I am trying to understand why I'm seeing TCP retransmissions on a simple
program. I created a small program based on the 2.4.2 docs:
object HelloWorld {
def main(args: Array[String]): Unit = {
implicit val system = ActorSystem("Sys")
implicit val materializer = ActorMaterializer()
val connections: Source[IncomingConnection, Future[Tcp.ServerBinding]] =
Tcp().bind("localhost", 8888)
connections runForeach { connection =>
// server logic, parses incoming commands
val commandParser = Flow[String].takeWhile(_ != "BYE").map(_ + "!")
import connection._
val welcomeMsg = s"Welcome to: $localAddress, you are: $remoteAddress!"
val welcome = Source.single(welcomeMsg)
val serverLogic = Flow[ByteString]
.via(Framing.delimiter( ByteString("\n"), 256 ))
.map(_.utf8String)
.via(commandParser)
// merge in the initial banner after parser
.merge(welcome)
.map(_ + "\n")
.map(ByteString(_))
connection.handleWith(serverLogic)
}
}
}
But when I connect to localhost/8888 via telnet, type in a few lines, and
watch the traffic with wireshark, I'm seeing various retransmits and out of
order:
46482→8888 [SYN] Seq=0 Win=43690 Len=0 MSS=65495 SACK_PERM=1 TSval=47383693
TSecr=0 WS=128
8888→46482 [SYN, ACK] Seq=0 Ack=1 Win=43690 Len=0 MSS=65495 SACK_PERM=1
TSval=47383693 TSecr=47383693 WS=128
46482→8888 [ACK] Seq=1 Ack=1 Win=43776 Len=0 TSval=47383693 TSecr=47383693
8888→46482 [PSH, ACK] Seq=1 Ack=1 Win=43776 Len=56 TSval=47383719
TSecr=47383693
46482→8888 [ACK] Seq=1 Ack=57 Win=43776 Len=0 TSval=47383719 TSecr=47383719
[TCP Out-Of-Order] 46482→8888 [SYN] Seq=0 Win=43690 Len=0 MSS=65495
SACK_PERM=1 TSval=47383693 TSecr=0 WS=128
[TCP Retransmission] 8888→46482 [SYN, ACK] Seq=0 Ack=1 Win=43690 Len=0
MSS=65495 SACK_PERM=1 TSval=47383693 TSecr=47383693 WS=128
46482→8888 [ACK] Seq=1 Ack=1 Win=43776 Len=0 TSval=47383693 TSecr=47383693
[TCP Retransmission] 8888→46482 [PSH, ACK] Seq=1 Ack=1 Win=43776 Len=56
TSval=47383719 TSecr=47383693
46482→8888 [ACK] Seq=1 Ack=57 Win=43776 Len=0 TSval=47383719 TSecr=47383719
46482→8888 [PSH, ACK] Seq=1 Ack=57 Win=43776 Len=6 TSval=47384746
TSecr=47383719
8888→46482 [ACK] Seq=57 Ack=7 Win=43776 Len=0 TSval=47384746 TSecr=47384746
8888→46482 [PSH, ACK] Seq=57 Ack=7 Win=43776 Len=7 TSval=47384747
TSecr=47384746
46482→8888 [ACK] Seq=7 Ack=64 Win=43776 Len=0 TSval=47384747 TSecr=47384747
[TCP Retransmission] 46482→8888 [PSH, ACK] Seq=1 Ack=57 Win=43776 Len=6
TSval=47384746 TSecr=47383719
8888→46482 [ACK] Seq=57 Ack=7 Win=43776 Len=0 TSval=47384746 TSecr=47384746
[TCP Retransmission] 8888→46482 [PSH, ACK] Seq=57 Ack=7 Win=43776 Len=7
TSval=47384747 TSecr=47384746
46482→8888 [ACK] Seq=7 Ack=64 Win=43776 Len=0 TSval=47384747 TSecr=47384747
46482→8888 [PSH, ACK] Seq=7 Ack=64 Win=43776 Len=6 TSval=47386128
TSecr=47384747
8888→46482 [PSH, ACK] Seq=64 Ack=13 Win=43776 Len=7 TSval=47386129
TSecr=47386128
46482→8888 [ACK] Seq=13 Ack=71 Win=43776 Len=0 TSval=47386129 TSecr=47386129
[TCP Retransmission] 46482→8888 [PSH, ACK] Seq=7 Ack=64 Win=43776 Len=6
TSval=47386128 TSecr=47384747
[TCP Retransmission] 8888→46482 [PSH, ACK] Seq=64 Ack=13 Win=43776 Len=7
TSval=47386129 TSecr=47386128
46482→8888 [ACK] Seq=13 Ack=71 Win=43776 Len=0 TSval=47386129 TSecr=47386129
Full trace at http://pastebin.com/09xi1kGa
Could someone explain to me if I'm doing something wrong, or what's going
on?
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