Problem

Experiencing partial file downloads from a Globalscape EFT server via SFTP protocol. Files are reported as successful transfers, however the received file size is around 20MBs in size (when its real size is above 20MBs for example).

Resolution

We have a history of similar issues where SecureTransport could not pull entire files from a Globalscape EFT server. We have narrowed down the issue to be due to incorrect behavior of the Globalscape server when it handles more than 35 outstanding read requests.

When the number of outstanding read requests (those which the Globalscape server has not served) exceeds 35, the new requests that it receives remain unserved and no data is returned.

SecureTransport can send up to 100 outstanding read requests. This explains why we receive usually about 1/3 (~ 35 per cent) of the file we downloaded. When the file to be downloaded is 1.2 MB or smaller the limit of 35 outstanding read requests is not exceeded and the file is received without truncation (this way we can test the theory, by pulling a smaller than 1.2 MB file from the Globalscape EFT server). Files bigger than 1.2 MB are always truncated.

The experienced issue is due to a change in the Globalscape EFT application. Previously, SFTP users were allowed to request as much data as they wanted without increasing the TCP Window Size, however the change was made to only allow the user to request 1 byte without increasing the window size.

In order to fix this issue, the a change should be done on the GlobalScape EFT server, where a specific registry entry should be created in order to allow larger limit on the outstanding read requests.

The complete details about the change can be read in the following 3rd party articles:

1. GlobalScape KB

2. IBM KB