DNA Working Group S. Narayanan
Internet-Draft Panasonic
Expires: December 21, 2004 G. Daley
Monash University CTIE
N. Montavont
LSIIT - ULP
June 22, 2004
Detecting Network Attachment in IPv6 - Best Current Practices
draft-narayanan-dna-bcp-00.txt
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Abstract
Hosts experiencing rapid link-layer changes may require further
configuration change detection procedures than more traditional fixed
hosts. Detecting Network Attachment is a set of strategies for
determining configuration validity in wireless or rapidly changing
environments. This document details best current practice for
Detecting Network Attachment in IPv6 hosts using Neighbor Discovery
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procedures.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Structure of this Document . . . . . . . . . . . . . . . . 4
2. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 5
3. Background & Motivation for DNA . . . . . . . . . . . . . . . 5
4. Current Practice for Hosts Supporting DNA . . . . . . . . . . 6
4.1 IP configurations relevant to DNA . . . . . . . . . . . . 6
4.1.1 Router and Prefix Discovery . . . . . . . . . . . . . 6
4.1.2 Address Configuration . . . . . . . . . . . . . . . . 7
4.1.3 Neighbor Caches . . . . . . . . . . . . . . . . . . . 8
4.1.4 Multicast Listener State . . . . . . . . . . . . . . . 10
4.1.5 Mobility Management . . . . . . . . . . . . . . . . . 10
4.1.6 Other Configuration . . . . . . . . . . . . . . . . . 11
4.2 Validation using Neighbor Discovery . . . . . . . . . . . 11
4.3 Further Procedures on Detection of Network Attachment . . 12
5. Detecting Network Attachment Steps . . . . . . . . . . . . . . 12
5.1 Validity of configuration . . . . . . . . . . . . . . . . 12
5.2 Reachability detection . . . . . . . . . . . . . . . . . . 13
6. Initiation of DNA Procedures . . . . . . . . . . . . . . . . . 15
6.1 Hint Reception and Processing . . . . . . . . . . . . . . 17
6.2 Handling Hints from Other Layers . . . . . . . . . . . . . 17
6.3 Timer Based Hints . . . . . . . . . . . . . . . . . . . . 17
6.4 Simultaneous Hints . . . . . . . . . . . . . . . . . . . . 18
6.5 Hint Validity and Hysteresis . . . . . . . . . . . . . . . 19
6.6 Hint Management for Inactive Hosts . . . . . . . . . . . . 19
7. Validation of configuration . . . . . . . . . . . . . . . . . 20
7.1 Making use of Prior Information . . . . . . . . . . . . . 20
7.2 Transient Link Availability . . . . . . . . . . . . . . . 21
7.3 Link Change and Router Discovery . . . . . . . . . . . . . 21
7.4 Validating configuration in Constrained Topologies . . . . 22
7.5 Validating configuration in Arbitrary Topologies . . . . . 23
7.6 Dealing with Incomplete Information . . . . . . . . . . . 23
7.7 Configuration Algorithms . . . . . . . . . . . . . . . . . 24
8. Reachability Detection . . . . . . . . . . . . . . . . . . . . 27
8.1 Wireless propagation . . . . . . . . . . . . . . . . . . . 28
8.2 Asymmetric Reachability . . . . . . . . . . . . . . . . . 28
8.3 Specific (Neighbor Solicitation) Tests . . . . . . . . . . 28
8.4 Non-Specific (Router Solicitation) Tests . . . . . . . . . 29
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8.5 Trade-offs in Reachability Testing . . . . . . . . . . . . 29
9. Complications to Detecting Network Attachment . . . . . . . . 30
9.1 Tentative Addressing . . . . . . . . . . . . . . . . . . . 30
9.2 Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 31
9.3 Router Configurations . . . . . . . . . . . . . . . . . . 31
9.4 Overlapping Coverage . . . . . . . . . . . . . . . . . . . 31
9.5 Multicast Snooping . . . . . . . . . . . . . . . . . . . . 32
10. Current Practice for Routers supporting DNA . . . . . . . . 32
10.1 Router Advertisement Intervals . . . . . . . . . . . . . . 32
10.2 Unicast Response Transmission . . . . . . . . . . . . . . 34
10.3 Point-to-Point Links . . . . . . . . . . . . . . . . . . . 34
10.4 Prefix Advertisement . . . . . . . . . . . . . . . . . . . 34
10.5 Secured Neighbor Discovery . . . . . . . . . . . . . . . . 35
11. Security Considerations . . . . . . . . . . . . . . . . . . 35
11.1 Replay or impersonation of messages. . . . . . . . . . . . 35
11.2 Authorization and Detecting Network Attachment . . . . . . 36
11.3 Addressing . . . . . . . . . . . . . . . . . . . . . . . . 36
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 37
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.1 Normative References . . . . . . . . . . . . . . . . . . . . 37
13.2 Informative References . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 39
A. Summary of Recommendations . . . . . . . . . . . . . . . . . . 40
B. Example State Transition Diagram . . . . . . . . . . . . . . . 43
C. DNA With Fast Handovers for Mobile IPv6 . . . . . . . . . . . 43
D. DNA with Candidate Access Router Discovery . . . . . . . . . . 43
Intellectual Property and Copyright Statements . . . . . . . . 44
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1. Introduction
When operating in changing environments, IPv6 hosts may experience
variations in reachability or configuration state over time. For
hosts accessing the Internet over wireless media, such changes may be
caused by changes in wireless propagation or host motion.
Detecting Network Attachment (DNA) in IPv6 is the task of checking
for changes in the validity of a host's IP configuration. Changes
may occur on establishment or disconnection of a link-layer. For
newly connected interfaces, they may be on a different link to the
existing configuration's routers.
In these and other cases, IP addressing and default routing
configuration may be invalid, which prevents packet transfer.
DNA focuses on determining whether the current configuration is
invalid, leaving the actual practice of re-configuration to other
subsystems.
This document presents the best current practices for IPv6 hosts to
address the task of Detecting Network Attachment in changing and
wireless environments.
1.1 Structure of this Document
Section 3 of this document provides a background and motivation for
Detecting Network Attachment. Section 4 provides an overview of DNA
practices for hosts, and their place within the change detection and
configuration cycle.
Elaboration of specific practices for hosts continues in Section 5,
Section 6, Section 7, and Section 8. These sections describe how to
safely determine network attachment with minimal signaling, across a
range of environments.
Topological and host-based challenges which govern the operation of
DNA procedures are detailed in Section 9.
Router based configurations which assist hosts' ability to detect
network attachment are described in Section 10.
Section 11 Provides security considerations, and details a number of
issues which arise due to wireless connectivity and the changeable
nature of DNA hosts' internet connections.
This document has a number of appendixes.
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Appendix A lists the recommendations for systems wishing to support
Best Current Practice. Appendix B provides an example state machine
for DNA describing knowledge and belief based on the prior listed
recommendations. The final two (Appendix C and Appendix D) look at
existing experimental protocols that may be used to provide DNA
processes with access network information before arrival on a new
link.
2. Terms and Abbreviations
There is an existing DNA terminology draft [20]. At this stage, it
is unclear if this draft or the mobility terminology [21] draft need
to be referenced, or specific terms need to be placed in this
document.
While the mobility terminology draft may be applicable, the focus of
this draft upon mobile hosts may be distracting for DNA. Comments on
this issue are welcome.
3. Background & Motivation for DNA
Hosts on the Internet may be connected by various media. It has
become common that hosts have access through wireless media or are
mobile, and have a variety of interfaces, which may be used to
provide internet connectivity. The frequency of configuration change
for wireless and nomadic devices are elevated, due to the vagaries of
wireless propagation or the motion of the hosts themselves.
Detecting Network Attachment is a strategy to assist such rapid
configuration changes by determining when they are required, or not.
While DNA has applicability to wireless and wireline access networks,
these two sets of networks bring different sets of requirements to
the problem.
Verifying the validity of current IP configuration is needed when
either a wireless or wireline link-layer is in use. Conversely,
wireless hosts are more likely to change their link-layer
connections.
Due to these frequent link-layer changes, an IP configuration change
detection mechanism for DNA needs to be efficient and rapid.
Making such detection procedures rapid helps to avoid unnecessary
configuration delays upon link-change.
For wireless devices, there will typically be a trade-off between
configuration delays and the energy or bandwidth required to transmit
for configuration validity tests or re-configuration. DNA seeks to
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assist hosts by providing information about network state, which may
allow hosts to appropriately make decisions regarding such tradeoffs.
Even though DNA is restricted to determining whether change is
needed, the process of obtaining information for the new
configuration may occur simultaneously with the detection to improve
the efficiency of these two steps.
4. Current Practice for Hosts Supporting DNA
Various protocols within IPv6 provide their own configuration
processes. While Detecting Network Attachment seeks to provide
efficient change detection without undertaking configuration, it is
necessary to describe these protocols and their relationship to each
other.
Each of the protocols has a role to play in configuration of hosts,
but many maintain their own change discovery mechanisms. In rapidly
changing and wireless environments it is necessary to rationalize the
discovery techniques on a minimal subset of procedures and messages,
sufficient to determine change validity and authorization.
This section aims to allow appropriate background for discussing the
best existing procedures for use in Detecting Network Attachment.
The following discussion describes how these discovery processes
interrelate, and indicates how IPv6 Neighbor Discovery can be used
not only to detect link-change, but assist in determining
requirements for other configuration.
4.1 IP configurations relevant to DNA
Each of the following subsystems is described in terms of the
configuration detection or change detection mechanisms which they
support, the reliance of the service on other configuration and a
summary of the relevant configuration procedures undertaken upon
change discovery.
Indications are provided as to their performance and applicability to
DNA.
This discussion continues in Section 4.2.
4.1.1 Router and Prefix Discovery
Router Discovery is designed to provide hosts with a set of locally
configurable prefixes and default routers. These may then be
configured by hosts for access to the Internet [1].
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It allows hosts to discover routers and manage lists of eligible next
hop gateways, and is based on IPv6 Neighbor Discovery. When one of
the routers in the router list is determined to be no longer
reachable, its destination cache entry is removed, and new router is
selected from the list.
As indicated below in Section 4.1.3, router reachability is
principally managed through the validity of the neighbor cache entry
and the advertised ValidLifetime of the router. Before the router is
actively used, though, a router advertisement can place a router into
the default router list for 30 days after being received [1].
Clearly, if router reachability is to be used for the purposes of
link change detection in volatile environments, practical and fast
means for determining reachability and unreachability need to be used
(see Section 8).
As described in Section 6, detection of network attachment may be
initiated by such events as missed Router Advertisements (see Section
4.1.5) or lack of expected response packets passing through a router.
If the currently configured router is unreachable, it is quite likely
that other devices on the same link are no longer reachable.
On determining that link-change has occurred, the default router list
SHOULD have entries removed which are related to unreachable routers,
and consequently these routers' destination cache entries SHOULD be
removed [1]. If no eligible default routers are in the default
router list, Router Solicitations may be sent, in order to discover
new routers.
4.1.2 Address Configuration
Unicast addresses are required to send all packets on the Internet,
except a restricted subset of local signaling such as router and
neighbor solicitations.
Reception of packets at a global address, which have been received
from off-link are likely to be an indication of router reachability.
On broadcast media where packets interpreted by all nodes though, a
host may receive incorrectly believe it has a valid address if it
arrives on a link where the packets from this data stream are in
transit. This may happen for example if there is no unique
link-layer addressing in a network.
In such cases, hosts MUST NOT assume that they are receiving packets
at a topologically correct address unless they can uniquely identify
the last hop transmitter of the packets as a configured router.
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As indicated below (in Section 4.1.3), where such information feeds
into neighbor cache reachability state, even packets which don't pass
through a router may indicate validity of an address, through
indication that a set of neighbors is present (including appropriate
routers).
In dynamic environments, hosts need to undertake automatic
configuration of addresses, and select which addresses to use based
on prefix information advertised in Router Advertisements. Such
configurations may be based on either Stateless Address
Autoconfiguration [3] or DHCPv6 [12].
For any configured address, Duplicate Address Detection (DAD) MUST be
performed [3]. DAD defines that an address is treated tentatively
until either series of timeouts expire after probe transmissions or
an address owner defends its address. Tentative addresses cannot
modify peers' neighbor cache entries, nor can they receive packets.
Additionally, IPv6 requires configuration of link-local addresses
which are to be used for signaling within a link. Possession of a
non-tentative link-local address allows transmission of all neighbor
and router discovery messages, as well as unicast reception of
configuration data. It is notable that while Stateless Address
Autoconfiguration needs only DAD, DHCPv6 relies upon having a
non-tentative link-local address to send its messages.
IPv6 routing assumes that IP subnets are available at a single
location, for any particular global prefix [13]. When a host leaves
a link, it therefore leaves its global prefix behind and cannot
receive packets on the link using that address. Link-local
addresses, while being available on every link also may change if a
link changes. This is because the scope of the individual address's
uniqueness is confined to a single link, and tests for uniqueness
MUST be undertaken for each link upon which a host arrives.
When undertaking DNA procedures, the host may suspect that it doesn't
have valid configuration. In this case, it SHOULD undertake
Duplicate Address Detection (DAD) for the link-local address, or
treat it as tentative until the host determines if its configuration
is valid or not (or the DAD procedure completes).
The motivations and implications for this practice are described in
Section 9.1.
4.1.3 Neighbor Caches
Neighbor caches allow for delivery of packets to peers on the same
link. Neighbor cache entries are created by router or neighbor
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discovery signaling, and may be updated either by upper-layer
reachability confirmations or explicit neighbor discovery exchanges.
In order to determine which link-layer address a peer is at, nodes
send solicitations to the link-local solicited-node multicast address
of their peer. If hosts are reachable on this address, then they
will respond to the solicitation with a unicast response.
This reliance on multicast packet delivery may mean that MLD
(Multicast Listener Discovery) reporting needs to be completed before
solicited-node's packet reception can occur (see Section 4.1.4), if
multicast delivery within a link requires group signaling.
By default, neighbor cache entries exist for at least 30 seconds
after reachability confirmation, before becoming STALE. They may
exist for a number for any length of time in the STALE state until
they are used, and then a Neighbor Unreachability Detection test is
performed (taking up to 8 seconds). After this, a neighbor cache
entry is deleted.
When link change occurs, the reachability of all existing neighbor
cache entries is likely to be invalidated, if link change prevents
packet reception from an old link. For these links, the neighbor
cache entries SHOULD be removed when a host moves to a new link
(although it MAY be possible to keep keying and authorization
information for such hosts cached on a least-recently-used basis
[7]).
For some wireless media, it may be possible to reach all the nodes on
two different access networks simultaneously. In this case, neighbor
cache entries for a link entries MAY be removed when routers on the
link are no-longer directly reachable.
In both forms of networks, the reachability of the set of routers on
a link is a good indicator for reachability to the rest of the link.
Hosts communicating using a particular medium SHOULD be aware of the
reachability conditions which prevail for a particular medium, and
make decisions accordingly.
Detecting that link change has occurred can be performed by actively
probing reachability with neighboring nodes or routers. (This is
detailed in Section 8).
Reachability of a single node may support the likelihood of reaching
the rest of the link, for example if a particular access technology
relays such messages through wireless base stations.
Even for such networks, link partitions may still cause router
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unreachability, and hosts SHOULD check for router unreachability in
the case that they lack expected packet receptions through or from a
router.
4.1.4 Multicast Listener State
Multicast routers on a link are aware of which groups are in use
within a link. This information is used to undertake initiation of
multicast routing for off-link multicast sources to the link [8][10].
When a node arrives on a link, it may need to send Multicast Listener
Discovery (MLD) reports, if the multicast stream is not already being
received on the link. If it is an MLDv2 node it SHOULD send state
change reports upon arrival on a link [10].
Since the identity of the link is tied to the presence and identity
of routers, initiation of these procedures may be undertaken when DNA
procedures have been completed. In the absence of received data
packets from a multicast stream, it is unlikely that a host will be
able to determine if the multicast stream is being received on a new
link, and will have to send state change reports, in addition to
responses to periodic multicast queries [8][10].
For link scoped multicast, reporting needs to be done to ensure that
packet reception in the link is available due to multicast snoopers.
Some interaction is possible when sending messages for the purpose of
DNA on a network where multicast snooping is in use. This issue is
described in Section 9.5.
While [8] specifies that routers may ignore messages from unspecified
source addresses [9] indicates that for the benefit of snooping
switches such messages MAY be transmitted.
Since DNA procedures are likely to force link-local addresses to be
tentative, this means MLD messages may need to be transmitted with
unspecified source addresses while link-locals are tentative, in
order to complete DNA. This is discussed further in Section 9.5
4.1.5 Mobility Management
Mobile IPv6 provides global mobility signaling for hosts wishing to
preserve sessions when their configured address becomes topologically
incorrect [5]. This system relies upon signaling messages and tunnel
movement to provide reachability at a constant address, while
directing packets to its visited network.
[5] defines 'movement detection' procedures, which themselves rely
upon Neighbor Discovery, to initiate mobility signaling. These
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procedures allow for some modification of Neighbor Discovery to
enable faster change or movement detection. While this document will
reference parameters defined in [5], hosts are not required to
undertake global mobility signaling or tunneling in order to benefit
from detecting network attachment.
Of benefit is an option which allows routers to advertise the
regularity of their unsolicited advertisements. This can be used to
determine if multiple advertisements have been missed by a host. In
these circumstances, DNA procedures may be initiated as described in
Section 6.
After change detection occurs, a MIPv6 mobile node still needs to
undertake global address configuration, and then mobility signaling
as specified in [5].
4.1.6 Other Configuration
When attempting to access the Internet, several other configuration
services may be required, dependent on the requirements of the access
networks.
In this case, it is likely that configuration parameters can be
passed to hosts using DHCPv6 [12]. The availability of such
additional configuration information can be advertised using Router
Advertisements [1].
IPv6 Stateless Address Autoconfiguration [3] describes an
OtherConfigFlag which is set when either the 'O' or the 'M' flags are
set in the Router Advertisement header. Hosts use the
OtherConfigFlag to determine if they need to undertake further
(non-addressing related) DHCPv6 procedures.
If the Advertisement's 'O' flag was set, but not the 'M' flag, the
host sends a DHCPv6 Information-Request message asking for such
configuration.
This signaling may occur after completion of DNA procedures.
4.2 Validation using Neighbor Discovery
Most of the procedures required for detection of network attachment
(router reachability, prefix validity) are provided in [1].
Handling rapid change may require specific initiation and
interpretation of message exchange procedures, but may be achieved
without new messages or options.
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Timing of messages and performance constraints will be described in
this document, and explicit indications of practical differences
between particular settings for these timers will be provided.
4.3 Further Procedures on Detection of Network Attachment
Detection of network attachment does not define or prescribe
configuration procedures. The actual configuration is therefore left
to the procedures which are invoked upon arrival on a new link.
While DNA does not undertake configuration, it does learn about the
state of the network using neighbor and router discovery. Where it
is safe to do so, such state SHOULD be made available to
configuration processes.
Particularly, state gained from change detection procedures SHOULD
NOT be discarded, such that discovery processes need to be undertaken
for each configuration protocol used.
5. Detecting Network Attachment Steps
Two different situations may lead a node to engage a network
attachment detection procedure. Either a node receives an indication
that its link may have changed or it may detect that is configuration
is not valid any more.
If a host receives an indication (such as a link-layer hint) that its
link may have changed, it has to verify the validity of its current
configuration and confirm the reachability with its default router.
If one of these two actions do not succeed, initiation of new
configuration is required.
If the host detects that its configuration is not valid any more, for
example because a timer has expired, the node should engage in
detection of its network attachment in order determine to what extent
it needs to create a new configuration.
While the initiation of the DNA procedures is described in the next
section, the different steps involved in detecting network attachment
are described below. Once the DNA procedure is engaged, two major
steps are to be done: check the validity of the current configuration
and confirm the reachability with the default router. Depending on
the method used, these two steps could be performed independently or
during the same procedure.
5.1 Validity of configuration
When link change occurs, an IPv6 host is likely to have one or more
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IPv6 configurations for the interface in its internal cache. Upon
initiation of DNA procedures (as specified in Section 6), the first
step for the host would be to verify if any of these configurations
is still valid.
The validity of a host's configuration can be inferred by determining
its presence on a particular link. The host can verify presence on a
particular link by learning the ranges of valid addresses and routers
associated with that link and comparing this information with its
cached configuration. Learning the routers available on the link and
the prefix supported by them can be done either passively by
listening to the Router Advertisements (RA) periodically sent by
routers, or actively by sending Router Solicitation (RS) [1].
Otherwise, the host may send Neighbor Solicitation (NS) in order to
check if its current default router is still reachable [1].
Router Discovery is used initially to begin gathering a set of
prefixes associated with a link, and determine the preference and
authorization of default routers [1], [7]. Neighbor Solicitations in
this case provide only a confirmation of reachability of a currently
configured router, as detailed in Section 8.
In addition to reachability information, routing authorization needs
to be determined for each router. In SEND [7], routing authorization
is delegated by certification authorities. Certificate authorities
delegate a portion of their routing authority to the router, tying
them to a public/private key-pair. While SEND Router Advertisement
packets contain the public key used to sign the message, the routing
certificate is not present in that message. Hosts need to test the
router's authority to provide routing for the source addresses
advertised in its Router Advertisement in order to ensure safe
configuration.
It might be noticed that the reception of a new Router Advertisement
on a link does not necessarily mean that the current default prefix
of a host is not valid anymore. Several prefixes as well as several
routers might be present on a link. This leads to the fact that the
non-presence of the current default router should be determined
before considering that the link has changed. This is even more
important with mobile hosts, which update their localization
according to their position in the Internet. Considering that the
current default router/prefix has changed upon the reception of a new
IPv6 prefix may lead to excessive Binding Update transmission.
Reachability detection is discussed in the next subsection.
5.2 Reachability detection
Reachability information usually relies on timers associated with
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received information. Reachability detection can be triggered when a
host has some indications that the link might have changed or when a
timer is expired. Reachability detection is very critical, since if
the peer, e.g. the current default router, is not reachable anymore,
the host will have to change its IP configuration to be able to
communicate. As we will see, reachability detection can be very fast
if the peer is still reachable. However, when the peer is not
reachable anymore, the unreachability detection relies on timeout
after transmission of solicitations. These timeouts introduce
important delays.
Presence of the current router can be validated by an unsolicited
Router Advertisement (RA) received on the link. To send RAs, a
router uses three main timers : MaxRtrAdvInterval, MinRtrAdvInterval
and MIN_DELAY_BETWEEN_RAS [1]. By default, MinRtrAdvinterval is 0.33
times MaxRtrAdvInterval (i.e. 200 sec. if MaxRtrAdvInterval is 600
sec.). A host can send a Router Solicitation if it detects that it
didn't get an RA during 3 times MaxRtrAdvInterval. So, if we
consider default values, a host might wait for 30 minutes before
sending an RS. We can also notice that by default, the AR can not
send a new multicast RA within 3 seconds after having sent one. The
reply of a RS must also be delayed for a random time between 0 and
MaxRADelayTime (0.5 sec by default). The specification of Mobile
IPv6 [5] identified that these defaults are too long to support
mobility of host and proposes to reduce the RA transmission between
30 and 70ms and to allow a host to send a RS if it didn't receive a
RA within the last second.
Absence of RA from the current default router MAY require
verification and acquisition of configuration using one of the active
mechanisms listed below. Non-presence will be detected either when
RA from the router is not received for a period of time or by
neighbor reachability test. Three different possible message
exchanges can be used to test reachability and actively learn about
the on-link information and they are presented below.
1. Neighbor Discovery:
The IP node sends NS message to the default router and waits for the
NA from the router. This mechanism is efficient if the current
default router is still reachable : only a round trip time between
the host and the router is needed to validate the configuration.
This method will allow the node to find out whether the current
configuration information is valid. Conversely, if the default
router is not available, the host will timeout without receiving a
NA. By default, the host can send three NS (one every second) before
considering that the pair is not reachable. However, if the host has
indication that the link may have changed, this delay may be reduced
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without significant damage for the network. If the current router is
considered unreachable (timeout on NS message), the host will have to
execute router discovery procedure to obtain new configuration and
reachability information. The host will send a RS message to the
All-Routers multicast address. In summary, this method works well
both when the current default router is available and when the
current default router is not available. When the current default
router is not available though, the delay introduced in doing ND
before switching RD could become a problem in deploying real time
applications in wireless networks. Reducing the timer associated
with the unreachability testing through the exchange of NS/NA might
be an issue.
2. Router Discovery:
Send a RS message to the All-Routers multicast address. A RA message
in response to the RS will be received from one of the available
routers on the link. This method will lead to quick configuration of
the interface because if the current default router is not
accessible, new configuration information can be received from the
responding router. But, this can lead to erroneous re-configuration
of the interface because a response from a new router doesn't
necessarily mean that the current router is not accessible. In
RFC2461 [1], the node may have to wait for 3 times MAX_RA_DELAY_TIME
to confirm that the current default router is not serving the default
IPv6 prefix. By considering the default values of RFC2461, 3 times
MAX_RA_DELAY_TIME is 30 minutes, while with the values defined in
MIPv6, this time is 1 second (due to the shorter Router Advertisement
frequency).
3. Neighbor Discovery and Router Discovery in parallel:
Send a NS to the current default router and a RS to the All-Routers
multicast address in parallel. If the response to the NS is received
within the timeout period, any response to the RS can be ignored. If
no NA is received, the RA received in response to the RS will be used
to configure the IP interface. The method works well in both cases
when the current router is still available and when not, and avoids
the delay of exchanging RS and RA after the ND timeouts. When the
current default router is available, the RS and RA messages are
unnecessarily transmitted, wasting network resources.
6. Initiation of DNA Procedures
Link change detection procedures are initiated when information is
received either directly from the network or from other protocol
layers within the host. This information indicates that network
reachability or configuration is suspect and is called a hint.
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Hints MAY be used to update a wireless host's timers or probing
behavior in such a way as to assist detection of network attachment.
Hints SHOULD NOT be considered to be authoritative for detecting IP
configuration change by themselves.
In some cases, hints will carry significant information (for example
a hint indicating PPP IPv6CP open state [4]), although details of the
configuration parameters may be available only after other IP
configuration procedures. Implementers are encouraged to treat hints
as though they may be incorrect, and require confirmation.
The signaling which causes a hint may be due to network-layer
messages such as unexpected Router Advertisements, multicast listener
queries or ICMPv6 error messages [1][8][6]. In these cases, caution
must be exerted.
Hosts MUST ensure that untrusted messages do not cause unnecessary
configuration changes, or significant consumption of host resources
or bandwidth.
In order to achieve this aim, a host MAY implement hysteresis
mechanisms such as token buckets, hint weighting or holddown timers
in order to limit the effect of excessive hints (see Section 6.5).
Transport and Link-Layers may also provide hints, caused by state
change in their own layer. Two examples of such state change are TCP
retransmission timeout and completion of link-layer access
procedures.
While hints from other protocol layers originate from within the
host's own stack, the network layer SHOULD NOT treat hints from other
protocol layers as authoritative indications of link change.
This is because state changes within other protocol layers may be
triggered by untrusted messages, come from compromised sources (for
example 802.11 WEP Encryption [19]) or be inaccurate with regard to
network-layer state. In most cases, additional procedures such as
those defined in Section 7.3 and Section 8 will be required to
confirm changes in network layer configuration.
State changes within the network-layer itself may initiate
link-change detection procedures. Existing subsystems for router and
neighbor discovery, address leasing and multicast reception maintain
their own timers and state variables. Changes to the state of one or
more of these mechanisms may hint that link change has occurred, and
initiate detection of network attachment.
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6.1 Hint Reception and Processing
Hints received due to external IP packets will typically be due to
multicast messages, when a host has arrived on a new link. A delay
before receiving these messages is likely as in most cases intervals
between All-Hosts multicast messages are tightly controlled [1][6].
Regardless of this, actions based on multicast reception from
untrusted sources are dangerous due to the threat of transmitter
impersonation. This issue is discussed further in Section 11.
6.2 Handling Hints from Other Layers
Timeouts and state change at other protocol layers may provide hints
of link change to detection of network attachment. While the hints
come from the host's own stack, the causes for such hints may be due
to packet reception or non-reception events at the originating
layers.
As such, it may be possible for other devices to instigate hint
delivery on a host or multiple hosts deliberately, in order to prompt
packet transmission, or configuration modification. This ability to
create hints may even extend to the parameters supplied with a hint
that give indications of the network's status.
Therefore, hosts SHOULD NOT change their configuration state based on
hints from other protocol layers. A host MAY choose to adopt an
appropriate link change detection strategy based upon hints received
from other layers, with suitable caution and hysteresis, as described
in Section 6.5.
6.3 Timer Based Hints
When receiving messages from upper and lower layers, or when
maintaining reachability information for routers or hosts, timers may
expire due to non-reception of messages. In some cases the expiry of
these timers may be a good hint that DNA procedures are necessary.
Hosts SHOULD NOT start DNA procedures simply because a data link is
idle, in accordance with [1]. Hosts MAY act on hints associated with
non-reception of expected signaling or data.
Since DNA is likely to be used when communicating with devices over
wireless links, suitable resilience to packet loss SHOULD be
incorporated into either the hinting mechanism, or the DNA initiation
system.
Notably, non-reception of data associated with end-to-end
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communication over the Internet may be caused by reception errors at
either end or because of network congestion. Hosts SHOULD NOT act
immediately on packet loss indications, delaying until it is clear
that the packet losses aren't caused by transient events.
Use of the Advertisement Interval Option specified in [5] follows
these principles. Routers sending this option indicate the maximum
interval between successive router advertisements. Hosts receiving
this option monitor for multiple successive packet losses and
initiate change discovery.
6.4 Simultaneous Hints
It is possible that hints arrive synchronously on multiple hosts at
the same time. As described in [1][6] , a host SHOULD delay randomly
before acting on a widely receivable advertisement, in order to avoid
response implosion.
Since a host's detection of network attachment may include Router
Solicitations sent to multicast addresses, a host may receive
responses from each of multiple routers on a link. Therefore, Router
Solicitations may eventually cause additional bandwidth consumption,
and require additional constraint.
Where a host considers it may be on a new link and learns this from a
hint generated by a multicast message, the host SHOULD defer 0-1000ms
in accordance with [1][3]. Please note though that a single
desynchronization is required for any number of transmissions
subsequent to a hint, regardless of which messages need to be sent.
Additional delays are only required if in response to messages
received from the network which are themselves multicast, and it is
not possible to identify which of the receivers the packet is in
response to.
While some link-layer hints may be generated by individual actions,
other events which generate hints may affect a number of devices
simultaneously.
For example, if a wireless base station goes down, all the hosts on
that base station are likely to initiate link-layer configuration
strategies after losing track of the last beacon or pilot signal from
the base station.
Hence there is some chance of network layer message synchronization,
if each device attempts the same link-layer procedures. Such
synchronization effects are reliant upon the nature of the particular
link-layer technology.
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In link-layers where sufficient serialization occurs after an event
experienced by multiple hosts, each host MAY avoid the random delays
before sending solicitations specified in [1].
6.5 Hint Validity and Hysteresis
Anecdotal evidence from the initial Detecting Network Attachment BoF
indicated that hints received at the network layer often did not
correspond to changes in IP connectivity [16].
In some cases, hints could be generated at an elevated rate, which
didn't reflect actual changes in IP configuration. In other cases,
hints were received prior to the availability of the medium for
network-layer packets.
Additionally, since packet reception at the network and other layers
are a source for hints, it is possible for traffic patterns on the
link to create hints, through chance or malicious intent.
Therefore, it may be necessary to classify hint sources and types for
their relevance and recent behavior.
When experiencing a large number of hints, a host SHOULD employ
hysteresis techniques to prevent excessive use of network resources.
The host MAY change the weight of particular hints, to devalue them
if their accuracy has been poor, suggests invalid configurations, or
are suspicious (refer to Section 11).
It is notable, that such hysteresis may cause sub-optimal change
detection performance, and may themselves be used to block legitimate
hint reception. It is notable that mechanisms that cause hints to
not be acted upon may affect the timeliness of detection of network
attachment, in the case that they subsequently work correctly.
6.6 Hint Management for Inactive Hosts
If a host does not expect to send or receive packets soon, it MAY
choose to defer detection of network attachment. This may preserve
resources on latent hosts, by removing any need for packet
transmission when a hint is received.
These hosts SHOULD delay 0-1000ms before sending a solicitation, and
MAY choose to wait up to twice the advertised Router Advertisement
Interval (plus the random delay) before sending [5].
When deferring this signaling, the host therefore relies upon the
regular transmission of unsolicited advertisements for timely
detection of link change. It is notable that when hosts take this
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approach the effect of simultaneous configuration is limited to those
hosts actively polling for information.
When a device begins sending packets, it will be necessary to test
bidirectional reachability with the router (whose current Neighbor
Cache state is STALE). As described in [1], the host will delay
before probing to allow for the probability that upper layer packet
reception confirms reachability.
If no packet transmission on the wireless link has occurred, before
the new IP configuration is used for upper layer protocols, hosts MAY
choose not to delay the reachability probe to the router, if the
transmission time is unrelated to received multicast packets.
This is because the initial delay is unlikely to be synchronized with
other devices on this link, and timely liveness detection for the
configuration may then be required.
7. Validation of configuration
Detecting changes in IP configuration requires either knowledge
gathered from the network upon attachment using such methods as
Router Discovery, or that known from prior information.
The current foci of work in DNA Working Group are procedures
subsequent to attachment. Some procedures that describe how
information may be gathered prior to arrival are summarized below.
7.1 Making use of Prior Information
A device that has recently been attached to a particular wireless
base station may still have state regarding the IP configuration
valid for use on that link. This allows a host to begin any
configuration procedures before checking the ongoing validity and
security of the parameters. The experimental protocols FMIPv6 [17]
and CARD [18] each provide ways to generate such information using
network-layer signaling, before arrival on a link. These are
respectively described in Appendix C and Appendix D. Additionally,
the issue is the same when you disconnect from one L2 Access Point
and return to it immediately, or movement between a pair of access
points (the ping-pong effect).
Care must be taken when there is a chance of an error or change in
the configuration. Otherwise, if the assumptions due to the stored
configuration are incorrect the configuration cost may be increased,
or even harm to other devices.
Hosts MUST ensure that they will cause no harm to other devices due
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to the invalidity or staleness of their configuration.
In the case that the host arrives back on the same link in time less
than the DAD completion time (minus a packet transmission and
propagation time), the host MAY reclaim the address by sending
Neighbor Advertisement messages as if another host had attempted DAD
while the host was away. This will prevent DAD completion by another
node upon NA reception.
If a host has not been present on a link to defend its address, and
has been absent for a full DAD delay (minus transmission time) the
host MUST undertake the full DAD procedure for each address from that
link it wishes to configure [3].
7.2 Transient Link Availability
Wireless Internet hosts can experience connectivity changes that may
or may not be associated with IP configuration change.
While some wireless base-station transitions are almost
instantaneous, other cell change procedures take hundreds of
milliseconds. In the interval between disconnection at one cell and
attachment at another, packets sent by the host may be discarded or
delayed.
In some cases, upper layer data with addressing incorrect for the new
link may remain queued for transmission in the link-layer. The
subsequent queuing of signaling packets can affect timers, and
therefore the success of reachability confirmation procedures if DNA
procedures are aware of link-detachment and attachment. Also if
signaling packets are sent when the link is unavailable, the packet
may be discarded. This will lead to timer expiry in the case a
solicitation is sent.
If a host knows that connectivity has been lost at the link-layer, it
SHOULD pause transmission of upper-layer packets to the lower-layer,
until general data frames can be send on the new cell. This may help
to avoid packet loss or the queuing of signaling packets for change
detection behind data frames. A host SHOULD also stop sending
signaling for the purpose of DNA to a link-layer it has been reliably
informed is unavailable. It is suggested that implementers utilize
possible prioritization of the DNA signaling packets over other data
packets while queuing them to the link-layer.
7.3 Link Change and Router Discovery
Determining the identity of a link in IPv6 relies upon Router
Discovery. A link contains a set of mutually reachable interfaces on
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routers, and media connecting them between which there is no
forwarding hop.
Monitoring the link-local source address of the Router Advertisement
is insufficient to prove that a device is still on an IP link, since
a router may share a single link-local address across multiple
interfaces.
Therefore the identity of the link may be determined by monitoring
the set of routers and IPv6 prefixes advertised on the link. Any
router advertising one of the prefixes previously received within an
advertisement may be assumed to belong to the same link, if the new
advertisement was received within the Valid Lifetime of the prefix
[1].
Reception of Router Advertisements that do not contain any prefixes
in common with the previously advertised set MAY be an indicator that
link change has occurred. IPv6 Neighbor Discovery [1] explicitly
allows such configurations to exist though, and additionally allows
omission of prefix information options in unsolicited Router
Advertisements.
In order to determine validity of configuration in such topologies,
reachability testing MAY be required. Additionally, during reception
of unsolicited Router Advertisements some heuristic SHOULD be used,
where possible, to ensure that complete prefix information is
received by the host. This may limit the false detection of link
change due to omitted prefixes.
7.4 Validating configuration in Constrained Topologies
In environments where only one router may exist within an IP link,
changed router and prefix advertisement implies link change. A
mobile host implementation MAY use the knowledge of such environment,
for example when it is on a PPP link or on a base-station/router, to
infer link change every time a new prefix advertisement is received.
If a host has recently received a solicited Router Advertisement from
the configured router, it SHOULD see all prefixes configured on the
router's interface at the time [1]. Subsequent reception of a Router
Advertisement with a prefix not in the set MAY mean that the current
IP configuration is invalid, and addressing and routing configuration
procedures MAY be started.
Also, some networks enforce IP address changes when link-layer change
occurs. Devices that are aware of such procedures SHOULD start IP
configuration immediately on attachment to a new link-layer.
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7.5 Validating configuration in Arbitrary Topologies
Many multi-access LAN-like media in the Internet may be bridged into
arbitrary topologies on the wired network. For these environments,
many wireless cells may be on the same IP hop, and multiple routers
may be reachable from each cell.
While most wireless access networks today contain one advertising
router, hosts SHOULD NOT immediately assume that only one router is
on a link.
Importantly, a host SHOULD NOT change its configuration if a new
router advertises a prefix known to be used by another router on the
same IP link. For such cases, hosts SHOULD undertake reachability
testing with the previously configured router before updating their
routing configuration [1].
7.6 Dealing with Incomplete Information
When determining if network attachment has occurred, it may be
difficult to determine if a received multicast Router Advertisement
is in response to a solicitation, or not.
In this case it is also difficult for Router Advertisement receivers
to determine which solicitation caused transmission of the RA.
Timing issues before Router Advertisement responses and between
multicast advertisements may make it difficult to match Router
Solicitations and Advertisements using timing.
These issues are only relevant if Secured Neighbor Discovery (SEND)
is not in use, since SEND provides explicit copies of Nonces, which
allow response matching [7].
The delays between Router Advertisements on each router depend on
random delays and the recentness of advertisement by the routers.
Therefore, it is difficult for a device that is newly arrived on a
link to determine if more routers are present on a link when it
receives the first advertisement.
When the host still has to wait for other packet reception and
processing to complete (such as for the router's delegation chain
authorization [7]), it is useful to continue monitoring Router
Advertisement packets in case the next responding router is one
currently known to the host.
A host SHOULD accept a response from a previously known and
authorized router if it is received while awaiting completion of
authorization checks for a new router. Note that previously known
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but unsecured routers MUST NOT override routers whose authorization
is being assessed, as their advertisement may constitute a denial of
service attack.
7.7 Configuration Algorithms
Hosts that travel in wireless IPv6 networks of unknown topology must
determine appropriate procedures in order to minimize detection
latency or preserve energy resources. If a host is prepared to
accept any received Router Advertisement for configuring a default
router, then it will complete link change detection more quickly than
a device that explicitly checks for the current router's
unreachability.
This mechanism is called Eager Configuration Switching [14]. It may
cause unnecessary configuration operations, especially if unsolicited
advertisements from multiple routers on a link are received
containing disjoint sets of prefixes. In this case, a configuration
per distinct set will result [1].
Additionally, use of only unsolicited Router Advertisements may cause
detection or configuration of links where routers are unable to
receive packets from the host. Reachability testing SHOULD be done
in accordance with [1].
Another alternative, which reduces the problem associated with
disjoint prefixes, only allows eager switching after link-layer hint
indicating that a cell change has occurred. Although another router
on the link may respond faster than the currently configured default
router, it will not lead to erroneous detection if the router was
previously seen before the link-layer hint was processed.
An alternative mechanism is called Lazy Configuration Switching [14].
This algorithm checks that the currently configured router is
reachable before indicating configuration change. In this case, new
configuration will be delayed until a timeout occurs, if the
currently configured router is unreachable.
Since the duration of such a timeout will significantly extend the
duration to detect link change, this procedure is best used if the
cell change to link change ratio is very low.
For example:
If the expected time to:
Successfully test reachability (with NS/NA) is N
Test unreachability with a timeout is T
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Receive a Router Advertisement is R
Reconfigure the host is C
The probability of link change is
p = (# of L3 links)/(# of L2 Point of attachment)
The probability of received RA being from a router different from the
current access router is
p1 = (sum of all (nr - 1)/ NR)
Where nr is the number of routers in each L3 link and NR is the total
number of routers.
Note that if you don't have multiple routers in the same L3 link,
then all the (nr - 1) will be zero leading to
p1 = 0
Then the mean cost of Eager Configuration switching is:
Cost(ECS)= R + ( (p + p1) * C )
And the cost of Lazy switching is:
Cost(LCS)= (T + R + C) * p + (1 - p) * N
The mean cost due to Lazy Configuration Switching is lower than that
of Eager Configuration Switching if:
( T + R + C ) * p + (1 - p) * N < R + C * (p + p1)
Using the indicative figures:
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N=100ms
T=1000ms
R=250ms
C=100ms
The values for p where LCS is better than ECS are:
p * (1000 + 250 + 100)ms + < 250ms +
(1 - p)*100ms (p + p1)*100ms
1350ms * p + 100ms - 100ms * p < 250ms + 100 * (p + p1)
1250ms * p - 100 * (p + p1) < 150ms
when p1 = 0
1150 ms * p < 150
p < 150/1150
p < 0.131
p ~< 0.125 (=0.130)
when p1 = 20%
1250 * p - 100 * p - 20 < 150
1150 * p < 170
p < 170/1150
p < 0.147
For these parameters, the Lazy Configuration Switching has better
performance if the mean number of cells a device resides in before it
has a link change is > 8.
It may be noted that these costs are indicative of a system which
requires a retransmission timeout of 1000ms to test unreachability,
routers respond with unicast Router Advertisements, and DAD is
neglected or has only 100ms of cost.
If the reconfiguration cost is C=1000ms you will have
2150 * p - 1000 * (p + p1) < 150
if p1 = 20 %
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1150 * p - 200 < 150
1150 * p < 350
p < 0.304
For these parameters, the Lazy Configuration Switching has better
performance if the mean number of cells a device resides in before it
has a link change is between 3 & 4.
This system describes a similar one to that above, except that in
this case, the delays due to DAD or configuration are the default
value of 1000ms.
8. Reachability Detection
The three different methods suggested for validating current
configuration have implied reachability confirmations in the messages
exchanged. This section identifies the reachability implications
from the hosts' perspective for the four different message exchanges
that are possible. The table below presents this implication for the
two directions involved, upstream referring to the direction from the
host to the router and downstream from the router to the host. The
host can confirm bi-directional reachability from any of the three
message exchanges except when a multicast RA is received at the host
for its RS message. In this case, the host cannot know whether the
multicast RA is in response to its solicitation message or whether it
is a periodic un-solicited transmission from the router.
+-----------------+----+----+----+-----+
| Exchanges: |Upstream |Downstream|
+-----------------+----+----+----+-----+
| multicast NS/NA | Y | Y |
+-----------------+----+----+----+-----+
| unicast NS/NA | Y | Y |
+-----------------+----+----+----+-----+
| RS/multicast RA | | Y |
+-----------------+----+----+----+-----+
| RS/unicast RA | Y | Y |
+-----------------+----+----+----+-----+
Successful exchange of the messages listed in the table indicate the
corresponding links to be operational. Lack of reception of response
from the router may indicate that reachability is broken for one or
both of the transmission directions or it may indicate ordinary
packet loss event in either direction.
If bi-directional reachability can not be confirmed using one of the
three message exchanges, the host SHOULD attempt to find a better
connection with possibly another router in the link. Because of the
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transient nature of the wireless link, the reachability may change
during communication after reachability is verified. If multiple
routers were available, the host MAY defer test of reachability until
the interface is to be actively used for transmission and reception.
Hosts should also be aware of particular issues regarding their own
wireless access technology which impinge on the reliability of
reachability tests. Particularly, where unicast and multicast
propagation behaviors are significantly different, hosts SHOULD
attempt to test both multicast and unicast reachability, to ensure
that each works. Hosts MAY defer such additional tests where either
communications method is not likely to be used soon.
8.1 Wireless propagation
Wireless channel characteristics change both in space and time. Even
when a wireless host is not moving, its connectivity to the access
router can change due to factors like interference from other
objects, temperature etc. When the host is moving, the changes can
be more pronounced because of change in distance, introduction of new
objects in the LOS (line of sight) etc. The change to the
connectivity may affect both directions or it can be only in either
one of the directions. Hence, in wireless links, reachability in one
direction does not guarantee reachability in the other. Also, these
variations in the wireless channel can be very short lived, creating
rapid hints about the status of the link-layer. It is important to
consider the transient nature of the wireless links in design the DNA
mechanism for such channels.
8.2 Asymmetric Reachability
As mentioned in the previous section, wireless channels can provide
asymmetric reachability that requires reachability testing on both
directions. Even previously verified bi-directional reachability can
not guaranteed at a later time if there are other (higher layer or
link-layer) hints implying the loss of bi-directional reachability.
The frequency of initiation of reachability testing MUST be
controlled in order to avoid flooding of the network. Implementers
are advised to build in rate-limiting mechanisms to responding to the
hints to avoid switching IP configuration frequently when the quality
of the wireless channel is fluctuating (see Section 6.5).
8.3 Specific (Neighbor Solicitation) Tests
Bi-directional reachability can be verified using the Neighbor
Discovery test to the current access router. Since ND involves the
use of two unicast messages exchanged between the host and the access
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router, a successful ND procedure verifies bi-directional
reachability.
8.4 Non-Specific (Router Solicitation) Tests
Initiating Router Discovery procedure can sometimes lead to
verification of the bi-directional reachability. It does not always
confirm bi-directional reachability because if the router responds
for the RS with a multicast RA message, there is no way for the host
to identify whether the RA is in response to the RS or whether it is
a periodic RA transmission. Even with multicast RA response, if SEND
is used, bi-directional reachability can confirmed because SEND uses
a unique NONCE to match request and response messages [7]. If the
router chooses to respond to a RS with a unicast RA message, again,
the host will be able to match the RS and RA and hence confirm
bi-directional reachability.
8.5 Trade-offs in Reachability Testing
There unique advantages and dis-advantages in using either the
Specific or the Non-specific test to confirm reachability.
Specific tests:
Advantages:
Confirmation of bi-directional reachability.
Dis-advantages:
If the ND test fails, the host has no potential configuration
information it can use.
Non-Specific tests:
Advantages:
Even when the current access router is not reachable, an RA message
from any available router will provide information that can used to
configure the host.
Confirmation of bi-directional reachability if SEND is used or if the
router chooses to respond with an unicast RA message.
Dis-advantages:
If multicast RA response is received, the host may have to execute an
ND test to confirm bi-directional reachability. Such a test may be
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avoided if upper layer confirmations are received within the DELAY
period prescribed by IPv6 Neighbor Discovery [1].
Even when the current access router is reachable, the response may
arrive from a different access router leading to erroneous
re-configuration of the host.
9. Complications to Detecting Network Attachment
Detection of network attachment procedures can be delayed or may be
incorrect due to different factors. As the reachability testing
mainly relies on timeout, packet loss or different router
configurations may lead to erroneous conclusions. This section gives
some examples where complications may interfere with DNA processing.
9.1 Tentative Addressing
When a host connects to a new link, it needs to create a link-local
address. But as the link-local address must be unique on a link,
Duplication Address Detection (DAD) must be performed [3] by sending
NS to the target link-local address. An address that is being
validated is said to be a tentative address. The host that only has
a tentative address must not accept packets intended to this
destination, neither may they send packets with it. If the host does
not get any reply to its DAD Neighbor Solicitations, the tentative
link-local address can be allocated to the interface of the host.
From that point, the link-local address can be used and the prefix
and router discovery can then take place.
Several NS's can be sent to perform DAD on a tentative link local
address. However, the default number of transmissions of Neighbor
Solicitations is 1. If an NA is not received within one second after
the NS transmission, the tentative address is considered as unique.
However, if the NS or NA are lost for some reason, the tentative
address will be considered as unique while another node might have
the same address. Notably though, each additional transmission of an
NS introduces a delay of one second in the configuration
establishment, which has an important impact on IP configuration
latency.
While hosts performing DNA do not know if they have arrived on a new
link, they SHOULD treat their addresses as if they were. This means
that link-local addresses SHOULD be treated as tentative, and
globally unique addresses SHOULD NOT be used in a way which creates
neighbor cache state on their peers, while DNA procedures are
underway. The different treatment of IP addressing comes from the
fact that on the global addresses cannot have an address conflict if
they move to a topologically incorrect network where link-local
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addresses may. Even though global addresses will not collide, the
incorrect creation of neighbor cache entries on legacy peers may
cause them some harm.
9.2 Packet Loss
Generally, packet loss while a host is validating or discovering an
IP configuration introduces significant delays. Because most of the
protocols rely on timeout to consider that a peer is not reachable
anymore, packet loss may lead to erroneous conclusions. Assume a
wireless host that connects to a new access point attached to the
same IPv6 subnet. The connexion establishment with the new access
point generates a link-layer hint trigger which tells the mobile node
that it may have moved to a new IPv6 link. Upon the reception of
this trigger, the mobile node may want to check reachability with its
current router and sends a NS. If the NS or the NA in response is
lost, the mobile node could conclude that it has changed link and
that its current default router is unreachable.
9.3 Router Configurations
Each router can have its own configuration with respect to sending
RA, and the treatment of router and neighbor solicitations.
Different timers and constants might be used by different routers,
such as the delay between Router Advertisements or delay before
replying to an RS. If a host is changing is IPv6 link, new router on
that link may have a different configuration and may introduce more
delay than the previous default router of the host. The time needed
to discover the new link can then be longer than expected by the
host.
9.4 Overlapping Coverage
If a host can be attached to different links at the same time with
the same interface, the host will probably listen to different
routers, at least one on each link. To be simultaneously attached to
several links may be very valuable for a MN when it moves from one
access network to another. If the node can still be reachable
through its old link while configuring the interface for its new
link, packet loss can be minimized. Such a situation may happen in a
wireless environment if the link layer technology allows the MN to be
simultaneously attached to several points of attachment and if the
coverage area of access points are overlapping. For the DNA purpose,
the different links should not be classified as a unique link.
Because if one router or an entire link where the node is attached
comes down, it doesn't mean that the other link are also down. In
particular, the routers on the other links might still be reachable
for the host.
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9.5 Multicast Snooping
When a host is participating in DNA on a link where multicast
snooping is in use, multicast packets may not be delivered to the
LAN-segment to which the host is attached until MLD signaling has
been performed [8][10][15]. Where DNA relies upon multicast packet
delivery (for example, if a router needs to send a Neighbor
Solicitation to the host), its function will be degraded until after
an MLD report is sent.
Where it is possible that multicast snooping is in operation, hosts
should consider sending MLD group joins (MLD reports) for solicited
nodes' addresses swiftly after starting DNA procedures.
10. Current Practice for Routers supporting DNA
This section provides guidance for those routers which wish to
support hosts undertaking detection of network attachment using
existing router and neighbor discovery techniques.
It should be noted that many deployed routers will not support these
recommendations, and that hosts SHOULD NOT rely on their being in
place, unless they have particular reason to do so.
10.1 Router Advertisement Intervals
The router discovery mechanism defined in RFC 2461 [1] recommends a
set of interval timers that affect the performance of the router
discovery procedure. The following table summarizes the values and
their effect.
+-----------------------+----+----+----+-----+----+-----+
| Timer |Maximum |Default |Minimum |
+-----------------------+----+----+----+-----+----+-----+
| MaxRtrAdvInterval | 1800 | 600 | 4 |
+-----------------------+----+----+----+-----+----+-----+
| MinRtrAdvInterval | 594 | 198 | 3 |
+-----------------------+----+----+----+-----+----+-----+
|Avg. Multicast RA time | 1197 | 399 | 3.5 |
+-----------------------+----+----+----+-----+----+-----+
|RA MCast response time | 3.5 | NA | 0 |
+-----------------------+----+----+----+-----+----+-----+
|RA Ucast response time | 0.5 | NA | 0 |
+-----------------------+----+----+----+-----+----+-----+
Assuming Poisson arrival of router solicitation messages at the rate
of 0.05 messages per second, the average multicast RA response time
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can be calculated as follows
Probability of at least one arrival in MIN_DELAY_BETWEEN_RAS time is:
(p) = 1 - exp (lambda * t) (i.e. 1 - P[X(t) == 0])
where lambda is the arrival rate of RS messages and t is
MIN_DELAY_BETWEEN_RAS
Given a Poisson occurrences in an interval, these occurrences are
uniformly located in that interval. Hence the delay introduced by
arrival in MIN_DELAY_BETWEEN_RAS time is p * t/2. Adding 0.250 for
the random delay introduced, the average multicast RA response time
is 0.458938 seconds.
The average unicast RA response time of course is 0.250 seconds.
The table gets modified as follows based on the recommendation of RFC
3775 [5]
+-----------------------+----+----+----+-----+----+-----+
| Timer |Maximum |Default |Minimum |
+-----------------------+----+----+----+-----+----+-----+
| MaxRtrAdvInterval | 1800 | 600 | 0.07 |
+-----------------------+----+----+----+-----+----+-----+
| MinRtrAdvInterval | 594 | 198 | 0.03 |
+-----------------------+----+----+----+-----+----+-----+
|Avg. Multicast RA time | 1197 | 399 | 0.05 |
+-----------------------+----+----+----+-----+----+-----+
|RA MCast response time | 0.5 | NA | 0 |
+-----------------------+----+----+----+-----+----+-----+
|RA UCast response time | 0.5 | NA | 0 |
+-----------------------+----+----+----+-----+----+-----+
Assuming Poisson arrival of router solicitation messages at the rate
of 0.05 messages per second, the average multicast RA response time =
0.250022 seconds. The average unicast RA response time is the same
0.250 seconds. Changing the minimum values for these protocol
constants as recommended by RFC 3775 [5] reduces the average time
delay introduced by both solicited and un-solicited Router
Advertisements. But, adopting these values for the unsolicited
Router Advertisements will increase the bandwidth overhead for these
RA messages. With the minimum allowed average for un-solicited
Router Advertisements there would be 20 message per second, assuming
the minimum packet size for an RA with one prefix as 88 bytes, the
bandwidth used the RAs alone will be 14 kbps. If SEND packets are
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used for these Router Advertisements, with the keys and certificates
the average packet size and hence the bandwidth used will increase
dramatically.
10.2 Unicast Response Transmission
The IPv6 Neighbor Discovery RFC allows the routers to respond to RS
message with a unicast RA, but does not mandate it [1]. The
advantage in responding with an unicast RA message is to allow the IP
host to infer bi-directional reachability from the RS-RA exchange.
The dis-advantage is that the router will not be able to aggregate
its response for multiple RS messages received during the wait
period.
It is important to note that the MIN_DELAY_BETWEEN_RAS restriction
required by the Neighbor Discovery RFC is applicable only to
multicast RAs [1]. Routers MAY choose to respond to RS messages with
a unicast RA response to reduce the response delay if it sent a
multicast RA within the last MIN_DELAY_BETWEEN_RAS seconds. This
difference may not be very high if the value (0.03) suggested in
Mobile IPv6 is implemented [5].
10.3 Point-to-Point Links
IPv6 Neighbor Discovery RFC mandates the delay of RA responses by
stating (in section 6.2.6 of [1]):
"In all cases, Router Advertisements sent in response to a
Router Solicitation MUST be delayed by a random time
between 0 and MAX_RA_DELAY_TIME seconds."
Cases where the router is on a point-to-point link, this restriction
is too stringent as the router in question will be the only router on
the link. Routers on such point-to-point links MAY avoid the delay
by not waiting for the prescribed random time before responding for
the Router Solicitation message.
10.4 Prefix Advertisement
A router MAY choose to split the options in the RA and send multiple
RAs to reduce bandwidth overhead or to reduce the size of the RA to
below the link MTU (see section 6.2.3 of [1]).
If such a choice is made, average multicast RA time discussed in
Section 10.1 increases for each subset of the prefixes included in
the split RA messages. Routers SHOULD consistently include one of
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the prefixes in each of its RA messages to provide the host with a
unique identifier based on the combination of link-local address and
the constant prefix, to identify the router every time a RA message
is received from the router.
TBD: Checking for consistency?
10.5 Secured Neighbor Discovery
Routers supporting DNA SHOULD provide secured router discovery
services [7].
This requires not only digitally signed messaging from hosts to
provide immutability of neighbour discovery messages on the access
link, but delegated authority from the network infrastructure to
route particular prefixes.
This delegation chain is transmitted and received in Delegation Chain
Solicitation and Delegation Chain Advertisement Messages. These
messages are rate limited in a similar fashion to Router
Advertisements and RS/RA exchanges [7].
The certificates can be used by hosts to identify routers and their
owner organizations, as well as to determine their authorization to
route for the advertised prefixes.
11. Security Considerations
Detecting Network Attachment is a mechanism which allows network
messages to change the node's belief about its configuration state.
As such, it is important that the need for rapid testing of link
change does not lead to situations where configuration is invalidated
by malicious third parties, nor that information passed to
configuration processes exposes the host to attack.
Since DNA relies heavily upon IPv6 Neighbor Discovery, the threats
which are applicable to those procedures also affect Detecting
Network Attachment. These threats are described in [11].
Some additional threats are outlined below.
11.1 Replay or impersonation of messages.
If a host receives a series of messages which authenticate the
transmission of Router Advertisements, but aren't sent in response to
the host's own solicitation, there is no guarantee that the router is
actually present on the link.
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If an attacking device can replay advertisements elsewhere, such a
router may be accepted based on the freshness of its timestamps in
accordance with [7], until the host attempts bidirectional
reachability tests with the false router.
Where the bidirectional reachability attempts may be replayed by the
attacker between a pair of links, a device with the ability to forge
link-layer addresses may be able to fool both the router and host to
believing they are directly adjacent.
Additionally, when moving between several different networks, timing
state as perceived by routers and hosts may become mismatched. In
this case it may be possible to allow a wider window of attacks by
determining if a host has a delayed clock, and replaying signaling
messages.
Additionally, if hosts' clocks can be made to slow down or speed up,
secured neighbor discovery messages can be forced outside the valid
window, and prevent correctly configured SEND messages from being
processed. In this case, it may be possible to bid down the host to
choose an unsecured Router Advertisement that doesn't perform SEND to
a valid router that does.
11.2 Authorization and Detecting Network Attachment
Hosts connecting to the Internet over wireless media may be exposed
to a variety of network configurations with differing robustness,
controls and security.
When a host is determining if link change has occurred, it may
receive messages from devices with no advertised security mechanisms
purporting to be routers, nodes sending signed router advertisements
but with unknown delegation, or routers whose credentials need to be
checked [11]. Where a host wishes to configure an unsecured router,
it SHOULD at least confirm bidirectional reachability with the
device, and it MUST mark the device as unsecured as described in [7].
In any case, a secured router SHOULD be preferred over an unsecured
one, except where other factors (unreachability) make the router
unsuitable. Since secured routers' advertisement services may be
subject to attack, alternative (secured) reachability mechanisms from
upper layers, or secured reachability of other devices known to be on
the same link may be used to check reachability in the first
instance.
11.3 Addressing
While a DNA host is checking attachment, and observing DAD, it may
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receive a DAD defense NA from an unsecured source.
SEND says that DAD defenses MAY be accepted even from non SEND nodes
for the first configured address [7].
While this is a valid action in the case where a host collides with
another address owner after arrival on a new link, In the case that
the host returns immediately to the same link, such a DAD defense NA
message can only be a denial-of-service attempt.
If a non-SEND node forges a DAD defense for an address which is still
in peers' neighbor cache entries, a host may send a SEND protected
unicast neighbor solicitation without a source link-layer address
option to one its peers (which also uses SEND). If this peer is
reachable, it will not have registered the non-SEND DAD defense NA in
its neighbor cache, and will send a direct NA back to the soliciting
host. Such an NA reception disproves the DAD defense NA's validity.
Therefore, a SEND host performing DNA which receives a DAD defense
from a non-SEND node SHOULD send a unicast Neighbor Solicitation to a
STALE or REACHABLE secured neighbor cache entry, omitting source
link-layer address options. In this case, the host should pick an
entry which is likely to have a corresponding entry on the peer. If
the host responds within a RetransTimer interval, then the DAD
defense was an attack, and the host SHOULD inform its systems
administrator without disabling the address.
12. Acknowledgments
JinHyeock Choi has done lots of work regarding inference of link
identity through sets of prefixes. Bernard Aboba's work on DNA for
IPv4 significantly influenced this document.
13. References
13.1 Normative References
[1] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC2462 2462, December 1998.
[4] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
December 1998.
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[5] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[6] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC2463 2463, December 1998.
[7] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P. Nikander,
"SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-05
(work in progress), April 2004.
13.2 Informative References
[8] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[9] Haberman, B., "Source Address Selection for the Multicast
Listener Discovery (MLD) Protocol", RFC 3590, September 2003.
[10] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
(MLDv2) for IPv6", RFC 3810, June 2004.
[11] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[12] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[13] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[14] Fikouras, N A., K"onsgen, A J. and C. G"org, "Accelerating
Mobile IP Hand-offs through Link-layer Information",
Proceedings of the International Multiconference on
Measurement, Modelling, and Evaluation of
Computer-Communication Systems (MMB) 2001, September 2001.
[15] Christensen, M., Kimball, K. and F. Solensky, "Considerations
for IGMP and MLD Snooping Switches", draft-ietf-magma-snoop-11
(work in progress), May 2004.
[16] Kniveton, T J. and B C. Pentland, "Session minutes of the
Detecting Network Attachment (DNA) BoF", Proceedings of the
fifty-seventh Internet Engineering Task Force Meeting IETF57,
July 2003.
[17] Koodli, R., "Fast Handovers for Mobile IPv6",
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draft-ietf-mipshop-fast-mipv6-01 (work in progress), February
2004.
[18] Liebsch, M., "Candidate Access Router Discovery",
draft-ietf-seamoby-card-protocol-06 (work in progress), January
2004.
[19] O'Hara, B. and G. Ennis, "Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE Std
802.11, 1999.
[20] Yamamoto, S., "Detecting Network Attachment Terminology",
draft-yamamoto-dna-term-00 (work in progress), February 2004.
[21] Manner, J. and M. Kojo, "Mobility Related Terminology",
draft-ietf-seamoby-mobility-terminology-06 (work in progress),
February 2004.
Authors' Addresses
Sathya Narayanan
Panasonic Digital Networking Lab
Two Research Way, 3rd Floor
Princeton, NJ 08536
USA
Phone: 609 734 7599
EMail: sathya@Research.Panasonic.COM
URI:
Greg Daley
Centre for Telecommunications and Information Engineering
Department of Electrical adn Computer Systems Engineering
Monash University
Clayton, Victoria 3800
Australia
Phone: +61 3 9905 4655
EMail: greg.daley@eng.monash.edu.au
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Nicolas Montavont
LSIIT - Univerity Louis Pasteur
Pole API, bureau C444
Boulevard Sebastien Brant
Illkirch 67400
FRANCE
Phone: (33) 3 90 24 45 87
EMail: montavont@dpt-info.u-strasbg.fr
URI: http://www-r2.u-strasbg.fr/~montavont/
Appendix A. Summary of Recommendations
The signaling which causes a hint may be due to network-layer
messages such as unexpected Router Advertisements, multicast listener
queries or ICMPv6 error messages [1][8][10][6]. In these cases,
caution must be exerted.
Hosts MUST ensure that untrusted messages do not cause unnecessary
configuration changes, or significant consumption of host resources
or bandwidth.
Care must be taken when there is a chance of an error or change in
the configuration. Otherwise, if the assumptions due to the stored
configuration are incorrect the configuration cost may be increased,
or even harm to other devices.
Hosts MUST ensure that they will cause no harm to other devices due
to the invalidity or staleness of their configuration.
If a host has not been present on a link to defend its address, and
has been absent for a full DAD delay (minus transmission time) the
host MUST undertake the full DAD procedure for each address from that
link it wishes to configure [3].
A host SHOULD accept a response from a previously known and
authorized router if it is received while awaiting completion of
authorization checks for a new router. Note that previously known
but unsecured routers MUST NOT override routers whose authorization
is being assessed, as their advertisement may constitute a denial of
service attack.
Hosts that travel in wireless IPv6 networks of unknown topology must
determine appropriate procedures in order to minimize detection
latency or preserve energy resources.
Where a host wishes to configure an unsecured router, it SHOULD at
least confirm bidirectional reachability with the device, and it MUST
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mark the device as unsecured [7].
Hints SHOULD NOT be considered to be authoritative for detecting IP
configuration change by themselves.
Therefore, hosts SHOULD NOT change their configuration state based on
hints from other protocol layers.
While hints from other protocol layers originate from within the
host's own stack, the network layer SHOULD NOT treat hints from other
protocol layers as authoritative indications of link change.
Hosts SHOULD NOT start DNA procedures simply because a data link is
idle, in accordance with [1]. Hosts MAY act on hints associated with
non-reception of expected signaling or data.
Since DNA is likely to be used when communicating with devices over
wireless links, suitable resilience to packet loss SHOULD be
incorporated into either the hinting mechanism, or the DNA initiation
system.
Hosts SHOULD NOT act immediately on packet loss indications, delaying
until it is clear that the packet losses aren't caused by transient
events.
It is possible that hints arrive synchronously on multiple hosts at
the same time. As described in [1][3], a host SHOULD delay randomly
before acting on a widely receivable advertisement, in order to avoid
response implosion.
Where a host considers it may be on a new link and learns this from a
hint generated by a multicast message, the host SHOULD defer 0-1000ms
in accordance with [1].
When experiencing a large number of hints, a host SHOULD employ
hysteresis techniques to prevent excessive use of network resources.
The host MAY change the weight of particular hints, to devalue them
if their accuracy has been poor, or suggests invalid configurations.
These (inactive) hosts SHOULD delay 0-1000ms before sending a
solicitation, and MAY choose to wait up to twice the advertised
Router Advertisement Interval (plus the random delay) before sending
[5].
A host MAY choose to adopt an appropriate link change detection
strategy based upon hints received from other layers, with suitable
caution and hysteresis.
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If a host knows that connectivity has been lost at the link-layer, it
SHOULD pause transmission of upper-layer packets to the lower-layer,
until general data frames can be send on the new cell.
A host SHOULD also stop sending signaling for the purpose of DNA to a
link-layer it has been reliably informed is unavailable.
In order to determine validity of configuration in such topologies,
reachability testing MAY be required. Additionally, reception of
solicited Router Advertisements some heuristic SHOULD be used, where
possible, to ensure that complete prefix information is received by
the host. This may limit the false detection of link change due to
omitted prefixes.
If a host has recently received a solicited Router Advertisement from
the configured router, it SHOULD see all prefixes configured on the
router's interface at the time [1]. Subsequent reception of a Router
Advertisement with a prefix not in the set means that the current IP
configuration is invalid, and addressing and routing configuration
procedures SHOULD be started.
Also, some networks enforce IP address changes when link-layer change
occurs. Devices that are aware of such procedures SHOULD start IP
configuration immediately on attachment to a new link-layer.
While most wireless access networks today contain one advertising
router, hosts SHOULD NOT immediately assume that only one router is
on a link.
Importantly, a host SHOULD NOT change its configuration if a new
router advertises a prefix known to be used by another router on the
same IP link. For such cases, hosts SHOULD undertake reachability
testing with the previously configured router before updating their
routing configuration [1].
Additionally, use of only unsolicited Router Advertisements may cause
detection or configuration of links where routers are unable to
receive packets from the host. Reachability testing SHOULD be done
in accordance with [1].
In any case, a secured router SHOULD be preferred over an unsecured
one, except where other factors (unreachability) make the router
unsuitable.
When using the passive method, absence of Router Advertisements (RA)
from the current default router MAY require verification and
acquisition of configuration using one of the active mechanisms
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Hints MAY be used to update a wireless host's timers or probing
behavior in such a way as to assist detection of network attachment.
A host MAY choose to adopt an appropriate link change detection
strategy based upon hints received from other layers, with suitable
caution and hysteresis.
Hosts MAY act on hints associated with non-reception of expected
signaling or data.
If a host does not expect to send or receive packets soon, it MAY
choose to defer detection of network attachment.
If no packet transmission on the wireless link has occurred, before
the new IP configuration is used for upper layer protocols, hosts MAY
choose not to delay the reachability probe to the router, if the
transmission time is unrelated to received multicast packets.
In the case that the host arrives back on the same link in time less
than the DAD completion time (minus a packet transmission and
propagation time), the host MAY reclaim the address by sending
Neighbor Advertisement messages as if another host had attempted DAD
while the host was away. This will prevent DAD completion by another
node upon NA reception.
Reception of Router Advertisements that do not contain any prefixes
in common with the previously advertised set MAY be an indicator that
link change has occurred. IPv6 Neighbor Discovery [1] explicitly
allows such configurations to exist though, and additionally allows
omission of prefix information options in unsolicited Router
Advertisements. In order to determine validity of configuration in
such topologies, reachability testing MAY be required.
Appendix B. Example State Transition Diagram
TBD
Appendix C. DNA With Fast Handovers for Mobile IPv6
TBD
Appendix D. DNA with Candidate Access Router Discovery
TBD
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Narayanan, et al. Expires December 21, 2004 [Page 44]