📄 rfc1793.txt
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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:
Moy [Page 15]
RFC 1793 OSPF over Demand Circuits April 1995
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.
Moy [Page 16]
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.
Moy [Page 17]
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.
Moy [Page 18]
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.
Moy [Page 19]
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.
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