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RFC 8300

Network Service Header (NSH)

Pages: 40
Proposed Standard
Updated by:  9451
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Internet Engineering Task Force (IETF)                     P. Quinn, Ed.
Request for Comments: 8300                                         Cisco
Category: Standards Track                                  U. Elzur, Ed.
ISSN: 2070-1721                                                    Intel
                                                       C. Pignataro, Ed.
                                                            January 2018

                      Network Service Header (NSH)


This document describes a Network Service Header (NSH) imposed on packets or frames to realize Service Function Paths (SFPs). The NSH also provides a mechanism for metadata exchange along the instantiated service paths. The NSH is the Service Function Chaining (SFC) encapsulation required to support the SFC architecture (defined in RFC 7665). Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at Copyright Notice Copyright (c) 2018 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 ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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Table of Contents

1. Introduction ....................................................3 1.1. Applicability ..............................................4 1.2. Requirements Language ......................................4 1.3. Definition of Terms ........................................4 1.4. Problem Space ..............................................6 1.5. NSH-Based Service Chaining .................................6 2. Network Service Header ..........................................7 2.1. Network Service Header Format ..............................7 2.2. NSH Base Header ............................................8 2.3. Service Path Header .......................................11 2.4. NSH MD Type 1 .............................................12 2.5. NSH MD Type 2 .............................................13 2.5.1. Optional Variable-Length Metadata ..................13 3. NSH Actions ....................................................15 4. NSH Transport Encapsulation ....................................16 5. Fragmentation Considerations ...................................17 6. Service Path Forwarding with NSH ...............................18 6.1. SFFs and Overlay Selection ................................18 6.2. Mapping the NSH to Network Topology .......................21 6.3. Service Plane Visibility ..................................21 6.4. Service Graphs ............................................22 7. Policy Enforcement with NSH ....................................22 7.1. NSH Metadata and Policy Enforcement .......................22 7.2. Updating/Augmenting Metadata ..............................24 7.3. Service Path Identifier and Metadata ......................25 8. Security Considerations ........................................26 8.1. NSH Security Considerations from Operators' Environments ..27 8.2. NSH Security Considerations from the SFC Architecture .....28 8.2.1. Integrity ..........................................29 8.2.2. Confidentiality ....................................31 9. IANA Considerations ............................................32 9.1. NSH Parameters ............................................32 9.1.1. NSH Base Header Bits ...............................32 9.1.2. NSH Version ........................................32 9.1.3. NSH MD Types .......................................33 9.1.4. NSH MD Class .......................................33 9.1.5. NSH IETF-Assigned Optional Variable-Length Metadata Types .....................................34 9.1.6. NSH Next Protocol ..................................35 10. NSH-Related Codepoints ........................................35 10.1. NSH Ethertype ............................................35 11. References ....................................................36 Acknowledgments ...................................................38 Contributors ......................................................39 Authors' Addresses ................................................40
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1. 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 WAN, data center, and so forth. Prior to development of the SFC architecture [RFC7665] and the protocol specified in this document, current Service Function deployment models have been 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 demands an agile service insertion model that supports dynamic and elastic service delivery. Specifically, the following functions are necessary: 1. The movement of Service Functions and application workloads in the network. 2. The ability to easily bind service policy to granular information, such as per-subscriber state. 3. The capability to steer traffic to the requisite Service Function(s). This document, the Network Service Header (NSH) specification, defines a new data-plane protocol, which is an encapsulation for SFCs. The NSH is designed to encapsulate an original packet or frame and, in turn, be encapsulated by an outer transport encapsulation (which is used to deliver the NSH to NSH-aware network elements), as shown in Figure 1: +------------------------------+ | Transport Encapsulation | +------------------------------+ | Network Service Header (NSH) | +------------------------------+ | Original Packet / Frame | +------------------------------+ Figure 1: Network Service Header Encapsulation
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   The NSH is composed of the following elements:

   1.  Service Function Path identification.

   2.  Indication of location within a Service Function Path.

   3.  Optional, per-packet metadata (fixed-length or variable).

   [RFC7665] provides an overview of a service chaining architecture
   that clearly defines the roles of the various elements and the scope
   of a SFC encapsulation.  Figure 3 of [RFC7665] depicts the SFC
   architectural components after classification.  The NSH is the SFC
   encapsulation referenced in [RFC7665].

1.1. Applicability

The NSH is designed to be easy to implement across a range of devices, both physical and virtual, including hardware platforms. The intended scope of the NSH is for use within a single provider's operational domain. This deployment scope is deliberately constrained, as explained also in [RFC7665], and limited to a single network administrative domain. In this context, a "domain" is a set of network entities within a single administration. For example, a network administrative domain can include a single data center, or an overlay domain using virtual connections and tunnels. A corollary is that a network administrative domain has a well-defined perimeter. An NSH-aware control plane is outside the scope of this document.

1.2. Requirements Language

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

1.3. Definition of Terms

Byte: All references to "bytes" in this document refer to 8-bit bytes, or octets. Classification: Defined in [RFC7665]. Classifier: Defined in [RFC7665].
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   Metadata (MD):  Defined in [RFC7665].  The metadata, or context
      information shared between Classifiers and SFs, and among SFs, is
      carried on the NSH's Context Headers.  It allows summarizing a
      classification result in the packet itself, avoiding subsequent
      re-classifications.  Examples of metadata include classification
      information used for policy enforcement and network context for
      forwarding after service delivery.

   Network Locator:  Data-plane address, typically IPv4 or IPv6, used to
      send and receive network traffic.

   Network Node/Element:  Device that forwards packets or frames based
      on an outer header (i.e., transport encapsulation) information.

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

   NSH-aware:  NSH-aware means SFC-encapsulation-aware, where the NSH
      provides the SFC encapsulation.  This specification uses NSH-aware
      as a more specific term from the more generic term "SFC-aware"

   Service Classifier:  Logical entity providing classification
      function.  Since they are logical, Classifiers may be co-resident
      with SFC elements such as SFs or SFFs.  Service Classifiers
      perform classification and impose the NSH.  The initial Classifier
      imposes the initial NSH and sends the NSH packet to the first SFF
      in the path.  Non-initial (i.e., subsequent) classification can
      occur as needed and can alter, or create a new service path.

   Service Function (SF):  Defined in [RFC7665].

   Service Function Chain (SFC):  Defined in [RFC7665].

   Service Function Forwarder (SFF):  Defined in [RFC7665].

   Service Function Path (SFP):  Defined in [RFC7665].

   Service Plane:  The collection of SFFs and associated SFs creates a
      service-plane overlay in which all SFs and SFC Proxies reside

   SFC Proxy:  Defined in [RFC7665].
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1.4. Problem Space

The NSH addresses several limitations associated with Service Function deployments. [RFC7498] provides a comprehensive review of those issues.

1.5. NSH-Based Service Chaining

The NSH creates a dedicated service plane; more specifically, the NSH enables: 1. Topological Independence: Service forwarding occurs within the service plane, so the underlying network topology does not require modification. The NSH provides an identifier used to select the network overlay for network forwarding. 2. Service Chaining: The NSH enables service chaining per [RFC7665]. The NSH contains path identification information needed to realize a service path. Furthermore, the NSH provides the ability to monitor and troubleshoot a service chain, end-to-end via service-specific Operations, Administration, and Maintenance (OAM) messages. The NSH fields can be used by administrators (for example, via a traffic analyzer) to verify the path specifics (e.g., accounting, ensuring correct chaining, providing reports, etc.) of packets being forwarded along a service path. 3. The NSH provides a mechanism to carry shared metadata between participating entities and Service Functions. The semantics of the shared metadata are communicated via a control plane (which is outside the scope of this document) to participating nodes. Section 3.3 of [SFC-CONTROL-PLANE] provides an example of this. Examples of metadata include classification information used for policy enforcement and network context for forwarding post service delivery. 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. 4. The NSH offers a common and standards-based header for service chaining to all network and service nodes. 5. Transport Encapsulation Agnostic: The NSH is transport encapsulation independent: meaning it can be transported by a variety of encapsulation protocols. An appropriate (for a given deployment) encapsulation protocol can be used to carry NSH- encapsulated traffic. This transport encapsulation may form an
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       overlay network; and if an existing overlay topology provides the
       required service path connectivity, that existing overlay may be

2. Network Service Header

An NSH is imposed on the original packet/frame. This NSH contains service path information and, optionally, metadata that are added to a packet or frame and used to create a service plane. Subsequently, an outer transport encapsulation is imposed on the NSH, which is used for network forwarding. A Service Classifier adds the NSH. The NSH is removed by the last SFF in the service chain or by an SF that consumes the packet.

2.1. Network Service Header Format

The NSH is composed of a 4-byte Base Header, a 4-byte Service Path Header, and optional Context Headers, as shown in Figure 2. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Base Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Path Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Context Header(s) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Network Service Header Base Header: Provides information about the service header and the payload protocol. Service Path Header: Provides path identification and location within a service path. Context Header: Carries metadata (i.e., context data) along a service path.
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2.2. NSH Base Header

Figure 3 depicts the NSH Base Header: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: NSH Base Header The field descriptions are as follows: Version: The Version field is used to ensure backward compatibility going forward with future NSH specification updates. It MUST be set to 0x0 by the sender, in this first revision of the NSH. If a packet presumed to carry an NSH header is received at an SFF, and the SFF does not understand the version of the protocol as indicated in the base header, the packet MUST be discarded, and the event SHOULD be logged. Given the widespread implementation of existing hardware that uses the first nibble after an MPLS label stack for Equal-Cost Multipath (ECMP) decision processing, this document reserves version 01b. This value MUST NOT be used in future versions of the protocol. Please see [RFC7325] for further discussion of MPLS-related forwarding requirements. O bit: Setting this bit indicates an OAM packet (see [RFC6291]). The actual format and processing of SFC OAM packets is outside the scope of this specification (for example, see [SFC-OAM-FRAMEWORK] for one approach). The O bit MUST be set for OAM packets and MUST NOT be set for non-OAM packets. The O bit MUST NOT be modified along the SFP. SF/SFF/SFC Proxy/Classifier implementations that do not support SFC OAM procedures SHOULD discard packets with O bit set, but MAY support a configurable parameter to enable forwarding received SFC OAM packets unmodified to the next element in the chain. Forwarding OAM packets unmodified by SFC elements that do not support SFC OAM procedures may be acceptable for a subset of OAM functions, but it can result in unexpected outcomes for others; thus, it is recommended to analyze the impact of forwarding an OAM packet for all OAM functions prior to enabling this behavior. The configurable parameter MUST be disabled by default.
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   TTL:  Indicates the maximum SFF hops for an SFP.  This field is used
      for service-plane loop detection.  The initial TTL value SHOULD be
      configurable via the control plane; the configured initial value
      can be specific to one or more SFPs.  If no initial value is
      explicitly provided, the default initial TTL value of 63 MUST be
      used.  Each SFF involved in forwarding an NSH packet MUST
      decrement the TTL value by 1 prior to NSH forwarding lookup.
      Decrementing by 1 from an incoming value of 0 shall result in a
      TTL value of 63.  The packet MUST NOT be forwarded if TTL is,
      after decrement, 0.

      This TTL field is the primary loop-prevention mechanism.  This TTL
      mechanism represents a robust complement to the Service Index (see
      Section 2.3), as the TTL is decremented by each SFF.  The handling
      of an incoming 0 TTL allows for better, although not perfect,
      interoperation with pre-standard implementations that do not
      support this TTL field.

   Length:  The total length, in 4-byte words, of the NSH including the
      Base Header, the Service Path Header, the Fixed-Length Context
      Header, or Variable-Length Context Header(s).  The length MUST be
      0x6 for MD Type 0x1, and it MUST be 0x2 or greater for MD Type
      0x2.  The length of the Network Service Header MUST be an integer
      multiple of 4 bytes; thus, variable-length metadata is always
      padded out to a multiple of 4 bytes.

   Unassigned bits:  All other flag fields, marked U, are unassigned and
      available for future use; see Section 9.1.1.  Unassigned bits MUST
      be set to zero upon origination, and they MUST be ignored and
      preserved unmodified by other NSH supporting elements.  At
      reception, all elements MUST NOT modify their actions based on
      these unknown bits.

   Metadata (MD) Type:  Indicates the format of the NSH beyond the
      mandatory NSH Base Header and the Service Path Header.  MD Type
      defines the format of the metadata being carried.  Please see the
      IANA Considerations in Section 9.1.3.

      This document specifies the following four MD Type values:

      0x0:  This is a reserved value.  Implementations SHOULD silently
            discard packets with MD Type 0x0.

      0x1:  This indicates that the format of the header includes a
            Fixed-Length Context Header (see Figure 5 below).
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      0x2:  This does not mandate any headers beyond the Base Header and
            Service Path Header, but may contain optional Variable-
            Length Context Header(s).  With MD Type 0x2, a length of 0x2
            implies there are no Context Headers.  The semantics of the
            Variable-Length Context Header(s) are not defined in this
            document.  The format of the optional Variable-Length
            Context Headers is provided in Section 2.5.1.

      0xF:  This value is reserved for experimentation and testing, as
            per [RFC3692].  Implementations not explicitly configured to
            be part of an experiment SHOULD silently discard packets
            with MD Type 0xF.

      The format of the Base Header and the Service Path Header is
      invariant and not affected by MD Type.

      The NSH MD Type 1 and MD Type 2 are described in detail in
      Sections 2.4 and 2.5, respectively.  NSH implementations MUST
      support MD Types 0x1 and 0x2 (where the length is 0x2).  NSH
      implementations SHOULD support MD Type 0x2 with length greater
      than 0x2.  Devices that do not support MD Type 0x2 with a length
      greater than 0x2 MUST ignore any optional Context Headers and
      process the packet without them; the Base Header Length field can
      be used to determine the original payload offset if access to the
      original packet/frame is required.  This specification does not
      disallow the MD Type value from changing along an SFP; however,
      the specification of the necessary mechanism to allow the MD Type
      to change along an SFP are outside the scope of this document and
      would need to be defined for that functionality to be available.
      Packets with MD Type values not supported by an implementation
      MUST be silently dropped.

   Next Protocol:  Indicates the protocol type of the encapsulated data.
      The NSH does not alter the inner payload, and the semantics on the
      inner protocol remain unchanged due to NSH SFC.  Please see the
      IANA Considerations in Section 9.1.6.

      This document defines the following Next Protocol values:

      0x1: IPv4
      0x2: IPv6
      0x3: Ethernet
      0x4: NSH
      0x5: MPLS
      0xFE: Experiment 1
      0xFF: Experiment 2
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      The functionality of hierarchical NSH using a Next Protocol value
      of 0x4 (NSH) is outside the scope of this specification.  Packets
      with Next Protocol values not supported SHOULD be silently dropped
      by default, although an implementation MAY provide a configuration
      parameter to forward them.  Additionally, an implementation not
      explicitly configured for a specific experiment [RFC3692] SHOULD
      silently drop packets with Next Protocol values 0xFE and 0xFF.

2.3. Service Path Header

Figure 4 shows the format of the Service Path Header: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Path Identifier (SPI) | Service Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Service Path Identifier (SPI): 24 bits Service Index (SI): 8 bits Figure 4: NSH Service Path Header The meaning of these fields is as follows: Service Path Identifier (SPI): Uniquely identifies a Service Function Path (SFP). Participating nodes MUST use this identifier for SFP selection. The initial Classifier MUST set the appropriate SPI for a given classification result. Service Index (SI): Provides location within the SFP. The initial Classifier for a given SFP SHOULD set the SI to 255; however, the control plane MAY configure the initial value of the SI as appropriate (i.e., taking into account the length of the SFP). The Service Index MUST be decremented by a value of 1 by Service Functions or by SFC Proxy nodes after performing required services; the new decremented SI value MUST be used in the egress packet's NSH. The initial Classifier MUST send the packet to the first SFF in the identified SFP for forwarding along an SFP. If re-classification occurs, and that re-classification results in a new SPI, the (re-)Classifier is, in effect, the initial Classifier for the resultant SPI. The SI is used in conjunction with the Service Path Identifier for SFP selection and for determining the next SFF/SF in the path. The SI is also valuable when troubleshooting or reporting service paths. While the TTL provides the primary SFF-based loop prevention for this mechanism, SI decrement by SF serves as a limited loop-prevention
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   mechanism.  NSH packets, as described above, are discarded when an
   SFF decrements the TTL to 0.  In addition, an SFF that is not the
   terminal SFF for an SFP will discard any NSH packet with an SI of 0,
   as there will be no valid next SF information.

2.4. NSH MD Type 1

When the Base Header specifies MD Type 0x1, a Fixed-Length Context Header (16-bytes) MUST be present immediately following the Service Path Header, as per Figure 5. The value of a Fixed-Length Context Header that carries no metadata MUST be set to zero. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Path Identifier | Service Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Fixed-Length Context Header | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: NSH MD Type 0x1 This specification does not make any assumptions about the content of the 16-byte Context Header that must be present when the MD Type field is set to 1, and it does not describe the structure or meaning of the included metadata. An SFC-aware SF or SFC Proxy needs to receive the data structure and semantics first in order to process the data placed in the mandatory context field. The data structure and semantics include both the allocation schema and order as well as the meaning of the included data. How an SFC-aware SF or SFC Proxy gets the data structure and semantics is outside the scope of this specification. An SF or SFC Proxy that does not know the format or semantics of the Context Header for an NSH with MD Type 1 MUST discard any packet with such an NSH (i.e., MUST NOT ignore the metadata that it cannot process), and MUST log the event at least once per the SPI for which the event occurs (subject to thresholding). [NSH-DC-ALLOCATION] and [NSH-BROADBAND-ALLOCATION] provide specific examples of how metadata can be allocated.
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2.5. NSH MD Type 2

When the Base Header specifies MD Type 0x2, zero or more Variable- Length Context Headers MAY be added, immediately following the Service Path Header (see Figure 6). Therefore, Length = 0x2, indicates that only the Base Header and Service Path Header are present (and in that order). The optional Variable-Length Context Headers MUST be of an integer number of 4-bytes. The Base Header Length field MUST be used to determine the offset to locate the original packet or frame for SFC nodes that require access to that information. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver|O|U| TTL | Length |U|U|U|U|MD Type| Next Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Path Identifier | Service Index | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Variable-Length Context Headers (opt.) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: NSH MD Type 0x2

2.5.1. Optional Variable-Length Metadata

The format of the optional Variable-Length Context Headers, is as depicted in Figure 7. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Metadata Class | Type |U| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Variable-Length Metadata | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Variable-Length Context Headers Metadata Class (MD Class): Defines the scope of the Type field to provide a hierarchical namespace. Section 9.1.4 defines how the MD Class values can be allocated to standards bodies, vendors, and others.
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   Type:  Indicates the explicit type of metadata being carried.  The
      definition of the Type is the responsibility of the MD Class

   Unassigned bit:  One unassigned bit is available for future use.
      This bit MUST NOT be set, and it MUST be ignored on receipt.

   Length:  Indicates the length of the variable-length metadata, in
      bytes.  In case the metadata length is not an integer number of
      4-byte words, the sender MUST add pad bytes immediately following
      the last metadata byte to extend the metadata to an integer number
      of 4-byte words.  The receiver MUST round the Length field up to
      the nearest 4-byte-word boundary, to locate and process the next
      field in the packet.  The receiver MUST access only those bytes in
      the metadata indicated by the Length field (i.e., actual number of
      bytes) and MUST ignore the remaining bytes up to the nearest
      4-byte-word boundary.  The length may be 0 or greater.

      A value of 0 denotes a Context Header without a Variable-Length
      Metadata field.

   This specification does not make any assumption about Context Headers
   that are mandatory to implement or those that are mandatory to
   process.  These considerations are deployment specific.  However, the
   control plane is entitled to instruct SFC-aware SFs with the data
   structure of the Context Header together with its scoping (see e.g.,
   Section 3.3.3 of [SFC-CONTROL-PLANE]).

   Upon receipt of a packet that belongs to a given SFP, if a mandatory-
   to-process Context Header is missing in that packet, the SFC-aware SF
   MUST NOT process the packet and MUST log an error at least once per
   the SPI for which the mandatory metadata is missing.

   If multiple mandatory-to-process Context Headers are required for a
   given SFP, the control plane MAY instruct the SFC-aware SF with the
   order to consume these Context Headers.  If no instructions are
   provided and the SFC-aware SF will make use of or modify the specific
   Context Header, then the SFC-aware SF MUST process these Context
   Headers in the order they appear in an NSH packet.

   If multiple instances of the same metadata are included in an NSH
   packet, but the definition of that Context Header does not allow for
   it, the SFC-aware SF MUST process the first instance and ignore
   subsequent instances.  The SFC-aware SF MAY log or increase a counter
   for this event.
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3. NSH Actions

NSH-aware nodes (which include Service Classifiers, SFFs, SFs, and SFC Proxies) may alter the contents of the NSH headers. These nodes have several possible NSH-related actions: 1. Insert or remove the NSH: These actions can occur respectively at the start and end of a service path. Packets are classified, and if determined to require servicing, an NSH will be imposed. A Service Classifier MUST insert an NSH at the start of an SFP. An imposed NSH MUST contain both a valid Base Header and Service Path Header. At the end of an SFP, an SFF MUST remove the NSH before forwarding or delivering the un-encapsulated packet. Therefore, it is the last node operating on the service header. Multiple logical Classifiers may exist within a given service path. Non-initial Classifiers may re-classify data, and that re-classification MAY result in the selection of a different SFP. When the logical Classifier performs re-classification that results in a change of service path, it MUST replace the existing NSH with a new NSH with the Base Header and Service Path Header reflecting the new service path information and MUST set the initial SI. The O bit, the TTL field, and unassigned flags MUST be copied transparently from the old NSH to a new NSH. Metadata MAY be preserved in the new NSH. 2. Select service path: The Service Path Header provides service path information and is used by SFFs to determine correct service path selection. SFFs MUST use the Service Path Header for selecting the next SF or SFF in the service path. 3. Update the NSH: SFs MUST decrement the service index by one. If an SFF receives a packet with an SPI and SI that do not correspond to a valid next hop in a valid SFP, that packet MUST be dropped by the SFF. Classifiers MAY update Context Headers if new/updated context is available. If an SFC proxy is in use (acting on behalf of an NSH-unaware Service Function for NSH actions), then the proxy MUST update the Service Index and MAY update contexts. When an SFC Proxy receives an NSH-encapsulated packet, it MUST remove the NSH before forwarding it to an NSH-unaware SF. When the SFC Proxy receives a packet back from an NSH-unaware SF, it MUST re-encapsulate it with the correct NSH, and it MUST decrement the Service Index by one.
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   4.  Service policy selection: Service Functions derive policy (i.e.,
       service actions such as permit or deny) selection and enforcement
       from the NSH.  Metadata shared in the NSH can provide a range of
       service-relevant information such as traffic classification.

   Figure 8 maps each of the four actions above to the components in the
   SFC architecture that can perform it.

   |           | Insert, remove, or    |Forward| Update        |Service|
   |           | replace the NSH       |the NSH| the NSH       |policy |
   |           |                       |packets|               |sel.   |
   |Component  +-------+-------+-------+       +-------+-------+       |
   |           |       |       |       |       |Dec.   |Update |       |
   |           |Insert |Remove |Replace|       |Service|Context|       |
   |           |       |       |       |       |Index  |Header |       |
   |           |  +    |       |   +   |       |       |   +   |       |
   |Classifier |       |       |       |       |       |       |       |
   |Service    |       |   +   |       |   +   |       |       |       |
   |Function   |       |       |       |       |       |       |       |
   |Forwarder  |       |       |       |       |       |       |       |
   |(SFF)      |       |       |       |       |       |       |       |
   |Service    |       |       |       |       |   +   |   +   |   +   |
   |Function   |       |       |       |       |       |       |       |
   |(SF)       |       |       |       |       |       |       |       |
   |           |  +    |   +   |       |       |   +   |   +   |       |
   |SFC Proxy  |       |       |       |       |       |       |       |

                   Figure 8: NSH Action and Role Mapping

4. NSH Transport Encapsulation

Once the NSH is added to a packet, an outer transport encapsulation is used to forward the original packet and the associated metadata to the start of a service chain. The encapsulation serves two purposes: 1. Creates a topologically independent services plane. Packets are forwarded to the required services without changing the underlying network topology.
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   2.  Transit network nodes simply forward the encapsulated packets
       without modification.

   The service header is independent of the transport encapsulation
   used.  Existing transport encapsulations can be used.  The presence
   of an NSH is indicated via a protocol type or another indicator in
   the outer transport encapsulation.

5. Fragmentation Considerations

The NSH and the associated transport encapsulation header are "added" to the encapsulated packet/frame. This additional information increases the size of the packet. Within a managed administrative domain, an operator can ensure that the underlay MTU is sufficient to carry SFC traffic without requiring fragmentation. Given that the intended scope of the NSH is within a single provider's operational domain, that approach is sufficient. However, although explicitly outside the scope of this specification, there might be cases where the underlay MTU is not large enough to carry the NSH traffic. Since the NSH does not provide fragmentation support at the service plane, the transport encapsulation protocol ought to provide the requisite fragmentation handling. For instance, Section 9 of [RTG-ENCAP] provides exemplary approaches and guidance for those scenarios. When the transport encapsulation protocol supports fragmentation, and fragmentation procedures needs to be used, such fragmentation is part of the transport encapsulation logic. If, as it is common, fragmentation is performed by the endpoints of the transport encapsulation, then fragmentation procedures are performed at the sending NSH entity as part of the transport encapsulation, and reassembly procedures are performed at the receiving NSH entity during transport de-encapsulation handling logic. In no case would such fragmentation result in duplication of the NSH header. For example, when the NSH is encapsulated in IP, IP-level fragmentation coupled with Path MTU Discovery (PMTUD) (e.g., [RFC8201]) is used. Since PMTUD relies on ICMP messages, an operator should ensure ICMP packets are not blocked. When, on the other hand, the underlay does not support fragmentation procedures, an error message SHOULD be logged when dropping a packet too big. Lastly, NSH-specific fragmentation and reassembly methods may be defined as well, but these methods are outside the scope of this document and subject for future work.
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6. Service Path Forwarding with NSH

6.1. SFFs and Overlay Selection

As described above, the NSH contains a Service Path Identifier (SPI) and a 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 provides a level of indirection between the service path / topology and the network transport encapsulation. Furthermore, there is no requirement for, or expectation of, an SPI being bound to a predetermined or static network path. The Service Index provides an indication of location within a service path. The combination of SPI and SI provides the identification of a logical SF and its order within the service plane. This combination is used to select the appropriate network locator(s) for overlay forwarding. The logical SF may be a single SF or a set of eligible SFs that are equivalent. In the latter case, the SFF provides load distribution amongst the collection of SFs as needed. SI serves as a mechanism for detecting invalid SFPs. In particular, an SI value of zero indicates that forwarding is incorrect and the packet must be discarded. This indirection -- SPI 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.). SFs need to be able to return a packet to an appropriate SFF (i.e., has the requisite NSH information) when service processing is complete. This can be via the overlay or underlay and, in some cases, can require additional configuration on the SF. As mentioned above, an existing overlay topology may be used, provided it offers the requisite connectivity. The mapping of SPI to transport encapsulation occurs on an SFF (as discussed above, the first SFF in the path gets an NSH encapsulated packet from the Classifier). The SFF consults the SPI/ID values to determine the appropriate overlay transport encapsulation protocol (several may be used within a given network) and next hop for the requisite SF. Table 1 depicts an example of a single next-hop SPI/ SI-to-network overlay network locator mapping.
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      | SPI  | SI   | Next Hop(s)         | Transport Encapsulation |
      | 10   | 255  |           | VXLAN-gpe               |
      |      |      |                     |                         |
      | 10   | 254  |       | GRE                     |
      |      |      |                     |                         |
      | 10   | 251  |       | GRE                     |
      |      |      |                     |                         |
      | 40   | 251  |       | GRE                     |
      |      |      |                     |                         |
      | 50   | 200  | 01:23:45:67:89:ab   | Ethernet                |
      |      |      |                     |                         |
      | 15   | 212  | Null (end of path)  | None                    |

                     Table 1: SFF NSH Mapping Example

   Additionally, further indirection is possible: the resolution of the
   required SF network locator may be a localized resolution on an SFF,
   rather than an SFC control plane responsibility, as per Tables 2 and

   Please note: VXLAN-gpe and GRE in the above table refer to
   [VXLAN-GPE] and [RFC2784] [RFC7676], respectively.

                      | SPI  | SI  | Next Hop(s)    |
                      | 10   | 3   | SF2            |
                      |      |     |                |
                      | 245  | 12  | SF34           |
                      |      |     |                |
                      | 40   | 9   | SF9            |

                    Table 2: NSH-to-SF Mapping Example
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          | SF   | Next Hop(s)       | Transport Encapsulation |
          | SF2  |         | VXLAN-gpe               |
          |      |                   |                         |
          | SF34 |     | UDP                     |
          |      |                   |                         |
          | SF9  | 2001:db8::1       | GRE                     |

                    Table 3: 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)
   paths to be used (for load distribution, redundancy, or policy); see
   Table 4.  The metric depicted in Table 4 is an example to help
   illustrate 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 the state of a Service Function
   (including load, session state, capacity, etc.).

                  | SPI  | SI  | NH           | Metric  |
                  | 10   | 3   |  | 1       |
                  |      |     |              |         |
                  |      |     |  | 1       |
                  |      |     |              |         |
                  | 20   | 12  |    | 1       |
                  |      |     |              |         |
                  |      |     |  | 1       |
                  |      |     |              |         |
                  | 30   | 7   |   | 10      |
                  |      |     |              |         |
                  |      |     | | 5       |

                (encapsulation type omitted for formatting)

                    Table 4: NSH Weighted Service Path

   The information contained in Tables 1-4 may be received from the
   control plane, but the exact mechanism is outside the scope of this
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6.2. Mapping the NSH to Network Topology

As described above, the mapping of the SPI to network topology may result in a single path, or it might result in a more complex topology. Furthermore, the SPI-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 exists. 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 2001:db8::1 SFFa mapping: SPI=10 --> VXLAN-gpe, dst-ip: 2001:db8::1 2. Next SF is located at SFFc with multiple network locators for load-distribution purposes: SFFb mapping: SPI=10 --> VXLAN-gpe, dst_ip:,,, equal cost 3. Next SF is located at SFFd with two paths from SFFc, one for redundancy: SFFc mapping: SPI=10 --> VXLAN-gpe, dst_ip: cost=10,, 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 network paths as required. For example, the overlay path between SFFs 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 plane.

6.3. 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 NSH information (packet capture, IP Flow Information Export (IPFIX), etc.). The information can be used for service scheduling and placement decisions, troubleshooting, and compliance verification.
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6.4. Service Graphs

While a given realized SFP is a specific sequence of Service Functions, the service, as seen by a user, can actually be a collection of SFPs, with the interconnection provided by Classifiers (in-service path, non-initial re-classification). These internal re- Classifiers examine the packet at relevant points in the network, and, if needed, SPI and SI are updated (whether this update is a re- write, or the imposition of a new NSH with new values is implementation specific) to reflect the "result" of the classification. These Classifiers may, of course, also modify the metadata associated with the packet. Section 2.1 of [RFC7665] describes Service Graphs in detail.

(page 22 continued on part 2)

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