draft-ietf-6lo-6lobac-08.txt   rfc8163.txt 
6Lo Working Group K. Lynn, Ed. Internet Engineering Task Force (IETF) K. Lynn, Ed.
Internet-Draft Verizon Labs Request for Comments: 8163 Verizon Labs
Intended status: Standards Track J. Martocci Category: Standards Track J. Martocci
Expires: September 11, 2017 Johnson Controls ISSN: 2070-1721 Johnson Controls
C. Neilson C. Neilson
Delta Controls Delta Controls
S. Donaldson S. Donaldson
Honeywell Honeywell
March 10, 2017 May 2017
Transmission of IPv6 over MS/TP Networks Transmission of IPv6 over Master-Slave/Token-Passing (MS/TP) Networks
draft-ietf-6lo-6lobac-08
Abstract Abstract
Master-Slave/Token-Passing (MS/TP) is a medium access control method Master-Slave/Token-Passing (MS/TP) is a medium access control method
for the RS-485 physical layer and is used primarily in building for the RS-485 physical layer and is used primarily in building
automation networks. This specification defines the frame format for automation networks. This specification defines the frame format for
transmission of IPv6 packets and the method of forming link-local and transmission of IPv6 packets and the method of forming link-local and
statelessly autoconfigured IPv6 addresses on MS/TP networks. statelessly autoconfigured IPv6 addresses on MS/TP networks.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
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 This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on September 11, 2017. 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.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Profile for IPv6 over MS/TP . . . . . . . . . . . . . . . . . 5 2. Profile for IPv6 over MS/TP . . . . . . . . . . . . . . . . . 6
3. Addressing Modes . . . . . . . . . . . . . . . . . . . . . . 6 3. Addressing Modes . . . . . . . . . . . . . . . . . . . . . . 7
4. Maximum Transmission Unit (MTU) . . . . . . . . . . . . . . . 7 4. Maximum Transmission Unit (MTU) . . . . . . . . . . . . . . . 8
5. LoBAC Adaptation Layer . . . . . . . . . . . . . . . . . . . 7 5. LoBAC Adaptation Layer . . . . . . . . . . . . . . . . . . . 8
6. Stateless Address Autoconfiguration . . . . . . . . . . . . . 8 6. Stateless Address Autoconfiguration . . . . . . . . . . . . . 9
7. IPv6 Link Local Address . . . . . . . . . . . . . . . . . . . 9 7. IPv6 Link-Local Address . . . . . . . . . . . . . . . . . . . 10
8. Unicast Address Mapping . . . . . . . . . . . . . . . . . . . 9 8. Unicast Address Mapping . . . . . . . . . . . . . . . . . . . 10
9. Multicast Address Mapping . . . . . . . . . . . . . . . . . . 10 9. Multicast Address Mapping . . . . . . . . . . . . . . . . . . 11
10. Header Compression . . . . . . . . . . . . . . . . . . . . . 10 10. Header Compression . . . . . . . . . . . . . . . . . . . . . 11
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
12. Security Considerations . . . . . . . . . . . . . . . . . . . 11 12. Security Considerations . . . . . . . . . . . . . . . . . . . 12
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 Appendix A. Abstract MAC Interface . . . . . . . . . . . . . . . 15
Appendix A. Abstract MAC Interface . . . . . . . . . . . . . . . 14 Appendix B. Consistent Overhead Byte Stuffing (COBS) . . . . . . 17
Appendix B. Consistent Overhead Byte Stuffing [COBS] . . . . . . 17 Appendix C. Encoded CRC-32K (CRC32K) . . . . . . . . . . . . . . 20
Appendix C. Encoded CRC-32K [CRC32K] . . . . . . . . . . . . . . 20
Appendix D. Example 6LoBAC Frame Decode . . . . . . . . . . . . 22 Appendix D. Example 6LoBAC Frame Decode . . . . . . . . . . . . 22
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
Master-Slave/Token-Passing (MS/TP) is a medium access control (MAC) 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 protocol for the RS-485 [TIA-485-A] physical layer and is used
primarily in building automation networks. This specification primarily in building automation networks. This specification
defines the frame format for transmission of IPv6 [RFC2460] packets defines the frame format for transmission of IPv6 [RFC2460] packets
and the method of forming link-local and statelessly autoconfigured and the method of forming link-local and statelessly autoconfigured
IPv6 addresses on MS/TP networks. The general approach is to adapt IPv6 addresses on MS/TP networks. The general approach is to adapt
elements of the 6LoWPAN specifications [RFC4944], [RFC6282], and elements of the 6LoWPAN specifications ([RFC4944], [RFC6282], and
[RFC6775] to constrained wired networks, as noted below. [RFC6775]) to constrained wired networks, as noted below.
An MS/TP device is typically based on a low-cost microcontroller with An MS/TP device is typically based on a low-cost microcontroller with
limited processing power and memory. These constraints, together limited processing power and memory. These constraints, together
with low data rates and a small MAC address space, are similar to with low data rates and a small MAC address space, are similar to
those faced in 6LoWPAN networks. MS/TP differs significantly from those faced in 6LoWPAN networks. MS/TP differs significantly from
6LoWPAN in at least three respects: a) MS/TP devices are typically 6LoWPAN in at least three respects: a) MS/TP devices are typically
mains powered, b) all MS/TP devices on a segment can communicate 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) directly so there are no hidden node or mesh routing issues, and c)
the latest MS/TP specification provides support for large payloads, the latest MS/TP specification provides support for large payloads,
eliminating the need for fragmentation and reassembly below IPv6. eliminating the need for fragmentation and reassembly below IPv6.
The following sections provide a brief overview of MS/TP, then The following sections provide a brief overview of MS/TP and then
describe how to form IPv6 addresses and encapsulate IPv6 packets in describe how to form IPv6 addresses and encapsulate IPv6 packets in
MS/TP frames. This specifcation (subsequently referred to as MS/TP frames. This specification (subsequently referred to as
"6LoBAC") includes a REQUIRED header compression mechanism that is "6LoBAC") includes a REQUIRED header compression mechanism that is
based on LOWPAN_IPHC [RFC6282] and improves MS/TP link utilization. based on LOWPAN_IPHC [RFC6282] and improves MS/TP link utilization.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
1.2. Abbreviations Used 1.2. Abbreviations Used
ASHRAE: American Society of Heating, Refrigerating, and Air- ASHRAE: American Society of Heating, Refrigerating, and Air-
Conditioning Engineers (http://www.ashrae.org) 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 BACnet: An ISO/ANSI/ASHRAE Standard Data Communication Protocol for
Building Automation and Control Networks
MAC: Medium Access Control CRC: Cyclic Redundancy Code
MSDU: MAC Service Data Unit (MAC client data) MAC: Medium Access Control
MTU: Maximum Transmission Unit; the size of the largest network MSDU: MAC Service Data Unit (MAC client data)
layer protocol data unit that can be communicated in a single MTU: Maximum Transmission Unit; the size of the largest data unit
network transaction at the network-layer protocol that can be communicated in a
single network transaction
UART: Universal Asynchronous Transmitter/Receiver UART: Universal Asynchronous Transmitter/Receiver
1.3. MS/TP Overview 1.3. MS/TP Overview
This section provides a brief overview of MS/TP, as specified in This section provides a brief overview of MS/TP, as specified in
ANSI/ASHRAE Standard 135-2016 [BACnet] Clause 9. The latest version Clause 9 of the ANSI/ASHRAE Standard 135-2016 [BACnet]. The latest
of [BACnet] integrates changes to legacy MS/TP (approved as version of [BACnet] integrates changes to legacy MS/TP (approved as
[Addendum_an]) that provide support for larger frame sizes and [Addendum_an]) that provide support for larger frame sizes and
improved error handling. [BACnet] Clause 9 also covers physical improved error handling. [BACnet], Clause 9 also covers physical-
layer deployment options. layer deployment options.
MS/TP is designed to enable multidrop networks over shielded twisted MS/TP is designed to enable multidrop networks over shielded twisted
pair wiring. It can support network segments up to 1000 meters in 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 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 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 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 disabled, and a 5 ms resolution timer. The MS/TP MAC is typically
implemented in software. implemented in software.
The differential signaling used by [TIA-485-A] requires a contention- 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. 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 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 data, and only when it holds the token. After sending at most a
configured maximum number of data frames, a master node passes the configured maximum number of data frames, a master node passes the
token to the next master node (as determined by MAC address). If token to the next master node (as determined by the MAC address). If
present on the link, legacy MS/TP implementations (including any present on the link, legacy MS/TP implementations (including any
slave nodes) ignore the frame format defined in this specification. slave nodes) ignore the frame format defined in this specification.
[BACnet] Clause 9 defines a range of Frame Type values used to [BACnet], Clause 9 defines a range of Frame Type values used to
designate frames that contain data and data CRC fields encoded using designate frames that contain Data and Data CRC fields encoded using
Consistent Overhead Byte Stuffing [COBS] (see Appendix B). The Consistent Overhead Byte Stuffing [COBS] (see Appendix B). The
purpose of COBS encoding is to eliminate preamble sequences from the purpose of COBS encoding is to eliminate preamble sequences from the
Encoded Data and Encoded CRC-32K fields. The Encoded Data is covered Encoded Data and Encoded CRC-32K fields. The Encoded Data field is
by a 32-bit CRC [CRC32K] (see Appendix C) that is also COBS encoded. covered by a 32-bit CRC [CRC32K] (see Appendix C) that is also COBS
encoded.
MS/TP COBS-encoded frames have the following format: MS/TP COBS-encoded frames have the following format:
0 1 2 3 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 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 | | 0x55 | 0xFF | Frame Type | DA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SA | Length (MS octet first) | Header CRC | | SA | Length (MS octet first) | Header CRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 5, line 27 skipping to change at page 6, line 11
ASHRAE. Frame Types 32 - 127 designate COBS-encoded frames that ASHRAE. Frame Types 32 - 127 designate COBS-encoded frames that
convey Encoded Data and Encoded CRC-32K fields. See Section 2 for convey Encoded Data and Encoded CRC-32K fields. See Section 2 for
additional details. additional details.
The Destination and Source Addresses are each one octet in length. The Destination and Source Addresses are each one octet in length.
See Section 3 for additional details. See Section 3 for additional details.
For COBS-encoded frames, the Length field indicates the size of the For COBS-encoded frames, the Length field indicates the size of the
[COBS] Encoded Data field in octets, plus three. (This adjustment is [COBS] Encoded Data field in octets, plus three. (This adjustment is
required in order for legacy MS/TP devices to ignore COBS-encoded required in order for legacy MS/TP devices to ignore COBS-encoded
frames.) See Section 4 and Appendices for additional details. frames.) See Section 4 and the Appendices for additional details.
The Header CRC field covers the Frame Type, Destination Address, The Header CRC field covers the Frame Type, Destination Address,
Source Address, and Length fields. The Header CRC generation and Source Address, and Length fields. The Header CRC generation and
check procedures are specified in [BACnet] Annex G.1. check procedures are specified in [BACnet], Annex G.1.
Use of the optional 0xFF trailer octet is discussed in [BACnet] Use of the optional 0xFF trailer octet is discussed in [BACnet],
Clause 9. Clause 9.
1.4. Goals and Constraints 1.4. Goals and Constraints
The main goals of this specification are a) to enable IPv6 directly The main goals of this specification are a) to enable IPv6 directly
on wired end devices in building automation and control networks by on wired end devices in building automation and control networks by
leveraging existing standards to the greatest extent possible, and b) leveraging existing standards to the greatest extent possible, and b)
to co-exist with legacy MS/TP implementations. Co-existence allows to co-exist with legacy MS/TP implementations. Co-existence allows
MS/TP networks to be incrementally upgraded to support IPv6. MS/TP networks to be incrementally upgraded to support IPv6.
In order to co-exist with legacy devices, no changes are permitted to In order to co-exist with legacy devices, no changes are permitted to
the MS/TP addressing modes, frame header format, control frames, or the MS/TP addressing modes, frame header format, control frames, or
Master Node state machine as specified in [BACnet] Clause 9. Master Node state machine as specified in [BACnet], Clause 9.
2. Profile for IPv6 over MS/TP 2. Profile for IPv6 over MS/TP
ASHRAE has assigned an MS/TP Frame Type value of 34 to indicate 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 over MS/TP (LoBAC) Encapsulation. This falls within the range of
values that designate COBS-encoded data frames. values that designate COBS-encoded data frames.
2.1. Mandatory Features 2.1. Mandatory Features
[BACnet] Clause 9 specifies mandatory to implement features of MS/TP [BACnet], Clause 9 specifies mandatory-to-implement features of MS/TP
devices. E.g., it is mandatory that all MS/TP nodes respond to a devices. For example, it is mandatory that all MS/TP nodes respond
Test_Request with a Test_Response frame. All MS/TP master nodes must to a Test_Request with a Test_Response frame. All MS/TP master nodes
implement the Master Node state machine and handle Token, Poll For must implement the Master Node state machine and handle Token, Poll
Master, and Reply to Poll For Master control frames. 6LoBAC nodes For Master, and Reply To Poll For Master control frames. 6LoBAC
are MS/TP master nodes that implement a Receive Frame state machine nodes are MS/TP master nodes that implement a Receive Frame state
capable of handling COBS-encoded frames. machine capable of handling COBS-encoded frames.
6LoBAC nodes must support a data rate of 115.2 kbit/s and may support 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 lower data rates as specified in [BACnet], Clause 9. The method of
selecting the data rate is outside the scope of this specification. selecting the data rate is outside the scope of this specification.
2.2. Configuration Constants 2.2. Configuration Constants
The following constants are used by the Receive Frame state machine. The following constants are used by the Receive Frame state machine.
Nmin_COBS_length The minimum valid Length value of any LoBAC Nmin_COBS_length The minimum valid Length value of any LoBAC-
encapsulated frame: 5 encapsulated frame: 5
Nmax_COBS_length The maximum valid Length value of any LoBAC Nmax_COBS_length The maximum valid Length value of any LoBAC-
encapsulated frame: 1509 encapsulated frame: 1509
2.3. Configuration Parameters 2.3. Configuration Parameters
The following parameters are used by the Master Node state machine. The following parameters are used by the Master Node state machine.
Nmax_info_frames The default maximum number of information frames Nmax_info_frames The default maximum number of information frames
the node may send before it must pass the token: 1 the node may send before it must pass the token: 1
Nmax_master The default highest allowable address for master Nmax_master The default highest allowable address for master
nodes: 127 nodes: 127
The mechanisms for setting parameters or monitoring MS/TP performance The mechanisms for setting parameters or monitoring MS/TP performance
are outside the scope of this specification. are outside the scope of this specification.
3. Addressing Modes 3. Addressing Modes
MS/TP node (MAC) addresses are one octet in length and assigned MS/TP node (MAC) addresses are one octet in length and are assigned
dynamically. The method of assigning MAC addresses is outside the dynamically. The method of assigning MAC addresses is outside the
scope of this specification. However, each MS/TP node on the link scope of this specification. However, each MS/TP node on the link
MUST have a unique address in order to ensure correct MAC operation. MUST have a unique address in order to ensure correct MAC operation.
[BACnet] Clause 9 specifies that addresses 0 through 127 are valid [BACnet], Clause 9 specifies that addresses 0 through 127 are valid
for master nodes. The method specified in Section 6 for creating a for master nodes. The method specified in Section 6 for creating a
MAC-address-derived Interface Identifier (IID) ensures that an IID of MAC-address-derived Interface Identifier (IID) ensures that an IID of
all zeros can never be generated. all zeros can never be generated.
A Destination Address of 255 (all nodes) indicates a MAC-layer A Destination Address of 255 (all nodes) indicates a MAC-layer
broadcast. MS/TP does not support multicast, therefore all IPv6 broadcast. MS/TP does not support multicast; therefore, all IPv6
multicast packets MUST be broadcast at the MAC layer and filtered at multicast packets MUST be broadcast at the MAC layer and filtered at
the IPv6 layer. A Source Address of 255 MUST NOT be used. the IPv6 layer. A Source Address of 255 MUST NOT be used.
Hosts learn IPv6 prefixes via router advertisements according to Hosts learn IPv6 prefixes via router advertisements according to
[RFC4861]. [RFC4861].
4. Maximum Transmission Unit (MTU) 4. Maximum Transmission Unit (MTU)
Upon transmission, the network layer MTU is formatted according to Upon transmission, the network-layer MTU is formatted according to
Section 5 and becomes the MAC service data unit (MSDU). The MSDU is 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. then COBS encoded by MS/TP. Upon reception, the steps are reversed.
[BACnet] Clause 9 supports MSDUs up to 2032 octets in length. [BACnet], Clause 9 supports MSDUs up to 2032 octets in length.
IPv6 [RFC2460] requires that every link in the internet have an MTU IPv6 [RFC2460] requires that every link in an internet have an MTU of
of 1280 octets or greater. Additionally, a node must be able to 1280 octets or greater. Additionally, a node must be able to accept
accept a fragmented packet that, after reassembly, is as large as a fragmented packet that, after reassembly, is as large as 1500
1500 octets. This specification defines an MTU length of at least octets. This specification defines an MTU length of at least 1280
1280 octets and at most 1500 octets. Support for an MTU length of octets and at most 1500 octets. Support for an MTU length of 1500
1500 octets is RECOMMENDED. octets is RECOMMENDED.
5. LoBAC Adaptation Layer 5. LoBAC Adaptation Layer
This section specifies an adaptation layer to support compressed IPv6 This section specifies an adaptation layer to support compressed IPv6
headers as specified in Section 10. IPv6 header compression MUST be headers as specified in Section 10. IPv6 header compression MUST be
implemented on all nodes. Implementations MAY also support Generic implemented on all 6LoBAC nodes. Implementations MAY also support
Header Compression [RFC7400] for transport layer headers. Generic Header Compression [RFC7400] for transport layer headers.
The LoBAC encapsulation format defined in this section describes the 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 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 packet) follows an encapsulation header stack. LoBAC is a subset of
the LoWPAN encapsulation defined in [RFC4944] as updated by [RFC6282] the LoWPAN encapsulation defined in [RFC4944], as updated by
so the use of "LOWPAN" in literals below is intentional. The primary [RFC6282], so the use of "LOWPAN" in literals below is intentional.
difference between LoWPAN and LoBAC encapsulation is omission of the The primary difference between LoWPAN and LoBAC encapsulation is
Mesh, Broadcast, Fragmentation, and LOWPAN_HC1 headers in the latter. omission of the Mesh, Broadcast, Fragmentation, and LOWPAN_HC1
headers in the latter.
All LoBAC encapsulated datagrams transmitted over MS/TP are prefixed All LoBAC-encapsulated datagrams transmitted over MS/TP are prefixed
by an encapsulation header stack consisting of a Dispatch value by an encapsulation header stack consisting of a Dispatch value
followed by zero or more header fields. The only sequence currently followed by zero or more header fields. The only sequence currently
defined for LoBAC is the LOWPAN_IPHC header followed by payload, as defined for LoBAC is the LOWPAN_IPHC header followed by payload, as
shown below: shown below:
+---------------+---------------+------...-----+ +---------------+---------------+------...-----+
| IPHC Dispatch | IPHC Header | Payload | | IPHC Dispatch | IPHC Header | Payload |
+---------------+---------------+------...-----+ +---------------+---------------+------...-----+
Figure 2: A LoBAC Encapsulated LOWPAN_IPHC Compressed IPv6 Datagram Figure 2: A LoBAC-Encapsulated LOWPAN_IPHC Compressed IPv6 Datagram
The Dispatch value is treated as an unstructured namespace. Only a The Dispatch value is treated as an unstructured namespace. Only a
single pattern is used to represent current LoBAC functionality. single pattern is used to represent current LoBAC functionality.
Pattern Header Type Pattern Header Type
+------------+-----------------------------------------------------+ +------------+-----------------------------------------------------+
| 01 1xxxxx | LOWPAN_IPHC - LOWPAN_IPHC compressed IPv6 [RFC6282] | | 01 1xxxxx | LOWPAN_IPHC - LOWPAN_IPHC compressed IPv6 [RFC6282] |
+------------+-----------------------------------------------------+ +------------+-----------------------------------------------------+
Figure 3: LoBAC Dispatch Value Bit Pattern Figure 3: LoBAC Dispatch Value Bit Pattern
Other IANA-assigned 6LoWPAN Dispatch values do not apply to 6LoBAC Other IANA-assigned 6LoWPAN Dispatch values do not apply to 6LoBAC
unless otherwise specified. unless otherwise specified.
6. Stateless Address Autoconfiguration 6. Stateless Address Autoconfiguration
This section defines how to obtain an IPv6 Interface Identifier. This section defines how to obtain an IPv6 Interface Identifier.
This specification distinguishes between two types of IID, MAC- This specification distinguishes between two types of IIDs, MAC-
address-derived and semantically opaque. address-derived and semantically opaque.
A MAC-address-derived IID is the RECOMMENDED type for use in forming A MAC-address-derived IID is the RECOMMENDED type for use in forming
a link-local address, as it affords the most efficient header a link-local address, as it affords the most efficient header
compression provided by the LOWPAN_IPHC [RFC6282] format specified in compression provided by the LOWPAN_IPHC [RFC6282] format specified in
Section 10. The general procedure for creating a MAC-address-derived Section 10. The general procedure for creating a MAC-address-derived
IID is described in [RFC4291] Appendix A, "Creating Modified EUI-64 IID is described in Appendix A of [RFC4291], "Creating Modified
Format Interface Identifiers", as updated by [RFC7136]. EUI-64 Format Interface Identifiers", as updated by [RFC7136].
The Interface Identifier for link-local addresses SHOULD be formed by The Interface Identifier for link-local addresses SHOULD be formed by
concatenating the node's 8-bit MS/TP MAC address to the seven octets concatenating the node's 8-bit MS/TP MAC address to the seven octets
0x00, 0x00, 0x00, 0xFF, 0xFE, 0x00, 0x00. For example, an MS/TP MAC 0x00, 0x00, 0x00, 0xFF, 0xFE, 0x00, and 0x00. For example, an MS/TP
address of hexadecimal value 0x4F results in the following IID: MAC address of hexadecimal value 0x4F results in the following IID:
|0 1|1 3|3 4|4 6| |0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3| |0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
|0000000000000000|0000000011111111|1111111000000000|0000000001001111| |0000000000000000|0000000011111111|1111111000000000|0000000001001111|
+----------------+----------------+----------------+----------------+ +----------------+----------------+----------------+----------------+
A semantically opaque IID having 64 bits of entropy is RECOMMENDED A semantically opaque IID having 64 bits of entropy is RECOMMENDED
for each globally scoped address and MAY be locally generated for each globally scoped address and MAY be locally generated
according to one of the methods cited in Section 12. A node that 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 generates a 64-bit semantically opaque IID MUST register the IID with
its local router(s) by sending a Neighbor Solicitation (NS) message its local router(s) by sending a Neighbor Solicitation (NS) message
with the Address Registration Option (ARO) and process Neighbor with the Address Registration Option (ARO) and process Neighbor
Advertisements (NA) according to [RFC6775]. Advertisements (NAs) according to [RFC6775].
An IPv6 address prefix used for stateless autoconfiguration [RFC4862] An IPv6 address prefix used for stateless autoconfiguration [RFC4862]
of an MS/TP interface MUST have a length of 64 bits. of an MS/TP interface MUST have a length of 64 bits.
7. IPv6 Link Local Address 7. IPv6 Link-Local Address
The IPv6 link-local address [RFC4291] for an MS/TP interface is The IPv6 link-local address [RFC4291] for an MS/TP interface is
formed by appending the Interface Identifier, as defined above, to formed by appending the Interface Identifier, as defined above, to
the prefix FE80::/64. the prefix FE80::/64.
10 bits 54 bits 64 bits 10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+ +----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Identifier | |1111111010| (zeros) | Interface Identifier |
+----------+-----------------------+----------------------------+ +----------+-----------------------+----------------------------+
8. Unicast Address Mapping 8. Unicast Address Mapping
The address resolution procedure for mapping IPv6 non-multicast The address resolution procedure for mapping IPv6 non-multicast
addresses into MS/TP MAC-layer addresses follows the general addresses into MS/TP MAC-layer addresses follows the general
description in Section 7.2 of [RFC4861], unless otherwise specified. description in Section 7.2 of [RFC4861], unless otherwise specified.
The Source/Target Link-layer Address option has the following form The Source/Target Link-Layer Address option has the following form
when the addresses are 8-bit MS/TP MAC-layer (node) addresses. when the addresses are 8-bit MS/TP MAC-layer (node) addresses.
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length=1 | | Type | Length=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | MS/TP Address | | 0x00 | MS/TP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+ Padding (all zeros) + + Padding (all zeros) +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option fields: Option fields:
Type: Type:
1: for Source Link-layer address. 1: for Source Link-Layer address.
2: for Target Link-layer address. 2: for Target Link-Layer address.
Length: This is the length of this option (including the type and 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 Length fields) in units of 8 octets. The value of this field
for 8-bit MS/TP MAC addresses. is 1 for 8-bit MS/TP MAC addresses.
MS/TP Address: The 8-bit address in canonical bit order [RFC2469]. MS/TP Address: The 8-bit address in canonical bit order [RFC2469].
This is the unicast address the interface currently responds to. This is the unicast address the interface currently responds
to.
9. Multicast Address Mapping 9. Multicast Address Mapping
All IPv6 multicast packets MUST be sent to MS/TP Destination Address All IPv6 multicast packets MUST be sent to MS/TP Destination Address
255 (broadcast) and filtered at the IPv6 layer. When represented as 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 a 16-bit address in a compressed header (see Section 10), it MUST be
formed by padding on the left with a zero octet: formed by padding on the left with a zero octet:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
skipping to change at page 10, line 25 skipping to change at page 11, line 25
| 0x00 | 0xFF | | 0x00 | 0xFF |
+-+-+-+-+-+-+-+-+---------------+ +-+-+-+-+-+-+-+-+---------------+
10. Header Compression 10. Header Compression
6LoBAC REQUIRES LOWPAN_IPHC IPv6 compression, which is specified in 6LoBAC REQUIRES LOWPAN_IPHC IPv6 compression, which is specified in
[RFC6282] and included herein by reference. This section will simply [RFC6282] and included herein by reference. This section will simply
identify substitutions that should be made when interpreting the text identify substitutions that should be made when interpreting the text
of [RFC6282]. of [RFC6282].
In general the following substitutions should be made: In general, the following substitutions should be made:
- Replace instances of "6LoWPAN" with "MS/TP network" - Replace instances of "6LoWPAN" with "MS/TP network"
- Replace instances of "IEEE 802.15.4 address" with "MS/TP address" - 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 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 address"), it MUST be formed by padding the MS/TP address to the left
with a zero octet: with a zero octet:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | MS/TP address | | 0x00 | MS/TP address |
+-+-+-+-+-+-+-+-+---------------+ +-+-+-+-+-+-+-+-+---------------+
If LOWPAN_IPHC compression [RFC6282] is used with context, the If LOWPAN_IPHC compression [RFC6282] is used with context, the
router(s) directly attached to the MS/TP segment MUST disseminate the router(s) directly attached to the MS/TP segment MUST disseminate the
6LoWPAN Context Option (6CO) according to [RFC6775], Section 7.2. 6LoWPAN Context Option (6CO) according to Section 7.2 of [RFC6775].
11. IANA Considerations 11. IANA Considerations
This document uses values previously reserved by [RFC4944] and This document uses values previously reserved by [RFC4944] and
[RFC6282] and makes no further requests of IANA. [RFC6282]; it does not require any IANA actions.
Note to RFC Editor: this section may be removed upon publication.
12. Security Considerations 12. Security Considerations
See [RFC8065] for a general discussion of privacy threats faced by See [RFC8065] for a general discussion of privacy threats faced by
constrained nodes. constrained nodes.
[RFC8065] makes a distinction between "stable" and "temporary" [RFC8065] makes a distinction between "stable" and "temporary"
addresses. The former are long-lived and typically advertised by addresses. The former are long-lived and typically advertised by
servers. The latter are typically used by clients and SHOULD be servers. The latter are typically used by clients and SHOULD be
changed frequently to mitigate correlation of activities over time. changed frequently to mitigate correlation of activities over time.
Nodes that engage in both activities SHOULD support simultaneous use Nodes that engage in both activities SHOULD support simultaneous use
of multiple addresses per device. of multiple addresses per device.
Globally scoped addresses that contain MAC-address-derived IIDs may Globally scoped addresses that contain MAC-address-derived IIDs may
expose a network to address scanning attacks. For this reason, it is expose a network to address-scanning attacks. For this reason, it is
RECOMMENDED that a 64-bit semantically opaque IID be generated for RECOMMENDED that a 64-bit semantically opaque IID be generated for
each globally scoped address in use according to, for example, each globally scoped address in use according to, for example,
[RFC3315], [RFC3972], [RFC4941], [RFC5535], or [RFC7217]. [RFC3315], [RFC3972], [RFC4941], [RFC5535], or [RFC7217].
13. Acknowledgments 13. References
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.
Alexandru Petrescu, Brian Frank, Geoff Mulligan, and Don Sturek
offered valuable comments.
14. References
14.1. Normative References 13.1. Normative References
[BACnet] American Society of Heating, Refrigerating, and Air- [BACnet] ASHRAE, "BACnet-A Data Communication Protocol for Building
Conditioning Engineers, "BACnet - A Data Communication Automation and Control Networks", ANSI/ASHRAE Standard
Protocol for Building Automation and Control Networks", 135-2016, January 2016,
ANSI/ASHRAE Standard 135-2016, January 2016,
<http://www.techstreet.com/ashrae/standards/ <http://www.techstreet.com/ashrae/standards/
ashrae-135-2016?product_id=1918140#jumps>. ashrae-135-2016?product_id=1918140#jumps>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
skipping to change at page 13, line 26 skipping to change at page 14, line 10
Interface Identifiers with IPv6 Stateless Address Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217, Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014, DOI 10.17487/RFC7217, April 2014,
<http://www.rfc-editor.org/info/rfc7217>. <http://www.rfc-editor.org/info/rfc7217>.
[RFC7400] Bormann, C., "6LoWPAN-GHC: Generic Header Compression for [RFC7400] Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
IPv6 over Low-Power Wireless Personal Area Networks IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
2014, <http://www.rfc-editor.org/info/rfc7400>. 2014, <http://www.rfc-editor.org/info/rfc7400>.
14.2. Informative References 13.2. Informative References
[Addendum_an] [Addendum_an]
American Society of Heating, Refrigerating, and Air- ANSI/ASHRAE, "Addenda: BACnet -- A Data Communication
Conditioning Engineers, "ANSI/ASHRAE Addenda an, at, au, Protocol for Building Automation and Control Networks",
av, aw, ax, and az to ANSI/ASHRAE Standard 135-2012, ANSI/ASHRAE Addenda an, at, au, av, aw, ax, and az
BACnet - A Data Communication Protocol for Building to ANSI/ASHRAE Standard 135-2012, July 2014,
Automation and Control Networks", July 2014,
<https://www.ashrae.org/File%20Library/docLib/StdsAddenda/ <https://www.ashrae.org/File%20Library/docLib/StdsAddenda/
07-31-2014_135_2012_an_at_au_av_aw_ax_az_Final.pdf>. 07-31-2014_135_2012_an_at_au_av_aw_ax_az_Final.pdf>.
[COBS] Cheshire, S. and M. Baker, "Consistent Overhead Byte [COBS] Cheshire, S. and M. Baker, "Consistent Overhead Byte
Stuffing", IEEE/ACM TRANSACTIONS ON NETWORKING, VOL.7, Stuffing", IEEE/ACM Transactions on Networking, Volume 7,
NO.2 , April 1999, Issue 2, DOI 10.1109/90.769765, April 1999,
<http://www.stuartcheshire.org/papers/COBSforToN.pdf>. <http://www.stuartcheshire.org/papers/COBSforToN.pdf>.
[CRC32K] Koopman, P., "32-Bit Cyclic Redundancy Codes for Internet [CRC32K] Koopman, P., "32-Bit Cyclic Redundancy Codes for Internet
Applications", IEEE/IFIP International Conference on Applications", Proceedings of the International Conference
Dependable Systems and Networks (DSN 2002) , June 2002, on Dependable Systems and Networks (DSN 2002), June 2002,
<https://users.ece.cmu.edu/~koopman/networks/dsn02/ <https://users.ece.cmu.edu/~koopman/networks/dsn02/
dsn02_koopman.pdf>. dsn02_koopman.pdf>.
[IEEE.802.3_2012] [IEEE.802.3]
IEEE, "802.3-2012", IEEE 802.3-2012, IEEE, "IEEE Standard for Ethernet", IEEE 802.3-2015, DOI
DOI 10.1109/ieeestd.2012.6419735, January 2013, 10.1109/IEEESTD.2016.7428776,
<http://ieeexplore.ieee.org/servlet/ <http://standards.ieee.org/getieee802/
opac?punumber=6419733>. download/802.3-2015.zip>.
[RFC2469] Narten, T. and C. Burton, "A Caution On The Canonical [RFC2469] Narten, T. and C. Burton, "A Caution On The Canonical
Ordering Of Link-Layer Addresses", RFC 2469, Ordering Of Link-Layer Addresses", RFC 2469,
DOI 10.17487/RFC2469, December 1998, DOI 10.17487/RFC2469, December 1998,
<http://www.rfc-editor.org/info/rfc2469>. <http://www.rfc-editor.org/info/rfc2469>.
[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation- [RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065, Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <http://www.rfc-editor.org/info/rfc8065>. February 2017, <http://www.rfc-editor.org/info/rfc8065>.
[TIA-485-A] [TIA-485-A]
Telecommunications Industry Association, "TIA-485-A, TIA, "Electrical Characteristics of Generators and
Electrical Characteristics of Generators and Receivers for Receivers for Use in Balanced Digital Multipoint Systems",
Use in Balanced Digital Multipoint Systems (ANSI/TIA/EIA- TIA-485-A (Revision of TIA-485), March 2003,
485-A-98) (R2003)", March 2003, <https://global.ihs.com/ <https://global.ihs.com/
doc_detail.cfm?item_s_key=00032964>. doc_detail.cfm?item_s_key=00032964>.
Appendix A. Abstract MAC Interface Appendix A. Abstract MAC Interface
This Appendix is informative and not part of the standard. This Appendix is informative and not part of the standard.
[BACnet] Clause 9 provides support for MAC-layer clients through its [BACnet], Clause 9 provides support for MAC-layer clients through its
SendFrame and ReceivedDataNoReply procedures. However, it does not SendFrame and ReceivedDataNoReply procedures. However, it does not
define a network-protocol independent abstract interface for the MAC. define a network-protocol independent abstract interface for the MAC.
This is provided below as an aid to implementation. This is provided below as an aid to implementation.
A.1. MA-DATA.request A.1. MA-DATA.request
A.1.1. Function A.1.1. Function
This primitive defines the transfer of data from a MAC client entity 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 to a single peer entity or multiple peer entities in the case of a
skipping to change at page 15, line 26 skipping to change at page 16, line 14
A.1.3. When Generated A.1.3. When Generated
This primitive is generated by the MAC client entity whenever data This primitive is generated by the MAC client entity whenever data
shall be transferred to a peer entity or entities. This can be in shall be transferred to a peer entity or entities. This can be in
response to a request from higher protocol layers or from data response to a request from higher protocol layers or from data
generated internally to the MAC client, such as a Token frame. generated internally to the MAC client, such as a Token frame.
A.1.4. Effect on Receipt A.1.4. Effect on Receipt
Receipt of this primitive will cause the MAC entity to insert all MAC Receipt of this primitive will cause the MAC entity to insert all
specific fields, including Destination Address, Source Address, Frame MAC-specific fields, including Destination Address, Source Address,
Type, and any fields that are unique to the particular media access Frame Type, and any fields that are unique to the particular media
method, and pass the properly formed frame to the lower protocol access method, and pass the properly formed frame to the lower
layers for transfer to the peer MAC sublayer entity or entities. protocol layers for transfer to the peer MAC sublayer entity or
entities.
A.2. MA-DATA.indication A.2. MA-DATA.indication
A.2.1. Function A.2.1. Function
This primitive defines the transfer of data from the MAC sublayer This primitive defines the transfer of data from the MAC sublayer
entity to the MAC client entity or entities in the case of a entity to the MAC client entity or entities in the case of a
broadcast address. broadcast address.
A.2.2. Semantics of the Service Primitive A.2.2. Semantics of the Service Primitive
skipping to change at page 16, line 22 skipping to change at page 17, line 10
length of the data unit. length of the data unit.
The 'type' parameter is the value of the MS/TP Frame Type field of The 'type' parameter is the value of the MS/TP Frame Type field of
the incoming frame. the incoming frame.
A.2.3. When Generated A.2.3. When Generated
The MA_DATA.indication is passed from the MAC sublayer entity to the 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 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. the local MAC sublayer entity that is destined for the MAC client.
Such frames are reported only if they are validly formed, received Such frames are reported only if they are validly formed and received
without error, and their destination address designates the local MAC without error, and their Destination Address designates the local MAC
entity. Frames destined for the MAC Control sublayer are not passed entity. Frames destined for the MAC Control sublayer are not passed
to the MAC client. to the MAC client.
A.2.4. Effect on Receipt A.2.4. Effect on Receipt
The effect of receipt of this primitive by the MAC client is The effect of receipt of this primitive by the MAC client is
unspecified. unspecified.
Appendix B. Consistent Overhead Byte Stuffing [COBS] Appendix B. Consistent Overhead Byte Stuffing (COBS)
This Appendix is informative and not part of the standard. This Appendix is informative and not part of the standard.
[BACnet] Clause 9 corrects a long-standing issue with the MS/TP [BACnet], Clause 9 corrects a long-standing issue with the MS/TP
specification; namely that preamble sequences were not escaped specification, namely that preamble sequences were not escaped
whenever they appeared in the Data or Data CRC fields. In rare whenever they appeared in the Data or Data CRC fields. In rare
cases, this resulted in dropped frames due to loss of frame cases, this resulted in dropped frames due to loss-of-frame
synchronization. The solution is to encode the Data and 32-bit Data synchronization. The solution is to encode the Data and 32-bit Data
CRC fields before transmission using Consistent Overhead Byte CRC fields before transmission using Consistent Overhead Byte
Stuffing [COBS] and decode these fields upon reception. Stuffing [COBS] and decode these fields upon reception.
COBS is a run-length encoding method that nominally removes '0x00' COBS is a run-length encoding method that nominally removes '0x00'
octets from its input. Any selected octet value may be removed by octets from its input. Any selected octet value may be removed by
XOR'ing that value with each octet of the COBS output. [BACnet] XOR'ing that value with each octet of the COBS output. [BACnet],
Clause 9 specifies the preamble octet '0x55' for removal. Clause 9 specifies the preamble octet '0x55' for removal.
The minimum overhead of COBS is one octet per encoded field. The 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 worst-case overhead in long fields is bounded to one octet per 254 as
described in [COBS]. described in [COBS].
Frame encoding proceeds logically in two passes. The Encoded Data Frame encoding proceeds logically in two passes. The Encoded Data
field is prepared by passing the MSDU through the COBS encoder and 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 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 Encoded CRC-32K field is then prepared by calculating a CRC-32K over
the Encoded Data field and formatting it for transmission as the Encoded Data field and formatting it for transmission as
described in Appendix C. The combined length of these fields, minus described in Appendix C. The combined length of these fields, minus
two octets for compatibility with legacy MS/TP devices, is placed in two octets for compatibility with legacy MS/TP devices, is placed in
the MS/TP header Length field before transmission. the MS/TP header Length field before transmission.
Example COBS encoder and decoder functions are shown below for Example COBS encoder and decoder functions are shown below for
illustration. Complete examples of use and test vectors are provided illustration. Complete examples of use and test vectors are provided
in [BACnet] Annex T. in [BACnet], Annex T.
<CODE BEGINS> <CODE BEGINS>
#include <stddef.h> #include <stddef.h>
#include <stdint.h> #include <stdint.h>
/* /*
* Encodes 'length' octets of data located at 'from' and * Encodes 'length' octets of data located at 'from' and
* writes one or more COBS code blocks at 'to', removing any * writes one or more COBS code blocks at 'to', removing any
* 'mask' octets that may present be in the encoded data. * 'mask' octets that may be present in the encoded data.
* Returns the length of the encoded data. * Returns the length of the encoded data.
*/ */
size_t size_t
cobs_encode (uint8_t *to, const uint8_t *from, size_t length, cobs_encode (uint8_t *to, const uint8_t *from, size_t length,
uint8_t mask) uint8_t mask)
{ {
size_t code_index = 0; size_t code_index = 0;
size_t read_index = 0; size_t read_index = 0;
size_t write_index = 1; size_t write_index = 1;
skipping to change at page 18, line 36 skipping to change at page 18, line 51
* copied the maximum number (254) of non-zero octets, store * copied the maximum number (254) of non-zero octets, store
* the code octet and reset the encoder state variables. * the code octet and reset the encoder state variables.
*/ */
last_code = code; last_code = code;
to[code_index] = code ^ mask; to[code_index] = code ^ mask;
code_index = write_index++; code_index = write_index++;
code = 1; code = 1;
} }
/* /*
* If the last chunk contains exactly 254 non-zero octets, then * If the last chunk contains exactly 254 non-zero octets, then
* this exception is handled above (and returned length must be * this exception is handled above (and the returned length must
* adjusted). Otherwise, encode the last chunk normally, as if * be adjusted). Otherwise, encode the last chunk normally, as if
* a "phantom zero" is appended to the data. * a "phantom zero" is appended to the data.
*/ */
if ((last_code == 255) && (code == 1)) if ((last_code == 255) && (code == 1))
write_index--; write_index--;
else else
to[code_index] = code ^ mask; to[code_index] = code ^ mask;
return write_index; return write_index;
} }
#include <stddef.h> #include <stddef.h>
#include <stdint.h> #include <stdint.h>
/* /*
* Decodes 'length' octets of data located at 'from' and * Decodes 'length' octets of data located at 'from' and
* writes the original client data at 'to', restoring any * writes the original client data at 'to', restoring any
* 'mask' octets that may present in the encoded data. * 'mask' octets that may present in the encoded data.
* Returns the length of the encoded data or zero if error. * Returns the length of the encoded data or zero if error.
*/ */
size_t size_t
skipping to change at page 19, line 41 skipping to change at page 20, line 4
read_index++; read_index++;
while (--code > 0) while (--code > 0)
to[write_index++] = from[read_index++] ^ mask; to[write_index++] = from[read_index++] ^ mask;
/* /*
* Restore the implicit zero at the end of each decoded block * Restore the implicit zero at the end of each decoded block
* except when it contains exactly 254 non-zero octets or the * except when it contains exactly 254 non-zero octets or the
* end of data has been reached. * end of data has been reached.
*/ */
if ((last_code != 255) && (read_index < length)) if ((last_code != 255) && (read_index < length))
to[write_index++] = 0; to[write_index++] = 0;
} }
return write_index; return write_index;
} }
<CODE ENDS> <CODE ENDS>
Appendix C. Encoded CRC-32K [CRC32K] Appendix C. Encoded CRC-32K (CRC32K)
This Appendix is informative and not part of the standard. This Appendix is informative and not part of the standard.
Extending the payload of MS/TP to 1500 octets required upgrading the 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 Data CRC from 16 bits to 32 bits. P. Koopman has authored several
papers on evaluating CRC polynomials for network applications. In papers on evaluating CRC polynomials for network applications. In
[CRC32K], he surveyed the entire 32-bit polynomial space and noted [CRC32K], he surveyed the entire 32-bit polynomial space and noted
some that exceed the [IEEE.802.3_2012] polynomial in performance. some that exceed the [IEEE.802.3] polynomial in performance.
[BACnet] Clause 9 specifies one of these, the CRC-32K (Koopman) [BACnet], Clause 9 specifies one of these, the CRC-32K (Koopman)
polynomial. polynomial.
The specified use of the calc_crc32K() function is as follows. The specified use of the calc_crc32K() function is as follows.
Before a frame is transmitted, 'crc_value' is initialized to all Before a frame is transmitted, 'crc_value' is initialized to all
ones. After passing each octet of the [COBS] Encoded Data through ones. After passing each octet of the [COBS] Encoded Data field
the function, the ones complement of the resulting 'crc_value' is through the function, the ones complement of the resulting
arranged in LSB-first order and is itself [COBS] encoded. The length 'crc_value' is arranged in LSB-first order and is itself [COBS]
of the resulting Encoded CRC-32K field is always five octets. 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. Upon reception of a frame, 'crc_value' is initialized to all ones.
The octets of the Encoded Data field are accumulated by the The octets of the Encoded Data field are accumulated by the
calc_crc32K() function before decoding. The Encoded CRC-32K field is calc_crc32K() function before decoding. The Encoded CRC-32K field is
then decoded and the resulting four octets are accumulated by the then decoded and the resulting four octets are accumulated by the
calc_crc32K() function. If the result is the expected residue value calc_crc32K() function. If the result is the expected residue value
'CRC32K_RESIDUE', then the frame was received correctly. 'CRC32K_RESIDUE', then the frame was received correctly.
An example CRC-32K function in shown below for illustration. An example CRC-32K function is shown below for illustration.
Complete examples of use and test vectors are provided in [BACnet] Complete examples of use and test vectors are provided in [BACnet],
Annex G.3. Annex G.3.
<CODE BEGINS> <CODE BEGINS>
#include <stdint.h> #include <stdint.h>
/* See BACnet Addendum 135-2012an, section G.3.2 */ /* See ANSI/ASHRAE Standard 135-2016 [BACnet], Section G.3.2 */
#define CRC32K_INITIAL_VALUE (0xFFFFFFFF) #define CRC32K_INITIAL_VALUE (0xFFFFFFFF)
#define CRC32K_RESIDUE (0x0843323B) #define CRC32K_RESIDUE (0x0843323B)
/* CRC-32K polynomial, 1 + x**1 + ... + x**30 (+ x**32) */ /* CRC-32K polynomial, 1 + x**1 + ... + x**30 (+ x**32) */
#define CRC32K_POLY (0xEB31D82E) #define CRC32K_POLY (0xEB31D82E)
/* /*
* Accumulate 'data_value' into the CRC in 'crc_value'. * Accumulate 'data_value' into the CRC in 'crc_value'.
* Return updated CRC. * Return updated CRC.
* *
skipping to change at page 27, line 5 skipping to change at page 27, line 5
70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f pqrstuvwxyz{|}~. 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 ................ 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 ................ 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 ................ 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 ................ 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 ................ 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 ................ 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 ................ 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 .............. 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.
Alexandru Petrescu, Brian Frank, Geoff Mulligan, and Don Sturek
offered valuable comments.
Authors' Addresses Authors' Addresses
Kerry Lynn (editor) Kerry Lynn (editor)
Verizon Labs Verizon Labs
50 Sylvan Rd 50 Sylvan Rd
Waltham , MA 02451 Waltham, MA 02451
USA United States of America
Phone: +1 781 296 9722 Phone: +1 781 296 9722
Email: kerlyn@ieee.org Email: kerlyn@ieee.org
Jerry Martocci Jerry Martocci
Johnson Controls, Inc. Johnson Controls, Inc.
507 E. Michigan St 507 E. Michigan St
Milwaukee , WI 53202 Milwaukee, WI 53202
USA United States of America
Email: jpmartocci@sbcglobal.net Email: jpmartocci@sbcglobal.net
Carl Neilson Carl Neilson
Delta Controls, Inc. Delta Controls, Inc.
17850 56th Ave 17850 56th Ave
Surrey , BC V3S 1C7 Surrey, BC V3S 1C7
Canada Canada
Phone: +1 604 575 5913 Phone: +1 604 575 5913
Email: cneilson@deltacontrols.com Email: cneilson@deltacontrols.com
Stuart Donaldson Stuart Donaldson
Honeywell Automation & Control Solutions Honeywell Automation & Control Solutions
6670 185th Ave NE 6670 185th Ave NE
Redmond , WA 98052 Redmond, WA 98052
USA United States of America
Email: stuart.donaldson@honeywell.com Email: stuart.donaldson@honeywell.com
 End of changes. 92 change blocks. 
196 lines changed or deleted 190 lines changed or added

This html diff was produced by rfcdiff 1.45. The latest version is available from http://tools.ietf.org/tools/rfcdiff/