Missing ACKs - no Retransmissions - is this a problem?

Giganews Newsgroups
Subject: Missing ACKs - no Retransmissions - is this a problem?
Posted by:  david.f.jenki…@usa.net
Date: 28 Dec 2005

(Also posted in comp.protocols.tcp-ip)

I am struggling with a Windows XP SMTP problem which is detailed
elsewhere. Since I am a devout newbie/neophyte at all things TCP, I
need inordinate amounts of help on this thing.

In examining Ethereal output for my failing conditions (lost
connections) I see instances where an SMTP message is sent, never
ACKed, and a subsequent TCP retransmission takes place for a missing
packet that was sent later than the original unACKed packet.  It's at
the point of the [TCP Retransmission] that my connection drops.

Are such sequences normal and to be expected?  Or something more
insidious?  Is this something I should be concerned about?  If so,
what steps might I take to reduce the probabliity of this occurring?

Here's a highly edited sample of Ethereal capture data that illustrates
what I'm talking about (I've inserted some packet numbers for clarity):

    No.  Time            Protocol  Info
  3007 21.627582    SMTP    Message Body
  3008 21.627597    SMTP    Message Body
  3009 21.627610    SMTP    Message Body
  3010 21.627624    SMTP    Message Body
  3011 21.627635    SMTP    Message Body
  3012 21.669225    TCP        smtp > 1102 [ACK] {3007} Seq=0
Ack=2430104 Win=62780 Len=0
  3013 21.675264    TCP        smtp > 1102 [ACK] {3009} Seq=0
Ack=2433024 Win=62780 Len=0
  3014 21.696797    TCP        smtp > 1102 [ACK] {3010} 0Seq=0
Ack=2434484 Win=62780 Len=0
  3015 21.950194    SMTP    [TCP Retransmission {3011}] Message Body

[and then... nothing]