Transport Layer

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Transport Layer
TCP Connection
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TCP Segment

• Source port destination port
  process-to-process
• Sequence number the position of the first data
  byte of a segment
• Acknowledgement number the next data byte that
  the receiver expects to receive
• URG if it is set, the urgent pointer is valid
• ACK if it is set, the acknowledgement number is
  valid

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TCP Segment

• RST some abnormal conditions
• SYN request for a connection
• FIN does not have any more data
• Receive Window specify the number of bytes the
  sender wants to receive
• Options other functions that are not covered by
  the header an example MSS

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Process in terminating a TCP connection

• A is a client, B is a server
• A sends the last segment with FIN
• B sends an acknowledgement ACK
• The TCP in A then enters TIME_WAIT state and
  starts the TIME_WAIT timer with an initial value
  set two twice maximum segment lifetime (2MSL)
• While A in the TIME_WAIT, the only valid segment
  is FIN segment from B if it is received ACK is
  sent.
• TIME_WAIT timer is restarted
• When TIME_WAIT timer expires the connection is
  closed

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Flow Control

• To eliminate the possibility of the sender
  overflowing the receivers buffer
• The rate at which the sender is sending must
  match to the rate at which the receiving
  application is reading

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Flow Control

• The receiving host maintains Receive Window
  (RcvWindow)
• RcvWindow tells the sending host how much free
  buffer space is available at the receiving host
• The Application Process at the receiving host
  reads from the buffer

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Flow Control

• Two important variables related to the buffer at
  the receiving host
• LastByteRead
• LastByteRcvd
• To avoid overflow at the Receiving host
• LastByteRcvd LastByteRead lt RcvBuffer
• RcvWindow RcvBuffer LastByteRcvd -
  LastByteRead
• RcvWindow is sent to the sender

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Flow Control

• The sending host keeps unacknowledged data less
  than the value of RcvWindow
• LastByteSent LastByteAcked lt RcvWindow
• Interesting Scenarios
• When the receiving advertises RcvWindow 0
• When buffer at the receiving is emptied

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Flow Control

• In UDP flow control is not provided
• The process reads one entire segment at a time
  from the buffer.
• What happen if the process does not read the
  segment fast enough from the buffer?
• The buffer will overflow and segments will be
  dropped