Add the payload directly to the ICMPv4 type

This makes handling inbound fragmented packets easier, because a fragmented
packet might not have an actual ICMP header but only a payload. After this
change, the ICMPv4 is the last layer you can get because the payload is
embedded in it.

Note that this makes it consistent with the ICMPv6 implementation.

While I'm here, I've also added the Ident and Sequence fields on the ICMPv4
type. Defaults are still zero.

PiperOrigin-RevId: 339577094

test/packetimpact/testbench/layers.go

@@ -879,6 +879,9 @@ type ICMPv4 struct {
 	Type     *header.ICMPv4Type
 	Code     *header.ICMPv4Code
 	Checksum *uint16
+	Ident    *uint16
+	Sequence *uint16
+	Payload  []byte
 }
 
 func (l *ICMPv4) String() string {
@@ -887,7 +890,7 @@ func (l *ICMPv4) String() string {
 
 // ToBytes implements Layer.ToBytes.
 func (l *ICMPv4) ToBytes() ([]byte, error) {
-	b := make([]byte, header.ICMPv4MinimumSize)
+	b := make([]byte, header.ICMPv4MinimumSize+len(l.Payload))
 	h := header.ICMPv4(b)
 	if l.Type != nil {
 		h.SetType(*l.Type)
@@ -895,15 +898,33 @@ func (l *ICMPv4) ToBytes() ([]byte, error) {
 	if l.Code != nil {
 		h.SetCode(*l.Code)
 	}
+	if copied := copy(h.Payload(), l.Payload); copied != len(l.Payload) {
+		panic(fmt.Sprintf("wrong number of bytes copied into h.Payload(): got = %d, want = %d", len(h.Payload()), len(l.Payload)))
+	}
+	if l.Ident != nil {
+		h.SetIdent(*l.Ident)
+	}
+	if l.Sequence != nil {
+		h.SetSequence(*l.Sequence)
+	}
+
+	// The checksum must be handled last because the ICMPv4 header fields are
+	// included in the computation.
 	if l.Checksum != nil {
 		h.SetChecksum(*l.Checksum)
-		return h, nil
+	} else {
+		// Compute the checksum based on the ICMPv4.Payload and also the subsequent
+		// layers.
+		payload, err := payload(l)
+		if err != nil {
+			return nil, err
+		}
+		var vv buffer.VectorisedView
+		vv.AppendView(buffer.View(l.Payload))
+		vv.Append(payload)
+		h.SetChecksum(header.ICMPv4Checksum(h, vv))
 	}
-	payload, err := payload(l)
-	if err != nil {
-		return nil, err
-	}
-	h.SetChecksum(header.ICMPv4Checksum(h, payload))
+
 	return h, nil
 }
 
@@ -915,8 +936,11 @@ func parseICMPv4(b []byte) (Layer, layerParser) {
 		Type:     ICMPv4Type(h.Type()),
 		Code:     ICMPv4Code(h.Code()),
 		Checksum: Uint16(h.Checksum()),
+		Ident:    Uint16(h.Ident()),
+		Sequence: Uint16(h.Sequence()),
+		Payload:  h.Payload(),
 	}
-	return &icmpv4, parsePayload
+	return &icmpv4, nil
 }
 
 func (l *ICMPv4) match(other Layer) bool {

test/packetimpact/tests/tcp_network_unreachable_test.go

@@ -74,7 +74,9 @@ func TestTCPSynSentUnreachable(t *testing.T) {
 	}
 	var icmpv4 testbench.ICMPv4 = testbench.ICMPv4{
 		Type: testbench.ICMPv4Type(header.ICMPv4DstUnreachable),
-		Code: testbench.ICMPv4Code(header.ICMPv4HostUnreachable)}
+		Code: testbench.ICMPv4Code(header.ICMPv4HostUnreachable),
+	}
+
 	layers = append(layers, &icmpv4, ip, tcp)
 	rawConn.SendFrameStateless(t, layers)