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Internet Engineering Task Force (IETF)                          A. Malis
Request for Comments: 8596                                     S. Bryant
Category: Informational                                        Futurewei
ISSN: 2070-1721                                               J. Halpern
                                                           W. Henderickx
                                                               June 2019

  MPLS Transport Encapsulation for the Service Function Chaining (SFC)
                      Network Service Header (NSH)


   This document describes how to use a Service Function Forwarder (SFF)
   Label (similar to a pseudowire label or VPN label) to indicate the
   presence of a Service Function Chaining (SFC) Network Service Header
   (NSH) between an MPLS label stack and the NSH original packet/frame.
   This allows SFC packets using the NSH to be forwarded between SFFs
   over an MPLS network.  The label is also used to select between
   multiple SFFs in the destination MPLS node.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 8596                  MPLS for the SFC NSH                 June 2019

Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
   2. MPLS Encapsulation Using an SFF Label ...........................3
      2.1. MPLS Label Stack Construction at the Sending Node ..........4
      2.2. SFF Label Processing at the Destination Node ...............5
   3. Equal-Cost Multipath (ECMP) Considerations ......................5
   4. Operations, Administration, and Maintenance (OAM)
      Considerations ..................................................6
   5. IANA Considerations .............................................6
   6. Security Considerations .........................................6
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................8
   Acknowledgements ...................................................9
   Authors' Addresses .................................................9

1.  Introduction

   As discussed in [RFC8300], a number of transport encapsulations for
   the Service Function Chaining (SFC) Network Service Header (NSH)
   already exist, such as Ethernet, UDP, GRE, and others.

   This document describes an MPLS transport encapsulation for the NSH
   and how to use a Service Function Forwarder (SFF) [RFC7665] Label to
   indicate the presence of the NSH in the MPLS packet payload.  This
   allows SFC packets using the NSH to be forwarded between SFFs in an
   MPLS transport network, where MPLS is used to interconnect the
   network nodes that contain one or more SFFs.  The label is also used
   to select between multiple SFFs in the destination MPLS node.

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   From an SFC perspective, this encapsulation is equivalent to other
   transport encapsulations of packets using the NSH.  This can be
   illustrated by adding an additional line to the example of a next-hop
   SPI / SI-to-network ("SPI" and "SI" stand for "Service Path
   Identifier" and "Service Index") overlay network locator mapping in
   Table 1 of [RFC8300]:

     | SPI  | SI   | Next Hop(s)         | Transport Encapsulation |
     | 25   | 220  | Label 5467          | MPLS                    |

                 Table 1: Extension to Table 1 in RFC 8300

   SFF Labels are similar to other service labels at the bottom of an
   MPLS label stack that denote the contents of the MPLS payload being
   other than a normally routed IP packet, such as a Layer 2 pseudowire,
   an IP packet that is routed in a VPN context with a private address,
   or an Ethernet virtual private wire service.

   This informational document follows well-established MPLS procedures
   and does not require any actions by IANA or any new protocol

   Note that using the MPLS label stack as a replacement for the SFC
   NSH, covering use cases that do not require per-packet metadata, is
   described in [RFC8595].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  MPLS Encapsulation Using an SFF Label

   The encapsulation is a standard MPLS label stack [RFC3032] with an
   SFF Label at the bottom of the stack, followed by an NSH as defined
   by [RFC8300] and the NSH original packet/frame.

   Much like a pseudowire label, an SFF Label MUST be allocated by the
   downstream receiver of the NSH from its per-platform label space,
   since the meaning of the label is identical, independent of which
   incoming interface it is received from [RFC3031].

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   If a receiving node supports more than one SFF (i.e., more than one
   SFC forwarding instance), then the SFF Label can be used to select
   the proper SFF, by having the receiving node advertise more than one
   SFF Label to its upstream sending nodes as appropriate.

   The method used by the downstream receiving node to advertise SFF
   Labels to the upstream sending node is out of scope for this
   document.  That said, a number of methods are possible, such as via a
   protocol exchange, or via a controller that manages both the sender
   and the receiver using the Network Configuration Protocol
   (NETCONF) / YANG, BGP, the Path Computation Element Communication
   Protocol (PCEP), etc.  One such BGP-based method has already been
   defined and is documented in [BGP-NSH-SFC].  This does not constrain
   the further definition of other such advertisement methods in the

   While the SFF Label will usually be at the bottom of the label stack,
   there may be cases where there are additional label stack entries
   beneath it.  For example, when an Associated Channel Header (ACH) is
   carried that applies to the SFF, a Generic Associated Channel Label
   (GAL) [RFC5586] will be in the label stack below the SFF.  Similarly,
   an Entropy Label Indicator / Entropy Label (ELI/EL) [RFC6790] may be
   carried below the SFF in the label stack.  This is identical to the
   situation with VPN labels.

   This document does not define the setting of the Traffic Class (TC)
   field [RFC5462] (formerly known as the Experimental Use (EXP) bits
   [RFC3032]) in the SFF Label.

2.1.  MPLS Label Stack Construction at the Sending Node

   When one SFF wishes to send an SFC packet with an NSH to another SFF
   over an MPLS transport network, a label stack needs to be constructed
   by the MPLS node that contains the sending SFF in order to transport
   the packet to the destination MPLS node that contains the receiving
   SFF.  The label stack is constructed as follows:

   1.  Push zero or more labels that are interpreted by the destination
       MPLS node on to the packet, such as the GAL [RFC5586] (see
       Section 4).  The TTL for these labels is set according to the
       relevant standards that define these labels.

   2.  Push the SFF Label to identify the desired SFF in the receiving
       MPLS node.  The TTL for this MPLS label MUST be set to 1 to avoid

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   3.  Push zero or more additional labels such that (a) the resulting
       label stack will cause the packet to be transported to the
       destination MPLS node, and (b) when the packet arrives at the
       destination node, either:

       *  the SFF Label will be at the top of the label stack (this is
          typically the case when penultimate hop popping is used at the
          penultimate node), or

       *  as a part of normal MPLS processing, the SFF Label becomes the
          top label in the stack before the packet is forwarded to
          another node and before the packet is dispatched to a higher

      The TTL for these labels is set by configuration or set to the
      defaults for normal MPLS operation in the network.

2.2.  SFF Label Processing at the Destination Node

   The destination MPLS node performs a lookup on the SFF Label to
   retrieve the next-hop context between the SFF and SF, e.g., to
   retrieve the destination Media Access Control (MAC) address in the
   case where native Ethernet encapsulation is used between the SFF and
   SF.  How the next-hop context is populated is out of scope for this

   The receiving SFF SHOULD check that the received SFF Label has a TTL
   of 1 upon receipt.  Any other values indicate a likely error
   condition and SHOULD result in discarding the packet.

   The receiving MPLS node then pops the SFF Label (and any labels
   beneath it) so that the destination SFF receives the SFC packet with
   the NSH at the top of the packet.

3.  Equal-Cost Multipath (ECMP) Considerations

   As discussed in [RFC4928] and [RFC7325], there are ECMP
   considerations for payloads carried by MPLS.

   Many existing routers use deep packet inspection to examine the
   payload of an MPLS packet.  If the first nibble of the payload is
   equal to 0x4 or 0x6, these routers (sometimes incorrectly, as
   discussed in [RFC4928]) assume that the payload is IPv4 or IPv6,
   respectively and, as a result, perform ECMP load balancing based on
   (presumed) information present in IP/TCP/UDP payload headers or in a
   combination of MPLS label stack and (presumed) IP/TCP/UDP payload
   headers in the packet.

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   For SFC, ECMP may or may not be desirable.  To prevent ECMP when it
   is not desired, the NSH Base Header was carefully constructed so that
   the NSH could not look like IPv4 or IPv6 based on its first nibble.
   See Section 2.2 of [RFC8300] for further details.  Accordingly, the
   default behavior for MPLS-encapsulated SFC is to not use ECMP other
   than by using entropy derived from the MPLS label stack.  This
   results in all packets going to the same SF taking the same path
   regardless of the use of ECMP in the network.

   If ECMP is desired when SFC is used with an MPLS transport network,
   there are two possible options: entropy labels [RFC6790] and
   flow-aware transport [RFC6391] labels.  A recommendation regarding
   choosing between these options, and their proper placement in the
   label stack, is left for future study.

4.  Operations, Administration, and Maintenance (OAM) Considerations

   OAM at the SFC layer is handled by SFC-defined mechanisms [RFC8300].
   However, OAM may be required at the MPLS transport layer.  If so,
   then standard MPLS-layer OAM mechanisms such as the GAL [RFC5586] may
   be used at the transport label layer.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   This document describes a method for transporting SFC packets using
   the NSH over an MPLS transport network.  It follows well-established
   MPLS procedures in widespread operational use.  It does not define
   any new protocol elements or allocate any new code points, and it is
   no more or less secure than carrying any other protocol over MPLS.
   To the MPLS network, the NSH and its contents are simply an opaque

   In addition, the security considerations in [RFC8595] also apply to
   this document.

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7.  References

7.1.  Normative References

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

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462,
              February 2009, <https://www.rfc-editor.org/info/rfc5462>.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,

   [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", RFC 8300,
              DOI 10.17487/RFC8300, January 2018,

   [RFC8595]  Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based
              Forwarding Plane for Service Function Chaining", RFC 8595,
              DOI 10.17487/RFC8595, June 2019,

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7.2.  Informative References

              Farrel, A., Drake, J., Rosen, E., Uttaro, J., and L.
              Jalil, "BGP Control Plane for NSH SFC", Work in Progress,
              draft-ietf-bess-nsh-bgp-control-plane-11, May 2019.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, DOI 10.17487/RFC4928, June 2007,

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,

   [RFC6391]  Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
              Regan, J., and S. Amante, "Flow-Aware Transport of
              Pseudowires over an MPLS Packet Switched Network",
              RFC 6391, DOI 10.17487/RFC6391, November 2011,

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,

   [RFC7325]  Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
              and C. Pignataro, "MPLS Forwarding Compliance and
              Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
              August 2014, <https://www.rfc-editor.org/info/rfc7325>.

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   The authors would like to thank Jim Guichard, Eric Rosen, Med
   Boucadair, Alexander (Sasha) Vainshtein, Jeff Tantsura, Anoop
   Ghanwani, John Drake, Loa Andersson, Carlos Pignataro, Christian
   Hopps, and Benjamin Kaduk for their reviews and comments.

Authors' Addresses

   Andrew G. Malis

   Email: agmalis@gmail.com

   Stewart Bryant

   Email: stewart.bryant@gmail.com

   Joel M. Halpern

   Email: joel.halpern@ericsson.com

   Wim Henderickx

   Email: wim.henderickx@nokia.com

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