PerformanceResourceTiming: nextHop

[avadacatavra]

The nextHopProtocol attribute is intended to identify the network protocol in use for the fetch regardless of how it was actually negotiated; that is, even if ALPN is not used to negotiate the network protocol, this attribute still uses the ALPN Protocol ID's to indicate the protocol in use.

On getting, the attribute nextHopProtocol returns the network protocol used to fetch the resource, as identified by the ALPN Protocol ID [RFC7301]; resources retrieved from relevant application caches or local resources, return an empty string. When a proxy is configured, if a tunnel connection is established then this attribute MUST return the ALPN Protocol ID of the tunneled protocol, otherwise it MUST return the ALPN Protocol ID of the first hop to the proxy. In order to have precisely one way to represent any ALPN protocol ID, the following additional constraints apply: octets in the ALPN protocol MUST NOT be percent-encoded if they are valid token characters except "%", and when using percent-encoding, uppercase hex digits MUST be used.

Formally registered ALPN protocol IDs are documented by IANA. In case the user agent is using an experimental, non-registered protocol, the user agent MUST use the ALPN negotiated value if any. If ALPN was not used for protocol negotiations, the user agent MAY use another descriptive string.

Spec: https://w3c.github.io/resource-timing/#dom-performanceresourcetiming-nexthopprotocol