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TCP Sliding Window Acknowledgment System For Data Transport, Reliability and Flow Control
(Page 6 of 9)
Sequence Number Assignment and Synchronization
The sender and receiver must agree
on the sequence numbers to assign to the bytes in the stream. This is
called synchronization and is
done when the TCP connection is established.
For simplicity, let's assume the first byte was sent with sequence number
1 (this is not normally the case). Thus, in our example the byte ranges
for the four categories are:
- Bytes Sent And Acknowledged:
Bytes 1 to 31.
- Bytes Sent But Not Yet Acknowledged:
Bytes 32 to 45.
- Bytes Not Yet Sent For Which Recipient
Is Ready: Bytes 46 to 51.
- Bytes Not Yet Sent For Which Recipient
Is Not Ready: Bytes 52 to 95.
The Send Window and Usable Window
The key to the operation of the entire
process is the number of bytes that the recipient is allowing the transmitter
to have unacknowledged at one time. This is called the send window,
or often, just the window. The window is what determines how
many bytes the sender is allowed to transmit, and is equal to the sum
of the number of bytes in Category #2 and Category #3. Thus, the dividing
line between the last two categories (bytes not sent that recipient
is ready for and ones it is not ready for) is determined
by adding the window to the byte number of the first unacknowledged
byte in the stream. In our example above, the first unacknowledged byte
is #32. The total window size is 20.
Figure 207: TCP Transmission Stream Categories and Send Window Terminology This diagram shows the same categories as Figure 206, with the send window indicated as well. The black box is the overall send window (categories #2 and #3 combined); the gray represents the bytes already sent (category #2) and the red box is the usable window (category #3).
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The term usable window
is defined as the amount of data the transmitter is still allowed to
send given the amount of data that is outstanding. It is thus exactly
equal to the size of Category #3. You may also commonly hear mention
of the edges of the window. The left edge marks the first
byte in the window (byte 32 above). The right edge marks the
last byte in the window (byte 51). Please see Figure 207
for a graphical view of these concepts.
 Key Concept: The send window is the key to the entire TCP sliding window system: it represents the maximum number of unacknowledged bytes a device is allowed to have outstanding at once. The usable window is the amount of the send window that the sender is still allowed to send at any point in time; it is equal to the size of the send window less the number of unacknowledged bytes already transmitted.
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