The IPv6 Compact Routing Header (CRH)

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Table of Contents

1. Introduction

IPv6 [RFC8200] source nodes use Routing headers to specify the path that a packet takes to its destination(s). The IETF has defined several Routing types [IANA-RH]. This document defines two new Routing types. Collectively, they are called the Compact Routing Headers (CRH). Individually, they are called CRH-16 and CRH-32.¶

The CRH allows IPv6 source nodes to specify the path that a packet takes to its destination. The CRH can be encoded in relatively few bytes. The following are reasons for encoding the CRH in as few bytes as possible:¶

• Many ASIC-based forwarders copy headers from buffer memory to on-chip memory. As header sizes increase, so does the cost of this copy.¶

• Because Path MTU Discovery (PMTUD) [RFC8201] is not entirely reliable, many IPv6 hosts refrain from sending packets larger than the IPv6 minimum link MTU (i.e., 1280 bytes). When packets are small, the overhead imposed by large Routing Headers is excessive.¶

This document describes an experiment whose purposes are:¶

• To demonstrate that the CRH can be implemented and deployed.¶

• To demonstrate that the security considerations, described in this document, can be addressed with access control lists.¶

• To encourage replication of the experiment.¶

2. Requirements Language

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.¶

4. The CRH Forwarding Information Base (CRH-FIB)

Each CRH SID identifies a CRH-FIB entry.¶

Each CRH-FIB entry contains:¶

• An IPv6 address.¶

• A topological function.¶

• Arguments for the topological function. (Optional).¶

The IPv6 address can be a Global Unicast Address (GUA), a Link Local Unicast address (LLU), or a Unique Local Address (ULA). When the IPv6 address is the final address in a path, it can also be a multicast address.¶

The topological function specifies how the processing node forwards the packet to the interface identified by the IPv6 address. The following are examples:¶

• Forward the packet through the least-cost path to the interface identified by the IPv6 address (i.e., loose source routing).¶

• Forward the packet through a specified interface to the interface identified by the IPv6 address (i.e.,strict source routing)¶

Some topological functions require parameters. For example, a topological function might require a parameter that identifies the interface through which the packet is forwarded.¶

The CRH-FIB can be populated:¶

• By an operator, using a Command Line Interface (CLI).¶

• By a controller, using the Path Computation Element (PCE) Communication Protocol (PCEP) [RFC5440] or the Network Configuration Protocol (NETCONF) [RFC6241].¶

• By a distributed routing protocol [ISO10589-Second-Edition], [RFC5340], [RFC4271].¶

The above-mentioned mechanisms are not defined here and are beyond the scope of this document¶

9. Textual Representation

A 16-bit CRH SID can be represented by four lower-case hexadecimal digits. Leading zeros SHOULD be omitted. However, the all-zeros CRH SID MUST be represented by a single 0. The following are examples:¶

• beef¶

• eef¶

• 0¶

A 16-bit CRH SID also can be represented in dotted-decimal notation. The following are examples:¶

• 192.0¶

• 192.51¶

A 32-bit CRH SID can be represented by four lower-case hexadecimal digits, a colon (:), and another four lower-case hexadecimal digits. Leading zeros MUST be omitted. The following are examples:¶

• dead:beef¶

• ead:eef¶

• :beef¶

• beef:¶

• :¶

A 32-bit CRH SID can also be represent in dotted-decimal notation. The following are examples:¶

• 192.0.2.1¶

• 192.0.2.2¶

• 192.0.2.3¶

10. Security Considerations

In this document, one node trusts another only if both nodes are operated by the same party. A node determines whether it trusts another node by examining its IP address. In many networks, operators number their nodes from a small number of prefixes. This facilitates identification of trusted nodes.¶

A node can encounter security vulnerabilities when it processes a Routing Header that originated on an untrusted node [RFC5095]. Therefore, nodes MUST deploy ACLs that discard packets containing the CRH when both of the following conditions are true:¶

• The Source Address does not identify an interface on a trusted node.¶

• The Destination Address identifies an interface on the local node.¶

The above-mentioned ACLs do not protect the node from attack packets that contain a forged (i.e., spoofed) Source Address. In order to mitigate this risk, nodes MAY also discard packets containing the CRH when all of the following conditions are true:¶

• The Source Address identifies an interface on a trusted node.¶

• The Destination Address identifies an interface on the local node.¶

• The packet does not pass an Enhanced Feasible-Path Unicast Reverse Path Forwarding (RPF) [RFC8704],¶

The RPF check eliminates some, but not all packets with forged source addresses. Therefore, a network operator that deploys CRH MUST implement Access Control Lists (ACL) on each of its edge nodes. The ACL discards packets whose source address identifies an interface on a trusted node.¶

The CRH is compatible with end-to-end IPv6 Authentication Header (AH) [RFC4302] processing. This is becasue the source node calculates the Integrity Check Value (ICV) over the packet as it arrives at the destination node.¶

14. Acknowledgements

Thanks to Dr. Vanessa Ameen, Dale Carder, Brian Carpenter, Adrian Farrel, Fernando Gont, Naveen Kottapalli, Joel Halpern, Mark Smith, Reji Thomas, Tony Li, Xing Li, Gerald Schmidt, Nancy Shaw, Ketan Talaulikar, and Chandra Venkatraman for their contributions to this document.¶

Appendix A. CRH Processing Examples

This appendix demonstrates CRH processing in the following scenarios:¶

• The CRH SID list contains one entry for each segment in the path (Appendix A.1).¶

• The CRH SID list omits the first entry in the path (Appendix A.2).¶

 -----------                 -----------                 -----------
|Node: S    |               |Node: I1   |               |Node: I2   |
|Loopback:  |---------------|Loopback:  |---------------|Loopback:  |
|2001:db8::a|               |2001:db8::1|               |2001:db8::2|
 -----------                 -----------                 -----------
      |                                                       |
      |                      -----------                      |
      |                     |Node: D    |                     |
       ---------------------|Loopback:  |---------------------
                            |2001:db8::b|
                             -----------

Figure 3 provides a reference topology that is used in all examples.¶

Table 1 describes two entries that appear in each node's CRH-FIB.¶

Authors' Addresses