tlsext_server_parse
if (!tlsext_server_parse(s, SSL_TLSEXT_MSG_CH, &cbs, &al)) {
int tlsext_server_parse(SSL *s, uint16_t msg_type, CBS *cbs, int *alert);
if (!tlsext_server_parse(s, SSL_TLSEXT_MSG_CH, cbs, &alert_desc)) {
CHECK(c_val == tlsext_server_parse(s, SSL_TLSEXT_MSG_CH, &cbs, &al)); \