        let full_len_without_crc = pdu_header.verify_length_and_checksum(buf)?;
+        generic_length_checks_pdu_deserialization(buf, current_idx + 3, full_len_without_crc)?;
+        let directive_type = FileDirectiveType::try_from(buf[current_idx]).map_err(|_| {
+            PduError::InvalidDirectiveType {
+                found: buf[current_idx],
+                expected: Some(FileDirectiveType::AckPdu),
+            }
+        })?;
+        if directive_type != FileDirectiveType::AckPdu {
+            return Err(PduError::WrongDirectiveType {
+                found: directive_type,
+                expected: FileDirectiveType::AckPdu,
+            });
+        }
+        current_idx += 1;
+        let acked_directive_type =
+            FileDirectiveType::try_from(buf[current_idx] >> 4).map_err(|_| {
+                PduError::InvalidDirectiveType {
+                    found: buf[current_idx],
+                    expected: None,
+                }
+            })?;
+        if acked_directive_type != FileDirectiveType::EofPdu
+            && acked_directive_type != FileDirectiveType::FinishedPdu
+        {
+            return Err(PduError::InvalidDirectiveType {
+                found: acked_directive_type as u8,
+                expected: None,
+            });
+        }
+        current_idx += 1;
+        let condition_code = ConditionCode::try_from((buf[current_idx] >> 4) & 0b1111)
+            .map_err(|_| PduError::InvalidConditionCode((buf[current_idx] >> 4) & 0b1111))?;
+        let transaction_status = TransactionStatus::try_from(buf[current_idx] & 0b11).unwrap();
+        Self::new(
+            pdu_header,
+            acked_directive_type,
+            condition_code,
+            transaction_status,
+        )
+    }
+}
+
+impl CfdpPdu for AckPdu {
+    fn pdu_header(&self) -> &PduHeader {
+        &self.pdu_header
+    }
+
+    fn file_directive_type(&self) -> Option<FileDirectiveType> {
+        Some(FileDirectiveType::AckPdu)
+    }
+}
+
+impl WritablePduPacket for AckPdu {
+    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::AckPdu as u8;
+        current_idx += 1;
+
+        buf[current_idx] = (self.directive_code_of_acked_pdu as u8) << 4;
+        if self.directive_code_of_acked_pdu == FileDirectiveType::FinishedPdu {
+            // This is the directive subtype code. It needs to be set to 0b0001 if the ACK PDU
+            // acknowledges a Finished PDU, and to 0b0000 otherwise.
+            buf[current_idx] |= 0b0001;
+        }
+        current_idx += 1;
+        buf[current_idx] = ((self.condition_code as u8) << 4) | (self.transaction_status as u8);
+        current_idx += 1;
+        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 {
+    use crate::cfdp::{
+        pdu::tests::{common_pdu_conf, verify_raw_header, TEST_DEST_ID, TEST_SEQ_NUM, TEST_SRC_ID},
+        LargeFileFlag, PduType, TransmissionMode,
+    };
+
+    use super::*;
+
+    #[test]
+    fn test_basic() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let ack_pdu = AckPdu::new(
+            pdu_header,
+            FileDirectiveType::FinishedPdu,
+            ConditionCode::NoError,
+            TransactionStatus::Active,
+        )
+        .expect("creating ACK PDU failed");
+        assert_eq!(
+            ack_pdu.directive_code_of_acked_pdu(),
+            FileDirectiveType::FinishedPdu
+        );
+        assert_eq!(ack_pdu.condition_code(), ConditionCode::NoError);
+        assert_eq!(ack_pdu.transaction_status(), TransactionStatus::Active);
+
+        assert_eq!(ack_pdu.crc_flag(), CrcFlag::NoCrc);
+        assert_eq!(ack_pdu.file_flag(), LargeFileFlag::Normal);
+        assert_eq!(ack_pdu.pdu_type(), PduType::FileDirective);
+        assert_eq!(
+            ack_pdu.file_directive_type(),
+            Some(FileDirectiveType::AckPdu)
+        );
+        assert_eq!(ack_pdu.transmission_mode(), TransmissionMode::Acknowledged);
+        assert_eq!(ack_pdu.direction(), Direction::TowardsReceiver);
+        assert_eq!(ack_pdu.source_id(), TEST_SRC_ID.into());
+        assert_eq!(ack_pdu.dest_id(), TEST_DEST_ID.into());
+        assert_eq!(ack_pdu.transaction_seq_num(), TEST_SEQ_NUM.into());
+    }
+
+    fn generic_serialization_test(
+        condition_code: ConditionCode,
+        transaction_status: TransactionStatus,
+    ) {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let ack_pdu = AckPdu::new_for_finished_pdu(pdu_header, condition_code, transaction_status);
+        let mut buf: [u8; 64] = [0; 64];
+        let res = ack_pdu.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let written = res.unwrap();
+        assert_eq!(written, ack_pdu.len_written());
+        verify_raw_header(ack_pdu.pdu_header(), &buf);
+
+        assert_eq!(buf[7], FileDirectiveType::AckPdu as u8);
+        assert_eq!((buf[8] >> 4) & 0b1111, FileDirectiveType::FinishedPdu as u8);
+        assert_eq!(buf[8] & 0b1111, 0b0001);
+        assert_eq!(buf[9] >> 4 & 0b1111, condition_code as u8);
+        assert_eq!(buf[9] & 0b11, transaction_status as u8);
+        assert_eq!(written, 10);
+    }
+
+    #[test]
+    fn test_serialization_no_error() {
+        generic_serialization_test(ConditionCode::NoError, TransactionStatus::Active);
+    }
+
+    #[test]
+    fn test_serialization_fs_error() {
+        generic_serialization_test(ConditionCode::FileSizeError, TransactionStatus::Terminated);
+    }
+
+    #[test]
+    fn test_deserialization() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let ack_pdu = AckPdu::new_for_finished_pdu(
+            pdu_header,
+            ConditionCode::NoError,
+            TransactionStatus::Active,
+        );
+        let ack_vec = ack_pdu.to_vec().unwrap();
+        let ack_deserialized =
+            AckPdu::from_bytes(&ack_vec).expect("ACK PDU deserialization failed");
+        assert_eq!(ack_deserialized, ack_pdu);
+    }
+
+    #[test]
+    fn test_with_crc() {
+        let pdu_conf = common_pdu_conf(CrcFlag::WithCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let ack_pdu = AckPdu::new_for_finished_pdu(
+            pdu_header,
+            ConditionCode::NoError,
+            TransactionStatus::Active,
+        );
+        let ack_vec = ack_pdu.to_vec().unwrap();
+        assert_eq!(ack_vec.len(), ack_pdu.len_written());
+        assert_eq!(ack_vec.len(), 12);
+        let ack_deserialized =
+            AckPdu::from_bytes(&ack_vec).expect("ACK PDU deserialization failed");
+        assert_eq!(ack_deserialized, ack_pdu);
+    }
+}

src/cfdp/pdu/eof.rs

@@ -63,6 +63,9 @@ impl EofPdu {
         if let Some(fault_location) = self.fault_location {
             len += fault_location.len_full();
         }
+        if self.crc_flag() == CrcFlag::WithCrc {
+            len += 2;
+        }
         len
     }
 
@@ -146,7 +149,7 @@ impl WritablePduPacket for EofPdu {
         if let Some(fault_location) = self.fault_location {
             current_idx += fault_location.write_to_be_bytes(buf)?;
         }
-        if self.pdu_header.pdu_conf.crc_flag == CrcFlag::WithCrc {
+        if self.crc_flag() == CrcFlag::WithCrc {
             current_idx = add_pdu_crc(buf, current_idx);
         }
         Ok(current_idx)
@@ -160,9 +163,11 @@ impl WritablePduPacket for EofPdu {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::cfdp::pdu::tests::{common_pdu_conf, verify_raw_header};
+    use crate::cfdp::pdu::tests::{
+        common_pdu_conf, verify_raw_header, TEST_DEST_ID, TEST_SEQ_NUM, TEST_SRC_ID,
+    };
     use crate::cfdp::pdu::{FileDirectiveType, PduHeader};
-    use crate::cfdp::{ConditionCode, CrcFlag, LargeFileFlag};
+    use crate::cfdp::{ConditionCode, CrcFlag, LargeFileFlag, PduType, TransmissionMode};
 
     #[test]
     fn test_basic() {
@@ -173,6 +178,19 @@ mod tests {
         assert_eq!(eof_pdu.file_checksum(), 0x01020304);
         assert_eq!(eof_pdu.file_size(), 12);
         assert_eq!(eof_pdu.condition_code(), ConditionCode::NoError);
+
+        assert_eq!(eof_pdu.crc_flag(), CrcFlag::NoCrc);
+        assert_eq!(eof_pdu.file_flag(), LargeFileFlag::Normal);
+        assert_eq!(eof_pdu.pdu_type(), PduType::FileDirective);
+        assert_eq!(
+            eof_pdu.file_directive_type(),
+            Some(FileDirectiveType::EofPdu)
+        );
+        assert_eq!(eof_pdu.transmission_mode(), TransmissionMode::Acknowledged);
+        assert_eq!(eof_pdu.direction(), Direction::TowardsReceiver);
+        assert_eq!(eof_pdu.source_id(), TEST_SRC_ID.into());
+        assert_eq!(eof_pdu.dest_id(), TEST_DEST_ID.into());
+        assert_eq!(eof_pdu.transaction_seq_num(), TEST_SEQ_NUM.into());
     }
 
     #[test]
@@ -215,8 +233,7 @@ mod tests {
         let mut buf: [u8; 64] = [0; 64];
         eof_pdu.write_to_bytes(&mut buf).unwrap();
         let eof_read_back = EofPdu::from_bytes(&buf);
-        if eof_read_back.is_err() {
-            let e = eof_read_back.unwrap_err();
+        if let Err(e) = eof_read_back {
             panic!("deserialization failed with: {e}")
         }
         let eof_read_back = eof_read_back.unwrap();
@@ -233,4 +250,24 @@ mod tests {
         let pdu_vec = eof_pdu.to_vec().unwrap();
         assert_eq!(buf[0..written], pdu_vec);
     }
+
+    #[test]
+    fn test_with_crc() {
+        let pdu_conf = common_pdu_conf(CrcFlag::WithCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let eof_pdu = EofPdu::new_no_error(pdu_header, 0x01020304, 12);
+        let mut buf: [u8; 64] = [0; 64];
+        let written = eof_pdu.write_to_bytes(&mut buf).unwrap();
+        assert_eq!(written, eof_pdu.len_written());
+        let eof_from_raw = EofPdu::from_bytes(&buf).expect("creating EOF PDU failed");
+        assert_eq!(eof_from_raw, eof_pdu);
+        buf[written - 1] -= 1;
+        let crc: u16 = ((buf[written - 2] as u16) << 8) as u16 | buf[written - 1] as u16;
+        let error = EofPdu::from_bytes(&buf).unwrap_err();
+        if let PduError::ChecksumError(e) = error {
+            assert_eq!(e, crc);
+        } else {
+            panic!("expected crc error");
+        }
+    }
 }

src/cfdp/pdu/file_data.rs

@@ -144,7 +144,7 @@ impl<'seg_meta, 'file_data> FileDataPdu<'seg_meta, 'file_data> {
             len += self.segment_metadata.as_ref().unwrap().written_len()
         }
         len += self.file_data.len();
-        if self.pdu_header.pdu_conf.crc_flag == CrcFlag::WithCrc {
+        if self.crc_flag() == CrcFlag::WithCrc {
             len += 2;
         }
         len
@@ -225,7 +225,7 @@ impl WritablePduPacket for FileDataPdu<'_, '_> {
         )?;
         buf[current_idx..current_idx + self.file_data.len()].copy_from_slice(self.file_data);
         current_idx += self.file_data.len();
-        if self.pdu_header.pdu_conf.crc_flag == CrcFlag::WithCrc {
+        if self.crc_flag() == CrcFlag::WithCrc {
             current_idx = add_pdu_crc(buf, current_idx);
         }
         Ok(current_idx)
@@ -239,17 +239,14 @@ impl WritablePduPacket for FileDataPdu<'_, '_> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::cfdp::pdu::tests::{TEST_DEST_ID, TEST_SEQ_NUM, TEST_SRC_ID};
     use crate::cfdp::pdu::{CommonPduConfig, PduHeader};
-    use crate::cfdp::{SegmentMetadataFlag, SegmentationControl};
-    use crate::util::UbfU8;
-
-    const SRC_ID: UbfU8 = UbfU8::new(1);
-    const DEST_ID: UbfU8 = UbfU8::new(2);
-    const SEQ_NUM: UbfU8 = UbfU8::new(3);
+    use crate::cfdp::{Direction, SegmentMetadataFlag, SegmentationControl, TransmissionMode};
 
     #[test]
     fn test_basic() {
-        let common_conf = CommonPduConfig::new_with_byte_fields(SRC_ID, DEST_ID, SEQ_NUM).unwrap();
+        let common_conf =
+            CommonPduConfig::new_with_byte_fields(TEST_SRC_ID, TEST_DEST_ID, TEST_SEQ_NUM).unwrap();
         let pdu_header = PduHeader::new_for_file_data_default(common_conf, 0);
         let file_data: [u8; 4] = [1, 2, 3, 4];
         let fd_pdu = FileDataPdu::new_no_seg_metadata(pdu_header, 10, &file_data);
@@ -260,11 +257,22 @@ mod tests {
             fd_pdu.len_written(),
             fd_pdu.pdu_header.header_len() + core::mem::size_of::<u32>() + 4
         );
+
+        assert_eq!(fd_pdu.crc_flag(), CrcFlag::NoCrc);
+        assert_eq!(fd_pdu.file_flag(), LargeFileFlag::Normal);
+        assert_eq!(fd_pdu.pdu_type(), PduType::FileData);
+        assert_eq!(fd_pdu.file_directive_type(), None);
+        assert_eq!(fd_pdu.transmission_mode(), TransmissionMode::Acknowledged);
+        assert_eq!(fd_pdu.direction(), Direction::TowardsReceiver);
+        assert_eq!(fd_pdu.source_id(), TEST_SRC_ID.into());
+        assert_eq!(fd_pdu.dest_id(), TEST_DEST_ID.into());
+        assert_eq!(fd_pdu.transaction_seq_num(), TEST_SEQ_NUM.into());
     }
 
     #[test]
     fn test_serialization() {
-        let common_conf = CommonPduConfig::new_with_byte_fields(SRC_ID, DEST_ID, SEQ_NUM).unwrap();
+        let common_conf =
+            CommonPduConfig::new_with_byte_fields(TEST_SRC_ID, TEST_DEST_ID, TEST_SEQ_NUM).unwrap();
         let pdu_header = PduHeader::new_for_file_data_default(common_conf, 0);
         let file_data: [u8; 4] = [1, 2, 3, 4];
         let fd_pdu = FileDataPdu::new_no_seg_metadata(pdu_header, 10, &file_data);
@@ -295,7 +303,8 @@ mod tests {
 
     #[test]
     fn test_write_to_vec() {
-        let common_conf = CommonPduConfig::new_with_byte_fields(SRC_ID, DEST_ID, SEQ_NUM).unwrap();
+        let common_conf =
+            CommonPduConfig::new_with_byte_fields(TEST_SRC_ID, TEST_DEST_ID, TEST_SEQ_NUM).unwrap();
         let pdu_header = PduHeader::new_for_file_data_default(common_conf, 0);
         let file_data: [u8; 4] = [1, 2, 3, 4];
         let fd_pdu = FileDataPdu::new_no_seg_metadata(pdu_header, 10, &file_data);
@@ -307,7 +316,8 @@ mod tests {
 
     #[test]
     fn test_deserialization() {
-        let common_conf = CommonPduConfig::new_with_byte_fields(SRC_ID, DEST_ID, SEQ_NUM).unwrap();
+        let common_conf =
+            CommonPduConfig::new_with_byte_fields(TEST_SRC_ID, TEST_DEST_ID, TEST_SEQ_NUM).unwrap();
         let pdu_header = PduHeader::new_for_file_data_default(common_conf, 0);
         let file_data: [u8; 4] = [1, 2, 3, 4];
         let fd_pdu = FileDataPdu::new_no_seg_metadata(pdu_header, 10, &file_data);
@@ -319,13 +329,33 @@ mod tests {
         assert_eq!(fd_pdu_read_back, fd_pdu);
     }
 
+    #[test]
+    fn test_with_crc() {
+        let mut common_conf =
+            CommonPduConfig::new_with_byte_fields(TEST_SRC_ID, TEST_DEST_ID, TEST_SEQ_NUM).unwrap();
+        common_conf.crc_flag = true.into();
+        let pdu_header = PduHeader::new_for_file_data_default(common_conf, 0);
+        let file_data: [u8; 4] = [1, 2, 3, 4];
+        let fd_pdu = FileDataPdu::new_no_seg_metadata(pdu_header, 10, &file_data);
+        let mut buf: [u8; 64] = [0; 64];
+        let written = fd_pdu.write_to_bytes(&mut buf).unwrap();
+        assert_eq!(written, fd_pdu.len_written());
+        let finished_pdu_from_raw = FileDataPdu::from_bytes(&buf).unwrap();
+        assert_eq!(finished_pdu_from_raw, fd_pdu);
+        buf[written - 1] -= 1;
+        let crc: u16 = ((buf[written - 2] as u16) << 8) | buf[written - 1] as u16;
+        let error = FileDataPdu::from_bytes(&buf).unwrap_err();
+        if let PduError::ChecksumError(e) = error {
+            assert_eq!(e, crc);
+        } else {
+            panic!("expected crc error");
+        }
+    }
+
     #[test]
     fn test_with_seg_metadata_serialization() {
-        let src_id = UbfU8::new(1);
-        let dest_id = UbfU8::new(2);
-        let transaction_seq_num = UbfU8::new(3);
         let common_conf =
-            CommonPduConfig::new_with_byte_fields(src_id, dest_id, transaction_seq_num).unwrap();
+            CommonPduConfig::new_with_byte_fields(TEST_SRC_ID, TEST_DEST_ID, TEST_SEQ_NUM).unwrap();
         let pdu_header = PduHeader::new_for_file_data(
             common_conf,
             0,
@@ -386,11 +416,8 @@ mod tests {
 
     #[test]
     fn test_with_seg_metadata_deserialization() {
-        let src_id = UbfU8::new(1);
-        let dest_id = UbfU8::new(2);
-        let transaction_seq_num = UbfU8::new(3);
         let common_conf =
-            CommonPduConfig::new_with_byte_fields(src_id, dest_id, transaction_seq_num).unwrap();
+            CommonPduConfig::new_with_byte_fields(TEST_SRC_ID, TEST_DEST_ID, TEST_SEQ_NUM).unwrap();
         let pdu_header = PduHeader::new_for_file_data(
             common_conf,
             0,

src/cfdp/pdu/finished.rs

@@ -120,14 +120,17 @@ impl<'fs_responses> FinishedPdu<'fs_responses> {
     }
 
     fn calc_pdu_datafield_len(&self) -> usize {
-        let mut base_len = 2;
+        let mut datafield_len = 2;
         if let Some(fs_responses) = self.fs_responses {
-            base_len += fs_responses.len();
+            datafield_len += fs_responses.len();
         }
         if let Some(fault_location) = self.fault_location {
-            base_len += fault_location.len_full();
+            datafield_len += fault_location.len_full();
         }
-        base_len
+        if self.crc_flag() == CrcFlag::WithCrc {
+            datafield_len += 2;
+        }
+        datafield_len
     }
 
     /// Generates [Self] from a raw bytestream.
@@ -246,7 +249,7 @@ impl WritablePduPacket for FinishedPdu<'_> {
         if let Some(fault_location) = self.fault_location {
             current_idx += fault_location.write_to_be_bytes(&mut buf[current_idx..])?;
         }
-        if self.pdu_header.pdu_conf.crc_flag == CrcFlag::WithCrc {
+        if self.crc_flag() == CrcFlag::WithCrc {
             current_idx = add_pdu_crc(buf, current_idx);
         }
         Ok(current_idx)
@@ -260,9 +263,11 @@ impl WritablePduPacket for FinishedPdu<'_> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::cfdp::pdu::tests::{common_pdu_conf, verify_raw_header};
+    use crate::cfdp::pdu::tests::{
+        common_pdu_conf, verify_raw_header, TEST_DEST_ID, TEST_SEQ_NUM, TEST_SRC_ID,
+    };
     use crate::cfdp::pdu::{FileDirectiveType, PduHeader};
-    use crate::cfdp::{ConditionCode, CrcFlag, Direction, LargeFileFlag};
+    use crate::cfdp::{ConditionCode, CrcFlag, Direction, LargeFileFlag, TransmissionMode};
 
     fn generic_finished_pdu(
         crc_flag: CrcFlag,
@@ -292,6 +297,22 @@ mod tests {
         assert_eq!(finished_pdu.filestore_responses(), None);
         assert_eq!(finished_pdu.fault_location(), None);
         assert_eq!(finished_pdu.pdu_header().pdu_datafield_len, 2);
+
+        assert_eq!(finished_pdu.crc_flag(), CrcFlag::NoCrc);
+        assert_eq!(finished_pdu.file_flag(), LargeFileFlag::Normal);
+        assert_eq!(finished_pdu.pdu_type(), PduType::FileDirective);
+        assert_eq!(
+            finished_pdu.file_directive_type(),
+            Some(FileDirectiveType::FinishedPdu)
+        );
+        assert_eq!(
+            finished_pdu.transmission_mode(),
+            TransmissionMode::Acknowledged
+        );
+        assert_eq!(finished_pdu.direction(), Direction::TowardsSender);
+        assert_eq!(finished_pdu.source_id(), TEST_SRC_ID.into());
+        assert_eq!(finished_pdu.dest_id(), TEST_DEST_ID.into());
+        assert_eq!(finished_pdu.transaction_seq_num(), TEST_SEQ_NUM.into());
     }
 
     fn generic_serialization_test_no_error(delivery_code: DeliveryCode, file_status: FileStatus) {
@@ -371,4 +392,27 @@ mod tests {
         let read_back = read_back.unwrap();
         assert_eq!(finished_pdu, read_back);
     }
+
+    #[test]
+    fn test_with_crc() {
+        let finished_pdu = generic_finished_pdu(
+            CrcFlag::WithCrc,
+            LargeFileFlag::Normal,
+            DeliveryCode::Complete,
+            FileStatus::Retained,
+        );
+        let mut buf: [u8; 64] = [0; 64];
+        let written = finished_pdu.write_to_bytes(&mut buf).unwrap();
+        assert_eq!(written, finished_pdu.len_written());
+        let finished_pdu_from_raw = FinishedPdu::from_bytes(&buf).unwrap();
+        assert_eq!(finished_pdu_from_raw, finished_pdu);
+        buf[written - 1] -= 1;
+        let crc: u16 = ((buf[written - 2] as u16) << 8) as u16 | buf[written - 1] as u16;
+        let error = FinishedPdu::from_bytes(&buf).unwrap_err();
+        if let PduError::ChecksumError(e) = error {
+            assert_eq!(e, crc);
+        } else {
+            panic!("expected crc error");
+        }
+    }
 }

src/cfdp/pdu/metadata.rs

@@ -187,7 +187,7 @@ impl<'src_name, 'dest_name, 'opts> MetadataPdu<'src_name, 'dest_name, 'opts> {
         if let Some(opts) = self.options {
             len += opts.len();
         }
-        if self.pdu_header.pdu_conf.crc_flag == CrcFlag::WithCrc {
+        if self.crc_flag() == CrcFlag::WithCrc {
             len += 2;
         }
         len
@@ -288,7 +288,7 @@ impl WritablePduPacket for MetadataPdu<'_, '_, '_> {
             buf[current_idx..current_idx + opts.len()].copy_from_slice(opts);
             current_idx += opts.len();
         }
-        if self.pdu_header.pdu_conf.crc_flag == CrcFlag::WithCrc {
+        if self.crc_flag() == CrcFlag::WithCrc {
             current_idx = add_pdu_crc(buf, current_idx);
         }
         Ok(current_idx)
@@ -453,6 +453,7 @@ pub mod tests {
                 + dest_filename.len_full()
                 + 2
         );
+        assert_eq!(written, metadata_pdu.len_written());
         let pdu_read_back = MetadataPdu::from_bytes(&buf).unwrap();
         assert_eq!(pdu_read_back, metadata_pdu);
     }

src/cfdp/pdu/mod.rs

@@ -9,10 +9,12 @@ use core::fmt::{Display, Formatter};
 #[cfg(feature = "std")]
 use std::error::Error;
 
+pub mod ack;
 pub mod eof;
 pub mod file_data;
 pub mod finished;
 pub mod metadata;
+pub mod nak;
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, TryFromPrimitive, IntoPrimitive)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
@@ -41,15 +43,20 @@ pub enum PduError {
         src_id_len: usize,
         dest_id_len: usize,
     },
+    /// Wrong directive type, for example when parsing the directive field for a file directive
+    /// PDU.
     WrongDirectiveType {
         found: FileDirectiveType,
         expected: FileDirectiveType,
     },
-    /// The directive type field contained a value not in the range of permitted values.
+    /// The directive type field contained a value not in the range of permitted values. This can
+    /// also happen if an invalid value is passed to the ACK PDU constructor.
     InvalidDirectiveType {
         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
@@ -70,9 +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,

src/cfdp/pdu/nak.rs

@@ -0,0 +1,793 @@
+use crate::{
+    cfdp::{CrcFlag, Direction, LargeFileFlag},
+    ByteConversionError,
+};
+use core::{marker::PhantomData, mem::size_of};
+
+use super::{
+    add_pdu_crc, generic_length_checks_pdu_deserialization, CfdpPdu, FileDirectiveType, PduError,
+    PduHeader, WritablePduPacket,
+};
+
+/// Helper type to encapsulate both normal file size segment requests and large file size segment
+/// requests.
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum SegmentRequests<'a> {
+    U32Pairs(&'a [(u32, u32)]),
+    U64Pairs(&'a [(u64, u64)]),
+}
+
+impl SegmentRequests<'_> {
+    pub fn is_empty(&self) -> bool {
+        match self {
+            SegmentRequests::U32Pairs(pairs) => pairs.is_empty(),
+            SegmentRequests::U64Pairs(pairs) => pairs.is_empty(),
+        }
+    }
+}
+
+/// NAK PDU abstraction specialized in the creation of NAK PDUs.
+///
+/// It exposes a specialized API which simplifies to generate these NAK PDUs with the
+/// format according to CFDP chapter 5.2.6.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct NakPduCreator<'seg_reqs> {
+    pdu_header: PduHeader,
+    start_of_scope: u64,
+    end_of_scope: u64,
+    segment_requests: Option<SegmentRequests<'seg_reqs>>,
+}
+
+impl<'seg_reqs> NakPduCreator<'seg_reqs> {
+    /// Please note that the start of scope and the end of scope need to be smaller or equal
+    /// to [u32::MAX] if the large file flag of the passed PDU configuration is
+    /// [LargeFileFlag::Normal].
+    ///
+    /// ## Errrors
+    ///
+    pub fn new_no_segment_requests(
+        pdu_header: PduHeader,
+        start_of_scope: u64,
+        end_of_scope: u64,
+    ) -> Result<NakPduCreator<'seg_reqs>, PduError> {
+        Self::new_generic(pdu_header, start_of_scope, end_of_scope, None)
+    }
+
+    /// Default constructor for normal file sizes.
+    pub fn new(
+        pdu_header: PduHeader,
+        start_of_scope: u32,
+        end_of_scope: u32,
+        segment_requests: &'seg_reqs [(u32, u32)],
+    ) -> Result<NakPduCreator, PduError> {
+        let mut passed_segment_requests = None;
+        if !segment_requests.is_empty() {
+            passed_segment_requests = Some(SegmentRequests::U32Pairs(segment_requests));
+        }
+        Self::new_generic(
+            pdu_header,
+            start_of_scope.into(),
+            end_of_scope.into(),
+            passed_segment_requests,
+        )
+    }
+
+    pub fn new_large_file_size(
+        pdu_header: PduHeader,
+        start_of_scope: u64,
+        end_of_scope: u64,
+        segment_requests: &'seg_reqs [(u64, u64)],
+    ) -> Result<NakPduCreator, PduError> {
+        let mut passed_segment_requests = None;
+        if !segment_requests.is_empty() {
+            passed_segment_requests = Some(SegmentRequests::U64Pairs(segment_requests));
+        }
+        Self::new_generic(
+            pdu_header,
+            start_of_scope,
+            end_of_scope,
+            passed_segment_requests,
+        )
+    }
+
+    fn new_generic(
+        mut pdu_header: PduHeader,
+        start_of_scope: u64,
+        end_of_scope: u64,
+        segment_requests: Option<SegmentRequests<'seg_reqs>>,
+    ) -> Result<NakPduCreator, PduError> {
+        // Force correct direction flag.
+        pdu_header.pdu_conf.direction = Direction::TowardsSender;
+        if let Some(ref segment_requests) = segment_requests {
+            match segment_requests {
+                SegmentRequests::U32Pairs(_) => {
+                    if start_of_scope > u32::MAX as u64 || end_of_scope > u32::MAX as u64 {
+                        return Err(PduError::InvalidStartOrEndOfScopeValue);
+                    }
+                    pdu_header.pdu_conf.file_flag = LargeFileFlag::Normal;
+                }
+                SegmentRequests::U64Pairs(_) => {
+                    pdu_header.pdu_conf.file_flag = LargeFileFlag::Large;
+                }
+            }
+        };
+        let mut nak_pdu = Self {
+            pdu_header,
+            start_of_scope,
+            end_of_scope,
+            segment_requests,
+        };
+        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 segment_requests(&self) -> Option<&SegmentRequests> {
+        self.segment_requests.as_ref()
+    }
+
+    pub fn num_segment_reqs(&self) -> usize {
+        match &self.segment_requests {
+            Some(seg_reqs) => match seg_reqs {
+                SegmentRequests::U32Pairs(pairs) => pairs.len(),
+                SegmentRequests::U64Pairs(pairs) => pairs.len(),
+            },
+            None => 0,
+        }
+    }
+
+    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
+    }
+}
+
+impl CfdpPdu for NakPduCreator<'_> {
+    fn pdu_header(&self) -> &PduHeader {
+        &self.pdu_header
+    }
+
+    fn file_directive_type(&self) -> Option<FileDirectiveType> {
+        Some(FileDirectiveType::NakPdu)
+    }
+}
+
+impl WritablePduPacket for NakPduCreator<'_> {
+    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;
+
+        let mut write_start_end_of_scope_normal = || {
+            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;
+        };
+        if let Some(ref seg_reqs) = self.segment_requests {
+            match seg_reqs {
+                SegmentRequests::U32Pairs(pairs) => {
+                    // Unwrap is okay here, the API should prevent invalid values which would trigger a
+                    // panic here.
+                    write_start_end_of_scope_normal();
+                    for (next_start_offset, next_end_offset) in *pairs {
+                        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;
+                    }
+                }
+                SegmentRequests::U64Pairs(pairs) => {
+                    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;
+                    for (next_start_offset, next_end_offset) in *pairs {
+                        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 {
+            write_start_end_of_scope_normal();
+        }
+
+        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()
+    }
+}
+
+/// Special iterator type for the NAK PDU which allows to iterate over both normal and large file
+/// segment requests.
+#[derive(Debug)]
+pub struct SegmentRequestIter<'a, T> {
+    seq_req_raw: &'a [u8],
+    current_idx: usize,
+    phantom: core::marker::PhantomData<T>,
+}
+
+pub trait SegReqFromBytes {
+    fn from_bytes(bytes: &[u8]) -> Self;
+}
+
+impl SegReqFromBytes for u32 {
+    fn from_bytes(bytes: &[u8]) -> u32 {
+        u32::from_be_bytes(bytes.try_into().unwrap())
+    }
+}
+
+impl SegReqFromBytes 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: SegReqFromBytes,
+{
+    type Item = (T, T);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let value = self.next_at_offset(self.current_idx);
+        self.current_idx += 2 * size_of::<T>();
+        value
+    }
+}
+
+impl<'a, 'b> PartialEq<SegmentRequests<'a>> for SegmentRequestIter<'b, u32> {
+    fn eq(&self, other: &SegmentRequests) -> bool {
+        match other {
+            SegmentRequests::U32Pairs(pairs) => self.compare_pairs(pairs),
+            SegmentRequests::U64Pairs(pairs) => {
+                if pairs.is_empty() && self.seq_req_raw.is_empty() {
+                    return true;
+                }
+                false
+            }
+        }
+    }
+}
+
+impl<'a, 'b> PartialEq<SegmentRequests<'a>> for SegmentRequestIter<'b, u64> {
+    fn eq(&self, other: &SegmentRequests) -> bool {
+        match other {
+            SegmentRequests::U32Pairs(pairs) => {
+                if pairs.is_empty() && self.seq_req_raw.is_empty() {
+                    return true;
+                }
+                false
+            }
+            SegmentRequests::U64Pairs(pairs) => self.compare_pairs(pairs),
+        }
+    }
+}
+
+impl<'a, T> SegmentRequestIter<'a, T>
+where
+    T: SegReqFromBytes + PartialEq,
+{
+    fn compare_pairs(&self, pairs: &[(T, T)]) -> bool {
+        if pairs.is_empty() && self.seq_req_raw.is_empty() {
+            return true;
+        }
+        let size = size_of::<T>();
+        if pairs.len() * 2 * size != self.seq_req_raw.len() {
+            return false;
+        }
+
+        for (i, pair) in pairs.iter().enumerate() {
+            let next_val = self.next_at_offset(i * 2 * size).unwrap();
+            if next_val != *pair {
+                return false;
+            }
+        }
+
+        true
+    }
+}
+
+impl<T: SegReqFromBytes> SegmentRequestIter<'_, T> {
+    fn next_at_offset(&self, mut offset: usize) -> Option<(T, T)> {
+        if offset + size_of::<T>() * 2 > self.seq_req_raw.len() {
+            return None;
+        }
+
+        let start_offset = T::from_bytes(&self.seq_req_raw[offset..offset + size_of::<T>()]);
+        offset += size_of::<T>();
+
+        let end_offset = T::from_bytes(&self.seq_req_raw[offset..offset + size_of::<T>()]);
+        Some((start_offset, end_offset))
+    }
+}
+
+/// NAK PDU abstraction specialized in the reading NAK PDUs from a raw bytestream.
+///
+/// This is a zero-copy class where the segment requests can be read using a special iterator
+/// API without the need to copy them.
+///
+/// The NAK format is expected to be conforming to CFDP chapter 5.2.6.
+#[derive(Debug, PartialEq, Eq)]
+pub struct NakPduReader<'seg_reqs> {
+    pdu_header: PduHeader,
+    start_of_scope: u64,
+    end_of_scope: u64,
+    seg_reqs_raw: &'seg_reqs [u8],
+}
+
+impl CfdpPdu for NakPduReader<'_> {
+    fn pdu_header(&self) -> &PduHeader {
+        &self.pdu_header
+    }
+
+    fn file_directive_type(&self) -> Option<FileDirectiveType> {
+        Some(FileDirectiveType::NakPdu)
+    }
+}
+
+impl<'seg_reqs> NakPduReader<'seg_reqs> {
+    pub fn new(buf: &'seg_reqs [u8]) -> Result<NakPduReader, PduError> {
+        Self::from_bytes(buf)
+    }
+
+    pub fn from_bytes(buf: &'seg_reqs [u8]) -> Result<NakPduReader, 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 =
+                u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap()) as u64;
+            current_idx += 4;
+            end_of_scope =
+                u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap()) as u64;
+            current_idx += 4;
+        }
+        Ok(Self {
+            pdu_header,
+            start_of_scope,
+            end_of_scope,
+            seg_reqs_raw: &buf[current_idx..full_len_without_crc],
+        })
+    }
+
+    pub fn start_of_scope(&self) -> u64 {
+        self.start_of_scope
+    }
+
+    pub fn end_of_scope(&self) -> u64 {
+        self.end_of_scope
+    }
+
+    pub fn num_segment_reqs(&self) -> usize {
+        if self.seg_reqs_raw.is_empty() {
+            return 0;
+        }
+        if self.file_flag() == LargeFileFlag::Normal {
+            self.seg_reqs_raw.len() / 8
+        } else {
+            self.seg_reqs_raw.len() / 16
+        }
+    }
+
+    /// This function returns [None] if this NAK PDUs contains segment requests for a large file.
+    pub fn get_normal_segment_requests_iterator(&self) -> Option<SegmentRequestIter<'_, u32>> {
+        if self.file_flag() == LargeFileFlag::Large {
+            return None;
+        }
+        Some(SegmentRequestIter {
+            seq_req_raw: self.seg_reqs_raw,
+            current_idx: 0,
+            phantom: PhantomData,
+        })
+    }
+
+    /// This function returns [None] if this NAK PDUs contains segment requests for a normal file.
+    pub fn get_large_segment_requests_iterator(&self) -> Option<SegmentRequestIter<'_, u64>> {
+        if self.file_flag() == LargeFileFlag::Normal {
+            return None;
+        }
+        Some(SegmentRequestIter {
+            seq_req_raw: self.seg_reqs_raw,
+            current_idx: 0,
+            phantom: PhantomData,
+        })
+    }
+}
+
+impl<'a, 'b> PartialEq<NakPduCreator<'a>> for NakPduReader<'b> {
+    fn eq(&self, other: &NakPduCreator<'a>) -> bool {
+        if self.pdu_header() != other.pdu_header()
+            || self.end_of_scope() != other.end_of_scope()
+            || self.start_of_scope() != other.start_of_scope()
+        {
+            return false;
+        }
+
+        // Check if both segment requests are empty or None
+        match (self.seg_reqs_raw.is_empty(), other.segment_requests()) {
+            (true, None) => true,
+            (true, Some(seg_reqs)) => seg_reqs.is_empty(),
+            (false, None) => false,
+            _ => {
+                // Compare based on file_flag
+                if self.file_flag() == LargeFileFlag::Normal {
+                    let normal_iter = self.get_normal_segment_requests_iterator().unwrap();
+                    normal_iter == *other.segment_requests().unwrap()
+                } else {
+                    let large_iter = self.get_large_segment_requests_iterator().unwrap();
+                    large_iter == *other.segment_requests().unwrap()
+                }
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::cfdp::{
+        pdu::tests::{common_pdu_conf, verify_raw_header, TEST_DEST_ID, TEST_SEQ_NUM, TEST_SRC_ID},
+        PduType, TransmissionMode,
+    };
+
+    use super::*;
+
+    fn check_generic_fields(nak_pdu: &impl CfdpPdu) {
+        assert_eq!(nak_pdu.crc_flag(), CrcFlag::NoCrc);
+        assert_eq!(nak_pdu.file_flag(), LargeFileFlag::Normal);
+        assert_eq!(nak_pdu.pdu_type(), PduType::FileDirective);
+        assert_eq!(
+            nak_pdu.file_directive_type(),
+            Some(FileDirectiveType::NakPdu),
+        );
+        assert_eq!(nak_pdu.transmission_mode(), TransmissionMode::Acknowledged);
+        assert_eq!(nak_pdu.direction(), Direction::TowardsSender);
+        assert_eq!(nak_pdu.source_id(), TEST_SRC_ID.into());
+        assert_eq!(nak_pdu.dest_id(), TEST_DEST_ID.into());
+        assert_eq!(nak_pdu.transaction_seq_num(), TEST_SEQ_NUM.into());
+    }
+
+    #[test]
+    fn test_seg_request_api() {
+        let seg_req = SegmentRequests::U32Pairs(&[]);
+        assert!(seg_req.is_empty());
+        let seg_req = SegmentRequests::U64Pairs(&[]);
+        assert!(seg_req.is_empty());
+    }
+
+    #[test]
+    fn test_basic_creator() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu = NakPduCreator::new_no_segment_requests(pdu_header, 0, 0)
+            .expect("creating NAK PDU creator failed");
+        assert_eq!(nak_pdu.start_of_scope(), 0);
+        assert_eq!(nak_pdu.end_of_scope(), 0);
+        assert_eq!(nak_pdu.segment_requests(), None);
+        assert_eq!(nak_pdu.num_segment_reqs(), 0);
+        check_generic_fields(&nak_pdu);
+    }
+
+    #[test]
+    fn test_serialization_empty() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu = NakPduCreator::new_no_segment_requests(pdu_header, 100, 300)
+            .expect("creating NAK PDU creator failed");
+        assert_eq!(nak_pdu.start_of_scope(), 100);
+        assert_eq!(nak_pdu.end_of_scope(), 300);
+        let mut buf: [u8; 64] = [0; 64];
+        nak_pdu
+            .write_to_bytes(&mut buf)
+            .expect("writing NAK PDU to buffer failed");
+        verify_raw_header(nak_pdu.pdu_header(), &buf);
+        let mut current_idx = nak_pdu.pdu_header().header_len();
+        assert_eq!(current_idx + 9, nak_pdu.len_written());
+        assert_eq!(buf[current_idx], FileDirectiveType::NakPdu as u8);
+        current_idx += 1;
+        let start_of_scope =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(start_of_scope, 100);
+        current_idx += 4;
+        let end_of_scope =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(end_of_scope, 300);
+        current_idx += 4;
+        assert_eq!(current_idx, nak_pdu.len_written());
+    }
+
+    #[test]
+    fn test_serialization_two_segments() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu = NakPduCreator::new(pdu_header, 100, 300, &[(0, 0), (32, 64)])
+            .expect("creating NAK PDU creator failed");
+        let mut buf: [u8; 64] = [0; 64];
+        nak_pdu
+            .write_to_bytes(&mut buf)
+            .expect("writing NAK PDU to buffer failed");
+        verify_raw_header(nak_pdu.pdu_header(), &buf);
+        let mut current_idx = nak_pdu.pdu_header().header_len();
+        assert_eq!(current_idx + 9 + 16, nak_pdu.len_written());
+        assert_eq!(buf[current_idx], FileDirectiveType::NakPdu as u8);
+        current_idx += 1;
+        let start_of_scope =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(start_of_scope, 100);
+        current_idx += 4;
+        let end_of_scope =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(end_of_scope, 300);
+        current_idx += 4;
+        let first_seg_start =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(first_seg_start, 0);
+        current_idx += 4;
+        let first_seg_end =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(first_seg_end, 0);
+        current_idx += 4;
+        let second_seg_start =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(second_seg_start, 32);
+        current_idx += 4;
+        let second_seg_end =
+            u32::from_be_bytes(buf[current_idx..current_idx + 4].try_into().unwrap());
+        assert_eq!(second_seg_end, 64);
+        current_idx += 4;
+        assert_eq!(current_idx, nak_pdu.len_written());
+    }
+
+    #[test]
+    fn test_deserialization_empty() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu = NakPduCreator::new_no_segment_requests(pdu_header, 100, 300)
+            .expect("creating NAK PDU creator failed");
+        let mut buf: [u8; 64] = [0; 64];
+        nak_pdu
+            .write_to_bytes(&mut buf)
+            .expect("writing NAK PDU to buffer failed");
+        let nak_pdu_deser = NakPduReader::from_bytes(&buf).expect("deserializing NAK PDU failed");
+        assert_eq!(nak_pdu_deser, nak_pdu);
+        check_generic_fields(&nak_pdu_deser);
+    }
+
+    #[test]
+    fn test_deserialization_large_segments() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Large);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu =
+            NakPduCreator::new_large_file_size(pdu_header, 100, 300, &[(50, 100), (200, 300)])
+                .expect("creating NAK PDU creator failed");
+        let mut buf: [u8; 128] = [0; 128];
+        nak_pdu
+            .write_to_bytes(&mut buf)
+            .expect("writing NAK PDU to buffer failed");
+        let nak_pdu_deser = NakPduReader::from_bytes(&buf).expect("deserializing NAK PDU failed");
+        assert_eq!(nak_pdu_deser, nak_pdu);
+        assert_eq!(nak_pdu_deser.start_of_scope(), 100);
+        assert_eq!(nak_pdu_deser.end_of_scope(), 300);
+        assert_eq!(nak_pdu_deser.num_segment_reqs(), 2);
+        assert!(nak_pdu_deser
+            .get_large_segment_requests_iterator()
+            .is_some());
+        assert!(nak_pdu_deser
+            .get_normal_segment_requests_iterator()
+            .is_none());
+        assert_eq!(
+            nak_pdu_deser
+                .get_large_segment_requests_iterator()
+                .unwrap()
+                .count(),
+            2
+        );
+        for (idx, large_segments) in nak_pdu_deser
+            .get_large_segment_requests_iterator()
+            .unwrap()
+            .enumerate()
+        {
+            if idx == 0 {
+                assert_eq!(large_segments.0, 50);
+                assert_eq!(large_segments.1, 100);
+            } else {
+                assert_eq!(large_segments.0, 200);
+                assert_eq!(large_segments.1, 300);
+            }
+        }
+    }
+
+    #[test]
+    fn test_deserialization_normal_segments() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu = NakPduCreator::new(pdu_header, 100, 300, &[(50, 100), (200, 300)])
+            .expect("creating NAK PDU creator failed");
+        let mut buf: [u8; 128] = [0; 128];
+        nak_pdu
+            .write_to_bytes(&mut buf)
+            .expect("writing NAK PDU to buffer failed");
+        let nak_pdu_deser = NakPduReader::from_bytes(&buf).expect("deserializing NAK PDU failed");
+        assert_eq!(nak_pdu_deser, nak_pdu);
+        assert_eq!(nak_pdu_deser.start_of_scope(), 100);
+        assert_eq!(nak_pdu_deser.end_of_scope(), 300);
+        assert_eq!(nak_pdu_deser.num_segment_reqs(), 2);
+        assert!(nak_pdu_deser
+            .get_normal_segment_requests_iterator()
+            .is_some());
+        assert!(nak_pdu_deser
+            .get_large_segment_requests_iterator()
+            .is_none());
+        assert_eq!(
+            nak_pdu_deser
+                .get_normal_segment_requests_iterator()
+                .unwrap()
+                .count(),
+            2
+        );
+        for (idx, large_segments) in nak_pdu_deser
+            .get_normal_segment_requests_iterator()
+            .unwrap()
+            .enumerate()
+        {
+            if idx == 0 {
+                assert_eq!(large_segments.0, 50);
+                assert_eq!(large_segments.1, 100);
+            } else {
+                assert_eq!(large_segments.0, 200);
+                assert_eq!(large_segments.1, 300);
+            }
+        }
+    }
+
+    #[test]
+    fn test_empty_is_empty() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu_0 =
+            NakPduCreator::new(pdu_header, 100, 300, &[]).expect("creating NAK PDU creator failed");
+        let nak_pdu_1 = NakPduCreator::new_no_segment_requests(pdu_header, 100, 300)
+            .expect("creating NAK PDU creator failed");
+        assert_eq!(nak_pdu_0, nak_pdu_1);
+        // Assert the segment request is mapped to None.
+        assert!(nak_pdu_0.segment_requests().is_none());
+    }
+
+    #[test]
+    fn test_new_generic_invalid_input() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let u32_list = SegmentRequests::U32Pairs(&[(0, 50), (50, 100)]);
+        if let Err(PduError::InvalidStartOrEndOfScopeValue) = NakPduCreator::new_generic(
+            pdu_header,
+            u32::MAX as u64 + 1,
+            u32::MAX as u64 + 2,
+            Some(u32_list),
+        ) {
+        } else {
+            panic!("API call did not fail");
+        }
+    }
+
+    #[test]
+    fn test_target_buf_too_small() {
+        let pdu_conf = common_pdu_conf(CrcFlag::NoCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu = NakPduCreator::new_no_segment_requests(pdu_header, 100, 300)
+            .expect("creating NAK PDU creator failed");
+        assert_eq!(nak_pdu.start_of_scope(), 100);
+        assert_eq!(nak_pdu.end_of_scope(), 300);
+        let mut buf: [u8; 5] = [0; 5];
+        let error = nak_pdu.write_to_bytes(&mut buf);
+        assert!(error.is_err());
+        let e = error.unwrap_err();
+        match e {
+            PduError::ByteConversionError(conv_error) => match conv_error {
+                ByteConversionError::ToSliceTooSmall { found, expected } => {
+                    assert_eq!(expected, nak_pdu.len_written());
+                    assert_eq!(found, 5);
+                }
+                _ => panic!("unexpected error {conv_error}"),
+            },
+            _ => panic!("unexpected error {e}"),
+        }
+    }
+
+    #[test]
+    fn test_with_crc() {
+        let pdu_conf = common_pdu_conf(CrcFlag::WithCrc, LargeFileFlag::Normal);
+        let pdu_header = PduHeader::new_no_file_data(pdu_conf, 0);
+        let nak_pdu = NakPduCreator::new_no_segment_requests(pdu_header, 0, 0)
+            .expect("creating NAK PDU creator failed");
+        let mut nak_vec = nak_pdu.to_vec().expect("writing NAK to vector failed");
+        assert_eq!(nak_vec.len(), pdu_header.header_len() + 9 + 2);
+        assert_eq!(nak_vec.len(), nak_pdu.len_written());
+        let nak_pdu_deser = NakPduReader::new(&nak_vec).expect("reading NAK PDU failed");
+        assert_eq!(nak_pdu_deser, nak_pdu);
+        nak_vec[nak_pdu.len_written() - 1] -= 1;
+        let nak_pdu_deser = NakPduReader::new(&nak_vec);
+        assert!(nak_pdu_deser.is_err());
+        if let Err(PduError::ChecksumError(raw)) = nak_pdu_deser {
+            assert_eq!(
+                raw,
+                u16::from_be_bytes(nak_vec[nak_pdu.len_written() - 2..].try_into().unwrap())
+            );
+        }
+    }
+}