How to configure the SMPP Replace if Present Field

What is the Replace If Present Field?

The Replace If Present field in SMPP is a 1-byte flag in PDUs like submit_sm that instructs the SMSC to replace an existing message stored on the recipient's device or SMSC queue. Key applications include:

  • Updating time-sensitive content (e.g., OTPs, status alerts)
  • Preventing duplicate messages
  • Managing device storage constraints

Field Values and Interpretation

The field uses a simple boolean-like structure:

Value (Hex) Description
0x00 Do NOT replace existing messages (default)
0x01 Replace existing messages if present
Note: Message replacement logic depends on SMSC implementation. Common matching criteria include:
  • Source/Destination addresses
  • Message ID (if provided)
  • Protocol ID (PID=0x41 often triggers replacement)

Use Cases

1. OTP Message Update

replace_if_present: 0x01  // Replace prior OTP message
PID: 0x41                 // Protocol ID for replacement
payload: "Your new OTP: 5678"

2. Campaign Message Refresh

replace_if_present: 0x01  // Update promotional content
PID: 0x00                 // Default protocol
payload: "Flash Sale: New prices!"

3. Queue Management

replace_if_present: 0x01  // Overwrite queued message
scheduled_delivery_time: (future timestamp)

Example SMPP PDUs

Example 1: Replacement Disabled (0x00)

0000001D  // Command Length (29 bytes)
00000004  // Command ID (SubmitSM)
00000001  // Sequence Number
...
00        // Replace If Present (0x00: Disabled)
...
07        // SM Length (7 septets)
C8329BFD06DDDF72  // Payload ("Hello!")

Example 2: Replacement Enabled (0x01)

0000001D  // Command Length (29 bytes)
00000004  // Command ID (SubmitSM)
00000002  // Sequence Number
...
01        // Replace If Present (0x01: Enabled)
...
07        // SM Length (7 septets)
E8329BFD0E...  // Payload ("Updated: 3 PM")

Example 3: Combined with PID=0x41

0000001D  // Command Length (29 bytes)
00000004  // Command ID (SubmitSM)
00000003  // Sequence Number
...
41        // Protocol ID (PID=0x41: Replace)
01        // Replace If Present (0x01)
...
07        // SM Length (7 septets)
C8329BFD06...  // Payload ("OTP: 9876")

Interactions with Other Fields

  • Protocol ID (PID): PID=0x41 explicitly triggers replacement in many SMSCs
  • message_id: Some SMSCs use this to identify messages for replacement
  • registered_delivery: Receipts indicate whether replacement succeeded

Common Pitfalls

  • Enabling replacement without SMSC support
  • Not setting message_id for targeted replacement
  • Assuming all SMSCs use the same replacement logic
  • Mismatching PID and replace_if_present values
SMSC Behavior Note:
Replacement policies vary widely. Some SMSCs:
  • Only replace messages with matching source/destination
  • Ignore replace_if_present if PID≠0x41
  • Limit replacement to messages in the SMSC queue (not delivered)

Conclusion

The Replace If Present field provides critical control over message lifecycle management but requires careful coordination with SMSC capabilities. Always verify replacement behavior with your provider and use PID=0x41 for explicit replacement requests. For detailed implementations, refer to SMPP v3.4 Section 5.2.20 and GSM 03.40 documentation on message handling.

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