6Lo Working Group                                                  G. Li
Internet-Draft                                                    D. Lou
Intended status: Experimental                                 L. Iannone
Expires: 3 December 2022                                          Huawei
                                                                  P. Liu
                                                                 R. Long
                                                            China Mobile
                                                             1 June 2022

   Native Short Addressing for Low power and Lossy Networks Expansion


   This document specifies a topological addressing scheme, Native Short
   Address (NSA) that enables IP packet transmission over links where
   the transmission of a full length address may not be desirable.
   Furthermore, packet forwarding is stateless, meaning that no routing
   table needs to be built, rather, the forwarding decision is based
   solely on the destination address structure.  This document focuses
   on carrying IP packets across an LLN (Low power and Lossy Network),
   in which the topology is static, where nodes' location is fixed, and
   the connection between nodes is also rather stable.  This
   specifications details the NSA architecture, address allocation,
   forwarding mechanism, header format design, including length-variable
   fields, and IPv6 interconnection support.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 3 December 2022.

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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   4
   4.  NSA Allocation  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  NSA Addresses and IPv6 Addresses  . . . . . . . . . . . .  11
     4.2.  Limitation of Number of Children Nodes  . . . . . . . . .  12
   5.  Forwarding in a NSA Network . . . . . . . . . . . . . . . . .  12
     5.1.  Forwarding toward an NSA endpoint . . . . . . . . . . . .  13
     5.2.  Forwarding toward an external IPv6 node . . . . . . . . .  15
   6.  NSA Header Format . . . . . . . . . . . . . . . . . . . . . .  15
   7.  NSA Control Message . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  New Control Message . . . . . . . . . . . . . . . . . . .  17
     7.2.  Address Configuration based on 6LOWPAN-ND . . . . . . . .  18
       7.2.1.  NSA Request Address Option (NRAO) Format  . . . . . .  18
       7.2.2.  NSA Assign Address Option (NAAO) Format . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  Dispatch Type Field . . . . . . . . . . . . . . . . . . .  20
     8.2.  Allocation Function Registry  . . . . . . . . . . . . . .  20
     8.3.  ICMP NSA Control Message  . . . . . . . . . . . . . . . .  21
     8.4.  NSA Neighbor Discovery Options  . . . . . . . . . . . . .  21
   9.  Reliability Considerations  . . . . . . . . . . . . . . . . .  21
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     12.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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1.  Introduction

   There is an ongoing massive expansion of the network edge that is
   driven by the "Internet of Things" (IoT), especially over low-power
   links which often, in the past, did not support IP packet

   Particularly driven by the requirements stemming from Industry 4.0
   and Smart City deployments, more and more devices/things are
   connected to the Internet.  Sensors in plants/parking bays/mines,
   temperature/humidity/flash sensors in museums, normally are located
   in a fixed position and are networked by low power and lossy links
   even in hard-wired networks.  Comparing with traditional scenarios,
   scalability of the (edge) network along with lower power consumption
   are key technical requirements.  Moreover, large-scale Low power
   Lossy Networks (LLNs) are expected to be able to carry IPv6 packets
   over their links, together with an efficient access to native IPv6

   The work in [SIXLOWPAN]/[SIXLO]/[LPWAN] Working Groups addresses many
   fundamental issues for those type of deployments.  Those deployments
   can be considered an instantiation of what [RFC8799] defines as
   "limited domains".  For instance, the 6lowpan compression technology
   ([RFC4944] and [RFC6282]) addresses the problem of IPv6 transmission
   over LLNs, making it possible to interconnect IPv6-based IoT networks
   and the Internet.  [RFC8138] introduces a framework for implementing
   multi-hop routing on an LLN using a compressed routing header, which
   works also with RPL (Routing Protocol for LLNs [RFC6550]).  This
   technique enables the ability to forward IPv6 packets within the
   domain without the need of decompression.  In addition, SCHC (Generic
   Framework for Static Context Header Compression and Fragmentation
   [RFC8724]) enables even more compression by using a common static

   Although aforementioned technologies are suitable in general for all
   IoT scenarios, there could be more simplified solutions for those
   scenarios and applications with static network topologies and stable
   network connections leveraging on wired technologies
   [I-D.ietf-6lo-use-cases] (e.g.  PLC [I-D.ietf-6lo-plc] or MS/TP
   [RFC8163], and Industrial IoT technologies like [RS485], etc.).  In
   those kinds of deployments, topologies are planned in advance and
   well provisioned, with sensor nodes usually fixed in specific
   locations.  This draft presents a topology based addressing mechanism
   with shorter packet header and simpler forwarding rules for those
   static IoT networks.

   The specifications in this document leverage on the dispatch type
   field ([RFC4944], [RFC8025]) that allows to accommodate the proposed

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   address format.  This means that except the addresses (source and
   destination) the other fields of the header will be compressed mostly
   according to LOWPAN_IPHC.  The proposed addressing is independent of
   Unique Local Addresses [RFC4193], which has a dependency on specific
   link-layer conventions [RFC6282].  It is also different from stateful
   address allocation that requires all nodes to obtain addresses from a
   centralized DHCP server, which leads to increased network startup
   time and consumption of extra bandwidth.  Compared to RPL-based
   routing [RFC6550], NSA avoids the extra overhead of address
   assignment by integrating address assignment and tree forming
   together.  Furthermore, NSA provides much smaller forwarding table
   size than storing mode RPL.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] and [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Architectural Overview

   Native Short Address (NSA) is an efficient topology-based network
   layer address assignment and packet forwarding mechanism.  The NSA
   nodes are aware of their own IPv6 address, constructed by IPv6 prefix
   and the NSA (see Section 4.1 and Section 5.2).  Inside the NSA
   domain, nodes communicate with each other by using only NSA
   addresses.  It is a smaller addressing space compared to the huge
   IPv6 addressing space.  The NSA enables stateless forwarding.  When
   IPv6 communication occurs between nodes inside the NSA domain and
   external IPv6 nodes, the border router, which plays as well the role
   of "root" in the addressing tree, performs network address
   translation (as per Section 5.2 and [RFC6282]).  The architecture of
   NSA network is showed in Figure 1.

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                  /|\               Internet (IPv6)
                   |               --------+--------
     IPv6 Domain   |                       |
                   |                       |
                   |               +-------+-------+
   ----------------------------    | Border Router |
                   |               |   (NSA Root)  |
                   |               +---------------+
                   |                         O
                   |                O         O   O
                   |                  O  O
                   |             O                    O
     NSA Domain    |                           O
                   |          O      O  O          O    O   O
                   |               O
                   |                      O   O
                   |                                  O
                   |               O
                               Low-Power and Lossy Network

            Figure 1: The architecture of general NSA networks.

   In the NSA network, there are 3 types of nodes, the root node, the
   forwarder node and the leaf node.  There is typically only one root
   node in the NSA network.

   *  Root Node: The root node is responsible for the management of the
      whole NSA network and routing/forwarding both internal and
      external traffic.  It stores the IPv6 prefix of the domain in
      order to perform the network address translation for external
      communications.  It also stores the address Allocation Function
      (AF) and performs the address assignment for its children.  After
      successful address assignment, the root will keep the state of its
      direct children.  The root node functions as gateway between the
      NSA domain and the Internet.  As such it also operates the
      translation between NSA header and IPv6 header (cf.  Section 5).

   *  Forwarder Nodes: A forwarder is a node, different from the root
      node, containing at least one child.  A forwarder node is
      basically the root of a subtree and its role is to forward traffic
      between its parent and its children according to the addressing.
      When handling a packet, if the destination is in one of its

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      subtrees, it forwards the packet to the right child, otherwise it
      simply sends it to its parent.

   *  Leaf Nodes: A leaf node is a node with no children.  Its operation
      is simple since it is either a destination or source of every
      packet it handles.  If it is the source of packets, it simply
      sends the packets to its parent.

   Each node acquiring a native short address needs to send an Address
   Request (AR) message to its link layer neighbors and wait for the
   response.  In the AR message, the node needs to designate a 'role'
   value (forwarder or leaf) and the "node-id".  The latter is a unique
   identifier of each NSA node, including root, forwarders, and leaves.
   This document assumes the use of the link-layer address of the node
   as 'node-id'.

   Forwarder and Leaf roles can be assigned similarly to IEEE 802.15.4,
   which distinguishes between Full-Function Devices (FFD) and reduced
   function devices (RFD) (cf., [ZigBee]).  If a neighbor is neither a
   forwarder nor the root, it will drop the AR message silently.
   Otherwise, the neighbor will calculate an address based on parameters
   in the AR message.  After the neighbor node assigns an address to the
   node, using a Allocation function (AF), it stores the suffix of that
   address as the interface ID towards the node.  Then, it generates and
   sends Address Assignment (AA) message back and becomes the parent

   This address assignment relies on the base mechanism described in
   6lowpan-ND ([RFC6775]), but defines two new options of ND message,
   whose format is defined in Section 7.2.1 and Section 7.2.2.

   The acceptance of the address assignment follows "first come first
   serve" principle.  Once a node receives a valid AA response, it uses
   that assigned address as its own network layer address, thus becomes
   a child of the address assigner.  It will then ignore replies from
   other neighbors.

   If a node does not receive any response after
   RTR_SOLICITATION_INTERVAL (10 seconds defined in [RFC6775]), it will
   send the AR message again.  It is RECOMMENDED that nodes re-send the
   AR message up to MAX_RTR_SOLICITATIONS (3 transmissions defined in
   [RFC6775]), if no answer is received, they SHOULD stop.

   The overall design objective is centered on reducing the size (or
   completely avoid the usage) of routing/forwarding table with a
   topological addressing scheme.  NSA eliminates compression/
   decompression of the address and also reduces the amount of
   information synchronization messages, so it actually reduces

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   computation complexity during packets parsing and forwarding.  As
   such, NSA may save communication energy in an IoT LLN network.

   NSA uses a context-independent address encoding mechanism.  It does
   not carry any field about address context in the packet.  It carries
   source and destination addresses as variable length fields whose size
   can be reduced to one octet each in the best case.  This allows the
   NSA packet header to be smaller than LOWPAN_IPHC's 7 octets (see
   Figure 2), down to 4 octets, representing around 40% reduction in the
   header size.

     0   1   2   3   4   5   6   7      0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+  +---+---+---+---+---+---+---+---+
   | 0 | 1 | 0 | 1 |   TF  |NH |HLM|  | 0 | 1 | 1 |  TF   |NH | HLIM  |
   +---+---+---+---+---+---+---+---+  +---+---+---+---+---+---+---+---+
   |Payload Length(variable length)|  |CID|SAC|  SAM  | M |DAC|  DAM  |
   +---+---+---+---+---+---+---+---+  +---+---+---+---+---+---+---+---+
   |I/O|AM |     Src(var len)         |      SCI      |      DCI      |
   +---+---+---+---+---+---+---+---+  +---+---+---+---+---+---+---+---+
           |   Dst (var len)          |                               |
   + ~ ~ ~ +---+---+---+---+ ~ ~ ~ +  +         Source Address        +
                                      |                               |
                                      |                               |
                                      +       Destination Address     +
                                      |                               |

                                        b. IPHC best case header
      a. NSA best case header              with context-based encoding
                                           and global unicast address

         Figure 2: Best case of NSA and LOWPAN_IPHC packet header.

   There are three distinct NSA features that allow NSA to be efficient,

   1.  Native Short Address allocation (see Section 4),

   2.  Stateless forwarding (see Section 5),

   3.  Compact header format design (see Section 6) that avoids context
       and compression.

4.  NSA Allocation

   The basic rules of allocation include:

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   *  Each node's address is prefixed by their parent's address.

   *  The root/forwarder runs an AF (Allocation Function) to generate
      its children's addresses.

   *  All nodes run the same AF in the same network instance.

   *  The maximum length of the NSA address should not exceed 64-bit.

   Normally, the root role is assigned to the border router when the LLN
   bootstraps.  An example of a possible result of an NSA deployment is
   shown in Figure 3.

                            root           +--------------------------+
                                 1         | append more bits to form |
                                 O ----+   | brother's address        |
                              /  |  \   \  +--------------------------+
                             /   |   \    \
                            /    |    \     \
         +---------+      /      |     \      \
         |forwarder| 10 /       11   110\       \  111
         |node     |  O -        O       O        O
         +---------+/ |\ \               | \
                  /   | \  \             |  \
                /     |  \    \          O   O
              /       |    \    \
          100/    1010|   101   1011  +--------------+
            O         O      O      O  |Prefix is '10'|
           /|        /|                +--------------+
          / |       / |
         O  O      O  O
      1001 10011 10101 101011

             Figure 3: An example of NSA addresses allocation.

   The allocation function AF(role,i) used in this document is defined
   in Figure 4.  Every forwarder node stores and maintain two indexes,
   one for the children that are forwarders and one for the children
   that are leaves (starting at 0 for the first child in each role).
   Let's call the first index 'f', as of forwarder, and the second 'l'
   as for leaves.  The '+' symbol indicates a concatenation operation.
   The b() operation indicates the binary string of '1' with length
   equal to its argument, for instance b(3) returns '111'.

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        AF(role, f, l) = 'address of the node performing the function'
                       + (role == leaf? b(l++):b(f++))
                       + (role == leaf?'1':'0'),
        in which, f and l are the indexes of respectively the forwarders
        and the leaves at this layer (starting at 0).

          Figure 4: Definition of the Allocation Function (AF) of
                           forwarder/root nodes.

   Taking the example of the topology in Figure 3, the proposed AF works
   as follows.

   At the top level, there are 4 children of root, two are forwarders
   and the other two are leaves.  Starting from the left most node and
   moving to the right, the root node applies the AF as follows:

   *  For the first child, which is a forwarder:

      -  A('forwarder', 0, 0) = '1'(root address) + b(0) + '0' = '1' +
         '' + '0' = 10

      -  Index f is increased by one and is now equal 1 (f=1)

   *  For the second child, which is a leaf:

      -  A('leaf', 1, 0) = '1'(root address) + b(0) + '1' = '1' + '' +
         '1' = 11

      -  Index l is increased by one and is now equal 1 (l=1)

   *  For the third child, which is a forwarder:

      -  A('forwarder', 1, 1) = '1'(root address) + b(1) + '0' = '1' +
         '1' + '0' = 110

      -  Index f is increased by one and is now equal 2 (f=2)

   *  For the fourth child, which is a leaf:

      -  A('leaf', 2, 1) = '1'(root address) + b(1) + '1' = '1' + '1' +
         '1' = 111

      -  Index l is increased by one and is now equal 2 (l=2)

   The first level addresses have now been assigned.  Let's now have a
   look how the node 10 (the first forwarder child of the root) applies
   the same Allocation Function.  Note that node 10 will use its own 'f'

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   and 'l' indexes initialized to 0.  Starting again from the left most
   node, node 10 applies the AF as follows:

   *  For the first child, which is a forwarder:

      -  A('forwarder', 0, 0) = '10'(node address) + b(0) + '0' = '10' +
         '' + '0' = 100

      -  Index f is increased by one and is now equal 1 (f=1)

   *  For the second child, which is a leaf:

      -  A('leaf', 1, 0) = '10'(node address) + b(0) + '1' = '10' + '' +
         '1' = 101

      -  Index l is increased by one and is now equal 1 (l=1)

   *  For the third child, which is a forwarder:

      -  A('forwarder', 1, 1) = '10'(node address) + b(1) + '0' = '10' +
         '1' + '0' = 1010

      -  Index f is increased by one and is now equal 2 (f=2)

   *  For the fourth child, which is a leaf:

      -  A('leaf', 2, 1) = '10'(node address) + b(1) + '1' = '10' + '1'
         + '1' = 1011

      -  Index l is increased by one and is now equal 2 (l=2)

   Note how the children of the same parent all have the same prefix (10
   in this example).  The proposed AF algorithmically assigns addresses
   to the different nodes without the need to know the topology in
   advance.  However, the largest address of the network will depend on
   the actual topology.  Indeed, the maximum length of an address with
   the proposed AF grows linearly at each level of the tree with the
   number of siblings from the same parent.  Let's take again the
   example in Figure 3 and let's assume that the children of node 10 are
   all leaves, for the largest address we need 2 bits to encode the
   parent node prefix (10 in this case) to which we need to add a number
   of '1' equal to the value of the l index which is the number of
   leaves minus one (because the first leaf has index 0), in this case
   since there are 4 leaves, the index value is 3 and we add the '111'
   string, hence the address length would be 6 (2 for the prefix, 3 to
   encode the 4th leaf address, and one for the final 1 the ends all
   leaves addresses).  In a more formal way the maximum address length
   at each level can be calculated as:

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   Max_Length = length(Parent address)
                + 1

   Where f and l are the indexes counting respectively the forwarders
        and the leaves at this level.

   The Allocation Function can be different from the one defined in
   Figure 4, but all nodes know which one to use by configuration.  The
   use of one and only one AF is allowed in an NSA domain.  It is
   RECOMMENDED that implementations support at least the AF proposed in
   this document (cf.  Section 8).

   Different allocation functions may, for example, leverage on a priori
   knowledge of the topology in order to optimize the maximum address
   size and make it smaller.  For instance, because the order of address
   allocation has an impact on the size, the address of children with
   the largest subtree should be allocated in the first place so to
   reduce the average address length of the whole subtree.  Also,
   knowing the traffic in advance, or being able to have an estimation,
   can help to minimize the size of addresses that have a lot of
   traffic.  This kind of optimization can be an option, the
   specification of optimizations is out of the scope of this document
   and may be defined in new Allocation Functions to be added to the
   "Allocation Function Registry" (see Section 8).

4.1.  NSA Addresses and IPv6 Addresses

   Obtaining a full IPv6 address from a NSA address is pretty
   straightforward.  First the NSA address is concatenated to the
   configured IPv6 prefix.  Since the length of the NSA address is
   smaller than or equal to 64 bits (the interface ID length in IPv6),
   the node needs to pad it with zeros ('0') used as most significant
   bits.  The full IPv6 address will look like: IPv6 prefix +
   "000...000" + NSA (or in IPv6 notation <IPv6 Prefix>::<NSA>).  The
   NSA is assigned by the root/forwarder as previously described.

   In an IPv6 communication, the node will derive the NSA address as the
   short source address from its own IPv6 address by simply removing the
   IPv6 prefix and all leading zeros before the NSA part.  The node will
   compare the destination IPv6 address with its own IPv6 address.  If
   they have the same prefix, it means that the destination is in the
   local NSA domain and its corresponding NSA address will be extracted
   as the short destination address (and the I/O Flag can be set
   accordingly).  Otherwise, it will be a communication towards the
   Internet.  In that case, a mapping mechanism implemented in the root
   node will generate a short address to be mapped to the full IPv6
   destination address.  For instance, the mapped short address can be

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   generated using the least significant bits of the original IPv6
   address.  As previously stated, the mapping mechanism is out of the
   scope of this document.

   Since the short mapped address is generated on the root, when the
   node first opens the connection toward the external site, with a
   first packet, the destination address is set to the full,
   uncompressed, IPv6 address.  Once the packet arrives at the root
   node, by performing the destination address lookup, the root will
   notice that a full IPv6 address is being used and will trigger the
   short address generation mechanism and create a new mapping.  Such a
   mapping is communicated to the source node via a new dedicated ICMP
   message (see Section 7).  Once the node originating the communication
   receives such a message it SHOULD use the mapped short address for
   any further communication.

   NSA does not prevent the normal checksum calculation for the
   transport layer (namely TCP or UDP) or IPSec encapsulation.  Indeed,
   any NSA node is aware of its full IP address, which can be used for
   the calculation.  For communication to/from the Internet, NSA nodes
   store the mappings between the external remote address and the short
   mapped address, hence checksum calculation can be performed as usual.

4.2.  Limitation of Number of Children Nodes

   The maximum number of child nodes is determined by the specific AF
   used.  IEEE 802.15.5 has explored the use of a per-branch setup,
   which, however, incurs scalability problems [LEE10].  NSA allocation
   design is more flexible and extensible than the one proposed in IEEE
   802.15.5.  The AF used as example in this document does not need any
   specific setup network by network, though it is still limited by the
   maximum length of addresses.  For the special case of the parent
   connecting to huge amount of children, a variant of the proposed AF
   can be designed to fulfill the requirement and optimize the address
   allocation (as previously described).

5.  Forwarding in a NSA Network

   Internal and external communications in an NSA network work slightly
   different.  For internal communications, among NSA endpoints, packets
   carry native short addresses and no special operation is needed.  For
   external communications, the root is responsible to perform the
   translation between native short addresses and IPv6 addresses.  For
   instance, for a packet entering into the NSA domain, the root will
   extract the native short address of the destination from the suffix
   of the IPv6 address, by removing all leading '0's.  It will also map
   the source IPv6 address to a mapped native short address, in order to
   make it more efficient for communication inside the NSA domain.

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   The root has to store the mapping between external IPv6 addresses and
   their assigned mapped Native Short Addresses.  The method of
   generating those mapping is out of scope of this document, however,
   the addressing space for the external NSA has to be maintained
   separate from the internal NSA address space.  Overlap are allowed
   since the two addressing space are distinguishable in the packets by
   the use of the I/O field, as explained later on.

   The following paragraphs will detail the forwarding operations for
   both internal and external communication.  The intra-network
   forwarding procedure depends on the specific AF used.  Here we will
   use the AF previously introduced (see Figure 4) to illustrate the
   forwarding procedure.

5.1.  Forwarding toward an NSA endpoint

   To perform forwarding operations, NSA nodes access the I/O field in
   the NSA header (see Section 6).  When its value is 1, the packet is
   destined to an internal NSA node, so it is an inner-domain packet.
   Otherwise, the packet is destined to an external IPv6 node.  It is
   called an outer-domain packet.  Intra-domain packets carry a native
   short addresses in the source and the destination address fields.
   More specifically the destination address field is the address of
   another node in the same NSA domain.  As such an NSA node performs
   the following sequence of actions (also see Figure 5):

   1.  Get destination address from packet (abbreviated to DA) and the
       current node's address (abbreviated to CA).  Go to step 2.

   2.  If length of DA is smaller than length of CA, send the packet to
       parent node, exit.  Otherwise, go to step 3.

   3.  If length of DA equals to length of CA, go to step 4.  Otherwise,
       go to step 5.

   4.  If DA and CA are the same, the packet arrived at destination,
       exit.  Otherwise, send the packet to parent node, exit.

   5.  Check whether CA is equal to the prefix of DA.  If yes, go to
       step 6.  Otherwise, send the packet to parent node, exit.

   6.  Calculate which child is the next hop address and forward packet
       to it.  With the AF propose in this document such operation
       reduces to reading the DA's bits starting from the position
       equals to the length of CA, then skip all '1' until the first '0'
       or the last bit of DA.  The sub-string obtained in such a way is
       the address of direct child of current node.

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   7.  If any exception happens in the above steps, drop the packet and
       send error notification.

              /*\       DA:Destination Address
             |***|      CA:Current Node's Address
      |Parse DA from pkt|
      /                \  yes
     | Len(DA)<Len(CA)? |-------------------------------+
      \                /                                |
       +-------+------+                                 |
               | no                                     |
              \|/                                       |
       +-------+------+           +--------------+      |
      /                \  yes    /                \  no |
     | Len(DA)=Len(CA)? |------>|     CA == DA ?   |--->+
      \                /         \                /     |
       +-------+------+           +-------+------+      |
               | no                       | yes         |
              \|/                        /*\            |
       +-------+------+                 |***|           |
      /                \  no             \*/            |
     | CA==PrefixOf(DA)?|------------------------------>+
      \                /                                |
       +-------+------+                                 |
               | yes                                    |
              \|/                                      \|/
     +---------+---------+                    +---------+---------+
     | Calculate next-hop|                    | Forward to Parent |
     |         &         |                    +---------+---------+
     |      Forward      |                              |
     +---------+---------+                              |

           Figure 5: Flow Chart of Internal Forwarding Procedure

   In the case of packet arriving from the Internet (external IPv6
   domain toward the local NSA domain) header adaptation operation is

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   performed by the root node.  Concerning the destination address, the
   root builds the native short address of the destination by removing
   the prefix and the leading '0's of the suffix of the destination
   address.  Meanwhile, it checks whether it exists already a mapping
   between the source address and a mapped NSA address to be used as
   source address in the NSA packet.  If not it creates one.  Then the
   root creates the inner-domain packet.  It uses the NSA address as
   destination setting the I/O field to 1 so to route the packet to as
   described above to the destination node.  The mapped NSA address is
   used as source address and the fact that is a Mapped Address is
   signaled by setting to 1 the MA field.

5.2.  Forwarding toward an external IPv6 node

   In the case that the I/O field (cf.  Section 6) is set to 0, the
   packet is destined to an external IPv6 node, it is an outer-domain
   packet.  As such the destination address is either a full IPv6
   address (for the first packet of a communication) or a mapped short
   address generated by the root node and not belonging to any node
   inside the NSA domain.

   All NSA nodes (except root) just send packets that are destined
   outside the local domain (I/O field equal 0) to their parent, not
   even looking at the actual destination address.  Eventually all
   packets will reach the root node, which acts as gateway.  The root
   node is able to map the destination NSA address to the corresponding
   full IPv6 address.  Also, the root node is able to rebuild the full
   source IPv6 address by concatenating the IPv6 prefix and the NSA
   address as explained in Section 5.2.  Other fields of the header are
   also decompressed as described in Section 6.  A full IPv6 header
   replaces the original NSA header in the packet, which is then
   forwarded according to traditional IPv6 protocol.

6.  NSA Header Format

   As explained in Section 4, the addresses in NSA are of variable
   length, in this section, we outline the design of the header format
   partially based on the format of 6lowPAN, accommodating the variable
   length property in the packet.  The header format is shown in
   Figure 6.

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      0                                       1
      0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
    | 0 | 1 | 0 | 1 |  TF   |NH |HL | Payload Length(Variable Len)  |
    |I/O|MA |    SA(Variable Len)   |         DA(Variable Len)      |
    |                        In-line fields ...                     |

                   Figure 6: Header format of NSA packets

   The first 4 bits are new dispatch types that will be introduced in
   Section 8.

   *  TF: The same definition as in [RFC6282] Section 3.1.1.

   *  NH: The same definition as in [RFC6282] Section 3.1.1.

   *  HL: This field indicates the hop limit.  When HL is 1, a hop limit
      field defined in [RFC2460] locates in in-line fields, while HL is
      0 means no hop limit header in packet.

   *  Payload length is a variable length field.  It normally occupies
      an octet assuming most packets are smaller than 252 octets.  For
      larger packets, payload length may expand to 2 to 3 octets.  The
      encoding method is defined as follows.  When the first octet has
      value of:

      -  0~252: Indicates how many octets the payload consist of.

      -  253: Indicates that there is an extra octet for payload length,
         with the actual length value equal to the last octet value plus

      -  254: Indicates that there is an extra two octets for payload
         length, with the actual length value obtained from the second
         and third octets interpreted as a 16 bits unsigned integer plus
         252 (from the first octet).

      -  255: Reserved.

   *  I/O: Indicates whether this packet is destined to a inner-domain
      node (value '1') or an outer-domain node (value '0'), where the
      former means from an NSA or IPv6 node to a NSA destination, while
      the latter means to an external IPv6 node.

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   *  MA: Indicates the source address is actually a Mapped Address
      generated by the root.  When it is '1', the source address of the
      packet is a mapped address of an external IPv6 address, while if
      it is '0', the source address of the packet is an NSA address.

   For length variable native short address encoding, for both Source
   Address (SA) and Destination Address (DA), the definition is:

   *  0~252: if the address value locates in this interval, one octet is
      used to encode the value

   *  253: indicates that the following 2 octets encode the address.

   *  254: indicates that the following 4 octets encode the address.

   *  255: indicates that the following octet defines the length of
      address in octets, followed by the address octets.

   The sequence of in-line fields is as per [RFC8200] section 3.

7.  NSA Control Message

7.1.  New Control Message

   This documents specifies only one new NSA Control Message, namely the
   NSA Mapped Address Advertisement described in Section 4.  The purpose
   of such a message is advertise the mapping of an IPv6 address into a
   NSA address.  The map is performed by the root node and sent to the
   node originating the communication.  The root keeps a copy of the
   mapping to be used for future packets.  The format is as follows:

     0                   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
     |  Type         | Code = 0x00   | Reserved      | NSA Length    |
     |                                                               |
     |          Target IPv6 Address (Fixed length 128 bits)          |
     |                                                               |
     |                                                               |
     | Target NSA Address (Variable length) .... |

   *  Type: Type value identifying NSA Control Message.  Value to be
      assigned by IANA (cf.  Section 8)

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   *  Code: This field identifies the specific control message.  In this
      case it is set to the value 0x00 "NSA Mapped Address for External
      IPv6 Address".

   *  Reserved: Set as 0 on transmission and ignored on reception.

   *  NSA Length: This field indicates the length of the Target NSA
      Address at the end of the message, expressed in octets.

   The "NSA Mapped Address for External IPv6 Address" is a variable
   length message, however, the first five fields of the message, namely
   Type, Code Reserved, NSA Length, and Target IPv6 address, have a
   fixed length of 160 bits (20 octets), hence the length of the NSA
   address is sufficient to calculate the length of the entire packet:
   20 octets + "NSA length".

7.2.  Address Configuration based on 6LOWPAN-ND

   According to [RFC6775], neighbor discovery is available in 6LoWPANs.
   This document specifies NSA address configuration mechanism based on
   RS (Router Solicitation) and solicited RA (Router Advertisement)
   defined in [RFC4861].  In order for an NSA node to request an
   address, it uses a newly defined 'Request Address Option (NRAO)' in
   RS messages.  The corresponding solicited RA will contain the 'NSA
   Assign Address Option (NAAO)' with the assigned address.

7.2.1.  NSA Request Address Option (NRAO) Format

   This option will be carried in RS messages [RFC4861] when node
   initializes.  The same RS messages MUST carry the Source Link-Layer
   Address Option (SLLAO) ([RFC4861], [RFC6775]) as well.  The link-
   layer address in SLLAO (Source Link-Layer Address Option will be used
   to identify unique NSA node.  The NRAO option, respecting the
   specifications in [RFC6775], has the following format:

    |     Type      |    Length    |   Expected Address Lifetime   |
    |                          Reserved                            |

   *  Type: 136

   *  Length: 8-bit unsigned integer.  The length of the option
      (including the Type and Length fields) in units of 8 octets.  This
      field is always set to 1.

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   *  Expected Address Lifetime: The sender of RS notify the node that
      assigns the address for how long is expected to be valid.  The
      receiver may ignore this field.  As for [RFC6775] the unit is set
      to 60 seconds (1 minute).  This field MUST be set to zero by
      sender if there is no requirement on the lifetime.

   *  Reserved: This field is not used.  It MUST be initialized to zero
      by the sender and MUST be ignored by the receiver.

7.2.2.  NSA Assign Address Option (NAAO) Format

   This option will be carried in the RA message solicited by the an RS
   message as for the usual Neighbor Discovery workflow.  The NAAO
   option, respecting the specifications in [RFC6775], has the following

    |     Type      |    Length    |        Address Lifetime       |
    | Prefix Length |                   Reserved                   |
    |                                                              |
    |                                                              |
    |                                                              |
    |                    NSA with IPv6 Prefix                      |
    |                                                              |
    |                                                              |
    |                                                              |

   *  Type: 137

   *  Length: 8-bit unsigned integer.  The length of the option
      (including the Type and Length fields) in units of 8 octets.  This
      field is always set to the value 3.

   *  Address Lifetime: The maximum time for the NSA being valid.  As
      for [RFC6775] the unit is set to 60 seconds (1 minute).  The node
      with this address MUST stop using this address for packet
      transmission when the life time expires.  When the Address
      Lifetime is zero, the node must drop the address immediately.
      When the lifetime field is 0xFFFF, the address will be valid
      forever until the node sends another NAAO to update the lifetime.

   *  Prefix Length: This field will notifies the receiver the length of
      the IPv6 prefix, expressed in octets.

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   *  Reserved: This field is not used.  It MUST be initialized to zero
      by the sender and MUST be ignored by the receiver.

   *  NSA with IPv6 Prefix: This field is filled by the node with the
      IPv6 prefix (according with the length field), the NSA address as
      the least significant bits of the IPv6 address, and padding the
      remaining bits in the middle with zeros.

8.  IANA Considerations

8.1.  Dispatch Type Field

   This document requires IANA to assign the range 01010000 to 01011111
   in page 10 of the "Dispatch Type Field" registry as follows:

   | Bit Pattern |Page|         Header Type         | Reference     |
   | 0101TTNH    | 10 |  LOWPAN NSA IP(LOWPAN_NIP)  |[This Document]|

         Figure 7: LOWPAN Dispatch Type Field requested allocation

8.2.  Allocation Function Registry

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of values related to the NSA
   specification, in accordance with BCP 26 [RFC8126].

   IANA is asked to create a registry named "Native Short Addresses
   (NSA) Parameters".

   Such registry should be populated with a one octet sub registry named
   "Allocation Function" and used to identify the AF used in a NSA
   deployment.  The sub registry is populated as follows:

         | Value   | AF Name                    | Reference       |
         | 0x00    | Native Allocation Function | [This Document] |
         |0x01-0xFF| Un-assigned                |                 |

   Values can be assigned by IANA on a "First Come, First Served" basis
   according to [RFC8126].

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8.3.  ICMP NSA Control Message

   IANA is requested to allocate an ICMPv6 type value from the "ICMPv6
   Parameters" registry to be used by "NSA Control Message".

   Also IANA is requested to create an "NSA Control Codes" sub registry,
   for the Code field of the ICMPv6 NSA Control Message.

   New codes may be allocated through the "Specification Required"
   procedure as defined in [RFC8126].  The following code is currently
   defined (the others are to be marked as un-assigned):

   | Code | Description                                | Reference     |
   | 0x00 |NSA Mapped Address for External IPv6 Address|[This Document]|

8.4.  NSA Neighbor Discovery Options

   IANA is requested to allocate two values from the "IPv6 Neighbor
   Discovery Option Formats" registry to be used by NRAO and NAAO.
   Suggested values are respectively 136 and 137.  [Note to RFC Editor:
   If IANA assign different values the authors will update the document

   | Code | Description                                | Reference     |
   | 136  |       NSA  Request Address Option          |[This Document]|
   | 137  |       NSA  Assign Address Option           |[This Document]|

9.  Reliability Considerations

   Because NSA uses algorithmically generated addresses based on the
   network topology, nodes do not generate and store forwarding table
   entries in the normal case.  One of the potential issues is the risk
   of renumbering of addresses in case of topology changes.  Because of
   the applicability domain of NSA, the common case of topology change
   is known in advance and can be planned, so to reduce disruption due
   to renumbering.  Another case is temporary link failures where the
   underlying technology is still able to provide connectivity through
   alternative links, which is strictly related to the underlying
   technology, the network topology, the deployed redundancy, and the
   expected reliability.

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   More complex reliability scenarios and alternative solutions are
   beyond the scope of this document, which is focused only on the
   address allocation framework.  Furthermore, specific reliability
   solutions can depend as well on the specific Allocation Function used
   (different from the one presented in this document).  Reliability is
   discussed in more details in [I-D.li-nsa-reliability-00].

10.  Security Considerations

   An extended security analysis will be provided in future revision of
   this document.  As of this point we consider that the security
   considerations of [RFC4944], [RFC6282] apply.

11.  Acknowledgements

   This document received many discussion and help from community
   people.  Pascal Thubert's technical questions steers this document
   being improved.  Brian Carpenter reminds key issues about IPv6
   address usage.  Dominique Barthel, Adnan Rashid, Michael Richardson,
   provide technical comments for this document.  There are other people
   helped on improving this document who want to be unnamed.  The
   authors would present thanks to all of them.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,

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              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,

   [RFC8025]  Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
              RFC 8025, DOI 10.17487/RFC8025, November 2016,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

12.2.  Informative References

              Hou, J., Liu, B., Hong, Y., Tang, X., and C. E. Perkins,
              "Transmission of IPv6 Packets over PLC Networks", Work in
              Progress, Internet-Draft, draft-ietf-6lo-plc-11, 18 May
              2022, <https://www.ietf.org/archive/id/draft-ietf-6lo-plc-

              Hong, Y., Gomez, C., Choi, Y., Sangi, A. R., and S.
              Chakrabarti, "IPv6 over Constrained Node Networks (6lo)
              Applicability & Use cases", Work in Progress, Internet-
              Draft, draft-ietf-6lo-use-cases-12, 25 January 2022,

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              Li, G., Lou, Z., and L. Iannone, "Reliability
              Considerations of Native Short Addressing", Work in
              Progress, Internet-Draft, draft-li-nsa-reliability-00, 1
              June 2022, <https://www.ietf.org/archive/id/draft-li-nsa-

   [LEE10]    Lee, M., Zhang, R., Zheng, J., Ahn, G., Zhu, C., Park, T.,
              Cho, S., Shin, C., and J. Ryu, "IEEE 802.15.5 WPAN mesh
              standard-low rate part: Meshing the wireless sensor
              networks", DOI 10.1109/jsac.2010.100902, IEEE Journal on
              Selected Areas in Communications Vol. 28, pp. 973-983,
              September 2010,

   [LPWAN]    "IPv6 over Low Power Wide-Area Networks (lpwan) WG", n.d.,

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

   [RFC8138]  Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
              "IPv6 over Low-Power Wireless Personal Area Network
              (6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
              April 2017, <https://www.rfc-editor.org/info/rfc8138>.

   [RFC8163]  Lynn, K., Ed., Martocci, J., Neilson, C., and S.
              Donaldson, "Transmission of IPv6 over Master-Slave/Token-
              Passing (MS/TP) Networks", RFC 8163, DOI 10.17487/RFC8163,
              May 2017, <https://www.rfc-editor.org/info/rfc8163>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,

   [RFC8799]  Carpenter, B. and B. Liu, "Limited Domains and Internet
              Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,

   [RS485]    "TIA-485-A Revision of EIA-485", n.d..

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   [SIXLO]    "IPv6 over Networks of Resource-constrained Nodes (6lo)
              WG", n.d., <https://datatracker.ietf.org/wg/6lo/about/>.

              "IPv6 over Low power WPAN (6lowpan) - Concluded WG", n.d.,

   [ZigBee]   "ZigBee Wireless Networks and Transceivers",
              DOI 10.1016/b978-0-7506-8393-7.x0001-5, Elsevier book,

Authors' Addresses

   Guangpeng Li
   Huawei Technologies
   Beiqing Road, Haidian District

   Email: liguangpeng@huawei.com

   David Lou
   Huawei Technologies Duesseldorf GmbH
   Riesstrasse 25
   80992 Munich

   Email: zhe.lou@huawei.com

   Luigi Iannone
   Huawei Technologies France S.A.S.U.
   18, Quai du Point du Jour
   92100 Boulogne-Billancourt

   Email: luigi.iannone@huawei.com

   Peng Liu
   China Mobile
   No. 53, Xibianmen Inner Street, Xicheng District

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   Email: liupengyjy@chinamobile.com

   Rong Long
   China Mobile
   No. 53, Xibianmen Inner Street, Xicheng District

   Email: longrong@chinamobile.com

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