wdiff draft-ietf-ospf-demand-00.txt draft-ietf-ospf-demand-01.txt

Network Working Group J. Moy
Internet Draft Proteon, Inc. Cascade
Expiration Date: November 1994 May 1995 November 1994
File name: draft-ietf-ospf-demand-00.txt draft-ietf-ospf-demand-01.txt

Extending OSPF to support demand circuits

Status of this Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts
months and may be updated, replaced, or obsoleted by other documents
at any time. It is not appropriate inappropriate to use
Internet-Drafts Internet- Drafts as
reference material or to cite them other than as
a "working draft" or "work in progress".

To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt
"1id-abstracts.txt" listing contained in the Internet-Drafts Internet- Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, ds.internic.net (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or
munnari.oz.au. munnari.oz.au (Pacific
Rim).

Abstract

This memo defines enhancements to the OSPF protocol that allow
efficient operation over "demand circuits". Demand circuits are
those
network segments whose costs vary with usage; charges can be based
both on connect time and on bytes/packets transmitted. Examples of such
demand circuits include ISDN circuits, X.25 SVCs, and dial-up lines.
The periodic nature of OSPF routing traffic (as defined in [1]) requires
such circuits has until now required a
demand circuit's underlying data-link connection to be open constantly, constantly
open, resulting in unwanted usage charges. With the modifications
described herein, OSPF Hellos and the refresh of OSPF routing
information are suppressed, allowing suppressed on demand circuits circuits, allowing the
underlying data-link connections to be closed when not carrying
application traffic.

Demand circuits and regular network segments (e.g., leased lines)
are allowed to be combined in any manner. In other words, there are
no topological restrictions on the demand circuit support. However,
while any OSPF network segment can be defined as a demand circuit,
only point-to-point networks receive the full benefit. When
broadcast and NBMA networks are declared demand circuits, routing
update traffic is reduced but the periodic sending of Hellos is not,
which in effect still requires that the data-link circuits connections be
constantly open.

While mainly intended for use with cost-conscious network links such
as ISDN, X.25 and dial-up, the modifications in this memo may also
prove useful over bandwidth-limited network links such as slow-speed
leased lines and packet radio.

The enhancements defined in this memo are backward-compatible with
the OSPF specification defined in [1], and with the OSPF extensions
defined in [3] (OSPF NSSA areas), [4] (MOSPF) and [8] (OSPF Point-
to-MultiPoint Interface).

This memo provides functionality comparable to that specified for
RIP in [2]. However, because OSPF is a employs link-state routing protocol
rather than a
technology as opposed to RIP's Distance Vector protocol, base, the mechanisms
used to achieve that the functionality are quite different.

Please send comments to ospf@gated.cornell.edu.

Acknowledgments

The author would like to acknowledge the helpful comments of Fred
Baker, Rob Coltun, Dawn Li, Gerry Meyer, and Tom Pusateri. Pusateri and Zhaohui
Zhang. This memo is a product of the OSPF Working Group.

Table of Contents

1 Model for demand circuits .............................. 4
2 Modifications to all OSPF routers ...................... 4 5
2.1 The OSPF Options field ................................. 5
2.2 The LS age field ....................................... 5
2.2
2.3 Removing old, non-aging stale DoNotAge LSAs ........................... 6
2.3
2.4 A change to the flooding algorithm ..................... 7
2.5 Interoperability with unmodified OSPF routers .......... 6 8
2.5.1 Indicating across area boundaries ...................... 9
2.5.1.1 Limiting indication-LSA origination ................... 10
3 Modifications to demand circuit endpoints .............. 7 ............. 11
3.1 Configuring demand circuits ............................ 7 Interface State machine modifications ................. 11
3.2 Sending and Receiving OSPF Hellos ...................... 8
3.3 Interface State ..................... 11
3.2.1 Negotiating Hello suppression ......................... 12
3.2.2 Neighbor state machine modifications .................. 8
3.4 12
3.3 Flooding over demand circuits .......................... 9 ......................... 13
3.4 Virtual link support .................................. 14
3.5 Point-to-MultiPoint Interface support ................. 15
4 Examples .............................................. 10 16
4.1 Example 1: Sole connectivity through demand circuits .. 10 16
4.2 Example 2: Demand and non-demand circuits in parallel . 14 20
4.3 Example 3: Operation when suffering SVC shortage ...... 18 oversubscribed .............. 24
5 Topology recommendations .............................. 20 25
6 Lost functionality .................................... 20 26
7 Future work: Oversubscription ......................... 26
A The Format of the OSPF Options field ..................................... 22 ...................... 29
B Configurable Parameters ............................... 23 30
C Architectural Constants ............................... 23 30
References ............................................ 24 31
Security Considerations ............................... 24 31
Author's Address ...................................... 24 31

1. Model for demand circuits

In this memo, demand circuits refer to those network segments whose
cost depends on either connect time or on and/or usage (expressed in terms
of bytes or packets). Examples include ISDN circuits and X.25 SVCs.
On these circuits, it is desirable for a routing protocol to send as
little routing traffic as possible. In fact, when there is no change
in network topology it is desirable for a routing protocol to send
no routing traffic at all; this allows the underlying data-link
connection to be closed when not needed for application data
traffic.

The model used within this memo for the maintenance of demand
circuits is as follows. If there is no data to send (either routing
protocol traffic or application data), the data-link connection
remains closed. As soon as there is data to be sent, an attempt to
open the data-link connection is made (e.g., an ISDN or X.25 call is
placed). If When/if the data-link connection is successful, established, the data
is sent, and the
circuit connection stays open until some period of time
elapses without more data to send. At this point the data-link
connection is again closed, in order to conserve cost and resources
(see Section 1 of [2]).

The "Presumption of Reachability" described in [2] is also used.
Even though a circuit's data-link connection may be closed at any
particular time, it is assumed by the routing layer (i.e., OSPF)
that it the circuit is available unless other information, such as a
discouraging diagnostic code resulting from an attempted data-link
connection, is present.

It may be possible that a data-link connection cannot be opened established
due to resource shortages. For example, a router with a single basic
rate ISDN interface cannot open more than two simultaneous ISDN
data-link connections (one for each B channel), and limitations in
interface firmware and/or switch capacity may limit the number of
X.25 SVCs simultaneously supported. When a router cannot
simultaneously open all of its circuits' underlying data-link
connections due to resource limitations, we say that the router is
oversubscribed. In these cases, datagrams to be forwarded out these the
(temporarily unopenable) data-link connections are discarded,
instead of being queued. Note also that this temporary inability to
open data-link connections due to oversubscription is NOT reported
by the OSPF routing system as unreachability; see Section 4.3 for
more information.

This memo assumes that either end of a demand circuit can open the
underlying data-link connection. Note that this assumption is not
true for certain dial-up modems. Also, for some dial network
technologies, call collisions can result when both ends of a circuit
simultaneously attempt to establish the data-link connection. This
memo does not address how such collisions are handled, assuming
instead that they are resolved at the data-link level.

2. Modifications to all OSPF routers

While most of the modifications to support demand circuits are
isolated to the demand circuit endpoints, endpoints (see Section 3), there are
changes required of all OSPF routers. These changes are described in
the subsections below. Routers

2.1. The OSPF Options field

A new bit is added to the OSPF Options field to support the
demand circuit extensions. This bit is called the "DC-bit". The
resulting format of the Options field is described in Appendix
A.

A router implementing these changes set the functionality described in Section 2
of this memo sets the DC-bit in the
options Options field of their router-LSA (see Section 2.3 all LSAs
that it originates. This is regardless of the LSAs' LS type, and Appendix A),
even if they do not implement
also regardless of whether the router implements the more
substantial modifications required of demand circuit endpoints
(see Section 3). Setting the DC-bit in self-originated LSAs
tells the rest of the routing domain that the router can
correctly process DoNotAge LSAs (see Sections 2.2, 2.3 and 2.5).

There is a single exception to the above rule. A router
implementing Section 2 of this memo may sometimes originate an
"indication-LSA"; these LSAs always have the DC-bit clear.
Indication-LSAs are described in used to convey across area boundaries the
existence of routers incapable of DoNotAge processing; see
Section
3.

2.1. 2.5.1 for details.

2.2. The LS age field

The semantics of the LSA's LS age field are changed, allowing
the high bit (DoNotAge; see Appendix C) to be set. Previously
the LS age field could not exceed the value of MaxAge. This bit is called "DoNotAge"; see
Appendix C for its formal definition. LSAs whose LS age field
have the DoNotAge bit set are not aged while they are held in
the link state database, which means that they do not have to be
refreshed every LSRefreshInterval as is done with all other OSPF
LSAs.

By convention, in the rest of this memo we will express LS age
fields having the DoNotAge set as "DoNotAge+x", while an LS age
expressed as just "x" is assumed not to not have the DoNotAge bit
set. LSAs having DoNotAge set are also sometimes referred to as
"DoNotAge LSAs".

When comparing two LSA instances to see which one is most
recent, the two LSAs' LS age fields are compared whenever the LS
sequence numbers and LS checksums are found identical (see
Section 13.1 of [1]). Before comparing, the LS age fields should must
have their DoNotAge bits masked off. For example, in
determining which LSA is more recent, LS ages of 1 and
DoNotAge+1 should be are considered equivalent; an LSA flooded with LS age
of 1 may be acknowledged with a Link State Acknowledgement
listing an LS age of DoNotAge+1, or vice versa.
As a special case, In particular,
DoNotAge+MaxAge is equivalent to MaxAge, and
either MaxAge; however for backward-
compatibility the MaxAge form can should always be used to flush when
flushing LSAs from the routing domain (see Section 14.1 of [1]).

Thus, the set of allowable values for the LS age field fall into
the two ranges: 0 through MaxAge and DoNotAge through
DoNotAge+MaxAge. (Previously the LS age field could not exceed
the value of MaxAge.) Any LS age field not falling into these
two ranges should be considered to be equal to MaxAge.

When an LSA is flooded out a non-demand an interface, the constant
InfTransDelay is added to the LSA's LS age field. This happens
even if the DoNotAge bit is set; in this case the LS age field
is not allowed to exceed DoNotAge+MaxAge, at which time DoNotAge+MaxAge. If the LS age field
reaches DoNotAge+MaxAge during flooding, the LSA is flushed from
the routing domain. This preserves the protection in [1]
afforded against flooding loops. Flooding out
demand interfaces is covered in Section 3.4.

Thus, the set of allowable values for the LS age field fall into
the two ranges: 0 through MaxAge and DoNotAge through
DoNotAge+MaxAge. Any LS age field not falling into these two
ranges should be considered to be equal to MaxAge.

The LS age field is not checksum protected. Errors in a router's
memory may mistakenly set an LSA's DoNotAge bit, stopping the
aging of the LSA. However, a router should note that its own
self-originated LSAs should never have the DoNotAge bit set in
its own database. This means that in any case the router's
self-originated LSAs will be refreshed every LSRefreshInterval.
As this refresh is flooded throughout the OSPF routing domain,
it will replace any LSA copies in other routers' databases whose
DoNotAge bits were mistakenly set.

2.2.

2.3. Removing old, non-aging stale DoNotAge LSAs

Because LSAs with the DoNotAge bit set are never aged, they can
stay in the link state database even when the originator of the
LSA no longer exists. To ensure that these LSAs are eventually
flushed from the routing domain, and that the size of the link
state database doesn't grow without bound, routers are required
to flush an a DoNotAge LSA if both of the following conditions are
met:

(1) The LSA has been in the router's database for at least
MaxAge seconds.

(2) The originator of the LSA has been unreachable (according to
the routing calculations specified by Section 16 of [1]) for the time MaxAge.
at least MaxAge seconds.

For an example, see Time T8 in the example of Section 4.1. This Note
that the above functionality is an exception to the general OSPF
rule that a router can only flush (i.e., prematurely age; see
Section 14.1 of [1]) its own self-originated LSAs. The above
functionality pertains only to DoNotAge LSAs. An LSA having
DoNotAge clear still can be prematurely aged only by its
originator; otherwise, the LSA must age naturally to MaxAge
before being removed from the routing domain.

An interval as long as MaxAge has been chosen to avoid any
possibility of thrashing (i.e., flushing an advertisement LSA only to have it
reoriginated soon afterwards).

2.3. Note that by the above rules, a
DoNotAge LSA will be removed from the routing domain no faster
than if it were being aged naturally (i.e., if DoNotAge were not
set).

2.4. A change to the flooding algorithm

The following change is made to the OSPF flooding algorithm.
When a Link State Update Packet is received that contains an LSA
instance which is actually less recent than the the router's
current database copy, the router must now respond by sending
its database copy (encapsulated in a Link State Update Packet)
back to the sending neighbor. When doing so, the LSA is NOT
added to the neighbor's link state retransmission list. The
previous behavior was to ignore the flood of the less recent LSA
instance; see Step 8 of Section 13 in [1].

This change is necessary to support flooding over demand
circuits. For example, see Time T4 in the example of Section
4.2.

However, this change is beneficial when flooding over non-demand
interfaces as well. For this reason, the flooding change
pertains to all interfaces, not just interfaces to demand
circuits. The main example involves MaxAge LSAs. There are times
when MaxAge LSAs stay in a router's database for extended
intervals: 1) when they are stuck in a retransmission queue on a
slow link or 2) when a router is not properly flushing them from
its database, due to software bugs. The prolonged existence of
these MaxAge LSAs can inhibit the flooding of new instances of
the LSA. New instances typically start with the initial LS
sequence number, and are treated as less recent (and hence
discarded) by routers still holding MaxAge instances. However,
with the above change to flooding, a router with a MaxAge
instance will respond back with the MaxAge instance. This will
get back to the LSA's originator, which will then pick the next
highest LS sequence number and reflood, overwriting the MaxAge
instance.

This change will be included in future revisions of the base
OSPF specification [1].

2.5. Interoperability with unmodified OSPF routers

Unmodified OSPF routers will probably treat LSAs with the DoNotAge bit set LSAs as if
they had LS age of MaxAge. At the very worst, this will cause
continual retransmissions of LSAs with the DoNotAge bit set. LSAs. (An example
scenario follows. Suppose Routers A and B are connected by a
point-to-point link. Router A implements the demand circuit
extensions, Router B does not. Neither one treats their
connecting link as a demand circuit. At some point in time,
Router A receives from another neighbor via flooding a DoNotAge
LSA. The DoNotAge LSA is then flooded by Router A to Router B.
Router B, not understanding DoNotAge LSAs, treats it as a MaxAge
LSA and acknowledges it as such to Router A. Router A receives
the acknowledgment, but notices that the acknowledgment is for a
different instance, and so starts retransmitting the LSA.)

However, to avoid this confusion, advertisements with DoNotAge
set LSAs will be allowed
in an OSPF area if and only if, in the area's link state
database, all router-LSAs and type-4-summary-LSAs
(location of ASBRs) LSAs have their the DC-bit set (indicating in their
ability to process DoNotAge). Options field
(see Section 2.1). Note that it is not required that the LSAs'
Advertising Router is be reachable; if any LSA is found not having
its DC-bit set (regardless of reachability), then the router
should flush (i.e., prematurely age; see Section 14.1 of [1])
from the area all advertisements having DoNotAge set; LSAs. These LSAs will then be
reoriginated at their sources, this time with DoNotAge clear.
Like the change in Section 2.3, this change is an exception to
the general OSPF rule that a router can only flush its own
self-originated LSAs. When
flushing, the LSAs' Both changes pertain only to DoNotAge
LSAs, and in both cases a flushed LSA's LS age field should be
set to MaxAge and not DoNotAge+MaxAge.

(As an implementation suggestion, a new variable "DCBitLessLSAs"
could be added to the OSPF

2.5.1. Indicating across area data structure in Section 6 of
[1]. This variable would count the number of boundaries

AS-external-LSAs are flooded throughout the area's router-
LSAs entire OSPF
routing domain, excepting only OSPF stub areas and type-4-summary-LSAs NSSAs.
For that do reason, if an OSPF router that is incapable of
DoNotAge processing exists in any "regular" area (i.e., an
area that is not a stub nor an NSSA), no AS-external-LSA can
have the DC-bit DoNotAge set. This variable would potentially increment/decrement any time a
new LSA was received or an old LSA was replaced or flushed.)

In particular, AS-external-LSAs with memo simplifies that requirement by
broadening it to the following rule: LSAs in regular OSPF
areas are allowed to have DoNotAge bit set cannot
exist if and only if every
router in the routing OSPF domain unless all routers (excepting those in all "regular
OSPF areas" (all areas that are neither stub areas nor and
NSSAs)
are is capable of DoNotAge processing. The rest of this
section describes how the rule is implemented.

As described above in Sections 2.1 and 2.5, a router
indicates that it is capable of DoNotAge processing DoNotAge. In order by
setting the DC-bit in the LSAs that it originates. However,
there is a problem. It is possible that, in all areas to convey this
which Router X directly attaches, all the routers are
capable of DoNotAge processing, yet there is some router in
a remote "regular" area that cannot process DoNotAge LSAs.
This information must then be conveyed to Router X, so that
it does not mistakenly flood/create DoNotAge LSAs.

The solution is as follows. Area border routers transmit the
existence of DoNotAge-incapable routers across area
boundaries, using "indication-LSAs". Indication-LSAs are
type-4-summary LSAs (also called ASBR-summary-LSAs), listing
the area border routers must router itself as the described ASBR, with
the LSA's cost set to LSInfinity and the DC-bit in a type-4-summary-LSA clear. Note
that indication-LSAs convey no additional information; in
particular, they originate are used even if the area border router is
not really an AS boundary router (ASBR).

Taking indication-LSAs into account, the rule as to whether
DoNotAge LSAs are allowed in any particular area is EXACTLY
the same as given previously in Section 2.5, namely:
DoNotAge LSAs will be allowed in an OSPF area if and only if
if, in the described ASBR has area's link state database, all LSAs have the
DC-bit set in its original
router-LSA. Additionally, sometimes in may their Options field.

Through origination of indication-LSAs, the existence of
DoNotAge-incapable routers can be necessary viewed as going from non-
backbone regular areas, to
convey across areas the backbone area and from there
to all other regular areas. The following two cases
summarize the requirements for an area border router to
originate indication-LSAs:

(1) Suppose an area border router (Router X) is connected to
a regular non-backbone OSPF area (Area A). Furthermore,
assume that Area A has LSAs with the existence DC-bit clear, other
than indication-LSAs. Then Router X should originate
indication-LSAs into all other directly-connected
"regular" areas, including the backbone area, keeping
the guidelines of Section 2.5.1.1 in mind.

(2) Suppose an area border router (Router X) is connected to
the backbone OSPF area (Area 0.0.0.0). Furthermore,
assume that the backbone has LSAs with the DC-bit clear
that are either a) not indication-LSAs or b)
indication-LSAs that have been originated by routers
other than Router X itself. Then Router X should
originate indication-LSAs into all other directly-
connected "regular" non-backbone areas, keeping the
guidelines of Section 2.5.1.1 in mind.

2.5.1.1. Limiting indication-LSA origination

To limit the number of indication-LSAs originated, the
following guidelines should be observed by an area
border router (Router X) when originating indication-
LSAs. First, indication-LSAs are not originated into an
Area A when A already has LSAs with DC-bit clear other
than indication-LSAs. Second, if another area border
router has originated a non-ASBR indication-LSA into Area A, and
that cannot
process DoNotAge. area border router has a higher OSPF Router ID than
Router X (same tie-breaker as for forwarding address
origination; see Section 12.4.5 of [1]), then Router X
should not originate an indication-LSA into Area A.

As an example, suppose that three regular OSPF areas
(Areas A, B and C) are connected by routers X, Y and Z
(respectively) to the backbone area. Furthermore,
suppose that all routers are capable of DoNotAge
processing, except for routers in Areas A and B.
Finally, suppose that Router Z has a higher Router ID
than Y, which in turn has a higher Router ID than X. In
this case, a type-4-summary-LSA should two indication-LSAs will be
originated with cost generated (if the
rules of LSInfinity Section 2.5.1 and DC-bit clear. the guidelines of the
preceding paragraph are followed): Router Y will
originate an indication-LSA into the backbone, and
Router Z will originate an indication-LSA into Area C.

3. Modifications to demand circuit endpoints

The following subsections detail the modifications required of the
routers at the endpoints of demand circuits. This consists These consist of
modifications to two main pieces of OSPF: 1) sending and receiving
hellos
Hello Packets over demand circuits and 2) flooding LSAs over demand
circuits.

3.1. Configuring demand circuits

An additional OSPF interface configuration parameter,
DemandInterface, is defined to indicate whether an OSPF interface
connects to a demand circuit (see Appendix B). On
point-to-point Two routers
connecting to a common network segment need not agree on that
segment's demand circuits, circuit status. However, to get full benefit of the neighboring router
demand circuit extensions, the two ends of a point-to-point link
must both agree that to treat the point-to-point link as a demand circuit (see Section
3.2).

3.1. Interface State machine modifications

An OSPF point-to-point interface connecting to a demand circuit
is considered to be in state "Point-to-point" if and only if its
associated neighbor is in state "1-Way" or greater; otherwise
the interface is considered to be in state "Down". Hellos are
sent out such an interface when it is in "Down" state, at the
reduced interval of PollInterval. If the negotiation in Section
3.2.1 succeeds, Hellos will cease to be sent out the interface
whenever the associated neighbor reaches state "Full".

Note that as a result, an "LLDown" event for the point-to-point
demand circuit. circuit's neighbor forces both the neighbor and the
interface into state "Down" (see Section 3.2.2).

For OSPF broadcast and NBMA networks that have been configured
as demand circuits, there are no changes to the Interface State
Machine.

3.2. Sending and Receiving OSPF Hellos

The following sections describe the required modifications to
OSPF Hello Packet processing on point-to-point demand circuits.

For OSPF broadcast and NBMA networks that have been configured
as demand circuits, there is no change to the sending and
receiving of Hellos, nor are there any changes to the Neighbor
State Machine. This is because the proper operation of the
Designated Router election algorithm requires periodic exchange
of Hello Packets.

3.2.1. Negotiating Hello suppression

On point-to-point demand circuits, both endpoints must agree
to suppress the sending of Hello Packets. To ensure this
agreement, the a router sets the DC-bit in OSPF Hellos
(and and
Database Description Packets) Packets sent out the demand interface.
Receiving an Hello or a Database Description Packet with the
DC-bit set indicates agreement. Receiving an Hello with the
DC-bit clear and also listing the router's Router ID in the
body of the Hello message, or a Database Description Packet
with the DC-bit clear (either one indicating bidirectional
connectivity) indicates that the other end refuses to treat the
link as a demand circuit.
suppress Hellos. In these latter cases, the router reverts
to
treating the link as a leased line.

The above procedure indicates that a normal periodic sending of Hello Packets out the
interface (see Section 9.5 of [1]).

A demand point-to-point circuit need be configured in only
one of the two endpoints (see Section 4.1). If the negotiation fails, a router
implementing Sections 2 and 3 of this memo receives an Hello
Packet with the router is forced to DC-bit set, it should treat the
line point-to-
point link as a leased line, demand circuit, making the appropriate
changes to its Hello Processing (see Section 3.2.2) and
flooding (see Section 3.3).

Even if the above negotiation fails, the router can always renegotiate should
continue setting the
link's demand status DC-bit in its Hellos and Database
Descriptions (the neighbor will just ignore the bit). The
router will then automatically attempt to renegotiate Hello
suppression whenever the link goes down. down and comes back up.
For example, if the neighboring router is rebooted with
software that is capable of operating over demand circuits, circuits
(i.e., implements Sections 2 and 3 of this memo), a future
negotiation will succeed.

For more information on sending OSPF

Also, even if the negotiation to suppress Hellos over demand
circuits, see fails, the
flooding modifications described in Section 3.2.

3.2. Sending and Receiving OSPF Hellos

Over point-to-point demand circuits OSPF 3.3 are still
performed over the link.

3.2.2. Neighbor state machine modifications

When the above negotiation succeeds, Hello packets Packets are sent
over point-to-point demand circuits only until initial
link-state database synchronization is achieved with the
neighbor (i.e., the state of the neighbor connection reaches
"Full", as defined in Section 10.1 of [1]). After this,
Hellos are suppressed and the data-link connection to the
neighbor is assumed available until evidence is received to
the contrary. When the router finds that the neighbor is no
longer available, presumably from something like a
diagnostic code contained in a response to a failed call
request, the neighbor connection transitions back to "Down"
and Hellos are sent periodically (at Intervals of
PollInterval) in an attempt to restart synchronization with
the neighbor.

This requires changes to the OSPF Neighbor State Machine
(see Section 10.3 of [1]). The receipt of Hellos from demand
circuit neighbors in state "Loading" or higher cannot "Full" can no longer
be required. In other words, the InactivityTimer event
defined in Section 10.2 of [1] has no effect on demand
circuit neighbors in state "Loading" or higher. "Full". An
additional clarification is needed in the Neighbor State
Machine's LLDown event. This For demand circuits, this event
should be mapped into the "discouraging diagnostic code"
discussed above previously in Section 1, and should not be
generated when the data-link connection has been closed
simply to save resources.

3.3. Interface State machine modifications

OSPF interfaces to point-to-point demand circuits are considered
to Nor should LLDown be in "Point-to-point" state if and only generated if they have
a
neighbor in state "1-Way" or greater, otherwise they are
considered data-link connection fails due to be in state "Down". However, as discussed above in
Section 3.2, Hellos are sent out interfaces in "Down" state, at
the reduced interval temporary lack of PollInterval. Hellos cease to be sent
out the interface whenever the associated neighbor reaches state
"Full".

Note that as a result, an "LLDown" event for the point-to-point
demand circuit's neighbor forces both the neighbor and the
interface into state "Down".

For OSPF broadcast and NBMA networks that have been configured
as demand circuits, there are no changes to the Interface State
Machine.

3.4.
resources.

3.3. Flooding over demand circuits

Flooding over demand circuits (point-to-point or otherwise) is
modified if and only if all routers have indicated that they can
process LSAs having DoNotAge set. This is determined by
examining the link state database of the OSPF area containing
the demand circuit. All LSAs in the router-LSAs and type-4-summary-LSAs database must have the DC-bit DC-
bit set. If one or more LSAs have the DC-bit clear, flooding
over demand circuits is unchanged from [1]. (In
particular, router-LSAs or type-4-summary-LSAs that do not have
the DC-bit set are flooded unchanged from [1], because their
reception inhibits the flooding behavior defined below). Otherwise, flooding
is changed as follows.

First, only

(1) Only truly changed LSAs are flooded over demand circuits.
When a router receives a new LSA instance, it checks first
to see whether the contents have changed. If not, the new
LSA is simply a periodic refresh and it is not flooded out
attached demand circuits (it is still flooded out other
interfaces however). This check should be performed in Step
5b of Section 13 in [1]. When comparing an LSA to its
previous instance, the following are all considered to be
changes in contents:

o The LSA's Options field has changed.

o One or both of the LSA instances has LS age set to
MaxAge (or DoNotAge+MaxAge).

o The length field in the LSA header has changed.

o The contents of the LSA, excluding the 20-byte link
state header, have changed. Note that this excludes
changes in LS Sequence Number and LS Checksum.

Second, when

(2) When it has been decided to flood an LSA over a demand
circuit, DoNotAge should be set in the copy of the LSA that
is flooded out the demand interface. (There is one
exception: DoNotAge should not be set if the LSA's LS age field before
flooding. This is
equal to MaxAge.) Setting DoNotAge will cause the routers on
the other side of the demand circuit to hold the LSA in
their database databases indefinitely, removing the need for periodic
refresh. Note that it is perfectly possible that DoNotAge
will already be set. This simply indicates that the LSA has
already been flooded over demand circuits. In any case, the
flooded copy's LS age field must also be incremented by
InfTransDelay before flooding (see Step 5 of Section 13.3 in [1]), [1], and
Section 2.2 of this memo), as protection against flooding
loops.

The above previous paragraph also pertains to LSAs flooded over
demand circuits in response to Link State Requests.

Third, when receiving a Link State Update from a It also
pertains to LSAs that are retransmitted over demand
circuits.

3.4. Virtual link support

OSPF virtual links are essentially unnumbered point-to-point
links (see Section 15 of [1]). Accordingly, demand circuit
neighbor that contains an LSA instance
support for virtual links resembles that described for point-
to-point links in the previous sections. The main difference is actually older
than
that a router implementing Sections 2 and 3 of this memo, and
supporting virtual links, always treats virtual links as if they
were demand circuits. Otherwise, when a virtual link's
underlying physical path contains one or more demand circuits,
periodic OSPF protocol exchanges over the virtual link would
unnecessarily keep the router's current copy, underlying demand circuits open.

Demand circuit support on virtual links can be summarized as
follows:

o Instead of modifying the router must respond by
flooding its current LSA copy directly to Interface state machine for virtual
links as was done for point-to-point links in Section 3.1,
the neighbor (without
putting Interface state machine for virtual links remains
unchanged. A virtual link is considered to be in state
"Point-to-point" if an intra-area path (through the LSA on virtual
link's transit area) exists to the neighbor's Link State retransmission
list). This other endpoint. Otherwise
it is instead considered to be in state "Down". See Section 15 of
[1] for more details.

o Virtual links are always treated as demand circuits. In
particular, over virtual links a router always negotiates to
suppress the sending of Hellos. See Sections 3.2.1 and 3.2.2
for details.

o In the behavior specified demand circuit support over virtual links, there is
no "discouraging diagnostic code" as described in Step 8 of Section 13 1.
Instead, the connection is considered to exist if and only
if an intra-area path (through the virtual link's transit
area) exists to the other endpoint. See Section 15 of [1]
for more details.

o Since virtual links are always treated as demand circuits,
flooding over virtual links always proceeds as in [1], Section
3.3.

3.5. Point-to-MultiPoint Interface support

The OSPF Point-to-MultiPoint interface has recently been
developed for use with non-mesh-connected network segments. A
common example is a Frame Relay subnet where PVCs are
provisioned between some pairs of routers, but not all pairs. In
this case the Point-to-Multipoint interface represents the
single physical interface to the Frame relay network, over which would effectively ignore
multiple point-to-point OSPF conversations (one on each PVC) are
taking place. For more information on the flood Point-to-MultiPoint
interface, see [8].

Since an OSPF Point-to-MultiPoint interface essentially consists
of multiple point-to-point connections, demand circuit support
on the older advertisement. To see Point-to-Multipoint interface strongly resembles demand
circuit support for point-to-point links. However, since the necessity
Point-to-MultiPoint interface requires commonality of responding its
component point-to-point links' configurations, there are some
differences.

Demand circuit support on Point-to-Multipoint interfaces can be
summarized as follows:

o Instead of modifying the Interface state machine for Point-
to-Multipoint interfaces as was done for point-to-point
links in Section 3.1, the Interface state machine for
Point-to-Multipoint interfaces remains unchanged.

o When a Point-to-MultiPoint interface is configured as a
DemandInterface, it tries to
old LSAs, see Time T4 negotiate Hello suppression
separately on each of its component point-to-point links.
This negotiation proceeds as in Section 4.2. 3.2.1. Negotiation
may fail on some component point-to-point links, and succeed
on others. This is acceptable. On those component links
where the negotiation fails, Hellos will always be sent;
otherwise, Hellos will cease to be sent when the Database
Description process completes on the component link (see
Section 3.2.2).

o Section 3.3 defines the demand circuit flooding behavior for
all OSPF interface types. This includes Point-to-Multipoint
interfaces.

4. Examples

This section gives three examples of the operation over demand
circuits. The first example is probably the most common and
certainly the most basic. It shows a single point-to-point demand
circuit connecting two routers. The second illustrates what happens
when demand circuits and leased lines are used in parallel. The
third explains what happens when a router has multiple demand
circuits and cannot keep them all open (for resource reasons) at the
same time.

4.1. Example 1: Sole connectivity through demand circuits

Figure 1 shows a sample internetwork with a single demand
circuit providing connectivity to the LAN containing Host H2.
Assume that all three routers (RTA, RTB and RTC) have
implemented the functionality in Section 2 of this memo, and
thus will be setting the DC-bit in their router-LSAs. LSAs. Furthermore
assume that Router RTB has been configured to treat the link to
Router RTC as a demand circuit, but Router RTC has not been so
configured. Finally assume that the LAN interface connecting
Router RTA to Host H1 is initially down.

The following sequence of events may then transpire, starting
with Router RTB booting and bringing up its link to Router RTC:

+ + +
| +---+ | |
+--+ |---|RTA|---| | +--+
|H1|---| +---+ | |---|H2|
+--+ | | +---+ ODL +---+ | +--+
|LAN Y |---|RTB|-------------|RTC|---|
+ | +---+ +---+ |
+ +

Figure 1: A single demand circuit (labeled
ODL) bisecting an internetwork.

Time T0: RTB negotiates demand circuit status Hello suppression

Router RTB will start sending Hellos over the demand circuit
with the DC-bit set in the Hello's Options field. Because
RTC is not configured to treat the link as a demand circuit,
the first Hello received from RTC may will not have the DC-bit
set. However, subsequent Hellos and Database Description
+ + +
| +---+ | |
+--+ |---|RTA|---| | +--+
|H1|---| +---+ | |---|H2|
+--+ | | +---+ ODL +---+ | +--+
|LAN Y |---|RTB|-------------|RTC|---|
+ | +---+ +---+ |
+ +

Figure 1: In the example of Section 4.1,
a single demand circuit (labeled
ODL) bisects an internetwork.

Packets received from RTC will have the DC-bit set,
indicating that the two routers have agreed that the link
will be treated as a demand circuit. The entire negotiation
is pictured in Figure 2. Note that if RTC were unable or
unwilling to treat suppress Hellos on the link as a demand circuit, link, the initial
Database Description sent from Router RTC to RTB would have
the DC-bit clear, forcing treatment of Router RTB to revert to the link as a leased
line.

+---+ +---+
|RTB| |RTC|
+---+ +---+
Hello (DC-bit set)
------------------------------------->
Hello (DC-bit clear)
<-------------------------------------
Hello (DC-bit set, RTC seen)
------------------------------------->
Database Description (DC-bit set)
<-------------------------------------

Figure 2: Successful negotiation
periodic sending of a link's
demand circuit status. Hellos specified in Section 9.5 of [1].

Time T1: Database exchange over demand circuit

The initial synchronization of link state databases (the
Database Exchange Process) over the demand circuit then
occurs as over any point-to-point link, with one exception.

Figure 2: Successful negotiation of Hello
suppression.

LSAs included in Link state updates State Updates Packets sent over the
demand circuit (in response to Link State Request Packets),
will have the DoNotAge bit set in their LS age field. So,
after the Database Exchange Process is finished, all routers
will have 3 LSAs in their link state databases (router-LSAs
for Routers RTA, RTB and RTC), but the LS age fields
belonging to the LSAs will vary depending on which side of
the demand circuit they were originated from (see Table 1).
For example, all routers other than Router RTC have the
DoNotAge bit set in Router RTC's router-LSA; this removes
the need for Router RTC to refresh its router-LSA over the
demand circuit.

Time T2: Hello traffic ceases

After the Database Exchange Process has completed, no Hellos
are sent over the demand circuit. If there is no application
data to be sent over the demand circuit, the circuit will be
idle.

Time T3: Underlying data-link connection torn down

After some period of inactivity, the underlying data-link
connection will be torn down (e.g., an ISDN call will would be
cleared) in order to save connect charges. This will be
transparent to the OSPF routing; no LSAs or routing table
entries will change as a result.

LS age
LSA in RTB in RTC
______________________________________________
RTA's Router-LSA 1000 DoNotAge+1001
RTB's Router-LSA 10 DoNotAge+11
RTC's Router-LSA DoNotAge+11 10

Table 1: After Time T1 in Section 4.1,
possible LS age fields on either
side of the demand circuit

Time T4: Router RTA's LSA is refreshed

At some point Router RTA will refresh its own router-LSA
(i.e., when the LSA's LS age hits LSRefreshInterval). This
refresh will be flooded to Router RTB, who will look at it
and decide NOT to flood it over the demand circuit to Router
RTC, because the LSA's contents have not really changed
(only the LS Sequence Number). At this point, the LS
sequence numbers that the routers have for RTA's router-LSA
differ depending on which side of the demand circuit the
routers lie. Because there is still no application traffic,
the underlying data-link connection remains disconnected.

Time T5: Router RTA's LAN interface comes up

When Router RTA's LAN interface (connecting to Host H1)
comes up, RTA will originate a new router-LSA. This router-
LSA WILL be flooded over the demand circuit because its
contents have now changed. The underlying data-link
connection will have to be brought up to flood the LSA.
After flooding, routers on both sides of the demand circuit
will again agree on the LS Sequence Number for RTA's
router-LSA.

Time T6: Underlying data-link connection is torn down again

Assuming that there is still no application traffic
transiting the demand circuit, the underlying data-link
connection will again be torn down after some period of
inactivity.

Time T7: File transfer started between Hosts H1 and H2

As soon as application data needs to be sent across the
demand circuit the underlying data-link connection is
brought back up.

Time T8: Physical link becomes inoperative

If an indication is received from the data-link or physical
layers indicating that the demand circuit can no longer be
established, Routers RTB and RTC declare their point-to-
point interfaces down, and originate new router-LSAs. Both
routers will attempt to bring the connection back up by
sending Hellos at the reduced rate of PollInterval. Note
that while the connection is inoperative, Routers RTA and
RTB will continue to have an old router-LSA for RTC in their
link state database, and this LSA will not age out because
it has the DoNotAge bit set. However, according to Section
2.2
2.3 they will flush Router RTC's router-LSA if the demand
circuit remains inoperative for longer than MaxAge.

4.2. Example 2: Demand and non-demand circuits in parallel

This example demonstrates the demand circuit functionality when
both demand circuits and non-demand circuits (e.g., leased
lines) are used to interconnect regions of an internetwork. Such
an internetwork is shown in Figure 3. Host H1 can communicate
with Host H2 either over the demand link between Routers RTB and
RTC, or over the leased line between Routers RTB and RTD.

Because the basic properties of the demand circuit functionality
were presented in the previous example, this example will only
address the unique issues involved when using both demand and
non-demand circuits in parallel.

Assume that Routers RTB and RTY are powered off, but that all
other routers and their attached links are both operational and
implement the demand circuit modifications to OSPF. Throughout
the example, a TCP connection between Hosts H1 and H2 is
transmitting data. Furthermore, assume that the cost of the
demand circuit from RTB to RTC has been set considerably higher
than the cost of the leased line between RTB and RTD; for this
reason traffic between Hosts H1 and H2 will always be sent over
the leased line when it is operational.

The following events may then transpire:

Time T0: Router RTB comes up.

Assume RTB supports the demand circuit OSPF modifications.
When Router RTB comes up and establishes links to Routers
RTC and RTD, it will flood the same information over both.
However, LSAs sent over the demand circuit (to Router RTC)
will have the DoNotAge bit set, while those sent over the
leased line to Router RTD will not. Because the DoNotAge bit
is not taken into account when comparing LSA instances, the
routers on the right side of RTB (RTC, RTE and RTD) may or
may not have the DoNotAge bit set in the their database copies
of the RTA RTA's and RTB router-LSAs contained in their database. RTB's router-LSAs. This depends on whether the
LSAs sent over the demand link reach the routers before
those sent over the leased line. One possibility is pictured
in Table 2.

+
+---+ |
|RTC|--| +
+---+ | +---+ |
+ / |--|RTE|--| +--+
+--+ | /ODL | +---+ |--|H2|
|H1|----| +---+ +---+/ | + +--+
+--+ |--|RTA|-------|RTB| |
| +---+ +---+\ | +
+ \ | +---+ |
\ |--|RTY|--|
+---+ | +---+ |
|RTD|--| +
+---+ |
+

Figure 3: A sample Example 2's internetwork.

Vertical lines are LAN segments. Six routers
are pictured, Routers RTA-RTE and RTY.
RTB has three serial line interfaces, two of
which are leased lines and the third (connecting to
RTC) a demand circuit. Two hosts, H1 and
H2, are pictured to illustrate the effect of
application traffic.

LS age
LSA in RTC in RTD in RTE
________________________________________________
RTA's Router-LSA DoNotAge+20 21 21
RTB's Router-LSA DoNotAge+5 6 6

Table 2: After Time T0, T0 in Example 2, LS age
fields on the right side of Router RTB.

LS age
LSA in RTC in RTD in RTE
_______________________________________________
RTA's Router-LSA 5 6 6
RTB's Router-LSA DoNotAge+5 1785 1785

Table 3: After Time T2, T2 in Example 2, LS age
fields on the right side of Router RTB.

LS age
LSA in RTC in RTD in RTE
_______________________________________________________
RTA's Router-LSA 325 326 326
RTB's Router-LSA DoNotAge+5 DoNotAge+6 DoNotAge+6

Table 4: After Time T3, T3 in Example 2, LS age
fields on the right side of Router RTB.

LS age
LSA in RTC in RTD in RTE
_______________________________________________________
RTA's Router-LSA DoNotAge+7 DoNotAge+8 DoNotAge+8
RTB's Router-LSA DoNotAge+5 DoNotAge+6 DoNotAge+6

Table 5: After Time T4, T4 in Example 2, LS age
fields on the right side of Router RTB.

Time T1: Underlying data-link connection is torn down.

All application traffic is flowing over the leased line
connecting Routers RTB and RTD instead of the demand
circuit, due to the leased line's lesser OSPF cost. After
some period of inactivity, the data-link connection
underlying the demand circuit will be torn down. This does
not affect the OSPF database or the routers' routing tables.

Time T2: Router RTA refreshes its router-LSA.

When Router RTA refreshes its router-LSA (as all routers do
every LSRefreshInterval), Router RTB floods the refreshed
LSA over the leased line but not over the demand circuit,
because the contents of the LSA have not changed. This new
LSA will not have the DoNotAge bit set, and will replace the
old instances (whether or not they have the DoNotAge bit
set) by virtue of its higher LS Sequence number. This is
pictured in Table 3.

Time T3: Leased line becomes inoperational.

When the leased line becomes operational, inoperational, the data-link
connection underlying the demand circuit will be reopened,
in order to flood a new (and changed) router-LSA for RTB and
also to carry the application traffic between Hosts H1 and
H2. At this point, After flooding the new LSA, all routers on the right
side of the demand circuit will have DoNotAge set in their
copy of RTB's router-LSA and DoNotAge clear in their copy of
RTA's router-LSA (see Table 4).

Time T4: In Router RTE, Router RTA's router-LSA times out.

Refreshes of Router RTA's router-LSA are not being flooded
over the demand circuit. However, RTA's router-LSA is aging
in all of the routers to the right of the demand circuit.
For this reason, the router-LSA will eventually be flushed aged out
and reflooded (by router RTE in our example). Because this flushed
aged out LSA constitutes a real change (see Section 3.4), 3.3), it
is flooded over the demand circuit from Router RTC to RTB.
There are then two possible scenarios. First, the LS
Sequence numbers number for RTA's router-LSA may have diverged be larger on either RTB's
side of the demand link. In this case, when router RTB
receives the flushed LSA it will respond by flooding back
the more recent instance (see Section 3.4). 2.4). If instead the
LS sequence numbers are the same, the flushed LSA will be
flooded all the way back to Router RTA, which will then be
forced to reoriginate the advertisement. LSA.

In any case, after a small period all the routers on the
right side of the demand link will have the DoNotAge bit set
in their copy of RTA's router-LSA (see Table 5). In the
small interval between the flushing and waiting for a new
instance of the LSA, there will be a temporary loss of
connectivity between Hosts H1 and H2.

Time T5: A non-supporting router joins.

Suppose Router RTY now becomes operational, and does not
support the demand circuit OSPF extensions. Router RTY's
router-LSA then does will not have the DC-bit set in its Options
field, and as the router-LSA is flooded throughout the
internetwork it flushes all LSAs having the DoNotAge bit set
and causes the flooding behavior over the demand circuit to
revert back to the normal flooding behavior defined in [1].
However, although all LSAs will now be flooded over the
demand circuit, regardless of whether their contents have
really changed, Hellos will still continue to be suppressed
on the demand circuit (see Section 3.2). 3.2.2).

4.3. Example 3: Operation when suffering SVC shortage oversubscribed

Figure 4 shows a single Router (RT1) connected via demand
circuits to three other routers (RT2-RT4). Assume that RT1 can
only have two out of three underlying data-link connections open
at once. This may be due to one of the following reasons. reasons:
Router RT1 may be using a single Primary rate Basic Rate ISDN interface (2 B
channels) to support all three demand circuits. Or, circuits, or, RT1 may be
connected a data-link switch (e.g., X.25 or Frame relay) that is
only capable of so many simultaneous data-link connections.

The following events may transpire, starting with Router RT1
coming up.

Time T0: Router RT1 comes up.

Router RT1 attempts to establish neighbor connections and
synchronize OSPF databases with routers RT2-RT4. But,
because it cannot have data-link connections open to all
three at once, it will synchronize with RT2 and RT3, while
Hellos sent to RT4 will be discarded (see Section 1).

Time T1: Data-link connection to RT2 closed due to inactivity.

Assuming that no application traffic is being sent to/from
Host H3, the underlying data-link connection to RT2 will
eventually close due to inactivity. Then, the next Hello
+ +--+
+---+ |--|H3|
+---------|RT2|--| +--+
/ +---+ |
/ ODL +
+--+ + /
|H1|--| / +
+--+ | +---+ ODL +---+ |
|--|RT1|------------|RT3|--| +--+
|--|RT1|------------|RT3|--|--|H4|
| +---+ +---+ | +--+
| \ +
+ \ODL
\ + +--+
\ +---+ |--|H2|
+--------|RT4|--| +--+
+---+ |
+

Figure 4: Behavior when not all
of the demand circuits' data-
link connections can be opened
at once. Example 3's internetwork.

that RT1 attempts to send to RT4 will cause that data-link
connection to open and synchronization with RT4 will ensue.
Note that, until this time, H2 will be considered
unreachable by OSPF routing. However, data traffic would not
have been deliverable to H2 until now in any case.

Time T2: RT2's LAN interface becomes inoperational

This causes RT2 to reissue its router-LSA. However, it may
be unable to flood it to RT1 if RT1 already has data-link
connections opened open to RT3 and RT4. While the data-link
connection from RT2 to RT1 cannot be opened due to resource
shortages, the new router-LSA will be continually
retransmitted (and dropped by RT2's ISDN interface; see the
last paragraph of
Section 1). This means that the routing routers RT1, RT3 and RT4
will not detect the unreachability of H4 Host H3 until a data-link data-
link connection on RT1 becomes available.

Increasing the OSPF cost of demand circuits that are currently
discarding application packets, due to underlying data-link
shortage, may help the routing find paths that can actually
deliver the packets. This however would create more routing
traffic, and is an issue for future study.

5. Topology recommendations

Because LSAs indicating topology changes are still flooded over
demand circuits, it is still advantageous to design networks so that
the demand circuits are isolated from as many topology changes as
possible. In OSPF, this is done by encasing the demand circuits
within OSPF stub areas or within NSSAs (see [3]). In both cases,
this isolates the demand circuits from AS external routing changes,
which in many networks are the most frequent (see [6]). Stub areas
can even isolate the demand circuits from changes in other OSPF
areas.

Also, considering the interoperation of OSPF routers supporting
demand circuits and those that do not (see Section 2.3), 2.5), isolated
stub areas or NSSAs can be converted independently to support demand
circuits. In contrast, regular OSPF areas must all be converted
before the functionality can take effect in any particular regular
OSPF area.

6. Lost functionality

The enhancements defined in this memo to support demand circuits
come at some cost. Although we gain an efficient use of demand
circuits, holding them open only when there is actual application
data to send, we lose the following:

Robustness
In regular OSPF [1], all LSAs are refreshed every
LSRefreshInterval. This provides protection against routers
losing LSAs from (or LSAs getting corrupted in) their link state
databases due to software errors, etc. Over demand circuits
this periodic refresh is removed, and we depend on routers
correctly holding LSAs marked with DoNotAge in their databases
indefinitely.

Database Checksum
OSPF supplies network management variables, namely
ospfExternLSACksumSum and ospfAreaLSACksumSum in [7], allowing a
network management station to verify OSPF database
synchronization among routers. However, these variables are sums
of the individual LSAs' LS checksum fields, which are no longer
guaranteed to be identical across demand circuits (because the
LS checksum covers the LS Sequence Number, which will in general
differ across demand circuits). This means that these variables
can no longer be used to verify database synchronization in OSPF
networks containing demand circuits.

7. Future work: Oversubscription

An internetwork is oversubscribed when not all of its demand
circuits' underlying connections can be open at once, due to
resource limitations. These internetworks were addressed in Section
4.3. However, when all possible sources in the internetwork are
+--+ + + +--+
|H1|--| +---+ ODL +---+ |--|H2|
+--+ |--|RT1|-------|RT2|--| +--+
| +---+ +---+ |
+ | \ / | +
| \ / |
| \ / |
|ODL / |ODL
| / \ODL |
| / \ |
+ | /ODL \ | +
+--+ | +---+ +---+ | +--+
|H4|--|--|RT4|-------|RT3|--|--|H3|
+--+ | +---+ ODL +---+ | +--+
+ +

Figure 5: Example of an oversubscribed
internetwork

+---+ +---+ +---+ +---+
|RT1|-------|RT2| |RT1| |RT2|
+---+ +---+ +---+ +---+
| | | \
| | | \
| | | \
| | | \
| | | \
| | | \
| | | \
+---+ +---+ +---+ +---+
|RT4|-------|RT3| |RT4|-------|RT3|
+---+ +---+ +---+ +---+

Figure 5a: One possible Figure 5b: Another possible
pattern of data-link pattern of data-link
connections connections

active at once, problems can occur which are not addressed in this
memo:

(1) There is a network design problem. Does a subset of demand
circuits exist such that a) their data-link connections can be
open simultaneously and b) they can provide connectivity for all
possible sources? This requires that (at least) a spanning tree
be formed out of established connections. Figure 4 shows an
example where this is not possible; Hosts H1 through H4 cannot
simultaneously talk, since Router RT1 is limited to two
simultaneously open demand circuits.

(2) Even if it is possible that a spanning tree can form, will one?
Given the model in Section 1, demand circuits are brought up
when needed for data traffic, and stay established as long as
data traffic is present. One example is shown in Figure 5. Four
routers are interconnected via demand circuits, with each router
being able to establish a circuit to any other. However, we
assume that each router can only have two circuits open at once
(e.g., the routers could be using Basic Rate ISDN). In this
case, one would hope that the data-link connections in Figure 5a
would form. But the connections in Figure 5b are equally
likely, which leave Host H2 unable to communicate.

One possible approach to this problem would be for a) the OSPF
database to indicate which demand circuits have actually been
established and b) implement a distributed spanning tree
construction (see for example Chapter 5.2.2 of [9]) when
necessary.

(3) Even when a spanning tree has been built, will it be used?
Routers implementing the functionality described in this memo do
not necessarily know which data-link connections are established
at any one time. In fact, they view all demand circuits as being
equally available, whether or not they are established. So for
example, even when the established connections form the pattern
in Figure 5a, Router RT1 may still believe that the best path to
Router RT3 is through the direct demand circuit. However, this
circuit cannot be established due to resource shortages.

On possible approach to this problem is to increase the OSPF
cost of demand circuits that are currently discarding
application packets (i.e., can't be established) due to resource
shortages. This may help the routing find paths that can
actually deliver the packets. On the downside, it would create
more routing traffic. Also, unwanted routing oscillations may
result when you start varying routing metrics to reflect dynamic
network conditions; see [10].

A. The Format of the OSPF Options field

The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all LSAs. The Options field enables OSPF
routers to support (or not support) optional capabilities, and to
communicate their capability level to other OSPF routers. Through
this mechanism routers of differing capabilities can be mixed within
an OSPF routing domain.

The memo defines one of the Option bits: the DC-bit (for Demand
Circuit capability). The DC-bit is set in a router's router-LSA self-originated
LSAs if and only if it supports the functionality defined in Section 3
2 of this memo. Note that this does not necessarily mean that the
router can be the endpoint of a demand circuit, but only that it can
properly process LSAs having the DoNotAge bit set. In contrast, the
DC-bit is set in Hello Packets and Database Description Packets sent
out an interface if and only if the router wants to treat the
attached point-to-point network as a demand circuit (see Section 3.1).
3.2.1).

The addition of the DC-bit makes the current assignment of the OSPF
Options field as follows:

+------------------------------------+
| * | * | DC | EA | N/P | MC | E | T |
+------------------------------------+

Figure 5: The OSPF Options field

T-bit
This bit describes TOS-based routing capability, as specified in
[1].

E-bit
This bit describes the way AS-external-LSAs are flooded, as
described in [1].

MC-bit
This bit describes whether IP multicast datagrams are forwarded
according to the specifications in [4].

N/P-bit
This bit describes the handling of Type-7 LSAs, as specified in
[3].

EA-bit
This bit describes the router's willingness to receive and
forward External Attributes LSAs, as specified in [5].

DC-bit
This bit describes the handling of demand circuits, as specified
in this memo. Its setting in Hellos and Database Description
Packets is described in Section 3.1. Sections 3.2.1 and 3.2.2. Its setting in
LSAs is described in Section 2.3. Sections 2.1 and 2.5.

B. Configurable Parameters

This memo defines a single additional configuration parameter for
OSPF interfaces. In addition, the OSPF Interface configuration
parameter PollInterval, previously used only on NBMA networks, is
now also used on point-to-point networks (see Section 3.3). Sections 3.1 and
3.2.2).

DemandInterface
Indicates whether the interface connects to a demand circuit.
When set to TRUE, the procedures described in Section 3 of this
memo are followed, in order to send a minimum of routing traffic
over the demand circuit. On point-to-point networks, this allows
the circuit to be closed when not carrying application traffic.
When the demand interface is configured to be a broadcast or NBMA network is configured to be a demand
interface (see Section 1.2 of [1]), the circuit will be kept
open constantly due to OSPF Hello traffic, but the amount of
flooding traffic will still be greatly reduced.

C. Architectural Constants

This memo defines a single additional OSPF architectural constant.

DoNotAge
Equal to the hexadecimal value 0x8000, or which is the high bit of
the 16-bit LS Age field in OSPF LSAs. When this bit is set in
the LS age field, the LSA is not aged as it is held in the
router's link state database. This allows the elimination of the
periodic LSA refresh over demand circuits. See Section 2.1 2.2 for
more information on processing the DoNotAge bit.

References

[1] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994.

[2] Meyer, G., "Extensions to RIP to Support Demand Circuits", RFC
1582, Spider Systems, February 1994.

[3] Coltun, R. and V. Fuller, "The OSPF NSSA Option", RFC 1587,
RainbowBridge Communications, Stanford University, March 1994.

[4] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon,
Inc., March 1994.

[5] Ferguson, D., "The OSPF External Attributes LSA", Internet
Draft, draft-ietf-ospf-extattr-00.txt, March 1993. work in
progress.

[6] Moy, J., editor, "OSPF protocol analysis", RFC 1245, Proteon,
Inc., July 1991.

[7] Baker F. and R. Coltun, "OSPF Version 2 Management Information
Base", RFC 1253, ACC, University of Maryland, August 1991.

[8] Baker F., "OSPF Point-to-MultiPoint Interface", Internet Draft,
draft-ietf-ospf-pmp-if-00.txt, ACC, March 1994. work in
progress.

[9] Bertsekas, D. and Gallager R., "Data Networks", Prentice Hall,
Inc., 1992.

[10] Khanna, A., "Short-Term Modifications to Routing and Congestion
Control", BBN Report 6714, BBN, February 1988.

Security Considerations

Security issues are not discussed in this memo.

Author's Address

John Moy
Proteon, Inc.
9 Technology Drive
Westborough,
Cascade Communications Corp.
5 Carlisle Road
Westford, MA 01581 01886

Phone: 508-898-2800 508-692-2600 Ext. 394
Fax: 508-898-3176 508-692-9214
Email: jmoy@proteon.com jmoy@casc.com

This document expires in November 1994. May 1995.