TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size
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TCP segments are the messages that carry data between TCP devices. The Data field is where the actual data being transmitted is carried, and since the length of the Data field in TCP is variable, this raises an interesting question: how much data should we put into each segment? With protocols that accept data in blocks from the higher layers there isn't as much of a question, but TCP accepts data as a constant stream from the applications that use it. This means it must decide how many bytes to put into each message that it sends.
A primary determinant of how much data to send in a segment is the current status of the sliding window mechanism on the part of the receiver. When Device A receives a TCP segment from Device B, it examines value of the Window field to know the limit on how much data Device B is allowing Device A to send in its next segment. There are also important issues in the selection and adjustment of window size that impact the operation of the TCP system as a whole, which are discussed in the reliability section.
In addition to the dictates of the current window size, each TCP device also has associated with it a ceiling on TCP sizea segment size that will never be exceeded regardless of how large the current window is. This is called the maximum segment size (MSS). When deciding how much data to put into a segment, each device in the TCP connection will choose the amount based on the current window size, in conjunction with the various algorithms described in the reliability section, but it will never be so large that the amount of data exceeds the MSS of the device to which it is sending.
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