Internet-Draft SRv6 Path Segment November 2021
Li, et al. Expires 31 May 2022 [Page]
SPRING Working Group
Intended Status:
Standards Track
C. Li
Huawei Technologies
W. Cheng
China Mobile
M. Chen
Huawei Technologies
D. Dhody
Huawei Technologies
Y. Zhu
China Telecom

Path Segment for SRv6 (Segment Routing in IPv6)


Segment Routing (SR) allows for a flexible definition of end-to-end paths by encoding an ordered list of instructions, called "segments". The SR architecture can be implemented over an MPLS data plane as well as an IPv6 data plane.

Currently, Path Segment has been defined to identify an SR path in SR-MPLS networks, and is used for various use-cases such as end-to-end SR Path Protection and Performance Measurement (PM) of an SR path. This document defines the Path Segment to identify an SRv6 path in an IPv6 network.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 31 May 2022.

Table of Contents

1. Introduction

Segment routing (SR) [RFC8402] is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node by inserting an ordered list of instructions, called segments.

When segment routing is deployed on an MPLS data plane, called SR-MPLS [RFC8660], a segment identifier (SID) is present as an MPLS label. When segment routing is deployed on an IPv6 data plane, a SID is presented as a 128-bit value, and it can be an IPv6 address of a local interface but it does not have to be. To support SR in an IPv6 network, a Segment Routing Header (SRH) [RFC8754] is used.

In an SR-MPLS network, when a packet is transmitted along an SR path, the labels in the MPLS label stack will be swapped or popped, so no label or only the last label may be left in the MPLS label stack when the packet reaches the egress node. Thus, the egress node can not determine from which ingress node or SR path the packet came from. Therefore, to identify an SR-MPLS path, a Path Segment is defined in [I-D.ietf-spring-mpls-path-segment].

Likewise, a path needs to be identified in an SRv6 network for several use cases such as binding bidirectional paths [I-D.ietf-pce-sr-bidir-path] and end-to-end performance measurement [I-D.gandhi-spring-udp-pm].

An SRv6 path MAY be identified by the content of a segment list. However, the segment list may not be a good key, since the length of a segment list is flexible according to the number of required SIDs. Also, the length of a segment list may be too long to be a key when it contains many SIDs. For instance, if packet A uses an SRH with 3 SIDs while Packet B uses an SRH with 10 SIDs, the key to identify these two paths will be a 384-bits value and a 1280-bits value, respectively. Further, an SRv6 path cannot be identified by the information carried by the SRH in reduced mode [RFC8754] as the first SID is not present.

Furthermore, different SRv6 policies may use the same segment list for different candidate paths, so the traffic of different SRv6 policies are merged, resulting in the inability to measure the performance of the specific path.

To solve the above issues, this document defines a new SRv6 segment called "SRv6 Path Segment", which is a 128-bits value, to identify an SRv6 path.

When the SRv6 Path Segment is used in reduced mode SRH [RFC8754], the entire path information is indicated by the Path Segment, and the performance will be better than using the entire segment list as the path identifier, while the overhead is equivalent to the SRH in normal mode. Furthermore, with SRv6 Path Segment, each SRv6 candidate path can be identified and measured, even when they use the same segment list.

An SRv6 Path Segment MUST NOT be copied to the IPv6 destination address, so it is not routable.

1.1. 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.

1.2. Terminology

MPLS: Multiprotocol Label Switching.

PM: Performance Measurement.

SID: Segment ID.

SR: Segment Routing.

SR-MPLS: Segment Routing with MPLS data plane.

SRH: Segment Routing Header.

PSID: Path Segment Identifier.

PSP: Penultimate Segment Popping.

Further, this document makes use of the terms defined in [RFC8402] and [RFC8986].

2. Use Cases for SRv6 Path Segment

Similar to SR-MPLS Path Segment [I-D.ietf-spring-mpls-path-segment], SRv6 Path Segment may also be used to identify an SRv6 Path in some use cases:

3. SRv6 Path Segment

As defined in [RFC8986], an SRv6 segment is a 128-bit value.

To identify an SRv6 path, this document defines a new segment called SRv6 Path Segment.

Depending on the use case, an SRv6 Path Segment identifies:

Note that, based on the use-case, a SRv6 Path Segment can be used for different SID-Lists within an SR Policy.

3.1. Format of an SRv6 Path Segment

This document proposes two types of SRv6 Path Segment format.

3.1.1. SRv6 Path Segment: Locator and Local ID

As per [RFC8986], an SRv6 segment is a 128-bit value, which can be represented as LOC:FUNCT, where LOC is the L most significant bits and FUNCT is the 128-L least significant bits. L is called the locator length and is flexible. Each network operator is free to use the locator length it chooses. Most often the LOC part of the SID is routable and leads to the node which instantiates that SID. The FUNCT part of the SID is an opaque identification of a local function bound to the SID. The FUNCT value zero is invalid.

SRv6 Path Segment can follow the format, where the LOC part identifies the egress node that allocates the Path Segment, and the FUNCT part is a unique local ID to identify an SRv6 Path and its endpoint behavior.

The Function Type of an SRv6 Path Segment is END.PSID (End Function with Path Segment Identifier).

 |  Locator              |        Function ID                   |

 |<-------------------------128 bits--------------------------->|

              Figure 2. PSID in Format LOC:FUNCT

3.1.2. SRv6 Path Segment: Global ID

An SRv6 Path Segment ID can be a Global ID, and its format depends on the use case.

The SRv6 Path Segment will not be copied to the IPv6 Destination Address, so the SRv6 Path Segment ID can be allocated from an independent 128-bits ID Space. In this case, a new table should be maintained at the node for SRv6 Path Segment.

 |                         Global ID/PSID                       |

 |<-------------------------128 bits--------------------------->|

             Figure 3. A Global ID as an PSID

4. Encoding of an SRv6 Path Segment

This section describes the SRv6 Path Segment encoding in SRH.

The SRv6 Path Segment MUST appear only once in a segment list, and it MUST appear as the last entry in the segment list.

4.1. SRH.P-flag

To indicate the existence of a Path Segment in the SRH, this document defines a P-flag in the SRH flag field. The encapsulation of SRv6 Path Segment is shown below.

        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 |
    |  Last Entry   |     Flags   |P|              Tag              |
    |                                                               |
    |            Segment List[0] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    |                                                               |
    |                                                               |
    |                                                               |
    |                                                               |
    |                                                               |
    |          Segment List[n-1] (128 bits IPv6 address)            |
    |                                                               |
    |                                                               |
    |                                                               |
    |    SRv6 Path Segment (Segment List[n],128 bits IPv6 value)    |
    |                                                               |
    |                                                               |
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //

                Figure 1. SRv6 Path Segment in SID List

SRH.P-bit processing can be enabled or disabled by configuration on devices, it can be done by CLI, NETCONF YANG or other ways, and this is out of the scope of this document.

The pseudo code of SRH.P-bit processing is described as below.

    S01.  if SRH.P-flag processing is enabled:
    S02.     if SRH.P-flag is set:
    S03.        SRv6 Path Segment processing       ;;ref1

Ref1: The SRv6 Path Segment processing is accosiated with the specific application, such as SRv6 Path Segment based Performance measurement, so this is out of the scope of this document.

In some use cases, only the egress need to process the SRv6 Path Segment, therefore, the P-bit processing can be done at the egress node only while the intermediate nodes do not need to process it. This feature can be enabled by configuration like CLI , NETCONF YANG or other ways. In this case, the pseudo code is described as below.

    S01.  if SRH.P-flag processing is enabled:
    S02.    if intermediate node processing is disabled:
    S03.        if SRH.P-flag is set and SRH.SL == 0:
    S03.            SRv6 Path Segment processing
    S04     else:
    S05.        if SRH.P-flag is set:
    S06.            SRv6 Path Segment processing

5. SRv6 Path Segment Allocation

A Path Segment is a local segment allocated by an egress node. A Path Segment can be allocated through several ways, such as CLI, BGP [I-D.ietf-idr-sr-policy-path-segment], PCEP [I-D.ietf-pce-sr-path-segment] or other ways. The mechanisms through which a Path Segment is allocated are out of scope of this document.

When a Path Segment is allocated by the egress, it MUST be distributed to the ingress node of the path that identified by the path segment. In this case, only the egress will process the Path Segment, and other nodes specified by SIDs in the segment list do not know how to process the Path Segment.

Depending on the use case, a Path Segment may be distributed to the SRv6 nodes along the SRv6 path. In this case, the SRv6 nodes that learned the Path Segment may process the Path Segment depending on the use case.

6. Processing of SRv6 Path Segment

When the SRv6 Path Segment is used, the following rules apply:

7. IANA Considerations

This I-D requests the IANA to allocate, within the "SRv6 Endpoint Behaviors" sub-registry belonging to the top-level "Segment-routing with IPv6 data plane (SRv6) Parameters" registry, the following allocations:

   Value      Description                               Reference
   TBA1       End.PSID - SRv6 Path Segment                [This.ID]

This document also requests IANA to allocate bit position TBA within the "Segment Routing Header Flags" registry defined in [RFC8402].

8. Security Considerations

This document does not introduce additional security requirements and mechanisms other than the ones described in [RFC8402].

9. Contributors

   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095


   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095


10. Acknowledgements

The authors would like to thank Stefano Previdi and Zafar Ali for their valuable comments and suggestions.

11. References

11.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, , <>.
Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header (SRH)", RFC 8754, DOI 10.17487/RFC8754, , <>.
Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 (SRv6) Network Programming", RFC 8986, DOI 10.17487/RFC8986, , <>.

11.2. Informative References

Gandhi, R., Filsfils, C., Voyer, D., Salsano, S., Ventre, P. L., and M. Chen, "UDP Path for In-band Performance Measurement for Segment Routing Networks", Work in Progress, Internet-Draft, draft-gandhi-spring-udp-pm-02, , <>.
Li, C., Li, Z., Yin, Y., Cheng, W., and K. Talaulikar, "SR Policy Extensions for Path Segment and Bidirectional Path", Work in Progress, Internet-Draft, draft-ietf-idr-sr-policy-path-segment-04, , <>.
Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong, "Path Computation Element Communication Protocol (PCEP) Extensions for Associated Bidirectional Segment Routing (SR) Paths", Work in Progress, Internet-Draft, draft-ietf-pce-sr-bidir-path-08, , <>.
Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong, "Path Computation Element Communication Protocol (PCEP) Extension for Path Segment in Segment Routing (SR)", Work in Progress, Internet-Draft, draft-ietf-pce-sr-path-segment-04, , <>.
Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler, "Path Segment in MPLS Based Segment Routing Network", Work in Progress, Internet-Draft, draft-ietf-spring-mpls-path-segment-05, , <>.
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", Work in Progress, Internet-Draft, draft-ietf-spring-segment-routing-policy-14, , <>.
Morton, A., "Active and Passive Metrics and Methods (with Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799, , <>.
Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing with the MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, , <>.

Authors' Addresses

Cheng Li
Huawei Technologies
Weiqiang Cheng
China Mobile
Mach(Guoyi) Chen
Huawei Technologies
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore 560066
Yongqing Zhu
China Telecom