AIX Version 4.3 Differences Guide

7.9.1 Reducing the Number of TCP Packages
There are some modifications to the TCP protocol implementation in AIX 4.3.2
that improves the performances of HTTP-like transactions on Web servers and
clients.
A performance improvement for Web servers and clients can be achieved by
reducing the total number of TCP packets that are exchanged, without violating
the TCP protocol. For Web servers and clients, this can be done by delaying
certain ACK messages and piggybacking them with the next packet that will be
sent and also by sending the FIN message along with the last data packet.
A typical HTTP transaction requires the exchange of nine TCP packets:
1. The client sends a SYN TCP packet to connect to the server.
2. The server acknowledges the client’s SYN and sends a SYN to accept the
connection.
3. The client acknowledges the server’s SYN.
4. The client sends its HTTP request in a data packet.
5. The server sends a data packet containing the acknowledgment and the
answer to the client’s request.
6. The server shuts down its side of the connection by sending a FIN packet.
7. The client acknowledges the server’s FIN
8. The client shuts down its side of the connection by sending a FIN packet.
9. The server acknowledges the client’s FIN.
In AIX 4.3.2, the TCP exchange packets are reduced from nine to seven by using
no command and to six by using send_file() system call with the SF_CLOSE flag.
7.9.2 Commands Affected
Reducing the TCP package number can be set by the user from the command
line or by an application on a per process basis.
Two new options, delayack and delayackports, have been added into the no
command.
delayack
The delayack options delays ACKs for certain TCP packets and attempts to
piggyback them with the next packet sent instead. It enables the user to specify
whether the delay will be performed for the SYN ACKs, the FIN ACKs, or both.
This will only be performed for connections whose destination port is specified in
the list of the delayackports attribute.
The delayack option may have one of the following four values:
0 No delay, This is the default value.
1 Delay the ACK for the SYN only
2 Delay the ACK for the FIN only
3 Delay the ACK for both.
Setting delayack to any other value will result in a failure with EINVAL error code.
174 AIX Version 4.3 Differences Guide