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

[Guy Harris <gharris@xxxxxxxxx>]

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.