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SPRING Working Group                                           R. Bonica
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                               Y. Kamite
Expires: January 2, 2020                  NTT Communications Corporation
                                                               A. Alston
                                                            D. Henriques
                                                          Liquid Telecom
                                                              J. Halpern
                                                                Ericsson
                                                              J. Linkova
                                                                  Google
                                                            July 1, 2019


                IPv6 Support for Segment Routing: SRv6+
                    draft-bonica-spring-srv6-plus-00

Abstract

   This document describes SRv6+. SRv6+ is a Segment Routing (SR)
   solution that leverages IPv6.  It supports a wide variety of use-
   cases while remaining in strict compliance with IPv6 specifications.
   SRv6+ is optimized for for ASIC-based forwarding devices that operate
   at high data rates.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 2, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Paths, Segments And Instructions  . . . . . . . . . . . . . .   4
   4.  Segment Types . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Strictly Routed . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Loosely Routed  . . . . . . . . . . . . . . . . . . . . .   7
   5.  Segment Identifiers (SID) . . . . . . . . . . . . . . . . . .   8
     5.1.  Range . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Assigning SIDs to Strictly Routed Segments  . . . . . . .  10
     5.3.  Assigning SIDs to Loosely Routed Segments . . . . . . . .  10
   6.  Service Instructions  . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Per-Segment . . . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Per-Path  . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  The IPv6 Data Plane . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  The Routing Header  . . . . . . . . . . . . . . . . . . .  12
     7.2.  The Destination Options Header  . . . . . . . . . . . . .  13
   8.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Differences Between SRv6 and SRv6+  . . . . . . . . . . . . .  14
     9.1.  Routing Header Size . . . . . . . . . . . . . . . . . . .  14
     9.2.  Decoupling of Topological and Service Instructions  . . .  15
     9.3.  Authentication  . . . . . . . . . . . . . . . . . . . . .  16
     9.4.  Traffic Engineering Capability  . . . . . . . . . . . . .  16
     9.5.  IP Addressing Architecture  . . . . . . . . . . . . . . .  17
   10. Compliance  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   11. Operational Considerations  . . . . . . . . . . . . . . . . .  18
     11.1.  Ping and Traceroute  . . . . . . . . . . . . . . . . . .  18
     11.2.  ICMPv6 Rate Limitting  . . . . . . . . . . . . . . . . .  18
     11.3.  SID Lengths And SID Length Transitions . . . . . . . . .  18
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     15.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22




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1.  Overview

   Network operators deploy Segment Routing (SR) [RFC8402] so that they
   can forward packets through SR paths.  An SR path provides
   unidirectional connectivity from its ingress node to its egress node.
   While an SR path can follow the least cost path from ingress to
   egress, it can also follow any other path.

   An SR path contains one or more segments.  A segment provides
   unidirectional connectivity from its ingress node to its egress node.
   It includes a topological instruction that controls its behavior.

   The topological instruction is executed on the segment ingress node.
   It determines the segment egress node and the method by which the
   segment ingress node forwards packets to the segment egress node.

   Per-segment service instructions can augment a segment.  Per-segment
   service instructions, if present, are executed on the segment egress
   node.

   Likewise, a per-path service instruction can augment a path.  The
   per-path service instruction, if present, is executed on the path
   egress node.  Section 3 of this document illustrates the relationship
   between SR paths, segments and instructions.

   A Segment Identifier (SID) identifies each segment.  Because there is
   a one-to-one mapping between segments and the topological
   instructions that control them, the SID that identifies a segment
   also identifies the topological instruction that controls it.

   A SID is different from the topological instruction that it
   identifies.  While a SID identifies a topological instruction, it
   does not contain the topological instruction that it identifies.
   Therefore, a SID can be encoded in relatively few bits, while the
   topological instruction that it identifies may require many more bits
   for encoding.

   An SR path can be represented by its ingress node as an ordered
   sequence of SIDs.  In order to forward a packet through an SR path,
   the SR ingress node encodes the SR path into the packet as an ordered
   sequence of SIDs.  It can also augment the packet with service
   instructions.

   Because the SR ingress node is also the first segment ingress node,
   it executes the topological instruction associated with the first
   segment.  This causes the packet to be forwarded to the first segment
   egress node.  When the first segment egress node receives the packet,




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   it executes any per-segment service instructions that augment the
   first segment.

   If the SR path contains exactly one segment, the first segment egress
   node is also the path egress node.  In this case, that node executes
   any per-path service instruction that augments the path, and SR
   forwarding is complete.

   If the SR path contains multiple segments, the first segment egress
   node is also the second segment ingress node.  In this case, that
   node executes the topological instruction associated with the second
   segment.  The above-described procedure continues until the packet
   arrives at the SR egress node.

   In the above-described procedure, only the SR ingress node maintains
   path information.  Segment ingress and egress nodes maintain
   information regarding the segments in which they participate, but
   they do not maintain path information.

   The SR architecture, described above, can leverage either an MPLS
   [RFC3031] data plane or an IPv6 [RFC8200] data plane.  SR-MPLS
   [I-D.ietf-spring-segment-routing-mpls] leverages MPLS.  SRv6
   [I-D.ietf-spring-srv6-network-programming]
   [I-D.ietf-6man-segment-routing-header] leverages IPv6.

   This document describes SRv6+. SRv6+ is another SR variant that
   leverages IPv6.  It supports a wide variety of use-cases while
   remaining in strict compliance with IPv6 specifications.  SRv6+ is
   optimized for ASIC-based forwarding devices that operate at high data
   rates.  Section 9 of this document highlights differences between
   SRv6 and SRv6+.

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.

3.  Paths, Segments And Instructions

   An SRv6+ path is determined by the segments that it contains.  It can
   be represented by its ingress node as an ordered sequence of SIDs.

   A segment is determined by its ingress node and by the topological
   instruction that controls its behavior.  The topological instruction




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   determines the segment egress node and the method by which the
   segment ingress node forwards packets to the segment egress node.

   Per-segment service instructions augment, but do not determine,
   segments.  A segment ingress node can:

   o  Send one packet through a segment with one per-segment service
      instruction.

   o  Send another packet through the same segment with a different per-
      segment service instruction.

   o  Send another packet through the same segment without any per-
      segment service instructions.

   Likewise, per-path service instructions augment, but do not
   determine, paths.

     ----      ----      ----      ----      ----      ----
    |Node|----|Node|----|Node|----|Node|----|Node|----|Node|
    | A  |    | B  |    | C  |    | D  |    | E  |    | F  |
     ----      ----      ----      ----      ----      ----
       |                   |         |                   |
        -------------------|         |-------------------|
           Segment A-C     |---------|    Segment D-F
                           Segment C-D
       |                                                 |
        -------------------------------------------------
                             SRv6+ Path

                Figure 1: Paths, Segments And Instructions

   Figure 1 depicts an SRv6+ path.  The path provides unidirectional
   connectivity from its ingress node (i.e., Node A) to its egress node
   (i.e., Node F).  It contains Segment A-C, Segment C-D and Segment
   D-F.

   In Segment A-C, Node A is the ingress node, Node B is a transit node,
   and Node C is the egress node.  Therefore, the topological
   instruction that controls the segment is executed on Node A, while
   per-segment service instructions that augment the segment (if any
   exist) are executed on Node C.

   In Segment C-D, Node C is the ingress node and Node D is the egress
   node.  Therefore, the topological instruction that controls the
   segment is executed on Node C, while per-segment service instructions
   that augment the segment (if any exist) are executed on Node D.




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   In Segment D-F, Node D is the ingress node, Node E is a transit node,
   and Node F is the egress node.  Therefore, the topological
   instruction that controls the segment is executed on Node D, while
   per-segment service instructions that augment the segment (if any
   exist) are executed on Node F.

   Node F is also the path egress node.  Therefore, if a per-path
   service instruction augments the path, it is executed on Node F.

   Segments A-C, C-D and D-F are also contained by other paths that are
   not included in the figure.

4.  Segment Types

   SRv6+ supports the following segment types:

   o  strictly routed

   o  loosely routed

   Strictly routed segments forward packets through a specified link
   that connects the segment ingress node to the segment egress node.
   Loosely routed segments forward packets through the least cost path
   from the segment ingress node to the segment egress node.

   Each segment type is described below.

4.1.  Strictly Routed

   When a packet is submitted to a strictly routed segment, the
   topological instruction associated with that segment operates upon
   the packet.  The topological instruction executes on the segment
   ingress node and accepts the following parameters:

   o  An IPv6 address that identifies an interface on the segment egress
      node.

   o  A primary interface identifier.

   o  Zero or more secondary interface identifiers.

   The topological instruction behaves as follows:

   o  If none of the interfaces identified by the above-mentioned
      parameters are operational, discard the packet and send an ICMPv6
      [RFC4443] Destination Unreachable message (Code: 5, Source Route
      Failed) to the packet's source node.




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   o  Decrement the packet's Hop Count.

   o  If the Hop Count has expired, discard the packet and send an
      ICMPv6 Time Expired message to the packet's source node.

   o  Overwrite the packet's Destination Address with the IPv6 address
      that was received as a parameter.

   o  If the primary interface is active, forward the packet through the
      primary interface.

   o  If the primary interface is not active and any of the secondary
      interfaces are active, forward the packet through one of the
      secondary interfaces.  Execute procedures so that all packets
      belonging to a flow are forwarded through the same secondary
      interface.

4.2.  Loosely Routed

   When a packet is submitted to a loosely routed segment, the
   topological instruction associated with that segment operates upon
   the packet.  The topological instruction executes on the segment
   ingress node and accepts an IPv6 address as a parameter.  The IPv6
   address identifies an interface on the segment egress node.

   The topological instruction behaves as follows:

   o  If the segment ingress node does not have a viable route to the
      IPv6 address included as a parameter, discard the packet and send
      an ICMPv6 Destination Unreachable message (Code:1 Net Unreachable)
      to the packet's source node.

   o  Decrement the packet's Hop Count.

   o  If the Hop Count has expired, discard the packet and send an
      ICMPv6 Time Expired message to the packet's source node.

   o  Overwrite the packet's Destination Address with the destination
      address that was included as a parameter.

   o  Forward the packet to the next hop along the least cost path the
      segment egress node.  If there are multiple least cost paths to
      the segment egress node (i.e., Equal Cost Multipath), execute
      procedures so that all packets belonging to a flow are forwarded
      through the same next hop.






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5.  Segment Identifiers (SID)

   A Segment Identifier (SID) is an unsigned integer that identifies a
   segment.  Because there is a one-to-one mapping between segments and
   the topological instructions that control them, the SID that
   identifies a segment also identifies the topological instruction that
   controls it.

   A SID is different from the topological instruction that it
   identifies.  While a SID identifies a topological instruction, it
   does not contain the topological instruction that it identifies.
   Therefore, a SID can be encoded in relatively few bits, while the
   topological instruction that it identifies may require many more bits
   for encoding.

   SIDs have node-local significance.  This means that a segment ingress
   node MUST identify each segment that it originates with a unique SID.
   However, a SID that is used by one segment ingress node to identify a
   segment that it originates can be used by another segment ingress
   node to identify another segment.

   Although SIDs have node-local significance, an SRv6+ path can be
   uniquely identified by its ingress node and an ordered sequence of
   SIDs.  This is because the topological instruction associated with
   each segment determines the ingress node of the next segment (i.e.,
   the node upon which the next SID has significance.)

   Although SIDs have node-local significance, they can be assigned in a
   manner that facilitates debugging.  See Section 5.2 and Section 5.3
   for details.

5.1.  Range

   SID values range from 0 to a configurable Maximum SID Value (MSV).
   The values 0 through 15 are reserved for future use.  The following
   are valid MSVs:

   o  65,535 (i.e., 2**16 minus 1)

   o  4,294,967,295 (i.e., 2**32 minus 1)

   In order to optimize packet encoding (Section 7.1), network operators
   can configure all nodes within an SRv6+ domain to have the smallest
   feasible MSV.  The following paragraphs explain how an operator
   determines the smallest feasible MSV.

   Consider an SRv6+ domain that contains 5,000 nodes connected to one
   another by point-to-point infrastructure links.  The network topology



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   is not a full-mesh.  In fact, each node supports 200 point-to-point
   infrastructure links or fewer.  Given this SRv6+ domain, we will
   determine the smallest feasible MSV under the following conditions:

   o  The SRv6+ domain contains strictly routed segments only.

   o  The SRv6+ domain contains loosely routed segments only.

   o  The SRv6+ domain contains both strictly and loosely routed
      segments.

   If an SRv6+ domain contains strictly routed segments only, and each
   node creates a strictly routed segment to each of its neighbors, each
   node will create 200 segments or fewer and consume 200 SIDs or fewer.
   This is because each node has 200 neighbors or fewer.  Because SIDs
   have node-local significance (i.e., they can be reused across nodes),
   the smallest feasible MSV is 65,535.

   Adding nodes to this SRv6+ domain will not increase the smallest
   feasible MSV, so long as each node continues to support 65,519 point-
   to-point infrastructure links or fewer.  If a single node is added to
   the domain and that node supports 240 infrastructure links, the
   smallest feasible MSV will increase to 65,535.

   If an SRv6+ domain contains loosely routed segments only, and every
   node creates a loosely routed segment to every other node, every node
   will create 4,999 segments and consume 4,999 SIDs.  This is because
   the domain contains 5,000 nodes.  Because SIDs have node-local
   significance (i.e., they can be reused across nodes), the smallest
   feasible MSV is 65,535.

   Adding nodes to this SRv6+ domain will not increase the smallest
   feasible MSV until the number of nodes exceeds 65,519.  When the
   smallest feasible MSV increases, it becomes 4,294,967,295.

   If an SRv6+ domain contains both strictly and loosely routed
   segments, each node will create 5,199 segments or fewer and consume
   5,199 SIDs or fewer.  This value is the sum of the following:

   o  The number of loosely routed segments that each node will create,
      given that every node creates a loosely routed segment to every
      other node (i.e., 4,999).

   o  The number of strictly routed segments that each node will create,
      given that each node creates a strictly routed segment to each of
      its neighbors (i.e., 200 or fewer).





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   Because SIDs have node-local significance (i.e., they can be reused
   across nodes), the smallest feasible MSV is 65,535.

   Adding nodes to this SRv6+ domain will not increase the smallest
   feasible MSV until the number of nodes plus the maximum number of
   infrastructure links per node exceeds 65,519.  When the smallest
   feasible MSV increases, it becomes 4,294,967,295.

5.2.  Assigning SIDs to Strictly Routed Segments

   Network operators can establish conventions by which they assign SIDs
   to strictly routed segments.  These conventions can facilitate
   debugging.

   For example, a network operator can reserved a range of SIDs for
   strictly routed segments.  It can further divide that range into
   subranges, so that all segments sharing a common egress node are
   identified by SIDs from the same subrange.

5.3.  Assigning SIDs to Loosely Routed Segments

   In order to facilitate debugging, all loosely routed segments that
   share a common egress node are identified by the same SID.  In order
   to maintain this discipline, network wide co-ordination is required.

   For example, assume that an SRv6+ domain contains N nodes.  Network
   administrators reserve a block of N SIDs and configure one of those
   SIDs on each node.  Each node advertises its SID into the control
   plane.  When another node receives that advertisement, it creates a
   loosely routed segment between itself and the advertising node.  It
   also associates the SID that it received in the advertisement with
   the newly created segment.  See [I-D.bonica-lsr-crh-isis-extensions]
   for details.

6.  Service Instructions

   SRv6+ supports the following service instruction types:

   o  Per-segment

   o  Per-path

   Each is described below.








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6.1.  Per-Segment

   Per-segment service instructions can augment a segment.  Per-segment
   service instructions, if present, are executed on the segment egress
   node.  Because the path egress node is also a segment egress node, it
   can execute per-segment service instructions.

   The following are examples of per-segment service instructions:

   o  Expose a packet to a firewall policy.

   o  Expose a packet to a sampling policy.

   Per-segment Service Instruction Identifiers identify a set of service
   instructions.  Per-segment Service Instruction Identifiers are
   allocated and distributed by a controller.  They have domain-wide
   significance.

6.2.  Per-Path

   A per-path service instruction can augment a path.  The per-path
   service instruction, if present, is executed on the path egress node.

   The following are examples of per-path service instructions:

   o  De-encapsulate a packet and forward its newly exposed payload
      through a specified interface.

   o  De-encapsulate a packet and forward its newly exposed payload
      using a specified routing table.

   Per-path Service Instruction Identifiers identify per-path service
   instructions.  Per-path Service Instruction Identifiers are allocated
   and distributed by the processing node (i.e., the path egress node).
   They have node-local significance.  This means that the path egress
   node MUST allocate a unique Per-path Service Instruction Identifier
   for each per-path service instruction that it instantiates.

7.  The IPv6 Data Plane

   SRv6+ ingress nodes generate IPv6 header chains that represent SRv6+
   paths.  An IPv6 header chain contains an IPv6 header.  It can also
   contain one or more extension headers.

   An extension header chain that represents an SRv6+ path can contain
   any valid combination of IPv6 extension headers.  The following
   bullet points describe how SRv6+ leverages IPv6 extension headers:




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   o  If an SRv6+ path contains multiple segments, the IPv6 header chain
      that represents it MUST contain a Routing header.  The SRv6+ path
      MUST be encoded in the Routing header as an ordered sequence of
      SIDs.

   o  If an SRv6+ path is augmented by a per-path service instruction,
      the IPv6 header chain that represents it MUST contain a
      Destination Options header.  The Destination Options header MUST
      immediately precede an upper-layer header and it MUST include a
      Per-Path Service Instruction Identifier.

   o  If an SRv6+ path contains a segment that is augmented by a per-
      segment service instruction, the IPv6 chain that represents it
      MUST contain a Routing header and a Destination Options header.
      The Destination Options header MUST immediately precede a Routing
      header and it MUST include the Per-Segment Service Instruction
      Identifier.

   The following subsections describe how SRv6+ uses the Routing header
   and the Destination Options header.

7.1.  The Routing Header

   SRv6+ defines a new Routing header type, called the Compressed
   Routing Header (CRH) [I-D.bonica-6man-comp-rtg-hdr].  The CRH
   contains the following fields:

   o  Next Header - Identifies the header immediately following the CRH.

   o  Hdr Ext Len - Length of the CRH.

   o  Routing Type - Identifies the Routing header variant (i.e., CRH)

   o  Segments Left - The number of segments still to be traversed
      before reaching the path egress node.

   o  Last Entry - Represents the index of the last element of the
      Segment List.

   o  Com (Compression) - Represents the length of each entry in the SID
      List.  Values are reserved (0), sixteen bits (1), thirty-two bits
      (2), and reserved (3).  In order to maximize header compression,
      this value should reflect the smallest feasible MSV (Section 5.1).

   o  SID List - Represents the SRv6+ path as an ordered list of SIDs.
      SIDs are listed in reverse order, with SID[0] representing the
      final segment, SID[1] representing the penultimate segment, and so
      forth.  SIDs are listed in reverse order so that Segments Left can



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      be used as an index to the SID List.  The SID indexed by Segments
      Left is called the current SID.

   As per [RFC8200], when an IPv6 node receives a packet, it examines
   the packet's destination address.  If the destination address
   represents an interface belonging to the node, the node processes the
   next header.  If the node encounters and recognizes the CRH, it
   processes the CRH as follows:

   o  If Segments Left equal 0, skip over the CRH and process the next
      header in the packet.

   o  Decrement Segments Left.

   o  Search for the current SID in a local table that maps SID's to
      topological instructions.  If the current SID cannot be found in
      that table, send an ICMPv6 Parameter Problem message to the
      packet's Source Address and discard the packet.

   o  Execute the topological instruction found in the table as
      described in Section 4.  This causes the packet to be forwarded to
      the segment egress node.

   When the packet arrives at the segment egress node, the above-
   described procedure is repeated.

7.2.  The Destination Options Header

   According to [RFC8200], the Destination Options header contains one
   or more IPv6 options.  It can occur twice within a packet, once
   before a Routing header and once before an upper-layer header.  The
   Destination Options header that occurs before a Routing header is
   processed by the first destination that appears in the IPv6
   Destination Address field plus subsequent destinations that are
   listed in the Routing header.  The Destination Options header that
   occurs before an upper-layer header is processed by the packet's
   final destination only.

   Therefore, SRv6+ defines the following new IPv6 options:

   o  The SRv6+ Per-Segment Service Instruction Option
      [I-D.bonica-6man-seg-end-opt]

   o  The SRv6+ Per-Path Service Instruction Option
      [I-D.bonica-6man-vpn-dest-opt]

   The SRv6+ Per-Segment Service Instruction Option is encoded in a
   Destination Options header that precedes the CRH.  Therefore, it is



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   processed by every segment egress node.  It includes a Per-Segment
   Service Instruction Identifier and causes segment egress nodes to
   execute per-segment service instructions.

   The SRv6+ Per-Path Service Instruction Option is encoded in a
   Destination Options header that precedes the upper-layer header.
   Therefore, it is processed by the path egress node only.  It includes
   a Per-Path Service Instruction Identifier and causes the path egress
   node to execute a per-path service instruction.

8.  Control Plane

   IS-IS extensions [I-D.bonica-lsr-crh-isis-extensions] have been
   defined for the following purposes:

   o  So that SRv6+ segment ingress nodes can flood information
      regarding strictly routed segments that they originate

   o  So that SRv6+ segment egress nodes can flood information regarding
      loosely routed segments that they terminate

   BGP extensions [RFC4271] are being defined so that SRv6+ path egress
   nodes can associated path-terminating service instructions with
   Network Layer Reachability Information (NLRI).

9.  Differences Between SRv6 and SRv6+

9.1.  Routing Header Size

   SRv6 defines a Routing header type, called the Segment Routing Header
   (SRH).  The SRH contains a field that represents the SRv6 path as an
   ordered sequence of SIDs.  Each SID contained by that field is 128
   bits long.

   Likewise, SRv6+ defines a Routing Header Type, called the Compressed
   Routing Header (CRH).  The CRH contains a field that represents the
   SRv6+ path as an ordered sequence of SIDs.  Within that field, SIDs
   can be 16 or 32 bits long.













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   +------+-------------------+-------------------+--------------------+
   | SIDs | SRv6 SRH (128-bit | SRv6+ CRH (16-bit | SRv6+ CRH (32-bit  |
   |      |        SID)       |        SID)       |        SID)        |
   +------+-------------------+-------------------+--------------------+
   |  1   |         24        |         16        |         16         |
   |  2   |         40        |         16        |         16         |
   |  3   |         56        |         16        |         24         |
   |  4   |         72        |         16        |         24         |
   |  5   |         88        |         24        |         32         |
   |  6   |        104        |         24        |         32         |
   |  7   |        120        |         24        |         40         |
   |  8   |        136        |         24        |         40         |
   |  9   |        152        |         32        |         48         |
   |  10  |        168        |         32        |         48         |
   |  11  |        184        |         32        |         56         |
   |  12  |        200        |         32        |         56         |
   |  13  |        216        |         40        |         64         |
   |  14  |        232        |         40        |         64         |
   |  15  |        248        |         40        |         72         |
   |  16  |        264        |         40        |         72         |
   +------+-------------------+-------------------+--------------------+

     Table 1: Routing Header Size (in Bytes) As A Function Of Routing
                      Header Type and Number Of SIDs

   Table 1 reflects Routing header size as a function of Routing header
   type and Number of SIDs contained by the Routing header.

   Large Routing headers are undesirable for the following reasons:

   o  Many ASIC-based forwarders copy the entire IPv6 extension header
      chain from buffer memory to on-chip memory.  As the size of the
      IPv6 extension header chain increases, so does the cost of this
      copy.

   o  Because Path MTU Discovery (PMTUD) [RFC8201] is not entirely
      reliable, many IPv6 hosts refrain from sending packets larger than
      the IPv6 minimum link MTU (i.e., 1280 bytes).  When packets are
      small, the overhead imposed by large Routing headers becomes
      pronounced.

9.2.  Decoupling of Topological and Service Instructions

   SRv6+ decouples topological instructions from service instructions.
   Topological instructions are invoked at the segment ingress node, as
   a result of CRH processing, while service instructions are invoked at
   the segment egress node, as a result of Destination Option




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   processing.  Therefore, network operators can use SRv6+ mechanisms to
   support topological instructions, service instructions, or both.

                  ----------           ----------          ----------
                 | Ethernet |         | Ethernet |        | Ethernet |
                  ----------           ----------          ----------
    Service      |  VXLAN   |         |  Dest    |        |   Dest   |
    Instruction   ----------          |          |        |          |
                 |   UDP    |         |  Option  |        |  Option  |
                  ----------           ----------          ----------
    Topological  |                    |          |        |          |
    Instructions |   CRH    |         |          |        |    CRH   |
                  ----------          |          |         ----------
                 |   IPv6   |         |   IPv6   |        |   IPv6   |
                  ----------           ----------          ----------

                  Option 1             Option 2             Option 3

                    Figure 2: EVPN Design Alternatives

   Figure 2 illustrates this point by depicting design options available
   to network operators offering Ethernet Virtual Private Network
   [RFC7432] services over Virtual eXtensible Local Area Network (VXLAN)
   [RFC7348].  In Option 1, the network operator encodes topological
   instructions in the CRH, while encoding service instructions in a
   VXLAN header.  In Option 2, the network operator encodes service
   instructions in a Destination Options header, while allowing traffic
   to traverse the least cost path between the ingress and egress
   Provider Edge (PE) routers.  In Option 3, the network operator
   encodes topological instructions in the CRH, and encodes service
   instructions in a Destination Options header.

9.3.  Authentication

   The IPv6 Authentication Header (AH) [RFC4302] can be used to
   authenticate SRv6+ packets.  However, AH processing is not defined in
   SRv6.

9.4.  Traffic Engineering Capability

   SRv6+ supports traffic engineering solutions that rely exclusively
   upon strictly routed segments.  For example, consider an SRv6+
   network whose diameter is 12 hops and whose minimum feasible MSV is
   65,525.  In that network, in the worst case, SRv6+ overhead is 72
   bytes (i.e., a 40-byte IPv6 header and a 32-byte CRH).

   SRv6 also supports traffic engineering solutions that rely
   exclusively upon strictly routed segments (i.e., END.X SIDs).



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   However, SRv6 overhead may be prohibitive.  For example, consider an
   SRv6 network whose diameter is 12 hops.  In the worst case, SRv6
   overhead is 240 bytes (i.e., a 40 byte IPv6 header and a 200-byte
   SRH).

9.5.  IP Addressing Architecture

   In SRv6, an IPv6 address can represent either of the following:

   o  A network interface

   o  An instruction instantiated on a node (i.e., an SRv6 SID)

   In SRv6+ an IPv6 address always represents a network interface, as
   per [RFC4291].

10.  Compliance

   In order to be compliant with this specification, an SRv6+
   implementation MUST:

   o  Be able to process IPv6 options as described in Section 4.2 of
      [RFC8200].

   o  Be able to process the Routing header as described in Section 4.4
      of [RFC8200].

   o  Be able to process the Destination Options header as described in
      Section 4.6 of [RFC8200].

   o  Recognize the CRH.

   o  Be able to encode an SRv6+ path in the CRH as an ordered sequence
      of 32-bit SIDs.

   o  Be able to process a CRH that includes 32-bit SIDs.

   Additionally, an SRv6+ implementation MAY:

   o  Be able to encode an SRv6+ path in the CRH as an ordered sequence
      of 16-bit SIDs.

   o  Be able to process a CRH that includes 16-bit SIDs.

   o  Recognize the Per-Segment Service Instruction Option.

   o  Recognize the Per-Path Service Instruction Option.




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11.  Operational Considerations

11.1.  Ping and Traceroute

   Ping and Traceroute [RFC2151] both operate correctly in SRv6+ (i.e.,
   in the presence of the CRH).

11.2.  ICMPv6 Rate Limitting

   As per [RFC4443], SRv6+ nodes rate limit the ICMPv6 messages that
   they emit.

11.3.  SID Lengths And SID Length Transitions

   An SRv6+ implementation MAY include a configuration option that
   determines how it encodes SIDs (i.e., in 16 or 32 bits).  In order to
   reduce operational complexity, network operators typically configure
   their networks so that every node encodes SIDs identically.

   As a network grows, its minimum feasible MSV may increase.  In this
   case, the network may need to migrate from one SID encoding to
   another.  The following bullet points describe a migration strategy
   for an SRv6+ network that is migrating from 16-bit SIDs to 32-bit
   SIDs:.

   o  Ensure that all nodes can process a CRH that includes 32-bit SIDs.

   o  Configure each nodes so that encodes SIDs in 32-bits.

   o  Configure SIDs whose value exceeds 65,535.

12.  IANA Considerations

   SID values 0-15 are reserved for future use.  They may be assigned by
   IANA, based on IETF Consensus.

   IANA is requested to establish a "Registry of SRv6+ Reserved SIDs".
   Values 0-15 are reserved for future use.

13.  Security Considerations

   SRv6+ domains MUST NOT span security domains.  In order to enforce
   this requirement, security domain edge routers MUST do one of the
   following:

   o  Discard all inbound SRv6+ packets

   o  Authenticate [RFC4302] [RFC4303] all inbound SRv6+ packets



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14.  Acknowledgements

   The authors wish to acknowledge Dr. Vanessa Ameen and John Scudder.

15.  References

15.1.  Normative References

   [I-D.bonica-6man-comp-rtg-hdr]
              Bonica, R., Kamite, Y., Niwa, T., Alston, A., Henriques,
              D., So, N., Xu, F., Chen, G., Zhu, Y., Yang, G., and Y.
              Zhou, "The IPv6 Compressed Routing Header (CRH)", draft-
              bonica-6man-comp-rtg-hdr-04 (work in progress), May 2019.

   [I-D.bonica-6man-seg-end-opt]
              Bonica, R., Halpern, J., So, N., Xu, F., Chen, G., Zhu,
              Y., Yang, G., and Y. Zhou, "The IPv6 Segment Endpoint
              Option", draft-bonica-6man-seg-end-opt-03 (work in
              progress), March 2019.

   [I-D.bonica-6man-vpn-dest-opt]
              Bonica, R., Lenart, C., So, N., Xu, F., Presbury, G.,
              Chen, G., Zhu, Y., Yang, G., and Y. Zhou, "The IPv6
              Virtual Private Network (VPN) Context Information Option",
              draft-bonica-6man-vpn-dest-opt-05 (work in progress),
              March 2019.

   [I-D.bonica-lsr-crh-isis-extensions]
              Kaneriya, P., Shetty, R., Hegde, S., and R. Bonica, "IS-IS
              Extensions To Support The IPv6 Compressed Routing Header
              (CRH)", draft-bonica-lsr-crh-isis-extensions-00 (work in
              progress), May 2019.

   [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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.





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   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [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>.

   [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>.

15.2.  Informative References

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and d. daniel.voyer@bell.ca, "IPv6 Segment
              Routing Header (SRH)", draft-ietf-6man-segment-routing-
              header-21 (work in progress), June 2019.

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-22
              (work in progress), May 2019.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J.,
              daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6
              Network Programming", draft-ietf-spring-srv6-network-
              programming-00 (work in progress), April 2019.

   [RFC2151]  Kessler, G. and S. Shepard, "A Primer On Internet and TCP/
              IP Tools and Utilities", FYI 30, RFC 2151,
              DOI 10.17487/RFC2151, June 1997,
              <https://www.rfc-editor.org/info/rfc2151>.







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   [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>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <https://www.rfc-editor.org/info/rfc4761>.

   [RFC6624]  Kompella, K., Kothari, B., and R. Cherukuri, "Layer 2
              Virtual Private Networks Using BGP for Auto-Discovery and
              Signaling", RFC 6624, DOI 10.17487/RFC6624, May 2012,
              <https://www.rfc-editor.org/info/rfc6624>.

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
              <https://www.rfc-editor.org/info/rfc7348>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.








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Authors' Addresses

   Ron Bonica
   Juniper Networks
   Herndon, Virginia  20171
   USA

   Email: rbonica@juniper.net


   Yuji Kamite
   NTT Communications Corporation
   3-4-1 Shibaura, Minato-ku
   Tokyo  108-8118
   Japan

   Email: : y.kamite@ntt.com


   Andrew Alston
   Liquid Telecom
   Nairobi
   Kenya

   Email: Andrew.Alston@liquidtelecom.com


   Daniam Henriques
   Liquid Telecom
   Johannesburg
   South Africa

   Email: daniam.henriques@liquidtelecom.com


   Joel Halpern
   Ericsson
   P. O. Box 6049
   Leesburg, Virginia  20178
   USA

   Email: joel.halpern@ericsson.com









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   Jen Linkova
   Google
   Mountain View, California  94043
   USA

   Email: furry@google.com













































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