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Appendix C. State Machine for the Reachability State

This appendix provides a state machine diagram and detailed explanation of the Neighbor Cache reachability states defined in this specification.

C.1. Overview​

The Neighbor Cache entry for a neighbor transitions through several states as the node learns about and verifies the neighbor's reachability. These states are:

  • INCOMPLETE: Address resolution is in progress
  • REACHABLE: Forward and return paths are working
  • STALE: Information is outdated but usable
  • DELAY: Waiting to verify reachability
  • PROBE: Actively probing for reachability

C.2. State Transition Diagram​

                    INCOMPLETE
                         |
                         | Address resolved
                         v
                    REACHABLE
                         |
                         | Reachability timeout
                         v
                      STALE
                         |
                         | Traffic sent
                         v
                      DELAY
                         |
                         | Probe timer expires
                         v
                      PROBE
                         |
            +------------+------------+
            |                         |
    Reachability confirmed      Max retries reached
            |                         |
            v                         v
        REACHABLE                  (deleted)

C.3. State Descriptions​

C.3.1. INCOMPLETE State​

Entry Conditions:

  • A Neighbor Cache entry is created when address resolution is needed
  • No link-layer address is known yet

Actions:

  • Send multicast Neighbor Solicitation
  • Start retransmit timer
  • Queue packets pending address resolution

Exit Conditions:

  • To REACHABLE: Neighbor Advertisement received with link-layer address
  • To (deleted): Max retransmissions reached without response

Timers:

  • Retransmit timer: RETRANS_TIMER (typically 1000ms)
  • Max retries: MAX_MULTICAST_SOLICIT (typically 3)

C.3.2. REACHABLE State​

Entry Conditions:

  • Address resolution successfully completed, OR
  • Reachability confirmation received (solicited Neighbor Advertisement, upper-layer confirmation)

Actions:

  • Normal packet forwarding
  • No probing needed
  • Traffic flows normally

Exit Conditions:

  • To STALE: ReachableTime expires without reachability confirmation

Timers:

  • ReachableTime: Randomized value based on BaseReachableTime (typically 15-45 seconds)

C.3.3. STALE State​

Entry Conditions:

  • ReachableTime expired in REACHABLE state, OR
  • Unsolicited Neighbor Advertisement received updating link-layer address, OR
  • Entry created by received Neighbor Discovery message (not solicited NA)

Actions:

  • Use cached link-layer address
  • Do not send probes
  • Consider information as potentially out-of-date

Exit Conditions:

  • To DELAY: Upper-layer protocols send traffic to this neighbor
  • To REACHABLE: Solicited Neighbor Advertisement received confirming reachability

Timers:

  • None (can remain in STALE indefinitely)

C.3.4. DELAY State​

Entry Conditions:

  • Traffic sent to a neighbor in STALE state

Actions:

  • Wait for upper-layer reachability confirmation
  • Do not send probes yet (give upper layers time to confirm)
  • Continue using cached link-layer address

Exit Conditions:

  • To REACHABLE: Reachability confirmed by upper-layer protocol
  • To PROBE: Delay timer expires without reachability confirmation

Timers:

  • Delay timer: DELAY_FIRST_PROBE_TIME (typically 5 seconds)

C.3.5. PROBE State​

Entry Conditions:

  • Delay timer expired in DELAY state without reachability confirmation

Actions:

  • Send unicast Neighbor Solicitation probes
  • Continue using cached link-layer address
  • Retransmit probes if no response

Exit Conditions:

  • To REACHABLE: Solicited Neighbor Advertisement received
  • To (deleted): Max unicast solicitations sent without response

Timers:

  • Retransmit timer: RETRANS_TIMER (typically 1000ms)
  • Max retries: MAX_UNICAST_SOLICIT (typically 3)

C.4. Events and Triggers​

C.4.1. Reachability Confirmations​

Reachability is confirmed by:

  1. Receipt of a solicited Neighbor Advertisement
  2. Upper-layer protocol indication (e.g., TCP ACK)
  3. Receipt of an Neighbor Discovery message with the Solicited flag set

When a Neighbor Advertisement updates the link-layer address:

  • If Override flag is set: Update address, move to STALE
  • If Override flag is clear and addresses differ: Move to STALE, do not update
  • If Override flag is clear and addresses match: No state change

C.4.3. Timeout Events​

  • ReachableTime expiry in REACHABLE: Move to STALE
  • DELAY_FIRST_PROBE_TIME expiry in DELAY: Move to PROBE
  • Retransmit timer in INCOMPLETE/PROBE: Retransmit or delete

C.5. Implementation Considerations​

C.5.1. State Tracking​

Implementations SHOULD track:

  • Current state
  • Link-layer address (if known)
  • IsRouter flag
  • Pending packets queue (for INCOMPLETE)
  • Retransmission count
  • Applicable timers

C.5.2. Upper-Layer Hints​

Upper-layer protocols can provide hints about reachability:

  • Positive hints: Connection establishment, data acknowledgment
  • Negative hints: Connection failures, timeouts

These hints can trigger state transitions or expedite probing.

C.5.3. Performance Optimizations​

Optimistic DAD (RFC 4429):

  • Allow address use before DAD completes
  • Transition to REACHABLE faster in some scenarios

Fast Recovery:

  • Prioritize reachability checks for actively used neighbors
  • Use parallel probing for critical neighbors

C.5.4. Security Considerations​

State transitions should be protected:

  • Validate source addresses of Neighbor Discovery messages
  • Use SEND (RFC 3971) for cryptographic protection
  • Rate-limit state transitions to prevent DoS attacks

C.6. Example Scenarios​

C.6.1. Successful Address Resolution​

  1. Create entry in INCOMPLETE state
  2. Send Neighbor Solicitation
  3. Receive Neighbor Advertisement → REACHABLE
  4. After ReachableTime → STALE
  5. Send traffic → DELAY
  6. Upper-layer confirms reachability → REACHABLE

C.6.2. Neighbor Becomes Unreachable​

  1. Entry in REACHABLE state
  2. ReachableTime expires → STALE
  3. Send traffic → DELAY
  4. No reachability confirmation → PROBE
  5. Send unicast probes (×3)
  6. No response → Delete entry
  1. Entry in REACHABLE/STALE state
  2. Receive unsolicited NA with new address, Override=1
  3. Update address → STALE
  4. Future traffic triggers verification

Note: This state machine is designed to balance prompt detection of unreachable neighbors with minimal overhead for stable, reachable neighbors.

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