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 Out-of-band LSDB resynchronization
draft-nguyen-ospf-oob-resync-06.txt
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Copyright (C) The Internet Society (2006).
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Abstract
OSPF is a link-state intra-domain routing protocol used in IP
networks. LSDB synchronization in OSPF is achieved via two methods--
initial LSDB synchronization when an OSPF router has just been
connected to the network and asynchronous flooding that ensures
continuous LSDB synchronization in the presence of topology changes
after the initial procedure was completed. It may sometime be
necessary for OSPF routers to resynchronize their LSDBs. OSPF
standard, however, does not allow routers to do so without actually
changing the topology view of the network.
This memo describes a vendor specific mechanism to perform such form
of out-of-band LSDB synchronization. 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. The LR bit . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. OSPF Neighbor Data Structure . . . . . . . . . . . . . . . 5
2.3. Hello Packets . . . . . . . . . . . . . . . . . . . . . . 6
2.4. DBD Packets . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Neighbor State Treatment . . . . . . . . . . . . . . . . . 8
2.6. Initiating OOB LSDB Resynchronization . . . . . . . . . . 9
3. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
According to the OSPF standard [RFC2328], after two OSPF routers have
established an adjacency (the neighbor FSMs have reached Full state),
routers announce the adjacency states in their router-LSAs.
Asynchronous flooding algorithm ensures routers' LSDBs stay in sync
in the presence of topology changes. However, if routers need (for
some reason) to resynchronize their LSDBs, they cannot do that
without actually putting the neighbor FSMs into the ExStart state.
This effectively causes the adjacencies to be removed from the
router-LSAs, which may not be acceptable if the desire is to prevent
routing table flaps during database resynchronization. In this
document, we provide the means for so-called out-of-band (OOB) LSDB
resynchronization.
The described mechanism can be used in a number of situations
including those where the routers are picking the adjacencies up
after a reload. The process of adjacency preemption is outside the
scope of this document. Only the details related to LSDB
resynchronization are mentioned herein.
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 Out-Of-Band Resynchronization Solution, the format of the
OSPF Database Description packet is changed to include a new R-bit
indicating OOB LSDB resynchronization. All DBD packets sent during
the OOB resynchronization procedure are sent with the R-bit set.
Also, two new fields are added to the neighbor data structure. The
first field indicates neighbor's OOB resynchronization capability.
The second indicates that OOB LSDB resynchronization is in process.
The latter field allows OSPF implementations to utilize the existing
neighbor FSM code.
A bit is occupied in the Extended Options TLV (see [LLS]). Routers
set this bit to indicate their capability to support the described
technique.
2.1. The LR bit
A new bit, called LR (LR stands for LSDB Resynchronization) is
introduced to the LLS Extended Options TLV (see [LLS]). The value of
the bit is 0x00000001; see Figure 1. See the "IANA Considerations"
section of [LLS] for more information on the Extended Options bit
definitions. Routers set LR bit to announce OOB LSDB
resynchronization capability.
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
| * | * | * | * | * | * | * |...| * | * | * | * | * | * | * | LR|
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
Figure 1. The Options field
Routers supporting the OOB LSDB resynchronization technique set the
LR bit in the EO-TLV in the LLS block attached to both Hello and DBD
packets. Note that no bit is set in the standard OSPF Options field,
neither in OSPF packets, nor in LSAs.
2.2. OSPF Neighbor Data Structure
A field is introduced into OSPF neighbor data structure, as described
below. The name of the field is OOBResync and it is a flag
indicating that the router is currently performing OOB LSDB
resynchronization with the neighbor.
OOBResync flag is set when the router is initiating the OOB LSDB
resynchronization (see Section 2.6 for more details).
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Routers clear OOBResync flag on the following conditions:
o The neighbor data structure is first created
o The neighbor FSM transitions to any state lower than ExStart
o The neighbor FSM transitions to ExStart state because a DBD packet
with R-bit clear has been received
o The neighbor FSM reaches state Full.
Note that OOBResync flag may have TRUE value only if the neighbor FSM
is in states ExStart, Exchange, or Loading. As indicated above, if
the FSM transitions to any other state, the OOBResync flag should be
cleared.
It is important to mention that operation of OSPF neighbor FSM is not
changed by this document. However, depending on the state of the
OOBResync flag, the router sends either normal DBD packets or DBD
packets with the R-bit set.
2.3. Hello Packets
Routers capable of performing OOB LSDB resynchronization should
always set the LR bit in their Hello packets.
2.4. DBD Packets
Routers supporting the described technique should always set the LR
bit in the DBD packets. Since the Options field of the initial DBD
packet is stored in corresponding neighbor data structure, the LR bit
may be used later to check if a neighbor is capable of performing OOB
LSDB resynchronization.
The format of type-2 (DBD) OSPF packets is changed to include a flag
indicating OOB LSDB resynchronization procedure. Figure 2
illustrates the new packet format.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 2 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface MTU | Options |0|0|0|0|R|I|M|MS
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DD sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| |
+- An LSA Header -+
| |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Figure 2. Modified DBD packet
The R-bit in OSPF type-2 packets is set when the OOBResync flag for
the specific neighbor is set to TRUE. If a DBD packets with R-bit
clear is received for a neighbor with active OOBResync flag, the OOB
LSDB resynchronization process is cancelled and normal LSDB
synchronization procedure is initiated.
When a DBD packet is received with R-bit set and the sender is known
to be OOB-incapable, the packet should be dropped and a SeqNumber-
Mismatch event should be generated for the neighbor.
Processing of DBD packets is modified as follows.
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1. If the OOBResync flag for the neighbor is set (the LSDB
resynchronization process has been started) and received DBD
packet does not have the R bit set, ignore the packet and gen-
erate a SeqNumberMismatch event for the neighbor FSM.
2. Otherwise, if OOBResync flag for the neighbor is clear and
received DBD packet has the R bit set, perform the following
steps:
* If the neighbor FSM is in state Full and bits I, M, and MS are
set in the DBD packet, set the OOBResync flag for the
neighbor, put the FSM in ExStart state and continue process-
ing the DBD packet as described in [RFC2328].
* Otherwise, ignore received DBD packet (no OOB DBD packets are
allowed with OOBResync flag clear and FSM in state other than
Full.) Also, if the state of the FSM is Exchange or higher,
generate a SeqNumberMismatch event for the neighbor FSM.
3. Otherwise, process the DBD packet as described in [RFC2328].
During normal processing of the initial OOB DBD packet (with bits R,
I, M, and MS set), if the receiving router is selected to be the Mas-
ter, it may speed up the resynchronization process by immediately
replying to the received packet.
It is also necessary to limit the time an adjacency can spend in
ExStart, Exchange, and Loading states with OOBResync flag set to a
finite period of time (e.g., by limiting the number of times DBD and
link state request packets can be retransmitted). If the adjacency
does not proceed to Full state before the timeout, it is indicative
that the neighboring router cannot resynchronize its LSDB with the
local router. The requesting router may decide to stop trying to
resynchronize the LSDB over this adjacency (if, for example, it can
be resynchronized via another neighbor on the same segment) or to
resynchronize using the legacy method by clearing the OOBResync flag
and leaving the FSM in ExStart state. The neighboring router may
decide to cancel the OOB procedure for the neighbor.
2.5. Neighbor State Treatment
OSPF implementation supporting the described technique should modify
the logic consulting the state of a neighbor FSM as described below.
o FSM state transitioning from and to the Full state with OOBResync
flag set should not cause origination of a new version of router-
LSA or network-LSA.
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o Any explicit checks for the Full state of a neighbor FSM for the
purposes other than LSDB synchronization and flooding should treat
states ExStart, Exchange, and Loading as state Full, pro- vided
that OOBResync flag is set for the neighbor. (Flooding and
MaxAge-LSA-specific procedures should not check the state of
OOBResync flag, but should continue consulting only the FSM
state.)
2.6. Initiating OOB LSDB Resynchronization
To initiate out-of-band LSDB resynchronization, the router must first
make sure that the corresponding neighbor supports this technology
(by checking the LR bit in Options field of the neighbor data struc-
ture). If the neighboring router is capable, the OOBResync flag for
the neighbor should be set to TRUE and the FSM state should be forced
to ExStart.
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3. Backward Compatibility
Because OOB-capable routers explicitly indicate their capability by
setting the corresponding bit in the Options field, no DBD packets
with R-bit set are sent to OOB-incapable routers.
The LR bit itself is transparent for OSPF implementations and does
not affect communication between routers.
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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.
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Appendix A. Acknowledgments
The authors would like to thank Acee Lindem, 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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