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Internet Engineering Task Force                                    T. Li
Internet-Draft                                                   S. Chen
Intended status: Experimental                               V. Ilangovan
Expires: January 26, 2021                                Arista Networks
                                                               G. Mishra
                                                            Verizon Inc.
                                                           July 25, 2020


                          Area Proxy for IS-IS
                   draft-ietf-lsr-isis-area-proxy-02

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 would 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 January 26, 2021.

Copyright Notice

   Copyright (c) 2020 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 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 . . . . . . . . . . . . . . . . . .   4
   2.  Area Proxy  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Segment Routing . . . . . . . . . . . . . . . . . . . . .   6
   3.  Inside Router Functions . . . . . . . . . . . . . . . . . . .   6
     3.1.  The Area Proxy TLV  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Level 2 SPF Computation . . . . . . . . . . . . . . . . .   7
   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  . . . . . . . . . .   8
       4.3.2.  The Area SID Sub-TLV  . . . . . . . . . . . . . . . .   9
         4.3.2.1.  Flags . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Proxy LSP Generation  . . . . . . . . . . . . . . . . . .  10
       4.4.1.  The Protocols Supported TLV . . . . . . . . . . . . .  10
       4.4.2.  The Area Address TLV  . . . . . . . . . . . . . . . .  11
       4.4.3.  The Dynamic Hostname TLV  . . . . . . . . . . . . . .  11
       4.4.4.  The IS Neighbors TLV  . . . . . . . . . . . . . . . .  11
       4.4.5.  The Extended IS Neighbors TLV . . . . . . . . . . . .  11
       4.4.6.  The MT Intermediate Systems TLV . . . . . . . . . . .  12
       4.4.7.  Reachability TLVs . . . . . . . . . . . . . . . . . .  12
       4.4.8.  The Router Capability TLV . . . . . . . . . . . . . .  13
       4.4.9.  The Multi-Topology TLV  . . . . . . . . . . . . . . .  13
       4.4.10. The SID/Label Binding and The Multi-Topology
               SID/Label Binding SID TLV . . . . . . . . . . . . . .  13
       4.4.11. The SRv6 Locator TLV  . . . . . . . . . . . . . . . .  13
       4.4.12. Traffic Engineering Information . . . . . . . . . . .  14
       4.4.13. The Area SID  . . . . . . . . . . . . . . . . . . . .  14
   5.  Inside Edge Router Functions  . . . . . . . . . . . . . . . .  15
     5.1.  Generating L2 IIHs to Outside Routers . . . . . . . . . .  15
     5.2.  Filtering LSP information . . . . . . . . . . . . . . . .  15
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17



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     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   The IS-IS routing protocol IS-IS [ISO10589] currently 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, 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 to 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 [1] [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown
   here.

2.  Area Proxy

   To address this, we propose to 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 ensure 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 is 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
      represent the Level 1 area by injecting the Proxy LSP into the



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      Level 2 LSDB.  There may be multiple candidates for Area Leader,
      but only one is elected at a given time.

   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

                       An example of router classes

   All Inside Edge Routers learn the Area Proxy System Identifier from
   the Level 1 LSDB and use that as the system identifier in their Level
   2 IS-IS Hello PDUs (IIHs) on all Outside interfaces.  Outside Edge
   Routers should 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.

   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



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   Router LSPs on this link.  Boundary LANs SHOULD NOT be enabled for
   Level 1.  An Inside Edge Router may be elected the 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.

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.  The range advertised for the area will be the minimum of
   all Inside Nodes.

   To support Segment Routing, the Area Leader will take the global SID
   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 would be 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.

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.






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      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 its L2 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: YYY

      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
   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.





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4.  Area Leader Functions

   The Area Leader has several responsibilities.  First, it MUST inject
   the Area Proxy System Identifier into the Level 1 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.

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 and 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 ID        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Proxy System Identifier continued               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: AAA

      Length: length of a system ID (6)



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      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 MUST be advertising
   the same system identifier.  Multiple proxy system identifiers in a
   single area is a misconfiguration and each unique occurrence SHOULD
   be logged.

   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.

4.3.2.  The Area SID Sub-TLV

   The Area SID Sub-TLV allows the Area Leader to advertise a 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)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: BBB in the Area Proxy TLV, ZZZ in TLV 149 or 150.

      Length: variable (1 + SID length)

      Flags: 1 octet.




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

4.3.2.1.  Flags

   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.

      L: Local Flag.  If set, then the value/index carried by the proxy
      SID has local significance.

      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 MAY 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.





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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.

4.4.4.  The IS Neighbors TLV

   The Area Leader MAY 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.

4.4.5.  The Extended IS Neighbors TLV

   The Area Leader MAY 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.





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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 a MT Intermediate Systems TLV into the
   Proxy LSP.

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
   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.








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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 value 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 own Router Id in the Router Capability TLV.

   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 common 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 [I-D.ietf-lsr-isis-srv6-extensions] advertised by
   Inside Routers to the Proxy LSP.






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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.

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 Binding/
   MT Binding TLVs described in RFC 8667 Section 2.4 are used to
   advertise such a SID.

   The following extensions to the Binding TLV are defined in order to
   support Area SID:

      A new flag is defined:

         T-flag: The SID directs traffic to an area.  (Bit 5)

         When T-flag is set:

            M and A flag MUST be clear

            Range and Prefix are ignored

      Section 2.4.4 of RFC 8667 is altered to say:

         "The Prefix-SID sub-TLV MUST be present in the SID/Label
         Binding TLV when the M-Flag and T-flag are both clear.  The
         Prefix-SID sub-TLV MUST NOT be present when either the M-Flag
         or T-flag are set."

      Regarding the SID/Label sub-TLV Section 2.4.5 of RFC 8667 is
      altered to say:

         "It MUST be present in the SID/Label Binding TLV when either
         the M-Flag or T-flag is set in the Flags field of the parent
         TLV."

   When used in support of Area Proxy, the SID advertised MUST be
   identical to the Area SID (Section 4.3.2).  Other uses of the Area



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   SID 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.

   If the Area Leader is advertising an Area SID in the Area SID sub-TLV
   of the Area Proxy TLV, then the Area Leader SHOULD advertise the Area
   SID in the Proxy LSP.  The advertisement in the Proxy LSP informs the
   remainder of the network that packets directed to the SID will be
   forwarded by one of the Inside Edge Nodes and the Area SID will be
   consumed.

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 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 and subsequently flap.

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.



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      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 one code point from
   the IS-IS TLV Codepoints registry for the Area Proxy TLV (YYY).  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 YYY (Area Proxy TLV)

      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.
      We propose the initial experts be Chris Hopps, Tony Li, and Sarah
      Chen.

      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:




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         +-------+------------------------------+---------------+
         | Value |             Name             |   Reference   |
         +-------+------------------------------+---------------+
         |  AAA  | Area Proxy System Identifier | This document |
         |  BBB  |           Area SID           | This document |
         +-------+------------------------------+---------------+

   This memo also requests that IANA allocate a code point (ZZZ) for the
   Area SID subTLV in the registry for Sub-TLVs for TLVs 149 and 150.

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.

9.  References

9.1.  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-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-08
              (work in progress), April 2020.

   [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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   [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>.

   [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>.

   [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>.

   [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>.






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   [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>.

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>.

9.3.  URIs

   [1] https://tools.ietf.org/html/bcp14

Authors' Addresses

   Tony Li
   Arista Networks
   5453 Great America Parkway
   Santa Clara, California  95054
   USA

   Email: tony.li@tony.li


   Sarah Chen
   Arista Networks
   5453 Great America Parkway
   Santa Clara, California  95054
   USA

   Email: sarahchen@arista.com


   Vivek Ilangovan
   Arista Networks
   5453 Great America Parkway
   Santa Clara, California  95054
   USA

   Email: ilangovan@arista.com



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   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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