+ /*
+ * Some protocol versions (namely NFSv4) require some form of
+ * congestion control. (See RFC 7530 section 3.1 paragraph 2)
+ * In other words, UDP is not allowed. We mark those when setting
+ * up the svc_xprt, and verify that here.
+ *
+ * The spec is not very clear about what error should be returned
+ * when someone tries to access a server that is listening on UDP
+ * for lower versions. RPC_PROG_MISMATCH seems to be the closest
+ * fit.
+ */
+ if (versp->vs_need_cong_ctrl &&
+ !test_bit(XPT_CONG_CTRL, &rqstp->rq_xprt->xpt_flags))
+ goto err_bad_vers;
+