Safe(r) Limited Domains

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Table of Contents

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.¶

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 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        |       |
+-----------+-----------+ - - - -+----------+--------- - - -+-------|

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.¶

  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

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.¶

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

Acknowledgments

Much thanks to Brian Carpenter, for his review and comments.¶

Also much thanks to Deborah Brungard, for her review and comments.¶

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