Here's what I'm imagining:
sequenceDiagram
participant wc as WHEP Client
participant ws as WHEP Server
wc->>+ws: GET /whep/stream123
ws-->>wc: HTTP Response with SDP Offer body, Link header "/whep/stream123/session456", optional header with ICE servers
wc->>ws: POST /whep/stream123/session456 with SDP Answer body
ws-->>wc: 201 Created