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Versions: 00 01 02 03 04 05 06 07 RFC 6860

OSPF Working Group                                               Y. Yang
Internet-Draft                                                 A. Retana
Updates: 2328, 5340 (if approved)                                 A. Roy
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: June 20, 2013                                 December 17, 2012


                  Hiding Transit-only Networks in OSPF
                 <draft-ietf-ospf-prefix-hiding-07.txt>

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 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
   remote attack vulnerability.

   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 RFC 2328 and RFC 5340.


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 http://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 17, 2013.







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Copyright Notice

   Copyright (c) 2012 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.





































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Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1. Requirements notation . . . . . . . . . . . . . . . . . . .  4
   2. Hiding IPv4 Transit-only Networks in OSPFv2 . . . . . . . . . .  4
     2.1. Point-to-Point Networks . . . . . . . . . . . . . . . . . .  4
       2.1.1. Advertising Point-to-Point Networks . . . . . . . . . .  4
       2.1.2. Hiding Point-to-Point Networks  . . . . . . . . . . . .  5
     2.2. Broadcast Networks  . . . . . . . . . . . . . . . . . . . .  5
       2.2.1. Advertising Broadcast Networks  . . . . . . . . . . . .  6
       2.2.2. Hiding Broadcast Networks . . . . . . . . . . . . . . .  6
         2.2.2.1. Sending Network-LSA . . . . . . . . . . . . . . . .  6
         2.2.2.2. Receiving Network-LSA . . . . . . . . . . . . . . .  7
           2.2.2.2.1. Backward Compatibility  . . . . . . . . . . . .  7
     2.3. Non-Broadcast Networks  . . . . . . . . . . . . . . . . . .  7
       2.3.1. NBMA  . . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.3.2. Point-to-MultiPoint . . . . . . . . . . . . . . . . . .  8
         2.3.2.1. Advertising Point-to-MultiPoint Networks  . . . . .  9
         2.3.2.2. Hiding Point-to-MultiPoint Networks . . . . . . . .  9
   3. Hiding IPv6 Transit-only Networks in OSPFv3 . . . . . . . . . . 10
     3.1. Hiding AF Enabled Transit-only Networks in OSPFv3 . . . . . 10
   4. Operational Considerations  . . . . . . . . . . . . . . . . . . 11
     4.1. Forwarding Address  . . . . . . . . . . . . . . . . . . . . 11
     4.2. Virtual Links . . . . . . . . . . . . . . . . . . . . . . . 11
     4.3. Unnumbered Interfaces . . . . . . . . . . . . . . . . . . . 12
   5. Security Considerations . . . . . . . . . . . . . . . . . . . . 12
   6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 13
   7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 13
   8. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1. Normative References  . . . . . . . . . . . . . . . . . . . 13
     8.2. Informative References  . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14


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 remote attack vulnerability.

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

   In the context of this document, 'hiding' implies that the prefixes



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   are not installed in the routing tables on OSPF routers. In [OSPFv2],
   the IPv4 interface addresses are still visible in the Router-Links-
   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


2.1.1. Advertising Point-to-Point Networks

   For each numbered point-to-point network, a router has 2 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 router-LSA originated by RT1 would look like

      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



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      #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 router-LSA originated by RT1, hiding the point-to-point
   network depicted in Figure 1, would look like

      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


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
   router RT3, RT4, and RT5.

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



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                                  +---+
                                  |RT5|
                                  +---+

                       Figure 2 Broadcast network

2.2.1. Advertising Broadcast Networks

   For each broadcast network, a designated router (DR) describes it in
   its network-LSA. Assuming 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 2-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.

   As there is no change of the Link State ID, the 2-way connectivity
   check would proceed normally.

   An example of network-LSA originated by RT3, hiding the broadcast
   network depicted in Figure 2, 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 = 255.255.255.255    ;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



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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 not-yet-upgraded router 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 better than the current result,
   which exposes the whole subnet.

   In a partial deployment scenario, upgraded routers and not-yet-
   upgraded routers 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 the 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.


2.3. Non-Broadcast Networks

   A non-broadcast networks 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 a NBMA transit-only network, OSPF adopts the same
   modification as over the broadcast transit-only network (see Section
   2.2.2.).




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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 router RT6, RT7,
   RT8, and RT9. In this network, all routers can communicate directly,
   except for routers RT7 and RT8.











































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                      +---+                   +---+
                      |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.

   An example of router-LSA originated by RT7 would look like

      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.




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

      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


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 address 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




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   [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 to not
   reduce the functionality and capability of the overall network, it is
   recommended that extensions such as [RFC5837] be also implemented.

   Note that the extension defined in [RFC5837] 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 NSSA-LSAs [NSSA] to achieve optimal routing to 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



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   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 to) in routing
   updates.  If needed, the numbered interface's address can be hidden
   using the mechanisms described in this document, or simply not
   advertising it.

   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 remote attack
   vulnerability 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 to
   transit-only networks only, they could be used to hide other networks
   as well.  It is expected that the same care that users put on the
   configuration of other routing protocol parameters is used in the
   configuration of this extension.



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6. IANA Considerations

   No actions are required from IANA as result of the publication of
   this document.


7. Acknowledgments

   The draft text was produced using Stefan Santesson's NroffEdit
   application.

   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.


8. References

8.1. 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", 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., Mirtorabi, S., Roy, A., Barnes, M., and R.
              Aggarwal, "Support of Address Families in OSPFv3",
              RFC5838, April 2010.

8.2. Informative References

   [RFC5837]  Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR.
              Rivers, "Extending ICMP for Interface and Next-Hop
              Identification", RFC 5837, April 2010.





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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
























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