RTP performance problems over EE with multipath routing enabled
If multipath routing is enabled on the TCP/IP stack, and multiple equal-cost routes exist to the partner EE node, then TCP/IP sends batches of EE packets across each of these routes using a round-robin schedule. If one of these routes cannot reach the partner EE node, then EE might not activate, or if it does, there is likely to be significant performance impacts.
- Multipath routing is enabled in HostA
- Two QDIO OSA interfaces are defined and active in HostA
- Two static and equally weighted IP routes to destination HostB
are defined in HostA
- The IP route through IP Cloud1 has a router in the path; the router has incorrect routing definitions to HostB. A router in IP Cloud1 with IP address 9.67.1.21 is the router that is unable to route to 9.67.1.6 (HostB).
- The IP route through IP Cloud2 has correct routing definitions to 9.67.1.6 (HostB).
In Figure 1, an RTP pipe is successfully established from HostA over EE to HostB. However, the sessions using this RTP pipe are experiencing poor performance. The operator issues a D NET,EEDIAG,REXMIT command, which reveals that this EE connection is experiencing a high percentage of retransmissions. In this example, when the HPR traffic is routed over the path that uses the router with IP address 9.67.1.3, the HPR packet is correctly routed to HostB. When the HPR packets are transmitted over the route that uses the router with IP address 9.67.1.2, the packet is incorrectly routed and is subsequently discarded. The high percentage of lost packets causes the RTP endpoints to report lost packets, which causes subsequent retransmissions. Excessive retransmissions significantly degrade HPR throughput, and can lead to HPR path switches, or in some cases HPR connection deactivation.
To disable multipath for EE without affecting other IP applications, code the VTAM® start option MULTPATH=NO or allow it to default. This will disable the multipath function in the stack for EE connections only. The multipath behavior for other IP applications will remain unchanged.