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Versions: (draft-dolson-sfc-hierarchical) 00 01 02 03 04

Service Function Chaining                                      D. Dolson
Internet-Draft                                                  Sandvine
Intended status: Informational                                  S. Homma
Expires: January 17, 2018                                            NTT
                                                                D. Lopez
                                                          Telefonica I+D
                                                            M. Boucadair
                                                                  Orange
                                                                  D. Liu
                                                           Alibaba Group
                                                                   T. Ao
                                                         ZTE Corporation
                                                                   V. Vu
                                                     Soongsil University
                                                           July 16, 2017


             Hierarchical Service Function Chaining (hSFC)
                     draft-ietf-sfc-hierarchical-04

Abstract

   Hierarchical Service Function Chaining (hSFC) is a network
   architecture allowing an organization to decompose a large-scale
   network into multiple domains of administration.

   The goals of hSFC are to make a large-scale network easier to reason
   about, simpler to control and to support independent functional
   groups within large network operators.

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 January 17, 2018.





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Copyright Notice

   Copyright (c) 2017 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Hierarchical Service Function Chaining (hSFC) . . . . . . . .   4
     2.1.  Top Level . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Lower Levels  . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Internal Boundary Node (IBN)  . . . . . . . . . . . . . . . .   7
     3.1.  IBN Path Configuration  . . . . . . . . . . . . . . . . .   8
       3.1.1.  Flow-Stateful IBN . . . . . . . . . . . . . . . . . .   8
       3.1.2.  Encoding Upper-Level Paths in Metadata  . . . . . . .  10
       3.1.3.  Using Unique Paths per Upper-Level Path . . . . . . .  11
       3.1.4.  Nesting Upper-Level NSH within Lower-Level NSH  . . .  11
       3.1.5.  Stateful / Metadata Hybrid  . . . . . . . . . . . . .  12
     3.2.  Gluing Levels Together  . . . . . . . . . . . . . . . . .  14
     3.3.  Decrementing Service Index  . . . . . . . . . . . . . . .  14
     3.4.  Managing TTL  . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Sub-domain Classifier . . . . . . . . . . . . . . . . . . . .  15
   5.  Control Plane Elements  . . . . . . . . . . . . . . . . . . .  15
   6.  Extension for Adapting to NSH-Unaware Service Functions . . .  16
     6.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.2.  Requirements for IBN  . . . . . . . . . . . . . . . . . .  18
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     9.1.  Control Plane . . . . . . . . . . . . . . . . . . . . . .  20
     9.2.  Infinite Forwarding Loops . . . . . . . . . . . . . . . .  20
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Examples of Hierarchical Service Function Chaining .  21
     A.1.  Reducing the Number of Service Function Paths . . . . . .  22
     A.2.  Managing a Distributed Data-Center Network  . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26



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1.  Introduction

   Service Function Chaining (SFC) is a technique for prescribing
   differentiated traffic forwarding policies within an SFC-enabled
   domain.  SFC is described in detail in the SFC architecture document
   [RFC7665], and is not repeated here.

   This document focuses on the difficult problem of implementing SFC
   across a large, geographically dispersed network, potentially
   comprised of millions of hosts and thousands of network forwarding
   elements, and which may involve multiple operational teams (with
   varying functional responsibilities).  We recognize that some
   stateful Service Functions (SFs) require bidirectional traffic for
   transport-layer sessions (e.g., NATs, firewalls).  We assume that
   some Service Function Paths (SFPs) need to be selected on the basis
   of application-specific data visible to the network, with transport-
   layer coordinate (typically, 5-tuple) stickiness to specific stateful
   SF instances.

   Difficult problems are often made easier by decomposing them in a
   hierarchical (nested) manner.  So instead of considering a single SFC
   Control Plane ([I-D.ietf-sfc-control-plane]) that can manage (create,
   withdraw, supervise, etc.) complete SFPs from one end of the network
   to the other, we decompose the network into smaller domains operated
   by as many SFC control plane components.  Coordination between such
   components is further discussed in the document.  Each sub-domain may
   support a subset of the network applications or a subset of the
   users.  Decomposing a network into multiple SFC-enabled domains
   should permit end-to-end visibility of SFs and SFPs.  Also,
   decomposing should be done with care to ease monitoring and
   troubleshooting of the network and services as a whole.  The criteria
   for decomposition a domain into multiple SFC-enabled sub-domains are
   beyond the scope of this document.  These criteria are deployment-
   specific.

   An example of simplifying a network by using multiple SFC-enabled
   domains is further discussed in [I-D.ietf-sfc-dc-use-cases].

   We assume the SFC-aware nodes use NSH [I-D.ietf-sfc-nsh] or a similar
   labeling mechanism.  Sample examples are described in Appendix A.

   The "domains" discussed in this document are assumed to be under
   control of a single organization, such that there is a strong trust
   relationship between the domains.  The intention of creating multiple
   domains is to improve the ability to operate a network.  It is
   outside of the scope of the document to consider domains operated by
   different organizations.




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2.  Hierarchical Service Function Chaining (hSFC)

   A hierarchy has multiple levels: the top-most level encompasses the
   entire network domain to be managed, and lower levels encompass
   portions of the network.  These levels are discussed in the following
   sub-sections.

2.1.  Top Level

   Considering the example depicted in Figure 1, a top-level network
   domain includes SFC data plane components distributed over a wide
   area, including:

   o  Classifiers (CFs),

   o  Service Function Forwarders (SFFs) and

   o  Sub-domains.

   For the sake of clarity, components of the underlay network are not
   shown; an underlay network is assumed to provide connectivity between
   SFC data plane components.

   Top-level SFPs carry packets from classifiers through a set of SFFs
   and sub-domains, with the operations within sub-domains being opaque
   to the higher levels.

   We expect the system to include a top-level control plane having
   responsibility for configuring forwarding policies and traffic
   classification rules (see for example, [I-D.ietf-sfc-control-plane]).
   The top-level Service Chaining control plane manages end-to-end
   service chains and associated service function paths from network
   edge points to sub-domains and configures top-level classifiers at a
   coarse level (e.g., based on source or destination host) to forward
   traffic along paths that will transit across appropriate sub-domains.

   Figure 1 shows one possible service chain passing from edge, through
   two sub-domains, to network egress.  The top-level control plane does
   not configure traffic classification rules or forwarding policies
   within the sub-domains.

   At this network-wide level, the number of SFPs required is a linear
   function of the number of ways in which a packet is required to
   traverse different sub-domains and egress the network.  Note that the
   various paths which may be followed within a sub-domain are not
   represented by distinct network-wide SFPs; specific policies at the
   ingress nodes of each sub-domain bind flows to sub-domain paths.




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   Packets are classified at the edge of the network to select the paths
   by which sub-domains are to be traversed.  At the ingress of each
   sub-domain, packets are reclassified to paths directing them to the
   required SFs of the sub-domain.  At the egress of each sub-domain,
   packets are returned to the top-level paths.  Contrast this with an
   approach requiring the top-level classifier to select paths to
   specify all of the SFs in each sub-domain.

   It should be assumed that some SFs require bidirectional symmetry of
   paths (see more in Section 4).  Therefore the classifiers at the top
   level must be configured with policies ensuring outgoing packets take
   the reverse path of incoming packets through sub-domains.

                    +------------+
                    |Sub-domain#1|
                    |  in DC1    |
                    +----+-------+
                         |
                  .---- SFF1 ------.   +--+
          +--+   /     /  |         \--|CF|
      --->|CF|--/---->'   |          \ +--+
          +--+ /  SC#1    |           \
               |          |            |
               |          V    .------>|--->
               |         /    /        |
               \         |   /        /
          +--+  \        |  /        /  +--+
          |CF|---\       | /        /---|CF|
          +--+    '---- SFF2 ------'    +--+
                         |
                    +----+-------+
                    |Sub-domain#2|
                    |   in DC2   |
                    +------------+

   One path is shown from edge classifier to SFF1 to Sub-domain#1
   (residing in data-center1) to SFF1 to SFF2 (residing in data-center
   2) to Sub-domain#2 to SFF2 to network egress.

           Figure 1: Network-wide view of top level of hierarchy

2.2.  Lower Levels

   Each of the sub-domains in Figure 1 is an SFC-enabled domain.

   Figure 2 shows a sub-domain interfaced with a higher-level domain by
   means of an Internal Boundary Node (IBN).  An IBN acts as an SFC-
   aware SF in the higher-level domain and as a classifier in the lower-



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   level domain.  As such, data packets entering the sub-domain are
   already SFC-encapsulated.  Also, it is the purpose of the IBN to
   apply classification rules and direct the packets to the selected
   local SFPs terminating at an egress IBN.  The egress IBN finally
   restores packets to the original SFC shim and hands them off to SFFs.

   Each sub-domain intersects a subset of the total paths that are
   possible in the higher-level domain.  An IBN is concerned with
   higher-level paths, but only those traversing its sub-domain.  A top-
   level control element may configure the IBN as an SF (i.e., the IBN
   plays the SF role in the top-level domain).

   Each sub-domain is likely to have a control plane that can operate
   independently of the top-level control plane, managing
   classification, forwarding paths, etc. within the level of the sub-
   domain, with the details being opaque to the upper-level control
   elements.  Section 3 provides more details about the behavior of an
   IBN.

   The sub-domain control plane configures the classification rules in
   the IBN, where SFC encapsulation of the top-level domain is converted
   to/from SFC encapsulation of the lower-level domain.  The sub-domain
   control plane also configures the forwarding rules in the SFFs of the
   sub-domain.



























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     +----+    +-----+  +----------------------+   +-----+
     |    |    | SFF |  |   IBN 1  (in DC 1)   |   | SFF |
     |    |SC#1|     |  |  +----------------+  |   |     |
   ->|    |===============>|      SFF       |================>
     |    |    +-----+  |  +----------------+  |   +-----+
     | CF |             |   |              ^   |
     |    |             |   v              |   |
     |    |             |+--------------------+|   Top domain
     |    |             ||CF, fwd/rev mapping ||
     |    |    * * * * *||  and "glue"        || * * * * *
     |    |    *        |+--------------------+|         *
     +----+    *        | | |              | | |    Sub  *
               *        +-o-o--------------o-o-+   domain*
               *     SC#2 | |SC#1          ^ ^       #1  *
               *    +-----+ |              | |           *
               *    |       V              | |           *
               *    |     +---+  +------+  | |           *
               *    |     |SFF|->|SF#1.1|--+ |           *
               *    |     +---+  +------+    |           *
               *    V                        |           *
               *  +---+  +------+  +---+  +------+       *
               *  |SFF|->|SF#2.1|->|SFF|->|SF#2.2|       *
               *  +---+  +------+  +---+  +------+       *
               * * * * * * * * * * * * * * * * * * * * * *
   Legend:
        *** Sub-domain boundary
        === top-level chain
        --- low-level chain

             Figure 2: Sub-domain within a higher-level domain

   If desired, the pattern can be applied recursively.  For example,
   SF#1.1 in Figure 2 could be a sub-domain of the sub-domain.

3.  Internal Boundary Node (IBN)

   As mentioned in the previous section, a network element termed
   "Internal Boundary Node" (IBN) is responsible for bridging packets
   between higher and lower layers of SFC-enabled domains.  It behaves
   as an SF to the higher level (Section 2.1), and looks like a
   classifier and end-of-chain to the lower level (Section 2.2).

   To achieve the benefits of hierarchy, the IBN should be applying more
   granular traffic classification rules at the lower level than the
   traffic passed to it.  This means that the number of SFPs within the
   lower level is greater than the number of SFPs arriving to the IBN.





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   The IBN is also the termination of lower-level SFPs.  This is because
   the packets exiting lower-level SF paths must be returned to the
   higher-level SF paths and forwarded to the next hop in the higher-
   level domain.

   When different metadata schemes are used at different levels, the IBN
   has further responsibilities: when packets enter the sub-domain, the
   IBN translates upper-level metadata into lower-level metadata; and
   when packets leave the sub-domain at the termination of lower-level
   SFPs, the IBN translates lower-level metadata into upper-level
   metadata.

   Appropriately configuring IBNs is key to ensure the consistency of
   the overall SFC operation within a given domain that enables hSFC.
   Classification rules (or lack thereof) in the IBN classifier can of
   course impact higher levels.

3.1.  IBN Path Configuration

   The lower-level domain may be provisioned with valid high-level paths
   or may allow any high-level paths.

   When packets enter the sub-domain, the Service Path Identifier (SPI)
   and Service Index (SI) are re-marked according to the path selected
   by the (sub-domain) classifier.

   At the termination of an SFP in the sub-domain, packets can be
   restored to an original upper-level SFP by implementing one of these
   methods:

   1.  Saving SPI and SI in transport-layer flow state (Section 3.1.1).

   2.  Pushing SPI and SI into a metadata header (Section 3.1.2).

   3.  Using unique lower-level paths per upper-level path coordinates
       (Section 3.1.3).

   4.  Nesting NSH headers, encapsulating the higher-level NSH headers
       within the lower-level NSH headers (Section 3.1.4).

   5.  Saving upper-level by a flow identifier (ID) and placing an hSFC
       flow ID into a metadata header (Section 3.1.5).

3.1.1.  Flow-Stateful IBN

   An IBN can be flow-aware, returning packets to the correct higher-
   level SFP on the basis, for example, of the transport-layer




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   coordinates (typically, a 5-tuple) of packets exiting the lower-level
   SFPs.

   When packets are received by the IBN on a higher-level path, the
   classifier parses encapsulated packets for IP and transport-layer
   (TCP, UDP, etc.) coordinates.  State is created, indexed by some or
   all transport-coordinates ({source-IP, destination-IP, source-port,
   destination-port and transport protocol} typically).  The state
   contains at minimum the critical fields of the encapsulating SFC
   header (SPI, SI, MD Type, flags); additional information carried in
   the packet (metadata, TTL) may also be extracted and saved as state.
   Note, that the some fields of a packet may be altered by an SF of the
   sub-domain (e.g., source IP address).

   Note that this state is only accessed by the classifier and
   terminator functions of the sub-domain.  Neither the SFFs nor SFs
   have knowldge of this state; in fact they may be agnostic about being
   in a sub-domain.

   One approach is to ensure that packets are terminated at the same IBN
   at the end of the chain that classified the packet at the start of
   the chain.  If the packet is returned to a different egress IBN,
   state must be synchronized between the IBNs.

   When a packet returns to the IBN at the end of a chain (which is the
   SFP terminating node of the lower-level chain), the SFC header is
   removed, the packet is parsed for IP and transport-layer coordinates,
   and state is retrieved from them.  The state contains the information
   required to forward the packet within the higher-level service chain.

   State cannot be created by packets arriving from the lower-level
   chain; when state cannot be found for such packets, they must be
   dropped.

   This stateful approach is limited to use with SFs that retain the
   transport coordinates of the packet.  This approach cannot be used
   with SFs that modify those coordinates (e.g., NATs) or otherwise
   create packets for new coordinates other than those received (e.g.,
   as an HTTP cache might do to retrieve content on behalf of the
   original flow).  In both cases, the fundamental problem is the
   inability to forward packets when state cannot be found for the
   packet transport-layer coordinates.

   In the stateful approach, there are issues caused by having state,
   such as how long the state should be maintained, as well as whether
   the state needs to be replicated to other devices to create a highly
   available network.




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   It is valid to consider the state to be disposable after failure,
   since it can be re-created by each new packet arriving from the
   higher-level domain.  For example, if an IBN loses all flow state,
   the state is re-created by an end-point retransmitting a TCP packet.

   If an SFC domain handles multiple network regions (e.g., multiple
   private networks), the coordinates may be augmented with additional
   parameters, perhaps using some metadata to identify the network
   region.

   In this stateful approach, it is not necessary for the sub-domain's
   control plane to modify paths when higher-level paths are changed.
   The complexity of the higher-level domain does not cause complexity
   in the lower-level domain.

   Since it doesn't depend on NSH in the lower domain, this flow-
   stateful approach can be applied to translation methods of converting
   NSH to other forwarding techniques (refer to Section 6).

3.1.2.  Encoding Upper-Level Paths in Metadata

   An IBN can push the upper-level SPI and SI (or encoding thereof) into
   a metadata field of the lower-level encapsulation (e.g., placing
   upper-level path information into a metadata field of NSH).  When
   packets exit the lower-level path, the upper-level SPI and SI can be
   restored from the metadata retrieved from the packet.

   This approach requires the SFs in the path to be capable of
   forwarding the metadata and appropriately attaching metadata to any
   packets injected for a flow.

   Using new metadata header may inflate packet size when variable-
   length metadata (type 2 from NSH [I-D.ietf-sfc-nsh]) is used.

   It is conceivable that the MD-type 1 Mandatory Context Header fields
   of NSH [I-D.ietf-sfc-nsh] are not all relevant to the lower-level
   domain.  In this case, one of the metadata slots of the Mandatory
   Context Header could be repurposed within the lower-level domain, and
   restored when leaving.

   If flags or TTL (see Section 3.4) from the original header also need
   to be saved, more metadata space will be consumed.

   In this metadata approach, it is not necessary for the sub-domain's
   control element to modify paths when higher-level paths are changed.
   The complexity of the higher-level domain does not increase
   complexity in the lower-level domain.




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3.1.3.  Using Unique Paths per Upper-Level Path

   This approach assumes that paths within the sub-domain are
   constrained so that a SPI (of the sub-domain) unambiguously indicates
   the egress SPI and SI (of the upper domain).  This allows the
   original path information to be restored at sub-domain egress from a
   look-up table using the sub-domain SPI.

   Whenever the upper-level domain provisions a path via the lower-level
   domain, the lower-level domain control plane must provision
   corresponding paths to traverse the lower-level domain.

   A down-side of this approach is that the number of paths in the
   lower-level domain is multiplied by the number of paths in the
   higher-level domain that traverse the lower-level domain.  I.e., a
   sub-path must be created for each combination of upper SPI/SI and
   lower chain.

   A further down-side of this approach is that it requires upper and
   lower levels to utilize the same metadata configuration.

   Furthermore, this approach does not allow any information to be
   stashed away in state or embedded in metadata.  E.g., the TTL
   modifications by the lower level cannot be hidden from the upper
   level.

3.1.4.  Nesting Upper-Level NSH within Lower-Level NSH

   When packets arrive at an IBN in the top-level domain, the classifier
   in the IBN determines the path for the lower-level domain and pushes
   the new NSH header in front of the original NSH header.

   As shown in Figure 3 the Lower-NSH header used to forward packets in
   the lower-level domain precedes the Upper-NSH header from the top-
   level domain.

                    +---------------------------------+
                    |  Outer-transport Encapsulation  |
                    +---------------------------------+
                    |        Lower-NSH Header         |
                    +---------------------------------+
                    |        Upper-NSH Header         |
                    +---------------------------------+
                    |          Original Packet        |
                    +---------------------------------+


                 Figure 3: Encapsulation of NSH within NSH



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   The traffic with the above stack of two NSH headers is to be
   forwarded according to the Lower-NSH header in the lower-level SFC
   domain.  The Upper-NSH header is preserved in the packets but not
   used for forwarding.  At the last SFF of the chain of the lower-level
   domain (which resides in the IBN), the Lower-NSH header is removed
   from the packet, and then the packet is forwarded by the IBN to an
   SFF of the upper-level domain.  The packet will be forwarded in the
   top-level domain according to the Upper-NSH header.

   With such encapsulation, Upper-NSH information is carried along the
   extent of the lower-level chain without modification.

   A benefit of this approach is that it does not require state in the
   IBN or configuration to encode fields in meta-data.  All header
   fields, including flags and TTL are easily restored when the chains
   of the sub-domain terminate.

   However, the down-side is it does require SFC-aware SFs in the lower-
   level domain to be able to parse multiple NSH layers.  If an SFC-
   aware SF injects packets, it must also be able to deal with adding
   appropriate multiple layers of headers to injected packets.

   By increasing packet overhead, nesting may lead to fragmentation or
   decreased MTU in some networks.

3.1.5.  Stateful / Metadata Hybrid

   The basic idea of this approach is for the IBN to save upper domain
   encapsulation information such that it can be retrieved by a unique
   identifier, termed an "hSFC Flow ID".  An example is shown in
   Table 1.

   +-----------+-----+-----+----------+----------+----------+----------+
   | hSFC Flow | SPI | SI  | Context1 | Context2 | Context3 | Context4 |
   | ID        |     |     |          |          |          |          |
   +-----------+-----+-----+----------+----------+----------+----------+
   | 1         | 45  | 254 | 100      | 2112     | 12345    | 7        |
   +-----------+-----+-----+----------+----------+----------+----------+

     Table 1: Example Mapping of an hSFC Flow ID to Upper-Level Header

   The ID is placed in the metadata in NSH headers of the packet in the
   lower domain, as shown in Figure 4.  When packets exit the lower
   domain, the IBN uses the ID to retrieve the appropriate NSH
   encapsulation for returning the packet to the upper domain.






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      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|O|R|    TTL    |   Length  |R|R|R|R|MD Type| Next Protocol |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Service Path Identifer               | Service Index |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      hSFC Flow ID                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Mandatory Context Header                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Mandatory Context Header                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Mandatory Context Header                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


          Figure 4: Storing hSFC Flow ID in lower-level metadata

   Advantages of this approach include:

   o  Does not require state based on 5-tuple, so it works with SFs that
      change the IP addresses or ports of a packet such as NATs.

   o  Does not require all domains to have the same metadata scheme.

   o  Can be used to restore any upper-domain information, including
      metadata, flags and TTL, not just service path.

   o  The lower domain only requires a single item of metadata
      regardless of the number of items of metadata used in the upper
      domain.  (For MD-Type 1, this leaves 3 slots for use in the lower
      domain.)

   o  No special functionality is required to be supported by an SFC-
      aware SF, other than the usual ability to preserve metadata and to
      apply metadata to injected packets.

   Disadvantages include those of other stateful approaches, including
   state timeout and replication mentioned in Section 3.1.1.

   There may be a large number of unique NSH encapsulations to be
   stored, given that the hSFC Flow ID must represent all of the bits in
   the upper-level encapsulation.  This might consume a lot of memory or
   create out-of-memory situations in which IDs cannot be created or old
   IDs are discarded while still in use.






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3.2.  Gluing Levels Together

   The SPI or metadata included in a packet received by the IBN may be
   used as input to reclassification and path selection within a lower-
   level domain.

   In some cases the meanings of the various path IDs and metadata must
   be coordinated between domains for the sake of proper end-to-end SFC
   operation.

   One approach is to use well-known identifier values in metadata,
   maintained in a global registry.

   Another approach is to use well-known labels for chain identifiers or
   metadata, as an indirection to the actual identifiers.  The actual
   identifiers can be assigned by control-plane systems.  For example, a
   sub-domain classifier could have a policy, "if pathID=classA then
   chain packet to path 1234"; the higher-level controller would be
   expected to configure the concrete higher-level pathID for classA.

3.3.  Decrementing Service Index

   Because the IBN acts as an SFC-aware SF to the higher-level domain,
   it must decrement the Service Index in the NSH headers of the higher-
   level path.  This operation should be undertaken when the packet is
   first received by the IBN, before applying any of the strategies of
   Section 3.1, immediately prior to classification.

3.4.  Managing TTL

   The NSH base header contains a TTL field [I-D.ietf-sfc-nsh].  There
   is a choice:

      a sub-domain may appear as a pure service function, which should
      not decrement the TTL from the perspective of the higher-level
      domain,

      or all of the TTL changes within the sub-domain may be visible to
      the higher-level domain.

   Some readers may recognize this as a choice between "pipe" and
   "uniform" models, respectively [RFC3443].

   The network operator should be given control of this behavior,
   choosing whether to expose the lower-level topology to the higher
   layer.  An implementation may support per-packet policy, allowing
   some users to perform a layer-transcending trace-route, for example.




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   The choice affects whether the methods of restoring the paths in the
   sub-sections of Section 3.1 restore a saved version of TTL or
   propagate it with the packet.  The method of Section 3.1.3 does not
   permit topology-hiding.  The other methods of Section 3.1.1,
   Section 3.1.2, Section 3.1.4, and Section 3.1.5 have unique methods
   for restoring saved versions of TTL.

4.  Sub-domain Classifier

   Within the sub-domain (referring to Figure 2), as the classifier
   receives incoming packets, the high-level encapsulation is treated
   according to one of the methods described in Section 3.1 to either
   statefully store, encode, or nest header information.  The classifier
   then selects the path and metadata for the packet within the sub-
   domain.

   One of the goals of the hierarchical approach is to make it easy to
   have transport-flow-aware service chaining with bidirectional paths.
   For example, it is desired that for each TCP flow, the client-to-
   server packets traverse the same SF instances as the server-to-client
   packets, but in the opposite sequence.  We call this bidirectional
   symmetry.  If bidirectional symmetry is required, it is the
   responsibility of the control plane to be aware of symmetric paths
   and configure the classifier to chain the traffic in a symmetric
   manner.

   Another goal of the hierarchical approach is to simplify the
   mechanisms of scaling in and scaling out SFs.  All of the
   complexities of load-balancing among multiple SFs can be handled
   within a sub-domain, under control of the classifier, allowing the
   higher-level domain to be oblivious to the existence of multiple SF
   instances.

   Considering the requirements of bidirectional symmetry and load-
   balancing, it is useful to have all packets entering a sub-domain to
   be received by the same classifier or a coordinated cluster of
   classifiers.  There are both stateful and stateless approaches to
   ensuring bidirectional symmetry.

5.  Control Plane Elements

   Although SFC control protocols have not yet been standardized (2016),
   from the point of view of hierarchical service function chaining we
   have these expectations:

   o  Each control-plane instance manages a single level of hierarchy of
      a single domain.




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   o  Each control plane is agnostic about other levels of hierarchy.
      This aspect allows humans to reason about the system within a
      single domain and allows control-plane algorithms to use only
      domain-local inputs.  Top-level control does not need visibility
      to sub-domain policies, nor does sub-domain control need
      visibility to higher-level policies.  (Top-level control considers
      a sub-domain as though it were an SF.)

   o  Sub-domain control planes are agnostic about control planes of
      other sub-domains.  This allows both humans and machines to
      manipulate sub-domain policy without considering policies of other
      domains.

   Recall that the IBN acts as an SFC-aware SF in the higher-level
   domain (receiving SF instructions from the higher-level control
   plane) and as a classifier in the lower-level domain (receiving
   classification rules from the sub-domain control plane).  In this
   view, it is the IBN that glues the layers together.

   The above expectations are not intended to prohibit network-wide
   control.  A control hierarchy can be envisaged to distribute
   information and instructions to multiple domains and sub-domains.
   Control hierarchy is outside the scope of this document.

6.  Extension for Adapting to NSH-Unaware Service Functions

   The hierarchical approach can be used for dividing networks into NSH-
   aware and NSH-unaware domains by converting NSH encapsulation to
   other forwarding techniques (e.g., 5-tuple-based routing with
   OpenFlow), as shown in Figure 5.





















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                    * * * * * * * * * * * * * * * * * *
                  *   NSH-aware domain                 *
                  *       +-------+       +-------+    *
                  *       | SF#1  |       | SF#5  |    *
                  *       +-o---o-+       +-o---o-+    *
                  *         ^   |           ^   |      *
                  *       +-|---|-+       +-|---|-+    *
                  *       | |SFF| |       | |SFF| |    *
                  *       +-|---|-+       +-|---|-+    *
                  *         .   |           |   .      *
                  * +--+   /    |           |    \     *
                 -->|CF|--'     |           |     '------->
                  * +--+        v           |          *
                  *         +---o-----------o---+      *
                   .*.*.*.*.|  / |   IBN   | \  |*.*.*.
                  .         +-o--o---------o--o-+      .
                  .           |  |         ^  ^        .
                  .           |  +-+     +-+  |        .
                  .       +---+    v     |    +---+    .
                  .       |      +-o-----o-+      |    .
                  .       |      |  SF#2   |      |    .
                  .       |      +---------+      |    .
                  .       +--+                 +--+    .
                  .          |   +---------+   |       .
                  .          v   |         v   |       .
                  .        +-o---o-+     +-o---o-+     .
                  .        | SF#3  |     | SF#4  |     .
                  .        +-------+     +-------+     .
                  .   NSH-unaware domain               .
                   . . . . . . . . . . . . . . . . . .


   SF#1 and SF#5 are NSH-aware and SF#2, SF#3 and SF#4 are NSH-unaware.
   In the NSH-unaware domain, packets are conveyed in a format supported
   by SFs which are deployed there.

           Figure 5: Dividing NSH-aware and NSH-unaware domains

6.1.  Purpose

   This approach is expected to facilitate service chaining in networks
   in which NSH-aware and NSH-unaware SFs coexist.  Some examples of
   such situations are:

   o  In a period of transition from legacy SFs to NSH-aware SFs, and

   o  Supporting multi-tenancy.




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6.2.  Requirements for IBN

   In this usage, an IBN classifier is required to have an NSH
   conversion table for applying packets to appropriate lower-level
   paths and returning packets to the correct higher-level paths.  For
   example, the following methods would be used for saving/restoring
   upper-level path information:

   o  Saving SPI and SI in transport-layer flow state (refer to
      Section 3.1.1) and

   o  Using unique lower-level paths per upper-level NSH coordinates
      (refer to Section 3.1.3).

   Especially, the use of unique paths approach would be good for
   translating NSH to a different forwarding technique in the lower
   level.  A single path in the upper level may be branched to multiple
   paths in the lower level such that any lower-level path is only used
   by one upper-level path.  This allows unambiguous restoration to the
   upper-level path.

   In addition, an IBN might be required to convert metadata contained
   in NSH to the format appropriate to the packet in the lower-level
   path.  For example, some legacy SFs identify subscriber based on
   information of network topology, such as VID, and IBN would be
   required to create VLAN to packets from metadata if subscriber
   identifier is conveyed as metadata in higher-level domains.

   Other fundamental functions required as IBN (e.g., maintaining
   metadata of upper level or decrementing Service Index) are same as
   normal usage.

   It is useful to permit metadata to be transferred between levels of a
   hierarchy.  Metadata from a higher level may be useful within a sub-
   domain and a sub-domain may augment metadata for consumption in an
   upper domain.  However, allowing uncontrolled metadata between
   domains may lead to forwarding failures.

      In order to prevent SFs of low-level SFC-enabled domains from
      supplying (illegitimate) metadata, IBNs may be instructed to
      permit specific metadata types to exit the sub-domain.  Such
      control over the metadata in the upper level is the responsibility
      of the upper-level control plane.

      To limit unintentional metadata reaching SFs of low-level SFC-
      enabled sub-domains, IBNs may be instructed to permit specific
      metadata types into the sub-domain.  Such control of metadata in




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      the low-level domain is the responsibility of the lower-level
      control plane.

7.  Acknowledgements

   The concept of Hierarchical Service Path Domains was introduced in
   [I-D.homma-sfc-forwarding-methods-analysis] as a means to improve
   scalability of service chaining in large networks.

   The concept of nested NSH headers was introduced in
   [I-D.ao-sfc-for-dc-interconnect] as a means of creating hierarchical
   SFC in a data center.

   The authors would like to thank the following individuals for
   providing valuable feedback:

      Ron Parker

      Christian Jacquenet

      Jie Cao

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   Hierarchical service function chaining makes use of service chaining
   architecture, and hence inherits the security considerations
   described in the architecture document [RFC7665].

   Furthermore, hierarchical service function chaining inherits security
   considerations of the data-plane protocols (e.g., NSH) and control-
   plane protocols used to realize the solution.

   The systems described in this document bear responsibility for
   forwarding Internet traffic.  In some cases the systems are
   responsible for maintaining separation of traffic in private
   networks.

   This document describes systems within different domains of
   administration that must have consistent configurations in order to
   properly forward traffic and to maintain private network separation.
   Any protocol designed to distribute the configurations must be secure
   from tampering.





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   All of the systems and protocols must be secure from modification by
   untrusted agents.

9.1.  Control Plane

   Security considerations related to the control plane should be
   discussed in the corresponding control specification documents (e.g.,
   [I-D.ietf-bess-nsh-bgp-control-plane],
   [I-D.wu-pce-traffic-steering-sfc], or [I-D.maglione-sfc-nsh-radius]).

   Generic security considerations related to the control plane are
   discussed in [I-D.ietf-sfc-control-plane].  These considerations
   apply for both high-level and low-level domains.

9.2.  Infinite Forwarding Loops

   Distributing policies among multiple domains may lead to forwarding
   loops.  NSH supports the ability to detect loops (Section 3.3 and
   Section 3.4), but means to ensure the consistency of the policies
   should be enabled at all levels of a domain.  Within the context of
   hSFC, it is the responsibility of the Control Elements at all levels
   to prevent such (unwanted) loops.

10.  References

10.1.  Normative References

   [I-D.ietf-sfc-nsh]
              Quinn, P. and U. Elzur, "Network Service Header", draft-
              ietf-sfc-nsh-05 (work in progress), May 2016.

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <http://www.rfc-editor.org/info/rfc7665>.

10.2.  Informative References

   [I-D.ao-sfc-for-dc-interconnect]
              Ao, T. and W. Bo, "Hierarchical SFC for DC
              Interconnection", draft-ao-sfc-for-dc-interconnect-01
              (work in progress), October 2015.









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   [I-D.homma-sfc-forwarding-methods-analysis]
              Homma, S., Naito, K., Lopez, D., Stiemerling, M., Dolson,
              D., Gorbunov, A., Leymann, N., Bottorff, P., and d.
              don.fedyk@hpe.com, "Analysis on Forwarding Methods for
              Service Chaining", draft-homma-sfc-forwarding-methods-
              analysis-05 (work in progress), January 2016.

   [I-D.ietf-bess-nsh-bgp-control-plane]
              Farrel, A., Drake, J., Rosen, E., Uttaro, J., and L.
              Jalil, "BGP Control Plane for NSH SFC", draft-ietf-bess-
              nsh-bgp-control-plane-00 (work in progress), March 2017.

   [I-D.ietf-sfc-control-plane]
              Boucadair, M., "Service Function Chaining (SFC) Control
              Plane Components & Requirements", draft-ietf-sfc-control-
              plane-06 (work in progress), May 2016.

   [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.maglione-sfc-nsh-radius]
              Maglione, R., Trueba, G., and C. Pignataro, "RADIUS
              Attributes for NSH", draft-maglione-sfc-nsh-radius-01
              (work in progress), October 2016.

   [I-D.wu-pce-traffic-steering-sfc]
              Wu, Q., Dhody, D., Boucadair, M., Jacquenet, C., and J.
              Tantsura, "PCEP Extensions for Service Function Chaining
              (SFC)", draft-wu-pce-traffic-steering-sfc-12 (work in
              progress), June 2017.

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, DOI 10.17487/RFC3443, January 2003,
              <http://www.rfc-editor.org/info/rfc3443>.

Appendix A.  Examples of Hierarchical Service Function Chaining

   The advantage of hierarchical service function chaining compared with
   normal or flat service function chaining is that it can reduce the
   management complexity significantly.  This section discusses examples
   that show those advantages.






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A.1.  Reducing the Number of Service Function Paths

   In this case, hierarchical service function chaining is used to
   simplify service function chaining management by reducing the number
   of Service Function Paths.

   As shown in Figure 6, there are two domains, each with different
   concerns: a Security Domain that selects Service Functions based on
   network conditions and an Optimization Domain that selects Service
   Functions based on traffic protocol.

   In this example there are five security functions deployed in the
   Security Domain.  The Security Domain operator wants to enforce the
   five different security policies, and the Optimization Domain
   operator wants to apply different optimizations (either cache or
   video optimization) to each of these two types of traffic.  If we use
   flat SFC (normal branching), 10 SFPs are needed in each domain.  In
   contrast, if we use hierarchical SFC, only 5 SFPs in Security Domain
   and 2 SFPs in Optimization Domain will be required, as shown in
   Figure 7.

   In the flat model, the number of SFPs is the product of the number of
   functions in all of the domains.  In the hSFC model, the number of
   SFPs is the sum of the number of functions.  For example, adding a
   "bypass" path in the Optimization Domain would cause the flat model
   to require 15 paths (5 more), but cause the hSFC model to require one
   more path in the Optimization Domain.
























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              . . . . . . . . . . . .   . . . . . . . . . . . . .
              . Security Domain     .   .  Optimization Domain  .
              .                     .   .                       .
              .    +-1---[     ]----------------->[Cache  ]------->
              .    |     [ WAF ]    .   .                       .
              .    +-2-->[     ]----------------->[Video Opt.]---->
              .    |                .   .                       .
              .    +-3---[Anti ]----------------->[Cache  ]------->
              .    |     [Virus]    .   .                       .
              .    +-4-->[     ]----------------->[Video Opt.]---->
              .    |                .   .                       .
              .    +-5-->[     ]----------------->[Cache  ]------->
   [DPI]--->[CF]---|     [ IPS ]    .   .                       .
              .    +-6-->[     ]----------------->[Video Opt.]---->
              .    |                .   .                       .
              .    +-7-->[     ]----------------->[Cache  ]------->
              .    |     [ IDS ]    .   .                       .
              .    +-8-->[     ]----------------->[Video Opt.]---->
              .    |                .   .                       .
              .    +-9-->[Traffic]--------------->[Cache  ]------->
              .    |     [Monitor]  .   .                       .
              .    +-10->[       ]--------------->[Video Opt.]---->
              . . . . . . . . . . . .   . . . . . . . . . . . . .

   The classifier must select paths that determine the combination of
   Security and Optimization concerns. 1:WAF+Cache, 2:WAF+VideoOpt,
   3:AntiVirus+Cache, 4:AntiVirus+VideoOpt, 5: IPS+Cache,
   6:IPS+VideoOpt, 7:IDS+Cache, 8:IDS+VideoOpt, 9:TrafficMonitor+Cache,
   10:TrafficMonitor+VideoOpt

                   Figure 6: Flat SFC (normal branching)




















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        . . . . . . . . . . . . . . .    . . . . . . . . . . . . . . .
        .     Security Domain       .    .   Optimization Domain     .
        .                           .    .                           .
   [CF]---->[  [CF]    IBN      ]---------->[  [CF]   IBN         ]---->
        .    |                  ^   .    .  |                     ^  .
        .    +----->[ WAF ]-----+   .    .  +-->[ Cache ]---------+  .
        .    |                  |   .    .  |                     |  .
        .    +-->[Anti-Virus]---+   .    .  +-->[Video Opt]-------+  .
        .    |                  |   .    .                           .
        .    +----->[ IPS ]-----+   .    . . . . . . . . . . . . . . .
        .    |                  |   .
        .    +----->[ IDS ]-----+   .
        .    |                  |   .
        .    +-->[ Traffic ]----+   .
        .        [ Monitor ]        .
        . . . . . . . . . . . . . . .

        Figure 7: Simplified path management with Hierarchical SFC

A.2.  Managing a Distributed Data-Center Network

   Hierarchical service function chaining can be used to simplify inter-
   data-center SFC management.  In the example of Figure 8, shown below,
   there is a central data center (Central DC) and multiple local data
   centers (Local DC#1, #2, #3) that are deployed in a geographically
   distributed manner.  All of the data centers are under a single
   administrative domain.

   The central DC may have some service functions that the local DC
   needs, such that the local DC needs to chain traffic via the central
   DC.  This could be because:

   o  Some service functions are deployed as dedicated hardware
      appliances, and there is a desire to lower the cost (both CAPEX
      and OPEX) of deploying such service functions in all data centers.

   o  Some service functions are being trialed, introduced or otherwise
      handle a relatively small amount of traffic.  It may be cheaper to
      manage these service functions in a single central data center and
      steer packets to the central data center than to manage these
      service functions in all data centers.










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                   +-----------+
                   |Central DC |
                   +-----------+
                      ^  ^   ^
                      |  |   |
                  .---|--|---|----.
                 /   /   |   |      \
                /   /    |    \      \
     +-----+   /   /     |     \      \    +-----+
     |Local|  |   /      |      \     |    |Local|
     |DC#1 |--|--.       |       .----|----|DC#3 |
     +-----+  |          |            |    +-----+
               \         |            /
                \        |           /
                 \       |          /
                  '----------------'
                         |
                      +-----+
                      |Local|
                      |DC#2 |
                      +-----+


                Figure 8: Simplify inter-DC SFC management

   For large data center operators, one local DC may have tens of
   thousands of servers and hundred of thousands of virtual machines.
   SFC can be used to manage user traffic.  For example, SFC can be used
   to classify user traffic based on service type, DDoS state etc.

   In such large scale data center, using flat SFC is very complex,
   requiring a super-controller to configure all data centers.  For
   example, any changes to Service Functions or Service Function Paths
   in the central DC (e.g., deploying a new SF) would require updates to
   all of the Service Function Paths in the local DCs accordingly.
   Furthermore, requirements for symmetric paths add additional
   complexity when flat SFC is used in this scenario.

   Conversely, if using hierarchical SFC, each data center can be
   managed independently to significantly reduce management complexity.
   Service Function Paths between data centers can represent abstract
   notions without regard to details within data centers.  Independent
   controllers can be used for the top level (getting packets to pass
   the correct data centers) and local levels (getting packets to
   specific SF instances).






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Authors' Addresses

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

   Phone: +1 519 880 2400
   Email: ddolson@sandvine.com


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

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


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

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


   Mohamed Boucadair
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com


   Dapeng Liu
   Alibaba Group
   Beijing  100022
   China

   Email: max.ldp@alibaba-inc.com






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   Ting Ao
   ZTE Corporation
   No.889,Bibo Rd.,Zhangjiang Hi-tech Park
   Shanghai   201203
   China

   Phone: +86-21-688976442
   Email: ao.ting@zte.com.cn


   Vu Anh Vu
   Soongsil University
   369 Sangdo-ro
   Seoul, Dongjak-gu  06978
   Korea

   Email: vuva@dcn.ssu.ac.kr


































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