Network Working Group P. Pillay-Esnault
Internet-Draft A. Lindem (Editor)
Expires: November 9, 2006 Cisco Systems, Inc
May 8, 2006
OSPFv3 Graceful Restart
draft-ietf-ospf-ospfv3-graceful-restart-04.txt
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Copyright (C) The Internet Society (2006).
Abstract
This memo describes the OSPFv3 graceful restart. For OSPFv3,
graceful restart is identical to OSPFv2 except for the differences
described in this memo. These differences include the format of the
grace Link State Advertisements (LSA) and other considerations.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Grace Link State Advertisement . . . . . . . . . . . . . . . . 4
2.1 Grace LSA - LS Type . . . . . . . . . . . . . . . . . . . 4
2.2 Grace LSA Format . . . . . . . . . . . . . . . . . . . . . 5
3. Additional Considerations for OSPFv3 Graceful Restart . . . . 7
3.1 Preservation of LSA ID to Prefix Correspondence . . . . . 7
3.2 Preservation of Interface IDs for Link-LSAs, Network
LSAs, and Router-LSAs . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Introduction
Graceful OSPF restart [GRACE] describes a mechanism to restart the
control plane of an OSPFv2 [OSPFv2] router which still has its
forwarding plane intact with a minimum of disruption to the network.
In general, the methods described in [GRACE] work for OSPFv3 [OSPFv3]
as well. However, OSPFv3 will use a different grace LSA to signal
that a router is (or is about) to attempt a graceful restart. This
document describes other OSPFv3 differences as well.
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].
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2. Grace Link State Advertisement
Grace-LSAs are originated by an OSPFv3 router that wishes to execute
a graceful restart of its OSPFv3 software. A grace-LSA requests that
the router's neighbors aid in its graceful restart by continuing to
advertise the router as fully adjacent during a specified grace
period. The grace-LSA contains the restarting router grace-period
and the reason code indicate the reason for the graceful restart.
In OSPFv3 (refer 2.11 of [OSPFv3]), neighboring routers on a given
link are always identified by router ID. This contrasts with the
IPv4 behavior where neighbors on point-to-point networks and virtual
links are identified by their Router IDs, and neighbors on broadcast,
NBMA and point-to-multipoint links are identified by their IPv4
interface addresses. Consequently, there is no requirement for the
router-address TLV used for OSPFv3 graceful restart [GRACE].
The grace-LSA body format will remain the same as described in
[GRACE].
2.1 Grace LSA - LS Type
A grace-LSA is defined as link-local scope LSA with the LS type equal
to 0x000b.
LSA function code LS Type Description
------------------------------------------
11 0x000b Grace LSA
The U-bit is set to 0 to since this is a link local scoped LSA and
the flooding scope is not impacted by whether or not the LSA is
known. The S2-bit and S1-bit are also set to 0 to indicate link-
local flooding scope.
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2.2 Grace LSA Format
The format of a grace LSA format is:
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 |0|0|0| 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
The Link State ID of a grace-LSA in OSPFv3 is the interface ID of the
interface the LSA is originated on.
The Length field defines the length of the value portion in octets
(thus a TLV with no value portion would have a length of zero). The
TLV is padded to four-octet alignment; padding is not included in the
length field (so a three octet value would have a length of three,
but the total size of the TLV would be eight octets). Nested TLVs
are also 32-bit aligned. For example, a one byte value would have
the length field set to 1, and three bytes of padding would be added
to the end of the value portion of the TLV. Unrecognized types are
ignored.
The format of each TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the TLVs within the body of a grace-LSA is the same as
the TLV format used by the Traffic Engineering Extensions to OSPF
[OSPF-TE]. The TLV header consists of a 16-bit Type field and a 16-
bit length field. The header is followed by zero or more bytes of
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value. The length field indicates the length of the value portion in
bytes. The value portion is padded to four-octet alignment, but the
padding is not included in the length field.
The following is the list of TLVs that can appear in the body of a
grace-LSA.
Grace Period (Type=1, length=4). The number of seconds that the
router's neighbors should continue to advertise the router as
fully adjacent, regardless of the state of database
synchronization between the router and its neighbors. This TLV
MUST always appear in a grace-LSA.
Graceful restart reason (Type=2, length=1). Encodes the reason
for the router restart, as one of the following: 0 (unknown), 1
(software restart), 2 (software reload/upgrade) or 3 (switch to
redundant control processor). This TLV MUST always appear in a
grace-LSA.
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3. Additional Considerations for OSPFv3 Graceful Restart
This section describes OSPFv3 unique considerations in addition to
those described in [GRACE].
3.1 Preservation of LSA ID to Prefix Correspondence
In OSPFv2 there is a direct correspondence between type 3 and type 5
LSA IDs and the prefixes being advertised. For OSPFv3, the LSA ID
for inter-area prefix LSAs and external LSAs is simply an unsigned 32
bit integer. To avoid network churn during graceful restart, the
restarting router SHOULD preserve the LSA ID to prefix correspondence
across graceful restarts.
3.2 Preservation of Interface IDs for Link-LSAs, Network LSAs, and
Router-LSAs
The OSPFv3 interface ID, as described in section 3.1.2 [OSPFv3], MUST
be preserved by the restarting router across restarts. It is used as
the LSA ID for link-LSAs and network-LSAs and is included in the link
descriptions in router-LSAs. Failure to preserve interface IDs would
result in a mismatch between the restarting router's pre-restart LSAs
and its neighbor adjacency state. This, in turn, would make
synchronizing an interface ID change between the restarting router
and its helping routers much more difficult (if not impossible) and
would most likely result in unreachability or premature graceful
restart termination. Placing the burden on the restarting router to
preserve interface IDs across restarts provides for a more robust,
more deterministic, and simpler mechanism.
Many implementations are using the interface's MIB-II IfIndex
([INTFMIB]) for Interface ID and are already preserving it across
restarts.
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4. Security Considerations
This document doesn't raise any new security concerns other than
those covered in [OSPFv3], [OSPFv3-AUTH], and [GRACE]. This is based
on the fact that [OSPFv3-AUTH] relies on manually key distribution
which precludes the use of replay protection utilizing sequence
numbers.
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5. IANA Considerations
A new LSA function code will be required for the OSPFv3 grace LSA.
Assignment of 0x000b has been suggested herein. Grace LSA TLVs and
sub-TLVs will share the same IANA registry as the TLVs and sub-TLVs
used by the OSPFv2 grace opaque LSA.
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6. Acknowledgments
Many thanks to Kireeti Kompella with whom much of this was discussed.
The authors also wish to thank Kunihiro Ishiguro and Vivek Dubey for
their comments.
The RFC text was produced using Marshall Rose's xml2rfc tool.
7. Normative References
[GRACE] Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful OSPF
Restart", RFC 3623, November 2003.
[OSPF-TE] Katz, D., Yeung, D., and K. Kompella, "Traffic Engineering
Extensions to OSPF", RFC 3630, September 2003.
[OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[OSPFv3] Moy, J., Ferguson, D., and R. Colton, "OSPF for IPv6",
RFC 2740, December 1999.
[OSPFv3-AUTH]
Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", draft-ietf-ospf-ospfv3-auth-08.txt (work in
progress).
[RFC2119] Bradner, S., "Key words for use in RFC's to Indicate
Requirement Levels", RFC 2119, March 1997.
Authors' Addresses
Padma Pillay-Esnault
Cisco Systems, Inc
3750 Cisco Way
San Jose, CA 95134
USA
Email: ppe@cisco.com
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Acee Lindem
Cisco Systems, Inc
7025 Kit Creek Road
Research Triangle Park, NC 27709
USA
Email: acee@cisco.com
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