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Problem Statement for Service Function Chaining

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7498.
Authors Paul Quinn , Thomas Nadeau
Last updated 2018-12-20 (Latest revision 2015-02-19)
Replaces draft-quinn-sfc-problem-statement
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Joel M. Halpern
Shepherd write-up Show Last changed 2014-08-12
IESG IESG state Became RFC 7498 (Informational)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Alia Atlas
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
Network Working Group                                      P. Quinn, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                            T. Nadeau, Ed.
Expires: August 23, 2015                                         Brocade
                                                       February 19, 2015

              Service Function Chaining Problem Statement


   This document provides an overview of the issues associated with the
   deployment of service functions (such as firewalls, load balancers,
   etc.) in large-scale environments.  The term service function
   chaining is used to describe the definition and instantiation of an
   ordered list of instances of such service functions, and the
   subsequent "steering" of traffic flows through those service

   The set of enabled service function chains reflect operator service
   offerings and is designed in conjunction with application delivery
   and service and network policy.

   This document also identifies several key areas that the SFC working
   group will investigate to guide its architectural and protocol work
   and associated drafts.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 23, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Definition of Terms  . . . . . . . . . . . . . . . . . . .  3
   2.  Problem Space  . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Topological Dependencies . . . . . . . . . . . . . . . . .  6
     2.2.  Configuration complexity . . . . . . . . . . . . . . . . .  7
     2.3.  Constrained High Availability  . . . . . . . . . . . . . .  7
     2.4.  Consistent Ordering of Service Functions . . . . . . . . .  7
     2.5.  Application of Service Policy  . . . . . . . . . . . . . .  7
     2.6.  Transport Dependence . . . . . . . . . . . . . . . . . . .  8
     2.7.  Elastic Service Delivery . . . . . . . . . . . . . . . . .  8
     2.8.  Traffic Selection Criteria . . . . . . . . . . . . . . . .  8
     2.9.  Limited End-to-End Service Visibility  . . . . . . . . . .  8
     2.10. Per-Service Function (re)Classification  . . . . . . . . .  8
     2.11. Symmetric Traffic Flows  . . . . . . . . . . . . . . . . .  9
     2.12. Multi-vendor Service Functions . . . . . . . . . . . . . .  9
   3.  Service Function Chaining  . . . . . . . . . . . . . . . . . . 10
     3.1.  Service Overlay  . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Service Classification . . . . . . . . . . . . . . . . . . 10
     3.3.  SFC Encapsulation  . . . . . . . . . . . . . . . . . . . . 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

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1.  Introduction

   The delivery of end-to-end services often require various service
   functions including traditional network service functions (for
   example firewalls and server load balancers), as well as application-
   specific features such as http header manipulation.  Service
   functions may be delivered within the context of an isolated user
   (e.g. a tenant), or shared amongst many users/user groups.

   Current service function deployment models are often tightly coupled
   to network topology and physical resources resulting in relatively
   rigid and static deployments.  The static nature of such deployments
   greatly reduces, and in many cases, limits the ability of an operator
   to introduce new or modify existing services and/or service
   functions.  Furthermore there is a cascading effect: changing one (or
   more) elements of a service function chain often affects other
   elements in the chain and/or the network elements used to construct
   the chain.

   This issue is particular acute in elastic service environments that
   require relatively rapid creation, destruction or movement of
   physical or virtual service functions or network elements.
   Additionally, the transition to virtual platforms requires an agile
   service insertion model that supports elastic and very granular
   service delivery, post-facto modification and the movement of service
   functions and application workloads in the existing network.  The
   service insertion model must also retain the network and service
   policies and the ability to easily bind service policy to granular
   information such as per-subscriber state.

   This document outlines the problems encountered with existing service
   deployment models for Service Function Chaining (SFC) (often referred
   to simply as service chaining (in this document the terms will be
   used interchangeably), as well as the problems of service chain
   creation, deletion, modification/update, policy integration with
   service chains, and policy enforcement within the network
   infrastructure.  The document highlights three key areas of WG focus
   for addressing the issues highlighted in this draft that will form
   the basis for the possible WG solutions that address the current

1.1.  Definition of Terms

   Classification:  Locally instantiated matching of traffic flows
      against policy for subsequent application of the required set of
      network service functions.  The policy may be customer/network/
      service specific.

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   Network Overlay:  A logical network built, via virtual links or
      packet encapsulation, over an existing network (the underlay).

   Network Service:  An offering provided by an operator that is
      delivered using one or more service functions.  This may also be
      referred to as a composite service.  The term "service" is used to
      denote a "network service" in the context of this document.

      Note: Beyond this document, the term "service" is overloaded with
      varying definitions.  For example, to some a service is an
      offering composed of several elements within the operator's
      network, whereas for others a service, or more specifically a
      network service, is a discrete element such as a "firewall".
      Traditionally, such services (in the latter sense) host a set of
      service functions and have a network locator where the service is

   Service Function:  A function that is responsible for specific
      treatment of received packets.  A Service Function can act at
      various layers of a protocol stack (e.g., at the network layer or
      other OSI layers).  As a logical component, a Service Function can
      be realized as a virtual element or be embedded in a physical
      network element.  One or more Service Functions can be embedded in
      the same network element.  Multiple occurrences of the Service
      Function can exist in the same administrative domain.

      A non-exhaustive list of service functions includes: firewalls,
      WAN and application acceleration, Deep Packet Inspection (DPI),
      server load balancers, NAT44 [RFC3022], NAT64 [RFC6146], HTTP
      Header Enrichment functions, TCP optimizer.

      The generic term "L4-L7 services" is often used to describe many
      service functions.

   Service Function Chain (SFC):  A service function chain defines an
      ordered or partially ordered set of abstract service functions
      (SFs) and ordering constraints that must be applied to packets
      and/or frames and/or flows selected as a result of classification.
      An example of an abstract service function is "a firewall".  The
      implied order may not be a linear progression as the architecture
      allows for SFCs that copy to more than one branch, and also allows
      for cases where there is flexibility in the order in which service
      functions need to be applied.  The term service chain is often
      used as shorthand for service function chain.

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   Service Overlay:  An overlay network created for the purpose of
      forwarding data to required service functions.

   Service Topology:  The service overlay connectivity forms a service

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2.  Problem Space

   The following points describe aspects of existing service deployments
   that are problematic, and that the Service Function Chaining (SFC)
   working group aims to address.

2.1.  Topological Dependencies

   Network service deployments are often coupled to network topology,
   whether it be physical or virtualized, or a hybrid of the two.  For
   example, use of a firewall requires that traffic flow through the
   firewall, which require means placing the firewall on the network
   path (often via creation of VLANs), or architecting the network
   topology to steer traffic through the firewall.  Such dependency
   imposes constraints on service delivery, potentially inhibiting the
   network operator from optimally utilizing service resources, and
   reduces flexibility.  This limits scale, capacity, and redundancy
   across network resources.

   These topologies serve only to "insert" the service function (i.e.,
   ensure that traffic traverses a service function); they are not
   required from a native packet delivery perspective.  For example,
   firewalls often require an "in" and "out" layer-2 segment and adding
   a new firewall requires changing the topology (i.e., adding new
   layer-2 segments and/or IP subnets).

   As more service functions are required - often with strict ordering -
   topology changes are needed in "front" and "behind" each service
   function resulting in complex network changes and device
   configuration.  In such topologies, all traffic, whether a service
   function needs to be applied or not, often passes through the same
   strict order.

   The topological coupling limits placement and selection of service
   functions: service functions are "fixed" in place by topology and
   therefore placement and service function selection taking into
   account network topology information such as load, new links, or
   traffic engineering is often not possible.

   A common example is web servers using a server load balancer as the
   default gateway.  When the web service responds to non-load balanced
   traffic (e.g., administrative or backup operations) all traffic from
   the server must traverse the load balancer forcing network
   administrators to create complex routing schemes or create additional
   interfaces to provide an alternate topology.

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2.2.  Configuration complexity

   A direct consequence of topological dependencies is the complexity of
   the entire configuration, specifically in deploying service function
   chains.  Simple actions such as changing the order of the service
   functions in a service function chain require changes to the logical
   and/or physical topology.  However, network operators are hesitant to
   make changes to the network once services are installed, configured
   and deployed in production environments for fear of misconfiguration
   and consequent downtime.  All of this leads to very static service
   delivery deployments.  Furthermore, the speed at which these
   topological changes can be made is not rapid or dynamic enough as it
   often requires manual intervention, or use of slow provisioning

2.3.  Constrained High Availability

   Since traffic reaches many service functions based on network
   topology, alternate, or redundant service functions must be placed in
   the same topology as the primary service.

   An effect of topological dependency is constrained service function
   high availability.  Worse, when modified, inadvertent non-high
   availability or downtime can result.

2.4.  Consistent Ordering of Service Functions

   Service functions are typically independent; service function_1
   (SF1)...service function_n (SFn) are unrelated and there is no notion
   at the service layer that SF1 occurs before SF2.  However, to an
   administrator many service functions have a strict ordering that must
   be in place, yet the administrator has no consistent way to impose
   and verify the ordering of the service functions that are used to
   deliver a given service.  Furthermore, altering the order of a
   deployed chain is complex and cumbersome.

2.5.  Application of Service Policy

   Service functions rely on topology information such as VLANs or
   packet (re)classification to determine service policy selection,
   i.e., the service function specific action taken.  Topology
   information is increasingly less viable due to scaling, tenancy and
   complexity reasons.  Topology-centric information often does not
   convey adequate information to the service functions, forcing
   functions to individually perform more granular classification.  In
   other words, the topology information is not granular enough, and its
   semantics often overloaded.

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2.6.  Transport Dependence

   Service functions can and will be deployed in networks with a range
   of network transports, including network under and overlays, such as
   Ethernet, GRE, VXLAN, MPLS, etc.  The coupling of service functions
   to topology may require service functions to support many transport
   encapsulations or for a transport gateway function to be present.

2.7.  Elastic Service Delivery

   Given that the current state of the art for adding/removing service
   functions largely centers around VLANs and routing changes, rapid
   changes to the deployed service capacity (increasing or decreasing)
   can be hard to realize due to the risk and complexity of VLANs and/or
   routing modifications.

2.8.  Traffic Selection Criteria

   Traffic selection is coarse, that is, all traffic on a particular
   segment traverses all service functions whether the traffic requires
   service enforcement or not.  This lack of traffic selection is
   largely due to the topological nature of service deployment since the
   forwarding topology dictates how (and what) data traverses which
   service function(s).  In some deployments, more granular traffic
   selection is achieved using policy routing or access control
   filtering.  This results in operationally complex configurations and
   is still relatively coarse and inflexible.

2.9.  Limited End-to-End Service Visibility

   Troubleshooting service related issues is a complex process that
   involve both network-specific and service-specific expertise.  This
   is especially the case when service function chains span multiple
   DCs, or across administrative boundaries.  Furthermore, the physical
   and virtual environments (network and service), can be highly
   divergent in terms of topology and that topological variance adds to
   these challenges.

2.10.  Per-Service Function (re)Classification

   Classification occurs at each service function independent from
   previously applied service functions since there are limited
   mechanisms to share the detailed classification information between
   services.  The classification functionality often differs between
   service functions, and service functions may not leverage the
   classification results from other service functions.

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2.11.  Symmetric Traffic Flows

   Service function chains may be unidirectional or bidirectional
   depending on the state requirements of the service functions.  In a
   unidirectional chain traffic is passed through a set of service
   functions in one forwarding direction only.  Bidirectional chains
   require traffic to be passed through a set of service functions in
   both forwarding directions.  Many common service functions such as
   DPI and firewall often require bidirectional chaining in order to
   ensure flow state is consistent.

   Existing service deployment models provide a static approach to
   realizing forward and reverse service function chain association most
   often requiring complex configuration of each network device
   throughout the SFC.  In other words, the same complex network
   configuration must be in place for both "directions" of the traffic,
   effectively doubling the configuration and associated testing.
   Further, if partial symmetry is required (i.e. only some of the
   services in the chain required symmetry), the network configuration
   complexity increases since the operator must ensure that the
   exceptions -- the services that do not need the symmetry flow -- are
   handled correctly via unique configuration to account for their

2.12.  Multi-vendor Service Functions

   Deploying service functions from multiple vendors often require per-
   vendor expertise: insertion models differ, there are limited common
   attributes and inter-vendor service functions do not share
   information, hence the need for standards to ensure interoperability.

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3.  Service Function Chaining

   Service Function Chaining aims to address the aforementioned problems
   associated with service deployment.  Concretely, the SFC working
   group will investigate solutions that address the following elements:

3.1.  Service Overlay

   Service function chaining utilizes a service specific overlay that
   creates the service topology.  The service overlay provides service
   function connectivity, built "on top" of the existing network
   topology and allows operators to use whatever overlay or underlay
   they prefer to create a path between service functions, and to locate
   service functions in the network as needed.

   Within the service topology, service functions can be viewed as
   resources for consumption and an arbitrary topology constructed to
   connect those resources in a required order.  Adding new service
   functions to the topology is easily accomplished, and no underlying
   network changes are required.

   Lastly, the service overlay can provide service specific information
   needed for troubleshooting service related issues.

3.2.  Service Classification

   Classification is used to select which traffic enters a service
   overlay.  The granularity of the classification varies based on
   device capabilities, customer requirements, and services offered.
   Initial classification determines the service function chain required
   to process the traffic.  Subsequent classification can be used within
   a given service function chain to alter the sequence of service
   functions applied.  Symmetric classification ensures that forward and
   reverse chains are in place.  Similarly, asymmetric -- relative to
   required service function -- chains can be achieved via service

3.3.  SFC Encapsulation

   The SFC encapsulation enables the creation of a service chain in the
   data plane and can convey information about the chain such as chain
   identification and OAM status.

   The SFC encapsulation also carries data plane metadata which provides
   the ability to exchange information between logical classification
   points and service functions (and vice versa) and between service
   functions.  Metadata is not used as forwarding information to deliver
   packets along the service overlay.

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   Metadata can include the result of antecedent classification and/or
   information from external sources.  Service functions utilize
   metadata, as required, for localized policy decisions.

   In addition to sharing of information, the use of metadata addresses
   several of the issues raised in section 2, most notably by decoupling
   policy from the network topology, and by removing the need for per-
   service function classification (and re-classification) described in
   section 2.10.

   A common approach to service metadata creates a common foundation for
   interoperability between service functions, regardless of vendor.

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4.  IANA Considerations

   This document makes no request to IANA.

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5.  Security Considerations

   Although this problem statement does not introduce any protocols,
   when considering service function chaining, the three main areas
   begin investigated (see section 3) by the WG have security aspects
   that warrant consideration.

   Service Overlay:  The service overlay will be constructed using
      existing transport protocols (e.g.  MPLS, VXLAN) and as such is
      subject to the security specifics of the transport selected.  If
      an operator requires authenticity and/or confidentiality in the
      service overlay, a transport (e.g.  IPSec) that provides such
      functionally can be used.

   Classification:  Since classification is used to select the
      appropriate service overlay, and required service encapsulation
      details, classification policy must be both accurate and trusted.
      Conveying the policy to a SFC-edge device node may be done via a
      multitude of methods depending on an operator's existing
      provisioning practices and security posture.

      Additionally, traffic entering the SFC domain and being classified
      may be encrypted thus limiting the granularity of classification.
      The use of pervasive encryption varies based on type of traffic,
      environment and level of operator control.  For instance a large
      enterprise can mandate how encryption is used by its users,
      whereas a broadband provider likely does not have the ability to
      do so.

      The use of encrypted traffic however does not obviate the need for
      SFC (nor the problems associated with current deployment models
      described herein), rather when encrypted traffic must be
      classified, the granularity of such classification must adapt.  In
      such cases, service overlay selection might occur, for example,
      using outer (i.e. unencrypted) header information, on the presence
      of encryption, or via external information about the packets.

   SFC Encapsulation:  As described in section 3, the SFC encapsulation
      carries information about the SFC, and data plane metadata.
      Depending on environment and security posture, the SFC
      encapsulation might need to be authenticated and/or encrypted.
      The use of an appropriate overlay transport as described above can
      provide data plane confidentially and authenticity.

      The exchange of SFC encapsulation data such as metadata must
      originate from trusted source(s) and, if needed, be subject to
      authenticity and confidentiality during the exchange to the
      various SFC nodes.

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   SFC and Multi-tenancy:  If tenant isolation is required in an SFC
      deployment, an appropriate network transport overlay that provides
      adequate isolation and identification can be used.  Additionally,
      tenancy might be used in the selection of the appropriate service
      chain, however, as stated, the network overlay is still required
      to provide transport isolation.  SF deployment and how specific
      SFs might or might not be allocated per tenant is outside the
      scope of this document.

   The SFC Architecture draft present a more complete review of the
   security implications of a complete SFC architecture.

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6.  Contributors

   The following people are active contributors to this document and
   have provided review, content and concepts (listed alphabetically by

   Puneet Agarwal

   Mohamed Boucadair
   France Telecom

   Abhishek Chauhan

   Uri Elzur

   Kevin Glavin

   Ken Gray
   Cisco Systems

   Jim Guichard
   Cisco Systems

   Christian Jacquenet
   France Telecom

   Surendra Kumar
   Cisco Systems

   Nic Leymann
   Deutsche Telekom

   Darrel Lewis
   Cisco Systems

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   Rajeev Manur

   Brad McConnell

   Carlos Pignataro
   Cisco Systems

   Michael Smith
   Cisco Systems

   Navindra Yadav
   Cisco Systems

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7.  Acknowledgments

   The authors would like to thank David Ward, Rex Fernando, David
   McDysan, Jamal Hadi Salim, Charles Perkins, Andre Beliveau, Joel
   Halpern and Jim French for their reviews and comments.

   Additionally, the authors would like to thank the IESG and Benjamin
   Kaduk for their detailed reviews and suggestions.

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8.  Informative References

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

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Authors' Addresses

   Paul Quinn (editor)
   Cisco Systems, Inc.


   Thomas Nadeau (editor)


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