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RFC 6845 - OSPF Hybrid Broadcast and Point-to-Multipoint Interfa


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Internet Engineering Task Force (IETF)                          N. Sheth
Request for Comments: 6845                              Contrail Systems
Updates: 2328, 5340                                              L. Wang
Category: Standards Track                                       J. Zhang
ISSN: 2070-1721                                         Juniper Networks
                                                            January 2013

      OSPF Hybrid Broadcast and Point-to-Multipoint Interface Type

Abstract

   This document describes a mechanism to model a broadcast network as a
   hybrid of broadcast and point-to-multipoint networks for purposes of
   OSPF operation.  Neighbor discovery and maintenance as well as Link
   State Advertisement (LSA) database synchronization are performed
   using the broadcast model, but the network is represented using the
   point-to-multipoint model in the router-LSAs of the routers connected
   to it.  This allows an accurate representation of the cost of
   communication between different routers on the network, while
   maintaining the network efficiency of broadcast operation.  This
   approach is relatively simple and requires minimal changes to OSPF.

   This document updates both OSPFv2 (RFC 2328) and OSPFv3 (RFC 5340).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6845.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  Interface Parameters  . . . . . . . . . . . . . . . . . . . 4
     4.2.  Neighbor Data Structure . . . . . . . . . . . . . . . . . . 4
     4.3.  Neighbor Discovery and Maintenance  . . . . . . . . . . . . 5
     4.4.  Database Synchronization  . . . . . . . . . . . . . . . . . 5
     4.5.  Generating Network-LSAs . . . . . . . . . . . . . . . . . . 5
     4.6.  Generating Router and Intra-Area-Prefix-LSAs  . . . . . . . 5
       4.6.1.  Stub Links in OSPFv2 Router-LSA . . . . . . . . . . . . 6
       4.6.2.  OSPFv3 Intra-Area-Prefix-LSA  . . . . . . . . . . . . . 6
     4.7.  Next-Hop Calculation  . . . . . . . . . . . . . . . . . . . 6
     4.8.  Graceful Restart  . . . . . . . . . . . . . . . . . . . . . 6
   5.  Compatibility Considerations  . . . . . . . . . . . . . . . . . 6
   6.  Scalability and Deployment Considerations . . . . . . . . . . . 7
   7.  Management Considerations . . . . . . . . . . . . . . . . . . . 7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   10. Normative References  . . . . . . . . . . . . . . . . . . . . . 8

1.  Introduction

   OSPF [RFC2328] operation on broadcast interfaces takes advantage of
   the broadcast capabilities of the underlying medium for doing
   neighbor discovery and maintenance.  Further, it uses a Designated
   Router (DR) and Backup Designated Router (BDR) to keep the Link State
   Advertisement (LSA) databases of the routers on the network
   synchronized in an efficient manner.  However, it has the limitation
   that a router cannot advertise different costs to each of the
   neighboring routers on the network in its router-LSA.

   Consider a radio network that supports true broadcast, yet the
   metrics between different pairs of terminals could be different for
   various reasons (e.g., different signal strength due to placement).
   When running OSPF over the radio network, for a router to advertise
   different costs to different neighbors, the interface must be treated
   as point-to-multipoint (P2MP), even though the network has true
   broadcast capability.

   Operation on point-to-multipoint interfaces could require explicit
   configuration of the identity of neighboring routers.  It also
   requires the router to send separate Hellos to each neighbor on the
   network.  Further, it mandates establishment of adjacencies to all
   configured or discovered neighbors on the network.  However, it gives
   the routers the flexibility to advertise different costs to each of
   the neighboring routers in their router-LSAs.

   This document proposes a new interface type that can be used on
   networks that have broadcast capability.  In this mode, neighbor
   discovery and maintenance, as well as database synchronization are
   performed using existing procedures for broadcast mode.  The network
   is modeled as a collection of point-to-point links in the router-LSA,
   just as it would be in point-to-multipoint mode.  This new interface
   type is referred to as hybrid-broadcast-and-P2MP in the rest of this
   document.

2.  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 [RFC2119].

3.  Motivation

   There are some networks that are broadcast capable but have a
   potentially different cost associated with communication between any
   given pair of nodes.  The cost could be based on the underlying

   topology as well as various link quality metrics such as bandwidth,
   delay, and jitter, among others.

   It is not accurate to treat such networks as OSPF broadcast networks
   since that does not allow a router to advertise a different cost to
   each of the other routers.  Using OSPF point-to-multipoint mode would
   satisfy the requirement to correctly describe the cost to reach each
   router.  However, it would be inefficient in the sense that it would
   require forming O(N^2) adjacencies when there are N routers on the
   network.

   It is advantageous to use the hybrid-broadcast-and-P2MP type for such
   networks.  This combines the flexibility of point-to-multipoint type
   with the advantages and efficiencies of broadcast interface type.

4.  Operation

   OSPF routers supporting the capabilities described herein should have
   support for an additional hybrid-broadcast-and-P2MP type for the Type
   data item described in Section 9 of [RFC2328].

   The following sub-sections describe salient aspects of OSPF operation
   on routers configured with a hybrid-broadcast-and-P2MP interface.

4.1.  Interface Parameters

   The "Router Priority" interface parameter as specified in OSPFv2
   [RFC2328] and OSPFv3 [RFC5340] applies to a hybrid-broadcast-and-P2MP
   interface.

   The "LinkLSASuppression" interface parameter as specified in OSPFv3
   [RFC5340] applies to a hybrid-broadcast-and-P2MP interface.  The
   default value is "disabled".  It may be set to "enabled" via
   configuration.

4.2.  Neighbor Data Structure

   An additional field called the Neighbor Output Cost is added to the
   neighbor data structure.  This is the cost of sending a data packet
   to the neighbor, expressed in the link state metric.  The default
   value of this field is the Interface output cost.  It may be set to a
   different value using mechanisms that are outside the scope of this
   document, like static per-neighbor configuration, or any dynamic
   discovery mechanism that is supported by the underlying network.

4.3.  Neighbor Discovery and Maintenance

   Routers send and receive Hellos so as to perform neighbor discovery
   and maintenance on the interface using the procedures specified for
   broadcast interfaces in [RFC2328] and [RFC5340].

4.4.  Database Synchronization

   Routers elect a DR and BDR for the interface and use them for initial
   and ongoing database synchronization using the procedures specified
   for broadcast interfaces in [RFC2328] and [RFC5340].

4.5.  Generating Network-LSAs

   Since a hybrid-broadcast-and-P2MP interface is described in router-
   LSAs using a collection of point-to-point links, the DR MUST NOT
   generate a network-LSA for the interface.

4.6.  Generating Router and Intra-Area-Prefix-LSAs

   Routers describe the interface in their router-LSA as specified for a
   point-to-multipoint interface in Section 12.4.1.4 of [RFC2328] and
   Section 4.4.3.2 of [RFC5340], with the following modifications for
   Type 1 links:

   o  If a router is not the DR and does not have a full adjacency to
      the DR, it MUST NOT add any Type 1 links.

   o  If a router is not the DR and has a full adjacency to the DR, and
      both the DR and this router agree on the DR role, it MUST add a
      Type 1 link corresponding to each neighbor that is in state 2-Way
      or higher and to which the DR's router-LSA includes a link.

   o  The cost for a Type 1 link corresponding to a neighbor SHOULD be
      set to the value of the Neighbor Output Cost field as defined in
      Section 4.2.

4.6.1.  Stub Links in OSPFv2 Router-LSA

   Routers MUST add a Type 3 link for their own IP address to the
   router-LSA as described in Section 12.4.1.4 of [RFC2328].  Further,
   they MUST also add a Type 3 link with the Link ID set to the IP
   subnet address, Link Data set to the IP subnet mask, and cost equal
   to the configured output cost of the interface.

4.6.2.  OSPFv3 Intra-Area-Prefix-LSA

   Routers MUST add globally scoped IPv6 addresses on the interface to
   the intra-area-prefix-LSA as described for point-to-multipoint
   interfaces in Section 4.4.3.9 of [RFC5340].  In addition, they MUST
   also add all globally scoped IPv6 prefixes on the interface to the
   LSA by specifying the PrefixLength, PrefixOptions, and Address Prefix
   fields.  The Metric field for each of these prefixes is set to the
   configured output cost of the interface.

   The DR MUST NOT generate an intra-area-prefix-LSA for the transit
   network for this interface since it does not generate a network-LSA
   for the interface.  Note that the global prefixes associated with the
   interface are advertised in the intra-area-prefix-LSA for the router
   as described above.

4.7.  Next-Hop Calculation

   Next-hops to destinations that are directly connected to a router via
   the interface are calculated as specified for a point-to-multipoint
   interface in Section 16.1.1 of [RFC2328].

4.8.  Graceful Restart

   The following modifications to the procedures defined in Section 2.2,
   item 1, of [RFC3623] are required in order to ensure that the router
   correctly exits graceful restart.

   o  If a router is the DR on the interface, the pre-restart network-
      LSA for the interface MUST NOT be used to determine the previous
      set of adjacencies.

   o  If a router is in state DROther on the interface, an adjacency to
      a non-DR or non-BDR neighbor is considered as reestablished when
      the neighbor state reaches 2-Way.

5.  Compatibility Considerations

   All routers on the network must support the hybrid-broadcast-and-P2MP
   interface type for successful operation.  Otherwise, the interface
   should be configured as a standard broadcast interface.

   If some routers on the network treat the interface as broadcast and
   others as hybrid-broadcast-and-P2MP, neighbors and adjacencies will
   still get formed as for a broadcast interface.  However, due to the
   differences in how router and network-LSAs are built for these two

   interface types, there will be no traffic traversing certain pairs of
   routers.  Note that this will not cause any persistent loops or
   black-holing of traffic.

   To detect and flag possible mismatched configurations, an
   implementation of this specification SHOULD log a message if a
   network-LSA is received for a locally configured hybrid interface.

6.  Scalability and Deployment Considerations

   Treating a broadcast interface as hybrid-broadcast-and-P2MP results
   in O(N^2) links to represent the network instead of O(N), when there
   are N routers on the network.  This will increase memory usage and
   have a negative impact on route calculation performance on all the
   routers in the area.  Network designers should carefully weigh the
   benefits of using the new interface type against the disadvantages
   mentioned here.

7.  Management Considerations

   The following MIB variable/value should be added to the appropriate
   OSPFv2 and OSPFv3 MIBs ([RFC4750], [RFC5643]).

   o  For ospfIfType/ospfv3IfType, a new value broadcast-P2MP-hybrid (X)
      for the hybrid interface type (X to be defined when the revised
      MIB documents are approved).

   o  For ospfNbrEntry/ospfv3NbrEntry, an ospfNbrMetricValue/
      ospfv3NbrMetricValue attribute for per-neighbor metrics.  In case
      of non-hybrid interfaces, the value is the same as the interface
      metric.

   This section is not normative.

8.  Security Considerations

   This document raises no new security issues for OSPF.  Security
   considerations for the base OSPF protocol are covered in [RFC2328],
   [RFC5340], and [RFC6506].

9.  Acknowledgements

   The authors would like to thank Acee Lindem and Richard Ogier for
   their comments and suggestions.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC3623]  Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful OSPF
              Restart", RFC 3623, November 2003.

   [RFC4750]  Joyal, D., Galecki, P., Giacalone, S., Coltun, R., and F.
              Baker, "OSPF Version 2 Management Information Base",
              RFC 4750, December 2006.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

   [RFC5643]  Joyal, D. and V. Manral, "Management Information Base for
              OSPFv3", RFC 5643, August 2009.

   [RFC6506]  Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 6506,
              February 2012.

Authors' Addresses

   Nischal Sheth
   Contrail Systems
   2350 Mission College Blvd, #1140
   Santa Clara, CA  95054
   US

   EMail: nsheth@contrailsystems.com

   Lili Wang
   Juniper Networks
   10 Technology Park Dr.
   Westford, MA  01886
   US

   EMail: liliw@juniper.net

   Jeffrey Zhang
   Juniper Networks
   10 Technology Park Dr.
   Westford, MA  01886
   US

   EMail: zzhang@juniper.net

 

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