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Versions: (draft-ietf-6man-6lobac) 00 01 02 03 04 05 06 07 08 RFC 8163

6Lo Working Group                                           K. Lynn, Ed.
Internet-Draft                                              Verizon Labs
Intended status: Standards Track                             J. Martocci
Expires: January 7, 2016                                Johnson Controls
                                                              C. Neilson
                                                          Delta Controls
                                                            S. Donaldson
                                                               Honeywell
                                                            July 6, 2015


                Transmission of IPv6 over MS/TP Networks
                        draft-ietf-6lo-6lobac-02

Abstract

   Master-Slave/Token-Passing (MS/TP) is a medium access control method
   for the RS-485 physical layer, which is used extensively 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 7, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  MS/TP Mode for IPv6 . . . . . . . . . . . . . . . . . . . . .   5
   3.  Addressing Modes  . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Maximum Transmission Unit (MTU) . . . . . . . . . . . . . . .   6
   5.  LoBAC Adaptation Layer  . . . . . . . . . . . . . . . . . . .   6
   6.  Stateless Address Autoconfiguration . . . . . . . . . . . . .   7
   7.  IPv6 Link Local Address . . . . . . . . . . . . . . . . . . .   8
   8.  Unicast Address Mapping . . . . . . . . . . . . . . . . . . .   8
   9.  Multicast Address Mapping . . . . . . . . . . . . . . . . . .   9
   10. Header Compression  . . . . . . . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Abstract MAC Interface . . . . . . . . . . . . . . .  13
   Appendix B.  Consistent Overhead Byte Stuffing [COBS] . . . . . .  15
   Appendix C.  Encoded CRC-32K [CRC32K] . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Master-Slave/Token-Passing (MS/TP) is a medium access control (MAC)
   protocol for the RS-485 [TIA-485-A] physical layer, which is used
   extensively 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.

   An MS/TP device is typically based on a low-cost microcontroller with
   limited processing power and memory.  Together with low data rates
   and a small MAC address space, these constraints are similar to those
   faced in 6LoWPAN networks and suggest some elements of that solution
   might be leveraged.  MS/TP differs significantly from 6LoWPAN in at
   least three respects: a) MS/TP devices typically have a continuous
   source of power, b) all MS/TP devices on a segment can communicate
   directly so there are no hidden node or mesh routing issues, and c)
   recent changes to MS/TP provide support for large payloads,
   eliminating the need for fragmentation and reassembly below IPv6.



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   The following sections provide a brief overview of MS/TP, then
   describe how to form IPv6 addresses and encapsulate IPv6 packets in
   MS/TP frames.  This document also specifies a header compression
   mechanism, based on [RFC6282], that is REQUIRED in order to reduce
   latency and make IPv6 practical on MS/TP networks.

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 Check

   MAC:     Medium Access Control

   MSDU:    MAC Service Data Unit (MAC client data)

   MTU:     Maximum Transmission Unit

   UART:    Universal Asynchronous Transmitter/Receiver

1.3.  MS/TP Overview

   This section provides a brief overview of MS/TP, which is specified
   in ANSI/ASHRAE 135-2012 (BACnet) Clause 9 [Clause9] and included
   herein by reference.  BACnet [Clause9] also covers physical layer
   deployment options.

   MS/TP is designed to enable multidrop networks over shielded twisted
   pair wiring.  It can support a data rate of 115,200 baud on segments
   up to 1000 meters in length, or segments up to 1200 meters in length
   at lower baud rates.  An MS/TP link requires only a UART, an RS-485
   [TIA-485-A] transceiver with a driver that can be disabled, and a 5ms
   resolution timer.  These features make MS/TP a cost-effective field
   bus for the most numerous and least expensive devices in a building
   automation network.

   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.



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   A master node may initiate the transmission of a data frame when it
   holds the token.  After sending at most a configured maximum number
   of data frames, a master node passes the token to the next master
   node (as determined by MAC address).  Slave nodes do not support the
   frame format required to convey IPv6 over MS/TP and therefore SHALL
   NOT be considered part of this specification.

   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

   *NOTE: BACnet Addendum 135-2012an [Addendum_an] defines a range of
    Frame Type values 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
    maximum length of an MSDU as defined by this specification is 1500
    octets (before encoding).  The Encoded Data is covered by a 32-bit
    CRC [CRC32K] (see Appendix C), which is itself then COBS encoded.

   MS/TP COBS-encoded frame fields have the following descriptions:

     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 Appendix B)
     Encoded CRC-32K       five octets (see Appendix C)
     (pad)                 (optional) at most one octet of trailer: 0xFF




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   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 Master
      2  Reply To Poll For Master
         ...
     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 and MUST
   convey Encoded Data and Encoded CRC-32K fields.  All master nodes
   MUST understand Token, Poll For Master, and Reply to Poll For Master
   control frames.  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 specifies the combined
   length of the [COBS] Encoded Data and Encoded CRC-32K fields in
   octets, minus two.  (This adjustment is required for backward
   compatibility with legacy MS/TP devices.)  See Section 4 and
   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 [Clause9].

1.4.  Goals and Constraints

   The primary goal of this specification is to enable IPv6 directly on
   wired end devices in building automation and control networks by
   leveraging existing standards to the greatest extent possible.  A
   secondary goal is to co-exist with legacy MS/TP implementations.
   Only the minimum changes necessary to support IPv6 over MS/TP were
   specified in BACnet [Addendum_an] (see Note in Section 1.3).

   In order to co-exist with legacy devices, no changes are permitted to
   the MS/TP addressing modes, frame header format, control frames, or
   Master Node state machine as specified in BACnet [Clause9].

2.  MS/TP Mode for IPv6

   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.

   All MS/TP master nodes (including those that support IPv6) must



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   understand Token, Poll For Master, and Reply to Poll For Master
   control frames and support the Master Node state machine as specified
   in BACnet [Clause9].  MS/TP master nodes that support IPv6 must also
   support the Receive Frame state machine as specified in [Clause9] and
   extended by BACnet [Addendum_an].

3.  Addressing Modes

   MS/TP node (MAC) addresses are one octet in length.  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 [Clause9] specifies that addresses 0 through 127 are valid for
   master nodes.  The method specified in Section 6 for creating a MAC-
   layer-derived Interface Identifier (IID) ensures that an IID of all
   zeros can never result.

   A Destination Address of 255 (all nodes) indicates a MAC-layer
   broadcast.  MS/TP does not support multicast, therefore all IPv6
   multicast packets SHOULD be broadcast at the MAC layer and filtered
   at the IPv6 layer.  A Source Address of 255 MUST NOT be used.

   This specification assumes that at most one unique local and/or
   global IPv6 prefix is assigned to each MS/TP segment.  Hosts learn
   IPv6 prefixes via router advertisements according to [RFC4861].

4.  Maximum Transmission Unit (MTU)

   BACnet [Addendum_an] supports MSDUs up to 2032 octets in length.
   This specification defines an MSDU length of at least 1280 octets and
   at most 1500 octets (before encoding).  This is sufficient to convey
   the minimum MTU required by IPv6 [RFC2460] without the need for link-
   layer fragmentation and reassembly.

5.  LoBAC Adaptation Layer

   The relatively low data rates of MS/TP indicate header compression as
   a means to reduce latency.  This section specifies an adaptation
   layer to support compressed IPv6 headers and the compression format
   is specified in Section 10.

   Implementations MAY also support Generic Header Compression (GHC)
   [RFC7400] for transport layer headers.  A node implementing [RFC7400]
   MUST probe its peers for GHC support before applying GHC.

   The encapsulation format defined in this section (subsequently
   referred to as the "LoBAC" encapsulation) comprises the MSDU of an



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   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] and extended by [RFC6282],
   therefore the use of "LOWPAN" in literals below is intentional.  The
   primary difference between LoWPAN and LoBAC is omission of the Mesh,
   Broadcast, Fragmentation, and LOWPAN_HC1 headers.

   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 may be 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 this
   specification.

6.  Stateless Address Autoconfiguration

   This section defines how to obtain an IPv6 Interface Identifier.  The
   general procedure for creating a MAC-address-derived IID is described
   in [RFC4291] Appendix A, "Creating Modified EUI-64 Format Interface
   Identifiers", as updated by [RFC7136].

   The IID SHOULD NOT embed an [EUI-64] or any other globally unique
   hardware identifier assigned to a device (see Section 12).

   The Interface Identifier for link-local addresses SHOULD be formed by
   concatenating its 8-bit MS/TP MAC address to the seven octets 0x00,
   0x00, 0x00, 0xFF, 0xFE, 0x00, 0x00.  For example, an MS/TP MAC
   address of hexadecimal value 0x4F results in the following IID:





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   |0              1|1              3|3              4|4              6|
   |0              5|6              1|2              7|8              3|
   +----------------+----------------+----------------+----------------+
   |0000000000000000|0000000011111111|1111111000000000|0000000001001111|
   +----------------+----------------+----------------+----------------+

   This is the RECOMMENDED method of forming an IID for use in link-
   local addresses, as it affords the most efficient header compression
   provided by the LOWPAN_IPHC [RFC6282] format specified in Section 10.

   A 64-bit privacy IID is RECOMMENDED for routable addresses and SHOULD
   be locally generated according to [I-D.ietf-6man-default-iids].  A
   node that generates a 64-bit privacy IID MUST register it with its
   local router(s) by sending a Neighbor Solicitation (NS) message with
   the Address Registration Option (ARO) and process Neighbor
   Advertisements (NA) 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   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +      Padding (all zeros)      +
   |                               |
   +               +-+-+-+-+-+-+-+-+
   |               | MS/TP Address |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 SHOULD 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:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     0x00      |     0xFF      |
   +-+-+-+-+-+-+-+-+---------------+

10.  Header Compression

   LoBAC uses 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].

   In general the following substitutions should be made:



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    - 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:

    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 border
   router(s) directly attached to the MS/TP segment MUST disseminate the
   6LoWPAN Context Option (6CO) according to [RFC6775], Section 7.2.

11.  IANA Considerations

   This document uses values previously reserved by [RFC4944] and
   [RFC6282] and makes no further requests of IANA.

   Note to RFC Editor: this section may be removed upon publication.

12.  Security Considerations

   The security and privacy implications of embedding a link-layer
   address in an IPv6 IID are discussed in
   [I-D.ietf-6man-ipv6-address-generation-privacy].  The issue most
   relevant to MS/TP networks is address scanning.  This is mainly an
   issue for routable addresses and probably only for those hosted on
   the global Internet.  This specification RECOMMENDS mitigating this
   threat according to [I-D.ietf-6man-default-iids].

13.  Acknowledgments

   We are grateful to the authors of [RFC4944] and members of the IETF
   6LoWPAN working group; this document borrows liberally from their
   work.

14.  References

14.1.  Normative References







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   [Addendum_an]
              ASHRAE, "ANSI/ASHRAE Addenda an, at, au, av, aw, ax, and
              az to ANSI/ASHRAE Standard 135-2012, BACnet - A Data
              Communication Protocol for Building Automation and Control
              Networks", July 2014,
              <https://www.ashrae.org/File%20Library/docLib/StdsAddenda/
              07-31-2014_135_2012_an_at_au_av_aw_ax_az_Final.pdf>.

   [Clause9]  American Society of Heating, Refrigerating, and Air-
              Conditioning Engineers, "BACnet - A Data Communication
              Protocol for Building Automation and Control Networks",
              ANSI/ASHRAE 135-2012 (Clause 9), March 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

   [RFC6775]  Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
              "Neighbor Discovery Optimization for IPv6 over Low-Power
              Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
              November 2012.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, February 2014.

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, November 2014.



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14.2.  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,
              <http://www.ece.cmu.edu/~koopman/networks/dsn02/
              dsn02_koopman.pdf>.

   [EUI-64]   IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
              Registration Authority", March 1997,
              <http://standards.ieee.org/regauth/oui/tutorials/
              EUI64.html>.

   [I-D.ietf-6man-default-iids]
              Gont, F., Cooper, A., Thaler, D., and S. LIU,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-04 (work in progress), June
              2015.

   [I-D.ietf-6man-ipv6-address-generation-privacy]
              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              draft-ietf-6man-ipv6-address-generation-privacy-07 (work
              in progress), June 2015.

   [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
              (CMSA/CD) Access Method and Physical Layer
              Specifications", IEEE Std 802.3-2012, December 2012,
              <http://standards.ieee.org/getieee802/802.3.html>.

   [RFC2469]  Narten, T. and C. Burton, "A Caution On The Canonical
              Ordering Of Link-Layer Addresses", RFC 2469, December
              1998.

   [TIA-485-A]
              Telecommunications Industry Association, "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.



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Appendix A.  Abstract MAC Interface

   This Appendix is informative and not part of the standard.

   BACnet [Clause9] defines 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,
                      priority,
                      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 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 'priority' parameter specifies the priority desired for the data
   unit transfer.  The priority parameter is ignored by MS/TP.

   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.



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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

   The semantics of the primitive are as follows:

     MA-DATA.indication (
                         destination_address,
                         source_address,
                         data,
                         priority,
                         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 'priority' parameter specifies the priority desired for the data
   unit transfer.  The priority parameter is ignored by MS/TP.



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   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, 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.

   BACnet [Addendum_an] 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
   [Addendum_an] specifies the preamble octet '0x55' for removal.

   The minimum overhead of COBS is one ectet per encoded field.  The
   worst-case overhead in long fields is bounded to one octet in 254, or
   less than 0.4%, 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 existing MS/TP devices, is placed
   in the MS/TP header Length field before transmission.




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   Example COBS encoder and decoder functions are shown below for
   illustration.  Complete examples of use and test vectors are provided
   in BACnet [Addendum_an].

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

   #define CRC32K_INITIAL_VALUE (0xFFFFFFFF)
   #define MSTP_PREAMBLE_X55 (0x55)

   /*
    * Encodes 'length' octets of data located at 'from' and
    * writes one or more COBS code blocks at 'to', removing any
    * 'mask' octets that may present be 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;

     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;



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     }
     /*
      * If the last chunk contains exactly 254 non-zero octets, then
      * this exception is handled above (and 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;
   }





































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

   #define CRC32K_INITIAL_VALUE (0xFFFFFFFF)
   #define MSTP_PREAMBLE_X55 (0x55)

   /*
    * 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 (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;
   }








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Appendix C.  Encoded CRC-32K [CRC32K]

   This Appendix is informative and not part of the standard.

   Extending the payload of MS/TP to 1500 octets required 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
   [Addendum_an] specifies 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 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 in shown below for illustration.
   Complete examples of use and test vectors are provided in BACnet
   [Addendum_an].























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   #include <stdint.h>

   /* See BACnet Addendum 135-2012an, 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: crcValue 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;
   }



















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Authors' Addresses

   Kerry Lynn (editor)
   Verizon Labs
   50 Sylvan Rd
   Waltham , MA   02451
   USA

   Phone: +1 781 296 9722
   Email: kerlyn@ieee.org


   Jerry Martocci
   Johnson Controls, Inc.
   507 E. Michigan St
   Milwaukee , WI   53202
   USA

   Phone: +1 414 524 4010
   Email: jerald.p.martocci@jci.com


   Carl Neilson
   Delta Controls, Inc.
   17850 56th Ave
   Surrey , BC   V3S 1C7
   Canada

   Phone: +1 604 575 5913
   Email: cneilson@deltacontrols.com


   Stuart Donaldson
   Honeywell Automation & Control Solutions
   6670 185th Ave NE
   Redmond , WA   98052
   USA

   Email: stuart.donaldson@honeywell.com












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