Skip to main content

SCION Overview
draft-dekater-panrg-scion-overview-01

Document Type Active Internet-Draft (individual)
Authors Corine de Kater , Nicola Rustignoli , Adrian Perrig
Last updated 2022-05-19
Stream (None)
Intended RFC status (None)
Formats
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
draft-dekater-panrg-scion-overview-01
PANRG                                                        C. de Kater
Internet-Draft                                             N. Rustignoli
Intended status: Informational                                 A. Perrig
Expires: 20 November 2022                                    ETH Zuerich
                                                             19 May 2022

                             SCION Overview
                 draft-dekater-panrg-scion-overview-01

Abstract

   The Internet has been successful beyond even the most optimistic
   expectations and is intertwined with many aspects of our society.
   But although the world-wide communication system guarantees global
   reachability, the Internet has not primarily been built with security
   and high availability in mind.  The next-generation inter-network
   architecture SCION (Scalability, Control, and Isolation On Next-
   generation networks) aims to address these issues.  SCION was
   explicitly designed from the outset to offer security and
   availability by default.  The architecture provides route control,
   failure isolation, and trust information for end-to-end
   communication.  It also enables multi-path routing between hosts.

   This document discusses the motivations behind the SCION architecture
   and gives a high-level overview of its fundamental components,
   including its authentication model and the setup of the control- and
   data plane.  As SCION is already in production use today, the
   document concludes with an overview of SCION deployments.

About This Document

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

   The latest revision of this draft can be found at
   https://scionassociation.github.io/scion-overview_I-D/draft-dekater-
   panrg-scion-overview.html.  Status information for this document may
   be found at https://datatracker.ietf.org/doc/draft-dekater-panrg-
   scion-overview/.

   Discussion of this document takes place on the WG Working Group
   mailing list (mailto:panrg@irtf.org), which is archived at
   https://datatracker.ietf.org/rg/panrg.

   Source for this draft and an issue tracker can be found at
   https://github.com/scionassociation/scion-overview_I-D.

de Kater, et al.        Expires 20 November 2022                [Page 1]
Internet-Draft                  SCION I-D                       May 2022

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 20 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Why SCION - Motivation  . . . . . . . . . . . . . . . . .   3
       1.1.1.  Scope of SCION  . . . . . . . . . . . . . . . . . . .   5
       1.1.2.  Practical Considerations Based on Related RFCs  . . .   5
       1.1.3.  Why Now?  . . . . . . . . . . . . . . . . . . . . . .   7
     1.2.  SCION Overview  . . . . . . . . . . . . . . . . . . . . .   7
       1.2.1.  Network Architecture and Naming . . . . . . . . . . .   7
       1.2.2.  Routing . . . . . . . . . . . . . . . . . . . . . . .   9
       1.2.3.  Infrastructure Components . . . . . . . . . . . . . .  10
       1.2.4.  Formal Verification . . . . . . . . . . . . . . . . .  11
     1.3.  Conventions and Definitions . . . . . . . . . . . . . . .  11
   2.  Key Concepts  . . . . . . . . . . . . . . . . . . . . . . . .  11
     2.1.  Authentication  . . . . . . . . . . . . . . . . . . . . .  12
       2.1.1.  The Control-Plane PKI (CP-PKI)  . . . . . . . . . . .  12
       2.1.2.  TRC Update and Verification . . . . . . . . . . . . .  13
       2.1.3.  Dissemination of TRC Updates  . . . . . . . . . . . .  14
       2.1.4.  Grace Period  . . . . . . . . . . . . . . . . . . . .  14
       2.1.5.  Revocation and Recovery from a Catastrophic Event . .  14

de Kater, et al.        Expires 20 November 2022                [Page 2]
Internet-Draft                  SCION I-D                       May 2022

     2.2.  SCION Control Plane . . . . . . . . . . . . . . . . . . .  15
       2.2.1.  Path Exploration  . . . . . . . . . . . . . . . . . .  15
       2.2.2.  Path Registration . . . . . . . . . . . . . . . . . .  18
       2.2.3.  Path Lookup . . . . . . . . . . . . . . . . . . . . .  19
       2.2.4.  Link Failures . . . . . . . . . . . . . . . . . . . .  20
     2.3.  SCION Data Plane  . . . . . . . . . . . . . . . . . . . .  21
       2.3.1.  Path Construction via Segment Combination . . . . . .  21
       2.3.2.  Path Authorization  . . . . . . . . . . . . . . . . .  24
       2.3.3.  Forwarding  . . . . . . . . . . . . . . . . . . . . .  24
       2.3.4.  Intra-AS Communication  . . . . . . . . . . . . . . .  24
   3.  Deployment  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     3.1.  Autonomous System Deployment  . . . . . . . . . . . . . .  25
     3.2.  Internet Exchange Points  . . . . . . . . . . . . . . . .  26
     3.3.  End Hosts and Incremental Deployability . . . . . . . . .  26
       3.3.1.  Native End Hosts  . . . . . . . . . . . . . . . . . .  27
       3.3.2.  SCION to IP Gateway (SIG) . . . . . . . . . . . . . .  27
     3.4.  Deployment Experiences  . . . . . . . . . . . . . . . . .  27
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  29
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   The Introduction section explores the motivation to develop SCION,
   followed by a short description of SCION's main elements.  The
   sections after the Introduction provide further insight into SCION's
   key concepts and deployment scenarios.  The document concludes with
   some concrete case studies where SCION has been successfully deployed
   in production.

1.1.  Why SCION - Motivation

   Since its inception, the Internet has continued to expand,
   encompassing new uses over time.  The continuous expansion has
   brought many issues to light, including a lack of control,
   limitations in scalability, performance and security, occurrences of
   severe outages, weak fault isolation, and energy consumption.  With
   the core focus on functionality and operation, the current Internet
   offers little protection against attacks such as spoofing, IP-address
   hijacking, denial-of-service, and combinations of these.  For more
   background information, see [SCHUCHARD2011], [LABOVITZ2000],
   [GRIFFIN1999], [SAHOO2009], and [RFC4264].

de Kater, et al.        Expires 20 November 2022                [Page 3]
Internet-Draft                  SCION I-D                       May 2022

   There have been numerous initiatives to address the above issues.
   Although these initiatives have brought many improvements, concerns
   regarding security and scalability still remain.  For more details,
   see, e.g., [RFC4033], [RFC6480], [RFC8205], and [RFC8446], as well as
   [LYCHEV2013], [LI2014], [COOPER2013], [ROTHENBERGER2017],
   [MORILLO2021], and [KING2022].

   As a consequence, today's Internet fails to fulfil many users'
   requirements.  This especially pertains to the demands of enterprises
   globally exchanging sensitive information, such as financial
   institutions, healthcare providers, universities, multinationals,
   governments, critical and transportation infrastructure operators.
   These users require the Internet to be highly available at all times.
   They expect reliable operation of the global network also in case of
   failures.  They need availability guarantees across multiple routing
   domains, even in the presence of attacks.  They further want to rely
   on an Internet that can be multilaterally governed and is free from
   global kill-switches.

   SCION has been developed in order to meet the above-mentioned
   requirements.  SCION aims to reach the following goals:

   *  Provide high-availability architecture (also in the presence of
      adversaries)

   *  Provide fast failover in the case of inter-domain link or router
      failures

   *  Prevent from IP-address hijacking, DoS, and other attacks

   *  Enable path transparency as well as application-specific path
      optimizations

   *  Improve the inter-domain control plane's scalability

   *  Prepare the Internet for tomorrow's applications, such as virtual
      reality, Internet of Things (IoT), and all other applications
      requiring high-performance connectivity.

   *Note*: A more detailed motivation for developing SCION will be
   described in a separate gap analysis internet draft.

de Kater, et al.        Expires 20 November 2022                [Page 4]
Internet-Draft                  SCION I-D                       May 2022

1.1.1.  Scope of SCION

   The above section describes SCION's aspiration to improve _inter_-AS
   routing and to focus on providing end-to-end connectivity.  However,
   SCION does not solve _intra_-AS routing issues, nor does it provide
   end-to-end payload encryption, and identity authentication.  These
   topics, which are equally important for the Internet to perform well,
   lie outside the scope of SCION.

1.1.2.  Practical Considerations Based on Related RFCs

   The SCION inter-domain routing concept has initially been developed
   by researchers of the ETH Zuerich [PERRIG2017], and could in the
   meantime also gain attention and recognition in the international
   academic world.  However, for an IT architecture to be successful, it
   must work well in practice, too.  This section pays attention to the
   implementation considerations of a conceptual framework such as SCION
   in real life, on the basis of some RFCs exploring this topic.  It
   also verifies in how far SCION meets the requirements mentioned and
   questions raised in these RFCs.

1.1.2.1.  Avoiding Pitfalls

   [RFC9049] describes why previous proposals to tackle some of the
   Internet's fundamental issues did not manage to succeed.  SCION seems
   to avoid the pitfalls mentioned in that document.  For example, SCION
   does not have to be adopted by the entire Internet to be effective:
   The routing architecture provides benefits already to early adapters.
   Even if only a small part of the global network works with SCION,
   adapters will still take advantage of using the SCION routing
   technology.  Moreover, not only users of SCION benefit from it, also
   ISPs and operators benefit from deploying SCION: early deployments
   showed that providers can charge the use of SCION as premium
   connectivity, with users who are willing to pay for it.  Furthermore,
   SCION can be installed on top of and function alongside the existing
   routing infrastructure and protocols--there is no need for high-
   impact changes in an operational network.

de Kater, et al.        Expires 20 November 2022                [Page 5]
Internet-Draft                  SCION I-D                       May 2022

   Another RFC that must be mentioned in this context is [RFC5218],
   "What Makes for a Successful Protocol?".  SCION seems to fulfil most
   factors that contribute to the success of a protocol, as described in
   section 2.1 of the RFC.  This includes such factors as offering a
   positive net value (i.e., the benefits of deploying SCION outweigh
   the costs), incremental deployability, and open source code
   availability.  More importantly, SCION averts the failure criteria
   mentioned in section 1.4 of the RFC: SCION is already deployed and in
   use by many actors of the Swiss financial and academic ecosystems,
   and allows for multiple implementations, both open and closed source.
   As existing SCION implementations are easily portable, adoption in
   mainstream platforms is also possible.

1.1.2.2.  Answering Questions

   SCION can be considered a _path-aware internetworking_ architecture,
   as described in [RFC9217].  This RFC poses (open) questions that must
   be answered in order to realize such a path-aware networking system.
   It was originally written to frame discussions in the Path Aware
   Networking Research Group (PANRG), and has been published to snapshot
   current thinking in this space.

   SCION intends to answer the questions raised in this RFC.  This
   especially pertains to the second, third, seventh, and eighth
   question:

   *  How do endpoints and applications get access to accurate, useful,
      and trustworthy path properties?

   *  How can endpoints select paths to use for traffic in a way that
      can be trusted by the network, the endpoints, and the applications
      using them?

   *  How can a path-aware network in a path-aware internetwork be
      effectively operated, given control inputs from network
      administrators, application designers, and end users?

   *  How can the incentives of network operators and end users be
      aligned to realize the vision of path-aware networking, and how
      can the transition from current ("path-oblivious") to path-aware
      networking be managed?

   SCION's answers to these questions can be found in Key Concepts
   (Section 2) and Deployments (Section 3.4), respectively.

de Kater, et al.        Expires 20 November 2022                [Page 6]
Internet-Draft                  SCION I-D                       May 2022

1.1.3.  Why Now?

   The emergence of multiple SCION implementations and early deployments
   highlight the need for standardization.  The time seems therefore
   ripe to take SCION to the IETF, also in order to contribute to the
   important discussion regarding path-aware networking.

1.2.  SCION Overview

   SCION has been designed to address the fundamental issues of today's
   Internet depicted in Why SCION - Motivation (Section 1.1).  The
   following section gives a high-level overview of SCION's main
   elements, providing a basic understanding of this next-generation
   inter-network architecture.

1.2.1.  Network Architecture and Naming

   SCION's main goal is to offer highly available and efficient inter-
   domain packet delivery--even in the presence of actively malicious
   entities.  To achieve scalability and sovereignty, SCION organizes
   existing ASes into groups of independent routing planes, called
   *Isolation Domains (ISD)*. An AS can be a member of multiple ISDs.
   All ASes in an ISD agree on a set of trust roots, called the *Trust
   Root Configuration (TRC)*. The ISD is governed by a set of *core
   ASes*, which provide connectivity to other ISDs and manage the trust
   roots.  Typically, a few distinguished ASes within an ISD form the
   ISD's core.

   Isolation domains serve the following purposes:

   *  They allow SCION to support trust heterogeneity, as each ISD can
      independently define its roots of trust;

   *  They provide transparency for trust relationships;

   *  They isolate the routing process within an ISD from external
      influences such as attacks and misconfigurations; and

   *  They improve the scalability of the routing protocol by separating
      it into a process within and one between ISDs.

   ISDs provide natural isolation of routing failures and
   misconfigurations, provide meaningful and enforceable trust, enable
   endpoints to optionally restrict traffic forwarding to trusted parts
   of the Internet infrastructure only, and enable scalable routing
   updates with high path-freshness.

de Kater, et al.        Expires 20 November 2022                [Page 7]
Internet-Draft                  SCION I-D                       May 2022

1.2.1.1.  Links

   There are three types of links in SCION: core links, parent-child
   links, and peering links.

   *  A *core link* can only exist between two core ASes.

   *  A *parent-child link* requires that at least one of the two
      connected ASes is a non-core AS.  ASes with a parent-child link
      usually belong to the same entity or have a provider-customer
      relationship.

   *  A *peering link* also includes at least one non-core AS.

   See Figure 1 for a high-level overview of the SCION network
   structure.

       ...............................
      :                               :
     :      [TRC]                      :
    :          (::::::::::::::)         :      ......................
   :        (::::: ISD core :::::)       :    :                      :
   :    (:: +---+ ::::::::: +---+ ::)    :   :    [TCR]               :
   : (::::: |CAS|===+---+ : |CAS| :::::) :  :        (: ISD core :)    :
   :    (:: +---+ : |CAS|===+---+====)===:==:=====(=+---+ :: +---+ :)  :
   :       /(:::::: +---+ ::::::) \      :  :    (: |CAS| == |CAS| :)  :
   :      /  (::::::: | :::::::)   \     :  :     ( +---+ :: +---+ )   :
   :     /            |             o    :  :       /(::::::::::::)    :
   :    o             |           +---+  :  :      /         \         :
   :  +---+           |          /|ASb|  :  :     /           \        :
   :  |ASa|           |         / +---+  :  :    o             o       :
   :  +---+           |        /    |    :  :  +---+          +---+    :
   :    |             |       /     |    :  :  |ASx| ---------|ASy|    :
   :    |             |      /      o    :  :  +---+          +---+    :
   :    o             o     /     +---+  :  :    |                     :
   :  +---+         +---+  /      |ASe|  :  :    o                     :
   :  |ASc|---------|ASd| o       +---+ -:--:--+---+                   :
   :  +---+         +---+                :  :  |ASz|      ISD 2        :
    :                                   :    : +---+                  :
     :            ISD 1                :      ........................
      .................................
   Legend:
                         |
                         |
   Parent AS - child AS: o
   Peering link: ----
   Core link: ===
   Core AS: CAS

de Kater, et al.        Expires 20 November 2022                [Page 8]
Internet-Draft                  SCION I-D                       May 2022

                     Figure 1: SCION network structure

1.2.2.  Routing

   SCION operates on two routing levels: intra-ISD and inter-ISD.  Both
   levels use *path-segment construction beacons (PCBs)* to explore
   network paths.  A PCB is initiated by a core AS and then disseminated
   either within an ISD (to explore intra-ISD paths) or among core ASes
   (to explore core paths across different ISDs).  The PCBs accumulate
   cryptographically protected path and forwarding information on AS-
   level, and store this information in the form of *hop fields (HFs)*.
   End hosts use information from these hop fields to create end-to-end
   forwarding paths for data packets, who carry this information in
   their packet headers.  This concept is called *packet-carried
   forwarding state (PCFS)*. The concept also supports multi-path
   communication among end hosts.

   The process of creating an end-to-end forwarding path consists of the
   following steps:

   1.  First, an AS discovers paths to other ASes, during the _path
       exploration_ (or beaconing) phase.

   2.  The AS then selects a few PCBs according to defined policies,
       transforms the selected PCBs into path segments, and registers
       these segments with its path infrastructure, thus making them
       available to other ASes.  This happens during the _path
       registration_ phase.

   3.  During the _path resolution_ phase, the actual creation of an
       end-to-end forwarding path to the destination takes place.  For
       this, an end host performs (a) a _path lookup_ step, to obtain
       path segments, and (b) a _path combination_ step, to combine the
       forwarding path from the segments.

   Figure 2 below shows the SCION routing process in a nutshell.

de Kater, et al.        Expires 20 November 2022                [Page 9]
Internet-Draft                  SCION I-D                       May 2022

   +------------------+             +------------------+
   | Path Exploration |             |                  |
   |   (Beaconing)    |------------>|Path Registration |
   |                  |             |                  |
   +------------------+             +--------+---------+
                                             |
                           +-----------------+
                           |
        +------------------v-----------------------+
        |            Path Resolution               |
        |+--------------+     +-------------------+|
        || Path Lookup  |---->| Path Combination  ||
        |+--------------+     +-------------------+|
        +------------------------------------------+

                   Figure 2: SCION routing in a nutshell

1.2.2.1.  ISD and AS numbering

   SCION decouples end-host addressing from inter-domain routing.
   Routing is based on the <ISD, AS> tuple, agnostic of end-host
   addressing.  Existing AS numbers are inherited from the current
   Internet, but a 48-bit namespace allows for additional SCION AS
   numbers beyond the 32-bit space in use today.  The end host local
   address is not used for inter-domain routing or forwarding, does not
   need to be globally unique, and can thus be an IPv4, IPv6, or MAC
   address, for example.  A SCION address is therefore composed of the
   <ISD, AS, local address> 3-tuple.

1.2.3.  Infrastructure Components

   The *beacon service*, the *path service*, and the *certificate
   service* are the main control-plane infrastructure components within
   a SCION AS.  Each service can be deployed redundantly, depending on
   the AS's size and type.  Existing Internal routers are used to
   forward packets inside the AS, while _SCION border routers_ provide
   interconnectivity between ASes.

   *  The _beacon service_ discovers path information.  It is
      responsible for generating, receiving, and propagating PCBs.
      Periodically, the beacon service generates a set of PCBs, which
      are forwarded to its child ASes or neighboring core ASes.  The
      PCBs are flooded over policy-compliant paths to discover multiple
      paths between any pair of core ASes.

   *  The _path service_ stores mappings from AS identifiers to sets of
      announced path segments.  The path service is organized as a
      hierarchical caching system similar to that of DNS.  Through the

de Kater, et al.        Expires 20 November 2022               [Page 10]
Internet-Draft                  SCION I-D                       May 2022

      beacon service, ASes select the set of path segments through which
      they want to be reached, and they register them to the path
      service in the ISD core.

   *  The _certificate service_ keeps cached copies of certificates and
      manages keys and certificates for securing inter-AS communication.
      The certificate service is queried by the beacon service when
      validating the authenticity of PCBs (i.e., when the beacon service
      lacks a certificate).

   _Border routers_ are deployed at the edge of SCION ASes.  The main
   task of border routers is to forward packets to a neighbor border
   router or to the destination host within the AS.  While SCION takes
   care of inter-domain routing, it relies on existing routing protocols
   (e.g., IS-IS, OSPF, SR) and communication fabric (e.g., IP, MPLS) for
   intra-domain forwarding. _Internal routers_, therefore, do not need
   to be changed to support SCION.

1.2.4.  Formal Verification

   An additional feature of SCION is its formal verification.  The SCION
   network system consists of a variety of components such as routers,
   servers, and edge devices.  Such a complex system eludes the mental
   capacities of human beings for considering all possible states and
   interactions.  That is why SCION includes a formal verification
   framework developed by the Department of Computer Science of the ETH
   Zurich [KLENZE2021].  This guarantees that packet forwarding as well
   as SCION's authentication mechanisms and implementations are correct
   and consistent.

1.3.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Key Concepts

   This section explains the SCION key concepts, including
   authentication, control- and data plane.

de Kater, et al.        Expires 20 November 2022               [Page 11]
Internet-Draft                  SCION I-D                       May 2022

2.1.  Authentication

   SCION's control plane relies on a public-key infrastructure called
   the *control-plane PKI (CP-PKI)*, which is organized on ISD-level.
   Each ISD can independently build its own roots of trust, defined in a
   file called *trust root configuration (TRC)*.

   *Note*: This section describes the SCION authentication concept on a
   very high level.  A much more detailed description of SCION's
   authentication will follow in a separate internet draft.

2.1.1.  The Control-Plane PKI (CP-PKI)

   Trust within each isolation domain is anchored in the trust root
   configuration (TRC) file.  Each TRC contains a collection of signed
   root certificates, which are used to sign CA certificates, which are
   in turn used to sign AS certificates.  The TRC also includes ISD-
   policies that specify, for example, the TRC's usage, validity, and
   future updates.  A TRC is a fundamental component of an CP-PKI.

   The initial TRC in an ISD is called the *base TRC*. This base TRC
   constitutes the ISD's trust anchor.  It is signed during a signing
   ceremony and then distributed throughout the ISD.  All entities
   within the ISD obtain the initial TRC with an offline mechanism such
   as a USB flash drive provided by a trusted AS, like the relevant ISP,
   or with an online mechanism that relies on a trust-on-first-use
   (TOFU) approach.  However, all updates to the base TRCs are performed
   in a straightforward process that does not require any manual or out-
   of-band action (such as a software update), see TRC Update and
   Verification (Section 2.1.2).

   Figure 3 shows the TRC trust chain and associated certificates.  TRC
   1 is the base TRC, and TRC 2 and 3 constitute updates to this base
   TRC.  TRC 2 must be verified using the voting certificates in TRC 1.
   Control-plane (CP) root certificates are used to verify other CP
   certificates (which are in turn used to verify path-segment
   construction beacons PCBs).

   Each SCION AS must hold a private key (to sign PCBs) and a
   certificate attesting that it is the rightful owner of the
   corresponding public key.  One of the main roles of the TRC is thus
   enabling the verification of *AS certificates* and PCBs.

de Kater, et al.        Expires 20 November 2022               [Page 12]
Internet-Draft                  SCION I-D                       May 2022

                                  TRC 2
                  +--------------------------------------+
                  |+------------------------------------+|
                  ||- Version       - Core ASes         ||
   +--------+     ||- ID            - Description       ||    +--------+
   | TRC 1  |     ||- Validity      - No Trust Reset    ||    | TRC 3  |
   | (Base  |---->||- Grace Period  - Voting Quorum     ||--->|        |
   |  TRC)  |     ||- ...                               ||    |        |
   +--------+     |+------------------------------------+|    +--------+
                  |+----------------+  +----------------+|
                  || Regular Voting |  |Sensitive Voting||
                  ||  Certificate   |  |  Certificate   ||
                  |+----------------+  +----------------+|
                  |+----------------+  +----------------+|
                  ||     Votes      |  |   Signatures   ||
                  |+----------------+  +----------------+|
                  |+------------------------------------+|
                  ||        CP Root Certificates        ||
                  |+----------+-------------+-----------+|
                  |           |             |            |
                  +-----------+-------------+------------+
                              |             |
                              |             |
                              v             v
                    +-----------+         +-----------+
                    |   CP CA   |         |   CP CA   |
                    |Certificate|         |Certificate|
                    +-+-------+-+         +-----+-----+
                      |       |                 |
                      |       |                 |
                      v       v                 v
             +-----------+ +-----------+      +-----------+
             |   CP AS   | |   CP AS   |      |   CP AS   |
             |Certificate| |Certificate|      |Certificate|
             +-----------+ +-----------+      +-----------+

                   Figure 3: TRC contents and trust chain

2.1.2.  TRC Update and Verification

   With a base TRC as trust anchor, TRCs can be updated in a verifiable
   manner.  There are two kinds of TRC updates: regular and sensitive
   updates.  A _regular_ TRC update happens when the TRC's validity
   period expires.  This period is defined by the _validity_ parameter
   in the TRC.  The default is one year.  A TRC update is _sensitive_ if
   and only if critical sections of the TRC are affected (for example,
   if the set of core ASes is modified).  For both regular and sensitive
   TRC updates, a number of votes (in the form of signatures) must be

de Kater, et al.        Expires 20 November 2022               [Page 13]
Internet-Draft                  SCION I-D                       May 2022

   cast to approve the update.  This number of votes is dictated by the
   voting quorum and set in each TRC with the _voting quorum_ parameter.

2.1.3.  Dissemination of TRC Updates

   Information about a TRC update is disseminated via the SCION's
   beaconing process, through the path-segment constructions beacons.
   Each PCB contains the version number of the currently active TRC.  If
   an AS receives a PCB with a TRC version number higher than the
   locally stored TRC, it requests the PCB-sending AS for the new TRC.
   The new TRC is verified on the basis of the current one, and is
   accepted if it contains at least the required quorum of correct
   signatures by trust roots defined in the current TRC.  This simple
   dissemination mechanism has two advantages: It is very efficient (as
   fresh PCBs rapidly reach all ASes), and it avoids circular
   dependencies with regard to the verification of PCBs and the
   beaconing process itself (as no server needs to be contacted over
   unknown paths in order to fetch the updated TRC).

2.1.4.  Grace Period

   At most two TRCs per ISD can be active at the same time.  The TRC
   parameter _grace period_ indicates for how long the currently active
   TRC can still be active after a new TRC is disseminated.  This so-
   called *grace period* starts at the beginning of the validity period
   of the new TRC.  An older TRC can only be active until either (1) the
   grace period has passed, or (2) yet a newer TRC is announced.  AS
   certificates are validated by following the chain of trust up to an
   active TRC.

2.1.5.  Revocation and Recovery from a Catastrophic Event

   The TRC dissemination mechanism also enables rapid revocation of
   trust roots.  When a trust root is compromised, the other trust roots
   can remove it from the TRC and disseminate a new TRC alongside a PCB
   with a new version number.

   In case of a catastrophic event--such as several private root keys
   being disclosed due to a critical vulnerability in a cryptographic
   library--SCION is equipped with a recovery procedure called *trust
   reset*. This procedure consists of creating a new TRC with fresh,
   trustworthy keys (and potentially new algorithms), and redistributing
   this TRC out-of-band.  A trust reset effectively establishes a new
   base TRC for the ISD.  It is possible for ISDs to disable trust
   resets by setting the _no trust reset_ Boolean parameter to "true" in
   their TRC, with the effect that the entire ISD would have to be
   abandoned in the event of such a catastrophic compromise (this
   abandonment would also have to be announced out-of-band).

de Kater, et al.        Expires 20 November 2022               [Page 14]
Internet-Draft                  SCION I-D                       May 2022

   The partition of the SCION network into ISDs guarantees that no
   single entity can take down the entire network.  Even if several
   entities formed a coalition to carry out an attack, the effects of
   that attack would be limited to one or a few ISDs.

2.2.  SCION Control Plane

   The SCION control plane is responsible for discovering path segments
   and making them available to end hosts.  This process includes path
   exploration, registration, and lookup; it involves the path service,
   beacon service, and certificate service, both in core ASes and non-
   core ASes.

   *Note*: This section describes the SCION control plane on a very high
   level.  A much more detailed description of SCION's control plane
   will follow in a separate internet draft.

2.2.1.  Path Exploration

   In SCION, the path segment construction process is referred to as
   *beaconing*. The _beacon service_ of each AS is responsible for the
   beaconing process.  The beacon service generates, receives, and
   propagates the *path-segment construction beacons (PCBs)* on a
   regular basis, to iteratively construct path segments.

   There are three types of path segments (note that all path segments
   can be used to send data traffic in both directions):

   *  A path segment from a non-core AS to a core AS is an _up-path
      segment_.

   *  A path segment from a core AS to a non-core AS is a _down-path
      segment_.

   *  A path segment between core ASes is a _core-path segment_.

   All path segments are invertible: A core-path segment can be used
   bidirectional, and an up-path segment can be converted into a down-
   path segment, or vice versa, depending on the direction of the end-
   to-end path.

   Path segment construction is conducted hierarchically on two levels:

   *  _Core beaconing_ is the process of constructing path segments
      between core ASes.  During core beaconing, the beacon service of a
      core AS either initiates PCBs or propagates PCBs received from
      neighboring core ASes to all other neighboring core ASes.  Core
      beaconing in SCION is similar to BGP's route-advertising process,

de Kater, et al.        Expires 20 November 2022               [Page 15]
Internet-Draft                  SCION I-D                       May 2022

      although in SCION the process is periodic and PCBs are flooded
      over policy-compliant paths to discover multiple paths between any
      pair of core ASes.

   *  _Intra-ISD beaconing_ creates path segments from core ASes to non-
      core ASes.  For this, the beacon service of a core AS creates PCBs
      and sends them to the non-core child ASes (typically customer
      ASes).  The beacon service of a non-core child AS receives these
      PCBs and forwards them to its child ASes, and so on.  This
      procedure continues until the PCB reaches an AS without any
      customer (leaf AS).  As a result, all ASes receive path segments
      to reach the core ASes of their ISD.

   On its way down, a PCB accumulates cryptographically protected path-
   and forwarding information per traversed AS.  At every AS, metadata
   as well as information about the AS's ingress and egress interfaces
   (i.e., link identifiers) is added to the PCB.  The ingress and egress
   interface IDs identify connections to neighboring ASes.  These IDs
   only need to be unique within each AS.  Therefore, they can be chosen
   and encoded by each AS independently and without any need for
   coordination.

   SCION also supports shortcuts and peering links.  In a _shortcut_, a
   path only contains an up-path and a down-path segment, which can
   cross over at a non-core AS that is common to both paths. _Peering
   links_ can be added to up- or down-path segments, resulting in an
   operation similar to today's Internet.

   To reduce beaconing overhead and prevent possible forwarding loops,
   PCBs do not traverse peering links.  Instead, peering links are
   announced along with a regular path in a PCB.  If the path segments
   of both ASes at the end of a peering link contain this peering link,
   then it is possible to use the peering link to shortcut the end-to-
   end path (i.e., without going through the core).  SCION also supports
   peering links that cross ISD boundaries, according to SCION's path
   transparency property.

   Figure 4 shows how intra-ISD PCB propagation works, from the ISD's
   core AS down to child ASes.  For the sake of illustration, the
   interfaces of each AS are numbered with integer values.  In practice,
   each AS can choose any encoding for its interfaces; in fact, only the
   AS itself needs to understand its encoding.  Here, AS F receives two
   different PCBs via two different links from core AS X.  Moreover, AS
   F uses two different links to send two different PCBs to AS G, each
   containing the respective egress interfaces.  AS G extends the two
   PCBs and forwards both of them over a single link to a child AS.

de Kater, et al.        Expires 20 November 2022               [Page 16]
Internet-Draft                  SCION I-D                       May 2022

                                     .-----.
                                    ; Core  :
                           +-----+  : AS X  ;
                           |PCB  |   \ 2 1 / +-----+
                           |Core |    `+-+'  |pcb  |
                           |Out:2|     | |   |core |
                           +--+--+   +-+ |   |out:1|
                              |      |   |   +--+--+
                              v      |   |      |
                                   .-+---+.     v
                      .---.       /  2   3 \             .---.
                     (  J  )- - -; 1      4 :- - - - - -(  H  )
                      `---'      :   AS F   ;            `---'
                               +--\7       /
   +----------+ +----------+ <-+     6  5
   |PCB       | |pcb       |        `+--+'
   |Core      | |core      |         |  |
   |Out:2     | |out:1     |         |  |
   |----------| |----------|         |  |
   |AS F      | |as f      |         |  |
   |In:2 Out:7| |in:3 out:7|         |  |
   |Peer J:1  | |peer j:1  |         |  | +----------+ +----------+
   |Peer H:4  | |peer h:4  |         |  | |PCB       | |pcb       |
   |          | |          |         |  | |Core      | |core      |
   +--+-------+ +--+-------+         |  | |Out:2     | |out:1     |
      |            |                 |  | |----------| |----------|
     <+           <+                 |  | |AS F      | |as f      |
                                     |  | |In:2 Out:5| |in:3 out:5|
            +----------+ +----------+|  | |Peer J:1  | |peer j:1  |
            |PCB       | |pcb       ||  | |Peer H:4  | |peer h:4  |
            |Core      | |core      ||  | |          | |          |
            |Out:2     | |out:1     ||  | +----+-----+ +----+-----+
            |----------| |----------||  |      |            |
            |AS F      | |as f      ||  |      v            v
            |In:2 Out:6| |in:3 out:6||  |
            |Peer J:1  | |peer j:1  ||  |
            |Peer H:4  | |peer h:4  ||  |
            |          | |          ||  |
            +----+-----+ +----+-----+|  |
                 |            |     .+--+-.
                 v            v   ,' 5  1  `.
                                 ;           :
                                 :   AS G    ;
                                  \         /
                               +---` 4  3 ,'
                             <-+     `--+'
                                        |  +----------+ +----------+
                                        |  |PCB       | |pcb       |

de Kater, et al.        Expires 20 November 2022               [Page 17]
Internet-Draft                  SCION I-D                       May 2022

                                        |  |Core      | |core      |
                                        |  |Out:2     | |out:1     |
              +----------+ +----------+ |  |----------| |----------|
              |PCB       | |pcb       | |  |AS F      | |as f      |
              |Core      | |core      | |  |In:2 Out:5| |in:3 out:5|
              |Out:2     | |out:1     | |  |Peer J:1  | |peer j:1  |
              |----------| |----------| |  |Peer H:4  | |peer h:4  |
              |AS F      | |as f      | |  |----------| |----------|
              |In:2 Out:6| |in:3 out:6| |  |AS G      | |as g      |
              |Peer J:1  | |peer j:1  | |  |In:1 Out:3| |in:1 out:3|
              |Peer H:4  | |peer h:4  | |  |          | |          |
              |----------| |----------| |  +----+-----+ +----+-----+
              |AS G      | |as g      | |       |            |
              |In:5 Out:3| |in:5 out:3| v       v            v
              |          | |          |
              +----+-----+ +----+-----+
                   |            |
                   v            v

       Figure 4: Intra-ISD PCB propagation from the ISD core down to
                                 child ASes

2.2.1.1.  Security

   Each PCB contains signatures of all on-path ASes.  Every time a
   beacon service receives a PCB, it validates the PCB's authenticity.
   During this process, the beacon service can query the certificate
   service, for example, when it lacks an intermediate certificate.

2.2.1.2.  Policies

   Each AS can independently set policies dictating which PCBs are sent
   in which time intervals, and to which neighbors.  In particular, PCBs
   do not need to be propagated immediately upon arrival.  However,
   during bootstrapping and if the AS obtains a PCB containing a
   previously unknown path, the AS should forward the PCB immediately,
   to ensure quick connectivity establishment.

2.2.2.  Path Registration

   Both the beacon service and the path service are involved in the path
   registration process.  A non-core AS typically receives several PCBs
   representing several path segments to various core ASes.  Out of
   these PCBs, the non-core AS must select those down-path segments
   through which it wants to be reached.  It is the task of the AS's
   beacon service to make this selection, according to the criteria
   described in Path-Segment Selection (Section 2.2.2.1).  The beacon
   service then registers these path segments both at the local path

de Kater, et al.        Expires 20 November 2022               [Page 18]
Internet-Draft                  SCION I-D                       May 2022

   service and at the path service of all core ASes.  When links fail,
   segments expire, or better segments become available, the beacon
   service updates the down-path segments registered for its AS.

   As a result, a core AS's path service contains all intra-ISD path
   segments registered by the leaf ASes of its ISD.  In addition, a core
   AS path service also stores the preferred core-path segments to other
   core ASes.

2.2.2.1.  Path-Segment Selection

   Among the received PCBs, the beacon service of an AS must choose (1)
   a set of PCBs to propagate further, and (2) a set of path segments to
   register.  The selection of these PCBs and path segments is based on
   a path quality metric.  This metric aims at identifying consistent,
   diverse, efficient, and policy-compliant paths:

   *  _Consistency_ implies that at least one property along the path is
      uniform, such as an AS capability (e.g., high bandwidth).

   *  _Diversity_ means that the set of paths announced over time are as
      path-disjoint as possible, in order to provide high-quality
      multipath options.

   *  _Efficiency_ refers to the length, bandwidth, latency,
      utilization, and availability of a path, where more efficient
      paths are naturally preferred.

   *  _Policy compliance_ implies that the path adheres to the AS's
      routing policy.

   Based on past PCBs, the AS beacon service assigns scores to the
   current set of candidate path segments, and sends the best segments
   in the next beaconing interval.

   Core beaconing operates similarly to intra-ISD beaconing, except that
   core PCBs only traverse core ASes.  The same path selection metrics
   apply, where a core AS attempts to forward the set of most desirable
   paths to its neighbors.

2.2.3.  Path Lookup

   An end host (source) who wants to start communication with another
   host (destination), requires up to three path segments: An up-path
   segment to reach the ISD core, a core-path segment to reach the
   destination ISD, and a down-path segment to reach the destination AS.
   The source host queries the path service in its AS for such segments.
   The path service has up-path segments stored in its database and

de Kater, et al.        Expires 20 November 2022               [Page 19]
Internet-Draft                  SCION I-D                       May 2022

   furthermore checks if it has appropriate core- and down-path segments
   in its cache; in this case it returns them immediately.

   If not, the path service in the source AS queries core path services
   (using locally stored up-path segments) in the source ISD for core-
   path segments to the destination ISD.  Then, it combines up-path
   segments with the newly retrieved core-path segments, and queries
   core path services in the remote ISD to fetch remote down-path
   segments.  To improve overall efficiency, the local path service
   caches the returned path segments and uses parallelism when
   requesting path segments from core path services.  Finally, the local
   path service returns all path segments to the source host.

   This recursive lookup significantly simplifies the process for end
   hosts (which only have to send a single query, similar to stub DNS
   resolvers).  The caching strategy ensures that path lookups are fast
   for frequently used destinations (similar to caching in recursive DNS
   resolvers).

2.2.4.  Link Failures

   Unlike in the current Internet, link failures are not automatically
   resolved by the network, but require active handling by end hosts.
   Since SCION forwarding paths are static, they break when one of the
   links fails.  Link failures are handled by a two-pronged approach
   that typically masks link failures without any outage to the
   application and rapidly re-establishes fresh working paths:

   *  The SCION Control Message Protocol (SCMP) (the SCION equivalent of
      ICMP) is used for signaling connectivity problems.  Instead of
      relying on application- or transport-layer timeouts, end hosts get
      immediate feedback from the network if a path stops working, and
      can quickly switch to an alternative path.

   *  SCION end hosts are encouraged to use multipath communication by
      default, thus masking a link failure with another working path.
      As multipath communication can increase availability (even in
      environments with very limited path choices), SCION beacon
      services attempt to create disjoint paths, SCION path services
      attempt to select and announce disjoint paths, and end hosts
      compose path segments to achieve maximum resilience to path
      failure.  Consequently, most link failures in SCION remain
      unnoticed by the application, unlike the frequent (albeit mostly
      brief) outages in the current Internet.  See also [ANDERSEN2001],
      [KATZ2012], [KUSHMAN2007], and [HITZ2021].

de Kater, et al.        Expires 20 November 2022               [Page 20]
Internet-Draft                  SCION I-D                       May 2022

2.3.  SCION Data Plane

   While the control plane is responsible for providing end-to-end
   paths, the data plane ensures that packets are forwarded on the
   selected path.  SCION border routers forward packets to the next AS
   based on the AS-level path in the packet header (which is extended
   with ingress and egress interface identifiers for each AS), without
   inspecting the destination address and also without consulting an
   inter-domain forwarding table.  Only the border router at the
   destination AS needs to inspect the destination address to forward it
   to the appropriate local end host.

   Because SCION splits the information about the locator (the path
   towards the destination AS) and the identifier (the destination
   address), the identifier can have any format that the destination AS
   can interpret--only the destination needs to consider that local
   identifier (see also [RFC6830]).  In other words, an AS can select an
   arbitrary addressing format for its hosts, e.g., a 4-byte IPv4,
   6-byte media access control (MAC) address, 16-byte IPv6, or any other
   up to 16-byte addressing scheme.  A valuable consequence is that
   hosts with different address types can directly communicate.

   The next two sections describe how an end host combines path segments
   into an end-to-end forwarding path, and how border routers forward
   packets efficiently.

   *Note*: This section describes the SCION data plane on a very high
   level.  A much more detailed description of SCION's data plane will
   follow in a separate internet draft.

2.3.1.  Path Construction via Segment Combination

   Through the path lookup, the end host obtains path segments that must
   be combined into an end-to-end path.  A valid SCION *forwarding path*
   can be created by combining up to three path segments, in the
   following ways:

   *  *Immediate combination of path segments*: The last AS on the up-
      path segment is also the first AS on the down-path segment.  In
      this case, the simple combination of an up-path segment and a
      down-path segment creates a valid forwarding path.

   *  *AS shortcut*: The up-path segment and down-path segment intersect
      at a non-core AS.  In this case, a shorter forwarding path can be
      created by removing the extraneous part of the path.

de Kater, et al.        Expires 20 November 2022               [Page 21]
Internet-Draft                  SCION I-D                       May 2022

   *  *Peering shortcut*: A peering link exists between the two
      segments, so a shortcut via the peering link is possible.  As in
      the AS shortcut case, the extraneous path segment is cut off.  The
      peering link could be traversing to a different ISD.

   *  *Combination with a core-path segment*: The last AS on the up-path
      segment is different from the first AS on the down-path segment.
      This case requires an additional core-path segment to connect the
      up- and down-path segment.  If the communication remains within
      the same ISD, a local ISD core-path segment is needed; otherwise,
      an inter-ISD core-path segment is required.

   *  *On-path*: The destination AS is part of the up-path segment or
      the source AS is part of the down-path segment; in this case, a
      single up- or down-path segment, respectively, is sufficient to
      create a forwarding path.

   Once a forwarding path is chosen, it is encoded in the SCION packet
   header.  This makes inter-domain routing tables unnecessary for
   border routers: Both the ingress and the egress interface of each AS
   on the path are encoded as *packet-carried forwarding state (PCFS)*
   in the packet header.  The destination can respond to the source by
   reversing the end-to-end path from the packet header, or it can
   perform its own path lookup and combination.

   The SCION packet header contains of a sequence of *hop fields (HFs)*,
   one HF for each AS that is traversed on the end-to-end path.  Each
   hop field contains the encoded numbers of the ingress and egress
   links, and thus defines which interfaces may be used to enter and
   leave an AS.  In addition to the hop fields, each path segment
   contains an *info field (INF)* with basic information about the
   segment.  A host can create an end-to-end forwarding path by
   extracting info fields and hop fields from path segments, as depicted
   in Figure 5.  The additional meta header (META) contains pointers to
   the currently active INF and HF.

   up-path segment        core-path segment        down-path segment

   +-------+              +-------+                +-------+
   |+-----+|              |+-----+|                |+-----+|
   |+ INF ||----------+   |+ INF ||---+            |+ INF ||-+
   |+-----+|          |   |+-----+|   |            |+-----+| |
   |+-----+|          |   |+-----+|   |            |+-----+| |
   || hf  ||--------+ |   || hf  ||---+--+         || hf  ||-+--+
   |+-----+|        | |   |+-----+|   |  |         |+-----+| |  |
   |+-----+|        | |   |+-----+|   |  |         |+-----+| |  |
   || hf  ||-----+  | |   || hf  ||---+--+--+      || hf  ||-+--+--+
   |+-----+|     |  | |   |+-----+|   |  |  |      |+-----+| |  |  |

de Kater, et al.        Expires 20 November 2022               [Page 22]
Internet-Draft                  SCION I-D                       May 2022

   |+-----+|     |  | |   +-------+   |  |  |      +-------+ |  |  |
   || hf  ||--+  |  | |               |  |  |                |  |  |
   |+-----+|  |  |  | |   +--------+  |  |  |                |  |  |
   +-------+  |  |  | |   |++-----+|  |  |  |                |  |  |
              |  |  | |   |++ Meta||  |  |  |                |  |  |
              |  |  | |   |++-----+|  |  |  |                |  |  |
              |  |  | |   |+-----+ |  |  |  |                |  |  |
              |  |  | +-->|+ INF | |  |  |  |                |  |  |
              |  |  |     |+-----+ |  |  |  |                |  |  |
              |  |  |     |+-----+ |  |  |  |                |  |  |
              |  |  |     |+ INF | |<-+  |  |                |  |  |
              |  |  |     |+-----+ |     |  |                |  |  |
              |  |  |     |+-----+ |     |  |                |  |  |
              |  |  |     |+ INF | |<----+--+----------------+  |  |
              |  |  |     |+-----+ |     |  |                   |  |
              |  |  |     |+-----+ |     |  |                   |  |
              |  |  +---->|| hf  | |     |  |                   |  |
              |  |        |+-----+ |     |  |                   |  |
              |  |        |+-----+ |     |  |                   |  |
              |  +------->|| hf  | |     |  |                   |  |
              |           |+-----+ |     |  |                   |  |
              |           |+-----+ |     |  |                   |  |
              +---------->|| hf  | |     |  |                   |  |
                          |+-----+ |     |  |                   |  |
                          |+-----+ |     |  |                   |  |
                          || hf  | |<----+  |                   |  |
                          |+-----+ |        |                   |  |
                          |+-----+ |        |                   |  |
                          || hf  | |<-------+                   |  |
                          |+-----+ |                            |  |
                          |+-----+ |                            |  |
                          || hf  | |<---------------------------+  |
                          |+-----+ |                               |
                          |+-----+ |                               |
                          || hf  | |<------------------------------+
                          |+-----+ |
                          +--------+
                        forwarding path

       Figure 5: Combining three path segments into a forwarding path

de Kater, et al.        Expires 20 November 2022               [Page 23]
Internet-Draft                  SCION I-D                       May 2022

2.3.2.  Path Authorization

   It is crucial for the data plane that end hosts only use paths
   constructed and authorized by ASes in the control plane.  In
   particular, end hosts should not be able to craft HFs themselves,
   modify HFs in authorized path segments, or combine HFs of different
   path segments (path splicing).  This property is called *path
   authorization* (see [KLENZE2021] and [LEGNER2020]).

   SCION achieves path authorization by creating message-authentication
   codes (MACs) during the beaconing process.  Each AS calculates these
   MACs using a local secret key (that is only shared between SCION
   infrastructure elements within the AS) and chains them to the
   previous HFs.  The MACs are then included in the forwarding path as
   part of the respective HFs.

2.3.3.  Forwarding

   Routers can efficiently forward packets in the SCION architecture.
   In particular, the absence of inter-domain routing tables and of
   complex longest-IP-prefix matching performed by current routers
   enables the construction of more efficient routers.

   During packet forwarding, a SCION border router at the ingress point
   of the AS verifies that:

   *  the packet entered through the correct ingress interface
      corresponding to the information in the HF,

   *  the HF is still valid, and

   *  the MAC in the HF is correct.

   If the packet has not yet reached the destination AS, the egress
   interface number in the HF of the non-destination AS refers to the
   egress SCION border router of this AS.  In this case, the packet can
   be sent from the ingress SCION border router to the egress SCION
   border router via native intra-domain forwarding (e.g., IP or MPLS).
   In case the packet has arrived at the destination AS, the destination
   AS's border router inspects the destination address and sends the
   packet to the corresponding host.

2.3.4.  Intra-AS Communication

   SCION routers use IP to communicate within an AS, therefore they rely
   on existing intra-domain routing protocols, such as Multiprotocol
   Label Switching (MPLS) or others.

de Kater, et al.        Expires 20 November 2022               [Page 24]
Internet-Draft                  SCION I-D                       May 2022

3.  Deployment

   Adoption of a next-generation architecture is a challenging task, as
   it needs to be integrated with, and operate alongside existing
   infrastructure.  SCION is designed to coexist with existing intra-
   domain routing infrastructure, and comprises coexistence and
   transition mechanisms that facilitate adoption, in accordance to
   principles defined in [RFC8170].  The following section discusses
   practical considerations for deploying SCION and briefly touches on
   some of the transition mechanisms, with focus on:

   *  Autonomous Systems (Section 3.1),

   *  Internet Exchange Points (Section 3.2), and

   *  end hosts (Section 3.3), covering both native SCION hosts and
      SCION to IP encapsulation.

   We then describe some of the early adopters deployment experiences.
   A more detailed adoption plan is to be outlined in dedicated
   documents.

3.1.  Autonomous System Deployment

   A SCION AS needs to deploy the SCION infrastructure components
   (Section 1.2.3) and border routers.  Within an AS, SCION is often
   deployed as an IP overlay on top of the existing network.  This way
   SCION allows to reuse the existing intra-domain network and equipment
   (e.g., IP, MPLS).  Customer-side SCION border routers directly
   connect to the provider-side border routers using last-mile
   connections.  The SCION design assumes that AS's internal entities
   are considered to be trustworthy, therefore the IP overlay or the
   first-hop routing does not compromise or degrade any security
   properties SCION delivers.  When it comes to inter-domain
   communication, an overlay deployment on top of today's Internet is
   not desirable, as SCION would inherit issues from its weak underlay.
   Thus, inter-AS SCION links are usually deployed in parallel to
   existing links, in order to preserve its security properties.  That
   is, two SCION border routers from neighbour ASes are directly
   connected via a layer-2 cross-connection at a common point-of-
   presence.

de Kater, et al.        Expires 20 November 2022               [Page 25]
Internet-Draft                  SCION I-D                       May 2022

   All SCION AS components can be deployed on standard x86 commercial
   off-the-shelf servers or virtual machines.  In fact, SCION border
   routers do not rely on forwarding tables, therefore they do not
   require specialized hardware.  Practice shows that off-the-shelf
   hardware can handle up to 100 Gbps links, while a prototype P4
   implementation [DERUITER2021] showed that it is possible to forward
   SCION traffic even at terabit speeds.

   Overall, an AS can be connected to SCION without high-impact changes
   to its network.  In addition, use of commodity hardware for both
   control and data-plane components reduces initial deployment costs.

3.2.  Internet Exchange Points

   Internet Exchange Points (IXP) play as important a role for SCION as
   they do in today's Internet.  SCION can be deployed at existing IXPs
   following a "big switch" model, where the IXP provides a large L2
   switch between multiple SCION ASes.  SCION has been deployed
   following this model at the Swiss Internet Exchange (SwissIX),
   currently interconnecting major SCION Swiss ISPs and enterprises
   through bi-lateral peering over dedicated SCION ports.

   Additionally, thanks to its path-awareness, SCION offers the option
   of an enhanced deployment model, i.e., to expose the internal
   topology of an IXP within the SCION control plane.  This enables IXP
   customers to use SCION's multi-path and fast failover capabilities to
   leverage the IXP's internal links (including backup links) and to
   select paths depending on the application's needs.  IXPs have
   therefore an incentive to expose their rich internal connectivity, as
   the benefits from SCION's multi-path capabilities would increase
   their value for customers and provide them with a competitive
   advantage.

3.3.  End Hosts and Incremental Deployability

   End users can leverage SCION in two different ways: (1) using SCION-
   aware applications on a SCION native end host (Section 3.3.1), or (2)
   using transparent IP-to-SCION conversion (Section 3.3.2).  The
   benefit of using SCION natively is that the full range of advantages
   becomes available to applications, at the cost of installing the
   SCION endpoint stack and making the application SCION-aware.  In
   early deployments, the second approach is often preferred, so that no
   changes are needed within applications or end hosts.

de Kater, et al.        Expires 20 November 2022               [Page 26]
Internet-Draft                  SCION I-D                       May 2022

3.3.1.  Native End Hosts

   A SCION native end host's stack consists of a dispatcher, which
   handles all incoming and outgoing SCION packets, and of a SCION
   daemon, which handles control-plane messages.  The latter fetches
   paths to remote ASes and provides an API for applications and
   libraries to interact with the SCION control plane (i.e., for path
   lookup, SCION extensions).  The current SCION implementation uses an
   UDP/IP underlay for communication between end hosts and SCION
   routers.  This allows reuse of existing intra-domain networking
   infrastructure.  SCION end hosts can optionally use automated
   bootstrapping mechanisms to retrieve configuration from the network
   and establish SCION connectivity.  This way, clients require no pre-
   existing network-specific configurations.

3.3.2.  SCION to 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.  A SIG can be deployed close to the end user (i.e., at
   branches of an enterprise, on a CPE), or within an ISP's network.  In
   the latter case, the SIG is called carrier-grade SIG, as it serves
   multiple customers within the AS where it is deployed.  This approach
   has the advantage that it does not require any changes at the
   customer's premises.  In order to allow incremental deployability and
   to ease transition from legacy IP-based Internet to SCION, SIGs can
   be augmented with mechanisms allowing them to coordinate and
   automatically exchange IP prefix information.  A more detailed
   description of the SIG and its coordination mechanisms is to be
   presented in dedicated documents.

3.4.  Deployment Experiences

   SCION has been deployed in production by multiple entities, growing
   its acceptance among industry.  While early deployments started on
   academic and research networks, SCION has expanded to serve the
   financial industry, government, and it is being evaluated for the
   healthcare sector.

   In 2017, SCION was evaluated for production use by a central bank,
   with the goal of modernising the network interconnecting banks and
   their branches.  SCION was chosen, as it allows moving away from a
   dedicated private network to a reliable Internet-based solution.
   SCION connectivity was later extended to support system-critical
   applications, like the national real-time gross settlement (RTGS)
   system, connecting all country's banks to exchange real-time payment
   information.  The network, called Secure Swiss Finance Network or
   SSFN (https://perma.cc/PU5L-ALPM), is implemented as a SCION ISD,

de Kater, et al.        Expires 20 November 2022               [Page 27]
Internet-Draft                  SCION I-D                       May 2022

   where a federation of three ISPs forms the ISD core.  Financial
   institutions are themselves SCION ASes and directly connect to one or
   more of the core ASes.  Institutions deploy SCION-IP gateways (SIGs),
   transparently enabling their traditional IP-based applications to use
   the SCION network.  The concept of the SCION ISD also provides a
   mechanism to implement strict governance and access control (through
   the issuance of AS certificates).

   Besides the SSFN, SCION connectivity has also been adopted by
   government entities for their international communications.  In
   addition, Swiss higher education institutions are connected thanks to
   the SCI-ED (http://scied.scion-architecture.net/) network.

   In addition to productive deployments, SCION also comprises a global
   SCION research testbed called SCIONLab (https://www.scionlab.org).
   It is composed of dozens of globally distributed infrastructure ASes,
   mostly run by academic institutions.  The testbed is open to any user
   who can easily set up their own AS with the aid of a web-based UI,
   connect to the network, and run experiments.  The setup has been the
   earliest global deployment of SCION and it has been supporting
   research and development of path-aware networking and SCION.

4.  IANA Considerations

   Currently, this document has no request for action to IANA.  However,
   when full specification of SCION is available, requests for IANA
   actions are expected regarding the registration of optional packet
   header fields as well as the coordination of SCION ISD and AS number
   assignments.

5.  Security Considerations

   SCION has been designed from the outset to offer security by default,
   and thus there are manifold security considerations.  As a matter of
   fact, SCION's protocol design has been formally verified and the open
   source router implementation is undergoing formal verification (see
   also [KLENZE2021]).  Describing all security considerations here,
   therefore, would go beyond the scope of this document.  A separate
   document including all security implications and considerations will
   follow later.

6.  References

6.1.  Normative References

de Kater, et al.        Expires 20 November 2022               [Page 28]
Internet-Draft                  SCION I-D                       May 2022

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

6.2.  Informative References

   [ANDERSEN2001]
              Andersen, D., Balakrishnan, H., Kaashoek, F., and R.
              Morris, "Resilient overlay networks", Proceedings of the
              eighteenth ACM symposium on Operating systems principles,
              DOI 10.1145/502034.502048, October 2001,
              <https://doi.org/10.1145/502034.502048>.

   [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,
              <https://doi.org/10.1007/978-3-031-05288-0>.

   [COOPER2013]
              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,
              <https://doi.org/10.1145/2535771.2535787>.

   [DERUITER2021]
              de Ruiter, J. and C. Schutijser, "Next-generation internet
              at terabit speed: SCION in P4", Proceedings of the 17th
              International Conference on emerging Networking
              EXperiments and Technologies, DOI 10.1145/3485983.3494839,
              December 2021, <https://doi.org/10.1145/3485983.3494839>.

   [GRIFFIN1999]
              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,
              <https://doi.org/10.1145/316194.316231>.

   [HITZ2021] Hitz, S., "Demonstrating the reliability and resilience of
              Secure Swiss Finance Network", 2021,
              <https://perma.cc/4H3Q-WZNG>.

de Kater, et al.        Expires 20 November 2022               [Page 29]
Internet-Draft                  SCION I-D                       May 2022

   [KATZ2012] Katz-Bassett, E., Scott, C., Choffnes, D., Cunha, Í.,
              Valancius, V., Feamster, N., Madhyastha, H., Anderson, T.,
              and A. Krishnamurthy, "LIFEGUARD: practical repair of
              persistent route failures", ACM SIGCOMM Computer
              Communication Review Vol. 42, pp. 395-406,
              DOI 10.1145/2377677.2377756, September 2012,
              <https://doi.org/10.1145/2377677.2377756>.

   [KING2022] King, D., Farrel, A., and C. Jacquenet, "Challenges for
              the Internet Routing Systems Introduced by Semantic
              Routing", 2022, <https://datatracker.ietf.org/doc/draft-
              king-irtf-challenges-in-routing/>.

   [KLENZE2021]
              Klenze, T., Sprenger, C., and D. Basin, "Formal
              Verification of Secure Forwarding Protocols", 2021 IEEE
              34th Computer Security Foundations Symposium (CSF),
              DOI 10.1109/csf51468.2021.00018, June 2021,
              <https://doi.org/10.1109/csf51468.2021.00018>.

   [KUSHMAN2007]
              Kushman, N., Kandula, S., and D. Katabi, "Can you hear me
              now?!: it must be BGP", ACM SIGCOMM Computer Communication
              Review Vol. 37, pp. 75-84, DOI 10.1145/1232919.1232927,
              March 2007, <https://doi.org/10.1145/1232919.1232927>.

   [LABOVITZ2000]
              Labovitz, C., Ahuja, A., Bose, A., and F. Jahanian,
              "Delayed Internet routing convergence", Proceedings of the
              conference on Applications, Technologies, Architectures,
              and Protocols for Computer Communication - SIGCOMM '00,
              DOI 10.1145/347059.347428, 2000,
              <https://doi.org/10.1145/347059.347428>.

   [LEGNER2020]
              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,
              <https://www.usenix.org/conference/usenixsecurity20/
              presentation/legner>.

   [LI2014]   Li, Q., Hu, Y., and X. Zhang, "Even Rockets Cannot Make
              Pigs Fly Sustainably: Can BGP be Secured with BGPsec?",
              Proceedings 2014 Workshop on Security of Emerging
              Networking Technologies, DOI 10.14722/sent.2014.23001,
              2014, <https://doi.org/10.14722/sent.2014.23001>.

de Kater, et al.        Expires 20 November 2022               [Page 30]
Internet-Draft                  SCION I-D                       May 2022

   [LYCHEV2013]
              Lychev, R., Goldberg, S., and M. Schapira, "BGP security
              in partial deployment: is the juice worth the squeeze?",
              ACM SIGCOMM Computer Communication Review Vol. 43, pp.
              171-182, DOI 10.1145/2534169.2486010, September 2013,
              <https://doi.org/10.1145/2534169.2486010>.

   [MORILLO2021]
              Morillo, R., Furuness, J., Morris, C., Breslin, J.,
              Herzberg, A., and B. Wang, "ROV++: Improved Deployable
              Defense against BGP Hijacking", Proceedings 2021 Network
              and Distributed System Security Symposium,
              DOI 10.14722/ndss.2021.24438, 2021,
              <https://doi.org/10.14722/ndss.2021.24438>.

   [PERRIG2017]
              Perrig, A., Szalachowski, P., Reischuk, R., and L. Chuat,
              "SCION: A Secure Internet Architecture",
              ISBN 978-3-319-67079-9, 2017,
              <https://doi.org/10.1007/978-3-319-67080-5>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/rfc/rfc4033>.

   [RFC4264]  Griffin, T. and G. Huston, "BGP Wedgies", RFC 4264,
              DOI 10.17487/RFC4264, November 2005,
              <https://www.rfc-editor.org/rfc/rfc4264>.

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
              <https://www.rfc-editor.org/rfc/rfc5218>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/rfc/rfc6480>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/rfc/rfc6830>.

   [RFC8170]  Thaler, D., Ed., "Planning for Protocol Adoption and
              Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8170>.

de Kater, et al.        Expires 20 November 2022               [Page 31]
Internet-Draft                  SCION I-D                       May 2022

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <https://www.rfc-editor.org/rfc/rfc8205>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC9049]  Dawkins, S., Ed., "Path Aware Networking: Obstacles to
              Deployment (A Bestiary of Roads Not Taken)", RFC 9049,
              DOI 10.17487/RFC9049, June 2021,
              <https://www.rfc-editor.org/rfc/rfc9049>.

   [RFC9217]  Trammell, B., "Current Open Questions in Path-Aware
              Networking", RFC 9217, DOI 10.17487/RFC9217, March 2022,
              <https://www.rfc-editor.org/rfc/rfc9217>.

   [ROTHENBERGER2017]
              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,
              <https://doi.org/10.1145/3065913.3065922>.

   [SAHOO2009]
              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,
              <https://doi.org/10.1016/j.comcom.2009.03.009>.

   [SCHUCHARD2011]
              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,
              <https://doi.org/10.1145/1866307.1866411>.

de Kater, et al.        Expires 20 November 2022               [Page 32]
Internet-Draft                  SCION I-D                       May 2022

Acknowledgments

   Many thanks go to Cyrill Kraehenbuehl and Juan A.  Garcia-Pardo for
   reviewing this document.  We are also indebted to Laurent Chuat,
   Markus Legner, David Basin, David Hausheer, Samuel Hitz, and Peter
   Mueller, for writing the book "The Complete Guide to SCION" (see
   [CHUAT22]), which provides the background information needed to write
   this informational draft.

Authors' Addresses

   Corine de Kater
   ETH Zuerich
   Email: corine.dekatermuehlhaeuser@inf.ethz.ch

   Nicola Rustignoli
   ETH Zuerich
   Email: nicola.rustignoli@inf.ethz.ch

   Adrian Perrig
   ETH Zuerich
   Email: adrian.perrig@inf.ethz.ch

de Kater, et al.        Expires 20 November 2022               [Page 33]