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Internet Engineering Task Force                                  H. Chen
Internet-Draft                                                     R. Li
Intended status: Experimental                                  Futurewei
Expires: March 18, 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
                                                      September 14, 2020


                    IS-IS Topology-Transparent Zone
                     draft-ietf-lsr-isis-ttz-00.txt

Abstract

   This document presents a topology-transparent zone in an area.  A
   zone is a 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 March 18, 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  . . . . . . . . . . . . . . . . . . . . . .   4
   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 and Termination . . . . . . .   8
       4.1.5.  Computation of Routes . . . . . . . . . . . . . . . .  10
       4.1.6.  Extensions to Protocols . . . . . . . . . . . . . . .  11
     4.2.  Zone as Edges Full Mesh . . . . . . . . . . . . . . . . .  14
       4.2.1.  Extensions to IS-IS . . . . . . . . . . . . . . . . .  14
     4.3.  Advertisement of LSs  . . . . . . . . . . . . . . . . . .  15
       4.3.1.  Advertisement of LSs within Zone  . . . . . . . . . .  15
       4.3.2.  Advertisement of LSs through Zone . . . . . . . . . .  16
   5.  Seamless Migration  . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Transfer Zone to a Single Node  . . . . . . . . . . . . .  16
     5.2.  Roll Back from Zone as a Single Node  . . . . . . . . . .  16
   6.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Security  Considerations  . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  20



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   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     11.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   [ISO10589] describes two levels of areas, which are level 1 and level
   2 areas in IS-IS.  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 areas, 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
   is a very challenging and time consuming task since it is involved in
   significant network architecture changes.

   These issues can be resolved by using topology-transparent zone
   (TTZ), which abstracts a zone (i.e., a block/piece 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 area (i.e.,
   the entire piece of an area).

   This document presents a topology-transparent zone and describes
   extensions to IS-IS for supporting the topology-transparent zone.

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 RFC 2119 [RFC2119].

1.2.  Terminology

   LSP:  A Link State Protocol Data Unit (PDU) in IS-IS.

   LS:   A Link Sate, which is short for LSP in IS-IS.

   TTZ:  A Topology-Transparent Zone.

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

   Zone External Node:  A node outside of a zone.

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




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   Zone Edge/Border:  A node 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 of a zone).

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

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

2.  Requirements

   Topology-Transparent Zone (TTZ) may be deployed for resolving some
   critical issues such as scalability in existing networks and future
   networks.  The requirements for TTZ are listed 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 TTZ.

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

   o  Abstracting a zone as a 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 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.

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

   o  The changes on the existing protocols for supporting TTZ SHOULD be
      minimum.

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 single node
   is connected to all the neighbors of the zone, and is in the same
   area as the neighbors.



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   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
   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 will be used exchangably 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 an LS (i.e., an LSP in IS-IS) for
   the zone as a single virtual node and sends it to its neighbors.

   The LS comprises all the links connecting the zone neighbors.  The LS
   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, the LS may contain the stub links for the routes such as
   the loopback addresses inside the zone to be accessed by zone
   external nodes (i.e., nodes outside of the zone).

4.1.4.  Adjacency Establishment and Termination

   A zone edge node, acting as a single virtual node for the zone, forms
   an adjacency with a node outside of the zone in a way described
   below.

   Case 1 for a new adjacency (i.e., no adjacency exists between the
   edge and the node outside of the zone also called zone neighbor):

   The edge node originates and sends the zone neighbor every protocol
   packet such as Hello, which contains the virtual node ID as Source
   ID.

   When the edge node synchronizes its link state database (LSDB) with
   the zone neighbor, it sends the zone neighbor the information about
   all the link states except for the link states belonging to the zone
   that are hidden from any node outside of the zone.

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

   Case 2 for an existing adjacency (i.e., an adjacency already exists
   between the zone edge and the zone neighbor):

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




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   And then, the edge terminates the existing adjacency between the edge
   and the external node after the zone has been transferred to the
   virtual node.  It stops sending Hellos for the adjacency to the zone
   external node.  Without receiving Hellos from the edge node for a
   given time such as hold-timer interval, the zone external node
   removes the adjacency to the edge node.  Even though this adjacency
   terminates, the edge node keeps the link to the external node in its
   LS.

   In another option, the zone edge sends Hellos to the zone neighbor
   with additional information, including a flag T-bit set to one and a
   TLV with the virtual node ID.  This information requests the zone
   neighbor to transfer the existing adjacency to the new adjacency
   smoothly through working together with the zone edge in following
   steps.

        Zone Edge                                Zone Neighbor
     (Transfer Zone
     to Virtual Node)    Hello(T=1, Virtual ID)
                        ----------------------> OK for Transfer
                                                Adjacency
                         Hello(T=1, Virtual ID)
      Remote Ready for <----------------------
      Transfer
                       Hello(Source=Virtual ID)
      Start Transfer   -----------------------> Transfer to New
                                                Adjacency
                        Hello
      Transfer to New  <-----------------------
      Adjacency                  . . .

   Step 1:  When "Transfer Zone to Virtual Node" is triggered, the zone
      edge sends the zone neighbor a Hello containing additional
      information T=1 and Virtual node ID.

   Step 2:  After receiving the Hello with T=1 and virtual node ID from
      the zone edge, the zone neighbor sends the zone edge a Hello with
      T=1 and virtual node ID, which means ok for transfer to the new
      adjacency.

   Step 3:  The edge sends the zone neighbor a Hello containing the
      virtual node ID as Source ID after receiving the Hello with T=1
      and virtual node ID from the zone neighbor, which starts to
      transfer to the new adjacency.

   Step 4:  The zone neighbor changes the existing adjacency to the new
      adjacency after receiving the Hello containing the virtual node ID
      as Source ID from the zone edge; and sends the zone edge a Hello



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      without the additional information, which means that it
      transferred to the new adjacency.

   Step 5:  The zone edge changes the existing adjacency to the new
      adjacency after receiving the Hello without the additional
      information from the zone neighbor; and continues to send the zone
      neighbor a Hello containing the virtual node ID as Source ID.  At
      this point, the old adjacency is transferred to the new one.

   For the zone neighbor, changing the existing adjacency to the new one
   includes:

   o  Changing the existing adjacency ID from the edge node ID to the
      virtual node ID through either removing the existing adjacency and
      adding a new adjacency with the virtual node ID or just changing
      the existing adjacency ID from the edge node ID to the virtual
      node ID,

   o  Removing the link to the zone edge node from its LS and adding a
      new link to the virtual node (or just changing the link to the
      edge node to the link to the virtual node in its LS), and

   o  Continuing sending the zone edge Hellos without additional
      information.

   For the zone edge, changing the existing adjacency to the new one
   includes:

   o  Keeping the link to the zone neighbor in its LS, and

   o  Continuing sending the zone neighbor Hellos containing the virtual
      node ID as Source ID.

4.1.5.  Computation of Routes

   After a zone edge migrates to zone as a virtual node, it computes the
   routes (i.e., shortest paths to the destinations) in the zone using
   the zone topology (i.e., the topology of the zone without the virtual
   node).

   For the routes outside of the zone, it computes them using the zone
   topology, the topology outside of the zone without the virtual node
   and the connections between each zone edge and its zone neighbor.

   After a zone internal node migrates to zone as a virtual node, it
   computes the routes using the zone topology, the topology outside of
   the zone without the virtual node and the connections between each
   zone edge and its zone neighbor.



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4.1.6.  Extensions to Protocols

   The following TLVs are defined in IS-IS.

   o  Adjacent Node ID TLV: containing an adjacent node ID, to which an
      adjacency is transferred or rolled back.  In case of transfer, the
      TLV contains the virtual node ID; in case of roll back, the TLV
      contains the edge node ID.

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

4.1.6.1.  Adjacent Node ID TLV

   The format of Adjacent Node ID TLV is illustrated 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 (TBD1)  |  Length (8)   |     Virtual/Edge Node ID      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Virtual/Edge Node ID (Continue)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Flags         |N|T|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type (1 byte): To be assigned by IANA.

   Length (1 byte): Its value is 8.

   Virtual/Edge Node ID (6 bytes): An adjacent node ID, to which an
   adjacency is transferred or rolled back.

   Flags field (16 bits): two new flag bits are defined as follows:

   o  T-bit: Short for Transfer Adjacency bit.  The T-bit set to one
      indicates a request for transferring to a new 'virtual' adjacency
      from the existing adjacency and the new adjacency is identified by
      the virtual node ID (or say abstract node ID).

   o  N-bit: Short for Roll Back to Normal Adjacency bit.  The N-bit set
      to one indicates a request for rolling back to a Normal adjacency
      from the existing 'virtual' adjacency and the normal adjacency is
      identified by the edge node ID.






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4.1.6.2.  Zone TLV

   The format of IS-IS Zone TLV is illustrated below.  It may be added
   into an LSP or a Hello PDU for a zone node.  When a node in a zone
   receives a CLI command triggering zone information distribution for
   migration, it updates its LSP by adding an IS-IS Zone TLV with T set
   to 1.  When a node in a zone receives a CLI command activating
   migration zone to an abstracted entity, it sets M to 1 in the Zone
   TLV in its LSP.

       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 (TBD2)  |     Length    |            Zone ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Zone ID (Continue)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Flags             |E|Z|S|  OP |            Sub TLVs           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: IS-IS Zone TLV

   Type (1 byte): To be assigned by IANA.

   Length (1 byte): Its value is variable.

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

   Flags field (16 bits): Three flag bits E, Z and S, and OP of 3 bits
   are defined.

      E = 1: Indicating a node is a zone edge node
      Z = 1: Indicating a node has migrated to Zone as a virtual entity
      S = 1: Indicating the virtual entity is a Single virtual node

   When a zone node originates an LS containing a zone TLV, it MUST set
   flag E to 1 if it is a zone edge node and to 0 if it is a zone-
   internal node.  It MUST set flag Z to 1 after it has migrated to zone
   as a virtual entity and to 0 before it migrates zone to the virtual
   entity or after it rolls back from zone as a virtual entity.  When
   the entity abstracted from a zone is a Single virtual node, flag S
   MUST be set to 1.







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     OP Value    Meaning (Operation)
     0x001 (T):  Advertising Zone Topology Information for Migration
     0x010 (M):  Migrating Zone to a Virtual Entity
     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, it is ignored.

   When a node in a zone receives a CLI command triggering zone
   information distribution for migration, it updates its LSP by adding
   an IS-IS Zone TLV with T set to 1.  When a node in a zone receives a
   CLI command activating migration zone to a virtual entity, it sets M
   to 1 in the Zone TLV in its LSP.

   Two new sub-TLVs are defined, which may be added into an IS-IS Zone
   TLV in an LSP.  One is Zone IS Neighbor sub-TLV, or Zone ISN sub-TLV
   for short.  The other is 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 router.
   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    |DefaultMetric(i| DelayMetric(i)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |ExpenseMetric(i| ErrorMetric(i)|        Neighbor ID(i)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 4: Zone ISN Sub TLV

   A Zone ISN Sub TLV has 1 byte of Type, 1 byte of Length of
   n*(IDLength + 4), which is followed by n tuples of Default Metric,
   Delay Metric, Expense Metric, Error Metric and Neighbor ID.

   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.










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       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    |Default Metric |  DelayMetric  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Expense Metric |  Error Metric |        Neighbor ID(i)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 5: Zone ESN Sub TLV

4.2.  Zone as Edges Full Mesh

   OSPF Topology-Transparent Zone [RFC8099] describes the zone as edges'
   full mesh and the extensions to OSPF for supporting zone as edges'
   full mesh.  Based on these extensions, IS-IS is extended by a few new
   TLVs or Sub-TLVs.

4.2.1.  Extensions to IS-IS

4.2.1.1.  Updating LSPs for Zone

   A zone internal node adds an IS-IS Zone TLV into its LSP after it
   receives an LSP containing an IS-IS Zone TLV with T = 1 or a CLI
   command triggering zone information distribution for migration.  The
   TLV has a zone ID set to the ID of the zone and E bit in Flags set to
   0 indicating zone internal node.  The node floods its LSP to its
   neighbors in the zone.

   When a node inside the zone receives an LSP containing an IS-IS Zone
   TLV from a neighboring node in the zone, it stores the LSP and floods
   the LSP to the other neighboring nodes in the zone.

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

   At first, the zone edge node adds an IS-IS Zone TLV into its LSP
   after it receives an LSP containing an IS-IS Zone TLV with T = 1 or a
   CLI command triggering zone information distribution for migration.
   The TLV has a zone ID set to the ID of the zone, E bit in Flags set
   to 1 indicating zone edge node and a Zone ISN Sub TLV.  The Sub TLV
   contains the information about the zone IS neighbors connected to the
   zone edge node.  In addition, the TLV may has a Zone ESN Sub TLV
   comprising the information about the zone end systems connected to
   the zone edge node.





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   Secondly, it adds each of the other zone edge nodes as an IS neighbor
   into the Intermediate System Neighbors TLV in the LSP after it
   receives an LSP containing an IS-IS Zone TLV with M = 1 or a CLI
   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 a Prefix Neighbors TLV into its LSP.  The TLV
   contains a number of address 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 TLV (i.e., in the Zone ISN sub TLV) from
   Intermediate System Neighbors TLV in the LSP, and the ES neighbors
   corresponding to the ES neighbors in the Zone 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.

4.3.  Advertisement of LSs

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

4.3.1.  Advertisement of LSs within Zone

   Any LS 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 router LS generated for a zone internal router is
   advertised only within the zone.

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

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






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4.3.2.  Advertisement of LSs through Zone

   Any LS 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 LS from a node outside of the zone, it floods
   the LS to its neighbors both inside and outside of the zone.  This LS
   may be any LS such as a router LSA that is advertised within an OSPF
   area.

   The nodes in the zone continue to flood the LS.  When another zone
   edge receives the LS, it floods the LS 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.  The seamless migration between a zone and its
   edges' full mesh for IS-IS is similar to that described in OSPF
   Topology-Transparent Zone [RFC8099] for OSPF.

5.1.  Transfer Zone to a Single Node

   After transfer a Zone to a Single Virtual Node is triggered, the zone
   is abstracted as a single virtual node in two steps:

   Step 1:  Every zone edge node works together with each of its zone
      neighbor nodes to create a new adjacency between the virtual node
      and the neighbor node in the way described in Section 4.1.4 for
      Adjacency Establishment and Termination procedure for case 2.
      After creating the adjacency, each of the zone neighbor nodes
      update its LS by adding the adjacency/link into its LS.

   Step 2:  The zone leader originates an LS for the virtual node after
      receiving the updated LSes originated by all the zone neighbor
      nodes, where the updated LSes contain all the zone neighbors.

   Step 3:  After receiving the LS for the virtual node, every zone edge
      does not send any LS inside the zone to any zone neighbors.  It
      advertises its LS without any links inside the zone to the nodes
      outside of the zone and terminates its adjacency to each of its
      zone neighbors in the way described in Section 4.1.4 for Adjacency
      Establishment and Termination procedure for case 2.

5.2.  Roll Back from Zone as a Single Node

   After roll back from Zone as a Signle Virtual Node is triggered,
   rolling back is done in following steps:




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   Step 1:  Every zone edge creates an adjacency to each of its zone
      neighbors in a normal way.

   Step 2:  After all the adjacencies between the zone edges and the
      zone neighbors are created, the zone leader updates the LS for the
      virtual node by changing every link metric to the maximum metric
      in the LS.

   Step 3:  Every zone edge sends its LS with the links inside the zone
      and all the LSes inside the zone to its zone neighbors.  Every
      zone edge acting as the virtual node terminates the adjacency
      between the virtual node and each of its zone neighbors through
      stopping Hellos to the neighbors.

   In another option, rolling back is done as follows:

   Step 1:  Using the procedure described in the following, every zone
      edge rolls back the existing virtual adjacency between the edge
      node acting as the virtual node and the zone neighbor node to a
      normal adjacency between the edge node and the neighbor.

   Step 2:  The zone leader may flush the LS for the virtual node.
      Every zone edge sends Hello and other packets to its zone
      neighbors, where the packets contain the edge node ID as Source
      ID.

   The procedure below smoothly rolls back the existing virtual
   adjacency between the edge node acting as the virtual node and the
   zone neighbor node to a normal adjacency between the edge node and
   the neighbor node.

   The edge node sends the neighbor node Hellos with additional
   information, including a flag N-bit set to one and a TLV with the
   edge node ID such as the Adjacent Node ID TLV with the edge node ID.
   This information requests the neighbor node to roll back the existing
   virtual adjacency to the normal adjacency smoothly through working
   together with the edge node.

   The following steps will roll back the existing virtual adjacency to
   the normal one:











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     zone Edge                                  zone Neighbor
     (Roll Back to
     Normal Adjacency)   Hello (N=1, Edge ID)
                        ----------------------> OK to Roll Back to
                                                Normal Adjacency
                         Hello (N=1, Edge ID)
      Remote Ready for <----------------------
      Rolling Back
                         Hello(Source=Edge ID)
      Start Roll Back  -----------------------> Roll Back to
                                                Normal Adjacency
                         Hello
      Roll Back to     <-----------------------
      Normal Adjacency           . . .

   Step 1:  When "Roll Back from Zone as a Single Node" is triggered,
      the edge node sends the neighbor node a Hello with the additional
      information N=1 and Edge ID as normal adjacency ID in order to
      roll back to the normal adjacency from the virtual adjacency.

   Step 2:  After receiving the Hello with the additional information
      from the edge node, the neighbor node sends the edge node a Hello
      with the additional information (i.e., N=1 and Edge ID as normal
      adjacency ID), which means ok for rolling back to the normal
      adjacency.

   Step 3:  The edge sends the neighbor a Hello containing the edge node
      ID as Source ID after receiving the Hello with the additional
      information from the neighbor, which starts to roll back to the
      normal adjacency.

   Step 4:  The neighbor node changes the existing adjacency to the
      normal adjacency after receiving the Hello containing the edge
      node ID as Source ID from the edge node; and sends the edge node a
      Hello without the additional information, which means that it
      rolled back to the normal adjacency.

   Step 5:  The edge node changes the existing adjacency to the normal
      adjacency after receiving the Hello without the additional
      information from the neighbor node; and continues to send the
      neighbor Hello containing the edge node ID as Source ID.  At this
      point, the virtual adjacency is rolled back to the normal
      adjacency.

   For the neighbor node, changing the existing virtual adjacency to the
   normal one includes:





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   o  Changing the existing adjacency ID from the virtual node ID to the
      edge node ID through either removing the existing adjacency and
      adding a new adjacency with the edge node ID or just changing the
      existing adjacency ID from the virtual node ID to the edge node
      ID,

   o  Removing the link to the virtual node from its LS and adding a new
      link to the edge node (or just changing the link to the virtual
      node to the link to the edge node in its LS), and

   o  Continuing sending the edge node Hellos without additional
      information.

   For the edge node, changing the existing virtual adjacency to the
   normal one includes:

   o  Sending its LS to the neighbor, and

   o  Continuing sending the neighbor node Hellos containing the edge
      node ID as Source ID without additional information.

6.  Operations

   The Operations on TTZ described in OSPF Topology-Transparent Zone
   [RFC8099] are for Zone as Edges Full Mesh in OSPF.  They can be used
   for Zone as Edges Full Mesh in IS-IS.  They can also be used for Zone
   as a Single Virtual Node in IS-IS.

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 new registry types for Adjacent Node ID, Zone ID and Zone
   Options as follows:

     +==============+===================+=====================+
     |  TLV Type    |      TLV Name     |    reference        |
     +==============+===================+=====================+
     | 26(suggested)| Adjacent Node ID  |    This document    |
     +--------------+-------------------+---------------------+
     | 27(suggested)| Zone              |    This document    |
     +--------------+-------------------+---------------------+





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9.  Contributors

        Alvaro Retana
        Futurewei
        Raleigh, NC
        USA

        Email: alvaro.retana@futurewei.com

10.  Acknowledgement

   The authors would like to thank Acee Lindem, Abhay Roy, Christian
   Hopps, Dean Cheng, Russ White, Tony Przygienda, Wenhu Lu, Lin Han,
   Kiran Makhijani, 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>.




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   [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>.

   [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>.

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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