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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

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:
   The list of Internet-Draft Shadow Directories can be accessed at:


   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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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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   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
      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
      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

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

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

   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

   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

   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

   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                         |   |

                     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

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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

   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

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

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

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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

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

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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
                                     <------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

   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

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

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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

   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

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
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   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

   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

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

   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


   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

   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

   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

   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

   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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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

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