Port 1321: PIP — Before presence was everywhere

Port 1321 carries PIP (Presence Information Protocol), an experimental protocol created in 1998 to enable presence information sharing and instant messaging across the Internet.

What PIP Does

Before Slack's green dots, Discord's status indicators, or WhatsApp's "last seen," someone had to figure out the basic question: how do you tell the Internet whether you're available to talk?

PIP was one of the early attempts. It provided:

• Presence broadcasting — Advertise whether you're online, away, or busy using XML-based status documents
• Instant messaging — Send simple text messages directly to someone's "instant inbox"
• Subscription system — Subscribe to someone's presence so you get updates when their status changes
• Lease-based updates — Status declarations expire automatically, so if you disconnect, your presence doesn't stay "online" forever

The protocol used HTTP/1.1-style message formatting and ran on port 1321 for receiving presence notifications and subscription requests.¹

The Problem It Tried to Solve

In August 1998, millions of people were online, but there was no standard way to answer two basic questions:

1. Is this person available right now?
2. Can I send them a quick message?

Various proprietary systems existed—ICQ, AIM, IRC—but they didn't talk to each other. PIP aimed to create an open, interoperable protocol for presence and messaging that would work across different platforms.¹

The protocol specification itself is refreshingly honest about its limitations. The author, Gordon Mohr, explicitly stated that PIP "makes NO attempt to be: (1) Secure (2) Scalable/efficient; or (3) Rigorously specified."¹ It was a testbed for ideas, not a production system.

Why PIP Didn't Become Widespread

PIP remained an experimental protocol that never saw widespread deployment. The problem it tried to solve was real—presence and instant messaging became fundamental to Internet communication—but different solutions won.

Proprietary systems like AIM, MSN Messenger, and later Skype dominated the instant messaging landscape through the 2000s. Eventually, standards like XMPP (Jabber) emerged for presence and messaging, and modern applications built their own presence systems on top of HTTP and WebSocket protocols.

But the core insight—that people need to know who's available and be able to reach them quickly—became ubiquitous. Every communication platform now has some form of presence indicator. The green dot is everywhere.

Checking for PIP

Port 1321 is officially assigned to PIP, but you're unlikely to encounter it in modern networks. To check if anything is listening on port 1321 on your system:

Linux/macOS:

sudo lsof -i :1321
# or
sudo netstat -tulpn | grep 1321

Windows:

netstat -ano | findstr :1321

If something appears, it's probably not PIP—it's more likely a modern application that happens to use the same port number for unrelated purposes.

The Port Range

Port 1321 belongs to the registered ports range (1024-49151). These ports are assigned by IANA to specific services upon application by a requesting entity. Unlike well-known ports (0-1023) which require root/administrator privileges to bind to, registered ports can be used by regular user applications.

Being in this range means PIP could be deployed by normal users without requiring special system permissions—fitting for an experimental protocol meant to be tested and iterated on.

What PIP Represents

PIP is a time capsule from the late 1990s Internet, when fundamental questions about online communication were still being worked out. The protocol itself never became important, but the problem it addressed—presence and instant messaging—became central to how we use the Internet.

Port 1321 is assigned, but largely unused. The protocol registered here was honest about being experimental, which makes it more interesting than the many protocols that promised production readiness but never delivered.