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SFC Working Group                                              A. Farrel
Internet-Draft                                                  J. Drake
Intended status: Standards Track                        Juniper Networks
Expires: December 31, 2017                                 June 29, 2017


  Operating the Network Service Header (NSH) with Next Protocol "None"
                      draft-farrel-sfc-convent-02

Abstract

   This document describes the use of the Network Service Header (NSH)
   in a Service Function Chaining (SFC) enabled network with no payload
   data and carrying only metadata.  This is achieved by defining a new
   NSH "Next Protocol" type value of "None".

   This document illustrates some of the functions that may be achieved
   or enhanced by this mechanism, but it does not provide an exhaustive
   list of use cases, nor is it intended to be definitive about the
   functions it describes.  It is expected that other documents will
   describe specific use cases in more detail and will define the
   protocol mechanics for each use case.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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 December 31, 2017.






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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
   (http://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
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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Network Service Header  . . . . . . . . . . . . . . . . .   3
     2.1.  Next Protocol 'None'  . . . . . . . . . . . . . . . . . .   4
   3.  Processing Rules  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   5
   5.  Overview of Use Cases . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Per-SFP Metadata  . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Per-Flow Metadata . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Coordination Between SFC-Aware SFIs . . . . . . . . . . .   6
     5.4.  Operations, Administration, and Maintenance (OAM) . . . .   7
     5.5.  Control Plane and Management Plane Uses . . . . . . . . .   8
     5.6.  Non-Applicable Use Cases  . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   An architecture for Service Function Chaining (SFC) is presented in
   [RFC7665].  That architecture enables packets to be forwarded along
   Service Function Paths (SFPs) to pass through various Service
   Functions (SFs) that act on the packets.  Each packet is encapsulated
   with a Network Service Header (NSH) [I-D.ietf-sfc-nsh] identify the
   SFP that the packet travels along (by means of a Service Path
   Identifier - SPI) and the hop (i.e., the next SF to be executed)




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   along the SFP that the packet has reached (by means of a Service
   Index - SI).  The SPI and SI are fields encoded in the NSH.

   Packets are classified at the SFC ingress boundaries (section 4.4 of
   [RFC7665]) and have an NSH applied to them.  Such packets are
   forwarded between Service Function Forwarders (SFFs) using tunnels
   across the underlay network, and each SFF may hand the packet off to
   one or more Service Function Instances (SFIs) according to the
   definition of the SFP.

   The SFC classifier or any SFC-aware SFI may wish to share information
   (possibly state information) about the SFP, the traffic flow, or a
   specific packet, and they may do this by adding "metadata" to packets
   as part of the NSH.  Metadata may be used to enhance or enable the
   function preformed by SFC-aware SFs, may enable coordination and data
   exchange between SFIs, or may be used to assist a network operator in
   the diagnosis and monitoring of an SFP.  The nature of metadata to be
   supplied and consumed is implementation- and deployment-specific.

   This document defines a mechanism for metadata to be carried on an
   SFP without the need for payload data.  This may enable diagnosis and
   monitoring of SFPs, and coordination between SFC-aware SFIs, without
   the need for traffic to be flowing, and without the need to rewrite
   data packets to insert what might be substantial amounts of metadata.

   This function is achieved by defining a new value for the NSH "Next
   Protocol" field to indicate "None".  Such packets are contained
   within the SFC-enabled domain.

   This document illustrates some of the functions that may be achieved
   or enhanced by this mechanism, but it does not provide an exhaustive
   list of use cases, nor is it intended to be definitive about the
   functions it describes.  It is expected that other documents will
   describe specific use cases in more detail and will define the
   protocol mechanics for each use case.

2.  The Network Service Header

   The NSH is defined in [I-D.ietf-sfc-nsh].  It includes a field called
   "Next Protocol" that is used to indicate the nature of the payload
   data that follows the NSH.  The field can be used by any component
   that processes the NSH (for example, to understand how to interpret
   and parse the payload) and by nodes at the end of the SFP that remove
   the NSH and forward the payload data.







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2.1.  Next Protocol 'None'

   This document defines a new value for the "Next Protocol" field.
   When set to TBD1, the field indicates that the next protocol is
   "None" meaning that there is no user/payload data following the NSH.

   When the next protocol is "None" the rest of the NSH still has
   meaning and, in particular, the metadata carried in the NSH may still
   be present.

3.  Processing Rules

   An SFC-aware node wishing to send metadata without a data packet:

   o  MUST create a packet carrying an NSH and the desired metadata

   o  MUST set the "Next Protocol" field to TBD1

   o  SHOULD ensure that there are no bytes following the end of the NSH
      (i.e., that there is no payload data)

   o  MUST encapsulate and send the packet as normal for tunneling to
      the next hop on the SFP as normal for an NSH packet.

   A packet with no payload data may be simply inserted at the head end
   of an SFP (such as a Classifier) and may be easily forwarded by an
   SFF or SFI on the SFP using the normal processing rules defined in
   [I-D.ietf-sfc-nsh].

   A packet with no payload may also be generated by an SFC-aware SFI as
   a result of processing an incoming packet (i.e., triggered by a
   condition arising from processing a normal NSH packet with a
   payload).  In such cases, the SPI/SI can be inherited from the
   original packet or can be set according to information supplied
   through the control plane or management plane.  This document does
   not further specify the triggers to generate an NSH packet with a
   "Next Protocol" set to "None".

   A transit node (SFF, SFI, or classifier) receiving a packet with
   "Next Protocol" indicating "None" MUST NOT attempt to parse or
   process beyond the end of the NSH, but can process the NSH and
   especially the metadata as normal.

   A node that is the egress of an SFP would normally strip the NSH and
   forward the payload according to the setting of the "Next Protocol"
   field.  Such nodes MUST NOT forward packets with "Next Protocol"
   indicating "None" even if there some bytes after the NSH.




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4.  Backward Compatibility

   This section describes procedures for default handling on unknown
   "Next Protocol" field values.  This material updates the procedures
   described in [I-D.ietf-sfc-nsh] and may be transferred to that
   document.

   SFC-aware nodes that do not understand the meaning of a value
   contained in the "Next Protocol" field of the NSH are unable to parse
   the payload.  Such nodes are not obliged to discard the packet unless
   they are specifically called upon to be able to examine the payload.

   Thus:

   o  Transit SFFs will normally not inspect the "Next Protocol" field
      or the packet payload and will forward the packets based solely on
      the SPI/SI

   o  An SFC Proxy must not pass to an SFI a packet of type where it
      cannot indicate the packet type to the SFI

   o  An SFC Proxy must not pass to an SFI a packet of type that the SFI
      does not support

   o  An SFC Proxy should not return to the SFF a packet it has not
      passed to the SFI

   o  An SFI should not return to the SFF a packet it hasn't processed
      unless local policy defines "process" for this SF to mean "do not
      process" in this case.

   o  Reclassifiers would normally require to understand the payload
      packet type, but it is possible to imagine reclassifiers that take
      action based on unknown values of the "Next Protocol" field or
      that perform protocol-independent actions (such as hashing the
      whole packet).

   All this means that legacy SFC-aware nodes that are unaware of the
   meaning of the "Next Protocol" value "None" will act as follows:

   o  SFFs will forward the packets

   o  SFC Proxies will drop the packets

   o  SFIs will most likely drop the packets

   o  Reclassifiers will most likely drop the packets




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   SFC-aware nodes at the end of an SFP possibly forward packets with no
   knowledge of the payload in a "pop and forward" form of processing
   where the NSH is removed and the packet is simply put on an interface
   and the payload protocol is known a priori (or assumed).  It is a
   general processing rule for all forwarders that they SHOULD NOT
   attempt to send packets with zero length, and since packets with the
   NSH "Next Protocol" set "None" are expected to have zero payload
   length.

5.  Overview of Use Cases

5.1.  Per-SFP Metadata

   Per-SFP metadata is metadata that applies to an SFP and any data
   packets on that SFP.  It does not need to be transmitted with every
   packet, but can be installed at the SFIs on the SFP and applied to
   all packets on the SFP.

   Per-SFP metadata may be sent along the path of an SFP simply by
   setting the correct SPI in the NSH, and setting the SI to the correct
   value for the hop of the SFP at which the metadata is to be
   introduced.  Classifiers and reclassifiers will know the correct SI
   values to used from information supplied by the control or management
   plane as is the case for NSH packets with payload data.

5.2.  Per-Flow Metadata

   Per-flow metadata is metadata that applies to a subset of the packets
   on an SFP, such as packets matching a particular 5 tuple of source
   address, destination address, source port, destination port, and
   payload protocol.  This metadata also does not need to be transmitted
   with every packet, but can be installed at the SFIs on the SFP and
   applied to the packets that match the flow description.

   If there is just one flow on an SFP then there is no difference
   between per-flow metadata and per-SFP metadata as described in .

   In normal processing, the flow to which per-flow metadata applies can
   be deduced by looking at the payload data in the context of the value
   of the "Next Protocol" field.  However, when "Next Protocol"
   indicates "None" this cannot be done.  In this case the identity of
   the flow is carried in the metadata.

5.3.  Coordination Between SFC-Aware SFIs

   A pair of SFC-aware SFIs (adjacent or not) on an SFP may desire to
   coordinate state and may do this by sending information encoded in
   metadata.



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   To do this using the mechanisms defined in this document:

   o  There must be an SFP that passes through the two SFIs in the
      direction of sender to receiver

   o  The sender must know the correct SPI to use

   o  The sender must know the correct SI to use for the point at which
      it resides on the SFP

   o  Ideally the receiver will know to remove the packet from the SFP
      and not forward it further as this might share metadata wider than
      desirable and would cause unnecessary packets in the network.
      Note, however, that continued forwarding of such packets would not
      be substantially harmful in its own right.

   Note that technically (according to the SFC architecture) the process
   of inserting a packet into an SFP is performed by a Classifier.
   However, a Classifier may be co-resident with an SFI so an
   implementation of an SF may also be able to generate NSH packets as
   described in this document.

   Note also that a system with SFIs that need to coordinate between
   each other may be configured so that there is a specific, dedicated
   SFP between those service functions that is used solely for this
   purpose.  Thus, such an SFI does not need to insert NSH packets onto
   SFPs used to carry payload data, but can use (and know the SPI of)
   this special, dedicated SFP.

5.4.  Operations, Administration, and Maintenance (OAM)

   Requirements for Operations, Administration, and Maintenance (OAM) in
   SFC networks are discussed in [I-D.ietf-sfc-oam-framework].  The NSH
   definition in [I-D.ietf-sfc-nsh] includes an O-bit that indicates
   that packet contains OAM information.

   Since OAM information will be carried in packets that also include
   payload data, that information must be carried in metadata.
   Therefore, the mechanism defined in this document can be used to
   carry OAM information independent of payload data.

   Sending OAM separate from (but interleaved with) packets that carry
   payload data may have several advantages including:

   o  Sending OAM when there is no other traffic flowing.

   o  Sending OAM at predictable intervals.




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   o  Measuring path qualities distinct from behavior of SFIs.

   o  Sending OAM without needing to rewrite payload data buffers.

   o  Keeping OAM processing components separate from other processing
      components.

5.5.  Control Plane and Management Plane Uses

   As described in Section 5.3, SFPs can be established specifically to
   carry metadata-only packets.  And as described in Section 5.1,
   metadata-only packets can be sent down existing SFPs.  This means
   that metadata-only packets can be used to carry control plane and
   management plane messages used to control and manage the SFC network.

   In effect, SFPs can be established to serve as a Data Control Network
   (DCN) or Management Control Network (MCN).  Further details of this
   process are out of scope of this document, but it should be
   understood that, just as for OAM, an essential feature of using a
   control channel is that the various speakers are assigned identifiers
   (i.e., addresses).  In this case, those identifiers could be SPI/SI
   pairs, or could be IP addresses as used in the normal control and
   management plane of the SFC network.

5.6.  Non-Applicable Use Cases

   Per-packt metadata is metadata that applies specifically to a payload
   packet.  It informs an SFI how to handle the payload packet, and does
   not apply to any other packets.

   The mechanisms described in this document are not applicable to per-
   packet metadata because, by definition, if the "Next Protocol"
   indicates "None" then there is no packet following the NSH for the
   metadata to be associated with.

6.  Security Considerations

   Metadata-only packets as enabled by this document provide a covert
   channel.  However, this is only different from the metadata feature
   in the normal NSH in that it can be sent without the presence of a
   data flow.

   Metadata may, of course, contain sensitive data and may also contain
   information used to control the behavior of SFIs in the network.  As
   such, this data needs to be protected according to its value and
   according to the perceived vulnerabilities of the network.
   Protection of metadata may be achieved by using encrypted transport
   between SFC entities or by encrypting the metadata in its own right.



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   The need to protect the metadata is not modified by this document and
   forms part of the NSH definition found in [I-D.ietf-sfc-nsh].

   The mechanism described in this document might possibly be used to
   introduce packets into the SFC overlay network.  Therefore measures
   SHOULD be taken to ensure authorization of sources of such packets,
   and tunneling of such packets into the network SHOULD be prevented.
   The amount of packets with "Next Protocol" set to "None" on an SFP
   MAY be rate limited at any point on the SFP to provide additional
   security.

   Further discussion of NSH security is presented in
   [I-D.ietf-sfc-nsh].

7.  IANA Considerations

   IANA has been requested to create a registry of "Next Protocol"
   values in [I-D.ietf-sfc-nsh].  This document requests IANA to
   allocate a value from that registry to indicate "None" (TBD1 in this
   document).

   It is strongly suggested that a value of 0 (zero) be assigned.

8.  Contributors


   Lucy Yong
   Retired


9.  Acknowledgements

   Thanks to the attendees at the SFC interim meeting in Westford in
   January 2017 for discussions that suggested the value of this
   document.

   Thanks to Eric Rosen and Med Boucadair for valuable review comments.

10.  References

10.1.  Normative References

   [I-D.ietf-sfc-nsh]
              Quinn, P. and U. Elzur, "Network Service Header", draft-
              ietf-sfc-nsh-12 (work in progress), February 2017.






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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

10.2.  Informative References

   [I-D.ietf-sfc-oam-framework]
              Aldrin, S., Krishnan, R., Akiya, N., Pignataro, C., and A.
              Ghanwani, "Service Function Chaining Operation,
              Administration and Maintenance Framework", draft-ietf-sfc-
              oam-framework-01 (work in progress), February 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>.

Authors' Addresses

   Adrian Farrel
   Juniper Networks

   Email: afarrel@juniper.net


   John Drake
   Juniper Networks

   Email: jdrake@juniper.net





















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