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Area Proxy for IS-IS
draft-ietf-lsr-isis-area-proxy-12

Document Type Active Internet-Draft (lsr WG)
Authors Tony Li , Sarah Chen , Vivek Ilangovan , Gyan Mishra
Last updated 2024-03-12 (Latest revision 2024-01-18)
Replaces draft-li-lsr-isis-area-proxy
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Experimental
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Stream WG state Submitted to IESG for Publication
Document shepherd Christian Hopps
Shepherd write-up Show Last changed 2023-05-23
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Send notices to Christian Hopps <chopps@chopps.org>
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Details
draft-ietf-lsr-isis-area-proxy-12
Internet Engineering Task Force                                    T. Li
Internet-Draft                                          Juniper Networks
Intended status: Experimental                                    S. Chen
Expires: 21 July 2024                                       V. Ilangovan
                                                         Arista Networks
                                                               G. Mishra
                                                            Verizon Inc.
                                                         18 January 2024

                          Area Proxy for IS-IS
                   draft-ietf-lsr-isis-area-proxy-12

Abstract

   Link state routing protocols have hierarchical abstraction already
   built into them.  However, when lower levels are used for transit,
   they must expose their internal topologies to each other, leading to
   scale issues.

   To avoid this, this document discusses extensions to the IS-IS
   routing protocol that allow level 1 areas to provide transit, yet
   only inject an abstraction of the level 1 topology into level 2.
   Each level 1 area is represented as a single level 2 node, thereby
   enabling greater scale.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

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

   This Internet-Draft will expire on 21 July 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Area Proxy  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Segment Routing . . . . . . . . . . . . . . . . . . . . .   6
   3.  Inside Router Functions . . . . . . . . . . . . . . . . . . .   6
     3.1.  The Area Proxy TLV  . . . . . . . . . . . . . . . . . . .   7
     3.2.  Level 2 SPF Computation . . . . . . . . . . . . . . . . .   7
     3.3.  Responsibilities concerning the Proxy LSP . . . . . . . .   8
   4.  Area Leader Functions . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Area Leader Election  . . . . . . . . . . . . . . . . . .   8
     4.2.  Redundancy  . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Distributing Area Proxy Information . . . . . . . . . . .   8
       4.3.1.  The Area Proxy System ID Sub-TLV  . . . . . . . . . .   9
       4.3.2.  The Area SID Sub-TLV  . . . . . . . . . . . . . . . .  10
     4.4.  Proxy LSP Generation  . . . . . . . . . . . . . . . . . .  11
       4.4.1.  The Protocols Supported TLV . . . . . . . . . . . . .  11
       4.4.2.  The Area Address TLV  . . . . . . . . . . . . . . . .  11
       4.4.3.  The Dynamic Hostname TLV  . . . . . . . . . . . . . .  11
       4.4.4.  The IS Neighbors TLV  . . . . . . . . . . . . . . . .  12
       4.4.5.  The Extended IS Neighbors TLV . . . . . . . . . . . .  12
       4.4.6.  The MT Intermediate Systems TLV . . . . . . . . . . .  12
       4.4.7.  Reachability TLVs . . . . . . . . . . . . . . . . . .  13
       4.4.8.  The Router Capability TLV . . . . . . . . . . . . . .  13
       4.4.9.  The Multi-Topology TLV  . . . . . . . . . . . . . . .  14
       4.4.10. The SID/Label Binding and The Multi-Topology SID/Label
               Binding SID TLV . . . . . . . . . . . . . . . . . . .  14
       4.4.11. The SRv6 Locator TLV  . . . . . . . . . . . . . . . .  14
       4.4.12. Traffic Engineering Information . . . . . . . . . . .  14
       4.4.13. The Area SID  . . . . . . . . . . . . . . . . . . . .  15
   5.  Inside Edge Router Functions  . . . . . . . . . . . . . . . .  15
     5.1.  Generating L2 IIHs to Outside Routers . . . . . . . . . .  15
     5.2.  Filtering LSP information . . . . . . . . . . . . . . . .  16
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17

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     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   The IS-IS routing protocol IS-IS [ISO10589] supports a two-level
   hierarchy of abstraction.  The fundamental unit of abstraction is the
   'area', which is a (hopefully) connected set of systems running IS-IS
   at the same level.  Level 1, the lowest level, is abstracted by
   routers that participate in both Level 1 and Level 2, and they inject
   area information into Level 2.  Level 2 systems seeking to access
   Level 1, use this abstraction to compute the shortest path to the
   Level 1 area.  The full topology database of Level 1 is not injected
   into Level 2, only a summary of the address space contained within
   the area, so the scalability of the Level 2 Link State Database
   (LSDB) is protected.

   This works well if the Level 1 area is tangential to the Level 2
   area.  This also works well if there are several routers in both
   Level 1 and Level 2 and they are adjacent to one another, so Level 2
   traffic will never need to transit Level 1 only routers.  Level 1
   will not contain any Level 2 topology, and Level 2 will only contain
   area abstractions for Level 1.

   Unfortunately, this scheme does not work so well if the Level 1 only
   area needs to provide transit for Level 2 traffic.  For Level 2
   shortest path first (SPF) computations to work correctly, the transit
   topology must also appear in the Level 2 LSDB.  This implies that all
   routers that could provide transit, plus any links that might also
   provide Level 2 transit must also become part of the Level 2
   topology.  If this is a relatively tiny portion of the Level 1 area,
   this is not overly painful.

   However, with today's data center topologies, this is problematic.  A
   common application is to use a Layer 3 Leaf-Spine (L3LS) topology,
   which is a folded 3-stage Clos [Clos] fabric.  It can also be thought
   of as a complete bipartite graph.  In such a topology, the desire is
   to use Level 1 to contain the routing dynamics of the entire L3LS
   topology and then use Level 2 for the remainder of the network.
   Leaves in the L3LS topology are appropriate for connection outside of
   the data center itself, so they would provide connectivity for Level
   2.  If there are multiple connections to Level 2 for redundancy, or
   other areas, these too would also be made to the leaves in the
   topology.  This creates a difficulty because there are now multiple
   Level 2 leaves in the topology, with connectivity between the leaves
   provided by the spines.

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   Following the current rules of IS-IS, all spine routers would
   necessarily be part of the Level 2 topology, plus all links between a
   Level 2 leaf and the spines.  In the limit, where all leaves need to
   support Level 2, it implies that the entire L3LS topology becomes
   part of Level 2.  This is seriously problematic as it more than
   doubles the LSDB held in the L3LS topology and eliminates any
   benefits of the hierarchy.

   This document discusses the handling of IP traffic.  Supporting MPLS-
   based traffic is a subject for future work.

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 BCP 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown
   here.

2.  Area Proxy

   To address this, in this specification we completely abstract away
   the details of the Level 1 area topology within Level 2, making the
   entire area look like a single proxy system directly connected to all
   of the area's Level 2 neighbors.  By only providing an abstraction of
   the topology, Level 2's requirement for connectivity can be satisfied
   without the full overhead of the area's internal topology.  It then
   becomes the responsibility of the Level 1 area to provide the
   forwarding connectivity that's advertised.

   For this discussion, we'll consider a single Level 1 IS-IS area to be
   the Inside Area, and the remainder of the Level 2 area to be the
   Outside Area.  All routers within the Inside Area speak Level 1 and
   Level 2 IS-IS on all of the links within the topology.  We propose to
   implement Area Proxy by having a Level 2 Proxy Link State Protocol
   Data Unit (PDU, LSP) that represents the entire Inside Area.  We will
   refer to this as the Proxy LSP.  This is the only LSP from the area
   that will be flooded into the overall Level 2 LSDB.

   There are four classes of routers that we need to be concerned with
   in this discussion:

   Inside Router  A router within the Inside Area that runs Level 1 and
      Level 2 IS-IS.  A router is recognized as an Inside Router by the
      existence of its LSP in the Level 1 LSDB.

   Area Leader  The Area Leader is an Inside Router that is elected to

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      represent the Level 1 area by injecting the Proxy LSP into the
      Level 2 LSDB.  There may be multiple candidates for Area Leader,
      but only one is elected at a given time.  Any Inside Router can be
      Area Leader.

   Inside Edge Router  An Inside Edge Router is an Inside Area Router
      that has at least one Level 2 interface outside of the Inside
      Area.  An interface on an Inside Edge Router that is connected to
      an Outside Edge Router is an Area Proxy Boundary.

   Outside Edge Router  An Outside Edge Router is a Level 2 router that
      is outside of the Inside Area that has an adjacency with an Inside
      Edge Router.

                               Inside Area

                  +--------+                 +--------+
                  | Inside |-----------------| Inside |
                  | Router |                 |  Edge  |
                  +--------+    +------------| Router |
                      |        /             +--------+
                      |       /                   |
                  +--------+ /       =============|======
                  | Area   |/        ||           |
                  | Leader |         ||      +---------+
                  +--------+         ||      | Outside |
                                     ||      |  Edge   |
                                     ||      | Router  |
                                     ||      +---------+

                                             Outside Area

                   Figure 1: An example of router classes

   All Inside Edge Routers learn the Area Proxy System Identifier from
   the Area Proxy TLV advertised by the Area Leader and use that as the
   system identifier in their Level 2 IS-IS Hello PDUs (IIHs) on all
   Outside interfaces.  Outside Edge Routers will then advertise an
   adjacency to the Area Proxy System Identifier.  This allows all
   Outside Routers to use the Proxy LSP in their SPF computations
   without seeing the full topology of the Inside Area.

   Area Proxy functionality assumes that all circuits on Inside Routers
   are either Level 1-2 circuits within the Inside Area, or Level 2
   circuits between Outside Edge Routers and Inside Edge Routers.

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   Area Proxy Boundary multi-access circuits (i.e., Ethernets in LAN
   mode) with multiple Inside Edge Routers on them are not supported.
   The Inside Edge Router on any boundary LAN MUST NOT flood Inside
   Router LSPs on this link.  Boundary LANs SHOULD NOT be enabled for
   Level 1.  An Inside Edge Router may be elected the Designated
   Intermediate System (DIS) for a Boundary LAN.  In this case, using
   the Area Proxy System ID as the basis for the LAN pseudonode
   identifier could create a collision, so the Insider Edge Router
   SHOULD compose the pseudonode identifier using its native system
   identifier.  This choice of pseudonode identifier may confuse
   neighbors with an extremely strict implementation, in which case the
   Inside Edge Router may be configured with priority 0, causing an
   Outside Router to be elected DIS.

2.1.  Segment Routing

   If the Inside Area supports Segment Routing [RFC8402], then all
   Inside Nodes MUST advertise an SR Global Block (SRGB).  The first
   value of the SRGB advertised by all Inside Nodes MUST start at the
   same value.  If the Area Leader detects SRGBs that do not start with
   the same value, it MUST log an error and not advertise an SRGB in the
   Proxy LSP.  The range advertised for the area will be the minimum of
   that advertised by all Inside Nodes.

   To support Segment Routing, the Area Leader will take the SRGB
   information found in the L1 LSDB and convey that to L2 through the
   Proxy LSP.  Prefixes with SID assignments will be copied to the Proxy
   LSP.  Adjacency SIDs for Outside Edge Nodes will be copied to the
   Proxy LSP.

   To further extend Segment Routing, it is helpful to have a segment
   that refers to the entire Inside Area.  This allows a path to refer
   to an area and have any node within that area accept and forward the
   packet.  In effect, this becomes an anycast SID that is accepted by
   all Inside Edge Nodes.  The information about this SID is distributed
   in the Area SID Sub-TLV, as part of the Area Leader's Area Proxy TLV
   (Section 4.3.2).  The Inside Edge Nodes MUST establish forwarding
   based on this SID.  The Area Leader SHALL also include the Area SID
   in the Proxy LSP so that the remainder of L2 can use it for path
   construction.  (Section 4.4.13).

3.  Inside Router Functions

   All Inside Routers run Level 1-2 IS-IS and must be explicitly
   instructed to enable the Area Proxy functionality.  To signal their
   readiness to participate in Area Proxy functionality, they will
   advertise the Area Proxy TLV in their L2 LSP.

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3.1.  The Area Proxy TLV

   The Area Proxy TLV serves multiple functions:

      The presence of the Area Proxy TLV in a node's LSP indicates that
      the node is enabled for Area Proxy.

      An LSP containing the Area Proxy TLV is also an Inside Node.  All
      Inside Nodes, including pseudonodes, MUST advertise the Area Proxy
      TLV.

      It is a container for sub-TLVs with Area Proxy information.

   A node advertises the Area Proxy TLV in fragment 0 of its L2 LSP.
   Nodes MUST NOT advertise the Area Proxy TLV in an L1 LSP.  Nodes MUST
   ignore the Area Proxy TLV if it is found in an L1 LSP.  The Area
   Proxy TLV is not used in the Proxy LSP.  The format of the Area Proxy
   TLV is:

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | TLV Type      | TLV Length    |  Sub-TLVs ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      TLV Type: 20

      TLV Length: length of the sub-TLVs

3.2.  Level 2 SPF Computation

   When Outside Routers perform a Level 2 SPF computation, they will use
   the Proxy LSP for computing a path transiting the Inside Area.
   Because the topology has been abstracted away, the cost for
   transiting the Inside Area will be zero.

   When Inside Routers perform a Level 2 SPF computation, they MUST
   ignore the Proxy LSP.  Further, because these systems do see the
   Inside Area topology, the link metrics internal to the area are
   visible.  This could lead to different and possibly inconsistent SPF
   results, potentially leading to forwarding loops.

   To prevent this, the Inside Routers MUST consider the metrics of
   links outside of the Inside Area (inter-area metrics) separately from
   the metrics of the Inside Area links (intra-area metrics).  Intra-
   area metrics MUST be treated as less than any inter-area metric.
   Thus, if two paths have different total inter-area metrics, the path
   with the lower inter-area metric would be preferred, regardless of

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   any intra-area metrics involved.  However, if two paths have equal
   inter-area metrics, then the intra-area metrics would be used to
   compare the paths.

   Point-to-point links between two Inside Routers are considered to be
   Inside Area links.  LAN links which have a pseudonode LSP in the
   Level 1 LSDB are considered to be Inside Area links.

3.3.  Responsibilities concerning the Proxy LSP

   The Area Leader will generate a Proxy LSP that will be flooded across
   the Inside Area.  Inside Routers MUST ignore the contents of the
   Proxy LSP other than for flooding.  The Proxy LSP uses the Area Proxy
   System Identifier as its Source ID.

4.  Area Leader Functions

   The Area Leader has several responsibilities.  First, it MUST inject
   the Area Proxy System Identifier into the Level 2 LSDB.  Second, the
   Area Leader MUST generate the Proxy LSP for the Inside Area.

4.1.  Area Leader Election

   The Area Leader is selected using the election mechanisms and TLVs
   described in Dynamic Flooding for IS-IS
   [I-D.ietf-lsr-dynamic-flooding].

4.2.  Redundancy

   If the Area Leader fails, another candidate may become Area Leader
   and MUST regenerate the Proxy LSP.  The failure of the Area Leader is
   not visible outside of the area and appears to simply be an update of
   the Proxy LSP.

   For consistency, all Area Leader candidates SHOULD be configured with
   the same Proxy System ID, Proxy Hostname, and any other information
   that may be inserted into the Proxy LSP.

4.3.  Distributing Area Proxy Information

   The Area Leader is responsible for distributing information about the
   area to all Inside Nodes.  In particular, the Area Leader distributes
   the Proxy System ID and the Area SID.  This is done using two sub-
   TLVs of the Area Proxy TLV.

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4.3.1.  The Area Proxy System ID Sub-TLV

   The Area Proxy System ID Sub-TLV MUST be used by the Area Leader to
   distribute the Area Proxy System ID.  This is an additional system
   identifier that is used by Inside Nodes as an indication that Area
   Proxy is active.  The format of this sub-TLV is:

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |     Length    |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    Proxy System Identifier    |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 1

      Length: length of a system ID (6)

      Proxy System Identifier: the Area Proxy System Identifier.

   The Area Leader MUST advertise the Area Proxy System Identifier Sub-
   TLV when it observes that all Inside Routers are advertising the Area
   Proxy TLV.  Their advertisements indicate that they are individually
   ready to perform Area Proxy functionality.  The Area Leader then
   advertises the Area Proxy System Identifier TLV to indicate that the
   Inside Area MUST enable Area Proxy functionality.

   Other candidates for Area Leader MAY also advertise the Area Proxy
   System Identifier when they observe that all Inside Routers are
   advertising the Area Proxy Router Capability.  All candidates
   advertising the Area Proxy System Identifier TLV SHOULD be
   advertising the same system identifier.  Multiple proxy system
   identifiers in a single area is a misconfiguration and each unique
   occurrence SHOULD be logged.  Systems should use the proxy system
   identifier advertised by the Area Leader.

   The Area Leader and other candidates for Area Leader MAY withdraw the
   Area Proxy System Identifier when one or more Inside Routers are not
   advertising the Area Proxy Router Capability.  This will disable Area
   Proxy functionality.  However, before withdrawing the Area Proxy
   System Identifier, an implementation SHOULD protect against
   unnecessary churn from transients by delaying the withdrawal.  The
   amount of delay is implementation dependent.

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4.3.2.  The Area SID Sub-TLV

   The Area SID Sub-TLV allows the Area Leader to advertise a prefix and
   SID that represents the entirety of the Inside Area to the Outside
   Area.  This sub-TLV is learned by all of the Inside Edge Nodes who
   should consume this SID at forwarding time.  The Area SID Sub-TLV has
   the format:

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Length    |     Flags     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  SID/Index/Label (variable)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |    Prefix (variable)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: 2

      Length: variable (1 + SID length)

      Flags: 1 octet.

      SID/Index/Label: as defined in [RFC8667] Section 2.1.1.1

      Prefix Length: 1 octet

      Prefix: 0-16 octets

   The Flags octet is defined as follows:

              0 1 2 3 4 5 6 7
              +-+-+-+-+-+-+-+-+
              |F|V|L|         |
              +-+-+-+-+-+-+-+-+

   where:

      F: Address-Family Flag.  If unset, then this proxy SID is used
      when forwarding IPv4-encapsulated traffic.  If set, then this
      proxy SID is used when forwarding IPv6-encapsulated traffic.

      V: Value Flag.  If set, then the proxy SID carries a value, as
      defined in [RFC8667] Section 2.1.1.1.

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      L: Local Flag.  If set, then the value/index carried by the proxy
      SID has local significance, as defined in [RFC8667]
      Section 2.1.1.1.

      Other bits: MUST be zero when originated and ignored when
      received.

4.4.  Proxy LSP Generation

   Each Inside Router generates a Level 2 LSP, and the Level 2 LSPs for
   the Inside Edge Routers will include adjacencies to Outside Edge
   Routers.  Unlike normal Level 2 operations, these LSPs are not
   advertised outside of the Inside Area and MUST be filtered by all
   Inside Edge Routers to not be flooded to Outside Routers.  Only the
   Proxy LSP is injected into the overall Level 2 LSDB.

   The Area Leader uses the Level 2 LSPs generated by the Inside Edge
   Routers to generate the Proxy LSP.  This LSP is originated using the
   Area Proxy System Identifier.  The Area Leader can also insert the
   following additional TLVs into the Proxy LSP for additional
   information for the Outside Area.  LSPs generated by unreachable
   nodes MUST NOT be considered.

4.4.1.  The Protocols Supported TLV

   The Area Leader SHOULD insert a Protocols Supported TLV (129)
   [RFC1195] into the Proxy LSP.  The values included in the TLV SHOULD
   be the protocols supported by the Inside Area.

4.4.2.  The Area Address TLV

   The Area Leader SHOULD insert an Area Addresses TLV (1) [ISO10589]
   into the Proxy LSP.

4.4.3.  The Dynamic Hostname TLV

   It is RECOMMENDED that the Area Leader insert the Dynamic Hostname
   TLV (137) [RFC5301] into the Proxy LSP.  The contents of the hostname
   may be specified by configuration.  The presence of the hostname
   helps to simplify debugging the network.

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4.4.4.  The IS Neighbors TLV

   The Area Leader can insert the IS Neighbors TLV (2) [ISO10589] into
   the Proxy LSP for Outside Edge Routers.  The Area Leader learns of
   the Outside Edge Routers by examining the LSPs generated by the
   Inside Edge Routers copying any IS Neighbors TLVs referring to
   Outside Edge Routers into the Proxy LSP.  Since the Outside Edge
   Routers advertise an adjacency to the Area Proxy System Identifier,
   this will result in a bi-directional adjacency.

   An entry for a neighbor in both the IS Neighbors TLV and the Extended
   IS Neighbors would be functionally redundant, so the Area Leader
   SHOULD NOT do this.  The Area Leader MAY omit either the IS Neighbors
   TLV or the Extended IS Neighbors TLV, but it MUST include at least
   one of them.

4.4.5.  The Extended IS Neighbors TLV

   The Area Leader can insert the Extended IS Reachability TLV (22)
   [RFC5305] into the Proxy LSP.  The Area Leader SHOULD copy each
   Extended IS Reachability TLV advertised by an Inside Edge Router
   about an Outside Edge Router into the Proxy LSP.

   If the Inside Area supports Segment Routing and Segment Routing
   selects a SID where the L-Flag is unset, then the Area Lead SHOULD
   include an Adjacency Segment Identifier sub-TLV (31) [RFC8667] using
   the selected SID.

   If the inside area supports SRv6, the Area Leader SHOULD copy the
   "SRv6 End.X SID" and "SRv6 LAN End.X SID" sub-TLVs of the extended IS
   reachability TLVs advertised by Inside Edge Routers about Outside
   Edge Routers.

   If the inside area supports Traffic Engineering (TE), the Area Leader
   SHOULD copy TE-related sub-TLVs [RFC5305] Section 3 to each Extended
   IS Reachability TLV in the Proxy LSP.

4.4.6.  The MT Intermediate Systems TLV

   If the Inside Area supports Multi-Topology, then the Area Leader
   SHOULD copy each Outside Edge Router advertisement that is advertised
   by an Inside Edge Router in an MT Intermediate Systems TLV into the
   Proxy LSP.

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4.4.7.  Reachability TLVs

   The Area Leader SHOULD insert additional TLVs describing any routing
   prefixes that should be advertised on behalf of the area.  These
   prefixes may be learned from the Level 1 LSDB, Level 2 LSDB, or
   redistributed from another routing protocol.  This applies to all of
   the various types of TLVs used for prefix advertisement:

      IP Internal Reachability Information TLV (128) [RFC1195]

      IP External Reachability Information TLV (130) [RFC1195]

      Extended IP Reachability TLV (135) [RFC5305]

      IPv6 Reachability TLV (236) [RFC5308]

      Multi-Topology Reachable IPv4 Prefixes TLV (235) [RFC5120]

      Multi-Topology Reachable IPv6 Prefixes TLV (237) [RFC5120]

   For TLVs in the Level 1 LSDB, for a given TLV type and prefix, the
   Area Leader SHOULD select the TLV with the lowest metric and copy
   that TLV into the Proxy LSP.

   When examining the Level 2 LSDB for this function, the Area Leader
   SHOULD only consider TLVs advertised by Inside Routers.  Further, for
   prefixes that represent Boundary links, the Area Leader SHOULD copy
   all TLVs that have unique sub-TLV contents.

   If the Inside Area supports Segment Routing and the selected TLV
   includes a Prefix Segment Identifier sub-TLV (3) [RFC8667], then the
   sub-TLV SHOULD be copied as well.  The P-Flag SHOULD be set in the
   copy of the sub-TLV to indicate that penultimate hop popping should
   not be performed for this prefix.  The E-Flag SHOULD be reset in the
   copy of the sub-TLV to indicate that an explicit NULL is not
   required.  The R-Flag SHOULD simply be copied.

4.4.8.  The Router Capability TLV

   The Area Leader MAY insert the Router Capability TLV (242) [RFC7981]
   into the Proxy LSP.  If Segment Routing is supported by the inside
   area, as indicated by the presence of an SRGB being advertised by all
   Inside Nodes, then the Area Leader SHOULD advertise an SR-
   Capabilities sub-TLV (2) [RFC8667] with an SRGB.  The first value of
   the SRGB is the same as the first value advertised by all Inside
   Nodes.  The range advertised for the area will be the minimum of all
   ranges advertised by Inside Nodes.  The Area Leader SHOULD use its
   Router ID in the Router Capability TLV.

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   If SRv6 Capability sub-TLV [RFC7981] is advertised by all Inside
   Routers, the Area Leader should insert an SRv6 Capability sub-TLV in
   the Router Capability TLV.  Each flag in the SRv6 Capability sub-TLV
   should be set if the flag is set by all Inside Routers.

   If the Node Maximum SID Depth (MSD) sub-TLV [RFC8491] is advertised
   by all Inside Routers, the Area Leader should advertise the
   intersection of the advertised MSD types and the smallest supported
   MSD values for each type.

4.4.9.  The Multi-Topology TLV

   If the Inside Area supports multi-topology, then the Area Leader
   SHOULD insert the Multi-Topology TLV (229) [RFC5120], including the
   topologies supported by the Inside Nodes.

   If any Inside Node is advertising the 'O' (Overload) bit for a given
   topology, then the Area Leader MUST advertise the 'O' bit for that
   topology.  If any Inside Node is advertising the 'A' (Attach) bit for
   a given topology, then the Area Leader MUST advertise the 'A' bit for
   that topology.

4.4.10.  The SID/Label Binding and The Multi-Topology SID/Label Binding
         SID TLV

   If an Inside Node advertises the SID/Label Binding or Multi-Topology
   SID/Label Binding SID TLV [RFC8667], then the Area Leader MAY copy
   the TLV to the Proxy LSP.

4.4.11.  The SRv6 Locator TLV

   If the inside area supports SRv6, the Area Leader SHOULD copy all
   SRv6 locator TLVs [RFC9352] advertised by Inside Routers to the Proxy
   LSP.

4.4.12.  Traffic Engineering Information

   If the inside area supports TE, the Area Leader SHOULD advertise a TE
   Router ID TLV (134) [RFC5305] in the Proxy LSP.  It SHOULD copy the
   Shared Risk Link Group (SRLS) TLVs (138) [RFC5307] advertised by
   Inside Edge Routers about links to Outside Edge Routers.

   If the inside area supports IPv6 TE, the Area Leader SHOULD advertise
   an IPv6 TE Router ID TLV (140) [RFC6119] in the Proxy LSP.  It SHOULD
   also copy the IPv6 SRLG TLVs (139) [RFC6119] advertised by Inside
   Edge Routers about links to Outside Edge Routers.

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4.4.13.  The Area SID

   When SR is enabled, it may be useful to advertise an Area SID which
   will direct traffic to any of the Inside Edge Routers.  The
   information for the Area SID is distributed to all Inside Edge
   Routers using the Area SID sub-TLV (Section 4.3.2) by the Area
   Leader.

   The Area Leader SHOULD advertise the Area SID information in the
   Proxy LSP as a Node SID as defined in [RFC8667] Section 2.1.  The
   advertisement in the Proxy LSP informs the Outside Area that packets
   directed to the SID will be forwarded to one of the Inside Edge Nodes
   and the Area SID will be consumed.

   Other uses of the Area SID and area SID prefix are outside the scope
   of this document.  Documents which define other use cases for the
   Area SID MUST specify whether the SID value should be the same or
   different from that used in support of Area Proxy.

5.  Inside Edge Router Functions

   The Inside Edge Router has two additional and important functions.
   First, it MUST generate IIHs that appear to have come from the Area
   Proxy System Identifier.  Second, it MUST filter the L2 LSPs, Partial
   Sequence Number PDUs (PSNPs), and Complete Sequence Number PDUs
   (CSNPs) that are being advertised to Outside Routers.

5.1.  Generating L2 IIHs to Outside Routers

   The Inside Edge Router has one or more Level 2 interfaces to the
   Outside Routers.  These may be identified by explicit configuration
   or by the fact that they are not also Level 1 circuits.  On these
   Level 2 interfaces, the Inside Edge Router MUST NOT send an IIH until
   it has learned the Area Proxy System ID from the Area Leader.  Then,
   once it has learned the Area Proxy System ID, it MUST generate its
   IIHs on the circuit using the Proxy System ID as the source of the
   IIH.

   Using the Proxy System ID causes the Outside Router to advertise an
   adjacency to the Proxy System ID, not to the Inside Edge Router,
   which supports the proxy function.  The normal system ID of the
   Inside Edge Router MUST NOT be used as it will cause unnecessary
   adjacencies to form.

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5.2.  Filtering LSP information

   For the area proxy abstraction to be effective the L2 LSPs generated
   by the Inside Routers MUST be restricted to the Inside Area.  The
   Inside Routers know which system IDs are members of the Inside Area
   based on the advertisement of the Area Proxy TLV.  To prevent
   unwanted LSP information from escaping the Inside Area, the Inside
   Edge Router MUST perform filtering of LSP flooding, CSNPs, and PSNPs.
   Specifically:

      A Level 2 LSP with a source system identifier that is found in the
      Level 1 LSDB MUST NOT be flooded to an Outside Router.

      A Level 2 LSP that contains the Area Proxy TLV MUST NOT be flooded
      to an Outside Router.

      A Level 2 CSNP sent to an Outside Router MUST NOT contain any
      information about an LSP with a system identifier found in the
      Level 1 LSDB.  If an Inside Edge Router filters a CSNP and there
      is no remaining content, then the CSNP MUST NOT be sent.  The
      source address of the CSNP MUST be the Area Proxy System ID.

      A Level 2 PSNP sent to an Outside Router MUST NOT contain any
      information about an LSP with a system identifier found in the
      Level 1 LSDB.  If an Inside Edge Router filters a PSNP and there
      is no remaining content, then the PSNP MUST NOT be sent.  The
      source address of the PSNP MUST be the Area Proxy System ID.

6.  Acknowledgments

   The authors would like to thank Bruno Decraene and Gunter Van De
   Velde for their many helpful comments.  The authors would also like
   to thank a small group that wishes to remain anonymous for their
   valuable contributions.

7.  IANA Considerations

   This memo requests that IANA allocate and assign code point 20 from
   the IS-IS TLV Codepoints registry for the Area Proxy TLV.  The
   registry fields should be: IIH:n, LSP:y, SNP:n, Purge:n.

   In association with this, this memo requests that IANA create a
   registry for code points for the sub-TLVs of the Area Proxy TLV.

      Name of the registry: Sub-TLVs for TLV 20 (Area Proxy TLV)

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      Required information for registrations: Temporary registrations
      may be made under the Early IANA Allocation of Standards Track
      Code Points policy.  [RFC7120] Permanent registrations require the
      publication of an RFC describing the usage of the code point.

      Applicable registration policy: RFC Required and Expert Review.

      Size, format, and syntax of registry entries: Value (0-255), Name,
      and Reference

      Initial assignments and reservations: IANA is requested to assign
      the following code points:

         +=======+==============================+===============+
         | Value |             Name             |   Reference   |
         +=======+==============================+===============+
         |   1   | Area Proxy System Identifier | This document |
         +-------+------------------------------+---------------+
         |   2   |           Area SID           | This document |
         +-------+------------------------------+---------------+

                                 Table 1

8.  Security Considerations

   This document introduces no new security issues.  Security of routing
   within a domain is already addressed as part of the routing protocols
   themselves.  This document proposes no changes to those security
   architectures.  Security for IS-IS is provided by [RFC5304] and
   [RFC5310].

9.  References

9.1.  Normative References

   [I-D.ietf-lsr-dynamic-flooding]
              Li, T., Psenak, P., Chen, H., Jalil, L., and S. Dontula,
              "Dynamic Flooding on Dense Graphs", Work in Progress,
              Internet-Draft, draft-ietf-lsr-dynamic-flooding-14, 8 June
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              lsr-dynamic-flooding-14>.

   [ISO10589] ISO, "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, October
              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>.

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

   [RFC5301]  McPherson, D. and N. Shen, "Dynamic Hostname Exchange
              Mechanism for IS-IS", RFC 5301, DOI 10.17487/RFC5301,
              October 2008, <https://www.rfc-editor.org/info/rfc5301>.

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

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

   [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>.

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

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

   [RFC6119]  Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
              Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
              February 2011, <https://www.rfc-editor.org/info/rfc6119>.

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   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,
              <https://www.rfc-editor.org/info/rfc7981>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

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

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC9352]  Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
              and Z. Hu, "IS-IS Extensions to Support Segment Routing
              over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
              February 2023, <https://www.rfc-editor.org/info/rfc9352>.

9.2.  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>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/info/rfc7120>.

Authors' Addresses

   Tony Li
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, California 94089
   United States of America

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   Email: tony.li@tony.li

   Sarah Chen
   Arista Networks
   5453 Great America Parkway
   Santa Clara, California 95054
   United States of America
   Email: sarahchen@arista.com

   Vivek Ilangovan
   Arista Networks
   5453 Great America Parkway
   Santa Clara, California 95054
   United States of America
   Email: ilangovan@arista.com

   Gyan S. Mishra
   Verizon Inc.
   13101 Columbia Pike
   Silver Spring, MD 20904
   United States of America
   Phone: 301 502-1347
   Email: gyan.s.mishra@verizon.com

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