MPLS Label Stacks in Tunnel Encapsulation Attribute
draft-zzhang-idr-tunnel-encapsulation-label-stack-01

idr                                                             Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                                S. Hares
Expires: 14 April 2023                                   11 October 2022

          MPLS Label Stacks in Tunnel Encapsulation Attribute
          draft-zzhang-idr-tunnel-encapsulation-label-stack-01

Abstract

   RFC 9012 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, 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-TLV), and defines two new
   Segment sub-TLVs to encode a segment list in a compact format.

Status of This Memo

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   This Internet-Draft will expire on 14 April 2023.

Copyright Notice

   Copyright (c) 2022 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
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Traffic Steering after Tunnel Endpoint  . . . . . . . . . . .   2
   2.  Traffic Steering to the Tunnel Endpoint . . . . . . . . . . .   3
     2.1.  Tunnel Label Stack sub-TLV  . . . . . . . . . . . . . . .   3
   3.  Compact Segment List  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Segment Type L  . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Segment Type M  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Assignments  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Traffic Steering after Tunnel Endpoint

   [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
   case:

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

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   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 within 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.  Traffic Steering to the Tunnel Endpoint

   Notice that in the above example, it may be desired that PE2 or the
   controller wants PE1 to send service traffic to PE2 via a specific
   path through the underlay network.  The path may be an Segment
   Routing path identified as a label stack encoded in the Tunnel
   Encapsulation Attribute of the service routes that PE1 receives.

   PE1 needs to impose the label stack AFTER it imposes other labels
   like service labels or the labels for traffic steering at site2 after
   the traffic arrives at PE2.  Obviously, a new sub-TLV is needed to
   encode the label stack for steering traffic to the tunnel endpoint,
   as the existing MPLS Label Stack sub-TLV is for steering after
   traffic reaches the tunnel endpoint.

2.1.  Tunnel Label Stack sub-TLV

   This document defines a new Tunnel Label Stack sub-TLV for that
   purpose.  It has exactly the same syntax as the existing MPLS Label
   Stack sub-TLV.  Section 3.6 of [RFC9012] applies to this new sub-TLV,
   with the following differences:

   *  A new tunnel type will be allocated by IANA

   *  The label stack MUST be imposed AFTER other labels are pushed.

   Both the existing MPLS Label Stack sub-TLV and the new Tunnel Label
   Stack sub-TLV MAY be present in a tunnel TLV.

3.  Compact Segment List

   Section 2.4.4 of [I-D.ietf-idr-segment-routing-te-policy] specifies a
   Segment List sub-TLV that is optional in a tunnel TLV.  It encodes a
   segment list in an SR Policy tunnel, containing zero or more Segment
   sub-TLVs.

   Each Segment sub-TLV specifies a segment of various types defined in
   Section 4 of [RFC9256].  For example, if a segment list is a 10-label
   stack, then the Segment List sub-TLV for it has 10 sub-TLVs, each
   being a Type A Segment as defined in 2.4.4.2.1 of
   [I-D.ietf-idr-segment-routing-te-policy]:

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     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |   Length      |     Flags     |   RESERVED    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Label                        | TC  |S|       TTL     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   It is clear that this is not efficient on-the-wire encoding format,
   and it involves additional encoding/decoding processing.

   To address this inefficiency, this document specifies two new types
   of Segment sub-TLVs, each encoding a label stack or an SRv6 SID list
   respectively.  A new segment type may be added in a future revision
   to encode a compressed SRv6 SID list.

   Note that, while each new type is called a Segment sub-TLV in a
   Segment List sub-TLV, it actually encodes a segment list (a label
   stack or an SRv6 SID list).  A Segment List sub-TLV MAY have a mixed
   Segment sub-TLVs of any types, e.g., a Type A segment that encodes
   one label and another new segment type that encodes a label stack.
   The actual segment list is a concatenation of all the labels in this
   example.

3.1.  Segment Type L

   The Type L Segment Sub-TLV encodes multiple SR-MPLS SIDs.  The format
   is as follows and is used to encode MPLS Label fields as specified in
   [RFC3032] [RFC5462]:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type TBD1   |   Length      |     Flags     |   RESERVED  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Label                        | TC  |S|       TTL    //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //          Repeated Label Entries                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Type TBD1 is to be assigned by IANA from the "SR Policy Segment
   List Sub-TLVs" under the "BGP Tunnel Encapsulation" registry.

   The Length value is (4 * number of labels + 2).

   Other fields are as defined in 2.4.4.2.1 of
   [I-D.ietf-idr-segment-routing-te-policy].

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3.2.  Segment Type M

   The Type M Segment Sub-TLV encodes multiple SRv6 SIDs with optional
   SRv6 Endpoint Behavior and SID Structure:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type TBD2     |   Length      |     Flags     |   RESERVED  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                       SRv6 SID (16 octets)                  //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                       Repeated SRv6 SIDs                    //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //     SRv6 Endpoint Behavior and SID Structure (optional)     //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Type TBD2 is to be assigned by IANA from the "SR Policy Segment
   List Sub-TLVs" under the "BGP Tunnel Encapsulation" registry.

   The Length value is (16 * number of SIDs + 2) when SRv6 Endpoint
   Behavior and SID Structure is not present.  If it is present, the
   Length value is increased by 8.

   Other fields are as defined in 2.4.4.2.2 of
   [I-D.ietf-idr-segment-routing-te-policy].

4.  Security Considerations

   This document does not introduce any new security issues besides what
   is already discussed in RFC9012 and
   [I-D.ietf-idr-segment-routing-te-policy].

5.  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.

   IANA is requested to assign two new sub-TLV types from the "SR Policy
   Segment List Sub-TLVs" under the "BGP Tunnel Encapsulation" registry,
   for Type L and Type M segments respectively.

6.  Acknowledgements

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

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7.  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,
              <https://www.rfc-editor.org/info/rfc9012>.

   [I-D.ietf-idr-segment-routing-te-policy]
              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-20, 27 July 2022,
              <https://www.ietf.org/archive/id/draft-ietf-idr-segment-
              routing-te-policy-20.txt>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
              2009, <https://www.rfc-editor.org/info/rfc5462>.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net

   Susan Hares
   Email: skh@ndzh.com

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