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Versions: 00                                                            
INTERNET DRAFT                                  Margaret Wasserman
IPng Working Group                             Epilogue Technology
<draft-wasserman-ipv6-nd-division-00.txt>             January 1998
                                                 Expires July 1998


  Division of IPv6 Neighbor Discovery Into Separable Mechanisms


Status of this Memo

This document is an Internet-Draft. 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
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This memo provides information for the Internet community.  This memo
does not specify an Internet standard of any kind, but proposes
changes to existing Internet Drafts.  Distribution of this memo is
unlimited.

Abstract

This memo proposes a formal division of the existing IPv6 Neighbor
Discovery protocol into eight separable, related mechanisms: Address
Resolution, Duplicate Address Detection (DAD), Router Discovery,
Prefix/Parameter Discovery, Address Autoconfiguration, Router
Unreachability Detection (RUD), Neighbor Unreachability Detection
(NUD) and ICMPv6 Redirects.  These mechanisms all depend upon a common
set of ICMPv6 Neighbor Discovery messages, but can be enabled and
disabled independently, subject to the restrictions and
recommendations outlined in this draft.

It is not the intention of this memo to propose substantive changes to
the existing IPv6 Neighbor Discovery protocol, but to allow the
separate portions of IPv6 Neighbor Discovery to be unambiguously
identified, making it possible to specify or configure different
portions of IPv6 Neighbor Discovery for use on specific link-types,
links or interfaces.

1.  Introduction

In discussions within the IETF IPng Working Group and on the mailing
list, it has become clear that different portions of the IPv6 Neighbor
Discovery (ND) protocol[1] may or may not be applicable to specific
situations (e.g. tunnels).  For example, it might be desireable to
perform DAD on a link for which Router Discovery is unnecessary.  Or,
RD could be valuable in a situation where Address Resolution is not
needed.

It has also become clear that the use of some portions of ND, such as
Address Resolution, could be specified in the link-type specifications
("IPv6 over XXXX"), whereas it might be useful to configure some
portions of ND on a per-link or per-interface basis (e.g. DAD).

In order to advance these discussions, it is necessary to view ND as a
group of separable, related mechanisms -- mechanisms which share a
common set of conceptual datastructures and ICMPv6 ND messages but
which can be used (or not used) independently.  Although this view is
already discussed in the IPv6 ND specification, the current
specification does not separately describe the behaviour of these
mechanisms or divide them with sufficiently fine granularity.  It is
the purpose of this draft to specify eight separable ND mechanisms,
identify their dependencies, determine restrictions on their
configuration and use, make recommendations regarding their default
state and propose their requirement levels within compliant IPv6
implementations.  This draft is highly derivative of the IPv6 Neighbor
Discovery specification[1] and does not describe the ND mechanisms in
their entirety. This draft attempts to clarify, not substantially
modify, the mechanisms defined in the ND specification.

2.  Terminology

This draft relies on the terminology included in the current IPv6
Neighbor Discovery Internet Draft [1].

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY and OPTIONAL, when they appear in this
document are to be interpreted as described in [2].

3.  ND Messages and Conceptual Datastructures

Although the basic mechanisms of IPv6 Neighbor Discovery remain
unchanged, the ability to selectively enable specific mechanisms will
result in some nodes which only require a subset of the messages or
datastructures required for a full IPv6 ND implementation.  The
messages, options, datastructures and states used by each portion of
IPv6 ND are described in the mechanism-specific and summary sections
below.

The five ICMPv6 messages described in the Neighbor Discovery
specification and their options remain unchanged.  Depending upon
which ND mechanisms are in use, however, a node could support sending
or receiving only a subset of the ND messages or none at all.
Unsupported messages MUST be silently discarded in most cases.

The conceptual datastructures described in the ND specification are
also unmodified.  However, a full set of conceptual datastructures may
not be needed on a node which only implements a subset of IPv6 ND.

3.1 Levels of Neighbor Cache Support

The full set of Neighbor Cache states and the corresponding semantics
may not be necessary on all nodes (e.g. those which do not have both
Address Resolution and Neighbor Unreachability Detection enabled).  In
order to specify which portions of the full Neighbor Cache are
necessary on different systems, it is useful to define three different
levels of Neighbor Cache implementation:

        Level 0:  No Neighbor Cache.

                This level would be applicable to a host which
                does not implement either Address Resolution or
                Neighbor Unreachability Detection.  In this
                case, no Neighbor Cache lookup is necessary to
                send an IPv6 packet, so no Neighbor Cache need
                be maintained.

        Level 1:  2-State Neighbor Cache.

                This level would be applicable to a host which
                implements Address Resolution but does not implement
                Neighbor Unreachability Detection.  This datastructure
                would be roughly equivalent to the ARP cache on IPv4
                hosts.

                This cache would have only two states:  INCOMPLETE
                and COMPLETE.

                Entries in the INCOMPLETE state do not have
                any valid link-layer address information associated
                with them.  They may have one or more queued packets
                awaiting address resolution.

                Entries in the COMPLETE state do have link-layer
                address information suitable for use in sending
                packets.  They can be updated by subsequent
                Neighbor Advertisements with new link-layer
                address information.

                COMPLETE entries periodically timeout.  An
                implementation MAY choose whether to detect such
                a timeout immediately, on a periodic sweep, or
                when the next packet is sent to the corresponding
                destination.  When a timeout is detected, the
                timed-out entry MAY be returned to the INCOMPLETE
                state or removed from the cache.

                These two states can be viewed as a degenerate
                version of the states in the full Neighbor Cache
                described in the current ND specification as follows:

                INCOMPLETE      INCOMPLETE
                PROBE

                STALE           COMPLETE
                REACHABLE
                DELAY

                Viewing the Neighbor Cache as a two-state cache
                when Neighbor Unreachability Detection is not in
                use can greatly simplify the implementation
                of Address Resolution for minimal systems which
                do not provide any support for Neighbor Unreachability
                Detection.

                If there is significant resistance to having different
                cache semantics for different combinations of ND
                features, the same functionality can be achieved by
                adjusting the default timer values based upon the
                specific ND options currently enabled.

        Level 2:  5-State Neighbor Cache.

                This level would be applicable to a host which
                implements both Address Resolution and Neighbor
                Unreachability Detection.  This is the full
                Neighbor Cache currently described in the
                ND specification.

4.  Address Resolution

Address Resolution is one of the core mechanisms of IPv6 Neighbor
Discovery.  It allows IPv6 addresses to be mapped to link-layer
addresses via the exchange of Neighbor Solicitation and Neighbor
Advertisement ICMPv6 messages containing Source Link-Layer Address and
Target Link-Layer Address options.  Information regarding the address
mapping is stored in a Neighbor Cache conceptual datastructure (Level
1 or higher).  IPv6 ND Address Resolution is not dependent upon the
use of other IPv6 ND mechanisms.

IPv6 ND Address Resolution is only useful on multicast-capable
link-types which require a network-layer mechanism for mapping IPv6
addresses to link-layer addresses.

        For multicast-capable links that require an address
        resolution mechanism, the link-type specification SHOULD
        mandate the use of IPv6 ND Address Resolution.

        For multicast-capable links that do not require an address
        resolution mechanism (e.g. some point-to-point links) the
        link-type specification SHOULD specifically state that
        IPv6 ND Address Resolution will not be used.

        For non-multicast-capable links, the link-type specification
        MAY specify modifications to the IPv6 ND Address Resolution
        mechanism or MAY specify a different address resolution
        mechanism.

>From a node's perspective, use of IPv6 ND Address Resolution is
enabled or disabled on a per-interface basis based solely upon the
link-type of the interface's link.  Interfaces to links which support
IPv6 ND Address Resolution MUST have Address Resolution support
enabled while others MUST have support for IPv6 ND Address Resolution
disabled.  ICMPv6 Neighbor Solicitation or Advertisement messages
received on links which have IPv6 ND Address Resolution disabled MUST
be silently discarded, unless they are of interest to other enabled ND
mechanisms.

The basic mechanism for IPv6 ND Address Resolution is unchanged from
the ND specification, except for the possible changes in the Neighbor
Cache states described in the previous section.

All compliant IPv6 implementations MUST support IPv6 ND Address
Resolution for any link-types which require it.

5.  Duplicate Address Detection

Duplicate Address Detection (DAD) involves sending an ICMPv6 Neighbor
Solicitation message (with or without link-layer address options) when
a new address token is added to an interface in order to detect
duplicate address tokens in use on the link.  DAD can be very valuable
when it is possible for two or more nodes to accidentally (or through
human error) choose the same interface token for use on a shared link.
DAD's usefulness is sharply curtailed on a given link if it is not
implemented for all nodes on the link.  DAD requires support for
both the ICMPv6 Neighbor Solicitation and Advertisement messages, but
does not require support for either of the link-layer address options.
If DAD is enabled and IPv6 ND Address Resolution is not enabled, any
received link-layer address options MAY be ignored.

DAD does, however, introduce some traffic overhead and a delay in
interface initialization.  For links that do not implement IPv6 ND
Address Resolution, DAD also introduces additional ICMPv6 messages.
So the benefits may not justify the costs in some cases.  Therefore, a
link-type specification SHOULD specify whether DAD is or is not
REQUIRED for a specific link-type.  If the link-type specification
indicates that use of DAD is optional for a given link-type, it SHOULD
specify what mechanism will be used to determine whether or not DAD is
in use on a given link at a given time (e.g. automatic mechanism, manual
configuration, etc.).

Because DAD may introduce too much overhead in specific situations,
implementations MAY allow DAD to be disabled on a per-node or
per-interface basis via stateful or manual configuration (within the
restrictions imposed by the link-type specifications). Implementations
which support DAD SHOULD default to using DAD for all new interface
tokens on all interfaces for which it has not be explicitly disabled.

DAD does not depend upon any other ND mechanisms and can be implemented
without support for any of the ND conceptual datastructures.  Compliant
IPv6 implementations MUST support the use of DAD on all link-types
for which it is not explicitly disabled by the link-type specification.

6.  Router Discovery

Router Discovery allows IPv6 hosts to dynamically discover routers
which can be used to forward packets off-link.  This is accomplished
through the use of ICMPv6 Router Solicitations and Router
Advertisements.  In general hosts send Router Solicitations and
process received Advertisements while routers send Router
Advertisements periodically or in response to Solicitations.  Hosts
which support Router Discovery store the information received in
Router Advertisements in a Default Router List.  Routers which support
Router Discovery MAY or MAY NOT store information based upon received
Router Advertisements, but the use of that information lies outside the
scope of the Neighbor Discovery protocol.

Router Discovery is a complex process which can be extremely valuable
when hosts are configured on complex or changing networks, but it may
be unnecessary for some link-types (e.g. Point-to-point links where
all traffic is sent through the remote end-point).  There are also
security issues with Router Discovery that may make it undesireable in
some environments.  Therefore, Router Discovery MAY be explicitly
enabled or disabled for a particular link-type in the link-type
specification.  Implementations SHOULD also allow Router Discovery to be
disabled on a per-interface basis (via stateful or manual
configuration).

Nodes MUST silently discard Router Solicitations and Advertisements
received on interfaces which do not have Router Discovery enabled,
unless those messages are of interest to other enabled ND mechanisms.

The term Router Disocovery refers only to the process of receiving
Router Advertisements and adding and deleting advertised routers
to/from the Default Router List.  Processing the additional
configuration information obtained in Router Advertisments,
configuring addresses based upon that information and maintaining
reachability information for routers are covered by Prefix/Parameter
Discovery, Address Autoconfiguration and Router Unreachability
Detection, respectively.  It is possible to enable Router Discovery to
dynamically create and maintain a Default Router List without enabling
any of these separate, related mechanisms.

When a router is placed in the Default Router List, its lifetime is
also included.  This lifetime may be updated by subsequent Router
Advertisements, pursuant to the restrictions in the ND specification.
When the lifetime of a Router Advertisement expires, the corresponding
router will be removed from the Default Router List.  It is also
RECOMMENDED that nodes which implement Router Discovery include support
for Router Unreachability Detection to allow ongoing communications
using the "unreachable" router to choose a new next-hop address.

It is REQUIRED that all compliant IPv6 routers support Router
Discovery.  It is also RECOMMENDED that a compliant IPv6 host support
Router Discovery, and hosts which do implement Router Discovery SHOULD
default to using it for all interfaces for which it has not been
explicitly disabled.

7.  Prefix/Parameter Discovery

Prefix/Parameter Discovery is the mechanism by which hosts obtain
information about an attached link to use in configuration of the
interface.  This information might include MTU information and the site
local and/or global prefixes in use on the link and their associated
lifetimes.  Although this information is contained in Router
Advertisement messages, it is possible to use this information for
interface configuration without choosing to place the supplied router
address(es) in the Default Router List.

Prefix/Parameter Discovery is the mechanism used to obtain this
link-specific configuration information and store it in the Prefix
List and other interface-specific configuration structures.  The
mechanism used for creating IPv6 addresses from this information is
handled by a separate mechanism called Address Autoconfiguration.

Since Prefix/Parameter Discovery is required for Address
Autoconfiguration, it is automatically enabled whenever Address
Autoconfiguration is in use.  It is also possible to configure
Prefix/Parameter Discovery separately to allow configuration of
link-specific information (e.g. MTU) when the IPv6 addresses will be
obtained elsewhere (e.g. through either stateful or manual
configuration).

It is possible for a link-type specification to explicitly disable
the use of Prefix/Parameter Discovery for a specific link-type.
The use of Prefix/Parameter Discovery MAY also be configurable on
a per-interface basis.

A compliant IPv6 host SHOULD implement Prefix/Parameter Discovery
on any applicable link-type.  Routers MUST implement the ability
to send Prefix and Parameter information in Router Advertisements
on any applicable link-types, although they MAY allow use of
this mechanism to be disabled on a per-interface basis.

8.  Address Autoconfiguration

Address Autoconfiguration is an extremely powerful and desireable
Neighbor Discovery mechanism for use in many situations.  It is
described in a separate ND-related document [3], but relies upon
information contained in ND Router Advertisement messages and upon the
Prefix/Parameter Discovery mechanism.  After discovering prefix
information via Prefix/Parameter Discovery, a host can automatically
generate site-local and global IPv6 addresses to allow for global
communication without any explicit configuration (manual or stateful).

This mechanism SHOULD be supported for all link-types which do not
inherently allow discovery of address information through other means.
When Address Autoconfiguration is in use for a particular link-type,
the link-type specification SHOULD also specify use of DAD, unless
another mechanism is provided for preventing or discovering duplicate
IPv6 addresses.

A compliant IPv6 host SHOULD implement Address Autoconfiguration for
applicable link-types.  However, a host which supports Address
Autoconfiguration SHOULD allow it to be disabled on a per-host or
per-interface basis.  Address Autoconfiguration is a host-only
mechanism; routers obtain their address information through other
means outside the scope of the Neighbor Discovery protocol.

9.  Router Unreachability Detection

Router Unreachability Detection is dependent upon the use of Router
Discovery.  When a "discovered" router times-out and is removed from
the Default Router List, the same router may also be currently in-use
as a next-hop for communication with any number of destinations, either
because it was originally chosen as the default router for that
communication or due to subsequent ICMPv6 Redirects.  Router Unreachability
Detection is the process of looking through the destination cache, and
causing all communications through the "unreachable" router to perform
a new next-hop determination.

Although Router Discovery can be somewhat useful without the
implementation of Router Unreachability Detection, it is highly
RECOMMENDED that this mechanism be enabled when Router Discovery is
enabled, as it introduces very little additional overhead and allows
existing communications to recover smoothly when a router becomes
unreachable.

10. Neighbor Unreachability Detection

Neighbor Unreachability Detection (NUD) uses Neighbor Solicitations,
Neighbor Advertisements and on-going advice from upper-layer protocols
to determine the two-way reachability of a particular neighbor.  When
a neighbor becomes unreachable, a node can perform Address Resolution
again to determine if the neighbor is now reachable at a new
link-layer address.

Neighbor Unreachability Detection is an interesting concept, the uses
for which have not been completely established.  When Address
Resolution is in use, NUD allows for fairly quick recovery when a node
changes to a new link-layer address.  Also, when upper layer advice is
available, this mechanism can help to eliminate unnecessary Address
Resolution traffic.

However, on a link which does not use ND Address Resolution
(e.g. point-to-point links), in situations where upper-layer advice
may not be available (e.g. an SNMP agent or a transaction processing
system), this mechanism may actually result in more traffic than
otherwise necessary, determining two-way reachability on an ongoing
basis when the information is not of interest.

Ultimately, Neighbor Unreachability Detection may prove very useful to
allow for dynamically switching between treating a particular node as
on-link or off-link for particular link-types with assymetric
reachability (e.g. for RF links), but insufficient experience is
available to be certain at this time.

For certain link-types, reachability may already be established by
a link-layer mechanism before any packets are sent.  In this case,
Neighbor Unreachability Detection is redundant and should be disabled
by the link-type specification.

For other situations, there are two factors that most influence
whether Neighbor Unreachability Detection is useful for a given link:

        - Whether ND Address Resolution is in-use.
        - Whether upper-layer advice is available for
                a significant portion of the communications.

If ND Address Resolution is in use, and upper-layer advice is likely
to be available, it is RECOMMENDED that Neighbor Unreachability
Detection be used to cut down on superfluous Address Resolution
traffic.  Although use of Address Resolution is specified on a
per-link-type basis, it may only be possible to determine if
upper-layer advice will be available on a per-node basis.  Therefore,
implementations should allow Neighbor Discovery to be configured on a
per-node or per-interface basis.

On links which do not use ND Address Resolution, it is RECOMMENDED
that Neighbor Unreachability Detection be disabled unless specifically
overridden by the link-layer specification.

Given the uncertainty of the trade-offs between the complexity of
Neighbor Unreachability Detection and its benefits, it is OPTIONAL for
an implementation to support Neighbor Unreachability Detection unless
the implementation supports a link-type for which Neighbor
Unreachability Detection is REQUIRED by the link-type specification.

11. ICMPv6 Redirects

It is expected that the use of ICMPv6 Redirects will be supported on
most, if not all, IPv6 interfaces.  This mechanism allows a router to
inform a host that IP datagrams destined for a particular node or
subnet should be sent to a different next-hop address than the one
currently in-use.  This is accomplished via sending an ICMPv6 Redirect
message.

The use of ICMPv6 Redirects on a particular link-type MAY be disabled
by a link-type specification.  A host implementation MAY allow ICMPv6
Redirects to be disabled on a per-interface basis.  When ICMPv6
Redirects are not in use on a particular link, routers SHOULD NOT send
Redirect messages.  Any Redirect messages received by a host on an
interface which has ICMPv6 Redirects disabled MUST be silently
discarded.

For all applicable link-types, ICMPv6 Redirects MUST be supported by
compliant IPv6 implementations.

12. Levels of Compliance

After separating the IPv6 Neighbor Discovery protocol into separate
mechanisms, it becomes possible to discuss a minimal compliant IPv6
implementation which does not implement the full set of IPv6 Neighbor
Discovery Mechanisms.  This section attempts to summarize the
requirement levels for the individual mechanisms discussed in this
document.

12.1 Host Compliance

It is possible, on a link-type that does not require Address
Resolution, DAD or support for ICMPv6 Redirects (e.g. a point-to-point
link with a fixed end-point address), to have a compliant IPv6 host
implementation that does not implement any ND mechanisms at all.
This is not likely to be the case for a majority of IPv6 implementations,
however.

To allow for minimal IPv6 host implementations, only those ND
mechanisms which rely upon cooperation from all hosts on a given link
have been strictly REQUIRED.  It is anticipated that most IPv6
implementations will also include some or all of the RECOMMENDED
mechanisms, but their usefulness to the nodes that implement them
will not diminished if some nodes do not participate.

Each of the requirement levels discussed in this document may be
overridden, in either direction, for a particular link-type by the
link-type specification.

Compliant IPv6 hosts are REQUIRED to implement the host portions of
the following mechanisms if they support any applicable link-types:

        Address Resolution
        Duplicate Address Detection
        ICMPv6 Redirects

It is strongly RECOMMENDED that IPv6 hosts implement the host portions
of the following mechanisms, as applicable to their supported
link-types:

        Router Discovery
        Prefix/Parameter Discovery
        Router Unreachability Detection
        Address Autoconfiguration

The following IPv6 ND mechanism is OPTIONAL for a compliant
IPv6 host implementations, unless explicitly required by a
supported link-type:

        Neighbor Unreachability Detection

12.2 Router Compliance

Compliant IPv6 routers are REQUIRED to implement the router portions
of the following IPv6 ND mechanisms if applicable to any of their
supported link-types:

        Address Resolution
        Duplicate Address Detection
        ICMPv6 Redirects
        Router Discovery
        Prefix/Parameter Discovery

The following ND mechanisms are not applicable to routers:

        Address Autoconfiguration
        Router Unreachability Detection
        Neighbor Unreachability Detection

Routers MAY choose to use information contained in ND messages normally
processed only by hosts (e.g. Router Advertisements), but use of that
information is outside the scope of the Neighbor Discovery protocol.

13. Summary of Separable ND Mechanisms

Each of the eight separable ND mechanisms has been summarized
below.  These summaries include the methods by which the mechanisms
are configurable, the default state of each mechanism, the requirement
level for implementation of each mechanism in a compliant IPv6 node
and the messages and datastructures used by each mechanism.

12.1 ADDRESS RESOLUTION

Enabled Scope:          By Link-Type (in link-type specification)
Default:                Enabled on all interfaces
Requirement Level:      REQUIRED (for applicable link-types)
Dependencies:           None
Messages Required:      Neighbor Solicitation
                                with Source Link-Layer Address Option
                        Neighbor Advertisement
                                with Target Link-Layer Address Option
Datastructures:         Neighbor Cache, Level 1 or higher

12.2 DUPLICATE ADDRESS DETECTION (DAD)

Enabled Scope:          By Link-Type (in link-type spec)
                          with OPTIONAL Per-Node or Per-Interface override
Default:                Enabled on all interfaces
Requirement Level:      REQUIRED (for applicable link-types)
Dependencies:           None
Messages Required:      Neighbor Solicitation
                        Neighbor Advertisement
Datastructures:         None

12.3 ROUTER DISCOVERY

Enabled Scope:          By Link-Type (in link-type spec)
                          with RECOMMENDED Per-Interface override
Default:                Enabled on all interfaces, if supported
Requirement Level:      Router:  REQUIRED (for applicable link-types)
                        Host:    RECOMMENDED (for applicable link-types)
Dependencies:           None
Messages Required:      Router Solicitation
                        Router Advertisement
Datastructures:         Default Router List

12.4 PREFIX/PARAMETER DISCOVERY

Enabled Scope:          By Link-Type (in link-type spec)
                          with RECOMMENDED Per-Interface override
Default:                Enabled on all interfaces, if supported
Requirement Level:      Router:  REQUIRED (for applicable link-types)
                        Host:    RECOMMENDED (for applicable link-types)
Dependencies:           None
Messages Required:      Router Solicitation
                        Router Advertisement
Datastructures:         Prefix List

12.5 ADDRESS AUTOCONFIGURATION

Enabled Scope:          By Link-Type (in link-type spec)
                          with RECOMMENDED Per-Interface override
Default:                Router:  N/A
                        Host:    Enabled on all interfaces, if supported
Requirement Level:      Host:    RECOMMMENDED
                        (N/A for routers)
Dependencies:           Requires Prefix/Parameter Discovery
Messages Required:      Router Solicitation
                        Router Advertisement
Datastructures:         Prefix List

12.6 ROUTER UNREACHABILITY DETECTION

Enabled Scope:          By Link-Type (in link-type spec)
                          with RECOMMENDED Per-Interface override
Default:                Router:  N/A
                        Host:    Enabled on all interfaces, if supported
Requirement Level:      Host:    RECOMMENDED
                        (N/A for routers)
Dependencies:           Requires Router Discovery
Messages Required:      Router Solicitation
                        Router Advertisement
Datastructures:         Default Router List

12.7 NEIGHBOR UNREACHABILITY DETECTION

Enabled Scope:          By Link-Type (in link-type spec)
                          with RECOMMENDED Per-Interface override
Default:                Router:  N/A
                        Host:    Disabled on all interfaces
Requirement Level:      Host:    OPTIONAL
                        (N/A for routers)
Dependencies:           None
Messages Required:      Neighbor Advertisement
                        Neighbor Solicitation
Datastructures:         Neighbor Cache, Level 2

12.8 ICMPv6 REDIRECTS

Enabled Scope:          By Link-Type (in link-type spec) or
                          with OPTIONAL Per-Node or Per-Interface override
Default:                Enabled on all interfaces
Requirement Level:      REQUIRED (for applicable link-types)
Dependencies:           None
Required Messages:      ICMPv6 Redirect Message
Datastructures:         Destination Cache

14. Impact on other documents

If the working group agrees with the text of this document, or a
modified version thereof, it would be desireable to incorporate this
information into the main IPv6 Neighbor Discovery specification.  It
is also possible that the current ND specification could be broken up
into multiple documents to allow some mechanisms (which have remained
constant for some time, are widely implemented and have no known
problems) to advance in the standards process while we continue to
gather experience with other mechanisms.

Link-type specifications SHOULD be modified, if necessary, to
explicitly state whether IPv6 ND Address Resolution, DAD or ICMPv6
Redirects are in use for their link-type.  In addition, link-type
specifications SHOULD specify requirement levels and/or defaults for the
other ND mechanisms individually if different from those discussed in
this document.  If a link-type specification does not explicitly state
otherwise, it is assumed that all unmentioned ND mechanisms are
applicable to the interface type and are subject to the requirements
and defaults outlined above.

15. Security considerations

This draft introduces no new protocols or mechanisms and, therefore,
does not introduce new security concerns.  There are some existing
security issues with the IPv6 Neighbor Discovery protocol, as
discussed in [1], which are unaffected by the organizational change
proposed in this draft.

16. Acknowledgments

The author would like to acknowledge the input of the IPng Working
Group.  This draft reflects ideas put forth at the IETF meeting in
Washington, D.C., December 1997 by several working group members.

16. References

[1]     T. Narten, E. Nordmark, W. Simpson, "Neighbor Discovery for
        IP Version 6 (IPv6)", draft-ietf-ipngwg-discovery-v2-01.txt.

[2]     S. Bradner, "Key words for use in RFCs to Indicate
        Requirement Levels", RFC 2119, March 1997.

[3]     S. Thompson, T. Narten, "IPv6 Address Autoconfiguration",
        draft-ietf-ipngwg-addrconf-v2-00.txt.

17. Author's contact information

Please address all comments or questions regarding this draft to:

Margaret Wasserman                             mrf@epilogue.com
Director of IP Development                        (617)245-1805
Epilogue Technology Corporation             FAX: (617) 245-0804