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Versions: (draft-boucadair-service-chaining-framework) 00 01 02 draft-merged-sfc-architecture

SFC                                                         M. Boucadair
Internet-Draft                                              C. Jacquenet
Intended status: Standards Track                          France Telecom
Expires: August 16, 2014                                       R. Parker
                                                       Affirmed Networks
                                                                D. Lopez
                                                          Telefonica I+D
                                                             J. Guichard
                                                            C. Pignataro
                                                     Cisco Systems, Inc.
                                                       February 12, 2014


          Service Function Chaining: Framework & Architecture
                    draft-boucadair-sfc-framework-02

Abstract

   IP networks rely more and more on the combination of advanced
   functions (besides the basic routing and forwarding functions) for
   the delivery of added value services.  This document defines a
   reference architecture and a framework to enforce Service Function
   Chaining (SFC) with minimum requirements on the physical topology of
   the network.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

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

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

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

   This Internet-Draft will expire on August 16, 2014.




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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  On the Proliferation of Service Functions . . . . . . . .   3
     1.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Objectives  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.4.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   4
     1.5.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Functional Elements . . . . . . . . . . . . . . . . . . . . .   7
   4.  SFC Provisioning  . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Assign Service Function Identifiers . . . . . . . . . . .   8
     4.2.  Service Function Locator  . . . . . . . . . . . . . . . .   8
     4.3.  Service Function Discovery  . . . . . . . . . . . . . . .   8
     4.4.  Building Service Function Maps  . . . . . . . . . . . . .   9
     4.5.  Building Service Function Chaining (SFC) Policy Tables  .   9
   5.  Theory Of Operation . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  SFC Boundary Node . . . . . . . . . . . . . . . . . . . .  11
     5.2.  SFC Classifier  . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  SFC Ingress Node  . . . . . . . . . . . . . . . . . . . .  12
     5.4.  SFC Egress Node . . . . . . . . . . . . . . . . . . . . .  13
     5.5.  SF Node . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.6.  Intermediate Nodes  . . . . . . . . . . . . . . . . . . .  14
   6.  Fragmentation Considerations  . . . . . . . . . . . . . . . .  14
   7.  Differentiated Services . . . . . . . . . . . . . . . . . . .  14
   8.  ECN (Explicit Congestion Notification) Considerations . . . .  14
   9.  Design Considerations . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Transmit A SFC Map Index In A Packet  . . . . . . . . . .  15
       9.1.1.  SFC Map Index . . . . . . . . . . . . . . . . . . . .  15
       9.1.2.  Where To Store SFC Map Indexes In A Packet? . . . . .  15
     9.2.  Steer Paths To Cross Specific SF Nodes  . . . . . . . . .  15
   10. Deployment Considerations . . . . . . . . . . . . . . . . . .  15
     10.1.  Generic Requirements . . . . . . . . . . . . . . . . . .  15



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     10.2.  Deployment Models  . . . . . . . . . . . . . . . . . . .  16
       10.2.1.  1.1. Proxy Node for Legacy Service Functions . . . .  16
     10.3.  On Service Function Profiles (a.k.a., Contexts)  . . . .  17
     10.4.  SF Node is also a Classifier . . . . . . . . . . . . . .  18
     10.5.  SFs within the Same Subnet . . . . . . . . . . . . . . .  19
     10.6.  Service Function Loops . . . . . . . . . . . . . . . . .  19
     10.7.  Lightweight SFC Policy Table . . . . . . . . . . . . . .  20
     10.8.  Liveness Detection Of SFs By The  PDP  . . . . . . . . .  21
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     15.2.  Informative References . . . . . . . . . . . . . . . . .  22

1.  Introduction

1.1.  On the Proliferation of Service Functions

   IP networks rely more and more on the combination of advanced
   functions (besides the basic routing and forwarding functions) for
   the delivery of added value services.  Typical examples of such
   functions include firewall (e.g., [RFC6092]), DPI (Deep Packet
   Inspection), LI (Lawful Intercept) module, NAT44 [RFC3022], NAT64
   [RFC6146], DS-Lite AFTR [RFC6333], NPTv6 [RFC6296], HOST_ID
   injection, HTTP Header Enrichment function, TCP tweaking and
   optimization function, transparent caching, charging function, load-
   balancer, etc.

   Such advanced functions are denoted SF (Service Function) in this
   document.

   The dynamic enforcement of a SF-derived, adequate forwarding policy
   for packets entering a network that supports such advanced Service
   Functions has become a key challenge for operators and service
   providers.  SF-inferred differentiated forwarding is ensured by
   tweaking the set of Service Functions to be invoked.  How to bind a
   flow of packets that share at least one common characteristic to a
   forwarding plane is policy-based, and subject to the set of SF
   functions that need to be solicited for the processing of this
   specific flow.

   Service Providers need to rationalize their service delivery logics
   and master its underlying complexity.






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   The overall problem space is described in
   [I-D.ietf-sfc-problem-statement].  A companion document that lists a
   set of requirements is available at [I-D.boucadair-sfc-requirements].

1.2.  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 allows for differentiated
   forwarding: 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 activated 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 (e.g.,
   to rationalize the proliferation of advanced features at the Gi
   Interface [RFC6459]).

   Considerations related to the chaining of Service Functions that span
   domains owned by multiple administrative entities is out of scope.
   Note, a single administrative entity may manage multiple domains.

1.3.  Objectives

   The main objectives of the proposed framework are listed below:

   o  Create service-inferred forwarding planes.
   o  Efficiently master the chained activation of Service functions,
      regardless of the network topology and routing policies.
   o  Allow packets to be forwarded to the required Service Functions
      without changing the network topology or overlay transports
      necessary for packet delivery to/from Service Functions.
   o  Allow for differentiated packet forwarding by selecting the set of
      Service functions to be invoked.
   o  Allow to easily change the sequentiality of the activation of
      Service functions to be invoked.
   o  Allow to easily change the set of Service functions to be invoked.
   o  Ease management (including withdrawal) of Service functions and
      minimize any subsequent topology update.
   o  Automate the overall process of generating and enforcing policies
      to accommodate a set of network connectivity service objectives.

1.4.  Assumptions

   The following assumptions are made:





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   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 nor 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
      environment.
   o  There is no global nor standard SF chaining logic.  The ordered
      set of SFs that need to be activated to deliver a given
      connectivity service is specific to each administrative entity.
   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 logics 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
      populated.
   o  How to bind the traffic to a given SF chaining is policy-based.

1.5.  Rationale

   Given the assumptions listed in Section 1.4, the rationale of the
   framework is as follows:

   o  The framework separates the dynamic provisioning of required SF
      functions from packet handling operations (e.g., forwarding
      decisions).
   o  The technical characterization of each SF is not required to
      design the SFC architecture and SFC operations.
   o  No IANA registry is required to store the list of SFs.  In
      particular, assignment of identifiers, header fields, or any other
      indication of the Service Function Chain, are all strictly local
      in scope.  An identifier assigned in one administrative domain
      will not indicate the same set of SFs in another administrative
      domain.
   o  No IANA registry is required to store the SF chaining candidates.
      The set of SFCs are local to each administrative domain, and are
      as such not global.
   o  No specific SF chaining is assumed.  The description of SF chains
      is an information that will be processed by the nodes that
      participate to the delivery of a network service.  The set of
      listed/chained SF functions is generated by each administrative
      entity operating the network.
   o  SF handling is policy-based: SF chains can be updated or deleted,
      new SFs can be added without any impact on existing SFs, etc.  In




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      particular, this design is compliant with the global framework
      discussed in [I-D.sin-sdnrg-sdn-approach].
   o  For the sake of efficiency, policy enforcement is automated (but
      policies can be statically enforced, for example).
   o  To minimize fragmentation, a minimal set of information needs to
      be signaled (possibly in data packets).
   o  Advanced features (e.g., load balancing) are also described and
      may be configured according to policies that can be service-
      specific.  Policy decisions are made by a Policy Decision Point
      [RFC2753] and the solicited enforcement points are responsible for
      applying these decisions, whatever the objective to achieve.
   o  SFs can be embedded in nodes that intervene in the transport
      service or supported by dedicated nodes (e.g., dedicated servers).
      The decision to implement one of these two models (or a
      combination thereof) is deployment-specific and it is orthogonal
      to the overall procedure.
   o  Multiple SFC-enabled domains can be deployed within the same
      administrative domain.  Nodes are provisioned with the policy
      table of the SFC-enabled domain they belong to.
   o  The overall consistency of the differentiated forwarding policy is
      ensured by the PDP.
   o  The PDP can be responsible to enforce other policies than those
      described in the SFC Policy Tables.

2.  Terminology

   This document makes use of the following terms:

   o  SF (Service Function): refers to a function which is enabled in
      the network operated by an administrative entity.  One or many
      Service Functions can be involved in the delivery of added-value
      services.  A non-exhaustive list of Service Functions include:
      firewall (e.g., [RFC6092]), DPI (Deep Packet Inspection), LI
      (Lawful Intercept) module, NAT44 [RFC3022], NAT64 [RFC6146], DS-
      Lite AFTR [RFC6333], NPTv6 [RFC6296], HOST_ID injection, HTTP
      Header Enrichment function, TCP optimizer, load-balancer, etc.
      This document does not make any assumption in the OSI Layer on
      which the Service Function acts on; the exact definition of each
      Service Function is deployment-specific.

   o  SFC-enabled domain: denotes a network (or a region thereof) that
      implements SFC.

   o  SF Identifier: is a unique identifier that unambiguously
      identifies a SF within a SFC-enabled domain.  SF Identifiers are
      assigned, configured and managed by the administrative entity that
      operates the SFC-enabled domain.  SF identifiers can be structured
      as strings; other formats can be used.  SF Identifiers are not



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      required to be globally unique nor be exposed to or used by
      another SF-enabled domain.

   o  SF Map: refers to an ordered list of SF identifiers.  Each SF Map
      is identified with a unique identifier called SF Map Index.

   o  SFC Policy Table: is a table containing a list of SF Maps, SFC
      classification rules and Locators for all SF Nodes.  A SFC Policy
      Table may contain a default SF Map.

   o  SF Locator: A SF Node identifier used to reach the said SF node.
      A locator is typically an IP address or a FQDN.

   o  Legacy Node (Node for short): refers to any node that is not a SF
      Node nor a SFC Boundary Node.  This node can be located within a
      SFC-enabled domain or outside a SFC-enabled domain.

   o  SF Proxy Node: a Network Element along the data path, to enforce
      SFC functions on behalf of legacy SF nodes.

3.  Functional Elements

   The following functional elements are defined in this document:

   o  SFC Boundary Node (or 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-unaware.

   o  SFC Egress Node (or Egress Node): denotes a SFC Boundary Node that
      handles traffic which leaves the SFC-enabled domain the Egress
      Node belongs to.

   o  SFC Ingress Node (or Ingress Node): denotes a SFC Boundary Node
      that handles traffic which enters the SFC-enabled domain the
      ingress Node belongs to.

   o  SF Node: denotes any node within an SFC-enabled domain that embeds
      one or multiple SFs.

   o  SFC Classifier (or Classifier): an entity that classifiers packets
      for service chaining according to classification rules defined in
      a SFC Policy Table.  Packets are then marked with the
      corresponding SF Map Index.  SFC Classifier is embedded in a SFC
      boundary (Ingress) Node.  A SFC Classifier may be considered as a
      Service Function, and therefore be uniquely identified by a
      dedicated SF Identifier.





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4.  SFC Provisioning

   It is out of scope of this document to discuss SF-specific policy
   enforcement; only SFC considerations are elaborated.

4.1.  Assign Service Function Identifiers

   The administrative entity that operates a SFC-enabled domain
   maintains a local repository that lists the enabled SFs.  This
   administrative entity assigns a unique SF identifier for each SF
   type.

   SF identifiers are structured as character strings.  SF identifiers
   are case-sensitive.

   The main constraint on the format is that two SFs MUST be assigned
   with different SF identifiers if they do not provide the exact same
   function, or do provide the same function but are unable to
   differentiation packets based on policies provisioned to the SF using
   an appropriate mechanism.

4.2.  Service Function Locator

   A SF may be embedded in one or several SF Nodes.  The SF locator is
   typically the IP address or the FQDN to reach a given SF.

   The use of an IP address is RECOMMENDED to avoid any extra complexity
   related to the support of name resolution capabilities in SF Nodes.
   Resolution capabilities are supported by the PDP (Policy Decision
   Point).  In the rest of the document, we assume a SF locator is
   structured as an IP address (IPv4 or IPv6).

   A SF can be reached by one or more locators.  When multiple SF
   locators are in use, the locator to be used to reach a given SF can
   be driven by the PDP, a SF in a SFC, result of a load-balancing
   heuristic, etc.

4.3.  Service Function Discovery

   The local repository that lists the enabled SFs within an SFC-enabled
   domain may be built as a direct input from the administrative entity,
   or they may be discovered dynamically through appropriate protocol
   discovery means.

   Whichever method is selected by the administrative entity is a local
   decision and is therefore outside the scope of this document.  Any
   Service Function Discovery solution must comply with the requirements
   identified in [I-D.boucadair-sfc-requirements].



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4.4.  Building Service Function Maps

   Added-value services delivered to the end-user rely on the invocation
   of several SFs.  For each of these services, the administrative
   entity that operates an SFC-enabled domain builds one or several SF
   Maps.  Each of these maps characterizes the list of SFs to be invoked
   with their exact invocation order.

   Each SF Map is unambiguously identified with a unique identifier
   called the SF Map Index.  The SF Map Index MUST be described as an
   unsigned integer.

   Distinct chains can be applied for inbound and outbound traffic.  The
   directionality of traffic is not included as an attribute of the SF
   Map, but it may be implicitly described by using two SF Maps
   installed and maintained in the SFC Policy Table.  In such case,
   incoming packets would be marked with Index_1 for example, while
   outgoing packets would be forwarded according to a distinct SF Map
   identified with Index_2.

   An example of SF Map to handle IPv6 traffic destined to an IPv4
   remote server is defined as follows:

      {15, {IPv6_Firewall, HOST_ID_Inject, NAT64}}.

   To handle incoming packets destined to the same IPv6 host, the
   following SF Map can be defined:

      {10, {IPv4_Firewall, NAT64}}.

4.5.  Building Service Function Chaining (SFC) Policy Tables

   A PDP (Policy Decision Point, [RFC2753]) is the central entity which
   is responsible for maintaining SFC Policy Tables (Figure 1), and
   enforcing appropriate policies in SF Nodes and SFC Boundary Nodes
   (Figure 1).  PDP-made decisions can be forwarded to the participating
   nodes by using a variety of protocols (e.g., NETCONF [RFC6241]).

   One or multiple SFC-enabled domains may be under the responsibility
   of the same PDP.  Delimiting the scope of each SFC-enabled domain is
   under the responsibility of the administrative entity that operates
   the SF-enabled network.









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   o . . . . . . . . . . . . . . . . . . . . . . . o
   . SFC Policy Enforcement                        .
   .             +-------+                         .
   .             |       |-----------------+       .
   .     +-------|  PDP  |                 |       .
   .     |       |       |-------+         |       .
   .     |       +-------+       |         |       .
   o . . | . . . . . | . . . . . | . . . . | . . . o
   o . . | . . . . . | . . . . . | . . . . | . . . o
   .     |           |           |         |       .
   .     v           v           v         v       .
   . +---------+ +---------+ +-------+ +-------+   .
   . |SFC_BN_1 | |SFC_BN_n | | SF_1  | | SF_m  |   .
   . +---------+ +---------+ +-------+ +-------+   .
   . SFC-enabled Domain                            .
   o . . . . . . . . . . . . . . . . . . . . . . . o

                 Figure 1: SFC Policy Enforcement Scheme.

   The SF Node MUST be provisioned with the following information:

   o  Local SF Identifier(s): This information is required for an SF to
      identify itself within an SF Map.
   o  List of SF Maps: The PDP may configure the full list (default
      mode) or only as subset of SF Maps in which SF(s) supported by the
      SF Node is involved (see Section 10.7).
   o  List of SF Locators: The PDP may configure the full list of
      locators (default mode) or only the locators of next hop SFs of SF
      Maps in which SF(s) supported by the local SF node is involved
      (see Section 10.7).

      [DISCUSSION NOTE: Discuss if we maintain both forms of the SFC
      Policy table (full and lite) or select only one of them.]

   Likewise, the SFC Boundary Node MUST be provisioned with the
   following information:

   o  List of SF Maps
   o  List of SF Locators
   o  List of SF Map Classification Rules (see Section 5.2).

   In addition to the SFC Policy Table, other SF-specific policies can
   be installed by the PDP (e.g., configure distinct user profiles,
   activate specific traffic filters, configure traffic conditioners,
   etc.).

   Policies managed by the PDP may be statically instantiated or
   dynamically triggered by external means (e.g., a AAA server).



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   In the event of any update (e.g., define a new SF Map, delete an SF
   Map, add a new SF Locator, update classification policy), the PDP
   MUST forward the updated policy configuration information in all
   relevant SF Nodes and SFC Boundary Nodes.

   Distributing the load among several SF Nodes supporting the same SF
   can be driven by the PDP.  Indeed, the PDP can generate multiple
   classification rules and SF Maps to meet some load-balancing
   objectives.

   Load balancing may also be achieved locally by an SF Node.  If the SF
   Node, SF Classifier, or SF Boundary Node has a table that provides
   the SF locator(s) of SF Nodes that provide a particular SF then it is
   possible to make that local load balancing decision.

   The processing of packets by the nodes that belong to a SFC-enabled
   domain does not necessarily require any interaction with the PDP,
   depending on the nature of the SF supported by the nodes and the
   corresponding policies to be enforced.  For example, traffic
   conditioning capabilities [RFC2475] are typical SF functions that may
   require additional solicitation of the PDP for the SF node to decide
   what to do with some out-of-profile traffic.

5.  Theory Of Operation

   The behavior of each node of a SFC-enabled domain is specified in the
   following sections.  We assume that the provisioning operations
   discussed in Section 4 have been successful (i.e., SF functions have
   been adequately configured according to the SFC-specific policy to be
   enforced).

5.1.  SFC Boundary Node

   SFC Boundary Nodes act both as a SFC Ingress Node and as a SFC Egress
   Node for the respective directions of the traffic.

   Traffic enters a SFC-enabled domain at a SFC Ingress Node
   (Section 5.3) and exits the domain at a SFC Egress Node
   (Section 5.4).

5.2.  SFC Classifier

   The SFC Classifier classifies packets based on (some of) the contents
   of the packet.  Particularly, it classifies packets based on the
   possible combination of one or more header fields, such as source
   address, destination address, DS field, protocol ID, source port and
   destination port numbers, and any other information.




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   Each SF Map Classification Rule MUST be bound to one single SF Map
   (i.e., the classification rule must include only one SF Map Index).

5.3.  SFC Ingress Node

   When a packet is received through an interface of the SFC Ingress
   Node that connects to the outside of the SFC domain, the Ingress Node
   MUST:

   o  Inspect the received packet and check whether any existing SF Map
      Index is included in the packet.

      *  The SFC Ingress Node SHOULD be configurable with a parameter to
         indicate whether received SF Map Index is to be preserved or
         striped.  The default behavior is to strip any received SF Map
         Index.
      *  Unless explicitly configured to trust SF Map index, The SFC
         Ingress Node MUST strip any existing SF Map Index if the packet
         is received from an SFC-enabled domain that has not explicitly
         been designated as "trusted".
   o  Check whether the received packet matches an existing
      classification rule (see Section 5.2).
   o  If no rule matches, forward the packet to the next hop according
      to legacy forwarding behavior (e.g., based upon the IP address
      conveyed in the DA field of the header).
   o  If a rule matches, proceed with the following operations:

      *  Retrieve the locator of the first SF as indicated in the SF Map
         entry the rule matches.  If multiple locators are available,
         the selection can be based on local criteria (e.g., the closest
         /best path).
      *  Check whether the corresponding SF node is an immediate (L3)
         neighbor.

         +  If so, update the packet with the SF Map Index of SF Map
            entry it matches and then forward the packet to the
            corresponding SF Node.
         +  If not, (1) encapsulate the original packet into a new one
            that will be forwarded to the corresponding SF node, (2)
            update the encapsulated packet with the SF Map Index of SF
            Map entry it matches, and (3) forward the packet to the next
            hop to reach the first SF node.

   As a result of this process, the packet will be sent to an SF Node or
   an Intermediate Node.






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5.4.  SFC Egress Node

   When a packet is received through an interface that connects the SFC
   Egress Node to its SFC domain, the Egress Node MUST:

   o  Strip any existing SF Map Index.
   o  Forward the packet according to legacy forwarding policies.

5.5.  SF Node

   This section assumes the default behavior is each SF Node does not
   embed a Classifier as discussed in Section 10.4.

   When a packet is received by a SF Node, the SF Node MUST:

   o  Check whether the packet conveys a SF Map Index.
   o  If no SF Map Index is included, forward the packet according to
      legacy forwarding policies.
   o  If the packet conveys a SF Map Index,

      *  Retrieve the corresponding SF Map from the SFC Policy Table.
         If no entry is found in the table, forward the packet according
         to legacy forwarding policies.

            [DISCUSSION NOTE: Another design choice is to drop the
            packet and send a notification to the PDP.  The
            justification for avoiding to drop the packet is that an SF
            can be part of the forwarding path of an SFC to which it
            does not belong to.]
      *  If an entry is found in the SFC Policy Table, check whether the
         local SF Identifier is present in the SF Map:

         +  If not, forward the packet according to legacy forwarding
            policies.

               [DISCUSSION NOTE: One would argue the packet should be
               dropped.  The justification for avoiding to drop the
               packet is that an SF can be part of the forwarding path
               of an SFC to which it does not belong to + the SF node is
               provisioned with the full SFC Policy Table.]
         +  If so, the packet is decapsulated (if needed) and then
            presented as an input to the local SF.  In case several SFs
            are co-located in the same node, the packet is processed by
            all SFs indicated in the SF Map. Once the packet is
            successfully handled by local SF(s), the packet is forwarded
            to the next SF Node in the list or to an intermediate node
            (if the local SF Node is the last element in the SF Map).
            If the local SF node is not the last one in the SF Map, it



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            retrieves the next SF Node from the list, retrieve its
            locator for the SFC Policy Table, and forwards the packet to
            the next hop.  If the local SF Node is the last element in
            the SF Map, it forwards the packet to the next hop according
            to legacy forwarding policies.

5.6.  Intermediate Nodes

   An Intermediate Node is any node that does not support any Service
   Function and which is located within a SFC-enabled domain.

   No modification is required to intermediate nodes to handle incoming
   packets.  In particular, routing and forwarding are achieved using
   legacy procedures.

6.  Fragmentation Considerations

   If adding the Service Chaining Header would result in a fragmented
   packet, the classifier should include a Service Chaining Header in
   each fragment.  Doing so would prevent SF Nodes to dedicate resource
   to handle fragments.

7.  Differentiated Services

   When encapsulating an IP packet, the Ingress Node and each SF Node
   SHOULD use its Diffserv Codepoint (DSCP, [RFC2474]) to derive the
   DSCP (or MPLS Traffic-Class Field) of the encapsulated packet.

   Generic considerations related to Differentiated Services and tunnels
   are further detailed in [RFC2983].

8.  ECN (Explicit Congestion Notification) Considerations

   When encapsulating an IP packet, the Ingress Node and each SF Node
   SHOULD follow [RFC6040] for ECN re-marking purposes.

9.  Design Considerations

   This section discusses two main protocol issues to be handled in
   order to deploy SFC.

   A detailed design analysis is documented in
   [I-D.boucadair-sfc-design-analysis].








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9.1.  Transmit A SFC Map Index In A Packet

9.1.1.  SFC Map Index

   A SF Map Index is an integer that points to a SF Map.

   In order to avoid all nodes of a SFC-enabled domain to be SF-aware,
   this specification recommends to undertake classifiers at boundary
   nodes while intermediate nodes forward the packets according to the
   SF Map Index conveyed in the packet (SF Node) or according to typical
   forwarding policies (any SF-unaware node).

   An 8-bit field would be sufficient to accommodate deployment contexts
   that assume a reasonable set of SF Maps.  A 16-bit (or 32-bit) field
   would be more flexible (e.g., to accommodate the requirement
   discussed in Section 10.3).

9.1.2.  Where To Store SFC Map Indexes In A Packet?

   SF Map Indexes can be conveyed in various locations of a packet:

   o  At L2 level
   o  Define a new IP option or a new IPv6 extension header
   o  Use IPv6 Flow Label
   o  Use MPLS Label
   o  Re-use an existing field (e.g., DS field)
   o  TCP option
   o  GRE Key
   o  Define a new shim
   o  Etc.

9.2.  Steer Paths To Cross Specific SF Nodes

   A SFC Ingress Node or a SF Node MUST be able to forward a packet that
   matches an existing SF Map to the relevant next hop SF Node.  The
   locator of the next SF is retrieved from the SFC Policy Table.  In
   case the next SF Node in the list is not an immediate (L3) neighbor,
   a solution to force the packet to cross that SF Node MUST be
   supported.

10.  Deployment Considerations

10.1.  Generic Requirements

   The following deployment considerations should be taken into account:

   o  Avoid inducing severe path stretch compared to the path followed
      if no SF is involved.



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   o  Minimize path computation delays: due to the enforcement of
      classification rules in all participating nodes, misconception of
      Service function chaining, inappropriate choice of nodes elected
      to embed Service functions, etc., must be avoided.
   o  Avoid SF invocation loops: the design of SF chainings should
      minimize as much as possible SF invocation loops.  In any case,
      forwarding loops must be avoided.

10.2.  Deployment Models

   Below are listed some deployment model examples:

   1.  A full marking mechanism: Ingress nodes perform the
       classification and marking functions.  Then, involved SF Nodes
       process received packets according to their marking.

   2.  SF node mechanism, in which every SF Node embeds also a
       classifier, and the ingress node only decides the first node to
       forward to.  Packets are forwarded at each node according to
       local policies.  No marking is required when all SFs are co-
       located with a classifier.  This model suffers from some
       limitations (see Section 10.4).

   3.  A router-based mechanism: All SF Nodes forward packets once
       processed to their default router.  This default routes is
       responsible for deciding how the packet should be progressed at
       each step in the chain.  One or multiple routers can be involved
       in the same Service Function Chain.

   4.  A combination thereof.

10.2.1.  1.1.  Proxy Node for Legacy Service Functions

   It is not uncommon to have multiple legacy service nodes located in
   close vicinity with a service chain proxy node, or one to two links
   away.  The following figure depicts typical network architecture for
   chaining those service nodes that are not aware of service layer
   encapsulation.













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                        |1  -----   |n        |21   ---- |2m
                    +---+---+   +---+---+   +-+---+   +--+----+
                    |  SF1  |   |  SF2  |   | SF3 |   |  SF4  |
                    +---+---+   +---+---+   +--+--+   +--+--+-+
                        :           :          :         :  :
                        :           :          :         :  :
                         \         /            \       /
       +--------------+   +--------+             +---------+
   -- >|     SFC      | ->| Proxy  |--------->   | Proxy   | ---->
       |  Classifier  |   | Node-1 |             |  Node-i |
       +--------------+   +----+---+             +----+--+-+
                                 |--                  |  |
                                 V                    +--->
                            +--------+
                            | Proxy  |
                            |   -j   |----->
                            +--------+

   Various deployment options can be envisaged:

   1.  Upgrade legacy service nodes to support required SFC
       functionalities.

   2.  Enable Proxy Service Nodes to involve these legacy nodes in
       instantiated SFCs.

   3.  Exclude legacy service nodes from a SFC domain.

   It is up to the responsibility of each Service Provider to decide
   which option to deploy within its networks.

10.3.  On Service Function Profiles (a.k.a., Contexts)

   Service Functions may often enforce multiple differentiated policy
   sets.  These policy sets may be coarsely-grained or fine-grained.  An
   example of coarsely-grained policy sets would be an entity that
   performs HTTP content filtering where one policy set may be
   appropriate for child users whereas another is appropriate for adult
   users.  An example of finely-grained policy sets would be PCEF (3GPP
   Policy Control Enforcement Function) that has a large number of
   differentiated QoS and charging profiles that are mapped on a per-
   subscriber basis.

   The Service Function Chaining mechanism directly support coarsely-
   grained differentiated policy sets and indirectly support finely-
   grained differentiated policy sets.





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   From a Service Function Chaining perspective, each coarsely-grained
   policy set for a Service Function will be considered as a distinct
   logical instance of that Service Function.  Consider the HTTP content
   filtering example where one physical or virtual entity provides both
   child and adult content filtering.  The single entity is represented
   as two distinct logical Service Functions, each with their own
   Service Function Identifier from a chaining perspective.  The two
   (logical) Service Functions may share the same IP address or may have
   distinct IP addresses.

   Finely-grained policy sets, on the other hand, would unacceptably
   explode the number of distinct Service Chains that were required with
   an administrative domain.  For this reason, Service Functions that
   utilize finely-grained policy sets are represented as a single
   Service Function that has its own internal classification mechanism
   in order to determine which of its differentiated policy sets to
   apply.  Doing so avoids from increasing the size of the SFC Policy
   Table.

   The threshold, in terms of number of policies, between choosing the
   coarsely-grained policy or finely-grained policy technique is left to
   the administrative entity managing a given domain.

      [DISCUSSION NOTE: This section will be updated to reflect the
      conclusions of the discussions from the design analysis draft.]

10.4.  SF Node is also a Classifier

   If SF Nodes are also configured to behave as Classifiers, the SF Map
   Index is not required to be explicitly signalled in each packet.
   Concretely, the SFC Policy Table maintained by the SF Node includes
   classification rules.  These classification rules are enforced to
   determine whether the local SF must be involved.  If an incoming
   packet matches at least one classification rule pointing to an SF Map
   in which the SF Identifier is listed, the SF Node retrieves the next
   hop SF from the SF Map indicated in the classification rule.

   The packet is then handled by the local SF, and the SF Node
   subsequently forwards the packet to the next hop SF.  If not, the
   packet is forwarded to the next hop according to a typical IP
   forwarding policy.

   Let us consider the example shown in Figure 2.  The local SF Node
   embeds SFa.  Once classification rules and the SF Maps are checked,
   the SF Node concludes SFa must be invoked only when a packet matches
   Rules 1 and 3.  If a packet matches Rule 1, the next SF is SFC.  If a
   packet matches Rule 3, the next SF is SFh.




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   +-----------------------------------------------+
   |                SFC Policy Table               |
   +-----------------------------------------------+
   |Local SF Identifier: SFa                       |
   +-----------------------------------------------+
   |Classification Rules                           |
   | Rule 1: If DEST=IP1; then SFC_MAP_INDEX1      |
   | Rule 2: If DEST=IP2; then SFC_MAP_INDEX2      |
   | Rule 3: IF DEST=IP3; then SFC_MAP_INDEX3      |
   +-----------------------------------------------+
   |SF Maps                                        |
   | SFC_MAP_INDEX1: {SFa, SFc}                    |
   | SFC_MAP_INDEX2: {SFd, SFb}                    |
   | SFC_MAP_INDEX3: {SFa, SFh}                    |
   +-----------------------------------------------+

                    Figure 2: SFC Policy Table Example.

10.5.  SFs within the Same Subnet

   SF Nodes may be enabled in a SFC-enabled domain so that each of them
   has a direct L3 adjacency with other SF Nodes.  In such
   configuration, no encapsulation scheme is required to exchange
   traffic between these nodes.

10.6.  Service Function Loops

   SF Nodes use the SFC Policy Table to detect whether the local SF was
   already applied to the received packet (i.e., detect SF Loop).  The
   SF Node MUST invoke the local SF only if the packet is received from
   a SFC Boundary Node or a SF Node having an identifier listed before
   the local SF in the SF Map matched by the packet.  SF Loop detection
   SHOULD be a configurable feature.

   Figure 3 shows an example of a SFC Policy Table of a SF Node
   embedding SFa.  Assume a packet received from Locb that matches Rule
   2.  SFa must not be invoked because SFb is listed after SFa (see the
   SF Map list).  That packet will be forwarded without invoking SFa.













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   +-----------------------------------------------+
   |                SFC Policy Table               |
   +-----------------------------------------------+
   |Local SF Identifier: SFa                       |
   +-----------------------------------------------+
   |SF Maps                                        |
   | SFC_MAP_INDEX1: {SFa, SFc}                    |
   | SFC_MAP_INDEX2: {SFd, SFa, SFb, SFh}          |
   +-----------------------------------------------+
   |SFC Locators                                   |
   | Locator_SFb: Locb                             |
   | Locator_SFC: Locc                             |
   | Locator_SFd: Locd                             |
   | Locator_SFh: Loch                             |
   +-----------------------------------------------+

                     Figure 3: Dealing With SF Loops.

10.7.  Lightweight SFC Policy Table

   If SF loop detection is not activated in an SFC-enabled domain, the
   PDP may provision SF nodes with a "lightweight" SFC Policy Table.  A
   lightweight SFC Policy Table is a subset of the full SFC Policy
   Table that includes:

   o  Only the SF Maps in which the local SF is involved.
   o  Only the next hop SF instead of the full SF chain.

   An example of a lightweight SFC Policy Table is shown in Figure 4.

   +-----------------------------------------------+
   |                SFC Policy Table               |
   +-----------------------------------------------+
   |Local SF Identifier: SFa                       |
   +-----------------------------------------------+
   |Lite SF Maps                                   |
   | SFC_MAP_INDEX1, Next_Hop_SF = SFc             |
   | SFC_MAP_INDEX2, Next_Hop_SF = SFb             |
   +-----------------------------------------------+
   |SFC Locators                                   |
   | Locator_SFb: Locb                             |
   | Locator_SFC: Locc                             |
   +-----------------------------------------------+

                  Figure 4: Lightweight SFC Policy Table.






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10.8.  Liveness Detection Of SFs By The PDP

   The ability of the PDP to check the liveness of each SF invoked in a
   service chain has several advantages, including:

   o  Enhanced status reporting by the PDP (i.e., an operational status
      for any given service chain derived from liveness state of its
      SFs).
   o  Ability to support various resiliency policies (i.e., bypass SF
      Node, use alternate SF Node, use alternate chain, drop traffic,
      etc.) .
   o  Ability to support load balancing capabilities to solicit multiple
      SF instances that provide equivalent functions.

   In order to determine the liveness of any particular SF Node,
   standard protocols such as ICMP or BFD (both single-hop [RFC5881] and
   multi-hop [RFC5883]) may be utilized between the PDP and the SF
   Nodes.

   Because an SF Node can be responsive from a reachability standpoint
   (e.g., IP level) while the function its provides may be broken (e.g.,
   a NAT module may be down), additional means to assess whether an SF
   is up and running are required.  These means may be service-specific
   (e.g., [RFC6849], [I-D.tsou-softwire-bfd-ds-lite]).

   For more sophisticated load-balancing support, protocols that allow
   for both liveness determination and the transfer of application-
   specific data, such as SNMP and NETCONF may be utilized between the
   PDP and the SF Nodes.

11.  IANA Considerations

   This document does not require any IANA actions.

12.  Security Considerations

   Means to protect SFC Boundary Nodes and SF Nodes against various
   forms of DDoS attacks MUST be supported.  For example, mutual PDP and
   SF node authentication should be supported.  Means to protect SF
   nodes against malformed, poorly configured (deliberately or not) SFC
   Policy Tables should be supported.

   SFC Boundary Nodes MUST strip any existing SF Map Index when handling
   an incoming packet.  A list of authorized SF Map Indexes are
   configured in the SFC elements.

   NETCONF-related security considerations are discussed in [RFC6146].




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   Means to prevent SF loops should be supported.

   Nodes involved in the same SFC-enabled domain MUST be provisioned
   with the same SFC Policy Table.  Possible table inconsistencies may
   result in forwarding errors.

13.  Contributors

   The following individuals contributed to the document:

      Parviz Yegani
      Juniper Networks
      1194 N. Mathilda Ave.
      Sunnyvale,  CA 94089
      USA
      Email: pyegani@juniper.net

      Paul Quinn
      Cisco Systems, Inc.
      USA
      Email: paulq@cisco.com


      Linda Dunbar
      Huawei Technologies
      1700 Alma Drive, Suite 500
      Plano, TX 75075, USA
      Phone: (469) 277 5840
      Email: ldunbar@huawei.com

14.  Acknowledgments

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

15.  References

15.1.  Normative References

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

15.2.  Informative References








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   [I-D.boucadair-sfc-design-analysis]
              Boucadair, M., Jacquenet, C., Parker, R., and L. Dunbar,
              "Service Function Chaining: Design Considerations,
              Analysis & Recommendations", draft-boucadair-sfc-design-
              analysis-02 (work in progress), February 2014.

   [I-D.boucadair-sfc-requirements]
              Boucadair, M., Jacquenet, C., Jiang, Y., Parker, R.,
              Pignataro, C., and K. Kengo, "Requirements for Service
              Function Chaining", draft-boucadair-sfc-requirements-03
              (work in progress), February 2014.

   [I-D.ietf-sfc-problem-statement]
              Quinn, P. and T. Nadeau, "Service Function Chaining
              Problem Statement", draft-ietf-sfc-problem-statement-00
              (work in progress), January 2014.

   [I-D.sin-sdnrg-sdn-approach]
              Boucadair, M. and C. Jacquenet, "Software-Defined
              Networking: A Perspective From Within A Service Provider",
              draft-sin-sdnrg-sdn-approach-09 (work in progress),
              January 2014.

   [I-D.tsou-softwire-bfd-ds-lite]
              Tsou, T., Li, B., Zhou, C., Schoenwaelder, J., Penno, R.,
              and M. Boucadair, "DS-Lite Failure Detection and
              Failover", draft-tsou-softwire-bfd-ds-lite-06 (work in
              progress), February 2014.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, December
              1998.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2753]  Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
              for Policy-based Admission Control", RFC 2753, January
              2000.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels", RFC
              2983, October 2000.

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



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   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
              2010.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, June 2010.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, November 2010.

   [RFC6092]  Woodyatt, J., "Recommended Simple Security Capabilities in
              Customer Premises Equipment (CPE) for Providing
              Residential IPv6 Internet Service", RFC 6092, January
              2011.

   [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.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)", RFC
              6241, June 2011.

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

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

   [RFC6459]  Korhonen, J., Soininen, J., Patil, B., Savolainen, T.,
              Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, January 2012.

   [RFC6849]  Kaplan, H., Hedayat, K., Venna, N., Jones, P., and N.
              Stratton, "An Extension to the Session Description
              Protocol (SDP) and Real-time Transport Protocol (RTP) for
              Media Loopback", RFC 6849, February 2013.

Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes  35000
   France

   EMail: mohamed.boucadair@orange.com



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   Christian Jacquenet
   France Telecom
   Rennes  35000
   France

   EMail: christian.jacquenet@orange.com


   Ron Parker
   Affirmed Networks
   Acton,  MA
   USA

   EMail: Ron_Parker@affirmednetworks.com


   Diego R. Lopez
   Telefonica I+D
   Don Ramon de la Cruz, 82
   Madrid  28006
   Spain

   Phone: +34 913 129 041
   EMail: diego@tid.es


   Jim Guichard
   Cisco Systems, Inc.
   USA

   EMail: jguichar@cisco.com


   Carlos Pignataro
   Cisco Systems, Inc.
   USA

   EMail: cpignata@cisco.com













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