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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 4944

Network Working Group                                      G. Montenegro
Internet-Draft                                     Microsoft Corporation
Expires: January 12, 2006                                 N. Kushalnagar
                                                              Intel Corp
                                                           July 11, 2005


        Transmission of IPv6 Packets over IEEE 802.15.4 Networks
                      draft-ietf-6lowpan-format-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on January 12, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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
   using shared context and provisions for packet delivery in IEEE
   802.15.4 meshes.




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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.  Maximum Transmission Unit  . . . . . . . . . . . . . . . . . .  4
   4.  Adaptation Layer and Frame Format  . . . . . . . . . . . . . .  5
     4.1   Link Fragmentation . . . . . . . . . . . . . . . . . . . .  5
     4.2   Reassembly . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Stateless Address Autoconfiguration  . . . . . . . . . . . . .  9
   6.  IPv6 Link Local Address  . . . . . . . . . . . . . . . . . . .  9
   7.  Unicast Address Mapping  . . . . . . . . . . . . . . . . . . .  9
   8.  Header Compression . . . . . . . . . . . . . . . . . . . . . . 10
     8.1   Encoding of IPv6 Header Fields . . . . . . . . . . . . . . 11
     8.2   Encoding of UDP Header Fields  . . . . . . . . . . . . . . 13
     8.3   Non-Compressed Fields  . . . . . . . . . . . . . . . . . . 14
       8.3.1   Non-Compressed IPv6 Fields . . . . . . . . . . . . . . 14
       8.3.2   Non-Compressed and partially compressed UDP fields . . 15
   9.  Packet Delivery in a Link-Layer Mesh . . . . . . . . . . . . . 15
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 17
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 17
   12.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
   13.   Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   14.   References . . . . . . . . . . . . . . . . . . . . . . . . . 18
     14.1  Normative References . . . . . . . . . . . . . . . . . . . 18
     14.2  Informative References . . . . . . . . . . . . . . . . . . 19
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20
       Intellectual Property and Copyright Statements . . . . . . . . 21






















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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.  Likewise, the provisions required for packet
   delivery in IEEE 802.15.4 meshes is defined.  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.

   IEEE 802.15.4 defines several addressing modes.  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.

   IEEE 802.15.4 allows the use of either IEEE 64 bit extended addresses
   or (after an association event) 16 bit addresses unique within the
   PAN.  This document assumes use of 64 bit extended addresses, but 16
   bit address support may be added in a future revision.

   This document assumes that a PAN maps to a specific IPv6 link, hence
   it implies a unique prefix.  If the PAN ID (16 bits) is included in
   the IEEE 802.15.4 headers, it may be possible to use it to
   automatically map 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 link prefix.  Whichever method is used, the assumption in
   this document is that a given PAN ID maps to a unique IPv6 prefix.
   This 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 in IEEE 802.15.4.  Hence, IPv6 level multicast packets
   MUST be carried as link-layer broadcast frames in IEEE 802.15.4
   networks.  As usual, hosts learn IPv6 prefixes via router
   advertisements ([I-D.ietf-ipv6-2461bis]).

3.  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.



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   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 4.

   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.

   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 8.

4.  Adaptation Layer and Frame Format

4.1  Link Fragmentation

   All IP datagrams transported over IEEE 802.15.4 are prefixed by an
   encapsulation header with one of the formats illustrated below.  In
   all cases, the encapsulation header size is 2 octets.  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.  Alternatively, if
   the 'M' bit is on, before this actual payload, a "Final Destination"
   field will be present (Section 9).



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   If an entire IP datagram may be transmitted within a single 802.15.4
   packet, it is unfragmented and the LoWPAN encapsulation SHALL conform
   to the format illustrated below.

   NOTE: All fields marked "reserved" or "rsv" SHALL be set to zero upon
   transmission, and ignored upon reception.

                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LF|    prot_type        |M|rsv| Payload (or Final Destination)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 1: Unfragmented encapsulation header format

   Field definitions are as follows:

   LF: This 2 bit field SHALL be zero.

   prot_type: This 11 bit field SHALL indicate the nature of the
      datagram that follows.  In particular, the prot_type for IPv6 is 1
      hexadecimal.  The value 2 hexadecimal is defined below for header
      compression (Section 8).  Other protocols may use this
      encapsulation format, but such use is outside the scope of this
      document.  Subsequent assignments are to be handled by IANA
      (Section 10).

   M: This bit is used to signal whether there is a "Final Destination"
      field as used for ad hoc or mesh routing.  If set to 1, a "Final
      Destination" field precedes the IPv6 packet  (Section 9).

   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
   conform to the format shown below.

                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LF|      prot_type      |M|datagram_tag |   datagram_size     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 2: First fragment encapsulation header format

   The second and subsequent link fragments (up to and including the
   last) SHALL conform 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LF|   datagram_offset   |M|datagram_tag |  datagram_size      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 3: Subsequent fragment(s) encapsulation header format

   Field definitions are as follows:

   LF: This 2 bit field SHALL specify the relative position of the link
      fragment within the IP datagram, as encoded by the following
      table.

             LF      Position
          +-------------------------------------------+
          |  00   |  Unfragmented         (Figure 1)  |
          |  01   |  First Fragment       (Figure 2)  |
          |  10   |  Last Fragment        (Figure 3)  |
          |  11   |  Interior Fragment    (Figure 3)  |
          +-------------------------------------------+

                     Figure 4: Link Fragment Bit Pattern


   datagram_size: This 11 bit field encodes the size of the entire IP
      datagram.  The value of datagram_size SHALL be the same for all
      link fragments of an IP datagram and SHALL be 40 octets more (the
      size of the IPv6 header) than the value of Payload Length in the
      datagram's 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.







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   prot_type: This 11 bit field is present only in the first link
      fragment.  For possible values, see Section 10.

   M: This bit present to allow delivery of link fragment in a mesh.  If
      set to 1, a "Final Destination" field is present as per Section 9.

   fragment_offset: This field is present only in the second and
      subsequent link fragments and SHALL specify the offset, in octets,
      of the fragment from the beginning of the IP datagram.  The first
      octet of the datagram (e.g., the start of the IP header) has an
      offset of zero; the implicit value of fragment_offset in the first
      link fragment is zero.  This field is 11 bits long, as per the
      datagram_size explanation above.

   datagram_tag: The value of datagram_tag (datagram tag) SHALL be the
      same for all link fragments of an IP datagram.  The sender SHALL
      increment datagram_tag for successive, fragmented datagrams; the
      incremented value of datagram_tag SHALL wrap from 127 back to
      zero.  Initial value is not defined.



   All protocol datagrams (e.g., IPv6) SHALL be preceded by one of the
   LoWPAN encapsulation headers described above.  This permits uniform
   software treatment of datagrams without regard to the mode of their
   transmission.

4.2  Reassembly

   The recipient of an IP datagram transmitted via more than one
   802.15.4 packet SHALL use both the sender's 802.15.4 source address
   and datagram_tag to identify all the link fragments from a single
   datagram.

   Upon receipt of a link fragment, the recipient may place the data
   payload (except the encapsulation header) within an IP datagram
   reassembly buffer at the location specified by fragment_offset.  The
   size of the reassembly buffer SHALL be determined from datagram_size.

   If a link fragment is received that overlaps another fragment
   identified by the same source address and datagram_tag, 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 fragment_offset from the encapsulation header and
   data_length from the 802.15.4 packet header.

   Upon detection of a IEEE 802.15.4 Disassociation event, the



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   recipient(s) SHOULD discard all link fragments of all partially
   reassembled IP datagrams, and the sender(s) SHOULD discard all not
   yet transmitted link fragments of all partially transmitted IP
   datagrams.

5.  Stateless Address Autoconfiguration

   The Interface Identifier [RFC3513] for an IEEE 802.15.4 interface is
   based on the EUI-64 identifier [EUI64] assigned to the IEEE 802.15.4
   device.  The Interface Identifier is formed from the EUI-64 according
   to the "IPv6 over Ethernet" specification [RFC2464].

   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.

6.  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 5


7.  Unicast Address Mapping

   The procedure for mapping IPv6 unicast addresses into IEEE 802.15.4
   link-layer addresses is described in [I-D.ietf-ipv6-2461bis].

   The Source/Target Link-layer Address option has the following form
   when the link layer is IEEE 802.15.4.







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                       0                   1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |     Type      |    Length     |
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |                               |
                      +-        IEEE 802.15.4        -+
                      |                               |
                      +-                             -+
                      |                               |
                      +-         Address             -+
                      |                               |
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |                               |
                      +-         Padding             -+
                      |                               |
                      +-        (all zeros)          -+
                      |                               |
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 6

   Option fields:

   Type:
      1: for Source Link-layer address.
      2: for Target Link-layer address.

   Length: 2.  This is the length of this option (including the type and
      length fields) in units of 8 octets.

   IEEE 802.15.4 Address: The 64 bit IEEE 802.15.4 address, 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.


8.  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:

      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.



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      Given the very limited packet sizes, it is highly desirable to
      integrate layer 2 with layer 3 compression, something typically
      not done.

      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 4 by using different values
   for the prot_type (defined below).

8.1  Encoding of IPv6 Header Fields

   However, it is possible to use header compression even in advance of
   setting up the customary state.  Thus, the following common IPv6
   header values may be compressed from the onset: Version is IPv6, both
   IPv6 source and destination are link local, the IPv6 bottom 64 bits
   can be inferred from the layer two source and destination, the packet
   length can be inferred from the layer two, both the Traffic Class and
   the Flow Label are zero, and the Next Header is UDP, ICMP or TCP.
   Thus, the IPv6 header info that always needs to be carried is the Hop
   Limit (8 bits).  Depending on how closely the packet matches this
   common case, different fields may not be compressible thus needing to
   be carried "in-line" as well (Section 8.3.1).  Thus this common IPv6
   header 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 (assigned a prot_type value
   of 2 hexadecimal).  It uses the "HC1 encoding" field (8 bits) to
   encode the different combinations as 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | HC1 encoding  |     Non-Compressed fields follow...           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 7: 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 8.3.

   The address fields encoded by "HC1 encoding" are interpreted as
   follows:

      PI: Prefix carried in-line (Section 8.3.1).
      PC: Prefix compressed (link-local prefix assumed).
      II: Interface identifier carried in-line (Section 8.3.1).
      IC: Interface identifier elided (to be derived from the
         corresponding link-layer address).  If applied to the
         destination interface identifier when routing in a mesh
         (Section 9), the corresponding link-layer address is that found
         in the "Final Destination" field (Figure 9).

   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):
         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








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      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).

8.2  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 8)
   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 8.3.2).

                           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 8: 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):








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      UDP source port (bit 0):
         0: Not compressed, carried "in-line" (Section 8.3.2)
         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 8.3.2)

      UDP destination port (bit 1):
         0: Not compressed, carried "in-line" (Section 8.3.2)
         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 8.3.2)

      Length (bit 2):
         0: not compressed, carried "in-line" (Section 8.3.2)
         1: compressed, length computed from IPv6 header length
            information (similar to how the length of the header is
            calculated in TCP

      Reserved (bit 3 through 7)

   Note: TCP, ICMP HC2 formats TBD.

8.3  Non-Compressed Fields

8.3.1  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.

   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 7), 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 8.1): 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.





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8.3.2  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.

9.  Packet Delivery in a Link-Layer Mesh

   IEEE 802.15.4-2003 [ieee802.15.4] does not define a mesh routing
   capability.  Nevertheless, it is expected that most 802.15.4 networks
   will use mesh routing.  In such cases, an ad hoc or mesh routing
   procotol populates the devices' routing tables.  A device that wishes
   to send a packet may, in such cases, use other intermediate devices
   as forwarders towards the final destination.  In order to achieve
   such packet delivery using unicast, it is necessary to include the
   final destination in addition to the hop-by-hop destination.  This
   final destination may be expressed either as a layer 2 or as an IP
   (layer 3) address.

   In the latter case, there is no need to provide any additional header
   support in this document (i.e., at the sub-IP layer).  The link-layer
   destination address points to the next hop destination address while
   the IP destination address points to the final destination (IP)
   address (that may be multiple hops away from the source).  Thus,
   while forwarding data, the single-hop destination address changes
   hop-by-hop pointing to the "best" next hop, while the destination IP
   address remains unchanged.

   If creating a mesh at the link-layer (layer 2), there is a need to
   include the link-layer final destination address within the packet.
   The advantage of expressing the final destination as a layer 2
   addresses is that the IPv6 destination address can be compressed as
   per the header compression specified in Section 8, thus saving 8
   octets.  Another advantage is that the the number of octets needed to
   maintain routing tables is reduced.  A disadvantage is that
   applications do not address packets to link-layer destination
   addresses, but to IP (layer 3) addresses.  Thus, given an IP address,
   there is a need to resolve the corresponding link-layer address.  A
   mesh routing specification needs to clarify the Neighbor Discovery
   implications, although in some special cases, it may be possible to
   derive one address from the other.  Such complete specification is
   outside the scope of this document.



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   This document merely defines how to effect packet delivery in a mesh,
   given a target link-layer address.

   This is the purpose of the 'M' bit that immediately follows the
   'prot_type' field.  If the 'M' bit is set, there is a "Final
   Destination" field included in the packet immediately following the
   current header (e.g., possibly preceding any existing header
   compression fields).  This implies that the "Final Destination" field
   will immediately follow an unfragmented packet as per Figure 1 (i.e.,
   preceding the IPv6 Header), or immediately following the first
   fragment header as per Figure 2.

   If a node wishes to use a forwarder to deliver a packet, it puts the
   forwarder's link-layer address in the link-layer destination address
   field.  It must also set the 'M' bit, and include a "Final
   Destination" field with the final destination's link-layer address.
   Similarly, if a node receives a frame with the 'M' bit set, it must
   look at the "Final Destination" field to determine the real
   destination.  Upon consulting its routing table, it determines what
   the next hop towards that destination should be.  The node then
   reduces the "Hops Left" field.  If the result is zero, the node
   discards the packet.  Otherwise, it puts the next hop's address in
   the link-layer destination address field, and transmits the packet.
   If upon examining the "Final Destination" field the node determines
   that it has direct reachability, it removes the "Final Destination"
   field, sets that final address as the link-layer destination address,
   and transmits the packet.

   The "Final Destination" field is defined as follows:

                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |S| Hops Left   |      Address of final destination             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 9: Final Destination Field

   Field definitions are as follows:

   S: This bit field SHALL be zero.  Future revisions will use this bit
      to signal the use of a short 16 bit address instead of the default
      IEEE extended 64 bit address format.








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   Hops Left: This 7 bit field SHALL be decremented by each forwarding
      node before sending this packet towards its next hop.  The packet
      is discarded if Hops Left is decremented to 0.

   Address: This is the final destination's link-layer address.  This
      document assumes that this field is 64 bits long, but a future
      revision may add support for short addresses (16 bits).

10.  IANA Considerations

   This document creates a new IANA registry for the prot_type (Protocol
   Type) field shown in the packet formats in Section 4.  This document
   defines the values 1 and 2 hexadecimal for IPv6 and the LOWPAN_HC1
   header compression format, respectively.  Future assignments in this
   field 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 defines this field to be 5 bits long.
   The value 0 being reserved and not used, this allows for a total of
   31 different values.  If there is a need for more assignments, future
   specifications may lengthen this field, e.g., by overloading the
   packet format in Figure 2 (Section 4).

11.  Security Considerations

   The method of derivation of Interface Identifiers from 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



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   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).

12.  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 and Samita Chakrabarti.

13.  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
      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.

14.  References

14.1  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)",



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              draft-ietf-ipv6-2461bis-03 (work in progress), May 2005.

   [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.

14.2  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-v3-06 (work in progress),
              November 2004.

   [I-D.ietf-ipv6-node-requirements]
              Loughney, J., "IPv6 Node Requirements",
              draft-ietf-ipv6-node-requirements-11 (work in progress),
              August 2004.

   [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.



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   [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.


Authors' Addresses

   Gabriel Montenegro
   Microsoft Corporation

   Email: gabriel_montenegro_2000@yahoo.com


   Nandakishore Kushalnagar
   Intel Corp

   Email: nandakishore.kushalnagar@intel.com






















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