Lost PPP Connection between Debian <-> PDA / HandHeld using SynCE

I am currently able to connect a handheld to the PC running Linux with a very old Ubuntu 14.04, adding repository PPA for SynCE to source list. Now, the goal is to update the operating system, I chose Debian Buster where I have successfully installed the DEB packages from the same repository in its version for Ubuntu Xenial. In Debian, USB serial device is recognized and connects with ttyUSB bus, obtains an IP address and disconnects after a few seconds (two, max three). I have done a PPP debug (options debug dump & kdebug), everything seems normal: pppd options in effect: debug # (from /etc/ppp/options) kdebug 7 # (from /etc/ppp/options) logfile /tmp/pppd.log # (from /etc/ppp/options) linkname synce-device0 # (from command line) dump # (from /etc/ppp/options) noauth # (from command line) /dev/ttyUSB0 # (from command line) 115200 # (from command line) lock # (from /etc/ppp/options) connect /usr/lib/synce-core/synce-serial-chat # (from command line) record /tmp/pppd.pcap # (from /etc/ppp/options) crtscts # (from command line) local # (from command line) asyncmap 0 # (from /etc/ppp/options) lcp-echo-failure 4 # (from /etc/ppp/options) lcp-echo-interval 30 # (from /etc/ppp/options) hide-password # (from /etc/ppp/options) ms-dns xxx # [don't know how to print value] # (from command line) nodefaultroute # (from command line) 192.168.131.1:192.168.131.129 # (from command line) noipx # (from /etc/ppp/options) Removed stale lock on ttyUSB0 (pid 4366) Script /usr/lib/synce-core/synce-serial-chat finished (pid 4435), status = 0x0 Serial connection established. using channel 14 Using interface ppp0 Connect: ppp0 /dev/pts/3 rcvd [LCP ConfReq id=0x0 ] sent [LCP ConfReq id=0x1 ] sent [LCP ConfAck id=0x0 ] rcvd [LCP ConfAck id=0x1 ] sent [LCP EchoReq id=0x0 magic=0xf230f2eb] sent [CCP ConfReq id=0x1 ] sent [IPCP ConfReq id=0x1 ] rcvd [CCP ConfReq id=0x0 ] sent [CCP ConfRej id=0x0 ] rcvd [IPCP ConfReq id=0x2 ] sent [IPCP ConfRej id=0x2 ] rcvd [IPV6CP ConfReq id=0x0 ] Unsupported protocol 'IPv6 Control Protocol' (0x8057) received sent [LCP ProtRej id=0x2 80 57 01 00 00 0e 01 0a 42 83 de ff fe ae de c9] rcvd [LCP EchoRep id=0x0 magic=0x0] rcvd [CCP ConfRej id=0x1 ] sent [CCP ConfReq id=0x2] rcvd [IPCP ConfAck id=0x1 ] rcvd [CCP ConfReq id=0x1] sent [CCP ConfAck id=0x1] rcvd [IPCP ConfReq id=0x3 ] sent [IPCP ConfNak id=0x3 ] rcvd [CCP ConfNak id=0x2 ] sent [CCP ConfReq id=0x3] rcvd [IPCP ConfReq id=0x4 ] sent [IPCP ConfAck id=0x4 ] local IP address 192.168.131.1 remote IP address 192.168.131.129 Script /etc/ppp/ip-up started (pid 4444) rcvd [CCP ConfAck id=0x3] Script /etc/ppp/ip-up finished (pid 4444), status = 0x0 In UDEV, I see that after the "add" event, from the kernel, it fires a "remove", without having disconnected the device. It is striking that the event is associated only with the ppp0 network device.

KERNEL[13133.932298] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2 (usb)
KERNEL[13133.941859] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0 (usb)
KERNEL[13133.946512] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0 (usb-serial)
KERNEL[13133.946847] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0 (tty)
KERNEL[13133.946887] bind     /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0 (usb-serial)
KERNEL[13133.946930] bind     /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0 (usb)
KERNEL[13133.947016] bind     /devices/pci0000:00/0000:00:06.0/usb2/2-2 (usb)
UDEV  [13133.951550] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2 (usb)
UDEV  [13133.967665] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0 (usb)
UDEV  [13133.969926] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0 (usb-serial)
UDEV  [13133.993251] add      /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0 (tty)
UDEV  [13134.002291] bind     /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0 (usb-serial)
UDEV  [13134.003803] bind     /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0 (usb)
UDEV  [13134.004620] bind     /devices/pci0000:00/0000:00:06.0/usb2/2-2 (usb)
KERNEL[13134.445488] add      /devices/virtual/net/ppp0 (net)
KERNEL[13134.445509] add      /devices/virtual/net/ppp0/queues/rx-0 (queues)
KERNEL[13134.445519] add      /devices/virtual/net/ppp0/queues/tx-0 (queues)
UDEV  [13134.498959] add      /devices/virtual/net/ppp0 (net)
UDEV  [13134.510588] add      /devices/virtual/net/ppp0/queues/rx-0 (queues)
UDEV  [13134.514145] add      /devices/virtual/net/ppp0/queues/tx-0 (queues)
>
KERNEL[13137.624110] remove   /devices/virtual/net/ppp0/queues/rx-0 (queues)
KERNEL[13137.624144] remove   /devices/virtual/net/ppp0/queues/tx-0 (queues)
KERNEL[13137.624169] remove   /devices/virtual/net/ppp0 (net)
UDEV  [13137.644633] remove   /devices/virtual/net/ppp0/queues/tx-0 (queues)
UDEV  [13137.644791] remove   /devices/virtual/net/ppp0/queues/rx-0 (queues)
UDEV  [13137.659488] remove   /devices/virtual/net/ppp0 (net)

The event is related to the network interface or the network protocol, but I have not seen anything that causes it. Looked at the network traffic with Whireshark (record to pcap file), I have not found any clue. I have compared with the connection in Ubuntu and I do not see anything. In Ubuntu, despite the RSTs, the connection is never lost (see below image) On Debian, it is lost abruptly. The objective is to have a TCP/IP stable connection between the USB serial device and Linux PC: ppp0 /dev/ttyUSB0 Currently, with Debian, the connection is lost in a few seconds (two or three)... Jan 18 20:26:56 debian10 pppd: local IP address 192.168.131.1 Jan 18 20:26:56 debian10 pppd: remote IP address 192.168.131.129 Jan 18 20:26:56 debian10 dccm: DEBUG: synce_device_manager_check_interface_cb: address ready Jan 18 20:26:56 debian10 dccm: DEBUG: synce_device_manager_create_device: found device interface for /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0 Jan 18 20:26:56 debian10 dccm: DEBUG: synce_device_manager_create_device: listening for device /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0 Jan 18 20:26:56 debian10 dccm: DEBUG: synce_device_manager_create_device: NOT triggering connection Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_manager_client_connected_cb: have a connection to port 5679 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_manager_client_connected_cb: creating device object for /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_set_property: running for device /devices/pci0000:00/0000:00:06.0/usb2/2-2/2-2:1.0/ttyUSB0/tty/ttyUSB0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_initable_init: connecting to udev Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: info buffer length = 138 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 0: unknown: guint32 ?: 40 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 4: os_major: guint8: 6 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 5: os_minor: guint8: 0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 6: unknown, build number ?: guint16 ?: 0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 8: cpu_type: guint16: 2577 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 10: unknown: guint16 ?: 0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 12: unknown: guint32 ?: 0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 16: cur_partner_id: guint32: 0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 20: second partner id ?: guint32 ?: 0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 24: offset to device name: guint32: 40 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 28: offset to platform name: guint32: 74 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: offset 32: offset to model name: guint32: 104 Jan 18 20:26:57 debian10 dccm: Message: synce_device_dbus_init: registering object path '/org/synce/dccm/Device/_devices_pci0000_00_0000_00_06_0_usb2_2_2_2_2_1_0_ttyUSB0_tty_ttyUSB0' Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_manager_device_obj_path_changed_cb: sending connected signal for /org/synce/dccm/Device/_devices_pci0000_00_0000_00_06_0_usb2_2_2_2_2_1_0_ttyUSB0_tty_ttyUSB0 Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_dbus_init: obj_path set to /org/synce/dccm/Device/_devices_pci0000_00_0000_00_06_0_usb2_2_2_2_2_1_0_ttyUSB0_tty_ttyUSB0 Jan 18 20:26:57 debian10 dccm: DEBUG: get_password_flag_text: setting password flags unset Jan 18 20:26:57 debian10 dccm: DEBUG: synce_device_legacy_info_received: setting CTRL_STATE_CONNECTED Jan 18 20:26:58 debian10 dccm: DEBUG: gudev_uevent_callback: received uevent **remove** for device /sys/devices/virtual/net/ppp0/queues/tx-0 Jan 18 20:26:58 debian10 dccm: DEBUG: gudev_uevent_callback: received uevent **remove** for device /sys/devices/virtual/net/ppp0/queues/rx-0 Jan 18 20:26:58 debian10 dccm: DEBUG: gudev_uevent_callback: received uevent **remove** for device /sys/devices/virtual/net/ppp0 How can I detect what is happening?, What can be the cause for the kernel to remove the ppp0 device? Any answer that helps to solve this problem will be highly appreciated, even if there is another way to connect a USB serial device, I don't know if there is.