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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                     P. Quinn, Ed.
Request for Comments: 8300                                         Cisco
Category: Standards Track                                  U. Elzur, Ed.
ISSN: 2070-1721                                                    Intel
                                                       C. Pignataro, Ed.
                                                                   Cisco
                                                            January 2018


                      Network Service Header (NSH)

Abstract

   This document describes a Network Service Header (NSH) imposed on
   packets or frames to realize Service Function Paths (SFPs).  The NSH
   also provides a mechanism for metadata exchange along the
   instantiated service paths.  The NSH is the Service Function Chaining
   (SFC) encapsulation required to support the SFC architecture (defined
   in RFC 7665).

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8300.

Copyright Notice

   Copyright (c) 2018 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.



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Table of Contents

   1. Introduction ....................................................3
      1.1. Applicability ..............................................4
      1.2. Requirements Language ......................................4
      1.3. Definition of Terms ........................................4
      1.4. Problem Space ..............................................6
      1.5. NSH-Based Service Chaining .................................6
   2. Network Service Header ..........................................7
      2.1. Network Service Header Format ..............................7
      2.2. NSH Base Header ............................................8
      2.3. Service Path Header .......................................11
      2.4. NSH MD Type 1 .............................................12
      2.5. NSH MD Type 2 .............................................13
           2.5.1. Optional Variable-Length Metadata ..................13
   3. NSH Actions ....................................................15
   4. NSH Transport Encapsulation ....................................16
   5. Fragmentation Considerations ...................................17
   6. Service Path Forwarding with NSH ...............................18
      6.1. SFFs and Overlay Selection ................................18
      6.2. Mapping the NSH to Network Topology .......................21
      6.3. Service Plane Visibility ..................................21
      6.4. Service Graphs ............................................22
   7. Policy Enforcement with NSH ....................................22
      7.1. NSH Metadata and Policy Enforcement .......................22
      7.2. Updating/Augmenting Metadata ..............................24
      7.3. Service Path Identifier and Metadata ......................25
   8. Security Considerations ........................................26
      8.1. NSH Security Considerations from Operators' Environments ..27
      8.2. NSH Security Considerations from the SFC Architecture .....28
           8.2.1. Integrity ..........................................29
           8.2.2. Confidentiality ....................................31
   9. IANA Considerations ............................................32
      9.1. NSH Parameters ............................................32
           9.1.1. NSH Base Header Bits ...............................32
           9.1.2. NSH Version ........................................32
           9.1.3. NSH MD Types .......................................33
           9.1.4. NSH MD Class .......................................33
           9.1.5. NSH IETF-Assigned Optional Variable-Length
                  Metadata Types .....................................34
           9.1.6. NSH Next Protocol ..................................35
   10. NSH-Related Codepoints ........................................35
      10.1. NSH Ethertype ............................................35
   11. References ....................................................36
   Acknowledgments ...................................................38
   Contributors ......................................................39
   Authors' Addresses ................................................40




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1.  Introduction

   Service Functions are widely deployed and essential in many networks.
   These Service Functions provide a range of features such as security,
   WAN acceleration, and server load balancing.  Service Functions may
   be instantiated at different points in the network infrastructure
   such as the WAN, data center, and so forth.

   Prior to development of the SFC architecture [RFC7665] and the
   protocol specified in this document, current Service Function
   deployment models have been relatively static and bound to topology
   for insertion and policy selection.  Furthermore, they do not adapt
   well to elastic service environments enabled by virtualization.

   New data-center network and cloud architectures require more flexible
   Service Function deployment models.  Additionally, the transition to
   virtual platforms demands an agile service insertion model that
   supports dynamic and elastic service delivery.  Specifically, the
   following functions are necessary:

   1.  The movement of Service Functions and application workloads in
       the network.

   2.  The ability to easily bind service policy to granular
       information, such as per-subscriber state.

   3.  The capability to steer traffic to the requisite Service
       Function(s).

   This document, the Network Service Header (NSH) specification,
   defines a new data-plane protocol, which is an encapsulation for
   SFCs.  The NSH is designed to encapsulate an original packet or frame
   and, in turn, be encapsulated by an outer transport encapsulation
   (which is used to deliver the NSH to NSH-aware network elements), as
   shown in Figure 1:

                     +------------------------------+
                     |    Transport Encapsulation   |
                     +------------------------------+
                     | Network Service Header (NSH) |
                     +------------------------------+
                     |    Original Packet / Frame   |
                     +------------------------------+

              Figure 1: Network Service Header Encapsulation






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   The NSH is composed of the following elements:

   1.  Service Function Path identification.

   2.  Indication of location within a Service Function Path.

   3.  Optional, per-packet metadata (fixed-length or variable).

   [RFC7665] provides an overview of a service chaining architecture
   that clearly defines the roles of the various elements and the scope
   of a SFC encapsulation.  Figure 3 of [RFC7665] depicts the SFC
   architectural components after classification.  The NSH is the SFC
   encapsulation referenced in [RFC7665].

1.1.  Applicability

   The NSH is designed to be easy to implement across a range of
   devices, both physical and virtual, including hardware platforms.

   The intended scope of the NSH is for use within a single provider's
   operational domain.  This deployment scope is deliberately
   constrained, as explained also in [RFC7665], and limited to a single
   network administrative domain.  In this context, a "domain" is a set
   of network entities within a single administration.  For example, a
   network administrative domain can include a single data center, or an
   overlay domain using virtual connections and tunnels.  A corollary is
   that a network administrative domain has a well-defined perimeter.

   An NSH-aware control plane is outside the scope of this document.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.3.  Definition of Terms

   Byte:  All references to "bytes" in this document refer to 8-bit
      bytes, or octets.

   Classification:  Defined in [RFC7665].

   Classifier:  Defined in [RFC7665].





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   Metadata (MD):  Defined in [RFC7665].  The metadata, or context
      information shared between Classifiers and SFs, and among SFs, is
      carried on the NSH's Context Headers.  It allows summarizing a
      classification result in the packet itself, avoiding subsequent
      re-classifications.  Examples of metadata include classification
      information used for policy enforcement and network context for
      forwarding after service delivery.

   Network Locator:  Data-plane address, typically IPv4 or IPv6, used to
      send and receive network traffic.

   Network Node/Element:  Device that forwards packets or frames based
      on an outer header (i.e., transport encapsulation) information.

   Network Overlay:  Logical network built on top of an existing network
      (the underlay).  Packets are encapsulated or tunneled to create
      the overlay network topology.

   NSH-aware:  NSH-aware means SFC-encapsulation-aware, where the NSH
      provides the SFC encapsulation.  This specification uses NSH-aware
      as a more specific term from the more generic term "SFC-aware"
      [RFC7665].

   Service Classifier:  Logical entity providing classification
      function.  Since they are logical, Classifiers may be co-resident
      with SFC elements such as SFs or SFFs.  Service Classifiers
      perform classification and impose the NSH.  The initial Classifier
      imposes the initial NSH and sends the NSH packet to the first SFF
      in the path.  Non-initial (i.e., subsequent) classification can
      occur as needed and can alter, or create a new service path.

   Service Function (SF):  Defined in [RFC7665].

   Service Function Chain (SFC):  Defined in [RFC7665].

   Service Function Forwarder (SFF):  Defined in [RFC7665].

   Service Function Path (SFP):  Defined in [RFC7665].

   Service Plane:  The collection of SFFs and associated SFs creates a
      service-plane overlay in which all SFs and SFC Proxies reside
      [RFC7665].

   SFC Proxy:  Defined in [RFC7665].







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1.4.  Problem Space

   The NSH addresses several limitations associated with Service
   Function deployments.  [RFC7498] provides a comprehensive review of
   those issues.

1.5.  NSH-Based Service Chaining

   The NSH creates a dedicated service plane; more specifically, the NSH
   enables:

   1.  Topological Independence: Service forwarding occurs within the
       service plane, so the underlying network topology does not
       require modification.  The NSH provides an identifier used to
       select the network overlay for network forwarding.

   2.  Service Chaining: The NSH enables service chaining per [RFC7665].
       The NSH contains path identification information needed to
       realize a service path.  Furthermore, the NSH provides the
       ability to monitor and troubleshoot a service chain, end-to-end
       via service-specific Operations, Administration, and Maintenance
       (OAM) messages.  The NSH fields can be used by administrators
       (for example, via a traffic analyzer) to verify the path
       specifics (e.g., accounting, ensuring correct chaining, providing
       reports, etc.) of packets being forwarded along a service path.

   3.  The NSH provides a mechanism to carry shared metadata between
       participating entities and Service Functions.  The semantics of
       the shared metadata are communicated via a control plane (which
       is outside the scope of this document) to participating nodes.
       Section 3.3 of [SFC-CONTROL-PLANE] provides an example of this.
       Examples of metadata include classification information used for
       policy enforcement and network context for forwarding post
       service delivery.  Sharing the metadata allows Service Functions
       to share initial and intermediate classification results with
       downstream Service Functions saving re-classification, where
       enough information was enclosed.

   4.  The NSH offers a common and standards-based header for service
       chaining to all network and service nodes.

   5.  Transport Encapsulation Agnostic: The NSH is transport
       encapsulation independent: meaning it can be transported by a
       variety of encapsulation protocols.  An appropriate (for a given
       deployment) encapsulation protocol can be used to carry NSH-
       encapsulated traffic.  This transport encapsulation may form an





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       overlay network; and if an existing overlay topology provides the
       required service path connectivity, that existing overlay may be
       used.

2.  Network Service Header

   An NSH is imposed on the original packet/frame.  This NSH contains
   service path information and, optionally, metadata that are added to
   a packet or frame and used to create a service plane.  Subsequently,
   an outer transport encapsulation is imposed on the NSH, which is used
   for network forwarding.

   A Service Classifier adds the NSH.  The NSH is removed by the last
   SFF in the service chain or by an SF that consumes the packet.

2.1.  Network Service Header Format

   The NSH is composed of a 4-byte Base Header, a 4-byte Service Path
   Header, and optional Context Headers, as shown in Figure 2.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Base Header                                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Service Path Header                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                Context Header(s)                              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2: Network Service Header

   Base Header:  Provides information about the service header and the
      payload protocol.

   Service Path Header:  Provides path identification and location
      within a service path.

   Context Header:  Carries metadata (i.e., context data) along a
      service path.









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2.2.  NSH Base Header

   Figure 3 depicts the NSH Base Header:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|O|U|    TTL    |   Length  |U|U|U|U|MD Type| Next Protocol |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: NSH Base Header

   The field descriptions are as follows:

   Version:  The Version field is used to ensure backward compatibility
      going forward with future NSH specification updates.  It MUST be
      set to 0x0 by the sender, in this first revision of the NSH.  If a
      packet presumed to carry an NSH header is received at an SFF, and
      the SFF does not understand the version of the protocol as
      indicated in the base header, the packet MUST be discarded, and
      the event SHOULD be logged.  Given the widespread implementation
      of existing hardware that uses the first nibble after an MPLS
      label stack for Equal-Cost Multipath (ECMP) decision processing,
      this document reserves version 01b.  This value MUST NOT be used
      in future versions of the protocol.  Please see [RFC7325] for
      further discussion of MPLS-related forwarding requirements.

   O bit:  Setting this bit indicates an OAM packet (see [RFC6291]).
      The actual format and processing of SFC OAM packets is outside the
      scope of this specification (for example, see [SFC-OAM-FRAMEWORK]
      for one approach).

      The O bit MUST be set for OAM packets and MUST NOT be set for
      non-OAM packets.  The O bit MUST NOT be modified along the SFP.

      SF/SFF/SFC Proxy/Classifier implementations that do not support
      SFC OAM procedures SHOULD discard packets with O bit set, but MAY
      support a configurable parameter to enable forwarding received SFC
      OAM packets unmodified to the next element in the chain.
      Forwarding OAM packets unmodified by SFC elements that do not
      support SFC OAM procedures may be acceptable for a subset of OAM
      functions, but it can result in unexpected outcomes for others;
      thus, it is recommended to analyze the impact of forwarding an OAM
      packet for all OAM functions prior to enabling this behavior.  The
      configurable parameter MUST be disabled by default.






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   TTL:  Indicates the maximum SFF hops for an SFP.  This field is used
      for service-plane loop detection.  The initial TTL value SHOULD be
      configurable via the control plane; the configured initial value
      can be specific to one or more SFPs.  If no initial value is
      explicitly provided, the default initial TTL value of 63 MUST be
      used.  Each SFF involved in forwarding an NSH packet MUST
      decrement the TTL value by 1 prior to NSH forwarding lookup.
      Decrementing by 1 from an incoming value of 0 shall result in a
      TTL value of 63.  The packet MUST NOT be forwarded if TTL is,
      after decrement, 0.

      This TTL field is the primary loop-prevention mechanism.  This TTL
      mechanism represents a robust complement to the Service Index (see
      Section 2.3), as the TTL is decremented by each SFF.  The handling
      of an incoming 0 TTL allows for better, although not perfect,
      interoperation with pre-standard implementations that do not
      support this TTL field.

   Length:  The total length, in 4-byte words, of the NSH including the
      Base Header, the Service Path Header, the Fixed-Length Context
      Header, or Variable-Length Context Header(s).  The length MUST be
      0x6 for MD Type 0x1, and it MUST be 0x2 or greater for MD Type
      0x2.  The length of the Network Service Header MUST be an integer
      multiple of 4 bytes; thus, variable-length metadata is always
      padded out to a multiple of 4 bytes.

   Unassigned bits:  All other flag fields, marked U, are unassigned and
      available for future use; see Section 9.1.1.  Unassigned bits MUST
      be set to zero upon origination, and they MUST be ignored and
      preserved unmodified by other NSH supporting elements.  At
      reception, all elements MUST NOT modify their actions based on
      these unknown bits.

   Metadata (MD) Type:  Indicates the format of the NSH beyond the
      mandatory NSH Base Header and the Service Path Header.  MD Type
      defines the format of the metadata being carried.  Please see the
      IANA Considerations in Section 9.1.3.

      This document specifies the following four MD Type values:

      0x0:  This is a reserved value.  Implementations SHOULD silently
            discard packets with MD Type 0x0.

      0x1:  This indicates that the format of the header includes a
            Fixed-Length Context Header (see Figure 5 below).






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      0x2:  This does not mandate any headers beyond the Base Header and
            Service Path Header, but may contain optional Variable-
            Length Context Header(s).  With MD Type 0x2, a length of 0x2
            implies there are no Context Headers.  The semantics of the
            Variable-Length Context Header(s) are not defined in this
            document.  The format of the optional Variable-Length
            Context Headers is provided in Section 2.5.1.

      0xF:  This value is reserved for experimentation and testing, as
            per [RFC3692].  Implementations not explicitly configured to
            be part of an experiment SHOULD silently discard packets
            with MD Type 0xF.

      The format of the Base Header and the Service Path Header is
      invariant and not affected by MD Type.

      The NSH MD Type 1 and MD Type 2 are described in detail in
      Sections 2.4 and 2.5, respectively.  NSH implementations MUST
      support MD Types 0x1 and 0x2 (where the length is 0x2).  NSH
      implementations SHOULD support MD Type 0x2 with length greater
      than 0x2.  Devices that do not support MD Type 0x2 with a length
      greater than 0x2 MUST ignore any optional Context Headers and
      process the packet without them; the Base Header Length field can
      be used to determine the original payload offset if access to the
      original packet/frame is required.  This specification does not
      disallow the MD Type value from changing along an SFP; however,
      the specification of the necessary mechanism to allow the MD Type
      to change along an SFP are outside the scope of this document and
      would need to be defined for that functionality to be available.
      Packets with MD Type values not supported by an implementation
      MUST be silently dropped.

   Next Protocol:  Indicates the protocol type of the encapsulated data.
      The NSH does not alter the inner payload, and the semantics on the
      inner protocol remain unchanged due to NSH SFC.  Please see the
      IANA Considerations in Section 9.1.6.

      This document defines the following Next Protocol values:

      0x1: IPv4
      0x2: IPv6
      0x3: Ethernet
      0x4: NSH
      0x5: MPLS
      0xFE: Experiment 1
      0xFF: Experiment 2





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      The functionality of hierarchical NSH using a Next Protocol value
      of 0x4 (NSH) is outside the scope of this specification.  Packets
      with Next Protocol values not supported SHOULD be silently dropped
      by default, although an implementation MAY provide a configuration
      parameter to forward them.  Additionally, an implementation not
      explicitly configured for a specific experiment [RFC3692] SHOULD
      silently drop packets with Next Protocol values 0xFE and 0xFF.

2.3.  Service Path Header

   Figure 4 shows the format of the Service Path Header:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Service Path Identifier (SPI)        | Service Index |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Service Path Identifier (SPI): 24 bits
     Service Index (SI): 8 bits

                     Figure 4: NSH Service Path Header

   The meaning of these fields is as follows:

   Service Path Identifier (SPI): Uniquely identifies a Service Function
   Path (SFP).  Participating nodes MUST use this identifier for SFP
   selection.  The initial Classifier MUST set the appropriate SPI for a
   given classification result.

   Service Index (SI): Provides location within the SFP.  The initial
   Classifier for a given SFP SHOULD set the SI to 255; however, the
   control plane MAY configure the initial value of the SI as
   appropriate (i.e., taking into account the length of the SFP).  The
   Service Index MUST be decremented by a value of 1 by Service
   Functions or by SFC Proxy nodes after performing required services;
   the new decremented SI value MUST be used in the egress packet's NSH.
   The initial Classifier MUST send the packet to the first SFF in the
   identified SFP for forwarding along an SFP.  If re-classification
   occurs, and that re-classification results in a new SPI, the
   (re-)Classifier is, in effect, the initial Classifier for the
   resultant SPI.

   The SI is used in conjunction with the Service Path Identifier for
   SFP selection and for determining the next SFF/SF in the path.  The
   SI is also valuable when troubleshooting or reporting service paths.
   While the TTL provides the primary SFF-based loop prevention for this
   mechanism, SI decrement by SF serves as a limited loop-prevention



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   mechanism.  NSH packets, as described above, are discarded when an
   SFF decrements the TTL to 0.  In addition, an SFF that is not the
   terminal SFF for an SFP will discard any NSH packet with an SI of 0,
   as there will be no valid next SF information.

2.4.  NSH MD Type 1

   When the Base Header specifies MD Type 0x1, a Fixed-Length Context
   Header (16-bytes) MUST be present immediately following the Service
   Path Header, as per Figure 5.  The value of a Fixed-Length Context
   Header that carries no metadata MUST be set to zero.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|O|U|    TTL    |   Length  |U|U|U|U|MD Type| Next Protocol |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Service Path Identifier              | Service Index |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 Fixed-Length Context Header                   |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 5: NSH MD Type 0x1

   This specification does not make any assumptions about the content of
   the 16-byte Context Header that must be present when the MD Type
   field is set to 1, and it does not describe the structure or meaning
   of the included metadata.

   An SFC-aware SF or SFC Proxy needs to receive the data structure and
   semantics first in order to process the data placed in the mandatory
   context field.  The data structure and semantics include both the
   allocation schema and order as well as the meaning of the included
   data.  How an SFC-aware SF or SFC Proxy gets the data structure and
   semantics is outside the scope of this specification.

   An SF or SFC Proxy that does not know the format or semantics of the
   Context Header for an NSH with MD Type 1 MUST discard any packet with
   such an NSH (i.e., MUST NOT ignore the metadata that it cannot
   process), and MUST log the event at least once per the SPI for which
   the event occurs (subject to thresholding).

   [NSH-DC-ALLOCATION] and [NSH-BROADBAND-ALLOCATION] provide specific
   examples of how metadata can be allocated.





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2.5.  NSH MD Type 2

   When the Base Header specifies MD Type 0x2, zero or more Variable-
   Length Context Headers MAY be added, immediately following the
   Service Path Header (see Figure 6).  Therefore, Length = 0x2,
   indicates that only the Base Header and Service Path Header are
   present (and in that order).  The optional Variable-Length Context
   Headers MUST be of an integer number of 4-bytes.  The Base Header
   Length field MUST be used to determine the offset to locate the
   original packet or frame for SFC nodes that require access to that
   information.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|O|U|    TTL    |   Length  |U|U|U|U|MD Type| Next Protocol |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Service Path Identifier              | Service Index |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~              Variable-Length Context Headers  (opt.)          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 6: NSH MD Type 0x2

2.5.1.  Optional Variable-Length Metadata

   The format of the optional Variable-Length Context Headers, is as
   depicted in Figure 7.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Metadata Class       |      Type     |U|    Length   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Variable-Length Metadata                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 7: Variable-Length Context Headers

   Metadata Class (MD Class):  Defines the scope of the Type field to
      provide a hierarchical namespace.  Section 9.1.4 defines how the
      MD Class values can be allocated to standards bodies, vendors, and
      others.






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   Type:  Indicates the explicit type of metadata being carried.  The
      definition of the Type is the responsibility of the MD Class
      owner.

   Unassigned bit:  One unassigned bit is available for future use.
      This bit MUST NOT be set, and it MUST be ignored on receipt.

   Length:  Indicates the length of the variable-length metadata, in
      bytes.  In case the metadata length is not an integer number of
      4-byte words, the sender MUST add pad bytes immediately following
      the last metadata byte to extend the metadata to an integer number
      of 4-byte words.  The receiver MUST round the Length field up to
      the nearest 4-byte-word boundary, to locate and process the next
      field in the packet.  The receiver MUST access only those bytes in
      the metadata indicated by the Length field (i.e., actual number of
      bytes) and MUST ignore the remaining bytes up to the nearest
      4-byte-word boundary.  The length may be 0 or greater.

      A value of 0 denotes a Context Header without a Variable-Length
      Metadata field.

   This specification does not make any assumption about Context Headers
   that are mandatory to implement or those that are mandatory to
   process.  These considerations are deployment specific.  However, the
   control plane is entitled to instruct SFC-aware SFs with the data
   structure of the Context Header together with its scoping (see e.g.,
   Section 3.3.3 of [SFC-CONTROL-PLANE]).

   Upon receipt of a packet that belongs to a given SFP, if a mandatory-
   to-process Context Header is missing in that packet, the SFC-aware SF
   MUST NOT process the packet and MUST log an error at least once per
   the SPI for which the mandatory metadata is missing.

   If multiple mandatory-to-process Context Headers are required for a
   given SFP, the control plane MAY instruct the SFC-aware SF with the
   order to consume these Context Headers.  If no instructions are
   provided and the SFC-aware SF will make use of or modify the specific
   Context Header, then the SFC-aware SF MUST process these Context
   Headers in the order they appear in an NSH packet.

   If multiple instances of the same metadata are included in an NSH
   packet, but the definition of that Context Header does not allow for
   it, the SFC-aware SF MUST process the first instance and ignore
   subsequent instances.  The SFC-aware SF MAY log or increase a counter
   for this event.






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3.  NSH Actions

   NSH-aware nodes (which include Service Classifiers, SFFs, SFs, and
   SFC Proxies) may alter the contents of the NSH headers.  These nodes
   have several possible NSH-related actions:

   1.  Insert or remove the NSH: These actions can occur respectively at
       the start and end of a service path.  Packets are classified, and
       if determined to require servicing, an NSH will be imposed.  A

       Service Classifier MUST insert an NSH at the start of an SFP.  An
       imposed NSH MUST contain both a valid Base Header and Service
       Path Header.  At the end of an SFP, an SFF MUST remove the NSH
       before forwarding or delivering the un-encapsulated packet.
       Therefore, it is the last node operating on the service header.

       Multiple logical Classifiers may exist within a given service
       path.  Non-initial Classifiers may re-classify data, and that
       re-classification MAY result in the selection of a different SFP.
       When the logical Classifier performs re-classification that
       results in a change of service path, it MUST replace the existing
       NSH with a new NSH with the Base Header and Service Path Header
       reflecting the new service path information and MUST set the
       initial SI.  The O bit, the TTL field, and unassigned flags MUST
       be copied transparently from the old NSH to a new NSH.  Metadata
       MAY be preserved in the new NSH.

   2.  Select service path: The Service Path Header provides service
       path information and is used by SFFs to determine correct service
       path selection.  SFFs MUST use the Service Path Header for
       selecting the next SF or SFF in the service path.

   3.  Update the NSH: SFs MUST decrement the service index by one.  If
       an SFF receives a packet with an SPI and SI that do not
       correspond to a valid next hop in a valid SFP, that packet MUST
       be dropped by the SFF.

       Classifiers MAY update Context Headers if new/updated context is
       available.

       If an SFC proxy is in use (acting on behalf of an NSH-unaware
       Service Function for NSH actions), then the proxy MUST update the
       Service Index and MAY update contexts.  When an SFC Proxy
       receives an NSH-encapsulated packet, it MUST remove the NSH
       before forwarding it to an NSH-unaware SF.  When the SFC Proxy
       receives a packet back from an NSH-unaware SF, it MUST
       re-encapsulate it with the correct NSH, and it MUST decrement the
       Service Index by one.



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   4.  Service policy selection: Service Functions derive policy (i.e.,
       service actions such as permit or deny) selection and enforcement
       from the NSH.  Metadata shared in the NSH can provide a range of
       service-relevant information such as traffic classification.

   Figure 8 maps each of the four actions above to the components in the
   SFC architecture that can perform it.

   +-----------+-----------------------+-------+---------------+-------+
   |           | Insert, remove, or    |Forward| Update        |Service|
   |           | replace the NSH       |the NSH| the NSH       |policy |
   |           |                       |packets|               |sel.   |
   |Component  +-------+-------+-------+       +-------+-------+       |
   |           |       |       |       |       |Dec.   |Update |       |
   |           |Insert |Remove |Replace|       |Service|Context|       |
   |           |       |       |       |       |Index  |Header |       |
   +-----------+-------+-------+-------+-------+-------+-------+-------+
   |           |  +    |       |   +   |       |       |   +   |       |
   |Classifier |       |       |       |       |       |       |       |
   +-----------+-------+-------+-------+-------+-------+-------+-------+
   |Service    |       |   +   |       |   +   |       |       |       |
   |Function   |       |       |       |       |       |       |       |
   |Forwarder  |       |       |       |       |       |       |       |
   |(SFF)      |       |       |       |       |       |       |       |
   +-----------+-------+-------+-------+-------+-------+-------+-------+
   |Service    |       |       |       |       |   +   |   +   |   +   |
   |Function   |       |       |       |       |       |       |       |
   |(SF)       |       |       |       |       |       |       |       |
   +-----------+-------+-------+-------+-------+-------+-------+-------+
   |           |  +    |   +   |       |       |   +   |   +   |       |
   |SFC Proxy  |       |       |       |       |       |       |       |
   +-----------+-------+-------+-------+-------+-------+-------+-------+

                   Figure 8: NSH Action and Role Mapping

4.  NSH Transport Encapsulation

   Once the NSH is added to a packet, an outer transport encapsulation
   is used to forward the original packet and the associated metadata to
   the start of a service chain.  The encapsulation serves two purposes:

   1.  Creates a topologically independent services plane.  Packets are
       forwarded to the required services without changing the
       underlying network topology.







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   2.  Transit network nodes simply forward the encapsulated packets
       without modification.

   The service header is independent of the transport encapsulation
   used.  Existing transport encapsulations can be used.  The presence
   of an NSH is indicated via a protocol type or another indicator in
   the outer transport encapsulation.

5.  Fragmentation Considerations

   The NSH and the associated transport encapsulation header are "added"
   to the encapsulated packet/frame.  This additional information
   increases the size of the packet.

   Within a managed administrative domain, an operator can ensure that
   the underlay MTU is sufficient to carry SFC traffic without requiring
   fragmentation.  Given that the intended scope of the NSH is within a
   single provider's operational domain, that approach is sufficient.

   However, although explicitly outside the scope of this specification,
   there might be cases where the underlay MTU is not large enough to
   carry the NSH traffic.  Since the NSH does not provide fragmentation
   support at the service plane, the transport encapsulation protocol
   ought to provide the requisite fragmentation handling.  For instance,
   Section 9 of [RTG-ENCAP] provides exemplary approaches and guidance
   for those scenarios.

   When the transport encapsulation protocol supports fragmentation, and
   fragmentation procedures needs to be used, such fragmentation is part
   of the transport encapsulation logic.  If, as it is common,
   fragmentation is performed by the endpoints of the transport
   encapsulation, then fragmentation procedures are performed at the
   sending NSH entity as part of the transport encapsulation, and
   reassembly procedures are performed at the receiving NSH entity
   during transport de-encapsulation handling logic.  In no case would
   such fragmentation result in duplication of the NSH header.

   For example, when the NSH is encapsulated in IP, IP-level
   fragmentation coupled with Path MTU Discovery (PMTUD) (e.g.,
   [RFC8201]) is used.  Since PMTUD relies on ICMP messages, an operator
   should ensure ICMP packets are not blocked.  When, on the other hand,
   the underlay does not support fragmentation procedures, an error
   message SHOULD be logged when dropping a packet too big.  Lastly,
   NSH-specific fragmentation and reassembly methods may be defined as
   well, but these methods are outside the scope of this document and
   subject for future work.





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6.  Service Path Forwarding with NSH

6.1.  SFFs and Overlay Selection

   As described above, the NSH contains a Service Path Identifier (SPI)
   and a Service Index (SI).  The SPI is, as per its name, an
   identifier.  The SPI alone cannot be used to forward packets along a
   service path.  Rather, the SPI provides a level of indirection
   between the service path / topology and the network transport
   encapsulation.  Furthermore, there is no requirement for, or
   expectation of, an SPI being bound to a predetermined or static
   network path.

   The Service Index provides an indication of location within a service
   path.  The combination of SPI and SI provides the identification of a
   logical SF and its order within the service plane.  This combination
   is used to select the appropriate network locator(s) for overlay
   forwarding.  The logical SF may be a single SF or a set of eligible
   SFs that are equivalent.  In the latter case, the SFF provides load
   distribution amongst the collection of SFs as needed.

   SI serves as a mechanism for detecting invalid SFPs.  In particular,
   an SI value of zero indicates that forwarding is incorrect and the
   packet must be discarded.

   This indirection -- SPI to overlay -- creates a true service plane.
   That is, the SFF/SF topology is constructed without impacting the
   network topology, but, more importantly, service-plane-only
   participants (i.e., most SFs) need not be part of the network overlay
   topology and its associated infrastructure (e.g., control plane,
   routing tables, etc.).  SFs need to be able to return a packet to an
   appropriate SFF (i.e., has the requisite NSH information) when
   service processing is complete.  This can be via the overlay or
   underlay and, in some cases, can require additional configuration on
   the SF.  As mentioned above, an existing overlay topology may be
   used, provided it offers the requisite connectivity.

   The mapping of SPI to transport encapsulation occurs on an SFF (as
   discussed above, the first SFF in the path gets an NSH encapsulated
   packet from the Classifier).  The SFF consults the SPI/ID values to
   determine the appropriate overlay transport encapsulation protocol
   (several may be used within a given network) and next hop for the
   requisite SF.  Table 1 depicts an example of a single next-hop SPI/
   SI-to-network overlay network locator mapping.







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      +------+------+---------------------+-------------------------+
      | SPI  | SI   | Next Hop(s)         | Transport Encapsulation |
      +------+------+---------------------+-------------------------+
      | 10   | 255  | 192.0.2.1           | VXLAN-gpe               |
      |      |      |                     |                         |
      | 10   | 254  | 198.51.100.10       | GRE                     |
      |      |      |                     |                         |
      | 10   | 251  | 198.51.100.15       | GRE                     |
      |      |      |                     |                         |
      | 40   | 251  | 198.51.100.15       | GRE                     |
      |      |      |                     |                         |
      | 50   | 200  | 01:23:45:67:89:ab   | Ethernet                |
      |      |      |                     |                         |
      | 15   | 212  | Null (end of path)  | None                    |
      +------+------+---------------------+-------------------------+

                     Table 1: SFF NSH Mapping Example

   Additionally, further indirection is possible: the resolution of the
   required SF network locator may be a localized resolution on an SFF,
   rather than an SFC control plane responsibility, as per Tables 2 and
   3.

   Please note: VXLAN-gpe and GRE in the above table refer to
   [VXLAN-GPE] and [RFC2784] [RFC7676], respectively.

                      +------+-----+----------------+
                      | SPI  | SI  | Next Hop(s)    |
                      +------+-----+----------------+
                      | 10   | 3   | SF2            |
                      |      |     |                |
                      | 245  | 12  | SF34           |
                      |      |     |                |
                      | 40   | 9   | SF9            |
                      +------+-----+----------------+

                    Table 2: NSH-to-SF Mapping Example














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          +------+-------------------+-------------------------+
          | SF   | Next Hop(s)       | Transport Encapsulation |
          +------+-------------------+-------------------------+
          | SF2  | 192.0.2.2         | VXLAN-gpe               |
          |      |                   |                         |
          | SF34 | 198.51.100.34     | UDP                     |
          |      |                   |                         |
          | SF9  | 2001:db8::1       | GRE                     |
          +------+-------------------+-------------------------+

                    Table 3: SF Locator Mapping Example

   Since the SPI is a representation of the service path, the lookup may
   return more than one possible next hop within a service path for a
   given SF, essentially a series of weighted (equally or otherwise)
   paths to be used (for load distribution, redundancy, or policy); see
   Table 4.  The metric depicted in Table 4 is an example to help
   illustrate weighing SFs.  In a real network, the metric will range
   from a simple preference (similar to routing next-hop) to a true
   dynamic composite metric based on the state of a Service Function
   (including load, session state, capacity, etc.).

                  +------+-----+--------------+---------+
                  | SPI  | SI  | NH           | Metric  |
                  +------+-----+--------------+---------+
                  | 10   | 3   | 203.0.113.1  | 1       |
                  |      |     |              |         |
                  |      |     | 203.0.113.2  | 1       |
                  |      |     |              |         |
                  | 20   | 12  | 192.0.2.1    | 1       |
                  |      |     |              |         |
                  |      |     | 203.0.113.4  | 1       |
                  |      |     |              |         |
                  | 30   | 7   | 192.0.2.10   | 10      |
                  |      |     |              |         |
                  |      |     | 198.51.100.1 | 5       |
                  +------+-----+--------------+---------+

                (encapsulation type omitted for formatting)

                    Table 4: NSH Weighted Service Path

   The information contained in Tables 1-4 may be received from the
   control plane, but the exact mechanism is outside the scope of this
   document.






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6.2.  Mapping the NSH to Network Topology

   As described above, the mapping of the SPI to network topology may
   result in a single path, or it might result in a more complex
   topology.  Furthermore, the SPI-to-overlay mapping occurs at each SFF
   independently.  Any combination of topology selection is possible.
   Please note, there is no requirement to create a new overlay topology
   if a suitable one already exists.  NSH packets can use any (new or
   existing) overlay, provided the requisite connectivity requirements
   are satisfied.

   Examples of mapping for a topology:

   1.  Next SF is located at SFFb with locator 2001:db8::1
       SFFa mapping: SPI=10 --> VXLAN-gpe, dst-ip: 2001:db8::1

   2.  Next SF is located at SFFc with multiple network locators for
       load-distribution purposes:
       SFFb mapping: SPI=10 --> VXLAN-gpe, dst_ip:203.0.113.1,
       203.0.113.2, 203.0.113.3, equal cost

   3.  Next SF is located at SFFd with two paths from SFFc, one for
       redundancy:
       SFFc mapping: SPI=10 --> VXLAN-gpe, dst_ip:192.0.2.10 cost=10,
       203.0.113.10, cost=20

   In the above example, each SFF makes an independent decision about
   the network overlay path and policy for that path.  In other words,
   there is no a priori mandate about how to forward packets in the
   network (only the order of services that must be traversed).

   The network operator retains the ability to engineer the network
   paths as required.  For example, the overlay path between SFFs may
   utilize traffic engineering, QoS marking, or ECMP, without requiring
   complex configuration and network protocol support to be extended to
   the service path explicitly.  In other words, the network operates as
   expected, and evolves as required, as does the service plane.

6.3.  Service Plane Visibility

   The SPI and SI serve an important function for visibility into the
   service topology.  An operator can determine what service path a
   packet is "on" and its location within that path simply by viewing
   NSH information (packet capture, IP Flow Information Export (IPFIX),
   etc.).  The information can be used for service scheduling and
   placement decisions, troubleshooting, and compliance verification.





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6.4.  Service Graphs

   While a given realized SFP is a specific sequence of Service
   Functions, the service, as seen by a user, can actually be a
   collection of SFPs, with the interconnection provided by Classifiers
   (in-service path, non-initial re-classification).  These internal re-
   Classifiers examine the packet at relevant points in the network,
   and, if needed, SPI and SI are updated (whether this update is a re-
   write, or the imposition of a new NSH with new values is
   implementation specific) to reflect the "result" of the
   classification.  These Classifiers may, of course, also modify the
   metadata associated with the packet.
   Section 2.1 of [RFC7665] describes Service Graphs in detail.

7.  Policy Enforcement with NSH

7.1.  NSH Metadata and Policy Enforcement

   As described in Section 2, NSH provides the ability to carry metadata
   along a service path.  This metadata may be derived from several
   sources.  Common examples include:

      Network nodes/devices: Information provided by network nodes can
      indicate network-centric information (such as VPN Routing and
      Forwarding (VRF) or tenant) that may be used by Service Functions
      or conveyed to another network node post service path egress.

      External (to the network) systems: External systems, such as
      orchestration systems, often contain information that is valuable
      for Service Function policy decisions.  In most cases, this
      information cannot be deduced by network nodes.  For example, a
      cloud orchestration platform placing workloads "knows" what
      application is being instantiated and can communicate this
      information to all NSH nodes via metadata carried in the Context
      Header(s).

      Service Functions: A Classifier co-resident with Service Functions
      often performs very detailed and valuable classification.

   Regardless of the source, metadata reflects the "result" of
   classification.  The granularity of classification may vary.  For
   example, a network switch, acting as a Classifier, might only be able
   to classify based on a 2-tuple, or based on a 5-tuple, while a
   Service Function may be able to inspect application information.
   Regardless of granularity, the classification information can be
   represented in the NSH.





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   Once the data is added to the NSH, it is carried along the service
   path.  NSH-aware SFs receive the metadata, and can use that metadata
   for local decisions and policy enforcement.  Figures 9 and 10
   highlight the relationship between metadata and policy.

                +-------+        +-------+        +-------+
                |  SFF  )------->(  SFF  |------->|  SFF  |
                +---+---+        +---+---+        +---+---+
                    ^                |                |
                  ,-|-.            ,-|-.            ,-|-.
                 /     \          /     \          /     \
                ( Class )        (  SF1  )        (  SF2  )
                 \ ify /          \     /          \     /
                  `---'            `---'            `---'
                 5-tuple:        Permit             Inspect
                 Tenant A        Tenant A           AppY
                 AppY

                       Figure 9: Metadata and Policy

               +-----+           +-----+            +-----+
               | SFF |---------> | SFF |----------> | SFF |
               +--+--+           +--+--+            +--+--+
                  ^                 |                  |
                ,-+-.             ,-+-.              ,-+-.
               /     \           /     \            /     \
              ( Class )         (  SF1  )          (  SF2  )
               \ ify /           \     /            \     /
                `-+-'             `---'              `---'
                  |              Permit            Deny AppZ
              +---+---+          employees
              |       |
              +-------+
              External
              system:
              Employee
              AppZ

                  Figure 10: External Metadata and Policy

   In both of the examples above, the Service Functions perform policy
   decisions based on the result of the initial classification: the SFs
   did not need to perform re-classification; instead, they rely on an
   antecedent classification for local policy enforcement.

   Depending on the information carried in the metadata, data privacy
   impact needs to be considered.  For example, if the metadata conveys
   tenant information, that information may need to be authenticated



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   and/or encrypted between the originator and the intended recipients
   (which may include intended SFs only); one approach to an optional
   capability to do this is explored in [NSH-ENCRYPT].  The NSH itself
   does not provide privacy functions, rather it relies on the transport
   encapsulation/overlay.  An operator can select the appropriate set of
   transport encapsulation protocols to ensure confidentiality (and
   other security) considerations are met.  Metadata privacy and
   security considerations are a matter for the documents that define
   metadata format.

7.2.  Updating/Augmenting Metadata

   Post-initial metadata imposition (typically, performed during initial
   service path determination), the metadata may be augmented or
   updated:

   1.  Metadata Augmentation: Information may be added to the NSH's
       existing metadata, as depicted in Figure 11.  For example, if the
       initial classification returns the tenant information, a
       secondary classification (perhaps co-resident with deep packet
       inspection (DPI) or server load balancing (SLB)) may augment the
       tenant classification with application information, and impose
       that new information in NSH metadata.  The tenant classification
       is still valid and present, but additional information has been
       added to it.

   2.  Metadata Update: Subsequent Classifiers may update the initial
       classification if it is determined to be incorrect or not
       descriptive enough.  For example, the initial Classifier adds
       metadata that describes the traffic as "Internet", but a security
       Service Function determines that the traffic is really "attack".
       Figure 12 illustrates an example of updating metadata.



















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               +-----+           +-----+            +-----+
               | SFF |---------> | SFF |----------> | SFF |
               +--+--+           +--+--+            +--+--+
                  ^                 |                  |
                ,---.             ,---.              ,---.
               /     \           /     \            /     \
              ( Class )         (  SF1  )          (  SF2  )
               \     /           \     /            \     /
                `-+-'             `---'              `---'
                  |              Inspect           Deny
              +---+---+          employees         employee+
              |       |          Class=AppZ        appZ
              +-------+
              External
              system:
              Employee

                     Figure 11: Metadata Augmentation

                +-----+           +-----+            +-----+
                | SFF |---------> | SFF |----------> | SFF |
                +--+--+           +--+--+            +--+--+
                   ^                 |                  |
                 ,---.             ,---.              ,---.
                /     \           /     \            /     \
               ( Class )         (  SF1  )          (  SF2  )
                \     /           \     /            \     /
                 `---'             `---'              `---'
              5-tuple:            Inspect             Deny
              Tenant A            Tenant A            attack
                                   --> attack

                        Figure 12: Metadata Update

7.3.  Service Path Identifier and Metadata

   Metadata information may influence the service path selection since
   the Service Path Identifier values can represent the result of
   classification.  A given SPI can be defined based on classification
   results (including metadata classification).  The imposition of the
   SPI and SI results in the packet being placed on the newly specified
   SFP at the position indicated by the imposed SPI and SI.

   This relationship provides the ability to create a dynamic service
   plane based on complex classification, without requiring each node to
   be capable of such classification or requiring a coupling to the
   network topology.  This yields Service Graph functionality as




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   described in Section 6.4.  Figure 13 illustrates an example of this
   behavior.

               +-----+           +-----+            +-----+
               | SFF |---------> | SFF |------+---> | SFF |
               +--+--+           +--+--+      |     +--+--+
                  |                 |         |        |
                ,---.             ,---.       |      ,---.
               /     \           / SF1 \      |     /     \
              (  SCL  )         (   +   )     |    (  SF2  )
               \     /           \SCL2 /      |     \     /
                `---'             `---'    +-----+   `---'
             5-tuple:            Inspect   | SFF |    Original
             Tenant A            Tenant A  +--+--+    next SF
                                  --> DoS     |
                                              V
                                            ,-+-.
                                           /     \
                                          (  SF10 )
                                           \     /
                                            `---'
                                             DoS
                                          "Scrubber"

             Legend:
             SCL = Service Classifier

                      Figure 13: Path ID and Metadata

   Specific algorithms for mapping metadata to an SPI are outside the
   scope of this document.

8.  Security Considerations

   NSH security must be considered in the contexts of the SFC
   architecture and operators' environments.  One important
   characteristic of NSH is that it is not an end-to-end protocol.  As
   opposed to a protocol that "starts" on a host and "ends" on a server
   or another host, NSH is typically imposed by a network device on
   ingress to the SFC domain and removed at the egress of the SFC
   domain.  As such, and as with any other network-centric protocols
   (e.g., IP Tunneling, Traffic Engineering, MPLS, or Provider-
   Provisioned Virtual Private Networks), there is an underlying trust
   in the network devices responsible for imposing, removing, and acting
   on NSH information.

   The following sections detail an analysis and present a set of
   requirements and recommendations in those two areas.



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8.1.  NSH Security Considerations from Operators' Environments

   Trusted Devices

      All Classifiers, SFFs and SFs (hereinafter referred to as "SFC
      devices") within an operator's environment are assumed to have
      been selected, vetted, and actively maintained; therefore, they
      are trusted by that operator.  This assumption differs from the
      oft held view that devices are untrusted, often referred to as the
      "zero-trust model".  Operators SHOULD regularly monitor (i.e.,
      continuously audit) these devices to help ensure compliant
      behavior.  This trust, therefore, extends into NSH operations: SFC
      devices are not, themselves, considered to be attack vectors.
      This assumption, and the resultant conclusion is reasonable since
      this is the very basis of an operator posture; the operator
      depends on this reality to function.  If these devices are not
      trusted, and indeed are compromised, almost the entirety of the
      operator's standard-based IP and MPLS protocol suites are
      vulnerable; therefore, the operation of the entire network is
      compromised.  Although there are well-documented monitoring-based
      methods for detecting compromise (such as included continuous
      monitoring and audit and log review), these may not be sufficient
      to contain damage by a completely compromised element.

      Methods and best practices to secure devices are also widely
      documented and outside the scope of this document.

   Single Domain Boundary

      As per [RFC7665], NSH is designed for use within a single
      administrative domain.  This scoping provides two important
      characteristics:

      i) Clear NSH boundaries

      NSH egress devices MUST strip the NSH headers before they send the
      users' packets or frames out of the NSH domain.

      Means to prevent leaking privacy-related information outside an
      administrative domain are natively supported by the NSH given that
      the last SFF of a service path will systematically remove the NSH
      encapsulation before forwarding a packet exiting the service path.

      The second step in such prevention is to filter the transport
      encapsulation protocol used by NSH at the domain edge.  The
      transport encapsulation protocol MUST be filtered and MUST NOT
      leave the domain edge.




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      Depending upon the transport encapsulation protocol used for NSH,
      this can be done either by completely blocking the transport
      encapsulation (e.g., if MPLS is the chosen NSH transport
      encapsulation protocol, it is therefore never allowed to leave the
      domain) or by examining the carried protocol with the transport
      encapsulation (e.g., if VXLAN-gpe is used as the NSH transport
      encapsulation protocol, all domain edges need to filter based on
      the carried protocol in the VXLAN-gpe.)

      The other consequence of this bounding is that ingress packets
      MUST also be filtered to prevent attackers from sending in NSH
      packets with service path identification and metadata of their own
      selection.  The same filters as described above for both the NSH
      at SFC devices and for the transport encapsulation protocol as
      general edge protections MUST be applied on ingress.

      In summary, packets originating outside the SFC-enabled domain
      MUST be dropped if they contain an NSH.  Similarly, packets
      exiting the SFC-enabled domain MUST be dropped if they contain an
      NSH.

      ii) Mitigation of external threats

      As per the trusted SFC device points raised above, given that NSH
      is scoped within an operator's domain, that operator can ensure
      that the environment and its transitive properties comply with
      that operator's required security posture.  Continuous audits for
      assurance are recommended with this reliance on a fully trusted
      environment.  The term "continuous audits" describes a method
      (automated or manual) of checking security-control compliance on a
      regular basis, at some set period of time.

8.2.  NSH Security Considerations from the SFC Architecture

   The SFC architecture defines functional roles (e.g., SFF), as well as
   protocol elements (e.g., Metadata).  This section considers each role
   and element in the context of threats posed in the areas of integrity
   and confidentiality.  As with routing, the distributed computation
   model assumes a distributed trust model.

   An important consideration is that NSH contains mandatory-to-mute
   fields, and further, the SFC architecture describes cases where other
   fields in NSH change, all on a possible SFP hop-by-hop basis.  This
   means that any cryptographic solution requires complex key
   distribution and life-cycle operations.






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8.2.1.  Integrity

   SFC devices

      SFC devices MAY perform various forms of verification on received
      NSH packets such as only accepting NSH packets from expected
      devices, checking that NSH SPI and SI values received from
      expected devices conform to expected values and so on.
      Implementation of these additional checks are a local matter and,
      thus, out of scope of this document.

   NSH Base and Service Path Headers

      Attackers who can modify packets within the operator's network may
      be able to modify the SFP, path position, and/or the metadata
      associated with a packet.

      One specific concern is an attack in which a malicious
      modification of the SPI/SI results in an alteration of the path to
      avoid security devices.  The options discussed in this section
      help thwart that attack, and so does the use of the optional
      "Proof of Transit" method [PROOF-OF-TRANSIT].

      As stated above, SFC devices are trusted; in the case where an SFC
      device is compromised, NSH integrity protection would be subject
      to forging (in many cases) as well.

      NSH itself does not mandate protocol-specific integrity
      protection.  However, if an operator deems protection is required,
      several options are viable:

      1.  SFF/SF NSH verification

          Although, strictly speaking, not integrity protection, some of
          the techniques mentioned above, such as checking expected NSH
          values are received from expected SFC device(s), can provide a
          form of verification without incurring the burden of a full-
          fledged integrity-protection deployment.

      2.  Transport Security

          NSH is always encapsulated by an outer transport encapsulation
          as detailed in Section 4 of this specification, and as
          depicted in Figure 1.  If an operator deems cryptographic
          integrity protection necessary due to their risk analysis,
          then an outer transport encapsulation that provides such
          protection [RFC6071], such as IPsec, MUST be used.




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          Although the threat model and recommendations of Section 5 of
          BCP 72 [RFC3552] would normally require cryptographic data
          origin authentication for the header, this document does not
          mandate such mechanisms in order to reflect the operational
          and technical realities of deployment.

          Given that NSH is transport independent, as mentioned above, a
          secure transport, such as IPsec can be used for carry NSH.
          IPsec can be used either alone or in conjunction with other
          transport encapsulation protocols, in turn, encapsulating NSH.

          Operators MUST ensure the selected transport encapsulation
          protocol can be supported by the transport encapsulation/
          underlay of all relevant network segments as well as SFFs,
          SFs, and SFC Proxies in the service path.

          If connectivity between SFC-enabled devices traverses the
          public Internet, then such connectivity MUST be secured at the
          transport encapsulation layer.  IPsec is an example of such a
          transport.

      3.  NSH Variable Header-Based Integrity

          Lastly, NSH MD Type 2 provides, via variable-length headers,
          the ability to append cryptographic integrity protection to
          the NSH packet.  The implementation of such a scheme is
          outside the scope of this document.

   NSH metadata

      As with the Base and Service Path Headers, if an operator deems
      cryptographic integrity protection needed, then an existing,
      standard transport protocol MUST be used since the integrity
      protection applies to entire encapsulated NSH packets.  As
      mentioned above, a risk assessment that deems data-plane traffic
      subject to tampering will apply not only to NSH but to the
      transport information; therefore, the use of a secure transport is
      likely needed already to protect the entire stack.

      If an MD Type 2 variable header integrity scheme is in place, then
      the integrity of the metadata can be ensured via that mechanism as
      well.









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8.2.2.  Confidentiality

   SFC devices

      SFC devices can "see" (and need to use) NSH information.

   NSH Base and Service Path Headers

      SPI and other base / service path information does not typically
      require confidentiality; however, if an operator does deem
      confidentiality to be required, then, as with integrity, an
      existing transport encapsulation that provides encryption MUST be
      utilized.

   NSH metadata

      An attacker with access to the traffic in an operator's network
      can potentially observe the metadata NSH carries with packets,
      potentially discovering privacy-sensitive information.

      Much of the metadata carried by NSH is not sensitive.  It often
      reflects information that can be derived from the underlying
      packet or frame.  Direct protection of such information is not
      necessary, as the risks are simply those of carrying the
      underlying packet or frame.

      Implementers and operators MUST be aware that metadata can have
      privacy implications, and those implications are sometimes hard to
      predict.  Therefore, attached metadata should be limited to that
      necessary for correct operation of the SFP.  Further, [RFC8165]
      defines metadata considerations that operators can take into
      account when using NSH.

      Protecting NSH metadata information between SFC components can be
      done using transport encapsulation protocols with suitable
      security capabilities, along the lines discussed above.  If a
      security analysis deems these protections necessary, then security
      features in the transport encapsulation protocol (such as IPsec)
      MUST be used.

      One useful element of providing privacy protection for sensitive
      metadata is described under the "SFC Encapsulation" area of the
      Security Considerations of [RFC7665].  Operators can and should
      use indirect identification for metadata deemed to be sensitive
      (such as personally identifying information), significantly
      mitigating the risk of a privacy violation.  In particular,
      subscriber-identifying information should be handled carefully,
      and, in general, SHOULD be obfuscated.



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      For those situations where obfuscation is either inapplicable or
      judged to be insufficient, an operator can also encrypt the
      metadata.  An approach to an optional capability to do this was
      explored in [NSH-ENCRYPT].  For other situations where greater
      assurance is desired, optional mechanisms such as
      [PROOF-OF-TRANSIT] can be used.

9.  IANA Considerations

9.1.  NSH Parameters

   IANA has created a new "Network Service Header (NSH) Parameters"
   registry.  The following subsections detail new registries within the
   "Network Service Header (NSH) Parameters" registry.

9.1.1.  NSH Base Header Bits

   There are five unassigned bits (U bits) in the NSH Base Header, and
   one assigned bit (O bit).  New bits are assigned via Standards Action
   [RFC8126].

   Bit 2 - O (OAM) bit
   Bit 3 - Unassigned
   Bits 16-19 - Unassigned

9.1.2.  NSH Version

   IANA has set up the "NSH Version" registry.  New values are assigned
   via Standards Action [RFC8126].

       +-------------+---------------------------------+-----------+
       | Version     | Description                     | Reference |
       +-------------+---------------------------------+-----------+
       | Version 00b | Protocol as defined by RFC 8300 | RFC 8300  |
       | Version 01b | Reserved                        | RFC 8300  |
       | Version 10b | Unassigned                      |           |
       | Version 11b | Unassigned                      |           |
       +-------------+---------------------------------+-----------+

                           Table 5: NSH Version











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9.1.3.  NSH MD Types

   IANA has set up the "NSH MD Types" registry, which contains 4-bit
   values.  MD Type values 0x0, 0x1, 0x2, and 0xF are specified in this
   document; see Table 6.  Registry entries are assigned via the "IETF
   Review" policy defined in RFC 8126 [RFC8126].

                +-----------+-----------------+-----------+
                | MD Type   | Description     | Reference |
                +-----------+-----------------+-----------+
                | 0x0       | Reserved        | RFC 8300  |
                |           |                 |           |
                | 0x1       | NSH MD Type 1   | RFC 8300  |
                |           |                 |           |
                | 0x2       | NSH MD Type 2   | RFC 8300  |
                |           |                 |           |
                | 0x3 - 0xE | Unassigned      |           |
                |           |                 |           |
                | 0xF       | Experimentation | RFC 8300  |
                +-----------+-----------------+-----------+

                          Table 6: MD Type Values

9.1.4.  NSH MD Class

   IANA has set up the "NSH MD Class" registry, which contains 16-bit
   values.  New allocations are to be made according to the following
   policies:

   0x0000 to 0x01ff: IETF Review
   0x0200 to 0xfff5: Expert Review

   IANA has assigned the values as follows:

        +------------------+------------------------+------------+
        | Value            | Meaning                | Reference  |
        +------------------+------------------------+------------+
        | 0x0000           | IETF Base NSH MD Class | RFC 8300   |
        |                  |                        |            |
        | 0xfff6 to 0xfffe | Experimental           | RFC 8300   |
        |                  |                        |            |
        | 0xffff           | Reserved               | RFC 8300   |
        +------------------+------------------------+------------+

                           Table 7: NSH MD Class

   A registry for Types for the MD Class of 0x0000 is defined in
   Section 9.1.5.



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   Designated Experts evaluating new allocation requests from the
   "Expert Review" range should principally consider whether a new MD
   class is needed compared to adding MD Types to an existing class.
   The Designated Experts should also encourage the existence of an
   associated and publicly visible registry of MD Types although this
   registry need not be maintained by IANA.

   When evaluating a request for an allocation, the Expert should verify
   that the allocation plan includes considerations to handle privacy
   and security issues associated with the anticipated individual MD
   Types allocated within this class.  These plans should consider, when
   appropriate, alternatives such as indirection, encryption, and
   limited-deployment scenarios.  Information that can't be directly
   derived from viewing the packet contents should be examined for
   privacy and security implications.

9.1.5.  NSH IETF-Assigned Optional Variable-Length Metadata Types

   The Type values within the IETF Base NSH MD Class, i.e., when the MD
   Class is set to 0x0000 (see Section 9.1.4), are the Types owned by
   the IETF.  Per this document, IANA has created a registry for the
   Type values for the IETF Base NSH MD Class called the "NSH IETF-
   Assigned Optional Variable-Length Metadata Types" registry, as
   specified in Section 2.5.1.

   The type values are assigned via Standards Action [RFC8126].

   No initial values are assigned at the creation of the registry.























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9.1.6.  NSH Next Protocol

   IANA has set up the "NSH Next Protocol" registry, which contains
   8-bit values.  Next Protocol values 0, 1, 2, 3, 4, and 5 are defined
   in this document (see Table 8).  New values are assigned via "Expert
   Review" as per [RFC8126].

               +---------------+--------------+-----------+
               | Next Protocol | Description  | Reference |
               +---------------+--------------+-----------+
               | 0x00          | Unassigned   |           |
               |               |              |           |
               | 0x01          | IPv4         | RFC 8300  |
               |               |              |           |
               | 0x02          | IPv6         | RFC 8300  |
               |               |              |           |
               | 0x03          | Ethernet     | RFC 8300  |
               |               |              |           |
               | 0x04          | NSH          | RFC 8300  |
               |               |              |           |
               | 0x05          | MPLS         | RFC 8300  |
               |               |              |           |
               | 0x06 - 0xFD   | Unassigned   |           |
               |               |              |           |
               | 0xFE          | Experiment 1 | RFC 8300  |
               |               |              |           |
               | 0xFF          | Experiment 2 | RFC 8300  |
               +---------------+--------------+-----------+

               Table 8: NSH Base Header Next Protocol Values

   Expert Review requests MUST include a single codepoint per request.
   Designated Experts evaluating new allocation requests from this
   registry should consider the potential scarcity of codepoints for an
   8-bit value, and check both for duplications and availability of
   documentation.  If the actual assignment of the Next Protocol field
   allocation reaches half of the range (that is, when there are 128
   unassigned values), IANA needs to alert the IESG.  At that point, a
   new more strict allocation policy SHOULD be considered.

10.  NSH-Related Codepoints

10.1.  NSH Ethertype

   An IEEE Ethertype, 0x894F, has been allocated for NSH.






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11.  References

11.1.  Normative References

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

   [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>.

   [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>.

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

11.2.  Informative References

   [NSH-BROADBAND-ALLOCATION]
              Napper, J., Kumar, S., Muley, P., Henderickx, W., and M.
              Boucadair, "NSH Context Header Allocation -- Broadband",
              Work in Progress, draft-napper-sfc-nsh-broadband-
              allocation-04, November 2017.

   [NSH-DC-ALLOCATION]
              Guichard, J., Smith, M., Kumar, S., Majee, S., Agarwal,
              P., Glavin, K., Laribi, Y., and T. Mizrahi, "Network
              Service Header (NSH) MD Type 1: Context Header Allocation
              (Data Center)", Work in Progress,
              draft-guichard-sfc-nsh-dc-allocation-07, August 2017.

   [NSH-ENCRYPT]
              Reddy, T., Patil, P., Fluhrer, S., and P. Quinn,
              "Authenticated and encrypted NSH service chains", Work in
              Progress, draft-reddy-sfc-nsh-encrypt-00, April 2015.









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   [PROOF-OF-TRANSIT]
              Brockners, F., Bhandari, S., Dara, S., Pignataro, C.,
              Leddy, J., Youell, S., Mozes, D., and T. Mizrahi, "Proof
              of Transit", Work in Progress, draft-brockners-proof-
              of-transit-04, October 2017.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <https://www.rfc-editor.org/info/rfc2784>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692,
              DOI 10.17487/RFC3692, January 2004,
              <https://www.rfc-editor.org/info/rfc3692>.

   [RFC6071]  Frankel, S. and S. Krishnan, "IP Security (IPsec) and
              Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
              DOI 10.17487/RFC6071, February 2011,
              <https://www.rfc-editor.org/info/rfc6071>.

   [RFC6291]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
              D., and S. Mansfield, "Guidelines for the Use of the "OAM"
              Acronym in the IETF", BCP 161, RFC 6291,
              DOI 10.17487/RFC6291, June 2011,
              <https://www.rfc-editor.org/info/rfc6291>.

   [RFC7325]  Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
              and C. Pignataro, "MPLS Forwarding Compliance and
              Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
              August 2014, <https://www.rfc-editor.org/info/rfc7325>.

   [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>.

   [RFC7676]  Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
              for Generic Routing Encapsulation (GRE)", RFC 7676,
              DOI 10.17487/RFC7676, October 2015,
              <https://www.rfc-editor.org/info/rfc7676>.





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   [RFC8165]  Hardie, T., "Design Considerations for Metadata
              Insertion", RFC 8165, DOI 10.17487/RFC8165, May 2017,
              <https://www.rfc-editor.org/info/rfc8165>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [RTG-ENCAP]
              Nordmark, E., Tian, A., Gross, J., Hudson, J., Kreeger,
              L., Garg, P., Thaler, P., and T. Herbert, "Encapsulation
              Considerations", Work in Progress,
              draft-ietf-rtgwg-dt-encap-02, October 2016.

   [SFC-CONTROL-PLANE]
              Boucadair, M., "Service Function Chaining (SFC) Control
              Plane Components & Requirements", Work in Progress,
              draft-ietf-sfc-control-plane-08, October 2016.

   [SFC-OAM-FRAMEWORK]
              Aldrin, S., Pignataro, C., Kumar, N., Akiya, N., Krishnan,
              R., and A. Ghanwani, "Service Function Chaining (SFC)
              Operation, Administration and Maintenance (OAM)
              Framework", Work in Progress,
              draft-ietf-sfc-oam-framework-03, September 2017.

   [VXLAN-GPE]
              Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
              Extension for VXLAN", Work in Progress,
              draft-ietf-nvo3-vxlan-gpe-05, October 2017.

Acknowledgments

   The authors would like to thank Sunil Vallamkonda, Nagaraj Bagepalli,
   Abhijit Patra, Peter Bosch, Darrel Lewis, Pritesh Kothari, Tal
   Mizrahi, and Ken Gray for their detailed reviews, comments, and
   contributions.

   A special thank you goes to David Ward and Tom Edsall for their
   guidance and feedback.

   Additionally, the authors would like to thank Larry Kreeger for his
   invaluable ideas and contributions, which are reflected throughout
   this document.

   Loa Andersson provided a thorough review and valuable comments; we
   thank him for that.



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   Reinaldo Penno deserves a particular thank you for his architecture
   and implementation work that helped guide the protocol concepts and
   design.

   The editors also acknowledge comprehensive reviews and respective
   useful suggestions by Med Boucadair, Adrian Farrel, Juergen
   Schoenwaelder, Acee Lindem, and Kathleen Moriarty.

   Lastly, David Dolson has provided significant review, feedback, and
   suggestions throughout the evolution of this document.  His
   contributions are very much appreciated.

Contributors

   This WG document originated as draft-quinn-sfc-nsh; the following are
   its coauthors and contributors along with their respective
   affiliations at the time of WG adoption.  The editors of this
   document would like to thank and recognize them and their
   contributions.  These coauthors and contributors provided invaluable
   concepts and content for this document's creation.

   o  Jim Guichard, Cisco Systems, Inc.
   o  Surendra Kumar, Cisco Systems, Inc.
   o  Michael Smith, Cisco Systems, Inc.
   o  Wim Henderickx, Alcatel-Lucent
   o  Tom Nadeau, Brocade
   o  Puneet Agarwal
   o  Rajeev Manur, Broadcom
   o  Abhishek Chauhan, Citrix
   o  Joel Halpern, Ericsson
   o  Sumandra Majee, F5
   o  David Melman, Marvell
   o  Pankaj Garg, Microsoft
   o  Brad McConnell, Rackspace
   o  Chris Wright, Red Hat, Inc.
   o  Kevin Glavin, Riverbed
   o  Hong (Cathy) Zhang, Huawei US R&D
   o  Louis Fourie, Huawei US R&D
   o  Ron Parker, Affirmed Networks
   o  Myo Zarny, Goldman Sachs
   o  Andrew Dolganow, Alcatel-Lucent
   o  Rex Fernando, Cisco Systems, Inc.
   o  Praveen Muley, Alcatel-Lucent
   o  Navindra Yadav, Cisco Systems, Inc.







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Authors' Addresses

   Paul Quinn (editor)
   Cisco Systems, Inc.

   Email: paulq@cisco.com


   Uri Elzur (editor)
   Intel

   Email: uri.elzur@intel.com


   Carlos Pignataro (editor)
   Cisco Systems, Inc.

   Email: cpignata@cisco.com

































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