Section 11.3. ICMPv6 and Mobile IPv6

11.3. ICMPv6 and Mobile IPv6

This section describes two new ICMPv6 message pairs and some modifications to Neighbor Discovery (ND).

11.3.1. Home Agent Address Discovery

The Home Agent Address Discovery mechanism is used by the mobile node to determine the address of its home agent on its home link. There are two ICMPv6 message pairs and the home agents list, which is a list to be maintained by each home agent.

11.3.1.1. ICMPv6 Home Agent Address Discovery messages

This message pair consists of a Home Agent Address Discovery Request and Reply message. As the name implies, the mobile node uses these messages to find its home agent on the home link dynamically. Normally mobile nodes are configured statically with a home agent address. In the case where a home agent is renumbered or goes down and is replaced by another home agent with a different IP address, dynamic discovery of the home agent address may be a useful mechanism.

The mobile node sends a Discovery message to the Home Agent Anycast address (Anycast ID decimal 126, hexadecimal 0x7E; see Chapter 3) on its home link. The Source address field in the IPv6 header carries the care-of address of the mobile node.

The home agents on the home link that are configured for the Home Agent Anycast address respond with a Home Agent Address Discovery Reply message.

The Discovery message has a type value of 150; the Reply message has a type value of 151. The Code field is always set to 0. The Identifier field is inserted by the mobile node and copied over by the home agent for the Reply. This allows to identify corresponding messages. The Reply carries a Home Address field, which can carry one or more home agent addresses. This address or list of addresses is generated from the home agents list (described next).

For a detailed description of the header fields in an ICMPv6 message, please refer to Chapter 4.

11.3.1.2. Home agents list

Every home agent needs to maintain a home agents list. In this list, every router must be listed that sits on the same link and provides home agent services. A router advertises itself as a home agent by setting the H-bit in the Router Advertisement. A router maintains a home agents list for each link on which it acts as a home agent. The list is updated through Router Advertisements and contains the following information:

• Link-local address of the HAs on the link. This address is learned from the Source address field in the IPv6 header of Router Advertisements.

• One or more global IPv6 unicast addresses for these HAs. These addresses are learned from the Prefix Option in the Router Advertisements.

• Remaining lifetime for this HA entry. When the lifetime expires, the HA has to be deleted from the list.

• Preference for this HA. Higher values means higher preference. This value is learned from the Home Agent Preference field in the Home Agent Information option in a Router Advertisement (if present). If not present, this value is set to 0. A HA uses this preference to sort the HA list when sending out an Home Agent Address Discovery Reply message.

The HA sending out a Home Agent Address Discovery Reply must list all HAs on the link, sorted by preference. Only the global IPv6 unicast addresses of the home agents are contained in the home agent address field of the home agent address discovery reply message. The reply must not be larger than 1280 bytes. Sorting by preference ensures that HAs with a high priority are listed in this packet.

11.3.2. Mobile Prefix Solicitation

The Mobile Prefix Solicitation message is sent by a mobile node away from home to determine changes in the prefix configuration of its home link (i.e., home network renumbering). The HA answers the Solicitation message with a Prefix Advertisement. Based on this reply, the mobile node can adjust its home address.

The mobile node sends an ICMPv6 Mobile Prefix Solicitation message to its HA. The IPv6 header carries the care-of address as Source address. A Home Address Destination option is inserted. IPsec headers are supported and should be used. The Mobile Prefix Solicitation can carry options that have to follow the format described in Chapter 4 (RFC 2461). Currently, there are no specific options defined. The type value field of an Mobile Prefix Solicitation message is set to 152; the Code field is set to 0.

The HA replies with an ICMPv6 Mobile Prefix Advertisement message to the care-of address of the mobile node. The HA can also send out unsolicited Advertisements at regular intervals. The Advertisement carries a Routing header type 2. The reply is sent to the Source address of the Solicitation message. If the Advertisement is unsolicited, it is sent to the care-of address of registered mobile nodes. The type value for the Advertisement is set to 153. If it is the answer to a Solicitation, the Identifier is copied over from the Solicitation. The Advertisement contains the Prefix option described in Chapter 4. It carries all prefixes that should be used by the mobile node to configure its home addresses.

11.3.3. Changes in Neighbor Discovery (ND)

Some changes and new options have been defined in ND for the use with Mobile IPv6.

11.3.3.1. Modified Router Advertisement format

As already mentioned in Chapter 4, there is a new flag in the Router Advertisement. The M-flag and the O-flag are followed by the H-flag, which allows a router to advertise that it acts as a home agent on this link.

11.3.3.2. Modified Prefix option

In order to build an updated HA list based on Router Advertisements, a mobile node must know the global unicast address of the routers. A regular Router Advertisement lists only the link-local address of the router. For this purpose, the Prefix Option has been modified. The Prefix option now carries an additional flag, the R-flag (Router Address). When this flag is set, it indicates that the Prefix option field does not contain a Prefix, but rather a global IPv6 unicast address for the router.

11.3.3.3. New Advertisement Interval option

The Advertisement Interval option is used in Router Advertisements. It indicates the interval at which the router will send unsolicited multicast Router Advertisements. The option follows the TLV format (Time, Length, Value) and has a type value of 7. The Advertisement Interval field has 4 bytes and carries the time in milliseconds between unsolicited Router Advertisements. The mobile node uses this information in its Movement Detection Algorithm (described later in this chapter).

11.3.3.4. New Home Agent Information option

The Home Agent Information option is used in Router Advertisements and follows the TLV format. The type value is 8.

The HA Preference field has a length of 2 bytes. In a Router Advertisement, the HA can use this field to indicate which level of preference should be associated to it. A higher value means a higher preference. When this option is not set, the HA has a preference of 0. This field can be used by the HA to dynamically adjust to different situations, e.g., to the number of mobile nodes currently connected or based on how many ressources are available to serve additional mobile nodes. Alternatively, the preference can be configured manually.

The Home Agent Lifetime field also has a length of 2 bytes. It indicates the lifetime for this HA in seconds. The default value corresponds to the value in the basic Router Advertisement header. The maximum possible value is 18.2 hours. A value of 0 is not accepted. The HA Lifetime field only relates to the HA service of this router, so it can be present only in a Router Advertisement with the H-bit (home agent) set.

11.3.3.5. Changes in the Router Advertisement Interval

The Neighbor Discovery specification specifies a minimum interval of three seconds for unsolicited multicast Router Advertisements. Mobile nodes are dependent on learning as fast as possible when moving to a new network to create new care-of addresses accordingly and send out Binding Updates. They detect their movement to a new network based on Router Advertisements from routers they don't know yet. As a consequence, routers supporting Mobile IPv6 must be configurable with a shorter Router Advertisement Interval. Alternatively mobility information from lower layers (i.e., layer 2) can be used to aid the mobile node in achieving faster movement detection.