commit 70d3daeccbed9abc7601833e48aefaf4e94d89e8
Author: Peter J. Holzer <hjp@hjp.at>
Date:   Sun Dec 20 14:51:54 2020 +0100

    Time packets sent from server to client

diff --git a/tcpdelay.go b/tcpdelay.go
new file mode 100644
index 0000000..3f7a952
--- /dev/null
+++ b/tcpdelay.go
@@ -0,0 +1,95 @@
+package main
+
+import (
+	"encoding/binary"
+	"gopkg.in/alecthomas/kingpin.v2"
+	"fmt"
+	"log"
+	"net"
+	"os"
+	"time"
+)
+
+var (
+	app = kingpin.New("tcpdelay", "Measure TCP delays")
+
+	server        = app.Command("server", "Run as server")
+	listenAddress = server.Arg("address", "Address:port to listen on").String()
+
+	client = app.Command("client", "Request measurement")
+	serverAddress = client.Arg("address", "Address:port to connect to").String()
+)
+
+func tcpdelayServer() {
+	ln, err := net.Listen("tcp", *listenAddress)
+	if err != nil {
+		log.Fatal("Cannot listen on ", *listenAddress, err)
+	}
+	log.Println("Listening on ", ln.Addr())
+	for {
+		conn, err := ln.Accept()
+		log.Println("accepted connection")
+		if err != nil {
+			log.Fatal("Cannot accept on ", listenAddress, err)
+		}
+		o := 0
+		t0 := time.Now()
+		d, _ := time.ParseDuration("1s")
+		te := t0.Add(d)
+		buffer := make([]byte, 1024)
+		for t := time.Now(); t.Before(te); t = time.Now() {
+			copy(buffer[0:8], "tcpdelay")
+			binary.BigEndian.PutUint64(buffer[8:], uint64(o))
+			binary.BigEndian.PutUint64(buffer[16:], uint64(t.UnixNano()))
+			n, err := conn.Write(buffer)
+			if err != nil {
+				log.Fatal("Cannot write on ", listenAddress, err)
+			}
+			if n != len(buffer) {
+				log.Fatal("Wrote ", n, " bytes intead of ", len(buffer))
+			}
+			o += n
+		}
+		conn.Close()
+		log.Println("closed connection")
+	}
+}
+
+func tcpdelayClient() {
+	conn, err := net.Dial("tcp", *serverAddress)
+	if err != nil {
+		log.Fatal("Cannot connect to ", *serverAddress, err)
+	}
+	o := 0
+	buffer := make([]byte, 1024)
+	for {
+		n, err := conn.Read(buffer)
+		if err != nil {
+			log.Fatal("Cannot read from ", *serverAddress, err)
+		}
+		if n != len(buffer) {
+			log.Fatal("Read ", n, " bytes intead of ", len(buffer))
+		}
+		tr := time.Now().UnixNano()
+		if string(buffer[0:8]) != "tcpdelay" {
+			log.Fatal("Unexpected magic ", buffer[0:8])
+		}
+		oSent := int(binary.BigEndian.Uint64(buffer[8:]))
+		if oSent != o {
+			log.Fatal("Unexpected offset ", oSent, " instead of ", o)
+		}
+		ts := binary.BigEndian.Uint64(buffer[16:])
+		fmt.Println(o, ts, tr) // XXX - buffer for performance
+		o += n
+	}
+
+}
+
+func main() {
+	switch kingpin.MustParse(app.Parse(os.Args[1:])) {
+	case server.FullCommand():
+		tcpdelayServer()
+	case client.FullCommand():
+		tcpdelayClient()
+	}
+}