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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.

Glossary​

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:

  • Packet Black Hole: 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).

  • ICMP Black Hole: 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".

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.

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).

Terminology Relationships​

                    MAX_PLPMTU
                         │
                         ↓
    PLPMTU (Current) ←──── PROBED_SIZE (Testing)
         │
         ↓
    BASE_PLPMTU (Baseline)
         │
         ↓
    MIN_PLPMTU (Minimum)

Key Concept Explanations​

PMTU vs PLPMTU​

  • PMTU: Network layer concept, includes all IP layer headers
  • PLPMTU: Packetization layer concept, only PL-controllable portion

MPS vs PLPMTU​

  • PLPMTU: Packet size including PL headers
  • MPS: Application data size (PLPMTU - PL header size)

Black Hole Types​

  1. Packet Black Hole: Packets dropped without notification
  2. ICMP Black Hole: PTB messages fail to reach sender

These terms are used throughout the document and understanding them is essential for implementing DPLPMTUD.

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