Network Working Group                                    L. Iannone, Ed.
Internet-Draft                                                    Huawei
Intended status: Informational                           23 October 2023
Expires: 25 April 2024

                      IP Addressing Considerations


   The Internet Protocol (IP) has been the major technological success
   in information technology of the last half century.  As the Internet
   becomes pervasive, IP has been replacing communication technology for
   many domain-specific solutions, but it also has been extended to
   better fit the specificities of the different use cases.  For
   Internet addressing in particular, as it is defined in RFC 791 for
   IPv4 and RFC 8200 for IPv6, respectively, there exist many
   extensions.  Those extensions have been developed to evolve the
   addressing capabilities beyond the basic properties of Internet
   addressing.  This document discusses the properties the IP addressing
   model, showcasing the continuing need to extend it and the methods
   used for doing so.

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Current Properties of Internet Protocol Addressing  . . . . .   5
     3.1.  Property 1: Fixed Address Length  . . . . . . . . . . . .   5
     3.2.  Property 2: Ambiguous Address Semantic  . . . . . . . . .   5
     3.3.  Property 3: Limited Address Semantic Support  . . . . . .   6
   4.  Perceived IP Addressing Shortcomings  . . . . . . . . . . . .   6
   5.  Existing IP Addressing Extensions . . . . . . . . . . . . . .  10
     5.1.  Length Extensions . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Identity Extensions . . . . . . . . . . . . . . . . . . .  13
     5.3.  Semantic Extensions . . . . . . . . . . . . . . . . . . .  17
     5.4.  IP Addressing Extensions Overall Summary  . . . . . . . .  21
   6.  Concerns Raised by IP Addressing Extensions . . . . . . . . .  23
     6.1.  Limiting Address Semantics  . . . . . . . . . . . . . . .  23
     6.2.  Complexity and Efficiency . . . . . . . . . . . . . . . .  23
     6.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  26
     6.4.  Fragility . . . . . . . . . . . . . . . . . . . . . . . .  27
     6.5.  Summary of Concerns . . . . . . . . . . . . . . . . . . .  28
   7.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  29
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  31
   Informative References  . . . . . . . . . . . . . . . . . . . . .  31
   Appendix A.  Desirable Networking Features  . . . . . . . . . . .  50
   Appendix B.  IP Addressing Extensions driven by Use Cases . . . .  53
     B.1.  Communication in Constrained Environments . . . . . . . .  53
     B.2.  Communication within Dynamically Changing Topologies  . .  54
     B.3.  Communication among Moving Endpoints  . . . . . . . . . .  56
     B.4.  Communication Across Services . . . . . . . . . . . . . .  59
     B.5.  Communication Traffic Steering  . . . . . . . . . . . . .  60
     B.6.  Communication with built-in security  . . . . . . . . . .  61
     B.7.  Communication protecting user privacy . . . . . . . . . .  62
     B.8.  Communication in Alternative Forwarding Architectures . .  62
   Appendix C.  Examples of Internet Addressing Properties
           Extensions  . . . . . . . . . . . . . . . . . . . . . . .  64
     C.1.  Length Extensions . . . . . . . . . . . . . . . . . . . .  64
     C.2.  Identity Extensions . . . . . . . . . . . . . . . . . . .  66

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     C.3.  Semantic Extensions . . . . . . . . . . . . . . . . . . .  68
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  72
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  74

1.  Rationale

   The IETF community has, at various times, discussed the IP addressing
   model and its possible evolution, while keeping its structure
   unchanged, so to accommodate future use cases and existing
   deployments.  This document does (or at least tries to) capture the
   discussion that the IETF community held about IP addressing model in
   the early 2020s.  The discussion originated from two memos proposing
   an analysis of the extensions developed to better adapt the IP
   addressing model to specific use cases
   [I-D.iannone-internet-addressing-considerations] and a (shorter)
   companion memo trying to formalize a related problem statement
   [I-D.iannone-scenarios-problems-addressing].  Further, an informal
   side meeting was organized during IETF 112 [SIDE112] with a panel of
   experts, which had a lively discussion.  That discussion continued,
   with a very large volume of messages, on the INTArea mailing list and
   other mailing lists, like architectural discuss, honing into the
   related question on what desired features a network should provide in
   the first place (see Appendix A for a summary of the feature listed
   in that discussion).  The IAB also touched briefly the topic in one
   of their retreats in 2022.  The momentum and the amplitude of the
   discussion did raise the question whether or not to go for a formal
   Working Group, however, the community failed to converge on a
   specific direction that could eventually lead to an evolution of the
   IP addressing model and at the same time the steam diminished.

   This document does not provide a definite answer nor does it propose
   or promote specific solutions to the issues it portrays.  Instead,
   this document, which includes a large portion of last revision of the
   aforementioned individual submissions, captures the discussion on the
   perceived needs for addressing, with the possibility to fundamentally
   re-think the addressing in the Internet beyond the objectives of
   IPv6, in order to provide the flexibility to suitably support the
   many new forms of communication that will emerge.

   Although some of the discussions hinted at "something should be
   done", those same discussions never converged to answer the "what
   should be done" aspect.  However, we assert from experiences in the
   past that the community may at some point in the future re-open
   discussions surrounding the IP addressing model and its possible
   evolution, in which case this document will be useful.

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2.  Introduction

   The Internet Protocol (IP), positioned as the unified protocol at the
   (Internet) network layer, is seen by many as key to the innovation
   stemming from Internet-based applications and services.  Even more
   so, with the success of the TCP/IP protocol stack, IP has been
   gradually replacing existing domain-specific protocols, evolving into
   the core protocol of the ever-growing communication eco-system

   The Internet addressing system [RFC0791], represented in the form of
   the IP address and its locator-based (topological) semantics, has
   brought about the notion of a 'common namespace for all
   communications at the IP layer'.  Compared to proprietary technology-
   specific solutions, such unified namespace ensures end-to-end
   communication from any device connected to the Internet to another.

   As the Internet Protocol adoption has grown towards the global
   communication system we know today, its characteristics have evolved
   subtly, with [RFC6250] documenting various aspects of the IP service
   model and its frequent misconceptions, including Internet addressing.
   Use cases, associated services, node behaviors, and requirements on
   packet delivery have since been significantly extended, with suitable
   Internet technology being developed to accommodate them in the
   framework of addressing that stood at the aforementioned beginning of
   the Internet's development.

   This continuing evolution includes addressing and, therefore, the
   address structure, as well as the semantic that is being used for
   packet forwarding (e.g., service identification, content location,
   device type).  In this, the topological semantic of IP is fundamental
   when reconciling the often-differing semantics for 'addressing' that
   can be found in new use cases.  Due to this centrality, use cases
   have to adopt specific solutions, e.g., translating/mapping/
   converting concepts, semantics, and ultimately, solution-specific
   addressing, and integrate them into the common IP addressing model.

   This per-use-case extension approach has implications that go beyond
   addressing, nevertheless, in this document the discussion only
   focuses on the addressing viewpoint, identifying shortcomings
   perceived from this perspective, in particular with respect to IP
   addressing properties.  The key properties of Internet addressing,
   outlined in Section 3, are (i) the fixed length of the IP addresses,
   (ii) the ambiguity of IP addresses semantic, while still (iii)
   providing limited IP address semantic support.  Those properties are
   derived directly as a consequence of the respective standards that
   provide the basis for Internet addressing, most notably [RFC0791] for
   IPv4 and [RFC8200] for IPv6, respectively.  The limitations of the IP

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   addressing properties are discussed in Section 4, including the
   various use cases and scenarios where such limitations actually show

   What is interesting to note is that different use cases may actually
   been handled with the same type of extension.  This shows that, based
   on an architectural approach, evolving the properties discussed in
   Section 3 is possible and even desirable since it has the advantage
   to be designed in a coherent fashion, avoiding point-solutions which
   potentially create contention when deployed.  To this end, Section 5
   discusses Internet addressing properties extensions, associating the
   different use cases that take advantage of the property's extensions.

   While the various extensions proposed through the years certainly did
   a fine job in solving the problem at hand, this "patching" approach
   raises also concerns.  Section 6 outlines considerations and concerns
   that arise with such extension-driven approach, arguing that any
   requirements for solutions that would revise the basic Internet
   addressing would require to address those concerns.

3.  Current Properties of Internet Protocol Addressing

   In this section, the three most acknowledged properties related to
   Internet addressing are detailed.  Those are (i) fixed IP address
   length, (ii) ambiguous IP address semantic, and (iii) limited IP
   address semantic support.

3.1.  Property 1: Fixed Address Length

   The fixed IP address length is specified as a key property of the
   design of Internet addressing, with 32 bits for IPv4 [RFC0791], and
   128 bits for IPv6 [RFC8200], respectively.  Given the capability of
   the hardware at the time of IPv4 design, a fixed length address was
   considered as a more appropriate choice for efficient packet
   forwarding.  Although the address length was once considered to be
   variable during the design of Internet Protocol Next Generation
   ("IPng", cf., [RFC1752]) in the 1990s, it finally inherited the
   design of IPv4 and adopted a fixed length address towards the current
   IPv6.  As a consequence, the 128-bit fixed address length of IPv6 is
   regarded as a balance between fast forwarding (i.e., fixed length)
   and practically boundless cyberspace (i.e., enabled by using 128-bit

3.2.  Property 2: Ambiguous Address Semantic

   Initially, the meaning of an IP address has been to identify an
   interface on a network device, although, when [RFC0791] was written,
   there were no explicit definitions of the IP address semantic.

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   With the global expansion of the Internet protocol, the semantic of
   the IP address is commonly believed to contain at least two notions,
   i.e., the explicit 'locator', and the implicit 'identifier'.  Because
   of the increasing use of IP addresses to both identify a node and to
   indicate the physical (or virtual) location of the node, the
   intertwined address semantics of identifier and locator was then
   gradually observed and first documented in [RFC2101] as 'locator/
   identifier overload' property.  With this, the IP address is used as
   an identification for hosts and servers.

3.3.  Property 3: Limited Address Semantic Support

   Although IPv4 [RFC0791] did not add any semantic to IP addresses
   beyond interface identification (and location), time has proven that
   additional semantics are desirable (c.f., the history of 127/8
   [HISTORY127] or the introduction of private addresses [RFC1918]).
   Later on, IPv6 [RFC4291] introduced some form of additional semantics
   based on specific prefix values, for instance link-local addresses or
   a more structured multicast addressing.  Nevertheless, systematic
   support for rich address semantics remains limited and basically

4.  Perceived IP Addressing Shortcomings

   What follows is the list of the most relevant perceived shortcomings
   identified during the various exchanges, which is however not to be
   considered exhaustive.

   1.  Limiting Alternative Address Semantics: Several communication
       scenarios pursue the use of alternative semantics (e.g., for
       privacy, for service identification, or for content
       identification) preserving what constitute an 'address' of a
       packet traversing the Internet, which falls foul of the defined
       network interface semantic of IP addresses.

   2.  Hampering Security: Aligning with the semantic and length
       limitations of IP addressing hampers the security objectives of
       any new semantic, possibly leading to detrimental effects and
       possible other workarounds (at the risk of introducing fragility
       rather than security).

   3.  Hampering Privacy: The simple use of IP addresses as global
       stable interface identifiers raises clear privacy concerns.  It
       goes beyond profiling the traffic of end users, since it can even
       be easily used to obtain the real identity of individuals.

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   4.  Complicating Traffic Engineering: Utilizing a plethora of non-
       address inputs (e.g., port numbers, segments ID, payload) into
       the traffic steering decision in real networks complicates
       traffic engineering in that it makes the development of suitable
       policies more complex, while also leading to possible contention
       between methods being used.

   5.  Hampering Efficiency: Extending IP addressing through point-wise
       solutions also hampers efficiency, e.g., through needed re-
       encapsulation (therefore increasing the header processing
       overhead as well as header-to-payload ratio), through introducing
       path stretch, or through requiring compression techniques to
       reduce the header proportion of large addresses when operating in
       constrained environments.

   6.  Fragility: The introduction of point solutions, each of which
       comes with possibly own usages of address or packet fields,
       together with extension-specific operations, increases the
       overall fragility of the resulting system, caused, for instance,
       through contention between feature extensions that were neither
       foreseen in the design nor tested during the implementation

   The above shortcomings are not apparent in every possible use case,
   rather they appear, in a more or less severe form, in specific use
   cases.  Hereafter, a set of such kind of use cases, for which
   extensions to the IP addressing model have been already proposed on a
   case-by-case basis, is listed.  Further details about these use cases
   and related extensions can be found in Appendix B, where for each use
   case there is an entire section.  Here, for each use case, a very
   short description and the issues they relate to is provided, also
   summarized inTable 1.

   *  Communication in Constrained Environments: Resource constrained
      networks like Internet of Things (IoT), Industrial IoT, avionics.
      When resources are strongly constrained the use of the single IP
      addressing space becomes an hindrance.  Proposed solutions relay
      on some form of adaptation that reduces resource consumption but
      complicates traffic engineering (Issue 4), reduces efficiency
      (Issue 5), and increases fragility (Issue 6).

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   *  Communication within Dynamically Changing Topologies: Networks
      that exhibit dynamically changing, e.g. satellite networks,
      vehicular networks, Flying Ad-hoc NETworks (FANETs).  The IP
      addressing model has been conceived for networks that do not
      change their topology that often, hence their semantic is not
      adapted to dynamic networks (Issue 1).  This clearly complicates
      traffic engineering (Issue 4) and reduces efficiency (Issue 5),
      leading to increased fragility (Issue 6).

   *  Communication among Moving Endpoints: The huge progress in
      wireless communications (WiFi, 3G/4G/5G, etc) enables ubiquitous
      endpoint mobility.  The implicit locator semantic (Issue 1) of the
      addresses does not match the endpoint mobility use case, because
      of its continuous location change, exposing user location (Issue
      3), complicating traffic steering (Issue 4), which reduces
      efficiency (Issue 5), making end-to-end connectivity more fragile
      (Issue 6).

   *  Communication Across Services: Communication among services and
      resources from various aspects such as remote collaboration,
      shopping, content production, delivery, education, etc.  The IP
      address has no notion of service (Issue 1), while proposed
      solutions introduce some form of service identification over the
      IP layer, which reduces efficiency (Issue 5) and complicates
      traffic engineering (Issue 4), introducing some fragility in the
      mapping function between IP addresses and service identifiers
      (Issue 6) and opening privacy concerns (Issue 3) if the services
      is accessing are exposed.

   *  Communication Traffic Steering: The ability to control where the
      traffic goes through (beyond the simple best-effort shortest-
      path).  The limited semantic of IP addresses translates to limited
      traffic engineering capabilities (Issue 1), which has been solved
      by considering other information beside IP addresses, hence using
      more complex and less efficient solutions (Issues 4 and 5).

   *  Communication with built-in security: AAA (Authentication,
      Authorization, Accountability), end-to-end encryption.  The
      limited semantic of IP addresses do not facilitate the
      implementation of security solutions (Issues 1 and 2), and the
      introduction of encryptions complicates traffic engineering
      because some information is now not available anymore (Issue 4),
      hence reducing efficiency (Issue 5) and adding fragility (Issue
      6), because of the workarounds introduced to cope with the lack of

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   *  Communication protecting user privacy: Private communication and
      fingerprinting avoidance is cumbersome in the IP addressing model
      (Issue 3), while the introduction of additional operations to
      protect user privacy reduces forwarding efficiency (Issue 5).

   *  Communication in Alternative Forwarding Architectures: Non-
      Internet Protocol based networks.  Alternative forwarding
      paradigms do not necessarily leverage on IP addressing, because of
      its limited semantic (Issue 1), also trying to simplify traffic
      steering (Issue 4) by leveraging on a reduced set of fields (if
      not just one).  However, while certainly boosting efficiency
      inside their own deployments, such solutions introduce some
      fragility (Issue 6) at the boundaries, where translations/
      adaptions need to be performed to restore native IP forwarding.

     |               | Issue | Issue | Issue | Issue | Issue | Issue |
     |               |   1   |   2   |   3   |   4   |   5   |   6   |
     | Constrained   |       |       |       |   x   |   x   |   x   |
     | Environments  |       |       |       |       |       |       |
     | Dynamically   |   x   |       |       |   x   |   x   |   x   |
     | Changing      |       |       |       |       |       |       |
     | Topologies    |       |       |       |       |       |       |
     | Moving        |   x   |       |   x   |   x   |   x   |   x   |
     | Endpoints     |       |       |       |       |       |       |
     | Across        |   x   |       |   x   |   x   |   x   |   x   |
     | Services      |       |       |       |       |       |       |
     | Traffic       |   x   |       |       |   x   |   x   |       |
     | Steering      |       |       |       |       |       |       |
     | Built-in      |   x   |   x   |       |   x   |   x   |   x   |
     | Security      |       |       |       |       |       |       |
     | User Privacy  |       |       |   x   |       |   x   |       |
     | Alternative   |   x   |       |       |   x   |       |   x   |
     | Forwarding    |       |       |       |       |       |       |
     | Architectures |       |       |       |       |       |       |

             Table 1: Issues Involved in Challenging Use Cases.

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5.  Existing IP Addressing Extensions

   As already stated, during the years various technologies have been
   developed that circumvent some IP addressing shortcomings, basically
   extending the properties defined in Section 3.

   Accommodating new requirements through ever new extensions as an
   extensibility approach to addressing complicates engineering due to
   the clearly missing boundaries against which contentions with other
   extensions could be managed.  It complicates standardization since
   extension-based extensibility requires independent, and often
   lengthy, standardization processes.  And ultimately, deployments are
   complicated due to backward compatibility testing required for any
   new extension being integrated into the deployed system.

   Hereafter, an overview of existing extensions is provided, grouped by
   property.  For each group, a general description and the methodology
   used by the various extensions is provided.  Details about the cited
   technologies relates to properties extension can be found in
   Appendix C.

5.1.  Length Extensions

   Extensions in this section aim at extending the property described in
   Section 3.1, i.e., the fixed IP address length.

   When IPv6 was designed, the main objective was to create an address
   space that would not lead to the same situation as IPv4, namely to
   address exhaustion.  To this end, while keeping the same addressing
   model like IPv4, IPv6 adopted a 128-bit address length with the aim
   of providing a sufficient and future-proof address space.  The choice
   was also founded on the assumption that advances in hardware and
   Moore's law would still allow to make routing and forwarding faster,
   and the IPv6 routing table manageable.

   We observe, however, that the rise of new use cases but also the
   number of new devices, e.g., industrial/home or small footprint
   devices, was possibly unforeseen.  Sensor networks and more generally
   the Internet of Things (IoT) emerged after the core body of work on
   IPv6, thus different from IPv6 assumptions, 128-bit addresses were
   costly in certain scenarios.  On the other hand, given the
   investments that IPv6 deployment involved, certain solutions are
   expected to increase the addressing space of IPv4 in a compatible
   way, and thus extend the lifespan of the sunk investment on IPv4.

   At the same time, it is also possible to use variable and longer
   address lengths to satisfy current networking demands.  For example,
   in content delivery networks, longer addresses such as URLs are

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   required to fetch content, an approach that Information-Centric
   Networking (ICN) applied for any data packet sent in the network,
   using information-based addressing at the network layer.
   Furthermore, as an approach to address the routing challenges faced
   in the Internet, structured addresses are a possible solution in
   order to avoid the need for routing protocols.  Using variable length
   addresses allow as well to have shorter addresses.  So, for
   requirements for smaller network layer headers, shorter addresses
   could be used, alleviating the need to compress other fields of the
   header.  Furthermore, transport layer port numbers can be considered
   short addresses, where the high order bits of the extended address
   are the public IP of a NAT.  Hence, in IoT deployments, the addresses
   of the devices can be really small and based on the port number, but
   they all share the global address of the gateway to make each one
   having a globally unique address.

5.1.1.  Shorter Address Length  Description

   In the context of constrained networks [RFC7228], where bandwidth and
   energy are very scarce resources, the static length of 128-bit for an
   IP address is more a hindrance than a benefit since 128-bit for an IP
   address is a lot of space, even to the point of being the dominant
   part of a packet.  In order to use bandwidth more efficiently and use
   less energy in end-to-end communication, solutions have been proposed
   that allow for very small network layer headers instead.  Methodology

   Header Compression/Translation:  One of the main approaches to reduce
      header size is by compressing it.  Such technique is based on a
      stateful approach, utilizing what is usually called a 'context' on
      the small constrained device and the gateway for communications
      between an the device and a server placed somewhere in the
      Internet - from the edge to the cloud.

      The role of the 'context' is to provide a way to 'compress' the
      original IP header into a smaller one, using shorter address
      information and/or dropping some field(s); the context here serves
      as a kind of dictionary.

   Separate device from locator identifier:  Approaches that are able to
      offer customized address length that is adequate for use in such
      constrained domains are preferred.  Using different namespaces for
      the 'device identifier' and the 'routing' or 'locator identifier'
      is one such approach.

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5.1.2.  Longer Address Length  Description

   Historically, obtaining adequate address space is considered as the
   primary and raw motivation to invent IPv6.  Longer address (more than
   32-bit of IPv4 address), which can accommodate almost inexhaustible
   devices, used to be considered as the surest direction in 1990s.
   Nevertheless, to protect the sunk cost of IPv4 deployment, certain
   efforts focus on IPv4 address space depletion question but engineer
   IPv4 address length in a more practical way.  Such effort, i.e., NAT
   (Network Address Translation), unexpectedly and significantly slows
   IPv6 deployment because of its high cost-effectiveness in practice.

   Another crucial need for longer address lengths comes from "semantic
   extensions" to IP addresses, where the extensions themselves do not
   fit within the length limitation of the IP address.  This section
   focuses on address length extensions that aim at reducing the IPv4
   addresses depletion, while Section 5.3 discusses when longer address
   length are suitable to accommodate different address semantic.  Methodology

   Split address zone by network realm:  This methodology first split
      the network realm into two types: one public realm (i.e., the
      Internet), and innumerable private realms (i.e., local networks,
      which may be embedded and/or having different scope).  Then, it
      splits the IP address space into two type of zones: global address
      zone (i.e., public address) and local address zone (e.g., private
      address, reserved address).  Based on this, it is assumed that in
      public realm, all devices attached to it should be assigned an
      address that belongs to the global address zone.  While for
      devices attached to private realms, only addresses belonging to
      the local address zone will be assigned.  In the local realms,
      addresses can be used for pure identification purpose (e.g. in a
      single hop WiFi network or a single hop personal area network).

      Given that the local address zone is not globally unique, certain
      mechanisms are designed to express the relationship between the
      global address zone (in public realm) and the local address zone
      (in any private realm).  In this case, global addresses are used
      for forwarding when a packet is in the public realm, and local
      addresses are used for forwarding when a packet is in a private

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5.1.3.  Examples

   Table 2 summarizes methodologies and lists examples of IP address
   length extensions.

      |                        | Methodology          | Examples    |
      | Shorter Address Length | Header compression/  | 6LoWPAN,    |
      |                        | translation          | ROHC, SCHC  |
      |                        | Separate device from | EIBP, LISP, |
      |                        | locator identifier   | ILNP, HIP   |
      | Longer Address Length  | Split address zone   | NAT, EzIP   |
      |                        | by network realm     |             |

                     Table 2: Length Extensions Summary

5.2.  Identity Extensions

   Extensions in this section attempt extending the property described
   in Section 3.2, i.e., 'locator/identifier overload' of the ambiguous
   address semantic.

   From the perspective of Internet users, on the one hand, the implicit
   identifier semantic results in a privacy concern due to network
   behavior tracking and association.  Despite that IP address
   assignments may be dynamic, they are nowadays considered as 'personal
   data' and as such undergoes privacy protection regulations.  Hence,
   additional mechanisms are necessary in order to protect end user

   For network regulation of sensitive information, on the other hand,
   dynamically allocated IP addresses are not sufficient to guarantee
   device or user identification.  As such, different address allocation
   systems, with stronger identification properties are necessary where
   security and authentication are at highest priority.  Hence, in order
   to protect information security within a network, additional
   mechanisms are necessary to identify the users or the devices
   attached to the network.

5.2.1.  Anonymous Address Identity

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   As discussed in Section 3.2, IP addresses reveal both 'network
   locations' as well as implicit 'identifier' information to both
   traversed network elements and destination nodes alike.  This enables
   recording, correlation, and profiling of user behaviors and
   historical network traces, possibly down to individual real user
   identity.  The IETF, e.g., in [RFC7258], has taken a clear stand on
   such pervasive surveillance by classifying it as an attack on end
   users' right to be left alone (i.e., privacy).  Regulations such as
   the EU's General Data Protection Regulation (GDPR) classifies, for
   instance, the 'online identifier' as personal data which must be
   carefully protected; this includes end users' IP addresses [VOIGT17].

   Even before pervasive surveillance [RFC7258], IP addresses have been
   seen as something that some organizational owners of networked system
   do not want to reveal at the individual level towards any non-member
   of the organization.  Beyond that, if forwarding is based on semantic
   extensions, like other fields of the header, extension headers, or
   any other possible extension, if not adequately protected it risks to
   introduce privacy leakage and/or new attack vectors.  Methodology

   Traffic Proxy:  Since nodes between trusted proxy and destination
      (including the destination per se) can only observe the source
      address of the proxy, the 'identification' of the origin source is
      thereby hidden.  To obfuscate information to the nodes between
      origin and the proxy, the traffic on such route would be encrypted
      via a key negotiated either in-band or off-band.  Considering that
      all applications' traffic in such route is seen as a unique flow
      directed to the same trusted proxy, eavesdroppers have to make
      more efforts to correlate user behavior through statistical
      analysis even if they are capable of identifying the users via
      their source addresses.  The protection lays in the inability to
      isolate single application-specific flows.  According to the
      methodology, such approach is IP version independent and works for
      both IPv4 and IPv6.

   Source Address Rollover:  Privacy concerns related to address
      'identifier' semantic can be mitigated through regular change
      (beyond the typical 24 hours lease of DHCP).  Due to the semantics
      of 'identifier' that an IP address carries, such approach promotes
      to change the source IP address at a certain frequency.  Under
      such methodology, the refresh cycling window has to reach a
      balance between privacy protection and address update cost.  Due
      to the limited space that IPv4 contains, such approach usually
      works for IPv6 only.

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   Private Address Spaces:  The introduction of private addresses
      (assigned to specific address spaces by IANA) allowed to
      communicate purely locally, e.g., within an enterprise, by
      separating private from public IP addresses ([RFC1597],
      [RFC1918]).  Considering that private addresses are never directly
      reachable from the Internet, hosts adopting private addresses are
      invisible and thus 'anonymous' for the Internet.  Besides, hosts
      for purely local communication used the latter while hosts
      requiring public Internet service access would still use public IP

   Address Translation:  The aforementioned original intention for using
      private IP addresses, namely for purely local communication,
      resulted in a lack of flexibility in changing from local to public
      Internet access on the basis of what application would require
      which type of service.

      If eventually every end-system in an organization would require
      some form of public Internet access in addition to local one, an
      adequate number of public Internet addresses would be required.
      Instead, address translation enables to utilize many private IP
      addresses within an organization, while only relying on one (or
      few) public IP addresses for the overall organization.

      In principle, address translation can be applied recursively.
      This can be seen in modern broadband access where some Internet
      providers rely on carrier-grade address translation for all their
      broadband customers, who in turn employ address translation of
      their internal home or office addresses to those (private again)
      IP addresses assigned to them by their network provider.

      Two benefits arise from the use of (private to public IP) address
      translation, namely (i) the hiding of local end systems at the
      level of the (address) assigned organization (e.g. in
      [GNATCATCHER]), and (ii) the reduction of public IP addresses
      necessary for communication across the Internet.  While the latter
      has been seen for long as a driver for address translation, here,
      we focus on the first one.

   Separate device from locator identifier:  Solutions that make a clear
      separation between the routing locator and the identifier, allow a
      device ID of any size, which in turn can be encrypted by a network
      element deployed at the border of routing domain (e.g., access/
      edge router).  Both source and end-domain addresses are encrypted
      and transported, as in the routing domain, only the routing
      locator is used.

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5.2.2.  Authenticated Address Identity  Description

   In some scenarios (e.g., corporate networks or [RFC7039]) it is
   desirable to being able authenticate IP addresses in order to prevent
   malicious attackers spoofing IP addresses.  This is usually achieved
   by using a mechanism that allows to prove ownership of the IP
   address.  Another growing use case where identity verification is
   necessary for security and safety reasons is in the aeronautical
   context, for both manned and unmanned aerial vehicles ([RFC9153],
   [I-D.haindl-lisp-gb-atn]).  Methodology

   Self-certified addresses:  This method is usually based on the use of
      public/private keys.  A node creates its own interface ID (IID) by
      using a cryptographic hash of its public key (with some additional
      parameters).  Messages are then signed using the nodes' private
      key.  The destination of the message will verify the signature
      through the information in the IP address.  Self-certification has
      the advantage that no third party or additional security
      infrastructure is needed.  Any node can generate its own address
      locally and then only the address and the public key are needed
      for verification.

   Collision-resistant addresses:  When self-certification cannot be
      used, an alternative approach is to generate addresses in a way
      that is statistically unique (collision-resistant).
      Authentication of the address then occurs in an out-of-band
      protocol, where the unique identifier is resolved to
      authenticating information.

   Third party granted addresses:  DHCP (Dynamic Host Configuration
      Protocol) is widely used to provide IP addresses, however, in its
      basic form, it does not perform any check and even an unauthorized
      user without the right to use the network can obtain an IP
      address.  To solve this problem, a trusted third party has to
      grant access to the network before generating an address (via DHCP
      or other) that identifies the user.  User authentication done
      securely either based on physical parameters like MAC addresses or
      based on an explicit login/password mechanism.

5.2.3.  Examples

   Table 3, summarize the methodologies and lists examples of identity

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    |                            | Methodology          | Examples    |
    | Anonymous Address Identity | Traffic Proxy        | VPN, TOR,   |
    |                            |                      | ODoH, oHTTP |
    |                            | Source Address       | SLAAC       |
    |                            | Rollover             |             |
    |                            | Private Address      | ULA         |
    |                            | Spaces               |             |
    |                            | Address Translation  | NAT         |
    |                            | Separate device from | EIBP, LISP  |
    |                            | locator identifier   |             |
    | Authenticated Address      | Self-certified       | CGA         |
    | Identity                   | Addresses            |             |
    |                            | Third party granted  | DHCP-Option |
    |                            | addresses            |             |

                    Table 3: Identity Extensions Summary

5.3.  Semantic Extensions

   Extensions in this section relate to the property described in
   Section 3.3, i.e., limited address semantic support.

   As explained in Section 3.2, IP addresses carry both locator and
   identification semantic.  Some efforts exist that try to separate
   these semantics either in different address spaces or through
   different address formats.  Beyond just identification, location, and
   the fixed address size, other efforts extended the semantic through
   existing or additional header fields (or header options) outside the
   Internet address.

   How much unique and globally routable an address should be?  With the
   effect of centralization, edges communicate with (rather) local DCs,
   hence a unique address globally routable is not a requirement
   anymore.  There is no need to use globally unique addresses all the
   time for communication, however, there is the need of having a unique
   address as a general way to communicate to any connected entity
   without caring what transmission networks the packets traverse.

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5.3.1.  Extended Address Semantics  Description

   Several extensions have been developed to extend beyond the limited
   IPv6 semantics.  Those approaches include the definition of structure
   to the address, utilize specific prefixes, or entirely utilize the
   IPv6 address for different semantics, while re-encapsulating the
   original packet to restore the semantics in another part of the
   network.  For instance, structured addresses have the capability to
   introduce delimiters to identify semantic information in the header,
   therefore not constraining any semantic by size limitations of the
   address fields.

   We note here that extensions often start out as being proposed as an
   extended header semantic, while standardization drives the solution
   to adopt an approach to accommodate their semantic within the
   limitations of an IP address.  This section does include examples of
   this kind.  Methodology

   Semantic prefixes:  Semantic prefixes are used to separate the IPv6
      address space.  Through this, new address families, such as for
      information-centric networking [CAROFIGLIO19], service routing or
      other semantically rich addressing, can be defined, albeit limited
      by the prefix length and structure as well as the overall length
      limitation of the IPv6 address.

   Separate device/resource from locator identifier:  The option to use
      separate namespaces for the device address would offer more
      freedom for the use of different semantics.  For instance, the
      static binding of IP addresses to servers creates a strong binding
      between IP addresses and service/resources, which is a limitation
      for large Content Distribution networks (CDNs) [FAYED21].

      As an extreme form of separating resource from locator identifier,
      recent engineering approaches, described in [FAYED21], decouple
      web service (semantics) from the routing address assignments by
      using virtual hosting capabilities, thereby effectively mapping
      possibly millions of services onto a single IP address.

   Structured addressing:  One approach to address the routing
      challenges faced in the Internet is the use of structured
      addresses, e.g., to void the need for routing protocols.  Benefits
      of this approach are significant, with the structured addresses
      capturing the relative physical or virtual position of routers in
      the network as well as being variable in length.  Key to the

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      approach, however, is that the structured addresses capturing the
      relative physical or virtual position of routers in the network,
      or networks in an internetwork, may end up not fitting within the
      fixed and limited IP address length (cf., Section 5.1.2).

   Localized forwarding semantics:  Layer 2 hardware, such as SDN
      switches, are limited to the use of specific header fields for
      forwarding decisions.  Hence, devising new localized forwarding
      mechanisms may be based on re-using differently existing header
      fields, such as the IPv6 source/destination fields, to achieve the
      desired forwarding behavior, while encapsulating the original
      packets in order to be restored at the local forwarding network
      boundary.  Networks in those solutions are limited by the size of
      the utilized address field, e.g., 256 bits for IPv6, thereby
      limiting the way such techniques could be used.

5.3.2.  Existing or Extended Header Semantics  Description

   While the former section explored extended address semantic, thereby
   limiting any such extended semantic with that of the existing IPv6
   semantic and length, additional semantics are also placed into the
   header of the packet or the packet itself, utilized for the
   forwarding decision to the appropriate endpoint according to the
   extended semantic.

   Reasons for embedding such new semantics is related to traffic
   engineering since it has long been shown that the IP address itself
   is not enough to steer traffic properly since the IP address itself
   is not semantically rich enough to adequately describe the forwarding
   decision to be taken in the network, not only impacting "where" the
   packet will need to go, but also "how" it will need to be sent.  Methodology

   In-Header extensions:  One way to add additional semantics besides
      the address fields is to use other fields already present in the

   Headers option extensions:  Another mechanism to add additional
      semantics is to actually add additional fields, e.g., through
      Header Options in IPv4 or through Extension Headers in IPv6.

   Re-encapsulation extension:  A more radical approach for additional
      semantics is the use of a completely new header that is designed
      so to carry the desired semantics in an efficient manner (e.g., as
      a shim header).

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   Structured addressing:  Similar to the methodology that structures
      addresses within the limitations of the IPv6 address length,
      outlined in the previous sections, structured addressing can also
      be applied within existing or extended header semantics, e.g.,
      utilizing a dedicated (extension) header to carry the structured
      address information.

   Localized forwarding semantics:  This set of solutions applies
      capabilities of newer (programmable) forwarding technology, such
      as [BOSSHART14], to utilize any header information for a localized
      forwarding decision.  This removes any limitation to use existing
      header or address information for embedding a new address semantic
      into the transferred packet.

5.3.3.  Examples

   Table 4, summarize the methodologies and lists examples of semantic

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    |                           | Methodology          | Examples    |
    | Utilizing Extended        | Semantic prefixes    | HICN        |
    | Address Semantics         |                      |             |
    |                           | Separate device from | EIBP, ILNP, |
    |                           | locator identifier   | LISP, HIP   |
    |                           | Structured           | EIBP, ILNP  |
    |                           | addressing           |             |
    |                           | Localized forwarding | REED        |
    |                           | semantics            |             |
    | Utilizing Existing or     | In-Header extensions | DetNet      |
    | Extended Header Semantics |                      |             |
    |                           | Headers option       | SHIM6,      |
    |                           | extensions           | SRv6, HIP   |
    |                           | Re-encapsulation     | VxLAN,      |
    |                           | extension            | ICNIP       |
    |                           | Structured           | EIBP        |
    |                           | addressing           |             |
    |                           | Localized forwarding | REED        |
    |                           | semantics            |             |

                   Table 4: Semantic Extensions Summary

5.4.  IP Addressing Extensions Overall Summary

   The following Table 5 describes the objectives of the extensions
   discussed in this memo with respect to the properties of Internet
   addressing (Section 3).  As summarized, extensions aim to extend one
   property of the Internet addressing, or extend other properties at
   the same time.

   |                          |     Length    |  Identity |  Semantic |
   |                          |   Extension   | Extension | Extension |
   | 6LoWPAN ([RFC6282],      |       x       |           |           |
   | [RFC7400], [BADENHOP15], |               |           |           |
   | [RFC8376], [RFC8724])    |               |           |           |

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   | ROHC [RFC5795]           |       x       |           |           |
   | EzIP [EZIP]              |       x       |           |           |
   | TOR [TOR]                |               |     x     |           |
   | ODoH [RFC9230], oHTTP    |               |     x     |           |
   | [I-D.ietf-ohai-ohttp]    |               |           |           |
   | SLAAC [RFC8981]          |               |     x     |           |
   | CGA [RFC3972]            |               |     x     |     x     |
   | NAT [RFC3083]            |       x       |     x     |           |
   | HICN [CAROFIGLIO19]      |               |     x     |     x     |
   | ICNIP [ICNIP]            |       x       |     x     |     x     |
   | CCNx names               |       x       |     x     |     x     |
   | EIBP [SHENOY21]          |       x       |     x     |     x     |
   | Geo addressing           |       x       |           |     x     |
   | REED [REED16]            |   x (with P4  |           |     x     |
   |                          | [BOSSHART14]) |           |           |
   | DetNet [DETNETWG]        |               |     x     |           |
   | Mobile IP [RFC6275]      |               |           |     x     |
   | SHIM6 [RFC5533]          |               |           |     x     |
   | SRv6 [RFC8402]           |               |           |     x     |
   | HIP [RFC7401]            |               |     x     |     x     |
   | VxLAN [RFC7348]          |               |     x     |     x     |
   | LISP ([RFC9300],         |               |     x     |     x     |
   | [RFC8060])               |               |           |           |
   | SFC [RFC7665]            |               |     x     |     x     |

     Table 5: Relationship between Extensions and Internet Addressing

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6.  Concerns Raised by IP Addressing Extensions

   While the extensions to the original Internet properties, discussed
   in Section 5, demonstrate that flexibility in the addressing model is
   desirable in certain circumstances, they also raise a number of
   concerns, which are discussed in the following sections.  To this
   end, the problems outlined hereafter link to the approaches to
   extensions summarized in Section 5.4.  These considerations are not
   present all the time and everywhere, since extensions are developed
   and deployed in different part of the Internet, which may worsen

6.1.  Limiting Address Semantics

   Many approaches changing the semantics of communication, e.g.,
   through separating host identification from network node
   identification [RFC7401], separating the device identifier from the
   routing locator ([SHENOY21], [RFC9299]), or through identifying
   content and services directly [CAROFIGLIO19], are limited by the
   existing packet size and semantic constraints of IPv6, e.g., in the
   form of its source and destination network addresses.

   While approaches such as ICNIP [I-D.trossen-icnrg-internet-icn-5glan]
   overrides the addressing semantics, e.g., by replacing IPv6 source
   and destination information with path identification, a possible
   unawareness of endpoints still requires the carrying of other address
   information as part of the payload, through a procedure that always
   provides the addressing information in the address fields to those

   Other approaches, like for instance [CAROFIGLIO19] and [REED16], use
   an hybrid approach preserving the existing addressing fields, while
   using them in a different way, but the limited number of available
   bits limits the benefits introduced by these proposals.

6.2.  Complexity and Efficiency

   Realizing the additional addressing semantics introduces additional
   complexity.  This is particularly a concern since those additional
   semantics are observed particularly at the edge of the Internet,
   utilizing the existing addressing semantic of the Internet to
   interconnect the domains that require those additional semantics.

   Furthermore, any additional complexity comes with an efficiency and/
   or cost penalty, particularly at the edge of the network, where
   resource constraints play a significant role.  Compression processes,

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   taking [FITZEK05] as an example, require additional resources both
   for the sender generating the compressed header but also the gateway
   linking to the general Internet by re-establishing the full IP

   Conversely, the performance requirements of core networks, in terms
   of packet processing speed, makes the accommodation of extensions to
   addressing not possible.  This is not only due to the necessary extra
   processing that is specific to the extension, but also due to the
   complexity that will need to be managed in doing so at significantly
   higher speeds than at the edge of the network.  The observations on
   the dropping of packets with IPv6 extension headers in the real world
   is (partially) due to such an implementation complexity [RFC7872].

   Another example for lowering the efficiency of packet forwarding is
   the routing in systems like Tor [TOR].  Traffic in Tor, for anonymity
   purposes, should be handed over by at least three intermediates
   before reaching the destination.  Frequent relaying enhances the
   privacy [CHAUM81], however, because such kind of solutions are
   implemented at application level, they come at the cost of lower
   communication efficiency.  A different privacy enhanced address
   semantic enables efficient implementation of Tor-like solutions at
   network layer.

   Repetitive Encapsulation:  Repetitive encapsulation is a concern
      since it bloats the packets size due to additional encapsulation
      headers.  Addressing proposals such as those in
      [I-D.trossen-icnrg-internet-icn-5glan] utilize path identification
      within an alternative forwarding architecture that acts upon the
      provided path identification.  However, due to the limitation of
      existing flow-based architectures with respect to the supported
      header structures (in the form of IPv4 or IPv6 headers), the new
      routing semantics are being inserted into the existing header
      structure, while repeating the original, sender-generated header
      structure, in the payload of the packet as it traverses the local
      domain, effectively doubling the per-packet header overhead.

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      The problem is also present in a number of solutions tackling
      different use cases, e.g., mobility [I-D.ietf-lisp-mn], data
      center networking ([RFC8926], [RFC7348], [I-D.ietf-intarea-gue]),
      traffic engineering [RFC8986], and privacy ([TOR], [DANEZIS09]).
      Certainly, these solutions are able to avoid issues like path
      lengthening or privacy concerns, as described before, but they
      come at the price of multiple encapsulations that reduce the
      effective payload.  This, not only hampers efficiency in terms of
      header-to-payload ratio, but also introduces 'encapsulation
      points', which in turn add complexity to the (edge) network as
      well as fragility due to the addition of possible failure points;
      this aspect is discussed in further details in Section 6.4.

   Compounding Concerns with Header Compression:  IP header overhead
      requires header compression in constrained environments, such as
      wireless sensor networks and IoT in general.  Together with
      fragmentation, both tasks constitute significant energy
      consumption, as shown in [MESRINEJAD11], negatively impacting
      resource limited devices, especially those that rely on battery
      for operation.  Further, the reliance on the compression/
      decompression points creates a dependence on such gateways, which
      is a problem for intermittent scenarios.

      According to [AYERS20] the implementation of the 6LowPAN protocol
      stack requires, once compiled, between 6.2 and 26.6 Kilobytes
      (Kb), depending on the implementation and supported features.  On
      extremely constrained devices, contiki-ng [CONTIKI], is the best
      choice because of its very small size (only 6.2 Kb), however, it
      offers less features.

   Introducing Path Stretch:  Mobile IP [RFC6275], which was designed
      for connection continuity in the face of moving endpoints, is a
      typical case for path stretch.  Since traffic must follow a
      triangular route before arriving at the destination, such detour
      routing inevitably impacts transmission efficiency as well as
      latency.  Mobile IP is not the only technology introducing path
      stretch.  Privacy preserving protocols like Tor, but also more
      classic VPNs introduce path stretch.

   Complicating Traffic Engineering:  While many extensions to the
      original IP address semantic target to enrich the decisions that
      can be taken to steer traffic, according to requirements like QoS,
      mobility, chaining, compute/network metrics, flow treatment, path
      usage, etc., the realization of the mechanisms as individual
      solutions complicates the original goal of traffic engineering
      when individual solutions are being used in combination.
      Ultimately, this may even prevent the combined use of more than
      one mechanism and/or policy with a need to identify and prevent

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      incompatibilities of mechanisms.  Key here is not the concerns
      arising from using conflicting traffic engineering policies,
      rather conflicting realizations of policies that should generally
      work well alongside ([CANINI15], [CURIC18]).

      This not only increases fragility, as discussed separately in
      Section 6.4, but also requires careful planning of which
      mechanisms to use and in which combination, needing human-in-the-
      loop approaches alongside possible automation approaches for the
      individual solutions.

6.3.  Security

   The properties described in Section 5 have, obviously, also
   consequences in terms of security and privacy related concerns, as
   already mentioned in other parts of this document.

   For instance, in the effort of being somehow backward compatible, HIP
   [RFC7401] uses a 128-bit Host Identity, which will be not
   sufficiently cryptographically strong in the future, because of the
   limited size (future computational power will erode 128-bit
   security).  Similarly, CGA [RFC3972] also aligns to the 128-bit
   limit, but uses only 59 bits of them, hence, the packet signature is
   not sufficiently robust to attacks [I-D.rafiee-6man-cga-attack].

   IP addresses, even temporary ones meant to protect privacy, have been
   long recognized as a 'Personal Identification Information' that
   allows even to geolocate the communicating endpoints [RFC8280].
   Depending on the renewal rate, some issues arise, like the large
   overhead due to the Duplicate Address Detection, the impact on the
   Neighbor Discovery mechanism, in particular the cache, potentially
   leading to communication disruption.  With such drawbacks, the
   extensions defeat their target, actually lowering security rather
   than increasing it.

   The introduction of alternative addressing semantics has also been
   used to help in (D)DoS attacks mitigation.  This leverages on
   changing the service identification model so to avoid topological
   information exposure, making the potential disruptions remain limited
   [HAO21].  However, this increased robustness for ongoing
   communications to DDoS on the servers comes at the price of important
   communication setup latency and fragility, as discussed next.

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6.4.  Fragility

   From the extensions discussed in Section 5, it is evident that having
   alternative or additional address semantic and formats available for
   making routing as well as forwarding decisions dependent on these, is
   common place in the Internet.  This, however, adds many extension-
   specific translation/adaptation points, mapping the semantic and
   format in one context into what is meaningful in another context, but
   also, more importantly, creating a dependency towards an additional
   component without explicit exposure to the endpoints that originally
   intended to communicate.

   For instance, the re-writing of IP addresses to facilitate the use of
   private address spaces throughout the public Internet, realized
   through network address translators (NATs), conflicts with the end-
   to-end nature of communication between two endpoints.  Additional
   (flow) state is required at the NAT middle-box to smoothly allow
   communication, which in turn creates a dependency between the NAT and
   the end-to-end communication between those endpoints, thus increasing
   the fragility of the communication relation.

   A similar situation arises when supporting constrained environments
   through a header compression mechanism, adding the need for, e.g., a
   ROHC [RFC5795] element in the communication path, with communication-
   related compression state being held outside the communicating
   endpoints.  Failure will introduce some inefficiencies due to context
   regeneration, which will affect the communicating endpoints,
   increasing fragility of the system overall.

   Such translation/adaptation between semantic extensions to the
   original 'semantic' of an IP address is generally not avoidable when
   accommodating more than a single universal semantic.  However, the
   solution-specific nature of every single extension risk to noticeably
   increase the fragility of the overall system, since individual
   extensions will need to interact with other extensions that are
   deployed in parallel, but were not designed taking into account such
   deployment scenario (cf., [I-D.ietf-intarea-tunnels]).  Considering
   that extensions to traditional per-hop-behavior (based on IP
   addresses) can essentially be realized over almost 'any' packet
   field, the possible number of conflicting behaviors or diverging
   interpretation of the semantic and/or content of such fields, among
   different extensions, will at some point become an issue, requiring
   careful testing and delineation at the boundaries of the network
   within which the specific extension has been realized.

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6.5.  Summary of Concerns

   Table 6, derived from the previous sections, summarizes the concerns
   discussed in this section related to each extension listed in
   Section 5.4.  While each extension involves at least one concern,
   some others, like ICNIP [I-D.trossen-icnrg-internet-icn-5glan],
   create several at the same time.

   |            |     Limiting     | Complexity | Security | Fragility |
   |            |     Address      |    and     |          |           |
   |            |    Semantics     | Efficiency |          |           |
   | 6LoWPAN    |                  |     x      |          |     x     |
   | ROHC       |                  |     x      |          |     x     |
   | EzIP       |                  |     x      |          |           |
   | TOR        |                  |     x      |          |     x     |
   | ODoH       |                  |     x      |          |           |
   | oHTTP      |                  |     x      |          |           |
   | SLAAC      |                  |     x      |          |           |
   | CGA        |        x         |            |    x     |           |
   | NAT        |                  |     x      |          |     x     |
   | HICN       |        x         |            |          |           |
   | ICNIP      |        x         |     x      |          |           |
   | CCNx name  |        x         |            |          |           |
   | EIBP       |                  |            |          |     x     |
   | Geo        |        x         |            |          |     x     |
   | addressing |                  |            |          |           |
   | REED       |        x         |            |          |           |
   | DetNet     |                  |     x      |          |           |
   | Mobile IP  |                  |     x      |          |     x     |

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   | SRv6       |                  |     x      |          |     x     |
   | HIP        |                  |            |    x     |     x     |
   | VxLAN      |                  |     x      |          |           |
   | LISP       |                  |     x      |          |     x     |
   | SFC        |                  |     x      |          |     x     |

           Table 6: Concerns in Extensions to Internet Addressing

7.  Discussion

   The examples of extensions discussed in Section 5 to the original
   Internet addressing scheme show that extensibility beyond the
   original model (and its underlying per-hop behavior) is a desired
   capability for networking technologies and has been so for a long
   time.  Generally, we can observe that those extensions are driven by
   the requirements of stakeholders, derived from the aforementioned
   problems and communication scenarios, thus, expecting a desirable
   extended functionality from the introduction of the specific
   extension.  If interoperability is required, those extensions require
   standardization of possibly new fields, new semantics as well as
   (network and/or end system) operations alike.

   This points to the conclusion that the existence of the many
   extensions to the original Internet addressing is clear evidence for
   wanting to develop evolution paths over time by the wider Internet
   community, each of which come with a raft of issues that we need to
   deal with daily.  This makes it desirable to develop an architectural
   and, more importantly, a sustainable approach to make Internet
   addressing extensible in order to capture the many new use cases that
   will still be identified for the Internet to come.

   This is not to 'second guess' the market and its possible evolution,
   but to outline clear features from which to derive clear principles
   for a design.  Any such design must not skew the technical
   capabilities of addressing to the current economic situation of the
   Internet and its technical realization, e.g., being a mere ephemeral
   token for accessing PoP-based services, since this bears the danger
   of locking down innovation capabilities as an outcome of those
   technical limitations introduced.  Instead, addressing must be
   aligned with enabling the model of permissionless innovation that the
   IETF has been promoting, ultimately enabling the serendipity of new
   applications that has led to many of those applications currently
   deployed in the Internet.

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   Having a more systematic approach, rather than point extensions,
   would allow the Internet community to identify an overall
   evolutionary path able to accommodate existing and future use cases,
   without disruptive solutions breaking existing deployments, rather
   with a well-thought out set of incremental steps.

   An architectural evolution of the IP addressing model allows bring
   clear benefits in various scenarios.  Examples of such benefits are
   provided hereafter, for a short sample of use cases.  An extensive
   discussion about these use cases can be found in Appendix B.

   *  Communication in Constrained Environments Potential Benefits:
      Avoid complex and energy hungry operations, like header
      compression and fragmentation, necessary to translate protocol
      headers from one limited domain to another, while enabling
      semantics different from locator-based addressing allows to better
      support the communication that occurs in those environments.

   *  Communication within Dynamically Changing Topologies Potential
      Benefits: Allow for accommodating such geographic address
      semantics into the overall Internet addressing, while also
      enabling name/content-based addressing, utilizing the redundancy
      of many network locations providing the possible data.

   *  Communication among Moving Endpoints Potential Benefits: Enable
      better mobility, e.g., through an augmented semantic that fulfils
      the mobility requirements [RFC7429] in a more efficient way or
      through moving from a locator- to a content or service-centric
      semantic for addressing.

   *  Communication Across Services Potential Benefits: Allow for
      incorporating different information, e.g., service as well as
      chaining semantics, into the overall Internet addressing.

   *  Communication Traffic Steering Potential Benefits: More semantic
      rich encoding schemes help in steering traffic at hardware level
      and speed, without complex mechanisms usually resulting in
      handling packets in the slow path of routers.

   *  Communication with built-in security Potential Benefits: Security-
      related key, certificate, or identifier could be included in a
      suitable address structure without any information loss, which
      weakens security and trust.

   *  Communication protecting user privacy Potential Benefits: Enable
      easy mechanism to obfuscate IP addresses to entities not involved
      in the communication.

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   *  Communication in Alternative Forwarding Architectures Potential
      Benefits: Reduce the wastage by accommodating Internet addressing
      in the light of alternative forwarding architectures, instead
      enabling the direct use of the alternative forwarding information.

   Finally, it is important to remark that any change in the addressing,
   hence at the data plane level, leads to changes and challenges at the
   control plane level, i.e., routing.  The latter is an even harder
   problem than just addressing and might need more research efforts
   that are beyond what is discussed in this document, which focuses
   solely on the data plane.

8.  Security Considerations

   The present memo does not introduce any new technology and/or
   mechanism and as such does not introduce any new security threat to
   the TCP/IP protocol suite.

   As an additional note, and as discussed in this document, security
   and privacy aspects were not considered as part of the key properties
   for Internet addressing, which led to the introduction of a number of
   extensions intending to fix those gaps.  The analysis presented in
   this memo (non-exhaustively) shows those concerns are either solved
   in an ad-hoc manner at application level, or at transport layer,
   while at network level only few extensions tackling specific aspects
   exist, albeit with limitations due to the adherence to the Internet
   addressing model and its properties.

9.  IANA Considerations

   This document does not include any IANA request.


   Thanks to all the people that shared insightful comments both
   privately to the authors as well as on various mailing list,
   especially on the INTArea Mailing List.  Thanks as well, for the
   interesting discussions, to Carsten Borman, Brian E.  Carpenter, and
   Eric Vyncke.  Thanks to Eliot Lear for his thorough review of this

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   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
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   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,

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   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,

   [RFC3083]  Woundy, R., "Baseline Privacy Interface Management
              Information Base for DOCSIS Compliant Cable Modems and
              Cable Modem Termination Systems", RFC 3083,
              DOI 10.17487/RFC3083, March 2001,

   [RFC3118]  Droms, R., Ed. and W. Arbaugh, Ed., "Authentication for
              DHCP Messages", RFC 3118, DOI 10.17487/RFC3118, June 2001,

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC4014]  Droms, R. and J. Schnizlein, "Remote Authentication Dial-
              In User Service (RADIUS) Attributes Suboption for the
              Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Information Option", RFC 4014, DOI 10.17487/RFC4014,
              February 2005, <>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <>.

   [RFC4581]  Bagnulo, M. and J. Arkko, "Cryptographically Generated
              Addresses (CGA) Extension Field Format", RFC 4581,
              DOI 10.17487/RFC4581, October 2006,

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   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,

   [RFC4982]  Bagnulo, M. and J. Arkko, "Support for Multiple Hash
              Algorithms in Cryptographically Generated Addresses
              (CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007,

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              DOI 10.17487/RFC5061, September 2007,

   [RFC5177]  Leung, K., Dommety, G., Narayanan, V., and A. Petrescu,
              "Network Mobility (NEMO) Extensions for Mobile IPv4",
              RFC 5177, DOI 10.17487/RFC5177, April 2008,

   [RFC5275]  Turner, S., "CMS Symmetric Key Management and
              Distribution", RFC 5275, DOI 10.17487/RFC5275, June 2008,

   [RFC5517]  HomChaudhuri, S. and M. Foschiano, "Cisco Systems' Private
              VLANs: Scalable Security in a Multi-Client Environment",
              RFC 5517, DOI 10.17487/RFC5517, February 2010,

   [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533,
              June 2009, <>.

   [RFC5795]  Sandlund, K., Pelletier, G., and L. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,

   [RFC5944]  Perkins, C., Ed., "IP Mobility Support for IPv4, Revised",
              RFC 5944, DOI 10.17487/RFC5944, November 2010,

   [RFC6158]  DeKok, A., Ed. and G. Weber, "RADIUS Design Guidelines",
              BCP 158, RFC 6158, DOI 10.17487/RFC6158, March 2011,

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   [RFC6182]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
              Iyengar, "Architectural Guidelines for Multipath TCP
              Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,

   [RFC6250]  Thaler, D., "Evolution of the IP Model", RFC 6250,
              DOI 10.17487/RFC6250, May 2011,

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC6626]  Tsirtsis, G., Park, V., Narayanan, V., and K. Leung,
              "Dynamic Prefix Allocation for Network Mobility for Mobile
              IPv4 (NEMOv4)", RFC 6626, DOI 10.17487/RFC6626, May 2012,

   [RFC6740]  Atkinson, RJ. and SN. Bhatti, "Identifier-Locator Network
              Protocol (ILNP) Architectural Description", RFC 6740,
              DOI 10.17487/RFC6740, November 2012,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, DOI 10.17487/RFC7039, October 2013,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

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   [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
              Routable Cryptographic Hash Identifiers Version 2
              (ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September
              2014, <>.

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
              2014, <>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,

   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <>.

   [RFC7429]  Liu, D., Ed., Zuniga, JC., Ed., Seite, P., Chan, H., and
              CJ. Bernardos, "Distributed Mobility Management: Current
              Practices and Gap Analysis", RFC 7429,
              DOI 10.17487/RFC7429, January 2015,

   [RFC7476]  Pentikousis, K., Ed., Ohlman, B., Corujo, D., Boggia, G.,
              Tyson, G., Davies, E., Molinaro, A., and S. Eum,
              "Information-Centric Networking: Baseline Scenarios",
              RFC 7476, DOI 10.17487/RFC7476, March 2015,

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,

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   [RFC7687]  Farrell, S., Wenning, R., Bos, B., Blanchet, M., and H.
              Tschofenig, "Report from the Strengthening the Internet
              (STRINT) Workshop", RFC 7687, DOI 10.17487/RFC7687,
              December 2015, <>.

   [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,

   [RFC8060]  Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", RFC 8060, DOI 10.17487/RFC8060,
              February 2017, <>.

   [RFC8061]  Farinacci, D. and B. Weis, "Locator/ID Separation Protocol
              (LISP) Data-Plane Confidentiality", RFC 8061,
              DOI 10.17487/RFC8061, February 2017,

   [RFC8105]  Mariager, P., Petersen, J., Ed., Shelby, Z., Van de Logt,
              M., and D. Barthel, "Transmission of IPv6 Packets over
              Digital Enhanced Cordless Telecommunications (DECT) Ultra
              Low Energy (ULE)", RFC 8105, DOI 10.17487/RFC8105, May
              2017, <>.

   [RFC8163]  Lynn, K., Ed., Martocci, J., Neilson, C., and S.
              Donaldson, "Transmission of IPv6 over Master-Slave/Token-
              Passing (MS/TP) Networks", RFC 8163, DOI 10.17487/RFC8163,
              May 2017, <>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC8280]  ten Oever, N. and C. Cath, "Research into Human Rights
              Protocol Considerations", RFC 8280, DOI 10.17487/RFC8280,
              October 2017, <>.

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,

   [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, <>.

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   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8595]  Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based
              Forwarding Plane for Service Function Chaining", RFC 8595,
              DOI 10.17487/RFC8595, June 2019,

   [RFC8609]  Mosko, M., Solis, I., and C. Wood, "Content-Centric
              Networking (CCNx) Messages in TLV Format", RFC 8609,
              DOI 10.17487/RFC8609, July 2019,

   [RFC8677]  Trossen, D., Purkayastha, D., and A. Rahman, "Name-Based
              Service Function Forwarder (nSFF) Component within a
              Service Function Chaining (SFC) Framework", RFC 8677,
              DOI 10.17487/RFC8677, November 2019,

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,

   [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,

   [RFC8763]  Rahman, A., Trossen, D., Kutscher, D., and R. Ravindran,
              "Deployment Considerations for Information-Centric
              Networking (ICN)", RFC 8763, DOI 10.17487/RFC8763, April
              2020, <>.

   [RFC8926]  Gross, J., Ed., Ganga, I., Ed., and T. Sridhar, Ed.,
              "Geneve: Generic Network Virtualization Encapsulation",
              RFC 8926, DOI 10.17487/RFC8926, November 2020,

   [RFC8928]  Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
              "Address-Protected Neighbor Discovery for Low-Power and
              Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November
              2020, <>.

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   [RFC8939]  Varga, B., Ed., Farkas, J., Berger, L., Fedyk, D., and S.
              Bryant, "Deterministic Networking (DetNet) Data Plane:
              IP", RFC 8939, DOI 10.17487/RFC8939, November 2020,

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [RFC9153]  Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements and Terminology", RFC 9153,
              DOI 10.17487/RFC9153, February 2022,

   [RFC9230]  Kinnear, E., McManus, P., Pauly, T., Verma, T., and C.A.
              Wood, "Oblivious DNS over HTTPS", RFC 9230,
              DOI 10.17487/RFC9230, June 2022,

   [RFC9268]  Hinden, R. and G. Fairhurst, "IPv6 Minimum Path MTU Hop-
              by-Hop Option", RFC 9268, DOI 10.17487/RFC9268, August
              2022, <>.

   [RFC9299]  Cabellos, A. and D. Saucez, Ed., "An Architectural
              Introduction to the Locator/ID Separation Protocol
              (LISP)", RFC 9299, DOI 10.17487/RFC9299, October 2022,

   [RFC9300]  Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
              Cabellos, Ed., "The Locator/ID Separation Protocol
              (LISP)", RFC 9300, DOI 10.17487/RFC9300, October 2022,

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   [RFC9301]  Farinacci, D., Maino, F., Fuller, V., and A. Cabellos,
              Ed., "Locator/ID Separation Protocol (LISP) Control
              Plane", RFC 9301, DOI 10.17487/RFC9301, October 2022,

   [RFC9354]  Hou, J., Liu, B., Hong, Y., Tang, X., and C. Perkins,
              "Transmission of IPv6 Packets over Power Line
              Communication (PLC) Networks", RFC 9354,
              DOI 10.17487/RFC9354, January 2023,

   [RFC9374]  Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
              "DRIP Entity Tag (DET) for Unmanned Aircraft System Remote
              ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023,

   [RFC9414]  Gont, F. and I. Arce, "Unfortunate History of Transient
              Numeric Identifiers", RFC 9414, DOI 10.17487/RFC9414, July
              2023, <>.

   [RFC9428]  Choi, Y., Ed., Hong, Y., and J. Youn, "Transmission of
              IPv6 Packets over Near Field Communication", RFC 9428,
              DOI 10.17487/RFC9428, July 2023,

   [RFC9434]  Card, S., Wiethuechter, A., Moskowitz, R., Zhao, S., Ed.,
              and A. Gurtov, "Drone Remote Identification Protocol
              (DRIP) Architecture", RFC 9434, DOI 10.17487/RFC9434, July
              2023, <>.

   [RFC9453]  Hong, Y., Gomez, C., Choi, Y., Sangi, A., and S.
              Chakrabarti, "Applicability and Use Cases for IPv6 over
              Networks of Resource-constrained Nodes (6lo)", RFC 9453,
              DOI 10.17487/RFC9453, September 2023,

   [SHENOY21] Shenoy, N., Chandraiah, S., and P. Willis, "A Structured
              Approach to Routing in the Internet", 2021 IEEE 22nd
              International Conference on High Performance Switching and
              Routing (HPSR), DOI 10.1109/hpsr52026.2021.9481818, June
              2021, <>.

   [SIDE112]  IETF 112 Side Meetings, "Internet Addressing: problems and
              gap analysis", 2021,

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              "Deutsche Telekom tests TeraStream, the network of the
              future, in Croatia", n.d.,

   [TOR]      "The Tor Project", n.d., <>.

              Trossen, D., Sarela, M., and K. Sollins, "Arguments for an
              information-centric internetworking architecture", ACM
              SIGCOMM Computer Communication Review vol. 40, no. 2, pp.
              26-33, DOI 10.1145/1764873.1764878, April 2010,

   [VOIGT17]  Voigt, P. and A. von dem Bussche, "The EU General Data
              Protection Regulation (GDPR)", Springer International
              Publishing book, DOI 10.1007/978-3-319-57959-7, 2017,

   [WANG19]   Wang, P., Zhang, J., Zhang, X., Yan, Z., Evans, B., and W.
              Wang, "Convergence of Satellite and Terrestrial Networks:
              A Comprehensive Survey", IEEE Access vol. 8, pp.
              5550-5588, DOI 10.1109/access.2019.2963223, 2020,

   [WION19]   Wion, A., Bouet, M., Iannone, L., and V. Conan,
              "Distributed Function Chaining with Anycast Routing",
              Proceedings of the 2019 ACM Symposium on SDN Research,
              DOI 10.1145/3314148.3314355, April 2019,

   [YU22]     Yu, Q., Liu, H., Xu, P., Li, J., Zhang, L., and H. Chen,
              "Next Generation Wireless Avionics Intra-Communications:
              Challenges and Research Topics", IEEE Wireless
              Communications pp. 1-21, DOI 10.1109/mwc.003.2200087,
              2022, <>.

Appendix A.  Desirable Networking Features

   The present section outlines the general features that are desirable
   in a networked system at large, i.e., not specific to any
   application/usage.  Such list is a "by-product" of the addressing

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   1.  Always-On: The world is getting more and more connected, leading
       to being connected to the Internet, anywhere, by any technology
       (e.g., cable, fiber, or radio), even simultaneously, "all the
       time", and, most importantly, automatically (without any switch
       turning).  However, when defining "all the time" there is a clear
       and important difference to be made between availability and
       reliability vs "desired usage".  From an end user perspective,
       clearly the former is of importance, not necessarily leading to
       an "always on" system notion but instead "always-app-available",
       merely requiring the needed availability and reliability to
       realize the perception of being "always on" (e.g., for earthquake
       alerts), possibly complemented by app-specific methods to realize
       the "always on" perception (e.g., using local caching rather than
       communication over the network).

   2.  Transparency: Being agnostic with respect to local domains
       network protocols (Bluetooth, ZigBee, Thread, Airdrop, Airplay,
       or any others) is key to provide an easy and straightforward
       method for contacting people and devices without any knowledge of
       network issues, particularly those related to network-specific
       solutions.  While having a flexible addressing model that
       accommodates a wide range of use cases is important, the
       centrality of the IP protocol remains key as a mean to provide
       global connectivity.

   3.  Multi-homing: Seamless multi-homing capability for the host is
       key to best use the connectivity options that is available to an
       end user, e.g., for increasing resilience in cases of failures of
       one available option.  Protocols like LISP, SHIM6, QUIC, MPTCP,
       SCTP (to cite a few) have been successful at providing this
       capability in an incremental way, but too much of that capability
       is realized within the specific use case, making it hard to
       leverage across all applications.  While today each transport
       protocol has its own way to perform multi-address discovery, the
       network layer should provide the multi-homing feature (e.g.,
       SHIM6 can be used to discover all addresses on both ends), and
       then leave the address selection to the transport.  With that,
       multi-address discovery remains a network feature exposed to the
       upper layers.  This also means to update the Socket API (which
       may be actually the first thing to do), which does not
       necessarily mean to expose more network details to the
       applications, rather to be more address agnostic, yet, more

   4.  Mobility: A lot of work has been put in MobileIP ([RFC5944],
       [RFC6275]) to provide seamless and lossless communications for
       moving nodes (vehicle, satellites).  However, it has never been
       widely deployed for several reasons, like complexity of the

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       protocol and the fact that the problem has also been tackled at
       higher layers, with applications resilient to address changes.
       However, similar to multi-homing, solving the problem at higher
       layers means that each and every transport protocol and
       application have their own way to deal with mobility, leading to
       similar observations as those for the previous multi-homing

   5.  Security and Privacy: The COVID-19 pandemic has boosted end
       users' desire to be protected and protect their privacy.  The
       balance among privacy, security, and accountability is not simple
       to achieve.  There exist different views on what those properties
       should be, however the network should provide the means to
       provide what is felt as the best trade-off for the specific use

   6.  Performance: While certainly desirable, "performance" is hard to
       define since it depends on the objectives of those building for
       but also paying for performance.  Examples are (i) speed (shorter
       paths/direct communications), (ii) bandwidth (10petabit/s for a
       link), (iii) efficiency (less overlays/encapsulations), (iv) high
       efficacy or sustainability (avoid waste).  From an addressing
       perspective, length/format/semantics that adapt to the specific
       use case (e.g. use short addresses for low power IoT, or, where
       needed, longer for addresses embedding certificates for strong
       authentication, authorization and accountability) contribute to
       the performance aspects that end users desire, such as reducing
       waste through not needed encapsulation or needed conversion at
       network boundaries.

   7.  Availability, Reliability, Predictability: These three properties
       are important to enable wide-range of services and applications
       according to the desired usage (cf. point 1).

   8.  Do not do harm: Access to the Internet is considered a human
       right [RFC8280].  Access to and expression through it should
       align with this core principle.  This issue transcends through a
       variety of previously discussed 'features' that are desired, such
       as privacy, security but also availability and reliability.
       However, lifting the feature of network access onto a basic
       rights level also brings in the aspect of "do not do harm"
       through the use of the Internet with respect to wider societal
       objectives.  Similar to other industries, such as electricity or
       cars, preventing harm usually requires an interplay of
       commercial, technological, and regulatory efforts, such as the
       enforcement of seat belt wearing to reduce accident mortality.
       As a first step, the potential harmfulness of a novel method must
       be recognized and weighted against the benefits of its

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       introduction and use.  One increasingly important consideration
       in the technology domain is "sustainability" of resource usage
       for an end user's "consumption of" and "participation in"
       Internet services.  As an example, Distributed Ledger
       Technologies (DLT) are seen as an important tool for a variety of
       applications, including Internet decentralization [DINRG].
       However, the non-linear increase in energy consumption means that
       extending proof-of-work systems to the entire population of the
       planet would not only be impractical but also possibly highly
       wasteful, not just at the level of computation but also
       communication resource usage [I-D.trossen-rtgwg-impact-of-dlts].
       This poses the question on how novel methods for addressing may
       improve on sustainability of such technologies, particularly if
       adopted more widely.

   9.  Maximum Transmission Unit (MTU): One long standing issue in the
       Internet is related to the MTU and how to discover the path MTU
       in order to avoid fragmentation ([RFC9268],
       [I-D.templin-6man-aero]).  While it makes sense to always
       leverage as much performance from local systems as possible, this
       should come without sacrificing the ability to communicate with
       all systems.  Having a solid solution to solve the issue would
       make the overall interconnection of systems more robust.

Appendix B.  IP Addressing Extensions driven by Use Cases

   Over the years, a plethora of extensions has been proposed in order
   to move beyond the native properties of IP addresses.  The
   development of those extensions are, in a certain way, attempts, in a
   limited scope, to go beyond the original properties of Internet
   addressing and desired new capabilities that those developing the
   extensions identified as being missing and yet needed and desirable.

   The following sections provide a detailed and in-depth analysis of
   the use cases listed in Section 4 and how they relates to the
   shortcomings listed in that very same section.

B.1.  Communication in Constrained Environments

   In the Internet of Things (IoT) scenario, a simple, communication
   network demanding minimal resources is required, allowing for a group
   of IoT network devices to form a network of constrained nodes, with
   the participating network and end nodes requiring as little
   computational power as possible and having small memory requirements.
   Furthermore, in the context of industrial IoT, real-time requirements
   and scalability make IP technology not naturally suitable as
   communication technology [OCADO].

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   The end-to-end principle [RFC2775] requires IP addresses to be used
   on such constrained nodes, allowing IoT devices to talk on the
   Internet.  Given the constraints imposed on the computational and
   possibly also lower layer communication technology, the usage of a
   single addressing semantic in the form of a 128-bit endpoint
   identifier, i.e., IPv6 address, poses a challenge when operating such
   networks, e.g. because of limited Maximum Transmission Unit or to
   reduce power consumtion.  As a consequence, devices located at the
   edge of constrained networks act as gateway performing header
   compression [RFC4919] or other forms of protocol adaptation
   ([RFC9453], [RFC8928], [RFC8724]).

   Another type of (differently) constrained environment is an aircraft,
   which encompasses not only passenger communication but also the
   integration of real-time data exchange to ensure that processes and
   functions in the cabin are automatically monitored or actuated.  The
   proposal for having an Wireless Avionics Intra-Communications (WAIC
   [YU22]) system promises reduction in the complexity of electrical
   wiring harness design and fabrication, reduction in wiring weight,
   increased configuration, and potential monitoring of otherwise
   inaccessible moving or rotating aircraft parts.  Similar to the IoT
   concept, WAIC systems consist of short-range communications and are a
   potential candidate for passenger entertainment systems, smoke
   detectors, engine health monitors, tire pressure monitoring systems,
   and other kinds of aircraft maintenance systems.  Most of the
   aircraft applications and services are focused on the data (e.g.
   temperature of gas tank on left wing) and not on the topological
   location of the data source.  This means that the current topological
   location semantic of IP addresses is not beneficial for aircraft
   applications and services.

B.2.  Communication within Dynamically Changing Topologies

   Communication may occur over networks that exhibit dynamically
   topologies.  One such example is that of satellite networks,
   providing global Internet connections through a combination of inter-
   satellite and ground station communication.  With the convergence of
   space-based and terrestrial networks, users experience seamless
   broadband access, e.g., on cruise ships, flights, and within cars,
   seamlessly switching between Wi-Fi, cellular, or satellite based
   networks at any time [WANG19].  With large scale LEO (Low Earth
   Orbit) satellites, the involved topologies of the satellite network
   will be changing constantly while observing a regular flight pattern
   in relation to other satellites and predictable overflight patterns
   to ground users [CHEN21].

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   Although satellite bearer services are capable of transporting IPv4
   and IPv6 [CCSDS-702.1-B-1], as well as associated protocols such as
   IP Multicast, DNS services, and routing information; no IP
   functionality is implemented on-board of the spacecraft, limiting the
   capability of leveraging for instance on large scale satellite

   Moreover, due to the current IP addressing scheme and its focus on IP
   unicast addressing with extended deployment of IP multicast and some
   IP anycast, current deployments do not take advantage of the
   broadcast nature of satellite networks.

   As a result of these constraints, the Consultative Committee for
   Space Data Systems (CCSDS) has produced its own communication
   standards distinct from those of the IETF.  The conceptual model
   shares many similarities with the Open Systems Interconnection model,
   and individual CCSDS protocols address comparable concerns to those
   standardized by the IETF, but always under the distinct concerns that
   connectivity is intermittent, and while throughput rates is high, so
   is latency.

   Concerning the vehicular networks use case, the communication
   includes Road Side Units (RSU) with the possibility to create
   ephemeral connections to those RSUs for the purpose of workload
   offloading, joint computation over multiple (vehicular) inputs, and
   other purposes [I-D.ietf-lisp-nexagon].  Communication here exhibits
   a multi-hop nature, not just involving the vehicle and the RSU over a
   direct link.

   Those topologies are naturally changing constantly due to the dynamic
   nature of the involved communication nodes.

   The advent of Flying Ad-hoc NETworks (FANETs) has opened up an
   opportunity to create new added-value services [CHRIKI19].  Due to
   high mobility of FANET nodes, the network topology changes more
   frequently than in a typical vehicular ad hoc network.  From a
   routing point of view, although ad-hoc reactive and proactive routing
   approaches can be used, there are other type of routing protocols
   that have been developed for FANETS, such as hybrid routing protocols
   and position based routing protocols, aiming to increase efficiency
   in large scale networks with dynamic topologies.

   Both type of protocols challenge the current Internet addressing
   semantic: in the case of hybrid protocols, two different routing
   strategies are used inside and outside a network zone.  While inside
   a zone packets are routed to a specific destination IP address,
   between zones, query packets are routed to a subset of neighbors as
   determined by a broadcast algorithm.  In the case of position based

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   routing protocol, the IP addressing scheme is not used at all, since
   packets are routed to a different identifier, corresponding to the
   geographic location of the destination and not its topological
   location.  Hence, what is needed is to consolidate the geo-spatial
   addressing with that of a locator-based addressing in order to
   optimize routing policies across the zones.

   In the aforementioned network technologies, there is a significant
   difference between the high dynamics of the underlying network
   topologies, compared to the relative static nature of terrestrial
   network topology, as reported in [HANDLEY18].  As a consequence, the
   notion of a topological network location becomes restrictive in the
   sense that not only the relation between network nodes and user
   endpoint may change, but also the relation between the nodes that
   form the network itself.  This leads to the challenge of maintaining
   and updating the topological addresses in this constantly changing
   network topology.

   In attempts to utilize entirely different semantics for the
   addressing itself, geographic-based routing, such as in [HUGHES03],
   has been proposed for MANETs (Mobile Ad-hoc NETworks) through
   providing geographic coordinates based addresses to achieve better
   routing performance, lower overhead, and lower latency [ABDALLAH16].

B.3.  Communication among Moving Endpoints

   When packet switching was first introduced, back in the 60s/70s, it
   was intended to replace the rigid circuit switching with a
   communication infrastructure that was more resilient to failures.  As
   such, the design never really considered communication endpoints as
   mobile.  Even in the pioneering ALOHA [KUO95] system, despite
   considering wireless and satellite links, the network was considered
   static (with the exception of failures and satellites, which fall in
   what is discussed in Appendix B.2).  Ever since, a lot of efforts
   have been devoted to overcome such limitations once it became clear
   that endpoint mobility will become a main (if not THE main)
   characteristic of ubiquitous communication systems.

   The IETF has for a long time worked on solutions that would allow
   extending the IP layer with mobility support.  Because of the
   topological semantic of IP addresses, endpoints need to change
   addresses each time they visit a different network.  However, because
   routing and endpoint identification is also IP address based, this
   leads to communication disruption.

   The lack of an efficient mobility management solution at network
   layer enabled the involvement of the transport layer in mobility
   solutions, either by introducing explicit in-band signaling to allow

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   for communicating IP address changes (e.g., in SCTP [RFC5061] and
   MPTCP [RFC6182]), or by introducing some form of connection ID that
   allows for identifying a communication independently from IP
   addresses (e.g., the connection ID used in QUIC [RFC9000]).

   Concerning network layer only solutions, anchor-based Mobile IP
   mechanisms have been introduced in the past ([RFC5177], [RFC6626]
   [RFC5944], [RFC5275]).  Mobile IP is based on a relatively complex
   and heavy mechanism that makes it hard to deploy and it is not very
   efficient.  Furthermore, it is even less suitable than native IP in
   constrained environments like the ones discussed in Appendix B.1.

   Some of the alternative approaches to Mobile IP leverage the
   introduction of some form of overlay.  LISP [RFC9299], by separating
   the topological semantic from the identification semantic of IP
   addresses, is able to cope with endpoint mobility by dynamically
   mapping endpoint identifiers with routing locators
   [I-D.ietf-lisp-mn].  This comes at the price of an overlay that needs
   its own additional control plane [RFC9301].

   Similarly, the NVO3 (Network Virtualization Overlays) Working Group,
   while focusing on Data Center environments, also explored an overlay-
   based solution for multi-tenancy purposes, but also resilient to
   mobility since relocating Virtual Machines (VMs) is common practice.
   NVO3 considered for a long time several data planes that implement
   slightly different flavors of overlays ([RFC8926], [RFC7348],
   [I-D.ietf-intarea-gue]), but lacks an efficient control plane
   specifically tailored for DCs.

   Alternative mobility architectures have also been proposed in order
   to cope with endpoint mobility outside the IP layer itself.  The Host
   Identity Protocol (HIP) [RFC7401] introduced a new namespace in order
   to identify endpoints, namely the Host Identity (HI), while
   leveraging the IP layer for topological location.  On the one hand,
   such an approach needs to revise the way applications interact with
   the network layer, by modifying the DNS (now returning an HI instead
   of an IP address) and applications to use the HIP socket extension.
   On the other hand, early adopters do not necessarily gain any benefit
   unless all communicating endpoints upgrade to use HIP.

   Another alternative approach is adopted by Information-Centric
   Networking (ICN) [RFC7476].  By making content a first class citizen
   of the communication architecture, the "what" rather than the "where"
   becomes the real focus of the communication.  However, as explained
   in the next section, ICN can run either over the IP layer or
   completely replace it, which in turn can be seen as running the
   Internet and ICN as logically separated limited domains.

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   Unmanned Aircraft Systems (UAS) are another example of moving devices
   that require a stable mobility management scheme since they consist
   of a number of Unmanned Aerial Vehicles (UAV; or drones) subordinated
   to a Ground Control Station (GCS) [MAROJEVIC20].  There exist a
   variety of specialized UAVs that, although having redundant links to
   maintain communications in long-range missions (e.g., satellite),
   perform most of the communications with the GCS over wireless data
   links, e.g., based on a radio line-of-sight technology such as Wi-Fi
   or 3G/4G/5G.  In particular, in Beyond Visual Line of Sight (BVLOS)
   operations, legal requirements include the use of multiple redundant
   radio links (even employing different radio bands), but still require
   unique identification of the vehicle.  This implies that some
   resolution mechanism is required that securely resolves drone
   identifiers to link locators.

   To this end, Drone Remote Identification Protocol [RFC9434] uses
   hierarchical DRIP Entity Tags, which are hierarchical versions of
   Host Identity Tags, and thus compatible with HIP [RFC7401].  DRIP
   does not mandate the use of HIP, but suggests its use in several
   places.  Using the mobility extensions of HIP provides for one way to
   ensure secure identifier resolution.

   In addition to such connectivity considerations, data-centric
   communication plays an increasing role, where information is named
   and decoupled from its location, and applications/services operate
   over these named data rather than on host-to-host communications.

   In this context, the Data Distribution Service [ALMADANI20] has
   emerged as an industry-oriented open standard.  The space and time
   decoupling allowed by DDS is very relevant in any dynamic and
   distributed system, since interacting entities are not forced to know
   each other and are not forced to be simultaneously present to
   exchange data.  Time decoupling significantly simplifies the
   management of intermittent data-links, in particular for wireless
   connectivity between UAS.  This model of communication, in turn,
   questions the locator-based addressing used in IP using instead a
   data-centric naming.

   When it comes to link reliability, this translates into an end-point
   selection problem, as multiple underlying links are available, but
   the determination of the "best" link depends on specific radio
   characteristics [FINKHAUSER21] or even the vehicle's spatial

   Scenarios from research projects such as [COMP4DRONES] and [ADACORSA]
   regarding connectivity assume worse conditions.  Consider an
   emergency scenario in which 3GPP towers are inoperable.  Emergency
   services need to deploy a mobile ground control station that issues

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   emergency landing overrides to all UAV in the area.  UAV must be able
   to authenticate this mobile GCS to prevent malicious interference
   with their opreations, but must be able to do so without access to
   internet-connected authentication databases.  HIP provides a means to
   secure communications to this mobile GCS, with no means for
   establishing its authority.  While such considerations are not
   directly part of the mechanism by which identifiers are mapped to
   locators, they illustrate the need for carrying authenticating and
   authorizing information within identifiers.

   Mobility management solutions increase the complexity of the
   deployment impacting the performance of data distribution, both in
   terms of signaling/data overhead and communication path delay.
   Considering the specific case of multicast data streams, mobility of
   content producers and consumers is inherently handled by multicast
   routing protocols, which are able to react to changes of location of
   mobile nodes by reconstructing the corresponding multicast delivery
   trees.  Nevertheless, this comes with a cost in terms of signaling
   and data overhead (data may still flow through branches of a
   multicast delivery tree where there are no receivers while the
   routing protocol is still converging).

   Another alternative is to perform the mobility management of
   producers and consumers not at the application layer based on IP
   multicast trees, but on the network layer based on an Information
   Centric Network approach, which was already mentioned in this

B.4.  Communication Across Services

   As a communication infrastructure spanning many facets of life, the
   Internet integrates services and resources from various aspects such
   as remote collaboration, shopping, content production as well as
   delivery, education, and many more.  Accessing those services and
   resources directly through URIs has been proposed by methods such as
   those defined in ICN [RFC7476].  Users access required services and
   resources by virtue of using the URI-based identification, with an
   ephemeral relationship built between user and provider, while the
   building of such relationship may be constrained with user- as well
   as service-specific requirements, such as proximity (finding nearest
   provider), load (finding fastest provider), and others.

   While systems like ICN [JACOBSON09] provide an alternative to the
   topological addressing of IP, its deployment requires an overlay
   (over IP) or native deployment (alongside IP), the latter with
   dedicated gateways needed for translation.  Underlay deployments are
   also envisioned in [RFC8763], where ICN solutions are being used to
   facilitate communication between IP addressed network endpoints or

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   URI-based service endpoints, still requiring gateway solutions for
   interconnection with ICN-based networks as well as IP routing based
   networks (cf.,

   Another aspect of communication across services is that of chaining
   individual services to a larger service.  Here, an identifier would
   be used that serves as a link to next hop destination within the
   chain of single services, as done in the work on Service Function
   Chaining (SFC).  With this, services are identified at the level of
   Layer 2/3 ([RFC7665], [RFC8754], [RFC8595]) or at the level of name-
   based service identifiers like URLs [RFC8677] although the service
   chain identification is carried as a Network Service header (NSH)
   [RFC7665], separate to the packet identifiers.  The forwarding with
   the chain of services utilizes individual locator-based IP addressing
   (for L3 chaining) to communicate the chained operations from one
   Service Function Forwarder [RFC7665] to another, leading to concerns
   regarding overhead incurred through the stacking of those chained
   identifiers in terms of packet overhead and therefore efficiency in
   handling in the intermediary nodes.

B.5.  Communication Traffic Steering

   Steering traffic within a communication scenario involves at least
   two aspects, namely (i) limiting certain traffic towards a certain
   set of communication nodes and (ii) restraining the sending of
   packets towards a given destination (or a chain of destinations) with
   metrics that would allow the selection among one or more possible

   One possibility for limiting traffic inside limited domains, towards
   specific objects, e.g., devices, users, or group of them, is subnet
   partition with techniques such as VLAN [RFC5517], VxLAN [RFC7348], or
   more evolved solution like TeraStream [TERASTREAM] realizing such
   partitioning.  Such mechanisms usually involve significant
   configuration, and even small changes in network and user nodes could
   result in a repartition and possibly additional configuration
   efforts.  Another key aspect is the complete lack of correlation of
   the topological address and any more semantic-rich identification
   that could be used to make policy decisions regarding traffic
   steering.  Suitably enriching the semantics of the packet address,
   either that of the sender or receiver, so that such decision could be
   made while minimizing the involvement of higher layer mechanisms, is
   a crucial challenge for improving on network operations and speed of
   such limited domain traffic.

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   When making decisions to select one out of a set of possible
   destinations for a packet, IP anycast semantics is applied albeit
   being limited to the locator semantic of the IP address itself.
   Recent work in [WION19] suggests utilizing the notion of IP anycast
   address to encode a "service identifier", which is dynamically mapped
   onto network locations where service instances fulfilling the service
   request are located.  Scenarios where this capability is utilized are
   provided in [WION19] and include, but are not limited to, scenarios
   such as edge-assisted VR/AR, transportation, smart cities, smart
   homes, smart wearables, and digital twins.

   The challenge here lies in the possible encoding of not only the
   service information itself, but the constraint information that helps
   the selection of the "best" service instance, which is a service-
   specific constraint in relation to the particular scenario.  The
   notion of an address here is a conditional (on those constraints) one
   where this conditional part is an essential aspect of the forwarding
   action to be taken.  It needs therefore consideration in the
   definition of what an address is, what is its semantic, and how the
   address structure ought to look like.

   As outlined in the previous section, chaining services are another
   aspect of steering traffic along a chain of constituent services,
   where the chain is identified through either a stack of individual
   identifiers, such as in Segment Routing [RFC8402], or as an
   identifier that serves as a link to next hop destination within the
   chain, such as in Service Function Chaining (SFC).  The latter can be
   applied to services identified at the level of Layer 2/3 ([RFC7665],
   [RFC8754], [RFC8595]) or at the level of name-based service
   identifiers like URLs [RFC8677].  However, the overhead incurred
   through the stacking of those chained identifiers is a concern in
   terms of packet overhead and therefore efficiency in handling in the
   intermediary nodes.

B.6.  Communication with built-in security

   Today, strong security in the Internet is usually implemented as a
   general network service ([KRAHENBUHL21], [RFC6158]).  Among the
   various reasons for such approach is the limited semantic of current
   IP addresses, which do not allow to natively express security
   features or trust relationships.  In specific contexts strong
   identification and tracking is necessary for safety and security
   purposes, like for instance for UAS [RFC9153] or aeronautical
   telecommunications networks [I-D.haindl-lisp-gb-atn].  This becomes
   very cumbersome when communication goes beyond limited domains and in
   the public Internet, where security and trust associated to those
   identifier is lost or just impossible to verify.

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   Efforts like Cryptographically Generated Addresses (CGA) [RFC3972],
   provide some security features by embedding a truncated public key in
   the last 57-bit of IPv6 address, thereby greatly enhancing
   authentication and security within an IP network via asymmetric
   cryptography (known to be not as efficient as symmetric
   ecryptography) and IPsec [RFC4301].  The development of the Host
   Identity Protocol (HIP) [RFC7401] saw the introduction of
   cryptographic identifiers for the newly introduced Host Identity (HI)
   to allow for enhanced accountability, and therefore trust.  The use
   of those HIs, however, is limited by the size of IPv6 128bit

B.7.  Communication protecting user privacy

   The last decade has witnessed increasing concerns for user privacy
   ([RFC7258], [RFC6973]).  IP Addresses are particularly exposed
   because they can easily be associated to end users, allowing
   fingerprinting and cross-site linking ([BUJLOW17], [MISHRA20],
   [I-D.irtf-pearg-ip-address-privacy-considerations-01]).  Indeed,
   while encryption is widely used to conceal the traffic payload, the
   IP header remain, and particularly IP addresses, must be transmitted
   in clear in order to forward packets.  Like mobility, privacy
   solutions have been developped with the help of higher layers, like
   for instance [I-D.ietf-ohai-ohttp] or [APPLEPRIV].

   Specific to the network layer, one widely used approach to obfuscate
   the mapping between end users and IP addresses is the use of
   temporary addresses [RFC8981].  The idea here is to reduce the time
   window during which eavesdroppers and information collectors can
   correlate network activity based on the simple IP address.  Ephemeral
   IP addresses have been in the working for more than 30 years
   [RFC9414], showing that having a temporal semantic in IP addresses
   can provide improved privacy protection.

   A more radical approch leverages on recursively encrypt packets on a
   per segment basis, so that source and destination is not directly
   accessible[GOLDSCHLAG99].  Such kind of solution offers strong
   privacy properties, but comes at the price of reduced forwarding
   performance due to cryptographic operations involved.

B.8.  Communication in Alternative Forwarding Architectures

   The performance of communication networks has long been a focus for
   optimization, due to the immediate impact on cost of ownership for
   communication service providers.  For instance, technologies like
   MPLS [RFC3031] have been introduced to optimize lower layer
   communication, e.g., by mapping L3 traffic into aggregated labels of
   forwarding traffic for the purposes of, e.g., traffic engineering.

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   Even further, other works have emerged in recent years that have
   replaced the notion of packets with other concepts for the same
   purpose of improved traffic engineering and therefore efficiency
   gains.  One such area is that of Software Defined Networks (SDN)
   [RFC7426], which has highlighted how a majority of Internet traffic
   is better identified by flows, rather than packets.  Based on such
   observation, alternate forwarding architectures have been devised
   that are flow-based or path-based.  With this approach, all data
   belonging to the same traffic stream is delivered over the same path,
   and traffic flows are identified by some connection or path
   identifier rather than by complete routing information, possibly
   enabling fast hardware based switching (e.g.  [DETNETWG], [PANRG]).

   On the one hand, such a communication model may be more suitable for
   real-time traffic like in the context of Deterministic Networks
   [DETNETWG], where indeed a lot of work has focused on how to
   "identify" packets belonging to the same DETNET flow in order to
   jointly manage the forwarding within the desired deterministic

   On the other hand, it may improve the communication efficiency in
   constrained wireless environments (cf., Appendix B.1), by reducing
   the overhead, hence increasing the number of useful bits per second
   per Hertz.

   Another opportunity to improve communication efficiency is being
   pursued in ongoing IETF/IRTF work to deliver IP- or HTTP-level
   packets directly over path-based or flow-based transport network
   solutions, such as in [I-D.ietf-bier-multicast-http-response],
   [TROSSEN10], [I-D.trossen-icnrg-internet-icn-5glan], and
   [I-D.irtf-icnrg-5gc-icn], with the capability to bundle unicast
   forward communication streams flexibly together in return path
   multipoint relations.  Such capability is particularly opportune in
   scenarios such as chunk-based video retrieval or distributed data
   storage.  However, those solutions currently require gateways to
   "translate" the flow communication into the packet-level addressing
   semantic in the peering IP networks.  Furthermore, the use of those
   alternative forwarding mechanisms often require the encapsulation of
   Internet addressing information, leading to wastage of bandwidth as
   well as processing resources.

   Providing an alternative way of forwarding data has also been the
   motivation for the efforts created in the European Telecommunication
   Standards Institute (ETSI), which formed an Industry Specification
   Group (ISG) named Non-IP Networking (NIN) [ETSI-NIN].  This group
   sets out to develop and standardize a set of protocols leveraging an
   alternative forwarding architecture, such as provided by a flow-based
   switching paradigm.  The deployment of such protocols may be seen to

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   form limited domains, still leaving the need to interoperate with the
   (packet-based forwarding) Internet; a situation possibly enabled
   through a greater flexibility of the addressing used across Internet-
   based and alternative limited domains alike.

   As an alternative to IP routing, EIBP (Extended Internet Bypass
   Protocol) [SHENOY21] offers a communications model that works with IP
   in parallel and entirely transparent and independent to any operation
   at network layer.  For this, EIBP proposes the use of physical and/or
   virtual structures in networks and among networks to auto assign
   routable addresses that capture the relative position of routers in a
   network or networks in a connected set of networks, which is used to
   route packets between end domains.  EIBP operates at Layer 2.5 and
   provides encapsulation (at source domain), routing, and de-
   encapsulation (at destination domain).  A resolver to map the domain
   ID to EIBP's edge router addresses is required.  When queried for a
   specific domain, the resolver will return the corresponding edge
   router structured address.

   EIBP decouples routing operations from end domain operations,
   offering to serve any domain, without point solutions to specific
   domains.  EIBP also decouples routing IDs or addresses from end
   device/domain addresses.  This allows for accommodation of new and
   upcoming domains.  A domain can extend EIBP's structured addresses
   into the domain, by joining as a nested domain under one or more edge
   routers, or by extending the edge router's structure addresses to its

Appendix C.  Examples of Internet Addressing Properties Extensions

C.1.  Length Extensions

C.1.1.  Shorter Address Length Examples

   *  Header Compression/Translation: Considering one base station is
      supposed to serve hundreds of user devices, maximizing the
      effectiveness for specific spectrum directly improves user quality
      of experience.  To achieve the optimal utilization of the spectrum
      resource in the wireless area, the RObust Header Compression
      (ROHC) [RFC5795] mechanism, which has been widely adopted in
      cellular networks like WCDMA, LTE, and 5G, utilizes header
      compression to shrink existing IPv6 headers onto shorter ones.

      Similarly, header compression techniques for IPv6 over Low-Power
      Wireless Personal Area Networks (6LoWPAN) have been around for
      several years now, constituting a main example of using the notion
      of a 'shared context' in order to reduce the size of the network
      layer header ([RFC6282], [RFC7400], [BADENHOP15]).  More recently,

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      other compression solutions have been proposed for Low Power Wide
      Area Networks (LPWAN - [RFC8376]).  Among them, the Static Context
      Header Compression (SCHC - [RFC8724]) generalized the compression
      mechanism developed by 6lo.  Instead of a standard compression
      behavior implemented in all 6lo nodes, SCHC introduces the notion
      of rules shared by two nodes.  The SCHC compression technique is
      generic and can be applied to IPv6 and layers above.  Regarding
      the nature of the traffic, IPv6 addresses (source and destination)
      can be elided, partially sent, or replaced by a small index.
      Instead of the versatile IP packet, SCHC defines new packet
      formats dedicated to specific applications.  SCHC rules are
      equivalence functions mapping this format to standard IP packets.

      Also, constraints coming from either devices or carrier links
      would lead to mixed scenarios and compound requirements for
      extraordinary header compression.  For native IPv6 communications
      on DECT ULE and MS/TP Networks [RFC6282], dedicated compression
      mechanisms are specified in [RFC8105] and [RFC8163], while the
      transmission of IPv6 packets over NFC and PLC, specifications are
      being developed in [RFC9428] and [RFC9354].

   *  Separate device from locator identifier: Solutions such as
      proposed in Expedited Internet Bypass Protocol [SHENOY21] and
      [RFC9300] use a separation of device from locator, where only the
      latter is used for routing between the different domains using the
      same technology, therefore enabling the use of shorter addresses
      in the (possibly constrained) local environment.  Device IDs used
      within such domains are carried as part of the payload by EIBP and
      hence it is possible to use addresses of shorter size, suited to
      the domain.  In LISP a flexible address encoding [RFC8060] allows
      shorter addresses to be supported in the LISP control plane

C.1.2.  Longer Address Length Examples

   *  Split address zone by network realm: Network Address Translation
      (NAT), which was first laid out in [RFC2663], using private
      address and a stateful address binding to translate between the
      realms.  As outlined in [RFC2663], basic address translation is
      usually extended to include port number information in the
      translation process, supporting bidirectional or simple outbound
      traffic only.  Because the 16-bits port number is used in the
      address translation, NAT theoretically increase IPv4 address
      length from 32-bit to 48-bit, i.e., 281 trillion address space.
      [CHERITON00] also proposed to revise the Internet architecture so
      to make NAT natively part of it.

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      Similarly, EzIP [I-D.chen-ati-adaptive-ipv4-address-space] expects
      to utilize a reserved address block, i.e., 240/4, and an IPv4
      header option to include it.  Based on this, EzIP is carrying a
      hierarchical address with two parts, where each part is a partial
      32-bit IPv4 address.  The first part is a public address residing
      in the "address field" of the header from globally routable IPv4
      pool [IPv4pool], i.e., ca. 3.84 billion address space.  The second
      part is the reserved address residing in "option field" and
      belongs to the 240/4 prefix, i.e., ca. 2^28=268 million.  Based on
      that, each EzIP deployment is tethered on the existing Internet
      via one single IPv4 address, and EzIP then have 3.84B * 268M
      address, ca. 1,000,000 trillion.  Collectively, the 240/4 can also
      be used as end point identifier and form an overlay network
      providing services parallel to the current Internet, yet
      independent of the latter in other aspects.

      Compared to NAT, EzIP is able to establish a communication session
      from either side of it, hence being completely transparent, and
      facilitating a full end-to-end networking configuration.

C.2.  Identity Extensions

C.2.1.  Anonymous Address Identity Examples

   *  Traffic Proxy: Although not initially designed as a traffic proxy
      approach, a Virtual Private Network (VPN [KHANVILKAR04]) is widely
      utilized for packets origin hiding as a traffic detouring
      methodology.  As it evolved, VPN derivatives like WireGuard
      [DONENFELD17] have become a mainstream instance for user privacy
      and security enhancement.

      With such methodology in mind, onion routing [GOLDSCHLAG99],
      instantiated in the Tor Project [TOR], achieves high anonymity
      through traffic hand over via intermediates, before reaching the
      destination.  Since the architecture of Tor requires at least
      three proxies, none of them is aware of the entire route.  Given
      that the proxies themselves can be deployed all over the
      cyberspace, trust is not the prerequisite if proxies are randomly

      In addition, dedicated protocols are also expected to be
      customized for privacy improvement via traffic proxy, as
      originally discussed during the the Strengthening the Internet
      (STRINT) IAB Workshop [RFC7687].  For example, Oblivious DNS over
      HTTPS (ODoH [RFC9230]) uses a third-party proxy to obscure
      identifications of user source addresses during DNS over HTTPS
      (DoH [RFC8484]) resolution.  Similarly, Oblivious HTTP (oHTTP
      [I-D.ietf-ohai-ohttp]) involves proxy in the HTTP environment.

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   *  Source Address Rollover: As for source address rollover, it has
      been standardized that IP addresses for Internet users should be
      dynamic and temporary every time they are being generated
      [RFC8981].  This benefits from the available address space in the
      case of IPv6, through which address generation or assignment
      should be unpredictable and stochastic for outside observers.

      More radically, [I-D.gont-v6ops-ipv6-addressing-considerations]
      advocates an 'ephemeral address', changing over time, for each
      process.  Through this, correlating user behaviors conducted by
      different identifiers (i.e., source address) becomes much harder,
      if not impossible, if based on the IP packet header alone.

   *  Private Addresses: The use and assignment of private addresses for
      IPv4 is laid out in [RFC1918], while Unique Local Addresses (ULAs)
      in IPv6 [RFC4193] take over the role of private address spaces.

   *  Network Address Translation: NATs exist as part of existing
      customer premise equipment (CPE), such as a cable or an Ethernet
      router, with private wired/wireless connectivity, or it can be
      provided in a carrier environment to further translate ISP-
      internal private addresses to a pool of (assigned) public IP

   *  Separate device from locator identifier: EIBP [SHENOY21] separates
      the routing ID from the device ID, where only the former is used
      for routing.  As such, it is possible to encrypt the device IDs,
      protecting the end device identity.  Similarly, LISP uses separate
      namespaces for routing and identification allowing to 'hide'
      identifiers in encrypted LISP packets that expose only known
      routing information [RFC8061].

C.2.2.  Authenticated Address Identity Examples

   *  Self-certified Addresses: As an example of this methodology,
      [RFC3972] defines IPv6 cryptographically Generated Addresses
      (CGA).  A Cryptographically Generated Address is formed by
      replacing the least-significant 64 bits of an IPv6 address with
      the cryptographic hash of the public key of the address owner.
      Packets are then signed with the private key of the sender.  The
      receiveer authehticates packets by using the public key and
      address of the sender.  The original specifications have been
      already amended (cf., [RFC4581] and [RFC4982]) in order to support
      multiple (stronger) cryptographic algorithms.

   *  Collision-resistant addresses: In order to provide a mechanism for
      IP mobility considerations, [RFC7343] defines Overlay Routable
      Cryptographic Hash Identifiers (ORCHIDv2).  ORCHIDs use a

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      determinstic scheme for generating statistically unique addresses
      by concatenating a designated IPv6 prefix, a hash function
      identifier, and a truncated hash.  The hash input is a unique,
      statically assigned context identifier concatenated with random
      data.  A variation of this scheme is proposed to solve
      requirements of [RFC9153] in identification of unmanned aerial
      vehicles using Drone Remote Identification Protocol Entity Tags
      (DRIP Entity Tag - DET) [RFC9374].  This variation proposes a
      distinct IPv6 prefix and new hash functions, but the major change
      is to further truncate the hash, and use the freed bits for a two-
      level registration authority hierarchy.

   *  Third party granted addresses: [RFC3118] defines a DHCP option
      through which authorization tickets are generated and newly
      attached hosts with proper authorization can be automatically
      configured from an authenticated DHCP server.  Solutions exist
      where separate servers are used for user authentication like
      [KOMORI02] and [RFC4014].  The former proposing to enhance the
      DHCP system using registered user login and password before
      actually providing an IP address lease and recording the MAC
      address of the device the user used to sign-in.  The latter,
      couples the RADIUS authentication protocol [RFC2865] with DHCP,
      basically piggybacking RADIUS attributes in a DHCP option, with
      the DHCP server contacting the RADIUS server to authenticate the

C.3.  Semantic Extensions

C.3.1.  Extended Address Semantics Examples

   *  Semantic prefixes: Newer approaches to IP anycast suggest the use
      of service identification in combination with a binding IP address
      model [WION19] as a way to allow for metric-based traffic steering
      decisions; approaches for Service Function Chaining (SFC)
      [RFC7665] utilize the Network Service Header (NSH) information and
      packet classification to determine the destination of the next

      Another example of the usage of different packet header extensions
      based on IP addressing is Segment Routing.  In this case, the
      source chooses a path and encodes it in the packet header as an
      ordered list of segments.  Segments are encoded using new Routing
      Extensions Header type, the Segment Routing Header (SRH), which
      contains the Segment List, similar to what is already specified in
      [RFC8200], i.e., a list of segment ID (SID) that dictate the path
      to follow in the network.  Such segment IDs are coded as 128 bit
      IPv6 addresses [RFC8986].

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      Approaches such as [CAROFIGLIO19] utilize semantic prefixing to
      allow for ICN forwarding behavior within an IPv6 network.  In this
      case, an HICN name is the hierarchical concatenation of a name
      prefix and a name suffix, in which the name prefix is encoded as
      an IPv6 128 bits word and carried in IPv6 header fields, while the
      name suffix is encoded in transport headers fields such as TCP.
      However, it is a challenge to determine which IPv6 prefixes should
      be used as name prefixes.  In order to know which IPv6 packets
      should be interpreted based on an ICN semantic, it is desirable to
      be able to recognize that an IPv6 prefix is a name prefix, e.g. to
      define a specific address family (AF_HICN, b0001::/16).  This
      establishment of a specific address family allows the management
      and control plane to locally configure HICN prefixes and announce
      them to neighbors for interconnection.

   *  Separate device from locator identifier: Separating the identity
      from routing is an idea that goes back to PIP [RFC1621], which
      defined "PIP addresses" for routing and "PIP ID" for end device
      identification.  More recently, EIBP [SHENOY21] separates the
      routing locator from the device identifier, relaxing therefore any
      semantic constraints on the device identifier.  Similarly, LISP
      uses a flexible encoding named LISP Canonical Address Format (LCAF
      [RFC8061]), which allows to associate to routing locators any
      possible form (and length) of identifier.  ILNP [RFC6740]
      introduces as well a different semantic of IP addresses, while
      aligning to the IPv6 address format (128 bits).  Basically, ILNP
      introduces a sharper logical separation between the 64 most
      significant bits and the 64 least significant bits of an IPv6
      address.  The former being a global locator, while the latter
      being an identifier that can have different semantics (rather than
      just being an interface identifier).

   *  Structured addressing: Network topology captures the physical
      connectivity among devices in the network.  There is a structure
      associated with the topology.  Examples are the core-distribution-
      access router structure commonly used in enterprise networks and
      clos topologies that are used to provide multiple connections
      between Top of Rack (ToR) devices and multiple layers of spine
      devices.  Internet service providers use a tier structure that
      defines their business relationships.  A clear structure of
      connected networks can be noticed in the Internet.  EIBP
      [SHENOY21] proposes to leverage the physical structure (or a
      virtual structure overlaid on the physical structure) to auto
      assign addresses to routers in a network or networks in an
      internetwork to capture their relative position in the physical/
      virtual topology.  EIBP proposes to administratively identify
      routers/networks with a tier value based on the structure.

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   *  Localized forwarding semantics: Approaches such as those outlined
      in [REED16] suggest using a novel forwarding semantic based on
      path information carried in the packet itself, said path
      information consists in a fixed size bit-field (see [REED16] for
      more information on how to represent the path information in said
      bit-field).  In order to utilize existing, e.g., SDN-based,
      forwarding switches, the direct use of the IPv6 source/destination
      address is suggested for building appropriate match-action rules
      (over the suitable binary information representing the local
      output ports), while preserving the original IPv6 information in
      the encapsulated packet.  As mentioned above, such use of the
      existing IPv6 address fields limits the size of the network to a
      maximum of 256 bits (therefore paths in the network over which
      such packets can be forwarded).
      [I-D.trossen-icnrg-internet-icn-5glan], however, goes a step
      further by suggesting to use the local forwarding as direct
      network layer mechanism, removing the IP packet and only leaving
      the transport/application layer, with the path identifier
      constituting the network-level identifier albeit limited by using
      the existing IP header for backward compatibility reasons (the
      next section outlines the removal of this limitation).

C.3.2.  Existing or Extended Header Semantics Examples

   *  In-Header extensions: In order to allow additional semantic with
      respect to the pure Internet addressing, the original design of
      IPv4 included the field 'Type of Service' [RFC2474], while IPv6
      introduced the 'Flow label' and the 'Traffic Class' [RFC8200]
      fields.  In a certain way, those fields can be considered
      'semantic extensions' of IP addresses, and they are 'in-header'
      because natively present in the IP header (differently from
      options and extension headers).  However, they proved not to be
      sufficient.  Indeed, a variety of network operations are performed
      on the well-known 5-tuple (source and destination addresses;
      source and destination port number; and protocol number).  In some
      contexts all of the above-mentioned fields are used in order to
      have a very fine grained solution [RFC8939].

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   *  Headers option extensions: Header options have been largely under-
      exploited in IPv4.  However, the introduction of the more
      efficient extension header model in IPv6 along with technology
      progress made the use of header extensions more widespread in
      IPv6.  Segment Routing re-introduced the possibility to add path
      semantic to the packet by encoding a loosely defined source
      routing [RFC8402].  Similarly, in the aim to overcome the inherent
      shortcoming of the multi-homing in the IP context, SHIM6 [RFC5533]
      also proposed the use of an extension header able to carry multi-
      homing information which cannot be accommodated natively in the
      IPv6 header.

      To serve a moving endpoint, mechanisms like Mobile IPv6 [RFC6275]
      are used for maintaining connection continuity by a dedicated IPv6
      extension header.  In such case, the IP address of the home agent
      in Mobile IPv6 is basically an identification of the on-going
      communication.  In order to go beyond the interface identification
      model of IP, the Host Identity Protocol (HIP) tries to introduce
      an identification layer to provide (as the name says) host
      identification.  The architecture here relies on the use of
      another type of extension header [RFC7401].

   *  Re-encapsulation extension: Differently from the previous
      approach, re-encapsulation prepends complete new IP headers to the
      original packet introducing a completely custom shim header
      between the outer and inner header.  This is the case for LISP,
      adding a LISP specific header right after an IP+UDP header
      [RFC9300].  A similar design is used by VxLAN [RFC7348] and GENEVE
      [RFC8926], even if they are designed for a data center context.
      IP packets can also be wrapped with headers using more generic and
      semantically rich names, for instance with ICN

   *  Structured addressing: Solutions such as those described in the
      previous section, e.g., EIBP [SHENOY21], provide structured
      addresses that are not limited to the IPv6 address length but
      instead carry the information in an extension header to remove
      such limitation.

      Also, Information-Centric Networking (ICN) naming approaches
      usually introduce structures in the (information) names without
      limiting themselves to the IP address length; more so, ICN
      proposes its own header format and therefore radically breaks with
      not only IP addressing semantic but the format of the packet
      header overall.  For this, approaches such as those described in
      [RFC8609] define a TLV-based binary application component
      structure that is carried as a 'name' part of the CCN messages.
      Such a name is a hierarchical structure for identifying and

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      locating a data object, which contains a sequence of name
      components.  For textual representation, URIs are normally used to
      represent names, as defined in [RFC3986].

      In geographic addressing, position based routing protocols use the
      geographic location of nodes as their addresses, and packets are
      forwarded when possible in a greedy manner towards the
      destination.  For this purpose, the packet header includes a field
      coding the geographic coordinates (x, y, z) of the destination
      node, as defined in [RFC2009].  Some proposals also rely on extra
      fields in the packet header to code the distance towards the
      destination, in which case only the geographic coordinates of
      neighbors are exchanged.  This way the location of the destination
      is protected even if routing packets are eavesdropped.

   *  Localized forwarding semantics: Unlike the original suggestion in
      [REED16] to use existing SDN switches, the proliferation of P4
      [BOSSHART14] opens up the possibility to utilize a locally limited
      address semantic, e.g., expressed through the path identifier, as
      an entirely new header (including its new address) with an
      encapsulation of the IP packet for E2E delivery (including further
      delivery outside the localized forwarding network) or positioning
      the limited address semantic directly as the network address
      semantic for the packet, i.e., removing any IP packet
      encapsulation from the forwarded packet, as done in
      [I-D.trossen-icnrg-internet-icn-5glan].  Removing the IPv6 address
      size limitation by not utilizing the existing IP header for the
      forwarding decision also allows for extensible length approaches
      for building the path identifier with the potential for increasing
      the supported network size.  On the downside, this approach
      requires to encapsulate the original IP packet header for
      communication beyond the local domain in which the new header is
      being used, such as discussed in the previous point above on 're-
      encapsulation extension'.


   Dirk Trossen
   Huawei Technologies Duesseldorf GmbH
   Riesstr. 25C
   80992 Munich

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   Paulo Mendes
   Willy-Messerschmitt Strasse 1
   81663 Munich

   Nirmala Shenoy
   Rochester Institute of Technology
   New-York,  14623
   United States of America

   Laurent Toutain
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex

   Abraham Y. Chen
   Avinta Communications, Inc.
   142 N. Milpitas Blvd.
   Milpitas, CA,  95035-4401
   United States of America

   Dino Farinacci
   United States of America

   Jens Finkhaeuser
   Interpeer gUG
   Feldgereuth 8
   86926 Greifenberg

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   Peng Liu
   China Mobile
   32 Xuanwumen West Ave
   Xicheng, Beijing
   P.R. China

   Yihao Jia

   Donald E. Eastlake 3rd
   Futurewei Technologies
   2386 Panoramic Circle
   Apopka, FL,  32703
   United States of America

Author's Address

   Luigi Iannone (editor)
   Huawei Technologies France S.A.S.U.
   18, Quai du Point du Jour
   92100 Boulogne-Billancourt

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