Network Service Header (NSH)
draft-ietf-sfc-nsh-27

Service Function Chaining                                  P. Quinn, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                           U. Elzur, Ed.
Expires: April 8, 2018                                             Intel
                                                       C. Pignataro, Ed.
                                                                   Cisco
                                                         October 5, 2017


                      Network Service Header (NSH)
                         draft-ietf-sfc-nsh-26

Abstract

   This document describes a Network Service Header (NSH) imposed on
   packets or frames to realize service function paths.  The NSH also
   provides a mechanism for metadata exchange along the instantiated
   service paths.  The NSH is the SFC encapsulation required to support
   the Service Function Chaining (SFC) architecture (defined in
   RFC7665).

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-
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   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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   This Internet-Draft will expire on April 8, 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



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   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.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Applicability . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Definition of Terms . . . . . . . . . . . . . . . . . . .   5
     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 . . . . . . . . . . . . . . . . . . . . . .  12
       2.5.1.  Optional Variable Length Metadata . . . . . . . . . .  13
   3.  NSH Actions . . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  NSH Transport Encapsulation . . . . . . . . . . . . . . . . .  16
   5.  Fragmentation Considerations  . . . . . . . . . . . . . . . .  17
   6.  Service Path Forwarding with NSH  . . . . . . . . . . . . . .  17
     6.1.  SFFs and Overlay Selection  . . . . . . . . . . . . . . .  17
     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.  Transport Encapsulation Protocol Security . . . . . . . .  27
     8.2.  Boundary Protection . . . . . . . . . . . . . . . . . . .  27
     8.3.  Metadata Considerations . . . . . . . . . . . . . . . . .  28
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  29
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  30
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
     11.1.  Network Service Header (NSH) Parameters  . . . . . . . .  31
       11.1.1.  NSH Base Header Bits . . . . . . . . . . . . . . . .  31
       11.1.2.  NSH Version  . . . . . . . . . . . . . . . . . . . .  31
       11.1.3.  MD Type Registry . . . . . . . . . . . . . . . . . .  31
       11.1.4.  MD Class Registry  . . . . . . . . . . . . . . . . .  32
       11.1.5.  New IETF Assigned Optional Variable Length Metadata
                Type Registry  . . . . . . . . . . . . . . . . . . .  33
       11.1.6.  NSH Base Header Next Protocol  . . . . . . . . . . .  33
   12. NSH-Related Codepoints  . . . . . . . . . . . . . . . . . . .  34



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     12.1.  NSH EtherType  . . . . . . . . . . . . . . . . . . . . .  34
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     13.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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 wide area network, 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).

   The Network Service Header (NSH) specification defines a new protocol
   and associated encapsulation for the creation of dynamic service
   chains, operating at the service plane.  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:










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

              Figure 1: Network Service Header Encapsulation

   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 service function chaining encapsulation.  Figure 3 of [RFC7665]
   depicts the SFC architectural components after classification.  The
   NSH is the SFC encapsulation referenced in [RFC7665].

1.1.  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 RFC 2119 [RFC2119].

1.2.  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.






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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].

   Metadata:  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 post 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 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].




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   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].

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 OAM messages.  The NSH fields can be used by
       administrators (via, for example, a traffic analyzer) to verify
       (account, ensure correct chaining, provide reports, etc.) the
       path specifics 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 is communicated via a control plane, which is
       outside the scope of this document, to participating nodes.
       [I-D.ietf-sfc-control-plane] provides an example of such in
       Section 3.3.  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



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       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
       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 below.

      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

   Base Header Field Descriptions:

   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 understnad 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 Operations, Administration, and
   Maintenance (OAM, see [RFC6291]) packet.  The actual format and
   processing of SFC OAM packets is outside the scope of this
   specification (see for example [I-D.ietf-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 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 decrement by each SFF.  The handling of
   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 equal to 0x1, and MUST be 0x2 or greater for MD Type equal to
   0x2.  The length of the NSH 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 11.1.1.  Unassigned bits MUST
   be set to zero upon origination, and 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 Base Header and the Service Path Header.  MD Type defines
   the format of the metadata being carried.  Please see the IANA
   Considerations Section 11.1.3.

   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).

   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



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   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 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 service function chaining.
   Please see the IANA Considerations section below, Section 11.1.6.

   This document defines the following Next Protocol values:

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

   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.



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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): Identifies a service path.
   Participating nodes MUST use this identifier for Service Function
   Path 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 SI as appropriate
   (i.e., taking into account the length of the service function path).
   The Service Index MUST be decremented by a value of 1 by Service
   Functions or by SFC Proxy nodes after performing required services
   and 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 the with Service Path Identifier for
   Service Function Path 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 mechanism.  NSH packets, as described
   above, are discarded when an SFF decrements the TTL to 0.  In
   addition, an SFF which is not the terminal SFF for a Service Function
   Path will discard any NSH packet with an SI of 0, as there will be no
   valid next SF information.






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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 does not describe the structure or meaning of
   the included metadata.

   An SFC-aware SF or SFC Proxy needs to receive the data semantics
   first in order to process the data placed in the mandatory context
   field.  The data semantics include both the allocation schema and the
   meaning of the included data.  How an SFC-aware SF or SFC Proxy gets
   the data 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).

   [I-D.guichard-sfc-nsh-dc-allocation] and
   [I-D.napper-sfc-nsh-broadband-allocation] provide specific examples
   of how metadata can be allocated.

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 followed by the Service Path



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   Header are present.  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 Metadata                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 7: Variable Context Headers

   Metadata Class (MD Class): Defines the scope of the 'Type' field to
   provide a hierarchical namespace.  The IANA Considerations
   Section 11.1.4 defines how the MD Class values can be allocated to
   standards bodies, vendors, and others.

   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 MUST be ignored on receipt.

   Length: Indicates the length of the variable metadata, in bytes.  In
   case the metadata length is not an integer number of 4-byte words,



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   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 up the length field 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 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 context header together with its scoping (see e.g.,
   Section 3.3.3 of [I-D.ietf-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.

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



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       Path Header.  At the end of a service function path, an SFF MUST
       remove the NSH before forwarding or delivering the un-
       encapsulated packet.  It is therefore 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 Service
       Function Path.  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, as well
       as 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 Service Function Path,
       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
       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 MUST decrement the
       Service Index by one.



   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.





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   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.

   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.




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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 [I-D.ietf-rtgwg-dt-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.

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.



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   Furthermore, there is no requirement, or expectation of an SPI being
   bound to a pre-determined 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, and 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 service function
   paths.  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 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 below 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 a service function chain control plane responsibility, as
   per Table 2 and Table 3 below.

   Please note: VXLAN-gpe and GRE in the above table refer to
   [I-D.ietf-nvo3-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
   illustrated 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 some service function-centric state
   (including load, sessions 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 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, 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 service function path is a specific sequence
   of service functions, the service as seen by a user can actually be a
   collection of service function paths, with the interconnection
   provided by classifiers (in-service path, non-initial
   reclassification).  These internal reclassifiers 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.
   [RFC7665], Section 2.1 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 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 perform 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.  Figure 9 and Figure 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 a
   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 [I-D.reddy-sfc-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 DPI or 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"

                      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 is designed for use within a single operator environment.  As
   such, it does not include any mandatory security mechanisms.  As with
   many other protocols, by itself and without extensions, the NSH
   encapsulation can be spoofed and is subject to snooping and
   modification in transit.  However, the deployment scope (as defined
   in [RFC7665]) of the NSH encapsulation is limited to a single network
   administrative domain as a controlled environment, with trusted
   devices (e.g., a data center) hence mitigating the risk of
   unauthorized manipulation of the encapsulation headers or metadata.
   This controlled environment is an important assumption for NSH.
   There is one additional important assumption: All of the service
   functions used by an operator in service chains are assumed to be
   selected and vetted by the operator.

   An attacker with access to the traffic in an operators network can
   potentially observe the metadata NSH carries with packets,
   potentially discovering privacy sensitive information.  Similarly,



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   attackers who can modify packets within the operators network may be
   able to modify the service function path, path position, and / or the
   metadata associated with a packet.  If an attacker can compromise SFC
   Classifiers, Service Function Forwarders, or Service Functions, then
   they can inspect any of the NSH information.

8.1.  Transport Encapsulation Protocol Security

   NSH is always encapsulated in a transport encapsulation protocol
   between SFC components (as detailed in Section 4 of this
   specification).  In selecting the transport encapsulation protocol to
   use in a particular deployment, operators SHOULD evaluate the degree
   of protection from e.g., intermediate observation and modification
   that is needed.  Operators SHOULD then select a transport
   encapsulation protocol such as one that supports [RFC6071] to provide
   the needed protection (e.g., authenticity, confidentiality) for the
   traffic between SFC components.  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.

   One example where transport encapsulation protocol security is highly
   applicable is when an operator is using the public Internet to
   provide communication between two parts of their own administrative
   domain.

   [I-D.ietf-rtgwg-dt-encap] provides additional transport encapsulation
   considerations.

8.2.  Boundary Protection

   Given the potential sensitivity of NSH information, it is important
   that operators ensure that NSH encapsulated packets do not leave the
   operator domain.  The first step in such is that 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 doamin edge.  Depending
   upon the transport encapsulation protocol used for NSH, this can
   either be done by completely blocking the transport encapsulation
   (for example if MPLS is the chosen NSH transport encapsulation
   protocol, and is never allowed to leave the domain) or by examing the
   carried protocol with the transport encapsulation (for example if



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   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 sensitivity 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
   devices and for the 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.

8.3.  Metadata Considerations

   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.  Protection of traffic at that level is the responsibility of
   the end systems.

   Having said that, some of the metadata can be either privacy or
   operationally sensitive.  For example, mdofiying the metadata
   indicating the charging category of packets can cause subscribers to
   underpay or overpay the operator in certain environments.  The
   service path identification and position is often itself sensitive,
   since modification of that information can cause packets to avoid
   service functions the customer or operator intend the packet to
   visit.

   Protecting such information between SFC components can be done using
   transport encapsulation protocols with suitable security capabilites,
   along the lines discussed above.  Protecting the information at SFC
   components is more complicated as re-classifers are permitted to
   modify NSH fields (with the caveats noted above regarding the flag
   bits); SFFs read the service function path information and modify the
   service function path index; and in general service functions need to
   read, and potentially modify NSH metadata.

   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 privacy violation.  In particular, subscriber identifying



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   information should be handled carefully, and in general should be
   obfuscated.

   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
   [I-D.reddy-sfc-nsh-encrypt].  For other situations where greater
   assurance is desired, optional mechanisms such as
   [I-D.brockners-proof-of-transit] can be used.

   Lastly, SF security, although out of scope of this document, should
   be considered, particularly if an SF needs to access, authenticate,
   or update the NSH encapsulation or metadata.  However, again, the
   selection and placement of SFs is assumed to be bounded within the
   scope of a single administrative domain and therefore under direct
   control of the operator.

9.  Contributors

   This WG document originated as draft-quinn-sfc-nsh; the following are
   its co-authors 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 co-authors 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




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   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.

10.  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 review, 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.

   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
   suggestions by Med Boucadair, Adrian Farrel, Juergen Schoenwaelder,
   and Acee Lindem.



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

11.  IANA Considerations

11.1.  Network Service Header (NSH) Parameters

   IANA is requested to create a new "Network Service Header (NSH)
   Parameters" registry.  The following sub-sections request new
   registries within the "Network Service Header (NSH) Parameters "
   registry.

11.1.1.  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

11.1.2.  NSH Version

   IANA is requested to setup a registry of "NSH Version".  New values
   are assigned via Standards Action [RFC8126].

   Version 00b: Protocol as defined by this document.
   Version 01b: Reserved.  This document.
   Version 10b: Unassigned.
   Version 11b: Unassigned.

11.1.3.  MD Type Registry

   IANA is requested to set up a registry of "MD Types".  These are
   4-bit values.  MD Type values 0x0, 0x1, 0x2, and 0xF are specified in
   this document, see Table 5.  Registry entries are assigned by using
   the "IETF Review" policy defined in RFC 8126 [RFC8126].












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

                          Table 5: MD Type Values

11.1.4.  MD Class Registry

   IANA is requested to set up a registry of "MD Class".  These are
   16-bit values.  New allocations are to be made according to the
   following policies:

   0x0000 to 0x01ff: IETF Review
   0x0200 to 0xfff5: Expert Review
   0xfff6 to 0xfffe: Experimental
   0xffff: Reserved

   IANA is requested to assign the values as per Table 6::

         +-----------+-----------------------------+------------+
         | MD Class  | Meaning                     | Reference  |
         +-----------+-----------------------------+------------+
         | 0x0000    | IETF Base NSH MD Class      | This.I-D   |
         +-----------+-----------------------------+------------+

                          Table 6: MD Class Value

   A registry for Types for the MD Class of 0x0000 is defined in
   Section 11.1.5.

   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.





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   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.

11.1.5.  New IETF Assigned Optional Variable Length Metadata Type
         Registry

   The Type values within the IETF Base NSH MD Class, i.e., when the MD
   Class is set to 0x0000 (see Section 11.1.4), are the Types owned by
   the IETF.  This document requests IANA to create a registry for the
   Type values for the IETF Base NSH MD Class called the "IETF Assigned
   Optional Variable Length Metadata Type 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.

11.1.6.  NSH Base Header Next Protocol

   IANA is requested to set up a registry of "Next Protocol".  These are
   8-bit values.  Next Protocol values 0, 1, 2, 3, 4 and 5 are defined
   in this document (see Table 7.  New values are assigned via "Expert
   Reviews" as per [RFC8126].






















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             +---------------+--------------+---------------+
             | Next Protocol | Description  | Reference     |
             +---------------+--------------+---------------+
             | 0x0           | Unassigned   |               |
             |               |              |               |
             | 0x1           | IPv4         | This document |
             |               |              |               |
             | 0x2           | IPv6         | This document |
             |               |              |               |
             | 0x3           | Ethernet     | This document |
             |               |              |               |
             | 0x4           | NSH          | This document |
             |               |              |               |
             | 0x5           | MPLS         | This document |
             |               |              |               |
             | 0x6..0xFD     | Unassigned   |               |
             |               |              |               |
             | 0xFE          | Experiment 1 | This document |
             |               |              |               |
             | 0xFF          | Experiment 2 | This document |
             +---------------+--------------+---------------+

               Table 7: NSH Base Header Next Protocol Values

   Expert Review requests MUST include a single code point per request.
   Designated Experts evaluating new allocation requests from this
   registry should consider the potential scarcity of code points for an
   8-bit value, and check both for duplications as well as 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 this point,
   a new more strict allocation policy SHOULD be considered.

12.  NSH-Related Codepoints

12.1.  NSH EtherType

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

13.  References

13.1.  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>.




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   [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>.

13.2.  Informative References

   [I-D.brockners-proof-of-transit]
              Brockners, F., Bhandari, S., Dara, S., Pignataro, C.,
              Leddy, J., Youell, S., Mozes, D., and T. Mizrahi, "Proof
              of Transit", draft-brockners-proof-of-transit-03 (work in
              progress), March 2017.

   [I-D.guichard-sfc-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)", draft-guichard-sfc-nsh-dc-allocation-07
              (work in progress), August 2017.

   [I-D.ietf-nvo3-vxlan-gpe]
              Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
              Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-04 (work
              in progress), April 2017.

   [I-D.ietf-rtgwg-dt-encap]
              Nordmark, E., Tian, A., Gross, J., Hudson, J., Kreeger,
              L., Garg, P., Thaler, P., and T. Herbert, "Encapsulation
              Considerations", draft-ietf-rtgwg-dt-encap-02 (work in
              progress), October 2016.

   [I-D.ietf-sfc-control-plane]
              Boucadair, M., "Service Function Chaining (SFC) Control
              Plane Components & Requirements", draft-ietf-sfc-control-
              plane-08 (work in progress), October 2016.

   [I-D.ietf-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", draft-ietf-sfc-oam-framework-03 (work in
              progress), September 2017.




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   [I-D.napper-sfc-nsh-broadband-allocation]
              Napper, J., Kumar, S., Muley, P., Henderickx, W., and M.
              Boucadair, "NSH Context Header Allocation -- Broadband",
              draft-napper-sfc-nsh-broadband-allocation-03 (work in
              progress), July 2017.

   [I-D.reddy-sfc-nsh-encrypt]
              Reddy, T., Patil, P., Fluhrer, S., and P. Quinn,
              "Authenticated and encrypted NSH service chains", draft-
              reddy-sfc-nsh-encrypt-00 (work in progress), April 2015.

   [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>.

   [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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   [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>.

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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