jpcodes@TYCHO.ARPA (Jim P. Codespote) (01/27/88)
I am trying to implement TCP in Ada. I am following the Military
Standard 1778 and I am having trouble following some of the events and actions.
My problem is what happens in a "net deliver" in an established state? (9
I found that the decision table is switched with the "net deliver" table
in the close wait state. Assuming I am useing the right table now, what should
happen when; seq num status is valid,
OA
-------jpcodes@TYCHO.ARPA (Jim P. Codespote) (01/27/88)
I am trying to implement TCP in Ada. I am following the Military
Standard 1778 and I am having some trouble in understanding it. My problem
is what happens in a "net deliver" in an established state? (9.4.6.1.5)
I found that the decision table is switched with the "net deliver"
table in the close wait state. Assuming I am using the right table now,
what should happen when; seq num status is valid, rst is on, sec prec
match is yes, syn in window is no, and ack status test2 is none? Does
this mean only data is being delivered? If so, why does the standard
say "no action"? Or, does this mean the only way data gets accepted is
if the segment contains an "ack"?
Is the standard in error or am I just missing the point? I would
greatly appreciate any suggestions. Thank you.
Please respond at jpcodes@TYCHO.ARPA
Jim Codespote
-------jonab@CAM.UNISYS.COM (Jonathan P. Biggar) (01/28/88)
> I found that the decision table is switched with the "net deliver" >table in the close wait state. Assuming I am using the right table now, >what should happen when; seq num status is valid, rst is on, sec prec >match is yes, syn in window is no, and ack status test2 is none? Does >this mean only data is being delivered? If so, why does the standard >say "no action"? Or, does this mean the only way data gets accepted is >if the segment contains an "ack"? You are right about the tables being switched. For your second question, I am assuming that you really mean rst is off. (If you do mean rst is on, then the last line of the table should be used and the connection should be reset and closed.) In this case what is happening is that the connection is probably receiving an old duplicate SYN packet from the other end. The only packets that do not have ACK set are an SYN packet generated from an active open request, or a RST packet in reponse to a SYN-ACK packet received on a closed connection. Since RST is not on, the only valid packet that can be received here is a duplicate SYN. Note that even though this packet may contains a SYN, the SYN is not in the window because the 'sv.rcv_next' variable was already incremented past the SYN sequence number by the action procedure record_syn. The proper response to a packet containing a duplicate SYN is just to ignore it. By the way, I have just completed a CRAD contract from DCA to develop MIL-STD implementations of TCP and IP in Ada. The code has been tested for conformance to MIL_STD 1777 and 1778. I believe that the code is available for free from DCA. If you are interested in it, let me know and I will find out how you can acquire it. Jon Biggar jonab@cam.unisys.com Distributed Architectures Branch Unisys Defense Systems Camarillo, CA