Network Working Group                                      P. Quinn, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                           U. Elzur, Ed.
Expires: September 25, 2015                                        Intel
                                                          March January 24, 2016                                          Intel
                                                           July 23, 2015

                         Network Service Header
                       draft-ietf-sfc-nsh-00.txt
                       draft-ietf-sfc-nsh-01.txt

Abstract

   This draft describes a Network Service Header (NSH) inserted onto
   encapsulated packets or frames to realize service function paths.
   NSH also provides a mechanism for metadata exchange along the
   instantiated service path.  NSH is the SFC encapsulation as per SFC
   Architecture [SFC-arch]

1.  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 RFC 2119 [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 September 25, 2015. January 24, 2016.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  2
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Definition of Terms  . . . . . . . . . . . . . . . . . . .  4
     2.2.  Problem Space  . . . . . . . . . . . . . . . . . . . . . .  6  7
     2.3.  NSH-based Service Chaining . . . . . . . . . . . . . . . .  8
   3.  Network Service Header . . . . . . . . . . . . . . . . . . . .  8 10
     3.1.  Network Service Header Format  . . . . . . . . . . . . . .  8 10
     3.2.  NSH Base Header  . . . . . . . . . . . . . . . . . . . . .  8 10
     3.3.  Service Path Header  . . . . . . . . . . . . . . . . . . . 10 12
     3.4.  NSH MD-type 1  . . . . . . . . . . . . . . . . . . . . . . 10
       3.4.1.  Mandatory Context Header Allocation Guidelines . . . . 11 13
     3.5.  NSH MD-type 2  . . . . . . . . . . . . . . . . . . . . . . 12 13
       3.5.1.  Optional Variable Length Metadata  . . . . . . . . . . 13 14
   4.  NSH Actions  . . . . . . . . . . . . . . . . . . . . . . . . . 15 16
   5.  NSH Encapsulation  . . . . . . . . . . . . . . . . . . . . . . 17
   6.  NSH Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  NSH Proxy Nodes  . . . . . . . . . . . . . . . . . . . . . . . 19
   8.
   6.  Fragmentation Considerations . . . . . . . . . . . . . . . . . 20
   9. 19
   7.  Service Path Forwarding with NSH . . . . . . . . . . . . . . . 21
     9.1. 20
     7.1.  SFFs and Overlay Selection . . . . . . . . . . . . . . . . 21
     9.2. 20
     7.2.  Mapping NSH to Network Overlay . . . . . . . . . . . . . . 23
     9.3. 22
     7.3.  Service Plane Visibility . . . . . . . . . . . . . . . . . 24
     9.4. 23
     7.4.  Service Graphs . . . . . . . . . . . . . . . . . . . . . . 24
   10. 23
   8.  Policy Enforcement with NSH  . . . . . . . . . . . . . . . . . 26
     10.1.
     8.1.  NSH Metadata and Policy Enforcement  . . . . . . . . . . . 26
     10.2.
     8.2.  Updating/Augmenting Metadata . . . . . . . . . . . . . . . 27
     10.3.
     8.3.  Service Path ID and Metadata . . . . . . . . . . . . . . . 29
   11.
   9.  NSH Encapsulation Examples . . . . . . . . . . . . . . . . . . 30
     11.1. 31
     9.1.  GRE + NSH  . . . . . . . . . . . . . . . . . . . . . . . . 30
     11.2. 31
     9.2.  VXLAN-gpe + NSH  . . . . . . . . . . . . . . . . . . . . . 30
     11.3. 31
     9.3.  Ethernet + NSH . . . . . . . . . . . . . . . . . . . . . . 31
   12. 32
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   13. 33
   11. Open Items for WG Discussion . . . . . . . . . . . . . . . . . 33
   14. 34
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 34
   15. 35
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 37
   16. 38
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     16.1. 39
     14.1. NSH EtherType  . . . . . . . . . . . . . . . . . . . . . . 38
     16.2. 39
     14.2. Network Service Header (NSH) Parameters  . . . . . . . . . 38
       16.2.1. 39
       14.2.1. NSH Base Header Reserved Bits  . . . . . . . . . . . . 38
       16.2.2. 39
       14.2.2. MD Type Registry . . . . . . . . . . . . . . . . . . . 38
       16.2.3. 39
       14.2.3. TLV Class Registry . . . . . . . . . . . . . . . . . . 39
       16.2.4. 40
       14.2.4. NSH Base Header Next Protocol  . . . . . . . . . . . . 39
   17. 40
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     17.1. 41
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     17.2. 41
     15.2. Informative References . . . . . . . . . . . . . . . . . . 40 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42 43

2.  Introduction

   Service functions are widely deployed and essential in many networks.
   These service functions provide a range of features such as security,
   WAN acceleration, and server load balancing.  Service functions may
   be instantiated at different points in the network infrastructure
   such as the wide area network, data center, campus, and so forth.

   The current service function deployment models are relatively static,
   and bound to topology for insertion and policy selection.
   Furthermore, they do not adapt well to elastic service environments
   enabled by virtualization.

   New data center network and cloud architectures require more flexible
   service function deployment models.  Additionally, the transition to
   virtual platforms requires an agile service insertion model that
   supports dynamic and elastic service delivery; the movement of
   service functions and application workloads in the network and the
   ability to easily bind service policy to granular information such as
   per-subscriber state and steer traffic to the requisite service
   function(s) are necessary.

   The approach taken by

   NSH defines a new dataplane protocol specifically for the creation of
   dynamic service chains and is composed of the following elements:

   1.  Service path Function Path identification

   2.  Transport independent per-packet/frame service metadata. function chain

   3.  Optional  Per-packet network and service metadata or optional variable TLV
       metadata.

   NSH is designed to be easy to implement across a range of devices,
   both physical and virtual, including hardware platforms.

   An NSH aware control plane is outside the scope of this document.

   The SFC Architecture document [SFC-arch] provides an overview of a
   service chaining architecture that clearly defines the roles of the
   various elements and the scope of a service function chaining
   encapsulation.  NSH is the SFC encapsulation defined in that draft.

2.1.  Definition of Terms
   Classification:  Locally instantiated policy and customer/network/
      service profile matching of traffic flows
      against policy for identification subsequent application of the required set of
      appropriate outbound forwarding actions.

   SFC Network Forwarder (NF):  SFC network forwarders provide
      network
      connectivity for service functions forwarders and service functions.  SFC network forwarders participate in the network
      overlay used for  The policy may be customer/network/
      service function chaining as well as in the SFC
      encapsulation. specific.

   Service Function Forwarder (SFF):  A service function forwarder is
      responsible for delivering forwarding traffic received from the NF to one or more connected
      service functions, and functions according to information carried in the NSH, as
      well as handling traffic coming back from the SF.  Additionally, a
      service functions function forwarder is responsible for transporting traffic
      to another SFF (in the same or different type of overlay), and
      terminating the NF. SFP.

   Service Function (SF):  A function that is responsible for specific
      treatment of received packets.  A service function Service Function can act at
      various layers of a protocol stack (e.g., at the network layer or
      other OSI layers.  A service function layers).  As a logical component, a Service Function can
      be realized as a virtual instance element or be embedded in a physical
      network element.  One of multiple service functions or more Service Functions can be embedded in
      the same network element.  Multiple instances occurrences of the service function Service
      Function can
      be enabled exist in the same administrative domain.

   Service Node (SN):  Physical or virtual element that hosts one

      One or more service functions Service Functions can be involved in the delivery of
      added-value services.  A non-exhaustive list of abstract Service
      Functions includes: firewalls, WAN and has one or more network locators
      associated with application acceleration,
      Deep Packet Inspection (DPI), LI (Lawful Intercept), server load
      balancing, NAT44 [RFC3022], NAT64 [RFC6146], NPTv6 [RFC6296],
      HOST_ID injection, HTTP Header Enrichment functions, TCP
      optimizer.

      An SF may be NSH-aware, that is it for reachability receives and service delivery. acts on
      information in the NSH.  The SF may also be NSH-unaware in which
      case data forwarded to the SF does not contain NSH.

   Service Function Chain (SFC):  A service function chain defines an
      ordered set of abstract service functions (SFs) and ordering
      constraints that must be applied to packets and/or frames and/or
      flows selected as a result of classification.  An example of an
      abstract service function is "a firewall".  The implied order may
      not be a linear progression as the architecture allows for nodes SFCs
      that copy to more than one branch. branch, and also allows for cases where
      there is flexibility in the order in which service functions need
      to be applied.  The term service chain is often used as shorthand
      for service function chain.

   Service Function Path (SFP):  The instantiation of Service Function Path is a SFC in the
      network.  Packets follow
      constrained specification of where packets assigned to a certain
      service function path from a classifier
      through must go.  While it may be so constrained as
      to identify the requisite service functions

   Network Node/Element:  Device that forwards packets or frames based
      on outer header information.  In most cases is not aware exact locations, it can also be less specific.
      The SFP provides a level of indirection between the
      presence fully abstract
      notion of NSH. service chain as a sequence of abstract service
      functions to be delivered, and the fully specified notion of
      exactly which SFF/SFs the packet will visit when it actually
      traverses the network.  By allowing the control components to
      specify this level of indirection, the operator may control the
      degree of SFF/SF selection authority that is delegated to the
      network.

   Network Node/Element:  Device that forwards packets or frames based
      on outer header information.

   Network Overlay:  Logical network built on top of existing network
      (the underlay).  Packets are encapsulated or tunneled to create
      the overlay network topology.

   Network Service Header:  Data plane header added to frames/packets.
      The header contains information required for service chaining, as
      well as metadata added  provides SFP identification, and consumed is used by network nodes
      the NSH-aware functions, such as the Classifier, SFF and service
      elements. NSH-aware
      SFs.  In addition to SFP identification, the NSH may carry data
      plane metadata.

   Service Classifier:  Function  Logical function that performs classification
      and imposes an NSH.  Creates a service  The initial classifier imposes the initial
      NSH and sends the NSH packet to the first SFF in the path.  Non-initial  Non-
      initial (i.e. subsequent) classification can occur as needed and
      can alter, or create a new service path.

   Service Hop:  NSH aware node, akin

   Network Locator:  dataplane address, typically IPv4 or IPv6, used to an IP hop but in the service
      overlay.

   Service Path Segment:  A segment of a service path overlay.
      send and receive network traffic.

   NSH Proxy:  Acts as a gateway: removes  Removes and inserts NSH on behalf of an NSH-unaware
      service function.  The proxy node removes the NSH header and
      delivers the original packet/frame via a local attachment circuit
      to the service function that is function.  Examples of a local attachment circuit
      include, but are not limited to: VLANs, IP in IP, GRE, VXLAN.
      When complete, the Service Function returns the packet to the NSH
      proxy via the same or different attachment circuit.  The NSH
      Proxy, in turn, re-imposes NSH aware. on the returned packets.  Often, an
      SFF will act as an NSH-proxy when required.

2.2.  Problem Space

   Network Service Header (NSH) addresses several limitations associated
   with service function deployments today. today (i.e. prior to use of NSH).
   A short reference is included below, RFC 7498 [RFC7498], provides a
   more comprehensive review of the SFC Problem Statement.

   1.  Topological Dependencies: Network service deployments are often
       coupled to network topology.  Such a dependency imposes
       constraints on the service delivery, potentially inhibiting the
       network operator from optimally utilizing service resources, and
       reduces the flexibility.  This limits scale, capacity, and
       redundancy across network resources.

   2.  Service Chain Construction: Service function chains today are
       most typically built through manual configuration processes.
       These are slow and error prone.  With the advent of newer dynamic
       service deployment models models, the control/management planes provide
       not only connectivity state, but will also be increasingly
       utilized for the creation of network services.  Such a control/management control/
       management planes could be centralized, or be distributed.

   3.  Application of Service Policy: Service functions rely on topology
       information such as VLANs or packet (re) classification to
       determine service policy selection, i.e. the service function
       specific action taken.  Topology information is increasingly less
       viable due to scaling, tenancy and complexity reasons.  The
       topological information is often stale, providing the operator
       with inaccurate service Function (SF) placement that can result
       in suboptimal resource utilization.  Furthermore topology-centric
       information often does not convey adequate information to the
       service functions, forcing functions to individually perform more
       granular classification.

   4.  Per-Service (re)Classification: Classification occurs at each
       service function independent from previously applied service
       functions.  More importantly, the classification functionality
       often differs per service function and service functions may not
       leverage the results from other service functions.

   5.  Common Header Format: Various proprietary methods are used to
       share metadata and create service paths.  An open header  A standardized protocol
       provides a common format for all network and service devices.

   6.  Limited End-to-End Service Visibility: Troubleshooting service
       related issues is a complex process that involve both network-
       specific and service-specific expertise.  This is especially the
       case
       case, when service function chains span multiple DCs, or across
       administrative boundaries.  Furthermore, the physical and virtual
       environments (network and service) can be highly divergent in
       terms of topology and that topological variance adds to these
       challenges.

   7.  Transport Dependence: Service functions can and will be deployed
       in networks with a range of transports requiring service
       functions to support and participate in many transports (and
       associated control planes) or for a transport gateway function to
       be present.

   Please see the

2.3.  NSH-based Service Function Chaining Problem Statement [SFC-PS]
   for

   The NSH creates a more detailed analysis of dedicated service function deployment problem
   areas.

3.  Network plane, that addresses many of the
   limitations highlighted in Section 2.2.  More specifically, NSH
   enables:

   1.  Topological Independence: Service Header

   A Network forwarding occurs within the
       service plane, via a network overlay, the underlying network
       topology does not require modification.  NSH provides an
       identifier used to select the network overlay for network
       forwarding.

   2.  Service Header (NSH) Chaining: NSH contains metadata and service path identification information that are added
       needed to realize a packet or frame and used service path.  Furthermore, NSH provides the
       ability to create monitor and troubleshoot a service plane. chain, end-to-end
       via service-specific OAM messages.  The packets and the NSH are then encapsulated in an
   outer header for transport.

   The service header is added fields can be used by
       administrators (via, for example a traffic analyzer) to verify
       (account, ensure correct chaining, provide reports, etc.) the
       path specifics of packets being forwarded along a service classification function - path.

   3.  NSH provides a
   device or application - that determines which packets require
   servicing, mechanism to carry shared metadata between network
       devices and correspondingly which service path to follow to apply function, and between service functions.  The
       semantics of the appropriate service.

3.1.  Network Service Header Format

   An NSH shared metadata is composed of a 4-byte base header, communicated via a 4-byte service path
   header control
       plane to participating nodes.  Examples of metadata include
       classification information used for policy enforcement and
       network context headers, as shown in Figure 1 below.

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 for forwarding post service delivery.

   4.  Classification and re-classification: sharing the metadata allows
       service functions to share initial and intermediate
       classification results with downstream service functions saving
       re-classification, where enough information was enclosed.

   5.  NSH offers a common and standards based header for service
       chaining to all network and service nodes.

   6.  Transport Agnostic: NSH is transport independent and is carried
       in an overlay, over existing underlays.  If an existing overlay
       topology provides the required service path connectivity, that
       existing overlay may be used.

3.  Network Service Header

   A Network Service Header (NSH) contains service path information and
   optionally metadata that are added to a packet or frame and used to
   create a service plane.  The original packets preceded by NSH, are
   then encapsulated in an outer header for transport.

   NSH is added by a Service Classifier.  The NSH header is removed by
   the last SFF in the chain or by a SF that consumes the packet.

3.1.  Network Service Header Format

   A NSH is composed of a 4-byte Base Header, a 4-byte Service Path
   Header and Context Headers, as shown in Figure 1 below.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Base Header                                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Service Path Header                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                Context Headers                                ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: Network Service Header

   Base header: provides information about the service header and the
   payload protocol.

   Service Path Header: provide path identification and location within
   a path.

   Context headers: carry opaque metadata and variable length encoded
   information.

3.2.  NSH Base Header

      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    | Next Protocol |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         Figure 2: NSH Base Header

   Base Header Field Descriptions Descriptions:

   Version: The version field is used to ensure backward compatibility
   going forward with future NSH updates.  It MUST be set to 0x0 by the
   sender, in this first revision of NSH.

   O bit: Indicates when set to 0x1 indicates that this packet is an operations
   and management (OAM) packet.  The receiving SFF and SFs nodes MUST
   examine the payload and take appropriate action (e.g. return status
   information).

   OAM message specifics and handling details are outside the scope of
   this document.

   C bit: Indicates that a critical metadata TLV is present (see Section
   3.4.2).  This bit acts as an indication for hardware implementers to
   decide how to handle the presence of a critical TLV without
   necessarily needing to parse all TLVs present.  The C bit MUST be set
   to 1 0x0 when MD Type= 0x01 and MAY be used with MD Type = 0x2 and MUST
   be set to 0x1 if one or more critical TLVs are present.

   All other flag fields are reserved.

   Length: total length, in 4-byte words, of the NSH header, including the Base
   Header, the Service Path Header and the optional variable TLVs.

   MD Type: indicates the format  The
   Length MUST be of NSH beyond the base header value 0x6 for MD Type = 0x1 and the
   type MUST be of metadata being carried.  This typing is used to describe the
   use value
   0x2 or higher for MD Type = 0x2.  The NSH header length MUST be an
   integer number of 4 bytes.

   MD Type: indicates the metadata. format of NSH beyond the mandatory Base Header
   and the Service Path Header.  MD Type defines the format of the
   metadata being carried.  A new registry will be requested from IANA
   for the MD Type.

   NSH defines two MD types:

   0x1 - which indicates that the format of the header includes fixed
   length context headers. headers (see Figure 4 below).

   0x2 - which does not mandate any headers beyond the base header Base Header and
   service path header,
   Service Path Header, and may contain optional variable length context
   information.

   The format of the base header and the service path header is
   invariant, and not described affected by MD Type.

   NSH implementations MUST support MD-Type = 0x1, and SHOULD support
   MD- Type = 0x2.

   Next Protocol: indicates the protocol type of the original packet.  A
   new IANA registry will be created for protocol type.

   This draft defines the following Next Protocol values:

   0x1 : IPv4
   0x2 : IPv6
   0x3 : Ethernet
   0x253: Experimental

3.3.  Service Path Header

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Service Path ID                      | Service Index |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Service path ID (SPI): 24 bits
   Service index (SI): 8 bits

                     Figure 3: NSH Service Path Header

   Service Path Identifier (SPI): identifies a service path.
   Participating nodes MUST use this identifier for path Service Function
   Path selection.  An
   administrator can use the service path value for reporting and
   troubleshooting packets along a specific path.

   Service Index (SI): provides location within the SFP.  The first
   Classifier (i.e. at the boundary of the NSH domain)in the NSH Service
   Function Path, SHOULD set the SI to 255, however the control plane
   MAY configure the initial value of SI as appropriate (i.e. taking
   into account the length of the service path. function path).  A Classifier
   MUST send the packet to the first SFF in the chain.  Service index
   MUST be decremented by service functions or proxy nodes after
   performing required services. services and the new decremented SI value MUST be
   reflected in the egress NSH packet.  SI MAY be used in conjunction
   with
   service path Service Path ID for path Service Function Path selection.  Service
   Index (SI) is also valuable when troubleshooting/reporting service
   paths.  In addition to indicating the location within a path, Service
   Function Path, SI can be used for loop detection.

3.4.  NSH MD-type 1

   When the base header Base Header specifies MD Type 1, NSH defines = 0x1, four Context Header,
   4-byte
   mandatory context headers, each, MUST be added immediately following the Service Path
   Header, as per Figure 4.  These headers must  Context Headers that carry no metadata MUST
   be
   present and the format is opaque as depicted in Figure 5. set to zero.

     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 ID                      | Service Index |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Mandatory Context Header                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Mandatory Context Header                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Mandatory Context Header                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Mandatory Context Header                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 4: NSH MD-type=0x1

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

   Draft-dc [dcalloc] and draft-mobility [moballoc] provide specific
   examples of how metadata can be allocated.

3.5.  NSH MD-type 2 3 4 5 6 7

   When the base header specifies MD Type= 0x2, zero or more Variable
   Length Context Headers MAY be added, immediately following the
   Service Path Header.  Therefore, Length = 0x2, indicates that only
   the Base Header followed by the Service Path Header are present.  The
   optional Variable Length Context Headers MUST be of an integer number
   of 4-bytes.

        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=0x2  | Next Protocol |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Service Path ID                      | Service Index |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~              Variable Length Context data Headers  (opt.)          ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 5: Context Header

3.4.1.  Mandatory Context Header Allocation Guidelines NSH MD-type=0x2

3.5.1.  Optional Variable Length Metadata

   The format of the optional variable length context headers, is as
   described below.

        0                   1                   2                   3
        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Network Platform Context                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Network Shared Context                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Service Platform Context          TLV Class            |C|    Type     |R|R|R|   Len   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Service Shared Context                      Variable Metadata                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 6: Variable Context Data Significance

   Figure 6, above, and Headers

   TLV Class: describes the following examples of context header
   allocation are guidelines that illustrate how various forms scope of
   information can the "Type" field.  In some cases,
   the TLV Class will identify a specific vendor, in others, the TLV
   Class will identify specific standards body allocated types.  A new
   IANA registry will be carried and exchanged via NSH.

   Network platform context: provides platform-specific metadata shared
   between network nodes.  Examples include (but are not limited to)
   ingress port information, forwarding context and encapsulation created for TLV Class type.

   Network shared context: metadata relevant to any network node such as

   Type: the result specific type of edge classification.  For example, application
   information, identity information or tenancy information can be
   shared using this context header.

   Service platform context: provides service platform specific metadata
   shared between service functions.  This context header being carried, within the
   scope of a given TLV Class.  Value allocation is analogous
   to the network platform context, enabling service platforms to
   exchange platform-centric information such as an identifier used for
   load balancing decisions.

   Service shared context: metadata relevant to, and shared, between
   service functions.  As with responsibility
   of the shared network context,
   classification information such as application type can be conveyed
   using this context.

   The data center[dcalloc] and mobility[moballoc] context header
   allocation drafts provide guidelines for TLV Class owner.

   Encoding the semantics criticality of NSH fixed
   context headers in each respective environment.

3.5.  NSH MD-type 2

   When the base header specifies MD TLV within the Type 2, NSH defines variable length
   only context headers.  There may be zero or more field is
   consistent with IPv6 option types: the most significant bit of these headers as
   per the length field.

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
   Type field indicates whether the TLV is mandatory for the receiver to
   understand/process.  This effectively allocates Type values 0 1 2 3 4 5 6 to 127
   for non-critical options and Type values 128 to 255 for critical
   options.  Figure 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|O|C|R|R|R|R|R|R|   Length  |  MD-type=0x2  | Next Protocol |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Service Path ID                      | Service Index |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~           Optional Variable Length Context Headers            ~
     | below illustrates the placement of the Critical
   bit within the Type field.

     +-+-+-+-+-+-+-+-+
     |C|     Type    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+

        Figure 7: NSH MD-type=0x2

3.5.1.  Optional Variable Length Metadata

   NSH MD Type 2 MAY contain optional variable length context headers.
   The format of these headers is as described below.

      0                   1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          TLV Class            |      Type     |R|R|R|   Len   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Variable Metadata                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 8: Variable Context Headers

   TLV Class: describes the scope of the "Type" field.  In some cases,
   the TLV Class will identify a specific vendor, in others, the TLV
   Class will identify specific standards body allocated types.

   Type: the specific type of information being carried, within the
   scope of a given TLV Class.  Value allocation is the responsibility
   of the TLV Class owner.

   The most significant bit of the Type field indicates whether the TLV
   is mandatory for the receiver to understand/process.  This
   effectively allocates Type values 0 to 127 for non-critical options
   and Type values 128 to 255 for critical options.  Figure 7 below
   illustrates the placement of the Critical bit within the Type field.

     +-+-+-+-+-+-+-+-+
     |C|     Type    |
     +-+-+-+-+-+-+-+-+

        Figure 9: Critical Bit Placement Within the TLV Critical Bit Placement Within the TLV Type Field

   Encoding the criticality of the TLV within the Type field is
   consistent with IPv6 option types.

   If a receiver receives an encapsulated packet containing a TLV with
   the Critical bit set to 0x1 in the Type field and it does not
   understand how to process the Type, it MUST drop the packet.  Transit
   devices MUST NOT drop packets based on the setting of this bit.

   Reserved bits: three reserved bit are present for future use.  The
   reserved bits MUST be zero. set to 0x0.

   Length: Length of the variable metadata, in 4-byte words.  A value of
   0x0 or higher can be used.  A value of 0x0 denotes a TLV header
   without a Variable Metadata field.

4.  NSH Actions

   Service header aware

   NSH-aware nodes - are the only nodes that MAY alter the content of the
   NSH headers.  NSH-aware nodes include: service classifiers, SFF, SF
   and NSH
   proxies, proxies.  These nodes have several possible header related
   actions:

   1.  Insert or remove service header: NSH: These actions can occur at the start and
       end respectively of a service path.  Packets are classified, and
       if determined to require servicing, a service
       header NSH will be imposed.  The last node in a service path, an SFF, removes
       the NSH.  A
       service classifier MUST insert NSH at the start of an NSH. SFP.  An
       imposed NSH MUST contain valid Base Header and Service Path
       Header.  At the end of a service function chain, path, a SFF, MUST be
       the last node operating on the service header and MUST remove it.

       A

       Multiple logical classifiers may exist within a given service function can
       path.  Non-initial classifiers may re-classify data as required and that re-
       classification might MAY result in a new service path.  In this case,
       the SF acts as a logical classifier as well. Service Function Path.  When
       the logical classifier performs re-classification that results in
       a change of service path, it MUST remove the existing NSH and
       MUST impose a new NSH with the base header Base Header and Service Path
       Header reflecting the new path.

   2.  Select service path: The base header provides service chain path information and is used by SFFs to determine correct service path
       selection.  SFFs MUST use the base header for selecting the next
       service in the service path.

   3.  Update a service header: NSH aware service functions MUST
       decrement the service index.  A service index = 0 indicates that
       a packet MUST be dropped by set the SFF performing NSH-based
       forwarding.

       Service functions SI
       to 255.  Metadata MAY update context headers if new/updated
       context is available.

       If an NSH proxy (see Section 7) is be preserved in use (acting on behalf of a
       non-NSH-aware service function for NSH actions), then the proxy
       MUST update service index and MAY update contexts.  When an NSH
       proxy receives an NSH-encapsulated packet, it removes the NSH
       before forwarding it to an NSH unaware SF.  When it receives a
       packet back from an NSH unaware SF, it re-encapsulates it with
       the NSH, decrementing the new NSH.

   2.  Select service index.

   4.  Service policy selection: path: The Service function instances derive
       policy selection from the service header.  Context shared in the Path Header provides service header can provide a range of service-relevant
       chain information such as traffic classification.  Service functions
       SHOULD use NSH to select local service policy.

   Figure 10 maps each of the four actions above to the components in
   the SFC architecture that can perform it.

 +----------------+--------------------+-------+---------------+-------+
 |                |  Insert or remove  |Select |   Update a    |Service|
 |                |   service header   |service|service header |Policy |
 |                +------+------+------+ path  +---------------+Select-|
 |                |Insert|Remove|Remove|       | Dec.  |Update |ion    |
 |                |      |      | and  |       |Service|Context|       |
 | Component      |      |      |Insert|       | Index |Header |       |
 +----------------+------+------+------+-------+-------+-------+-------+
 |Service Classif-|  +   |      |      |       |       |   +   |       |
 |ication Function|      |      |      |       |       |       |       |
 + -------------- + ---- + ---- + ---- + ----- + ----- + ----- + ----- +
 |Service Function|      |  +   |      |   +   |       |   +   |       |
 |Forwarder(SFF)  |      |      |      |       |       |       |       |
 + -------------- + ---- + ---- + ---- + ----- + ----- + ----- + ----- +
 |Service         |      |      |      |       |   +   |   +   |   +   |
 |Function  (SF)  |      |      |      |       |       |       |       |
 + -------------- + ---- + ---- + ---- + ----- + ----- + ----- + ----- +
 |NSH Proxy       |  +   |  +   |      |       |   +   |   +   |       |
 +----------------+------+------+------+-------+-------+-------+-------+

                  Figure 10: NSH Action and Role Mapping

5.  NSH Encapsulation

   Once NSH is added to a packet, an outer encapsulation and is used by SFFs to
   forward determine correct
       service path selection.  SFFs MUST use the original packet and Service Path Header
       for selecting the associated metadata to next SF or SFF in the start
   of a service chain.  The encapsulation serves two purposes:

   1.  Creates path.

   3.  Update a topologically independent services plane.  Packets are
       forwarded to the required services without changing the
       underlying network topology.

   2.  Transit network nodes simply forward Service Path Header: NSH aware service functions (SF)
       MUST decrement the encapsulated packets as
       is.

   The service header is independent of index.  A service index = 0x0
       indicates that a packet MUST be dropped by the encapsulation used and SFF.

       Classifier(s) MAY update Context Headers if new/updated context
       is
   encapsulated in existing transports.  The presence of available.

       If an NSH proxy (see Section 7) is
   indicated via protocol type or other indicator in the outer
   encapsulation.

   See Section 11 use (acting on behalf of a
       non-NSH-aware service function for NSH encapsulation examples.

6.  NSH Usage

   The actions), then the proxy
       MUST update Service Index and MAY update contexts.  When an NSH creates a dedicated service plane, that addresses many of
       proxy receives an NSH-encapsulated packet, it MUST remove the
   limitations highlighted in Section 2.2.  More specifically, NSH
   enables:

   1.  Topological Independence: Service
       headers before forwarding occurs within the
       service plane, via a network overlay, the underlying network
       topology does not require modification.  Service functions have
       one or more network locators (e.g.  IP address) it to receive/send
       data within the service plane, an NSH unaware SF.  When the NSH contains an identifier
       that is used to uniquely identify
       Proxy receives a service path packet back from an NSH unaware SF, it MUST re-
       encapsulate it with the correct NSH, and MUST also decrement the services
       within that path.

   2.
       Service Chaining: NSH contains path identification information
       needed to realize a Index.

   4.  Service policy selection: Service Function instances derive
       policy (i.e. service path.  Furthermore, NSH provides the
       ability to monitor actions such as permit or deny) selection
       and troubleshoot a enforcement from the service chain, end-to-end
       via service-specific OAM messages.  The NSH fields header.  Metadata shared in the
       service header can be used by
       administrators (via, for example a traffic analyzer) to verify
       (account, ensure correct chaining, provide reports, etc.) the
       path specifics of packets being forwarded along a service path.

   3.  Metadata Sharing: range of service-relevant
       information such as traffic classification.  Service functions
       SHOULD use NSH provides a mechanism to carry shared
       metadata between network devices and service function, and
       between select local service functions.  The semantics policy.

   Figure 8 maps each of the shared metadata
       is communicated via a control plane four actions above to participating nodes.
       Examples of metadata include classification information used for
       policy enforcement and network context for forwarding post
       service delivery.

   4.  Transport Agnostic: the components in the
   SFC architecture that can perform it.

 +---------------+------------------+-------+----------------+---------+
 |                |  Insert         |Select |   Update       |Service  |
 |                |  or remove NSH  |Service|    NSH         |policy   |
 |                |                 |Function|               |selection|
 | Component      +--------+--------+Path   +----------------+         |
 |                |        |        |       | Dec.   |Update |         |
 |                | Insert | Remove |       |Service |Context|         |
 |                |        |        |       | Index  |Header |         |
 +----------------+--------+--------+-------+--------+-------+---------+
 |                |   +    |   +    |       |        |   +   |         |
 |Classifier      |        |        |       |        |       |         |
 +--------------- +--------+--------+-------+--------+-------+---------+
 |Service Function|        |   +    |  +    |        |       |         |
 |Forwarder(SFF)  |        |        |       |        |       |         |
 +--------------- +--------+--------+-------+--------+-------+---------+
 |Service         |        |        |       |   +    |       |   +     |
 |Function  (SF)  |        |        |       |        |       |         |
 +--------------- +--------+--------+-------+--------+-------+---------+
 |NSH Proxy       |   +    |   +    |       |   +    |       |         |
 +----------------+--------+--------+-------+--------+-------+---------+

                   Figure 8: NSH is transport independent Action and is carried
       in an overlay, over existing underlays.  If an existing overlay
       topology provides the required service path connectivity, that
       existing overlay may be used.

7. Role Mapping

5.  NSH Proxy Nodes

   In order Encapsulation

   Once NSH is added to support NSH-unaware service functions, a packet, an NSH proxy outer encapsulation is
   used.  The proxy node removes used to
   forward the NSH header original packet and delivers the
   original packet/frame via a local attachment circuit associated metadata to the service
   function.  Examples start
   of a local attachment circuit include, but are
   not limited to: VLANs, IP in IP, GRE, VXLAN.  When complete, the service function returns the packet chain.  The encapsulation serves two purposes:

   1.  Creates a topologically independent services plane.  Packets are
       forwarded to the NSH proxy via the same or
   different attachment circuit.

   NSH is re-imposed on packets returned to required services without changing the proxy from
       underlying network topology

   2.  Transit network nodes simply forward the non-NSH-
   aware service.

   Typically, an SFF will act encapsulated packets as an NSH-proxy when required.

   An NSH proxy MUST perform
       is.

   The service header is independent of the encapsulation used and is
   encapsulated in existing transports.  The presence of NSH actions as described is
   indicated via protocol type or other indicator in the outer
   encapsulation.

   See Section 4.

8. 9 for NSH encapsulation examples.

6.  Fragmentation Considerations

   Work in progress

9. progress: discussion of jumbo frames and PMTUD implications.

7.  Service Path Forwarding with NSH

9.1.

7.1.  SFFs and Overlay Selection

   As described above, NSH contains a service path identifier Service Path Identifier (SPI) and
   a service index Service Index (SI).  The SPI is, as per its name, an identifier.
   The SPI alone cannot be used to forward packets along a service path.
   Rather the SPI provide a level of indirection between the service
   path/topology and the network transport.  Furthermore, there is no
   requirement, or expectation of an SPI being bound to a pre-determined
   or static network path.

   The service index

   The Service Index provides an indication of location within a service
   path.  The combination of SPI and SI provides the identification and
   location of a
   logical SF (locator and order). its order within the service plane, and is used to
   select the appropriate network locator(s) for overlay forwarding.
   The logical SF may be a single SF, or a set of SFs that are
   equivalent.  In the latter case, the SFF provides load distribution
   amongst the collection of SFs as needed.  SI may also serve as a
   mechanism for loop detection
   with in within a service path since each SF in
   the path decrements the index; an index Service Index of 0 indicates that a
   loop occurred and packet must be discarded.

   This indirection -- path ID to overlay -- creates a true service
   plane.  That is the SFF/SF topology is constructed without impacting
   the network topology but more importantly service plane only
   participants (i.e. most SFs) need not be part of the network overlay
   topology and its associated infrastructure (e.g. control plane,
   routing tables, etc.).  As mentioned above, an existing overlay
   topology may be used provided it offers the requisite connectivity.

   The mapping of SPI to transport occurs on an SFF. SFF (as discussed above,
   the first SFF in the path gets a NSH encapsulated packet from the
   Classifier).  The SFF consults the SPI/ID values to determine the
   appropriate overlay transport protocol (several may be used within a
   given network) and next hop for the requisite SF.  Figure 10 9 below
   depicts an a simple, single next-hop SPI/SI to network overlay network
   locator mapping.

   +-------------------------------------------------------+
   |  SPI |  SI |  NH                 |   Transport        |
   +-------------------------------------------------------+
   |  10  |  3 255 |  1.1.1.1            |   VXLAN-gpe        |
   |  10  |  2 254 |  2.2.2.2            |   nvGRE            |
   |  245 |  12 |  192.168.45.3       |   VXLAN-gpe        |
    |  10  |  9 251 |  10.1.2.3           |   GRE              |
   |  40  |  9 251 |  10.1.2.3           |   GRE              |
   |  50  |  7 200 |  01:23:45:67:89:ab  |   Ethernet         |
   |  15  |  1 212 |  Null (end of path) |   None             |
   +-------------------------------------------------------+

                     Figure 11: 9: SFF NSH Mapping Example

   Additionally, further indirection is possible: the resolution of the
   required SF function network locator may be a localized resolution on an
   SFF,rather SFF,
   rather than a service function chain control plane responsibility, as
   per figures 11 10 and 12 11 below.

    +-------------------+
    | SPI |  SI |  NH   |
    +-------------------+
    | 10  |  3  |  SF2  |
    | 245 |  12 |  SF34 |
    | 40  |  9  |  SF9  |
    +-------------------+

                   Figure 12: 10: NSH to SF Mapping Example

    +-----------------------------------+
    |  SF  |  NH          |  Transport  |
    +-----------------------------------|
    |  SF2 |  10.1.1.1    |  VXLAN-gpe  |
    |  SF34|  192.168.1.1 |  UDP        |
    |  SF9 |  1.1.1.1     |  GRE        |
    +-----------------------------------+

                   Figure 13: 11: SF Locator Mapping Example

   Since the SPI is a representation of the service path, the lookup may
   return more than one possible next-hop within a service path for a
   given SF, essentially a series of weighted (equally or otherwise)
   overlay links to be used (for load distribution, redundancy or
   policy), see Figure 13. 12.  The metric depicted in Figure 13 12 is an
   example to help illustrated weighing SFs.  In a real network, the
   metric will range from a simple preference (similar to routing next-
   hop), to a true dynamic composite metric based on some service
   function-centric state (including load, sessions sate, state, capacity,
   etc.)

    +----------------------------------+
    | SPI | SI |   NH        |  Metric |
    +----------------------------------+
    | 10  |  3 | 10.1.1.1    |  1      |
    |     |    | 10.1.1.2    |  1      |
    |     |    |             |         |
    | 20  | 12 | 192.168.1.1 |  1      |
    |     |    | 10.2.2.2    |  1      |
    |     |    |             |         |
    | 30  |  7 | 10.2.2.3    |  10     |
    |     |    | 10.3.3.3    |  5      |
    +----------------------------------+
     (encap type omitted for formatting)

                   Figure 14: 12: NSH Weighted Service Path

9.2.

7.2.  Mapping NSH to Network Overlay

   As described above, the mapping of SPI to network topology may result
   in a single overlay path, or it might result in a more complex
   topology.  Furthermore, the SPIx to overlay mapping occurs at each
   SFF independently.  Any combination of topology selection is
   possible.  Please note, there is no requirement to create a new
   overlay topology if a suitable one already existing.  NSH packets can
   use any (new or existing) overlay provided the requisite connectivity
   requirements are satisfied.

   Examples of mapping for a topology:

   1.  Next SF is located at SFFb with locator 10.1.1.1
       SFFa mapping: SPI=10 --> VXLAN-gpe, dst-ip: 10.1.1.1

   2.  Next SF is located at SFFc with multiple locator network locators for
       load distribution purposes:
       SFFb mapping: SPI=10 --> VXLAN-gpe, dst_ip:10.2.2.1, 10.2.2.2,
       10.2.2.3, equal cost

   3.  Next SF is located at SFFd with two path paths to SFFc, one for
       redundancy:
       SFFc mapping: SPI=10 --> VXLAN-gpe, dst_ip:10.1.1.1 cost=10,
       10.1.1.2, cost=20

   In the above example, each SFF makes an independent decision about
   the network overlay path and policy for that path.  In other words,
   there is no a priori mandate about how to forward packets in the
   network (only the order of services that must be traversed).

   The network operator retains the ability to engineer the overlay
   paths as required.  For example, the overlay path between service
   functions forwarders may utilize traffic engineering, QoS marking, or
   ECMP, without requiring complex configuration and network protocol
   support to be extended to the service path explicitly.  In other
   words, the network operates as expected, and evolves as required, as
   does the service function plane.

9.3.

7.3.  Service Plane Visibility

   The SPI and SI serve an important function for visibility into the
   service topology.  An operator can determine what service path a
   packet is "on", and its location within that path simply by viewing
   the NSH information (packet capture, IPFIX, etc.).  The information
   can be used for service scheduling and placement decisions,
   troubleshooting and compliance verification.

9.4.

7.4.  Service Graphs

   In some cases, a service path is exactly that -- a linear list of
   service functions that must be traversed.  However, increasingly, the "path" is
   actually a true directed graph.  Furthermore, within a given service
   topology several directed graphs may exist with packets moving
   between graphs based on non-initial classification (usually
   performed by a service function).  Note: strictly speaking a path is
   a form of graph; (in Figure 13, co-
   located with the intent is to distinguish between a directed
   graph and a path. SFs).

                     ,---.          ,---.            ,---.
                   /     \        /     \          /     \
                  (  SF2  )      (  +------+  SF7  )        (  +--------+  SF3  )
            ,------\     +.       \     /        \     /       /-+      /
           ;        |---'  `-.          `---'\      /    `-+-'
           |        :         :                    \            ;    /
           |         \        |           :          ;                /---:---
         ,-+-.        `.     ,+--.     ,---.   /     :          |
        /     \         '---+     \/        \        \         ;
       (  SF1  )           (  SF6  )        \       /
        \     /             \     +--.       :     /
         `---'               `---'    `-.  ,-+-.  /
                                         `+     +'     \
                                         (  SF4  )
                                          \     /
                                           `---'

                     Figure 15: 13: Service Graph Example

   The SPI/SI combination provides a simple representation of a directed
   graph, the SPI represents a graph ID; and the SI a node ID.  The
   service topology formed by SPI/SI support cycles, weighting, and
   alternate topology selection, all within the service plane.  The
   realization of the network topology occurs as described above: SPI/ID
   mapping to an appropriate transport and associated next network hops.

   NSH-aware services receive the entire header, including the SPI/SI.
   An SF non-initial logical classifier (in many deployment, this
   classifier will be co-resident with a SF) can now, based on local
   policy, alter the SPI, which in turn effects both the service graph,
   and in turn the selection of overlay at the SFF.  The figure below
   depicts the policy associated with the graph in Figure 14 13 above.
   Note: this illustrates multiple graphs and their representation; it
   does not depict the use of metadata within a single service function
   graph.

 +---------------------------------------------------------------------+
 |

   SF1:
       SPI: 21 Bob: SF7     |
 | 10
           NH: SF2
   SF2:
       Class: Bad
               SPI: 20 Bad : SF2 -->
               NH: SF6 -->
        Class: Good
               SPI: 30
               NH: SF7
   SF6:
        Class: Employee
               SPI: 21
               NH: SF4                  |
 |SPI: 10 SF1 --> SF2 --> SF6
        Class: Guest
               SPI: 22 Alice:
               NH: SF3   |
 |
   SF7:
        Class: Employee
               SPI: 30 Good: 31
               NH: SF4                                  |
 |                             SPI:31 Bob: SF7                         |
 |                             SPI:32 Alice:
        Class: Guest
                  SPI: 32
              NH: SF3                       |
 +---------------------------------------------------------------------+

                    Figure 16: 14: Service Graphs Using SPI

   This example above does not show the mapping of the service topology
   to the network overlay topology.  As discussed in the sections above,
   the overlay selection occurs as per network policy.

10.

8.  Policy Enforcement with NSH

10.1.

8.1.  NSH Metadata and Policy Enforcement

   As described in Section 3, NSH provides the ability to carry metadata
   along a service path.  This metadata may be derived from several
   sources, common examples include:

      Network nodes: nodes/devices: Information provided by network nodes can
      indicate network-centric information (such as VRF or tenant) that
      may be used by service functions, or conveyed to another network
      node
      post-service pathing. post service path egress.

      External (to the network) systems: External systems, such as
      orchestration systems, often contain information that is valuable
      for service function policy decisions.  In most cases, this
      information cannot be deduced by network nodes.  For example, a
      cloud orchestration platform placing workloads "knows" what
      application is being instantiated and can communicate this
      information to all NSH nodes via metadata. metadata carried in the context
      header(s).

      Service functions: Functions: A classifier co-resident with Service functions Functions
      often perform very detailed and valuable classification.  In some
      cases they may terminate, and be able to inspect encrypted
      traffic.  SFs may update, alter
      or impose metadata information.

   Regardless of the source, metadata reflects the "result" of
   classification.  The granularity of classification may vary.  For
   example, a network switch switch, acting as a classifier, might only be able
   to classify based on a 5-tuple, whereas, a service function may be
   able to inspect application information.  Regardless of granularity,
   the classification information can be represented in NSH.

   Once the data is added to NSH, it is carried along the service path,
   NSH-aware SFs receive the metadata, and can use that metadata for
   local decisions and policy enforcement.  The following two examples
   highlight the relationship between metadata and policy:

    +-------------------------------------------------+
    |   ,---.             ,---.              ,---.

    +-------+        +-------+        +-------+
    |  SFF  )------->(  SFF  |------->|  SFF  |
    +---^---+        +---|---+        +---|---+
      ,-|-.            ,-|-.            ,-|-.
     /     \          /     \          /     \   |
    |
    (  SCL  )-------->( Class )           SF1  )--------->(  )        (  SF2  )  |
    |
     \ ify /          \     /          \     /   |
    |
      `---'            `---'            `---'    |
    |5-tuple:
     5-tuple:        Permit             Inspect   |
    |Tenant
     Tenant A        Tenant A           AppY      |
    |AppY                                             |
    +-------------------------------------------------+
     AppY

                      Figure 17: 15: Metadata and Policy

    +-------------------------------------------------+

       +-----+           +-----+            +-----+
       |    ,---.             ,---.              ,---. SFF |---------> | SFF |----------> | SFF |
       +--+--+           +--+--+            +--+--+
          ^                 |                  |
        ,-+-.             ,-+-.              ,-+-.
       /     \           /     \            /     \  |
    |
      (  SCL  )-------->( Class )         (  SF1  )--------->(  )          (  SF2  ) |
    |
       \ ify /           \     /            \     /  |
    |
        `-+-'             `---'              `---'
          |
    |      |              Permit            Deny AppZ |
    |
      +---+---+          employees
      |       |  |       |                                      |
    |
      +-------+                                      |
    |
      external                                       |
    |
      system:                                        |
    |
      Employee                                       |
    |  App Z                                          |
    +-------------------------------------------------+
      AppZ

                  Figure 18: 16: External Metadata and Policy

   In both of the examples above, the service functions perform policy
   decisions based on the result of the initial classification: the SFs
   did not need to perform re-classification, rather they relied rely on a
   antecedent classification for local policy enforcement.

10.2.

8.2.  Updating/Augmenting Metadata

   Post-initial metadata imposition (typically performed during initial
   service path determination), metadata may be augmented or updated:

   1.  Metadata Augmentation: Information may be added to NSH's existing
       metadata, as depicted in Figure 18. 17.  For example, if the initial
       classification returns the tenant information, a secondary
       classification (perhaps a co-resident with DPI or SLB) may augment
       the tenant classification with application information. information, and
       impose that new information in the NSH metadata.  The tenant
       classification is still valid and present, but additional
       information has been added to it.

   2.  Metadata Update: Subsequent classifiers may update the initial
       classification if it is determined to be incorrect or not
       descriptive enough.  For example, the initial classifier adds
       metadata that describes the trafic traffic as "internet" but a security
       service function determines that the traffic is really "attack".
       Figure 19 18 illustrates an example of updating metadata.

     +-------------------------------------------------+

        +-----+           +-----+            +-----+
        | SFF |---------> | SFF |----------> | SFF |
        +--+--+           +--+--+            +--+--+
          ^                 |                  |
         ,---.             ,---.              ,---.   |
    |
        /     \           /     \            /     \  |
    |
       (  SCL  )-------->( Class )         (  SF1  )--------->(  )          (  SF2  ) |
    |
        \     /           \     /            \     /  |
    |
         `-+-'             `---'              `---'
          |
    |      |              Inspect           Deny      |
    |
       +---+---+          employees         employee+
       |       |  |       |          Class=AppZ        appZ      |
    |
       +-------+                                      |
    |
       external                                       |
    |
       system:
       Employee

                     Figure 17: Metadata Augmentation

       +-----+           +-----+            +-----+
       | SFF |---------> |  Employee SFF |----------> | SFF |
       +--+--+           +--+--+            +--+--+
          ^                 |
    +-------------------------------------------------+

                     Figure 19: Metadata Augmentation

    +-------------------------------------------------+                  |
        ,---.             ,---.              ,---.    |
    |
       /     \           /     \            /     \   |
    |
      (  SCL  )-------->( Class )         (  SF1  )--------->(  )          (  SF2  )  |
    |
       \     /           \     /            \     /   |
    |
        `---'             `---'              `---'    |
    |5-tuple:
     5-tuple:            Inspect             Deny     |
    |Tenant
     Tenant A            Tenant A            attack   |
    |
                          --> attack                  |
    +-------------------------------------------------+

                        Figure 20: 18: Metadata Update

10.3.

8.3.  Service Path ID and Metadata

   Metadata information may influence the service path selection since
   the service path identifier Service Path Identifier can represent the result of
   classification.  A given SPI can represent all or some of the
   metadata, and be updated based on metadata classification results.
   This relationship provides the ability to create a dynamic services
   plane based on complex classification without requiring each node to
   be capable of such classification, or requiring a coupling to the
   network topology.  This yields service graph functionality as
   described in Section 9.4. 7.4.  Figure 20 19 illustrates an example of this
   behavior.

    +----------------------------------------------------+

       +-----+           +-----+            +-----+
       | SFF |---------> | SFF |------+---> | SFF |
       +--+--+           +--+--+      |     +--+--+
          |                 |         |        |   ,---.
        ,---.             ,---.       |
    |      ,---.
       /     \           /     \      |     /     \      |
    |
      (  SCL  )-------->(  )         (  SF1  )--------->(  SF2  )     |
    |    (  SF2  )
       \     /           \     /      |     \     /      |
    |
        `---'             `---' \    +-----+   `---'     |
    |5-tuple:
     5-tuple:            Inspect \            Original   |
    |Tenant SFF |    Original
     Tenant A            Tenant A \  +--+--+    next SF    |
    |
                          --> DoS  \                     |
    |                                 \                  |
    |                                  ,---.             |     |
                                      V
                                    ,-+-.
                                   /     \            |
    |
                                  (  SF10 )           |
    |
                                   \     /            |
    |
                                    `---'             |
    |
                                     DoS              |
    |
                                  "Scrubber"          |
    +----------------------------------------------------+

                      Figure 21: 19: Path ID and Metadata

   Specific algorithms for mapping metadata to an SPI are outside the
   scope of this draft.

11.

9.  NSH Encapsulation Examples

11.1.

9.1.  GRE + NSH

    IPv4 Packet:
   +----------+--------------------+--------------------+
   |L2 header | L3 header, proto=47|GRE header,PT=0x894F|
   +----------+--------------------+--------------------+
   --------------+----------------+
   NSH, NP=0x1   |original packet |
   --------------+----------------+

    L2 Frame:
    +----------+--------------------+--------------------+
    |L2 header | L3 header, proto=47|GRE header,PT=0x894F|
    +----------+--------------------+--------------------+
    ---------------+---------------+
    NSH, NP=0x3    |original frame |
    ---------------+---------------+

                           Figure 22: 20: GRE + NSH

11.2.

9.2.  VXLAN-gpe + NSH

    IPv4 Packet:
    +----------+------------------------+---------------------+
    |L2 header | IP + UDP dst port=4790 |VXLAN-gpe NP=0x4(NSH)|
    +----------+------------------------+---------------------+
    --------------+----------------+
    NSH, NP=0x1   |original packet |
    --------------+----------------+

    L2 Frame:
    +----------+------------------------+---------------------+
    |L2 header | IP + UDP dst port=4790 |VXLAN-gpe NP=0x4(NSH)|
    +----------+------------------------+---------------------+
    ---------------+---------------+
    NSH,NP=0x3     |original frame |
    ---------------+---------------+

                        Figure 23: 21: VXLAN-gpe + NSH

11.3.

9.3.  Ethernet + NSH

  IPv4 Packet:
  +-------------------------------+---------------+--------------------+
  |Outer Ethernet, ET=0x894F      | NSH, NP = 0x1 | original IP Packet |
  +-------------------------------+---------------+--------------------+

  L2 Frame:
  +-------------------------------+---------------+----------------+
  |Outer Ethernet, ET=0x894F      | NSH, NP = 0x3 | original frame |
  +-------------------------------+---------------+----------------+

                         Figure 24: 22: Ethernet + NSH

12.

10.  Security Considerations

   As with many other protocols, NSH data can be spoofed or otherwise
   modified.  In many deployments, NSH will be used in a controlled
   environment, with trusted devices (e.g. a data center) thus
   mitigating the risk of unauthorized header manipulation.

   NSH is always encapsulated in a transport protocol and therefore,
   when required, existing security protocols that provide authenticity
   (e.g.  RFC 2119 [RFC6071]) can be used.

   Similarly if confidentiality is required, existing encryption
   protocols can be used in conjunction with encapsulated NSH.

13.

11.  Open Items for WG Discussion

   1.  MD type 1 metadata semantics specifics

   2.  Bypass bit in NSH.

   3.  Rendered Service Path ID (RSPID).

14.

12.  Contributors

   This WG document originated as draft-quinn-sfc-nsh and had the
   following co-authors and contributors.  The editors of this document
   would like to thank and recognize them and their contributions.
   These co-authors and contributors provided invaluable concepts and
   content for this document's creation.

   Surendra Kumar
   Cisco Systems
   smkumar@cisco.com

   Michael Smith
   Cisco Systems
   michsmit@cisco.com

   Jim Guichard
   Cisco Systems
   jguichar@cisco.com

   Rex Fernando
   Cisco Systems
   Email: rex@cisco.com

   Navindra Yadav
   Cisco Systems
   Email: nyadav@cisco.com

   Wim Henderickx
   Alcatel-Lucent
   wim.henderickx@alcatel-lucent.com

   Andrew Dolganow
   Alcaltel-Lucent
   Email: andrew.dolganow@alcatel-lucent.com

   Praveen Muley
   Alcaltel-Lucent
   Email: praveen.muley@alcatel-lucent.com

   Tom Nadeau
   Brocade
   tnadeau@lucidvision.com

   Puneet Agarwal
   puneet@acm.org

   Rajeev Manur
   Broadcom
   rmanur@broadcom.com

   Abhishek Chauhan
   Citrix
   Abhishek.Chauhan@citrix.com

   Joel Halpern
   Ericsson
   joel.halpern@ericsson.com

   Sumandra Majee
   F5
   S.Majee@f5.com

   David Melman
   Marvell
   davidme@marvell.com

   Pankaj Garg
   Microsoft
   Garg.Pankaj@microsoft.com

   Brad McConnell
   Rackspace
   bmcconne@rackspace.com

   Chris Wright
   Red Hat Inc.
   chrisw@redhat.com

   Kevin Glavin
   Riverbed
   kevin.glavin@riverbed.com

   Hong (Cathy) Zhang
   Huawei US R&D
   cathy.h.zhang@huawei.com

   Louis Fourie
   Huawei US R&D
   louis.fourie@huawei.com

   Ron Parker
   Affirmed Networks
   ron_parker@affirmednetworks.com

   Myo Zarny
   Goldman Sachs
   myo.zarny@gs.com

15.

13.  Acknowledgments

   The authors would like to thank Nagaraj Bagepalli, Abhijit Patra,
   Peter Bosch, Darrel Lewis, Pritesh Kothari, Tal Mizrahi and Ken Gray
   for their detailed review, comments and contributions.

   A special thank you goes to David Ward and Tom Edsall for their
   guidance and feedback.

   Additionally the authors would like to thank Carlos Pignataro and
   Larry Kreeger for their invaluable ideas and contributions which are
   reflected throughout this draft.

   Lastly, Reinaldo Penno deserves a particular thank you for his
   architecture and implementation work that helped guide the protocol
   concepts and design.

16.

14.  IANA Considerations

16.1.

14.1.  NSH EtherType

   An IEEE EtherType, 0x894F, has been allocated for NSH.

16.2.

14.2.  Network Service Header (NSH) Parameters

   IANA is requested to create a new "Network Service Header (NSH)
   Parameters" registry.  The following sub-sections request new
   registries within the "Network Service Header (NSH) Parameters "
   registry.

16.2.1.

14.2.1.  NSH Base Header Reserved Bits

   There are ten bits at the beginning of the NSH Base Header.  New bits
   are assigned via Standards Action [RFC5226].

   Bits 0-1 - Version
   Bit 2 - OAM (O bit)
   Bits 2-9 - Reserved

16.2.2.

14.2.2.  MD Type Registry

   IANA is requested to set up a registry of "MD Types".  These are
   8-bit values.  MD Type values 0, 1, 2, 254, and 255 are specified in
   this document.  Registry entries are assigned by using the "IETF
   Review" policy defined in RFC 5226 [RFC5226].

                +---------+--------------+---------------+
                | MD Type | Description  | Reference     |
                +---------+--------------+---------------+
                | 0       | Reserved     | This document |
                |         |              |               |
                | 1       | NSH          | This document |
                |         |              |               |
                | 2       | NSH          | This document |
                |         |              |               |
                | 3..253  | Unassigned   |               |
                |         |              |               |
                | 254     | Experiment 1 | This document |
                |         |              |               |
                | 255     | Experiment 2 | This document |
                +---------+--------------+---------------+

                                  Table 1

16.2.3.

14.2.3.  TLV Class Registry

   IANA is requested to set up a registry of "TLV Types".  These are 16-
   bit values.  Registry entries are assigned by using the "IETF Review"
   policy defined in RFC 5226 [RFC5226].

16.2.4.

14.2.4.  NSH Base Header Next Protocol

   IANA is requested to set up a registry of "Next Protocol".  These are
   8-bit values.  Next Protocol values 0, 1, 2 and 3 are defined in this
   draft.  New values are assigned via Standards Action [RFC5226].

              +---------------+-------------+---------------+
              | Next Protocol | Description | Reference     |
              +---------------+-------------+---------------+
              | 0             | Reserved    | This document |
              |               |             |               |
              | 1             | IPv4        | This document |
              |               |             |               |
              | 2             | IPv6        | This document |
              |               |             |               |
              | 3             | Ethernet    | This document |
              |               |             |               |
              | 4..253        | Unassigned  |               |
              +---------------+-------------+---------------+

                                  Table 2

17.

15.  References

17.1.

15.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981. 1981,
              <http://www.rfc-editor.org/info/rfc791>.

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

17.2. 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

15.2.  Informative References

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000. 2000,
              <http://www.rfc-editor.org/info/rfc2784>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008. 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6071]  Frankel, S. and S. Krishnan, "IP Security (IPsec) and
              Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
              DOI 10.17487/RFC6071, February 2011.

   [SFC-PS] 2011,
              <http://www.rfc-editor.org/info/rfc6071>.

   [RFC7498]  Quinn, P., Ed. and T. Nadeau, Ed., "Service "Problem Statement for
              Service Function
              Chaining Problem Statement", 2014, <http://
              datatracker.ietf.org/doc/
              draft-ietf-sfc-problem-statement/>. Chaining", RFC 7498, DOI 10.17487/
              RFC7498, April 2015,
              <http://www.rfc-editor.org/info/rfc7498>.

   [SFC-arch]
              Quinn, P., Ed. and J. Halpern, Ed., "Service Function
              Chaining (SFC) Architecture", 2014,
              <http://datatracker.ietf.org/doc/draft-quinn-sfc-arch>.

   [VXLAN-gpe]
              Quinn, P., Manur, R., Agarwal, P., Kreeger, L., Lewis, D.,
              Maino, F., Smith, M., Yong, L., Xu, X., Elzur, U., and P. Garg,
              P., and D. Melman, "Generic Protocol Extension for VXLAN",
              <https://datatracker.ietf.org/doc/draft-quinn-vxlan-gpe/>.
              <https://datatracker.ietf.org/doc/
              draft-ietf-nvo3-vxlan-gpe/>.

   [dcalloc]  Guichard, J., Smith, M., and S. Kumar, "Network Service
              Header (NSH) Context Header Allocation (Data Center)",
              2014, <https://datatracker.ietf.org/doc/
              draft-guichard-sfc-nsh-dc-allocation/>.

   [moballoc]
              Napper, J. and S. Kumar, "NSH Context Header Allocation --
              Mobility", 2014, <https://datatracker.ietf.org/doc/
              draft-napper-sfc-nsh-mobility-allocation/>.

Authors' Addresses

   Paul Quinn (editor)
   Cisco Systems, Inc.

   Email: paulq@cisco.com

   Uri Elzur (editor)
   Intel

   Email: uri.elzur@intel.com