pub const ALLOW: &str = "INVITE, ACK, CANCEL, BYE, OPTIONS, UPDATE";Expand description
The Allow value sipx advertises (RFC 3311 §4, RFC 3261 §20.5).
One constant rather than a literal at each site that writes the header, because §4 makes this list the only way a peer is permitted to decide it may send an UPDATE at all. A copy that drifts is a peer that silently falls back to a re-INVITE forever, on a path no test looks at — the failure is invisible from this side, which is exactly the kind that survives.