The IPv6 Compressed Routing Header (CRH)
draft-bonica-6man-comp-rtg-hdr-06

Abstract

This document defines two new IPv6 Routing header types. Generically, they are called the Compressed Routing Header (CRH). More specifically, the 16-bit version of the CRH is called the CRH-16, while the 32-bit version of the CRH is called the CRH-32. SRv6+ nodes use the CRH to steer packets from segment to segment along SRv6+ paths.

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. The Compressed Routing Header (CRH)
• 4. The Segment Forwarding Information Base (SFIB)
• 5. Processing Rules
• 5.1. General
• 5.2. CRH Specific
• 5.2.1. Computing Minimum CRH Length
• 5.2.2. Topological Instructions That Control Adjacency Segments
• 5.2.3. Topological Instructions That Control Node Segments
• 6. Mutability
• 7. Compliance
• 8. Management Considerations
• 9. Security Considerations
• 10. IANA Considerations
• 11. Acknowledgements
• 12. References
• 12.1. Normative References
• 12.2. Informative References
• Appendix A. CRH Processing Examples
• A.1. SR Path Contains Node Segments Only
• A.2. SR Path Contains Node Segments Only And Preserves The First SID
• A.3. SR Path Contains Adjacency Segments Only
• Authors' Addresses

1. Introduction

This document defines two new IPv6 Routing header types. Generically, they are called the Compressed Routing Header (CRH). More specifically, the 16-bit version of the CRH is called the CRH-16, while the 32-bit version of the CRH is called the CRH-32. SRv6+ nodes use the CRH to steer packets from segment to segment along SRv6+ paths.

For details regarding SRv6+ paths, segments, Segment Identifiers (SIDs) and instructions, see [I-D.bonica-spring-srv6-plus].

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 [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. The Compressed Routing Header (CRH)

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

• Next Header - Defined in [RFC8200]. Identifies the type of header immediately following the CRH.
• Hdr Ext Len - Defined in [RFC8200]. Length of the CRH in 8-octet units, not including the first 8 octets.
• Routing Type - Defined in [RFC8200]. Value TBD by IANA. (For CRH-16, the suggested value is 5. For CRH-32, the suggested value is 6.)
• Segments Left - Defined in [RFC8200]. Number of route segments remaining, i.e., number of explicitly listed intermediate nodes still to be visited before reaching the final destination.
• SID List - Represents the SRv6+ path as an ordered list of SIDs. SIDs are listed in reverse order, with SID[0] representing the final segment, SID[1] representing the penultimate segment, and so forth. SIDs are listed in reverse order so that Segments Left can be used as an index to the SID List. The SID indexed by Segments Left is called the current SID.

In the CRH-16, each SID list entry is encoded in 16-bits. In the CRH-32, each SID list entry is encoded in 32-bits. In networks where the smallest feasible Maximum SID Value (MSV) is greater than 65,635, CRH-32 is required. Otherwise, CRH-16 is preferred.

In all cases, the CRH MUST end on a 64-bit boundary. Therefore, the CRH MAY be padded with zeros.

     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]               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
    |                          .........
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-        
   

Figure 1: CRH-16

     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]                            +
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   

Figure 2: CRH-32

4. The Segment Forwarding Information Base (SFIB)

A segment ingress node MUST maintain one Segment Forwarding Information Base (SFIB) entry for each segment that it originates. Each SFIB entry contains the following information:

• A SID.
• A segment type.
• Topological instruction parameters.

The following are valid segment types:

• Adjacency.
• Node.

The following parameters are associated with topological instructions that control adjacency segments:

• An IPv6 address that identifies an interface on the segment egress node.
• An interface identifier.

Node segments are associated with a single topological instruction parameter. This parameter is an IPv6 address that identifies an interface on the segment egress node.

5. Processing Rules

5.1. General

[RFC8200] defines rules that apply to IPv6 extension headers, in general, and IPv6 Routing headers, in particular. All of these rules apply to the CRH.

For example:

• Extension headers (except for the Hop-by-Hop Options header) are not processed, inserted, or deleted by any node along a packet's delivery path, until the packet reaches the node (or each of the set of nodes, in the case of multicast) identified in the Destination Address field of the IPv6 header.
• If, while processing a received packet, a node encounters a Routing header with an unrecognized Routing Type value, the required behavior of the node depends on the value of the Segments Left field. If Segments Left is zero, the node must ignore the Routing header and proceed to process the next header in the packet, whose type is identified by the Next Header field in the Routing header. If Segments Left is non-zero, the node must discard the packet and send an ICMPv6 Parameter Problem, Code 0, message to the packet's Source Address, pointing to the unrecognized Routing Type.
• If, after processing a Routing header of a received packet, an intermediate node determines that the packet is to be forwarded onto a link whose link MTU is less than the size of the packet, the node must discard the packet and send an ICMPv6 Packet Too Big message to the packet's Source Address.

5.2. CRH Specific

When a node recognizes and processes a CRH, it executes the following procedure:

Appendix A.

• If the IPv6 Source Address is a link-local address, discard the packet.
• If the IPv6 Source Address is a multicast address, discard the packet.
• 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 Parameter Problem, Code 0, message to the Source Address, pointing to the Hdr Ext Len field.
• Compute L, the minimum CRH length (See Section 5.2.1).
• 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 the packet's Hop Count.
• If the Hop Count has expired, discard the packet and send an ICMPv6 Time Expired message to the packet's source node.
• Decrement Segments Left
• Search for the current SID in the SFIB.
• If the above-mentioned search does not return an SFIB entry, discard the packet and send an ICMPv6 Parameter Problem, Code 0, message to the Source Address, pointing to the current SID.
• If the above-mentioned search returns an SFIB entry that represents an adjacency segment, execute the topological instruction described in Section 5.2.2.
• If the above-mentioned search returns an SFIB entry that represents a node segment, execute the topological instruction described in Section 5.2.3.

The above stated rules are demonstrated in

5.2.1. Computing Minimum CRH Length

The algorithm described in this section accepts the following CRH fields as its input parameters:

• Routing Type (i.e., CRH-16 or CRH-32).
• Segments Left.

It yields L, the minimum CRH length. The minimum CRH length is measured in 8-octet units, not including the first 8 octets.

<CODE BEGINS>

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

if (sidsBeyondFirstWord <= 0)
    return(0)

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

return(words)


<CODE ENDS>

5.2.2. Topological Instructions That Control Adjacency Segments

A topological instruction that controls an adjacency segment accepts the following parameters:

• An IPv6 address that identifies an interface on the segment egress node.
• An interface identifier.

The instruction behaves as follows:

• If the interface that was received as a parameter is not operational, discard the packet and send an ICMPv6 Destination Unreachable message (Code: 5, Source Route Failed) to the packet's source node.
• Overwrite the packet's Destination Address with the IPv6 address that was received as a parameter.
• Forward the packet through the above-mentioned interface.

5.2.3. Topological Instructions That Control Node Segments

A topological instruction that controls a node segment accepts a single parameter. This parameter is an IPv6 address that identifies an interface on the segment egress node.

The instruction behaves as follows:

• If the segment ingress node does not have a viable route to the IPv6 address included as a parameter, discard the packet and send an ICMPv6 Destination Unreachable message (Code:1 Net Unreachable) to the packet's source node.
• Overwrite the packet's Destination Address with the destination address that was included as a parameter.
• Forward the packet to the next hop along the least cost path to the segment egress node. If there are multiple least cost paths to the segment egress node (i.e., Equal Cost Multipath), execute procedures so that all packets belonging to a flow are forwarded through the same next hop.

6. Mutability

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

7. Compliance

In order to be compliant with this specification, an SRv6+ implementation MUST:

• Be able to process IPv6 options as described in Section 4.2 of [RFC8200].
• Be able to process the Routing header as described in Section 4.4 of [RFC8200].
• Support the CRH-16 and the CRH-32.

8. Management Considerations

PING and TRACEROUTE both operate correctly in the presence of the CRH.

9. Security Considerations

SRv6+ domains MUST NOT span security domains. In order to enforce this requirement, security domain edge routers MUST do one of the following:

• Discard all inbound SRv6+ packets whose IPv6 destination address represents domain infrastructure.
• Authenticate [RFC4303] all inbound SRv6+ packets whose IPv6 destination address represents domain infrastructure.

10. IANA Considerations

IANA is requested to make the following entries in the Internet Protocol Version 6 (IPv6) Parameters "Routing Type" registry:

     Suggested    
     Value            Description                           Reference
   -----------------------------------------------------------------------
       5      Compressed Routing Header (16-bit) (CRH-16)   This document
       6      Compressed Routing Header (32-bit) (CRH-32)   This document

11. Acknowledgements

Thanks to Joel Halpern, Tony Li, Gerald Schmidt, Nancy Shaw, and Chandra Venkatraman for their comments.

12. References

12.1. Normative References

12.2. Informative References

Appendix A. CRH Processing Examples

This appendix demonstrates CRH processing in the following scenarios:

• SR path contains node segments only.
• SR path contains node segments only and preserves the first SID.
• SR path contains adjacency segments only.

                             -----------
         2001:db8:0:2/64    |Node: I2   |   2001:db8:0:4/64
      ----------------------|Loopback:  |--------------------         
     |                  ::2 |2001:db8::2| ::1                |
     |                       -----------                     |
     | ::1                                               :: 2|
 -----------                 -----------                 -----------                    
|Node: S    |2001:db8:0:1/64|Node: I1   |2001:db8:0:3/64|Node: I3   |      
|Loopback   |---------------|Loopback:  |---------------|Loopback:  |                          
|2001:db8::a| ::1       ::2 |2001:db8::1| ::1       ::2 |2001:db8::3|               
 -----------                 -----------                 -----------                     
                                                              | ::1
                             -----------                      |
                            |Node: D    |   2001:db8:0:b/64   |
                            |Loopback:  |---------------------
                            |2001:db8::b| ::2
                             -----------

Figure 3: Reference Topology

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

Table 1 describes SFIB entries that are instantiated on all nodes. All of these SFIB entries represent node segments.

Table 2 describes SFIB entries that are instantiated on specific nodes. All of these SFIB entries represent adjacency segments.

A.1. SR Path Contains Node Segments Only

In this example, Node S sends a packet to Node D, though a node segment that terminates on I3. In this example, I3 does not appear in the CRH segment list. Therefore, the destination node may not be able to send return traffic through the same path.

A.2. SR Path Contains Node Segments Only And Preserves The First SID

In this example, Node S sends a packet to Node D, through a node segment that terminates on I3. In this example, I3 appears in the CRH segment list. Therefore, the destination node can send return traffic through the same path.

A.3. SR Path Contains Adjacency Segments Only

In this example, Node S sends a packet to Node D, via two adjacency segments..

Authors' Addresses