fix

Reviewed-on: #129

CHANGELOG.md

@@ -8,6 +8,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 # [unreleased]
 
+## Added
+
+- `PusTcCreatorWithReservedAppData` and `PusTmCreatorWithReservedSourceData` constructor variants
+  which allow writing source/app data into the serialization buffer directly without
+  requiring an extra buffer.
+
 # [v0.14.0] 2025-05-10
 
 ## Changed

src/ecss/mod.rs

@@ -377,7 +377,7 @@ pub trait WritablePusPacket {
     fn write_to_bytes_no_crc(&self, slice: &mut [u8]) -> Result<usize, PusError>;
 
     /// First uses [Self::write_to_bytes_no_crc] to write the packet to the given slice and then
-    /// uses the [CRC_CCITT_FALS] to calculate the CRC and write it to the slice.
+    /// uses the [CRC_CCITT_FALSE] to calculate the CRC and write it to the slice.
     fn write_to_bytes(&self, slice: &mut [u8]) -> Result<usize, PusError> {
         let mut curr_idx = self.write_to_bytes_no_crc(slice)?;
         let mut digest = CRC_CCITT_FALSE.digest();

src/ecss/tc.rs

@@ -372,6 +372,49 @@ impl<'app_data> PusTcCreator<'app_data> {
         vec.extend_from_slice(&digest.finalize().to_be_bytes());
         appended_len
     }
+
+    /// Write the raw PUS byte representation to a provided buffer.
+    pub fn write_to_bytes(&self, slice: &mut [u8]) -> Result<usize, ByteConversionError> {
+        let writer_unfinalized = self.common_write(slice)?;
+        Ok(writer_unfinalized.finalize())
+    }
+
+    /// Write the raw PUS byte representation to a provided buffer.
+    pub fn write_to_bytes_crc_no_table(
+        &self,
+        slice: &mut [u8],
+    ) -> Result<usize, ByteConversionError> {
+        let writer_unfinalized = self.common_write(slice)?;
+        Ok(writer_unfinalized.finalize_crc_no_table())
+    }
+
+    /// Write the raw PUS byte representation to a provided buffer.
+    pub fn write_to_bytes_no_crc(&self, slice: &mut [u8]) -> Result<usize, ByteConversionError> {
+        let writer_unfinalized = self.common_write(slice)?;
+        Ok(writer_unfinalized.finalize_no_crc())
+    }
+
+    fn common_write<'a>(
+        &self,
+        slice: &'a mut [u8],
+    ) -> Result<PusTcCreatorWithReservedAppData<'a>, ByteConversionError> {
+        if self.len_written() > slice.len() {
+            return Err(ByteConversionError::ToSliceTooSmall {
+                found: slice.len(),
+                expected: self.len_written(),
+            });
+        }
+        let mut writer_unfinalized = PusTcCreatorWithReservedAppData::write_to_bytes_partially(
+            slice,
+            self.sp_header,
+            self.sec_header,
+            self.app_data.len(),
+        )?;
+        writer_unfinalized
+            .app_data_mut()
+            .copy_from_slice(self.app_data);
+        Ok(writer_unfinalized)
+    }
 }
 
 impl WritablePusPacket for PusTcCreator<'_> {
@@ -380,31 +423,17 @@ impl WritablePusPacket for PusTcCreator<'_> {
         PUS_TC_MIN_LEN_WITHOUT_APP_DATA + self.app_data.len()
     }
 
-    /// Writes the packet to the given slice without writing the CRC.
-    ///
-    /// The returned size is the written size WITHOUT the CRC.
+    /// Write the raw PUS byte representation to a provided buffer.
     fn write_to_bytes_no_crc(&self, slice: &mut [u8]) -> Result<usize, PusError> {
-        let mut curr_idx = 0;
-        let tc_header_len = size_of::<zc::PusTcSecondaryHeader>();
-        let total_size = self.len_written();
-        if total_size > slice.len() {
-            return Err(ByteConversionError::ToSliceTooSmall {
-                found: slice.len(),
-                expected: total_size,
-            }
-            .into());
-        }
-        self.sp_header.write_to_be_bytes(slice)?;
-        curr_idx += CCSDS_HEADER_LEN;
-        let sec_header = zc::PusTcSecondaryHeader::try_from(self.sec_header).unwrap();
-        sec_header
-            .write_to(&mut slice[curr_idx..curr_idx + tc_header_len])
-            .map_err(|_| ByteConversionError::ZeroCopyToError)?;
+        Ok(Self::write_to_bytes_no_crc(self, slice)?)
+    }
 
-        curr_idx += tc_header_len;
-        slice[curr_idx..curr_idx + self.app_data.len()].copy_from_slice(self.app_data);
-        curr_idx += self.app_data.len();
-        Ok(curr_idx)
+    fn write_to_bytes(&self, slice: &mut [u8]) -> Result<usize, PusError> {
+        Ok(Self::write_to_bytes(self, slice)?)
+    }
+
+    fn write_to_bytes_crc_no_table(&self, slice: &mut [u8]) -> Result<usize, PusError> {
+        Ok(Self::write_to_bytes_crc_no_table(self, slice)?)
     }
 }
 
@@ -450,6 +479,133 @@ impl GenericPusTcSecondaryHeader for PusTcCreator<'_> {
 
 impl IsPusTelecommand for PusTcCreator<'_> {}
 
+/// A specialized variant of [PusTcCreator] designed for efficiency when handling large source
+/// data.
+///
+/// Unlike [PusTcCreator], this type does not require the user to provide the application data
+/// as a separate slice. Instead, it allows writing the application data directly into the provided
+/// serialization buffer. This eliminates the need for an intermediate buffer and the associated
+/// memory copy, improving performance, particularly when working with large payloads.
+///
+/// **Important:** The total length of the source data must be known and specified in advance
+/// to ensure correct serialization behavior.
+///
+/// Note that this abstraction intentionally omits certain trait implementations that are available
+/// on [PusTcCreator], as they are not applicable in this optimized usage pattern.
+pub struct PusTcCreatorWithReservedAppData<'buf> {
+    buf: &'buf mut [u8],
+    app_data_offset: usize,
+    full_len: usize,
+}
+
+impl<'buf> PusTcCreatorWithReservedAppData<'buf> {
+    /// Generates a new instance with reserved space for the user application data.
+    ///
+    /// # Arguments
+    ///
+    /// * `sp_header` - Space packet header information. The correct packet type and the secondary
+    ///   header flag are set correctly by the constructor.
+    /// * `sec_header` - Information contained in the secondary header, including the service
+    ///   and subservice type
+    /// * `app_data_len` - Custom application data length
+    #[inline]
+    pub fn new(
+        buf: &'buf mut [u8],
+        mut sp_header: SpHeader,
+        sec_header: PusTcSecondaryHeader,
+        app_data_len: usize,
+    ) -> Result<Self, ByteConversionError> {
+        sp_header.set_packet_type(PacketType::Tc);
+        sp_header.set_sec_header_flag();
+        let len_written = PUS_TC_MIN_LEN_WITHOUT_APP_DATA + app_data_len;
+        if len_written > buf.len() {
+            return Err(ByteConversionError::ToSliceTooSmall {
+                found: buf.len(),
+                expected: len_written,
+            });
+        }
+        sp_header.data_len = len_written as u16 - size_of::<crate::zc::SpHeader>() as u16 - 1;
+        Self::write_to_bytes_partially(buf, sp_header, sec_header, app_data_len)
+    }
+
+    fn write_to_bytes_partially(
+        buf: &'buf mut [u8],
+        sp_header: SpHeader,
+        sec_header: PusTcSecondaryHeader,
+        app_data_len: usize,
+    ) -> Result<Self, ByteConversionError> {
+        let mut curr_idx = 0;
+        sp_header.write_to_be_bytes(&mut buf[0..CCSDS_HEADER_LEN])?;
+        curr_idx += CCSDS_HEADER_LEN;
+        let sec_header_len = size_of::<zc::PusTcSecondaryHeader>();
+        let sec_header_zc = zc::PusTcSecondaryHeader::try_from(sec_header).unwrap();
+        sec_header_zc
+            .write_to(&mut buf[curr_idx..curr_idx + sec_header_len])
+            .map_err(|_| ByteConversionError::ZeroCopyToError)?;
+        curr_idx += sec_header_len;
+        let app_data_offset = curr_idx;
+        curr_idx += app_data_len;
+        Ok(Self {
+            buf,
+            app_data_offset,
+            full_len: curr_idx + 2,
+        })
+    }
+
+    #[inline]
+    pub const fn len_written(&self) -> usize {
+        self.full_len
+    }
+
+    /// Mutable access to the application data buffer.
+    #[inline]
+    pub fn app_data_mut(&mut self) -> &mut [u8] {
+        &mut self.buf[self.app_data_offset..self.full_len - 2]
+    }
+
+    /// Access to the source data buffer.
+    #[inline]
+    pub fn app_data(&self) -> &[u8] {
+        &self.buf[self.app_data_offset..self.full_len - 2]
+    }
+
+    #[inline]
+    pub fn app_data_len(&self) -> usize {
+        self.full_len - 2 - self.app_data_offset
+    }
+
+    /// Finalize the TC packet by calculating and writing the CRC16.
+    ///
+    /// Returns the full packet length.
+    pub fn finalize(self) -> usize {
+        let mut digest = CRC_CCITT_FALSE.digest();
+        digest.update(&self.buf[0..self.full_len - 2]);
+        self.buf[self.full_len - 2..self.full_len]
+            .copy_from_slice(&digest.finalize().to_be_bytes());
+        self.full_len
+    }
+
+    /// Finalize the TC packet by calculating and writing the CRC16 using a table-less
+    /// implementation.
+    ///
+    /// Returns the full packet length.
+    pub fn finalize_crc_no_table(self) -> usize {
+        let mut digest = CRC_CCITT_FALSE_NO_TABLE.digest();
+        digest.update(&self.buf[0..self.full_len - 2]);
+        self.buf[self.full_len - 2..self.full_len]
+            .copy_from_slice(&digest.finalize().to_be_bytes());
+        self.full_len
+    }
+
+    /// Finalize the TC packet without writing the CRC16.
+    ///
+    /// Returns the length WITHOUT the CRC16.
+    #[inline]
+    pub fn finalize_no_crc(self) -> usize {
+        self.full_len - 2
+    }
+}
+
 /// This class can be used to read a PUS TC telecommand from raw memory.
 ///
 /// This class also derives the [serde::Serialize] and [serde::Deserialize] trait if the
@@ -627,8 +783,6 @@ impl PartialEq<PusTcReader<'_>> for PusTcCreator<'_> {
 
 #[cfg(all(test, feature = "std"))]
 mod tests {
-    use std::error::Error;
-
     use super::*;
     use crate::ecss::PusVersion::PusC;
     use crate::ecss::{PusError, PusPacket, WritablePusPacket};
@@ -675,6 +829,32 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_serialization_with_trait_1() {
+        let pus_tc = base_ping_tc_simple_ctor();
+        let mut test_buf: [u8; 32] = [0; 32];
+        let size = WritablePusPacket::write_to_bytes(&pus_tc, test_buf.as_mut_slice())
+            .expect("Error writing TC to buffer");
+        assert_eq!(size, 13);
+        assert_eq!(
+            pus_tc.opt_crc16().unwrap(),
+            u16::from_be_bytes(test_buf[size - 2..size].try_into().unwrap())
+        );
+    }
+
+    #[test]
+    fn test_serialization_with_trait_2() {
+        let pus_tc = base_ping_tc_simple_ctor();
+        let mut test_buf: [u8; 32] = [0; 32];
+        let size = WritablePusPacket::write_to_bytes_crc_no_table(&pus_tc, test_buf.as_mut_slice())
+            .expect("Error writing TC to buffer");
+        assert_eq!(size, 13);
+        assert_eq!(
+            pus_tc.opt_crc16().unwrap(),
+            u16::from_be_bytes(test_buf[size - 2..size].try_into().unwrap())
+        );
+    }
+
     #[test]
     fn test_serialization_crc_no_table() {
         let pus_tc = base_ping_tc_simple_ctor();
@@ -701,6 +881,17 @@ mod tests {
         assert_eq!(test_buf[12], 0);
     }
 
+    #[test]
+    fn test_serialization_no_crc_with_trait() {
+        let pus_tc = base_ping_tc_simple_ctor();
+        let mut test_buf: [u8; 32] = [0; 32];
+        let size = WritablePusPacket::write_to_bytes_no_crc(&pus_tc, test_buf.as_mut_slice())
+            .expect("error writing tc to buffer");
+        assert_eq!(size, 11);
+        assert_eq!(test_buf[11], 0);
+        assert_eq!(test_buf[12], 0);
+    }
+
     #[test]
     fn test_deserialization() {
         let pus_tc = base_ping_tc_simple_ctor();
@@ -718,6 +909,28 @@ mod tests {
         verify_crc_no_app_data(&test_buf);
     }
 
+    #[test]
+    fn test_deserialization_alt_ctor() {
+        let sph = SpHeader::new_for_unseg_tc_checked(0x02, 0x34, 0).unwrap();
+        let tc_header = PusTcSecondaryHeader::new_simple(17, 1);
+        let mut test_buf: [u8; 32] = [0; 32];
+        let mut pus_tc =
+            PusTcCreatorWithReservedAppData::new(&mut test_buf, sph, tc_header, 0).unwrap();
+        assert_eq!(pus_tc.len_written(), 13);
+        assert_eq!(pus_tc.app_data_len(), 0);
+        assert_eq!(pus_tc.app_data(), &[]);
+        assert_eq!(pus_tc.app_data_mut(), &[]);
+        let size = pus_tc.finalize();
+        assert_eq!(size, 13);
+        let tc_from_raw =
+            PusTcReader::new(&test_buf).expect("Creating PUS TC struct from raw buffer failed");
+        assert_eq!(tc_from_raw.total_len(), 13);
+        verify_test_tc_with_reader(&tc_from_raw, false, 13);
+        assert!(tc_from_raw.user_data().is_empty());
+        verify_test_tc_raw(&test_buf);
+        verify_crc_no_app_data(&test_buf);
+    }
+
     #[test]
     fn test_deserialization_no_table() {
         let pus_tc = base_ping_tc_simple_ctor();
@@ -757,6 +970,7 @@ mod tests {
         tc.update_ccsds_data_len();
         assert_eq!(tc.data_len(), 6);
     }
+
     #[test]
     fn test_deserialization_with_app_data() {
         let pus_tc = base_ping_tc_simple_ctor_with_app_data(&[1, 2, 3]);
@@ -859,22 +1073,17 @@ mod tests {
         let res = pus_tc.write_to_bytes(test_buf.as_mut_slice());
         assert!(res.is_err());
         let err = res.unwrap_err();
-        if let PusError::ByteConversion(e) = err {
-            assert_eq!(
-                e,
-                ByteConversionError::ToSliceTooSmall {
-                    found: 12,
-                    expected: 13
-                }
-            );
-            assert_eq!(
-                err.to_string(),
-                "pus error: target slice with size 12 is too small, expected size of at least 13"
-            );
-            assert_eq!(err.source().unwrap().to_string(), e.to_string());
-        } else {
-            panic!("unexpected error {err}");
-        }
+        assert_eq!(
+            err,
+            ByteConversionError::ToSliceTooSmall {
+                found: 12,
+                expected: 13
+            }
+        );
+        assert_eq!(
+            err.to_string(),
+            "target slice with size 12 is too small, expected size of at least 13"
+        );
     }
 
     #[test]

src/ecss/tm.rs

@@ -392,12 +392,8 @@ impl<'time, 'src_data> PusTmCreator<'time, 'src_data> {
 
     /// Write the raw PUS byte representation to a provided buffer.
     pub fn write_to_bytes(&self, slice: &mut [u8]) -> Result<usize, ByteConversionError> {
-        let mut curr_idx = self.write_to_bytes_no_crc(slice)?;
-        let mut digest = CRC_CCITT_FALSE.digest();
-        digest.update(&slice[0..curr_idx]);
-        slice[curr_idx..curr_idx + 2].copy_from_slice(&digest.finalize().to_be_bytes());
-        curr_idx += 2;
-        Ok(curr_idx)
+        let writer_unfinalized = self.common_write(slice)?;
+        Ok(writer_unfinalized.finalize())
     }
 
     /// Write the raw PUS byte representation to a provided buffer.
@@ -405,39 +401,36 @@ impl<'time, 'src_data> PusTmCreator<'time, 'src_data> {
         &self,
         slice: &mut [u8],
     ) -> Result<usize, ByteConversionError> {
-        let mut curr_idx = self.write_to_bytes_no_crc(slice)?;
-        let mut digest = CRC_CCITT_FALSE_NO_TABLE.digest();
-        digest.update(&slice[0..curr_idx]);
-        slice[curr_idx..curr_idx + 2].copy_from_slice(&digest.finalize().to_be_bytes());
-        curr_idx += 2;
-        Ok(curr_idx)
+        let writer_unfinalized = self.common_write(slice)?;
+        Ok(writer_unfinalized.finalize_crc_no_table())
     }
 
     /// Write the raw PUS byte representation to a provided buffer.
     pub fn write_to_bytes_no_crc(&self, slice: &mut [u8]) -> Result<usize, ByteConversionError> {
-        let mut curr_idx = 0;
-        let total_size = self.len_written();
-        if total_size > slice.len() {
+        let writer_unfinalized = self.common_write(slice)?;
+        Ok(writer_unfinalized.finalize_no_crc())
+    }
+
+    fn common_write<'a>(
+        &self,
+        slice: &'a mut [u8],
+    ) -> Result<PusTmCreatorWithReservedSourceData<'a>, ByteConversionError> {
+        if self.len_written() > slice.len() {
             return Err(ByteConversionError::ToSliceTooSmall {
                 found: slice.len(),
-                expected: total_size,
+                expected: self.len_written(),
             });
         }
-        self.sp_header
-            .write_to_be_bytes(&mut slice[0..CCSDS_HEADER_LEN])?;
-        curr_idx += CCSDS_HEADER_LEN;
-        let sec_header_len = size_of::<zc::PusTmSecHeaderWithoutTimestamp>();
-        let sec_header = zc::PusTmSecHeaderWithoutTimestamp::try_from(self.sec_header).unwrap();
-        sec_header
-            .write_to(&mut slice[curr_idx..curr_idx + sec_header_len])
-            .map_err(|_| ByteConversionError::ZeroCopyToError)?;
-        curr_idx += sec_header_len;
-        slice[curr_idx..curr_idx + self.sec_header.timestamp.len()]
-            .copy_from_slice(self.sec_header.timestamp);
-        curr_idx += self.sec_header.timestamp.len();
-        slice[curr_idx..curr_idx + self.source_data.len()].copy_from_slice(self.source_data);
-        curr_idx += self.source_data.len();
-        Ok(curr_idx)
+        let mut writer_unfinalized = PusTmCreatorWithReservedSourceData::write_to_bytes_partially(
+            slice,
+            self.sp_header,
+            self.sec_header,
+            self.source_data.len(),
+        )?;
+        writer_unfinalized
+            .source_data_mut()
+            .copy_from_slice(self.source_data);
+        Ok(writer_unfinalized)
     }
 
     /// Append the raw PUS byte representation to a provided [alloc::vec::Vec]
@@ -471,9 +464,18 @@ impl WritablePusPacket for PusTmCreator<'_, '_> {
     fn write_to_bytes_no_crc(&self, slice: &mut [u8]) -> Result<usize, PusError> {
         Ok(Self::write_to_bytes_no_crc(self, slice)?)
     }
+
+    fn write_to_bytes(&self, slice: &mut [u8]) -> Result<usize, PusError> {
+        Ok(Self::write_to_bytes(self, slice)?)
+    }
+
+    fn write_to_bytes_crc_no_table(&self, slice: &mut [u8]) -> Result<usize, PusError> {
+        Ok(Self::write_to_bytes_crc_no_table(self, slice)?)
+    }
 }
 
 impl PartialEq for PusTmCreator<'_, '_> {
+    #[inline]
     fn eq(&self, other: &Self) -> bool {
         self.sp_header == other.sp_header
             && self.sec_header == other.sec_header
@@ -525,6 +527,136 @@ impl GenericPusTmSecondaryHeader for PusTmCreator<'_, '_> {
 
 impl IsPusTelemetry for PusTmCreator<'_, '_> {}
 
+/// A specialized variant of [PusTmCreator] designed for efficiency when handling large source
+/// data.
+///
+/// Unlike [PusTmCreator], this type does not require the user to provide the source data
+/// as a separate slice. Instead, it allows writing the source data directly into the provided
+/// serialization buffer. This eliminates the need for an intermediate buffer and the associated
+/// memory copy, improving performance, particularly when working with large payloads.
+///
+/// **Important:** The total length of the source data must be known and specified in advance
+/// to ensure correct serialization behavior.
+///
+/// Note that this abstraction intentionally omits certain trait implementations that are available
+/// on [PusTmCreator], as they are not applicable in this optimized usage pattern.
+pub struct PusTmCreatorWithReservedSourceData<'buf> {
+    buf: &'buf mut [u8],
+    source_data_offset: usize,
+    full_len: usize,
+}
+
+impl<'buf> PusTmCreatorWithReservedSourceData<'buf> {
+    /// Generates a new instance with reserved space for the user source data.
+    ///
+    /// # Arguments
+    ///
+    /// * `sp_header` - Space packet header information. The correct packet type and the secondary
+    ///   header flag are set correctly by the constructor.
+    /// * `sec_header` - Information contained in the secondary header, including the service
+    ///   and subservice type
+    /// * `src_data_len` - Custom source data length
+    #[inline]
+    pub fn new(
+        buf: &'buf mut [u8],
+        mut sp_header: SpHeader,
+        sec_header: PusTmSecondaryHeader,
+        src_data_len: usize,
+    ) -> Result<Self, ByteConversionError> {
+        sp_header.set_packet_type(PacketType::Tm);
+        sp_header.set_sec_header_flag();
+        let len_written =
+            PUS_TM_MIN_LEN_WITHOUT_SOURCE_DATA + sec_header.timestamp.len() + src_data_len;
+        if len_written > buf.len() {
+            return Err(ByteConversionError::ToSliceTooSmall {
+                found: buf.len(),
+                expected: len_written,
+            });
+        }
+        sp_header.data_len = len_written as u16 - size_of::<crate::zc::SpHeader>() as u16 - 1;
+        Self::write_to_bytes_partially(buf, sp_header, sec_header, src_data_len)
+    }
+
+    fn write_to_bytes_partially(
+        buf: &'buf mut [u8],
+        sp_header: SpHeader,
+        sec_header: PusTmSecondaryHeader,
+        src_data_len: usize,
+    ) -> Result<Self, ByteConversionError> {
+        let mut curr_idx = 0;
+        sp_header.write_to_be_bytes(&mut buf[0..CCSDS_HEADER_LEN])?;
+        curr_idx += CCSDS_HEADER_LEN;
+        let sec_header_len = size_of::<zc::PusTmSecHeaderWithoutTimestamp>();
+        let sec_header_zc = zc::PusTmSecHeaderWithoutTimestamp::try_from(sec_header).unwrap();
+        sec_header_zc
+            .write_to(&mut buf[curr_idx..curr_idx + sec_header_len])
+            .map_err(|_| ByteConversionError::ZeroCopyToError)?;
+        curr_idx += sec_header_len;
+        buf[curr_idx..curr_idx + sec_header.timestamp.len()].copy_from_slice(sec_header.timestamp);
+        curr_idx += sec_header.timestamp.len();
+        let source_data_offset = curr_idx;
+        curr_idx += src_data_len;
+        Ok(Self {
+            buf,
+            source_data_offset,
+            full_len: curr_idx + 2,
+        })
+    }
+
+    #[inline]
+    pub const fn len_written(&self) -> usize {
+        self.full_len
+    }
+
+    /// Mutable access to the source data buffer.
+    #[inline]
+    pub fn source_data_mut(&mut self) -> &mut [u8] {
+        &mut self.buf[self.source_data_offset..self.full_len - 2]
+    }
+
+    /// Access to the source data buffer.
+    #[inline]
+    pub fn source_data(&self) -> &[u8] {
+        &self.buf[self.source_data_offset..self.full_len - 2]
+    }
+
+    #[inline]
+    pub fn source_data_len(&self) -> usize {
+        self.full_len - 2 - self.source_data_offset
+    }
+
+    /// Finalize the TM packet by calculating and writing the CRC16.
+    ///
+    /// Returns the full packet length.
+    pub fn finalize(self) -> usize {
+        let mut digest = CRC_CCITT_FALSE.digest();
+        digest.update(&self.buf[0..self.full_len - 2]);
+        self.buf[self.full_len - 2..self.full_len]
+            .copy_from_slice(&digest.finalize().to_be_bytes());
+        self.full_len
+    }
+
+    /// Finalize the TM packet by calculating and writing the CRC16 using a table-less
+    /// implementation.
+    ///
+    /// Returns the full packet length.
+    pub fn finalize_crc_no_table(self) -> usize {
+        let mut digest = CRC_CCITT_FALSE_NO_TABLE.digest();
+        digest.update(&self.buf[0..self.full_len - 2]);
+        self.buf[self.full_len - 2..self.full_len]
+            .copy_from_slice(&digest.finalize().to_be_bytes());
+        self.full_len
+    }
+
+    /// Finalize the TM packet without writing the CRC16.
+    ///
+    /// Returns the length WITHOUT the CRC16.
+    #[inline]
+    pub fn finalize_no_crc(self) -> usize {
+        self.full_len - 2
+    }
+}
+
 /// This class models the PUS C telemetry packet. It is the primary data structure to read
 /// a telemetry packet from raw bytes.
 ///
@@ -962,7 +1094,41 @@ mod tests {
             .write_to_bytes(&mut buf)
             .expect("Serialization failed");
         assert_eq!(ser_len, 22);
-        verify_raw_ping_reply(pus_tm.opt_crc16().unwrap(), &buf);
+        verify_raw_ping_reply(pus_tm.opt_crc16(), &buf);
+    }
+
+    #[test]
+    fn test_serialization_no_source_data_alt_ctor() {
+        let timestamp = dummy_timestamp();
+        let sph = SpHeader::new_for_unseg_tm_checked(0x123, 0x234, 0).unwrap();
+        let tm_header = PusTmSecondaryHeader::new_simple(17, 2, timestamp);
+        let mut buf: [u8; 32] = [0; 32];
+        let mut pus_tm =
+            PusTmCreatorWithReservedSourceData::new(&mut buf, sph, tm_header, 0).unwrap();
+        assert_eq!(pus_tm.source_data_len(), 0);
+        assert_eq!(pus_tm.source_data(), &[]);
+        assert_eq!(pus_tm.source_data_mut(), &[]);
+        let ser_len = pus_tm.finalize();
+        assert_eq!(ser_len, 22);
+        verify_raw_ping_reply(None, &buf);
+    }
+
+    #[test]
+    fn test_serialization_no_source_data_alt_ctor_no_crc() {
+        let timestamp = dummy_timestamp();
+        let sph = SpHeader::new_for_unseg_tm_checked(0x123, 0x234, 0).unwrap();
+        let tm_header = PusTmSecondaryHeader::new_simple(17, 2, timestamp);
+        let mut buf: [u8; 32] = [0; 32];
+        let mut pus_tm =
+            PusTmCreatorWithReservedSourceData::new(&mut buf, sph, tm_header, 0).unwrap();
+        assert_eq!(pus_tm.source_data_len(), 0);
+        assert_eq!(pus_tm.source_data(), &[]);
+        assert_eq!(pus_tm.source_data_mut(), &[]);
+        let ser_len = pus_tm.finalize_no_crc();
+        assert_eq!(ser_len, 20);
+        verify_raw_ping_reply_no_crc(&buf);
+        assert_eq!(buf[20], 0);
+        assert_eq!(buf[21], 0);
     }
 
     #[test]
@@ -974,7 +1140,7 @@ mod tests {
             .write_to_bytes_crc_no_table(&mut buf)
             .expect("Serialization failed");
         assert_eq!(ser_len, 22);
-        verify_raw_ping_reply(pus_tm.opt_crc16().unwrap(), &buf);
+        verify_raw_ping_reply(pus_tm.opt_crc16(), &buf);
     }
 
     #[test]
@@ -1004,6 +1170,46 @@ mod tests {
         assert_eq!(buf[22], 3);
     }
 
+    #[test]
+    fn test_serialization_with_source_data_alt_ctor() {
+        let src_data = &[1, 2, 3];
+        let mut buf: [u8; 32] = [0; 32];
+        let sph = SpHeader::new_for_unseg_tm_checked(0x123, 0x234, 0).unwrap();
+        let tc_header = PusTmSecondaryHeader::new_simple(3, 5, dummy_timestamp());
+        let mut hk_reply_unwritten =
+            PusTmCreatorWithReservedSourceData::new(&mut buf, sph, tc_header, 3).unwrap();
+        assert_eq!(hk_reply_unwritten.source_data_len(), 3);
+        assert_eq!(hk_reply_unwritten.source_data(), &[0, 0, 0]);
+        assert_eq!(hk_reply_unwritten.source_data_mut(), &[0, 0, 0]);
+        let source_data_mut = hk_reply_unwritten.source_data_mut();
+        source_data_mut.copy_from_slice(src_data);
+        let ser_len = hk_reply_unwritten.finalize();
+        assert_eq!(ser_len, 25);
+        assert_eq!(buf[20], 1);
+        assert_eq!(buf[21], 2);
+        assert_eq!(buf[22], 3);
+    }
+
+    #[test]
+    fn test_serialization_with_source_data_alt_ctor_no_table() {
+        let src_data = &[1, 2, 3];
+        let mut buf: [u8; 32] = [0; 32];
+        let sph = SpHeader::new_for_unseg_tm_checked(0x123, 0x234, 0).unwrap();
+        let tc_header = PusTmSecondaryHeader::new_simple(3, 5, dummy_timestamp());
+        let mut hk_reply_unwritten =
+            PusTmCreatorWithReservedSourceData::new(&mut buf, sph, tc_header, 3).unwrap();
+        assert_eq!(hk_reply_unwritten.source_data_len(), 3);
+        assert_eq!(hk_reply_unwritten.source_data(), &[0, 0, 0]);
+        assert_eq!(hk_reply_unwritten.source_data_mut(), &[0, 0, 0]);
+        let source_data_mut = hk_reply_unwritten.source_data_mut();
+        source_data_mut.copy_from_slice(src_data);
+        let ser_len = hk_reply_unwritten.finalize_crc_no_table();
+        assert_eq!(ser_len, 25);
+        assert_eq!(buf[20], 1);
+        assert_eq!(buf[21], 2);
+        assert_eq!(buf[22], 3);
+    }
+
     #[test]
     fn test_setters() {
         let timestamp = dummy_timestamp();
@@ -1029,6 +1235,7 @@ mod tests {
         assert_eq!(tm_vec.len(), tm_deserialized.total_len());
         verify_ping_reply_with_reader(&tm_deserialized, false, 22, dummy_timestamp());
     }
+
     #[test]
     fn test_deserialization_no_source_data() {
         let timestamp = dummy_timestamp();
@@ -1046,6 +1253,22 @@ mod tests {
         verify_ping_reply_with_reader(&tm_deserialized, false, 22, dummy_timestamp());
     }
 
+    #[test]
+    fn test_deserialization_no_source_data_with_trait() {
+        let timestamp = dummy_timestamp();
+        let pus_tm = base_ping_reply_full_ctor(timestamp);
+        let mut buf: [u8; 32] = [0; 32];
+        let ser_len =
+            WritablePusPacket::write_to_bytes(&pus_tm, &mut buf).expect("Serialization failed");
+        assert_eq!(ser_len, 22);
+        let tm_deserialized = PusTmReader::new(&buf, 7).expect("Deserialization failed");
+        assert_eq!(ser_len, tm_deserialized.total_len());
+        assert_eq!(tm_deserialized.user_data(), tm_deserialized.source_data());
+        assert_eq!(tm_deserialized.raw_data(), &buf[..ser_len]);
+        assert_eq!(tm_deserialized.crc16(), pus_tm.opt_crc16().unwrap());
+        verify_ping_reply_with_reader(&tm_deserialized, false, 22, dummy_timestamp());
+    }
+
     #[test]
     fn test_deserialization_no_table() {
         let timestamp = dummy_timestamp();
@@ -1130,7 +1353,7 @@ mod tests {
         let res = pus_tm.append_to_vec(&mut vec);
         assert!(res.is_ok());
         assert_eq!(res.unwrap(), 22);
-        verify_raw_ping_reply(pus_tm.opt_crc16().unwrap(), vec.as_slice());
+        verify_raw_ping_reply(pus_tm.opt_crc16(), vec.as_slice());
     }
 
     #[test]
@@ -1146,7 +1369,7 @@ mod tests {
         assert_eq!(vec.len(), 26);
     }
 
-    fn verify_raw_ping_reply(crc16: u16, buf: &[u8]) {
+    fn verify_raw_ping_reply_no_crc(buf: &[u8]) {
         // Secondary header is set -> 0b0000_1001 , APID occupies last bit of first byte
         assert_eq!(buf[0], 0x09);
         // Rest of APID 0x123
@@ -1167,12 +1390,19 @@ mod tests {
         assert_eq!(buf[12], 0x00);
         // Timestamp
         assert_eq!(&buf[13..20], dummy_timestamp());
+    }
+
+    fn verify_raw_ping_reply(crc16: Option<u16>, buf: &[u8]) {
+        verify_raw_ping_reply_no_crc(buf);
         let mut digest = CRC_CCITT_FALSE.digest();
         digest.update(&buf[0..20]);
         let crc16_calced = digest.finalize();
-        assert_eq!(((crc16 >> 8) & 0xff) as u8, buf[20]);
-        assert_eq!((crc16 & 0xff) as u8, buf[21]);
-        assert_eq!(crc16, crc16_calced);
+        let crc16_read = u16::from_be_bytes([buf[20], buf[21]]);
+        assert_eq!(crc16_read, crc16_calced);
+        if let Some(crc16) = crc16 {
+            assert_eq!(((crc16 >> 8) & 0xff) as u8, buf[20]);
+            assert_eq!((crc16 & 0xff) as u8, buf[21]);
+        }
     }
 
     fn verify_ping_reply(

src/lib.rs

@@ -73,6 +73,7 @@ pub mod crc;
 pub mod ecss;
 pub mod seq_count;
 pub mod time;
+pub mod uslp;
 pub mod util;
 
 mod private {

src/uslp/mod.rs

@@ -0,0 +1,685 @@
+/// # Support of the CCSDS Unified Space Data Link Protocol (USLP)
+use crate::{ByteConversionError, crc::CRC_CCITT_FALSE};
+
+/// Only this version is supported by the library
+pub const USLP_VERSION_NUMBER: u8 = 0b1100;
+
+/// Identifies the association of the data contained in the transfer frame.
+#[derive(
+    Debug, Copy, Clone, PartialEq, Eq, num_enum::TryFromPrimitive, num_enum::IntoPrimitive,
+)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum SourceOrDestField {
+    /// SCID refers to the source of the transfer frame.
+    Source = 0,
+    /// SCID refers to the destination of the transfer frame.
+    Dest = 1,
+}
+
+#[derive(
+    Debug, Copy, Clone, PartialEq, Eq, num_enum::TryFromPrimitive, num_enum::IntoPrimitive,
+)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum BypassSequenceControlFlag {
+    /// Acceptance of this frame on the receiving end is subject to normal frame acceptance
+    /// checks of FARM.
+    SequenceControlledQoS = 0,
+    /// Frame Acceptance Checks of FARM by the receiving end shall be bypassed.
+    ExpeditedQoS = 1,
+}
+
+#[derive(
+    Debug, Copy, Clone, PartialEq, Eq, num_enum::TryFromPrimitive, num_enum::IntoPrimitive,
+)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum ProtocolControlCommandFlag {
+    TfdfContainsUserData = 0,
+    TfdfContainsProtocolInfo = 1,
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum UslpError {
+    ByteConversion(ByteConversionError),
+    HeaderIsTruncated,
+    InvalidProtocolId(u8),
+    InvalidConstructionRule(u8),
+    InvalidVersionNumber(u8),
+    InvalidVcid(u8),
+    InvalidMapId(u8),
+    ChecksumFailure(u16),
+}
+
+impl From<ByteConversionError> for UslpError {
+    fn from(value: ByteConversionError) -> Self {
+        Self::ByteConversion(value)
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct InvalidValueForLen {
+    value: u64,
+    len: u8,
+}
+
+#[derive(Debug, Copy, Clone, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct PrimaryHeader {
+    pub spacecraft_id: u16,
+    pub source_or_dest_field: SourceOrDestField,
+    pub vc_id: u8,
+    pub map_id: u8,
+    frame_len_field: u16,
+    pub sequence_control_flag: BypassSequenceControlFlag,
+    pub protocol_control_command_flag: ProtocolControlCommandFlag,
+    pub ocf_flag: bool,
+    vc_frame_count_len: u8,
+    vc_frame_count: u64,
+}
+
+impl PrimaryHeader {
+    pub fn new(
+        spacecraft_id: u16,
+        source_or_dest_field: SourceOrDestField,
+        vc_id: u8,
+        map_id: u8,
+        frame_len: u16,
+    ) -> Result<Self, UslpError> {
+        if vc_id > 0b111111 {
+            return Err(UslpError::InvalidVcid(vc_id));
+        }
+        if map_id > 0b1111 {
+            return Err(UslpError::InvalidMapId(map_id));
+        }
+        Ok(Self {
+            spacecraft_id,
+            source_or_dest_field,
+            vc_id,
+            map_id,
+            frame_len_field: frame_len.saturating_sub(1),
+            sequence_control_flag: BypassSequenceControlFlag::SequenceControlledQoS,
+            protocol_control_command_flag: ProtocolControlCommandFlag::TfdfContainsUserData,
+            ocf_flag: false,
+            vc_frame_count_len: 0,
+            vc_frame_count: 0,
+        })
+    }
+
+    pub fn set_vc_frame_count(
+        &mut self,
+        count_len: u8,
+        count: u64,
+    ) -> Result<(), InvalidValueForLen> {
+        if count > 2_u64.pow(count_len as u32 * 8) - 1 {
+            return Err(InvalidValueForLen {
+                value: count,
+                len: count_len,
+            });
+        }
+        self.vc_frame_count_len = count_len;
+        self.vc_frame_count = count;
+        Ok(())
+    }
+
+    #[inline(always)]
+    pub fn vc_frame_count(&self) -> u64 {
+        self.vc_frame_count
+    }
+
+    #[inline(always)]
+    pub fn vc_frame_count_len(&self) -> u8 {
+        self.vc_frame_count_len
+    }
+
+    pub fn from_bytes(buf: &[u8]) -> Result<Self, UslpError> {
+        if buf.len() < 4 {
+            return Err(ByteConversionError::FromSliceTooSmall {
+                found: buf.len(),
+                expected: 4,
+            }
+            .into());
+        }
+        // Can only deal with regular frames for now.
+        if (buf[3] & 0b1) == 1 {
+            return Err(UslpError::HeaderIsTruncated);
+        }
+        // We could check this above, but this is a better error for the case where the user
+        // tries to read a truncated frame.
+        if buf.len() < 7 {
+            return Err(ByteConversionError::FromSliceTooSmall {
+                found: buf.len(),
+                expected: 7,
+            }
+            .into());
+        }
+        let version_number = (buf[0] >> 4) & 0b1111;
+        if version_number != USLP_VERSION_NUMBER {
+            return Err(UslpError::InvalidVersionNumber(version_number));
+        }
+        let source_or_dest_field = match (buf[2] >> 3) & 1 {
+            0 => SourceOrDestField::Source,
+            1 => SourceOrDestField::Dest,
+            _ => unreachable!(),
+        };
+        let vc_frame_count_len = buf[6] & 0b111;
+        if buf.len() < 7 + vc_frame_count_len as usize {
+            return Err(ByteConversionError::FromSliceTooSmall {
+                found: buf.len(),
+                expected: 7 + vc_frame_count_len as usize,
+            }
+            .into());
+        }
+        let vc_frame_count = match vc_frame_count_len {
+            1 => buf[7] as u64,
+            2 => u16::from_be_bytes(buf[7..9].try_into().unwrap()) as u64,
+            4 => u32::from_be_bytes(buf[7..11].try_into().unwrap()) as u64,
+            len => {
+                let mut vcf_count = 0u64;
+                let mut end = len;
+                for byte in buf[7..7 + len as usize].iter() {
+                    vcf_count |= (*byte as u64) << ((end - 1) * 8);
+                    end -= 1;
+                }
+                vcf_count
+            }
+        };
+        Ok(Self {
+            spacecraft_id: (((buf[0] as u16) & 0b1111) << 12)
+                | ((buf[1] as u16) << 4)
+                | ((buf[2] as u16) >> 4) & 0b1111,
+            source_or_dest_field,
+            vc_id: ((buf[2] & 0b111) << 3) | (buf[3] >> 5) & 0b111,
+            map_id: (buf[3] >> 1) & 0b1111,
+            frame_len_field: ((buf[4] as u16) << 8) | buf[5] as u16,
+            sequence_control_flag: ((buf[6] >> 7) & 0b1).try_into().unwrap(),
+            protocol_control_command_flag: ((buf[6] >> 6) & 0b1).try_into().unwrap(),
+            ocf_flag: ((buf[6] >> 3) & 0b1) != 0,
+            vc_frame_count_len,
+            vc_frame_count,
+        })
+    }
+
+    pub fn write_to_be_bytes(&self, buf: &mut [u8]) -> Result<usize, ByteConversionError> {
+        if buf.len() < self.len_header() {
+            return Err(ByteConversionError::ToSliceTooSmall {
+                found: buf.len(),
+                expected: self.len_header(),
+            });
+        }
+        buf[0] = (USLP_VERSION_NUMBER << 4) | ((self.spacecraft_id >> 12) as u8) & 0b1111;
+        buf[1] = (self.spacecraft_id >> 4) as u8;
+        buf[2] = (((self.spacecraft_id & 0b1111) as u8) << 4)
+            | ((self.source_or_dest_field as u8) << 3)
+            | (self.vc_id >> 3) & 0b111;
+        buf[3] = ((self.vc_id & 0b111) << 5) | (self.map_id << 1);
+        buf[4..6].copy_from_slice(&self.frame_len_field.to_be_bytes());
+        buf[6] = ((self.sequence_control_flag as u8) << 7)
+            | ((self.protocol_control_command_flag as u8) << 6)
+            | ((self.ocf_flag as u8) << 3)
+            | self.vc_frame_count_len;
+        let mut packet_idx = 7;
+        for idx in (0..self.vc_frame_count_len).rev() {
+            buf[packet_idx] = ((self.vc_frame_count >> (idx * 8)) & 0xff) as u8;
+            packet_idx += 1;
+        }
+        Ok(self.len_header())
+    }
+
+    #[inline(always)]
+    pub fn set_frame_len(&mut self, frame_len: usize) {
+        // 4.1.2.7.2
+        // The field contains a length count C that equals one fewer than the total octets
+        // in the transfer frame.
+        self.frame_len_field = frame_len.saturating_sub(1) as u16;
+    }
+
+    #[inline(always)]
+    pub fn len_header(&self) -> usize {
+        7 + self.vc_frame_count_len as usize
+    }
+
+    #[inline(always)]
+    pub fn len_frame(&self) -> usize {
+        // 4.1.2.7.2
+        // The field contains a length count C that equals one fewer than the total octets
+        // in the transfer frame.
+        self.frame_len_field as usize + 1
+    }
+}
+
+/// Custom implementation which skips the check whether the VC frame count length field is equal.
+/// Only the actual VC count value is compared.
+impl PartialEq for PrimaryHeader {
+    fn eq(&self, other: &Self) -> bool {
+        self.spacecraft_id == other.spacecraft_id
+            && self.source_or_dest_field == other.source_or_dest_field
+            && self.vc_id == other.vc_id
+            && self.map_id == other.map_id
+            && self.frame_len_field == other.frame_len_field
+            && self.sequence_control_flag == other.sequence_control_flag
+            && self.protocol_control_command_flag == other.protocol_control_command_flag
+            && self.ocf_flag == other.ocf_flag
+            && self.vc_frame_count == other.vc_frame_count
+    }
+}
+
+#[derive(
+    Debug, Copy, Clone, PartialEq, Eq, num_enum::TryFromPrimitive, num_enum::IntoPrimitive,
+)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+#[non_exhaustive]
+pub enum UslpProtocolId {
+    SpacePacketsOrEncapsulation = 0b00000,
+    /// COP-1 control commands within the TFDZ.
+    Cop1ControlCommands = 0b00001,
+    /// COP-P control commands within the TFDZ.
+    CopPControlCommands = 0b00010,
+    /// SDLS control commands within the TFDZ.
+    Sdls = 0b00011,
+    UserDefinedOctetStream = 0b00100,
+    /// Proximity-1 Supervisory Protocol Data Units (SPDUs) within the TFDZ.
+    Spdu = 0b00111,
+    /// Entire fixed-length TFDZ contains idle data.
+    Idle = 0b11111,
+}
+
+#[derive(
+    Debug, Copy, Clone, PartialEq, Eq, num_enum::TryFromPrimitive, num_enum::IntoPrimitive,
+)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum ConstructionRule {
+    /// Indicated fixed-length TFDZ whose contents are CCSDS packets concatenated together, which
+    /// span transfer frame boundaries. The First Header Pointer (FHP) is required for packet
+    /// extraction.
+    PacketSpanningMultipleFrames = 0b000,
+    StartOfMapaSduOrVcaSdu = 0b001,
+    ContinuingPortionOfMapaSdu = 0b010,
+    OctetStream = 0b011,
+    StartingSegment = 0b100,
+    ContinuingSegment = 0b101,
+    LastSegment = 0b110,
+    NoSegmentation = 0b111,
+}
+
+impl ConstructionRule {
+    pub const fn applicable_to_fixed_len_tfdz(&self) -> bool {
+        match self {
+            ConstructionRule::PacketSpanningMultipleFrames => true,
+            ConstructionRule::StartOfMapaSduOrVcaSdu => true,
+            ConstructionRule::ContinuingPortionOfMapaSdu => true,
+            ConstructionRule::OctetStream => false,
+            ConstructionRule::StartingSegment => false,
+            ConstructionRule::ContinuingSegment => false,
+            ConstructionRule::LastSegment => false,
+            ConstructionRule::NoSegmentation => false,
+        }
+    }
+}
+
+pub struct TransferFrameDataFieldHeader {
+    /// Construction rule for the TFDZ.
+    construction_rule: ConstructionRule,
+    uslp_protocol_id: UslpProtocolId,
+    /// First header or last valid octet pointer. Only present if the constuction rule indicated
+    /// a fixed-length TFDZ.
+    fhp_or_lvo: Option<u16>,
+}
+
+impl TransferFrameDataFieldHeader {
+    pub fn len_header(&self) -> usize {
+        if self.construction_rule.applicable_to_fixed_len_tfdz() {
+            3
+        } else {
+            1
+        }
+    }
+
+    pub fn construction_rule(&self) -> ConstructionRule {
+        self.construction_rule
+    }
+
+    pub fn uslp_protocol_id(&self) -> UslpProtocolId {
+        self.uslp_protocol_id
+    }
+
+    pub fn fhp_or_lvo(&self) -> Option<u16> {
+        self.fhp_or_lvo
+    }
+
+    pub fn from_bytes(buf: &[u8]) -> Result<Self, UslpError> {
+        if buf.is_empty() {
+            return Err(ByteConversionError::FromSliceTooSmall {
+                found: 0,
+                expected: 1,
+            }
+            .into());
+        }
+        let construction_rule = ConstructionRule::try_from((buf[0] >> 5) & 0b111)
+            .map_err(|e| UslpError::InvalidConstructionRule(e.number))?;
+        let mut fhp_or_lvo = None;
+        if construction_rule.applicable_to_fixed_len_tfdz() {
+            if buf.len() < 3 {
+                return Err(ByteConversionError::FromSliceTooSmall {
+                    found: buf.len(),
+                    expected: 3,
+                }
+                .into());
+            }
+            fhp_or_lvo = Some(u16::from_be_bytes(buf[1..3].try_into().unwrap()));
+        }
+        Ok(Self {
+            construction_rule,
+            uslp_protocol_id: UslpProtocolId::try_from(buf[0] & 0b11111)
+                .map_err(|e| UslpError::InvalidProtocolId(e.number))?,
+            fhp_or_lvo,
+        })
+    }
+}
+
+/// Simple USLP transfer frame reader.
+///
+/// Currently, only insert zone lengths of 0 are supported.
+pub struct TransferFrameReader<'buf> {
+    primary_header: PrimaryHeader,
+    data_field_header: TransferFrameDataFieldHeader,
+    data: &'buf [u8],
+    operational_control_field: Option<u32>,
+}
+
+impl<'buf> TransferFrameReader<'buf> {
+    /// This function assumes an insert zone length of 0.
+    pub fn from_bytes(
+        buf: &'buf [u8],
+        has_fecf: bool,
+    ) -> Result<TransferFrameReader<'buf>, UslpError> {
+        let primary_header = PrimaryHeader::from_bytes(buf)?;
+        if primary_header.len_frame() > buf.len() {
+            return Err(ByteConversionError::FromSliceTooSmall {
+                expected: primary_header.len_frame(),
+                found: buf.len(),
+            }
+            .into());
+        }
+        let data_field_header =
+            TransferFrameDataFieldHeader::from_bytes(&buf[primary_header.len_header()..])?;
+        let data_idx = primary_header.len_header() + data_field_header.len_header();
+        let frame_len = primary_header.len_frame();
+        let mut operational_control_field = None;
+        let mut data_len = frame_len - data_idx;
+        if has_fecf {
+            data_len -= 2;
+        }
+        if primary_header.ocf_flag {
+            data_len -= 4;
+            operational_control_field = Some(u32::from_be_bytes(
+                buf[data_idx + data_len..data_idx + data_len + 4]
+                    .try_into()
+                    .unwrap(),
+            ));
+        }
+        let data_end = data_idx + data_len;
+        let mut digest = CRC_CCITT_FALSE.digest();
+        digest.update(&buf[0..frame_len]);
+        if digest.finalize() != 0 {
+            return Err(UslpError::ChecksumFailure(u16::from_be_bytes(
+                buf[frame_len - 2..frame_len].try_into().unwrap(),
+            )));
+        }
+        Ok(Self {
+            primary_header,
+            data_field_header,
+            data: buf[data_idx..data_end].try_into().unwrap(),
+            operational_control_field,
+        })
+    }
+
+    pub fn len_frame(&self) -> usize {
+        self.primary_header.len_frame()
+    }
+
+    pub fn primary_header(&self) -> &PrimaryHeader {
+        &self.primary_header
+    }
+
+    pub fn data_field_header(&self) -> &TransferFrameDataFieldHeader {
+        &self.data_field_header
+    }
+
+    pub fn data(&self) -> &'buf [u8] {
+        self.data
+    }
+
+    pub fn operational_control_field(&self) -> &Option<u32> {
+        &self.operational_control_field
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::println;
+
+    use super::*;
+
+    fn common_basic_check(buf: &[u8]) {
+        assert_eq!(buf[0] >> 4, USLP_VERSION_NUMBER);
+        // First four bits SCID.
+        assert_eq!(buf[0] & 0b1111, 0b1010);
+        // Next eight bits SCID.
+        assert_eq!(buf[1], 0b01011100);
+        // Last four bits SCID.
+        assert_eq!(buf[2] >> 4, 0b0011);
+        assert_eq!((buf[2] >> 3) & 0b1, SourceOrDestField::Dest as u8);
+        // First three bits VCID.
+        assert_eq!(buf[2] & 0b111, 0b110);
+        // Last three bits VCID.
+        assert_eq!(buf[3] >> 5, 0b101);
+        // MAP ID
+        assert_eq!((buf[3] >> 1) & 0b1111, 0b1010);
+        // End of primary header flag
+        assert_eq!(buf[3] & 0b1, 0);
+        assert_eq!(u16::from_be_bytes(buf[4..6].try_into().unwrap()), 0x2345);
+    }
+    #[test]
+    fn test_basic_0() {
+        let mut buf: [u8; 8] = [0; 8];
+        // Should be all zeros after writing.
+        buf[6] = 0xff;
+        let primary_header = PrimaryHeader::new(
+            0b10100101_11000011,
+            SourceOrDestField::Dest,
+            0b110101,
+            0b1010,
+            0x2345,
+        )
+        .unwrap();
+        // Virtual channel count 0.
+        assert_eq!(primary_header.write_to_be_bytes(&mut buf).unwrap(), 7);
+        common_basic_check(&buf);
+        // Bypass / Sequence Control Flag.
+        assert_eq!(
+            (buf[6] >> 7) & 0b1,
+            BypassSequenceControlFlag::SequenceControlledQoS as u8
+        );
+        // Protocol Control Command Flag.
+        assert_eq!(
+            (buf[6] >> 6) & 0b1,
+            ProtocolControlCommandFlag::TfdfContainsUserData as u8
+        );
+        // OCF flag.
+        assert_eq!((buf[6] >> 3) & 0b1, false as u8);
+        // VCF count length.
+        assert_eq!(buf[6] & 0b111, 0);
+    }
+
+    #[test]
+    fn test_basic_1() {
+        let mut buf: [u8; 16] = [0; 16];
+        // Should be all zeros after writing.
+        buf[6] = 0xff;
+        let mut primary_header = PrimaryHeader::new(
+            0b10100101_11000011,
+            SourceOrDestField::Dest,
+            0b110101,
+            0b1010,
+            0x2345,
+        )
+        .unwrap();
+        primary_header.sequence_control_flag = BypassSequenceControlFlag::ExpeditedQoS;
+        primary_header.protocol_control_command_flag =
+            ProtocolControlCommandFlag::TfdfContainsProtocolInfo;
+        primary_header.ocf_flag = true;
+        primary_header.set_vc_frame_count(4, 0x12345678).unwrap();
+        // Virtual channel count 4.
+        assert_eq!(primary_header.write_to_be_bytes(&mut buf).unwrap(), 11);
+        common_basic_check(&buf);
+        // Bypass / Sequence Control Flag.
+        assert_eq!(
+            (buf[6] >> 7) & 0b1,
+            BypassSequenceControlFlag::ExpeditedQoS as u8
+        );
+        // Protocol Control Command Flag.
+        assert_eq!(
+            (buf[6] >> 6) & 0b1,
+            ProtocolControlCommandFlag::TfdfContainsProtocolInfo as u8
+        );
+        // OCF flag.
+        assert_eq!((buf[6] >> 3) & 0b1, true as u8);
+        // VCF count length.
+        assert_eq!(buf[6] & 0b111, 4);
+        assert_eq!(
+            u32::from_be_bytes(buf[7..11].try_into().unwrap()),
+            0x12345678
+        );
+    }
+
+    #[test]
+    fn test_reading_0() {
+        let mut buf: [u8; 8] = [0; 8];
+        let primary_header = PrimaryHeader::new(
+            0b10100101_11000011,
+            SourceOrDestField::Dest,
+            0b110101,
+            0b1010,
+            0x2345,
+        )
+        .unwrap();
+        assert_eq!(primary_header.write_to_be_bytes(&mut buf).unwrap(), 7);
+        let parsed_header = PrimaryHeader::from_bytes(&buf).unwrap();
+        assert_eq!(parsed_header, primary_header);
+    }
+
+    #[test]
+    fn test_reading_1() {
+        let mut buf: [u8; 16] = [0; 16];
+        let mut primary_header = PrimaryHeader::new(
+            0b10100101_11000011,
+            SourceOrDestField::Dest,
+            0b110101,
+            0b1010,
+            0x2345,
+        )
+        .unwrap();
+        primary_header.sequence_control_flag = BypassSequenceControlFlag::ExpeditedQoS;
+        primary_header.protocol_control_command_flag =
+            ProtocolControlCommandFlag::TfdfContainsProtocolInfo;
+        primary_header.ocf_flag = true;
+        primary_header.set_vc_frame_count(4, 0x12345678).unwrap();
+        assert_eq!(primary_header.write_to_be_bytes(&mut buf).unwrap(), 11);
+        let parsed_header = PrimaryHeader::from_bytes(&buf).unwrap();
+        assert_eq!(parsed_header, primary_header);
+    }
+
+    #[test]
+    fn test_invalid_vcid() {
+        let error = PrimaryHeader::new(
+            0b10100101_11000011,
+            SourceOrDestField::Dest,
+            0b1101011,
+            0b1010,
+            0x2345,
+        );
+        assert!(error.is_err());
+        let error = error.unwrap_err();
+        matches!(error, UslpError::InvalidVcid(0b1101011));
+    }
+
+    #[test]
+    fn test_invalid_mapid() {
+        let error = PrimaryHeader::new(
+            0b10100101_11000011,
+            SourceOrDestField::Dest,
+            0b110101,
+            0b10101,
+            0x2345,
+        );
+        assert!(error.is_err());
+        let error = error.unwrap_err();
+        matches!(error, UslpError::InvalidMapId(0b10101));
+    }
+
+    #[test]
+    fn test_invalid_vc_count() {
+        let mut primary_header = PrimaryHeader::new(
+            0b10100101_11000011,
+            SourceOrDestField::Dest,
+            0b110101,
+            0b1010,
+            0x2345,
+        )
+        .unwrap();
+        matches!(
+            primary_header.set_vc_frame_count(0, 1).unwrap_err(),
+            InvalidValueForLen { value: 1, len: 0 }
+        );
+        matches!(
+            primary_header.set_vc_frame_count(1, 256).unwrap_err(),
+            InvalidValueForLen { value: 256, len: 1 }
+        );
+    }
+
+    #[test]
+    fn test_frame_parser() {
+        let mut buf: [u8; 32] = [0; 32];
+        // Build a variable frame manually.
+        let mut primary_header =
+            PrimaryHeader::new(0x01, SourceOrDestField::Dest, 0b110101, 0b1010, 0).unwrap();
+        let header_len = primary_header.len_header();
+        buf[header_len] = ((ConstructionRule::NoSegmentation as u8) << 5)
+            | (UslpProtocolId::UserDefinedOctetStream as u8) & 0b11111;
+        buf[header_len + 1] = 0x42;
+        // 1 byte TFDH, 1 byte data, 2 bytes CRC.
+        primary_header.set_frame_len(header_len + 4);
+        primary_header.write_to_be_bytes(&mut buf).unwrap();
+        // Calculate and write CRC16.
+        let mut digest = CRC_CCITT_FALSE.digest();
+        digest.update(&buf[0..header_len + 2]);
+        buf[header_len + 2..header_len + 4].copy_from_slice(&digest.finalize().to_be_bytes());
+        println!("Buffer: {:x?}", buf);
+        // Now parse the frame.
+        let frame = TransferFrameReader::from_bytes(&buf, true).unwrap();
+        assert_eq!(frame.data().len(), 1);
+        assert_eq!(frame.data()[0], 0x42);
+        assert_eq!(
+            frame.data_field_header().uslp_protocol_id,
+            UslpProtocolId::UserDefinedOctetStream
+        );
+        assert_eq!(
+            frame.data_field_header().construction_rule,
+            ConstructionRule::NoSegmentation
+        );
+        assert!(frame.data_field_header().fhp_or_lvo().is_none());
+        assert_eq!(frame.len_frame(), 11);
+    }
+}