Transmission of IPv6 over Multidrop Serial Bus/Token Passing (MS/TP) Networks

Status of This Memo

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Table of Contents

2. Profile for IPv6 over MS/TP

ASHRAE has assigned an MS/TP Frame Type value of 34 to indicate IPv6 over MS/TP (LoBAC) Encapsulation. This falls within the range of values that designate COBS-encoded data frames.¶

Nmin_COBS_length  The minimum valid Length value of any LoBAC-
                  encapsulated frame: 5

Nmax_COBS_length  The maximum valid Length value of any LoBAC-
                  encapsulated frame: 1509

Nmax_info_frames  The default maximum number of information frames
                  the node may send before it must pass the token: 1

Nmax_manager      The default highest allowable address for master
                  nodes: 127

3. Addressing Modes

MS/TP node (MAC) addresses are one octet in length and are assigned dynamically. The method of assigning MAC addresses is outside the scope of this specification. However, each MS/TP node on the link MUST have a unique address in order to ensure correct MAC operation.¶

[BACnet], Clause 9 specifies that addresses 0 through 127 are valid for manager nodes. The method specified in Section 6 for creating a MAC-address-derived Interface Identifier (IID) ensures that an IID of all zeros can never be generated.¶

A Destination Address of 255 (all nodes) indicates a MAC-layer broadcast. MS/TP does not support multicast; therefore, all IPv6 multicast packets MUST be broadcast at the MAC layer and filtered at the IPv6 layer. A Source Address of 255 MUST NOT be used.¶

Hosts learn IPv6 prefixes via router advertisements according to [RFC4861].¶

4. Maximum Transmission Unit (MTU)

Upon transmission, the network-layer MTU is formatted according to Section 5 and becomes the MAC service data unit (MSDU). The MSDU is then COBS encoded by MS/TP. Upon reception, the steps are reversed. [BACnet], Clause 9 supports MSDUs up to 2032 octets in length.¶

IPv6 [RFC2460] requires that every link in an internet have an MTU of 1280 octets or greater. Additionally, a node must be able to accept a fragmented packet that, after reassembly, is as large as 1500 octets. This specification defines an MTU length of at least 1280 octets and at most 1500 octets. Support for an MTU length of 1500 octets is RECOMMENDED.¶

5. LoBAC Adaptation Layer

This section specifies an adaptation layer to support compressed IPv6 headers as specified in Section 10. IPv6 header compression MUST be implemented on all 6LoBAC nodes. Implementations MAY also support Generic Header Compression [RFC7400] for transport layer headers.¶

The LoBAC encapsulation format defined in this section describes the MSDU of an IPv6 over MS/TP frame. The LoBAC payload (i.e., an IPv6 packet) follows an encapsulation header stack. LoBAC is a subset of the LoWPAN encapsulation defined in [RFC4944], as updated by [RFC6282], so the use of "LOWPAN" in literals below is intentional. The primary difference between LoWPAN and LoBAC encapsulation is omission of the Mesh, Broadcast, Fragmentation, and LOWPAN_HC1 headers in the latter.¶

All LoBAC-encapsulated datagrams transmitted over MS/TP are prefixed by an encapsulation header stack consisting of a Dispatch value followed by zero or more header fields. The only sequence currently defined for LoBAC is the LOWPAN_IPHC header followed by payload, as shown below:¶

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

The Dispatch value is treated as an unstructured namespace. Only a single pattern is used to represent current LoBAC functionality.¶

  Pattern      Header Type
+------------+-----------------------------------------------------+
| 01  1xxxxx | LOWPAN_IPHC - LOWPAN_IPHC compressed IPv6 [RFC6282] |
+------------+-----------------------------------------------------+

Other IANA-assigned 6LoWPAN Dispatch values do not apply to 6LoBAC unless otherwise specified.¶

6. Stateless Address Autoconfiguration

This section defines how to obtain an IPv6 Interface Identifier. This specification distinguishes between two types of IIDs, MAC- address-derived and semantically opaque.¶

A MAC-address-derived IID is the RECOMMENDED type for use in forming a link-local address, as it affords the most efficient header compression provided by the LOWPAN_IPHC [RFC6282] format specified in Section 10. The general procedure for creating a MAC-address-derived IID is described in Appendix A of [RFC4291], "Creating Modified EUI-64 Format Interface Identifiers", as updated by [RFC7136].¶

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

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

A semantically opaque IID having 64 bits of entropy is RECOMMENDED for each globally scoped address and MAY be locally generated according to one of the methods cited in Section 12. A node that generates a 64-bit semantically opaque IID MUST register the IID with its local router(s) by sending a Neighbor Solicitation (NS) message with the Address Registration Option (ARO) and process Neighbor Advertisements (NAs) according to [RFC6775].¶

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

10. Header Compression

6LoBAC REQUIRES LOWPAN_IPHC IPv6 compression, which is specified in [RFC6282] and included herein by reference. This section will simply identify substitutions that should be made when interpreting the text of [RFC6282].¶

In general, the following substitutions should be made:¶

-

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

-

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

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

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

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

12. Security Considerations

See [RFC8065] for a general discussion of privacy threats faced by constrained nodes.¶

[RFC8065] makes a distinction between "stable" and "temporary" addresses. The former are long-lived and typically advertised by servers. The latter are typically used by clients and SHOULD be changed frequently to mitigate correlation of activities over time. Nodes that engage in both activities SHOULD support simultaneous use of multiple addresses per device.¶

Globally scoped addresses that contain MAC-address-derived IIDs may expose a network to address-scanning attacks. For this reason, it is RECOMMENDED that a 64-bit semantically opaque IID be generated for each globally scoped address in use according to, for example, [RFC3315], [RFC3972], [RFC4941], [RFC5535], or [RFC7217].¶

Appendix A. Abstract MAC Interface

This Appendix is informative and not part of the standard.¶

[BACnet], Clause 9 provides support for MAC-layer clients through its SendFrame and ReceivedDataNoReply procedures. However, it does not define a network-protocol independent abstract interface for the MAC. This is provided below as an aid to implementation.¶

The semantics of the primitive are as follows:

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

The semantics of the primitive are as follows:

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

Appendix B. Consistent Overhead Byte Stuffing (COBS)

This Appendix is informative and not part of the standard.¶

[BACnet], Clause 9 corrects a long-standing issue with the MS/TP specification, namely that preamble sequences were not escaped whenever they appeared in the Data or Data CRC fields. In rare cases, this resulted in dropped frames due to loss of frame synchronization. The solution is to encode the Data and 32-bit Data CRC fields before transmission using Consistent Overhead Byte Stuffing [COBS] and decode these fields upon reception.¶

COBS is a run-length encoding method that nominally removes '0x00' octets from its input. Any selected octet value may be removed by XOR'ing that value with each octet of the COBS output. [BACnet], Clause 9 specifies the preamble octet '0x55' for removal.¶

The minimum overhead of COBS is one octet per encoded field. The worst-case overhead in long fields is bounded to one octet per 254 as described in [COBS].¶

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

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

<CODE BEGINS>

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

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

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

  while (read_index < length) {
    data = from[read_index++];
    /*
     * In the case of encountering a non-zero octet in the data,
     * simply copy input to output and increment the code octet.
     */
    if (data != 0) {
      to[write_index++] = data ^ mask;
      code++;
      if (code != 255)
        continue;
    }
    /*
     * In the case of encountering a zero in the data or having
     * copied the maximum number (254) of non-zero octets, store
     * the code octet and reset the encoder state variables.
     */
    last_code = code;
    to[code_index] = code ^ mask;
    code_index = write_index++;
    code = 1;
  }
  /*
   * If the last chunk contains exactly 254 non-zero octets, then
   * this exception is handled above (and the returned length must
   * be adjusted). Otherwise, encode the last chunk normally, as if
   * 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;
}


<CODE ENDS>

<CODE BEGINS>

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

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

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


<CODE ENDS>

Appendix C. Encoded CRC-32K [CRC32K]

This Appendix is informative and not part of the standard.¶

Extending the payload of MS/TP to 1500 octets requires upgrading the Data CRC from 16 bits to 32 bits. P.Koopman has authored several papers on evaluating CRC polynomials for network applications. In [CRC32K], he surveyed the entire 32-bit polynomial space and noted some that exceed the [IEEE.802.3] polynomial in performance. [BACnet], Clause 9 specifies one of these, the CRC-32K (Koopman) polynomial.¶

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

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

An example CRC-32K function is shown below for illustration. Complete examples of use and test vectors are provided in [BACnet], Annex G.3.¶

<CODE BEGINS>

#include <stdint.h>

/* See ANSI/ASHRAE Standard 135-2024 [BACnet], Section G.3.2 */
#define CRC32K_INITIAL_VALUE (0xFFFFFFFF)
#define CRC32K_RESIDUE (0x0843323B)

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

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

  for (b = 0; b < 8; b++) {
    if ((data_value & 1) ^ (crc_value & 1)) {
      crc_value >>= 1;
      crc_value ^= CRC32K_POLY;
    } else {
      crc_value >>= 1;
    }
    data_value >>= 1;
  }
  return crc_value;
}


<CODE ENDS>

Appendix D. Example 6LoBAC Frame Decode

This Appendix is informative and not part of the standard.¶

BACnet MS/TP, Src (2), Dst (1), IPv6 Encapsulation
    Preamble 55: 0x55
    Preamble FF: 0xff
    Frame Type: IPv6 Encapsulation (34)
    Destination Address: 1
    Source Address: 2
    Length: 537
    Header CRC: 0x1c [correct]
    Extended Data CRC: 0x9e7259e2 [correct]
6LoWPAN
    IPHC Header
        011. .... = Pattern: IP header compression (0x03)
        ...1 1... .... .... = Traffic class and flow label:
                              Version, traffic class, and flow label
                              compressed (0x0003)
        .... .0.. .... .... = Next header: Inline
        .... ..00 .... .... = Hop limit: Inline (0x0000)
        .... .... 1... .... = Context identifier extension: True
        .... .... .1.. .... = Source address compression: Stateful
        .... .... ..01 .... = Source address mode:
                              64-bits inline (0x0001)
        .... .... .... 0... = Multicast address compression: False
        .... .... .... .1.. = Destination address compression:
                              Stateful
        .... .... .... ..10 = Destination address mode:
                              16-bits inline (0x0002)
        0000 .... = Source context identifier: 0x00
        .... 0000 = Destination context identifier: 0x00
        [Source context: aaaa:: (aaaa::)]
        [Destination context: aaaa:: (aaaa::)]
    Next header: ICMPv6 (0x3a)
    Hop limit: 63
    Source: aaaa::1 (aaaa::1)
    Destination: aaaa::ff:fe00:1 (aaaa::ff:fe00:1)
Internet Protocol Version 6, Src: aaaa::1 (aaaa::1),
                             Dst: aaaa::ff:fe00:1 (aaaa::ff:fe00:1)
    0110 .... .... .... .... .... .... .... = Version: 6
    .... 0000 0000 .... .... .... .... .... = Traffic class:
                                              0x00000000
    .... 0000 00.. .... .... .... .... .... = Differentiated
                                              Services Field:
                                              Default (0x00000000)
    .... .... ..0. .... .... .... .... .... = ECN-Capable Transport
                                              (ECT): Not set
    .... .... ...0 .... .... .... .... .... = ECN-CE: Not set
    .... .... .... 0000 0000 0000 0000 0000 = Flowlabel: 0x00000000
    Payload length: 518
    Next header: ICMPv6 (58)
    Hop limit: 63
    Source: aaaa::1 (aaaa::1)
    Destination: aaaa::ff:fe00:1 (aaaa::ff:fe00:1)
Internet Control Message Protocol v6
    Type: Echo (ping) request (128)
    Code: 0
    Checksum: 0x783f [correct]
    Identifier: 0x2ee5
    Sequence: 2
    [Response In: 5165]
    Data (510 bytes)
        Data: e4dbe8553ba0040008090a0b0c0d0e0f1011121314151617...
        [Length: 510]

Frame (547 bytes):
55 ff 22 01 02 02 19 1c 56 2d 83 56 6f 6a 54 54   U.".....V-.VojTT
54 54 54 54 57 54 56 54 d5 50 2d 6a 7b b0 5c 57   TTTTWTVT.P-j{.\W
b1 8e bd 00 6e f5 51 ac 5d 5c 5f 5e 59 58 5b 5a   ....n.Q.]\_^YX[Z
45 44 47 46 41 40 43 42 4d 4c 4f 4e 49 48 4b 4a   EDGFA@CBMLONIHKJ
75 74 77 76 71 70 73 72 7d 7c 7f 7e 79 78 7b 7a   utwvqpsr}|.~yx{z
65 64 67 66 61 60 63 62 6d 6c 6f 6e 69 68 6b 6a   edgfa`cbmlonihkj
15 14 17 16 11 10 13 12 1d 1c 1f 1e 19 18 1b 1a   ................
05 04 07 06 01 00 03 02 0d 0c 0f 0e 09 08 0b 0a   ................
35 34 37 36 31 30 33 32 3d 3c 3f 3e 39 38 3b 3a   54761032=<?>98;:
25 24 27 26 21 20 23 22 2d 2c 2f 2e 29 28 2b 2a   %$'&! #"-,/.)(+*
d5 d4 d7 d6 d1 d0 d3 d2 dd dc df de d9 d8 db da   ................
c5 c4 c7 c6 c1 c0 c3 c2 cd cc cf ce c9 c8 cb ca   ................
f5 f4 f7 f6 f1 f0 f3 f2 fd fc ff fe f9 f8 fb fa   ................
e5 e4 e7 e6 e1 e0 e3 e2 ed ec ef ee e9 e8 eb ea   ................
95 94 97 96 91 90 93 92 9d 9c 9f 9e 99 98 9b 9a   ................
85 84 87 86 81 80 83 82 8d 8c 8f 8e 89 88 8b 8a   ................
b5 b4 b7 b6 b1 b0 b3 b2 bd bc bf be b9 b8 bb ba   ................
a5 a4 a7 a6 a1 a0 a3 a2 ad ac af ae a9 a8 ab aa   ................
ab 54 57 56 51 50 53 52 5d 5c 5f 5e 59 58 5b 5a   .TWVQPSR]\_^YX[Z
45 44 47 46 41 40 43 42 4d 4c 4f 4e 49 48 4b 4a   EDGFA@CBMLONIHKJ
75 74 77 76 71 70 73 72 7d 7c 7f 7e 79 78 7b 7a   utwvqpsr}|.~yx{z
65 64 67 66 61 60 63 62 6d 6c 6f 6e 69 68 6b 6a   edgfa`cbmlonihkj
15 14 17 16 11 10 13 12 1d 1c 1f 1e 19 18 1b 1a   ................
05 04 07 06 01 00 03 02 0d 0c 0f 0e 09 08 0b 0a   ................
35 34 37 36 31 30 33 32 3d 3c 3f 3e 39 38 3b 3a   54761032=<?>98;:
25 24 27 26 21 20 23 22 2d 2c 2f 2e 29 28 2b 2a   %$'&! #"-,/.)(+*
d5 d4 d7 d6 d1 d0 d3 d2 dd dc df de d9 d8 db da   ................
c5 c4 c7 c6 c1 c0 c3 c2 cd cc cf ce c9 c8 cb ca   ................
f5 f4 f7 f6 f1 f0 f3 f2 fd fc ff fe f9 f8 fb fa   ................
e5 e4 e7 e6 e1 e0 e3 e2 ed ec ef ee e9 e8 eb ea   ................
95 94 97 96 91 90 93 92 9d 9c 9f 9e 99 98 9b 9a   ................
85 84 87 86 81 80 83 82 8d 8c 8f 8e 89 88 8b 8a   ................
b5 b4 b7 b6 b1 b0 b3 b2 bd bc bf be b9 b8 bb ba   ................
a5 a4 a7 a6 a1 a0 a3 a2 ad ac af ae a9 a8 50 cb   ..............P.
27 0c b7                                          '..

Decoded Data and CRC32K (537 bytes):
78 d6 00 3a 3f 00 00 00 00 00 00 00 01 00 01 80   x..:?...........
00 78 3f 2e e5 00 02 e4 db e8 55 3b a0 04 00 08   .x?.......U;....
09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18   ................
19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28   ....... !"#$%&'(
29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37 38   )*+,-./012345678
39 3a 3b 3c 3d 3e 3f 40 41 42 43 44 45 46 47 48   9:;<=>?@ABCDEFGH
49 4a 4b 4c 4d 4e 4f 50 51 52 53 54 55 56 57 58   IJKLMNOPQRSTUVWX
59 5a 5b 5c 5d 5e 5f 60 61 62 63 64 65 66 67 68   YZ[\]^_`abcdefgh
69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 78   ijklmnopqrstuvwx
79 7a 7b 7c 7d 7e 7f 80 81 82 83 84 85 86 87 88   yz{|}~..........
89 8a 8b 8c 8d 8e 8f 90 91 92 93 94 95 96 97 98   ................
99 9a 9b 9c 9d 9e 9f a0 a1 a2 a3 a4 a5 a6 a7 a8   ................
a9 aa ab ac ad ae af b0 b1 b2 b3 b4 b5 b6 b7 b8   ................
b9 ba bb bc bd be bf c0 c1 c2 c3 c4 c5 c6 c7 c8   ................
c9 ca cb cc cd ce cf d0 d1 d2 d3 d4 d5 d6 d7 d8   ................
d9 da db dc dd de df e0 e1 e2 e3 e4 e5 e6 e7 e8   ................
e9 ea eb ec ed ee ef f0 f1 f2 f3 f4 f5 f6 f7 f8   ................
f9 fa fb fc fd fe ff 00 01 02 03 04 05 06 07 08   ................
09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18   ................
19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28   ....... !"#$%&'(
29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37 38   )*+,-./012345678
39 3a 3b 3c 3d 3e 3f 40 41 42 43 44 45 46 47 48   9:;<=>?@ABCDEFGH
49 4a 4b 4c 4d 4e 4f 50 51 52 53 54 55 56 57 58   IJKLMNOPQRSTUVWX
59 5a 5b 5c 5d 5e 5f 60 61 62 63 64 65 66 67 68   YZ[\]^_`abcdefgh
69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 78   ijklmnopqrstuvwx
79 7a 7b 7c 7d 7e 7f 80 81 82 83 84 85 86 87 88   yz{|}~..........
89 8a 8b 8c 8d 8e 8f 90 91 92 93 94 95 96 97 98   ................
99 9a 9b 9c 9d 9e 9f a0 a1 a2 a3 a4 a5 a6 a7 a8   ................
a9 aa ab ac ad ae af b0 b1 b2 b3 b4 b5 b6 b7 b8   ................
b9 ba bb bc bd be bf c0 c1 c2 c3 c4 c5 c6 c7 c8   ................
c9 ca cb cc cd ce cf d0 d1 d2 d3 d4 d5 d6 d7 d8   ................
d9 da db dc dd de df e0 e1 e2 e3 e4 e5 e6 e7 e8   ................
e9 ea eb ec ed ee ef f0 f1 f2 f3 f4 f5 f6 f7 f8   ................
f9 fa fb fc fd 9e 72 59 e2                        ......rY.

Decompressed 6LoWPAN IPHC (558 bytes):
60 00 00 00 02 06 3a 3f aa aa 00 00 00 00 00 00   `.....:?........
00 00 00 00 00 00 00 01 aa aa 00 00 00 00 00 00   ................
00 00 00 ff fe 00 00 01 80 00 78 3f 2e e5 00 02   ..........x?....
e4 db e8 55 3b a0 04 00 08 09 0a 0b 0c 0d 0e 0f   ...U;...........
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f   ................
20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f    !"#$%&'()*+,-./
30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f   0123456789:;<=>?
40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f   @ABCDEFGHIJKLMNO
50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f   PQRSTUVWXYZ[\]^_
60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f   `abcdefghijklmno
70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f   pqrstuvwxyz{|}~.
80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f   ................
90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f   ................
a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af   ................
b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf   ................
c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf   ................
d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df   ................
e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef   ................
f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd fe ff   ................
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f   ................
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f   ................
20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f    !"#$%&'()*+,-./
30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f   0123456789:;<=>?
40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f   @ABCDEFGHIJKLMNO
50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f   PQRSTUVWXYZ[\]^_
60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f   `abcdefghijklmno
70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f   pqrstuvwxyz{|}~.
80 81 82 83 84 85 86 87 88 89 8a 8b 8c 8d 8e 8f   ................
90 91 92 93 94 95 96 97 98 99 9a 9b 9c 9d 9e 9f   ................
a0 a1 a2 a3 a4 a5 a6 a7 a8 a9 aa ab ac ad ae af   ................
b0 b1 b2 b3 b4 b5 b6 b7 b8 b9 ba bb bc bd be bf   ................
c0 c1 c2 c3 c4 c5 c6 c7 c8 c9 ca cb cc cd ce cf   ................
d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 da db dc dd de df   ................
e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 ea eb ec ed ee ef   ................
f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 fa fb fc fd         ..............

Acknowledgements

We are grateful to the authors of [RFC4944] and members of the IETF 6LoWPAN working group; this document borrows liberally from their work. Ralph Droms and Brian Haberman provided indispensable guidance and support from the outset. Peter van der Stok, James Woodyatt, Carsten Bormann, and Dale Worley provided detailed reviews. Stuart Cheshire invented the very clever COBS encoding. Michael Osborne made the critical observation that encoding the data and CRC32K fields separately would allow the CRC to be calculated on-the-fly. Alexandru Petrescu, Brian Frank, Geoff Mulligan, and Don Sturek offered valuable comments.¶

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