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Versions: 00 01

Internet Engineering Task Force                                  H. Chen
Internet-Draft                                                     R. Li
Intended status: Experimental                                  Futurewei
Expires: April 6, 2021                                           Y. Yang
                                                                     IBM
                                                            A. Kumar S N
                                                                 RtBrick
                                                                  Y. Fan
                                                            Casa Systems
                                                                   N. So

                                                                  V. Liu

                                                                  M. Toy
                                                                 Verizon
                                                                  L. Liu
                                                                 Fujitsu
                                                            K. Makhijani
                                                               Futurewei
                                                         October 3, 2020


                    IS-IS Topology-Transparent Zone
                     draft-ietf-lsr-isis-ttz-01.txt

Abstract

   This document specifies a topology-transparent zone in an area.  A
   zone is a subset (block/piece) of an area, which comprises a group of
   routers and a number of circuits connecting them.  It is abstracted
   as a virtual entity such as a single virtual node or zone edges mesh.
   Any router outside of the zone is not aware of the zone.  The
   information about the circuits and routers inside the zone is not
   distributed to any router outside of the zone.

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




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   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 April 6, 2021.

Copyright Notice

   Copyright (c) 2020 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.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Zone Abstraction  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Topology-Transparent Zone . . . . . . . . . . . . . . . . . .   5
     4.1.  Zone as a Single Node . . . . . . . . . . . . . . . . . .   5
       4.1.1.  An Example of Zone as a Single Node . . . . . . . . .   5
       4.1.2.  Zone Leader Election  . . . . . . . . . . . . . . . .   7
       4.1.3.  LS Generation for Zone as a Single Node . . . . . . .   8
       4.1.4.  Adjacency Establishment . . . . . . . . . . . . . . .   8
       4.1.5.  Computation of Routes . . . . . . . . . . . . . . . .   9
     4.2.  Extensions to Protocols . . . . . . . . . . . . . . . . .   9
       4.2.1.  Zone ID TLV . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Zone as Edges Full Mesh . . . . . . . . . . . . . . . . .  11
       4.3.1.  Updating LSPs for Zone as Edges' Mesh . . . . . . . .  12
     4.4.  Advertisement of LSPs . . . . . . . . . . . . . . . . . .  12
       4.4.1.  Advertisement of LSPs within Zone . . . . . . . . . .  12
       4.4.2.  Advertisement of LSPs through Zone  . . . . . . . . .  13
   5.  Seamless Migration  . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Transfer Zone to a Single Node  . . . . . . . . . . . . .  13
     5.2.  Roll Back from Zone as a Single Node  . . . . . . . . . .  14
   6.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Configuring Zone  . . . . . . . . . . . . . . . . . . . .  15
     6.2.  Transferring Zone to Node . . . . . . . . . . . . . . . .  16
     6.3.  Rolling back Node to Zone . . . . . . . . . . . . . . . .  16



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   7.  Security  Considerations  . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  18
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   [ISO10589] describes two levels of areas in IS-IS, level 1 and level
   2 areas.  There are scalability issues in using areas as the number
   of routers in a network becomes larger and larger.

   Through splitting the network into multiple level 1 areas connected
   by level 2, we may extend the network further.  However, dividing a
   network from one area into multiple areas or from a number of
   existing areas to even more areas can be a challenging and time
   consuming task since it involves significant network architecture
   changes.

   These issues can be resolved by using topology-transparent zone
   (TTZ), which abstracts a zone (i.e., a subset of an area) as a single
   virtual node or zone edges' mesh with minimum efforts and minimum
   service interruption.  Note that a zone can be an entire area.

   This document presents a topology-transparent zone and specifies
   extensions to IS-IS that support topology-transparent zones.

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

1.2.  Terminology

   TTZ:  A Topology-Transparent Zone.

   Zone:  A subset (block or piece) of an area.  In a special case, a
       zone is an entire area.

   Zone External Node:  A node outside of a zone.

   Zone Internal Node:  A node within a zone without any connection to a
       node outside of the zone.



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   Zone Edge/Border Node:  A node that is part of a zone connecting to a
       node outside of the zone.

   Zone Node:  A zone internal node or a zone edge/border node (i.e., a
       node that is part of a zone).

   Zone Link:  A link connecting zone nodes (i.e., a link that is part
       of a zone).

   Zone Neighbor Node:   A node outside of a zone that is a neighbor of
       a zone edge/border node.

   CLI:  Command Line Interface.

   LSP:  A Link State Protocol Data Unit (PDU) in IS-IS.  An LSP
       contains link state information.  In general, a router/node
       originates multiple LSPs, distinguished by LSP fragment number,
       to carry the link state information about it and the links
       attached to it.

   LS: Link State.  In general, the LS for a node is all the LSPs that
       the node originates.  The LS for a zone is the set of LSPs that
       all the nodes in the zone originate to carry the information
       about them and the links attached to them inside the zone.

2.  Requirements

   A Topology-Transparent Zone (TTZ) may be deployed to resolve some
   critical issues such as scalability in existing networks and future
   networks.  The requirements for a TTZ are as follows:

   o  TTZ MUST be backward compatible.  When a TTZ is deployed on a set
      of routers in a network, the routers outside of the TTZ in the
      network do not need to know or support the TTZ.

   o  TTZ MUST support at least one more levels of network hierarchy, in
      addition to the hierarchies supported by existing IS-IS.

   o  Abstracting a zone as a TTZ virtual entity, which is a single
      virtual node or zone edges' mesh, SHOULD be smooth with minimum
      service interruption.

   o  De-abstracting (or say rolling back) a TTZ virtual entity to a
      zone SHOULD be smooth with minimum service interruption.

   o  Users SHOULD be able to easily set up an end-to-end service
      crossing TTZs.




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   o  The configuration for a TTZ in a network SHOULD be minimal.

   o  The changes on the existing protocols to support TTZ SHOULD be
      minimal.

3.  Zone Abstraction

   A zone can be abstracted as a single virtual node or the zone edges'
   full mesh.

   When a zone is abstracted as a single virtual node, this node appears
   to be connected to all the neighbors of the zone, and to be in the
   same area as those neighbors.

   When a zone is abstracted as its edges' full mesh, there is a full
   mesh connections among the edges and each edge is also connected to
   its neighbors outside of the zone.

4.  Topology-Transparent Zone

   A Topology-Transparent Zone (TTZ) comprises an Identifier (ID) and a
   subset (piece/block) of an area such as a Level 2 area in IS-IS.  It
   is abstracted as a single virtual node or its edges' full mesh.  TTZ
   and zone as well as node and router will be used interchangeably
   below.

4.1.  Zone as a Single Node

   After a zone is abstracted as a single virtual node having a virtual
   node ID, every node outside of the zone sees a number of links
   connected to this single node.  Each of these links connects a zone
   neighbor.  The link states inside the zone are not advertised to any
   node outside of the zone.  The virtual node ID may be derived from
   the zone ID.

4.1.1.  An Example of Zone as a Single Node

   The figure below shows an example of an area containing a TTZ: TTZ
   600.












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                 TTZ 600
                   \
                    \ ^~^~^~^~^~^~^~^~^~^~^~^~
                     (                        )
    ===[R15]========(==[R61]------------[R63]==)======[R29]===
        ||         (   |    \          /    |   )       ||
        ||         (   |     \        /     |   )       ||
        ||         (   |      \      /      |   )       ||
        ||         (   |    ___\    /       |   )       ||
        ||         (   |   /   [R71]        |   )       ||
        ||         (   | [R73] /    \       |   )       ||
        ||         (   |      /      \      |   )       ||
        ||         (   |     /        \     |   )       ||
        ||         (   |    /          \    |   )       ||
    ===[R17]========(==[R65]------------[R67]==)======[R31]===
         \\          (//                    \\)       //
          ||         //v~v~v~v~v~v~v~v~v~v~v~\\      ||
          ||        //                        \\     ||
          ||       //                          \\    ||
           \\     //                            \\  //
       ======[R23]==============================[R25]=====
             //                                     \\
            //                                       \\


                      Figure 1: An Example of TTZ 600

   The area comprises routers R15, R17, R23, R25, R29 and R31.  It also
   contains TTZ 600, which comprises routers R61, R63, R65, R67, R71 and
   R73, and the circuits connecting them.

   There are two types of routers in a TTZ: TTZ internal routers and TTZ
   edge/border routers.  A TTZ internal router is a router inside the
   TTZ and its adjacent routers are inside the TTZ.  A TTZ edge/border
   router is a router inside the TTZ and has at least one adjacent
   router that is outside of the TTZ.

   The TTZ in the figure above comprises four TTZ edge/border routers
   R61, R63, R65 and R67.  Each TTZ edge/border router is connected to
   at least one router outside of the TTZ.  For instance, router R61 is
   a TTZ edge/border router since it is connected to router R15, which
   is outside of the TTZ.

   In addition, the TTZ comprises two TTZ internal routers R71 and R73.
   A TTZ internal router is not connected to any router outside of the
   TTZ.  For instance, router R71 is a TTZ internal router since it is
   not connected to any router outside of the TTZ.  It is just connected
   to routers R61, R63, R65, R67 and R73 inside the TTZ.



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   A TTZ MUST hide the information inside the TTZ from the outside.  It
   MUST NOT directly distribute any internal information about the TTZ
   to a router outside of the TTZ.

   For instance, the TTZ in the figure above MUST NOT send the
   information about TTZ internal router R71 to any router outside of
   the TTZ in the routing domain; it MUST NOT send the information about
   the circuit between TTZ router R61 and R65 to any router outside of
   the TTZ.

   From a router outside of the TTZ, a TTZ is seen as a single node
   (refer to the Figure below).  For instance, router R15, which is
   outside of TTZ 600, sees TTZ 600 as a single node Rz, which has
   normal connections to R15, R29, R17 and R23, R25 and R31.

                 TTZ 600
                   \
                    \ ^~^~^~^~^~^~^~^~^~^~^~^~
                     (                        )
    ===[R15]========(                          )======[R29]===
        ||         (                            )       ||
        ||         (                            )       ||
        ||         (                            )       ||
        ||         (                            )       ||
        ||         (             Rz             )       ||
        ||         (                            )       ||
        ||         (                            )       ||
        ||         (                            )       ||
        ||         (                            )       ||
    ===[R17]========(                          )======[R31]===
         \\          (                        )       //
          ||         //v~v~v~v~v~v~v~v~v~v~v~\\      ||
          ||        //                        \\     ||
          ||       //                          \\    ||
           \\     //                            \\  //
       ======[R23]==============================[R25]=====
             //                                     \\
            //                                       \\


                    Figure 2: TTZ 600 as Single Node Rz

4.1.2.  Zone Leader Election

   A node in a zone is elected as a leader for the zone, which is the
   node with the highest priority (and the highest node ID when there
   are more than one nodes having the same highest priority) in the
   zone.  The leader election mechanism described in



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   [I-D.ietf-lsr-dynamic-flooding] may be used to elect the leader for
   the zone.

4.1.3.  LS Generation for Zone as a Single Node

   The leader for the zone originates the LS (i.e., set of LSPs) for the
   zone as a single virtual node and sends it to its neighbors.

   This LS comprises all the adjacencies between the virtual node and
   the zone neighbors.  The System ID of each LSP ID is the ID of the
   virtual node for the zone.  The Source ID or Advertising Node/Router
   ID is the ID of the virtual node.

   In addition, this LS may contain the IP prefixes such as the loopback
   IP addresses inside the zone to be accessed by zone external nodes
   (i.e., nodes outside of the zone).  These IP prefixes are included in
   the IP internal reachability TLV.

4.1.4.  Adjacency Establishment

   A zone edge node X, acting as a proxy for the single virtual node for
   the zone, forms an adjacency between the virtual node and a node Y
   that is outside of the zone and in node X's area as described below.

   For a new adjacency (i.e., no adjacency exists between X and Y):

   Every IS-IS protocol packet, such as Hello, that edge node X
   originates and sends node Y, uses the virtual node ID as Source ID.

   When node X synchronizes its link state database (LSDB) with node Y,
   it sends Y all the link state information except for the link state
   belonging to the zone that is hidden from the nodes outside of the
   zone.

   At the end of the LSDB synchronization, the LS for the zone as a
   single virtual node is originated by the zone leader and distributed
   to node Y.  This LS contains the adjacencies between the virtual node
   and all the zone neighbors, including this newly formed zone neighbor
   Y.

   For an existing adjacency (i.e., an adjacency already exists between
   X and Y):

   At first, edge node X acting as a proxy for the virtual node creates
   a new adjacency between the virtual node for the zone and node Y in a
   normal way.  It sends Hellos and other packets containing the virtual
   node ID as Source ID to node Y.  Node Y establishes an adjacency with
   the virtual node in the normal way.



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   Then, node X terminates the existing adjacency between node X and
   node Y after the zone has transitioned to be the virtual node.  It
   stops sending Hellos for the adjacency to node Y.  Without receiving
   Hellos from node X for a given time such as hold-timer interval, node
   Y removes the adjacency to node X.  Even though this adjacency
   terminates, node X keeps the link to node Y in its LS.

   In the case where node Y is not in node X's area, is in the backbone
   and connected to node X, node X, acting as a proxy for the virtual
   node, creates a new adjacency between the virtual node and node Y in
   a normal way and sends the LS for the virtual node to node Y if the
   zone includes all the nodes in its area.

4.1.5.  Computation of Routes

   After a zone is transferred/migrated to a single virtual node, every
   zone node computes the routes (i.e., shortest paths to the
   destinations) using the zone topology, the connections between each
   zone edge and its zone neighbor, and the topology outside of the zone
   without the virtual node.  The metric of a link outside of the zone
   is one order of magnitude larger than the metric of a link inside the
   zone.

4.2.  Extensions to Protocols

   The following TLV is defined in IS-IS.

   o  Zone ID TLV: containing a zone ID, a flags field and optional sub-
      TLVs.

4.2.1.  Zone ID TLV

   The format of IS-IS Zone ID TLV is illustrated below.  It may be
   added into an LSP for a zone node.

       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 (TBD1)  |     Length    |            Zone ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Zone ID (Continue)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  RESV                 |E|  OP |            Sub TLVs           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 3: IS-IS Zone ID TLV



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   Type (1 byte): TBD1.

   Length (1 byte): Its value is variable with a minimum of 8.  A value
   larger than 8 means that sub-TLVs are present.  If length is less
   than 8, the TLV MUST be ignored.

   Zone ID (6 bytes): It is the identifier (ID) of a zone.

   Flags field (16 bits): one flag bit E, OP of 3 bits, and a reserved
   subfield are as follows:

      RESV: Reserved. MUST be send as zero and ignored on receipt.
      E = 1: Indicating a node is a zone edge node
      E = 0: Indicating a node is a zone internal node

   When a Zone ID is configured on a zone node (refer to Section 6.1),
   the node updates its LSP by adding an IS-IS Zone ID TLV with the Zone
   ID.  If it is a zone internal node, the TLV has its flag E = 0;
   otherwise (i.e., it is a zone edge node) the TLV has its flag E = 1
   and may include a Zone ISN Sub TLV containing the zone links
   configured.  Every link of a zone internal node is a zone link.  If
   every link of a zone edge node is a zone link, the TLV with E = 1
   does not include any Zone ISN Sub TLV; otherwise (i.e., some of its
   links are zone links), it includes the Zone ISN Sub TLV containing
   the zone links.

     OP Value    Meaning (Operation)
     0x001 (T):  Advertising Zone Topology Information for Migration
     0x010 (M):  Migrating Zone to a Virtual Entity such as Virtual Node
     0x011 (N):  Advertising Normal Topology Information for Rollback
     0x100 (R):  Rolling Back from the Virtual Entity

   The value of OP indicates one of the four operations above.  When any
   of the other values is received, the TLV MUST be ignored.

   When a zone node, such as the zone leader, receives a command via
   management, such as a CLI command, to advertise zone information for
   migration, or determines to advertise, it sets OP = T (i.e., 0x001)
   in the Zone ID TLV of its LSP.  When a zone node receives a command
   to migrate zone to a virtual entity, or determines to migrate, it
   sets OP = M (i.e., 0x010) in the Zone ID TLV of its LSP.  When a zone
   node receives a command to advertise Normal topology information for
   roll back, it sets OP = N (i.e., 0x011) in the Zone ID TLV of its
   LSP.  When a zone node receives a command to roll back or determines
   to roll back, it sets OP = R (i.e., 0x100) in the Zone ID TLV of its
   LSP.





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   Two new sub-TLVs are defined, which may be added to an IS-IS Zone ID
   TLV.  One is the Zone IS Neighbor sub-TLV, or Zone ISN sub-TLV for
   short.  The other is the Zone ES Neighbor sub-TLV, or Zone ESN sub-
   TLV for short.  A Zone ISN sub-TLV contains the information about a
   number of IS neighbors in the zone connected to a zone edge node.  It
   has the format below.

       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 (TBD)   |    Length     |        Neighbor ID(i)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      +               +-----------------------------------------------+
      |               |           Metric (i)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 4: Zone ISN Sub TLV

   A Zone ISN Sub TLV has 1 byte of Type, 1 byte of Length of
   n*(IDLength + 3), which is followed by n tuples of Neighbor ID and
   Metric.

   A Zone ESN sub-TLV contains the information about a number of ES
   neighbors in the zone connected to a zone edge node.  It has the
   format below.

       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 (TBD)   |    Length     |        Neighbor ID(i)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      +               +-----------------------------------------------+
      |               |           Metric (i)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 5: Zone ESN Sub TLV

4.3.  Zone as Edges Full Mesh







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4.3.1.  Updating LSPs for Zone as Edges' Mesh

   For every zone edge node, it updates its LSP in three steps and
   floods the LSP to all its neighbors.

   At first, it adds each of the other zone edge nodes as an IS neighbor
   into the Intermediate System Neighbors TLV in its LSP after it
   receives an LSP containing an IS-IS Zone ID TLV with OP = M or a
   command activating migration zone to an abstracted entity.  The
   metric to the neighbor is the metric of the shortest path to the edge
   node within the zone.

   In addition, it adds an IP internal reachability TLV into its LSP.
   The TLV contains a number of IP prefixes in the zone to be reachable
   from outside of the zone.

   And then it removes the IS neighbors corresponding to the IS
   neighbors in the Zone ID TLV (i.e., in the Zone ISN sub TLV) from
   Intermediate System Neighbors TLV in its LSP, and the ES neighbors
   corresponding to the ES neighbors in the Zone ID TLV (i.e., in the
   Zone ESN sub TLV) from End System Neighbors TLV in the LSP.  This
   SHOULD be done after it receives the LSPs for virtualizing zone from
   the other zone edges for a given time.

4.4.  Advertisement of LSPs

   LSPs can be divided into a couple of classes according to their
   Advertisements.  The first class of LSPs is advertised within a zone.
   The second is advertised through a zone.

4.4.1.  Advertisement of LSPs within Zone

   Any LSP about a link state in a zone is advertised only within the
   zone.  It is not advertised to any router outside of the zone.  For
   example, a LSP generated for a zone internal node is advertised only
   within the zone.

   Any LSP generated for a broadcast network in a zone is advertised
   only within the zone.  It is not advertised outside of the zone.

   After migrating to a zone as a single virtual node or edges' full
   mesh, every zone edge MUST NOT advertise any LSP belonging to the
   zone or any information in a LSP belonging to the zone to any node
   outside of the zone.  The zone edge determines whether an LSP is
   about a zone internal link state by checking if the originating node
   of the LSP is a zone internal node.





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   For any LSP originated by a node within the zone, every zone edge
   node MUST NOT advertise it to any node outside of the zone.

4.4.2.  Advertisement of LSPs through Zone

   Any LSP about a link state outside of a zone received by a zone edge
   is advertised using the zone as transit.  For example, when a zone
   edge node receives an LSP from a node outside of the zone, it floods
   the LSP to its neighbors both inside and outside of the zone.

   The nodes in the zone continue to flood the LSP.  When another zone
   edge receives the LSP, it floods the LSP to its neighbors both inside
   and outside of the zone.

5.  Seamless Migration

   This section presents the seamless migration between a zone and its
   single virtual node.

5.1.  Transfer Zone to a Single Node

   Transferring a zone to a single virtual node smoothly takes a few
   steps or stages.

   At first, a user configures the zone on every node of the zone (refer
   to Section 6.1).  Every zone node updates its LSP by including a Zone
   ID TLV.  For a zone edge node, the TLV has the Zone ID configured,
   its flag E = 1 and a Zone ISN Sub TLV containing the zone links
   configured.  For a zone internal node, the TLV has the Zone ID
   configured and its flag E = 0.

   Second, after finishing the configuration of the zone, a user may
   issue a command, such as a CLI command, on a zone node, such as the
   zone leader, to trigger transferring the zone to the single virtual
   node.  When the node receives the command, it updates its LSP by
   setting OP = T in its Zone ID TLV, which is distributed to every zone
   node.  After receiving the Zone ID TLV with OP = T, every zone edge
   node, acting as a proxy of the virtual node, establishes a new
   adjacency between the virtual node and each of its zone neighbor
   nodes.

   The command may be replaced by the determination made by a zone node,
   such as the zone leader.  After determining that the configuration of
   the zone is finished for a given time such as 10 seconds, it updates
   its LSP by setting OP = T in its Zone ID TLV.  The configuration is
   complete if every zone link configured is bidirectional.  For every
   zone internal node configured with the Zone ID, there is an LSP
   containing its Zone ID TLV with E = 0 in the LSDB, which indicates



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   that each link from the node (one direction) is a zone link.  For
   every zone edge node, each of its zone links configured from the edge
   node (one direction) is included in its LSP containing its Zone ID
   TLV with E = 1 and Zone ISN Sub TLV in the LSDB.

   Third, after receiving the updated LSPs from all the zone neighbor
   nodes, the zone leader checks if all the new adjacencies between the
   virtual node and the zone neighbor nodes have been established.  If
   so, it originates an LS for the virtual node and updates its LSP
   (i.e., the LSP for itself zone leader) by setting OP = M in its Zone
   ID TLV, which is distributed to every zone node.

   After receiving the Zone ID TLV with OP = M, every zone node migrates
   to zone as virtual node.  Every zone edge node does not send any LS
   inside the zone to any zone neighbors.  It advertises its LSP without
   any zone links to the nodes outside of the zone or purges its LSP
   outside of the zone, terminates its adjacency to each of its zone
   neighbors, but contains the adjacency in its LSP that is distributed
   within the zone.  Every zone node computes the routes according to
   Section 4.1.5.

5.2.  Roll Back from Zone as a Single Node

   After abstracting a zone to a single virtual node, we may want to
   roll back the node to the zone smoothly in some cases.  The process
   of rolling back has a few steps or stages.

   At first, a user issues a command, such as a CLI command, on a zone
   node, such as the zone leader, to start (or prepare) for roll back.
   When receiving the command, the node updates its LSP by setting OP =
   N in its Zone ID TLV, which will be distributed to every node in the
   zone.  After receiving the Zone ID TLV with OP = N, every zone edge
   node establishes a normal adjacency between the edge node and each of
   its zone neighbor nodes, and advertises the link state of the zone
   over the adjacency if it crosses the adjacency, but holds off its LSP
   containing the normal adjacency.

   Second, a user may issue a command, such as a CLI command, on a zone
   node, such as the zone leader, to roll back from the virtual node to
   the zone if the following conditions are met.

   Condition 1:  All the normal adjacencies between every zone edge node
      and each of its zone neighbor nodes have been established.

   Condition 2:  All the link state about the zone that is supposed to
      be advertised outside of the zone has been advertised.





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   After receiving the command, the node updates its LSP by setting OP =
   R in its Zone ID TLV, which is distributed to every zone node.  After
   receiving the Zone ID TLV with OP = R,

   o  every zone edge node, acting as a proxy of the virtual node,
      terminates the adjacency between the virtual node and each of its
      zone neighbor nodes and advertises its LSP containing the normal
      adjacencies between it and each of its zone neighbor nodes;

   o  The zone leader purges the LS for the virtual node abstracted from
      the zone; and

   o  Every zone node rolls back to normal.

   The command may be replaced by the determination made by a zone node,
   such as the zone leader.  After determining that all the conditions
   are met, it updates its LSP by setting OP = R in its Zone ID TLV,
   which is distributed to every zone node.

   Condition 1 is met if it has its LSDB containing the link from each
   zone neighbor node to its zone edge node.  That is that for every
   link from a zone neighbor node to the virtual node in the LSDB, there
   is a corresponding link from the zone neighbor to a zone edge node.

   Condition 2 is met after Condition 1 has been met for a given time,
   such as maximum LSP advertisement time (MaxLSPAdvTime) crossing a
   network.  We may assume that MaxLSPAdvTime is 5 seconds.

6.  Operations

6.1.  Configuring Zone

   In general, a zone is a subset of an area and has a zone ID.  It
   consists of some zone internal nodes and zone edge nodes.  To
   configure it, a user configures this zone ID on every zone internal
   node and on every zone link of each zone edge node.

   A node configured with the zone ID has all its links to be the zone
   links.  The zone internal nodes and all their links plus the zone
   edge nodes and their zone links constitute the zone.

   In a special case, a zone is an entire area and has a zone ID.  All
   the links in the area are the zone links of the zone.  To configure
   this zone, a user configures the zone ID on every zone node.







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6.2.  Transferring Zone to Node

   Transferring a zone to a single virtual node smoothly may take a few
   steps or stages.

   At first, a user configures the zone on every node of the zone.

   After finishing the configuration of the zone, the user may issue a
   command, such as a CLI command, on a zone node, such as the zone
   leader, to trigger transferring the zone to the node.  When receiving
   the command, the node distributes it to every zone node.  After
   receiving it, every zone edge node, acting as a proxy of the virtual
   node, establishes a new adjacency between the virtual node and each
   of its zone neighbor nodes.

   If automatic transferring zone to node is enabled, the user does not
   need to issue the command.  A zone node, such as the zone leader,
   will distribute the "command" to every zone node after determining
   that the configuration of the zone has been finished.

   Then, all the zone nodes, including the zone leader, zone edge nodes
   and zone internal nodes, work together to make the zone to appear as
   a single virtual node smoothly in a couple of steps.

6.3.  Rolling back Node to Zone

   After abstracting a zone to a single virtual node, we may want to
   roll back the node to the zone smoothly in some cases.  The process
   of rolling back has a few steps or stages.

   At first, a user issues a command, such as a CLI command, on a zone
   node, such as the zone leader, to start (or prepare) for roll back.
   When receiving the command, the node distributes it to every node in
   the zone.  After receiving it, every zone edge node establishes a
   normal adjacency between the edge node and each of its zone neighbor
   nodes, and advertises the link state of the zone over the adjacency
   if it crosses the adjacency, but holds off its LSP containing the
   normal adjacency.

   Second, a user may issue a command, such as a CLI command, on a zone
   node, such as the zone leader, to roll back from the virtual node to
   the zone if it is ready for roll back.

   After receiving the command, the node distributes it to every node in
   the zone.  After receiving it, all the zone nodes work together to
   roll back from the virtual node to the zone.





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   If automatic roll back Node to Zone is enabled, the user does not
   need to issue the command.  A zone node, such as the zone leader,
   will distribute the "command" to every zone node after determining
   that it is ready for roll back.

7.  Security Considerations

   The mechanism described in this document does not raise any new
   security issues for the IS-IS protocols.

8.  IANA Considerations

   Under the registry name "IS-IS TLV Codepoints", IANA is requested to
   assign a new registry type for Zone ID as follows:

     +==============+===================+=====================+
     |  TLV Type    |      TLV Name     |    reference        |
     +==============+===================+=====================+
     |  TBD1        | Zone ID           |    This document    |
     +--------------+-------------------+---------------------+

   IANA is requested to create a new sub-registry "Adjacent Node ID Sub-
   TLVs" on the IANA IS-IS TLV Codepoints web page as follows:

     +==============+===================+=====================+
     |     Type     |      Name         |    reference        |
     +==============+===================+=====================+
     |     0        |                Reserved                 |
     +--------------+-------------------+---------------------+
     |     1        |    Zone ISN       |    This document    |
     +--------------+-------------------+---------------------+
     |     2        |    Zone ESN       |    This document    |
     +--------------+-------------------+---------------------+
     |     3 - 255  |                Unassigned               |
     +--------------+-------------------+---------------------+


9.  Contributors

        Alvaro Retana
        Futurewei
        Raleigh, NC
        USA

        Email: alvaro.retana@futurewei.com






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10.  Acknowledgement

   The authors would like to thank Acee Lindem, Abhay Roy, Christian
   Hopps, Dean Cheng, Russ White, Tony Przygienda, Wenhu Lu, Lin Han,
   Donald Eastlake, Tony Li, Robert Raszuk, Padmadevi Pillay Esnault,
   and Yang Yu for their valuable comments on TTZ.

11.  References

11.1.  Normative References

   [I-D.ietf-lsr-dynamic-flooding]
              Li, T., Psenak, P., Ginsberg, L., Chen, H., Przygienda,
              T., Cooper, D., Jalil, L., Dontula, S., and G. Mishra,
              "Dynamic Flooding on Dense Graphs", draft-ietf-lsr-
              dynamic-flooding-07 (work in progress), June 2020.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [ISO10589]
              International Organization for Standardization,
              "Intermediate System to Intermediate System Intra-Domain
              Routing Exchange Protocol for use in Conjunction with the
              Protocol for Providing the Connectionless-mode Network
              Service (ISO 8473)", ISO/IEC 10589:2002, Nov. 2002.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [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>.

   [RFC5029]  Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
              Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
              September 2007, <https://www.rfc-editor.org/info/rfc5029>.

   [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>.





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   [RFC7142]  Shand, M. and L. Ginsberg, "Reclassification of RFC 1142
              to Historic", RFC 7142, DOI 10.17487/RFC7142, February
              2014, <https://www.rfc-editor.org/info/rfc7142>.

   [RFC8099]  Chen, H., Li, R., Retana, A., Yang, Y., and Z. Liu, "OSPF
              Topology-Transparent Zone", RFC 8099,
              DOI 10.17487/RFC8099, February 2017,
              <https://www.rfc-editor.org/info/rfc8099>.

   [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>.

11.2.  Informative References

   [Clos]     Clos, C., "A Study of Non-Blocking Switching Networks",
              The Bell System Technical Journal Vol. 32(2), DOI
              10.1002/j.1538-7305.1953.tb01433.x, March 1953,
              <http://dx.doi.org/10.1002/j.1538-7305.1953.tb01433.x>.

   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <https://www.rfc-editor.org/info/rfc5307>.

Authors' Addresses

   Huaimo Chen
   Futurewei
   Boston, MA
   USA

   Email: huaimo.chen@futurewei.com


   Richard Li
   Futurewei
   2330 Central expressway
   Santa Clara, CA
   USA

   Email: richard.li@futurewei.com









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   Yi Yang
   IBM
   Cary, NC
   United States of America

   Email: yyietf@gmail.com


   Anil Kumar S N
   RtBrick
   Bangalore
   India

   Email: anil.ietf@gmail.com


   Yanhe Fan
   Casa Systems
   USA

   Email: yfan@casa-systems.com


   Ning So
   Plano, TX  75082
   USA

   Email: ningso01@gmail.com


   Vic Liu
   USA

   Email: liu.cmri@gmail.com


   Mehmet Toy
   Verizon
   USA

   Email: mehmet.toy@verizon.com


   Lei Liu
   Fujitsu
   USA

   Email: liulei.kddi@gmail.com



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   Kiran Makhijani
   Futurewei
   USA

   Email: kiranm@futurewei.com














































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