4. Message Formats
Neighbor Discovery messages are sent as ICMPv6 [ICMPv6] type 133-137 messages.
4.1. Router Solicitation Message Format
Hosts send Router Solicitations in order to prompt routers to generate Router Advertisements quickly.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
-
Source Address: An IP address assigned to the sending interface, or the unspecified address if no address is assigned to the sending interface.
-
Destination Address: Typically the all-routers multicast address.
-
Hop Limit: 255
ICMP Fields:
-
Type: 133
-
Code: 0
-
Checksum: The ICMP checksum. See [ICMPv6].
-
Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
Valid Options:
- Source link-layer address: The link-layer address of the sender, if known. MUST NOT be included if the Source Address is the unspecified address. Otherwise, it SHOULD be included on link layers that have addresses.
Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.
4.2. Router Advertisement Message Format
Routers send out Router Advertisement messages periodically, or in response to Router Solicitations.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cur Hop Limit |M|O| Reserved | Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
-
Source Address: MUST be the link-local address assigned to the interface from which this message is sent.
-
Destination Address: Typically the Source Address of an invoking Router Solicitation or the all-nodes multicast address.
-
Hop Limit: 255
ICMP Fields:
-
Type: 134
-
Code: 0
-
Checksum: The ICMP checksum. See [ICMPv6].
-
Cur Hop Limit: 8-bit unsigned integer. The default value that should be placed in the Hop Count field of the IP header for outgoing IP packets. A value of zero means unspecified (by this router).
-
M: 1-bit "Managed address configuration" flag. When set, it indicates that addresses are available via Dynamic Host Configuration Protocol [DHCPv6].
-
O: 1-bit "Other configuration" flag. When set, it indicates that other configuration information is available via DHCPv6. Examples of such information are DNS-related information or information on other servers within the network.
-
Reserved: A 6-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
Router Lifetime: 16-bit unsigned integer. The lifetime associated with the default router in units of seconds. The field can contain values up to 65535 and receivers should handle any value, while the sending rules in Section 6 limit the lifetime to 9000 seconds. A Lifetime of 0 indicates that the router is not a default router and SHOULD NOT appear on the default router list. The Router Lifetime applies only to the router's usefulness as a default router; it does not apply to information contained in other message fields or options. Options that need time limits for their information include their own lifetime fields.
-
Reachable Time: 32-bit unsigned integer. The time, in milliseconds, that a node assumes a neighbor is reachable after having received a reachability confirmation. Used by the Neighbor Unreachability Detection algorithm (see Section 7.3). A value of zero means unspecified (by this router). See Section 6.2.1.
-
Retrans Timer: 32-bit unsigned integer. The time, in milliseconds, between retransmitted Neighbor Solicitation messages. Used by address resolution and the Neighbor Unreachability Detection algorithm (see Sections 7.2 and 7.3). A value of zero means unspecified (by this router).
Valid Options:
-
Source link-layer address: The link-layer address of the interface from which the Router Advertisement is sent. Only used on link layers that have addresses. A router MAY omit this option in order to enable inbound load sharing across multiple link-layer addresses.
-
MTU: SHOULD be sent on links that have a variable MTU (as specified in the document that describes how to run IP over the particular link type). MAY be sent on other links.
-
Prefix Information: These options specify the prefixes that are on-link and/or are used for stateless address autoconfiguration. A router SHOULD include all its on-link prefixes (except the link-local prefix) so that multihomed hosts have complete prefix information about on-link destinations for the links to which they attach. If complete information is lacking, a host with multiple interfaces may not be able to choose the correct outgoing interface when sending traffic to its neighbors.
Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.
4.3. Neighbor Solicitation Message Format
Nodes send Neighbor Solicitations to request the link-layer address of a target node while also providing their own link-layer address to the target. Neighbor Solicitations are multicast when the node needs to resolve an address and unicast when the node seeks to verify the reachability of a neighbor.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
-
Source Address: Either an address assigned to the interface from which this message is sent or (if Duplicate Address Detection is in progress) the unspecified address.
-
Destination Address: Either the solicited-node multicast address corresponding to the target address, or the target address.
-
Hop Limit: 255
ICMP Fields:
-
Type: 135
-
Code: 0
-
Checksum: The ICMP checksum. See [ICMPv6].
-
Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
Target Address: The IP address of the target of the solicitation. It MUST NOT be a multicast address.
Valid Options:
- Source link-layer address: The link-layer address for the sender. MUST NOT be included when the source IP address is the unspecified address. Otherwise, on link layers that have addresses this option MUST be included in multicast solicitations and SHOULD be included in unicast solicitations.
Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.
4.4. Neighbor Advertisement Message Format
A node sends Neighbor Advertisements in response to Neighbor Solicitations and sends unsolicited Neighbor Advertisements in order to (unreliably) propagate new information quickly.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|S|O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
-
Source Address: An address assigned to the interface from which the advertisement is sent.
-
Destination Address: For solicited advertisements, the Source Address of an invoking Neighbor Solicitation or, if the solicitation's Source Address is the unspecified address, the all-nodes multicast address.
For unsolicited advertisements typically the all-nodes multicast address.
- Hop Limit: 255
ICMP Fields:
-
Type: 136
-
Code: 0
-
Checksum: The ICMP checksum. See [ICMPv6].
-
R: Router flag. When set, the R-bit indicates that the sender is a router. The R-bit is used by Neighbor Unreachability Detection to detect a router that changes to a host.
-
S: Solicited flag. When set, the S-bit indicates that the advertisement was sent in response to a Neighbor Solicitation from the Destination address. The S-bit is used as a reachability confirmation for Neighbor Unreachability Detection. It MUST NOT be set in multicast advertisements or in unsolicited unicast advertisements.
-
O: Override flag. When set, the O-bit indicates that the advertisement should override an existing cache entry and update the cached link-layer address. When it is not set the advertisement will not update a cached link-layer address though it will update an existing Neighbor Cache entry for which no link-layer address is known. It SHOULD NOT be set in solicited advertisements for anycast addresses and in solicited proxy advertisements. It SHOULD be set in other solicited advertisements and in unsolicited advertisements.
-
Reserved: 29-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
Target Address: For solicited advertisements, the Target Address field in the Neighbor Solicitation message that prompted this advertisement. For an unsolicited advertisement, the address whose link-layer address has changed. The Target Address MUST NOT be a multicast address.
Valid Options:
- Target link-layer address: The link-layer address for the target, i.e., the sender of the advertisement. This option MUST be included on link layers that have addresses when responding to multicast solicitations. When responding to a unicast Neighbor Solicitation this option SHOULD be included.
Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.
4.5. Redirect Message Format
Routers send Redirect packets to inform a host of a better first hop for a destination.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
-
Source Address: MUST be the link-local address assigned to the interface from which this message is sent.
-
Destination Address: The Source Address of the packet that triggered the redirect.
-
Hop Limit: 255
ICMP Fields:
-
Type: 137
-
Code: 0
-
Checksum: The ICMP checksum. See [ICMPv6].
-
Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
Target Address: An IP address that is a better first hop to use for the ICMP Destination Address. When the target is the actual endpoint of communication, i.e., the destination is a neighbor, the Target Address field MUST contain the same value as the ICMP Destination Address field. Otherwise, the target is a better first-hop router and the Target Address MUST be the router's link-local address so that hosts can uniquely identify routers.
-
Destination Address: The IP address of the destination that is redirected to the target. The ICMP Destination Address field is copied from the Destination Address field of the invoking packet.
Valid Options:
-
Target link-layer address: The link-layer address for the target. It SHOULD be included (if known). Note that on NBMA links, hosts may rely on the presence of the Target Link-Layer Address option in Redirect messages as the means for determining the link-layer addresses of neighbors. In such cases, the option MUST be included in Redirect messages.
-
Redirected Header: As much as possible of the IP packet that triggered the sending of the Redirect without making the redirect packet exceed the minimum MTU required to support IPv6 as specified in [IPv6].
Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.
4.6. Option Formats
Neighbor Discovery messages include zero or more options, some of which may appear multiple times in the same message. Options should be padded when necessary to ensure that they end on their natural 64-bit boundaries. All options are of the form:
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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 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
-
Type: 8-bit identifier of the type of option.
-
Length: 8-bit unsigned integer. The length of the option (including the type and length fields) in units of 8 octets. The value 0 is invalid. Nodes MUST silently discard an ND packet that contains an option with length zero.
4.6.1. Source/Target Link-Layer Address
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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 | Link-Layer Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
-
Type: 1 for Source Link-layer Address; 2 for Target Link-layer Address.
-
Length: The length of the option (including the type and length fields) in units of 8 octets. For example, the length for IEEE 802 addresses is 1.
-
Link-Layer Address: The variable length link-layer address. The length and format of this field is link type specific. For example, for IEEE 802 addresses, the length is 6 octets.
Description: The Source Link-Layer Address option contains the link-layer address of the sender of the packet. It is used in the Neighbor Solicitation, Router Solicitation, and Router Advertisement packets. The Target Link-Layer Address option contains the link-layer address of the target. It is used in Neighbor Advertisement and Redirect packets.
These options MUST be silently ignored for other Neighbor Discovery messages.
4.6.2. Prefix Information
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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 | Prefix Length |L|A| Reserved1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
-
Type: 3
-
Length: 4
-
Prefix Length: 8-bit unsigned integer. The number of leading bits in the Prefix that are valid. The value ranges from 0 to 128. The prefix length field provides necessary information for on-link determination (when combined with the L flag in the prefix information option). It also assists with address autoconfiguration as specified in [ADDRCONF], for which there may be more restrictions on the prefix length.
-
L: 1-bit on-link flag. When set, indicates that this prefix can be used for on-link determination. When not set the advertisement makes no statement about on-link or off-link properties of the prefix. In other words, if the L flag is not set a host MUST NOT conclude that an address derived from the prefix is off-link. That is, it MUST NOT update a previous indication that the address is on-link.
-
A: 1-bit autonomous address-configuration flag. When set indicates that this prefix can be used for stateless address configuration as specified in [ADDRCONF].
-
Reserved1: 6-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
Valid Lifetime: 32-bit unsigned integer. The length of time in seconds (relative to the time the packet is sent) that the prefix is valid for the purpose of on-link determination. A value of all one bits (0xffffffff) represents infinity. The Valid Lifetime is also used by [ADDRCONF].
-
Preferred Lifetime: 32-bit unsigned integer. The length of time in seconds (relative to the time the packet is sent) that addresses generated from the prefix via stateless address autoconfiguration remain preferred [ADDRCONF]. A value of all one bits (0xffffffff) represents infinity. See [ADDRCONF]. Note that the value of this field MUST NOT exceed the Valid Lifetime field to avoid preferring addresses that are no longer valid.
-
Reserved2: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
Prefix: An IP address or a prefix of an IP address. The Prefix Length field contains the number of valid leading bits in the prefix. The bits in the prefix after the prefix length are reserved and MUST be initialized to zero by the sender and ignored by the receiver. A router SHOULD NOT send a prefix option for the link-local prefix and a host SHOULD ignore such a prefix option.
Description: The Prefix Information option provide hosts with on-link prefixes and prefixes for Address Autoconfiguration. The Prefix Information option appears in Router Advertisement packets and MUST be silently ignored for other messages.
4.6.3. Redirected Header
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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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IP header + data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
-
Type: 4
-
Length: The length of the option in units of 8 octets.
-
Reserved: These fields are unused. They MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
IP header + data: The original packet truncated to ensure that the size of the redirect message does not exceed the minimum MTU required to support IPv6 as specified in [IPv6].
Description: The Redirected Header option is used in Redirect messages and contains all or part of the packet that is being redirected.
This option MUST be silently ignored for other Neighbor Discovery messages.
4.6.4. MTU
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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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
-
Type: 5
-
Length: 1
-
Reserved: This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.
-
MTU: 32-bit unsigned integer. The recommended MTU for the link.
Description: The MTU option is used in Router Advertisement messages to ensure that all nodes on a link use the same MTU value in those cases where the link MTU is not well known.
This option MUST be silently ignored for other Neighbor Discovery messages.
In configurations in which heterogeneous technologies are bridged together, the maximum supported MTU may differ from one segment to another. If the bridges do not generate ICMPv6 Packet Too Big messages, communicating nodes will incorrectly assume that the path MTU is larger than what it is. Proper implementation of the algorithm for Performing Path MTU Discovery [PMTU] will identify the problem and allow the nodes to adjust. However, the algorithm converges much more quickly when routers send the correct MTU option in each segment.
The MTU value placed in the MTU option MUST be less than or equal to the maximum MTU value derived from the link layer information. A node MUST NOT place an MTU option in a Router Advertisement containing an MTU value greater than the MTU configured for the advertising interface or greater than the maximum transmission unit for the link as specified in the document that covers operation of IP over the particular link type.