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Versions: 00 01 02                                                      
Service Function Chaining                                      Vu Anh Vu
Internet-Draft                                              Younghan Kim
Intended status: Informational                             Kyoungjae Sun
Expires: January 4, 2018                                   Van-Ca Nguyen
                                                     Soongsil University
                                                          July 3, 2017

     Controlling Service Function Access to Network Service Header


   This document describes a mechanism to control Service Function
   access to the Network Service Header (NSH).  It addresses the Service
   Function trust issue and provide a method to enforce predefined
   access control lists to limit Service Function access to Service
   Function Chain information in the NSH in NSH-based Service Chaining.

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
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   This Internet-Draft will expire on September 12, 2017.

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   Copyright (c) 2017 IETF Trust and the persons identified as the
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Problem Statement . . . . . . . . . . . . . . . . . . . .   2
     1.3.  Definition Of Terms . . . . . . . . . . . . . . . . . . .   3
   2.  SF Access Control List  . . . . . . . . . . . . . . . . . . .   4
   3.  Access Control Enforcing Mechanisms . . . . . . . . . . . . .   4
   4.  Consideration for NSH Concealment . . . . . . . . . . . . . .   7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.2.  Problem Statement

   SFC Architecture document [RFC7665] defines architectural concepts
   and core components, including Service Functions (SFs), Service
   Function Forwarder (SFF), Classifier (CF), SFC Proxy.  These
   terminologies will be used in this documents.

   It is argued that whether or not we should trust the SFs in SFC.  In
   SFC general use cases, SFs vary from virtual services hosted in
   general-purpose servers to legacy service functions with dedicated
   hardware.  Most of the time, these SFs are deployed and operated by
   their service provider.  Therefore they are highly trusted.  Despite
   being in private and relatively safe service provider networks, SFs
   are not invulnerable to all security threats.  Indeed, several
   reasons cause the misbehavior of SFs. For instance, they can still
   be manipulated by multiple types of malware.  Furthermore,
   malfunctioned and misconfigured SFs can have anomaly behaviors as

   Aside from their own SFs, service providers may use SFs from other
   sources such as third party service providers (in case they want to
   outsource their SFs), SFs on customer premise, and legacy black-box

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   SFs.  In addition, enterprises are also trying to outsource their SFs
   to service providers, while still manage the SFC by themselves.
   Although service providers always have some SLAs for each SF, these
   SFs need to be verified and security checked frequently.

   Even if the SFs are trusted and secured, we still need to concern
   about the security of transportation layer.  Traffic between SFs and
   forwarding components (SFF, Classifier, SFC proxy) can be harmed by
   other threats such as man-in-the-middle attacks and spoofing,
   especially in geographically distributed data centers.

   As described in [I-D.ietf-sfc-nsh], NSH can be used to encapsulate
   SFC information to the packets in NSH-based Service Chaining.  There
   have been considerations about the security of this encapsulation.
   Problem Statement for Service Function Chaining [RFC7498] and SFC
   Architecture document [RFC7665] express concerns about SFC
   Encapsulation Security, which emphasize the importance of securing
   sensitive metadata carried by the encapsulation and state the
   requirement of an "appropriate protective treatment of NSH
   information".  Specifically, the NSH document [I-D.ietf-sfc-nsh]
   suggests some options (e.g.  IP Sec) to provide NSH metadata
   authenticity and confidentiality, most of them involve NSH

   As described in [I-D.ietf-sfc-control-plane], control-plane store
   and manage the access policies for SFs. Classifiers follow the
   policies to push/pop NSH for packets.

   [I-D.mglt-sfc-security-environment-req] lists requirements for SFC
   security, including data plane requirements, which emphasize the need
   of preventing the privacy leakage in SFC metadata.  The document also
   recommends some solutions such as metadata encryption and replacing
   metadata by its reference, which is our method in this document.

   Certainly, NSH encryption can provide rather strong security for the
   SFC metadata in an SFC-enabled domain, but it is also costly.  Both
   header encryption and key distribution require lots of resources and
   probably cause performance penalties to the SFC.  In this document,
   we describe an inexpensive mechanism to protect the sensitive
   metadata in NSH from either corrupted SFs or underlay networks
   security threats in NSH-based Service Chaining.

1.3.  Definition Of Terms

   o  Access-Controlled Segment (ACS): an ACS is an area/field within
      NSH that carries a piece of sensitive SFC information needed to be
      protected.  The access to this information from SFs should be
      limited and be controlled.

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   o  SF Access Control List (SF ACL): a list describes the access
      permission of an SF to each ACS in the packet passing through it.
      Each SF should have an SF ACL.

   o  Ingress Subsequent Classifier (Ingress S-CF): a logical classifier
      located BEFORE an access-controlled SF.  S-CFs are responsible for
      classify packets going into the SF and update their NSH.

   o  Egress Subsequent Classifier (Egress S-CF): a logical classifier
      located AFTER an access-controlled SF.  S-CFs are responsible for
      classify packets going out of the SF and update their NSH.

   o  NSH-state: a set of value/information stored in the NSH of a
      packet at a particular moment.  For example, the value of Service
      Index, Service Path Index, Mandatory Context Header 1-4.  An NSH-
      state usually consists of some ACS values.

2.  SF Access Control List

   Currently, without packet encryption, all SFs have full access to the
   packets they process and, in particular, the SFC information.
   Consequently, an SF can read and modify any unencrypted information
   within the NSH during its packet handling process.  Depend on what
   metadata stored in NSH, a corrupted SF can manipulate a part of
   information for harmful purposes such as changing service path, SF
   spoofing, gathering tenant information.

   In most situations, a SF might not need to know all SFC
   information to process its incoming packets.  An Access Control List
   (ACL) of an SF contains access permissions of the SF to each ACS in
   the NSH, including NSH base header, Service Path Index (SPI),
   Service Index (SI),and Metadata (MD).  For MD type 1, each of the 4
   Mandatory Context Header(MCH) can become an ACS.  MD type 2 has
   variable length MCH, therefore SF ACL should be defined according
   to the MD structure.  Wepropose three levels of permission to
   access an ACS of NSH:

   o  Hidden: the SF cannot view the information in this ACS

   o  Read-only: the SF can view, but cannot modify the information in
      this ACS

   o  Full access: the SF can view and modify the information in this

3.  Access Control Enforcing Mechanisms

   In this section, we propose a mechanism to enforce SF ACLs in SFC.
   The mechanism has two principles:

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   1.  Only give SFs what information they need to access.

   2.  SFFs cannot control SFs not to modify SFC information, but they
       can choose not to accept the modification.

   Figure 1 shows the components involved in the mechanism.
   Occasionally, an SF is attached to an SFF.  However, according [I-
   D.ietf-sfc-nsh], SFFs CANNOT perform NSH updating, which is the
   essential requirement of this mechanism.  Therefore, Ingress/Egress
   S-CFs are added and coordinate with SFF to classify packets and
   update NSH.

   When a packet come to an SFF on its way to the next SF in the SFP,
   the SFF will forward it to the ingress S-CF corresponds to the SF.
   Next, the ingress S-CF will check the SF ACL to get the SF permission
   to each ACS.  If the SF has Hidden permission to an ACS, the data
   contained in that ACS will be stored by the S-CF and erased from the
   packet (i.e. set all byte to zero).  As a result, sensitive
   information will be obscured from the SF, hence reduce the
   possibility of leaking information.  Besides, the ingress S-CF does
   not store only the erased ACS but also the ACSes that the SF has
   read-only permission for later use.

   After processing the packet, the SF forwards it back to an egress
   S-CF, which could be the same one as the aforementioned ingress S-CF.
   This S-CF checks the SF ACL and 1) With Hidden ACS, it gets the
   stored NSH-state (consists of ACS values) from the ingress S-CF and
   put it back to the packet, 2) With read-only ACS, it also gets the
   value from the NSH-state stored in the ingress S-CF and compares to
   the current value.  If the value is changed, the SF has tried to
   modify the ACS and egress S-CF would recover the ACS from the NSH-
   state stored by ingress S-CF.  Using this method, we can preserve the
   original SFC information, as well as detect abnormal SF behaviors.

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                             |                            |
                        +----+       Control Plane        +----+
                        |    |                            |    |
                        |    +-+------------------------+-+    |
                        |      |                        |      |
                        |      |                        |      |
                        |      |                        |      |
                        |      |                        |      |
                        |      |                        |      |
                        v      |                        |      |
  +---------+       +---+---+  |                        |  +---v---+
  |         |       |       |  |                        |  |       |
  | Initial |  ...  |  SFF  |  |                        |  |  SFF  | ...
  |   CF    |       |       |  |                        |  |       |
  +---------+       +---+---+  |                        |  +---+---+
                        |      |                        |      ^
                        |      v                        v      |
                        | +----+----+   +------+   +----+----+ |
                        | |         |   |      |   |         | |
                        +-> Ingress +--->  SF  +---> Egress  +-+
                          |  S-CF   |   |      |   |  S-CF   |
                          +---------+   +------+   +---------+

        Figure 1: Controlling SFs access to NSH in SFC architecture

   In this mechanism, the ingress and egress S-CF must exchange stored
   ACS data in either way:

   o  Shared storage

   o  Send the data to the SFC control plane

   Another key point of this mechanism is the method to track packets
   between ingress and egress S-CFs to recover the appropriate NSH
   metadata.  In detail, a packet encapsulation (including NSH) and
   payload might be changed after it traverses the SF between ingress
   and egress S-CFs.  The egress S-CF must know which NSH-state saved by
   the ingress SFF corresponds with a packet it receives.  Current
   solutions for this include:

   o  Reclassification: The packet will be classified (i.e.  Using
      5-tuple) after it exits the SF and goes to the egress S-CF.  The
      appropriate NSH will be determined based on the classification
      result.  Nevertheless, this approach cannot work with SFs which
      change 5-tuple (such as NAT)

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   o  Using Metadata: In this approach, the ingress S-CF put a unique ID
      named NSH-state ID into the NSH metadata of the packets.  The
      egress S-CF get the ID from a packet and use it to determine the
      packet's original NSH-state.  Figure 2 presents an example of NSH
      having Metadata type 1 with NSH-state ID.  Also, NSH-state ID can
      be defined per-flow or per-packet.  Nevertheless, the disadvantage
      is that we have to reserve a part of Metadata, which also can be
      access by SFs, for NSH-state ID.

      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|C|R|R|R|R|R|R|   Length  |  MD-type=0x1  | Next Protocol |
     |          Service Path Identifier              | Service Index |
     |                      NSH-State ID                             |
     |                Mandatory Context Header                       |
     |                Mandatory Context Header                       |
     |                Mandatory Context Header                       |

               Figure 2: NSH-state ID in NSH Metadata type 1

   This mechanism guarantees the consistency of sensitive SFC data
   within NSH when packets travel from an ingress SFF to an egress SFF
   through an SF, which means it can eliminate potential threats from
   the SF as well as the transportation networks between SFs and SFFs.

4.  Consideration for NSH Concealment

   In the previous section, we describe a mechanism to obscure ACS in
   NSH from SFs.  However, it is not limited to controlling the access
   to NSH of only SFs but other components as well.  Indeed, the
   mechanism can be extended to conceal ACS between two any points on
   the service function path of a packet.  In other words, an ingress
   and an egress SFFs can be any SFF on the SFP, not just the SFF which
   is directly attached to an SF.

   Moreover, the mechanism can be used to perform simple and low-cost
   NSH encryptions.  For example, the ingress SFF will save an ACS value
   and replace it with another value.  The mapping table which maps
   those two values will be given to the egress SFF and the all
   authorized SFs.

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5.  Acknowledgements


6.  IANA Considerations

   This document does not require any IANA actions.

7.  Security Considerations

   Secure communications between SFC control plane and components is
   required, as described in [I-D.ietf-sfc-control-plane], in order to
   secure access control policies during policy propagation from the
   control plane to enforcing components (such as SFFs and classifiers).

   Furthermore, if the NSH state table of an ingress S-CF is leaked, the
   controlling mechanism can be easily bypassed by spoofing.  Therefore,
   NSH state exchange process, which is either between CF-control plane
   or CF-CF, should be secured as well.

8.  Normative References

              Boucadair, M., "Service Function Chaining (SFC) Control
              Plane Components & Requirements", draft-ietf-sfc-control-
              plane-08 (work in progress), October 2016.

              Quinn, P. and U. Elzur, "Network Service Header", draft-
              ietf-sfc-nsh-13 (work in progress), June 2017.

              Migault, D., Pignataro, C., Reddy, T., and C. Inacio, "SFC
              environment Security requirements", draft-mglt-sfc-
              security-environment-req-02 (work in progress), October

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7498]  Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
              Service Function Chaining", RFC 7498,
              DOI 10.17487/RFC7498, April 2015,

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   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,

Authors' Addresses

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

   Email: vuva@dcn.ssu.ac.kr

   Younghan Kim
   Soongsil University
   369 Sangdo-ro
   Dongjak-gu, Seoul  06978
   South Korea

   Email: younghak@ssu.ac.kr

   Kyoungjae Sun
   Soongsil University
   369 Sangdo-ro
   Dongjak-gu, Seoul  06978
   South Korea

   Email: gomjae@ssu.ac.kr

   Van-Ca Nguyen
   Soongsil University
   369 Sangdo-ro
   Dongjak-gu, Seoul  06978
   South Korea

   Email: canguyen@dcn.ssu.ac.kr

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