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SCION Components Analysis

Document Type Active Internet-Draft (individual)
Authors Nicola Rustignoli , Corine de Kater
Last updated 2023-11-04 (Latest revision 2023-09-10)
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Path Aware Networking RG                                   N. Rustignoli
Internet-Draft                                               C. de Kater
Intended status: Informational                         SCION Association
Expires: 13 March 2024                                 10 September 2023

                       SCION Components Analysis


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

   This document analyzes its core components from a functionality
   perspective, describing their dependencies, outputs, and properties
   provided.  The goal is to answer the following questions:

   *  What are the main components of SCION and their dependencies?  Can
      they be used independently?

   *  What existing protocols are reused or extended?  Why (or why not)?

   In addition, it focuses on the properties achievable, motivating
   cases when a greenfield approach is used.  It then briefly touches on
   the maturity level of components and some extensions.

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  Subscribe

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Design Goals  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Minimal Stack - Core Components . . . . . . . . . . . . . . .   4
     2.1.  Authentication - SCION CP-PKI . . . . . . . . . . . . . .   6
       2.1.1.  Key Properties  . . . . . . . . . . . . . . . . . . .   6
       2.1.2.  Dependencies  . . . . . . . . . . . . . . . . . . . .   7
       2.1.3.  Provided to Other Components  . . . . . . . . . . . .   7
       2.1.4.  Relationship to Existing Protocols  . . . . . . . . .   8
     2.2.  Routing - Control Plane . . . . . . . . . . . . . . . . .   9
       2.2.1.  Key Properties  . . . . . . . . . . . . . . . . . . .   9
       2.2.2.  Dependencies  . . . . . . . . . . . . . . . . . . . .  11
       2.2.3.  Provided to Other Components  . . . . . . . . . . . .  11
       2.2.4.  Relationship to Existing Protocols  . . . . . . . . .  12
     2.3.  Forwarding - Data Plane . . . . . . . . . . . . . . . . .  13
       2.3.1.  Key Properties  . . . . . . . . . . . . . . . . . . .  14
       2.3.2.  Dependencies  . . . . . . . . . . . . . . . . . . . .  15
       2.3.3.  Provided to Other Components  . . . . . . . . . . . .  15
       2.3.4.  Relationship to Existing Protocols  . . . . . . . . .  16

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   3.  Additional Components . . . . . . . . . . . . . . . . . . . .  16
     3.1.  Transition Mechanisms . . . . . . . . . . . . . . . . . .  17
     3.2.  Extensions and Other Components . . . . . . . . . . . . .  18
   4.  Component Dependencies Summary  . . . . . . . . . . . . . . .  18
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  Informative References  . . . . . . . . . . . . . . . . . . .  19
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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 consists of a system of related components, some of
   which are essential to set up end-to-end SCION connectivity.  Core
   components are the data plane, the control plane, and the PKI.
   Extensions provide additional functionality, security, or backward
   compatibility.  Discussions at PANRG [PANRG-INTERIM-Min] showed the
   need to describe the relationships between components.  This
   document, therefore, takes a look at each core component individually
   and independently from others.  It focuses on describing its
   dependencies, outputs, functionality, and properties.  It then
   touches on 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 and work.

   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.  Each
   component is described in-depth in dedicated drafts: see
   [I-D.dekater-scion-pki] for the SCION PKI,
   [I-D.dekater-scion-controlplane] for the control plane.  The data
   plane will be available within weeks from the last update of this
   draft.  For any other components, please 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.

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   *  _Availability_. SCION aims to provide highly available
      communication.  Its focus is not only on quickly 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.

   *  _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.  SCION
      strongly focuses on preventing routing attacks, traffic
      hijackings, and on providing stronger guarantees than the existing
      Internet.  Routing information can be unambiguously attributed to
      an AS, and packets are only forwarded along authorized sections of
      the network.  Payload encryption, on the other hand, is not within
      the scope of SCION, as existing protocols can be reused.  Security
      is tightly related to trust.  SCION, therefore, offers a new trust
      model, transparency, and control to endpoints over 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 upon.

   *  _Scalability_. Security and high availability should not result in
      compromises on scalability.  At the same time, a next-generation
      Internet architecture should scale with global network growth and
      avoid limitations related to 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 (as described later in the document).

   Many research efforts have analyzed whether such properties could be
   achieved by extending the existing Internet architecture.  As
   described for each core component in the following paragraphs,
   tradeoffs between properties would be unavoidable when exclusively
   relying on or extending existing protocols.

2.  Minimal Stack - Core Components

   To establish end-to-end connectivity, SCION relies on three main

   *  Data plane: it carries out secure packet forwarding, providing
      path-aware inter-domain connectivity.

   *  Control plane: it performs inter-domain routing by discovering and
      securely disseminating path information.

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   *  PKI: it handles cryptographic material and provides a unique trust

   A SCION network is formed of multiple interconnected administrative
   domains, called SCION autonomous systems (AS).  Each AS deploys all
   of the three components above.  Implementations of all of the above
   components are deployed in production (e.g., they are in use within
   the SSFN, the Swiss Finance Network).  There are commercial
   implementations (including a high-performance data plane).

   A SCION packet is sent through a SCION network by SCION endpoints
   (i.e., a network host).  It is then forwarded between ASes by the
   SCION data plane, which authenticates packets at each hop.  The
   control plane is responsible for discovering and disseminating
   routing information.  Path discovery is performed by each AS thanks
   to an authenticated path-exploration mechanism called beaconing.
   SCION endpoints query their respective AS control plane and obtain
   authenticated and authorized network paths, in the form of path
   segments.  Endpoints select one or more of the end-to-end network
   paths, based on the application requirements (i.e., latency).
   Endpoints then craft SCION packets containing the end-to-end path to
   the destination.

   The control plane relies on the control-plane PKI (CP-PKI) for
   authentication (e.g., of path segments).  SCION's authentication
   mechanisms aim at protecting the whole end-to-end path at each hop.
   Such mechanisms are based on a trust model that is provided by the
   concept of Isolation Domains (ISDs).  An ISD is a group of Autonomous
   Systems that independently defines its own roots of trust.  ISD
   members share therefore a uniform trust 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 trust model, therefore, differs from the one provided by other
   PKI architectures.  The motivation behind this design choice is
   clarified in Section 2.1.

   The following paragraphs look at each component individually.  Rather
   than describing how each component works, they focus on each
   component's dependencies and properties provided to other components.
   The idea is to try to think of each component as a black box, and
   look at its "inputs" and "outputs".

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2.1.  Authentication - SCION CP-PKI

   SCION's control plane messages and path information are all
   authenticated.  This helps SCION avoid 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 hosts and the intermediate network.  The
   verification of 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).  A detailed
   specification of the PKI is available in [I-D.dekater-scion-pki].

2.1.1.  Key Properties

   One might ask why SCION requires its own PKI, rather than reusing
   some of the existing PKI architectures to issue AS certificates.
   Several properties distinguish the CP-PKI from others, and motivate
   SCION's distinct approach.

   *  _Locally scoped and flexible trust._ SCION is designed to securely
      connect ASes that do not necessarily share mutual trust.  This
      requires a trust model that is different from the ones that are
      behind commonly deployed PKIs.  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.  In the SCION CP-PKI, trust is locally scoped within
      each ISD, and the capabilities of each ISD (authentication-wise)
      are limited to the communication channels in which they are
      involved.  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.

   *  _Resilience to compromised entities and keys._ Compromised or
      malicious trust roots outside an ISD cannot affect operations that
      stay within that ISD.  Moreover, as trust roots (in the form of a
      TRC) can only be updated through a voting process, each ISD can be
      configured to withstand the compromise of a number of its root

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   *  _Multilateral governance._ The voting mechanism mentioned above
      makes sure that fundamental changes to the trust policies are only
      allowed with the consent of multiple entities administering an
      ISD.  Within an ISD, no single entity is in full control, or owns
      a cryptographic "kill-switch".

   *  _Support for versioning & updates._ Trust within an ISD is
      normally bootstrapped with an initial ceremony.  Subsequent
      updates to the root of trust (TRC) are handled automatically.  The
      PKI design makes sure that certificate rollover can be automated
      so that certificates can be rotated frequently (e.g., every few
      days for AS certificates).

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

2.1.2.  Dependencies

   Setting up the PKI in a freshly created Isolation Domain requires an
   initial trust bootstrapping process among some of the ISD members
   (i.e. a key exchange ceremony, and manual distribution of the initial
   ISD trust anchor).  As updates to the later roots of trust are
   automated, this process is in principle only required once.  In
   addition, certificate verification requires that PKI components can
   mutually communicate and have coarsely synchronized time.

   The CP PKI enables the verification of signatures, e.g., on path-
   segment construction beacons (PCBs).  It is built on top of a
   peculiar trust model, where entities are able to select their roots
   of trust.  It constitutes the most independent and self-contained
   core component, as it does not have significant dependencies on other
   SCION components.

2.1.3.  Provided to Other Components

   The PKI makes trust information available to the control plane
   through two elements:

   *  _Trust Root Configuration (TRC)_: The PKI provides well-defined
      per-ISD trust policies, in the form of a per-ISD Trust Root
      Configuration (TRC).  The TRC contains the ISD trust roots, and it
      is co-signed by multiple entities in a multilateral process called

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   *  _AS certificates_: For each Autonomous System that is part of an
      ISD, the PKI provides an AS certificate that is used by other
      components for authentication.  It also provides a validation path
      up to the ISD trust root, through intermediate CA certificates.

   SCION CP-PKI comprises an optional extension called DRKey, which
   enables efficient symmetric key derivation between any two entities
   in possession of AS certificates.  Such symmetric keys are used for
   additional authentication mechanisms for high-rate data-plane traffic
   and some control messages.  As authentication based on digital
   signatures only scales well for relatively low message rates, using
   symmetric keys makes sure that the performance requirements for the
   high message rate of the data plane can be met.  For more
   information, refer to the extension draft [I-D.garciapardo-drkey].

   The trust model and certificates provided could be used not only by
   the SCION control plane but also other systems and protocols.

2.1.4.  Relationship to Existing Protocols

   The CP-PKI is based on certificates that use the X.509v3 standard
   [RFC5280].  There are already several professional industry-grade

   The SCION trust model differs from existing PKIs in two ways.  First,
   no entity is globally omnipotent, as Isolation Domains elect their
   own locally scoped root of trust.  Second, changes to the trust roots
   require a voting process, making governance multilateral and each
   trust root resilient to the compromise of some of its keys.

   These properties would be lost if SCION were to rely on an existing
   PKI (i.e., the web PKI, the RPKI, ...).  For example, if SCION were
   to use the RPKI instead of the CP-PKI, its control plane would lack
   the trust model required to support Isolation Domains.  This is
   because RPKI's trust model follows the same structure as the IP
   allocation hierarchy, where the five RIRs represent the trust roots.
   Within SCION, RPKI is instead used to secure some of its transition
   mechanisms, as later explained in Section 3.1.

   In conclusion, SCION is built around a unique trust model, justifying
   the existence of the CP-PKI.

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2.2.  Routing - Control Plane

   The SCION control plane's main purpose is to securely discover and
   disseminate routing information.  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
   endpoints via its path service, part of the control plane.

   Overall, the control plane takes an unexplored topology and AS
   certificates as input, it then discovers the inter-domain topology
   and makes routing information available to endpoints.

   The following section describes the core properties provided by the
   SCION control plane, its relationships with existing protocols, and
   its dependencies on the PKI.  For an in-depth description of the
   control plane, including its sub-components, as the beacon service,
   responsible for path discovery, and the path service, responsible for
   path dissemination, refer to [I-D.dekater-scion-controlplane].

2.2.1.  Key Properties

   *  _Massively multipath._ When exploring paths through beaconing,
      SCION ASes can select PCBs according to their policies, and
      register the corresponding path segments, making them available to
      other ASes and endpoints inside their network.  SCION endpoints
      can leverage a wide range of (possibly disjoint) inter-domain
      paths, based on application requirements or path conditions.  This
      goes beyond the capabilities of existing multipath mechanisms,
      such as BGP ADD-PATH [RFC7911], that is focusing on advertising
      multiple paths for the same prefix to provide a backup path.

   *  _Scalability._ The SCION's beaconing algorithm is scalable and
      efficient due to the following reasons: The routing process is
      divided into 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 IP
      prefix-based announcements.  Scalability of the routing process is
      fundamental not only to support network size growth but also to
      quickly react to failures.  An in-depth study of SCION's
      scalability in comparison to BGP is available in [KRAHENBUHL2022].

   *  _Convergence time._ Since routing decisions are decoupled from the
      dissemination of path information, SCION features faster
      convergence times than path-vector protocols.  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

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      helps in speeding up global distribution of routing information.
      This means that SCION can restore global reachability, even after
      catastrophic failures, within tens of seconds.

   *  _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 nested MACs,
      which are then verified during forwarding, giving ASes strong
      guarantees about the path where the data is routed, with minimal
      overhead and resource requirements on routers.  Giving endpoints
      strong guarantees about the full inter-domain path is important to
      avoid traffic interception, and to enable geofencing (i.e.,
      keeping data in transit within a well-defined trusted area of the
      SCION network).  This facilitated early adoption in the finance

   *  _Host addressing agnostic._ SCION decouples routing from host
      addressing: inter-domain routing is based on ISD-AS tuples rather
      than on host addresses.  This design decision has two outcomes:
      First of all, SCION can reuse existing host addressing schemes,
      such as IPv6, IPv4, or others.  Second, the control plane does not
      carry prefix information.  Since packets contain forwarding state,
      routers do not need to look up routing tables (avoiding the need
      for dedicated hardware).

   *  _Transparency._ SCION endpoints have full visibility of 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 endpoints 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.

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   *  _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 endpoints upon detecting failures or eventually
      receiving an authenticated signal from the data plane.  This
      separation of control and data plane prevents the control plane
      from cutting off an existing communication or having a global

2.2.2.  Dependencies

   The SCION control plane requires the control-plane PKI to
   authenticate path information.  It heavily relies on certificates
   provided by the CP-PKI for beaconing (i.e., for authenticating
   routing information).  Each Isolation Domain requires its own root of
   trust, in the form of a TRC, in order to carry out path exploration
   and dissemination.

   While in principle the control plane could use certificates provided
   by another PKI, it would be severely affected by a lack of the ISD
   concept.  All security properties related to the trust model would be
   affected.  The concept of ISD is also necessary for scalability and
   fault isolation to organize the routing process into a two-tiered

   In conclusion, the control plane depends on the CP-PKI.  If it were
   to be used with another PKI, it would lose several of its fundamental

2.2.3.  Provided to Other Components

   In SCION, an endpoint sending a packet must specify, in the header,
   the full SCION forwarding path the packet takes towards the
   destination.  Rather than having knowledge of the network topology,
   an endpoint's data plane relies on the control plane for getting such
   information.  The endpoint's SCION stack queries path segments, then
   it selects them and combines them into a full forwarding path to the

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   The control plane is responsible, therefore, for providing an
   authenticated (multipath) view of the explored global topology to
   endpoints (and, in turn, to the data plane).  In addition, it
   provides the data plane the ability to send authenticated control
   messages.  The "interfaces" towards the data plane are represented

   *  _Path segments_, that are provided to endpoints and used by SCION
      routers for forwarding.  Segments are designed so that each AS
      data plane can independently verify its own segments, while
      globally achieving full path authorization.

   *  _SCMP._ SCION control-plane messages are by default all
      authenticated.  In addition to beacons, the control plane offers
      the SCION Control Message Protocol (SCMP).  It is analogous to
      ICMP, and it provides functionality for network diagnostics, such
      as ping and traceroute, and authenticated error messages that
      signal packet processing or network layer problems.  SCMP is the
      first control message protocol that supports the authentication of
      network control messages, preventing unauthenticated control
      messages from potentially being used to affect or even prevent
      traffic forwarding.  SCMP is used, for example, by the data plane
      to achieve path revocation.

2.2.4.  Relationship to Existing Protocols

   At first sight, it might seem that the SCION control plane takes care
   of similar duties as existing routing protocols.  While both focus on
   disseminating routing information, there are substantial differences
   in their mechanisms and properties offered.

   The SCION control plane was designed to carry out inter-domain
   routing, while intra-domain routing (and forwarding) are
   intentionally left out of scope.  Existing IGPs are used within an
   AS, allowing the reuse of existing intra-domain routing
   infrastructure and reducing the amount of changes required for

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   End-host addressing is decoupled from routing.  Similar to LISP
   [RFC6830], SCION separates routing, that is based on locator (an ISD-
   AS tuple), and host identifiers (e.g., IPv6, IPv4, ...).  While the
   two architectures have this concept in common, there are notable
   differences.  SCION brings improvements to inter-domain routing and
   provides secure multipath, while LISP provides a framework to build
   overlays on top of the existing Internet.  In addition, LISP security
   proposals focus on protecting identifier to locator mappings, while
   SCION focuses on securing inter-domain routing.  Lastly, identifier
   to locator mapping in SCION not part of the core components, rather
   it is left to some of its transition mechanisms, later described in
   Section 3.1.

   The above-mentioned decoupling also implies that SCION does not
   provide, by design, IP prefix origin validation, which is currently
   provided by RPKI [RFC8210].  As prefix origin validation is outside
   of SCION's scope, IP-to-SCION's coexistence mechanisms (SIAM, SBAS)
   later discussed in Section 3.1 build on top of RPKI for IP origin

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

   One last point is that 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
   achieving transparency, routing scalability, fault isolation, and
   fast propagation of routing information.  No existing protocol
   provides such a concept, motivating the existence of the control

2.3.  Forwarding - Data Plane

   The SCION data plane is responsible for inter-domain packet
   forwarding between ASes.  SCION routers are deployed at an AS network
   edge.  They receive and validate SCION packets from neighbors, then
   they use their intra-domain forwarding information to transmit
   packets to the next SCION border router or to a SCION endpoint inside
   the AS.

   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 host address, and can therefore

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   carry any address (IPv4, IPv6, ...).  In addition, host addresses
   only need to be unique within an AS, and can be, in principle,

2.3.1.  Key Properties

   *  _Path selection._ In SCION, endpoints select inter-domain network
      paths, rather than routers.  The endpoints 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 keep 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.  The 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
      a 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 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 authorization._ SCION routers validate per-hop MACs 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
      hijacking attacks are avoided.

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   In conclusion, in comparison to today's Internet, the SCION's data
   plane takes some of the responsibilities away from routers and places
   them on endpoints (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 specialized routing
   table lookup hardware.  Routers validate network paths so that
   packets are only forwarded on previously authorized paths.

2.3.2.  Dependencies

   The data plane is generally decoupled from the control plane.  To be
   able to transmit data, endpoints need to fetch path information from
   their AS control plane.  In addition, some operations (such as path
   revocation) require the data plane to be able to use an authenticated
   control-plane mechanism, such as SCMP.

   Path information is assumed to be fresh and validated by the control
   plane, which in turn relies on the CP-PKI for validation.  The data
   plane, therefore, relies on both the control plane and indirectly on
   the CP-PKI to function.

   Should the data plane be used independently, without end-to-end path
   authorization, SCION would lose 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.

2.3.3.  Provided to Other Components

   The SCION data plane provides path-aware connectivity to
   applications.  The SCION stack on an endpoint, therefore, takes
   application requirements as an input (i.e., latency, bandwidth, a
   list of trusted ASes, ... ), and crafts packets containing an
   appropriate path to a given destination.

   How to expose capabilities of path-aware networking to upper layers
   remains an open question.  PANAPI (Path-Aware Networking API)
   [slides-113-taps-panapi] is being evaluated as a way of making path-
   awareness and multipath available to the transport layer at
   endpoints, using the TAPS abstraction layer.

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2.3.4.  Relationship to Existing Protocols

   SCION is an inter-domain network architecture and as such its data
   plane does not interfere with intra-domain forwarding.  It re-uses
   the existing intra-domain data and control plane to provide
   connectivity among its infrastructure services, border routers, and
   endpoints, minimizing changes to the internal infrastructure.  This
   corresponds to the practice today where ASes use an intra-domain
   protocol of their choice (i.e., OSPF, IS-IS, MPLS, ...).

   Given its path-aware properties, some of SCION's data plane
   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 compete, 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.  Possible integration of its path-aware properties with
   SR remains for now an open question.

   In SCION's current implementation and early deployments, intra-AS
   SCION packets are encapsulated into an IP/UDP datagram for AS-local
   packet delivery, reusing the AS existing IGP and IP-based data plane.
   This design decision eased early deployments of SCION in IP-based
   networks.  In the long term, it is not excluded that SCION's data
   plane could be better integrated with IP.  For example, SCION path
   information could be included in a custom IPv6 routing extension
   header ([RFC8200] section 4.4).  Such approach requires further
   exploration on its impact on intra-domain forwarding and on
   addressing, so further discussion on the topic is left to future
   revisions of this draft.

3.  Additional Components

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

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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 production-grade to research prototype).  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

   *  _SCION-IP-Gateway (SIG)._ A SCION-IP-Gateway (SIG) is a SCION
      endpoint that encapsulates regular IP packets into SCION packets.
      A corresponding SIG at the destination performs the decapsulation.
      This mechanism enables IP 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 can 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 authorizations 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
      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 [BIRGLEE2022].

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3.2.  Extensions and Other Components

   In addition to transition mechanisms, there are other proposed
   extensions, that build upon the three SCION core components described
   earlier in this document.  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.  COLIBRI
   [GIULIARI2021] is SCION's inter-domain bandwidth reservation system.
   EPIC [LEGNER2020] is a proposal that extends the data plane to
   provide full path validation, with different levels of guarantees.
   These additional components are briefly mentioned here in order to
   provide additional context and some of them are experimental.

4.  Component Dependencies Summary

   Figure 1 briefly summarises on a high level the dependencies between
   SCION's core components discussed in the previous paragraphs.

                                     * Initial trust ceremony
                                     * Loose time synchronization
                                     * Communication
                     │     Control plane PKI      │
                                    │ * TRC
                                    ▼ * AS Certificates
                     │       Control plane        │
                                    │ * Path segments
                                    ▼ * SCMP
                     │         Data plane         │
                                    │ * Secure  inter-domain paths
                                    ▼ to destination
                     │  Applications on endpoint  │

                      Figure 1: Dependencies overview

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   Overall, the control plane PKI represents the most independent
   building block, as it does not rely on other SCION components.  The
   control plane relies on the trust model and on certificate material
   provided by the PKI.  It provides the data plane with path segments,
   that are then used at forwarding, and with SCMP, that is used for
   secure error messages.  The data plane makes multipath communication
   available to applications on SCION endpoints.

5.  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, to foster a
   discussion on the standardization of key components.  The authors
   welcome feedback from the IETF community for future iterations.

6.  Informative References

              Birge-Lee, H., Wanner, J., Cimaszewski, G. H., Kwon, J.,
              Wang, L., Wirz, F., Mittal, P., Perrig, A., and Y. Sun,
              "Creating a Secure Underlay for the Internet", 2022,

   [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,

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

              de Kater, C., Rustignoli, N., and M. Frei, "SCION Control
              Plane", 2023, <

              de Kater, C., Rustignoli, N., and A. Perrig, "SCION
              Overview", 2023, <

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              de Kater, C. and N. Rustignoli, "SCION Control-Plane PKI",
              2023, <

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

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

              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,

              Legner, M., Klenze, T., Wyss, M., Sprenger, C., and A.
              Perrig, "EPIC: Every Packet Is Checked in the Data Plane
              of a Path-Aware Internet", 2020,

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

   [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,

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   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,

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

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

   [RFC8210]  Bush, R. and R. Austein, "The Resource Public Key
              Infrastructure (RPKI) to Router Protocol, Version 1",
              RFC 8210, DOI 10.17487/RFC8210, 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,

              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", ACM, Proceedings of the 17th ACM conference on
              Computer and communications security,
              DOI 10.1145/1866307.1866411, October 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,

   [SSFN]     "Secure Swiss Finance Network (SSFN)", September 2023,

              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.
   Many thanks also to François Wirz, Juan A.  Garcia-Pardo and Matthias
   Frei for reviewing this document.

Authors' Addresses

   Nicola Rustignoli
   SCION Association

   Corine de Kater
   SCION Association

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