Internet Engineering Task Force (IETF) B. Sarikaya

Internet Engineering Task Force (IETF) B. Sarikaya
Request for Comments: 8979
Category: Standards Track D. von Hugo
ISSN: 2070-1721 Deutsche Telekom
M. Boucadair
Orange
February 2021

Subscriber and Performance Policy Identifier Context Headers in the
Network Service Header (NSH)

Abstract

This document defines the Subscriber and Performance Policy
Identifier Context Headers. These Variable-Length Context Headers
can be carried in the Network Service Header (NSH) and are used to
inform Service Functions (SFs) of subscriber- and performance-related
information for the sake of policy enforcement and appropriate
Service Function Chaining (SFC) operations. The structure of each
Context Header and their use and processing by NSH-aware nodes are
described.

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/rfc8979.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

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

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. In particular, 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. As
discussed in Section 3, the use of obfuscated and non-persistent
identifiers is recommended.

Also, traffic steering by means of SFC may be driven, for example, by
Quality of Service (QoS) 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 the
identification, location, and sequencing of SF instances. Hence, the
need arises to provide downstream SFs with a performance policy
identifier in order for them to appropriately meet the QoS
requirements. This document also specifies a new NSH Context Header
(Section 4) to convey such policy identifiers.

The context information defined in this document can be applicable in
the context of mobile networks (particularly in the 3GPP-defined
(S)Gi interface) [CASE-MOBILITY]. Typically, because of the
widespread use of private IPv4 addresses in those networks, if the
SFs to be invoked are located after a NAT function, the
identification based on the internal IPv4 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, a mechanism to pass
the internal information past the NAT boundary may optimize 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 outside the scope of this document to include a
comprehensive list of deployments that may make use of the Context
Headers defined in the document.

Since subscriber 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
being multiplexed in one single Context Header. This approach avoids
a requirement for additional internal structure in the Context
Headers to specify whether an identifier refers to a subscriber or to
a policy.

This document does not make any assumptions 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 each SFC-enabled domain. This
document focuses on the data plane behavior. Control plane
considerations are out of the scope.

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

This document assumes the NSH is used exclusively within a single
administrative domain. This document follows the recommendations in
[RFC8300] for handling the Context Headers at both ingress and egress
SFC boundary nodes (i.e., to strip the entire NSH, including Context
Headers). Revealing any subscriber-related information to parties
outside the SFC-enabled domain is avoided by design. Accordingly,
the scope for privacy breaches and user tracking is limited to within
the SFC-enabled domain where the NSH is used. It is assumed that
appropriate mechanisms to monitor and audit an SFC-enabled domain to
detect misbehavior and to deter misuse are in place.

MTU considerations are discussed in Section 5.

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 Identifier 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 fields are described as follows:

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

Type: 0x00 (see Section 6).

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

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

Subscriber Identifier: Carries an opaque local identifier that is
assigned to a subscriber by a network operator.

While this document does not specify an internal structure for
these identifiers, it also does not provide any cryptographic
protection for them; any internal structure to the identifier
values chosen will thus be visible on the wire if no secure
transport encapsulation is used. Accordingly, in alignment with
Section 8.2.2 of [RFC8300], identifier values SHOULD be
obfuscated.

The Subscriber Identifier Context Header is used by SFs to enforce
per-subscriber policies (e.g., resource quota, customized filtering
profile, accounting). To that aim, network operators may rely on
identifiers that are generated from those used in legacy deployments
(e.g., Section 3.3 of [CASE-MOBILITY]). Alternatively, network
operators may use identifiers that are associated with customized
policy profiles that are preconfigured on SFs using an out-of-band
mechanism. Such a mechanism can be used to rotate the identifiers,
thus allowing for better unlinkability (Section 3.2 of [RFC6973]).
Such alternative methods may be suboptimal (e.g., scalability issues
induced by maintaining and processing finer granular profiles) or
inadequate for providing some per-subscriber policies. The
assessment of whether a method for defining a subscriber identifier
provides the required functionality and whether it is compatible with
the capabilities of the SFs at the intended performance level is
deployment specific.

The classifier and NSH-aware SFs MAY inject a Subscriber Identifier
Context Header as a function of a local policy. This local policy
should indicate the SFP(s) for which the Subscriber Identifier
Context Header will be added. 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) might depend on the nature
of the information in the Context Header. Parameters for sending
alarms, such as frequency, thresholds, and content of the alarm,
should be configurable.

The default behavior of intermediary NSH-aware nodes is 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.,
accepting only some lengths or types). These policies may also
indicate the behavior to be followed by an NSH-aware SF if the
validation checks fail (e.g., removing 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 will discard Subscriber Identifier Context Headers conveying
identifiers in all formats that are different from the one the SF is
instructed to expect.

Multiple Subscriber Identifier Context Headers MAY be present in the
NSH, each carrying a distinct opaque value but all pointing to the
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
[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 Identifier 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 selection of suitably distributed
SFC paths, SF instances, or applicable policy 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 Context Header allows for the
distributed enforcement of a per-service policy such as requiring an
SFP to only include specific SF instances (e.g., SFs located within
the same Data Center (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 standby operation of backup
capacity or the presence of SFs deployed in multiple instances may be
requested.

In an environment characterized by frequent changes of link and path
behavior (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.

The default behavior of intermediary NSH-aware nodes is 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 Context Header 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
needs 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 fields are described as follows:

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

Type: 0x01 (see Section 6).

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

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

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

5. MTU Considerations

As discussed in Section 5.6 of [RFC7665], the SFC architecture
prescribes that additional information be added to packets to:

* Identify SFPs. This is typically the NSH Base Header (Section 2.2
of [RFC8300]) and Service Path Header (Section 2.3 of [RFC8300]).

* Carry metadata such those defined in Sections 3 and 4.

* Steer the traffic along the SFPs: This is realized by means of
transport encapsulation.

This added information increases the size of the packet to be carried
along an SFP.

Aligned with Section 5 of [RFC8300], it is RECOMMENDED for network
operators to increase the underlying MTU so that NSH traffic is
forwarded within an SFC-enabled domain without fragmentation. The
available underlying MTU should be taken into account by network
operators when providing SFs with the required Context Headers to be
injected per SFP and the size of the data to be carried in these
Context Headers.

If the underlying MTU cannot be increased to accommodate the NSH
overhead, network operators may rely upon a transport encapsulation
protocol with the required fragmentation handling. The impact of
activating such feature on SFFs should be carefully assessed by
network operators (Section 5.6 of [RFC7665]).

When dealing with MTU issues, network operators should consider the
limitations of various transport encapsulations such as those
discussed in [INTAREA-TUNNELS].

6. IANA Considerations

IANA has assigned the following types from the "NSH IETF-Assigned
Optional Variable-Length Metadata Types" subregistry (0x0000 IETF
Base NSH MD Class) available at: <https://www.iana.org/assignments/
nsh>.

+=======+===============================+===========+
| Value | Description | Reference |
+=======+===============================+===========+
| 0x00 | Subscriber Identifier | [RFC8979] |
+-------+-------------------------------+-----------+
| 0x01 | Performance Policy Identifier | [RFC8979] |
+-------+-------------------------------+-----------+

Table 1: NSH IETF-Assigned Optional Variable-
Length Metadata Types Additions

7. Security Considerations

Data plane SFC-related security considerations, including privacy,
are discussed in Section 6 of [RFC7665] and Section 8 of [RFC8300].
In particular, Section 8.2.2 of [RFC8300] states that attached
metadata (i.e., Context Headers) should be limited to that necessary
for correct operation of the SFP. Section 8.2.2 of [RFC8300]
indicates that metadata considerations that operators can take into
account when using NSH are discussed in [RFC8165].

As specified in [RFC8300], means to prevent leaking privacy-related
information outside an SFC-enabled domain are natively supported by
the NSH given that the last SFF of an SFP will systematically remove
the NSH (and therefore the identifiers defined in this specification)
before forwarding a packet exiting the SFP.

Nodes that are involved in an SFC-enabled domain are assumed to be
trusted (Section 1.1 of [RFC8300]). Discussion of means to check
that only authorized nodes are traversed when a packet is crossing an
SFC-enabled domain is out of scope of this document.

Both Subscriber Identifier and Performance Policy Identifier Context
Headers carry opaque data. In particular, the Subscriber Identifier
Context Header is locally assigned by a network provider and can be
generated from some of the information that is already conveyed in
the original packets from a host (e.g., internal IP address) or other
information that is collected from various sources within an SFC-
enabled domain (e.g., line identifier). The structure of the
identifiers conveyed in these Context Headers is communicated only to
entitled NSH-aware nodes. Nevertheless, some structures may be
easily inferred from the headers if trivial structures are used
(e.g., IP addresses). As persistent identifiers facilitate tracking
over time, the use of indirect and non-persistent identification is
thus RECOMMENDED.

Moreover, the presence of multiple Subscriber Identifier Context
Headers in the same NSH allows a misbehaving node from within the
SFC-enabled domain to bind these identifiers to the same subscriber.
This can be used to track that subscriber more effectively. The use
of non-persistent (e.g., regularly randomized) identifiers as well as
the removal of the Subscriber Identifier Context Headers from the NSH
by the last SF making use of such headers soften this issue (see
"data minimization" discussed in Section 3 of [RFC8165]). Such
behavior is especially strongly recommended in case no encryption is
enabled.

A misbehaving node from within the SFC-enabled domain may alter the
content of Subscriber Identifier and Performance Policy Identifier
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] are to be followed. A
mechanism for NSH integrity is specified in [NSH-INTEGRITY].

If no secure transport encapsulation is enabled, the use of trivial
subscriber identifier structures, together with the presence of
specific SFs in a Service Function Chain, may reveal sensitive
information to every on-path device. Also, operational staff in
teams managing these devices could gain access to such user privacy-
affecting data. Such disclosure can be a violation of legal
requirements because such information should be available to very few
network operator personnel. Furthermore, 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 Subscriber and Performance Policy Identifier Context Headers
unless the SF actually uses the content of these headers for its
operation. As discussed in Section 8.2.2 of [RFC8300], subscriber-
identifying information should be obfuscated, and, if an operator
deems cryptographic integrity protection is needed, security features
in the transport encapsulation protocol (such as IPsec) must be used.
A mechanism for encrypting sensitive NSH data is specified in
[NSH-INTEGRITY]. This mechanism can be considered by network
operators when enhanced SF-to-SF security protection of NSH metadata
is required (e.g., to protect against compromised SFFs).

Some events are logged locally with notification alerts sent by NSH-
aware nodes to a Control Element. These events SHOULD be rate
limited.

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

[CASE-MOBILITY]
Haeffner, W., Napper, J., Stiemerling, M., Lopez, D. R.,
and J. Uttaro, "Service Function Chaining Use Cases in
Mobile Networks", Work in Progress, Internet-Draft, draft-
ietf-sfc-use-case-mobility-09, 1 January 2019,
<https://tools.ietf.org/html/draft-ietf-sfc-use-case-
mobility-09>.

[INTAREA-TUNNELS]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", Work in Progress, Internet-Draft, draft-
ietf-intarea-tunnels-10, 12 September 2019,
<https://tools.ietf.org/html/draft-ietf-intarea-tunnels-
10>.

[NSH-INTEGRITY]
Boucadair, M., Reddy.K, T., and D. Wing, "Integrity
Protection for the Network Service Header (NSH) and
Encryption of Sensitive Context Headers", Work in
Progress, Internet-Draft, draft-ietf-sfc-nsh-integrity-03,
22 January 2021, <https://tools.ietf.org/html/draft-ietf-
sfc-nsh-integrity-03>.

[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.

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

[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>.

[TS23401] 3GPP, "General Packet Radio Service (GPRS) enhancements
for Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access, Release 16", Version 16.5.0, TS 23.401,
December 2019.

[TS23501] 3GPP, "System architecture for the 5G System (5GS),
Release 16", Version 16.3.0, TS 23.501, December 2019.

Acknowledgements

Comments from Joel Halpern on a previous draft version and from
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, Erik Kline, Éric Vyncke, Robert Wilton, and
Magnus Westerlund for the IESG review.

Special thanks to Benjamin Kaduk for the careful review and
suggestions that enhanced this specification.

Authors' Addresses

Behcet Sarikaya

Email: [email protected]

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

Email: [email protected]

Mohamed Boucadair
Orange
3500 Rennes
France

Email: [email protected]