DNA Working Group S. Narayanan
Internet-Draft Panasonic
Expires: December 25, 2005 G. Daley
Monash University CTIE
N. Montavont
LSIIT - ULP
June 23, 2005
Detecting Network Attachment in IPv6 - Best Current Practices for
Routers
draft-ietf-dna-routers-00.txt
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Abstract
Hosts experiencing rapid link-layer changes may require to do
configuration change detection procedures more frequently than
traditional fixed hosts. This document describes practices available
to network deployers in order to support such hosts in Detecting
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Network Attachment in IPv6 networks.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terms and Abbreviations . . . . . . . . . . . . . . . . . 3
1.2 Relevant Host Issues . . . . . . . . . . . . . . . . . . . 4
1.3 Relevant Router Issues . . . . . . . . . . . . . . . . . . 4
1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Configuration Practices for DNAv6 Routers . . . . . . . . . . 5
2.1 Multicast and Unicast RA Responses . . . . . . . . . . . . 5
2.1.1 Recommendations . . . . . . . . . . . . . . . . . . . 7
2.2 Router Advertisement Parameters . . . . . . . . . . . . . 7
2.2.1 Recommendations . . . . . . . . . . . . . . . . . . . 8
2.3 Router Advertisement Options . . . . . . . . . . . . . . . 8
2.3.1 Recommendations . . . . . . . . . . . . . . . . . . . 9
2.4 Triggered Router Advertisements . . . . . . . . . . . . . 9
2.5 Split Advertisements . . . . . . . . . . . . . . . . . . . 9
2.6 Router Configurations . . . . . . . . . . . . . . . . . . 10
3. Topological Practices for DNAv6 Networks . . . . . . . . . . . 10
3.1 Link Extent and Composition . . . . . . . . . . . . . . . 10
3.2 Wireless cell coverage . . . . . . . . . . . . . . . . . . 10
3.3 Multiple Router Links . . . . . . . . . . . . . . . . . . 11
3.4 Point-to-point Links . . . . . . . . . . . . . . . . . . . 12
3.5 Disjoint Administration . . . . . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
4.1 Supporting host security . . . . . . . . . . . . . . . . . 12
4.2 Providing Router Authorization . . . . . . . . . . . . . . 13
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1 Normative References . . . . . . . . . . . . . . . . . . . 14
5.2 Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
A. Summary of Recommendations . . . . . . . . . . . . . . . . . . 16
B. Analysis of the response time . . . . . . . . . . . . . . . . 18
C. Router Advertisement Rates . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . 22
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1. Introduction
Hosts on the Internet may be connected by various media. It has
become common that hosts have access through wireless media and are
mobile. 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.
Such hosts need to determine if they have moved to a new IPv6 link
rapidly, in order that configuration procedures may be run and
application packet delivery services restored. Detecting Network
Attachment (DNA) is a strategy to assist such configuration changes
by rapidly determining whether they are required.
Several network-side factors may impact on the effectiveness and
speed of DNA procedures. This document provides guidelines embodying
the best current practice for network deployers wishing to support
detection of network attachment by IPv6 hosts.
It should be noted that many already 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.
1.1 Terms and Abbreviations
Access network: A network where hosts are present. Especially, a
network used for the support of visiting wireless hosts.
Link: A link is the range across which communications can pass
without being forwarded through a router [1].
Link-Change: Link-Change occurs when a host moves from a point-of-
attachment on a link, to another point-of-attachment where it is
unable to reach devices belonging to the previous link, without
being forwarded through a router.
Point-of-Attachment: A link-layer base-station, VLAN or port through
which a device attempts to reach the network. Changes to a
host's point-of-attachment may cause link-change.
Reachability Detection: Determination that a device (such as a
router) is currently reachable, over both a wireless medium, and
any attached fixed network. This is typically achieved using
Neighbor Unreachability Detection procedure [1].
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Wireless Medium: A physical layer which incorporates free space
electromagnetic or optical propagation. Such media are
susceptible to mobility and interference effects, potentially
resulting in high packet loss probabilities.
1.2 Relevant Host Issues
Hosts attempting to discover link change are likely to send Router
Solicitations (RSs) in order to identify the routers and prefixes
available on a link. Additionally, they may wish to send Neighbour
Solicitations (NSs) to known routers for reachability detection
purposes.
The following is a list of critical issues for hosts undertaking link
change detection in IPv6:
Hosts require Router Advertisements (RAs) rapidly in order to
minimize reconfiguration latencies in the case of link change or
link failure.
Host wishes to identify if their current prefix is still valid on
a link before the prefix expires. Existing IPv6 Neighbour
Discovery procedures make this difficult. If the host can
determine that the target router is still reachable through a
NS/NA exchange, it does not mean that the prefix is still valid.
Conversely, if host sends an RS, the RA received in response may
not contain the prefix of interest for the hosts.
Hosts wish to detect if a particular router is reachable in order
to use it for routing.
Hosts may require some assurance that a device is actually
present, and is authorized to act as a router.
Consideration for these issues underlie the practices recommended in
this document.
1.3 Relevant Router Issues
The IPv6 Neighbour Discovery RFC [1] provides mechanisms where
hosts can send Router Solicitations and receive Router
Advertisements, from each of the routers on a link.
Responses may either be unicast or multicast, but in all cases, a
random delay of between 0 and 500 milliseconds is required before
responses are sent. This is to prevent multiple routers
responding at the same time, and also may mitigate the effects of
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simultaneous solicitations. This results in a basic time delay
incurred by hosts receiving response RAs, which cannot be avoided
within current standards [1].
As described in Section 2.1, additional delays may occur if
multicast responses are required.
Routers should also be careful not to increase the network
overhead by frequently transmitting router advertisements (see
Section 2.4).
Routers should include all advertised prefixes within the same RA
and indicate whether a previous advertised prefix is not valid
anymore. Multiple prefixes advertised in different RAs by a
single router may lead to host configurations errors.
1.4 Summary
The practices embodied in this document are considered to provide
minimal support for hosts wishing to detect network attachment.
Current work within the DNA working group aims to provide
substantially improved performance for link change detection.
Existing limitations in base protocols such as IPv6 Neighbour
Discovery preclude support of real-time applications in some
environments. Future deployers and implementers are encouraged to
consider the protocols under development at this time in order to
provide a generic service to support hosts detecting change.
2. Configuration Practices for DNAv6 Routers
Routers which are being deployed to support hosts' change detection
procedures should attempt to use appropriate configurations, which
limit advertisement latency, and provide appropriate service
considering the constraints of the deployed access network
technology.
This section describes several configuration parameters which may
exist on IPv6 routers, and how their tuning may affect DNA hosts.
2.1 Multicast and Unicast RA Responses
While IPv6 Neighbour Discovery assumes that responses to
solicitations will be sent multicast, the specification allows any
router to respond to RS message with a unicast RA if it desires [1].
The advantage in responding with an unicast RA message is to allow
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the IP host to conclusively infer bi-directional reachability from
the RS-RA exchange. Neighbour Discovery does not provide any
mechanism to match multicast RA responses with their solicitation,
and therefore it is not possible for the hosts to find out whether at
least one of its RS messages was received and processed by the
router. Since unicast RAs are only sent in response to solicitation,
a host can infer that at least one of its Router Solicitations
reached the router.
The dis-advantage in sending unicast RA is that the router will not
be able to aggregate its response for multiple RS messages from
multiple hosts received during the waiting period before RA
transmission.
Where many unicast responses are scheduled awaiting transmission,
Routers MAY consider aggregating them into a single multicast
response if a multicast advertisement may be sent before the
advertisements' scheduled transmission time.
It is noted that computational requirements for SEND may preclude
this subsequent aggregation in some environments.
Where multiple unicast transmissions for the same destination await
transmission, routers MAY remove all transmissions after the first
without ill-effect, if a multicast RA is scheduled for the next
possible response time.
For multicast Router Advertisements, a minimum separating delay
exists so that these RAs may not be scheduled close to each other.
When a host solicits and attempts to schedule a multicast RA within
MIN_DELAY_BETWEEN_RAS (or MinDelayBetweenRAs from Mobile IPv6 [3]) of
the previous multicast Router Advertisement, the scheduling of a
response will be deferred until the minimum separation expires.
This separation delay does not affect unicast Router Advertisement
responses. Routers MAY choose to respond to RS messages with a
unicast RA response to avoid the delay introduced by the
MIN_DELAY_BETWEEN_RAS restriction [1].
In some cases it is not possible to provide unicast responses, since
solicitations may be sent with an unspecified address, or
solicitations do not provide enough link-layer addressing information
to send an unicast response without neighbour discovery exchange. In
these cases, a router may need to send multicast responses, even if
the expected delay is greater.
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2.1.1 Recommendations
Routers SHOULD respond to a RS message with unicast RS message.
Routers SHOULD aggregate RA messages into a multi-cast RA message
if more than 3 unicast RA messages are queued for transmission.
Where multiple unicast transmissions for the same destination
await transmission, routers MAY remove all transmissions after the
first without ill-effect.
2.2 Router Advertisement Parameters
Where hosts have changeable connectivity, there may be a number of
prefixes or routers stored in the host's memory, which are no longer
directly reachable. This additional storage may make movement
detection slower where hosts rapidly pass through networks, or pass
through networks which have very long advertised timeouts.
Routers SHOULD be configured to advertise non-default Valid and
Preferred lifetimes in order to provide DNA hosts with link-specific
address lifetime information.
Administrators are advised to set the advertised Preferred and Valid
timers of prefixes to the maximum duration for which any host may be
required to continue functioning without receiving a particular
advertised prefix.
Where hosts with ongoing transactions, or well known services (such
as DNS) are present on a network, this duration SHOULD be greater
than the maximum expected outage time (for example 9 hours for 0.999
uptime, with a single outage/year).
Upon links where fixed hosts are unlikely to be present,
administrators SHOULD reduce the Router Lifetime, and Prefix Valid
and Preferred Lifetimes on routers used to support DNA.
One potential configuration heuristic would be to configure lifetimes
to be a low number (for example: 15) of times the MaxRtrAdvInterval,
or greater than the lower quartile cell residence time of hosts on
the network (if known). This allows reuse of configuration in the
case where hosts are moving back and forth rapidly between links, but
allows rapid timeouts of old configurations.
The Router Lifetime MUST NOT be advertised as less than the
MaxRtrAdvInterval unless the router is not to be used as a default
[1].
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Routers MUST NOT be configured with Valid or Preferred lifetime
values lower than the MaxRtrAdvInterval.
These minima ensure that lifetimes do not expire in between periodic
Router Advertisements.
2.2.1 Recommendations
Routers SHOULD be configured to advertise non-default Valid and
Preferred lifetimes in order to provide DNA hosts with link-
specific address lifetime information.
Upon links where fixed hosts are unlikely to be present,
administrators SHOULD reduce the Router Lifetime, and Prefix Valid
and Preferred Lifetimes on routers used to support DNA.
The Router Lifetime MUST NOT be advertised as less than the
MaxRtrAdvInterval unless the router is not to be used as a default
[1].
Routers MUST NOT be configured with Valid or Preferred lifetime
values lower than the MaxRtrAdvInterval.
2.3 Router Advertisement Options
When receiving a Router Advertisement from a particular router for
the first time, a host needs to determine if the information
contained in the RA indicates link change or that the transmitting
router is part of the same link as another router it has already
seen. It is not possible to do this unless global prefix information
is included in the advertisement.
Routers SHOULD include at least one global Prefix Information Option
in every Router Advertisement.
Mobile IPv6 introduced a new option for Router Advertisements, which
indicates the current MaxRtrAdvInterval of router [3]. Reception of
this option allows hosts to estimate whether they have missed Router
Advertisements, and allows them to check reachability or discover new
routers.
Routers SHOULD include Advertisement Interval options in Router
Advertisements.
Mobile IPv6 adds the Router Address 'R' Flag to Prefix Information
options [3]. This flag, when set indicates that the router's entire
global address is configured and sent in the prefix information
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option. Bits beyond those specified in the prefix length field
identify the router's Interface Identifier [6].
Hosts which are detecting network attachment can use a global router
address to uniquely identify the router and link, rather than using
link-local source addresses, which may be present on multiple links.
Routers SHOULD advertise at least one global address consistently in
a Prefix Information Option, by setting the Router Address 'R' Flag.
2.3.1 Recommendations
Routers SHOULD include at least one global Prefix Information
Option in every Router Advertisement.
Routers SHOULD include Advertisement Interval options in Router
Advertisements.
Routers SHOULD advertise at least one global address consistently
in a Prefix Information Option, by setting the Router Address 'R'
Flag.
2.4 Triggered Router Advertisements
There are proposals for IPv6 Router Advertisements to be sent to
hosts based on network side link-layer information.
Where these mechanisms exist they can provide Router Advertisements
in the quickest possible time without need for Router Solicitation.
These systems rely upon link-layer facilities are not available in
all environments. Therefore, interested readers are referred to the
individual methods' documentation [15].
2.5 Split Advertisements
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 (section 6.2.3 of [1]).
If such a choice is made, average multicast RA time discussed in
Appendix C increases for each subset of the prefixes included in the
split RA messages.
Routers SHOULD consistently include one prefix in both sets of its RA
messages. This 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.
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2.6 Router Configurations
Each router can have its own configuration with respect to sending
RAs, and the treatment of router and neighbour solicitations.
Different timers and constants might be used by different routers,
such as the delay between Router Advertisements or delay before
replying to a multicast RS. If a host is changing its IPv6 link, a
newly seen router on that link may have a different configuration and
may introduce more delay than the previous default router of the
host.
While transitions between links under different administrative
control are considered to be common, it is RECOMMENDED that network
deployers adopt uniform configuration practices across routers on
different links within the same logical domain, in order to provide
consistent performance.
3. Topological Practices for DNAv6 Networks
IPv6 does not prefer one particular network topology over another and
allows multiple routers and subnet prefixes to exist on one link.
Different deployments of network elements and their configuration may
impact on link change detection though. Effects and recommended
practices for dealing with different network topologies are presented
below.
3.1 Link Extent and Composition
Most of today's access networks deploy link-layer bridging
technologies in order to extend their logical range beyond a single
Medium Access Control domain.
Consequently, while many routers will come with traditional wired or
optic-fibre interfaces, packets travelling within the same link may
have been bridged across from a wired segment to a wireless segment.
In many of cases, the router will not have accurate information about
the transmission rates or media of particular segments on the link.
Routers with interfaces whose technology is bridgeable SHOULD NOT
assume that all segments and devices on the link have the same
bandwidth available.
3.2 Wireless cell coverage
Where the topology of a set of access networks is known to have a
single cell per link or subnet, IPv6 hosts may immediately solicit
for Router Advertisements upon notification of cell change. If link
design is also constrained to a single router per cell, RA response
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delays may be reduced as described in Section 3.4.
Where link design is not constrained, many cells may exist within the
same link.
The scalability of a subnet in terms of wireless cell coverage is
likely to be limited by broadcast/multicast traffic between hosts on
the link. Where multicast packets may pass from one cell to another
in the link (even if no multicast listeners for the group exist),
constrained wireless segments' performance may become degraded.
In this case, it may be necessary to deploy multicast snooping or
split the link into smaller broadcast domains [13].
3.3 Multiple Router Links
IPv6 Neighbour Discovery allows multiple routers to be advertising on
the same link [1]. These routers are not required to advertise the
same prefixes as each other. This section provides some guidelines
for deploying multiple routers on the same link.
While many routers may exist on a link, it is preferable to limit the
number of advertising routers. There SHOULD NOT be more than three
(3) routers advertising on a link. This will provide robustness in
the case of RA packet loss, but provides a bound for bandwidth
consumption.
Multiple routers responding to Router Solicitation will reduce the
mean delay for solicitation, at the cost of additional traffic. For
unicast responses, the delays may be halved for three responding
routers.
+-----------------------+---------+----------+----------+
|Num advertising routers| 1 | 2 | 3 |
+-----------------------+---------+----------+----------+
|Expected Unicast Delay | 0.250s | 0.167s | 0.125s |
+-----------------------+---------+----------+----------+
If using advertising intervals lower than those specified in IPv6
Neighbour Discovery, only one router MAY advertise at the elevated
rate. Other routers beyond the first SHOULD NOT have
MinDelayBetweenRAs, MinRtrAdvInterval or MaxRtrAdvInterval less than
the minima specified in IPv6 Neighbour Discovery [1][3].
Where it is possible, routers SHOULD include at least one common
prefix in all of their Router Advertisement messages. This allows
hosts to immediately see that both routers are on the same link.
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3.4 Point-to-point Links
IPv6 Router Discovery 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 [8] [14].
3.5 Disjoint Administration
It is possible that multiple sets of routers may be administered by
different organizations, both advertising on a link.
Routers administered by different organizations are likely to have
different trust models, especially for routing and renumbering. This
may prevent alien routers from learning about changes in
configuration. Consequently, re-advertisements of learned prefixes
in router advertisements may cause problems for hosts trying to
detect link-change. It is important for deployers to remember that
prefixes belonging to another organization, but valid within a link,
SHOULD NOT be advertised if up-to-date prefix validity or lifetime
data are not available.
4. Security Considerations
When operating a network in support of hosts performing link change
detection, both the operational security of the hosts and network
infrastructure are important. DNA procedures rely upon rapid
delivery of information to hosts using IPv6 Neighbour Discovery.
Neighbour Discovery as a critical service in IPv6 networks is subject
to various attacks as described in [7].
The following sections describe issues and practices to provide
additional functional security for both operators and hosts.
4.1 Supporting host security
In DNA, some hosts will begin configuration procedures based on a
single message transmitted by a router. As such the ability of
routing infrastructure to prove its authenticity and authorization is
important to support correct operation of hosts. As described in
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Section 4.2, authentication and authorization mechanisms exist which
allow hosts to check security of routers when they receive Router
Advertisements indicating link change.
Today these mechanisms require additional message exchanges and
public key operations to check the authorization chain back to a
trusted root. Considering the computational cost for verifying
certificates, it will useful for administrators to attempt to
minimize the length of these authorization chains.
Where a Router Advertisement is sent by a router, it SHOULD contain
sufficient information to prove that the router is on the same link
as previously seen advertisers, or is indeed the same router. This
may prevent expensive checks by hosts which will not need to
immediately test the authenticity of the router through signature
verification, or additional transmissions.
As described in section Section 3.3, advertising common prefixes
achieves this goal.
4.2 Providing Router Authorization
Hosts which wish to have secured exchanges with neighbours and on-
link routers may use Secured Neighbour Discovery (SEND) [4]. SEND
provides authenticity as well as response matching, using nonces
copied from solicitations into advertisements.
Responses which contain nonces may be matched to one or more
solicitations (dependent on the number of Nonce Options contained in
the Advertisement), and may be used to provide authenticated
bidirectional reachability confirmation even if received in a
multicast advertisement.
The router digitally signs its advertisements with a key-pair which
was also used to generate the router's transmitting address. This
guarantees the origin, as well as the freshness of the Router
Advertisement transmission.
DNA relies particularly heavily upon router discovery in order to
test the validity of routing and addressing configuration. In
addition to reachability and configuration parameters, routing
authorization needs to be determined for each router. In SEND [4],
routing authorization is delegated by certificate authorities.
SEND Router Advertisement packets contain the router's public key
from a key pair used to sign the message. Certificate authorities
delegate a portion of their routing authority to the router, tying
them to the public key of the router. Hosts need to test the
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router's authority to provide routing for the prefixes advertised in
its RAs in order to ensure safe configuration.
While it may be onerous to organize and manage key distribution and
certificates for routing authorization, the security and robustness
afforded by secured neighbour discovery provides a key advantage for
IPv6 networks over what is available in IPv4.
Routers supporting DNA SHOULD provide secured router discovery
services using SEND [4].
5. References
5.1 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] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[4] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-06
(work in progress), July 2004.
5.2 Informative References
[5] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[6] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[7] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[8] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
December 1998.
[9] O'Hara, B. and G. Ennis, "Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE
Std 802.11, 1999.
[10] Yamamoto, S., "Detecting Network Attachment Terminology",
draft-yamamoto-dna-term-00 (work in progress), February 2004.
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[11] Manner, J. and M. Kojo, "Mobility Related Terminology",
draft-ietf-seamoby-mobility-terminology-06 (work in progress),
February 2004.
[12] Moore, N., "Optimistic Duplicate Address Detection for IPv6",
draft-ietf-ipv6-optimistic-dad-02 (work in progress),
September 2004.
[13] Christensen, M., Kimball, K., and F. Solensky, "Considerations
for IGMP and MLD Snooping Switches", draft-ietf-magma-snoop-12
(work in progress), February 2005.
[14] "3GPP TS 29.061 V5.5.0 (2003-03) Interworking between the
Public Land Mobile Network (PLMN) supporting packet based
services and Packet Data Networks (PDN) (Release 5)",
TS 29.061, March 2003.
[15] Choi, J., "Fast Router Discovery with RA Caching",
draft-jinchoi-dna-frd-00 (work in progress), July 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
It should be noted that many already 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.
Where many unicast responses are scheduled awaiting transmission,
Routers MAY consider aggregating them into a single multicast
response if a multicast advertisement may be sent before the
advertisements' scheduled transmission time.
Where multiple unicast transmissions for the same destination await
transmission, routers MAY remove all transmissions after the first
without ill-effect, if a multicast RA is scheduled for the next
possible response time.
Routers MAY choose to respond to RS messages with a unicast RA
response to avoid the delay introduced by the MIN_DELAY_BETWEEN_RAS
restriction [1].
Routers SHOULD be configured to advertise non-default Valid and
Preferred lifetimes in order to provide DNA hosts with link-specific
address lifetime information.
Where hosts with ongoing transactions, or well known services are
present on a network, this duration SHOULD be greater than the
maximum expected outage time.
Upon links where fixed hosts are unlikely to be present,
administrators SHOULD reduce the Router Lifetime, and Prefix Valid
and Preferred Lifetimes on routers used to support DNA.
The Router Lifetime MUST NOT be advertised as less than the
MaxRtrAdvInterval unless the router is not to be used as a default
[1].
Routers MUST NOT be configured with Valid or Preferred lifetime
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values lower than the MaxRtrAdvInterval.
Routers SHOULD include at least one global Prefix Information Option
in every Router Advertisement.
Routers SHOULD include Advertisement Interval options in Router
Advertisements.
Routers SHOULD advertise at least one global address consistently in
a Prefix Information Option, by setting the Router Address 'R' Flag.
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]).
While transitions between links under different administrative
control are considered to be common, it is RECOMMENDED that network
deployers adopt uniform configuration practices across routers on
different links within the same logical domain, in order to provide
consistent performance.
Routers with interfaces whose technology is bridgeable SHOULD NOT
assume that all segments and devices on the link have the same
bandwidth available.
There SHOULD NOT be more than three (3) routers advertising on a
link.
If using advertising intervals lower than those specified in IPv6
Neighbour Discovery, only one router MAY advertise at the elevated
rate. Other routers beyond the first SHOULD NOT have
MinDelayBetweenRAs, MinRtrAdvInterval or MaxRtrAdvInterval less than
the minima specified in IPv6 Neighbour Discovery [1][3].
Where it is possible, routers SHOULD include at least one common
prefix in all of their Router Advertisement messages.
Routers on point-to-point links MAY avoid delay by not waiting for
the prescribed random time before responding for the Router
Solicitation message [8] [14].
Considering the computational cost for verifying certificates,
administrators SHOULD attempt to minimize the length of authorization
chains.
Where a Router Advertisement is sent by a router, it SHOULD contain
sufficient information to prove that the router is on the same link
as previously seen advertisers, or is indeed the same router.
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Routers supporting DNA SHOULD provide secured router discovery
services using SEND [4].
On access networks supporting Detecting Network Attachment,
administrators SHOULD configure routers to advertise at the shortest
safe intervals.
Appendix B. Analysis of the response time
In IPv6 Neighbour Discovery, the delay induced by the
MIN_DELAY_BETWEEN_RAS restriction is up to 3 seconds. This delay may
not be very high if the minimum value (MinDelayBetweenRAs=0.03s)
suggested in Mobile IPv6 is implemented [3].
The fraction of time which MinDelayBetweenRAs occupies in the Router
Advertisement interval determines the probability of scheduling
delay.
Assuming that the arrival of RS messages is independent of each
other, and that the arrival of any particular RS is equally likely
across an interval, The probability of delay occurring to a
particular RS is:
P_mcdelay = MinDelayBetweenRAs
----------------------------
(MinRtrAdvInterval + MaxRtrAdvInterval)/2
Where the mean interval is defined as the mean delay before
scheduling an unsolicited Router Advertisement. The probability of
delay with routers using the minimum values from IPv6 Neighbour
Discovery is: 0.851 [1]
Considering the above values, it is also necessary to remember that
all responses will be delayed by a random time between 0 and
MAX_RA_DELAY_TIME (0.5 s) [1].
In this case, the expected delay E_mcrsra for a single arriving RS
is:
E_mcrsra = P_mcdelay * MinDelayBetweenRAs/2 + MAX_RA_DELAY_TIME/2
Again for RFC2461 minima, the expected delay is thus: 1.535 s.
The average unicast RA response time of course is 0.250 seconds.
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Assumptions underpinning this approximation hold only if the
likelihood of more than one RS arriving within a multicast delay
interval is very low. This depends on the arrival rate (lambda) of
Router Solicitations, and is based on a Poisson distribution.
The probability of more than one RS arriving in the interval t of
MinDelayBetweenRAs (defaulting to MIN_DELAY_BETWEEN_RAS) is:
P[X(t) > 1] = 1 - P[X(t) == 1] - P[X(t) == 0]
= 1 - (lambda*t)*exp(-lambda*t)/1! - exp(-lambda*t)/0!
= 1 - ( 1 + lambda*t)* exp (-lambda*t)
For a given load (lambda)=0.05 RS/sec, the probability of multiple RS
arrival is 1.01%.
Adopting the MinDelayBetweenRAs = 0.03s and MaxRtrAdvinterval= 4s the
probability of arrival in the MinDelayBetweenRAs interval is 0.0148.
The estimated expected delay for multicast RS/RA exchange is
therefore 0.25022 seconds.
This is comparable to the unicast response delay.
In this case, the chance of additional solicitation arrival during
MinDelayBetweenRAs is very low (around 1 in 10^6 for t=0.03,
lambda=0.05).
Please note that if the MaxRtrAdvInterval is also low (making the
mean interval duration low), the solicitor is likely to get get an
unsolicited RA due to early scheduling of the RA during the RS/RA
delay period (see Appendix C).
As described in [3], some systems may not provide tunable
MinDelayBetweenRAs except that it assumes the value of the minimum
time difference between unsolicited RAs (MinRtrAdvInterval) when
MinRtrAdvInterval is less than 3 seconds. Reducing MinRtrAdvInterval
to will increase the rate of RA transmission, but this doesn't need
to be a dramatic increase. For example, even if the lowest value for
MinRtrAdvInterval is selected (0.03 s), if the MaxRtrAdvInterval is
kept at its RFC2461 minimum (4 seconds) the mean interval between RAs
is over 2 seconds. This compares with a mean interval of 3.5 seconds
for advertisement only using the RFC2461 minima.
Where the MinDelayBetweenRAs is correspondingly lowered to the
minimum values, the rate of solicited multicast RAs may be elevated
to around 33 per second. This raises the potential for abuse by
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attackers which can force uniform resource consumption across all
cells in a link.
It is possible to choose delay values which provide significantly
improved expected and worst case delays over RFC 2461 values, and
have well defined upper bound traffic costs for constrained links.
For example, if a link is able to sustain a maximum of two multicast
RAs per second, a safe value for MinRtrAdvInterval and consequently
MinDelayBetweenRAs is t=0.5s. With similar load values to those
presented above, the probability of arrival within the interval
P_mcdelay = 0.222, with an expected RS/RA delay of E_mcrsra = 0.305s.
As mentioned above the probability of multiple RS arrival in the
interval would be 3.07 * 10^-4 with a load (lambda)=0.05.
This MinDelayBetweenRAs allows fairly good multicast response
performance but bounds the number of multicast RAs to two per second.
Regardless of load, the worst case delay for RA response in this case
is no greater than 2*MIN_DELAY_BETWEEN_RAs (1 second).
Appendix C. Router Advertisement Rates
Unsolicited Router Advertisements are scheduled to be transmitted at
a time between MinRtrAdvInterval and MaxRtrAdvInterval after the last
multicast Router Advertisement. These parameters may be configured
in the way which best suits the network. The table below summarizes
the parameters as described by IPv6 Neighbour Discovery [1].
+-----------------------+----+----+----+-----+----+-----+
| Timer |Maximum |Default |Minimum |
+-----------------------+----+----+----+-----+----+-----+
| MaxRtrAdvInterval | 1800 | 600 | 4 |
+-----------------------+----+----+----+-----+----+-----+
| MinRtrAdvInterval | 594 | 198 | 3 |
+-----------------------+----+----+----+-----+----+-----+
|Avg. Multicast RA time | 1197 | 399 | 3.5 |
+-----------------------+----+----+----+-----+----+-----+
The load on the network, and the timeliness of any received
information updates are therefore influenced by the router's
advertisement parameters.
On access networks supporting Detecting Network Attachment,
administrators SHOULD configure routers to advertise at the shortest
safe intervals. Determination of the shortest safe intervals depends
on topology, and the composition of the link, as described in
Section 3.1.
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Mobile IPv6 attempts to address the delays associated with hosts'
movement and change detection by reducing the minimum settings for
MinRtrAdvInterval to 30ms and MaxRtrAdvInterval to 70ms. Not all
IPv6 routers support these configuration values today. Where hosts
have no reactive way of detecting change, and do not solicit for
Router Advertisements, these intervals may allow change detection
sufficiently fast to support real-time applications.
The effect of these timers are summarized in the table below.
+-----------------------+----+----+----+-----+----+-----+
| Timer |Maximum |Default |Minimum |
+-----------------------+----+----+----+-----+----+-----+
| MaxRtrAdvInterval | 1800 | 600 | 0.07 |
+-----------------------+----+----+----+-----+----+-----+
| MinRtrAdvInterval | 594 | 198 | 0.03 |
+-----------------------+----+----+----+-----+----+-----+
|Avg. Multicast RA time | 1197 | 399 | 0.05 |
+-----------------------+----+----+----+-----+----+-----+
Where Mobile IPv6 is supported, the minimum values change, but the
default timers are unmodified. If administrators wish to take
advantage of shorter intervals between unsolicited RAs, explicit
configuration is required. This is because the elevated rate of
multicast RA transmission can have detrimental effects on some
constrained links [3].
The minimum average for un-solicited Router Advertisements would be
20 messages per second. Assuming the minimum packet size for an RA
with one prefix as 88 bytes, the bandwidth used will be 14 kbps.
With SEND Options, and (somewhat weak) 1024-bit RSA keys, a single RA
could be around 432 octets. This would consume approximately 69 kbps
without considering link-layer overheads [4].
As described in Section 2.1, parameters may be chosen to optimize
solicited behaviour in a way which limits the mean bandwidth overhead
for unsolicited RAs.
A good example would be setting a MinRtrAdvInterval (along with
MinDelayBetweenRAs) as 0.5 s, and the MaxRtrAdvInterval to 4s. This
makes the mean delay before receiving an unsolicited RA 2.25 seconds,
and limits the bandwidth utilization for unsolicited RAs (using the
SEND example above) to 1.5 kbps, and the maximum multicast solicited
rate to 6.9 kbps (one multicast RA each 0.5s).
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