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Versions: 00 01 02 03 04 05 06 07                                       
SPRING                                                          W. Cheng
Internet-Draft                                              China Mobile
Intended status: Informational                                    C. Xie
Expires: January 10, 2022                                  China Telecom
                                                               R. Bonica
                                                                 Juniper
                                                                D. Dukes
                                                           Cisco Systems
                                                                   C. Li
                                                                  Huawei
                                                               P. Shaofu
                                                                     ZTE
                                                           W. Henderickx
                                                                   Nokia
                                                           July 09, 2021


                 Compressed SRv6 SID List Requirements
            draft-srcompdt-spring-compression-requirement-07

Abstract

   This document specifies requirements for solutions to compress SRv6
   SID lists.

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 10, 2022.

Copyright Notice

   Copyright (c) 2021 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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     2.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  SRv6 SID List Compression Requirements  . . . . . . . . . . .   4
     3.1.  Dataplane Efficiency and Performance Requirements . . . .   4
       3.1.1.  Encapsulation Header Size . . . . . . . . . . . . . .   5
       3.1.2.  Forwarding Efficiency . . . . . . . . . . . . . . . .   5
       3.1.3.  State Efficiency  . . . . . . . . . . . . . . . . . .   6
   4.  SRv6 Specific Requirements  . . . . . . . . . . . . . . . . .   6
     4.1.  SRv6 Based  . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Functional Requirements . . . . . . . . . . . . . . . . .   7
       4.2.1.  SRv6 Functionality  . . . . . . . . . . . . . . . . .   7
       4.2.2.  Heterogeneous SID lists . . . . . . . . . . . . . . .   8
       4.2.3.  SID list length . . . . . . . . . . . . . . . . . . .   8
       4.2.4.  SID summarization . . . . . . . . . . . . . . . . . .   8
     4.3.  Operational Requirements  . . . . . . . . . . . . . . . .   9
       4.3.1.  Lossless Compression  . . . . . . . . . . . . . . . .   9
       4.3.2.  Preservation of non-routing information . . . . . . .   9
       4.3.3.  Address Planning  . . . . . . . . . . . . . . . . . .  10
     4.4.  Scalability Requirements  . . . . . . . . . . . . . . . .  10
       4.4.1.  Adjacency segment scale . . . . . . . . . . . . . . .  10
       4.4.2.  Prefix segment scale  . . . . . . . . . . . . . . . .  11
       4.4.3.  Service Scale . . . . . . . . . . . . . . . . . . . .  11
       4.4.4.  Compression Levels  . . . . . . . . . . . . . . . . .  11
   5.  Protocol Design Requirements  . . . . . . . . . . . . . . . .  11
     5.1.  SRv6 Base Coexistence . . . . . . . . . . . . . . . . . .  11
   6.  Security Requirements . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Security Mechanisms . . . . . . . . . . . . . . . . . . .  12
     6.2.  SR Domain Protection  . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  13
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  13
   Appendix A.  Proposed Requirements  . . . . . . . . . . . . . . .  14
     A.1.  IPv6 Based  . . . . . . . . . . . . . . . . . . . . . . .  14



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     A.2.  Point to Multipoint . . . . . . . . . . . . . . . . . . .  15
     A.3.  Parsability . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The SPRING working group defined SRv6, with [RFC8402] describing how
   the Segment Routing (SR) architecture is instantiated on two data-
   planes: SR over MPLS (SR-MPLS) and SR over IPv6 (SRv6).  SRv6 uses a
   routing header called the SR Header (SRH) [RFC8754] and defines SRv6
   SID behaviors and a registry for identifying them in [RFC8986].  SRv6
   is a proposed standard and is deployed today.

   The SPRING working group has observed that some use cases, such as
   strict path TE, may require long SRv6 SID lists.  There are several
   proposed methods to reduce the resulting SRv6 encapsulation size by
   compressing the SID list.

   The SPRING working group formed a design team to define requirements
   for, and analyze proposals to, compress SRv6 SID lists.

   It is a goal of the design team to identify solutions to SRv6 SID
   list compression that are based on the SRv6 standards.  As such, this
   document provides requirements for SRv6 SID list compression
   solutions that utilize the existing SRv6 data plane and control
   plane.

   It is also a goal of the design team to consider proposals that are
   not based on the SRv6 data plane and control plane.  As such, this
   document includes requirements to evaluate whether a compression
   proposal provides all the functionality of SRv6 (section "SRv6
   Functionality") in addition to satisfying compression specific
   requirements.

   For each requirement, a description, rationale and metrics are
   described.

   The design team will produce a separate document to analyze the
   proposals.

   This document is a draft; additional requirements are under review,
   additional requirements will be added, and current requirements may
   change.  Appendix A contains a subset of requirements without
   unanimous consensus.  Additional requirements without unanimous
   consensus are not in the appendix.






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2.  Conventions used in this document

2.1.  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.

2.2.  Terminology

   SR: Segment Routing

   SRH: Segment Routing Header

   MPLS: Multiprotocol Label Switching

   SR-MPLS: Segment Routing over MPLS data plane

   SID: Segment Identifier

   SRv6: Segment Routing over IPv6

   SRv6 SID List: A list of SRv6 SIDs

   Compression proposal: A proposal to compress SRv6 SID lists

   SRv6 base: SRv6 as defined in [RFC8402], [RFC8754], [RFC8986]

   SID numbering space: may be implemented as

   o  a single IGP instance
   o  a single IGP level or area
   o  two or more autonomous systems that coordinate SID numbering space
   o  two or more IGP instances that coordinate SID numbering space

   SRv6 Encapsulation Header: The IPv6 header, and any extension headers
   preceding a payload, used to implement a SRv6 base or compression
   proposal.

3.  SRv6 SID List Compression Requirements

3.1.  Dataplane Efficiency and Performance Requirements







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3.1.1.  Encapsulation Header Size

   Description: The compression proposal MUST reduce the size of the
   SRv6 encapsulation header.

   Rationale: A smaller SRv6 encapsulation results in better MTU
   efficiency.

   Metric: Compression is the ratio of the IPv6 encapsulation size of
   SRv6 as defined in [RFC8402], [RFC8754], [RFC8986] vs the IPv6
   encapsulation size of a given proposal.  The encapsulation savings of
   a compression proposal vs the SRv6 base is a useful measurement to
   compare proposals.

   The encapsulation metric (E) records the number of bytes required for
   a proposal to encapsulate a packet given a specific segment list.

   o  E(proposal, segment list).

   The encapsulation savings(ES)records the encapsulation savings for a
   proposal to encapsulate a packet given a specific segment list.

   o  ES(proposal, segment list) = 1 - E(proposal, segment list)/E(SRv6
      base, segment list).

3.1.2.  Forwarding Efficiency

   Description: The compression proposal SHOULD minimize the number of
   required hardware resources accessed to process a segment.

   Rationale: Efficiency in bits on the wire and processing efficiency
   are both important.  Optimizing one at the expense of the other may
   lead to significant performance impact.

   Metric: The data plane efficiency metric (D) records the data plane
   forwarding efficiency of the proposed solution.  Two metrics are used
   and recorded at each segment endpoint:

   o  D.PRS(segment list): number of headers parsed during processing of
      the segment list, starting from and including the IPv6 header.
   o  D.LKU(segment list): number of FIB lookups during processing of
      the segment list.  The type of lookup is also recorded as longest
      prefix match (LPM) or exact match (EM)








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3.1.3.  State Efficiency

   Description: The compression proposal SHOULD minimize the amount of
   additional forwarding state stored at a node.

   Rationale: Additional state increases the complexity of the control
   plane and data plane.  It can also result in an increase in memory
   usage.

   Metric: The state efficiency metric (S) records the amount of
   additional forwarding state required by the proposed solution.

   o  S(node parameters): the number of additional forwarding states
      that need to be stored at a node, given a set of node parameters
      consisting of the number of nodes in the network, number of local
      interfaces, number of adjacencies.  The forwarding state is
      counted as entries required in a Forwarding Information Base (FIB)
      at a node.

4.  SRv6 Specific Requirements

4.1.  SRv6 Based

   Description: A solution to compress SRv6 SID Lists SHOULD be based on
   the SRv6 architecture, control plane and data plane.  The compression
   solution MAY be based on a different data plane and control plane,
   provided that it derives sufficient benefit.

   Rationale: A compression proposal built on existing IETF standards is
   preferable to creating new standards with equivalent functionality
   and performance.

   Metric: The utilization metric (U) records whether a proposal
   utilizes the SRv6 specifications.

   Utilization is recorded in a table, with a column per proposal and
   rows consisting of the following metrics:

   o  U.RFC8402: utilizes [RFC8402].
   o  U.RFC8754: utilizes [RFC8754].
   o  U.PGM: utilizes [RFC8986].
   o  U.IGP: utilizes [I-D.ietf-lsr-isis-srv6-extensions].
   o  U.BGP: utilizes [I-D.ietf-bess-srv6-services].
   o  U.POL: utilizes [I-D.ietf-spring-segment-routing-policy].
   o  U.BLS: utilizes [I-D.ietf-idr-bgpls-srv6-ext].
   o  U.SVC: utilizes [I-D.ietf-spring-sr-service-programming].
   o  U.OAM: utilizes [I-D.ietf-6man-spring-srv6-oam].
   o  U.ALG: utilizes [I-D.ietf-lsr-flex-algo].



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   Each cell contains "yes" for utilizes, or "no" for does not utilize.

4.2.  Functional Requirements

4.2.1.  SRv6 Functionality

   Description: A solution to compress an SRv6 SID list MUST support the
   functionality of SRv6.  This requirement ensures no SRv6
   functionality is lost.  It is particularly important to understand
   how a proposal, as evaluated in section "SRv6 Based", provides this
   functionality.

   Rationale: Operators require SRv6 functionality.  Evaluating the
   extent to which a proposal supports SRv6 functionality is important
   for operators and implementors to understand the impact on network
   operations.

   Metric: The Functionality metric (F) records whether a proposal
   supports SRv6 functionality and how the functionality is provided.

   Functionality is recorded in a table with columns for each proposal
   and rows consisting of the following metrics:

   o  F.SID: Supports SRv6 SID functionality as described in [RFC8402]
   o  F.SCOPE: Supports globally and locally scoped SID functionality as
      described in [RFC8402]
   o  F.PFX: Supports prefix SID functionality as described in [RFC8402]
      and [RFC8986]
   o  F.ADJ: Supports adjacency SID functionality as described in
      [RFC8402] and [RFC8986]
   o  F.BIND: Supports binding SID functionality as described in
      [RFC8402] and [RFC8986]
   o  F.PEER: Supports BGP peering SID functionality as described in
      [RFC8402] and [RFC8986]
   o  F.SVC: Supports L3 and L2 VPN service SID functionality as
      described in [RFC8986]
   o  F.ALG: Supports flexible algorithms functionality as described in
      [I-D.ietf-lsr-flex-algo]
   o  F.TILFA: Supports TI-LFA functionality as described in
      [I-D.ietf-rtgwg-segment-routing-ti-lfa]
   o  F.SEC: Supports securing an SR domain with ingress filtering as
      functionally defined in [RFC8754]
   o  F.IGP: Supports distributing topological SIDs and behaviors via
      ISIS as functionally described in
      [I-D.ietf-lsr-isis-srv6-extensions]
   o  F.BGP: Supports BGP VPNs as functionally described in
      [I-D.ietf-bess-srv6-services]




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   o  F.POL: Supports SR policies and steering traffic over those
      policies as funcitonally described in
      [I-D.ietf-spring-segment-routing-policy]
   o  F.BLS: Supports Link State distribution via BGP as functionally
      described in [I-D.ietf-idr-bgpls-srv6-ext]
   o  F.SFC: Supports stateless service programming as functionally
      described in [I-D.ietf-spring-sr-service-programming]
   o  F.PING: Supports pinging a SID to verify the SID is implemented as
      functionally described in [I-D.ietf-6man-spring-srv6-oam]

   Each cell contains the specification name documenting the
   functionality.

4.2.2.  Heterogeneous SID lists

   Description: The compression proposal SHOULD support a combination of
   compressed and non-compressed segments in a single path.  As an
   example, a solution may satisfy this requirement without being SRv6
   based by using a binding SID to impose an additional SRv6 header
   (IPv6 header plus optional SRH) with non-compressed SID.

   Rationale: Support of SID lists with compressed and non-compressed
   SIDs reduces encapsulation size when not all SRv6 nodes deploy the
   compression proposal or 128-bit SIDs are required.

   Metric: A compliant compression proposal supports both:

   o  classic 128-bit SRv6 SIDs in the IPv6 Destination Address field
   o  segment lists (i.e., paths) with both compressed and 128-bit SRv6
      SIDs.

4.2.3.  SID list length

   Description: The compression proposal MUST be able to represent SR
   paths that contain up to 16 segments.

   Rationale: Strict TE paths require SID list lengths proportional to
   the diameter of the SR domain.

   Metric: The compression proposal must be able to steer a packet
   through an SR path that contains up to sixteen segments.

4.2.4.  SID summarization

   Description: The solution MUST be compatible with segment
   summarization.





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   Rationale: Summarization of segments is a key benefit of SRv6 vs SR
   MPLS.  In interdomain deployments, any node can reach any other node
   via a single prefix segment.  Without summarization, border router
   SIDs must be leaked, and an additional global prefix segment is
   required for each domain border to be traversed.

   Metric: A solution supports summarization when segments can be
   summarized for advertisement into other IGP domains or levels.

4.3.  Operational Requirements

4.3.1.  Lossless Compression

   Description: A path traversed using a compessed SID list MUST always
   be the same as the path traversed using the uncompressed SID list if
   no compression was applied.

   Rationale: In SRv6, we can represent a path to meet certain
   objectives.  A compression proposal needs to support the objectives
   with the same path.

   Metric: Information present in the pre-compression segment list MUST
   also be present in the post-compression SID list.

4.3.2.  Preservation of non-routing information

   Description:The compression mechanism MUST NOT cause the loss of non-
   routing information when delivering a packet from the SR ingress node
   to the egress/penultimate SR node

   Rationale: SRv6 ingress nodes encode non-routing information in the
   IPv6 header chain.  This information can be encoded in the following
   fields:

   o  Hop Count
   o  DSCP bits
   o  ECN bits
   o  Flow label
   o  HBH Options Extension header
   o  Fragment Extension header
   o  Authentication Extension header
   o  Encrypted Security Payload Extension header
   o  Destination Options Extension header

   Some of these fields are mutable (e.g., Hop Count) while others are
   immutable (e.g., Fragment Extension Header).





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   Some of these fields contain information that is required by every
   node along a packet's delivery path (e.g., Hop Count).  Others
   contain information that is required only by the packet's ultimate
   destination (e.g., Fragment Extension Header).

   Therefore, the compression mechanism MUST NOT prevent this
   information from being delivered, in an IPv6 header chain, to any
   node that needs it.

   Metric: The SR source node encapsulates its payload (e.g.., Ethernet,
   IP, TCP) in an IPv6 header.  The SRv6 header contains both routing
   and non-routing information.  The compression mechanism MUST NOT
   cause the loss of non-routing information when delivering a packet
   from the SR ingress node to the egress/penultimate SR node.

4.3.3.  Address Planning

   Description: Network operators require addressing plan flexibility,
   The compression mechanism MUST support flexible IPv6 address
   planning, it MUST support deployment by using GUA from different
   address blocks.

   Rationale: The address planning of the network may vary based on the
   addressing scheme of the operator, so the solution MUST support a
   flexible addressing scheme.  Operators need to deploy the solution
   based on their own address planning.

   Metric: The compression proposal supports locators drawn from
   different prefixes with the solutions analysis indicating efficiency.

4.4.  Scalability Requirements

4.4.1.  Adjacency segment scale

   Description: The compression proposal MUST be capable of representing
   65000 adjacency segments per node

   Rationale: Typically, network operators deploy networks with tens or
   hundreds of adjacency segments per node, but some network operators
   may deploy networks that use more adjacency segments per node.

   Metric: A proposal that allows 65000 adjacency segments per node
   satisfies this requirement.








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4.4.2.  Prefix segment scale

   Description: The compression proposal MUST be capable of representing
   1 million prefix segments per SID numbering space.

   Rationale: Typically, network operators deploy networks with
   thousands of prefix segments per SID numbering space, but some
   network operators may deploy networks that use more prefix segments
   per SID numbering space.

   Metric: A proposal that allows 1 million prefix segments per SID
   numbering space satisfies this requirement.

4.4.3.  Service Scale

   Description: The compression proposal MUST be capable of representing
   1 million services per node.

   Rationale: Typically, network operators deploy networks with tens to
   hundreds of thousands of services per node, but some network
   operators may deploy networks that use more services per node.

   Metric: A proposal that allows 1 million services per node satisfies
   this requirement.

4.4.4.  Compression Levels

   Description: The compression proposal SHOULD be able to support
   multiple levels of compression.

   Rationale: The compression proposal will be deployed in networks of
   varying size with SID numbering spaces of varying size.  Network and
   service scale can directly impact SID length and the ability of a
   proposal to compress the SID list.

   Metric: A compression proposal that supports relatively better
   compression for smaller SID numbering spaces and service scale
   satisfies this requirement.

5.  Protocol Design Requirements

5.1.  SRv6 Base Coexistence

   Description: The compression proposal MUST support simultaneous
   deployment with SRv6 networks.

   Rationale: SRv6 is deployed today.  A compression proposal that
   interoperates well with SRv6, as deployed, will reduce the overhead



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   and simplify operations.  For Network operators who would migrate to
   compressed SRv6 SID lists, the migration is expected to gradually
   occur over a period of time as they upgrade networks, domains, device
   families and software instances.

   Metric: A compliant compression proposal provides the following

   o  Supports simultaneous deployment at a node with current SRv6 SIDs.
   o  Supports simultaneous deployment at a node with current SRv6
      control plane.
   o  Supports simultaneous operation of current SRv6 paths with
      compressed paths.
   o  Supports the behaviors in [RFC8986].
   o  Does not require removal of existing IPv6 address planning.

6.  Security Requirements

6.1.  Security Mechanisms

   Description: The compression solution SHOULD be able to address
   security issues that it introduces, using existing security
   mechanisms.

   Rationale: It is important to identify security issues and how to
   address them in any specification.

   Metric: A compression solution that does not introduce unresolved
   security issues meets this requirement.

6.2.  SR Domain Protection

   Description: A compression solution must not require nodes outside
   the SR domain to know SID values within the SR domain, and it must
   provide the ability to block nodes outside an SR domain from
   accessing SIDS.

   Rationale: The unauthorized use of SIDs within the SR domain by nodes
   outside the domain can disrupt an operators' network.

   Metric: A compliant solution describes how access to SIDs within the
   SR domain is denied to nodes outside the SR domain.

7.  IANA Considerations

   This document has no requests to IANA.






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8.  Security Considerations

   TBD

9.  Contributors

   The following individuals contributed to this draft

   Sanders Steffann, SJM Steffann Consultancy, sander@steffann.nl

10.  Normative References

   [I-D.ietf-6man-spring-srv6-oam]
              Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M.
              Chen, "Operations, Administration, and Maintenance (OAM)
              in Segment Routing Networks with IPv6 Data plane (SRv6)",
              draft-ietf-6man-spring-srv6-oam-10 (work in progress),
              April 2021.

   [I-D.ietf-bess-srv6-services]
              Dawra, G., Filsfils, C., Talaulikar, K., Raszuk, R.,
              Decraene, B., Zhuang, S., and J. Rabadan, "SRv6 BGP based
              Overlay Services", draft-ietf-bess-srv6-services-07 (work
              in progress), April 2021.

   [I-D.ietf-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              Bernier, D., and B. Decraene, "BGP Link State Extensions
              for SRv6", draft-ietf-idr-bgpls-srv6-ext-07 (work in
              progress), March 2021.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-15 (work in progress), April 2021.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-14
              (work in progress), April 2021.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", draft-ietf-rtgwg-segment-
              routing-ti-lfa-06 (work in progress), February 2021.




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   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-11 (work in progress),
              April 2021.

   [I-D.ietf-spring-sr-service-programming]
              Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C.,
              Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and
              S. Salsano, "Service Programming with Segment Routing",
              draft-ietf-spring-sr-service-programming-04 (work in
              progress), March 2021.

   [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>.

   [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>.

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

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

Appendix A.  Proposed Requirements

   This appendix contains requirements that the design team discussed
   but could not be agreed upon.

A.1.  IPv6 Based

   Description: The compression mechanism requires every node along the
   packet's delivery path to be IPv6-capable.  It MUST not require any




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   node along the packet's forwarding path to support any other
   forwarding plane (e.g., IPv4, MPLS)

   Rational: According to RFC 8402, SRv6 is an instantiation of the SR
   Architecture over the IPv6 data plane.

   Metric: A compliant solution requires every node along the packet's
   delivery path to be IPv6-capable.  It does not require any node along
   the packet's forwarding path to support any other forwarding plane
   (e.g., IPv4, MPLS)

A.2.  Point to Multipoint

   Description: The compression mechanism SHOULD support point-to-
   multipoint SR paths.

   Rationale: Many VPN services require point-to-multipoint SR paths.

   Metric: A compliant proposal can encode a multicast address in the
   ultimate segment of the segment list.

A.3.  Parsability

   Description: The compression mechanism MUST be parsable.  That is,
   the node that consumes the compressed SID list must be able to decode
   the active and next segment.  Parsing information MAY be conveyed in
   either the forwarding or control plane.

   Rationale: Failure to parse the compressed SID list leads to
   undesired behaviors.

   Metric: In the nominal case the producer and consumer of the SID list
   agree on the active segment and next segment.  In forseeable failure
   modes it is possible to determine why the producer and consumer don't
   agree.

Authors' Addresses

   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com


   Chongfeng Xie
   China Telecom

   Email: xiechf@chinatelecom.cn



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   Ron Bonica
   Juniper

   Email: rbonica@juniper.net


   Darren Dukes
   Cisco Systems

   Email: ddukes@cisco.com


   Cheng Li
   Huawei

   Email: c.l@huawei.com


   Peng Shaofu
   ZTE

   Email: peng.shaofu@zte.com.cn


   Wim Henderickx
   Nokia

   Email: wim.henderickx@nokia.com























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