spring                                                          Z. Zhang
Internet-Draft                                                  S. Hegde
Intended status: Standards Track                        Juniper Networks
Expires: 14 September 2023                                 13 March 2023


                MPLS/SRv6 Service Interworking Option BC
              draft-zzhang-spring-service-interworking-00

Abstract

   Draft-bonica-spring-srv6-end-dtm specifies SRv6/MPLS transport
   interworking procedures, and draft-agrawal-spring-srv6-mpls-
   interworking specifies SRv6/MPLS transport and service interworking
   procedures.  For service interworking, the latter draft defines two
   modes, similar to VPN Inter-AS Option A and Option B.  This document
   specifies another Option BC for service interworking which has much
   better scaling property.

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
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   This Internet-Draft will expire on 14 September 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.











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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Re-advertising Service Routes from MPLS to SRv6 Domain  .   3
     1.2.  Re-advertising Service Routes from SRv6 to MPLS Domain  .   4
     1.3.  EVPN ESI Label  . . . . . . . . . . . . . . . . . . . . .   5
   2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   [I-D.bonica-spring-srv6-end-dtm] specifies SRv6/MPLS transport
   interworking procedures, and
   [I-D.agrawal-spring-srv6-mpls-interworking] specifies SRv6/MPLS
   transport and service interworking procedures.  For service
   interworking, the latter draft defines two styles, similar to VPN
   Inter-AS Option A and Option B [RFC4364].

   Specifically, for Option B style interworking, an InterWorking (IW)
   node does the following:

   *  For service routes received from MPLS domain, re-advertise to SRv6
      domain with an SRv6 SID (whose bits may be transposed in NLRI and
      in the Prefix SID attribute) and with nexthop set to itself.  The
      SID maps to the <service label, nexthop> tuple as received from
      the MPLS domain.  For service traffic from SRv6 domain, the
      incoming SID maps to <base tunnel to nexthop, service label>
      forwarding state in the MPLS domain.









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   *  For service routes received from SRv6 domain, re-advertise to MPLS
      domain with an MPLS label and with nexthop set to itself.  The
      label maps to the service SID (whose bits may be transposed in
      NLRI and in the Prefix SID attribute) as received from the SRv6
      domain.  For service traffic from MPLS domain, the incoming
      service label maps to corresponding forwarding state in the SRv6
      domain.

   This is a straightforward solution that does not require service
   instances on the IW node.  However, it does require per-service
   label/SID forwarding state on the IW node, that Inter-AS Option C VPN
   does not require.

   For a true Option C style MPLS/SRv6 service interworking, the SRv6
   service PEs must support MPLSoverUDP or MPLSoverIP, and as such there
   is no "service interworking" for Option C - it's just MPLS based
   services over interworked MPLS/SRv6 transport.

   This document proposes an Option BC style service interworking that
   does not require per-service-label/SID state on the IW nodes, and the
   service PEs can be single plane - MPLS or SRv6 only.

   The key behind the Option BC style interworking is that the SRv6
   Service SID is encoded in two parts of a service route - in the
   "label" field of the NLRI and the Prefix SID attribute.  The SRv6 SID
   Structure sub-sub-TLV specifies the LOC:FUNCT:ARG encoding scheme of
   the Service SID, and specifies the which part of the Service SID the
   "label" field of the NLRI fits into.  In most cases, the ARG part is
   not used, with the exception of EVPN multi-homing support with label-
   based split-horizon filtering [RFC7432] [RFC9252].  This is discussed
   in Section 6.1.1 of [RFC9252] and in Section 1.3 of this document.

1.1.  Re-advertising Service Routes from MPLS to SRv6 Domain

   When the IW node re-advertises a service route from MPLS domain to
   SRv6 domain, it attach a Prefix SID attribute but does not change the
   label field of the NLRI.  An SRv6 SID Structure Sub-Sub-TLV is
   included in the L2/L3 SRv6 Service TLV's SRv6 SID Information sub-
   TLV, specifying the LOC:FUNCT:ARG encoding scheme and which part of
   the SID (in the SRv6 SID value field of the SRv6 SID Information sub-
   TLV) that the label field of the NLRI fits into.

   A receiving SRv6 PE sends corresponding service traffic using the
   SRv6 Service SID resulting from superimposing the label value in the
   NLRI(s) to the SID in SRv6 SID Information sub-TLV.






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   On the IW node, the FUNCT bits of the Service SID in the Prefix SID
   attribute indicate a new End.DBS behavior, where DBS stands for
   Decapsulate, Binding, and Shifting.  The FUNCT bits also map/bind to
   a particular MPLS router, which is the BGP protocol nexthop in the
   service route received from the MPLS domain.  Note that the MPLS
   router could either be an MPLS service PE, or an ASBR implementing
   Inter-AS Option B.

   When a service packet arrives from the SRv6 domain, the IW node
   identifies an MPLS router (that advertised the service route to the
   IW node) based on the FUNCT bits.  Since the FUNCT bits identify the
   End.DBS end behavior, the packet is decapsulated, the incoming SID's
   certain bits are extracted as MPLS service label, and then the packet
   is sent to the corresponding MPLS router with the <base tunnel label,
   service label> label stack.

   For example, the LOC:FUNCT:ARG encoding of the SRv6 SID that the IW
   node advertises could be 64:44:20 where the numbers represent the
   number of bits in each part.  The 20-bit ARG part is not used except
   in the case of EVPN ESI label based split-horizon filtering
   (Section 1.3).  The lower 20 bits of the 44-bit FUNCT part are for
   the MPLS label received from the MPLS side, X number of bits of the
   44-bit FUNCT part are used to identify End.DBS behavior for each MPLS
   PE/ASBR - only 10 bits are needed for 1k of PEs/ASBRs on MPLS side.
   The rest of FUNCT space can still be used for other purposes.  The
   IWS only needs to maintain IPv6 1k SIDs in the forwarding path to
   switch traffic to those 1k MPLS PEs/ASBRs using the End.DBS behavior,
   no matter how many service labels are advertised from those PEs/
   ASBRs.

1.2.  Re-advertising Service Routes from SRv6 to MPLS Domain

   When the IW node re-advertises a service route from SRv6 domain to
   MPLS domain, the Prefix SID attribute is removed but the label field
   in the NLRI does not change.  The nexthop is set to the SID value in
   the SRv6 SID Information Sub-TLV of the L2/L3 SRv6 Service TLV in the
   Prefix SID attribute.

   In addition, the IW node advertises a labeled underlay prefix route
   for the SID value (which is the BGP protocol nexthop in the re-
   advertised service route), with a per-prefix binding label and with
   the nexthop set to the IW node itself.

   When an MPLS PE (or ASBR in case of Inter-AS Option B) receives the
   service route, it resolves the protocol nexthop via the underlay
   labeled prefix route.  As a result, service traffic is sent with a
   label stack <underlay tunnel label used to reach the IW node, binding
   label for the underlay prefix (i.e. nexthop), service label in NLRI>.



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   When the IW node gets service traffic from the MPLS domain, the
   binding label for the underlay prefix (which is the SID in the Prefix
   SID attribute of the service route from the SRv6 domain) leads to the
   following processing:

   *  Pop the next label, which is the service label

   *  Find the underlay prefix that is associated with the binding label
      (note that the underlay prefix is the SID in the Prefix SID
      attribute), and super impose the popped service label to it
      according to the Transposition offset/length in the SID Structure
      sub-sub-TLV in the Prefix SID attribute.

   *  Send packet after encapsulating it in IPv6 with the resulting SID

1.3.  EVPN ESI Label

   Typically, if there are separate SRv6 and MPLS domains for an EVPN
   network, multihoming is unlikely to span across the domains.  If that
   does happen, the following method can be used to achieve label based
   split-horizon filtering across the domains.

   When the IW node re-advertises the EVPN Ethernet A-D per ES Route
   from MPLS domain to SRv6 domain, a Prefix SID attribute is attached,
   with the SID Structure sub-sub-TLV specifying the transposition
   length and offset for the ESI label, as specified in Section 6.1.1 of
   [RFC9252].

   When SRv6 service traffic arrives at the IW node, if the end behavior
   for the SID is End.DBS and the ARG part is not 0, the IW node
   extracts the ARG bits into an ESI label that is imposed before the
   service label (that is extracted from the FUNCT bits) is imposed.

   When the IW node re-advertise the EVPN Ethernet A-D per ES Route from
   SRv6 domain to MPLS domain, the Prefix SID attribute is simply
   removed but the transposition information is saved locally.

   When MPLS service traffic arrives at the IW node, if there is another
   label after the service label, that label is also popped and
   superimposed to the SRv6 Service SID that is bound to the binding
   label described in Section 1.2, in addition to that the service label
   is popped and superimposed to the same SRv6 Service SID.

2.  Procedures

   Normative procedures will be specified in future revisions of the
   document.




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3.  Security Considerations

   To be provided.

4.  References

4.1.  Normative References

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC9252]  Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
              B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
              Based on Segment Routing over IPv6 (SRv6)", RFC 9252,
              DOI 10.17487/RFC9252, July 2022,
              <https://www.rfc-editor.org/info/rfc9252>.

4.2.  Informative References

   [I-D.agrawal-spring-srv6-mpls-interworking]
              Agrawal, S., Ali, Z., Filsfils, C., Voyer, D., and Z. Li,
              "SRv6 and MPLS interworking", Work in Progress, Internet-
              Draft, draft-agrawal-spring-srv6-mpls-interworking-10, 24
              October 2022, <https://datatracker.ietf.org/doc/html/
              draft-agrawal-spring-srv6-mpls-interworking-10>.

   [I-D.bonica-spring-srv6-end-dtm]
              Hegde, S., Kaneriya, P., Bonica, R., Peng, S., Mirsky, G.,
              Zhang, Z., and B. Decraene, "SR-MPLS / SRv6 Transport
              Interworking", Work in Progress, Internet-Draft, draft-
              bonica-spring-srv6-end-dtm-08, 21 October 2022,
              <https://datatracker.ietf.org/doc/html/draft-bonica-
              spring-srv6-end-dtm-08>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

Contributors

   Krzysztof Szarkowicz
   Juniper Networks
   Email: kszarkowicz@juniper.net






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Authors' Addresses

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net


   Shraddha Hegde
   Juniper Networks
   Email: shraddha@juniper.net









































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