diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 07cb175..6c10992 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -589,11 +589,11 @@
 	// error if the server doesn't reply with the renegotiation extension.
 	RequireRenegotiationInfo bool
 
-	// SequenceNumberIncrement, if non-zero, causes outgoing sequence
-	// numbers in DTLS to increment by that value rather by 1. This is to
-	// stress the replay bitmap window by simulating extreme packet loss and
-	// retransmit at the record layer.
-	SequenceNumberIncrement uint64
+	// SequenceNumberMapping, if non-nil, is the mapping function to apply
+	// to the sequence number of outgoing packets. For both TLS and DTLS,
+	// the two most-significant bytes in the resulting sequence number are
+	// ignored so that the DTLS epoch cannot be changed.
+	SequenceNumberMapping func(uint64) uint64
 
 	// RSAEphemeralKey, if true, causes the server to send a
 	// ServerKeyExchange message containing an ephemeral key (as in
diff --git a/ssl/test/runner/conn.go b/ssl/test/runner/conn.go
index ea9f3bb..b755c46 100644
--- a/ssl/test/runner/conn.go
+++ b/ssl/test/runner/conn.go
@@ -12,6 +12,7 @@
 	"crypto/ecdsa"
 	"crypto/subtle"
 	"crypto/x509"
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
@@ -87,6 +88,8 @@
 	c.out.isDTLS = c.isDTLS
 	c.in.config = c.config
 	c.out.config = c.config
+
+	c.out.updateOutSeq()
 }
 
 // Access to net.Conn methods.
@@ -134,6 +137,7 @@
 	cipher  interface{} // cipher algorithm
 	mac     macFunction
 	seq     [8]byte // 64-bit sequence number
+	outSeq  [8]byte // Mapped sequence number
 	bfree   *block  // list of free blocks
 
 	nextCipher interface{} // next encryption state
@@ -189,10 +193,6 @@
 	if hc.isDTLS {
 		// Increment up to the epoch in DTLS.
 		limit = 2
-
-		if isOutgoing && hc.config.Bugs.SequenceNumberIncrement != 0 {
-			increment = hc.config.Bugs.SequenceNumberIncrement
-		}
 	}
 	for i := 7; i >= limit; i-- {
 		increment += uint64(hc.seq[i])
@@ -206,6 +206,8 @@
 	if increment != 0 {
 		panic("TLS: sequence number wraparound")
 	}
+
+	hc.updateOutSeq()
 }
 
 // incNextSeq increments the starting sequence number for the next epoch.
@@ -241,6 +243,22 @@
 			hc.seq[i] = 0
 		}
 	}
+
+	hc.updateOutSeq()
+}
+
+func (hc *halfConn) updateOutSeq() {
+	if hc.config.Bugs.SequenceNumberMapping != nil {
+		seqU64 := binary.BigEndian.Uint64(hc.seq[:])
+		seqU64 = hc.config.Bugs.SequenceNumberMapping(seqU64)
+		binary.BigEndian.PutUint64(hc.outSeq[:], seqU64)
+
+		// The DTLS epoch cannot be changed.
+		copy(hc.outSeq[:2], hc.seq[:2])
+		return
+	}
+
+	copy(hc.outSeq[:], hc.seq[:])
 }
 
 func (hc *halfConn) recordHeaderLen() int {
@@ -460,7 +478,7 @@
 
 	// mac
 	if hc.mac != nil {
-		mac := hc.mac.MAC(hc.outDigestBuf, hc.seq[0:], b.data[:3], b.data[recordHeaderLen-2:recordHeaderLen], b.data[recordHeaderLen+explicitIVLen:])
+		mac := hc.mac.MAC(hc.outDigestBuf, hc.outSeq[0:], b.data[:3], b.data[recordHeaderLen-2:recordHeaderLen], b.data[recordHeaderLen+explicitIVLen:])
 
 		n := len(b.data)
 		b.resize(n + len(mac))
@@ -478,7 +496,7 @@
 		case *tlsAead:
 			payloadLen := len(b.data) - recordHeaderLen - explicitIVLen
 			b.resize(len(b.data) + c.Overhead())
-			nonce := hc.seq[:]
+			nonce := hc.outSeq[:]
 			if c.explicitNonce {
 				nonce = b.data[recordHeaderLen : recordHeaderLen+explicitIVLen]
 			}
@@ -486,7 +504,7 @@
 			payload = payload[:payloadLen]
 
 			var additionalData [13]byte
-			copy(additionalData[:], hc.seq[:])
+			copy(additionalData[:], hc.outSeq[:])
 			copy(additionalData[8:], b.data[:3])
 			additionalData[11] = byte(payloadLen >> 8)
 			additionalData[12] = byte(payloadLen)
diff --git a/ssl/test/runner/dtls.go b/ssl/test/runner/dtls.go
index 538bf51..5c59dea 100644
--- a/ssl/test/runner/dtls.go
+++ b/ssl/test/runner/dtls.go
@@ -298,13 +298,13 @@
 	b.data[1] = byte(vers >> 8)
 	b.data[2] = byte(vers)
 	// DTLS records include an explicit sequence number.
-	copy(b.data[3:11], c.out.seq[0:])
+	copy(b.data[3:11], c.out.outSeq[0:])
 	b.data[11] = byte(len(data) >> 8)
 	b.data[12] = byte(len(data))
 	if explicitIVLen > 0 {
 		explicitIV := b.data[recordHeaderLen : recordHeaderLen+explicitIVLen]
 		if explicitIVIsSeq {
-			copy(explicitIV, c.out.seq[:])
+			copy(explicitIV, c.out.outSeq[:])
 		} else {
 			if _, err = io.ReadFull(c.config.rand(), explicitIV); err != nil {
 				return
diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 9fa394f..ff43678 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -158,6 +158,8 @@
 	// messageLen is the length, in bytes, of the test message that will be
 	// sent.
 	messageLen int
+	// messageCount is the number of test messages that will be sent.
+	messageCount int
 	// certFile is the path to the certificate to use for the server.
 	certFile string
 	// keyFile is the path to the private key to use for the server.
@@ -221,7 +223,7 @@
 
 var testCases []testCase
 
-func doExchange(test *testCase, config *Config, conn net.Conn, messageLen int, isResume bool) error {
+func doExchange(test *testCase, config *Config, conn net.Conn, isResume bool) error {
 	var connDebug *recordingConn
 	var connDamage *damageAdaptor
 	if *flagDebug {
@@ -379,6 +381,7 @@
 		connDamage.setDamage(false)
 	}
 
+	messageLen := test.messageLen
 	if messageLen < 0 {
 		if test.protocol == dtls {
 			return fmt.Errorf("messageLen < 0 not supported for DTLS tests")
@@ -387,45 +390,52 @@
 		_, err := io.Copy(ioutil.Discard, tlsConn)
 		return err
 	}
-
 	if messageLen == 0 {
 		messageLen = 32
 	}
-	testMessage := make([]byte, messageLen)
-	for i := range testMessage {
-		testMessage[i] = 0x42
-	}
-	tlsConn.Write(testMessage)
 
-	for i := 0; i < test.sendEmptyRecords; i++ {
-		tlsConn.Write(nil)
+	messageCount := test.messageCount
+	if messageCount == 0 {
+		messageCount = 1
 	}
 
-	for i := 0; i < test.sendWarningAlerts; i++ {
-		tlsConn.SendAlert(alertLevelWarning, alertUnexpectedMessage)
-	}
-
-	buf := make([]byte, len(testMessage))
-	if test.protocol == dtls {
-		bufTmp := make([]byte, len(buf)+1)
-		n, err := tlsConn.Read(bufTmp)
-		if err != nil {
-			return err
+	for j := 0; j < messageCount; j++ {
+		testMessage := make([]byte, messageLen)
+		for i := range testMessage {
+			testMessage[i] = 0x42 ^ byte(j)
 		}
-		if n != len(buf) {
-			return fmt.Errorf("bad reply; length mismatch (%d vs %d)", n, len(buf))
-		}
-		copy(buf, bufTmp)
-	} else {
-		_, err := io.ReadFull(tlsConn, buf)
-		if err != nil {
-			return err
-		}
-	}
+		tlsConn.Write(testMessage)
 
-	for i, v := range buf {
-		if v != testMessage[i]^0xff {
-			return fmt.Errorf("bad reply contents at byte %d", i)
+		for i := 0; i < test.sendEmptyRecords; i++ {
+			tlsConn.Write(nil)
+		}
+
+		for i := 0; i < test.sendWarningAlerts; i++ {
+			tlsConn.SendAlert(alertLevelWarning, alertUnexpectedMessage)
+		}
+
+		buf := make([]byte, len(testMessage))
+		if test.protocol == dtls {
+			bufTmp := make([]byte, len(buf)+1)
+			n, err := tlsConn.Read(bufTmp)
+			if err != nil {
+				return err
+			}
+			if n != len(buf) {
+				return fmt.Errorf("bad reply; length mismatch (%d vs %d)", n, len(buf))
+			}
+			copy(buf, bufTmp)
+		} else {
+			_, err := io.ReadFull(tlsConn, buf)
+			if err != nil {
+				return err
+			}
+		}
+
+		for i, v := range buf {
+			if v != testMessage[i]^0xff {
+				return fmt.Errorf("bad reply contents at byte %d", i)
+			}
 		}
 	}
 
@@ -595,7 +605,7 @@
 
 	conn, err := acceptOrWait(listener, waitChan)
 	if err == nil {
-		err = doExchange(test, &config, conn, test.messageLen, false /* not a resumption */)
+		err = doExchange(test, &config, conn, false /* not a resumption */)
 		conn.Close()
 	}
 
@@ -625,7 +635,7 @@
 		var connResume net.Conn
 		connResume, err = acceptOrWait(listener, waitChan)
 		if err == nil {
-			err = doExchange(test, &resumeConfig, connResume, test.messageLen, true /* resumption */)
+			err = doExchange(test, &resumeConfig, connResume, true /* resumption */)
 			connResume.Close()
 		}
 	}
@@ -3303,19 +3313,38 @@
 	testCases = append(testCases, testCase{
 		protocol:     dtls,
 		name:         "DTLS-Replay",
+		messageCount: 200,
 		replayWrites: true,
 	})
 
-	// Test the outgoing sequence number skipping by values larger
+	// Test the incoming sequence number skipping by values larger
 	// than the retransmit window.
 	testCases = append(testCases, testCase{
 		protocol: dtls,
 		name:     "DTLS-Replay-LargeGaps",
 		config: Config{
 			Bugs: ProtocolBugs{
-				SequenceNumberIncrement: 127,
+				SequenceNumberMapping: func(in uint64) uint64 {
+					return in * 127
+				},
 			},
 		},
+		messageCount: 200,
+		replayWrites: true,
+	})
+
+	// Test the incoming sequence number changing non-monotonically.
+	testCases = append(testCases, testCase{
+		protocol: dtls,
+		name:     "DTLS-Replay-NonMonotonic",
+		config: Config{
+			Bugs: ProtocolBugs{
+				SequenceNumberMapping: func(in uint64) uint64 {
+					return in ^ 31
+				},
+			},
+		},
+		messageCount: 200,
 		replayWrites: true,
 	})
 }