Service Function Chaining: Subscriber and Performance Policy Identification Variable-Length Network Service Header (NSH) Context Headers
draft-ietf-sfc-serviceid-header-12

SFC                                                          B. Sarikaya
Internet-Draft
Intended status: Standards Track                             D. von Hugo
Expires: May 3, 2021                                    Deutsche Telekom
                                                            M. Boucadair
                                                                  Orange
                                                        October 30, 2020

      Service Function Chaining: Subscriber and Performance Policy
  Identification Variable-Length Network Service Header (NSH) Context
                                Headers
                   draft-ietf-sfc-serviceid-header-12

Abstract

   This document defines two Variable-Length Context Headers that can be
   carried in Network Service Header: Subscriber and Performance Policy
   Identifiers.  These Context Headers are used to inform Service
   Functions about subscriber- and performance-related information for
   the sake of policy enforcement and appropriate service function
   chaining operations.  The structure of each Context Header, their use
   and processing by NSH-aware nodes are described.

Status of This Memo

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   This Internet-Draft will expire on May 3, 2021.

Copyright Notice

   Copyright (c) 2020 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

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  Subscriber Identification NSH Variable-Length Context Header    4
   4.  Performance Policy Identification NSH Variable-Length Context
       Headers . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document discusses how to inform Service Functions (SFs)
   [RFC7665] about subscriber and service policy information, when
   required, for the sake of policy enforcement within a single
   administrative domain.  Particularly, subscriber-related information
   may be required to enforce subscriber-specific SFC-based traffic
   policies.  However, the information carried in packets may not be
   sufficient to unambiguously identify a subscriber.  This document
   fills this void by specifying a new Network Service Header (NSH)
   [RFC8300] Context Header to convey and disseminate such information
   within the boundaries of a single administrative domain (Section 3).

   Also, traffic steering by means of SFC may be driven, for example, by
   QoS (Quality of Service) considerations.  Typically, QoS information
   may serve as an input for the computation, establishment, and
   selection of the Service Function Path (SFP).  Furthermore, the
   dynamic structuring of service function chains and their subsequent
   SFPs may be conditioned by QoS requirements that will affect SF
   instance(s) identification, location, and sequencing.  Hence, the
   need to supply a performance policy identifier to downstream SFs to
   appropriately meet the service requirements arises.  This information
   is proposed to be included as a Context Header in the NSH
   (Section 4).

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   The context information defined in this document can be applicable in
   the context of mobile networks (particularly, in the 3GPP defined
   (S)Gi Interface) [I-D.ietf-sfc-use-case-mobility].  Typically,
   because of the widespread use of private addressing in those
   networks, if SFs to be invoked are located after a NAT function, the
   identification based on the internal IP address is not possible once
   the NAT has been crossed.  NAT functionality can reside in a distinct
   node.  For a 4G 3GPP network, that node can be the Packet Data
   Network (PDN) Gateway (PGW) as specified in [TS23401].  For a 5G 3GPP
   network, it can be the User Plane Function (UPF) facing the external
   Data Network (DN) [TS23501].  As such, means to allow passing the
   internal information may optimise packet traversal within an SFC-
   enabled mobile network domain.  Furthermore, some SFs that are not
   enabled on the PGW/UPF may require a subscriber identifier to
   properly operate (see, for example, those listed in [RFC8371]).  It
   is out of the scope of this document to include a comprehensive list
   of deployments which may make use of the Context Headers defined in
   the document.

   Since subscribers identifiers are distinct from those used to
   identify a performance policy and given that multiple policies may be
   associated with a single subscriber within a service function chain,
   these identifiers are carried in distinct Context Headers rather than
   multiplexing them in one single Context Header.  This approach avoids
   the requirement of an additional internal structures of the Context
   Headers to decide unambiguously whether an identifier refers to a
   subscriber or to a policy.

   This document does not make any assumption about the structure of
   subscriber or performance policy identifiers; each such identifier is
   treated as an opaque value.  The semantics and validation of these
   identifiers are policies local to an SFC-enabled domain.  This
   document focuses on the data plane behaviour.  Control plane
   considerations are out of the scope.

   The reader may refer to Section 3 of [RFC8300] for MTU
   considerations.  Such considerations are not reiterated here.

   This document assumes the NSH is used exclusively within a single
   administrative domain.

   This document adheres to the SFC data plane architecture defined in
   [RFC7665].  This document assumes the reader is familiar with
   [RFC8300].

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2.  Conventions and Terminology

   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.

   The reader should be familiar with the terms defined in [RFC7665].

   SFC Control Element refers to a logical entity that instructs one or
   more SFC data plane functional elements on how to process packets
   within an SFC-enabled domain.

3.  Subscriber Identification NSH Variable-Length Context Header

   Subscriber Identifier is defined as an optional variable-length NSH
   Context Header.  Its structure is shown in Figure 1.

       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   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                      Subscriber Identifier                    ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1: Subscriber Identifier Variable-Length Context Header

   The description of the fields is as follows:

   o  Metadata Class: MUST be set to 0x0 [RFC8300].

   o  Type: TBD1 (See Section 5).

   o  U bit: Unassigned bit (see Section 2.5.1 of [RFC8300]).

   o  Length: Indicates the length of the Subscriber Identifier, in
      bytes (see Section 2.5.1 of [RFC8300]).

   o  Subscriber Identifier: Carries an opaque subscriber identifier.

   The Subscriber Identifier Context Header is used to convey an
   identifier assigned by the service provider to uniquely identify a
   subscriber.  This subscriber identifier can be used by service
   functions to enforce per-subscriber policies (e.g., apply resource
   quota).

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   The classifier and NSH-aware SFs MAY inject or strip a Subscriber
   Identifier Context Header as a function of a local policy.  In order
   to prevent interoperability issues, the type and format of the
   identifiers to be injected in a Subscriber Identifier Context Header
   should be configured to nodes authorized to inject and consume such
   headers.  For example, a node can be instructed to insert such data
   following a type/set scheme (e.g., node X should inject subscriber ID
   type Y).  Other schemes may be envisaged.

   Failures to inject such headers should be logged locally while a
   notification alarm may be sent to a Control Element.  The details of
   sending notification alarms (i.e., the parameters affecting the
   transmission of the notification alarms depend on the information in
   the Context Header such as frequency, thresholds, and content of the
   alarm (full header, timestamp, etc.)) should be configurable.

   This document adheres to the recommendations in [RFC8300] for
   handling the Context Headers at both ingress and egress SFC boundary
   nodes (i.e., to strip such Context Headers).  Revealing any personal
   and subscriber-related information to third parties is avoided by
   design to prevent privacy breaches in terms of user tracking.

   Intermediary NSH-aware nodes have to preserve Subscriber Identifier
   Context Headers (i.e., the information can be passed to next hop NSH-
   aware nodes), but local policy may require an intermediary NSH-aware
   node to strip a Subscriber Identifier Context Header after processing
   it.

   NSH-aware SFs MUST ignore Context Headers carrying unknown subscriber
   identifiers.

   Local policies at NSH-aware SFs may require running additional
   validation checks on the content of these Context Headers (e.g.,
   accept only some lengths or types).  These policies may also indicate
   the behavior to follow by an NSH-aware SF if the validation checks
   fail (e.g., remove the Context Header from the packet).  These
   additional validation checks are deployment-specific.  If validation
   checks fail on a Subscriber Identifier Context Header, an NSH-aware
   SF MUST ignore that Context Header.  The event should be logged
   locally while a notification alarm may be sent to a Control Element
   if the NSH-aware SF is instructed to do so.  For example, an SF that
   expects an internal IP address as subscriber identifier will discard
   Subscriber Identifier Context Headers conveying Mobile Subscriber
   ISDN Number (MSISDN), International Mobile Subscriber Identity
   (IMSI), or malformed IP addresses.

   Multiple Subscriber Identifier Context Headers MAY be present in the
   NSH, each carrying a distinct opaque value but all pointing to the

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   same subscriber.  This may be required, e.g., by policy enforcement
   mechanisms in a mobile network where some SFs rely on IP addresses as
   subscriber Identifiers, while others use non-IP specific identifiers
   such as those listed in [RFC8371] and Section 3.3.2 of
   [I-D.ietf-sfc-use-case-mobility].  When multiple subscriber
   identifier Context Headers are present and an SF is instructed to
   strip the Subscriber Identifier Context Header, that SF MUST remove
   all Subscriber Identifier Context Headers.

4.  Performance Policy Identification NSH Variable-Length Context
    Headers

   Dedicated service-specific performance identifiers are defined to
   differentiate between services that require specific treatment in
   order to exhibit a performance characterized by, e.g., ultra-low
   latency (ULL) or ultra-high reliability (UHR).  Other policies can be
   considered when instantiating a service function chain within an SFC-
   enabled domain.  They are conveyed in the Performance Policy
   Identifier Context Header.

   The Performance Policy Identifier Context Header is inserted in an
   NSH packet so that downstream NSH-aware nodes can make use of the
   performance information for proper distributed SFC path selection, SF
   instance selection, or policy selection at SFs.  Note that the use of
   the Performance Policy Identifier is not helpful if the path
   computation is centralized and a strict SFP is presented as local
   policy to SF Forwarders (SFFs).

   The Performance Policy Identifier allows for the distributed
   enforcement of a per-service policy such as a service function path
   to only include specific SFs instances (e.g., SFs located within the
   same DC or those that are exposing the shortest delay from an SFF).
   Details of this process are implementation-specific.  For
   illustration purposes, an SFF may retrieve the details of usable SFs
   based upon the corresponding performance policy identifier.  Typical
   criteria for instantiating specific SFs include location,
   performance, or proximity considerations.  For the particular case of
   UHR services, the stand-by operation of back-up capacity or the
   deployment of multiple SF instances may be requested.

   In an environment characterised by frequent changes of link and path
   behaviour, for example due to variable load or availability caused by
   propagation conditions on a wireless link, the SFP may have to be
   adapted dynamically by on-the-move SFC path and SF instance
   selection.

   Performance Policy Identifier is defined as an optional variable
   length Context Header.  Its structure is shown in Figure 2.

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   Intermediary NSH-aware nodes have to preserve such Context Headers
   (i.e., the information can be passed to next hop NSH-aware nodes),
   but local policy may require an intermediary NSH-aware node to strip
   one after processing it.

   Multiple Performance Policy Identifier Context Headers MAY be present
   in the NSH, each carrying an opaque value for a distinct policy that
   need to be enforced for a flow.  Supplying conflicting policies may
   complicate the SFP computation and SF instance location.
   Corresponding rules to detect conflicting policies may be provided as
   a local policy to the NSH-aware nodes.  When such conflict is
   detected by an NSH-aware node, the default behavior of the node is to
   discard the packet and send a notification alarm to a Control
   Element.

       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   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                     Performance Policy Identifier             ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 2: Performance Policy Identifier Variable-Length Context
                                  Header

   The description of the fields is as follows:

   o  Metadata Class: MUST be set to 0x0 [RFC8300].

   o  Type: TBD2 (See Section 5).

   o  U bit: Unassigned bit (see Section 2.5.1 of [RFC8300]).

   o  Length: Indicates the length of the Performance Policy Identifier,
      in bytes (see Section 2.5.1 of [RFC8300]).

   o  Performance Policy Identifier: Represents an opaque value pointing
      to specific performance policy to be enforced.  The structure and
      semantics of this field are deployment-specific.

5.  IANA Considerations

   This document requests IANA to assign the following types from the
   "NSH IETF- Assigned Optional Variable-Length Metadata Types" (0x0000
   IETF Base NSH MD Class) registry available at:

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   https://www.iana.org/assignments/nsh/nsh.xhtml#optional-variable-
   length-metadata-types.

        +-------+-------------------------------+----------------+
        | Value | Description                   | Reference      |
        +-------+-------------------------------+----------------+
        | TBD1  | Subscriber Identifier         | [ThisDocument] |
        | TBD2  | Performance Policy Identifier | [ThisDocument] |
        +-------+-------------------------------+----------------+

6.  Security Considerations

   Data plane SFC-related security considerations, including privacy,
   are discussed in [RFC7665] and [RFC8300].

   Nodes that are involved in an SFC-enabled domain are assumed to be
   trusted ([RFC8300]).  Means to check that only authorized nodes are
   solicited when a packet is crossing an SFC-enabled domain are out of
   scope of this document.

   A misbehaving node within from the SFC-enabled domain may alter the
   content of Subscriber Identifier and Performance Policy Context
   Headers which may lead to service disruption.  Such attack is not
   unique to the Context Headers defined in this document; measures
   discussed in [RFC8300] are to be followed.

   Access to subscriber data usually requires specific access privilege
   levels.  To maintain that protection, an SF keeping operational logs
   should not log the content of a Subscriber and Performance Policy
   Context Headers unless the SF actually uses the content of these
   headers for its operation.

7.  Acknowledgements

   Comments from Joel Halpern on a previous version and by Carlos
   Bernardos are appreciated.

   Contributions and review by Christian Jacquenet, Danny Lachos,
   Debashish Purkayastha, Christian Esteve Rothenberg, Kyle Larose,
   Donald Eastlake, Qin Wu, Shunsuke Homma, and Greg Mirsky are
   thankfully acknowledged.

   Many thanks to Robert Sparks for the secdir review.

   Thanks to Barry Leiba for the IESG review.

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8.  References

8.1.  Normative References

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

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

   [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>.

8.2.  Informative References

   [I-D.ietf-sfc-use-case-mobility]
              Haeffner, W., Napper, J., Stiemerling, M., Lopez, D., and
              J. Uttaro, "Service Function Chaining Use Cases in Mobile
              Networks", draft-ietf-sfc-use-case-mobility-09 (work in
              progress), January 2019.

   [RFC8371]  Perkins, C. and V. Devarapalli, "Mobile Node Identifier
              Types for MIPv6", RFC 8371, DOI 10.17487/RFC8371, July
              2018, <https://www.rfc-editor.org/info/rfc8371>.

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

   [TS23401]  3GPP 23.401 16.5.0, "General Packet Radio Service (GPRS)
              enhancements for Evolved Universal Terrestrial Radio
              Access Network (E-UTRAN) access,", December 2019.

   [TS23501]  3GPP 23.501 16.3.0, "System architecture for the 5G System
              (5GS),", December 2019.

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Authors' Addresses

   Behcet Sarikaya

   Email: sarikaya@ieee.org

   Dirk von Hugo
   Deutsche Telekom
   Deutsche-Telekom-Allee 9
   D-64295 Darmstadt
   Germany

   Email: Dirk.von-Hugo@telekom.de

   Mohamed Boucadair
   Orange
   Rennes 3500
   France

   Email: mohamed.boucadair@orange.com

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