Configuring for Redundant VIC Operation
The list of VTS Internet Servers and the redundant operation parameters are configured using the Server Setup tab of the Internet Client/Server Setup dialog:
The VTS Internet Servers are listed in order of descending connection priority. Under redundant operation, when the connection is lost to the current server, it will be picked up by the next in order.
The parameters for redundant operation are (briefly):
• The read timeout. If a client does not receive a response from a server within this time, it assumes a loss of connection.
• The session timeout. If the server does not receive communication from the client within this time, it will close the connection and clean up resources that the connection had been using.
• The VIC mode. Determines whether redundant operation is enabled and if so, how it will operate.
A complete description of each parameter follows.
Detecting a Lost Connection - Read Timeout
Under normal operation, the VTS Internet Client works by sending requests to a VTS Internet Server. The server will respond with data if there is any to send, or with an empty response is there is none.
If no response at all is sent to the VIC by the end of the read timeout period, then the VIC detects that a server has failed to respond. A loss of connection is assumed.
Therefore:
• A shorter read timeout results in more rapid detection of loss of the server, at the expense of increased network traffic.
• Too short a timeout may result in false detection of server failure.
If the VIC detects that the server has failed to respond, it closes its connection to that server and makes one attempt at re-establishing the connection. If this fails, and the mode is not set to legacy, the VIC will attempt to connect to the next highest-ranked server available, according to the order of the server list.
A read timeout of 15 seconds is recommended as the maximum setting for a LAN, and the minimum setting for a WAN. If your network serves both LAN and WAN connections then 15 seconds is recommended. Otherwise, adust the value up or down accordingly.
Since there is always one attempt to reconnect after a timeout, the time required for the VIC detect irrecoverable loss of communication with its server is twice the read timeout value.
Orphaned Sessions - Session Timeout
Loss of a connection between the VIC and the server will result in an orphaned session taking up resources on the server. The session timeout exists to let the server know when it should recover those resources and clean up such orphaned sessions.
When the server detects that there has been no communication from the VIC for the period of time specified in the Session Timeout, it destroys all resources used by the orphaned session.
The minimum session timeout that you can specify is four times the read timeout value. An orphaned session will remain visible in the Internet Client Monitor display until the server destroys it at the end of the session timeout.
Mode of Operation
The Mode of Operation determines how the VIC will behave when:
• The VIC loses communication with its current server.
• A higher-ranked server than the one the VIC is currently connected to becomes available.
There are three choices for the VIC Mode; Priority, Sticky and Legacy.
Priority Mode: The VIC will always attempt to establish communication with the highest-ranked available server when loss of communication to its current server is detected.
When the VIC detects that a server of higher priority than the one it is currently connected to becomes available, it will connect to that server and terminate the older connection.
Sticky Mode: The VIC will always attempt to establish communication with the highest-ranked available server when loss of communication to its current server is detected.
The VIC will not switch connections if it detects that a server of higher priority than the one it is currently connected to becomes available.
Legacy Mode: The VIC will make no attempt to establish a connection to a different server when its current connection is lost. The VIC will not continue attempting to connect to a server if the primary is not available.
Note that the timeouts and VIC Mode are transmitted to the VIC at the start of each session, therefore the VIC will operate with whatever is configured on the server when it connects to it.
When the VIC makes its initial connection, it simultaneously attempts to connect to each server in the list and uses the server nearest the top of the list that it can successfully establish a session with. The VIC waits for an initial period of time, or until the highest-order server responds – whichever is shorter – then it selects and connects to a server. This initial timeout is left up to the TCP subsystem of the client computer and is normally around 20 seconds.
If the mode is set to Priority or Sticky, then if there are no servers available, the VIC will continue to try to connect to each server. In Legacy mode, the VIC will not make continuous attempts.