Not only is it legal, but every packet in a data transfer should have the ACK flag set. A PSH flag is not required to send data, but when used, tells the receiver to immediately "push" whatever data it currently has in the TCP receive buffer up to the receiving application.
The TCP repeated retransmissions you are seeing are called an exponential backoff. The receiver is not acknowledging the packets being sent by the sender, therefore the sender is assuming that these packets have been lost and is retransmitting them. You can also tell this from the delta time column. If you filter out the single conversation in question, you will see the delta time doubling between each retransmission. The default number of retries before giving up on a TCP connection is 5 on the Windows operating system.
To find out more info, first check out the TCP RFC 793. There are several great books out there on TCP/IP like Stevens "TCP/IP Illustrated", Hall's "Internet Core Protocols", and Kozierok's newly published "The TCP/IP Guide".
Andrew
-----Original Message-----
From: ethereal-users-bounces@xxxxxxxxxxxx [mailto:ethereal-users-bounces@xxxxxxxxxxxx] On Behalf Of andy@xxxxxxxxxxxxxxxxx
Sent: Monday, January 09, 2006 4:23 AM
To: ethereal-users@xxxxxxxxxxxx
Subject: [Ethereal-users] Help with the Rules of TCP/IP
Hi Folks
Is it Legal to send a TCP segment with the ACK flag set and data in the payload.
I have a malfunctioning application and when it fails I see a continuos TCP
retransmission (5 attempts) of a packet that has an ACK flag only set and a
payload length of 1460 bytes.
Forgive me if I miss understand, but I though you had to have the PSH flag set
to send data? Is there a good document that would explain these rules?
Many Thanks for your help
Andy Harcup
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"If you discovered how one wheel in the 'clock' turns - you may speculate how
the rest move, but you . . . better leave alone the question of who wound up
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