diff --git a/.gitignore b/.gitignore
index c5d9618..473fc44 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@ vendor/
 *.so
 *.dylib
 *.hex
+**__debug_bin*
 # `__debug_bin` Debug binary generated in VSCode when using the built-in debugger.
 *bin
 
diff --git a/internal/ring.go b/internal/ring.go
index 819aae2..439620d 100644
--- a/internal/ring.go
+++ b/internal/ring.go
@@ -28,10 +28,14 @@ type Ring struct {
 // SizeLimited returns the amount of bytes that can be written up to the
 // argument offset limitOffset. See [Ring.WriteLimited]
 func (r *Ring) FreeLimited(limitOffset int) (free int) {
-	if limitOffset > r.End {
-		free = limitOffset - r.End
+	end := r.End
+	if r.End == 0 {
+		end = r.Off
+	}
+	if limitOffset > end {
+		free = limitOffset - end
 	} else {
-		free = len(r.Buf) - r.End + limitOffset
+		free = len(r.Buf) - end + limitOffset
 	}
 	return free
 }
@@ -57,7 +61,8 @@ func (r *Ring) WriteString(s string) (int, error) {
 	return r.Write(unsafe.Slice(unsafe.StringData(s), len(s)))
 }
 
-// Write appends data to the ring buffer that can then be read back in order with [Ring.Read] methods. An error is returned if length of data too large for buffer.
+// Write appends data to the ring buffer that can then be read back in order with [Ring.Read] methods.
+// An error is returned if length of data too large for buffer. Write is guaranteed to start at buffer index [Ring.Off].
 func (r *Ring) Write(b []byte) (int, error) {
 	free := r.Free()
 	if len(b) > free {
@@ -70,6 +75,10 @@ func (r *Ring) Write(b []byte) (int, error) {
 		n := copy(r.Buf[r.End:r.Off], b)
 		r.End += n
 		return n, nil
+	} else if r.End == 0 {
+		// To ensure Write begins on r.Off.
+		// Specialised for when user controls Off manually instead of by internal calls to [Ring.onReadEnd] or calls to [Ring.Reset].
+		r.End = r.Off
 	}
 	// start       off       end      len(buf)
 	//   |  sfree   |  used   |  efree   |
@@ -86,14 +95,14 @@ func (r *Ring) Write(b []byte) (int, error) {
 // ReadDiscard is a performance auxiliary method that performs a dummy read or no-op read
 // for advancing the read pointer n bytes without actually copying data.
 // This method panics if amount of bytes is more than buffered (see [Ring.Buffered]).
-func (r *Ring) ReadDiscard(n int) {
-	if n < 0 {
-		panic("negative discard amount")
+func (r *Ring) ReadDiscard(n int) error {
+	if n <= 0 {
+		return errors.New("invalid discard amount")
 	}
 	buffered := r.Buffered()
 	switch {
 	case n > buffered:
-		panic("discard exceeds length")
+		return errors.New("discard exceeds length")
 	case n == buffered:
 		r.Reset()
 	case n+r.Off > len(r.Buf):
@@ -101,6 +110,7 @@ func (r *Ring) ReadDiscard(n int) {
 	default:
 		r.Off += n
 	}
+	return nil
 }
 
 // ReadAt reads data at an offset from start of readable data but does not advance read pointer. [io.EOF] returned when no data available.
@@ -171,7 +181,7 @@ func (r *Ring) Buffered() int {
 
 // Free returns amount of bytes that can be read into ring buffer before reaching maximum capacity given by [ring.Size]. Always less than [ring.Size].
 func (r *Ring) Free() int {
-	if r.Off == 0 || r.End == 0 {
+	if r.End == 0 || r.Off == 0 {
 		return len(r.Buf) - r.End
 	}
 	if r.Off < r.End {
@@ -201,11 +211,15 @@ func (r *Ring) onReadEnd(totalRead int) {
 	newOff := r.addOff(r.Off, totalRead)
 	if newOff == r.End {
 		r.Reset()
+	} else if newOff == len(r.Buf) {
+		r.Off = 0 // Optimization case.
 	} else {
 		r.Off = newOff
 	}
 }
 
+// addOff sums a and b to return an index within 1..[Ring.Size] supposing a and b are each less-equal than [Ring.Size].
+// Result will never be 0 unless both a and b are 0.
 func (r *Ring) addOff(a, b int) int {
 	result := a + b
 	if result > len(r.Buf) {
diff --git a/internal/ring_test.go b/internal/ring_test.go
index da3849a..a5ae339 100644
--- a/internal/ring_test.go
+++ b/internal/ring_test.go
@@ -175,6 +175,7 @@ func TestRingEmpty(t *testing.T) {
 					if buf2 != 0 {
 						t.Fatalf("want 0 bytes buffered(second call), got buf=%d->%d for off=%d->%d, end=0->%d size=%d", buf, buf2, off, r.Off, r.End, r.Size())
 					}
+					testRingSanity(t, r)
 					for _, read := range readCalls {
 						n, err := read(data)
 						if err != io.EOF {
@@ -182,8 +183,8 @@ func TestRingEmpty(t *testing.T) {
 						} else if n != 0 {
 							t.Fatalf("expected no bytes read, got %d", n)
 						}
+						testRingSanity(t, r)
 					}
-					testRingSanity(t, r)
 				}
 			})
 		}
@@ -243,6 +244,42 @@ func TestRingNonEmpty(t *testing.T) {
 	}
 }
 
+func TestRing_OffWrite(t *testing.T) {
+	const bufSize = 8
+	var rawbuf, auxbuf, readback [bufSize]byte
+	r := &Ring{Buf: rawbuf[:]}
+	for n := 1; n < bufSize+1; n++ {
+		for off := 0; off < bufSize+1; off++ {
+			r.Off = off // Start write at off.
+			r.End = 0   // Reset to use no data.
+			for i := 0; i < n; i++ {
+				rawbuf[(off+i)%len(rawbuf)] = 0
+				auxbuf[i] = byte(i) + 1
+			}
+			ngot, err := r.Write(auxbuf[:n])
+			if err != nil {
+				t.Fatal(err)
+			} else if ngot != n {
+				t.Fatal(n, ngot)
+			}
+			for i := 0; i < n; i++ {
+				offz := (off + i) % len(rawbuf)
+				if rawbuf[offz] != auxbuf[i] {
+					t.Fatalf("mismatch pos=%d off=%d %q!=%q", i, offz, rawbuf[offz], auxbuf[i])
+				}
+			}
+			ngot, err = r.Read(readback[:])
+			if err != nil {
+				t.Fatal(err)
+			} else if ngot != n {
+				t.Fatal(n, ngot)
+			} else if !bytes.Equal(readback[:n], auxbuf[:n]) {
+				t.Fatalf("want readback %q, got %q", auxbuf[:n], readback[:n])
+			}
+		}
+	}
+}
+
 func TestRing_TwoWrite(t *testing.T) {
 	const bufSize = 8
 	rng := rand.New(rand.NewSource(1))
@@ -441,24 +478,6 @@ func testRing1_loopback(t *testing.T, rng *rand.Rand, ringbuf, data, auxbuf []by
 	return !t.Failed()
 }
 
-func fragmentReadInto(r io.Reader, buf []byte) (n int, _ error) {
-	maxSize := len(buf) / 4
-	for {
-		ntop := min(n+rand.Intn(maxSize)+1, len(buf))
-		ngot, err := r.Read(buf[n:ntop])
-		n += ngot
-		if err != nil {
-			if err == io.EOF {
-				return n, nil
-			}
-			return n, err
-		}
-		if n == len(buf) {
-			return n, nil
-		}
-	}
-}
-
 func setRingData(t *testing.T, r *Ring, offset int, data []byte) {
 	t.Helper()
 	sz := r.Size()
@@ -520,7 +539,7 @@ func testRingSanity(t *testing.T, r *Ring) {
 
 func canonRing(r *Ring) {
 	if r.Buffered() == 0 {
-		// r.End = r.addOff(r.Off, 1)
-		// r.onReadEnd(1)
+		r.End = r.addOff(r.Off, 1)
+		r.onReadEnd(1)
 	}
 }
diff --git a/tcp/txqueue.go b/tcp/txqueue.go
index 83612ee..44268d1 100644
--- a/tcp/txqueue.go
+++ b/tcp/txqueue.go
@@ -17,9 +17,9 @@ type ringTx struct {
 	rawbuf []byte
 	// packets contains
 	packets []ringidx
-	// unsentOff is the offset of start of unsent data into rawbuf.
+	// unsentOff is the offset of start of unsent data in rawbuf.
 	unsentoff int
-	// unsentend is the offset of end of unsent data in rawbuf.
+	// unsentend is the offset of end of unsent data in rawbuf. If zero then unsent buffer is empty.
 	unsentend int
 	seq       Value
 	// always empty ring.
@@ -71,32 +71,36 @@ func (rx *ringTx) ResetOrReuse(buf []byte, maxQueuedPackets int, ack Value) erro
 	return rx.Reset(buf, maxQueuedPackets, ack)
 }
 
+// Size returns the total storage space of the transmission buffer.
+func (tx *ringTx) Size() int { return len(tx.rawbuf) }
+
+// Free returns the total available space for Write calls.
+func (tx *ringTx) Free() int {
+	freeStart, freeEnd, _ := tx.lims()
+	r := tx.ring(freeEnd, freeStart)
+	return r.Free()
+}
+
 // Buffered returns the amount of unsent bytes.
 func (tx *ringTx) Buffered() int {
-	r := tx.unsentRing()
+	r, _ := tx.unsentRing()
 	return r.Buffered()
 }
 
 // BufferedSent returns the total amount of bytes sent but not acked.
 func (tx *ringTx) BufferedSent() int {
-	r := tx.sentRing()
+	r, _ := tx.sentRing()
 	return r.Buffered()
 }
 
 // Write writes data to the underlying unsent data ring buffer.
 func (tx *ringTx) Write(b []byte) (n int, err error) {
-	first := tx.pkt(tx.firstPkt())
-	r := tx.unsentRing()
-	if !first.sent() {
-		// No packets in queue case.
-		n, err = r.Write(b)
-	} else {
-		n, err = r.WriteLimited(b, first.off)
-	}
+	r, lim := tx.unsentRing()
+	n, err = r.WriteLimited(b, lim)
 	if err != nil {
 		return 0, err
 	}
-	tx.unsentend = tx.addOff(tx.unsentend, n)
+	tx.unsentend = tx.addEnd(tx.unsentend, n)
 	return n, err
 }
 
@@ -107,7 +111,7 @@ func (tx *ringTx) MakePacket(b []byte) (int, Value, error) {
 	if tx.nextPkt() < 0 {
 		return 0, 0, errors.New("queue full")
 	}
-	r := tx.unsentRing()
+	r, _ := tx.unsentRing()
 	start := r.Off
 	n, err := r.Read(b)
 	if err != nil {
@@ -115,12 +119,14 @@ func (tx *ringTx) MakePacket(b []byte) (int, Value, error) {
 	}
 	plen := Value(n)
 	seq := tx.seq
-	tx.unsentoff = tx.addOff(tx.unsentoff, n)
+	tx.unsentoff = tx.addEnd(tx.unsentoff, n)
+	if tx.unsentoff == tx.unsentend {
+		tx.unsentend = 0 // Mark unsent as being empty.
+	}
 	tx.seq += plen
-
 	pkt := &tx.packets[nxtpkt]
 	pkt.off = start
-	pkt.end = tx.addOff(start, n)
+	pkt.end = tx.addEnd(start, n)
 	pkt.seq = seq + plen
 	return n, seq, nil
 }
@@ -133,37 +139,43 @@ func (tx *ringTx) RecvACK(ack Value) error {
 			pkt.markRcvd()
 		}
 	}
+
 	return nil
 }
 
-func (tx *ringTx) unsentRing() internal.Ring {
-	off := tx.unsentoff
-	if off == tx.unsentend && off != 0 {
-		off--
+func (tx *ringTx) unsentRing() (internal.Ring, int) {
+	freeStart, freeEnd, sentEnd := tx.lims()
+	if tx.unsentend == 0 {
+		freeStart = 0 // Unsent is empty.
 	}
-	return tx.ring(off, tx.unsentend)
+	return tx.ring(sentEnd, freeStart), freeEnd
 }
 
-func (tx *ringTx) sentRing() internal.Ring {
-	first := tx.pkt(tx.firstPkt())
-	if !first.sent() {
-		return internal.Ring{}
-	}
-	last := tx.pkt(tx.lastPkt())
-	return tx.ring(first.off, last.end)
+func (tx *ringTx) sentRing() (internal.Ring, int) {
+	freeStart, freeEnd, sentEnd := tx.lims()
+	return tx.ring(freeEnd, sentEnd), freeStart
 }
 
 func (tx *ringTx) ring(off, end int) internal.Ring {
 	return internal.Ring{Buf: tx.rawbuf, Off: off, End: end}
 }
 
-// addOff adds two integers together and wraps the value around the ring's buffer size.
-func (tx *ringTx) addOff(a, b int) int {
-	off := a + b
-	if off >= len(tx.rawbuf) {
-		off -= len(tx.rawbuf)
+// addEnd adds two integers together and wraps the value around the ring's buffer size.
+// Result of addEnd will never be 0 unless arguments are (0,0).
+func (tx *ringTx) addEnd(a, b int) int {
+	result := a + b
+	if result > len(tx.rawbuf) {
+		result -= len(tx.rawbuf)
 	}
-	return off
+	return result
+}
+
+func (tx *ringTx) addOff(a, b int) int {
+	result := a + b
+	if result >= len(tx.rawbuf) {
+		result -= len(tx.rawbuf)
+	}
+	return result
 }
 
 func (tx *ringTx) pkt(i int) *ringidx {
@@ -180,14 +192,9 @@ func (tx *ringTx) firstPkt() int {
 	idx := -1
 	for i := 0; i < len(tx.packets); i++ {
 		pkt := &tx.packets[i]
-		if pkt.sent() {
-			if idx == -1 {
-				seq = pkt.seq
-			}
-			if seq.LessThan(pkt.seq) {
-				seq = pkt.seq
-				idx = i
-			}
+		if pkt.sent() && (idx == -1 || seq.LessThan(pkt.seq)) {
+			seq = pkt.seq
+			idx = i
 		}
 	}
 	return idx
@@ -198,14 +205,9 @@ func (tx *ringTx) lastPkt() int {
 	idx := -1
 	for i := 0; i < len(tx.packets); i++ {
 		pkt := &tx.packets[i]
-		if pkt.sent() {
-			if idx == -1 {
-				seq = pkt.seq
-			}
-			if pkt.seq.LessThan(seq) {
-				seq = pkt.seq
-				idx = i
-			}
+		if pkt.sent() && (idx == -1 || pkt.seq.LessThan(seq)) {
+			seq = pkt.seq
+			idx = i
 		}
 	}
 	return idx
@@ -223,6 +225,31 @@ func (tx *ringTx) nextPkt() int {
 	return idx
 }
 
+// lims returns the limits of free|sent|unsent buffers.
+// Example:
+//
+//	|   acked(free)  |          sent         |          unsent          |             free       |
+//	0       freeEnd=first.off       last.end==unsent.off        freeStart=unsent.end         Size()
+func (tx *ringTx) lims() (freeStart, freeEnd, sentEndorUnsentStart int) {
+	freeStart = tx.unsentend
+	if freeStart == 0 {
+		freeStart = tx.unsentoff
+	}
+	first := tx.pkt(tx.firstPkt())
+	if first.sent() {
+		freeEnd = first.off
+		sentEndorUnsentStart = tx.unsentoff
+	} else if tx.unsentend != 0 {
+		// sent section empty and unsent not empty.
+		freeEnd = tx.unsentoff
+		sentEndorUnsentStart = tx.unsentoff
+	} else {
+		freeEnd = tx.unsentoff
+		sentEndorUnsentStart = tx.unsentoff
+	}
+	return freeStart, freeEnd, sentEndorUnsentStart
+}
+
 func (pkt *ringidx) sent() bool {
 	return pkt.end != 0 || pkt.off != 0
 }
diff --git a/tcp/txqueue_test.go b/tcp/txqueue_test.go
index 9daf248..0905a86 100644
--- a/tcp/txqueue_test.go
+++ b/tcp/txqueue_test.go
@@ -11,21 +11,37 @@ func TestTxQueue(t *testing.T) {
 	rng := rand.New(rand.NewSource(1))
 
 	var rtx ringTx
-	t.Run("SequentialMessages", func(t *testing.T) {
-		for i := 0; i < 10; i++ {
-			rng.Read(msgBuf[:])
-			msgs := bytes.SplitAfter(msgBuf[:], []byte{0})
-			testTxQueue_SequentialMessages(t, &rtx, msgs, buf[:], aux[:], rng.Intn(4)+1, Value(rng.Int()))
+	increasingComplexityTests := []struct {
+		name string
+		test func(*testing.T)
+	}{
+		0: {
+			name: "SequentialMessages",
+			test: func(t *testing.T) {
+				for i := 0; i < 10; i++ {
+					rng.Read(msgBuf[:])
+					msgs := bytes.SplitAfter(msgBuf[:], []byte{0})
+					testTxQueue_SequentialMessages(t, &rtx, msgs, buf[:], aux[:], rng.Intn(4)+1, 0)
+				}
+			},
+		},
+		1: {
+			name: "N-Messages",
+			test: func(t *testing.T) {
+				for i := 0; i < 10; i++ {
+					rng.Read(msgBuf[:])
+					msgs := bytes.SplitAfter(msgBuf[:], []byte{0})
+					testTxQueue_NMessages(t, &rtx, msgs, buf[:], aux[:], len(msgs), 0)
+				}
+			},
+		},
+	}
+	for i, test := range increasingComplexityTests {
+		t.Run(test.name, test.test)
+		if t.Failed() {
+			t.Fatalf("subtest %d/%d %q failed, not running more complex tests until fixed", i+1, len(increasingComplexityTests), test.name)
 		}
-	})
-	t.Run("N-Messages", func(t *testing.T) {
-		for i := 0; i < 10; i++ {
-			rng.Read(msgBuf[:])
-			msgs := bytes.SplitAfter(msgBuf[:], []byte{0})
-
-			testTxQueue_NMessages(t, &rtx, msgs, buf[:], aux[:], len(msgs), Value(rng.Int()))
-		}
-	})
+	}
 }
 
 func testTxQueue_NMessages(t *testing.T, rtx *ringTx, msgs [][]byte, buf, aux []byte, maxPkt int, startAck Value) {
@@ -53,10 +69,12 @@ func testTxQueue_NMessages(t *testing.T, rtx *ringTx, msgs [][]byte, buf, aux []
 		} else if n != len(msg) {
 			t.Fatalf("want %d written, got %d", len(msg), n)
 		}
+		testQueueSanity(t, rtx)
 		unsent := rtx.Buffered()
 		if unsent != n {
 			t.Fatalf("want unset %d, got %d", n, unsent)
 		}
+		testQueueSanity(t, rtx)
 		n, seq, err := rtx.MakePacket(aux[sent : sent+len(msg)])
 		if err != nil {
 			t.Fatal(err)
@@ -65,10 +83,12 @@ func testTxQueue_NMessages(t *testing.T, rtx *ringTx, msgs [][]byte, buf, aux []
 		} else if n != len(msg) {
 			t.Fatalf("want full message %d sent, got %d", len(msg), n)
 		}
+		testQueueSanity(t, rtx)
 		gotSent := rtx.BufferedSent()
 		if gotSent != sent+n {
 			t.Fatalf("want sent %d, got %d", sent+n, gotSent)
 		}
+		testQueueSanity(t, rtx)
 		packets = append(packets, aux[sent:sent+n])
 		prevSeq += Value(n)
 		sent += n
@@ -91,14 +111,17 @@ func testTxQueue_SequentialMessages(t *testing.T, rtx *ringTx, msgs [][]byte, bu
 		} else if n != len(msg) {
 			t.Fatalf("want %d written, got %d", len(msg), n)
 		}
+		testQueueSanity(t, rtx)
 		unsent := rtx.Buffered()
 		if len(msg) != unsent {
-			t.Fatalf("want %d unsent buffered, got %d", unsent, len(msg))
+			t.Fatalf("want %d unsent buffered, got %d", len(msg), unsent)
 		}
+		testQueueSanity(t, rtx)
 		sent := rtx.BufferedSent()
 		if sent != 0 {
 			t.Fatalf("want 0 bytes sent, got %d", sent)
 		}
+		testQueueSanity(t, rtx)
 		n, seq, err := rtx.MakePacket(aux[:])
 		data := aux[:n]
 		if err != nil {
@@ -110,14 +133,43 @@ func testTxQueue_SequentialMessages(t *testing.T, rtx *ringTx, msgs [][]byte, bu
 		} else if seq != prevSeq {
 			t.Fatalf("want seq %d, got %d", prevSeq, seq)
 		}
+		testQueueSanity(t, rtx)
 		sent = rtx.BufferedSent()
 		if sent != len(msg) {
 			t.Fatalf("want %d sent, got %d", len(msg), sent)
 		}
+		testQueueSanity(t, rtx)
 		prevSeq += Value(n)
 		err = rtx.RecvACK(prevSeq)
 		if err != nil {
 			t.Fatal(err)
 		}
+		testQueueSanity(t, rtx)
+	}
+}
+
+func testQueueSanity(t *testing.T, rtx *ringTx) {
+	t.Helper()
+	if rtx.emptyRing != (ringidx{}) {
+		t.Fatalf("empty ring not empty")
+	}
+	free := rtx.Free()
+	sent := rtx.BufferedSent()
+	unsent := rtx.Buffered()
+	sz := rtx.Size()
+	gotSz := free + sent + unsent
+	if gotSz != sz {
+		t.Fatalf("want size=%d, got size=%d (free+sent+unsent=%d+%d+%d)", sz, gotSz, free, sent, unsent)
+	}
+	freeStart, freeEnd, sentEnd := rtx.lims()
+	gotFreeEnd := rtx.addOff(freeStart, free)
+	gotSentEnd := rtx.addOff(freeEnd, sent)
+	gotUnsentEnd := rtx.addOff(sentEnd, unsent)
+	if free != 0 && gotFreeEnd != freeEnd {
+		t.Fatalf("want freeEnd=%d, got %d", freeEnd, gotFreeEnd)
+	} else if sent != 0 && gotSentEnd != sentEnd {
+		t.Fatalf("want sentEnd=%d, got %d", sentEnd, gotSentEnd)
+	} else if unsent != 0 && gotUnsentEnd != freeStart {
+		t.Fatalf("want unsentEnd=%d, got %d (freeStart)", freeStart, gotUnsentEnd)
 	}
 }