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Versions: 00 01 RFC 1793

Network Working Group                                             J. Moy
Internet Draft                                                   Cascade
Expiration Date: May 1995                                  November 1994
File name: 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,
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    Rim).

Abstract

    This memo defines enhancements to the OSPF protocol that allow
    efficient operation over "demand circuits". Demand circuits are
    network segments whose costs vary with usage; charges can be based
    both on connect time and on bytes/packets transmitted. Examples of
    demand circuits include ISDN circuits, X.25 SVCs, and dial-up lines.
    The periodic nature of OSPF routing traffic has until now required a
    demand circuit's underlying data-link connection to be constantly
    open, resulting in unwanted usage charges. With the modifications
    described herein, OSPF Hellos and the refresh of OSPF routing
    information are suppressed on demand 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



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    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 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 employs link-state routing
    technology as opposed to RIP's Distance Vector base, the mechanisms
    used to achieve 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, Tom Pusateri and Zhaohui
    Zhang. This memo is a product of the OSPF Working Group.
























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Table of Contents

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



















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1.  Model for demand circuits

    In this memo, demand circuits refer to those network segments whose
    cost depends on either connect time 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). When/if the data-link connection is established, the data
    is sent, and the 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 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 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 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



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    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 (see Section 3), there are
    changes required of all OSPF routers. These changes are described in
    the subsections below.

    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 the functionality described in Section 2
        of this memo sets the DC-bit in the Options field of all LSAs
        that it originates. This is regardless of the LSAs' LS type, and
        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 used to convey across area boundaries the
        existence of routers incapable of DoNotAge processing; see
        Section 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 to be set. 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 to not have the DoNotAge bit



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        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 must
        have their DoNotAge bits masked off.  For example, in
        determining which LSA is more recent, LS ages of 1 and
        DoNotAge+1 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. In particular,
        DoNotAge+MaxAge is equivalent to MaxAge; however for backward-
        compatibility the MaxAge form should always be used 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 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. 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.

        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.

    2.3.  Removing 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 a DoNotAge LSA if both of the following conditions are



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        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
            at least MaxAge seconds.

        For an example, see Time T8 in the example of Section 4.1. 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 LSA only to have it
        reoriginated soon afterwards). 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



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        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 DoNotAge LSAs as if
        they had LS age of MaxAge. At the very worst, this will cause
        continual retransmissions of the DoNotAge 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, DoNotAge LSAs will be allowed
        in an OSPF area if and only if, in the area's link state
        database, all LSAs have the DC-bit set in their Options field
        (see Section 2.1). Note that it is not required that the LSAs'
        Advertising Router 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 DoNotAge 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. 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.







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        2.5.1.  Indicating across area boundaries

            AS-external-LSAs are flooded throughout the entire OSPF
            routing domain, excepting only OSPF stub areas and NSSAs.
            For that 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 DoNotAge set. This memo simplifies that requirement by
            broadening it to the following rule: LSAs in regular OSPF
            areas are allowed to have DoNotAge set if and only if every
            router in the OSPF domain (excepting those in stub areas and
            NSSAs) 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 by
            setting the DC-bit in the LSAs that it originates. However,
            there is a problem. It is possible that, in all areas to
            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 router itself as the described ASBR, with
            the LSA's cost set to LSInfinity and the DC-bit clear. Note
            that indication-LSAs convey no additional information; in
            particular, they 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, in the area's link state database, all LSAs have the
            DC-bit set in their Options field.

            Through origination of indication-LSAs, the existence of
            DoNotAge-incapable routers can be viewed as going from non-
            backbone regular areas, to 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:




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            (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 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 indication-LSA into Area A, and
                that 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, two indication-LSAs will be generated (if the
                rules of Section 2.5.1 and 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.







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3.  Modifications to demand circuit endpoints

    The following subsections detail the modifications required of the
    routers at the endpoints of demand circuits. These consist of
    modifications to two main pieces of OSPF: 1) sending and receiving
    Hello Packets over demand circuits and 2) flooding LSAs over 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). Two routers
    connecting to a common network segment need not agree on that
    segment's demand circuit status. However, to get full benefit of the
    demand circuit extensions, the two ends of a point-to-point link
    must both agree to treat the 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'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.




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        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, a router sets the DC-bit in OSPF Hellos and
            Database Description 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
            suppress Hellos. In these latter cases, the router reverts
            to the 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 a router
            implementing Sections 2 and 3 of this memo receives an Hello
            Packet with the DC-bit set, it should treat the point-to-
            point link as a 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 should
            continue setting the 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 and comes back up.
            For example, if the neighboring router is rebooted with
            software that is capable of operating over demand circuits
            (i.e., implements Sections 2 and 3 of this memo), a future
            negotiation will succeed.

            Also, even if the negotiation to suppress Hellos fails, the
            flooding modifications described in Section 3.3 are still
            performed over the link.

        3.2.2.  Neighbor state machine modifications

            When the above negotiation succeeds, Hello 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



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            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 "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 "Full".  An
            additional clarification is needed in the Neighbor State
            Machine's LLDown event. For demand circuits, this event
            should be mapped into the "discouraging diagnostic code"
            discussed previously in Section 1, and should not be
            generated when the data-link connection has been closed
            simply to save resources. Nor should LLDown be generated if
            a data-link connection fails due to temporary lack of
            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 database must have the DC-
        bit set.  If one or more LSAs have the DC-bit clear, flooding
        over demand circuits is unchanged from [1].  Otherwise, flooding
        is changed as follows.

        (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).




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            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.

        (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 is
            equal to MaxAge.) Setting DoNotAge will cause the routers on
            the other side of the demand circuit to hold the LSA in
            their 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 (see Step 5 of Section 13.3 in [1], and
            Section 2.2 of this memo), as protection against flooding
            loops.

            The previous paragraph also pertains to LSAs flooded over
            demand circuits in response to Link State Requests. 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
        support for virtual links resembles that described for point-
        to-point links in the previous sections. The main difference is
        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 underlying demand circuits open.

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

        o   Instead of modifying the Interface state machine for virtual
            links as was done for point-to-point links in Section 3.1,
            the 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 virtual
            link's transit area) exists to the other endpoint. Otherwise



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            it is 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 demand circuit support over virtual links, there is
            no "discouraging diagnostic code" as described in Section 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 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
        multiple point-to-point OSPF conversations (one on each PVC) are
        taking place. For more information on the 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 Point-to-Multipoint interface strongly resembles demand
        circuit support for point-to-point links. However, since the
        Point-to-MultiPoint interface requires commonality of 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.





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        o   When a Point-to-MultiPoint interface is configured as a
            DemandInterface, it tries to negotiate Hello suppression
            separately on each of its component point-to-point links.
            This negotiation proceeds as in Section 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 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:

        Time T0: RTB negotiates 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 will not have the DC-bit
            set. However, subsequent Hellos and Database Description



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               +           +                             +
               |   +---+   |                             |
        +--+   |---|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 suppress Hellos on the link, the initial
            Database Description sent from Router RTC to RTB would have
            the DC-bit clear, forcing Router RTB to revert to the
            periodic sending of 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.

            +---+                                        +---+
            |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 of Hello
                              suppression.





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            LSAs included in Link 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 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






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        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



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            it has the DoNotAge bit set. However, according to Section
            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 their database copies
            of RTA's and 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.





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

                       Figure 3: 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.
























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                                          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 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 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 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 in Example 2, LS age
                fields on the right side of Router RTB.






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        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 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. 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 aged out
            and reflooded (by router RTE in our example).  Because this
            aged out LSA constitutes a real change (see Section 3.3), it
            is flooded over the demand circuit from Router RTC to RTB.
            There are then two possible scenarios. First, the LS
            Sequence number for RTA's router-LSA may be larger on 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 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 LSA.



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            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 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 3.2.2).

    4.3.  Example 3: Operation when 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:
        Router RT1 may be using a single Basic Rate ISDN interface (2 B
        channels) to support all three demand 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



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                                                 +  +--+
                                          +---+  |--|H3|
                                +---------|RT2|--|  +--+
                               /          +---+  |
                              / ODL              +
                +--+  +      /
                |H1|--|     /                    +
                +--+  |  +---+    ODL     +---+  |  +--+
                      |--|RT1|------------|RT3|--|--|H4|
                      |  +---+            +---+  |  +--+
                      |      \                   +
                      +       \ODL
                               \                 +  +--+
                                \         +---+  |--|H2|
                                 +--------|RT4|--|  +--+
                                          +---+  |
                                                 +


                     Figure 4: 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 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
            Section 1). This means that the routers RT1, RT3 and RT4
            will not detect the unreachability of Host H3 until a data-
            link connection on RT1 becomes available.

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



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    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 2.5), 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



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                     +--+  +                     +  +--+
                     |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








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    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].



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    A. 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 self-originated
    LSAs if and only if it supports the functionality defined in Section
    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
    3.2.1).

    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



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        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 Sections 3.2.1 and 3.2.2. Its setting in
        LSAs is described in 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 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 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, 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 2.2 for
        more information on processing the DoNotAge bit.













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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", 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", 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
    Cascade Communications Corp.
    5 Carlisle Road
    Westford, MA 01886

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

    This document expires in May 1995.




Moy                                                            [Page 31]


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