Port 2775: SMPP — The Invisible Backbone of Global SMS

What Port 2775 Carries

Port 2775 is the standard TCP port for SMPP — Short Message Peer-to-Peer Protocol. It connects applications that want to send SMS messages to the message centers (SMSCs) that actually deliver them to mobile phones.

When a service sends you a verification code, that text almost certainly didn't come from a web server making a phone call. It came from an application connecting to an SMS gateway on port 2775, handing off a binary PDU, and waiting for an acknowledgment. The gateway takes it from there.

The Origin Story

SMPP wasn't designed to be infrastructure. It was designed to skip infrastructure.

In the early 1990s, an Irish company called Aldiscon was building SMS Message Centers. Developer Ian J. Chambers needed to test them — but the proper way to do that meant SS7 signaling equipment, which was expensive, complex, and slow to work with. So he built a simpler alternative: a lightweight protocol over TCP/IP that could talk to an SMSC directly, without the telco hardware.

That internal testing tool worked well enough that Aldiscon started including it in their products. Logica acquired Aldiscon, and in 1997 released the first public specification: SMPP v3.3. By 1999, the protocol had outgrown its authors — Logica handed it to the SMPP Developers Forum, which became the SMS Forum. SMPP v3.4 arrived that same year. Version 5, the current spec, in 2003.

The SMS Forum disbanded in 2007. By then it didn't matter much — SMPP was already everywhere.¹

How It Works

Despite the "peer-to-peer" in the name, SMPP is a client-server protocol. The SMSC listens on port 2775. Applications — called ESMEs (External Short Message Entities) — connect and bind.

That bind step is how a session announces its direction. Three options:

• bind_transmitter — the application can only submit messages outbound
• bind_receiver — the application can only receive incoming messages
• bind_transceiver — bidirectional; introduced in SMPP 3.4

Once bound, everything travels as binary PDUs (Protocol Data Units). Each PDU has a fixed 16-octet header — command length, command ID, status, and a sequence number for matching async requests to responses — followed by a variable body.

The design is deliberately simple. No XML, no JSON, no HTTP overhead. Binary, compact, fast. It was built for the throughput demands of SMS gateways routing millions of messages a day.

Security

SMPP has no built-in authentication beyond a shared system ID and password sent in the bind PDU. In plaintext. Over port 2775.

For most of SMPP's life this was considered acceptable because connections happened over private leased lines between trusted parties. Today, production deployments use SMPP over TLS — typically on port 3550 — or tunnel through a VPN. Running unencrypted SMPP over the public Internet is not a good idea.

Checking What's on Port 2775

On your own system:

# Linux/macOS - see what's listening
ss -tlnp | grep 2775
# or
lsof -i :2775

# Windows
netstat -ano | findstr :2775

If you see something on this port on a server, you're almost certainly running an SMS gateway or a service that talks to one.