Port 2737: SRP Feedback — A Registered Port with a Quiet History

What Port 2737 Is

Port 2737 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 services like HTTP, SSH, and DNS. Dynamic ports (49152–65535) are handed out temporarily to client applications. Registered ports are the middle ground: officially catalogued by IANA, historically associated with specific applications, but open to other uses if no registered service is running.

This port has no current active assignment in the IANA registry.¹ Older port databases list it under the name srp-feedback, pointing to Cisco's Spatial Reuse Protocol.

The "SRP Feedback" Story

Spatial Reuse Protocol (SRP) was a MAC-layer technology Cisco developed for ring-based fiber optic networks in the late 1990s, formalized in RFC 2892 in 2000.² It ran on Cisco's 12000 series routers using Dynamic Packet Transport (DPT) line cards — a technology designed to carry IP traffic efficiently over SONET/SDH rings by letting multiple nodes reuse bandwidth around the ring simultaneously.

The "feedback" part of SRP was how nodes communicated congestion information to their neighbors, allowing fair bandwidth distribution around the ring. Port 2737 appears in legacy sources as the TCP endpoint for that feedback mechanism.

Whether it was ever widely deployed as a TCP service — rather than operating purely at the MAC layer — is unclear. Cisco's SRP/DPT technology itself never became dominant; it was largely eclipsed by MPLS and Ethernet-based optical transport, and the Cisco 12000 DPT platform has since been discontinued.³

The result: a port name that shows up in port databases compiled years ago, tied to hardware that's mostly been decommissioned.

Security Posture

No known malware or trojans are currently associated with port 2737.⁴ Because it has no active registered service, any process listening here is application-specific — which is either intentional (a developer chose it arbitrarily, or a niche application uses it) or unexpected (worth investigating).

How to Check What's Using This Port

If you see traffic on port 2737 and want to know what's behind it:

On Linux/macOS:

# Show what process is listening on port 2737
sudo ss -tlnp | grep 2737

# Or with lsof
sudo lsof -i :2737

On Windows:

netstat -ano | findstr :2737

The PID in the last column can be matched to a process in Task Manager or with tasklist.

With nmap (from another host):

nmap -sV -p 2737 <target-ip>

Why Unassigned Ports Matter

The registered port range contains 48,127 possible ports. Many have active assignments — databases, application servers, monitoring tools, developer frameworks. But many others are like port 2737: listed under names that describe software nobody runs anymore, or never formally assigned at all.

This matters for two reasons. First, security scanners use port databases to identify services — a process running on an unassigned port won't be auto-identified, so unexpected activity here deserves scrutiny. Second, when developers need a port for a new application and don't want to conflict with anything well-known, unassigned registered ports are the right place to look.

Port 2737 isn't dangerous. It's just quiet — a numbered door in a long corridor, holding a name from a chapter of networking history that most networks have closed.