When you create a class-of-service description, you can define one of three transmission priorities for each class of service. You can specify, using the transmission priority (TMSPTY) parameter, that the transmission priority for any class of service is high, medium, or low.
The transmission priority you specify is carried in the session activation request at session establishment. The transmission priority allows each logical unit on the session and each routing entry along the session path to store the same transmission priority. By assigning an appropriate mode (which includes a class of service) at session activation time, you can ensure a better response time for the applications that require it. Generally, interactive traffic should have a high priority and batch traffic a low priority.
Route addition resistance (RAR) is a relative value that indicates how desirable one network node is, as compared to other network nodes, for having intermediate sessions routed through it.
Changing this value and working with the different class-of-service descriptions can control route sessions.
The RAR value is defined in the network attributes for the local iSeries system.
When a session is requested to a remote location that matches a network node control-point name, a directory search is not performed by the node that calculates the route. This is true if the session request is being started by a user on the network node, or on an end node that the network node is providing services for. Session start requests for remote locations in end nodes and remote locations in network nodes that do not match the control-point name of the network nodes take longer. These session start requests take longer because the directory search needs to be sent and the replies need to be received.
The Change Network Attributes (CHGNETA) command specifies the maximum number of intermediate sessions that are allowed on a network node. When the number of intermediate sessions reaches 90% of the maximum value, the node is marked as congested. A node that is congested may or may not be used for intermediate sessions that depend on the class-of-service definition. The node is not congested when the number of intermediate sessions drops below 80% of the configured value. Also, if the maximum number of intermediate sessions is reached (100%), then intermediate sessions will not be allowed through this network node until the value drops. You can limit the effect of intermediate sessions on local processing by setting an appropriate value.
On iSeries™, some IOPs that support the local area network protocols, such as token ring and Ethernet, have the ability to perform segmentation and reassembly of SNA Request Units. Performing this function in the IOPs offloads this work from the server CPU. The server CPU is free to perform other tasks.
With
APPN, any network congestion control is handled on a hop-by-hop basis by using pacing values. It is possible to over-drive
connections in an APPN environment. A particular system might receive more
data over a communications link than it can handle based on buffer space.
The system requires the node that sends the data to retransmit all of the
frames that were sent following the last successfully acknowledged frame.
This retransmission occurs at the data link control (DLC) layer.
APPN requires link-level error recovery to cause retransmission of lost frames. This link-level error recovery can only survive short and temporary outages (several seconds). If a link outage or node outage occurs, that is of longer duration, APPN has no recovery mechanisms for keeping the affected sessions active. The applications must handle any session recovery.
The following matrix shows how HPR traffic is supported between two systems that are based on their HPR link-level error recovery settings. The HPR link-level error settings are exchanged between the systems:
| System 1 | System 2 | ||
| No link-level ERP allowed | Link-level ERP required | Prefer no link-level ERP but might run using link-level ERP | |
| No link-level ERP allowed | HPR supported (no ERP) | HPR not used | HPR supported (no ERP) |
| Link-level ERP required | HPR not used | HPR supported (uses ERP) | HPR supported (uses ERP) |
| Prefer no link-level ERP but might run using link-level ERP | HPR supported (no ERP) | HPR supported (uses ERP) | HPR supported (no ERP) |