Network Working Group L. Nguyen
Internet-Draft A. Roy
Intended status: Informational Cisco Systems
Expires: April 29, 2007 A. Zinin
Alcatel
October 26, 2006
OSPF Restart Signaling
draft-nguyen-ospf-restart-06.txt
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Copyright (C) The Internet Society (2006).
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Abstract
Abstract OSPF is a link-state intra-domain routing protocol used in
IP networks. Routers find new and detect unreachable neighbors via
Hello subprotocol. Hello OSPF packets are also used to ensure two-
way connectivity within time. When a router restarts its OSPF
software, it may not know its neighbors. If such a router sends a
hello packet on an interface, its neighbors are going to reset the
adjacency, which may not be desirable in certain conditions.
This memo describes a vendor specific mechanism that allows OSPF
routers to inform their neighbors about the restart process. Note
that this mechanism requires support from neighboring routers. The
mechanism described in this document was proposed before Graceful
OSPF Restart [RFC3623] came into existence. It is implemented/
supported by at least one major vendor and is currently deployed in
the field. The purpose of this document is to capture the details of
this mechanism for public use. This mechanism is not an IETF
standard.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 4
2. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Sending Hello Packets with the RS-bit set . . . . . . . . 5
2.2. Receiving Hello Packets with RS-bit set . . . . . . . . . 5
2.3. Insuring topology stability . . . . . . . . . . . . . . . 6
3. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
While performing a graceful restart of OSPF software [OSPF], routers
need to prevent their neighbors from resetting their adjacencies.
However, after a reload, routers may not be aware of the neighbors
they had adjacencies with in their previous incarnations. If such a
router sends a Hello packet on an interface and this packet does not
list some neighbors, those neighbors will reset the adjacency with
restarting router.
This document describes a technique that allows restarting routers to
inform their neighbors that they may not know about some neighbors
yet and the absence of some router-IDs in the Hello packets should be
ignored.
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 RFC2119 [RFC2119].
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2. Proposed Solution
With this Restart Signaling Solution, A new bit, called RS (restart
signal), is introduced into Extended Options TLV in the LLS block
(see [LLS]). The value of this bit is 0x00000002; see Figure 1
below.
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
| * | * | * | * | * | * | * |...| * | * | * | * | * | * | RS| LR|
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
Figure 1. Bits in Extended Options TLV
For a definition of the LR bit, see [OOB].
2.1. Sending Hello Packets with the RS-bit set
OSPF routers should set the RS-bit in the EO-TLV attached to a Hello
packet when it is not known that all neighbors are listed in this
packet, but the restarting router wants them to preserve their
adjacencies. The RS-bit must not be set in Hello packets longer than
RouterDeadInterval seconds.
2.2. Receiving Hello Packets with RS-bit set
When an OSPF router receives a Hello packet, containing the LLS block
with the EO-TLV which has the RS-bit set, the router should skip the
two-way connectivity check with the announcing neighbor (i.e., the
router should not generate a 1-WayReceived event for the neighbor if
it does not find its own router ID in the list of neighbors as
described in 10.5 of [RFC2328]), provided that the neighbor FSM for
this neighbor is in the Full state.
The router should also send a unicast Hello back to the sender in
reply to a Hello packet with RS bit set. This is to speed up
learning of previously known neighbors. When sending such a reply
packet, care must be taken to ensure that the RS bit is clear in it.
Two additional fields are introduced in the neighbor data structure:
RestartState flag and ResyncTimeout timer. RestartState flag
indicates that a Hello packet with RS-bit set has been received and
the local router expects its neighbor to go through the LSDB
resynchronization procedure using [OOB]. ResyncTimeout is a single-
shot timer limiting the delay between the first seen Hello packet
with RS-bit set and initialization of the LSDB resynchronization
procedure. The length of ResyncTimeout timer is RouterDeadInterval
seconds.
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When a Hello packet with RS-bit set is received and RestartState flag
is not set for the neighbor, the router sets RestartState flag and
starts ResyncTimeout timer. If ResyncTimeout expires, RestartState
flag is cleared and a 1-WayReceived event is generated for the
neighbor. If, while ResyncTimeout timer is running, the neighbor
starts LSDB resynchronization procedure using [OOB], ResyncTimeout
timer is cancelled. The router also clears RestartState flag on
completion of the LSDB resynchronization process.
Two or more routers on the same segment cannot have Hello packets
with the RS-bit set at the same time, as can be the case when two or
more routers restart at about the same time. In such scenario, the
routers should clear the RestartState flag, cancel the ResyncTimeout
timer, and generate a 1-WayReceived event.
2.3. Insuring topology stability
Under certain circumstances it might be desirable to stop announcing
the restarting router as fully adjacent if this may lead to possible
routing loops. In order to provide this functionality, a
configurable option is provided on the neighboring routers that
instructs the OSPF process to follow the logics described below.
When an OSPF router schedules a routing table calculation due to a
change in the contents of its LSDB, it should also reset all
adjacencies with restarting routers (those with RestartState set to
TRUE) by clearing the RestartState neighbor flags, canceling
ResyncTimeout timers (if running), and generating the 1-WayReceived
events for the neighbor FSMs.
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3. Backward Compatibility
The described technique requires cooperation from neighboring
routers. However, if neighbors do not support this technique, they
will just reset the adjacency.
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4. Security Considerations
The described technique does not introduce any new security issues
into OSPF protocol.
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5. IANA Considerations
Please refer to the "IANA Considerations" section of [LLS] for more
information on the Extended Options bit definitions.
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFC's to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[RFC3623] Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful OSPF
Restart", RFC 3623, November 2003.
6.2. Informative References
[LLS] Friedman, B., Nguyen, L., Roy, A., Yeung, D., and A. Zinin,
"OSPF Link-local Signaling", Work in progress , October 2006.
[OOB] Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-band LSDB
resynchronization", Work in progress , October 2006.
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Appendix A. Acknowledgments
The authors would like to thank John Moy, Russ White, Don Slice, and
Alvaro Retana for their valuable comments.
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Authors' Addresses
Liem Nguyen
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
Email: lhnguyen@cisco.com
Abhay Roy
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
Email: akr@cisco.com
Alex Zinin
Alcatel
Sunnyvale, CA
USA
Email: zinin@psg.com
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