IPv6 Mobility Messages and Options

The following options, messages, and modifications to existing messages are needed to facilitate the processes of IPv6 mobility.

Destination Options Header Options

The Destination Options extension header is used to contain the following IPv6 mobility-related options:

• Binding Update
• Binding Acknowledgement
• Binding Request
• Home Address

For information on the structure of these options, see Chapter 4, "The IPv6 Header."

Binding Update Option

The Binding update option is used by a mobile IPv6 node that is away from home to update another node with its new care-of address. The Binding Update option is an option used within the Destination Options extension header for the destination node and can be included in an existing packet sent to the destination or in a packet that contains just the Destination Options header. In this latter case, the Next Header field in the Destination Options header is set to 59, indicating no next header. A binding update is a packet that contains the Binding Update option. A binding update always contains the Home Address option (described later in this chapter).

The Binding Update option is used for the following:

• To update the home agent with a new primary care-of address. This is known as a home registration binding update. The home agent uses the home address in the Home Address option and the source address of the packet to update its Home Address/Primary Care-of Address binding cache entry for the mobile node.
• To update a correspondent node with which the mobile node is actively communicating with a new binding that maps the home address of the mobile node to its care-of address. The correspondent node uses the home address in the Home Address option and the source address of the packet to update its Home Address/Care-of Address binding cache entry for the mobile node.

The mobile node can use the Alternate Care-of Address sub-option in the Binding Update option to specify a care-of address that is different than the source address of the binding update.

Binding Acknowledgement Option

The Binding Acknowledgement option is used to acknowledge the receipt of a binding update whose Acknowledge (A) flag has been set, and to report errors in the binding update. The mobile node sets the A flag when it wants to receive confirmation that the Binding Update message was received. The Binding Acknowledgement option can be included in an existing packet sent to the destination, or in a packet that contains just the Destination Options header. In this latter case, the Next Header field in the Destination Options header is set to 59, indicating no next header. A binding acknowledgement is a packet that contains the Binding Acknowledgement option.

Included in the binding acknowledgement is an indication of how long the node will cache the binding. For home agents, this lifetime indicates how long the home agent will be in service as the home agent for the mobile node. To refresh the binding, either the mobile node sends a new binding update or the correspondent nodes and home agent send a request to update the binding.

The binding acknowledgement also includes an indication of how often the mobile node should send binding updates.

Binding Request Option

A Binding Request option is used to request the current binding from a mobile node. If a mobile node receives a binding request, it responds with a binding update. A correspondent node sends a binding request when the binding cache entry is in active use and the lifetime of the binding cache entry approaches expiration. A home agent sends a binding request when the lifetime of the binding cache entry approaches expiration.

The Binding Request option can be included in an existing packet sent to the destination or in a packet that contains just the Destination Options header. In this latter case, the Next Header field in the Destination Options header is set to 59, indicating no next header. A binding request is a packet that contains the Binding Request option.

Home Address Option

The Home Address option is used to indicate the home address of the mobile node. The Home Address option is included in any packet sent to correspondent nodes and home agents by a mobile node when it is away from home (with the exception of a tunneled Router Solicitation message sent to the home agent). When a mobile node sends a packet, the source address in the IPv6 header is set to the care-of address. If the source address in the IPv6 header is set to the home address, then the router on the foreign link might discard the packet because the source address does not match the prefix of the link on which the mobile node is located. To help minimize Internet attacks in which the source address of attack packets is spoofed with an address that is not assigned to the attacking computer, peripheral routers can implement ingress filtering and silently discard packets that do not have topologically correct source addresses. Ingress in this instance is defined relative to the Internet for packets entering the Internet, rather than packets entering an intranet from the Internet.

By using the care-of address as the source address in the packet (a topologically correct address on the foreign link), and including the Home Address destination option, the packet is forwarded by the router on the foreign link to its destination. When the packet is received at the destination, the correspondent node processes the Destination Options header, and before passing the payload to the upper layer protocol, logically replaces the source address of the packet with the address in the Home Address option. As far as the upper layer protocol is concerned, the packet was sent from the home address.

The Home Address option is also included with the binding update so that the home address for the binding is indicated to the receiving node.

ICMPv6 Messages

The following ICMPv6 messages are used by the mobile node for dynamic home agent address discovery:

• Home Agent Address Discovery Request
• Home Agent Address Discovery Reply

Dynamic home agent address discovery is a process through which the mobile node dynamically discovers the global address of the home agent on the home link. This process is needed only if the mobile node is not already configured with the address of its home agent or if the current home agent becomes unavailable.

Home Agent Address Discovery Request

The ICMPv6 Home Agent Address Discovery Request message is used by a mobile node to begin dynamic home agent address discovery. This message is sent to the Mobile IPv6 Home-Agents anycast address that is described in RFC 2526. The Mobile IPv6 Home-Agents anycast address is composed of the 64-bit home subnet prefix and the interface ID of ::FEFF:FFFF:FFFF:FFFE. All home agents are configured automatically with this anycast address. The home agent that is topologically closest to the mobile node receives the request message.

Figure 12-2 shows the structure of the ICMPv6 Home Agent Address Discovery Request message.

Figure 12-2. The structure of ICMPv6 Home Agent Address Discovery Request message

In the Home Agent Address Discovery Request message, the Type field is set to 150 and the Code field is set to 0. Following the Checksum field is a 16-bit Identifier field. The value of the Identifier field is chosen by the sending node and copied to the Identifier field of the Home Agent Address Discovery Reply message to match a reply with its request. Following the Identifier field is an 80-bit Reserved field that is set to 0 by the sender and a 128-bit Home Address field. The Home Address field contains the home address of the mobile node.

The Home Agent Address Discovery Request message is sent with the source address in the IPv6 header set to the mobile node's care-of address. The Home Address destination option is not included.

Home Agent Address Discovery Reply

The ICMPv6 Home Agent Address Discovery Reply message is used by a home agent to complete the dynamic home agent address discovery process by informing the mobile node of the addresses of the set of routers attached to the mobile node's home link that are capable of being a home agent.

Figure 12-3 shows the structure of the ICMPv6 Home Agent Address Discovery Reply message.

Figure 12-3. The structure of the ICMPv6 Home Agent Address Discovery Reply message

In the Home Agent Address Discovery Reply message, the Type field is set to 151 and the Code field is set to 0. Following the Checksum field is a 16-bit Identifier field. The value of the Identifier field is set to the same value as the Identifier field of the received Home Agent Address Discovery Request message. Following the Identifier field is an 80-bit Reserved field that is set to 0 by the sender, and one or more 128-bit Home Agent Address fields. The Home Agent Address fields contain the global addresses of home agents on the home link in preference order (highest preference first).

The Home Agent Address Discovery Reply message is sent with the source address in the IPv6 header set to the global address of the answering home agent, and the destination address set to the mobile node's care-of address. A Routing extension header is not included.

Modifications to Neighbor Discovery Messages and Options

IPv6 mobility defines the following changes to ND messages and options:

• Modified Router Advertisement message
• Modified Prefix Information option
• New Advertisement Interval option
• New Home Agent Information option

For more information about the structure of these ND messages and options, see Chapter 6, "Neighbor Discovery."

Modifications to the Router Advertisement Message

IPv6 mobility defines an additional flag in the Router Advertisement message to help facilitate home agent discovery by the home agents and mobile nodes on a home link. The new flag, known as the Home Agent (H) flag, indicates whether the advertising router is capable of being a home agent. Each of the home agents on the home link set this flag when they send their router advertisements, and each home agent and mobile node receives each router advertisement. Therefore, each home agent and mobile node can compile the list of possible home agents.

Additionally, IPv6 mobility allows a router advertisement to be sent more frequently than every 3 seconds, as specified in RFC 2461. By sending router advertisements more frequently, IPv6 mobile nodes can use a newly received router advertisement to detect movement to a foreign link more quickly. Recommended values for the pseudo-periodic router advertisement process for routers that might provide connectivity for mobile IPv6 nodes are a minimum of 0.5 seconds and a maximum of 1.5 seconds.

Modified Prefix Information Option

To indicate the global address of the advertising router, IPv6 mobility defines an additional flag and a redefined use of the Prefix field in the Prefix Information option.

As per RFC 2461, which defines Neighbor Discovery, router advertisements are sent from the link-local address. However, the global address for a home agent must be indicated in the router advertisement it sends so that a list of home agents can be compiled by each home agent and mobile node. IPv6 mobility defines the Router Address (R) flag in the Prefix Information option. When set, the R flag indicates to the receiver that the Prefix field contains the global address of the advertising router. In the originally defined Prefix field, the high-order bits corresponding to the value of the Prefix Length field are set to the appropriate values for the advertised prefix and the bits beyond the indicated prefix length are set to 0. With this new definition, the Prefix Length field is advertised in the same way, except the Prefix field contains the entire 128-bit global address of the advertising router.

Advertisement Interval Option

The Advertisement Interval option is sent in Router Advertisement messages to specify how often the router sends unsolicited multicast router advertisements. A mobile node that receives a router advertisement with the Advertisement Interval option can use the advertisement interval to detect whether it has moved to another link.

The Advertisement Interval option contains a 32-bit field that indicates the maximum number of milliseconds between consecutive unsolicited multicast Router Advertisement messages sent by the router using the pseudo-periodic advertising scheme described in section 6.2.4 of RFC 2461.

Home Agent Information Option

The Home Agent Information option is sent in Router Advertisement messages sent by a home agent to specify the home agent's configuration. Included in the Home Agent Information option are the home agent preference (a number indicating a preference level for the advertising router to be a home agent) and the home agent lifetime (how long the home agent is acting as a home agent).

The home agents on a home link use the home agent preference values to order the list of home agents sent to the mobile node during home agent address discovery. The mobile nodes on a home link use the home agent preference values to select the home agent that has the highest preference value.