This preview shows page 1. Sign up to view the full content.
Unformatted text preview: es we see in Figure 22.1.
The window probes contain 1 byte of data (sequence number 9217). TCP is always
allowed to send 1 byte of data beyond the end of a closed window. Notice, however, that
the acknowledgments returned with the window size of 0 do not ACK this byte. (They
ACK the receipt of all bytes through and including byte number 9216.) Therefore this
byte keeps being retransmitted.
The characteristic of the persist state that is different from the retransmission timeout in
Chapter 21 is that TCP never gives up sending window probes. These window probes
continue to be sent at 60-second intervals until the window opens up or either of the
applications using the connection is terminated. 22.3 Silly Window Syndrome
Window-based flow control schemes, such as the one used by TCP, can fall victim to a
condition known as the silly window syndrome (SWS). When it occurs, small amounts of
data are exchanged across the connection, instead of full-sized segments [dark 1982].
It can be caused by either end: the receiver can advertise small windows (instead of
waiting until a larger window could be advertised) and the sender can transmit small
amounts of data (instead of waiting for additional data, to send a larger segment). Correct
avoidance of the silly window syndrome is performed on both ends.
1. The receiver must not advertise small windows. The normal algorithm is for the
receiver not to advertise a larger window than it is currently advertising (which
can be 0) until the window can be increased by either one full-sized segment (i.e„
the MSS being received) or by one-half the receiver's buffer space, whichever is
2. Sender avoidance of the silly window syndrome is done by not transmitting unless
one of the following conditions is true: (a) a full-sized segment can be sent, (b) we file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_pers.htm (4 of 11) [12/09/2001 14.47.29] Chapter 22. TCP Persist Timer can send at least one-half of the maximum sized wind...
View Full Document
- Spring '12