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Internet Engineering Task Force                                  H. Chen
Internet-Draft                                                 A. Retana
Intended status: Standards Track                                   R. Li
Expires: April 11, 2020                                        Futurewei
                                                            A. Kumar S N
                                                                 RtBrick
                                                                   N. So

                                                                  V. Liu

                                                                  M. Toy
                                                                 Verizon
                                                                  L. Liu
                                                                 Fijitsu
                                                         October 9, 2019


                       Topology-Transparent Zone
                       draft-chen-isis-ttz-07.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 pseudo node or zone edges mess.  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 to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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

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

   This Internet-Draft will expire on April 11, 2020.





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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.

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 Pseudo 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 . . . . . . .   7
       4.1.4.  Adjacency Establishment . . . . . . . . . . . . . . .   8
       4.1.5.  Computation of Routes . . . . . . . . . . . . . . . .  10
       4.1.6.  Extensions to Protocols . . . . . . . . . . . . . . .  10
     4.2.  Zone as Edges Full Mess . . . . . . . . . . . . . . . . .  12
       4.2.1.  Extensions to IS-IS . . . . . . . . . . . . . . . . .  13
     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  . . . . . . . . . .  17
   6.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Security  Considerations  . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21








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1.  Introduction

   [ISO10589] and [RFC2328] describe two levels of areas, which are
   level 1 and level 2 areas in IS-IS, and backbone and non-backbone
   areas in OSPF.  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.

   Furthermore, the services carried by the network may be interrupted
   while the network is being split from one area into multiple areas or
   from a few existing areas into even more areas.

   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 pseudo node or zone edges mess with minimum efforts and
   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 and OSPF 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.

   LSA:  A Link State Advertisement in OSPF.

   LS:   A Link Sate, which is an LSA in OSPF or 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.




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

   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  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 pseudo node or the zone edges'
   full mess.

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

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



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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 and a backbone
   area in OSPF.  It is abstracted as a single pseudo node or its edges'
   full mess.  TTZ and zone will be used exchangably below.

4.1.  Zone as a Single Pseudo Node

   After a zone is abstracted as a single pseudo node having a pseudo
   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 pseudo 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.

                 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




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   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.

   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.















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                 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 Sinlge 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
   [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 a LS (i.e., a LSP in IS-IS and a
   LSA in OSPF) for the zone as a single Pseudo node and sends it to its
   neighbors.  This is the only LS to be flooded to the nodes outside of
   the zone.

   The LS comprises all the links connecting the zone neighbors.  The LS
   ID is the ID of the Pseudo node for the zone.  The Source ID or
   Advertising Node/Router ID is the ID of the Pseudo node.





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   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

   A zone edge node, acting as a single Pseudo 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 Pseudo 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 pseudo 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):

   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
   Pseudo 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.

















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

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

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

   Step 3:  The edge sends the zone neighbor a Hello containing the
      Pseudo node ID as Source ID after receiving the Hello with T=1 and
      Pseudo 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 Pseudo node ID
      as Source ID from the zone edge; and sends the zone edge a Hello
      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 Pseudo 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
      Pseudo node ID through either removing the existing adjacency and
      adding a new adjacency with the Pseudo node ID or just changing




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      the existing adjacency ID from the edge node ID to the Pseudo node
      ID,

   o  Removing the link to the zone edge node from its LS and adding a
      new link to the Pseudo node (or just changing the link to the edge
      node to the link to the Pseudo 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 Pseudo
      node ID as Source ID.

4.1.5.  Computation of Routes

   After a zone edge migrates to zone as a pseudo node, it computes the
   routes (i.e., shortest paths to the destinations) in the zone in the
   same way as that described in RFC 2328.  That is, it computes the
   routes in the normal way using the zone topology (i.e., the topology
   of the zone without the pseudo node).

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

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

4.1.6.  Extensions to Protocols

   The following TLVs are defined in IS-IS and OSPF.

   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 Pseudo node ID; in case of roll back, the TLV
      contains the edge node ID.

   o  Zone ID TLV: containing an zone ID.

   o  Zone Options TLV: containing one of some operation options.




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   In addition, two new flag bits are defined in a TLV.  In OSPF, this
   TLV is the Extended Options and Flags (EOF) TLV of LLS Type 1, which
   is defined in OSPF Link-Local Signaling [RFC5613].  In IS-IS, this
   TLV may be the IS-IS Flooding Request TLV, which is defined in
   Dynamic Flooding on Dense Graphs [I-D.ietf-lsr-dynamic-flooding].

   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 Pseudo node ID (or say abstract node ID), which is included in
      a TLV, called Adjacent Node ID TLV.

   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, which is included in an Adjacent
      Node ID TLV.

4.1.6.1.  Extensions to IS-IS

   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 (TBD)   |  Length (6)   |     Pseudo/Edge Node ID       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Pseudo/Edge Node ID (Continue)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of Zone 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |ZoneID TLV Type| TLV-Length (8)|            Zone ID            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Zone ID (Continue)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Reserved (MUST be zero)   |E|Z|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   E = 1: Indicating a node is a zone edge node
   Z = 1: Indicating a node has migrated to Zone as an abstracted entity

   When a zone node originates a zone LS containing a zone ID TLV, it
   MUST set flag E to 1 in the zone ID TLV 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



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   has migrated to zone as an abstracted entity and to 0 before it
   migrates zone to the abstracted entity or after it rolls back from
   zone as an abstracted entity.

   The format of Zone Options 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |ZoneOP TLV Type| TLV-Length (2)|  OP | Reserved (MUST be zero) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   An OP field of 3 bits is defined.  It may have a value of 0x001 for
   T, 0x010 for M, 0x011 for N, or 0x100 for R, which indicates one of
   the four operations above.  When any of the other values is received,
   it is ignored.  The details on these OPs are described in OSPF
   Topology-Transparent Zone [RFC8099].

4.1.6.2.  Extensions to OSPF

   The format of Adjacent Node ID TLV in OSPF 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 (TBD)           |            Length (4)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Pseudo/Edge Node ID (i.e., Pseudo/Edge Router ID)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Zone ID TLV and Zone Options TLV in OSPF are defined in OSPF
   Topology-Transparent Zone [RFC8099].

4.2.  Zone as Edges Full Mess

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






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4.2.1.  Extensions to IS-IS

4.2.1.1.  Zone TLV

   A new TLV, which is called Zone TLV, may be added into an LSP or a
   Hello PDU for a zone node.  It 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 (TBD)   |     Length    |             Flags             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Zone ID                           |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |            Sub TLVs           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 3: Zone TLV

   A Zone TLV has 1 byte of Type, 1 byte of Length of the value field of
   the TLV, 2 bytes of Flags and 6 bytes of Zone ID.  A Zone TLV in an
   LSP may contains a number of Sub TLVs and have Flags defined as
   follows.

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |E|T|M|N|R|    0                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      E = 1: Edge Node of Zone
      T = 1: Distributing Zone Topology Information for Migration
      M = 1: Migrating Zone to an Abstracted Entity
      N = 1: Distributing Normal Topology Information for Rollback
      R = 1: Rolling back from Zone as an Abstracted Entity

   When a node in a zone receives a CLI command triggering zone
   information distribution for migration, it updates its LSP by adding
   a 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.

   Two new sub-TLVs are defined, which may be added into a 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




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   number of Zone IS neighbors 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 Zone ES
   neighbors 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    |Default Metric |  DelayMetric  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Expense Metric |  Error Metric |        Neighbor ID(i)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 5: Zone ESN Sub TLV

4.2.1.2.  Updating LSPs for Zone

   A zone internal node adds a Zone TLV into its LSP after it receives
   an LSP containing a 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 a 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.



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   At first, the zone edge node adds a Zone TLV into its LSP after it
   receives an LSP containing a 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.

   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 a 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 pseudo node or edges full mess,
   every zone edge MUST NOT advertise any LS belonging to the zone or



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   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.

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 pseudo node.  The seamless migration between a zone and its
   edges full mess 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 Node is triggered, the zone is abstracted
   as a single Pseudo node in two steps:

   Step 1:  Every zone edge node works together with each of its zone
      neighbor nodes to smoothly transfer the existing adjacency between
      the zone edge and the zone neighbor to a new adjacency between the
      Pseudo node and the neighbor node in the way described in
      Section 4.1.4 for Adjacency Establishment procedure for case 2.

   Step 2:  The zone leader originates a LS for the Pseudo node after
      receiving the updated LSs originated by all the zone neighbor
      nodes, where the updated LSs contain all the zone neighbors.
      Every zone edge does not send any LS inside the zone to any zone
      neighbors.







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5.2.  Roll Back from Zone as a Single Node

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

   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 Pseudo 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 Pseudo node.  Every
      zone edge sends Hello and other packages such as CSNP in IS-IS to
      its zone neighbors, where the packages 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 Pseudo 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:

     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 Node" is triggered, the edge
      node sends the neighbor node a Hello with the additional




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      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:

   o  Changing the existing adjacency ID from the Pseudo 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 Pseudo node ID to the edge node ID,

   o  Removing the link to the Pseudo node from its LS and adding a new
      link to the edge node (or just changing the link to the Pseudo
      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





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   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 Mess in OSPF.  They can be used
   for Zone as Edges Full Mess in IS-IS.  They can also be used for Zone
   as a Single Pseudo Node in OSPF and IS-IS.

7.  Security Considerations

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

8.  IANA Considerations

9.  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.

10.  References

10.1.  Normative References

   [I-D.ietf-lsr-dynamic-flooding]
              Li, T., Psenak, P., Ginsberg, L., Chen, H., Przygienda,
              T., Cooper, D., Jalil, L., and S. Dontula, "Dynamic
              Flooding on Dense Graphs", draft-ietf-lsr-dynamic-
              flooding-03 (work in progress), June 2019.

   [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.





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   [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>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [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>.

   [RFC5613]  Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
              Yeung, "OSPF Link-Local Signaling", RFC 5613,
              DOI 10.17487/RFC5613, August 2009,
              <https://www.rfc-editor.org/info/rfc5613>.

   [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>.

10.2.  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>.





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Authors' Addresses

   Huaimo Chen
   Futurewei
   Boston, MA
   USA

   Email: huaimo.chen@futurewei.com


   Alvaro Retana
   Futurewei
   Raleigh, NC
   USA

   Email: alvaro.retana@futurewei.com


   Richard Li
   Futurewei
   2330 Central expressway
   Santa Clara, CA
   USA

   Email: richard.li@futurewei.com


   Anil Kumar S N
   RtBrick
   Bangalore
   India

   Email: anil.ietf@gmail.com


   Ning So
   Plano, TX  75082
   USA

   Email: ningso01@gmail.com


   Vic Liu
   USA

   Email: liu.cmri@gmail.com





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   Mehmet Toy
   Verizon
   USA

   Email: mehmet.toy@verizon.com


   Lei Liu
   Fijitsu
   USA

   Email: liulei.kddi@gmail.com







































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