CSCE 515: Computer Network Programming

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CSCE 515Computer Network Programming

• Chin-Tser Huang
• huangct_at_cse.sc.edu
• University of South Carolina

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TCP Reno

• Additive increase and multiplicative decrease
  (AIMD)
• Increase cwnd by 1 MSS every RTT in the absence
  of loss events
• Cut cwnd in half after a message loss, and apply
  additive increase again

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TCP Persist Timer

• When window size goes to 0, sender cannot
  transmit more data
• If the ACK that opens window is lost, can end up
  in a deadlock in which receiver waiting for data
  and sender waiting for window update
• Sender uses a persist timer to send window probes
  to receiver every 60 seconds
• Until window opens up or either of applications
  using this connection is terminated

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TCP Keepalive Timer

• An idle connection may occupy some resources on
  server side
• Server sends a probe packet after connection has
  been idle for 2 hours
• Controversial because it may cause a good
  connection to be terminated due to temporary loss
  of network connectivity

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TCP Keepalive Timer

• Four scenarios
• Other end is still up receive response from
  other end and reset keepalive timer back to 2
  hours
• Other end has crashed no response from other
  end send 10 more probes every 75 seconds and
  terminate connection if no response
• Other end has crashed and rebooted receive reset
  from other end and terminate connection
• Other end is currently unreachable receive ICMP
  host unreachable error send 10 more probes every
  75 seconds and terminate connection if unreachable

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TCP Window Scale Option

• Apply a scaling to 16-bit window size field
• Shift count is between 0 and 14
• Can specify a window of up to 65536214 bytes

kind3
len3
shift count
1
1
1
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TCP Timestamp Option

• Let more segments be accurately timed
• Sender places a 32-bit value in timestamp value
  field
• Receiver echoes it in reply field
• Timestamp is monotonically increasing

kind8
len10
timestamp value
timestamp echo reply
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4
1
1
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Wrapped Sequence Numbers

• TCP Sequence and ACK fields contain 32-bit values
  
• If using a 1GB window, could conceivably wrap the
  sequence number space to a point where old
  packets containing a valid sequence number could
  reappear
• A monotonically increasing timestamp extends
  sequence number can resolve this condition

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T/TCP Extension for Transaction

• In transaction service, it is desirable to avoid
  overhead of connection establishment and
  termination, and to reduce latency to RTTSPT
• TCP is too much and UDP is too little
• T/TCP is an alternative solution
• Use connection count (CC) option to identify
  connections opened to a certain host
• Client packs SYN, request, FIN, CC in first
  segment
• If received CC is larger than cached CC, server
  passes data to application without three-way
  handshake
• Reduce transaction sequence to three segments

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Next Class

• HyperText Transfer Protocol (HTTP)
• Read JNP Ch. 14, 17