6Lo Working Group                                                  G. Li
Internet-Draft                                                    D. Lou
Intended status: Experimental                                 L. Iannone
Expires: 19 June 2023                                             Huawei
                                                                  P. Liu
                                                                 R. Long
                                                            China Mobile
                                                            K. Makhijani
                                                               Futurewei
                                                              P. Thubert
                                                                   Cisco
                                                        16 December 2022


 Path-Aware Semantic Addressing (PASA) for Low power and Lossy Networks
             draft-li-6lo-path-aware-semantic-addressing-01

Abstract

   This document specifies a topological addressing scheme, Path-Aware
   Semantic Addressing (PASA) that enables IP packet stateless
   forwarding.
   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, the location of the nodes
   is fixed, and the connection between the nodes is also rather stable.
   This specifications describes the PASA architecture, along with PASA
   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 19 June 2023.




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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.  Comprehensive Use Cases . . . . . . . . . . . . . . . . . . .   4
     3.1.  Smart Grid  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Smart Home  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Data Center Monitoring  . . . . . . . . . . . . . . . . .   6
     3.4.  Industrial Operational Technology Networks  . . . . . . .   8
   4.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   9
   5.  PASA Allocation . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  PASA Addresses and IPv6 Addresses . . . . . . . . . . . .  15
     5.2.  Limitation of Number of Child Nodes . . . . . . . . . . .  16
   6.  The PASA-6LoRH Header . . . . . . . . . . . . . . . . . . . .  16
     6.1.  PASA-6LoRH Sequence . . . . . . . . . . . . . . . . . . .  16
     6.2.  PASA-6LoRH Format . . . . . . . . . . . . . . . . . . . .  17
     6.3.  PASA-6LoRH and LOWPAN_IPHC co-existence . . . . . . . . .  17
   7.  Forwarding in a PASA Network  . . . . . . . . . . . . . . . .  19
     7.1.  Forwarding toward a local PASA endpoint . . . . . . . . .  19
     7.2.  Forwarding toward an external IPv6 address  . . . . . . .  21
   8.  PASA Address Configuration  . . . . . . . . . . . . . . . . .  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Critical 6LoWPAN Routing Header Type for PASA-6LoRH . . .  23
     9.2.  Allocation Function Registry  . . . . . . . . . . . . . .  24
     9.3.  Address Registration Option Flags . . . . . . . . . . . .  24
   10. Reliability Considerations  . . . . . . . . . . . . . . . . .  25
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  25
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     Normative References  . . . . . . . . . . . . . . . . . . . . .  26
     Informative References  . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28





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1.  Introduction

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

   Particularly driven by the requirements stemming from Industry 4.0,
   Smart Grid and Smart City deployments, more and more devices/things
   are connected to the Internet.  Sensors in plants/parking bays/mines/
   data-centers, temperature/humidity/flash sensors in buildings/
   museums, normally are located in a fixed position and are networked
   by low power and lossy links even in hardwired 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 domains.

   The work in [SIXLOWPAN]/[SIXLO]/[LPWAN] Working Groups addresses many
   fundamental issues for those type of deployments, which can be
   considered an instantiation of what [RFC8799] defines as "limited
   domains".  For instance, the 6lowpan compression ([RFC4944],
   [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 stateful static
   context.

   The aforementioned technologies, which leverage on the presence of a
   routing protocol, are suitable in generic IoT scenarios and LLN
   networks.  The above technologies leverage topology discovery or
   routing mechanisms, whereas there are several special-purpose
   networks, where routing protocols are not deployed and the networks
   are statically manageable [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 in fixed locations.  This document introduces a
   topology-based addressing mechanism with that allows to avoid the use
   of routing protocol in favor of a topological stateless forwarding
   algorithm (see Section 3).





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   This specification document leverages on the 6Lo Routing Header
   (6LoRH) af defined in [RFC8138] and LOWPAN_IPHC header compression
   [RFC6282].  The use of other compression techniques is out of the
   scope of this document, and may be the object of separate
   specifications.  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], PASA avoids the extra overhead of address
   assignment by integrating address assignment and tree forming
   together.  Furthermore, PASA provides much smaller forwarding table
   size than storing mode RPL.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.  Comprehensive Use Cases

   As mentioned in Section 1, the [I-D.ietf-6lo-use-cases] provides some
   6lo use cases with wired connectivity, tree-based topology, and no
   mobility requirement (cf.  Table 2 of [I-D.ietf-6lo-use-cases]).
   These use cases, where PASA can be used, include Smart Grid, Smart
   Building, etc.  The PASA solution utilizes stable and static topology
   information to allocate addresses for nodes, which enables stateless
   forwarding.  It saves overhead of messages triggered by routing
   protocols and reduces RAM footprint for routing table storage.  Thus,
   it will reduce the overall energy consumption.  The PASA forwarding
   logic is extreme simple, few lines of code is sufficient to implement
   the stack.  It enables the solution being ported onto extreme
   constrained nodes.  In the following paragraphs, we will dive deeper
   into a few use cases to demo the applicability of the PASA solution.

3.1.  Smart Grid

   A typical smart grid network topology whose purpose is to distribute
   electricity to homes in a residential area consists of Smart Circuit
   Breaker (SCB), Phase Change Switch (PCS), Cable Branch Box (CBB) and
   Power Distribution Cabinet (PDC), as shown in Figure 1.  The PDC
   containing a few SCBs, phase compensation units, sensors and
   actuators is responsible for the power distribution towards CBB.  The
   CBB containing SCBs and sensors further distributes the power to PCS



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   and eventually to the home.  The smart grid power distribution
   network forms a typical tree topology, where the PLC communication
   technology is used to collect data (meter numbers, phases, etc.) and
   perform control/management of the overall system.

                             +---Voltage Transformer
                             |
                  +----------+-----------+
                  | PDC    +-+-+         |   SCB:Smart Circuit Breaker
                  |        |SCB|         |   PCS:Phase Change Switch
                  |        +-+-+         |   CBB:Cable Branch Box
                  |   +------+-------+   |   PDC:Power Distribution
                  | +-+-+  +-+-+   +-+-+ |       Cabinet
                  | |SCB|  |SCB|   |SCB| |
                  | +-+-+  +-+-+   +-+-+ |
                  +-+---------+-------+--+
                   /          |       +-------------------------+
                  /           +----------+                      |
                 /                       |                      |
    +-----------+----------+ +-----------+----------+           |
    | CBB       |          | | CBB       |          |  Chargers |
    |   +-------+------+   | |   +-------+------+   |      ++   |
    | +-+-+   +-+-+  +-+-+ | | +-+-+   +-+-+  +-+-+ |      ||---+
    | |SCB|   |SCB|  |SCB| | | |SCB|   |SCB|  |SCB| |      ++   |
    | +-+-+   +-+-+  +-+-+ | | +-+-+   +-+-+  +-+-+ |      ++   |
    +---+-------+------+---+ +---+-------+------+---+      ||---+
       |        |      |         |       |      |          ++   |
       |        |      |       +-++    +-++ +--++
     +-+-+    +-+-+  +-+-+     +--+    +--+ +--+|
     |PCS|    |PCS|  |PCS|     Monitors for end |
     +---+    +---+  +---+                      |
                                     +CBB-------+----------+
                                     |  +-------+-------+  |
                                     |+-+-+   +-+-+   +-+-+|
                                     ||SCB|   |SCB|   |SCB||
                                     |+---+   +---+   +---+|
                                     +---------------------+

                   Figure 1: The topology of smart grid.

3.2.  Smart Home

   Smart home or home domotica is another example, as shown in figure
   Figure 2, where a PLC router (PLC-R) in each room is used to connect
   home appliances (boiler, dishwasher, fridge, etc.) and devices
   (lights, doorbell, sound boxes, etc.) to home network and sometimes
   to the Internet.  The network can be further extended if a switch/
   router is connected.  As it leverages the power line distribution,



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   the network forms a typical tree topology as well.  Some observations
   and considerations are:

   *  Usually a Home Gateway bridges the smart home to the Internet.

   *  The Home Gateway, the PLC routers, and most of the home appliance
      are fixed in different locations.  They rarely move after setup.

   *  The smart home automation requires any to any communication.

   *  Lightweight communication stack with limited MCU and RAM
      consumption is desired.

                               /----------\
                              |  Internet  |
                               \-----+----/
                                     |
                              +------+------+
                              | Home Gateway|
                              +------+------+
                                     |
                      +--------------+----------------------+
   +------------------|----++--------|-----------++---------|---------+
   |                  |    ||        |           ||         |  Kitchen|
   |  Living       +--+---+||    +---+--+ Bedroom||     +---+--+      |
   |  Room         |PLC-R |||    |PLC-R |        ||     |PLC-R |      |
   |               +---+--+||    +--+---+        ||     +---+--+      |
   |                   |   ||       |            ||         |         |
   |  +-----+-----+----+   ||  +----+--+------+  ||  +------+------+  |
   |  |     |     |    |   ||  |       |      |  ||  |      |      |  |
   |  |     |     |    |   ||  |       |      |  ||  |      |      |  |
   | /+\   /+\   /+\  /+\  || /+\     /+\    /+\ || /+\    /+\    /+\ |
   ||   | |   | |   ||   | |||   |   |   |  |   ||||   |  |   |  |   ||
   | \-/   \-/   \-/  \-/  || \-/     \-/    \-/ || \-/    \-/    \-/ |
   |      Switches    Door ||Strip  Voice   Sound||Boiler Fridge  Dish|
   |Light     door bell    ||Light  Command Boxes||             Washer|
   +-----------------------+|       Device       |+-------------------+
                            +--------------------+

                   Figure 2: The topology of smart home.

3.3.  Data Center Monitoring

   Data centers is a significant infrastructure, which requires numerous
   safeguards in place to protect hardware assets against cyber-attacks.
   Besides, environmental issues such as extreme temperature, high
   humidity, water leakage and high dust concentration can cause device
   failures as well.  Therefore, it is critical to deploy sensors to



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   monitor environmental factors to make sure data center is running
   efficiently.

   The network topology of the data center supervision system is
   hierarchical, and mainly consists of Network Management System (NMS),
   Supervision Center (SC), Field Supervision Unit (FSU), dumb and smart
   devices, as shown in theFigure 3.  The smart devices refer to smart
   air conditioner, smart door lock and power equipment with embedded
   sensors to report their working status.  The dumb devices refer to
   the many devices without embedded sensors, which require additional
   sensors to collect and update information of environment.

   NMS:Network Management System  /----\             //------\\
   SC :Supervisor Center         /      \          ||          ||
   FSU:Field Supervisor Unit    |   SC   +---------+|    NMS   ||
                                 \      /            \\------//
                                  \----/
                                 /      \
                               /         \
                           /----\         \
                          /      \          \
                         |   SC   |          \
                          \      /            \
                           \--X-/              \
                          /     \               \
                        /         \               \
                      /             \              \
                   /-/-\           /-\-\           /---\
                  | FSU |         | FSU |         | FSU |
                   \-X-/           \-X-/           \-X-/
                   /   \           /   \           /   \
                  /     \         /     \         /     \
               +---+   /--\    +---+   /--\    +---+   /--\
               |   |  |    |   |   |  |    |   |   |  |    |
               |   |  |    |   |   |  |    |   |   |  |    |
               +---+   \--/    +---+   \--/    +---+   \--/
               Smart   dumb    Smart   dumb    Smart   dumb
              Device  Device  Device  Device  Device  Device

      Figure 3: The topology of Power & Environment Supervisor System.

   Both dumb and smart devices are connected to the FSU, which monitors
   and connects all devices of the whole floor.  The number of ports on
   FSU is limited, where one FSU usually contains 8 analog input ports,
   16 digital input ports, 4 digital output ports, 8 RS485 ports and 4
   IP ports.  The terminal devices report working status and
   environmental information to FSUs every 3 second.  If values that are
   abnormal or above a certain threshold are detected, the FSU reports



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   it to the SC immediately and keeps on reporting it in real-time for
   next couple of hours, until the manager issues new commands.  The SC
   can be constructed as required.  The FSU reports to the local SC
   first, then relay the message to the central SC for data analyzing
   and management.

   In this scenario, deployed devices (usually 600-1000 sensors per
   floor), due to the shortage of ports and limitation of voltage
   supply, use additional power supply or batteries.  Since battery
   replacement and maintenance is costly, it is desired to have low
   energy consumption for longer service life.  We should not only
   reduce the power consumption on the device level, but also on the
   data transmission level.  The data transmission also causes huge
   power consumption, which can be reduced by leveraging low power
   transmission protocol.  The FSU connects to sensors with wired
   technology, such as AI/DI/RS232/RS485/single pair ethernet.  Multiple
   FSUs will connect to hierarchical supervision centers and then make
   data communication with supervision platform by IPv6.

3.4.  Industrial Operational Technology Networks

   The Operational Technology (OT) networks are not pure IP networks.
   Shop floors deploy fieldbus protocols such as Modbus, Profinet/IP,
   BacNET, CAN etc. for process control using field devices (sensors and
   actuators).  To improve automation, Industry 4.0 is looking at means
   to integrate process control in OT domain with the applications
   residing in IPv6 domains (the enterprise networks).  This leads to
   three primary requirements:

   *  Continuity in connectivity between the end devices and
      applications, both of which follow different address structures.

   *  The OT networks are traditionally designed as layer-2 and OT
      operators are not expected to deploy or maintain IT style routing
      infrastructure, hence auto-configuration mechanisms for device
      addresses and reachability are preferred.

   *  The OT networks are also delay-intolerant; therefore, compact and
      lean message structures are favored over encapsulations to
      minimize processing and translation overheads.

   Using PASA, as described in details later in this document, the
   following applies:

   *  The OT network is represented as PASA domain, interfacing with
      native IPv6 applications, e.g., Human-Machine Interface (HMI),
      Manufacturing Execution System (MES).  In general on shop floors,




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      devices are at fixed locations or cell-sites and the PASA tree
      hierarchy described in Figure 4 applies suitably.

   *  In an idealized PASA-based OT domain, a leaf-node could be a field
      device (sensor or actuator) that always connects to PLC serving as
      last node forwarding traffic to/from the leaves, i.e. sensors and
      actuators.

   *  The border node may be at the root for any IT application
      requirement.  Then the packet communication inside the PASA domain
      will strictly follow PASA structure whereas communications with
      IPv6 domain networks will use the Border router for translations.

                   IPV6   +------+------------+
          +---------------| HMI/MES/FW/Gateway|----------+
          | PASA          +------+------------+          |
          |                                              |
     +----|------------------+                           | PASA
     | +--+---+              |                           |
     | |PLC   |--------------+-----------+               |
     | +---+--+              |           |      +--------+---------+
     |    | profinet         |  +--------+----+ |        |         |
     |    |                  |  |   +-----+   | |     +--+--+      |
     |  +-----+-----+----+   |  |   | PLC |   | |     | PLC |      |
     |  |     |     |    |   |  |   +--+--+   | |     +--+--+      |
     |  |     |     |    |   |  |      |      | |        |         |
     | /+\   /+\   /+\  /+\  |  | profi|bus   | |        |  modbus |
     | \-/   \-/   \-/  \-/  |  |  +---+---+  | |  +-----+------+  |
     |  sensors/actuators    |  |  |       |  | |  |     |      |  |
     | cell-site-A           |  |  |       |  | |  |     |      |  |
     +-----------------------+  | /+\     /+\ | | /+\   /+\    /+\ |
                                | \-/     \-/ | | \-/   \-/    \-/ |
                                |             | | sensors/actuators|
                                | cell site B | | cell site C      |
                                +-------------+ +------------------+

       Figure 4: Industrial Operational Technology Network topology.

4.  Architectural Overview

   Path-Aware Semantic Addressing (PASA) is an efficient topology-based
   network layer address assignment and packet forwarding mechanism.
   Each PASA node is aware of its own IPv6 address, constructed by IPv6
   prefix and the PASA itself (see Section 5.1).  Inside the PASA
   domain, nodes communicate with each other by using only PASA
   addresses.  It is a smaller addressing space compared to the huge
   IPv6 addressing space, but enabling stateless forwarding using the
   PASA-6LoRH header (see Section 6).  When IPv6 communication occurs



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   between nodes inside the PASA domain and external IPv6 nodes, the
   border router, which plays as well the role of "root" in the
   addressing tree, performs packet decompression (as per Section 7.2
   and [RFC6282]).  Note that packets destined outside the PASA domain
   do not need to use the PASA-6LoRh header, since they can be easily
   forwarded to the root following the default gateway (see
   Section 7.2).  However, an IP-in-IP header, as for [RFC8138], is used
   to avoid compression/decompression at each hop.  The architecture of
   PASA network is shown in Figure 5.

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

            Figure 5: The architecture of general PASA networks.

   In the PASA network, there are 3 types of nodes, the PASA Root, the
   PASA Router and the PASA Host.  There is typically only one root node
   in the PASA network.

   *  PASA Root: The root node is the router responsible for the
      management of the whole PASA network and routing/forwarding both
      internal and external traffic.  It uses 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 PASA domain and the Internet.  As such it also
      operates the translation between LOWPAN_IPHC and IPv6 formats (cf.
      Section 7).



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   *  PASA Router: A PASA Router is an internal node, different from the
      root, having least one child.  It is basically the root of a
      subtree and as such it is a router forwarding traffic between its
      parent and its children according to the addressing.  When
      handling a packet, if the destination is in one of its subtrees,
      it forwards the packet to the right child, otherwise it simply
      sends it to its parent.

   *  PASA Host: A PASA Host is a node with no children, hence a leaf.
      It operates as an host, since it is either destination or source
      of every packet it handles.  If it is the source of packets, it
      simply sends the packets to its parent.

   PASA Routers and Hosts roles can be assigned similarly to IEEE
   802.15.4, which distinguishes between Full-Function Devices (FFD) and
   reduced function devices (RFD) (cf., [ZigBee]).

   The address assignment described in this document relies on the
   address registration mechanism described in [RFC8505] (see
   Section 8).  Each node acquiring a PASA address firstly needs to
   select a parent node by choosing among the nodes that replied with an
   Router Advertisement (RA) after an initial Router Solicitation (RS).
   A "first come first served" selection policy is sufficient.  Then it
   registers its link-local address to the selected parent, asking at
   the same time for a PASA address.  In its reply the parent will
   propose an address according to the node's role (indicated in the
   request).  The proposed address is algorithmically calculated using
   an Allocation Function (AF).  The address assigner is the parent of
   the node and becomes as well the default gateway from a routing
   perspective (used for destinations that are not in the local PASA
   domain).  The node will then ignore replies from other neighbors.

   The overall design objective is centered on reducing the size (or
   completely avoid the usage) of routing/forwarding table by using a
   topological addressing scheme.  PASA reduces the amount of
   information synchronization messages, so it actually reduces
   computation complexity during packets parsing and forwarding.  As
   such, PASA may save communication energy in an IoT LLN network.

   There are two distinct PASA features that allow PASA to be efficient,
   namely:

   1.  PASA Address allocation (see Section 5),

   2.  Stateless forwarding (see Section 7),






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5.  PASA Allocation

   The basic rules of allocation include:

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

   *  Routers (Root and routers) run 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 PASA address MUST NOT exceed 64 bits.

   Normally, the root role is assigned to the border router when the LLN
   bootstraps.  An example of a possible result of a PASA deployment is
   shown in Figure 6.

                            root           +--------------------------+
                                 1         | append more bits to form |
                                 O ----+   | brother's address        |
                              /  |  \   \  +--------------------------+
                             /   |   \    \
                            /    |    \     \
     +-------------+      /      |     \      \
     | PASA Router | 10 /       11   110\       \  111
     +-------------+  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  | PASA Host |
                               +-----------+

             Figure 6: An example of PASA addresses allocation.

   Every router node stores and maintain two indexes, one for the
   children that are also routers and one for the children that are
   hosts (starting at 0 for the first child in each role).  The first
   index is named 'r', as of routers, and the second 'h' as for hosts.
   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'.  The allocation function
   AF(role,i) used in this document is defined as:



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   AF(role, r, h) = 'address of the node performing the function'
                     + (role == host? b(h++):b(r++))
                     + (role == host?'1':'0')

   Where 'r' and 'h' are the indexes of respectively the routers and the
   hosts at this layer (starting at 0).  Taking the example of the
   topology in Figure 6, the proposed AF works as follows.

   At the top level, there are 4 children of root, two are routers and
   the other two are hosts.  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 router:

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

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

   *  For the second child, which is a host:

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

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

   *  For the third child, which is a router:

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

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

   *  For the fourth child, which is a host:

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

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

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




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

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

   *  For the second child, which is a host:

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

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

   *  For the third child, which is a ruoter:

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

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

   *  For the fourth child, which is a host:

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

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

   Note how the children of the same parent all have the same prefix (10
   in this example) and such parent will be their default gateway.  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 6 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)
                length(b(max(r,h)))
                + 1

   Where 'r' and 'h' are the indexes counting respectively the routers
        and the hosts at this level.

   The Allocation Function can be different from the one defined in this
   specifications, where all nodes know which one to use by
   configuration (cf.  Section 9).  The use of one and only one AF is
   allowed in a PASA domain and MUST be the same for all nodes.  It is
   RECOMMENDED that implementations support at least the AF proposed in
   this document.

   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 9).

5.1.  PASA Addresses and IPv6 Addresses

   Obtaining a full IPv6 address from a PASA address is pretty
   straightforward.  First the PASA address is concatenated to the
   configured IPv6 prefix.  Since the length of the PASA 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" + PASA (or in IPv6 notation <IPv6 Prefix>::<PASA>).  This
   is equivalent of doing a coalescence operation as described in
   [RFC8138] (see as well Section 6.3).  The PASA is assigned by the
   root or router as previously described.

   PASA does not prevent the normal checksum calculation for the
   transport layer (namely TCP or UDP) or IPSec encapsulation.  Indeed,
   any PASA node is aware of its full IP address, which can be used for
   the calculation.







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5.2.  Limitation of Number of Child 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].  PASA 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).

6.  The PASA-6LoRH Header

   The PASA encodes path information into addresses to enable stateless
   forwarding.  Such operation can be performed without touching the
   stateful forwarding procedure (based on the presence of a routing
   protocol like RPL), aka without modifying the 6LowPAN architecture,
   rather leveraging on mechanism already defined.  In particular, by
   using the 6LowPAN Routing Header in Page 1, defined in [RFC8138], it
   is possible to define a new Critical 6LowPAN Routing Header Type,
   named PASA-6LoRH, that will be used by nodes to perform stateless
   PASA forwarding as described in Section 7.

6.1.  PASA-6LoRH Sequence

   The extension octets typical sequence for a compressed 6LowPAN packet
   with PASA Routing Header is shown in Figure 7, following the
   specification of [RFC8138].

   +-----------+----....----+--------...------+----...----+
   |  11110001 | PASA-6LoRH |   LOWPAN_IPHC   |  Payload  |
   |  Page 1   |   Type 8   |                 |           |
   +-----------+----....----+--------...------+----...----+

       Figure 7: A lowPAN encapsulated IPv6 header compressed packet
                  with PASA-6LoRH and LOWPAN_IPHC headers.

   Where:

   *  PASA-6LoRH: is the PASA specific extension.  See Section 6.2 for
      details.

   *  LOWPAN_IPHC: IPv6 compressed header according to [RFC6282].

   These two fields are followed by the packet payload.




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   All nodes of a PASA domain MUST recognize the PASA critical 6LoWPAN
   Routing Header and be able to handle the packets according to these
   specifications.  Otherwise, packets can be dropped, hence disrupting
   communications.

6.2.  PASA-6LoRH Format

   The format of the PASA-6LoRH header, is shown in Figure 8.

     0                                       1
     0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 1 | 0 | 0 | Rsvd  |   Size    |          6LoRH Type           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |              Octet 1          |          Octet 2              |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   ~                        ...                                    ~
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |              Octet N-1        |         Octet N               |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
               Where N = Size + 1, and 6LoRH Type = PASA

               Figure 8: The PASA 6Lo Routing Header format.

   Where:

   *  Reserved (Rsvd): Reserved for future use.  It MUST be initialized
      to zero by the sender and MUST be ignored by the receiver.

   *  Size: indicates the length of the PASA address in octets.  The
      length N equals Size plus 1, which indicates that the length of
      the PASA address in PASA-6LoRH is at least 1 octet and no more
      than 8 octets.

   *  Octet 1 .. Octet N: the PASA destination address used for
      forwarding purposes.  See Section 7 for detailed forwarding
      operation.  PASA addresses are aligned on the least significant
      bits.  For instance, to encode the address b1011, which is the
      address of a host node since it terminates with '1', the
      corresponding octet would be b00001011 (or in hexadecimal: 0x0B).

6.3.  PASA-6LoRH and LOWPAN_IPHC co-existence

   In a PASA domain every node has to use PASA and being able to
   compress/uncompress PASA addresses according to this specification.
   The reference prefix of the PASA domain represents a context that can
   be used to compress addresses in accordance to [RFC6282] and
   decompress using the context and the coalescence procedure in



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   [RFC8138].  As such the simplest mode of co-existence of PASA-6LoRH
   with LOWPAN_IPHC is to use stateful address compression in the
   LOWPAN_IPHC header using the PASA context, then the PASA engine can
   just read the destination address from the LOWPAN_IPHC header,
   encoding it in the PASA_6LoRH header according to format previously
   described in Section 6.2.  However, this mode of operation is sub-
   optimal because PASA-6LoRH already includes the destination address,
   hence, it can be completely elided from the LOWPAN_IPHC header.

   For nodes sending packets, the first step is to create a compressed
   packet using [RFC6282], where the source PASA address is statefully
   compressed using the context and the destination PASA address
   statefully completely elided.  The destination address is then
   encoded in the PASA-6LoRH in its shorter form.

   In case of the destination address is an address outside the PASA
   domain, there is not need to use the the PASA-6LoRH header, since the
   packet just need to follow the default route until it reaches the
   root node (more details in Section 7.2).

   The root node, when relaying a packet coming from outside the PASA
   domain, compresses the source address in the LOWPAN_IPHC header
   according to [RFC6282] specifications.

   The opposite operations need to be performed on the receiving node.
   Since the destination address is completely elided in LOWPAN_IPHC the
   IID is obtained by its encapsulation, in this case the PASA-6LoRH.
   The full destination address, including the IID, can be obtained via
   a coalescence operation with the PASA prefix in the context as
   described in Section&nbsp;4.3.1 of [RFC8138].  The source address is
   handled as defined in [RFC6282].  As an example, let's assume that
   the PASA IPv6 prefix is 2001:db8::/64, as for [RFC8138] the reference
   address will be 2001:db8:0:0.  Let the PASA address in the PASA-6LoRH
   header be 111110, which in hexadecimal is 0x3E, then the complete
   IPv6 address is:

   2001:db8:0:0:0:0:0:0    Reference address
                      3E   Compressed address

   2001:db8:0:0:0:0:0:3E   Coalesced address

   In compact notation the address is: 2001:db8::3E.









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7.  Forwarding in a PASA Network

   Internal and external communications in a PASA network work slightly
   differently.  For internal communications, among PASA endpoints,
   packets carry PASA destination addresses in the PASA-6LoRH Header.
   For external communications, the root is responsible to perform the
   translation between PASA addresses and IPv6 addresses.  For instance,
   for a packet entering into the PASA domain, the root will extract the
   PASA of the destination from the suffix of the IPv6 address, reducing
   it to the smallest set of quad that can contain the address, by
   removing all leading octets that are just equal to 0x00.  The the
   root will compress the original IPv6 and transport header according
   to [RFC6282] and prepend the PASA-6LoRH header according to
   [RFC8138].

   The following details the forwarding operations for both internal and
   external communication.  The intra-network forwarding decision
   depends on the specific AF used.  Here we will use the AF previously
   introduced (see Section 5) to illustrate the forwarding procedure.

7.1.  Forwarding toward a local PASA endpoint

   Inner-domain packets carry a PASA destination address in the PASA-
   6LoRH header.  More specifically the destination address field is the
   address of another node in the same PASA domain.  As such a PASA node
   performs the following sequence of actions (also see Figure 9):

   1.  Get destination address from the PASA-6LoRH (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 proposed in this document, such operation is
       reduced to reading the DA's bits starting from the position
       equals to the length of CA, then skip all '1' until the first '0'




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       or the last bit of DA.  The sub-string obtained in such a way is
       the address of direct child of current node.

   7.  If any exception happens in the above steps, drop the packet and
       send an ICMPv6 "No Route to Host" notification back to the source
       address.

              /*\       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      |                              |
     +---------+---------+                              |
               |<---------------------------------------+
              \|/
              /*\
             |***|
              \*/




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           Figure 9: Flow Chart of Internal Forwarding Procedure

   In the case of packets arriving from the Internet (external IPv6
   domain toward the local PASA domain) header adaptation operation is
   performed by the root node.  It first compresses the IPv6 header
   according to [RFC6282] and also described in Section 6.3.  The root
   builds the PASA address of the destination by removing the prefix and
   the leading '0's octets of the suffix of the destination address.
   Then the root creates the inner-domain packet with the PASA-6LoRH
   header.  It uses the PASA address as destination, so to route the
   packet as described above to the destination node.

7.2.  Forwarding toward an external IPv6 address

   When the packet is destined to an external IPv6 address, it is an
   outer-domain packet.  In this case there is no need to use the PASA-
   6LoRH encapsulation.  Indeed, since each node has a default gateway
   entry in the routing table, namely its parent, so all PASA nodes
   (except root) just send packets that are destined outside the local
   domain to their parent.  Eventually all packets will reach the root
   node, which acts as border gateway.

   When the network forwarding operation are based on RFC 8138, the
   source node encapsulates the the LOWPAN_IPHC packet with the IP-in-IP
   6LoRH Header defined in Section&nbsp;7 of [RFC8138].  Where the
   encapsulator address is always the source address in the LOWPAN_IPHC
   header and the destination is always implicitly the root node.  The
   latter will decapsulate and decompress the packet.  Hence, according
   to [RFC8138] the IP-in-IP 6LoRH will have the form depicted in
   Figure 10.

     0                   1                   2
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1|0|1| Length  | 6LoRH Type 6  |  Hop Limit    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 10: IP-in-IP 6LoRH in a PASA domain.

   Where the Length field is set to 1 to indicate that only the Hop
   Limit field is present.  Such a header is positioned before
   LOWPAN_IPHC as shown in Figure 11.

    +-----------+----....----+--------...------+----...----+
    |  11110001 |  IP-in-IP  |   LOWPAN_IPHC   |  Payload  |
    |  Page 1   |   6LoRH    |                 |           |
    +-----------+----....----+--------...------+----...----+




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       Figure 11: A lowPAN encapsulated IPv6 header compressed packet
                   with IP-in-IP and LOWPAN_IPHC headers.

8.  PASA Address Configuration

   [RFC8505] Registration Extensions for IPv6 over 6LowPAN Neighbor
   Discovery can be further extended to accommodate PASA address
   configuration.  In order for a PASA node to request an address, the
   Extended Address Registration Option (EARO) message is uses,
   exploiting two of the reserved bits.  The format of the EARO message
   is shown in Figure 12.

     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      |     Length    |    Status     |    Opaque     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Rsd|P|H| I |R|T|     TID       |     Registration Lifetime     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
   ...            Registration Ownership Verifier (ROVR)           ...
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 12: EARO Format.

   All the fields in EARO message are defined in [RFC8505], except for
   bits P and H that are allocated by this document (see Section 9) and
   are defined as follows:

   *  PASA bit (P): If set, this flag indicates that the registration
      message is requesting or delivering a PASA address as part of the
      link-local address registration procedure.

   *  Host bit (H): If set, this flag indicates that the node is acting
      as a PASA Host, otherwise, it means that the node is acting as a
      PASA Router (cf.  Section 4).

   When a PASA node bootstraps, it typically does multicast a Routing
   Solicitation(RS) and receives one or more unicast Routing
   Advertisements (RA) messages from potential parents. it can choose a
   parent on a "first come first served" basis and send a Neighbor
   Solicitation (NS) with a EARO message to register its link-local
   address to the selected parent.  In this EARO message it will set the
   P bit, to indicate that it is also requesting a PASA address.  It
   will set the H accordingly to its intended role.  The parent, acting
   as routing registrar will process the received EARO message and act
   according to [RFC8505], and the corresponding EARO message for the NA



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   packet is generated.  The NA message will carry the EARO message with
   the bits P and H set exactly as in the corresponding EARO message of
   the NS packet.  If the returning status is 0, meaning "success"
   according to [RFC6775], the returning EARO message will carry as well
   the PASA address that the parent assigns to its child using the
   procedures described in Section 5.  The PASA address is appended to
   the EARO message (whose length is now set to 3), so the returning
   format becomes the one depicted in Figure 13.

     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      |   Length = 3  |  Status = 0   |    Opaque     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Rsd|P|H| I |R|T|     TID       |     Registration Lifetime     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Link-Local Address                       |
    |                          of the child                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Proposed  PASA Address                    |
    |                          of the child                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 13: NA EARO message example.

   At this point, the child MUST register the PASA address to the same
   parent, but not using the P and L bits.  This is in order to be in
   line with Section&nbsp;5.6 of [RFC8505], requesting global unique
   addresses to be registered.  Furthermore, the registration procedure
   has the nice property to confirm that the child accepted and will use
   the proposed address.

   If the node that made the request is a router, it can start acting as
   a routing registrar so to allow other nodes to select it as a parent.

9.  IANA Considerations

9.1.  Critical 6LoWPAN Routing Header Type for PASA-6LoRH

   This document requires IANA to assign one value of the "Critical
   6LoWPAN Routing Header Type" registry, to be used according to the
   specification in this document, as shown in Table 1.  [Note to RFC
   Editor: If IANA assign different values the authors will update the
   document accordingly]







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             +---------------+-------------+-----------------+
             | Value         | Description | Reference       |
             +===============+=============+=================+
             | 8 (suggested) | PASA-6LoRH  | [This Document] |
             +---------------+-------------+-----------------+

               Table 1: Critical 6LoWPAN Routing Header Type
                                  for PASA

9.2.  Allocation Function Registry

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

   IANA is asked to create a registry named "Path-Aware Semantic
   Addressing (PASA) Parameters".

   Such registry should be populated with a one octet sub registry named
   "Allocation Function" and used to identify the AF used in a PASA
   deployment.  The sub registry is populated as shown in Table 2:

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

                Table 2: Allocation Function sub-registry

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

9.3.  Address Registration Option Flags

   IANA is requested to add the content show in Table 3 to the existing
   sub-registry "Address Registration Option Flags" under "Internet
   Control Message Protocol version 6".











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                  +-----+-------------+-----------------+
                  | Bit | Description | Reference       |
                  +=====+=============+=================+
                  | 2   | P Flag      | [This Document] |
                  +-----+-------------+-----------------+
                  | 3   | H Flag      | [This Document] |
                  +-----+-------------+-----------------+

                     Table 3: New Address Registration
                                Option Flags

10.  Reliability Considerations

   Because PASA 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 PASA, 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.

   More complex reliability scenarios and alternative solutions are
   beyond the scope of this document, which is focused only on the
   address allocation framework and stateless forwarding.  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-6lo-pasa-reliability].

11.  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], [RFC8138], and [RFC8505]
   apply.

Acknowledgements

   This document received many discussion and help from community
   people.  Tommaso Pecorella, Esko Dijk, Dominique Barthel, Adnan
   Rashid, Michael Richardson, Brian Carpenter, did provide technical
   comments for this document.  The authors would like to thank all of
   them.




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References

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc4944>.

   [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,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [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,
              <https://www.rfc-editor.org/info/rfc6775>.

   [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,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [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>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power



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              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

Informative References

   [I-D.ietf-6lo-plc]
              Hou, J., Liu, B. R., 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-11.txt>.

   [I-D.ietf-6lo-use-cases]
              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-14, 24 October 2022,
              <https://www.ietf.org/archive/id/draft-ietf-6lo-use-cases-
              14.txt>.

   [I-D.li-6lo-pasa-reliability]
              Li, G., Lou, Z., and L. Iannone, "Reliability
              Considerations of Path-Aware Semantic Addressing", Work in
              Progress, Internet-Draft, draft-li-6lo-pasa-reliability-
              00, 24 October 2022, <https://www.ietf.org/archive/id/
              draft-li-6lo-pasa-reliability-00.txt>.

   [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, no. 7, pp.
              973-983, September 2010,
              <https://doi.org/10.1109/jsac.2010.100902>.

   [LPWAN]    "IPv6 over Low Power Wide-Area Networks (lpwan) WG", n.d.,
              <https://datatracker.ietf.org/wg/lpwan/about/>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [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>.




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   [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,
              <https://www.rfc-editor.org/info/rfc8724>.

   [RFC8799]  Carpenter, B. and B. Liu, "Limited Domains and Internet
              Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
              <https://www.rfc-editor.org/info/rfc8799>.

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

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

   [SIXLOWPAN]
              "IPv6 over Low power WPAN (6lowpan) - Concluded WG", n.d.,
              <https://datatracker.ietf.org/wg/6lowpan/about/>.

   [ZigBee]   "ZigBee Wireless Networks and Transceivers",
              DOI 10.1016/b978-0-7506-8393-7.x0001-5, Elsevier book,
              2008,
              <https://doi.org/10.1016/b978-0-7506-8393-7.x0001-5>.

Authors' Addresses

   Guangpeng Li
   Huawei Technologies
   Beiqing Road, Haidian District
   Beijing
   100095
   China

   Email: liguangpeng@huawei.com


   David Lou
   Huawei Technologies Duesseldorf GmbH
   Riesstrasse 25
   80992 Munich
   Germany

   Email: zhe.lou@huawei.com


   Luigi Iannone
   Huawei Technologies France S.A.S.U.
   18, Quai du Point du Jour



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   92100 Boulogne-Billancourt
   France

   Email: luigi.iannone@huawei.com


   Peng Liu
   China Mobile
   No. 53, Xibianmen Inner Street, Xicheng District
   Beijing
   100053
   China

   Email: liupengyjy@chinamobile.com


   Rong Long
   China Mobile
   No. 53, Xibianmen Inner Street, Xicheng District
   Beijing
   100053
   China

   Email: longrong@chinamobile.com


   Kiran Makhijani
   Futurewei
   United States of America

   Email: kiranm@futurewei.com


   Pascal Thubert
   Cisco Systems, Inc.
   France

   Email: pthubert@cisco.com













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