Network Working Group J. Hui
Internet-Draft Arch Rock Corporation
Intended status: Standards Track October 6, 2008
Expires: April 9, 2009
Compression Format for IPv6 Datagrams in 6LoWPAN Networks
draft-ietf-6lowpan-hc-00
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This Internet-Draft will expire on April 9, 2009.
Abstract
This document specifies an IPv6 header compression format for IPv6
packet delivery in 6LoWPAN networks. The compression format relies
on shared context to allow compression of arbitrary prefixes. This
document specifies compression of well-known multicast addresses and
a framework for compressing next headers. UDP compression is
specified within this framework.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. IPv6 Header Compression . . . . . . . . . . . . . . . . . . . 4
2.1. LOWPAN_IPHC Encoding Format . . . . . . . . . . . . . . . 5
2.2. IPv6 Unicast Address Compression . . . . . . . . . . . . . 6
2.3. IPv6 Multicast Address Compression . . . . . . . . . . . . 7
2.4. 16-bit Compressed Address Ranges . . . . . . . . . . . . . 8
3. IPv6 Next Header Compression . . . . . . . . . . . . . . . . . 9
3.1. LOWPAN_NHC Format . . . . . . . . . . . . . . . . . . . . 9
3.2. LOWPAN_UDP Header Compression . . . . . . . . . . . . . . 9
3.3. ISA100_UDP Header Compression . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
The IEEE 802.15.4 standard specifies an MTU of 128 bytes, (including
the length byte) on a wireless link with a link throughput of 250
kbps or less[ieee802.15.4]. The 6LoWPAN adaptation format [RFC4944]
was specified to carry IPv6 datagrams over IEEE 802.15.4 links,
taking into account limited bandwidth, memory, or energy resources
that are expected in IEEE 802.15.4 applications. The 6LoWPAN
adaptation format defines a Mesh Addressing header to support sub-IP
forwarding, a Fragmentation header to support the IPv6 minimum MTU
requirement [RFC2460], and stateless header compression for IPv6
datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce the relatively large
IPv6 and UDP headers down to (in the best case) several bytes.
LOWPAN_HC1 is most effective for link-local unicast communication,
where IPv6 addresses carry the link-local prefix and an Interface
Identifier (IID) directly derived from IEEE 802.15.4 addresses. In
this case, both addresses may be completely elided. This scenario is
most effective when communication remains local to a mesh-under
network where any forwarding occurs below IP and all 6LoWPAN nodes
are connected by a single IP hop. Even so, LOWPAN_HC1 cannot elide
the IPv6 Hop Limit. In cases where communication only occurs over a
single IP hop, there may be cases where a common IPv6 Hop Limit is
used.
Routable addresses must be used when communicating in a route-over
network where forwarding occurs at IP or when communicating with
devices external to the 6LoWPAN network. In this scenario,
LOWPAN_HC1 requires both IPv6 source and destination addresses to
carry the prefix in-line. Furthermore, in route-over networks, the
Mesh Addressing header may not be used and the IID must be carried
in-line. However LOWPAN_HC1 requires 64-bits for the IID when
carried in-line and cannot be shortened even when it is derived
directly from the IEEE 802.15.4 16-bit short address. When sending
to an IPv6 multicast address, LOWPAN_HC1 requires the full 128-bit
multicast address to be carried in-line. Multicast addresses are
commonly used for neighbor discovery, such as in IPv6 ND.
LOWPAN_HC1 can be extended to include a LOWPAN_HC2 octet to support
compression of UDP, TCP, or ICMPv6. RFC 4944 [RFC4944] only defines
compression for UDP, where UDP ports may be compressed and the UDP
Length may be elided. However, LOWPAN_HC1 also does not provide any
flexibility in supporting future compression mechanisms for next
headers other than UDP, TCP or ICMPv6.
This document specifies a header compression format for IPv6
datagrams. This format improves on the header compression format
defined in RFC 4944 [RFC4944] by generalizing it to support a broader
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range of communication paradigms, including both mesh-under and
route-over configurations; communication to nodes internal and
external to the 6LoWPAN network; and multicast communication. This
document also defines a flexible framework for compressing arbitrary
next headers and defines UDP header compression within this
framework. This compression format carries forward the design
concepts in RFC 4944 [RFC4944], minimizing any state and relying on
shared context among all nodes in a 6LoWPAN network.
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 RFC 2119 [RFC2119].
2. IPv6 Header Compression
In this section, we define the LOWPAN_IPHC encoding format for
compressing the IPv6 header. To enable effective compression
LOWPAN_IPHC relies on information pertaining to the entire 6LoWPAN
network. LOWPAN_IPHC assumes the following will be the common case
for 6LoWPAN communication: Version is 6; Traffic Class and Flow Label
are both zero; Payload Length can be inferred from lower layers from
either the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header;
Hop Limit will be set to a well-known value by the source; addresses
assigned to 6LoWPAN interfaces will be formed using the link-local
prefix or a single routable prefix assigned to the entire 6LoWPAN
network; addresses assigned to 6LoWPAN interfaces are formed with an
IID derived directly from either the 64-bit extended or 16-bit short
IEEE 802.15.4 addresses.
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 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LOWPAN_IPHC | Uncompressed fields follow...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: LOWPAN_IPHC Header
The LOWPAN_IPHC encoding utilizes a two octets, with uncompressed
fields following, as shown in Figure 1. With the above scenario, the
LOWPAN_IPHC can compress the IPv6 header down to two octets (the
LOWPAN_IPHC encoding) with link-local communication. When
communicating over multiple IP hops, LOWPAN_IPHC can compress the
IPv6 header down to 7 octets (2-octet LOWPAN_IPHC, 1-octet Hop Limit,
2-octet Source Address, and 2-octet Destination Address).
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2.1. LOWPAN_IPHC Encoding Format
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| T |VF |NH | HLIM | rsv | SAM | SAC | DAM | DAC |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 2: LOWPAN_IPHC Encoding
T: Traffic Class (bit 0):
0: Full 8 bits for Traffic Class are carried in-line.
1: Traffic Class is elided and implicitly 0.
VF: Version and Flow Label (bit 1):
0: Full 4 bits for Version and 20 bits for Flow Label are carried
in-line.
1: Version and Flow Label are elided. Version is implicitly 6.
Traffic Class and Flow Label are implicitly 0.
NH: Next Hop (bit 2):
0: Full 8 bits for Next Hop are carried in-line.
1: Next Hop is elided and the next header is compressed using
LOWPAN_NHC, which is discussed in Section 3.
HLIM: Hop Limit (bits 3-4):
00: All 8 bits of Hop Limit are carried in-line.
01: All 8 bits of Hop Limit are elided and the Hop Limit is
assumed to be 1.
10: All 8 bits of Hop Limit are elided and the Hop Limit is
assumed to be 64.
11: All 8 bits of Hop Limit are elided and the Hop Limit is
assumed to be 255.
rsv: Reserved (bit 5-7)
SAC: Source Address Mode (bits 8-9):
00: All 128 bits of Source Address are carried in-line.
01: 64-bit Compressed IPv6 address.
10: 16-bit Compressed IPv6 address.
11: All 128 bits of Source Address are elided.
SAC: Source Address Context (bits 10-11): Identifies the compression
context when the source address is compressed. The value '00' is
reserved and indicates a link-local address.
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DAM: Destination Address Mode (bits 12-13):
00: All 128 bits of Destination Address are carried in-line.
01: 64-bit Compressed IPv6 address.
10: 16-bit Compressed IPv6 address.
11: All 128 bits of Destination Address are elided.
DAC: Destination Address Context (bits 14-15): Identifies the
compression context when the destination address is compressed.
The value '00' is reserved and indicates a link-local address.
Fields carried in-line (in part or in whole) appear in the same order
as they do in the IPv6 header format [RFC2460]. IPv6 addresses may
be compressed to 64 or 16 bits or completely elided. The IPv6
Payload Length field MUST always be elided and inferred from lower
layers using the 6LoWPAN Fragmentation header or the IEEE 802.15.4
header.
2.2. IPv6 Unicast Address Compression
IPv6 unicast addresses may be compressed to 64, 16, or 0 bits. When
an IPv6 unicast address is compressed, the compression context
identifies the value of the elided bits. A compression value of '00'
indicates the link-local prefix. The mapping between a specific
context and prefix may be obtained through simple modifications to
IPv6 Neighbor Discovery. However, the specification of those
mechanisms are out of scope of this document. Care should be taken
when renumbering a network. Nodes SHOULD only use a context after
all of its neighbors have been configured with the same context
information with high probability. New information within a context
SHOULD only be assigned after all nodes in the network have received
notification of its deprecation with high probability.
There may be cases where the compressor and decompressor are out of
sync within a context. In this cases, the decompressor may
reconstruct the IPv6 address using the incorrect prefix. To prevent
such errors, upper-layer integrity checks (e.g. psuedo-header
checksum) that cover both source and destination addresses SHOULD be
used.
When an IPv6 unicast address is compressed to 64 bits, the last 64
bits are carried in-line. When an IPv6 unicast address is compressed
to 16 bits, the last 16 bits are carried in line. Because the 16-bit
compressed form is also used for IPv6 multicast address compression,
the 16-bit address space is divided into multiple ranges. For
unicast addresses, the first bit carried in-line must be zero.
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1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Last 15 bits of IPv6 Addr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: 16-bit Compressed IPv6 Unicast Address Encoding
When an address is completely elided, the IID is inferred from lower
layers (either from the 6LoWPAN Mesh Addressing header or from the
IEEE 802.15.4 header). The prefix is inferred from the identified
context. Any remaining bits in between are implicitly zero.
To elide the IID, it MUST be derivable from IEEE 802.15.4 addresses.
An IID may be derived from the IEEE EUI-64 address by creating a
Modified EUI-64 IID from the IEEE EUI-64 address, as defined in RFC
4291 [RFC4291]. The universal/local bit in the Modified IEEE EUI-64
IID must be set to '1', indicating universal scope. An IID may also
be derived from the 16-bit short address and PAN ID, as defined in
RFC 4944 [RFC4944]. Note, however, that the most significant bit in
the short address must be zero.
2.3. IPv6 Multicast Address Compression
IPv6 multicast addresses may be compressed to 16 bits by utilizing a
different 6LoWPAN short address range. This document allocates
another range of 8192 values to be used for well-known IPv6 multicast
addresses.
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Range| Scope | Mapped Group ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Compressed IPv6 Multicast Address Encoding
Range (bits 0-2): Must be set to '101' (TBD), which identifies the
6LoWPAN short address range for compressed IPv6 multicast
addresses.
Scope (bits 3-6): 4-bit multicast scope as specified in RFC 4007
[RFC4007].
Mapped Group ID (bits 7-15): 9-bit mapped multicast group
identifier.
The full 128-bit multicast address can be reconstructed from the 16-
bit mapped multicast address. By definition, the 3-bit range
identifier indicates the well-known multicast prefix (0xFF) in
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addition to a flags field set to all zeros (indicating a permanently
assigned multicast address, that the multicast address is not
assigned based on the network prefix, and that it doesn't embed the
address of a Rendezvous Point). The 4-bit scope is carried in-line
and the 112-bit group ID is derived from the 9-bit mapped group ID
using a well-known mapping maintained by the Internet Assigned
Numbers Authority (IANA).
Nodes MUST accept both the compressed and uncompressed form of well-
known multicast addresses that they subscribe to. Doing so removes
any ambiguity of which form to use as both will work. Conversely,
nodes MUST NOT subscribe to well-known multicast addresses that are
not defined by the well-known mapping.
This document defines an initial mapping. Additional mappings
between 9-bit mapped group IDs and 112-bit group IDs may be specified
in the future.
+-------+---------+-----------------------+
| 9-bit | 112-bit | Description |
+-------+---------+-----------------------+
| 1 | 1 | All Nodes Addresses |
| 2 | 2 | All Routers Addresses |
+-------+---------+-----------------------+
9-bit to 112-bit Group ID Mapping
2.4. 16-bit Compressed Address Ranges
To use the 16-bit compressed address format for different kinds of
addresses (e.g. unicast or multicast), LOWPAN_IPHC utilizes the 16-
bit short address ranges as specified in RFC 4944. This document
specifies another range, for compressed multicast addresses.
Range 0, 0xxxxxxxxxxxxxxx: As specified in RFC 4944.
Range 2, 100xxxxxxxxxxxxx: As specified in RFC 4944.
Range 1, 101xxxxxxxxxxxxx: The remaining 13 bits represent a
compressed IPv6 multicast address, as described in Section 2.3.
Range 3, 110xxxxxxxxxxxxx: Reserved.
Range 4, 111xxxxxxxxxxxxx: Reserved.
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3. IPv6 Next Header Compression
LOWPAN_IPHC elides the IPv6 Next Header field when the NH bit is set
to 1. It also indicates the use of 6LoWPAN next header compression,
LOWPAN_NHC. The value of IPv6 Next Header is recovered from the
first bits in the LOWPAN_NHC encoding. The following bits are
specific to the IPv6 Next Header value. Figure 5 shows the structure
of an IPv6 datagram compressed using LOWPAN_IPHC and LOWPAN_NHC.
+-------------+-------------+-------------+-----------------+--------
| LOWPAN_IPHC | In-line | LOWPAN_NHC | In-line Next | Payload
| Encoding | IP Fields | Encoding | Header Fields |
+-------------+-------------+-------------+-----------------+--------
Figure 5: Typical LOWPAN_IPHC/LOWPAN_NHC Header Configuration
3.1. LOWPAN_NHC Format
Compression formats for different next headers are identified by a
variable length bit-pattern immediately following the LOWPAN_IPHC
compressed header. When defining a next header compression format,
the number of bits used SHOULD be determined by the perceived
frequency of using that format. However, the number of bits and any
remaining encoding bits SHOULD respect octet alignment. The
following bits are specific to the next header compression format.
In this document, we define a compression format for UDP headers.
+----------------+---------------------------
| var-len NHC ID | compressed next header...
+----------------+---------------------------
Figure 6: LOWPAN_NHC Encoding
3.2. LOWPAN_UDP Header Compression
This document defines a compression format for UDP headers using
LOWPAN_NHC. The LOWPAN_UDP compression format is shown in Figure 7.
Bits 0 through 5 represent the NHC ID and '111110' indicates the
specific UDP header compression encoding defined in this section.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 1 | 1 | 1 | 1 | 0 | S | D |
+---+---+---+---+---+---+---+---+
Figure 7: Compressed UDP Header Encoding
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S: Source Port (bit 6):
0: All 16 bits of Source Port are carried in-line.
1: First 12 bits of Source Port are elided and the remaining 4
bits are carried in-line. The Source Port is recovered by: P +
short_port, where P is 61616 (0xF0B0).
D: Destination Port (bit 7):
0: All 16 bits of Destination Port are carried in-line.
1: First 12 bits of Destination Port are elided and the remaining
4 bits are carried in-line. The Destination Port is recovered
by: P + short_port, where P is 61616 (0xF0B0).
Fields carried in-line (in part or in whole) appear in the same order
as they do in the IPv6 header format [RFC0768]. IPv6 addresses may
be compressed to 64 or 16 bits or completely elided. The UDP Length
field MUST always be elided and is inferred from lower layers using
the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header.
3.3. ISA100_UDP Header Compression
This document defines a compression format for UDP headers using
LOWPAN_NHC. The LOWPAN_UDP compression format is shown in Figure 8.
Bits 0 through 4 represent the NHC ID and '11110' indicates the
specific UDP header compression encoding defined in this section.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 1 | 1 | 1 | 0 | C | S | D |
+---+---+---+---+---+---+---+---+
Figure 8: Compressed ISA100.11a UDP Header Encoding
C: Checksum (bit 5):
0: All 16 bits of Checksum are carried in-line. The Checksum MUST
be included if there are no other end-to-end integrity checks
that are stronger than what is provided by the UDP checksum.
Such an integrity check MUST be end-to-end and cover the IPv6
pseudo-header, UDP header, and UDP payload.
1: All 16 bits of Checksum are elided. The Checksum is recovered
by recomputing it.
S: Source Port (bit 6):
0: All 16 bits of Source Port are carried in-line.
1: First 12 bits of Source Port are elided and the remaining 4
bits are carried in-line. The Source Port is recovered by: P +
short_port, where P is 61616 (0xF0B0).
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D: Destination Port (bit 7):
0: All 16 bits of Destination Port are carried in-line.
1: First 12 bits of Destination Port are elided and the remaining
4 bits are carried in-line. The Destination Port is recovered
by: P + short_port, where P is 61616 (0xF0B0).
Fields carried in-line (in part or in whole) appear in the same order
as they do in the IPv6 header format [RFC0768]. IPv6 addresses may
be compressed to 64 or 16 bits or completely elided. The UDP Length
field MUST always be elided and is inferred from lower layers using
the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header.
4. IANA Considerations
This document defines a new IPv6 header compression format for
6LoWPAN networks. The document allocates a new Dispatch type value
of 0x03 (TBD) for LOWPAN_IPHC.
This document reserves another 16-bit short address range from RFC
4944 for use with 16-bit compressed well-known IPv6 multicast
addresses.
This document creates a new IANA registry for mapped well-known
multicast addresses, mapping 112-bit group identifiers to compressed
9-bit ones. The registry MUST include the All Nodes Address (1) and
the All Routers Address (2).
5. Security Considerations
The definition of LOWPAN_IPHC permits the compression of header
information on communication that could take place in its absence,
albeit in a less efficient form. It recognizes that a IEEE 802.15.4
PAN may have associated with it a global prefix. How that global
prefix is assigned and managed is beyond the scope of this document.
6. Acknowledgements
Thanks to Pascal Thubert for useful discussions in helping shape the
header compression mechanisms. Thanks to Carsten Bormann for useful
feedback and discussion.
7. References
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7.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[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.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998.
[RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
March 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[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.
[ieee802.15.4]
IEEE Computer Society, "IEEE Std. 802.15.4-2006",
October 2006.
7.2. Informative References
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
July 2003.
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Author's Address
Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
San Francisco, California 94107
USA
Phone: +415 692 0828
Email: jhui@archrock.com
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Full Copyright Statement
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