One of the references is the [rsync protocol][1] ([short overview][2]):

- Not directly validated by either ISO, OASIS, W3C, IETF(RFC), ECMA, IEC, but rsync has an [official TCP port][3] (873) and there is one RFC on the [rsync URI Scheme](https://www.rfc-editor.org/rfc/rfc5781). (note that an ISO standard is typically charged [160 USD][4] to be accessed to...).
- Plenty of clients and servers, the reference implementation being the [rsync](http://rsync.samba.org/) application.
- File synchronization,  but [no conflict handling][5] 
- Bandwidth as low as possible:

[The rsync algorithm](http://rsync.samba.org/tech_report/):

> 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.


  [1]: http://rsync.samba.org/how-rsync-works.html
  [2]: http://tutorials.jenkov.com/rsync/network-protocol.html
  [3]: https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
  [4]: https://math.stackexchange.com/q/658965/24265
  [5]: http://doc.opensuse.org/products/draft/SLES/SLES-admin_sd_draft/cha.net.sync.html