Using Subnet-Specific Link-Local Addresses to Improve SLAAC Robustness
draft-link-v6ops-gulla-01
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| Author | Jen Linkova | ||
| Last updated | 2024-02-25 | ||
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draft-link-v6ops-gulla-01
IPv6 operations J. Linkova
Internet-Draft Google
Intended status: Informational 25 February 2024
Expires: 28 August 2024
Using Subnet-Specific Link-Local Addresses to Improve SLAAC Robustness
draft-link-v6ops-gulla-01
Abstract
This document suggests that a link-local address used by a router as
a source address for Router Advertisement packets is calculated as a
function of prefixes listed in the Prefix Information Option of the
Router Advertisement. The proposed approach, combined with the Rule
5.5 of the Default Source Address Selection algorithm (RFC6724) and
first-hop selection requirements for hosts (RFC 8028) improves the
robustness of the SLAAC by allowing the hosts to detect the IPv6
subnet changes much faster and select the correct source address.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 28 August 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Subnet Change Scenarios . . . . . . . . . . . . . . . . . . . 3
5. Default Address Selection Rule 5.5, Default Router Selection
and Renumbering . . . . . . . . . . . . . . . . . . . . . 4
6. Outage Duration During Renumbering Event . . . . . . . . . . 6
6.1. Receiving New Configuration Information from Routers . . 6
6.2. Hosts Deprecating the Outdated Configuration . . . . . . 6
7. Generating Subnet-Specific Link-Local Addresses for Router
Interfaces . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Subnet-Specific and Stable Link-Local Addresses . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
IPv6 Stateless Address AutoConfugration (SLAAC, [RFC4862] provides
IPv6 hosts with a mechanism to configure their IPv6 stack based on
the information (such as an IPv6 prefix and the default router
address) provided by the on-link routers. If that information
changes (e.g. a prefix assigned to the link is changed), the routers
need to explicitly invalidate the outdated information (e.g. by
sending a Router Advertisement packet which deprecates the old
prefix). In the absence of an explicit signal the host would be
using the outdated information until its lifetime expires. Multiple
documents discuss the SLAAC renumbering problem and proposed various
improvements to the host and router behaviour (see [RFC9096] and
draft-ietf-6man-slaac-renum).
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This document recommends that the link-local address the router sends
the router advertisement from should depend on the network prefix(es)
assigned to the router interface. As a result, Router Advertisements
containing different sets of PIOs are sent from different link-local
addresses. That allows the hosts to select the source address from
the prefix advertized by the reachable next-hop and recover from a
renumbering or network segment change events much faster.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
PIO: Prefix Information Option, [RFC4861].
RA: Router Advertisement, [RFC4861].
SLAAC: StateLess Address AutoConfiguration, [RFC4862].
4. Subnet Change Scenarios
There are multiple scenarios when an IPv6 subnet assigned to the link
can change without any explicit signals received by hosts. When it
happens, the hosts can end up with outdated IPv6 address
configuration, which leads to broken connectivity and degraded user
experience. Examples include but are not limited to:
* A prefix delegated to a CPE router changes, and the router does
not send RAs to the connected hosts, deprecating the old prefix.
The hosts would be using addresses from both old and new prefixes
until the prefix lifetime expires.
* A host is connected to a wired port, and a VLAN (and the
corresponding IPv6 subnet) changed. This often happens if the
VLAN is configured as a result of 802.1x or MAC-based
authentication, so the VLAN is provided by RADIUS. Some OSes do
not correctly reset IPv6 stack when the wired interface 802.1x
state changes from ‘unauthenticated’ to ‘authenticated’ (or vice
versa).
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In all those scenarios a host might move between IPv6 subnets without
complete disconnection and without detecting the network change. As
a result the following sequence of events may occur, leading to
broken connectivity:
* The host is connected to a network A, receives an RA from the
router with a PIO containing pref_a, forms IPv6 addresses from
that prefix using SLAAC.
* The host attachment changes from network A to network B or an IPv6
prefix configured on the network changes from pref_a to pref_b.
The host doesn’t detect the network change and doesn’t clear the
IPv6 stack.
* The host receives an RA from the router with a new PIO for pref_b
and forms new addresses from that prefix.
* Now the host has two sets of IPv6 addresses - one from pref_a and
one from pref_b. Addresses from pref_a are unusable: even if the
outgoing packets are not dropped by anti-spoofing filters,the
return traffic wouldn’t be able to reach the host. So if the host
selects an address from pref_a as a source address for outgoing
communication (as per RFC6724 or by using any other custom
algorithms), the traffic would be dropped, causing user-visible
outages.
It should be noted that the Detecting Network Attachment algorithm
defined in [RFC6059] relies on the combination of the link-layer
address and the link-local IPv6 address of a router to be unique
across links. However that assumption doesn't often hold. For
example, network administrators prefer to configure the same, easy to
remember link-local address (e.g. 'fe80::1') to router interfaces on
different links. Some router implementations are known to use the
virtual router MAC address to create the Modified Extended Unique
Identifier (EUI)-64 identifiers for VRRPV3 virtual link-local
addresses (violating Section 7.4 of [RFC5798]). As a result, all
links with the same VRRP ID (and hence the same virtual router MAC
address) would have the same virtual link-local address as well.
5. Default Address Selection Rule 5.5, Default Router Selection and
Renumbering
Rule 5.5 of the Default Source Address Selection ([RFC6724]) requires
the host to prefer addresses in a prefix advertised by the next-hop.
It allows the multihomed host to select the source address correctly:
when two routers advertize different prefixes, the host will be
sending packets with source address from a given prefix to the router
the prefix was received from.
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In case of renumbering if both old and new prefixes are advertized by
the same router (received from a router with the same link-local
address), then Rule 5.5 doesn't help selecting the correct (working)
source address. However if the subnet change also leads to the
default router address change, then a host implementing Rule 5.5
could recover from the renumbering quickly:
* The host is connected to a network A, receives an RA from the
router (link-local address LLA_A) with a PIO containing pref_a,
forms IPv6 addresses from that prefix using SLAAC.
* The host attachment changes from network A to network B or an IPv6
prefix configured on the network changes from pref_a to pref_b.
The host doesn’t detect the network change and doesn’t clear the
IPv6 stack.
* The host receives an RA from the router (link-local address LLA_B)
with a new PIO for pref_b and forms new addresses from that
prefix.
* Link-local address LLA_A is not reachable anymore, as the host
changes the network attachment point. Neighbor Unreachability
Detection ([RFC4861]) detects it and removes LLA_A from the list
of default routers.
* The host is using LLA_B as a next-hop for outgoing traffic, so
addresses from the pref_b are selected, and addresses from pref_a
are not used.
As per [RFC8028], "A host SHOULD select default routers for each
prefix it is assigned an address in. Routers that have advertised
the prefix in their Router Advertisement message SHOULD be preferred
over routers that do not advertise the prefix.". If the host
complies with [RFC8028], then the proposed mechanism would work even
better, and would provide fast recovery from a renumbering event.
* The host selects a default router (LLA_A) for pref_a.
* When the host receives an RA from LLA_B, containing pref_b, the
host selects another default gateway, LLA_B.
* Neighbor Unreachability Detection detects that LLA_A is not
reachable, and removes it from the neighbor cache table, so the
host can not use it as a default gateway anymore. The host
switches to LLA_B and, in accordance with Rule 5.5, starts using
addresses from pref_b.
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6. Outage Duration During Renumbering Event
When the IPv6 subnet changes (either because the given link has been
renumbered, or because the client has moved to another link), there
are two factors contributing to the duration of the outage:
* Time required for the host to receive new configuration
information (RAs containing new PIOs).
* Time required for the host to deprecate the old configuration
information.
6.1. Receiving New Configuration Information from Routers
There is always a delay between the network subnet change
(renumebring event or link change event) and arrival of an RA. To
reduce that delay in case of undetected link change, the network
administrators SHOULD reduce MinRtrAdvInterval and MaxRtrAdvInterval
([RFC4861]) to ensure that RAs are sent often. In case of the link
renumbering, the router SHOULD notify hosts by sending an RA with the
new information. While Requirement L-13 of [RFC7084] requires that
the IPv6 CE router MUST send an RA immidiately after the delegated
prefix changes, that section does not explicitly requires the new
prefix to be included. Also, that behaviour is only mandatory for
CPE devices, while other routers (Enterprise-grade devices, for
example) comply with Section 6.2.4 of [RFC4861] which says:
The information contained in Router Advertisements may change through
actions of system management. For instance, the lifetime of
advertised prefixes may change, new prefixes could be added, a router
could cease to be a router (i.e., switch from being a router to being
a host), etc. In such cases, the router MAY transmit up to
MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the
same rules as when an interface becomes an advertising interface.
To reduce the time required for hosts to detect renumbering event, it
might be desirable to upgrade that "MAY" to "SHOULD".
6.2. Hosts Deprecating the Outdated Configuration
Without changes proposed in this document, a host might be using the
outdated prefix for the duration of the PIO preferred lifetime. As
per [RFC4861], the default value for preferred lifetime is 604800
secs (7 days). The expired draft draft-ietf-6man-slaac-renum
proposes to reduce that value to 14400 seconds (4 hours).
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The solution proposed in this document allows hosts which implement
Rule 5.5 of the source address selection ([RFC6724]) to stop using
the outdated prefix much faster. The time required for the host to
detect that the old prefix shouldn't be used for initiating new
session is the time required for Neighbor Unreachability Detection
(NUD, [RFC4861]) to remove an unreachable entry for the old link-
local address of the default router. The default value would be
(without taking randomisation factors into account): ReachableTime
milliseconds (to move from REACHABLE to STALE) +
DELAY_FIRST_PROBE_TIME + MAX_UNICAST_SOLICIT*RetransTimer = 30
seconds + 5 second + 3*1 = 38 seconds.
7. Generating Subnet-Specific Link-Local Addresses for Router
Interfaces
* The router SHOULD send router advertisement packets from a
dedicated link-local address.
* That dedicated link-local address SHOULD change if the set of
prefixes advertized in the Router Advertisement changes. In other
words, when the set of prefixes advertized on a given interface
changes, the router SHOULD generate a new link-local address and
use that address as a source address for Router Advertisements.
As soon as a new link-local address is generated, the router
SHOULD transmit Router Advertisements as specified in
Section 6.2.4 of [RFC4861].
* The router SHOULD stop responding to Neighbor Solicitation packets
to the old link-local address. This is required for Neighbor
Unreachability Detection mechanism on hosts to mark the old
address as unreachable and make the hosts to use the new address
as a default router.
Routers which act as DHCPv6-PD clients need to implement some
algorithm to generate the interface ID based on the set of prefixes
advertised on a given interface. The exact algorithm is outside of
scope of this document - it could be some form of hashing function
which consumes a list of network prefixes and generates a 64-bits
interface ID. For example, the router MAY use the algorithm defined
in [RFC7217] and use the list of PIOs as Network_ID.
If the interface subnets are configured statically, the network
administrator can configure link-local addresses statically as well.
In some cases it might be possible to just utilize the global subnet
prefix as an interface ID. For example, if the router has two
interfaces configured with 2001:db8:1:1::/64 and 2001:db8:2:2::/64
subnets respectively, it is sufficient to configure
fe80::2001:db8:1:1 and fe80::2001:db8:2:2 as the link-local addresses
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for those interfaces. If the first-hop redundancy is provided by
VRRPv3, there is no need to configure the static link-local interface
addresses but the virtual link-local address SHOULD be configured
instead.
Discussion: if a given router's interface has multiple prefixes
configured, it's possible than only one prefix changes. For example:
* An interface has both GUA and ULA prefixes configured, and the
global prefix delegated via DHCPv6-PD changes, while ULA stays the
same.
* In a multihomed environment, an interface migth have two prefixes
delegated by two upstream networks. Those prefixes can change
independently.
If all PIOs are advertised in a single RA, and the link-local address
used as an RA source, is generated as described in this document,
changing just one prefix would impact traffic sent from all addresses
(as the previous default gateway becomes unreachable). Therefore it
might be desirable for a router to send multiple RAs - one per PIO,
and use a dedicated PIO-specific source address for each of those
RAs. In that case, if the host complies with [RFC8028] and selects
default routers for each prefix it is assigned an address in, then
only traffic from source addresses in the renumbered prefix is
impacted (as only the default gateway for that prefix becomes
unreachable and needs to be reselected). On the other hand such
proposal would lead to increased numbers of RAs being sent, which
might negatively impact hosts battery life. It should be noted that
an expired draft draft-ietf-6man-slaac-renum recommends against such
behaviour and requires all PIOs to be advertized in a single RA.
7.1. Subnet-Specific and Stable Link-Local Addresses
In many cases it might be beneficial for a router to have a stable
link-local address (e.g. if that address is advertized as a DNS
server, or for management purposes. Router MAY generate subnet-
specific link-local addresses in addition to a stable link-local
address. Alternatively, if the router is not capable of supporting
multiple link-local addresses, it MAY generate a new stable link-
local address every time the set of prefixes on the interface
changes.
It should be noted that the proposed mechanism assumes that the
router does not use the modified EUI-64 format for generating
interface ID. As per Section 3 of [RFC8064], nodes SHOULD NOT use
the modified EUI-64 format, and SHOULD use the algorithm defined in
[RFC7217] instead.
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8. Security Considerations
To be added.
9. Privacy Considerations
This document does not introduce any privacy considerations.
10. IANA Considerations
This memo does not introduce any requests to IANA.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>.
[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
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[RFC8028] Baker, F. and B. Carpenter, "First-Hop Router Selection by
Hosts in a Multi-Prefix Network", RFC 8028,
DOI 10.17487/RFC8028, November 2016,
<https://www.rfc-editor.org/info/rfc8028>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8781] Colitti, L. and J. Linkova, "Discovering PREF64 in Router
Advertisements", RFC 8781, DOI 10.17487/RFC8781, April
2020, <https://www.rfc-editor.org/info/rfc8781>.
[RFC8925] Colitti, L., Linkova, J., Richardson, M., and T.
Mrugalski, "IPv6-Only Preferred Option for DHCPv4",
RFC 8925, DOI 10.17487/RFC8925, October 2020,
<https://www.rfc-editor.org/info/rfc8925>.
11.2. Informative References
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC5798] Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC 5798,
DOI 10.17487/RFC5798, March 2010,
<https://www.rfc-editor.org/info/rfc5798>.
[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059,
DOI 10.17487/RFC6059, November 2010,
<https://www.rfc-editor.org/info/rfc6059>.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>.
[RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu,
"Recommendation on Stable IPv6 Interface Identifiers",
RFC 8064, DOI 10.17487/RFC8064, February 2017,
<https://www.rfc-editor.org/info/rfc8064>.
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[RFC9096] Gont, F., Žorž, J., Patterson, R., and B. Volz, "Improving
the Reaction of Customer Edge Routers to IPv6 Renumbering
Events", BCP 234, RFC 9096, DOI 10.17487/RFC9096, August
2021, <https://www.rfc-editor.org/info/rfc9096>.
Acknowledgements
Thanks to Dale W. Carder, Brian Carpenter, Lorenzo Colitti, Fernando
Gont, Alexandre Petrescu, Mark Smith, Ole Troan, Eduard Vasilenko,
Eric Vyncke for the discussions, the input and all contribution.
Author's Address
Jen Linkova
Google
1 Darling Island Rd
Pyrmont NSW 2009
Australia
Email: furry13@gmail.com, furry@google.com
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