Host Router Support for OSPFv2
draft-ietf-ospf-ospfv2-hbit-09
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (lsr WG) | |
|---|---|---|---|
| Authors | Keyur Patel , Padma Pillay-Esnault , Manish Bhardwaj , Serpil Bayraktar | ||
| Last updated | 2019-09-20 (Latest revision 2019-09-13) | ||
| Replaces | draft-keyupate-ospf-ospfv2-hbit | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
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| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Yingzhen Qu | ||
| Shepherd write-up | Show Last changed 2018-12-05 | ||
| IESG | IESG state | AD Evaluation::Revised I-D Needed | |
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| Responsible AD | Alvaro Retana | ||
| Send notices to | Yingzhen Qu <yingzhen.ietf@gmail.com>, aretana.ietf@gmail.com |
draft-ietf-ospf-ospfv2-hbit-09
OSPF K. Patel
Internet-Draft Arrcus
Updates: 6987 (if approved) P. Pillay-Esnault
Intended status: Standards Track PPE Consulting
Expires: March 16, 2020 M. Bhardwaj
S. Bayraktar
Cisco Systems
September 13, 2019
Host Router Support for OSPFv2
draft-ietf-ospf-ospfv2-hbit-09
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 assigns a new bit (Host-bit) in the
OSPF Router-LSA bit registry and in the OSPF Router Informational
Capability Bits Registry that enables a host router to advertise that
it is a non-transit router. It also describes the changes needed to
support the Host-bit in the domain. In addition, this document
updates OSPF Stub Router Advertisement (RFC6987) to advertise for
type-2 External and NSSA LSAs with a high cost in order to repel
traffic effectively.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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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 March 16, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://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 . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . 8
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 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.
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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 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 router for transit
traffic in OSPFv2 routing domains. This document defines the Host-
bit in the OSPFv2 Router Properties Registry and 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 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 host-bit is NOT set routers MUST act transit routers as
described in [RFC2328] ensuring backward compatibility.
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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
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 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 use
the host router only for its local destinations.
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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 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 behind 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 of other
routing protocols. Therefore, non-local IPv4 prefixes, e.g., those
exported from other routing protocols, MUST NOT be advertised in AS-
external-LSAs for routers acting permanently as a host. However, in
use cases such as an overloaded router or a router being gracefully
isolated, these routers are only temporarily acting as host routers
and therefore SHOULD continue to advertise their External LSAs but
ensure that they do not attract traffic. In addition to the
procedure described above, temporary host routers advertising type
2-metric External LSAs MUST set the metrics 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 avoid the possibility of any routing loops due to partial
deployment, this document defines a OSPF Router Information (RI) LSA
[RFC7770] with and an area flooding scope and a new bit assigned in
the OSPF Router Informational Capability Bits Registry. 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 MUST only take effect if all the routers in a given OSPF
area support this functionality.
In normal operations, there is no guarantee that the RI LSA will
reach all routers in an area in a timely manner that may result in
rooting loops in partial deployments. For example, in a new router
joins an area which previous 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
functionality in partial deployments where the topology guarantees
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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-local links with a metric equal to MaxLinkMetric 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 have the H-Bit support then
all routers in the areas MUST run the normal SPF.
o Any router not supporting the H-bit capability is detected (by
examination of RI- LSA or RTR LSA in the area database) then all
routers in the area MUST revert back to normal operations.
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 with Type-2 metrics.
Consequently, OSPF routers implementing [RFC6987] and required to be
the last resort transit then they MUST advertise a Type-2 metric of
LSInfinity-1 for any self-originated type 2 AS-External-LSAs or NSSA-
LSAs. However, in situations, the router needs to repel traffic and
acts as a host router then, in addition of the host bit procedure
described in this document they MUST advertise a Type-2 metric of
LSInfinity for any self-originated type 2 AS-External-LSAs or NSSA-
LSAs.
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
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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. [RFC7770]
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 except for its local
destinations. This is a subset of the operations of a normal router
and therefore should not introduce new security considerations beyond
those already known in OSPF. The feature, however does introduce the
flooding of a capability information that allows discovery and
verification that all routers in an area are capable before turning
on the feature. In the event that a rogue or buggy router advertises
incorrectly its capability there are two possible cases:
o The router does not have the capability but sends 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 to 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.
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
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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.
Authors' Addresses
Keyur Patel
Arrcus
Email: keyur@arrcus.com
Padma Pillay-Esnault
PPE Consulting
Email: padma.ietf@gmail.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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