IPv6 Operations (v6ops) Working Group X. Xiao
Internet Draft E. Vasilenko
Intended status: Informational Huawei Technologies
Expires: October 2025 E. Metz
KPN
G. Mishra
Verizon Inc.
N. Buraglio
Energy Sciences Network
April 8, 2025
Neighbor Discovery Considerations in IPv6 Deployments
draft-ietf-v6ops-nd-considerations-10
Abstract
The Neighbor Discovery (ND) protocol is a critical component of the
IPv6 architecture. The protocol uses multicast extensively. It also
assumes a security model where all nodes on a link are trusted. Such
a design might be inefficient in some scenarios (e.g., use of
multicast in wireless networks) or when nodes are not trustworthy
(e.g., public access networks). These security and operational
issues and the associated mitigation solutions are documented in
more than 20 RFCs. There is a need to track these issues and
solutions in a single document.
To that aim, this document summarizes the published ND issues and
then describes how all these issues originate from three causes.
Addressing the issues is made simpler by addressing the causes. This
document also analyzes the mitigation solutions and demonstrates
that isolating hosts into different subnets and links can help to
address the three causes. Guidance is provided for selecting a
suitable isolation method to prevent potential ND issues.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
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at any 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 in Oct. 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction...................................................3
1.1. Terminology...............................................5
2. Review of Inventoried ND Issues................................6
2.1. Multicast May Cause Performance and Reliability Issues....6
2.2. Trusting-all-hosts May Cause On-link Security Issues......7
2.3. Router-NCE-on-Demand May Cause Forwarding Delay, NCE
Exhaustion, and Address Accountability Issues..................7
2.4. Summary of ND Issue.......................................8
3. Review of ND Mitigation Solutions..............................9
3.1. ND Solution in Mobile Broadband IPv6.....................10
3.2. ND Solution in Fixed Broadband IPv6......................11
3.3. Unique IPv6 Prefix per Host (UPPH).......................12
3.4. Wireless ND and Subnet ND................................13
3.5. Scalable Address Resolution Protocol.....................13
3.6. ARP and ND Optimization for TRILL:.......................14
3.7. Proxy ARP/ND in Ethernet Virtual Private Networks (EVPN).14
3.8. Gratuitous Neighbor Discovery (GRAND)....................15
3.9. Reducing Router Advertisements...........................15
3.10. Source Address Validation Improvement (SAVI) and Router
Advertisement Guard...........................................16
3.11. RFC 6583 Dealing with NCE Exhaustion Attacks............16
3.12. Registering Self-generated IPv6 Addresses using DHCPv6..16
3.13. Enhanced DAD............................................17
3.14. ND Mediation for IP Interworking of Layer 2 VPNs........17
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3.15. ND Solutions Defined before the Latest Versions of ND...17
3.15.1. Secure Neighbor Discovery (SeND)...................17
3.15.2. Cryptographically Generated Addresses (CGA)........18
3.15.3. ND Proxy...........................................18
3.15.4. Optimistic DAD.....................................18
4. Guidelines for Prevention of Potential ND Issues..............19
4.1. Learning Host Isolation from the Existing Solutions......19
4.2. Applicability of Various Isolation Methods...............20
4.2.1. Applicability of L3 & L2 Isolation..................20
4.2.2. Applicability of L3 Isolation.......................21
4.2.3. Applicability of Partial L2 Isolation...............22
4.3. Guidelines for Applying Isolation Methods................23
5. Security Considerations.......................................23
6. IANA Considerations...........................................23
7. References....................................................24
7.1. Informative References...................................24
8. Acknowledgments...............................................27
1. Introduction
Neighbor Discovery (ND) [RFC4861] defines the protocol mechanisms
that nodes (hosts and routers) on a link interact with each other.
Stateless Address Autoconfiguration (SLAAC) [RFC4862] defines how
nodes use the ND protocol mechanisms to auto-configure IPv6
addresses. To understand what ND issues nodes may encounter, it is
useful to understand the ND mechanisms they use during their
lifetime, starting with the SLAAC process. For a host, the overall
procedure is as follows:
1. LLA DAD: The host forms a Link-Local Address (LLA) and performs
Duplicate Address Detection (DAD) using multicast Neighbor
Solicitations (NSs).
2. Router Discovery: The host sends multicast Router Solicitations
(RSs) to discover a router on the link. The router responds
with Router Advertisements (RAs), providing subnet prefixes and
other information. The host installs a Neighbor Cache Entry
(NCE) for that router upon receiving the RAs. In contrast, the
router cannot install an NCE for the host at this moment of the
exchange because the host's global IP address is still unknown.
When the router later needs to forward a packet to the host's
global address, it will perform address resolution and install
an NCE for the host.
3. GUA DAD: The host forms a Global Unicast Address (GUA) or a
Unique Local Address (ULA) and uses multicast NSs for DAD. For
description simplicity, this document will not further
distinguish GUA and ULA.
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4. Next-hop determination and address resolution: When the host
needs to send a packet, it will first determine whether the
next-hop is a router or an on-link host (which is the
destination). If the next-hop is a router, the host already has
the NCE for that router. If the next-hop is an on-link host, it
will use multicast NSs to perform address resolution for the
destination host. As a result, the source host installs an NCE
for the destination host.
5. Node Unreachability Detection (NUD): The host uses unicast NSs
to determine whether another node with an NCE is still
reachable.
6. Link-layer address change announcement: If a host's link-layer
address changes, it may use multicast NAs to announce its new
link-layer address to other nodes.
For a router, the procedure is similar except that there is no
Router Discovery. Instead, routers perform a Redirect procedure that
hosts do not have. A router sends a Redirect to inform a host of a
better next-hop for the host's traffic.
ND uses multicast in many messages, trusts messages from all nodes,
and routers may install NCEs for hosts on demand when they are to
forward packets to these hosts. These may lead to issues.
Concretely, various ND issues and mitigation solutions have been
published in more than 20 RFCs, including:
. ND Trust Models and Threats [RFC3756],
. Secure ND [RFC3971],
. Cryptographically Generated Addresses [RFC3972],
. ND Proxy [RFC4389],
. Optimistic ND [RFC4429],
. ND for mobile broadband [RFC6459][RFC7066],
. ND for fixed broadband [TR177],
. ND Mediation [RFC6575],
. Operational ND Problems [RFC6583],
. Wireless ND (WiND) [RFC6775][RFC8505][RFC8928][RFC8929][SND],
. DAD Proxy [RFC6957],
. Source Address Validation Improvement [RFC7039],
. Router Advertisement Guard [RFC6105][RFC7113],
. Enhanced Duplicate Address Detection [RFC7527],
. Scalable ARP [RFC7586],
. Reducing Router Advertisements [RFC7772],
. Unique Prefix Per Host [RFC8273],
. ND Optimization for Transparent Interconnection of Lots of
Links (TRILL) [RFC8302],
. Gratuitous Neighbor Discovery [RFC9131],
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. Proxy ARP/ND for EVPN [RFC9161], and
. Using DHCPv6 Prefix Delegation (DHCPv6-PD) to Allocate Unique
IPv6 Prefixes per Client in Large Broadcast Networks [RFC9663].
This document summarizes these RFCs into a one-stop reference (as of
the time of writing) for easier access. This document also
identifies three causes of the issues and defines three host
isolation methods to address the causes and prevent potential ND
issues.
1.1. Terminology
This document uses the terms defined in [RFC4861]. Additional terms
are defined in this section.
MAC - To avoid confusion with link-local addresses, link-layer
addresses are referred to as MAC addresses in this document.
Host Isolation - separating hosts into different subnets or links.
L3 Isolation - allocating a unique prefix per host
[RFC8273][RFC9663] so that every host is in a different
subnet. Given that a unique prefix can be allocated per host
on shared media, hosts in different subnets may be in the
same link.
L2 Isolation - taking measures to prevent a host from reaching other
hosts directly in Layer 2 (L2) so that every host is in a
different link. Due to the existence of Multi-Link Subnet
[RFC4903], hosts in different links may be in the same
subnet. Therefore, L2 Isolation does not imply L3 Isolation,
and L3 Isolation does not imply L2 Isolation either.
L3 & L2 Isolation - applying L3 Isolation and L2 Isolation
simultaneously so that every host is in a different subnet
and on a different link.
Partial L2 Isolation - using an L3 ND proxy [RFC4389] device to
represent the hosts behind it to other hosts in the same
subnet. Within the subnet, ND multicast exchange is
segmented into multiple smaller scopes, each represented by
an ND proxy device.
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2. Review of Inventoried ND Issues
2.1. Multicast May Cause Performance and Reliability Issues
In some cases, ND uses multicast for Node Solicitations NSs), Node
Advertisements (NAs), Router Solicitations (RSs), and Router
Advertisements (RAs). While multicast can be highly efficient in
certain scenarios, e.g., in wired networks, multicast can also be
inefficient in other scenarios, e.g., in large L2 networks or
wireless networks.
Typically, multicast can create a large amount of protocol traffic
in large L2 networks. This can consume network bandwidth, increase
processing overhead, and degrade network performance [RFC7342].
In wireless networks, multicast can be inefficient or even
unreliable due to a higher probability of interference, lower data
rate, and lack of acknowledgements. [RFC9119].
Multicast-related performance issues of the various ND messages are
summarized below:
. Issue 1 LLA DAD Degrading Performance: In an L2 network of N
addresses (which can be much larger than the number of hosts,
as each host can have multiple addresses), there can be N such
multicast messages. This may cause performance issues when N is
large.
. Issue 2 Router's Periodic Unsolicited RAs Draining Hosts'
Battery: Multicast RAs are generally limited to one packet
every MIN_DELAY_BETWEEN_RAS (3 seconds), and there are usually
only one or two routers on the link, so it is unlikely to cause
a performance issue. However, for battery-powered hosts, such
messages may wake them up and create battery life issues
[RFC7772].
. Issue 3 GUA DAD Degrading Performance: same as in Issue 1.
. Issue 4 Router's Address Resolution for Hosts Degrading
Performance: same as in Issue 1.
. Issue 5 Host's Address Resolution for Hosts Degrading
Performance: same as in Issue 1.
. (For Further Study) Hosts' MAC Address Change NAs Degrading
Performance: with randomized and changing MAC addresses
[MADINAS], there may be many such multicast messages.
In wireless networks, multicast is more likely to cause packet loss.
Because DAD treats no response as no duplication, packet loss may
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cause duplicate addresses to be undetected. Multicast reliability
issues are summarized below:
. Issue 6 LLA DAD Not Completely Reliable in Wireless Networks.
. Issue 7 GUA DAD Not Completely Reliable in Wireless Networks.
Note: IPv6 address collisions are extremely unlikely. As a result,
these two issues are largely theoretical rather than practical.
2.2. Trusting-all-hosts May Cause On-link Security Issues
In scenarios such as public access networks, some nodes may not be
trustworthy. An attacker on the link can cause the following
security issues [RFC3756][RFC9099]:
. Issue 8 Source IP Address Spoofing: An attacker can use another
node's IP address as the source address of its ND message to
pretend to be that node. The attacker can then launch various
Redirect or Denial-of-Service (DoS) attacks.
. Issue 9 Denial of DAD: An attacker can repeatedly reply to a
victim's DAD messages, causing the victim's address
configuration procedure to fail, resulting in a DoS to the
victim.
. Issue 10 Forged RAs: An attacker can send RAs to victim hosts
to pretend to be a router. The attacker can then launch various
Redirect or DoS attacks.
. Issue 11 Spoofed Redirects: An attacker can send forged
Redirects to victim hosts to redirect their traffic to the
legitimate router itself.
. Issue 12 Replay Attacks: An attacker can capture valid ND
messages and replay them later.
2.3. Router-NCE-on-Demand May Cause Forwarding Delay, NCE Exhaustion,
and Address Accountability Issues
When a router needs to forward a packet to a node but does not yet
have a Neighbor Cache Entry (NCE) for that node, it first creates an
NCE with the state set to INCOMPLETE. The router then multicasts
Neighbor Solicitations (NS) to all nodes and waits for the
destination node to respond with its MAC address, completing the
NCE. This process is referred to as Router-NCE-on-Demand in this
document.
Router-NCE-on-Demand can cause the following issues:
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. Issue 13 NCE Exhaustion: Router-NCE-on-Demand introduces a
security vulnerability: an attacker can send a high volume of
packets targeting non-existent IP addresses, causing the router
to create numerous NCEs in the INCOMPLETE state. The resulting
resource exhaustion may render the router unable to function.
This vulnerability, described as "NCE Exhaustion" in this
document, does not require the attacker to be on-link.
. Issue 14 Router Forwarding Delay: When a packet arrives at a
router, the router buffers it while attempting to determine the
host's MAC address. This buffering delays forwarding and,
depending on the router's buffer size, may lead to packet loss.
This delay is referred to as "Router-NCE-on-Demand Forwarding
Delay" in this document.
. Issue 15 Lack of Address Accountability: With SLAAC, hosts
generate their IP addresses. The router does not become aware
of a host's IP address until an NCE entry is created. With
DHCPv6 [RFC8415], the router may not know the host's addresses
unless it performs DHCPv6 snooping. In public access networks,
where subscriber management often relies on IP address (or
prefix) identification, this lack of address accountability
poses a challenge [AddrAcc]. Without knowledge of the host's IP
address, network administrators are unable to effectively
manage subscribers, which is particularly problematic in public
access networks. Additionally, once NCE entries are created on
a router, there is no standardized method to retrieve these
entries for management purposes, as highlighted in Section
2.6.1 of [RFC9099].
2.4. Summary of ND Issue
The ND issues, as discussed in Sections 2.1 to 2.3, are summarized
below. These issues stem from three primary causes: multicast,
Trusting-all-nodes, and Router-NCE-on-Demand. Eliminating any of
these causes would also mitigate the corresponding issues. These
observations provide guidance for addressing and preventing ND-
related issues.
(1) Multicast-related issues:
. Performance issues
o Issue 1: LLA DAD Degrading Performance.
o Issue 2: Unsolicited RA Draining Host Battery Life.
o Issue 3: GUA DAD degrading performance.
o Issue 4: Router Address Resolution for Hosts Degrading
Performance.
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o Issue 5: Host Address Resolution for Other Hosts Degrading
Performance.
. Reliability issues
o Issue 6: LLA DAD Not Completely Reliable in Wireless
Networks
o Issue 7: GUA DAD Not Completely Reliable in Wireless
Networks
(2) Trusting-all-nodes related issues:
o Issue 8: Source IP Address Spoofing
o Issue 9: Denial of DAD
o Issue 10: Forged RAs
o Issue 11: Spoofed Redirects
o Issue 12: Replay Attacks
(3) Router-NCE-on-Demand related issues:
o Issue 13: NCE Exhaustion
o Issue 14: Router Forwarding Delay
o Issue 15: Lack of Address Accountability
These issues are potential vulnerabilities and may not manifest in
all usage scenarios.
When these issues may occur in a specific deployment, it is
advisable to consider the mitigation solutions available. They are
described in the following section.
3. Review of ND Mitigation Solutions
Table 1 summarizes ND mitigation solutions available for each issue.
Similar solutions are grouped, beginning with those that address the
most issues. Unrelated solutions are ordered based on the issues
(listed in Section 2.4) they address. Each solution corresponds to a
section below, where abbreviations in the table are described.
+-----+-------------------+--------+--------+--------+------+-----+
| | Multicast | Reli- |On-link |NCE |Fwd. |No A.|
| | performance | ability|security|Exhaust.|Delay |Acct.|
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|Issue| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8-12 | 13 | 14 | 15 |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|MBBv6| All issues solved |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|FBBv6| All issues solved |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
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|UPPH | | X | | X | X | | X | | X | X | X |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|WiND | All issues solved for Low-Power and Lossy Networks (LLNs) |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|SARP | | | | | X | | | | | | |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|ND | | | | | X | | | | | | |
|TRILL| | | | | | | | | | | |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|ND | | | | | X | | | | | | |
|EVPN | | | | | | | | | | | |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|7772 | | X | | | | | | | | | |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|GRAND| | | | X | | | | | |Partly| |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|SAVI/| | | | | | | | | | | |
|RA | | | | | | | | X | | | |
|G/G+ | | | | | | | | | | | |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|6583 | | | | | | | | | X | | |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
|AddrR| | | | | | | | | | | X |
+-----+---+---+---+---+---+---+----+--------+--------+------+-----+
Table 1. Solutions for identified issues
3.1. ND Solution in Mobile Broadband IPv6
The IPv6 solution defined in "IPv6 in 3GPP EPS" [RFC6459], "IPv6 for
3GPP Cellular Hosts" [RFC7066], and "Extending an IPv6 /64 Prefix
from a Third Generation Partnership Project (3GPP) Mobile Interface
to a LAN Link" [RFC7278] is called Mobile Broadband IPv6 (MBBv6) in
this document. The key points are:
. Putting every host, e.g., the mobile User Equipment (UE), in a
Point-to-Point (P2P) link with the router, e.g., the mobile
gateway. Consequently:
o All multicast is effectively turned into unicast.
o The P2P links do not have a MAC address. Therefore, Router-
NCE-on-Demand is not needed.
o Trusting-all-nodes is only relevant to the router. By
applying filtering at the router, e.g., dropping RAs from
the hosts, even malicious hosts cannot cause harm.
. Assigning a unique /64 prefix to each host. Together with the
P2P link, this puts each host on a separate link and subnet.
. Maintaining (prefix, interface) binding at the router for
forwarding purposes.
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Since all the three causes of ND issues are addressed, all the
issues discussed in Section 2.4 are addressed.
3.2. ND Solution in Fixed Broadband IPv6
The IPv6 solution defined in "IPv6 in the context of TR-101" [TR177]
is called Fixed Broadband IPv6 (FBBv6) in this document. FBBv6 has
two flavors:
. P2P: Every host, e.g., the Residential Gateway (RG), is in a
P2P link with the router, e.g., the Broadband Network Gateway
(BNG). In this case, the solution is functionally similar to
MBBv6. All ND issues discussed in Section 2.4 are solved.
. Point-to-Multi-Point (P2MP): All hosts, e.g., the RGs,
connected to an access device, e.g., the Optical Line Terminal
(OLT), are in a P2MP link with the router, e.g., the BNG. This
is implemented by aggregating all hosts into a single VLAN at
the router and implementing L2 Split Horizon at the OLT to
prevent direct host communication.
The following summarizes the key aspects of the FBBv6-P2MP
architecture as described in [TR177]:
. Implementing DAD Proxy [RFC6957]:
In a P2MP architecture with Split Horizon, the normal ND DAD
procedure is disrupted. Since all hosts appear on the same
interface from the router's perspective:
o The router must ensure that all hosts have unique LLAs and
GUAs, as address duplication would prevent the router from
distinguishing between hosts.
o Due to the inability of hosts to reach each other directly,
normal DAD cannot function. To address this, the router
participates in the DAD process as a DAD Proxy to resolve
address duplication
With P2MP Links and DAD Proxy:
o Multicast traffic from all hosts to the router is
effectively converted into unicast, as hosts can only
communicate directly with the router.
o The Trusting-all-nodes model is limited to the router. By
applying simple filtering, e.g., dropping RAs from hosts,
the router can mitigate security risks, even from
malicious hosts
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. Assigning a unique /64 prefix to each host:
Assigning each host a unique /64 prefix results in several
operational improvements:
o The router can proactively install a forwarding entry for
that prefix towards the host, eliminating the need for
Router-NCE-on-Demand.
o Since each host resides in a different subnet, traffic
between hosts is routed through the router, eliminating
the need for hosts to perform address resolution for one
another.
o Without address resolution, router multicast to hosts is
limited to unsolicited RAs. As each host resides in its
own subnet, these RAs are sent as unicast packets to
individual hosts. This follows the approach specified in
[RFC6085], where the host's MAC address replaces the
multicast MAC address in the RA.
Since all three causes of ND issues are addressed, all ND issues
(Section 2.4) are also addressed.
3.3. Unique IPv6 Prefix per Host (UPPH)
UPPH solutions are described in [RFC8273] and [RFC9663]. Although
[RFC8273] relies on SLAAC for unique prefix allocation while
[RFC9663] relies on DHCP-PD, ND issues with these two solutions are
the same because SLAAC is mandatory for all nodes regardless of
[RFC8273] or [RFC9663]. Therefore, it suffices to discuss only
[RFC8273].
[RFC8273] "improves host isolation and enhanced subscriber
management on shared network segments" such as Wi-Fi or Ethernet.
The key points are:
. When a prefix is allocated to the host, the router can
proactively install a forwarding entry for that prefix towards
the host. There is no more Router-NCE-on-Demand.
. Without address resolution, router multicast to hosts consists
only of unsolicited RAs. They will be sent to hosts one by one
in unicast because the prefix for every host is different.
. Since different hosts are in different subnets, hosts will send
traffic to other hosts via the router. There is no host-to-host
address resolution.
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Therefore, ND issues caused by Router-NCE-on-Demand and router
multicast to hosts are prevented.
[RFC8273] indicates that a "network implementing a unique IPv6
prefix per host can simply ensure that devices cannot send packets
to each other except through the first-hop router". But when hosts
are on a shared medium like Ethernet, ensuring "devices cannot send
packets to each other except through the first-hop router" requires
additional measures that are not easy to implement. This can limit
the applicability of UPPH and may not be worth the effort. This
tradeoff will be further discussed in Section 4.2. Without such
additional measures, on a shared medium, hosts can still reach each
other in L2 as they belong to the same Solicited-Node Multicast
Group. Therefore, Trusting-all-nodes and host multicast to routers
may cause issues. Of the host multicast issues (i.e., Issues 1, 3,
5, 6, and 7), Unique Prefix per Host prevents Issues 5 and 7,
because there is no need for address resolution among hosts (Issue
5) and there is no possibility of GUA duplication (Issue 7). But
Issues 1, 3, and 6 may occur.
3.4. Wireless ND and Subnet ND
Wireless ND (WiND) [RFC6775][RFC8505][RFC8928][RFC8929] defines a
fundamentally different ND solution for Low-Power and Lossy Networks
(LLNs) [RFC7102]. WiND changes host and router behaviors to use
multicast only for router discovery. The key points are:
. Hosts use unicast to proactively register their addresses at
the routers. Routers use unicast to communicate with hosts and
become an abstract registrar and arbitrator for address
ownership.
. The router also proactively installs NCEs for the hosts. This
avoids the need for address resolution for the hosts.
. The router sets PIO L-bit to 0. Each host communicates only
with the router.
. Other functionalities that are relevant only to LLNs.
WiND addresses all ND issues (Section 2.4) in LLNs. However, WiND
support is not mandatory for general-purpose hosts. Therefore, it
cannot be relied upon as a deployment option without imposing
additional constraints on the participating nodes.
3.5. Scalable Address Resolution Protocol
Scalable Address Resolution Protocol [RFC7586] was an experimental
solution that ended in 2017. The usage scenario is Data Centers
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(DCs), where large L2 domains span across multiple sites. In each
site, multiple hosts are connected to a switch. The hosts can be
Virtual Machines (VMs) so the number can be large. The switches are
interconnected by a native or overlay L2 network.
The switch will snoop and install (IP, MAC address) proxy table for
the local hosts. The switch will also reply to address resolution
requests from other sites to its hosts with its own MAC address. In
doing so, all hosts within a site will appear to have a single MAC
address to other sites. As such, a switch only needs to build a MAC
address table for the local hosts and the remote switches, not for
all the hosts in the L2 domain. Consequently, the MAC address table
size of the switches is significantly reduced. A switch will also
add the (IP, MAC address) replies from remote switches to its proxy
ND table so that it can reply to future address resolution requests
from local hosts for such IPs directly. This greatly reduces the
number of address resolution multicast in the network.
Unlike MBBv6, FBBv6, and UPPH, which try to address all ND issues
(Section 2.4), SARP focuses on reducing address resolution multicast
to improve the performance and scalability of large L2 domains in
DCs.
3.6. ARP and ND Optimization for TRILL:
ARP and ND Optimization for TRILL [RFC8302] is similar to SARP
(Section 3.5). It can be considered an application of SARP in the
TRILL environment.
Like SARP, ARP, and ND Optimization for TRILL focuses on reducing
multicast address resolution. That is, it addresses Issue 5 (Section
2.1).
3.7. Proxy ARP/ND in Ethernet Virtual Private Networks (EVPN)
Proxy ARP/ND in EVPN is specified in [RFC9161]. The usage scenario
is DCs where large L2 domains span across multiple sites. In each
site, multiple hosts are connected to a Provider Edge (PE) router.
The PEs are interconnected by EVPN tunnels.
PE of each site snoops the local address resolution NAs to build
(IP, MAC address) Proxy ND table entries. PEs then propagate such
Proxy ND entries to other PEs via the Border Gateway Protocol (BGP).
Each PE also snoops local hosts' address resolution NSs for remote
hosts. If an entry exists in its Proxy ND table for the remote
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hosts, the PE will reply directly. Consequently, the number of
multicast address resolution messages is significantly reduced.
Like SARP, Proxy ARP/ND in EVPN also focuses on reducing address
resolution multicast.
3.8. Gratuitous Neighbor Discovery (GRAND)
GRAND [RFC9131] changes ND in the following ways:
. A node sends unsolicited NAs upon assigning a new IPv6 address
to its interface.
. A router creates a new NCE for the node and sets its state to
STALE.
Later, when the router receives traffic to the node, the existence
of the NCE entry in the STALE state will cause the router to send
unicast NS to the node to verify its reachability rather than
sending multicast NS to resolve its MAC address. This can shorten
the time the NCE entry reaches the REACHABLE state and improve
forwarding performance. Therefore, GRAND provides an improvement
but does not fully solve the Router-NCE-on-Demand issues. For
example, NCE exhaustion can still happen.
3.9. Reducing Router Advertisements
Maintaining IPv6 connectivity requires that hosts be able to receive
periodic multicast RAs [RFC4861]. Hosts that process unicast
packets while they are asleep must also process multicast RAs while
they are asleep. An excessive number of RAs can significantly reduce
the battery life of mobile hosts. [RFC7772] specifies a solution to
reduce RAs:
. The router should respond to RS with unicast RA (rather than
the normal multicast RA) if the host's source IP address is
specified and the host's MAC address is valid. This way, other
hosts will not receive this RA.
. The router should reduce multicast RA frequency
[RFC7772] addresses Issue 2 (Section 2.1).
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3.10. Source Address Validation Improvement (SAVI) and Router
Advertisement Guard
SAVI [RFC7039] binds an address to a port on an L2 switch and
rejects claims from other ports for that address. Therefore, a node
cannot spoof the IP address of another node.
Router Advertisement Guard (RA-Guard) [RFC6105][RFC7113] only allows
RAs from a port that a router is connected to. Therefore, nodes on
other ports cannot pretend to be a router.
SAVI and RA-Guard address the on-link security issues.
3.11. RFC 6583 Dealing with NCE Exhaustion Attacks
[RFC6583] deals with the NCE Exhaustion attack issue (Section 2.3).
[RFC6583] recommends that:
. Operators should
o Filter unused address space so that messages to such
addresses can be dropped rather than triggering NCE
creation.
o Implement rate-limiting mechanisms for ND message
processing to prevent CPU and memory resources from being
overwhelmed.
. Vendors should
o Prioritizing NDP processing for existing NCEs over creating
new NCEs
[RFC6583] acknowledges that "some of these options are 'kludges',
and can be operationally difficult to manage". [RFC6583] partially
addresses the Router NCE Exhaustion issue. In practical scenarios,
network equipment vendors typically limit the number of NCEs on a
router interface to prevent NCE Exhaustion. But this can have a
side-effect. When more addresses are connected to that interface
than the limit, irregular packet drops may result because the router
does not maintain NCEs for all those IPv6 addresses [RFC9663].
3.12. Registering Self-generated IPv6 Addresses using DHCPv6
In IPv4, network administrators can retrieve a host's IP address
from the DHCP server and use it for subscriber management. In IPv6
and SLAAC, this is not possible (Section 2.3).
[RFC9686] defines a method for informing a DHCPv6 server that a host
has one or more self-generated or statically configured addresses.
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This enables network administrators to retrieve the IPv6 addresses
for each host from the DHCPv6 server. [RFC9686] provides a solution
for Issue 15 (Section 2.3).
3.13. Enhanced DAD
Enhanced DAD [RFC7527] addresses a DAD failure issue in a specific
situation: looped back interface. DAD will fail in a looped-back
interface because the sending host will receive the DAD message back
and will interpret it as another host is trying to use the same
address. The solution is to include a Nonce option [RFC3971] in each
DAD message so that the sending host can detect that the looped-back
DAD message is sent by itself.
Enhanced DAD does not solve any ND issue. It extends ND to work in a
new scenario: looped-back interface. It is reviewed here only for
completeness.
3.14. ND Mediation for IP Interworking of Layer 2 VPNs
ND mediation is specified in [RFC6575]. When two Attachment Circuits
(ACs) are interconnected by a Virtual Private Wired Service (VPWS),
and the two ACs are of different media (e.g., one is Ethernet while
the other is Frame Relay), the two PEs must interwork to provide
mediation service so that a Customer Edge (CE) can resolve the MAC
address of the remote end. [RFC6575] specifies such a solution.
ND Mediation does not address any ND issue. It extends ND to work in
a new scenario: two ACs of different media interconnected by a VPWS.
It is reviewed here only for completeness.
3.15. ND Solutions Defined before the Latest Versions of ND
The latest versions of ND and SLAAC are specified in [RFC4861] and
[RFC4862]. Several ND mitigation solutions were published before
[RFC4861]. They are reviewed in this section only for completeness.
3.15.1. Secure Neighbor Discovery (SeND)
The purpose of SeND [RFC3971] is to ensure that hosts and routers
are trustworthy. SeND defined three new ND options, i.e.,
Cryptographically Generated Addresses (CGA) [RFC3972], RSA public-
key cryptosystem, and Timestamp/Nonce, an authorization delegation
discovery process, an address ownership proof mechanism, and
requirements for the use of these components in the ND protocol.
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3.15.2. Cryptographically Generated Addresses (CGA)
The purpose of CGA is to associate a cryptographic public key with
an IPv6 address in the SeND protocol. The key point is to generate
the Interface Identifier (IID) of an IPv6 address by computing a
cryptographic hash of the public key. The resulting IPv6 address is
called a CGA. The corresponding private key can then be used to
sign messages sent from the address.
CGA assumes that a legitimate host does not care about the bit
combination of the IID that would be created by some hash procedure.
The attacker needs an exact IID to impersonate the legitimate hosts,
but then the attacker is challenged to do a reverse hash calculation
that is a strong mathematical challenge.
CGA is part of SeND. There is no reported deployment.
3.15.3. ND Proxy
ND Proxy [RFC4389] is an Experimental solution. The objective is to
enable multiple links joined by an ND Proxy device to work as a
single link.
. When an ND Proxy receives an ND request from a host on a link,
it will proxy the message out the "best" (defined in the next
paragraph) outgoing interface. If there is no best interface,
the ND Proxy will proxy the message to all other links. Here,
proxy means acting as if the ND message originates from the ND
Proxy itself. That is, the ND Proxy will change the ND
message's source IP and source MAC address to the ND Proxy's
outgoing interface's IP and MAC address, and create an NCE
entry at the outgoing interface accordingly.
. When ND Proxy receives an ND reply, it will act as if the ND
message is destined for itself, and update the NCE entry state
at the receiving interface. Based on such state information,
the ND Proxy can determine the "best" outgoing interface for
future ND requests. The ND Proxy then proxies the ND message
back to the requesting host.
ND Proxy is widely used in SARP (Sections 3.5), ND Optimization for
TRILL (Sections 3.6), and Proxy ARP/ND in EVPN (Sections 3.7).
3.15.4. Optimistic DAD
Optimistic DAD [RFC4429] seeks to minimize address configuration
delays in the successful case and to reduce disruption as far as
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possible in the failure case. That is, Optimistic DAD lets hosts
immediately use the newly formed address to communicate before DAD
completes, assuming that DAD will succeed anyway. If the address
turns out to be duplicate, Optimistic DAD provides a set of
mechanisms to minimize the impact. Optimistic DAD modified the
original ND [RFC2461] and SLAAC [RFC2462], but the solution was not
incorporated into the latest specifications of [RFC4861] and
[RFC4862]. However, implementations of Optimistic DAD exist.
Optimistic DAD does not solve any ND issue (Section 2). It is
reviewed here only for completeness.
4. Guidelines for Prevention of Potential ND Issues
By knowing the potential ND issues and associated mitigation
solutions, network administrators of existing IPv6 deployments can
assess whether these issues may occur in their networks and, if so,
whether to deploy the mitigation solutions proactively. Deploying
these solutions may take time and additional resources. Therefore,
it is advisable to plan.
Network administrators planning to start their IPv6 deployments can
use the issue-solution information to help plan their deployments.
Moreover, they can take proactive action to prevent potential ND
issues.
4.1. Learning Host Isolation from the Existing Solutions
While various ND solutions may initially appear unrelated,
categorizing them into four distinct groups highlights an important
observation: "host isolation" is an effective strategy for
mitigating ND-related issues.
Group 1: L3 and L2 Isolation
This group includes MBBv6 and FBBv6, which isolate hosts at both L3
and L2 by placing each host within its subnet and link. This
prevents ND issues caused by multicast and Trusting-all-nodes, as
each host operates within its isolated domain. Furthermore, since
routers can route packets to a host based on its unique prefix, the
need for Router-NCE-on-Demand is also eliminated. Therefore, L3 and
L2 Isolation prevents all ND issues.
Group 2: L3 Isolation
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This group includes UPPH solutions like [RFC8273] and [RFC9663],
which isolate hosts into separate subnets while potentially leaving
them on the same shared medium. This approach
mitigates ND issues caused by router multicast to hosts and
eliminates the need for "Router-NCE-on-Demand", as detailed in
Section 3.3.
Group 3: Partial L2 Isolation
This group encompasses solutions such as WiND, SARP, ND Optimization
for TRILL, and Proxy ND in EVPN. These solutions use a proxy device
to represent the hosts behind it, effectively isolating those hosts
into distinct multicast domains. While hosts are still located
within the same subnet, their separation into different multicast
domains reduces the scope of ND issues related to multicast-based
address resolution.
Group 4: Non-Isolating Solutions
The final group includes remaining solutions that do not implement
host isolation. These solutions do not prevent ND issues but instead
focus on addressing specific ND problems.
The analysis demonstrates that the stronger the isolation of hosts,
the more ND issues can be mitigated. This correlation is intuitive,
as isolating hosts reduces the multicast scope, minimizes the number
of nodes that must be trusted, and may eliminate the need for
"Router-NCE-on-Demand", the three primary causes of ND issues.
This understanding can be used to prevent ND issues.
4.2. Applicability of Various Isolation Methods
4.2.1. Applicability of L3 & L2 Isolation
Benefits:
o All ND issues (Section 2.4) can be effectively mitigated.
Constraints:
1. L2 Isolation:
Actions must be taken to isolate hosts in L2. In many scenarios,
this can be difficult.
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2. Unique Prefix Allocation:
A large number of prefixes will be required, with one prefix
assigned per host. This is generally not a limitation for IPv6. For
instance, members of a Regional Internet Registry (RIR) can obtain a
/29 prefix allocation [RIPE738], which provides 32 billion /64
prefixes - sufficient for any foreseeable deployment scenarios.
Practical implementations, such as MBBv6 assigning /64 prefixes to
billions of mobile UEs [RFC6459] and FBBv6 assigning /56 prefixes to
hundreds of millions of routed RGs [TR177], demonstrate the
feasibility of this approach.
3. Privacy Issue from Unique Prefix Identifiability:
Assigning a unique prefix to each host may theoretically reduce
privacy, as hosts can be directly identified by their assigned
prefix. However, alternative host identification methods, such as
cookies, are commonly used. Therefore, unique prefix identifiability
may not make much difference. The actual impact on privacy is
therefore likely to be limited.
4. Router Support for L3 Isolation:
The router must support an L3 Isolation solution, e.g., [RFC8273] or
[RFC9663].
5. Increased Router Interface Requirements:
A large number of interfaces will be required at the router, with
one interface dedicated to each host.
6. Router as a Bottleneck:
Since all communication between hosts is routed through the router,
the router may become a performance bottleneck in high-traffic
scenarios.
7. Incompatibility with Host-Based Multicast Services:
Services that rely on multicast communication among hosts, such as
Multicast Domain Name System [RFC6762], will be disrupted.
4.2.2. Applicability of L3 Isolation
Benefits:
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. All ND issues (Section 2.4) are mitigated, with the exception
of:
o LLA DAD multicast degrading performance,
o LLA DAD not reliable in wireless networks, and
o On-link security
These remaining issues depend on the characteristics of the
shared medium:
o If the shared medium is Ethernet, the issues related to LLA
DAD multicast are negligible.
o If nodes can be trusted, such as in private networks, On-
link security concerns are not significant.
. No need for L2 Isolation. Consequently, this method can be
applied in a wide range of scenarios, making it possibly the
most practical host isolation method.
Constraints, as discussed in Section 4.2.1:
1. Unique Prefix Allocation
2. Router Support for L3 Isolation
3. Router as a Bottleneck
4. Privacy Issue from Unique Prefix Identifiability.
4.2.3. Applicability of Partial L2 Isolation
Benefits:
. Reduced Multicast Traffic:
This method reduces multicast traffic, particularly for address
resolution, by dividing the subnet into multiple multicast
domains.
Constraint:
. Router Support for Partial L2 Isolation:
The router must implement a Partial L2 Isolation solution such as
WiND, SARP, ND Optimization for TRILL, and Proxy ND in EVPN to
support this method.
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4.3. Guidelines for Applying Isolation Methods
Based on the applicability analysis provided in the preceding
sections, network administrators can determine whether to implement
an isolation method and, if so, which method is most appropriate for
their specific deployment.
A simple guideline is to consider the isolation methods in the order
listed in the preceding sections, progressing from the strongest
isolation to the weakest:
o Stronger isolation methods can prevent more ND issues, but
may also impose higher entry requirements.
o Weaker isolation methods have fewer entry requirements but
may leave some ND issues unmitigated.
L3 Isolation is often a practical tradeoff:
o It provides significant benefits by addressing most ND
issues.
o Its entry requirements, such as prefix allocation and
router support for L3 Isolation, are generally manageable.
Selecting an isolation method that is either too strong or too weak
does not result in serious consequences:
. Choosing an overly strong isolation method may require the
network administrator to meet higher entry requirements
initially, such as measures for L2 Isolation, additional
prefixes, or additional router capabilities.
. Choosing a "weaker isolation method" may necessitate deploying
supplemental ND mitigation techniques to address any unresolved
ND issues.
In either case, the resulting solution can be functional and
effective.
5. Security Considerations
This document is a review of known ND issues and solutions,
including security. It does not introduce any new solutions.
Therefore, it does not introduce new security issues.
6. IANA Considerations
This document has no request to IANA.
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7. References
7.1. Informative References
[AddrAcc] T. Chown, C. Cummings, D.Carder, "IPv6 Address
Accountability Considerations", Internet draft, Oct. 2024.
[MADINAS] J. Henry, Y. Lee, "Randomized and Changing MAC Address:
Context, Network Impacts, and Use Cases", draft-ietf-
madinas-use-cases-19.
[RFC2461] T. Narten, E. Nordmark, W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, obsoleted by RFC 4861.
[RFC2462] S. Thomson, T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, obsoleted by RFC 4862.
[RFC3756] P. Nikander, J. Kempf, E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756.
[RFC3971] J. Arkko, J. Kempf, B. Zill, P. Nikander, "SEcure Neighbor
Discovery (SEND)", RFC3971.
[RFC3972] T. Aura, "Cryptographically Generated Addresses (CGA)",
RFC3972.
[RFC4389] D. Thaler, M. Talwar, C. Patel, "Neighbor Discovery
Proxies (ND Proxy)", RFC 4389.
[RFC4429] N. Moore, "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862.
[RFC4903] D. Thaler, "Multi-Link Subnet Issues", RFC 4903.
[RFC6085] S. Gundavelli, M. Townsley, O. Troan, W. Dec, "Address
Mapping of IPv6 Multicast Packets on Ethernet", RFC 6085.
[RFC6105] E. Levy-Abegnoli, G. Van de Velde, C. Popoviciu, J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105.
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[RFC6459] J. Korhonen, J. Soininen, B. Patil, T. Savolainen, G.
Bajko, K. Iisakkila, "IPv6 in 3rd Generation Partnership
Project (3GPP) Evolved Packet System (EPS)", RFC 6459.
[RFC6575] H. Shah, E. Rosen, G. Heron, V. Kompella, "Address
Resolution Protocol (ARP) Mediation for IP Interworking of
Layer 2 VPNs", RFC 6575.
[RFC6583] I. Gashinsky, J. Jaeggli, W. Kumari, "Operational Neighbor
Discovery Problems", RFC 6583.
[RFC6762] S. Cheshire, M. Krochmal, "Multicast DNS", RFC 6762.
[RFC6775] Z. Shelby, S. Chakrabarti, E. Nordmark, C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775.
[RFC6957] F. Costa, J-M. Combes, X. Pougnard, H. Li, "Duplicate
Address Detection Proxy", RFC 6957
[RFC7039] J. Wu, J. Bi, M. Bagnulo, F. Baker, C. Vogt, "Source
Address Validation Improvement (SAVI) Framework", RFC
7039.
[RFC7066] J. Korhonen, J. Arkko, T. Savolainen, S. Krishnan, "IPv6
for Third Generation Partnership Project (3GPP) Cellular
Hosts", RFC 7066.
[RFC7102] JP. Vasseur, "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102.
[RFC7113] F. Gont, "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)", RFC 7113.
[RFC7278] Extending an IPv6 /64 Prefix from a Third Generation
Partnership Project (3GPP) Mobile Interface to a LAN
Link", RFC7278.
[RFC7342] L. Dunbar, W. Kumari, I. Gashinsky, "Practices for Scaling
ARP and Neighbor Discovery (ND) in Large Data Centers",
RFC 7342.
[RFC7527] R. Asati, H. Singh, W. Beebee, C. Pignataro, E. Dart, W.
George, "Enhanced Duplicate Address Detection", RFC 7527.
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[RFC7586] Y. Nachum, L. Dunbar, I. Yerushalmi, T. Mizrahi, "The
Scalable Address Resolution Protocol (SARP) for Large Data
Centers", RFC7586.
[RFC7772] A. Yourtchenko, L. Colitti, "Reducing Energy Consumption
of Router Advertisements", RFC 7772.
[RFC8273] J. Brzozowski, G. Van de Velde, "Unique IPv6 Prefix per
Host", RFC 8273.
[RFC8302] Y. Li, D. Eastlake 3rd, L. Dunbar, R. Perlman, M. Umair,
"Transparent Interconnection of Lots of Links (TRILL): ARP
and Neighbor Discovery (ND) Optimization", RFC 8302.
[RFC8415] T. Mrugalski, M. Siodelski, A. Yourtchenko, M. Richardson,
S. Jiang, T. Lemon, T. Winters, "Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 8415.
[RFC8505] P. Thubert, E. Nordmark, S. Chakrabarti, C. Perkins,
"Registration Extensions for IPv6 over Low-Power Wireless
Personal Area Network (6LoWPAN) Neighbor Discovery", RFC
8505.
[RFC8928] P. Thubert, B. Sarikaya, M. Sethi, R. Struik, "Address-
Protected Neighbor Discovery for Low-Power and Lossy
Networks", RFC 8928.
[RFC8929] P. Thubert, C.E. Perkins, E. Levy-Abegnoli, "IPv6 Backbone
Router", RFC 8929.
[RFC9099] E. Vyncke, K. Chittimaneni, M. Kaeo, E. Rey, "Operational
Security Considerations for IPv6 Networks", RFC 9099.
[RFC9119] C. Perkins, M. McBride, D. Stanley, W. Kumari, JC. Zuniga,
"Multicast Considerations over IEEE 802 Wireless Media",
RFC 9119.
[RFC9131] J. Linkova, "Gratuitous Neighbor Discovery: Creating
Neighbor Cache Entries on First-Hop Routers", RFC 9131.
[RFC9161] J. Rabadan, S. Sathappan, K. Nagaraj, G. Hankins, T. King,
"Operational Aspects of Proxy ARP/ND in Ethernet Virtual
Private Networks", RFC 9161.
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[RFC9663] L. Colitti, J. Linkova, X. Ma, "Using DHCP-PD to Allocate
Unique IPv6 Prefix per Client in Large Broadcast
Networks", RFC 9663.
[RFC9686] W. Kumari, S. Krishnan, R. Asati, L. Colitti, J. Linkova,
S. Jiang, "Registering Self-generated IPv6 Addresses using
DHCPv6", RFC 9686.
[RIPE738] IPv6 Address Allocation and Assignment Policy,
https://www.ripe.net/publications/docs/ripe-738
[SND] P. Thubert, M. Richardson, "Architecture and Framework for
IPv6 over Non-Broadcast Access", Internet draft, June
2023.
[TR177] S. Ooghe, B. Varga, W. Dec, D. Allan, "IPv6 in the context
of TR-101", Broadband Forum, TR-177.
8. Acknowledgments
The authors would like to thank Eric Vyncke, Gunter Van de Velde,
Lorenzo Colitti, Erik Kline, Warren Kumari, Mohamed Boucadair,
Pascal Thubert, Jen Linkova, Brian Carpenter, Mike Ackermann, Nalini
Elkins, Ed Horley, Ole Troan, David Thaler, Chongfeng Xie, Chris
Cummings, Dale Carder, Tim Chown, Priyanka Sinha, Aijun Wang, Ines
Robles, Magnus Westerlund, Barry Leiba, and Paul Wouters for their
reviews and comments. The authors would also like to thank Tim
Winters for being the document shepherd.
Authors' Addresses
XiPeng Xiao
Huawei Technologies Dusseldorf
Hansaallee 205, 40549 Dusseldorf, Germany
Email: xipengxiao@huawei.com
Eduard Vasilenko
Huawei Technologies
17/4 Krylatskaya st, Moscow, Russia 121614
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Email: vasilenko.eduard@huawei.com
Eduard Metz
KPN N.V.
Email: eduard.metz@kpn.com
Gyan Mishra
Verizon Inc.
Email: gyan.s.mishra@verizon.com
Nick Buraglio
Energy Sciences Network
Email: buraglio@es.net
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