Expand Configuration and Management Manual (H06.21+, J06.10+)

Tuning
Expand Configuration and Management Manual — 529522-013
19 - 10
Layer 2 Window Size
Expand-Over-IP Connections
Expand-over-IP line-handler processes use the User Datagram Protocol (UDP)
services provided by a NonStop TCP/IP process to transmit data across an Internet
Protocol (IP) network. Because data transfer with UDP is not guaranteed, the Expand
End-to-End protocol is used to achieve reliable communications for Expand-over-IP
connections. You can avoid congestion and improve error-recovery by enabling the
congestion control feature on Expand-over-IP connections.
Congestion Control Configuration
You can enable the congestion control feature for outgoing transmissions on a per-
connection basis using the L4CONGCTRL_ON modifier. HP recommends that you
enable the congestion control feature for all connections in your Expand network.
If your network contains some Expand line-handler processes that support the
congestion control feature and some that do not, HP recommends that you enable the
feature for all connections that are capable of congestion control and set the out-of-
sequence (OOS) timeout value (OSTIMEOUT modifier) to the default value of 3
seconds for all Expand line-handler processes.
If all nodes in the network are running D30 or later versions of the Expand subsystem,
then HP recommends that the congestion control feature be enabled for all Expand
line-handler processes and that the OOS timeout value be set to 10 seconds.
For more information on configuring the congestion control feature, see Congestion
Control Feature on page 17-69.
Layer 2 Window Size
At the OSI Data Link Layer (Layer 2), the mechanisms for flow control and windowing
are provided by the particular Layer 2 protocol used (for example, HDLC or, if a
network access method (NAM) is used, SNAX/APN, X25AM, or ServerNet).
Small Layer 2 windows (as specified with the TXWINDOW modifier) tend to use
communications lines inef
ficiently. Large Layer 2 windows can be used effectively
when propagation delay is long or line quality is very high.
For Expand-over-IP connections, the TXWINDOW modifier specifies the number of
packets the Expand-over-IP line-handler process can send to the NonStop TCP/IP
process (with which it is associated) before waiting for a reply from the NonStop
TCP/IP process; it does not control the transfer of data across the link.
Note. A small Layer 2 window size can be used to cope with poor quality lines when line
efficiency is not important.