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Versions: 00 01 02 03 04 05                                             
Service Function Chaining                                       S. Homma
Internet-Draft                                                  K. Naito
Intended status: Informational                                       NTT
Expires: July 27, 2015                                       D. R. Lopez
                                                          Telefonica I+D
                                                          M. Stiemerling
                                                                NEC/H-DA
                                                               D. Dolson
                                                                Sandvine
                                                        January 23, 2015


          Analysis on Forwarding Methods for Service Chaining
             draft-homma-sfc-forwarding-methods-analysis-01

Abstract

   Some working groups of the IETF and other Standards Developing
   Organizations are now discussing use cases of a technology that
   enables data packets to traverse appropriate service functions
   located remotely through networks.  This is called Service Chaining
   in this document.  (Also, in Network Functions Virtualisation (NFV),
   a subject that forwarding packets to required service functions in
   appropriate order is called VNF Forwarding Graph.)  This draft does
   not focus only on SFC method, and thus, use the term "Service
   Chaining".  SFC may be one of approaches to realize Service Chaining.
   There are several Service Chaining methods to forward data packets to
   service functions, and the applicable methods will vary depending on
   the service requirements of individual networks.

   This document presents the results of analyzing packet forwarding
   methods and path selection patterns for achieving Service Chaining.
   For forwarding data packets to the appropriate service functions,
   distribution of route information and steering data packets following
   the route information, are required.  Examples of route information
   are packet identifier and the routing configurations based on the
   identifier.  Also, forwarding functions are required to decide the
   path according to the route information.

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/.



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   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 July 27, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   4
   3.  Classification of Forwarding Methods and SP Decision Patterns   5
     3.1.  Forwarding Methods  . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Method 1: Forwarding Based on Flow Identifiable
               Information . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Method 2: Forwarding with Stacked Transport Headers .   6
       3.1.3.  Method 3: Forwarding Based on Service Chain
               Identifiable Tags . . . . . . . . . . . . . . . . . .   8
     3.2.  Service Path Selection Patterns . . . . . . . . . . . . .   9
       3.2.1.  Pattern 1: Static Selection of End to End Service
               Path  . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.2.2.  Pattern 2: Dynamic Selection of Segmented Service
               Path  . . . . . . . . . . . . . . . . . . . . . . . .  12
   4.  Consideration of Service Chaining Methods and Architecture
       Patterns  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  Analysis of 3.1. Forwarding Methods . . . . . . . . . . .  18
       4.1.1.  Analysis of Method 1  . . . . . . . . . . . . . . . .  18
       4.1.2.  Analysis of Method 2  . . . . . . . . . . . . . . . .  18
       4.1.3.  Analysis of Method 3  . . . . . . . . . . . . . . . .  19
     4.2.  Analysis of 3.2. Service Paths Selection Patterns . . . .  19
       4.2.1.  Analysis of Pattern 1 . . . . . . . . . . . . . . . .  19
       4.2.2.  Analysis of Pattern 2 . . . . . . . . . . . . . . . .  20
     4.3.  Example of selecting Methods and Patterns . . . . . . . .  24



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       4.3.1.  Example A: Datacenter Network . . . . . . . . . . . .  24
       4.3.2.  Example B: Current Mobile Carrier's Network . . . . .  24
       4.3.3.  Example C: Fixed and Mobile Converged Network . . . .  25
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  25
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   Service Chaining is a technology that enables data packets to
   traverse the appropriate service functions deployed in networks.
   This draft assumes that Service Chaining is achieved in the following
   steps:

   a. A classification function identifies data packets and determines
      the set of services that will be provided for the packets and in
      which order.

   b. The path, that the packets will traverse for reaching the required
      service functions, is established based on the result of step a.

   c. Forwarding functions determine the appropriate destination and
      forward each packet to the next hop according to the path.

   d. A service function provides services to received packets and
      return each packet to the forwarding function.

   e. Steps c and d are repeated until each packet has been transferred
      to all required service functions.

   f. After a packet has been transferred to all required Service
      Functions, it is forwarded to its original destination.

   There are several forwarding methods for Service Chaining, and they
   can be classified into certain categories in terms of distribution of
   information for setting the paths and decision of the paths.  The
   methods used to distribute the information and the patterns used to
   decide the paths will affect the mechanism of Service Chaining as
   well as service flexibility.

   The applicable methods vary depending on network requirements, and
   thus, classifying and determining forwarding methods will be
   important in designing the architecture of Service Function Chaining
   (SFC).  This document provides the results of analyzing forwarding
   methods for Service Chaining.




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   OAM, security, and redundancy are outside the scope of this draft.

2.  Definition of Terms

   Term "Classification", "Classifier" referred to
   [I-D.ietf-sfc-architecture].  Term "Service Function", "Service Node"
   referred to [I-D.ietf-sfc-dc-use-cases].

   Service Chaining:  A technology that lets data packets traverse a
      series of service functions.

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

   Classifier (CF):  The entity that performs classification.

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

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

   Service Node (SN):  A virtual or physical device that hosts one or
      more service functions, which can be accessed via the network
      location associated with it.

   Forwarder (FWD):  The entity, responsible for forwarding data packets
      along the service path, which includes delivery of traffic to the
      connected service functions.  FWD handles Forwarding Tables, which
      is used for forwarding packets.

   Control Entity (CE):  The entity responsible for managing service
      topology and indicating forwarding configurations to Forwarders.

   Service Chain (SC):  A service chain defines an ordered list of
      service functions that must be applied to user packets selected as



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      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.

   Service Path (SP):  The instantiation of a service chain in the
      network.  Packets follow a service path through the requisite
      service functions.  Service path shows a specific path of
      traversing SF instance.  For example, SC is written as SF#1 ->
      SF#2 -> SF#3 (This shows an ordered list of SFs), and SP is
      written as SF#1_1(1_1 means instance 1 of SF1) -> SF#2_1 ->
      SF#3_1.

   Service Chaining Domain (SC Domain):  The domain managed by one or a
      set of CEs.

   Service Path Information (SPI):  The information used to forward
      packets to The appropriate SFs based on the selected service.
      Examples of SPI include routing configurations for Forwarders,
      transport headers for forwarding packets to required SFs, and
      service/flow identifiable tags.

3.  Classification of Forwarding Methods and SP Decision Patterns

3.1.  Forwarding Methods

   In Service Chaining, data packets are transferred to service
   functions, which can be located outside the regular computed path to
   the original destination.  Therefore, a routing mechanism that is
   different from general L2/L3 switching/routing may be required.  The
   routing mechanism can be classified into three methods in terms of
   distribution of SPI and packet forwarding.

3.1.1.  Method 1: Forwarding Based on Flow Identifiable Information

   The mechanism of method 1 is shown in Figure 1.  In this method,
   routing configurations based on flow identifiable information, such
   as 5-tuple (e.g. dst IP, src IP, dst port, src port, tcp) are
   indicated to the CF and each FWD.  There may be an CE to handle this.
   The flow identifiable information can be constructed with some fields
   of L2 or L3 or combination of those.  The information can be
   configured either before packets arrive, or at the time packets
   arrive at CF and FWD.  Each FWD identifies the packets with flow
   identifiable information and forwards the packets to the SFs
   according to the configuration.  This method does not require
   changing any fields of the original packet frame.






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*Distribution model of SPI*

      +----------------+
      | Control Entity |
      +----------------+
           ^ |     indication of routing configuration
           | |           based on packet identifiable information
           | +---------------+-------------------------------+--------->
           | |               |                               |
           | |               |                               |
           | v               v                               v
       +--------+        +-------+        +------+       +-------+
------>|   CF   |------> |  FWD  |------> | SF#1 |------>|  FWD  |----->
       +--------+        +-------+        +------+       +-------+

////////////////////////////////////////////////////////////////////////
*Forwarding Tables*

Locate:     [CF]             [FWD]                           [FWD]

Table:   192.168.1.1       192.168.1.1                    192.168.1.1
          ->FWD#1           ->SF#1                         ->SF#2
         10.0.1.1          10.0.1.1                       10.0.1.1
          ->FWD#1           ->FWD#2                        ->SF#2
         ...               ...                            ...

////////////////////////////////////////////////////////////////////////
*Condition of Packet*

Locate:     [CF]             [FWD]           [SF#1]          [FWD]

         +-------+         +-------+        +-------+      +-------+
Packet:  |  PDU  |         |  PDU  |        |  PDU  |      |  PDU  |
         +-------+         +-------+        +-------+      +-------+


        Figure 1: Forwarding Based on Flow Identifiable Information

3.1.2.  Method 2: Forwarding with Stacked Transport Headers

   The mechanism of method 2 is shown in Figure 2.  In this method, the
   CF classifies packets and stacks transport headers in which actual
   network address is included, e.g., MPLS or GRE headers, onto the
   packets based on the classification.  This method does not require
   any forwarding function for forwarding packets based on the service
   information.  Forwarding functions of underlay networks forward the
   packets to SFs following the outermost header.  The outermost header




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   is removed after service process of the SF.  The actions are repeated
   until all headers are removed.

*Distribution model of SPI*

        +----------------+
        | Control Entity |
        +----------------+
            ^ |
            | |    indication of
            | |      stacking headers
            | v
         +--------+       +-------+       +------+       +------+
-------->|   CF   |------>| SF#1  |------>| SF#2 |------>| SF#3 |------>
         +--------+       +-------+       +------+       +------+

////////////////////////////////////////////////////////////////////////
*Forwarding Tables*

Locate:       [CF]

Table:    192.168.1.1           __/__/__/__/__/__/__/__/__/__/__/__/__/
           ->Stack #1,2,3       __/ Packets are forwarded to SFs by __/
          10.0.1.1 FWD1         __/ the outermost transport header. __/
           ->Stack #1,3         __/__/__/__/__/__/__/__/__/__/__/__/__/
          ...

////////////////////////////////////////////////////////////////////////
*Condition of Packet*

Locate:       [CF]           [SF#1]          [SF#2]         [SF#3]

           +--------+
Header:    |To SF#1 |
           +--------+       +--------+
           |To SF#2 |       |To SF#2 |
           +--------+       +--------+     +--------+
           |To SF#3 |       |To SF#3 |     |To SF#3 |
           +--------+       +--------+     +--------+
               :                :              :              :
           +--------+       +--------+     +--------+      +--------+
Packet:    |  PDU   |       |  PDU   |     |  PDU   |      |  PDU   |
           +--------+       +--------+     +--------+      +--------+


       Figure 2: Forwarding with Stacked Multiple Transport Headers





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3.1.3.  Method 3: Forwarding Based on Service Chain Identifiable Tags

   The mechanism of this method is shown in Figure 3.  In this method, a
   CF classifies each packet and attaches a tag for identifying the
   service or flow on the packets based on the classification.  The
   routing configuration based on the tags is sent to each FWD (from
   some CE) in advance.  Each FWD forwards packets to the SFs following
   the configuration and the tag.  After a packet has traversed all SFs,
   the tag is removed and the packet is transported to the original
   destination.









































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  *Distribution model of SPI*

      +----------------+
      | Control Entity |
      +----------------+
           ^ |     indication of attached tag
           | |       and routing configuration based on tags
           | +----------------+------------------------------+--------->
           | |                |                              |
           | |                |                              |
           | v                v                              v
        +--------+        +-------+       +------+       +-------+
  ----->|   CF   |------> |  FWD  |------>| SF#1 |------>|  FWD  |----->
        +--------+        +-------+       +------+       +-------+

  //////////////////////////////////////////////////////////////////////
  *Forwarding Tables*

  Locate:  [CF]             [FWD]                          [FWD]

  Table: 192.168.1.1        IF ID#1,3                   IF ID#1,2,5
          ->Stack ID#1       ->SF#1                       ->SF#2
         10.0.1.1 FWD1
          ->Stack ID#2
         ...                ...                         ...

  //////////////////////////////////////////////////////////////////////
  *Condition of Packet*

  Locate:  [CF]             [FWD]         [SF#1]           [FWD]

         +-------+        +-------+      +-------+       +-------+
  Tag:   | ID#1  |        | ID#1  |      | ID#1  |       | ID#1  |
         +-------+        +-------+      +-------+       +-------+
  Packet:|  PDU  |        |  PDU  |      |  PDU  |       |  PDU  |
         +-------+        +-------+      +-------+       +-------+


       Figure 3: Forwarding Based on Service Chain Identifiable Tags

3.2.  Service Path Selection Patterns

   Since SC contains only logical information (e.g. series of services
   that are applied to flows and their sequences), the actual instances,
   which are called SPs, are needed in order for the forwarding process
   to work.  In this process, an instance of SP is created at certain
   points during a packet's delivery.  Therefore, to forward packets,
   the SC needs to be turned into an SP, which indicates specific FWDs



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   (or switches, routers) and SFs that the packets will be forwarded to.
   In the points translating SC to SP, the paths that determine the
   Service Chaining are classified into two patterns.

3.2.1.  Pattern 1: Static Selection of End to End Service Path

   The translation point is only a CF; that is, the SP is statically
   pre-established as an end-to-end path and a CF inserts packets into
   the appropriate path based on the result of the classification.  Each
   FWD on the route has a routing table to uniquely determine the next
   destination of packets, and each FWD statically forwards the received
   packets to the next destination.  FWD requires only a function to
   receive indications of routing configurations from the CE.  Pattern 1
   can be achieved in the following ways.

3.2.1.1.  SF Shared Model

   Figure 4 shows the mechanism of this way.  An SF is shared by
   multiple SPs.  In this way, the FWDs require a function to identify
   SP for each packet and insert the packets into the next appropriate
   hop.






























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*Path Structure*

  +----+    +-----+   +----+   +-----+   +------+   +-----+   +----+
  |    |SC#1| FWD |   |SF#1|   | FWD |   |SF#2_1|   | FWD |   |SF#3| SP#1
  |    |================================================================>
  |    |SC#2|     |   |    |   |     |   +------+   |     |   |    | SP#2
  |    |===============================# +------+ #=====================>
  |    |    |     |   +----+   |     | # |SF#2_2| # |     |   +----+
  |    |    |     |            |     | #==========# |     |
->| CF |    +-----+            +-----+   +------+   +-----+
  |    |
    .         .
    .         .
    .         .
            +-----+   +----+                        +-----+   +----+
  |    |SC#n| FWD |   |SF#4|                        | FWD |   |SF#5| SP#n
  |    |================================================================>
  +----+    +-----+   +----+                        +-----+   +----+

                                                         SC:Service Chain
/////////////////////////////////////////////////////////////////////////
*Packet Flow*

Service Chain#1:
SP#1
  [ CF ]--->[ FWD ]-->[SF#1]-->[ FWD ]-->[SF#2_1]-->[ FWD ]-->[SF#3]-->

Service Chain#2:
SP#2
  [ CF ]--->[ FWD ]-->[SF#1]-->[ FWD ]-->[SF#2_2]-->[ FWD ]-->[SF#3]-->
    :
Service Chain#n:
SP#n
  [ CF ]--->[ FWD ]-->[SF#4]----------------------->[ FWD ]-->[SF#5]-->


                         Figure 4: SF Shared Model

3.2.1.2.  SF Dedicated Model

   Figure 5 shows the mechanism of this style.  An SF (instance) is used
   by only one single SP; in other words, there is an SF instance per
   SP.  At each FWD, incoming packets are statically routed to a single
   predefined next hop.







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*Path Structure*

  +----+    +-----+  +------+  +-----+  +------+  +-----+  +------+
  |    |SC#1| FWD |  |SF#1_1|  | FWD |  |SF#2_1|  | FWD |  |SF#3_1| SP#1
  |    |================================================= =============>
  |    |    +-----+  +------+  +-----+  +------+  +-----+  +------+
  |    |    +-----+  +------+  +-----+  +------+  +-----+  +------+
  |    |SC#2| FWD |  |SF#1_2|  | FWD |  |SF#2_2|  | FWD |  |SF#3_2| SP#2
  |    |===============================================================>
->| CF |    +-----+  +------+  +-----+  +------+  +-----+  +------+
  |    |
    .           .
    .           .
    .           .
            +-----+  +------+                    +-----+   +------+
  |    |SC#n| FWD |  | SF#4 |                    | FWD |   | SF#5 | SP#n
  |    |===============================================================>
  +----+    +-----+  +------+                    +-----+   +------+

                                                        SC:Service Chain
////////////////////////////////////////////////////////////////////////
*How packets traverse*

Service Chain#1:
SP#1
  [ CF ]--->[ FWD ]->[SF#1_1]->[ FWD ]->[SF#2_1]->[ FWD ]->[SF#3_1]-->

Service Chain#2:
SP#2
  [ CF ]--->[ FWD ]->[SF#1_2]->[ FWD ]->[SF#2_2]->[ FWD ]->[SF#3_2]-->
    :
Service Chain#n:
SP#n
  [ CF ]--->[ FWD ]->[ SF#4 ]-------------------->[ FWD ]->[ SF#5 ]-->


                       Figure 5: SF Dedicated Model

3.2.2.  Pattern 2: Dynamic Selection of Segmented Service Path

   The mechanism of this style is shown in Figure 6.  The translation
   points are a CF and some FWDs.  The SP is established by a series of
   multiple paths, which are sectioned by CFs and FWDs.  The path, which
   is sectioned by CFs and FWDs, is referred to as a segmented path in
   this draft.  CFs or FWDs that select the next segmented path may
   require notification of routing configurations from the CE.
   Moreover, some FWDs require functions to select the destination of
   packets from various alternatives and to retrieve the information for



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   selecting the next path.  For example, each FWD obtains metric
   information or load conditions of servers and selects an optimal
   segmented path based on the information.  The CE may have the
   selection mechanism and may notify CFs or FWDs of it.


*Path Structure*

  +----+    +-----+   +----+   +-----+   +------+   +-----+   +----+
  |    |SC#1| FWD |   |SF#1|   | FWD |   |SF#2_1|   | FWD |   |SF#3| SP#1
  |    |==========================*=====================================>
  |    |    |     |   |    |   |  #  |   +------+   |     |   |    | SP#2
  |    |    |     |   |    |   |  #  |   +------+ #=====================>
  |    |    |     |   +----+   |  #  |   |SF#2_2| # |     |   +----+
  |    |    |     |            |  #===============# |     |
->| CF |    +-----+            +-----+   +------+   +-----+
  |    |
    .         .
    .         .
    .         .
            +-----+   +----+                        +-----+   +----+
  |    |SC#n| FWD |   |SF#4|                        | FWD |   |SF#5| SP#m
  |    |================================================================>
  +----+    +-----+   +----+                        +-----+   +----+
                                                         SC:Service Chain
/////////////////////////////////////////////////////////////////////////

*How packets traverse*

Service Chain#1:
SP#1
  [ CF ]--->[ FWD ]-->[SF#1]-->[ FWD ]-->[SF#2_1]-->[ FWD ]-->[SF#3]-->

SP#2
  [ CF ]--->[ FWD ]-->[SF#1]-->[ FWD ]-->[SF#2_2]-->[ FWD ]-->[SF#3]-->
    :
Service Chain#n:
SP#m
  [ CF ]--->[ FWD ]-->[SF#4]----------------------->[ FWD ]-->[SF#5]-->


           Figure 6: Dynamic Selection of Segmented Service Path

   In addition, this pattern accepts establishment of hierarchical
   domains as following:






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3.2.2.1.  Hierarchical Service Path Domains

   Complex problems often become manageable with a hierarchical
   approach.  This pattern allows network-wide orchestration of Service
   Chaining to be relatively simple, while hiding the complexities of
   fine-grained policy-based path selection within sub-domains.  Each
   sub-domain can be independently administered and orchestrated.

   Figure 7 shows two levels of hierarchy in a service provider's
   network.  At the top level in the hierarchy, Service Chaining
   components are:

   1.  Edge-classifiers (Edge CF) that reside near the edge of a service
       provider's domain and

   2.  SF sub-domains that reside in data centers.

   3.  SF Domain Proxies that reside in data centers, linking together
       the levels of the hierarchy.  To the higher level, this is an SF.
       To the lower level, this is a classifier and FWD.































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   *How packets traverse*

     +----+    +-----+  +----------------------+   +-----+
     |    |SC#1| FWD |  |SF Domain Proxy #1    |   | FWD |
   ->|    |================*                *===============>
     |    |    +-----+  |  # (in DC#1)      #  |   +-----+
     |    |             |  V                #  |
     |Edge|             |+---+            +---+|   Top domain
     | CF |    * * * * *||CF | * * * * * *|FWD|| * * * * *
     |    |    *        |+---+            +-+-+|         *
     |    |    *        | | |              | | |    Sub  *
     |    |    *        +-o-o--------------o-o-+   domain*
     |    |    *   SC#1.2 | |SC#1.1        ^ ^       #1  *
     |    |    *    +-----+ |              | |           *
     |    |    *    |       V              | |           *
     |    |    *    |     +---+  +------+  | |           *
     |    |    *    |     |FWD|->|SF#1_1|--+ |           *
     |    |    *    |     +---+  +------+    |           *
     |    |    *    V                        |           *
     |    |    *  +---+  +------+  +---+  +------+       *
     |    |    *  |FWD|->|SF#1_2|->|FWD|->|SF#2_1|       *
     |    |    *  +---+  +------+  +---+  +------+       *
       .       * * * * * * * * * * * * * * * * * * * * * *
       .
     |    |    +-----+   +-------------------+       +-----+
     |    |SC#n| FWD |   | SF Domain Proxy#q |       | FWD |
     |    |=======================================================>
     |    |    +-----+   |     (in DC#m)     |       +-----+
     +----+              +-------------------+
                     (Details of sub-domain #q not shown)

       Figure 7: Service Chain Hierarchy in Service Provider Network

   The components within an SF sub-domain are opaque at the top level;
   each SF domain proxy acts as a single SF node in the top-level
   domain.  A service path in the top-level domain may visit multiple
   sub-domains.

   At the lower level in the hierarchy, each sub-domain contains an
   independently administrated Service Chaining network, generally
   comprised of multiple instances of multiple types of hosts, most
   likely (but not necessarily) within the same data center.  There is
   no need for knowledge of the "big picture" at the level of the SF-
   sub-domain except as required to forward packets to the other SFs
   that are the next hop of each chain.

   Note that different encapsulation methods can be used at each layer
   in the hierarchy, provided the SF domain-Proxy can translate between



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   them.  For example, MPLS could be used to deliver packets from
   network edge to the SF clusters within data centers, and NSH
   [I-D.quinn-sfc-nsh] could be used within the data center.

   Details of Top Level of Hierarchy

   In this pattern, referring to Figure 8, network-wide Service Chaining
   orchestration is only concerned with creating service paths from
   network edge points to sub-domains within data centers and
   configuring classifiers at a coarse level to get the correct hosts'
   traffic onto paths that will arrive at appropriate sub-domains.  The
   figure shows one possible service chain passing from edge, through
   two sub-domains, to network egress.

   This top level of orchestration may attach meta-data to provide
   context from the network edge into the data center.

                          +------------+
                          |Sub-domain#1|
                          |  in DC1    |
                          +----+-------+
                               |
                        .------+---------.   +--+
                +--+   /     /  |         \--|CF|
            --->|CF|--/---->'   |          \ +--+
                +--+ /  SC#1    |           \
                     |          |            |
                     |          |    .------>|--->
                     |         /    /        |
                     \         |   /        /
                +--+  \        |  /        /  +--+
                |CF|---\       V /        /---|CF|
                +--+    '------+---------'    +--+
                               |
                          +----+-------+
                          |Sub-domain#2|
                          |   in DC2   |
                          +------------+

           Figure 8: Network-wide view of Top Level of Hierarchy

   The orchestration at this top level must ensure bidirectional path
   symmetry so that inbound packets traverse sub-domains in the reverse
   order as outbound packets.

   Because classifiers must have rules to handle any traffic passing
   through the network, we believe that a useful approach to
   classification will be to assign traffic to service function paths on



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   the basis of coarse classification like subscriber tier, tenant or
   VRF identifier.  These classification rules could be relatively
   static, changing in response to provisioning but not in response to
   traffic.

   In some networks it might be possible to create a rule per
   residential subscriber, resulting in rule updates when subscribers
   are assigned IP addresses.  However, with judicious allocation of IP
   blocks, entire classes of subscribers could be classified with IP-
   prefix rules.  Similarly, in a mobile network path selection could be
   based on APN.

   Hence, there are methods of globally managing very large networks by
   choosing a suitable classification granularity.

   Details of Lower Level of Hierarchy

   Within each SF sub-domain, there are:

   1.  An SF domain-proxy to receive incoming data packets on any of the
       configured service chains and load-balance (if necessary) traffic
       to classifiers,

   2.  Classifier(s) to select internal service chain to use,
       potentially based on stateful flow analysis, DPI, etc.

   3.  Service components comprised of FWD and SF.

   Local Service Chaining orchestration is concerned with providing
   viable paths to various functions, providing failure recovery, NFV
   elasticity, etc.

   Classification within each sub-domain can be concerned with
   determining the local service paths for individual transport-layer
   flows based on ports, DPI and meta-data provided by the higher-level
   chain.

   For any classifier that is transport-layer-stateful, it is most
   efficient for the same classifier instance to handle traffic in both
   directions of a bidirectional connection.  State tracking may require
   that service function paths begin and end at the same node with the
   flow state, where the same classifier instance can be used for both
   directions of traffic.








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4.  Consideration of Service Chaining Methods and Architecture Patterns

   This chapter presents the results of analyzing the forwarding methods
   and architecture patterns in chapter 3.

4.1.  Analysis of 3.1.  Forwarding Methods

4.1.1.  Analysis of Method 1

   This method can achieve Service Chaining without adding any headers
   to packets, so it may not cause any increase in packet size or be
   subject to MTU restrictions.  Furthermore, this method does not
   require additional functions within SFs to be applied to any headers
   because data packets are transported in original format.  Therefore,
   it will be easier to use legacy SFs for network operators.

   However, forwarding entries or static configuration for flows at each
   FWD is required.  For example, if there are 10,000 flows to be
   handled at a CF/FWD, the routing table for each CF/FWD uses 10,000
   flow entries at most.  Therefore, it might not be feasible for large-
   scale networks such as carrier networks that handle a SC per user
   (which means that individual users have their own policies), because
   some large carriers have over a million users and even more flows.
   Another concern is the traffic increase in the control plane because
   route setting is required for each flow.  Moreover, it may be hard to
   use this method if some service functions modify header fields of a
   packet or frame, for example, NAT/NAPT, in a chain.  For example, if
   a NAT changes the IP address of packets dynamically, the FWDs that
   follow need to renew their routing tables.  The results of the above
   analysis suggest that this method may be suitable for networks with a
   limited number of flows.

4.1.2.  Analysis of Method 2

   In this method, none of the FWDs require any specific routing tables
   for Service Chaining, but they require a function to forward packets
   based on header information, and to remove the outermost header from
   the received packets.  Therefore, the control plane would be simple
   because the SC controller would not be required to manage the routing
   configuration of FWDs.  Also, there are already several technologies
   proposed that can be used to achieve this method, such as MPLS.

   However, the more the SFs packets traverse, the more headers have to
   be added to the packet and this in turn means that the packet size
   increases.  But packet sizes are restricted by the minimal available
   MTU of any link in the network path and exceeding the MTU will
   require to fragment the original packet before starting to add more
   headers required to the service chaining.  This requires more



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   complexity in processing due to the fragmentation, adds a new source
   of errors, as fragments of packets can get lost and so the whole
   original packet will get discarded, and also will cause an increase
   in traffic as more packets have to be processed by the network.
   Moreover, from a hardware point of view, it might be challenging for
   FWDs or SFs to process packets with variable length headers.  In
   terms of SF equipment, if fragmented packets need to be reassembled
   at every SF, this would be very wasteful of CPU resources, and some
   equipment has restricted resources and memory for reassembly.  The
   results of the above analysis indicate that this method would be
   appropriate when the number of SFs in an SC is small, or most packets
   are forwarded to a static SP.  On the other hand, it may be
   unsuitable in cases where there are many SFs in a chain.

4.1.3.  Analysis of Method 3

   In this method, a tag is defined for each Service Chain.  By adopting
   single fixed-length tags, this method can prevent an increase in the
   amount of traffic in the data plane, and can provide an upper bound
   on packet size.  (Problems which happen as a result of exceeding MTU
   are stated in 4.1.2.)  This method also enables FWDs to save
   resources for flow tables and all SPs may be established in advance
   in most of cases.  It enables CFs and FWDs which are located on the
   SP to change the following SP, because the CFs and FWDs have only to
   change the tag attached on the packet.  Therefore, this method has
   many advantages in terms of scalability, and it might be appropriate
   for use in large-scale networks.

   However, this method might require renewal of equipment, or Operating
   Systems (OSes) installed in hardware, or software, or any other
   components to realize the method in network which includes SFs, if
   this tag handling is an entirely new mechanism.  Furthermore
   discussion might be required to deploy such standardized
   technologies.

4.2.  Analysis of 3.2.  Service Paths Selection Patterns

4.2.1.  Analysis of Pattern 1

   In this pattern, the mechanism of FWDs would be simpler than the one
   in pattern 2 because FWDs do not require any functions to select
   paths or retrieve any information for determination of the next hop.
   Moreover, it is not necessary to maintain the state of each flow.
   Therefore, existing protocols for virtualizing networks, such as
   VxLAN or MPLS, can be used to achieve Service Chaining in this
   pattern.





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   However, this pattern will impact the flexibility of the SCs, as
   adding new SFs to a SC, removing SFs from a SC, or migrating SFs to
   other locations requires an update or new creation of a path in the
   Service Path.  Furthermore, unified management of FWDs and SFs in an
   SC domain would be required in setting end-to-end paths.  Therefore,
   the management system of SPs, for example, a CE, for wide-area
   networks that include several segments may be massive and complex.
   Figure 9 shows the case in which SPs are established across multiple
   datacenters in pattern 1.  In Figure 9, a CE manages multiple
   datacenters as a single SC domain for establishing SPs across
   multiple datacenters.

   In pattern 4.2.1.2 (SF Dedicated Model), the number of flow entries
   that FWDs hold can be extremely small, as FWDs hold only static next-
   hop information.  Also, the CF function would be simple, as the CF
   only determines the gateway of each SP.  However, because the SF
   (instance) is settled for each SP, resource usage would be high if
   there were many SPs.


                            +--------------+
        . . . . . . . . . . |Control Entity| . . . . .
        .         .         +--------------+         .
        .         .                                  .
   * *  . * * * * . * * * * * * * * * * * * * * * *  . * * * * * * * * *
   *    .         .                                  .                 *
   *    .         .                                  .                 *
   *    .      .-----.        .-----------.       .-----.              *
   * +----+   /  DC#1 \      /     WAN     \     /  DC#2 \             *
   * |    |=====================================================> SP#1 *
   * | CF |=====================================================> SP#2 *
   *   :                            :                              :   *
   * |    |=====================================================> SP#n *
   * +----+   \       /      \             /     \       /             *
   *           '-----'        '-----------'       '-----'              *
   *                                                                   *
   *                           SC Domain                               *
   * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *


     Figure 9: Establishment of SPs Across Multiples DCs in Pattern 1

4.2.2.  Analysis of Pattern 2

   In this pattern, SPs are established with a combination of segmented
   paths, so it enables SPs to be established flexibly (which means, CEs
   do not need to constantly manage the entire end-to-end SP) based on
   additional information such as the load condition of SFs.



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   Furthermore, as it is described in the previous section, in cases
   where some SPs traverse multiple datacenters across a WAN, SPs could
   be established with a combination of segmented paths that each
   datacenter determines independently based on the Service Chain
   information.  Therefore, it might be possible to separate SC domains
   into several small areas for WANs, which would enable a simpler
   configuration of each CE.  Figure 10 shows the case in which SPs are
   established across multiple datacenters in pattern 2.  In Figure 10,
   each CE manages a single datacenter independently, and the CEs
   synchronize the Service Chain information for establishing and
   determining the appropriate segmented SPs in each domain.

   However, the (fault) monitoring of the whole SC can get harder as
   multiple domains are part of the SC.  On the other hand each domain
   can perform its fault management as required (and probably better as
   it is more specific).  This will require an overarching (fault)
   monitoring where information from multiple SC domains is collected
   and aggregated to get a full view of the end-to-end service of the
   SC.

   Moreover, in this pattern, some FWDs may require additional
   mechanisms to select the next segmented path, and the FWDs must
   maintain the states of each flow because some SFs require a stateful
   process, and the FWDs need to insert packets into the same SF
   instances in the same session.

   In case that SC information is conveyed to some components via data
   plane as any encapsulation, a new protocol such as SFC
   [I-D.ietf-sfc-architecture] will be required.






















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                          Synchronization of
                           Service Chain info.
                 +--------------------------------------+
                 |                                      |
                 v                                      v
            +--------+                              +--------+
            |  CE#1  |                              |  CE#2  |
            +--------+                              +--------+
                 .                                      .
    * * * * * *  .  * * * * * *            * * * * * *  .  * * * * * *
    *            .            *            *            .            *
    *     .-------------.     *            *     .------------.      *
    *    /     DC#1      \    *  .------.  *    /     DC#2     \     *
    * +----+          +-----+ * /  WAN   \ * +-----+            |    *
    * |    |=========>|     | * |        | * | CF/ |==========> SP#1 *
    * | CF |=========>| FWD |===============>| FWD |==========> SP#2 *
    *   :       :        :    * |        | *    :         :       :  *
    * |    |=========>|     | * \        / * |     |==========> SP#n *
    * +----+          +-----+ *  '------'  * +-----+            |    *
    *    \               /    *            *     \             /     *
    *     '-------------'     *            *      '-----------'      *
    *       SC Domain#1       *            *      SC Domain#2        *
    * * * * * * * * * * * * * *            * * * * * * * * * * * * * *


     Figure 10: Establishment of SPs Across Multiples DCs in pattern 2

   Also, the detail analysis of establishment of "Hierarchical Service
   Path domains" is shown in the following section.

4.2.2.1.  Analysis of Hierarchical Service Path domains

   The dynamic selection of SPs pattern allows multiple independent
   domains of administration.  (In the example, two levels were shown,
   but the pattern could be extended to multiple levels.)

   This pattern allows even the largest networks to implement SC from
   the edges of the network by using coarse-grained classification.
   Classification choices can be made that are feasible within the
   constraints of the edge classifiers and FWDs.  There is no need to
   maintain flow state or react to traffic at the top level.

   This pattern allows control of sub-domains to be delegated to
   different owners.  Each domain is simpler to comprehend than would be
   the case by dealing with a single flat network.  Furthermore,
   failures and errors are localized.  (See Figure 11.)





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   +----------+
   |Top-level | . . . . . . . . . . . . . . . . . . . . .
   |Control   |                                         .
   |Entity    |          +------------+     +--------+  .
   +----------+          |sub-domain#1|. . .|  CE#1  |  .
        .                +-----+------+     +--------+  .
        .                      |                        .
        .               .------+---------.   +---+      .
        .      +---+   /                  \--|CF |. . . .
        . . . .|CF |--/                    \ |FWD|      .
        .      |FWD| /                      \+---+      .
        .      +---+ |                       |          .
        .            |                       |          .
        .            |                       |          .
        .      +---+ \                      /           .
        .      |CF |  \                    /  +---+     .
        . . . .|FWD|---\                  /---|CF | . . .
               +---+    '------+---------'    |FWD|
                               |              +---+
          +--------+     +------------+
          |  CE#2  |. . .|sub-domain#2|
          +--------+     +-----+------+

   Figure 11: Multiple Control Entities in Hierarchical Service Chaining

   This hierarchical model supports management of large networks by
   adhering to these principles:

   1.  At higher levels of hierarchy packet classification is coarse, to
       minimize state and control-plane chatter.

   2.  At lower levels of hierarchy packet classification can be more
       granular because classifiers in the lower levels deal with a
       subset of the entire network: fewer flows, lower bit-rate and a
       subset of network policy.

   However, in this model, a new component that can proxy between the
   different domains, termed "SF Domain Proxy," will be required.  It
   has some commonality with the legacy SF proxy discussed in
   [I-D.song-sfc-legacy-sf-mapping].

   This model also requires some coordination of path information within
   the SF Domain Proxy component, since the proxy must map packets back
   and forth between domains.  Solving this probably requires sharing
   metadata dictionaries among controllers and inventing a scheme that
   provides a level of indirection by naming path identifiers and
   metadata values.




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4.3.  Example of selecting Methods and Patterns

   In this section, clarifications about the most suitable method and
   pattern are made for the following example networks based on the
   results of the above analysis.

4.3.1.  Example A: Datacenter Network

   The conditions of network A are as follows:

   1. The network is used for several business offices as a single DC.

   2. Service Chain varies per office (not per user).

   3. The number of SF included in for each Service Chain is few. (e.g.
      within 5.)

   4. SF (instance) cost is not so high.

   5. MTU should not be restricted.

   6. Service Chains do not fork paths through end-to-end.  (As
      monitoring, or controlling will be harder, some operators may not
      want to change paths after packets got into a service chain.)

   On the basis of conditions 4 and 6, Pattern 1 (SF Dedicated Model)
   would be selected.  In this case, any method would be applicable.
   (Even if method 2 is selected, only one header that shows the gateway
   to the specific SC is stacked on packets.  This does not restrict the
   MTU.)

4.3.2.  Example B: Current Mobile Carrier's Network

   The conditions of network B are as follows:

   1. The network handles millions of users.

   2. Service Chain (SF set and order) is predefined and limited.

   3. The number of SF, included in for each Service Chain, is few.
      (e.g. within 5.)

   4. The user chooses or the provider can choose for the user a
      predefined Service Chain to adopt to their traffic.

   5. SFs are located in (S)Gi-LAN.(Term referred to
      [I-D.ietf-sfc-use-case-mobility])




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   6. Service Chains do not require to fork paths through end-to-end.

   On the basis of conditions 1, 2, and 5, Pattern 1 (SF Shared Model)
   would be selected because the architecture would be simple.

   On the basis of conditions 3 and 4, method 1 (unless the
   configuration or forwarding table does not increase explosively) or 3
   would be applicable.

4.3.3.  Example C: Fixed and Mobile Converged Network

   Conditions of the network A is as follows:

   1. The network handles millions of users.

   2. The user chooses or the provider can choose for the user multiple
      SFs to adopt to their traffic.

   3. Many SFs (e.g. 5 or more,) are included in for each Service Chain.

   4. SFs are located in multiple DCs.(e.g.  Some delay sensitive SFs,
      or SFs which should be placed near users' locations are installed
      in DCs located locally, and added-value SFs are installed in DCs
      located centrally.)

   5. There are some expansive SFs (instance) that should be shared by
      several SPs.

   6. Service Chains may be forked according to the process of SF.

   On the basis of conditions 1, 2, 3, 4, and 5, Method 3 would be
   applicable in terms of scalability.  Pattern 2 should be selected
   based on conditions 1 and 6.  Although the operation would be
   complex, there may be a case in which some carriers set multiple DCs
   and separate SC domains according to their network or service policy.
   The use case and architecture pattern is introduced in
   [I-D.ietf-sfc-dc-use-cases].

5.  Acknowledgements

   The authors would like to thank Konomi Mochizuki and Lily Guo for
   their reviews and comments.

6.  Contributors

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



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   Hiroshi Dempo
   NEC

   Ron Parker
   Affirmed Networks

   Paul Quinn
   Cisco Systems

7.  IANA Considerations

   This memo includes no request to IANA.

8.  References

   [I-D.ietf-sfc-architecture]
              Halpern, J. and C. Pignataro, "Service Function Chaining
              (SFC) Architecture", draft-ietf-sfc-architecture-04 (work
              in progress), January 2015.

   [I-D.ietf-sfc-dc-use-cases]
              Surendra, S., Tufail, M., Majee, S., Captari, C., and S.
              Homma, "Service Function Chaining Use Cases In Data
              Centers", draft-ietf-sfc-dc-use-cases-02 (work in
              progress), January 2015.

   [I-D.ietf-sfc-problem-statement]
              Quinn, P. and T. Nadeau, "Service Function Chaining
              Problem Statement", draft-ietf-sfc-problem-statement-10
              (work in progress), August 2014.

   [I-D.ietf-sfc-use-case-mobility]
              Haeffner, W., Napper, J., Stiemerling, M., Lopez, D., and
              J. Uttaro, "Service Function Chaining Use Cases in Mobile
              Networks", draft-ietf-sfc-use-case-mobility-03 (work in
              progress), January 2015.

   [I-D.quinn-sfc-nsh]
              Quinn, P., Guichard, J., Surendra, S., Smith, M.,
              Henderickx, W., Nadeau, T., Agarwal, P., Manur, R.,
              Chauhan, A., Majee, S., Elzur, U., Melman, D., Garg, P.,
              McConnell, B., Wright, C., and K. Kevin, "Network Service
              Header", draft-quinn-sfc-nsh-04 (work in progress),
              December 2014.







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   [I-D.song-sfc-legacy-sf-mapping]
              Song, H., You, J., Yong, L., Jiang, Y., Dunbar, L.,
              Bouthors, N., and D. Dolson, "SFC Header Mapping for
              Legacy SF", draft-song-sfc-legacy-sf-mapping-04 (work in
              progress), December 2014.

   [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.

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

Authors' Addresses

   Shunsuke Homma
   NTT, Corp.
   3-9-11, Midori-cho
   Musashino-shi, Tokyo  180-8585
   Japan

   Email: homma.shunsuke@lab.ntt.co.jp


   Kengo Naito
   NTT, Corp.
   3-9-11, Midori-cho
   Musashino-shi, Tokyo  180-8585
   Japan

   Email: naito.kengo@lab.ntt.co.jp


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

   Phone: +34 913 129 041
   Email: diego.r.lopez@telefonica.com






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   Martin Stiemerling
   NEC Laboratories Europe / Hochschule Darmstadt
   Kurfuerstenanlage 36
   Heidelberg  69115
   Germany

   URI:   ietf.stiemerling.org


   David Dolson
   Sandvine
   408 Albert Street
   Waterloo, Ontario  N2L 3V3
   Canada

   Email: ddolson@sandvine.com



































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