Function first_hop_end 

pub fn first_hop_end(value: &[u8]) -> usize

The index just past the first via-parm in a header value that may carry several comma-separated hops.

A server has to add received and rport to the topmost hop and only that one, so it needs to know where the first hop ends without reserializing the rest — the other hops belong to other elements and must go back out exactly as they arrived.

Commas inside quoted parameter values are not separators, which is why this is not a call to position.