What This Port Is
Port 3687 sits in the registered port range (1024–49151), the middle tier of the port numbering system. Well-known ports (0–1023) are reserved for foundational protocols — HTTP, SSH, DNS. Registered ports are where organizations and developers stake claims for their own services. The range is governed by IANA, which maintains the official registry of what's supposed to run where.
Port 3687 has an entry in that registry. IANA lists it for both TCP and UDP under the name simple-push. Port 3688 is listed as simple-push-s — the secure variant.1
The Problem: The Protocol Was Never Published
That's where the trail goes cold.
"simple-push" has no RFC. No published specification. No working group, no mailing list archive, no GitHub repository, no documentation that explains what the protocol does or how it works. The IANA entry provides a service name and transport protocol — and nothing else.
This happens more than you'd expect. The IANA registration process requires a name and a contact; it doesn't require a finished protocol. Someone reserved ports 3687 and 3688 for a push notification system they intended to build — or did build, privately — and the reservation made it into the official record while the documentation didn't.
The result: a port with a name and no instructions.
What You'll Actually Find on Port 3687
In practice, if you see traffic on port 3687, it's almost certainly not "simple-push." It's more likely:
- An application that picked a high-numbered port because it seemed unused
- A development server, proxy, or internal service that chose this port arbitrarily
- A game, IoT device, or proprietary tool using it as a convenience
The registered name carries no enforcement. Any application can listen on any unblocked port — the registry is a coordination mechanism, not a lock.
How to Check What's Listening
If you see port 3687 active on your system and want to know what's using it:
On macOS or Linux:
On Windows:
Then match the PID to a process name using Task Manager or ps.
On Linux (alternative):
Why Registered-But-Undocumented Ports Matter
The registered port range exists to prevent collisions — two applications accidentally choosing the same port and fighting over it. When a protocol gets registered without documentation, it occupies that coordination space without contributing to it. Other developers don't know what's reserved or why, and the value of the registry erodes a little.
It's not a crisis. There are 48,127 registered ports and plenty of room. But it's a reminder that the port registry is a human system, maintained by humans, with all the incompleteness that implies.
If you expected something to be running on 3687 and nothing is, that's probably correct. This port is effectively unoccupied.
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