Understanding Ipv6

Before understanding the various processes used for IPv6 mobility, it is important to understand how packets containing mobility options and Application layer data are sent in a mobility-enabled environment. The following are the types of IPv6 mobility communication:

• Between a mobile node and a correspondent node
• Between a mobile node and a home agent

Communication Between a Mobile Node and a Correspondent Node

Communication between a mobile node and a correspondent node is one of the following:

• From the mobile node to the correspondent node
• From the correspondent node to the mobile node

From the Mobile Node to the Correspondent Node

The mobile node sends the correspondent node the following types of packets:

• Binding updates
• Data

Binding Updates

Binding updates sent from the mobile node to the correspondent node are shown in Figure 12-4.

Figure 12-4. Binding updates sent from the mobile node to the correspondent node

The packet contains the following:

• IPv6 header

  In the IPv6 header, the source address is the care-of address of the mobile node (indicated by CoA in Figure 12-4) and the destination address is the correspondent node's address (indicated by CNA in Figure 12-4). By using the care-of address rather than the home address, ingress filtering by the foreign link router does not prevent the packet from being forwarded.

• Destination Options header

  The Destination Options extension header contains two options: the Home Address option and the Binding Update option. By including the Home Address option, the home address (indicated by HA in Figure 12-4) for the binding is indicated to the correspondent node.

The binding update can be sent either with data (an upper layer PDU) or in a separate packet. Figure 12-4 shows the binding update sent as a separate packet.

Version 13 of the IPv6 mobility draft (draft-ietf-mobileip-ipv6-13.txt in the \RFCs_and_Drafts folder on the companion CD-ROM) requires the use of an Authentication Header (AH) to provide sender authentication, data integrity, and replay protection for binding updates. Many implementations allow you to disable security for IPv6 mobility messages. If an AH is present, there are two different Destination Options headers: one before the AH and one after the AH. The first Destination Options header contains the Home Address option and the second Destination Options header contains the Binding Update option. This is not shown in Figure 12-4. If an AH is not present, both the Home Address and Binding Acknowledgement options are included in a single Destination Options header. This is shown in Figure 12-4.

If the correspondent node is also mobile, the destination address in the IPv6 header is set to the correspondent node's care-of address and the packet includes a Routing header with the correspondent node's home address. The Routing header is placed before the Destination Options header. If an AH is present, there are two different Destination Options headers: one before the AH and one after the AH. The first Destination Options header contains the Home Address option and the second Destination Options header contains the Binding Update option. In this case, the order of the extension headers is: Routing, Destination Options (w/Home Address option), AH, Destination Options (w/Binding Update). If an AH is not present, both the Home Address and Binding Acknowledgement options are included in a single Destination Options header that is placed after the Routing header. This is not shown in Figure 12-4.

Data

When the mobile node is away from home, it can choose to either send data from its home address using mobility options, or its care-of address without using mobility options, based on the following:

• For Transport layer connection data (such as TCP sessions) that is long-term and being sent to a correspondent node, the mobile node sends the data from its home address and includes the Home Address option.
• For short-term communication that does not require a logical connection, such as the exchange of DNS Name Query and DNS Name Query Response messages for DNS name resolution, the mobile node can send data from its care-of address and not use a Home Address option. In this case, the mobile node is sending and receiving packets normally from its care-of address.

Packets containing Transport layer connection data sent from the mobile node to the correspondent node are shown in Figure 12-5.

Figure 12-5. Data sent from the mobile node to the correspondent node

The packet contains the following:

• IPv6 header

  In the IPv6 header, the source address is the care-of address of the mobile node and the destination address is the correspondent node's address. By using the care-of address rather than the home address, ingress filtering by the foreign link router does not prevent the packet from being forwarded.

• Destination Options header
• In the Destination Options extension header, the Home Address option contains the home address of the mobile node. When the correspondent node processes the Home Address option, it indicates to upper layer protocols that the source address of the packet is the home address, rather than the care-of address.
• Upper layer PDU

  The upper layer PDU contains the Application layer data sent from the mobile node to the correspondent node. From the Application layer perspective, the data was addressed from the home address to the correspondent node address.

If the correspondent node is also mobile, the destination address in the IPv6 header is set to the correspondent node's care-of address and the packet includes a Routing header with the correspondent node's home address. The Routing header is placed before the Destination Options header. This is not shown in Figure 12-5.

From the Correspondent Node to the Mobile Node

The correspondent node sends the mobile node the following types of packets:

• Binding maintenance
• Data

Binding Maintenance

Binding maintenance packets sent from the correspondent node to the mobile node are either binding acknowledgments or binding requests and are shown in Figure 12-6.

The packets contain the following:

• IPv6 header

  In the IPv6 header, the source address is the correspondent node's address and the destination address is the mobile node's care-of address.

• Routing header

  In the Routing extension header, the Routing Type field is set to 0, the Segments Left field is set to 1, and the Address 1 field (the final destination address of the packet) is set to the mobile node's home address. When the mobile node receives the packet, it processes the Routing header and notes that the next destination address (the address in the Address 1 field) is its own home address. The mobile node removes the Routing header and logically replaces the care-of address with the home address as the destination address in the IPv6 header. When the packet is passed to the upper layer protocol, it appears to have been addressed to the mobile node's home address.

• Destination Options header

  The Destination Options extension header contains either a Binding Acknowledgement option (if a received binding update had the Acknowledge [A] flag set) or a Binding Request option.

The binding acknowledgement or binding request can be sent either with data (an upper layer PDU) or in a separate packet. Figure 12-6 shows the binding acknowledgement or binding request sent as a separate packet.

The IPv6 mobility draft requires the use of an AH to provide data authentication, data integrity, and replay protection for binding acknowledgements. Many implementations allow you to disable security for IPv6 mobility messages. The AH is placed between the Routing header and the Destination Options header and is not shown in Figure 12-6.

Figure12-6. Binding maintenance packets sent from the correspondent node to the mobile node

If the correspondent node is also mobile, the source address in the IPv6 header is set to the correspondent node's care-of address and the packet includes the Home Address option containing the correspondent node's home address. If an AH is present, there are two different Destination Options headers: one before the AH and one after the AH. The first Destination Options header contains the Home Address option and the second Destination Options header contains the Binding Acknowledgement option. In this case, the order of the extension headers is: Routing, Destination Options (w/Home Address option), AH, Destination Options (w/Binding Acknowledgement). If an AH is not present, both the Home Address and Binding Acknowledgement options are included in a single Destination Options header that is placed after the Routing header. This is not shown in Figure 12-6.

Data with a Binding Cache Entry Present

The form of data packets sent from the correspondent node to mobile nodes depends on whether the correspondent node has a binding cache entry for the mobile node's home address. A packet containing an upper layer PDU sent from the correspondent node to the mobile node when a binding cache entry for the mobile node's care-of address is present is shown in Figure 12-7.

Figure12-7. Data sent from the correspondent node when a binding cache entry for the mobile node is present

The packet contains the following:

• IPv6 header

  In the IPv6 header, the source address is the correspondent node's address and the destination address is the mobile node's care-of address. By using the care-of address rather than the home address, the packet is delivered to the mobile node's current location on the IPv6 Internet.

• Routing header

  In the Routing extension header, the Routing Type field is set to 0, the Segments Left field is set to 1, and the Address 1 field (the final destination address of the packet) is set to the mobile node's home address. When the mobile node receives the packet, it processes the Routing header and notes that the next destination address (the address in the Address 1 field) is its own home address. The mobile node removes the Routing header and logically replaces the care-of address with the home address as the destination address in the IPv6 header. When the packet is passed to the upper layer protocol, it appears to have been addressed to the mobile node's home address.

• Upper layer PDU

  The upper layer PDU contains the Application layer data sent from the correspondent node to the mobile node. From the Application layer perspective, the data was addressed from the correspondent node address to the home address.

If the correspondent node is also mobile, the source address in the IPv6 header is set to the correspondent node's care-of address and the packet includes a Destination Options header with the Home Address option containing the correspondent node's home address. The Destination Options header is placed after the Routing header. This is not shown in Figure 12-7.

Data with a Binding Cache Entry Not Present

A packet containing an upper layer PDU sent from the correspondent node to the mobile node when a binding cache entry for the mobile node is not present is shown in Figure 12-8.

Figure 12-8. Data sent from the correspondent node when a binding cache entry for the mobile node is not present

The packet contains the following:

• IPv6 header

  In the IPv6 header, the source address is the correspondent node address and the destination address is the mobile node's home address. Because a binding cache entry does not exist, the correspondent node sends the packet as if the mobile node were physically attached to the home link.

• Upper layer PDU

  The upper layer PDU contains the Application layer data sent from the correspondent node to the mobile node.

  While addressed to the mobile node's home address (represented by the Virtual Mobile Node in Figure 12-8), the home agent, which has a binding cache entry for the mobile node, intercepts the packet and forwards it to the mobile node by encapsulating the IPv6 packet with an IPv6 header. This is known as IPv6-over-IPv6 tunneling. For more information, see the "Communication Between a Mobile Node and Its Home Agent" section in this chapter.

If the correspondent node is also mobile, the source address in the IPv6 header is set to the correspondent node's care-of address and the packet includes a Destination Options header with the Home Address option containing the correspondent node's home address. The Destination Options header is placed after the IPv6 header. This is not shown in Figure 12-8.

Communication Between a Mobile Node and Its Home Agent

Communication between a mobile node and a home agent is one of the following:

• From the mobile node to its home agent
• From the home agent to the mobile node

From the Mobile Node to its Home Agent

The mobile node sends the home agent the following types of packets:

• Binding update
• ICMPv6 Home Agent Address Discovery Request message

Binding Update

Binding updates sent from the mobile node to the home agent are shown in Figure 12-9.

Figure 12-9. Binding updates sent from the mobile node to the home agent

The binding update contains the following:

• IPv6 header

  In the IPv6 header, the source address is the mobile node's care-of address and the destination address is the home agent's address (indicated by HAA in Figure 12-9). By using the care-of address rather than the home address, ingress filtering by the foreign link router does not prevent the packet from being forwarded.

• Destination Options header

  The Destination Options extension header contains two options: the Home Address option and the Binding Update option.

  The Home Address option contains the home address of the mobile node. By including the Home Address option, the home address for the binding is indicated to the home agent.

  In the Binding Update option, the Home Registration (H) flag is set, indicating that the sender is requesting that the receiver be the home agent for the mobile node. The Acknowledge (A) flag is also set to request a binding acknowledgement from the home agent.

If security for binding updates is enabled and an AH is present, there are two different Destination Options headers: one before the AH and one after the AH. The first Destination Options header contains the Home Address option and the second Destination Options header contains the Binding Update option. This is not shown in Figure 12-9.

ICMPv6 Home Agent Address Discovery Request Message

When the mobile node sends an ICMPv6 Home Agent Address Discovery Request message, it has the form shown in Figure 12-10.

Figure 12-10. ICMPv6 Home Agent Address Discovery Request message sent fr

• IPv6 header

  In the IPv6 header, the source address is the care-of address of the mobile node and the destination address is the Mobile IPv6 Home-Agents anycast address corresponding to the home link prefix.

• ICMPv6 Home Agent Address Discovery Request message

  The ICMPv6 Home Agent Address Discovery Request message is used by the mobile node to query the home link for a list of home agents. For more information, see the "ICMPv6 Messages" section in this chapter.

From the Home Agent to the Mobile Node

Communication from the home agent to the mobile node takes the following forms:

• Binding maintenance
• ICMPv6 Home Agent Address Discovery Reply message
• Tunneled data

Binding Maintenance

Binding maintenance packets sent from the home agent to the mobile node are either binding acknowledgments or binding requests and are shown in Figure 12-11.

Figure 12-11. Binding maintenance packets sent from the home agent to the mobile node

The packet contains the following:

• IPv6 header

  In the IPv6 header, the source address is the home agent's address and the destination address is the mobile node's care-of address.

• Routing header

  In the Routing extension header, the Routing Type field is set to 0, the Segments Left field is set to 1, and the Address 1 field (the final destination address of the packet) is set to the mobile node's home address. When the mobile node receives the packet, it processes the Routing header and notes that the next destination address (the address in the Address 1 field) is its own home address. The mobile node removes the Routing header and logically replaces the care-of address with the home address as the destination in the IPv6 header.

• Destination Options header

  The Destination Options extension header contains either a Binding Acknowledgement option (if a received binding update had the Acknowledge [A] flag set) or a Binding Request option.

If security for binding updates is enabled, an AH is present between the Routing header and the Destination Options header for the binding acknowledgement. This is not shown in Figure 12-11.

ICMPv6 Home Agent Address Discovery Reply Message

When the home agent sends an ICMPv6 Home Agent Address Discovery Reply message, it has the form shown in Figure 12-12.

Figure 12-12. ICMPv6 Home Agent Address Discovery Reply message sent from the home agent

• IPv6 header

  In the IPv6 header, the source address is the home agent's address and the destination address is the mobile node's care-of address.

• ICMPv6 Home Agent Address Discovery Reply message

  The ICMPv6 Home Agent Address Discovery Reply message contains the list of home agents on the home link in order of preference. For more information, see the "ICMPv6 Messages" section in this chapter.

Tunneled Packet

When the home agent intercepts a packet sent directly to a mobile node's home address when the mobile node is away from home, it forwards the packet to the mobile node by using the form shown in Figure 12-13.

Figure 12-13. Intercepted packet tunneled to a mobile node by its home agent

• IPv6 header (outer)

  In the outer IPv6 header, the source address is the home agent's address and the destination address is the mobile node's care-of address.

• IPv6 header (inner)
• In the inner IPv6 header, the source address is the correspondent node's address and the destination address is the mobile node's home address.
• Upper layer PDU

  The upper layer PDU contains the Application layer data sent from the correspondent node to the mobile node at its home address. From the Application layer perspective, the data was addressed from the correspondent node address to the home address.

Notice that this packet is the original packet sent by the correspondent node that did not have a binding cache entry for the mobile node with an additional IPv6 header addressed from the home agent's address to the mobile node's care-of address. The original packet is described in the "Data with a Binding Cache Entry Not Present" section of this chapter.