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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Message Formatting and Data Transfer

TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size 1 2 3 4 5 6 TCP Immediate Data Transfer: "Push" Function

The Relationship Between Send and Receive Pointers

The SND and RCV pointers are complementary, of course, just as the categories are, with each device managing both the sending of its data and receiving of data from its peer. Assuming we have a client and a server, then:

• Client: The SND pointers keep track of the client's outgoing data stream; the RCV pointers refer to the data coming in from the server. The client’s SND categories correspond to the server’s RCV categories.
  
  
• Server: The SND pointers keep track of the server's outgoing data stream; the RCV pointers refer to the data being received from the client. The server’s SND categories correspond to the client’s RCV categories.

Since the SND and RCV values are complementary, the send window of one device is the receive window of the other, and vice-versa. Note, however, that the values of the pointers do not always line up exactly between the two devices, because at any given time, some bytes may be in transit between the two. Figure 220, for example, shows the receive pointers of the recipient prior to receiving bytes 32 to 45, which are shown in transit in Figure 219.

Both SND and RCV pointers are all maintained in the transmission control block (TCB) for the connection held by each device. As data is exchanged the pointers are updated, and communication about the state of the send and receive streams is exchanged using control fields in the TCP segment format. The three most important ones are:

• Sequence Number: Identifies the sequence number of the first byte of data in the segment being transmitted. This will normally be equal to the value of the SND.UNA pointer at the time that data is sent.
  
  
• Acknowledgment Number: Acknowledges the receipt of data by specifying the sequence number that the sender of the segment expects in the segment recipient's next transmission. This field will normally be equal to the RCV.NXT pointer of the device that sends it.
  
  
• Window: The size of the receive window of the device sending the segment (and thus, the send window of the device receiving the segment.)

The Acknowledgment Number field is critical because this is what a device uses to tell its peer what segments it has received. The system is cumulative: the Acknowledgment Number field says “I have received all data bytes with sequence numbers less than this value”. This means if a client receives many segments of data from a server in rapid succession, it can acknowledge all of them using a single number, as long as they are contiguous. If they are not contiguous, then things get more complicated.

TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size 1 2 3 4 5 6 TCP Immediate Data Transfer: "Push" Function