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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
    Commented Apr 18, 2014 at 13:06
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    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
    Commented May 16, 2014 at 5:31
  • @Caleb: Git is not well suited for managing large binaries, because it wastes time trying to compress them, and it requires, at the very least, 2X + delta space. There are hacks to address that, but they are even more brittle. standardsinsight.com/ieee_company_detail/… I am looking for 40-year perennity here. Git is "de facto" and might change drastically within 20 years. That's my requirement, sorry.
    – Nicolas Raoul ♦
    Commented Sep 1, 2014 at 3:03

4 Answers 4

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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 ♦
    Commented Apr 17, 2014 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
    Commented Apr 17, 2014 at 2:23
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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 ♦
    Commented Apr 17, 2014 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
    Commented May 16, 2014 at 5:25
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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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  • Correct, FTP does not handle conflict resolution at all. Also, standard RFC959 protocol is not well suited for anything other than complete file transfers (well, you could use SIZE/REST to do partial file transfers but that get's chatty with multiple data channel connects & disconnects). Also, FTP in general is quite chatty, having to do port/pasv negotiation for data transfers and directory listings)
    – WebDrive
    Commented Feb 28, 2015 at 22:24
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Are you looking for a cloud-based service, or an on-premise solution? There are a few options under both configurations.

On premise, we offer a server solution called Cornerstone that supports WebDAV, SFTP and http/s. You can share files as links, much like Dropbox, or you can use the WebDrive client to map a network drive letter to the Cornerstone Server through WebDAV. This will give you file locking, offline access and syncing, and the ability for a group of users to work directly from a shared folder.

For cloud-based solutions, there’s a Cornerstone AMI that lets you run Cornerstone in an Amazon EC2 cloud. Again, you would use WebDrive as the client, or the Web Interface that’s included with the Cornerstone server.

If you don’t want to host your own server in EC2, we have a lot of customers that have had success with FilesAnywhere, which is WebDAV-based. FilesAnywhere offers their own browser-based client that you can use, or they recommend that their customers use WebDrive, which works very nicely with their service.

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  • on-premise. Is WebDrive compatible with ANY WebDAV server? Is FilesAnywhere compatible with ANY WebDAV server?
    – Nicolas Raoul ♦
    Commented Feb 27, 2015 at 15:49
  • @NicolasRaoul, Yes, WebDrive implements the WebDAV RFC so it will be compatible with any RFC compliant WebDAV Server. WRT FilesAnyWhere, it's an online storage service which has a WebDAV interface for connecting a WebDAV client to it, but check their website, WebDAV support does not appear to be free, it's included in their $$ plans.
    – WebDrive
    Commented Mar 2, 2015 at 11:37

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