iperf_sctp: disable delayed-SACK by default to fix slow single-flow SCTP throughput

configure

@@ -16895,6 +16895,16 @@ then :
 printf '%s\n' "#define HAVE_STRUCT_SCTP_ASSOC_VALUE 1" >>confdefs.h
 
 
+fi
+
+ac_fn_c_check_type "$LINENO" "struct sctp_sack_info" "ac_cv_type_struct_sctp_sack_info" "#include <netinet/sctp.h>
+"
+if test "x$ac_cv_type_struct_sctp_sack_info" = xyes
+then :
+
+printf '%s\n' "#define HAVE_STRUCT_SCTP_SACK_INFO 1" >>confdefs.h
+
+
 fi
 
 fi

configure.ac

@@ -152,6 +152,8 @@ AC_CHECK_HEADERS([netinet/sctp.h],
 		 AC_DEFINE([HAVE_SCTP_H], [1], [Have SCTP support.])
 		 AC_SEARCH_LIBS(sctp_bindx, [sctp])
 		 AC_CHECK_TYPES([struct sctp_assoc_value], [], [],
+				[[#include <netinet/sctp.h>]])
+		 AC_CHECK_TYPES([struct sctp_sack_info], [], [],
 				[[#include <netinet/sctp.h>]]),
 		 [],
 		 [#ifdef HAVE_SYS_SOCKET_H

src/iperf_api.h

@@ -506,6 +506,7 @@ enum {
     IESETCNTLKACOUNT = 158,    // Unable to set/get socket keepalive TCP number of retries (TCP_KEEPCNT) option
     IEPTHREADSIGMASK=159,      // Unable to initialize sub thread signal mask (check perror)
     IESERVERTESTDURATIONEXPIRED = 160, // Server test duration expired
+    IESETSCTPDELAYEDSACK = 161, // Unable to set SCTP_DELAYED_SACK (check perror)
     /* Stream errors */
     IECREATESTREAM = 200,   // Unable to create a new stream (check herror/perror)
     IEINITSTREAM = 201,     // Unable to initialize stream (check herror/perror)

src/iperf_config.h.in

@@ -117,6 +117,9 @@
 /* Define to 1 if the system has the type 'struct sctp_assoc_value'. */
 #undef HAVE_STRUCT_SCTP_ASSOC_VALUE
 
+/* Define to 1 if the system has the type 'struct sctp_sack_info'. */
+#undef HAVE_STRUCT_SCTP_SACK_INFO
+
 /* Define to 1 if you have the <sys/endian.h> header file. */
 #undef HAVE_SYS_ENDIAN_H
 

src/iperf_error.c

@@ -469,6 +469,10 @@ iperf_strerror(int int_errno)
             snprintf(errstr, len, "unable to set SCTP_DISABLE_FRAGMENTS");
             perr = 1;
             break;
+        case IESETSCTPDELAYEDSACK:
+            snprintf(errstr, len, "unable to set SCTP_DELAYED_SACK");
+            perr = 1;
+            break;
         case IESETSCTPNSTREAM:
             snprintf(errstr, len, "unable to set SCTP_INIT num of SCTP streams\n");
             perr = 1;

src/iperf_sctp.c

@@ -125,6 +125,38 @@ iperf_sctp_accept(struct iperf_test * test)
         return -1;
     }
 
+    if (test->no_delay) {
+        int opt = 1;
+        if (setsockopt(s, IPPROTO_SCTP, SCTP_NODELAY, &opt, sizeof(opt)) < 0) {
+            i_errno = IESETNODELAY;
+            close(s);
+            return -1;
+        }
+    }
+
+#ifdef HAVE_STRUCT_SCTP_SACK_INFO
+    /*
+     * Apply SCTP_DELAYED_SACK on the accepted association.  On Linux the
+     * SCTP_DELAYED_SACK setting on the listener does NOT propagate to
+     * accepted associations, so the per-association setting must be applied
+     * here for the receiver-side fix to take effect.  ENOPROTOOPT is
+     * non-fatal so this is safe on platforms without support.
+     */
+    {
+        struct sctp_sack_info sack;
+        memset(&sack, 0, sizeof(sack));
+        sack.sack_assoc_id = 0;
+        sack.sack_delay = 0;
+        sack.sack_freq = 1;
+        if (setsockopt(s, IPPROTO_SCTP, SCTP_DELAYED_SACK, &sack, sizeof(sack)) < 0 &&
+            errno != ENOPROTOOPT) {
+            i_errno = IESETSCTPDELAYEDSACK;
+            close(s);
+            return -1;
+        }
+    }
+#endif /* HAVE_STRUCT_SCTP_SACK_INFO */
+
     if (Nread(s, cookie, COOKIE_SIZE, Psctp) < 0) {
         i_errno = IERECVCOOKIE;
         close(s);
@@ -248,6 +280,52 @@ iperf_sctp_listen(struct iperf_test *test)
         return -1;
     }
 
+    if (test->no_delay) {
+        opt = 1;
+        if (setsockopt(s, IPPROTO_SCTP, SCTP_NODELAY, &opt, sizeof(opt)) < 0) {
+            saved_errno = errno;
+            close(s);
+            freeaddrinfo(res);
+            errno = saved_errno;
+            i_errno = IESETNODELAY;
+            return -1;
+        }
+    }
+
+#ifdef HAVE_STRUCT_SCTP_SACK_INFO
+    /*
+     * Disable SCTP delayed-SACK on future accepted associations.  With the
+     * default sack_freq=2/sack_delay=200ms, a sender that writes blocks
+     * producing exactly one DATA chunk stalls for 200 ms waiting for a SACK
+     * that the receiver is in turn delaying.  iperf3 is a throughput
+     * benchmark, so request an immediate SACK on every packet.  On Linux
+     * the endpoint default set here does not propagate to accepted
+     * associations -- iperf_sctp_accept() repeats the call on the per-
+     * association socket.  ENOPROTOOPT (platform without support) is
+     * non-fatal.
+     */
+    {
+        struct sctp_sack_info sack;
+        memset(&sack, 0, sizeof(sack));
+#ifdef SCTP_FUTURE_ASSOC
+        sack.sack_assoc_id = SCTP_FUTURE_ASSOC;
+#else
+        sack.sack_assoc_id = 0;
+#endif
+        sack.sack_delay = 0;
+        sack.sack_freq = 1;
+        if (setsockopt(s, IPPROTO_SCTP, SCTP_DELAYED_SACK, &sack, sizeof(sack)) < 0 &&
+            errno != ENOPROTOOPT) {
+            saved_errno = errno;
+            close(s);
+            freeaddrinfo(res);
+            errno = saved_errno;
+            i_errno = IESETSCTPDELAYEDSACK;
+            return -1;
+        }
+    }
+#endif /* HAVE_STRUCT_SCTP_SACK_INFO */
+
     /* servers must call sctp_bindx() _instead_ of bind() */
     if (!TAILQ_EMPTY(&test->xbind_addrs)) {
         if (iperf_sctp_bindx(test, s, IPERF_SCTP_SERVER)) {
@@ -541,6 +619,33 @@ iperf_sctp_connect(struct iperf_test *test)
         return -1;
     }
 
+#ifdef HAVE_STRUCT_SCTP_SACK_INFO
+    /*
+     * Disable SCTP delayed-SACK on this association (see iperf_sctp_listen()
+     * for rationale).  Applied here as well so that --reverse, which makes
+     * the client the receiver, still gets immediate SACKs.  The socket is
+     * already connected and 1:1-style, so sack_assoc_id=0 unambiguously
+     * targets the current association on every platform (RFC 6458 §8.1).
+     * ENOPROTOOPT is treated as non-fatal.
+     */
+    {
+        struct sctp_sack_info sack;
+        memset(&sack, 0, sizeof(sack));
+        sack.sack_assoc_id = 0;
+        sack.sack_delay = 0;
+        sack.sack_freq = 1;
+        if (setsockopt(s, IPPROTO_SCTP, SCTP_DELAYED_SACK, &sack, sizeof(sack)) < 0 &&
+            errno != ENOPROTOOPT) {
+            saved_errno = errno;
+            close(s);
+            freeaddrinfo(server_res);
+            errno = saved_errno;
+            i_errno = IESETSCTPDELAYEDSACK;
+            return -1;
+        }
+    }
+#endif /* HAVE_STRUCT_SCTP_SACK_INFO */
+
     freeaddrinfo(server_res);
     return s;
 #else