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Dsack?

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Dsack?

Subject: Dsack?
From: Steve Modica <email@hidden>
Date: Fri, 18 May 2007 17:14:30 -0500
Organization: Small Tree Communications

Hi All,

Does the current Mac TCP stack handle duplicate SACKS?

We're looking at a problem where the receiver (a linux machine) has an ordering issue and a false retransmit occurs. The mac gets 3 dup acks and so we dutifully resend the packet. Then the receiver sends us a duplicate SACK to let us know this was a false retransit (as per RFC 2883).

The mac responds very strangely to this:

1626 28.44484 [TCP Out-Of-Order] 49159 > iscsi-target [ACK] Seq=3913751031 Ack=3461081386 Win=58112 Len=1460 1627 28.44484 iscsi-target > 49159 [ACK] Seq=3461081386 Ack=3913796291 Win=126 Len=0 SLE=3913751031 SRE=3913752491 1628 28.44485 [TCP Dup ACK 1626#1] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1629 28.44485 [TCP Dup ACK 1626#2] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1630 28.44486 [TCP Dup ACK 1626#3] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1631 28.44486 [TCP Dup ACK 1626#4] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1632 28.44487 [TCP Dup ACK 1626#5] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1633 28.44488 [TCP Dup ACK 1626#6] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1634 28.44488 [TCP Dup ACK 1626#7] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1635 28.44489 [TCP Dup ACK 1626#8] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1636 28.44490 [TCP Dup ACK 1626#9] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1637 28.44490 [TCP Dup ACK 1626#10] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1638 28.44491 [TCP Dup ACK 1626#11] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1639 28.44492 [TCP Dup ACK 1626#12] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1640 28.44492 iscsi-target > 49159 [PSH, ACK] Seq=3461081386 Ack=3913796291 Win=126 Len=48 1641 28.44492 [TCP Dup ACK 1626#13] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1642 28.44493 [TCP Dup ACK 1626#14] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1643 28.44494 [TCP Dup ACK 1626#15] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1644 28.44495 [TCP Dup ACK 1626#16] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1645 28.44496 [TCP Dup ACK 1626#17] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1646 28.44496 [TCP Dup ACK 1626#18] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1647 28.44497 [TCP Dup ACK 1626#19] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1648 28.44497 [TCP Dup ACK 1626#20] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1649 28.44498 [TCP Dup ACK 1626#21] 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=0 1650 28.44500 49159 > iscsi-target [ACK] Seq=3913796291 Ack=3461081386 Win=58112 Len=1460 1651 28.44500 49159 > iscsi-target [ACK] Seq=3913797751 Ack=3461081434 Win=58100 Len=0 1652 28.48438 iscsi-target > 49159 [ACK] Seq=3461081434 Ack=3913797751 Win=126 Len=0 1653 28.48453 49159 > iscsi-target [ACK] Seq=3913797751 Ack=3461081434 Win=58112 Len=1460

Note the extremely long delay at the bottom here. The last two lines lost 39msec.

I'm guessing the mac has interpreted this as a problem since it didn't get a complete ack of the entire window (per RENO) and it's sort of "catching up" and dropping into slow start.

Steve

--
Steve Modica
CTO -  Small Tree Communications
www.small-tree.com
phone: 651-389-9951
mobile: 651-261-3201

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