IDR                                                               Y. Liu
Internet-Draft                                                   S. Peng
Intended status: Standards Track                                     ZTE
Expires: 2 September 2022                                   1 March 2022

            BGP Extension for SR-MPLS Entropy Label Position


   This document proposes extensions for BGP to indicate the entropy
   label position in the SR-MPLS label stack when delivering SR Policy
   via BGP.

Status of This Memo

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

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   This Internet-Draft will expire on 2 September 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     2.2.  Terminology and Acronyms  . . . . . . . . . . . . . . . .   3
   3.  Entropy Label Position in SR-MPLS with the Controller . . . .   3
   4.  BGP Extensions for ELP in SR Policy . . . . . . . . . . . . .   5
   5.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Segment Routing (SR) leverages the source routing paradigm.  Segment
   Routing can be instantiated on MPLS data plane which is referred to
   as SR-MPLS [RFC8660].  SR-MPLS leverages the MPLS label stack to
   construct the SR path.

   Entropy labels (ELs) [RFC6790] are used in the MPLS data plane to
   provide entropy for load-balancing.  The idea behind the entropy
   label is that the ingress router computes a hash based on several
   fields from a given packet and places the result in an additional
   label named "entropy label".  Then, this entropy label can be used as
   part of the hash keys used by an LSR.  Using the entropy label as
   part of the hash keys reduces the need for deep packet inspection in
   the LSR while keeping a good level of entropy in the load-balancing.

   [RFC8662] proposes to use entropy labels for SR-MPLS networks and
   multiple <ELI, EL> pairs may be inserted in the SR-MPLS label stack.
   The ingress node may decide the number and position of the ELI/ELs
   which need to be inserted into the label stack, that is termed as ELP
   (Entropy Label Position) in this document.  But in some cases, the
   controller (e.g.  PCE) can be used to perform the TE path computation
   as well as the Entropy Label Position which is useful for inter-
   domain scenarios.

   [I-D.ietf-idr-segment-routing-te-policy] specifies the way to use BGP
   to distribute one or more of the candidate paths of an SR Policy to
   the headend of that policy.

   This document proposes extensions for BGP to indicate the ELP in the
   segment list when delivering SR Policy via BGP in SR-MPLS networks.

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2.  Conventions used in this document

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.2.  Terminology and Acronyms

   EL: Entropy Label

   ELI: Entropy Label Indicator

   ELC: Entropy Label Capability

   ERLD: Entropy Readable Label Depth

   ELP: Entropy Label Position

   MSD: Maximum SID Depth

3.  Entropy Label Position in SR-MPLS with the Controller

   As described in [RFC8662] section 7, ELI/EL placement is not an easy
   decision, multiple criteria may be taken into account.

   First is the Maximum SID Depth (MSD), it defines the maximum number
   of labels that a particular node can impose on a packet, and it is a
   limit when the ingress node imposing ELI/EL pairs on the SR label

   The Entropy Readable Label Depth(ERLD) value is another important
   parameter to consider when inserting an ELI/EL.  The ERLD is defined
   as the number of labels a router can both read in an MPLS packet
   received on its incoming interface(s) and use in its load-balancing
   function.  An ELI/EL pair must be within the ERLD of the LSR in order
   for the LSR to use the EL during load-balancing.  It's necessary to
   get the ERLD of the nodes along the SR path to achieve efficient

   An implementation MAY try to evaluate if load-balancing is really
   expected at a particular node based on the segment type of its label,
   which also influences the ELP of a segment list.

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   Other criteria includes maximizing number of LSRs that will load-
   balance, preference for a part of the path, and etc.  Using which
   criteria and how to decide the ELP based on the criteria is a matter
   of implementation.

   As shown in Figure 1, in the inter-domain scenario, a path from A to
   Z is required, a centralized controller performs the computation of
   the end-to-end path, along which traffic load-balancing is required.

    ....................   ....................    .....................
    .                  .   .                  .    .                   .
    .+---+       +---+ .   . +---+      +---+ .    .+---+      +----+  .
    .| A |-------| B |------ | C |------| X |-------| Y |------| Z  |  .
    .+---+       +---+ .   . +---+      +---+ .    .+---+      +----+  .
    .     domain 1     .   .      domain 2    .    .     domain 3      .
    ....................   ....................    .....................

         Figure 1: Entropy Labels in SR-MPLS Inter-Domain Scenario

   When the headend node in the first domain can't get the information
   of the nodes/SIDs in other domains, e.g, the ERLD of each node or the
   type of the SID bounded to a node/link, it's difficult for the
   headend node to decide the ELP of the segment list for the path.

   Performing the computation of the ELP by the controller is an
   alternate, since it's easier for the controller to get the required
   information along the segment list prescribed by itself.

   For example, the ERLD value can be advertised via IS-
   IS[I-D.ietf-isis-mpls-elc] and OSPF[I-D.ietf-ospf-mpls-elc] within
   the domain, in each domain, one or more nodes are configured with
   BGP-LS so the controller can get the ERLD value of all the nodes
   through BGP-LS[RFC9085].  The controller can acquire the MSD of the
   headend node or the Binding SID anchor node via BGP-LS[RFC8814] or

   Another benefit of utilizing the controller to calculate ELP is that
   if the criteria or calculation algorithm is changed, the
   corresponding modification only needs to be made on the controller
   instead of each headend node in the network.

   When the controller performs the computation of the the ELP for a
   segment list, the considerations for the placement of ELI/ELs
   introduced in [RFC8662] are still applicable.  How the controller
   computes the ELP is out of scope of the document.

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   After the ELP of an SR path is decided, the controller SHOULD inform
   the result to the headend node of the path, so the node knows where
   to insert the ELI/ELs when needed.  Section 4 proposes the detailed
   extensions for BGP to carry this information.

4.  BGP Extensions for ELP in SR Policy

   The Segment Flags for Segment Sub-TLVs are defined in
   Section of [I-D.ietf-idr-segment-routing-te-policy].  In
   this document, the ELP information is transmitted by extending the
   flags of Segment Sub-TLVs.

                               0 1 2 3 4 5 6 7
                              |V|A|S|B|E|     |

   E-Flag: This flag, when set, indicates that presence of < ELI, EL>
   label pairs which are inserted after this segment.  E-Flag is
   applicable to Segment Types A, C, D, E, F, G and H.  If E-Flag
   appears with Segment Types B, I, J and K, it MUST be ignored.

5.  Operations

   Node A receives an SR Policy NLRI with an Segment List sub-TLV from
   the controller.  The Segment List sub-TLV contains multiple Segment
   sub-TLVs, e.g, <S1, S2, S3, S4, S5, S6>, the E-Flags of S3 and S6 are
   set, it indicates that if load-balancing is required, two <ELI, EL>
   pairs SHOULD be inserted into the label stack of the SR-TE forwarding
   entry, respectively after the Label for S3 and Label for S6.

   The value of EL is supplemented by the ingress node according to
   load-balancing function of the appropriate keys extracted from a
   given packet.  After inserting ELI/ELs, the label stack on the
   ingress node would be <S1, S2, S3, ELI, EL, S4, S5, S6, ELI, EL>.

6.  IANA Considerations

   This document requests bit 4 for Entropy Label Flag in "SR Policy
   Segment Flags" under the "BGP Tunnel Encapsulation" registry.

       Bit     Description                                Reference
        4     Entropy Label Position Flag(E-Flag)         This document

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7.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   introduce any new security considerations beyond those already listed
   in [RFC8662] and [I-D.ietf-idr-segment-routing-te-policy].

8.  References

8.1.  Normative References

              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
              Jain, D., and S. Lin, "Advertising Segment Routing
              Policies in BGP", Work in Progress, Internet-Draft, draft-
              ietf-idr-segment-routing-te-policy-14, 10 November 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8662]  Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
              Shakir, R., and J. Tantsura, "Entropy Label for Source
              Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
              DOI 10.17487/RFC8662, December 2019,

8.2.  Informative References

              Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using IS-IS", Work in

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              Progress, Internet-Draft, draft-ietf-isis-mpls-elc-13, 28
              May 2020, <

              Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using OSPF", Work in
              Progress, Internet-Draft, draft-ietf-ospf-mpls-elc-15, 1
              June 2020, <

   [RFC8476]  Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
              DOI 10.17487/RFC8476, December 2018,

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,

   [RFC8660]  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, December 2019,

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,

   [RFC8814]  Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
              and N. Triantafillis, "Signaling Maximum SID Depth (MSD)
              Using the Border Gateway Protocol - Link State", RFC 8814,
              DOI 10.17487/RFC8814, August 2020,

   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,

Authors' Addresses

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   Yao Liu

   Shaofu Peng

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