Internet Architecture Board Steve King, Bay Networks
INTERNET DRAFT Ruth Fax, Bay Networks
Dimitry Haskin, Bay Networks
Wenken Ling, Bay Networks
Tom Meehan, Bay Networks
Robert Fink, LBNL
25 June 2000 Charles E. Perkins, Nokia Research Center
The Case for IPv6
draft-iab-case-for-ipv6-06.txt
Status of This Memo
This document is a submission by the Internet Architecture Board
(IAB) of the Internet Engineering Task Force (IETF). Comments should
be submitted to the iab@isi.edu mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at:
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at:
http://www.ietf.org/shadow.html.
Abstract
This document outlines the business and technical case for IPv6. It
is intended to acquaint both the existing IPv4 community with IPv6,
to encourage its support for change, and to attract potential future
users of Internet technology.
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Contents
Status of This Memo i
Introduction 2
1. The Business Case for IPv6 3
1.1. IPv6: Standardization and Productization Status . . . . 3
1.2. IPv6 Design Goals . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Addressing and Routing . . . . . . . . . . . . . 5
1.2.2. Eliminating Special Cases . . . . . . . . . . . . 6
1.2.3. Minimizing Administrative Workload . . . . . . . 8
1.2.4. Security . . . . . . . . . . . . . . . . . . . . 9
1.2.5. Mobility . . . . . . . . . . . . . . . . . . . . 10
1.3. The IPv6 solution . . . . . . . . . . . . . . . . . . . . 11
1.3.1. Address Autoconfiguration . . . . . . . . . . . . 11
1.3.2. IPv6 Header Format . . . . . . . . . . . . . . . 12
1.3.3. Multicast . . . . . . . . . . . . . . . . . . . . 13
1.3.4. Anycast . . . . . . . . . . . . . . . . . . . . . 14
1.3.5. Quality of Service . . . . . . . . . . . . . . . 16
1.3.6. The Transition to IPv6 . . . . . . . . . . . . . 16
1.3.7. IPv6 DNS . . . . . . . . . . . . . . . . . . . . 17
1.3.8. Application Modification for IPv6 . . . . . . . . 18
1.3.9. Routing in IPv6/IPv4 Networks . . . . . . . . . . 18
1.3.10. The Dual-Stack Transition Method . . . . . . . . 20
1.3.11. Automatic Tunneling . . . . . . . . . . . . . . . 20
2. The Technical Case for IPv6 21
2.1. IPv6 Headers vs. IPv4 Headers . . . . . . . . . . . . . . 21
2.2. Extension Headers . . . . . . . . . . . . . . . . . . . . 23
2.3. Hop-by-Hop Options Header . . . . . . . . . . . . . . . . 24
2.4. Destination Options Headers . . . . . . . . . . . . . . . 24
2.5. Routing Headers . . . . . . . . . . . . . . . . . . . . . 25
2.6. Fragmentation Header . . . . . . . . . . . . . . . . . . 25
2.7. IPv6 Security . . . . . . . . . . . . . . . . . . . . . . 27
2.8. IPv6 Authentication Header . . . . . . . . . . . . . . . 27
2.9. IPv6 Encryption Header . . . . . . . . . . . . . . . . . 29
2.10. The IPv6 Address Architecture . . . . . . . . . . . . . . 31
2.11. The IPv6 Address Hierarchy . . . . . . . . . . . . . . . 32
2.12. Host Address Autoconfiguration . . . . . . . . . . . . . 35
2.13. Other Protocols and Services . . . . . . . . . . . . . . 39
3. Transition Scenarios 40
3.1. First Scenario: No Need to NAT . . . . . . . . . . . . . 40
3.2. Second Scenario: IPv6 from the Edges to the Core . . . . 42
3.3. Other mechanisms . . . . . . . . . . . . . . . . . . . . 43
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4. Security Considerations 44
5. Acknowledgments 44
A. Myths 45
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Introduction
This document provides an overview of the benefits of the IETF's
next-generation IP protocol known as IP Version 6 (IPv6). The core
protocols of IPv6 are stable and have been standardized by the
IETF; they are unlikely to change significantly at this point. The
intended audience for this document includes enterprise network
administrators and decision makers, router vendors, host vendors,
Internet Service Providers (ISPs) managers, and protocol engineers
who are as yet unfamiliar with the basic aspects of IPv6. This
document was revised based on an existing original, at the request of
the IAB.
The Internet Protocol (IP) has its roots in early research networks
of the 1970s, but within the past decade has become the leading
network-layer protocol. This means that IP is a primary vehicle for
a vast array of client/server and peer-to-peer communications, and
the current scale of deployment is straining many aspects of its
twenty-year old design [7].
The Internet Engineering Task Force (IETF) has produced
specifications (see section 1.1) that define the next-generation
IP protocol known as "IPng," or "IPv6." IPv6 is both a near-term
and long-range concern for network owners and service providers.
IPv6 products have already come to market; on the other hand, IPv6
development work will likely continue for years to come. Though it
is based on much-needed enhancements to IPv4 standards, IPv6 should
be viewed as a new protocol that will provide a firmer base for the
continued growth of today's internetworks.
Because it is intended to replace IP (hereafter called IPv4) IPv6
is of considerable importance to businesses, consumers, and network
access providers of all sizes. IPv6 is designed to improve upon
IPv4's scalability, security, ease-of-configuration, and network
management; these issues are central to the competitiveness and
performance of all types of network-dependent businesses. IPv4 can
be modified to perform some of these functions, but the expectation
within the IAB is that the results are likely to be far less useful
than what could be obtained by widespread deployment of IPv6. On
the other hand IPv6 aims to preserve existing investment as much as
possible. End users, industry executives, network administrators,
protocol engineers, and many others will benefit from understanding
the ways that IPv6 will affect future internetworking and distributed
computing applications.
By early 1998 a worldwide IPv6 testing and pre-production deployment
network, called the 6BONE, had already reached approximately
400 sites and networks in 40 countries. There are over 50 IPv6
implementations completed or underway worldwide, and over 25 in test
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or production use on the 6BONE. The 6BONE has been built by an active
population of protocol inventors, designers and programmers. They
have worked together to solve the questions and problems that might
be expected to arise during such a huge project. Their experience
has served to validate the expectations of the protocol designers.
This document presents IPv6 issues in several parts:
- The Business Case for IPv6, giving a high-level view of business
issues, protocol basics, and current status, and
- The Technical Case for IPv6, which describes more of the
functional and technical aspects of IPv6.
- Transition Scenarios, which discusses mechanisms that have been
designed to ease the transition from IPv4 to IPv6.
This document has been revised from an existing document originally
written by employees of Bay Networks (see Acknowledgements,
section 5). Since the original document was written, many things
have changed surrounding the standardization, implementation, and
deployment of IPv6. Recent developments involving IPv6 in telephony
standards illustrate the power and the essential need for IPv6's
features. Now that IPv6 is specified to be a required, mandatory
to implement network layer protocol by 3GPP, 3GPP2 and MWIF, we
can expect to see hundreds of millions, or more likely billions,
of IPv6-capable devices populating the Internet within 5 years.
This eventuality has been brought about because telephone equipment
manufacturers have recognized the advantages of the IPv6 architecture
as detailed in this document.
1. The Business Case for IPv6
Given the remarkable growth of the Internet, and business opportunity
represented by the Internet, IPv6 is of major interest to business
interests, enterprise internetworks, and the global Internet. IPv6
presents all networking interests with an opportunity for global
improvements, which is now receiving the collective action that is
needed to realize the benefits.
1.1. IPv6: Standardization and Productization Status
The base protocols of IPv6 have been approved as Draft Standards,
so that it is known to be highly stable and appropriate for
productization. A large number of end-user organizations, standards
groups, and network vendors have been working together on the
specification and testing of early IPv6 implementations. A number
of IETF working groups have produced IPv6 specifications that are
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finished or well underway. Current Draft Standards at the time of
this writing include:
RFC 1981 Path MTU Discovery for IP version 6
RFC 2374 An IPv6 Aggregatable Global Unicast Address Format
RFC 2460 Internet Protocol, Version 6 (IPv6) Specification
RFC 2461 Neighbor Discovery for IP Version 6 (IPv6)
RFC 2462 IPv6 Stateless Address Autoconfiguration
RFC 2463 Internet Control Message Protocol (ICMPv6) for the
Internet Protocol Version 6 (IPv6) Specification
Current Proposed Standards include:
RFC 1886 DNS Extensions to support IP version 6
RFC 1887 An Architecture for IPv6 Unicast Address Allocation
RFC 2080 RIPng for IPv6
RFC 2373 IP Version 6 Addressing Architecture
RFC 2452 IP Version 6 Management Information Base for the
Transmission Control Protocol
RFC 2454 IP Version 6 Management Information Base for the User
Datagram Protocol
RFC 2464 Transmission of IPv6 Packets over Ethernet Networks
RFC 2465 Management Information Base for IP Version 6: Textual
Conventions and General Group
RFC 2466 Management Information Base for IP Version 6: ICMPv6
Group
RFC 2467 Transmission of IPv6 Packets over FDDI Networks
RFC 2470 Transmission of IPv6 Packets over Token Ring Networks
RFC 2472 IP Version 6 over PPP
RFC 2473 Generic Packet Tunneling in IPv6 Specification
RFC 2491 IPv6 over Non-Broadcast Multiple Access (NBMA) Networks
RFC 2492 IPv6 over ATM Networks
RFC 2507 IP Header Compression
RFC 2526 Reserved IPv6 Subnet Anycast Addresses
RFC 2529 Transmission of IPv6 over IPv4 Domains without Explicit
Tunnels
RFC 2545 Use of BGP-4 Multiprotocol Extensions for IPv6
Inter-Domain Routing
RFC 2590 Transmission of IPv6 Packets over Frame Relay
RFC 2675 IPv6 Jumbograms
RFC 2710 Multicast Listener Discovery (MLD) for IPv6
RFC 2711 IPv6 Router Alert Option
There are too many related RFCs and Internet Drafts to list them all
here, but among them are included the following:
RFC 1888 OSI NSAPs and IPv6
RFC 2292 Advanced Sockets API for IPv6
RFC 2375 IPv6 Multicast Address Assignments
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RFC 2450 Proposed TLA and NLA Assignment Rules
RFC 2471 IPv6 Testing Address Allocation
RFC 2553 Basic Socket Interface Extensions for IPv6
RFC 2740 OSPF for IPv6 [9]
RFC 29xx Router Renumbering for IPv6 [11]
work in progress Mobility Support in IPv6 [23]
work in progress DHCP for IP Version 6 [5, 35]
work in progress Site prefixes in Neighbor Discovery [33]
work in progress Routing of Scoped Addresses in the Internet
Protocol Version 6 (IPv6)
To RFC-editor: please replace "RFC 29xx" with the
appropriate value
Standards work on IPv6 and related components is far enough along
that numerous vendors have developed either prototype implementations
or products. For current information on implementations and vendor
activities, see the following URLS:
- http://playground.sun.com/pub/ipng/html/ipng-main.html
- http://www.ipv6forum.com
1.2. IPv6 Design Goals
IPv6 has been designed to enable high-performance, scalable
internetworks that should operate as needed for decades. Part of the
design process involved correcting the inadequacies of IPv4. IPv6
offers a number of enhanced features, such as a larger address space
and improved packet formats. Scalable networking requires careful
utilization of human resources as well as network resources; so, a
great deal of attention has been given to creating autoconfiguration
protocols for IPv6, minimizing the need for human intervention
when assigning IP addresses and relevant network parameters such
as link MTU. Other benefits relate to the fresh start that IPv6
gives to those who build and administer networks. For instance,
a well-structured, efficient and adaptable routing hierarchy
will be possible. The following sections give an overview of the
improvements that IPv6 brings to enterprise networking and the global
Internet.
1.2.1. Addressing and Routing
IPv6 helps to solve a number of problems that currently exist within
and between enterprises. On the global scale, IPv6 will allow
Internet backbone designers to create a flexible and expandable
global routing hierarchy. The Internet backbone, where major
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enterprises and Internet Service Provider (ISP) networks come
together, depends upon the maintenance of a hierarchical address
system, similar to that of the national and international telephone
systems. Large central-office phone switches, for instance, only
need a three-digit national area code prefix to route a long-distance
telephone call toward the correct local exchange. The current
IPv4 system also uses an address hierarchy to sort traffic towards
networks attached to the Internet backbone.
Without an address hierarchy, backbone routers would be forced to
store route table information on the reachability of every network
in the world. Given the current number of IP subnets in the world
and the growth of the Internet, it is not feasible to manage route
tables and updates for so many routes. With a hierarchy, backbone
routers can use IP address prefixes to determine how traffic should
be routed through the backbone. In recent years, IPv4 has begun to
use a technique called Classless InterDomain Routing (CIDR) [38, 19],
which uses bit masks to allocate a variable portion of the 32-bit
IPv4 address to a network, subnet, or host. CIDR permits "route
aggregation" at various levels of the Internet hierarchy, whereby
backbone routers can store a single route table entry that provides
reachability to many lower- level networks.
But CIDR does not guarantee an efficient and scalable hierarchy.
In order to avoid maintaining a separate entry for each route
individually, it is important for routes at lower levels of the
routing hierarchy, that naturally have longer prefixes, to be
collected together (or "summarized", or "aggregated") into fewer and
less specific routes at higher levels of the routing hierarchy.
Legacy IPv4 address assignments that originated before CIDR and
the current access provider hierarchy often do not facilitate
summarization. The lack of uniformity of the current hierarchical
system, coupled with the difficulty in obtaining IPv4 addresses,
makes Internet addressing and routing quite complicated. These
issues affect high-level service providers and consequently
individual end users in all types of businesses. Finally,
renumbering IPv4 sites when changing from one ISP to another,
in order to maintain and improve address/route aggregation, is
significantly more expensive and difficult compared with IPv6's site
renumbering capabilities (see section 1.2.3).
1.2.2. Eliminating Special Cases
Many of the same problems that exist today in the Internet backbone
are also being felt at the level of the enterprise and the individual
business user. When an enterprise can't summarize its routes
effectively, it puts a larger load on the backbone route tables.
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If an enterprise can't present globally unique addresses to the
Internet, it may be forced to deploy private, isolated address space
that isn't visible to the Internet.
Users in private address spaces with non-unique addresses typically
require gateways, and possibly Network Address Translators
(NATs) [39], to manage their connectivity to the outside world. In
such situations, some services are simply not available. A NAT
is meant to allow an enterprise to have whatever internal address
structure it desires, without concern for integrating internal
addresses with the global Internet. This is seen as particularly
convenient in the existing IPv4 world, with its more cumbersome
address space management. The NAT device sits on the border
between the enterprise and the Internet, converting private internal
addresses to a smaller pool of globally unique addresses that are
passed to the backbone and vice versa (see Figure 1).
|
|
Private address space | Unique global addresses
|
|
--------------- |
/ \ +-----+ +----------+
| Enterprise | | | | |
| |----| NAT |-----| Internet |
| Network | | | | |
\ / +-----+ +----------+
--------------- |
|
|
|
Figure 1: Network Address Translator (NAT)
NAT may be appropriate in some organizations, particularly if
full connectivity with the outside world is not desired. But for
enterprises that require robust interaction with the Internet, NAT
devices often get in the way. The NAT technique of substituting
address fields in each and every packet that leaves and enters the
enterprise is very demanding, and presents a bottleneck between
the enterprise and the Internet. A NAT may keep up with address
conversion in a small network, but as the enterprise's Internet
access increases, the NAT's performance must increase in parallel.
The bottleneck effect is exacerbated by the difficulty of integrating
and synchronizing multiple NAT devices within a single enterprise.
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Enterprises with NAT are less likely to achieve the reliable
high-performance Internet connectivity that is common today with
multiple routers attached to an ISP backbone in an arbitrary mesh
fashion. Furthermore, use of NAT devices takes away the additional
element of reliability afforded by the possibility for asymmetric
routing, since NAT devices require control of traffic directions both
to and from internally addressed network nodes.
NAT translators also run into trouble when applications embed IP
addresses in the packet payload, above the network layer. This
is the case for a number of applications, including certain File
Transfer Protocol (FTP) programs, Mobile IP, and the Windows Internet
Name Service (WINS) registration process of Windows 95 and Windows
NT. Unless a NAT parses every packet all the way to the application
level, it is likely to fail to translate some embedded addresses,
which will lead to application failures. NAT can also break Domain
Name Servers, because they work above the network layer. NATs
prevent the use of some types of IP-level security between the
endpoints of a transaction. Today, NAT devices are helpful in
certain limited scenarios for smaller enterprises, but are considered
by many to be generally disadvantageous for the long-term health of
the Internet. See [20] for a fuller discussion about the effects of
NAT use on the Internet.
1.2.3. Minimizing Administrative Workload
A major component of today's network administration involves the
assignment of networking parameters to computers and other network
nodes, that are needed before they can begin any sort of network
operation. Information such as an IP address, DNS server, default
router, and other configuration details have to be installed at
each network node. In many cases, this is still done by manual
configuration, either by the network administration, or worse yet by
the users themselves. Recent efforts to shift this administrative
load onto departmental servers have focussed on deployment of the
Dynamic Host Configuration Protocol (DHCP) [16, 1], but this comes
along with its own administrative difficulties.
IPv4's limitations also aggravate the occasional need in many
organizations to renumber network devices -- i.e., assign new IP
addresses to them. When an enterprise changes ISPs, it may have
to either renumber all addresses to match the new ISP-assigned
prefix, or implement Network Address Translation devices (NATs).
Renumbering may be indicated when a corporation undergoes a merger
or an acquisition with consequent network consolidation. Since
routing prefixes are assigned to reflect the routing topology of
the enterprise networks and the number of nodes attached to the
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particular network links, there are two ways that the choice of
routing prefixes can become inconvenient or incorrect:
1. The routing prefix can become too long for the administration to
be able to increase the number of nodes that can be attached to
the particular link, and
2. The ways that the network links are connected together, or are
connected to the outside world, can change.
Either of these occurrence would indicate the need to renumber one or
more enterprise networks. It would be quite profitable to be able to
renumber enterprise networks without requiring expensive downtime for
the networks and or the nodes on the network.
Address shortages and routing hierarchy problems threaten the network
operations of larger enterprises, but they also affect small sites
-- even the home worker who dials in to the office via the Internet.
Smaller networks can be completely dropped from Internet backbone
route tables if they do not adapt to the address hierarchy, while
larger networks may refuse to renumber and cause a larger routing
problem for the backbone providers of the Internet. With today's
IPv4 address registries, ISPs with individual dial-in clients
cannot allocate IP numbers as freely as they wish. Consequently,
many dial-in users must use an address allocated from a pool on a
temporary basis. In other cases, small dial-in sites are forced to
share a single IP address among multiple end systems.
A unique IP address sets the stage for users to gain direct
connectivity to other users on the Internet, as determined by local
policy. It also simplifies a wide range of productive interactive
applications, of which telecommuting and remote diagnostics are only
two examples. Today's hierarchy of limited and poorly allocated IPv4
addresses has already caused problems, and will continue to do so
as more and more devices of varying capabilities are added to the
Internet.
1.2.4. Security
Encryption, authentication, and data integrity safeguards are needed
for enterprise internetworking and virtual private networks (VPNs).
For these purposes, IPv6 offers security header extensions.
The IPv6 authentication extension header allows a receiver to
determine with a high degree of certainty whether or not a packet
originated from the host indicated in its source address. This
prevents malicious users from configuring an IP host to impersonate
another, to gain access to secure resources. Such source-address
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masquerading (spoofing) is among the techniques that could be used
to obtain valuable financial and corporate data, or could give
adversaries of the enterprise control of servers for malicious
purposes. Spoofing might fool a server into granting access to
valuable data, passwords, or network control utilities. IP spoofing
is known to be one of the most common forms of denial-of-service
attack; with IPv4 it is typically impossible for a server to
determine whether packets are being received from the legitimate
end node. Some enterprises have responded by installing firewalls,
but these devices introduce a number of new problems, including
performance bottlenecks, restrictive network policies, and limited
connectivity to the Internet or even between divisions of the same
company.
IPv6 uses a standard method to determine the authenticity of packets
received at the network layer, ensuring that network products from
different vendors can use interoperable authentication services.
IPv6 implementations are required to support the MD5 [26] and
SHA-1 [27] algorithms for authentication and integrity checking to
insure that any two IPv6 nodes can interoperate securely. Since the
specification is algorithm-independent, other techniques may be used
as well.
Along with packet spoofing, another major hole in Internet security
is the widespread deployment of traffic analyzers and network
"sniffers" which can surreptitiously eavesdrop on network traffic.
These generally helpful diagnostic devices can be misused by those
seeking access to credit card and bank account numbers, passwords,
trade secrets, and other valuable data. In IPv6 privacy (data
confidentiality) is provided by a standard header extension for
end-to-end encryption at the network layer. IPv6 encryption headers
indicate which encryption keys to use, and carry other handshaking
information. IPv4 network-layer extensions for this have been
defined and are compatible with those for IPv6, but are not yet in
wide use.
Both IPv6 security headers can be used directly between hosts
or in conjunction with a specialized security gateway that adds
an additional level of security with its own packet signing and
encryption methods.
1.2.5. Mobility
IPv4 has difficulties managing mobile computers, for several reasons:
- A mobile computer needs to make use of a forwarding address at
each new point of attachment to the Internet, and it's not always
so easy to get such an address with IPv4
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- Informing any agent in the routing infrastructure about
the mobile node's new location requires good authentication
facilities which are not commonly deployed in IPv4 nodes.
- In IPv4, it may be difficult for mobile nodes to determine
whether or not they are attached to the same network.
- It is unlikely in IPv4 that mobile nodes would be able to inform
their communication partners about any change in location.
Each of these problems is solved in a natural way by using features
in IPv6. The benefits for mobile computing are apparent in quite a
number of aspects of the IPv6 protocol design, and go beyond merely
providing dial-up support for road warriors. The improvements
in option processing for destination options, autoconfiguration,
routing headers, encapsulation, security, and anycast addresses all
contribute to the natural design of mobility for IPv6 [23].
1.3. The IPv6 solution
IPv6, with its immensely larger address space, defines a multi-level
hierarchical global routing architecture. Using CIDR-style
prefixes [38], the IPv6 address space can be allocated in a way that
facilitates route summarization, and controls expansion of route
tables in backbone routers. The vastly greater availability of IPv6
addresses eliminates the need for private address spaces. ISPs
will have enough addresses to allocate to smaller businesses and
dial-in users that need globally unique addresses to fully exploit
the Internet. Using an example from crowded telephone networks, one
might say that IPv6 eliminates the need for "extensions", so that all
offices have direct communication lines and do not need operators
(automatic or otherwise) to redirect calls.
1.3.1. Address Autoconfiguration
Each IPv6 node initially creates a local IPv6 address for itself
using "stateless" address autoconfiguration, not requiring a manually
configured server. Stateless autoconfiguration further makes it
possible for nodes to configure their own globally routable addresses
in cooperation with a local IPv6 router. Typically, the node
combines its 48 or 64 bit MAC (i.e., layer-2) address, assigned by
the equipment manufacturer, with a network prefix it learns from a
neighboring router. This keeps end user costs down by not requiring
knowledgeable staff to properly configure each workstation before
it can be deployed. These costs are currently part of the initial
equipment expense for almost all IPv4 computing platforms. With the
possibility of low or zero administrative costs, and the possibility
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of extremely low cost network interfaces, new market possibilities
can be created for control of embedded computer systems. This
feature will also help when residential networks emerge as an
important market segment.
IPv4 networks often employ the Dynamic Host Configuration Protocol
(DHCP) to reduce the effort associated with manually assigning
addresses to end nodes. DHCP is termed a "stateful" address
configuration tool because it maintains static tables that determine
which addresses are assigned to newly connected network nodes.
A new version of DHCP has been developed for IPv6 to provide
similar stateful address assignment as may be desired by many
network administrators. DHCPv6 [5, 35] also assists with efficient
reconfiguration in addition to initial address configuration, by
using multicast from the DHCP server to any desired population of
clients.
The robust autoconfiguration capabilities of IPv6 will benefit
internetwork users at many levels. When an enterprise needs to
renumber, IPv6 autoconfiguration will allow hosts to be given
new prefixes, without even requiring manual reconfiguration
of workstations or DHCP clients. A key aspect of IPv6's eased
renumbering capability is its built-in support for multiple
simultaneous addresses. This capability allows a site to migrate
to a new numbering scheme slowly while continuing to support the
previous numbering scheme. Only when migration to the new addresses
is complete, are the old addresses retired. In contrast, with IPv4,
renumbering a network involves having a ``flag day'' -- that is, a
day when work stops and the network administrators are faced with
making a huge conversion. This function also assists enterprises
in keeping up with dynamic end-user populations. Autoconfiguration
allows mobile computers to receive valid forwarding addresses
automatically, no matter where they connect to the network.
1.3.2. IPv6 Header Format
IPv6 regularizes and enhances the basic header layout of the IP
packet (see Figures 5,6 in section 2.1). In IPv6, some of the IPv4
header information was dropped or made optional. The simplified
packet structure is expected to offset the bandwidth cost of the
longer IPv6 address fields. The 16-byte (128-bit) IPv6 addresses are
four times longer than the 4-byte IPv4 addresses, but as a result of
the retooling, the total IPv6 header size is only twice as large;
many processing aspects are substantially more efficient. Note
that a number of other designs were considered, including variable
length addresses; in the end, simplicity won out over infinite
extensibility, partially because 128 bits offers such a huge total
address space. Recent work [15] in IP header compression promises to
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reduce or perhaps even effectively eliminate any additional network
load associated with the use of 128-bit addresses over low-bandwidth
links.
IPv6 encodes IP header options in a way that streamlines the
forwarding process. Optional IPv6 header information is conveyed
in independent "extension headers" located after the IPv6 header
and before the transport-layer header in each packet. Most IPv6
extension headers are not examined or processed by intermediate
nodes (in contrast with IPv4). This enables a big improvement
in the deployability of optional IPv6 features, compared to IPv4
where IP options typically cause a major performance loss for the
packet at every intermediate router. IPv6 header extensions are
variable in length and can contain more information than before.
Network protocol designers can introduce new header options in a
straightforward manner. More details about the comparisons between
the IPv4 and IPv6 headers are discussion in section 2.1.
So far, option fields have been specified for carrying explicit
routing information created by the source node, as well as for
mobility, authentication, encryption, and fragmentation control.
At the application level, header extensions are available for
specialized end-to-end network applications that require their own
header fields within the IP packet.
1.3.3. Multicast
Modern internetworks need to transmit streams of video, audio,
animated graphics, news, financial, or other timely data to groups
of functionally related but dispersed endstations. This is best
achieved by network layer multicast. Typically, a server sends out a
single stream of multimedia or time-sensitive data to be received by
subscribers. A multicast-capable network routes the server's packets
to each subscriber in the multicast group using an efficient path
(see Figure 2), replicating only as needed. In the figure, a single
packet from the source will be received by all the multicast group
members. When there are multiple networks containing multicast group
members, a packet distribution "tree" is created for the multicast
group.
Routers use multicast protocols such as DVMRP (Distance Vector
Multicast Routing Protocol) [44] and PIM (Protocol Independent
Multicast) [18] or MOSPF (Multicast Open Shortest Path First) [31]
to dynamically construct the packet distribution tree that connects
all members of a group with the multicast server. Only members that
have joined the multicast group perform the processing to receive
the data. A new member becomes part of a multicast group by sending
a "join" message to a nearby router. The distribution tree is then
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Multicast Source
+---+
| |
| |
+-+-+
|
|
|
---+------+----+----------+---+----+-----+--------+------+-----+-
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| +-+-+ | | +-+-+ | | | |
| | | | | | | | +-+-+ | |
| | | | | | | | | | | +-+-+
+-+-+ +---+ | +-+-+ +---+ | | | | | |
| | | | | | +---+ | | |
| | +-+-+ | | +-+-+ Multicast | +---+
+---+ | | +---+ | | Group |
Multicast | | Multicast | | Member +-+-+
Group +---+ Group +---+ | |
Member Member | |
+---+
Multicast
Group
Member
Figure 2: Multicast in Action
adjusted to include the new route. Servers can then multicast a
single packet, and it will be replicated as needed and forwarded
through the internetwork to the multicast group. This conserves both
server and network resources and, hence, is superior to unicast and
broadcast solutions. Multicast applications have been developed
for IPv4, but IPv6 extends IP multicasting capabilities by defining
a much larger multicast address space. All IPv6 hosts and routers
are required to support multicast. In fact, IPv6 has no broadcast
address as such; it has various multicast addresses of various
scopes. The improved scoping offered in IPv6 promises to simplify
the use and administration of multicast in many applications.
1.3.4. Anycast
Anycast services, supported in the IPv6 specification, are not
defined architecturally in IPv4. Conceptually, anycast is a cross
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----- ----- -----
| X | | Y | | Z |
----- ----- -----
\ | / ------- ISP transit domain ---------
\ | / | |
------- | ------- |
| rtr |---------------------------------| rtr | |
------- | ------- |
/ \ | / \ |
/ \ | / \ |
------- ------- | ------- ------- |
| rtr | Enterprise| rtr |---------------| rtr | Anycast | rtr | |
------- Network ------- | ------- Group ------- |
\ / | \ / |
\ / | \ / |
------- | ------- |
| rtr |---------------------------------| rtr | |
------- | ------- |
| | |
----- | |
| Q | ------- ISP transit domain ---------
-----
Figure 3: Anycast Addressing
between unicast and multicast: an arbitrary collection of nodes may
be designated as an anycast group [34]. A packet addressed to the
group's anycast address is delivered to only one of the nodes in the
group, typically the node with the "nearest" interface in the group,
according to current routing protocol metrics. This is in contrast
with multicast services, which deliver packets to all members of the
multicast group. Nodes in an anycast group are specially configured
to recognize anycast addresses, which are drawn from the unicast
address space [22].
Anycasting is a new service, and its applications have not been fully
developed. Using anycast, an enterprise could forward packets to
exactly one of the routers on its ISP's backbone (see Figure 3). If
all of a provider's routers have the same anycast address, traffic
from the enterprise will have several redundant access points to the
Internet. And if one of the backbone routers goes down, the next
nearest device automatically will receive the traffic.
In figure 3, suppose some hosts Q, X, Y, and Z in an Enterprise
Network send data to the anycast address served by the backbone
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routers in the Anycast Group of the ISP Transit Domain. The border
routers in the Enterprise Network forward the data just as they would
for data sent to a unicast address. Then, any one of the backbone
routers in the Anycast Group may receive the data, eliminating the
overhead which would have been incurred if the backbone routers were
instead configured to form a multicast group. If there are multiple
home agents for mobile nodes on a single home network, they also
join an anycast group. In that way, a mobile node can register with
exactly one home agent even in the case when it doesn't know the
address of any specific one.
1.3.5. Quality of Service
IPv4 carries a "differentiated services" byte and IPv6 carries an
equivalent "traffic class" byte, intended for support of simple
differentiated services. Both IPv4 and IPv6 can support the RSVP
protocol for more complex quality of service implementations.
Additionally, the IPv6 packet format contains a new 20-bit
traffic-flow identification field that will be of great value to
vendors who implement quality-of-service (QoS) network functions.
Such QoS products are still in the planning stage, but IPv6 lays the
foundation so that a wide range of QoS functions (including bandwidth
reservation and delay bounds) may be made available in a open and
interoperable manner.
An additional benefit for QoS in IPv6 is that a flow label has been
allocated within the IPv6 header that can be used to distinguish
traffic flows for optimized routing. Furthermore, the flow label can
be used to identify flows even when the payload is encrypted (i.e.,
the port numbers are hidden).
1.3.6. The Transition to IPv6
The transition from IPv4 to IPv6 could take one of several paths.
Some are lobbying for rapid adoption of IPv6 as soon as possible.
Others prefer to defer IPv6 deployment until the IPv4 address space
is exhausted, or until other issues leave no other choice. Either
way, given the millions of existing IPv4 network nodes, IPv4 and IPv6
will coexist for an extended period of time.
Therefore, IETF protocol designers have gone to great lengths to
ensure that hosts and routers can be upgraded to IPv6 in a graceful,
incremental manner. The transition will prevent isolation of
IPv4 nodes, and also prevent "fork-lift" upgrades for entire user
populations. Transition mechanisms have been engineered to allow
network administrators flexibility in how and when they upgrade hosts
and intermediate nodes. IPv6 can be deployed in hosts first, in
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routers first, or, alternatively, in a limited number of adjacent or
remote hosts and routers. The nodes that are upgraded initially do
not have to be colocated in the same local area network or campus.
Many upgraded hosts and routers will need to retain downward
compatibility with IPv4 devices for an extended time period (possibly
years or even indefinitely). It is also assumed that upgraded
devices should have the option of retaining their IPv4 addresses. To
accomplish these goals, IPv6 transition relies on several special
functions that have been specified by the ``ngtrans'' working group
of the IETF, including dual-stack hosts and routers, and tunneling
IPv6 via IPv4. A dual-stack host is a computer able to handle both
IPv4 and IPv6 packets. Such a computer can deliver packetized data
to a single application that has been equipped to ask for data from
both addressing domains. This facilitates easy transition from IPv4
to IPv6 since the application can then still receive data from its
current communications partners, without change in any way noticeable
to the users.
1.3.7. IPv6 DNS
Domain Name Service (DNS) is something that administrators must
consider before deploying IPv6 or dual-stack hosts. In response
to this issue, IETF designers have defined "DNS Extensions to
Support IP Version 6" [41] and "DNS Extensions to Support IPv6
Address Aggregation and Renumbering" [12]. These specifications
create new DNS record types (A6 and AAAA ("quad A") record types)
that map a domain name to an IPv6 address. Domain name lookups
(reverse lookups) based on 128-bit addresses also are defined. Once
an IPv6-capable DNS is in place, dual-stack hosts can interact
interchangeably with IPv6 nodes. If a dual-stack host queries DNS
and receives back a 32-bit address, IPv4 is used; if a 128-bit
address is received, then IPv6 is used.
In the current DNS, addresses are stored within DNS Resource Records
that contain complete addresses. Changing any of the bits of
an address (e.g., the bits defining the specific ISP the host is
connected to) requires updating every Resource Record for every one
of a site's hosts. This poses an administrative burden and requires
generating new cryptographic signatures for the records as well, when
DNSSEC is used [17]. For a site with tens of thousands of nodes, the
effort can be significant.
The new DNS extensions [12] defined by IPv6 will ease the process
of managing a site's addresses and in particular simplify the DNS
updates required should the site switch providers or otherwise need
renumbering. The new "A6" DNS records allow for the division of
an address into its logical components, each stored as a separate
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component in the DNS. Should a site renumber, it only needs to update
the DNS records that define the new ISP, without modifying any of the
individual Resource Records associated with each specific host. A
change in a single DNS record is inherited transparently by all of
the site's hosts.
The new, renumbering-friendly DNS mechanisms allow a prefix change
for all of a site's hosts to be accomplished with as little as one
DNS record change in each of the forward and reverse data sets. It
also allows a typical site the option of storing the forward and
reverse data in the same DNS zone. It is expected that this scheme
will soon supplant the older, AAAA-based methods.
IPv6 autoconfiguration and IPv6 DNS can be linked by using dynamic
DNS updates, coupled with secure DNS. By these means DNS servers can
be securely and automatically updated whenever an IPv6 node acquires
a new address, enabling an additional measure of convenience compared
with renumbering in IPv4 today.
1.3.8. Application Modification for IPv6
Applications that do not directly access network functions (do not
call a socket or DNS API and do not handle numeric IP addresses in
any way) need no modifications to run in the dual-stack environment.
Applications that use certain interface APIs to communicate with the
network stack will require updating before using IPv6. For example,
applications that access DNS or use sockets must be enhanced with
the capability to handle A6 or AAAA records and 128-bit addresses.
Applications which are expected to run both IPv4 and IPv6, as well
as using IPv6 security, quality of service, and other features, will
need more extensive updating.
Adding such a dual-stack architecture to all the existing hosts
is, in fact, a significant effort. This effort has to be balanced
against the benefits of IPv6, and against the effort to renumber the
existing hosts if the network deployment grows past the restrictions
resulting from insufficient address space.
1.3.9. Routing in IPv6/IPv4 Networks
Routers running both IPv6 and IPv4 can be administered in much the
same fashion that IPv4-only networks are currently administered.
Multi-protocol extensions to BGP4 [43] have been defined by the IETF;
one of them carries IPv6 prefixes. The IPv6 extension has been
used widely in the 6bone since early 1997. IPv6 versions of other
popular routing protocols, such as Open Shortest Path First (OSPF)
and Routing Information Protocol (RIP), are also available.
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Administrators may choose to keep the IPv6 topology and addressing
logically separate from the IPv4 network, even though both run on the
same physical infrastructure, allowing the two to be administered
separately. Alternatively, it may be advantageous to align the two
architectures by using the same domain boundaries, areas, and subnet
organization. Both approaches have their advantages. A separate
IPv6 architecture can be used to replace the inefficient IPv4
topologies burdening many of today's enterprises. An independent
IPv6 architecture presents the opportunity to build a fresh,
hierarchical network address plan that will facilitate connection to
one or more ISPs. This simplifies renumbering, route aggregation
(summarization), and other goals of a routing hierarchy.
Initially, many IPv6 hosts may have direct connectivity to each other
only via IPv4 routers. Such hosts will exist in islands of IPv6
topology surrounded by an ocean of IPv4. So, there are transition
mechanisms that allow IPv6 hosts to communicate over intervening
IPv4 networks. The essential technique of these mechanisms is IPv6
over IPv4 tunneling, which carries IPv6 packets within IPv4 packets
(see Figure 4). Tunneling allows early IPv6 implementations to take
+-------------------+
+-----------+ | IPv4 Network | +-----------+
| Dual-stack| | | | Dual-stack|
| IPv4/IPv6 ========tunnel through======== IPv4/IPv6 |
| router | | | | router |
+-----------+ | | +-----------+
/ | \ +-------------------+ / | \
/ | \ / | \
/ | \ / | \
+--+ +--+ +--+ +--+ +--+ +--+
| | | | | | | | | | | |
+--+ +--+ +--+ +--+ +--+ +--+
IPv6 endstations IPv6 endstations
Figure 4: IPv6 over IPv4 Tunneling
advantage of existing IPv4 infrastructure without any change to IPv4
components. A dual-stack router or host on the "edge" of the IPv6
topology simply inserts an IPv4 header in front of ("encapsulates")
each IPv6 packet and sends it as native IPv4 traffic through existing
links. IPv4 routers forward this traffic without knowledge that IPv6
is involved. On the other side of the tunnel, another dual-stack
router or host "decapsulates" (removes the extra IP header from) the
IPv6 packet and routes it to the ultimate destination using standard
IPv6.
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To accommodate different administrative needs, IPv6 transition
mechanisms include two types of tunneling: automatic and configured.
To build configured tunnels, administrators manually define IPv6-to-
IPv4 address mappings at tunnel endpoints. Outside of the tunnel,
traffic is forwarded with full 128-bit addresses. At the tunnel
entry point, a manually configured router table entry dictates
which IPv4 address is used to traverse the tunnel. This requires
a certain amount of manual administration at the tunnel endpoints,
but traffic is routed through the IPv4 topology dynamically, without
the knowledge of IPv4 routers. The 128-bit addresses do not have to
align with 32-bit addresses in any way.
Mbone deployment using IP-within-IP tunneling has been quite
successful, and validates this design approach as well as supporting
the likelihood of smooth transition.
1.3.10. The Dual-Stack Transition Method
Initial users of IPv6 machines will require continued interaction
with existing IPv4 nodes. This is accomplished with the dual-stack
IPv4/IPv6 approach. Many hosts and routers in today's multivendor,
multiplatform networking environment already support multiple network
stacks. For instance, the majority of routers in enterprise networks
are multiprotocol routers. Many workstations run some combination
of IPv4, IPX, AppleTalk, NetBIOS, SNA, DECnet, or other protocols.
The inclusion of one additional protocol (IPv6) on an endstation or
router is a well-understood problem. When running a dual IPv4/IPv6
stack, a host has access to both IPv4 and IPv6 resources. Routers
running both protocols can forward traffic for both IPv4 and IPv6 end
nodes.
Dual-stack machines can use totally independent IPv4 and IPv6
addresses, or they can be configured with an IPv6 address that
is IPv4-compatible. Dual-stack nodes can use conventional IPv4
autoconfiguration services (DHCP) to obtain their IPv4 addresses
while using IPv6 autoconfiguration mechanisms to obtain their IPv6
addresses.
1.3.11. Automatic Tunneling
Automatic tunnels use "IPv4-compatible" addresses, which are hybrid
IPv4/IPv6 addresses. A compatible address is created by adding
leading zeros to a 32-bit IPv4 address to pad it out to 128 bits.
When traffic is forwarded with a compatible address, the device at
the tunnel entry point can automatically address encapsulated traffic
by simply converting the IPv4-compatible 128-bit address to a 32-bit
IPv4 address. On the other side of the tunnel, the IPv4 header is
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removed to reveal the original IPv6 address. Automatic tunneling
allows IPv6 hosts to dynamically exploit IPv4 networks, but it does
require the use of IPv4-compatible addresses, which do not bring the
full benefits of the 128-bit address space.
IPv6 nodes using IPv4-compatible addresses cannot take advantage
of the extended address space, but they can exploit the other IPv6
enhancements, including flow labels, authentication, encryption,
multicast, and anycast. Once a node is migrated to IPv6 with IPv4
compatibility, the door is open for a fairly painless move to the
full IPv6 address space. IPv4-compatible addressing means that
administrators can add IPv6 nodes while initially preserving their
basic address and subnet architecture. Automatic tunnels are
available when needed, but they may not be necessary when major
backbone routers are upgraded to include the IPv6 stack. Upgrades
can be achieved quickly and efficiently when backbone routers support
full remote configuration and upgrade capabilities.
2. The Technical Case for IPv6
In this section, the technical aspects of IPv6 are discussed. In
many cases, the technical details illustrate the concepts of the
previous section. Other features are introduced as needed to help
provide a fuller understanding of the protocol.
2.1. IPv6 Headers vs. IPv4 Headers
To start the technical look at IPv6, we compare the IPv6 header
with the IPv4 header. Both headers carry version numbers and
source/destination addresses, but as Figure 6 shows, the IPv6 header
is considerably simplified, which makes for more efficient processing
by routing nodes. Whereas IPv4 headers are potentially variable in
length, IPv6 headers have a fixed length of 40 bytes. This allows
router software designers to optimize the parsing of IPv6 headers
along fixed boundaries. Additional processing efficiencies have been
realized by reducing the number of required header fields in IPv6.
An IPv4 header, illustrated in figure 5 contains at least 12 fields,
depending on how they are counted, and can also contain additional
(and hard to parse) option fields, not illustrated in the figure.
IPv6, on the other hand, only uses 8 fields.
One of the first IPv4 components to be discarded was the header
length field, which is clearly no longer required due to the fixed
header length of all IPv6 packets. The total length field of IPv4
has been retained in the guise of the IPv6 payload length field. But
this field does not include the length of the IPv6 header, which is
always assumed to be 40 bytes. The new payload length field can
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+-------+-------+---------------+-------------------------------+
|Version| 4 bits| 8 bits | 16 bits |
| == 4 | IHL |Type of Service| Total Length |
+-------+-------+---------------+-------------------------------+
| 16 bits | 4 bits| 12 bits |
| Identification | Flags | Fragment Offset |
+-------------------------------+-------------------------------+
| 8 bits | 8 bits | 16 bits |
| Time to Live | Protocol | Header Checksum |
+-------------------------------+-------------------------------+
| 32 bits |
| Source Address |
+---------------------------------------------------------------+
| 32 bits |
| Destination Address |
+---------------------------------------------------------------+
: 0 or more bits :
: IP options :
+---------------------------------------------------------------+
Figure 5: IPv4 Header Format
+-------+---------------+---------------------------------------+
|Version| 8 bits | 20 bits |
| == 6 | Traffic Class | Flow Label |
+-------+---------------+-------+---------------+---------------+
| 16 bits | 8 bits | 8 bits |
| Payload Length | Next Header | Hop Limit |
+-------------------------------+---------------+---------------+
| 128 bits |
| |
| Source Address |
+---------------------------------------------------------------+
| 128 bits |
| |
| Destination Address |
+---------------------------------------------------------------+
Figure 6: IPv6 Header Format
accommodate packets up to 64 KB in length. Even larger packets,
called "jumbograms", can be passed between IPv6 nodes if the payload
length field is set to zero and a special extension header is added,
as discussed below.
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The time-to-live (TTL) field of IPv4 has been renamed the IPv6 ``hop
limit'' field, to describe more accurately its actual function. The
field is used to break loops, by decrementing a maximum hop value by
1 for each hop of the end-to-end route. The hop-limit field is set
to the appropriate value by the source node. When the value in the
hop limit field is decremented to zero, the packet is discarded. The
IPv6 hop-count field allows up to 255 hops, which exceeds the needs
for even the largest of networks, as best we can calculate today.
In addition to the header length field, a number of basic IPv4
fields were eliminated from the IPv6 header: fragment offset,
identification, flags, checksum. The IPv4 type-of-service field is
replaced by the IPv4 traffic class field, plus the all-new flow label
field. The IPv4 fragmentation fields (offset, identification, and
flags) have been moved to optional headers in IPv6, as discussed in
section 2.6. Finally, the IPv4 checksum field has been abandoned in
IPv6, since error checking typically is duplicated at other levels
of the protocol stack. Bad packets will be detected below, at the
link-layer, or above, at the transport layer. Requiring routers to
perform error checking has caused reduced performance in today's
Internet.
2.2. Extension Headers
IPv4 headers include an options field, which conveys information
about security, source routing, and other optional parameters.
Unfortunately, options are poorly utilized because routers typically
offer degraded performance to packets that contained options.
The IPv4 options field has been replaced in IPv6 by extension
headers that are located after the primary IPv6 header and before the
transport header and application payload. IPv6 extension headers
provide security, fragmentation, source routing, and other functions.
There is no set limit on the number of extension headers between the
initial header and the higher layer payload. Since IPv6 separates
options into modular headers, processing should be simpler and thus
can remain on the fast path as needed. Figure 7 shows encryption and
fragmentation headers occurring after the primary IPv6 header and
before the Transmission Control Protocol (TCP) header.
+----------+-------------------+----------------+-----------------
| IPv6 Hdr | Fragmentation Hdr | Encryption Hdr | Transport, etc
+----------+-------------------+----------------+-----------------
Figure 7: IPv6 Extension Headers
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The protocol type field (e.g., TCP or User Datagram Protocol (UDP)),
is has been replaced by the "Next Header" field; each header field
indicates the type of the next header, which can be a TCP/UDP header,
or another IPv6 extension header. IETF working groups have already
defined a number of extension headers for IPv6 and have suggested
guidelines for the order of header insertion. The suggested order
for extension headers, if any are present, is as follows:
- (Primary IPv6 header)
- Hop-by-Hop options header
- Destination options header-1
- Source Routing header
- Fragmentation header
- Authentication header
- IPv6 Encryption header
- Destination options header-2
followed by the upper layer headers and payload.
Each extension header typically occurs only once within a given
packet, except for the destination options header (as explained in
Section 2.4).
2.3. Hop-by-Hop Options Header
When present, this header carries options that are examined by
intermediate nodes along the forwarding path. It must be the first
extension header after the initial IPv6 header. Since this header
is read by all routers along the path, it is useful for transmitting
management information or debugging commands to routers. One
currently defined application of the hop-by-hop extension header
is the Router Alert option, which informs routers that the packet
should be processed completely by a router before it is forwarded to
the next hop. An example of such a packet is an RSVP [6] resource
reservation message for QoS. Another hop-by-hop extension header
allows ``jumbograms'', or packets larger than 65,536 bytes [4].
2.4. Destination Options Headers
There are two variations of this header, each with a different
position in the packet. A Destination Options header appearing
before a Routing Header will be processed by every node listed in
the latter. A Destination Header appearing after a Routing Header,
or without a Routing Header, will be processed only by the final
destination.
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2.5. Routing Headers
IPv6, in [14], defines a "Type 0" (zero) routing header, which gives
a sending node a great deal of control over each packet's route. The
IPv6 routing extension header replaces the loose source route (LSR)
option supported currently by IPv4. This optional header allows a
source node to specify a list of IP addresses that determine which
routing path a packet will traverse.
IPv6's loose source routing (LSR)) is illustrated in Figure 8. In
"loose" forwarding, unlisted routers can be visited by a packet. So,
for example in figure 8 the packet could be routed from router 3
through router 4 and then to router 5, even though router 4 was not
specified in the routing information field of the routing header.
The source routing feature works in conjunction with another routing
header field that contains a value equal to the total number of
segments remaining in the source route. Each time a hop is made,
this "segments left" field is decremented.
IPv6 corrects another deficiency in the specification of IPv4 source
routing options, by relaxing the requirement that destination nodes
reverse the source route for transmitting packets back to the node
originating the source route. This requirement is among the reasons
that IPv4 source routing has almost entirely fallen out of use,
because it opens up a big security hole. If a source route were to
be reversed, without being sure that the source route was in fact
originated by the indicated source node, then any other node within
the Internet could easily masquerade as that indicated source node.
IPv6 source routes, on the other hand, do not carry with them the
same security exposure, since the recipient of such a routing header
is not required to use the information for sending packets back to
the source.
When Type 0 routing headers are used, the initial IPv6 header
contains the destination addresses of the first router in the
source route, not the final destination address. At each hop,
the intermediate node replaces this destination address with the
address of the next routing node, and the "segments left" field is
decremented.
2.6. Fragmentation Header
IPv4 has the ability to fragment packets at any point in the
path, depending on the transmission capabilities of the links
involved. This feature has been dropped in IPv6 in favor of
end-to-end fragmentation/reassembly, which is executed only by
IPv6 source and destination nodes. Packet fragmentation is not
permitted in intermediate IPv6 nodes. The elimination of the
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IPv6 Packet
+----------+-----+-------------------------------+- -- -- -- -- --
| IPv6 Hdr | ... | Route Information: 1, 2, 3, 5 | ...
+----------+-----+-------------------------------+- -- -- -- -- --
+---+
| X | +-------+ +-------+ +---+
+---+ ---| rtr 4 |------------| rtr 5 |------| Y |
\ / +-------+ +-------+ +---+
\ / \
+-------+ \ +-------+
| rtr 1 | \--| rtr 3 |
+-------+ +-------+
\ /
\ /
+-------+ /
| rtr 2 |--
+-------+
Figure 8: Source Routing Extension Header
fragmentation field allows a simplified packet header design and
better router performance for the great majority of cases where
fragmentation is not required. Today's networks generally support
frame sizes that are large enough to carry typical IP packets without
fragmentation. In the event that fragmentation is required, IPv6
provides an optional extension header that is used by source nodes
to divide packets into smaller units. If higher level protocols
are using larger payloads, the source node can make use of the IPv6
fragmentation extension header to divide large packets into 1500-byte
units for network transmission. The IPv6 destination node will
reassemble these fragments in a manner that is transparent to upper
layer protocols and applications.
The IPv6 fragmentation header contains fields that identify a group
of fragments as a packet and assigns them sequence numbers. The
source node is responsible for sizing packets correctly, so it has to
determine the Maximum Transmission Unit (MTU) of the end-to-end path,
which is the smallest MTU of each link along the path. For instance,
if two FDDI networks with 4500-byte MTUs are connected by an Ethernet
with an MTU of 1500, then the source node must send packets that are
no larger than 1500.
End nodes can determine the smallest MTU of a path with the MTU
path discovery process [30]. Typically, with this technique, the
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+--ICMP Datagram Too Big--<--+
v |
+---+ FDDI +-----+ FDDI +-----+ Ethernet +-----+ FDDI +---+
| X |--------| rtr |--------| rtr |--------------| rtr |--------| Y |
+---+ +-----+ +-----+ MTU = 1500 +-----+ +---+
| |
+-->-MTU Discovery Message->-+
Figure 9: MTU Discovery Process
source node probes the MTU by transmitting a packet with an MTU as
large as the local interface can handle (see Figure 9). If this
MTU is too large for some link along the path, an ICMP "Datagram
too big" message will be sent back to the source. This message
will contain a packet-too-big indicator and the MTU of the affected
link. The source can then adjust the packet size downward (fragment)
and retransmit another packet. This process is repeated until a
packet gets all the way to the destination node. The discovered
MTU is then used for fragmentation purposes. Although source-based
fragmentation is fully supported in IPv6, it is recommended that
network applications adjust packet size to accommodate the smallest
MTU of the path. This will avoid the drawbacks associated with
fragmentation/reassembly on source and destination nodes.
2.7. IPv6 Security
IPv6 has two security extension headers, one that enables the
authentication of IP traffic for security purposes, and another that
fully or partially encrypts IP packets. Implementation of security
at the IP level can benefit "security aware" applications, as well as
"security ignorant" applications that don't take explicit advantage
of security features.
2.8. IPv6 Authentication Header
Using IPv6 authentication headers [24], hosts can verify the
authenticity and integrity of the IPv6 payload data. The
authentication header makes use of an established security
association, that may, for instance, be based on the exchange of
algorithm-independent secret keys. In a client/server session, for
instance, both the client and the server need to have knowledge of
the key. Before each packet is sent, IPv6 authentication creates a
Message Integrity Code (MIC) (using, e.g., MD5 [26]) based on the key
cryptographically digestified with the entire contents of the packet
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including data within the Authentication Extension to eliminate
replay attacks. The MIC is then recomputed on the receiving side
and compared. This approach provides authentication of the sender
and guarantees that data within the packet has not been modified or
replayed by an intervening party. Authentication can take place
between clients, or clients and servers on the corporate backbone.
It can also be deployed between remote nodes and corporate dial-in
servers to ensure that the perimeter of the corporate security is not
breached.
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2.9. IPv6 Encryption Header
<-------------- Unencrypted ---------------> <----- Encrypted ----...
+-------------+----------------+------------+------------------------
| IPv6 Header | Extension Hdrs | ESP Header | Transport Hdr & Payload
+-------------+----------------+------------+------------------------
Figure 10: Transport Mode of IPv6 Encryption
<-----Unencrypted--------> <--------- Encrypted ----------------...
+--------+--------+-------+--------+--------+-------+---------------
|IPv6 Hdr|Ext.Hdrs|ESP Hdr|IPv6 Hdr|Ext.Hdrs|ESP Hdr|Transpt/Payload
+--------+--------+-------+--------+--------+-------+---------------
<-Encapsulating Headers--> <--------- Original Packet -------.......
Figure 11: Tunnel Mode of IPv6 Encryption
Authentication headers eliminate a number of host spoofing and packet
modification attacks, but they do not prevent passively reading of
data traversing the Internet and corporate backbone networks. This
protection is offered by the Encapsulating Security Payload (ESP)
service of IPv6 -- another optional extension header [25]. Packets
protected by the ESP encryption techniques can have very high levels
of privacy and integrity -- something that is not widely available
with the current Internet, except with certain secure applications
(e.g., private electronic mail and secure HTTP Web servers). ESP
provides encryption at the network layer, making it available to all
applications in a standardized fashion.
IPv6 ESP is used to encrypt the transport-layer header and payload
(e.g., TCP, UDP), or else the appropriate IPv6 header fields along
with the payload. Both these methods are accomplished with an ESP
extension header that carries encryption parameters end-to-end. When
just the transport payload is to be encrypted, the ESP header is
inserted in the packet directly before the TCP or other transport
header. In this case, the headers before the ESP header are not
encrypted and the headers and payload after the ESP header are
encrypted. This is referred to as "transport-mode" encryption, and
is illustrated in figure 10. If it is desirable to encrypt the
entire IP datagram, a new IPv6 and an ESP header are wrapped around
all the fields (including the initial address fields) of the packet.
Full datagram encryption is sometimes called "tunnel-mode" encryption
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because the payload of the datagram is unintelligible except at the
endpoints of the security tunnel (see Figure 11).
Fully encrypted datagrams are somewhat more secure than transport
mode encryption because the headers of the fully encrypted packet are
not available for traffic analysis.
For instance, full tunnel-mode encryption allows the addresses
contained in IPv6 source routing headers to be hidden from packet
sniffing devices for the public portion of a path. This is in
addition to the typical use of tunnel-mode encryption for the
purposes of creating a private link. There is a considerable
performance penalty for full encryption, due to the overhead and
processing cost of adding an additional IPv6 header to each datagram.
In spite of its cost, full ESP encryption is particularly valuable
to create a security tunnel (steel pipe) between the firewalls of
two remote sites (see Figure 12). The full datagram encryption
in the tunnel ensures that the various headers and address fields
of encrypted packets will not be visible as traffic traverses the
public Internet. Within the tunnel, only the temporary encapsulating
address header is visible. Once through the tunnel and safely within
a firewall, the leading ESP headers are stripped off and the packet
is again visible, including any source routing headers required to
finish the path.
~~ ~~
F~ ~F
+--------+ i~ +--------------------+ ~i +--------+
| | r~ | | ~r | |
| Site 1 | e~ | Public Internet | ~e | Site 2 |
| | ---------------------------------------- | |
| <-------( - - - - - - ESP Steel Pipe - - - - - -()<-----<-- |
| | ---------------------------------------- | |
| | w~ | | ~w | |
| | a~ | | ~a | |
| | l~ +--------------------+ ~l | |
+--------+ l~ ~l +--------+
~~ ~~
Figure 12: Firewalls and Steel Pipe
The encryption and authentication services of IPv6 together
create the security solution often needed by business and military
applications. An authentication header is typically carried
inside an encrypted datagram, providing an additional layer of data
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integrity and verification of the sender's identification. The
authentication header should be placed in front of the encrypted
transport-mode portion of the packet. This approach enables the
authentication to take place before decryption on the receiving
end, which is important for security purposes. Taken together, the
authentication and encryption services of IPv6 provide a robust,
standards-based security mechanism that will play a decisive role in
the continuing expansion of commerce and corporate operations onto
IP-based network fabrics.
2.10. The IPv6 Address Architecture
Much of the discussion of IPv4 versus IPv6 focuses on the relative
size of the address fields of the two protocols (32 bits versus
128 bits). But an equally important difference is the relative
abilities of IPv6 and IPv4 to provide a hierarchical address space
that facilitates efficient routing architectures. IPv4 was initially
designed with class A, class B, and class C addresses, which divided
address bits between network and host but did not create a hierarchy
that would allow a single high-level address to represent many
lower-level addresses. Hierarchical address systems work in much
the same way as telephony country codes or area codes, which allow
long-haul phone switches to route calls efficiently to the correct
country or region using only a portion of the full phone number.
As the Internet grew, the non-hierarchical nature of the original
IPv4 address space proved inadequate. This problem has been improved
by use of CIDR (see section 1.2.1), but legacy address assignments
still hamper routing within the Internet. These legacy assignments
limit both local and global levels of internetworking. To combat
IPv4 deficiencies at the local area network level, the subnetting
technique has been developed to create a more manageable division of
large networks. Using subnets, a single network address can stand
for a number of physical networks, a technique that conserves address
space considerably. For example, a single Class B address can be
used to access hundreds of physical networks, each of which itself
could have dozens or hundreds of individual hosts.
At the level of large internet backbones and global routing,
classful IPv4 addresses can be more efficiently aggregated with
supernetting, a form of hierarchical addressing. With supernetting,
backbone routers store a single address that represents the path
to a number of lower level networks. This can considerably reduce
the size of routing tables in backbone routers, which increases
backbone performance and lowers the amount of memory and number of
route processors required. Subnetting and supernetting have been
particularly useful in extending the viability of the IPv4 Class C
addresses. Both of these techniques are made possible by associating
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addresses stored in routers to bit masks that indicate which bits in
an address are valid at the various levels of the hierarchy.
The process of creating an IPv4 routing hierarchy was formalized
in CIDR, as discussed in Section 1.2.1. For instance, CIDR allows
a number of (plentiful) Class C addresses to be summarized by a
single prefix address, allowing Class C addresses to function in
a similar way to hard-to-get Class A and Class B addresses. CIDR
has extended the life of IPv4 and helped the Internet scale to its
current size, but it has not been implemented in a consistent way
across the Internet and enterprise networks. Consequently, the route
table efficiencies and address space conservation advantages of CIDR
are not today fully realized, nor will they ever be fully realized,
due to the legacy nature of IPv4 networks and the difficulty of
restructuring them. IPv4 will continue to waste a larger proportion
of its address space, and to burden routers with inefficient routes
and excessively large routing tables.
At the departmental and workgroup level of internetworking, IPv4
engenders a high administrative workload associated with maintaining
subnet bit masks and host addresses within the subnet structure,
particularly where there are large, dynamic populations of end users.
When an end user is moved in the subnetting environment, careful
attention must be paid to ensure that the host renumbering process
does not disrupt the ability of the user to make effective use of the
network. The complexities and pitfalls of current subnetting methods
can eventually make IPv4 less than viable in large organizations that
experience growth of internetwork user populations (especially at
current rates of growth). IPv6, with its greater subnetting space,
makes the job of aggregating and administering networks much easier
and more flexible.
2.11. The IPv6 Address Hierarchy
Motivated by the experience gained from IPv4, IPv6 designers made
sure from the very beginning to provide a scalable address space that
can be partitioned into a efficient global routing hierarchy. At
the top of this hierarchy, several international registries assign
blocks of addresses to top level aggregators (TLA). TLAs allocate
blocks of addresses to Next Level Aggregators (NLA), which represent
large providers and global corporate networks. When an NLA is a
provider, it further allocates its addresses to its subscribers.
Routing is efficient because NLAs that are under the same TLA will
have addresses with a common TLA prefix. Subscribers with the same
provider have IP addresses with an NLA common prefix. See Figure 13
for an example of Aggregation-based Allocation Structures. Although
a number of allocation schemes are possible within IPv6's huge
address space, an aggregation-based hierarchy is favored by IETF
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+-----------+ +-----------+
| Long-Haul | - - - - - - - - - - - - -| Long-Haul |
| Provider | | Provider |
+-----------+ +-----------+
| \ /
\--------\ /------------------/
| +---------------+
| Interexchange | - - - - - - ---> To other
| | (TLA) | interexchanges
+---------------+
| /--------/ | \ \---------------\
| / | \ \
+-----------+ +--------+ \ +-----------+
| Long-Haul | |Provider| \ | Long-Haul |
| Provider | +--------+ | | Provider |
+-----------+ | | +-----------+
| | | | |
+----------+ | | +----------+ +----------+
|Subscriber| | | |Subscriber| | Provider |
+----------+ | \ +----------+ +----------+
+----------+ \ |
|Subscriber| \ |
+----------+ +----------+ +----------+
|Subscriber| |Subscriber|
+----------+ +----------+
Figure 13: Aggregation-based Allocation Structures
designers because it allows a choice between various allocation
approaches. Provider allocation divides the hierarchy along lines of
large service providers, regardless of their location. Geographic
allocation divides the hierarchy strictly on the basis of the
location of providers/subscribers (as does the telephony system
of country and area codes). Both of these approaches have their
drawbacks because large backbone networks often don't conform
strictly to geographic or provider boundaries. Some large networks,
for instance, may connect to several ISPs; many large networks span
numerous countries and geographical regions.
Aggregation-based allocation is based on the existence today of a
limited number of high-level exchange points, where large long-haul
service providers and telephone networks interconnect. The use
of these exchange points to divide the IPv6 address hierarchy has
a geographical component because exchanges are distributed around
the globe. It also has a provider orientation because all large
providers are represented at one or more exchange points.
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| 3| 13 | 8 | 24 | 16 | 64 bits |
+--+-----+---+--------+--------+--------------------------------+
|FP| TLA |RES| NLA | SLA | Interface ID |
| | ID | | ID | ID | |
+--+-----+---+--------+--------+--------------------------------+
<--Public Topology---> Site
<-------->
Topology
<------Interface Identifier----->
Figure 14: Aggregation-based IPv6 Addresses
As shown in Figure 14, the first 3 address bits indicate what type
of address follows (unicast, multicast, etc.). The next 13 bits
are allocated to the various TLAs around the world. Eight bits are
reserved for future use, and the following 24 bits are allocated to
the next lower level of providers and subscribers.
Next level aggregators can divide the NLA address field to create
their own hierarchy, one that maps well to the current ISP industry,
in which smaller ISPs subscribe to higher level ISPs, and so on.
This is accomplished by the further subdivision of the 32-bit
NLA field (see Figure 15). Following the NLA ID are fields for
<------------ 32 bits -----------> <--16 bits-> <---- 64 bits ---->
+-------+-------------------------+------------+-------------------+
| NLA 1 | Site | SLA | Interface ID |
+-------+-------------------------+------------+-------------------+
+-------+-----------------+------------+-------------------+
| NLA 2 | Site | SLA | Interface ID |
+-------+-----------------+------------+-------------------+
+-----------------+------------+-------------------+
| NLA 3 | Site | SLA | Interface ID |
+-----------------+------------+-------------------+
Figure 15: Subdividing the NLA Address Space
subscriber site networking information: Site Level Aggregator (SLA)
and Interface ID. Typically, service providers supply subscribers
with blocks of contiguous addresses, which are then used by
individual organizations to create their own local address hierarchy
and identify subnets and hosts. The 16-bit SLA field supports up to
65,535 individual subnets. The 64-bit Interface ID, which is used
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to identify an IPv6 interface on a network link, will typically be
derived from the installed MAC address. Large sites are expected to
get an entire SLA.
Internet backbone routers must maintain 75,000 or more routes. As
the Internet continues to grow in size, IPv6's uniform application
of hierarchical routing will likely be the only viable method for
keeping the size of backbone router tables under control. With an
aggregator-based address hierarchy, all of a subscriber's internal
network segments can be reached through one or more high-level
aggregation points. This allows backbone routers around the globe
to efficiently summarize the routes to a customer's networks with
high-level TLA address prefixes. Routing tables in the highest-level
backbones can be calculated quickly and efficiently because of
the relatively small number of aggregated routes compared with
IPv4. IPv6's large hierarchical address space also allows a more
decentralized approach to IP address allocation. Service providers
can allocate addresses independently from central authorities,
encouraging global network growth and eliminating bureaucratic
bottlenecks in the growth process.
Aggregation-based addresses are just part of the total address
space that has been defined for IPv6. Other address ranges have
been assigned to multicasting and to nodes that only require
unique addresses within a limited area (site-local and link-local
addresses).
Site-local and link-local addresses are available for private,
internal use by all enterprises, and are not allocated by public
registry authorities. Site-local addresses enable two separate
domains to use the same non-unique addresses that never collide
because site-local routing restrictions keep them apart. This has
an advantage: if an ISP changes, site local addresses can remain
the same because they do not directly connect to the outside world.
Link local addresses operate only over a single link, and can be used
for temporary "bootstrapping" of network nodes before they receive a
globally unique address (more on this in section 2.12).
2.12. Host Address Autoconfiguration
IPv6 has an address architecture [17] that can accommodate Internet
expansion for many decades to come. Furthermore, IPv6 hosts can
have their addresses automatically configured and reconfigured in a
cost-effective and manageable way. Automatic address configuration
is necessary in hierarchical routing because it supports scalable
(and thus cost-effective) numbering and renumbering of large
populations of IP hosts. Even a small renumbering cost, if incurred
tens of thousands of times for every ISP connection, adds up to a
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major administrative headache. Conversely, scalable renumbering
techniques will enable business enterprises to shop for the best
connectivity solutions with reduced renumbering costs of reconnection
to a new provider.
Autoconfiguration capabilities are important regardless of which
style of address allocation is in effect. Occasionally, it may
be necessary to renumber every host within an organization, as
would be the case with a company that relocated its operations
(with geographic addressing) or changed to another service provider
(with provider-based addressing). Configuration of IP addresses
is a fact of life at the workgroup and department levels of large
networked organizations. IP addresses need to be configured for new
hosts, for hosts that change location, and for hosts connected to
physical networks that must be renumbered in response to external
considerations (e.g., when a site changes ISPs). In addition to
these traditional requirements for configuration, new requirements
are emerging as large numbers of hosts become mobile. These
requirements for reduced static configuration of router addresses,
route parameters, and server addresses, are basically not met in any
meaningful way for use with the existing IPv4 installed base.
The process of autoconfiguration under IPv6 starts with the Neighbor
Discovery (ND) protocol [32]. ND combines and refines the services
provided in the IPv4 environment by Address Resolution Protocol
(ARP) [36], Internet Control Message Protocol (ICMP) [37], and Router
Advertisement [13]. Although it has a new name, ND is actually just
a set of complementary ICMPv6 [10] messages that allow IPv6 nodes
on the same link to discover link-layer addresses and to obtain and
advertise various network parameters and reachability information.
In a typical scenario, a host starts the process of autoconfiguration
by creating a link-local address [42]. This address can be formed
by prepending a generic local address prefix to a unique token
(typically derived from the host's IEEE LAN interface address [21]).
Once this address is formed, the host sends out an ND message to
ensure that the address is unique. If no ICMP Neighbor Advertisement
message comes back, the address is presumed unique. If a message
comes back indicating that the link-local address is already in use,
then a different token can be used (e.g., an administrative token,
centrally generated, or a randomly generated token).
Using the new link local address as a source address, the host then
sends out an ND router solicitation, or waits for a periodic router
advertisement. The solicitation is sent out using the IPv6 multicast
service. Unlike the broadcast ARPs of IPv4, IPv6 ND multicast
solicitations are not necessarily processed by all nodes on the link,
which can conserve processing resources in hosts. IPv6 currently
defines several permanent multicast groups for finding resources on
the local node or link, including an all-routers group, an all-hosts
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group, and a DHCP server group. Routers respond to solicitation
messages from hosts with a router advertisement that contains,
among other things, prefix information that indicates a valid range
of addresses for the subnet. The ND message exchange is shown in
Figure 16. Routers also send unsolicited advertisements periodically
to local multicast groups.
+---+ +---+
| Y |---------------------| Z |
+---+ +---+
/ \
----/ \-----
/ \
+---+ ----- Router Solicitation ------> +-----+
| X | | rtr |====To Internet
+---+ <----- Router Advertisement ----- +-----+
\ /
---- -----
\ /
\ /
+---+ +---+
| W |---------------------| V |
+---+ +---+
Figure 16: Neighbor Discovery (ND) Router Message Exchange
The router advertisement message controls whether hosts use stateless
or stateful autoconfiguration methods. In the case of stateful
autoconfiguration, the host will contact a stateful address server,
which will assign an address from a manually administered list.
DHCP [16] is the protocol of choice for autoconfiguration in IPv4
networks and has been reformulated for the IPv6 environment [5, 35].
With the stateless approach [42], a host can automatically configure
its own IPv6 address without the help of a stateful address server
or any human intervention. The host uses the globally valid address
prefix information in the router advertisement message to create its
own IPv6 address. This process involves the concatenation of a valid
prefix with the host's link-layer address or a similar unique token.
As long as the token is unique on the link and the prefix received
from the router is correct, the newly configured IP address should
provide reachability for the host extending to the entire enterprise
and the Internet at large.
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The advantages of stateless autoconfiguration are many. For
instance, if an enterprise changes service providers, the prefix
information from the new provider can be propagated to routers
throughout the enterprise, and hence to all stateless autoconfiguring
hosts. Hypothetically, if all hosts in the enterprise use IPv6
stateless autoconfiguration, the entire enterprise could be
renumbered without the manual configuration of a single non-router
host. At a more modest level, workgroups with substantial
move/change activity also benefit from stateless autoconfiguration
because hosts can receive a freshly configured and valid IP number
each time they connect and reconnect to the network.
+-------+
| Home |
| Agent |\
+-------+ \ +---------------------+
\ | |
----------+ | +---+
| | /------------------| X |
----------+ <----/ | +---+
/ | |
/ +---------------------+
/
+--------+/
| Mobile |
| Node |
+--------+
Figure 17: Forwarding IP Traffic for Mobile IPv6 Nodes
Address autoconfiguration plays an essential role in the support
for mobile nodes within IPv6. Each mobile node can configure an
appropriate address, no matter which network it is attached to; it
uses this address as a kind of forwarding address (or, as it is
called, a "care-of address"). Then, the mobile node can receive
all of its data from its home network by asking a router (called a
"home agent") to forward packets to it at its care-of address. This
process is illustrated in figure 17. Better yet, the mobile node
can also instruct any other node (e.g., node 'X' in the figure) to
forward data to its care-of address, so that the data never traverses
the home network. Although not shown by the figure, the mobile
node is identified by its home address, even though it is receiving
packets sent to its care-of address. This is important so that the
mobile node can maintain its connections even when it is wireless
and undergoing handoff operations during continued operation of its
network applications.
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To facilitate dynamic host renumbering, IPv6 has a built-in
mechanism to create a graceful transition from old to new addresses.
Fundamental to this mechanism is the ability of IPv6 nodes to support
multiple addresses per interface. IPv6 addresses assigned to an
interface can be identified as valid, deprecated, or invalid. In
the renumbering process, an interface's IPv6 address would become
deprecated when a new address was automatically assigned (e.g., in
the case of network renumbering). For a period of time after the new
(valid) address is configured, the deprecated address continues to
send and receive traffic. This allows sessions and communications
based on the older address to be finished gracefully. Eventually
the deprecated address becomes invalid and the valid address is used
exclusively. Issuing multiple IP addresses allows renumbering to
occur dynamically and transparently to end users and applications.
Besides simplifying host renumbering, IPv6 has work underway to help
with reconfiguring routers [11].
The above described stateless autoconfiguration process is
particularly suited to conventional IP/LAN environments with 48-bit
or 64-bit link-level addressing [21] and native multicast services.
Other network environments with different link characteristics may
require modified or alternative configuration techniques. For
instance, current ATM networks do not inherently support multicast
services or IEEE MAC addresses, due to the use of virtual circuits
and telephony-style calling numbers. Multicasting solutions for
ATM are seen in the emerging Multicast Address Resolution Server
(MARS) [40] developed for IPv4 multicast over ATM. Plans have
been devised to use MARS-style functionality to extend the IPv6
Neighbor Discovery protocol across ATM networks. This allows network
renumbering and stateless autoconfiguration to take place seamlessly
in hybrid ATM/IPv6 fabrics [3, 2].
2.13. Other Protocols and Services
The preceding discussion focuses on some of the more innovative
and radical changes that IPv6 brings to internetworking. In many
other areas, protocols and services will operate much the same as
they do in the current IPv4 regime. As the industry moves to IPv6,
PPP, DHCP and DNS servers are being modified to accommodate 128-bit
addresses, but in terms of basic functionality, there will be little
change. This is also generally true for interior and exterior
routing protocols.
For example, OSPF has been updated with full support for IPv6 [9],
allowing routers to be addressed with 128-bit addresses. The 32-bit
link-state records of current OSFP will be replaced by 128-bit
records. In general, the OSPF IPv6 link-state database of backbone
routers will run in parallel with the database for IPv4 topologies.
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In this sense, the two versions of OSPF will operate as "ships in the
night," just as the routing engines for IPv4, OSI and proprietary
protocols may coexist in the same router without major interaction.
Given the limited nature of the OSPF IPv6 upgrade, those engineers
and administrators who are proficient in OSPF for IPv4 should have
little trouble adapting to the new version. An updated version of
RIP is also available [28].
As with the interior gateway protocols, RFC 2545 provides a
IPv6-compatible version of the exterior gateway protocols that
are used by routers to establish reachability across the Internet
backbone between large enterprises, providers, and other autonomous
systems. Today's backbone routers use the Border Gateway Protocol
(BGP) to distribute CIDR-based routing information throughout the
Internet. BGP is known by providers and enterprises and has a large
installed base. BGP extensions [29] have been defined to exchange
reachability information based on the new IPv6 hierarchical address
space.
3. Transition Scenarios
Section 1 of this paper provided an overview of the major transition
mechanisms that are integral to the IPv6 design effort. These
techniques include dual-stack IPv4 /IPv6 hosts and routers, tunneling
of IPv6 via IPv4, and a number of IPv6 services, including IPv6 DNS,
DHCP, MIBs, and so on. The flexibility and usefulness of the IPv6
transition mechanisms are best gauged through scenarios that address
real-world networking requirements.
3.1. First Scenario: No Need to NAT
Take, for instance, the case of two large, network-dependent
organizations that must interface operations due to a merger and
acquisition (M&A), or a new business partnership. Suppose both
of the enterprises have large IPv4-based networks that have grown
from small beginnings. Both of the original enterprises have a
substantial number of private IPv4 addresses that are not necessarily
unique within the current global IPv4 address space. Combining these
two non-unique address spaces could require costly renumbering and
restructuring of routers, host addresses, domains, areas, exterior
routing protocols, and so on. This scenario is common in the current
business climate, not only for Merger and Acquisition (M&A) projects,
but also for large outsourcing and customer/supplier networking
relationships, where many hosts from the parent, outsourcer,
supplier, or partner must be integrated into one existing enterprise
address structure. For these situations, IPv6 offers a convenient
solution.
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-------------- --------------
/ \ / \
| Enterprise | +----------+ | Enterprise |
| A |-------| IPv6 rtr |-------| B |
\ / +----------+ \ /
-------------- --------------
^ |
| |
| v
+-------+ +-------+
|IPv4 + | IPv6 communication |IPv4 + |
| IPv6| - - - - - - - - - - - - - > | IPv6|
| Host | | Host |
+-------+ +-------+
Figure 18: IPv6 Unites Private Address Spaces
The task of logically merging two enterprise networks into a single
autonomous domain can be expensive and disruptive. To avoid the
cost and disruption of comprehensive renumbering, enterprises
may be tempted to opt for the stopgap solution of a network
address translator (NAT). In the M&A scenario, a NAT could allow
the two enterprises to maintain their private addresses more or
less unchanged. To accomplish this, a NAT must conduct address
translation in real time for all packets that move between the two
organizations. Unfortunately, this solution introduces all the
problems associated with NATs that were discussed in section 1.2.2,
including performance bottlenecks, lack of scalability, lack of
standards, and lack of universal connectivity among all the nodes in
the new enterprise and the Internet.
In contrast with NAT, IPv6 seamlessly integrates the two physical
networks (see Figure 18). Suppose the two originally independent
enterprises are known as Enterprise A and Enterprise B. The first
step is to determine which hosts need access to both sides of the
new organization. These hosts are outfitted with dual IPv4/IPv6
stacks, which allow them to maintain connectivity to their original
IPv4 network while also participating in a new IPv6 logical
network that will be created "on top" of the existing IPv4 physical
infrastructure.
The accounting department of the combined enterprise will often have
financial applications on servers that will need to be accessed
by accounting employees in both Enterprise A and Enterprise B.
Both servers and clients will run IPv6, but they will also retain
their IPv4 stacks. The IPv6 sessions of the accounting department
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will traverse the existing local and remote links as "just another
protocol," requiring no changes to the physical network. The only
requirement for IPv6 connectivity is that routers that are adjacent
to accounting department users must be upgraded to run IPv6. Where
end-to-end IPv6 connectivity can't be achieved, one of the IPv4/IPv6
tunneling techniques can be employed.
As integration continues, other departments in the newly merged
enterprises will also be given IPv4/IPv6 hosts. As new departments
and workgroups are added, they may be given dual-stack hosts, or in
some cases, IPv6-only hosts. Hosts that require communications to
the outside world via the Internet will likely receive dual stacks to
maintain compatibility with IPv4 nodes exterior to the enterprise.
But in some cases, hosts that only require access to internal servers
and specific outside partners may be able to achieve connectivity
with IPv6-only hosts. A migration to IPv6 presents the opportunity
for a fresh start in terms of address allocation and routing protocol
structure. IPv6 hosts and routers can immediately take advantage
of IPv6 features such as stateless autoconfiguration, encryption,
authentication, and so on.
3.2. Second Scenario: IPv6 from the Edges to the Core
For corporate users, connectivity requirements typically focus
primarily on access to local e-mail, WWW, database, and applications
servers. In this case, it may be best to initially upgrade only
isolated workgroups and departments to IPv6, with backbone router
upgrades implemented at a slower rate. As shown in Figure 19,
independent workgroups can upgrade their clients and servers
to dual-stack IPv4/IPv6 hosts or IPv6-only hosts. This creates
"islands" of IPv6 functionality.
After the first few IPv6 routers are in place, it may be desirable
to connect IPv6 islands together with router-to-router tunnels. In
this case, one or more routers in each island would be configured
as tunnel endpoints. As illustrated in section 1, in figure 4,
when hosts use full IPv6 128-bit addressing, tunnels are manually
configured so that the routers participating in tunnels know the
address of the endpoints of the tunnel. With IPv4-compatible IPv6
addresses, automatic, nonconfigured tunneling is possible.
Routing protocols treat tunnels as a single IPv6 hop, even if
the tunnel is comprised of many IPv4 hops across a number of
different media. IPv6 routers running OSPF can propagate link-state
reachability advertisements through tunnels, just as they would
across conventional point-to-point links. In the IPv6 environment,
OSPF can ensure that each tunnel is weighted properly within the
topology. Routers generally make packet-forwarding decisions in the
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IPv6 "Island" IPv6 "Island"
-------------------- --------------------
| | | |
| Dual Stack Hosts | | Dual Stack Hosts |
| +---+ +---+ | | +---+ +---+ |
| | | | | | | | | | | |
| +---+ +---+ | | +---+ +---+ |
| | | | | | | |
| \ / | | \ / |
| +-------+ | | +-------+ |
| | Dual | | | | Dual | |
| | Stack | | | | Stack | |
| | Router| | | | Router| |
| +-------+ | | +-------+ |
| | | |
-------------------- --------------------
\ /
\ /
+------+ +------+
IPv4 | IPv4 |-------------------| IPv4 | IPv4
Hosts | rtr | | rtr | Hosts
+---+ +------+ IPv4 +------+ +---+
| X |-\ / \ infrastructure / \ /-| W |
+---+ \ / \-------\ /--------/ \ / +---+
\ / \ / \ /
+---+ \ +-----+ +-----+ +-----+ / +---+
| Y |------| rtr |----------| rtr |---------| rtr |------| Z |
+---+ +-----+ +-----+ +-----+ +---+
Figure 19: Islands of IPv6
tunneling environment in the same way as in the IPv6-only network.
The underlying IPv4 connections are essentially transparent to IPv6
routing protocols.
3.3. Other mechanisms
Additional mechanisms for transition or for IPv4/IPv6 coexistence
are also under discussion. For example, IPv4 multicast can be used
to support neighbor discovery by isolated IPv6 nodes [8]. There are
several proposals on how to support transactions between IPv4-only
nodes and IPv6 nodes that do not have IPv4-compatible addresses.
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IETF members are putting intense effort into transition, as well
as the basic IPv6 protocol specification. The combination of
tunnels, compatible addresses, and dual-stack nodes gives network
administrators the range of flexibility and interoperability they
need to deploy IPv6. Transition services allow organizations
depending upon current IPv4 networks to take advantage of the more
technical IPv6 features.
4. Security Considerations
Sections 1.2.4, 2.8, and 2.9 of this paper emphasize the security
benefits that IPv6 offers. By adopting IPv6, the Internet and the
enterprise-specific applications will be much better able to satisfy
their security needs by making use of standardized network features.
Expediting the deployment for IPv6 will bring these security features
into service sooner. Furthermore, the Internet will be able to
avoid the security pitfalls made more likely by the deployment of
NAT devices, as discussed in Section 1.2.2, and arising from any
applications using IPv4 source routing (see section 2.5).
5. Acknowledgments
This work is derived from a Bay Networks white paper on IPv6
(published in 1997) that was co-authored by Steve King, Ruth Fax,
Dimitri Haskin, Wenken Ling, and Tom Meehan. They were all employed
by Bay Networks at that time. Thanks to Steve Deering and Bob Hinden
for their many efforts as chairs of the IPng working group. Thanks
to Matt Crawford and Thomas Narten for their additional detailed
comments.
Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However,
this document itself may not be modified in any way, such as by
removing the copyright notice or references to the Internet Society
or other Internet organizations, except as needed for the purpose
of developing Internet standards in which case the procedures
for copyrights defined in the Internet Standards process must be
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
A. Myths
Because of its potential for future dominance and the number of
detailed technical choices that had to be made, the birth of IPv6
has been attended by some controversy, and by a number of somewhat
misleading stories that can distract network owners who are in
the process of crafting their forward-looking network strategy.
Confusion is to be expected, considering the implications of
migrating our global internetwork infrastructure to an updated
protocol. But if the IPv6 myths are perpetuated indefinitely,
there's a risk that the Internet will not be able to progress
beyond a patched-up version of IPv4. In these appendices, we try to
counteract some of these myths.
Myth #1: The only driving force behind IPv6 is address space
depletion.
Many of the discussions about a new Internet protocol focus on the
fact that we will sooner or later run out of globally unique network
layer addresses, due to IPv4's fixed 32-bit address space. The
various address registries that assign blocks of IP addresses to
large network service providers and network operators have become
quite cautious about the way these addresses are handed out, though
most predictions for IPv4 address exhaustion target a time frame that
starts well into the this decade.
With the long-haul in mind, IPv6 has been outfitted with a 128-bit
address space that should guarantee globally unique addresses for
every conceivable variety of network device for the foreseeable
future (i.e., decades). IPv6 has 16 byte addresses, or
340,282,366,920,938,463,463,374,607,431,768,211,456
addresses (over a third of a duodecillion of them, in fact). The
number of addresses gets a lot of attention but it is only one of
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many important issues that IPv6 designers have tackled. Other IPv6
capabilities have been developed in direct response to current
business requirements for more scalable network architectures,
mandatory security and data integrity, extended quality-of-service
(QoS), autoconfiguration, and more efficient network route
aggregation at the global backbone level. These features are all
specified with IPv6 in a way that would be difficult to realize as
effectively in IPv4.
Myth #2: Extensions to IPv4 can replicate IPv6 functionality.
There have been multiple efforts to extend the life of IPv4
incrementally with evolutionary changes to the protocol standards and
various proprietary techniques. One such example is the development
of network address translators (NAT) that preserve IPv4 address space
by intercepting traffic and converting private intra-enterprise
addresses into one or a few globally unique Internet addresses.
Other examples include the various QoS and security enhancements to
IPv4, which are in general scaled-back or identical to mechanisms
specified in IPv6.
We do not know how long IPv4's life can be extended by these
techniques. What is certain is that the widespread introduction
of NAT devices negatively affects the end-to-end viability of
emerging Internet applications; in practice only a limited set of
well-known applications can be correctly handled by NAT devices or
by application level gateways associated with them. In particular
NAT devices prevent the deployment of end-to-end IPv4 security.
Furthermore, the development of new and innovative Internet
applications is burdened with the design constraints posed by
NATs [20]. Since NAT is strictly unnecessary for IPv6, standard
end-to-end IPv6 security can be deployed, and a future enlivened
by new lightweight and more fully functional applications can be
envisioned. NAT translation is also known to create great difficulty
in the construction of Virtual Private Networks (VPNs), since it
makes address space administration difficult and interferes with
standard security mechanisms.
NAT also only works in a "flat universe" for a site accessing the
global Internet - even moderately-sized enterprises are not flat
internally, with nested multi-party relationships. Realistic NAT
deployment solutions would have to include routing via multiple
ingress/egress NATs for load balancing, multi-NAT-hop routes and
so on - all this would create in miniature the v4 (or in fact v6)
architecture, since it is solving the same problem, but piecewise and
badly.
It is hard to compare the costs of converting to IPv6 with those of
remaining with IPv4 and its upgrades. Every network manager will
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have to make this comparison; but staying with IPv4 has been likened
to the situation of a lobster in a pot of water, as the temperature
slowly increases - at first, it feels comfortable.
Myth #3: IPv6 support for a large diversity of network devices is
not an end-user or business concern.
Over the next few years, conventional computers on the Internet will
be joined by a myriad of new devices, including palmtop personal
data assistants (PDA), hybrid mobile phone technology with data
processing capabilities, smart set-top boxes with integrated Web
browsers, and embedded network components in equipment ranging from
office copy machines to kitchen appliances. Some of the new devices
requiring IP addresses and connectivity will be consumer-oriented,
but many will become integral to the information management functions
of corporations and institutions of all sizes. These new devices
require features not fully understood by most protocol designers
during the initial growth of the IPv4 Internet.
IPv6's 128-bit address space will allow businesses to deploy a huge
array of new desktop, mobile, and embedded network devices in a
cost-effective, manageable way. Further, IPv6's autoconfiguration
features will make it feasible for large numbers of devices to attach
dynamically to the network, without incurring unsupportable costs for
the administration for an ever-increasing number of adds, moves, and
changes.
The business requirement for IPv6 will be driven by end-user
applications. Applications for mobile nodes, electronic commerce,
and those needing specialized routing features will be easier
to design and implement using IPv6, especially as compared to
IPv4 patched by NAT. To remain competitive in the coming era of
high-density networking, businesses should exploit IPv6 to create a
highly scalable address space and robust autoconfiguration services
that will remain viable in the face of an explosion of end-user
networking needs.
Myth #4: IPv6 is primarily relevant to backbone routers, not
end-user applications.
It is true that IPv6 address aggregation allows efficient multitiered
routing hierarchies that prevent the uncontrolled growth of backbone
router tables. But many of the advanced features of IPv6 also
bring direct benefits to end-user applications at the workgroup
and departmental levels. For instance, applications will have
available the mandatory IPv6 encryption and authentication services
as an integral part of the IP stack. For mobile business users
and changing organizations, IPv6 autoconfiguration will allow the
efficient assignment of IP addresses without the delays and cost
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associated with manual address administration or even traditional
DHCP, which takes place in many current IP networks. IPv6 is very
much both an end-user concern and a business concern. This concern
will become increasingly important as QoS flows and QoS routing
become important architectural components of the Internet.
Myth #5: Asynchronous Transfer Mode (ATM) cell switching will negate
the need for IPv6.
ATM and other switching methods offer interesting technology for
present and future internetworks, but ATM is, by itself, not a
replacement for packet routing Internet architecture. ATM is better
understood as a link-layer technology over a non-broadcast multiple
access (NBMA) medium. It gives some isolation properties, and
offers the promise for offering improved Quality of Service (QoS)
connections for applications that need it. Even these hypothetical
advantages are not yet fully developed for ATM, and it is possible
that these advantages will be equally well available in future IPv6
networks not running over ATM.
Fortunately, network owners do not have to make a choice between ATM
or IPv6 because the two protocols will continue to serve different
and complementary roles in corporate networking. Large networks
will make use of both protocols. For many network designers,
ATM is a useful transmission medium for high-speed IPv6 backbone
networks. Standards and development work has integrated ATM and IPv6
environments. IPv6, like its predecessor IPv4, provides network
layer services over all major link types, including ATM, Ethernet,
Token Ring, ISDN, Frame Relay, and T1.
Myth #6: IPv6 is something that only large telephone companies or
the government should worry about.
Some Internet pundits have characterized IPv6 as a concern that's
outside the corporate network and outside the current time frame.
In reality, IPv6 is a standards track and mainstream solution
for the operation and continued efficiency of day-to-day business
activities. But the only way that IPv6 will take hold and succeed is
if businesses and institutions of all types come to terms with the
inadequacies of IPv4 and begin to lay plans for migration. In the
past few years, Internet protocols have enabled a whole new style of
distributed commerce that brings people together inside enterprises
and gives enterprises access to the entire world. In fact, the
sustained and impressive growth of the Internet, which has inspired
the current engineering efforts for IPv6, is in large measure due to
the penetration of the World Wide Web to business and consumer end
users. Offering services to such end users is of interest to many
more institutions than merely governments and telephone companies.
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Myth #7: IPv6 requires extensive modifications to existing operating
systems, applications, and programming techniques.
IPv6 obviously requires certain modifications to the network protocol
handling modules installed on the relevant computers. However,
this typically requires little or no change to the base operating
system outside the network protocol stack. Simple and natural
modifications, typically confined to fewer than a dozen lines of the
programs, can be made to enable applications to use IPv6 addresses
directly. Since IPv6 reserves a part of its address space for
compatibility with IPv4 addresses, applications modified to handle
IPv6 addresses can still communicate with existing IPv4 clients and
servers.
Moreover, the transition strategies defined for IPv6 deployment
within the IPv4 Internet should make the gradual adoption of IPv6 a
smooth process that allows existing applications to be converted for
native IPv6 operation in a gradual, controlled manner.
Myth #8: Too Little, Too Soon
IPv6 appears as an incremental enhancement to IPv4, and some
people say that if we are going to go to all the trouble to switch
network-layer protocols, we really ought to go all out for some
really futuristic feature-full new protocol. This argument ignores
the following simple facts:
- The purpose of a network-layer protocol is to hook together
networks, and
- IPv6 builds on the amazing success of IP, by not forgetting the
successful parts, and by repairing the known faults. This is far
different than starting over again with something unknown and
untested.
Those who claim that it is too early for IPv6 ignore the facts that
existing solutions extending the life of IPv4 are clearly stopgap
measures, and that one can put IPv6 into service now.
Myth #9: Site Renumbering is fixed in IPv6
Although IPv6 has gone a long way to enable more convenient
renumbering operations, it is a mistake to say that renumbering is
a completely solved problem. IPv6 engineers are still considering
designs for renumbering routers, and for renumbering collections of
computers larger than a single network. Furthermore, applications
that have been ported from IPv4 to IPv6 do not automatically become
more able to support renumbering. Some applications will require
small design improvements in order to support renumbering. Lastly,
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the biggest impediment to renumbering seems typically to be the
institution of administrative practices that key information directly
on IP addresses instead of some more appropriate indexing method.
These administrative practices require attention and adherence to
more modern guidelines for Internet administration before the problem
of renumbering can be considered to be solved.
Myth #10: Routing is fixed in IPv6
Because IPv6 Routing is fundamentally the same as IPv4's CIDR, some
have asserted that IPv6 routing has all the same problems as IPv4
routing, and thus there is no benefit in moving to IPv6. However,
IPv6 does offer improvements for routing in a number of ways. It
allows for allocation of IPv6 addresses in a way that is more
favorable for aggregation than existing IPv4 allocations. It allows
for more streamlined packet forwarding than IPv4 routers can do,
especially when IP options are used. IPv6's larger address space
offers opportunities for more optimal network planning, since the
constraints for planning out network connectivity have been relaxed
to such a great extent. Furthermore, since every IPv6 router can be
presumed to have security processing enabled, it is much easier to
institute the appropriate security measures for authentication and
keeping private data private.
However, there are still many operational issues that need attention.
IPv6 routing protocols are largely adapted from almost identical
IPv4 routing protocols, and thus inherit some of the same problems.
Improvements continue to be made to routing protocols to improve
their stability, convergence time, and configurability. One of the
hardest problems is to make routing protocols more human-friendly,
so that it does not take a genius to make the routing fabric work
reliably. There are remaining issues surrounding multi-homing that
have not been solved. All of these issues will continue to receive
the attention of engineers involved with the creation of IPv6. The
scoped addresses and native security are expected to make their
solution much easier.
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[43] K. Varadhan, S. Hares, and Y. Rekhter. BGP4/IDRP for IP---OSPF
Interaction. Request for Comments (Proposed Standard) 1745,
Internet Engineering Task Force, December 1994.
[44] D. Waitzman, C. Partridge, and S. E. Deering. Distance Vector
Multicast Routing Protocol. Request for Comments (Experimental)
1075, Internet Engineering Task Force, November 1988.
King, et.al. Expires 25 December 2000 [Page 54]
Internet Draft The Case for IPv6 25 June 2000
Editors' Addresses
Questions about this memo can also be directed to the authors:
Robert Fink Charles E. Perkins
Esnet R&D Communications Systems Lab
Lawrence Berkeley Nat'l Laboratory Nokia Research Center
1 Cyclotron Road 313 Fairchild Drive
Bldg. 50A, Room 3139
Mail-Stop 50A-3111
Berkeley, CA 94720 Mountain View, California 94043
USA USA
Phone: +1 510 486-5692 Phone: +1-650 625-2986
Fax: +1 510 486-4790 Fax: +1 650 625-2502
E-mail: rlfink@lbl.gov EMail: charliep@iprg.nokia.com
www.iprg.nokia.com/~charliep
King, et.al. Expires 25 December 2000 [Page 55]
