RFC 9927: Fixing the C-Flag in the Extended Address Registration Option (EARO)

Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9927.¶

Copyright Notice

Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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1. Introduction

The Address-Protected Neighbor Discovery for Low-Power and Lossy Networks (AP-ND) [RFC8928] defined the C-flag in EARO. It is used to indicate that the Registration Ownership Verifier (ROVR) field contains a Crypto-ID and that the 6LoWPAN Node (6LN) may be challenged for ownership of the registered address. Initially, [RFC8928] defined the C-flag in the EARO in bit position 3; later, [RFC9685] defined the P-Field in bits 2 and 3 of the EARO flags field with proper IANA registration, causing an overlap with Figure 1 of [RFC8928], which depicts the location of the C-flag.¶

This specification updates [RFC8928] by repositioning the C-flag as bit 1 of the EARO flags field, thereby preventing conflicts.¶

3. Updating RFC 8928

[RFC8928] incorrectly refers to the Extended Address Registration Option (EARO) as the Enhanced Address Registration Option. This specification corrects this terminology throughout the document.¶

In [RFC8928], the C-flag is specified in the EARO flags field at bit position 3 (as depicted in Figure 1 of [RFC8928]); however, [RFC8928] fails to register its position with IANA. Later, [RFC9685] defined the P-Field in bits 2 and 3 of the EARO flags field and obtained proper IANA registration, but this introduced an overlap with the representation in [RFC8928]. To resolve the conflict, this specification updates [RFC8928] by repositioning the C-flag to bit 1 of the EARO flags field, ensuring there are no overlapping definitions.¶

Figure 1 replaces Figure 1 in [RFC8928] in the case of an EARO used in an NS message.¶

   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    |F|Prefix Length|    Opaque     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |r|C| P | I |R|T|     TID       |     Registration Lifetime     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
 ...            Registration Ownership Verifier (ROVR)           ...
  |                  (64, 128, 192, or 256 bits)                  |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2 replaces Figure 1 in [RFC8928] in the case of an EARO used in an NA message. The difference between the two formats is in the usage of bits 16 to 23.¶

   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    | r |  Status   |    Opaque     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |r|C| P | I |R|T|     TID       |     Registration Lifetime     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
 ...            Registration Ownership Verifier (ROVR)           ...
  |                  (64, 128, 192, or 256 bits)                  |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Option fields of interest for this specification:¶

Type:

33¶

Length:

Defined in [RFC8505]¶

F:

Defined in [RFC9926]¶

Prefix Length

Defined in [RFC9926]¶

Status:

6-bit unsigned integer. This field is used in NA(EARO) response messages only to indicate the status of a registration. This field is defined in [RFC8505] and resized by [RFC9010]. The values for the Status field are available in [IANA.ICMP.ARO.STAT]. This field MUST be set to 0 in NS(EARO) messages unless the registration is for a prefix, in which case the F-flag is set and the prefix length is provided.¶

Opaque:

Defined in [RFC8505]¶

r (reserved):

1-bit reserved field in NS(EARO) and NA(EARO) as depicted in Figure 1 and Figure 2. 2-bit reserved field (most significant bits of Status filed) in NA(EARO) as depicted in Figure 2. All reserved field MUST be set to zero by the sender and MUST be ignored by the receiver.¶

C:

1-bit flag, moved from its position in Figure 1 of [RFC8928]. It is set to indicate that the ROVR field contains a Crypto-ID and that the 6LN MAY be challenged for ownership.¶

P:

2-bit field for Registered Address Type Indicator (RATInd). Indicates whether the registered address is unicast, multicast, anycast, or derived from the registered unicast prefix. Used to transport the RATInd in different protocols. The values for the RATInd field are available in [IANA.ICMP.ARO.P-FIELD].¶

I:

Defined in [RFC8505]¶

R:

Defined in [RFC8505]¶

T:

Defined in [RFC8505]¶

TID (Transaction ID):

Defined in [RFC8505]¶

Registration Lifetime:

Defined in [RFC8505]¶

Registration Ownership Verifier (ROVR):

Defined in [RFC8505]. Variable-length field used to verify who "owns" a registered IPv6 address. When the C-flag is set, this field contains a Crypto-ID [RFC8928].¶

Authors' Addresses