i used this quite extensively to work on tcp
     SYNOPSIS
          nettest -l [ -s sleepms ][ -n nblocks ]
          nettest [ -a addr ][ -s sleepms ]
          nettest -r raddr -n nblocks
    DESCRIPTION
          Nettest tests goodput of reliable, in-order transports at
          full speed, and with receiver throttling.  Both sender and
[...]

Reference: /n/sources/patch/maybe/net-test
Date: Mon Dec 24 17:38:40 CET 2012
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/nettest.c	Thu Jan  1 00:00:00 1970
+++ /sys/src/cmd/nettest.c	Mon Dec 24 17:37:33 2012
@@ -0,0 +1,179 @@
+#include <u.h>
+#include <libc.h>
+
+char	*service	= "17766";
+char	*laddr	= "*";
+char	*raddr;
+
+uchar	flag[127];
+ulong	n;
+long	sleepms;
+vlong	sleepns;
+
+#define dprint(...) {if(flag['d']) print(__VA_ARGS__);}
+
+enum {
+	Blen = 8*1024,
+};
+
+void
+usage(void)
+{
+	fprint(2, "usage: %s [-l] [-s sleepms] [-n nblocks]\n"
+		"	%s [-a addr] [-s sleepms]\n"
+		"	%s -r addr -n nblocks\n", argv0, argv0, argv0);
+	exits("usage");
+}
+
+void
+die(void)
+{
+	sysfatal("%s: %r", argv0);
+}
+
+int
+notef(void*, char *note)
+{
+	if(strstr(note, "alarm") != nil)
+		return 1;
+	return 0;
+}
+
+char*
+·netmkaddr(char *linear, char *net, char *service)
+{
+	char *r;
+	static QLock stupid;
+
+	qlock(&stupid);		/* protect stupid static memory */
+	r = netmkaddr(linear, net, service);
+	if(r != nil)
+		r = strdup(r);
+	qunlock(&stupid);
+
+	return r;
+}
+
+void
+client(void)
+{
+	char *dst, b[Blen];
+	int fd, i;
+
+	atnotify(notef, 1);
+	sleep(1);
+	dprint("client\n");
+	dst = ·netmkaddr(raddr, "tcp", service);
+	alarm(3*1000);
+	if((fd = dial(dst, 0, 0, 0)) < 0)
+		die();
+	alarm(0);
+	free(dst);
+	dprint("dialed\n");
+	for(i = 0; i < n; ++i){
+		if(write(fd, b, sizeof b) < 0)
+			die();
+	}
+	close(fd);
+	dprint("wrote\n");
+}
+				
+void
+server(void)
+{
+	int acfd, lcfd, dfd;
+	char adir[40], ldir[40], b[Blen], *a;
+	long c;
+	uvlong i, t;
+	double ratemb, maxr, delta;
+	NetConnInfo *nc;
+
+	dprint("server\n");
+	a = ·netmkaddr(laddr, "tcp", service);
+	acfd = announce(a, adir);
+	if(acfd == -1)
+		die();
+	free(a);
+
+	dprint("listen\n");
+	if((lcfd = listen(adir, ldir)) < 0)
+		die();
+	dprint("listened\n");
+	if((dfd = accept(lcfd, ldir)) < 0)
+		die();
+	nc = getnetconninfo(ldir, dfd);
+	dprint("accepted\n");
+	t = -nsec();
+	for(i = 0;; i++){
+		c = readn(dfd, b, sizeof b);
+		if(c != sizeof b)
+			break;
+		if(sleepms)
+			sleep(sleepms);
+	}
+	t += nsec();
+	close(dfd);
+	close(lcfd);
+
+	print("%s count %llud; ", nc? nc->raddr: "(unknown)", i);
+	free(nc);
+	i *= Blen;
+	delta = t/1e9;
+	ratemb = i/delta/1024/1024;
+	if(sleepms > 0){
+		maxr = Blen*1000. / sleepms / 1e6;
+		print("%llud bytes in %g s @ %.2g MB/s (%ldms; limit %.2g MB/s)\n", i, delta, ratemb, sleepms, maxr);
+	}
+	else
+		print("%llud bytes in %g s @ %.2g MB/s (0ms)\n", i, delta, ratemb);
+}
+
+void
+main(int argc, char **argv)
+{
+	ARGBEGIN{
+	case 'r':
+		raddr = EARGF(usage());
+		break;
+	case 'a':
+		laddr = EARGF(usage());
+		break;
+	case 'n':
+		n = strtoul(EARGF(usage()), nil, 0);
+		break;
+	case 's':
+		sleepms = strtoul(EARGF(usage()), nil, 0);
+		break;
+	case 'd':
+	case 'l':
+		flag[ARGC()] = 1;
+		break;
+	default:
+		usage();
+	}ARGEND
+	if(argc != 0)
+		usage();
+
+	if(flag['l']){
+		/* loopback */
+		raddr = getenv("sysname");
+		if(raddr == nil)
+			sysfatal("%s: can't find my address", argv0);
+		switch(fork()){
+		case -1:
+			die();
+			break;
+		case 0:
+			server();
+			break;
+		default:
+			client();
+			wait();
+			break;
+		}
+	}else if(raddr != nil)
+		client();
+	else
+		server();
+	exits(nil);
+}
--- /sys/man/8/nettest	Thu Jan  1 00:00:00 1970
+++ /sys/man/8/nettest	Mon Dec 24 17:37:34 2012
@@ -0,0 +1,94 @@
+.TH NETTEST 8
+.SH NAME
+nettest \- test network performance
+.SH SYNOPSIS
+.B nettest
+.B -l
+[
+.B -s
+.I sleepms
+][
+.B -n
+.I nblocks
+]
+.br
+.B nettest
+[
+.B -a
+.I addr
+][
+.B -s
+.I sleepms
+]
+.br
+.B nettest
+.B -r
+.I raddr
+.B -n
+.I nblocks
+.SH DESCRIPTION
+.I Nettest
+tests goodput of reliable, in-order transports
+at full speed, and with receiver throttling.  Both sender
+and receiver do i/o in 8 KB chunks.
+Goodput in MB/s and if throttled, the theoretical
+maximum goodput in MB/s are reported.
+.PP
+As a client the options are
+.TF "-s sleepms"
+.PD 0
+.TP
+.B "-r \fIraddr\fP"
+remote address
+.I addr
+.TP
+.B "-n \fInblocks\fP"
+send
+.I nblocks
+8-KiB blocks
+.PP
+Server options are
+.TF "-s sleepms"
+.PD 0
+.TP
+.B "-a \fIaddr\fP"
+listen on network address
+.I addr
+.TP
+.B "-s \fIsleepms\fP"
+sleep
+.I sleepms
+between 8KiB reads
+.PP
+In loopback mode the options are
+.TF "-s sleepms"
+.PD 0
+.TP
+.B -l
+specify loopback mode
+.TP
+.B "-s \fIsleepms\fP"
+sleep
+.I sleepms
+between 8KiB reads
+.TP
+.B "-n \fInblocks\fP"
+send
+.I nblocks
+8-KiB blocks
+.SH EXAMPLE
+To test a reader limited to 8MB/s
+.IP
+.EX
+nettest -l -n 5000 -s 1
+.EE
+.PP
+To do the same test using two independent IP stacks
+.IP
+.EX
+nettest -s 1 -a /net.a/tcp!* &
+nettest -n 5000 -r /net.b/tcp!mymachine-neta
+.EE
+nettest 
+.SH "SEE ALSO"
+.IR ping (8)