4. New Neighbor Discovery Options and Messages
4. New Neighbor Discovery Options and Messages
This section defines new Neighbor Discovery message options used by this specification. The Address Registration Option is used by hosts, whereas the Authoritative Border Router Option and 6LoWPAN Context Option are used in the substitutable router-to-router interaction. This section also defines the new router-to-router Duplicate Address Request and Duplicate Address Confirmation messages.
4.1. Address Registration Option
The routers need to know the set of host IP addresses that are directly reachable and their corresponding link-layer addresses. This needs to be maintained as the radio reachability changes. For this purpose, an Address Registration Option (ARO) is introduced, which can be included in unicast NS messages sent by hosts. Thus, it can be included in the unicast NS messages that a host sends as part of NUD to determine that it can still reach a default router. The ARO is used by the receiving router to reliably maintain its Neighbor Cache. The same option is included in corresponding NA messages with a Status field indicating the success or failure of the registration. This option is always host initiated.
The information contained in the ARO is also included in the multihop DAR and DAC messages used between 6LRs and 6LBRs, but the option itself is not used in those messages.
The ARO is required for reliability and power saving. The lifetime field provides flexibility to the host to register an address that should be usable (continue to be advertised by the 6LR in the routing protocol, etc.) during its intended sleep schedule.
The sender of the NS also includes the EUI-64 [EUI64] of the interface from which it is registering an address. This is used as a unique ID for the detection of duplicate addresses. It is used to tell the difference between the same node re-registering its address and a different node (with a different EUI-64) registering an address that is already in use by someone else. The EUI-64 is also used to deliver an NA carrying an error Status code to the EUI-64-based link-local IPv6 address of the host (see Section 6.5.2).
When the ARO is used by hosts, an SLLAO (Source Link-Layer Address Option) [RFC4861] MUST be included, and the address that is to be registered MUST be the IPv6 source address of the NS message.
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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 = 2 | Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ EUI-64 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 33
Length: 8-bit unsigned integer. The length of the option in units of 8 bytes. Always 2.
Status: 8-bit unsigned integer. Indicates the status of a registration in the NA response. MUST be set to 0 in NS messages. See below.
Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Registration Lifetime: 16-bit unsigned integer. The amount of time in units of 60 seconds that the router should retain the NCE for the sender of the NS that includes this option.
EUI-64: 64 bits. This field is used to uniquely identify the interface of the Registered Address by including the EUI-64 identifier [EUI64] assigned to it unmodified.
The Status values used in NAs are:
+--------+--------------------------------------------+
| Status | Description |
+--------+--------------------------------------------+
| 0 | Success |
| 1 | Duplicate Address |
| 2 | Neighbor Cache Full |
| 3-255 | Allocated using Standards Action [RFC5226] |
+--------+--------------------------------------------+
Table 1
4.2. 6LoWPAN Context Option
The 6LoWPAN Context Option (6CO) carries prefix information for LoWPAN header compression and is similar to the PIO of [RFC4861]. However, the prefixes can be remote as well as local to the LoWPAN, since header compression potentially applies to all IPv6 addresses. This option allows for the dissemination of multiple contexts identified by a CID for use as specified in [RFC6282]. A context may be a prefix of any length or an address (/128), and up to 16 6COs may be carried in an RA message.
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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 |Context Length | Res |C| CID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Context Prefix .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: 6LoWPAN Context Option Format
Type: 34
Length: 8-bit unsigned integer. The length of the option (including the Type and Length fields) in units of 8 bytes. May be 2 or 3, depending on the length of the Context Prefix field.
Context Length: 8-bit unsigned integer. The number of leading bits in the Context Prefix field that are valid. The value ranges from 0 to 128. If it is more than 64, then the Length MUST be 3.
C: 1-bit context Compression flag. This flag indicates if the context is valid for use in compression. A context that is not valid MUST NOT be used for compression but SHOULD be used in decompression in case another compressor has not yet received the updated context information. This flag is used to manage the context life cycle based on the recommendations in Section 7.2.
CID: 4-bit Context Identifier for this prefix information. The CID is used by context-based header compression as specified in [RFC6282]. The list of CIDs for a LoWPAN is configured on the 6LBR that originates the context information for the 6LoWPAN.
Res, Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Valid Lifetime: 16-bit unsigned integer. The length of time in units of 60 seconds (relative to the time the packet is received) that the context is valid for the purpose of header compression or decompression. A value of all zero bits (0x0) indicates that this context entry MUST be removed immediately.
Context Prefix: The IPv6 prefix or address corresponding to the CID field. The valid length of this field is included in the Context Length field. This field is padded with zeros in order to make the option a multiple of 8 bytes.
4.3. Authoritative Border Router Option
The Authoritative Border Router Option (ABRO) is needed when RA messages are used to disseminate prefixes and context information across a route-over topology. In this case, 6LRs receive PIOs from other 6LRs. This implies that a 6LR can't just let the most recently received RA win. In order to be able to reliably add and remove prefixes from the 6LoWPAN, we need to carry information from the authoritative 6LBR. This is done by introducing a version number that the 6LBR sets and that 6LRs propagate as they propagate the prefix and context information with this ABRO. When there are multiple 6LBRs, they would have separate version number spaces. Thus, this option needs to carry the IP address of the 6LBR that originated that set of information.
The ABRO MUST be included in all RA messages in the case when RAs are used to propagate information between routers (as described in Section 8.2).
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length = 3 | Version Low |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version High | Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ 6LBR Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 35
Length: 8-bit unsigned integer. The length of the option in units of 8 bytes. Always 3.
Version Low, Version High: Together, Version Low and Version High constitute the Version Number field, a 32-bit unsigned integer where Version Low is the least significant 16 bits and Version High is the most significant 16 bits. The version number corresponding to this set of information contained in the RA message. The authoritative 6LBR originating the prefix increases this version number each time its set of prefix or context information changes.
Valid Lifetime: 16-bit unsigned integer. The length of time in units of 60 seconds (relative to the time the packet is received) that this set of border router information is valid. A value of all zero bits (0x0) assumes a default value of 10,000 (~one week).
Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
6LBR Address: IPv6 address of the 6LBR that is the origin of the included version number.
4.4. Duplicate Address Messages
For the multihop DAD exchanges between a 6LR and 6LBR as specified in Section 8.2, there are two new ICMPv6 message types called the Duplicate Address Request (DAR) and the Duplicate Address Confirmation (DAC). We avoid reusing the NS and NA messages for this purpose, since these messages are not subject to the hop limit=255 check as they are forwarded by intermediate 6LRs. The information contained in the messages is otherwise the same as would be in an NS carrying an ARO, with the message format inlining the fields that are in the ARO.
The DAR and DAC use the same message format with different ICMPv6 type values, and the Status field is only meaningful in the DAC message.
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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 | Reserved | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ EUI-64 +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Registered Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields:
IPv6 Source: A non-link-local address of the sending router.
IPv6 Destination: In a DAR, a non-link-local address of a 6LBR. In a DAC, this is just the source from the DAR.
Hop Limit: Set to MULTIHOP_HOPLIMIT on transmit. MUST be ignored on receipt.
ICMP Fields:
Type: 157 for the DAR and 158 for the DAC.
Code: Set to zero on transmit. MUST be ignored on receipt.
Checksum: The ICMP checksum. See [RFC4443].
Status: 8-bit unsigned integer. Indicates the status of a registration in the DAC. MUST be set to 0 in the DAR. See Table 1.
Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Registration Lifetime: 16-bit unsigned integer. The amount of time in units of 60 seconds that the 6LBR should retain the DAD table entry (Section 8.2.2) for the Registered Address. A value of 0 indicates in a DAR that the DAD table entry should be removed.
EUI-64: 64 bits. This field is used to uniquely identify the interface of the Registered Address by including the EUI-64 identifier [EUI64] assigned to it unmodified.
Registered Address: 128-bit field. Carries the host address that was contained in the IPv6 Source field in the NS that contained the ARO sent by the host.