fix backwards compatibility

src/tags/media_segment/map.rs

@@ -138,7 +138,12 @@ impl Encrypted for ExtXMap {
 
 /// This tag requires [`ProtocolVersion::V6`].
 impl RequiredVersion for ExtXMap {
+    // this should return ProtocolVersion::V5, if it does not contain an
+    // EXT-X-I-FRAMES-ONLY!
+    // http://alexzambelli.com/blog/2016/05/04/understanding-hls-versions-and-client-compatibility/
     fn required_version(&self) -> ProtocolVersion { ProtocolVersion::V6 }
+
+    fn introduced_version(&self) -> ProtocolVersion { ProtocolVersion::V5 }
 }
 
 impl fmt::Display for ExtXMap {

src/traits.rs

@@ -120,7 +120,14 @@ pub trait Encrypted {
 /// ```
 pub trait RequiredVersion {
     /// Returns the protocol compatibility version that this tag requires.
+    ///
+    /// # Note
+    /// This is for the latest working [`ProtocolVersion`] and a client, that
+    /// only supports an older version would break.
     fn required_version(&self) -> ProtocolVersion;
+
+    /// The protocol version, in which the tag has been introduced.
+    fn introduced_version(&self) -> ProtocolVersion { self.required_version() }
 }
 
 impl<T: RequiredVersion> RequiredVersion for Vec<T> {

src/types/decryption_key.rs

@@ -482,6 +482,38 @@ mod test {
             ProtocolVersion::V1
         );
 
+        assert_eq!(
+            DecryptionKey::builder()
+                .method(EncryptionMethod::Aes128)
+                .uri("https://www.example.com/")
+                .key_format(KeyFormat::Identity)
+                .key_format_versions(vec![1, 2, 3])
+                .build()
+                .unwrap()
+                .required_version(),
+            ProtocolVersion::V1
+        );
+
+        assert_eq!(
+            DecryptionKey::builder()
+                .method(EncryptionMethod::Aes128)
+                .uri("https://www.example.com/")
+                .iv([1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7])
+                .build()
+                .unwrap()
+                .required_version(),
+            ProtocolVersion::V2
+        );
+    }
+
+    #[test]
+    fn test_introduced_version() {
+        assert_eq!(
+            DecryptionKey::new(EncryptionMethod::Aes128, "https://www.example.com/")
+                .required_version(),
+            ProtocolVersion::V1
+        );
+
         assert_eq!(
             DecryptionKey::builder()
                 .method(EncryptionMethod::Aes128)