add a type for arbitrary length Connection IDs, and parsing function

RFC 8999 allows Connection IDs of up to 255 bytes. Current QUIC versions
only use up to 20 bytes.

internal/wire/header.go

@@ -35,6 +35,44 @@ func ParseConnectionID(data []byte, shortHeaderConnIDLen int) (protocol.Connecti
 	return protocol.ConnectionID(data[6 : 6+destConnIDLen]), nil
 }
 
+// ParseArbitraryLenConnectionIDs parses the most general form of a Long Header packet,
+// using only the version-independent packet format as described in Section 5.1 of RFC 8999:
+// https://datatracker.ietf.org/doc/html/rfc8999#section-5.1.
+// This function should only be called on Long Header packets for which we don't support the version.
+func ParseArbitraryLenConnectionIDs(data []byte) (bytesParsed int, dest, src protocol.ArbitraryLenConnectionID, _ error) {
+	r := bytes.NewReader(data)
+	remaining := r.Len()
+	src, dest, err := parseArbitraryLenConnectionIDs(r)
+	return remaining - r.Len(), src, dest, err
+}
+
+func parseArbitraryLenConnectionIDs(r *bytes.Reader) (dest, src protocol.ArbitraryLenConnectionID, _ error) {
+	r.Seek(5, io.SeekStart) // skip first byte and version field
+	destConnIDLen, err := r.ReadByte()
+	if err != nil {
+		return nil, nil, err
+	}
+	destConnID := make(protocol.ArbitraryLenConnectionID, destConnIDLen)
+	if _, err := io.ReadFull(r, destConnID); err != nil {
+		if err == io.ErrUnexpectedEOF {
+			err = io.EOF
+		}
+		return nil, nil, err
+	}
+	srcConnIDLen, err := r.ReadByte()
+	if err != nil {
+		return nil, nil, err
+	}
+	srcConnID := make(protocol.ArbitraryLenConnectionID, srcConnIDLen)
+	if _, err := io.ReadFull(r, srcConnID); err != nil {
+		if err == io.ErrUnexpectedEOF {
+			err = io.EOF
+		}
+		return nil, nil, err
+	}
+	return destConnID, srcConnID, nil
+}
+
 // IsLongHeaderPacket says if this is a Long Header packet
 func IsLongHeaderPacket(firstByte byte) bool {
 	return firstByte&0x80 > 0