\
MPLS WG                                                      K. Kompella
Internet-Draft                                               V.P. Beeram
Intended status: Standards Track                                 T. Saad
Expires: 12 August 2022                                 Juniper Networks
                                                               I. Meilik
                                                                Broadcom
                                                         8 February 2022


       Multi-purpose Special Purpose Label for Forwarding Actions
                     draft-kompella-mpls-mspl4fa-02

Abstract

   The MPLS architecture introduced Special Purpose Labels (SPLs) to
   indicate special forwarding actions and offered a few simple
   examples, such as Router Alert.  In the two decades since the
   original architecture was crafted, the range, complexity and sheer
   number of such actions has grown; in addition, there now is need for
   "associated data" for some of the forwarding actions.  Likewise, the
   capabilities and scale of forwarding engines has also improved vastly
   over the same time period.  There is a pressing need to match the
   needs with the capabilities to deliver the next generation of MPLS
   architecture.

   In this memo, we propose an alternate mechanism whereby a single SPL
   can encode multiple forwarding actions and carry associated data,
   some in the label stack and some after the label stack.  This
   proposal also solves the problem of scarcity of base SPLs.

   This approach can immediately address several use cases:

   *  to carry a Slice Selector for IETF network slicing;

   *  to signal that further fast reroute may have harmful consequences;

   *  to indicate that there is relevant data after the label stack;

   *  among others.

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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
     1.2.  Revision History  . . . . . . . . . . . . . . . . . . . .   3
       1.2.1.  Changes from -00 to -01 . . . . . . . . . . . . . . .   4
       1.2.2.  Changes from -01 to -02 . . . . . . . . . . . . . . .   4
     1.3.  Slice Selector  . . . . . . . . . . . . . . . . . . . . .   5
   2.  Multi-purpose bSPL: the Forwarding Actions Indicator  . . . .   5
     2.1.  The FAI bSPL  . . . . . . . . . . . . . . . . . . . . . .   6
       2.1.1.  ISD vs PSD  . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Format of the FAI bSPL  . . . . . . . . . . . . . . . . .   6
       2.2.1.  Definitions of the FAI Flag Bits  . . . . . . . . . .   7
       2.2.2.  Processing the FAI Flags and the ISD  . . . . . . . .   9
       2.2.3.  Example of the FAI  . . . . . . . . . . . . . . . . .   9
   3.  Issues to be Resolved . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Preventing FAI From Reaching Top of Stack . . . . . . . .  10
     3.2.  Repeating the FAI at "Readable Stack Depth" . . . . . . .  11
     3.3.  PSD . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12



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   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Base Special Purpose Labels (bSPLs) are a precious commodity; there
   are only 16 such values, of which 8 have already been allocated.
   There are currently five requests for bSPLs that the authors are
   aware of; this document proposes another use case for a bSPL, in all
   consuming nearly all the remaining values.  This document suggests a
   method whereby a single bSPL can be used for all the purposes
   currently requested.  This leads to perhaps the more valuable long-
   term contribution of this document: an approach to the definition and
   use of bSPLs (and SPLs in general) whereby a single value can be used
   for multiple purposes, and provide a flexible yet efficient means of
   carrying associated data.

1.1.  Conventions and Definitions

   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.

   FAI:  Forwarding Actions Indicator

   FFB:  Forwarding Flags Block

   ISD:  In-Stack Data

   sISD:  Standard ISD

   uISD:  User-Defined ISD

   PSD:  Post-Stack Data

   SPL:  Special-purpose label

   bSPL:  Base special-purpose label

1.2.  Revision History

   This section (to be removed before publication) offers highlights
   from the draft's revision history.



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1.2.1.  Changes from -00 to -01

   1.  This section added.

   2.  Added a section discussing when data should be put in the LS FAD
       vs in the PL FAD.

   3.  Tweaked the bits in the FAI.  Added a field "edist".

   4.  Elaborated on the use of the H bit and the FAH data.

   5.  Updated the processing of the LS FAD.

   6.  Added processing of edist.

   7.  Updated the FAI example.

   8.  Updated the Issues section.

1.2.2.  Changes from -01 to -02

   1.  Updated Abstract and Introduction to focus on FAI; moved
       description of use cases to separate section.

   2.  Added terminology.

   3.  Changed terminology: LS FAD and PL FAD to ISD and PSD,
       respectively.

   4.  Updated text on criteria for putting associated data in ISD.

   5.  Introduced the terms FAI Block, FFB Block, sISD Block and uISD
       Block.  Introduced an "end of block" bit, s.  Updated flag bits;
       updated processing of ISD.

   6.  Removed field edist.

   7.  Updated the section on preventing the FAI from reaching the Top
       of Stack.

   8.  Updated the section on Readable Stack Depth










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1.3.  Slice Selector

   Network slicing is an important ongoing effort both for network
   design, as well as for standardization, in particular at the IETF
   [I-D.nsdt-teas-ns-framework].  A key issue is identifying which slice
   a packet belongs to, by means of a "slice selector" carried in the
   packet header.  [I-D.bestbar-teas-ns-packet] describes several such
   methods for MPLS networks, of which the Global Identifier for Slice
   Selector (GISS) is one of the more practical solutions.  This
   document shows how to realize the GISS using a base special purpose
   label (bSPL).

   In MPLS networks, a GISS is a data plane construct identifying
   packets belonging to a slice aggregate (the set of packets that
   belong to the slice).  The GISS dictates forwarding actions for the
   slice aggregate: QoS behavior and next hop selection.  The purpose of
   the GISS is detailed in [I-D.bestbar-teas-ns-packet].  To embed a
   GISS in a label stack, one must preface it with a bSPL identifying it
   as such.  For reasons that will become apparent, this bSPL is called
   the Forwarding Actions Indicator (FAI).

2.  Multi-purpose bSPL: the Forwarding Actions Indicator

   This document proposes the use of a single bSPL to tell routers one
   or more forwarding actions they should take on a packet, e.g.:

   *  to treat a packet according to its slice, given its GISS;

   *  to load balance a packet, given its entropy;

   *  whether or not to perform fast reroute on a failure
      [I-D.kompella-mpls-nffrr];

   *  whether or not a packet has metadata relevant to intermediate hops
      along the path;

   *  and perhaps other functions in the future.

   This bSPL is called the "Forwarding Actions Indicator" (FAI).  There
   are other suggestions for this name, including "Network Functions
   Indicator" and "Network Actions Indicator".  We'll let WG consensus
   determine the final choice of name, but for now, we'll continue to
   use FAI.








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   The FAI uses the label's TC bits and TTL field to inform the
   forwarding plane of the required actions.  Each of these actions may
   have associated data.  This data may be carried in the label stack as
   "In-Stack Data" (ISD) or after the label stack as "Post-Stack Data"
   (PSD).

2.1.  The FAI bSPL

   The design of the bSPL hinges on two key insights: forwarding engines
   do not interpret the TC bits or the TTL field for labels that are not
   at the top of the label stack (ToS); nor do they do so for SPLs.  For
   non-ToS labels, the important bit fields are the label value field
   (to compute entropy and identify SPLs) and the End of Stack (S) bit
   (to know when the label stack ends).  [If you know of a forwarding
   engine that looks at other bit fields of labels below the ToS, please
   contact the authors.]  This means that for a bSPL that will never
   appear at the ToS, the TC bits and the TTL bits can be used to carry
   additional information.  Furthermore, for the ISD, the entire 4-octet
   label word, the S bit excepted, can be used to carry data.  We use
   this technique to make the FAI bSPL multipurpose, and to make the ISD
   words compact and efficient.

2.1.1.  ISD vs PSD

   A pertinent question is when one should put data in the ISD versus in
   the PSD.  One alternative is to put all such data in the PSD.
   However, this would mean that accessing such information would
   require finding the End of Stack, and parsing the PSD.  For certain
   types of data, this would be a severe burden on the packet forwarding
   engine.  Examples of such data are the Entropy label (needed for
   efficient load balancing) and the GISS (needed for accurate packet
   forwarding).  Having any of this data in the PSD would hurt
   forwarding performance.

   This memo suggests that data that is required for accurate and
   optimal forwarding should be put in the ISD, and data that is
   optional from a forwarding point of view should be put in the PSD.
   Furthermore, each flag bit should have no more than one word of
   associated ISD.  The EG flag can thus have up to 2 words of
   associated data.

   By the above criteria, this memo suggests that in-situ OAM data and
   the Flow ID be carried in the PSD.

2.2.  Format of the FAI bSPL






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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
                                             TC b S       TTL
                                            -----   ---------------
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         (previous forwarding label    | TC  |0|      TTL      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Forwarding Actions Indicator  |s|u|0|0|h|N|E G|x|y|z|a|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Forwarding Actions Header         |0|0|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Forwarding Actions Header         |1|0|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Standard In-Stack Data (sISD)     |0|0|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Last word of sISD                 |1|0|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         User-defined ISD (uISD)           |0|0|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Last word of User-defined ISD     |1|0|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Other labels                        |0|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         End of Stack label                  |1|               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |b b b b| Payload (potentially, PSD)                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Payload                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: Format for FAI, ISD and PSD

   The FAI's label value MUST be the IANA allocated value.  The S bit
   MUST be reflect whether the label stack ends at this label or not.

2.2.1.  Definitions of the FAI Flag Bits

   The TC and TTL bits are used as flags, defined as follows:

   s:  sISD is present (1) or not (0).

   u:  uISD is present (1) or not (0).

   b:  this is the "end of block" bit that indicates the end of the
      Forwarding Flags Block and the end of the ISD Block.

   S:  MUST be set if the FAI is the end of stack, and clear otherwise.




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   h:  If set, the PSD contains hop-by-hop information.  Every node in
      the path SHOULD attempt to process the hop-by-hop information, but
      not at the expense of exceeding the processing time budget, which
      could cause this (or other) packets to be dropped.  If clear, no
      hop-by-hop data exists in the PSD: either the PSD is empty, or it
      contains only end-to-end data (to be processed by the egress).

   N:  If set, do not do fast reroute (NFFRR).

   EG:  this is a 2-bit flag indicating whether the ISD carries Entropy
      and/or GISS information.

   The FAI Block consists of a Forwarding Flags Block, an sISD Block and
   a uISD Block.  The two ISD Blocks are optional; their presence is
   indicated by the s and u bits.  Each of these three blocks end when
   the b bit is set.

   The Forwarding Flags Block extends from the FAI bSPL up to (and
   including) the first label that has the b bit set.  If the FFB
   consists of just the bSPL, then its b bit must be set.

   The sISD Block extends from the label after the FFB up to (and
   including) the label with the b bit set.  If there is no sISD, the s
   bit in the FFB MUST be clear.

   The uISD Block extends from the label after the sISD Block up to (and
   including) the label with the b bit set.  If there is no uISD, the u
   bit in the FFB MUST be clear.

   The EG field is used as follows:

   00:  No Entropy or GISS present

   01:  ISD 0 contains 16 bits of Entropy in the high order 16 bits and
      14 bits of GISS in the low order 16 bits (S and b bits excepted).

   10:  ISD 0 contains 20 bits of Entropy in the high order 20 bits and
      10 bits of GISS in the low order 12 bits (S and b bits excepted).

   11:  ISD 0 contains the 30-bit Entropy; ISD 1 contains the 30-bit
      GISS.  In ISD 0, the S and b bits MUST be 0; the packet forwarding
      engine may choose to use the S and b bits as part of the Entropy,
      as it doesn't affect the outcome.  In ISD 1, the S bit may be 0 or
      1.







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2.2.2.  Processing the FAI Flags and the ISD

   Here's how the Standard ISD is parsed.  One must keep track of the s
   bit to know when the Standard ISD Block end, and the S bit to know
   when the stack ends.  The Standard ISD data appears in the order of
   the corresponding flags.

   It is an error if the label stack ends while there are more ISD words
   to process.  In particular, it is an error if the FAI's S bit is set,
   but the b bit is clear.

   1.  If s and u are both 0, done: there is no associated ISD.

   2.  Set CL ("current label") to the FAI label.  LL is the last label
       (End of Stack); PL ("payload") is the first 4-octet word of the
       payload.

   3.  While b is clear:

       1.  increment CL

   4.  Process N.  CL is unchanged.

   5.  If s is set, Standard ISD is present: process standard flags.

       1.  Process EG:

       2.  If EG is 00, CL is unchanged.

       3.  If EG is 01 or 10, increment CL.  CL now contains both GISS
           and Entropy.

       4.  If EG is 11, CL+1 contains Entropy; CL+2 contains GISS.
           Increment CL by 2.

       5.  Process other standard data-bearing flags; increment CL by 1
           for each.

   6.  If u is set, uISD is present.

       1.  Process uISD until b is set.

   Note that how the uISD is used is not defined here; this is up to the
   user.  All that is included here is how a forwarding engine can tell
   where the uISD block ends.

2.2.3.  Example of the FAI




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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
                                                 TC b S       TTL
                                                -----   ---------------
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Tunnel Label-1                   | TC  |0|      TTL      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Tunnel Label-2                   | TC  |0|      TTL      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Forwarding Actions Indicator     |1|1|1|0|1|1|0|1|0|0|0|0|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Entropy                  |   GISS ...|1|0|      ...      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      VPN Label                        |TC   |1|      TTL      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |b b b b|                    PSD                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | real payload starts ...

   s  =  1: there is standard ISD.
   u  =  0: there is no user-defined ISD.
   N  =  1: NFFRR is set.
   EG = 01: ISD 0 contains Entropy + GISS.
   h  =  1: There is hop-by-hop PSD.

              Figure 2: Example of FAI + ISD + hop-by-hop PSD

   The real payload starts after the PSD.

3.  Issues to be Resolved

   This section captures issues to be resolved, in this memo and others.
   As the issues are fixed, they should be removed from here; ideally,
   this section should be empty before publication.

3.1.  Preventing FAI From Reaching Top of Stack

   As was said earlier, the FAI MUST NOT be at the top of stack, since
   its TC and TTL bits have been repurposed.  There are two ways to
   prevent this.  If an LSR X pops a label and the next label is the
   FAI, X can pop the FAI and all ISD words.  This version of the memo
   introduces the "end-of-block" (s) bit, whereby a forwarding engine
   that knows the FAI can detect the entire FAI block, even if it
   doesn't know some of the flags.  This can be used in conjunction with
   Section 3.2.






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   In case it is desired to preserve the FAI+FAD until the egress, X
   should push an explicit NULL (label value 0 or 2) onto the stack
   above the FAI, with the correct TC and TTL values.

   Other options may be pursued; however, we believe this is an adequate
   resolution.

3.2.  Repeating the FAI at "Readable Stack Depth"

   For LSRs which cannot parse the entire label stack, or would prefer
   not to unless needed, it is possible to repeat the FAI at "readable
   stack depth" (rsd).  Say the rsd is 10 labels, and the FAI block is 3
   labels.  Then, the FAI block can be repeated every 7 labels, allowing
   all forwarding engines in the path to process it.  When a forwarding
   label is popped and the FAI block exposed, it is deleted in its
   entirety, since the same (or potentially different) FAI block is
   again within the rsd.

   Note that the s or u bits set to 0 can be used to indicate that the
   corresponding ISD is absent.  Only the last FAI would contain the
   full information, reducing the size of the label stack.  However, in
   this case, LSRs that don't process the whole stack may not load
   balance less effectively, and potentially not adhere to the slice
   service level objectives.

   Other options will be described in future versions of this document.

3.3.  PSD

   The format of the PSD, whether or not a Control Word is present, and
   handling of the first nibble, is outside the scope of this document.
   The FAI will not contain details about the contents of the PSD,
   besides the single flag on whether or not the PSD contains
   information relevant to (most) intermediate hops.  It is assumed that
   another memo will document the format of the PSD, and that that memo
   will provide a means of parsing the PSD (e.g., a TLV structure) and
   thus determining its contents.

   The PSD memo should also comment on the impact of processing the PSD
   on forwarding performance, especially in the case of hop-by-hop info.

4.  Contributors

   Many thanks to Colby Barth, Chandra Ramachandran and Srihari Sangli
   for their contributions to this draft.






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5.  Acknowledgments

   We'd like to acknowledge the helpful discussions with Swamy SRK and
   folks from the Broadcom team on the impacts to existing and future
   forwarding engines.

   The edist field was added thanks to Haoyu Song, who suggested the
   optimization to find End of Stack.

6.  IANA Considerations

   If this draft is deemed useful and adopted as a WG document, the
   authors request the allocation of a bSPL for the FAI.  We suggest the
   early allocation of label 8 for this.

7.  Security Considerations

   A malicious or compromised LSR can insert the FAI and associated data
   into a label stack, preventing (for example) FRR from occurring.  If
   so, protection will not kick in for failures that could have been
   protected, and there will be unnecessary packet loss.  Similarly,
   inserting or removing a Fragmentation Header means that a packet's
   contents cannot be accurately reconstructed.  Inserting or changing a
   GISS means that the packet will be misclassified, perhaps leaving or
   entering a high-value slice and causing damage.

8.  References

8.1.  Normative References

   [I-D.bestbar-teas-ns-packet]
              Saad, T., Beeram, V. P., Wen, B., Ceccarelli, D., Halpern,
              J., Peng, S., Chen, R., Liu, X., Contreras, L. M., Rokui,
              R., and L. Jalil, "Realizing Network Slices in IP/MPLS
              Networks", Work in Progress, Internet-Draft, draft-
              bestbar-teas-ns-packet-07, 11 January 2022,
              <https://www.ietf.org/archive/id/draft-bestbar-teas-ns-
              packet-07.txt>.

   [I-D.kompella-mpls-nffrr]
              Kompella, K. and W. Lin, "No Further Fast Reroute", Work
              in Progress, Internet-Draft, draft-kompella-mpls-nffrr-02,
              12 July 2021, <https://www.ietf.org/archive/id/draft-
              kompella-mpls-nffrr-02.txt>.







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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [I-D.nsdt-teas-ns-framework]
              Gray, E. and J. Drake, "Framework for IETF Network
              Slices", Work in Progress, Internet-Draft, draft-nsdt-
              teas-ns-framework-05, 2 February 2021,
              <https://www.ietf.org/archive/id/draft-nsdt-teas-ns-
              framework-05.txt>.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States

   Email: kireeti.ietf@gmail.com


   Vishnu Pavan Beeram
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States

   Email: vbeeram@juniper.net


   Tarek Saad
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States

   Email: tsaad@juniper.net






Kompella, et al.         Expires 12 August 2022                [Page 13]


Internet-Draft                 MSPL for FA                 February 2022


   Israel Meilik
   Broadcom

   Email: israel.meilik@broadcom.com















































Kompella, et al.         Expires 12 August 2022                [Page 14]