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Definition

The 6LoWPAN (IPv6 over Low-power Wireless Personal Area Networks) is an adaptation Layer that enables the use of IPv6 over low-power IEEE 802.15.4 radio links. In most cases, the 6LoWPAN network is connected to the IPv6 network using an edge router.

Table of Contents

Header Format

The 6LoWPAN protocol uses the first byte of the IEEE 802.15.4 encapsulation header stack as a special Dispatch Byte. This Dispatch Byte indicates that the following bytes are a LoWPAN header instead of an IPv6 header.

Bin Header Description
00 xxxxxx NLAP Not a LoWPAN frame.
01 000001 IPv6 Uncompressed IPv6
01 000010 HC1 HC1 compressed IPv6
01 1xxxxx IPHC IPHC compressed IPv6
10 xxxxxx MESH Mesh header
11 000xxx FRAG1 Fragmentation Header

Compression Schemes

IPHC/NHC compression was defined in RFC 6282 in 2011 and updates the older HC1/HC2 compression.

Structure of a typical 6LoWPAN frame with IPv6+UDP:

+--------+---  -  -  ---+----------+---  -  -  ---+--------
|  IPHC  | Inline IPv6  | UDP NHC  | Inline UDP   | Payload
|(2 byte)|  (variable)  | (1 Byte) |  (variable)  |
+--------+---  -  -  ---+----------+---  -  -  ---+--------

IPHC Format (2 Bytes)

The frame starts with 2 IPHC bytes, where the first byte is the LoWPAN dispatch byte. Therefore the first byte starts with 011 to indicate IPHC compression. The remaining bits specify the IPv6 compression.

  0                                       1
  0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0 | 1 | 1 |  TF   |NH | HLIM  |CID|SAC|  SAM  | M |DAC|  DAM  |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Explanation of IPHC dispatch byte:

TF – 2 bit
Traffic Class and Flow Label (TF), 2 bit
  • 00: Carried Inline (ECN+DSCP+Flow)
  • 01: ECN+Flow
  • 10: ECN+DSCP
  • 11: Traffic Class and Flow Label are elided.
NH – 1 bit
Next Header compression (NH)
  • 0: Carried Inline
  • 1: next header is compressed using LOWPAN_NHC
HLIM – 2 bit
Hop Limit (HLIM)
  • 00: Carried Inline.
  • 01: 1 and elided.
  • 10: 64 and elided.
  • 11: 255 and elided.
CID – 1 bit
Context Identifier Extension (CID)
  • 0: No 1-byte CID identifier
  • 1: 1-byte identifier follows
SAC/DAC – 1 bit each
Source/Destination Address Compression
  • 0: Stateless
  • 1: Context-based
SAM/DAM – 2 bit each
Source/Destination Address Mode
If SAC/DAC=0:
  • 00: 128 b / 16 B. The full address is carried in-line.
  • 01: 64 bits. The first 64-bits of the address are are the first 64 bits of the link-local prefix padded with zeros. The remaining 64 bits are carried in-line.
  • 10: 16 bits. The first 112 bits of the address are the first 64 bits of the link-local prefix padded with zeros. The following 64 bits are 0000:00ff:fe00:XXXX, where XXXX are the 16 bits carried in-line.
  • 11: 0 bits. The address is fully elided.

If SAC/DAC=1:

  • 00: The UNSPECIFIED address, ::
  • 01: 64 bits. The address is derived using context information and the 64 bits carried in-line.
  • 10: 16 bits. The address is derived using context information and the 16 bits carried in-line.
  • 11: 0 bits. The address is fully elided and is derived using context information and the encapsulating header.
M – 1 bit
Multicast Destination (M)
  • 0: Destination is not multicast
  • 1: Destination is multicast

Context Identifier Extension (CIE)

In case CID = 1 in the IPHC bytes, an additional CIE byte directly follows the two IPHC bytes. This CIE byte is used to identify previously exchanged IPv6 contexts by a 4 bit Source Context Identifier (SCI) and a 4 bit Destination Context Identifier (DCI) instead of 128 bit for each address.

  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
|      SCI      |      DCI      |
+---+---+---+---+---+---+---+---+

NHC Format

Compression formats for different next headers are identified by a variable-length bit-pattern (within one Byte) immediately following the IPHC compressed header.

UDP LOWPAN_NHC Format

Bits 0 through 4 represent the NHC ID and '11110' indicates the specific UDP header compression ID.

  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| 1 | 1 | 1 | 1 | 0 | C |   P   |
+---+---+---+---+---+---+---+---+
Explanation of UDP LOWPAN_NHC Byte

C
Checksum:
  • 0: All 16 bits of Checksum are carried in-line.
  • 1: All 16 bits of Checksum are elided with higher-layer end-to-end integrity checks.
P
Ports:
  • 00: 16 bits. Source Port and Destination Port are carried in-line.
  • 01: All 16 bits for Source Port are carried in-line. First 8 bits of Destination Port is 0xf0 and elided. The remaining 8 bits of Destination Port are carried in-line.
  • 10: First 8 bits of Source Port are 0xf0 and elided. The remaining 8 bits of Source Port are carried in-line. All 16 bits for Destination Port are carried in-line.
  • 11: First 12 bits of both Source Port and Destination Port are 0xf0b and elided. The remaining 4 bits for each are carried in-line.

The HC1/HC2 compression was defined in RFC 4944 in 2007 and is not recommended anymore. It uses two bytes – HC1 for IPv6 and HC2 for UDP – to specify how headers are compressed.

With this compression it is possible to achieve a very small 7 byte frame for IPv6 + UDP if all compression mechanisms are used:

  1. Dispatch Byte (1 Byte)
  2. HC1 Byte (1 Byte)
  3. HC2 Byte (1 Byte)
  4. HopLimit (1 Byte)
  5. UDP Header (3 Byte)

HC1 Compression Byte

The HC1 byte is used to compress the IPv6 header.

  0   1   2   3   4   5   6   7    
+---+---+---+---+---+---+---+---+ 
| IP-SA | IP-DA |TF |  NH   |HC2| 
+---+---+---+---+---+---+---+---+ 
Explanation of HC1 Byte

IP-SA – 2 bit
IPv6 source address (bits 0 and 1):
  • 00: Prefix in-line, Interface identifier
  • 01: Prefix compressed (link-local prefix assumed).
  • 10: Interface identifier carried in-line
  • 11: Interface identifier elided
IP-DA – 2 bit
IPv6 destination address (bits 2 and 3):
  • 00: Prefix carried in-line
  • 01: Prefix compressed (link-local prefix assumed).
  • 10: Interface identifier carried in-line
  • 11: Interface identifier elided
TF – 1 bit
Traffic Class and Flow Label (bit 4):
  • 0: not compressed; full 8 bits for TC and 20 bits for FL
  • 1: Traffic Class and Flow Label are zero
NH – 2 bit
Next Header (bits 5 and 6):
  • 00: not compressed; full 8 bits are sent
  • 01: UDP
  • 10: ICMP
  • 11: TCP
HC2 – 1 bit
HC2 encoding(bit 7):
  • 0: no more header compression bits
  • 1: HC1 compression byte follows immediately

HC2 compression byte

The HC2 byte is used to compress the UDP header.

  0   1   2   3   4   5   6   7  
+---+---+---+---+---+---+---+---+
|SP |DP |LC | - | - | - | - | - |
+---+---+---+---+---+---+---+---+
Explanation of HC2 Byte

SP – 1 bit
UDP source port (bit 0): - 0: Not compressed, carried "in-line" - 1: Compressed to 4 bits.
DP – 1 bit
UDP destination port (bit 1): - 0: Not compressed, carried "in-line" - 1: Compressed to 4 bits.
LC – 1 bit
Length (bit 2): - 0: Not compressed, carried "in-line" - 1: length computed from IPv6 header length
Rest – bit 3 to 7
Reserved for future use.


Routing

6LoWPAN supports two routing methods on different layers.

References

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