By: Henri Wohlfarth
Date: December 7, 2005
IETF64 saw the establishment of two new working groups related to mobility and wireless:
MONAMI6 – working on a problem statement and standards track specifications addressing issues associated with the simultaneous use of multiple addresses for mobile hosts using Mobile IPv6 or mobile routers using NEMO Basic Support.
AUTOCONF – working to standardize mechanisms by which ad hoc network nodes can configure locally or globally routable IPv6 addresses.
In addition, there were BoFs related to mobility and wireless:
NETLMM – second BoF on network-based, localized mobility management;
16NG – discussed 802.16 wireless link architectures and work needed to run IPv6 over 802.16;
EMU – standardizing Extensible Authentication Protocol (EAP) methods.
This article will discuss the results of the first meeting of the AUTOCONF working group and progress in the DNA (Detecting Network Attachment) working group. Both working groups are in the Internet Area.
The AUTOCONF working group met for the first time at IETF64, after two BoFs. The objective of the working group is to standardize a way to configure locally routable and globally routable addresses within an ad hoc network. Part of this problem may involve discovery of routers that connect a collection of ad hoc nodes to the Internet. The working group will focus on IPv6 only, to take advantage of the more powerful local link configuration protocols available in IPv6.
One of the major issues that came up during the chartering of the working group was that the IETF has really not considered what a Mobile, Ad hoc NETwork (MANET) is and what makes it different from a standard IP network. The first working group meeting discussed the architecture of MANETs in some detail. One presentation characterized the difference very succulently as follows: in other IP networks, the links form the network, while in MANETs, the network forms the links. MANETs tend to be characterized by half broadcast links, little or no specialized infrastructure for routing (the hosts act as routers themselves), and relatively flat routing control structures.
In the past, addresses within MANETs could be local (i.e. valid only within the collection of ad hoc hosts forming the MANET), but there is now increasing interest in hybrid MANETs, where the MANET is connected to the Internet through a gateway router. Because the nodes participating in the MANET and the links between nodes are quite fluid, MANETs see network partitions and the joining of two networks more often than other IP networks, and such operations can be thought of as a common part of a MANET’s operation rather than an error condition. This kind of shifting network structure is difficult to accommodate with traditional IP network address and routing configuration. Up until now MANETs have been thought of as principally a routing problem, but work in the ad hoc research community, which is well-represented among the AUTOCONF working group members, has come up with some additional areas were standardization is necessary for good interoperability.
The DNA working group has been chartered to devise a more robust network attachment and movement detection protocol for IPv6 than currently is available. RFC 3775, the Mobile IPv6 specification, specifies passive movement detection as the default. The mobile node waits until it hears a multicast Router Advertisement, then checks whether the router was seen before. If not, the mobile node infers that it has moved. The frequency of Router Advertisements is increased to 50 ms. This technique of movement detection has many disadvantages. Besides generating lots of Router Advertisement traffic, the requirement to wait until a Router Advertisement beacon is seen slows down the process of handover. In addition, if a link is configured with multiple routers, the protocol could cause the mobile node to conclude that it had moved to a new link when it really only is seeing a router advertisement from another router.
In order to improve movement detection, the working group is developing Best Current Practice (BCP) specifications for configuring hosts and routers without any additional protocol support to facilitate better network attachment and movement detection.
The DNA protocol design itself was finished by the design team. The design is based on having the host respond to a Layer 2 hint indicating that it has changed to a new access point. The host then multicasts a Router Solicitation to the All Routers Muticast Address, and receives unicast Router Advertisements from routers on the link. Both the Router Solicitation and Router Advertisement are enhanced for DNA, with additional options.
These options allow the host to indicate the link it thinks it is on, and for the router to reply indicating if the host is correct. If the router indicates to the host that it is on a new link, the router returns enough information so that the host can quickly autoconfigure a new IPv6 address on the new link and otherwise become established. The Router Advertisements are returned without the delay required by RFC 2461. Such delays can significantly hamper the ability of a mobile node to quickly configure on the new link. The protocol also contains security features to limit the ability of an attacker to subject the link to a Denial of Service attack.