Information Network 1 TCP 1/2. Youki Kadobayashi NAIST

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1 Information Network 1 TCP 1/2 Youki Kadobayashi NAIST 1

2 Transport layer: a birds-eye view Hosts maintain state for each transport-layer endpoint Routers don t maintain per-host state H R R R R H Transport IP IP IP IP IP Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Copyright(C)2013 Youki Kadobayashi. All rights reserved. 2

3 Functions provided by the transport layer Communication between processes Identify process Identify inter-process channel P (P, Q) Q Interface for upper layer Connection-oriented (virtual circuit) Connectionless (datagram) Competition and arbitration of network resource Flow control Congestion control Next lecture Copyright(C)2013 Youki Kadobayashi. All rights reserved. 3

4 Transport protocols in the Internet protocol suite TCP (RFC793) Transmission Control Protocol Connection-oriented Multiple functions for reliability UDP (RFC768) User Datagram Protocol Connectionless IP & Process identification Packets can be lost For self study SCTP (RFC4960) DCCP (RFC4340) Advanced topic Copyright(C)2013 Youki Kadobayashi. All rights reserved. 4

5 Identifying process and connection in TCP Unique identification of process (IP, port) Unique identification of TCP connection (source IP, source port, destination IP, destination port) ( , 1040) ( , 22) 1040 connection 22 process 80 connection Copyright(C)2013 Youki Kadobayashi. All rights reserved. 5

6 TCP service model (1) Connection-oriented Virtual Circuit Looks like a circuit, but virtual: no physical wires between specific endpoints Adapts to speed Adapts to speed of intermediate networks as well as the speed of endpoints Copyright(C)2013 Youki Kadobayashi. All rights reserved. 6

7 Establishing a TCP connection 3-way handshake SYN, SYN-ACK, ACK Ensure full-duplex communication SYN URG ACK PSH RST SYN FIN SYN-ACK 16bit source port 16bit destination port 32bit sequence number 32bit acknowledgment number 4bit hlen reserved flags 16bit TCP checksum 16bit window size 16bit urgent pointer ACK Copyright(C)2013 Youki Kadobayashi. All rights reserved. 7

8 Establishing TCP connection: a concrete example with Wireshark # tshark -i en0 -n -f 'port 80' tcpdump: listening on de0 Capturing on en > TCP > 80 [SYN] Seq=0 Win=65535 Len=0 MSS=1460 WS=16 TSval= TSecr=0 SACK_PERM= > TCP 80 > [SYN, ACK] Seq=0 Ack=1 Win=50137 Len=0 TSval= TSecr= MSS=1460 WS=8 SACK_PERM= > TCP > 80 [ACK] Seq=1 Ack=1 Win= Len=0 TSval= TSecr= Reply with Sequence number + 1 as an ACK implies acknowledgment Copyright(C)2013 Youki Kadobayashi. All rights reserved. 8

9 Meanings of Wireshark output time source IP -> dest IP TCP source port > dest port [ flags ] Seq=n Ack=n > TCP 80 > [SYN, ACK] Seq=0 Ack=1 Win=50137 Len=0 TSval= TSecr= MSS=1460 WS=8 SACK_PERM=1 Copyright(C)2013 Youki Kadobayashi. All rights reserved. 9

10 Virtual Circuit(2):TCP connection release close FIN Ack of FIN FIN close Ack of FIN > TCP 80 > [FIN, ACK] Seq=659 Ack=2449 Win= Len=0 TSval= TSecr= > TCP > 80 [ACK] Seq=2449 Ack=660 Win= Len=0 TSval= TSecr= > TCP > 80 [FIN, ACK] Seq=2449 Ack=660 Win= Len=0 TSval= TSecr= > TCP 80 > [ACK] Seq=660 Ack=2450 Win= Len=0 TSval= TSecr= Copyright(C)2013 Youki Kadobayashi. All rights reserved. 10

11 TCP connection reset RST Abortive release Nonexistent port (e.g., dead process) telnet to sh.naist.jp, port 80 will result in connection reset: > TCP > 80 [SYN] Seq=0 Win=65535 Len=0 MSS=1460 WS=16 TSval= TSecr=0 SACK_PERM= > TCP 80 > [RST, ACK] Seq=1 Ack=1 Win=0 Len=0 If you kill Dropbox : > TCP > 80 [RST] Seq=310 Win=0 Len=0 Copyright(C)2013 Youki Kadobayashi. All rights reserved. 11

12 Virtual Circuit Summary: TCP state transition diagram State transition: trigger / response Copyright(C)2013 Youki Kadobayashi. All rights reserved. 12

13 Troubleshooting TCP with state machine netstat $ netstat Active Internet connections Proto Recv-Q Send-Q Local Address Foreign Address (state) tcp http SYN_SENT tcp ey-in-f101.1e100.http ESTABLISHED tcp http ESTABLISHED SYN sent, awaiting SYN+ACK response Many other open-source tools are available: lsof, trpt, tcptraceroute, tcptrace, tcpflow, etc. Try some of these tools in the provided VM image. Copyright(C)2013 Youki Kadobayashi. All rights reserved. 13

14 Hands on: observe connection setup/release In the provided virtual machine, 1.Observe connection setup/release; e.g., by using browser within VM 2.Observe connection reset by terminating program; e.g., by skill firefox 3.Observe connection reset by connecting to nonexistent port on the working machine. e.g., by telnet sh.naist.jp 80 Try many tools: wireshark, tshark, tshark -V Copyright(C)2013 Youki Kadobayashi. All rights reserved. 14

15 Buffered transfer: adapts to varying speed of endpoints, as well as intermediate networks Process Process Write() Read() block/unblock Write() Read() send buffer recv buffer send buffer recv buffer TCP connection OS kernel OS kernel Copyright(C)2013 Youki Kadobayashi. All rights reserved. 15

16 TCP service model (2) Byte-stream service Upper layer can t see boundaries between packets no boundary: structuring (framing) is needed at upper layer O L L E H OK TCP being viewed as byte-stream service O L L E H OK Full duplex independent two streams in single connection Reliable masks packet reordering, duplication, discard and bit error Copyright(C)2013 Youki Kadobayashi. All rights reserved. 16

17 How does TCP implement reliable stream service? ACK: Acknowledgment Active acknowledgment Explicitly acknowledge the receipt of packet Duplicate ACK Implicitly communicate the packet loss information Timeout and Retransmission Whenever sender doesn t receive ACK after fixed time, a Timeout event is triggered Retransmission: assuming that transmission has failed Exponential back-off: 3, 6, 12, Copyright(C)2013 Youki Kadobayashi. All rights reserved. 17

18 ACK: Acknowledgment Sender Sent and acknowledged Sent but unacknowledged Nara Institute of Science and Technology User data arrives Packets in transit Receiver Nara Insti Copyright(C)2013 Youki Kadobayashi. All rights reserved. 18

19 Piggybacking: Exploiting full-duplex channel Note: rough sketch Sender Receiver Sent and acknowledged Sent but unacknowledged Nara Institute of Science and Technology User data arrives Graduate S Packets in transit Receiver Sender Nara Insti Graduate School of Information Science User data arrives Sent and acknowledged Sent but unacknowledged Copyright(C)2013 Youki Kadobayashi. All rights reserved. 19

20 Duplicate ACK: implicit communication of packet loss Sender Sent and acknowledged Sent but unacknowledged Nara Institute of Science and Technology User data arrives Packet loss Packets in transit Receiver Nara Institute o Copyright(C)2013 Youki Kadobayashi. All rights reserved. 20

21 From byte-stream to packets: TCP header Chop appropriate length from byte-stream buffer & add TCP header IP Header TCP Header TCP segment TCP data 16bit source port 16bit destination port 32bit sequence number 32bit acknowledgment number 4bit hlen reserved flags 16bit window size 16bit TCP checksum 16bit urgent pointer (options) 20 octets (TCP data) Copyright(C)2013 Youki Kadobayashi. All rights reserved. 21

22 Nagle algorithm Q. If you added 20byte+20byte size header to 1byte data, isn t that overhead big? Nagle algorithm (RFC896) There is only one small segment which is unacknowledged in the network. In case of short RTT: acceptable overhead, as LAN bandwidth is abundant send packets with small buffering In case of long RTT reduce overhead, due to WAN bandwidth constraints Copyright(C)2013 Youki Kadobayashi. All rights reserved. 22

23 Hands on: observe packet loss & retransmission Within provided virtual machine, 1.Observe duplicate ACK; Emulate lossy network with: # tc qdisc change dev eth0 root netem loss 10% 1.Observe the effect of Nagle algorithm; Emulate satellite network with: # tc qdisc add dev eth0 root netem delay 500ms Copyright(C)2013 Youki Kadobayashi. All rights reserved. 23

24 Questions? Copyright(C)2013 Youki Kadobayashi. All rights reserved. 24

25 Summary thus far Transport layer The transport protocol on the internet TCP TCP: service model, and features Efficiency: ACK, piggybacking, Nagle algorithm TCP connection establishment and release Diagnosis: tools + state machine knowledge Copyright(C)2013 Youki Kadobayashi. All rights reserved. 25

26 Transport protocol challenges With a number of unknown parameters: Number of active communications Bottleneck bandwidth Error rate how can we accommodate as much communications as possible, without collapsing network itself? 1 1 n k Key ideas: probing, estimation, self-policing, macro-level stability Copyright(C)2013 Youki Kadobayashi. All rights reserved. 26

27 Flow control and congestion control: Competition and Arbitration of Network Resource Flow control Absorb difference of transmission rate Recovery from sequence error Recovery from duplication, packets drop and bit error Congestion control Sharing the bandwidth while suppressing the congestion Fair sharing of bandwidth Copyright(C)2013 Youki Kadobayashi. All rights reserved. 27

28 Characteristics of TCP Flow Control End to end No global assignment of resource Estimate available bandwidth at individual hosts Routers do not explicitly allocate resource Implicit signaling through packet drops Scalable Host-based autonomous method has better scalability, because it doesn t need state management inside network. Autonomous Less state management Scalable Copyright(C)2013 Youki Kadobayashi. All rights reserved. 28

29 TCP: key characteristics Very simple algorithm Macroscopic self-stabilization TCP doesn't assume the presence of greedy nodes. Elimination of global control system Reject the idea of intermediate policing system Modest performance across almost all data-links As opposed to optimal performance in specific condition Stepwise improvements over 30 years Copyright(C)2013 Youki Kadobayashi. All rights reserved. 29

30 Flow Control on TCP Control available bandwidth Sliding window Using sequence number, not packet number Window size Control transmission interval of packets ACK clocking Miscellaneous Error detection with TCP checksum Packet drop detection with duplicate ACK, timeout Copyright(C)2013 Youki Kadobayashi. All rights reserved. 30

31 Sliding window Sent and acknowledged Sent but unacknowledged Sender Nara Institute of Science and Technology Window size User data arrives Sequence number Packets in flight Receiver Nara Insti Copyright(C)2013 Youki Kadobayashi. All rights reserved. 31

32 Congestion Control on TCP Fair-share model: fair distribution of bandwidth End to end Increase and decrease of window size additive increase multiplicative decrease AIMD is known to be self-stabilizing (R. Jain, et al., Analysis of the increase and decrease algorithms for congestion avoidance in computer networks, 1989) Switch increasing strategy of window size Detect congestion through packet drops Copyright(C)2013 Youki Kadobayashi. All rights reserved. 32

33 Increasing Window size TCP switches increasing strategy of congestion window (cwnd) by slow start threshold (ssthresh) (Outline of the algorithm) On receiving an ACK: if (cwnd < ssthresh) { /* slow start: exponential increase */ cwnd += 1; } else { /* congestion avoidance: additive increase */ cwnd += 1 / cwnd; } Copyright(C)2013 Youki Kadobayashi. All rights reserved. 33

34 Increasing Window size Slow start exponential increase Congestion avoidance additive increase Source: TCP/IP Illustrated, Vol.1 Effectiveness: see V. Jacobson, Congestion Avoidance and Control, SIGCOMM 88. Copyright(C)2013 Youki Kadobayashi. All rights reserved. 34

35 Decreasing Window size (Outline of algorithm ) On detecting packet drop: ssthresh = cwnd / 2; if (timeout) { cwnd = 1; } Source: TCP/IP Illustrated, Vol.1 Copyright(C)2013 Youki Kadobayashi. All rights reserved. 35

36 Questions? Copyright(C)2013 Youki Kadobayashi. All rights reserved. 36

37 Packet drop Detection with 2 methods: Duplicate ACK Retransmission Time Out (RTO) How do I judge the time out? RTO ~ RTT Estimator Copyright(C)2013 Youki Kadobayashi. All rights reserved. 37

38 RTO Calculation Source: TCP/IP Illustrated, Vol.1 Err = M A A <- A + gerr D <- D + h( Err - D) RTO = A + 4D A: smoothed RTT D: smoothed mean deviation g: gain for the average (1/8) h: gain for the deviation (1/4) Copyright(C)2013 Youki Kadobayashi. All rights reserved. 38

39 RTO calculation Source: A Quick Tour Around TCP, Copyright(C)2013 Youki Kadobayashi. All rights reserved. 39

40 Summary Transport layer The transport protocol on the internet TCP TCP: service model, and features Efficiency: ACK, piggybacking, Nagle algorithm TCP connection establishment and release Flow control and congestion control in TCP Copyright(C)2013 Youki Kadobayashi. All rights reserved. 40