diff --git a/Cargo.toml b/Cargo.toml
index 2d03fa7..86162b2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -22,6 +22,7 @@ derive-new = ">=0.6, <=0.7"
 hashbrown = { version = ">=0.14, <=0.15", optional = true }
 spacepackets = { git = "https://egit.irs.uni-stuttgart.de/rust/spacepackets.git", version = "0.16", default-features = false }
 thiserror = { version = "2", default-features = false }
+heapless = "0.9"
 serde = { version = "1", optional = true }
 defmt  = { version = "1", optional = true }
 
diff --git a/src/dest.rs b/src/dest.rs
index b36f648..587c8dd 100644
--- a/src/dest.rs
+++ b/src/dest.rs
@@ -30,7 +30,10 @@
 //!  4. A Finished PDU has been sent back to the remote side.
 //!  5. A Finished PDU ACK was received.
 use crate::{DummyPduProvider, GenericSendError, PduProvider, user::TransactionFinishedParams};
-use core::str::{Utf8Error, from_utf8, from_utf8_unchecked};
+use core::{
+    cell::Cell,
+    str::{Utf8Error, from_utf8, from_utf8_unchecked},
+};
 
 use super::{
     CountdownProvider, EntityType, LocalEntityConfig, PacketTarget, PduSendProvider,
@@ -94,11 +97,11 @@ struct TransferState<Countdown: CountdownProvider> {
     // TODO: Can we delete this for good?
     // file_size_eof: u64,
     metadata_only: bool,
-    condition_code: ConditionCode,
-    delivery_code: DeliveryCode,
-    fault_location_finished: Option<EntityIdTlv>,
+    condition_code: Cell<ConditionCode>,
+    delivery_code: Cell<DeliveryCode>,
+    fault_location_finished: Cell<Option<EntityIdTlv>>,
     file_status: FileStatus,
-    completion_disposition: CompletionDisposition,
+    completion_disposition: Cell<CompletionDisposition>,
     checksum: u32,
     current_check_count: u32,
     current_check_timer: Option<Countdown>,
@@ -112,11 +115,11 @@ impl<CheckTimer: CountdownProvider> Default for TransferState<CheckTimer> {
             progress: Default::default(),
             // file_size_eof: Default::default(),
             metadata_only: false,
-            condition_code: ConditionCode::NoError,
-            delivery_code: DeliveryCode::Incomplete,
-            fault_location_finished: None,
+            condition_code: Cell::new(ConditionCode::NoError),
+            delivery_code: Cell::new(DeliveryCode::Incomplete),
+            fault_location_finished: Cell::new(None),
             file_status: FileStatus::Unreported,
-            completion_disposition: CompletionDisposition::Completed,
+            completion_disposition: Cell::new(CompletionDisposition::Completed),
             checksum: 0,
             current_check_count: 0,
             current_check_timer: None,
@@ -182,8 +185,8 @@ impl<CheckTimer: CountdownProvider> Default for TransactionParams<CheckTimer> {
 
 impl<CheckTimer: CountdownProvider> TransactionParams<CheckTimer> {
     fn reset(&mut self) {
-        self.tstate.condition_code = ConditionCode::NoError;
-        self.tstate.delivery_code = DeliveryCode::Incomplete;
+        self.tstate.condition_code.set(ConditionCode::NoError);
+        self.tstate.delivery_code.set(DeliveryCode::Incomplete);
         self.tstate.file_status = FileStatus::Unreported;
     }
 }
@@ -263,7 +266,7 @@ pub struct DestinationHandler<
     CheckTimerProvider: CountdownProvider,
 > {
     local_cfg: LocalEntityConfig<UserFaultHook>,
-    step: TransactionStep,
+    step: core::cell::Cell<TransactionStep>,
     state: State,
     tparams: TransactionParams<CheckTimerProvider>,
     packet_buf: alloc::vec::Vec<u8>,
@@ -321,7 +324,7 @@ impl<
     ) -> Self {
         Self {
             local_cfg,
-            step: TransactionStep::Idle,
+            step: Cell::new(TransactionStep::Idle),
             state: State::Idle,
             tparams: Default::default(),
             packet_buf: alloc::vec![0; max_packet_len],
@@ -387,7 +390,7 @@ impl<
                     EntityIdTlv::new(self.local_cfg.id),
                 );
 
-                self.step = TransactionStep::TransferCompletion;
+                self.set_step(TransactionStep::TransferCompletion);
                 return true;
             }
         }
@@ -511,7 +514,7 @@ impl<
             }
         }
         self.state = State::Busy;
-        self.step = TransactionStep::TransactionStart;
+        self.set_step(TransactionStep::TransactionStart);
         Ok(())
     }
 
@@ -520,9 +523,10 @@ impl<
         user: &mut impl CfdpUser,
         raw_packet: &[u8],
     ) -> Result<(), DestError> {
+        let step = self.step.get();
         if self.state == State::Idle
-            || (self.step != TransactionStep::ReceivingFileDataPdus
-                && self.step != TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling)
+            || (step != TransactionStep::ReceivingFileDataPdus
+                && step != TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling)
         {
             return Err(DestError::WrongStateForFileData);
         }
@@ -547,7 +551,11 @@ impl<
             fd_pdu.offset(),
             fd_pdu.file_data(),
         ) {
-            self.declare_fault(ConditionCode::FilestoreRejection);
+            if self.declare_fault(ConditionCode::FilestoreRejection)
+                == FaultHandlerCode::AbandonTransaction
+            {
+                self.abandon_transaction();
+            }
             return Err(e.into());
         }
         self.tstate_mut().progress += fd_pdu.file_data().len() as u64;
@@ -559,7 +567,7 @@ impl<
         cfdp_user: &mut impl CfdpUser,
         raw_packet: &[u8],
     ) -> Result<(), DestError> {
-        if self.state == State::Idle || self.step != TransactionStep::ReceivingFileDataPdus {
+        if self.state == State::Idle || self.step() != TransactionStep::ReceivingFileDataPdus {
             return Err(DestError::WrongStateForEof);
         }
         let eof_pdu = EofPdu::from_bytes(raw_packet)?;
@@ -578,9 +586,15 @@ impl<
             );
             self.tparams.tstate.progress = eof_pdu.file_size();
             if eof_pdu.file_size() > 0 {
-                self.tparams.tstate.delivery_code = DeliveryCode::Incomplete;
+                self.tparams
+                    .tstate
+                    .delivery_code
+                    .set(DeliveryCode::Incomplete);
             } else {
-                self.tparams.tstate.delivery_code = DeliveryCode::Complete;
+                self.tparams
+                    .tstate
+                    .delivery_code
+                    .set(DeliveryCode::Complete);
             }
             // TODO: The cancel EOF also supplies a checksum and a progress number. We could cross
             // check that checksum, but how would we deal with a checksum failure? The standard
@@ -595,27 +609,43 @@ impl<
     }
 
     fn trigger_notice_of_completion_cancelled(
-        &mut self,
+        &self,
         cond_code: ConditionCode,
         fault_location: EntityIdTlv,
     ) {
-        self.tparams.tstate.completion_disposition = CompletionDisposition::Cancelled;
-        self.tparams.tstate.condition_code = cond_code;
-        self.tparams.tstate.fault_location_finished = Some(fault_location);
+        self.tparams
+            .tstate
+            .completion_disposition
+            .set(CompletionDisposition::Cancelled);
+        self.tparams.tstate.condition_code.set(cond_code);
+        self.tparams
+            .tstate
+            .fault_location_finished
+            .set(Some(fault_location));
         // For anything larger than 0, we'd have to do a checksum check to verify whether
         // the delivery is really complete, and we need the EOF checksum for that..
         if self.tparams.tstate.progress == 0 {
-            self.tparams.tstate.delivery_code = DeliveryCode::Complete;
+            self.tparams
+                .tstate
+                .delivery_code
+                .set(DeliveryCode::Complete);
         }
     }
 
     /// Returns whether the transfer can be completed regularly.
     fn handle_no_error_eof_pdu(&mut self, eof_pdu: &EofPdu) -> Result<bool, DestError> {
         // CFDP 4.6.1.2.9: Declare file size error if progress exceeds file size
-        if self.tparams.tstate.progress > eof_pdu.file_size()
-            && self.declare_fault(ConditionCode::FileSizeError) != FaultHandlerCode::IgnoreError
-        {
-            return Ok(false);
+        if self.tparams.tstate.progress > eof_pdu.file_size() {
+            match self.declare_fault(ConditionCode::FileSizeError) {
+                FaultHandlerCode::IgnoreError => (),
+                FaultHandlerCode::AbandonTransaction => {
+                    self.abandon_transaction();
+                    return Ok(false);
+                }
+                FaultHandlerCode::NoticeOfCancellation | FaultHandlerCode::NoticeOfSuspension => {
+                    return Ok(false);
+                }
+            }
         } else if (self.tparams.tstate.progress < eof_pdu.file_size())
             && self.tparams.transmission_mode() == TransmissionMode::Acknowledged
         {
@@ -645,10 +675,10 @@ impl<
 
     fn file_transfer_complete_transition(&mut self) {
         if self.tparams.transmission_mode() == TransmissionMode::Unacknowledged {
-            self.step = TransactionStep::TransferCompletion;
+            self.set_step(TransactionStep::TransferCompletion);
         } else {
             // TODO: Prepare ACK PDU somehow.
-            self.step = TransactionStep::SendingAckPdu;
+            self.set_step(TransactionStep::SendingAckPdu);
         }
     }
 
@@ -675,33 +705,54 @@ impl<
                 Ok(checksum_success) => {
                     file_delivery_complete = checksum_success;
                     if !checksum_success {
-                        self.tparams.tstate.delivery_code = DeliveryCode::Incomplete;
-                        self.tparams.tstate.condition_code = ConditionCode::FileChecksumFailure;
+                        self.tparams
+                            .tstate
+                            .delivery_code
+                            .set(DeliveryCode::Incomplete);
+                        self.tparams
+                            .tstate
+                            .condition_code
+                            .set(ConditionCode::FileChecksumFailure);
                     }
                 }
                 Err(e) => match e {
                     FilestoreError::ChecksumTypeNotImplemented(_) => {
-                        self.declare_fault(ConditionCode::UnsupportedChecksumType);
+                        if self.declare_fault(ConditionCode::UnsupportedChecksumType)
+                            == FaultHandlerCode::AbandonTransaction
+                        {
+                            self.abandon_transaction();
+                        }
                         // For this case, the applicable algorithm shall be the the null checksum,
                         // which is always succesful.
                         file_delivery_complete = true;
                     }
                     _ => {
-                        self.declare_fault(ConditionCode::FilestoreRejection);
+                        if self.declare_fault(ConditionCode::FilestoreRejection)
+                            == FaultHandlerCode::AbandonTransaction
+                        {
+                            self.abandon_transaction();
+                        }
+
                         // Treat this equivalent to a failed checksum procedure.
                     }
                 },
             };
         }
         if file_delivery_complete {
-            self.tparams.tstate.delivery_code = DeliveryCode::Complete;
-            self.tparams.tstate.condition_code = ConditionCode::NoError;
+            self.tparams
+                .tstate
+                .delivery_code
+                .set(DeliveryCode::Complete);
+            self.tparams
+                .tstate
+                .condition_code
+                .set(ConditionCode::NoError);
         }
         file_delivery_complete
     }
 
     fn start_check_limit_handling(&mut self) {
-        self.step = TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling;
+        self.set_step(TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling);
         self.tparams.tstate.current_check_timer = Some(self.check_timer_creator.create_countdown(
             TimerContext::CheckLimit {
                 local_id: self.local_cfg.id,
@@ -725,7 +776,11 @@ impl<
             if self.tparams.tstate.current_check_count + 1
                 >= self.tparams.remote_cfg.unwrap().check_limit
             {
-                self.declare_fault(ConditionCode::CheckLimitReached);
+                if self.declare_fault(ConditionCode::CheckLimitReached)
+                    == FaultHandlerCode::AbandonTransaction
+                {
+                    self.abandon_transaction();
+                }
             } else {
                 self.tparams.tstate.current_check_count += 1;
                 self.tparams
@@ -744,7 +799,7 @@ impl<
 
     fn fsm_busy(&mut self, cfdp_user: &mut impl CfdpUser) -> Result<u32, DestError> {
         let mut sent_packets = 0;
-        if self.step == TransactionStep::TransactionStart {
+        if self.step() == TransactionStep::TransactionStart {
             let result = self.transaction_start(cfdp_user);
             if let Err(e) = result {
                 // If we can not even start the transaction properly, reset the handler.
@@ -754,28 +809,30 @@ impl<
                 return Err(e);
             }
         }
-        if self.step == TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling {
+        if self.step() == TransactionStep::ReceivingFileDataPdusWithCheckLimitHandling {
             self.check_limit_handling();
         }
-        if self.step == TransactionStep::TransferCompletion {
+        if self.step() == TransactionStep::TransferCompletion {
             sent_packets += self.transfer_completion(cfdp_user)?;
         }
-        if self.step == TransactionStep::SendingAckPdu {
+        if self.step() == TransactionStep::SendingAckPdu {
             return Err(DestError::NotImplemented);
         }
-        if self.step == TransactionStep::SendingFinishedPdu {
+        if self.step() == TransactionStep::SendingFinishedPdu {
             self.reset();
         }
         Ok(sent_packets)
     }
 
     /// Get the step, which denotes the exact step of a pending CFDP transaction when applicable.
+    #[inline]
     pub fn step(&self) -> TransactionStep {
-        self.step
+        self.step.get()
     }
 
     /// Get the step, which denotes whether the CFDP handler is active, and which CFDP class
     /// is used if it is active.
+    #[inline]
     pub fn state(&self) -> State {
         self.state
     }
@@ -846,7 +903,8 @@ impl<
             self.vfs.create_file(dest_path_str)?;
         }
         self.tparams.tstate.file_status = FileStatus::Retained;
-        self.step = TransactionStep::ReceivingFileDataPdus;
+        drop(msgs_to_user);
+        self.set_step(TransactionStep::ReceivingFileDataPdus);
         Ok(())
     }
 
@@ -858,12 +916,12 @@ impl<
         {
             sent_packets += self.send_finished_pdu()?;
         }
-        self.step = TransactionStep::SendingFinishedPdu;
+        self.set_step(TransactionStep::SendingFinishedPdu);
         Ok(sent_packets)
     }
 
     fn notice_of_completion(&mut self, cfdp_user: &mut impl CfdpUser) -> Result<(), DestError> {
-        if self.tstate().completion_disposition == CompletionDisposition::Completed {
+        if self.tstate().completion_disposition.get() == CompletionDisposition::Completed {
             // TODO: Execute any filestore requests
         } else if self
             .tparams
@@ -871,7 +929,7 @@ impl<
             .as_ref()
             .unwrap()
             .disposition_on_cancellation
-            && self.tstate().delivery_code == DeliveryCode::Incomplete
+            && self.tstate().delivery_code.get() == DeliveryCode::Incomplete
         {
             // Safety: We already verified that the path is valid during the transaction start.
             let dest_path = unsafe {
@@ -888,15 +946,17 @@ impl<
         let tstate = self.tstate();
         let transaction_finished_params = TransactionFinishedParams {
             id: tstate.transaction_id.unwrap(),
-            condition_code: tstate.condition_code,
-            delivery_code: tstate.delivery_code,
+            condition_code: tstate.condition_code.get(),
+            delivery_code: tstate.delivery_code.get(),
             file_status: tstate.file_status,
         };
         cfdp_user.transaction_finished_indication(&transaction_finished_params);
         Ok(())
     }
 
-    fn declare_fault(&mut self, condition_code: ConditionCode) -> FaultHandlerCode {
+    // When the fault handler code [FaultHandlerCode::AbandonTransaction] is returned, the caller
+    // must call [Self::abandon_transaction] as soon as it is possible.
+    fn declare_fault(&self, condition_code: ConditionCode) -> FaultHandlerCode {
         // Cache those, because they might be reset when abandoning the transaction.
         let transaction_id = self.tstate().transaction_id.unwrap();
         let progress = self.tstate().progress;
@@ -910,25 +970,24 @@ impl<
             }
             FaultHandlerCode::NoticeOfSuspension => self.notice_of_suspension(),
             FaultHandlerCode::IgnoreError => (),
-            FaultHandlerCode::AbandonTransaction => self.abandon_transaction(),
+            FaultHandlerCode::AbandonTransaction => (),
         }
         self.local_cfg
             .fault_handler
             .report_fault(transaction_id, condition_code, progress)
     }
 
-    fn notice_of_cancellation(
-        &mut self,
-        condition_code: ConditionCode,
-        fault_location: EntityIdTlv,
-    ) {
-        self.step = TransactionStep::TransferCompletion;
-        self.tstate_mut().condition_code = condition_code;
-        self.tstate_mut().fault_location_finished = Some(fault_location);
-        self.tstate_mut().completion_disposition = CompletionDisposition::Cancelled;
+    fn notice_of_cancellation(&self, condition_code: ConditionCode, fault_location: EntityIdTlv) {
+        self.set_step_internal(TransactionStep::TransferCompletion);
+        let tstate = self.tstate();
+        tstate.condition_code.set(condition_code);
+        tstate
+            .completion_disposition
+            .set(CompletionDisposition::Cancelled);
+        tstate.fault_location_finished.set(Some(fault_location));
     }
 
-    fn notice_of_suspension(&mut self) {
+    fn notice_of_suspension(&self) {
         // TODO: Implement suspension handling.
     }
 
@@ -936,12 +995,22 @@ impl<
         self.reset();
     }
 
+    #[inline]
+    fn set_step_internal(&self, step: TransactionStep) {
+        self.step.set(step);
+    }
+
+    #[inline]
+    fn set_step(&mut self, step: TransactionStep) {
+        self.set_step_internal(step);
+    }
+
     /// This function is public to allow completely resetting the handler, but it is explicitely
     /// discouraged to do this. CFDP has mechanism to detect issues and errors on itself.
     /// Resetting the handler might interfere with these mechanisms and lead to unexpected
     /// behaviour.
     pub fn reset(&mut self) {
-        self.step = TransactionStep::Idle;
+        self.set_step(TransactionStep::Idle);
         self.state = State::Idle;
         // self.packets_to_send_ctx.packet_available = false;
         self.tparams.reset();
@@ -951,18 +1020,22 @@ impl<
         let tstate = self.tstate();
 
         let pdu_header = PduHeader::new_no_file_data(self.tparams.pdu_conf, 0);
-        let finished_pdu = if tstate.condition_code == ConditionCode::NoError
-            || tstate.condition_code == ConditionCode::UnsupportedChecksumType
+        let finished_pdu = if tstate.condition_code.get() == ConditionCode::NoError
+            || tstate.condition_code.get() == ConditionCode::UnsupportedChecksumType
         {
-            FinishedPduCreator::new_default(pdu_header, tstate.delivery_code, tstate.file_status)
+            FinishedPduCreator::new_default(
+                pdu_header,
+                tstate.delivery_code.get(),
+                tstate.file_status,
+            )
         } else {
             FinishedPduCreator::new_generic(
                 pdu_header,
-                tstate.condition_code,
-                tstate.delivery_code,
+                tstate.condition_code.get(),
+                tstate.delivery_code.get(),
                 tstate.file_status,
                 &[],
-                tstate.fault_location_finished,
+                tstate.fault_location_finished.get(),
             )
         };
         finished_pdu.write_to_bytes(&mut self.packet_buf)?;
@@ -974,14 +1047,17 @@ impl<
         Ok(1)
     }
 
+    #[inline]
     pub fn local_cfg(&self) -> &LocalEntityConfig<UserFaultHook> {
         &self.local_cfg
     }
 
+    #[inline]
     fn tstate(&self) -> &TransferState<Countdown> {
         &self.tparams.tstate
     }
 
+    #[inline]
     fn tstate_mut(&mut self) -> &mut TransferState<Countdown> {
         &mut self.tparams.tstate
     }
diff --git a/src/lib.rs b/src/lib.rs
index adef6df..d9ef1cf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -93,6 +93,7 @@ extern crate std;
 #[cfg(feature = "alloc")]
 pub mod dest;
 pub mod filestore;
+pub mod lost_segments;
 pub mod request;
 #[cfg(feature = "alloc")]
 pub mod source;
diff --git a/src/lost_segments.rs b/src/lost_segments.rs
new file mode 100644
index 0000000..bd2af38
--- /dev/null
+++ b/src/lost_segments.rs
@@ -0,0 +1,753 @@
+//! # Lost Segment Store Module
+//!
+//! The core abstraction provided by this module in the [LostSegmentStore].
+//!
+//! The two concrete implementations provided are:
+//!
+//! * [LostSegmentsMap]: A hash set based implementation which can grow dynamically andcan
+//!   optionally be bounded. Suitable for systems where dynamic allocation is allowed.
+//! * [LostSegmentsList]: A fixed-size list based implementation where the size
+//!   of the lost segment list is statically known at compile-time. Suitable for resource
+//!   constrained devices where dyanamic allocation is not allowed or possible.
+#[derive(Debug, PartialEq, Eq, thiserror::Error)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[non_exhaustive]
+pub enum LostSegmentError {
+    #[error("store is full")]
+    StoreFull,
+    #[error("segment is empty")]
+    EmptySegment,
+    #[error("segment start is larger than segment end")]
+    StartLargerThanEnd((u64, u64)),
+    #[error("large file segments are not supported")]
+    LargeFileSegmentNotSupported,
+}
+
+/// Generic trait to model a lost segment store.
+///
+/// The destination handler can use this store to keep track of lost segments and re-requesting
+/// them. This abstraction allow using different data structures as a backend.
+pub trait LostSegmentStore {
+    // Iteration
+    type Iter<'a>: Iterator<Item = (u64, u64)> + 'a
+    where
+        Self: 'a;
+
+    /// Iterate over all lost segments stored.
+    fn iter(&self) -> Self::Iter<'_>;
+
+    fn len(&self) -> usize;
+
+    /// Implementations may explicitely omit support for large file segments to save memory if
+    /// large file sizes are not used.
+    fn supports_large_file_size(&self) -> bool;
+    fn capacity(&self) -> Option<usize>;
+    fn reset(&mut self);
+
+    fn add_lost_segment(&mut self, lost_seg: (u64, u64)) -> Result<(), LostSegmentError>;
+    fn remove_lost_segment(
+        &mut self,
+        segment_to_remove: (u64, u64),
+    ) -> Result<bool, LostSegmentError>;
+
+    /// The lost segment store may additionally have the capability to coalesce overlapping or
+    /// adjacent segments.
+    fn coalesce_lost_segments(&mut self) -> Result<(), LostSegmentError>;
+
+    #[inline]
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+}
+
+/// Implementation based on a [hashbrown::HashSet] which can grow dynamically.
+///
+/// Optionally, a maximum capacity can be specified at creation time. This container allocates at
+/// run-time!
+#[cfg(feature = "alloc")]
+#[derive(Debug, Default)]
+pub struct LostSegmentsMap {
+    map: hashbrown::HashSet<(u64, u64)>,
+    opt_capacity: Option<usize>,
+}
+
+#[cfg(feature = "alloc")]
+impl LostSegmentsMap {
+    pub fn new(opt_capacity: Option<usize>) -> Self {
+        Self {
+            map: hashbrown::HashSet::new(),
+            opt_capacity,
+        }
+    }
+}
+
+#[cfg(feature = "alloc")]
+impl LostSegmentStore for LostSegmentsMap {
+    type Iter<'a>
+        = core::iter::Cloned<hashbrown::hash_set::Iter<'a, (u64, u64)>>
+    where
+        Self: 'a;
+
+    fn iter(&self) -> Self::Iter<'_> {
+        self.map.iter().cloned()
+    }
+
+    #[inline]
+    fn len(&self) -> usize {
+        self.map.len()
+    }
+
+    #[inline]
+    fn supports_large_file_size(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn capacity(&self) -> Option<usize> {
+        self.opt_capacity
+    }
+
+    #[inline]
+    fn reset(&mut self) {
+        self.map.clear();
+    }
+
+    #[inline]
+    fn add_lost_segment(&mut self, lost_seg: (u64, u64)) -> Result<(), LostSegmentError> {
+        if lost_seg.1 == lost_seg.0 {
+            return Err(LostSegmentError::EmptySegment);
+        }
+        if lost_seg.0 > lost_seg.1 {
+            return Err(LostSegmentError::StartLargerThanEnd(lost_seg));
+        }
+        if let Some(capacity) = self.opt_capacity
+            && self.map.len() == capacity
+        {
+            return Err(LostSegmentError::StoreFull);
+        }
+        self.map.insert(lost_seg);
+        Ok(())
+    }
+
+    fn coalesce_lost_segments(&mut self) -> Result<(), LostSegmentError> {
+        if self.map.len() <= 1 {
+            return Ok(());
+        }
+
+        // Move out segments, discard empty/invalid ones
+        let mut segs: alloc::vec::Vec<(u64, u64)> = self
+            .map
+            .drain()
+            .filter(|(s, e)| e > s) // keep only non-empty [s, e)
+            .collect();
+
+        // Sort by start, then by end
+        segs.sort_unstable_by(|a, b| a.0.cmp(&b.0).then(a.1.cmp(&b.1)));
+
+        // Merge
+        let mut merged: alloc::vec::Vec<(u64, u64)> = alloc::vec::Vec::with_capacity(segs.len());
+        for (s, e) in segs {
+            if let Some(last) = merged.last_mut() {
+                // For half-open ranges, this merges overlapping or adjacent:
+                // [last.0, last.1) with [s, e) if s <= last.1
+                if s <= last.1 {
+                    if e > last.1 {
+                        last.1 = e;
+                    }
+                    continue;
+                }
+            }
+            merged.push((s, e));
+        }
+
+        // Rebuild the set
+        self.map.extend(merged);
+        Ok(())
+    }
+
+    #[inline]
+    fn remove_lost_segment(
+        &mut self,
+        segment_to_remove: (u64, u64),
+    ) -> Result<bool, LostSegmentError> {
+        if segment_to_remove.1 == segment_to_remove.0 {
+            return Err(LostSegmentError::EmptySegment);
+        }
+        if segment_to_remove.0 > segment_to_remove.1 {
+            return Err(LostSegmentError::StartLargerThanEnd(segment_to_remove));
+        }
+        Ok(self.map.remove(&segment_to_remove))
+    }
+}
+
+/// Implementation based on a [heapless::Vec] with a statically known container size.
+#[derive(Default, Debug)]
+pub struct LostSegmentsList<const N: usize, T> {
+    list: heapless::vec::Vec<(T, T), N>,
+}
+
+/// Type definition for segment list which only supports normal file sizes. This can be used
+/// to save memory required for the lost segment list.
+pub type LostSegmentsListSmall<const N: usize> = LostSegmentsList<N, u32>;
+
+impl<const N: usize, T> LostSegmentsList<N, T> {
+    pub fn new() -> Self {
+        Self {
+            list: heapless::Vec::new(),
+        }
+    }
+
+    #[inline]
+    fn num_lost_segments(&self) -> usize {
+        self.list.len()
+    }
+
+    #[inline]
+    fn capacity(&self) -> Option<usize> {
+        Some(N)
+    }
+
+    #[inline]
+    fn reset(&mut self) {
+        self.list.clear();
+    }
+}
+
+impl<const N: usize, T: Copy + Clone + Ord> LostSegmentsList<N, T> {
+    fn coalesce_lost_segments(&mut self) -> Result<(), LostSegmentError> {
+        // Remove empty/invalid ranges
+        self.list.retain(|&(s, e)| e > s);
+        if self.list.len() <= 1 {
+            return Ok(());
+        }
+
+        // Sort by start, then end (no extra allocs)
+        self.list
+            .as_mut_slice()
+            .sort_unstable_by(|a, b| a.0.cmp(&b.0).then(a.1.cmp(&b.1)));
+
+        // In-place merge; merges overlapping or adjacent [s, e) where next.s <= prev.e
+        let mut w = 0usize;
+        for i in 0..self.list.len() {
+            if w == 0 {
+                self.list[w] = self.list[i];
+                w = 1;
+                continue;
+            }
+
+            let (prev_s, mut prev_e) = self.list[w - 1];
+            let (s, e) = self.list[i];
+
+            if s <= prev_e {
+                // Extend previous
+                if e > prev_e {
+                    prev_e = e;
+                    self.list[w - 1] = (prev_s, prev_e);
+                }
+            } else {
+                // Start new merged interval
+                self.list[w] = (s, e);
+                w += 1;
+            }
+        }
+
+        // Truncate to merged length
+        self.list.truncate(w);
+        Ok(())
+    }
+}
+
+impl<const N: usize> LostSegmentStore for LostSegmentsList<N, u64> {
+    type Iter<'a>
+        = core::iter::Cloned<core::slice::Iter<'a, (u64, u64)>>
+    where
+        Self: 'a;
+
+    fn iter(&self) -> Self::Iter<'_> {
+        self.list.iter().cloned()
+    }
+
+    fn add_lost_segment(&mut self, lost_seg: (u64, u64)) -> Result<(), LostSegmentError> {
+        if lost_seg.1 == lost_seg.0 {
+            return Err(LostSegmentError::EmptySegment);
+        }
+        if lost_seg.0 > lost_seg.1 {
+            return Err(LostSegmentError::StartLargerThanEnd(lost_seg));
+        }
+        if self.list.is_full() {
+            return Err(LostSegmentError::StoreFull);
+        }
+        self.list.push(lost_seg).ok();
+        Ok(())
+    }
+
+    fn remove_lost_segment(
+        &mut self,
+        segment_to_remove: (u64, u64),
+    ) -> Result<bool, LostSegmentError> {
+        if segment_to_remove.1 == segment_to_remove.0 {
+            return Err(LostSegmentError::EmptySegment);
+        }
+        if segment_to_remove.0 > segment_to_remove.1 {
+            return Err(LostSegmentError::StartLargerThanEnd(segment_to_remove));
+        }
+        let current_len = self.list.len();
+        self.list.retain(|val| val != &segment_to_remove);
+        Ok(self.list.len() != current_len)
+    }
+
+    fn coalesce_lost_segments(&mut self) -> Result<(), LostSegmentError> {
+        self.coalesce_lost_segments()
+    }
+
+    fn len(&self) -> usize {
+        self.num_lost_segments()
+    }
+
+    fn supports_large_file_size(&self) -> bool {
+        true
+    }
+
+    fn capacity(&self) -> Option<usize> {
+        self.capacity()
+    }
+
+    fn reset(&mut self) {
+        self.reset();
+    }
+}
+
+impl<const N: usize> LostSegmentStore for LostSegmentsList<N, u32> {
+    type Iter<'a>
+        = core::iter::Map<core::slice::Iter<'a, (u32, u32)>, fn(&(u32, u32)) -> (u64, u64)>
+    where
+        Self: 'a;
+
+    fn iter(&self) -> Self::Iter<'_> {
+        self.list.iter().map(|pair| (pair.0 as u64, pair.1 as u64))
+    }
+
+    fn add_lost_segment(&mut self, lost_seg: (u64, u64)) -> Result<(), LostSegmentError> {
+        if lost_seg.1 == lost_seg.0 {
+            return Err(LostSegmentError::EmptySegment);
+        }
+        if lost_seg.0 > lost_seg.1 {
+            return Err(LostSegmentError::StartLargerThanEnd(lost_seg));
+        }
+        if lost_seg.1 > u32::MAX as u64 || lost_seg.0 > u32::MAX as u64 {
+            return Err(LostSegmentError::LargeFileSegmentNotSupported);
+        }
+        if self.list.is_full() {
+            return Err(LostSegmentError::StoreFull);
+        }
+        self.list.push((lost_seg.0 as u32, lost_seg.1 as u32)).ok();
+        Ok(())
+    }
+
+    fn remove_lost_segment(
+        &mut self,
+        segment_to_remove: (u64, u64),
+    ) -> Result<bool, LostSegmentError> {
+        if segment_to_remove.1 == segment_to_remove.0 {
+            return Err(LostSegmentError::EmptySegment);
+        }
+        if segment_to_remove.0 > segment_to_remove.1 {
+            return Err(LostSegmentError::StartLargerThanEnd(segment_to_remove));
+        }
+        if segment_to_remove.1 > u32::MAX as u64 || segment_to_remove.0 > u32::MAX as u64 {
+            return Err(LostSegmentError::LargeFileSegmentNotSupported);
+        }
+        let current_len = self.list.len();
+        self.list
+            .retain(|val| (val.0 as u64, val.1 as u64) != segment_to_remove);
+        Ok(self.list.len() != current_len)
+    }
+
+    fn coalesce_lost_segments(&mut self) -> Result<(), LostSegmentError> {
+        self.coalesce_lost_segments()
+    }
+
+    #[inline]
+    fn len(&self) -> usize {
+        self.num_lost_segments()
+    }
+
+    #[inline]
+    fn supports_large_file_size(&self) -> bool {
+        false
+    }
+
+    #[inline]
+    fn capacity(&self) -> Option<usize> {
+        self.capacity()
+    }
+
+    #[inline]
+    fn reset(&mut self) {
+        self.reset();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::vec::Vec;
+
+    use super::*;
+
+    fn generic_basic_state_test(
+        store: &impl LostSegmentStore,
+        supports_large_file_size: bool,
+        capacity: Option<usize>,
+    ) {
+        assert_eq!(store.supports_large_file_size(), supports_large_file_size);
+        assert_eq!(store.len(), 0);
+        assert!(store.is_empty());
+        assert_eq!(store.capacity(), capacity);
+        assert_eq!(store.iter().count(), 0);
+    }
+
+    fn generic_error_tests(store: &mut impl LostSegmentStore) {
+        matches!(
+            store.add_lost_segment((0, 0)).unwrap_err(),
+            LostSegmentError::EmptySegment
+        );
+        matches!(
+            store.add_lost_segment((10, 0)).unwrap_err(),
+            LostSegmentError::StartLargerThanEnd((10, 0))
+        );
+        matches!(
+            store.remove_lost_segment((0, 0)).unwrap_err(),
+            LostSegmentError::EmptySegment
+        );
+        matches!(
+            store.remove_lost_segment((10, 0)).unwrap_err(),
+            LostSegmentError::StartLargerThanEnd((10, 0))
+        );
+    }
+
+    fn generic_add_segments_test(store: &mut impl LostSegmentStore) {
+        store.add_lost_segment((0, 20)).unwrap();
+        assert_eq!(store.len(), 1);
+        for segment in store.iter() {
+            assert_eq!(segment, (0, 20));
+        }
+        store.add_lost_segment((20, 40)).unwrap();
+        let mut segments: Vec<(u64, u64)> = store.iter().collect();
+        segments.sort_unstable();
+        assert_eq!(segments.len(), 2);
+        assert_eq!(segments[0], (0, 20));
+        assert_eq!(segments[1], (20, 40));
+    }
+
+    fn generic_reset_test(store: &mut impl LostSegmentStore) {
+        store.add_lost_segment((0, 20)).unwrap();
+        assert_eq!(store.len(), 1);
+        store.reset();
+        assert_eq!(store.len(), 0);
+        assert!(store.is_empty());
+        assert_eq!(store.iter().count(), 0);
+    }
+
+    fn generic_removal_test(store: &mut impl LostSegmentStore) {
+        store.add_lost_segment((0, 20)).unwrap();
+        store.add_lost_segment((20, 40)).unwrap();
+        assert_eq!(store.len(), 2);
+        assert!(store.remove_lost_segment((0, 20)).unwrap());
+        assert_eq!(store.len(), 1);
+        assert!(!store.remove_lost_segment((0, 20)).unwrap());
+        assert_eq!(store.len(), 1);
+        assert!(store.remove_lost_segment((20, 40)).unwrap());
+        assert_eq!(store.len(), 0);
+    }
+
+    fn generic_coalescing_simple_test(store: &mut impl LostSegmentStore) {
+        store.add_lost_segment((0, 20)).unwrap();
+        store.add_lost_segment((20, 40)).unwrap();
+        store.add_lost_segment((40, 60)).unwrap();
+        store.coalesce_lost_segments().unwrap();
+        for segment in store.iter() {
+            assert_eq!(segment, (0, 60));
+        }
+        assert_eq!(store.len(), 1);
+    }
+
+    fn generic_coalescing_simple_with_gaps_test(store: &mut impl LostSegmentStore) {
+        store.add_lost_segment((0, 20)).unwrap();
+        store.add_lost_segment((20, 40)).unwrap();
+        store.add_lost_segment((40, 60)).unwrap();
+
+        store.add_lost_segment((80, 100)).unwrap();
+        store.add_lost_segment((110, 120)).unwrap();
+        store.add_lost_segment((120, 130)).unwrap();
+        store.coalesce_lost_segments().unwrap();
+        let mut segments: Vec<(u64, u64)> = store.iter().collect();
+        segments.sort_unstable();
+        assert_eq!(segments.len(), 3);
+        assert_eq!(segments[0], (0, 60));
+        assert_eq!(segments[1], (80, 100));
+        assert_eq!(segments[2], (110, 130));
+    }
+
+    fn generic_coalescing_overlapping_simple_test(store: &mut impl LostSegmentStore) {
+        store.add_lost_segment((0, 20)).unwrap();
+        store.add_lost_segment((10, 30)).unwrap();
+        store.coalesce_lost_segments().unwrap();
+        for segment in store.iter() {
+            assert_eq!(segment, (0, 30));
+        }
+        assert_eq!(store.len(), 1);
+    }
+
+    fn generic_coalescing_overlapping_adjacent_test(store: &mut impl LostSegmentStore) {
+        store.add_lost_segment((0, 20)).unwrap();
+        store.add_lost_segment((10, 30)).unwrap();
+        store.add_lost_segment((20, 40)).unwrap();
+        store.coalesce_lost_segments().unwrap();
+        for segment in store.iter() {
+            assert_eq!(segment, (0, 40));
+        }
+        assert_eq!(store.len(), 1);
+    }
+
+    fn generic_useless_coalescing_test(store: &mut impl LostSegmentStore) {
+        // Is okay, does nothing.
+        assert!(store.coalesce_lost_segments().is_ok());
+        assert_eq!(store.len(), 0);
+        assert!(store.is_empty());
+        store.add_lost_segment((10, 20)).unwrap();
+        // Is okay, does nothing.
+        assert!(store.coalesce_lost_segments().is_ok());
+        for segment in store.iter() {
+            assert_eq!(segment, (10, 20));
+        }
+    }
+
+    #[test]
+    fn test_basic_map_state_map() {
+        let store = LostSegmentsMap::default();
+        generic_basic_state_test(&store, true, None);
+    }
+
+    #[test]
+    fn test_basic_errors_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_error_tests(&mut store);
+    }
+
+    #[test]
+    fn test_add_segments_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_add_segments_test(&mut store);
+    }
+
+    #[test]
+    fn test_reset_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_reset_test(&mut store);
+    }
+
+    #[test]
+    fn test_removal_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_removal_test(&mut store);
+    }
+
+    #[test]
+    fn test_cap_limit_map() {
+        let mut store = LostSegmentsMap::new(Some(4));
+        for i in 0..4 {
+            store.add_lost_segment((i * 20, (i + 1) * 20)).unwrap();
+        }
+        matches!(
+            store.add_lost_segment((80, 100)),
+            Err(LostSegmentError::StoreFull)
+        );
+    }
+
+    #[test]
+    fn test_basic_list_state_list() {
+        let store = LostSegmentsListSmall::<12>::default();
+        generic_basic_state_test(&store, false, Some(12));
+        let store = LostSegmentsListSmall::<12>::new();
+        generic_basic_state_test(&store, false, Some(12));
+    }
+    #[test]
+    fn test_basic_errors_list() {
+        let mut store = LostSegmentsListSmall::<12>::default();
+        generic_error_tests(&mut store);
+    }
+
+    #[test]
+    fn test_add_segments_list() {
+        let mut store = LostSegmentsListSmall::<12>::default();
+        generic_add_segments_test(&mut store);
+    }
+
+    #[test]
+    fn test_reset_list() {
+        let mut store = LostSegmentsListSmall::<12>::default();
+        generic_reset_test(&mut store);
+    }
+
+    #[test]
+    fn test_removal_list() {
+        let mut store = LostSegmentsListSmall::<12>::default();
+        generic_removal_test(&mut store);
+    }
+
+    #[test]
+    fn test_cap_limit_list() {
+        let mut store = LostSegmentsListSmall::<4>::default();
+        for i in 0..4 {
+            store.add_lost_segment((i * 20, (i + 1) * 20)).unwrap();
+        }
+        matches!(
+            store.add_lost_segment((80, 100)),
+            Err(LostSegmentError::StoreFull)
+        );
+    }
+
+    #[test]
+    fn test_large_file_size_unsupported() {
+        let mut store = LostSegmentsListSmall::<4>::default();
+        matches!(
+            store.add_lost_segment((0, u32::MAX as u64 + 1)),
+            Err(LostSegmentError::LargeFileSegmentNotSupported)
+        );
+    }
+
+    #[test]
+    fn test_large_file_size_unsupported_2() {
+        let mut store = LostSegmentsListSmall::<4>::default();
+        matches!(
+            store.remove_lost_segment((0, u32::MAX as u64 + 1)),
+            Err(LostSegmentError::LargeFileSegmentNotSupported)
+        );
+    }
+
+    #[test]
+    fn test_basic_list_state_list_large() {
+        let store = LostSegmentsList::<12, u64>::default();
+        generic_basic_state_test(&store, true, Some(12));
+    }
+    #[test]
+    fn test_basic_errors_list_large() {
+        let mut store = LostSegmentsList::<12, u64>::default();
+        generic_error_tests(&mut store);
+    }
+
+    #[test]
+    fn test_add_segments_list_large() {
+        let mut store = LostSegmentsList::<12, u64>::default();
+        generic_add_segments_test(&mut store);
+    }
+
+    #[test]
+    fn test_reset_list_large() {
+        let mut store = LostSegmentsList::<12, u64>::default();
+        generic_reset_test(&mut store);
+    }
+
+    #[test]
+    fn test_removal_list_large() {
+        let mut store = LostSegmentsList::<12, u64>::default();
+        generic_removal_test(&mut store);
+    }
+
+    #[test]
+    fn test_cap_limit_list_large() {
+        let mut store = LostSegmentsList::<4, u64>::default();
+        for i in 0..4 {
+            store.add_lost_segment((i * 20, (i + 1) * 20)).unwrap();
+        }
+        matches!(
+            store.add_lost_segment((80, 100)),
+            Err(LostSegmentError::StoreFull)
+        );
+    }
+
+    #[test]
+    fn test_coalescing_simple_in_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_coalescing_simple_test(&mut store);
+    }
+
+    #[test]
+    fn test_useless_coalescing_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_useless_coalescing_test(&mut store);
+    }
+
+    #[test]
+    fn test_useless_coalescing_list() {
+        let mut store = LostSegmentsListSmall::<4>::default();
+        generic_useless_coalescing_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_simple_in_list() {
+        let mut store = LostSegmentsListSmall::<4>::default();
+        generic_coalescing_simple_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_simple_in_list_large() {
+        let mut store = LostSegmentsList::<4, u64>::default();
+        generic_coalescing_simple_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_overlapping_simple_in_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_coalescing_overlapping_simple_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_overlapping_simple_in_list() {
+        let mut store = LostSegmentsListSmall::<4>::default();
+        generic_coalescing_overlapping_simple_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_overlapping_simple_in_list_large() {
+        let mut store = LostSegmentsList::<4, u64>::default();
+        generic_coalescing_overlapping_simple_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_overlapping_adjacent_in_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_coalescing_overlapping_adjacent_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_overlapping_adjacent_in_list() {
+        let mut store = LostSegmentsListSmall::<4>::default();
+        generic_coalescing_overlapping_adjacent_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_overlapping_adjacent_in_list_large() {
+        let mut store = LostSegmentsList::<4, u64>::default();
+        generic_coalescing_overlapping_adjacent_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_simple_with_gaps_in_map() {
+        let mut store = LostSegmentsMap::default();
+        generic_coalescing_simple_with_gaps_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_simple_with_gaps_in_list() {
+        let mut store = LostSegmentsListSmall::<8>::default();
+        generic_coalescing_simple_with_gaps_test(&mut store);
+    }
+
+    #[test]
+    fn test_coalescing_simple_with_gaps_in_list_large() {
+        let mut store = LostSegmentsList::<8, u64>::default();
+        generic_coalescing_simple_with_gaps_test(&mut store);
+    }
+}
diff --git a/src/source.rs b/src/source.rs
index 3eb46a7..1344971 100644
--- a/src/source.rs
+++ b/src/source.rs
@@ -27,7 +27,7 @@
 //! 4. A Finished PDU will be awaited, for example one generated using
 //!    [spacepackets::cfdp::pdu::finished::FinishedPduCreator].
 //!
-//! ### Acknowledged transfer (*not implemented yet*)
+//! ### Acknowledged transfer
 //!
 //! 4. A EOF ACK packet will be awaited, for example one generated using
 //!    [spacepackets::cfdp::pdu::ack::AckPdu].