Transmission of IPv6 over Multidrop Serial Bus/Token Passing (MS/TP) Networks

Transmission of IPv6 over Multidrop Serial Bus/Token Passing (MS/TP) Networks Consultant

USA kerlyn@ieee.org

Internet 6Lo Working Group Multidrop Serial Bus/Token Passing (MS/TP) is a medium access control method for the RS-485 physical layer and is used primarily in building automation networks. This specification defines the frame format for transmission of IPv6 packets and the method of forming link-local and statelessly autoconfigured IPv6 addresses on MS/TP networks. It obsoletes RFC 8163.

Introduction Multidrop Serial Bus/Token Passing (MS/TP) is a Medium Access Control (MAC) protocol for the RS-485 physical layer and is used primarily in building automation networks. This specification defines the frame format for transmission of IPv6 packets and the method of forming link-local and statelessly autoconfigured IPv6 addresses on MS/TP networks. The general approach is to adapt elements of the 6LoWPAN specifications (, , and ) to constrained wired networks, as noted below. An MS/TP device is typically based on a low-cost microcontroller with limited processing power and memory. These constraints, together with low data rates and a small MAC address space, are similar to those faced in 6LoWPAN networks. MS/TP differs significantly from 6LoWPAN in at least three respects: a) MS/TP devices are typically mains powered, b) all MS/TP devices on a segment can communicate directly so there are no hidden node or mesh routing issues, and c) the latest MS/TP specification provides support for large payloads, eliminating the need for fragmentation and reassembly below IPv6. The following sections provide a brief overview of MS/TP and then describe how to form IPv6 addresses and encapsulate IPv6 packets in MS/TP frames. This specification (subsequently referred to as "6LoBAC") includes a REQUIRED header compression mechanism that is based on LOWPAN_IPHC and improves MS/TP link utilization.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

Abbreviations Used ASHRAE: American Society of Heating, Refrigerating, and Air- Conditioning Engineers <http://www.ashrae.org> BACnet: An ISO/ANSI/ASHRAE Standard Data Communication Protocol for Building Automation and Control Networks CRC: Cyclic Redundancy Code MAC: Medium Access Control MSDU: MAC Service Data Unit (MAC client data) MTU: Maximum Transmission Unit; the size of the largest data unit at the network-layer protocol that can be communicated in a single network transaction UART: Universal Asynchronous Transmitter/Receiver

MS/TP Overview This section provides a brief overview of MS/TP, as specified in Clause 9 of the ANSI/ASHRAE Standard 135-2024 . Versions of since 2016 include changes to legacy MS/TP that provide support for larger frame sizes and improved error handling. , Clause 9 also covers physical-layer deployment options. MS/TP is designed to enable multidrop serial bus networks over shielded twisted pair wiring. It can support network segments up to 1000 meters in length at a data rate of 115.2 kbit/s or segments up to 1200 meters in length at lower bit rates. An MS/TP interface requires only a UART, an RS-485 transceiver with a driver that can be disabled, and a 5 ms resolution timer. The MS/TP MAC is typically implemented in software. The differential signaling used by requires a contention- free MAC. MS/TP uses a token to control access to a multidrop bus. Only an MS/TP manager node can initiate the unsolicited transfer of data, and only when it holds the token. After sending at most a configured maximum number of data frames, a manager node passes the token to the next manager node (as determined by the MAC address). If present on the link, legacy MS/TP implementations (including any subordinate nodes) ignore the frame format defined in this specification. , Clause 9 defines a range of Frame Type values used to designate frames that contain Data and Data CRC fields encoded using Consistent Overhead Byte Stuffing (see Appendix B). The purpose of COBS encoding is to eliminate preamble sequences from the Encoded Data and Encoded CRC-32K fields. The Encoded Data field is covered by a 32-bit CRC (see Appendix C) that is also COBS encoded. MS/TP COBS-encoded frames have the following format:

MS/TP COBS-Encoded Frame Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x55 | 0xFF | Frame Type | DA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SA | Length (MS octet first) | Header CRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Encoded Data (2 - 1507 octets) . . . + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Encoded CRC-32K (5 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ | | optional 0xFF | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ MS/TP COBS-encoded frame fields are defined as follows: Preamble two octet preamble: 0x55, 0xFF Frame Type one octet Destination Address one octet address Source Address one octet address Length two octets, most significant octet first Header CRC one octet Encoded Data 2 - 1507 octets (see Section 4 and Appendix B) Encoded CRC-32K five octets (see Appendix C) (pad) (optional) at most one octet of padding: 0xFF The Frame Type is used to distinguish between different types of MAC frames. The types relevant to this specification (in decimal) are: 0 Token 1 Poll For Manager 2 Reply To Poll For Manager 3 Test_Request 4 Test_Response ... 34 IPv6 over MS/TP (LoBAC) Encapsulation Frame Types 8 - 31 and 35 - 127 are reserved for assignment by ASHRAE. Frame Types 32 - 127 designate COBS-encoded frames that convey Encoded Data and Encoded CRC-32K fields. See for additional details. The Destination and Source Addresses are each one octet in length. See for additional details. For COBS-encoded frames, the Length field indicates the size of the Encoded Data field in octets, plus three. (This adjustment is required in order for legacy MS/TP devices to ignore COBS-encoded frames.) See and the Appendices for additional details. The Header CRC field covers the Frame Type, Destination Address, Source Address, and Length fields. The Header CRC generation and check procedures are specified in , Annex G.1. Use of the optional 0xFF padding octet is discussed in , Clause 9.

Goals and Constraints The main goals of this specification are a) to enable IPv6 directly on wired end devices in building automation and control networks by leveraging existing standards to the greatest extent possible, and b) to co-exist with legacy MS/TP implementations. Co-existence allows MS/TP networks to be incrementally upgraded to support IPv6. In order to co-exist with legacy devices, no changes are permitted to the MS/TP addressing modes, frame header format, control frames, or Manager Node state machine as specified in , Clause 9.

Profile for IPv6 over MS/TP ASHRAE has assigned an MS/TP Frame Type value of 34 to indicate IPv6 over MS/TP (LoBAC) Encapsulation. This falls within the range of values that designate COBS-encoded data frames.

Mandatory Features , Clause 9 specifies mandatory-to-implement features of MS/TP devices. For example, it is mandatory that all MS/TP nodes respond to a Test_Request with a Test_Response frame. All MS/TP manager nodes must implement the Manager Node state machine and handle Token, Poll For Manager, and Reply To Poll For Manager control frames. 6LoBAC nodes are MS/TP manager nodes that implement a Receive Frame state machine capable of handling COBS-encoded frames. 6LoBAC nodes must support a data rate of 115.2 kbit/s and may support lower data rates as specified in , Clause 9. The method of selecting the data rate is outside the scope of this specification.

Configuration Constants The following constants are used by the Receive Frame state machine. Nmin_COBS_length The minimum valid Length value of any LoBAC- encapsulated frame: 5 Nmax_COBS_length The maximum valid Length value of any LoBAC- encapsulated frame: 1510 (see Section 4)

Configuration Parameters The following parameters are used by the Manager Node state machine. Nmax_info_frames The default maximum number of information frames the node may send before it must pass the token: 1 Nmax_manager The default highest allowable address for manager nodes: 127 The mechanisms for setting parameters or monitoring MS/TP performance are outside the scope of this specification.

Addressing Modes MS/TP node (MAC) addresses are one octet in length and are assigned dynamically. The method of assigning MAC addresses is outside the scope of this specification. However, each MS/TP node on the link MUST have a unique address in order to ensure correct MAC operation. , Clause 9 specifies that addresses 0 through 127 are valid for manager nodes. The method specified in for creating a MAC-address-derived Interface Identifier (IID) ensures that an IID of all zeros can never be generated. A Destination Address of 255 (all nodes) indicates a MAC-layer broadcast. MS/TP does not support multicast; therefore, all IPv6 multicast packets MUST be broadcast at the MAC layer and filtered at the IPv6 layer. A Source Address of 255 MUST NOT be used. Hosts learn IPv6 prefixes via router advertisements according to .

Maximum Transmission Unit (MTU) Upon transmission, the network-layer MTU is formatted according to and becomes the MAC service data unit (MSDU). The MSDU is then COBS encoded by MS/TP. Upon reception, the steps are reversed. , Clause 9 supports MSDUs up to 2032 octets in length. IPv6 requires that every link in an internet have an MTU of 1280 octets or greater. Additionally, a node must be able to accept a fragmented packet that, after reassembly, is as large as 1500 octets. This specification defines an MTU length of at least 1280 octets and at most 1500 octets. Support for an MTU length of 1500 octets is RECOMMENDED. It is possible for the LOWPAN_IPHC encoding () to generate an MSDU longer than the MTU. With the Context Identifier Extension present and no fields elided, the 40-octet IPv6 header is encoded in 41 octets. The MSDU can therefore be at most 1501 octets for an MTU of 1500 octets. COBS adds at most one octet per 254 octets of MSDU, rounded up (see ), so the Encoded Data field is at most 1507 octets and the Length field is at most 1510 octets.

LoBAC Adaptation Layer This section specifies an adaptation layer to support compressed IPv6 headers as specified in . IPv6 header compression MUST be implemented on all 6LoBAC nodes. Implementations MAY also support Generic Header Compression for transport layer headers. The LoBAC encapsulation format defined in this section describes the MSDU of an IPv6 over MS/TP frame. The LoBAC payload (i.e., an IPv6 packet) follows an encapsulation header stack. LoBAC is a subset of the LoWPAN encapsulation defined in , as updated by , so the use of "LOWPAN" in literals below is intentional. The primary difference between LoWPAN and LoBAC encapsulation is omission of the Mesh, Broadcast, Fragmentation, and LOWPAN_HC1 headers in the latter. All LoBAC-encapsulated datagrams transmitted over MS/TP are prefixed by an encapsulation header stack consisting of a Dispatch value followed by zero or more header fields. The only sequence currently defined for LoBAC is the LOWPAN_IPHC header followed by payload, as shown below:

A LoBAC-Encapsulated LOWPAN_IPHC Compressed IPv6 Datagram +---------------+---------------+------...-----+ | IPHC Dispatch | IPHC Header | Payload | +---------------+---------------+------...-----+ The Dispatch value is treated as an unstructured namespace. Only a single pattern is used to represent current LoBAC functionality.

LoBAC Dispatch Value Bit Pattern Pattern Header Type +------------+-----------------------------------------------------+ | 01 1xxxxx | LOWPAN_IPHC - LOWPAN_IPHC compressed IPv6 [RFC6282] | +------------+-----------------------------------------------------+ Other IANA-assigned 6LoWPAN Dispatch values do not apply to 6LoBAC unless otherwise specified.

Stateless Address Autoconfiguration This section defines how to obtain an IPv6 Interface Identifier. This specification distinguishes between two types of IIDs, MAC- address-derived and semantically opaque. A MAC-address-derived IID is the RECOMMENDED type for use in forming a link-local address, as it affords the most efficient header compression provided by the LOWPAN_IPHC format specified in . The general procedure for creating a MAC-address-derived IID is described in Appendix A of , "Creating Modified EUI-64 Format Interface Identifiers", as updated by . The Interface Identifier for link-local addresses SHOULD be formed by concatenating the node's 8-bit MS/TP MAC address to the seven octets 0x00, 0x00, 0x00, 0xFF, 0xFE, 0x00, and 0x00. For example, an MS/TP MAC address of hexadecimal value 0x4F results in the following IID: |0 1|1 3|3 4|4 6| |0 5|6 1|2 7|8 3| +----------------+----------------+----------------+----------------+ |0000000000000000|0000000011111111|1111111000000000|0000000001001111| +----------------+----------------+----------------+----------------+ A semantically opaque IID having 64 bits of entropy is RECOMMENDED for each globally scoped address and MAY be locally generated according to one of the methods cited in . A node that generates a 64-bit semantically opaque IID MUST register the IID with its local router(s) by sending a Neighbor Solicitation (NS) message with the Address Registration Option (ARO) and process Neighbor Advertisements (NAs) according to . An IPv6 address prefix used for stateless autoconfiguration of an MS/TP interface MUST have a length of 64 bits.

IPv6 Link-Local Address The IPv6 link-local address for an MS/TP interface is formed by appending the Interface Identifier, as defined above, to the prefix FE80::/64. 10 bits 54 bits 64 bits +----------+-----------------------+----------------------------+ |1111111010| (zeros) | Interface Identifier | +----------+-----------------------+----------------------------+

Unicast Address Mapping The address resolution procedure for mapping IPv6 non-multicast addresses into MS/TP MAC-layer addresses follows the general description in Section 7.2 of , unless otherwise specified. The Source/Target Link-Layer Address option has the following form when the addresses are 8-bit MS/TP MAC-layer (node) addresses. 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length=1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 | MS/TP Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Padding (all zeros) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option fields: Type: 1: for Source Link-Layer address. 2: for Target Link-Layer address. Length: This is the length of this option (including the Type and Length fields) in units of 8 octets. The value of this field is 1 for 8-bit MS/TP MAC addresses. MS/TP Address: The 8-bit address in canonical bit order [RFC2469]. This is the unicast address the interface currently responds to.

Multicast Address Mapping All IPv6 multicast packets MUST be sent to MS/TP Destination Address 255 (broadcast) and filtered at the IPv6 layer. When represented as a 16-bit address in a compressed header (see ), it MUST be formed by padding on the left with a zero octet: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 | 0xFF | +-+-+-+-+-+-+-+-+---------------+

Header Compression 6LoBAC REQUIRES LOWPAN_IPHC IPv6 compression, which is specified in and included herein by reference. This section will simply identify substitutions that should be made when interpreting the text of . In general, the following substitutions should be made:

-: Replace instances of "6LoWPAN" with "MS/TP network"
-: Replace instances of "IEEE 802.15.4 address" with "MS/TP address"

When a 16-bit address is called for (i.e., an IEEE 802.15.4 "short address"), it MUST be formed by padding the MS/TP address to the left with a zero octet: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 | MS/TP address | +-+-+-+-+-+-+-+-+---------------+ If LOWPAN_IPHC compression is used with context, the router(s) directly attached to the MS/TP segment MUST disseminate the 6LoWPAN Context Option (6CO) according to Section 7.2 of .

IANA Considerations This document uses values previously reserved by and ; it does not require any IANA actions.

Security Considerations See for a general discussion of privacy threats faced by constrained nodes. makes a distinction between "stable" and "temporary" addresses. The former are long-lived and typically advertised by servers. The latter are typically used by clients and SHOULD be changed frequently to mitigate correlation of activities over time. Nodes that engage in both activities SHOULD support simultaneous use of multiple addresses per device. Globally scoped addresses that contain MAC-address-derived IIDs may expose a network to address-scanning attacks. For this reason, it is RECOMMENDED that a 64-bit semantically opaque IID be generated for each globally scoped address in use according to, for example, , , , , or .

Implementation Status [RFC Editor: please remove this section, and the reference to RFC 7942, before publication.] This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

RIOT

Description:: An MS/TP driver for the RIOT operating system, with example applications for a 6LBR (mstp_6lbr), a 6LN (mstp_6lo), and a bus monitor (mstp_monitor). It implements the Manager Node and Receive Frame state machines of Clause 9, Frame Type 34 with COBS and CRC-32K, and 6LoWPAN Neighbor Discovery and LOWPAN_IPHC through RIOT's GNRC network stack, with an IPv6 MTU of 1500 octets.
Source:: https://github.com/kerlyn-bsd3/RIOT, branch 6lobac-mstp, tag 6lobac-2026-11-02 or newer. The test bed, a soak test, and the steps to reproduce are described in examples/networking/mstp_6lbr/TESTBED.md.
Platforms:: Nordic nRF52840-DK with a MAX485-based RS-485 shield (DFRobot DFR0259).
Maturity:: Prototype, to be proposed for inclusion in RIOT. A test bed of a 6LBR, a 6LN, and a bus monitor at 115.2 kbit/s completed 60,000 ICMPv6 echo exchanges of 1500-octet IPv6 packets without loss and without any MS/TP frame errors.
License:: BSD-3-Clause (driver and examples); RIOT is LGPL-2.1.
Contact:: Kerry Lynn (kerlyn@ieee.org).

Wireshark

Description:: Changes to the Wireshark MS/TP and 6LoWPAN dissectors: decode all COBS-encoded Frame Types (32-127), verify the CRC-32K, check the Length limits, dissect Frame Type 34 as 6LoWPAN, and derive 6LoWPAN Interface Identifiers from MS/TP addresses as in . The decode in was produced with these changes.
Source:: Merge requests wireshark/wireshark!27569, !27570, and !27571 (https://gitlab.com/wireshark/wireshark/-/merge_requests), under review.
License:: GPL-2.0-or-later.
Contact:: Kerry Lynn (kerlyn@ieee.org).

References Normative References BACnet - A Data Communication Protocol for Building Automation and Control Networks ASHRAE Informative References Consistent Overhead Byte Stuffing 32-Bit Cyclic Redundancy Codes for Internet Applications RFC 8163 Erratum 5996 RFC Errata IEEE Standard for Ethernet Institute of Electrical and Electronics Engineers Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems TIA

Abstract MAC Interface This Appendix is informative and not part of the standard. , Clause 9 provides support for MAC-layer clients through its SendFrame and ReceivedDataNoReply procedures. However, it does not define a network-protocol independent abstract interface for the MAC. This is provided below as an aid to implementation.

MA-DATA.request

Function This primitive defines the transfer of data from a MAC client entity to a single peer entity or multiple peer entities in the case of a broadcast address.

Semantics of the Service Primitive The semantics of the primitive are as follows: MA-DATA.request ( destination_address, source_address, data, type ) The 'destination_address' parameter may specify either an individual or a broadcast MAC entity address. It must contain sufficient information to create the Destination Address field (see ) that is prepended to the frame by the local MAC sublayer entity. The 'source_address' parameter, if present, must specify an individual MAC address. If the source_address parameter is omitted, the local MAC sublayer entity will insert a value associated with that entity. The 'data' parameter specifies the MAC service data unit (MSDU) to be transferred by the MAC sublayer entity. There is sufficient information associated with the MSDU for the MAC sublayer entity to determine the length of the data unit. The 'type' parameter specifies the value of the MS/TP Frame Type field that is prepended to the frame by the local MAC sublayer entity.

When Generated This primitive is generated by the MAC client entity whenever data shall be transferred to a peer entity or entities. This can be in response to a request from higher protocol layers or from data generated internally to the MAC client, such as a Token frame.

Effect on Receipt Receipt of this primitive will cause the MAC entity to insert all MAC-specific fields, including Destination Address, Source Address, Frame Type, and any fields that are unique to the particular media access method, and pass the properly formed frame to the lower protocol layers for transfer to the peer MAC sublayer entity or entities.

MA-DATA.indication

Function This primitive defines the transfer of data from the MAC sublayer entity to the MAC client entity or entities in the case of a broadcast address.

Semantics of the Service Primitive The semantics of the primitive are as follows: MA-DATA.indication ( destination_address, source_address, data, type ) The 'destination_address' parameter may be either an individual or a broadcast address as specified by the Destination Address field of the incoming frame. The 'source_address' parameter is an individual address as specified by the Source Address field of the incoming frame. The 'data' parameter specifies the MAC service data unit (MSDU) as received by the local MAC entity. There is sufficient information associated with the MSDU for the MAC sublayer client to determine the length of the data unit. The 'type' parameter is the value of the MS/TP Frame Type field of the incoming frame.

When Generated The MA_DATA.indication is passed from the MAC sublayer entity to the MAC client entity or entities to indicate the arrival of a frame to the local MAC sublayer entity that is destined for the MAC client. Such frames are reported only if they are validly formed and received without error, and their Destination Address designates the local MAC entity. Frames destined for the MAC Control sublayer are not passed to the MAC client.

Effect on Receipt The effect of receipt of this primitive by the MAC client is unspecified.

Consistent Overhead Byte Stuffing (COBS) This Appendix is informative and not part of the standard. , Clause 9 corrects a long-standing issue with the MS/TP specification, namely that preamble sequences were not escaped whenever they appeared in the Data or Data CRC fields. In rare cases, this resulted in dropped frames due to loss-of-frame synchronization. The solution is to encode the Data and 32-bit Data CRC fields before transmission using Consistent Overhead Byte Stuffing and decode these fields upon reception. COBS is a run-length encoding method that nominally removes '0x00' octets from its input. Any selected octet value may be removed by XOR'ing that value with each octet of the COBS output. , Clause 9 specifies the preamble octet '0x55' for removal. The minimum overhead of COBS is one octet per encoded field. The worst-case overhead in long fields is bounded to one octet per 254 as described in . Frame encoding proceeds logically in two passes. The Encoded Data field is prepared by passing the MSDU through the COBS encoder and XOR'ing the preamble octet '0x55' with each octet of the output. The Encoded CRC-32K field is then prepared by calculating a CRC-32K over the Encoded Data field and formatting it for transmission as described in . The combined length of these fields, minus two octets for compatibility with legacy MS/TP devices, is placed in the MS/TP header Length field before transmission. Example COBS encoder and decoder functions are shown below for illustration. Complete examples of use and test vectors are provided in , Annex T. #include /* * Encodes 'length' octets of data located at 'from' and * writes one or more COBS code blocks at 'to', removing any * 'mask' octets that may be present in the encoded data. * Returns the length of the encoded data. */ size_t cobs_encode (uint8_t *to, const uint8_t *from, size_t length, uint8_t mask) { size_t code_index = 0; size_t read_index = 0; size_t write_index = 1; uint8_t code = 1; uint8_t data, last_code = 0; while (read_index < length) { data = from[read_index++]; /* * In the case of encountering a non-zero octet in the data, * simply copy input to output and increment the code octet. */ if (data != 0) { to[write_index++] = data ^ mask; code++; if (code != 255) continue; } /* * In the case of encountering a zero in the data or having * copied the maximum number (254) of non-zero octets, store * the code octet and reset the encoder state variables. */ last_code = code; to[code_index] = code ^ mask; code_index = write_index++; code = 1; } /* * If the last chunk contains exactly 254 non-zero octets, then * this exception is handled above (and the returned length must * be adjusted). Otherwise, encode the last chunk normally, as if * a "phantom zero" is appended to the data. */ if ((last_code == 255) && (code == 1)) write_index--; else to[code_index] = code ^ mask; return write_index; } /* * Decodes 'length' octets of data located at 'from' and * writes the original client data at 'to', restoring any * 'mask' octets that may present in the encoded data. * Returns the length of the encoded data or zero if error. */ size_t cobs_decode (uint8_t *to, const uint8_t *from, size_t length, uint8_t mask) { size_t read_index = 0; size_t write_index = 0; uint8_t code, last_code; while (read_index < length) { code = from[read_index] ^ mask; last_code = code; /* * Sanity check the encoding to prevent the while() loop below * from overrunning the output buffer. See [Err5996]. */ if (code == 0 || read_index + code > length) return 0; read_index++; while (--code > 0) to[write_index++] = from[read_index++] ^ mask; /* * Restore the implicit zero at the end of each decoded block * except when it contains exactly 254 non-zero octets or the * end of data has been reached. */ if ((last_code != 255) && (read_index < length)) to[write_index++] = 0; } return write_index; } ]]>

Encoded CRC-32K (CRC32K) This Appendix is informative and not part of the standard. Extending the payload of MS/TP to 1500 octets requires upgrading the Data CRC from 16 bits to 32 bits. P. Koopman has authored several papers on evaluating CRC polynomials for network applications. In , he surveyed the entire 32-bit polynomial space and noted some that exceed the polynomial in performance. , Clause 9 specifies one of these, the CRC-32K (Koopman) polynomial. The specified use of the calc_crc32K() function is as follows. Before a frame is transmitted, 'crc_value' is initialized to all ones. After passing each octet of the Encoded Data field through the function, the ones complement of the resulting 'crc_value' is arranged in LSB-first order and is itself encoded. The length of the resulting Encoded CRC-32K field is always five octets. Upon reception of a frame, 'crc_value' is initialized to all ones. The octets of the Encoded Data field are accumulated by the calc_crc32K() function before decoding. The Encoded CRC-32K field is then decoded and the resulting four octets are accumulated by the calc_crc32K() function. If the result is the expected residue value 'CRC32K_RESIDUE', then the frame was received correctly. An example CRC-32K function is shown below for illustration. Complete examples of use and test vectors are provided in , Annex G.3. /* See ANSI/ASHRAE Standard 135-2024 [BACnet], Clause G.3.1 */ #define CRC32K_INITIAL_VALUE (0xFFFFFFFF) #define CRC32K_RESIDUE (0x0843323B) /* CRC-32K polynomial, 1 + x**1 + ... + x**30 (+ x**32) */ #define CRC32K_POLY (0xEB31D82E) /* * Accumulate 'data_value' into the CRC in 'crc_value'. * Return updated CRC. * * Note: crc_value must be set to CRC32K_INITIAL_VALUE * before initial call. */ uint32_t calc_crc32K (uint8_t data_value, uint32_t crc_value) { int b; for (b = 0; b < 8; b++) { if ((data_value & 1) ^ (crc_value & 1)) { crc_value >>= 1; crc_value ^= CRC32K_POLY; } else { crc_value >>= 1; } data_value >>= 1; } return crc_value; } ]]>

Example 6LoBAC Frame Decode This Appendix is informative and not part of the standard. The frame below is an ICMPv6 Echo Request forwarded by a 6LBR (MS/TP address 2) to a 6LN (MS/TP address 3). The IPv6 packet was sent from a host on the 6LBR's other interface with every field fixed, so the frame can be reproduced exactly:

Source fdea:dbee:f::1, Destination aaaa::ff:fe00:3, Hop Limit 64 (63 after forwarding).
ICMPv6 Echo Request, Identifier 0x6f49, Sequence 1.
767 octets of data, in which octet i has the value i modulo 256.

For example, with Scapy: , dst=<6LBR MAC>) / IPv6(src='fdea:dbee:f::1', dst='aaaa::ff:fe00:3', hlim=64) / ICMPv6EchoRequest(id=0x6f49, seq=1, data=bytes(i & 0xff for i in range(767))), iface=) ]]> The 6LBR advertises aaaa::/64 as 6LoWPAN context 0, which LOWPAN_IPHC uses to elide the destination address. Other LOWPAN_IPHC encoders may make different, equally valid compression choices for the same IPv6 packet; these result in a different MSDU and therefore a different frame. Given the MSDU shown here, any correct implementation of and produces exactly this frame. The decode below was produced by Wireshark with context 0 configured, for example with tshark: -V -x -o 6lowpan.context0:aaaa::/64 \ -Y "icmpv6.echo.identifier == 0x6f49 && mstp.src == 2" ]]> The data and the Identifier were chosen so that the frame exercises every case of the COBS encoder and decoder in . The Encoded Data field contains zero octets, consecutive zero octets, runs of 254 non-zero octets in the middle of the data (code 0xFF, which is not followed by an implicit zero), and a final block of exactly 254 non-zero octets. The CRC-32K, c4 31 32 00 in transmission order, ends in a zero, so the Encoded CRC-32K field is two blocks (codes 4 and 1). 98;:%$'&! #"-,/ 2e 29 28 2b 2a d5 d4 d7 d6 d1 d0 d3 d2 dd dc df .)(+*........... de d9 d8 db da c5 c4 c7 c6 c1 c0 c3 c2 cd cc cf ................ ce c9 c8 cb ca f5 f4 f7 f6 f1 f0 f3 f2 fd fc ff ................ fe f9 f8 fb fa e5 e4 e7 e6 e1 e0 e3 e2 ed ec ef ................ ee e9 e8 eb ea 95 94 97 96 91 90 93 92 9d 9c 9f ................ 9e 99 98 9b 9a 85 84 87 86 81 80 83 82 8d 8c 8f ................ 8e 89 88 8b 8a b5 b4 b7 b6 b1 b0 b3 b2 bd bc bf ................ be b9 b8 bb ba a5 a4 a7 a6 a1 a0 a3 a2 ad ac af ................ ae a9 a8 ab 57 aa aa 54 57 56 51 50 53 52 5d 5c ....W..TWVQPSR]\ 5f 5e 59 58 5b 5a 45 44 47 46 41 40 43 42 4d 4c _^YX[ZEDGFA@CBML 4f 4e 49 48 4b 4a 75 74 77 76 71 70 73 72 7d 7c ONIHKJutwvqpsr}| 7f 7e 79 78 7b 7a 65 64 67 66 61 60 63 62 6d 6c .~yx{zedgfa`cbml 6f 6e 69 68 6b 6a 15 14 17 16 11 10 13 12 1d 1c onihkj.......... 1f 1e 19 18 1b 1a 05 04 07 06 01 00 03 02 0d 0c ................ 0f 0e 09 08 0b 0a 35 34 37 36 31 30 33 32 3d 3c ......54761032=< 3f 3e 39 38 3b 3a 25 24 27 26 21 20 23 22 2d 2c ?>98;:%$'&! #"-, 2f 2e 29 28 2b 2a d5 d4 d7 d6 d1 d0 d3 d2 dd dc /.)(+*.......... df de d9 d8 db da c5 c4 c7 c6 c1 c0 c3 c2 cd cc ................ cf ce c9 c8 cb ca f5 f4 f7 f6 f1 f0 f3 f2 fd fc ................ ff fe f9 f8 fb fa e5 e4 e7 e6 e1 e0 e3 e2 ed ec ................ ef ee e9 e8 eb ea 95 94 97 96 91 90 93 92 9d 9c ................ 9f 9e 99 98 9b 9a 85 84 87 86 81 80 83 82 8d 8c ................ 8f 8e 89 88 8b 8a b5 b4 b7 b6 b1 b0 b3 b2 bd bc ................ bf be b9 b8 bb ba a5 a4 a7 a6 a1 a0 a3 a2 ad ac ................ af ae a9 a8 ab 57 aa aa 54 57 56 51 50 53 52 5d .....W..TWVQPSR] 5c 5f 5e 59 58 5b 5a 45 44 47 46 41 40 43 42 4d \_^YX[ZEDGFA@CBM 4c 4f 4e 49 48 4b 4a 75 74 77 76 71 70 73 72 7d LONIHKJutwvqpsr} 7c 7f 7e 79 78 7b 7a 65 64 67 66 61 60 63 62 6d |.~yx{zedgfa`cbm 6c 6f 6e 69 68 6b 6a 15 14 17 16 11 10 13 12 1d lonihkj......... 1c 1f 1e 19 18 1b 1a 05 04 07 06 01 00 03 02 0d ................ 0c 0f 0e 09 08 0b 0a 35 34 37 36 31 30 33 32 3d .......54761032= 3c 3f 3e 39 38 3b 3a 25 24 27 26 21 20 23 22 2d 98;:%$'&! #"- 2c 2f 2e 29 28 2b 2a d5 d4 d7 d6 d1 d0 d3 d2 dd ,/.)(+*......... dc df de d9 d8 db da c5 c4 c7 c6 c1 c0 c3 c2 cd ................ cc cf ce c9 c8 cb ca f5 f4 f7 f6 f1 f0 f3 f2 fd ................ fc ff fe f9 f8 fb fa e5 e4 e7 e6 e1 e0 e3 e2 ed ................ ec ef ee e9 e8 eb ea 95 94 97 96 91 90 93 92 9d ................ 9c 9f 9e 99 98 9b 9a 85 84 87 86 81 80 83 82 8d ................ 8c 8f 8e 89 88 8b 8a b5 b4 b7 b6 b1 b0 b3 b2 bd ................ bc bf be b9 b8 bb ba a5 a4 a7 a6 a1 a0 a3 a2 ad ................ ac af ae a9 a8 ab 51 91 64 67 54 ......Q.dgT Decoded Data and CRC32K (799 bytes): 78 07 3a 3f fd ea db ee 00 0f 00 00 00 00 00 00 x.:?............ 00 00 00 01 80 00 4a 1c 6f 49 00 01 00 01 02 03 ......J.oI...... 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ................ 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 ............ !"# 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 $%&'()*+,-./0123 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 40 41 42 43 456789:;<=>?@ABC 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f 50 51 52 53 DEFGHIJKLMNOPQRS 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f 60 61 62 63 TUVWXYZ[\]^_`abc 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 defghijklmnopqrs 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f 80 81 82 83 tuvwxyz{|}~..... 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f 90 91 92 93 ................ 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f a0 a1 a2 a3 ................ a4 a5 a6 a7 a8 a9 aa ab ac ad ae af b0 b1 b2 b3 ................ b4 b5 b6 b7 b8 b9 ba bb bc bd be bf c0 c1 c2 c3 ................ c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf d0 d1 d2 d3 ................ d4 d5 d6 d7 d8 d9 da db dc dd de df e0 e1 e2 e3 ................ e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef f0 f1 f2 f3 ................ f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff 00 01 02 03 ................ 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ................ 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 ............ !"# 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 $%&'()*+,-./0123 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 40 41 42 43 456789:;<=>?@ABC 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f 50 51 52 53 DEFGHIJKLMNOPQRS 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f 60 61 62 63 TUVWXYZ[\]^_`abc 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 defghijklmnopqrs 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f 80 81 82 83 tuvwxyz{|}~..... 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f 90 91 92 93 ................ 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f a0 a1 a2 a3 ................ a4 a5 a6 a7 a8 a9 aa ab ac ad ae af b0 b1 b2 b3 ................ b4 b5 b6 b7 b8 b9 ba bb bc bd be bf c0 c1 c2 c3 ................ c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf d0 d1 d2 d3 ................ d4 d5 d6 d7 d8 d9 da db dc dd de df e0 e1 e2 e3 ................ e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef f0 f1 f2 f3 ................ f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff 00 01 02 03 ................ 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ................ 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 ............ !"# 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 $%&'()*+,-./0123 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 40 41 42 43 456789:;<=>?@ABC 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f 50 51 52 53 DEFGHIJKLMNOPQRS 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f 60 61 62 63 TUVWXYZ[\]^_`abc 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 defghijklmnopqrs 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f 80 81 82 83 tuvwxyz{|}~..... 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f 90 91 92 93 ................ 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f a0 a1 a2 a3 ................ a4 a5 a6 a7 a8 a9 aa ab ac ad ae af b0 b1 b2 b3 ................ b4 b5 b6 b7 b8 b9 ba bb bc bd be bf c0 c1 c2 c3 ................ c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf d0 d1 d2 d3 ................ d4 d5 d6 d7 d8 d9 da db dc dd de df e0 e1 e2 e3 ................ e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef f0 f1 f2 f3 ................ f4 f5 f6 f7 f8 f9 fa fb fc fd fe c4 31 32 00 ............12. Decompressed 6LoWPAN IPHC (815 bytes): 60 00 00 00 03 07 3a 3f fd ea db ee 00 0f 00 00 `.....:?........ 00 00 00 00 00 00 00 01 aa aa 00 00 00 00 00 00 ................ 00 00 00 ff fe 00 00 03 80 00 4a 1c 6f 49 00 01 ..........J.oI.. 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ................ 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f ................ 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f !"#$%&'()*+,-./ 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 0123456789:;<=>? 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f @ABCDEFGHIJKLMNO 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f PQRSTUVWXYZ[\]^_ 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f `abcdefghijklmno 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f pqrstuvwxyz{|}~. 80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f ................ 90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f ................ a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af ................ b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf ................ c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf ................ d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df ................ e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef ................ f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff ................ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ................ 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f ................ 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f !"#$%&'()*+,-./ 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 0123456789:;<=>? 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f @ABCDEFGHIJKLMNO 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f PQRSTUVWXYZ[\]^_ 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f `abcdefghijklmno 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f pqrstuvwxyz{|}~. 80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f ................ 90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f ................ a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af ................ b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf ................ c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf ................ d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df ................ e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef ................ f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff ................ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ................ 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f ................ 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f !"#$%&'()*+,-./ 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 0123456789:;<=>? 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f @ABCDEFGHIJKLMNO 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f PQRSTUVWXYZ[\]^_ 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f `abcdefghijklmno 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f pqrstuvwxyz{|}~. 80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f ................ 90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f ................ a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af ................ b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf ................ c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf ................ d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df ................ e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef ................ f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ............... ]]>

Changes since RFC 8163

-: Title and body text changed to use inclusive terminology after [BACnet].
-: Updated the BACnet reference from ANSI/ASHRAE Standard 135-2016 to Standard 135-2024.
-: Updated the IPv6 specification reference from RFC 2460 to RFC 8200.
-: Updated the IEEE 802.3 reference from IEEE 802.3-2015 to IEEE 802-3-2022.
-: Updated security considerations references: replaced RFC 3315 with RFC 8415 and RFC 4941 with RFC 8981.
-: Incorporated Erratum 5996 , which adds a sanity check in the COBS decoder to prevent a potential buffer overrun, in Appendix B.
-: Initialized last_code in the example COBS encoder in Appendix B, which could otherwise be read before it was set.
-: Corrected the maximum Length of a LoBAC-encapsulated frame from 1509 to 1510 (Encoded Data 2-1507 octets), derived from a maximum MSDU of 1501 octets in Section 4. Every frame that was valid under RFC 8163 is still valid.
-: Replaced the example in Appendix D with a frame that can be reproduced exactly and that exercises every case of the COBS encoder and decoder in Appendix B.
-: Added an Implementation Status section (to be removed before publication).
-: Made additional terminology corrections.
-: NOTE: There are no changes to the frame format or encoding against RFC 8163. The maximum Length of a LoBAC-encapsulated frame is corrected from 1509 to 1510 (see above); every frame that was valid under RFC 8163 remains valid.

Contributors Jerry Martocci, Carl Neilsen, and Stuart Donaldson (all since retired) were co-authors of RFC 8163.

Acknowledgements We are grateful to the authors of and members of the IETF 6LoWPAN working group; this document borrows liberally from their work. Ralph Droms and Brian Haberman provided indispensable guidance and support from the outset. Peter van der Stok, James Woodyatt, Carsten Bormann, and Dale Worley provided detailed reviews. Stuart Cheshire invented the very clever COBS encoding. Michael Osborne made the critical observation that encoding the data and CRC32K fields separately would allow the CRC to be calculated on the fly. Alexandru Petrescu, Brian Frank, Geoff Mulligan, and Don Sturek offered valuable comments.