Ethereal-users: Re: [Ethereal-users] Help with the Rules of TCP/IP

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From: Guy Harris <gharris@xxxxxxxxx>
Date: Tue, 10 Jan 2006 00:51:43 -0800
andy@xxxxxxxxxxxxxxxxx wrote:


Is it Legal to send a TCP segment with the ACK flag set and data in the payload.
As others have said, yes. That's called "piggybacked ACK" - the system sending the ACK also has data to send, so it sends the ACK with the data.
In fact, RFC 793 says that, once a connection is established, *all* packets will have ACK set: 

  Acknowledgment Number:  32 bits

    If the ACK control bit is set this field contains the value of the
    next sequence number the sender of the segment is expecting to
    receive.  Once a connection is established this is always sent.


I have a malfunctioning application and when it fails I see a continuos TCP
retransmission (5 attempts) of a packet
Somehow either the machine to which that packet is being sent isn't receiving it (so it's not ACKing it), or the ACK it's sending isn't being received.