Segment Routing SR, SRv6
Modern source routing — the sender determines the entire path through a list of "segments" in the packet header, instead of per-hop signalling (RSVP-TE).
Two variants: SR-MPLS (segments as MPLS labels) and SRv6 (segments as IPv6 addresses via a Segment Routing Header). Powers modern telecom backbones and 5G transport. Dramatically simplifies traffic engineering — no state in intermediate routers for LSPs. Programmable networks: your "intent" can be encoded directly in the packet's segment list. Preferred replacement for MPLS-RSVP in new designs. IETF SPRING WG owns the spec.