在TLS协议中，0-RTT（零往返时间）是指在某些情况下，客户端可以在第一个往返（RTT）中就开始发送加密的应用数据，而不需要等待完整的TLS握手完成。这种机制允许客户端在重新连接到服务器时，利用之前会话中缓存的信息（如会话票据、密钥等），来减少连接建立的延迟。

0-RTT主要分为session和session ticket两种方式。在实际应用中，我们经常需要统计0-RTT比率和0-RTT建连失败的原因。如果你使用的是boringssl库进行ssl加解密，可以使用boringssl读取early_data_reason，来定位0-RTT是否成功以及失败的原因。

enum ssl_early_data_reason_t BORINGSSL_ENUM_INT {
  // The handshake has not progressed far enough for the 0-RTT status to be
  // known.
  ssl_early_data_unknown = 0,
  // 0-RTT is disabled for this connection.
  ssl_early_data_disabled = 1,
  // 0-RTT was accepted.
  ssl_early_data_accepted = 2,
  // The negotiated protocol version does not support 0-RTT.
  ssl_early_data_protocol_version = 3,
  // The peer declined to offer or accept 0-RTT for an unknown reason.
  ssl_early_data_peer_declined = 4,
  // The client did not offer a session.
  ssl_early_data_no_session_offered = 5,
  // The server declined to resume the session.
  ssl_early_data_session_not_resumed = 6,
  // The session does not support 0-RTT.
  ssl_early_data_unsupported_for_session = 7,
  // The server sent a HelloRetryRequest.
  ssl_early_data_hello_retry_request = 8,
  // The negotiated ALPN protocol did not match the session.
  ssl_early_data_alpn_mismatch = 9,
  // The connection negotiated Channel ID, which is incompatible with 0-RTT.
  ssl_early_data_channel_id = 10,
  // Value 11 is reserved. (It has historically |ssl_early_data_token_binding|.)
  // The client and server ticket age were too far apart.
  ssl_early_data_ticket_age_skew = 12,
  // QUIC parameters differ between this connection and the original.
  ssl_early_data_quic_parameter_mismatch = 13,
  // The application settings did not match the session.
  ssl_early_data_alps_mismatch = 14,
  // The value of the largest entry.
  ssl_early_data_reason_max_value = ssl_early_data_alps_mismatch,
};

这边使用常用的状态码举例：

（1）ssl_early_data_accepted，表示0-rtt建立按成功

（2）ssl_early_data_alpn_mismatch，表示请求的alpn没有匹配上

（3）ssl_early_data_no_session_offered，表示请求没有提供相关的session信息，无法建立0-RTT连接