Packet Ordering Function for TSN/DetNet Out-of-Order PDUs
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Solution Overview
Problem
Current networks fail to address out-of-order packet delivery at OSI Layer-2 or Layer-3 levels, particularly in Time-Sensitive Networking (TSN) and Deterministic Networking (DetNet), where frame/packet order correction is not specified, leading to inefficiencies in real-time applications.
Innovation Solution
A Packet Ordering Function (POF) is implemented at Layer-2 or Layer-3 levels to restore the proper order of Protocol Data Units (PDUs) by storing the sequence number of the last forwarded PDU and comparing it with newly received PDUs, buffering those out of order, and using path-dependent maximum buffering times to maintain latency and delay budgets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If per-packet replication and elimination functions are used to achieve extremely low packet loss, then packet loss is reduced, but out-of-order delivery of frames/packets occurs due to differing latencies of different paths
Solution Approach 1:
A Packet Ordering Function (POF) is introduced as an intermediary component between the replication/elimination functions and the higher layers. The POF receives out-of-order packets from the elimination function and reorders them before forwarding to the next stage, thus mediating the conflict between achieving low packet loss through replication and maintaining packet order for real-time applications.
2Stability of the object's composition
If packet ordering is implemented at Layer-2 or Layer-3 levels, then out-of-order packet delivery is corrected, but device complexity increases
Solution Approach 1:
The packet ordering function is segmented into simple, manageable components: a buffer for storing out-of-order packets, a sequence number comparison mechanism, and a forwarding decision logic. This segmentation allows the complex ordering task to be broken down into basic operations that can be implemented efficiently at Layer-2 or Layer-3 without requiring complex centralized control.
Solution Approach 2:
The POF operates autonomously using locally available information (sequence numbers in packet headers) to make ordering decisions. The function self-manages the buffering and forwarding process based on simple comparison logic, eliminating the need for complex external control mechanisms or sophisticated algorithms, thus reducing overall device complexity while achieving reliable packet ordering.
3Stability of the object's composition
If packets are buffered to correct out-of-order delivery, then packet order is restored, but latency increases
Solution Approach 1:
The packet ordering function applies partial buffering only when necessary - specifically when packets arrive out of order according to their sequence numbers. In-order packets are forwarded immediately without buffering, thus avoiding unnecessary latency. The buffering mechanism is activated only for the subset of packets that require reordering, minimizing the overall latency impact while still achieving reliable packet order restoration.
Data Source
AI summary
A PDU ordering function (POF) restores the proper order of PDUs in a stream, at the Layer-2 or 3 level of a TSN or DetNet. The sequence number of the last PDU forwarded is stored, and compared to the sequence number of a newly received PDU. If the new PDU is an earlier or the “next” PDU in the stream, it is forwarded and the value of the last PDU forwarded is updated. If the newly received PDU is two or more sequence numbers ahead of the last PDU forwarded, it is buffered. Other PDUs are received and processed, and the value of the last PDU forwarded is updated. When the buffered PDU is the next one to the last PDU forwarded, it is retrieved from the buffer and forwarded. To bound the total latency the POF can impose on aPDU stream, a predetermined maximum buffering time timer is started upon buffering the PDU. If the timer expires before the buffered PDU becomes “next” to the last PDU forwarded, it is nevertheless retrieved and forwarded.