# TransferReassemblyErrorID.UNEXPECTED\_TRANSFER\_ID **URL:** \ **Category:** Libcanard\ **Created:** [April 5, 2025, 10:58pm UTC](https://forum.opencyphal.org/t/transferreassemblyerrorid-unexpected-transfer-id/2359 "2025-04-05T22:58:23Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Scoeerg](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/scoeerg/32/1713_2.png) [@Scoeerg](https://forum.opencyphal.org/u/Scoeerg)\ **Post date:** [April 5, 2025, 10:58pm UTC](https://forum.opencyphal.org/t/transferreassemblyerrorid-unexpected-transfer-id/2359/1 "2025-04-05T22:58:23Z")
Hi, When is UNEXPECTED\_TRANSFER\_ID usually thrown? Context: using Libcanard and [BXCan](https://github.com/OpenCyphal-Garage/platform_specific_components/tree/master/stm32/libcanard/bxcan) on STM32 and [Post-Mortem CAN Dump Analyzer](https://forum.opencyphal.org/t/postmortem-dump-analyzer-tool/2184/3) to debug. Trying to answer to a [GetInfo.1.0 Request](https://github.com/OpenCyphal/public_regulated_data_types/blob/master/uavcan/node/430.GetInfo.1.0.dsdl) I will show you the human-readable .json of the candump: ```bash {"_meta_": {"ts_system": 1743866278.796648, "ts_monotonic": 1364.192972352, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "optional", "transfer_id": 4, "source_node_id": 49, "destination_node_id": 125, "service_id": 430, "role": "request", "dtype": "uavcan.node.GetInfo.Request.1.0", "payload_hex": ""}} ``` and then (mixed with a couple heartbeat messages in between) `TransferReassemblyErrorID.UNEXPECTED_TRANSFER_ID`. ```bash {"_meta_": {"ts_system": 1743866278.797218, "ts_monotonic": 1364.193137026, "transport": {"kind": "can", "source_iface": "can0"}, "error": "CANErrorTrace(timestamp=Timestamp(system_ns=1743866278797218000, monotonic_ns=1364193137026), error=)"}} {"_meta_": {"ts_system": 1743866278.899989, "ts_monotonic": 1364.193852735, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "optional", "transfer_id": 6, "source_node_id": 33, "destination_node_id": null, "subject_id": 7510, "dtype": null, "payload_hex": "060000000102551d561d060000000102551d561d40000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000040000000000000000000004000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000003000000004000000000000000000000"}} {"_meta_": {"ts_system": 1743866278.900267, "ts_monotonic": 1364.193924927, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 29, "source_node_id": 33, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "3d00000000003c"}, "uptime": 61, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 60} {"_meta_": {"ts_system": 1743866279.070962, "ts_monotonic": 1364.19405874, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 13, "source_node_id": 49, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "cd00000000002a"}, "uptime": 205, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 42} {"_meta_": {"ts_system": 1743866279.104825, "ts_monotonic": 1364.194189198, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 14, "source_node_id": 1, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "8e000000000016"}, "uptime": 142, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 22} {"_meta_": {"ts_system": 1743866279.797341, "ts_monotonic": 1364.194317781, "transport": {"kind": "can", "source_iface": "can0"}, "error": "CANErrorTrace(timestamp=Timestamp(system_ns=1743866279797341000, monotonic_ns=1364194317781), error=)"}} {"_meta_": {"ts_system": 1743866279.89335, "ts_monotonic": 1364.194376017, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 30, "source_node_id": 33, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "3e00000000003c"}, "uptime": 62, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 60} {"_meta_": {"ts_system": 1743866280.078343, "ts_monotonic": 1364.194524186, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 14, "source_node_id": 49, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "ce00000000002a"}, "uptime": 206, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 42} {"_meta_": {"ts_system": 1743866280.106402, "ts_monotonic": 1364.194658323, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 15, "source_node_id": 1, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "8f000000000016"}, "uptime": 143, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 22} {"_meta_": {"ts_system": 1743866280.799132, "ts_monotonic": 1364.194788259, "transport": {"kind": "can", "source_iface": "can0"}, "error": "CANErrorTrace(timestamp=Timestamp(system_ns=1743866280799132000, monotonic_ns=1364194788259), error=)"}} {"_meta_": {"ts_system": 1743866280.893382, "ts_monotonic": 1364.194846453, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 31, "source_node_id": 33, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "3f00000000003c"}, "uptime": 63, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 60} {"_meta_": {"ts_system": 1743866281.071019, "ts_monotonic": 1364.194974466, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 15, "source_node_id": 49, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "cf00000000002a"}, "uptime": 207, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 42} {"_meta_": {"ts_system": 1743866281.105233, "ts_monotonic": 1364.195106052, "transport": {"kind": "can", "source_iface": "can0"}, "priority": "nominal", "transfer_id": 16, "source_node_id": 1, "destination_node_id": null, "subject_id": 7509, "dtype": "uavcan.node.Heartbeat.1.0", "payload_hex": "90000000000016"}, "uptime": 144, "health": {"value": 0}, "mode": {"value": 0}, "vendor_specific_status_code": 22} {"_meta_": {"ts_system": 1743866281.801047, "ts_monotonic": 1364.195233659, "transport": {"kind": "can", "source_iface": "can0"}, "error": "CANErrorTrace(timestamp=Timestamp(system_ns=1743866281801047000, monotonic_ns=1364195233659), error=)"}} ``` Obviously the message is verbose and tells me something is off with the transfer-ID. My logical order of functions is: ```bash canPoll() └── handle_getInfo_Request() │ └── serialize_info_Response() │ └── canardTxPush() └── bxcan's bxCANPush() [continues until canardTxPeek returns Null] ``` See [bxCANPush() in bxcan.h line 152](https://github.com/OpenCyphal-Garage/platform_specific_components/blob/master/stm32/libcanard/bxcan/src/bxcan.h) Now the meta-data of the response is: ```bash const CanardTransferMetadata transfer_metadata = { .priority = CanardPriorityNominal, .transfer_kind = CanardTransferKindResponse, .port_id = uavcan_node_GetInfo_1_0_FIXED_PORT_ID_, .remote_node_id = requester_node_id, .transfer_id = get_info_request_transfer_id, }; ``` based on [Cyphal Specifictation.pdf](https://opencyphal.org/specification/Cyphal_Specification.pdf) p. 8/153 and 53/153: > Service response matching – when a server responds to a request, it uses the same transfer-ID for the response transfer as in the request transfer, allowing the client to emit concurrent requests to the same server > while being able to match each response with the corresponding local request state. For the deadline in bxCANPush() I set values up to 100.000µs. When is `UNEXPECTED_TRANSFER_ID` usually thrown? It’s used a couple times in PyCyphal’s `_transfer_reassembler.py`, see [\_transfer\_reassembler.py](https://github.com/OpenCyphal/pycyphal/blob/master/pycyphal/transport/can/_session/_transfer_reassembler.py) ---
**Author:** ![Scoeerg](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/scoeerg/32/1713_2.png) [@Scoeerg](https://forum.opencyphal.org/u/Scoeerg)\ **Post date:** [April 5, 2025, 10:58pm UTC](https://forum.opencyphal.org/t/transferreassemblyerrorid-unexpected-transfer-id/2359/2 "2025-04-05T22:58:55Z")
**Answer** : [see related git issue I should not have opened](https://github.com/OpenCyphal/libcanard/issues/242#issuecomment-2781030904): > It means there is some transfer that is being repeatedly transmitted by some node with the same transfer-ID value. In your case, that value is three. You can find the offending transfer in the raw candump using the timestamps reported in the error traces; e.g., one is at system\_ns=1743866281801047000. by @pavel.kirienko