Port 3550: Secure SMPP — Encrypted SMS in Transit

What This Port Carries

Port 3550 is the default port for SSMPP — Secure SMPP (Short Message Peer-to-Peer over TLS). SMPP is the protocol that telecom operators and SMS service providers use to route text messages between systems: from an application, through an SMS gateway, to a carrier's Short Message Service Center (SMSC), and out to your phone.

Port 2775 does this in plaintext. Port 3550 does it encrypted.

The two ports work as a pair. When an SMPP client connects to a gateway and doesn't specify a port, it defaults to 2775 for unencrypted sessions and 3550 for TLS-wrapped ones.¹

The Protocol Behind It

SMPP was designed in the early 1990s by Aldiscon, an Irish telecom company, to give applications a standardized way to talk to SMS centers. It's a binary protocol — compact, fast, built for high-volume message throughput. An SMS aggregator sending a million messages a day doesn't want HTTP overhead.

The protocol defines a set of PDUs (Protocol Data Units): bind_transmitter, submit_sm, deliver_sm, unbind. An SMPP client binds to the SMSC, submits messages, receives delivery receipts, and unbinds when done. It's more like a persistent session than a request-response cycle.

Secure SMPP wraps this binary exchange in TLS. The session is identical — same PDUs, same flow — just encrypted in transit. This protects:

• Authentication credentials (system ID and password sent during bind)
• Message content
• Delivery receipts and status updates

Who Uses Port 3550

SMS aggregators, telecom operators, and enterprise messaging platforms. Any system that sends transactional SMS — two-factor authentication codes, delivery notifications, appointment reminders — likely touches SMPP somewhere in the chain. Whether it uses port 2775 or 3550 depends on whether the gateway supports TLS and whether the operator enforces it.

Port 3550 also appears in GeoVision surveillance systems, which use it for RemoteView remote camera access.² This is an entirely separate, unrelated use — a reminder that port assignments don't always prevent collisions between applications.

Checking What's on This Port

If you see traffic on port 3550 and want to know what's running:

# See what process is listening on port 3550
sudo lsof -i :3550

# Or with ss (Linux)
sudo ss -tlnp | grep 3550

# Or with netstat
sudo netstat -tlnp | grep 3550

If you're watching network traffic rather than a local process:

# Capture traffic on port 3550
sudo tcpdump -n port 3550

If the connection is TLS (which SSMPP should be), the captured payload will be encrypted. You'll see the TLS handshake but not the message content — which is the point.

Why Unassigned Ports Matter

The registered port range runs from 1024 to 49151. IANA maintains the registry, but registration is voluntary and not perfectly enforced. Port 3550's SSMPP association appears in multiple port databases and SMPP client documentation as a de facto standard, even where formal IANA assignment records are ambiguous.

This happens throughout the registered range. Applications pick ports, document them, publish them, and the port becomes "theirs" through convention rather than formal process. The registry captures the official record; convention fills the gaps.