Area Proxy for IS-IS
draft-li-lsr-isis-area-proxy-00

Internet Engineering Task Force                                    T. Li
Internet-Draft                                                   S. Chen
Intended status: Standards Track                         Arista Networks
Expires: April 5, 2020                                   October 3, 2019

                          Area Proxy for IS-IS
                    draft-li-lsr-isis-area-proxy-00

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
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   This Internet-Draft will expire on April 5, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Area Proxy  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Inside Router Functions . . . . . . . . . . . . . . . . . . .   5
     3.1.  The Area Proxy Router Capability  . . . . . . . . . . . .   5
     3.2.  Level 2 SPF Computation . . . . . . . . . . . . . . . . .   5
   4.  Area Leader Functions . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Area Leader Election  . . . . . . . . . . . . . . . . . .   6
     4.2.  Redundancy  . . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Area Proxy System Identifier TLV  . . . . . . . . . . . .   6
     4.4.  Area Proxy LSP Generation . . . . . . . . . . . . . . . .   7
   5.  Inside Edge Router Functions  . . . . . . . . . . . . . . . .   8
     5.1.  Generating L2 IIHs to Outside Routers . . . . . . . . . .   8
     5.2.  Filtering LSP information . . . . . . . . . . . . . . . .   8
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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

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   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.

   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 documment 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.

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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.  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.

   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
      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.

   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.

   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.

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   Area Proxy functionality assumes that all circuits are either Level
   1-2 circuits within the Inside Area, or Level 2 circuits between
   Outside Routers and a single Inside Edge Router.  Multi-access
   circuits (i.e.  Ethernets in LAN mode) with multiple Inside Edge
   Routers and an Outside Router are not supported.

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 Router Capability as part of its Level 1
   Router Capability TLV.

3.1.  The Area Proxy Router Capability

   The Area Proxy Router Capability is a sub-TLV of the Router
   Capability TLV [RFC7981] and has the following format:

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

      TLV Type: YYY

      TLV Length: 0

   A router advertising this TLV indicates that it is running Level 1-2
   and is prepared to perform Area Proxy functions.

3.2.  Level 2 SPF Computation

   When Outside Routers perform a Level 2 SPF computation, they will use
   the Area 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 Area 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-

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   area metrics are always 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.

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 Area Proxy LSP.  The failure of the Area
   Leader is not visible outside of the area and appears to simply be an
   update of the Area Proxy LSP.

4.3.  Area Proxy System Identifier TLV

   The Area Proxy System Identifier TLV allows the Area Leader to
   advertise the existence of an Area Proxy System Identifier.  This TLV
   is injected into the Area Leader's Level 1 LSP.

   The format of the Area Proxy System Identifier 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    |  Proxy SysID  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Proxy System Identifier continued ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      TLV Type: XXX

      TLV Length: length of a system ID (6)

      Proxy System Identifier: the Area Proxy System Identifier.

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   The Area Leader MAY advertise the Area Proxy System Identifier TLV
   when it observes that all Inside Routers are advertising the Area
   Proxy Router Capability.  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 should 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.

   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.

4.4.  Area 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.

   The Area Leader uses the Level 2 LSPs generated by the Inside Edge
   Routers to generate the Area Proxy LSP.  This LSP is originated using
   the Area Proxy System Identifier and includes adjacencies for all of
   the Outside Edge Routers that have been advertised by the Inside Edge
   Routers.  Since the Outside Edge Routers also advertise an adjacency
   to the proxy system identifier, this will result in a bi-directional
   adjacency.  The Area Proxy LSP is the only LSP that is injected into
   the overall Level 2 LSDB, with all other Level 2 LSPs from the Inside
   Area being filtered out at the Inside Area boundary.

   The Area Leader may also insert additional TLVs into the Area Proxy
   LSP for additional information for the Outside Area.  It is
   RECOMMENDED that the Area Leader insert the Dynamic Hostname TLV
   [RFC5301] into the Area Proxy LSP.  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, statically configured, or redistributed from

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   another routing protocol, using the usual TLVs for prefix
   advertisement.  [RFC5305] [RFC5308] [RFC5120]

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 should 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 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 Level 1 LSDB.  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 should never 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 should 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

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      is no remaining content, then the PSNP MUST NOT be sent.  The
      source address of the PSNP should 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 Pseudonode TLV (XXX).

   IANA is also requested to allocate and assign one code point from the
   IS-IS Router Capability TLV sub-TLV registry for the Area Proxy
   Capability (YYY).

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., and S. Dontula, "Dynamic
              Flooding on Dense Graphs", draft-ietf-lsr-dynamic-
              flooding-03 (work in progress), June 2019.

   [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.

   [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>.

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

   [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>.

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>.

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

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

   Email: sarahchen@arista.com

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