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SCION Components Analysis

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Authors Nicola Rustignoli , Corine de Kater
Last updated 2022-07-11
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Path Aware Networking RG                                   N. Rustignoli
Internet-Draft                                               C. de Kater
Intended status: Informational                               ETH Zuerich
Expires: 12 January 2023                                    11 July 2022

                       SCION Components Analysis


   SCION is a future Internet architecture that focuses on security and
   availability.  Its fundamental functions are carried out by a number
   of components.

   This document illustrates the dependencies between its core
   components and extensions.  It also discusses the relationship
   between SCION and existing protocols, with focus on illustrating
   which existing protocols are reused or extended.  Additionally, it
   describes the motivations behind cases where a greenfield approach is
   needed, and the properties that can be achieved thanks to it.  It
   then briefly touches on the maturity level of components.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   rustignoli-panrg-scion-components.html.  Status information for this
   document may be found at

   Discussion of this document takes place on the Path Aware Networking
   RG Research Group mailing list (, which is
   archived at

   Source for this draft and an issue tracker can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Design Goals  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Minimal Stack - Core Components . . . . . . . . . . . . . . .   4
     2.1.  Routing - Control Plane . . . . . . . . . . . . . . . . .   5
       2.1.1.  Key Properties in Relationship to Existing
               Protocols . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Forwarding - Data Plane . . . . . . . . . . . . . . . . .   8
       2.2.1.  Key Properties in Relationship to Existing
               Protocols . . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Authentication - SCION PKI  . . . . . . . . . . . . . . .  10
       2.3.1.  Key Properties  . . . . . . . . . . . . . . . . . . .  10
   3.  Additional Components . . . . . . . . . . . . . . . . . . . .  12
     3.1.  Transition Mechanisms . . . . . . . . . . . . . . . . . .  12
     3.2.  Extensions and Other Components . . . . . . . . . . . . .  13
   4.  Related Work  . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  SCION and RPKI  . . . . . . . . . . . . . . . . . . . . .  14
     4.2.  SCION and Segment Routing . . . . . . . . . . . . . . . .  14
     4.3.  SCION and Other Routing Approaches  . . . . . . . . . . .  14
   5.  Dependency Analysis . . . . . . . . . . . . . . . . . . . . .  15
   6.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  16
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   While SCION was initially developed in academia, the architecture has
   now "slipped out of the lab" and counts its early productive
   deployments (including the Swiss inter-banking network SSFN).  The
   architecture is composed of a system of related components, some of
   which are essential to set up end-to-end SCION connectivity.  Add-ons
   provide additional functionality, security, or backwards
   compatibility.  Discussions at PANRG [PANRG-INTERIM-Min] showed the
   need to describe the relationships between SCION's core components.
   This document, therefore focuses on each component, describing its
   functionality, properties, dependencies and relationships to existing
   protocols.  The goal is not to describe each component's
   specification, but to illustrate the engineering decisions that made
   SCION what it is and to provide a basis for further discussions.

   Before reading this document, please refer to
   [I-D.dekater-scion-overview] for a generic overview of SCION and its
   components, the problems it solves, and existing deployments.  For an
   in-depth description of SCION, refer to [CHUAT22].

1.1.  Design Goals

   SCION was created from the start with the intention to provide the
   following properties for inter-domain communication.

   *  _Availability_. SCION aims to provide highly available
      communication.  Its focus is not only on handling failures (both
      on the last hop or anywhere along the path), but also on allowing
      communication in the presence of adversaries.  Availability is
      fundamental as applications move to cloud data centers, and
      enterprises increasingly rely on the Internet for mission-critical
      communication.  For example, as highlighted in
      [I-D.rtgwg-net2cloud-problem-statement], achieving reliable inter-
      domain Internet connectivity remains an open challenge for cloud

   *  _Security_. SCION comes with an arsenal of mechanisms, designed by
      security researchers with the goal of making most network-based
      and routing attacks either impossible or easy to mitigate.  The
      relevance of Internet's routing security issues is testified by
      the fact that these issues now have the attention of policymakers,
      while previously they were only well known in industry and
      academia.  One example is the 2022 FCC inquiry on routing security
      [FCC2022].  SCION strongly focuses on preventing routing attacks,
      IP prefix hijackings, DoS, providing stronger guarantees than the
      existing Internet.  Security is tightly related to trust.  SCION
      therefore offers end-hosts transparency and control over

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      forwarding paths.  In addition, SCION's design starts from the
      assumption that any two entities on the global Internet do not
      mutually trust each other.  SCION therefore enables trust agility,
      allowing its users to decide the roots of trust they wish to rely

   *  _Scalability_.  Security and high availability should not result
      in compromises on scalability.  At the same time, a next-
      generation Internet architecture should not suffer from
      scalability issues due to network growth or forwarding table size.
      The S in SCION, indeed, stands for scalability.  The architecture
      proposes a design that is scalable both in the control plane and
      in the data plane (making secure forwarding efficient).

   Many research efforts have analysed whether such properties could be
   achieved by extending the existing Internet architecture.  But as
   described in Section 2.2.1, tradeoffs between properties would be
   unavoidable when exclusively relying on or extending existing

   The following paragraphs describe the key properties of SCION's core
   components.  They then describe the components' mutual dependencies
   and their relation with existing protocols.

2.  Minimal Stack - Core Components

   In order to establish end-to-end connectivity, SCION relies on three
   main components.  SCION's data plane carries out secure path-aware
   forwarding.  Its control plane performs routing and provides a
   selection of path segments.  The Control Plane PKI then handles
   cryptographic material.

   The control plane is responsible for discovering and disseminating
   routing information.  Path discovery is performed by each autonomous
   system (AS) thanks to an authenticated path-exploration mechanism
   called beaconing.  SCION end hosts query their respective AS control
   plane and obtain authenticated and authorized network paths, in the
   form of path segments.  End hosts select one or more of the end-to-
   end network paths, based on the application requirements (i.e.,
   latency).  End hosts then craft SCION packets containing the end-to-
   end path to the destination.  The data plane is responsible for
   forwarding SCION packets while authenticating them at each hop.

   Both the control and data plane rely on the Control-Plane PKI (CP-
   PKI) for authentication.  SCION's authentication mechanisms aim at
   protecting the whole end-to-end path at each hop.  SCION Autonomous
   Systems are organised in Isolation Domains (ISDs), that independently
   define their own roots of trust.  ISD members share a uniform trust

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   environment (i.e., a common jurisdiction).  They can transparently
   define trust relationships between parts of the network by deciding
   whether to trust other ISDs.  SCION therefore relies on a unique
   trust model, which differs from other PKIs.  The motivation behind
   this design choice is clarified in Section 2.3.

   All above mentioned core components are deployed in production (e.g.,
   they are in use within the SSFN, the Swiss Finance Network).  There
   are commercial implementations of all core components (including a
   high performance data-plane).

2.1.  Routing - Control Plane

   The SCION control plane's main purpose is to discover and disseminate
   routing information, in the form of path segments.  Path exploration
   is based on path-segment construction beacons (PCBs), which are
   initiated by a subset of ASes and accumulate cryptographically
   protected path forwarding information.  Each AS selects a few PCBs
   and makes them available to end hosts via its path service.  End
   hosts query the control plane for path segments, and combine them
   into forwarding paths to transmit packets in the data plane.  For an
   overview of the process to create and disseminate path information,
   refer to [I-D.dekater-scion-overview], section 1.2.2.

2.1.1.  Key Properties in Relationship to Existing Protocols

   On first sight, it might seem that the SCION control plane takes care
   of similar duties as BGP.  While both focus on disseminating routing
   information, there are substantial differences in their mechanisms
   and properties offered.  This section describes the core properties
   provided by the SCION control plane, and its relationships with
   existing protocols.

   *  _Host addressing._ SCION decouples routing from end-host
      addressing: inter-domain routing is based on ISD-AS tuples rather
      than on end-host addresses, making SCION agnostic to end-host
      addressing.  This design decision has two outcomes: First of all,
      SCION can reuse existing host addressing schemes, as IPv6, IPv4,
      or others.  Secondly, its control plane does not carry prefix
      information, avoiding known issues of using routing tables (i.e.,
      scalability, the need for dedicated hardware).

   *  _Multipath._ SCION ASes can select PCBs according to their
      policies, and register the corresponding path segments, making
      them available to other ASes and end hosts.  SCION hosts can
      leverage a wide range of inter-domain paths, selecting them at
      each hop based on application requirements or path conditions.
      One existing mechanism is BGP ADD-PATH [RFC7911], focusing on

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      advertising multiple paths for the same prefix in order to provide
      a backup path.  However, BGP multipath does not allow end hosts to
      select the whole end-to-end paths, therefore traffic cannot be
      routed based on application requirements.  In addition, it faces
      scalability concerns typical for BGP (i.e., increased resource
      requirement on routers), as discussed in the above-mentioned RFC.
      Similarly to BGP multipath, other approaches based on BGP either
      are only able to provide backup paths that can solely be activated
      in case of failure (i.e., "Diverse BGP Paths" [RFC6774]), or they
      face scalability limitations.  Such concerns motivate an
      alternative approach, such as SCION.

   *  _Hop-by-hop path authorization._ SCION packets can only be
      forwarded along authorized path segments.  This is achieved thanks
      to message authentication codes (MACs) within each hop field.
      During beaconing, each AS's control plane creates MACs, which are
      then verified during forwarding.  This gives end hosts strong
      guarantees about the path where the data is routed.  Other
      approaches, such as BGPSec ([RFC8205]), suffer from challenges
      with scalability, introduce circular dependencies [COOPER2013] and
      global kill switches [ROTHENBERGER2017].  Giving end hosts
      guarantees about the full inter-domain path is important in order
      to avoid traffic interception, and to enable geofencing (i.e.,
      keeping data in transit within a well-defined trusted area of the
      global Internet).

   *  _Scalability._ The SCION's beaconing algorithm is around two
      orders of magnitude more efficient than BGP due to the following
      reasons: The routing process is divided in a process within each
      ISD (intra-ISD) and one between ISDs (inter-ISD), SCION beaconing
      does not need to iteratively converge, and SCION makes AS-based
      announcements instead of BGP's IP prefix-based announcements.
      Scalability of the routing process is fundamental not only in
      order to support network size growth, but also in order to quickly
      react to failures.  Refer to [KRAHENBUHL2022] for an in-depth
      study of SCION's scalability in comparison to BGP.

   *  _Convergence time._ Since routing decisions are decoupled from the
      dissemination of path information, SCION features faster
      convergence times than path-vector protocols such as BGP.  Path
      information is propagated across the network by PCBs in times that
      are within the same order of magnitude of network round trip time.
      In addition, the division of the beaconing process into intra- and
      inter-ISD helps in speeding up global distribution of routing
      information.  This means that SCION has the capability to restore
      global reachability, even after catastrophic failures, within tens
      of seconds.  This is in contrast to BGP, which in certain
      situations will never converge to a stable state, or converge only

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      non-deterministically (see [GRIFFIN1999] and [RFC4264]).
      Convergence under BGP may also simply take too much time

   *  _Transparency._ SCION end hosts have full visibility about the
      inter-domain path where their data is forwarded.  This is a
      property that is missing in traditional IP networks, where routing
      decisions are made by each hop, therefore end hosts have no
      visibility nor guarantees on where their traffic is going.
      Additionally, SCION users have visibility on the roots of trust
      that are used to forward traffic.  SCION therefore makes it harder
      to redirect traffic through an adversary's vantage point.
      Moreover, SCION gives end users the ability to select which parts
      of the Internet to trust.  This is particularly relevant for
      workloads that currently use segregated networks.

   *  _Fault isolation._ As the SCION routing process is hierarchically
      divided into intra-ISD and inter-ISD, faults have a generally
      limited and localized impact.  Misconfigurations, such as an
      erroneous path policy, may suppress some paths.  However, as long
      as an alternative path exists, communication is possible.  In
      addition, while the control plane is responsible for creating new
      paths, it does not invalidate existing paths.  The latter function
      is handled by end hosts upon detecting failures or eventually
      receiving a SCMP message from the data plane.  This separation of
      control and data plane prevents the control plane from cutting off
      an existing communication.

   *  _Authenticated control messages._ BGP has no built-in security
      mechanisms and does not provide any tools for ASes to authenticate
      the information they receive through BGP update messages.  This
      opens up a multitude of attack opportunities.  SCION control-plane
      messages, instead, are all authenticated, avoiding pitfalls that
      could possibly prevent deployment, as discussed in [RFC9049].  In
      addition, currently the Internet Control Message Protocol (ICMP)
      lacks authentication support, see [RFC4443] and [RFC0791].
      Unauthenticated ICMP messages can potentially be used to affect or
      even prevent traffic forwarding.  SCION therefore provides the
      SCION Control Message Protocol (SCMP), which is analogous to ICMP.
      It provides functionality for network diagnostics, such as ping
      and traceroute, and error messages that signal packet processing
      or network layer problems.  SCMP is the first control message
      protocol that supports the authentication of network control

   Additionally, the SCION control plane design takes into account some
   of the lessons learned discussed in [RFC9049]: It does not try to
   outperform end-to-end mechanisms, as path selection is performed by

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   end hosts.  SCION, therefore, can leverage existing end-to-end
   mechanisms to switch paths, rather than competing with them.  In
   addition, there is no component in the architecture that needs to
   keep connection state, as this task is pushed to end hosts.

   Overall, several of the SCION control plane properties and key
   mechanisms depend on the fact that SCION ASes are grouped into
   Isolation Domains (ISDs).  For example, ISDs are fundamental to
   achieve transparency, routing scalability, fault isolation, and fast
   propagation of routing information.  The SCION control plane
   therefore is built around the concept of ISDs, and relies on the
   SCION Control-Plane PKI (see Section 2.3) for authenticating control

2.2.  Forwarding - Data Plane

   SCION is an inter-domain network architecture and as such does not
   interfere with intra-domain forwarding.  This corresponds to the
   practice today where BGP is used for inter-domain routing, while ASes
   use an intra-domain protocol of their choice (i.e., OSPF, IS-IS,
   MPLS, ...).  SCION therefore re-uses the intra-domain network fabric
   to provide connectivity among its infrastructure services, border
   routers, and end hosts, minimising changes to the internal

   SCION routers are deployed at the network edge.  They receive and
   validate SCION packets from neighbours, then they use their intra-
   domain forwarding information to transmit packets to the next border
   router or SCION end host.

   SCION packets are at the network layer (layer-3), and the SCION
   header sits in between the transport and link layer.  The header
   contains a variable type and length end-host address, and can
   therefore carry any address (IPv4, IPv6, ...).  In addition, end-host
   addresses only need to be unique within an AS, and can be, in
   principle, reused.  In early deployments, intra-AS SCION packets are
   sometimes encapsulated into an IP packet, for backwards

2.2.1.  Key Properties in Relationship to Existing Protocols

   Thanks to its data plane, SCION achieves properties that are
   difficult to achieve when exclusively extending existing protocols.

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   *  _Path selection._ In SCION, end hosts select inter-domain network
      paths, rather than routers.  The end hosts are empowered to make
      end-to-end path choices based on application requirements.  This
      means that routers do not carry the burden of making enhanced
      routing or forwarding decisions.

   *  _Scalability._ SCION routers can efficiently forward packets
      without the need to look up forwarding tables or keeping per-
      connection state.  Routers only need to verify MACs in hop fields.
      This operation is based on modern block ciphers such as AES, can
      be computed faster than performing a memory lookup and is widely
      supported in modern CPUs.  Routers, therefore, do not require
      expensive and energy-intensive dedicated hardware, and can be
      deployed on off-the-shelf hardware.  Lack of forwarding tables
      also implies that the growing size of forwarding tables is of no
      concern to SCION.  Additionally, routers that keep state of
      network information can suffer from denial-of-service (DoS)
      attacks exhausting the router's state [SCHUCHARD2011], which is
      less of a problem to SCION.

   *  _Recovery from failures._ SCION hosts usually receive more than
      one path to a given destination.  Each host can select
      (potentially disjoint) backup paths that are available in case of
      failure.  In contrast to the IP-based Internet, SCION packets are
      not dynamically rerouted by the network in case of failures.
      Routers use BFD [RFC5880] to detect link failures, and in case
      they cannot forward a packet, they send an authenticated SCMP
      message triggering path revocation.  End hosts can use this
      information, or alternatively perform active monitoring, to
      quickly reroute traffic in case of failures.  There is therefore
      no need to wait for inter-domain routing protocol convergence.

   *  _Extensibility._ SCION, similarly to IPv6, supports extensions in
      its header.  Such extensions can be hop-by-hop (and are processed
      at each hop), or end-to-end.

   *  _Path validation._ SCION routers validate network paths in packets
      at each hop, so that they are only forwarded along paths that were
      authorized by all on-path ASes in the control plane.  Thanks to a
      system of nested message authentication codes, traffic hijackings
      attacks are avoided.

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   In conclusion, in comparison to today's Internet, the SCION's data
   plane pushes some of the responsibilities away from routers onto end
   hosts (such as selecting paths or reacting to failures).  This
   contributes to creating a data plane that is more efficient and
   scalable, and that does not require routers with specialised routing
   table lookup hardware.  Routers validate network paths so that
   packets are only forwarded on previously authorized packets.

2.3.  Authentication - SCION PKI

   SCION's control plane messages are all authenticated.  The
   verification of those messages relies on a public-key infrastructure
   (PKI) called the Control-Plane PKI or CP-PKI.  It consists of a set
   of mechanisms, roles, and policies related to the management and
   usage of certificates, which enables the verification of signatures
   of, e.g., path-segment construction beacons (PCBs).

2.3.1.  Key Properties

   One might ask why SCION requires its own PKI, rather than reusing
   some of the existing PKI architectures.  There are several properties
   that distinguish the CP-PKI from others, and motivate SCION's
   distinct approach.

   *  _Unique decentralised trust model._ SCION is designed to enable
      global secure connectivity, where ASes do not necessarily share
      mutual trust.  This requires a trust model that is different from
      existing ones that are behind commonly deployed PKIs in today's
      Internet.  In a monopolistic model, all entities trust one or a
      small number of roots of trust.  In an oligopolistic model, there
      are multiple equally trusted roots (e.g., in the Web PKI).  In
      both models, some or all certification authorities are omnipotent.
      If their key is compromised, then the security of the entire
      system collapses.  Both models do not scale well to a global
      environment, because mutually distrustful entities cannot agree on
      a single root of trust (monopoly) and because in the oligopoly
      model, the security is as strong as its weakest root.  The SCION
      trust model differs from classic PKIs in two ways.  First, no
      entity is omnipotent, as Isolation Domains elect their own root of
      trust, and the capabilities of each ISD (authentication-wise) are
      limited to communication channels in which they are involved.
      Second, the trust roots of each ISD are located in a single file,
      the TRC, which is co-signed by multiple entities in a process
      called voting.

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   *  _Resilience to compromised entities and keys._ Compromised or
      malicious trust roots outside an ISD cannot affect operations that
      stay within that ISD.  Moreover, each ISD can be configured to
      withstand the compromise of any single voting key.

   *  _Trust flexibility._ Each ISD can define its own trust policy.
      ASes must accept the trust policy of the ISD(s) in which they
      participate, but they can decide which ISDs they want to join, and
      they can participate in multiple ISDs.

   *  _Scalability._ The authentication infrastructure scales to the
      size of the Internet and is adapted to the heterogeneity of
      today's Internet constituents.

   *  _A basis for authentication of data-plane messages._
      Authentication based on digital signatures works well for the
      relatively low message rates in the control plane, but it does not
      meet the performance requirements for the high message rate of the
      data plane.  The authentication of data-plane traffic and control
      messages requires a highly efficient and ideally stateless system
      to achieve high bandwidths on commodity hardware, and to avoid
      creating opportunities for DoS attacks.  SCION comprises a
      component called DRKey, which enables high-speed data-plane
      elements, like border routers, to derive symmetric cryptographic
      keys from local secrets only.  This DRKey component is used to
      authenticate SCMP messages.  Today's Internet also lacks a
      fundamental mechanism to share a secret key between two end hosts
      for secure end-to-end communication.  Existing approaches (i.e.,
      SSH) resort to trust-on-first-use (TOFU), where a host's initial
      public key is accepted without verification.  DRKey addresses this
      issue as well.  For more information, refer to the draft

   The CP-PKI is based on certificates that follow the X.509v3 standard
   [RFC5280].  There are already several professional industry-grade
   implementations.  Trust within an ISD is normally bootstrapped with
   an initial ceremony.  Subsequent updates to the root of trust are
   handled automatically.

   SCION is built around a unique trust model, allowing mutually
   distrustful entities to communicate.  This justifies the existence of
   the CP-PKI, which differs from existing PKI architectures.  Thanks to
   the CP-PKI, control and data plane packets are authenticated.  This
   helps avoiding some of the obstacles to deployment mentioned in
   [RFC9049], where several path-aware methods failed to achieve
   deployment because of lack of authentication or lack of mutual trust
   between end hosts and the intermediate network.

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3.  Additional Components

   This document mainly focuses on describing the fundamental components
   needed to run a minimal SCION network.  Beyond that, SCION comprises
   a number of extensions and transition mechanisms that provide
   additional properties, as improved incremental deployability,
   security, additional features.  For the sake of completeness, this
   paragraph briefly mentions some of these transition mechanisms and

3.1.  Transition Mechanisms

   As presented in [I-D.dekater-scion-overview], incremental
   deployability is a focus area of SCION's design.  It comprises
   transition mechanisms that allow partial deployment and coexistence
   with existing protocols.  These mechanisms require different levels
   of changes in existing systems, and have different maturity levels
   (from research to production).  Rather than describing how each
   mechanism works, this document provides a short summary of each
   approach, focusing on its functions and properties, as well as on how
   it reuses, extends or interacts with existing protocols.

   *  _SCION-IP-Gateway (SIG)._ A SCION-IP-Gateway (SIG) encapsulates
      regular IP packets into SCION packets with a corresponding SIG at
      the destination that performs the decapsulation.  This mechanism
      enables IP end hosts to benefit from a SCION deployment by
      transparently obtaining improved security and availability
      properties.  SCION routing policies can be configured on SIGs, in
      order to select appropriate SCION paths based on application
      requirements.  SIGs have the ability to dynamically exchange
      prefix information, currently using their own encapsulation and
      prefix exchange protocol.  This does not exclude reusing existing
      protocols in the future.  SIGs are deployed in production SCION
      networks, and there are commercial implementations.

   *  _SIAM._ To make SIGs a viable transition mechanism in an Internet-
      scale network with tens of thousands of ASes, an automatic
      configuration system is required.  SIAM creates mappings between
      IP prefixes and SCION addresses, relying on the authorisations in
      the Resource Public Key Infrastructure (RPKI).  SIAM is currently
      a research prototype, further described in [SUPRAJA2021].

   *  _SBAS_ is an experimental architecture aiming at extending the
      benefits of SCION (in terms of performance and routing security)
      to potentially any IP host on the Internet.  SBAS consists of a
      federated backbone of entities.  SBAS appears on the outside
      Internet as a regular BGP-speaking AS.  Customers of SBAS can
      leverage the system to route traffic across the SCION network

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      according to their requirements (i.e., latency, geography, ... ).
      SBAS contains globally distributed PoPs that advertise its
      customer's announcements.  SBAS relies on RPKI to validate IP
      prefix authorization.  Traffic is therefore routed as close as
      possible to the source onto the SCION network.  The system is
      further described in chapter 13 of [CHUAT22].

3.2.  Extensions and Other Components

   In addition to the components mentioned above, there are others that
   aim at facilitating deployment or at better integrating SCION with
   existing networks.  As an example, PANAPI (Path-Aware Networking API)
   [slides-113-taps-panapi] aims at making path-awareness and multipath
   to the transport layer at end hosts.  DRKey [I-D.garciapardo-drkey]
   is a SCION extension that provides an Internet-wide key-establishment
   system allowing any two hosts to efficiently derive a symmetric key.
   This extension can be leveraged by other components to provide
   additional security properties.  For example, LightningFilter
   [slides-111-panrg-lightning-filter] leverages DRKey to provide high-
   speed packet filtering between trusted SCION ASes.  The SCION Control
   Message Protocol (SCMP) provides authenticated error messages and
   network diagnostics.  COLIBRI [GIULIARI2021] is SCION's inter-domain
   bandwidth reservation system.  RHINE (Robust and High-performance
   Internet Naming for End-to-end security, formerly RAINS) is a secure-
   by-design naming system that provides a set of desired security,
   reliability, and performance properties beyond what the DNS security
   infrastructure offers today.  It is further described in chapter 19
   of [CHUAT22].

   These additional components are briefly mentioned here in order to
   provide additional context.  As extensions, they build upon the three
   SCION core components described earlier in this document.  They are
   therefore unlikely to be the first components being standardised.

4.  Related Work

   A question that is often asked is whether SCION could simply reuse or
   extend existing protocols.  This section tries to clarify this
   question, giving an overview of the relationships between SCION and
   other approaches.  This section discusses what properties can be
   achieved by extending existing protocols, already deployed in the
   wild, and what properties can only be achieved with an approach like

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4.1.  SCION and RPKI

   One might ask why SCION could not just rely on RPKI.  Summarising the
   points discussed in this document, the CP-PKI distinguishes itself
   because of its trust model, which comprises independent trust roots
   that are a fundamental building block for SCION's Isolation Domains.
   RPKI's trust model follows the same structure as the IP allocation
   hierarchy, where the five RIRs run a CA.  This clashes with the trust
   model required for SCION's Isolation Domains, therefore the SCION
   control plane would not be able to leverage RPKI instead of the CP-
   PKI.  In addition, RPKI is only meant to provide authorisation, but
   not authentication.  SCION indeed does not provide, by design, IP
   authorisation.  Rather, one of IP-to-SCION's coexistence mechanisms
   mentioned earlier (SIAM) relies on RPKI for IP origin attestation.

4.2.  SCION and Segment Routing

   Given its path-aware properties, some of SCION's characteristics
   might seem similar to the ones provided by Segment Routing (SR)
   [RFC8402].  There are, however, fundamental differences that
   distinguish and motivate SCION.  The most salient one is that Segment
   Routing is designed to be deployed across a single trusted domain.
   SR therefore does not focus on security, which remains an open
   question, as outlined in [I-D.spring-srv6-security-consideration].
   SCION, instead, is designed from the start to facilitate inter-domain
   communication between (potentially mutually distrustful) entities.
   It comes, therefore, with built-in security measures to prevent
   attacks (i.e., authenticating all control-plane messages and all
   critical fields in the data-plane header).  Rather than competing,
   SCION and SR complement each other.  SCION relies on existing intra-
   domain routing protocols, therefore SR can be one of the possible
   intra-domain forwarding mechanisms.  A possible integration of its
   path-aware properties remains for now an open question.

4.3.  SCION and Other Routing Approaches

   There is an increasing motivation to extend inter-domain routing
   beyond mere reachability.  See for example
   [I-D.trossen-routing-beyond-reachability], which provides a summary
   of some of the existing methods, and states that wider architectural
   approaches are needed.  One proposed approach is semantic routing
   [I-D.irtf-introduction-to-semantic-routing], which adds support for
   advanced routing and forwarding into packets and into the data-plane.
   SCION takes a different approach: Path selection is carried out by
   end hosts, which have the ability to select network paths based on
   application requirements.  This means that there is no need to
   include semantics in packets.  This comes with the benefit that the
   SCION data plane can provide advanced routing without increased

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   complexity or strain on routers.  BGP ADD-PATH [RFC7911] extends BGP
   to allow additional path announcements for a certain prefix, without
   implicitly revoking the existing path.  However, when additional
   paths are advertised for a large number of prefixes, router memory
   consumption is significantly increased.  Furthermore, the additional
   path diversity is not exposed to end-hosts, therefore the additional
   paths can only be used for redundancy.

5.  Dependency Analysis

   This section briefly discusses dependencies between SCION's core
   components, with the goal of facilitating a discussion on whether it
   is possible to implement each of SCION's core components on its own,
   independently from other core components.

   *  _Control-plane PKI._ The CP PKI enables the verification of
      signatures, e.g., on path-segment construction beacons (PCBs).  As
      discussed in Section 2.3, it is built on top of a peculiar trust
      model, where entities are able to select their roots of trust.
      Overall, it constitutes the most independent and self-contained
      building block, as it could potentially be leveraged by SCION or
      other protocols.  The PKI itself does not have significant
      dependencies on other SCION components, therefore it could
      represent a good starting point for standardisation.  Its unique
      trust properties, interfaces, and processes (as voting), could be
      a good candidate for a first draft.

   *  _Control plane._ The SCION control plane is built around the
      concept of Isolation Domains, being the routing process divided
      into an intra- and inter-ISD one.  It heavily relies on the CP-PKI
      for beaconing (i.e., for authenticating routing information).
      Each Isolation Domain requires its own root of trust in order to
      carry out path exploration and dissemination.  Decoupling the
      control plane from the CP-PKI would severely affect the properties
      and guarantees that can be provided by the control plane.  The
      control plane could, therefore, be specified in parallel with the
      CP-PKI.  The control plane is internally formed by multiple sub-
      components (as the beacon service, responsible for path discovery,
      and the path service, responsible for path dissemination).
      Processes and interfaces between these sub-components could be
      topic for one or multiple drafts.

   *  _Data plane._ In order to be able to transmit data, end hosts need
      to fetch path information from their AS control plane, as
      discussed in Section 2.2.  In addition, the SCION data plane
      requires that hosts validate paths, and that routers authenticate
      path information at each hop.  This authentication mechanism
      relies on the control-plane PKI.  It is what allows SCION to

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      distinguish itself from other proposals, gaining many of the
      security and availability proprieties discussed earlier.  The data
      plane, therefore, relies on both the control plane and the
      control-plane PKI in order to function.  Should the data plane be
      used independently, without end-to-end path validation, SCION
      would loose many of its security properties, which are fundamental
      in an inter-domain scenario where entities are mutually
      distrustful.  As discussed in [RFC9049], lack of authentication
      has often been the cause for path-aware protocols never being
      adopted because of security concerns.  SCION should avoid such
      pitfalls and therefore its data plane should rely on the
      corresponding control plane and control-plane PKI.

6.  Conclusions

   This document describes the three fundamental SCION core components,
   together with their properties and dependencies.  It highlights how
   such components allow SCION to provide unique properties.  It then
   discusses how the main components are interlinked, with the goal of
   fostering a discussion on the standardisation of key components.  As
   this document is an early draft, the authors welcome feedback from
   the IETF community for future iterations.

7.  Informative References

   [CHUAT22]  Chuat, L., Legner, M., Basin, D., Hausheer, D., Hitz, S.,
              Mueller, P., and A. Perrig, "The Complete Guide to SCION",
              ISBN 978-3-031-05287-3, 2022,

              Cooper, D., Heilman, E., Brogle, K., Reyzin, L., and S.
              Goldberg, "On the risk of misbehaving RPKI authorities",
              Proceedings of the Twelfth ACM Workshop on Hot Topics
              in Networks, DOI 10.1145/2535771.2535787, November 2013,

   [FCC2022]  Federal Communications Commission, "Notice of Inquiry on
              Secure Internet Routing", 2022,

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              Giuliari, G., Roos, D., Wyss, M., García-Pardo, J.,
              Legner, M., and A. Perrig, "Colibri: A Cooperative
              Lightweight Inter-domain Bandwidth-Reservation
              Infrastructure", 2022,

              Griffin, T. and G. Wilfong, "An analysis of BGP
              convergence properties", ACM SIGCOMM Computer
              Communication Review Vol. 29, pp. 277-288,
              DOI 10.1145/316194.316231, October 1999,

              de Kater, C., Rustignoli, N., and A. Perrig, "SCION
              Overview", 2022, <

              Pardo, J., Krähenbühl, C., Rothenberger, B., and A.
              Perrig, "Dynamically Recreatable Keys", 2022,

              Farrel, A. and D. King, "An Introduction to Semantic
              Routing", 2022, <

              Dunbar, L., Malis, A., Jacquenet, C., Toy, M., and K.
              Majumdar, "SCION Overview", 2022,

              Li, C., Li, Z., Xie, C., Tian, H., and J. Mao, "Security
              Considerations for SRv6 Networks", 2022,

              Trossen, D., Lou, D., and S. Jiang, "Continuing to Evolve
              Internet Routing Beyond 'Mere' Reachability", 2022,

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              Krähenbühl, C., Tabaeiaghdaei, S., Glοοr, C., Kwon, J.,
              Perrig, A., Hausheer, D., and D. Roos, "Deployment and
              Scalability of an Inter-Domain Multi-Path Routing
              Infrastructure", 2022,

              "Path Aware Networking Research Group - Interim 106
              Minutes", June 2022,

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [RFC4264]  Griffin, T. and G. Huston, "BGP Wedgies", RFC 4264,
              DOI 10.17487/RFC4264, November 2005,

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC6774]  Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
              and K. Kumaki, "Distribution of Diverse BGP Paths",
              RFC 6774, DOI 10.17487/RFC6774, November 2012,

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,

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   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <>.

   [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, <>.

   [RFC9049]  Dawkins, S., Ed., "Path Aware Networking: Obstacles to
              Deployment (A Bestiary of Roads Not Taken)", RFC 9049,
              DOI 10.17487/RFC9049, June 2021,

              Rothenberger, B., Asoni, D., Barrera, D., and A. Perrig,
              "Internet Kill Switches Demystified", Proceedings of the
              10th European Workshop on Systems Security,
              DOI 10.1145/3065913.3065922, April 2017,

              Sahoo, A., Kant, K., and P. Mohapatra, "BGP convergence
              delay after multiple simultaneous router failures:
              Characterization and solutions", Computer
              Communications Vol. 32, pp. 1207-1218,
              DOI 10.1016/j.comcom.2009.03.009, May 2009,

              Schuchard, M., Mohaisen, A., Foo Kune, D., Hopper, N.,
              Kim, Y., and E. Vasserman, "Losing control of the
              internet: using the data plane to attack the control
              plane", Proceedings of the 17th ACM conference on Computer
              and communications security - CCS '10,
              DOI 10.1145/1866307.1866411, 2010,

              Garcia Pardo, J. A., "Lightning Filter: High-Speed Traffic
              Filtering based on DRKey", 2021,

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              Krüger, T., "PANAPI, a Path-Aware Networking API", 2022,

              Supraja, S., Wirz, F., de Ruiter, J., Schutijser, C.,
              Legner, M., and A. Perrig, "Global Distributed Secure
              Mapping of Network Addresses", 2021,


   The authors are indebted to Adrian Perrig, Laurent Chuat, Markus
   Legner, David Basin, David Hausheer, Samuel Hitz, and Peter Mueller,
   for writing the book "The Complete Guide to SCION" [CHUAT22], which
   provides the background information needed to write this document.

Authors' Addresses

   Nicola Rustignoli
   ETH Zuerich

   Corine de Kater
   ETH Zuerich

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