What is the proper classification of an IS-IS link state PDU?
Here are some suggestions:
Cell: Not applicable because TLVs are used. A cell must have a fixed length, according to ITU-T I.113 1997-06 and RFC3985.
Datagram: According to RFC1983, a datagram carries enough information to be routed from source to destination without depending on earlier exchanges between that source and destination. But IS-IS LSP are only communicated between intermediate systems.
Frame: According to ITU-T I.113 1997-06, a frame is a block of variable length at L2. However, since IS-IS is encapsulated by a L2 protocol, it seems redundant.
Message: Way too ambiguous. Sometimes a message is an opcode of an application layer protocol. Sometimes a message is defined by setting a flag (control bit).
Packet: Not applicable because according to ITU-T I.113 1997-06, a packet resides at L3.
Segment: Not applicable because it is connectionless. A segment is a packet that contains TCP/IP data or an acknowledgement, according to RFC5681. Apart from TCP, a segment implies sequence numbering.