[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04

LSR Working Group                                            U. Chunduri
Internet-Draft                                                     R. Li
Intended status: Standards Track                               Futurewei
Expires: January 9, 2020                                        R. White
                                                        Juniper Networks
                                                             J. Tantsura
                                                             Apstra Inc.
                                                            L. Contreras
                                                              Telefonica
                                                                   Y. Qu
                                                               Futurewei
                                                            July 8, 2019


                 Preferred Path Routing (PPR) in IS-IS
           draft-chunduri-lsr-isis-preferred-path-routing-04

Abstract

   This document specifies Preferred Path Routing (PPR), a routing
   protocol extension to simplify the path description on the packet for
   Segment Routing (SR) deployments and beyond.  PPR aims to mitigate
   the MTU and data plane processing issues that may result from SR
   packet overhead; and also supports further extensions along the
   paths.

Requirements Language

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

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."



Chunduri, et al.         Expires January 9, 2020                [Page 1]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   This Internet-Draft will expire on January 9, 2020.

Copyright Notice

   Copyright (c) 2019 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 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
     1.1.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Preferred Path Routing (PPR)  . . . . . . . . . . . . . . . .   4
     2.1.  PPR-ID and Data Plane Extensibility . . . . . . . . . . .   4
     2.2.  PPR Path Description  . . . . . . . . . . . . . . . . . .   4
     2.3.  ECMP Considerations . . . . . . . . . . . . . . . . . . .   5
     2.4.  Scalability and PPR Graphs  . . . . . . . . . . . . . . .   5
   3.  PPR Related TLVs  . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  PPR-Prefix Sub-TLV  . . . . . . . . . . . . . . . . . . .   8
     3.2.  PPR-ID Sub-TLV  . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  PPR-PDE Sub-TLV . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  PPR-Attributes Sub-TLV  . . . . . . . . . . . . . . . . .  13
   4.  PPR Processing Procedure Example  . . . . . . . . . . . . . .  13
     4.1.  PPR TLV Processing  . . . . . . . . . . . . . . . . . . .  15
     4.2.  Path Fragments  . . . . . . . . . . . . . . . . . . . . .  16
   5.  PPR Data Plane aspects  . . . . . . . . . . . . . . . . . . .  16
     5.1.  SR-MPLS with PPR  . . . . . . . . . . . . . . . . . . . .  16
     5.2.  PPR Native IP Data Planes . . . . . . . . . . . . . . . .  17
     5.3.  SRv6 with PPR . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  PPR Sub-TLVs  . . . . . . . . . . . . . . . . . . . . . .  18
     7.2.  IGP Parameters  . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Appendix . . . . . . . . . . . . . . . . . . . . . .  22
     A.1.  Challenges with Increased SID Depth . . . . . . . . . . .  22



Chunduri, et al.         Expires January 9, 2020                [Page 2]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


     A.2.  Mitigation with MSD . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   In a network implementing Segment Routing (SR), packets are steered
   through the network using Segment Identifiers (SIDs) carried in the
   packet header.  Each SID uniquely identifies a segment as defined in
   [RFC8402] . SR capabilities are defined for MPLS and IPv6 data planes
   called SR-MPLS and SRv6 respectively.  Appendix A.1 and Appendix A.2
   describe performance, hardware capabilities and various associated
   issues which may result in SR deployments.

   The above motivate the proposed solution, Preferred Path Routing
   (PPR).  In brief, PPR involves, associating path descriptions to IS-
   IS advertised prefixes, mapping those to a data-plane identifier and
   specifying a mechanism to route packets with the abstracted
   identifier (PPR-ID), as opposed to individual segments on the packet.
   This is specified in detail in Section 2 and Section 3.

1.1.  Acronyms

   EL       -  Entropy Label

   ELI      -  Entropy Label Indicator

   LSP      -  IS-IS Link State PDU

   MPLS     -  Multi Protocol Label Switching

   MSD      -  Maximum SID Depth

   MTU      -  Maximum Transferrable Unit

   NH       -  Next-Hop

   PPR      -  Preferred Path Routing/Route

   PPR-ID   -  Preferred Path Route Identifier, a data plane identifier

   SID      -  Segment Identifier

   SPF      -  Shortest Path First

   SR-MPLS  -  Segment Routing with MPLS data plane

   SRH      -  Segment Routing Header - IPv6 routing Extension header




Chunduri, et al.         Expires January 9, 2020                [Page 3]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   SRv6     -  Segment Routing with IPv6 data plane with SRH

   TE       -  Traffic Engineering

2.  Preferred Path Routing (PPR)

   PPR mitigates the issues described in Appendix A.1, while continuing
   to allow the direction of traffic along an engineered path through
   the network by replacing the label stack with a PPR-ID.  The PPR-ID
   can either be a single label or a native destination address.  To
   facilitate the use of a single label to describe an entire path, a
   new TLV is added to IS-IS, as described below in Section 3.

   A PPR could be an SR path, a traffic engineered path computed based
   on some constraints, an explicitly provisioned Fast Re-Route (FRR)
   path or a service chained path.  A PPR can be signaled by any node,
   computed by a central controller, or manually configured by an
   operator.  PPR extends the source routing and path steering
   capabilities to native IP (IPv4 and IPv6) data planes without
   hardware upgrades; see Section 5.

2.1.  PPR-ID and Data Plane Extensibility

   The PPR-ID describes a path through the network.  A data plane type
   and corresponding data plane identifier as specified in Section 3.2
   is mapped to PPR-ID to allow data plane extensibility.

   For SR-MPLS, PPR-ID is mapped to an MPLS Label/SID and for SRv6, this
   is mapped to an IPv6-SID.  For native IP data planes, this is mapped
   to either IPv4 or IPv6 address/prefix.

2.2.  PPR Path Description

   The path identified by the PPR-ID is described as a set of Path
   Description Elements (PDEs), each of which represents a segment of
   the path.  Each node determines its location in the path as
   described, and forwards to the next segment/hop or label of the path
   description (see the Forwarding Procedure Example later in this
   document).

   These PPR-PDEs as defined in Section 3.3, like SR SIDs, can represent
   topological elements like links/nodes, backup nodes, as well as non-
   topological elements such as a service, function, or context on a
   particular node.

   A PPR path can be described as a Strict-PPR or a Loose-PPR.  In a
   Strict-PPR all nodes/links on the path are described with SR SIDs for
   SR data planes or IPv4/IPV6 addresses for native IP data planes.  In



Chunduri, et al.         Expires January 9, 2020                [Page 4]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   a Loose-PPR only some of the nodes/links from source to destination
   are described.  More specifics and restrictions around Strict/Loose
   PPRs are described in respective data planes in Section 5.  Each PDE
   is described as either an MPLS label towards the Next-Hop (NH) in
   MPLS enabled networks, or as an IP NH, in the case of either
   "plain"/"native" IP or SRv6 enabled networks.  A PPR path is related
   to a set of PDEs using the TLVs as specified in Section 3.

2.3.  ECMP Considerations

   PPR inherently supports Equal Cost Multi Path (ECMP) for both strict
   and loose paths.  If a path is described using nodes, would have ECMP
   NHs established for PPR-ID along the path.  However, one can avoid
   ECMP on any segment of the path by pinning the path using link
   identifier to the next segment.

2.4.  Scalability and PPR Graphs

   In a network of N nodes O(N^2) total (unidirectional) paths are
   necessary to establish any-to-any connectivity, and multiple (k) such
   path sets may be desirable if multiple path policies are to be
   supported (lowest latency, highest throughput etc.).

   In many solutions and topologies, N may be small enough and/or only a
   small set of paths need to be preferred paths, for example for high
   value traffic (DetNet, some of the defined 5G slices), and then the
   technology specified in this document can support these deployments.

   However, to address the scale needed when a larger number of PPR
   paths are required, the PPR TREE structure can be used [I-D.draft-ce-
   ppr-graph-00].  Each PPR Tree uses one label/SID and defines paths
   from any set of nodes to one destination, thus reduces the number of
   entries needed in SRGB at each node (for SR-MPLS data plane
   Section 5.1).  These paths form a tree rooted in the destination.  In
   other word, PPR Tree identifiers are destination identifiers and PPR
   Treed are path engineered destination routes (like IP routes) and it
   scaling simplifies to linear in N i.e., O(k*N).

3.  PPR Related TLVs

   This section describes the encoding of PPR TLV.  This TLV can be seen
   as having 4 logical sections viz, encoding of the PPR-Prefix (IS-IS
   Prefix), encoding of PPR-ID, encoding of path description with an
   ordered PDE Sub-TLVs and a set of optional PPR attribute Sub-TLVs,
   which can be used to describe one or more parameters of the path.
   Multiple instances of this TLV MAY be advertised in IS-IS LSPs with
   different PPR-ID Type (data plane) and with corresponding PDE Sub-




Chunduri, et al.         Expires January 9, 2020                [Page 5]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   TLVS.  The PPR TLV has Type TBD (suggested value xxx), and has the
   following format:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |     Length    |  PPR-Flags                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Fragment-ID   |             MT-ID             | Algorithm     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          PPR-Prefix Sub-TLV (variable size)                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          PPR-ID Sub-TLV (variable size)                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          PPR-PDE Sub-TLVs (variable)                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          PPR-Attribute Sub-TLVs (variable)                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 1: PPR TLV Format

   o  Type: 155 (Suggested Value, TBD IANA) from IS-IS top level TLV
      registry.

   o  Length: Total length of the value field in bytes.

   o  PPR-Flags: 2 Octet bit-field of flags for this TLV; described
      below.

   o  Fragment-ID: This is an 8-bit Identifier value (0-255) of the TLV
      fragment.  If fragments are not needed to represent the complete
      path, 'U' bit MUST be set and this value MUST be set to 0.

   o  MT-ID: The multi-topology identifier defined in [RFC5120]; the 4
      most significant bits MUST be set to 0 on transmit and ignored on
      receive.  The remaining 12-bit field contains the MT-ID.

   o  Algorithm: 1 octet value represents the route computation
      algorithm.  Algorithm registry is as defined in
      [I-D.ietf-isis-segment-routing-extensions].  Computation towards
      PPR-ID (Section 3.2) happens per MT-ID/Algorithm pair.

   o  PPR-Prefix: A variable size Sub-TLV representing the destination
      of the path being described.  This is defined in Section 3.1.

   o  PPR-ID: A variable size Sub-TLV representing the data plane or
      forwarding identifier of the PPR.  Defined in Section 3.2.




Chunduri, et al.         Expires January 9, 2020                [Page 6]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   o  PPR-PDEs: Variable number of ordered PDE Sub-TLVs which represents
      the path.  This is defined in Section 3.3.

   o  PPR-Attributes: Variable number of PPR-Attribute Sub-TLVs which
      represent the path attributes.  These are defined in Section 3.4.

   The Flags field has the following flag bits defined:

        PPR TLV Flags Format

            0 1 2 3 4 5 6 7               15
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |F|D|A|U|Reserved               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   1.  F: Flood bit.  If set, the PPR TLV MUST be flooded across the
       entire routing domain.  If the F bit is not set, the PPR TLV MUST
       NOT be leaked between IS-IS levels.  This bit MUST NOT be altered
       during the TLV leaking

   2.  D: Down Bit. When the PPR TLV is leaked from IS-IS level-2 to
       level-1, the D bit MUST be set.  Otherwise, this bit MUST be
       clear.  PPR TLVs with the D bit set MUST NOT be leaked from
       level-1 to level-2.  This is to prevent TLV looping across
       levels.

   3.  A: Attach bit.  The originator of the PPR TLV MUST set the A bit
       in order to signal that the prefix and PPR-ID advertised in the
       PPR TLV are directly connected to the originators.  If this bit
       is not set, this allows any other node in the network to
       advertise this TLV on behalf of the originating node of the PPR-
       Prefix.  If PPR TLV is leaked to other areas/levels the A-flag
       MUST be cleared.  In case if the originating node of the prefix
       must be disambiguated for any reason including, if it is a Multi
       Homed Prefix (MHP) or leaked to a different IS-IS level or
       because [RFC7794] X-Flag is set, then PPR-Attribute Sub-TLV
       Source Router ID SHOULD be included.

   4.  U: Ultimate fragment bit. bit MUST be set if a path has only one
       fragment or if it is the last Fragment of the path.  PPR-ID value
       for all fragments of the same path MUST be same.

   5.  Reserved: For future use; MUST be set to 0 on transmit and
       ignored on receive.

   PPR path description for each IS-IS level is computed and given to
   one of the nodes for L1 and L2 respectively.  Similarly path



Chunduri, et al.         Expires January 9, 2020                [Page 7]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   information when crossing the level boundaries MUST be relevant to
   the destination level.  If there is no path information available for
   the destination level PPR TLV MUST NOT be leaked regardless of F and
   D bits as defined above.

   The following Sub-TLVs draw from a new registry for Sub-TLV numbers
   as specified in Section 7.1 and Section 7.2.

3.1.  PPR-Prefix Sub-TLV

   The structure of PPR-Prefix is:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Type          |  Length       | Prefix Length |  Mask Length  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //           IS-IS Prefix (variable)                           //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: PPR-Prefix Sub-TLV Format

   o  Type: 1 (IANA to assign from Sub-TLV registry described above).

   o  Length: Total length of the value field in bytes.

   o  Prefix Length: The length of the IS-IS Prefix being encoded in
      bytes.  For IPv4 it MUST be 4 and IPv6 it MUST be 16 bytes.

   o  Mask Length: The length of the prefix in bits.  Only the most
      significant octets of the Prefix are encoded.

   o  IS-IS Prefix: The IS-IS prefix at the tail-end of the advertised
      PPR.  This corresponds to a routable prefix of the originating
      node and it MAY have one of the [RFC7794] flags set (X-Flag/R-
      Flag/N-Flag) in the IS-IS reachability TLVs.  Length of this field
      MUST be as per "Prefix Length".  Encoding is same as TLV 135
      [RFC5305] and TLV 236 [RFC5308] or MT-Capable [RFC5120] IPv4 (TLV
      235) and IPv6 Prefixes (TLV 237) respectively.

3.2.  PPR-ID Sub-TLV

   This is the actual data plane identifier in the packet header and
   could be of any data plane as defined in PPR-ID Type field.  Both
   PPR-Prefix and PPR-ID belongs to a same node in the network.






Chunduri, et al.         Expires January 9, 2020                [Page 8]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Type          |  Length       |PPR-ID Flags                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | PPR-ID Type   | PPR-ID Length |PPR-ID Mask Len|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //                 PPR-ID (variable size)                      //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: PPR-ID Sub-TLV Format

   o  Type: 2 (IANA to assign from Sub-TLV registry described above).

   o  Length: Total length of the value field in bytes.

   o  PPR-ID Flags: 2 Octet field for PPR-ID flags:

        PPR-ID Flags Format

            0 1 2 3 4 5 6 7..             15
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |    Reserved                   |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


        Reserved: For future use; MUST be set to 0 on transmit and
        ignored on receive.

   o  PPR-ID Type: Data plane type of PPR-ID.  This is a new registry
      (TBD IANA - Suggested values as below) for this Sub-TLV and the
      defined types are as follows:

        Type: 1 SR-MPLS SID/Label

        Type: 2 Native IPv4 Address/Prefix

        Type: 3 Native IPv6 Address/Prefix

        Type: 4 IPv6 SID in SRv6 with SRH

   o  PPR-ID Length: Length of the PPR-ID field in octets and this
      depends on the PPR-ID type.

   o  PPR-ID Mask Len: It is applicable for only for PPR-ID Type 2, 3
      and 4.  For Type 1 this value MUST be set to zero.  It contains
      the length of the PPR-ID Prefix in bits.  Only the most
      significant octets of the Prefix are encoded.  This is needed, if



Chunduri, et al.         Expires January 9, 2020                [Page 9]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


      PPR-ID followed is an IPv4/IPv6 Prefix instead of 4/16 octet
      Address respectively.

   o  PPR-ID: This is the Preferred Path forwarding identifier that
      would be on the data packet.  The value of this field is variable
      and it depends on the PPR-ID Type - for Type 1, this is encoded as
      SR-MPLS SID/Label.  For Type 2 this is a 4 byte IPv4 address.  For
      Type 3 this is a 16 byte IPv6 address.  For Type 2 and Type 3
      encoding is similar to "IS-IS Prefix" as specified in Section 3.1.
      For Type 4, this is encoded as 16 byte SRv6 SID.

   For PPR-ID Type 2, 3 or 4, PPR-ID MUST NOT be advertised as a
   routable prefix in TLV 135, TLV 235, TLV 236 and TLV 237.  PPR-ID
   MUST belong to the node, from where the PPR-Prefix (Section 3.1) is
   advertised.

3.3.  PPR-PDE Sub-TLV

   This Sub-TLV represents the PPR Path Description Element (PDE).  PPR-
   PDEs are used to describe the path in the form of set of contiguous
   and ordered Sub-TLVs, where first Sub-TLV represents (the top of the
   stack in MPLS data plane or) first node/segment of the path.  These
   set of ordered Sub-TLVs can have both topological elements and non-
   topological elements (e.g., service segments).

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Type          |  Length       | PPR-PDE Type  | PDE-ID Type   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | PDE-ID Length | PPR-PDE Flags                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //                 PDE-ID Value (variable size)                //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Sub-TLV Length |  PPR-PDE  Sub-TLVs (variable)                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: PPR-PDE Sub-TLV Format

   o  Type: 3 (See IANA for suggested value) from IS-IS PPR TLV
      Section 3 Sub-TLV registry.

   o  Length: Total length of the value field in bytes.

   o  PPR-PDE Type: A new registry (TBD IANA) for this Sub-TLV and the
      defined types are as follows:

        Type: 1 Topological



Chunduri, et al.         Expires January 9, 2020               [Page 10]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


        Type: 2 Non-Topological

   o  PDE-ID Type: 1 Octet PDE-forwarding IDentifier Type.  A new
      registry (Suggested Values as listed, IANA TBD) for this Sub-TLV
      and the defined types and corresponding PDE-ID Length, PDE-ID
      Value are as follows:

        Type 0: This value MUST be set only when PPR-PDE Type is Non-
        Topological.  PDE-ID Length indicates the length of the PDE-ID
        Value field in bytes.  For this type, PDE-ID value represents a
        service/function.  This information is provisioned on the
        immediate topological PDE preceding to this PDE based on the 'E'
        bit.

        Type 1: SID/Label type as defined in
        [I-D.ietf-isis-segment-routing-extensions].  PDE-ID Length and
        PDE-ID Value fields are per Section 2.3 of the referenced
        document.

        Type 2: SR-MPLS Prefix SID.  PDE-ID Length and PDE-ID Value are
        same as Type 1.

        Type 3: SR-MPLS Adjacency SID.  PDE-ID Length and PDE-ID Value
        are same as Type 1.

        Type 4: IPv4 Node Loopback Address.  PDE-ID Length 4 bytes and
        PDE-ID Value is full 4 bytes IPv4 address encoded as specified
        in "4-octet IPv4 address" of Sub-TLV 6/TLV 22 in [RFC5305].

        Type 5: IPv4 Interface Address.  PDE-ID Length is 4 bytes and
        PDE-ID Value is full 4 bytes IPv4 address encoded as specified
        in "4-octet IPv4 address" of Sub-TLV 6/TLV 22 in [RFC5305].
        This PDE-ID in the path description represents the egress
        interface of the path segment and correponding adjacency is set
        as nexthop for the PPR-ID.

        Type 6: IPv6 Node Loopback Address.  PDE-ID Length and PDE-ID
        Value are encoded as specified in "Prefix Len" and "prefix"
        portion of TLV 236 in [RFC5308] respectively.

        Type 7: IPv6 Interface Address.  PDE-ID Length is 16 bytes and
        PDE-ID Value is full 16 bytes IPv6 address encoded as specified
        in "Interface Address 1" portion of TLV 232 in [RFC5308].  This
        PDE-ID in the path description represents the egress interface
        of the path segment and correponding adjacency is set as nexthop
        for the PPR-ID.





Chunduri, et al.         Expires January 9, 2020               [Page 11]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


        Type 8: SRv6 Node SID as defined in
        [I-D.bashandy-isis-srv6-extensions].  PDE-ID Length and PDE-ID
        Value are as defined in SRv6 SID from the referenced draft.

        Type 9: SRv6 Adjacency-SID.  PDE-ID Length and PDE-ID Values are
        similar to SRv6 Node SID above.

   o  PPR-PDE Flags: 2 Octet bit-field of flags; described below:

        PPR-PDE Flags Format

            0 1 2 3 4 5 6 7 ..            15
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |L|N|E|    Reserved             |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


        L: Loose Bit. Indicates the type of next "Topological PDE-ID" in
        the path description.  This bit MUST be set for only Node/Prefix
        PDE type.  If this flag is unset, the next Topological PDE is
        Strict Type.

        N: Node Bit. By default this bit MUST be unset.  This bit MUST
        be set only for PPR-PDE Type is 1 i.e., Topological and this PDE
        represents the node, where PPR-Prefix (Section 3.1) belongs to
        (if there is no further PDE specific Sub-TLVs to override PPR-
        Prefix and PPR-ID values).

        E: Egress Bit. By default this bit MUST be unset.  This bit MUST
        be set only for PPR-PDE Type is 2 i.e., Non-Topological and the
        service needs to be applied on the egress side of the
        topological PDE preceding this PDE.

        Reserved: Reserved bits for future use.  Reserved bits MUST be
        reset on transmission and ignored on receive.

   o  Sub-TLV Length: 1 byte length of all Sub-TLVs followed.  It MUST
      be set to 0 if no further Sub-TLVs are present.

   o  PPR-PDE Sub-TLVs: These have 1 octet type, 1 octet length and
      value field is defined per the type field.  Types are as defined
      in PPR-TLV Sub-TLVs (Section 7), encoded further as sub-sub-TLVs
      of PPR-PDE and the length field represents the total length of the
      value field in bytes.

        IS-IS System-ID Sub-TLV: Type 4 (IANA TBD), Length Total length
        of value field in bytes, Value: IS-IS System-ID of length "ID
        Length" as defined in [ISO.10589.1992].  This Sub-TLV MUST NOT



Chunduri, et al.         Expires January 9, 2020               [Page 12]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


        be present, if the PPR-PDE Type is not Topological.  Though the
        type for this come from the PPR Sub-TLV registry, here this is a
        sub-sub-TLV and is part of PPR-ID/PPR-PDE Sub-TLV.

3.4.  PPR-Attributes Sub-TLV

   PPR-Attribute Sub-TLVs describe the attributes of the path.  This
   document defines the following optional PPR-Attribute Sub-TLVs:

   o  Type 5 (Suggested Value - IANA TBD): PPR-Prefix originating node's
      IPv4 Router ID Sub-TLV.  Length and Value field are as specified
      in [RFC7794].

   o  Type 6 (Suggested Value - IANA TBD): PPR-Prefix originating node's
      IPv6 Router ID Sub-TLV.  Length and Value field are as specified
      in [RFC7794].

   o  Type 7 (Suggested Value - IANA TBD): PPR-Metric Sub-TLV.  Length 4
      bytes, and Value is metric of this path represented through the
      PPR-ID.  Different nodes can advertise the same PPR-ID for the
      same Prefix with a different set of PPR-PDE Sub-TLVs and the
      receiving node MUST consider the lowest metric value.

4.  PPR Processing Procedure Example

   As specified in Section 2, a PPR can be a TE path, locally
   provisioned by the operator or by a controller.  Consider the
   following IS-IS network to describe the operation of PPR TLV as
   defined in Section 3:






















Chunduri, et al.         Expires January 9, 2020               [Page 13]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


                                           1
                                        _______
                                       /   1   \
                                   +---R2-------R3---+
                                  /    \_______/      \
                                 /         1           \
                              1 /                       \ 1
                               /      1__R13__1          \
                              /       /       \           \
                            R1------R6        R7-----------R4
                              \ 2     \__R14__/       2   /\
                               \      2       2          /  \
                              3 \                       / 3  \1
                                 \                 4   /      \
                                  +----R8------R9-----R10------R12
                                                \          1   /
                                               1 \            / 1
                                                  +----R11---+


                          Figure 5: IS-IS Network

   In the (Figure 5), consider node R1 as an ingress node, or a head-end
   node, and the node R4 may be an egress node or another head-end node.
   The numbers shown on links between nodes indicate the bi-directional
   IS-IS metric as provisioned.  R1 may be configured to receive TE
   source routed path information from a central entity (PCE [RFC5440],
   Netconf [RFC6241] or a Controller) that comprises of PPR information
   which relates to sources that are attached to R1.  It is also
   possible to have a PPR provisioned locally by the operator for non-TE
   needs (e.g.  FRR or for chaining certain services).

   The PPR TLV (as specified in Section 3) is encoded as an ordered list
   of PPR-PDEs from source to a destination node in the network and is
   represented with a PPR-ID (Section 3.2).  The PPR TLV includes PPR-
   PDE Sub-TLVs Section 3.3, which represent both topological and non-
   topological elements and specifies the actual path towards a PPR-
   Prefix at R4.

   o  The shortest path towards R4 from R1 are through the following
      sequence of nodes: R1-R2-R3-R4 based on the provisioned metrics.

   o  The central entity can define a few PPRs from R1 to R4 that
      deviate from the shortest path based on other network
      characteristic requirements as requested by an application or
      service.  For example, the network characteristics or performance
      requirements may include bandwidth, jitter, latency, throughput,
      error rate, etc.



Chunduri, et al.         Expires January 9, 2020               [Page 14]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   o  A first PPR may be identified by PPR-ID = 1 (value) and may
      include the path of R1-R6-R7-R4 for a Prefix advertised by R4.
      This is an example for a Loose-PPR and 'L' bit MUST be set
      appropriately at Section 3.3.

   o  A second PPR may be identified by PPR-ID = 2 (value) and may
      include the path of R1-R8-R9-R10-R4.  This is an example for a
      Strict-PPR and 'L' bit MUST be unset appropriately at Section 3.3.
      Though this example shows PPR with all nodal SIDs, it is possible
      to have a PPR with combination of node and adjacency SIDs (local
      or global) or with PPR-PDE Type set to Non-Topological as defined
      in Section 3.3 elements along with these.

4.1.  PPR TLV Processing

   The first topological sub-object or PDE (Section 3.3) relative to the
   beginning of PPR Path contains the information about the first node
   (e.g. in SR-MPLS it's the topmost label).  The last topological sub-
   object or PDE contains information about the last node (e.g. in SR-
   MPLS it's the bottommost label).

   Each receiving node, determines whether an advertised PPR includes
   information regarding the receiving node.  Before processing any
   further, validation MUST be done to see if any PPR topological PDE is
   seen more than once (possible loop), if yes, this PPR TLV MUST be
   ignored.  Processing of PPR TLVs may be done, during the end of the
   SPF computation (for MTID that is advertised in this TLV) and for
   each prefix described through PPR TLV.  For example, node R9 receives
   the PPR information, and ignores the PPR-ID=1 (Section 4) because
   this PPR TLV does not include node R9 in the path description/ordered
   PPR-PDE list.

   However, node R9 may determine that the second PPR identified by PPR-
   ID = 2 does include the node R9 in its PDE list.  Therefore, node R9
   updates the local forwarding database to include an entry for the
   destination address that R4 indicates, so that when a data packet
   comprising a PPR-ID of 2 is received, forward the data packet to node
   R10 instead of R11.  This is done, even though from R9 the shortest
   path to reach R4 via R11 (Cost 3: R9-R11-R12-R4) it chooses the NH to
   R10 to reach R4 as specified in the PPR path description.  Same
   process happens to all nodes or all topological PDEs as described in
   the PPR TLV.

   In summary, the receiving node checks first, if this node is on the
   path by checking the node's topological elements (with PPR-PDE Type
   set to Topological) in the path list.  If yes, it adds/adjusts the
   PPR-ID's shortest path NH towards the next topological PDE in the
   PPR's Path.



Chunduri, et al.         Expires January 9, 2020               [Page 15]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


4.2.  Path Fragments

   A complete PPR path may not fit into maximum allowable size of the
   IS-IS TLV.  To overcome this a 7 bit Fragment-ID field is defined in
   Section 3 .  With this, a single PPR path is represented via one or
   more fragmented PPR path TLVs, with all having the same PPR-ID.  Each
   fragment carries the PPR-ID as well as a numeric Fragment-ID from 0
   to (N-1), when N fragments are needed to describe the PPR Graph
   (where N>1).  In this case Fragment (N-1) MUST set the 'U' bit (PPR-
   Flags) to indicate it is the last fragment.  If Fragment-ID is non
   zero in the TLV, then it MUST not carry PPR-Prefix Sub-TLV.  The
   optional PPR Attribute Sub-TLVs which describe the path overall MUST
   be included in the last fragment only (i.e., when the 'U' bit is
   set).

5.  PPR Data Plane aspects

   Data plane for PPR-ID is selected by the entity (e.g., a controller,
   locally provisioned by operator), which selects a particular PPR in
   the network.  Section 3.2 defines various data plane identifier types
   and a corresponding data plane identifier is selected by the entity
   which selects the PPR.

5.1.  SR-MPLS with PPR

   If PPR-ID Type is 1, then the PPR belongs to SR-MPLS data plane and
   the complete PPR stack is represented with a unique SR SID/Label and
   this gets programmed on the data plane of each node, with the
   appropriate NH computed as specified in Section 4.  PPR-ID here is a
   label/index from the SRGB (like another node SID or global ADJ-SID).
   PPR path description here is a set of ordered SIDs represented with
   PPR-PDE (Section 3.2) Sub-TLVs.  Non-Topological segments also
   programmed in the forwarding to enable specific function/service,
   when the data packet hits with corresponding PPR-ID.

   Based on 'L' flag in PPR-ID Flags (Section 3.2), for SR-MPLS data
   plane either 1 label or 2 labels need to be provisioned on individual
   nodes on the path description.  For the example network in Section 4,
   for PPR-ID=1, which is a loose path, node R6 programs the bottom
   label as PPR-ID and the top label as the next topological PPR-PDE in
   the path, which is a node SID of R7.  The NH computed at R6 would be
   the shortest path towards R7 i.e., the interface towards R13.  If 'L'
   flag is unset only PPR-ID is programmed on the data plane with NH set
   to the shortest path towards next topological PPR-PDE.







Chunduri, et al.         Expires January 9, 2020               [Page 16]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


5.2.  PPR Native IP Data Planes

   If PPR-ID Type is 2 then source routing and packet steering can be
   done in IPv4 data plane (PPR-IPv4), along the path as described in
   PPR Path description.  This is achieved by setting the destination IP
   address as PPR-ID, which is an IPv4 address in the data packet
   (tunneled/encapsulated).  There is no data plane change or upgrade
   needed to support this.

   Similarly for PPR-ID Type is 3, then source routing and packet
   steering can be done in IPv6 data plane (PPR-IPv6), along the path as
   described in PPR Path description.  Whatever specified above for IPv4
   applies here too, except that destination IP address of the data
   packet is an IPv6 Address (PPR-ID).  This doesn't require any IPv6
   extension headers (EH), if there is no metadata/TLVs need to be
   carried in the data packet.

   Based on 'L' flag in PPR-ID Flags (Section 3.2), for PPR-ID Type 2 or
   3 (Native IPv4 or IPv6 data planes respectively) the packet has to be
   encapsulated using the capabilities (either dynamically signaled
   through [I-D.ietf-isis-encapsulation-cap] or statically provisioned
   on the nodes) of the next loose PDE in the path description.

   For the example network in Section 4, for PPR-ID=1, which is a loose
   path, node R6 programs to encapsulate the data packet towards the
   next loose topological PPR-PDE in the path, which is R7.  The NH
   computed at R6 would be the shortest path towards R7 i.e., the
   interface towards R13.  If 'L' flag is unset only PPR-ID is
   programmed on the data plane with NH set to the shortest path towards
   next topological PPR-PDE, with no further encapsulation of the data
   packet.

5.3.  SRv6 with PPR

   If PPR-ID Type is 4, the PPR belongs to SRv6 with SRH data plane and
   the complete PPR stack is represented with IPv6 SIDs and this gets
   programmed on the data plane with the appropriate NH computed as
   specified in Section 4.  PPR-ID here is a SRv6 SID.  PPR path
   description here is a set of ordered SID TLVs similar to as specified
   in Section 5.1.  One way PPR-ID would be used in this case is by
   setting it as the destination IPv6 address and SL field in SRH would
   be set to 0; however SRH [I-D.ietf-6man-segment-routing-header] can
   contain any other TLVs and non-topological SIDs as needed.








Chunduri, et al.         Expires January 9, 2020               [Page 17]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


6.  Acknowledgements

   Thanks to Alex Clemm, Lin Han, Toerless Eckert, Asit Chakraborti,
   Stewart Bryant and Kiran Makhijani for initial discussions on this
   topic.  Thanks to Kevin Smith and Stephen Johnson for various
   deployment scenarios applicability from ETSI WGs perspective.
   Authors also acknowledge Alexander Vainshtein for detailed
   discussions and few suggestions on this topic.

   Earlier versions of draft-ietf-isis-segment-routing-extensions have a
   mechanism to advertise EROs through Binding SID.

7.  IANA Considerations

   This document requests the following new TLV in IANA IS-IS TLV code-
   point registry.

        TLV #   Name
        -----   --------------
        155     PPR TLV (Suggested Value, IANA TBD)


7.1.  PPR Sub-TLVs

   This document requests IANA to create a new Sub-TLV registry for PPR
   TLV Section 3 with the following initial entries (suggested values).
   Though these are defined as Sub-TLVs of PPR TLV, these can be part of
   another Sub-TLV as a nested sub-sub-TLV (e.g.  IS-IS System-ID).

   Sub-TLV #   Sub-TLV Name
   ---------   ---------------------------------------------------------

    1          PPR-Prefix (Section 3.1)

    2          PPR-ID (Section 3.2)

    3          PPR-PDE (Section 3.3)

    4          IS-IS System-ID (Section 3.3)

    5          PPR-Prefix Source IPv4 Router ID (Section 3.4)

    6          PPR-Prefix Source IPv6 Router ID (Section 3.4)

    7          PPR-Metric (Section 3.4)






Chunduri, et al.         Expires January 9, 2020               [Page 18]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


7.2.  IGP Parameters

   This document requests additional IANA registries in an IANA managed
   registry "Interior Gateway Protocol (IGP) Parameters" for various PPR
   TLV parameters.  The registration procedure is based on the "Expert
   Review" as defined in [RFC8126].  The suggested registry names are:

   o  "PPR-Type" - Types are an unsigned 8 bit numbers.  Values are as
      defined in Section 3 of this document.

   o  "PPR-Flags" - 1 Octet.  Bits as described in Section 3 of this
      document.

   o  "PPR-ID Type" - Types are an unsigned 8 bit numbers.  Values are
      as defined in Section 3.2 of this document.

   o  "PPR-ID Flags" - 1 Octet.  Bits as described in Section 3.2 of
      this document.

   o  "PPR-PDE Type" - Types are an unsigned 8 bit numbers.  Values are
      as defined in Section 3.3 of this document.

   o  "PPR-PDE Flags" - 1 Octet.  Bits as described in Section 3.3 of
      this document.

   o  "PDE-ID Type" - Types are an unsigned 8 bit numbers.  Values are
      as defined in Section 3.3 of this document.

8.  Security Considerations

   Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310].
   Further security analysis for IS-IS protocol is done in [RFC7645]
   with detailed analysis of various security threats and why [RFC5304]
   should not be used in the deployments.  Advertisement of the
   additional information defined in this document introduces no new
   security concerns in IS-IS protocol.  However, for extensions related
   ro SR-MPLS and SRH data planes, those particular data plane security
   considerations does apply here.

9.  References

9.1.  Normative References









Chunduri, et al.         Expires January 9, 2020               [Page 19]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   [ISO.10589.1992]
              International Organization for Standardization,
              "Intermediate system to intermediate system intra-domain-
              routing routine information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)",
              ISO Standard 10589, 1992.

   [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>.

9.2.  Informative References

   [I-D.bashandy-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extensions to Support Routing over IPv6
              Dataplane", draft-bashandy-isis-srv6-extensions-05 (work
              in progress), March 2019.

   [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-isis-encapsulation-cap]
              Xu, X., Decraene, B., Raszuk, R., Chunduri, U., Contreras,
              L., and L. Jalil, "Advertising Tunnelling Capability in
              IS-IS", draft-ietf-isis-encapsulation-cap-01 (work in
              progress), April 2017.

   [I-D.ietf-isis-mpls-elc]
              Xu, X., Kini, S., Psenak, P., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label Depth Using IS-IS", draft-ietf-isis-mpls-
              elc-07 (work in progress), May 2019.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., and B. Decraene, "IS-IS Extensions for
              Segment Routing", draft-ietf-isis-segment-routing-
              extensions-25 (work in progress), May 2019.



Chunduri, et al.         Expires January 9, 2020               [Page 20]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   [I-D.ietf-mpls-sfc]
              Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based
              Forwarding Plane for Service Function Chaining", draft-
              ietf-mpls-sfc-07 (work in progress), March 2019.

   [I-D.xuclad-spring-sr-service-chaining]
              Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca,
              d., Li, C., Decraene, B., Ma, S., Yadlapalli, C.,
              Henderickx, W., and S. Salsano, "Segment Routing for
              Service Chaining", draft-xuclad-spring-sr-service-
              chaining-01 (work in progress), March 2018.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.







Chunduri, et al.         Expires January 9, 2020               [Page 21]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC7645]  Chunduri, U., Tian, A., and W. Lu, "The Keying and
              Authentication for Routing Protocol (KARP) IS-IS Security
              Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015,
              <https://www.rfc-editor.org/info/rfc7645>.

   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <https://www.rfc-editor.org/info/rfc7794>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

Appendix A.  Appendix

A.1.  Challenges with Increased SID Depth

   SR label stacks carried in the packet header create challenges in the
   design and deployment of networks and networking equipment.
   Following examples illustrates the need for increased SID depth in
   various use cases:

   (a).  Consider the following network where SR-MPLS data plane is in
   use and with same SRGB (5000-6000) on all nodes i.e., A1 to A11 and
   B1 to B7 for illustration:









Chunduri, et al.         Expires January 9, 2020               [Page 22]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


 SID:10   SID:20   SID:30   SID:40  SID:50 SID:300(Ax)  SID:60    SID:70
 A1--------A2-------A3-------A4-------A5===============A6-- ----------A7
 |          \               /  \5     5/ \   SID:310(Ay) \            /
 | 5         \ 10        10/    +-A10-+   \               \10      10/
 |            \  SID:80   /     |SID:100   \               \        /
 A11 SID:111   \A8-----A9/      |           \  40           \      /
 |             /  SID:90 \      +-----+      +---+           \    /
 | 5          /10         \10     5    \          \           \  /
 |           /SID:125(B2x) \            \          \           \/
 B1-------B2==============B3----B4------B5-------=B6----------B7
             SID:127(B2y)
 SID:110 SID:120      SID:130 SID:140   SID:150  SID:160  SID:170

    === = Path with Parallel Adjacencies and ADJ-SIDs
    --- = Shortest Path Nodal SID


                         Figure 6: SR-MPLS Network

      Global ADJ-SIDs are provisioned between A5-A6 and B2-B3 (with
      parallel adjecencies).  All other SIDs shown are nodal SID
      indices.

      All metrics of the links are set to 1, unless marked otherwise.

      Shortest Path from A1 to A7: A2-A3-A4-A5-A6-A7

      Path-x: From A1 to A7 - A2-A8-B2-B2x-A9-A10-Ax-A7; Pushed Label
      Stack @A1: 5020:5080:5120:5125:5090:5100:5300:5070 (where B2x is a
      local ADJ-SID and Ax is a global ADJ-SID).

      In this example, the traffic engineered path is represented with a
      combination of Adjacency and Node SIDs with a stack of 8 labels.
      However, this value can be larger, if the use of entropy label
      [RFC6790] is desired and based on the Readable Label Depth
      (Appendix A.2) capabilities of each node and additional labels
      required to insert ELI/EL at appropriate places.

      Though above network is shown with SR-MPLS data plane, if the
      network were to use SRv6 data plane, path size would be increased
      even more because of the size of the IPv6 SID (16 bytes) in SRH.

   (b).  Apart from the TE case above, when deploying
   [I-D.ietf-mpls-sfc] or [I-D.xuclad-spring-sr-service-chaining], with
   the inclusion of services, or non-topological segments on the label
   stack, can also make the size of the stack much larger.





Chunduri, et al.         Expires January 9, 2020               [Page 23]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   Overall the additional path overhead in various SR deployments may
   cause the following issues:

   a.  HW Capabilities: Not all nodes in the path can support the
       ability to push or read label stack (with additional non-
       topological and special labels) needed to satisfy user/operator
       requirements.  Alternate paths, which meet these user/operator
       requirements may not be available.

   b.  Line Rate: Potential performance issues in deployments, which use
       data plane with extension header as both size of the SIDs in the
       extension header and the fixed extension header size itself needs
       to be factored by the hardware.

   c.  MTU: Larger SID stacks on the data packet can cause potential
       MTU/fragmentation issues (SRH).

   d.  Header Tax: Some deployments, such as 5G, require minimal packet
       overhead in order to conserve network resources.  Carrying 40 or
       50 octets of data in a packet with hundreds of octet of header
       would be an unacceptable use of available bandwidth.

   With the solution proposed in this document (Section 2), for Path-x
   in Figure 6 above, SID stack would be reduced from 8 SIDs to a single
   SID witout any additional overhead.

A.2.  Mitigation with MSD

   The number of SIDs in the stack a node can impose is referred as
   Maximum SID Depth (MSD) capability [RFC8491], which must be taken
   into consideration when computing a path to transport a data packet
   in a network implementing segment routing.  [I-D.ietf-isis-mpls-elc]
   defines another MSD type, Readable Label Depth (RLD) that is used by
   a head-end to insert Entropy Label pair (ELI/EL) at appropriate
   depth, so it could be read by transit nodes.  There are situations
   where the source routed path can be excessive as path represented by
   SR SIDs need to describe all the nodes and ELI/EL based on the
   readability of the nodes in that path.  Registries setforth in
   [RFC8491] applicable for MPLS data plane and for IPv6 data plane with
   SRH.

   MSDs (and RLD type) capabilities advertisement help mitigate the
   problem for a central entity to create the right source routed path
   per application/operator requirements.  However the availability of
   actual paths meeting these requirements are still limited by the
   underlying hardware and their MSD capabilities in the data path.





Chunduri, et al.         Expires January 9, 2020               [Page 24]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


Authors' Addresses

   Uma Chunduri
   Futurewei
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Email: umac.ietf@gmail.com


   Richard Li
   Futurewei
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Email: richard.li@futurewei.com


   Russ White
   Juniper Networks
   Oak Island, NC  28465
   USA

   Email: russ@riw.us


   Jeff Tantsura
   Apstra Inc.
   333 Middlefield Road
   Menlo Park, CA  94025
   USA

   Email: jefftant.ietf@gmail.com


   Luis M. Contreras
   Telefonica
   Sur-3 building, 3rd floor
   Madrid  28050
   Spain

   Email: luismiguel.contrerasmurillo@telefonica.com







Chunduri, et al.         Expires January 9, 2020               [Page 25]


Internet-Draft    Preferred Path Routing (PPR) in IS-IS        July 2019


   Yingzhen Qu
   Futurewei
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Email: yingzhen.qu@futurewei.com












































Chunduri, et al.         Expires January 9, 2020               [Page 26]

Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/