HP-UX 11iV1 - TCP-IP : how is ACK sending triggered on received segments?

[Antonio Cardoso_1]

Hi all,
Looking at some ethereal traces, I see that empty ACK messages are sent to some (not all) received segments of long (> MTU) messages.
Does anyone here know how these messages are triggered:
- after some timer expires, if no response was sent to carry the ACK flag?
- after some amount of data is received ?

I've been searching in ndd's TCP parameters, but found nothing relevant.

thanks.
antonio.

[Steven E. Protter]

Shalom Antonio,

Can you share a sample of the ethereal trace? It would be helpful.

SEP

[Antonio Cardoso_1]

OK, here attached is a short sample.

In this sample 10.4.128.42 is a HP-UX box, we find:
- frames 1,2 are 2 first segments of a corba request (last segment is frame4) frame 3 : ACK for frames 1,2 sent by 10.4.128.42
=> what triggers the frame 3 tp be sent?

- frames 7,8,9 are 3 segments of 1 corba request, frame 10 (empty) ACKs frame 7, frame 11 (application response) ACKs frames 8,9

- ...

I'd like to understand what conditions cause empty ACKs to be or not be sent.

thanks,
antonio.

[rick jones · Accepted solution]

In broad handwaving terms, a "standalone" or "bare" ACK will be send by the receiver when one or more of the following happens:

1) The arriving segment is "out of order" - this triggers an immediate ACK to say which sequence number was expected.

2) It is time to send a window update because the receiving application has consumed enough data from the socket to warrant a window update

3) The standalone ACK timer has expired (tcp_deferred_ack_interval, IIRC)

4) (HP-UX specific) we have received tcp_deferred_ack_max segments in a row

Is there a specific situation you are trying to debug, or are you merely curious?

[Antonio Cardoso_1]

rick,

Thanks for your answers, it helps my understanding.
The question was raised in the context of debugging an interworking issue with another system, but the HP-UX box is not suspected.

[Antonio Cardoso_1]

Rick,
the tcp_deferred_ack_max is definedas "Upper limit on the number of bytes of data (measured in MSS) that can be received without sending an an ACK. [2-32] Default: 22 MSS"

=> what is MSS ? is it the value of tcp_mss_def parameter?

thanks again.

[rick jones]

MSS - Maximum Segment Size - exchanged in each direction on the SYNchronize segments that establish a TCP connection. What each side sends will be based on their link-local MTU (Maximum Transmission Unit), and whether or not Path MTU discovery is being used.

If not MSS option appears in a SYN segment, 536 bytes is to be ass-u-med.

[Antonio Cardoso_1]

OK, it's all clear now,
I appreciate quality of responses,
thanks Rick.