Mobility support is one of the requirements for IPv6. This paper proposes a protocol for mobility support in IPv6. The proposed protocol is based on the mechanism of Virtual Internet Protocol (VIP), which provides mobility in IPv4.
Two Hop-by-Hop options are added to support mobility, one for user data and another for control. The proposed protocol provides routing optimi- zation and authentication mechanisms as well as seamless mobility.
2. The Proposed Protocol
The basic concept of the protocol is to separate addresses and identifiers. The identifier never changes regardless of the point of attachment to the Internet while the address changes when a node moves to another subnet. Both the identifier and the address have the same format. The subnet specified by the identifier of a mobile node is called the home subnet. The identifier can be used as the default address. TCP/UDP communicates with other nodes by using identifiers.
The protocol resolves the identifier into the address by using a cache called Address Mapping Table (AMT). An AMT entry propagates along the communication path. Most packets travel the optimal route since a node learns the current address of another node once it receives a packet.
Two options are added in the Hop-by-Hop Options Header of IPv6. The reason why these options should be categorized in the Hop-by-Hop Options Header is to create/update AMT entries on every node along the communi- cation path. One option is for user data while another is for control.
The proposed protocol uses two authentication mechanisms, keyed MD5 and RSA digital signature. Keyed MD5 is mainly used in data packet for end-to-end authentication since it requires a shared secret key between the authenticated node and the authenticating node. RSA digital signature is mainly used in control packet for hop-by-hop authentica- tion. Key distribution is beyond the scope of this protocol.
A mobile node obtains a temporary address when it is connected to a sub- net, and then it transmits a packet with the mobility option for control to its home subnet to register the current address with the home subnet. An intermediate node forwarding this packet creates/updates the AMT entry for the mobile node after authenticating the packet.
When a node transmits a packet to a mobile node, it uses the identifier of the mobile node as the address if it does not have an AMT entry for the mobile node. Address resolution for this packet is executed on either a node in the home subnet of the mobile node or an intermediate node which has the AMT entry for the mobile node. If the source node has the AMT entry for the mobile node, address resolution is executed on the source node, and then the packet travels the optimal route.
In IPv4, a mobile protocol (Mobile-IP) is also under development in IETF. It is possible to apply the mechanism of Mobile-IP to IPv6. How- ever, since a goal of Mobile-IP is to minimize changes to the current Internet, there are several problems: it violates the subnet model in the subnet to which mobile nodes are connected, it does not support routing optimization, and so on.
The proposed protocol is more excellent than Mobile-IP from a network architecture viewpoint. In addition, it supports routing optimization and scales well.
The Internet is being restructured by migrating from IPv4 to IPv6. It is the time to develop a protocol for mobile nodes, considering not backward compatibility but the network architecture of the Internet.
 R. Hinden. "Internet Protocol, Version 6 (IPv6) Specification", Internet-Draft, October 1994.
 F. Kastenholz and C. Partridge. "Technical Criteria for Choosing IP: The Next Generation (IPng)", RFC 1726, December 1994.
 F. Teraoka, K. Uehara, H. Sunahara, and J. Murai. "VIP: A Protocol Providing Host Mobility," in CACM, Vol. 37, No. 8, August 1994.