rfc1793.txt

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            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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RFC 1793               OSPF over Demand Circuits              April 1995


            +---+                                        +---+
            |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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RFC 1793               OSPF over Demand Circuits              April 1995


        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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RFC 1793               OSPF over Demand Circuits              April 1995


        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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RFC 1793               OSPF over Demand Circuits              April 1995


      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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RFC 1793               OSPF over Demand Circuits              April 1995


                                          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