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TCP's (send/receive) buffers and its data transmission algorithm

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• TCP data segment

  • TCP data segment

    TCP data segment = another name for a TCP message

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  • Facts about a TCP data segment:

    A TCP data segment contains zero or more bytes A TCP ACK message may contains zero bytes of data !!! Important note: A zero byte message (= an ACK message) does not need to be acknowledged !!! because: you do not acknowledge an ACK message, otherwise, you have an infinite loop....

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• The TCP send buffer
  • Transmitting data using TCP:

    A user application transmits data with TCP by using: a write system call in C/C++ (We will learn more about network programming later)

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  • Effect of the write system call on TCP:

    The data written by the write operation is: stored in a (TCP) send buffer

    Note:

    The data written by a write system call is not transmitted immediately !!!! The data written by several write operation may be transmitted inside a single TCP segment This will help improve efficiency/performance !!! Also, the data written by a single write operation can be transmitted in multiple TCP segments E.g., when a write( ) operation write a large amount of data

    Graphically illustrated:

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• TCP's Maximum Segment Size (MSS)
  • Maximum Segment Size:

    Maximum Segment Size = the largest TCP segment size that the TCP protocol will transmit Small MSS values will reduce/eliminate IP fragmentation Small MSS values will also result in transmitting more IP packets and thus higher overhead.

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  • Note:

    MSS is set by the Operating System (system administrator)

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• TCP transmission timer
  • Transmission timer:

    Transmission timer = a count down timer used by TCP to transmit data segments

    Operation:

    The transmission timer is reset (to about 3 sec) when TCP transmits a data segment When the transmission timer expires: All data in the TCP send buffer are transmitted immediately

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• The TCP segment transmission algorithm
  • TCP segment transmission algorithm:

    if ( write( data ) operation ) { Append data to the TCP send buffer; /* ================================================= Transmit data if buffer exceeds MMS ================================================= */ while ( length(TCP send buffer) ≥ MSS ) { Transmit MSS bytes from send buffer in a TCP segment; Reset TCP transmission timer; } } else if ( push( ) operation ) // C/C++'s flush( ) call { Transmit all data from send buffer in a TCP segment; Reset TCP transmission timer; } else if ( TCP timer expired ) { Transmit all data from send buffer in a TCP segment; Reset TCP transmission timer; }

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• The TCP receive buffer
  • The TCP receive buffer:

    Receive buffer = kernel data structure used to stored received data (Similar to the data structure used to implement a pipe)

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  • Extracting data from the TCP receive buffer:

    The read( ) system call is used to extract data from the TCP receive buffer

    Behavior of the read( N bytes, ...) system call:

    if ( length(TCP receive buffer) ≥ N bytes ) { return N bytes to user program; } else { block until TCP receive buffer has ≥ N bytes; } (I.e.: exactly like a pipe)

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  • Note:

    The read pattern is independent from how the data was trasnmitted by the sender. Example: