





Internet Engineering Task Force (IETF)                         A. Farrel
Request for Comments: 8595                            Old Dog Consulting
Category: Standards Track                                      S. Bryant
ISSN: 2070-1721                                                Futurewei
                                                                J. Drake
                                                        Juniper Networks
                                                               June 2019


      An MPLS-Based Forwarding Plane for Service Function Chaining

Abstract

   This document describes how Service Function Chaining (SFC) can be
   achieved in an MPLS network by means of a logical representation of
   the Network Service Header (NSH) in an MPLS label stack.  That is,
   the NSH is not used, but the fields of the NSH are mapped to fields
   in the MPLS label stack.  This approach does not deprecate or replace
   the NSH, but it acknowledges that there may be a need for an interim
   deployment of SFC functionality in brownfield networks.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8595.

















Farrel, et al.               Standards Track                    [Page 1]

RFC 8595                        MPLS SFC                       June 2019


Copyright Notice

   Copyright (c) 2019 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
   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 and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Requirements Language ...........................................4
   3. Choice of Data-Plane SPI/SI Representation ......................4
   4. Use Case Scenarios ..............................................5
      4.1. Label Swapping for Logical NSH .............................5
      4.2. Hierarchical Encapsulation .................................5
      4.3. Fine Control of Service Function Instances .................6
      4.4. Micro Chains and Label Stacking ............................6
      4.5. SFC and Segment Routing ....................................6
   5. Basic Unit of Representation ....................................6
   6. MPLS Label Swapping .............................................7
   7. MPLS Label Stacking ............................................10
   8. Mixed-Mode Forwarding ..........................................12
   9. A Note on Service Function Capabilities and SFC Proxies ........13
   10. Control-Plane Considerations ..................................14
   11. Use of the Entropy Label ......................................14
   12. Metadata ......................................................15
      12.1. Indicating Metadata in User Data Packets .................16
      12.2. In-Band Programming of Metadata ..........................18
           12.2.1. Loss of In-Band Metadata ..........................21
   13. Worked Examples ...............................................22
   14. Implementation Notes ..........................................26
   15. Security Considerations .......................................26
   16. IANA Considerations ...........................................28
   17. References ....................................................29
      17.1. Normative References .....................................29
      17.2. Informative References ...................................30
   Acknowledgements ..................................................31
   Contributors ......................................................31
   Authors' Addresses ................................................32




Farrel, et al.               Standards Track                    [Page 2]

RFC 8595                        MPLS SFC                       June 2019


1.  Introduction

   Service Function Chaining (SFC) is the process of directing packets
   through a network so that they can be acted on by an ordered set of
   abstract Service Functions (SFs) before being delivered to the
   intended destination.  An architecture for SFC is defined in
   [RFC7665].

   When applying a particular service function chain to the traffic
   selected by a service classifier, the traffic needs to be steered
   through an ordered set of SFs in the network.  This ordered set of
   SFs is termed a Service Function Path (SFP), and the traffic is
   passed between Service Function Forwarders (SFFs) that are
   responsible for delivering the packets to the SFs and for forwarding
   them onward to the next SFF.

   In order to steer the selected traffic between SFFs and to the
   correct SFs, the service classifier needs to attach information to
   each packet.  This information indicates the SFP on which the packet
   is being forwarded and hence the SFs to which it must be delivered.
   The information also indicates the progress the packet has already
   made along the SFP.

   The Network Service Header (NSH) [RFC8300] has been defined to carry
   the necessary information for SFC in packets.  The NSH can be
   inserted into packets and contains various information, including a
   Service Path Identifier (SPI), a Service Index (SI), and a Time To
   Live (TTL) counter.

   Multiprotocol Label Switching (MPLS) [RFC3031] is a widely deployed
   forwarding technology that uses labels placed in a packet in a label
   stack to identify the forwarding actions to be taken at each hop
   through a network.  Actions may include swapping or popping the
   labels as well as using the labels to determine the next hop for
   forwarding the packet.  Labels may also be used to establish the
   context under which the packet is forwarded.  In many cases, MPLS
   will be used as a tunneling technology to carry packets through
   networks between SFFs.

   This document describes how SFC can be achieved in an MPLS network by
   means of a logical representation of the NSH in an MPLS label stack.
   This approach is applicable to all forms of MPLS forwarding (where
   labels are looked up at each hop and are swapped or popped
   [RFC3031]).  It does not deprecate or replace the NSH, but it
   acknowledges that there may be a need for an interim deployment of
   SFC functionality in brownfield networks.  The mechanisms described
   in this document are a compromise between the full function that can
   be achieved using the NSH and the benefits of reusing the existing



Farrel, et al.               Standards Track                    [Page 3]

RFC 8595                        MPLS SFC                       June 2019


   MPLS forwarding paradigms (the approach defined here does not include
   the O bit defined in [RFC8300] and has some limitations to the use of
   metadata as described in Section 12).

   Section 4 provides a short overview of several use case scenarios
   that help to explain the relationship between the MPLS label
   operations (swapping, popping, stacking) and the MPLS encoding of the
   logical NSH described in this document.

   It is assumed that the reader is fully familiar with the terms and
   concepts introduced in [RFC7665] and [RFC8300].

   Note that one of the features of the SFC architecture described in
   [RFC7665] is the "SFC proxy", which exists to include legacy SFs that
   are not able to process NSH-encapsulated packets.  This issue is
   equally applicable to the use of MPLS-encapsulated packets that
   encode a logical representation of an NSH.  It is discussed further
   in Section 9.

2.  Requirements Language

   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.

3.  Choice of Data-Plane SPI/SI Representation

   While [RFC8300] defines the NSH that can be used in a number of
   environments, this document provides a mechanism to handle situations
   in which the NSH is not ubiquitously deployed.  In this case, it is
   possible to use an alternative data-plane representation of the
   SPI/SI by carrying the identical semantics in MPLS labels.

   In order to correctly select the mechanism by which SFC information
   is encoded and carried between SFFs, it may be necessary to configure
   the capabilities and choices either within the whole Service Function
   Overlay Network or on a hop-by-hop basis.  It is a requirement that
   both ends of a tunnel over the underlay network (i.e., a pair of SFFs
   adjacent in the SFP) know that the tunnel is used for SFC and know
   what form of NSH representation is used.  A control-plane signaling
   approach to achieve these objectives is provided using BGP in
   [BGP-NSH-SFC].







Farrel, et al.               Standards Track                    [Page 4]

RFC 8595                        MPLS SFC                       June 2019


   Note that the encoding of the SFC information is independent of the
   choice of tunneling technology used between SFFs.  Thus, an MPLS
   representation of the logical NSH (as defined in this document) may
   be used even if the tunnel between a pair of SFFs is not an MPLS
   tunnel.  Conversely, MPLS tunnels may be used to carry other
   encodings of the logical NSH (specifically, the NSH itself).

4.  Use Case Scenarios

   There are five scenarios that can be considered for the use of an
   MPLS encoding in support of SFC.  These are set out in the following
   subsections.

4.1.  Label Swapping for Logical NSH

   The primary use case for SFC is described in [RFC7665] and delivered
   using the NSH, which, as described in [RFC8300], uses an
   encapsulation with a position indicator that is modified at each SFC
   hop along the chain to indicate the next hop.

   The label-swapping use case scenario effectively replaces the NSH
   with an MPLS encapsulation as described in Section 6.  The MPLS
   labels encode the same information as the NSH to form a logical NSH.
   The labels are modified (swapped per [RFC3031]) at each SFC hop along
   the chain to indicate the next hop.  The processing and the
   forwarding state for a chain (i.e., the actions to take on a received
   label) are programmed into the network using a control plane or
   management plane.

4.2.  Hierarchical Encapsulation

   [RFC8459] describes an architecture for hierarchical encapsulation
   using the NSH.  It facilitates partitioning of SFC domains for
   administrative reasons and allows concatenation of service function
   chains under the control of a service classifier.

   The same function can be achieved in an MPLS network using an MPLS
   encoding of the logical NSH, and label stacking as defined in
   [RFC3031] and described in Section 7.  In this model, swapping is
   used per Section 4.1 to navigate one chain, and when the end of the
   chain is reached, the final label is popped, revealing the label for
   another chain.  Thus, the primary mode is swapping, but stacking is
   used to enable the ingress classifier to control concatenation of
   service function chains.







Farrel, et al.               Standards Track                    [Page 5]

RFC 8595                        MPLS SFC                       June 2019


4.3.  Fine Control of Service Function Instances

   It may be that a service function chain (as described in Section 4.1)
   allows some leeway in the choice of service function instances along
   the chain.  However, it may be that a service classifier wishes to
   constrain the choice and this can be achieved using chain
   concatenation so that the first chain ends at the point of choice,
   the next label in the stack indicates the specific service function
   instance to be executed, and the next label in the stack starts a new
   chain.  Thus, a mixture of label swapping and stacking is used.

4.4.  Micro Chains and Label Stacking

   The scenario in Section 4.2 may be extended to its logical extreme by
   making each concatenated chain as short as it can be: one SF.  Each
   label in the stack indicates the next SF to be executed, and the
   network is programmed through the control plane or management plane
   to know how to route to the next (i.e., first) hop in each chain just
   as it would be to support the scenarios in Sections 4.1 and 4.2.

   This scenario is functionally identical to the use of Segment Routing
   (SR) in an MPLS network (known as SR-MPLS) for SFC, as described in
   Section 4.5, and the discussion in that section applies to this
   section as well.

4.5.  SFC and Segment Routing

   SR-MPLS uses a stack of MPLS labels to encode information about the
   path and network functions that a packet should traverse.  SR-MPLS is
   achieved by applying control-plane and management-plane techniques to
   program the MPLS forwarding plane and by imposing labels on packets
   at the entrance to the SR-MPLS network.  An implementation proposal
   for achieving SFC using SR-MPLS can be found in [SR-Srv-Prog] and is
   not discussed further in this document.

5.  Basic Unit of Representation

   When an MPLS label stack is used to carry a logical NSH, a basic unit
   of representation is used.  This unit comprises two MPLS labels, as
   shown below.  The unit may be present one or more times in the label
   stack as explained in subsequent sections.

   In order to convey the same information as is present in the NSH, two
   MPLS label stack entries are used.  One carries a label to provide
   context within the SFC scope (the SFC Context Label), and the other
   carries a label to show which SF is to be actioned (the SF Label).
   This two-label unit is shown in Figure 1.




Farrel, et al.               Standards Track                    [Page 6]

RFC 8595                        MPLS SFC                       June 2019


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           SFC Context Label           | TC  |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           SF Label                    | TC  |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 1: The Basic Unit of MPLS Label Stack for SFC

   The fields of these two label stack entries are encoded as follows:

   Label:  The Label fields contain the values of the SFC Context Label
      and the SF Label encoded as 20-bit integers.  The precise
      semantics of these Label fields are dependent on whether the label
      stack entries are used for MPLS label swapping (see Section 6) or
      MPLS label stacking (see Section 7).

   TC:  The TC bits have no meaning in this case.  They SHOULD be set to
      zero in both label stack entries when a packet is sent and MUST be
      ignored on receipt.

   S: The "Bottom of Stack" bit has its usual meaning in MPLS.  It MUST
      be clear in the SFC Context Label stack entry.  In the SF Label
      stack entry, it MUST be clear in all cases except when the label
      is the bottom of the stack, when it MUST be set.

   TTL:  The TTL field in the SFC Context Label stack entry SHOULD be
      set to 1.  The TTL in the SF Label stack entry (called the SF TTL)
      is set according to its use for MPLS label swapping (see
      Section 6) or MPLS label stacking (see Section 7) and is used to
      mitigate packet loops.

   The sections that follow show how this basic unit of MPLS label stack
   may be used for SFC in the MPLS label-swapping case and in the MPLS
   label-stacking case.  For simplicity, these sections do not describe
   the use of metadata; that topic is covered separately in Section 12.

6.  MPLS Label Swapping

   This section describes how the basic unit of MPLS label stack for SFC
   (introduced in Section 5) is used when MPLS label swapping is in use.
   The use case scenario for this approach is introduced in Section 4.1.

   As can be seen in Figure 2, the top of the label stack comprises the
   labels necessary to deliver the packet over the MPLS tunnel between
   SFFs.  Any MPLS encapsulation may be used (i.e., MPLS, MPLS in UDP,
   MPLS in GRE, and MPLS in Virtual Extensible Local Area Networks



Farrel, et al.               Standards Track                    [Page 7]

RFC 8595                        MPLS SFC                       June 2019


   (VXLANs) or the Generic Protocol Extension for VXLAN (GPE)); thus,
   the tunnel technology does not need to be MPLS, but MPLS is shown
   here for simplicity.

   An entropy label [RFC6790] may also be present, as described in
   Section 11.

       ---------------
      ~ Tunnel Labels ~
      +---------------+
      ~   Optional    ~
      ~ Entropy Label ~
      +---------------+ - - -
      |   SPI Label   |
      +---------------+  Basic unit of MPLS label stack for SFC
      |   SI Label    |
      +---------------+ - - -
      |               |
      ~    Payload    ~
      |               |
       ---------------

                    Figure 2: The MPLS SFC Label Stack

   Under these labels (or other encapsulation) comes a single instance
   of the basic unit of MPLS label stack for SFC.  In addition to the
   interpretation of the fields of these label stack entries (provided
   in Section 5), the following meanings are applied:

   SPI Label:  The Label field of the SFC Context Label stack entry
      contains the value of the SPI encoded as a 20-bit integer.  The
      semantics of the SPI are exactly as defined in [RFC8300].  Note
      that an SPI as defined by [RFC8300] can be encoded in 3 octets
      (i.e., 24 bits), but that the Label field allows for only 20 bits
      and reserves the values 0 through 15 as "special-purpose labels"
      [RFC7274].  Thus, a system using MPLS representation of the
      logical NSH MUST NOT assign SPI values greater than 2^20 - 1 or
      less than 16.

   SI Label:  The Label field of the SF Label stack entry contains the
      value of the SI exactly as defined in [RFC8300].  Since the SI
      requires only 8 bits, and to avoid overlap with the
      special-purpose label range of 0 through 15 [RFC7274], the SI is
      carried in the top (most significant) 8 bits of the Label field
      with the low-order 12 bits set to zero.

   TC:  The TC fields are as described in Section 5.




Farrel, et al.               Standards Track                    [Page 8]

RFC 8595                        MPLS SFC                       June 2019


   S: The S bits are as described in Section 5.

   TTL:  The TTL field in the SPI Label stack entry SHOULD be set to 1
      as stated in Section 5.  The TTL in the SF Label stack entry is
      decremented once for each forwarding hop in the SFP, i.e., for
      each SFF transited, and so mirrors the TTL field in the NSH.

   The following processing rules apply to the Label fields:

   o  When a classifier inserts a packet onto an SFP, it sets the SPI
      Label to indicate the identity of the SFP and sets the SI Label to
      indicate the first SF in the path.

   o  When a component of the SFC system processes a packet, it uses the
      SPI Label to identify the SFP and the SI Label to determine which
      SFF or instance of an SF (an SFI) to deliver the packet to.  Under
      normal circumstances (with the exception of branching and
      reclassification -- see [BGP-NSH-SFC]), the SPI Label value is
      preserved on all packets.  The SI Label value is modified by SFFs
      and through reclassification to indicate the next hop along
      the SFP.

   The following processing rules apply to the TTL field of the SF Label
   stack entry and are derived from Section 2.2 of [RFC8300]:

   o  When a classifier places a packet onto an SFP, it MUST set the TTL
      to a value between 1 and 255.  It SHOULD set this according to the
      expected length of the SFP (i.e., the number of SFs on the SFP),
      but it MAY set it to a larger value according to local
      configuration.  The maximum TTL value supported in an NSH is 63,
      and so the practical limit here may also be 63.

   o  When an SFF receives a packet from any component of the SFC system
      (classifier, SFI, or another SFF), it MUST discard any packets
      with TTL set to zero.  It SHOULD log such occurrences but MUST
      apply rate limiting to any such logs.

   o  An SFF MUST decrement the TTL by one each time it performs a
      lookup to forward a packet to the next SFF.

   o  If an SFF decrements the TTL to zero, it MUST NOT send the packet
      and MUST discard the packet.  It SHOULD log such occurrences but
      MUST apply rate limiting to any such logs.

   o  SFIs MUST ignore the TTL but MUST mirror it back to the SFF
      unmodified along with the SI (which may have been changed by local
      reclassification).




Farrel, et al.               Standards Track                    [Page 9]

RFC 8595                        MPLS SFC                       June 2019


   o  If a classifier along the SFP makes any change to the intended
      path of the packet, including for looping, jumping, or branching
      (see [BGP-NSH-SFC]), it MUST NOT change the SI TTL of the packet.
      In particular, each component of the SFC system MUST NOT increase
      the SI TTL value; otherwise, loops may go undetected.

7.  MPLS Label Stacking

   This section describes how the basic unit of MPLS label stack for SFC
   (introduced in Section 5) is used when MPLS label stacking is used to
   carry information about the SFP and SFs to be executed.  The use case
   scenarios for this approach are introduced in Section 4.

   As can be seen in Figure 3, the top of the label stack comprises the
   labels necessary to deliver the packet over the MPLS tunnel between
   SFFs.  Any MPLS encapsulation may be used.

       -------------------
      ~   Tunnel Labels   ~
      +-------------------+
      ~     Optional      ~
      ~   Entropy Label   ~
      +-------------------+ - - -
      | SFC Context Label |
      +-------------------+  Basic unit of MPLS label stack for SFC
      |     SF Label      |
      +-------------------+ - - -
      | SFC Context Label |
      +-------------------+  Basic unit of MPLS label stack for SFC
      |     SF Label      |
      +-------------------+ - - -
      ~                   ~
      +-------------------+ - - -
      | SFC Context Label |
      +-------------------+  Basic unit of MPLS label stack for SFC
      |     SF Label      |
      +-------------------+ - - -
      |                   |
      ~      Payload      ~
      |                   |
       -------------------

           Figure 3: The MPLS SFC Label Stack for Label Stacking

   An entropy label [RFC6790] may also be present, as described in
   Section 11.





Farrel, et al.               Standards Track                   [Page 10]

RFC 8595                        MPLS SFC                       June 2019


   Under these labels comes one or more instances of the basic unit of
   MPLS label stack for SFC.  In addition to the interpretation of the
   fields of these label stack entries (provided in Section 5), the
   following meanings are applied:

   SFC Context Label:  The Label field of the SFC Context Label stack
      entry contains a label that delivers SFC context.  This label
      contains the SPI, encoded as a 20-bit integer using the semantics
      exactly as defined in [RFC8300].  Note that in this case a system
      using MPLS representation of the logical NSH MUST NOT assign SPI
      values greater than 2^20 - 1 or less than 16.  This label may also
      be used to convey other SFC context-specific semantics, such as
      indicating how to interpret the SF Label or how to forward the
      packet to the node that offers the SF if so configured and
      coordinated with the controller that programs the labels for
      the SFP.

   SF Label:  The Label field of the SF Label stack entry contains a
      value that identifies the next SFI to be actioned for the packet.
      This label may be scoped globally or within the context of the
      preceding SFC Context Label and comes from the range
      16 ... 2^20 - 1.

   TC:  The TC fields are as described in Section 5.

   S: The S bits are as described in Section 5.

   TTL:  The TTL fields in the SFC Context Label stack entry and in the
      SF Label stack entry SHOULD be set to 1 as stated in Section 5 but
      MAY be set to larger values if the label indicated a forwarding
      operation towards the node that hosts the SF.

   The following processing rules apply to the Label fields:

   o  When a classifier inserts a packet onto an SFP, it adds a stack
      comprising one or more instances of the basic unit of MPLS label
      stack for SFC.  Taken together, this stack defines the SFs to be
      actioned and so defines the SFP that the packet will traverse.

   o  When a component of the SFC system processes a packet, it uses the
      top basic unit of label stack for SFC to determine to which SFI to
      next deliver the packet.  When an SFF receives a packet, it
      examines the top basic unit of MPLS label stack for SFC to
      determine where to send the packet next.  If the next recipient is
      a local SFI, the SFF strips the basic unit of MPLS label stack for
      SFC before forwarding the packet.





Farrel, et al.               Standards Track                   [Page 11]

RFC 8595                        MPLS SFC                       June 2019


8.  Mixed-Mode Forwarding

   The previous sections describe homogeneous networks where SFC
   forwarding is either all label swapping or all label popping
   (stacking).  This simplification helps to clarify the explanation of
   the mechanisms.

   However, as described in Section 4.2, some use cases may use label
   swapping and stacking at the same time.  Furthermore, it is also
   possible that different parts of the network utilize swapping or
   popping such that an end-to-end service chain has to utilize a
   combination of both techniques.  It is also worth noting that a
   classifier may be content to use an SFP as installed in the network
   by a control plane or management plane and so would use label
   swapping, but that there may be a point in the SFP where a choice of
   SFIs can be made (perhaps for load balancing) and where, in this
   instance, the classifier wishes to exert control over that choice by
   use of a specific entry on the label stack as described in
   Section 4.3.

   When an SFF receives a packet containing an MPLS label stack, it
   checks from the context of the incoming interface, and from the SFP
   indicated by the top label, whether it is processing an {SPI, SI}
   label pair for label swapping or a {context label, SFI index} label
   pair for label stacking.  It then selects the appropriate SFI to
   which to send the packet.  When it receives the packet back from the
   SFI, it has four cases to consider.

   o  If the current hop requires an {SPI, SI} and the next hop requires
      an {SPI, SI}, it sets the SPI Label according to the SFP to be
      traversed, selects an instance of the SF to be executed at the
      next hop, sets the SI Label to the SI value of the next hop, and
      tunnels the packet to the SFF for that SFI.

   o  If the current hop requires an {SPI, SI} and the next hop requires
      a {context label, SFI Label}, it pops the {SPI, SI} from the top
      of the MPLS label stack and tunnels the packet to the SFF
      indicated by the context label.













Farrel, et al.               Standards Track                   [Page 12]

RFC 8595                        MPLS SFC                       June 2019


   o  If the current hop requires a {context label, SFI Label}, it pops
      the {context label, SFI Label} from the top of the MPLS label
      stack.

      *  If the new top of the MPLS label stack contains an {SPI, SI}
         label pair, it selects an SFI to use at the next hop and
         tunnels the packet to the SFF for that SFI.

      *  If the new top of the MPLS label stack contains a {context
         label, SFI Label}, it tunnels the packet to the SFF indicated
         by the context label.

9.  A Note on Service Function Capabilities and SFC Proxies

   The concept of an "SFC proxy" is introduced in [RFC7665].  An SFC
   proxy is logically located between an SFF and an SFI that is not
   "SFC aware".  Such SFIs are not capable of handling the SFC
   encapsulation (whether that be NSH or MPLS) and need the
   encapsulation stripped from the packets they are to process.  In many
   cases, legacy SFIs that were once deployed as "bumps in the wire" fit
   into this category until they have been upgraded to be SFC aware.

   The job of an SFC proxy is to remove and then reimpose SFC
   encapsulation so that the SFF is able to process as though it was
   communication with an SFC-aware SFI, and so that the SFI is unaware
   of the SFC encapsulation.  In this regard, the job of an SFC proxy is
   no different when NSH encapsulation is used and when MPLS
   encapsulation is used as described in this document, although (of
   course) it is different encapsulation bytes that must be removed and
   reimposed.

   It should be noted that the SFC proxy is a logical function.  It
   could be implemented as a separate physical component on the path
   from the SFF to the SFI, but it could be co-resident with the SFF or
   it could be a component of the SFI.  This is purely an implementation
   choice.

   Note also that the delivery of metadata (see Section 12) requires
   specific processing if an SFC proxy is in use.  This is also no
   different when NSH functionality or the MPLS encoding defined in this
   document is in use, and how it is handled will depend on how (or if)
   each non-SFC-aware SFI can receive metadata.









Farrel, et al.               Standards Track                   [Page 13]

RFC 8595                        MPLS SFC                       June 2019


10.  Control-Plane Considerations

   In order that a packet may be forwarded along an SFP, several
   functional elements must be executed.

   o  Discovery/advertisement of SFIs.

   o  Computation of the SFP.

   o  Programming of classifiers.

   o  Advertisement of forwarding instructions.

   Various approaches may be taken.  These include a fully centralized
   model where SFFs report to a central controller the SFIs that they
   support, the central controller computes the SFP and programs the
   classifiers, and (if the label-swapping approach is taken) the
   central controller installs forwarding state in the SFFs that lie on
   the SFP.

   Alternatively, a dynamic control plane may be used, such as that
   described in [BGP-NSH-SFC].  In this case, the SFFs use the control
   plane to advertise the SFIs that they support, a central controller
   computes the SFP and programs the classifiers, and (if the
   label-swapping approach is taken) the central controller uses the
   control plane to advertise the SFPs so that SFFs that lie on the SFP
   can install the necessary forwarding state.

11.  Use of the Entropy Label

   Entropy is used in ECMP situations to ensure that packets from the
   same flow travel down the same path, thus avoiding jitter or
   reordering issues within a flow.

   Entropy is often determined by hashing on specific fields in a packet
   header, such as the "five-tuple" in the IP and transport headers.
   However, when an MPLS label stack is present, the depth of the stack
   could be too large for some processors to correctly determine the
   entropy hash.  This problem is addressed by the inclusion of an
   entropy label as described in [RFC6790].











Farrel, et al.               Standards Track                   [Page 14]

RFC 8595                        MPLS SFC                       June 2019


   When entropy is desired for packets as they are carried in MPLS
   tunnels over the underlay network, it is RECOMMENDED that an entropy
   label be included in the label stack immediately after the tunnel
   labels and before the SFC Labels, as shown in Figures 2 and 3.

   If an entropy label is present in an MPLS payload, it is RECOMMENDED
   that the initial classifier use that value in an entropy label
   inserted in the label stack when the packet is forwarded (on the
   first tunnel) to the first SFF.  In this case, it is not necessary to
   remove the entropy label from the payload.

12.  Metadata

   Metadata is defined in [RFC7665] as providing "the ability to
   exchange context information between classifiers and SFs, and among
   SFs."  [RFC8300] defines how this context information can be directly
   encoded in fields that form part of the NSH encapsulation.

   Sections 12.1 and 12.2 describe how metadata is associated with user
   data packets, and how metadata may be exchanged between SFC nodes in
   the network, when using an MPLS encoding of the logical
   representation of the NSH.

   It should be noted that the MPLS encoding is less functional than the
   direct use of the NSH.  Both methods support metadata that is
   "per-SFP" or "per-flow" (see [RFC8393] for definitions of these
   terms), but "per-packet" metadata (where the metadata must be carried
   on each packet because it differs from one packet to the next even on
   the same flow or SFP) is only supported using the NSH and not using
   the mechanisms defined in this document.





















Farrel, et al.               Standards Track                   [Page 15]

RFC 8595                        MPLS SFC                       June 2019


12.1.  Indicating Metadata in User Data Packets

   Metadata is achieved in the MPLS realization of the logical NSH by
   the use of an SFC Metadata Label, which uses the extended
   special-purpose label construct [RFC7274].  Thus, three label stack
   entries are present, as shown in Figure 4:

   o  The Extension Label (value 15).

   o  An extended special-purpose label called the Metadata Label
      Indicator (MLI) (value 16).

   o  The Metadata Label (ML).

                              ----------------
                             | Extension = 15 |
                             +----------------+
                             |      MLI       |
                             +----------------+
                             | Metadata Label |
                              ----------------

                   Figure 4: The MPLS SFC Metadata Label

   The Metadata Label value is an index into a table of metadata that is
   programmed into the network using in-band or out-of-band mechanisms.
   Out-of-band mechanisms potentially include management-plane and
   control-plane solutions (such as [BGP-NSH-SFC]) but are out of scope
   for this document.  The in-band mechanism is described in
   Section 12.2.

   The SFC Metadata Label (as a set of three labels as indicated in
   Figure 4) may be present zero, one, or more times in an MPLS SFC
   packet.  For MPLS label swapping, the SFC Metadata Labels are placed
   immediately after the basic unit of MPLS label stack for SFC, as
   shown in Figure 5.  For MPLS label stacking, the SFC Metadata Labels
   are placed at the bottom of the label stack, as shown in Figure 6.














Farrel, et al.               Standards Track                   [Page 16]

RFC 8595                        MPLS SFC                       June 2019


                            ----------------
                           ~ Tunnel Labels  ~
                           +----------------+
                           ~   Optional     ~
                           ~ Entropy Label  ~
                           +----------------+
                           |   SPI Label    |
                           +----------------+
                           |   SI Label     |
                           +----------------+
                           | Extension = 15 |
                           +----------------+
                           |     MLI        |
                           +----------------+
                           | Metadata Label |
                           +----------------+
                           ~     Other      ~
                           |    Metadata    |
                           ~  Label Triples ~
                           +----------------+
                           |                |
                           ~    Payload     ~
                           |                |
                            ----------------

           Figure 5: The MPLS SFC Label Stack for Label Swapping
                            with Metadata Label
























Farrel, et al.               Standards Track                   [Page 17]

RFC 8595                        MPLS SFC                       June 2019


                           -------------------
                          ~   Tunnel Labels   ~
                          +-------------------+
                          ~     Optional      ~
                          ~   Entropy Label   ~
                          +-------------------+
                          | SFC Context Label |
                          +-------------------+
                          |     SF Label      |
                          +-------------------+
                          ~                   ~
                          +-------------------+
                          | SFC Context Label |
                          +-------------------+
                          |     SF Label      |
                          +-------------------+
                          |   Extension = 15  |
                          +-------------------+
                          |        MLI        |
                          +-------------------+
                          |  Metadata Label   |
                          +-------------------+
                          ~       Other       ~
                          |      Metadata     |
                          ~   Label Triples   ~
                          +-------------------+
                          |                   |
                          ~      Payload      ~
                          |                   |
                           -------------------

           Figure 6: The MPLS SFC Label Stack for Label Stacking
                            with Metadata Label

12.2.  In-Band Programming of Metadata

   A mechanism for sending metadata associated with an SFP without a
   payload packet is described in [RFC8393].  The same approach can be
   used in an MPLS network where the NSH is logically represented by an
   MPLS label stack.











Farrel, et al.               Standards Track                   [Page 18]

RFC 8595                        MPLS SFC                       June 2019


   The packet header is formed exactly as previously described in this
   document so that the packet will follow the SFP through the SFC
   network.  However, instead of payload data, metadata is included
   after the bottom of the MPLS label stack.  An extended
   special-purpose label is used to indicate that the metadata is
   present.  Thus, three label stack entries are present:

   o  The Extension Label (value 15).

   o  An extended special-purpose label called the Metadata Present
      Indicator (MPI) (value 17).

   o  The Metadata Label (ML) that is associated with this metadata on
      this SFP and can be used to indicate the use of the metadata as
      described in Section 12.

   The MPI, if present, is placed immediately after the last basic unit
   of MPLS label stack for SFC.  The resultant label stacks are shown in
   Figure 7 for the MPLS label-swapping case and Figure 8 for the MPLS
   label-stacking case.

                              ---------------
                             ~ Tunnel Labels ~
                             +---------------+
                             ~   Optional    ~
                             ~ Entropy Label ~
                             +---------------+
                             |   SPI Label   |
                             +---------------+
                             |   SI Label    |
                             +---------------+
                             | Extension = 15|
                             +---------------+
                             |     MPI       |
                             +---------------+
                             | Metadata Label|
                             +---------------+
                             |               |
                             ~    Metadata   ~
                             |               |
                              ---------------

           Figure 7: The MPLS SFC Label Stack for Label Swapping
                             Carrying Metadata







Farrel, et al.               Standards Track                   [Page 19]

RFC 8595                        MPLS SFC                       June 2019


                            -------------------
                           ~   Tunnel Labels   ~
                           +-------------------+
                           ~     Optional      ~
                           ~   Entropy Label   ~
                           +-------------------+
                           | SFC Context Label |
                           +-------------------+
                           |     SF Label      |
                           +-------------------+
                           | SFC Context Label |
                           +-------------------+
                           |     SF Label      |
                           +-------------------+
                           ~                   ~
                           +-------------------+
                           | SFC Context Label |
                           +-------------------+
                           |     SF Label      |
                           +-------------------+
                           |   Extension = 15  |
                           +-------------------+
                           |        MPI        |
                           +-------------------+
                           |  Metadata Label   |
                           +-------------------+
                           |                   |
                           ~    Metadata       ~
                           |                   |
                            -------------------

           Figure 8: The MPLS SFC Label Stack for Label Stacking
                             Carrying Metadata

   In both cases, the metadata is formatted as a TLV, as shown in
   Figure 9.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Length              |        Metadata Type          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                         Metadata                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 9: The Metadata TLV





Farrel, et al.               Standards Track                   [Page 20]

RFC 8595                        MPLS SFC                       June 2019


   The fields of this TLV are interpreted as follows:

   Length:  The length of the metadata carried in the Metadata field in
      octets, not including any padding.

   Metadata Type:  The type of the metadata present.  Values for this
      field are taken from the "NSH MD Types" registry maintained by
      IANA and defined in [RFC8300] and encoded with the most
      significant bit first.

   Metadata:  The actual metadata formatted as described in whatever
      document defines the metadata.  This field is end-padded with zero
      to 3 octets of zeroes to take it up to a 4-octet boundary.

12.2.1.  Loss of In-Band Metadata

   Note that in-band exchange of metadata is vulnerable to packet loss.
   This is both a risk arising from network faults and an attack
   vulnerability.

   If packets that arrive at an SFF use an MLI that does not have an
   entry in the metadata table, an alarm can be raised and the packet
   can be discarded or processed without the metadata according to local
   configuration.  This provides some long-term mitigation but is not an
   ideal solution.

   Further mitigation of loss of metadata packets can be achieved by
   retransmitting them at a configurable interval.  This is a relatively
   cheap, but only partial, solution because there may still be a window
   during which the metadata has not been received.

   The concern of lost metadata may be particularly important when the
   metadata applicable to a specific MPI is being changed.  This could
   result in out-of-date metadata being applied to a packet.  If this is
   a concern, it is RECOMMENDED that a new MPI be used to install a new
   entry in the metadata table, and the packets in the flow should be
   marked with the equivalent new MLI.

   Finally, if an application that requires metadata is sensitive to
   this potential loss or attack, it SHOULD NOT use in-band metadata
   distribution but SHOULD rely on control-plane or management-plane
   mechanisms, because these approaches can use a more sophisticated
   protocol that includes confirmation of delivery and can perform
   verification or inspection of entries in the metadata table.







Farrel, et al.               Standards Track                   [Page 21]

RFC 8595                        MPLS SFC                       June 2019


13.  Worked Examples

   This section reverts to the simplified descriptions of networks that
   rely wholly on label swapping or label stacking.  As described in
   Section 4, actual deployment scenarios may depend on the use of both
   mechanisms and utilize a mixed mode as described in Section 8.

   Consider the simplistic MPLS SFC overlay network shown in Figure 10.
   A packet is classified for an SFP that will see it pass through two
   SFs (SFa and SFb) that are accessed through two SFFs (SFFa and SFFb,
   respectively).  The packet is ultimately delivered to the
   destination, D.

            +---------------------------------------------------+
            |                   MPLS SFC Network                |
            |                                                   |
            |            +---------+       +---------+          |
            |            |   SFa   |       |   SFb   |          |
            |            +----+----+       +----+----+          |
            |               ^ | |             ^ | |             |
            |            (2)| | |(3)       (5)| | |(6)          |
            |       (1)     | | V     (4)     | | V    (7)      |
       +----------+ ---> +----+----+ ----> +----+----+ ---> +-------+
       |Classifier+------+  SFFa   +-------+  SFFb   +------+   D   |
       +----------+      +---------+       +---------+      +-------+
            |                                                   |
            +---------------------------------------------------+

          Figure 10: Service Function Chaining in an MPLS Network

   Let us assume that the SFP is computed and assigned an SPI value of
   239.  The forwarding details of the SFP are distributed (perhaps
   using the mechanisms of [BGP-NSH-SFC]) so that the SFFs are
   programmed with the necessary forwarding instructions.

   The packet progresses as follows:

   1.  The classifier assigns the packet to the SFP and imposes two
       label stack entries comprising a single basic unit of MPLS SFC
       representation:

       *  The higher label stack entry contains a label carrying the SPI
          value of 239.

       *  The lower label stack entry contains a label carrying the SI
          value of 255.





Farrel, et al.               Standards Track                   [Page 22]

RFC 8595                        MPLS SFC                       June 2019


       Further labels may be imposed to tunnel the packet from the
       classifier to SFFa.

   2.  When the packet arrives at SFFa, SFFa strips any labels
       associated with the tunnel that runs from the classifier to SFFa.
       SFFa examines the top labels and matches the SPI/SI to identify
       that the packet should be forwarded to SFa.  The packet is
       forwarded to SFa unmodified.

   3.  SFa performs its designated function and returns the packet
       to SFFa.

   4.  SFFa modifies the SI in the lower label stack entry (to 254) and
       uses the SPI/SI to look up the forwarding instructions.  It sends
       the packet with two label stack entries:

       *  The higher label stack entry contains a label carrying the SPI
          value of 239.

       *  The lower label stack entry contains a label carrying the SI
          value of 254.

       Further labels may be imposed to tunnel the packet from SFFa
       to SFFb.

   5.  When the packet arrives at SFFb, SFFb strips any labels
       associated with the tunnel from SFFa.  SFFb examines the top
       labels and matches the SPI/SI to identify that the packet should
       be forwarded to SFb.  The packet is forwarded to SFb unmodified.

   6.  SFb performs its designated function and returns the packet
       to SFFb.

   7.  SFFb modifies the SI in the lower label stack entry (to 253) and
       uses the SPI/SI to look up the forwarding instructions.  It
       determines that it is the last SFF in the SFP, so it strips the
       two SFC Label stack entries and forwards the payload toward D
       using the payload protocol.













Farrel, et al.               Standards Track                   [Page 23]

RFC 8595                        MPLS SFC                       June 2019


   Alternatively, consider the MPLS SFC overlay network shown in
   Figure 11.  A packet is classified for an SFP that will see it pass
   through two SFs (SFx and SFy) that are accessed through two SFFs
   (SFFx and SFFy, respectively).  The packet is ultimately delivered to
   the destination, D.

           +---------------------------------------------------+
           |                   MPLS SFC Network                |
           |                                                   |
           |            +---------+       +---------+          |
           |            |   SFx   |       |   SFy   |          |
           |            +----+----+       +----+----+          |
           |               ^ | |             ^ | |             |
           |            (2)| | |(3)       (5)| | |(6)          |
           |       (1)     | | V     (4)     | | V    (7)      |
      +----------+ ---> +----+----+ ----> +----+----+ ---> +-------+
      |Classifier+------+  SFFx   +-------+  SFFy   +------+   D   |
      +----------+      +---------+       +---------+      +-------+
           |                                                   |
           +---------------------------------------------------+

      Figure 11: Service Function Chaining Using MPLS Label Stacking

   Let us assume that the SFP is computed and assigned an SPI value of
   239.  However, the forwarding state for the SFP is not distributed
   and installed in the network.  Instead, it will be attached to the
   individual packets using the MPLS label stack.

   The packet progresses as follows:

   1.  The classifier assigns the packet to the SFP and imposes two
       basic units of MPLS SFC representation to describe the full SFP:

       *  The top basic unit comprises two label stack entries as
          follows:

          +  The higher label stack entry contains a label carrying the
             SFC context.

          +  The lower label stack entry contains a label carrying the
             SF indicator for SFx.










Farrel, et al.               Standards Track                   [Page 24]

RFC 8595                        MPLS SFC                       June 2019


       *  The lower basic unit comprises two label stack entries as
          follows:

          +  The higher label stack entry contains a label carrying the
             SFC context.

          +  The lower label stack entry contains a label carrying the
             SF indicator for SFy.

       Further labels may be imposed to tunnel the packet from the
       classifier to SFFx.

   2.  When the packet arrives at SFFx, SFFx strips any labels
       associated with the tunnel from the classifier.  SFFx examines
       the top labels and matches the context/SF values to identify that
       the packet should be forwarded to SFx.  The packet is forwarded
       to SFx unmodified.

   3.  SFx performs its designated function and returns the packet
       to SFFx.

   4.  SFFx strips the top basic unit of MPLS SFC representation,
       revealing the next basic unit.  It then uses the revealed
       context/SF values to determine how to route the packet to the
       next SFF, SFFy.  It sends the packet with just one basic unit of
       MPLS SFC representation comprising two label stack entries:

       *  The higher label stack entry contains a label carrying the SFC
          context.

       *  The lower label stack entry contains a label carrying the SF
          indicator for SFy.

       Further labels may be imposed to tunnel the packet from SFFx
       to SFFy.

   5.  When the packet arrives at SFFy, SFFy strips any labels
       associated with the tunnel from SFFx.  SFFy examines the top
       labels and matches the context/SF values to identify that the
       packet should be forwarded to SFy.  The packet is forwarded to
       SFy unmodified.

   6.  SFy performs its designated function and returns the packet
       to SFFy.

   7.  SFFy strips the top basic unit of MPLS SFC representation,
       revealing the payload packet.  It forwards the payload toward D
       using the payload protocol.



Farrel, et al.               Standards Track                   [Page 25]

RFC 8595                        MPLS SFC                       June 2019


14.  Implementation Notes

   It is not the job of an IETF specification to describe the internals
   of an implementation, except where that directly impacts upon the
   bits on the wire that change the likelihood of interoperability or
   where the availability of configuration or security options directly
   affects the utility of an implementation.

   However, in view of the objective of this document to acknowledge
   that there may be a need for an interim deployment of SFC
   functionality in brownfield MPLS networks, this section provides some
   observations about how an SFF might utilize MPLS features that are
   available in existing routers.  This section is not intended to be
   definitive or technically complete; rather, it is indicative.

   Consider the mechanism used to indicate to which Virtual Routing and
   Forwarding (VRF) system an incoming MPLS packet should be routed in a
   Layer 3 Virtual Private Network (L3VPN) [RFC4364].  In this case, the
   top MPLS label is an indicator of the VRF system that is to be used
   to route the payload.

   A similar approach can be taken with the label-swapping SFC technique
   described in Section 6 such that the SFC Context Label identifies a
   routing table specific to the SFP.  The SF Label can be looked up in
   the context of this routing table to determine to which SF to direct
   the packet and how to forward it to the next SFF.

   Advanced features (such as metadata) are not inspected by SFFs.  The
   packets are passed to SFIs that are MPLS-SFC aware or to SFC proxies,
   and those components are responsible for handling all metadata
   issues.

   Of course, an actual implementation might make considerable
   optimizations on this approach, but this section should provide hints
   about how MPLS-based SFC might be achieved with relatively small
   modifications to deployed MPLS devices.

15.  Security Considerations

   Discussion of the security properties of SFC networks can be found in
   [RFC7665].  Further security discussion for the NSH and its use is
   provided in [RFC8300].  Those documents provide analysis and present
   a set of requirements and recommendations for security, and the
   normative security requirements from those documents apply to this
   specification.  However, it should be noted that those documents do
   not describe any mechanisms for securing NSH systems.





Farrel, et al.               Standards Track                   [Page 26]

RFC 8595                        MPLS SFC                       June 2019


   It is fundamental to the SFC design that the classifier is a fully
   trusted element.  That is, the classification decision process is not
   visible to the other elements, and its output is treated as accurate.
   As such, the classifier has responsibility for determining the
   processing that the packet will be subject to, including, for
   example, firewall functions.  It is also fundamental to the MPLS
   design that packets are routed through the network using the path
   specified by the node imposing the labels and that the labels are
   swapped or popped correctly.  Where an SF is not encapsulation aware,
   the encapsulation may be stripped by an SFC proxy such that a packet
   may exist as a native packet (perhaps IP) on the path between the SFC
   proxy and the SF; however, this is an intrinsic part of the SFC
   design, which needs to define how a packet is protected in that
   environment.

   SFC components are configured and enabled through a management system
   or a control plane.  This document does not make any assumptions
   about what mechanisms are used.  Deployments should, however, be
   aware that vulnerabilities in the management plane or control plane
   of an SFC system imply vulnerabilities in the whole SFC system.
   Thus, control-plane solutions (such as [BGP-NSH-SFC]) and management-
   plane mechanisms must include security measures that can be enabled
   by operators to protect their SFC systems.

   An analysis of the security of MPLS systems is provided in [RFC5920],
   which also notes that the MPLS forwarding plane has no built-in
   security mechanisms.  Some proposals to add encryption to the MPLS
   forwarding plane have been suggested [MPLS-Opp-Sec], but no
   mechanisms have been agreed upon at the time of publication of this
   document.  Additionally, MPLS does not provide any cryptographic
   integrity protection on the MPLS headers.  That means that procedures
   described in this document rely on three basic principles:

   o  The MPLS network is often considered to be a closed network such
      that insertion, modification, or inspection of packets by an
      outside party is not possible.  MPLS networks are operated with
      closed boundaries so that MPLS-encapsulated packets are not
      admitted to the network, and MPLS headers are stripped before
      packets are forwarded from the network.  This is particularly
      pertinent in the SFC context because [RFC7665] notes that "The
      architecture described herein is assumed to be applicable to a
      single network administrative domain."  Furthermore, [RFC8300]
      states that packets originating outside the SFC-enabled domain
      MUST be dropped if they contain an NSH and packets exiting the
      SFC-enabled domain MUST be dropped if they contain an NSH.  These
      constraints apply equally to the use of MPLS to encode a logical
      representation of the NSH.




Farrel, et al.               Standards Track                   [Page 27]

RFC 8595                        MPLS SFC                       June 2019


   o  The underlying transport mechanisms (such as Ethernet) between
      adjacent MPLS nodes may offer security mechanisms that can be used
      to defend packets "on the wire".

   o  The SFC-capable devices participating in an SFC system are
      responsible for verifying and protecting payload packets and their
      contents as well as providing other security capabilities that
      might be required in the particular system.

   Additionally, where a tunnel is used to link two non-MPLS domains,
   the tunnel design needs to specify how the tunnel is secured.

   Thus, this design relies on the component underlying technologies to
   address the potential security vulnerabilities, and it documents the
   necessary protections (or risk of their absence) above.  It does not
   include any native security mechanisms in-band with the MPLS encoding
   of the NSH functionality.

   Note that configuration elements of this system (such as the
   programming of the table of metadata; see Section 12) must also be
   adequately secured, although such mechanisms are not in scope for
   this protocol specification.

   No known new security vulnerabilities over the SFC architecture
   [RFC7665] and the NSH specification [RFC8300] are introduced by this
   design, but if issues are discovered in the future, it is expected
   that they will be addressed through modifications to control/
   management components of any solution or through changes to the
   underlying technology.

16.  IANA Considerations

   IANA has made allocations from the "Extended Special-Purpose MPLS
   Label Values" subregistry of the "Special-Purpose Multiprotocol Label
   Switching (MPLS) Label Values" registry as follows:

      Value  | Description                       | Reference
      -------+-----------------------------------+--------------
       16    | Metadata Label Indicator (MLI)    | RFC 8595
       17    | Metadata Present Indicator (MPI)  | RFC 8595











Farrel, et al.               Standards Track                   [Page 28]

RFC 8595                        MPLS SFC                       June 2019


17.  References

17.1.  Normative References

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274,
              DOI 10.17487/RFC7274, June 2014,
              <https://www.rfc-editor.org/info/rfc7274>.

   [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>.

   [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", RFC 8300,
              DOI 10.17487/RFC8300, January 2018,
              <https://www.rfc-editor.org/info/rfc8300>.

   [RFC8393]  Farrel, A. and J. Drake, "Operating the Network Service
              Header (NSH) with Next Protocol "None"", RFC 8393,
              DOI 10.17487/RFC8393, May 2018,
              <https://www.rfc-editor.org/info/rfc8393>.


















Farrel, et al.               Standards Track                   [Page 29]

RFC 8595                        MPLS SFC                       June 2019


17.2.  Informative References

   [BGP-NSH-SFC]
              Farrel, A., Drake, J., Rosen, E., Uttaro, J., and L.
              Jalil, "BGP Control Plane for NSH SFC", Work in Progress,
              draft-ietf-bess-nsh-bgp-control-plane-11, May 2019.

   [MPLS-Opp-Sec]
              Farrel, A. and S. Farrell, "Opportunistic Security in
              MPLS Networks", Work in Progress,
              draft-ietf-mpls-opportunistic-encrypt-03, March 2017.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [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>.

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
              <https://www.rfc-editor.org/info/rfc5920>.

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <https://www.rfc-editor.org/info/rfc7665>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8459]  Dolson, D., Homma, S., Lopez, D., and M. Boucadair,
              "Hierarchical Service Function Chaining (hSFC)", RFC 8459,
              DOI 10.17487/RFC8459, September 2018,
              <https://www.rfc-editor.org/info/rfc8459>.

   [SR-Srv-Prog]
              Clad, F., Ed., Xu, X., Ed., Filsfils, C., Bernier, D., Li,
              C., Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W.,
              and S. Salsano, "Service Programming with Segment
              Routing", Work in Progress,
              draft-xuclad-spring-sr-service-programming-02, April 2019.





Farrel, et al.               Standards Track                   [Page 30]

RFC 8595                        MPLS SFC                       June 2019


Acknowledgements

   This document derives ideas and text from [BGP-NSH-SFC].  The authors
   are grateful to all those who contributed to the discussions that led
   to that work: Loa Andersson, Andrew G. Malis, Alexander (Sasha)
   Vainshtein, Joel Halpern, Tony Przygienda, Stuart Mackie, Keyur
   Patel, and Jim Guichard.  Loa Andersson provided helpful review
   comments.

   Thanks to Loa Andersson, Lizhong Jin, Matthew Bocci, Joel Halpern,
   and Mach Chen for reviews of this text.  Thanks to Russ Mundy for his
   Security Directorate review and to S Moonesamy for useful
   discussions.  Thanks also to Benjamin Kaduk, Alissa Cooper, Eric
   Rescorla, Mirja Kuehlewind, Alvaro Retana, and Martin Vigoureux for
   comprehensive reviews during IESG evaluation.

   The authors would like to be able to thank the authors of
   [SR-Srv-Prog] and [RFC8402] whose original work on service chaining
   and the identification of services using Segment Identifiers (SIDs),
   and conversation with whom, helped clarify the application of SR-MPLS
   to SFC.

   Particular thanks to Loa Andersson for conversations and advice about
   working group process.

Contributors

   The following individual contributed text to this document:

      Andrew G. Malis
      Email: agmalis@gmail.com




















Farrel, et al.               Standards Track                   [Page 31]

RFC 8595                        MPLS SFC                       June 2019


Authors' Addresses

   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk


   Stewart Bryant
   Futurewei

   Email: stewart.bryant@gmail.com


   John Drake
   Juniper Networks

   Email: jdrake@juniper.net

































Farrel, et al.               Standards Track                   [Page 32]

