target = "https://www.rfc-editor.org/rfc/rfc8899#section-2"

# 2.  Terminology
#
# The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
# "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
# "OPTIONAL" in this document are to be interpreted as described in
# BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
# capitals, as shown here.
# 
# The following terminology is defined.  Relevant terms are directly
# copied from [RFC4821], and the definitions in [RFC1122] apply.
# 
# Acknowledged PL:  A PL that includes a mechanism that can confirm
#    successful delivery of datagrams to the remote PL endpoint (e.g.,
#    SCTP).  Typically, the PL receiver returns acknowledgments
#    corresponding to the received datagrams, which can be utilized to
#    detect black-holing of packets (c.f., Unacknowledged PL).
# 
# Actual PMTU:  The actual PMTU is the PMTU of a network path between a
#    sender PL and a destination PL, which the DPLPMTUD algorithm seeks
#    to determine.
# 
# Black Hole:  A black hole is encountered when a sender is unaware
#    that packets are not being delivered to the destination endpoint.
#    Two types of black hole are relevant to DPLPMTUD:
# 
#    *  Packets encounter a packet black hole when packets are not
#       delivered to the destination endpoint (e.g., when the sender
#       transmits packets of a particular size with a previously known
#       effective PMTU, and they are discarded by the network).
# 
#    *  An ICMP black hole is encountered when the sender is unaware
#       that packets are not delivered to the destination endpoint
#       because PTB messages are not received by the originating PL
#       sender.
# 
# Classical Path MTU Discovery:  Classical PMTUD is a process described
#    in [RFC1191] and [RFC8201] in which nodes rely on PTB messages to
#    learn the largest size of unfragmented packet that can be used
#    across a network path.
# 
# Datagram:  A datagram is a transport-layer protocol data unit,
#    transmitted in the payload of an IP packet.
# 
# DPLPMTUD:  Datagram Packetization Layer Path MTU Discovery
#    (DPLPMTUD), PLPMTUD performed using a datagram transport protocol.
# 
# Effective PMTU:  The effective PMTU is the current estimated value
#    for PMTU that is used by a PMTUD.  This is equivalent to the
#    PLPMTU derived by PLPMTUD plus the size of any headers added below
#    the PL, including the IP layer headers.
# 
# EMTU_S:  The effective MTU for sending (EMTU_S) is defined in
#    [RFC1122] as "the maximum IP datagram size that may be sent, for a
#    particular combination of IP source and destination addresses...".
# 
# EMTU_R:  The effective MTU for receiving (EMTU_R) is designated in
#    [RFC1122] as "the largest datagram size that can be reassembled".
# 
# Link:  A link is a communication facility or medium over which nodes
#    can communicate at the link layer, i.e., a layer below the IP
#    layer.  Examples are Ethernet LANs and Internet (or higher) layer
#    tunnels.
# 
# Link MTU:  The link Maximum Transmission Unit (MTU) is the size in
#    bytes of the largest IP packet, including the IP header and
#    payload, that can be transmitted over a link.  Note that this
#    could more properly be called the IP MTU, to be consistent with
#    how other standards organizations use the acronym.  This includes
#    the IP header but excludes link layer headers and other framing
#    that is not part of IP or the IP payload.  Other standards
#    organizations generally define the link MTU to include the link
#    layer headers.  This specification continues the requirement in
#    [RFC4821] that states, "All links MUST enforce their MTU: links
#    that might non-deterministically deliver packets that are larger
#    than their rated MTU MUST consistently discard such packets."
# 
# MAX_PLPMTU:  The MAX_PLPMTU is the largest size of PLPMTU that
#    DPLPMTUD will attempt to use (see the constants defined in
#    Section 5.1.2).
# 
# MIN_PLPMTU:  The MIN_PLPMTU is the smallest size of PLPMTU that
#    DPLPMTUD will attempt to use (see the constants defined in
#    Section 5.1.2).
# 
# MPS:  The Maximum Packet Size (MPS) is the largest size of
#    application data block that can be sent across a network path by a
#    PL using a single datagram (see Section 4.4).
# 
# MSL:  The Maximum Segment Lifetime (MSL) is the maximum delay a
#    packet is expected to experience across a path, taken as 2 minutes
#    [BCP145].
# 
# Packet:  A packet is the IP header(s) and any extension headers/
#    options plus the IP payload.
# 
# Packetization Layer (PL):  The PL is a layer of the network stack
#    that places data into packets and performs transport protocol
#    functions.  Examples of a PL include TCP, SCTP, SCTP over UDP,
#    SCTP over DTLS, or QUIC.
# 
# Path:  The path is the set of links and routers traversed by a packet
#    between a source node and a destination node by a particular flow.
# 
# Path MTU (PMTU):  The Path MTU (PMTU) is the minimum of the link MTU
#    of all the links forming a network path between a source node and
#    a destination node, as used by PMTUD.
# 
# PTB:  In this document, the term PTB message is applied to both IPv4
#    ICMP Unreachable messages (Type 3) that carry the error
#    Fragmentation Needed (Type 3, Code 4) [RFC0792] and ICMPv6 Packet
#    Too Big messages (Type 2) [RFC4443].
# 
# PTB_SIZE:  The PTB_SIZE is a value reported in a validated PTB
#    message that indicates next-hop link MTU of a router along the
#    path.
# 
# PL_PTB_SIZE:  The size reported in a validated PTB message, reduced
#    by the size of all headers added by layers below the PL.
# 
# PLPMTU:  The Packetization Layer PMTU is an estimate of the largest
#    size of PL datagram that can be sent by a path, controlled by
#    PLPMTUD.
# 
# PLPMTUD:  Packetization Layer Path MTU Discovery (PLPMTUD), the
#    method described in this document for datagram PLs, which is an
#    extension to Classical PMTU Discovery.
# 
# Probe packet:  A probe packet is a datagram sent with a purposely
#    chosen size (typically the current PLPMTU or larger) to detect if
#    packets of this size can be successfully sent end-to-end across
#    the network path.
# 
# Unacknowledged PL:  A PL that does not itself provide a mechanism to
#    confirm delivery of datagrams to the remote PL endpoint (e.g.,
#    UDP), and therefore requires DPLPMTUD to provide a mechanism to
#    detect black-holing of packets (c.f., Acknowledged PL).

[[spec]]
level = "MUST"
quote = '''
This specification continues the requirement in
[RFC4821] that states, "All links MUST enforce their MTU: links
that might non-deterministically deliver packets that are larger
than their rated MTU MUST consistently discard such packets."
'''

[[spec]]
level = "MUST"
quote = '''
This specification continues the requirement in
[RFC4821] that states, "All links MUST enforce their MTU: links
that might non-deterministically deliver packets that are larger
than their rated MTU MUST consistently discard such packets."
'''