One document matched: draft-ietf-6lo-6lobac-01.xml
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<rfc category="std" ipr="trust200902" docName="draft-ietf-6lo-6lobac-01">
<front>
<title abbrev=" IPv6 over MS/TP "> Transmission of IPv6 over MS/TP Networks </title>
<author fullname="Kerry Lynn" initials="K.E." surname="Lynn" role="editor">
<organization> Verizon </organization>
<address>
<postal>
<street> 50 Sylvan Rd </street>
<city> Waltham </city>
<region> MA </region>
<code> 02451 </code>
<country> USA </country>
</postal>
<phone> +1 781 296 9722 </phone>
<email> kerlyn@ieee.org </email>
</address>
</author>
<author fullname="Jerry Martocci" initials="J.P." surname="Martocci">
<organization abbrev="Johnson Controls"> Johnson Controls, Inc. </organization>
<address>
<postal>
<street> 507 E. Michigan St </street>
<city> Milwaukee </city>
<region> WI </region>
<code> 53202 </code>
<country> USA </country>
</postal>
<phone> +1 414 524 4010 </phone>
<email> jerald.p.martocci@jci.com </email>
</address>
</author>
<author fullname="Carl Neilson" initials="C." surname="Neilson">
<organization abbrev="Delta Controls"> Delta Controls, Inc. </organization>
<address>
<postal>
<street> 17850 56th Ave </street>
<city> Surrey </city>
<region> BC </region>
<code> V3S 1C7 </code>
<country> Canada </country>
</postal>
<phone> +1 604 575 5913 </phone>
<email> cneilson@deltacontrols.com </email>
</address>
</author>
<author fullname="Stuart Donaldson" initials="S." surname="Donaldson">
<organization abbrev="Honeywell"> Honeywell Automation & Control Solutions </organization>
<address>
<postal>
<street> 6670 185th Ave NE </street>
<city> Redmond </city>
<region> WA </region>
<code> 98052 </code>
<country> USA </country>
</postal>
<email> stuart.donaldson@honeywell.com </email>
</address>
</author>
<date day="9" month="March" year="2015"/>
<area> Internet </area>
<workgroup> 6Lo Working Group </workgroup>
<abstract>
<t>
Master-Slave/Token-Passing (MS/TP) is a contention-free access 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.
</t>
</abstract>
</front>
<middle>
<!-- section anchor="sec-1" title="Introduction" -->
<section title="Introduction">
<t>
Master-Slave/Token-Passing (MS/TP) is a contention-free access method
for the RS-485 <xref target="TIA-485-A"/> physical layer, which is used extensively
in building automation networks. This specification defines the
frame format for transmission of IPv6 <xref target="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 <xref target="RFC4944"/> specification to constrained wired
networks.
</t><t>
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 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.
</t><t>
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 <xref target="RFC6282"/>, that is RECOMMENDED in
order to make IPv6 practical on low speed MS/TP networks.
</t>
<!-- section anchor="sec-1.1" title="Requirements Language" -->
<section title="Requirements Language">
<t>
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 <xref target="RFC2119"/>.
</t>
</section>
<!-- section anchor="sec-1.2" title="Abbreviations Used" -->
<section title="Abbreviations Used">
<figure>
<artwork>
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
</artwork>
</figure>
</section>
<section anchor="sec-1.3" title="MS/TP Overview" >
<!-- section title="MS/TP Overview" -->
<t>
This section provides a brief overview of MS/TP, which is specified
in ANSI/ASHRAE 135-2012 (BACnet) Clause 9 <xref target="Clause9"/> and included
herein by reference. BACnet <xref target="Clause9"/> also covers physical layer
deployment options.
</t><t>
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
<xref target="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.
</t><t>
The differential signaling used by <xref target="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 address). Slave nodes transmit only when
polled and SHALL NOT be considered part of this specification.
</t><t>
MS/TP COBS-encoded* frames have the following format:
</t>
<figure title="Figure 1: MS/TP COBS-Encoded Frame Format" align="center">
<artwork>
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 octets) .
. .
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Encoded CRC-32K* (5 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | optional 0xFF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<figure>
<artwork>
*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
octets (before encoding). The Encoded Data is covered by a 32-bit
CRC [CRC32K] (see Appendix C), which is then itself COBS encoded.
</artwork>
</figure>
<t>
MS/TP COBS-encoded frame fields have the following descriptions:
</t>
<figure>
<artwork>
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 octets (see Appendix B)
Encoded CRC-32K five octets (see Appendix C)
(pad) (optional) at most one octet of trailer: 0xFF
</artwork>
</figure>
<figure>
<artwork>
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
</artwork>
</figure>
<t>
Frame Types 8 through 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 <xref target="sec-2"/> for additional details.
</t><t>
The Destination and Source Addresses are each one octet in length.
See <xref target="sec-3"/> for additional details.
</t><t>
For COBS-encoded frames, the Length field specifies the combined
length of the <xref target="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 <xref target="sec-4"/> and
Appendices for additional details.
</t><t>
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 <xref target="Clause9"/>.
</t>
</section>
<!-- section anchor="sec-1.4" title="Goals and Non-goals" -->
<section title="Goals and Non-goals">
<t>
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
specified in BACnet <xref target="Addendum_an"/> (see Note in <xref target="sec-1.3"/>).
</t><t>
Non-goals include making changes to the MS/TP frame header format,
control frames, 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.
</t>
</section>
</section>
<section anchor="sec-2" title="MS/TP Mode for IPv6">
<!-- section title="MS/TP Mode for IP" -->
<t>
ASHRAE must assign a new MS/TP Frame Type to indicate IPv6 over MS/TP
Encapsulation from the range reserved for designating COBS-encoded
frames. The Frame Type requested for IPv6 over MS/TP Encapsulation
is 34 (0x22).
</t><t>
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 <xref target="Clause9"/>. MS/TP master nodes that support IPv6 must also
support the Receive Frame state machine as specified in <xref target="Clause9"/> and
extended by BACnet <xref target="Addendum_an"/>.
</t>
</section>
<section anchor="sec-3" title="Addressing Modes">
<!-- section title="Addressing Modes" -->
<t>
MS/TP link-layer (node) addresses are one octet in length. The
method of assigning a node address is outside the scope of this
document. However, each MS/TP node on the link MUST have a unique
address or a mis-configuration condition exists.
</t><t>
BACnet <xref target="Clause9"/> specifies that addresses 0 through 127 are valid for master
nodes. The method specified in <xref target="sec-6"/> for creating the Interface
Identifier (IID) ensures that an IID of all zeros can never result.
</t><t>
A Destination Address of 255 (0xFF) denotes a link-level broadcast
(all nodes). A Source Address of 255 MUST NOT be used. MS/TP
does not support multicast, therefore all IPv6 multicast packets
MUST be sent as link-level broadcasts and filtered at the IPv6 layer.
</t><t>
This specification assumes that a unique IPv6 subnet prefix is
assigned to each MS/TP segment. Hosts learn IPv6 prefixes via router
advertisements according to <xref target="RFC4861"/>.
</t>
</section>
<section anchor="sec-4" title="Maximum Transmission Unit (MTU)">
<!--section title="Maximum Transmission Unit (MTU)"-->
<t>
BACnet <xref target="Addendum_an"/> supports MSDUs up to 2032 octets in length.
This specification defines an MSDU length of at least 1281 octets and
at most 1501 octets. This is sufficient to convey the minimum MTU
required by IPv6 <xref target="RFC2460"/> without the need for link-layer
fragmentation and reassembly.
</t><t>
The relatively low data rates of MS/TP, however, still make a
compelling case for header compression. An adaptation layer to
indicate compressed or uncompressed IPv6 headers is specified in
<xref target="sec-5"/> and the compression scheme is specified in <xref target="sec-10"/>.
</t>
</section>
<section anchor="sec-5" title="LoBAC Adaptation Layer">
<!-- section title="LoBAC Adaptation Layer" -->
<t>
The encapsulation formats defined in this section (subsequently
referred to as the "LoBAC" encapsulation) comprise the MSDU (payload)
of an 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 <xref target="RFC4944"/>, therefore the use of "LOWPAN"
in literals below is intentional. The primary differences between
LoBAC and LoWPAN are: a) omission of the Fragmentation, Mesh, and
Broadcast headers, and b) use of LOWPAN_IPHC <xref target="RFC6282"/> in place of
LOWPAN_HC1 header compression (which is deprecated by <xref target="RFC6282"/>).
</t><t>
All LoBAC encapsulated datagrams transmitted over MS/TP are prefixed
by an encapsulation header stack. Each header in the stack consists
of a header type 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 <xref target="RFC2460"/>; in a LoBAC encapsulation the
analogous sequence is (optional) header compression and payload. The
header stacks that are valid in a LoBAC network are shown below.
</t>
<figure align="left">
<artwork>
A LoBAC encapsulated IPv6 datagram:
+---------------+-------------+---------+
| IPv6 Dispatch | IPv6 Header | Payload |
+---------------+-------------+---------+
A LoBAC encapsulated LOWPAN_IPHC compressed IPv6 datagram:
+---------------+-------------+---------+
| IPHC Dispatch | IPHC Header | Payload |
+---------------+-------------+---------+
</artwork>
</figure>
<t>
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.
</t><t>
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.
</t>
<!-- section anchor="sec-5.1" title="Dispatch Value and Header" -->
<section title="Dispatch Value and Header">
<t>
The LoBAC Dispatch value begins with a "0" bit followed by a "1" bit.
The Dispatch value and header are shown here:
</t>
<figure>
<artwork>
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
The Dispatch value may be treated as an unstructured namespace. Only
a few symbols are required 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.
Pattern Header Type
+------------+-----------------------------------------------------+
| 00 xxxxxx | NALP - Not a LoWPAN (LoBAC) frame |
| 01 000000 | ESC - Additional Dispatch octet follows |
| 01 000001 | IPv6 - Uncompressed IPv6 Addresses |
| ... | reserved - Defined or reserved by [RFC4944] |
| 01 1xxxxx | LOWPAN_IPHC - LOWPAN_IPHC compressed IPv6 [RFC6282] |
| 1x xxxxxx | reserved - Defined or reserved by [RFC4944] |
+------------+-----------------------------------------------------+
Figure 3: 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 Dispatch
value of 00xxxxxx shall discard the packet. Non-LoBAC protocols
that wish 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.
</artwork>
</figure>
</section>
</section>
<section anchor="sec-6" title="Stateless Address Autoconfiguration">
<!-- section title="Stateless Address Autoconfiguration" -->
<t>
This section defines how to obtain an IPv6 Interface Identifier. The
general procedure is described in Appendix A of <xref target="RFC4291"/>, "Creating
Modified EUI-64 Format Interface Identifiers", as updated by <xref target="RFC7136"/>.
</t><t>
The Interface Identifier MAY be based on an <xref target="EUI-64"/> 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 <xref target="RFC4291"/> MUST be used. This will
result in a globally unique Interface Identifier.
</t><t>
If the device does not have an EUI-64, then the Interface Identifier
SHOULD be formed by concatenating its 8-bit MS/TP node address to the
seven octets 0x00, 0x00, 0x00, 0xFF, 0xFE, 0x00, 0x00. For example,
an MS/TP node address of hexadecimal value 0x4F results in the
following Interface Identifier:
</t>
<figure align="center">
<artwork>
|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|0000000000000000|0000000011111111|1111111000000000|0000000001001111|
+----------------+----------------+----------------+----------------+
</artwork>
</figure>
<t>
This is the RECOMMENDED method of forming an IID, as it supports the
most efficient header compression provided by the LOWPAN_IPHC
<xref target="RFC6282"/> scheme specified in <xref target="sec-10"/>.
</t><t>
Optionally, an opaque 64-bit IID (for non-link-local addresses) MAY
be formed by the technique discussed in <xref target="I-D.ietf-6man-default-iids"/>
or alternate method. A node that generates a non-link-local address
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 <xref target="RFC6775"/>.
</t><t>
An IPv6 address prefix used for stateless autoconfiguration <xref target="RFC4862"/> of an MS/TP
interface MUST have a length of 64 bits.
</t><t>
<vspace blankLines='100' /> <!-- page break -->
</t>
</section>
<!-- section anchor="sec-7" title="IPv6 Link Local Address" -->
<section title="IPv6 Link Local Address">
<t>
The IPv6 link-local address <xref target="RFC4291"/> for an MS/TP interface is
formed by appending the Interface Identifier, as defined above, to
the prefix FE80::/64.
</t>
<figure align="center">
<artwork>
10 bits 54 bits 64 bits
+----------+-----------------------+----------------------------+
|1111111010| (zeros) | Interface Identifier |
+----------+-----------------------+----------------------------+
</artwork>
</figure>
</section>
<!-- section anchor="sec-8" title="Unicast Address Mapping" -->
<section title="Unicast Address Mapping">
<t>
The address resolution procedure for mapping IPv6 non-multicast
addresses into MS/TP link-layer addresses follows the general
description in Section 7.2 of <xref target="RFC4861"/>, unless otherwise specified.
</t><t>
The Source/Target Link-layer Address option has the following form
when the addresses are 8-bit MS/TP link-layer (node) addresses.
</t>
<figure align="center">
<artwork>
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<figure>
<artwork>
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 addresses.
MS/TP Address: The 8-bit address in canonical bit order [RFC2469].
This is the unicast address the interface currently responds to.
</artwork>
</figure>
</section>
<!-- section anchor="sec-9" title="Multicast Address Mapping" -->
<section title="Multicast Address Mapping">
<t>
All IPv6 multicast packets MUST be sent to MS/TP Destination Address
255 (broadcast) and filtered at the IPv6 layer. When represented as
a 16-bit address in a compressed header (see <xref target="sec-10"/>), it MUST be
formed by padding on the left with a zero:
</t>
<figure align="center">
<artwork>
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0xFF |
+-+-+-+-+-+-+-+-+---------------+
</artwork>
</figure>
</section>
<section anchor="sec-10" title="Header Compression">
<!-- section title="Header Compression" -->
<t>
LoBAC uses LOWPAN_IPHC IPv6 compression, which is specified in
<xref target="RFC6282"/> and included herein by
reference. This section will simply identify substitutions that
should be made when interpreting the text of <xref target="RFC6282"/>.
</t><t>
In general the following substitutions should be made:
</t><t>
<list style="hanging">
<t hangText=" -">Replace instances of "6LoWPAN" with "MS/TP network"</t>
<t hangText=" -">Replace instances of "IEEE 802.15.4 address" with "MS/TP address"</t>
</list>
</t><t>
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:
</t>
<figure align="center">
<artwork>
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | MS/TP address |
+-+-+-+-+-+-+-+-+---------------+
</artwork>
</figure>
<t>
If LOWPAN_IPHC compression <xref target="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 <xref target="RFC6775"/>.
</t>
</section>
<!-- section anchor="IANA" title="IANA Considerations" -->
<section title="IANA Considerations">
<t>
This document uses values previously reserved by <xref target="RFC4944"/> and
<xref target="RFC6282"/> and makes no further requests of IANA.
</t><t>
Note to RFC Editor: this section may be removed upon publication.
</t>
</section>
<!-- section anchor="Security" title="Security Considerations" -->
<section title="Security Considerations">
<t>
The method of deriving Interface Identifiers from MAC addresses is
intended to preserve global uniqueness when possible. However, there
is no protection from duplication through accident or forgery.
</t>
</section>
<!-- section anchor="Acknowledgments" title="Acknowledgments" -->
<section title="Acknowledgments">
<t>
We are grateful to the authors of <xref target="RFC4944"/> and members of the IETF
6LoWPAN working group; this document borrows liberally from their
work.
</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2119;
&RFC2460;
&RFC4291;
&RFC4861;
&RFC4862;
&RFC4944;
&RFC6282;
&RFC6775;
&RFC7136;
<reference anchor="Clause9">
<front>
<title>
BACnet - A Data Communication Protocol for Building Automation and Control Networks
</title>
<author>
<organization>American Society of Heating, Refrigerating, and Air-Conditioning Engineers</organization>
</author>
<date month="March" year="2013"/>
</front>
<seriesInfo name="ANSI/ASHRAE" value="135-2012 (Clause 9)"/>
</reference>
<reference anchor="Addendum_an" target="https://www.ashrae.org/File%20Library/docLib/StdsAddenda/135_2012_an_at_au_av_aw_ax_az_Final.pdf">
<front>
<title>
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
</title>
<author>
<organization>ASHRAE</organization>
</author>
<date month="July" year="2014"/>
</front>
</reference>
</references>
<?rfc compact="no" ?>
<references title="Informative References">
<reference anchor="COBS" target="http://www.stuartcheshire.org/papers/COBSforToN.pdf">
<front>
<title>Consistent Overhead Byte Stuffing</title>
<author initials="S." surname="Cheshire" fullname="Stuart Cheshire"/>
<author initials="M." surname="Baker" fullname="Mary Baker"/>
<date year="1999" month="April"/>
</front>
<seriesInfo name="IEEE/ACM TRANSACTIONS ON NETWORKING, VOL.7, NO.2" value=""/>
</reference>
<reference anchor="CRC32K" target="http://www.ece.cmu.edu/~koopman/networks/dsn02/dsn02_koopman.pdf">
<front>
<title>32-Bit Cyclic Redundancy Codes for Internet Applications</title>
<author initials="P." surname="Koopman" fullname="Philip Koopman"/>
<date year="2002" month="June"/>
</front>
<seriesInfo name="IEEE/IFIP International Conference on Dependable Systems and Networks (DSN 2002)" value=""/>
</reference>
<reference anchor="EUI-64" target="http://standards.ieee.org/regauth/oui/tutorials/EUI64.html">
<front>
<title>
Guidelines for 64-bit Global Identifier (EUI-64) Registration Authority
</title>
<author>
<organization>IEEE</organization>
</author>
<date month="March" year="1997"/>
</front>
</reference>
&I-D.ietf-6man-default-iids;
<!-- Patterned after http://xml.resource.org/public/rfc/bibxml2/_reference.IEEE.802-3.1998.xml -->
<reference anchor="IEEE.802.3" target="http://standards.ieee.org/getieee802/802.3.html">
<front>
<title>
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
</title>
<author/>
<date year="2008" month="December"/>
</front>
<seriesInfo name="IEEE" value="Std 802.3-2008"/>
</reference>
&RFC2469;
<reference anchor="TIA-485-A">
<front>
<title>
TIA-485-A, Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems (ANSI/TIA/EIA-485-A-98) (R2003)
</title>
<author>
<organization>Telecommunications Industry Association</organization>
</author>
<date month="March" year="2003"/>
</front>
</reference>
</references>
<section anchor="app-a" title="Abstract MAC Interface">
<t>
This Appendix is informative and not part of the standard.
</t><t>
BACnet <xref target="Clause9"/> defines support for MAC-layer clients through its
SendFrame and ReceivedDataNoReply procedures. However, it does not
define a protocol independent abstract interface for the data link.
This is provided below as an aid to implementation.
</t>
<section title="MA-DATA.request">
<section title="Function">
<t>
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.
</t>
</section>
<section title="Semantics of the Service Primitive">
<figure>
<artwork>
The semantics of the primitive are as follows:
MA-DATA.request (
destination_address,
source_address,
data,
priority,
type
)
</artwork>
</figure>
<t>
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 <xref target="sec-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.
</t><t>
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.
</t><t>
The 'priority' parameter specifies the priority desired for the data
unit transfer. The priority parameter is ignored by MS/TP.
</t><t>
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.
</t>
</section>
<section title="When Generated">
<t>
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.
</t>
</section>
<section title="Effect on Receipt">
<t>
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.
</t>
</section>
</section>
<section title="MA-DATA.indication">
<section title="Function">
<t>
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.
</t>
</section>
<section title="Semantics of the Service Primitive">
<figure>
<artwork>
The semantics of the primitive are as follows:
MA-DATA.indication (
destination_address,
source_address,
data,
priority,
type
)
</artwork>
</figure>
<t>
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.
</t><t>
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.
</t><t>
The 'priority' parameter specifies the priority desired for the data
unit transfer. The priority parameter is ignored by MS/TP.
</t><t>
The 'type' parameter is the value of the MS/TP Frame Type field of
the incoming frame.
</t>
</section>
<section title="When Generated">
<t>
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.
</t>
</section>
<section title="Effect on Receipt">
<t>
The effect of receipt of this primitive by the MAC client is unspecified.
</t>
</section>
</section>
</section>
<section anchor="app-b" title="Consistent Overhead Byte Stuffing [COBS]">
<t>
This Appendix is informative and not part of the standard.
</t><t>
BACnet <xref target="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 <xref target="COBS"/> and decode these fields upon reception.
</t><t>
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 <xref target="Addendum_an"/> specifies
the preamble octet '0x55' for removal.
</t><t>
The minimum overhead of COBS is one ectet per encoded field. The
worst-case overhead is bounded to one octet in 254, or less than
0.5%, as described in <xref target="COBS"/>.
</t><t>
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' 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 <xref target="app-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.
</t><t>
Example COBS encoder and decoder functions are shown below for
illustration. Complete examples of use and test vectors are provided in
BACnet <xref target="Addendum_an"/>.
</t>
<figure>
<artwork><![CDATA[
#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;
}
]]></artwork>
</figure>
<t>
<vspace blankLines='2' /> <!-- page break -->
</t>
<figure>
<artwork><![CDATA[
#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;
}
]]></artwork>
</figure>
</section>
<section anchor="app-c" title="Encoded CRC-32K [CRC32K]">
<t>
This Appendix is informative and not part of the standard.
</t><t>
Extending the payload of MS/TP to 1501 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
<xref target="CRC32K"/>, he surveyed the entire 32-bit polynomial space and noted
some that exceed the <xref target="IEEE.802.3"/> polynomial in performance. BACnet
<xref target="Addendum_an"/> specifies the CRC-32K (Koopman) polynomial.
</t><t>
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 <xref target="COBS"/> Encoded Data through
the function, the ones complement of the resulting 'crc_value' is
arranged in LSB-first order and is itself <xref target="COBS"/> encoded. The length
of the resulting Encoded CRC-32K field is always five octets.
</t><t>
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.
</t><t>
An example CRC-32K function in shown below for illustration. Complete
examples of use and test vectors are provided in BACnet <xref target="Addendum_an"/>.
</t>
<figure>
<artwork><![CDATA[
#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); 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;
}
]]></artwork>
</figure>
<t>
<vspace blankLines='100' /> <!-- page break -->
</t>
</section>
</back>
</rfc>
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