The IPv6 Compact Routing Header (CRH)

Abstract

This document describes an experiment in which two new IPv6 Routing headers are implemented and deployed. Collectively, they are called the Compact Routing Headers (CRH). Individually, they are called CRH-16 and CRH-32.¶

One purpose of this experiment is to demonstrate that the CRH can be implemented and deployed in a production network. Another purpose is to demonstrate that the security considerations, described in this document, can be addressed with access control lists. Finally, this document encourages replication of the experiment.¶

3. The Compressed Routing Headers (CRH)

Both CRH versions (i.e., CRH-16 and CRH-32) contain the following fields:¶

• Next Header - Defined in [RFC8200].¶

• Hdr Ext Len - Defined in [RFC8200].¶

• Routing Type - Defined in [RFC8200]. (CRH-16 value is 5. CRH-32 value is 6).¶

• Segments Left - Defined in [RFC8200].¶

• Type-specific Data - Described in [RFC8200].¶

In the CRH, the Type-specific data field contains a list of Segment Identifiers (SIDs). Each SID identifies an entry in the CRH Forwarding Information Base (CRH-FIB) (Section 4). Each CRH-FIB entry identifies an interface on the path that the packet takes to its destination.¶

SIDs are listed in reverse order. So, the first SID in the list represents the final interface in the path. Because segments are listed in reverse order, the Segments Left field can be used as an index into the SID list. In this document, the "current SID" is the SID list entry referenced by the Segments Left field.¶

The first segment in the path can be omitted from the list. See Appendix A an example.¶

In the CRH-16 (Figure 1), each SID is encoded in 16-bits. In the CRH-32 (Figure 2), each SID is encoded in 32-bits.¶

In all cases, the CRH MUST end on a 64-bit boundary. So, the Type- specific data field MUST be padded with zeros if the CRH would otherwise not end on a 64-bit boundary.¶

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Next Header  |  Hdr Ext Len  | Routing Type  | Segments Left |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             SID[0]            |          SID[1]               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
  |                          .........
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Next Header  |  Hdr Ext Len  | Routing Type  | Segments Left |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  +                             SID[0]                            +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  +                             SID[1]                            +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  //                                                              //
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  +                             SID[n]                            +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5. Processing Rules

The following rules describe CRH processing:¶

• If Segments Left equals 0, skip over the CRH and process the next header in the packet.¶

• If Hdr Ext Len indicates that the CRH is larger than the implementation can process, discard the packet and send an ICMPv6 [RFC4443] Parameter Problem, Code 0, message to the Source Address, pointing to the Hdr Ext Len field.¶

• Compute L, the minimum CRH length ( Section 5.1).¶

• If L is greater than Hdr Ext Len, discard the packet and send an ICMPv6 Parameter Problem, Code 0, message to the Source Address, pointing to the Segments Left field.¶

• Decrement Segments Left.¶

• Search for the current SID in the CRH-FIB. In this document, the "current SID" is the SID list entry referenced by the Segments Left field.¶

• If the search does not return a CRH-FIB entry, discard the packet and send an ICMPv6 Parameter Problem, Code 0, message to the Source Address, pointing to the current SID.¶

• If Segments Left is greater than 0 and the CRH-FIB entry contains a multicast address, discard the packet and send an ICMPv6 Parameter Problem, Code 0, message to the Source Address, pointing to the current SID.¶

• Copy the IPv6 address from the CRH-FIB entry to the Destination Address field in the IPv6 header.¶

• Decrement the IPv6 Hop Limit.¶

• Submit the packet, its topological function and its parameters to the IPv6 module. See NOTE.¶

NOTE: By default, the IPv6 module determines the next-hop and forwards the packet. However, the topological function may elicit another behavior. For example, the IPv6 module may forward the packet through a specified interface.¶

<CODE BEGINS>

switch(Routing Type) {
    case CRH-16:
        if (Segments Left <= 2)
            return(0)
        sidsBeyondFirstWord = Segments Left - 2;
        sidPerWord = 4;
    case CRH-32:
        if (Segments Left <= 1)
            return(0)
        sidsBeyondFirstWord = Segments Left - 1;
        sidsPerWord = 2;
    case default:
        return(0xFF);
    }

words = sidsBeyondFirstWord div sidsPerWord;
if (sidsBeyondFirstWord mod sidsPerWord)
    words++;

return(words)



<CODE ENDS>

6. Mutability

In the CRH, the Segments Left field is mutable. All remaining fields are immutable.¶

7. Applications And SIDs

A CRH contains one or more SIDs. Each SID is processed by exactly one node.¶

Therefore, a SID is not required to have domain-wide significance. Applications can:¶

• Allocate SIDs so that they have domain-wide significance.¶

• Allocate SIDs so that they have node-local significance.¶

8. Management Considerations

PING and TRACEROUTE [RFC2151] both operate correctly in the presence of the CRH. TCPDUMP and Wireshark have been extended to support the CRH.¶

9. Textual Representation

A 16-bit SID is represented by a colon (:) followed by four hexadecimal digits. Leading zeros can be omitted. The following are examples:¶

• :1¶

• :fef3¶

• :1234¶

• :34¶

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

• ::13¶

• :123:abcd¶

• :fffe:ffaa¶

• :1234:5678¶

11. Implementation and Deployment Status

Juniper Networks has produced experimental implementations of the CRH on the MX-series (ASIC-based) router¶

Liquid Telecom has produced experimental implementations of the CRH on software based routers.¶

The CRH has carried non-production traffic in CERNET and Liquid Telecom.¶

12. IANA Considerations

This document makes the following registrations in the "Internet Protocol Version 6 (IPv6) Parameters" "Routing Types" subregistry maintained by IANA:¶

         +-------+------------------------------+---------------+
         | Value | Description                  | Reference     |
         +=======+==============================+===============+
         | 5     | CRH-16                       | This document |
         +-------+------------------------------+---------------+
         | 6     | CRH-32                       | This document |
         +-------+------------------------------+---------------+

14. Contributors

• Gang Chen¶

• Baidu¶

• No.10 Xibeiwang East Road Haidian District¶

• Beijing 100193 P.R. China¶

• Email: phdgang@gmail.com¶

• ¶

• Yifeng Zhou¶

• ByteDance¶

• Building 1, AVIC Plaza, 43 N 3rd Ring W Rd Haidian District¶

• Beijing 100000 P.R. China¶

• Email: yifeng.zhou@bytedance.com¶

• ¶

• Gyan Mishra¶

• Verizon¶

• Silver Spring, Maryland, USA¶

• Email: hayabusagsm@gmail.com¶

15. References