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10.3. The Neighbor state machine Connected: An Internet Encyclopedia
10.3. The Neighbor state machine

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Up: RFC 1583
Up: 10. The Neighbor Data Structure
Prev: 10.2. Events causing neighbor state changes
Next: 10.4. Whether to become adjacent

10.3. The Neighbor state machine

10.3. The Neighbor state machine

A detailed description of the neighbor state changes follows. Each state change is invoked by an event (Section 10.2). This event may produce different effects, depending on the current state of the neighbor. For this reason, the state machine below is organized by current neighbor state and received event. Each entry in the state machine describes the resulting new neighbor state and the required set of additional actions.

When a neighbor's state changes, it may be necessary to rerun the Designated Router election algorithm. This is determined by whether the interface NeighborChange event is generated (see Section 9.2). Also, if the Interface is in DR state (the router is itself Designated Router), changes in neighbor state may cause a new network links advertisement to be originated (see Section 12.4).

When the neighbor state machine needs to invoke the interface state machine, it should be done as a scheduled task (see Section 4.4). This simplifies things, by ensuring that neither state machine will be executed recursively.

         State(s):  Down

            Event:  Start

        New state:  Attempt

           Action:  Send an Hello Packet to the neighbor (this neighbor
                    is always associated with a non-broadcast network)
                    and start the Inactivity Timer for the neighbor.
                    The timer's later firing would indicate that
                    communication with the neighbor was not attained.

         State(s):  Attempt

            Event:  HelloReceived

        New state:  Init

           Action:  Restart the Inactivity Timer for the neighbor, since
                    the neighbor has now been heard from.

         State(s):  Down

            Event:  HelloReceived

        New state:  Init

           Action:  Start the Inactivity Timer for the neighbor.  The
                    timer's later firing would indicate that the
                    neighbor is dead.

         State(s):  Init or greater

            Event:  HelloReceived

        New state:  No state change.

           Action:  Restart the Inactivity Timer for the neighbor, since
                    the neighbor has again been heard from.

         State(s):  Init

            Event:  2-WayReceived

        New state:  Depends upon action routine.

           Action:  Determine whether an adjacency should be established
                    with the neighbor (see Section 10.4).  If not, the
                    new neighbor state is 2-Way.

                    Otherwise (an adjacency should be established) the
                    neighbor state transitions to ExStart.  Upon
                    entering this state, the router increments the DD
                    sequence number for this neighbor.  If this is the
                    first time that an adjacency has been attempted, the
                    DD sequence number should be assigned some unique
                    value (like the time of day clock).  It then
                    declares itself master (sets the master/slave bit to
                    master), and starts sending Database Description
                    Packets, with the initialize (I), more (M) and
                    master (MS) bits set.  This Database Description
                    Packet should be otherwise empty.  This Database
                    Description Packet should be retransmitted at
                    intervals of RxmtInterval until the next state is
                    entered (see Section 10.8).

         State(s):  ExStart

            Event:  NegotiationDone

        New state:  Exchange

           Action:  The router must list the contents of its entire area
                    link state database in the neighbor Database summary
                    list.  The area link state database consists of the
                    router links, network links and summary links
                    contained in the area structure, along with the AS
                    external links contained in the global structure.
                    AS external link advertisements are omitted from a
                    virtual neighbor's Database summary list.  AS
                    external advertisements are omitted from the
                    Database summary list if the area has been
                    configured as a stub (see Section 3.6).
                    Advertisements whose age is equal to MaxAge are
                    instead added to the neighbor's Link state
                    retransmission list.  A summary of the Database
                    summary list will be sent to the neighbor in
                    Database Description packets.  Each Database
                    Description Packet has a DD sequence number, and is
                    explicitly acknowledged.  Only one Database
                    Description Packet is allowed outstanding at any one
                    time.  For more detail on the sending and receiving
                    of Database Description packets, see Sections 10.8
                    and 10.6.

         State(s):  Exchange

            Event:  ExchangeDone

        New state:  Depends upon action routine.

           Action:  If the neighbor Link state request list is empty,
                    the new neighbor state is Full.  No other action is
                    required.  This is an adjacency's final state.

                    Otherwise, the new neighbor state is Loading.  Start
                    (or continue) sending Link State Request packets to
                    the neighbor (see Section 10.9).  These are requests
                    for the neighbor's more recent advertisements (which
                    were discovered but not yet received in the Exchange
                    state).  These advertisements are listed in the Link
                    state request list associated with the neighbor.

         State(s):  Loading

            Event:  Loading Done

        New state:  Full

           Action:  No action required.  This is an adjacency's final
                    state.

         State(s):  2-Way

            Event:  AdjOK?

        New state:  Depends upon action routine.

           Action:  Determine whether an adjacency should be formed with
                    the neighboring router (see Section 10.4).  If not,
                    the neighbor state remains at 2-Way.  Otherwise,
                    transition the neighbor state to ExStart and perform
                    the actions associated with the above state machine
                    entry for state Init and event 2-WayReceived.

         State(s):  ExStart or greater

            Event:  AdjOK?

        New state:  Depends upon action routine.

           Action:  Determine whether the neighboring router should
                    still be adjacent.  If yes, there is no state change
                    and no further action is necessary.

                    Otherwise, the (possibly partially formed) adjacency
                    must be destroyed.  The neighbor state transitions
                    to 2-Way.  The Link state retransmission list,
                    Database summary list and Link state request list
                    are cleared of link state advertisements.
         State(s):  Exchange or greater

            Event:  SeqNumberMismatch

        New state:  ExStart

           Action:  The (possibly partially formed) adjacency is torn
                    down, and then an attempt is made at
                    reestablishment.  The neighbor state first
                    transitions to ExStart.  The Link state
                    retransmission list, Database summary list and Link
                    state request list are cleared of link state
                    advertisements.  Then the router increments the DD
                    sequence number for this neighbor, declares itself
                    master (sets the master/slave bit to master), and
                    starts sending Database Description Packets, with
                    the initialize (I), more (M) and master (MS) bits
                    set.  This Database Description Packet should be
                    otherwise empty (see Section 10.8).

         State(s):  Exchange or greater

            Event:  BadLSReq

        New state:  ExStart

           Action:  The action for event BadLSReq is exactly the same as
                    for the neighbor event SeqNumberMismatch.  The
                    (possibly partially formed) adjacency is torn down,
                    and then an attempt is made at reestablishment.  For
                    more information, see the neighbor state machine
                    entry that is invoked when event SeqNumberMismatch
                    is generated in state Exchange or greater.

         State(s):  Any state

            Event:  KillNbr

        New state:  Down

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of link
                    state advertisements.  Also, the Inactivity Timer is
                    disabled.

         State(s):  Any state

            Event:  LLDown

        New state:  Down

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of link
                    state advertisements.  Also, the Inactivity Timer is
                    disabled.

         State(s):  Any state

            Event:  InactivityTimer

        New state:  Down

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of link
                    state advertisements.

         State(s):  2-Way or greater

            Event:  1-WayReceived

        New state:  Init

           Action:  The Link state retransmission list, Database summary
                    list and Link state request list are cleared of link
                    state advertisements.

         State(s):  2-Way or greater

            Event:  2-WayReceived

        New state:  No state change.

           Action:  No action required.

         State(s):  Init

            Event:  1-WayReceived

        New state:  No state change.

           Action:  No action required.


Next: 10.4. Whether to become adjacent

Connected: An Internet Encyclopedia
10.3. The Neighbor state machine

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