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

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

    Abstract

       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
       https://www.rfc-editor.org/info/rfc8596.

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    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
       extensions.

       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",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "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
       future.

       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
           mis-forwarding.

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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
              layer.

          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
       document.

       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
       payload.

       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,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
                  Label Switching Architecture", RFC 3031,
                  DOI 10.17487/RFC3031, January 2001,
                  <https://www.rfc-editor.org/info/rfc3031>.

       [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,
                  <https://www.rfc-editor.org/info/rfc3032>.

       [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,
                  <https://www.rfc-editor.org/info/rfc7665>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
                  RFC 2119 Key Words", BCP 14, RFC 8174,
                  DOI 10.17487/RFC8174, May 2017,
                  <https://www.rfc-editor.org/info/rfc8174>.

       [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
                  "Network Service Header (NSH)", RFC 8300,
                  DOI 10.17487/RFC8300, January 2018,
                  <https://www.rfc-editor.org/info/rfc8300>.

       [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,
                  <https://www.rfc-editor.org/info/rfc8595>.

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    7.2.  Informative References

       [BGP-NSH-SFC]
                  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,
                  <https://www.rfc-editor.org/info/rfc4928>.

       [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
                  "MPLS Generic Associated Channel", RFC 5586,
                  DOI 10.17487/RFC5586, June 2009,
                  <https://www.rfc-editor.org/info/rfc5586>.

       [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,
                  <https://www.rfc-editor.org/info/rfc6391>.

       [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,
                  <https://www.rfc-editor.org/info/rfc6790>.

       [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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    Acknowledgements

       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
       Futurewei

       Email: agmalis@gmail.com

       Stewart Bryant
       Futurewei

       Email: stewart.bryant@gmail.com

       Joel M. Halpern
       Ericsson

       Email: joel.halpern@ericsson.com

       Wim Henderickx
       Nokia

       Email: wim.henderickx@nokia.com

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