target = "https://www.rfc-editor.org/rfc/rfc8899#section-4.6.2"

# 4.6.2.  Use of PTB Messages
#
# PTB messages that have been validated MAY be utilized by the DPLPMTUD
# algorithm but MUST NOT be used directly to set the PLPMTU.
# 
# Before using the size reported in the PTB message, it must first be
# converted to a PL_PTB_SIZE.  The PL_PTB_SIZE is smaller than the
# PTB_SIZE because it is reduced by headers below the PL, including any
# IP options or extensions added to the PL packet.
# 
# A method that utilizes these PTB messages can improve the speed at
# which the algorithm detects an appropriate PLPMTU by triggering an
# immediate probe for the PL_PTB_SIZE (resulting in a network-layer
# packet of size PTB_SIZE), compared to one that relies solely on
# probing using a timer-based search algorithm.
# 
# A set of checks are intended to provide protection from a router that
# reports an unexpected PTB_SIZE.  The PL also needs to check that the
# indicated PL_PTB_SIZE is less than the size used by probe packets and
# at least the minimum size accepted.
# 
# This section provides a summary of how PTB messages can be utilized,
# using the set of constants defined in Section 5.1.2.  This processing
# depends on the PL_PTB_SIZE and the current value of a set of
# variables:
# 
# PL_PTB_SIZE < MIN_PLPMTU
#    *  Invalid PL_PTB_SIZE, see Section 4.6.1.
# 
#    *  PTB message ought to be discarded without further processing
#       (i.e., PLPMTU is not modified).
# 
#    *  The information could be utilized as an input that triggers the
#       enabling of a resilience mode (see Section 5.3.3).
# 
# MIN_PLPMTU < PL_PTB_SIZE < BASE_PLPMTU
#    *  A robust PL MAY enter an error state (see Section 5.2) for an
#       IPv4 path when the PL_PTB_SIZE reported in the PTB message is
#       larger than or equal to 68 bytes [RFC0791] and when this is
#       less than the BASE_PLPMTU.
# 
#    *  A robust PL MAY enter an error state (see Section 5.2) for an
#       IPv6 path when the PL_PTB_SIZE reported in the PTB message is
#       larger than or equal to 1280 bytes [RFC8200] and when this is
#       less than the BASE_PLPMTU.
# 
# BASE_PLPMTU <= PL_PTB_SIZE < PLPMTU
#    *  This could be an indication of a black hole.  The PLPMTU SHOULD
#       be set to BASE_PLPMTU (the PLPMTU is reduced to the BASE_PLPMTU
#       to avoid unnecessary packet loss when a black hole is
#       encountered).
# 
#    *  The PL ought to start a search to quickly discover the new
#       PLPMTU.  The PL_PTB_SIZE reported in the PTB message can be
#       used to initialize a search algorithm.
# 
# PLPMTU < PL_PTB_SIZE < PROBED_SIZE
#    *  The PLPMTU continues to be valid, but the size of a packet used
#       to search (PROBED_SIZE) was larger than the actual PMTU.
# 
#    *  The PLPMTU is not updated.
# 
#    *  The PL can use the reported PL_PTB_SIZE from the PTB message as
#       the next search point when it resumes the search algorithm.
# 
# PL_PTB_SIZE >= PROBED_SIZE
#    *  Inconsistent network signal.
# 
#    *  PTB message ought to be discarded without further processing
#       (i.e., PLPMTU is not modified).
# 
#    *  The information could be utilized as an input to trigger the
#       enabling of a resilience mode.

[[spec]]
level = "MUST"
quote = '''
PTB messages that have been validated MAY be utilized by the DPLPMTUD
algorithm but MUST NOT be used directly to set the PLPMTU.
'''

[[spec]]
level = "MAY"
quote = '''
MIN_PLPMTU < PL_PTB_SIZE < BASE_PLPMTU
*  A robust PL MAY enter an error state (see Section 5.2) for an
IPv4 path when the PL_PTB_SIZE reported in the PTB message is
larger than or equal to 68 bytes [RFC0791] and when this is
less than the BASE_PLPMTU.
'''

[[spec]]
level = "MAY"
quote = '''
*  A robust PL MAY enter an error state (see Section 5.2) for an
IPv6 path when the PL_PTB_SIZE reported in the PTB message is
larger than or equal to 1280 bytes [RFC8200] and when this is
less than the BASE_PLPMTU.
'''

[[spec]]
level = "SHOULD"
quote = '''
The PLPMTU SHOULD
be set to BASE_PLPMTU (the PLPMTU is reduced to the BASE_PLPMTU
to avoid unnecessary packet loss when a black hole is
encountered).
'''