• Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint
Share this Page URL
Help

Chapter 20. Server Performance > Look for Excessive TCP Retransmits

20.10. Look for Excessive TCP Retransmits

TCP has many parameters that can be adjusted to affect network performance. A primary concern is avoiding retransmissions of packets when they have been received intact, only delayed because of the latency inherent in the Internet. The default settings may work just fine on a low-latency LAN but then wind up causing unneeded retransmission on the Internet. Under Solaris, many TCP/IP parameters can be changed in a running kernel with the ndd command. The ndd timing parameters are in milliseconds. You can see ndd parameters even if you are not root, using the command ndd /dev/tcp \?, but you can't change anything with ndd unless you are root.

If TCP does not receive a segment acknowledgment within a certain interval of time, it considers the segment lost and sends another copy. The default timeout is 200 milliseconds, which is fine on a LAN but sometimes inadequate for the much larger latencies of the Internet. If you simply use the default, you may end up sending several identical packets when you could just be sending one. A reasonable setting for a high-latency environment like the Internet is 1000 milliseconds. The Unix snoop utility can give you a feel for how your network is running and whether you are retransmitting unnecessarily. The Mac TCP Monitor tool is a good way to see retransmissions on the client side if your clients are Macs. The retransmission delay parameter is known as RTOmax on Windows.


PREVIEW

                                                                          

Not a subscriber?

Start A Free Trial


  
  • Creative Edge
  • Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint