| lpwan | S. Farrell, Ed. |
| Internet-Draft | Trinity College Dublin |
| Intended status: Informational | December 5, 2016 |
| Expires: June 8, 2017 |
LPWAN Overview
draft-ietf-lpwan-overview-00
Low Power Wide Area Networks (LPWAN) are wireless technologies with characteristics such as large coverage areas, low bandwidth, possibly very small packet and application layer data sizes and long battery life operation. This memo is an informational overview of the set of LPWAN technologies being considered in the IETF and of the gaps that exist between the needs of those technologies and the goal of running IP in LPWANs.
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[[Ed: Editor comments/queries are in double square brackets like this. Note that the eventual fate of this draft is a topic for the WG to consider - it might end up as a useful RFC, or it might be best maintained as a draft only until its utility has dissapated. FWIW, the editor doesn't mind what outcome the WG choose.]]
This document provides background material and an overview of the technologies being considered in the IETF's Low Power Wide-Area Networking (LPWAN) working group. We also provide a gap analysis between the needs of these technologies and currently available IETF specifications.
Most technologies in this space aim for similar goals of supporting large numbers of low-cost, low-throughput devices at very low-cost and with very-low power consumption, so that even battery-powered devices can be deployed for years. And as the name implies, coverage of large areas is also a common goal. So, by and large, the different technologies aim for deployment in very similar circumstances.
Existing pilot deployments have shown huge potential and created much industrial interest in these technolgies. As of today, [[Ed: with the possible exception of Wi-SUN devices?]] essentially no LPWAN devices have IP capabilities. Connecting LPWANs to the Internet would provide significant benefits to these networks in terms of interoperability, application deployment, and management, among others. The goal of the LPWAN WG is to adapt IETF defined protocols, addressing schemes and naming to this particular constrained environment.
This document is largely the work of the people listed in Section 7. Discussion of this document should take place on the lp-wan@ietf.org list.
This section provides an overview of the set of LPWAN technologies that are being considered in the LPWAN working group. The text for each was mainly contributed by proponents of each technology.
Note that this text is not intended to be normative in any sesne, but simply to help the reader in finding the relevant layer 2 specifications and in understanding how those integrate with IETF-defined technologies. Similarly, there is no attempt here to set out the pros and cons of the relevant technologies. [[Ed: I assume that's the right target here. Please comment if you disagree.]]
[[Ed: the goal here is 2-3 pages per technology. If there's much more needed then we could add appendices I guess depending on what text the WG find useful to include.]]
[[Ed: A lot of the radio frequency related details below could disappear I think - for the purposes of this WG, I think a lot of that is extraneous detail. Haven't yet done that though, in case I'm missing something. It might also further imbalance the level of description of the different technologies, to the extent that the WG care explicitly about that.]]
[[Ed: Text here is from [I-D.farrell-lpwan-lora-overview]]]
LoRaWAN is a wireless technology for long-range low-power low-data-rate applications developed by the LoRa Alliance, a membership consortium. <https://www.lora-alliance.org/> This draft is based on version 1.0.2 [LoRaSpec] of the LoRa specification. (Version 1.0.2 is expected to be published in a few weeks. We will when that has happened. For now, version 1.0 is available at [LoRaSpec1.0])
In LoRaWAN networks, end-device transmissions may be received at multiple gateways, so during nominal operation a network server may see multiple instances of the same uplink message from an end-device.
The LoRaWAN network infrastructure manages the data rate and RF output power for each end-device individually by means of an adaptive data rate (ADR) scheme. End-devices may transmit on any channel allowed by local regulation at any time, using any of the currently available data rates.
LoRaWAN networks are typically organized in a star-of-stars topology in which gateways relay messages between end-devices and a central "network server" in the backend. Gateways are connected to the network server via IP links while end-devices use single-hop LoRaWAN communication that can be received at one or more gateways. All communication is generally bi-directional, although uplink communication from end-devices to the network server are favoured in terms of overall bandwidth availability.
Figure 1 shows the entities involved in a LoRaWAN network.
+----------+
|End-device| * * *
+----------+ * +---------+
* | Gateway +---+
+----------+ * +---------+ | +---------+
|End-device| * * * +---+ Network +--- Application
+----------+ * | | Server |
* +---------+ | +---------+
+----------+ * | Gateway +---+
|End-device| * * * * +---------+
+----------+
Key: * LoRaWAN Radio
+---+ IP connectivity
Figure 1: LoRaWAN architecture
LoRaWAN radios make use of ISM bands, for example, 433MHz and 868MHz within the European Union and 915MHz in the Americas.
The end-device changes channel in a pseudo-random fashion for every transmission to help make the system more robust to interference and/or to conform to local regulations.
Figure 2 below shows that after a transmission slot a Class A device turns on its receiver for two short receive windows that are offset from the end of the transmission window. End-devices can only transmit a subsequent uplink frame after the end of the associated receive windows. When a device joins a LoRaWAN network, there are similar timeouts on parts of that process.
|----------------------------| |--------| |--------|
| Tx | | Rx | | Rx |
|----------------------------| |--------| |--------|
|---------|
Rx delay 1
|------------------------|
Rx delay 2
Figure 2: LoRaWAN Class A transmission and reception window
Given the different regional requirements the detailed specification for the LoRaWAN physical layer (taking up more than 30 pages of the specification) is not reproduced here. Instead and mainly to illustrate the kinds of issue encountered, in Table 1 we present some of the default settings for one ISM band (without fully explaining those here) and in Table 2 we describe maxima and minima for some parameters of interest to those defining ways to use IETF protocols over the LoRaWAN MAC layer.
| Parameters | Default Value |
|---|---|
| Rx delay 1 | 1 s |
| Rx delay 2 | 2 s (must be RECEIVE_DELAY1 + 1s) |
| join delay 1 | 5 s |
| join delay 2 | 6 s |
| 868MHz Default channels | 3 (868.1,868.2,868.3), data rate: 0.3-5 kbps |
| Parameter/Notes | Min | Max |
|---|---|---|
| Duty Cycle: some but not all ISM bands impose a limit in terms of how often an end-device can transmit. In some cases LoRaWAN is more stringent in an attempt to avoid congestion. | 1% | no-limit |
| EU 868MHz band data rate/frame-size | 250 bits/s : 59 octets | 50000 bits/s : 250 octets |
| US 915MHz band data rate/frame-size | 980 bits/s : 19 octets | 21900 bits/s : 250 octets |
Note that in the case of the smallest frame size (19 octets), 8 octets are required for LoRa MAC layer headers leaving only 11 octets for payload (including MAC layer options). However, those settings do not apply for the join procedure - end-devices are required to use a channel that can send the 23 byte Join-request message for the join procedure.
Uplink and downlink higher layer data is carried in a MACPayload. There is a concept of "ports" (an optional 8 bit value) to handle different applications on an end-device. Port zero is reserved for LoRaWAN specific messaging, such as the join procedure.
In addition to carrying higher layer PDUs there are Join-Request and Join-Response (aka Join-Accept) messages for handling network access. And so-called "MAC commands" (see below) up to 15 bytes long can be piggybacked in an options field ("FOpts").
There are a number of MAC commands for: Link and device status checking, ADR and duty-cycle negotiation, managing the RX windows and radio channel settings. For example, the link check response message allows the network server (in response to a request from an end-device) to inform an end-device about the signal attenuation seen most recently at a gateway, and to also tell the end-device how many gateways received the corresponding link request MAC command.
Some MAC commands are initiated by the network server. For example, one command allows the network server to ask an end-device to reduce it's duty-cycle to only use a proportion of the maximum allowed in a region. Another allows the network server to query the end-device's power status with the response from the end-device specifying whether it has an external power source or is battery powered (in which case a relative battery level is also sent to the network server).
A LoRaWAN network has a short network identifier ("NwkID") which is a seven bit value. A private network (common for LoRaWAN) can use the value zero. If a network wishes to support "foreign" end-devices then the NwkID needs to be registered with the LoRA Alliance, in which case the NwkID is the seven least significant bits of a registered 24-bit NetID. (Note however, that the methods for "roaming" are currently being enhanced within the LoRA Alliance, so the situation here is somewhat fluid.)
In order to operate nominally on a LoRaWAN network, a device needs a 32-bit device address, which is the catentation of the NwkID and a 25-bit device-specific network address that is assigned when the device "joins" the network (see below for the join procedure) or that is pre-provisioned into the device.
End-devices are assumed to work with one or a quite limited number of applications, identified by a 64-bit AppEUI, which is assumed to be a registered IEEE EUI64 value. In addition, a device needs to have two symmetric session keys, one for protecting network artefacts (port=0), the NwkSKey, and another for protecting appliction layer traffic, the AppSKey. Both keys are used for 128 bit AES cryptographic operations. So, one option is for an end-device to have all of the above, plus channel information, somehow (pre-)provisioned, in which case the end-device can simply start transmitting. This is achievable in many cases via out-of-band means given the nature of LoRaWAN networks. Table 3 summarises these values.
| Value | Description |
|---|---|
| DevAddr | DevAddr (32-bits) = NwkId (7-bits) + device-specific network address (25 bits) |
| AppEUI | IEEE EUI64 naming the application |
| NwkSKey | 128 bit network session key for use with AES |
| AppSKey | 128 bit application session key for use with AES |
As an alternative, end-devices can use the LoRaWAN join procedure in order to setup some of these values and dynamically gain access to the network. To use the join procedure, an end-device must still know the AppEUI, and in addition, a different (long-term) symmetric key that is bound to the AppEUI - this is the application key (AppKey), and is distinct from the application session key (AppSKey). The AppKey is required to be specific to the device, that is, each end-device should have a different AppKey value. And finally the end-device also needs a long-term identifier for itself, syntactically also an EUI-64, and known as the device EUI or DevEUI. Table 4 summarises these values.
| Value | Description |
|---|---|
| DevEUI | IEEE EUI64 naming the device |
| AppEUI | IEEE EUI64 naming the application |
| AppKey | 128 bit long term application key for use with AES |
The join procedure involves a special exchange where the end-device asserts the AppEUI and DevEUI (integrity protected with the long-term AppKey, but not encrypted) in a Join-request uplink message. This is then routed to the network server which interacts with an entity that knows that AppKey to verify the Join-request. All going well, a Join-accept downlink message is returned from the network server to the end-device that specifies the 24-bit NetID, 32-bit DevAddr and channel information and from which the AppSKey and NwkSKey can be derived based on knowledge of the AppKey. This provides the end-device with all the values listed in Table 3.
All payloads are encrypted and have data integrity. MAC commands, when sent as a payload (port zero), are therefore protected. MAC commands piggy-backed as frame options ("FOpts") are however sent in clear. Any MAC commands sent as frame options and not only as payload, are visible to a passive attacker but are not malleable for an active attacker due to the use of the MIC.
For LoRaWAN version 1.0.x, the NWkSkey session key is used to provide data integrity between the end-device and the network server. The AppSKey is used to provide data confidentiality between the end-device and network server, or to the application "behind" the network server, depending on the implementation of the network.
All MAC layer messages have an outer 32-bit Message Integrity Code (MIC) calculated using AES-CMAC calculated over the ciphertext payload and other headers and using the NwkSkey. Payloads are encrypted using AES-128, with a counter-mode derived from IEEE 802.15.4 using the AppSKey. Gateways are not expected to be provided with the AppSKey or NwkSKey, all of the infrastructure-side cryptography happens in (or "behind") the network server. When session keys are derived from the AppKey as a result of the join procedure the Join-accept message payload is specially handled.
The long-term AppKey is directly used to protect the Join-accept message content, but the function used is not an aes-encrypt operation, but rather an aes-decrypt operation. The justification is that this means that the end-device only needs to implement the aes-encrypt operation. (The counter mode variant used for payload decryption means the end-device doesn't need an aes-decrypt primitive.)
The Join-accept plaintext is always less than 16 bytes long, so electronic code book (ECB) mode is used for protecting Join-accept messages. The Join-accept contains an AppNonce (a 24 bit value) that is recovered on the end-device along with the other Join-accept content (e.g. DevAddr) using the aes-encrypt operation. Once the Join-accept payload is available to the end-device the session keys are derived from the AppKey, AppNonce and other values, again using an ECB mode aes-encrypt operation, with the plaintext input being a maximum of 16 octets.
[[Ed: Text here is from [I-D.ratilainen-lpwan-nb-iot].]]
Narrowband Internet of Things (NB-IoT) is developed and standardized by 3GPP. The standardization of NB-IoT was finalized with 3GPP Release-13 in June 2016, but further enhancements for NB-IoT are worked on in the following releases, for example in the form of multicast support. For more information of what has been specified for NB-IoT, 3GPP specification 36.300 [TGPP36300] provides an overview and overall description of the E-UTRAN radio interface protocol architecture, while specifications 36.321 [TGPP36321], 36.322 [TGPP36322], 36.323 [TGPP36323] and 36.331 [TGPP36331] give more detailed description of MAC, RLC, PDCP and RRC protocol layers respectively.
[[Ed: Not clear what minimum/worst-case MTU might be. There are many 3GPP acronyms/terms to eliminate or explain.]]
Specific targets for NB-IoT include: Less than 5$ module cost, extended coverage of 164 dB maximum coupling loss, battery life of over 10 years, ~55000 devices per cell and uplink reporting latency of less than 10 seconds.
NB-IoT supports Half Duplex FDD operation mode with 60 kbps peak rate in uplink and 30 kbps peak rate in downlink, and a maximum size MTU of 1600 bytes. As the name suggests, NB-IoT uses narrowbands with the bandwidth of 180 kHz in both, downlink and uplink. The multiple access scheme used in the downlink is OFDMA with 15 kHz sub-carrier spacing. On uplink multi-tone SC-FDMA is used with 15 kHz tone spacing or as a special case of SC-FDMA single tone with either 15kHz or 3.75 kHz tone spacing may be used.
NB-IoT can be deployed in three ways. In-band deployment means that the narrowband is multiplexed within normal LTE carrier. In Guard-band deployment the narrowband uses the unused resource blocks between two adjacent LTE carriers. Also standalone deployment is supported, where the narrowband can be located alone in dedicated spectrum, which makes it possible for example to refarm the GSM carrier at 850/900 MHz for NB-IoT. All three deployment modes are meant to be used in licensed bands. The maximum transmission power is either 20 or 23 dBm for uplink transmissions, while for downlink transmission the eNodeB may use higher transmission power, up to 46 dBm depending on the deployment.
For signaling optimization, two options are introduced in addition to legacy RRC connection setup, mandatory Data-over-NAS (Control Plane optimization, solution 2 in [TGPP23720]) and optional RRC Suspend/Resume (User Plane optimization, solution 18 in [TGPP23720]). In the control plane optimization the data is sent over Non Access Stratum, directly from Mobility Management Entity (MME) in core network to the UE without interaction from base station. This means there are no Access Stratum security or header compression, as the Access Stratum is bypassed, and only limited RRC procedures.
The RRC Suspend/Resume procedures reduce the signaling overhead required for UE state transition from Idle to Connected mode in order to have a user plane transaction with the network and back to Idle state by reducing the signaling messages required compared to legacy operation
With extended DRX the RRC Connected mode DRX cycle is up to 10.24 seconds and in RRC Idle the DRX cycle can be up to 3 hours.
NB-IoT has no channel access restrictions allowing up to a 100% duty-cycle.
3GPP access security is specified in [TGPP33203].
+--+
|UE| \ +------+ +------+
+--+ \ | MME |------| HSS |
\ / +------+ +------+
+--+ \+-----+ / |
|UE| ----| eNB |- |
+--+ /+-----+ \ |
/ \ +--------+
/ \| | +------+ Service PDN
+--+ / | S-GW |----| P-GW |---- e.g. Internet
|UE| | | +------+
+--+ +--------+
Figure 3: 3GPP network architecture
Mobility Management Entity (MME) is responsible for handling the mobility of the UE. MME tasks include tracking and paging UEs, session management, choosing the Serving gateway for the UE during initial attachment and authenticating the user. At MME, the Non Access Stratum (NAS) signaling from the UE is terminated.
Serving Gateway (S-GW) routes and forwards the user data packets through the access network and acts as a mobility anchor for UEs during handover between base stations known as eNodeBs and also during handovers between other 3GPP technologies.
Packet Data Node Gateway (P-GW) works as an interface between 3GPP network and external networks.
Home Subscriber Server (HSS) contains user-related and subscription-related information. It is a database, which performs mobility management, session establishment support, user authentication and access authorization.
E-UTRAN consists of components of a single type, eNodeB. eNodeB is a base station, which controls the UEs in one or several cells.
The illustration of 3GPP radio protocol architecture can be seen from Figure 4.
+---------+ +---------+
| NAS |----|-----------------------------|----| NAS |
+---------+ | +---------+---------+ | +---------+
| RRC |----|----| RRC | S1-AP |----|----| S1-AP |
+---------+ | +---------+---------+ | +---------+
| PDCP |----|----| PDCP | SCTP |----|----| SCTP |
+---------+ | +---------+---------+ | +---------+
| RLC |----|----| RLC | IP |----|----| IP |
+---------+ | +---------+---------+ | +---------+
| MAC |----|----| MAC | L2 |----|----| L2 |
+---------+ | +---------+---------+ | +---------+
| PHY |----|----| PHY | PHY |----|----| PHY |
+---------+ +---------+---------+ +---------+
LTE-Uu S1-MME
UE eNodeB MME
Figure 4: 3GPP radio protocol architecture
The radio protocol architecture of NB-IoT (and LTE) is separated into control plane and user plane. Control plane consists of protocols which control the radio access bearers and the connection between the UE and the network. The highest layer of control plane is called Non-Access Stratum (NAS), which conveys the radio signaling between the UE and the EPC, passing transparently through radio network. It is responsible for authentication, security control, mobility management and bearer management.
Access Stratum (AS) is the functional layer below NAS, and in control plane it consists of Radio Resource Control protocol (RRC) [TGPP36331], which handles connection establishment and release functions, broadcast of system information, radio bearer establishment, reconfiguration and release. RRC configures the user and control planes according to the network status. There exists two RRC states, RRC_Idle or RRC_Connected, and RRC entity controls the switching between these states. In RRC_Idle, the network knows that the UE is present in the network and the UE can be reached in case of incoming call. In this state the UE monitors paging, performs cell measurements and cell selection and acquires system information. Also the UE can receive broadcast and multicast data, but it is not expected to transmit or receive singlecast data. In RRC_Connected the UE has a connection to the eNodeB, the network knows the UE location on cell level and the UE may receive and transmit singlecast data. RRC_Connected mode is established, when the UE is expected to be active in the network, to transmit or receive data. Connection is released, switching to RRC_Idle, when there is no traffic to save the UE battery and radio resources. However, a new feature was introduced for NB-IoT, as mentioned earlier, which allows data to be transmitted from the MME directly to the UE, while the UE is in RRC_Idle transparently to the eNodeB.
Packet Data Convergence Protocol's (PDCP) [TGPP36323] main services in control plane are transfer of control plane data, ciphering and integrity protection.
Radio Link Control protocol (RLC) [TGPP36322] performs transfer of upper layer PDUs and optionally error correction with Automatic Repeat reQuest (ARQ), concatenation, segmentation and reassembly of RLC SDUs, in-sequence delivery of upper layer PDUs, duplicate detection, RLC SDU discard, RLC-re-establishment and protocol error detection and recovery.
Medium Access Control protocol (MAC) [TGPP36321] provides mapping between logical channels and transport channels, multiplexing of MAC SDUs, scheduling information reporting, error correction with HARQ, priority handling and transport format selection.
Physical layer [TGPP36201] provides data transport services to higher layers. These include error detection and indication to higher layers, FEC encoding, HARQ soft-combining. Rate matching and mapping of the transport channels onto physical channels, power weighting and modulation of physical channels, frequency and time synchronization and radio characteristics measurements.
User plane is responsible for transferring the user data through the Access Stratum. It interfaces with IP and consists of PDCP, which in user plane performs header compression using Robust Header Compression (RoHC), transfer of user plane data between eNodeB and UE, ciphering and integrity protection. Lower layers in user plane are similarly RLC, MAC and physical layer performing tasks mentioned above.
Under worst-case conditions, NB-IoT may achieve data rate of roughly 200 bps. For downlink with 164 dB coupling loss, NB-IoT may achieve higher data rates, depending on the deployment mode. Stand-alone operation may achieve the highest data rates, up to few kbps, while in-band and guard-band operations may reach several hundreds of bps. NB-IoT may even operate with higher maximum coupling loss than 170 dB with very low bit rates.
[[Ed: Text here is from [I-D.zuniga-lpwan-sigfox-system-description].]]
The SIGFOX LPWAN is in line with the terminology and specifications being defined by the ETSI ERM TG28 Low Throughput Networks (LTN) group [etsi_ltn]. As of today, SIGFOX's network has been fully deployed in 6 countries, with ongoing deployments on 18 other countries, in total a geography containing 397M people.
SIGFOX LPWAN autonomous battery-operated devices send only a few bytes per day, week or month, in principle allowing them to remain on a single battery for up to 10-15 years. The capacity of a SIGFOX base station mainly depends on the number of messages generated by the devices, and not on the number of devices. The battery life of devices also depends on the number of messages generated by the device, but it is important to keep in mind that these devices are designed to last several years, some of them even buried underground. The coverage of the cell also depends on the link budget and on the type of deployment (urban, rural, etc.), which can vary from sending less than one message per device per day to dozens of messages per device per day.
The radio interface is compliant with the following regulations:
The SIGFOX LTN radio interface is also compliant with the local regulations of the following countries: Australia, Brazil, Canada, Kenya, Lebanon, Mauritius, Mexico, New Zealand, Oman, Peru, Singapore, South Africa, South Korea, and Thailand.
The radio interface is based on Ultra Narrow Band (UNB) communications, which allow an increased transmission range by spending a limited amount of energy at the device. Moreover, UNB allows a large number of devices to coexist in a given cell without significantly increasing the spectrum interference.
Both uplink and downlink communications are possible with the UNB solution. Due to spectrum optimizations, different uplink and downlink frames and time synchronization methods are needed.
The main radio characteristics of the UNB uplink transmission are:
In Europe, the UNB uplink frequency band is limited to 868,00 to 868,60 MHz, with a maximum output power of 25 mW and a maximum mean transmission time of 1%.
The format of the uplink frame is the following:
+--------+--------+--------+------------------+-------------+-----+ |Preamble| Frame | Dev ID | Payload |Msg Auth Code| FCS | | | Sync | | | | | +--------+--------+--------+------------------+-------------+-----+
Figure 5: Uplink Frame Format
The uplink frame is composed of the following fields:
The main radio characteristics of the UNB downlink transmission are:
In Europe, the UNB downlink frequency band is limited to 869,40 to 869,65 MHz, with a maximum output power of 500 mW with 10% duty cycle.
The format of the downlink frame is the following:
+------------+-----+---------+------------------+-------------+-----+ | Preamble |Frame| ECC | Payload |Msg Auth Code| FCS | | |Sync | | | | | +------------+-----+---------+------------------+-------------+-----+
Figure 6: Downlink Frame Format
The downlink frame is composed of the following fields:
The radio interface is optimized for uplink transmissions, which are asynchronous. Downlink communications are achieved by querying the network for existing data from the device.
A device willing to receive downlink messages opens a fixed window for reception after sending an uplink transmission. The delay and duration of this window have fixed values. The LTN network transmits the downlink message for a given device during the reception window. The LTN network selects the BS for transmitting the corresponding downlink message.
Uplink and downlink transmissions are unbalanced due to the regulatory constraints on the ISM bands. Under the strictest regulations, the system can allow a maximum of 140 uplink messages and 4 downlink messages per device. These restrictions can be slightly relaxed depending on system conditions and the specific regulatory domain of operation.
+--+
|EP| * +------+
+--+ * | RA |
* +------+
+--+ * |
|EP| * * * * |
+--+ * +----+ |
* | BS | \ +--------+
+--+ * +----+ \ | |
DA -----|EP| * * * | SC |----- NA
+--+ * / | |
* +----+ / +--------+
+--+ * | BS |/
|EP| * * * * +----+
+--+ *
*
+--+ *
|EP| * *
+--+
Figure 7: ETSI LTN architecture
Figure 7 depicts the different elements of the SIGFOX architecture.
SIGFOX has a "one-contract one-network" model allowing devices to connect in any country, without any notion of roaming.
The architecture consists of a single core network, which allows global connectivity with minimal impact on the end device and radio access network. The core network elements are the Service Center (SC) and the Registration Authority (RA). The SC is in charge of the data connectivity between the Base Station (BS) and the Internet, as well as the control and management of the BSs and End Points. The RA is in charge of the End Point network access authorization.
The radio access network is comprised of several BSs connected directly to the SC. Each BS performs complex L1/L2 functions, leaving some L2 and L3 functionalities to the SC.
The devices or End Points (EPs) are the objects that communicate application data between local device applications (DAs) and network applications (NAs).
EPs (or devices) can be static or nomadic, as they associate with the SC and they do not attach to a specific BS. Hence, they can communicate with the SC through one or many BSs.
Due to constraints in the complexity of the EP, it is assumed that EPs host only one or very few device applications, which communicate to one single network application at a time.
The radio protocol provides mechanisms to authenticate and ensure integrity of the message. This is achieved by using a unique device ID and a message authentication code, which allow ensuring that the message has been generated and sent by the device with the ID claimed in the message.
Security keys are independent for each device. These keys are associated with the device ID and they are pre-provisioned. Application data can be encrypted by the application provider.
[[Ed: Text here is via personal communication from Bob Heile (bheile@ieee.org) and was authored by Bob and Sum Chin Sean. Many references to specifications are still needed here.]]
The Wi-SUN Alliance <https://www.wi-sun.org/> is an industry alliance for smart city, smart grid, smart utility, and a broad set of general IoT applications. The Wi-SUN Alliance Field Area Network (FAN) profile is open standards based (primarily on IETF and IEEE802 standards) and was developed to address applications like smart municipality/city infrastructure monitoring and management, electric vehicle (EV) infrastructure, advanced metering infrastructure (AMI), distribution automation (DA), supervisory control and data acquisition (SCADA) protection/management, distributed generation monitoring and management, and many more IoT applications. Additionally, the Alliance has created a certification program to promote global multi-vendor interoperability.
The FAN profile [[Ed: reference needed!]] is an IPv6 frequency hopping wireless mesh network with support for enterprise level security. The frequency hopping wireless mesh topology aims to offer superior network robustness, reliability due to high redundancy, good scalability due to the flexible mesh configuration and good resilience to interference. Very low power modes are in development permitting long term battery operation of network nodes. [[Ed: details welcome.]]
[[Ed: this really needs the references.]] The FAN profile is based on various open standards in IETF, IEEE802 and ANSI/TIA for low power and lossy networks. The FAN profile specification provides an application-independent IPv6-based transport service for both connectionless (i.e. UDP) and connection-oriented (i.e. TCP) services. There are two possible methods for establishing the IPv6 packet routing: mandatory Routing Protocol for Low-Power and Lossy Networks (RPL) at the Network layer or optional Multi-Hop Delivery Service (MHDS) at the Data Link layer. Table 5 provides an overview of the FAN network stack.
The Transport service is based on User Datagram Protocol (UDP) defined in RFC768 or Transmission Control Protocol (TCP) defined in RFC793.
The Network service is provided by IPv6 defined in RFC2460 with 6LoWPAN adaptation as defined in RC4944 and RFC6282. Additionally, ICMPv6 as defined in RFC4443 is used for control plane in information exchange.
The Data Link service provides both control/management of the Physical layer and data transfer/management services to the Network layer. These services are divided into Media Access Control (MAC) and Logical Link Control (LLC) sub-layers. The LLC sub-layer provides a protocol dispatch service which supports 6LoWPAN and an optional MAC sub-layer mesh service. The MAC sub-layer is constructed using data structures defined in IEEE802.15.4-2015. Multiple modes of frequency hopping are defined. The entire MAC payload is encapsulated in an IEEE802.15.9 Information Element to enable LLC protocol dispatch between upper layer 6LoWPAN processing, MAC sublayer mesh processing, etc. These areas will be expanded once IEEE802.15.12 is completed
The PHY service is derived from a sub-set of the SUN FSK specification in IEEE802.15.4-2015. The 2-FSK modulation schemes, with channel spacing range from 200 to 600 kHz, are defined to provide data rates from 50 to 300 kbps, with Forward Error Coding (FEC) as an optional feature. Towards enabling ultra-low-power applications, the PHY layer design is also extendable to low energy and critical infrastructure monitoring networks, such as IEEE802.15.4k.
| Layer | Description |
|---|---|
| IPv6 protocol suite | TCP/UDP |
| 6LoWPAN Adaptation + Header Compression | |
| DHCPv6 for IP address management. | |
| Routing using RPL. | |
| ICMPv6. | |
| Unicast and Multicast forwarding. | |
| MAC based on IEEE 802.15.4e + IE extensions | Frequency hopping |
| Discovery and Join | |
| Protocol Dispatch (IEEE 802.15.9) | |
| Several Frame Exchange patterns | |
| Optional Mesh Under routing (ANSI 4957.210). | |
| PHY based on 802.15.4g | Various data rates and regions |
| Security | 802.1X/EAP-TLS/PKI Authentication. |
| 802.11i Group Key Management | |
| Optional ETSI-TS-102-887-2 Node 2 Node Key Management |
The FAN security supports Data Link layer network access control, mutual authentication, and establishment of a secure pairwise link between a FAN node and its Border Router, which is implemented with an adaptation of IEEE802.1X and EAP-TLS as described in RFC5216 using secure device identity as described in IEEE802.1AR. Certificate formats are based upon RFC5280. A secure group link between a Border Router and a set of FAN nodes is established using an adaptation of the IEEE802.11 Four-Way Handshake. A set of 4 group keys are maintained within the network, one of which is the current transmit key. Secure node to node links are supported between one-hop FAN neighbors using an adaptation of ETSI-TS-102-887-2. FAN nodes implement Frame Security as specified in IEEE802.15.4-2015.
[[Ed: Text here is from [I-D.minaburo-lpwan-gap-analysis].]]
LPWAN technologies, such as those discussed above, have similar architectures but different terminology. We can identify different types of entities in a typical LPWAN network:
+---------------------------------------------------------------------+ | Function/ | | | | | | Technology | LORAWAN | NB-IOT | SIGFOX | IETF | +--------------+-----------+------------+-------------+---------------+ | Sensor, | | | | | | Actuator, | End | User | End | Thing | |device, object| Device | Equipment | Point | (HOST) | +--------------+-----------+------------+-------------+---------------+ | Transceiver | | Evolved | Base | RADIO | | Antenna | Gateway | Node B | Station | GATEWAY | +--------------+-----------+------------+-------------+---------------+ | Server | Network | Serving- | Service |Network Gateway| | | Server | Gateway | Center | (ROUTER) | +--------------+-----------+------------+-------------+---------------+ | Security | Join | Home |Registration | | | Server | Server | Subscriber | Authority | AAA | | | | Server | | SERVER | +--------------+-----------+------------+-------------+---------------+ | Application |Application| Packet Data| Network | APPLICATION | | | Server |Node Gateway| Application | SERVER | +---------------------------------------------------------------------+
Figure 8: LPWAN Architecture Terminology
() () () | +------+ () () () () / \ +---------+ | AAA | () () () () () () / \========| /\ |====|Server| +-----------+ () () () | | <--|--> | +------+ |Application| () () () () / \============| v |==============| Server | () () () / \ +---------+ +-----------+ HOSTS Radio Gateways Network Gateway
Figure 9: LPWAN Architecture
[[Ed: Text here is from [I-D.minaburo-lpwan-gap-analysis].]]
IPv6 [RFC2460] has been designed to allocate addresses to all the nodes connected to the Internet. Nevertheless, the header overhead of at least 40 bytes introduced by the protocol is incompatible with LPWAN constraints. If IPv6 with no further optimization were used, several LPWAN frames would be needed just to carry the IP header. Another problem arises from IPv6 MTU requirements, which require the layer below to support at least 1280 byte packets [RFC2460].
IPv6 needs a configuration protocol (neighbor discovery protocol, NDP [RFC4861]) for a node to learn network parameters NDP generates regular traffic with a relatively large message size that does not fit LPWAN constraints.
In some LPWAN technologies, layer two multicast is not supported. In that case, if the network topology is a star, the solution and considerations of section 3.2.5 of [RFC7668] may be applied.
[[Ed: other things to maybe mention: IPsec, DHCPv6, anything with even 1 regular RTT needed, e.g. DNS.]]
Several technologies that exhibit significant constraints in various dimensions have exploited the 6LoWPAN suite of specifications [RFC4944], [RFC6282], [RFC6775] to support IPv6 [I-D.hong-6lo-use-cases]. However, the constraints of LPWANs, often more extreme than those typical of technologies that have (re)used 6LoWPAN, constitute a challenge for the 6LoWPAN suite in order to enable IPv6 over LPWAN. LPWANs are characterised by device constraints (in terms of processing capacity, memory, and energy availability), and specially, link constraints, such as:
6LoWPAN header compression reduces IPv6 (and UDP) header overhead by eliding header fields when they can be derived from the link layer, and by assuming that some of the header fields will frequently carry expected values. 6LoWPAN provides both stateless and stateful header compression. In the latter, all nodes of a 6LoWPAN are assumed to share compression context. In the best case, the IPv6 header for link-local communication can be reduced to only 2 bytes. For global communication, the IPv6 header may be compressed down to 3 bytes in the most extreme case. However, in more practical situations, the smallest IPv6 header size may be 11 bytes (one address prefix compressed) or 19 bytes (both source and destination prefixes compressed). These headers are large considering the link layer payload size of LPWAN technologies, and in some cases are even bigger than the LPWAN PDUs. 6LoWPAN has been initially designed for IEEE 802.15.4 networks with a frame size up to 127 bytes and a throughput of up to 250 kb/s, which may or may not be duty-cycled.
Traditionally, Interface Identifiers (IIDs) have been derived from link layer identifiers [RFC4944] . This allows optimisations such as header compression. Nevertheless, recent guidance has given advice on the fact that, due to privacy concerns, 6LoWPAN devices should not be configured to embed their link layer addresses in the IID by default.
As stated above, IPv6 requires the layer below to support an MTU of 1280 bytes [RFC2460]. Therefore, given the low maximum payload size of LPWAN technologies, fragmentation is needed.
If a layer of an LPWAN technology supports fragmentation, proper analysis has to be carried out to decide whether the fragmentation functionality provided by the lower layer or fragmentation at the adaptation layer should be used. Otherwise, fragmentation functionality shall be used at the adaptation layer.
6LoWPAN defined a fragmentation mechanism and a fragmentation header to support the transmission of IPv6 packets over IEEE 802.15.4 networks [RFC4944]. While the 6LoWPAN fragmentation header is appropriate for IEEE 802.15.4-2003 (which has a frame payload size of 81-102 bytes), it is not suitable for several LPWAN technologies, many of which have a maximum payload size that is one order of magnitude below that of IEEE 802.15.4-2003. The overhead of the 6LoWPAN fragmentation header is high, considering the reduced payload size of LPWAN technologies and the limited energy availability of the devices using such technologies. Furthermore, its datagram offset field is expressed in increments of eight octets. In some LPWAN technologies, the 6LoWPAN fragmentation header plus eight octets from the original datagram exceeds the available space in the layer two payload. In addition, the MTU in the LPWAN networks could be variable which implies a variable fragmentation solution.
6LoWPAN Neighbor Discovery [RFC6775] defined optimizations to IPv6 Neighbor Discovery [RFC4861], in order to adapt functionality of the latter for networks of devices using IEEE 802.15.4 or similar technologies. The optimizations comprise host-initiated interactions to allow for sleeping hosts, replacement of multicast-based address resolution for hosts by an address registration mechanism, multihop extensions for prefix distribution and duplicate address detection (note that these are not needed in a star topology network), and support for 6LoWPAN header compression.
6LoWPAN Neighbor Discovery may be used in not so severely constrained LPWAN networks. The relative overhead incurred will depend on the LPWAN technology used (and on its configuration, if appropriate). In certain LPWAN setups (with a maximum payload size above ~60 bytes, and duty-cycle-free or equivalent operation), an RS/RA/NS/NA exchange may be completed in a few seconds, without incurring packet fragmentation.
In other LPWANs (with a maximum payload size of ~10 bytes, and a message rate of ~0.1 message/minute), the same exchange may take hours or even days, leading to severe fragmentation and consuming a significant amount of the available network resources. 6LoWPAN Neighbor Discovery behavior may be tuned through the use of appropriate values for the default Router Lifetime, the Valid Lifetime in the PIOs, and the Valid Lifetime in the 6CO, as well as the address Registration Lifetime. However, for the latter LPWANs mentioned above, 6LoWPAN Neighbor Discovery is not suitable.
The 6lo WG has been reusing and adapting 6LoWPAN to enable IPv6 support over link layer technologies such as Bluetooth Low Energy (BTLE), ITU-T G.9959, DECT-ULE, MS/TP-RS485, NFC or IEEE 802.11ah. These technologies are similar in several aspects to IEEE 802.15.4, which was the original 6LoWPAN target technology. [[Ed: refs?]]
6lo has mostly used the subset of 6LoWPAN techniques best suited for each lower layer technology, and has provided additional optimizations for technologies where the star topology is used, such as BTLE or DECT-ULE.
The main constraint in these networks comes from the nature of the devices (constrained devices), whereas in LPWANs it is the network itself that imposes the most stringent constraints. [[Ed: I'm not sure that conclusion follows from the information provided in this section - is more needed?.]]
The 6tisch solution is dedicated to mesh networks that operate using 802.15.4e MAC with a deterministic slotted channel. The TSCH [[Ed: expand on 1st use]] can help to reduce collisions and to enable a better balance over the channels. It improves the battery life by avoiding the idle listening time for the return channel.
A key element of 6tisch is the use of synchronization to enable determinism. TSCH and 6TiSCH may provide a standard scheduling function. The LPWAN networks probably will not support synchronization like the one used in 6tisch.
RoHC [[Ed: expand on 1st use]] header compression mechanisms were defined for point to point multimedia channels, to reduce the header overhead of RTP flows. RoHC can also reduce the overhead of IPv4 or IPv6 or UDP headers. It is based on shared context which does not require any state but compressed packets are not routable. The context is initialised at the beginning of the communication or when it [[Ed: which "it"?]] is lost. The compression is managed using a sequence number (SN) which is encoded using a windowing algorithm allowing for reduction of the SN to 4 bits instead of 2 bytes. [[Ed: is that the 2 bytes as per 6lowPAN?]] But this window needs to be updated each 15 packets which implies larger headers. When RoHC is used we talk about an average header compression size to give the performance of compression. For example, RoHC starts sending bigger packets than the original (52 bytes) to reduce the header up to 4 bytes (it stays here only for 15 packets, which correspond to the window size). Each time the context is lost or needs to be synchronised, packets of about 15 to 43 bytes are sent. [[Ed: the above isn't that cleaar to me.]]
RoHC is not adapted to the constrained nodes of the LPWAN networks: it does not take into account the energy limitations and the transmission rate, and context is synchronised during the transmission, which does not allow a better compression. [[Ed: this seems to conflict a bit with what was said about 6tisch which puzzled me.]]
Most technologies considered by the lpwan WG are based on a star topology, which eliminates the need for routing at that layer. Future work may address additional use-cases that may require adaptation of existing routing protocols or the definition of new ones. As of the time of writing, work similar to that done in the ROLL WG and other routing protocols are out of scope of the LPWAN WG.
CoAP [RFC7252] provides a RESTful framework for applications intended to run on constrained IP networks. It may be necessary to adapt CoAP or related protocols to take into account for the extreme duty cycles and the potentially extremely limited throughput of LPWANs.
For example, some of the timers in CoAP may need to be redefined. Taking into account CoAP acknowledgements may allow the reduction of L2 acknowledgements. On the other hand, the current work in progress in the CoRE WG where the COMI/CoOL network management interface which, uses Structured Identifiers (SID) to reduce payload size over CoAP proves to be a good solution for the LPWAN technologies. The overhead is reduced by adding a dictionary which matches a URI to a small identifier and a compact mapping of the YANG model into the CBOR binary representation.
LPWANs nodes can be mobile. However, LPWAN mobility is different from the one specified for Mobile IP. LPWAN implies sporadic traffic and will rarely be used for high-frequency, real-time communications. The applications do not generate a flow, they need to save energy and most of the time the node will be down. The mobility will imply most of the time a group of devices, which represent a network itself. The mobility concerns more the gateway than the devices.
NEMO [[Ed: refs?]] Mobility solutions may be used in the case where some hosts belonging to the same Network gateway will move from one point to another and that they are not aware of this mobility.
The purpose of the DNS is to enable applications to name things that have a global unique name. Lots of protocols are using DNS to identify the objects, especially REST and applications using CoAP. Therefore, hosts (things), or the named services they use, should be registered in DNS. DNS is probably a good topic of research for LPWAN technologies, while the matching of the name and the IP information can be used to configure the LPWAN devices. [[Ed: I'm not sure what that last bit means.]]
[[Ed: be good to add stuff here about a) privacy and b) difficulties with getting current security protocols to work in this context. For a) maybe try find nice illustrations, e.g. extremecom instrumeted-igloo traces (temperature change allowing one to infer when someone took a pee:-). For b) things like IPsec/(D)TLS/OCSP and NTP to work in these environments. Not sure how much of that is known or useful for the WG. Probably worth noting the IAB statement on confidentiality and to ponder the impact of more than one layer of encryption in this context. Text below is basically from the "gaps" draft.]]
Most LPWAN technologies integrate some authentication or encryption mechanisms that were defined outside the IETF. The working group may need to do work to integrate these mechanisms to unify management. A standardized Authentication, Accounting and Authorization (AAA) infrastructure [RFC2904] may offer a scalable solution for some of the security and management issues for LPWANs. AAA offers centralized management that may be of use in LPWANs, for example [I-D.garcia-dime-diameter-lorawan] and [I-D.garcia-radext-radius-lorawan] suggest possible security processes for a LoRaWAN network. Similar mechanisms may be useful to explore for other LPWAN technologies.
There are no IANA considerations related to this memo.
As stated above this document is mainly a collection of content developed by the full set of contributors listed below. The main input documents and their authors were:
The full list of contributors are:
Jon Crowcroft University of Cambridge JJ Thomson Avenue Cambridge, CB3 0FD United Kingdom Email: jon.crowcroft@cl.cam.ac.uk Carles Gomez UPC/i2CAT C/Esteve Terradas, 7 Castelldefels 08860 Spain Email: carlesgo@entel.upc.edu Bob Heile Wi-Sun Alliance 11 Robert Toner Blvd, Suite 5-301 North Attleboro, MA 02763 USA Phone: +1-781-929-4832 Email: bheile@ieee.org Ana Minaburo Acklio 2bis rue de la Chataigneraie 35510 Cesson-Sevigne Cedex France Email: ana@ackl.io Josep PAradells UPC/i2CAT C/Jordi Girona, 1-3 Barcelona 08034 Spain Email: josep.paradells@entel.upc.edu Benoit Ponsard SIGFOX 425 rue Jean Rostand Labege 31670 France Email: Benoit.Ponsard@sigfox.com URI: http://www.sigfox.com/ Antti Ratilainen Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: antti.ratilainen@ericsson.com Chin-Sean SUM Wi-Sun Alliance 20, Science Park Rd Singapore 117674 Phone: +65 6771 1011 Email: sum@wi-sun.org Laurent Toutain Institut MINES TELECOM ; TELECOM Bretagne 2 rue de la Chataigneraie CS 17607 35576 Cesson-Sevigne Cedex France Email: Laurent.Toutain@telecom-bretagne.eu Alper Yegin Actility Paris, Paris FR Email: alper.yegin@actility.com Juan Carlos Zuniga SIGFOX 425 rue Jean Rostand Labege 31670 France Email: JuanCarlos.Zuniga@sigfox.com URI: http://www.sigfox.com/
Thanks to all those listed in Section 7 for the excellent text. Errors in the handling of that are solely the editor's fault.
In addition to the contributors above, thanks are due to Jiazi Yi, [your name here] for comments.
Stephen Farrell's work on this memo was supported by the Science Foundation Ireland funded CONNECT centre <https://connectcentre.ie/>.