



SFC Working Group                                            D. Eastlake
Internet-Draft                                               Independent
Intended status: Standards Track                           13 April 2026
Expires: 15 October 2026


       Service Function Chaining (SFC) Parallelism and Diversions
                     draft-eastlake-sfc-parallel-12

Abstract

   Service Function Chaining (SFC) is the processing of packets through
   a sequence of Service Functions (SFs) within an SFC domain by the
   addition of path information and metadata on entry to that domain,
   the use and modification of that path information and metadata to
   step the packet through a sequence of SFs, and the removal of that
   path information and metadata on exit from that domain.  The IETF has
   standardized a method for SFC using the Network Service Header
   specified in RFC 8300.

   There are requirements for SFC to process packets through parallel
   sequences of service functions, rejoining thereafter, and to easily
   splice in additional service functions or splice service functions
   out of a service chain.  The IETF has received a liaison from
   International Telecommunication Union (ITU) indicating their interest
   in such requirements.  This document provides use cases and specifies
   extensions to SFC to support these requirements.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 15 October 2026.







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Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
   2.  Service Function Chaining Background  . . . . . . . . . . . .   4
     2.1.  The Network Service Header (NSH)  . . . . . . . . . . . .   6
     2.2.  NSH Metadata and Variable Length Context Headers  . . . .   7
   3.  Requirements for Parallelism and Diversions . . . . . . . . .   8
   4.  Diversion Points and Rendezvous Points  . . . . . . . . . . .  11
     4.1.  Rendezvous Point Information (RePIn)  . . . . . . . . . .  11
       4.1.1.  Packet Identifier . . . . . . . . . . . . . . . . . .  12
       4.1.2.  Packet Extent Modified  . . . . . . . . . . . . . . .  13
       4.1.3.  Saved Metadata  . . . . . . . . . . . . . . . . . . .  14
       4.1.4.  Saved TTL . . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Diversion Point (DP) Behavior . . . . . . . . . . . . . .  15
     4.3.  Rendezvous Point (RP) Behavior  . . . . . . . . . . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     5.1.  Variable Length Context Header Type . . . . . . . . . . .  19
     5.2.  RePIn VLCH Sub-Types  . . . . . . . . . . . . . . . . . .  19
   6.  Security Consideration  . . . . . . . . . . . . . . . . . . .  20
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  20
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  21
   Appendix A.  Relation to Hierarchical SFC . . . . . . . . . . . .  21
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Service Function Chaining (SFC) is the processing of packets through
   a sequence of Service Functions (SFs) within an SFC domain by the
   addition of path information and metadata on entry to that domain,
   the use and modification of that path information and metadata to
   step the packet through a sequence of SFs, and the removal of that
   path information and metadata on exit from that domain.  The IETF has
   standardized a method for SFC using the Network Service Header



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   specified in [RFC8300].

   There are requirements for SFC to process packets through parallel
   sequences of service functions, rejoining thereafter, and to easily
   splice in additional service functions or splice service functions
   out of a service chain.  The IETF has received a liaison from the ITU
   [Liaison] indicating their interest in such requirements.  This
   document provides use cases and specifies extensions to SFC to
   support these requirements.

1.1.  Conventions Used in This Document

   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.

   Acronyms and terms:

   downstream -  The direction from ingress to egress.

   diversion -  A reclassification of an SFC packet into one or multiple
      parallel packets with difference SPIs where this reclassified
      packet or packets are, in the normal case, combined at a
      downstream rendezvous point which restores the original SPI.

   DP -  Diversion Point - An SF implementing a diversion.

   ITU -  International Telecommunications Union (www.itu.int).

   MD -  Metadata - Part of the NSH.

   NSH -  Network Service Header [RFC8300].

   rendezvous -  The process of taking one or more corresponding SFC
      packets that have been diverted at an upstream DP, combining the
      packets if there are more than one, and restoring the original
      SPI.

   RePIn -  Rendezvous Point Information.  Metadata included in an SFC
      packet for use at an RP.

   RP -  Rendezvous Point - An SF implementing a rendezvous.

   SF -  Service Function [RFC7665].

   SFC -  Service Function Chaining [RFC7665].



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   SFF -  Service Function Forwarder [RFC7665] - A type of node that
      forwards packets based on the NSH.

   SFP -  Service Function Path.

   SI -  Service Index - Part of the NSH.

   SPI -  Service Path Identifier - Part of the NSH.

   TLV -  Type Length Value.

   upstream -  The direction from egress to ingress.

   VLCH -  Variable Length Context Header - A type of NSH header
      metadata.

2.  Service Function Chaining Background

   Service Function Chaining (SFC) calls for the encapsulation of
   traffic within a service function chaining domain using a Network
   Service Header (NSH [RFC8300]) added by the "Classifier" (ingress
   node) on entry to the domain and the NSH being removed on exit from
   the domain at the downstream egress node as shown in Figure 1.  The
   NSH controls the path of a packet in an SFC domain and includes
   additional information.


























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                         |
                         v
                    +----------+
                 . .|Classifier|. . . . . . . . . . . . . .
                 .  +----------+                          .
                 .       |          +----+                .
                 .       |        --+ SF |     Service    .
                 .       |       /  +----+     Function   .
                 .       v    ---              Chaining   .
                 .    +-----+/       +----+    domain     .
                 .    | SFF |--------+ SF |               .
                 .    +-----+\       +----+               .
                 .       |    ---                         .
                 .       |       \  +----+                .
                 .       |        --+ SF |                .
                 .       v          +----+                .
                 .    +-----+                 +----+      .
                 .    | SFF |-----------------+ SF |      .
                 .    +-----+                 +----+      .
                 .       |          +----+                .
                 .       |        --+ SF |                .
                 .       |       /  +----+                .
                 .       v    ---                         .
                 .    +-----+/       +----+               .
                 .    | SFF |--------+ SF |               .
                 .    +-----+\       +----+               .
                 .       |    ---                         .
                 .       |       \  +----+                .
                 .       |        --+ SF |                .
                 .       v          +----+                .
                 .    +------+                            .
                 . . .| Exit |. . . . . . . . . . . . . . .
                      +------+
                         |
                         v

                Figure 1: Example SFC Path Forwarding Nodes

   Traffic passes through a sequence of Service Function Forwarders
   (SFFs) each of which sends the traffic to one or, sequentially, more
   than one Service Functions (SFs).  Each SF performs some operation on
   the traffic, for example firewall or Network Address Translation
   (NAT) or load balancer, and then returns it to the SFF from which it
   was received.  There may be multiple instances of SFs performing the
   same function attached to the same or different SFFs.






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   Logically, during the transit of an SFF, the outer transport header
   that got the packet to the SFF is stripped (see Figure 2), the SFF
   decides on the next forwarding step either (1) adding a new transport
   header or (2) in case of error discarding or logging the packet and
   not forwarding it or (3) if the SFF is the exit/egress, removing the
   NSH header and then adding a new transport header.  The transport
   used may be different in different regions of the SFC domain.  For
   example, a version of the Internet Protocol (IP) could be used in
   some parts and Multi-Protocol Label Switching (MPLS) used in other
   parts of the SFC domain.

                   +-----------------------------------+
                   |   Outer Transport Header          |
                   +-----------------------------------+
                   |   Network Service Header (NSH)    |
                   | +------------------------------+  |
                   | | Base Header                  |  |
                   | +------------------------------+  |
                   | | Service Path Header          |  |
                   | +------------------------------+  |
                   | | Metadata (Context Header(s)) |  |
                   | +------------------------------+  |
                   +-----------------------------------+
                   | Original Packet / Frame / Payload |
                   +-----------------------------------+

                 Figure 2: Data Encapsulation with the NSH

   An SF can receive one or more SFC packets from an SFF and return to
   it a larger or smaller number of SFC packets; that is to say, SFC
   packets can be discarded or created by an SF.

2.1.  The Network Service Header (NSH)

   The NSH header is used to encapsulate traffic and control its
   subsequent path.  It consists of three parts, the initial 32-bit Base
   Header, the 32-bit Service Path Header, and any Context Headers
   holding metadata, as shown in more detail in Figure 3 and specified
   in [RFC8300].

   The Base Header includes a Length field whose value is the overall
   NSH length.  Because the Base Header and Service Path Header are
   fixed length, the length of the Metadata can be computed from this
   Length field.  The Base Header also includes a field indicating the
   type of metadata in the NSH.






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   The Service Path Header consists of a Service Path Identifier (SPI)
   and a Service Index (SI).  The SPI identifies the logical path the
   packet should follow while the SI indicates which step along that
   path the packet is at.

   An SF anywhere along a Service Function Path can re-classify an SFC
   packet by replacing the Service Path Identifier (SPI) and Service
   Index (SI) in the NSH.  SFFs can also insert, delete, or change
   metadata (Context Header(s)) in the NSH.

      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 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Optional Context Headers / Metadata                ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

          Figure 3: Network Service Header Details from [RFC8300]

2.2.  NSH Metadata and Variable Length Context Headers

   If the MD Type field in the NSH Base Header has the value 1, there is
   a single fixed length 128-bit Context Header whose format is not
   further defined by the IETF.  In that case, the NSH Length field has
   the value 6.

   If the MD Type field has the value 2, there are zero or more
   Variable-Length Context Headers (VLCHs) as shown in Figure 4 at the
   end of the NSH.  The absence of any Context Headers is indicated by
   using MD Type 2 and an NSH Length of 2.  MD Type 0 is reserved and MD
   Types 3 through 15 are unassigned.

      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 4: Variable Length Context Header

   The minimum size for a VLCH is 32 bits consisting of the Metadata
   Class, Type, one unused bit, and Length as shown in Figure 4.




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   The Metadata Class field is 16 bits, and its value specifies the
   organization under which the particular of VLCHs specified.  Metadata
   Class zero is the IETF base class.  The 8-bit Type field's value,
   along with the Metadata Class value, indicates the meaning of the
   Context Header and its Variable-Length Metadata.  The size of the
   Length field is 7 bits and its value gives, as an unsigned integer,
   the length in octets of the Variable-Length Metadata that follows the
   initial fixed length portion of the VLCH.  A VLCH with no Variable-
   Length Metadata is indicated by a Length field whose value is zero.
   VLCHs are padded so that they always start and end at a multiple of 4
   bytes from the beginning of the NSH.

3.  Requirements for Parallelism and Diversions

   There are requirements to split a Service Function Chain (SFC) into
   two or more parallel Service Function Paths (SFPs) that later rejoin
   as shown in Figure 5.

                                 +------+      +-----+
                           ----->|SFF 2a|<---->| SF 2|
                          /      +------+\     +-----+
                         /                \____
                        /                      \
                +-----+/         +------+       ->+-----+
           ---->|SFF 1|--------->|SFF 2b|-------->|SFF 3|----->
                +-----+\         +------+       ->+-----+
                  /|\   \          /|\         /    /|\
                   |     \          |         /      |
                  \|/     \        \|/       |      \|/
                +-----+    \      +-----+    |    +-----+
                | SF 1|     \     | SF 3|    /    | SF 5|
                | DP  |      \    +-----+   /     | RP  |
                +-----+       \            /      +-----+
                               \  +------+/     +-----+
                                ->|SFF 2c|<---->| SF 4|
                                  +------+      +-----+

                 Figure 5: Parallel Service Function Paths

   For example, there may be two or more Service Functions (SFs) that
   can be performed in parallel with the goal, for time critical traffic
   such as some financial or gaming traffic, of delaying the stream of
   packets only by the amount of time taken by the slowest of those SFs;
   if the packets went through the SFs sequentially, the delay would be
   the sum of the times taken by each of the SFs.  An example of such
   potential parallel processing might be that the SFs operate on
   different parts of the packet such as one SF operating on packet
   addressing while another operates on the information payload.



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   Another example might be that one SF creates a signature or integrity
   code over parts of the packet to be inserted into the packet payload
   while another SF encrypts parts of the packet (or alternatively, they
   verify and decrypt in parallel).  As indicated by their [Liaison],
   the ITU is interested in such use cases.

   Another example of desirable parallelism would be improved
   reliability or accuracy if the SFs executed in parallel where
   unreliable or where different implementations of doing the same
   processing.  For example, some quantum computers are currently
   unreliable so it would be desirable to perform some quantum process
   several times and compare the results to pick the most common value,
   or a vote could be taken between the results of different
   implementations of some process.

   In Figure 5 it could be that any of the parallel paths could have
   more or less than one SFF, although exactly one is shown in the
   example for simplicity and any of the SFFs in any of the parallel
   SFPs could process a packet through more than one SF, although they
   are shown using only one SF in Figure 5 for simplicity.  (Note that
   while SFFs implement an SFP, SFPs logically consists of the sequence
   of SFs.  Thus, for example, an SFP could divert into multiple
   parallel SFPs that rejoin at an RP all implemented through SFs off of
   one and the same SFF.)  It could also be that one or more of the
   parallel paths would themselves further split into parallel paths and
   so on.

   There are cases where it is desirable to divert an SFP so as to
   splice one or more added SFs into that SFP or to divert it so as to
   slice out one or more sequential SFs that were downstream in that
   SFP, as shown in Figure 6 and Figure 7.  Although SF 3 in each of
   those two cases could re-classify the packet with a new SPI and SI
   that include the remainder of the new diverted path, this would
   require that a new SFP with this new SPI already be configured in all
   the SFFs for the remainder of the Service Function Path after a
   diversion.  In the case of Figure 7, SF 3 could possibly just adjust
   the Service Index, but this would require relaxing any checking at SF
   5 of the SPI/SI or source address of packets on the main SFP or may
   otherwise be undesirable.












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                                 +-----+       +-----+
                               ->|SFF 3| <---->| SF 4|
                              /  +-----+\      +-----+
                             /           \
          +-----+      +-----+            ->+-----+      +-----+
      --->|SFF 1|----->|SFF 2| - - - - - - >|SFF 4|----->|SFF 5|--->
          +-----+      +-----+              +-----+      +-----+
            /|\          /|\                  /|\          /|\
             |            |                    |            |
            \|/          \|/                  \|/          \|/
          +-----+      +-----+              +-----+      +-----+
          | SF 1|      | SF 3|              | SF 5|      | SF 6|
          +-----+      | DP  |              | RP  |      +-----+
                       +-----+              +-----+

                   Figure 6: Splicing in One or More SFFs


                                   ------------\
                                  /             \
                                 /               \
              +-----+      +-----+      +-----+   \->+-----+
          --->|SFF 1|----->|SFF 2|- - ->|SFF 4|- - ->|SFF 5|--->
              +-----+      +-----+      +-----+      +-----+
                /|\          /|\          /|\          /|\
                 |            |            |            |
                \|/          \|/          \|/          \|/
              +-----+      +-----+      +-----+      +-----+
              | SF 1|      | SF 3|      | SF 5|      | SF 5|
              +-----+      | DP  |      +-----+      | RP  |
                           +-----+                   +-----+

                  Figure 7: Splicing out One or More SFFs

   Combinations of the cases shown in Figure 5, Figure 6, and Figure 7
   may be needed where a diversion such as in Figure 6 or Figure 7
   occurs within a parallel path as in Figure 5 or parallel paths as in
   Figure 5 occur within a diversion as in Figure 6.  Generalizing
   Figure 6 and Figure 7, a diversion might splice in a path with some
   number of SFs that cuts out a portion of the original SFP that had
   some number of SFs.

   Although DPs and RPs are logically Service Functions (SFs) and shown
   as separate boxes in the above figures, like any other SF they can be
   implemented as co-located with an SFF.






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4.  Diversion Points and Rendezvous Points

   SF 1 in Figure 5 and SF 3 in Figure 6 and Figure 7 are referred to as
   Diversion Points (DPs) because they are nodes at which an SFP is
   diverted to one or more SFPs with new SPIs that are intended to
   rejoin/return to the original SPI at a downstream Rendezvous Point.
   SF 5 in Figure 5, Figure 6, and Figure 7 is referred to as a
   Rendezvous Point (RP) because it is the node at which one or more
   SFPs from an upstream DP rejoin an original SFP and an original SPI
   is restored.  The corresponding packets so received at an RP are
   merged or coordinated.

   In general, an RP needs to be configured to expect SFC packets to
   arrive at that RP on diverted SFPs.  An RP may need additional
   information in the SFC packets, as discussed in Section 4.1, to be
   included in their NSH.  Divergence point behavior is discussed in
   Section 4.2 and RP behavior is discussed in Section 4.3.

4.1.  Rendezvous Point Information (RePIn)

   To recombine packets from divergent SFP(s) at a Rendezvous Point
   (RP), or rejoin a diverted SFP to the original SFP, additional
   information may be needed in the packets.  This is accomplished
   through inclusion of the RP Information (RePIn) Variable Length
   Context Header (VLCH), as shown in Figure 8, in the packet's NSH.

      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 = 0x0000       | Type=TBD      |U| Length      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Saved Service Path Identifier                 | Saved SI      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Variable Length Sub-TLVs                           ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                            Figure 8: RePIn VLCH

   The Saved Service Path Identifier and Saved SI are the SPI and SI in
   the NSH of the SFC packet being diverted after entry to the DP SF and
   the SI has been decremented.

   The Length field is the total length of the Variable Length Sub-TLVs
   in octets plus 4 for the length of the Saved SPI and SI.







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   The Variable Length Sub-TLVs consist of zero or more RePIn VLCH Sub-
   TLVs.  The format of a RePIn VLCH Sub-TLV is as shown in Figure 9
   except for Sub-Type 1 as discussed in Section 4.1.1; however, all
   RePIn VLCH Sub-TLVs are padded at the end up to an even multiple of 4
   octets.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Sub-Type      |X|Sub-Length   | Variable Length Metadata      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                    Figure 9: General RePIn VLCH Sub-TLV

   The Sub-Type field is an 8-bit unsigned integer that is always
   present and indicates the format of the Variable Length Metadata in
   the Sub-TLV.  The X bit may be assigned a meaning for particular Sub-
   Types; if no such meaning is assigned for a particular Sub-Type, the
   X bit MUST be sent as zero and ignored on receipt.  Sub-Length is an
   unsigned 7-bit integer giving the length of the variable length
   metadata in octets.

   Unless the specification for a RePIn VLCH Sub-TLV Sub-Type specifies
   that there may be multiple occurrences of that Sub-TLV, it may only
   be included once.  If there are multiple instances, the first
   occurrence is used and any subsequent occurrences are ignored.

4.1.1.  Packet Identifier

   When an SFC packet is replicated and diverted to more than one
   parallel path to be merged back together at a Rendezvous Point (RP),
   a method of matching packets is needed such as labeling each copy
   that originated with the same packet before the split using a packet
   identifier such as a packet counter, fine grained time stamp, or hash
   code.  Such an identifier might already exist in the packet, for
   example a TCP sequence number.  The requirement is that the packet
   identifier and SPI together uniquely identify the ingressed packet.
   If such an existing identifier cannot be trusted or there is none,
   the Packet Identifier sub-TLV shown below is included as a VLCH.  Use
   of the Packet Counter sub-TLV is RECOMMENDED.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Sub-Type=1    | Packet Identifier                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 10: Packet Identifier Special Sub-TLV



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   Because this is expected to be a common RePIn VLCH Sub-TLV, in order
   to save octets, for this Sub-Type only, the "Sub-TLV" X and Sub-
   Length fields are omitted and the Packet Identifier field expanded to
   use the area where they would have occurred.

4.1.2.  Packet Extent Modified

   If two or more SFPs used in parallel have modified parts of a packet,
   the RP may need additional information to be able to recombine the
   different copies of the packet it will be receiving.  As an example
   of the complexities involved, an SF could change the length of part
   of a packet in a way dependent on the content of that part such as by
   applying a data compression or de-compression algorithm to part of
   the packet or by conditionally inserting or removing a VLAN tag
   depending on addressing information.

   In simple cases such as parallel SFPs that modify fixed size disjoint
   parts of a packet without changing the size of those parts, it may be
   possible for an RP to be configured to recombine the results without
   added information.  But in more complex or variable length cases,
   parallel SFPs need to add information as to what part of the original
   packet they modified and how this may have changed the length of that
   part.  Also, with such additional information, in some cases only one
   of the parallel SFPs would need to forward all of the original packet
   with modifications to the RP; one or more other parallel SFPs could
   just forward their modified part and the RP would be able to
   recombine the results thus saving communications link capacity that
   would be used if they all sent full packets.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Sub-Type=2    |X|Sub-Length=6 | Offset                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Priority      | Original Size         | Modified Size         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 11: Packet Extent Modified Sub-TLV

   If the X bit is zero, the entire modified packet is present in the
   SFC packet.  If the X bit is a one, only the modified portion appears
   in the packet, which requires any SFs between the SF that modified
   the packet and the RP to be capable of handling such an abbreviated
   packet.

   Offset is the number of bits between the end of the NSH or the last
   NSH if there are multiple stacked NSHs and the portion of the packet
   being modified.  Original Size and Modified Size are the size in bits



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   of the portion being modified before and after that modification.
   Any of the Offset, Original Size, and Modified Size fields may have
   the value zero.

   If parallel SFPs have modified the same or overlapping parts of a
   packet, the RP may need some way to resolve this conflict which could
   include a relative priority for changes made by different SFs
   configured at the RP and/or indicated in the RP Information (RePIn)
   or from other sources.  The Priority field may be used for this
   purpose; it contains an unsigned integer where a larger magnitude
   value indicates a higher priority that would prevail over a lower
   priority.  If not used, the Priority field MUST be sent as zero and
   ignored on receipt.

   If a path has modified more than one portion of a packet, multiple
   instance of the Packet Extent Modified Sub-TLV can be included in the
   RePIn VLCH.  If any Packet Extent Modified Sub-TLV Sub-Length is any
   value other than 6, the metadata is corrupt, the packet is silently
   discarded, and an error SHOULD be logged.

4.1.3.  Saved Metadata

   A DP may need to save Metadata so it will not be seen inside a
   diversion and will be restored at the RP.  This Sub-TLV is used for
   that purpose.  See Section 4.2 and 4.3 for further details on the use
   of this sub-TLV.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Sub-Type=3    |X| Sub-Length  | MBZ                   |MetaTyp|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Variable Length Saved Metadata                  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                     Figure 12: Saved Metadata Sub-TLV

   The X and MBZ bits MUST be sent as zero and ignored on receipt.
   MetaTyp is the MD Type of Metadata saved.  The presence of the MBZ
   field causes the saved metadata to be aligned on a multiple of 4
   octets.










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4.1.4.  Saved TTL

   The TTL limits the number of SFs that can be traversed between
   ingress and egress.  The packet is discarded if the TTL is exhausted.
   This is a safety measure to defend against infinite or very large
   loops due to malfunctions, configuration error, or other reasons.
   Thus, the RECOMMENDED mode of operation is to use a TTL value that is
   decremented continuously from original SFC domain ingress to final
   SFC egress including throughout any diversions.  If the TTL is reset
   on entry to a diversion, then the Saved TTL Sub-TLV MUST be used so
   that the previous TTL can be restored at the diversion's RP.

   Note that resetting the TTL on entry to a diversion opens the
   possibility for loop where a diversion diverts to itself or there are
   two diversions X and Y where X diverts to Y and Y diverts to X or
   more complex scenarios all of which are made safe by using a
   continuous TTL and unsafe by resetting the TTL on diversion entry.
   Such loops will result in a growing amount of metadata which might
   safely lead to packet discard or unsafely cause repeated
   fragmentation.

   If, despite the above warning, it is desired to reset the TTL at the
   DP and restore it at the RP, the Saved TTL Sub-TLV as shown below is
   used.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Sub-Type=4    |X| Sub-Length=2| MBZ           | Saved TTL     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 13: Saved TTL Sub-TLV

   The X and MBZ bits MUST be sent as zero and ignored on receipt.  The
   Saved TTL is the value of the TTL field copied from the NSH after its
   initial decrement on entering the DP SF.  If the Saved TTL Sub-TLV
   Sub-Lenth is any value other than 2, the metadata is corrupt, the
   packet is silently discarded, and an error SHOULD be logged.

4.2.  Diversion Point (DP) Behavior

   If it is desired to simply skip some SFs in an SFP, a diversion may
   not be necessary.  The SI can simply be decreased to that for the
   next SF desired if the SFF to which the SF/DP that reduces the SI
   returns the packet can handle that reduced SI value and forward the
   packet to the appropriate SFF and that SFF does not check packet
   source and indicate an error.  Otherwise, the procedure below in this
   section is used and this procedure MAY be used even in cases where



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   simple reduction of the SI would work.

   If the RP can recognize diverted SFC packets and modify/merge them
   appropriately to restore them to the original SFP with appropriate
   Metadata, then inserting a RePIn VLCH might not be needed.  In other
   cases take the steps below.  This is a logical procedure and any
   procedure can be used that results in the same diverted SFC
   packet(s).

   1.  Construct a RePIn VLCH containing the SPI and SI from the NSH and
       then change those fields in the NSH to the diversion SPI and SI.
       If diversion is to multiple parallel SFPs, make a copy of the SFC
       packet for each diversion, then construct a VLCH and modify the
       NSH as in the previous sentence for each one of the parallel
       paths.  Then perform the following steps to one or more modified
       copies and its corresponding RePIn VLCH.

   2.  If it will be necessary for the RP to merge the modified copies
       of the original SFC packet sent over parallel paths or if the RP
       needs to restore a particular ordering to packets, then add a
       Packet Identification Sub-TLV to the RePIn VLCH unless sufficient
       trusted information is available in the packet payload for the RP
       to do so without a Packet Identification Sub-TLV.

   3.  Take any Metadata from the original NSH that should be hidden
       during the diversion and restored at the RP and add it to the
       RePIn VLCH as a Saved Metadata Sub-TLV.  If the NSH already has
       any RePIn VLCHs, they need not be saved as they will be masked by
       the new RePIn VLCH that will be inserted before them (this
       indicates that a diversion from a diversion is being created).
       To save space, any Metadata that has been saved in the RePIn VLCH
       and is not needed in the diversion SFP SHOULD be removed from the
       NSH if MD Type 2 and MUST be removed from the NSH if MD Type 1.
       (In the Type 1 case, this converts the NSH to MD Type 2 with no
       Metadata.)

   4.  If it is desired to use a new value for the NSH TTL in the
       diversion, with the old value restored at the RP, add a Saved TTL
       Sub-TLV to the RePIn VLCH and set the TTL in the NSH to a
       configured value which may be dependent on the diversion being
       entered.  This is NOT RECOMMENDED as discussed in Section 4.1.4.

   5.  The RePIn VLCH constructed as above is inserted into an NSH as in
       the subpoints below:

       5.a.  If, after the above step 3, the initial NSH in the SFC
       packet is MD Type 2, insert the RePIn VLCH constructed above
       before any existing RePIn VLCH that may be in the NSH.



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       5.b.  If, after the above step 3, the initial NSH in the SFC
       packet is MD Type 1, this implies that there is Type 1 metadata
       that may be needed by one or more SFs in the diversion.  If the
       SFs in the diversion can handle stacked NSHs, insert an MD Type 2
       NSH copied from the initial NSH except for metadata, after the
       initial MD Type 1 NSH to hold the RePIn VLCH.  Handling stacked
       NSHs means the SF (or its proxy) can parse through them to find
       the needed metadata and the payload to operate on and, if the SF
       generates packets, the SF can create them with appropriate
       stacked NSHs.  If the SFs in the diversion cannot handle stacked
       NSHes, the creation of the diversion where the initial HSH has MD
       Type 1 is beyond the scope of this document.

   6.  Perform such other functions or modifications to the metadata or
       other parts of the SFC packet as are appropriate based on the
       saved or new SPI or other factors.

   7.  Transmit the modified packet(s).

   The addition of Metadata and possible additional NSH header (see step
   5.b above) may lead to fragmentation or decreased payload Maximum
   Transmission Unit (MTU) in some networks.

4.3.  Rendezvous Point (RP) Behavior

   A RP will have been configured to know the SFC packet SPI and SI
   values in diverted packets for which it is to perform the RP service.
   The SI is decremented when an SFC packet is received by an SF; for an
   RP this might decrement the SI to zero.  The RP performs the steps
   below.  If the RP can restore diverted SFC packets to their former
   SFP and, to the extent necessary, match and merge diverted packets
   received over parallel paths and correctly order the resulting SFC
   packets, without the presence of a RePIn VLCH, it does so and the
   remainder of this section is inapplicable.  If not, the following
   logical procedure or any procedure resulting in the same SFC packet
   is used.

   1.  Steps 2 and 3 below are performed on each diverted packet
       received by the RP.  If the RP is merging parallel diversions,
       step 4 is then performed on the set of matching packets.  In this
       case and any case where the RP should restore packet order, the
       RP SHOULD be prepared to buffer packets until they can be
       processed and forwarded.  Overflow of such a buffer will result
       in lost packets and SHOULD be logged as an error.  How long to
       wait for missing diverted packets and what action to take if it
       is decided they have been lost are application and implementation
       dependent.  Finally, Step 5 is performed.




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   2.  Find and remove the first RePIn VLCH in the diverted packet.
       This is referred to below as the removed VLCH.  It might be in a
       second stacked NSH if the initial NSH has MD Type 1.

   3.  Restore the packet from the diversion through the sub-steps
       listed below.

       3.a.  If there is a Saved TTL in the removed VLCH, restore the
       old TTL into the initial NSH.

       3.b.  Restore the saved SPI and SI from the removed RePIn VLCH
       into the initial NSH.

       3.c.  Restore metadata as follows:

       3.c.1.  Restore any MD Type 2 Saved Metadata from the removed
       VLCH into the NSH from which that VLCH was removed.

       3.c.2.  If there is MD Type 1 Saved Metadata in the removed VLCH
       and there is an initial MD Type 1 NSH in the packet, replace the
       MD Type 1 metadata with the saved MD Type 1 metadata.

       3.c.3.  If there is MD Type 1 Saved Metadata in the removed VLCH
       and there is an initial MD Type 2 NSH in the packet, insert a new
       initial NSH into the packet which is a copy of that MD Type 2 NSH
       except that it is MD Type 1 with the saved MD Type 1 metadata.

       3.d.  If, at this point, the packet starts with an MD Type 1 NSH
       followed by an MD Type 2 NSH with no metadata, remove that 2nd
       NSH.

   4.  Match up SFC packets arriving at the RP through parallel paths
       using the Packet Identification Sub-TLV in the removed RePIn VLCH
       or some other technique.  For each matching set, perform the sub-
       steps below.  Arbitrarily select one of the matching diverted
       packets to modify into the merged packet unless configured to use
       some particular diverted packet such as the one received over a
       particular diversion.  This is referred to below as the merged
       packet even before the merger is complete.

       4.a For error checking, any saved SPI and SI in the matching
       packets SHOULD be compared and an error logged if they are not
       identical.

       4.b For safety, it is RECOMMENDED that the minimum of the NSH TTL
       values from the parallel SFC packets be copied into the merged
       packet.




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       4.c Depending on the application and implementation, the
       remaining metadata in the merged packet may be used or updated
       based on the remaining Metadata in the other packets being
       merged.  How to do this is beyond the scope of this document.

       4.d The payloads of the other packets being merged, that is the
       portion after any NSHs, are used to update the payload in the
       merged packet.  This may be based on RP configuration for the
       application or Packet Extent Modified Sub-TLVs in the removed
       RePIn VLCHs or a combination of these.  The details, including
       how to handle conflicting modifications or overlapping Packet
       Extent Modified regions, is out of scope for this document.

   5.  Perform such other functions or modifications to the metadata or
       other parts of the SFC packet as are appropriate based on the
       saved or new SPI or other factors.

   6.  Transmit the merged packet.

5.  IANA Considerations

   The following subsections provide IANA assignment considerations.

5.1.  Variable Length Context Header Type

   IANA is requested to assign a variable length context header type
   from the "NSH IETF-Assigned Optional Variable-Length Metadata Types"
   registry as follows:

    +=======+======================================+=================+
    | Value |             Description              |    Reference    |
    +=======+======================================+=================+
    |  TBD  | Rendezvous Point Information (RePIn) | [this document] |
    +-------+--------------------------------------+-----------------+

                                 Table 1

5.2.  RePIn VLCH Sub-Types

   IANA is requested to create a sub-registry under the "NSH IETF-
   Assigned Optional Variable-Length Metadata Types" registry as
   follows:

   Name: Sub-TLVs under the Type TBD Variable Length Context Header

   Registration Procedure: Expert Review

   Reference: [this document]



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          +==========+========================+=================+
          | Sub-Type | Description            | Reference       |
          +==========+========================+=================+
          |        0 | reserved               | [this document] |
          +----------+------------------------+-----------------+
          |        1 | Packet Identifier      | [this document] |
          +----------+------------------------+-----------------+
          |        2 | Packet Extent Modified | [this document] |
          +----------+------------------------+-----------------+
          |        3 | Saved Metadata         | [this document] |
          +----------+------------------------+-----------------+
          |        4 | Saved TTL              | [this document] |
          +----------+------------------------+-----------------+
          |    5-254 | unassigned             | [this document] |
          +----------+------------------------+-----------------+
          |      255 | reserved               | [this document] |
          +----------+------------------------+-----------------+

                                  Table 2

6.  Security Consideration

   For general SFC and NSH security considerations, see [RFC8300].

   The following should be considered:

   *  Intehrity protection of metadata influencing forwarding or
      recombination.

   *  Replay of packets on one or more paths between a DP and an RP.

   *  Risks due to limited RP buffer capacity such as buffer exhaustion
      or behavior on missing packets.

   *  Potention use of RePIn as a covert channel.

7.  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>.

   [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>.





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   [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>.

8.  Informative References

   [Liaison]  "LS on recent service function chaining related
              developments in Q4/SG11: two new draft Supplements",
              ITU-T SG11-LS-179, March 2021,
              <https://datatracker.ietf.org/liaison/1736/>.

   [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>.

   [RFC8459]  Dolson, D., Homma, S., Lopez, D., and M. Boucadair,
              "Hierarchical Service Function Chaining (hSFC)", RFC 8459,
              DOI 10.17487/RFC8459, September 2018,
              <https://www.rfc-editor.org/info/rfc8459>.

Appendix A.  Relation to Hierarchical SFC

   Experimental [RFC8459] describes "Hierarchical SFC" in which SFs in a
   higher level SFP can be entire lower level SFPs with a different SPI
   and where the higher level SPI is restored at the end of the lower
   level SFP.  This is similar to a diversion in this document.  The
   Internal Boundary Nodes (IBNs) in [RFC8459] that transition an SFC
   packet between the higher and lower levels are similar to DPs/RPs in
   the terminology of this document.

   Experimental [RFC8459] discusses a wide variety of mechanisms to
   implement Hierarchical SFC while this document looks toward
   specifying a more specific set of mechanisms as a Proposed Standard
   to support parallelism and other types of diversions.

Author's Address

   Donald E. Eastlake 3rd
   Independent
   2386 Panoramic Circle
   Apopka, Florida 32703
   United States of America
   Phone: +1-508-333-2270
   Email: d3e3e3@gmail.com





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