OSPF K. Patel
Internet-Draft Arrcus
Updates: 2328 (if approved) P. Pillay-Esnault
Intended status: Standards Track Huawei Technologies
Expires: November 20, 2019 M. Bhardwaj
S. Bayraktar
Cisco Systems
May 19, 2019
Host Router Support for OSPFv2
draft-ietf-ospf-ospfv2-hbit-07
Abstract
The OSPFv2 specifies an 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
only help to repel traffic if an alternative better route exists.
This document defines the Host-bit functionality to prevent other
OSPFv2 routers from using the router for transit traffic in OSPFv2
routing domains. This document updates the [RFC2328] by assigning a
new bit (Host-bit) in the OSPF Router-LSA bit registry. If the Host-
bit is set, the calculation of the shortest-path tree for an area, as
described in [RFC2328], is modified by including a new check to
verify that transit vertices have the Host-bit clear.
Status of This Memo
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This Internet-Draft will expire on November 20, 2019.
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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 . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The OSPFv2 specifies an 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
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is a transit vertex in the only existing or shortest path to the
destination. The use of metrics to make the node undesirable can
only help to repel traffic if an alternative better route exists.
This functionality is particularly useful for a number of use cases:
1. To 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 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
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 reachbility
resolution for its clients
[I-D.ietf-idr-bgp-optimal-route-reflection].
This document defines the Host-bit (H-Bit)functionality to prevent
other OSPFv2 routers from using the router for transit traffic in
OSPFv2 routing domains. This document updates the [RFC2328] by -
assigning the Host-bit in the OSPF Router-LSA bit registry - if the
host-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 new check to verify that transit vertices DO NOT have the
host-bit set.
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 to other OSPFv2 routers in the area supporting the
functionality that it MUST NOT be used as a transit router (see
section 4).
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If the host-bit is NOT set routers MUST act transit routers as
described in [RFC2328] ensuring backward compatibility.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Host Bit in router-LSA
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|H|0|0|N|W|V|E|B|
+-+-+-+-+-+-+-+-+
Host Bit
Bit H is the high-order bit of the OSPF as shown above. When set, an
OSPFv2 router is a non-transit router and is incapable of forwarding
transit traffic.
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An OSPFv2 router originating a router-LSA with the H-bit set MUST
advertise all its router links with a link cost of MaxLinkMetric
[RFC6987]. This is to increase the applicability of the H-bit to
partial deployments where it is the responsibility of the operator to
ensure that OSPFv2 routers not supporting the H-bit do not install
routes causing routing loops.
When the H-bit is set, an Area Border Router (ABR) MUST advertise a
consistent H-bit setting in its self-originated router-LSAs for all
attached areas. ONLY IPv4 prefixes associated with its local
interfaces MAY be advertised in summary LSAs.
When the H-bit is set cannot act as an AS Boundary Router (ASBR), as
non-local IPv4 prefixes, e.g., those exported from other routing
protocols, MUST NOT be advertised in AS-external-LSAs.
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:
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):
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5. Auto Discovery and Backward Compatibility
To avoid the possibility of any routing loops due to partial
deployment, this document defines a OSPF Router Information (RI) LSA
with a Router Functional Capability TLV that includes the following
Router Functional Capability Bit:
Bit Capabilities
7 Host Router Support capability
Auto Discovery via announcement of the Host Support Functional
Capability ensures that the H-bit functionality and its associated
SPF changes SHOULD only take effect if all the routers in a given
OSPF area support this functionality.
Implementations are encouraged to provide a configuration parameter
to manually override enforcement of the H-bit functionality in
partial deployments where the topology guarantees that OSPFv2 routers
not supporting the H-bit do not compute routes resulting in routing
loops. More precisely, the advertisement of MaxLinkMetric for the
router's non-local links will prevent OSPFv2 routers not supporting
the H-bit from attempting to use it for transit traffic.
6. OSPF AS-External-LSAs/NSSA LSAs with Type 2 Metrics
When calculating the path to an OSPF AS-External-LSA or NSSA-LSA 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. Consequently, OSPF routers implementing [RFC6987] or
this specification should advertise a Type-2 metric of LSInfinity for
any self-originated AS-External-LSAs or NSSA-LSAs in situations when
the OSPF router is acting as a stub router [RFC6987] or implementing
this specification.
7. IANA Considerations
IANA is requested to create the OSPF Router-LSA bit registry with the
following assignments:
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Value Description Reference
0x01 Area Border Router (B-bit) [RFC2328]
0x02 AS Boundary Router (E-bit) [RFC2328]
0x04 Virtual Link Endpoint (V-bit) [RFC2328]
0x08 Historic (W-bit) [RFC1584]
0x10 Unconditional NSSA Translator (Nt-bit) [RFC3101]
0x20 Unassigned
0x40 Unassigned
0x80 Host (H-bit) This Document
This document also defines a new Router Functional Capability
[RFC7770] known as the Host Router Support Functional Capability.
This document requests IANA to allocate the value of this capability
from the Router Functional Capability Bits TLV.
8. Security Considerations
This document introduces no new security considerations beyond those
already specified in [RFC6987], [RFC2328], and [RFC5340].
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.
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>.
[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>.
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[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-18 (work in
progress), April 2019.
[RFC1584] Moy, J., "Multicast Extensions to OSPF", RFC 1584,
DOI 10.17487/RFC1584, March 1994,
<https://www.rfc-editor.org/info/rfc1584>.
[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>.
Authors' Addresses
Keyur Patel
Arrcus
Email: keyur@arrcus.com
Padma Pillay-Esnault
Huawei Technologies
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: padma@huawei.com
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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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