how to resume an interrupted download - part 2

Question

This is a continuation of my previous question which I posted when I wasn\'t a registered user. As a refresher, I\'m trying to resume the downloading of a large file from m

Answer 1

I managed to implement http resume download on my client Java application, trick was to use "If-Range:original_ETag" request header as mentioned in an accepted answer. Here is a full header debug information it may help you understand how resume works.

Normal reply from server, file is read from the start to the end. ETag value is W/"filesize_bytes-modified_utc" metadata from the disk file. It could be something else but thats what Tomcat uses and usually is best for regular file content.

HTTP/1.1 200 OK 
ETag: W/"19097900-1410863319978" 
Last-Modified: Tue, 16 Sep 2014 10:28:39 GMT 
Content-Length: 19097900 
Content-Type: application/octet-stream 
Accept-Ranges: bytes 
Server: Apache-Coyote/1.1 
Date: Wed, 17 Sep 2014 10:39:52 GMT 

Client reads a given chunk from the file by adding Range and If-Range headers. Byte range can be open so server should read starting from given index to the end. Note Content-Length is not the total length of file but this chunk of bytes.

Range: bytes=10000000-
If-Range: W/"19097900-1410863319978"

HTTP/1.1 206 Partial Content 
ETag: W/"19097900-1410863319978" 
Last-Modified: Tue, 16 Sep 2014 10:28:39 GMT 
Content-Length: 9097900 
Content-Type: application/octet-stream 
Accept-Ranges: bytes 
Content-Range: bytes 10000000-19097899/19097900 
Server: Apache-Coyote/1.1 
Date: Wed, 17 Sep 2014 18:12:36 GMT 

If remote file was changed then server should not return 206-PartialContent but regular 200-OK reply. Server should use If-Range value to check for consistency. Some clients like VLCPlayer puts Range field without If-Range value.

When client is making an initial request without Range+If-Range headers it may add "If-Modified-Since: modified_http_date" header. Server may return 304-NotModified status and client is done. If client gives byteStartIdx>=filesize then server returns 416-RequestedRangeNotSatisfiable.

Additional debug dumps to clarify how http-resume works.

Range=bytes=0-0
If-Range=W/"19097900-1410863319978"

HTTP/1.1 206 Partial Content 
ETag=W/"19097900-1410863319978" 
Date=Fri, 19 Sep 2014 12:53:36 GMT 
Content-Length=1 
Last-Modified=Tue, 16 Sep 2014 10:28:39 GMT 
Content-Type=application/octet-stream 
Accept-Ranges=bytes 
Server=Apache-Coyote/1.1 
Content-Range=bytes 0-0/19097900 
- - - - -

Range=bytes=19097800-29097800
If-Range=W/"19097900-1410863319978"

HTTP/1.1 206 Partial Content 
ETag=W/"19097900-1410863319978" 
Date=Fri, 19 Sep 2014 12:59:24 GMT 
Content-Length=100 
Last-Modified=Tue, 16 Sep 2014 10:28:39 GMT 
Content-Type=application/octet-stream 
Accept-Ranges=bytes 
Server=Apache-Coyote/1.1 
Content-Range=bytes 19097800-19097899/19097900 
- - - - - - - - 

Range=bytes=19097899-19097899
If-Range=W/"19097900-1410863319978"

HTTP/1.1 206 Partial Content 
ETag=W/"19097900-1410863319978" 
Date=Fri, 19 Sep 2014 13:01:47 GMT 
Content-Length=1 
Last-Modified=Tue, 16 Sep 2014 10:28:39 GMT 
Content-Type=application/octet-stream 
Accept-Ranges=bytes 
Server=Apache-Coyote/1.1 
Content-Range=bytes 19097899-19097899/19097900