Fixing the C-Flag in EARO

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Table of Contents

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 the 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, thereby avoiding the overlapping definitions.¶

Figure 1 below 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 below 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:

1-bit flag; set to 1 to indicate that the sender expects other routers to forward packets to self when the packets are sourced within the registered prefix. For use in NS messages only [I-D.ietf-6lo-prefix-registration].¶

Prefix Length

7-bit integer. This field contains a prefix length expressed in bits if the P-field is set to 3 and the EARO is placed in an NS message. In that case, the value MUST be at least 16 and at most 120. The field contains a Status if the EARO is placed in an NA message regardless of the setting of the P-flag. In all other cases, it is reserved, so it MUST be set to 0 by the sender and ignored by the receiver. For use in NS messages only [I-D.ietf-6lo-prefix-registration].¶

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:

8-bit field opaque to ND. It MUST be set to 0 when not used. Defined in [RFC8505].¶

r (reserVed):

1-bit reserved field. It 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, or anycast, or derived from the unicast prefix that is being registered. Used to transport the RATInd in different protocols. The values for the RATInd field are available in [IANA.ICMP.ARO.P-FIELD].¶

I:

2-bit integer field that helps to decide how to use the 8-bit Opaque field. When 0, it means the Opaque field has info about the default routing path. Other values are reserved. Defined in [RFC8505].¶

R:

1-bit flag. Registering Node sets the R-flag to request reachability services for the Registered Address from a Routing Registrar. Defined in [RFC8505].¶

T:

1-bit flag. Set if the next octet is used as a TID. Defined in [RFC8505].¶

TID (Transaction ID):

8-bit unsigned integer that tracks transactions for registrations. It is incremented with each new transaction and ignored unless the T-flag is set. Defined in [RFC8505].¶

Registration Lifetime:

16-bit integer representing the lifetime in minutes of the Registered Address. Zero indicates the registration has ended, and the associated state MUST be removed. Defined in [RFC8505].¶

Registration Ownership Verifier (ROVR):

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

Authors' Addresses