OSPF R. Perlman
Internet Draft Sun microsystems
Intended status: Internet Draft February 18, 2008 A. Roy
Expires: August 2008 Cisco Systems
M. Thomas
University of Essex
Automatic Method for Minimization of Extraneous Pseudonodes in OSPF
draft-ospf-pseudo-perlman-roy-thomas-00.txt
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Abstract
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An automatic method is recommended for a link state protocol to
automatically decide whether an Ethernet link should be represented
by a pseudonode or not. Included is encoding recommendations for IS-
IS and recommendations for OSPF for implementing this feature.
Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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
Table of Contents
1. Introduction...................................................3
1.1. Method....................................................3
2. Generic changes................................................4
3. OSPF Considerations and process................................5
3.1. Extended Options TLV for OSPF V2 and V3...................5
3.2. Initial DR Election.......................................5
3.2.1. Changing link types to P2P/P2MP......................5
3.3. Overview of OSPF States...................................6
3.3.1. A router changes advertised support for Network-LSA
reduction...................................................6
3.3.2. Addition of a router that supports network-LSA reduction
............................................................6
3.3.3.......................................................6
4. Security Considerations........................................7
5. IANA Considerations............................................7
6. Acknowledgments................................................7
7. References.....................................................7
7.1. Normative References......................................7
7.2. Informative References....................................8
APPENDIX A: Packet Structures.....................................9
A.1. OSPF hello packet structure and flag locations............9
A.1.1. Extended Option LLS Data block TLV...................9
Author's Addresses...............................................10
Intellectual Property Statement..................................10
Disclaimer of Validity...........................................11
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1. Introduction
When Ethernets were originally introduced as a type of link, the
assumption was that an "Ethernet" was assumed to be a large cloud
with many directly connected nodes. Given that the overhead of a link
state protocol (such as IS-IS or OSPF) is proportional to the number
of links in the topology, a cloud with a lot of neighbors,
represented in the most straightforward way, as having all nodes on
the cloud being fully connected, would represent n^2 links. Thus to
make large shared links scalable, the concept of a "pseudonode"
(Network-LSA) was introduced. If there are n neighbors on an
Ethernet, rather than representing the connectivity between the
neighbors as n^2 links, one router on the Ethernet is elected
Designated Router, and that router impersonates the cloud itself,
issuing link state information on behalf of the cloud, and each
router on the cloud (in addition to the Designated Router) claims
only the pseudonode as a neighbor. The "pseudonode" is the cloud
itself. In this way the pseudonode will have n router neighbors, but
each router will only report a single neighbor (the pseudonode).
However, Ethernet has evolved into "switched Ethernet", where instead
of Ethernet being a large cloud, it is often as small as a single pt-
to-pt link connecting exactly two neighbors. An Ethernet might even
be a pt-to-pt link between a router and an endnode. It would be
wasteful to represent such a link as a pseudonode.
This paper presents a method that allows a Designated Router running
OSPF to decide when it is appropriate to represent an Ethernet as a
pseudonode, and generate a Network-LSA and when it is appropriate
instead to represent the topology as direct connections between all
the nodes on the cloud. This is automatic and non disruptive to the
rest of the network if routers on the cloud fail and reappear.
1.1. Method
To implement this technique, each link state router on a single
broadcast segment would set a bit indicating it supports the
technique as described in this document
If every router on the link claims the ability to represent the
broadcast link without a pseudonode, then the Designated Router
specifies to the routers on the link whether the link will be
represented by a pseudonode or not. The actual algorithm chosen by
the DR need not be the same for all routers. A router MAY be
configured with a different algorithm, or experience may show that a
different algorithm might be more optimal. Since the DR decides on
behalf of the link, whether it will be represented with a pseudonode,
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or as fully connected direct links between each pair of routers,
there is no need for all routers to have the same algorithm. The only
requirement is that a router follow the mandate of the DR.
The algorithm in this document is that the DR mandates use of a
pseudonode if there are at least 4 routers on the link (including the
DR), and mandates no pseudonode if there are only 2 routers on the
link (including the DR). If there are 3 routers on the link
(including the DR), the DR does not change the state of the link. So
if there are 4 routers on the link, the link will be represented by a
pseudonode. If one of those routers go down, the link will remain
represented by a pseudonode. Only when two of the four routers go
down, so that the DR has only one neighbor, will the DR revert to "no
pseudonode". When the DR has specified that the link is not to be
represented by a pseuonode, the link will remain as "no pseudonode"
until there are again 4 or more routers on the link.
2. Generic changes
1. The Hello message on a broadcast link must contain a flag
specifying the ability of the transmitting router to do pseudonode
minimization.
2. The Hello message on a broadcast link must contain a flag set by
the DR to indicate that the link is not to be represented by a
pseudonode. If the link is not to be represented by a pseudonode,
then there is no need to give a name to the pseudonode.
3. The DR must check the "pseudonode minimization capable" of all
routers on the link. If any router does not advertise this
ability, the DR MUST represent the link with a pseudonode.
4. If the link is not to be represented by a pseudonode, then all
routers on the link will report direct pt-to-pt / p2mp links about
each other router on the link. The DR remains in place and
flooding continues on the network itself as normal.
5. If all routers on the link are "pseudonode minimization capable",
then the DR specifies that the link will not use a pseudonode if
the number of routers on the link is 2 or fewer (i.e. the DR has
at most one router neighbor).
6. If there are at least 4 routers on the link (i.e., the DR has at
least 3 router neighbors), then the DR specifies use of a
pseudonode for the link.
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7. If there are 3 routers on the link, then the DR does not change
the state of the link.
8. If a non compatible router appears on the link then the DR signals
to the other routers to no longer perform pseudonode minimization
3. OSPF Considerations and process
3.1. Extended Options TLV for OSPF V2 and V3
In order to indicate compliance with this draft, a router needs to
also be compliant with link local signaling [zinin2007]. This draft
provides for TLV extensions incorporated into an LLS data block at
the end of the Hello packet. [zinin2007] recommends the registration
of a particular TLV called the EO-TLV (Extended Options TLV). See
Appendix A.1.1 It is our recommendation in this draft that we
register bit 3 (NS-Bit) and bit 4 (SS-Bit), for use within this
Extended options field.
3.2. Initial DR Election
It is envisioned that this will stay the same except for the
following setting of two newly allocated flags in the EO-TLV field.
1. The NS-Bit; Network-LSA suppression capability supported bit, is
set in the Extended Options field (EO-TLV) using Link Local
signaling [Zinin2007]. This EO-TLV field is added to the
Broadcast Hello. Setting of the NS-Bit implies compliance with
this pseudonode reduction draft
2. The SS-Bit; Network-LSA suppression signal, is set in the EO-TLV
field to allow the DR to indicate to the routers on the Broadcast
LAN that network-LSA reduction is required.
Hellos are sent as usual with DR election taking place. If all of the
routers on the LAN signify via NS-Bit that they are capable of this
feature, AND if the DR decides to implement network-LSA suppression,
then the DR signals this to the routers on the Ethernet via the
setting of the SS-Bit. If the other routers all respond with the SS-
Bit set, then the router LSA migration starts.
3.2.1. Changing link types to P2P/P2MP
The other routers must follow the DR and reset the network type
advertised in their router LSAs to P2P for each link (as when using
P2MP).
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3.3. Overview of OSPF States
State1: OSPF has elected a DR. The DR noted that the NS-Bit was set
on all neighbors indicating compliance with this draft. A network LSA
has NOT yet been generated or advertised by the Designated Router.
State2: If required, the DR signals network-LSA suppression by
setting the SS-Bit. If ALL of the routers respond to the DR by
setting the SS-Bit, then state 3.
State3:
o All routers attached to the network advertise the network type in
their router LSAs as multiple P2P connections(as in P2MP) and not
broadcast. (This ensures that remote routers are not expecting a
network-LSA. This allows the feature to be compatible with non
network-LSA reduction capable routers.)
o The DR suppresses advertising a network-LSA for the network.
o Despite remotely representing the Ethernet as a series of P2P
connections, the network itself still operates as a broadcast
Ethernet. The DR and BDR continue to control the flooding process
and the OSPF hellos on the network remain unchanged.
3.3.1. A router changes advertised support for Network-LSA reduction
Any changes to the advertised support for Network-LSA reduction /EO-
TLV fields should be handled in the same way as changes to the
options field [OSPFV2] section 10.2
3.3.2. Addition of a router that supports network-LSA reduction
If a further router is added to the network and indicates compliance
with network-LSA reduction by the setting of the NS-Bit in the hello
packets, then the DR responds with the SS-Bit set as expected. If the
new neighbor responds with the SS-Bit also set then operations
continue as in state 3. No timer is required, with the router
advertising the multiple P2P connections in the network type in the
router LSA.
3.3.3.
If a new router is added to a network that is implementing network-
LSA reduction and does NOT support this feature then:
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The DR signals to the other routers on the network the resetting of
the SS-Bit flag. A 30 second LSA-reduction timer starts. Routers
signify complicity with the change back to Broadcast type by
resetting their SS-Bit in accordance with the DR. The current graph
is retained for routing calculations while a new graph is built.
If after expiration of the LSA-reduction timer all routers are still
advertising reset SS-Bit in agreement, then the migration back is
considered as stable, and the network type is advertised
simultaneously in the router LSAs as a broadcast OSPF type.
If a router has failed to reset the SS-Bit flag within the LSA-
reduction timer, then neighbor state with the router is reset as per
Changes in advertised options Section 10.2 [OSPFV2]
4. Security Considerations
A group of 3 routers could maliciously come up and down as a group,
or a single router could pretend to be 3 routers, and cause the
pseudonode state of the link to oscillate. Also, a malicious DR can
oscillate the state of the link. A single router could oscillate the
state of the link by advertising first that it is capable of
pseudonode minimization, and then advertising that it is not capable.
However, any disruption to routing by a router using any of these
strategies can already be done trivially by, for instance, pretending
to be highest priority and taking over as DR, and then changing
identity and the pseudonode name. So the capability in this document
does not make it any easier for a malicious router on a link to cause
disruption.
5. IANA Considerations
The Assignment of bits 3 and 4 of the link local signaling EO-TLV.
6. Acknowledgments
We thank Mike Shand for doing the math to show that the link state
database in IS-IS becomes smaller with use of a pseudonode on a link
with at least 4 routers.
7. References
7.1. Normative References
[zinin2007] Zinin,A., Roy, A,. Nuguyen, L,. Friedman, B,. and Yeung,
D,. "OSPF Link-local signaling draft-ietf-ospf0lls-03.txt",
IETF working Group document August 2007
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[OSPFV2] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[OSPFV3] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6", RFC
2740, December 1999.
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2334, October
1998.
[KEY] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
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APPENDIX A: Packet Structures
A.1. OSPF hello packet structure and flag locations
A.1.1. Extended Option LLS Data block TLV
LLS Data Block in OSPFv2 and OSPFv3 as described in [zinin2007]
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+--+--+
| Extended Options SS NS RS LR|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+--+--+
Figure X: Format of Extended Options in EO TLV
NS-Bit: Network-LSA suppression capability supported
SS-Bit: Network-LSA suppression signal. If this bit is set, the DR
is requesting all of it's neighbors to use Network-LSA
suppression mode on the link
Figure 2: Format of EO-TLV showing proposed Flags
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Author's Addresses
Radia Perlman
Sun Microsystems
Email: radia.perlman@sun.com
Abhay Roy
Cisco Systems
E-mail: akr@cisco.com
Matthew Thomas
University of Essex
Email: mrthom@essex.ac.uk
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