> QUIC version 2 is not intended to deprecate version 1. Endpoints that support version 2 might continue support for version 1 to maximize compatibility with other endpoints. In particular, HTTP clients often use Alt-Svc [RFC7838] to discover QUIC support. As this mechanism does not currently distinguish between QUIC versions, HTTP servers SHOULD support multiple versions to reduce the probability of incompatibility and the cost associated with QUIC version negotiation or TCP fallback.
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