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Challenges for the Internet Routing Infrastructure Introduced by Semantic Routing
draft-king-irtf-challenges-in-routing-05

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Authors Daniel King , Adrian Farrel
Last updated 2022-01-17
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draft-king-irtf-challenges-in-routing-05
IRTF                                                             D. King
Internet-Draft                                      Lancaster University
Intended status: Informational                                 A. Farrel
Expires: 21 July 2022                                 Old Dog Consulting
                                                         17 January 2022

    Challenges for the Internet Routing Infrastructure Introduced by
                            Semantic Routing
                draft-king-irtf-challenges-in-routing-05

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 were enhanced to route packets according
   to additional information carried within the packets 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 to facilitate enhanced routing decisions based on these
   additional semantics in order to provide differentiated paths for
   different packet flows distinct from simple shortest path first
   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 protocols to
   make use of new or additional semantics.

   This document is presented as 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 21 July 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Current Challenges to IP Routing  . . . . . . . . . . . . . .   4
   3.  What is Semantic Routing? . . . . . . . . . . . . . . . . . .   7
     3.1.  Architectural Considerations  . . . . . . . . . . . . . .   8
   4.  Challenges for Internet Routing Research  . . . . . . . . . .   9
     4.1.  Research Principles . . . . . . . . . . . . . . . . . . .   9
     4.2.  Routing Research Questions to be Addressed  . . . . . . .  10
   5.  Security and Privacy Considerations . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Historically, the meaning of an IP address has been to identify an
   interface on a network device.  Routing protocols were developed to
   determine paths through the network toward destination addresses so
   that IP packets with a common destination address converged on that
   destination.  Anycast and multicast addresses were also defined and
   those address semantics necessitated variations to the routing
   protocols and the development of new protocols.

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   Over time, routing decisions were enhanced to route packets according
   to additional information carried within the packets and dependent on
   policy coded in, configured at, or signaled to the routers.  Perhaps
   the most obvious example is Equal-Cost Multipath (ECMP) where a
   router makes a consistent choice for forwarding packets 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 to facilitate enhanced routing decisions based on these
   additional semantics in order to provide differentiated paths for
   different packet flows distinct from simple shortest path first
   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.
   These 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 their own purposes.  Some proposals suggest variable
   address lengths, others offer hierarchical addresses, and some
   introduce a structure to addresses so that they can carry additional
   information in a common way.  Other approaches perform routing
   decisions on fields in the packet header (such as the IPv6 Flow
   Label, or the Traffic Class field), overload packet fields, or add
   new information to packet headers.

   A survey of ways in which routing 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
   limited domains [RFC8799] (networks) that are IP-based, 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 routing that
   are present in today's IP networks.  It then defines the concept of
   "Semantic Routing" and presents some of the challenges to the
   existing routing system that Semantic Routing may present.  Finally,
   this document presents a list of related research questions that
   offer opportunities for future research into new or modified routing
   protocols that make use of Semantic Routing.

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   In this document, the focus is on routing and forwarding at the IP
   layer.  A variety of overlay mechanisms exist 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 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.  These challenges include mobility, multihoming,
   programmable paths, scalability, and security, and were not the focus
   of the original design of the Internet.  Nevertheless, IP-based
   networks have, in general, coped well in an incremental manner as
   each new challenge has evolved.  This list is presented to give
   context to the continuing requirements that routing protocols must
   meet as new semantics are applied to the routing process.

   *  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.
      Mobility users may also consume network resources, while
      physically moving.  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) scenarios.

   *  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 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 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-based network topologies facilitate
      multiple paths each with different characteristics and with

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      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
      in order to provide delivery qualities or to avoid transiting
      specific areas of an IP-based network.  However, the way in which
      packets are forwarded using the best or 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 two "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 replication into the
      network to make more optimal use of the network resources can be
      complex to set up and manage requiring sophisticated protocols
      that can determine the best multicast delivery topologies, as well
      as hardware that can replicate packets within the network.  In
      order that packets can be addressed to a group of destinations and
      not be routed using the normal unicast approaches, parts of the
      addressing space (that is, address prefixes) have been reserved to
      indicate multicast.

   *  Programmable Paths - The ability to decouple IP-based network
      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 mainatin 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 owns the network, but in compliance with the
      requirements expressed by the user or the application, and which
      may have triggered a dynamic service parameter negotiation cycle
      to best accommodate such requirements by means of proper (network,
      CPU, storage) resource allocation.

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   *  End-Point 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 best use of these distributed services 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) or
      in the transport layer (such as using ALTO).  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-based network
      must maintain and exchange with their peers.  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 and are not always amenable to aggregation.  This
      problem also exists in specific limited domains (such as IoT),
      where, as more devices are added to the network, the size of the
      routing table may be affect the operation of certain routing
      protocols.  It may be noted that scaling issues are exacerbated by
      multihoming practices if a host that is multihomed is allocated a
      different address for each point of attachment.

   *  Security - Issues of security and privacy have been largely
      overlooked within 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 routed via 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 IABs "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].

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3.  What is Semantic Routing?

   Semantic Routing is the term applied to routing in an IP-based
   network that enhances decisions by considering information present in
   the packet and configured or programmed into the routers in addition
   to the routable part of the destination IP address (the prefix).
   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,
   configured through management protocols or as part of a software
   defined networking (SDN) system, or derived by a software component
   on the router that considers network conditions and traffic loads.
   The packet fields concerned may be the normal fields of the IP
   header, those same fields but with additional semantics, elements of
   the packet payload, or new fields defined for inclusion in the packet
   header.  In the 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-based
   network depending on which approach is taken.  These include:

   *  The continual growth of routing tables.

   *  Convergence times for larger 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.

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   *  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 preventiog mechanisms are required.

   *  Overloading of routing protocols causing stability and scaling
      problems.

   *  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-based networking and
      routing protocols.

3.1.  Architectural Considerations

   Semantic data may be applied in several ways to integrate with
   existing routing architectures.  An overlay can be built such that
   semantic routing is used to route 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 Internet.  In other
   cases, traffic from within the domain is exchanged with other domains
   that are connected together across an IP-based network using tunnels
   or via application gateways.  And in still another case traffic from
   the domain is routed 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-based networks.  In these cases, there
      is no risk to external networks from any semantic routing schemes
      carried out within the domain.  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

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      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
      routed 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-based 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 semantic 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 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-based 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 limited
          domain, and communications with the wider Internet will be
          handled by a protocol or application gateway.

       Domain:  The use of the semantic routing is entirely 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

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       systems react if such a packet escaped (accidentally or
       maliciously) from the planned scope of the proposal?  For
       example: how are cryptographically generated addresses (such as
       [RFC3972]) made routable and kept simple enough for management?;
       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?

   3.  What path characteristics are needed to describe the desired
       paths and as input to route computations?  Since one of the
       purposes of adding semantics to the 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?

       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
       characteristic 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 charateristics 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.

       *  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.

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       *  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 simple backwards-compatible modifications to
             existing protocols such that they work for today's IP
             addresses as well as enhanced-semantics?

          -  Do we need entirely new protocols or radical evolutions of
             existing protocols in order to deliver the functions that
             we need?

          -  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?

          Do we need new management tools and techniques?

   5.  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 made on
       upstream routers.

   6.  What is the security impact for 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 routing of flows to different security
          functions?

       *  Is semantic routing able to enhance 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?

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       *  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 for routing systems?  Scalability
       can be measured as:

       *  Routing table size.  How many entries need to be maintained in
          the routing tables at 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 forwading table may be
          less of an issue on 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 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 of routing scalability, research that presents
       real numbers 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.

   8.  To what extent can multicast be developed:

       *  To support programmable SDN systems such as P4 [P4]?

       *  To satisfy end-to-end applications?

       *  To apply per-packet multicasting to develop new services?

       *  As a separate network layer distinct from IP or by encoding
          group destinations into IP addresses?

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   9.  What aspects need to be standardized?  It is really important to
       understand the necessity of standardization within this research.
       What degree of interoperability is expected between devices and
       networks?  Is the limited 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.

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, Christian Jacquenet, David Hutchison, Jeffery He, Dino
   Farinacci, and Greg Mirsky 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

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               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-02, 14 January 2022,
              <https://www.ietf.org/archive/id/draft-farrel-irtf-
              introduction-to-semantic-routing-02.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>.

   [P4]       P4 and ONF, "P4 Open Source Programming Language", Web
              page, Programming Protocol-independent Packet Processors
              (P4), 2021, <https://p4.org/>.

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

   [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>.

   [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

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   Email: d.king@lancaster.ac.uk

   Adrian Farrel
   Old Dog Consulting
   United Kingdom

   Email: adrian@olddog.co.uk

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