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Versions: 00                                                            
idr                                                             Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                           July 07, 2021
Expires: January 8, 2022

          MPLS Label Stacks in Tunnel Encapsulation Attribute


   [RFC9012] defines an MPLS Label Stack sub-TLV for Tunnel
   Encapsulation Attribute, and specifies that it is to be pushed BEFORE
   other labels.  This document clarifies the use case for that, and
   defines a new Tunnel Label Stack sub-TLV for a label stack to be
   pushed AFTER other labels (e.g., the label embedded in the NLRI for a
   labeled address family, and/or the stack in an MPLS Label Stack sub-

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on January 8, 2022.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   [RFC9012] defines an MPLS Label Stack sub-TLV for Tunnel
   Encapsulation Attribute and specifies that:

   "If a packet is to be sent through the tunnel identified in a
   particular TLV, and if that TLV contains an MPLS Label Stack sub-TLV,
   then the label stack appearing in the sub-TLV MUST be pushed onto the
   packet before any other labels are pushed onto the packet."

   Specifically, the label stack in the sub-TLV is to be pushed BEFORE
   any other labels are pushed.  This may sound counter-intuitive, in
   that if a label stack (e.g. for Segment Routing) is to be used to
   steer traffic to the tunnel endpoint, the stack should be pushed
   AFTER other labels (e.g. the label embedded in the NLRI).

   This document clarifies that it is NOT for steering traffic to but
   for steering AFTER the tunnel endpoint.  Consider the following use

                .          .
               .            .
    site1 --- PE1 -------- PE2 --- site2

   Two sites are connected to two PEs respectively, and traffic steering
   is desired within each site not just among the PEs.  While PE2 could
   push the label stack used for steering withing site2, there may be
   situations where PE2 is not a device capable of pushing a large label
   stack so PE1 is tasked with pushing the label stack that is used
   after the tunnel end point PE2, within site2.

2.  Tunnel Label Stack sub-TLV

   Notice that in the above example, the route update that PE1 receives
   could be from the controller instead of PE2.  The controller may want
   to steer traffic both withing site2 and between PE1 and PE2, yet
   RFC9012 currently does not specify how to signal the label stack used
   to reach the tunnel end point.

   To be able to signal that, this document defines a new Tunnel Label
   Stack sub-TLV.  It has exactly the same syntax as the existing MPLS
   Label Stack sub-TLV, but with a semantics that it is pushed AFTER
   other labels are pushed.

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3.  Security Considerations

   This document does not introduce any new security issues besides what
   is already discussed in RFC9012.

4.  IANA Assignments

   IANA is requested to assign a new sub-TLV type for "Tunnel Label
   Stack" from "BGP Tunnel Encapsulation Attribute Sub-TLVs" registry,
   in the 0~127 range.

5.  Acknowledgements

   The authors thank Eric Rosen and John Scudder for their valuable
   comments and suggestions.

6.  Normative References

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,

Author's Address

   Zhaohui Zhang
   Juniper Networks

   Email: zzhang@juniper.net

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