Re: Who wants SACK? (Re: was My planned work on networking stack)

From: Harti Brandt (brandt_at_fokus.fraunhofer.de)
Date: 03/10/04

  • Next message: Hannes Persson: "Re: sendto() problem using T/TCP over IPv6" 
    Date: Wed, 10 Mar 2004 12:37:11 +0100 (CET)
To: Mike Silbersack <silby@silby.com>

    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.

    harti
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