Registration Extensions for 6LoWPAN Neighbor Discovery
draft-ietf-6lo-rfc6775-update-18

Abstract

This specification updates RFC 6775 - 6LoWPAN Neighbor Discovery, to clarify the role of the protocol as a registration technique, simplify the registration operation in 6LoWPAN routers, as well as to provide enhancements to the registration capabilities and mobility detection for different network topologies including the backbone routers performing proxy Neighbor Discovery in a low power network.

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 October 8, 2018.

Copyright Notice

Copyright (c) 2018 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
• 2. Terminology
• 2.1. BCP 14
• 2.2. Subset of a 6LoWPAN Glossary
• 2.3. References
• 2.4. New Terms
• 3. Applicability of Address Registration Options
• 4. Extended ND Options and Messages
• 4.1. Extended Address Registration Option (EARO)
• 4.2. Extended Duplicate Address Message Formats
• 4.3. New 6LoWPAN Capability Bits in the Capability Indication Option
• 5. Updating RFC 6775
• 5.1. Extending the Address Registration Option
• 5.2. Transaction ID
• 5.2.1. Comparing TID values
• 5.3. Registration Ownership Verifier
• 5.4. Extended Duplicate Address Messages
• 5.5. Registering the Target Address
• 5.6. Link-Local Addresses and Registration
• 5.7. Maintaining the Registration States
• 6. Backward Compatibility
• 6.1. Signaling EARO Capability Support
• 6.2. First Exchanges
• 6.3. RFC6775-only 6LoWPAN Node
• 6.4. RFC6775-only 6LoWPAN Router
• 6.5. RFC6775-only 6LoWPAN Border Router
• 7. Security Considerations
• 8. Privacy Considerations
• 9. IANA Considerations
• 9.1. ARO Flags
• 9.2. ICMP Codes
• 9.3. New ARO Status values
• 9.4. New 6LoWPAN capability Bits
• 10. Acknowledgments
• 11. References
• 11.1. Normative References
• 11.2. Terminology Related References
• 11.3. Informative References
• 11.4. External Informative References
• Appendix A. Applicability and Requirements Served (Not Normative)
• Appendix B. Requirements (Not Normative)
• B.1. Requirements Related to Mobility
• B.2. Requirements Related to Routing Protocols
• B.3. Requirements Related to the Variety of Low-Power Link types
• B.4. Requirements Related to Proxy Operations
• B.5. Requirements Related to Security
• B.6. Requirements Related to Scalability
• B.7. Requirements Related to Operations and Management
• B.8. Matching Requirements with Specifications
• Authors' Addresses

1. Introduction

The scope of this draft is an IPv6 Low-Power Network including star and mesh topologies. In that context, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks" (6LoWPAN ND) defines a registration mechanism that leverages a central registrar for the main purpose of Duplicate Address Detection (DAD), with the intention to reduce the dependency of the IPv6 Neighbor Discovery Protocol (IPv6 ND) [RFC4861][RFC4862] on network-layer multicast and link-layer broadcast operations.

This specification updates 6LoWPAN ND to simplify the registration operation in 6LoWPAN routers and to extend the protocol as a more generic registration technique. The specified updates enable other specifications to define new services such as Source Address Validation (SAVI) with [I-D.ietf-6lo-ap-nd], participation as an unaware leaf to an abstract routing protocol such as the "Routing Protocol for Low Power and Lossy Networks" (RPL) with [I-D.thubert-roll-unaware-leaves], and registration to a backbone routers performing proxy Neighbor Discovery in a Low-Power and Lossy Network (LLN) with [I-D.ietf-6lo-backbone-router].

In more details, this specification modifies and extends the behavior and protocol elements of 6LoWPAN ND to enable the following new capabilities: Appendix B.

• determining the freshest location in case of mobility (TID)
• Simplifying the registration flow for Link-Local Addresses
• Support of a Leaf Node in a Route-Over network
• Proxy registration in a Route-Over network
• Associating the registration with a variable-length Registration Ownership Verifier (ROVR)
• Registration to a IPv6 ND proxy over a Backbone Link (6BBR)
• Clarification of support for privacy and temporary addresses

A more comprehensive set of requirements is provided in

2. Terminology

2.1. BCP 14

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][RFC8174] when, and only when, they appear in all capitals, as shown here.

2.2. Subset of a 6LoWPAN Glossary

This document often uses the following acronyms:

6BBR:

6LoWPAN Backbone Router (proxy for the registration)

6LBR:

6LoWPAN Border Router (authoritative on DAD)

6LN:

6LoWPAN Node

6LR:

6LoWPAN Router (relay to the registration process)

6CIO:

Capability Indication Option

(E)ARO:

(Extended) Address Registration Option

(E)DAR:

(Extended) Duplicate Address Request

(E)DAC:

(Extended) Duplicate Address Confirmation

DAD:

Duplicate Address Detection

DODAG:

Destination-Oriented Directed Acyclic Graph

LLN:

Low-Power and Lossy Network (a typical IoT network)

NA:

Neighbor Advertisement

NCE:

Neighbor Cache Entry

ND:

Neighbor Discovery

NDP:

Neighbor Discovery Protocol

NS:

Neighbor Solicitation

ROVR:

Registration Ownership Verifier (pronounced rover)

RPL:

IPv6 Routing Protocol for LLNs (pronounced ripple)

RA:

Router Advertisement

RS:

Router Solicitation

TSCH:

Timeslotted Channel Hopping

TID:

Transaction ID (a sequence counter in the EARO)

2.3. References

The Terminology used in this document is consistent with and incorporates that described in Terms Used in Routing for Low-Power and Lossy Networks (LLNs)..

Other terms in use in LLNs are found in Terminology for Constrained-Node Networks.

Readers are expected to be familiar with all the terms and concepts that are discussed in

• "Neighbor Discovery for IP version 6",
• "IPv6 Stateless Address Autoconfiguration",
• "Problem Statement and Requirements for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Routing",
• "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and Goals" and
• "Neighbor Discovery Optimization for Low-power and Lossy Networks".

2.4. New Terms

This specification introduces the following terminology:

Backbone Link:

An IPv6 transit link that interconnects two or more Backbone Routers. It is expected to be of high speed compared to the LLN in order to carry the traffic that is required to federate multiple segments of the potentially large LLN into a single IPv6 subnet.

Backbone Router:

A logical network function in an IPv6 router that federates an LLN over a Backbone Link. In order to do so, the Backbone Router (6BBR) proxies the 6LoWPAN ND operations detailed in this document onto the matching operations that run over the backbone, typically IPv6 ND. Note that 6BBR is a logical function, just like 6LR and 6LBR, and that the same physical router may operate all three.

Extended LLN:

Multiple LLNs as defined in [RFC6550], interconnected by a Backbone Link via Backbone Routers, and forming a single IPv6 Multi-Link Subnet.

Registration:

The process during which a 6LN registers an IPv6 Address with a 6LR in order to obtain services such as DAD and routing back. In a Route-Over network, a router that provides connectivity to the LLN (typically a 6LBR, e.g., collocated with a RPL Root) may serve as proxy for the registration of the 6LN to the 6BBR so the 6BBR can provide IPv6 ND proxy services over the Backbone.

Binding:

The association between an IP address, a MAC address, a physical port on a switch, and other information about the node that owns the IP Address.

Registered Node:

The 6LN for which the registration is performed, and which owns the fields in the Extended ARO option.

Registering Node:

The node that performs the registration; this may be the Registered Node, or a proxy such as a 6LBR performing a registration to a 6BBR, on behalf of the Registered Node.

Registered Address:

An address owned by the Registered Node that was or is being registered.

RFC6775-only:

Applied to an implementation, a type of node, or a type of message, this adjective indicates a behavior that is strictly as specified by [RFC6775] as opposed to updated with this specification.

updated:

Qualifies a 6LN, a 6LR, or a 6LBR that supports this specification.

3. Applicability of Address Registration Options

The purpose of the Address Registration Option (ARO) in [RFC6775] is to facilitate duplicate address detection (DAD) for hosts as well as to populate Neighbor Cache Entries (NCEs) [RFC4861] in the routers. This reduces the reliance on multicast operations, which are often as intrusive as broadcast, in IPv6 ND operations.

With this specification, a failed or useless registration can be detected by a 6LR or a 6LBR for reasons other than address duplication. Examples include: the router having run out of space; a registration bearing a stale sequence number perhaps denoting a movement of the host after the registration was placed; a host misbehaving and attempting to register an invalid address such as the unspecified address [RFC4291]; or a host using an address that is not topologically correct on that link.

In such cases the host will receive an error to help diagnose the issue and may retry, possibly with a different address, and possibly registering to a different router, depending on the returned error. The ability to return errors to address registrations is not intended to be used to restrict the ability of hosts to form and use multiple addresses. Rather, the intention is to conform to "Host Address Availability Recommendations".

In particular, the freedom to form and register addresses is needed for enhanced privacy; each host may register a number of addresses using mechanisms such as "Privacy Extensions for Stateless Address Autoconfiguration (SLAAC) in IPv6".

In IPv6 ND [RFC4861], a router needs enough storage to hold NCEs for all directly connected addresses to which it is currently forwarding packets (entries that do not appear to be in use may be flushed). In contrast, a router serving the Address Registration mechanism needs enough storage to hold NCEs for all the addresses that may be registered to it, regardless of whether or not they are actively communicating. The number of registrations supported by a 6LoWPAN Router (6LR) or 6LoWPAN Border Router (6LBR) MUST be clearly documented by the vendor and the dynamic use of associated resources SHOULD be made available to the network operator, e.g., to a management console.

In order to deploy this, network administrators need to ensure that 6LR/6LBRs in their network support the number and type of devices that can register to them, based on the number of IPv6 addresses that those devices require and their address renewal rate and behavior.

4. Extended ND Options and Messages

This specification does not introduce new options, but it modifies existing ones and updates the associated behaviors as specified in the following subsections.

4.1. Extended Address Registration Option (EARO)

The Address Registration Option (ARO) is defined in section 4.1 of [RFC6775]. This specification introduces the Extended Address Registration Option (EARO) based on the ARO for use in NS and NA messages. The EARO conveys additional information such as a sequence counter called Transaction ID (TID) that is used to determine the latest location of a registering mobile device. A 'T' flag is added to indicate that the TID field is populated.

The EARO also signals whether the 6LN expects routing or proxy services from the 6LR using a new 'R' flag.

The EUI-64 field is overloaded and renamed ROVR in order to carry different types of information, e.g., cryptographic information of variable size. A larger ROVR size may be used if and only if backward compatibility is not an issue in the particular deployment.

Section 5.1 discusses those changes in depth.

An NS message with an EARO is a registration if and only if it also carries an SLLA Option [RFC6775]. The EARO is also used in NS and NA messages between Backbone Routers [I-D.ietf-6lo-backbone-router] over the Backbone Link to sort out the distributed registration state; in that case, it does not carry the SLLA Option and is not confused with a registration.

When using the EARO, the address being registered is found in the Target Address field of the NS and NA messages.

The EARO extends the ARO and is indicated by the 'T' flag being set.

The format of the EARO 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      |     Length    |    Status     |   Reserved    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Reserved  |R|T|     TID       |     Registration Lifetime     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
 ...             Registration Ownership Verifier                 ...
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    

Figure 1: EARO

Option Fields

Type:

33

Length:

8-bit unsigned integer. The length of the whole option in units of 8 bytes. It MUST be 2 when operating in a backward-compatible mode with a ROVR size of 64 bits. It MAY be 3, 4 or 5, denoting a ROVR size of 128, 192 and 256 bits respectively.

Status:

8-bit unsigned integer. Indicates the status of a registration in the NA response. MUST be set to 0 in NS messages. See Table 1 below.

Reserved:

This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.

R:

One-bit flag. If the 'R' flag is set, the Registering Node expects that the 6LR ensures reachability for the registered address, e.g., by injecting the address in a Route-Over routing protocol or proxying ND over a Backbone Link.

T:

One-bit flag. Set if the next octet is used as a TID.

TID:

One-byte integer; a Transaction ID that is maintained by the node and incremented with each transaction of one or more registrations performed at the same time to one or more respective 6LRs. This field MUST be ignored if the 'T' flag is not set.

Registration Lifetime:

16-bit integer; expressed in minutes. 0 means that the registration has ended and the associated state MUST be removed.

Registration Ownership Verifier (ROVR):

Enables the correlation between multiple attempts to register a same IPv6 Address. The ROVR is stored in the 6LR and the 6LBR in the state associated to the registration. This can be a unique ID of the Registering Node, such as the EUI-64 address of an interface. This can also be a token obtained with cryptographic methods which can be used in additional protocol exchanges to associate a cryptographic identity (key) with this registration to ensure that only the owner can modify it later. The scope of a ROVR is the registration of a particular IPv6 Address and it must not be used to correlate registrations of different addresses.

4.2. Extended Duplicate Address Message Formats

The DAR and DAC messages are defined in section 4.4 of [RFC6775]. Those messages follow a common base format, which enables information from the ARO to be transported over multiple hops.

Those messages are extended to adapt to the new EARO format, 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      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Status     |     TID       |     Registration Lifetime     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
  ...          Registration Ownership Verifier                    ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                       Registered Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     

Figure 2: Duplicate Address Messages Format

Modified Message Fields

Code:

The ICMP Code as defined in [RFC4443]. The ICMP Code MUST be set to 1 with this specification. An non-null value of the ICMP Code indicates support for this specification.

TID:

1-byte integer; same definition and processing as the TID in the EARO as defined in Section 4.1. This field MUST be ignored if the ICMP Code is null.

Registration Ownership Verifier (ROVR):

The size of the ROVR is computed from the overall size of the IPv6 packet. It MUST be 64bits long when operating in backward-compatible mode. This field has the same definition and processing as the ROVR in the EARO option as defined in Section 4.1.

4.3. New 6LoWPAN Capability Bits in the Capability Indication Option

This specification defines 5 new capability bits for use in the 6CIO, which was introduced by [RFC7400] for use in IPv6 ND RA messages.

This specification introduces the "E" flag to indicate that extended ARO can be used in a registration. A 6LR that supports this specification MUST set the "E" flag.

A similar flag "D" indicates the support of Extended Duplicate Address Messages by the 6LBR; A 6LBR that supports this specification MUST set the "D" flag. The "D" flag is learned from advertisements by a 6LBR, and is propagated down a graph of 6LRs as a node acting as 6LN registers to a 6LR (which could be the 6LBR), and in turn becomes a 6LR to which other 6LNs will register.

The new "L", "B", and "P" flags, indicate whether a router is capable of acting as 6LR, 6LBR, and 6BBR, respectively. These flags are not mutually exclusive and a node MUST set all the flags that are relevant to it.

As an example, a 6LBR sets the "B" and "D" flags. If it acts as a 6LR, then it sets the "L" and "E" flags. If it is collocated with a 6BBR, then it also sets the "P" flag.

    
    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 = 1  |     Reserved      |D|L|B|P|E|G|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Reserved                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    

Figure 3: New capability Bits L, B, P, E in the 6CIO

Option Fields

Type:

36

L:

Node is a 6LR.

B:

Node is a 6LBR.

P:

Node is a 6BBR.

E:

Node supports registrations based on EARO.

D:

6LBR supports EDA messages.

5. Updating RFC 6775

The Extended Address Registration Option (EARO) (see Section 4.1) replaces the ARO used within Neighbor Discovery NS and NA messages between a 6LN and its 6LR. Similarly, the EDA messages, EDAR and EDAC, replace the DAR and DAC messages so as to transport the new information between 6LRs and 6LBRs across an LLN mesh such as a 6TiSCH network.

The extensions to the ARO option are used in the Duplicate Address messages, the Duplicate Address Request (DAR) and Duplicate Address Confirmation (DAC), so as to convey the additional information all the way to the 6LBR. In turn the 6LBR may proxy the registration using IPv6 ND over a Backbone Link as illustrated in Figure 4. Note that this specification avoids the Duplicate Address message flow for Link-Local Addresses in a Route-Over [RFC6606] topology (see Section 5.6).

     6LN              6LR             6LBR            6BBR
      |                |               |                |
      |   NS(EARO)     |               |                |
      |--------------->|               |                |
      |                | Extended DAR  |                |
      |                |-------------->|                |
      |                |               |                |
      |                |               | proxy NS(EARO) |
      |                |               |--------------->|
      |                |               |                | NS(DAD)
      |                |               |                | ------>
      |                |               |                | <wait>
      |                |               |                |
      |                |               | proxy NA(EARO) |
      |                |               |<---------------|
      |                | Extended DAC  |                |
      |                |<--------------|                |
      |   NA(EARO)     |               |                |
      |<---------------|               |                |
      |                |               |                |
    

Figure 4: (Re-)Registration Flow

In order to support various types of link layers, this specification allows multiple registrations, including for privacy / temporary addresses and provides new mechanisms to help clean up stale registration state as soon as possible, e.g., after a movement (see Section 7).

Section 5 of [RFC6775] specifies how a 6LN bootstraps an interface and locates available 6LRs. A Registering Node prefers registering to a 6LR that is found to support this specification, as discussed in Section 6.1, over an RFC6775-only one, and operates in a backward-compatible fashion when attaching to an RFC6775-only 6LR.

5.1. Extending the Address Registration Option

The Extended ARO (EARO) replaces the ARO and is backward compatible with the ARO if and only if the Length of the option is set to 2. Its format is presented in Section 4.1. More details on backward compatibility can be found in Section 6.

The semantics of the Neighbor Solicitation (NS) and the ARO are modified as follows:

• The Target Address in the NS containing the EARO is now the field that indicates the address that is being registered, as opposed to the Source Address field as specified in [RFC6775] (see Section 5.5). This change enables a 6LBR to use one of its addresses as source of the proxy-registration of an address that belongs to a LLN Node to a 6BBR. This also limits the use of an address as source address before it is registered and the associated DAD process is complete.
• The EUI-64 field in the ARO Option is renamed Registration Ownership Verifier (ROVR) and is not required to be derived from a MAC address (see Section 5.3).
• The option Length MAY be different than 2 and take a value between 3 and 5, in which case the EARO is not backward compatible with an ARO. The increase of size corresponds to a larger ROVR field, so the size of the ROVR is inferred from the option Length.
• This document specifies a new flag in the EARO, the 'R' flag. If the 'R' flag is set, the Registering Node expects that the 6LR ensures reachability for the Registered Address, e.g., by means of routing or proxying ND. Conversely, when it is not set, the 'R' flag indicates that the Registering Node is a router, which for instance participates to a Route-Over routing protocol such as RPL [RFC6550] and that it will take care of injecting its Address over the routing protocol by itself. A 6LN that acts only as a host, when registering, MUST set the 'R' flag to indicate that it is not a router and that it will not handle its own reachability. A 6LR that manages its reachability SHOULD NOT set the 'R' flag; if it does, routes towards this router may be installed on its behalf and may interfere with those it injects.
• The specification introduces a Transaction ID (TID) field in the EARO (see Section 5.2). The TID MUST be provided by a node that supports this specification and another new flag, the 'T' flag, MUST be set to indicate so.
• Finally, this specification introduces new status codes to help diagnose the cause of a registration failure (see Table 1).

5.2. Transaction ID

The TID is a sequence number that is incremented by the 6LN with each re-registration to a 6LR. The TID is used to detect the freshness of the registration request and to detect one single registration by multiple 6LoWPAN border routers (e.g., 6LBRs and 6BBRs) supporting the same 6LoWPAN. The TID may also be used by the network to route to the current (freshest known) location of a moving node by spotting the most recent TID.

When a Registered Node is registered with multiple 6BBRs in parallel, the same TID MUST be used. This enables the 6BBRs to determine that the registrations are the same, and distinguish that situation from a movement (see section 4 of [I-D.ietf-6lo-backbone-router] and Section 5.7 below).

5.2.1. Comparing TID values

As a note to the implementer, the operation of the TID is fully compatible with that of the RPL Path Sequence counter as described in the "Sequence Counter Operation" section of the "IPv6 Routing Protocol for Low-Power and Lossy Networks" specification.

A TID is deemed to be fresher than another when its value is greater per the operations detailed in this section.

The TID range is subdivided in a 'lollipop' fashion ([Perlman83]), where the values from 128 and greater are used as a linear sequence to indicate a restart and bootstrap the counter, and the values less than or equal to 127 used as a circular sequence number space of size 128 as in [RFC1982]. Consideration is given to the mode of operation when transitioning from the linear region to the circular region. Finally, when operating in the circular region, if sequence numbers are detected to be too far apart then they are not comparable, as detailed below.

A window of comparison, SEQUENCE_WINDOW = 16, is configured based on a value of 2^N, where N is defined to be 4 in this specification.

For a given sequence counter,

1. The sequence counter SHOULD be initialized to an implementation defined value which is 128 or greater prior to use. A recommended value is 240 (256 - SEQUENCE_WINDOW).
2. When a sequence counter increment would cause the sequence counter to increment beyond its maximum value, the sequence counter MUST wrap back to zero. When incrementing a sequence counter greater than or equal to 128, the maximum value is 255. When incrementing a sequence counter less than 128, the maximum value is 127.
3. When comparing two sequence counters, the following rules MUST be applied:
   1. When a first sequence counter A is in the interval [128..255] and a second sequence counter B is in [0..127]:
      1. If (256 + B - A) is less than or equal to SEQUENCE_WINDOW, then B is greater than A, A is less than B, and the two are not equal.
      2. If (256 + B - A) is greater than SEQUENCE_WINDOW, then A is greater than B, B is less than A, and the two are not equal.

      For example, if A is 240, and B is 5, then (256 + 5 - 240) is 21. 21 is greater than SEQUENCE_WINDOW (16), thus 240 is greater than 5. As another example, if A is 250 and B is 5, then (256 + 5 - 250) is 11. 11 is less than SEQUENCE_WINDOW (16), thus 250 is less than 5.

   2. In the case where both sequence counters to be compared are less than or equal to 127, and in the case where both sequence counters to be compared are greater than or equal to 128:
      1. If the absolute magnitude of difference between the two sequence counters is less than or equal to SEQUENCE_WINDOW, then a comparison as described in [RFC1982] is used to determine the relationships greater than, less than, and equal.
      2. If the absolute magnitude of difference of the two sequence counters is greater than SEQUENCE_WINDOW, then a desynchronization has occurred and the two sequence numbers are not comparable.
4. If two sequence numbers are determined to be not comparable, i.e., the results of the comparison are not defined, then a node should give precedence to the sequence number that was most recently incremented. Failing this, the node should select the sequence number in order to minimize the resulting changes to its own state.

5.3. Registration Ownership Verifier

The ROVR field generalizes the EUI-64 field of the ARO defined in [RFC6775]. It is scoped to a registration and enables recognizing and blocking an attempt to register a duplicate address, which is characterized by a different ROVR in the conflicting registrations. It can also be used to protect the ownership of a Registered Address, if the proof-of-ownership of the ROVR can be obtained (more in Section 5.6).

The ROVR can be of different types, as long as the type is signaled in the message that carries the new type. For instance, the type can be a cryptographic string and used to prove the ownership of the registration as specified in "Address Protected Neighbor Discovery for Low-power and Lossy Networks". In order to support the flows related to the proof-of-ownership, this specification introduces new status codes "Validation Requested" and "Validation Failed" in the EARO.

Note on ROVR collision: different techniques for forming the ROVR will operate in different name-spaces. [RFC6775] operates on EUI-64(TM) addresses. [I-D.ietf-6lo-ap-nd] generates cryptographic tokens. While collisions are not expected in the EUI-64 name-space only, they may happen in the case of [I-D.ietf-6lo-ap-nd] and in a mixed situation. An implementation that understands the name-space MUST consider that ROVRs from different name-spaces are different even if they have the same value. An RFC6775-only 6LR or 6LBR will confuse the name-spaces, which slightly increases the risk of a ROVR collision. A collision of ROVR has no effect if the two Registering Nodes register different addresses, since the ROVR is only significant within the context of one registration. A ROVR is not expected to be unique to one registration, as this specification allows a node to use the same ROVR to register multiple IPv6 addresses. This is why the ROVR MUST NOT be used as a key to identify the Registering Node, or as an index to the registration. It is only used as a match to ensure that the node that updates a registration for an IPv6 address is the node that made the original registration for that IPv6 address. Also, when the ROVR is not an EUI-64 address, then it MUST NOT be used as the interface ID of the Registered Address. This way, a registration that uses that ROVR will not collision with that of an IPv6 Address derived from EUI-64 and using the EUI-64 as ROVR per [RFC6775].

The Registering Node SHOULD store the ROVR, or enough information to regenerate it, in persistent memory. If this is not done and an event such as a reboot causes a loss of state, re-registering the same address could be impossible until the 6LRs and the 6LBR time out the previous registration, or a management action is taken to clear the relevant state in the network.

5.4. Extended Duplicate Address Messages

In order to map the new EARO content in the Extended Duplicate Address (EDA) messages, a new TID field is added to the Extended DAR (EDAR) and the Extended DAC (EDAC) messages as a replacement of the Reserved field, and a non-null value of the ICMP Code indicates support for this specification. The format of the EDA messages is presented in Section 4.2.

As with the EARO, the Extended Duplicate Address messages are backward compatible with the RFC6775-only versions as long as the ROVR field is 64 bits long. Remarks concerning backwards compatibility for the protocol between the 6LN and the 6LR apply similarly between a 6LR and a 6LBR.

5.5. Registering the Target Address

The Registering Node is the node that performs the registration to the 6BBR. As in [RFC6775], it may be the Registered Node as well, in which case it registers one of its own addresses and indicates its own MAC Address as Source Link Layer Address (SLLA) in the NS(EARO).

This specification adds the capability to proxy the registration operation on behalf of a Registered Node that is reachable over an LLN mesh. In that case, if the Registered Node is reachable from the 6BBR over a Mesh-Under mesh, the Registering Node indicates the MAC Address of the Registered Node as the SLLA in the NS(EARO). If the Registered Node is reachable over a Route-Over mesh from the Registering Node, the SLLA in the NS(ARO) is that of the Registering Node. This enables the Registering Node to attract the packets from the 6BBR and route them over the LLN to the Registered Node.

In order to enable the latter operation, this specification changes the behavior of the 6LN and the 6LR so that the Registered Address is found in the Target Address field of the NS and NA messages as opposed to the Source Address. With this convention, a TLLA option indicates the link-layer address of the 6LN that owns the address.

If Registering Node expects packets for the 6LN, e.g., a 6LBR also acting as RPL Root, then it MUST place its own Link Layer Address in the SLLA Option that MUST always be placed in a registration NS(EARO) message. This maintains compatibility with RFC6775-only 6LoWPAN ND.

5.6. Link-Local Addresses and Registration

Considering that LLN nodes are often not wired and may move, there is no guarantee that a Link-Local Address stays unique between a potentially variable and unbounded set of neighboring nodes.

Compared to [RFC6775], this specification only requires that a Link-Local Address be unique from the perspective of the two nodes that use it to communicate (e.g., the 6LN and the 6LR in an NS/NA exchange). This simplifies the DAD process in a Route-Over topology for Link-Local Addresses by avoiding an exchange of EDA messages between the 6LR and a 6LBR for those addresses.

In more details:

An exchange between two nodes using Link-Local Addresses implies that they are reachable over one hop. A node MUST register a Link-Local Address to a 6LR in order to obtain reachability from that 6LR beyond the current exchange, and in particular to use the Link-Local Address as source address to register other addresses, e.g., global addresses.

If there is no collision with an address previously registered to this 6LR by another 6LN, then the Link-Local Address is unique from the standpoint of this 6LR and the registration is not a duplicate. Alternatively, two different 6LRs might expose the same Link-Local Address but different link-layer addresses. In that case, a 6LN MUST only interact with at most one of the 6LRs.

The DAD process between the 6LR and a 6LBR, which is based on an exchange of EDA messages, does not need to take place for Link-Local Addresses.

When registering to a 6LR that conforms to this specification (see Section 6.2, a node MUST use a Link-Local Address as the source address of the registration, whatever the type of IPv6 address that is being registered. That Link-Local Address MUST be either an address that is already registered to the 6LR, or the address that is being registered.

When a Registering Node does not have an already-registered Address, it MUST register a Link-Local Address, using it as both the Source and the Target Address of an NS(EARO) message. In that case, it is RECOMMENDED to use a Link-Local Address that is (expected to be) globally unique, e.g., derived from a globally unique EUI-64 address. A 6LR that supports this specification replies with an NA(EARO), setting the appropriate status.

Since there is no exchange of EDA messages for Link-Local Addresses, the 6LR may answer immediately to the registration of a Link-Local Address, based solely on its existing state and the Source Link-Layer Option that is placed in the NS(EARO) message as required in [RFC6775].

A node needs to register its IPv6 Global Unicast Addresses (GUAs) to a 6LR in order to establish global reachability for these addresses via that 6LR. When registering with an updated 6LR, a Registering Node does not use a GUA as Source Address, in contrast to a node that complies to [RFC6775]. For non-Link-Local Addresses, the exchange of EDA messages MUST conform to [RFC6775], but the extended formats described in this specification for the DAR and the DAC are used to relay the extended information in the case of an EARO.

5.7. Maintaining the Registration States

This section discusses protocol actions that involve the Registering Node, the 6LR, and the 6LBR. It must be noted that the portion that deals with a 6LBR only applies to those addresses that are registered to it; as discussed in Section 5.6, this is not the case for Link-Local Addresses. The registration state includes all data that is stored in the router relative to that registration, in particular, but not limited to, an NCE. 6LBRs and 6BBRs may store additional registration information in more complex abstract data structures and use protocols that are out of scope of this document to keep them synchronized when they are distributed.

When its resource available to store registration states are exhausted, a 6LR cannot accept a new registration. In that situation, the EARO is returned in an NA message with a Status Code of "Neighbor Cache Full" (Table 1), and the Registering Node may attempt to register to another 6LR.

If the registry in the 6LBR is saturated, then the 6LBR cannot decide whether a registration for a new address is a duplicate. In that case, the 6LBR replies to an EDAR message with an EDAC message that carries a new Status Code indicating "6LBR Registry saturated" (Table 1). Note: this code is used by 6LBRs instead of "Neighbor Cache Full" when responding to a Duplicate Address message exchange and is passed on to the Registering Node by the 6LR. There is no point for the node to retry this registration immediately via another 6LR, since the problem is global to the network. The node may either abandon that address, de-register other addresses first to make room, or keep the address in TENTATIVE state and retry later.

A node renews an existing registration by sending a new NS(EARO) message for the Registered Address. In order to refresh the registration state in the 6LBR, the registration MUST be reported to the 6LBR.

A node that ceases to use an address SHOULD attempt to de-register that address from all the 6LRs to which it has registered the address. This is achieved using an NS(EARO) message with a Registration Lifetime of 0. If this is not done, the associated state will remain in the network till the current Registration Lifetime expires and this may lead to a situation where the 6LR resources become saturated, even if they are correctly planned to start with. The 6LR may then take defensive measures that may prevent this node or some other nodes from owning as many addresses as they would expect (see Section 7).

A node that moves away from a particular 6LR SHOULD attempt to de-register all of its addresses registered to that 6LR and register to a new 6LR with an incremented TID. When/if the node shows up elsewhere, an asynchronous NA(EARO) or EDAC message with a Status Code of "Moved" SHOULD be used to clean up the state in the previous location. For instance, as described in [I-D.ietf-6lo-backbone-router], the "Moved" status can be used by a 6BBR in an NA(EARO) message to indicate that the ownership of the proxy state on the Backbone Link was transferred to another 6BBR as the consequence of a movement of the device. If the receiver of the message has a state corresponding to the related address, it SHOULD propagate the status down the forwarding path to the Registered node (e.g., reversing an existing RPL [RFC6550] path as prescribed in [I-D.ietf-roll-efficient-npdao]). Whether it could do so or not, the receiver MUST clean up said state.

Upon receiving an NS(EARO) message with a Registration Lifetime of 0 and determining that this EARO is the freshest for a given NCE (see Section 5.2), a 6LR cleans up its NCE. If the address was registered to the 6LBR, then the 6LR MUST report to the 6LBR, through a Duplicate Address exchange with the 6LBR, indicating the null Registration Lifetime and the latest TID that this 6LR is aware of.

Upon receiving the EDAR message, the 6LBR evaluates if this is the most recent TID it has received for that particular registry entry. If so, then the EDAR is answered with an EDAC message bearing a Status of "Success" and the entry is scheduled to be removed. Otherwise, a Status Code of "Moved" is returned instead, and the existing entry is maintained.

When an address is scheduled to be removed, the 6LBR SHOULD keep its entry in a DELAY state for a configurable period of time, so as to protect a mobile node that de-registered from one 6LR and did not register yet to a new one, or the new registration did not yet reach the 6LBR due to propagation delays in the network. Once the DELAY time is passed, the 6LBR silently removes its entry.

6. Backward Compatibility

This specification changes the behavior of the peers in a registration flow. To enable backward compatibility, a 6LN that registers to a 6LR that is not known to support this specification MUST behave in a manner that is backward-compatible with [RFC6775]. On the contrary, if the 6LR is found to support this specification, then the 6LN MUST conform to this specification when communicating with that 6LR.

A 6LN that supports this specification MUST always use an EARO as a replacement for an ARO in its registration to a router. This is backward-compatible since the 'T' flag and TID field are reserved in [RFC6775], and are ignored by an RFC6775-only router. A router that supports this specification MUST answer an NS(ARO) and an NS(EARO) with an NA(EARO). A router that does not support this specification will consider the ROVR as an EUI-64 address and treat it the same, which has no consequence if the Registered Addresses are different.

6.1. Signaling EARO Capability Support

"Generic Header Compression for IPv6 over 6LoWPANs" introduces the 6LoWPAN Capability Indication Option (6CIO) to indicate a node's capabilities to its peers. The 6CIO MUST be present in both Router Solicitation (RS) and Router Advertisement (RA) messages, unless the information therein was already shared. This can have happened in recent exchanges. The information can also be implicit, or pre-configured in all nodes in a network. In any case, a 6CIO MUST be placed in an RA message that is sent in response to an RS with a 6CIO.

Section 4.3 defines a new flag for the 6CIO to signal support for EARO by the issuer of the message and Section 6.2 specifies how the flag is to be used. New flags are also added to the 6CIO to signal the sender's capability to act as a 6LR, 6LBR, and 6BBR (see Section 4.3).

Section 4.3 also defines a new flag that indicates the support of EDA messages by the 6LBR. This flag is valid in RA messages but not in RS messages. More information on the 6LBR is found in a separate Authoritative Border Router Option (ABRO). The ABRO is placed in RA messages as prescribed by [RFC6775]; in particular, it MUST be placed in an RA message that is sent in response to an RS with a 6CIO indicating the capability to act as a 6LR, since the RA propagates information between routers.

6.2. First Exchanges

A typical flow when a node starts up is that it sends a multicast RS and receives one or more unicast RA messages. If the 6LR can process Extended ARO, then it places a 6CIO in its RA message back with the "E" Flag set as required in Section 6.1.

In order to ensure that it registers a first address successfully a 6LN MAY register a Link Local Address that is derived from an EUI-64, placing the same address in the Source and Target Address fields of the NS(EARO) message. For such an address, DAD is not required (see [RFC6775]) and using the SLLA Option in the NS is actually more consistent with existing ND specifications such as the "Optimistic Duplicate Address Detection (ODAD) for IPv6". The 6LN MAY then use that address to register one or more other addresses.

6.3. RFC6775-only 6LoWPAN Node

An RFC6775-only 6LN will use the Registered Address as the source address of the NS message and will not use an EARO. An updated 6LR MUST accept that registration if it is valid per [RFC6775], and it MUST manage the binding cache accordingly. The updated 6LR MUST then use the RFC6775-only EDA messages as specified in [RFC6775] to indicate to the 6LBR that the TID is not present in the messages.

The main difference from [RFC6775] is that the exchange of EDA messages for the purpose of DAD is avoided for Link-Local Addresses. In any case, the 6LR MUST use an EARO in the reply, and can use any of the Status codes defined in this specification.

6.4. RFC6775-only 6LoWPAN Router

An updated 6LN discovers the capabilities of the 6LR in the 6CIO in RA messages from that 6LR; if the 6CIO was not present in the RA, then the 6LR is assumed to be a RFC6775-only 6LoWPAN Router.

An updated 6LN MUST use an EARO in the request regardless of the type of 6LR, RFC6775-only or updated, which implies that the 'T' flag is set. It MUST use a ROVR of 64 bits if the 6LR is an RFC6775-only 6LoWPAN Router.

If an updated 6LN moves from an updated 6LR to an RFC6775-only 6LR, the RFC6775-only 6LR will send an RFC6775-only DAR message, which cannot be compared with an updated one for freshness. Allowing RFC6775-only DAR messages to replace a state established by the updated protocol in the 6LBR would be an attack vector and that cannot be the default behavior. But if RFC6775-only and updated 6LRs coexist temporarily in a network, then it makes sense for an administrator to install a policy that allows this, and the capability to install such a policy should be configurable in a 6LBR though it is out of scope for this document.

6.5. RFC6775-only 6LoWPAN Border Router

With this specification, the Duplicate Address messages are extended to transport the EARO information. Similarly to the NS/NA exchange, an updated 6LBR MUST always use the EDA messages.

Note that an RFC6775-only 6LBR will accept and process an EDAR message as if it were an RFC6775-only DAR, as long as the ROVR is 64 bits long. An updated 6LR discovers the capabilities of the 6LBR in the 6CIO in RA messages from the 6LR; if the 6CIO was not present in any RA, then the 6LBR si assumed to be a RFC6775-only 6LoWPAN Border Router.

If the 6LBR is RFC6775-only, and the ROVR in the NS(EARO) was more than 64 bits long, then the 6LR MUST truncate the ROVR to the 64 rightmost bit and place the result in the EDAR message to maintain compatibility. This way, the support of DAD is preserved.

7. Security Considerations

This specification extends [RFC6775], and the security section of that document also applies to this as well. In particular, it is expected that the link layer is sufficiently protected to prevent rogue access, either by means of physical or IP security on the Backbone Link and link-layer cryptography on the LLN.

[RFC6775] does not protect the content of its messages and expects a lower layer encryption to defeat potential attacks. This specification also expects that the LLN MAC provides secure unicast to/from the Backbone Router and secure Broadcast or Multicast from the Backbone Router in a way that prevents tampering with or replaying the Neighbor Discovery messages.

This specification recommends using privacy techniques (see Section 8) and protecting against address theft such as provided by "Address Protected Neighbor Discovery for Low-power and Lossy Networks", which guarantees the ownership of the Registered Address using a cryptographic ROVR.

The registration mechanism may be used by a rogue node to attack the 6LR or the 6LBR with a Denial-of-Service attack against the registry. It may also happen that the registry of a 6LR or a 6LBR is saturated and cannot take any more registrations, which effectively denies the requesting node the capability to use a new address. In order to alleviate those concerns, Section 5.7 provides a number of recommendations that ensure that a stale registration is removed as soon as possible from the 6LR and 6LBR. In particular, this specification recommends that:

• A node that ceases to use an address SHOULD attempt to de-register that address from all the 6LRs to which it is registered. See Section 5.2 for the mechanism to avoid replay attacks and avoiding the use of stale registration information.
• The Registration lifetimes SHOULD be individually configurable for each address or group of addresses. The nodes SHOULD be configured with a Registration Lifetime that reflects their expectation of how long they will use the address with the 6LR to which it is registered. In particular, use cases that involve mobility or rapid address changes SHOULD use lifetimes that are larger yet of a same order as the duration of the expectation of presence.
• The router (6LR or 6LBR) SHOULD be configurable so as to limit the number of addresses that can be registered by a single node, but as a protective measure only. In any case, a router MUST be able to keep a minimum number of addresses per node. That minimum depends on the type of device and ranges between 3 for a very constrained LLN and 10 for a larger device. A node may be identified by its MAC address, as long as it is not obfuscated by privacy measures. A stronger identification (e.g., by security credentials) is RECOMMENDED. When the maximum is reached, the router should use a Least-Recently-Used (LRU) algorithm to clean up the addresses, keeping at least one Link-Local Address. The router SHOULD attempt to keep one or more stable addresses if stability can be determined, e.g., because they are used over a much longer time span than other (privacy, shorter-lived) addresses.
• In order to avoid denial of registration for the lack of resources, administrators should take great care to deploy adequate numbers of 6LRs to cover the needs of the nodes in their range, so as to avoid a situation of starving nodes. It is expected that the 6LBR that serves an LLN is a more capable node than the average 6LR, but in a network condition where it may become saturated, a particular deployment should distribute the 6LBR functionality, for instance by leveraging a high speed Backbone Link and Backbone Routers to aggregate multiple LLNs into a larger subnet.

The LLN nodes depend on the 6LBR and the 6BBR for their operation. A trust model must be put in place to ensure that the right devices are acting in these roles so as to avoid threats such as black-holing or bombing attack whereby an impersonated 6LBR would destroy state in the network by using the "Removed" Status code. This trust model could be at a minimum based on a Layer-2 access control, or could provide role validation as well (see Req5.1 in Appendix B.5).

8. Privacy Considerations

As indicated in Section 3, this protocol does not inherently limit the number of IPv6 addresses that each device can form. However, to mitigate denial-of-service attacks, it can be useful as a protective measure to have a limit that is high enough not to interfere with the normal behavior of devices in the network. A host should be able to form and register any address that is topologically correct in the subnet(s) advertised by the 6LR/6LBR.

This specification does not mandate any particular way for forming IPv6 addresses, but it discourages using EUI-64 for forming the Interface ID in the Link-Local Address because this method prevents the usage of "SEcure Neighbor Discovery (SEND)", "Cryptographically Generated Addresses (CGA)", and that of address privacy techniques.

"Privacy Considerations for IPv6 Adaptation-Layer Mechanisms" explains why privacy is important and how to form privacy-aware addresses. All implementations and deployments must consider the option of privacy addresses in their own environments.

The IPv6 address of the 6LN in the IPv6 header can be compressed statelessly when the Interface Identifier in the IPv6 address can be derived from the Lower Layer address. When it is not critical to benefit from that compression, e.g., the address can be compressed statefully, or it is rarely used and/or it is used only over one hop, then privacy concerns should be considered. In particular, new implementations should follow the IETF "Recommendation on Stable IPv6 Interface Identifiers". [RFC8064] recommends the use of "A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)" for generating Interface Identifiers to be used in SLAAC.

9. IANA Considerations

Note to RFC Editor, to be removed: please replace "This RFC" throughout this document by the RFC number for this specification once it is allocated.

IANA is requested to make a number of changes under the "Internet Control Message Protocol version 6 (ICMPv6) Parameters" registry, as follows.

9.1. ARO Flags

IANA is requested to create a new subregistry for "ARO Flags". This specification defines 8 positions, bit 0 to bit 7, and assigns bit 6 for the 'R' flag and bit 7 for the 'T' flag (see Section 4.1). The policy is "IETF Review" or "IESG Approval" [RFC8126]. The initial content of the registry is as shown in Table 2.

New subregistry for ARO Flags under the "Internet Control Message Protocol version 6 (ICMPv6) [RFC4443] Parameters"

9.2. ICMP Codes

IANA is requested to create 2 new subregistries of the ICMPv6 "Code" Fields registry, which itself is a subregistry of the Internet Control Message Protocol version 6 (ICMPv6) Parameters for the ICMP codes. The new subregistries relate to the ICMP type 157, Duplicate Address Request (shown in Table 3), and 158, Duplicate Address Confirmation (shown in Table 4), respectively. The range of an ICMPv6 "Code" Field is 0..255 in all cases. The policy is "IETF Review" or "IESG Approval" [RFC8126] for both subregistries. The new subregistries are initialized as follows:

New entries for ICMP types 157 DAR message

New entries for ICMP types 158 DAC message

9.3. New ARO Status values

IANA is requested to make additions to the Address Registration Option Status Values Registry as follows:

Address Registration Option Status Values Registry

9.4. New 6LoWPAN capability Bits

IANA is requested to make additions to the Subregistry for "6LoWPAN capability Bits" as follows:

Subregistry for "6LoWPAN capability Bits" under the "Internet Control Message Protocol version 6 (ICMPv6) Parameters"

10. Acknowledgments

Kudos to Eric Levy-Abegnoli who designed the First Hop Security infrastructure upon which the first backbone router was implemented. Many thanks to Sedat Gormus, Rahul Jadhav, Tim Chown, Juergen Schoenwaelder, Chris Lonvick, Dave Thaler, Adrian Farrel, Peter Yee, Warren Kumari, Benjamin Kaduk, Mirja Kuhlewind, and Lorenzo Colitti for their various contributions and reviews. Also, many thanks to Thomas Watteyne for the world first implementation of a 6LN that was instrumental to the early tests of the 6LR, 6LBR and Backbone Router.

11. References

11.1. Normative References

11.2. Terminology Related References

11.3. Informative References

11.4. External Informative References

Appendix A. Applicability and Requirements Served (Not Normative)

This specification extends 6LoWPAN ND to provide a sequence number to the registration and serves the requirements expressed in Appendix B.1 by enabling the mobility of devices from one LLN to the next based on the complementary work in the "IPv6 Backbone Router" specification.

In the context of the Timeslotted Channel Hopping (TSCH) mode of IEEE Std. 802.15.4, the "6TiSCH architecture" introduces how a 6LoWPAN ND host could connect to the Internet via a RPL mesh network, but this requires additions to the 6LoWPAN ND protocol to support mobility and reachability in a secured and manageable environment. This specification details the new operations that are required to implement the 6TiSCH architecture and serves the requirements listed in Appendix B.2.

The term LLN is used loosely in this specification to cover multiple types of WLANs and WPANs, including Low-Power IEEE Std. 802.11 networking, Bluetooth Low Energy, IEEE Std. 802.11ah, and IEEE Std. 802.15.4 wireless meshes, so as to address the requirements discussed in Appendix B.3.

This specification can be used by any wireless node to associate at Layer-3 with a 6BBR and register its IPv6 addresses to obtain routing services including proxy-ND operations over a Backbone Link, effectively providing a solution to the requirements expressed in Appendix B.4.

This specification is extended by "Address Protected Neighbor Discovery for Low-power and Lossy Networks" to providing a solution to some of the security-related requirements expressed in Appendix B.5.

"Efficiency aware IPv6 Neighbor Discovery Optimizations" suggests that 6LoWPAN ND can be extended to other types of links beyond IEEE Std. 802.15.4 for which it was defined. The registration technique is beneficial when the Link-Layer technique used to carry IPv6 multicast packets is not sufficiently efficient in terms of delivery ratio or energy consumption in the end devices, in particular to enable energy-constrained sleeping nodes. The value of such extension is especially apparent in the case of mobile wireless nodes, to reduce the multicast operations that are related to IPv6 ND ([RFC4861], [RFC4862]) and affect the operation of the wireless medium [I-D.ietf-mboned-ieee802-mcast-problems] [I-D.perkins-intarea-multicast-ieee802]. This serves the scalability requirements listed in Appendix B.6.

Appendix B. Requirements (Not Normative)

This section lists requirements that were discussed by the 6lo WG for an update to 6LoWPAN ND. How those requirements are matched with existing specifications at the time of this writing is shown in Appendix B.8.

B.1. Requirements Related to Mobility

Due to the unstable nature of LLN links, even in an LLN of immobile nodes, a 6LN may change its point of attachment from 6LR-a to 6LR-b, and may not be able to notify 6LR-a. Consequently, 6LR-a may still attract traffic that it cannot deliver any more. When links to a 6LR change state, there is thus a need to identify stale states in a 6LR and restore reachability in a timely fashion, e.g., by using some signaling upon the detection of the movement, or using a keep-alive mechanism with a period that is consistent with the application needs.

Req1.1: Upon a change of point of attachment, connectivity via a new 6LR MUST be restored in a timely fashion without the need to de-register from the previous 6LR.

Req1.2: For that purpose, the protocol MUST enable differentiating between multiple registrations from one 6LoWPAN Node and registrations from different 6LoWPAN Nodes claiming the same address.

Req1.3: Stale states MUST be cleaned up in 6LRs.

Req1.4: A 6LoWPAN Node SHOULD also be able to register its Address concurrently to multiple 6LRs.

B.2. Requirements Related to Routing Protocols

The point of attachment of a 6LN may be a 6LR in an LLN mesh. IPv6 routing in an LLN can be based on RPL, which is the routing protocol that was defined by the IETF for this particular purpose. Other routing protocols are also considered by Standards Development Organizations (SDO) on the basis of the expected network characteristics. It is required that a 6LN attached via ND to a 6LR indicates whether it participates in the selected routing protocol to obtain reachability via the 6LR, or whether it expects the 6LR to manage its reachability.

Beyond the 6LBR unicast address registered by ND, other addresses including multicast addresses are needed as well. For example, a routing protocol often uses a multicast address to register changes to established paths. ND needs to register such a multicast address to enable routing concurrently with discovery.

Multicast is needed for groups. Groups may be formed by device type (e.g., routers, street lamps), location (Geography, RPL sub-tree), or both.

The Bit Index Explicit Replication (BIER) Architecture proposes an optimized technique to enable multicast in an LLN with a very limited requirement for routing state in the nodes.

Related requirements are:

Req2.1: The ND registration method SHOULD be extended so that the 6LR is instructed whether to advertise the Address of a 6LN over the selected routing protocol and obtain reachability to that Address using the selected routing protocol.

Req2.2: Considering RPL, the Address Registration Option that is used in the ND registration SHOULD be extended to carry enough information to generate a DAO message as specified in section 6.4 of [RFC6550], in particular the capability to compute a Path Sequence and, as an option, a RPLInstanceID.

Req2.3: Multicast operations SHOULD be supported and optimized, for instance, using BIER or MPL. Whether ND is appropriate for the registration to the 6BBR is to be defined, considering the additional burden of supporting the Multicast Listener Discovery Version 2 (MLDv2) for IPv6.

B.3. Requirements Related to the Variety of Low-Power Link types

6LoWPAN ND was defined with a focus on IEEE Std.802.15.4 and in particular the capability to derive a unique identifier from a globally unique EUI-64 address. At this point, the 6lo Working Group is extending the 6LoWPAN Header Compression (HC) technique to other link types including ITU-T G.9959, Master-Slave/Token-Passing, DECT Ultra Low Energy, Near Field Communication, IEEE Std. 802.11ah, as well as Bluetooth(R) Low Energy, and Power Line Communication (PLC) Networks.

Related requirements are:

Req3.1: The support of the registration mechanism SHOULD be extended to more LLN links than IEEE Std.802.15.4, matching at least the LLN links for which an "IPv6 over foo" specification exists, as well as Low-Power Wi-Fi.

Req3.2: As part of this extension, a mechanism to compute a unique identifier should be provided, with the capability to form a Link-Local Address that SHOULD be unique at least within the LLN connected to a 6LBR discovered by ND in each node within the LLN.

Req3.3: The Address Registration Option used in the ND registration SHOULD be extended to carry the relevant forms of unique Identifier.

Req3.4: The Neighbor Discovery should specify the formation of a site-local address that follows the security recommendations from [RFC7217].

B.4. Requirements Related to Proxy Operations

Duty-cycled devices may not be able to answer themselves to a lookup from a node that uses IPv6 ND on a Backbone Link and may need a proxy. Additionally, the duty-cycled device may need to rely on the 6LBR to perform registration to the 6BBR.

The ND registration method SHOULD defend the addresses of duty-cycled devices that are sleeping most of the time and not capable to defend their own addresses.

Related requirements are:

Req4.1: The registration mechanism SHOULD enable a third party to proxy register an address on behalf of a 6LoWPAN node that may be sleeping or located deeper in an LLN mesh.

Req4.2: The registration mechanism SHOULD be applicable to a duty-cycled device regardless of the link type and SHOULD enable a 6BBR to operate as a proxy to defend the Registered Addresses on its behalf.

Req4.3: The registration mechanism SHOULD enable long sleep durations, on the order of multiple days to a month.

B.5. Requirements Related to Security

In order to guarantee the operations of the 6LoWPAN ND flows, the spoofing of the 6LR, 6LBR, and 6BBRs roles should be avoided. Once a node successfully registers an address, 6LoWPAN ND should provide energy-efficient means for the 6LBR to protect that ownership even when the node that registered the address is sleeping.

In particular, the 6LR and the 6LBR then should be able to verify whether a subsequent registration for a given address comes from the original node.

In an LLN it makes sense to base security on Layer-2 security. During bootstrap of the LLN, nodes join the network after authorization by a Joining Assistant (JA) or a Commissioning Tool (CT). After joining, nodes communicate with each other via secured links. The keys for the Layer-2 security are distributed by the JA/CT. The JA/CT can be part of the LLN or be outside the LLN. In both cases it is needed that packets are routed between JA/CT and the joining node.

Related requirements are:

Req5.1: 6LoWPAN ND security mechanisms SHOULD provide a mechanism for the 6LR, 6LBR, and 6BBR to authenticate and authorize one another for their respective roles, as well as with the 6LoWPAN Node for the role of 6LR.

Req5.2: 6LoWPAN ND security mechanisms SHOULD provide a mechanism for the 6LR and the 6LBR to validate new registration of authorized nodes. Joining of unauthorized nodes MUST be prevented.

Req5.3: 6LoWPAN ND security mechanisms SHOULD NOT lead to large packet sizes. In particular, the NS, NA, DAR, and DAC messages for a re-registration flow SHOULD NOT exceed 80 octets so as to fit in a secured IEEE Std.802.15.4 [IEEEstd802154] frame.

Req5.4: Recurrent 6LoWPAN ND security operations MUST NOT be computationally intensive on the LoWPAN Node CPU. When a Key hash calculation is employed, a mechanism lighter than SHA-1 SHOULD be preferred.

Req5.5: The number of Keys that the 6LoWPAN Node needs to manipulate SHOULD be minimized.

Req5.6: The 6LoWPAN ND security mechanisms SHOULD enable the variation of CCM [RFC3610] called CCM* for use at both Layer 2 and Layer 3, and SHOULD enable the reuse of security code that has to be present on the device for upper layer security such as TLS. Algorithm agility and support for large keys (e.g., 256-bit key sizes) is also desirable, following at Layer-3 the introduction of those capabilities at Layer-2.

Req5.7: Public key and signature sizes SHOULD be minimized while maintaining adequate confidentiality and data origin authentication for multiple types of applications with various degrees of criticality.

Req5.8: Routing of packets should continue when links pass from the unsecured to the secured state.

Req5.9: 6LoWPAN ND security mechanisms SHOULD provide a mechanism for the 6LR and the 6LBR to validate whether a new registration for a given address corresponds to the same 6LN that registered it initially, and, if not, determine the rightful owner and deny or clean up the registration that is duplicate.

B.6. Requirements Related to Scalability

Use cases from Automatic Meter Reading (AMR, collection tree operations) and Advanced Metering Infrastructure (AMI, bi-directional communication to the meters) indicate the needs for a large number of LLN nodes pertaining to a single RPL DODAG (e.g., 5000) and connected to the 6LBR over a large number of LLN hops (e.g., 15).

Related requirements are:

Req6.1: The registration mechanism SHOULD enable a single 6LBR to register multiple thousands of devices.

Req6.2: The timing of the registration operation should allow for a large latency such as found in LLNs with ten to more hops.

B.7. Requirements Related to Operations and Management

Section 3.8 of "Architectural Principles of the Internet" recommends to: "avoid options and parameters whenever possible. Any options and parameters should be configured or negotiated dynamically rather than manually". This is especially true in LLNs where the number of devices may be large and manual configuration is infeasible. Capabilities for a dynamic configuration of LLN devices can also be constrained by the network and power limitation.

A Network Administrator should be able to validate that the network is operating within capacity, and that in particular a 6LBR does not get overloaded with an excessive amount of registration, so the administrator can take actions such as adding a Backbone Link with additional 6LBRs and 6BBRs to the network.

Related requirements are:

Req7.1: A management model SHOULD be provided that enables access to the 6LBR, monitor its usage vs. capacity, and alert in case of congestion. It is recommended that the 6LBR be reachable over a non-LLN link.

Req7.2: A management model SHOULD be provided that enables access to the 6LR and its capacity to host additional NCE. This management model SHOULD avoid polling individual 6LRs in a way that could disrupt the operation of the LLN.

Req7.3: Information on successful and failed registration SHOULD be provided, including information such as the ROVR of the 6LN, the Registered Address, the address of the 6LR, and the duration of the registration flow.

Req7.4: In case of a failed registration, information on the failure including the identification of the node that rejected the registration and the status in the EARO SHOULD be provided.

B.8. Matching Requirements with Specifications

I-drafts/RFCs addressing requirements

Authors' Addresses