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29a7a58a6   Rémi Emonet   cm6 and corr
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  <!DOCTYPE html>
  <html>
      <head>
          <meta charset="utf-8">
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          <title>Computer Networks (6)</title>
          <meta name="cours-n" content="6">
  
          <meta name="author" content="Rémi Emonet">
          <meta name="venue" content="DWA M1 WI/MLDM">
          <meta name="date" content="2017">
          <meta name="affiliation" content="Université Jean Monnet − Laboratoire Hubert Curien">
  
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  <section class="smart">
  
  # @chunk: chunks/title.md
  
  # @chunk: chunks/objectives.md
  
  <!-- PART 3 -->
  ## Computer Networks <span>{var-cours-n}</span>: Plan {#plan overview}
  @SVG: media/stack-index.svg 100px 400px {svg floatright appTrans margin-left-minus-100}
  
  - Goal: transport layer {shaded}
    - understand its main roles and mechanisms
    - understand the how TCP (and UDP) are implemented
  - Overview
    - Transport layer: context and services (role) {it1}
      // see how it fits between application and network
    - Multiplexing and demultiplexing {it3}
    - UDP {it4}
    - Reliable communications, please! {it5}
    - Pipelining: principle and algos {it6}
    - Implementation of TCP {it7}
    - Online timeout estimation {it8}
    - Congestion: principle and algos {it9}
    - TCP: optimality? equity? {it10}
  # @copy:#plan
  
  
  
  
  
  
  <!-- TCP IMPLEMENTATION -->
  # @copy:#plan: %+class:inred: .it7
  
  ## TCP: Transmission Control Protocol <br/><span style="font-size:70%">[RFC 793,1122,1323, 2018, 2581]</span> // 2018: selective repeat
  - TCP communication
    - between two processes
    - reliable, ordered
    - stream-based: no separation between messages
      // even if sent as segments
    - bi-directional
    - connection oriented, stateful, handshake at the beginning
      // initialize the state between sender and receiver
    - pipelined (with a variable window size)
      // congestion
    - with flow control
      // limits bitrate to the receiver
  
  
  ## TCP: formats of segment headers {libyli}
  @SVG: media/part3/segment-structure-tcp.svg 250px 500px {floatright withmargin}
  
  - Total size: variable (20 bytes minimum)
  - Source and dest. ports (16 bits each)
  - Sequence and ack numbers: **in bytes** // ! not the packet number
  - *offset*: size of the header (cf. options)
  - *flags*, booleans, including
    - ACK: this segment contains a ACK
    - SYN: connection initialization <br/> (exchange of the initial sequence number)
    - FIN: end of connection
  - *receive window*: size in bytes that the receiver wants
    // flow control
  - *checksum* as in UDP
  
  
  ## TCP: sequence and ack numbers {libyli}
  @SVG: media/part3/segment-structure-tcp.svg 150px 300px {floatright withmargin}
  
  - Reminder: TCP handles a stream of bytes
  - Sequence numbers
    - index in the stream of the first byte of the segment
  - Acknowledgment (ack)
    - cumulative acknowledgment (as in <i>go-back-N</i>)
    - stream index of the next byte to be received
    - ACK flag is set to 1
  
  @SVG: media/part3/tcp-sequence-number.svg 350px 100px {floatright clearright withmargin}
  - go-back-N or selective repeat?
    - cumulative acknowledgment
    - TCP, by default, does not specify what to do with out-of-order packets
    - RFC2018: option for using *selective repeat* (SACK)
  
  ## TCP: examples
  @SVG: media/part3/tcp-sequence-ex1.svg 350px 350px {floatleft}
  
  @SVG: media/part3/tcp-sequence-ex2.svg 450px 500px {floatright second}
  - @anim: #e11 |#e12 |#e13 |#e14 |#l2r |#r2l
  - @anim: .second, #s1 |#s2 |#a1 |#a2 |#s3 |#s4 |#a3 |#a4 |#s5
  
  ## TCP: multiples acknowledgment {libyli}
  - TCP acknowledgment (reminder)
    - ack with the index of the next expected byte
    - out-of-order packets &rArr; double ack
  - Using double acks for fast re-transmission
    - the timeout is generally long
    - if we receive multiple double acks, there was probably a loss
    - TCP strategy
        - on the 3<sup>rd</sup> double ack (4<sup>th</sup> identical)
        - send the packet again
  - @anim: .floatright
  
  @SVG: media/part3/tcp-sequence-ex3.svg 250px 250px {floatright}
  
  ## TCP Fast Retransmit
  @SVG: media/part3/tcp-sequence-ex3.svg 450px 500px {centered}
  
  - @anim: #send1 |#ac1 |#ac2 |#ac3 |#ac4 |#send2
  
  
  
  <!-- ??????? -->
  ## TCP Flow Control
  @SVG: media/part3/segment-structure-tcp.svg 200px 500px {floatright withmargin}
  
  - Goal: do not flood the receiver
  - TCP has a receive buffer
    - typical size: 4096 kB
    - the system can adapt it dynamically
  - rcv win {slide}
    - send in the tcp header of every segment
    - amount of available space in the buffer
  
  ## TCP Connection Opening
  @SVG: media/part3/tcp-sequence-handshake.svg 200px 500px {floatright withmargin}
  
  - Client {slide}
    - generation of a sequence number
    - emission of a SYN packet
  - Server {slide}
    - generation of a sequence number
    - emission of a SYNACK packet (ACK + SYN)
  - Client {slide}
    - emission of a ACK packet
    - can also start sending data in this packet
  
  
  
  
  
  
  <!-- ONLINE RTT ESTIMATION -->
  # @copy:#plan: %+class:inred: .it8
  
  
  ## What should be the timeout in TCP? <br/>(before retransmitting <br/> a non-ACK'd packet){question}
  
  ## Timeout and Round Trip Time
  - Choice of the timeout
    - longer than RTT
    - but RTT varies
    - if too short: useless retransmissions
    - if too long: long delay on loss
      // still we have the 3dup acks fast retransmit
  - Estimation of RTT{slide}
    - $\text{obsRTT}_i$: time between sending packet $i$ and receiving its ACK
    - $\text{obsRTT}_i$ varies and is unstable &rArr; moving average{slide}
  
  ## Timeout in TCP: RTT estimation
  @SVG: media/part3/running-average.svg 700px 300px {graph}
  
  - @anim: .truc | .graph
  - At each new ACK ($\text{obsRTT}_i$){truc}
    - $\text{avg} = (1-\alpha) \; \text{avg} + \alpha \; \text{obsRTT}_i$
    - moving/rolling/running average
    - exponential weighting{slide}
    - value for $\alpha$ : 0.125{slide}
  
  ## Timeout Computation in TCP
  - Estimation of RTT{slide}
    - $\text{avg} = (1-\alpha) \; \text{avg} + \alpha \; \text{obsRTT}_i$
  - Estimation of the mean deviation{slide}
    - $\text{dev} = (1-\beta) \; \text{dev} + \beta \; \left| \text{obsRTT}_i - \text{avg}\right|$ // absolute
  - Transmission delay $ = \text{avg} + 4 \cdot \text{dev}{}${slide}
  - Values: $\alpha = 0.125 \;\; \beta = 0.25${slide}
  - @anim: .centered
  
  @SVG: media/part3/tcp-timeout-estimation.svg 700px 200px {centered}
  
  
  
  <!-- CONGESTION -->
  # @copy:#plan: %+class:inred: .it9
  
  
  ## Congestion: principles (infinite queue)
  @SVG: media/part3/congestion-infinite.svg 800px 350px {centered}
  
  - Ideal case of an infinite queue {a}
  - Maximal bandwidth {b}
  - Unreasonable delays (time spend in the queue){c}
    // even before reaching C/2
  - @anim: #lin |#lout |#finf |#axes |#curve |.a |.b |.c
  
  ## Congestion: principles
  @SVG: media/part3/congestion-bounded.svg 800px 350px {centered}
  
  - More realistic case with limited buffers and packet loss{a}
  - Retransmitting packets due to timeout (loss, delay){b}
  - Reduced bandwidth due to retransmissions {c}
  - @anim: .centered |#lin |#lout |#finf |#axes |#curve |.a |.b |.c
  
  ## Congestion: in a network
  @SVG: media/part3/congestion-network.svg 800px 350px {centered}
  
  - Augmenting the rate of a connection can penalize the rest {a}
  - When a router drops packets, <br/>all the bandwidth used to bring the packet there is wasted {b}
  - @anim: #mask+#blue |#green |#red |#graph |.a |.b
  
  ## Congestion Control/avoidance: <br/>two kinds of approaches
  - Congestion-control assisted by the network{slide}
    - smart routers
    - router&rarr;host messages on congestion level {slide}
    - the network tells the host what bandwidth to use {slide}
    - disadvantages {slide}
        - expensive routers
        - difficult to get robustness
          // more complex
  - Congestion-control at a host level{slide}
    - the network is a black box
      // we still suppose its behavior (drops, ...)
    - based on observed delays and losses {slide}
    - used by TCP{slide}
      // we will see only the case of TCP
  
  
  
  
  
  <!-- OPTIMALITYCONGESTION -->
  # @copy:#plan: %+class:inred: .it10
  
  
  ## TCP Congestion Control (+reminders)
  @svg: media/part3/tcp-sequence-number.svg 400px 100px {floatright withmargin}
  
  - Sequence number in bytes
  - Sliding window {slide}
    - limitation on the sender side
    - (last byte sent - last byte ACK'd) &le; N{slide}
    - N is also denoted $cwnd$ (Congestion Window){slide}
  - TCP transmission rate{slide}
    - approximately: $\frac{cwnd}{RTT}{}$
      // explication...
  - Congestion control in TCP{slide}
    - dynamic adaptation of $cwnd$
    - as a function of the observed delays and losses
    - different possible algorithms (still evolving)
  
  ## TCP Congestion Control: principles
  - Given $MSS$: maximum TCP segment size{slide}
  - Increases the window size (emission rate){slide}
    - goal: use at best the available bandwidth
    - additive increase ($cwnd = cwnd + MSS$){slide}
    - reacts in case of packet loss {slide}
  - In case of loss {slide}
    - diminishes the window size
    - multiplicative decrease  ($cwnd = 0.5 \times cwnd$)
  - Concept of “slow start” {slide}
    - initial phase
    - goal: reach/find as fast as possible the bandwidth limit
    - multiplicative increase at the beginning ($cwnd = 2 \times cwnd$)
  
  ## TCP Slow Start
  @svg: media/part3/tcp-sequence-slowstart.svg 200px 500px {floatright withmargin}
  
  - Exponential increase of the window size
    - initialization: $cwnd = MSS$
    - $MSS$: maximum segment size
    - $cwnd = 2\times cwnd$ at each RTT{slide}
    - $cwnd = cwnd + MSS$ at each ACK{slide}
      // in practice
  - Slow start{b}
    - starts with a low rate
    - increases the rate exponentially
    - end of the slow start phase
        - in case of loss
        - or, when a threshold is reached: $cwnd \ge ssthres$
  - @anim: #g1 |#g2 |#g4 |#g8 |.b
  
  ## TCP Congestion Avoidance (CA)
  @svg: media/part3/tcp-congestion-graph.svg 700px 400px {centered}
  
  - @anim: #slowstart |#lin1 |#half1 |#rest + .a
  - Additive increase {a}
  - Multiplicative decrease{a}
  
  
  ## Loss Detection+Compensation (TCP Reno)
  - Timeout expiration {slide}
    // an actual problem, not normal
    - re-initialization: $cwnd = MSS$ ; then slow start again
  - Triple double-ACK (4 times the same ACK){slide}
    // less problematic: the network is operation, just bad luck or BEGIN of congestion
    - $cwnd = 0.5 \times cwnd$ ; then linear increase
    - TCP Tahoe (older): re-initializing cwnd
  - @anim: .centered
  
  @svg: media/part3/tcp-congestion-graph-precise.svg 700px 290px {centered}
  
  
  <!-- NB: need CSS -->
  ## Number of packets emitted… <br/> from the start to the 1<sup>st</sup> red arrow? <br/>up to the 2<sup>nd</sup> red arrow?{question bottom} // to validate understanding of the graph
  @svg: media/part3/tcp-congestion-graph-how-many-packets.svg 700px 290px {hum}
  
  
  <!-- optimality/equity -->
  
  ## Optimality and Equity of TCP
  @SVG: media/part3/tcp-fairness.svg 700px 400px {svg}
  
  @anim: #lienC | #graph | #equity |#gstart |#gs1 |#climit |#gs2 |#gs3 |#gs4 |#gs5 |#gs6 |#gs7
  
  
  ## (In)Equity of TCP
  - UDP has no congestion control {slide}
    - sends packets, interpolation/correction in case of packet loss
    - no emission reduction in case of congestion
    - some routers are blocking UDP?
  - Multiple TCP connections{slide}
    - TCP provides connection-equity
    - opening of multiple connections
        - common for web browsers, etc
    - example, if there are already 4 connections {slide}
        - the new application opens 1 connection &rArr; effective rate of $\frac{R}{5}{}$
        - the new application opens 4 connections &rArr; effective rate of $\frac{R}{2}{}$ {slide}
    - NB: routers can still do IP based drop {slide}
29a7a58a6   Rémi Emonet   cm6 and corr
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  </section>
  
  
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