Cisco PIX 520 - PIX Firewall 520 Online Help Manual page 253

TCP Sliding Window
A TCP sliding window provides more efficient use of network bandwidth than PAR because it enables hosts to
send multiple bytes or packets before waiting for an acknowledgment. In TCP, the receiver specifies the current
window size in every packet. Because TCP provides a byte-stream connection, window sizes are expressed in
bytes. This means that a window is the number of data bytes that the sender is allowed to send before waiting for
an acknowledgment. Initial window sizes are indicated at connection setup, but might vary throughout the data
transfer to provide flow control. A window size of zero, for instance, means "Send no data." In a TCP
sliding-window operation, for example, the sender might have a sequence of bytes to send (numbered 1 to 10) to
a receiver who has a window size of five. The sender then would place a window around the first five bytes and
transmit them together. It would then wait for an acknowledgment. The receiver would respond with an ACK = 6,
indicating that it has received bytes 1 to 5 and is expecting byte 6 next. In the same packet, the receiver would
indicate that its window size is 5. The sender then would move the sliding window five bytes to the right and
transmit bytes 6 to 10. The receiver would respond with an ACK = 11, indicating that it is expecting sequenced
byte 11 next. In this packet, the receiver might indicate that its window size is 0 (because, for example, its internal
buffers are full). At this point, the sender cannot send any more bytes until the receiver sends another packet with
a window size greater than 0.
TCP Packet Format
Twelve fields comprise a TCP packet:
1. Source Port—Identifies points at which upper-layer source and destination processes receive TCP
services.
2. Destination Port—Identifies points at which upper-layer source and destination processes receive TCP
services.
3. Sequence Number—Usually specifies the number assigned to the first byte of data in the current message.
In the connection-establishment phase, this field also can be usedto identify an initial sequence number to
be used in an upcoming transmission.
4. Acknowledgment Number—Contains the sequence number of the next byte of data the sender of the
packet expects to receive.
5. Data Offset—Indicates the number of 32-bit words in the TCP header.