Talk about the two or three things about deterministic networks

The IETF is working to provide deterministic services through IP routers and wireless networks in the Deterministic Network (DetNet) and RAW working groups.

For many emerging services and applications, low latency in the network is particularly important, such as drones, industrial automation, and self-driving cars. The International Standards Organization is currently developing new technologies to meet the requirements of these deterministic applications.

 

IEEE 802.1 is working to support deterministic Ethernet services in its Time Sensitive Networking (TSN) task group. 3GPP is committed to providing deterministic 5G to support ultra-reliable and low-latency communication (URLLC) usage scenarios. The IETF is working to provide deterministic services through IP routers and wireless networks in the Deterministic Network (DetNet) and RAW working groups.

 

The deterministic network provides a deterministic delay on the basis of each deterministic flow. The data flow of each deterministic flow is transmitted within a certain bounded delay and low delay variation constraints. The deterministic network aims to achieve zero data loss for all allowed deterministic flows, and may reject or lower the priority of certain flows to ensure the transmission of high-priority flows. The deterministic network supports a wide range of applications, and each application can have different QoS requirements.

In traditional networks, achieving lower latency means discarding more packets (or requiring a lot of over-configuration). In the case of deterministic services, the goal is to solve the long tail problem and provide bounded delays, see Figure 1.

IEEE 802.1 Delay Sensitive Network (TSN)

Standardization

The IEEE 802.1 Working Group (WG) focuses on standards and practices in the following areas: (1) 802 LAN/MAN architecture, (2) 802 LAN, interconnection between MAN and other WANs, (3) 802 Security, (4) 802 overall network management, and the protocol layer above the MAC and LLC layers.

 

The Time Sensitive Network (TSN) Task Group (TG) of the IEEE 802.1 Working Group is responsible for the deterministic services of IEEE 802 networks, including:

 

·         Guaranteed packet transmission

·         Low packet loss rate

·         Bounded low latency

·         Low packet delay variation

The TSN task force evolved from the Audio Video Bridging (AVB) task group.

 

TSN standards and projects are divided into three groups:

1) Basic technology (such as 802.1CB, 802.1Qbv, etc.)

2) Configuration (such as 802.1Qcp, 802.1Qcc, etc.)

3) Configuration files (for example, 802.1BA, 802.1CM, IEC/IEEE 60802, etc.)

Figure 2: IEEE 802.1 TSN components

 

TSN Function

IEEE 802.1 defines TSN flow as a unidirectional data flow from Talker to one or more Listeners. In the forwarding process of the bridge, QoS functions are applied to the frames of the TSN flow, such as filtering and policing, shaping and queuing.

 

IEEE 802.1 TSN TG defines a wide range of TSN functions. This article only discusses some functions. The main medium of TSN is IEEE 802.3 Ethernet. Work involving wireless is also in progress, such as the 5G-TSN integration work in 3GPP.

 

Scheduled Traffic (802.1Qbv) reduces the delay variation of known time frames. This is achieved through time-based control and bridge queue programming. Each queue is equipped with time-gates (time-gated queues), which can only serve the queue when the "gate" is open. The door opening/closing status changes according to a periodically repeating schedule. This feature requires end-to-end time synchronization.

 

Frame Preemption (802.3br and 802.1Qbu) enables so-called fast frames (that is, critical traffic) to suspend the transmission of preemptible frames (that is, non-critical traffic). As a result, the delay variation of fast traffic is reduced, and the available bandwidth that can be preempted for traffic is increased. Frame preemption is a link-local per-hop function, which means it is not multi-hop.

 

Per-Stream Filtering and Policing (802.1Qci) provides protection to prevent traffic from violating its bandwidth allocation, malfunctioning, participating in attacks, etc. Filtering and control decisions can be made on a per-flow, per-priority, etc. basis.

Asynchronous Traffic Shaping (ATS) (802.1Qcr) provides zero congestion loss and does not require time synchronization. The essence of the ATS function is to smooth the traffic pattern by reshaping at each hop so that urgent traffic takes precedence over less urgent or flexible traffic. ATS uses strict priority queues.

 

Frame Replication and Elimination for Reliability (FRER) (802.1CB) aims to avoid frame loss due to equipment failure. It is a 1+1 (or 1+n) redundant function per frame. No failure detection or switching mechanism is required. FRER sends frames on the two (or more) largest disjoint paths, then combines the streams and deletes the extra frames.

Explicit Trees by IS-IS Path Control & Reservation (802.1Qca, RFC 7813) adds non-shortest path or explicit path forwarding, and provides IS-IS control beyond the shortest path tree (SPT). The protocol has not changed, only a few new sub-TLVs are defined, and existing sub-TLVs are reused as much as possible. The concept is a hybrid software-defined network (SDN) approach, where IS-IS provides basic functions such as topology discovery and default paths, and one or more controllers control an explicit tree.

 

Stream Reservation Protocol (SRP) enhancements and performance improvements (802.1Qcc): Provides time-sensitive network (TSN) configuration related attributes. 802.1Qcc describes three models of TSN user and network configuration (fully distributed, centralized network/distributed user and fully centralized model). Each model specification defines the logical flow of user/network configuration information between different entities in the network.

 

TSN's Future Outlook

TSN standardization is still in progress. The IEC/IEEE 60802 TSN industrial automation specification is a joint project of IEC SC65C/WG18 and IEEE 802. This joint work will provide a dual marking standard that is both an International Electrotechnical Commission (IEC) and an IEEE standard.

 

OPC UA is based on TSN, DetNet and 5G. Several OPC UA work items related to TSN are in progress. One of them is the FLC (Field Level Communications) working group, which is mainly based on the IEC/IEEE 60802 specifications and related evaluation specifications.

 

3GPP Supports Deterministic Transmission (URLLC)

Standardization

The three major application scenarios of 5G include enhanced mobile broadband (eMMB), massive machine communication (mMTC), and ultra-high reliability and low-latency communication (uRLLC). Among them, URLLC makes 5G the best candidate to support wireless deterministic and time-sensitive communication applications.

 

5G R15 introduces multiple functions, the one-way delay of message transmission is as low as 1 millisecond, and the reliability is as high as 99.999%. R16 adds more URLLC features to support one-way delays as low as 0.5 milliseconds and reliability as high as 99.9999%.

 

URLLC function

At the beginning of the R15 research, a work project was established to study delay reduction technologies such as sub-carrier spacing, flexible frame structure, and short time slot scheduling. As of R16, 3GPP has successively completed the performance evaluation of URLLC use cases, the enhancement of the physical layer channels, and the research and standardization of URLLC and eMBB uplink multiplexing technologies, but there are still many optimizations expected to be left to R17 for research.

 

5G defines a powerful transmission mode to improve the reliability of the data and control radio channels. Multi-antenna transmission, the use of multiple carriers, and packet duplication on independent radio links all further improve reliability.

 

Time synchronization has been embedded in the cellular radio system as an important part of its operation. The equipment is time calibrated by the base station to compensate for their different propagation delays. The wireless network components themselves are also time synchronized. This is a good basis for providing synchronization for time-critical applications.

In addition to 5G RAN functions, the 5G system also provides core network (CN) solutions for Ethernet networking and URLLC. 5G CN supports local Ethernet Protocol Data Unit (PDU) sessions. For user plane redundancy at the 5G system level, 5G supports the establishment of redundant user plane paths through 5G systems including RAN, CN, and transmission networks. By using a single user equipment (UE) with RAN dual connectivity in the terminal device or by using multiple UEs in the terminal device, a redundant path is realized. In addition, 5G can also provide virtual networks (5G-VN) and LAN groups to allocate resources to members of specific groups.

 

All these new URLLC features of 5G provide a good design and a solid foundation for using 5G in deterministic scenarios, and can even be used as a stand-alone solution or part of a deterministic network.

Figure 3: System architecture view, 5GS is shown as a deterministic node (here is TSN bridge)

 

Figure 3 shows the 5G system architecture, where the 5G system is regarded as a TSN bridge. The figure specifies a new conversion function (called DS-TT and NW-TT) for saving and forwarding user plane data packets to eliminate jitter. The 5G system (5GS) is integrated as a bridge connecting the TSN network . 5GS includes the TSN Translator (TT) function, which is used to adapt 5GS to the TSN domain of the user plane and the control plane.

 

The Future of URLLC

The 5G URLLC function closely matches the TSN and deterministic network functions. Therefore, these three technologies can be integrated to provide an end-to-end deterministic connection, that is, the connection between the input/output device and its controller. The integration already includes data plane support for the necessary basic bridging/routing functions and TSN/DetNet add-on components, but the control and management planes require further standardization work.

 

IETF Deterministic Network (DetNet)

Standardization

IETF DetNet WG (Working Group) belongs to the routing area (Routing Area, RTG), which mainly studies routing protocols and signaling protocols. It focuses on deterministic data paths running on layer 2 bridging and layer 3 routing segments, which can provide limitations on delay, loss, and packet jitter, as well as high reliability. The scope of DetNet WG includes: overall architecture, data plane specification, data flow information model and related YANG models.

 

There is close cooperation between IETF DetNet WG and IEEE 802.1 TSN TG.

 

DetNet operates at the IP/MPLS layer, and its initial scope is to achieve deterministic protection under a single management control or within a closed management and control group of the network.

 

The solution document specifies the procedures and behaviors required for nodes supporting DetNet, and its specifications focus on interoperable implementations. The following two data planes are defined:

• IP: Use IP and transport protocol header information to support DetNet [RFC 8939]

• MPLS: Use labels to support DetNet [RFC 8964

The forwarding feature is achieved by allocating network resources (such as link bandwidth and buffer space) to the DetNet stream and protecting data packets. The unused reserved resources can be used for the transmission of non-DetNet data streams to realize the shared network transmission of different priority service streams.

 

The following defines the forwarding parameters from the source to the destination layer:

·         Minimum and maximum end-to-end delay: timely delivery, and bounded jitter (variation in packet delay) resulting from these constraints

·         Packet loss rate: Lost during transmission, extremely low packet loss value can be applied

·         The upper limit of out-of-order packet delivery: some deterministic network applications cannot tolerate any out-of-order delivery

There is a difference in deterministic networks (similar to TSN). It only focuses on the worst-case values ​​of end-to-end delay, delay variation, and disorder. The average or typical value is not important because they will not affect the performance of the real-time system. ability.

Deterministic network functions:

 ·         Congestion protection

·         Service Guarantee

·         Explicit routing

Congestion protection means allocating resources along the path of the DetNet flow, such as buffer space or link bandwidth.

 

Congestion protection eliminates congestion-related losses through the use of appropriately designed queues, so no packets will be dropped due to lack of buffer storage. It can also be used as a tool to reduce latency variations, for example, to converge sensitive non-IP networks onto public IP network infrastructure. Many functions of congestion protection require time synchronization of deterministic network nodes. However, time synchronization is not within the scope of deterministic network discussions because it does not affect interoperability. Time synchronization should be provided by an appropriate solution, for example, provided by a lower layer.

Service protection solves data packet errors and equipment failures, such as data packet duplication and elimination (to prevent failures), data packet encoding (to prevent data packet errors), and reordering (to ensure sequential delivery). Service protection can be ensured through these technologies. The PREOF defined by the deterministic network is: data packet copy function (PRF: sending copies of the same data packet with sorting information on multiple paths), redundancy elimination function (PEF: according to the sorting information of the received data packet and The history record discards duplicates), and the data packet sorting function (POF: restore the original data packet order, because out-of-order delivery will affect the amount of buffer at the destination to correctly process the received data). Data packet duplication and elimination will not react to and correct the failure, these functions are completely passive. Packet coding (also called network coding) encodes information into multiple transmission units, uses multiple paths to send them, and combines these units at the other end.

 

Explicit routing can be used to resolve the impact of routing or bridging protocol convergence (ie, temporary interruption).

 The deterministic network function is implemented in two adjacent sublayers of the protocol stack:

1) DetNet service sublayer: Provides DetNet services for higher layers in the protocol stack and applications (for example, service protection)

2) DetNet forwarding sublayer: support DetNet services in the underlying network (for example, by providing explicit routing and congestion protection) to DetNet flow

Figure 4: DetNet data plane protocol stack

 

The layer 3 equivalent of the TSN stream is called the DetNet stream. The DetNet flow is a unique sequence of data packets that conforms to the flow identifier and will provide deterministic network services to it. It includes any deterministic network headers added to support the DetNet service and forwarding sublayer.

 

Deterministic network-related mechanisms require two attributes:

 

• Flow-ID: Identifies the flow to which the data packet belongs

• Sequence number: identify duplicate packets and reorder packets

The Future of Deterministic Networks

The standardization of deterministic networks is still in progress. IETF DetNet and IEEE TSN will continue to work closely together to ensure interoperability and simplify the implementation of deterministic functions applicable to Layer 2 and Layer 3. For example, IEEE P802.1CBdb (FRER Extended Stream Identification Functions) focuses on extending the fields used for stream identification functions to arbitrary mask matching, which is essential for the combined network scenario of combining TSN and DetNet. Control and management plane related work is the next focus of DetNet WG.

In conclusion

In the past, packet-based networks were designed to carry all traffic except for very delay-sensitive/real-time application traffic. Over time, with the development of deterministic technology, packet-based networks are also evolving to integrate support for demanding applications.

 

TSN, DetNet, and 5G URLLC can meet the networking requirements of deterministic applications, and provide ultra-reliable, low-latency connections through converged networks. TSN and DetNet (for wired) and 5G (for wireless) technologies are perfect partners in deterministic transmission networks. A certain degree of overall integration of these technologies is required to provide end-to-end connectivity that meets deterministic requirements.

 

For example, time synchronization between the wireless 5G domain and the wired TSN/DetNet domain is necessary, because regardless of the network technology connecting them, a common reference time is essential for certain endpoints. Providing limited low latency may also require integration between TSN, DetNet, and 5G, depending on the deterministic tools used in the deployment. End-to-end ultra-reliability adjusts the characteristics of necessary disjoint forwarding paths. The first step to support overall integration is to use an SDN-based approach. The basic technologies of TSN, DetNet, and URLLC are ready, and their combined deployment is imminent.