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TCP Sliding Window Data Transfer and Acknowledgement Mechanics
(Page 4 of 6)
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.
TCP Segment Fields Used to Exchange Pointer Information
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.
 Key Concept: Three essential fields in the TCP segment format are used to implement the sliding windows system. The Sequence Number field indicates the number of the first byte of data being transmitted. The Acknowledgment Number is used to acknowledge data received by the device sending this segment. The Window field tells the recipient of the segment the size to which it should set its send window.
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