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As stated by IANA when one wants to register a port number and a service name in the User Ports range, one of the requirements from RFC 6335 section 8.1.2 is to have a reason:

"...why using a port number in the Dynamic Ports range is unsuitable for the given application."

as I did not find any given examples for valid reasons I tried to come up with one.

The only reason I can think of is the convenience of expecting your port to be free but since no one is really prevented from using your registered port you can't be sure about that. Therefore one needs to have some mechanic in place to resort to a free port from the Dynamic Ports range anyway. Besides that I don't think convenience is a reason they would accept but since there does not seem to be a list of invalid/inadequate reasons I can't be sure about that either.

Does someone know of a list of example reasons or a single example that was accepted in the past or examples that are inadequate?

What are generally adequate reasons why using a port number in the Dynamic Ports range is unsuitable?

EN20
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  • A reason why is already explained in that section in the earlier bullet point: _"On the other hand, application software MUST NOT assume that a specific port number in the Dynamic Ports range will always be available for communication at all times, and a port number in that range hence MUST NOT be used as a service identifier."_. In other words, if you need a service identifier, then you can't use a dynamic port. In any case this question is not really on-topic for Stack Overflow, because it is basically soliciting opinions. – Mark Rotteveel Jan 12 '23 at 08:28
  • I see where you are coming from regarding soliciting opinions but my intend is to understand why a registerd port in the user ports range would be needed on a technical level since as I saied before, no one is prohibiting the miss use of a registerd port and therefore you could run into the same issue as with a Dynamic Port. – EN20 Jan 12 '23 at 08:39
  • But basically when I understand your quoted section right any application that intended use is to identify a service should have some kind of reasoning to become registerd. Would it be possible to have a general purpose service identificattion port that is free for everyone to use as long as they follow a given protocoll or is there even somthin similar? – EN20 Jan 12 '23 at 08:46
  • It is not really about a technical level. The user ports range is intended for use as service identifiers, e.g. an application uses a port for its entire lifetime to accept requests, while dynamic ports are allocated at random (e.g. for individual socket connections), so while availability of user ports is generally deterministic and stable given a certain set of applications, the same cannot be said for the dynamic port range. With the same set of active applications at point N a port might be available in the dynamic range, and at N+1 it might be in use, and at N+2 it can be available again – Mark Rotteveel Jan 12 '23 at 09:59

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