OSPF Working Group R. Ogier
Internet-Draft December 16, 2007
Category: Informational
Expires: June 16, 2008
OSPF Database Exchange Summary List Optimization
draft-ogier-ospf-dbex-opt-03.txt
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Copyright (C) The IETF Trust (2007).
Abstract
This document describes a backward compatible optimization for the
Database Exchange process in OSPFv2 and OSPFv3. In this
optimization, a router does not list an LSA in Database Description
packets sent to a neighbor, if the same or a more recent instance of
the LSA was listed in a Database Description packet already received
from the neighbor. This optimization reduces Database Description
overhead by about 50% in large networks. This optimization does not
affect synchronization, since it only omits unnecessary information
from Database Description packets.
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1. Introduction
In OSPFv2 [RFC2328] and OSPFv3 [RFC2740], when two neighboring
routers become adjacent, they synchronize their link-state databases
via the Database Exchange process. Each router sends the other
router a set of Database Description (DD) packets that describes the
router's link-state database. This is done by listing each LSA
(i.e., including the header of each LSA) in one of the sent DD
packets. This procedure allows each router to determine whether the
other router has newer LSA instances that should be requested via
Link State Request packets.
The optimization simply observes that it is not necessary for a
router (master or slave) to list an LSA in a DD packet if it knows
the neighbor already has an instance of the LSA that is the same or
more recent (and therefore will not request the LSA). To avoid
listing such LSAs in DD packets, when an LSA is listed in a DD packet
received from the neighbor, and the Database summary list for the
neighbor has an instance of the LSA that is the same as or less
recent than the one received, the LSA is removed from the summary
list.
The optimization, called the Database Exchange summary list
optimization, does not affect synchronization, since the LSAs that
are omitted from DD packets are unnecessary. The optimization is
fully backward compatible with OSPF. The optimization reduces
Database Description overhead by about 50% in large networks in which
routers are usually already nearly synchronized when they become
adjacent, since it reduces the total number of LSA headers exchanged
by about one-half in such networks. The optimization is especially
beneficial in large networks with limited bandwidth, such as large
mobile ad hoc networks.
2. Specification of Optimization
The Database Exchange summary list optimization is defined by
modifying Section 10.6 (Receiving Database Description Packets) of
RFC 2328 as follows. The second-to-last paragraph of Section 10.6 is
replaced with the following augmented paragraph:
When the router accepts a received Database Description Packet as the
next in sequence, the packet contents are processed as follows. For
each LSA listed, the LSA's LS type is checked for validity. If the
LS type is unknown (e.g., not one of the LS types 1-5 defined by this
specification), or if this is an AS- external-LSA (LS type = 5) and
the neighbor is associated with a stub area, generate the neighbor
event SeqNumberMismatch and stop processing the packet. Otherwise,
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the router looks up the LSA in its database to see whether it also
has an instance of the LSA. If it does not, or if the database copy
is less recent, the LSA is put on the Link state request list so that
it can be requested (immediately or at some later time) in Link State
Request Packets. In addition, if the Database summary list contains
an instance of the LSA that is the same as or less recent than the
listed LSA, the LSA is removed from the Database summary list.
The above additional step (which updates the Database summary list)
may be performed either before or after the router looks up the
listed LSA in its database and possibly adds the LSA to the Link
state request list. However, to implement the optimization, the
additional step must be performed for each LSA listed in the received
DD packet (to fully update the Database summary list) before the next
DD packet is sent in response.
Although the optimization does not require that LSAs be listed in DD
packets in any particular order, faster lookup of LSAs in the
Database summary list may be possible if LSAs are listed in the same
order by all routers. If such an ordering is used, then to encourage
different implementations to use the same ordering, this document
recommends that LSAs be listed in lexicographically increasing order
of (LS type, Link State ID, Advertising Router) for OSPFv2 and (LS
type, Advertising Router, Link State ID) for OSPFv3.
3. Example
This section describes an example to illustrate the database exchange
summary list optimization. Assume that routers RT1 and RT2 already
have identical databases when they start database exchange. Also
assume that the list of LSA headers for the database fits into two DD
packets. Then the standard database exchange is as follows when RT1
is the first to change the neighbor state to ExStart. Note that each
router sends two full DD packets.
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RT1 (slave) RT2 (master)
ExStart Empty DD (Seq=x,I,M,Master)
------------------------------>
Empty DD (Seq=y,I,M,Master) ExStart
<------------------------------
Exchange Full DD (Seq=y,M,Slave)
------------------------------>
Full DD (Seq=y+1,M,Master) Exchange
<------------------------------
Full DD (Seq=y+1,Slave)
------------------------------>
Full DD (Seq=y+2, Master)
<------------------------------
Full Empty DD (Seq=y+2, Slave)
------------------------------>
Full
If the optimization is used, when RT2 receives the first full DD
packet from RT1, it removes from its summary list all LSAs that are
listed in the DD packet. Then RT2 sends a DD packet that lists the
remaining LSAs (since all of the LSA headers fit into two DD
packets). When RT1 receives this DD packet, it removes these
remaining LSAs from its summary list (causing it to be empty) and
sends an empty DD packet to RT2.
With the optimization, each router sends only one full DD packet
instead of two, as shown below.
RT1 (slave) RT2 (master)
ExStart Empty DD (Seq=x,I,M,Master)
------------------------------>
Empty DD (Seq=y,I,M,Master) ExStart
<------------------------------
Exchange Full DD (Seq=y,M,Slave)
------------------------------>
Full DD (Seq=y+1,Master) Exchange
<------------------------------
Full Empty DD (Seq=y+1, Slave)
------------------------------>
Full
4. Security Considerations
This document does not raise any new security concerns.
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5. IANA Considerations
This document specifies a simple backward compatible optimization for
OSPFv2 and OSPFv3 that does not require any new number assignment.
6. Normative References
[RFC2328] J. Moy. "OSPF Version 2", RFC 2328, April 1998.
[RFC2740] R. Coltun, D. Ferguson, and J. Moy. "OSPF for IPv6", RFC
2740, December 1999.
Appendix A. Acknowledgments
The recommendation to list LSAs in lexicographical order was
suggested by Acee Lindem.
Author's Address
Richard G. Ogier
Email: rich.ogier@earthlink.net
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