On Wed, Mar 10, 2004 at 12:37:11PM +0100, Harti Brandt wrote: > On Tue, 9 Mar 2004, Mike Silbersack wrote: > > MS> > MS>On Tue, 9 Mar 2004, Kevin Oberman wrote: > MS> > MS>> Selective ACKnowledgment (SACK) allows acknowledgment of received > MS>> packets in a TCP window so that only the missing/damaged packet needs to > MS>> be re-transmitted. This is normally of little value on a LAN where ACKs > MS>> arrive quickly and windows are smaller and no use on slow circuits. On > MS>> fat pipes with latency and big windows it is a huge win as it allows you to > MS>> recover much faster from a packet drop. If you don't have SACK, you need > MS>> to re-transmit all of the packets in flight within the window while > MS>> with SACK, you need only retransmit the dropped packet(s). If you have a > MS>> 10 or 20 MB window, this is a big deal. > MS> > MS>That's not correct. Non-SACK TCP doesn't drop any additional packets vs > MS>SACK. The difference is that SACK allows the transmitter to transmit the > MS>packet which fills the "hole" and then immediately start transmitting new > MS>data (or fill other holes.) Non-SACK senders have to wait to receive an > MS>ACK after retransmitting the hole in order to find out if there are other > MS>holes which must be filled or if new data can be transmitted. > MS> > MS>SACK itself really doesn't do much, it's all the new congestion control > MS>schemes (FACK, Rate Halving, etc) that come shipped with most SACK > MS>implementations that do the work and contain most of the complexity. > > For satellite pipes with drops that are not the result of congestion, but > of transmission errors SACK helps. With congestion control only you get no > throughput no matter what you do (I did some tests with a simulated > 50MBit/sec GEO link with errors). But this is a rather limited > application.
For that matter, there are sufficent drops on 10GbE from data errors to insure that two boxes connected back to back won't achieve line speed on a single TCP session. -- Brooks -- Any statement of the form "X is the one, true Y" is FALSE. PGP fingerprint 655D 519C 26A7 82E7 2529 9BF0 5D8E 8BE9 F238 1AD4
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