diff --git a/src/Frag.cc b/src/Frag.cc
index b0eb0d2127..3e3300efdd 100644
--- a/src/Frag.cc
+++ b/src/Frag.cc
@@ -185,24 +185,32 @@ void FragReassembler::Overlap(const u_char* b1, const u_char* b2, uint64_t n)
 		Weird("fragment_overlap");
 	}
 
-void FragReassembler::BlockInserted(const DataBlock* /* start_block */)
+void FragReassembler::BlockInserted(DataBlockMap::const_iterator /* it */)
 	{
-	const DataBlock* b = block_list.Head();
-	// TODO: review all iteration here to see if it can be done better
+	auto it = block_list.Begin();
 
-	if ( b->seq > 0 || ! frag_size )
+	if ( it->second->seq > 0 || ! frag_size )
 		// For sure don't have it all yet.
 		return;
 
+	auto next = std::next(it);
+
 	// We might have it all - look for contiguous all the way.
-	for ( ; b->next; b = b->next )
-		if ( b->upper != b->next->seq )
+	while ( next != block_list.End() )
+		{
+		if ( it->second->upper != next->second->seq )
 			break;
 
-	if ( b->next )
+		++it;
+		++next;
+		}
+
+	auto last = std::prev(block_list.End())->second;
+
+	if ( next != block_list.End() )
 		{
 		// We have a hole.
-		if ( b->upper >= frag_size )
+		if ( it->second->upper >= frag_size )
 			{
 			// We're stuck.  The point where we stopped is
 			// contiguous up through the expected end of
@@ -215,19 +223,19 @@ void FragReassembler::BlockInserted(const DataBlock* /* start_block */)
 			// We decide to analyze the contiguous portion now.
 			// Extend the fragment up through the end of what
 			// we have.
-			frag_size = b->upper;
+			frag_size = it->second->upper;
 			}
 		else
 			return;
 		}
 
-	else if ( block_list.Tail()->upper > frag_size )
+	else if ( last->upper > frag_size )
 		{
 		Weird("fragment_size_inconsistency");
-		frag_size = block_list.Tail()->upper;
+		frag_size = last->upper;
 		}
 
-	else if ( block_list.Tail()->upper < frag_size )
+	else if ( last->upper < frag_size )
 		// Missing the tail.
 		return;
 
@@ -248,12 +256,15 @@ void FragReassembler::BlockInserted(const DataBlock* /* start_block */)
 
 	pkt += proto_hdr_len;
 
-	for ( b = block_list.Head(); b; b = b->next )
+	for ( it = block_list.Begin(); it != block_list.End(); ++it )
 		{
+		auto b = it->second;
+		DataBlock* prev = it == block_list.Begin() ? nullptr : std::prev(it)->second;
+
 		// If we're above a hole, stop.  This can happen because
 		// the logic above regarding a hole that's above the
 		// expected fragment size.
-		if ( b->prev && b->prev->upper < b->seq )
+		if ( prev && prev->upper < b->seq )
 			break;
 
 		if ( b->upper > n )
@@ -265,8 +276,7 @@ void FragReassembler::BlockInserted(const DataBlock* /* start_block */)
 			return;
 			}
 
-		memcpy((void*) &pkt[b->seq], (const void*) b->block,
-			b->upper - b->seq);
+		memcpy(&pkt[b->seq], b->block, b->upper - b->seq);
 		}
 
 	delete reassembled_pkt;
diff --git a/src/Frag.h b/src/Frag.h
index ca65f21ae8..4b63de0bff 100644
--- a/src/Frag.h
+++ b/src/Frag.h
@@ -37,7 +37,7 @@ public:
 	const FragReassemblerKey& Key() const	{ return key; }
 
 protected:
-	void BlockInserted(const DataBlock* start_block) override;
+	void BlockInserted(DataBlockMap::const_iterator it) override;
 	void Overlap(const u_char* b1, const u_char* b2, uint64_t n) override;
 	void Weird(const char* name) const;
 
diff --git a/src/Reassem.cc b/src/Reassem.cc
index d4acc63e21..3f6dce1659 100644
--- a/src/Reassem.cc
+++ b/src/Reassem.cc
@@ -1,7 +1,6 @@
 // See the file "COPYING" in the main distribution directory for copyright.
 
 #include <algorithm>
-#include <vector>
 
 #include "zeek-config.h"
 
@@ -10,39 +9,22 @@
 uint64_t Reassembler::total_size = 0;
 uint64_t Reassembler::sizes[REASSEM_NUM];
 
-DataBlock::DataBlock(DataBlockList* list, DataBlockMap::const_iterator hint,
-                     const u_char* data, uint64_t size, uint64_t arg_seq,
-                     DataBlock* arg_prev, DataBlock* arg_next)
+DataBlock::DataBlock(const u_char* data, uint64_t size, uint64_t arg_seq)
 	{
-	// TODO: make a separate DataBlockList::Insert()
 	seq = arg_seq;
 	upper = seq + size;
 	block = new u_char[size];
-
-	memcpy((void*) block, (const void*) data, size);
-
-	prev = arg_prev;
-	next = arg_next;
-
-	if ( prev )
-		prev->next = this;
-	if ( next )
-		next->prev = this;
-
-	list->block_map.emplace_hint(hint, seq, this);
-	// TODO: no longer need a separate block count since the map can provide that
-	++list->total_blocks;
-	list->total_data_size += size;
-
-	Reassembler::sizes[list->reassembler->rtype] += pad_size(size) + padded_sizeof(DataBlock);
-	Reassembler::total_size += pad_size(size) + padded_sizeof(DataBlock);
+	memcpy(block, data, size);
 	}
 
 void DataBlockList::DataSize(uint64_t seq_cutoff, uint64_t* below, uint64_t* above) const
 	{
-	// TODO: just have book-keeping to track this info and avoid iterating ?
-	for ( auto b = head; b; b = b->next )
+	// TODO: add warnings that this is O(n) and slow
+
+	for ( const auto& e : block_map )
 		{
+		auto b = e.second;
+
 		if ( b->seq <= seq_cutoff )
 			*above += b->Size();
 		else
@@ -50,63 +32,47 @@ void DataBlockList::DataSize(uint64_t seq_cutoff, uint64_t* below, uint64_t* abo
 		}
 	}
 
-void DataBlockList::DeleteBlock(DataBlock* b)
+void DataBlockList::Delete(DataBlockMap::const_iterator it)
 	{
-	// TODO: Separate Remove / Delete ?
+	auto b = it->second;
 	auto size = b->Size();
 
-	--total_blocks;
+	block_map.erase(it);
 	total_data_size -= size;
 
-	Reassembler::total_size -= pad_size(size) + padded_sizeof(DataBlock);
-	Reassembler::sizes[reassembler->rtype] -= pad_size(size) + padded_sizeof(DataBlock);
-
-	// TODO: most of the time we'd better erase via iterator rather than search
-	//block_map.erase(b->seq);
-
 	delete b;
+	Reassembler::total_size -= size + sizeof(DataBlock);
+	Reassembler::sizes[reassembler->rtype] -= size + sizeof(DataBlock);
+	}
+
+DataBlock* DataBlockList::Remove(DataBlockMap::const_iterator it)
+	{
+	auto b = it->second;
+	auto size = b->Size();
+
+	block_map.erase(it);
+	total_data_size -= size;
+
+	return b;
 	}
 
 void DataBlockList::Clear()
 	{
-	// TODO: can be done more efficiently
-	while ( head )
-		{
-		auto next = head->next;
-		DeleteBlock(head);
-		head = next;
-		}
+	// TODO: maybe can just use clear()
+	while ( ! block_map.empty() )
+		Delete(block_map.begin());
 
 	block_map.clear();
-	tail = nullptr;
 	}
 
 void DataBlockList::Append(DataBlock* block, uint64_t limit)
 	{
-	++total_blocks;
 	total_data_size += block->Size();
-	block->next = nullptr;
-
-	if ( tail )
-		{
-		block->prev = tail;
-		tail->next = block;
-		}
-	else
-		{
-		block->prev = nullptr;
-		head = tail = block;
-		}
 
 	block_map.emplace_hint(block_map.end(), block->seq, block);
 
-	while ( head && total_blocks > limit )
-		{
-		auto next = head->next;
-		DeleteBlock(head);
-		block_map.erase(block_map.begin());
-		head = next;
-		}
+	while ( block_map.size() > limit )
+		Delete(block_map.begin());
 	}
 
 DataBlockMap::const_iterator DataBlockList::FindFirstBlockBefore(uint64_t seq) const
@@ -123,23 +89,35 @@ DataBlockMap::const_iterator DataBlockList::FindFirstBlockBefore(uint64_t seq) c
 	return std::prev(it);
 	}
 
-DataBlock* DataBlockList::Insert(uint64_t seq, uint64_t upper,
-                                 const u_char* data,
-                                 DataBlockMap::const_iterator* hint)
-    {
+DataBlockMap::const_iterator
+DataBlockList::Insert(uint64_t seq, uint64_t upper, const u_char* data,
+                      DataBlockMap::const_iterator hint)
+	{
+	auto size = upper - seq;
+	auto db = new DataBlock(data, size, seq);
+
+	auto rval = block_map.emplace_hint(hint, seq, db);
+
+	total_data_size += size;
+	Reassembler::sizes[reassembler->rtype] += size + sizeof(DataBlock);
+	Reassembler::total_size += size + sizeof(DataBlock);
+
+	return rval;
+	}
+
+DataBlockMap::const_iterator
+DataBlockList::Insert(uint64_t seq, uint64_t upper, const u_char* data,
+                      DataBlockMap::const_iterator* hint)
+	{
 	// Empty list.
-	if ( ! head )
-		{
-		head = tail = new DataBlock(this, block_map.end(), data, upper - seq, seq, 0, 0);
-		return head;
-		}
+	if ( block_map.empty() )
+		return Insert(seq, upper, data, block_map.end());
+
+	auto last = block_map.rbegin()->second;
 
 	// Special check for the common case of appending to the end.
-	if ( tail && seq == tail->upper )
-		{
-		tail = new DataBlock(this, block_map.end(), data, upper - seq, seq, tail, 0);
-		return tail;
-		}
+	if ( seq == last->upper )
+		return Insert(seq, upper, data, block_map.end());
 
 	// Find the first block that doesn't come completely before the new data.
 	DataBlockMap::const_iterator it;
@@ -160,40 +138,33 @@ DataBlock* DataBlockList::Insert(uint64_t seq, uint64_t upper,
 	DataBlock* b = it->second;
 
 	if ( b->upper <= seq )
-		{
 		// b is the last block, and it comes completely before the new block.
-		tail = new DataBlock(this, block_map.end(), data, upper - seq, seq, b, 0);
-		return tail;
-		}
-
-	DataBlock* new_b = 0;
+		return Insert(seq, upper, data, block_map.end());
 
 	if ( upper <= b->seq )
-		{
 		// The new block comes completely before b.
-		new_b = new DataBlock(this, it, data, upper - seq, seq, b->prev, b);
+		return Insert(seq, upper, data, it);
 
-		if ( b == head )
-			head = new_b;
-
-		return new_b;
-		}
+	DataBlock* new_b;
+	DataBlockMap::const_iterator rval;
 
 	// The blocks overlap.
 	if ( seq < b->seq )
 		{
 		// The new block has a prefix that comes before b.
 		uint64_t prefix_len = b->seq - seq;
-		new_b = new DataBlock(this, it, data, prefix_len, seq, b->prev, b);
 
-		if ( b == head )
-			head = new_b;
+		rval = Insert(seq, seq + prefix_len, data, it);
+		new_b = rval->second;
 
 		data += prefix_len;
 		seq += prefix_len;
 		}
 	else
+		{
+		rval = it;
 		new_b = b;
+		}
 
 	uint64_t overlap_start = seq;
 	uint64_t overlap_offset = overlap_start - b->seq;
@@ -207,17 +178,14 @@ DataBlock* DataBlockList::Insert(uint64_t seq, uint64_t upper,
 		data += overlap_len;
 		seq += overlap_len;
 
+		auto r = Insert(seq, upper, data, &it);
+
 		if ( new_b == b )
-			new_b = Insert(seq, upper, data, &it);
-		else
-			Insert(seq, upper, data, &it);
+			rval = r;
 		}
 
-	if ( new_b->prev == tail )
-		tail = new_b;
-
-	return new_b;
-    }
+	return rval;
+	}
 
 uint64_t DataBlockList::Trim(uint64_t seq, uint64_t max_old,
                              DataBlockList* old_list)
@@ -227,19 +195,19 @@ uint64_t DataBlockList::Trim(uint64_t seq, uint64_t max_old,
 	// Do this accounting before looking for Undelivered data,
 	// since that will alter last_reassem_seq.
 
-	if ( head )
+	if ( ! block_map.empty() )
 		{
-		if ( head->seq > reassembler->LastReassemSeq() )
-			// An initial hole.
-			num_missing += head->seq - reassembler->LastReassemSeq();
-		}
+		auto first = block_map.begin()->second;
 
+		if ( first->seq > reassembler->LastReassemSeq() )
+			// An initial hole.
+			num_missing += first->seq - reassembler->LastReassemSeq();
+		}
 	else if ( seq > reassembler->LastReassemSeq() )
-		{ // Trimming data we never delivered.
-		if ( ! head )
-			// We won't have any accounting based on blocks
-			// for this hole.
-			num_missing += seq - reassembler->LastReassemSeq();
+		{
+		// Trimming data we never delivered.
+		// We won't have any accounting based on blocks for this hole.
+		num_missing += seq - reassembler->LastReassemSeq();
 		}
 
 	if ( seq > reassembler->LastReassemSeq() )
@@ -248,56 +216,48 @@ uint64_t DataBlockList::Trim(uint64_t seq, uint64_t max_old,
 		reassembler->Undelivered(seq);
 		}
 
-	auto first_removed = head && head->upper <= seq ? block_map.begin() : block_map.end();
-	auto last_removed = first_removed;
-
-	while ( head && head->upper <= seq )
+	while ( ! block_map.empty() )
 		{
-		DataBlock* b = head->next;
+		auto first_it = block_map.begin();
+		auto first = first_it->second;
 
-		if ( b && b->seq <= seq )
+		if ( first->upper > seq )
+			break;
+
+		auto next_it = std::next(first_it);
+		auto next = next_it == block_map.end() ? nullptr : next_it->second;
+
+		if ( next && next->seq <= seq )
 			{
-			if ( head->upper != b->seq )
-				num_missing += b->seq - head->upper;
+			if ( first->upper != next->seq )
+				num_missing += next->seq - first->upper;
 			}
 		else
 			{
 			// No more blocks - did this one make it to seq?
 			// Second half of test is for acks of FINs, which
 			// don't get entered into the sequence space.
-			if ( head->upper != seq && head->upper != seq - 1 )
-				num_missing += seq - head->upper;
+			if ( first->upper != seq && first->upper != seq - 1 )
+				num_missing += seq - first->upper;
 			}
 
-		// NOTE: erasing the node from the map takes place over the full
-		// range getting removed (see below) rather than one node at a time.
 		if ( max_old )
-			{
-			--total_blocks;
-			total_data_size -= head->Size();
-			old_list->Append(head, max_old);
-			}
+			old_list->Append(Remove(first_it), max_old);
 		else
-			DeleteBlock(head);
-
-		++last_removed;
-		head = b;
+			Delete(first_it);
 		}
 
-	block_map.erase(first_removed, last_removed);
-
-	if ( head )
+	if ( ! block_map.empty() )
 		{
-		head->prev = 0;
+		auto first_it = block_map.begin();
+		auto first = first_it->second;
 
 		// If we skipped over some undeliverable data, then
 		// it's possible that this block is now deliverable.
 		// Give it a try.
-		if ( head->seq == reassembler->LastReassemSeq() )
-			reassembler->BlockInserted(head);
+		if ( first->seq == reassembler->LastReassemSeq() )
+			reassembler->BlockInserted(first_it);
 		}
-	else
-		tail = 0;
 
 	reassembler->SetTrimSeq(seq);
 	return num_missing;
@@ -317,25 +277,22 @@ void Reassembler::CheckOverlap(const DataBlockList& list,
 	if ( list.Empty() )
 		return;
 
-	auto head = list.Head();
-	auto tail = list.Tail();
+	auto last = list.LastBlock();
 
-	if ( seq == tail->upper )
+	if ( seq == last->upper )
 		// Special case check for common case of appending to the end.
 		return;
 
 	uint64_t upper = (seq + len);
 
 	auto it = list.FindFirstBlockBefore(seq);
-	const DataBlock* start;
 
-	if ( it == list.block_map.end() )
-		start = head;
-	else
-		start = it->second;
+	if ( it == list.End() )
+		it = list.Begin();
 
-	for ( auto b = start; b; b = b->next )
+	for ( ; it != list.End(); ++it )
 		{
+		auto b = it->second;
 		uint64_t nseq = seq;
 		uint64_t nupper = upper;
 		const u_char* ndata = data;
@@ -387,8 +344,8 @@ void Reassembler::NewBlock(double t, uint64_t seq, uint64_t len, const u_char* d
 		len -= amount_old;
 		}
 
-	auto start_block = block_list.Insert(seq, upper_seq, data);;
-	BlockInserted(start_block);
+	auto it = block_list.Insert(seq, upper_seq, data);;
+	BlockInserted(it);
 	}
 
 uint64_t Reassembler::TrimToSeq(uint64_t seq)
diff --git a/src/Reassem.h b/src/Reassem.h
index b003bb589e..31e9c02599 100644
--- a/src/Reassem.h
+++ b/src/Reassem.h
@@ -21,26 +21,24 @@ enum ReassemblerType {
 };
 
 class Reassembler;
-class DataBlock;
-class DataBlockList;
-using DataBlockMap = std::map<uint64_t, DataBlock*>;
 
 class DataBlock {
 public:
-	DataBlock(DataBlockList* list, DataBlockMap::const_iterator hint,
-	          const u_char* data, uint64_t size, uint64_t seq,
-	          DataBlock* prev, DataBlock* next);
+	DataBlock(const u_char* data, uint64_t size, uint64_t seq);
 
-	~DataBlock()	{ delete [] block; }
+	~DataBlock()
+		{ delete [] block; }
 
-	uint64_t Size() const	{ return upper - seq; }
+	uint64_t Size() const
+		{ return upper - seq; }
 
-	DataBlock* next;	// next block with higher seq #
-	DataBlock* prev;	// previous block with lower seq #
-	uint64_t seq, upper;
+	uint64_t seq;
+	uint64_t upper;
 	u_char* block;
 };
 
+using DataBlockMap = std::map<uint64_t, DataBlock*>;
+
 // TODO: add comments
 class DataBlockList {
 public:
@@ -54,17 +52,23 @@ public:
 	~DataBlockList()
 		{ Clear(); }
 
-	const DataBlock* Head() const
-		{ return head; }
+	DataBlockMap::const_iterator Begin() const
+		{ return block_map.begin(); }
 
-	const DataBlock* Tail() const
-		{ return tail; }
+	DataBlockMap::const_iterator End() const
+		{ return block_map.end(); }
+
+	DataBlock* FirstBlock() const
+		{ return block_map.begin()->second; }
+
+	DataBlock* LastBlock() const
+		{ return block_map.rbegin()->second; }
 
 	bool Empty() const
-		{ return head == nullptr; };
+		{ return block_map.empty(); };
 
 	size_t NumBlocks() const
-		{ return total_blocks; };
+		{ return block_map.size(); };
 
 	size_t DataSize() const
 		{ return total_data_size; }
@@ -73,8 +77,9 @@ public:
 
 	void Clear();
 
-	DataBlock* Insert(uint64_t seq, uint64_t upper, const u_char* data,
-	                  DataBlockMap::const_iterator* hint = nullptr);
+	DataBlockMap::const_iterator
+	Insert(uint64_t seq, uint64_t upper, const u_char* data,
+	       DataBlockMap::const_iterator* hint = nullptr);
 
 	void Append(DataBlock* block, uint64_t limit);
 
@@ -84,15 +89,15 @@ public:
 
 private:
 
-	void DeleteBlock(DataBlock* b);
+	DataBlockMap::const_iterator
+	Insert(uint64_t seq, uint64_t upper, const u_char* data,
+	       DataBlockMap::const_iterator hint);
 
-	friend class DataBlock;
-	friend class Reassembler;
+	void Delete(DataBlockMap::const_iterator it);
+
+	DataBlock* Remove(DataBlockMap::const_iterator it);
 
 	Reassembler* reassembler = nullptr;
-	DataBlock* head = nullptr;
-	DataBlock* tail = nullptr;
-	size_t total_blocks = 0;
 	size_t total_data_size = 0;
 	DataBlockMap block_map;
 };
@@ -138,12 +143,11 @@ public:
 protected:
 	Reassembler()	{ }
 
-	friend class DataBlock;
 	friend class DataBlockList;
 
 	virtual void Undelivered(uint64_t up_to_seq);
 
-	virtual void BlockInserted(const DataBlock* b) = 0;
+	virtual void BlockInserted(DataBlockMap::const_iterator it) = 0;
 	virtual void Overlap(const u_char* b1, const u_char* b2, uint64_t n) = 0;
 
 	void CheckOverlap(const DataBlockList& list,
@@ -152,7 +156,6 @@ protected:
 	DataBlockList block_list;
 	DataBlockList old_block_list;
 
-	// TODO: maybe roll some of these stats into DataBlockList ?
 	uint64_t last_reassem_seq;
 	uint64_t trim_seq;	// how far we've trimmed
 	uint32_t max_old_blocks;
diff --git a/src/analyzer/protocol/tcp/TCP_Reassembler.cc b/src/analyzer/protocol/tcp/TCP_Reassembler.cc
index a48fd81583..d38ee3bf64 100644
--- a/src/analyzer/protocol/tcp/TCP_Reassembler.cc
+++ b/src/analyzer/protocol/tcp/TCP_Reassembler.cc
@@ -66,11 +66,14 @@ void TCP_Reassembler::Done()
 
 	if ( record_contents_file )
 		{ // Record any undelivered data.
-		auto last_block = block_list.Tail();
+		if ( ! block_list.Empty() )
+			{
+			auto last_block = std::prev(block_list.End())->second;
 
-		if ( ! block_list.Empty() && last_reassem_seq < last_block->upper )
-			RecordToSeq(last_reassem_seq, last_block->upper,
-					record_contents_file);
+			if ( last_reassem_seq < last_block->upper )
+				RecordToSeq(last_reassem_seq, last_block->upper,
+				            record_contents_file);
+			}
 
 		record_contents_file->Close();
 		}
@@ -85,12 +88,11 @@ void TCP_Reassembler::SizeBufferedData(uint64_t& waiting_on_hole,
 
 uint64_t TCP_Reassembler::NumUndeliveredBytes() const
 	{
-	auto last_block = block_list.Tail();
+	if ( block_list.Empty() )
+		return 0;
 
-	if ( last_block )
-		return last_block->upper - last_reassem_seq;
-
-	return 0;
+	auto last_block = std::prev(block_list.End())->second;
+	return last_block->upper - last_reassem_seq;
 	}
 
 void TCP_Reassembler::SetContentsFile(BroFile* f)
@@ -107,7 +109,7 @@ void TCP_Reassembler::SetContentsFile(BroFile* f)
 	else
 		{
 		if ( ! block_list.Empty() )
-			RecordToSeq(block_list.Head()->seq, last_reassem_seq, f);
+			RecordToSeq(block_list.Begin()->second->seq, last_reassem_seq, f);
 		}
 
 	Ref(f);
@@ -235,14 +237,16 @@ void TCP_Reassembler::Undelivered(uint64_t up_to_seq)
 		if ( ! skip_deliveries )
 			{
 			// If we have blocks that begin below up_to_seq, deliver them.
-			auto b = block_list.Head();
-			while ( b )
+			auto it = block_list.Begin();
+
+			while ( it != block_list.End() )
 				{
-				// TODO: better way to do this iteration ?
+				auto b = it->second;
+
 				if ( b->seq < last_reassem_seq )
 					{
 					// Already delivered this block.
-					b = b->next;
+					++it;
 					continue;
 					}
 
@@ -255,10 +259,10 @@ void TCP_Reassembler::Undelivered(uint64_t up_to_seq)
 
 				Gap(gap_at_seq, gap_len);
 				last_reassem_seq += gap_len;
-				BlockInserted(b);
+				BlockInserted(it);
 				// Inserting a block may cause trimming of what's buffered,
 				// so have to assume 'b' is invalid, hence re-assign to start.
-				b = block_list.Head();
+				it = block_list.Begin();
 				}
 
 			if ( up_to_seq > last_reassem_seq )
@@ -285,8 +289,8 @@ void TCP_Reassembler::MatchUndelivered(uint64_t up_to_seq, bool use_last_upper)
 	if ( block_list.Empty() || ! rule_matcher )
 		return;
 
-	auto last_block = block_list.Tail();
-	ASSERT(last_block);
+	auto last_block = std::prev(block_list.End())->second;
+
 	if ( use_last_upper )
 		up_to_seq = last_block->upper;
 
@@ -304,38 +308,45 @@ void TCP_Reassembler::MatchUndelivered(uint64_t up_to_seq, bool use_last_upper)
 	// Skip blocks that are already delivered (but not ACK'ed).
 	// Question: shall we instead keep a pointer to the first undelivered
 	// block?
-	// TODO: better way to iterate ?
-	const DataBlock* b;
-	for ( b = block_list.Head(); b && b->upper <= last_reassem_seq; b = b->next )
-	      tcp_analyzer->Conn()->Match(Rule::PAYLOAD, b->block, b->Size(),
-						false, false, IsOrig(), false);
 
-	ASSERT(b);
+	for ( auto it = block_list.Begin(); it != block_list.End(); ++it )
+		{
+		auto b = it->second;
+
+		if ( b->upper > last_reassem_seq )
+			break;
+
+		tcp_analyzer->Conn()->Match(Rule::PAYLOAD, b->block, b->Size(),
+		                            false, false, IsOrig(), false);
+		}
 	}
 
 void TCP_Reassembler::RecordToSeq(uint64_t start_seq, uint64_t stop_seq, BroFile* f)
 	{
-	auto b = block_list.Head();
-	// TODO: better way to iterate ?
-	// Skip over blocks up to the start seq.
-	while ( b && b->upper <= start_seq )
-		b = b->next;
+	auto it = block_list.Begin();
 
-	if ( ! b )
+	// Skip over blocks up to the start seq.
+	while ( it != block_list.End() && it->second->upper <= start_seq )
+		++it;
+
+	if ( it == block_list.End() )
 		return;
 
 	uint64_t last_seq = start_seq;
-	while ( b && b->upper <= stop_seq )
+
+	while ( it != block_list.End() && it->second->upper <= stop_seq )
 		{
+		auto b = it->second;
+
 		if ( b->seq > last_seq )
 			RecordGap(last_seq, b->seq, f);
 
 		RecordBlock(b, f);
 		last_seq = b->upper;
-		b = b->next;
+		++it;
 		}
 
-	if ( b )
+	if ( it != block_list.End() )
 		// Check for final gap.
 		if ( last_seq < stop_seq )
 			RecordGap(last_seq, stop_seq, f);
@@ -375,8 +386,10 @@ void TCP_Reassembler::RecordGap(uint64_t start_seq, uint64_t upper_seq, BroFile*
 		}
 	}
 
-void TCP_Reassembler::BlockInserted(const DataBlock* start_block)
+void TCP_Reassembler::BlockInserted(DataBlockMap::const_iterator it)
 	{
+	auto start_block = it->second;
+
 	if ( start_block->seq > last_reassem_seq ||
 	     start_block->upper <= last_reassem_seq )
 		return;
@@ -387,10 +400,13 @@ void TCP_Reassembler::BlockInserted(const DataBlock* start_block)
 	// new stuff off into its own block(s), but in the following
 	// loop we have to take care not to deliver already-delivered
 	// data.
-	// TODO: better way to iterate ?
-	for ( auto b = start_block;
-	      b && b->seq <= last_reassem_seq; b = b->next )
+	while ( it != block_list.End() )
 		{
+		auto b = it->second;
+
+		if ( b->seq > last_reassem_seq )
+			break;
+
 		if ( b->seq == last_reassem_seq )
 			{ // New stuff.
 			uint64_t len = b->Size();
@@ -402,6 +418,8 @@ void TCP_Reassembler::BlockInserted(const DataBlock* start_block)
 
 			DeliverBlock(seq, len, b->block);
 			}
+
+		++it;
 		}
 
 	TCP_Endpoint* e = endp;
diff --git a/src/analyzer/protocol/tcp/TCP_Reassembler.h b/src/analyzer/protocol/tcp/TCP_Reassembler.h
index 1afd77a22a..4f93ef73fb 100644
--- a/src/analyzer/protocol/tcp/TCP_Reassembler.h
+++ b/src/analyzer/protocol/tcp/TCP_Reassembler.h
@@ -90,7 +90,7 @@ private:
 	void RecordBlock(const DataBlock* b, BroFile* f);
 	void RecordGap(uint64_t start_seq, uint64_t upper_seq, BroFile* f);
 
-	void BlockInserted(const DataBlock* b) override;
+	void BlockInserted(DataBlockMap::const_iterator it) override;
 	void Overlap(const u_char* b1, const u_char* b2, uint64_t n) override;
 
 	TCP_Endpoint* endp;
diff --git a/src/file_analysis/FileReassembler.cc b/src/file_analysis/FileReassembler.cc
index e41e41c8b2..5949c6adc3 100644
--- a/src/file_analysis/FileReassembler.cc
+++ b/src/file_analysis/FileReassembler.cc
@@ -26,18 +26,16 @@ uint64_t FileReassembler::Flush()
 	if ( flushing )
 		return 0;
 
-	auto last_block = block_list.Tail();
+	if ( block_list.Empty() )
+		return 0;
 
-	if ( last_block )
-		{
-		// This is expected to call back into FileReassembler::Undelivered().
-		flushing = true;
-		uint64_t rval = TrimToSeq(last_block->upper);
-		flushing = false;
-		return rval;
-		}
+	auto last_block = std::prev(block_list.End())->second;
 
-	return 0;
+	// This is expected to call back into FileReassembler::Undelivered().
+	flushing = true;
+	uint64_t rval = TrimToSeq(last_block->upper);
+	flushing = false;
+	return rval;
 	}
 
 uint64_t FileReassembler::FlushTo(uint64_t sequence)
@@ -52,22 +50,29 @@ uint64_t FileReassembler::FlushTo(uint64_t sequence)
 	return rval;
 	}
 
-void FileReassembler::BlockInserted(const DataBlock* start_block)
+void FileReassembler::BlockInserted(DataBlockMap::const_iterator it)
 	{
+	auto start_block = it->second;
+
 	if ( start_block->seq > last_reassem_seq ||
 	     start_block->upper <= last_reassem_seq )
 		return;
 
-	// TODO: better way to iterate ?
-	for ( auto b = start_block;
-	      b && b->seq <= last_reassem_seq; b = b->next )
+	while ( it != block_list.End() )
 		{
+		auto b = it->second;
+
+		if ( b->seq > last_reassem_seq )
+			break;
+
 		if ( b->seq == last_reassem_seq )
 			{ // New stuff.
 			uint64_t len = b->Size();
 			last_reassem_seq += len;
 			the_file->DeliverStream(b->block, len);
 			}
+
+		++it;
 		}
 
 	// Throw out forwarded data
@@ -77,15 +82,16 @@ void FileReassembler::BlockInserted(const DataBlock* start_block)
 void FileReassembler::Undelivered(uint64_t up_to_seq)
 	{
 	// If we have blocks that begin below up_to_seq, deliver them.
-	const DataBlock* b = block_list.Head();
+	auto it = block_list.Begin();
 
-	// TODO: better way to iterate ?
-	while ( b )
+	while ( it != block_list.End() )
 		{
+		auto b = it->second;
+
 		if ( b->seq < last_reassem_seq )
 			{
 			// Already delivered this block.
-			b = b->next;
+			++it;
 			continue;
 			}
 
@@ -97,10 +103,10 @@ void FileReassembler::Undelivered(uint64_t up_to_seq)
 		uint64_t gap_len = b->seq - last_reassem_seq;
 		the_file->Gap(gap_at_seq, gap_len);
 		last_reassem_seq += gap_len;
-		BlockInserted(b);
+		BlockInserted(it);
 		// Inserting a block may cause trimming of what's buffered,
 		// so have to assume 'b' is invalid, hence re-assign to start.
-		b = block_list.Head();
+		it = block_list.Begin();
 		}
 
 	if ( up_to_seq > last_reassem_seq )
diff --git a/src/file_analysis/FileReassembler.h b/src/file_analysis/FileReassembler.h
index e8ec2371f1..fd8effc402 100644
--- a/src/file_analysis/FileReassembler.h
+++ b/src/file_analysis/FileReassembler.h
@@ -51,7 +51,7 @@ protected:
 	FileReassembler();
 
 	void Undelivered(uint64_t up_to_seq) override;
-	void BlockInserted(const DataBlock* b) override;
+	void BlockInserted(DataBlockMap::const_iterator it) override;
 	void Overlap(const u_char* b1, const u_char* b2, uint64_t n) override;
 
 	File* the_file;