Added class to URLs in the response

htsget.md

@@ -162,6 +162,13 @@ The server SHOULD reply with an `UnsupportedFormat` error if the requested forma
 [^a]
 </td></tr>
 <tr markdown="block"><td>
+`class`  
+_optional string_
+</td><td>
+Allows clients to request the data header only, using `class=header`. It is not possible to return the data body only, because that would not constitute a valid response in any of the supported formats.
+Default: all
+</td></tr>
+<tr markdown="block"><td>
 `referenceName` 
 _optional_
 </td><td>
@@ -280,6 +287,14 @@ _optional object_
 </td><td>
 For HTTPS URLs, the server may supply a JSON object containing one or more string key-value pairs which the client MUST supply as headers with any request to the URL. For example, if headers is `{"Range": "bytes=0-1023", "Authorization": "Bearer xxxx"}`, then the client must supply the headers `Range: bytes=0-1023` and `Authorization: Bearer xxxx` with the HTTPS request to the URL.
 </td></tr>
+<tr markdown="block"><td>
+`class`
+_string_
+</td><td>
+For file formats whose specification describes a header and a body, the class indicates which of the two will be retrieved when querying this URL. The allowed values are `header` and `body`.
+
+Either all or none of the URLs in the response MUST have a class attribute. If it is absent, clients should assume data blocks include both header and body, possibly mixed in one of the blocks.
+</td></tr>
 </table>
 
 </td></tr>
@@ -300,24 +315,28 @@ An example of a JSON response is:
       "format" : "BAM",
       "urls" : [
          {
-            "url" : "data:application/vnd.ga4gh.bam;base64,QkFNAQ=="
+            "url" : "data:application/vnd.ga4gh.bam;base64,QkFNAQ==",
+            "class" : "header"
          },
          {
-            "url" : "https://htsget.blocksrv.example/sample1234/header"
+            "url" : "https://htsget.blocksrv.example/sample1234/header",
+            "class" : "header"
          },
          {
             "url" : "https://htsget.blocksrv.example/sample1234/run1.bam",
             "headers" : {
                "Authorization" : "Bearer xxxx",
                "Range" : "bytes=65536-1003750"
-             }
+             },
+            "class" : "body"
          },
          {
             "url" : "https://htsget.blocksrv.example/sample1234/run1.bam",
             "headers" : {
                "Authorization" : "Bearer xxxx",
                "Range" : "bytes=2744831-9375732"
-            }
+            },
+            "class" : "body"
          }
       ]
    }