I have a desktop application that needs to upload files to a webserver.
The protocol is HTTPS.
I'm thinking I should write an ashx that will handle the uploads; sending 4k chunks at a time. With a response of ok each time. Is this a good algorithm?
What is the best algorithm for uploading from a desktop application to a webserver over HTTPS?
How large of files are you looking to upload? As long as they're not huge (> 100 MB), you could generate a multipart MIME encoded upload (RFC 1867) from your application. This is the same format browsers use for uploads, so your handler would just pull the file from the Request.Files collection. Chunking isn't going to get you much unless you're uploading on a slow connection or uploading huge files.
Chunking is likely to slow you down if you take the simplistic approach of sending the next packet only after the acknowledgement of the previous packet is received. This approach has proven to be slow (over networks with nontrivial latency). Try something that's directly supported by the webserver rather than rolling your own (such as the suggestion made by Chris Hynes: multipart MIME encoded upload).