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Challenges for the Internet Routing Systems Introduced by Semantic Routing
draft-king-irtf-challenges-in-routing-08

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Authors Daniel King , Adrian Farrel , Christian Jacquenet
Last updated 2022-04-25
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draft-king-irtf-challenges-in-routing-08
RTGWG                                                            D. King
Internet-Draft                                      Lancaster University
Intended status: Informational                                 A. Farrel
Expires: 27 October 2022                              Old Dog Consulting
                                                            C. Jacquenet
                                                                  Orange
                                                           25 April 2022

   Challenges for the Internet Routing Systems Introduced by Semantic
                                Routing
                draft-king-irtf-challenges-in-routing-08

Abstract

   Historically, the meaning of an IP address has been to identify an
   interface on a network device.  Routing protocols were developed
   based on the assumption that a destination address had this semantic.

   Over time, routing decisions have been enhanced to determine paths on
   which packets could be forwarded according to additional information
   carried principally within the packet headers, and dependent on
   policy coded in, configured at, or signaled to the routers.

   Many proposals have been made to add semantics to IP packets by
   placing additional information into existing fields, by adding
   semantics to IP addresses, or by adding fields to the packets.  The
   intent is always to facilitate routing decisions based on these
   additional semantics in order to provide differentiated paths to
   enable forwarding of different packet flows on paths that may be
   distinct from those derived by shortest path first or path vector
   routing.  We call this approach "Semantic Routing".

   This document describes the challenges to the existing routing system
   that are introduced by Semantic Routing.  It then summarizes the
   opportunities for research into new or modified routing and
   forwarding approaches that make use of additional semantics.

   This document is presented as a study to support further research
   into clarifying and understanding the issues.  It does not pass
   comment on the advisability or practicality of any of the proposals
   and does not define any technical solutions.

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.  Current Challenges to IP Routing  . . . . . . . . . . . . . .   4
   3.  What is Semantic Routing? . . . . . . . . . . . . . . . . . .   7
     3.1.  Architectural Considerations  . . . . . . . . . . . . . .   9
   4.  Challenges for Internet Routing Research  . . . . . . . . . .  10
     4.1.  Research Principles . . . . . . . . . . . . . . . . . . .  10
     4.2.  Routing Research Questions to be Addressed  . . . . . . .  11
   5.  Security and Privacy Considerations . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  16
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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1.  Introduction

   Historically, the meaning of an IP address has been to identify an
   interface on a network device.  Routing protocols were to compute,
   establish, and maintain paths through networks toward destination
   prefixes until IP packets eventually reach their destination, and
   were based on the assumption that a destination address had this
   semantic.  Anycast and multicast addresses were also defined, and
   those address semantics sometimes required variations to the routing
   protocols or even encouraged the development of new protocols.

   Over time, the mechanisms that enabled routing decisions were
   enhanced to determine paths on which packets could be forwarded
   according to additional information carried principally within the
   packets headers or within 'shim' headers, and dependent on policy
   coded in, configured at, or signaled to the routers.  Perhaps one of
   the most iconic examples is Equal-Cost Multipath (ECMP) where a
   router makes a choice about how to forward a packet over a number of
   parallel links or paths based on the values of a set of fields in the
   packet header.

   Many proposals have been made to add semantics to IP packets by
   placing additional information into existing fields, by adding
   semantics to IP addresses, or by adding fields to the packets.  The
   intent is always to facilitate routing decisions based on these
   additional semantics in order to provide differentiated paths to
   enable forwarding of different packet flows on paths that may be
   distinct from those derived by shortest path first or path vector
   routing.  We call this approach "Semantic Routing"
   [I-D.farrel-irtf-introduction-to-semantic-routing].

   There are many approaches to adding semantics to packet headers: the
   additional information may be derived from the destination addresses,
   from other fields in the packet header, or the packet itself.
   Mechanisms for using the destination address range from assigning an
   address prefix to have a special purpose and meaning (such as is done
   for multicast addressing) through allowing the owner of a prefix to
   use the low-order bits of an address for specific purposes (e.g., to
   provide an indication of the nature of the service that is associated
   with these packets).  Some proposals suggest variable address
   lengths, others offer new hierarchical address formats, and some
   introduce a structure to addresses so that they can carry additional
   information in a common way.  Alternatively, forwarding decisions can
   be performed based on fields in the packet header (such as the IPv6
   Flow Label, or the Traffic Class field), overloading of existing
   packet fields, or new fields added to the packet headers.

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   A survey of ways in which routing and forwarding decisions have been
   made based on additional information carried in packets can be found
   in [I-D.king-irtf-semantic-routing-survey].

   Some Semantic Routing proposals are intended to be deployed in
   administratively scoped IP domains whose network components (routers,
   switches, etc.) are operated by a single administrative entity
   (sometimes referred to as 'limited domains' [RFC8799]), while other
   proposals are intended for use across the Internet.  The impact the
   proposals have on routing systems may require clean-slate solutions,
   hybrid solutions, extensions to existing routing protocols, or
   potentially no changes at all.

   This document describes some of the key challenges to the routing
   system that are already present in today's IP networks.  It then
   briefly outlines the concept of "Semantic Routing" with reference to
   [I-D.farrel-irtf-introduction-to-semantic-routing] and presents some
   of the additional challenges to the existing routing system that
   Semantic Routing may introduce.  Finally, this document presents a
   list of research questions that offer opportunities for future
   research into new or modified routing protocols and forwarding
   systems that make use of Semantic Routing.

   In this document, the focus is on routing and forwarding at the IP
   layer.  A variety of overlay mechanisms exists to perform service or
   path routing at higher layers, and those approaches may be based on
   similar extensions to packet semantics, but that is out of scope for
   this document.  Similarly, it is possible that Semantic Routing can
   be applied in a number of underlay network technologies, and that,
   too, is out of scope for this document.

   This document is presented as a study to support further research
   into clarifying and understanding the issues.  It does not pass
   comment on the advisability or practicality of any of the proposals
   and does not define any technical solutions.

2.  Current Challenges to IP Routing

   Today's IP routing faces several significant challenges which are a
   consequence of architectural design decisions and the continued
   exponential growth in traffic.  These challenges include mobility,
   multihoming, programmable paths, scalability, and security, and were
   not the focus of the original design of the Internet.  Nevertheless,
   IP networks have, in general, coped well in an incremental manner
   whenever a new challenge has arisen.  The following list is presented
   to give context to the continuing requirements that routing protocols
   must meet as new semantics are applied to the routing process.

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   *  Mobility - Mobility introduces several challenges, including
      maintaining a relationship between a sender and a receiver in
      cases where the sender or receiver changes their point of network
      attachment.  The network must always be informed about the mobile
      node's current location, to allow continuity of services.  Mobile
      users may also consume network resources, while in motion.  The
      mobile user's service instances and attachments will also change
      due to varying load or latency, e.g., in Multi-access Edge
      Computing (MEC) environments.

   *  Multihoming - Multihomed stations or multihomed networks are
      connected to the Internet via more than one access circuit or
      access network and, therefore, may be assigned multiple IP
      addresses or prefixes from different pools.  There are challenges
      concerning how traffic is forwarded back to the source if the
      source has originated its traffic using the wrong source address
      for a particular connection, or if one of the connections to the
      Internet is degraded.

   *  Multi-path - The Internet was initially designed to find the
      single, "best" path to a destination using a distributed routing
      algorithm.  Current IP network topologies can provide multiple
      paths to reach a destination, each with different characteristics
      and with different failure likelihoods.  It may be beneficial to
      send traffic over multiple paths to achieve reliability and
      enhance throughput, and it may be desirable to select one path or
      another because of QoS or security considerations for example, or
      to avoid transiting specific areas of an IP network, based (for
      example) on the reputation of transit provider for example.
      However, how packets are forwarded by using the shortest path
      means that distinguishing these alternate paths and directing
      traffic to them can be hard.  Further, problems concerning
      scalability, commercial agreements among Service Providers, and
      the design of BGP make the utilization of multi-path techniques
      difficult for inter-domain routing.  (Note that this discussion is
      distinct from Equal Cost Multi-path (ECMP) where packets are
      directed onto several "parallel" paths of identical least cost
      using a hash algorithm operated on some of the packets' header
      fields.)

   *  Multicast - Delivering the same packet to multiple destinations
      can place considerable load on a network.  Solutions that
      replicate the packet at the source or at the network edge may
      obviously cause multiple copies of the packet to flow along the
      same network links.  Solutions that move deterministic replication
      into the network to make more optimal use of the network resources
      can be complex to set up and manage since multicast network
      designs often assume dynamic tree computation where the multicast

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      distribution tree can be rooted at the source or in the multicast
      network, thereby leading to specific routing tables whose entries
      denote the tree structure.  More complicated hardware that can
      replicate packets may also be required within the network.  In
      order that packets can be addressed to a group of destinations and
      not be forwarded by means of unicast transmission, parts of the
      addressing space (that is, address prefixes) have been reserved
      for multicast addressing.

   *  Programmable Paths - The ability to decouple IP paths from routing
      protocols and agreements between Service Providers could allow
      users and applications to select network paths themselves, based
      on the required path characteristics.  Another option is to let
      the route computation logic select, establish, and maintain paths
      on behalf of the user or the application and as a function of
      their requirements so that Service Providers can participate in
      the route computation "service".  Currently, user and application
      packets follow the path selected by routing protocols and the way
      traffic is forwarded through a network is under the control of the
      Service Provider that operates the said network.  The
      corresponding traffic forwarding policies enforced by the service
      provider usually comply with the requirements expressed by the
      user or the application.  These requirements may have triggered a
      dynamic service parameter negotiation cycle that eventually leads
      to proper (network, CPU, storage) resource allocation.

   *  Endpoint Selection - As compute resources and content storage move
      closer to the edge of the network, there are often multiple points
      in the network that can satisfy user requests.  In order to make
      the best use of these distributed resources and so as to not
      overload parts of the network, user traffic needs to be steered to
      appropriate servers or data centres.  In many cases, this function
      may be achieved in the application layer (such as through DNS
      [RFC3467]) or in the transport layer (such as using ALTO
      [RFC5693]).  The challenge is to balance higher-layer decisions
      about which application layer resources to use with information
      from the lower layers about the availability and load of network
      resources.

   *  Scalability - There are many scaling concerns that pose critical
      challenges to the Internet.  Not least among these challenges is
      the size of the routing tables that routers in an IP network must
      maintain.  As the number of devices attached to the network grows,
      so the number of addresses in use also grows, and because of the
      schemes used to assign address prefixes, the mobility of devices,
      and the various connectivity options between networks, the routing
      table sizes also grow, even more so when prefixes are not always
      amenable to aggregation.  This problem is exacerbated by some

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      services (such as those supported by the IoT where several
      thousands of objects/sensors may be networked), where, as more
      devices are added to the network, the size of the routing table
      may affect the operation of certain routing protocols.  It may be
      noted that scaling issues are also exacerbated by multihoming
      practices if a host that is multihomed is allocated a different
      address for each point of attachment.

   *  Manageability, Maintainability, and Extensibility - Operational
      manageability is a key requirement for network technologies:
      network operators must be able to determine the status of their
      network and understand the causes of any disruptions or problems.
      Further, it must be possible to maintain the networks and the
      technologies running in them without disrupting the services being
      delivered by the networks.  Additionally, the network technologies
      developed and deployed need to be extensible so that new features
      can be added and new services supported without the need to invent
      whole new technologies.

   *  Security - Issues of security and privacy have been largely
      overlooked by the routing systems.  However, there is increasing
      concern that attacks on routing systems can not only be disruptive
      (for example, causing traffic to be dropped), but may cause
      traffic to be redirected to inspection points that can breach the
      security or privacy of the payloads.

   Some of the challenges outlined here were previously considered
   within the IETF by the IAB's "Routing and Addressing Workshop" held
   in Amsterdam, The Netherlands on October 18-19, 2006 [RFC4984].
   Several architectures and protocols have since been developed and
   worked on within and outside the IETF, and these are examined in
   [I-D.king-irtf-semantic-routing-survey].

3.  What is Semantic Routing?

   Semantic Routing is the term applied to routing in an IP network that
   relies upon additional information to feed the route computation
   process, to enhance route selection decisions, and to direct the
   forwarding process.  In addition to the routable part of the
   destination IP address (the prefix), such information may be present
   in other fields in the packet (chiefly the packet header) and
   configured or programmed into the routers/forwarders.  Semantic
   Routing includes mechanisms such as "Preferential Routing", "Policy-
   based Routing", and "Flow steering".

   In Semantic Routing, a packet forwarding engine may examine a variety
   of fields in a packet and match them against forwarding instructions.
   Those forwarding instructions may be installed by routing protocols,

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   configured through management protocols or a software defined
   networking (SDN) controller, or derived by a software component on
   the router that considers network conditions and traffic loads.  The
   packet fields concerned may be the fields of an IP header, those same
   fields but with additional semantics, elements of the packet payload,
   or new fields defined for inclusion in the packet header or as a
   "shim" between the header and payload.  In the case of additional
   semantics included in existing packet header fields, the approach
   implies some "overloading" of those fields to include meaning beyond
   the original definition.  In all cases, a well-known definition of
   the encoding of the additional information is required to enable
   consistent interpretation within the network.

   A more detailed description of Semantic routing can be found in
   [I-D.farrel-irtf-introduction-to-semantic-routing] and a survey of
   Semantic Routing proposals and research projects can be found in
   [I-D.king-irtf-semantic-routing-survey].

   Many technical challenges exist for Semantic Routing in IP networks
   depending on which approach is taken.  These challenges include (but
   are not limited to):

   *  The continual growth of routing tables.

   *  Convergence times for large networks.

   *  Granularity of routing decisions.

   *  Address consumption caused by lower address utility rate.  The
      wastage mainly comes from aligning finite allocation for semantic
      address blocks.

   *  Encoding too many semantics into prefixes will require evaluation
      of which to prioritize.

   *  Risk of privacy/information leakage.

   *  Lack of visibility of the Semantic Routing information when end-
      to-end or edge-to-edge encryption is used.

   *  Burdening the user, application, or prefix assignment node.

   *  Source address spoofing prevention mechanisms are required.

   *  Overloading of routing protocols causing stability and scaling
      problems.

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   *  Depending on encoding mechanisms, there may be challenges for data
      planes to scale the processes of finding, reading, and looking up
      semantic data in order to forward packets at line speed.

   *  Backwards compatibility with existing IP networking and routing
      protocols.

   *  Extensibility to support additional functions in the future.

   *  Manageability and network diagnostics to be able to determine how
      the network is functioning and to isolate the causes of any
      problems.

3.1.  Architectural Considerations

   Semantic data may be taken into account to integrate with existing
   routing architectures.  An overlay can be built such that Semantic
   Routing is used to forward traffic between nodes in the overlay, but
   regular IP is used in the underlay.  The application of semantics may
   also be constrained to within a limited domain.  In some cases, such
   a domain will use IP, but be disconnected from the Internet.  In
   other cases, traffic from within the domain is exchanged with other
   domains that are connected together across an IP network using
   tunnels or via application gateways.  And in still another case
   traffic from the domain is forwarded across the Internet to other
   nodes and this requires backward-compatible routing approaches.

   Isolated Domains:  Some IP network domains are entirely isolated from
      the Internet and other IP networks.  In these cases, packets
      cannot "escape" from the isolated domain into external networks
      and so the Semantic Routing schemes applied within the domain can
      have no detrimental effects on external domains.  Thus, the
      challenges are limited to enabling the desired function within the
      domain.

   Bridged Domains:  In some deployments, it will be desirable to
      connect together multiple isolated domains to build a larger
      network.  These domains may be connected (or bridged) over an IP
      network or even over the Internet, possibly using tunnels.  An
      alternative to tunneling is achieved using gateway functionality
      where packets from a domain are mapped at the domain boundary to
      produce regular IP packets that are sent across the IP network.

   Semantic Prefix Domains:  A semantic prefix domain is a portion of
      the Internet over which a consistent set of semantic-based
      policies are administered in a coordinated fashion.  This is
      achieved by assigning a routable address prefix (or a set of
      prefixes) for use with Semantic Routing so that packets may be

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      forwarded through the regular IP network (or the Internet).  Once
      delivered to the semantic prefix domain, a packet can be subjected
      to whatever Semantic Routing is enabled in the domain.

   Further discussion of architectures for Semantic Routing can be found
   in [I-D.farrel-irtf-introduction-to-semantic-routing].

4.  Challenges for Internet Routing Research

   It may not be possible to embrace all emerging scenarios with a
   single approach or solution.  Requirements such as 5G mobility, near-
   space-networking, and networking for outer-space (inter-planetary
   networking), may need to be handled using different network
   technologies.  Improving IP network capabilities and capacity to
   scale, and address a set of growing requirements presents significant
   research challenges, and will require contributions from the
   networking research community.  Solutions need to be both
   economically feasible and have the support of the networking
   equipment vendors as well as the network operators.

4.1.  Research Principles

   Research into Semantic Routing should be founded on regular
   scientific research principles [royalsoc].  Given the importance of
   the Internet today, it is critical that research is targeted,
   rigorous, and reproducible.

   The most valuable research will go beyond an initial hypothesis, a
   report of the work done, and the results observed.  Although that is
   a required foundation, networking research needs to be independently
   reproducible so that claims can be verified or falsified.  Further,
   the networks on which the research is carried out need to both
   reflect the characteristics that are being explicitly tested, and
   reproduce the variety of real networks that constitute the Internet.

   Thus, when conducting experiments and research to address the
   questions in Section 4.2, attention should be given to how the work
   is documented and how meaningful the test environment is, with a
   strong emphasis on making it possible for others to reproduce and
   validate the work.

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4.2.  Routing Research Questions to be Addressed

   As research into the scenarios and possible uses of Semantic Routing
   progresses, a number of questions need to be answered.  These
   questions go beyond "Why do we need this function?" and "What could
   we achieve by carrying additional semantics in an IP address?"  The
   questions are also distinct from issues of how the additional
   semantics can be encoded within an IP address.  All of those issues
   are, of course, important considerations in the debate about Semantic
   Routing, but they form only part of the essential groundwork of
   research into Semantic Routing itself.

   This section sets out some of the concerns about how the wider the
   use of Semantic Routing might impact a routing system.  These
   questions need to be answered in separate research work or folded
   into the discussion of each Semantic Routing proposal.

   1.   What is the scope of the Semantic Routing proposal?  This
        question may lead to various answers:

        Global:  It is intended to apply to all uses of IP.

        Backbone:  It is intended to apply to IP network connectivity.

        Overlay:  It is to be used as an overlay network using tunneling
           over IP or other underlay technologies.

        Gateway:  The Semantic Routing will be used within a specific
           domain, and communications with the wider Internet will be
           handled by IP and probably application gateways.

        Domain:  The use of the Semantic Routing is strictly limited to
           within a domain or private network.

        Underlying this question is a broader question about the
        boundaries of the use of IP, and the limit of "the Internet".
        If a limited domain is used, is it a semantic prefix domain
        [RFC8799] where a part of the IP address space identifies the
        domain so that an address is routable to the domain, but the
        additional semantics are used only within the domain, or is the
        address used exclusively within the domain so that the external
        impact of the routability of the address and the additional
        semantics is not important?

   2.   What will be the impact on existing routing systems?  What would
        happen if a packet carrying additional semantics was subjected
        to normal routing operations?  How would the existing routing
        systems react if such a packet escaped (accidentally or

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        maliciously) from the planned scope of the proposal?  For
        example: how are the semantic parts of an address distinguished
        from the routable parts (if, indeed, they are separable)?; is
        there an impact on the size and maintenance of routing tables
        due to the addition of semantics?; how are cryptographically
        generated addresses (such as [RFC3972]) made routable and kept
        simple enough for management?.

   3.   What path characteristics are needed to describe the desired
        paths and as input to route computation?  Since one of the
        implications of adding semantics to IP packets is to cause
        special processing by routers, it is important to understand
        what behaviors are wanted.  Such path characteristics include
        (but are not limited to):

        Quality:  Expressed in terms of throughput, latency, jitter,
           drop precedence, etc.

        Resilience:  Expressed in terms of survival of network failures
           and delivery guarantees.

        Destination:  How is a destination address to be interpreted if
           it encodes a choice of actual destinations?  Can traffic be
           forwarded over multiple distinct paths if multiple
           destination addresses are encoded?

        Security:  What choices of path reduce the vulnerability of the
           traffic to security or privacy attacks?

        In these cases, how do the routers utilize the additional
        semantics to determine the desired characteristics?  Or are such
        characteristics used to feed the route computation logic, for
        example, by means of metrics?  What additional information about
        the network do the routing protocols need to gather?  What
        changes to the routing algorithm are needed to deliver packets
        according to the desired characteristics?  How can routes be
        computed with characteristics that accommodate traffic patterns,
        requirements, and constraints?

   4.   Can we solve these routing challenges with existing routing
        tools and methods?  We can break this question into a set of
        more detailed questions.

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        *  Is new hardware needed?  Existing deployed hardware has
           certain assumptions about how forwarding is carried out based
           on IP addresses and routing tables.  But hardware is
           increasingly programmable so that it may be possible to
           instruct the forwarding components to act on a variety of
           elements of the packets.

        *  Do we need new routing protocols?  We might ask some
           subsidiary questions:

           -  Can we make do with existing protocols, possibly by tuning
              configuration parameters or using them out of the box?

           -  Can we make backwards-compatible modifications to existing
              protocols such that they work equally for today's IP
              addresses or addresses with extra semantics?

           -  Do we need entirely new protocols or radical evolutions of
              existing protocols in order to enforce advanced Semantic
              Routing policies?

           -  Should we focus on the benefits of routing solutions that
              are optimized for specific environments (network
              topologies, technologies, use cases), or should we attempt
              to generalize to enable wider applicability?

   5.   Do we need new management tools and techniques?  How practical
        is it to debug and operate the routing system?  Management of
        the routing system (especially diagnostic management) is a
        crucial and often neglected part of the problem space.  A
        critical part of this issue is how packets within the network
        can be inspected by diagnostic tools (or human operators) and
        mapped to the routing and forwarding decisions that were made
        within the network in order to understand the actions made at
        and by upstream routers.

   6.   What is the impact of Semantic Routing on the security of the
        routing system?

        *  Does the introduction of Semantic Routing provide a greater
           attack surface?

        *  Can Semantic Routing provide greater opportunities for
           security by fine-grain forwarding of flows to be inspected by
           different security functions?

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        *  Can Semantic Routing improve security and privacy by
           obscuring information in the packets, or does the inclusion
           of additional information risk compromising security and
           privacy?

        *  To what extent does deployment within a limited domain
           strengthen security or make it less of a concern?

        *  Does the use of Semantic Routing make it easier or harder to
           impose censorship, prohibit access to the Internet by
           specific parties, or block access to certain resources or
           types of service?

   7.   What is the scalability impact of Semantic Routing on routing
        systems?  Scalability can be measured as:

        *  Routing table size.  How many entries need to be maintained
           in the routing tables by different routers serving different
           roles in the network?  Some approaches to Semantic Routing
           may be explicitly intended to address this problem.

        *  Forwarding table size.  The size of the forwarding table may
           be less of an issue considering modern hardware, however the
           more granular the routing/forwarding decisions made in a
           router, the greater the size of this table.  The size of the
           forwarding table has implications for memory in the
           forwarding engine, but also for the lookup time for
           forwarding each packet.

        *  Routing performance.  Routing performance may be considered
           in terms of the volume of data that has to be exchanged both
           to construct and maintain the routing tables at the
           participating routers.  It may also be measured in terms of
           how much processing is required to compute new routes when
           there is a change in the network.

        *  Routing convergence.  This is the time that it takes for a
           routing protocol to discover changes (especially faults) in
           the network, to distribute the information about any changes
           to its peers, and to reach a stable state across the network
           such that packets are forwarded consistently.

        For all questions about routing scalability, research that
        presents figures based on credible example networks is highly
        desirable.  Similar questions may be asked about the amount of
        forwarding state that has to be maintained in the routers.

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   8.   To what extent can Semantic Routing be applied to multicast
        transmission schemes:

        *  Can Semantic Routing facilitate the computation and the
           establishment of (service-inferred) multicast distribution
           trees?

        *  Can specific semantics be carried in multicast addresses?

   9.   Is the approach extensible and maintainable?  Can new features
        be added without increasing the complexity and in a backward
        compatible way?  Could the approach be modified to handle
        evolutions in the rest of the networking infrastructure?
        Considerations might include the ability to encode additional
        options or variants within protocol fields, and the ability to
        add new fields.  Such considerations must be actively traded
        against the processing overhead associated with certain encoding
        types.

   10.  What aspects need to be standardized?  It is important to
        understand the necessity of standardization within this
        research.  What degree of interoperability is expected between
        devices and networks?  Is a given domain so constrained (for
        example, to a single equipment vendor) that standardization
        would be meaningless?  Is the application so narrow (for
        example, in niche hardware environments) such that
        interoperability is best handled by agreements among small
        groups of vendors such as in industry consortia?

5.  Security and Privacy Considerations

   Research into Semantic Routing must give full consideration to the
   security and privacy issues that are introduced by these mechanisms.
   Placing additional information into packet header fields might reveal
   details of what the packet is for, what function the user is
   performing, who the user is, etc.  Furthermore, in-flight
   modification of the additional information might not directly change
   the destination of the packet, but might change how the packet is
   handled within the network and at the destination.

   It should also be considered how packet encryption techniques that
   are increasingly popular for end-to-end or edge-to-edge security may
   obscure the semantic information carried in some fields of the packet
   header or found deeper in the packet.  This may render some semantic
   routing techniques impractical and may dictate other methods of
   carrying the necessary information to enable Semantic Routing.

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6.  IANA Considerations

   This document makes no requests for IANA action.

7.  Acknowledgements

   Thanks to Stewart Bryant for useful conversations.  Luigi Iannone,
   Robert Raszuk, Dirk Trossen, Ron Bonica, Marie-Jose Montpetit, Yizhou
   Li, Toerless Eckert, Tony Li, Joel Halpern, Stephen Farrell, Carsten
   Bormann, David Hutchison, Jeffery He, Dino Farinacci, Greg Mirsky,
   and Jeff Haas made helpful suggestions.

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement number 101015857 Secured autonomic
   traffic management for a Tera of SDN flows (Teraflow).

8.  Contributors

               Joanna Dang
               Email: dangjuanna@huawei.com

9.  Informative References

   [I-D.farrel-irtf-introduction-to-semantic-routing]
              Farrel, A. and D. King, "An Introduction to Semantic
              Routing", Work in Progress, Internet-Draft, draft-farrel-
              irtf-introduction-to-semantic-routing-03, 22 January 2022,
              <https://www.ietf.org/archive/id/draft-farrel-irtf-
              introduction-to-semantic-routing-03.txt>.

   [I-D.king-irtf-semantic-routing-survey]
              King, D. and A. Farrel, "A Survey of Semantic Internet
              Routing Techniques", Work in Progress, Internet-Draft,
              draft-king-irtf-semantic-routing-survey-03, 26 November
              2021, <https://www.ietf.org/archive/id/draft-king-irtf-
              semantic-routing-survey-03.txt>.

   [RFC3467]  Klensin, J., "Role of the Domain Name System (DNS)",
              RFC 3467, DOI 10.17487/RFC3467, February 2003,
              <https://www.rfc-editor.org/info/rfc3467>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

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   [RFC4984]  Meyer, D., Ed., Zhang, L., Ed., and K. Fall, Ed., "Report
              from the IAB Workshop on Routing and Addressing",
              RFC 4984, DOI 10.17487/RFC4984, September 2007,
              <https://www.rfc-editor.org/info/rfc4984>.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              DOI 10.17487/RFC5693, October 2009,
              <https://www.rfc-editor.org/info/rfc5693>.

   [RFC8799]  Carpenter, B. and B. Liu, "Limited Domains and Internet
              Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
              <https://www.rfc-editor.org/info/rfc8799>.

   [royalsoc] The Royal Society, "Evidence synthesis : Principles", Web
              page, Principles for good evidence synthesis, 19 September
              2018, <https://royalsociety.org/topics-policy/projects/
              evidence-synthesis/principles/>.

Authors' Addresses

   Daniel King
   Lancaster University
   United Kingdom
   Email: d.king@lancaster.ac.uk

   Adrian Farrel
   Old Dog Consulting
   United Kingdom
   Email: adrian@olddog.co.uk

   Christian Jacquenet
   Orange
   Rennes
   France
   Email: christian.jacquenet@orange.com

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