O'Reilly Network: Traffic ShapingAug 27, 2000, 21:50 (0 Talkback[s])
(Other stories by Terry Dawson)
"Most of us will at some time or another have experienced the effects of network latency and queuing. A common experience is the sometimes frustrating delay before characters are echoed when ssh- or Telnet-connected to a remote host. Any of you who have been forced to use a low speed dial-up PPP or SLIP connection will have seen the effect that a file transfer has on interactive traffic over the connection. The file transfer easily consumes most of the link bandwidth, forcing the Telnet or ssh data to be queued, waiting for a free slot before being transmitted across the link."
"This problem occurs because the datagrams containing the file transfer data are given equal priority on the link to the Telnet or ssh datagrams. No consideration is given to the type of data contained within the datagram when queuing it for transmission; queuing is performed on a "First In, First Out" (FIFO) basis, and the datagrams are scheduled for transmission on a "First Come, First Served" basis. When a new datagram arrives at the queue, it is added to the tail of the queue; when the link bandwidth becomes available, the datagram at the head of the queue is transmitted."
"Traffic shaping allows us to implement a specific policy that alters the way in which data is queued for transmission. Datagrams associated with file transfers are generally quite large, often MTU sized, while datagrams associated with interactive sessions like ssh or Telnet are often quite small. All data takes time to transmit over a network connection; the larger the datagram, the longer it takes. ... Ultimately, it will take the same amount of time to transmit all of a set of datagrams across a network link, no matter what order they are transmitted in, so at some point you may decide that it is worth trading off a small amount of delay in completing the file transfer for more normal response times for your interactive sessions like ssh or Telnet by giving them higher priority."
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