SFC B. Sarikaya
Internet-Draft Denpel Informatique
Intended status: Standards Track D. von Hugo
Expires: November 12, 2020 Deutsche Telekom
M. Boucadair
Orange
May 11, 2020
Service Function Chaining: Subscriber and Performance Policy
Identification Variable-Length Network Service Header (NSH) Context
Headers
draft-ietf-sfc-serviceid-header-07
Abstract
This document defines Subscriber and Performance Policy Identifiers
Network Service Header Variable-Length Context Headers to inform
Service Functions about subscriber- and service-related information
for the sake of policy enforcement and appropriate service function
chaining operations. The structure of each context header is
defined; their use and processing instructions by SFC-aware nodes are
explained.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 12, 2020.
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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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 . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This document discusses how to inform Service Functions (SFs,
[RFC7665]) about subscriber- and service-related 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. Nevertheless, 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.
Also, the enforcement of SFC-based differentiated traffic policies
may be inferred, 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 enforcement may be conditioned by QoS
requirements that will affect SF instance identification, location,
and sequencing. Hence, the need to supply a performance policy
identifier to downstream SFs to appropriately meet the service
requirements arises.
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SFs and SF Forwarders (SFFs) involved in a service chain have to
contribute to the respective service policy (QoS, for example)
requirements characterized by low transmission delay between each
other, by exposing a high availability of resources to process
function tasks, or by redundancy provided by stand-by machines for
seamless execution continuation in case of failures. These
requirements may be satisfied by means of control plane protocols,
but in some contexts, e.g., in networks where resources are very much
constrained, carrying QoS-related information directly in packets may
improve the overall SFC operation instead of relying upon the
potential complexity or adding overhead introduced by some SFC
control plane features. This information is typically included as a
context header in the NSH.
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 which for 3GPP network can be the Packet Data Network (PDN)
Gateway (PGW) as specified in [TS23401] in case of 4G or the User
Plane Function (UPF) facing the external Data Network (DN) [TS23501]
in case of 5G, respectively. 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. As such identifier one of already specified 3GPP
identifiers may serve (see, for example, those listed in [RFC8371]),
but it is out of scope of this document to include a comprehensive
list of deployment contexts which may make use of the context headers
defined in the document.
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 up to the control plane used for SFC within an SFC-
enabled domain. Expectations to SFC control plane protocols are laid
down, e.g., in [RFC8459], but specifications of SFC control plane
functions are also discussed in, for example,
[I-D.ietf-bess-nsh-bgp-control-plane]. Control plane related
considerations are out of scope.
The reader may refer to Section 3 of [RFC8300] for MTU
considerations. Such considerations are not reiterated here.
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This document assumes the NSH is used exclusively within a single
administrative domain.
This document adheres to the architecture defined in [RFC7665]. This
document assumes the reader is familiar with [RFC8300].
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.
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The subscriber identifier 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).
The classifier and SFC-aware SFs MAY be instructed via a control
interface to inject or strip a subscriber identifier context header.
Also, the data to be injected in such header SHOULD be configured to
nodes authorized to inject such headers. Typically, 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 by the
control plane.
This document adheres to the recommendations in [RFC8300] for
handling the context headers at both ingress and egress SFC boundary
nodes. That is, 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.
SFC-aware SFs and proxies MAY be instructed to strip a subscriber
identifier context header from the packet or to pass the data to the
next SF in the service chain after processing the content of the
context headers. If no instruction is provided, an intermediary SFC-
aware node MUST maintain such context headers so that the information
can be passed to next SFC-aware hops.
SFC-aware SFs MAY be instructed via the control plane about the
validation checks to run on the content of these context headers
(e.g., accept only some lengths) and the behavior to adopt. For
example, SFC-aware SFs may be instructed to ignore the context
header, to remove the context header from the packet, etc.
Nevertheless, this specification does not require nor preclude such
additional validation checks. These validation checks are
deployment-specific. If validation checks fail on a subscriber
identifier context header, an SFC-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 SFC-aware SF is
instructed to do so.
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Multiple subscriber Identifier context TLVs MAY be present in the NSH
each carrying a distinct opaque value but all pointing to the same
subscriber. When multiple subscriber identifier context TLVs are
present and an SF is instructed to strip the subscriber identifier
context header, that SF has to remove all subscriber identifier
context TLVs.
4. Performance Policy Identification NSH Variable-Length Context
Headers
Dedicated service-specific performance identifier is defined to
differentiate between services requiring specific treatment 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 is inserted in an NSH packet so
that downstream SFC-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
performance policy identifier is not helpful if the path computation
is centralized and a strict SFP is passed by means of SFC control
plane to 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 availablility 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 optional variable length
context header. Its structure is shown in Figure 2.
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Similar control plane considerations as those discussed in Section 3
are to be followed.
Multiple performance policy identifier context headers MAY be present
in the NSH; each carrying a distinct opaque value but all are
pointing to policies that need to be enforced for a flow. It is up
to the control plane to ensure that these policies are not
conflicting. When such conflict is detected by an SFC-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
semantic of this field is 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:
https://www.iana.org/assignments/nsh/nsh.xhtml#optional-variable-
length-metadata-types.
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+-------+-------------------------------+----------------+
| 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 and Performance Policy TLVs which may lead to
service disruption. Such attack is not unique to the TLVs defined in
this document; measures discussed in [RFC8300] are to be followed.
Also, integrity checks offered by the transport encapsulation can be
used to detect anomalies.
An SF maintaining logs for operational reasons MUST NOT log the
content of Subscriber and Performance Policy context headers received
in NSH packets if the SF does not use the content of that header 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.
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>.
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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>.
[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-bess-nsh-bgp-control-plane]
Farrel, A., Drake, J., Rosen, E., Uttaro, J., and L.
Jalil, "BGP Control Plane for NSH SFC", draft-ietf-bess-
nsh-bgp-control-plane-13 (work in progress), December
2019.
[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
Denpel Informatique
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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