3. Terminology
We use the following terms in this document:
IP: Either IPv4 [RFC0791] or IPv6 [RFC2460].
Node: A device that implements IP.
Upper layer: A protocol layer immediately above IP. Examples are transport protocols such as TCP and UDP, control protocols such as ICMP, routing protocols such as OSPF, and Internet or lower-layer protocols being "tunneled" over (i.e., encapsulated in) IP such as IPX, AppleTalk, or IP itself.
Link: A communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IP.
Interface: A node's attachment to a link.
Address: An IP layer identifier for an interface or a set of interfaces.
Packet: An IP header plus payload.
MTU: Maximum Transmission Unit, the size in bytes of the largest IP packet, including the IP header and payload, that can be transmitted on a link or path.
Link MTU: The Maximum Transmission Unit, i.e., maximum IP packet size in bytes, that can be conveyed in one piece over a link.
Path: The set of links traversed by a packet between a source node and a destination node.
Path MTU, or PMTU: The minimum link MTU of all the links in a path between a source node and a destination node.
Classical Path MTU Discovery: Process described in RFC 1191 and RFC 1981, in which nodes rely on ICMP Packet Too Big (PTB) messages to learn the MTU of a path.
Packetization Layer: The layer of the network stack that segments data into packets.
Effective PMTU: The current estimated value for PMTU used by a Packetization Layer for segmentation.
PLPMTUD: Packetization Layer Path MTU Discovery, the method described in this document, which is an extension to classical PMTU Discovery.
PTB (Packet Too Big) message: An ICMP message reporting that an IP packet is too large to forward.
Flow: A context in which MTU Discovery algorithms can be invoked.
MSS: The TCP Maximum Segment Size [RFC0793], the maximum payload size available to the TCP layer.
Probe packet: A packet that is being used to test a path for a larger MTU.
Probe size: The size of a packet being used to probe for a larger MTU, including IP headers.
Probe gap: The payload data that will be lost and need to be retransmitted if the probe is not delivered.
Leading window: Any unacknowledged data in a flow at the time a probe is sent.
Trailing window: Any data in a flow sent after a probe, but before the probe is acknowledged.
Search strategy: The heuristics used to choose successive probe sizes to converge on the proper Path MTU.
Full-stop timeout: A timeout where none of the packets transmitted after some event are acknowledged by the receiver, including any retransmissions.