Return Routability Procedure

The Return Routability Procedure enables the correspondent node to obtain some reasonable assurance that the mobile node is in fact addressable at its claimed care-of address as well as at its home address. Only with this assurance is the correspondent node able to accept Binding Updates from the mobile node which would then instruct the correspondent node to direct that mobile node’s data traffic to its claimed care-of address.

This is done by testing whether packets addressed to the two claimed addresses are routed to the mobile node. The mobile node can pass the test only if it is able to supply proof that it received certain data (the "keygen tokens") which the correspondent node sends to those addresses. These data are combined by the mobile node into a binding management key, denoted Kbm.

The figure below shows the message flow for the return routability procedure.

Mobile node Home agent Correspondent node | |

| Home Test Init (HoTI) | | |--->|--->|

| | | | Care-of Test Init (CoTI) | |--->|

| | | | Home Test (HoT) | |<---|<---|

| | | | Care-of Test (CoT) | |<---|

| |

The Home and Care-of Test Init messages are sent at the same time.

The procedure requires very little processing at the correspondent node, and the Home and Care-of Test messages can be returned quickly, perhaps nearly simultaneously. These four messages form the return routability procedure.

Home Test Init

A mobile node sends a Home Test Init message to the correspondent node (via the home agent) to acquire the home keygen token. The contents of the message can be summarized as follows:

* Source Address = home address

* Destination Address = correspondent * Parameters:

+ home init cookie

The Home Test Init message conveys the mobile node’s home address to the correspondent node. The mobile node also sends along a home init cookie that the correspondent node must return later.

The Home Test Init message is reverse tunneled through the home agent. (The headers and addresses related to reverse tunneling have been omitted from the above discussion of the message

contents.) The mobile node remembers these cookie values to obtain some assurance that its protocol messages are being processed by the desired correspondent node.

Care-of Test Init

The mobile node sends a Care-of Test Init message to the

correspondent node (directly, not via the home agent) to acquire the care-of keygen token. The contents of this message can be summarized as follows:

* Source Address = care-of address * Destination Address = correspondent * Parameters:

+ care-of init cookie

The Care-of Test Init message conveys the mobile node’s care-of address to the correspondent node. The mobile node also sends along a care-of init cookie that the correspondent node must return later. The Care-of Test Init message is sent directly to the correspondent node.

Home Test

The Home Test message is sent in response to a Home Test Init message. It is sent via the home agent. The contents of the message are:

* Source Address = correspondent * Destination Address = home address * Parameters:

+ home init cookie + home keygen token + home nonce index

When the correspondent node receives the Home Test Init message, it generates a home keygen token as follows:

home keygen token :=

First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0)))

where | denotes concatenation. The final "0" inside the HMAC_SHA1 function is a single zero octet, used to distinguish home and care-of cookies from each other.

The home keygen token is formed from the first 64 bits of the MAC.

The home keygen token tests that the mobile node can receive were messages sent to its home address. Kcn is used in the production of home keygen token in order to allow the correspondent node to verify that it generated the home and care-of nonces, without forcing the correspondent node to remember a list of all tokens it has handed out.

The Home Test message is sent to the mobile node via the home network, where it is presumed that the home agent will tunnel the message to the mobile node. This means that the mobile node needs to already have sent a Binding Update to the home agent, so that the home agent will have received and authorized the new care-of address for the mobile node before the return routability

procedure. For improved security, the data passed between the home agent and the mobile node is made immune to inspection and passive attacks. Such protection is gained by encrypting the home keygen token as it is tunneled from the home agent to the mobile node as specified in Section 10.4.6. The security properties of this additional security are discussed in Section 15.4.1.

The home init cookie from the mobile node is returned in the Home Test message, to ensure that the message comes from a node on the route between the home agent and the correspondent node.

The home nonce index is delivered to the mobile node to later allow the correspondent node to efficiently find the nonce value that it used in creating the home keygen token.

Care-of Test

This message is sent in response to a Care-of Test Init message.

This message is not sent via the home agent, it is sent directly to the mobile node. The contents of the message are:

* Source Address = correspondent

* Destination Address = care-of address

* Parameters:

+ care-of init cookie + care-of keygen token + care-of nonce index

When the correspondent node receives the Care-of Test Init message, it generates a care-of keygen token as follows:

care-of keygen token :=

First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1))) Here, the final "1" inside the HMAC_SHA1 function is a single octet containing the hex value 0x01, and is used to distinguish home and care-of cookies from each other. The keygen token is formed from the first 64 bits of the MAC, and sent directly to the mobile node at its care-of address. The care-of init cookie from the Care-of Test Init message is returned to ensure that the message comes from a node on the route to the correspondent node.

The care-of nonce index is provided to identify the nonce used for the care-of keygen token. The home and care-of nonce indices MAY be the same, or different, in the Home and Care-of Test messages.

When the mobile node has received both the Home and Care-of Test messages, the return routability procedure is complete. As a result of the procedure, the mobile node has the data it needs to send a Binding Update to the correspondent node. The mobile node hashes the tokens together to form a 20 octet binding key Kbm:

Kbm = SHA1 (home keygen token | care-of keygen token)

A Binding Update may also be used to delete a previously established binding (Section 6.1.7). In this case, the care-of keygen token is not used. Instead, the binding management key is generated as follows:

Kbm = SHA1(home keygen token)

Note that the correspondent node does not create any state specific to the mobile node, until it receives the Binding Update from that mobile node. The correspondent node does not maintain the value for