Chapter 9: Error Handling

9.1 Error Handling Model

DTP's error handling follows a three-phase "Detect-Notify-Recover" model:

1. Detect: Identify abnormal conditions
2. Notify: Send error information to the remote peer or upper layer
3. Recover: Take recovery measures based on the error type

9.2 Error Code System

DTP defines a unique error code for each error type, divided into eight ranges by functional module:

Error Category	Code Range	Handling Strategy
Frame Processing Errors	1xxx	Discard frame + notify sender + log
Encryption Errors	2xxx	Discard frame + notify sender + may trigger key renegotiation
Agreement Errors	3xxx	Discard Fragment + notify sender + may trigger renegotiation
DAG Errors	4xxx	Reject Fragment + notify sender, or cache and wait
Session Errors	5xxx	Attempt session recovery + if failed, close and notify upper layer
Resume Errors	6xxx	Pause sending + notify upper-layer application
Version Errors	7xxx	Send version incompatibility notification + attempt downgrade
Permission Errors	8xxx	Reject operation + notify requestor

9.3 Error Code Reference

Frame Processing Errors (1xxx)

Error Code	Name	Description
1001	FRAME_DESERIALIZATION_FAILED	Frame deserialization failed
1002	FRAME_INVALID_FORMAT	Invalid frame format

Encryption Errors (2xxx)

Error Code	Name	Description
2001	DECRYPTION_FAILED	Payload decryption failed
2002	KEY_NOT_READY	Key not ready (CAP not completed)

Agreement Errors (3xxx)

Error Code	Name	Description
3001	AGREEMENT_NOT_FOUND	Agreement not found
3002	AGREEMENT_EXPIRED	Agreement expired
3003	AGREEMENT_NEGOTIATION_FAILED	Agreement negotiation failed

DAG Errors (4xxx)

Error Code	Name	Description
4001	DAG_CYCLE_DETECTED	DAG cycle detected
4002	DAG_DEPENDENCY_UNRESOLVED	DAG dependency unresolved

Session Errors (5xxx)

Error Code	Name	Description
5001	SESSION_NOT_FOUND	Session not found
5002	SESSION_TIMEOUT	Session timeout
5003	SESSION_RESTORE_FAILED	Session restore failed

Resume Errors (6xxx)

Error Code	Name	Description
6001	BUFFER_FULL	Buffer full
6002	RETRANSMISSION_TIMEOUT	Retransmission timeout

Version Errors (7xxx)

Error Code	Name	Description
7001	VERSION_INCOMPATIBLE	Version incompatible

Permission Errors (8xxx)

Error Code	Name	Description
8001	PERMISSION_DENIED	Permission denied
8002	OBSERVER_WRITE_DENIED	Observer write operation denied

9.4 Error Notification Mechanism

Error notifications are conveyed via Control Frames, containing the following information:

Field	Description
errorCode	Error code
errorMessage	Error description message
relatedFrameId	ID of the frame that triggered the error (optional)
relatedAgreementId	Related Agreement ID (optional)
details	Additional details (optional)

9.5 Key Error Scenarios

Deserialization Failure

When a received Logical_Frame cannot be correctly deserialized:

1. Discard the frame
2. Send a FRAME_DESERIALIZATION_FAILED (1001) error notification to the sender

Decryption Failure

When a received Logical_Frame's payload cannot be correctly decrypted:

1. Discard the frame
2. Send a DECRYPTION_FAILED (2001) error notification to the sender
3. If consecutive failures exceed the threshold, trigger CAP key renegotiation

DAG Cycle Detection

When a Fragment's declared dependency relationships would form a cycle in the DAG:

1. Reject the Fragment
2. Return a DAG_CYCLE_DETECTED (4001) error

Unknown Agreement

When a Fragment references an Agreement_ID that does not exist on the receiver:

1. Discard the Fragment
2. Return an AGREEMENT_NOT_FOUND (3001) error

Key Not Ready

When an attempt is made to send data but CAP key exchange has not yet completed:

1. Refuse to send
2. Return a KEY_NOT_READY (2002) error to the upper-layer caller

Buffer Full

When the sender's unacknowledged Fragment cache reaches its capacity limit:

1. Pause sending new Fragments
2. Send a BUFFER_FULL (6001) notification to the upper-layer application

Observer Privilege Violation

When an Observer attempts to initiate a request or modify an agreement:

1. Reject the operation
2. Return an OBSERVER_WRITE_DENIED (8002) error