Safe(r) Limited Domains
draft-wkumari-intarea-safe-limited-domains-02
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Warren "Ace" Kumari , Andrew Alston , Éric Vyncke , Suresh Krishnan , Donald E. Eastlake 3rd | ||
| Last updated | 2024-10-11 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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draft-wkumari-intarea-safe-limited-domains-02
Internet Area Working Group W. Kumari
Internet-Draft Google, LLC
Intended status: Standards Track A. Alston
Expires: 14 April 2025 Liquid Intelligent Technologies
É. Vyncke
S. Krishnan
Cisco
D. Eastlake
Independent
11 October 2024
Safe(r) Limited Domains
draft-wkumari-intarea-safe-limited-domains-02
Abstract
There is a trend towards documents describing protocols that are only
intended to be used within "limited domains". These documents often
do not clearly define how the boundary of the limited domain is
implemented and enforced, or require that operators of these limited
domains //perfectly// add filters at all of the boundary nodes of the
domain to protect the rest of the global Internet from these
protocols and vice-versa.
This document discusses the concepts of "fail-open" versus "fail-
closed" protocols for limited domains. It further specifies how to
use layer-2 protocol identification mechanisms for designing limited
domain protocols that are safer to deploy.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Internet Area Working
Group Working Group mailing list (int-area@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/int-area/.
Source for this draft and an issue tracker can be found at
https://github.com/wkumari/draft-wkumari-intarea-safe-limited-
domains.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Fail-open versus Fail-closed . . . . . . . . . . . . . . . . 4
4. Making a layer-3 type limited-domain protocol fail-closed . . 4
5. Ethernet Protocol Identification . . . . . . . . . . . . . . 5
5.1. Extended EtherType Protocol Identification . . . . . . . 6
5.2. Specific EtherType Protocol Identification . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
[RFC8799] discusses the concept of "limited domains", provides
examples of limited domains, as well as Examples of Limited Domain
Solutions, including Service Function Chaining (SFC [RFC7665] ),
Segment Routing, "Creative uses of IPv6 features" (including
Extension headers, e.g., for in situ Operations, Administration, and
maintenance [RFC9378]).
In order to provide context, this document will quote extensively
from [RFC8799], but it is assumed that the reader will actually read
[RFC8799] in its entirety.
[RFC8799] Section 3, notes:
A common argument is that if a protocol is intended for limited
use, the chances are very high that it will in fact be used (or
misused) in other scenarios including the so-called open Internet.
This is undoubtedly true and means that limited use is not an
excuse for bad design or poor security. In fact, a limited use
requirement potentially adds complexity to both the protocol and
its security design, as discussed later.
Notably, in [RFC8799] Section 2, states:
Domain boundaries that are defined administratively (e.g., by
address filtering rules in routers) are prone to leakage caused by
human error, especially if the limited domain traffic appears
otherwise normal to the boundary routers. In this case, the
network operator needs to take active steps to protect the
boundary. This form of leakage is much less likely if nodes must
be explicitly configured to handle a given limited-domain
protocol, for example, by installing a specific protocol handler.
This document addresses the problem of "leakage" of limited domain
protocols by providing a mechanism so that nodes must be explicitly
configured to handle the given limited-domain protocol ("fail-
closed"), rather than relying on the network operator to take active
steps to protect the boundary ("fail-open").
2. Conventions and Definitions
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.
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3. Fail-open versus Fail-closed
Protocols can be broadly classified as either "fail-open" or "fail-
closed". Fail-closed protocols are those that require explicit
interface or device-wide configuration to enable them to be accepted
or processed when received on an interface. A classic example of a
fail-closed protocol is MPLS ([RFC3031]): In order to allow MPLS to
transit an interface, the operator must enable the MPLS protocol on
that interface and on the device itself. This ensures that outside
MPLS traffic does not leak in.
Fail-open protocols are those that require explicit configuration in
order to ensure that they do not leak out of a domain, for example,
through the application of filters. An example of a fail-open
protocol is SRv6 - in order to ensure that SRv6 traffic does not leak
out of a network, the operator must explicitly filter this traffic,
and, in order to ensure that SRv6 traffic does not leak in, the
operator must explicitly filter SRv6 traffic.
Fail-open protocols are inherently riskier than fail-closed
protocols, as they rely on perfect configuration of filters on all
interfaces at the boundary of a domain, and, if the filters are
removed for any reason (for example, during troubleshooting), there
is a risk of inbound or outbound leaks. In addition, some devices or
interfaces may have limitations in the size and complexity of filters
that can be applied, and so adding new filter entries to limit leaks
of a new protocol may not be possible.
Fail-closed protocols, on the other hand, do not require any explicit
filtering. In order for the protocol to be accepted and processed
when received on an interface, the operator must explicitly enable
the protocol on that interface and on the device itself. In
addition, there is less risk of operational mistakes, as it does not
rely on filters that may be limited in number and complexity.
Finally, fail-closed protocols do not require that operators of
networks outside of the limited domain implement filters to protect
their networks from the limited domain traffic.
4. Making a layer-3 type limited-domain protocol fail-closed
One way to make a limited-domain protocol fail-closed is to assign it
a unique layer-2 protocol identifier, usually an EtherType. This
mechanism is used by MPLS. In modern router and hosts, if such a
protocol identifier is not enabled on an interface, then the Ethernet
chipset will ignore the frame, and the node will not see or process
it. Thus, it is necessary to specifically enable the layer-2
protocol identifier on all relevant interfaces inside the limited
domain, and the protocol will be blocked at the domain boundary where
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the protocol has not been so enabled. This is a simple and effective
mechanism to ensure that the protocol does not leak out of the
limited domain if and when an operator makes a mistake in configuring
filters based on identifiers appearing deeper in the frame such as IP
addresses or IP protocol or header options.
This layer-2 protocol identifier technique only works for transport-
type limited domain protocols (i.e., protocols running at layer 3).
Higher layer protocols cannot necessarily be protected in this way,
and so cryptographically enforced mechanisms may need to be used
instead (e.g., as done used by ANIMA in [RFC8994] and [RFC8995]).
5. Ethernet Protocol Identification
Figure 1 shows the general format of Ethernet frames. The relevant
protocol identification field occurs after the destination and source
MAC addresses and any tags (such a VLAN tags). The alternatives for
protocol identification are discussed in Section 3 of [RFC9542].
+-----------+-----------+- - - - +----------+--------- - - -+-------+
|Destination| Source |Optional| Protocol | Body of |Trailer|
|MAC Address|MAC Address| Tags |Identifier| Frame | |
+-----------+-----------+ - - - -+----------+--------- - - -+-------|
Figure 1: Ethernet Frame Format
This document considers EtherType protocol identification. An
EtherType is an unsigned 16-bit field in an Ethernet frame with a
value in the range of 0x0600 to 0xFFFF, and so it is a somewhat
limited resource; however, there exists a special Extended EtherType
(0x88B7) that can be suffixed by an Organizationally Unique
Identifier (OUI) followed by a further 16-bits identifying the
protocol relative to that OUI as discussed in Section 3 of [RFC9542].
These alternatives of a direct EtherType or use of the Extended
EtherType for the case of the IANA OUI are illustrated in Figure 2.
The following subsections discuss the factors which may influence the
choice between these alternatives when use of such layer 2 protocol
identification, to make the isolation of a limited domain more
robust, is warranted.
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01234567 01234567
+--------+--------+
| EtherType |
+--------+--------+
Specific EtherType
01234567 01234567 01234567 01234567 01234567 01234567 01234567
+--------+--------+--------+--------+--------+--------+--------+
| 0x88 | 0xB7 | 0x00 | 0x00 | 0x5E | Protocol Number |
+--------+--------+--------+--------+--------+--------+--------+
Extended EtherType| IANA OUI |IANA Protocol Number
Figure 2: EtherType Based Protocol Identification
Because specific EtherTypes are a limited resource, an Extended
EtherType SHOULD be used unless there is a strong reason why it will
not work satisfactorily and a specific EtherType is required.
5.1. Extended EtherType Protocol Identification
The main advantage of using an Extended EtherType with an IANA
Protocol Number, as shown in Figure 2, is that such a number can be
allocated by IANA with Expert Review based on an Internet Draft and
are thus relatively easy to obtain. The main disadvantage is that
the protocol identification is 5 bytes longer than a specific
dedicated EtherType.
5.2. Specific EtherType Protocol Identification
The primary disadvantage of using a specific EtherType, as opposed to
an Extended EtherType, is that assignment of such an EtherType is
significantly more difficult than assignment of an Extended EtherType
IANA protocol number. As discussed in [RFC9542], a specific
EtherType can only be assigned by the IEEE Registration Authority
under the following policy: "Since EtherTypes are a fairly scarce
resource, the IEEE RAC has let us know that they will not assign a
new EtherType to a new IETF protocol specification until the IESG has
approved the protocol specification for publication as an RFC. In
exceptional cases, the IEEE RA is willing to consider "early
allocation" of an EtherType for an IETF protocol that is still under
development as long as the request comes from and has been vetted by
the IESG." ([RFC9542] Appendix B.1, citing [IESG_EtherType])
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During development and testing, a protocol can use a "Local
Experimental Ethertype" (0x88b5 and 0x88b6 - [IANA_EtherType]). Once
the protocol is approved for publication, the IESG can request an
EtherType from the IEEE. However, there is always a risk of some
implementation using a Local Experimental EtherType not getting
updated causing conflicts with a later different use of that
experimental EtherType.
The primary advantage of using a specific EtherType is the saving of
5 bytes relative to the use of the Extended EtherType with a protocol
number under the IANA OUI.
6. Security Considerations
Protocols are designated as "limited domain" because something
unexpected might happen if they leak outside of a domain with unified
management. For example, VLAN or VPN or overlay identifiers may be
misinterpreted resulting in the delivery of data to or the acceptance
of data from unauthorized network nodes violating intended security
constraints. The use of a layer-2 protocol identifier to provide a
"fail closed" barrier at the domain border can significantly improve
security by eliminating the opportunity for such misinterpretation.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. 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/rfc/rfc2119>.
[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/rfc/rfc8174>.
[RFC8799] Carpenter, B. and B. Liu, "Limited Domains and Internet
Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
<https://www.rfc-editor.org/rfc/rfc8799>.
8.2. Informative References
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[IANA_EtherType]
"IANA EtherType Registry", Web
<https://www.iana.org/assignments/ieee-802-numbers/ieee-
802-numbers.xhtml#ieee-802-numbers-1>.
[IESG_EtherType]
"IESG Statement on EtherTypes", Web
<https://www.ietf.org/about/groups/iesg/statements/
ethertypes>, 1 May 2023.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/rfc/rfc3031>.
[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/rfc/rfc7665>.
[RFC8994] Eckert, T., Ed., Behringer, M., Ed., and S. Bjarnason, "An
Autonomic Control Plane (ACP)", RFC 8994,
DOI 10.17487/RFC8994, May 2021,
<https://www.rfc-editor.org/rfc/rfc8994>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/rfc/rfc8995>.
[RFC9378] Brockners, F., Ed., Bhandari, S., Ed., Bernier, D., and T.
Mizrahi, Ed., "In Situ Operations, Administration, and
Maintenance (IOAM) Deployment", RFC 9378,
DOI 10.17487/RFC9378, April 2023,
<https://www.rfc-editor.org/rfc/rfc9378>.
[RFC9542] Eastlake 3rd, D., Abley, J., and Y. Li, "IANA
Considerations and IETF Protocol and Documentation Usage
for IEEE 802 Parameters", BCP 141, RFC 9542,
DOI 10.17487/RFC9542, April 2024,
<https://www.rfc-editor.org/rfc/rfc9542>.
Acknowledgments
Much thanks to Brian Carpenter, for his review and comments.
Also much thanks to Deborah Brungard, for her review and comments.
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Also much thanks to everyone else with whom we have discussed this
topic; I've had numerous discussions with many many people on this,
and I'm sure that I've forgotten some of them. Apologies if you were
one of them.
Changelog
* 01-02:
- Add Donald Eastlake as an author.
- Substantial re-write and expansion of material concerning
specific and Extended EtherType protocol identification.
- Add initial Security Considerations text.
* 00-01:
- Deborah pointed out that "this only works for transport-type
limited domain protocols (e.g., SRv6)" could be read as SRv6
fails-closed.
Authors' Addresses
Warren Kumari
Google, LLC
Email: warren@kumari.net
Andrew Alston
Liquid Intelligent Technologies
Email: andrew-ietf@liquid.tech
Éric Vyncke
Cisco
Email: evyncke@cisco.com
Suresh Krishnan
Cisco
Email: suresh.krishnan@gmail.com
Donald Eastlake
Independent
Email: d3e3e3@gmail.com
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