Information Network 1 Transport layer: TCP - iplab.naist.jp · " Congestion control Next lecture P...
Transcript of Information Network 1 Transport layer: TCP - iplab.naist.jp · " Congestion control Next lecture P...
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Information Network 1 Transport layer: TCP
Youki Kadobayashi NAIST�
Copyright(C)2015 Youki Kadobayashi. All rights reserved.
Transport layer: a birds-eye view�
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H HR�R�R� R�
IP�IP� IP� IP�IP�
Transport�
Routers don’t maintain per-host state�
Hosts maintain state for each transport-layer endpoint�
Application Presentation
Session Transport Network
Data Link Physical
Application Presentation
Session Transport Network
Data Link Physical
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Functions provided by the transport layer
! Communication between processes " Identify process " Identify inter-process channel
! Interface for upper layer
" Connection-oriented (virtual circuit) " Connectionless (datagram)
! Competition and arbitration of network resource
" Flow control " Congestion control
Next lecture
P Q
(P, Q)�
Copyright(C)2015 Youki Kadobayashi. All rights reserved.
Transport protocols in the Internet protocol suite�
! TCP (RFC793) " Transmission Control Protocol
" Connection-oriented " Multiple functions
! for reliability
! SCTP (RFC4960)
! UDP (RFC768) " User Datagram Protocol
" Connectionless " IP & Process identification�
! Packets can be lost
! DCCP (RFC4340)
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For self study
Advanced topic
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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
80
22
2137
(203.178.136.36, 22)
163.221.52.100 203.178.136.36
(163.221.52.100, 1040)
connection
connection
process
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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
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Establishing a TCP connection
! 3-way handshake ! SYN, SYN-ACK, ACK ! Ensure full-duplex communication
16bit source port 16bit destination port
32bit sequence number
32bit acknowledgment number
4bit hlen 16bit window size
16bit TCP checksum 16bit urgent pointer
reserved flags
URG ACK PSH RST SYN FIN
SYN
SYN-ACK
ACK
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Establishing TCP connection: a concrete example with Wireshark ! # tshark -i en0 -n -f 'port 80' ! tcpdump: listening on de0 ! Capturing on en0 ! 0.164720 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [SYN] Seq=0
Win=65535 Len=0 MSS=1460 WS=16 TSval=1135377929 TSecr=0 SACK_PERM=1
! 0.195154 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [SYN, ACK] Seq=0 Ack=1 Win=50137 Len=0 TSval=190559467 TSecr=1135377929 MSS=1460 WS=8 SACK_PERM=1
! 0.195322 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [ACK] Seq=1 Ack=1 Win=131760 Len=0 TSval=1135377958 TSecr=190559467
Reply with Sequence number + 1 as an ACK implies acknowledgment�
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Meanings of Wireshark output
! time source IP -> dest IP TCP source port > dest port [ flags ] Seq=n Ack=n …
! 0.195154 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [SYN, ACK] Seq=0 Ack=1 Win=50137 Len=0 TSval=190559467 TSecr=1135377929 MSS=1460 WS=8 SACK_PERM=1
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Virtual Circuit(2):TCP connection release
FIN
Ack of FIN
FIN
Ack of FIN
close
close
5.749366 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [FIN, ACK] Seq=659 Ack=2449 Win=401096 Len=0 TSval=190560021 TSecr=1135378482 5.749464 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [ACK] Seq=2449 Ack=660 Win=131104 Len=0 TSval=1135383482 TSecr=190560021 5.749650 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [FIN, ACK] Seq=2449 Ack=660 Win=131104 Len=0 TSval=1135383482 TSecr=190560021 5.765279 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [ACK] Seq=660 Ack=2450 Win=401096 Len=0 TSval=190560024 TSecr=1135383482
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TCP connection reset
! RST " Abortive release " Nonexistent port (e.g., dead process)
telnet to sh.naist.jp, port 80 will result in connection reset: ! 0.000000 10.0.1.148 -> 163.221.10.10 TCP 63436 > 80 [SYN] Seq=0 Win=65535
Len=0 MSS=1460 WS=16 TSval=1137289324 TSecr=0 SACK_PERM=1 ! 0.018841 163.221.10.10 -> 10.0.1.148 TCP 80 > 63436 [RST, ACK] Seq=1
Ack=1 Win=0 Len=0
If you kill “Dropbox”: ! 55.992214 10.0.1.148 -> 108.160.163.51 TCP 62991 > 80 [RST] Seq=310 Win=0
Len=0
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Virtual Circuit Summary: TCP state transition diagram�
State transition: trigger / response�
Copyright(C)2015 Youki Kadobayashi. All rights reserved.
$ netstat!Active Internet connections!Proto Recv-Q Send-Q Local Address Foreign Address (state)!tcp4 0 0 45.1.20.101.57456 74.125.235.138.http SYN_SENT!tcp4 0 0 45.1.20.101.57455 ey-in-f101.1e100.http ESTABLISHED!tcp4 0 0 45.1.20.101.57454 74.125.235.148.http ESTABLISHED!�
Troubleshooting TCP with state machine�
! netstat�
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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.�
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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
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Buffered transfer: adapts to varying speed of endpoints, as well as intermediate networks
TCP connection
send buffer
recv buffer
send buffer
recv buffer
Process
OS kernel
block/unblock Read() Write()
Process
Read() Write()
OS kernel
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TCP service model (2)
! Byte-stream service " Upper layer can’t see boundaries between packets " no boundary: structuring (framing) is needed at upper layer
! Full duplex " independent two streams in single connection
! Reliable " masks packet reordering, duplication, discard and bit error
O L L E H O L L E H TCP being viewed as byte-stream service OK OK
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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, …
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ACK: Acknowledgment
Packets in transit
Sent but unacknowledged Sent and acknowledged
User data arrives
Sender
Receiver
Nara Institute of Science and Technology
Nara Insti
10
16
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Piggybacking: Exploiting full-duplex channel
Packets in transit
Sent but unacknowledged Sent and acknowledged
User data arrives
Sender Receiver
Receiver Sender
Nara Institute of Science and Technology
Nara Insti
Graduate School of Information Science
Graduate S
User data arrives
Note: rough sketch �
Sent but unacknowledged Sent and acknowledged
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Duplicate ACK: implicit communication of packet loss
Packets in transit
Sent but unacknowledged Sent and acknowledged
User data arrives
Sender
Receiver
Nara Institute of Science and Technology
Nara Institute o
Packet loss
10
16
16
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From byte-stream to packets: TCP header
IP Header
TCP Header TCP data
TCP segment
16bit source port 16bit destination port
32bit sequence number
32bit acknowledgment number
4bit hlen 16bit window size
16bit TCP checksum 16bit urgent pointer
(options)
(TCP data)
reserved flags
20 octets
Chop appropriate length from byte-stream buffer & add TCP header�
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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
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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%
2. Observe the effect of Nagle algorithm; " Emulate satellite network with: " # tc qdisc add dev eth0 root netem delay 500ms
Copyright(C)2015 Youki Kadobayashi. All rights reserved. 24
Questions?
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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)2015 Youki Kadobayashi. All rights reserved.
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?�
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1� 1�
n� k�
…�
…�
Key ideas: probing, estimation, self-policing, macro-level stability�
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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�
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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
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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�
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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
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Sliding window
Packets in flight
Sent but unacknowledged Sent and acknowledged
User data arrives
Sender
Receiver
Nara Institute of Science and Technology
Nara Insti
10
16
Window size Sequence number
Copyright(C)2015 Youki Kadobayashi. All rights reserved. 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
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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; }
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Increasing Window size�
! Slow start " exponential increase
! Congestion avoidance " additive increase
! Effectiveness: see V. Jacobson, “Congestion Avoidance and Control”, SIGCOMM’88.�
Source: TCP/IP Illustrated, Vol.1�
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Decreasing Window size�
(Outline of algorithm ) ! On detecting packet drop:
ssthresh = cwnd / 2; if (timeout) { cwnd = 1; }
Source: TCP/IP Illustrated, Vol.1�
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Questions?
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Packet drop
! Detection with 2 methods: " Duplicate ACK " Retransmission Time Out (RTO)
! How do I judge the time out? → RTO ~ RTT Estimator
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RTO Calculation
! 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)
Source: TCP/IP Illustrated, Vol.1�
Copyright(C)2015 Youki Kadobayashi. All rights reserved.
RTO calculation�
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Source: A Quick Tour Around TCP, http://web.eecs.utk.edu/~dunigan/tcptour/
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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