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. The IETF has defined several Routing header types [IANA-RH]. This document defines two new Routing header 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.¶
• Is designed to operate within a network domain. (See Section 9).¶

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.¶

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 SID identifies a CRH-FIB entry.¶

Each CRH-FIB entry contains:¶

• An IPv6 address (optional).¶
• A topological function.¶
• Arguments for the topological function (optional).¶
• Flags.¶
• A service function (optional).¶
• Arguments for the service function (optional).¶

The IPv6 address can represent either:¶

• An interface on the next segment endpoint.¶
• An SRv6 SID [RFC8986], instantiated on the next segment endpoint.¶

The first ten bits of the IPv6 address MUST NOT be fe00. That prefix is reserved for link-local [RFC6890] addresses.¶

The topological function specifies how the processing node forwards the packet to the next segment endpoint. The following are examples:¶

• Forward the packet through the least-cost path to the next segment endpoint.¶
• Forward the packet through a specified interface.¶
• Encapsulate the packet in another IPv6 header of any type (e.g., MPLS, IPv6) and forward either through the least cost path or a specified interface.¶
• Recycle the packet, as if the node had forwarded to one of its own interfaces. When recycling is complete, process the next SID. See Appendix B for a packet recycling use-case.¶

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

The following flags are defined:¶

• The PSP flag indicates whether the penultimate segment endpoint (i.e., the node that sets Segments Left to 0) MAY remove the CRH.¶
• The OAM flag indicates whether the processing node should invoke OAM procedures for which it is configured.¶

The service function is optional. If present, it invokes a node specific procedure. The following are examples of node specific procedures:¶

• Emit telemetry.¶
• Subject the packet's payload to a firewall rule.¶
• Replicate the packet, forwarding one copy and retaining the other for sampling, analysis, or other purposes.¶

Node specific procedures are not subject to standardization. A node can support any number of node specific procedures and associate them with any SIDs.¶

Some service functions require parameters. For example, an instruction to emit telemetry might require an IP address to which telemetry should be sent.¶

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].¶

9. Security Considerations

Networks that process the CRH MUST NOT accept packets containing the CRH from untrusted sources. Their border routers SHOULD discard packets that satisfy the following criteria:¶

• The packet contains a CRH¶
• The Segments Left field in the CRH has a value greater than 0¶
• The Destination Address field in the IPv6 header represents an interface that resides inside of the network.¶

Many border routers cannot filter packets based upon the Segments Left value. These border routers MAY discard packets that satisfy the following criteria:¶

• The packet contains a CRH¶
• The Destination Address field in the IPv6 header represents an interface that resides inside of the network.¶

12. Acknowledgements

Thanks to Dr. Vanessa Ameen, Fernando Gont, Naveen Kottapalli, Joel Halpern, Tony 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 SID list contains one entry for each segment in the path (Appendix A.1).¶
• The 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.¶

Appendix B. A Packet Recycling Use-Case

             Network A--D1
            /     \     /
           /       \   /
          /         \ /
S  ----  P           +
          \        /  \
           \      /    \
            \    /      \
             Network B---DN

In Figure 4:¶

• The SR domain contains Node S, Node P, and a set of destination nodes (D1 through DN)¶
• S is connected to P¶
• P is connected to Network A and to Network B. Neither of these networks are SR-capable.¶
• The destination nodes connect to both Network A and Network B¶

S needs to reach each destination node through two SR paths. One SR path traverses Network A while the other traverses Network B.¶

Uncompressed SRv6 can encode this SR Path in two segments,with one segment instantiated on P and the other on the destination. To support this strategy, P instantiates two END.X SIDs (one per network).¶

CRH compressed SRv6 can encode this SR Path in two or three segments. When it encodes the path in two segments, one segment instantiated on P and the other on the destination. To support this strategy, P instantiates 2*N SIDs (one per network per destination). When CRH compressed SRv6 encodes the path in three segments, two segments are instantiated on P and the other on the destination. The first segment on P updates the IPv6 Destination address without forwarding the packet, while the other segment on P forwards the packet without updating the IPv6 destination address. To support this strategy, P instantiates 2+N SIDs (one per network and one per destination).¶

Authors' Addresses