



Independent Submission                                      B. Carpenter
Request for Comments: 8799                             Univ. of Auckland
Category: Informational                                           B. Liu
ISSN: 2070-1721                                      Huawei Technologies
                                                               July 2020


                 Limited Domains and Internet Protocols

Abstract

   There is a noticeable trend towards network behaviors and semantics
   that are specific to a particular set of requirements applied within
   a limited region of the Internet.  Policies, default parameters, the
   options supported, the style of network management, and security
   requirements may vary between such limited regions.  This document
   reviews examples of such limited domains (also known as controlled
   environments), notes emerging solutions, and includes a related
   taxonomy.  It then briefly discusses the standardization of protocols
   for limited domains.  Finally, it shows the need for a precise
   definition of "limited domain membership" and for mechanisms to allow
   nodes to join a domain securely and to find other members, including
   boundary nodes.

   This document is the product of the research of the authors.  It has
   been produced through discussions and consultation within the IETF
   but is not the product of IETF consensus.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8799.

Copyright Notice

   Copyright (c) 2020 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
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction
   2.  Failure Modes in Today's Internet
   3.  Examples of Limited Domain Requirements
   4.  Examples of Limited Domain Solutions
   5.  The Scope of Protocols in Limited Domains
   6.  Functional Requirements of Limited Domains
   7.  Security Considerations
   8.  IANA Considerations
   9.  Informative References
   Appendix A.  Taxonomy of Limited Domains
     A.1.  Domain as a Whole
     A.2.  Individual Nodes
     A.3.  Domain Boundary
     A.4.  Topology
     A.5.  Technology
     A.6.  Connection to the Internet
     A.7.  Security, Trust, and Privacy Model
     A.8.  Operations
     A.9.  Making Use of This Taxonomy
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   As the Internet continues to grow and diversify, with a realistic
   prospect of tens of billions of nodes being connected directly and
   indirectly, there is a noticeable trend towards network-specific and
   local requirements, behaviors, and semantics.  The word "local"
   should be understood in a special sense, however.  In some cases, it
   may refer to geographical and physical locality -- all the nodes in a
   single building, on a single campus, or in a given vehicle.  In other
   cases, it may refer to a defined set of users or nodes distributed
   over a much wider area, but drawn together by a single virtual
   network over the Internet, or a single physical network running in
   parallel with the Internet.  We expand on these possibilities below.
   To capture the topic, this document refers to such networks as
   "limited domains".  Of course, a similar situation may arise for a
   network that is completely disconnected from the Internet, but that
   is not our direct concern here.  However, it should not be forgotten
   that interoperability is needed even within a disconnected network.

   Some people have concerns about splintering of the Internet along
   political or linguistic boundaries by mechanisms that block the free
   flow of information.  That is not the topic of this document, which
   does not discuss filtering mechanisms (see [RFC7754]) and does not
   apply to protocols that are designed for use across the whole
   Internet.  It is only concerned with domains that have specific
   technical requirements.

   The word "domain" in this document does not refer to naming domains
   in the DNS, although in some cases, a limited domain might
   incidentally be congruent with a DNS domain.  In particular, with a
   "split horizon" DNS configuration [RFC6950], the split might be at
   the edge of a limited domain.  A recent proposal for defining
   definite perimeters within the DNS namespace [DNS-PERIMETER] might
   also be considered to be a limited domain mechanism.

   Another term that has been used in some contexts is "controlled
   environment".  For example, [RFC8085] uses this to delimit the
   operational scope within which a particular tunnel encapsulation
   might be used.  A specific example is GRE-in-UDP encapsulation
   [RFC8086], which explicitly states that "The controlled environment
   has less restrictive requirements than the general Internet."  For
   example, non-congestion-controlled traffic might be acceptable within
   the controlled environment.  The same phrase has been used to delimit
   the useful scope of quality-of-service protocols [RFC6398].  It is
   not necessarily the case that protocols will fail to operate outside
   the controlled environment, but rather that they might not operate
   optimally.  In this document, we assume that "limited domain" and
   "controlled environment" mean the same thing in practice.  The term
   "managed network" has been used in a similar way, e.g., [RFC6947].
   In the context of secure multicast, a "group domain of
   interpretation" is defined by [RFC6407].

   Yet more definitions of types of domains are to be found in the
   routing area, such as [RFC4397], [RFC4427], and [RFC4655].  We
   conclude that the notion of a limited domain is very widespread in
   many aspects of Internet technology.

   The requirements of limited domains will depend on the deployment
   scenario.  Policies, default parameters, and the options supported
   may vary.  Also, the style of network management may vary between a
   completely unmanaged network, one with fully autonomic management,
   one with traditional central management, and mixtures of the above.
   Finally, the requirements and solutions for security and privacy may
   vary.

   This document analyzes and discusses some of the consequences of this
   trend and how it may impact the idea of universal interoperability in
   the Internet.  First, we list examples of limited domain scenarios
   and of technical solutions for limited domains, with the main focus
   being the Internet layer of the protocol stack.  An appendix provides
   a taxonomy of the features to be found in limited domains.  With this
   background, we discuss the resulting challenge to the idea that all
   Internet standards must be universal in scope and applicability.  To
   the contrary, we assert that some protocols, although needing to be
   standardized and interoperable, also need to be specifically limited
   in their applicability.  This implies that the concepts of a limited
   domain, and of its membership, need to be formalized and supported by
   secure mechanisms.  While this document does not propose a design for
   such mechanisms, it does outline some functional requirements.

   This document is the product of the research of the authors.  It has
   been produced through discussions and consultation within the IETF
   but is not the product of IETF consensus.

2.  Failure Modes in Today's Internet

   Today, the Internet does not have a well-defined concept of limited
   domains.  One result of this is that certain protocols and features
   fail on certain paths.  Earlier analyses of this topic have focused
   either on the loss of transparency of the Internet [RFC2775]
   [RFC4924] or on the middleboxes responsible for that loss [RFC3234]
   [RFC7663] [RFC8517].  Unfortunately, the problems persist both in
   application protocols and even in very fundamental mechanisms.  For
   example, the Internet is not transparent to IPv6 extension headers
   [RFC7872], and Path MTU Discovery has been unreliable for many years
   [RFC2923] [RFC4821].  IP fragmentation is also unreliable
   [FRAG-FRAGILE], and problems in TCP MSS negotiation have been
   reported [IPV6-USE-MINMTU].

   On the security side, the widespread insertion of firewalls at domain
   boundaries that are perceived by humans but unknown to protocols
   results in arbitrary failure modes as far as the application layer is
   concerned.  There are operational recommendations and practices that
   effectively guarantee arbitrary failures in realistic scenarios
   [IPV6-EXT-HEADERS].

   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.

   Investigations of the unreliability of IP fragmentation
   [FRAG-FRAGILE] and the filtering of IPv6 extension headers [RFC7872]
   strongly suggest that at least for some protocol elements,
   transparency is a lost cause and middleboxes are here to stay.  In
   the following two sections, we show that some application
   environments require protocol features that cannot, or should not,
   cross the whole Internet.

3.  Examples of Limited Domain Requirements

   This section describes various examples where limited domain
   requirements can easily be identified, either based on an application
   scenario or on a technical imperative.  It is, of course, not a
   complete list, and it is presented in an arbitrary order, loosely
   from smaller to bigger.

   1.   A home network.  It will be mainly unmanaged, constructed by a
        non-specialist.  It must work with devices "out of the box" as
        shipped by their manufacturers and must create adequate security
        by default.  Remote access may be required.  The requirements
        and applicable principles are summarized in [RFC7368].

   2.   A small office network.  This is sometimes very similar to a
        home network, if whoever is in charge has little or no
        specialist knowledge, but may have differing security and
        privacy requirements.  In other cases, it may be professionally
        constructed using recommended products and configurations but
        operate unmanaged.  Remote access may be required.

   3.   A vehicle network.  This will be designed by the vehicle
        manufacturer but may include devices added by the vehicle's
        owner or operator.  Parts of the network will have demanding
        performance and reliability requirements with implications for
        human safety.  Remote access may be required to certain
        functions but absolutely forbidden for others.  Communication
        with other vehicles, roadside infrastructure, and external data
        sources will be required.  See [IPWAVE-NETWORKING] for a survey
        of use cases.

   4.   Supervisory Control And Data Acquisition (SCADA) networks and
        other hard real-time networks.  These will exhibit specific
        technical requirements, including tough real-time performance
        targets.  See, for example, [RFC8578] for numerous use cases.
        An example is a building services network.  This will be
        designed specifically for a particular building but using
        standard components.  Additional devices may need to be added at
        any time.  Parts of the network may have demanding reliability
        requirements with implications for human safety.  Remote access
        may be required to certain functions but absolutely forbidden
        for others.  An extreme example is a network used for virtual
        reality or augmented reality applications where the latency
        requirements are very stringent.

   5.   Sensor networks.  The two preceding cases will all include
        sensors, but some networks may be specifically limited to
        sensors and the collection and processing of sensor data.  They
        may be in remote or technically challenging locations and
        installed by non-specialists.

   6.   Internet-of-Things (IoT) networks.  While this term is very
        flexible and covers many innovative types of networks, including
        ad hoc networks that are formed spontaneously and some
        applications of 5G technology, it seems reasonable to expect
        that IoT edge networks will have special requirements and
        protocols that are useful only within a specific domain, and
        that these protocols cannot, and for security reasons should
        not, run over the Internet as a whole.

   7.   Constrained Networks.  An important subclass of IoT networks
        consists of constrained networks [RFC7228] in which the nodes
        are limited in power consumption and communications bandwidth
        and are therefore limited to using very frugal protocols.

   8.   Delay-tolerant networks.  These may consist of domains that are
        relatively isolated and constrained in power (e.g., deep space
        networks) and are connected only intermittently to the outside,
        with a very long latency on such connections [RFC4838].
        Clearly, the protocol requirements and possibilities are very
        specialized in such networks.

   9.   "Traditional" enterprise and campus networks, which may be
        spread over many kilometers and over multiple separate sites,
        with multiple connections to the Internet.  Interestingly, the
        IETF appears never to have analyzed this long-established class
        of networks in a general way, except in connection with IPv6
        deployment (e.g., [RFC7381]).

   10.  Unsuitable standards.  A situation that can arise in an
        enterprise network is that the Internet-wide solution for a
        particular requirement may either fail locally or be much more
        complicated than is necessary.  An example is that the
        complexity induced by a mechanism such as Interactive
        Connectivity Establishment (ICE) [RFC8445] is not justified
        within such a network.  Furthermore, ICE cannot be used in some
        cases because candidate addresses are not known before a call is
        established, so a different local solution is essential
        [RFC6947].

   11.  Managed wide-area networks run by service providers for
        enterprise services such as Layer 2 (Ethernet, etc.) point-to-
        point pseudowires, multipoint Layer 2 Ethernet VPNs using
        Virtual Private LAN Service (VPLS) or Ethernet VPN (EVPN), and
        Layer 3 IP VPNs.  These are generally characterized by service-
        level agreements for availability, packet loss, and possibly
        multicast service.  These are different from the previous case
        in that they mostly run over MPLS infrastructures, and the
        requirements for these services are well defined by the IETF.

   12.  Data centers and hosting centers, or distributed services acting
        as such centers.  These will have high performance, security,
        and privacy requirements and will typically include large
        numbers of independent "tenant" networks overlaid on shared
        infrastructure.

   13.  Content Delivery Networks (CDNs), comprising distributed data
        centers and the paths between them, spanning thousands of
        kilometers, with numerous connections to the Internet.

   14.  Massive Web Service Provider Networks.  This is a small class of
        networks with well-known trademarked names, combining aspects of
        distributed enterprise networks, data centers, and CDNs.  They
        have their own international networks bypassing the generic
        carriers.  Like CDNs, they have numerous connections to the
        Internet, typically offering a tailored service in each economy.

   Three other aspects, while not tied to specific network types, also
   strongly depend on the concept of limited domains:

   1.  Many of the above types of networks may be extended throughout
       the Internet by a variety of virtual private network (VPN)
       techniques.  Therefore, we argue that limited domains may overlap
       each other in an arbitrary fashion by use of virtualization
       techniques.  As noted above in the discussion of controlled
       environments, specific tunneling and encapsulation techniques may
       be tailored for use within a given domain.

   2.  Intent-Based Networking.  In this concept, a network domain is
       configured and managed in accordance with an abstract policy
       known as "Intent" to ensure that the network performs as required
       [IBN-CONCEPTS].  Whatever technologies are used to support this
       will be applied within the domain boundary, even if the services
       supported in the domain are globally accessible.

   3.  Network Slicing.  A network slice is a form of virtual network
       that consists of a managed set of resources carved off from a
       larger network [ENHANCED-VPN].  This is expected to be
       significant in 5G deployments [USER-PLANE-PROTOCOL].  Whatever
       technologies are used to support slicing will require a clear
       definition of the boundary of a given slice within a larger
       domain.

   While it is clearly desirable to use common solutions, and therefore
   common standards, wherever possible, it is increasingly difficult to
   do so while satisfying the widely varying requirements outlined
   above.  However, there is a tendency when new protocols and protocol
   extensions are proposed to always ask the question "How will this
   work across the open Internet?"  This document suggests that this is
   not always the best question.  There are protocols and extensions
   that are not intended to work across the open Internet.  On the
   contrary, their requirements and semantics are specifically limited
   (in the sense defined above).

   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.

   Nevertheless, because of the diversity of limited domains with
   specific requirements that is now emerging, specific standards (and
   ad hoc standards) will probably emerge for different types of
   domains.  There will be attempts to capture each market sector, but
   the market will demand standardized solutions within each sector.  In
   addition, operational choices will be made that can in fact only work
   within a limited domain.  The history of RSVP [RFC2205] illustrates
   that a standard defined as if it could work over the open Internet
   might not in fact do so.  In general, we can no longer assume that a
   protocol designed according to classical Internet guidelines will in
   fact work reliably across the network as a whole.  However, the "open
   Internet" must remain as the universal method of interconnection.
   Reconciling these two aspects is a major challenge.

4.  Examples of Limited Domain Solutions

   This section lists various examples of specific limited domain
   solutions that have been proposed or defined.  It intentionally does
   not include Layer 2 technology solutions, which by definition apply
   to limited domains.  It is worth noting, however, that with recent
   developments such as Transparent Interconnection of Lots of Links
   (TRILL) [RFC6325] or Shortest Path Bridging [SPB], Layer 2 domains
   may become very large.

   1.   Differentiated Services.  This mechanism [RFC2474] allows a
        network to assign locally significant values to the 6-bit
        Differentiated Services Code Point field in any IP packet.
        Although there are some recommended code point values for
        specific per-hop queue management behaviors, these are
        specifically intended to be domain-specific code points with
        traffic being classified, conditioned, and mapped or re-marked
        at domain boundaries (unless there is an inter-domain agreement
        that makes mapping or re-marking unnecessary).

   2.   Integrated Services.  Although it is not intrinsic in the design
        of RSVP [RFC2205], it is clear from many years' experience that
        Integrated Services can only be deployed successfully within a
        limited domain that is configured with adequate equipment and
        resources.

   3.   Network function virtualization.  As described in [RFC8568],
        this general concept is an open research topic in which virtual
        network functions are orchestrated as part of a distributed
        system.  Inevitably, such orchestration applies to an
        administrative domain of some kind, even though cross-domain
        orchestration is also a research area.

   4.   Service Function Chaining (SFC).  This technique [RFC7665]
        assumes that services within a network are constructed as
        sequences of individual service functions within a specific SFC-
        enabled domain such as a 5G domain.  As that RFC states:
        "Specific features may need to be enforced at the boundaries of
        an SFC-enabled domain, for example to avoid leaking SFC
        information".  A Network Service Header (NSH) [RFC8300] is used
        to encapsulate packets flowing through the service function
        chain: "The intended scope of the NSH is for use within a single
        provider's operational domain."

   5.   Firewall and Service Tickets (FAST).  Such tickets would
        accompany a packet to claim the right to traverse a network or
        request a specific network service [FAST].  They would only be
        meaningful within a particular domain.

   6.   Data Center Network Virtualization Overlays.  A common
        requirement in data centers that host many tenants (clients) is
        to provide each one with a secure private network, all running
        over the same physical infrastructure.  [RFC8151] describes
        various use cases for this, and specifications are under
        development.  These include use cases in which the tenant
        network is physically split over several data centers, but which
        must appear to the user as a single secure domain.

   7.   Segment Routing.  This is a technique that "steers a packet
        through an ordered list of instructions, called segments"
        [RFC8402].  The semantics of these instructions are explicitly
        local to a segment routing domain or even to a single node.
        Technically, these segments or instructions are represented as
        an MPLS label or an IPv6 address, which clearly adds a semantic
        interpretation to them within the domain.

   8.   Autonomic Networking.  As explained in [REF-MODEL], an autonomic
        network is also a security domain within which an autonomic
        control plane [ACP] is used by autonomic service agents.  These
        agents manage technical objectives, which may be locally
        defined, subject to domain-wide policy.  Thus, the domain
        boundary is important for both security and protocol purposes.

   9.   Homenet.  As shown in [RFC7368], a home networking domain has
        specific protocol needs that differ from those in an enterprise
        network or the Internet as a whole.  These include the Home
        Network Control Protocol (HNCP) [RFC7788] and a naming and
        discovery solution [HOMENET-NAMING].

   10.  Creative uses of IPv6 features.  As IPv6 enters more general
        use, engineers notice that it has much more flexibility than
        IPv4.  Innovative suggestions have been made for:

        *  The flow label, e.g., [RFC6294].

        *  Extension headers, e.g., for segment routing [RFC8754] or
           Operations, Administration, and Maintenance (OAM) marking
           [IPV6-ALT-MARK].

        *  Meaningful address bits, e.g., [EMBEDDED-SEMANTICS].  Also,
           segment routing uses IPv6 addresses as segment identifiers
           with specific local meanings [RFC8402].

        *  If segment routing is used for network programming
           [SRV6-NETWORK], IPv6 extension headers can support rather
           complex local functionality.

        The case of the extension header is particularly interesting,
        since its existence has been a major "selling point" for IPv6,
        but new extension headers are notorious for being virtually
        impossible to deploy across the whole Internet [RFC7045]
        [RFC7872].  It is worth noting that extension header filtering
        is considered an important security issue [IPV6-EXT-HEADERS].
        There is considerable appetite among vendors or operators to
        have flexibility in defining extension headers for use in
        limited or specialized domains, e.g., [IPV6-SRH], [BIGIP], and
        [APP-AWARE].  Locally significant hop-by-hop options are also
        envisaged, that would be understood by routers inside a domain
        but not elsewhere, e.g., [IN-SITU-OAM].

   11.  Deterministic Networking (DetNet).  The Deterministic Networking
        Architecture [RFC8655] and encapsulation [DETNET-DATA-PLANE] aim
        to support flows with extremely low data loss rates and bounded
        latency but only within a part of the network that is "DetNet
        aware".  Thus, as for Differentiated Services above, the concept
        of a domain is fundamental.

   12.  Provisioning Domains (PvDs).  An architecture for Multiple
        Provisioning Domains has been defined [RFC7556] to allow hosts
        attached to multiple networks to learn explicit details about
        the services provided by each of those networks.

   13.  Address Scopes.  For completeness, we mention that, particularly
        in IPv6, some addresses have explicitly limited scope.  In
        particular, link-local addresses are limited to a single
        physical link [RFC4291], and Unique Local Addresses [RFC4193]
        are limited to a somewhat loosely defined local site scope.
        Previously, site-local addresses were defined, but they were
        obsoleted precisely because of "the fuzzy nature of the site
        concept" [RFC3879].  Multicast addresses also have explicit
        scoping [RFC4291].

   14.  As an application-layer example, consider streaming services
        such as IPTV infrastructures that rely on standard protocols,
        but for which access is not globally available.

   All of these suggestions are only viable within a specified domain.
   Nevertheless, all of them are clearly intended for multivendor
   implementation on thousands or millions of network domains, so
   interoperable standardization would be beneficial.  This argument
   might seem irrelevant to private or proprietary implementations, but
   these have a strong tendency to become de facto standards if they
   succeed, so the arguments of this document still apply.

5.  The Scope of Protocols in Limited Domains

   One consequence of the deployment of limited domains in the Internet
   is that some protocols will be designed, extended, or configured so
   that they only work correctly between end systems in such domains.
   This is to some extent encouraged by some existing standards and by
   the assignment of code points for local or experimental use.  In any
   case, it cannot be prevented.  Also, by endorsing efforts such as
   Service Function Chaining, Segment Routing, and Deterministic
   Networking, the IETF is in effect encouraging such deployments.
   Furthermore, it seems inevitable, if the Internet of Things becomes
   reality, that millions of edge networks containing completely novel
   types of nodes will be connected to the Internet; each one of these
   edge networks will be a limited domain.

   It is therefore appropriate to discuss whether protocols or protocol
   extensions should sometimes be standardized to interoperate only
   within a limited-domain boundary.  Such protocols would not be
   required to interoperate across the Internet as a whole.  Various
   scenarios could then arise if there are multiple domains using the
   limited-domain protocol in question:

   A.  If a domain is split into two parts connected over the Internet
       directly at the IP layer (i.e., with no tunnel encapsulating the
       packets), a limited-domain protocol could be operated between
       those two parts regardless of its special nature, as long as it
       respects standard IP formats and is not arbitrarily blocked by
       firewalls.  A simple example is any protocol using a port number
       assigned to a specific non-IETF protocol.

       Such a protocol could reasonably be described as an "inter-
       domain" protocol because the Internet is transparent to it, even
       if it is meaningless except in the two limited domains.  This is,
       of course, nothing new in the Internet architecture.

   B.  If a limited-domain protocol does not respect standard IP formats
       (for example, if it includes a non-standard IPv6 extension
       header), it could not be operated between two domains connected
       over the Internet directly at the IP layer.

       Such a protocol could reasonably be described as an "intra-
       domain" protocol, and the Internet is opaque to it.

   C.  If a limited-domain protocol is clearly specified to be invalid
       outside its domain of origin, neither scenario A nor B applies.
       The only solution would be a single virtual domain.  For example,
       an encapsulating tunnel between two domains could be used to
       create the virtual domain.  Also, nodes at the domain boundary
       must drop all packets using the limited-domain protocol.

   D.  If a limited-domain protocol has domain-specific variants, such
       that implementations in different domains could not interoperate
       if those domains were unified by some mechanism as in scenario C,
       the protocol is not interoperable in the normal sense.  If two
       domains using it were merged, the protocol might fail
       unpredictably.  A simple example is any protocol using a port
       number assigned for experimental use.  Related issues are
       discussed in [RFC5704], including the complex example of
       Transport MPLS.

   To provide a widespread example, consider Differentiated Services
   [RFC2474].  A packet containing any value whatsoever in the 6 bits of
   the Differentiated Services Code Point (DSCP) is well formed and
   falls into scenario A.  However, because the semantics of DSCP values
   are locally significant, the packet also falls into scenario D.  In
   fact, Differentiated Services are only interoperable across domain
   boundaries if there is a corresponding agreement between the
   operators; otherwise, a specific gateway function is required for
   meaningful interoperability.  Much more detailed discussion is found
   in [RFC2474] and [RFC8100].

   To provide a provocative example, consider the proposal in [IPV6-SRH]
   that the restrictions in [RFC8200] should be relaxed to allow IPv6
   extension headers to be inserted on the fly in IPv6 packets.  If this
   is done in such a way that the affected packets can never leave the
   specific limited domain in which they were modified, scenario C
   applies.  If the semantic content of the inserted headers is locally
   defined, scenario D also applies.  In neither case is the Internet
   outside the limited domain disturbed.  However, inside the domain,
   nodes must understand the variant protocol.  Unless it is
   standardized as a formal version, with all the complexity that
   implies [RFC6709], the nodes must all be non-standard to the extent
   of understanding the variant protocol.  For the example of IPv6
   header insertion, that means non-compliance with [RFC8200] within the
   domain, even if the inserted headers are themselves fully compliant.
   Apart from the issue of formal compliance, such deviations from
   documented standard behavior might lead to significant debugging
   issues.  The possible practical impact of the header insertion
   example is explored in [IN-FLIGHT-IPV6].

   The FAST proposal mentioned in Section 4, Paragraph 2, Item 5 is also
   an interesting case study.  The semantics of FAST tickets [FAST] have
   limited scope.  However, they are designed in a way that, in
   principle, allows them to traverse the open Internet, as standardized
   IPv6 hop-by-hop options or even as a proposed form of IPv4 extension
   header [IPV4-EXT-HEADERS].  Whether such options can be used reliably
   across the open Internet remains unclear [IPV6-EXT-HEADERS].

   We conclude that it is reasonable to explicitly define limited-domain
   protocols, either as standards or as proprietary mechanisms, as long
   as they describe which of the above scenarios apply and they clarify
   how the domain is defined.  As long as all relevant standards are
   respected outside the domain boundary, a well-specified limited-
   domain protocol need not damage the rest of the Internet.  However,
   as described in the next section, mechanisms are needed to support
   domain membership operations.

   Note that this conclusion is not a recommendation to abandon the
   normal goal that a standardized protocol should be global in scope
   and able to interoperate across the open Internet.  It is simply a
   recognition that this will not always be the case.

6.  Functional Requirements of Limited Domains

   Noting that limited-domain protocols have been defined in the past,
   and that others will undoubtedly be defined in the future, it is
   useful to consider how a protocol can be made aware of the domain
   within which it operates and how the domain boundary nodes can be
   identified.  As the taxonomy in Appendix A shows, there are numerous
   aspects to a domain.  However, we can identify some generally
   required features and functions that would apply partially or
   completely to many cases.

   Today, where limited domains exist, they are essentially created by
   careful configuration of boundary routers and firewalls.  If a domain
   is characterized by one or more address prefixes, address assignment
   to hosts must also be carefully managed.  This is an error-prone
   method, and a combination of configuration errors and default routing
   can lead to unwanted traffic escaping the domain.  Our basic
   assumption is therefore that it should be possible for domains to be
   created and managed automatically, with minimal human configuration.
   We now discuss requirements for automating domain creation and
   management.

   First, if we drew a topology map, any given domain -- virtual or
   physical -- will have a well-defined boundary between "inside" and
   "outside".  However, that boundary in itself has no technical
   meaning.  What matters in reality is whether a node is a member of
   the domain and whether it is at the boundary between the domain and
   the rest of the Internet.  Thus, the boundary in itself does not need
   to be identified, but boundary nodes face both inwards and outwards.
   Inside the domain, a sending node needs to know whether it is sending
   to an inside or outside destination, and a receiving node needs to
   know whether a packet originated inside or outside.  Also, a boundary
   node needs to know which of its interfaces are inward facing or
   outward facing.  It is irrelevant whether the interfaces involved are
   physical or virtual.

   To underline that domain boundaries need to be identifiable, consider
   the statement from the Deterministic Networking Problem Statement
   [RFC8557] that "there is still a lack of clarity regarding the limits
   of a domain where a deterministic path can be set up".  This remark
   can certainly be generalized.

   With this perspective, we can list some general functional
   requirements.  An underlying assumption here is that domain
   membership operations should be cryptographically secured; a domain
   without such security cannot be reliably protected from attack.

   1.   Domain Identity.  A domain must have a unique and verifiable
        identifier; effectively, this should be a public key for the
        domain.  Without this, there is no way to secure domain
        operations and domain membership.  The holder of the
        corresponding private key becomes the trust anchor for the
        domain.

   2.   Nesting.  It must be possible for domains to be nested (see, for
        example, the network-slicing example mentioned above).

   3.   Overlapping.  It must be possible for nodes and links to be in
        more than one domain (see, for example, the case of PvDs
        mentioned above).

   4.   Node Eligibility.  It must be possible for a node to determine
        which domain(s) it can potentially join and on which
        interface(s).

   5.   Secure Enrollment.  A node must be able to enroll in a given
        domain via secure node identification and to acquire relevant
        security credentials (authorization) for operations within the
        domain.  If a node has multiple physical or virtual interfaces,
        individual enrollment for each interface may be required.

   6.   Withdrawal.  A node must be able to cancel enrollment in a given
        domain.

   7.   Dynamic Membership.  Optionally, a node should be able to
        temporarily leave or rejoin a domain (i.e., enrollment is
        persistent but membership is intermittent).

   8.   Role, implying authorization to perform a certain set of
        actions.  A node must have a verifiable role.  In the simplest
        case, the role choices are "interior node" and "boundary node".
        In a boundary node, individual interfaces may have different
        roles, e.g., "inward facing" and "outward facing".

   9.   Peer Verification.  A node must be able to verify whether
        another node is a member of the domain.

   10.  Role Verification.  A node should be able to learn the verified
        role of another node.  In particular, it should be possible for
        a node to find boundary nodes (interfacing to the Internet).

   11.  Domain Data.  In a domain with management requirements, it must
        be possible for a node to acquire domain policy and/or domain
        configuration data.  This would include, for example, filtering
        policy to ensure that inappropriate packets do not leave the
        domain.

   These requirements could form the basis for further analysis and
   solution design.

   Another aspect is whether individual packets within a limited domain
   need to carry any sort of indicator that they belong to that domain
   or whether this information will be implicit in the IP addresses of
   the packet.  A related question is whether individual packets need
   cryptographic authentication.  This topic is for further study.

7.  Security Considerations

   As noted above, a protocol intended for limited use may well be
   inadvertently used on the open Internet, so limited use is not an
   excuse for poor security.  In fact, a limited use requirement
   potentially adds complexity to the security design.

   Often, the boundary of a limited domain will also act as a security
   boundary.  In particular, it will serve as a trust boundary and as a
   boundary of authority for defining capabilities.  For example,
   segment routing [RFC8402] explicitly uses the concept of a "trusted
   domain" in this way.  Within the boundary, limited-domain protocols
   or protocol features will be useful, but they will in many cases be
   meaningless or harmful if they enter or leave the domain.

   The boundary also serves to provide confidentiality and privacy for
   operational parameters that the operator does not wish to reveal.
   Note that this is distinct from privacy protection for individual
   users within the domain.

   The security model for a limited-scope protocol must allow for the
   boundary and in particular for a trust model that changes at the
   boundary.  Typically, credentials will need to be signed by a domain-
   specific authority.

8.  IANA Considerations

   This document has no IANA actions.


9.  Informative References

   [ACP]      Eckert, T., Behringer, M., and S. Bjarnason, "An Autonomic
              Control Plane (ACP)", Work in Progress, Internet-Draft,
              draft-ietf-anima-autonomic-control-plane-27, 2 July 2020,
              <https://tools.ietf.org/html/draft-ietf-anima-autonomic-
              control-plane-27>.

   [APP-AWARE]
              Li, Z., Peng, S., Li, C., Xie, C., Voyer, D., Li, X., Liu,
              P., Liu, C., and K. Ebisawa, "Application-aware IPv6
              Networking (APN6) Encapsulation", Work in Progress,
              Internet-Draft, draft-li-6man-app-aware-ipv6-network-02, 2
              July 2020, <https://tools.ietf.org/html/draft-li-6man-app-
              aware-ipv6-network-02>.

   [BIGIP]    Li, R., "HUAWEI - Big IP Initiative", 2018,
              <https://www.iaria.org/announcements/HuaweiBigIP.pdf>.

   [DETNET-DATA-PLANE]
              Varga, B., Farkas, J., Berger, L., Malis, A., and S.
              Bryant, "DetNet Data Plane Framework", Work in Progress,
              Internet-Draft, draft-ietf-detnet-data-plane-framework-06,
              6 May 2020, <https://tools.ietf.org/html/draft-ietf-
              detnet-data-plane-framework-06>.

   [DNS-PERIMETER]
              Crocker, D. and T. Adams, "DNS Perimeter Overlay", Work in
              Progress, Internet-Draft, draft-dcrocker-dns-perimeter-01,
              11 June 2019, <https://tools.ietf.org/html/draft-dcrocker-
              dns-perimeter-01>.

   [EMBEDDED-SEMANTICS]
              Jiang, S., Qiong, Q., Farrer, I., Bo, Y., and T. Yang,
              "Analysis of Semantic Embedded IPv6 Address Schemas", Work
              in Progress, Internet-Draft, draft-jiang-semantic-prefix-
              06, 15 July 2013, <https://tools.ietf.org/html/draft-
              jiang-semantic-prefix-06>.

   [ENHANCED-VPN]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Networks (VPN+)
              Service", Work in Progress, Internet-Draft, draft-ietf-
              teas-enhanced-vpn-06, 13 July 2020,
              <https://tools.ietf.org/html/draft-ietf-teas-enhanced-vpn-
              06>.

   [FAST]     Herbert, T., "Firewall and Service Tickets", Work in
              Progress, Internet-Draft, draft-herbert-fast-04, 10 April
              2019, <https://tools.ietf.org/html/draft-herbert-fast-04>.

   [FRAG-FRAGILE]
              Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile", Work
              in Progress, Internet-Draft, draft-ietf-intarea-frag-
              fragile-17, 30 September 2019,
              <https://tools.ietf.org/html/draft-ietf-intarea-frag-
              fragile-17>.

   [HOMENET-NAMING]
              Lemon, T., Migault, D., and S. Cheshire, "Homenet Naming
              and Service Discovery Architecture", Work in Progress,
              Internet-Draft, draft-ietf-homenet-simple-naming-03, 23
              October 2018, <https://tools.ietf.org/html/draft-ietf-
              homenet-simple-naming-03>.

   [IBN-CONCEPTS]
              Clemm, A., Ciavaglia, L., Granville, L., and J. Tantsura,
              "Intent-Based Networking - Concepts and Definitions", Work
              in Progress, Internet-Draft, draft-irtf-nmrg-ibn-concepts-
              definitions-01, 9 March 2020,
              <https://tools.ietf.org/html/draft-irtf-nmrg-ibn-concepts-
              definitions-01>.

   [IN-FLIGHT-IPV6]
              Smith, M., Kottapalli, N., Bonica, R., Gont, F., and T.
              Herbert, "In-Flight IPv6 Extension Header Insertion
              Considered Harmful", Work in Progress, Internet-Draft,
              draft-smith-6man-in-flight-eh-insertion-harmful-02, 30 May
              2020, <https://tools.ietf.org/html/draft-smith-6man-in-
              flight-eh-insertion-harmful-02>.

   [IN-SITU-OAM]
              Bhandari, S., Brockners, F., Pignataro, C., Gredler, H.,
              Leddy, J., Youell, S., Mizrahi, T., Kfir, A., Gafni, B.,
              Lapukhov, P., Spiegel, M., Krishnan, S., and R. Asati,
              "In-situ OAM IPv6 Options", Work in Progress, Internet-
              Draft, draft-ietf-ippm-ioam-ipv6-options-02, 13 July 2020,
              <https://tools.ietf.org/html/draft-ietf-ippm-ioam-ipv6-
              options-02>.

   [IPV4-EXT-HEADERS]
              Herbert, T., "IPv4 Extension Headers and Flow Label", Work
              in Progress, Internet-Draft, draft-herbert-ipv4-eh-01, 2
              May 2019,
              <https://tools.ietf.org/html/draft-herbert-ipv4-eh-01>.

   [IPV6-ALT-MARK]
              Fioccola, G., Zhou, T., Cociglio, M., Qin, F., and R.
              Pang, "IPv6 Application of the Alternate Marking Method",
              Work in Progress, Internet-Draft, draft-ietf-6man-ipv6-
              alt-mark-01, 22 June 2020, <https://tools.ietf.org/html/
              draft-ietf-6man-ipv6-alt-mark-01>.

   [IPV6-EXT-HEADERS]
              Gont, F. and W. LIU, "Recommendations on the Filtering of
              IPv6 Packets Containing IPv6 Extension Headers", Work in
              Progress, Internet-Draft, draft-ietf-opsec-ipv6-eh-
              filtering-06, 2 July 2018, <https://tools.ietf.org/html/
              draft-ietf-opsec-ipv6-eh-filtering-06>.

   [IPV6-SRH] Voyer, D., Filsfils, C., Dukes, D., Matsushima, S., Leddy,
              J., Li, Z., and J. Guichard, "Deployments With Insertion
              of IPv6 Segment Routing Headers", Work in Progress,
              Internet-Draft, draft-voyer-6man-extension-header-
              insertion-09, 19 May 2020, <https://tools.ietf.org/html/
              draft-voyer-6man-extension-header-insertion-09>.

   [IPV6-USE-MINMTU]
              Andrews, M., "TCP Fails To Respect IPV6_USE_MIN_MTU", Work
              in Progress, Internet-Draft, draft-andrews-tcp-and-ipv6-
              use-minmtu-04, 18 October 2015,
              <https://tools.ietf.org/html/draft-andrews-tcp-and-ipv6-
              use-minmtu-04>.

   [IPWAVE-NETWORKING]
              Jeong, J., "IPv6 Wireless Access in Vehicular Environments
              (IPWAVE): Problem Statement and Use Cases", Work in
              Progress, Internet-Draft, draft-ietf-ipwave-vehicular-
              networking-16, 7 July 2020, <https://tools.ietf.org/html/
              draft-ietf-ipwave-vehicular-networking-16>.

   [REF-MODEL]
              Behringer, M., Carpenter, B., Eckert, T., Ciavaglia, L.,
              and J. Nobre, "A Reference Model for Autonomic
              Networking", Work in Progress, Internet-Draft, draft-ietf-
              anima-reference-model-10, 22 November 2018,
              <https://tools.ietf.org/html/draft-ietf-anima-reference-
              model-10>.

   [RFC2205]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, DOI 10.17487/RFC2205,
              September 1997, <https://www.rfc-editor.org/info/rfc2205>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
              DOI 10.17487/RFC2775, February 2000,
              <https://www.rfc-editor.org/info/rfc2775>.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,
              <https://www.rfc-editor.org/info/rfc2923>.

   [RFC3234]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
              Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,
              <https://www.rfc-editor.org/info/rfc3234>.

   [RFC3879]  Huitema, C. and B. Carpenter, "Deprecating Site Local
              Addresses", RFC 3879, DOI 10.17487/RFC3879, September
              2004, <https://www.rfc-editor.org/info/rfc3879>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4397]  Bryskin, I. and A. Farrel, "A Lexicography for the
              Interpretation of Generalized Multiprotocol Label
              Switching (GMPLS) Terminology within the Context of the
              ITU-T's Automatically Switched Optical Network (ASON)
              Architecture", RFC 4397, DOI 10.17487/RFC4397, February
              2006, <https://www.rfc-editor.org/info/rfc4397>.

   [RFC4427]  Mannie, E., Ed. and D. Papadimitriou, Ed., "Recovery
              (Protection and Restoration) Terminology for Generalized
              Multi-Protocol Label Switching (GMPLS)", RFC 4427,
              DOI 10.17487/RFC4427, March 2006,
              <https://www.rfc-editor.org/info/rfc4427>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, DOI 10.17487/RFC4838,
              April 2007, <https://www.rfc-editor.org/info/rfc4838>.

   [RFC4924]  Aboba, B., Ed. and E. Davies, "Reflections on Internet
              Transparency", RFC 4924, DOI 10.17487/RFC4924, July 2007,
              <https://www.rfc-editor.org/info/rfc4924>.

   [RFC5704]  Bryant, S., Ed., Morrow, M., Ed., and IAB, "Uncoordinated
              Protocol Development Considered Harmful", RFC 5704,
              DOI 10.17487/RFC5704, November 2009,
              <https://www.rfc-editor.org/info/rfc5704>.

   [RFC6294]  Hu, Q. and B. Carpenter, "Survey of Proposed Use Cases for
              the IPv6 Flow Label", RFC 6294, DOI 10.17487/RFC6294, June
              2011, <https://www.rfc-editor.org/info/rfc6294>.

   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
              <https://www.rfc-editor.org/info/rfc6325>.

   [RFC6398]  Le Faucheur, F., Ed., "IP Router Alert Considerations and
              Usage", BCP 168, RFC 6398, DOI 10.17487/RFC6398, October
              2011, <https://www.rfc-editor.org/info/rfc6398>.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
              October 2011, <https://www.rfc-editor.org/info/rfc6407>.

   [RFC6709]  Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              DOI 10.17487/RFC6709, September 2012,
              <https://www.rfc-editor.org/info/rfc6709>.

   [RFC6947]  Boucadair, M., Kaplan, H., Gilman, R., and S.
              Veikkolainen, "The Session Description Protocol (SDP)
              Alternate Connectivity (ALTC) Attribute", RFC 6947,
              DOI 10.17487/RFC6947, May 2013,
              <https://www.rfc-editor.org/info/rfc6947>.

   [RFC6950]  Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
              "Architectural Considerations on Application Features in
              the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013,
              <https://www.rfc-editor.org/info/rfc6950>.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,
              <https://www.rfc-editor.org/info/rfc7045>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7368]  Chown, T., Ed., Arkko, J., Brandt, A., Troan, O., and J.
              Weil, "IPv6 Home Networking Architecture Principles",
              RFC 7368, DOI 10.17487/RFC7368, October 2014,
              <https://www.rfc-editor.org/info/rfc7368>.

   [RFC7381]  Chittimaneni, K., Chown, T., Howard, L., Kuarsingh, V.,
              Pouffary, Y., and E. Vyncke, "Enterprise IPv6 Deployment
              Guidelines", RFC 7381, DOI 10.17487/RFC7381, October 2014,
              <https://www.rfc-editor.org/info/rfc7381>.

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
              <https://www.rfc-editor.org/info/rfc7556>.

   [RFC7663]  Trammell, B., Ed. and M. Kuehlewind, Ed., "Report from the
              IAB Workshop on Stack Evolution in a Middlebox Internet
              (SEMI)", RFC 7663, DOI 10.17487/RFC7663, October 2015,
              <https://www.rfc-editor.org/info/rfc7663>.

   [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/info/rfc7665>.

   [RFC7754]  Barnes, R., Cooper, A., Kolkman, O., Thaler, D., and E.
              Nordmark, "Technical Considerations for Internet Service
              Blocking and Filtering", RFC 7754, DOI 10.17487/RFC7754,
              March 2016, <https://www.rfc-editor.org/info/rfc7754>.

   [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
              2016, <https://www.rfc-editor.org/info/rfc7788>.

   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", RFC 7872,
              DOI 10.17487/RFC7872, June 2016,
              <https://www.rfc-editor.org/info/rfc7872>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <https://www.rfc-editor.org/info/rfc8086>.

   [RFC8100]  Geib, R., Ed. and D. Black, "Diffserv-Interconnection
              Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
              March 2017, <https://www.rfc-editor.org/info/rfc8100>.

   [RFC8151]  Yong, L., Dunbar, L., Toy, M., Isaac, A., and V. Manral,
              "Use Cases for Data Center Network Virtualization Overlay
              Networks", RFC 8151, DOI 10.17487/RFC8151, May 2017,
              <https://www.rfc-editor.org/info/rfc8151>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", RFC 8300,
              DOI 10.17487/RFC8300, January 2018,
              <https://www.rfc-editor.org/info/rfc8300>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8517]  Dolson, D., Ed., Snellman, J., Boucadair, M., Ed., and C.
              Jacquenet, "An Inventory of Transport-Centric Functions
              Provided by Middleboxes: An Operator Perspective",
              RFC 8517, DOI 10.17487/RFC8517, February 2019,
              <https://www.rfc-editor.org/info/rfc8517>.

   [RFC8557]  Finn, N. and P. Thubert, "Deterministic Networking Problem
              Statement", RFC 8557, DOI 10.17487/RFC8557, May 2019,
              <https://www.rfc-editor.org/info/rfc8557>.

   [RFC8568]  Bernardos, CJ., Rahman, A., Zuniga, JC., Contreras, LM.,
              Aranda, P., and P. Lynch, "Network Virtualization Research
              Challenges", RFC 8568, DOI 10.17487/RFC8568, April 2019,
              <https://www.rfc-editor.org/info/rfc8568>.

   [RFC8578]  Grossman, E., Ed., "Deterministic Networking Use Cases",
              RFC 8578, DOI 10.17487/RFC8578, May 2019,
              <https://www.rfc-editor.org/info/rfc8578>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [SPB]      "IEEE Standard for Local and metropolitan area networks -
              Bridges and Bridged Networks",
              DOI 10.1109/IEEESTD.2018.8403927, IEEE 802.1Q-2018, July
              2018, <https://ieeexplore.ieee.org/document/8403927>.

   [SRV6-NETWORK]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              Work in Progress, Internet-Draft, draft-ietf-spring-srv6-
              network-programming-16, 27 June 2020,
              <https://tools.ietf.org/html/draft-ietf-spring-srv6-
              network-programming-16>.

   [USER-PLANE-PROTOCOL]
              Homma, S., Miyasaka, T., Matsushima, S., and D. Voyer,
              "User Plane Protocol and Architectural Analysis on 3GPP 5G
              System", Work in Progress, Internet-Draft, draft-ietf-dmm-
              5g-uplane-analysis-03, 3 November 2019,
              <https://tools.ietf.org/html/draft-ietf-dmm-5g-uplane-
              analysis-03>.

Appendix A.  Taxonomy of Limited Domains

   This appendix develops a taxonomy for describing limited domains.
   Several major aspects are considered in this taxonomy:

   *  The domain as a whole

   *  The individual nodes

   *  The domain boundary

   *  The domain's topology

   *  The domain's technology

   *  How the domain connects to the Internet

   *  The security, trust, and privacy model

   *  Operations

   The following sub-sections analyze each of these aspects.

A.1.  Domain as a Whole

   *  Why does the domain exist? (e.g., human choice, administrative
      policy, orchestration requirements, technical requirements such as
      operational partitioning for scaling reasons)

   *  If there are special requirements, are they at Layer 2, Layer 3,
      or an upper layer?

   *  Where does the domain lie on the spectrum between completely
      managed by humans and completely autonomic?

   *  If managed, what style of management applies?  (Manual
      configuration, automated configuration, orchestration?)

   *  Is there a policy model?  (Intent, configuration policies?)

   *  Does the domain provide controlled or paid service or open access?

A.2.  Individual Nodes

   *  Is a domain member a complete node or only one interface of a
      node?

   *  Are nodes permanent members of a given domain, or are join and
      leave operations possible?

   *  Are nodes physical or virtual devices?

   *  Are virtual nodes general purpose or limited to specific
      functions, applications, or users?

   *  Are nodes constrained (by battery, etc.)?

   *  Are devices installed "out of the box" or pre-configured?

A.3.  Domain Boundary

   *  How is the domain boundary identified or defined?

   *  Is the domain boundary fixed or dynamic?

   *  Are boundary nodes special, or can any node be at the boundary?

A.4.  Topology

   *  Is the domain a subset of a Layer 2 or 3 connectivity domain?

   *  Does the domain overlap other domains?  (In other words, is a node
      allowed to be a member of multiple domains?)

   *  Does the domain match physical topology, or does it have a virtual
      (overlay) topology?

   *  Is the domain in a single building, vehicle, or campus?  Or is it
      distributed?

   *  If distributed, are the interconnections private or over the
      Internet?

   *  In IP addressing terms, is the domain Link local, Site local, or
      Global?

   *  Does the scope of IP unicast or multicast addresses map to the
      domain boundary?

A.5.  Technology

   *  What routing protocol(s) or different forwarding mechanisms (MPLS
      or other non-IP mechanism) are used?

   *  In an overlay domain, what overlay technique is used (L2VPN,
      L3VPN, etc.)?

   *  Are there specific QoS requirements?

   *  Link latency - Normal or long latency links?

   *  Mobility - Are nodes mobile?  Is the whole network mobile?

   *  Which specific technologies, such as those in Section 4, are
      applicable?

A.6.  Connection to the Internet

   *  Is the Internet connection permanent or intermittent?  (Never
      connected is out of scope.)

   *  What traffic is blocked, in and out?

   *  What traffic is allowed, in and out?

   *  What traffic is transformed, in and out?

   *  Is secure and privileged remote access needed?

   *  Does the domain allow unprivileged remote sessions?

A.7.  Security, Trust, and Privacy Model

   *  Must domain members be authorized?

   *  Are all nodes in the domain at the same trust level?

   *  Is traffic authenticated?

   *  Is traffic encrypted?

   *  What is hidden from the outside?

A.8.  Operations

   *  Safety level - Does the domain have a critical (human) safety
      role?

   *  Reliability requirement - Normal or 99.999%?

   *  Environment - Hazardous conditions?

   *  Installation - Are specialists needed?

   *  Service visits - Easy, difficult, or impossible?

   *  Software/firmware updates - Possible or impossible?

A.9.  Making Use of This Taxonomy

   This taxonomy could be used to design or analyze a specific type of
   limited domain.  For the present document, it is intended only to
   form a background to the scope of protocols used in limited domains
   and the mechanisms required to securely define domain membership and
   properties.

Acknowledgements

   Useful comments were received from Amelia Andersdotter, Edward
   Birrane, David Black, Ron Bonica, Mohamed Boucadair, Tim Chown,
   Darren Dukes, Donald Eastlake, Adrian Farrel, Tom Herbert, Ben Kaduk,
   John Klensin, Mirja Kuehlewind, Warren Kumari, Andy Malis, Michael
   Richardson, Mark Smith, Rick Taylor, Niels ten Oever, and others.

Contributors

   Sheng Jiang
   Huawei Technologies
   Q14, Huawei Campus
   No. 156 Beiqing Road
   Hai-Dian District, Beijing
   100095
   China

   Email: jiangsheng@huawei.com


Authors' Addresses

   Brian Carpenter
   The University of Auckland
   School of Computer Science
   University of Auckland
   PB 92019
   Auckland 1142
   New Zealand

   Email: brian.e.carpenter@gmail.com


   Bing Liu
   Huawei Technologies
   Q14, Huawei Campus
   No. 156 Beiqing Road
   Hai-Dian District, Beijing
   100095
   China

   Email: leo.liubing@huawei.com
