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The current version of the Internet Protocol, known as IPv4 or simply as IP, has
served the Internet and corporate Intranets in good stead for over twenty years.
Now, however, it is beginning to show its age as it struggles to cope with the
Internet's daunting growth rate and the demand for new services. With IP, configuring
networks and terminals is no easy task, the available address space is running out,
and there are no simple solutions to the problem of renumbering a network when Internet
Service Provider (ISP) is changed. Though a number of mechanisms for overcoming these
limitations have been developed over the years - including DHCP
(Dynamic Host Configuration Protocol) for automatic terminal configuration and NATs
(Network Address Translators) to make it possible to reuse addresses - all have major
drawbacks.
The IETF (Internet Engineering Task Force) tackled this problem in the early '90s,
embarking on a research project designed to specify a new-generation IP protocol
capable of overcoming the limitations of today's versions. After a series of proposals
which contributed to establishing the requirements for the new protocol, the candidate
replacement for the current IP, designated IPv6, was selected in 1994 . Since then, an
enormous amount of work has been done: specifications have reached a high level of
maturity, and many of the protocol have been completed including some by the major router
manufacturers. From 1996 onwards, a worldwide IPv6 protocol testbed, the 6Bone network, has
grown continuously and encompasses in July 2002 around 1200 nodes operated by manufacturers, ISPs,
universities and research centers in 59 countries. 6Bone has been a very useful testbed for manufacturers
and ISPs to make reliable implementations of IPv6 protocol and to acquire operational experience on IPv6
deployment. Anyway the IETF community decided to start the 6bone phase-out that will be definitely closed
on 6th June 2006 (06/06/06).
Address space exhaustion
There can be little doubt that the main reason for
introducing a new IP protocol is the progressive exhaustion of the IPv4 address space.
Though the theoretical number of available addresses is 232 or more than four billion,
how the address space can be used in practice to number terminals is limited by the need
for flexibility in network configuration. This leads us to conclude that there is a
maximum efficiency factor for address space usage, as has been determined empirically
by comparison with the capacity to make use of available addresses shown by other
telecommunications systems such as telephony, SNA, DECNET, and so forth. For IP, the
maximum number of addresses that can be used without interfering with flexible network
configuration is around 240 million.
Furthermore the shortage of addresses is hampering Internet's growth in
developing countries, which do not have the buying power needed to absorb the
rising costs that obtaining global and unique Internet addresses involves.
Features of the IPv6 protocol
The most important innovation introduced by the
IPv6 protocol is the use of a 128 bits address space instead of IPv4's 32 bits.
In addition to guaranteeing virtually unlimited growth margins, being able to use
longer addresses will make it possible to give the Internet a more flexible and
efficient structure than it has today. In particular, it will be possible to organize
the network into an arbitrary number of hierarchical levels, while adopting an address
assignment policy which reflects this hierarchy from the outset will ensure maximum
routing information aggregation and, consequently, guarantee that routing is scalable.
The larger address space, in any case, is only one of the distinguishing features
of the new IP protocol. Indeed, IPv6 has also provided an ideal opportunity to rationalize
the IPv4 protocol by eliminating little-used functions, and to respond to the needs
that Internet users have been expressing in the last few years by introducing a set
of innovative capabilities not contemplated by the current version of IP.
One of the changes introduced in the move from IPv4 to IPv6 is a significant
simplification of header format. Several of the IPv4 header fields have been dropped
or made optional in order to reduce the packet processing cost and reduce the header's
bandwidth occupation as much as possible, despite the fact that addresses are larger.
Thus, even though IPv6 addresses are four times longer than their IPv4 counterparts,
the IPv6 header is only twice as large as the IPv4 header.
In addition, substantial changes have been made in how header options are encoded
in order to ensure more efficient packet forwarding and provide greater leeway for
future protocol extensions. In IPv6, options are no longer an integral part of the
IP header, but are each stored in a separate header - called the extension header -
placed between the IPv6 header and the header for the overlying transport layer
(e.g., TCP or UDP). Furthermore, whereas under IPv4 each router crossed is required
to examine all of the options in each packet, most IPv6 extension headers are examined
only at the final destination. This, together with the fact that the IPv6 header has
a fixed size (which permits a higher degree of optimization for the hardware modules
responsible for forwarding), translates into a sizable increase in performance and
means that IPv6 options are actually usable in practice.
Over and above all these advantages, the real advances which IPv6 makes over IPv4
are to be found in the capabilities which will be integrated in the protocol in order
to facilitate network administration and support for new services and applications.
To this end, the final version of the protocol will include native support for:
- Differentiated services (best-effort and guaranteed quality)
- Automatic configuration of terminals, services and network equipment
- Multicast services (i.e., the ability to set up "multipoint-multipoint" communications)
- Terminal mobility
- Secure communication
Support for providing differentiated services is based on the new class and flow label
fields in the IPv6 header, which the source can use to mark packets belonging to traffic
flows requiring special handling on the part of the network (e.g., guarantees on
delay and/or loss en route to the destination).
As regards autoconfiguration mechanisms, on the other hand, IPv6 was designed to be
a true "Plug and Play" protocol. This means that each newly installed IPv6 terminal
will be able to bring itself to operative status without manual intervention on the
part of the network administrator. Powerful host and router autoconfiguration mechanisms
have already been defined for this purpose. In particular, these mechanisms include the
Neighbor Discovery (ND) protocol, whereby user terminals can independently configure
their interfaces' IPv6 addresses starting from information announced by neighboring
routers. This mechanism simplifies network administration to a considerable extent because,
unlike the autoconfiguration protocols now available for IPv4 (DHCP, Dynamic Host
Configuration Protocol), it does not require that a central server be manually
configured and maintained.
As mentioned above, the final version of IPv6 will be required to provide integrated
support for network services such as multicast transmission, terminal mobility and
security. To this end, mechanisms for providing authentication, confidentiality and
data integrity services are at an advanced stage of definition, as are the mobility
management protocols which enable each IPv6 terminal to seamlessly change its point
of access to the Internet without jeopardizing its level of network connectivity.
The problem of transition
Moving from today's Internet to a new, IPv6-based
Internet will not be easy, as the two protocols are not interoperable.
The differences in the header (i.e., the different address lengths and dissimilar
semantics) are such that IPv6 packets cannot be routed across IPv4 networks and vice versa.
Currently, IPv4-based applications for Intranet and Internet environments cannot generally
be used as-is on IPv6 networks, but must be modified so that longer addresses can be employed.
In addition, we can expect that the transition from IPv4 to IPv6 will be a fairly
lengthy one, lasting several years. During this period, the two protocols will coexist,
with IPv4 accounting for the lion's share of usage at the beginning. Subsequent evolution
will depend to a large extent on how quickly IPv6 is able to gain ground.
For these reasons, work has been going on for some time on transition mechanisms
which will permit interoperability between terminals and routers implementing the two
protocols. The simplest solution is the dual-stack approach, where both protocols are
present on all network nodes (hosts and routers). This solution is extremely advantageous,
especially for hosts (system requirements are quite modest by comparison with present-day
computing power and memory capacity).
However, a number of different needs may arise, including:
- IPv6 network interconnection via IPv4 backbones
- Interoperability between IPv6-only terminals and IPv4-only terminals
- IPv4 LAN interconnection via IPv6 backbones
Some of the first answers to these needs are to be sought in tunneling mechanisms
(e.g., for encapsulating IPv6 packets in IPv4 packets and carrying them across the current
Internet). Other solutions consist of placing equipment which performs address and protocol
translation at the borders between networks based on different protocols: this is a simple
extension of the NAT (Network Address Translator) concept, which thus progresses from
being an alternative to IPv6 to being an effective aid to transition. Yet other solutions
involve using application gateways or proxies to provide interoperability between
applications resident on terminals attached to non-homogeneous networks.
A final class of solutions, such as the Tunnel Broker service proposed by CSELT
(https://carmen.cselt.it/ipv6tb)
together with IMAG in Grenoble, aims at assisting individual users (including those
employing dial-up access) in the procedures involved in connecting to IPv6 networks
and services via the current Internet.
The role of experimental activities
The first laboratory tests on IPv6-based networking
solutions were quickly channeled into a larger, worldwide initiative with the creation
of the 6Bone network in 1996. The 6Bone (or IPv6 Backbone) is an IPv6 testbed
(for information, see http://www.6bone.net)
set up on the Internet by interconnecting IPv6 laboratories using the tunneling technique.
To date, the 6Bone has seen a continuous rise in the number of interconnected laboratories,
and is the world's major focal point for experimental work on the IPv6 protocol.
This work addresses such topics as implementation maturity, management of the address
spaces assigned to experimental providers, IPv6 routing, renumbering techniques, etc.
The network is organized as a three-layer hierarchical structure. The highest
hierarchical level consists of the backbone nodes, and is the portion of the network on
which the geographical connectivity enjoyed by all of the other connected nodes is chiefly
based. At the next-lower level are the 6Bone transit nodes, i.e., nodes connected to at
least one backbone node but which in turn act as points of access to the network for one
or more nodes that do not have a direct tunnel to the backbone. The latter nodes are the
lowest hierarchical level of the current 6Bone network structure,
and are called leaf nodes.
At the moment, the backbone consists of over 60 nodes (including two in Italy: CSELT
and INFN-CNAF) and accounts for most of the complexity of 6Bone routing. Connectivity
between backbone nodes is guaranteed by a large number of tunnels established through
the Internet and forming an arbitrary mesh topology in which IPv6 packet routing is
based on the BGP4+ dynamic routing protocol (a version of BGP4 capable of supporting
IPv6 as well as IPv6).
CSELT's connections in the
6bone backbone |
The figure shows a map of CSELT's connections in the 6Bone backbone (further
information about the work now being done at CSELT's IPv6 laboratory is available
on the site
http://carmen.cselt.it/ipv6).
Together with CSELT, other participants in the initiative include equipment
manufacturers (Cisco, Bay Networks, Digital, etc.), TLC operators (MCI, SPRINT, etc.),
Internet Service Providers (UUNET, ANSNET, SURFNET, etc.), research centers and
universities.
It should be noted that the 6Bone initiative is also giving birth to the first native
IPv6 research networks (CAIRN, vBNS/Internet2, WIDE and DARENet). In addition, a number
of major ISPs (UUNet, ANSNet) and governmental networks in the U.S. (ESNet) have begun
to install IPv6 routers alongside their backbone routers in order to create experimental
IPv6 networks. Though largely based on tunnels established on the existing IPv4
infrastructure, these experimental networks make it possible to provide "friendly"
users with interconnection service based on the new-generation IP protocol.
Several ISPs are showing signs of wanting to provide IPv6-based service in the fairly
near future. Recently, for instance, AT&T, UUNet/Worldcom and other operators asked
the Internet Assigned Numbers Authority (IANA) for an official IPv6 address space to
be used in their networks. In response to these needs, IANA has already assigned a subset
of IPv6 addresses to each of the registries responsible for allocating IP addresses in
the United States (ARIN), Europe (RIPE-NCC) and Asia (APNIC), who will manage these
addresses in their own areas of jurisdiction. This means that it will shortly be possible
for an ISP to request and obtain IPv6 addresses from the registries, and use them to
provide commercial services to its customers.
Thus, we can say that the Internet's transition towards the new-generation IP protocol
is already under way, and got its start precisely from the experimental activities.
There is every sign, moreover, that this transition will proceed "painlessly", and at
least at the beginning can be limited to flanking the existing IPv4 infrastructure with
IPv6 equipment.
Conclusions
IPv6 is a major change from the current version of the IP
protocol, and is still the only concrete solution for dealing with the problem of the
Internet's future growth in the long term. Introducing IPv6 in commercial networks will
involve a difficult period of transition, made even more difficult by the fact that
enormous business interests now rely on IPv4, and that many of the improvements (in terms
of network architectures and new services) defined for IPv6 can also be implemented with
IPv4. However, there are also a number of drivers which can be expected to spur transition,
including:
- The availability of a virtually unlimited address space
- The advanced level reached by standards.
- The fact that equipment is now available from all major manufacturers.
- The costs associated with non-transition, or in other words with supporting the
Internet's growth while continuing to use the current IP protocol and adding all of
the new services which IPv6 offers on a native basis.
In this context, IPv6 experimental activities play a key role in guiding the construction
of the new Internet and retracing the steps that have made the Internet what it is today.
More than anything else, though, IPv6's success or failure will hinge on whether or not
the forces (e.g., Microsoft) who develop and market Internet and Intranet applications
decide to adopt the new protocol.
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