path: root/doc/rfc/rfc1793.txt

Network Working Group                                             J. Moy
Request for Comments: 1793                                       Cascade
Category: Standards Track                                     April 1995


               Extending OSPF to Support Demand Circuits

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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 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 remain
   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-



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   to-MultiPoint Interface).

   This memo provides functionality similar to that specified for RIP in
   [2], with the main difference being the way the two proposals handle
   oversubscription (see Sections 4.3 and 7 below).  However, because
   OSPF employs link-state routing technology as opposed to RIP's
   Distance Vector base, the mechanisms used to achieve the demand
   circuit 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.

Table of Contents

    1.      Model for demand circuits .............................. 3
    2.      Modifications to all OSPF routers ...................... 4
    2.1     The OSPF Options field ................................. 4
    2.2     The LS age field ....................................... 5
    2.3     Removing stale DoNotAge LSAs ........................... 6
    2.4     A change to the flooding algorithm ..................... 6
    2.5     Interoperability with unmodified OSPF routers .......... 7
    2.5.1   Indicating across area boundaries ...................... 8
    2.5.1.1 Limiting indication-LSA origination .................... 9
    3.      Modifications to demand circuit endpoints ............. 10
    3.1     Interface State machine modifications ................. 10
    3.2     Sending and Receiving OSPF Hellos ..................... 11
    3.2.1   Negotiating Hello suppression ......................... 11
    3.2.2   Neighbor state machine modifications .................. 12
    3.3     Flooding over demand circuits ......................... 12
    3.4     Virtual link support .................................. 13
    3.5     Point-to-MultiPoint Interface support ................. 14
    4.      Examples .............................................. 15
    4.1     Example 1: Sole connectivity through demand circuits .. 15
    4.2     Example 2: Demand and non-demand circuits in parallel . 19
    4.3     Example 3: Operation when oversubscribed .............. 23
    5.      Topology recommendations .............................. 25
    6.      Lost functionality .................................... 25
    7.      Future work: Oversubscription ......................... 26
    8.      Unsupported capabilities .............................. 28
    A.      Format of the OSPF Options field ...................... 30
    B.      Configurable Parameters ............................... 31
    C.      Architectural Constants ............................... 31
            References ............................................ 32



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            Security Considerations ............................... 32
            Author's Address ...................................... 32

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.





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   Either end of a demand circuit may attempt to open the data-link
   connection. When both ends attempt to open the connection
   simultaneously, there is the possibility of call collision. Not all
   data-links specify how call collisions are handled. Also, while OSPF
   requires that all periodic timers be randomized to avoid
   synchronization (see Section 4.4 of [1]), if call attempts are
   strictly data-driven there may still be insufficient spacing of call
   attempts to avoid collisions on some data-links. For these reasons,
   for those data-links without collision detection/avoidance support,
   it is suggested (but not specified herein) that an exponential
   backoff scheme for call retries be employed at the data-link layer.
   Besides helping with call collisions, such a scheme could minimize
   charges (if they exist) for failed call attempts.

   As a result of the physical implementation of some demand circuits,
   only one end of the circuit may be capable of opening the data-link
   connection. For example, some async modems can initiate calls, but
   cannot accept incoming calls. In these cases, since connection
   initiation in this memo is data-driven, care must be taken to ensure
   that the initiating application party is located at the calling end
   of the demand circuit.

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



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      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 of the LS age field 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 bit set as "DoNotAge+x", while an LS
      age expressed as just "x" is assumed 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 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



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      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 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 process the LSA as
      follows (modifying Step 8 of Section 13 in [1] accordingly):





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        o   If the database copy has LS age equal to MaxAge and LS
            sequence number equal to MaxSequenceNumber, simply discard
            the received LSA without acknowledging it. (In this case,
            the LSA's sequence number is wrapping, and the
            MaxSequenceNumber LSA must be completely flushed before any
            new LSAs can be introduced). This is identical to the
            behavior specified by Step 8 of Section 13 in [1].

        o   Otherwise, send the database copy back to the sending
            neighbor, encapsulated within a Link State Update Packet. In
            so doing, do not put the database copy of the LSA on the
            neighbor's link state retransmission list, and do not
            acknowledge the received (less recent) LSA instance.

      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 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



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      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.

      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



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         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:

            (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



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            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.

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, ospfIfDemand,
   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).





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      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, 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.



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         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 longer available,
         presumably from something like a discouraging 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



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            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.

        (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



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      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
            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 links, 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.




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      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 ospfIfDemand is set on a Point-to-MultiPoint interface,
            the router tries to negotiate Hello suppression separately
            on each of interface's 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:




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        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 that RTB receives from RTC may not have the
            DC-bit set. However, subsequent Hellos and Database
            Description 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.
            LSAs included in Link State Updates Packets sent over the


               +           +                             +
               |   +---+   |                             |
        +--+   |---|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.













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            +---+                                        +---+
            |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.

            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.


                                          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 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.





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        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.

        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.







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        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
            2.3 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 initially 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.













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      The following events may then transpire:


                                             +
                                      +---+  |
                                      |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.


        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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                                          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

      The following example shows the behavior of the demand circuit
      extensions in the presence of oversubscribed interfaces. Note that
      the example's topology excludes the possibility of alternative
      paths. The combination of oversubscription and redundant topology
      (i.e., alternative paths) poses special problems for the demand
      circuit extensions. These problems are discussed later in Section
      7.

      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 to a data-link
      switch (e.g., an X.25 or Frame relay switch) that is only capable
      of so many simultaneous data-link connections.

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









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        Time T0: Router RT1 comes up.

            Router RT1 attempts to establish neighbor connections and
            synchronize OSPF databases with routers RT2-RT4. But,


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


                     Figure 4: Example 3's internetwork.



            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 H2, the underlying data-link connection to RT2 will
            eventually close due to inactivity. This will allow RT1 to
            finally synchronize with RT4; the next Hello 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, H4 will have been considered unreachable by
            OSPF routing. However, data traffic would not have been
            deliverable to H4 until now in any case.







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        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 H2 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
   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.






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    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 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.





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RFC 1793               OSPF over Demand Circuits              April 1995


    (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 currently
        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.





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


                     Figure 5: Example of an oversubscribed
                                internetwork

















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              +---+       +---+              +---+       +---+
              |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

   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].

8.  Unsupported capabilities

   The following possible capabilities associated with demand circuit
   routing have explicitly not been supported by this memo:

    o   When the topology of an OSPF area changes, the changes are
        flooded over the area's demand circuits, even if this requires
        (re)establishing the demand circuits' data-link connections. One
        might imagine a routing system where the flooding of topology
        changes over demand circuits were delayed until the demand
        circuits were (re)opened for application traffic. However, this
        capability is unsupported because delaying the flooding in this
        manner would sometimes impair the ability to discover new
        network destinations.

    o   Refining the previous capability, one might imagine that the
        network administrator would be able to configure for each demand
        interface whether flooding should be immediate, or whether it
        should be delayed until the data-link connection is established
        for application traffic. This would allow certain "application-
        specific" routing behaviors. For example, a demand circuit may
        connect a collection of client-based subnets to a collection of



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        server-based subnets. If the client end was configured to delay
        flooding, while the server end was configured to flood changes
        immediately, then new servers would be discovered promptly while
        clients might not be discovered until they initiate
        conversations. However, this capability is unsupported because
        of the increased complexity of (and possibility for error in)
        the network configuration.












































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RFC 1793               OSPF over Demand Circuits              April 1995


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].





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    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 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).

    ospfIfDemand
        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 interface is configured to connect to a
        demand circuit (see Section 1.2 of [1]), the data-link
        connections 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 R. Gallager, "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







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