Network Working Group                                      P. Quinn, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                           J. Halpern, Ed.
Expires: November 6, 2014                                       Ericsson
                                                             May 5, 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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   This Internet-Draft will expire on November 6, 2014.

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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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Definition of Terms  . . . . . . . . . . . . . . . . . . .  3
   2.  Architectural Concepts . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Service Function Chains  . . . . . . . . . . . . . . . . .  5
     2.2.  Service Function Chain Symmetry  . . . . . . . . . . . . .  6
     2.3.  Service Function Paths . . . . . . . . . . . . . . . . . .  6
   3.  Architecture Principles  . . . . . . . . . . . . . . . . . . .  7
   4.  Core SFC Architecture Components . . . . . . . . . . . . . . .  8
     4.1.  SFC Encapsulation  . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Service Function (SF)  . . . . . . . . . . . . . . . . . .  9
     4.3.  Service Function Forwarder (SFF) . . . . . . . . . . . . .  9
       4.3.1.  Transport Derived SFF  . . . . . . . . . . . . . . . . 11
     4.4.  Network Forwarder (NF) . . . . . . . . . . . . . . . . . . 11
     4.5.  Classification/Re-classification . . . . . . . . . . . . . 11
     4.6.  SFC Control Plane  . . . . . . . . . . . . . . . . . . . . 12
     4.7.  Shared Metadata  . . . . . . . . . . . . . . . . . . . . . 13
     4.8.  Resource Control . . . . . . . . . . . . . . . . . . . . . 13
   5.  The Role of Policy . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Load Balancing Considerations  . . . . . . . . . . . . . . . . 15
   7.  SFC Proxy  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   8.  MTU Considerations . . . . . . . . . . . . . . . . . . . . . . 19
   9.  SFC OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   10. Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 27
     15.2. Informative References . . . . . . . . . . . . . . . . . . 27
   Appendix A.  Existing Service Deployments  . . . . . . . . . . . . 28
   Appendix B.  Issues with Existing Deployments  . . . . . . . . . . 29
   Appendix C.  SFC Encapsulation Requirements  . . . . . . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31

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1.  Introduction

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

   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.

1.1.  Scope

   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.  Definition of Terms

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

   SFC Network Forwarder (NF):  SFC network forwarders provide network
      connectivity for service function forwarders (SFF) and service
      functions (SF).

   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 (SF):  A function that is responsible for specific
      treatment of received packets.  A Service Function can act at the
      network layer or other OSI layers.  A Service Function can be a
      virtual instance or be embedded in a physical network element.
      One of multiple Service Functions can be embedded in the same
      network element.  Multiple instances of the Service Function can
      be enabled in the same administrative domain.

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      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.

      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

   Service Function Identity (SFID):  A unique identifier that
      represents a service function.  SFIDs are unique for each SF
      within an SFC domain.

   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.

      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.

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

   Service Function Chain (SFC):  A service Function chain defines an
      ordered set of service functions 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.  The term service
      chain is often used as shorthand for service function chain.

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

   Service Function Path (SFP):  The instantiation of a SFC in the
      network.  Packets follow a service function path from a classifier
      through the requisite service functions

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2.  Architectural Concepts

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

2.1.  Service Function Chains

   In most networks services are constructed as a 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

   SFCs can start from the origination point of the service function
   graph (i.e.: node 1 in Figure 1), or from any subsequent SF 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.

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

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

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

                  Figure 1: Service Function Chain Graphs

   The architecture allows for two or more SFs to be co-resident on the

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   same service node.  In these cases, some implementations 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.  Implementation details of such
   mechanisms are considered out-of-scope for this document.

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.

2.3.  Service Function Paths

   When an SFC is instantiated into the network it is necessary to
   select the specific instances of SFs that will be used, and to create
   the service topology for that SFC using SF's network locator.  Thus,
   instantiation of the SFC results in the creation of a Service
   Function Path (SFP) and is used for forwarding packets through the
   SFC.  In other words, an SFP is the instantiation of the defined SFC.

   This abstraction enables the binding of SFCs to specific instances,
   or set of like instances of SFs based on a range of policy attributes
   defined by the operator.  For example, an SFC definition might
   specify that one of the SF elements is a firewall.  However, on the
   network, there might exist a number of instances of the same firewall
   (that is to say they enforce the same policy) and only when the SFP
   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.  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.  Consistent policy identifiers: a common identifier is used for SF
       policy selection.

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

   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.  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.

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4.  Core SFC Architecture Components

   The following sub-sections provide details on each logical component
   that form the basis of the SFC architecture.  An overview of how each
   of these components interact is provided in the following figure.

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

         Figure 2: Service Function Chain Architecture Components

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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 nodes -- such as router and switches --
   simply forward SFC encapsulated packets based on the outer (non-SFC)

   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.

4.2.  Service Function (SF)

   The concept of a SF evolves; rather than being viewed as a bump in
   the wire, a SF becomes a resource within a specified administrative
   domain that is available for consumption as part of a composite
   service.  As such, SFs have one or more network locators through
   which they are reachable, and a variable set of attributes that
   describe the function offered.  The combination of network locator
   and attributes are used to construct an SFP.  SFs send/receive SFC
   encapsulated data from one or more SFFs.

   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.

4.3.  Service Function Forwarder (SFF)

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

   The collection of SFFs 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.

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   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 SFC should be
   applied as traffic flows through the associated SFP.  Each SFF need
   only maintain SFC forwarding information that is relevant locally.
   The SFC forwarding state at all SFFs collectively represents the SFPs
   associated with each SFC in the SFC domain.

                | SFP  |   Ordered Service Functions      |
                | ID   | order1 | order2 | order3 | ...   |
                | SFP1 | SFID1  | SFID5  | SFID20 |       |
                | SFP4 | SFID100| SFID3  | SFID4  | SFID9 |
                | ...  |        |        |        |       |

                            Figure 3: SFF Table

   Figure 3 depicts a view of the service forwarding state for two SFPs
   - SFP1 and SFP4.  The SF columns of this table may come from
   different SFFs.

   The SFF component has the following primary responsibilities:

   1.  SFP forwarding : Traffic arrives at an SFF from one or more NFs.
       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 hop in the SFP, the SFF, encapsulates the traffic in the
       appropriate network transport and delivers it to the NF for
       delivery to the next SFF along the path.

   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 an NF for forwarding.

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   3.  Maintaining flow state: In some cases, the SFF may be stateful.
       It creates flows and stores flow-centric information.  When
       traffic arrives after being steered through an SFC-unaware SF,
       the SFF must perform re-classification of traffic to determine
       the SFP.  A state-full SFF simplifies such classification to a
       flow lookup.

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 a transport
   mechanism which carries the service path information from the SFC
   encapsulation, and information derived from the SFC encapsulation, to
   build transport information.

   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.  Network Forwarder (NF)

   This component is responsible for performing the overlay
   encapsulation/de-capsulation and forwarding of packets on the overlay
   network.  NF forwarding 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.

4.5.  Classification/Re-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

   The SFC architecture supports reclassification (or non-initial

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   classification) as well.  As packets traverse an SFP,
   reclassification 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.

   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 reclassifies 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 reclassified the traffic based on local
   application layer classification capabilities (that were not
   available during the initial classification step).

4.6.  SFC Control Plane

   The SFC control plane is responsible for constructing the SFPs;
   translating the SFCs 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 SFC construction may be static - using specific SF
   instances, or dynamic - choosing service explicit SF instances at the
   time of delivering traffic to the SF.  In SFC, SFs 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).  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 SF instances for a requested SFC, either
       statically (using specific SF instances) or dynamically (using
       service explicit SF instances at the time of delivering traffic
       to the SF).

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

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

4.7.  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.  This component is
   optional.  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

      *  Service function classification

4.8.  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 nodes, characteristics of the nodes themselves such as
   memory, number of virtual interfaces, routes, etc..., and
   configuration of the SFs running on the service nodes.

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5.  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.

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6.  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.

                   +----+----->web server
     source+------>|sf1 +----->web server
                   |----+----->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
   to account for this behavior - as long as the traffic coalesces to a
   common next-hop after the point of elasticity.

   In the following figure, we have a chain of five service functions
   between the traffic source and it's destination.

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

                         Figure 5: Load Balancing

   Either through an imbedded action in sf1 and sf3, or through external
   control, the service functions sf2 and sf4 are elastically expanded
   and contracted dynamically.  This would be represented as one chain:
   s1->s2->s3->s4->s5, but with multiple paths (not as a number of
   chains equal to the factorial combination of potential end-to-end
   paths).  The load distribution decision will be localized (in
   general, although there might be macro policy controlling that -
   which is out of scope for the sake of a simple example).  In this
   case, the control entity will push to the sf1 nodes, a table of
   sorts: sf2 with a series of next hops, and if needed some weighted or
   other metrics (these could also be decided locally by some policy,
   but sf1 would need to be aware of expand/contract triggers and
   actions). sf1 would use local logic -- hash, state table, etc. -- to
   distribute the chained packets to sf2.

   The addition of high availability should likewise not require a
   multitude of new chains.

                             +-----+-+                   +-----+-+
                         +-->| sf2 |-|+              +-->| sf4 |-|+
                     +------>|-----| ||          +------>|-----| ||
             +------+|---+   +-----+ |+-->+-----+|---+   +-----+ |+-->+-----+
             | sf1  ||       +-----+ +--->| sf3 ||       +-----+ +--->| sf5 |
         +-->|------||------>| sf2 |+---->|-----||------>| sf4 |+---->|-----|---+
         |   |      || +---->|-----|-+    |     || +---->|-----|-+    |     |   |
         |   +------+|-|-+   +-----+ |+-->+-----+|-|-+   +-----+ |+-->+-----+   |
source+--+           | | |   +-----+ ||          | | |   +-----+ ||             +----->
         |   +------++ | +-->| sf2 |-|+   +-----++ | +-->| sf4 |-|+   +-----+   |
         |   | sf1' |  | +-->|-----| +--->| sf3'|  | +-->|-----| +--->| sf5'|   |
         +-->|------|--+ |   +-----+----->|-----|--+ |   +-----+----->|-----|---+
             |      |    |                |     |    |                |     |
             +------+----+                +-----+----+                +-----+

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                      Figure 6: Load Balancing and HA

   In the figure, sf1, sf3 and sf5 have a redundant counterpart for high
   availability purposes (typical of stateful appliance/function
   redundancy strategies, these entities may have private connections
   for transferring state not shown).  Note that the elasticity of sf2
   and sf4 provide a separate high availability strategy for those
   functions.  In the case where sf1', sf3' and sf5' provide transparent
   dynamic replacement (they assert the addressing characteristics of
   their counterparts via an internal or external trigger), there is
   still a single chain (again, not a factorial explosion).

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7.  SFC Proxy

   In order for the SFC architecture to support SFC-unaware SF's, 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.  More specifically:

   For traffic received from a NF or SFF, destined to an SF, the SFC

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

   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, 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.

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8.  MTU Considerations

   Modern systems are expected to be able to cope gracefully with MTU
   issues that may arise from the application of additional headers to a
   packet.  Adopting the recommendations of other WG's who have recently
   tackled this issue (e.g.  [RFC6830]), there are several mechanisms
   for dealing with packets that are too large to transit the path from
   the point of service classification to the last function (SFn) in the

   In the "stateful" approach, the classifier keeps a per-path record of
   the maximum size allowed, and sends an ICMP Too Big message to the
   original source when a packet which is too large is seen (where "too
   large" implies after the imposition of the appropriate SFC

   In the "stateless" approach, for IPv4, packets without the 'DF' bit
   set, too-large packets are fragmented, and then the fragments are
   forwarded; all other packets are discarded and an ICMP Too Big
   message returned.

   A recommendation of a specific mechanism and/or its implementation is
   beyond the scope of this document.

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   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.
   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.

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

   Some of the detailed functions performed by SFC OAM include fault
   detection, continuity checks, connectivity verification, service path
   tracing, diagnostic and fault isolation, alarm reporting, performance
   measurement, locking and testing of service functions, and also allow
   for vendor-specific as well as experimental functions.  SFC should
   leverage, and if needed extend relevant existing OAM mechanisms.

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10.  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

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11.  Security Considerations

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

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12.  Contributors

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

   Puneet Agarwal

   Andre Beliveau

   Kevin Glavin

   Ken Gray
   Cisco Systems, Inc.

   Jim Guichard
   Cisco Systems, Inc.

   Surendra Kumar
   Cisco Systems, Inc.

   Darrel Lewis
   Cisco Systems, Inc.

   Nic Leymann
   Deutsche Telekom

   Rajeev Manur

   Thomas Nadeau

   Carlos Pignataro
   Cisco Systems, Inc.

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   Michael Smith
   Cisco Systems, Inc.

   Navindra Yadav
   Cisco Systems, Inc.

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13.  Acknowledgments

   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.

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14.  IANA Considerations

   This document creates no new requirements on IANA namespaces

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15.  References

15.1.  Normative References

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

15.2.  Informative References

   [NSCprob]  "Network Service Chaining Problem Statement", <http://

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [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.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              January 2013.

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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

   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

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

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Appendix C.  SFC Encapsulation Requirements


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Authors' Addresses

   Paul Quinn (editor)
   Cisco Systems, Inc.


   Joel Halpern (editor)


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