A DICOM Echo Server (Verification SCP) tutorial demonstrating network communication fundamentals. This builds on Level 1 concepts and introduces DICOM networking.

Learning Objectives

By completing this tutorial, you will understand:

Server Configuration - AE Title, port, timeouts, PDU size
Association Management - Connection states, negotiation flow
Service Class Provider (SCP) - Handling incoming DIMSE requests
C-ECHO Operation - Verification service for connectivity testing
Graceful Shutdown - Signal handling and resource cleanup

Prerequisites

Completed Level 1: Hello DICOM
C++20 compatible compiler (GCC 11+, Clang 14+, MSVC 2022+)
pacs_system library built with tutorials enabled
DCMTK installed (for testing with echoscu)

Build & Run

# Configure with tutorials enabled
cmake -B build -DPACS_BUILD_SAMPLES=ON
 
# Build this tutorial
cmake --build build --target sample_02_echo_server
 
# Run the server (default port 11112)
./build/examples/echo_server
 
# Or specify a custom port
./build/examples/echo_server 11113

Testing

With the server running, open another terminal and test with DCMTK:

# Single C-ECHO test
echoscu -v -aec ECHO_SCP localhost 11112
 
# Multiple tests (stress test)
for i in {1..10}; do echoscu -aec ECHO_SCP localhost 11112; done
 
# Verbose output showing DIMSE details
echoscu -d -aec ECHO_SCP localhost 11112

Expected Output

Server side:

[14:30:22.456] [CONNECT] ECHOSCU -> ECHO_SCP (active: 1)

[14:30:22.458] [RELEASE] ECHOSCU disconnected (active: 0)

Client side (echoscu -v):

I: Requesting Association
I: Association Accepted
I: Sending Echo Request
I: Received Echo Response (Status: Success)
I: Releasing Association

Key Concepts Explained

DICOM Association

An association is a logical connection between two DICOM applications:

SCU (Client)                          SCP (Server)
    │                                     │
    │  A-ASSOCIATE-RQ ─────────────────►  │  Request association
    │                                     │
    │  ◄───────────────── A-ASSOCIATE-AC  │  Accept association
    │                                     │
    │  C-ECHO-RQ ──────────────────────►  │  Ping request
    │                                     │
    │  ◄────────────────────── C-ECHO-RSP │  Ping response
    │                                     │
    │  A-RELEASE-RQ ───────────────────►  │  Request release
    │                                     │
    │  ◄─────────────────── A-RELEASE-RP  │  Confirm release
    │                                     │

Server Configuration

Key parameters for DICOM servers:

Parameter	Description	Typical Value
AE Title	Application Entity identifier	16 chars max
Port	TCP listening port	104 (standard), 11112 (alternate)
Max PDU Size	Maximum data unit size	16384 - 65536 bytes
Idle Timeout	Disconnect after inactivity	30 - 300 seconds
Max Associations	Concurrent connection limit	10 - 100

Service Registration

// Create server with configuration
net::dicom_server server{config};
 
// Register Verification SCP (handles C-ECHO)
server.register_service(std::make_shared<svc::verification_scp>());
 
// You can register multiple services:
// server.register_service(std::make_shared<svc::storage_scp>(path));
// server.register_service(std::make_shared<svc::query_scp>(db));

Event Callbacks

// Monitor association lifecycle
server.on_association_established([](const net::association& assoc) {
    std::cout << "Connected: " << assoc.calling_ae() << "\n";
});
 
server.on_association_released([](const net::association& assoc) {
    std::cout << "Disconnected: " << assoc.calling_ae() << "\n";
});
 
server.on_error([](const std::string& error) {
    std::cerr << "Error: " << error << "\n";
});

Troubleshooting

Port Already in Use

# Find process using the port
lsof -i :11112
 
# Or use a different port
./build/examples/echo_server 11113

Connection Refused

Ensure the server is running
Check firewall settings
Verify the port number matches

AE Title Mismatch

DCMTK's echoscu uses -aec to specify the called (server) AE Title:

echoscu -aec ECHO_SCP localhost 11112

Next Steps

Proceed to Level 3: Storage Server to learn:

C-STORE operation for receiving DICOM images
File storage and organization
Transfer syntax handling
IOD (Information Object Definition) validation