EnquireLink

EnquireLink is a simple, standard PDU used in the Short Message Peer-to-Peer (SMPP) protocol to test connectivity and keep sessions alive between an ESME (External Short Messaging Entity) and an SMSC (Short Message Service Center). It functions as a lightweight heartbeat to verify that the TCP connection and the SMPP session are still operational.

The Enquire Link PDU carries no user data or parameters. It consists of the fixed SMPP PDU header, which includes command_length, command_id (identifying the Enquire Link), command_status, and sequence_number. When an Enquire Link is received, the peer should respond with the corresponding Enquire Link response, typically indicating success (command_status OK) and echoing the sequence number of the request. The request and its response are designed to be lightweight and non-intrusive.

Usage and behavior: Enquire Link is generally sent periodically by either side of the connection to detect broken or stalled links and to prevent idle-timeouts from closing the session. The exact interval is implementation-dependent and may be tuned based on network reliability, latency, and organizational policies. If a response is not received within the configured timeout, the connection is usually considered broken, and reconciliation or re-establishment procedures may be initiated.

Implementation notes: Enquire Link should not be used for authentication or data transfer; it serves solely as a keep-alive mechanism. Some networks or intermediaries may affect keep-alives, so implementations often pair Enquire Link with appropriate timeouts and/or session management logic to avoid premature disconnects. It is part of the SMPP specification and is widely supported by both SMPP servers and clients.