IPv6 Maintenance WG K. Lynn, Ed.
Internet-Draft Consultant
Intended status: Standards Track July 04, 2011
Expires: January 5, 2012
Transmission of IPv6 over MS/TP Networks
draft-lynn-6man-6lobac-00
Abstract
MS/TP (Master-Slave/Token-Passing) is a contention-free access method
for the TIA-485-A physical layer that is used extensively in building
automation networks. This document describes 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
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This Internet-Draft will expire on January 5, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. MS/TP Mode for IP . . . . . . . . . . . . . . . . . . . . . . 6
3. Addressing Modes . . . . . . . . . . . . . . . . . . . . . . . 6
4. Maximum Transmission Unit . . . . . . . . . . . . . . . . . . 6
5. LoBAC Adaptation Layer . . . . . . . . . . . . . . . . . . . . 7
6. Stateless Address Autoconfiguration . . . . . . . . . . . . . 9
7. IPv6 Link Local Address . . . . . . . . . . . . . . . . . . . 10
8. Unicast Address Mapping . . . . . . . . . . . . . . . . . . . 10
9. Header Compression . . . . . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
13.1. Normative References . . . . . . . . . . . . . . . . . . 12
13.2. Informative References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
MS/TP (Master-Slave/Token-Passing) is a contention-free access method
for the [TIA-485-A] physical layer that is used extensively in
building automation networks. This document describes 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 to constrained wired networks.
An MS/TP device is typically based on a low-cost microcontroller with
limited processing power and/or 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 applied. 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)
proposed changes to MS/TP will support payloads of up to 1500 octets
without the need for MAC-layer fragmentation and reassembly.
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 defines mechanisms for header
compression required to make IPv6 practical on MS/TP networks based
on [I-D.ietf-6lowpan-hc].
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
MAC: Medium Access Control
MSDU: MAC Service Data Unit (MAC client data)
MTU: Maximum Transmission Unit (data link client data)
UART: Universal Asynchcronous Transmitter/Receiver
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1.3. MS/TP Overview
This section provides a brief overview of MS/TP, which is specified
in Clause 9 of ANSI/ASHRAE 135-2010 [BACnet] and included herein by
reference. [BACnet] also covers physical layer deployment options.
MS/TP is designed to enable multidrop networks over shielded twisted
pair wiring. It can support segments up to 1200 meters in length or
data rates up to 115,200 baud (at this data rate the segment length
is limited to 1000 meters). An MS/TP link requires only a UART, a
5ms resolution timer, and a [TIA-485-A] transceiver with a driver
that can be disabled. These features combine to 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 bus network. 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 pasess the token to the next master
node. (Slave nodes transmit only when polled and are not considered
part of this specification.)
MS/TP 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/
/ Data*
/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data CRC* (LS octet first) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| optional 0xFF |
+-+-+-+-+-+-+-+-+
Figure 1: MS/TP Frame Format
*Note: The Data and Data CRC fields are present only if Length is
non-zero. A BACnet proposal now under review assigns a new Frame
Type for IPv6, sets the maximum length of the Data field to 1500
octets, and replaces the 16-bit Data CRC with a 32-bit Data CRC.
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The MS/TP frame fields have the following descriptions:
Preamble two octet preamble: 0x55, 0xFF
Frame Type one octet
Destination Address one octet node identifier
Source Address one octet node identifier
Length two octets, most significant octet first
Header CRC one octet
Data 0 - 1500 octets*
(present only if Length is non-zero)
Data CRC four octets*, least significant octet first
(present only if Length is non-zero)
(pad) (optional) at most one octet of trailer: 0xFF
The Frame Type is used to distinguish between different types of MAC
frames. Currently defined types (in decimal) are:
00 Token
01 Poll For Master
02 Reply To Poll For Master
...
10 IPv6 over MS/TP Encapsulation*
Frame Types 11 through 127 are reserved for assignment by ASHRAE.
Frame Types 128 through 255 are available to vendors for proprietary
frames. Token, Poll For Master, and Reply to Poll For Master frames
MUST be understood by all master nodes. See Section 2 for additional
details.
The Destination and Source Addresses are each one octet in length.
See Section 3 for additional details.
The Length field specifies the length in octets of the Data field and
is transmitted most significant octet first. See Section 4 for
additional details.
The Header CRC field covers the Frame Type, Destination Address,
Source Address, and Length fields.
The Data and Data CRC fields are conditional on the Frame Type and
the Length. The Data and Data CRC fields are always present in
frames specified by this document. *See previous note regarding the
[BACnet] change proposal to support IPv6 over MS/TP.
The Header and Data CRC generation and check procedures are specified
in [BACnet] and exended by the proposal.
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2. MS/TP Mode for IP
The [BACnet] MS/TP change proposal now under review allocates a new
Frame Type from the reserved range to indicate IPv6 encapsulation.
The new Frame Type for IPv6 over MS/TP Encapsulation (LoBAC) is 10.
All MS/TP master devices (including those that support IPv6) must
understand Token, Poll For Master, and Reply to Poll For Master
frames and support the Receive Frame and Master Node state machines
specified in [BACnet] and extended by the proposal.
3. Addressing Modes
MS/TP Destination and Source Addresses are one octet in length. A
Destination Address of 255 (0xFF) denotes a link-level broadcast (all
nodes). All IPv6 multicast packets MUST be sent to Destination
Address 255 and filtered at the IPv6 layer. A Source Address of 255
MUST NOT be used.
[BACnet] specifies that addresses 0 to 127 are valid for master
nodes. However, this specification restricts the allowable unicast
address range to between 1 and 127, inclusive. Zero MUST NOT be used
as either a Destination Address or Source Address.
The assignment of node addresses (identifiers) is outside the scope
of this document. Each node must have a unique identifier on the
link or a misconfiguration condition exists.
This document assumes that each MS/TP link maps to a unique IPv6
subnet prefix. Hosts learn IPv6 prefixes via router advertisements
according to [RFC4861]. MS/TP does not support multicast, therefore
IPv6-level multicast packets MUST be carried as link-layer broadcast
frames in MS/TP networks.
4. Maximum Transmission Unit
The [BACnet] MS/TP change proposal now under review requires that the
MSDU be increased to 1500 octets and covered by a 32-bit CRC. This
is sufficient to convey an MTU of at least 1280 octets as required by
IPv6 and eliminates the need for MAC-layer fragmentation and
reassembly.
However, the relatively low data rates of MS/TP still make a
compelling case for header compression. Since it is expected that
many (if not all) applications of IPv6 over MS/TP will make use of
header compression, an adaptation layer to compress and decompress
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IPv6 headers is specified below in Section 5 and the compression
scheme is specified in Section 9.
5. LoBAC Adaptation Layer
The encapsulation formats defined in this section (subsequently
referred to as the "LoBAC encapsulation") comprise the payload (MSDU)
of an MS/TP frame. The LoBAC payload (e.g., an IPv6 packet) follows
an encapsulation header stack. LoBAC is a subset of the LoWPAN
encapsulation defined in [RFC4944]. Use of the "LOWPAN" prefix where
it appears below is therefore intentional. The primary differences
are a) elimination of the mesh, broadcast, and fragmentation headers,
and b) use of LOWPAN_IPHC compression [I-D.ietf-6lowpan-hc].
All LoBAC encapsulated datagrams transported over MS/TP are prefixed
by an encapsulation header stack. Each header in the header stack
contains 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 [RFC2460]; in a LoBAC header
the analogous header sequence is (optionally) compression and
payload. These examples show the header stacks that may be used in a
LoBAC network.
A LoBAC encapsulated IPv6 datagram:
+---------------+-------------+---------+
| IPv6 Dispatch | IPv6 Header | Payload |
+---------------+-------------+---------+
A LoBAC encapsulated LOWPAN_IPHC compressed IPv6 datagram:
+---------------+-------------+---------+
| IPHC Dispatch | IPHC Header | Payload |
+---------------+-------------+---------+
All protocol datagrams (e.g., IPv6, compressed IPv6 headers, etc.)
SHALL be preceded by one of the valid LoBAC encapsulation headers,
examples of which are given above. This permits uniform software
treatment of datagrams without regard to the mode of their
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
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purpose extensibility. This format document specifies a small set of
header types using the header stack for clarity, compactness, and
orthogonality.
5.1. Dispatch Type and Header
The dispatch type is defined by a zero bit as the first bit and a one
bit as the second bit. The dispatch type and header are shown here:
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 Header.
type-specific header A header determined by the Dispatch Header.
Figure 2: Dispatch Type 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 octet, 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 - Reserved for future use |
| 01 1xxxxx | LOWPAN_IPHC - LOWPAN_IPHC compressed IPv6 |
+------------+-------------------------------------------------+
Figure 3: Dispatch Value Bit Pattern
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. Other 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
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the Dispatch value. It allows support for Dispatch values larger
than 127.
IPv6: Specifies that the following header is an uncompressed IPv6
header [RFC2460].
LOWPAN_IPHC: A value of 001xxxxx specifies a LOWPAN_IPHC compression
header (see Section 9.)
6. Stateless Address Autoconfiguration
This section defines how to obtain an IPv6 Interface Identifier. The
general procedure is described in Appendix A of [RFC4291], "Creating
Modified EUI-64 Format Interface Identifiers".
The Interface Identifier may be based on an [EUI-64] identifier
assigned to the device. In this case, the Interface Identifier is
formed from the EUI-64 by inverting the "u" (universal/local) bit
according to [RFC4291]. This will result in a globally unique
Interface Identifier.
If the device does not have an EUI-64, then the Interface Identifier
is formed by taking the MS/TP node identifier and concatenating it to
the seven octets 0x00, 0x00, 0x00, 0xFF, 0xFE, 0x00, 0x00. For
example, an MS/TP node identifier of hexadecimal value 0x4F results
in the following Interface Identifier:
|0 1|1 3|3 4|4 6|
|0 5|6 1|2 7|8 3|
+----------------+----------------+----------------+----------------+
|0000000000000000|0000000011111111|1111111000000000|0000000001001111|
+----------------+----------------+----------------+----------------+
Note that this results in the universal/local bit set to "0" to
indicate local scope.
A different MAC address set manually or by software MAY be used to
derive the Interface Identifier. If such a MAC address is used, its
global uniqueness property should be reflected in the value of the
universal/local bit.
An IPv6 address prefix used for stateless autoconfiguration [RFC4862]
of an MS/TP interface MUST have a length of 64 bits.
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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 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 link layer is MS/TP and the addresses are 8-bit MS/TP node
addresses.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | MS/TP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Padding -+
| (all zeros) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option fields:
Type:
1: for Source Link-layer address.
2: for Target Link-layer address.
Length: This is the length of this option (including the type and
length fields) in units of 8 octets. The value of this field is 1
for 8-bit MS/TP addresses.
MS/TP Address: The 8-bit address in canonical bit order [RFC2469].
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This is the address the interface currently responds to.
9. Header Compression
LoBAC uses LOWPAN_IPHC IPv6 compression, which is specified in
[I-D.ietf-6lowpan-hc] and included herein by reference. This section
will simply identify substitutions that should be made when
interpreting the text of [I-D.ietf-6lowpan-hc].
In general the following substitutions should be made:
* Replace "6LoWPAN network" with "MS/TP network"
* Replace "IEEE 802.15.4 address" with "MS/TP address"
Where 16-bit addresses are called for (e.g., a short IEEE 802.15.4
address) they 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 |
+-+-+-+-+-+-+-+-+---------------+
10. IANA Considerations
This document uses values previously reserved by [RFC4944] and
[I-D.ietf-6lowpan-hc] and makes no further requests of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
11. Security Considerations
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.
12. Acknowledgments
Thanks are extended to the authors of [RFC4944] and members of the
IETF 6LoWPAN working group; this document borrows extensively from
their work.
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13. References
13.1. Normative References
[BACnet] American Society of Heating, Refrigerating, and Air-
Conditioning Engineers, "BACnet, A Data Communication
Protocol for Building Automation and Control Networks
(ANSI Approved)", ANSI/ASHRAE 135-2010, April 2011.
[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.
13.2. Informative References
[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-6lowpan-hc]
Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams in Low Power and Lossy Networks (6LoWPAN)",
draft-ietf-6lowpan-hc-15 (work in progress),
February 2011.
[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
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Use in Balanced Digital Multipoint Systems (ANSI/TIA/
EIA-485-A-98) (R2003)", March 2003.
Author's Address
Kerry Lynn (editor)
Consultant
Phone: +1 (978) 460-4253
Email: kerlyn@ieee.org
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