



6Lo Working Group                                                K. Lynn
Internet-Draft                                                Consultant
Obsoletes: 8163 (if approved)                            20 October 2025
Intended status: Standards Track                                        
Expires: 23 April 2026


  Transmission of IPv6 over Multidrop Serial Bus/Token Passing (MS/TP)
                                Networks
                      draft-lynn-6lo-6lobac-bis-00

Abstract

   Mutidrop 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on 23 April 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.










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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Profile for IPv6 over MS/TP . . . . . . . . . . . . . . . . .   6
   3.  Addressing Modes  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Maximum Transmission Unit (MTU) . . . . . . . . . . . . . . .   7
   5.  LoBAC Adaptation Layer  . . . . . . . . . . . . . . . . . . .   8
   6.  Stateless Address Autoconfiguration . . . . . . . . . . . . .   8
   7.  IPv6 Link-Local Address . . . . . . . . . . . . . . . . . . .   9
   8.  Unicast Address Mapping . . . . . . . . . . . . . . . . . . .   9
   9.  Multicast Address Mapping . . . . . . . . . . . . . . . . . .  10
   10. Header Compression  . . . . . . . . . . . . . . . . . . . . .  10
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     13.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Abstract MAC Interface . . . . . . . . . . . . . . .  14
   Appendix B.  Consistent Overhead Byte Stuffing (COBS) . . . . . .  16
   Appendix C.  Encoded CRC-32K [CRC32K] . . . . . . . . . . . . . .  19
   Appendix D.  Example 6LoBAC Frame Decode  . . . . . . . . . . . .  21
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   Mutidrop Serial Bus/Token Passing (MS/TP) is a Medium Access Control
   (MAC) protocol for the RS-485 [TIA-485-A] physical layer and is used
   primarily in building automation networks.  This specification
   defines the frame format for transmission of IPv6 [RFC2460] 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 ([RFC4944], [RFC6282], and
   [RFC6775]) 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



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   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 [RFC6282] and improves MS/TP link utilization.

1.1.  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 [RFC2119].

1.2.  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

1.3.  MS/TP Overview

   This section provides a brief overview of MS/TP, as specified in
   Clause 9 of the ANSI/ASHRAE Standard 135-2024 [BACnet].  This version
   of [BACnet] integrates changes to legacy MS/TP that provide support
   for larger frame sizes and improved error handling.  [BACnet], Clause
   9 also covers physical-layer deployment options.






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   MS/TP is designed to enable multidrop 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 [TIA-485-A] 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 [TIA-485-A] 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.

   [BACnet], 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 [COBS] (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 [CRC32K] (see Appendix C) that is also COBS
   encoded.

   MS/TP COBS-encoded frames have the following 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 - 1506 octets)                 .
     .                                                               .
     +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |  Encoded CRC-32K (5 octets)   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+
     |                                               | optional 0xFF |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: MS/TP COBS-Encoded Frame Format

   MS/TP COBS-encoded frame fields are defined as follows:




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    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 - 1506 octets (see Section 4 and Appendix B)
    Encoded CRC-32K       five octets (see Appendix C)
    (pad)                 (optional) at most one octet of trailer: 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 Section 2 for
   additional details.

   The Destination and Source Addresses are each one octet in length.
   See Section 3 for additional details.

   For COBS-encoded frames, the Length field indicates the size of the
   [COBS] Encoded Data field in octets, plus three.  (This adjustment is
   required in order for legacy MS/TP devices to ignore COBS-encoded
   frames.)  See Section 4 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 [BACnet], Annex G.1.

   Use of the optional 0xFF trailer octet is discussed in [BACnet],
   Clause 9.










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1.4.  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 [BACnet], Clause 9.

2.  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.

2.1.  Mandatory Features

   [BACnet], 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 [BACnet], Clause 9.  The method of
   selecting the data rate is outside the scope of this specification.

2.2.  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: 1509

2.3.  Configuration Parameters

   The following parameters are used by the Manager Node state machine.






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   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 master
                     nodes: 127

   The mechanisms for setting parameters or monitoring MS/TP performance
   are outside the scope of this specification.

3.  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.

   [BACnet], Clause 9 specifies that addresses 0 through 127 are valid
   for manager nodes.  The method specified in Section 6 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
   [RFC4861].

4.  Maximum Transmission Unit (MTU)

   Upon transmission, the network-layer MTU is formatted according to
   Section 5 and becomes the MAC service data unit (MSDU).  The MSDU is
   then COBS encoded by MS/TP.  Upon reception, the steps are reversed.
   [BACnet], Clause 9 supports MSDUs up to 2032 octets in length.

   IPv6 [RFC2460] 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.









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5.  LoBAC Adaptation Layer

   This section specifies an adaptation layer to support compressed IPv6
   headers as specified in Section 10.  IPv6 header compression MUST be
   implemented on all 6LoBAC nodes.  Implementations MAY also support
   Generic Header Compression [RFC7400] 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 [RFC4944], as updated by
   [RFC6282], 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:

                  +---------------+---------------+------...-----+
                  | IPHC Dispatch |  IPHC Header  |    Payload   |
                  +---------------+---------------+------...-----+

    Figure 2: A LoBAC-Encapsulated LOWPAN_IPHC Compressed IPv6 Datagram

   The Dispatch value is treated as an unstructured namespace.  Only a
   single pattern is used to represent current LoBAC functionality.

     Pattern      Header Type
   +------------+-----------------------------------------------------+
   | 01  1xxxxx | LOWPAN_IPHC - LOWPAN_IPHC compressed IPv6 [RFC6282] |
   +------------+-----------------------------------------------------+

                 Figure 3: LoBAC Dispatch Value Bit Pattern

   Other IANA-assigned 6LoWPAN Dispatch values do not apply to 6LoBAC
   unless otherwise specified.

6.  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.





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   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 [RFC6282] format specified in
   Section 10.  The general procedure for creating a MAC-address-derived
   IID is described in Appendix A of [RFC4291], "Creating Modified
   EUI-64 Format Interface Identifiers", as updated by [RFC7136].

   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 Section 12.  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 [RFC6775].

   An IPv6 address prefix used for stateless autoconfiguration [RFC4862]
   of an MS/TP interface MUST have a length of 64 bits.

7.  IPv6 Link-Local Address

   The IPv6 link-local address [RFC4291] 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    |
     +----------+-----------------------+----------------------------+

8.  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 [RFC4861], 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.



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                      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.

9.  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 Section 10), 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      |
                     +-+-+-+-+-+-+-+-+---------------+

10.  Header Compression

   6LoBAC REQUIRES LOWPAN_IPHC IPv6 compression, which is specified in
   [RFC6282] and included herein by reference.  This section will simply
   identify substitutions that should be made when interpreting the text
   of [RFC6282].




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   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 [RFC6282] 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 [RFC6775].

11.  IANA Considerations

   This document uses values previously reserved by [RFC4944] and
   [RFC6282]; it does not require any IANA actions.

12.  Security Considerations

   See [RFC8065] for a general discussion of privacy threats faced by
   constrained nodes.

   [RFC8065] 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,
   [RFC3315], [RFC3972], [RFC4941], [RFC5535], or [RFC7217].

13.  References

13.1.  Normative References





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   [BACnet]   ASHRAE, "BACnet - A Data Communication Protocol for
              Building Automation and Control Networks", ANSI/
              ASHRAE Standard 135-2024, 2024,
              <https://store.accuristech.com/standards/ashrae-
              135-2024?product_id=2926783>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.




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   [RFC5535]  Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
              DOI 10.17487/RFC5535, June 2009,
              <https://www.rfc-editor.org/info/rfc5535>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <https://www.rfc-editor.org/info/rfc7136>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <https://www.rfc-editor.org/info/rfc7217>.

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
              2014, <https://www.rfc-editor.org/info/rfc7400>.

13.2.  Informative References

   [COBS]     Cheshire, S. and M. Baker, "Consistent Overhead Byte
              Stuffing", IEEE/ACM TRANSACTIONS ON NETWORKING, VOL.7,
              NO.2 , April 1999,
              <http://www.stuartcheshire.org/papers/COBSforToN.pdf>.

   [CRC32K]   Koopman, P., "32-Bit Cyclic Redundancy Codes for Internet
              Applications", IEEE/IFIP International Conference on
              Dependable Systems and Networks (DSN 2002) , June 2002,
              <https://users.ece.cmu.edu/~koopman/networks/dsn02/
              dsn02_koopman.pdf>.

   [IEEE.802.3]
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              3: Carrier Sense Multiple Access with Collision Detection



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              (CMSA/CD) Access Method and Physical Layer
              Specifications", IEEE Std 802.3-2022, 2022,
              <https://standards.ieee.org/ieee/802.3/10422/>.

   [RFC2469]  Narten, T. and C. Burton, "A Caution On The Canonical
              Ordering Of Link-Layer Addresses", RFC 2469,
              DOI 10.17487/RFC2469, December 1998,
              <https://www.rfc-editor.org/info/rfc2469>.

   [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
              Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
              February 2017, <https://www.rfc-editor.org/info/rfc8065>.

   [TIA-485-A]
              TIA, "TIA-485-A, Electrical Characteristics of Generators
              and Receivers for Use in Balanced Digital Multipoint
              Systems (ANSI/TIA/EIA-485-A-98) (R2003)", March 2003,
              <https://global.ihs.com/
              doc_detail.cfm?item_s_key=00032964>.

Appendix A.  Abstract MAC Interface

   This Appendix is informative and not part of the standard.

   [BACnet], 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.

A.1.  MA-DATA.request

A.1.1.  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.

A.1.2.  Semantics of the Service Primitive

   The semantics of the primitive are as follows:

     MA-DATA.request (
                      destination_address,
                      source_address,
                      data,
                      type
                     )




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   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 Section 1.3)
   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.

A.1.3.  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.

A.1.4.  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.

A.2.  MA-DATA.indication

A.2.1.  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.

A.2.2.  Semantics of the Service Primitive









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   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.

A.2.3.  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.

A.2.4.  Effect on Receipt

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


Appendix B.  Consistent Overhead Byte Stuffing (COBS)

   This Appendix is informative and not part of the standard.









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   [BACnet], 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 [COBS] 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.  [BACnet],
   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 [COBS].

   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 Appendix C.  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 [BACnet], Annex T.

   <CODE BEGINS>
   #include <stddef.h>
   #include <stdint.h>

   /*
    * 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;



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     uint8_t code = 1;
     uint8_t data, last_code;

     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;
   }
   <CODE ENDS>












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   <CODE BEGINS>
   #include <stddef.h>
   #include <stdint.h>

   /*
    * 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.
        */
       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;
   }
   <CODE ENDS>


Appendix C.  Encoded CRC-32K [CRC32K]

   This Appendix is informative and not part of the standard.




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   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
   [CRC32K], he surveyed the entire 32-bit polynomial space and noted
   some that exceed the [IEEE.802.3] polynomial in performance.
   [BACnet], 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 [COBS] Encoded Data field
   through the function, the ones complement of the resulting
   'crc_value' is arranged in LSB-first order and is itself [COBS]
   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 [BACnet],
   Annex G.3.

























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   <CODE BEGINS>
   #include <stdint.h>

   /* See ANSI/ASHRAE Standard 135-2024 [BACnet], Section G.3.2 */
   #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;
   }
   <CODE ENDS>


Appendix D.  Example 6LoBAC Frame Decode

   This Appendix is informative and not part of the standard.

   BACnet MS/TP, Src (2), Dst (1), IPv6 Encapsulation
       Preamble 55: 0x55
       Preamble FF: 0xff
       Frame Type: IPv6 Encapsulation (34)
       Destination Address: 1
       Source Address: 2
       Length: 537
       Header CRC: 0x1c [correct]



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       Extended Data CRC: 0x9e7259e2 [correct]
   6LoWPAN
       IPHC Header
           011. .... = Pattern: IP header compression (0x03)
           ...1 1... .... .... = Traffic class and flow label:
                                 Version, traffic class, and flow label
                                 compressed (0x0003)
           .... .0.. .... .... = Next header: Inline
           .... ..00 .... .... = Hop limit: Inline (0x0000)
           .... .... 1... .... = Context identifier extension: True
           .... .... .1.. .... = Source address compression: Stateful
           .... .... ..01 .... = Source address mode:
                                 64-bits inline (0x0001)
           .... .... .... 0... = Multicast address compression: False
           .... .... .... .1.. = Destination address compression:
                                 Stateful
           .... .... .... ..10 = Destination address mode:
                                 16-bits inline (0x0002)
           0000 .... = Source context identifier: 0x00
           .... 0000 = Destination context identifier: 0x00
           [Source context: aaaa:: (aaaa::)]
           [Destination context: aaaa:: (aaaa::)]
       Next header: ICMPv6 (0x3a)
       Hop limit: 63
       Source: aaaa::1 (aaaa::1)
       Destination: aaaa::ff:fe00:1 (aaaa::ff:fe00:1)
   Internet Protocol Version 6, Src: aaaa::1 (aaaa::1),
                                Dst: aaaa::ff:fe00:1 (aaaa::ff:fe00:1)
       0110 .... .... .... .... .... .... .... = Version: 6
       .... 0000 0000 .... .... .... .... .... = Traffic class:
                                                 0x00000000
       .... 0000 00.. .... .... .... .... .... = Differentiated
                                                 Services Field:
                                                 Default (0x00000000)
       .... .... ..0. .... .... .... .... .... = ECN-Capable Transport
                                                 (ECT): Not set
       .... .... ...0 .... .... .... .... .... = ECN-CE: Not set
       .... .... .... 0000 0000 0000 0000 0000 = Flowlabel: 0x00000000
       Payload length: 518
       Next header: ICMPv6 (58)
       Hop limit: 63
       Source: aaaa::1 (aaaa::1)
       Destination: aaaa::ff:fe00:1 (aaaa::ff:fe00:1)
   Internet Control Message Protocol v6
       Type: Echo (ping) request (128)
       Code: 0
       Checksum: 0x783f [correct]
       Identifier: 0x2ee5



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       Sequence: 2
       [Response In: 5165]
       Data (510 bytes)
           Data: e4dbe8553ba0040008090a0b0c0d0e0f1011121314151617...
           [Length: 510]


   Frame (547 bytes):
   55 ff 22 01 02 02 19 1c 56 2d 83 56 6f 6a 54 54   U.".....V-.VojTT
   54 54 54 54 57 54 56 54 d5 50 2d 6a 7b b0 5c 57   TTTTWTVT.P-j{.\W
   b1 8e bd 00 6e f5 51 ac 5d 5c 5f 5e 59 58 5b 5a   ....n.Q.]\_^YX[Z
   45 44 47 46 41 40 43 42 4d 4c 4f 4e 49 48 4b 4a   EDGFA@CBMLONIHKJ
   75 74 77 76 71 70 73 72 7d 7c 7f 7e 79 78 7b 7a   utwvqpsr}|.~yx{z
   65 64 67 66 61 60 63 62 6d 6c 6f 6e 69 68 6b 6a   edgfa`cbmlonihkj
   15 14 17 16 11 10 13 12 1d 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 3c 3f 3e 39 38 3b 3a   54761032=<?>98;:
   25 24 27 26 21 20 23 22 2d 2c 2f 2e 29 28 2b 2a   %$'&! #"-,/.)(+*
   d5 d4 d7 d6 d1 d0 d3 d2 dd dc df de d9 d8 db da   ................
   c5 c4 c7 c6 c1 c0 c3 c2 cd cc cf ce c9 c8 cb ca   ................
   f5 f4 f7 f6 f1 f0 f3 f2 fd fc ff fe f9 f8 fb fa   ................
   e5 e4 e7 e6 e1 e0 e3 e2 ed ec ef ee e9 e8 eb ea   ................
   95 94 97 96 91 90 93 92 9d 9c 9f 9e 99 98 9b 9a   ................
   85 84 87 86 81 80 83 82 8d 8c 8f 8e 89 88 8b 8a   ................
   b5 b4 b7 b6 b1 b0 b3 b2 bd bc bf be b9 b8 bb ba   ................
   a5 a4 a7 a6 a1 a0 a3 a2 ad ac af ae a9 a8 ab aa   ................
   ab 54 57 56 51 50 53 52 5d 5c 5f 5e 59 58 5b 5a   .TWVQPSR]\_^YX[Z
   45 44 47 46 41 40 43 42 4d 4c 4f 4e 49 48 4b 4a   EDGFA@CBMLONIHKJ
   75 74 77 76 71 70 73 72 7d 7c 7f 7e 79 78 7b 7a   utwvqpsr}|.~yx{z
   65 64 67 66 61 60 63 62 6d 6c 6f 6e 69 68 6b 6a   edgfa`cbmlonihkj
   15 14 17 16 11 10 13 12 1d 1c 1f 1e 19 18 1b 1a   ................
   05 04 07 06 01 00 03 02 0d 0c 0f 0e 09 08 0b 0a   ................
   35 34 37 36 31 30 33 32 3d 3c 3f 3e 39 38 3b 3a   54761032=<?>98;:
   25 24 27 26 21 20 23 22 2d 2c 2f 2e 29 28 2b 2a   %$'&! #"-,/.)(+*
   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 50 cb   ..............P.
   27 0c b7                                          '..








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   Decoded Data and CRC32K (537 bytes):
   78 d6 00 3a 3f 00 00 00 00 00 00 00 01 00 01 80   x..:?...........
   00 78 3f 2e e5 00 02 e4 db e8 55 3b a0 04 00 08   .x?.......U;....
   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   )*+,-./012345678
   39 3a 3b 3c 3d 3e 3f 40 41 42 43 44 45 46 47 48   9:;<=>?@ABCDEFGH
   49 4a 4b 4c 4d 4e 4f 50 51 52 53 54 55 56 57 58   IJKLMNOPQRSTUVWX
   59 5a 5b 5c 5d 5e 5f 60 61 62 63 64 65 66 67 68   YZ[\]^_`abcdefgh
   69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 78   ijklmnopqrstuvwx
   79 7a 7b 7c 7d 7e 7f 80 81 82 83 84 85 86 87 88   yz{|}~..........
   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   )*+,-./012345678
   39 3a 3b 3c 3d 3e 3f 40 41 42 43 44 45 46 47 48   9:;<=>?@ABCDEFGH
   49 4a 4b 4c 4d 4e 4f 50 51 52 53 54 55 56 57 58   IJKLMNOPQRSTUVWX
   59 5a 5b 5c 5d 5e 5f 60 61 62 63 64 65 66 67 68   YZ[\]^_`abcdefgh
   69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 78   ijklmnopqrstuvwx
   79 7a 7b 7c 7d 7e 7f 80 81 82 83 84 85 86 87 88   yz{|}~..........
   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 9e 72 59 e2                        ......rY.
















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   Decompressed 6LoWPAN IPHC (558 bytes):
   60 00 00 00 02 06 3a 3f aa aa 00 00 00 00 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 01 80 00 78 3f 2e e5 00 02   ..........x?....
   e4 db e8 55 3b a0 04 00 08 09 0a 0b 0c 0d 0e 0f   ...U;...........
   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         ..............


Acknowledgements

   We are grateful to the authors of [RFC4944] 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.



Lynn                      Expires 23 April 2026                [Page 25]

Internet-Draft          IPv6 over MS/TP (6LoBAC)            October 2025


   Alexandru Petrescu, Brian Frank, Geoff Mulligan, and Don Sturek
   offered valuable comments.

Author's Address

   Kerry Lynn
   Consultant
   Phone: +1 978 460 4253
   Email: kerlyn@ieee.org










































Lynn                      Expires 23 April 2026                [Page 26]
