During an initial transfer, Aspera's FASP protocol has an internal mechanism that verifies that all the pieces of the file are written to the destination file before the transfer is deemed complete. When using encryption, each piece of the file is checked for integrity using an MD5 checksum.

If you perform a checksum operation on both copies of the file, you can ensure that they are identical. This test validates that the original file is the same as the destination file. You can download a tool called md5sum from http://www.etree.org/md5com.html

Simply copy this tool to both the source computer and the destination computer, and use the tool to check the files in question. From the command line, the syntax is:
md5sum filename

For example:
C:\Documents and Settings\aspera\>md5sum.exe g2m_dowload.exe
958066f0e0cb8b0631a0464a060ed6d5 *g2m_download.exe

The output is a 32-digit alphanumeric checksum. If you get the same checksum on both the original file and the destination file, you can be confident that they are identical.

If you are not using encryption, and if you are subject to an attack that changes contents of packets on the network, the Aspera receiver will not be able to tell. However, if you run encrypted transfers the corruption will be detected immediately and data will be retransmitted.

When a file is already at the destination or partially at the destination, Aspera performs one of the following verifications:

• File length and "last-modified" time
• Sparse MD5 checksum (a checksum of several samples in the file, not of the entire file)
• Full MD5 checksum (a checksum of the entire file) This option is set in the sync-conf.xml file.

The full checksum is CPU intensive and it may take as long as 30 seconds per GB.