XBee 3 Global LTE Cat 1 Transparent TCP Server stops answering new SYNs after silent network interruption(?)

XBee & XBee Gateway
,
1

We are using Digi XBee 3 Global LTE Cat 1 modules as Modbus TCP servers in Transparent Mode.

Configuration:

  • Module: XBee 3 Global LTE Cat 1 (XB3-C-G1-UT-001)
  • Firmware: 11521
  • Transparent TCP Server, listening on port 502
  • XBee server timeout TS = 0x4B (7.5 seconds)
  • Cellular private network / VPN with carrier-side NAT
  • Windows Modbus TCP polling client
  • Poll interval approximately 2.5 seconds, request timeout 1.5 seconds
  • The UART-attached Modbus MCU has no unsolicited traffic; it only replies to polls.

Observed failure sequence:

  1. The Windows client has an established TCP/Modbus connection to the XBee.
  2. A cellular-path disruption occurs, such as a mobility event, PDP-context update, or carrier/NAT routing change. There is no observed FIN or RST for the original TCP session.
  3. The Windows client times out, closes its local socket, and later attempts a fresh connection from a new ephemeral source port.
  4. In the failed state, the client sends valid TCP SYN packets to XBee_IP:502, but receives no SYN-ACK and no RST.
  5. The XBee remains unreachable on port 502 for a prolonged period. We have observed this for many hours.
  6. A module/network reset or LTE detach restores normal operation.

The problem appears consistent with the XBee retaining a stale internal TCP-server connection after the former cellular route has silently disappeared. Since Transparent TCP Server mode supports a single active connection, the listener may be occupied by that stale connection and ignore replacement SYNs.

We cannot use API-mode socket control in this product at present, and the remote MCU cannot initiate a keepalive because it has no unsolicited UART traffic.

Questions:

  1. Is this a known issue in XBee 3 Global LTE Cat 1 firmware 11521, particularly for Transparent TCP Server mode after cellular route/PDP/NAT changes?
  2. Is there a documented way in Transparent Mode to detect or clear a stale TCP server connection without resetting the module?
  3. Is there a recommended server timeout, connection-close delay, reconnect backoff, or watchdog/reset strategy for this deployment?
  4. Are there newer firmware versions that specifically improve stale TCP socket/listener recovery?

We can provide a filtered PCAP showing the old connection, the new SYN attempts, and the absence of any XBee reply.

2

Do you have the APN set on the module as provided by the Carrier?

3

Yes. The APN is configured per the carrier’s instructions, and the module has an active cellular data session. In fact the module is communicating normally for some time before this failure occurs. It may come back sometimes after 10 minutes or sometimes it never comes back while i run the application (16+hrs).

4

Please make sure that you have an open case with Digi Support regarding this issue.