diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..1dc9e55
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,11 @@
+
+all: client server
+
+client: client.c
+	gcc -o $@ $@.c -lpthread
+
+server: server.c
+	gcc -o $@ $@.c
+
+clean:
+	-rm -rf client server
diff --git a/client.c b/client.c
new file mode 100644
index 0000000..7c67952
--- /dev/null
+++ b/client.c
@@ -0,0 +1,97 @@
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <sys/time.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define BUFLEN sizeof(struct timeval)
+#define PORT 9876
+
+static int sock_fd, slen;
+
+void diep(char * s)
+{
+    perror(s);
+    exit(1);
+}
+
+static long long time_diff(struct timeval * one, struct timeval * two)
+{
+    long long t1 = ((long long) one->tv_sec) * 1000000
+        + (long long) one->tv_usec;
+    long long t2 = ((long long) two->tv_sec) * 1000000
+        + (long long) two->tv_usec;
+    return t1 - t2;
+}
+
+void * thread(void * arg)
+{
+    struct sockaddr_in si_other;
+    char time_str[100];
+    struct timeval tv_msg, tv_now, tv_last;
+    for (;;)
+    {
+        if (recvfrom(sock_fd, &tv_msg, BUFLEN, 0,
+                    (struct sockaddr *) &si_other, &slen) == -1)
+            diep("recvfrom()");
+        gettimeofday(&tv_now, NULL);
+        struct tm * lt = localtime(&tv_now.tv_sec);
+        strftime(time_str, 100, "%F %T", lt);
+        printf("Rcv: %s, rtt: %lld, elapsed: %lld\n",
+                time_str,
+                time_diff(&tv_now, &tv_msg),
+                time_diff(&tv_now, &tv_last));
+        tv_last.tv_sec = tv_now.tv_sec;
+        tv_last.tv_usec = tv_now.tv_usec;
+    }
+}
+
+int main(int argc, char * argv[])
+{
+    int interval;
+    struct timeval tv;
+    struct sockaddr_in si_other;
+    if (argc != 3)
+    {
+        printf("Usage: client server-address msec-interval\n");
+        exit(2);
+    }
+    interval = atoi(argv[2]) * 1000;
+
+    slen = sizeof(si_other);
+
+    if ((sock_fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
+        diep("socket");
+
+    memset((char *) &si_other, 0, sizeof(si_other));
+    si_other.sin_family = AF_INET;
+    si_other.sin_port = htons(PORT);
+    inet_aton(argv[1], &si_other.sin_addr);
+#if 0
+    if (bind(sock_fd, (struct sockaddr *) &si_other, sizeof(si_other)) == -1)
+        diep("bind");
+#endif
+
+    pthread_t t;
+    if (pthread_create(&t, NULL, thread, NULL) != 0)
+        diep("pthread_create()");
+
+    usleep(100000);
+    for (;;)
+    {
+        gettimeofday(&tv, NULL);
+        if (sendto(sock_fd, &tv, BUFLEN, 0,
+                    (struct sockaddr *) &si_other, slen) == -1)
+            diep("sendto()");
+        usleep(interval);
+    }
+
+    close(sock_fd);
+    return 0;
+}
+
diff --git a/server.c b/server.c
new file mode 100644
index 0000000..24da9d1
--- /dev/null
+++ b/server.c
@@ -0,0 +1,49 @@
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include <sys/time.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define BUFLEN sizeof(struct timeval)
+#define PORT 9876
+
+void diep(char * s)
+{
+    perror(s);
+    exit(1);
+}
+
+int main(void)
+{
+    struct sockaddr_in si_me, si_other;
+    int s, slen = sizeof(si_other);
+    struct timeval buf;
+
+    if ((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
+        diep("socket");
+
+    memset((char *) &si_me, 0, sizeof(si_me));
+    si_me.sin_family = AF_INET;
+    si_me.sin_port = htons(PORT);
+    si_me.sin_addr.s_addr = htonl(INADDR_ANY);
+    if (bind(s, (struct sockaddr *) &si_me, sizeof(si_me)) == -1)
+        diep("bind");
+
+    for (;;)
+    {
+        if (recvfrom(s, &buf, BUFLEN, 0,
+                    (struct sockaddr *) &si_other, &slen) == -1)
+            diep("recvfrom()");
+        if (sendto(s, &buf, BUFLEN, 0,
+                    (struct sockaddr *) &si_other, slen) == -1)
+            diep("sendto()");
+    }
+
+    close(s);
+    return 0;
+}
+