OSPF K. Patel
Internet-Draft Arrcus
Updates: 6987 (if approved) P. Pillay-Esnault
Intended status: Standards Track PPE Consulting
Expires: May 19, 2020 M. Bhardwaj
S. Bayraktar
Cisco Systems
November 16, 2019
Host Router Support for OSPFv2
draft-ietf-ospf-ospfv2-hbit-11
Abstract
The Open Shortest Path First Version 2 (OSPFv2) does not have a
mechanism for a node to repel transit traffic if it is on the
shortest path. This document defines a bit (Host-bit) that enables a
router to advertise that it is a non-transit router. It also
describes the changes needed to support the H-bit in the domain. In
addition, this document updates RFC 6987 to advertise type-2 External
and Not-So-Stubby-Area (NSSA) Link State Advertisements (LSAs) with a
high cost in order to repel traffic effectively.
Status of This Memo
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This Internet-Draft will expire on May 19, 2020.
Copyright Notice
Copyright (c) 2019 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Host-bit Support . . . . . . . . . . . . . . . . . . . . . . 3
4. SPF Modifications . . . . . . . . . . . . . . . . . . . . . . 5
5. Auto Discovery and Backward Compatibility . . . . . . . . . . 6
6. OSPF AS-External-LSAs/NSSA LSAs with Type 2 Metrics . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The OSPFv2 specifies a Shortest Path First (SPF) algorithm that
identifies transit vertices based on their adjacencies. Therefore,
OSPFv2 does not have a mechanism to prevent traffic transiting a
participating node if it is a transit vertex in the only existing or
shortest path to the destination. The use of metrics to make the
node undesirable can help to repel traffic only if an alternative
better route exists.
This functionality is particularly useful for a number of use cases:
1. To gracefully isolate a router to avoid blackhole scenarios when
there is a reload and possible long reconvergence times.
2. Closet Switches are usually not used for transit traffic but need
to participate in the topology.
3. Overloaded routers could use such a capability to temporarily
repel traffic until they stabilize.
4. BGP Route reflectors known as virtual Route Reflectors (vRRs),
that are not in the forwarding path but are in central locations
such as data centers. Such Route Reflectors typically are used
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for route distribution and are not capable of forwarding transit
traffic. However, they need to learn the OSPF topology to
perform SPF computation for optimal routes and reachability
resolution for its clients
[I-D.ietf-idr-bgp-optimal-route-reflection].
This document describes the Host-bit (H-bit) functionality that
prevents other OSPFv2 routers from using the host router by excluding
it in path calculations for transit traffic in OSPFv2 routing
domains. If the H-bit is set then the calculation of the shortest-
path tree for an area, as described in section 16.1 of [RFC2328], is
modified by including a check to verify that transit vertices DO NOT
have the H-bit set (see Section 4). Furthermore, in order to repel
traffic effectively, [RFC6987] is updated so that type-2 External and
NSSA LSAs are advertised with a high cost (see Section 6). Open
Shortest Path First Version 3 defines an option bit for router-LSAs
known as the R-bit in [RFC5340] to support a similar functionality.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Host-bit Support
This document defines a new router-LSA bit known as the Host Bit or
the H-bit. An OSPFv2 router advertising a router-LSA with the H-bit
set indicates that it MUST NOT be used as a transit router (see
Section 4) by other OSPFv2 routers in the area supporting the
functionality.
If the H-bit is not set then backwards compatibility is achieved as
the behavior will be the same as in [RFC2328].
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|H|0|0|N|W|V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Figure 1: OSPF Router-LSA
Bit H is the high-order bit of the OSPF flags as shown below.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|H|0|0|N|W|V|E|B|
+-+-+-+-+-+-+-+-+
Figure 2: OSPF Router-LSA Option bits
When the H-bit is set, the OSPFv2 router is a Host (non-transit)
router and is incapable of forwarding transit traffic. In this mode,
the other OSPFv2 routers in the area MUST NOT use the host router for
transit traffic, but may send traffic to its local destinations.
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An OSPFv2 router originating a router-LSA with the H-bit set MUST
advertise all its non-stub links with a link cost of MaxLinkMetric
[RFC6987].
When the H-bit is set, an Area Border Router (ABR) MUST advertise the
same H-bit setting in its self-originated router-LSAs for all
attached areas. The consistency of the setting will prevent inter-
area traffic transiting through the router by suppressing
advertisement of prefixes from other routers in the area in its
summary LSAs. Only IPv4 prefixes associated with its local
interfaces MUST be advertised in summary-LSAs to provide reachability
to end hosts attached to a router with the H-bit set.
When the H-bit is set the host router cannot act as an AS Boundary
Router (ASBR). Indeed, ASBR are transit routers to prefixes that are
typically imported through redistribution of prefixes from other
routing protocols. Therefore, non-local IPv4 prefixes, e.g., those
imported from other routing protocols, SHOULD NOT be advertised in
AS-external-LSAs if the H-bit is set. Some use cases, such as an
overloaded router or a router being gracefully isolated, may benefit
from continued advertisement of non-local prefixes. In these cases,
the type 2-metric in AS-external-LSAs MUST be set to LSInfinity to
repel traffic.(see Section 6 of this document).
4. SPF Modifications
The SPF calculation described in section 16.1 [RFC2328] will be
modified to ensure that the routers originating router-LSAs with the
H-bit set will not be used for transit traffic. Step 2 is modified
as follows:
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2) Call the vertex just added to the
tree vertex V. Examine the LSA
associated with vertex V. This is
a lookup in the Area A's link state
database based on the Vertex ID. If
this is a router-LSA, and the H-bit
of the router-LSA is set, and
vertex V is not the root, then the
router should not be used for transit
and step (3) should be executed
immediately. If this is a router-LSA,
and bit V of the router-LSA (see
Section A.4.2) is set, set Area A's
TransitCapability to TRUE. In any case,
each link described by the LSA gives
the cost to an adjacent vertex. For
each described link, (say it joins
vertex V to vertex W):
5. Auto Discovery and Backward Compatibility
To reduce the possibility of any routing loops due to partial
deployment, this document defines an OSPF Router Information (RI) LSA
[RFC7770] capability. The RI LSA MUST be area-scoped. Bit:
Bit Capabilities
7 Host Router Support capability
Table 1: OSPF Router Information LSA Capabilities
Auto Discovery via announcement of the Host Router Support Capability
ensures that the H-bit functionality and its associated SPF changes
MUST only take effect if all the routers in a given OSPF area support
this functionality.
In normal operation, there is no guarantee that the RI LSA will reach
all routers in an area in a timely manner, which may result in
forwarding loops in partial deployments. For example, if a new
router joins an area, which previously had only H-bit capable routers
with H-bit set then it may take some time for the RI to propagate to
all routers.
The following recommendations will mitigate transient routing loops:
o Implementations are RECOMMENDED to provide a configuration
parameter to manually override enforcement of the H-bit
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functionality in partial deployments where the topology guarantees
that OSPFv2 routers not supporting the H-bit do not compute routes
resulting in routing loops.
o All routers, with the H-bit set, MUST advertise all of the
router's non-stub links with a metric equal to MaxLinkMetric
[RFC6987] in its LSAs in order to avoid OSPFv2 (unless last
resort) routers not supporting the H-bit from attempting to use it
for transit traffic.
o All routers supporting H-Bit MUST check all the RI LSAs of nodes
in the area before actively running the modified SPF to account
for the H-bit in order to verify that all routers are in routing
capability. If any router does not advertise the Host Router
Support capability then the SPF Modifications (Section 4) MUST NOT
be used in the area.
6. OSPF AS-External-LSAs/NSSA LSAs with Type 2 Metrics
When calculating the path to an OSPF AS-External-LSA or NSSA-LSA
[RFC3101] with a Type-2 metric, the advertised Type-2 metric is taken
as more significant than the OSPF intra-area or inter-area path.
Hence, advertising the links with MaxLinkMetric as specified in
[RFC6987] does not discourage transit traffic when calculating AS
external or NSSA routes with Type-2 metrics.
Consequently, [RFC6987] is updated so that the Type-2 metric in any
self-originated AS-External-LSAs or NSSA-LSAs is advertised as
LSInfinity-1 [RFC2328]. If the H-bit is set, then the Type-2 metric
MUST be set to LSInfinity.
7. IANA Considerations
This document requests the IANA to assign the 0x80 value to the Host-
Bit (H-bit)in the OSPFv2 Router Properties Registry
Value Description Reference
0x80 Host (H-bit) This Document
This document requests the IANA to assign the Bit Number value of 7
to the Host Router Support Capability in the OSPF Router
Informational Capability Bits Registry.
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Bit Number Capability Name Reference
7 OSPF Host Router This Document
8. Security Considerations
This document introduces the H-bit which is a capability that
restricts the use of a router for transit, while only its local
destinations are reachable. This is a subset of the operations of a
normal router and therefore should not introduce new security
considerations beyond those already known in OSPFv2 [RFC2328]. The
feature introduces the advertising of a host router capability
information to all OSPFv2 routers in an area. This information can
be leveraged for discovery and verification that all routers in the
area support the capability before the feature is turned on. In the
event that a rogue or buggy router advertises incorrectly its
capability the possible cases are:
o The router does not have the capability but sends the H-Bit set in
its LSAs: In this case, there is a possibility of a routing loop.
However this is mitigated by the fact that this router should be
avoided anyway. Moreover, the link metrics cost (MaxLinkMetric)
of this router will mitigate this situation. In any case, a
router advertising the H-bit capability without its links cost
equal to MaxLinkMetric may be an indicator that this is a rogue
router and should be avoided.
o The router has the capability but sends the H-Bit clear in its
LSAs: In this case, the router merely prevents support of other
H-bit routers in the area and all the routers to run the modified
SPF. The impact is also mitigated as other H-Bit routers in the
area also advertise MaxLinkMetric cost so they will still be
avoided unless they are the last resort path.
o The rogue router is on the only transit path for some destinations
and sends the H-Bit set (for no good/valid reason) in its LSAs and
effectively partition the network. This case is indistinguishable
from the normal case where the operator may consciously decide to
set the H-bit to perform maintenance on a router that is on the
only transit path. The OSPF protocol will continue to function
within the partitioned domains.
9. Acknowledgements
The authors would like to acknowledge Hasmit Grover for discovery of
the limitation in [RFC6987], Acee Lindem, Abhay Roy, David Ward,
Burjiz Pithawala, and Michael Barnes for their comments.
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10. References
10.1. Normative References
[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>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC6987, September 2013,
<https://www.rfc-editor.org/info/rfc6987>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[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>.
10.2. Informative References
[I-D.ietf-idr-bgp-optimal-route-reflection]
Raszuk, R., Cassar, C., Aman, E., Decraene, B., and K.
Wang, "BGP Optimal Route Reflection (BGP-ORR)", draft-
ietf-idr-bgp-optimal-route-reflection-19 (work in
progress), July 2019.
[RFC3101] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
RFC 3101, DOI 10.17487/RFC3101, January 2003,
<https://www.rfc-editor.org/info/rfc3101>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
Authors' Addresses
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Keyur Patel
Arrcus
Email: keyur@arrcus.com
Padma Pillay-Esnault
PPE Consulting
Email: padma.ietf@gmail.com
Manish Bhardwaj
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: manbhard@cisco.com
Serpil Bayraktar
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: serpil@cisco.com
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