Transmission Control Protocol (TCP )

• TCP provides a reliable, error-free virtual connection for two-way stream-oriented transfer of data
  
• RFC 793
  
• Five basic features:
  • delivers streams of bits with guaranteed delivery in the same order as sent
  • virtual circuit connection
  • buffered transfer with push semantics
  • unstructured data stream
  • full duplex connection
    
• Reliability provided through positive acknowledgment with retransmission strategy
  
• Ideal communication
  • Sender sends Packet 1
  • Receiver receives Packet 1
  • Receiver sends acknowledgment (ACK) for Packet 1
  • Sender receives ACK 1
  • Sender sends Packet 2
  • Receiver receives Packet 2
  • Receiver sends ACK2
  • Sender receives ACK2
  • Etc.

Basic TCP send/receive transmission.

• Faulty communication
  • Sender sends Packet 1
  • Sender starts timer
  • Receiver does not receive Packet 1
  • Sender timer expires
  • Sender resends Packet 1
  • Sender starts timer
  • Receiver receives Packet 1
  • Receiver sends ACK1
  • Sender receives ACK1
  • Sender cancels timer
  • Etc.
    

TCP timeout

• Sliding Window
  • Sender sends Packet 1 and starts Timer 1
  • Sender sends Packet 2 and starts Timer 2
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  •  
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  • Sender sends Packet N and starts Timer N
  • Receiver receives Packet 1 and sends ACK 1
  • Receiver receives Packet 2 and sends ACK 2
  •  
  •  
  •  
  • Receiver receives Packet N and sends ACK N
  • Sender receives ACK 1 before Timer 1 expires
  • Sender receives ACK 2 before Timer 2 expires
  •  
  •  
  •  
  • Sender receives ACK N before Timer N expires
  • Etc.

  

  TCP send/receive using sliding window.

• Connection
  • Connection is a basic TCP abstraction defined by a pair of endpoints
  • Endpoint is a pair: (host, port)
    • host is the host's IP address
    • port is a TCP port on the host
  • Operations using connections described below
    
• TCP Segments
  • Data stream transmitted in segments, usually encapsulated within single IP packets
  • Concept of sliding window of octets used
    • Sender maintains pointers to octets in sequence sent, those ready to be sent but not sent, and those not ready to be sent
    • Receiver sends acknowledgment of octets received plus advertisement of number it is prepared to receive
      
  • Segment structure
    • Header
      • source port
      • destination port
        
        
      • sequence number
        
        
      • acknowledgment number
        
        
      • header length
      • reserved
      • code bits
        • URG
        • ACK
        • PSH
        • RST
        • SYN
        • FIN
      • window
        
        
      • checksum
      • urgent pointer
        
        
      • options
      • padding

      
      
      

    • data
      
      

  
  
  

  TCP Segment.
  Adapted from Comer,IWTCP/IP, Figure 13.7.
  
  

• Establishing TCP connection
  • Receiver sends passive open request to operating system, saying it is open for business
  • OS assigns Receiver TCP port number, often a well-known number, e.g., port 80
  • Time passes . . .
  • Sender sends message (TCP segment) requesting connection with SYN bit set and SEQ=x, where x is some random number that begins sequence of numbers it will use for synchronization.
  • Receiver receives message and returns message with SYN and ACK bits set; also saves SEQ=x, and sends SEQ=y, where y is some random number that begins sequence of numbers it will use for synchronization, and ACK=x+1(seq. number of first octet it expects to receive)
  • Sender receives ACK message and sends its own ACK message back to Receiver; also saves y and sends y+1, confirming receipt of y
  • Receiver receives Senders ACK message
  • Connection is, thus, established
    
• Closing TCP connection
  • Sender closes connection
  • Sender sends close connection message with FIN bit set
  • Receiver receives close message and returns message with FIN and ACK bits set
  • Sender receives ACK message
  • Receiver closes connection
  • Receiver sends message with FIN and ACK bits set
  • Sender receives FIN and ACK message
  • Sender sends ACK message
  • Receiver receives Senders ACK message
  • Connection is, thus, closed