NEW-PROTOCOL: document what needs to be done to add one

Closes #6263

docs/Makefile.am

@@ -74,6 +74,7 @@ EXTRA_DIST =                                    \
  KNOWN_BUGS                                     \
  MAIL-ETIQUETTE                                 \
  MQTT.md                                        \
+ NEW-PROTOCOL.md                                \
  options-in-versions                            \
  PARALLEL-TRANSFERS.md                          \
  README.md                                      \

docs/NEW-PROTOCOL.md

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+# Adding a new protocol?
+
+Every once in a while someone comes up with the idea of adding support for yet
+another protocol to curl. After all, curl already supports 25 something
+protocols and it is the Internet transfer machine for the world.
+
+In the curl project we love protocols and we love supporting many protocols
+and do it well.
+
+So how do you proceed to add a new protocol and what are the requirements?
+
+## No fixed set of requirements
+
+This document is an attempt to describe things to consider. There is no
+checklist of the twenty-seven things you need to cross off. We view the entire
+effort as a whole and then judge if it seems to be the right thing - for
+now. The more things that look right, fit our patterns and are done in ways
+that align with our thinking, the better are the chances that we will agree
+that supporting this protocol is a grand idea.
+
+## Mutual benefit is preferred
+
+curl is not here for your protocol. Your protocol is not here for curl. The
+best cooperation and end result occur when all involved parties mutually see
+and agree that supporting this protocol in curl would be good for everyone.
+Heck, for the world!
+
+Consider "selling us" the idea that we need an implementation merged in curl,
+to be fairly important. *Why* do we want curl to support this new protocol?
+
+## Protocol requirements
+
+### Client-side
+
+The protocol implementation is for a client's side of a "communication
+session".
+
+### Transfer oriented
+
+The protocol itself should be focused on *transfers*. Be it uploads or
+downloads or both. I should at least be possible to view the transfers as
+such, like we can view reading emails over POP3 as a downloading and sending
+emails over SMTP as an upload.
+
+If you cannot even shoehorn the protocol into a transfer focused view, then
+you are up for a tough argument.
+
+### URL
+
+There should be a documented URL format. If there is an RFC for it there is no
+question about it but the syntax doesn't have to be a published RFC. It could
+be enough if it is already in use by other implementations.
+
+If you make up the syntax just in order to be able to propose it to curl, then
+you are in a bad place. URLs are designed and defined for interoperability.
+There should at least be a good chance that other clients and servers can be
+implemented supporting the same URL syntax and work the same or similar way.
+
+URLs work on registered 'schemes'. There is a register of [all officially
+recognized
+schemes](https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml). If
+your protocol is not in there, is it really a protocol we want?
+
+### Wide and public use
+
+The protocol shall already be used or have an expectation of getting used
+widely. Experimental protocols are better off worked on in experiments first,
+to prove themselves before they are adopted by curl.
+
+## Code
+
+Of course the code needs to be written, provided, licensed agreeably and it
+should follow our code guidelines and review comments have to be dealt with.
+If the implementation needs third party code, that third party code should not
+have noticeably lesser standards than the curl project itself.
+
+## Tests
+
+As much of the protocol implementation as possible needs to be verified by
+curl test cases. We must have the implementation get tested by CI jobs,
+torture tests and more.
+
+We've experienced many times in the past how new implementations were brought
+to curl and immediately once the code had been merged, the originator vanished
+from the face of the earth. That is fine, but we need to take the necessary
+precautions so when it happens we are still fine.
+
+Our test infrastructure is powerful enough to test just about every possible
+protocol - but it might require a bit of an effort to make it happen.
+
+## Documentation
+
+We cannot assume that users are particularly familiar with specific details
+and peculiarities of the protocol. It needs documentation.
+
+Maybe it even needs some internal documentation so that the developers who
+will try to debug something five years from now can figure out functionality a
+little easier!
+
+The protocol specification itself should be freely available without requiring
+any NDA or similar.
+
+## Don't compare
+
+We are constantly raising the bar and we are constantly improving the
+project. A lot of things we did in the past would not be acceptable if done
+today.  Therefore, you might be tempted to use shortcuts or "hacks" you can
+spot other - existing - protocol implementations have used, but there is
+nothing to gain from that. The bar has been raised. Former "cheats" won't be
+tolerated anymore.