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Network Service Header (NSH) Metadata Type 2 Variable-Length Context Headers
RFC 9263

Document Type RFC - Proposed Standard (August 2022)
Authors Yuehua Wei , Uri Elzur , Sumandra Majee , Carlos Pignataro , Donald E. Eastlake 3rd
Last updated 2022-08-09
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Document shepherd Greg Mirsky
Shepherd write-up Show Last changed 2021-09-01
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RFC 9263


Internet Engineering Task Force (IETF)                       Y. Wei, Ed.
Request for Comments: 9263                               ZTE Corporation
Category: Standards Track                                       U. Elzur
ISSN: 2070-1721                                                    Intel
                                                                S. Majee
                                                  Individual Contributor
                                                            C. Pignataro
                                                                   Cisco
                                                         D. Eastlake 3rd
                                                  Futurewei Technologies
                                                             August 2022

  Network Service Header (NSH) Metadata Type 2 Variable-Length Context
                                Headers

Abstract

   Service Function Chaining (SFC) uses the Network Service Header (NSH)
   (RFC 8300) to steer and provide context metadata (MD) with each
   packet.  Such metadata can be of various types, including MD Type 2,
   consisting of Variable-Length Context Headers.  This document
   specifies several such Context Headers that can be used within a
   Service Function Path (SFP).

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/rfc9263.

Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions Used in This Document
     2.1.  Terminology
     2.2.  Requirements Language
   3.  NSH MD Type 2 Format
   4.  NSH MD Type 2 Context Headers
     4.1.  Forwarding Context
     4.2.  Tenant ID
     4.3.  Ingress Network Node Information
     4.4.  Ingress Network Source Interface
     4.5.  Flow ID
     4.6.  Source and/or Destination Groups
     4.7.  Policy ID
   5.  Security Considerations
     5.1.  Forwarding Context
     5.2.  Tenant ID
     5.3.  Ingress Network Node Information
     5.4.  Ingress Node Source Interface
     5.5.  Flow ID
     5.6.  Source and/or Destination Groups
     5.7.  Policy ID
   6.  IANA Considerations
     6.1.  MD Type 2 Context Types
     6.2.  Forwarding Context Types
     6.3.  Flow ID Context Types
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The Network Service Header (NSH) [RFC8300] is the Service Function
   Chaining (SFC) encapsulation that supports the SFC architecture
   [RFC7665].  As such, the NSH provides the following key elements:

   1.  Service Function Path (SFP) identification

   2.  indication of location within an SFP

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

   [RFC8300] further defines two metadata formats (MD Types): 1 and 2.
   MD Type 1 defines the fixed-length, 16-octet metadata, whereas MD
   Type 2 defines a variable-length context format for metadata.  This
   document defines several common metadata Context Headers for use
   within NSH MD Type 2.  These supplement the Subscriber Identifier and
   Performance Policy MD Type 2 metadata Context Headers specified in
   [RFC8979].

   This document does not address metadata usage, updating/chaining of
   metadata, or other SFP functions.  Those topics are described in
   [RFC8300].

2.  Conventions Used in This Document

2.1.  Terminology

   This document uses the terminology defined in the SFC architecture
   [RFC7665] and the NSH [RFC8300].

2.2.  Requirements Language

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

3.  NSH MD Type 2 Format

   An NSH is composed of a 4-octet Base Header, a 4-octet Service Path
   Header, and optional Context Headers.  The Base Header identifies the
   MD Type in use:

      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 1: NSH Base Header

   Please refer to the NSH [RFC8300] for a detailed header description.

   When the Base Header specifies MD Type = 0x2, zero or more Variable-
   Length Context Headers MAY be added, immediately following the
   Service Path Header.  Figure 2 below depicts the format of the
   Context Header as defined in Section 2.5.1 of [RFC8300].

      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 2: NSH Variable-Length Context Headers

4.  NSH MD Type 2 Context Headers

   [RFC8300] specifies Metadata Class 0x0000 as IETF Base NSH MD Class.
   In this document, metadata types are defined for the IETF Base NSH MD
   Class.  The Context Headers specified in the subsections below are as
   follows:

   1.  Forwarding Context

   2.  Tenant ID

   3.  Ingress Network Node Information

   4.  Ingress Node Source Interface

   5.  Flow ID

   6.  Source and/or Destination Groups

   7.  Policy ID

4.1.  Forwarding Context

   This metadata context carries a network forwarding context, used for
   segregation and forwarding scope.  Forwarding context can take
   several forms depending on the network environment, for example,
   Virtual eXtensible Local Area Network (VXLAN) / Generic Protocol
   Extension for VXLAN (VXLAN-GPE) Virtual Network Identifier (VNID),
   VPN Routing and Forwarding (VRF) identification, or VLAN.

      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 = 0x0000    |  Type = 0x04  |U|  Length = 4 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CT=0x0 |             Reserved          |        VLAN ID        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: VLAN Forwarding Context

      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 = 0x0000    |  Type = 0x04  |U|  Length = 4 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CT=0x1 |Resv   |     Service VLAN ID   |    Customer VLAN ID   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: QinQ Forwarding Context

      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 = 0x0000    |  Type = 0x04  |U|  Length = 4 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CT=0x2 |   Reserved    |              MPLS VPN Label           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: MPLS VPN Forwarding Context

      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 = 0x0000    |  Type = 0x04  |U|  Length = 4 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CT=0x3 | Resv  |            Virtual Network Identifier         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 6: VNI Forwarding Context

      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 = 0x0000    |  Type = 0x04  |U|  Length = 8 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CT=0x4 |             Reserved                                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Session ID                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 7: Session ID Forwarding Context

   The fields are described as follows:

   Context Type (CT):  This 4-bit field that defines the interpretation
      of the Forwarding Context field.  Please see the IANA
      considerations in Section 6.2.  This document defines these CT
      values:

      0x0:  12-bit VLAN identifier [IEEE.802.1Q_2018].  See Figure 3.

      0x1:  24-bit double tagging identifiers.  A service VLAN tag
            followed by a customer VLAN tag [IEEE.802.1Q_2018].  The two
            VLAN IDs are concatenated and appear in the same order that
            they appeared in the payload.  See Figure 4.

      0x2:  20-bit MPLS VPN label [RFC3032] [RFC4364].  See Figure 5.

      0x3:  24-bit virtual network identifier (VNI) [RFC8926].  See
            Figure 6.

      0x4:  32-bit Session ID [RFC3931].  This is called Key in GRE
            [RFC2890].  See Figure 7.

   Reserved (Resv):  These bits in the context fields MUST be sent as
      zero and ignored on receipt.

4.2.  Tenant ID

   Tenant identification is often used for segregation within a multi-
   tenant environment.  Orchestration system-generated Tenant IDs are an
   example of such data.  This Context Header carries the value of the
   Tenant ID.  Virtual Tenant Network (VTN) [OpenDaylight-VTN] is an
   application that provides multi-tenant virtual networks on a
   Software-Defined Networking (SDN) controller.

      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 = 0x0000    |  Type = 0x05  |U|    Length   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                         Tenant ID                             ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 8: Tenant ID List

   The fields are described as follows:

   Length:  Indicates the length of the Tenant ID in octets (see
      Section 2.5.1 of [RFC8300]).

   Tenant ID:  Represents an opaque value pointing to orchestration
      system-generated Tenant ID.  The structure and semantics of this
      field are specific to the operator's deployment across its
      operational domain and are specified and assigned by an
      orchestration function.  The specifics of that orchestration-based
      assignment are outside the scope of this document.

4.3.  Ingress Network Node Information

   This Context Header carries a Node ID of the network node at which
   the packet entered the SFC-enabled domain.  This node will
   necessarily be a classifier [RFC7665].  In cases where the Service
   Path Identifier (SPI) identifies the ingress node, this Context
   Header is superfluous.

      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 = 0x0000    |  Type = 0x06  |U|   Length    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                        Node ID                                ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 9: Ingress Network Node ID

   The fields are described as follows:

   Length:  Indicates the length of the Node ID in octets (see
      Section 2.5.1 of [RFC8300]).

   Node ID:  Represents an opaque value of the ingress network Node ID.
      The structure and semantics of this field are deployment specific.
      For example, Node ID may be a 4-octet IPv4 address Node ID, a
      16-octet IPv6 address Node ID, a 6-octet MAC address, an 8-octet
      MAC address (64-bit Extended Unique Identifier (EUI-64)), etc.

4.4.  Ingress Network Source Interface

   This context identifies the ingress interface of the ingress network
   node.  The l2vlan (135), l3ipvlan (136), ipForward (142), and mpls
   (166) in [IANAifType] are examples of source interfaces.

      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 = 0x0000    |  Type = 0x07  |U|    Length   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                     Source Interface                          ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 10: Ingress Network Source Interface

   The fields are described as follows:

   Length:  Indicates the length of the Source Interface in octets (see
      Section 2.5.1 of [RFC8300]).

   Source Interface:  Represents an opaque value of the identifier of
      the ingress interface of the ingress network node.

4.5.  Flow ID

   Flow ID provides a field in NSH MD Type 2 to label packets belonging
   to the same flow.  For example, [RFC8200] defines IPv6 Flow Label as
   Flow ID.  Another example of Flow ID is how [RFC6790] defines an
   entropy label that is generated based on flow information in the MPLS
   network.  Absence of this field or a value of zero denotes that
   packets have not been labeled with a Flow ID.

      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 = 0x0000    |  Type = 0x08  |U| Length = 4  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CT=0x0 |   Reserved    |           IPv6 Flow ID                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 11: IPv6 Flow ID

      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 = 0x0000    |  Type = 0x08  |U| Length = 4  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |CT=0x1 |   Reserved    |        MPLS entropy label             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 12: MPLS Entropy Label

   The fields are described as follows:

   Length:  Indicates the length of the Flow ID in octets (see
      Section 2.5.1 of [RFC8300]).  For example, the IPv6 Flow Label in
      [RFC8200] is 20 bits long.  An entropy label in the MPLS network
      in [RFC6790] is also 20 bits long.

   Context Type (CT):  This 4-bit field that defines the interpretation
      of the Flow ID field.  Please see the IANA considerations in
      Section 6.3.  This document defines these CT values:

      0x0:  20-bit IPv6 Flow Label in [RFC8200].  See Figure 11.

      0x1:  20-bit entropy label in the MPLS network in [RFC6790].  See
            Figure 12.

   Reserved:  These bits in the context fields MUST be sent as zero and
      ignored on receipt.

4.6.  Source and/or Destination Groups

   Intent-based systems can use this data to express the logical
   grouping of source and/or destination objects.  [OpenStack] and
   [OpenDaylight] provide examples of such a system.  Each is expressed
   as a 32-bit opaque object.

      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 = 0x0000    |  Type = 0x09  |U|  Length=8   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Source Group                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Destination Group                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 13: Source/Destination Groups

   If there is no group information specified for the Source Group or
   Destination Group field, the field MUST be sent as zero and ignored
   on receipt.

4.7.  Policy ID

   Traffic handling policies are often referred to by a system-generated
   identifier, which is then used by the devices to look up the policy's
   content locally.  For example, this identifier could be an index to
   an array, a lookup key, or a database ID.  The identifier allows
   enforcement agents or services to look up the content of their part
   of the policy.

      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 = 0x0000    |  Type = 0x0A  |U|    Length   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                           Policy ID                           ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 14: Policy ID

   The fields are described as follows:

   Length:  Indicates the length of the Policy ID in octets (see
      Section 2.5.1 of [RFC8300]).

   Policy ID:  Represents an opaque value of the Policy ID.

   This Policy ID is a general Policy ID, essentially a key to allow
   Service Functions (SFs) to know which policies to apply to packets.
   Those policies generally will not have much to do with performance
   but rather with what specific treatment to apply.  It may, for
   example, select a URL filter data set for a URL filter or select a
   video transcoding policy in a transcoding SF.  The Performance Policy
   ID in [RFC8979] is described there as having very specific use and,
   for example, says that fully controlled SFPs would not use it.  The
   Policy ID in this document is for cases not covered by [RFC8979].

5.  Security Considerations

   A misbehaving node from within the SFC-enabled domain may alter the
   content of the Context Headers, which may lead to service disruption.
   Such an attack is not unique to the Context Headers defined in this
   document.  Measures discussed in Section 8 of [RFC8300] describes the
   general security considerations for protecting the NSH.  [RFC9145]
   specifies methods of protecting the integrity of the NSH metadata.
   If the NSH includes the Message Authentication Code (MAC) and
   Encrypted Metadata Context Header [RFC9145], the authentication of
   the packet MUST be verified before using any data.  If the
   verification fails, the receiver MUST stop processing the Variable-
   Length Context Headers and notify an operator.

   The security and privacy considerations for the 7 types of Context
   Headers specified above are discussed below.  Since NSH-ignorant SFs
   will never see the NSH, then even if they are malign, they cannot
   compromise security or privacy based on the NSH or any of these
   Context Headers; however, they could cause compromise based on the
   rest of the packet.  To the extent that any of these headers are
   included when they would be unneeded or have no effect, they provide
   a covert channel for the entity adding the Context Header to
   communicate a limited amount of arbitrary information to downstream
   entities within the SFC-enabled domain.

5.1.  Forwarding Context

   All of the Forwarding Context variants specified in this document
   (those with CT values between 0 and 4) merely repeat a field that is
   available in the packet encapsulated by the NSH.  These variants
   repeat that field in the NSH for convenience.  Thus, there are no
   special security or privacy considerations in these cases.  Any
   future new values of CT for the Forwarding Context must specify the
   security and privacy considerations for those extensions.

5.2.  Tenant ID

   The Tenant ID indicates the tenant to which traffic belongs and might
   be used to tie together and correlate packets for a tenant that some
   monitoring function could not otherwise group, especially if other
   possible identifiers were being randomized.  As such, it may reduce
   security by facilitating traffic analysis but only within the SFC-
   enabled domain where this Context Header is present in packets.

5.3.  Ingress Network Node Information

   The SFC-enabled domain manager normally operates the initial ingress/
   classifier node and is thus potentially aware of the information
   provided by this Context Header.  Furthermore, in many cases, the SPI
   that will be present in the NSH identifies or closely constrains the
   ingress node.  Also, in most cases, it is anticipated that many
   entities will be sending packets into an SFC-enabled domain through
   the same ingress node.  Thus, under most circumstances, this Context
   Header is expected to weaken security and privacy to only a minor
   extent and only within the SFC-enabled domain.

5.4.  Ingress Node Source Interface

   This Context Header is likely to be meaningless unless the Ingress
   Network Node Information Context Header is also present.  When that
   node information header is present, this source interface header
   provides a more fine-grained view of the source by identifying not
   just the initial ingress/classifier node but also the port of that
   node on which the data arrived.  Thus, it is more likely to identify
   a specific source entity or at least to more tightly constrain the
   set of possible source entities than just the node information
   header.  As a result, inclusion of this Context Header with the node
   information Context Header is potentially a greater threat to
   security and privacy than the node information header alone, but this
   threat is still constrained to the SFC-enabled domain.

5.5.  Flow ID

   The variations of this Context Header specified in this document
   simply repeat fields already available in the packet and thus have no
   special security or privacy considerations.  Any future new values of
   CT for the Flow ID must specify the security and privacy
   considerations for those extensions.

5.6.  Source and/or Destination Groups

   This Context Header provides additional information that might help
   identify the source and/or destination of packets.  Depending on the
   granularity of the groups, it could either (1) distinguish packets as
   part of flows from and/or to objects where those flows could not
   otherwise be easily distinguished but appear to be part of one or
   fewer flows or (2) group packet flows that are from and/or to an
   object where those flows could not otherwise be easily grouped for
   analysis or another purpose.  Thus, the presence of this Context
   Header with non-zero source and/or destination groups can, within the
   SFC-enabled domain, erode security and privacy to an extent that
   depends on the details of the grouping.

5.7.  Policy ID

   This Context Header carries an identifier that nodes in the SFC-
   enabled domain can use to look up policy to potentially influence
   their actions with regard to the packet carrying this header.  If
   there are no such decisions regarding their actions, then the header
   should not be included.  If there are such decisions, the information
   on which they are to be based needs to be included somewhere in the
   packet.  There is no reason for inclusion in this Context Header to
   have any security or privacy considerations that would not apply to
   any other plaintext way of including such information.  It may
   provide additional information to help identify a flow of data for
   analysis.

6.  IANA Considerations

6.1.  MD Type 2 Context Types

   IANA has assigned the following types (Table 1) from the "NSH IETF-
   Assigned Optional Variable-Length Metadata Types" registry available
   at [IANA-NSH-MD2].

         +=======+==================================+===========+
         | Value |           Description            | Reference |
         +=======+==================================+===========+
         | 0x04  |        Forwarding Context        | RFC 9263  |
         +-------+----------------------------------+-----------+
         | 0x05  |            Tenant ID             | RFC 9263  |
         +-------+----------------------------------+-----------+
         | 0x06  |     Ingress Network Node ID      | RFC 9263  |
         +-------+----------------------------------+-----------+
         | 0x07  |    Ingress Network Interface     | RFC 9263  |
         +-------+----------------------------------+-----------+
         | 0x08  |             Flow ID              | RFC 9263  |
         +-------+----------------------------------+-----------+
         | 0x09  | Source and/or Destination Groups | RFC 9263  |
         +-------+----------------------------------+-----------+
         | 0x0A  |            Policy ID             | RFC 9263  |
         +-------+----------------------------------+-----------+

                           Table 1: Type Values

6.2.  Forwarding Context Types

   IANA has created a new subregistry for "Forwarding Context Types" at
   [IANA-NSH-MD2] as follows.

   The registration policy is IETF Review.

     +=========+=========================================+===========+
     | Value   |               Description               | Reference |
     +=========+=========================================+===========+
     | 0x0     |          12-bit VLAN identifier         | RFC 9263  |
     +---------+-----------------------------------------+-----------+
     | 0x1     |    24-bit double tagging identifiers    | RFC 9263  |
     +---------+-----------------------------------------+-----------+
     | 0x2     |          20-bit MPLS VPN label          | RFC 9263  |
     +---------+-----------------------------------------+-----------+
     | 0x3     | 24-bit virtual network identifier (VNI) | RFC 9263  |
     +---------+-----------------------------------------+-----------+
     | 0x4     |            32-bit Session ID            | RFC 9263  |
     +---------+-----------------------------------------+-----------+
     | 0x5-0xE |                Unassigned               |           |
     +---------+-----------------------------------------+-----------+
     | 0xF     |                 Reserved                | RFC 9263  |
     +---------+-----------------------------------------+-----------+

                     Table 2: Forwarding Context Types

6.3.  Flow ID Context Types

   IANA has created a new subregistry for "Flow ID Context Types" at
   [IANA-NSH-MD2] as follows.

   The registration policy is IETF Review.

    +=========+==========================================+===========+
    | Value   |               Description                | Reference |
    +=========+==========================================+===========+
    | 0x0     |          20-bit IPv6 Flow Label          | RFC 9263  |
    +---------+------------------------------------------+-----------+
    | 0x1     | 20-bit entropy label in the MPLS network | RFC 9263  |
    +---------+------------------------------------------+-----------+
    | 0x2-0xE |                Unassigned                |           |
    +---------+------------------------------------------+-----------+
    | 0xF     |                 Reserved                 | RFC 9263  |
    +---------+------------------------------------------+-----------+

                      Table 3: Flow ID Context Types

7.  References

7.1.  Normative References

   [IANA-NSH-MD2]
              IANA, "Network Service Header (NSH) Parameters",
              <https://www.iana.org/assignments/nsh>.

   [IEEE.802.1Q_2018]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Network -- Bridges and Bridged Networks", July 2018,
              <https://ieeexplore.ieee.org/document/8403927>.

   [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>.

   [RFC3931]  Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
              "Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
              RFC 3931, DOI 10.17487/RFC3931, March 2005,
              <https://www.rfc-editor.org/info/rfc3931>.

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

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

   [RFC9145]  Boucadair, M., Reddy.K, T., and D. Wing, "Integrity
              Protection for the Network Service Header (NSH) and
              Encryption of Sensitive Context Headers", RFC 9145,
              DOI 10.17487/RFC9145, December 2021,
              <https://www.rfc-editor.org/info/rfc9145>.

7.2.  Informative References

   [IANAifType]
              IANA, "IANAifType-MIB DEFINITIONS", 2021,
              <https://www.iana.org/assignments/ianaiftype-mib>.

   [OpenDaylight]
              OpenDaylight, "Group Based Policy User Guide", 2021,
              <https://docs.opendaylight.org/en/stable-fluorine/user-
              guide/group-based-policy-user-
              guide.html?highlight=group%20policy#>.

   [OpenDaylight-VTN]
              OpenDaylight, "OpenDaylight VTN", 2021, <https://nexus.ope
              ndaylight.org/content/sites/site/org.opendaylight.docs/mas
              ter/userguide/manuals/userguide/bk-user-guide/
              content/_vtn.html>.

   [OpenStack]
              OpenStack, "GroupBasedPolicy", 2021,
              <https://wiki.openstack.org/wiki/GroupBasedPolicy>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <https://www.rfc-editor.org/info/rfc2890>.

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

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8926]  Gross, J., Ed., Ganga, I., Ed., and T. Sridhar, Ed.,
              "Geneve: Generic Network Virtualization Encapsulation",
              RFC 8926, DOI 10.17487/RFC8926, November 2020,
              <https://www.rfc-editor.org/info/rfc8926>.

   [RFC8979]  Sarikaya, B., von Hugo, D., and M. Boucadair, "Subscriber
              and Performance Policy Identifier Context Headers in the
              Network Service Header (NSH)", RFC 8979,
              DOI 10.17487/RFC8979, February 2021,
              <https://www.rfc-editor.org/info/rfc8979>.

Acknowledgments

   The authors would like to thank Paul Quinn, Behcet Sarikaya, Dirk von
   Hugo, Mohamed Boucadair, Gregory Mirsky, and Joel Halpern for
   providing invaluable concepts and content for this document.

Authors' Addresses

   Yuehua Wei (editor)
   ZTE Corporation
   No.50, Software Avenue
   Nanjing
   210012
   China
   Email: wei.yuehua@zte.com.cn

   Uri Elzur
   Intel
   Email: uri.elzur@intel.com

   Sumandra Majee
   Individual Contributor
   Email: Sum.majee@gmail.com

   Carlos Pignataro
   Cisco
   Email: cpignata@cisco.com

   Donald E. Eastlake, 3rd
   Futurewei Technologies
   2386 Panoramic Circle
   Apopka, FL 32703
   United States of America
   Phone: +1-508-333-2270
   Email: d3e3e3@gmail.com