SID Space (5f00::/16) Inter-domain Addressing Recommendations

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Table of Contents

1. Introduction

[RFC9602] requested of IANA a dedicated prefix for Segment Routing over IPv6 [RFC8402] Segment Identifiers (SRv6 SIDs), with the aim of "improv[ing] security by making it simpler to filter traffic at the edge of the SR domains." The prefix 5f00::/16 was allocated for this purpose [IANA-IPv6Special]. No requirements were placed on the use of this prefix nor any recommendations made for structured use of this prefix.¶

This specification recommends a specific structured use of the SRv6 SIDs prefix in support of Inter-Domain SRv6 networks. The core of the proposal is to structure the address space by Autonomous System Number (ASN).¶

Use of this proposed structure is entirely voluntary. Voluntary use of this structure aids SRv6 operations while preserving the ability to use this prefix across cooperating SRv6 domains, but not across the general Internet.¶

The SID space prefix was allocated to improve ease of filtering. Where SRv6 traffic using these prefixes may be shared with cooperating partner networks, this proposal makes it easier to craft filters that permit only SRv6 traffic from identified ASNs.¶

As a point of historical interest, this proposal contains echos of the structure of the original 6bone test allocation [RFC1897].¶

2. Inter-domain SRv6 SIDs

An inter-domain SRv6 SID, as used in this document, means an SRv6 SID from the address space used by one SRv6 domain that is advertised to another SRv6 domain force inclusion an SRv6 Policy used by the second domain when forwarding policy-specific traffic to the advertising SRv6 domain.¶

3. Proposed Structure

The recommendation of this specification is for SRv6 domains to allocate SIDs from prefixes that are concatenations of the SRv6 SID prefix (5f00::/16) and an applicable ASN. Assuming 32-bit ASNs, this yields a /48 per ASN in use within an SRv6 domain, i.e. 5f00:as-hi16:as-lo16::/48.¶

5f00:0:fbf0::/48
...
5f00:0:fbff::/48

5f00:0:fc00::/48
...
5f00:0:fffe::/48

5f00:fa56:ea00::/48
...
5f00:ffff:fffe::/48

4. Routing and Filtering

As noted in [draft-bdmgct-spring-srv6-security], it is assumed that each ASN using this SRv6 SID space structure has deployed their respective SRv6 implementations within a limited domain [RFC8799] with appropriate filtering at the domain boundaries. Because this is intended for inter-domain use, the requisite filtering exceptions must be made between each SRv6 domain to allow for the desired Inter-Domain communication to occur. Care should be taken to allow only the desired and necessary communication between each SRv6 domain. The mechanisms used should be conformant with the given domain's security policy and may include, but are not limited to:¶

• routing filters such as BGP prefix-lists, route-maps, route-policies, or other analogous mechanisms, or¶

• access control filters at the domain edge¶

5. Example test case

One possible test case is the exchange of the IPv6 prefix SID between two autonomous systems with independent management domains. In this example, AS4294967294 exchanges their SRv6 SID prefix (5f00:ffff:fffe::/48) with AS4200000000 who announces their ASN derived SRv6 SID prefix (5f00:fa56:ea00::/48).¶

  ┌─────────────────────────────────┐           ┌──────────────────────────────────┐
  │                                 │           │                                  │
  │                                 │           │                                  │
  │                  eBGP speaker   │           │   eBGP speaker                   │
  │           5f00:ffff:fffe::/48   │           │   5f00:fa56:ea00::/48            │
  │   ┌─────┐               ┌────┐  │           │  ┌────┐                ┌─────┐   │
  │   │     ├──────┐        │    ├──┼───────────┼──┤    │        ┌───────┤     │   │
  │   │     │      │        │    │  │           │  │    │        │       │     │   │
  │   └─────┘   ┌──┴──┐     └─┬──┘  │           │  └──┬─┘     ┌──┴──┐    └─────┘   │
  │             │     │       │     │           │     │       │     │              │
  │             │     ├───────┘     │           │     └───────┤     │              │
  │             └─────┘             │           │             └─────┘              │
  │                                 │           │                                  │
  │                                 │           │                                  │
  │                                 │           │                                  │
  │ AS4294967294                    │           │                      AS4200000000│
  └─────────────────────────────────┘           └──────────────────────────────────┘

Within this structure, appropriate and agreed upon policy may be shared between the partner ASNs. Defining the policy or use cases is outside of the scope of this document.¶

6. Security Considerations

This document does not alter the inherent security posture of SRv6 [RFC8402], [RFC8754]. The SID space prefix was allocated to improve ease of filtering. Where SRv6 traffic using these prefixes may be shared with cooperating partner networks, this proposal makes it easier to craft filters that permit only SRv6 traffic from identified ASNs.¶

7. IANA Considerations

This document has no IANA actions.¶

8. References

Acknowledgments

TODO acknowledge.¶

Authors' Addresses