Network Working Group G. Montenegro
Internet-Draft Microsoft Corporation
Intended status: Standards Track N. Kushalnagar
Expires: July 22, 2007 Intel Corp
J. Hui
D. Culler
Arch Rock Corp
January 18, 2007
Transmission of IPv6 Packets over IEEE 802.15.4 Networks
draft-ietf-6lowpan-format-09
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes the frame format for transmission of IPv6
packets and the method of forming IPv6 link-local addresses and
statelessly autoconfigured addresses on IEEE 802.15.4 networks.
Additional specifications include a simple header compression scheme
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using shared context and provisions for packet delivery in IEEE
802.15.4 meshes.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
1.2. Terms used . . . . . . . . . . . . . . . . . . . . . . . . 3
2. IEEE 802.15.4 mode for IP . . . . . . . . . . . . . . . . . . 3
3. Addressing Modes . . . . . . . . . . . . . . . . . . . . . . . 4
4. Maximum Transmission Unit . . . . . . . . . . . . . . . . . . 5
5. LoWPAN Adaptation Layer and Frame Format . . . . . . . . . . . 6
5.1. Dispatch Type and Header . . . . . . . . . . . . . . . . . 8
5.2. Mesh Addressing Type and Header . . . . . . . . . . . . . 9
5.3. Fragmentation Type and Header . . . . . . . . . . . . . . 10
6. Stateless Address Autoconfiguration . . . . . . . . . . . . . 12
7. IPv6 Link Local Address . . . . . . . . . . . . . . . . . . . 13
8. Unicast Address Mapping . . . . . . . . . . . . . . . . . . . 13
9. Multicast Address Mapping . . . . . . . . . . . . . . . . . . 15
10. Header Compression . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Encoding of IPv6 Header Fields . . . . . . . . . . . . . . 16
10.2. Encoding of UDP Header Fields . . . . . . . . . . . . . . 18
10.3. Non-Compressed Fields . . . . . . . . . . . . . . . . . . 19
10.3.1. Non-Compressed IPv6 Fields . . . . . . . . . . . . . 19
10.3.2. Non-Compressed and partially compressed UDP fields . 20
11. Frame Delivery in a Link-Layer Mesh . . . . . . . . . . . . . 20
11.1. LOWPAN_BC0 Option . . . . . . . . . . . . . . . . . . . . 21
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
13. Security Considerations . . . . . . . . . . . . . . . . . . . 24
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
15.1. Normative References . . . . . . . . . . . . . . . . . . . 25
15.2. Informative References . . . . . . . . . . . . . . . . . . 25
Appendix A. Alternatives for Delivery of Frames in a Mesh . . . . 26
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . . . . 31
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1. Introduction
The IEEE 802.15.4 standard [ieee802.15.4] targets low power personal
area networks. This document defines the frame format for
transmission of IPv6 [RFC2460] packets as well as the formation of
IPv6 link-local addresses and statelessly autoconfigured addresses on
top of IEEE 802.15.4 networks. Since IPv6 requires support of packet
sizes much larger than the largest IEEE 802.15.4 frame size, an
adaptation layer is defined. This document also defines mechanisms
for header compression required to make IPv6 practical on IEEE
802.15.4 networks, and the provisions required for packet delivery in
IEEE 802.15.4 meshes. However, a full specification of mesh routing
(the specific protocol used, the interactions with neighbor
discovery, etc) is out of scope of this document.
1.1. Requirements notation
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. Terms used
AES: Advanced Encryption Scheme
CSMA/CA: Carrier Sense Multiple Access / Collision Avoidance
FFD: Full Function Device
GTS: Guaranteed Time Service
MTU: Maximum Transmission Unit
MAC: Media Access Control
PAN: Personal Area Network
RFD: Reduced Function Device
2. IEEE 802.15.4 mode for IP
IEEE 802.15.4 defines four types of frames: beacon frames, MAC
command frames, acknowledgement frames and data frames. IPv6 packets
MUST be carried on data frames. Data frames may optionally request
that they be acknowledged. In keeping with [RFC3819] it is
recommended that IPv6 packets be carried in frames for which
acknowledgements are requested so as to aid link-layer recovery.
IEEE 802.15.4 networks can either be nonbeacon-enabled or beacon-
enabled [ieee802.15.4]. The latter is an optional mode in which
devices are synchronized by a so-called coordinator's beacons. This
allows the use of superframes within which a contention-free
Guaranteed Time Service (GTS) is possible. This document does not
require that IEEE networks run in beacon-enabled mode. In nonbeacon-
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enabled networks, data frames (including those carrying IPv6 packets)
are sent via the contention-based channel access method of unslotted
CSMA/CA.
In nonbeacon-enabled networks, beacons are not used for
synchronization. However, they are still useful for link-layer
device discovery to aid in association and disassociation events.
This document recommends that beacons be configured so as to aid
these functions. A further recommendation is for these events to be
available at the IPv6 layer to aid in detecting network attachment, a
problem being worked on at the IETF at the time of this writing.
The specification allows for frames in which either the source or
destination addresses (or both) are elided. The mechanisms defined
in this document require that both source and destination addresses
be included in the IEEE 802.15.4 frame header. The source or
destination PAN ID fields may also be included.
3. Addressing Modes
IEEE 802.15.4 defines several addressing modes: it allows the use of
either IEEE 64-bit extended addresses or (after an association event)
16-bit addresses unique within the PAN [ieee802.15.4]. This document
supports both 64-bit extended addresses, and 16-bit short addresses.
For use within 6LoWPANs, this document imposes additional constraints
(beyond those imposed by IEEE 802.15.4) on the format of the 16-bit
short addresses, as specified in Section 12. Short addresses being
transient in nature, a word of caution is in order: since they are
doled out by the PAN coordinator function during an association
event, their validity and uniqueness is limited by the lifetime of
that association. This can be cut short by expiration of the
association or simply by any mishap occurring to the PAN coordinator.
Because of the scalability issues posed by such a centralized
allocation and single point of failure at the PAN coordinator,
deployers should carefully weigh the tradeoffs (and implement the
necessary mechanisms) of growing such networks based on short
addresses. Of course, IEEE 64-bit extended addresses may not suffer
from these drawbacks, but still share the remaining scalability
issues concerning routing, discovery, configuration, etc.
This document assumes that a PAN maps to a specific IPv6 link, hence
it implies a unique prefix. Knowledge of the 16-bit PAN ID (e.g., by
including it in the IEEE 802.15.4 headers) would enable automatically
mapping it to the corresponding IPv6 prefix. One possible method is
to concatenate the 16 bits of PAN ID to a /48 in order to obtain the
64-bit link prefix. Whichever method is used, the assumption in this
document is that a given PAN ID maps to a unique IPv6 prefix. This
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complies with the recommendation that shared networks support link-
layer subnet [RFC3819] broadcast. Strictly speaking, it is multicast
not broadcast that exists in IPv6. However, multicast is not
supported natively in IEEE 802.15.4. Hence, IPv6 level multicast
packets MUST be carried as link-layer broadcast frames in IEEE
802.15.4 networks. This MUST be done such that the broadcast frames
are only heeded by devices within the specific PAN of the link in
question. As per section 7.5.6.2 in [ieee802.15.4], this is
accomplished as follows:
1. A destination PAN identifier is included in the frame, and it
MUST match the PAN ID of the link in question.
2. A short destination address is included in the frame, and it MUST
match the broadcast address (0xffff).
Additionally, support for mapping of IPv6 multicast addresses MAY be
provided as per Section 9. A full specification of such
functionality is out of scope of this document.
As usual, hosts learn IPv6 prefixes via router advertisements as per
[I-D.ietf-ipv6-2461bis]. The working group may pursue additional
mechanisms as well.
4. Maximum Transmission Unit
The MTU size for IPv6 packets over IEEE 802.15.4 is 1280 octets.
However, a full IPv6 packet does not fit in an IEEE 802.15.4 frame.
802.15.4 protocol data units have different sizes depending on how
much overhead is present [ieee802.15.4]. Starting from a maximum
physical layer packet size of 127 octets (aMaxPHYPacketSize) and a
maximum frame overhead of 25 (aMaxFrameOverhead), the resultant
maximum frame size at the media access control layer is 102 octets.
Link-layer security imposes further overhead, which in the maximum
case (21 octets of overhead in the AES-CCM-128 case, versus 9 and 13
for AES-CCM-32 and AES-CCM-64, respectively) leaves only 81 octets
available. This is obviously far below the minimum IPv6 packet size
of 1280 octets, and in keeping with section 5 of the IPv6
specification [RFC2460], a fragmention and reassembly adaptation
layer must be provided at the layer below IP. Such a layer is
defined below in Section 5.
Furthermore, since the IPv6 header is 40 octets long, this leaves
only 41 octets for upper-layer protocols, like UDP. The latter uses
8 octets in the header which leaves only 33 octets for application
data. Additionally, as pointed out above, there is a need for a
fragmentation and reassembly layer, which will use even more octets.
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The above considerations lead to the following two observations:
1. The adaptation layer must be provided to comply with IPv6
requirements of minimum MTU. However, it is expected that (a)
most applications of IEEE 802.15.4 will not use such large
packets, and (b) small application payloads in conjunction with
proper header compression will produce packets that fit within a
single IEEE 802.15.4 frame. The justification for this
adaptation layer is not just for IPv6 compliance, as it is quite
likely that the packet sizes produced by certain application
exchanges (e.g., configuration or provisioning) may require a
small number of fragments.
2. Even though the above space calculation shows the worst case
scenario, it does point out the fact that header compression is
compelling to the point of almost being unavoidable. Since we
expect that most (if not all) applications of IP over IEEE
802.15.4 will make use of header compression, it is defined below
in Section 10.
5. LoWPAN Adaptation Layer and Frame Format
The encapsulation formats defined in this section, (subsequently
referred to as the "LoWPAN encapsulation") are the payload in the
IEEE 802.15.4 MAC protocol data unit (PDU). The LoWPAN payload
(e.g., an IPv6 packet) follows this encapsulation header.
All LoWPAN encapsulated datagrams transported over IEEE 802.15.4 are
prefixed by an encapsulation header stack. Each header in the header
stack contains a header type followed zero or more header fields.
Whereas in an IPv6 header the stack would contain, in order,
addressing, hop-by-hop options, routing, fragmentation, destination
options, and finally payload [RFC2460]; in a LoWPAN header the
analogous header sequence is mesh (L2) addressing, hop-by-hop options
(including L2 broadcast/multicast), fragmentation, and finally
payload. These examples show typical header stacks that may be used
in a LoWPAN network.
A LoWPAN encapsulated IPv6 datagram:
+---------------+-------------+---------+
| IPv6 Dispatch | IPv6 Header | Payload |
+---------------+-------------+---------+
A LoWPAN encapsulated LOWPAN_HC1 compressed IPv6 datagram:
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+--------------+------------+---------+
| HC1 Dispatch | HC1 Header | Payload |
+--------------+------------+---------+
A LoWPAN encapsulated LOWPAN_HC1 compressed IPv6 datagram that
requires mesh addressing:
+-----------+-------------+--------------+------------+---------+
| Mesh Type | Mesh Header | HC1 Dispatch | HC1 Header | Payload |
+-----------+-------------+--------------+------------+---------+
A LoWPAN encapsulated LOWPAN_HC1 compressed IPv6 datagram that
requires fragmentation:
+-----------+-------------+--------------+------------+---------+
| Frag Type | Frag Header | HC1 Dispatch | HC1 Header | Payload |
+-----------+-------------+--------------+------------+---------+
A LoWPAN encapsulated LOWPAN_HC1 compressed IPv6 datagram that
requires both mesh addressing and fragmentation:
+-------+-------+-------+-------+---------+---------+---------+
| M Typ | M Hdr | F Typ | F Hdr | HC1 Dsp | HC1 Hdr | Payload |
+-------+-------+-------+-------+---------+---------+---------+
A LoWPAN encapsulated LOWPAN_HC1 compressed IPv6 datagram that
requires both mesh addressing and a broadcast header to support mesh
broadcast/multicast:
+-------+-------+-------+-------+---------+---------+---------+
| M Typ | M Hdr | B Dsp | B Hdr | HC1 Dsp | HC1 Hdr | Payload |
+-------+-------+-------+-------+---------+---------+---------+
When more than one LoWPAN header is used in the same packet, they
MUST appear in the following order:
Mesh Addressing Header
Broadcast Header
Fragmentation Header
All protocol datagrams (e.g., IPv6, compressed IPv6 headers, etc)
SHALL be preceded by one of the valid LoWPAN 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 headers, other than mesh addressing and
fragmentation, in LoWPAN consists of the dispatch value, the
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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
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
othogonality. All headers used in a LOWPAN adaptation layer SHALL be
defined in this format document.
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 is 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 7: Dispatch Type and Header
The dispatch value may be treated as an unstructured namespace. Only
a few symbols are required to represent current LoWPAN functionality.
Although some additional savings could be achieved by encoding
additional functionality into the dispatch byte, these measures would
tend to constrain the ability to address future alternatives.
Pattern Header Type
+----------+-----------------------------------------------+
| 00xxxxxx | NALP - Not a LoWPAN frame |
| 01000001 | IPv6 - uncompressed IPv6 Addresses |
| 01000010 | LOWPAN_HC1 - LOWPAN_HC1 compressed IPv6 |
| ... | reserved - Reserved for future use |
| 01010000 | LOWPAN_BC0 - LOWPAN_BC0 broadcast |
| ... | reserved - Reserved for future use |
| 01111111 | ESC - Additional Dispatch byte follows |
+----------+-----------------------------------------------+
Figure 8: Dispatch Value Bit Pattern
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NALP: Specifies that the following bits are not a part of the LoWPAN
encapsulation, and any LoWPAN node that encounters a dispatch
value of 00xxxxxx shall discard the packet. Other non-LoWPAN
protocols that wish to coexist with LoWPAN nodes should include a
byte matching this pattern immediately following the 802.15.4.
header.
IPv6: Specifies that the following header is an uncompressed IPv6
header [RFC2460].
LOWPAN_HC1: Specifies that the following header is a LOWPAN_HC1
compressd IPv6 header. This header format is defined in
Figure 15.
LOWPAN_BC0: Specifies that the following header is a LOWPAN_BC0
header for mesh broadcast/multicast support and is descrbied in
Section 11.1.
ESC: Specifies that the following header is a single 8-bit field for
the Dispatch value. Allows support for Dispatch values larger
than 127.
5.2. Mesh Addressing Type and Header
The mesh type is defined by a one-bit and zero-bit as the first two
bits. The mesh type and header is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0|O|F|HopsLft| originator address, final address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Mesh Addressing Type and Header
Field definitions are as follows:
O: This 1-bit field SHALL be zero if the Originator Address is an
IEEE extended 64 bit address (EUI-64), or 1 if it is a short 16-
bit addresses.
F: This 1-bit field SHALL be zero if the Final Destination Address is
an IEEE extended 64 bit address (EUI-64), or 1 if it is a short
16-bit addresses.
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Hops Left: This 6-bit field SHALL be decremented by each forwarding
node before sending this packet towards its next hop. The packet
is not forwarded any further if Hops Left is decremented to 0.
The value 0xF is reserved and signifies an 8-bit Deep Hops Left
field immediately following, and allows a source node to specify a
hop limit greater than 14 hops.
Originator Address: This is the link-layer address of the
Originator.
Final Destination Address: This is the link-layer address of the
Final Destination.
Note that the 'O' and 'F' bits allow for a mix of 16 and 64-bit
addresses. This is useful at least to allow for mesh layer
"broadcast", as 802.15.4 broadcast addresses are defined as 16-bit
short addresses.
A further discussion of frame delivery within a mesh is in
Section 11.
5.3. Fragmentation Type and Header
If an entire payload (e.g., IPv6) datagram fits within a single
802.15.4 frame, it is unfragmented and the LoWPAN encapsulation
should contain no fragmentation header. If the datagram does not fit
within a single IEEE 802.15.4 frame, it SHALL be broken into link
fragments. The first link fragment SHALL contain the first fragment
header (defined by one-bit as the first two bits and a zero-bit as
the third bit) shown below.
1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 0| datagram_tag | datagram_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: First Fragment
The second and subsequent link fragments (up to and including the
last) SHALL contain a fragmentation header that conforms to the
format shown below.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1| datagram_tag | datagram_size |datagram_offset|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Subsequent Fragments
datagram_tag: The value of datagram_tag (datagram tag) SHALL be the
same for all link fragments of a payload (e.g., IPv6) datagram.
The sender SHALL increment datagram_tag for successive, fragmented
datagrams. The incremented value of datagram_tag SHALL wrap from
1023 back to zero. This field is 10 bits long, and its initial
value is not defined.
datagram_size: This 11 bit field encodes the size of the entire IP
payload datagram. The value of datagram_size SHALL be the same
for all link fragments of an IP payload datagram. For IPv6, this
SHALL be 40 octets (the size of the uncompressed IPv6 header) more
than the value of Payload Length in the IPv6 header [RFC2460].
Typically, this field needs to encode a maximum length of 1280
(IEEE 802.15.4 link MTU as defined in this document), and as much
as 1500 (the default maximum IPv6 packet size if IPv6
fragmentation is in use). Therefore, this field is 11 bits long,
which works in either case.
NOTE: This field does not need to be in every packet, as one could
send it with the first fragment and elide it subsequently.
However, including it in every link fragment eases the task of
reassembly in the event that a second (or subsequent) link
fragment arrives before the first. In this case, the guarantee of
learning the datagram_size as soon as any of the fragments arrives
tells the receiver how much buffer space to set aside as it waits
for the rest of the fragments. The format above trades off
simplicity for efficiency.
datagram_offset: This field is present only in the second and
subsequent link fragments and SHALL specify the offset, in
increments of 8 octets, of the fragment from the beginning of the
payload datagram. The first octet of the datagram (e.g., the
start of the IPv6 header) has an offset of zero; the implicit
value of datagram_offset in the first link fragment is zero. This
field is 8 bits long.
The recipient of link fragments SHALL use (1) the sender's 802.15.4
source address (or the Originator Address if a Mesh Addressing field
is present), (2) the destination's 802.15.4 address (or the Final
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Destination address if a Mesh Addressing field is present), (3)
datagram_size and (4) datagram_tag to identify all the link fragments
that belong to a given datagram.
Upon receipt of a link fragment, the recipient starts constructing
the original unfragmented packet whose size is datagram_size. It
uses the datagram_offset field to determine the location of the
individual fragments within the original unfragmented packet. For
example, it may place the data payload (except the encapsulation
header) within a payload datagram reassembly buffer at the location
specified by datagram_offset. The size of the reassembly buffer
SHALL be determined from datagram_size.
If a link fragment is received that overlaps another fragment as
identified above and differs in either the size or datagram_offset of
the overlapped fragment, the fragment(s) already accumulated in the
reassembly buffer SHALL be discarded. A fresh reassembly may be
commenced with the most recently received link fragment. Fragment
overlap is determined by the combination of datagram_offset from the
encapsulation header and "Frame Length" from the 802.15.4 PPDU packet
header.
Upon detection of a IEEE 802.15.4 Disassociation event, fragment
recipients SHOULD discard all link fragments of all partially
reassembled payload datagrams, and fragment senders SHOULD discard
all not yet transmitted link fragments of all partially transmitted
payload (e.g., IPv6) datagrams. Similarly, when a node first
receives a fragment with a given datagram_tag, it starts a reassembly
timer. When this time expires, if the entire packet has not been
reassembled, the existing fragments SHOULD be discarded and the
reassembly state SHOULD be flushed. The reassembly timeout MUST be
set to a maximum of 60 seconds (this is also the timeout in the IPv6
reassembly procedure [RFC2460] ).
6. Stateless Address Autoconfiguration
This section defines how to obtain an IPv6 interface identifier.
The Interface Identifier [RFC3513] for an IEEE 802.15.4 interface may
be based on the EUI-64 identifier [EUI64] assigned to the IEEE
802.15.4 device. In this case, the Interface Identifier is formed
from the EUI-64 according to the "IPv6 over Ethernet" specification
[RFC2464].
All 802.15.4 devices have an IEEE EUI-64 address, but 16-bit short
addresses (Section 3 and Section 12) are also possible. In these
cases, a "pseudo 48-bit address" is formed as follows. First, the
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left-most 32 bits are formed by concatenating 16 zero bits to the 16-
bit PAN ID (alternatively, if no PAN ID is known, 16 zero bits may be
used). This produces a 32-bit field as follows:
16_bit_PAN:16_zero_bits
Then, these 32 bits are concatenated with the 16-bit short address.
This produces a 48-bit address as follows:
32_bits_as_specified_previously:16_bit_short_address
The interface identifier is formed from this 48-bit address as per
the "IPv6 over Ethernet" specification. However, in the resultant
interface identifier, the "Universal/Local" (U/L) bit SHALL be set to
0 in keeping with the fact that this is not a globally unique value.
For either address format, all zero addresses MUST NOT be used.
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
U/L bit.
An IPv6 address prefix used for stateless autoconfiguration
[I-D.ietf-ipv6-rfc2462bis] of an IEEE 802.15.4 interface MUST have a
length of 64 bits.
7. IPv6 Link Local Address
The IPv6 link-local address [RFC3513] for an IEEE 802.15.4 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 |
+----------+-----------------------+----------------------------+
Figure 12
8. Unicast Address Mapping
The address resolution procedure for mapping IPv6 non-multicast
addresses into IEEE 802.15.4 link-layer addresses follows the general
description in section 7.2 of [I-D.ietf-ipv6-2461bis], unless
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otherwise specified.
The Source/Target Link-layer Address option has the following forms
when the link layer is IEEE 802.15.4 and the addresses are EUI-64 or
16-bit short addresses, respectively.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- IEEE 802.15.4 -+
| EUI-64 |
+- -+
| |
+- Address -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Padding -+
| |
+- (all zeros) -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 16-bit short Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Padding -+
| (all zeros) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13
Option fields:
Type:
1: for Source Link-layer address.
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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 2
if using EUI-64 addresses, or 1 if using 16-bit short addresses.
IEEE 802.15.4 Address: The 64-bit IEEE 802.15.4 address, or the 16-
bit short address (as per the format in Section 9), in canonical
bit order. This is the address the interface currently responds
to. This address may be different from the built-in address used
to derive the Interface Identifier, because of privacy or security
(e.g., of neighbor discovery) considerations.
9. Multicast Address Mapping
An IPv6 packet with a multicast destination address DST, consisting
of the sixteen octets DST[1] through DST[16], is transmitted to the
following 802.15.4 16-bit multicast address:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0 0|DST[15]* | DST[16] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14
Here, DST[15]* refers to the last 5 bits in octet DST[15], that is,
bits 3-7 within DST[15]. The initial 3-bit pattern of "100" follows
the 16-bit address format for multicast addresses (Section 12).
This allows for multicast support within 6LoWPAN networks, but the
full specification of such support is out of scope of this document.
Example mechanisms are: flooding, controlled flooding, unicasting to
the PAN coordinator, etc. It is expected that this would be
specified by the different mesh routing mechanisms.
10. Header Compression
There is much published and in-progress standardization work on
header compression. Nevertheless, header compression for IPv6 over
IEEE 802.15.4 has differing constraints summarized as follows:
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Existing work assumes that there are many flows between any two
devices. Here, we assume that most of the time there will be only
one flow, and this allows a very simple and low context flavor of
header compression.
Given the very limited packet sizes, it is highly desirable to
integrate layer 2 with layer 3 compression, something
traditionally not done (although now changing due to the ROHC
working group).
It is expected that IEEE 802.15.4 devices will be deployed in
multi-hop networks. However, header compression in a mesh departs
from the usual point-to-point link scenario in which the
compressor and decompressor are in direct and exclusive
communication with each other. In an IEEE 802.15.4 network, it is
highly desirable for a device to be able to send header compressed
packets via any of its neighbors, with as little preliminary
context-building as possible.
Preliminary context is often required. If so, it is highly
desirable to allow building it by not relying exclusively on the
in-line negotiation phase. For example, if we assume there is
some manual configuration phase that precedes deployment (perhaps
with human involvement), then one should be able to leverage this
phase to set up context such that the first packet sent will
already be compressed.
Any new packets formats required by header compression reuse the
basic packet formats defined in Section 5 by using different dispatch
values.
10.1. Encoding of IPv6 Header Fields
By virtue of having joined the same 6LoWPAN network, devices share
some state. This makes it possible to compress headers even in the
absence of the customary context-building phase. Thus, the following
common IPv6 header values may be compressed from the onset: Version
is IPv6; both IPv6 source and destination addresses are link local;
the IPv6 interface identifiers (bottom 64 bits) for the source or
destination addresses can be inferred from the layer two source and
destination addresses (of course, this is only possible for interface
identifiers derived from an underlying 802.15.4 MAC address); the
packet length can be inferred either from layer two ("Frame Length"
in the IEEE 802.15.4 PPDU) or from the "datagram_size" field in the
fragment header (if present); both the Traffic Class and the Flow
Label are zero; and the Next Header is UDP, ICMP or TCP. The only
field in the IPv6 header that always needs to be carried in full is
the Hop Limit (8 bits). Depending on how closely the packet matches
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this common case, different fields may not be compressible thus
needing to be carried "in-line" as well (Section 10.3.1). This
common IPv6 header (as mentioned above) can be compressed to 2 octets
(1 octet for the HC1 encoding and 1 octet for the Hop Limit), instead
of 40 octets. Such a packet is compressible via the LOWPAN_HC1
format by using a Dispatch value of LOWPAN_HC1 followed by a
LOWPAN_HC1 header "HC1 encoding" field (8 bits) to encode the
different combinations as shown below. This header may be preceded
by a fragmentation header, which may be preceded by a mesh header.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HC1 encoding | Non-Compressed fields follow...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: LOWPAN_HC1 (common compressed header encoding)
As can be seen below (bit 7), an HC2 encoding may follow an HC1
octet. In this case, the non-compressed fields follow the HC2
encoding field Section 10.3.
The address fields encoded by "HC1 encoding" are interpreted as
follows:
PI: Prefix carried in-line (Section 10.3.1).
PC: Prefix compressed (link-local prefix assumed).
II: Interface identifier carried in-line (Section 10.3.1).
IC: Interface identifier elided (derivable from the corresponding
link-layer address). If applied to the interface identifier of
either the source or destination address when routing in a mesh
(Section 11), the corresponding link-layer address is that
found in the "Mesh Addressing" field (Section 5.2).
The "HC1 encoding" is shown below (starting with bit 0 and ending at
bit 7):
IPv6 source address (bits 0 and 1):
00: PI, II
01: PI, IC
10: PC, II
11: PC, IC
IPv6 destination address (bits 2 and 3):
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00: PI, II
01: PI, IC
10: PC, II
11: PC, IC
Traffic Class and Flow Label (bit 4):
0: not compressed, full 8 bits for Traffic Class and 20 bits
for Flow Label are sent
1: Traffic Class and Flow Label are zero
Next Header (bits 5 and 6):
00: not compressed, full 8 bits are sent
01: UDP
10: ICMP
11: TCP
HC2 encoding(bit 7):
0: No more header compression bits
1: HC1 encoding immediately followed by more header compression
bits per HC2 encoding format. Bits 5 and 6 determine which
of the possible HC2 encodings apply (e.g., UDP, ICMP or TCP
encodings).
10.2. Encoding of UDP Header Fields
Bits 5 and 6 of the LOWPAN_HC1 allows compressing the Next Header
field in the IPv6 header (for UDP, TCP and ICMP). Further
compression of each of these protocol headers is also possible. This
section explains how the UDP header itself may be compressed. The
HC2 encoding in this section is the HC_UDP encoding, and it only
applies if bits 5 and 6 in HC1 indicate that the protocol that
follows the IPv6 header is UDP. The HC_UDP encoding (Figure 16)
allows compressing the following fields in the UDP header: source
port, destination port and length. The UDP header's checksum field
is not compressed and is therefore carried in full. The scheme
defined below allows compressing the UDP header to 4 octets instead
of the original 8 octets.
The only UDP header field whose value may be deduced from information
available elsewhere is the Length. The other fields must be carried
in-line either in full or in a partially compressed manner
(Section 10.3.2).
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|HC_UDP encoding| Fields carried in-line follow...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: HC_UDP (UDP common compressed header encoding)
The "HC_UDP encoding" for UDP is shown below (starting with bit 0 and
ending at bit 7):
UDP source port (bit 0):
0: Not compressed, carried "in-line" (Section 10.3.2)
1: Compressed to 4 bits. The actual 16-bit source port is
obtained by calculating: P + short_port value. P is a
predetermined port number with value TBD. The short_port is
expressed as a 4-bit value which is carried "in-line"
(Section 10.3.2)
UDP destination port (bit 1):
0: Not compressed, carried "in-line" (Section 10.3.2)
1: Compressed to 4 bits. The actual 16-bit destination port is
obtained by calculating: P + short_port value. P is a
predetermined port number with value TBD. The short_port is
expressed as a 4-bit value which is carried "in-line"
(Section 10.3.2)
Length (bit 2):
0: not compressed, carried "in-line" (Section 10.3.2)
1: compressed, length computed from IPv6 header length
information. The value of the UDP length field is equal to
the Payload Length from the IPv6 header, minus the length of
any extension headers present between the IPv6 header and
the UDP header.
Reserved (bit 3 through 7)
10.3. Non-Compressed Fields
10.3.1. Non-Compressed IPv6 Fields
This scheme allows the IPv6 header to be compressed to different
degrees. Hence, instead of the entire (standard) IPv6 header, only
non-compressed fields need to be sent. The subsequent header (as
specified by the Next Header field in the original IPv6 header)
immediately follows the IPv6 non-compressed fields.
Uncompressed IPv6 addressing is described by a dispatch type
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containing an IPv6 dispatch value followed by the uncompressed IPv6
header. This dispatch type may be preceded by additional LoWPAN
headers.
The non-compressed IPv6 field that MUST be always present is the Hop
Limit (8 bits). This field MUST always follow the encoding fields
(e.g., "HC1 encoding" as shown in Figure 15), perhaps including other
future encoding fields). Other non-compressed fields MUST follow the
Hop Limit as implied by the "HC1 encoding" in the exact same order as
shown above (Section 10.1): source address prefix (64 bits) and/or
interface identifier (64 bits), destination address prefix (64 bits)
and/or interface identifier (64 bits), Traffic Class (8 bits), Flow
Label (20 bits) and Next Header (8 bits). The actual next header
(e.g., UDP, TCP, ICMP, etc) follows the non-compressed fields.
10.3.2. Non-Compressed and partially compressed UDP fields
This scheme allows the UDP header to be compressed to different
degrees. Hence, instead of the entire (standard) UDP header, only
non-compressed or partially compressed fields need to be sent.
The non-compressed or partially compressed fields in the UDP header
MUST always follow the IPv6 header and any of its associated in-line
fields. Any UDP header in-line fields present MUST appear in the
same order as the corresponding fields appear in a normal UDP header
[RFC0768], e.g., source port, destination port, length and checksum.
If either the source or destination ports are in "short_port"
notation (as indicated in the compressed UDP header), then instead of
taking 16 bits, the inline port numbers take 4 bits.
11. Frame Delivery in a Link-Layer Mesh
Even though 802.15.4 networks are expected to commonly use mesh
routing, the IEEE 802.15.4-2003 specification [ieee802.15.4] does not
define such capability. In such cases, Full Function Devices (FFDs)
run an ad hoc or mesh routing protocol to populate their routing
tables (outside the scope of this document). In such mesh scenarios,
two devices do not require direct reachability in order to
communicate. Of these devices, the sender is known as the
"Originator", and the receiver is known as the "Final Destination".
An originator device may use other intermediate devices as forwarders
towards the final destination. In order to achieve such frame
delivery using unicast, it is necessary to include the link-layer
addresses of the originator and final destinations, in addition to
the hop-by-hop source and destination.
This section defines how to effect delivery of layer 2 frames in a
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mesh, given a target "Final Destination" link-layer address.
Mesh delivery is enabled by including a Mesh Addressing header prior
to any other headers of the LoWPAN encapsulation (Section 5),
unfragmented and fragmented, a full-blown IPv6 header, or a
compressed IPv6 header as per Section 10, or any others defined
elsewhere.
If a node wishes to use a default mesh forwarder to deliver a packet
(i.e., because it does not have direct reachability to the
destination), it MUST include a Mesh Addressing header with the
originator's link-layer address set to its own, and the final
destination's link-layer address set to the packet's ultimate
destination. It sets the source address in the 802.15.4 header to
its own link-layer address, and puts the forwarder's link-layer
address in the 802.15.4 header's destination address field. Finally,
it transmits the packet.
Similarly, if a node receives a frame with a Mesh Addressing header,
it must look at the Mesh Addressing header's "Final Destination"
field to determine the real destination. If the node is itself the
final destination, it consumes the packet as per normal delivery. If
it is not the final destination, the device then reduces the "Hops
Left" field, and if the result is zero, discards the packet.
Otherwise, the node consults its link-layer routing table, determines
what the next hop towards the final destination should be, and puts
that address in the destination address field of the 802.15.4 header.
Finally, the node changes the source address in the 802.15.4 header
to its own link-layer address and transmits the packet.
Whereas a node must participate in a mesh routing protocol to be a
forwarder, no such requirement exists for simply using mesh
forwarding. Only "Full Function Devices" (FFDs) are expected to
participate as routers in a mesh. "Reduced Function Devices" (RFDs)
limit themselves to discovering FFDs and using them for all their
forwarding, in a manner similar to how IP hosts typically use default
routers to forward all their off-link traffic. For an RFD using mesh
delivery, the "forwarder" is always the appropriate FFD.
11.1. LOWPAN_BC0 Option
Additional mesh routing functionality is encoded using additional
routing header immediately following the Mesh header. In particular,
a broadcast header consists of a LOWPAN_BC0 dispatch followed by a
broadcast header consisting of a sequence number.
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1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| LOWPAN_BC0 |Sequence Number|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: Broadcast Header
Field definitions are as follows:
Sequence Number: This 8-bit field SHALL be incremented by the
originator whenever it sends a new mesh broadcast or multicast
packet. Full specification of how to handle this field is out of
scope of this document.
Further implications of such mesh-layer broadcast, e.g., whether it
maps to a controlled flooding mechanism or its role in, say, topology
discovery, is out of scope of this document.
Additional mesh routing capabilities, such as specifying the mesh
routing protocol, source routing, and so on may be expressed by
defining additional routing headers that preceed the fragmentation or
addressing header in the header stack. The full specification of
such mesh routing capabilities are out of scope of this document.
12. IANA Considerations
This document creates two new IANA registries, as discussed below.
Future assignments in these registries are to be coordinated via IANA
under the policy of "Specification Required" [RFC2434]. It is
expected that this policy will allow for other (non-IETF)
organizations to more easily obtain assignments.
This document creates a new IANA registry for the Dispatch type field
shown in the header definitions Section 5. This document defines the
values IPv6, LOWPAN_HC1 header compression, BC0 broadcast and two
escape patterns (NALP to indicate not a LOWPAN frame and ESC to allow
additional dispatch bytes). This document defines this field to be 8
bits long. The values 00xxxxxx being reserved and not used, this
allows for a total of 192 different values, which should be more than
enough. If header compression formats in addition to HC1 are defined
or if additional TCP, ICMP HC2 formats are defined, it is expected
that these will use reserved dispatch values following LOWPAN_HC1.
If additional mesh delivery formats are defined these will use
reserved values following LOWPAN_BC0.
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This document creates a new IANA registry for the 16-bit short
address fields as used in 6LoWPAN packets.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 16-bit short Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18
This registry MUST include the addresses 0xffff (16-bit broadcast
address accepted by all devices currently listening to the channel)
and 0xfffe as defined in [ieee802.15.4]. Additionally, within
6LoWPAN networks, 16-bit short addresses MUST follow this format
(referring to bit fields in the order from 0 to 7), where "x" is a
place holder for an unspecified bit value:
Range 1, Oxxxxxxxxxxxxxxx: The first bit (bit 0) SHALL be zero if
the 16-bit address is a unicast address. This leaves 15 bits for
the actual address.
Range 2, 100xxxxxxxxxxxxx: Bits 0,1 and 2 SHALL follow this pattern
if the 16-bit address is a multicast address (see Section 9).
This leaves 13 bits for the actual multicast address.
Range 3, 101xxxxxxxxxxxxx: This pattern for bits 0,1 and 2 is
reserved. Any future assignment shall follow the policy mentioned
above.
Range 4, 110xxxxxxxxxxxxx: This pattern for bits 0,1 and 2 is
reserved. Any future assignment shall follow the policy mentioned
above.
Range 5, 111xxxxxxxxxxxxx: This pattern for bits 0,1 and 2 is
reserved. Any future assignment shall follow the policy mentioned
above.
This document requests an IANA assignment of a port number P, for use
with UDP header compression (Section 10.2). This port number P is
used as the base to which the "short_port" 4-bit values are added in
order to obtain the actual UDP port used.
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13. Security Considerations
The method of derivation of Interface Identifiers from EUI-64 MAC
addresses is intended to preserve global uniqueness when possible.
However, there is no protection from duplication through accident or
forgery.
Neighbor Discovery in IEEE 802.15.4 links may be susceptible to
threats as detailed in [RFC3756]. Mesh routing is expected to be
common in IEEE 802.15.4 networks. This implies additional threats
due to ad hoc routing as per [KW03]. IEEE 802.15.4 provides some
capability for link-layer security. Users are urged to make use of
such provisions if at all possible and practical. Doing so will
alleviate the threats referred to above.
A sizeable portion of IEEE 802.15.4 devices is expected to always
communicate within their PAN (i.e., within their link, in IPv6
terms). In response to cost and power consumption considerations,
and in keeping with the IEEE 802.15.4 model of "Reduced Function
Devices" (RFDs), these devices will typically implement the minimum
set of features necessary. Accordingly, security for such devices
may rely quite strongly on the mechanisms defined at the link-layer
by IEEE 802.15.4. The latter, however, only defines the AES modes
for authentication or encryption of IEEE 802.15.4 frames, and does
not, in particular, specify key management (presumably group
oriented). Other issues to address in real deployments relate to
secure configuration and management. Whereas such a complete picture
is out of scope of this document, it is imperative that IEEE 802.15.4
networks be deployed with such considerations in mind. Of course, it
is also expected that some IEEE 802.15.4 devices (the so-called "Full
Function Devices", or "FFDs") will implement coordination or
integration functions. These may communicate regularly with off-link
IPv6 peers (in addition to the more common on-link exchanges). Such
IPv6 devices are expected to secure their end-to-end communications
with the usual mechanisms (e.g., IPsec, TLS, etc).
14. Acknowledgements
Thanks to the authors of RFC 2464 and RFC 2734, as parts of this
document are patterned after theirs. Thanks to Geoff Mulligan for
useful discussions which helped shape this document. Erik Nordmark's
suggestions were instrumental for the header compression section.
Also thanks to Shoichi Sakane, Samita Chakrabarti, Vipul Gupta,
Carsten Bormann, Ki-Hyung Kim, Mario Mao, and Phil Levis.
15. References
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15.1. Normative References
[EUI64] "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
REGISTRATION AUTHORITY", IEEE http://standards.ieee.org/
regauth/oui/tutorials/EUI64.html.
[I-D.ietf-ipv6-2461bis]
Narten, T., "Neighbor Discovery for IP version 6 (IPv6)",
draft-ietf-ipv6-2461bis-10 (work in progress),
January 2007.
[I-D.ietf-ipv6-rfc2462bis]
Thomson, S., "IPv6 Stateless Address Autoconfiguration",
draft-ietf-ipv6-rfc2462bis-08 (work in progress),
May 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[ieee802.15.4]
IEEE Computer Society, "IEEE Std. 802.15.4-2003",
October 2003.
15.2. Informative References
[I-D.ietf-ipngwg-icmp-v3]
Conta, A., "Internet Control Message Protocol (ICMPv6) for
the Internet Protocol Version 6 (IPv6) Specification",
draft-ietf-ipngwg-icmp-v3-07 (work in progress),
July 2005.
[I-D.ietf-ipv6-node-requirements]
Loughney, J., "IPv6 Node Requirements",
draft-ietf-ipv6-node-requirements-11 (work in progress),
August 2004.
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[KW03] Karlof, Chris and Wagner, David, "Secure Routing in Sensor
Networks: Attacks and Countermeasures", Elsevier's AdHoc
Networks Journal, Special Issue on Sensor Network
Applications and Protocols vol 1, issues 2-3,
September 2003.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC1042] Postel, J. and J. Reynolds, "Standard for the transmission
of IP datagrams over IEEE 802 networks", STD 43, RFC 1042,
February 1988.
[RFC3439] Bush, R. and D. Meyer, "Some Internet Architectural
Guidelines and Philosophy", RFC 3439, December 2002.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
Appendix A. Alternatives for Delivery of Frames in a Mesh
Before settling on the mechanism finally adopted for delivery in a
mesh (Section 11), several alternatives were considered. In addition
to the hop-by-hop source and destination link-layer addresses,
delivering a packet in a LoWPAN mesh requires the end-to-end
originator and destination addresses. These could be expressed
either as layer 2 or as layer 3 (i.e., IP ) addresses. In the latter
case, there would be no need to provide any additional header support
in this document (i.e., within the LoWPAN header itself). The link-
layer destination address would point to the next hop destination
address while the IP header destination address would point to the
final destination (IP) address (possibly multiple hops away from the
source), and similarly for the source addresses. Thus, while
forwarding data, the single-hop source and destination addresses
would change at each hop (always pointing to the node doing the
forwarding and the "best" next link-layer hop, respectively), while
the source and destination IP addresses would remain unchanged.
Notice that if an IP packet is fragmented, the individual fragments
may arrive at any node out of order. If the initial fragment (which
contains the IP header) is delayed for some reason, a node that
receives a subsequent fragment would lack the required information.
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It would be forced to wait until it receives the IP header (within
the first fragment) before being able to forward the fragment any
further. This imposes some additional buffering requirements on
intermediate nodes. Additionally, such a specification would only
work for one type of LoWPAN payload: IPv6. In general, it would have
to be adapted for any other payload, and would require that payload
to provide its own end-to-end addressing information.
On the other hand, the approach finally followed (Section 11) creates
a mesh at the LoWPAN layer (below layer 3). Accordingly, link-layer
originator and final destination address are included within the
LoWPAN header. This enables mesh delivery for any protocol or
application layered on the LoWPAN adaptation layer (Section 5). For
IPv6 as supported in this document, another advantage of expressing
the originator and final destinations as layer 2 addresses is that
the IPv6 addresses can be compressed as per the header compression
specified in Section 10. Furthermore, the number of octets needed to
maintain routing tables is reduced due to the smaller size of
802.15.4 addresses (either 64 bits or 16 bits) as compared to IPv6
addresses (128 bits). A disadvantage is that applications on top of
IP do not address packets to link-layer destination addresses, but to
IP (layer 3) destination addresses. Thus, given an IP address, there
is a need to resolve the corresponding link-layer address.
Accordingly, a mesh routing specification needs to clarify the
Neighbor Discovery implications, although in some special cases, it
may be possible to derive a device's address at layer 2 from its
address at layer 3 (and viceversa). Such complete specification is
outside the scope of this document.
Appendix B. Changes
Changes up to version draft-ietf-6lowpan-format-09.txt are as
follows:
Editorial changes, typos, nits.
Changes up to version draft-ietf-6lowpan-format-08.txt are as
follows:
Clarification of dispatch header name space and Mesh Delivery
Header.
Changes up to version draft-ietf-6lowpan-format-07.txt are as
follows:
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Conversion to stacked header layout analogous to IPv6 headers.
Changes up to version draft-ietf-6lowpan-format-06.txt are as
follows:
Further clarification in the reassembly procedures.
Editorial nits and corrections.
Changes up to version draft-ietf-6lowpan-format-05.txt are as
follows:
Added some padding bits to the first and subsequent fragment
formats to align on an octet boundary.
Header compression may result in alignment not falling on an octet
boundary. Since hardware typically cannot transmit units less
than an octet, added text to the effect that one lays out the
contiguous compressed headers and then zero bits SHOULD BE added
as appropriate to align to an octet boundary.
Added how to distinguish between the multicast and the unicast
formats for the mesh delivery field. We use one of the 5 reserved
bits to signal if the bcast/mcast mesh delivery format is being
used, and we called it the 'B' ("broadcast") bit. So no change to
the mesh delivery fields is required. Since the reserved bits are
common to all three lowpan header formats, the 'B' bit applies to
all.
Changes from version draft-ietf-6lowpan-format-02.txt to version
draft-ietf-6lowpan-format-03.txt are as follows:
Interface Identifier derivation using 16-bit short addresses now
using the PAN ID as well.
Word of caution on the transient nature of 16 bit short addresses.
Reassembly now also keying on destination and datagram_size.
Mesh delivery header now allowing mix of 16/64 bit addresses.
This leaves 6 bits for hops_left (64 hops is plenty).
Added optional Multicast Address mapping patterned after that of
ethernet.
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Clarified that all zero addresses must not be used (for either 16
or 64 bit formats).
Added address format section to IANA considerations to define
unicast, multicast and reserved address formats.
Added Mesh Broadcast or Multicast Delivery Field.
Created a new section on Addressing Modes.
Sundry editorial changes.
Changes from version draft-ietf-6lowpan-format-01.txt to version
draft-ietf-6lowpan-format-02.txt are as follows:
Further details on broadcast by using PAN-specific broadcast.
Sundry editorial changes.
Changes from version draft-ietf-6lowpan-format-00.txt to version
draft-ietf-6lowpan-format-01.txt are as follows:
Added a reassembly timeout of 15 sec.
Added support for 16-bit "short" addresses.
datagram tag now at 10 bits protocol_type and datagram offset both
went from 11 to 8 bits (which is still enough for the format, and
which implies counting offset in units of 8 octets for the
latter).
Addition of the originator's link-layer source address to the
"Mesh Delivery" header.
Changed name of "Final Destination" header to "Mesh Delivery"
header.
Further clarification on mesh delivery.
Sundry editorial changes.
Changes from version
draft-montenegro-lowpan-ipv6-over-802.15.4-02.txt to version
draft-ietf-6lowpan-format-00.txt are as follows:
The LoWPAN encapsulation was modified to allow 11 bits of protocol
type (prot_type field). Because of this, the minimum overhead
grew from 1 octet to 2 octets. This was done in order to allow
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more protocol types as the previous format started with a field
only 5 bits wide. Whereas growing it to 7 bits was possible in
the future, this would always entail 2 octets of overhead for the
longer protocol types to be used.
The 'M' bit had been left out of the 3rd packet format (for
subsequent fragments). Corrected this oversight. This means that
the fragment tag lost one bit.
Sundry editorial changes.
Authors' Addresses
Gabriel Montenegro
Microsoft Corporation
Email: g_e_montenegro@yahoo.com
Nandakishore Kushalnagar
Intel Corp
Email: nandakishore.kushalnagar@intel.com
Jonathan W. Hui
Arch Rock Corp
Email: jhui@archrock.com
David E. Culler
Arch Rock Corp
Email: dculler@archrock.com
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