Lightweight GeneRic Autonomic Signaling Protocol

In the IoT that has developed rapidly in recent years, the traditional centralized and human-centered network management methods have gradually shown defects such as low efficiency and high operating costs due to the growth in the number, heterogeneity, diversity, and the increasingly uncertain distribution of devices. Autonomic Network empowers networks and devices with self-management capabilities, enabling them to self-configure, self-optimize, self-recover, and self-protect without human intervention, effectively improving the stability and reliability of the network, which meets the development needs and trends of the IoT and is essential for implementing IoT applications such as smart homes, smart cities, and industrial IoT. As a new network management solution for TCP/IP networks, the Autonomic Network does not intend to break the existing IP-based network architecture. So does the GRASP, the signaling protocol in the Autonomic Network. While located between the transport layer and the application layer, GRASP provides reliable and efficient services for nodes in the Autonomic Network, like parameter discovery, exchange, and negotiation, based on the TCP/IP protocols. Since it does not provide reliability mechanisms such as error detection, retransmission, and flow control, GRASP must depend on the reliability mechanisms provided by the transport layer, particularly its synchronization and negotiation procedures based on one or more round(s) of message interaction. It is specified in that GRASP unicast messages MUST use the reliable transport layer protocol, e.g., TCP. However, the reliability provided by TCP is not free. GRASP must tolerate the inevitable additional latency, control overhead, and memory consumption caused by complex reliability mechanisms of TCP, e.g., the resource consumption and control overhead associated with establishing, maintaining, and closing TCP connections. In addition, the size of the TCP/IP stack on which GRASP relies and the memory resources required to run it are not negligible, e.g., running a standard full TCP/IP stack requires at least tens to hundreds of KBs of data and code memory, and even TCP/IP stacks specifically designed and implemented for resource-constrained devices require tens of KBs of memory. However, the resource-constrained device typically has only about 50KB of memory. Obviously, in the IoT networks dominated by resource-constrained devices with limited CPU, memory, and power resources, the resource footprint of the TCP/IP stack and its execution, especially the TCP, is likely to be a limiting factor in the deployment of the Autonomic Network and GRASP. Therefore, making GRASP lightweight and removing its dependence on TCP or even IP is of great significance for the deployment and promotion of GRASP in the IoT. In addition, considering the generally short length of interaction messages between IoT nodes, it is also necessary to shorten the length of GRASP messages with the best efforts, especially the control fields, which can also reduce the overhead of nodes in processing, parsing, and sending GRASP messages. Considering the demand for self-management and the resource-constrained feature of IoT devices, this document proposes the UDP-based Lightweight GRASP (LW-GRASP). By reducing the length of fixed fields, and adding a built-in reliability mechanism with the acknowledgment and retransmission capability, LW-GRASP can provide reliable signaling services without relying on TCP. In addition, to better address the need for self-management of the IoT, the possible IP-independent extension is discussed, which can extend the use of LW-GRASP to networks without IP connectivity.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

LW-GRASP is designed to be UDP-based to avoid the additional control overhead and memory consumption caused by TCP, thus matching the capabilities of IoT nodes. Meanwhile, to ensure reliability, the LW-GRASP introduces a message-oriented built-in reliability mechanism. LW-GRASP uses the 16-bit random number called Nonce to implement the acknowledgment and retransmission mechanism for messages to avoid interaction failures caused by message losses. However, as discussed in , not all LW-GRASP messages require acknowledgment, such as multicast messages. The LW-GRASP messages that require acknowledgment are referred to in this document as confirmable messages, and the others that do not require acknowledgment are referred to as non-confirmable messages. The transmission of confirmable messages MUST use the reliability mechanism defined in this section, while non-confirmable messages do not.

When sending a confirmable message, the LW-GRASP sender MUST generate a 16-bit random Nonce and append the Nonce to the message. Upon receipt of a confirmable message, the receiver MUST acknowledge immediately using the same Nonce as that of the received, or wait for a post-order message in the same direction and piggyback acknowledge with this message within the LW_GRASP_ACK_DELAYED_TIME. The latter is the delayed acknowledgment, if there is no corresponding message to be sent within the LW_GRASP_ACK_DELAYED_TIME, an ACK message MUST be sent immediately. LW-GRASP defines two new options, i.e., the REQ-ACK option and the ACK option. The REQ-ACK option is used to carry the Nonce generated by LW-GRASP for a specific confirmable message and MUST be added to this message as an option. The ACK option also contains a Nonce for acknowledging a corresponding confirmable message, which MUST be added as an option to an ACK message (immediate acknowledgment) or a post-order message in the same direction (delayed acknowledgment). The REQ-ACK option, the ACK option, and the ACK message are defined in , , and , respectively. The Nonce can be regarded as the unique identifier of a confirmable message before it is acknowledged. Thus, the LW-GRASP nodes MUST avoid Nonce conflicts among unacknowledged confirmable messages. Specifically, the Nonce SHOULD be generated by a pseudo-random number generator (PRNG) based on the locally generated unique seed to avoid the conflict of Nonce generated by different nodes in the same network. Meanwhile, the LW-GRASP instance SHOULD create and maintain a Nonce cache to record the Nonce used by confirmable messages. After generating a Nonce for a message, the LW-GRASP MUST check whether it conflicts with an existing entry in the Nonce cache, and if it doesn't, it SHOULD record the Nonce in the cache. Otherwise, the Nonce for the confirmable message MUST be regenerated. After the GRASP node receives a message with an ACK option(or more than one ACK option), it SHOULD first extract the Nonce from it and check whether there is a corresponding entry with the same Nonce value in the Nonce cache; if not, the received message SHOULD be directly ignored. Otherwise, the GRASP node SHOULD mark the Nonce entry as acknowledged and delete it when the corresponding LW-GRASP session is completed. It is worth emphasizing that confirmable messages marked as acknowledged SHOULD also be considered by the aforementioned Nonce conflict detection. The LW-GRASP sender MUST set the retransmission timer when sending a confirmable message; see for details on setting the timeout. If the LW-GRASP confirmable message does not get an acknowledgment within the retransmission timeout, then the message MUST be retransmitted. The retransmission message SHOULD keep the Nonce the same as the original message. However, when a confirmable message has been accepted and processed by the receiver but is retransmitted due to lost acknowledgment, the LW-GRASP can not identify the retransmission message and will repeatedly process it, which can be dangerous. Thus, the LW-GRASP receiver SHOULD record and cache the Nonces of confirmable messages that have been received and processed for each LW-GRASP session until it is completed and check whether the Nonce of each arriving message conflicts with the cached Nonces, if it doesn't, then accept and process it. Otherwise, which means the message is a retransmission message, LW-GRASP SHOULD discard it and send acknowledgment, to avoid duplicated processing of the retransmission and original messages due to the loss of the acknowledgment. The delayed acknowledgment mechanism can reduce the communication cost caused by the ACK message, but its waiting time may cause unnecessary delay, which reduces the efficiency of communication. In the actual LW-GRASP implementation, users SHOULD be allowed to enable or completely disable delayed acknowledgment according to their needs.

The retransmission timeout for reliable transmission of LW-GRASP messages is LW_GRASP_RETRANS_TIMEOUT. If the LW-GRASP message is not acknowledged within the retransmission timeout and the number of retransmissions does not reach MAX_RETRANS, the message MUST be retransmitted and the retransmission timer SHOULD be reset, the retransmission timeout SHOULD be incremented to twice, and the number of retransmissions SHOULD be incremented by 1. If the LW-GRASP message is not acknowledged within the retransmission timeout and the number of retransmissions exceeds MAX_RETRANS, the retransmission MUST be discarded, and the transmission fails.

LW-GRASP has made modifications to the standard GRASP by reducing the fixed fields and introducing a message-oriented built-in reliability mechanism with the acknowledgment and retransmission capability based on Nonce. To achieve this, LW-GRASP redefines the Objective option in standard GRASP as the LW-Objective option and defines a new message named ACK message, along with two new options named REQ-ACK option and ACK option. However, LW-GRASP does not modify the discovery, negotiation, synchronization, and flooding procedures, as well as the defined options (except for the Objective option) of the standard GRASP. In addition, LW-GRASP still adheres to the High-Level Deployment Model and High-Level Design defined for GRASP, so as not to affect the signaling service provided by the protocol. In order to differentiate from standard GRASP, LW-GRASP instances SHOULD listen for messages using a new well-known port, LW_GRASP_LISTEN_PORT (TBD1).

Like standard GRASP, LW-GRASP messages continue to be transmitted in Concise Binary Object Representation (CBOR) and be described using Concise Data Definition Language (CDDL). The session-id in the LW-GRASP message is shortened from 32 bits to 16 bits to minimize the length of the message, while the meanings of the other fields are still consistent with the standard GRASP message. In fragmentary CDDL, a LW-GRASP message follows the pattern:

In fragmentary CDDL, a LW-GRASP Objective option follows the pattern:

Instead of using the text string with indefinite length (i.e., objective-name) as the unique identifier for the Objective option, the LW-Objective option is uniquely identified by an 8-bit number (i.e., objective-num), with the remaining fields keeping consistent with the Objective option in standard GRASP. The first two bits of objective-num indicate the LW-Objective type (00, 01, and 10 stand for generic LW-Objective; 11 stands for privately defined LW-Objective), and represent the number of LW-Objective together with the remaining six bits. Each generic LW-Objective MUST be assigned a unique objective number and be made public to all LW-GRASP nodes when it's registered. When a private LW-Objective is defined, it MUST also be assigned a uniquely distinguishable objective number and be made public within the specific private domain. In LW-GRASP, the identifier of the LW-Objective option is changed from the text string with indefinite length to the 8-bit number, which can minimize the length of the LW-Objective option, and also can avoid the additional communication cost caused by excessively long objective-name text strings, and the overhead of byte-by-byte comparison and identification of objective-name in the standard GRASP.

The REQ-ACK option is used to indicate that the message MUST be acknowledged by the receiver. When a message needs acknowledgment (i.e., the confirmable message), the sender MUST generate the REQ-ACK option and add it to the message to request the receiver to acknowledge. The REQ-ACK option MUST NOT be allowed to appear in the non-confirmable message (like the Discovery message and the Flood Synchronization message) to avoid a large number of ACK messages in a short time. In fragmentary CDDL, a REQ-ACK option follows the pattern:

Nonce is a 16-bit random number and MUST avoid local conflicts. The Nonce generation and conflict prevention mechanisms are described in .

LW-GRASP also defines an ACK option for acknowledging messages carrying a REQ-ACK option. Upon receiving a message with the REQ-ACK option, as specified in , the LW-GRASP receiver MUST either promptly send an ACK message with a corresponding ACK option; or wait a while for a post-order message in the same direction to be sent and add the ACK option to that message to piggyback acknowledge. The ACK option MUST only be allowed to appear in confirmable messages, as discussed in . In fragmentary CDDL, an ACK option follows the pattern:

Where, the Nonce MUST be the same as the Nonce in the corresponding REQ-ACK option.

LW-GRASP reserves all the message types and values of the standard GRASP, as well as the definitions of each related field. LW-GRASP extends its unicast messages to allow them to carry the REQ-ACK option or the ACK option, enabling LW-GRASP to implement a built-in reliability mechanism. All unicast messages used in the three procedures of discovery, negotiation, and synchronization of LW-GRASP MUST be acknowledged to ensure the reliability and operational efficiency of the interactions. At the same time, these unicast messages are allowed to carry zero or more ACK option(s) to acknowledge the confirmable message belonging to the same or different interaction session(s). In addition, Invalid messages are used to respond to invalid messages and contain related diagnostic information which if lost may affect the subsequent message interactions, thus its acknowledgment is necessary and MUST carry a REQ-ACK option. Similarly, the Invalid message can also carry zero or more ACK option(s) for acknowledgment. The Discovery message and Flood Synchronization message that is multicast, as well as the NOOP message that does not contain actual information, are not allowed to carry the REQ-ACK option or the ACK option, i.e., non-confirmable message, whose definition is the same as the standard GRASP and will not be repeated here. The CDDL definitions for messages with extension( i.e. the confirmable message) for reliability are defined as follows:

In addition, LW-GRASP defines an ACK message for immediate acknowledgment. In fragmentary CDDL, an ACK message follows the pattern:

The Nonce in the ACK option must be the same as the corresponding REQ-ACK option.

LW_GRASP_LISTEN_PORT(TBD1)A well-known UDP user port that every LW-GRASP-enabled network device MUST listen to for UDP-based messages. LW_GRASP_ACK_DELAYED_TIME(200 milliseconds)The default maximum waiting time for delayed acknowledgment. LW_GRASP_RETRANS_TIMEOUT(2000 milliseconds)The default timeout is used to determine that a LW-GRASP confirmable message needs to be resent. MAX_RETRANS(3)The default maximum times of retransmission for confirmable messages. In addition, the constants for LW-GRASP also contain the ALL_LW_GRASP_NEIGHBORS, LW_GRASP_DEF_TIMEOUT, LW_GRASP_DEF_LOOPCT, LW_GRASP_DEF_MAX_SIZE, whose definitions and values are respectively same as the ALL_GRASP_NEIGHBORS, GRASP_DEF_TIMEOUT, GRASP_DEF_LOOPCT, GRASP_DEF_MAX_SIZE in GRASP.

In some IoT scenarios where the need for self-management is urgent, resource-constrained devices in it may not or choose not to support IP connectivity. Therefore, to improve the generality of LW-GRASP and better support the self-management requirements of the IoT, it is necessary to further discuss how LW-GRASP adapts to networks without the IP connection.

The GRASP and its lightweight version LW-GRASP can only work in IP networks, due to the Locator options used by them. The Locator option is used to locate resources, services, devices, and interfaces on the network and is the basis for GRASP and LW-GRASP discovery, negotiation, and synchronization procedures. All the four Locator options defined in have unique identification capabilities only within an IP network: O_IPv6_LOCATOR, O_IPv4_LOCATOR, O_FQDN_LOCATOR, O_URI_LOCATOR, which respectively depend on the IPv6 address, IPv4 address, Fully Qualified Domain Name (FQDN), and Uniform Resource identifier (URI) for identification and location. Therefore, to enable the LW-GRASP to work without the IP connection and provide services to LW-GRASP-enabled nodes, it's necessary to select an identifier(such as the MAC address in the Ethernet) based on the environment and define a new Locator option in the LW-GRASP to identify and locate a device, interface, resource, or service that can remove dependence of the LW-GRASP on IP. Using LW-GRASP without the IP connection requires not only the definition of new Locator options but also the identification of LW-GRASP so that network nodes and devices can recognize LW-GRASP messages encapsulated in specific bearer protocol messages. For example, designs GRASP as a user program, using a well-known port to identify GRASP messages. In practice, the protocol identification of LW-GRASP should be chosen and extended by the bearer protocol on which it depends, which is out of the scope of this document.

In the IoT, where the need for self-management is more urgent, the memory, energy, and computation overheads associated with IP connectivity and transmission may be unacceptable for its resource-constrained devices. In addition, considering the episodic feature of information interactions between IoT devices, some resource-constrained devices may prefer to use low-power and low-bandwidth network connections based on technologies such as Bluetooth Low Energy and Zigbee rather than IP connections. This section discusses how LW-GRASP adapts to BLE environments without IP connectivity. The core protocol used to establish and manage communication between devices in BLE is the Generic Attribute Profile (GATT, Volume 3 PART G in ), which defines how data is transferred between two BLE devices based on the concepts of Services and Characteristics. In BLE, data is transferred and stored in the form of Characteristics, and the Service is a collection of Characteristics, both identified by a unique numeric ID called UUID. GATT is at the top layer of the BLE stack and can provide API interfaces directly to the upper-layer applications, so it is possible to discuss the LW-GRASP-over-GATT to exchange LW-GRASP over BLE. LW-GRASP-over-GATT can define and use one or more GATT Characteristic(s) to transport LW-GRASP messages. With the unique identification UUID of the GATT Characteristic, the device can easily recognize whether the transmitted data is a LW-GRASP message or not. Regarding address identification, BLE devices use a 48-bit device address as a device identifier. As described in , the LW-GRASP-over-GATT should define and register a new Locator option based on this identifier. However, since the read/write semantics of the GATT characteristic do not fully match the semantics of the actions associated with the LW-GRASP interaction procedures, how to bridge this gap is an important step in realizing LW-GRASP-over-GATT. In addition, BLE provides both reliable ("write without response", "notify") and unreliable ("write with response", "indicate") data transmission, and how to choose between the two modes of data transmission for LW-GRASP-over-GATT needs to be carefully considered.

This document defines the Lightweight GeneRic Autonomic Signaling Protocol (LW-GRASP). As specified in , the IANA is requested to assign a USER PORT(LW_GRASP_LISTEN_PORT, TBD1) for use by LW-GRASP over UDP. Like the standard GRASP, LW-GRASP also requires IANA to create the "Lightweight GeneRic Autonomic Signaling Protocol (LW-GRASP) Parameters" registry. The "Lightweight GeneRic Autonomic Signaling Protocol (LW-GRASP) Parameters" should also include two subregistries: "LW-GRASP Messages and Options" and "LW-GRASP Objective Numbers". The "LW-GRASP Messages and Options" MUST retain all the entries in the "GRASP Messages and Options" subregistry assigned for the standard GRASP, and MUST also add three entries for the new message named "M_ACK", and the two new options named "O_REQ_ACK" and "O_ACK", whose initial values assigned by this document are like the following:

The initial numbers for the "LW-GRASP Objective Numbers" subregistry assigned by this document are like the following:

As a lightweight version of GRASP, LW-GRASP must attach importance to the security considerations of GRASP discussed in . In addition, given the limited capabilities and weak tamper resistance of constrained nodes, as well as their possible exposure to insecure environments, security issues associated with constrained nodes must not be ignored by the external secure infrastructure (e.g., the ACP) on which the LW-GRASP is based, e.g., the constrained code space and CPU for implementing cryptographic primitives.