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There are many Dropbox alternatives using their own protocols (Owncloud, Seafile, etc) or a de-facto protocol (SparkleShare, etc).
How about alternatives using a rock-solid standard protocol?

Requirements:

  • For each protocol, list at least 1 server and 1 client, preferably open source, any operating system
  • The protocol must have been validated by either ISO, OASIS, W3C, IETF(RFC), ECMA, IEC
  • A protocol is not made standard just because it uses HTTP/XML/SSH/etc
  • File synchronization, conflict handling
  • Bandwidth as low as possible
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I guess SparkelShare using git as its backend also falls short of your “validated protocol” requirement even if it operates over ssh. –  MvG Apr 18 at 13:06
    
I fail to see the point of this. You want low bandwidth, reliable file synchronization with conflict handling that has open source implementations yet rule out anything based on git? Also ownCloud supports WebDAV which fits your criteria but does not play nearly as well with your other requirements as the osync backend which does not. –  Caleb May 16 at 5:31

3 Answers 3

FTP is a file transfer protocol validated by IETF as RFC959.

FTPbox works quite well, but the FTP protocol is not very suited to file synchronization:

  • No "changes" command, so FTPbox has to crawl the FTP server once in a while, which wastes bandwidth
  • No "move" command, so moving a file or directory is very inefficient.
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One of the references is the rsync protocol (short overview):

  • Not directly validated by either ISO, OASIS, W3C, IETF(RFC), ECMA, IEC, but rsync has an official TCP port (873) and there is one RFC on the rsync URI Scheme. (note that an ISO standard is typically charged 160 USD to be accessed to...).
  • Plenty of clients and servers, the reference implementation being the rsync application.
  • File synchronization, but no conflict handling
  • Bandwidth as low as possible:

The rsync algorithm:

The algorithm identifies parts of the source file which are identical to some part of the destination file, and only sends those parts which cannot be matched in this way. Effectively, the algorithm computes a set of differences without having both files on the same machine. The algorithm works best when the files are similar, but will also function correctly and reasonably efficiently when the files are quite different.

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Thanks but in this question it is mandatory that the protocol itself (with all verbs and structures) is a standard, not just the URI scheme. rsync has no specification, and only 1 popular implementation. That makes it very fragile, in 20 years the protocol will probably have silently changed and different versions will be incompatible. This is what I must avoid. ISO is expensive/old-school but W3C and OASIS are great and free to access. –  Nicolas Raoul Apr 17 at 1:52
    
@NicolasRaoul I understand what you mean. rsync is the most widely used as far as I know, that's why I felt the need to mention it. I added the recently published and as of now pretty unused WebDAV sync (but WebDAV is older and often used). –  Franck Dernoncourt Apr 17 at 2:23

You can use WebDAV, which is defined in RFC 4918 and was recently extended to support synchronization in RFC 6578.

The only application I have personally used that relied on WebDAV RFC 4918 was French OVH's online backup service Hubic, but it was total crap (server issues, not protocol issues) and the official forum got so much flooded by negative comments that OVH's CEO Octave Klaba decided to remove all derogatory comments (including 500 of mine, and thousands of others').

Anyway, some other applications seem to use WebDAV RFC 4918 such as BestSync, but I don't know who use RFC 6578, maybe Mavericks Server Admin or in the future Firefox OS's Gaia::Calendar.

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+1 Great protocol indeed, an example of longevity. I had a look at re.be/webdav_sync a very long time ago. Syncany can use WebDAV but it stores opaque blobs so it is an edge case. –  Nicolas Raoul Apr 17 at 4:12
    
ownCloud actually supports WebDAV (you can use any WebDAV client to access your files or even mount it as a drive). It's probably worth mentioning this here. –  Caleb May 16 at 5:25

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