Network Working Group                                    J. Halpern, Ed.
Internet-Draft                                                  Ericsson
Intended status: Standards Track                       C. Pignataro, Ed.
Expires: February 4, 2015                                          Cisco
                                                          August 3, 2014

              Service Function Chaining (SFC) Architecture


   This document describes an architecture for the specification,
   creation, and ongoing maintenance of Service Function Chains (SFC) in
   a network.  It includes architectural concepts, principles, and
   components used in the construction of composite services through
   deployment of SFCs.  This document does not propose solutions,
   protocols, or extensions to existing protocols.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Definition of Terms . . . . . . . . . . . . . . . . . . .   4
   2.  Architectural Concepts  . . . . . . . . . . . . . . . . . . .   6
     2.1.  Service Function Chains . . . . . . . . . . . . . . . . .   6
     2.2.  Service Function Chain Symmetry . . . . . . . . . . . . .   7
     2.3.  Service Function Paths  . . . . . . . . . . . . . . . . .   8
   3.  Architecture Principles . . . . . . . . . . . . . . . . . . .   9
   4.  Core SFC Architecture Components  . . . . . . . . . . . . . .  10
     4.1.  SFC Encapsulation . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Service Function (SF) . . . . . . . . . . . . . . . . . .  12
     4.3.  Service Function Forwarder (SFF)  . . . . . . . . . . . .  12
       4.3.1.  Transport Derived SFF . . . . . . . . . . . . . . . .  13
     4.4.  SFC-Enabled Domain  . . . . . . . . . . . . . . . . . . .  14
     4.5.  Network Components  . . . . . . . . . . . . . . . . . . .  14
     4.6.  SFC Proxy . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.7.  Classification  . . . . . . . . . . . . . . . . . . . . .  15
     4.8.  Re-Classification and Branching . . . . . . . . . . . . .  16
     4.9.  Shared Metadata . . . . . . . . . . . . . . . . . . . . .  16
   5.  Additional Architectural Concepts . . . . . . . . . . . . . .  17
     5.1.  The Role of Policy  . . . . . . . . . . . . . . . . . . .  17
     5.2.  SFC Control Plane . . . . . . . . . . . . . . . . . . . .  17
     5.3.  Resource Control  . . . . . . . . . . . . . . . . . . . .  18
     5.4.  Infinite Loop Detection and Avoidance . . . . . . . . . .  19
     5.5.  Load Balancing Considerations . . . . . . . . . . . . . .  19
     5.6.  MTU and Fragmentation Considerations  . . . . . . . . . .  20
     5.7.  SFC OAM . . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.8.  Resilience and Redundancy . . . . . . . . . . . . . . . .  22
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   7.  Contributors and Acknowledgments  . . . . . . . . . . . . . .  23
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   This document describes an architecture used for the creation and
   ongoing maintenance of Service Function Chains (SFC) in a network.
   It includes architectural concepts, principles, and components.

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   An overview of the issues associated with the deployment of end-to-
   end service function chains, abstract sets of service functions and
   their ordering constraints that create a composite service and the
   subsequent "steering" of traffic flows through said service
   functions, is described in [I-D.ietf-sfc-problem-statement].

   This architecture presents a model addressing the problematic aspects
   of existing service deployments, including topological independence
   and configuration complexity.

   Service function chains enable composite services that are
   constructed from one or more service functions.

1.1.  Scope

   This document defines a framework to enforce Service Function
   Chaining (SFC) with minimum requirements on the physical topology of
   the network.  The proposed solution relies on initial packet
   classification.  packets are initially classified at the entry point
   of an SFC-enabled network, and are then forwarded according to the
   ordered set of SF functions that need to be enabled to process these
   packets in the SFC-enabled domain.

   This document does not make any assumption on the deployment context.
   The proposed framework covers both fixed and mobile networks.

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

1.2.  Assumptions

   The following assumptions are made:

   o  Not all SFs can be characterized with a standard definition in
      terms of technical description, detailed specification,
      configuration, etc.

   o  There is no global or standard list of SFs enabled in a given
      administrative domain.  The set of SFs varies as a function of the
      service to be provided and according to the networking

   o  There is no global or standard SF chaining logic.  The ordered set
      of SFs that need to be enabled to deliver a given connectivity
      service is specific to each administrative entity.

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   o  The chaining of SFs and the criteria to invoke some of them are
      specific to each administrative entity that operates the SF-
      enabled network (also called administrative domain).

   o  SF chaining logic and related policies should not be exposed
      outside a given administrative domain.

   o  Several SF chaining policies can be simultaneously enforced within
      an administrative domain to meet various business requirements.

   o  No assumption is made on how FIBs and RIBs of involved nodes are

   o  How to bind the traffic to a given SF chaining is policy-based.

1.3.  Definition of Terms

   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: 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
        locator where the service is hosted.

   SFC Encapsulation:  The SFC Encapsulation provides at a minimum SFP
        identification, and is used by the SFC-aware functions, such as
        the SFF and SFC-aware SFs.  The SFC Encapsulation is not used
        for network packet forwarding.  In addition to SFP
        identification, the SFC encapsulation carries dataplane context
        information, also referred to as metadata.

   Classification:  Locally instantiated policy and customer/network/
        service profile matching of traffic flows for identification of
        appropriate outbound forwarding actions.

   Classifier:  An element that performs Classification.

   Service Function (SF):  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).  A Service Function can be a virtual
        element or be embedded in a physical network element.  One of

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        multiple Service Functions can be embedded in the same network
        element.  Multiple occurrences of the Service Function can be
        enabled in the same administrative domain.

        One or more Service Functions can be involved in the delivery of
        added-value services.  A non-exhaustive list of Service
        Functions includes: firewalls, WAN and application acceleration,
        Deep Packet Inspection (DPI),a LI (Lawful Intercept) module,
        server load balancers, NAT44 [RFC3022], NAT64 [RFC6146], NPTv6
        [RFC6296], HOST_ID injection, HTTP Header Enrichment functions,
        TCP optimizer, etc.

        An SF may be SFC encapsulation aware, that is it receives, and
        acts on information in the SFC encapsulation, or unaware in
        which case data forwarded to the service does not contain the
        SFC encapsulation.

   Service Function Forwarder (SFF):  A service function forwarder is
        responsible for delivering traffic received from the SFC network
        forwarder to one or more connected service functions via
        information carried in the SFC encapsulation.

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

   Service Function Path (SFP):  The SFP provides a level of indirection
        between the fully abstract notion of service chain as an
        abstract sequence of functions to be delivered, and the fully
        specified notion of exactly what SFF/SFs the packet will visit
        when it actually traverses the network.  By allowing the control
        components to specify the use of this level of indirection, the
        deployment may choose the degree of SFF/SF selection authority
        that is delegated to the network.

   Rendered Service Path (RSP):  The Service Function Path is a
        constrained specification of where packets using a certain
        service chain must go.  While it may be so constrained as to
        identify the exact locations, it can be not so specific.
        Packets themselves naturally are transmitted from and to
        specific places in the network, visiting a specific sequence of
        SFFs and SFs.  This sequence of actual visits by a packet to

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        specific SFFs and SFs in the network is known as the Rendered
        Service Path (RSP).  This definition is included here for use by
        later documents, such as when solutions may need to discuss this
        actual sequence of locations the packets visits.

   SFC-enabled Domain:  A network or region of a network that implements
        SFC.  An SFC-enabled Domain is limited to a single network
        administrative domain.

   SFC Proxy:  Removes and inserts SFC encapsulation on behalf of an
        SFC-unaware service function.  SFC proxies are logical elements.

2.  Architectural Concepts

   The following sections describe the foundational concepts of service
   function chaining and the SFC architecture.

   Service Function Chaining enables the creation of composite services
   that consist of an ordered set of Service Functions (SF) that must be
   applied to packets and/or frames selected as a result of
   classification.  Each SF is referenced using an identifier that is
   unique within an administrative domain.  No IANA registry is required
   to store the identity of SFs.

   Service Function Chaining is a concept that provides for more than
   just the application of an ordered set of SFs to selected traffic;
   rather, it describes a method for deploying SFs in a way that enables
   dynamic ordering and topological independence of those SFs as well as
   the exchange of metadata between participating entities.

2.1.  Service Function Chains

   In most networks services are constructed as an abstract sequence of
   SFs that represent an SFC.  At a high level, an SFC creates an
   abstracted view of a service and specifies the set of required SFs as
   well as the order in which they must be executed.  Graphs, as
   illustrated in Figure 1, define each SFC.  SFs can be part of zero,
   one, or many SFCs.  A given SF can appear one time or multiple times
   in a given SFC.

   SFCs can start from the origination point of the service function
   graph (i.e.: node 1 in Figure 1), or from any subsequent node in the
   graph.  SFs may therefore become branching nodes in the graph, with
   those SFs selecting edges that move traffic to one or more branches.
   SFCs can have more than one terminus.

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

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

     ,-+-.         ,---.          ,---.          ,---.          ,---.
    /     \       /     \        /     \        /     \        /     \
   (   1   )+--->(   7   )+---->(   8   )+---->(   4   )+---->(   7   )
    \     /       \     /        \     /        \     /        \     /
     `---'         `---'          `---'          `---'          `---'

                  Figure 1: Service Function Chain Graphs

2.2.  Service Function Chain Symmetry

   SFCs may be unidirectional or bidirectional.  A unidirectional SFC
   requires that traffic be forwarded through the ordered SFs in one
   direction (SF1 -> SF2 -> SF3), whereas a bidirectional SFC requires a
   symmetric path (SF1 -> SF2 -> SF3 and SF3 -> SF2 -> SF1).  A hybrid
   SFC has attributes of both unidirectional and bidirectional SFCs;
   that is to say some SFs require symmetric traffic, whereas other SFs
   do not process reverse traffic.

   SFCs may contain cycles; that is traffic may need to traverse more
   than once one or more SFs within an SFC.  Solutions will need to
   ensure suitable disambiguation for such situations.

   The architectural allowance that is made for SFPs that delegate
   choice to the network for which SFs or SFFs a packet will visit
   creates potential issues here.  A solution that allows such
   delegation needs to also describe how the solution ensures that those
   service chains that require service function chain symmetry can
   achieve that.

   Further, there are state tradeoffs in symmetry.  Symmetry may be
   realized in several ways depending on the SFF and classifier
   functionality.  In some cases, "mirrored" classification (S -> D and
   D -> S) policy may be deployed, whereas in others shared state
   between classifiers may be used to ensure that symmetric flows
   correctly identified, then steered along the required SFP.  At a
   high-level, there are various common cases.  In a non-exhaustive way,

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   there can be for example: a single classifier (or a small number of
   classifiers), in which case both incoming and outgoing flows could be
   recognized at a same classifier, so the synchronization would be
   feasible by internal mechanism implementation.  Another case is the
   one of stateful classifiers where several classifiers may be
   clustered and share state.  Lastly, another case entails fully
   distributed classifiers, where synchronization needs to be provided.
   This is a non-comprehensive list of common cases.

2.3.  Service Function Paths

   A service function path (SFP) is a mechanism used by service chaining
   to express the result of applying policy and operational constraints
   to the abstract requirements of a service chain (SFC).  This
   architecture does not mandate the degree of specificity of the SFP.
   Architecturally, within the same system some SFPs may be fully
   specified, selecting exactly which SFF and which SF packets using
   that SFP are to visit, while other SFPs may be quite vague, deferring
   to the SFF the decisions about the exact sequence steps to be used to
   realize the SFC.  The specificity may be anywhere in between these

   As an example of such an intermediate specificity, there may be two
   SFPs associated with a single SFC, where one SFP says essentially
   that any order of SFF and SF may be used as long as it is within data
   center 1, and where the second SFP allows the same latitude, but only
   within data center 2.

   Thus, the policies and logic of SFP selection or creation (depending
   upon the solution) produce what may be thought of as a constrained
   version of the original SFC.  Since there may be multiple policies
   which apply to different traffic that uses the same SFC, it also
   follows that there may be multiple SFP associated with a single SFC.

   The architecture allows for the same SF to be reachable through
   multiple SFFs.  In these cases, some SFPs may constrain which SFF are
   used to reach which SF, while some SFPs may leave that decision to
   the SFF themselves.

   Further, the architecture allows for two or more SFs to be attached
   to the same SFF, and possibly connected via internal means allowing
   more effective communication.  In these cases, some solutions or
   deployments may choose to use some form of internal inter-process or
   inter-VM messaging (communication behind the virtual switching
   element) that is optimized for such an environment.  This must be
   coordinated with the SFF handling so that the service function
   forwarding can properly perform is job.  Implementation details of
   such mechanisms are considered out-of-scope for this document, and

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   can include a spectrum of methods: for example situations including
   all next-hops explicitly, others where a list of possible next-hops
   is provided and the selection is local, or cases with just an
   indentifier, and all resolution is local.

   This architecture also allows the same SF to be part of multiple

3.  Architecture Principles

   Service function chaining is predicated on several key architectural

   1.  Topological independence: no changes to the underlay network
       forwarding topology - implicit, or explicit - are needed to
       deploy and invoke SFs or SFCs.

   2.  Plane separation: dynamic realization of SFPs is separated from
       packet handling operations (e.g., packet forwarding).

   3.  Classification: traffic that satisfies classification rules is
       forwarded according to a specific SFP.  For example,
       classification can be as simple as an explicit forwarding entry
       that forwards all traffic from one address into the SFP.
       Multiple classification points are possible within an SFC (i.e.
       forming a service graph) thus enabling changes/updates to the SFC
       by SFs.

       Classification can occur at varying degrees of granularity; for
       example, classification can use a 5-tuple, a transport port or
       set of ports, part of the packet payload, or it can come fro
       external systems.

   4.  Shared Metadata: Metadata/context data can be shared amongst SFs
       and classifiers, between SFs, and between external systems and
       SFs (e.g.  orchestration).

       Generally speaking, the metadata can be thought of as providing,
       and sharing the result of classification (that occurs with the
       SFC domain, or external to it) along an SFP.  For example, an
       external repository might provide user/subscriber information to
       a service chain classifier.  This classifier in turn imposes that
       information in the SFC encapsulation for delivery to the
       requisite SFs.  The SFs in turn utilize the user/subscriber
       information for local policy decisions.

   5.  Service definition independence: the technical characterization
       of each SF is not required to design the SFC architecture and SFC

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       data plane operations.  Consequently, no IANA registry is
       required to store the list of SFs.

   6.  Service function chain independence: The creation, modification,
       or deletion of a service chain have no impact on other service

   7.  Heterogeneous control/policy points: allowing SFs to use
       independent mechanisms (out of scope for this document) like IF-
       MAP or Diameter to populate and resolve local policy and (if
       needed) local classification criteria.

4.  Core SFC Architecture Components

   At a very high level, the logical architecture of an SFC-enabled
   Domain comprises:

      o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
      .  +--------------+                  +------------------~~~
      .  |   Service    |       SFC        |  Service  +---+   +---+
      .  |Classification|  Encapsulation   | Function  |sf1|...|sfn|
   +---->|   Function   |+---------------->|   Path    +---+   +---+
      .  +--------------+                  +------------------~~~
      . SFC-enabled Domain
      o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

               Figure 2: Service Function Chain Architecture

   The following sub-sections provide details on each logical component
   that form the basis of the SFC architecture.  A detailed overview of
   how each of these architectural components interact is provided in
   Figure 3:

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      +----------------+                        +----------------+
      |   SFC-aware    |                        |  SFC-unaware   |
      |Service Function|                        |Service Function|
      +-------+--------+                        +-------+--------+
              |                                         |
        SFC Encapsulation                       No SFC Encapsulation
              |                  SFC                    |
      +----+  +----------------+ Encapsulation     +---------+
      | SF |-----------------+  \     +------------|SFC Proxy|
      +----+ ... ----------+  \  \   /             +---------+
                            \  \  \ /
                           |   SF Forwarder |
                           |      (SFF)     |
                           SFC Encapsulation
                       ... SFC-enabled Domain ...
                       Network Overlay Transport
                            ,-'        `-.
                           /              `.
                          |     Network    |
                          `.              /
                            `.__     __,-'

         Figure 3: Service Function Chain Architecture Components

4.1.  SFC Encapsulation

   The SFC encapsulation enables service function path selection and the
   sharing of metadata/context information.

   The SFC encapsulation provides explicit information used to identify
   the SFP.  However, the SFC encapsulation is not a transport
   encapsulation itself: it is not used to forward packets within the
   network fabric.  The SFC encapsulation therefore, relies on an outer
   network transport.  Transit forwarders -- such as router and switches
   -- simply forward SFC encapsulated packets based on the outer (non-
   SFC) encapsulation.

   One of the key architecture principles of SFC is that the SFC
   encapsulation remain transport independent and as such any network
   transport protocol may be used to carry the SFC encapsulation.

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4.2.  Service Function (SF)

   The concept of an SF evolves; rather than being viewed as a bump in
   the wire, an SF becomes a resource within a specified administrative
   domain that is available for consumption as part of a composite
   service.  SFs send/receive data from one or more SFFs.  SFC aware SFs
   receive this data with the SFC encapsulation.

   While the SFC architecture defines a new encapsulation - the SFC
   encapsulation - and several logical components for the construction
   of SFCs, existing SF implementations may not have the capabilities to
   act upon or fully integrate with the new SFC encapsulation.  In order
   to provide a mechanism for such SFs to participate in the
   architecture a logical SFC proxy function is defined.  The SFC proxy
   acts a gateway between the SFC encapsulation and SFC unaware SFs.
   The integration of SFC-unaware service function is discussed in more
   detail in the SFC proxy section.

   This architecture allows an SF to be part of multiple SFPs and SFCs.

4.3.  Service Function Forwarder (SFF)

   The SFF is responsible for forwarding packets and/or frames received
   from the network to one or more SFs associated with a given SFF using
   information conveyed in the SFC encapsulation.

   The collection of SFFs and associated SFs creates a service plane
   using an overlay in which SFC-aware SFs, as well as SFC-unaware SFs
   reside.  Within this service plane, the SFF component connects
   different SFs that form a service function path.

   SFFs maintain the requisite SFP forwarding information.  SFP
   forwarding information is associated with a service path identifier
   that is used to uniquely identify an SFP.  The service forwarding
   state enables an SFF to identify which SF of a given SFP should be
   applied as traffic flows through the associated SFP.  While there may
   appear to the SFF to be only one available way to deliver the given
   SF, there may also be multiple choices allowed by the constraints of
   the SFP.

   If there are multiple choices, the SFF needs to preserve the property
   that all packets of a flow are handled the same way, since the SF may
   well be stateful.

   The SFF also has the information to allow it to forward packets to
   the next SFF after applying local service functions.  Again, while
   there may be only a single choice available, the architecture allows
   for multiple choices for next SFF.  As with SF, the solution needs to

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   operate such that the behavior flows (see the Rendered Service Path)
   is stable.  It should be noted that the selection of available SFs
   and next SFFs may be interwoven when an SFF support multiple distinct
   service functions and the same service function is available at
   multiple SFFs.  Solutions need to be clear about what is allowed in
   these cases.

   It should be noted that even when the SFF supports and utilizes
   multiple choices, the decision as to whether to use flow-specific
   mechanisms or coarser grained means to ensure that the behavior flows
   are stable is a matter for specific solutions and specific

   The SFF component has the following primary responsibilities:

   1.  SFP forwarding : Traffic arrives at an SFF from the network.  The
       SFF determines the appropriate SF the traffic should be forwarded
       to via information contained in the SFC encapsulation.  Post-SF,
       the traffic is returned to the SFF, and if needed forwarded to
       another SF associated with that SFF.  If there is another non-
       local (i.e., different SFF) hop in the SFP, the SFF encapsulates
       the traffic in the appropriate network transport and delivers it
       to the network for delivery to the next SFF along the path.
       Related to this forwarding responsibility, an SFF should be able
       to interact with metadata.

   2.  Terminating SFPs : An SFC is completely executed when traffic has
       traversed all required SFs in a chain.  When traffic arrives at
       the SFF after the last SF has finished servicing it, SFF fails to
       find the next SF or knows from the service forwarding state that
       the SFC is complete.  SFF removes the SFC encapsulation and
       delivers the packet to the network for forwarding.

   3.  Maintaining flow state: In some cases, the SFF may be stateful.
       It creates flows and stores flow-centric information.  This state
       information may be used for a range of SFP-related tasks such as
       ensuring symmetry or for state-aware SFC Proxy functionality (see
       Section 4.8).

4.3.1.  Transport Derived SFF

   Service function forwarding, as described above, directly depends
   upon the use of the service path information contained in the SFC
   encapsulation.  Existing implementations may not be able to act on
   the SFC encapsulation.  These platforms may opt to use existing
   transport information if it provides explicit service path

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   This results in the same architectural behavior and meaning for
   service function forwarding and service function paths.  It is the
   responsibility of the control components to ensure that the transport
   path executed in such a case is fully aligned with the path
   identified by the information in the service chaining encapsulation.

4.4.  SFC-Enabled Domain

   Specific features might need to be enforced at the boundaries of an
   SFC-enabled domain, for example to avoid leaking SFC information.
   Using the term node to refer generically to an entity that is
   performing a set of functions, in this context, an SFC Boundary Node
   denotes a node that connects one SFC-enabled domain to a node either
   located in another SFC-enabled domain or in a domain that is SFC-

   An SFC Boundary node can act as egress or ingress.  An SFC Egress
   Node denotes a SFC Boundary Node that handles traffic leaving the
   SFC-enabled domain the Egress Node belongs to.  Such a node is
   required to remove any information specific to the SFC Domain,
   typically the SFC Encapsulation.  An SFC Ingress Node denotes a SFC
   Boundary Node that handles traffic entering the SFC-enabled domain
   the ingress Node belongs to.  In most solutions and deployments this
   will need to include a classifier, and will be responsible for adding
   the SFC encapsulation to the packet.

4.5.  Network Components

   Underneath the SFF, there are components responsible for performing
   the overlay encapsulation/de-capsulation and forwarding of packets on
   the overlay network.  They may consult the SFC encapsulation or the
   inner payload of an incoming packet only in the necessary cases to
   achieve optimal forwarding in the network.

4.6.  SFC Proxy

   In order for the SFC architecture to support SFC-unaware SF's (e.g.,
   legacy service functions), an optional, logical SFC proxy function
   may be used.  This proxy removes the SFC encapsulation and then uses
   a local attachment circuit to deliver packets to SFC unaware SFs.

   Architecturally, the SFC Proxy along with an SFC-unaware Service
   Function make up an SF.  More specifically:

   For traffic received from an SFF, destined to an SF, the SFC proxy:

   o  Removes the SFC encapsulation from SFC encapsulated packets and/or

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   o  Identifies the required SF to be applied based on information
      carried in the SFC encapsulation.

   o  Selects the appropriate outbound local attachment circuit through
      which the next SF for this SFP is reachable.  This information is
      derived from the SFC encapsulation or from local configuration.
      Examples of a local attachment circuit include, but are not
      limited to, VLANs, IP-in-IP, L2TPv3, GRE, VXLAN.

   o  Forwards the original payload via a local attachment circuit to
      the appropriate SF.

   When traffic is returned from the SF:

   o  Applies the required SFC encapsulation.  The determination of the
      encapsulation details may be inferred by the local attachment
      circuit through which the packet and/or frame was received, or via
      packet classification, or other local policy.  In some cases,
      packet ordering or modification by the SF may necessitate
      additional classification in order to re-apply the correct SFC

   o  Imposes the appropriate SFC encapsulation based on the
      identification of the SFC to be applied.

   Alternatively, a service provider may decide to exclude legacy
   service functions from an SDC domain.

4.7.  Classification

   Traffic that satisfies classification criteria is directed into an
   SFP and forwarded to the requisite service function(s).
   Classification is handled by a logical service classification
   function, and initial classification occurs at the edge of the SFC
   domain.  The granularity of the initial classification is determined
   by the capabilities of the classifier and the requirements of the SFC
   policy.  For instance, classification might be relatively coarse: all
   packets from this port are directed into SFP A, or quite granular:
   all packets matching this 5-tuple are subject to SFP B.

   As a consequence of the classification decision, the appropriate SFC
   encapsulation is imposed on the data prior to forwarding along the
   SFP.  Classification results in attaching the traffic to a specific

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4.8.  Re-Classification and Branching

   The SFC architecture supports re-classification (or non-initial
   classification) as well.  As packets traverse an SFP, re-
   classification may occur - typically performed by a classification
   function co-resident with a service function.  Reclassification may
   result in the selection of a new SFP, an update of the associated
   metadata, or both.  This is referred to as "branching".

   For example, an initial classification results in the selection of
   SFP A: DPI_1 --> SLB_8.  However, when the DPI service function is
   executed "attack" traffic is detected at the application layer.
   DPI_1 re-classifies the traffic as "attack" and alters the service
   path, to SFP B, to include a firewall for policy enforcement:
   dropping the traffic: DPI_1 --> FW_4.  In this simple example, the
   DPI service function re-classified the traffic based on local
   application layer classification capabilities (that were not
   available during the initial classification step).

   When traffic arrives after being steered through an SFC-unaware SF,
   the SFC Proxy must perform re-classification of traffic to determine
   the SFP.  The SFC Proxy is concerned with re-attaching information
   for SFC-unaware SFs, and a state-full SFC Proxy simplifies such
   classification to a flow lookup.

4.9.  Shared Metadata

   Sharing metadata allows the network to provide network-derived
   information to the SFs, SF-to-SF information exchange and the sharing
   of service-derived information to the network.  Some SFCs may not
   require metadata exchange.  SFC infrastructure enables the exchange
   of this shared data along the SFP.  The shared metadata serves
   several possible roles within the SFC architecture:

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

   o  Encodes information about the network and/or data for post-
      service forwarding.

   o  Creates an identifier used for policy binding by SFs.

   o  Context information can be derived in several ways:

      *  External sources

      *  Network node classification

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      *  Service function classification

5.  Additional Architectural Concepts

   There are a number of issues which solutions need to address, and
   which the architecture informs but does not determine.  This section
   lays out some of those concepts.

5.1.  The Role of Policy

   Much of the behavior of service chains is driven by operator and
   customer policy.  This architecture is structured to isolate the
   policy interactions from the data plane and control logic.

   Specifically, it is assumed that service chaining control plane
   creates the service paths.  The service chaining data plane is used
   to deliver the classified packets along the service chains to the
   intended service functions.

   Policy, in contrast interacts with the system in other places.
   Policies, and policy engines, may monitor service functions to decide
   if additional (or fewer) instances of services are needed.  When
   applicable, those decisions may in turn result in interactions which
   direct the control logic to change the service chain placement or the
   packet classification rules.

   Similarly, operator service policy, often managed by operational or
   business support systems (OSS or BSS), will frequently determine what
   service functions are available.  Depending upon operator
   preferences, these policies may also determine which sequences of
   functions are valid and to be used or made available.

   The offering of service chains to customers, and the selection of
   which service chain a customer wishes to use are driven by a
   combination of operator and customer policies using appropriate
   portals in conjunction with the OSS and BSS tools.  These selections
   then drive the service chaining control logic which in turn
   establishes the appropriate packet classification rules.

5.2.  SFC Control Plane

   This is part of the overall architecture but outside the scope of

   The SFC control plane is responsible for constructing the SFPs;
   translating the SFCs to the forwarding paths and propagating path
   information to participating nodes to achieve requisite forwarding
   behavior to construct the service overlay.  For instance, an SFC

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   construction may be static; selecting exactly which SFF and which SF
   from those SFF are to be used, or may be dynamic; allowing the
   network to perform some or all of the choices of SFF or SF to use to
   deliver the selected service chain within the constraints represented
   by the service path.  In SFC, SFs are resources; the control plane
   manages and communicates their capabilities, availability and
   location in fashions suitable for the transport and SFC operations in
   use.  The control plane is also responsible for the creation of the
   context (see below).  The control plane may be distributed (using new
   or existing control plane protocols), or be centralized, or a
   combination of the two.

   The SFC control plane provides the following functionality:

   1.  An administrative domain wide view of all available service
       function resources as well as the network locator through which
       they are reachable.

   2.  Uses SFC policy to construct service function chains, and
       associated service function paths.

   3.  Selection of specific SFs for a requested SFC, either statically
       (using specific SFs) or dynamically (using service explicit SFs
       at the time of delivering traffic to them).

   4.  Provides requisite SFC data plane information to the SFC
       architecture components, most notably the SFF.

   5.  Allocation of metadata associated with a given SFP and
       propagation of metadata syntax to relevant SFs and/or SFC
       encapsulation-proxies or their respective policy planes.

5.3.  Resource Control

   The SFC system may be responsible for managing all resources
   necessary for the SFC components to function.  This includes network
   constraints used to plan and choose the network path(s) between
   service function forwarders, network communication paths between
   service function forwarders and their attach service functions,
   characteristics of the nodes themselves such as memory, number of
   virtual interfaces, routes, etc., and configuration of the running
   SFs running.

   The SFC system will also be required to reflect policy decisions
   about resource control, as expressed by other components in the

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   It should be noted that while all of these aspects are part of the
   overall system, they normal reside outside of the scope of this

5.4.  Infinite Loop Detection and Avoidance

   This SFC architecture is predicated on topological independence from
   the underlying forwarding topology.  Consequently, a service topology
   is created by Service Function Paths or by the local decisions of the
   Service Function Forwarders based on the constraints expressed in the
   SFP.  Due to the overlay constraints, the packet-forwarding path may
   need to visit the same SFF multiple times, and in some less common
   cases may even need to visit the same SF more than once.  The Service
   Chaining solution needs to permit these limited and policy-compliant
   loops.  At the same time, the solutions must ensure that indefinite
   and unbounded loops cannot be formed, as such would consume unbounded
   resources without delivering any value.

   In other words, this architecture prevents infinite Service Function
   Loops, even when Service Functions may be invoked multiple times in
   the same SFP.

5.5.  Load Balancing Considerations

   Supporting function elasticity and high-availability shouldn't overly
   complicate SFC or lead to unnecessary scalability problems.

   In the simplest case, where there is only a single function in the
   chain (the next hop is either the destination address of the flow or
   the appropriate next hop to that destination), one could argue that
   there may be no need for SFC.

   In the case where the classifier is separate from the single function
   or a function at the terminal address may need sub-prefix or per
   subscriber metadata, we would have a single chain (the metadata
   changes but the SFC chain does not), regardless of the number of
   potential terminal addresses for the flow.  This is the case of the
   simple load balancer.  See Figure 4.

                            +---+    +---++--->web server
                  source+-->|sff|+-->|sf1|+--->web server
                            +---+    +---++--->web server

                      Figure 4: Simple Load Balancing

   By extrapolation, in the case where intermediary functions within a
   chain had similar "elastic" behaviors, we do not need separate chains

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   to account for this behavior - as long as the traffic coalesces to a
   common next-hop after the point of elasticity.

   In Figure 5, we have a chain of five service functions between the
   traffic source and its destination.

                +---+ +---+ +---+   +---+ +---+ +---+
                |sf2| |sf2| |sf3|   |sf3| |sf4| |sf4|
                +---+ +---+ +---+   +---+ +---+ +---+
                  |     |     |       |     |     |
                  +-----+-----+       +-----+-----+
                        |                   |
                        +                   +
             +---+    +---+     +---+     +---+    +---+
             +---+    +---+     +---+     +---+    +---+
               +                  +                  +
               |                  |                  |
             +---+              +---+              +---+
             |sf1|              |sf3|              |sf5|
             +---+              +---+              +---+

                         Figure 5: Load Balancing

   This would be represented as one service function path:
   sf1->sf2->sf3->sf4->sf5.  The SFF is a logical element, which may be
   made up of one or multiple components.  In this architecture, the SFF
   handle load distribution based on policy.

   It can also be seen in the above that the same service function may
   be reachable through multiple SFF, as discussed earlier.  The
   selection of which SFF to use to reach SF3 may be made by the control
   logic in defining the SFP, or may be left to the SFF themselves,
   depending upon policy, solution, and deployment constraints.

5.6.  MTU and Fragmentation Considerations

   This architecture prescribes additional information being added to
   packets to represent service function paths and often metadata.  It
   also envisions adding transport information to carry packets along
   service function paths, at least between service function forwarders.
   This added information increases the size of the packet to be carried
   by service chaining.  Such additions could potentially increase the
   packet size beyond the MTU supported on some or all of the media used
   in the service chaining domain.

   Such packet size increases can thus cause operational MTU problems.
   Requiring fragmentation and reassembly in SFF would be a major

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   processing increase, and may be impossible with some transports.
   Expecting service functions to deal with packets fragmented by the
   SFC function may be onerous even when such fragmentation is possible.
   Thus, at the very least, solutions need to pay attention to the size
   cost of their approach.  There may be alternative or additional means
   available, although any solution needs to consider the impact of
   those techniques.

   In addition to these considerations applicable to any generic
   architecture that increases the header size, there are more specific
   MTU considerations: Effects to Path MTU Discovery (PMTUD) as well as
   deployment considerations.  Deployments within a single
   administrateive control or even a single Data Center complex can
   afford more flexibility in dealing with larger packets, and deploying
   existing mitigations that decrease the likelihood of fragmentation.

5.7.  SFC OAM

   Operations, Administration, and Maintenance (OAM) tools are an
   integral part of the architecture.  These serve various purposes,
   including fault detection and isolation, and performance management.
   For example, there are many advantages of SFP liveness detection,
   including status reporting, support for resiliency operations and
   policies, and an enhanced ability to load balance.

   Service Function Paths create a services topology, and OAM performs
   various functions within this service layer.  Furthermore, SFC OAM
   follows the same architectural principles of SFC in general.  For
   example, topological independence (including the ability to run OAM
   over various overlay technologies) and classification-based policy.

   We can subdivide the SFC OAM architecture in two parts:

   o  In-band: OAM packets run in-band fate sharing with the service
      topology.  For this, they also follow the architectural principle
      of consistent policy identifiers, and use the same path IDs as the
      service chain data packets.  Load balancing and SFC Encapslation
      encapsulation with packet forwarding are particularly important

   o  Out-of-band: reporting beyond the actual data plane.  An
      additional layer beyond the data-plane OAM, allows for additional
      alerting and measurements.

   This architecture prescribes end-to-end SFP OAM functions, which
   implies SFF understanding of whether an in-band packet is an OAM or
   user packet.  However, service function validation is outside of the

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   scope of this architecture, and application-level OAM is not what
   this architecture prescribes.

   Some of the detailed functions performed by SFC OAM include fault
   detection and isolation in a Service Function Path or a Service
   Function, verification that communication using SFPs is both
   effective and directing packets to the intended service functions,
   service path tracing, diagnostic and fault isolation, alarm
   reporting, performance measurement, locking and testing of service
   functions, validation with the control plane (see Section 5.2, and
   also allow for vendor-specific as well as experimental functions.
   SFC should leverage, and if needed extend relevant existing OAM

5.8.  Resilience and Redundancy

   As a practical operational requirement, any service chaining solution
   needs to be able to respond effectively, and usually very quickly, to
   failure conditions.  These failures may be failures of connectivity
   in the network between SFF, failures of SFF, or failures of SF.  Per-
   SF state, as for example stateful-firewall state, is the
   responsibility of the SF and not addressed in this section.

   There are multiple techniques available to address this issue, and
   solutions can describe both what they require and what they allow to
   address this.  Solutions can make use of the flexibility in the
   specificity of service function paths, if the SFF can be given enough
   information in a timely fashion to do this.  Solutions can also make
   use of MAC or IP level redundancy mechanisms such as VRRP.  Also,
   particularly for SF failures, load balancers co-located with the SFF
   or as part of the service function delivery mechanism can provide
   such robustness.

   Similarly, operational requirements will lead to the need for
   resilience in solutions in the face of load changes.  While
   mechanisms for managing (e.g., monitoring, instantiating, loading
   images, providing configuration to service function chaining control,
   deleting, etc.) virtual machines are out scope for this architecture,
   solutions can and are aided by describing how they can make use of
   scaling mechanisms.

6.  Security Considerations

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

   Security considerations apply to the realization of this
   architecture.  Such realization ought to provide means to protect the

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   SFC-enabled domain and its borders against various forms of attacks,
   including DDoS attacks.  Further, SFC OAM Functions need to not
   negatively affect the security considerations of an SFC-enabled
   domain.  Additionally, all entities (software or hardware)
   interacting with the service chaining mechanisms need to provide
   means of security against malformed, poorly configured (deliberate or
   not) protocol constructs and loops.

7.  Contributors and Acknowledgments

   The editors would like to thank Sam Aldrin, Linda Dunbar, Ken Gray,
   Nagendra Kumar, Xiaohu Xu, and L.  Yong for a thorough review and
   useful comments.

   The initial version of this "Service Function Chaining (SFC)
   Architecture" document is the result of merging two previous
   documents, and this section lists the aggregate of authors, editors,
   contributors and acknowledged participants, all who provided
   important ideas and text that fed into this architecture.



         Mohamed Boucadair
         Christian Jacquenet
         Ron Parker
         Diego R.  Lopez
         Jim Guichard
         Carlos Pignataro


         Parviz Yegani
         Paul Quinn
         Linda Dunbar


         Many thanks to D.  Abgrall, D.  Minodier, Y.  Le Goff, D.
         Cheng, R.  White, and B.  Chatras for their review and



         Paul Quinn (editor)

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         Joel Halpern (editor)


         Puneet Agarwal
         Andre Beliveau
         Kevin Glavin
         Ken Gray
         Jim Guichard
         Surendra Kumar
         Darrel Lewis
         Nic Leymann
         Rajeev Manur
         Thomas Nadeau
         Carlos Pignataro
         Michael Smith
         Navindra Yadav


         The authors would like to thank David Ward, Abhijit Patra,
         Nagaraj Bagepalli, Darrel Lewis, Ron Parker, Lucy Yong and
         Christian Jacquenet for their review and comments.

8.  IANA Considerations

   This document creates no new requirements on IANA namespaces

9.  References

9.1.  Normative References

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

9.2.  Informative References

              Boucadair, M., Jacquenet, C., Parker, R., Lopez, D.,
              Guichard, J., and C. Pignataro, "Service Function
              Chaining: Framework & Architecture", draft-boucadair-sfc-
              framework-02 (work in progress), February 2014.

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              Quinn, P. and T. Nadeau, "Service Function Chaining
              Problem Statement", draft-ietf-sfc-problem-statement-07
              (work in progress), June 2014.

              Quinn, P. and J. Halpern, "Service Function Chaining (SFC)
              Architecture", draft-quinn-sfc-arch-05 (work in progress),
              May 2014.

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

   [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.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, June 2011.

Authors' Addresses

   Joel Halpern (editor)


   Carlos Pignataro (editor)
   Cisco Systems, Inc.


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