Internet DRAFT - draft-ietf-v6ops-nd-cache-init
draft-ietf-v6ops-nd-cache-init
v6ops J. Linkova
Internet-Draft Google
Intended status: Informational September 6, 2020
Expires: March 10, 2021
Neighbor Cache Entries on First-Hop Routers: Operational Considerations
draft-ietf-v6ops-nd-cache-init-05
Abstract
Neighbor Discovery (RFC4861) is used by IPv6 nodes to determine the
link-layer addresses of neighboring nodes as well as to discover and
maintain reachability information. This document discusses how the
neighbor discovery state machine on a first-hop router is causing
user-visible connectivity issues when a new (not being seen on the
network before) IPv6 address is being used. The various approaches
to mitigate the problem are described, with the proposed solution
fully documented in I-D.ietf-6man-grand.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 10, 2021.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Solution Requirements . . . . . . . . . . . . . . . . . . 5
2.2. Solution Overview . . . . . . . . . . . . . . . . . . . . 5
3. Solutions Considered but Discarded . . . . . . . . . . . . . 6
3.1. Do Nothing . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Change to the Registration-Based Neighbor Discovery . . . 7
3.3. Host Sending NS to the Router Address from Its GUA . . . 7
3.4. Host Sending Router Solicitation from its GUA . . . . . . 8
3.5. Routers Populating Their Caches by Gleaning From Neighbor
Discovery Packets . . . . . . . . . . . . . . . . . . . . 9
3.6. Initiating Hosts-to-Routers Communication . . . . . . . . 9
3.7. Transit Dataplane Traffic From a New Address Triggering
Address Resolution . . . . . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The section 7.2.5 of [RFC4861] states: "When a valid Neighbor
Advertisement is received (either solicited or unsolicited), the
Neighbor Cache is searched for the target's entry. If no entry
exists, the advertisement SHOULD be silently discarded. There is no
need to create an entry if none exists, since the recipient has
apparently not initiated any communication with the target."
This approach is perfectly suitable for host-to-host communications,
which are in most cases bi-directional, and it could be expected that
if a host A has an neighbor cache entry for the host B IPv6 address,
host B also has the corresponding entry for the host A address in its
cache. However when a host communicates to off-link destinations via
its first-hop router, that logic does not apply. The most typical
scenario when the problem may arise is a host joining the network,
forming a new address and using that address for accessing the
Internet:
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1. A host joins the network and receives a Router Advertisement (RA)
packet from the first-hop router (either a periodic unsolicited
RA or a response to a Router Solicitation sent by the host). The
RA contains information the host needs to perform Stateless
Address Autoconfiguration ([RFC4862]) and to configure its
network stack. As in most cases the RA also contains the link-
layer address of the router, the host can populate its Neighbor
Cache with the router's link-local and link-layer addresses.
2. The host starts opening connections to off-link destinations. A
very common use case is a mobile device sending probes to detect
the Internet connectivity and/or the presence of a captive portal
on the network. To speed up that process many implementations
use Optimistic Duplicate Address Detection [RFC4429] which allows
them to send probes before the Duplicate Address Detection (DAD)
process is completed. At that moment the device neighbor cache
contains all information required to send those probes (such as
the default router link-local the link-layer addresses). The
router neighbor cache, however, might contain an entry for the
device link-local address (if the device has been performing the
address resolution for the router link-local address), but there
are no entries for the device global addresses.
3. Return traffic is received by the first-hop router. As the
router does not have any cache entry for the host global address
yet, the router starts the neighbor discovery process by creating
an INCOMPLETE cache entry and then sending a Neighbor Solictation
to the Solicited Node Multicast Address. Most router
implementations buffer only one data packet while resolving the
packet destination address, so it would drop all subsequent
packets for the host global address, until the address resolution
process is completed.
4. If the host sends multiple probes in parallel, it would consider
all but one of them failed. That leads to user-visible delay in
connecting to the network, especially if the host implements some
form of backoff mechanism and does not retransmit the probes as
soon as possible.
This scenario illustrates the problem occurring when the device
connects to the network for the first time or after a timeout long
enough for the device address to be removed from the router's
neighbor cache. However, the same sequence of events happen when the
host starts using a new global address previously unseen by the
router, such as a new privacy address [RFC4941] or if the router's
Neighbor Cache has been flushed.
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While in dual-stack networks this problem might be hidden by Happy
Eyeballs [RFC8305] it manifests quite clearly in IPv6-only
environments, especially wireless ones, leading to poor user
experience and contributing to a negative perception of IPv6-only
solutions as unstable and non-deployable.
This document discusses the operational implications of not
proactively creating Neighbor Cache entries on first-hop routers and
summarizes various approaches to mitigate the problem. The document
provides an overview of the proposed solution which is fully
described in [I-D.ietf-6man-grand].
1.1. 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.
1.2. Terminology
ND: Neighbor Discovery, [RFC4861].
SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862].
NS: Neighbor Solicitation, [RFC4861].
NA: Neighbor Advertisement, [RFC4861].
RS: Router Solicitation, [RFC4861].
RA: Router Advertisement, [RFC4861].
SLLAO: Source link-layer Address Option, an option in the ND packets
containing the link-layer address of the sender of the packet,
[RFC4861].
GUA: Global Unicast Address, [RFC4291].
DAD: Duplicate Address Detection, [RFC4862].
Optimistic DAD: a modification of DAD, [RFC4429].
FCFS SAVI: First-Come, First-Served Source Address Validation,
[RFC6620].
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2. Proposed Solution
2.1. Solution Requirements
It would be highly desirable to improve the Neighbor Discovery
mechanics so routers have a usable cache entry for a host address by
the time the router receives the first packet for that address. In
particular:
o If the router does not have a Neighbor Cache entry for the
address, a STALE entry needs to be created.
o The solution needs to work for Optimistic addresses as well.
Devices implementing the Optimistic DAD usually attempt to
minimize the delay in connecting to the network and therefore are
more likely to be affected by the problem described in this
document.
o In case of duplicate addresses present in the network, the
proposed solution MUST NOT override the existing entry.
o In topologies with multiple first-hop routers the cache needs to
be updated on all of them, as traffic might be asymmetric:
outgoing flows leaving the network via one router while the return
traffic enters the segment via another one.
In addition the solution MUST NOT exacerbate issues described in
[RFC6583] and MUST be compatible with the recommendations provided in
[RFC6583].
2.2. Solution Overview
The Neighbor Discovery is designed to allow IPv6 nodes to discover
neighboring nodes' reachability and learn IPv6 to link-layer
addresses mapping. Therefore ND seems to be the most appropriate
tool to inform the first-hop routers about addresses the host is
going to use.
Section 4.4 of [RFC4861] says:
"A node sends Neighbor Advertisements in response to Neighbor
Solicitations and sends unsolicited Neighbor Advertisements in order
to (unreliably) propagate new information quickly."
Propagating information about new GUA as quickly as possible is
exactly what is required to solve the problem outlined in this
document. Therefore the host might send an unsolicited NA with the
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target link-layer address option to advertise its GUA as soon as the
said address enters Optimistic or Preferred state.
The proposed solution is discussed in [I-D.ietf-6man-grand]. In
summary, the following changes to [RFC4861] are suggested:
o A node SHOULD send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited NA
packets with the Override flag cleared to all-routers multicast
address (ff02::2) as soon as one of the following events happens:
* (if Optimistic DAD is used): a new Optimistic address is
assigned to the node interface.
* (if Optimistic DAD is not used): an address changes the state
from tentative to preferred.
o Routers SHOULD create a new STALE ND cache entry upon receiving
unsolicited NAs.
It should be noted that some routing and switching platforms have
implemented such behaviour already. Administrators could enable the
creation of neighbor discovery cache entries based on unsolicited NA
packets sent from the previously unknown neighbors on that interface.
Network devices implementing FCFS SAVI might drop Neighbor
Advertisements received through a Validating Port which is in the
TENTATIVE state (see Section 2.3.2 of[RFC6620]). Therefore hosts
using Optimistic DAD might not benefit from the proposed solution if
FCFS SAVI is implemented on the network infrastructure.
[I-D.ietf-6man-grand] discusses in more details how the proposed
solution interacts with SAVI.
3. Solutions Considered but Discarded
The problem could be addressed from different angles. Possible
approaches are:
o Just do nothing.
o Migrate from the "reactive" Neighbor Discovery ([RFC4861]) to the
registration-based mechanisms ([RFC8505]).
o The router creates new entries in its Neighbor Cache by gleaning
from Neighbor Discovery DAD messages.
o The host initiates bidirectional communication to the router using
the host GUA.
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o Making the probing logic on hosts more robust.
o Increasing the buffer size on routers.
o Transit dataplane traffic from an unknown address (an address w/o
the corresponding neighbor cache entry) triggers an address
resolution process on the router.
It should be noted that some of those options are already implemented
by some vendors. The following sections discuss those approaches and
the reasons they were discarded.
3.1. Do Nothing
One of the possible approaches might be to declare that everything is
working as intended and let the upper-layer protocols to deal with
packet loss. The obvious drawbacks include:
o Unhappy users.
o Many support tickets.
o More resistance to deploy IPv6 and IPv6-Only networks.
3.2. Change to the Registration-Based Neighbor Discovery
The most radical approach would be to move away from the reactive ND
as defined in [RFC4861] and expand the registration-based ND
([RFC6775], [RFC8505]) used in Low-Power Wireless Personal Area
Networks (6LoWPANs) to the rest of IPv6 deployments. This option
requires some investigation and discussions and seems to be excessive
for the problem described in this document.
3.3. Host Sending NS to the Router Address from Its GUA
The host could force creating a STALE entry for its GUA in the router
ND cache by sending the following Neighbor Solicitation message:
o The NS source address is the host GUA.
o The destination address is the default router IPv6 address.
o The Source Link-Layer Address option contains the host link-layer
address.
o The target address is the host default router address (the default
router address the host received in the RA).
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The main disadvantages of this approach are:
o Would not work for Optimistic addresses as section 2.2 of
[RFC4429] explicitly prohibits sending Neighbor Solicitations from
an Optimistic Address.
o If first-hop redundancy is deployed in the network, the NS would
reach the active router only, so all backup routers (or all active
routers except one) would not get their neighbor cache updated.
o Some wireless devices are known to alter ND packets and perform
various non-obvious forms of ND proxy actions. In some cases,
unsolicited NAs might not even reach the routers.
3.4. Host Sending Router Solicitation from its GUA
The host could send a router solicitation message to 'all routers'
multicast address, using its GUA as a source. If the host link-layer
address is included in the Source Link-Layer Address option, the
router would create a STALE entry for the host GUA as per the section
6.2.6 of [RFC4861]. However, this approach can not be used if the
GUA is in optimistic state: section 2.2 of [RFC4429] explicitly
prohibits using an Optimistic Address as the source address of a
Router Solicitation with a SLLAO as it might disrupt the rightful
owner of the address in the case of a collision. So for the
optimistic addresses the host can send an RS without SLLAO included.
In that case the router may respond with either a multicast or a
unicast RA (only the latter would create a cache entry).
This approach has the following drawbacks:
o If the address is in the Optimistic state the RS can not contain
SLLAO. As a result the router would only create a cache entry if
the solicited RAs is sent as as a unicast. Routers sending
solicited RAs as multicast would not create a new cache entry as
they do not need to send a unicast packet back to the host.
o There might be a random delay between receiving an RS and sending
a unicast RA back (and creating a cache entry) which might
undermine the idea of creating the cache entry proactively.
o Some wireless devices are known to intercept ND packets and
perform various non-obvious forms of ND proxy actions. In some
cases the RS might not even reach the routers.
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3.5. Routers Populating Their Caches by Gleaning From Neighbor
Discovery Packets
Routers may be able to learn about new addresses by gleaning from the
DAD Neighbor Solicitation messages. The router could listen to all
solicited node multicast address groups and upon receiving a Neighbor
Solicitation from the unspecified address search its Neighbor Cache
for the solicitation's Target Address. If no entry exists, the
router may create an entry, set its reachability state to
'INCOMPLETE' and start the address resolution for that entry.
The same solution was proposed in
[I-D.halpern-6man-nd-pre-resolve-addr]. Some routing vendors support
such optimization already. However, this approach has a number of
drawbacks and therefore should not be used as the only solution:
o Routers need to receive all multicast Neighbor Discovery packets
which might negatively impact the routers CPU.
o If the router starts the address resolution as soon as it receives
the DAD Neighbor Solicitation the host might be still performing
DAD and the target address might be tentative. In that case, the
host SHOULD silently ignore the received Neighbor Solicitation
from the router as per the Section 5.4.3 of [RFC4862]. As a
result the router might not be able to complete the address
resolution before the return traffic arrives.
3.6. Initiating Hosts-to-Routers Communication
The host may force the router to start address resolution by sending
a data packet such as ping or traceroute to its default router link-
local address, using the GUA as a source address. As the RTT to the
default router is lower than RTT to any off-link destinations it's
quite likely that the router would start the neighbor discovery
process for the host GUA before the first packet of the returning
traffic arrives.
This approach has the following drawbacks:
o Data packets to the router link-local address could be blocked by
security policy or control plane protection mechanism.
o It introduces an additional overhead for routers control plane (in
addition to processing ND packets, the data packet needs to be
processed as well).
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o Unless the data packet is sent to 'all routers' ff02::2 multicast
address, if the network provides a first-hop redundancy then only
the active router would create a new cache entry.
3.7. Transit Dataplane Traffic From a New Address Triggering Address
Resolution
When a router receives a transit packet, it might check the presence
of the neighbor cache entry for the packet source address and if the
entry does not exist start address resolution process. This approach
does ensure that a Neighbor Cache entry is proactively created every
time a new, previously unseen GUA is used for sending offlink
traffic. However this approach has a number of limitations, in
particular:
o If traffic flows are asymmetrical the return traffic might not
transit the same router as the original traffic which triggered
the address resolution. So the neighbor cache entry is created on
the "wrong" router, not the one which actually needs the neighbor
cache entry for the host address.
o The functionality needs to be limited to explicitly configured
networks/interfaces, as the router needs to distinguish between
onlink addresses (ones the router needs to have Neighbor Cache
entries for) and the rest of the address space.
o Implementing such functionality is much more complicated than all
other solutions as it would involve complex data-control planes
interaction.
4. IANA Considerations
This memo asks the IANA for no new parameters.
5. Security Considerations
This memo documents the operational issue and does not introduce any
new security considerations. Security considerations of the proposed
solution are discussed in the corresponding section of
[I-D.ietf-6man-grand].
6. Acknowledgements
Thanks to the following people (in alphabetical order) for their
review and feedback: Mikael Abrahamsson, Stewart Bryant, Lorenzo
Colitti, Owen DeLong, Igor Gashinsky, Fernando Gont, Tatuya Jinmei,
Erik Kline, Warren Kumari, Barry Leiba, Jordi Palet Martinez, Michael
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Richardson, Dave Thaler, Pascal Thubert, Loganaden Velvindron, Eric
Vyncke.
7. References
7.1. Normative References
[I-D.ietf-6man-grand]
Linkova, J., "Gratuitous Neighbor Discovery: Creating
Neighbor Cache Entries on First-Hop Routers", draft-ietf-
6man-grand-01 (work in progress), July 2020.
[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>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
<https://www.rfc-editor.org/info/rfc4429>.
[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>.
[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>.
[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583,
DOI 10.17487/RFC6583, March 2012,
<https://www.rfc-editor.org/info/rfc6583>.
[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses",
RFC 6620, DOI 10.17487/RFC6620, May 2012,
<https://www.rfc-editor.org/info/rfc6620>.
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[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.
[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>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
Perkins, "Registration Extensions for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Neighbor
Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
<https://www.rfc-editor.org/info/rfc8505>.
7.2. Informative References
[I-D.halpern-6man-nd-pre-resolve-addr]
Chen, I. and J. Halpern, "Triggering ND Address Resolution
on Receiving DAD-NS", draft-halpern-6man-nd-pre-resolve-
addr-00 (work in progress), January 2014.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
Author's Address
Jen Linkova
Google
1 Darling Island Rd
Pyrmont, NSW 2009
AU
Email: furry@google.com
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