| RAW | P. Thubert, Ed. |
| Internet-Draft | Cisco Systems |
| Intended status: Informational | May 21, 2019 |
| Expires: November 22, 2019 |
Reliable and Available Wireless Technologies
draft-thubert-raw-technologies-00
This document presents a series of recent technologies that are capable of time synchronization and scheduling of transmission, making them suitable to carry time-sensitive flows with requirements of both reliable delivery in bounded time, and availability at all times, regardless of packet transmission or individual equipement failures.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 22, 2019.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
When used in math or philosophy, the term "deterministic" generally refers to a perfection where all aspect are understood and predictable. A perfectly Deterministic Network would ensure that every packet reach its destination following a predetermined path along a predefined schedule to be delivered at the exact due time. In a real and imperfect world, a Deterministic Network must highly predictable, which is a combination of reliability and availability. On the one hand the network must be reliable, meaning that it will perform as expected for all packets and in particular that it will always deliver the packet at the destination in due time. On the other hand, the network must be available, meaning that it is resilient to any single outage, whether the cause is a software, a hardware or a transmission issue.
RAW (Reliable and Available Wireless) is an effort to provide Deterministic Networking on across a path that include a a wireless physical layer. Making Wireless Reliable and Available is even more challenging than it is with wires, due to the numerous causes of loss in transmission that add up to the congestion losses and the delays caused by overbooked shared resources. In order to maintain a similar quality of service along a multihop path that is composed of wired and wireless hops, additional methods that are specific to wireless must be leveraged to combat the sources of loss that are also specific to wireless.
Such wireless-specific methods include per-hop retransmissions (HARQ) and P2MP overhearing whereby multiple receivers are scheduled to receive the same transmission, which balances the adverse effects of the transmission losses that are eperienced when a radio is used as pure P2P.
This specification uses a number of terms that are uncommon on protocols that ensure bets effort transmissions for stochastics flows, such as found in the traditional Internet and other statistically multiplexed packet networks.
The operations of a Deterministic Network often rely on precisely applying a tight schedule, in order to avoid collision loss and guarantee the worst case time of delivery . To achieve this, there must be a shared sense of time throughout the network. The sense of time is usually provided by the lower layer and is not in scope for RAW.
A network is reliable when the statistical effects that affect the packet transmission are eliminated. This involves maintaining at all time the amount of critical packets within the physical capabilities of the hardware and that of the radio medium. This is achieved by controlling the use of time-shared resources such as CPUs and buffers, by shaping the flows and by scheduling the time of transmission of the packets that compose the flow at every hop.
Equipment failure, such as an access point rebooting, a broken radio adapter, or a permanent obstacle to the transmission, is a secondary source of packet loss. When a breakage occurs, multiple packets are lost in a row before the flows are rerouted or the system may recover. This is not acceptable for critical applications such as related to safety. A typical process control loop will tolerate an occasional packet loss, but a loss of several packets in a row will cause an emergency stop (e.g., after 4 packets lost, within a period of 1 second).
Network Availability is obtained by making the transmission resilient against hardware failures and radio transmission losses due to uncontrolled events such as co-channel interferers, multipath fading or moving obstacles. The best results are typically achieved by pseudo randomly cumulating all forms of diversity, in the spatial domain with replication and elimination, in the time domain with ARQ and diverse scheduled transmissions, and in the frequency domain with frequency hopping or channel hopping between frames.
In addition to the benefits listed in Section 3.1, scheduling transmissions provides specific value to the wireless medium.
On the one hand, scheduling avoids collisions between scheduled transmissions and can ensure both time and frequency diversity between retries in order to defeat co-channel interference from uncontroller transmitters as well as multipath fading. Transmissions can be scheduled on multiple channels in parallel, which enables to use the full available spectrum while avoiding the hidden terminal problem, e.g., when the next packet in a same flow interferes on a same channel with the previous one that progressed a few hops farther.
On the other hand, scheduling optimizes the bandwidth usage: compared to classical Collision Avoidance techniques, there is no blank time related to inter-frame space (IFS) and exponential back-off in scheduled operations. A minimal Clear Channel Assessment may be needed to comply with the local regulations such as ETSI 300-328, but that will not detect a collision when the senders are synchronized. And because scheduling allows a time sharing operation, there is no limit to the ratio of isolated critical traffic.
Finally, scheduling plays a critical role to save energy. In IOT, energy is the foremost concern, and synchronizing sender and listener enables to maintain them in deep sleep at all times when there is no scheduled transmission. This avoids idle listening and long preambles and enables long sleep periods between traffic and resynchronization, allowing battery-operated nodes to operate in a mesh topology for multiple years.
This specification does not require IANA action.
Most RAW technologies integrate some authentication or encryption mechanisms that were defined outside the IETF.
Many thanks to the participants of the RAW WG where a lot of the work discussed here happened.
| [I-D.ietf-6tisch-architecture] | Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-20, March 2019. |
| [RFC8200] | Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017. |