It's not as simple as just looking at the window size. First, the "who" is the receiving station. The "how frequent" depends. Normally, a TCP stack implements what is called "delayed ACK", meaning it will not simply just send an ack the instant it receives a packet that requires an ack. It will normally wait some small amount of time (somewhere around 200-500ms) to see if it has any data to send that it can piggyback on the ack (this is good for slow networks). However, delayed acks don't come into play if the reciver gets multiple packets requiring an ack.
On a fast network, the receiver is always getting multiple segments that require an ack, so it won't wait and will typically send an ack after the first 2 or 3 segments requiring an ack, depending on how fast the TCP stack can process the segments. This is a good thing since if the receiver waited until the max number of TCP segments had arrived per the window size, the sender will be idle until it receives an ack of all of the outstanding segments (even a delay of 10-20ms can be "long" on a fast network). A sender cannot send additional TCP segments once it sends the max allowed by the window, so it will sit there until it gets an ack. By sending acks every 2 or 3 segments, the receiver ensures that the sender can continually put packets on the wire and keep the traffic flowing smoothly. If you have a long delay, high bandwidth network such as a satellite link, it's possible the sender could completely fill the window size before receiving an ack (this is why it's generally a good idea to have very large window sizes on these types of networks). This can lead to long delays waiting for the ack from the receiver. It's also possible for the sender to fill the window if the receiver is a very slow computer since the sender can send TCP segments faster than the receiver can process them. Again, this can lead to long delays while the sender waits for acks of it's already sent segments. This topic and many others related to TCP/IP are given excellent coverage in "TCP/IP Illustrated Vol 1" by the late, great Richard Stevens. I highly recommend it. HTH, Kent -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of z z Sent: Tuesday, November 20, 2001 9:28 AM To: [EMAIL PROTECTED] Subject: Tcp window size question [7:26861] Hi I used a sniffer to monitor my network traffic. I found even if the tcp window size is very big (around 32000), my ftp session is still getting one ack after every two pakets sent. So who is deciding how frequent the ack will be sent? I thought it should be decided by the TCP window size. Please correct me. __________________________________________________ Do You Yahoo!? Yahoo! GeoCities - quick and easy web site hosting, just $8.95/month. http://geocities.yahoo.com/ps/info1 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=27010&t=26861 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
