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Versions: 00 draft-quinn-sfc-arch

Network Working Group                                           P. Quinn
Internet-Draft                                               J. Guichard
Intended status: Informational                                  S. Kumar
Expires: March 27, 2014                                     C. Pignataro
                                                     Cisco Systems, Inc.
                                                      September 23, 2013


              Service Function Chaining (SFC) Architecture
                      draft-quinn-nsc-arch-00.txt

Abstract

   This document describes a standard architecture for the creation of
   Service Function Chains.  It includes architectural concepts,
   principles, and components used for the application of services in a
   network.  This document does not propose solutions or protocols.

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 http://datatracker.ietf.org/drafts/current/.

   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 March 27, 2014.

Copyright Notice

   Copyright (c) 2013 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
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Definition of Terms  . . . . . . . . . . . . . . . . . . .  3
     1.3.  Service Function Chaining  . . . . . . . . . . . . . . . .  4
   2.  Architectural Concepts . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Service Function Chains  . . . . . . . . . . . . . . . . .  5
     2.2.  Service Function Chain Symmetry  . . . . . . . . . . . . .  7
     2.3.  Service Function Paths . . . . . . . . . . . . . . . . . .  7
   3.  Service Function Chaining Architecture . . . . . . . . . . . .  8
     3.1.  Architecture Principles  . . . . . . . . . . . . . . . . .  8
     3.2.  Fundamental Components . . . . . . . . . . . . . . . . . .  8
   4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Existing Service Deployments  . . . . . . . . . . . . 15
   Appendix B.  Issues with Existing Deployments  . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17


























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1.  Introduction

   This document describes a standard architecture for the creation of
   Service Function Chains.  It includes architectural concepts,
   principles, and components used for the application of services in a
   network.  This document does not propose solutions or protocols.

1.1.  Scope

   The architecture described herein is assumed to be applicable to a
   single network administrative domain.  While it is possible for the
   principals and architectural components to be applied to inter-domain
   service function chains, these are left for future study.

1.2.  Definition of Terms

   Service Function (SF):  A network or application based packet
      treatment, application, compute or storage resource, used
      singularly or in concert with other service functions within a
      service chain to enable a service offered by a network operator.

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

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

   Service:  An offering provided by a network operator that is
      delivered using one or more service functions.  This may also be
      referred to as a composite service.

      Note: The term "service" is overloaded with varying definitions.
      For example, to some a service is an offering composed of several
      elements within the operators network whereas for others a
      service, or more specifically a network service, is a discrete
      element such as a firewall.  Traditionally, these network services
      host a set of service functions and have a network location where
      the service is delivered.

   Service Node (SN):  Physical or virtual element that hosts one or
      more service functions and has one or more network locaters
      associated with it for reachability and service delivery.







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   Service Function Chain (SFC):  The combination of a set of service
      functions that are to be applied to selected traffic in a specific
      order.

   Service Chain (SC):  A short form of Service Function Chain.

1.3.  Service Function Chaining

   Service Function Chaining is a concept that implies more than just an
   ordered set of service functions, rather it describes a method for
   deploying service functions that enables not only ordering but
   topological independence of those service functions as well as value
   addition.  A basic service function chain might simply utilize an
   existing overlay technology along with service specific forwarding in
   the network to steer traffic through the necessary service functions.
   However, additional information that is shared across a subset of
   service functions enables value added service functions and a richer
   service function chain.  For example, shared information, such as the
   results of a classification function, may be passed to downstream
   service functions to enable the offloading of service function
   processing.  As another example, sharing the information derived at
   one service function to the rest in the service chain would not only
   obviate the need to re-derive the same information but also
   simplifies the service as re-deriving may be impractical.



























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2.  Architectural Concepts

   The following sections describe the core principles of a service
   function chaining infrastructure.

2.1.  Service Function Chains

   In most networks services are constructed as a sequence of service
   functions that represent a Service Function Chain.  The collection of
   available service functions within an administrative domain forms a
   directed graph where the vertices represent an individual service
   function and the edges form the overlay connecting those vertices as
   partially represented in figure 1.


                                ,---.
                               /     \
                     +------->(   5   )
                     |         \     /
                     |          `---'
                     |
                     |
                ,---.+          ,---.               ,---.
               /     \         /     \             /     \
        +---->(  2   +------->(   6   )+--------->(   8   )
        |      \     /         \     /             \     /
        |       `-+-'           `---'               `---'
        |         |
        |         |
        |       ,-v-.
        |      /     \               ,---.
      ,-+-.   (   3   +             /     \
     /     \   \     /+------------>   7   +
    (   1   )   `---'<--------------+     /|
     \     /                         `---' |
      `---'                                |
                                           |
               ,---.                       +------->---.
              /     \                             /     \
             (   4   +--------------------------->   9   )
              \     /                             \     /
               +--^'                               `---'
               |  |
               +--+


              Figure 1: Service Function Chain Directed Graph




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   At a high level, service function chaining creates an abstracted view
   of a service and specifies the set of required service functions as
   well as the order in which they must be executed.  Sub-graphs, from
   the overall directed graph, define each Service Function Chain.
   Service functions can be part of none, one, or many service function
   chains.



      ,-+-.           ,---.            ,---.             ,---.
     /     \         /     \          /     \           /     \
    (   1   )+----->(   2   )+------>(   6   )+------> (   8   )
     \     /         \     /          \     /           \     /
      `---'           `---'            `---'             `---'

                 ,---.
                /     \
        +----->(   2   )
        |       \     /
        |        `---'
        |          +
      ,-+-.        |
     /     \       v
    (   1   )    ,---.          ,---.           ,---.
     \     /    /     \        /     \         /     \
      `---'    (   3   )+---->(   7   )+----->(   9   )
                \     /        \     /         \     /
                 `---'          `---'           `---'

                +---+
                |   |
                |   |
                |   v
                +---.
               /     \
        +----->   4   )
        |      \     /+
        |       `---' |
        |             |
      ,-+-.           |
     /     \          |
    (   1   )         |         ,---.
     \     /          |        /     \
      `---'           +------>(   9   )
                               \     /
                                `---'





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                Figure 2: Service Function Chain Sub-Graphs

2.2.  Service Function Chain Symmetry

   Service Function Chains may be unidirectional or bidirectional.  A
   unidirectional service function chain requires traffic to be
   forwarded through the ordered service functions in one direction (SF1
   -> SF2 -> SF3), whereas a bidirectional service function chain
   requires a symmetric path (SF1 -> SF2 -> SF3 and SF3 -> SF2 -> SF1).
   A hybrid service function chain has attributes of both bidirectional
   and unidirectional service function chains: some service functions
   require symmetric traffic, other service functions do not process
   reverse traffic.

2.3.  Service Function Paths

   Service function chains, when instantiated in the network, leads to
   the selection of specific instances of service functions at various
   SNs as well as the creation of the service topology using the network
   locator of each individual SN.  Thus, instantiation of the service
   function chain results in the creation of a Service Function Path and
   is used for forwarding packets through the service function chain.
   In other words, Service Function Path is the instantiation of the
   defined service function chain.

   This abstraction enables the binding of service function chains to
   specific service function instances based on a range of policy
   attributes defined by the operator.  For example, a service function
   chain definition might specify that one of the service function
   elements of the chain is a firewall.  However, on the network, there
   might exist a number of firewall service function elements (each
   providing the same policy enforcement) and only when the service
   function path is created is one of those firewall instances selected.
   The selection can be based on a range of policy attributes, ranging
   from simple to more elaborate criteria.
















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3.  Service Function Chaining Architecture

3.1.  Architecture Principles

   Service function chaining is predicated on several key architectural
   principles:

   1.  Topological independence: no changes to the underlying forwarding
       topology - implicit, or explicit - are needed to deploy service
       functions.

   2.  Consistent policy identifiers: per-service policy leverages
       policy identifier(s) that are consistent for the service function
       chain.

   3.  Classification: the start of a service function chain is defined
       by classification and only required traffic is forwarded through
       the service function chain.

   4.  Sharing of metadata/context: the network and service functions no
       longer exist in separate silos.  Metadata/context data can be
       shared amongst all participating nodes - network or service
       functions.

3.2.  Fundamental Components

   Service function chaining can be divided into several components that
   together form the basis of the architecture:

   1.  Service Functions as Resources: The concept of a service function
       evolves: rather than being viewed as a bump in the wire, a
       service function becomes a resource within a specified
       administrative domain that is available for consumption.  As
       such, service functions have a network locator and a variable set
       of attributes that describe the function offered.  The
       combination of locator and attributes are used to construct a
       service function chain.

   2.  Classifier: A component that performs traffic classification.
       Classification is the precursor to the start of a service
       function path: traffic that matches classification criteria is
       forwarded along a given service function path to realize the
       requirements of a service function chain.  The granularity of
       classification varies based on operator requirements and device
       capabilities.  While initial classification at a network node
       starts a service function path, subsequent classifications may
       occur along the service function chain and further alter the
       service function path.  This re-classification may also update



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       the context information (see below).

   3.  Overlay Service Topology: A service topology is created to
       interconnect the elements used to form the service function path.
       The overlay topology is specific to the service function path: it
       is created for the express purpose of steering the service
       packets through the service functions and optionally passing
       context data.  The overlay topology can be constructed using any
       existing transport, for example IP, MPLS, etc.

   4.  Control plane: The service function chaining control plane is
       responsible for constructing the service function paths:
       translating the service function chains to the forwarding paths
       and propagating path information to participating nodes - network
       and service - to achieve requisite forwarding behavior to
       construct the service overlay.  For instance, a service function
       chain construction may be static - using specific service
       function instances, or dynamic - choosing service function
       instances at the time of delivering traffic to the service
       function.  In service function chaining, service functions are
       resources; the control plane advertises their capabilities,
       availability and location.  The control plane is also responsible
       for the creation of the context (see below).

   5.  Shared context data: Sharing context data allows the network to
       provide network-derived information to the service functions, as
       well as enabling service function to service function information
       passing.  This component is optional.  Service function chaining
       infrastructure enables the exchange of this shared context along
       the service function path.  The shared context serves several key
       functions within the architecture:


       *  Allows elements that typically operate as ships-in-the-night
          to exchange information

       *  Encodes information about the network for post-service
          forwarding

       *  Creates an identifier used for policy binding by service
          functions


       Context information can be derived in several ways:

       *  External sources





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       *  Network node classification

       *  Service function classification

   The figure below provides a high level view of the components:



                                 +-------+
                     +----------+|control|+----------+
                     |           |plane  |           |
                     |           +---+---+           |
                     |               |               |
                     v               v               v
+----------+       ,---.           ,---.            ,---.           +----------+
|classifier|+---> /     \+------->/     \+-------->/     \+-------->|classifier|
|          |     (   1   )<-----+(   2   )<------+(   3   )<-------+|          |
+----------+      \     /         \     /          \     /          +----------+
                   `---'           `---'            `---'


             Figure 3: Service Function Chaining Architecture





























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4.  Summary

   Service function chains enable composite services that are
   constructed from one or more service functions.  This document
   provides a standard architecture, including architectural concepts,
   principles, and components, for the creation of Service function
   chains.












































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5.  Security Considerations

   This document does not define a new protocol and therefore creates no
   new security issues.















































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6.  Acknowledgments

   The authors would like to thank David Ward, Abhijit Patra and Nagaraj
   Bagepalli for their contributions.















































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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

7.2.  Informative References

   [NSCprob]  "Network Service Chaining Problem Statement", <http://
              datatracker.ietf.org/doc/
              draft-quinn-nsc-problem-statement/>.

   [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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Appendix A.  Existing Service Deployments

   Existing service insertion and deployment techniques fail to address
   new challenging requirements raised by modern network architectures
   and evolving technologies such as multi-tenancy, virtualization,
   elasticity, and orchestration.  Networks, servers, storage
   technologies, and applications, have all undergone significant change
   in recent years: virtualization, network overlays, and orchestration
   have increasingly become adopted techniques.  All of these have
   profound effects on network and services design.

   As network service functions evolve, operators are faced with an
   array of form factors - virtual and physical - as well as with a
   range of insertion methods that often vary by vendor and type of
   service.

   Such existing services are deployed using a range of techniques, most
   often associated with topology or forwarding modifications.  For
   example, firewalls often rely on layer-2 network changes for
   deployment: a VLAN is created for the "inside" interface, and another
   for the "outside" interface.  In other words, a new L2 segment was
   created simply to add a service function.  In the case of server load
   balancers, policy routing is often used to ensure traffic from
   server's returns to the load balancer.  As with the firewall example,
   the policy routing serves only to ensure that the network traffic
   ultimately flows to the service function(s).

   The network-centric information (e.g.  VLAN) is not limited to
   insertion; this information is often used as a policy identifier on
   the service itself.  So, on a firewall, the layer-2 segment
   identifies the local policy to be selected.  If more granular policy
   discrimination is required, more network identifiers must be created
   either per-hop, or communicated consistently to all services.


















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Appendix B.  Issues with Existing Deployments

   Due to the tight coupling of network and service function resources
   in existing networks, adding or removing service functions is a
   complex task that is fraught with risk and is tied to
   operationalizing topological changes leading to massively static
   configuration procedures for network service delivery or update
   purposes.  The inflexibility of such deployments limits (and in many
   cases precludes) dynamic service scaling (both horizontal and
   vertical) and requires hop-by-hop configuration to ensure that the
   correct service functions, and sequence of service functions are
   traversed.

   A non-exhaustive list of existing service deployment and insertion
   techniques as well as the issues associated with each may be found in
   [NSCprob].



































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Authors' Addresses

   Paul Quinn
   Cisco Systems, Inc.

   Email: paulq@cisco.com


   Jim Guichard
   Cisco Systems, Inc.

   Email: jguichar@cisco.com


   Surendra Kumar
   Cisco Systems, Inc.

   Email: smkumar@cisco.com


   Carlos Pignataro
   Cisco Systems, Inc.

   Email: cpignata@cisco.com



























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