Internet-Draft SCION DP July 2024
de Kater, et al. Expires 9 January 2025 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-dekater-scion-dataplane-02
Published:
Intended Status:
Informational
Expires:
Authors:
C. de Kater
SCION Association
N. Rustignoli
SCION Association
S. Hitz
Anapaya Systems

SCION Data Plane

Abstract

This document describes the data plane of the path-aware, inter-domain network architecture SCION (Scalability, Control, and Isolation On Next-generation networks). One of the basic characteristics of SCION is that it gives path control to endpoints. The SCION control plane is responsible for discovering these paths and making them available as path segments to the endpoints. The role of the SCION data plane is to combine the path segments into end-to-end paths, and forward data between endpoints according to the specified path.

The SCION data plane fundamentally differs from today's IP-based data plane in that it is path-aware: In SCION, interdomain forwarding directives are embedded in the packet header. This document first provides a detailed specification of the SCION data packet format as well as the structure of the SCION header. SCION also supports extension headers - these are described, too. The document then shows the life cycle of a SCION packet while traversing the SCION Internet. This is followed by a specification of the SCION path authorization mechanisms and the packet processing at routers.

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-dp_I-D/draft-dekater-scion-dataplane.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-dekater-scion-dataplane/.

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

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 9 January 2025.

▲

Table of Contents

1. Introduction

SCION is a path-aware internetworking routing architecture as described in [RFC9217]. It allows endpoints and applications to select paths across the network to use for traffic, based on trustworthy path properties. SCION is an inter-domain network architecture and is therefore not concerned with intra-domain forwarding.

The data transmission order for SCION is the same as for IPv6 as defined in Introduction of [RFC8200].

SCION has been developed with the following goals:

Availability - to provide highly available communication that can send traffic over paths with optimal or required characteristics, quickly handle inter-domain link or router failures (both on the last hop or anywhere along the path) and provide continuity in the presence of adversaries.

Security - to provide higher levels of trust in routing information in order to prevent traffic hijacking, reduce potential for denial-of-service and other attacks. Endpoints can decide the trust roots they wish to rely on, routing information can be unambiguously attributed to an AS, and packets are only forwarded along authorized path segments. A particular use case is to enable geofencing.

Scalability - to improve the scalability of the inter-domain control plane and data plane, avoiding existing limitations related to convergence and forwarding table size. The advertising of path segments is separated into a beaconing process within each Isolation Domain (ISD) and between ISDs which incurs minimal overhead and resource requirements on routers.

SCION relies on three main components:

PKI - To achieve scalability and trust, SCION organizes existing ASes into logical groups of independent routing planes called Isolation Domains (ISDs). All ASes in an ISD agree on a set of trust roots called the Trust Root Configuration (TRC) which is a collection of signed root certificates in X.509 v3 format [RFC5280]. The ISD is governed by a set of core ASes which typically manage the trust roots and provide connectivity to other ISDs. This is the basis of the public key infrastructure which the SCION control plane relies upon for the authentication of messages that is used for the SCION control plane. See [I-D.scion-cppki]

Control Plane - performs inter-domain routing by discovering and securely disseminating path information between ASes. The core ASes use Path-segment Construction Beacons (PCBs) to explore intra-ISD paths, or to explore paths across different ISDs. See [I-D.scion-cp]

Data Plane - carries out secure packet forwarding between SCION-enabled ASes over paths selected by endpoints. A SCION border router reuses existing intra-domain infrastructure to communicate to other SCION routers or SCION endpoints within its AS.

This document describes the SCION Data Plane component.

1.1. Terminology

Autonomous System (AS): An autonomous system is a network under a common administrative control. For example, the network of an Internet service provider, company, or university can constitute an AS.

Core AS: Each SCION isolation domain (ISD) is administered by a set of distinguished autonomous systems (ASes) called core ASes, which are responsible for initiating the path-discovery and -construction process (in SCION called "beaconing").

Data Plane: The data plane (sometimes also referred to as the forwarding plane) is responsible for forwarding data packets that endpoints have injected into the network. After routing information has been disseminated by the control plane, packets are forwarded according to such information by the data plane.

Endpoint: An endpoint is the start- or the end of a SCION path. For example, an endpoint can be a host as defined in [RFC1122], or a gateway bridging a SCION and an IP domain. This definition is based on the definition in [RFC9473].

Forwarding Key: A forwarding key is a symmetric key that is shared between the control service (control plane) and the routers (data plane) of an AS. It is used to authenticate hop fields in the end-to-end SCION path. The forwarding key is an AS-local secret and is not shared with other ASes.

Forwarding Path: A forwarding path is a complete end-to-end path between two SCION hosts, which is used to transmit packets in the data plane and can be created with a combination of up to three path segments (an up-segment, a core-segment, and a down-segment).

Hop Field (HF): As they traverse the network, path-segment construction beacons (PCBs) accumulate cryptographically protected AS-level path information in the form of hop fields. In the data plane, hop fields are used for packet forwarding: they contain the incoming and outgoing interface IDs of the ASes on the forwarding path.

Info Field (INF): Each path-segment construction beacon (PCB) contains a single info field, which provides basic information about the PCB. Together with hop fields (HFs), info fields are used to create forwarding paths.

Interface Identifier (Interface ID): 16 bit identifier that designates a SCION interface at the end of a link connecting two SCION ASes. Each interface belongs to one border router. Hop fields describe the traversal of an AS by a pair of interface IDs (the ingress and egress interfaces). The Interface ID MUST be unique within each AS. Interface ID 0 is not a valid identifier, implementations can use it as the "unspecified" value.

Isolation Domain (ISD): In SCION, Autonomous Systems (ASes) are organized into logical groups called isolation domains or ISDs. Each ISD consists of ASes that span an area with a uniform trust environment (e.g., a common jurisdiction). A possible model is for ISDs to be formed along national boundaries or federations of nations.

Leaf AS: An AS at the "edge" of an ISD, with no other downstream ASes.

MAC: Message Authentication Code. In the rest of this document, "MAC" always refers to "Message Authentication Code" and never to "Medium Access Control". When "Medium Access Control address" is implied, the phrase "Link Layer Address" is used.

Packet-Carried Forwarding State (PCFS): Rather than relying on inter-domain forwarding tables, SCION data packets contain all the necessary, cryptographically-protected path information. This property is referred to as packet-carried forwarding state.

Path Authorization: A requirement for the data plane is that endpoints can only use paths that were constructed and authorized by ASes in the control plane. This property is called path authorization. The goal of path authorization is to prevent endpoints from crafting hop fields (HFs) themselves, modifying HFs in authorized path segments, or combining HFs of different path segments.

Path Control: Path control is a property of a network architecture that gives endpoints the ability to select how their packets travel through the network. Path control is stronger than path transparency.

Path Segment: Path segments are derived from path-segment construction beacons (PCBs). A path segment can be (1) an up-segment (i.e., a path between a non-core AS and a core AS in the same ISD), (2) a down-segment (i.e., the same as an up-segment, but in the opposite direction), or (3) a core-segment (i.e., a path between core ASes). Up to three path segments can be used to create a forwarding path.

Path-Segment Construction Beacon (PCB): Core ASes generate PCBs to explore paths within their isolation domain (ISD) and among different ISDs. ASes further propagate selected PCBs to their neighboring ASes. As a PCB traverses the network, it carries path segments, which can subsequently be used for traffic forwarding.

Path Transparency: Path transparency is a property of a network architecture that gives endpoints full visibility over the network paths their packets are taking. Path transparency is weaker than path control.

Peering Link: A link between two SCION border routers of different ASes, where at least one of the two ASes is not core. Two peering ASes may be in different ISDs. A peering link can be seen as a short-cut on a normal path. Peering link information is added to segment information during the beaconing process and used to shorten paths while assembling them from segments.

Valley Route: A valley route contains ASes that do not profit economically from traffic on this route. The name comes from the fact that such routes go "down" (following parent-child links) before going "up" (following child-parent links).

1.2. 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.

1.3. Overview

The SCION data plane forwards inter-domain packets between SCION-enabled ASes. SCION routers are normally deployed at the edge of an AS, and peer with neighbor SCION routers. Inter-domain forwarding is based on end-to-end path information contained in the packet header. This path information consists of a sequence of hop fields (HFs). Each hop field corresponds to an AS on the path, and it includes an ingress- as well as an egress interface ID, which univocally identify the ingress and egress interfaces within the AS. The information is authenticated with a Message Authentication Code (MAC) to prevent forgery.

This concept allows SCION routers to forward packets to a neighbor AS without inspecting the destination address and also without consulting an inter-domain forwarding table. Intra-domain forwarding and routing are based on existing mechanisms (e.g., IP). A SCION border router reuses existing intra-domain infrastructure to communicate to other SCION routers or SCION endpoints within its AS. The last SCION router at the destination AS therefore uses the destination address to forward the packet to the appropriate local endpoint.

This SCION design choice has the following advantages:

  • It provides control and transparency over forwarding paths to endpoints.

  • It simplifies the packet-processing at routers: Instead of having to perform longest-prefix matching on IP addresses, which requires expensive hardware and substantial amounts of energy, a router can simply access the next hop from the packet header, after having verified the authenticity of the hop field's MAC.

1.3.1. Inter- and Intra-Domain Forwarding

As SCION is an inter-domain network architecture, it is not concerned with intra-domain forwarding. This corresponds to the general practice today where BGP and IP are used for inter-domain routing and forwarding, respectively, but ASes use an intra-domain protocol of their choice, for example OSPF or IS-IS for routing and IP, MPLS, and various layer-2 protocols for forwarding. In fact, even if ASes use IP-forwarding internally today, they typically encapsulate the original IP packet they receive at the edge of their network into another IP packet with the destination address set to the egress border router, to avoid full inter-domain forwarding tables at internal routers.

SCION emphasizes this separation, as SCION is used exclusively for inter-domain forwarding, and re-uses the intra-domain network fabric to provide connectivity among all SCION infrastructure services, border routers, and endpoints. As a consequence, minimal change to the infrastructure is required for ISPs when deploying SCION.

Although a complete SCION address is composed of the <ISD, AS, endpoint address> 3-tuple, the endpoint address is not used for inter-domain routing or forwarding. This implies that the endpoint addresses are not required to be globally unique or globally routable, they can be selected independently by the corresponding ASes. This means, for example, that an endpoint identified by a link-local IPv6 address in the source AS can directly communicate with an endpoint identified by a globally routable IPv4 address via SCION. Alternatively, it is possible for two SCION hosts with the same IPv4 address 10.0.0.42 but located in different ASes to communicate with each other via SCION ([RFC1918]).

1.3.2. Intra-Domain Forwarding Process

The full forwarding process for a packet transiting an intermediate AS consists of the following steps.

Note: In this context, a border router is called ingress border router when it refers to an entrance border router to an AS, as seen from the direction of travel of the SCION packet. A border router is called egress border router when it refers to an exit border router of an AS, as seen from the direction of travel of the SCION packet.

  1. The AS's SCION ingress router receives a SCION packet from the neighboring AS.

  2. The SCION router parses, validates, and authenticates the SCION header.

  3. The SCION router maps the egress interface ID in the current hop field of the SCION header to the destination address of the intra-domain protocol (e.g. MPLS or IP) on the egress border router.

  4. The packet is forwarded within the AS by routers and switches based on the header of the intra-domain protocol.

  5. Upon receiving the packet, the SCION egress router strips off the header of the intra-domain protocol, again validates and updates the SCION header, and forwards the packet to the neighboring SCION router.

  6. The last SCION router on the path forwards the packet to the packet's destination endpoint indicated by the field DstHostAddr of the Address Header (Section 2.2.2).

1.3.3. Configuration

Border routers require mappings from SCION interface IDs to underlay addresses. Such information MUST be supplied to each router in an out of band fashion (e.g in a configuration file). For each link to a neighbor, these values MUST be configured. A typical implementation will require:

  • Interface ID.

  • Link type (core, parent, child, peer). Link type depends on mutual agreements between the organizations operating the ASes at each end of each link.

  • Neighbor ISD-AS number.

  • For the router that manages the interface: the neighbor interface underlay address.

  • For the routers that do not manage the interface: the address of the intra-domain protocol on the router that does.

In order to forward traffic to a service endpoint addresse (DT/DS == 0b01 in the common header (Section 2.2.1)), a border router translates the service number into a specific destination address. The method used to accomplish the translation is not defined by this document. It only depends on the implementation and the choices of each AS's administrator. In current practice this is accomplished by way of a configuration file.

Note: The current SCION implementation runs over the UDP/IP protocol. However, the use of other lower layers protocols is possible.

1.4. Path Construction (Segment Combinations)

Paths are discovered by the SCION control plane, which makes them available to SCION endpoints in the form of path segments. As described in [I-D.scion-cp], there are three kinds of path segments: up, down, and core. In the data plane, a SCION endpoint creates end-to-end paths from the path segments, by combining multiple path segments. Depending on the network topology, a SCION forwarding path can consist of one, two, or three segments. Each path segment contains several hop fields representing the ASes on the segment as well as one info field with basic information about the segment, such as a timestamp.

Segments cannot be combined arbitrarily. To construct a valid forwarding path, the source endpoint MUST obey the following rules:

  • There can be at most one of each type of segment (up-, core-, and down-segment). Allowing multiple up- or down-segments would decrease efficiency and the ability of ASes to enforce path policies.

  • If an up-segment is present, it MUST be the first segment in the path.

  • If a down-segment is present, it MUST be the last segment in the path.

  • If there are two path segments (one up- and one down-segment) that both announce the same peering link, then a shortcut via this peering link is possible.

  • If there are two path segments (one up- and one down-segment) that share a common ancestor AS (in the direction of beaconing), then a shortcut via this common ancestor AS is possible.

  • Additionally, all segments without any peering possibility MUST consist of at least two hop fields.

Note: The type of segment is known to the endpoint but is not visible in the path header of data packets. Therefore, a SCION router needs to explicitly verify that these rules were followed correctly.

Besides enabling the enforcement of path policies, the above rules also protect the economic interest of ASes, as they prevent building "valley paths". A valley path contains ASes that do not profit economically from traffic on this route. The name comes from the fact that such paths go "down" (following parent-child links) before going "up" (following child-parent links).

Figure 1 below shows valid segment combinations.

Note: It is assumed that the source and destination endpoints are in different ASes (as endpoints from the same AS use an empty forwarding path to communicate with each other).

                                  ------- = end-to-end path
   C = Core AS                    - - - - = unused links
   * = source/destination AS      ------> = direction of beaconing


          Core                        Core                  Core
      ---------->                 ---------->           ---------->
     .-.       .-.               .-.       .-.         .-.       .-.
+-- ( C )-----( C ) --+     +-- ( C )-----(C/*)       (C/*)-----(C/*)
|    `+'       `+'    |     |    `+'       `-'         `-'       `-'
|     |    1a   |     |     |     |     1b                   1c
|     |         |     |     |     |
|     |         |     |     |     |
|    .+.       .+.    |     |    .+.                       Core
|   (   )     (   )   |     |   (   )                 -------------->
|    `+'       `+'    |     |    `+'                        .-.
|     |         |     |     |     |                   +----( C )----+
|     |         |     |     |     |                   |     `-'     |
|     |         |     |     |     |                   |             |
|    .+.       .+.    |     |    .+.                 .+.     1d    .+.
+-> ( * )     ( * ) <-+     +-> ( * )               (C/*)         (C/*)
     `-'       `-'               `-'                 `-'           `-'



          .-.                      .-.                   .-.
+--   +--( C )--+   --+      +--  (C/*)        +--    - ( C ) -    --+
|     |   `-'   |     |      |     `+'         |     |   `-'   |     |
|     |         |     |      |      |          |                     |
|     |    2a   |     |      |  2b  |          |     |    3a   |     |
|     |         |     |      |      |          |                     |
|    .+.       .+.    |      |     .+.         |    .+.       .+.    |
|   (   )     (   )   |      |    (   )        |   (   #-----#   )   |
|    `+'       `+'    |      |     `+'         |    `+'  Peer `+'    |
|     |         |     |      |      |          |     |         |     |
|     |         |     |      |      |          |     |         |     |
|     |         |     |      |      |          |     |         |     |
|    .+.       .+.    |      |     .+.         |    .+.       .+.    |
+-> ( * )     ( * ) <-+      +->  ( * )        +-> ( * )     ( * ) <-+
     `-'       `-'                 `-'              `-'       `-'


          Core                                    Core
      ---------->                              ---------->
     .-.       .-.             .-.            .-.       .-.        .-.
 +--( C )- - -( C )--+  +---- ( C ) ----+    ( C )- - -( C )  +-- ( C )
 |   `+'       `+'   |  |      `+'      |     `+'       `+'   |    `+'
 |         3b        |  |           4a  |          4b         |  5
 |    |         |    |  |       |       |      |         |    |     |
 |                   |  |               |                     |
 |   .+.       .+.   |  |      .+.      |      + - --. - +    |    .+.
 |  (   #-----#   )  |  |   +-(   )-+   |      +--(   )--+    |   ( * )
 |   `+'  Peer `+'   |  |   |  `-'  |   |      |   `-'   |    |    `+'
 |    |         |    |  |   |       |   |      |         |    |     |
 |    |         |    |  |   |       |   |      |         |    |     |
 |    |         |    |  |   |       |   |      |         |    |     |
 |   .+.       .+.   |  |  .+.     .+.  |     .+.       .+.   |    .+.
 +->( * )     ( * )<-+  +>( * )   ( * )<+    ( * )     ( * )  +-> ( * )
     `-'       `-'         `-'     `-'        `-'       `-'        `-'
Figure 1: Illustration of valid path-segment combinations. Each node represents a SCION Autonomous System.

Valid path-segment combinations:

  • Communication through core ASes:

    • Core-segment combination (Cases 1a, 1b, 1c, 1d in Figure 1): The up- and down-segments of source and destination do not have an AS in common. In this case, a core-segment is REQUIRED to connect the source's up- and the destination's down-segment (Case 1a). If either the source or the destination AS is a core AS (Case 1b) or both are core ASes (Cases 1c and 1d), then no up- or down-segment(s) are REQUIRED to connect the respective AS(es) to the core-segment.

    • Immediate combination (Cases 2a, 2b in Figure 1): The last AS on the up-segment (which is necessarily a core AS) is the same as the first AS on the down-segment. In this case, a simple combination of up- and down-segments creates a valid forwarding path. In Case 2b, only one segment is required.

  • Peering shortcut (Cases 3a and 3b): A peering link exists between the up- and down-segment. The extraneous path segments to the core are cut off. Note that the up- and down-segments do not need to originate from the same core AS and the peering link could also be traversing to a different ISD.

  • AS shortcut (Cases 4a and 4b): The up- and down-segments intersect at a non-core AS below the ISD core, thus creating a shortcut. In this case, a shorter path is made possible by removing the extraneous part of the path to the core. Note that the up- and down-segments do not need to originate from the same core AS and can even be in different ISDs (if the AS at the intersection is part of multiple ISDs).

  • On-path (Case 5): In the case where the source's up-segment contains the destination AS or the destination's down-segment contains the source AS, a single segment is sufficient to construct a forwarding path. Again, no core AS is on the final path.

1.5. Path Authorization

The SCION data plane provides path authorization. This property ensures that data packets always traverse the network using path segments that were explicitly authorized by the respective ASes, and prevents endpoints from constructing unauthorized paths or paths containing loops. SCION uses symmetric cryptography in the form of Message Authentication Codes (MACs) to authenticate the information encoded in hop fields. Such MACs are verified by routers at forwarding. For a detailed specification, see Section 4.

3. Life of a SCION Data Packet

This section gives a high-level description of the life cycle of a SCION packet: How it is crafted at its source endpoint, passes through a number of routers, and finally reaches its destination endpoint. It is assumed that both source and destination are native SCION endpoints (i.e., they both run a native SCION network stack).

To keep it brief, the example illustrates an intra-ISD case, i.e., all communication happens within a single ISD. As the sample ISD only consists of one core AS, the end-to-end path only includes an up-path and down-path segment. In the case of inter-ISD forwarding, the complete end-to-end path from source endpoint to destination endpoint would always require a core-path segment, too. But this makes no difference for the forwarding process, which works the same in an intra- and inter-ISD context. We therefore abstain from describing the inter-ISD forwarding.

3.1. Description 

                    +--------------------+
                    |                    |
                    |        AS 1        |
                    |                    |
                    |                    |
                    |     198.51.100.4 .-+. i1b (1-1,198.51.100.17)
                    |          +------( R3 )---+
                   .+-.        |       `-+'    |
          +-------( R2 )-------+         |     |
          |    i1a `+-' 198.51.100.1     |     |
          |         |                    |     |
          |         +--------------------+     | (1-3,198.51.100.18)
          |                                    | i3a
          |                                   .+-.
    i2a .-+.                          +------( R4 )--------+
+------( R1 )--------+                |       `-+'         |
|       `-+'         |                |         |192.0.2.34|
|         |203.0.113.17               |         |          |
|         |          |                |         |    AS 3  |
|         |    AS 2  |                |         |          |
|         |          |                |       +---+        |
|       +---+        |                |       | B |        |
|       | A |        |                |       +---+        |
|       +---+        |                |   1-3,192.0.2.7    |
|  1-2,203.0.113.6   |                |                    |
|                    |                +--------------------+
+--------------------+
Figure 17: Sample topology to illustrate the life cycle of a SCION packet. AS 1 is the core AS of ISD 1, and AS 2 and AS 3 are non-core ASes of ISD 1.

Based on the network topology in Figure 17 above, this example shows the path of a SCION packet sent from source endpoint A to destination endpoint B, and how it will be processed by each router on the path. This is done by means of simplified snapshots of the packet header after each such processing step. These snapshots, which are depicted in tables, show the most relevant information of the header, i.e., the SCION path and IP encapsulation for local communication.

3.2. Creating an End-to-End SCION Forwarding Path

In this example, source endpoint A in AS 2 wants to send a data packet to destination endpoint B in AS 3. Both AS 2 and AS 3 are part of ISD 1. To create an end-to-end SCION forwarding path, source endpoint A first requests its own AS-2 control service for up-segments to the core AS in its ISD. The AS-2 control service will return up-segments from AS 2 to the ISD core. Endpoint A will also query its AS-2 control service for a down-segment from its ISD core AS to AS 3, in which destination endpoint B is located. The AS-2 control service (possibly after connecting to the core control service) will return down-segments from the ISD core down to AS 3.

Note: For more details on the lookup of path segments, see the section "Path Lookup" in the Control Plane specification ([I-D.scion-cp]).

Based on its own selection criteria, source endpoint A selects the up-segment (0,i2a)(i1a,0) and the down-segment (0,i1b)(i3a,0) from the path segments returned by its own AS-2 control service. The path segments consist of hop fields that carry the ingress and egress interfaces of each AS (e.g., i2a, i1a, ...), as described in detail in Section 2.1 - (x,y) represents one hop field.

To obtain an end-to-end forwarding path from the source AS to the destination AS, source endpoint A combines the two path segments into the resulting SCION forwarding path, which contains the two info fields IF1 and IF2 and the hop fields (0,i2a), (i1a,0), (0,i1b), and (i3a,0).

Note: As this brief sample path does not contain a core-segment, the end-to-end path only consists of two path segments.

Source endpoint A now adds this end-to-end forwarding path to the header of the packet that A wants to send to destination endpoint B, and starts transferring the packet. The following section describes what happens with the SCION packet header on the packet's way from A to B.

3.3. Step-by-Step Explanation

This section explains the packet header modifications at each router, based on the network topology in Figure 17 above. Each step includes a table that represents a simplified snapshot of the packet header at the end of this specific step. Regarding the notation used in the figure/tables, each SRC and DST entry should be read as router (or endpoint) followed by its address. The current info field (with metadata on the current path segment) in the SCION header is depicted italic/cursive in the tables. The current hop field, representing the current AS, is shown bold. The snapshot tables also include references to IP/UDP addresses.

Note: In this context, a border router is called ingress border router when it refers to an entrance border router to an AS, as seen from the direction of travel of the SCION packet. So in the context here, the ingress border router is the (packet) incoming border router. A border router is called egress border router when it refers to an exit border router of an AS, as seen from the direction of travel of the SCION packet. So in this context, the egress border router is the (packet) leaving border router.

  • Step 1
    A->R1: The SCION-enabled source endpoint A in AS 2 creates a new SCION packet destined for destination endpoint B in AS 3, with payload P. Endpoint A sends the packet (for the chosen forwarding path) to the next SCION router as provided by its control service, which is in this case R1. A encapsulates the SCION packet into an underlay UDP/IPv4 header for the local delivery to R1, utilizing AS 2's internal routing protocol. The current info field is IF1. Upon receiving the packet, R1 will forward the packet on the egress interface that endpoint A has included into the first hop field of the SCION header.

Table 6: Snapshot header - step 1
A -> R1  
SCION SRC = 1-2,203.0.113.6 (source endpoint A)
  DST = 1-3,192.0.2.7 (dest. endpoint B)
  PATH =
  - IF1 (0,i2a) (i1a,0)
  - IF2 (0,i1b) (i3a,0)
UDP PS = 30041, PD = 30041
IP SRC = 203.0.113.6 (endpoint A)
  DST = 203.0.113.17 (router R1)
Link layer SRC=A, DST=R1

  • Step 2
    R1->R2: Router R1 inspects the SCION header and considers the relevant info field of the specified SCION path, which is the info field indicated by the current info field pointer. In this case, it is the first info field IF1. The current hop field is the first hop field (0,i2a), which instructs router R1 to forward the packet on its interface i2a. After reading the current hop field, R1 moves the pointer forward by one position to the second hop field (i1a,0). Note that, at this point, no underlay IP header is necessary, since the routers R1 and R2 are directly connected over layer 2.

    Note: Although technically there is no need for a UDP/IP underlay if two routers are directly connected, the SCION implementation always uses a UDP/IP underlay in practice. This is to enable a common interface for all routers.

Table 7: Snapshot header - step 2
R1 -> R2  
SCION SRC = 1-2,203.0.113.6 (source endpoint A)
  DST = 1-3,192.0.2.7 (dest. endpoint B)
  PATH =
  - IF1 (0,i2a) (i1a,0)
  - IF2 (0,i1b) (i3a,0)
Link layer SRC=R1, DST=R2

  • Step 3
    R2->R3: When receiving the packet, router R2 of core AS 1 checks whether the packet has been received through the ingress interface i1a as specified by the current hop field. Otherwise, the packet is dropped by router R2. The router notices that it has consumed the last hop field of the current path segment, and hence moves the pointer from the current info field to the next info field IF2. The corresponding current hop field is (0,i1b), which contains egress interface i1b. R2 maps the i1b interface ID to egress router R3, it therefore encapsulates the SCION packet inside an intra-AS underlay IP packet with the address of R3 as the underlay destination.

Table 8: Snapshot header - step 3
R2 -> R3  
SCION SRC = 1-2,203.0.113.6 (source endpoint A)
  DST = 1-3,192.0.2.7 (dest. endpoint B)
  PATH =
  - IF1 (0,i2a) (i1a,0)
  - IF2 (0,i1b) (i3a,0)
UDP PS = 30041, PD = 30041
IP SRC = 198.51.100.1 (router R2)
  DST = 198.51.100.4 (router R3)
Link layer SRC=R2, DST=R3

  • Step 4
    R3->R4: Router R3 inspects the current hop field in the SCION header, uses interface i1b to forward the packet to its neighbor SCION router R4 of AS 3, and moves the current hop-field pointer forward. It adds an IP header to reach R4.

Table 9: Snapshot header - step 4
R3 -> R4  
SCION SRC = 1-2,203.0.113.6 (source endpoint A)
  DST = 1-3,192.0.2.7 (dest. endpoint B)
  PATH =
  - IF1 (0,i2a) (i1a,0)
  - IF2 (0,i1b) (i3a,0)
UDP PS = 30041, PD = 30041
IP SRC = 1-1,198.51.100.17 (router R3)
  DST = 1-3,198.51.100.18 (router R4)
Link layer SRC=R3, DST=R4

  • Step 5
    R4->B: SCION router R4 first checks whether the packet has been received through the ingress interface i3a as specified by the current hop field. R4 will then also realize, based on the fields CurrHF and SegLen in the SCION header, that the packet has reached the last hop in its SCION path. Therefore, instead of stepping up the pointers to the next info or hop field, router R4 inspects the SCION destination address and extracts the endpoint address 192.0.2.7. It creates a fresh underlay UDP/IP header with this address as destination and with itself as source. The intra-domain forwarding can now deliver the packet to destination endpoint B.

Table 10: Snapshot header - step 5
R4 -> B  
SCION SRC = 1-2,203.0.113.6 (source endpoint A)
  DST = 1-3,192.0.2.7 (dest. endpoint B)
  PATH =
  - IF1 (0,i2a) (i1a,0)
  - IF2 (0,i1b) (i3a,0)
UDP PS = 30041, PD = 30041
IP SRC = 192.0.2.34 (router R4)
  DST = 192.0.2.7 (endpoint B)
Link layer SRC=R4, DST=B

When destination endpoint B wants to respond to source endpoint A, it can just swap the source and destination addresses in the SCION header, reverse the SCION path, and set the pointers to the info and hop fields at the beginning of the reversed path (see also Section 2.2.3.4).

4. Path Authorization

Path authorization guarantees that data packets always traverse the network along paths segments authorized by all on-path ASes in the control plane. In contrast to the IP-based Internet, where forwarding decisions are made by routers based on locally stored information, SCION routers base their forwarding decisions purely on the forwarding information carried in the packet header and set by endpoints.

SCION uses cryptographic mechanisms to efficiently provide path authorization. The mechanisms are based on symmetric cryptography in the form of Message Authentication Codes (MACs) in the data plane to secure forwarding information encoded in hop fields. This chapter first explains how hop field MACs are computed, then how they are validated as they traverse the network.

4.1. Authorizing Segments through Chained MACs

When authorizing SCION PCBs and path segments in the control plane and forwarding information in the data plane, an AS authenticates not only its own hop information but also an aggregation of all upstream hops. This section describes how this works.

4.1.1. Hop Field MAC Computation

The MAC in the hop fields of a SCION path has two purposes:

  • Preventing malicious endpoints from illegally adding, removing, or reordering hops within a path segment created during beaconing in the control plane. In particular, preventing path splicing, i.e. the combination of parts of different valid path segments into a new, unauthorized, path segment.

  • Authentication of the information contained in the hop field itself, in particular the ExpTime, ConsIngress, and ConsEgress.

To fulfill the above purposes, the MAC for the hop field of ASi includes both the components of the current hop field HFi and an aggregation of the path segment identifier and all preceding hop fields/entries in the path segment. The aggregation is a 16-bit XOR-sum of the path segment identifier and the hop field MACs.

When originating a path-segment construction beacon PCB in the control plane, a core AS chooses a random 16-bit value as segment identifier SegID for the path segment and includes it in the PCB's Segment Info component. In the control plane, each ASi on the path segment computes the MAC for the current hop HFi, based on the value of SegID and the MACs of the preceding hop entries. Here, the full XOR-sum is computed explicitly.

For high-speed packet processing in the data plane, computing even cheap operations such as the XOR-sum over a variable number of inputs is complicated, in particular for hardware router implementations. To avoid this overhead for the MAC-chaining in path authorization in the data plane, the XOR-sum is tracked incrementally for each (of the up to three) path segments in a path, as a separate, updatable accumulator field Acc. The routers update the accumulator field Acc by adding/subtracting only a single 16-bit value each.

When combining path segments to create a path to the destination endpoint, the source endpoint MUST also initialize the value of accumulator field Acc for each path segment. The Acc field MUST contain the correct XOR-sum of the path segment identifier and preceding hop field MACs expected by the first router that is traversed.

The aggregated 16-bit path segment identifier and preceding MACs prevents the splicing parts of different path segments, unless there is a per-chance collision of the Acc value among compatible path segments in on AS. See Section 5.1.3 for more details.

In the following, the computation of the hop field MAC as well as the accumulator field Acc is explained.

4.1.1.1. Hop Field MAC - Definition

  • Consider a path segment with "n" hops, containing ASes AS0, ... , ASn-1, with forwarding keys K0, ... , Kn-1 in this order.

  • AS0 is the core AS that created the PCB representing the path segment and that added a random initial 16-bit segment identifier SegID to the Segment Info field of the PCB.

The MACi of the hop field of ASi is now calculated as:

MACi =
Cki (SegID XOR MAC0 [:2] ... XOR MACi-1 [:2], Timestamp, ExpTimei, ConsIngressi, ConsEgressi)

where

  • ki = The forwarding key k of the current ASi

  • Cki (...) = Cryptographic checksum C over (...) computed with forwarding key ki

  • SegID = The random initial 16-bit segment identifier set by the core AS when creating the corresponding PCB

  • XOR = The bitwise "exclusive or" operation

  • MACi [:2] = The hop field MAC for ASi, truncated to 2 bytes

  • Timestamp = The timestamp set by the core AS when creating the corresponding PCB

  • ExpTimei, ConsIngressi, ConsEgressi = The content of the hop field HFi

Thus, the current MAC is based on the XOR-sum of the truncated MACs of all preceding hop fields in the path segment as well as the path segment's SegID. In other words, the current MAC is chained to all preceding MACs. In order to effectively prevent path-splicing, the cryptographic checksum function used MUST ensure that the truncation of the MACs is non-degenerate and roughly uniformly distributed (see Section 4.1.1.4).

4.1.1.2. Accumulator Acc - Definition

The accumulator Acci is an updatable counter introduced in the data plane to avoid the overhead caused by MAC-chaining for path authorization. This is achieved by incrementally tracking the XOR-sum of the previous MACs as a separate, updatable accumulator field Acc, which is part of the path segment's info field InfoField in the packet header (see also Section 2.2.3.2.3). Routers update this field by adding/subtracting only a single 16-bit value each.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|r r r r r r P C|      RSV      |             Acc               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Timestamp                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: The Info Field of a specific path segment in the packet header, with the updatable accumulator field `Acc`.

This is how it works:

Section 4.1.1.1 defines MACi as follows:

MACi =
Cki (SegID XOR MAC0 [:2] ... XOR MACi-1 [:2], Timestamp, ExpTimei, ConsIngressi, ConsEgressi)

In the data plane, the expression SegID XOR MAC_0 [:2] ... XOR MAC_i-1 [:2] is replaced by Acci. This results in the following alternative procedure for the computation of MACi used in the data plane:

MACi = Cki (Acci, Timestamp, ExpTimei, ConsIngressi, ConsEgressi)

During forwarding in the data plane, each ASi updates the Acc field in the packet header, such, that it contains the correct input value of the Accumulator Acc for the next AS in the path segment to be able to calculate the MAC over its hop field. Note that the correct input value of the Acc field depends on the direction of travel.

The value of the accumulator Acci+1 is calculated based on the following definition (in the direction of beaconing):

Acci+1 = Acci XOR MACi [:2]

  • XOR = The bitwise "exclusive or" operation

  • MACi [:2] = The hop field MAC for the current ASi, truncated to 2 bytes

4.1.1.3. Default Hop Field MAC Algorithm

The algorithm used to compute the hop field MAC is an AS-specific choice. The operator of an AS can freely choose any MAC algorithm without outside coordination. However, the control service and routers of the AS do need to agree on the algorithm used. All control service and router implementations MUST support the Default Hop Field MAC algorithm described below.

The default MAC algorithm is AES-CMAC ([RFC4493]) truncated to 48-bits, computed over the info field and the first 6 bytes of the hop field, with flags and reserved fields zeroed out. The input is padded to 16 bytes. The first 6 bytes of the AES-CMAC output are used as resulting hop field MAC. Figure 19 below shows the layout of the input data to calculate the hop field MAC.

0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----+
|               0               |           Acc                 |  Info|
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| Field|
|                           Timestamp                           |      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| -----+
|       0       |    ExpTime    |          ConsIngress          |   Hop|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Field|
|          ConsEgress           |               0               |      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----+
Figure 19: Input data to calculate the hop field MAC for the default hop-field MAC algorithm
4.1.1.4. Alternative Hop Field MAC Algorithms

For alternative algorithms, the following requirements MUST all be met: - The hop field MAC field is computed as a function of the secret forwarding key, the Acc, Timestamp fields of the info field and the ExpTime, ConsIngress and ConsEgress fields of the hop field. Function is used in the mathematical sense, that is, for any values of these inputs there is exactly one result. - The algorithm returns an unforgable 48-bit value. Unforgable specifically means "existentially unforgable under a chosen message attack" ([CRYPTOBOOK]). Informally, this means an attacker without access to the secret key has no computationally efficient means to create a valid MAC for some attacker chosen input values, even if it has access to an "oracle" providing a valid MAC for any other input values. - The truncation of the result value to the first 2 bytes / 16 bits of the result value: - is not degenerate, i.e. any small change in any input value SHOULD have an "avalanche effect" on these bits, and - is roughly uniformly distributed when considering all possible input values.

This additional requirment is naturally satisfied for MAC algorithms based on typical block ciphers or hash algorithms. It ensures that the MAC chaining via the Acc field is not degenerate.

4.2. Path Initialization and Packet Processing

As is described in Section 2.1, the path header of the data-plane packets only contains a sequence of info fields and hop fields without any additional data from the corresponding PCBs. Also, the SCION path does not contain any AS numbers (except for the source and destination ASes), and there is no field explicitly defining the type of each segment (up, core, or down). This chapter describes the required steps for the source endpoint and each SCION router to ensure that a data packet only traverses authorized segments. The chapter first specifies the initialization of a path at the source endpoint, followed by the steps that the SCION routers needs to perform when a data-plane packet traverses an AS on its way to the destination.

4.2.1. Initialization at Source Endpoint

The source endpoint needs to initialize a path correctly for the SCION routers to be able to verify the hop fields in the data plane. To this end, the source endpoint MUST perform the following steps:

  1. Combine the preferred end-to-end path from the path segments obtained during path lookup.

  2. Extract the info fields and hop fields from the different path segments that together build the end-to-end path to the destination endpoint. Then insert the relevant information from the path segments' info and hop fields into the corresponding InfoFields and Hopfields, respectively, in the data packet header.

  3. Each 8-byte info field InfoField in the packet header contains the updatable Acc field as well as a Peering flag P and a Construction Direction flag C (see also Section 2.2.3.2.3). As a next step in the path initialization process, the source MUST correctly set the flags and the Acc field of all InfoFields included in the path, according to the following rules:

    Note: As already stated above, the type of segment is not visible directly in the forwarding path but can be inferred from flags and other information. See also Section 4.2.2.

    • The Construction Direction flag C MUST be set to "1" whenever the corresponding segment is traversed in construction direction, i.e., for down-path segments and potentially for core-segments. It MUST be set to "0" for up-path segments and "reversed" core-segments.

    • The Peering flag P MUST be set to "1" for up- and down-segments if, and only if, the path contains a peering hop field.

    • The field Acc field is an updatable field. It is used to compute the MAC over the current hop field. The value of the field Acc field corresponds to the value of the Accumulator Acci for ASi. It is initialized based on the location of the sender in relation to path construction.

    The following InfoField settings are possible, based on the following possible use cases:

    • Use case 1
      The path segment is traversed in construction direction and includes no peering hop field. It starts at the i-th AS of the full segment discovered in beaconing. In this case:

      • The Peering flag P = "0"

      • The Construction Direction flag C = "1"

      • The value of the Acc = Acci. For more details, see Section 4.1.1.2.

    • Use case 2
      The path segment is traversed in construction direction and includes a peering hop field (which is the first hop field of the segment). It starts at the i-th AS of the full segment discovered in beaconing. In this case:

      • The Peering flag P = "1"

      • The Construction Direction flag C = "1"

      • The value of the Acc = Acci+1. For more details, see Section 4.1.1.2.

    • Use case 3
      The path segment is traversed against construction direction. The full segment has "n" hops. In this case:

      • The Peering flag P = "0" or "1" (depending on whether the last hop field in the up-segment is a peering hop field)

      • The Construction Direction flag C = "0"

      • The value of the Acc = Accn-1. This is because seen from the direction of beaconing, the source endpoint is the last AS in the path segment. For more details, see Section 4.1.1.1 and Section 4.1.1.2.

  4. Besides setting the flags and the Acc field, the source endpoint MUST also set the pointers in the CurrInf and CurrHF fields of the Path Meta Header PathMetaHdr (see Section 2.2.3.2.1). As the source endpoint builds the starting point of the forwarding, both pointers MUST be set to "0".

4.2.2. Processing at Routers

During forwarding, each ASi verifies the path contained in the packet header with the help of the current value of the MAC in the current hop field, and the current value of the Accumulator in the Acc field of the current info field. Additionally, each AS has to correctly set the value of the Accumulator in the Acc field for the next AS to be able to verify its hop field. The exact operations differ based on the location of the AS on the path.

The processing of SCION packets for ASes where a peering link is crossed between path segments is special cased. A path containing a peering link contains exactly two path segments, one against construction direction (up) and one in construction direction (down). On the path segment against construction direction (up), the peering hop field is the last hop of the segment. In construction direction (down), the peering hop field is the first hop of the segment.

The following sections describe the tasks to be performed by the ingress and egress border router of each on-path AS. Each operation is described from the perspective of ASi, where i belongs to [0 ... n-1], and n == the number of ASes in the path segment (counted from the first AS in the beaconing direction).

Note: In this context, a border router is called ingress border router when it refers to an entrance border router to an AS, as seen from the direction of travel of the SCION packet. So in the context here, the ingress border router is the (packet) incoming border router. A border router is called egress border router when it refers to an exit border router of an AS, as seen from the direction of travel of the SCION packet. So in this context, the egress border router is the (packet) leaving border router.

The following figure provides a simplified representation of the processing at routers both in construction direction and against construction direction.

                              .--.
                             ( RR )  = Router
Processing in                 `--'
construction
direction

      1. Verify MAC of AS1          1. Verify MAC of AS2
      2. Update Acc for AS2         2. Update Acc for AS3
                 |                            |
>>>--------------o----------------------------o---------------------->>>

+-------------+  |           +-------------+  |          +-------------+
|             |              |             |             |             |
|           .--. |          .--.         .--. |         .--.           |
|   AS1    ( RR )o---------( RR )  AS2  ( RR )o--------( RR )  AS3     |
|           `--' |          `--'         `--' |         `--'           |
|             |              |             |             |             |
+-------------+  |           +-------------+  |          +-------------+

                 |                            |
<<<--------------o----------------------------o----------------------<<<
                 |                            |
      1. Update Acc for AS1         1. Update Acc for AS2
      2. Verify MAC of AS1          2. Verify MAC of AS2

                                                      Processing against
                                                            construction
                                                               direction
Figure 20: A simplified representation of the processing at routers in both directions.
4.2.2.1. Steps Ingress Border Router

This section describes the steps that a SCION ingress border router MUST perform when it receives a SCION packet.

  1. Check that the interface through which the packet was received is equal to the ingress interface in the current hop field. If not, the router MUST drop the packet.

  2. Check if the current hop field is expired or originated in the future. That is, the current info field MUST NOT have a timestamp in the future, as defined in Section 2.2.3.2.3. If either is true, the router MUST drop the packet.

  3. The next steps depend on the direction of travel and whether this segment includes a peering hop field. Both features are indicated by the settings of the Construction Direction flag C and the Peering flag P in the current info field. Therefore, check the settings of both flags. The following combinations are possible:

    • The packet traverses the path segment in construction direction (C = "1" and P = "0" or "1"). In this case, proceed with step 4.

    • The packet traverses the path segment against construction direction (C = "0"). The following use cases are possible:

      • Use case 1
        The path segment includes no peering hop field (P = "0"). In this case, the ingress border router MUST take the following step(s):

        • Compute the value of the Accumulator Acc as follows:

          Acc = Acci+1 XOR MACi
          where
          Acci+1 = the current value of the field Acc in the current info field
          MACi = the value of MACi in the current hop field representing ASi

          Note: In the case described here the packet travels against direction of beaconing. That is, the packet comes from ASi+1 and is going to enter ASi. This means that the Acc field of this incoming packet represents the value of Accumulator Acci+1. However, to compute the MACi for the current ASi, we need the value of Accumulator Acci (see Section 4.1.1.2). Because the border router knows that the formula for Acci+1 = Acci XOR MACi [:2] (see also Section 4.1.1.2), and because the values of Acci+1 and MACi are known, the router will be able to recover the value Acci based on the just-mentioned formula for Acc.

        • Replace the current value of the field Acc in the current info field with the newly calculated value of Acc.

        • Compute the MACVerifyi over the hop field of the current ASi. For this, use the formula in Section 4.1.1.1, but replace SegID XOR MAC_0[:2] ... XOR MAC_i-1 [:2] in the formula with the value of the accumulator Acc as just set in the Acc field in the current info field.

        • Check that the MACi in the current hop field matches the just-calculated MACVerifyi. If yes, it is fine. Otherwise, drop the packet.

        • Check whether the current hop field is the last hop field in the path segment. For this, look at the value of the current SegLen and other metadata in the path meta header. If yes, increment both CurrInf and CurrHF in the path meta header. Proceed with step 4.

      • Use case 2
        The path segment includes a peering hop field (P = "1"), but the current hop is not the peering hop, that is, the current hop field is not the last hop field of the segment, seen from the direction of travel - this can be determined by looking at the value of the current SegLen and other metadata in the path meta header. In this case, the ingress border router needs to perform the steps previously described for the path segment without peering hop field. However, the border router MUST NOT increment CurrInf and MUST NOT increment CurrHF in the path meta header. Proceed with step 4.

      • Use case 3
        The path segment includes a peering hop field (P = "1"), and the current hop field is the peering hop field. This would be the case if the current hop field is the last hop field of the segment, seen from the direction of travel - to find out whether this is true, check the value of the current SegLen and other metadata in the path meta header. In this case, the ingress border router MUST take the following step(s):

        • Compute MACPeeri. For this, use the formula in Section 4.1.2, but replace SegID XOR MAC_0[:2] ... XOR MAC_i [:2] in the formula with the value of the accumulator Acc as set in the Acc field in the current info field (this is the value of the accumulator Acc as it comes with the packet).

        • Check that the MACi in the current hop field matches the just-calculated MACPeeri. If yes, it is fine. Otherwise, drop the packet.

        • Increment both CurrInf and CurrHF in the path meta header. Proceed with step 4.

  4. Forward the packet to the egress border router (based on the egress interface ID in the current hop field) or to the destination endpoint, if this is the destination AS.

Note: For more information on the path meta header, see Section 2.2.3.2.1.

4.2.2.2. Steps Egress Border Router

This section describes the steps that a SCION egress border router MUST perform when it receives a SCION packet.

  1. Parse the SCION packet.

  2. The next steps depend on the direction of travel and whether this segment includes a peering link. Both features are indicated by the settings of the Construction Direction flag C and the Peering flag P in the currently valid info field. Therefore, first check the settings of both flags. The following use cases are possible:

    • Use case 1
      The packet traverses the path segment in construction direction (C = "1"). The path segment either includes no peering hop field (P = "0"), or the path segment does include a peering hop field (P = "1"), but the current hop is not the peering hop, that is, the current hop field is not the first hop field of the segment, seen from the direction of travel. To check whether this is true, look at the value of the current SegLen and other metadata in the path meta header. In this case, the egress border router MUST take the following step(s):

      • Compute MACVerifyi over the hop field of the current ASi. For this, use the formula in Section 4.1.1.1, but replace SegID XOR MAC_0[:2] ... XOR MAC_i-1 [:2] in the formula with the value of the accumulator Acc as set in the Acc field in the current info field.

      • Check that the just-calculated MACVerifyi matches MACi in the hop field of the current ASi. If yes, it is fine. Otherwise, drop the packet.

      • Compute the value of Acci+1. For this, use the formula in Section 4.1.1.2. Replace Acci in the formula with the current value of the accumulator Acc as set in the Acc field of the current info field.

      • Replace the value of the Acc field in the current info field with the just-calculated value of Acci+1.

      • Proceed with step 3.

    • Use case 2
      The packet traverses the path segment in construction direction (C = "1"). The path segment includes a peering hop field (P = "1"), and the current hop field is the peering hop field. This would be the case if the current hop field is the first hop field of the segment, seen from the direction of travel - to find out whether this is true, check the value of the current SegLen and other metadata in the path meta header. In this case, the egress border router MUST take the following steps:

      • Compute MACPeeri. For this, use the formula in Section 4.1.2, but replace SegID XOR MAC_0 [:2] ... XOR MAC_i [:2] with the value in the Acc field of the current info field.

      • Check that the MACi in the hop field of the current ASi matches the just-calculated MACPeeri. If yes, it is fine - proceed with step 3. Otherwise, drop the packet.

    • Use case 3
      The packet traverses the path segment against construction direction (C = "0" and P = "0" or "1"). In this case, proceed with the next step, step 3.

  3. Increment CurrHF in the path meta header.

  4. Forward the packet to the neighbor AS.

Note: For more information on the path meta header, see Section 2.2.3.2.1.

4.2.2.3. Effects of Clock Inaccuracy

A PCB originated by a given control service is used to construct data plane paths. Specifically, the timestamp in the Info Field and the expiry time of hop fields are used for hop field MAC computation, see Section 4.1.1, which is used to validate paths at each on-path SCION router. A segment's originating control service and the routers that the segment refers to all have different clocks. Their differences affect the validation process:

  • A fast clock at origination or a slow clock at validation will yield a lengthened expiration time for hops, and possibly an origination time in the future.

  • A slow clock at origination or a fast clock at validation will yield a shortened expiration time for hops, and possibly an expiration time in the past.

This bias comes in addition to a structural delay: PCBs are propagated at a configurable interval (typically, one minute). As a result of this and the way they are iteratively constructed, PCBs with N hops may become available for path construction up to N intervals (so typically N minutes) after origination. This creates a constraint on the expiration of hops. Hops of the minimal expiration time (337.5 seconds - see Section 2.2.3.2.4) would render useless any path segment longer than 5 hops. For this reason, it is unadvisable to create hops with a short expiration time. The norm is 6 hours.

In comparison to these time scales, clock offsets in the order of minutes are immaterial.

Each administrator of SCION control services and routers is responsible for maintaining sufficient clock accuracy. No particular method is assumed by this specification.

5. Security Considerations

This section describes the possible security risks and attacks that SCION's data plane may be prone to, and how these attacks may be mitigated. It first discusses security risks that pertain to path authorization, followed by a section on other forwarding-related security considerations.

5.1. Path Authorization

A central property of the SCION path-aware data plane is path authorization. Path authorization guarantees that data packets always traverse the network along path segments authorized in the control plane by all on-path ASes. This section discusses how an adversary may attempt to violate the path-authorization property, as well as SCION's prevention mechanisms to these attacks; either an attacker can attempt to create unauthorized hop fields, or they can attempt to create illegitimate paths assembled from authentic individual hop fields.

The main protection mechanism here is the hop field MAC (see Section 4.1), authenticating the hop field content, consisting of ingress/egress interface identifiers, creation and expiration timestamp and, virtually, the preceding hop field MACs in the path segment. Recall that each hop field MAC is computed using the respective AS's secret forwarding key, which is shared across the SCION routers and control plane services within each AS.

5.1.1. Forwarding key compromise

For the current default MAC algorithm, AES-CMAC truncated to 48 bits, key recovery attacks from (any number of) known plaintext/MAC combinations is computationally infeasible, as far as publicly known. In addition, the MAC algorithm can be freely chosen by each AS, enabling algorithmic agility for MAC computations. Should a MAC algorithm be discovered to be weak or insecure, each AS can quickly switch to a secure algorithm without the need for coordination with other ASes.

A more realistic risk to the secrecy of the forwarding key is exfiltration from a compromised router or control plane service. An AS can optionally rotate its forwarding key at regular intervals to limit the exposure after a temporary device compromise. However, as is perhaps self-evident, such a key rotation scheme cannot mitigate the impact of an undiscovered, permanent compromise of a device.

When an AS's forwarding key is compromised, an attacker can forge hop field MACs, undermining path authorization. Recall that path segments are checked for validity and policy compliance during the path discovery phase, and during forwarding, routers only validate the MAC and basic validity of the current the hop field. Consequently, creating fraudulent hop fields with valid MACs allows an attacker to bypass most path segment validity checks, and to create path segments that violate the AS's local policy and/or general path segment validity requirements. In particular, an attacker could create paths that include loops (limited by the maximum number of hop fields of a path).

Unless an attacker has access to the forwarding keys of all ASes on the illegitimate path it wants to fabricate, it will need to splice fragments of two legitimate path segments with an illegitimate hop field. For this, it needs to create a hop field with a MAC that fits into the MAC chain expected by the second path segment fragment. The only input that the attacker can vary relatively freely is the 8 bit ExpTime, but the resulting MAC needs to match a specific 16 bit Acc value. While there is a low probability of this working for a specific attempt (1/256), the attack will succeed eventually if the attacker can keep retrying over a longer time period or with many different path segment fragments.

While a forwarding key compromise and the resulting loss of path authorization is a serious degradation of SCION's routing security properties, this does not affect, for example, access control or data security for the hosts in the affected AS. Unauthorized paths are available to the attacker, but the routing of packets from legitimate senders is not affected.

5.1.2. Forging hop field MAC

As a second method to break path authorization is to directly forge a hop field in an online attack, using the router as an oracle to determine the validity of the hop field MAC. The adversary needs to send one packet per guess for verification. For a 6-byte MAC, the adversary would need an expected 247 (~140 trillion) tries to successfully forge the MAC of a single hop field. As the router only checks MACs during the encoded validity period of the hop field, which is limited by the packet header format to at most 24 hours, these tries need to occur in a limited time period. This results in a seemingly infeasible number of ~1.6e9 guesses per second. In the unlikely case that an online brute-force attack succeeds, the obtained hop field can be used until its inevitable expiration after the just mentioned 24 hour limit.

5.1.3. Path Splicing

In a path-splicing attack, an adversary source endpoint takes valid hop fields of multiple path segments and splices them together to obtain a new unauthorized path.

Two candidates path segments for splicing must have at least one AS interface in common as a connection point. The path segments must have the same origination timestamp, as this is directly protected by the hop field MAC. This can occur by chance, or if the two candidate path segments were originated as the same segment that then diverged and converged back. Finally, the hop field MAC protects the 16-bit aggregation of path segment identifier and preceding MACs. For details, see Section 4.1. This MAC chaining prevents splicing even in the case that the AS interface and segment timestamp match.

As the segment identifier and aggregation of preceding MACs is only 16-bits wide, per-chance collision among compatible path segments can occur. With typical network sizes and numbers of paths of today, such collisions might occur rarely. Successful path splicing would allow an attacker to briefly use a path that violates an ASes path policy, e.g. making a special transit link available to a customer AS that is not billed accordingly, or violate general path segment validity requirements. In particular, a spliced path segment could traverse one or multiple links twice. However, creating a loop traversing a link an arbitrary number of times would involve multiple path splices and therefore multiple random collisions happening simultaneously, which is exceedingly unlikely. A wider security margin against path splicing could be obtained by increasing the width of the segment identifier / Acc field, e.g. by extending it into the 8-bit reserved field next to it in the info field.

5.2. On-Path Attacks

When an adversary sits on the path between the source and destination endpoint, it is able to intercept the data packets that are being forwarded. This would allow the adversary to hijack traffic onto a path that is different from the intended one selected by the source endpoint. Possible on-path attacks in the data plane are modifications of the SCION path header and SCION address header. In addition, an on-path adversary can always simply drop packets. This kind of attack is fundamental and generally cannot be prevented. However, in this case, the endpoint can use SCION's path-awareness to immediately select an alternate path if available.

5.2.1. Modification of the Path Header

An on-path adversary could modify the SCION path header, and replace the remaining part of path segments to the destination with different segments. Such replaced segments must include authorized segments, as otherwise the packet would be simply dropped on its way to the destination. The already traversed portion of the current segment and past segments can also be modified by the adversary (for instance, deleting and adding valid and invalid hop fields). On reply packets from the destination, the adversary can transparently revert the changes to the path header again. For instance, if an adversary M is an intermediate AS on the path of a packet from A to B, then M can replace the packet’s past path (leading up to, but not including M). The new path may not be a valid end-to-end path. However, when B reverses the path and sends a reply packet, that packet would go via M, which can then transparently change the invalid path back to the valid path to A. In addition, the endpoint address header can also be modified.

Modifications of the SCION path and address header can be discovered by the destination endpoint by a data integrity protection system. Such a data integrity protection system, loosely analogous to the IPSec Authentication Header, exists for SCION but is out of scope for this document. This is described as the SCION Packet Authentication Option (SPAO) in [CHUAT22].

Moreover, packet integrity protection is not enough if there are two colluding adversaries on the path. These colluding adversaries can forward the packet between them using a different path than selected by the source endpoint: The first on-path attacker remodels the packet header arbitrarily, and the second on-path attacker changes the path back to the original source-selected path, such that the integrity check by the destination endpoint succeeds. To prevent this attack and to defend against multiple on-path adversaries in general, proof of transit is required, which is not in scope for this document.

5.3. Off-Path Attacks

SCION's path-awareness limits the abilities of an off-path adversary to influence forwarding in the data plane. Once a packet is "in flight", it will follow its set route, no matter what an adversary may do. An adversary can attempt to disrupt the connectivity of said path by flooding a link with excessive traffic (see Section 5.4 below). After detecting congestion, the endpoint can switch to another, non-congested path for subsequent packets.

5.4. Volumetric Denial of Service Attacks

An adversary can attempt to disrupt the connectivity of a network path by flooding a link with excessive traffic. In this case, the endpoint can switch to another, non-congested path for subsequent packets.

SCION provides protection against certain reflection-based DoS attacks. Here, the adversary sends requests to a server with the source address set to the address of the victim. The server will send a reply, typically larger than the request, to the victim. As long as the attacker and the victim are located in different ASes, this can be prevented in SCION. The reply packets are simply returned along reversed path to the actual sender, regardless of the source address information. Thus, the reflected will be forwarded to the attackers AS (where it will be discarded because the destination AS does not match).

On the flip side, the path choice of the endpoint may possibly be exploited by an attacker to create intermittent congestion with relatively low send rate; the attacker can abuse the latency differences of the available paths, sending at precisely timed intervals to cause short, synchronized bursts of packets near the victim.

Note that SCION does not protect against two other types of DoS attacks, namely transport protocol attacks and application layer attacks. Such attacks are out of SCION's scope. However, the additional information contained in the SCION header enables more targeted filtering, e.g., by ISD, AS or path length.

6. IANA Considerations

This document has no IANA actions.

The SCION AS and ISD number are SCION-specific numbers. They are currently allocated by Anapaya Systems, a provider of SCION-based networking software and solutions (see Anapaya ISD AS assignments). This task is currently being transitioned from Anapaya to the SCION Association.

7. References

7.1. Normative References

[I-D.scion-cp]
de Kater, C., Rustignoli, N., and S. Hitz, "SCION Control Plane", , <https://datatracker.ietf.org/doc/draft-dekater-scion-controlplane/>.
[I-D.scion-cppki]
de Kater, C., Rustignoli, N., and S. Hitz, "SCION Control-Plane PKI", , <https://datatracker.ietf.org/doc/draft-dekater-scion-pki/>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC2474]
Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, , <https://www.rfc-editor.org/rfc/rfc2474>.
[RFC3168]
Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, , <https://www.rfc-editor.org/rfc/rfc3168>.
[RFC4493]
Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, , <https://www.rfc-editor.org/rfc/rfc4493>.
[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, , <https://www.rfc-editor.org/rfc/rfc5280>.
[RFC5880]
Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, , <https://www.rfc-editor.org/rfc/rfc5880>.
[RFC5881]
Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI 10.17487/RFC5881, , <https://www.rfc-editor.org/rfc/rfc5881>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8200]
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <https://www.rfc-editor.org/rfc/rfc8200>.

7.2. Informative References

[CHUAT22]
Chuat, L., Legner, M., Basin, D., Hausheer, D., Hitz, S., Mueller, P., and A. Perrig, "The Complete Guide to SCION", ISBN 978-3-031-05287-3, , <https://doi.org/10.1007/978-3-031-05288-0>.
[CRYPTOBOOK]
Boneh, D. and V. Shoup, "A Graduate Course in Applied Cryptography", , <https://toc.cryptobook.us/>.
[RFC1122]
Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, , <https://www.rfc-editor.org/rfc/rfc1122>.
[RFC1918]
Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. J., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, , <https://www.rfc-editor.org/rfc/rfc1918>.
[RFC2711]
Partridge, C. and A. Jackson, "IPv6 Router Alert Option", RFC 2711, DOI 10.17487/RFC2711, , <https://www.rfc-editor.org/rfc/rfc2711>.
[RFC9217]
Trammell, B., "Current Open Questions in Path-Aware Networking", RFC 9217, DOI 10.17487/RFC9217, , <https://www.rfc-editor.org/rfc/rfc9217>.
[RFC9473]
Enghardt, R. and C. Krähenbühl, "A Vocabulary of Path Properties", RFC 9473, DOI 10.17487/RFC9473, , <https://www.rfc-editor.org/rfc/rfc9473>.
[SCMP]
Anapaya, ETH, and SCION, "SCMP Documentation", , <https://docs.scion.org/en/latest/protocols/scmp.html>.

Acknowledgments

Many thanks go to Matthias Frei (SCION Association), Juan A. Garcia Prado (ETH Zurich), Kevin Meynell (SCION Association) and Jean-Christophe Hugly (SCION Association) for reviewing this document. We are also very grateful to Adrian Perrig (ETH Zurich), for providing guidance and feedback about each aspect of SCION. Finally, we are indebted to the SCION development teams of Anapaya and ETH Zurich, for their practical knowledge and for the documentation about the SCION Data Plane, as well as to the authors of [CHUAT22] - the book is an important source of input and inspiration for this draft.

Assigned SCION Protocol Numbers

This appendix lists the assigned SCION protocol numbers.

Considerations

SCION attempts to take the IANA's assigned Internet protocol numbers into consideration. Widely employed protocols have the same protocol number as the one assigned by IANA. SCION specific protocol numbers start at 200.

The protocol numbers are used in the SCION header to identify the upper layer protocol.

SCMP refers to the SCION Control Message Protocol, used for diagnostics and error messages. Support for this protocol is OPTIONAL. A work-in-progress specification is available at: [SCMP].

Assignment

Table 11: The assigned SCION protocol numbers
Decimal Keyword Protocol
0-5   Unassigned
6 TCP/SCION Transmission Control Protocol over SCION
7-16   Unassigned
17 UDP/SCION User Datagram Protocol over SCION
18-199   Unassigned
200 HBH SCION Hop-by-Hop Options
201 E2E SCION End-to-End Options
202 SCMP SCION Control Message Protocol
203 BFD/SCION BFD over SCION
204-252   Unassigned
253   Use for experimentation and testing
254   Use for experimentation and testing
255   Reserved

Authors' Addresses

Corine de Kater
SCION Association
Nicola Rustignoli
SCION Association
Samuel Hitz
Anapaya Systems