Unmarshal packets only once

Unmarshal of a packet should only be done once, since doing
it multiple times ends up in an incorrect list of chunks.
Also, perform checksum validation first.
For pion, the only chunks requiring a correct CRC32c are the
INIT and COOKIE-ECHO chunks, which need to be the first
chunk in a packet.

association.go

@@ -645,22 +645,9 @@ func (a *Association) marshalPacket(p *packet) ([]byte, error) {
 
 func (a *Association) unmarshalPacket(raw []byte) (*packet, error) {
 	p := &packet{}
-	if !a.useZeroChecksum {
-		if err := p.unmarshal(true, raw); err != nil {
-			return nil, err
-		}
-		return p, nil
-	}
-
-	if err := p.unmarshal(false, raw); err != nil {
+	if err := p.unmarshal(!a.useZeroChecksum, raw); err != nil {
 		return nil, err
 	}
-	if chunkMandatoryChecksum(p.chunks) {
-		if err := p.unmarshal(true, raw); err != nil {
-			return nil, err
-		}
-	}
-
 	return p, nil
 }
 

packet.go

@@ -70,11 +70,30 @@ func (p *packet) unmarshal(doChecksum bool, raw []byte) error {
 		return fmt.Errorf("%w: raw only %d bytes, %d is the minimum length", ErrPacketRawTooSmall, len(raw), packetHeaderSize)
 	}
 
+	offset := packetHeaderSize
+
+	// Check if doing CRC32c is required.
+	// Without having SCTP AUTH implemented, this depends only on the type
+	// og the first chunk.
+	if offset+chunkHeaderSize <= len(raw) {
+		switch chunkType(raw[offset]) {
+		case ctInit, ctCookieEcho:
+			doChecksum = true
+		default:
+		}
+	}
+	theirChecksum := binary.LittleEndian.Uint32(raw[8:])
+	if theirChecksum != 0 || doChecksum {
+		ourChecksum := generatePacketChecksum(raw)
+		if theirChecksum != ourChecksum {
+			return fmt.Errorf("%w: %d ours: %d", ErrChecksumMismatch, theirChecksum, ourChecksum)
+		}
+	}
+
 	p.sourcePort = binary.BigEndian.Uint16(raw[0:])
 	p.destinationPort = binary.BigEndian.Uint16(raw[2:])
 	p.verificationTag = binary.BigEndian.Uint32(raw[4:])
 
-	offset := packetHeaderSize
 	for {
 		// Exact match, no more chunks
 		if offset == len(raw) {
@@ -126,14 +145,6 @@ func (p *packet) unmarshal(doChecksum bool, raw []byte) error {
 		offset += chunkHeaderSize + c.valueLength() + chunkValuePadding
 	}
 
-	theirChecksum := binary.LittleEndian.Uint32(raw[8:])
-	if theirChecksum != 0 || doChecksum {
-		ourChecksum := generatePacketChecksum(raw)
-		if theirChecksum != ourChecksum {
-			return fmt.Errorf("%w: %d ours: %d", ErrChecksumMismatch, theirChecksum, ourChecksum)
-		}
-	}
-
 	return nil
 }