On Jul 25, 2008, at 12:10 PM, Troy Tate wrote:
I have an application where all packets seem to have the PSH/ACK
flags set (from both sides of the conversation). This is a barcode
application. Is this a typical behavior for these types of
applications? The user is reporting slow performance, but I am not
seeing any network issues other than these flags set in every packet.
It is typical behavior for the type of application called "TCP-based
application" to have ACK set on all but some of the initial packets;
to quote RFC 793:
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.
Whether PSH is set depends on whether the TCP implementation sets it;
that's probably not application-specific. In Mac OS X, for example,
there's nothing the application can do to explicitly request PSH;
looking at the versions of the {Free,Net,Open}BSD TCP code I had
handy, the same is true there. I don't have the Linux or Solaris code
handy so I don't know whether they can explicitly force a push, and I
don't know what other UN*Xes or Windows do.
