]> SCION Association

kme@scion.org

ETH Zürich

juan.garcia@inf.ethz.ch

ETH Zürich

tilmann.zaeschke@inf.ethz.ch

SCION Association

nic@scion.org

IRTF PANRG Internet-Draft TODO Abstract here About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the WG Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction SCION is an inter-domain network architecture. Its core components specification, as deployed by some of its early adopters, is outlined in , , , currently under ISE review. The goal of this draft is to explore how SCION and its early deployments try to address open research questions in . Specifically, there are still many open areas of research around path-aware networking, where SCION with its early deployment experiences and research efforts can provide a contribution. This can also be a starting point for discussions around long-term protocol evolution. This draft assumes the reader is familiar with some of the fundamental concepts defined in the components specification. Note: This is the very first version of the SCION research questions draft, and it merely contains a skeleton of potential topics to be further discussed in this draft. Any feedback is welcome and much appreciated. Thanks!

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 when, and only when, they appear in all capitals, as shown here.

Discovery, Distribution, and Trustworthiness of Path Properties

ISD, AS identity

• How many ASes do we expect in the SCION network model?
• How many ISDs? What is the ontology of an ISD? Per geographic area only?
• One AS belongs to many ISDs?

Segment Dissemination Control servers return a large number of path segments. This can cost considerable bandwidth / network egress while at the same time overloading clients with an unnecessarily large numbers of segments, mostly consisting of redundant information in terms of duplicate link and hops.

• This problem may be more problematic in ASes with many end hosts (e.g. IoT), or end hosts with little computing power or little spare bandwidth.
• Getting a full path to a remote endhost may require three round-trips with the control server.

There are multiple possible and independent solution steps here:

• Compression (idea suggested by Francois Wirz): Segments could be stored in a way that duplicate information (hops & links) is only stored once and the segments contain only references to the hops and links.
• Allow queries from start AS to end AS across multiple segments. This should be very easy to implement and would be compatible with the current wire protocol (protobuf).
  • This would reduce the number of round trips to one.
  • It would reduce the number of returned segments because only segments that actually connect to other segments would need to be returned.
• Predefine some policies that can be resolved by the control server, e.g. ANY, BEST_LATENCY, BEST_BANDWIDTH, BEST_PRICE, BEST_CO2. For these, a control server could simply calculate 5-10 good paths and return these. Moreover these could be cached for commonly requested remote ASs. If a user requires a custom policy they can still resort to requesting actual segments.

Doing path computation on the control server will initially increase computational cost. However, it would substantially decrease network egress. Caching of paths should reduce CPU cost, maybe even below the current cost for retrieving a large amount of segments from the local database and sending them over the network interface.

Routing Policies and Traffic Engineering Reduced adoption due to limited routing policy possibilities, such as a (core-)AS does not want to accept transit traffic unless it starts/ends in ASs with special properties. For example a GEANT AS does not want to allow transit traffic unless it originates or ends in another research AS. One solution could be to add a “confirm full path”-flag to certain segments. If this flag is set, the full path (all segments) needs to authorized by all ASes that insist on authorizing it. This is obviously less scalable but may be viable for ASes that insist on such policies. This also allows for “secret” policies. Collateral: this probably needs a data plane change. Conceptually, we have only a single resulting segment, and that segment needs to be used in full, e.g. no on-path trickery.

DRKey

• Is forward-secrecy DRKey useful and should we develop it?
• What are the properties of the control-plane?
  • Do we want to have any authorization of the data-plane transit undertaken at this stage?
  • Would this obsolete firewalls?
  • What do we mean when we say "authorize transit"?

For more info: .

SCMP Authentication

Proof of transit FABRID and EPIC .

NAT At this moment, the SCION implementation is not compatible out-of-the-box with NAT'ed devices, whether these devices are end-hosts, or even running SCION services. This is due to the (UDP-IP) underlay being modified by the NAT mechanism, but not the internal destination SCION address. Although this does not concern the SCION protocols themselves, we want to check that this will not be a problem.

• With IPv6 underlay, this problem disappears.
• Modify the SCION border router to also act as a STUN server.

Dataplane stability

Link load balancing Links may get overloaded because the SCION routing system fails to distribute load properly over different links. New/different links might be underutilized. If links become overloaded, there are several ways to handle that. Non comprehensively:

• Squeeze: send an SCMP message to trigger end-hosts to use an alternative path
• Steer: send and SCMP to trigger users to ask CS for a better path
• Reduce: hand over very short lived paths, let the end-hosts wait for the path to expire so that they request new paths and (hopefully) decide on a different path.
• Recommend: let the end-hosts know which paths are recommended by the AS at this time.

If a link has good properties, many AS will disseminate segments, therefore paths that go through this link and the link may become overloaded. See Simon Scherrer's work on Braess Paradox. Either there needs to be some constant control by all clients to not choose the best theoretical path, but the one that works best. Or we need to find a way that control servers do not disseminate “good” links to all end-hosts. The current consensus is that end-hosts can use multi-pathing and “automatically” converge on the best path, i.e. creating an equilibrium. Again, see Simon Scherrer's work on Braess Paradox.

Reverse Path Refreshment When a client contacts a server, it is usually understood that it wants the server to use the reverse path to answer back. It the server uses that path for a long period of time, the path will eventually expire. How to standardize the process of refreshment?

• The server must ask the CS for a path, regardless of the client's policy.
• The client (somehow) sends a new packet with a new path, prompting the server to use this path from now on.

There are some nuances: Usually the server's API will store the initial address of the client to be used through all the session. We might need to take this into account.

Hummingbird / QoS

• How many QoS flows to support at core routers?
• How does QoS interact with Net Neutrality?
• What proof of transit (or forwarding failure detection) is needed or wanted?
• What time synchronization precision should we expect at the border router level of every AS? How far can we go realistically?

Interfaces for Path Awareness

• IPv6 in the Data Plane
• SCION-IP translation

Implications of Path Awareness for the Transport and Application Layers To be discussed

Naming To be discussed

Security Considerations TODO Security

IANA Considerations This document has no IANA actions.

References Normative References In contrast to the present Internet architecture, a path-aware internetworking architecture has two important properties: it exposes the properties of available Internet paths to endpoints, and it provides for endpoints and applications to use these properties to select paths through the Internet for their traffic. While this property of "path awareness" already exists in many Internet-connected networks within single domains and via administrative interfaces to the network layer, a fully path-aware internetwork expands these concepts across layers and across the Internet. This document poses questions in path-aware networking, open as of 2021, that must be answered in the design, development, and deployment of path-aware internetworks. It was originally written to frame discussions in the Path Aware Networking Research Group (PANRG), and has been published to snapshot current thinking in this space. SCION Association SCION Association Anapaya Systems SCION Association SCION Association Anapaya Systems SCION Association SCION Association Anapaya Systems In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich ETH Zürich Armasuisse ETH Zürich Armasuisse

Acknowledgments TODO acknowledge.