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Internet Engineering Task Force (IETF) K. Lynn, Ed.
Request for Comments: 8163 Verizon Labs
Category: Standards Track J. Martocci
ISSN: 2070-1721 Johnson Controls
C. Neilson
Delta Controls
S. Donaldson
Honeywell
May 2017
Transmission of IPv6 over Master-Slave/Token-Passing (MS/TP) Networks
Abstract
Master-Slave/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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8163.
Lynn, et al. Standards Track [Page 1]
RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
Copyright Notice
Copyright (c) 2017 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
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Profile for IPv6 over MS/TP . . . . . . . . . . . . . . . . . 6
3. Addressing Modes . . . . . . . . . . . . . . . . . . . . . . 7
4. Maximum Transmission Unit (MTU) . . . . . . . . . . . . . . . 8
5. LoBAC Adaptation Layer . . . . . . . . . . . . . . . . . . . 8
6. Stateless Address Autoconfiguration . . . . . . . . . . . . . 9
7. IPv6 Link-Local Address . . . . . . . . . . . . . . . . . . . 10
8. Unicast Address Mapping . . . . . . . . . . . . . . . . . . . 10
9. Multicast Address Mapping . . . . . . . . . . . . . . . . . . 11
10. Header Compression . . . . . . . . . . . . . . . . . . . . . 11
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
12. Security Considerations . . . . . . . . . . . . . . . . . . . 12
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Abstract MAC Interface . . . . . . . . . . . . . . . 15
Appendix B. Consistent Overhead Byte Stuffing (COBS) . . . . . . 17
Appendix C. Encoded CRC-32K (CRC32K) . . . . . . . . . . . . . . 20
Appendix D. Example 6LoBAC Frame Decode . . . . . . . . . . . . 22
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
Master-Slave/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
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)
Lynn, et al. Standards Track [Page 3]
RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
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 135-2016 [BACnet]. The latest
version of [BACnet] integrates changes to legacy MS/TP (approved as
[Addendum_an]) that provide support for larger frame sizes and
improved error handling. [BACnet], Clause 9 also covers physical-
layer deployment options.
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 master 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 master node passes the
token to the next master node (as determined by the MAC address). If
present on the link, legacy MS/TP implementations (including any
slave 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.
Lynn, et al. Standards Track [Page 4]
RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
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:
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 Master
2 Reply To Poll For Master
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.
Lynn, et al. Standards Track [Page 5]
RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
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.
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
Master 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 master nodes
must implement the Master Node state machine and handle Token, Poll
For Master, and Reply To Poll For Master control frames. 6LoBAC
nodes are MS/TP master 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.
Lynn, et al. Standards Track [Page 6]
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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 Master Node state machine.
Nmax_info_frames The default maximum number of information frames
the node may send before it must pass the token: 1
Nmax_master 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 master 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].
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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.
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.
Lynn, et al. Standards Track [Page 8]
RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
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.
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.
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RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
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.
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.
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RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
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].
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.
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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
[BACnet] ASHRAE, "BACnet-A Data Communication Protocol for Building
Automation and Control Networks", ANSI/ASHRAE Standard
135-2016, January 2016,
<http://www.techstreet.com/ashrae/standards/
ashrae-135-2016?product_id=1918140#jumps>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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, <http://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, <http://www.rfc-editor.org/info/rfc3315>.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005,
<http://www.rfc-editor.org/info/rfc3972>.
Lynn, et al. Standards Track [Page 12]
RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://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,
<http://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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc4944>.
[RFC5535] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
DOI 10.17487/RFC5535, June 2009,
<http://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,
<http://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,
<http://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, <http://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,
<http://www.rfc-editor.org/info/rfc7217>.
Lynn, et al. Standards Track [Page 13]
RFC 8163 IPv6 over MS/TP (6LoBAC) May 2017
[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, <http://www.rfc-editor.org/info/rfc7400>.
13.2. Informative References
[Addendum_an]
ANSI/ASHRAE, "Addenda: BACnet -- A Data Communication
Protocol for Building Automation and Control Networks",
ANSI/ASHRAE Addenda an, at, au, av, aw, ax, and az
to ANSI/ASHRAE Standard 135-2012, July 2014,
<https://www.ashrae.org/File%20Library/docLib/StdsAddenda/
07-31-2014_135_2012_an_at_au_av_aw_ax_az_Final.pdf>.
[COBS] Cheshire, S. and M. Baker, "Consistent Overhead Byte
Stuffing", IEEE/ACM Transactions on Networking, Volume 7,
Issue 2, DOI 10.1109/90.769765, April 1999,
<http://www.stuartcheshire.org/papers/COBSforToN.pdf>.
[CRC32K] Koopman, P., "32-Bit Cyclic Redundancy Codes for Internet
Applications", Proceedings of the 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]
IEEE, "IEEE Standard for Ethernet", IEEE 802.3-2015, DOI
10.1109/IEEESTD.2016.7428776,
<http://standards.ieee.org/getieee802/
download/802.3-2015.zip>.
[RFC2469] Narten, T. and C. Burton, "A Caution On The Canonical
Ordering Of Link-Layer Addresses", RFC 2469,
DOI 10.17487/RFC2469, December 1998,
<http://www.rfc-editor.org/info/rfc2469>.
[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <http://www.rfc-editor.org/info/rfc8065>.
[TIA-485-A]
TIA, "Electrical Characteristics of Generators and
Receivers for Use in Balanced Digital Multipoint Systems",
TIA-485-A (Revision of TIA-485), March 2003,
<https://global.ihs.com/
doc_detail.cfm?item_s_key=00032964>.
Lynn, et al. Standards Track [Page 14]
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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
)
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.
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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,
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.
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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.
[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.
Lynn, et al. Standards Track [Page 17]
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<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;
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
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* 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;
}
#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 (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;
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}
return write_index;
}
<CODE ENDS>
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 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 135-2016 [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>
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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]
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)
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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
Sequence: 2
[Response In: 5165]
Data (510 bytes)
Data: e4dbe8553ba0040008090a0b0c0d0e0f1011121314151617...
[Length: 510]
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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 %