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IETF RFC 6860
Last modified on Thursday, January 17th, 2013
 Permanent link to RFC 6860
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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/RFC 6860.
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
RFC TOTAL SIZE: 26368 bytes
PUBLICATION DATE: Thursday, January 17th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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