Insights into the performance and configuration of TCP in Automotive Ethernet Networks

TCP has become an integral part of the Autosar communication stack located immediately below the Socket Adaptor layer. There are numerous use-cases for TCP in automotive networks. First TCP can be leveraged on by many higher-level protocols be it for diagnostic (DoIp), calibration protocols (XCP), service-oriented communications (SomeIP), FTP and HTTP transfer, or more specialized tasks like protocols for the control of electric vehicle charging. In addition, communication through standard TCP sockets simplifies the development and re-use of applications requiring reliable transmissions within the vehicle or with external end-systems (car2x, cloud-based services).If previous works focused on throughput as performance criterion, we here also consider the communication latencies and memory usage in the end-systems and switches. The first question explored by simulation is to quantify the importance of TCP configuration choices in the latencies that can be achieved with TCP. In particular, it is shown that turning off Nagle's algorithm in transmission and delayed acknowledgment in reception as Autosar is very beneficial in terms of latencies but at the expense of throughput. We then extend the experiments to integrate the interactions between the Socket Adaptor and TCP layers. Finally, we study the performance of TCP streams as part of a complete automotive TSN case-study made up of control traffic, audio/video streams and best-effort traffic to evaluate the impact of higher priority traffic classes on TCP streams throughput and delays. The contribution of this work is twofold. First, we draw up a panorama of the TCP features and configuration options available in Autosar. Second, through experiments on models increasingly close to an implementation, we provide quantified insights in what we can expect from TCP in terms of latencies and throughput with the different configuration options available to us

Fine Tuning the Scheduling of Tasks on Posix1003.1b Compliant Systems

Posix1003.1b compliant operating systems provide two real-time scheduling policies, namely sched_fifo and sched_rr, which under some limited hypotheses are respectively the equivalent of Fixed Priority Pre-emptive (FPP) and Round-Robin (RR). In the field of processor scheduling, schedulability analysis has been extensively studied and the problem of assessing the schedulability of multi-policy systems has been recently addressed. A schedulability analysis provides valuable help for the application designer but it simply asserts whether a given configuration is feasible or not, in general it does not propose any feasible configurations (1) and, as stated by Gerber and Hong "it can rarely help to tune the system (2), which is the inevitable next step". To address problems (1) and (2), we propose in this study an approach using a Genetic Algorithm (GA) to best set task priorities and scheduling policies, according to a chosen criterion, on Posix 1003.1b uniprocessor systems. Moreover, it will be shown that the use of RR, in conjunction with FPP, may improve the schedulability as well as the satisfaction of additional application-dependant criteria

Pre-Shaping Bursty Transmissions under IEEE802.1Q as a Simple and Efficient QoS Mechanism

International audienceThe automotive industry is swiftly moving towards Ethernet as the high-speed communication network for in-vehicle communication. There is nonetheless a need for protocols that go beyond what standard Ethernet has to offer in order to provide additional QoS to demanding applications such as ADAS systems or audio/video streaming. The main protocols currently considered for that purpose are IEEE802.1Q, AVB with the Credit Based Shaper mechanism (IEEE802.1Qav) and TSN with its Time-Aware Shaper (IEEE802.1Qbv). AVB/CBS and TSN/TAS both provide efficient QoS mechanisms and they can be used in a combined manner, which offers many possibilities to the designer. Their use however requires dedicated hardware and software components, and clock synchronization in the case of TAS. Previous studies have also shown that the efficiency of these protocols depends much on the application at hand and the value of the configuration parameters. In this work, we explore the use of "pre-shaping" strategies under IEEE802.1Q for bursty traffic such as audio/video streams as a simple and efficient alternative to AVB/CBS and TSN/TAS. Pre-shaping means inserting on the sender side "well-chosen" pauses between successive frames of a burst (e.g., a camera frame), all the other characteristics of traffic remaining unchanged. We show on an automotive case-study how the use of pre-shaping for audio/video streams leads to a drastic reduction of the communication latencies for the best-effort streams while enabling to meet the timing constraints for the rest of the traffic. We then discuss the limitations of the pre-shaping mechanism and future works needed to facilitate its adoption

Early-stage topological and technological choices for TSN-based communication architectures

A main issue in the design of automotive communication architectures is that the most important design choices pertaining to the topology of the networks and the technologies to use (protocols, data rate, hardware) have to be made at a time when the communication requirements are not entirely known. Indeed, many functions only becomes available along the development cycle, and vehicle platforms have to support incremental evolutions of the embedded system that may not be fully foreseeable at the time design choices are made. The problem is becoming even more difficult and crucial with the introduction of dynamically evolving communication requirements requiring network re-configuration at run-time. We present how the use of synthetic data, that is data generated programmatically based on past vehicle projects and what can be foreseen for the current project, enables the designers to make such early stage choices based on quantified metrics. The proposals are applied to Groupe Renault's FACE service-oriented E/E architecture with the use of a software-implemented function we called “Topology Stress Test”

Real-Time Scheduling: Non-Preemption, Critical Sections and Round Robin

The main property of a (hard) real-time system is feasibility. It is the guarantee that tasks do always meet their deadlines, when the system is running under a given scheduling policy. An optimal policy produces a feasible schedule whenever a feasible schedule exists. It is therefore the most appropriate choice if feasibility is the only matter of concern. However, if a set of tasks is feasible under several policies, it is possible to impose additional constraints, which improve in a certain way the quality of a system. A refined choice is only possible if feasibility tests or, more broadly, timing analysis is available for other policies than the optimal ones. In this document we derive timing analysis for policies obtained by combining known policies in hierarchical layers. These layered priorities are motivated by the Posix 1003.1c standard, which allows such a combination of Fixed Preemptive Priorities and the Round Robin scheduling policy. In this context we extend the trajectory based model developed in [8] for systems scheduled under real-time constraints to account for non-preemptive resources and the associated priority ceiling protocol. Furthermore, timing analysis of the Round Robin policy is derived

Reducing CAN latencies by use of weak synchronization between stations

Scheduling frames with offsets has been shown in the literature to be very beneficial for reducing response times in real-time networks because it allows the workload to be better spread over time and thus to reduce peaks of load. Maintaining a global synchronization amongst the stations induces substantial overhead and complexity in networks not providing a global time service such as CAN. Indeed, on CAN, a global clock is rarely implemented in practice and each station possesses its own local clock. Without a global clock, the de-synchronization between the streams of frames created by offsets remains local to each station and thus less efficient. In a previous paper [1], we developed a method to compute latency upper bounds for set of messages with offsets when the inter-node synchronization is not perfect. On a simplified test case, we obtained a reduction of 65% of the delay using a clock accuracy of only 1ms. In this article, we extend the method to consider a realistic case study (mixing periodic and asynchronous flows, considering errors and tacking into account the synchronization protocol)

Timing verification of real­time automotive Ethernet networks: what can we expect from simulation?

Switched Ethernet is a technology that is profoundly reshaping automotive communication architectures as it did in other application domains such as avionics with the use of AFDX backbones. Early stage timing verification of critical embedded networks typically relies on simulation and worst-case schedulability analysis. When the modeling power of schedulability analysis is not sufficient, there are typically two options: either make pessimistic assumptions or ignore what cannot be modeled. Both options are unsatisfactory because they are either inefficient in terms of resource usage or potentially unsafe. To overcome those issues, we believe it is a good practice to use simulation models, which can be more realistic, along with schedulability analysis. The two basic questions that we aim to study here is what can we expect from simulation, and how to use it properly? This empirical study explores these questions on realistic case-studies and provides methodological guidelines for the use of simulation in the design of switched Ethernet networks. A broader objective of the study is to compare the outcomes of schedulability analyses and simulation, and conclude about the scope of usability of simulation in the design of critical Ethernet networks

Comparaison de strategies de calcul de bornes sur NoC

The Kalray MPPA2-256 processor integrates 256 processing cores and 32 management cores on a chip. Theses cores are grouped into clusters, and clusters are connected by a high-performance network on chip (NoC). This NoC provides some hardware mechanisms (egress traffic limiters) that can be configured to offer bounded latencies. This paper presents how network calculus can be used to bound these latencies while computing the routes of data flows, using linear programming. Then, its shows how other approaches can also be used and adapted to analyze this NoC. Their performances are then compared on three case studies: two small coming from previous studies, and one realistic with 128 or 256 flows. On theses cases studies, it shows that modeling the shaping introduced by links is of major importance to get accurate bounds. And when packets are of constant size, the Total Flow Analysis gives, on average, bounds 20%-25% smaller than all other methods

Dual-Priority versus Background Scheduling: A Path-Wise Comparison

In this paper two well-known scheduling policies for Real Time Systems, namely Background Scheduling and Dual-Priority are compared in terms of response times for Soft Real Time traffic (SRT). It is proved that, when the SRT traffic is FIFO, the Dual-Priority policy always behaves better, in the preemptive case as well as in the non-preemptive case. The proof is based on a trajectorial method. As a complementary result, some non-FIFO examples where the Background Scheduling outperforms the Dual-Priority, are given

A simple and efficient class of functions to model arrival curve of packetised flows

International audienceNetwork Calculus is a generic theory conceived to compute upper bounds on network traversal times (WCTT - Worst Case Traversal Time). This theory models traffic constraints and service contracts with arrival curves and service curves. As usual in modelling, the more realistic the model is, the more accurate the results are, however a detailed model implies large running times which may not be the best option at each stage of the design cycle. Sometimes, a trade-off must be found between result accuracy and computation time. This paper proposes a combined use of two simple class of curves in order to produce accurate results with a low computational complexity. Experiments are then conducted on a realistic AFDX case-study to benchmark the proposal against two existing approaches