





Internet Engineering Task Force (IETF)                           Y. Yang
Request for Comments: 6860                                     A. Retana
Updates: 2328, 5340                                               A. Roy
Category: Standards Track                            Cisco Systems, Inc.
ISSN: 2070-1721                                             January 2013


                  Hiding Transit-Only Networks in OSPF

Abstract

   A transit-only network is defined as a network connecting routers
   only.  In OSPF, transit-only networks are usually configured with
   routable IP addresses, which are advertised in Link State
   Advertisements (LSAs) but are not needed for data traffic.  In
   addition, remote attacks can be launched against routers by sending
   packets to these transit-only networks.  This document presents a
   mechanism to hide transit-only networks to speed up network
   convergence and reduce vulnerability to remote attacks.

   In the context of this document, 'hiding' implies that the prefixes
   are not installed in the routing tables on OSPF routers.  In some
   cases, IP addresses may still be visible when using OSPFv2.

   This document updates RFCs 2328 and 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/rfc6860.












Yang, et al.                 Standards Track                    [Page 1]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


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
      1.1. Requirements Notation ......................................3
   2. Hiding IPv4 Transit-Only Networks in OSPFv2 .....................3
      2.1. Point-to-Point Networks ....................................3
           2.1.1. Advertising Point-to-Point Networks .................4
           2.1.2. Hiding Point-to-Point Networks ......................4
      2.2. Broadcast Networks .........................................5
           2.2.1. Advertising Broadcast Networks ......................5
           2.2.2. Hiding Broadcast Networks ...........................5
                  2.2.2.1. Sending Network-LSA ........................5
                  2.2.2.2. Receiving Network-LSA ......................6
                           2.2.2.2.1. Backward Compatibility ..........6
      2.3. Non-Broadcast Networks .....................................7
           2.3.1. NBMA ................................................7
           2.3.2. Point-to-Multipoint .................................7
                  2.3.2.1. Advertising Point-to-Multipoint Networks ...7
                  2.3.2.2. Hiding Point-to-Multipoint Networks ........8
   3. Hiding IPv6 Transit-Only Networks in OSPFv3 .....................9
      3.1. Hiding AF-Enabled Transit-Only Networks in OSPFv3 ..........9
   4. Operational Considerations ......................................9
      4.1. Forwarding Address ........................................10
      4.2. Virtual Links .............................................10
      4.3. Unnumbered Interfaces .....................................10
   5. Security Considerations ........................................11
   6. Acknowledgments ................................................11
   7. References .....................................................12
      7.1. Normative References ......................................12
      7.2. Informative References ....................................12






Yang, et al.                 Standards Track                    [Page 2]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


1.  Introduction

   A transit-only network is defined as a network connecting routers
   only.  In OSPF, transit-only networks are usually configured with
   routable IP addresses, which are advertised in LSAs but not needed
   for data traffic.  In addition, remote attacks can be launched
   against routers by sending packets to these transit-only networks.
   This document presents a mechanism to hide transit-only networks to
   speed up network convergence and reduce vulnerability to remote
   attacks.

   Hiding transit-only networks will not impact reachability to the end
   hosts.

   In the context of this document, 'hiding' implies that the prefixes
   are not installed in the routing tables on OSPF routers.  In
   [OSPFv2], the IPv4 interface addresses are still visible in the
   Router-LSA links and the network-LSA Link-State ID (LSID).  In
   [OSPFv3], the router-LSAs and network-LSAs do not contain IPv6
   addresses and are not visible.

   This document updates [OSPFv2] and [OSPFv3] by specifying a mechanism
   that can be used to hide transit-only networks.

1.1.  Requirements Notation

   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 [KEYWORD].

2.  Hiding IPv4 Transit-Only Networks in OSPFv2

   In [OSPFv2], networks are classified as point-to-point, broadcast, or
   non-broadcast.  In the following sections, we will review how these
   OSPF networks are being advertised and discuss how to hide them.

2.1.  Point-to-Point Networks

   A point-to-point network joins a single pair of routers.  Figure 1
   shows a point-to-point network connecting routers RT1 and RT2.

                  +---+.1    198.51.100.0/30    .2+---+
                  |RT1|---------------------------|RT2|
                  +---+                           +---+

                Figure 1.  Physical Point-to-Point Network





Yang, et al.                 Standards Track                    [Page 3]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


2.1.1.  Advertising Point-to-Point Networks

   For each numbered point-to-point network, a router has two link
   descriptions in its router-LSA: one Type 1 link (point-to-point)
   describing the neighboring router, and one Type 3 link (stub)
   describing the assigned IPv4 subnet.

   An example of a router-LSA originated by RT1 would look like the
   following:

        LS age = 0                        ;newly (re-)originated
        LS type = 1                       ;router-LSA
        Link State ID = 192.0.2.1         ;RT1's Router ID
        Advertising Router = 192.0.2.1    ;RT1's Router ID
        #links = 2
           Link ID = 192.0.2.2            ;RT2's Router ID
           Link Data = 198.51.100.1       ;Interface IP address
           Type = 1                       ;connects to RT2
           Metric = 10

           Link ID= 198.51.100.0          ;IP network/subnet number
           Link Data = 255.255.255.252    ;Subnet's mask
           Type = 3                       ;Connects to stub network
           Metric = 10

   The Type 1 link will be used for SPF calculation, while the Type 3
   link will be used to install a route to the corresponding subnet in
   the Routing Information Base (RIB).

2.1.2.  Hiding Point-to-Point Networks

   To hide a transit-only point-to-point network, the Type 3 link is
   omitted from the router-LSA.

   An example of a router-LSA originated by RT1, hiding the point-to-
   point network depicted in Figure 1, would look like the following:

        LS age = 0                        ;newly (re-)originated
        LS type = 1                       ;router-LSA
        Link State ID = 192.0.2.1         ;RT1's Router ID
        Advertising Router = 192.0.2.1    ;RT1's Router ID
        #links = 1
           Link ID = 192.0.2.2            ;RT2's Router ID
           Link Data = 198.51.100.1       ;Interface IP address
           Type = 1                       ;connects to RT2
           Metric = 10





Yang, et al.                 Standards Track                    [Page 4]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


2.2.  Broadcast Networks

   A broadcast network joins many (more than two) routers and supports
   the capability to address a single physical message to all of the
   attached routers.  Figure 2 shows a broadcast network connecting
   routers RT3, RT4, and RT5.

                       +---+                   +---+
                       |RT3|                   |RT4|
                       +---+                   +---+
                         |.3  198.51.100.0/24  .4|
                      +-----------------------------+
                                     |.5
                                   +---+
                                   |RT5|
                                   +---+

                       Figure 2.  Broadcast Network

2.2.1.  Advertising Broadcast Networks

   A Designated Router (DR) describes a broadcast network in a
   network-LSA.  Assuming that RT3 is elected as the DR in Figure 2, an
   example of the network-LSA originated by RT3 would look like

        LS age = 0                        ;newly (re)originated
        LS type = 2                       ;network-LSA
        Link State ID = 198.51.100.3      ;IP address of the DR (RT3)
        Advertising Router = 192.0.2.3    ;RT3's Router ID
        Network Mask = 255.255.255.0
           Attached Router = 192.0.2.3    ;RT3's Router ID
           Attached Router = 192.0.2.4    ;RT4's Router ID
           Attached Router = 192.0.2.5    ;RT5's Router ID

   OSPF obtains the IP network number from the combination of the Link
   State ID and the network mask.  In addition, the Link State ID is
   also being used for the two-way connectivity check.

2.2.2.  Hiding Broadcast Networks

2.2.2.1.  Sending Network-LSA

   A special subnet mask value of 255.255.255.255 MUST be used in the
   network-LSA to hide a transit-only broadcast network.  While a
   broadcast network connects more than routers, using 255.255.255.255
   will not hide an access broadcast network accidentally.





Yang, et al.                 Standards Track                    [Page 5]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


   As there is no change of the Link State ID, the two-way connectivity
   check would proceed normally.

   An example of a network-LSA originated by RT3, hiding the broadcast
   network depicted in Figure 2, would look like the following:

        LS age = 0                        ;newly (re-)originated
        LS type = 2                       ;network-LSA
        Link State ID = 198.51.100.3      ;IP address of the DR (RT3)
        Advertising Router = 192.0.2.3    ;RT3's Router ID
        Network Mask = 255.255.255.255    ;special subnet mask
           Attached Router = 192.0.2.3    ;RT3's Router ID
           Attached Router = 192.0.2.4    ;RT4's Router ID
           Attached Router = 192.0.2.5    ;RT5's Router ID

2.2.2.2.  Receiving Network-LSA

   It is RECOMMENDED that all routers in an area be upgraded at the same
   time to process the modified network-LSA correctly and consistently.

   When a router receives a network-LSA, it MUST calculate the routing
   table normally [OSPFv2].  However, if the network mask in the
   network-LSA is 255.255.255.255, the router MUST NOT install the route
   in the RIB.

2.2.2.2.1.  Backward Compatibility

   When a router that has not yet been upgraded receives a modified
   network-LSA, as specified in Section 2.2.2.1, a host route to the
   originating DR will be installed.  This is not ideal, but it is
   better than the current result, which exposes the whole subnet.

   In a partial-deployment scenario, upgraded routers and routers that
   have not yet been upgraded may coexist.  The former do not install
   the host route to the DR's interface, while the latter install it.
   Such inconsistencies create routing black holes, which should
   normally be avoided.  In this case, however, as packets destined for
   the transit-only networks are dropped somewhere in the network, the
   black holes actually help the DRs defend themselves from remote
   attacks.

   In summary, the modification of the network-LSA, as specified in
   Section 2.2.2.1, is backward compatible with the current
   specification of [OSPFv2], even in a partial-deployment scenario.







Yang, et al.                 Standards Track                    [Page 6]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


2.3.  Non-Broadcast Networks

   A non-broadcast network joins many (more than two) routers but does
   NOT support the capability to address a single physical message to
   all of the attached routers.  As mentioned in [OSPFv2], OSPF runs in
   one of two modes over non-broadcast networks: Non-Broadcast Multi-
   Access (NBMA) or point-to-multipoint.

2.3.1.  NBMA

   In NBMA mode, OSPF emulates operation over a broadcast network: a
   Designated Router is elected for the NBMA network, and the Designated
   Router originates an LSA for the network.

   To hide an NBMA transit-only network, OSPF adopts the same
   modification as that used over the broadcast transit-only network
   (see Section 2.2.2).

2.3.2.  Point-to-Multipoint

   In point-to-multipoint mode, OSPF treats the non-broadcast network as
   a collection of point-to-point links.

   Figure 3 shows a non-broadcast network connecting routers RT6, RT7,
   RT8, and RT9.  In this network, all routers can communicate directly,
   except for routers RT7 and RT8.

                       +---+                   +---+
                       |RT6|                   |RT7|
                       +---+                   +---+
                         |.6  198.51.100.0/24  .7|
                      +----------------------------+
                         |.8                   .9|
                       +---+                   +---+
                       |RT8|                   |RT9|
                       +---+                   +---+

                     Figure 3.  Non-Broadcast Network

2.3.2.1.  Advertising Point-to-Multipoint Networks

   For a point-to-multipoint network, a router has multiple link
   descriptions in its router-LSA, one Type 1 link (point-to-point) for
   EACH directly communicable router, and one Type 3 link (stub)
   advertising its interface IPv4 address with a subnet mask of
   255.255.255.255.





Yang, et al.                 Standards Track                    [Page 7]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


   An example of a router-LSA originated by RT7 would look like the
   following:

        LS age = 0                        ;newly (re-)originated
        LS type = 1                       ;router-LSA
        Link State ID = 192.0.2.7         ;RT7's Router ID
        Advertising Router = 192.0.2.7    ;RT7's Router ID
        #links = 3
           Link ID = 192.0.2.6            ;RT6's Router ID
           Link Data = 198.51.100.7       ;Interface IP address
           Type = 1                       ;connects to RT6
           Metric = 10

           Link ID = 192.0.2.9            ;RT9's Router ID
           Link Data = 198.51.100.7       ;Interface IP address
           Type = 1                       ;connects to RT9
           Metric = 10

           Link ID= 198.51.100.7          ;Interface IP address
           Link Data = 255.255.255.255    ;Subnet's mask
           Type = 3                       ;Connects to stub network
           Metric = 0

2.3.2.2.  Hiding Point-to-Multipoint Networks

   To hide a transit-only point-to-multipoint network, the Type 3 link
   is omitted from the router-LSA.

   An example of a router-LSA originated by RT7, hiding the point-to-
   point network depicted in Figure 3, would look like the following:

        LS age = 0                        ;newly (re-)originated
        LS type = 1                       ;router-LSA
        Link State ID = 192.0.2.7         ;RT7's Router ID
        Advertising Router = 192.0.2.7    ;RT7's Router ID
        #links = 2
           Link ID = 192.0.2.6            ;RT6's Router ID
           Link Data = 198.51.100.7       ;Interface IP address
           Type = 1                       ;connects to RT6
           Metric = 10

           Link ID = 192.0.2.9            ;RT9's Router ID
           Link Data = 198.51.100.7       ;Interface IP address
           Type = 1                       ;connects to RT9
           Metric = 10






Yang, et al.                 Standards Track                    [Page 8]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


3.  Hiding IPv6 Transit-Only Networks in OSPFv3

   In [OSPFv3], addressing semantics have been removed from the OSPF
   protocol packets and the main LSA types, leaving a network-protocol-
   independent core.

   More specifically, router-LSAs and network-LSAs no longer contain
   network addresses but simply express topology information.  Instead,
   two new LSA types, link-LSA and intra-area-prefix-LSA, have been
   introduced.  A link-LSA associates a list of IPv6 addresses to a link
   and has local-link flooding scope, and an intra-area-prefix-LSA
   either associates a list of IPv6 addresses with a router by
   referencing a router-LSA or associates a list of IPv6 addresses with
   a broadcast/NBMA network by referencing a network-LSA.  In the latter
   case, the prefixes in the link-LSAs from adjacent neighbors are
   copied into the intra-area-prefix-LSA by the Designated Router.

   To hide a transit-only network in [OSPFv3], the IPv6 address prefixes
   are omitted from the router-LSA.  Consequently, when a Designated
   Router builds an intra-area-prefix-LSA referencing a network-LSA,
   these IPv6 address prefixes will be omitted.

   In addition, when a router builds an intra-area-prefix-LSA that is
   referencing a router-LSA, the associated IPv6 address prefixes from
   the transit-only network MUST also be omitted from the intra-area-
   prefix-LSA.

3.1.  Hiding AF-Enabled Transit-Only Networks in OSPFv3

   [OSPF-AF] supports multiple Address Families (AFs) by mapping each AF
   to a separate Instance ID and OSPFv3 instance.

   In the meantime, each prefix advertised in OSPFv3 has a prefix length
   field [OSPFv3], which facilitates advertising prefixes of different
   lengths in different AFs.  The existing LSAs defined in [OSPFv3] are
   used for prefix advertising, and there is no need to define new LSAs.

   In other words, as link-LSAs and intra-area-prefix-LSAs are still
   being used, the same mechanism explained in Section 3 can be used to
   hide those AF-enabled transit-only networks as well.

4.  Operational Considerations

   By eliminating the ability to reach transit-only networks, the
   ability to manage these interfaces may be reduced.  In order not to
   reduce the functionality and capability of the overall network, it is
   recommended that extensions such as [UNNUMBERED] also be implemented.




Yang, et al.                 Standards Track                    [Page 9]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


   Note that the extension defined in [UNNUMBERED] may provide the user
   with the IP address of an interface.  If that address was hidden, as
   specified in this document, then even though the address is assigned
   to the interface, it will not be reachable.

4.1.  Forwarding Address

   A non-zero forwarding address can be advertised in AS-external-LSAs
   and Not-So-Stubby Area LSAs (NSSA-LSAs) [NSSA] to achieve optimal
   routing to Autonomous System (AS) external routes.  The matching
   routing table entry for the forwarding address must exist to
   facilitate the SPF calculation.

   In other words, when prefix-hiding is configured on the next-hop
   interface, the next-hop address MUST NOT be advertised as a
   forwarding address.

   Consequently, sub-optimal routing to these AS external routes may
   exist when prefix-hiding is configured.

4.2.  Virtual Links

   Virtual links are used to connect physically separate components of
   the backbone.  The virtual link's viability is determined by the
   existence of an intra-area path between two endpoints.  The matching
   routing table entries of the endpoints must exist to ensure the
   virtual link's operation.

   In other words, if prefix-hiding is configured on an interface, the
   virtual link endpoint MUST NOT use that interface's IP address as the
   virtual interface's IP address.

4.3.  Unnumbered Interfaces

   Note that no host route is generated for, and no IP packets can be
   addressed to, interfaces to unnumbered point-to-point networks
   [OSPFv2].  In other words, these addresses are already hidden.

   However, for manageability purposes, it may be common practice to
   manually include the numbered interface (for example, a loopback
   interface to which the unnumbered interface points) in routing
   updates.  If needed, the numbered interface's address can be hidden
   by using the mechanisms described in this document or by simply not
   advertising it.







Yang, et al.                 Standards Track                   [Page 10]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


   Before deciding to hide (or suppress the advertisement of) a numbered
   interface, it is very important to consider other uses that interface
   may have.  Examples of common uses may include virtual link endpoint,
   inter-domain routing peering point, etc.  In other words, it may not
   be possible to hide the address associated to an unnumbered interface
   due to other applications in the network.

5.  Security Considerations

   One motivation for this document is to reduce vulnerability to remote
   attacks by hiding transit-only networks.  The result should then be
   that fewer OSPF core networks will be exposed.

   The mechanisms described above result in reachability information
   from transit-only networks not being installed in the routers'
   forwarding tables.  The effect is that even if the address of a
   transit-only network is known, the forwarding information is not
   present in the routers to reach the destination.  Also, in some
   cases, the address information is completely omitted from the LSA.

   Some information in the LSA (such as the OSPF Router ID) cannot be
   omitted.  Even though the Router ID may be taken from an IPv4 address
   on the router, the configuration can be easily changed.  Note again
   that having an address doesn't guarantee reachability if the
   information is hidden from the forwarding tables.

   While the steps described in this document are meant to be applied
   only to transit-only networks, they could be used to hide other
   networks as well.  It is expected that the same care that users put
   into the configuration of other routing protocol parameters is used
   in the configuration of this extension.

6.  Acknowledgments

   The idea of using a special subnet mask to hide broadcast networks in
   OSPF was originally introduced in the US patent "Apparatus and method
   to hide transit only multi-access networks in OSPF" (patent number:
   7,929,524), by Yi Yang, Alvaro Retana, James Ng, Abhay Roy, Alfred
   Lindem, Sina Mirtorabi, Timothy Gage, and Khalid Raza.

   The authors would like to thank Acee Lindem, Shraddha Hegde, Rajesh
   Shetty, Marek Karasek, Michael Barnes, Paul Wells, Adrian Farrel, and
   Stephen Farrell for their feedback on the document.








Yang, et al.                 Standards Track                   [Page 11]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


7.  References

7.1.  Normative References

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

   [NSSA]        Murphy, P., "The OSPF Not-So-Stubby Area (NSSA)
                 Option", RFC 3101, January 2003.

   [OSPFv2]      Moy, J., "OSPF Version 2", STD 54, RFC 2328,
                 April 1998.

   [OSPFv3]      Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
                 for IPv6", RFC 5340, July 2008.

   [OSPF-AF]     Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M.,
                 and R. Aggarwal, "Support of Address Families in
                 OSPFv3", RFC 5838, April 2010.

7.2.  Informative References

   [UNNUMBERED]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed.,
                 Shen, N., and JR. Rivers, "Extending ICMP for Interface
                 and Next-Hop Identification", RFC 5837, April 2010.


























Yang, et al.                 Standards Track                   [Page 12]

RFC 6860          Hiding Transit-Only Networks in OSPF      January 2013


Authors' Addresses

   Yi Yang
   Cisco Systems, Inc.
   7025 Kit Creek Road
   RTP, NC  27709
   USA

   EMail: yiya@cisco.com


   Alvaro Retana
   Cisco Systems, Inc.
   7025 Kit Creek Road
   RTP, NC  27709
   USA

   EMail: aretana@cisco.com


   Abhay Roy
   Cisco Systems, Inc.
   225 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: akr@cisco.com
























Yang, et al.                 Standards Track                   [Page 13]

