v6ops Working Group L. Colitti
Internet-Draft Google, LLC
Intended status: Informational J. Linkova, Ed.
Expires: 31 August 2023 X. Ma, Ed.
Google
27 February 2023
Using DHCP-PD to Allocate /64 per Host in Broadcast Networks
draft-collink-v6ops-ent64pd-02
Abstract
This document discusses the IPv6 deployment scenario when individual
hosts connected to broadcast networks (like WiFi hotspots or
enterprise networks) are allocated /64 subnets via DHCP-PD.
Status of This Memo
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This Internet-Draft will expire on 31 August 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Design Principles . . . . . . . . . . . . . . . . . . . . . . 3
4. Prefix Length Considerations . . . . . . . . . . . . . . . . 4
5. Routing Considerations . . . . . . . . . . . . . . . . . . . 5
5.1. First-Hop Routers Requirements . . . . . . . . . . . . . 5
5.2. Topologies with Multiple First-Hop Routers . . . . . . . 5
5.3. Preventing Routing Loops . . . . . . . . . . . . . . . . 5
6. DHCPv6-PD Server Considerations . . . . . . . . . . . . . . . 6
7. Antispoofing and SAVI Interaction . . . . . . . . . . . . . . 6
8. Migration Strategies and Co-existence with SLAAC Using Prefixes
From PIO . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Applicability and Limitations . . . . . . . . . . . . . . . . 8
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
13. Security Considerations . . . . . . . . . . . . . . . . . . . 9
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
14.1. Normative References . . . . . . . . . . . . . . . . . . 9
14.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Unlike IPv4, IPv6 allows (and often requires) hosts to have multiple
addresses. At the very least, a host can be expected to have one
link-local address, one temporary address and, in most cases, one
stable global address. On an IPv6-only network the device would need
to have a dedicated 464XLAT address, which brings the total number of
addresses to 4. If the network is multihomed and uses two different
prefixes, or during graceful renumbering (when the old prefix is
deprecated), or if an enterprise uses ULAs, the number of global
addresses can easily double, bringing the total number of addresses
to 7. Devices running containers/namespaces might need even more
addresses per physical host. On one hand multiple addresses could be
considered as a significant advantage of IPv6. On the other hand,
however, they are sometimes seen as a drawback for the following
reasons:
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* Increased number of addresses introduces scalability issues on the
network infrastructure. Network devices need to maintain various
types of tables/hashes (Neighbor Cache on first-hop routers,
Neighbor Discovery Proxy caches on L2 devices etc). On VXLAN
[RFC7348] networks each address might be represented as a route so
8 addresses instead of 1 requires the devices to support 8 times
more routes, etc.
* An operator might need to know all addresses used by a given
device in the past for forensics or troubleshooting purposes.
* If an infrastructure device resources are exhausted, the device
might drop some IPv6 addresses from the corresponding tables. The
host, however, might be still using the address to send traffic.
This leads to traffic blackholing and degraded customer
experience.
[RFC7934] discusses this aspect and explicitly states that IPv6
deployments SHOULD NOT limit the number of IPv6 addresses a host can
have. However it seems inevitable that some limits might need to be
imposed by the network in an attempt to protect the network resources
and prevent DoS attacks (see [I-D.linkova-v6ops-ipmaclimi]).
Therefore it would be beneficial for IPv6 deployments to address the
above mentioned issues while still allowing hosts to have multiple
IPv6 addresses. One of the very promising approaches is allocating
an unique /64 prefix per host ([RFC8273]). The same principle has
been actively used in mobile IPv6 deployments. However it's very
uncommon in enterprise-style networks, where nodes are usually
connected to broadcast segments/VLANs and each segment has a single
/64 subnet assigned. This document expands the approach defined in
[RFC8273] to allocate an unique IPv6 /64 prefix per host using DHCP-
PD.
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. Design Principles
Instead of all hosts on a given segment forming addresses from the
same /64 assigned to that segment:
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* A host acts as DHCP-PD client and requests /64 via DHCPv6-PD by
sending IA_PD.
* The first-hop router acts as a DHCPv6-PD relay and sends the
request to the DHCPv6-PD servers. In smaller networks it's
entirely possible for the first-hop router to act as a DHCPv6-PD
server and assign /64 from a larger pool allocated for the given
segment or the whole site.
* The allocated /64 is installed into the first-hop router routing
table as a route pointing to the client's link-local address. For
the router and all other infrastructure devices that prefix is
considered off-link, so no traffic to that prefix does not trigger
any ND packets.
* The host uses the delegated /64 to allocate addresses to its
interfaces, containers etc. For example, the device can include
that /64 into Router Advertisements sent to virtual systems or to
any other devices connected to its downstream interface.
4. Prefix Length Considerations
As per [RFC7421] /64 is the only prefix size that will allow the host
to use SLAAC. As a result delegating a /64 prefix to the client
allows the client to provide limitless addresses to IPv6 nodes
connected to it (e.g., virtual machines, tethered devices), because
all IPv6 nodes are required to support SLAAC. In other words,
delegating /64 allows the hosts to extend the network arbitrarily,
similarily to using NAT in IPv4. Chosing longer prefixes would
require the host and any connected system to use some other form of
address assignment and therefore would drastically limit
applicability of the proposed solution.
Section 9.2 of [RFC7934] demonstrates that if a network uses
10.0.0.0/8 to address hosts, /40 would be sufficient to provide each
host with /64. In multi-site networks the calculations might get
more complex as each site IPv6 prefix needs to be larger enough to be
globally routable and accepted by eBGP peers, for example /48. Let's
consider an enterprise network which has 8000 sites (~2^13). Imagine
that site has up to 64 (2^6) different network types and each network
requires its own /48. So each network can provide /64 to 65K clients
(an equivalent of using /16 IPv4 subnet to address hosts). In that
case such an enterprise would need /29 (48 - 6 - 13) to provide /64
to its devices. Networks of such size usually have multiple
allocations from RIRs so /29 sounds reasonable. In real life there
are very few networks of that scale and a single /32 would be
sufficient for most deployments.
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5. Routing Considerations
5.1. First-Hop Routers Requirements
The design described in this document is targeted to large networks
were the number of clients combined with multiple IPv6 addresses per
client creates scalability issues. In such networks DHCPv6 servers
are usually deployed as dedicated systems, so the first-hop routers
act as DHCP relays. To delegate IPv6 prefixes to hosts the first hop
router needs to implement DHCPv6-PD relay functions and meet the
requirements defined in [RFC8987].
In particular, if the same DHCPv6-PD pool is used for clients
connected to multiple routers, dynamic routing protocols are required
to propagate the routes to the allocated prefixes. Each relay needs
to advertise the learned delegated leases as per requirement R-4
specified in Section 4.2 of [RFC8987].
5.2. Topologies with Multiple First-Hop Routers
Traditionally DHCPv6-PD is used in environments where a DHCPv6-PD
client (a home CPE, for example) is connected to a single router
which performs DHCPv6-PD relay functions. In the topology with
redundant first-hop routers, all those routers need to snoop DHCPv6
traffic, install the delegated prefixes into its routing table and,
if needed, advertise those prefixes to the network. That means that
all relays the host is connected to must be able to snoop DHCPv6-PD
traffic, in particular Reply messages sent by the server (as those
messages contain the delegated prefix). Normally the client uses
multicast to reach all servers or an individual server (see
Section 14 of [RFC8415]). As per Section 18.4 of [RFC8415] the
server is not supposed to accept unicast traffic when it is not
explicitly configured to do, and unicast transmission is only allowed
for some messages and only if the Server Unicast option ([RFC8415],
Section 21.12) is used. Therefore, in the topologies with multiple
first-hop routers the DHCPv6 servers MUST be configured not to use
the Server Unicast option (it should be noted that
[I-D.dhcwg-dhc-rfc8415bis] deprecates the Server Unicast option
exactly because it is not compatible with multiple relays topology).
Therefore as long as the Server Unicast option is not used, all
first-hop routers shall be able to install the route for the
delegates prefix.
5.3. Preventing Routing Loops
To prevent routing loops caused by traffic to unused addresses from
the delegated /64, the client MUST drop all packets to such addresses
(see the requirement WPD-5 in Section 4.2 of [RFC7084].
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6. DHCPv6-PD Server Considerations
The following requirements are applicable to the DHCPv6-PD server
delegating prefixes to endhosts:
* The server MUST follow [RFC8168] recommendations on processing
prefix-length hints. Specifically, the server MUST NOT delegate
prefixes longer than /64 if the client sets the prefix-length
field of the IA_PD option to 64. This is required so the host can
use SLAAC to assign addresses from the delegated /64 to its
interfaces or virtual instances. The client MAY ignore any
delegated prefixes longer that /64.
* Do not send the Server Unicast option to the client unless the
network topology guarantees that no client is connected to a
segment with multiple relays (see Section 5.2).
7. Antispoofing and SAVI Interaction
Enabling the unicast Reverse Path Forwarding (uRPF) on the first-hop
router interfaces towards clients provides the first layer of defence
agains spoofing. If the malicious client sends a spoofed packet it
would be dropped by the router unless the spoofed address belongs to
a prefix delegated to another client on the same interface.
Therefore the malicious client can only spoof addresses already
delegated to another client on the same segment or another host link-
local address.
Source Address Validation Improvement (SAVI, [RFC7039]) provides more
reliable protection against address spoofing. Administrators
deploying the proposed solution on the SAVI-enabled infastructure
should ensure that SAVI perimeter devices support DHCPv6-PD snooping
to create the correct binding for the delegated prefixes (see
[RFC7513]). Using FCFS SAVI ([RFC6620]) for protecting link-local
addresses and creating SAVI bindings for DHCPv6-PD assigned prefixes
would prevent spoofing.
It should be noted that using DHCPv6-PD makes it harder for an
attacker to select the spoofed source address. When all hosts are
using the same /64 to form addresses, the attacker might learn
addresses used by other hosts by listening to multicast Neighbor
Solicitations and Neighbour Advertisements. In DHCPv6-PD
environments, however, the attacker can only learn about other hosts
global addresses by listening to multicast DHCPv6 messages, which are
not transmitted so often.
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8. Migration Strategies and Co-existence with SLAAC Using Prefixes From
PIO
It would be beneficial for the network to explicitly indicate its
support of DHCPv6-PD for hosts.
* In small networks (e.g. home ones), where the number of devices is
not too high, the number of available /64 becomes a limiting
factor. If every phone or laptop in a home network would request
an unique /64, the home network might run out of /64s, if the
prefix allocated to the CPE by its ISP is too small (e.g. if an
ISP allocates /60, it would only allow 16 devices to request /64).
So while the enterprise network administrator might want all
phones in the network to request /64, it would be highly
undesirable for the same phone to request /64 when connecting to a
home network.
* When the network supports both /64 per host and /64 advertised in
PIO as address assignment methods, it's highly desirable for the
host NOT to use the PIO prefix tto form global addresses and only
use the prefix delegated via DHCPv6-PD. Starting both SLAAC using
the PIO prefix and DHCPv6-PD and deprecating that SLAAC addresses
after receiving a delegated prefix would be very disruptive for
applications. If the host continues to use addresses formed from
the PIO prefix it would not only undermine the benefits of the
proposed solution (see Section 9), but would also introduce
complexity and unpredictability in the source address selection.
Therefore the host needs to know what address assignment method to
use and whether to use the prefix in PIO or not, if the network
provides the PIO with A flag set.
To allow the network to signal DHCPv6-PD support a new PIO flag is
proposed (link to the 6man draft will be added here after this draft
is adopted).
9. Benefits
The proposed solution provides the following benefits:
* The network devices resources (e.g. memory) need to scale to the
number of devices, not the number of IPv6 addresses. The first-
hop routers have a single /64 route per host pointing to the
host's link-local address.
* If all devices connected to the given network segment support this
mode of operation and can generate addresses from the delegated
/64, there is no reason to advertise a common /64 assigned to that
segment in PIO with 'A' flag set. Therefore it is possible to
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remove the global /64 prefix from that network segment and the
router interface completely, so no global addresses are on-link
for the segment. This would lead to reducing the attack surface
for Neighbor Discovery attacks described in [RFC6583].
* DHCP-PD logs and first-hop routers routing tables provide complete
information on IPv6 to MAC mapping, which can be used for
forensics and troubleshooting. Such information is much less
dynamic than ND cache and therefore it's much easier for an
operator to collect and process it.
* The cost of having multiple addresses is offloaded to the
endpoint. Devices are free to create and use as many addresses as
they need.
* Fate sharing: all global addresses used by a given device are
routed as a single /64. Either all of them work or not, which
makes the failures easier to diagnoze and mitigate.
* Ability to extend the network transparently. If the hosts use
SLAAC, delegating /64 allows the host to provide connectivity to
other hosts, like it is possible in IPv4 with NAT.
10. Applicability and Limitations
The solution described in this document shouldn't be seen as an
attempt to deprecate SLAAC. Delegating /64 provides an alternative
for SLAAC rather than replacing it, so network administrators and OS
developers have a choice. This design would substantially benefit
some networks (see Section 9), so running an additional service and
allocation larger amount of address space is well justified.
Examples of such networks include but are not limited to:
* Large-scale networks where even 3-5 addresses per client might
intorduce scalability issues.
* Networks with high number of virtual hosts, so physical enpoints
require multiple addresses.
* Managed networks where extensive troubleshooting, host traffic
logging or forensics might be required.
Smaller networks (like home ones) with smaller address space and
lower number of clients, SLAAC might be a better and simpler option.
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11. Privacy Considerations
Eventually, if/when the vast majority of endpoints support the
proposed mechanism, an eavesdropper/information collector might be
able to correlate the prefix to the device. To mitigate the threat
the host might implement a mechanism similar to SLAAC temporary
extensions ([RFC8981]) but for delegated prefixes:
* The host requests another /64 prefix.
* Upon receiving the new prefix the host deprecates all addresses
from the old one.
* After some time (shall it be T2 from IA_PD for the original
prefix?) the host sends RELEASE for the old prefix.
12. IANA Considerations
This memo includes no request to IANA.
13. Security Considerations
A malicious or just misbehaving host might exhaust the DHCP-PD pool
by sending a large number of requests with various DUIDs. However
this is not a new issue as the same attack might be implemented in
DHCPv4 or DHCPv6 for IA_NA requests.
A malicious host might request a prefix and then release it very
quickly, causing routing convergence events on the relays. The
probability of such attack can be reduced if the network rate limits
the amount of broadcast and multicast messages from the client.
Delegating the same prefix for the same host introduces privacy
concerns. The proposed mitigation is discussed in Section 11.
Spoofing scenarios and prevention mechanisms are discussed in
Section 7.
14. References
14.1. 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>.
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[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>.
[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>.
[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>.
[RFC8168] Li, T., Liu, C., and Y. Cui, "DHCPv6 Prefix-Length Hint
Issues", RFC 8168, DOI 10.17487/RFC8168, May 2017,
<https://www.rfc-editor.org/info/rfc8168>.
[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>.
[RFC8273] Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix
per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017,
<https://www.rfc-editor.org/info/rfc8273>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.
[RFC8987] Farrer, I., Kottapalli, N., Hunek, M., and R. Patterson,
"DHCPv6 Prefix Delegating Relay Requirements", RFC 8987,
DOI 10.17487/RFC8987, February 2021,
<https://www.rfc-editor.org/info/rfc8987>.
14.2. Informative References
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[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>.
[RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
"Source Address Validation Improvement (SAVI) Framework",
RFC 7039, DOI 10.17487/RFC7039, October 2013,
<https://www.rfc-editor.org/info/rfc7039>.
[RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
Boundary in IPv6 Addressing", RFC 7421,
DOI 10.17487/RFC7421, January 2015,
<https://www.rfc-editor.org/info/rfc7421>.
[RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
Validation Improvement (SAVI) Solution for DHCP",
RFC 7513, DOI 10.17487/RFC7513, May 2015,
<https://www.rfc-editor.org/info/rfc7513>.
[RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
eXtensible Local Area Network (VXLAN): A Framework for
Overlaying Virtualized Layer 2 Networks over Layer 3
Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
<https://www.rfc-editor.org/info/rfc7348>.
[RFC7934] Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
"Host Address Availability Recommendations", BCP 204,
RFC 7934, DOI 10.17487/RFC7934, July 2016,
<https://www.rfc-editor.org/info/rfc7934>.
[I-D.linkova-v6ops-ipmaclimi]
Linkova, J., "Minimizing Damage of Limiting Number of IPv6
Addresses per Host", Work in Progress, Internet-Draft,
draft-linkova-v6ops-ipmaclimi-00, 7 November 2022,
<https://datatracker.ietf.org/doc/html/draft-linkova-
v6ops-ipmaclimi-00>.
[I-D.dhcwg-dhc-rfc8415bis]
Mrugalski, T., Volz, B., Richardson, M., Jiang, S., and T.
Winters, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", Work in Progress, Internet-Draft, draft-dhcwg-
dhc-rfc8415bis-00, 7 October 2022,
<https://datatracker.ietf.org/doc/html/draft-dhcwg-dhc-
rfc8415bis-00>.
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Acknowledgements
Thanks to Brian Carpenter, Gert Doering, Fernando Gont, Martin Hunek,
Erik Kline, Pascal Thubert, Eduard Vasilenko, Timothy Winters for the
discussions, the input and all contribution.
Contributors
Authors' Addresses
Lorenzo Colitti
Google, LLC
Shibuya 3-21-3,
Japan
Email: lorenzo@google.com
Jen Linkova (editor)
Google
1 Darling Island Rd
Pyrmont NSW 2009
Australia
Email: furry13@gmail.com, furry@google.com
Xiao Ma (editor)
Google
Shibuya 3-21-3,
Japan
Email: xiaom@google.com
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