SCION Data Plane
draft-dekater-scion-dataplane-03
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| Authors | Corine de Kater , Nicola Rustignoli , Jean-Christophe Hugly , Samuel Hitz | ||
| Last updated | 2024-10-19 (Latest revision 2024-07-08) | ||
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draft-dekater-scion-dataplane-03
Network Working Group C. de Kater
Internet-Draft N. Rustignoli
Intended status: Informational J. C. Hugly
Expires: 22 April 2025 SCION Association
S. Hitz
Anapaya Systems
19 October 2024
SCION Data Plane
draft-dekater-scion-dataplane-03
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 provides
a detailed specification of the SCION data packet format as well as
the structure of the SCION header. SCION also supports extension
headers, which are additionally described. The document continues
with the life cycle of a SCION packet while traversing the SCION
Internet, 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.
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Status of This Memo
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Copyright Notice
Copyright (c) 2024 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Conventions and Definitions . . . . . . . . . . . . . . . 8
1.3. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.1. Inter- and Intra-Domain Forwarding . . . . . . . . . 9
1.3.2. Intra-Domain Forwarding Process . . . . . . . . . . . 10
1.3.3. Configuration . . . . . . . . . . . . . . . . . . . . 11
1.4. Path Construction (Segment Combinations) . . . . . . . . 12
1.5. Path Authorization . . . . . . . . . . . . . . . . . . . 15
2. SCION Header Specification . . . . . . . . . . . . . . . . . 15
2.1. Common Header . . . . . . . . . . . . . . . . . . . . . . 16
2.2. Address Header . . . . . . . . . . . . . . . . . . . . . 19
2.3. Path Header . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.1. Empty Path Type . . . . . . . . . . . . . . . . . . . 20
2.3.2. SCION Path Type . . . . . . . . . . . . . . . . . . . 21
2.3.3. One-Hop Path Type . . . . . . . . . . . . . . . . . . 27
2.3.4. Path Reversal . . . . . . . . . . . . . . . . . . . . 28
2.4. Extension Headers . . . . . . . . . . . . . . . . . . . . 28
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2.4.1. Options Field . . . . . . . . . . . . . . . . . . . . 30
2.5. Pseudo Header for Upper-Layer Checksum . . . . . . . . . 32
3. Life of a SCION Data Packet . . . . . . . . . . . . . . . . . 33
3.1. Description . . . . . . . . . . . . . . . . . . . . . . . 33
3.2. Creating an End-to-End SCION Forwarding Path . . . . . . 34
3.3. Step-by-Step Explanation . . . . . . . . . . . . . . . . 35
4. Path Authorization . . . . . . . . . . . . . . . . . . . . . 40
4.1. Authorizing Segments through Chained MACs . . . . . . . . 41
4.1.1. Hop Field MAC Overview . . . . . . . . . . . . . . . 41
4.1.2. Peering Link MAC Computation . . . . . . . . . . . . 45
4.2. Path Initialization and Packet Processing . . . . . . . . 46
4.2.1. Initialization at Source Endpoint . . . . . . . . . . 46
4.2.2. Processing at Routers . . . . . . . . . . . . . . . . 48
5. Deployment Considerations . . . . . . . . . . . . . . . . . . 54
5.1. MTU . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5.2. Packet Fragmentation . . . . . . . . . . . . . . . . . . 55
5.3. SCION IP Gateway . . . . . . . . . . . . . . . . . . . . 55
6. SCMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6.1. General Format . . . . . . . . . . . . . . . . . . . . . 56
6.2. Message Types . . . . . . . . . . . . . . . . . . . . . . 56
6.3. Checksum Calculation . . . . . . . . . . . . . . . . . . 58
6.4. Processing Rules . . . . . . . . . . . . . . . . . . . . 58
6.5. Error Messages . . . . . . . . . . . . . . . . . . . . . 59
6.5.1. Packet Too Big . . . . . . . . . . . . . . . . . . . 59
6.5.2. External Interface Down . . . . . . . . . . . . . . . 59
6.5.3. Internal Connectivity Down . . . . . . . . . . . . . 60
6.6. Informational Messages . . . . . . . . . . . . . . . . . 62
6.6.1. Echo Request . . . . . . . . . . . . . . . . . . . . 62
6.6.2. Echo Reply . . . . . . . . . . . . . . . . . . . . . 63
6.6.3. Traceroute Request . . . . . . . . . . . . . . . . . 63
6.6.4. Traceroute Reply . . . . . . . . . . . . . . . . . . 65
6.7. SCMP Authentication . . . . . . . . . . . . . . . . . . . 66
7. Handling Link Failures . . . . . . . . . . . . . . . . . . . 66
7.1. Link Failure Detection - BFD . . . . . . . . . . . . . . 66
7.2. Link Failure Notification - SCMP . . . . . . . . . . . . 66
8. Security Considerations . . . . . . . . . . . . . . . . . . . 67
8.1. Path Authorization . . . . . . . . . . . . . . . . . . . 67
8.1.1. Forwarding key compromise . . . . . . . . . . . . . . 67
8.1.2. Forging Hop Field MAC . . . . . . . . . . . . . . . . 68
8.1.3. Path Splicing . . . . . . . . . . . . . . . . . . . . 69
8.2. On-Path Attacks . . . . . . . . . . . . . . . . . . . . . 70
8.2.1. Modification of the Path Header . . . . . . . . . . . 70
8.2.2. Payload Integrity . . . . . . . . . . . . . . . . . . 71
8.3. Off-Path Attacks . . . . . . . . . . . . . . . . . . . . 71
8.4. Volumetric Denial of Service Attacks . . . . . . . . . . 71
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 72
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 72
10.1. Normative References . . . . . . . . . . . . . . . . . . 72
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10.2. Informative References . . . . . . . . . . . . . . . . . 73
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 75
Deployment Testing: SCIONLab . . . . . . . . . . . . . . . . . . 75
Assigned SCION Protocol Numbers . . . . . . . . . . . . . . . . . 75
Considerations . . . . . . . . . . . . . . . . . . . . . . . . 75
Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
draft-dekater-scion-dataplane-03 . . . . . . . . . . . . . . . 76
draft-dekater-scion-dataplane-02 . . . . . . . . . . . . . . . 77
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 78
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 trusted
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:
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_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.dekater-scion-pki]
_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.dekater-scion-controlplane]
_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. It should be
read in conjunction with the other components [I-D.dekater-scion-pki]
and [I-D.dekater-scion-controlplane].
The SCION architecture was initially developed outside of the IETF by
ETH Zurich with significant contributions from Anapaya Systems. It
is deployed in the Swiss finance sector to provide resilient
connectivity between financial institutions. The aim of this
document is to document the existing protocol specification as
deployed, and to introduce new concepts that can potentially be
further improved to address particular problems with the current
Internet architecture.
==Note (to be removed before publication): this document, together
with the other components [I-D.dekater-scion-pki] and
[I-D.dekater-scion-controlplane], deprecates
[I-D.dekater-panrg-scion-overview]. This document provides an
extensive description of how the SCION Data Plane is implemented in
order to facilitate understanding, but could potentially be split
into separate documents if considered suitable for submission to the
Internet Standards Process.==
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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 or organization 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 path
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 by
the data plane in accordance with such information.
*Egress/Ingress*: refers to the direction of travel. In SCION, path
construction with beaconing happens in one direction, while actual
traffic might follow the opposite direction. This document clarifies
on a case-by-case basis whether 'egress' or 'ingress' refers to the
direction of travel of the SCION packet or to the direction of
beaconing.
*Endpoint*: An endpoint is the start or the end of a SCION path, as
defined 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 endpoints which is used to transmit packets in the
data plane. It 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.
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*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), these are used to create
forwarding paths.
*Interface Identifier (Interface ID)*: A 16-bit identifier that
designates a SCION interface at the end of a link connecting two
SCION ASes, with each interface belonging to one border router. Hop
fields describe the traversal of an AS by a pair of interface IDs
called ConsIngress and ConsEgress, as they refer to the ingress and
egress interfaces in the direction of path construction (beaconing).
The Interface ID MUST be unique within each AS. Interface ID 0 is
not a valid identifier as implementations MAY 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.
*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.
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*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 AS control planes
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 that can be used as a shortcut. Peering link information is
added to segment information during the beaconing process and used to
shorten paths while assembling them from segments. It is possible to
construct a path out of only two partial segments which top-most hops
are joined by a peering link. Two peering ASes may be in different
ISDs and may exist between any ASes, including core ASes.
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 interface ID as well as an egress interface ID, which
unequivocally identifies the ingress and egress interfaces within the
AS. The information is authenticated with a Message Authentication
Code (MAC) to prevent forgery.
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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 it is used exclusively for inter-
domain forwarding; re-using the intra-domain network fabric to
provide connectivity amongst all SCION infrastructure services,
border routers, and endpoints. As a consequence, minimal change to
the infrastructure is required for ISPs when deploying SCION. In
practice, in most existing SCION deployments, SCION routers
communicate among themselves and with endpoints by enclosing the
SCION header inside an UDP/IPv6 or UDP/IPv4 packet. The choice of
using an UDP/IP as an intra-domain protocol between routers was
driven by the need to maximize compatibility with existing networks.
This does not exclude that a SCION packet may be enclosed directly on
top of a L2 protocol, since the choice of intra-domain protocol is AS
specific.
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Figure 1 shows the SCION header within the protocol stack, in an AS
where the SCION deployment uses UDP/IP as an intra-domain protocol.
A similar model may be used for inter-domain links, depending on the
individual choice of the two interconnected SCION router operators.
A full example of the life of a SCION packet is later presented in
Section 3. A list of currently used upper layer protocols on top of
SCION is presented in Appendix "Assigned SCION Protocol Numbers".
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Payload (L4) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| SCION |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --------------+
| UDP | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Intra-domain |
| IP | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ protocol |
| Link Layer | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --------------+
Figure 1: The SCION header within the protocol stack in a typical
deployment.
A complete SCION address is composed of the <ISD, AS, endpoint
address> 3-tuple. The ISD-AS part is used for inter-domain routing.
The endpoint address part is only used for intra-domain forwarding at
the source and destination ASes. This implies that endpoint
addresses are only required to be globally unique within each SCION
AS. This means, for example, that an endpoint running a SCION stack
using a [RFC1918] could directly communicate with another SCION
endpoint using a [RFC1918] endpoint address in a different SCION AS.
1.3.2. Intra-Domain Forwarding Process
When transiting an intermediate SCION AS, a packet gets forwarded by
at most two SCION routers. The forwarding process consists of the
following steps.
1. The AS's SCION ingress router receives a SCION packet from the
neighboring AS.
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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) of the egress border
router.
4. The packet is forwarded within the AS by SCION-unaware 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).
1.3.3. Configuration
Border routers require mappings from SCION interface IDs to underlay
addresses and 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.
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In order to forward traffic to a service endpoint address (DT/DS ==
0b01 in the common header (Section 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
and is only dependent 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.dekater-scion-controlplane], 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 MUST be at most one of each type of segment (up, core, and
down). 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. The Up-then-
Down constraint still applies.
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* Additionally, all segments without any peering possibility MUST
consist of at least two Hop Fields.
Note that the type of segment is known to the endpoint but it is not
explicitly visible in the path header of data packets. Therefore, a
SCION router needs to explicitly verify that these rules were
followed correctly by performing checks described in Section 4.2.2.1.
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, with the name coming from
the fact that such paths go "down" (following parent-child links)
before going "up" (following child-parent links).
Figure 2 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 ) - --+
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| | `-' | | | `+' | | `-' | |
| | | | | | | |
| | 2a | | | 2b | | | 3a | |
| | | | | | | |
| .+. .+. | | .+. | .+. .+. |
| ( ) ( ) | | ( ) | ( #-----# ) |
| `+' `+' | | `+' | `+' Peer `+' |
| | | | | | | | | |
| | | | | | | | | |
| | | | | | | | | |
| .+. .+. | | .+. | .+. .+. |
+-> ( * ) ( * ) <-+ +-> ( * ) +-> ( * ) ( * ) <-+
`-' `-' `-' `-' `-'
Core Core
----------> ---------->
.-. .-. .-. .-. .-. .-.
+--( C )- - -( C )--+ +---- ( C ) ----+ ( C )- - -( C ) +-- ( C )
| `+' `+' | | `+' | `+' `+' | `+'
| 3b | | 4a | 4b | 5
| | | | | | | | | | |
| | | | |
| .+. .+. | | .+. | + - --. - + | .+.
| ( #-----# ) | | +-( )-+ | +--( )--+ | ( * )
| `+' Peer `+' | | | `-' | | | `-' | | `+'
| | | | | | | | | | | |
| | | | | | | | | | | |
| | | | | | | | | | | |
| .+. .+. | | .+. .+. | .+. .+. | .+.
+->( * ) ( * )<-+ +>( * ) ( * )<+ ( * ) ( * ) +-> ( * )
`-' `-' `-' `-' `-' `-' `-'
Figure 2: 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 2):
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 segment 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 segments are REQUIRED to connect the
respective ASes to the core segment.
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- *Immediate combination* (Cases 2a, 2b in Figure 2): 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, and 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.
* *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 and such MACs are verified by routers at
forwarding. For a detailed specification, see Section 4.
2. SCION Header Specification
The SCION packet header is aligned to 4 bytes. It is composed of a
common header, an address header, a path header, and an OPTIONAL
extension header, see Figure 3 below.
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+--------------------------------------------------------+
| Common header |
| |
+--------------------------------------------------------+
| Address header |
| |
+--------------------------------------------------------+
| Path header |
| |
+--------------------------------------------------------+
| Extension header (OPTIONAL) |
| |
+--------------------------------------------------------+
Figure 3: High-level SCION header structure
The _common header_ contains important meta information including
version number and the lengths of the header and payload. In
particular, it contains flags that control the format of subsequent
headers such as the address and path headers. For more details, see
Section 2.1.
The _address header_ contains the ISD, AS and endpoint addresses of
source and destination. The type and length of endpoint addresses
are variable and can be set independently using flags in the common
header. For more details, see Section 2.2.
The _path header_ contains the full AS-level forwarding path of the
packet. A path type field in the common header specifies the path
format used in the path header. For more details, see Section 2.3.
The OPTIONAL _extension_ header contains a variable number of hop-by-
hop and end-to-end options, similar to extensions in the IPv6 header
[RFC8200]. For more details, see Section 2.4.
2.1. Common Header
The SCION common header has the following packet format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| TraffCl | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NextHdr | HdrLen | PayloadLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathType |DT |DL |ST |SL | RSV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 4: The SCION common header packet format
* Version: The version of the SCION common header. Currently, only
version "0" is supported.
* TrafficClass (TraffCl in the image above): The 8-bit long
identifier of the packet's class or priority. The value of the
traffic class bits in a received packet might differ from the
value sent by the packet's source. The current use of the
TrafficClass field for Differentiated Services and Explicit
Congestion Notification is specified in [RFC2474] and [RFC3168].
* Flow Label: This 20-bit field labels sequences of packets to be
treated in the network as a single flow. Sources MUST set this
field. This serves the same purpose as what [RFC6437] describes
for IPv6 and is used in the same manner. Notably, a Flow Label of
zero does not imply that packet reordering is acceptable.
* NextHdr: Encodes the type of the first header after the SCION
header. This can be either a SCION extension or a Layer 4
protocol such as TCP or UDP. Values of this field respect the
Assigned SCION Protocol Numbers (see Appendix "Assigned SCION
Protocol Numbers").
* HdrLen: Specifies the entire length of the SCION header in bytes,
i.e. the sum of the lengths of the common header, the address
header, and the path header. The SCION header is aligned to a
multiple of 4 bytes. The SCION header length is computed as
HdrLen * 4 bytes. The 8 bits of the HdrLen field limit the SCION
header to a maximum of 255 * 4 = 1020 bytes.
* PayloadLen: Specifies the length of the payload in bytes. The
payload includes (SCION) extension headers and the L4 payload.
This field is 16 bits long, supporting a maximum payload size of
65'535 bytes.
* PathType: Specifies the type of the SCION path and is 8 bits long.
The format of one path type is independent of all other path
types. The currently defined SCION path types are Empty (0),
SCION (1), OneHopPath (2), EPIC (3) and COLIBRI (4). This
document only specifies the Empty, SCION and OneHopPath path
types. The other path types are currently experimental. For more
details, see Section 2.3.
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+=======+=============================+
| Value | Path Type |
+=======+=============================+
| 0 | Empty path (EmptyPath) |
+-------+-----------------------------+
| 1 | SCION (SCION) |
+-------+-----------------------------+
| 2 | One-hop path (OneHopPath) |
+-------+-----------------------------+
| 3 | EPIC path (experimental) |
+-------+-----------------------------+
| 4 | COLIBRI path (experimental) |
+-------+-----------------------------+
Table 1: SCION path types
* DT/DL/ST/SL: These fields define the endpoint address type and
endpoint address length for the source and destination endpoint.
DT and DL stand for Destination Type and Destination Length,
whereas ST and SL stand for Source Type and Source Length. The
possible endpoint address length values are 4 bytes, 8 bytes, 12
bytes, and 16 bytes. If an address has a length different from
the supported values, the next larger size SHALL be used and the
address can be padded with zeros. Table 2 below lists the
currently used values for address length. The "type" identifier
is only defined in combination with a specific address length.
For example, address type "0" is defined as IPv4 in combination
with address length 4, but is defined as IPv6 in combination with
address length 16. Per address length, several sub-types are
possible and Table 3 shows the currently assigned combinations of
lengths and types.
+=============+================+
| DL/SL Value | Address Length |
+=============+================+
| 0 | 4 bytes |
+-------------+----------------+
| 1 | 8 bytes |
+-------------+----------------+
| 2 | 12 bytes |
+-------------+----------------+
| 3 | 16 bytes |
+-------------+----------------+
Table 2: Address length values
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+================+======+=========================+==============+
| Length (bytes) | Type | DT/DL or ST/SL encoding | Conventional |
| | | | Use |
+================+======+=========================+==============+
| 4 | 0 | 0b0000 | IPv4 |
+----------------+------+-------------------------+--------------+
| 4 | 1 | 0b0100 | Service |
+----------------+------+-------------------------+--------------+
| 16 | 0 | 0b0011 | IPv6 |
+----------------+------+-------------------------+--------------+
| other | | | Unassigned |
+----------------+------+-------------------------+--------------+
Table 3: Allocations of type values to length values
A service address designates a set of endpoint addresses rather than
a singular one. A packet addressed to a service is redirected to any
one endpoint address that is known to be part of the set. Table 4
lists the known services.
* RSV: These bits are currently reserved for future use.
2.2. Address Header
The SCION address header has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DstISD | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| DstAS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SrcISD | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| SrcAS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DstHostAddr (variable Len) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SrcHostAddr (variable Len) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: The SCION address header packet format
* DstISD, SrcISD: The 16-bit ISD identifier of the destination/
source.
* DstAS, SrcAS: The 48-bit AS identifier of the destination/source.
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* DstHostAddr, SrcHostAddr: Specifies the variable length endpoint
address of the destination/source. The accepted type and length
are defined in the DT/DL/ST/SL fields of the common header.
If a service address is implied by the DT/DL or ST/SL field of the
common header, the corresponding address field has the following
format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Number | RSV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Service address format
* RSV: reserved for future use
The currently known service numbers are:
+======================+============+========================+
| Service Number (hex) | Short Name | Description |
+======================+============+========================+
| 0x0001 | DS | Discovery Service |
+----------------------+------------+------------------------+
| 0x0002 | CS | Control Service |
+----------------------+------------+------------------------+
| 0xFFFF | None | Reserved invalid value |
+----------------------+------------+------------------------+
Table 4: Known Service Numbers
*Note:* For more information on addressing in SCION, see the SCION
Control Plane Specification ([I-D.dekater-scion-controlplane]).
2.3. Path Header
The path header of a SCION packet differs for each SCION path type.
The path type is set in the PathType field of the SCION common
header.
SCION supports three path types:
2.3.1. Empty Path Type
The Empty path type (PathType=0) is used to send traffic within an
AS. It has no additional fields, i.e. it consumes 0 bytes on the
wire.
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One use case of the Empty path type lies in the context of
link-failure detection (Section 7.1).
2.3.2. SCION Path Type
The SCION path type (PathType=1) is the standard path type. A SCION
path has the following layout:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathMetaHdr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| InfoField |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| InfoField |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HopField |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HopField |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Layout of a standard SCION path
It consists of a path meta header, up to 3 Info Fields and up to 64
Hop Fields.
* PathMetaHdr indicates the currently valid Info Field and Hop Field
while the packet is traversing the network along the path, as well
as the number of Hop Fields per segment.
* InfoField equals the number of path segments that the path
contains - there is one Info Field per path segment. Each Info
Field contains basic information about the corresponding segment,
such as a timestamp indicating the creation time. There are also
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two flags: one specifies whether the segment is to be traversed in
construction direction, the other whether the first or last Hop
Field in the segment represents a peering Hop Field.
* HopField represents a hop through an AS on the path, with the
ingress and egress interface identifiers for this AS. This
information is authenticated with a Message Authentication Code
(MAC) to prevent forgery.
The SCION header is created by extracting the required Info Fields
and Hop Fields from the corresponding path segments. The process of
extracting is illustrated in Figure 8 below. Note that ASes at the
intersection of multiple segments are represented by two Hop Fields.
Be aware that these Hop Fields are not equal!
In the Hop Field that represents the last Hop in the first segment
(seen in the direction of travel), only the ingress interface will be
specified. However, in the hop Field that represents the first hop
in the second segment (also in the direction of travel), only the
egress interface will be defined. Thus, the two Hop Fields for this
one AS build a full hop through the AS, specifying both the ingress
and egress interface. As such, they bring the two adjacent segments
together.
+-----------------+
| ISD Core |
.--. .--. | .--. .--. | .--. .--.
(AS 3)--(AS 4)----|-(AS 1)---(AS 2)-|----(AS 5)--(AS 6)
`--' `--' | `--' `--' | `--' `--'
+-----------------+
Up-Segment Core-Segment Down-Segment
+---------+ +---------+ +---------+
| +-----+ | | +-----+ | | +-----+ |
| + INF + |--------+ | + INF + |---+ | + INF + |--+
| +-----+ | | | +-----+ | | | +-----+ | |
| +-----+ | | | +-----+ | | | +-----+ | |
| | HF | |------+ | | | HF | |---+-+ | | HF | |--+-+
| +-----+ | | | | +-----+ | | | | +-----+ | | |
| +-----+ | | | | +-----+ | | | | +-----+ | | |
| | HF | |----+ | | | | HF | |---+-+-+ | | HF | |--+-+-+
| +-----+ | | | | | +-----+ | | | | | +-----+ | | | |
| +-----+ | | | | +---------+ | | | | +-----+ | | | |
| | HF | |--+ | | | | | | | | HF | |--+-+-+-+
| +-----+ | | | | | +----------+ | | | | +-----+ | | | | |
+---------+ | | | | | ++++++++ | | | | +---------+ | | | |
| | | | | | Meta | | | | | | | | |
| | | | | ++++++++ | | | | | | | |
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| | | | | +------+ | | | | | | | |
| | | +->| + INF + | | | | | | | |
| | | | +------+ | | | | | | | |
| | | | +------+ | | | | | | | |
| | | | + INF + |<-+ | | | | | |
| | | | +------+ | | | | | | |
| | | | +------+ | | | | | | |
| | | | + INF + |<---+-+--------------+ | | |
| | | | +------+ | | | | | |
| | | | +------+ | | | | | |
| | +--->| | HF | | | | | | |
| | | +------+ | | | | | |
| | | +------+ | | | | | |
| +----->| | HF | | | | | | |
| | +------+ | | | | | |
| | +------+ | | | | | |
+------->| | HF | | | | | | |
| +------+ | | | | | |
| +------+ | | | | | |
| | HF | |<---+ | | | |
| +------+ | | | | |
| +------+ | | | | |
Forwarding Path | | HF | |<-----+ | | |
| +------+ | | | |
| +------+ | | | |
| | HF | |<----------------------+ | |
| +------+ | | |
| +------+ | | |
| | HF | |<------------------------+ |
| +------+ | |
| +------+ | |
| | HF | |<--------------------------+
| +------+ |
+----------+
Figure 8: Path construction example
2.3.2.1. Path Meta Header Field
The 4-byte Path Meta Header field (PathMetaHdr) defines meta
information about the SCION path that is contained in the path
header. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| C | CurrHF | RSV | Seg0Len | Seg1Len | Seg2Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 9: SCION path type - Format of the Path Meta Header field
* C (urrINF): Specifies a 2-bits index (0-based) pointing to the
current Info Field for the packet on its way through the network.
For details, see Section 2.3.2.2 below.
* CurrHF: Specifies a 6-bits index (0-based) pointing to the current
Hop Field for the packet on its way through the network. For
details, see Section 2.3.2.2 below. Note that the CurrHF index
MUST point to a Hop Field that is part of the current path
segment, as indicated by the CurrINF index.
Both indices are used by SCION routers when forwarding data traffic
through the network. The SCION routers also increment the indexes if
required. For more details, see Section 4.2.2.
* Seg{0,1,2}Len: The number of Hop Fields in a given segment.
Seg{i}Len > 0 implies that segment _i_ contains at least one Hop
Field, which means that Info Field _i_ exists. (If Seg{i}Len = 0
then segment _i_ is empty, meaning that this path does not include
segment _i_, and therefore there is no Info Field _i_.) The
following rules apply:
- The total number of Hop Fields in an end-to-end path MUST be
equal to the sum of all Seg{0,1,2}Len contained in this end-to-
end path.
- It is an error to have Seg{X}Len > 0 AND Seg{Y}Len == 0, where
2 >= _X_ > _Y_ >= 0. That is, if path segment Y is empty, the
following path segment X MUST also be empty.
* RSV: Unused and reserved for future use.
2.3.2.2. Path Offset Calculations
The path offset calculations are used by the SCION border routers to
determine the Info Field and Hop Field that are currently valid for
the packet on its way through the network.
The following rules apply when calculating the path offsets:
if Seg2Len > 0: NumINF = 3
else if Seg1Len > 0: NumINF = 2
else if Seg0Len > 0: NumINF = 1
else: invalid
The offsets of the current Info Field and current Hop Field (relative
to the end of the address header) are now calculated as:
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B = byte
InfoFieldOffset = 4B + 8B * CurrINF
HopFieldOffset = 4B + 8B.NumINF + 12B * CurrHF
To check that the current Hop Field is in the segment of the current
Info Field, the CurrHF needs to be compared to the SegLen fields of
the current and preceding Info Fields.
2.3.2.3. Info Field
The 8-byte Info Field (InfoField) has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RSV |P|C| RSV | Acc |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: SCION path type - Format of the Info Field
* RSV: Unused and reserved for future use.
* P: Peering flag. If the flag has value "1", the segment
represented by this Info Field contains a peering Hop Field, which
requires special processing in the data plane. For more details,
see Section 4.1.2 and Section 4.2.
* C: Construction direction flag. If the flag has value "1", the
Hop Fields in the segment represented by this Info Field are
arranged in the direction they have been constructed during
beaconing.
* Acc: Accumulator. This updatable field/counter is REQUIRED for
calculating the MAC in the data plane. For more details, see
Section 4.1.
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* Timestamp: Timestamp created by the initiator of the corresponding
beacon. The timestamp is defined as the number of seconds elapsed
since the POSIX Epoch (1970-01-01 00:00:00 UTC), encoded as a
32-bit unsigned integer. This timestamp enables the validation of
a Hop Field in the segment represented by this Info Field, by
verifying the expiration time and MAC set in the Hop Field - the
expiration time of a Hop Field is calculated relative to the
timestamp. A Info field with a timestamp in the future is
invalid. For the purpose of validation, a timestamp is considered
"future" if it is later than the locally available current time
plus 337.5 seconds (i.e. the minimum time to live of a hop).
2.3.2.4. Hop Field
The 12-byte Hop Field (HopField) has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RSV |I|E| ExpTime | ConsIngress |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ConsEgress | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| MAC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: SCION path type - Format of the Hop Field
* RSV: Unused and reserved for future use.
* I: The Ingress Router Alert flag. If this has value "1" and the
packet is received on the interface with ID corresponding to the
value of ConsIngress, the router SHOULD process the L4 payload in
the packet.
* E: The Egress Router Alert flag. If this has value "1" and the
packet is received on the interface with ID corresponding to the
value of ConsEgress, the router SHOULD process the L4 payload in
the packet.
* ExpTime: Expiration time of a Hop Field. This field is 1-byte
long, and the expiration time specified in this field is relative.
An absolute expiration time in seconds is computed in combination
with the Timestamp field (from the corresponding Info Field), as
follows:
- Timestamp + (1 + ExpTime) * (3600/256)
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* ConsIngress, ConsEgress: The 16-bits ingress/egress interface IDs
in construction direction, that is, the direction of beaconing.
* MAC: The 6-byte Message Authentication Code to authenticate the
Hop Field. For details on how this MAC is calculated, see
Section 4.1.1.
The Ingress Router (respectively Egress Router) is the router owning
the Ingress interface (respectively, Egress interface) when the
packet is traveling in the _construction direction_ of the path
segment (i.e. the direction of beaconing). When the packet is
traveling in the opposite direction, the meanings are reversed.
Router alert flags work similarly to [RFC2711] and allow a sender to
address a specific router on the path without knowing its address.
Processing the L4 payload in the packet means that the router will
treat the payload of the packet as a message to itself and parse it
according to the value of the NextHdr field. Such messages include
Traceroute Requests (Section 6.6.3)
Setting multiple router alert flags on a path SHOULD be avoided.
This is because the router for which the corresponding Router Alert
flag is set to "1" may process the request without further forwarding
it along the path. Use cases that require multiple routers/hops on
the path to process a packet SHOULD rely on a hop-by-hop extension
(see Section 2.4).
2.3.3. One-Hop Path Type
The OneHopPath path type (PathType=2) is currently used to bootstrap
beaconing between neighboring ASes. This is necessary as neighbor
ASes do not have a forwarding path before beaconing is started.
A one-hop path has exactly one Info Field and two Hop Fields with the
specialty that the second Hop Field is not known a priori, but is
instead created by the ingress SCION border router of the neighboring
AS while processing the one-hop path. Any entity with access to the
forwarding key of the source endpoint AS can create a valid info and
Hop Field as described in Section 2.3.2.3 and Section 2.3.2.4,
respectively.
Upon receiving a packet containing a one-hop path, the ingress border
router of the destination AS fills in the ConsIngress field in the
second Hop Field of the one-hop path with the ingress interface ID.
It sets the ConsEgress field to an invalid value (e.g. unspecified
value 0), ensuring the path cannot be used beyond the destination AS.
Then it calculates and appends the appropriate MAC for the Hop Field.
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Figure 12 below shows the layout of a SCION one-hop path type. There
is only a single Info Field; the appropriate Hop Field can be
processed by a border router based on the source and destination
address. In this context, the following rules apply:
* At the source endpoint AS, _CurrHF := 0_.
* At the destination endpoint AS, _CurrHF := 1_.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| InfoField |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HopField |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HopField |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Layout of the SCION one-hop path type
2.3.4. Path Reversal
When a destination endpoint receives a SCION packet, it MAY use the
path information in the SCION header for sending the reply packets.
To reverse a path, the destination endpoint MUST perform the
following steps:
1. Reverse the order of the Info Fields;
2. Reverse the order of the Hop Fields;
3. For each Info Field, negate the construction direction flag C; do
not change the accumulator field Acc.
4. In the PathMetaHdr field:
* Set the CurrINF and CurrHF to "0".
* Reverse the order of the non-zero SegLen fields.
2.4. Extension Headers
SCION provides two types of extension headers:
* The Hop-by-Hop Options header is used to carry OPTIONAL
information that MAY be examined and processed by every SCION
router along a packet's delivery path. The Hop-by-Hop Options
header is identified by value "200" in the NextHdr field of the
SCION common header (see Section 2.1).
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* The End-to-End Options header is used to carry OPTIONAL
information that MAY be examined and processed by the sender and/
or the receiving endpoints of the packet. The End-to-End Options
header is identified by value "201" in the NextHdr field of the
SCION common header (see Section 2.1).
If both headers are present, the Hop-by-Hop Options header MUST come
before the End-to-End Options header.
*Note:* The SCION extension headers are defined and used based on and
similar to the IPv6 extensions as specified in section 4 of
[RFC8200]. The SCION Hop-by-Hop Options header and End-to-End
Options header resemble the IPv6 Hop-by-Hop Options Header (section
4.3 in the RFC) and Destination Options Header (section 4.6),
respectively.
The SCION Hop-by-Hop Options and End-to-End Options headers are
aligned to 4 bytes and have the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NextHdr | ExtLen | Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: Extension headers: Options header
* NextHdr: Unsigned 8-bit integer. Identifies the type of header
immediately following the Hop-by-Hop/End-to-End Options header.
Values of this field respect the Assigned SCION Protocol Numbers
(see also Appendix "Assigned SCION Protocol Numbers").
* ExtLen: 8-bit unsigned integer. The length of the Hop-by-hop or
End-to-end options header in 4-octet units, not including the
first 4 octets. That is: ExtLen = uint8(((L + 2) / 4) - 1), where
L is the size of the header in bytes, assuming that L + 2 is a
multiple of 4.
* Options: This is a variable-length field. The length of this
field MUST be such that the complete length of the Hop-by-Hop/End-
to-End Options header is an integer multiple of 4 bytes. This can
be achieved by using options of type 0 or 1 (see Table 4) . The
Options field contains one or more Type-Length-Value (TLV) encoded
options. For details, see Section 2.4.1.
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2.4.1. Options Field
The Options field of the Hop-by-Hop Options and the End-to-End
Options headers carries a variable number of options that are type-
length-value (TLV) encoded. Each TLV-encoded option has the
following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptType | OptDataLen | OptData |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Options field: TLV-encoded options
* OptType: 8-bit identifier of the type of option. The following
option types are assigned to the SCION HBH/E2E Options header:
+=========+====================================+
| Decimal | Option Type |
+=========+====================================+
| 0 | Pad1 (see Section 2.4.1.1) |
+---------+------------------------------------+
| 1 | PadN (see Section 2.4.1.2) |
+---------+------------------------------------+
| 2 | SCION Packet Authenticator Option. |
| | Only used by the End-to-End |
| | Options header (experimental). |
+---------+------------------------------------+
| 253 | Used for experimentation and |
| | testing |
+---------+------------------------------------+
| 254 | Used for experimentation and |
| | testing |
+---------+------------------------------------+
| 255 | Reserved |
+---------+------------------------------------+
Table 5: Option types of SCION Options header
* OptDataLen: Unsigned 8-bit integer denoting the length of the
OptData field of this option in bytes.
* OptData: Variable-length field. Option-type specific data.
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The options within a header MUST be processed strictly in the order
they appear in the header. This is to prevent a receiver from, for
example, scanning through the header looking for a specific option
and processing this option prior to all preceding ones.
Individual options may have specific alignment requirements, to
ensure that multibyte values within the OptData fields have natural
boundaries. The alignment requirement of an option is specified
using the notation "xn+y". This means that the OptType MUST appear
at an integer multiple of x bytes from the start of the header, plus
y bytes. For example:
* 2n: means any 2-bytes offset from the start of the header.
* 4n+2: means any 4-bytes offset from the start of the header, plus
2 bytes.
There are two padding options to align subsequent options and to pad
out the containing header to a multiple of 4 bytes in length - for
details, see below. All SCION implementations MUST recognize these
padding options.
2.4.1.1. Pad1 Option
Alignment requirement: none.
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 |
+-+-+-+-+-+-+-+-+
Figure 15: TLV-encoded options - Pad1 option
*Note:* The format of the Pad1 option is a special case - it does not
have length and value fields.
The Pad1 option is used to insert 1 byte of padding into the Options
field of an extension header. If more than one byte of padding is
required, the PadN option MUST be used.
2.4.1.2. PadN Option
Alignment requirement: none.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | OptDataLen | OptData |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: TLV-encoded options - PadN option
The PadN option is used to insert two or more bytes of padding into
the Options field of an extension header. For N bytes of padding,
the OptDataLen field contains the value N-2, and the OptData consists
of N-2 zero-valued bytes.
2.5. Pseudo Header for Upper-Layer Checksum
The SCION Data Plane does not provide payload integrity protection,
as further clarified in Section 8.2.2. Should any transport or other
upper-layer protocols compute a checksum of the SCION header, then
they SHOULD use the following pseudo header:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----+
| DstISD | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + SCION|
| DstAS | ad-|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ dress|
| SrcISD | | hea-|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + der|
| SrcAS | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| DstHostAddr (variable Len) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| SrcHostAddr (variable Len) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----+
| Upper-Layer Packet Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero | Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: Layout of the pseudo header for the upper-layer checksum
* DstISD, SrcISD, DstAS, SrcAS, DstHostAddr, SrcHostAddr: These
values are taken from the SCION address header.
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* Upper-Layer Packet Length: The length of the upper-layer header
and data. Some upper-layer protocols define headers that carry
the length information explicitly (e.g. UDP). This information
is used as the upper-layer packet length in the pseudo header for
these protocols. The remaining protocols, which do not carry the
length information directly, use the value from the PayloadLen
field in the SCION common header, minus the sum of the extension
header lengths.
* Next Header: The protocol identifier associated with the upper-
layer protocol (e.g., 17 for UDP - see also Appendix "Assigned
SCION Protocol Numbers"). This field can differ from the NextHdr
field in the SCION common header, if extensions are present.
This pseudo-header is used in current implementations of UDP on top
of SCION. However, as checksums across layers are not recommended,
this should be re-evaluated in future revisions.
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 created at its source endpoint, passes
through a number of SCION 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).
This example illustrates an intra-ISD case, i.e. all communication
happening 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 as well, although this makes no
difference for the forwarding process which works the same in an
intra-ISD and inter-ISD context.
3.1. Description
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+--------------------+
| |
| AS1 |
| |
| |
| 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 | | |
| | | | | AS3 |
| | AS2 | | | |
| | | | +---+ |
| +---+ | | | B | |
| | A | | | +---+ |
| +---+ | | 1-3,192.0.2.7 |
| 1-2,203.0.113.6 | | |
| | +--------------------+
+--------------------+
Figure 18: Sample topology to illustrate the life cycle of a
SCION packet. AS1 is the core AS of ISD 1, and AS2 and AS3 are
non-core ASes of ISD 1.
Based on the network topology in Figure 18 above, this example shows
the path of a SCION packet sent from its source at Endpoint A to its
destination at Endpoint B, and how it will be processed by each
router on the path using simplified snapshots of the packet header
after each 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, Endpoint A in AS2 wants to send a data packet to
Endpoint B in AS3. Both AS2 and AS3 are part of ISD 1. To create an
end-to-end SCION forwarding path, Endpoint A first requests its own
AS2 control service for up segments to the core AS in its ISD. The
AS2 control service will return up segments from AS2 to the ISD core.
Endpoint A will also query its AS2 control service for a down segment
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from its ISD core AS to AS3, in which Endpoint B is located. The AS2
control service will return down segments from the ISD core down to
AS3.
*Note:* For more details on the lookup of path segments, see the
section "Path Lookup" in the Control Plane specification
([I-D.dekater-scion-controlplane]).
Based on its own selection criteria, 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 AS2 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 - (x,y) represents one Hop Field.
To obtain an end-to-end forwarding path from the source AS to the
destination AS, 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.
Endpoint A now adds this end-to-end forwarding path to the header of
the packet that it wants to send to Endpoint B, and starts
transferring the packet.
3.3. Step-by-Step Explanation
This section explains what happens with the SCION packet header at
each router, based on the network topology in described Figure 18
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 source and
destination 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 as 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. In this context, words "ingress" and "egress" refer to
the direction of travel of SCION data packets.
* _Step 1_
*A->R1*: The SCION-enabled Endpoint A in AS2 creates a new SCION
packet destined for destination endpoint B in AS3, 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
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in this case Router 1. Endpoint A encapsulates the SCION packet
into an underlay UDP/IPv4 header for the local delivery to Router
1, utilizing AS2's internal routing protocol. The current Info
Field is _IF1_. Upon receiving the packet, Router 1 will forward
the packet on the egress interface that endpoint A has included
into the first Hop Field of the SCION header.
+============+===========================================+
| 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 | P_S = 30041, P_D = 30041 |
+------------+-------------------------------------------+
| IP | SRC = 203.0.113.6 (Endpoint A) |
+------------+-------------------------------------------+
| | DST = 203.0.113.17 (Router 1) |
+------------+-------------------------------------------+
| Link layer | SRC=A, DST=R1 |
+------------+-------------------------------------------+
Table 6: Snapshot header - step 1
* _Step 2_
*R1->R2*: Router 1 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 1 to forward the
packet on its interface i2a. After reading the current Hop Field,
Router 1 moves the pointer forward by one position to the second
Hop Field (i1a,0).
The link shown here is an example of not using a UDP/IP underlay.
Although most implementations use such an encapsulation, SCION
only requires link-layer connectivity. What is used for one given
inter-AS link is a function of the available implementations at
each end, the available infrastructure, and the joint preference
of the two ASes administrators.
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+============+===========================================+
| 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 |
+------------+-------------------------------------------+
Table 7: Snapshot header - step 2
* _Step 3_
*R2->R3*: When receiving the packet, Router 2 of Core AS1 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 2. 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. Router maps the i1b interface ID
to egress Router 3, it therefore encapsulates the SCION packet
inside an intra-AS underlay IP packet with the address of Router 3
as the underlay destination.
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+============+===========================================+
| 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 | P_S = 30041, P_D = 30041 |
+------------+-------------------------------------------+
| IP | SRC = 198.51.100.1 (Router 2) |
+------------+-------------------------------------------+
| | DST = 198.51.100.4 (Router 3) |
+------------+-------------------------------------------+
| Link layer | SRC=R2, DST=R3 |
+------------+-------------------------------------------+
Table 8: Snapshot header - step 3
* _Step 4_
*R3->R4*: Router 3 inspects the current Hop Field in the SCION
header, uses interface i1b to forward the packet to its neighbor
SCION-enabled Router 4 of AS3, and moves the current hop-field
pointer forward. It adds an IP header to reach Router 4.
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+============+===========================================+
| 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 | P_S = 30041, P_D = 30041 |
+------------+-------------------------------------------+
| IP | SRC = 1-1,198.51.100.17 (Router 3) |
+------------+-------------------------------------------+
| | DST = 1-3,198.51.100.18 (Router 4) |
+------------+-------------------------------------------+
| Link layer | SRC=R3, DST=R4 |
+------------+-------------------------------------------+
Table 9: Snapshot header - step 4
* _Step 5_
*R4->B*: SCION-enabled Router 4 first checks whether the packet
has been received through the ingress interface i3a as specified
by the current Hop Field. Router 4 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 Field or Hop
Field, Router 4 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 its destination at Endpoint B.
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+============+===========================================+
| 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 | P_S = 30041, P_D = 30041 |
+------------+-------------------------------------------+
| IP | SRC = 192.0.2.34 (Router 4) |
+------------+-------------------------------------------+
| | DST = 192.0.2.7 (Endpoint B) |
+------------+-------------------------------------------+
| Link layer | SRC=R4, DST=B |
+------------+-------------------------------------------+
Table 10: Snapshot header - step 5
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
Fields and Hop Fields at the beginning of the reversed path (see also
Section 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 section
first explains how Hop Field MACs are computed, then how they are
validated as they traverse the network.
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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 Overview
The MAC in the Hop Fields of a SCION path has two purposes:
* Preventing malicious endpoints from 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
and unauthorized path segment.
* Authentication of the information contained in the Hop Field
itself, in particular the ExpTime, ConsIngress, and ConsEgress.
To fulfill these purposes, the MAC for the Hop Field of AS_i includes
both the components of the current Hop Field HF_i 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 AS_i on the path
segment computes the MAC for the current HF_i, 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
path segments in a path as a separate, updatable Accumulator Field
Acc. The routers update Acc by adding/subtracting only a single
16-bit value each.
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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
prevent splicing of different path segments unless there is a
collision of the Acc value among compatible path segments in an AS.
See Section 8.1.3 for more details.
4.1.1.1. Hop Field MAC - Calculation
The Hop Field MAC is generally calculated at a current AS_i as
follows:
* Consider a path segment with "n" hops, containing ASes AS_0, ... ,
AS_(n-1), with forwarding keys K_0, ... , K_(n-1) in this order.
* AS_0 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.
MAC_i =
Ck_i (SegID XOR MAC_0 [:2] ... XOR MAC_(i-1) [:2], Timestamp,
ExpTime_i, ConsIngress_i, ConsEgress_i)
where
* k_i = The forwarding key k of the current AS_i
* Ck_i (...) = Cryptographic checksum C over (...) computed with
forwarding key k_i
* SegID = The random initial 16-bit segment identifier set by the
core AS when creating the corresponding PCB
* XOR = The bitwise "exclusive or" operation
* MAC_i [:2] = The Hop Field MAC for AS_i, truncated to 2 bytes
* Timestamp = The timestamp set by the core AS when creating the
corresponding PCB
* ExpTime_i, ConsIngress_i, ConsEgress_i = The content of the Hop
Field HF_i
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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 Acc_i 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 2.3.2.3). Routers update this field
by adding/subtracting only a single 16-bit value each.
Section 4.1.1.1 provides a general formula to compute MAC_i, but at
each SCION router the expression SegID XOR MAC_0 [:2] ... XOR MAC_i-1
[:2] is replaced by Acc_i. This results in the following alternative
procedure for the computation of MAC_i at SCION routers:
MAC_i = Ck_i (Acc_i, Timestamp, ExpTime_i, ConsIngress_i,
ConsEgress_i)
During forwarding, SCION routers at each AS_i update the Acc field in
the packet header so that it contains the correct input value of 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 Acc_(i+1) is calculated based on the following
definition (in the direction of beaconing):
Acc_(i+1) = Acc_i XOR MAC_i [:2]
* XOR = The bitwise "exclusive or" operation
* MAC_i [:2] = The Hop Field MAC for the current AS_i, truncated to
2 bytes
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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
and the control service and routers of the AS do need to agree on the
algorithm used, but all 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 and 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 for for
any values of these inputs there is exactly one result.
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* 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 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 requirement 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.
4.1.2. Peering Link MAC Computation
The Hop Field MAC computation described in Section 4.1.1.1 does not
apply to a peering Hop Field, i.e. to a Hop Field that allows
transiting from a child interface/link to a peering interface/link.
The reason for this is that the MACs of the Hop Fields "after" the
peering Hop Field (in beaconing direction) are not chained to the MAC
of the peering Hop Field, but to the MAC of the main Hop Field in the
corresponding AS entry. To make this work, the MAC of the peering
Hop Field is also chained to the MAC of the main Hop Field - this
allows for the validation of the chained MAC for both the peering Hop
Field and the following Hop Fields by using the same Acc field value.
The peering Hop Field is defined as follows:
Hop Field^Peer_i = (ExpTime^Peer_i, ConsIngress^Peer_i,
ConsEgress^Peer_i, MAC^Peer_i)
See [I-D.dekater-scion-controlplane] for more information.
This results in the calculation of the MAC for the peering Hop
Field^Peer_i as follows:
MAC^Peer_i =
Ck^Peer_i (SegID XOR MAC_0 [:2] ... XOR MAC_i [:2], Timestamp,
ExpTime^Peer_i, ConsIngress^Peer_i, ConsEgress^Peer_i)
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*Note:* The XOR-sum of the MACs in the formula of the peering Hop
Field *also includes* the MAC of the main Hop Field (whereas for the
calculation of the MAC for the main Hop Field itself only the XOR-sum
of the _previous_ MACs is used).
4.2. Path Initialization and Packet Processing
As described in Section 2, 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. The SCION path also
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 SCION routers to
respectively craft and forward a data packet.
4.2.1. Initialization at Source Endpoint
The source endpoint MUST perform the following steps to correctly
initialize a path:
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 Info Fields and Hop Fields into the corresponding InfoFields
and Hopfields 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 2.3.2.3). 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:
* 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-segments and
down-segments if the path contains a peering Hop Field.
The following InfoField settings are possible, based on the
following cases:
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* *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 = Acc_i. For more details, see
Section 4.1.1.2.
* *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 = Acc_(i+1). For more details, see
Section 4.1.1.2.
* *UCase 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 = Acc_(n-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 2.3.2.1). As the
source endpoint builds the starting point of the forwarding, both
pointers MUST be set to "0".
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4.2.2. Processing at Routers
During forwarding, each SCION router verifies the path contained in
the packet header. Each SCION router also MUST correctly verify or
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 a special case. 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 routers (see ) of each on-path AS. Each
operation is described from the perspective of AS_i, 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).
The following figure provides a simplified representation of the
processing at routers both in construction direction and against
construction direction.
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.--.
( 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 at Ingress Border Router
A SCION ingress border router MUST perform the following steps 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. If there is a segment switch at the current router, check that
the ingress and egress interface links are either:
* Both core
* Parent-child or vice-versa
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* Peering-child or vice-versa
Link types above are defined in [I-D.dekater-scion-controlplane]
section "Paths and Links". This check prevents valley use of peering
links or hair-pin segments. 3. Check if the current Hop Field is
expired or originated in the future, i.e. the current Info Field MUST
NOT have a timestamp in the future, as defined in Section 2.3.2.3.
If either is true, the router MUST drop the packet.
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, so the settings of both flags MUST
be checked. 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 cases are possible:
- *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):
o Compute the value of the Accumulator Acc as follows:
Acc = Acc_(i+1) XOR MAC_i
where
Acc_(i+1) = the current value of the field Acc in the
current Info Field
MAC_i = the value of MAC_i in the current Hop Field
representing AS_i
*Note:* In the case described here, the packet travels
against direction of beaconing, i.e. the packet comes from
AS_(i+1) and will enter AS_i. This means that the Acc field
of this incoming packet represents the value of Acc_(i+1),
but to compute the MAC_i for the current AS_i, we need the
value of Acc_i (see Section 4.1.1.2). As the border router
knows that the formula for Acc_(i+1) = Acc_i XOR MAC_i [:2]
(see also Section 4.1.1.2), and because the values of
Acc_(i+1) and MAC_i are known, the router will be able to
recover the value Acc_i based on the aforementioned formula
for Acc.
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o Replace the current value of the field Acc in the current
Info Field with the newly calculated value of Acc.
o Compute the MAC^Verify_i over the Hop Field of the current
AS_i. 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 Acc as just set in the Acc field
in the current Info Field.
o If the MAC_i in the current Hop Field does not match the
just calculated MAC^Verify_i, drop the packet.
o If the current Hop Field is the last Hop Field in the path
segment as determined by the value of the current SegLen and
other metadata in the path meta header, increment both
CurrInf and CurrHF in the path meta header. Proceed with
step 4.
- *Case 2*
The path segment includes a *peering Hop Field* (P = "1"), but
the current hop is *not* the peering hop (i.e. the current hop
is *neither* the last hop of the first segment *nor* the first
hop of the second segment). In this case, the ingress border
router needs to perform the steps previously described for the
path segment without peering Hop Field, but the border router
MUST NOT increment CurrInf and MUST NOT increment CurrHF in the
path meta header. Proceed with step 4.
- *Case 3*
The path segment includes a *peering Hop Field* (P = "1"), and
the current Hop Field _is_ the peering Hop Field (i.e. the
current hop is *either* the last hop of the first segment *or*
the first hop of the second segment). In this case, the
ingress border router MUST take the following step(s):
o Compute MAC^Peer_i. 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 Acc as set in the Acc
field in the current Info Field (this is the value of Acc as
it comes with the packet).
o If the MAC_i in the current Hop Field does not match the
just calculated MAC^Peer_i, drop the packet.
o Increment both CurrInf and CurrHF in the path meta header.
Proceed with step 4.
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1. 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.
4.2.2.2. Steps at Egress Border Router
A SCION egress border router MUST perform the following steps when it
receives a SCION packet:
1. Check the settings of the Construction Direction flag C and the
Peering flag P in the currently valid Info Field. The following
cases are possible:
* *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 (i.e. the current hop is *neither* the last
hop of the first segment *nor* the first hop of the second
segment). In this case, the egress border router MUST take
the following step(s):
- Compute MAC^Verify_i over the Hop Field of the current
AS_i. 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 Acc as set in the Acc field in
the current Info Field.
- If the just calculated MAC^Verify_i does not match the
MAC_i in the Hop Field of the current AS_i, drop the
packet.
- Compute the value of Acc_(i+1). For this, use the formula
in Section 4.1.1.2. Replace Acc_i in the formula with the
current value of 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 Acc_(i+1).
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* *Case 2*
The packet traverses the path segment in *construction
direction* (C = "1") where the path segment includes a
*peering Hop Field* (P = "1") and the current Hop Field _is_
the peering Hop Field (i.e. the current hop is *either* the
last hop of the first segment *or* the first hop of the second
segment). In this case, the egress border router MUST take
the following steps:
- Compute MAC^Peer_i. 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.
- If the MAC_i in the Hop Field of the current AS_i does not
match the just calculated MAC^Peer_i, drop the packet.
* *Case 3*
The packet traverses the path segment *against construction
direction* (C = "0" and P = "0" or "1"). In this case,
proceed with Step 3.
2. Increment CurrHF in the path meta header.
3. Forward the packet to the neighbor AS.
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 4.1.1.1, 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
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intervals (so typically N minutes) after origination which creates a
constraint on the expiration of hops. Hops of the minimal expiration
time (337.5 seconds - see Section 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 for this.
5. Deployment Considerations
5.1. MTU
SCION requires its underlay protocol to provide a minimum MTU of 1232
bytes. This number results from 1280, the minimum IPv6 MTU as of
[RFC2460]), minus 48, assuming UDP/IPv6 as underlay. Higher layer
protocols such as SCMP rely only on such minimum MTU.
The MTU of a SCION path is defined as the minimum of the MTUs of the
links traversed by that path. The control plane disseminates such
values and makes them available to endpoints (see:
[I-D.dekater-scion-controlplane], Path MTU).
The MTU of each link may be discovered or administratively configured
(current practice is for it to be configured). It must be less than
or equal to the MTU of the link's underlay encapsulation or native
link-layer in either direction.
SCION assumes that the MTUs of a path segment remains correct for the
life time of that segment. This is generally a safe assumption
because:
* Intra-AS network MTUs are a result of the network configuration of
each AS and therefore predictable.
* Inter-AS links MTU are normally under the joint control of the
administrators of the two ASes involved and therefore equally
predictable.
Should the inter-AS link MTU be unpredictable (e.g. because the
inter-AS link is deployed as an overlay), then the link's MTU MUST be
configured statically to a conservative value. For a UDP/IP
underlay, 1232 is a safe value.
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5.2. Packet Fragmentation
The SCION network layer does not support packet fragmentation; not
even at the source endpoint. Upper layer protocols and applications
MUST comply with the MTU of the paths that they use.
SCION is agnostic to datagram fragmentation by the underlay network
layer, (e.g. used for intra-AS communication). Implementations
SHOULD allow MTU discovery mechanisms such as [RFC4821] to be enabled
in the underlay and avoid fragmentation. For inter-AS links, using a
different configuration is the joint decision of the administrators
of the two ASes involved. For intra-AS interfaces using a different
configuration is the choice of that AS' administrator alone.
5.3. SCION IP Gateway
The SCION IP Gateway (SIG) enables IP packets to be tunneled over
SCION to support communication between hosts that do not run a SCION
implementation. A SIG acts as a router from the perspective of IP,
whilst acting as SCION endpoint from the perspective of the SCION
network. It is typically deployed inside the same AS-internal
network as its non-SCION hosts, or at the edge of an enterprise
network. Tunneling IP traffic over SCION requires a pair of SIGs: at
the ingress and egress points of the SCION network.
IP tunneling over SCION is an application from the perspective of the
Data Plane and is outwith the scope of this document.
6. SCMP
The SCION Control Message Protocol (SCMP) is analogous to the
Internet Control Message Protocol (ICMP). It provides functionality
for network diagnostics, such as traceroute, and error messages that
signal packet processing or network-layer problems. SCMP is a
helpful tool for network diagnostics and, in the case of External
Interface Down and Internal Connectivity Down messages, a signal for
endpoints to detect network failures more rapidly and fail-over to
different paths. However, SCION nodes should not strictly rely on
the availability of SCMP, as this protocol may not be supported by
all devices and/or may be subject to rate limiting.
This document specifies only messages RECOMMENDED for the purposes of
path diagnosis and recovery. An extended specification, still a work
in progress, can be found in [SCMP].
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6.1. General Format
Every SCMP message is preceded by a SCION header, and zero or more
SCION extension headers. The SCMP header is identified by a NextHdr
value of 202 in the immediately preceding header.
The messages have the following general format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-dependent Block |
+ +
| (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 21: SCMP message format
* Type: it indicates the type of SCMP message. Its value determines
the format of the type-dependent block.
* Code: it provides additional granularity to the SCMP type.
* Checksum: it is used to detect data corruption.
* Type-dependent Block: optional field of variable length which
format is dependent on the message type.
6.2. Message Types
SCMP messages are grouped into two classes: error messages and
informational messages. Error messages are identified by a zero in
the high-order bit of the type value. I.e., error messages have a
type value in the range of 0-127. Informational messages have type
values in the range of 128-255.
This specification defines the message formats for the following SCMP
messages:
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+======+===============================================+
| Type | Meaning |
+======+===============================================+
| 1 | Reserved for future use |
+------+-----------------------------------------------+
| 2 | Packet Too Big (Section 6.5.1) |
+------+-----------------------------------------------+
| 3 | Reserved for future use |
+------+-----------------------------------------------+
| 4 | Reserved for future use |
+------+-----------------------------------------------+
| 5 | External Interface Down (Section 6.5.2) |
+------+-----------------------------------------------+
| 6 | Internal Connectivity Down (Section 6.5.3) |
+------+-----------------------------------------------+
+------+-----------------------------------------------+
| 100 | Private Experimentation |
+------+-----------------------------------------------+
| 101 | Private Experimentation |
+------+-----------------------------------------------+
+------+-----------------------------------------------+
| 127 | Reserved for expansion of SCMP error messages |
+------+-----------------------------------------------+
Table 11: error messages types
+======+=======================================================+
| Type | Meaning |
+======+=======================================================+
| 128 | Echo Request (Section 6.6.1) |
+------+-------------------------------------------------------+
| 129 | Echo Reply (Section 6.6.2) |
+------+-------------------------------------------------------+
| 130 | Traceroute Request (Section 6.6.3) |
+------+-------------------------------------------------------+
| 131 | Traceroute Reply (Section 6.6.4) |
+------+-------------------------------------------------------+
| 200 | Private Experimentation |
+------+-------------------------------------------------------+
| 201 | Private Experimentation |
+------+-------------------------------------------------------+
+------+-------------------------------------------------------+
| 255 | Reserved for expansion of SCMP informational messages |
+------+-------------------------------------------------------+
Table 12: informational messages types
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Type values 100, 101, 200, and 201 are reserved for private
experimentation.
All other values are reserved for future use.
6.3. Checksum Calculation
The checksum is the 16-bit one's complement of the one's complement
sum of the entire SCMP message, starting with the SCMP message type
field, and prepended with a "pseudo-header" consisting of the SCION
address header and the Layer 4 protocol type as defined in
Section 2.5.
6.4. Processing Rules
Implementations MUST respect the following rules when processing SCMP
messages:
* If an SCMP error message of unknown type is received at its
destination, it MUST be passed to the upper-layer process that
originated the packet that caused the error, if it can be
identified.
* If an SCMP informational message of unknown type is received, it
MUST be silently dropped.
* Every SCMP error message MUST include as much of the offending
SCION packet as possible. The error message packet - including
the SCION header and all extension headers - MUST NOT exceed *1232
bytes* in order to fit into the minimum MTU (see Section 5.1).
* In case the implementation is required to pass an SCMP error
message to the upper-layer process, the upper-layer protocol type
is extracted from the original packet in the body of the SCMP
error message and used to select the appropriate process to handle
the error. In case the upper-layer protocol type cannot be
extracted from the SCMP error message body, the SCMP message MUST
be silently dropped.
* An SCMP error message MUST NOT be originated in response to any of
the following:
- An SCMP error message.
- A packet which source address does not uniquely identify a
single node. E.g., an IPv4 or IPv6 multicast address.
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The maximum size 1232 bytes is chosen so that the entire datagram, if
encapsulated in UDP and IPv6, does not exceed 1280 bytes (L2 Header
excluded). 1280 bytes is the minimum MTU required by IPv6 and it is
assumed that, nowadays, this MTU can also be safely expected when
using IPv4.
6.5. Error Messages
6.5.1. Packet Too Big
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reserved | MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of the offending packet |
+ as possible without the SCMP packet +
| exceeding 1232 bytes. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 22: Packet-too-big format
+======+=====================================================+
| Name | Value |
+======+=====================================================+
| Type | 2 |
+------+-----------------------------------------------------+
| Code | 0 |
+------+-----------------------------------------------------+
| MTU | The Maximum Transmission Unit of the next-hop link. |
+------+-----------------------------------------------------+
Table 13: field values
A *Packet Too Big* message SHOULD be originated by a router in
response to a packet that cannot be forwarded because the packet is
larger than the MTU of the outgoing link. The MTU value is set to
the maximum size a SCION packet can have to still fit on the next-hop
link, as the sender has no knowledge of the underlay.
6.5.2. External Interface Down
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ISD | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AS +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Interface ID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of the offending packet |
+ as possible without the SCMP packet +
| exceeding 1232 bytes. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: External-interface-down format
+==============+==================================================+
| Name | Value |
+==============+==================================================+
| Type | 5 |
+--------------+--------------------------------------------------+
| Code | 0 |
+--------------+--------------------------------------------------+
| ISD | The 16-bit ISD identifier of the SCMP originator |
+--------------+--------------------------------------------------+
| AS | The 48-bit AS identifier of the SCMP originator |
+--------------+--------------------------------------------------+
| Interface ID | The interface ID of the external link with |
| | connectivity issue. |
+--------------+--------------------------------------------------+
Table 14: field values
A *External Interface Down* message SHOULD be originated by a router
in response to a packet that cannot be forwarded because the link to
an external AS is broken. The ISD and AS identifier are set to the
ISD-AS of the originating router. The interface ID identifies the
link of the originating AS that is down.
Recipients can use this information to route around broken data-plane
links.
6.5.3. Internal Connectivity Down
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ISD | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AS +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Ingress Interface ID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Egress Interface ID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of the offending packet |
+ as possible without the SCMP packet +
| exceeding 1232 bytes. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: Internal-connectivity-down format
+============+==================================================+
| Name | Value |
+============+==================================================+
| Type | 6 |
+------------+--------------------------------------------------+
| Code | 0 |
+------------+--------------------------------------------------+
| ISD | The 16-bit ISD identifier of the SCMP originator |
+------------+--------------------------------------------------+
| AS | The 48-bit AS identifier of the SCMP originator |
+------------+--------------------------------------------------+
| Ingress ID | The interface ID of the ingress link. |
+------------+--------------------------------------------------+
| Egress ID | The interface ID of the egress link. |
+------------+--------------------------------------------------+
Table 15: field values
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A *Internal Connectivity Down* message SHOULD be originated by a
router in response to a packet that cannot be forwarded inside the AS
because because the connectivity between the ingress and egress
routers is broken. The ISD and AS identifier are set to the ISD-AS
of the originating router. The ingress interface ID identifies the
interface on which the packet enters the AS. The egress interface ID
identifies the interface on which the packet is destined to leave the
AS, but the connection is broken to.
Recipients can use this information to route around a broken data
plane inside an AS.
6.6. Informational Messages
6.6.1. Echo Request
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data (variable Len) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: Echo-request format
+============+=================================+
| Name | Value |
+============+=================================+
| Type | 128 |
+------------+---------------------------------+
| Code | 0 |
+------------+---------------------------------+
| Identifier | A 16-bit identifier to aid |
| | matching replies with requests |
+------------+---------------------------------+
| Sequence | A 16-bit sequence number to aid |
| Nr. | matching replies with requests |
+------------+---------------------------------+
| Data | Variable length of arbitrary |
| | data |
+------------+---------------------------------+
Table 16: field values
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Every node SHOULD implement a SCMP Echo responder function that
receives Echo Requests and originates corresponding Echo replies.
6.6.2. Echo Reply
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data (variable Len) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26: Echo-reply format
+==============+=========================================+
| Name | Value |
+==============+=========================================+
| Type | 129 |
+--------------+-----------------------------------------+
| Code | 0 |
+--------------+-----------------------------------------+
| Identifier | The identifier of the Echo Request |
+--------------+-----------------------------------------+
| Sequence Nr. | The sequence number of the Echo Request |
+--------------+-----------------------------------------+
| Data | The data of the Echo Request |
+--------------+-----------------------------------------+
Table 17
Every node SHOULD implement a SCMP Echo responder function that
receives Echo Requests and originates corresponding Echo replies.
The data received in the SCMP Echo Request message MUST be returned
entirely and unmodified in the SCMP Echo Reply message.
6.6.3. Traceroute Request
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ISD | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AS +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Interface ID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27: Traceroute-request format
Given a SCION path constituted of hop fields, traceroute allows to
identify the corresponding on-path ISD-ASes.
+============+=================================+
| Name | Value |
+============+=================================+
| Type | 130 |
+------------+---------------------------------+
| Code | 0 |
+------------+---------------------------------+
| Identifier | A 16-bit identifier to aid |
| | matching replies with requests |
+------------+---------------------------------+
| Sequence | A 16-bit sequence number to aid |
| Nr. | matching replies with request |
+------------+---------------------------------+
| ISD | Place holder set to zero by |
| | SCMP sender |
+------------+---------------------------------+
| AS | Place holder set to zero by |
| | SCMP sender |
+------------+---------------------------------+
| Interface | Place holder set to zero by |
| ID | SCMP sender |
+------------+---------------------------------+
Table 18: field values
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A border router is alerted of a Traceroute Request message through
the Ingress or Egress Router Alert flag set to 1 in the hop field
that describes the traversal of that router in a packet's path (See
Section 2.3.2.4). When such a packet is received, the border router
SHOULD reply with a Traceroute Reply message (Section 6.6.4).
6.6.4. Traceroute Reply
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ISD | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ AS +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Interface ID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28: Traceroute-reply format
+==============+==================================================+
| Name | Value |
+==============+==================================================+
| Type | 131 |
+--------------+--------------------------------------------------+
| Code | 0 |
+--------------+--------------------------------------------------+
| Identifier | The identifier set in the Traceroute Request |
+--------------+--------------------------------------------------+
| Sequence Nr. | The sequence number of the Tracroute Request |
+--------------+--------------------------------------------------+
| ISD | The 16-bit ISD identifier of the SCMP originator |
+--------------+--------------------------------------------------+
| AS | The 48-bit AS identifier of the SCMP originator |
+--------------+--------------------------------------------------+
| Interface ID | The interface ID of the SCMP originating router |
+--------------+--------------------------------------------------+
Table 19: field values
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The identifier is set to the identifier value from the Traceroute
Request message (Section 6.6.3). The ISD and AS identifiers are set
to the ISD-AS of the originating border router.
6.7. SCMP Authentication
Authentication of SCMP packets is not specified here. In current
deployments it is still experimental. Endpoints should therefore
validate link down messages (External Interface Down (Section 6.5.2)
and Internal Connectivity Down (Section 6.5.3)) with additional
signals for reliable operations.
7. Handling Link Failures
7.1. Link Failure Detection - BFD
To detect link failures quickly and reliably, SCION uses the
Bidirectional Forwarding Detection (BFD) protocol ([RFC5880]) on
links between SCION routers. If a router does not receive a BFD
message from its peer at some regular interval, it considers the link
to be down (in both directions) until messages are received again.
A SCION BFD message consists of a SCION packet with a NextHdr value
of 203 (BFD/SCION) and a path type of either 00 (Empty - used on
intra-AS links) or 2 (OneHopPath - used on inter-AS links). The BFD
header itself is a BFD Control Header as described in [RFC5880].
More information on one-hop and empty paths is available in
Section 2.3.3 and Section 2.3.1.
A SCION router SHOULD accept BFD connections from its peers and
SHOULD attempt to establish BFD connections to its peers. While a
link is considered to be down, a SCION router should drop packets
destined to that link. In that case, it SHOULD send a notification
(Section 7.2) to the originator.
7.2. Link Failure Notification - SCMP
In SCION, an intermediate router cannot change the path followed by a
packet, only the source endpoint can chose a different path.
Therefore, to enable fast recovery, a router SHOULD signal forwarding
failures to the source, via a SCMP notification (Section 6.5). This
allows the source endpoint to quickly switch to a different path.
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Sending an SCMP error notification is OPTIONAL. Endpoints should
therefore implement additional mechanisms to validate or detect link
down signals. To reduce exposure to denial-of-service attacks, SCION
routers SHOULD employ rate limiting when sending recommended SCMP
notifications (especially identical ones). Rate limit policies are
up to each AS' administrator.
8. Security Considerations
This section describes the possible security risks and attacks that
the SCION 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.
8.1. 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; 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 is the Hop Field MAC (see Section 4.1)
that authenticates the Hop Field content comprised of ingress/egress
interface identifiers, creation and expiration timestamp and the
preceding Hop Field MACs in the path segment. Each Hop Field MAC is
computed using the secret forwarding key of the respective AS, which
is shared across the SCION routers and control plane services within
each AS.
8.1.1. 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
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limit the exposure after a temporary device compromise. However,
such a key rotation scheme cannot mitigate the impact of an
undiscovered compromise of a device.
When an AS's forwarding key is compromised, an attacker can forge Hop
Field MACs and undermine path authorization. As 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 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.
8.1.2. Forging Hop Field MAC
Another 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 and for 6-byte MAC, an adversary
would need an expected 2^47 (~140 trillion) tries to successfully
forge the MAC of a single Hop Field.
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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.
8.1.3. 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.
Candidate path segments for splicing must have at least one AS
interface in common as a connection point, and 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 diverged and converged back.
The Hop Field MAC protects the 16-bit aggregation of path segment
identifier and preceding MACs, 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, a chance collision among compatible path segments can
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.
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8.2. 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 and 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, or
by simply dropping packets. This kind of attack generally cannot be
prevented, although anendpoint can use SCION's path awareness to
immediately select an alternate path if available.
8.2.1. 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 (e.g. by 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)
where 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. However, such
an attack is of little value. An on-path adversary may inspect/copy/
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disrupt its traffic without diverting it away from the sender-chosen
path. For this reason proof-of-transit, which would be required to
detect such an attack, has marginal benefit in the context of SCION
and it is not in scope for this document.
8.2.2. Payload Integrity
An on-path attacker can modify the payload of a SCION packet.
Existing higher layer protocols can easily defend against such an
attack without any cooperation by the SCION network. For that
reason, payload integrity is not in scope for this specification.
However, there exists a proposal for an experimental extension to
authenticate addresses, provide integrity protection for payloads,
and replay protection: SPAO . This is still very experimental and it
not used in the production network.
8.3. 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 en-route
it will follow its determined path regardless of the actions of the
adversary. An adversary can attempt to disrupt the connectivity of
the path by flooding a link with excessive traffic (see Section 8.4
below), but after detecting congestion, the endpoint can switch to
another non-congested path for subsequent packets.
8.4. 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, and the server will
send a reply that is typically larger than the request to the victim.
This can be prevented in SCION as long as the attacker and the victim
are located in different ASes as the reply packets are simply
returned along reversed path to the actual sender regardless of the
source address information. Thus, the reply packets will be
forwarded to the attacker's AS where they will be discarded because
the destination AS does not match.
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However, the path choice of the endpoint may possibly be exploited by
an attacker to create intermittent congestion with a relatively low
send rate. The attacker can exploit the latency differences of the
available paths, sending at precisely timed intervals to cause short,
synchronized bursts of packets near the victim.
*Note* 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 although additional information
contained in the SCION header enables more targeted filtering, e.g.
by ISD, AS or path length.
9. 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 [ISD-AS-assignments]). This
task is currently being transitioned from Anapaya to the SCION
Association.
10. References
10.1. Normative References
[I-D.dekater-scion-controlplane]
de Kater, C., Rustignoli, N., and S. Hitz, "SCION Control
Plane", Work in Progress, Internet-Draft, draft-dekater-
scion-controlplane-06, 19 October 2024,
<https://datatracker.ietf.org/doc/html/draft-dekater-
scion-controlplane-06>.
[I-D.dekater-scion-pki]
de Kater, C., Rustignoli, N., and S. Hitz, "SCION Control
Plane PKI", Work in Progress, Internet-Draft, draft-
dekater-scion-pki-07, 19 October 2024,
<https://datatracker.ietf.org/doc/html/draft-dekater-
scion-pki-07>.
[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>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/rfc/rfc2460>.
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[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, December 1998,
<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, September 2001,
<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, June
2006, <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, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/rfc/rfc5880>.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/rfc/rfc6437>.
[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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/rfc/rfc8200>.
10.2. 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, 2022,
<https://doi.org/10.1007/978-3-031-05288-0>.
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[CRYPTOBOOK]
Boneh, D. and V. Shoup, "A Graduate Course in Applied
Cryptography", 2023, <https://toc.cryptobook.us/>.
[I-D.dekater-panrg-scion-overview]
de Kater, C., Rustignoli, N., and A. Perrig, "SCION
Overview", Work in Progress, Internet-Draft, draft-
dekater-panrg-scion-overview-06, 7 May 2024,
<https://datatracker.ietf.org/doc/html/draft-dekater-
panrg-scion-overview-06>.
[ISD-AS-assignments]
"SCION ISD and AS Assignments", 2024,
<https://docs.anapaya.net/en/latest/resources/isd-as-
assignments/>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<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,
February 1996, <https://www.rfc-editor.org/rfc/rfc1918>.
[RFC2711] Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
RFC 2711, DOI 10.17487/RFC2711, October 1999,
<https://www.rfc-editor.org/rfc/rfc2711>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/rfc/rfc4821>.
[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>.
[RFC9473] Enghardt, R. and C. Krähenbühl, "A Vocabulary of Path
Properties", RFC 9473, DOI 10.17487/RFC9473, September
2023, <https://www.rfc-editor.org/rfc/rfc9473>.
[SCIONLAB] Kown, J., García-Pardo, J., Legner, M., Wirz, F., Frei,
M., Hausheer, D., and A. Perrig, "SCIONLAB - A Next-
Generation Internet Testbed", 2020,
<https://ieeexplore.ieee.org/abstract/document/9259355>.
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[SCIONLAB_WEBSITE]
"SCIONLab website", 2024, <https://www.scionlab.org/>.
[SCMP] Anapaya, ETH, and SCION, "SCMP Documentation", 2023,
<https://docs.scion.org/en/latest/protocols/scmp.html>.
Acknowledgments
Many thanks go to Matthias Frei (SCION Association), Juan A. Garcia
Prado (ETH Zurich) and Kevin Meynell (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.
Deployment Testing: SCIONLab
SCIONLab is a global research network that is available to test the
SCION architecture. You can create and use your ASes using your own
computation resources which allows you to gain real-world experience
of deploying and managing a SCION network.
More information can be found at [SCIONLAB_WEBSITE] and in the
[SCIONLAB] paper.
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.
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Assignment
+=========+===========+==========================================+
| 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 |
+---------+-----------+------------------------------------------+
Table 20: The assigned SCION protocol numbers
Change Log
Changes made to drafts since ISE submission. This section is to be
removed before publication.
draft-dekater-scion-dataplane-03
Major changes:
* Introduction: clarified document goal and added Figure showing
SCION Header within the stack
* Added section with SCMP specification
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* Added section on Handling Link Failures and BFD
* Added sections on MTU and fragmentation
* Clarified router checks in Processing at Routers
* Security Considerations: add section on Payload Modifications
Minor changes:
* Added short section mentioning SCION IP Gateway
* Clarified the router alert flags and relationship to the
ConsIngress/Egress fields.
* Clarifications in the SCION Header Specification section (router
alert flags, service addresses, one-hop paths, text
clarifications, validity of peering links)
* Added mention of why proof of transit is not needed.
* Rename flow ID to Flow Label and document by reference to
[RFC6437].
* Added reference to SCIONLab as a testbed for implementors
* Added J. C. Hugly as author.
* Introduced this change log
* Clarify addressing and avoid confusing claim of communication
between two endpoints with the same IP (section 1.3.1)
draft-dekater-scion-dataplane-02
Major changes:
* Added overview of SCION components to Introduction section.
* Introduced AES-CMAC as default MAC algorithm and elaborated on MAC
chaining and path splicing.
* Added section to describe Effects of Clock Inaccuracy / time
synchronization requirements
* Added section to describe required router Configuration
* Added service field table.
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Minor changes:
* Removed forward references.
* General edits to make terminology consistent, remove duplication
and rationalize text.
* Added and capitalized RFC2119 compliant terminology.
* Clarified implications of AS forwarding key compromise and path
splicing in security considerations
* Clarified the computation of ExtLen.
Authors' Addresses
Corine de Kater
SCION Association
Email: c_de_kater@gmx.ch
Nicola Rustignoli
SCION Association
Email: nic@scion.org
Jean-Christophe Hugly
SCION Association
Email: jch@scion.org
Samuel Hitz
Anapaya Systems
Email: hitz@anapaya.net
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