Network Protocols: Design and Analysis

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Network Protocols Design and Analysis

• Polly Huang
• EE NTU
• http//cc.ee.ntu.edu.tw/phuang
• phuang_at_cc.ee.ntu.edu.tw

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TCP Overview
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What does TCP Provide?

• Connection establishment
• Connectionless communication
• Congestion avoidance
• Differentiated services
• Duplicate packet detection
• Flow control

• loss recovery
• message or record boundaries
• ordered data delivery to the application
• out-of-order data delivery to the application
• quality-of-service
• urgent data indication

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Where and Why is TCP Used?

• where anywhere reliable communication is needed
• file transfer/ftp, http, p2p e-mail smpt remote
  login/telnet db transfer/dns some real audio
• why?
• connection orientedcan be easier to manage (ex.
  firewall config)
• has the right features (congestion ctl, etc.)
• widely deployed gt interoperability, understood,
  exhaustively studied, pretty good implementations
• doing your own protocol is a lot of work

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TCP in a Nutshell

• abstraction
• reliable
• ordered
• point-to-point
• byte-stream

• mechanisms
• window-based flow control
• sequence numbers/ordering, 3-way handshake
• reliability (ACK, retx policies)
• congestion control
• RTT estimation

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TCP Header
Source port
Destination port
Sequence number
Flags
SYN FIN RESET PUSH URG ACK
Acknowledgement
Advertised window
Hdr len
Flags
0
Checksum
Urgent pointer
Options (variable)
Data
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Agenda

• connection setup and teardown
• initial sequence number selection
• passive/active open
• time-wait
• flow control
• congestion control practice and theory
• loss recovery
• security
• performance

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Three-Way Handshake

• why?
• passes buffer sizes
• connection startup
• set up initial seq number
• options
• is someone there?

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Connection Setup States
passive vs. active (and both active!)
(error recovery is not on this figure)
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Initial Sequence Number Selection

• Why not just start at 0?
• want to avoid accidental replay of old packets
  (tcp connection src dest ip address port)
• Approach
• randomly chosen
• OR semi-sequentally

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Tear-down Packet Exchange
Sender
Receiver
FIN
FIN-ACK
Data write
Data ack
FIN
FIN-ACK
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Connection Tear-down
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Connection Tear-down

• either side can close
• or one side can close and the other stay open
• one side must maintain state (TIME_WAIT) for 2
  minutes, why?
• suppress old packets

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Agenda

• connection setup and teardown
• flow control
• setting window sizes
• Nagles algorithm
• silly window syndrome
• protection against wrap-around
• congestion control practice and theory
• loss recovery
• security
• performance

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Flow Control

• Window sizes are passed in every packet
• beware implementations often have separate TCP
  and socket buffers
• effective window is the minimum of the two

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Flow Control

• Why?
• avoid overrunning receiver
• Solutions
• TCP have a sliding window w/how much data can be
  outstanding
• (OR could have a rate)

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Window Flow Control Sender
effective window
advertised window (from receiver)
Sent but not acked
Not yet sent
Sequence numbers
send buffer
last byte sent
last byte ACKed
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Window Flow Control Receiver
Receive buffer (possible window)
ACKed but not delivered to user
recvd but not ACKed
Sequence numbers
missing data
advertised window
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Window Advancement Issues

• What if window is full?
• sender sends full window, but ACK is lost
• sender sends 1-byte probes (solicits new ACK)
• Silly window syndrome (RFC-813)
• receiver dribbles out small window advances
• Silly Window Avoidance delay ACKing (receiver)
  or sending small segments (sender)
• Sender who dribbles out data (like telnet)
• Nagles algorithm (RFC-896) send 1st partial
  packet, but not more until its ACKed or you have
  a full packet

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Problem Rapid Wrap-Around

• Wraparound time vs. Link speed
• 1.5Mbps 6.4 hours
• 10Mbps 57 minutes
• 45Mbps 13 minutes
• 100Mbps 6 minutes
• 622Mbps 55 seconds
• 1.2Gbps 28 seconds
• Protection Against Wrapped Sequences (PAWS
  extension) Use timestamp to distinguish sequence
  number wraparound

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Agenda

• connection setup and teardown
• flow control
• congestion control theory
• what and why
• how
• congestion control practice
• loss recovery
• security
• performance

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Congestion Collapse

• If both sources send full speed, the router is
  completely overwhelmed
• congestion collapse senders lose data from
  congestion and they resend, causing more
  congestion (can be self-reinforcing)
• has been observed many times

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Congestion Control vs. Flow Control

• What does flow control do?
• avoids overrunning the receiver
• What does congestion control do?
• avoid overrunning router buffers avoid
  saturating the network
• What mechanism do they use?
• both use windows (flow control) wnd, (congestion
  control) cwnd actual window used is the MIN of
  wnd and cwnd

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Congestion Control Goals

• control network buffer usage
• avoid congestion collapse
• want to fairly allocate network resources
• make good use of network bandwidth power

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Power and Load
knee

• throughput and delay change due to load
• want to optimize power

(From Ramakrishnan90a)
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Fairness

• Also want fairness
• should treat all users equally
• but its not so easy
• what is a user? host, flow, person?
• if n flows through a link, each should get n-1
  of the bandwidth
• RJs fairness index (Sxi)2/n(Sxi2)
• but what if flows have different needs?
  different RTTs?

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Congestion Control Design

• Avoidance or control? (RJ)
• avoidance keeps system at knee of curve
• requires some congestion signal
• control responds to loss after the fact
• TCP
• Which is TCP?
• congestion control (according to RJs
  definition)
• How does TCP do it?
• slow start, congestion avoidance, exponential
  backoff

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How to Adjust Window?

• When to increase/decrease?
• A control theory problem
• observe network
• reduce window if congested
• increase window if not congested

• Constraints
• efficency
• fairness
• stability (too much oscillation is bad)
• out-of-date info
• RTT is fundamental limit to how quickly you can
  react

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Linear Control

• Xi(t 1) ai(t,f) bi(t,f) Xi(t)
• Formulation allows for the feedback signal
• to change additively ai(t)
• to change multiplicatively bi(t)
• can consider feedback
• What does TCP do and why?
• AIMD additive increase, multiplicative decrease
• Types of feedback in Internet?
• packet loss or ECN (explicit congestion
  notification) or in RJ binary feedback DECBit

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Agenda

• connection setup and teardown
• flow control
• congestion control theory
• congestion control practice (in TCP)
• loss recovery
• security
• performance
• gt next sets of slides