Integration of Network Service Header (NSH) and Segment Routing for Service Function Chaining (SFC)
draft-ietf-spring-nsh-sr-09

Document Type Active Internet-Draft (spring WG)
Authors Jim Guichard  , Jeff Tantsura 
Last updated 2021-07-26
Replaces draft-guichard-spring-nsh-sr
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SPRING                                                  J. Guichard, Ed.
Internet-Draft                                    Futurewei Technologies
Intended status: Standards Track                        J. Tantsura, Ed.
Expires: January 27, 2022                                      Microsoft
                                                           July 26, 2021

  Integration of Network Service Header (NSH) and Segment Routing for
                    Service Function Chaining (SFC)
                      draft-ietf-spring-nsh-sr-09

Abstract

   This document describes the integration of the Network Service Header
   (NSH) and Segment Routing (SR), as well as encapsulation details, to
   support Service Function Chaining (SFC) in an efficient manner while
   maintaining separation of the service and transport planes as
   originally intended by the SFC architecture.

   Combining these technologies allows SR to be used for steering
   packets between Service Function Forwarders (SFF) along a given
   Service Function Path (SFP) while NSH has the responsibility for
   maintaining the integrity of the service plane, the SFC instance
   context, and any associated metadata.

   This integration demonstrates that NSH and SR can work cooperatively
   and provide a network operator with the flexibility to use whichever
   transport technology makes sense in specific areas of their network
   infrastructure while still maintaining an end-to-end service plane
   using NSH.

Status of This Memo

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   This Internet-Draft will expire on January 27, 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  SFC Overview and Rationale  . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  SFC within Segment Routing Networks . . . . . . . . . . . . .   4
   3.  NSH-based SFC with SR-MPLS or SRv6 Transport Tunnel . . . . .   5
   4.  SR-based SFC with Integrated NSH Service Plane  . . . . . . .   9
   5.  Packet Processing for SR-based SFC  . . . . . . . . . . . . .  11
     5.1.  SR-based SFC (SR-MPLS) Packet Processing  . . . . . . . .  11
     5.2.  SR-based SFC (SRv6) Packet Processing . . . . . . . . . .  11
   6.  Encapsulation . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  NSH using SR-MPLS Transport . . . . . . . . . . . . . . .  12
     6.2.  NSH using SRv6 Transport  . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .  14
   9.  Caching Considerations  . . . . . . . . . . . . . . . . . . .  14
   10. MTU Considerations  . . . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Protocol Number for NSH  . . . . . . . . . . . . . . . .  14
     11.2.  SRv6 Endpoint Behavior for NSH . . . . . . . . . . . . .  15
   12. Contributing Authors  . . . . . . . . . . . . . . . . . . . .  15
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     13.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

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1.1.  SFC Overview and Rationale

   The dynamic enforcement of a service-derived and adequate forwarding
   policy for packets entering a network that supports advanced Service
   Functions (SFs) has become a key challenge for network operators.
   For instance, cascading SFs at the 3GPP (Third Generation Partnership
   Project) Gi interface (N6 interface in 5G architecture) has shown
   limitations such as 1) redundant classification features must be
   supported by many SFs to execute their function, 2) some SFs receive
   traffic that they are not supposed to process (e.g., TCP proxies
   receiving UDP traffic) which inevitably affects their dimensioning
   and performance, and 3) an increased design complexity related to the
   properly ordered invocation of several SFs.

   In order to solve those problems, and to decouple the services
   topology from the underlying physical network while allowing for
   simplified service delivery, Service Function Chaining (SFC)
   techniques have been introduced [RFC7665].

   SFC techniques are meant to rationalize the service delivery logic
   and master the resulting complexity while optimizing service
   activation time cycles for operators that need more agile service
   delivery procedures to better accommodate ever-demanding customer
   requirements.  SFC allows network operators to dynamically create
   service planes that can be used by specific traffic flows.  Each
   service plane is realized by invoking and chaining the relevant
   service functions in the right sequence.  [RFC7498] provides an
   overview of the overall SFC problem space and [RFC7665] specifies an
   SFC data plane architecture.  The SFC architecture does not make
   assumptions on how advanced features (e.g., load-balancing, loose or
   strict service paths) could be enabled within a domain.  Various
   deployment options are made available to operators with the SFC
   architecture and this approach is fundamental to accommodate various
   and heterogeneous deployment contexts.

   Many approaches can be considered for encoding the information
   required for SFC purposes (e.g., communicate a service chain pointer,
   encode a list of loose/explicit paths, or disseminate a service chain
   identifier together with a set of context information).  Likewise,
   many approaches can also be considered for the channel to be used to
   carry SFC-specific information (e.g., define a new header, re-use
   existing packet header fields, or define an IPv6 extension header).
   Among all these approaches, the IETF created a transport-independent
   SFC encapsulation scheme: NSH [RFC8300].  This design is pragmatic as
   it does not require replicating the same specification effort as a
   function of underlying transport encapsulation.  Moreover, this
   design approach encourages consistent SFC-based service delivery in

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   networks enabling distinct transport protocols in various network
   segments or even between SFFs vs SF-SFF hops.

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
   [RFC2119] [RFC8174] when, and only when, they appear in all capitals,
   as shown here.

2.  SFC within Segment Routing Networks

   As described in [RFC8402], SR leverages the source routing technique.
   Concretely, a node steers a packet through an SR policy instantiated
   as an ordered list of instructions called segments.  While initially
   designed for policy-based source routing, SR also finds its
   application in supporting SFC
   [I-D.ietf-spring-sr-service-programming].

   The two SR data plane encapsulations, namely SR-MPLS [RFC8660] and
   SRv6 [RFC8754], can both encode an SF as a segment so that an SFC can
   be specified as a segment list.  Nevertheless, and as discussed in
   [RFC7498], traffic steering is only a subset of the issues that
   motivated the design of the SFC architecture.  Further considerations
   such as simplifying classification at intermediate SFs and allowing
   for coordinated behaviors among SFs by means of supplying context
   information (a.k.a. metadata) should be considered when designing an
   SFC data plane solution.

   While each scheme (i.e., NSH-based SFC and SR-based SFC) can work
   independently, this document describes how the two can be used
   together in concert and complement each other through two
   representative application scenarios.  Both application scenarios may
   be supported using either SR-MPLS or SRv6:

   o  NSH-based SFC with SR-based transport plane: in this scenario SR-
      MPLS or SRv6 provides the transport encapsulation between SFFs
      while NSH is used to convey and trigger SFC policies.

   o  SR-based SFC with integrated NSH service plane: in this scenario
      each service hop of the SFC is represented as a segment of the SR
      segment-list.  SR is thus responsible for steering traffic through
      the necessary SFFs as part of the segment routing path while NSH
      is responsible for maintaining the service plane and holding the
      SFC instance context (including associated metadata).

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   It is of course possible to combine both of these two scenarios to
   support specific deployment requirements and use cases.

   A classifier MUST assign an NSH Service Path Identifier (SPI) per SR
   policy so that different traffic flows that use the same NSH Service
   Function Path (SFP) but different SR policy can coexist on the same
   SFP without conflict during SFF processing.

3.  NSH-based SFC with SR-MPLS or SRv6 Transport Tunnel

   Because of the transport-independent nature of NSH-based service
   function chains, it is expected that the NSH has broad applicability
   across different network domains (e.g., access, core).  By way of
   illustration the various SFs involved in a service function chain may
   be available in a single data center, or spread throughout multiple
   locations (e.g., data centers, different Points of Presense (POPs)),
   depending upon the network operator preference and/or availability of
   service resources.  Regardless of where the SFs are deployed it is
   necessary to provide traffic steering through a set of SFFs, and when
   NSH and SR are integrated, this is provided by SR-MPLS or SRv6.

   The following three figures provide an example of an SFC established
   flow F that has SF instances located in different data centers, DC1
   and DC2.  For the purpose of illustration, let the SFC's NSH SPI be
   100 and the initial Service Index (SI) be 255.

   Referring to Figure 1, packets of flow F in DC1 are classified into
   an NSH-based SFC and encapsulated after classification as <Inner
   Pkt><NSH: SPI 100, SI 255><Outer-transport> and forwarded to SFF1
   (which is the first SFF hop for this service function chain).

   After removing the outer transport encapsulation, SFF1 uses the SPI
   and SI carried within the NSH encapsulation to determine that it
   should forward the packet to SF1.  SF1 applies its service,
   decrements the SI by 1, and returns the packet to SFF1.  SFF1
   therefore has <SPI 100, SI 254> when the packet comes back from SF1.
   SFF1 does a lookup on <SPI 100, SI 254> which results in <next-hop:
   DC1-GW1> and forwards the packet to DC1-GW1.

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   +--------------------------- DC1 ----------------------------+
   |                          +-----+                           |
   |                          | SF1 |                           |
   |                          +--+--+                           |
   |                             |                              |
   |                             |                              |
   |        +------------+       |    +------------+            |
   |        | N(100,255) |       |    | N(100,254) |            |
   |        +------------+       |    +------------+            |
   |        | F:Inner Pkt|       |    | F:Inner Pkt|            |
   |        +------------+  ^    |  | +------------+            |
   |                    (2) |    |  | (3)                       |
   |                        |    |  v                           |
   |                  (1)        |         (4)                  |
   |+------------+   ---->    +--+---+    ---->     +---------+ |
   ||            |    NSH     |      |     NSH      |         | |
   || Classifier +------------+ SFF1 +--------------+ DC1-GW1 + |
   ||            |            |      |              |         | |
   |+------------+            +------+              +---------+ |
   |                                                            |
   |             +------------+       +------------+            |
   |             | N(100,255) |       | N(100,254) |            |
   |             +------------+       +------------+            |
   |             | F:Inner Pkt|       | F:Inner Pkt|            |
   |             +------------+       +------------+            |
   |                                                            |
   +------------------------------------------------------------+

                  Figure 1: SR for inter-DC SFC - Part 1

   Referring now to Figure 2, DC1-GW1 performs a lookup using the
   information conveyed in the NSH which results in <next-hop: DC2-GW1,
   encapsulation: SR>.  The SR encapsulation, which may be SR-MPLS or
   SRv6, has the SR segment-list to forward the packet across the inter-
   DC network to DC2.

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                     +----------- Inter DC ----------------+
              (4)    |                (5)                  |
   +------+  ---->   | +---------+   ---->     +---------+ |
   |      |   NSH    | |         |     SR      |         | |
   + SFF1 +----------|-+ DC1-GW1 +-------------+ DC2-GW1 + |
   |      |          | |         |             |         | |
   +------+          | +---------+             +---------+ |
                     |                                     |
                     |          +------------+             |
                     |          | S(DC2-GW1) |             |
                     |          +------------+             |
                     |          | N(100,254) |             |
                     |          +------------+             |
                     |          | F:Inner Pkt|             |
                     |          +------------+             |
                     +-------------------------------------+

                  Figure 2: SR for inter-DC SFC - Part 2

   When the packet arrives at DC2, as shown in Figure 3, the SR
   encapsulation is removed and DC2-GW1 performs a lookup on the NSH
   which results in next hop: SFF2.  When SFF2 receives the packet, it
   performs a lookup on <NSH: SPI 100, SI 254> and determines to forward
   the packet to SF2.  SF2 applies its service, decrements the SI by 1,
   and returns the packet to SFF2.  SFF2 therefore has <NSH: SPI 100, SI
   253> when the packet comes back from SF2.  SFF2 does a lookup on
   <NSH: SPI 100, SI 253> which results in the end of the service
   function chain.

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                   +------------------------ DC2 ----------------------+
                   |                         +-----+                   |
                   |                         | SF2 |                   |
                   |                         +--+--+                   |
                   |                            |                      |
                   |                            |                      |
                   |        +------------+      |    +------------+    |
                   |        | N(100,254) |      |    | N(100,253) |    |
                   |        +------------+      |    +------------+    |
                   |        | F:Inner Pkt|      |    | F:Inner Pkt|    |
                   |        +------------+  ^   |  | +------------+    |
                   |                    (7) |   |  | (8)               |
                   |                        |   |  v                   |
             (5)   |                 (6)        |     (9)              |
+---------+  --->  | +----------+   ---->    +--+---+ ---->            |
|         |   SR   | |          |    NSH     |      |  IP              |
+ DC1-GW1 +--------|-+ DC2-GW1  +------------+ SFF2 |                  |
|         |        | |          |            |      |                  |
+---------+        | +----------+            +------+                  |
                   |                                                   |
                   |           +------------+      +------------+      |
                   |           | N(100,254) |      | F:Inner Pkt|      |
                   |           +------------+      +------------+      |
                   |           | F:Inner Pkt|                          |
                   |           +------------+                          |
                   +---------------------------------------------------+

                  Figure 3: SR for inter-DC SFC - Part 3

   The benefits of this scheme are listed hereafter:

   o  The network operator is able to take advantage of the transport-
      independent nature of the NSH encapsulation, while the service is
      provisioned end-to-end.

   o  The network operator is able to take advantage of the traffic
      steering (traffic engineering) capability of SR where appropriate.

   o  Clear responsibility division and scope between NSH and SR.

   Note that this scenario is applicable to any case where multiple
   segments of a service function chain are distributed across multiple
   domains or where traffic-engineered paths are necessary between SFFs
   (strict forwarding paths for example).  Further note that the above
   example can also be implemented using end-to-end segment routing
   between SFF1 and SFF2.  (As such DC-GW1 and DC-GW2 are forwarding the

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   packets based on segment routing instructions and are not looking at
   the NSH header for forwarding.)

4.  SR-based SFC with Integrated NSH Service Plane

   In this scenario we assume that the SFs are NSH-aware and therefore
   it should not be necessary to implement an SFC proxy to achieve SFC.
   The operation relies upon SR-MPLS or SRv6 to perform SFF-SFF
   transport and NSH to provide the service plane between SFs thereby
   maintaining SFC context (e.g., the service plane path referenced by
   the SPI) and any associated metadata.

   When a service function chain is established, a packet associated
   with that chain will first carry an NSH that will be used to maintain
   the end-to-end service plane through use of the SFC context.  The SFC
   context is used by an SFF to determine the SR segment list for
   forwarding the packet to the next-hop SFFs.  The packet is then
   encapsulated using the SR header and forwarded in the SR domain
   following normal SR operations.

   When a packet has to be forwarded to an SF attached to an SFF, the
   SFF performs a lookup on the segment identifier (SID) associated with
   the SF.  In the case of SR-MPLS this will be a prefix SID [RFC8402].
   In the case of SRv6, the behavior described within this document is
   assigned the name END.NSH, and section 9.2 requests allocation of a
   code point by IANA.  The result of this lookup allows the SFF to
   retrieve the next hop context between the SFF and SF (e.g., the
   destination MAC address in case native Ethernet encapsulation is used
   between SFF and SF).  In addition the SFF strips the SR information
   from the packet, updates the SR information, and saves it to a cache
   indexed by the NSH Service Path Identifier (SPI) and the Service
   Index (SI) decremented by 1.  This saved SR information is used to
   encapsulate and forward the packet(s) coming back from the SF.

   The behavior of remembering the SR segment-list occurs at the end of
   the regularly defined logic.  The behavior of reattaching the
   segment-list occurs before the SR process of forwarding the packet to
   the next entry in the segment-list.  Both behaviors are further
   detailed in section 5.

   When the SF receives the packet, it processes it as usual.  When the
   SF is co-resident with a classifier, the already processed packet may
   be re-classified.  The SF sends the packet back to the SFF.  Once the
   SFF receives this packet, it extracts the SR information using the
   NSH SPI and SI as the index into the cache.  The SFF then pushes the
   retrieved SR header on top of the NSH header, and forwards the packet
   to the next segment in the segment-list.  The lookup in the SFF cache
   might fail if re-classification at the SF changed the NSH SPI and/or

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   SI to values that do not exist in the SFF cache.  In such a case, the
   SFF must generate an error and drop the packet.

   Figure 4 illustrates an example of this scenario.

                        +-----+                       +-----+
                        | SF1 |                       | SF2 |
                        +--+--+                       +--+--+
                           |                             |
                           |                             |
             +-----------+ | +-----------+ +-----------+ | +-----------+
             |N(100,255) | | |N(100,254) | |N(100,254) | | |N(100,253) |
             +-----------+ | +-----------+ +-----------+ | +-----------+
             |F:Inner Pkt| | |F:Inner Pkt| |F:Inner Pkt| | |F:Inner Pkt|
             +-----------+ | +-----------+ +-----------+ | +-----------+
                     (2) ^ | (3) |                 (5) ^ | (6) |
                         | |     |                     | |     |
                         | |     |                     | |     |
                 (1)     | |     v      (4)            | |     v (7)
+------------+   --->    +-+----+      ---->          +---+--+   -->
|            | NSHoverSR |      |    NSHoverSR        |      |    IP
| Classifier +-----------+ SFF1 +---------------------+ SFF2 |
|            |           |      |                     |      |
+------------+           +------+                     +------+

             +------------+        +------------+           +------------+
             |   S(SF1)   |        |   S(SF2)   |           | F:Inner Pkt|
             +------------+        +------------+           +------------+
             |   S(SFF2)  |        | N(100,254) |
             +------------+        +------------+
             |   S(SF2)   |        | F:Inner Pkt|
             +------------+        +------------+
             | N(100,255) |
             +------------+
             | F:Inner Pkt|
             +------------+

                       Figure 4: NSH over SR for SFC

   The benefits of this scheme include:

   o  It is economically sound for SF vendors to only support one
      unified SFC solution.  The SF is unaware of the SR.

   o  It simplifies the SFF (i.e., the SR router) by nullifying the
      needs for re-classification and SR proxy.

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   o  SR is also used for forwarding purposes including between SFFs.

   o  It takes advantage of SR to eliminate the NSH forwarding state in
      SFFs.  This applies each time strict or loose SFPs are in use.

   o  It requires no interworking as would be the case if SR-MPLS based
      SFC and NSH-based SFC were deployed as independent mechanisms in
      different parts of the network.

5.  Packet Processing for SR-based SFC

   This section describes the End.NSH behavior (SRv6), Prefix SID
   behavior (SR-MPLS), and NSH processing logic.

5.1.  SR-based SFC (SR-MPLS) Packet Processing

   When an SFF receives a packet destined to S and S is a local prefix
   SID associated with an SF, the SFF strips the SR segment-list (label
   stack) from the packet, updates the SR information, and saves it to a
   cache indexed by the NSH Service Path Identifier (SPI) and the
   Service Index (SI) decremented by 1.  This saved SR information is
   used to re-encapsulate and forward the packet(s) coming back from the
   SF.

5.2.  SR-based SFC (SRv6) Packet Processing

   This section describes the End.NSH behavior and NSH processing logic
   for SRv6.  The pseudo code is shown below.

   When N receives a packet destined to S and S is a local End.NSH SID,
   the processing is the same as that specified by [RFC8754] section
   4.3.1.1, up through line S.16.

   After S.15, if S is a local End.NSH SID, then:

   S15.1.  Remove and store IPv6 and SRH headers in local cache indexed
   by <NSH: service-path-id, service-index -1>

   S15.2.  Submit the packet to the NSH FIB lookup and transmit to the
   destination associated with <NSH: service-path-id, service-index>

   Note: The End.NSH behavior interrupts the normal SRH packet
   processing as described in [RFC8754] section 4.3.1.1, which does not
   continue to S16 at this time.

   When a packet is returned to the SFF from the SF, reattach the cached
   IPv6 and SRH headers based on the <NSH: service-path-id, service-

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   index> from the NSH header.  Then resume processing from [RFC8754]
   section 4.3.1.1 with line S.16.

6.  Encapsulation

6.1.  NSH using SR-MPLS Transport

   SR-MPLS instantiates Segment IDs (SIDs) as MPLS labels and therefore
   the segment routing header is a stack of MPLS labels.

   When carrying NSH within an SR-MPLS transport, the full encapsulation
   headers are as illustrated in Figure 5.

                          +------------------+
                          ~   MPLS-SR Labels ~
                          +------------------+
                          |   NSH Base Hdr   |
                          +------------------+
                          | Service Path Hdr |
                          +------------------+
                          ~     Metadata     ~
                          +------------------+

                   Figure 5: NSH using SR-MPLS Transport

   As described in [RFC8402], the IGP signaling extension for IGP-Prefix
   segment includes a flag to indicate whether directly connected
   neighbors of the node on which the prefix is attached should perform
   the NEXT operation or the CONTINUE operation when processing the SID.
   When NSH is carried beneath SR-MPLS it is necessary to terminate the
   NSH-based SFC at the tail-end node of the SR-MPLS label stack.  This
   can be achieved using either the NEXT or CONTINUE operation.

   If the NEXT operation is to be used, then at the end of the SR-MPLS
   path it is necessary to provide an indication to the tail-end that
   NSH follows the SR-MPLS label stack as described by [RFC8596].

   If the CONTINUE operation is to be used, this is the equivalent of
   MPLS Ultimate Hop Popping (UHP) and therefore it is necessary to
   ensure that the penultimate hop node does not pop the top label of
   the SR-MPLS label stack and thereby expose NSH to the wrong SFF.
   This is realized by setting No-PHP flag in Prefix-SID Sub-TLV
   [RFC8667], [RFC8665].  It is RECOMMENDED that a specific prefix-SID
   be allocated at each node for use by the SFC application for this
   purpose.

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6.2.  NSH using SRv6 Transport

   When carrying NSH within an SRv6 transport the full encapsulation is
   as illustrated in Figure 6.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Next Header   |  Hdr Ext Len  | Routing Type  | Segments Left |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Last Entry   |     Flags     |              Tag              | S
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e
     |                                                               | g
     |            Segment List[0] (128 bits IPv6 address)            | m
     |                                                               | e
     |                                                               | n
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ t
     |                                                               |
     |                                                               | R
     ~                              ...                              ~ o
     |                                                               | u
     |                                                               | t
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ i
     |                                                               | n
     |            Segment List[n] (128 bits IPv6 address)            | g
     |                                                               |
     |                                                               | S
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R
     //                                                             // H
     //         Optional Type Length Value objects (variable)       //
     //                                                             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|O|U|    TTL    |   Length  |U|U|U|U|MD Type| Next Protocol |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
     |          Service Path Identifier              | Service Index | S
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ H
     |                                                               |
     ~              Variable-Length Context Headers  (opt.)          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 6: NSH using SRv6 Transport

   Encapsulation of NSH following SRv6 is indicated by the IP protocol
   number for NSH in the Next Header of the SRH.

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7.  Security Considerations

   Generic SFC-related security considerations are discussed in
   [RFC7665].

   NSH-specific security considerations are discussed in [RFC8300].

   Generic segment routing related security considerations are discussed
   in section 7 of [RFC8754] and section 5 of [RFC8663].

8.  Backwards Compatibility

   For SRv6/IPv6, if a processing node does not recognize NSH it should
   follow the procedures described in section 4 of [RFC8200].  For SR-
   MPLS, if a processing node does not recognize NSH it should follow
   the procedures laid out in section 3.18 of [RFC3031].

9.  Caching Considerations

   The cache mechanism must remove cached entries at an appropriate time
   determined by the implementation.  Further, an implementation MAY
   allow network operators to set the said time value.  In the case a
   packet arriving from an SF does not have a matching cached entry, the
   SFF SHOULD log this event.

10.  MTU Considerations

   Aligned with Section 5 of [RFC8300] and Section 5.3 of [RFC8754], it
   is RECOMMENDED for network operators to increase the underlying MTU
   so that SR/NSH traffic is forwarded within an SR domain without
   fragmentation.

11.  IANA Considerations

11.1.  Protocol Number for NSH

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IANA is requested to assign a protocol number TBA1 for the NSH from the
"Assigned Internet Protocol Numbers" registry available at
https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml

   +---------+---------+--------------+---------------+----------------+
   | Decimal | Keyword |   Protocol   |      IPv6     |   Reference    |
   |         |         |              |   Extension   |                |
   |         |         |              |     Header    |                |
   +---------+---------+--------------+---------------+----------------+
   |   TBA1  |   NSH   |   Network    |       N       | [ThisDocument] |
   |         |         |   Service    |               |                |
   |         |         |    Header    |               |                |
   +---------+---------+--------------+---------------+----------------+

11.2.  SRv6 Endpoint Behavior for NSH

This I-D requests IANA to allocate, within the "SRv6 Endpoint Behaviors"
sub-registry belonging to the top-level "Segment-routing with IPv6 data
plane (SRv6) Parameters" registry, the following allocations:

      Value      Description                               Reference
      --------------------------------------------------------------
      TBA2       End.NSH  - NSH Segment                    [This.ID]

12.  Contributing Authors

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The following co-authors, along with their respective affiliations at
the time of publication, provided valuable inputs and text contributions
to this document.

Mohamed Boucadair
Orange
mohamed.boucadair@orange.com

Joel Halpern
Ericsson
joel.halpern@ericsson.com

Syed Hassan
Cisco System, inc.
shassan@cisco.com

Wim Henderickx
Nokia
wim.henderickx@nokia.com

Haoyu Song
Futurewei Technologies
haoyu.song@futurewei.com

13.  References

13.1.  Normative References

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

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

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <https://www.rfc-editor.org/info/rfc7665>.

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   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <https://www.rfc-editor.org/info/rfc8086>.

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

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

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

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8596]  Malis, A., Bryant, S., Halpern, J., and W. Henderickx,
              "MPLS Transport Encapsulation for the Service Function
              Chaining (SFC) Network Service Header (NSH)", RFC 8596,
              DOI 10.17487/RFC8596, June 2019,
              <https://www.rfc-editor.org/info/rfc8596>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8663]  Xu, X., Bryant, S., Farrel, A., Hassan, S., Henderickx,
              W., and Z. Li, "MPLS Segment Routing over IP", RFC 8663,
              DOI 10.17487/RFC8663, December 2019,
              <https://www.rfc-editor.org/info/rfc8663>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

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   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

13.2.  Informative References

   [I-D.ietf-spring-sr-service-programming]
              Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca,
              d., Li, C., Decraene, B., Ma, S., Yadlapalli, C.,
              Henderickx, W., and S. Salsano, "Service Programming with
              Segment Routing", draft-ietf-spring-sr-service-
              programming-03 (work in progress), September 2020.

   [RFC7498]  Quinn, P., Ed. and T. Nadeau, Ed., "Problem Statement for
              Service Function Chaining", RFC 7498,
              DOI 10.17487/RFC7498, April 2015,
              <https://www.rfc-editor.org/info/rfc7498>.

Authors' Addresses

   James N Guichard (editor)
   Futurewei Technologies
   2330 Central Express Way
   Santa Clara
   USA

   Email: james.n.guichard@futurewei.com

   Jeff Tantsura (editor)
   Microsoft
   USA

   Email: jefftant.ietf@gmail.com

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