target = "https://www.rfc-editor.org/rfc/rfc8899#section-5.2"

# 5.2.  State Machine
#
# A state machine for DPLPMTUD is depicted in Figure 5.  If multipath
# or multihoming is supported, a state machine is needed for each path.
# 
# Note: Not all changes are shown to simplify the diagram.
# 
#    |         |
#    | Start   | PL indicates loss
#    |         |  of connectivity
#    v         v
# +---------------+                                   +---------------+
# |    DISABLED   |                                   |     ERROR     |
# +---------------+               PROBE_TIMER expiry: +---------------+
#         | PL indicates     PROBE_COUNT = MAX_PROBES or    ^      |
#         | connectivity  PTB: PL_PTB_SIZE < BASE_PLPMTU    |      |
#         +--------------------+         +------------------+      |
#                              |         |                         |
#                              v         |       BASE_PLPMTU Probe |
#                           +---------------+          acked       |
#                           |      BASE     |--------------------->+
#                           +---------------+                      |
#                              ^ |    ^  ^                         |
#          Black hole detected | |    |  | Black hole detected     |
#         +--------------------+ |    |  +--------------------+    |
#         |                      +----+                       |    |
#         |                PROBE_TIMER expiry:                |    |
#         |             PROBE_COUNT < MAX_PROBES              |    |
#         |                                                   |    |
#         |               PMTU_RAISE_TIMER expiry             |    |
#         |    +-----------------------------------------+    |    |
#         |    |                                         |    |    |
#         |    |                                         v    |    v
# +---------------+                                   +---------------+
# |SEARCH_COMPLETE|                                   |   SEARCHING   |
# +---------------+                                   +---------------+
#    |    ^    ^                                         |    |    ^
#    |    |    |                                         |    |    |
#    |    |    +-----------------------------------------+    |    |
#    |    |            MAX_PLPMTU Probe acked or              |    |
#    |    |  PROBE_TIMER expiry: PROBE_COUNT = MAX_PROBES or  |    |
#    +----+            PTB: PL_PTB_SIZE = PLPMTU              +----+
# CONFIRMATION_TIMER expiry:                        PROBE_TIMER expiry:
# PROBE_COUNT < MAX_PROBES or               PROBE_COUNT < MAX_PROBES or
#      PLPMTU Probe acked                           Probe acked or PTB:
#                                    PLPMTU < PL_PTB_SIZE < PROBED_SIZE
# 
#              Figure 5: State Machine for Datagram PLPMTUD
# 
# The following states are defined:
# 
# DISABLED:  The DISABLED state is the initial state before probing has
#    started.  It is also entered from any other state, when the PL
#    indicates loss of connectivity.  This state is left once the PL
#    indicates connectivity to the remote PL.  When transitioning to
#    the BASE state, a probe packet of size BASE_PLPMTU can be sent
#    immediately.
# 
# BASE:  The BASE state is used to confirm that the BASE_PLPMTU size is
#    supported by the network path and is designed to allow an
#    application to continue working when there are transient
#    reductions in the actual PMTU.  It also seeks to avoid long
#    periods when a sender searching for a larger PLPMTU is unaware
#    that packets are not being delivered due to a packet or ICMP black
#    hole.
# 
#    On entry, the PROBED_SIZE is set to the BASE_PLPMTU size, and the
#    PROBE_COUNT is set to zero.
# 
#    Each time a probe packet is sent, the PROBE_TIMER is started.  The
#    state is exited when the probe packet is acknowledged, and the PL
#    sender enters the SEARCHING state.
# 
#    The state is also left when the PROBE_COUNT reaches MAX_PROBES or
#    a received PTB message is validated.  This causes the PL sender to
#    enter the ERROR state.
# 
# SEARCHING:  The SEARCHING state is the main probing state.  This
#    state is entered when probing for the BASE_PLPMTU completes.
# 
#    Each time a probe packet is acknowledged, the PROBE_COUNT is set
#    to zero, the PLPMTU is set to the PROBED_SIZE, and then the
#    PROBED_SIZE is increased using the search algorithm (as described
#    in Section 5.3).
# 
#    When a probe packet is sent and not acknowledged within the period
#    of the PROBE_TIMER, the PROBE_COUNT is incremented, and a new
#    probe packet is transmitted.
# 
#    The state is exited to enter SEARCH_COMPLETE when the PROBE_COUNT
#    reaches MAX_PROBES, a validated PTB is received that corresponds
#    to the last successfully probed size (PL_PTB_SIZE = PLPMTU), or a
#    probe of size MAX_PLPMTU is acknowledged (PLPMTU = MAX_PLPMTU).
# 
#    When a black hole is detected in the SEARCHING state, this causes
#    the PL sender to enter the BASE state.
# 
# SEARCH_COMPLETE:  The SEARCH_COMPLETE state indicates that a search
#    has completed.  This is the normal maintenance state, where the PL
#    is not probing to update the PLPMTU.  DPLPMTUD remains in this
#    state until either the PMTU_RAISE_TIMER expires or a black hole is
#    detected.
# 
#    When DPLPMTUD uses an unacknowledged PL and is in the
#    SEARCH_COMPLETE state, a CONFIRMATION_TIMER periodically resets
#    the PROBE_COUNT and schedules a probe packet with the size of the
#    PLPMTU.  If MAX_PROBES successive PLPMTUD-sized probes fail to be
#    acknowledged, the method enters the BASE state.  When used with an
#    acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to
#    generate PLPMTU probes in this state.
# 
# ERROR:  The ERROR state represents the case where either the network
#    path is not known to support a PLPMTU of at least the BASE_PLPMTU
#    size or when there is contradictory information about the network
#    path that would otherwise result in excessive variation in the MPS
#    signaled to the higher layer.  The state implements a method to
#    mitigate oscillation in the state-event engine.  It signals a
#    conservative value of the MPS to the higher layer by the PL.  The
#    state is exited when packet probes no longer detect the error.
#    The PL sender then enters the SEARCHING state.
# 
#    Implementations are permitted to enable endpoint fragmentation if
#    the DPLPMTUD is unable to validate MIN_PLPMTU within PROBE_COUNT
#    probes.  If DPLPMTUD is unable to validate MIN_PLPMTU, the
#    implementation will transition to the DISABLED state.
# 
#    Note: MIN_PLPMTU could be identical to BASE_PLPMTU, simplifying
#    the actions in this state.

[[spec]]
level = "SHOULD"
quote = '''
When used with an
acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to
generate PLPMTU probes in this state.
'''