2. Terminology

node
A device that implements IPv6.

router
A node that forwards IPv6 packets not explicitly addressed to itself.

host
Any node that is not a router.

upper layer
A protocol layer immediately above IPv6. Examples are transport protocols such as TCP and UDP, control protocols such as ICMPv6, routing protocols such as OSPF, and internet-layer or lower-layer protocols being "tunneled" over (i.e., encapsulated in) IPv6 such as Internetwork Packet Exchange (IPX), AppleTalk, or IPv6 itself.

link
A communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IPv6. Examples are Ethernets (simple or bridged); PPP links; X.25, Frame Relay, or ATM networks; and internet-layer or higher-layer "tunnels", such as tunnels over IPv4 or IPv6 itself.

interface
A node's attachment to a link.

address
An IPv6-layer identifier for an interface or a set of interfaces.

packet
An IPv6 header plus payload. The packet can have a size less than or equal to the PMTU. Alternatively, this can be a larger packet that is fragmented into a series of fragments each with a size less than or equal to the PMTU.

link MTU
The maximum transmission unit, i.e., maximum packet size in octets, 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
The minimum link MTU of all the links in a path between a source node and a destination node.

PMTU
Path MTU.

Path MTU Discovery
The process by which a node learns the PMTU of a path.

EMTU_S
Effective MTU for sending; used by upper-layer protocols to limit the size of IP packets they queue for sending [RFC6691] [RFC1122].

EMTU_R
Effective MTU for receiving; the largest packet that can be reassembled at the receiver [RFC1122].

flow
A sequence of packets sent from a particular source to a particular (unicast or multicast) destination for which the source desires special handling by the intervening routers.

flow id
A combination of a source address and a non-zero flow label.