wdiff draft-ietf-6lo-6lobac-01.txt draft-ietf-6lo-6lobac-02.txt

6Lo Working Group K. Lynn, Ed.
Internet-Draft Verizon Labs
Intended status: Standards Track J. Martocci
Expires: September 10, 2015 January 7, 2016 Johnson Controls
C. Neilson
Delta Controls
S. Donaldson
Honeywell
March 9,
July 6, 2015

Transmission of IPv6 over MS/TP Networks
draft-ietf-6lo-6lobac-01
draft-ietf-6lo-6lobac-02

Abstract

Master-Slave/Token-Passing (MS/TP) is a contention-free medium access control method
for the RS-485 physical layer, which is used extensively in building
automation networks. This specification defines the frame format for
transmission of IPv6 packets and the method of forming link-local and
statelessly autoconfigured IPv6 addresses on MS/TP networks.

Status of This Memo

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

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. MS/TP Mode for IPv6 . . . . . . . . . . . . . . . . . . . . . 5
3. Addressing Modes . . . . . . . . . . . . . . . . . . . . . . 6
4. Maximum Transmission Unit (MTU) . . . . . . . . . . . . . . . 6
5. LoBAC Adaptation Layer . . . . . . . . . . . . . . . . . . . 6
6. Stateless Address Autoconfiguration . . . . . . . . . . . . . 9 7
7. IPv6 Link Local Address . . . . . . . . . . . . . . . . . . . 10 8
8. Unicast Address Mapping . . . . . . . . . . . . . . . . . . . 10 8
9. Multicast Address Mapping . . . . . . . . . . . . . . . . . . 11 9
10. Header Compression . . . . . . . . . . . . . . . . . . . . . 11 9
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 10
12. Security Considerations . . . . . . . . . . . . . . . . . . . 12 10
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 10
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 10
Appendix A. Abstract MAC Interface . . . . . . . . . . . . . . . 14 13
Appendix B. Consistent Overhead Byte Stuffing [COBS] . . . . . . 16 15
Appendix C. Encoded CRC-32K [CRC32K] . . . . . . . . . . . . . . 20 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 21

1. Introduction

Master-Slave/Token-Passing (MS/TP) is a contention-free medium access method control (MAC)
protocol for the RS-485 [TIA-485-A] physical layer, which is used
extensively in building automation networks. This specification
defines the frame format for transmission of IPv6 [RFC2460] packets
and the method of forming link-local and statelessly autoconfigured
IPv6 addresses on MS/TP networks. The general approach is to adapt
elements of the 6LoWPAN [RFC4944] specification specifications [RFC4944], [RFC6282], and
[RFC6775] to constrained wired networks.

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

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

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

1.2. Abbreviations Used

ASHRAE: American Society of Heating, Refrigerating, and Air-
Conditioning Engineers (http://www.ashrae.org)

BACnet: An ISO/ANSI/ASHRAE Standard Data Communication Protocol
for Building Automation and Control Networks

CRC: Cyclic Redundancy Check

MAC: Medium Access Control

MSDU: MAC Service Data Unit (MAC client data)

MTU: Maximum Transmission Unit

UART: Universal Asynchronous Transmitter/Receiver

1.3. MS/TP Overview

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

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

The differential signaling used by [TIA-485-A] requires a contention-
free MAC. MS/TP uses a token to control access to a multidrop bus.

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

MS/TP COBS-encoded* frames have the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x55 | 0xFF | Frame Type* | DA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SA | Length (MS octet first) | Header CRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Encoded Data* (2 - 1507 1506 octets) .
. .
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Encoded CRC-32K* (5 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | optional 0xFF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: MS/TP COBS-Encoded Frame Format

*Note:

*NOTE: BACnet Addendum 135-2012an [Addendum_an] defines a range of
Frame Type values to designate frames that contain data and data CRC
fields encoded using Consistent Overhead Byte Stuffing [COBS] (see
Appendix B). The purpose of COBS encoding is to eliminate preamble
sequences from the Encoded Data and Encoded CRC-32K fields. The
maximum length of an MSDU as defined by this specification is 1501 1500
octets (before encoding). The Encoded Data is covered by a 32-bit
CRC [CRC32K] (see Appendix C), which is then itself then COBS encoded.

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

Preamble two octet preamble: 0x55, 0xFF
Frame Type one octet
Destination Address one octet address
Source Address one octet address
Length two octets, most significant octet first
Header CRC one octet
Encoded Data 2 - 1512 1506 octets (see 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
...
34 IPv6 over MS/TP (LoBAC) Encapsulation

Frame Types 8 through - 31 and 35 - 127 are reserved for assignment by
ASHRAE. Frame Types 32 through - 127 designate COBS-encoded frames and MUST
convey Encoded Data and Encoded CRC-32K fields. All master nodes
MUST understand Token, Poll For Master, and Reply to Poll For Master
control frames. See Section 2 for additional details.

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

For COBS-encoded frames, the Length field specifies the combined
length of the [COBS] Encoded Data and Encoded CRC-32K fields in
octets, minus two. (This adjustment is required for backward
compatibility with legacy MS/TP devices.) See Section 4 and
Appendices for additional details.

The Header CRC field covers the Frame Type, Destination Address,
Source Address, and Length fields. The Header CRC generation and
check procedures are specified in BACnet [Clause9].

1.4. Goals and Non-goals Constraints

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

Non-goals include making

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, or addressing modes.
Also, while the techniques described here may be applicable to other
data links, no attempt is made to define a general design pattern. machine as specified in BACnet [Clause9].

2. MS/TP Mode for IPv6

ASHRAE must assign a new has assigned an MS/TP Frame Type value of 34 to indicate IPv6
over MS/TP
Encapsulation from (LoBAC) Encapsulation. This falls within the range reserved for designating of
values that designate COBS-encoded data frames. The Frame Type requested for IPv6 over MS/TP Encapsulation
is 34 (0x22).

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

3. Addressing Modes

MS/TP link-layer (node) node (MAC) addresses are one octet in length. The method of
assigning a node address MAC addresses is outside the scope of this
document. specification.
However, each MS/TP node on the link MUST have a unique address or a mis-configuration condition exists. in
order to ensure correct MAC operation.

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

A Destination Address of 255 (0xFF) denotes a link-level broadcast (all nodes). A Source Address of 255 MUST NOT be used. nodes) indicates a MAC-layer
broadcast. MS/TP does not support multicast, therefore all IPv6
multicast packets MUST SHOULD be
sent as link-level broadcasts broadcast at the MAC layer and filtered
at the IPv6 layer. A Source Address of 255 MUST NOT be used.

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

4. Maximum Transmission Unit (MTU)

BACnet [Addendum_an] supports MSDUs up to 2032 octets in length.
This specification defines an MSDU length of at least 1281 1280 octets and
at most 1501 octets. 1500 octets (before encoding). This is sufficient to convey
the minimum MTU required by IPv6 [RFC2460] without the need for link-layer link-
layer fragmentation and reassembly.

5. LoBAC Adaptation Layer

The relatively low data rates of MS/TP, however, still make a
compelling case for MS/TP indicate header compression. An compression as
a means to reduce latency. This section specifies an adaptation
layer to
indicate support compressed or uncompressed IPv6 headers is specified in
Section 5 and the compression scheme format
is specified in Section 10.

5. LoBAC Adaptation Layer

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

The encapsulation formats format defined in this section (subsequently
referred to as the "LoBAC" encapsulation) comprise comprises the MSDU (payload) 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], [RFC4944] and extended by [RFC6282],
therefore the use of "LOWPAN" in literals below is intentional. The
primary differences difference between LoBAC
and LoWPAN are: a) and LoBAC is omission of the Fragmentation, Mesh,
Broadcast, Fragmentation, and Broadcast
headers, and b) use of LOWPAN_IPHC [RFC6282] in place of LOWPAN_HC1
header compression (which is deprecated by [RFC6282]). headers.

All LoBAC encapsulated datagrams transmitted over MS/TP are prefixed
by an encapsulation header stack. Each header in the stack consists consisting of a header type Dispatch value
followed by zero or more header fields. Whereas in
an IPv6 header the stack would contain, in the following order,
addressing, hop-by-hop options, routing, fragmentation, destination
options, and finally payload [RFC2460]; in a LoBAC encapsulation the
analogous The only sequence currently
defined for LoBAC is (optional) header compression and payload. The the LOWPAN_IPHC header stacks that are valid in a LoBAC network are followed by payload, as
shown below.

A LoBAC encapsulated IPv6 datagram:

+---------------+-------------+---------+
| IPv6 Dispatch | IPv6 Header | Payload |
+---------------+-------------+---------+

A LoBAC encapsulated LOWPAN_IPHC compressed IPv6 datagram:

+---------------+-------------+---------+ below:

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

All protocol datagrams (i.e., IPv6 or compressed IPv6 headers) SHALL
be preceded by one of the valid LoBAC encapsulation headers. This
permits uniform software treatment of datagrams without regard to
their mode of transmission.

The definition of LoBAC headers consists of the dispatch value, the
definition of the header fields that follow, and their ordering
constraints relative to all other headers. Although the header stack
structure provides a mechanism to address future demands on the LoBAC
(LoWPAN) adaptation layer, it is not intended to provided general
purpose extensibility. This format document specifies a small set of
header types using the header stack for clarity, compactness, and
orthogonality.

5.1. Dispatch Value and Header

The LoBAC Dispatch value begins with a "0" bit followed by a "1" bit.
The Dispatch value and header are shown here:

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| Dispatch | Type-specific header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Dispatch 6-bit selector. Identifies the type of header
immediately following the Dispatch value.

Type-specific header A header determined by the Dispatch value.
+---------------+---------------+------...-----+

Figure 2: Dispatch Value and Header A LoBAC Encapsulated LOWPAN_IPHC Compressed IPv6 Datagram

The Dispatch value may be treated as an unstructured namespace. Only
a few symbols are required single pattern is used to represent current LoBAC functionality.
Although some additional savings could be achieved by encoding
additional functionality into the dispatch value, these measures
would tend to constrain the ability to address future alternatives.

Figure 3: LoBAC Dispatch Value Bit Patterns

NALP: Specifies that the following bits are not a part of the LoBAC
encapsulation, and any LoBAC node that encounters a Pattern

Other IANA-assigned 6LoWPAN Dispatch
value of 00xxxxxx shall discard the packet. Non-LoBAC protocols
that wish values do not apply to coexist with LoBAC nodes should include an octet
matching this pattern immediately following the MS/TP header.

ESC: Specifies that the following header is a single 8-bit field for
the Dispatch value. It allows support for Dispatch values larger
than 127 (see Section 5 of [RFC6282]).

IPv6: Specifies that the following header is an uncompressed IPv6
header [RFC2460].

LOWPAN_IPHC: A value of 011xxxxx specifies a LOWPAN_IPHC compression
header (see Section 10.)

Reserved: A LoBAC node that encounters a Dispatch value in the range
01000010 through 01011111 or 1xxxxxxx SHALL discard the packet.
specification.

6. Stateless Address Autoconfiguration

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

The Interface Identifier MAY be based on IID SHOULD NOT embed an [EUI-64] or any other globally unique
hardware identifier assigned to the device but this is not typical for MS/TP. In this
case, the EUI-64 to IID transformation defined in the IPv6 addressing
architecture [RFC4291] MUST be used. This will result in a globally
unique Interface Identifier.

If the device does not have an EUI-64, then the (see Section 12).

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

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

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

Optionally, an opaque

A 64-bit privacy IID (for non-link-local addresses) MAY is RECOMMENDED for routable addresses and SHOULD
be formed by the technique discussed in [I-D.ietf-6man-default-iids]
or alternate method. locally generated according to [I-D.ietf-6man-default-iids]. A
node that generates a non-link-local address 64-bit privacy IID MUST register it with its
local router(s) by sending a Neighbor Solicitation (NS) message with
the Address Registration Option (ARO) and process Neighbor
Advertisements (NA) according to [RFC6775].

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

7. IPv6 Link Local Address

The IPv6 link-local address [RFC4291] for an MS/TP interface is
formed by appending the Interface Identifier, as defined above, to
the prefix FE80::/64.

10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Identifier |
+----------+-----------------------+----------------------------+

8. Unicast Address Mapping

The address resolution procedure for mapping IPv6 non-multicast
addresses into MS/TP link-layer 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 link-layer 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Padding (all zeros) +
| |
+ +-+-+-+-+-+-+-+-+
| | MS/TP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Option fields:

Type:

1: for Source Link-layer address.

2: for Target Link-layer address.

Length: This is the length of this option (including the type and
length fields) in units of 8 octets. The value of this field is 1
for 8-bit MS/TP node MAC addresses.

MS/TP Address: The 8-bit address in canonical bit order [RFC2469].
This is the unicast address the interface currently responds to.

9. Multicast Address Mapping

All IPv6 multicast packets MUST SHOULD be sent to MS/TP Destination
Address 255 (broadcast) and filtered at the IPv6 layer. When
represented as a 16-bit address in a compressed header (see
Section 10), it MUST be formed by padding on the left with a zero:

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

10. Header Compression

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

In general the following substitutions should be made:

- Replace instances of "6LoWPAN" with "MS/TP network"

- Replace instances of "IEEE 802.15.4 address" with "MS/TP address"

When a 16-bit address is called for (i.e., an IEEE 802.15.4 "short
address") it MUST be formed by padding the MS/TP address to the left
with a zero:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | MS/TP address |
+-+-+-+-+-+-+-+-+---------------+

If LOWPAN_IPHC compression [RFC6282] is used with context, the border
router(s) directly attached to the MS/TP segment MUST disseminate the
6LoWPAN Context Option (6CO) as specified in [RFC6775]. according to [RFC6775], Section 7.2.

11. IANA Considerations

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

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

12. Security Considerations

The method security and privacy implications of deriving Interface Identifiers from MAC addresses is
intended embedding a link-layer
address in an IPv6 IID are discussed in
[I-D.ietf-6man-ipv6-address-generation-privacy]. The issue most
relevant to preserve global uniqueness when possible. However, there MS/TP networks is no protection from duplication through accident or forgery. address scanning. This is mainly an
issue for routable addresses and probably only for those hosted on
the global Internet. This specification RECOMMENDS mitigating this
threat according to [I-D.ietf-6man-default-iids].

13. Acknowledgments

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

14. References

14.1. Normative References

[Addendum_an]
ASHRAE, "ANSI/ASHRAE Addenda an, at, au, av, aw, ax, and
az to ANSI/ASHRAE Standard 135-2012, BACnet - A Data
Communication Protocol for Building Automation and Control
Networks", July 2014,
<https://www.ashrae.org/File%20Library/docLib/
StdsAddenda/135_2012_an_at_au_av_aw_ax_az_Final.pdf>.
<https://www.ashrae.org/File%20Library/docLib/StdsAddenda/
07-31-2014_135_2012_an_at_au_av_aw_ax_az_Final.pdf>.

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

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

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

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

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

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

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

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

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

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

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

14.2. Informative References

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

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

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

[I-D.ietf-6man-default-iids]
Gont, F., Cooper, A., Thaler, D., and W. Will, S. LIU,
"Recommendation on Stable IPv6 Interface Identifiers",
draft-ietf-6man-default-iids-02
draft-ietf-6man-default-iids-04 (work in progress),
January June
2015.

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

[IEEE.802.3]
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific requirements - Part
3: Carrier Sense Multiple Access with Collision Detection
(CMSA/CD) Access Method and Physical Layer
Specifications", IEEE Std 802.3-2008, 802.3-2012, December 2008, 2012,
<http://standards.ieee.org/getieee802/802.3.html>.

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

[TIA-485-A]
Telecommunications Industry Association, "TIA-485-A,
Electrical Characteristics of Generators and Receivers for
Use in Balanced Digital Multipoint Systems (ANSI/TIA/EIA-
485-A-98) (R2003)", March 2003.

Appendix A. Abstract MAC Interface

This Appendix is informative and not part of the standard.

BACnet [Clause9] defines support for MAC-layer clients through its
SendFrame and ReceivedDataNoReply procedures. However, it does not
define a protocol network-protocol independent abstract interface for the data link. MAC.
This is provided below as an aid to implementation.

A.1. MA-DATA.request

A.1.1. Function

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

A.1.2. Semantics of the Service Primitive

The semantics of the primitive are as follows:

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

The 'destination_address' parameter may specify either an individual
or a broadcast MAC entity address. It must contain sufficient
information to create the Destination Address field (see Section 1.3)
that is prepended to the frame by the local MAC sublayer entity. The
'source_address' parameter, if present, must specify an individual
MAC address. If the source_address parameter is omitted, the local
MAC sublayer entity will insert a value associated with that entity.

The 'data' parameter specifies the MAC service data unit (MSDU) to be
transferred by the MAC sublayer entity. There is sufficient
information associated with the MSDU for the MAC sublayer entity to
determine the length of the data unit.

The 'priority' parameter specifies the priority desired for the data
unit transfer. The priority parameter is ignored by MS/TP.

The 'type' parameter specifies the value of the MS/TP Frame Type
field that is prepended to the frame by the local MAC sublayer
entity.

A.1.3. When Generated

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

A.1.4. Effect on Receipt

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

A.2. MA-DATA.indication

A.2.1. Function

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

A.2.2. Semantics of the Service Primitive

The semantics of the primitive are as follows:

MA-DATA.indication (
destination_address,
source_address,
data,
priority,
type
)

The 'destination_address' parameter may be either an individual or a
broadcast address as specified by the Destination Address field of
the incoming frame. The 'source_address' parameter is an individual
address as specified by the Source Address field of the incoming
frame.

The 'data' parameter specifies the MAC service data unit (MSDU) as
received by the local MAC entity. There is sufficient information
associated with the MSDU for the MAC sublayer client to determine the
length of the data unit.

The 'priority' parameter specifies the priority desired for the data
unit transfer. The priority parameter is ignored by MS/TP.

The 'type' parameter is the value of the MS/TP Frame Type field of
the incoming frame.

A.2.3. When Generated

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

A.2.4. Effect on Receipt

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

Appendix B. Consistent Overhead Byte Stuffing [COBS]

This Appendix is informative and not part of the standard.

BACnet [Addendum_an] corrects a long-standing issue with the MS/TP
specification; namely that preamble sequences were not escaped
whenever they appeared in the Data or Data CRC fields. In rare
cases, this resulted in dropped frames due to loss of frame
synchronization. The solution is to encode the Data and 32-bit Data
CRC fields before transmission using Consistent Overhead Byte
Stuffing [COBS] and decode these fields upon reception.

COBS is a run-length encoding method that nominally removes '0x00'
octets from its input. Any selected octet value may be removed by
XOR'ing that value with each octet of the COBS output. BACnet
[Addendum_an] specifies the preamble octet '0x55' for removal.

The minimum overhead of COBS is one ectet per encoded field. The
worst-case overhead in long fields is bounded to one octet in 254, or
less than
0.5%, 0.4%, as described in [COBS].

Frame encoding proceeds logically in two passes. The Encoded Data
field is prepared by passing the MSDU through the COBS encoder and
XOR'ing the preamble octet '0x055' '0x55' with each octet of the output. The
Encoded CRC-32K field is then prepared by calculating a CRC-32K over
the Encoded Data field and formatting it for transmission as
described in Appendix C. The combined length of these fields, minus
two octets for compatibility with existing MS/TP devices, is placed
in the MS/TP header Length field before transmission.

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

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

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

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

size_t
cobs_encode (uint8_t *to, const uint8_t *from, size_t length,
uint8_t mask)
{
size_t code_index = 0;
size_t read_index = 0;
size_t write_index = 1;
uint8_t code = 1;
uint8_t data, last_code;

while (read_index < length) {
data = from[read_index++];
/*
* In the case of encountering a non-zero octet in the data,
* simply copy input to output and increment the code octet.
*/
if (data != 0) {
to[write_index++] = data ^ mask;
code++;
if (code != 255)
continue;
}
/*
* In the case of encountering a zero in the data or having
* copied the maximum number (254) of non-zero octets, store
* the code octet and reset the encoder state variables.
*/
last_code = code;
to[code_index] = code ^ mask;
code_index = write_index++;
code = 1;

}
/*
* If the last chunk contains exactly 254 non-zero octets, then
* this exception is handled above (and returned length must be
* adjusted). Otherwise, encode the last chunk normally, as if
* a "phantom zero" is appended to the data.
*/
if ((last_code == 255) && (code == 1))
write_index--;
else
to[code_index] = code ^ mask;

return write_index;
}
#include <stddef.h>
#include <stdint.h>

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

/*
* Decodes 'length' octets of data located at 'from' and
* writes the original client data at 'to', restoring any
* 'mask' octets that may present in the encoded data.
* Returns the length of the encoded data or zero if error.
*/
size_t
cobs_decode (uint8_t *to, const uint8_t *from, size_t length,
uint8_t mask)
{
size_t read_index = 0;
size_t write_index = 0;
uint8_t code, last_code;

while (read_index < length) {
code = from[read_index] ^ mask;
last_code = code;
/*
* Sanity check the encoding to prevent the while() loop below
* from overrunning the output buffer.
*/
if (read_index + code > length)
return 0;

read_index++;
while (--code > 0)
to[write_index++] = from[read_index++] ^ mask;
/*
* Restore the implicit zero at the end of each decoded block
* except when it contains exactly 254 non-zero octets or the
* end of data has been reached.
*/
if ((last_code != 255) && (read_index < length))
to[write_index++] = 0;
}
return write_index;
}

Appendix C. Encoded CRC-32K [CRC32K]

This Appendix is informative and not part of the standard.

Extending the payload of MS/TP to 1501 1500 octets required upgrading the
Data CRC from 16 bits to 32 bits. P.Koopman has authored several
papers on evaluating CRC polynomials for network applications. In
[CRC32K], he surveyed the entire 32-bit polynomial space and noted
some that exceed the [IEEE.802.3] polynomial in performance. BACnet
[Addendum_an] specifies the CRC-32K (Koopman) polynomial.

The specified use of the calc_crc32K() function is as follows.
Before a frame is transmitted, 'crc_value' is initialized to all
ones. After passing each octet of the [COBS] Encoded Data through
the function, the ones complement of the resulting 'crc_value' is
arranged in LSB-first order and is itself [COBS] encoded. The length
of the resulting Encoded CRC-32K field is always five octets.

Upon reception of a frame, 'crc_value' is initialized to all ones.
The octets of the Encoded Data field are accumulated by the
calc_crc32K() function before decoding. The Encoded CRC-32K field is
then decoded and the resulting four octets are accumulated by the
calc_crc32K() function. If the result is the expected residue value
'CRC32K_RESIDUE', then the frame was received correctly.

An example CRC-32K function in shown below for illustration.
Complete examples of use and test vectors are provided in BACnet
[Addendum_an].

#include <stdint.h>

/* See BACnet Addendum 135-2012an, section G.3.2 */
#define CRC32K_INITIAL_VALUE (0xFFFFFFFF)
#define CRC32K_RESIDUE (0x0843323B)

/* CRC-32K polynomial, 1 + x**1 + ... + x**30 (+ x**32) */
#define CRC32K_POLY (0xEB31D82E)

/*
* Accumulate 'data_value' into the CRC in 'crc_value'.
* Return updated CRC.
*
* Note: crcValue must be set to CRC32K_INITIAL_VALUE
* before initial call.
*/
uint32_t
calc_crc32K (uint8_t data_value, uint32_t crc_value)
{
int b;

for (b = 0; b < sizeof(uint8_t); 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;
}

Authors' Addresses

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

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

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

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

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

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

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

Email: stuart.donaldson@honeywell.com