NUS.SOC.CS2105

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Application
Transport
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Network
Link
Physical
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TCP

• Transmission Control Protocol

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Application
Transport (process-to-process)
Network (unreliable, host-to-host)
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src port
dest port
checksum
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sequence number
acknowledgement number
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Data to Send
Data Received
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TCP

• TCP is a full-duplex, connection-oriented,
  reliable protocol. TCP grabs data from a send
  buffer (containing data from applications), add
  TCP header, and pass to network layer for
  transmission. TCP can grab MSS bytes (maximum
  segment size) at one time.
• TCP header is 20 bytes (maybe longer if includes
  option header). Each segment contains a 32-bit
  sequence number and acknowledgement number.
  Sequence number is the byte number of the first
  byte in the segment Acknowledgement number is
  the byte number of the first byte expected.

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GBN SR
ACK Cumulative Selective
Out-of-Order Throw Buffer
Retransmit All unACK 1 unACK
Timer One Many
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• call from above

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• recv ACK y

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• timeout

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Reliability in TCP

• TCP uses a mixed of techniques from GBN and SR.
• Cumulative ACK since ACK number tells sender
  which byte the receiver is expecting.
• Only one timer for oldest unacknowledged
  segment.
• Receiver buffers out-of-order segments.
• Retransmit only one packet when timeout occur.
• TCP retransmit if it receives duplicate ACKs, but
  only after 3 of them is received (fast retransmit)

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receiver window
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Receiver Buffer
Sender
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Flow Control in TCP

• To avoid buffer overflow at receiver, the
  receiver limits the number of bytes the sender
  can send through a receiver window (rcvwin),
  which is the size of the spare room in the
  receiver buffer.
• The sender ensures that the amount of outstanding
  bytes (sent but not acknowledged) is less than
  rcvwin.

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TCP Connection Management

• Other fields of interest in TCP header are bits
  to indicate type of packets (SYN, FIN, ACK, RST
  etc).
• To establish connection, TCP uses a 3-way
  handshake A send SYN to B, B replies with
  SYNACK, A replies with ACKdata (optional).
• To tear down connection, A sends FIN to B, B
  sends ACK back. B sends FIN to A, A sends ACK
  back to B.