BGP Control Plane for NSH SFC
draft-ietf-bess-nsh-bgp-control-plane-01

BESS Working Group                                             A. Farrel
Internet-Draft                                                  J. Drake
Intended status: Standards Track                                E. Rosen
Expires: March 29, 2018                                 Juniper Networks
                                                               J. Uttaro
                                                                    AT&T
                                                                L. Jalil
                                                                 Verizon
                                                      September 25, 2017


                     BGP Control Plane for NSH SFC
                draft-ietf-bess-nsh-bgp-control-plane-01

Abstract

   This document describes the use of BGP as a control plane for
   networks that support Service Function Chaining (SFC).  The document
   introduces a new BGP address family called the SFC AFI/SAFI with two
   route types.  One route type is originated by a node to advertise
   that it hosts a particular instance of a specified service function.
   This route type also provides "instructions" on how to send a packet
   to the hosting node in a way that indicates that the service function
   has to be applied to the packet.  The other route type is used by a
   Controller to advertise the paths of "chains" of service functions,
   and to give a unique designator to each such path so that they can be
   used in conjunction with the Network Service Header.

   This document adopts the SFC architecture described in RFC 7665.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 29, 2018.

Copyright Notice

   Copyright (c) 2017 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
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Functional Overview . . . . . . . . . . . . . . . . . . .   5
     2.2.  Control Plane Overview  . . . . . . . . . . . . . . . . .   7
   3.  BGP SFC Routes  . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Service Function Instance Route (SFIR)  . . . . . . . . .  10
     3.2.  Service Function Path Route (SFPR)  . . . . . . . . . . .  11
       3.2.1.  The SFP Attribute . . . . . . . . . . . . . . . . . .  12
       3.2.2.  General Rules For The SFP Attribute . . . . . . . . .  16
   4.  Mode of Operation . . . . . . . . . . . . . . . . . . . . . .  17
     4.1.  Route Targets . . . . . . . . . . . . . . . . . . . . . .  17
     4.2.  Service Function Instance Routes  . . . . . . . . . . . .  17
     4.3.  Service Function Path Routes  . . . . . . . . . . . . . .  18
     4.4.  Classifier Operation  . . . . . . . . . . . . . . . . . .  19
     4.5.  Service Function Forwarder Operation  . . . . . . . . . .  20
       4.5.1.  Processing With 'Gaps' in the SI Sequence . . . . . .  21
   5.  Selection in Service Function Paths . . . . . . . . . . . . .  22
   6.  Looping, Jumping, and Branching . . . . . . . . . . . . . . .  24
     6.1.  Protocol Control of Looping, Jumping, and Branching . . .  24
     6.2.  Implications for Forwarding State . . . . . . . . . . . .  25
   7.  Advanced Topics . . . . . . . . . . . . . . . . . . . . . . .  25
     7.1.  Preserving Entropy  . . . . . . . . . . . . . . . . . . .  25
     7.2.  Correlating Service Function Path Instances . . . . . . .  26
     7.3.  Considerations for Stateful Service Functions . . . . . .  27
     7.4.  VPN Considerations and Private Service Functions  . . . .  28



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     7.5.  Flow Spec for SFC Classifiers . . . . . . . . . . . . . .  28
     7.6.  Choice of Data Plane SPI/SI Representation  . . . . . . .  30
       7.6.1.  MPLS Representation of the SPI/SI . . . . . . . . . .  31
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     8.1.  Example Explicit SFP With No Choices  . . . . . . . . . .  34
     8.2.  Example SFP With Choice of SFIs . . . . . . . . . . . . .  34
     8.3.  Example SFP With Open Choice of SFIs  . . . . . . . . . .  35
     8.4.  Example SFP With Choice of SFTs . . . . . . . . . . . . .  36
     8.5.  Example Correlated Bidirectional SFPs . . . . . . . . . .  36
     8.6.  Example Correlated Asymmetrical Bidirectional SFPs  . . .  37
     8.7.  Example Looping in an SFP . . . . . . . . . . . . . . . .  37
     8.8.  Example Branching in an SFP . . . . . . . . . . . . . . .  38
     8.9.  Examples of SFPs with Stateful Service Functions  . . . .  39
       8.9.1.  Forward and Reverse Choice Made at the SFF  . . . . .  39
       8.9.2.  Parallel End-to-End SFPs with Shared SFF  . . . . . .  40
       8.9.3.  Parallel End-to-End SFPs with Separate SFFs . . . . .  41
       8.9.4.  Parallel SFPs Downstream of the Choice  . . . . . . .  43
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  46
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
     10.1.  New BGP AF/SAFI  . . . . . . . . . . . . . . . . . . . .  47
     10.2.  New BGP Path Attribute . . . . . . . . . . . . . . . . .  47
     10.3.  New SFP Attribute TLVs Type Registry . . . . . . . . . .  47
     10.4.  New SFP Association Type Registry  . . . . . . . . . . .  48
     10.5.  New Service Function Type Registry . . . . . . . . . . .  48
     10.6.  New Generic Transitive Experimental Use Extended
            Community Sub-Type . . . . . . . . . . . . . . . . . . .  49
     10.7.  SPI/SI Representation  . . . . . . . . . . . . . . . . .  49
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  50
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  50
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     13.2.  Informative References . . . . . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51

1.  Introduction

   As described in [RFC7498], the delivery of end-to-end services can
   require a packet to pass through a series of Service Functions (SFs)
   (e.g., classifiers, firewalls, TCP accelerators, and server load
   balancers) in a specified order: this is termed "Service Function
   Chaining" (SFC).  There are a number of issues associated with
   deploying and maintaining service function chaining in production
   networks, which are described below.

   Conventionally, if a packet needs to travel through a particular
   service chain, the nodes hosting the service functions of that chain
   are placed in the network topology in such a way that the packet
   cannot reach its ultimate destination without first passing through



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   all the service functions in the proper order.  This need to place
   the service functions at particular topological locations limits the
   ability to adapt a service function chain to changes in network
   topology (e.g., link or node failures), network utilization, or
   offered service load.  These topological restrictions on where the
   service functions can be placed raise the following issues:

   1.  The process of configuring or modifying a service function chain
       is operationally complex and may require changes to the network
       topology.

   2.  Alternate or redundant service functions may need to be co-
       located with the primary service functions.

   3.  When there is more than one path between source and destination,
       forwarding may be asymmetric and it may be difficult to support
       bidirectional service function chains using simple routing
       methodologies and protocols without adding mechanisms for traffic
       steering or traffic engineering.

   In order to address these issues, the SFC architecture describes
   Service Function Chains that are built in their own overlay network
   (the service function overlay network), coexisting with other overlay
   networks, over a common underlay network [RFC7665].  A Service
   Function Chain is a sequence of Service Functions through which
   packet flows that satisfy specified criteria will pass.

   This document describes the use of BGP as a control plane for
   networks that support Service Function Chaining (SFC).  The document
   introduces a new BGP address family called the SFC AFI/SAFI with two
   route types.  One route type is originated by a node to advertise
   that it hosts a particular instance of a specified service function.
   This route type also provides "instructions" on how to send a packet
   to the hosting node in a way that indicates that the service function
   has to be applied to the packet.  The other route type is used by a
   Controller to advertise the paths of "chains" of service functions,
   and to give a unique designator to each such path so that they can be
   used in conjunction with the Network Service Header.

   This document adopts the SFC architecture described in [RFC7665].

1.1.  Terminology

   This document uses the following terms from [RFC7665]:

   o  Bidirectional Service Function Chain

   o  Classifier



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   o  Service Function (SF)

   o  Service Function Chain (SFC)

   o  Service Function Forwarder (SFF)

   o  Service Function Instance (SFI)

   o  Service Function Path (SFP)

   o  SFC branching

   Additionally, this document uses the following terms from
   [I-D.ietf-sfc-nsh]:

   o  Network Service Header (NSH)

   o  Service Index (SI)

   o  Service Path Identifier (SPI)

   This document introduces the following terms:

   o  Service Function Instance Route (SFIR)

   o  Service Function Overlay Network

   o  Service Function Path Route (SFPR)

   o  Service Function Type (SFT)

2.  Overview

2.1.  Functional Overview

   In [I-D.ietf-sfc-nsh] a Service Function Chain (SFC) is an ordered
   list of Service Functions (SFs).  A Service Function Path (SFP) is an
   indication of which instances of SFs are acceptable to be traversed
   in an instantiation of an SFC in a service function overlay network.
   The Service Path Identifier (SPI) is a 24-bit number that identifies
   a specific SFP, and a Service Index (SI) is an 8-bit number that
   identifies a specific point in that path.  In the context of a
   particular SFP (identified by an SPI), an SI represents a particular
   Service Function, and indicates the order of that SF in the SFP.

   In fact, each SI is mapped to one or more SFs that are implemented by
   one or more Service Function Instances (SFIs) that support those
   specified SFs.  Thus an SI may represent a choice of SFIs of one or



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   more Service Function Types.  By deploying multiple SFIs for a single
   SF, one can provide load balancing and redundancy.

   A special Service Function, called a Classifier, is located at each
   ingress point to a service function overlay network.  It assigns the
   packets of a given packet flow to a specific Service Function Path.
   This may be done by comparing specific fields in a packet's header
   with local policy, which may be customer/network/service specific.
   The classifier picks an SFP and sets the SPI accordingly, it then
   sets the SI to the value of the SI for the first hop in the SFP, and
   then prepends a Network Services Header (NSH) [I-D.ietf-sfc-nsh]
   containing the assigned SPI/SI to that packet.  Note that the
   Classifier and the node that hosts the first Service Function in a
   Service Function Path need not be located at the same point in the
   service function overlay network.

   Note that the presence of the NSH can make it difficult for nodes in
   the underlay network to locate the fields in the original packet that
   would normally be used to constrain equal cost multipath (ECMP)
   forwarding.  Therefore, it is recommended, as described in
   Section 7.1, that the node prepending the NSH also provide some form
   of entropy indicator that can be used in the underlay network.

   The Service Function Forwarder (SFF) receives a packet from the
   previous node in a Service Function Path, removes the packet's link
   layer or tunnel encapsulation and hands the packet and the NSH to the
   Service Function Instance for processing.  The SFI has no knowledge
   of the SFP.

   When the SFF receives the packet and the NSH back from the SFI it
   must select the next SFI along the path using the SPI and SI in the
   NSH and potentially choosing between multiple SFIs (possibly of
   different Service Function Types) as described in Section 5.  In the
   normal case the SPI remains unchanged and the SI will have been
   decremented to indicate the next SF along the path.  But other
   possibilities exist if the SF makes other changes to the NSH through
   a process of re-classification:

   o  The SI in the NSH may indicate:

      *  A previous SF in the path: known as "looping" (see Section 6).

      *  An SF further down the path: known as "jumping" (see also
         Section 6).

   o  The SPI and the SI may point to an SF on a different SFP: known as
      "branching" (see also Section 6).




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   Such modifications are limited to within the same service function
   overlay network.  That is, an SPI is known within the scope of
   service function overlay network.  Furthermore, the new SI value is
   interpreted in the context of the SFP identified by the SPI.

   An unknown or invalid SPI SHALL be treated as an error and the SFF
   MUST drop the packet.  Such errors SHOULD be logged, and such logs
   MUST be subject to rate limits.

   An SFF receiving an SI that is unknown in the context of the SPI MAY
   reduce the value to the next meaningful SI value in the SFP indicated
   by the SPI.  If no such value exists or if the SFF does not support
   this function it MUST drop the packet and SHOULD log the event: such
   logs MUST be subject to rate limits.

   The SFF then selects an SFI that provides the SF denoted by the SPI/
   SI, and forwards the packet to the SFF that supports that SFI.

2.2.  Control Plane Overview

   To accomplish the function described in Section 2.1, this document
   introduces a new BGP AFI/SAFI [values to be assigned by IANA] for
   "SFC Routes".  Two SFC Route Types are defined by this document: the
   Service Function Instance Route (SFIR), and the Service Function Path
   Route (SFPR).  As detailed in Section 3, the route type is indicated
   by a sub-field in the NLRI.

   o  The SFIR is advertised by the node hosting the service function
      instance.  The SFIR describes a particular instance of a
      particular Service Function and the way to forward a packet to it
      through the underlay network, i.e., IP address and encapsulation
      information.

   o  The SFPRs are originated by Controllers.  One SFPR is originated
      for each Service Function Path.  The SFPR specifies:

      A.  the SPI of the path

      B.  the sequence of SFTs and/or SFIs of which the path consists

      C.  for each such SFT or SFI, the SI that represents it in the
          identified path.

   This approach assumes that there is an underlay network that provides
   connectivity between SFFs and Controllers, and that the SFFs are
   grouped to form one or more service function overlay networks through
   which SFPs are built.  We assume BGP connectivity between the




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   Controllers and all SFFs within each service function overlay
   network.

   In addition, we also introduce the Service Function Type (SFT) that
   is the category of SF that is supported by an SFF (such as
   "firewall").  An IANA registry of Service Function Types is
   introduced in Section 10.  An SFF may support SFs of multiple
   different SFTs, and may support multiple SFIs of each SF.

   When choosing the next SFI in a path, the SFF uses the SPI and SI as
   well as the SFT to choose among the SFIs, applying, for example, a
   load balancing algorithm or direct knowledge of the underlay network
   topology as described in Section 4.

   The SFF then encapsulates the packet using the encapsulation
   specified by the SFIR of the selected SFI and forwards the packet.
   See Figure 1.

   Thus the SFF can be seen as a portal in the underlay network through
   which a particular SFI is reached.































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      Packets
       | | |
       | | |
       | | |
    ------------
   |            |
   | Classifier |
   |            |
    ------------
          |
          |
       -------               -------
      |       |    Tunnel   |       |
      |  SFF  |=============|  SFF  |===========     .........
      |       |             |       |           #   : SFT     :
      |       |              -+---+-            #   :  -----  :
      |       |              /     \            #   : | SFI | :
      |       |       ....../.......\......     #   :  --+--  :
      |       |      :     /         \     :    #    ....|....
      |       |      :   -+---     ---+-   :    #        |
      |       |      :  | SFI |   | SFI |  :    #     ---+---
      |       |      :   -----     -----   :     ====|       |---
      |       |      :                     :         |  SFF  |--- Dests
      |       |      :        -----        :     ====|       |---
      |       |      :       | SFI |       :    #     -------
      |       |      :        --+--        :    #
      |       |      : SFT      |          :    #
      |       |       ..........|..........     #
      |       |                 |               #
      |       |                 |               #
      |       |              ---+---            #
      |       |             |       |           #
      |       |=============|  SFF  |===========
       -------              |       |
                             -------


              Figure 1: The SFC Architecture Reference Model

3.  BGP SFC Routes

   This document defines a new AFI/SAFI for BGP, known as "SFC", with an
   NLRI that is described in this section.

   The format of the SFC NLRI is shown in Figure 2.






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                    +---------------------------------------+
                    |  Route Type (2 octets)                |
                    +---------------------------------------+
                    |  Length (2 octets)                    |
                    +---------------------------------------+
                    |  Route Type specific (variable)       |
                    +---------------------------------------+


                   Figure 2: The Format of the SFC NLRI

   The Route Type field determines the encoding of the rest of the route
   type specific SFC NLRI.

   The Length field indicates the length in octets of the route type
   specific field of the SFC NLRI.

   This document defines the following Route Types:

   1.  Service Function Instance Route (SFIR)

   2.  Service Function Path Route (SFPR)

   A Service Function Instance Route (SFIR) is used to identify an SFI.
   A Service Function Path Route (SFPR) defines a sequence of Service
   Functions (each of which has at least one instance advertised in an
   SFIR) that form an SFP.

   The detailed encoding and procedures for these Route Types are
   described in subsequent sections.

   The SFC NLRI is carried in BGP [RFC4271] using BGP Multiprotocol
   Extensions [RFC4760] with an Address Family Identifier (AFI) of TBD1
   and a Subsequent Address Family Identifier (SAFI) of TBD2.  The NLRI
   field in the MP_REACH_NLRI/MP_UNREACH_NLRI attribute contains the SFC
   NLRI, encoded as specified above.

   In order for two BGP speakers to exchange SFC NLRIs, they must use
   BGP Capabilities Advertisements to ensure that they both are capable
   of properly processing such NLRIs.  This is done as specified in
   [RFC4760], by using capability code 1 (Multiprotocol BGP) with an AFI
   of TBD1 and a SAFI of TBD2.

3.1.  Service Function Instance Route (SFIR)

   Figure 3 shows the Route Type specific NLRI of the SFIR.





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                    +--------------------------------------------+
                    |  Route Distinguisher (RD) (8 octets)       |
                    +--------------------------------------------+
                    |  Service Function Type (2 octets)          |
                    +--------------------------------------------+


                  Figure 3: SFIR Route Type specific NLRI

   Per [RFC4364] the RD field comprises a two byte Type field and a six
   byte Value field.  Two SFIs of the same SFT must be associated with
   different RDs, where the association of an SFI with an RD is
   determined by provisioning.  If two SFIRs are originated from
   different administrative domains, they must have different RDs.  In
   particular, SFIRs from different VPNs (for different service function
   overlay networks) must have different RDs, and those RDs must be
   different from any non-VPN SFIRs.

   The Service Function Type identifies a service function, e.g.,
   classifier, firewall, load balancer, etc.  There may be several SFIs
   that can perform a given Service Function.  Each node hosting an SFI
   must originate an SFIR for each SFI that it hosts.  The SFIR
   representing a given SFI will contain an NLRI with RD field set to an
   RD as specified above, and with SFT field set to identify that SFI's
   Service Function Type.  The values for the SFT field are taken from a
   registry administered by IANA (see Section 10).  A BGP Update
   containing one or more SFIRs will also include a Tunnel Encapsulation
   attribute [I-D.ietf-idr-tunnel-encaps].  If a data packet needs to be
   sent to an SFI identified in one of the SFIRs, it will be
   encapsulated as specified by the Tunnel Encapsulation attribute, and
   then transmitted through the underlay network.

3.2.  Service Function Path Route (SFPR)

   Figure 4 shows the Route Type specific NLRI of the SFPR.


                +-----------------------------------------------+
                |  Route Distinguisher (RD) (8 octets)          |
                +-----------------------------------------------+
                |  Service Path Identifier (SPI) (3 octets)     |
                +-----------------------------------------------+


                  Figure 4: SFPR Route Type Specific NLRI

   Per [RFC4364] the RD field comprises a two byte Type field and a six
   byte Value field.  All SFPs must be associated with different RDs.



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   The association of an SFP with an RD is determined by provisioning.
   If two SFPRs are originated from different Controllers they must have
   different RDs.  Additionally, SFPRs from different VPNs (i.e., in
   different service function overlay networks) must have different RDs,
   and those RDs must be different from any non-VPN SFPRs.

   The Service Path Identifier is defined in [I-D.ietf-sfc-nsh] and is
   the value to be placed in the Service Path Identifier field of the
   NSH header of any packet sent on this Service Function Path.  It is
   expected that one or more Controllers will originate these routes in
   order to configure a service function overlay network.

   The SFP is described in a new BGP Path attribute, the SFP attribute.
   Section 3.2.1 shows the format of that attribute.

3.2.1.  The SFP Attribute

   [RFC4271] defines the BGP Path attribute.  This document introduces a
   new Path attribute called the SFP attribute with value TBD3 to be
   assigned by IANA.  The first SFP attribute MUST be processed and
   subsequent instances MUST be ignored.

   The common fields of the SFP attribute are set as follows:

   o  Optional bit is set to 1 to indicate that this is an optional
      attribute.

   o  The Transitive bit is set to 1 to indicate that this is a
      transitive attribute.

   o  The Extended Length bit is set according to the length of the SFP
      attribute as defined in [RFC4271].

   o  The Attribute Type Code is set to TBD3.

   The content of the SFP attribute is a series of Type-Length-Variable
   (TLV) constructs.  Each TLV may include sub-TLVs.  All TLVs and sub-
   TLVs have a common format that is:

   o  Type: A single octet indicating the type of the SFP attribute TLV.
      Values are taken from the registry described in Section 10.3.

   o  Length: A two octet field indicating the length of the data
      following the Length field counted in octets.

   o  Value: The contents of the TLV.





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   The formats of the TLVs defined in this document are shown in the
   following sections.  The presence rules and meanings are as follows.

   o  The SFP attribute contains a sequence of zero or more Association
      TLVs.  That is, the Association TLV is optional.  Each Association
      TLV provides an association between this SFPR and another SFPR.
      Each associated SFPR is indicated using the RD with which it is
      advertised (we say the SFPR-RD to avoid ambiguity).

   o  The SFP attribute contains a sequence of one or more Hop TLVs.
      Each Hop TLV contains all of the information about a single hop in
      the SFP.

   o  Each Hop TLV contains an SI value and a sequence of one or more
      SFT TLVs.  Each SFT TLV contains an SFI reference for each
      instance of an SF that is allowed at this hop of the SFP for the
      specific SFT.  Each SFI is indicated using the RD with which it is
      advertised (we say the SFIR-RD to avoid ambiguity).

3.2.1.1.  The Association TLV

   The Association TLV is an optional TLV in the SFP attribute.  It may
   be present multiple times.  Each occurrence provides an association
   with another SFP as advertised in another SFPR.  The format of the
   Association TLV is shown in Figure 5


                +--------------------------------------------+
                |  Type = 1 (1 octet)                        |
                +--------------------------------------------|
                |  Length (2 octets)                         |
                +--------------------------------------------|
                |  Association Type (1 octet)                |
                +--------------------------------------------|
                |  Associated SFPR-RD (8 octets)             |
                +--------------------------------------------|
                |  Associated SPI (3 octets)                 |
                +--------------------------------------------+


                Figure 5: The Format of the Association TLV

   The fields are as follows:

      Type is set to 1 to indicate an Association TLV.

      Length indicates the length in octets of the Association Type and
      Associated SFPR-RD fields.  The value of the Length field is 12.



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      The Association Type field indicate the type of association.  The
      values are tracked in an IANA registry (see Section 10.4).  Only
      one value is defined in this document: type 1 indicates
      association of two unidirectional SFPs to form a bidirectional
      SFP.  An SFP attribute SHOULD NOT contain more than one
      Association TLV with Association Type 1: if more than one is
      present, the first one MUST be processed and subsequent instances
      MUST be ignored.  Note that documents that define new Association
      Types must also define the presence rules for Association TLVs of
      the new type.

      The Associated SFPR-RD contains the RD of some other SFPR
      advertisement that contains the SFP with which this SFP is
      associated.

      The Associated SPI contains the SPI of the associated SFP as
      advertised in the SFPR indicated by the Associated SFPR-RD field.

   Association TLVs with unknown Association Type values SHOULD be
   ignored.  Association TLVs that contain an Associated SFPR-RD value
   equal to the RD of the SFPR in which they are contained SHOULD be
   ignored.  If the Associated SPI is not equal to the SPI advertised in
   the SFPR indicated by the Associated SFPR-RD then the Association TLV
   SHOULD be ignored.

   Note that when two SFPRs reference each other using the Association
   TLV, one SFPR advertisement will be received before the other.
   Therefore, processing of an association MUST NOT be rejected simply
   because the Associated SFPR-RD is unknown.

   Further discussion of correlation of SFPRs is provided in
   Section 7.2.

3.2.1.2.  The Hop TLV

   There is one Hop TLV in the SFP attribute for each hop in the SFP.
   The format of the Hop TLV is shown in Figure 6.  At least one Hop TLV
   must be present in an SFP attribute.













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                +--------------------------------------------+
                |  Type = 2 (1 octet)                        |
                +--------------------------------------------|
                |  Length (2 octets)                         |
                +--------------------------------------------|
                |  Service Index (1 octet)                   |
                +--------------------------------------------|
                |  Hop Details (variable)                    |
                +--------------------------------------------+


                    Figure 6: The Format of the Hop TLV

   The fields are as follows:

      Type is set to 2 to indicate a Hop TLV.

      Length indicates the length in octets of the Service Index and Hop
      Details fields.

      The Service Index is defined in [I-D.ietf-sfc-nsh] and is the
      value found in the Service Index field of the NSH header that an
      SFF will use to lookup to which next SFI a packet should be sent.

      The Hop Details consist of a sequence of one or more SFT TLVs.

3.2.1.3.  The SFT TLV

   There is one or more SFT TLV in each Hop TLV.  There is one SFT TLV
   for each SFT supported in the specific hop of the SFP.  The format of
   the SFT TLV is shown in Figure 7.


                +--------------------------------------------+
                |  Type = 3 (1 octet)                        |
                +--------------------------------------------|
                |  Length (2 octets)                         |
                +--------------------------------------------|
                |  Service Function Type (2 octets)          |
                +--------------------------------------------|
                |  SFIR-RD List (variable)                   |
                +--------------------------------------------+


                    Figure 7: The Format of the SFT TLV

   The fields are as follows:




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      Type is set to 3 to indicate an SFT TLV.

      Length indicates the length in octets of the Service Function Type
      and SFIR-RD List fields.

      The Service Function Type is used to identify a Service Function
      Instance Route in the service function overlay network which, in
      turn, will allow lookup of routes to SFIs implementing the SF.
      SFT values in the range 1-31 are Special Purpose SFT values and
      have meanings defined by the documents that describe them - the
      value 'Change Sequence' is defined in Section 6.1 of this
      document.

      The SFIR-RD List is made up of one or more SFIR-RD values from the
      advertisements of SFIs in SFIRs.  An SFIR-RD of value zero has
      special meaning as described in Section 5.  Each entry in the list
      is 8 octets long, and the number of entries in the list can be
      deduced from the value of the Length field.

3.2.2.  General Rules For The SFP Attribute

   It is possible for the same SFI, as described by an SFIR, to be used
   in multiple SFPRs.

   When two SFPRs have the same SPI but different SFPR-RDs there can be
   three cases:

   o  Two or more Controllers are originating SFPRs for the same SFP.
      In this case the content of the SFPRs is identical and the
      duplication is to ensure receipt and to provide Controller
      redundancy.

   o  There is a transition in content of the advertised SFP and the
      advertisements may originate from one or more Controllers.  In
      this case the content of the SFPRs will be different.

   o  The reuse of an SPI may result from a configuration error.

   In all cases, there is no way for the receiving SFF to know which
   SFPR to process, and the SFPRs could be received in any order.  At
   any point in time, when multiple SFPRs have the same SPI but
   different SFPR-RDs, the SFF MUST use the SFPR with the numerically
   lowest SFPR-RD.  The SFF SHOULD log this occurrence to assist with
   debugging.

   Furthermore, a Controller that wants to change the content of an SFP
   is RECOMMENDED to use a new SPI and so create a new SFP onto which
   the Classifiers can transition packet flows before the SFPR for the



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   old SFP is withdrawn.  This avoids any race conditions with SFPR
   advertisements.

   Additionally, a Controller SHOULD NOT re-use an SPI after it has
   withdrawn the SFPR that used it until at least a configurable amount
   of time has passed.  This timer SHOULD have a default of one hour.

4.  Mode of Operation

   This document describes the use of BGP as a control plane to create
   and manage a service function overlay network.

4.1.  Route Targets

   The main feature introduced by this document is the ability to create
   multiple service function overlay networks through the use of Route
   Targets (RTs) [RFC4364].

   Every BGP UPDATE containing an SFIR or SFPR carries one or more RTs.
   The RT carried by a particular SFIR or SFPR is determined by the
   provisioning of the route's originator.

   Every node in a service function overlay network is configured with
   one or more import RTs.  Thus, each SFF will import only the SFPRs
   with matching RTs allowing the construction of multiple service
   function overlay networks or the instantiation of Service Function
   Chains within an L3VPN or EVPN instance (see Section 7.4).  An SFF
   that has a presence in multiple service function overlay networks
   (i.e., imports more than one RT) may find it helpful to maintain
   separate forwarding state for each overlay network.

4.2.  Service Function Instance Routes

   The SFIR (see Section 3.1) is used to advertise the existence and
   location of a specific Service Function Instance and consists of:

   o  The RT as just described.

   o  A Service Function Type (SFT) that is the category of Service
      Function that is provided (such as "firewall").

   o  A Route Distinguisher (RD) that is unique to a specific instance
      of a service function.








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4.3.  Service Function Path Routes

   The SFPR (see Section 3.2) describes a specific path of a Service
   Function Chain.  The SFPR contains the Service Path Identifier (SPI)
   used to identify the SFP in the NSH in the data plane.  It also
   contains a sequence of Service Indexes (SIs).  Each SI identifies a
   hop in the SFP, and each hop is a choice between one of more SFIs.

   As described in this document, each Service Function Path Route is
   identified in the service function overlay network by an RD and an
   SPI.  The SPI is unique within a single VPN instance supported by the
   underlay network.

   The SFPR advertisement comprises:

   o  An RT as described in Section 4.1.

   o  A tuple that identifies the SFPR

      *  An RD that identifies an advertisement of an SFPR.

      *  The SPI that uniquely identifies this path within the VPN
         instance distinguished by the RD.  This SPI also appears in the
         NSH.

   o  A series of Service Indexes.  Each SI is used in the context of a
      particular SPI and identifies one or more SFs (distinguished by
      their SFTs) and for each SF a set of SFIs that instantiate the SF.
      The values of the SI indicate the order in which the SFs are to be
      executed in the SFP that is represented by the SPI.

   o  The SI is used in the NSH to identify the entries in the SFP.
      Note that the SI values have meaning only relative to a specific
      path.  They have no semantic other than to indicate the order of
      Service Functions within the path and are assumed to be
      monotonically decreasing from the start to the end of the path
      [I-D.ietf-sfc-nsh].

   o  Each Service Index is associated with a set of one or more Service
      Function Instances that can be used to provide the indexed Service
      Function within the path.  Each member of the set comprises:

      *  The RD used in an SFIR advertisement of the SFI.

      *  The SFT that indicates the type of function as used in the same
         SFIR advertisement of the SFI.





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   This may be summarized as follows where the notations "SFPR-RD" and
   "SFIR-RD" are used to distinguish the two different RDs:

      RT, {SFPR-RD, SPI}, m * {SI, {n * {SFT, p * SFIR-RD} } }

   Where:

      RT: Route Target

      SFPR-RD: The Route Descriptor of the Service Function Path Route
      advertisement

      SPI: Service Path Identifier used in the NSH

      m: The number of hops in the Service Function Path

      n: The number of choices of Service Function Type for a specific
      hop

      p: The number of choices of Service Function Instance for given
      Service Function Type in a specific hop

      SI: Service Index used in the NSH to indicate a specific hop

      SFT: The Service Function Type used in the same advertisement of
      the Service Function Instance Route

      SFIR-RD: The Route Descriptor used in an advertisement of the
      Service Function Instance Route

   Note that the values of SI are from the set {255, ..., 1} and are
   monotonically decreasing within the SFP.  SIs MUST appear in order
   within the SFPR (i.e., monotonically decreasing) and MUST NOT appear
   more than once.  Gaps MAY appear in the sequence as described in
   Section 4.5.1.  Malformed SFPRs MUST be discarded and MUST cause any
   previous instance of the SFPR (same SFPR-RD and SPI) to be discarded.

   The choice of SFI is explained further in Section 5.  Note that an
   SFIR-RD value of zero has special meaning as described in that
   Section.

4.4.  Classifier Operation

   As shown in Figure 1, the Classifier is a special Service Function
   that is used to assign packets to an SFP.

   The Classifier is responsible for determining to which packet flow a
   packet belongs (usually by inspecting the packet header), imposing an



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   NSH, and initializing the NSH to include the SPI of the selected SFPR
   and to include the SI from first hop of the selected SFP.

   The Classifier may also provide an entropy indicator as described in
   Section 7.1.

4.5.  Service Function Forwarder Operation

   Each packet sent to an SFF is transmitted encapsulated in an NSH.
   The NSH includes an SPI and SI: the SPI indicates the SFPR
   advertisement that announced the Service Function Path; the tuple
   SPI/SI indicates a specific hop in a specific path and maps to the
   RD/SFT of a particular SFIR advertisement.

   When an SFF gets an SFPR advertisement it will first determine
   whether to import the route by examining the RT.  If the SFPR is
   imported the SFF then determines whether it is on the SFP by looking
   for its own SFIR-RDs in the SFPR.  For each occurrence in the SFP,
   the SFF creates forwarding state for incoming packets and forwarding
   state for outgoing packets that have been processed by the specified
   SFI.

   The SFF creates local forwarding state for packets that it receives
   from other SFFs.  This state makes the association between the SPI/SI
   in the NSH of the received packet and one or more specific local SFIs
   as identified by the SFIR-RD/SFT.  If there are multiple local SFIs
   that match this is because a single advertisement was made for a set
   of equivalent SFIs and the SFF may use local policy (such as load
   balancing) to determine to which SFI to forward a received packet.

   The SFF also creates next hop forwarding state for packets received
   back from the local SFI that need to be forwarded to the next hop in
   the SFP.  There may be a choice of next hops as described in
   Section 4.3.  The SFF could install forwarding state for all
   potential next hops, or it could choose to only install forwarding
   state to a subset of the potential next hops.  If a choice is made
   then it will be as described in Section 5.

   The installed forwarding state may change over time reacting to
   changes in the underlay network and the availability of particular
   SFIs.

   Note that SFFs only create and store forwarding state for the SFPs on
   which they are included.  They do not retain state for all SFPs
   advertised.

   An SFF may also install forwarding state to support looping, jumping,
   and branching.  The protocol mechanism for explicit control of



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   looping, jumping, and branching is described in Section 6.1 using a
   special value of the SFT within an entry in an SFPR.

4.5.1.  Processing With 'Gaps' in the SI Sequence

   The behavior of an SF as described in [I-D.ietf-sfc-nsh] is to
   decrement the value of the SI field in the NSH by one before
   returning a packet to the local SFF for further processing.  This
   means that there is a good reason to assume that the SFP is composed
   of a series of SFs each indicated by an SI value one less than the
   previous.

   However, there is an advantage to having non-successive SIs in an
   SPI.  Consider the case where an SPI needs to be modified by the
   insertion or removal of an SF.  In the latter case this would lead to
   a "gap" in the sequence of SIs, and in the former case, this could
   only be achieved if a gap already existed into which the new SF with
   its new SI value could be inserted.  Otherwise, all "downstream" SFs
   would need to be renumbered.

   Now, of course, such renumbering could be performed, but would lead
   to a significant disruption to the SFC as all the SFFs along the SFP
   were "reprogrammed".  Thus, to achieve dynamic modification of an SFP
   (and even, in-service modification) it is desirable to be able to
   make these modifications without changing the SIs of the elements
   that were present before the modification.  This will produce much
   more consistent/predictable behavior during the convergence period
   where otherwise the change would need to be fully propagated.

   Another approach says that any change to an SFP simply creates a new
   SFP that can be assigned a new SPI.  All that would be needed would
   be to give a new instruction to the Classifier and traffic would be
   switched to the new SFP that contains the new set of SFs.  This
   approach is practical, but neglects to consider that the SFP may be
   referenced by other SFPs (through "branch" instructions) and used by
   many Classifiers.  In those cases the corresponding configuration
   resulting from a change in SPI may have wide ripples and give scope
   for errors that are hard to trace.

   Therefore, while this document requires that the SI values in an SFP
   are monotonic decreasing, it makes no assumption that the SI values
   are sequential.  Configuration tools may apply that rule, but they
   are not required to.  To support this, an SFF SHOULD process as
   follows when it receives a packet:

   o  If the SI indicates a known entry in the SFP, the SFF MUST process
      the packet as normal, looking up the SI and determining whether to
      deliver the packet to a local SFI or to forward it to another SFF.



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   o  If the SI does not match an entry in the SFP, the SFF MUST reduce
      the SI value to the next (smaller) value present in the SFP and
      process the packet using that SI.

   o  If there is no smaller SI (i.e., if the end of the SFP has been
      reached) the SFF MUST treat the SI value as invalid as described
      in [I-D.ietf-sfc-nsh].

   SFF implementations MAY choose to only support contiguous SI values
   in an SFP.  Such an implementation will not support receiving an SI
   value that is not present in the SFP and will discard the packets as
   described in [I-D.ietf-sfc-nsh].

5.  Selection in Service Function Paths

   As described in Section 2 the SPI/SI in the NSH passed back from an
   SFI to the SFF may leave the SFF with a choice of next hop SFTs, and
   a choice of SFIs for each SFT.  That is, the SPI indicates an SFPR,
   and the SI indicates an entry in that SFPR.  Each entry in an SFPR is
   a set of one or more SFT/SFIR-RD pairs.  The SFF must choose one of
   these, identify the SFF that supports the chosen SFI, and send the
   packet to that next hop SFF.

   The choice may offered for load balancing across multiple SFIs, or
   for discrimination between different actions necessary at a specific
   hop in the SFP.  Different SFT values may exist at a given hop in an
   SFP to support several cases:

   o  There may be multiple instances of similar service functions that
      are distinguished by different SFT values.  For example, firewalls
      made by vendor A and vendor B may need to be identified by
      different SFT values because, while they have similar
      functionality, their behavior is not identical.  Then, some SFPs
      may limit the choice of SF at a given hop by specifying the SFT
      for vendor A, but other SFPs might not need to control which
      vendor's SF is used and so can indicate that either SFT can be
      used.

   o  There may be an obvious branch needed in an SFP such as the
      processing after a firewall where admitted packets continue along
      the SFP, but suspect packets are diverted to a "penalty box".  In
      this case, the next hop in the SFP will be indicated with two
      different SFT values.

   In the typical case, the SFF chooses a next hop SFF by looking at the
   set of all SFFs that support the SFs identified by the SI (that set
   having been advertised in individual SFIR advertisements), finding
   the one or more that are "nearest" in the underlay network, and



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   choosing between next hop SFFs using its own load-balancing
   algorithm.

   An SFI may influence this choice process by passing additional
   information back along with the packet and NSH.  This information may
   influence local policy at the SFF to cause it to favor a next hop SFF
   (perhaps selecting one that is not nearest in the underlay), or to
   influence the load-balancing algorithm.

   This selection applies to the normal case, but also applies in the
   case of looping, jumping, and branching (see Section 6).

   Suppose an SFF in a particular service overlay network (identified by
   a particular import RT, RT-z) needs to forward an NSH-encapsulated
   packet whose SPI is SPI-x and whose SI is SI-y.  It does the
   following:

   1.  It looks for an installed SFPR that carries RT-z and that has
       SPI-x in its NLRI.  If there is none, then such packets cannot be
       forwarded.

   2.  From the SFP attribute of that SFPR, it finds the Hop TLV with SI
       value set to SI-y.  If there is no such Hop TLV, then such
       packets cannot be forwarded.

   3.  It then finds the "relevant" set of SFIRs by going through the
       list of SFT TLVs contained in the Hop TLV as follows:

       A.  An SFIR is relevant if it carries RT-z, the SFT in its NLRI
           matches the SFT value in one of the SFT TLVs, and the RD
           value in its NLRI matches an entry in the list of SFIR-RDs in
           that SFT TLV.

       B.  If an entry in the SFIR-RD list of an SFT TLV contains the
           value zero, then an SFIR is relevant if it carries RT-z and
           the SFT in its NLRI matches the SFT value in that SFT TLV.
           I.e., any SFIR in the service function overlay network
           defined by RT-z and with the correct SFT is relevant.

   Each of the relevant SFIRs identifies a single SFI, and contains a
   Tunnel Encapsulation attribute that specifies how to send a packet to
   that SFI.  For a particular packet, the SFF chooses a particular SFI
   from the set of relevant SFIRs.  This choice is made according to
   local policy.

   A typical policy might be to figure out the set of SFIs that are
   closest, and to load balance among them.  But this is not the only
   possible policy.



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6.  Looping, Jumping, and Branching

   As described in Section 2 an SFI or an SFF may cause a packets to
   "loop back" to a previous SF on a path in order that a sequence of
   functions may be re-executed.  This is simply achieved by replacing
   the SI in the NSH with a higher value instead of decreasing it as
   would normally be the case to determine the next hop in the path.

   Section 2 also describes how an SFI or an SFF may cause a packets to
   "jump forward" to an SF on a path that is not the immediate next SF
   in the SFP.  This is simply achieved by replacing the SI in the NSH
   with a lower value than would be achieved by decreasing it by the
   normal amount.

   A more complex option to move packets from one SFP to another is
   described in [I-D.ietf-sfc-nsh] and Section 2 where it is termed
   "branching".  This mechanism allows an SFI or SFF to make a choice of
   downstream treatments for packets based on local policy and output of
   the local SF.  Branching is achieved by changing the SPI in the NSH
   to indicate the new path and setting the SI to indicate the point in
   the path at which the packets should enter.

   Note that the NSH does not include a marker to indicate whether a
   specific packet has been around a loop before.  Therefore, the use of
   NSH metadata may be required in order to prevent infinite loops.

6.1.  Protocol Control of Looping, Jumping, and Branching

   If the SFT value in an SFT TLV in an SFPR has the Special Purpose SFT
   value "Change Sequence" (see Section 10) then this is an indication
   that the SFF may make a loop, jump, or branch according to local
   policy and information returned by the local SFI.

   In this case, the SPI and SI of the next hop is encoded in the eight
   bytes of an entry in the SFIR-RD list as follows:

      3 bytes SPI

      2 bytes SI

      3 bytes Reserved (SHOULD be set to zero and ignored)

   If the SI in this encoding is not part of the SFPR indicated by the
   SPI in this encoding, then this is an explicit error that SHOULD be
   detected by the SFF when it parses the SFPR.  The SFPR SHOULD NOT
   cause any forwarding state to be installed in the SFF and packets
   received with the SPI that indicates this SFPR SHOULD be silently
   discarded.



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   If the SPI in this encoding is unknown, the SFF SHOULD NOT install
   any forwarding state for this SFPR, but MAY hold the SFPR pending
   receipt of another SFPR that does use the encoded SPI.

   If the SPI matches the current SPI for the path, this is a loop or
   jump.  In this case, if the SI is greater than to the current SI it
   is a loop.  If the SPI matches and the SI is less than the next SI,
   it is a jump.

   If the SPI indicates anther path, this is a branch and the SI
   indicates the point at which to enter that path.

   The Change Sequence SFT is just another SFT that may appear in a set
   of SFI/SFT tuples within an SI and is selected as described in
   Section 5.

   Note that Special Purpose SFTs MUST NOT be advertised in SFIRs.

6.2.  Implications for Forwarding State

   Support for looping and jumping requires that the SFF has forwarding
   state established to an SFF that provides access to an instance of
   the appropriate SF.  This means that the SFF must have seen the
   relevant SFIR advertisements and known that it needed to create the
   forwarding state.  This is a matter of local configuration and
   implementation: for example, an implementation could be configured to
   install forwarding state for specific looping/jumping.

   Support for branching requires that the SFF has forwarding state
   established to an SFF that provides access to an instance of the
   appropriate entry SF on the other SFP.  This means that the SFF must
   have seen the relevant SFIR and SFPR advertisements and known that it
   needed to create the forwarding state.  This is a matter of local
   configuration and implementation: for example, an implementation
   could be configured to install forwarding state for specific
   branching (identified by SPI and SI).

7.  Advanced Topics

   This section highlights several advanced topics introduced elsewhere
   in this document.

7.1.  Preserving Entropy

   Forwarding decisions in the underlay network in the presence of equal
   cost multipath (ECMP) are usually made by inspecting key invariant
   fields in a packet header so that all packets from the same packet
   flow receive the same forwarding treatment.  However, when an NSH is



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   included in a packet, those key fields may be inaccessible.  For
   example, the fields may be too far inside the packet for a forwarding
   engine to quickly find them and extract their values, or the node
   performing the examination may be unaware of the format and meaning
   of the NSH and so unable to parse far enough into the packet.

   Various mechanisms exist within forwarding technologies to include an
   "entropy indicator" within a forwarded packet.  For example, in MPLS
   there is the entropy label [RFC6790], while for encapsulations in UDP
   the source port field is often used to carry an entropy indicator
   (such as for MPLS in UDP [RFC7510]).

   Implementations of this specification are RECOMMENDED to include an
   entropy indicator within the packet's underlay network header, and
   SHOULD preserve any entropy indicator from a received packet for use
   on the same packet when it is forwarded along the path but MAY choose
   to generate a new entropy indicator so long as the method used is
   constant for all packets.  Note that preserving per packet entropy
   may require that the entropy indicator is passed to and returned by
   the SFI to prevent the SFF from having to maintain per-packet state.

7.2.  Correlating Service Function Path Instances

   It is often useful to create bidirectional SFPs to enable packet
   flows to traverse the same set of SFs, but in the reverse order.
   However, packets on SFPs in the data plane (per [I-D.ietf-sfc-nsh])
   do not contain a direction indicator, so each direction must use a
   different SPI.

   As described in Section 3.2.1.1 an SFPR can contain one or more
   correlators encoded in Association TLVs.  If the Association Type
   indicates "Bidirectional SFP" then the SFP advertised in the SFPR is
   one direction of a bidirectional pair of SFPs where the other in the
   pair is advertised in the SFPR with RD as carried in the Associated
   SFPR-RD field of the Association TLV.  The SPI carried in the
   Associated SPI field of the Association TLV provides a cross-check
   and should match the SPI advertised in the SFPR with RD as carried in
   the Associated SFPR-RD field of the Association TLV.

   As noted in Section 3.2.1.1 SFPRs reference each other one SFPR
   advertisement will be received before the other.  Therefore
   processing of an association will require that the first SFPR is not
   rejected simply because the Associated SFPR-RD it carries is unknown.
   However, the SFP defined by the first SFPR is valid and SHOULD be
   available for use as a unidirectional SFP even in the absence of an
   advertisement of its partner.





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   Furthermore, in error cases where SFPR-a associates with SFPR-b, but
   SFPR-b associates with SFPR-c such that a bidirectional pair of SFPs
   cannot be formed, the individual SFPs are still valid and SHOULD be
   available for use as unidirectional SFPs.  An implementation SHOULD
   log this situation because it represents a Controller error.

   Usage of a bidirectional SFP may be programmed into the Classifiers
   by the Controller.  Alternatively, a Classifier may look at incoming
   packets on a bidirectional packet flow, extract the SPI from the
   received NSH, and look up the SFPR to find the reverse direction SFP
   to use when it sends packets.

   See Section 8 for an example of how this works.

7.3.  Considerations for Stateful Service Functions

   Some service functions are stateful.  That means that they build and
   maintain state derived from configuration or from the packet flows
   that they handle.  In such cases it can be important or necessary
   that all packets from a flow continue to traverse the same instance
   of a service function so that the state can be leveraged and does not
   need to be regenerated.

   In the case of bidirectional SFPs, it may be necessary to traverse
   the same instances of a stateful service function in both directions.
   A firewall is a good example of such a service function.

   This issue becomes a concern where there are multiple parallel
   instances of a service function and a determination of which one to
   use could normally be left to the SFF as a load-balancing or local
   policy choice.

   For the forward direction SFP, the concern is that the same choice of
   service function is made for all packets of a flow under normal
   network conditions.  It may be possible to guarantee that the load
   balancing functions applied in the SFFs are stable and repeatable,
   but a controller that constructs SFPs might not want to trust to
   this.  The controller can, in these cases, build a number of more
   specific SFPs each traversing a specific instance of the stateful
   SFs.  In this case, the load balancing choice can be left up to the
   Classifier.  Thus the Classifier selects which instance of a stateful
   SF is used by a particular flow by selecting the SFP that the flow
   uses.

   For bidirectional SFPs where the same instance of a stateful SF must
   be traversed in both directions, it is not enough to leave the choice
   of service function instance as a local choice even if the load
   balancing is stable because coordination would be required between



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   the decision points in the forward and reverse directions and this
   may be hard to achieve in all cases except where it is the same SFF
   that makes the choice in both directions.

   Note that this approach necessarily increases the amount of SFP state
   in the network (i.e., there are more SFPs).  It is possible to
   mitigate this effect by careful construction of SFPs built from a
   concatenation of other SFPs.

   Section 8.9 includes some simple examples of SFPs for stateful
   service functions.

7.4.  VPN Considerations and Private Service Functions

   Likely deployments include reserving specific instances of Service
   Functions for specific customers or allowing customers to deploy
   their own Service Functions within the network.  Building Service
   Functions in such environments requires that suitable identifiers are
   used to ensure that SFFs distinguish which SFIs can be used and which
   cannot.

   This problem is similar to how VPNs are supported and is solved in a
   similar way.  The RT field is used to indicate a set of Service
   Functions from which all choices must be made.

7.5.  Flow Spec for SFC Classifiers

   [RFC5575] defines a set of BGP routes that can be used to identify
   the packets in a given flow using fields in the header of each
   packet, and a set of actions, encoded as extended communities, that
   can be used to disposition those packets.  This document enables the
   use of RFC 5575 mechanisms by SFC Classifiers by defining a new
   action extended community called "Flow spec for SFC classifiers"
   identified by the value TBD4.  Note that other action extended
   communities may also be present.

   This extended community is encoded as an 8-octet value, as shown in
   Figure 8:













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     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Type=0x80     | Sub-Type=TBD4 |  SPI                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  SPI  (cont.) |   SI          |  SFT                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Figure 8: The Format of the Flow Spec for SFC Classifiers Extended
                                 Community

   The extended community contains the Service Path Identifier (SPI),
   Service Index (SI), and Service Function Type (SFT) as defined
   elsewhere in this document.  Thus, each action extended community
   defines the entry point (not necessarily the first hop) into a
   specific service function path.  This allows, for example, different
   flows to enter the same service function path at different points.

   Note that a given Flow Spec update according to [RFC5575] may include
   multiple of these action extended communities, and that if a given
   action extended community does not contain an installed SFPR with the
   specified [SPI, SI, SFT] it MUST NOT be used for dispositioning the
   packets of the specified flow.

   The normal case of packet classification for SFC will see a packet
   enter the SFP at its first hop.  In this case the SI in the extended
   community is superfluous and the SFT may also be unnecessary.  To
   allow these cases to be handled, a special meaning is assigned to a
   Service Index of zero (not a valid value) and an SFT of zero (a
   reserved value in the registry - see Section 10.5).

   o  If an SFC Classifiers Extended Community is received with SI = 0
      then it means that the first hop of the SFP indicated by the SPI
      MUST be used.

   o  If an SFC Classifiers Extended Community is received with SFT = 0
      then there are two sub-cases:

      *  If there is a choice of SFT in the hop indicated by the value
         of the SI (including SI = 0) then SFT = 0 means there is a free
         choice according to local policy of which SFT to use).

      *  If there is no choice of SFT in the hop indicated by the value
         of SI, then SFT = 0 means that the value of the SFT at that hop
         as indicated in the SPFR for the indicated SPI MUST be used.






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7.6.  Choice of Data Plane SPI/SI Representation

   This document ties together the control and data planes of an SFC
   overlay network through the use of the SPI/SI which is nominally
   carried in the NSH of a given packet.  However, in order to handle
   situations in which the NSH is not ubiquitously deployed, it is also
   possible to use alternative data plane representations of the SPI/SI
   by carrying the identical semantics in other protocol fields such as
   MPLS labels.

   This document defines a new sub-TLV for the Tunnel Encapsulation
   attribute, the SPI/SI Representation sub-TLV of type TBD5.  This sub-
   TLV MAY be present in each Tunnel TLV contained in a Tunnel
   Encapsulation attribute when the attribute is carried by an SFIR.
   The value field of this sub-TLV is a two octet field of flags, each
   of which describes how the originating SFF expects to see the SPI/SI
   represented in the data plane for packets carried in the tunnels
   described by the Tunnel TLV.

   The following bits are defined by this document:

   Bit 0:  If this bit is set the NSH is to be used to carry the SPI/SI
      in the data plane.

   Bit 1:  If this bit is set two labels in an MPLS label stack are to
      be used as described in Section 7.6.1.

   If a given Tunnel TLV does not contain an SPI/SI Representation sub-
   TLV then it MUST be processed as if such a sub-TLV is present with
   Bit 0 set and no other bits set.  That is, the absence of the sub-TLV
   SHALL be interpreted to mean that the NSH is to be used.

   If a given Tunnel TLV contains an SPI/SI Representation sub-TLV with
   value field that has no flag set then the tunnel indicated by the
   Tunnel TLV MUST NOT be used for forwarding SFC packets.  If a given
   Tunnel TLV contains an SPI/SI Representation sub-TLV with both bit 0
   and bit 1 set then the tunnel indicated by the Tunnel TLV MUST NOT be
   used for forwarding SFC packets.  The meaning and rules for presence
   of other bits is to be defined in future documents, but
   implementations of this specification MUST set other bits to zero and
   ignore them on receipt.

   If a given Tunnel TLV contains more than one SPI/SI Representation
   sub-TLV then the first one MUST be considered and subsequent
   instances MUST be ignored.

   Note that the MPLS representation of the logical NSH may be used even
   if the tunnel is not an MPLS tunnel.  Conversely, MPLS tunnels may be



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   used to carry other encodings of the logical NSH (specifically, the
   NSH itself).  It is a requirement that both ends of a tunnel over the
   underlay network know that the tunnel is used for SFC and know what
   form of NSH representation is used.  The signaling mechanism
   described here allows coordination of this information.

7.6.1.  MPLS Representation of the SPI/SI

   If bit 1 is set in the in the SPI/SI Representation sub-TLV then two
   labels in the label stack are used to indicate the SPI and SI to
   achieve the semantics of a logical NSH.  The label stack appears as
   shown in Figure 9.


    ---------------
   | Tunnel Labels |
   +---------------+
   ~   Optional    ~
   ~ Entropy Label ~
   +---------------+
   |   SPI Label   |
   +---------------+
   |   SI Label    |
   +---------------+
   |               |
   ~    Payload    ~
   |               |
    ---------------


              Figure 9: The MPLS Label Stack with Logical NSH

   As can be seen from the figure, the top of the label stack comprises
   the labels necessary to deliver the packet over the MPLS tunnel
   between SFFs.  Any of the MPLS encapsulations defined for the Tunnel
   Encapsulation may be used (i.e., MPLS, MPLS in GRE, and MPLS in VXLAN
   or GPE) and an entropy label ([RFC6790]) may also be present as
   described in Section 7.1.

   Under these labels comes the SPI label stack entry and the SI label
   stack entry.  Each of these label stack entries is formatted as
   normal (see [RFC3032] and the fields are encoded as follows:

   Label:  The Label field contains the value of the SPI or SI encoded
      as a 20 bit integer.  Since the SI only requires 8 bits, it is
      right-justified with the high order 12 bits set to zero.  Note
      that an SPI as defined by [I-D.ietf-sfc-nsh] can be encoded in 3
      octets (i.e., 24 bits) but that the Label field allows for only 20



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      bits.  Thus, a system using MPLS representation of the logical NSH
      MUST NOT assign SPI values greater than 2^20 - 1.

   TC:  The TC bits have no meaning.  They SHOULD be set to zero and
      MUTS be ignored.

   S: The bottom of stack flag has its usual meaning in MPLS.  It MUST
      be clear in the SPI label stack entry and MAY be set in the SI
      label stack entry depending on whether the payload is MPLS (not
      set) or not MPLS (set).

   TTL:  The TTL field in the SPI label stack entry SHOULD be set to 1.
      The TTL in SI label stack entry (called the SI TTL) is used to
      count progress along the SFP and to prevent looping.  It is
      decremented once for each hop in the SFP, i.e., for each SFI
      executed.

      *  When a Classifier places a packet onto an SFP it MUST set the
         SI TTL to a value between 1 and 255.  It should set this
         according to the expected length of the SFP, but it MAY set it
         to a larger value according to local configuration.

      *  When an SFF receives a packet from the Classifier or another
         SFF it MUST discard any packets with SI TTL set to zero.  It
         SHOULD log such occurrences, but MUST apply rate limiting to
         any such logs.

      *  An SFF MUST decrement the SI TTL by one each time it sends the
         packet to an SFI (local or remote) or to another SFF

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

      *  SFIs MUST ignore the SI TTL, but MUST mirror it back to the SFF
         unmodified along with the SI (which may have been changed).

      *  If a Classifier along the SFP makes any change to the intended
         path of the packet including for looping, jumping, or branching
         (see Section 6 and Section 6.1) it MUST NOT change the SI TTL
         of the packet.  In particular, every component of the SFC
         system MUST NOT increase the SI TTL value.

8.  Examples

   Assume we have a service function overlay network with four SFFs
   (SFF1, SFF3, SFF3, and SFF4).  The SFFs have addresses in the
   underlay network as follows:



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      SFF1 192.0.2.1
      SFF2 192.0.2.2
      SFF3 192.0.2.3
      SFF4 192.0.2.4


   Each SFF provides access to some SFIs from the four Service Function
   Types SFT=41, SFT=42, SFT=43, and SFT=44 as follows:


      SFF1 SFT=41 and SFT=42
      SFF2 SFT=41 and SFT=43
      SFF3 SFT=42 and SFT=44
      SFF4 SFT=43 and SFT=44


   The service function network also contains a Controller with address
   198.51.100.1.

   This example service function overlay network is shown in Figure 10.


          --------------
         |  Controller  |
         | 198.51.100.1 |   ------     ------    ------     ------
          --------------   | SFI  |   | SFI  |  | SFI  |   | SFI  |
                           |SFT=41|   |SFT=42|  |SFT=41|   |SFT=43|
                            ------     ------    ------     ------
                                 \     /              \     /
                                ---------            ---------
                  ----------   |   SFF1  |          |   SFF2  |
      Packet --> |          |  |192.0.2.1|          |192.0.2.2|
      Flows  --> |Classifier|   ---------            ---------  -->Dest
                 |          |                                   -->
                  ----------    ---------            ---------
                               |   SFF3  |          |   SFF4  |
                               |192.0.2.3|          |192.0.2.4|
                                ---------            ---------
                                 /     \              /     \
                            ------     ------    ------     ------
                           | SFI  |   | SFI  |  | SFI  |   | SFI  |
                           |SFT=42|   |SFT=44|  |SFT=43|   |SFT=44|
                            ------     ------    ------     ------


            Figure 10: Example Service Function Overlay Network





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   The SFFs advertise routes to the SFIs they support.  So we see the
   following SFIRs:


      RD = 192.0.2.1,1, SFT = 41
      RD = 192.0.2.1,2, SFT = 42
      RD = 192.0.2.2,1, SFT = 41
      RD = 192.0.2.2,2, SFT = 43
      RD = 192.0.2.3,7, SFT = 42
      RD = 192.0.2.3,8, SFT = 44
      RD = 192.0.2.4,5, SFT = 43
      RD = 192.0.2.4,6, SFT = 44


   Note that the addressing used for communicating between SFFs is taken
   from the Tunnel Encapsulation attribute of the SFIR and not from the
   SFIR-RD.

8.1.  Example Explicit SFP With No Choices

   Consider the following SFPR.


      SFP1:  RD = 198.51.100.1,101, SPI = 15,
             [SI = 255, SFT = 41, RD = 192.0.2.1,1],
             [SI = 250, SFT = 43, RD = 192.0.2.2,2]


   The Service Function Path consists of an SF of type 41 located at
   SFF1 followed by an SF of type 43 located at SFF2.  This path is
   fully explicit and each SFF is offered no choice in forwarding packet
   along the path.

   SFF1 will receive packets on the path from the Classifier and will
   identify the path from the SPI (15).  The initial SI will be 255 and
   so SFF1 will deliver the packets to the SFI for SFT 41.

   When the packets are returned to SFF1 by the SFI the SI will be
   decreased to 250 for the next hop.  SFF1 has no flexibility in the
   choice of SFF to support the next hop SFI and will forward the packet
   to SFF2 which will send the packets to the SFI that supports SFT 43
   before forwarding the packets to their destinations.

8.2.  Example SFP With Choice of SFIs







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      SFP2:  RD = 198.51.100.1,102, SPI = 16,
             [SI = 255, SFT = 41, RD = 192.0.2.1,],
             [SI = 250, SFT = 43, {RD = 192.0.2.2,2,
                                   RD = 192.0.2.4,5 } ]


   In this example the path also consists of an SF of type 41 located at
   SFF1 and this is followed by an SF of type 43, but in this case the
   SI = 250 contains a choice between the SFI located at SFF2 and the
   SFI located at SFF4.

   SFF1 will receive packets on the path from the Classifier and will
   identify the path from the SPI (16).  The initial SI will be 255 and
   so SFF1 will deliver the packets to the SFI for SFT 41.

   When the packets are returned to SFF1 by the SFI the SI will be
   decreased to 250 for the next hop.  SFF1 now has a choice of next hop
   SFF to execute the next hop in the path.  It can either forward
   packets to SFF2 or SFF4 to execute a function of type 43.  It uses
   its local load balancing algorithm to make this choice.  The chosen
   SFF will send the packets to the SFI that supports SFT 43 before
   forwarding the packets to their destinations.

8.3.  Example SFP With Open Choice of SFIs


      SFP3:  RD = 198.51.100.1,103, SPI = 17,
             [SI = 255, SFT = 41, RD = 192.0.2.1,1],
             [SI = 250, SFT = 44, RD = 0]


   In this example the path also consists of an SF of type 41 located at
   SFF1 and this is followed by an SI with an RD of zero and SF of type
   44.  This means that a choice can be made between any SFF that
   supports an SFI of type 44.

   SFF1 will receive packets on the path from the Classifier and will
   identify the path from the SPI (17).  The initial SI will be 255 and
   so SFF1 will deliver the packets to the SFI for SFT 41.

   When the packets are returned to SFF1 by the SFI the SI will be
   decreased to 250 for the next hop.  SFF1 now has a free choice of
   next hop SFF to execute the next hop in the path selecting between
   all SFFs that support SFs of type 44.  Looking at the SFIRs it has
   received, SFF1 knows that SF type 44 is supported by SFF3 and SFF4.
   SFF1 uses its local load balancing algorithm to make this choice.
   The chosen SFF will send the packets to the SFI that supports SFT 44
   before forwarding the packets to their destinations.



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8.4.  Example SFP With Choice of SFTs


      SFP4:  RD = 198.51.100.1,104, SPI = 18,
             [SI = 255, SFT = 41, RD = 192.0.2.1,1],
             [SI = 250, {SFT = 43, RD = 192.0.2.2,2,
                         SFT = 44, RD = 192.0.2.3,8 } ]


   This example provides a choice of SF type in the second hop in the
   path.  The SI of 250 indicates a choice between SF type 43 located
   through SF2 and SF type 44 located at SF3.

   SFF1 will receive packets on the path from the Classifier and will
   identify the path from the SPI (18).  The initial SI will be 255 and
   so SFF1 will deliver the packets to the SFI for SFT 41.

   When the packets are returned to SFF1 by the SFI the SI will be
   decreased to 250 for the next hop.  SFF1 now has a free choice of
   next hop SFF to execute the next hop in the path selecting between
   all SFF2 that support an SF of type 43 and SFF3 that supports an SF
   of type 44.  These may be completely different functions that are to
   be executed dependent on specific conditions, or may be similar
   functions identified with different type identifiers (such as
   firewalls from different vendors).  SFF1 uses its local policy and
   load balancing algorithm to make this choice, and may use additional
   information passed back from the local SFI to help inform its
   selection.  The chosen SFF will send the packets to the SFI that
   supports the chose SFT before forwarding the packets to their
   destinations.

8.5.  Example Correlated Bidirectional SFPs


     SFP5:  RD = 198.51.100.1,105, SPI = 19,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,106, Assoc-SPI = 20,
            [SI = 255, SFT = 41, RD = 192.0.2.1,1],
            [SI = 250, SFT = 43, RD = 192.0.2.2,2]

     SFP6:  RD = 198.51.100.1,106, SPI = 20,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,105, Assoc-SPI = 19,
            [SI = 254, SFT = 43, RD = 192.0.2.2,2],
            [SI = 249, SFT = 41, RD = 192.0.2.1,1]


   This example demonstrates correlation of two SFPs to form a
   bidirectional SFP as described in Section 7.2.




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   Two SFPRs are advertised by the Controller.  They have different SPIs
   (19 and 20) so they are known to be separate SFPs, but they both have
   Association TLVs with Association Type set to 1 indicating
   bidirectional SFPs.  Each has an Associated SFPR-RD fields containing
   the value of the other SFPR-RD to correlated the two SFPs as a
   bidirectional pair.

   As can be seen from the SFPRs in this example, the paths are
   symmetric: the hops in SFP5 appear in the reverse order in SFP6.

8.6.  Example Correlated Asymmetrical Bidirectional SFPs


     SFP7:  RD = 198.51.100.1,107, SPI = 21,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,108, Assoc-SPI = 22,
            [SI = 255, SFT = 41, RD = 192.0.2.1,1],
            [SI = 250, SFT = 43, RD = 192.0.2.2,2]

     SFP8:  RD = 198.51.100.1,108, SPI = 22,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,107, Assoc-SPI = 21,
            [SI = 254, SFT = 44, RD = 192.0.2.4,6],
            [SI = 249, SFT = 41, RD = 192.0.2.1,1]


   Asymmetric bidirectional SFPs can also be created.  This example
   shows a pair of SFPs with distinct SPIs (21 and 22) that are
   correlated in the same way as in the example in Section 8.5.

   However, unlike in that example, the SFPs are different in each
   direction.  Both paths include a hop of SF type 41, but SFP7 includes
   a hop of SF type 43 supported at SFF2 while SFP8 includes a hop of SF
   type 44 supported at SFF4.

8.7.  Example Looping in an SFP


      SFP9:  RD = 198.51.100.1,109, SPI = 23,
             [SI = 255, SFT = 41, RD = 192.0.2.1,1],
             [SI = 250, SFT = 44, RD = 192.0.2.4,5],
             [SI = 245, SFT = 1, RD = {SPI=23, SI=255, Rsv=0}],
             [SI = 245, SFT = 42, RD = 192.0.2.3,7]


   Looping and jumping are described in Section 6.  This example shows
   an SFP that contains an explicit loop-back instruction that is
   presented as a choice within an SFP hop.





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   The first two hops in the path (SI = 255 and SI = 250) are normal.
   That is, the packets will be delivered to SFF1 and SFF4 in turn for
   execution of SFs of type 41 and 44 respectively.

   The third hop (SI = 245) presents SFF4 with a choice of next hop.  It
   can either forward the packets to SFF3 for an SF of type 42 (the
   second choice), or it can loop back.

   The loop-back entry in the SFPR for SI = 245 is indicated by the
   special purpose SFT value 1 ("Change Sequence").  Within this hop,
   the RD is interpreted as encoding the SPI and SI of the next hop (see
   Section 6.1.  In this case the SPI is 23 which indicates that this is
   loop or branch: i.e., the next hop is on the same SFP.  The SI is set
   to 255: this is a higher number than the current SI (245) indicating
   a loop.

   SFF4 must make a choice between these two next hops.  Either the
   packets will be forwarded to SFF3 with the NSH SI decreased to 245 or
   looped back to SFF1 with the NSH SI reset to 255.  This choice will
   be made according to local policy, information passed back by the
   local SFI, and details in the packets' metadata that are used to
   prevent infinite looping.

8.8.  Example Branching in an SFP


      SFP10:  RD = 198.51.100.1,110, SPI = 24,
             [SI = 254, SFT = 42, RD = 192.0.2.3,7],
             [SI = 249, SFT = 43, RD = 192.0.2.2,2]

      SFP11:  RD = 198.51.100.1,111, SPI = 25,
             [SI = 255, SFT = 41, RD = 192.0.2.1,1],
             [SI = 250, SFT = 1, RD = {SPI=24, SI=254, Rsv=0}]


   Branching follows a similar procedure to that for looping (and
   jumping) as shown in Section 8.7 however there are two SFPs involved.

   SFP10 shows a normal path with packets forwarded to SFF3 and SFF2 for
   execution of service functions of type 42 and 43 respectively.

   SFP11 starts as normal (SFF1 for an SF of type 41), but then SFF1
   processes the next hop in the path and finds a "Change Sequence"
   Special Purpose SFT.  The SFIR-RD field includes an SPI of 24 which
   indicates SFP10, not the current SFP.  The SI in the SFIR-RD is 254,
   so SFF1 knows that it must set the SPI/SI in the NSH to 24/254 and
   send the packets to the appropriate SFF as advertised in the SFPR for
   SFP10 (that is, SFF3).



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8.9.  Examples of SFPs with Stateful Service Functions

   This section provides some examples to demonstrate establishing SFPs
   when there is a choice of service functions at a particular hop, and
   where consistency of choice is required in both directions.  The
   scenarios that give rise to this requirement are discussed in
   Section 7.3.

8.9.1.  Forward and Reverse Choice Made at the SFF

   Consider the topology shown in Figure 11.  There are three SFFs
   arranged neatly in a line, and the middle one (SFF2) supports three
   SFIs all of SFT 42.  These three instances can be used by SFF2 to
   load balance so that no one instance is swamped.


                   ------     ------   ------   ------    ------
                  | SFI  |   | SFIa | | SFIb | | SFIc |  | SFI  |
                  |SFT=41|   |SFT=42| |SFT=42| |SFT=42|  |SFT=43|
                   ------     ------\  ------  /------    ------
                        \            \   |    /           /
                       ---------     ---------     ---------
         ----------   |   SFF1  |   |   SFF2  |   |   SFF3  |
    --> |          |..|192.0.2.1|...|192.0.2.2|...|192.0.2.3|-->
    --> |Classifier|   ---------     ---------     ---------
        |          |
         ----------


            Figure 11: Example Where Choice is Made at the SFF

   This leads to the following SFIRs being advertised.


      RD = 192.0.2.1,11, SFT = 41
      RD = 192.0.2.2,11, SFT = 42  (for SFIa)
      RD = 192.0.2.2,12, SFT = 42  (for SFIb)
      RD = 192.0.2.2,13, SFT = 42  (for SFIc)
      RD = 192.0.2.3,11, SFT = 43


   The controller can create a single forward SFP giving SFF2 the choice
   of which SFI to use to provide function of SFT 42 as follows.  The
   load-balancing choice between the three available SFIs is assumed to
   be within the capabilities of the SFF and if the SFs are stateful it
   is assumed that the SFF knows this and arranges load balancing in a
   stable, flow-dependent way.




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      SFP12:  RD = 198.51.100.1,112, SPI = 26,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,113, Assoc-SPI = 27,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, {RD = 192.0.2.2,11,
                                        192.0.2.2,12,
                                        192.0.2.2,13 }],
             [SI = 253, SFT = 43, RD = 192.0.2.3,11]


   The reverse SFP in this case may also be created as shown below using
   association with the forward SFP and giving the load-balancing choice
   to SFF2.  This is safe, even in the case that the SFs of type 42 are
   stateful because SFF2 is doing the load balancing in both directions
   and can apply the same algorithm to ensure that packets associated
   with the same flow use the same SFI regardless of the direction of
   travel.


      SFP13:  RD = 198.51.100.1,113, SPI = 27,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,112, Assoc-SPI = 26,
             [SI = 255, SFT = 43, RD = 192.0.2.3,11],
             [SI = 254, SFT = 42, {RD = 192.0.2.2,11,
                                        192.0.2.2,12,
                                        192.0.2.2,13 }],
             [SI = 253, SFT = 41, RD = 192.0.2.1,11]


8.9.2.  Parallel End-to-End SFPs with Shared SFF

   The mechanism described in Section 8.9.1 might not be desirable
   because of the functional assumptions it places on SFF2 to be able to
   load balance with suitable flow identification, stability, and
   equality in both directions.  Instead, it may be desirable to place
   the responsibility for flow classification in the Classifier and let
   it determine load balancing with the implied choice of SFIs.

   Consider the network graph as shown in Figure 11 and with the same
   set of SFIRs as listed in Section 8.9.1.  In this case the controller
   could specify three forward SFPs with their corresponding associated
   reverse SFPs.  Each bidirectional pair of SFPs uses a different SFI
   for the SF of type 42.  The controller can instruct the Classifier
   how to place traffic on the three bidirectional SFPs, or can treat
   them as a group leaving the Classifier responsible for balancing the
   load.







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      SFP14:  RD = 198.51.100.1,114, SPI = 28,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,117, Assoc-SPI = 31,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, RD = 192.0.2.2,11],
             [SI = 253, SFT = 43, RD = 192.0.2.3,11]

      SFP15:  RD = 198.51.100.1,115, SPI = 29,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,118, Assoc-SPI = 32,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, RD = 192.0.2.2,12],
             [SI = 253, SFT = 43, RD = 192.0.2.3,11]

      SFP16:  RD = 198.51.100.1,116, SPI = 30,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,119, Assoc-SPI = 33,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, RD = 192.0.2.2,13],
             [SI = 253, SFT = 43, RD = 192.0.2.3,11]

      SFP17:  RD = 198.51.100.1,117, SPI = 31,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,114, Assoc-SPI = 28,
             [SI = 255, SFT = 43, RD = 192.0.2.3,11],
             [SI = 254, SFT = 42, RD = 192.0.2.2,11],
             [SI = 253, SFT = 41, RD = 192.0.2.1,11]

      SFP18:  RD = 198.51.100.1,118, SPI = 32,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,115, Assoc-SPI = 29,
             [SI = 255, SFT = 43, RD = 192.0.2.3,11],
             [SI = 254, SFT = 42, RD = 192.0.2.2,12],
             [SI = 253, SFT = 41, RD = 192.0.2.1,11]

      SFP19:  RD = 198.51.100.1,119, SPI = 33,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,116, Assoc-SPI = 30,
             [SI = 255, SFT = 43, RD = 192.0.2.3,11],
             [SI = 254, SFT = 42, RD = 192.0.2.2,13],
             [SI = 253, SFT = 41, RD = 192.0.2.1,11]


8.9.3.  Parallel End-to-End SFPs with Separate SFFs

   While the examples in Section 8.9.1 and Section 8.9.2 place the
   choice of SFI as subtended from the same SFF, it is also possible
   that the SFIs are ach subtended from a different SFF as shown in
   Figure 12.  In this case it is harder to coordinate the choices for
   forward and reverse paths without some form of coordination between
   SFF1 and SFF3.  Therefore it would be normal to consider end-to-end
   parallel SFPs as described in Section 8.9.2.





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                                        ------
                                       | SFIa |
                                       |SFT=42|
                                        ------
                         ------           |
                        | SFI  |      ---------
                        |SFT=41|     |   SFF5  |
                         ------    ..|192.0.2.5|..
                           |     ..:  ---------  :..
                       ---------.:                 :.---------
         ----------   |   SFF1  |     ---------     |   SFF3  |
    --> |          |..|192.0.2.1|....|   SFF6  |....|192.0.2.3| -->
    --> |Classifier|   ---------:    |192.0.2.6|    :---------
        |          |            :     ---------     :    |
         ----------             :         |         :  ------
                                :       ------      : | SFI  |
                                :..    | SFIb |   ..: |SFT=43|
                                  :..  |SFT=42| ..:    ------
                                    :   ------  :
                                    :.---------.:
                                     |   SFF7  |
                                     |192.0.2.7|
                                      ---------
                                          |
                                        ------
                                       | SFIc |
                                       |SFT=42|
                                        ------


          Figure 12: Second Example With Parallel End-to-End SFPs

   In this case, five SFIRs are advertised as follows:


      RD = 192.0.2.1,11, SFT = 41
      RD = 192.0.2.5,11, SFT = 42  (for SFIa)
      RD = 192.0.2.6,11, SFT = 42  (for SFIb)
      RD = 192.0.2.7,11, SFT = 42  (for SFIc)
      RD = 192.0.2.3,11, SFT = 43


   In this case the controller could specify three forward SFPs with
   their corresponding associated reverse SFPs.  Each bidirectional pair
   of SFPs uses a different SFF and SFI for middle hop (for an SF of
   type 42).  The controller can instruct the Classifier how to place
   traffic on the three bidirectional SFPs, or can treat them as a group
   leaving the Classifier responsible for balancing the load.



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      SFP20:  RD = 198.51.100.1,120, SPI = 34,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,123, Assoc-SPI = 37,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, RD = 192.0.2.5,11],
             [SI = 253, SFT = 43, RD = 192.0.2.3,11]

      SFP21:  RD = 198.51.100.1,121, SPI = 35,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,124, Assoc-SPI = 38,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, RD = 192.0.2.6,11],
             [SI = 253, SFT = 43, RD = 192.0.2.3,11]

      SFP22:  RD = 198.51.100.1,122, SPI = 36,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,125, Assoc-SPI = 39,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, RD = 192.0.2.7,11],
             [SI = 253, SFT = 43, RD = 192.0.2.3,11]

      SFP23:  RD = 198.51.100.1,123, SPI = 37,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,120, Assoc-SPI = 34,
             [SI = 255, SFT = 43, RD = 192.0.2.3,11],
             [SI = 254, SFT = 42, RD = 192.0.2.5,11],
             [SI = 253, SFT = 41, RD = 192.0.2.1,11]

      SFP24:  RD = 198.51.100.1,124, SPI = 38,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,121, Assoc-SPI = 35,
             [SI = 255, SFT = 43, RD = 192.0.2.3,11],
             [SI = 254, SFT = 42, RD = 192.0.2.6,11],
             [SI = 253, SFT = 41, RD = 192.0.2.1,11]

      SFP25:  RD = 198.51.100.1,125, SPI = 39,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,122, Assoc-SPI = 36,
             [SI = 255, SFT = 43, RD = 192.0.2.3,11],
             [SI = 254, SFT = 42, RD = 192.0.2.7,11],
             [SI = 253, SFT = 41, RD = 192.0.2.1,11]


8.9.4.  Parallel SFPs Downstream of the Choice

   The mechanism of parallel SFPs demonstrated in Section 8.9.3 is
   perfectly functional and may be practical in many environments.
   However, there may be scaling concerns because of the large amount of
   state (knowledge of SFPs, i.e., SFPR advertisements retained) if
   there is a very large amount of choice of SFIs (for example, tens of
   instances of the same stateful SF), or if there are multiple choices
   of stateful SF along a path.  This situation may be mitigated using
   SFP fragments that are combined to form the end to end SFPs.




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   The example presented here is necessarily simplistic, but should
   convey the basic principle.  The example presented in Figure 13 is
   similar to that in Section 8.9.3 but with an additional first hop.


                                             ------
                                            | SFIa |
                                            |SFT=43|
                                             ------
                  ------      ------           |
                 | SFI  |    | SFI  |      ---------
                 |SFT=41|    |SFT=42|     |   SFF5  |
                  ------      ------    ..|192.0.2.5|..
                    |           |     ..:  ---------  :..
                ---------   ---------.:                 :.---------
       ------  |   SFF1  | |   SFF2  |     ---------     |   SFF3  |
   -->|Class-|.|192.0.2.1|.|192.0.2.2|....|   SFF6  |....|192.0.2.3|-->
   -->| ifier|  ---------   ---------:    |192.0.2.6|    :---------
       ------                        :     ---------     :    |
                                     :         |         :  ------
                                     :       ------      : | SFI  |
                                     :..    | SFIb |   ..: |SFT=44|
                                       :..  |SFT=43| ..:    ------
                                         :   ------  :
                                         :.---------.:
                                          |   SFF7  |
                                          |192.0.2.7|
                                           ---------
                                               |
                                             ------
                                            | SFIc |
                                            |SFT=43|
                                             ------


        Figure 13: Example With Parallel SFPs Downstream of Choice

   The six SFIs are advertised as follows:


      RD = 192.0.2.1,11, SFT = 41
      RD = 192.0.2.2,11, SFT = 42
      RD = 192.0.2.5,11, SFT = 43  (for SFIa)
      RD = 192.0.2.6,11, SFT = 43  (for SFIb)
      RD = 192.0.2.7,11, SFT = 43  (for SFIc)
      RD = 192.0.2.3,11, SFT = 44





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   SFF2 is the point at which a load balancing choice must be made.  So
   "tail-end" SFPs are constructed as follows.  Each takes in a
   different SFF that provides access to an SF of type 43.


      SFP26:  RD = 198.51.100.1,126, SPI = 40,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,130, Assoc-SPI = 44,
             [SI = 255, SFT = 43, RD = 192.0.2.5,11],
             [SI = 254, SFT = 44, RD = 192.0.2.3,11]

      SFP27:  RD = 198.51.100.1,127, SPI = 41,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,131, Assoc-SPI = 45,
             [SI = 255, SFT = 43, RD = 192.0.2.6,11],
             [SI = 254, SFT = 44, RD = 192.0.2.3,11]

      SFP28:  RD = 198.51.100.1,128, SPI = 42,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,132, Assoc-SPI = 46,
             [SI = 255, SFT = 43, RD = 192.0.2.7,11],
             [SI = 254, SFT = 44, RD = 192.0.2.3,11]


   Now an end-to-end SFP with load balancing choice can be constructed
   as follows.  The choice made by SFF2 is expressed in terms of
   entering one of the three "tail end" SFPs.


      SFP29:  RD = 198.51.100.1,129, SPI = 43,
             [SI = 255, SFT = 41, RD = 192.0.2.1,11],
             [SI = 254, SFT = 42, RD = 192.0.2.2,11],
             [SI = 253, {SFT = 1, RD = {SPI=40, SI=255, Rsv=0},
                                  RD = {SPI=41, SI=255, Rsv=0},
                                  RD = {SPI=42, SI=255, Rsv=0} } ]


   Now, despite the load balancing choice being made other than at the
   initial classifier, it is possible for the reverse SFPs to be well-
   constructed without any ambiguity.  The three reverse paths appear as
   follows.













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      SFP30:  RD = 198.51.100.1,130, SPI = 44,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,126, Assoc-SPI = 40,
             [SI = 255, SFT = 44, RD = 192.0.2.4,11],
             [SI = 254, SFT = 43, RD = 192.0.2.5,11],
             [SI = 253, SFT = 42, RD = 192.0.2.2,11],
             [SI = 252, SFT = 41, RD = 192.0.2.1,11]

      SFP31:  RD = 198.51.100.1,131, SPI = 45,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,127, Assoc-SPI = 41,
             [SI = 255, SFT = 44, RD = 192.0.2.4,11],
             [SI = 254, SFT = 43, RD = 192.0.2.6,11],
             [SI = 253, SFT = 42, RD = 192.0.2.2,11],
             [SI = 252, SFT = 41, RD = 192.0.2.1,11]

      SFP32:  RD = 198.51.100.1,132, SPI = 46,
            Assoc-Type = 1, Assoc-RD = 198.51.100.1,128, Assoc-SPI = 42,
             [SI = 255, SFT = 44, RD = 192.0.2.4,11],
             [SI = 254, SFT = 43, RD = 192.0.2.7,11],
             [SI = 253, SFT = 42, RD = 192.0.2.2,11],
             [SI = 252, SFT = 41, RD = 192.0.2.1,11]



9.  Security Considerations

   This document inherits all the security considerations discussed in
   the documents that specify BGP, the documents that specify BGP
   Multiprotocol Extensions, and the documents that define the
   attributes that are carried by BGP UPDATEs of the SFC AFI/SAFI.  For
   more information look in [RFC4271], [RFC4760], and
   [I-D.ietf-idr-tunnel-encaps].

   Service Function Chaining provides a significant attack opportunity:
   packets can be diverted from their normal paths through the network,
   can be made to execute unexpected functions, and the functions that
   are instantiated in software can be subverted.  However, this
   specification does not change the existence of Service Function
   Chaining and security issues specific to Service Function Chaining
   are covered in [RFC7665] and [I-D.ietf-sfc-nsh].

   This document defines a control plane for Service Function Chaining.
   Clearly, this provides an attack vector for a Service Function
   Chaining system as an attack on this control plane could be used to
   make the system misbehave.  Thus, the security of the BGP system is
   critically important to the security of the whole Service Function
   Chaining system.





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10.  IANA Considerations

10.1.  New BGP AF/SAFI

   IANA maintains a registry of "Address Family Numbers".  IANA is
   requested to assign a new Address Family Number from the "Standards
   Action" range called "BGP SFC" (TBD1 in this document) with this
   document as a reference.

   IANA maintains a registry of "Subsequent Address Family Identifiers
   (SAFI) Parameters".  IANA is requested to assign a new SAFI value
   from the "Standards Action" range called "BGP SFC" (TBD2 in this
   document) with this document as a reference.

10.2.  New BGP Path Attribute

   IANA maintains a registry of "Border Gateway Protocol (BGP)
   Parameters" with a subregistry of "BGP Path Attributes".  IANA is
   requested to assign a new Path attribute called "SFP attribute" (TBD3
   in this document) with this document as a reference.

10.3.  New SFP Attribute TLVs Type Registry

   IANA maintains a registry of "Border Gateway Protocol (BGP)
   Parameters".  IANA is request to create a new subregistry called the
   "SFP Attribute TLVs" registry.

   Valid values are in the range 0 to 65535.

   o  Values 0 and 65535 are to be marked "Reserved, not to be
      allocated".

   o  Values 1 through 65524 are to be assigned according to the "First
      Come First Served" policy [RFC8126].

   This document should be given as a reference for this registry.

   The new registry should track:

   o  Type

   o  Name

   o  Reference Document or Contact

   o  Registration Date

   The registry should initially be populated as follows:



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       Type  | Name                  | Reference     | Date
       ------+-----------------------+---------------+---------------
       1     | Association TLV       | [This.I-D]    | Date-to-be-set
       2     | Hop TLV               | [This.I-D]    | Date-to-be-set
       3     | SFT TLV               | [This.I-D]    | Date-to-be-set


10.4.  New SFP Association Type Registry

   IANA maintains a registry of "Border Gateway Protocol (BGP)
   Parameters".  IANA is request to create a new subregistry called the
   "SFP Association Type" registry.

   Valid values are in the range 0 to 65535.

   o  Values 0 and 65535 are to be marked "Reserved, not to be
      allocated".

   o  Values 1 through 65524 are to be assigned according to the "First
      Come First Served" policy [RFC8126].

   This document should be given as a reference for this registry.

   The new registry should track:

   o  Association Type

   o  Name

   o  Reference Document or Contact

   o  Registration Date

   The registry should initially be populated as follows:


    Association Type | Name               | Reference  | Date
    -----------------+--------------------+------------+---------------
    1                | Bidirectional SFP  | [This.I-D] | Date-to-be-set


10.5.  New Service Function Type Registry

   IANA is request to create a new top-level registry called "Service
   Function Chaining Service Function Types".

   Valid values are in the range 0 to 65535.




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   o  Values 0 and 65535 are to be marked "Reserved, not to be
      allocated".

   o  Values 1 through 31 are to be assigned by "Standards Action"
      [RFC8126] and are referred to as the Special Purpose SFT values.

   o  Other values (32 through 65534) are to be assigned according to
      the "First Come First Served" policy [RFC8126].

   This document should be given as a reference for this registry.

   The new registry should track:

   o  Value

   o  Name

   o  Reference Document or Contact

   o  Registration Date

   The registry should initially be populated as follows:


       Value | Name                  | Reference     | Date
       ------+-----------------------+---------------+---------------
       1     | Change Sequence       | [This.I-D]    | Date-to-be-set


10.6.  New Generic Transitive Experimental Use Extended Community Sub-
       Type

   IANA maintains a registry of "Border Gateway Protocol (BGP)
   Parameters" with a subregistry of "Generic Transitive Experimental
   Use Extended Community Sub-Type".  IANA is requested to assign a new
   sub-type called "Flow spec for SFC classifiers" (TBD4 in this
   document) with this document as the reference.

10.7.  SPI/SI Representation

   IANA is requested to assign a codepoint from the "BGP Tunnel
   Encapsulation Attribute Sub-TLVs" registry for the "SPI/SI
   Representation Sub-TLV" (TBD5 in this document) with this document
   being the reference.







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11.  Contributors


      Stuart Mackie
      Juniper Networks

      Email: wsmackie@juinper.net

      Keyur Patel
      Arrcus, Inc.

      Email: keyur@arrcus.com


12.  Acknowledgements

   Thanks to Tony Przygienda for helpful comments, and to Joel Halpern
   for discussions that improved this document.

13.  References

13.1.  Normative References

   [I-D.ietf-idr-tunnel-encaps]
              Rosen, E., Patel, K., and G. Velde, "The BGP Tunnel
              Encapsulation Attribute", draft-ietf-idr-tunnel-encaps-07
              (work in progress), July 2017.

   [I-D.ietf-sfc-nsh]
              Quinn, P., Elzur, U., and C. Pignataro, "Network Service
              Header (NSH)", draft-ietf-sfc-nsh-23 (work in progress),
              September 2017.

   [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>.

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

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.




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   [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>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,
              <https://www.rfc-editor.org/info/rfc5575>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

13.2.  Informative References

   [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>.

   [RFC7498]  Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
              Service Function Chaining", RFC 7498,
              DOI 10.17487/RFC7498, April 2015,
              <https://www.rfc-editor.org/info/rfc7498>.

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510,
              DOI 10.17487/RFC7510, April 2015,
              <https://www.rfc-editor.org/info/rfc7510>.

   [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>.

Authors' Addresses

   Adrian Farrel
   Juniper Networks

   Email: afarrel@juniper.net




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   John Drake
   Juniper Networks

   Email: jdrake@juniper.net


   Eric Rosen
   Juniper Networks

   Email: erosen@juniper.net


   Jim Uttaro
   AT&T

   Email: ju1738@att.com


   Luay Jalil
   Verizon

   Email: luay.jalil@verizon.com





























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