IST2245I
XMIT STALL DETECTED FOR RTP puname TO cpname

Explanation

This unsolicited message is issued when HPR detects a transmission stall for an RTP pipe. A transmission stall occurs when the RTP partner continuously requests transmission of the same network layer packet (NLP). VTAM® honors the request each time by transmitting the NLP, but the partner does not receive it. This message is issued after VTAM retransmits the NLP for the sixth time and at least 4-10 seconds has elapsed since the NLP was first retransmitted.

In the message text:
puname
The name of the HPR PU.
cpname
The name of the CP at the other end of the pipe.

System action

When a specific NLP is retransmitted 6 times without being acknowledged by the RTP partner, and at least 10 seconds has elapsed since the NLP was first retransmitted, VTAM notifies the operator with this message. The system actions vary depending on whether the RTP pipe traverses an Enterprise Extender (EE) link.
  • If the RTP pipe traverses EE, VTAM provides RTP transmission stall recovery support. If the RTP connection is multiple hops with at least one refifo connection, an HPR path switch is started. If the RTP connection is one hop over EE or an EE connection network, the RTP endpoint immediately lowers its NLP size to the minimum 768 bytes allowed by the HPR architecture. All outbound data is then re-segmented to this value to resolve the transmission stall. If the stall persists, VTAM issues message IST2246I every 30 seconds until the stall is alleviated. If a path switch was initiated to alleviate the stall and the stall persists, VTAM lowers the NLP size to 768 bytes when message IST2246I is issued. If the stall is alleviated, VTAM issues message IST2247I and normal data flow resumes. After 20 minutes of operating with the reduced NLP size, the original NLP size of the RTP pipe is restored to increase RTP throughput. If the network still does not allow transport of the larger packets, the stall detection and alleviation process repeats.
  • If the RTP pipe does not traverse EE, VTAM does not provide RTP transmission stall recovery support. In these configurations, the maximum NLP size is calculated for the path when the RTP pipe is established. The underlying NLP size for the RTP path should not change during the life of the RTP pipe, unless a path switch occurs. VTAM alerts the operator of a transmission stall and continues to honor the retransmission requests sent by the RTP partner. If the stall persists, VTAM issues message IST2246I every 30 seconds until the stall is alleviated. If the stall is alleviated, VTAM issues message IST2247I and normal data flow resumes.

If the stall persists beyond the time limit specified by the HPRSTALL start option, VTAM automatically deactivates the RTP pipe. See the information about the HPRSTALL start option in z/OS Communications Server: SNA Resource Definition Reference.

Operator response

Perform the following steps to alleviate the transmission stall:

  1. If the RTP pipe traverses EE and the following conditions are true, verify that the firewalls are configured to allow ICMP errors to flow on all hops of the connection:
    If the firewalls are not allowing ICMP errors to flow, this could be the cause of the transmission stall. Disable path MTU discovery for EE connections until the firewalls are configured to allow ICMP errors to flow:
    • Issue the MODIFY procname,VTAMOPTS, PMTUD=NO command to temporarily disable path MTU discovery.
    • Contact the system programmer to permanently disable path MTU discovery for IPv4 and IPv6 EE connections.
  2. If the transmission stall persists, consider ending the stalled sessions by issuing the VARY net,INACT,ID=rtp_pu_name,TYPE=FORCE command to deactivate the HPR PU. If you want VTAM to automatically end stalled RTP pipes after a specified amount of time, use the HPRSTALL start option. See the HPRSTALL start option information in z/OS Communications Server: SNA Resource Definition Reference.

System programmer response

If you want path MTU discovery for IPv4 and IPv6 Enterprise Extender connections to be permanently disabled, specify PMTUD=NO in the appropriate ATCSTRxx VTAM start list. If you want path MTU discovery for IPv4 and IPv6 Enterprise Extender connections to be enabled, ensure that all firewalls in the network path are configured to allow ICMP errors to flow.

User response

Not applicable.

Problem determination

If transmission stalls are prevalent and persistent, contact VTAM support for instructions about gathering documentation for problem determination.
  • If you have access to IBMLink, search for known problems with similar symptoms.
  • If no applicable matches are found, or if you do not have access to IBMLink, obtain a diagnostic dump for this error and report the problem to IBM®. If you have access to IBMLink, the problem can be reported to IBM using the Electronic Technical Report (ETR) option on IBMLink.

Source

z/OS® Communications Server SNA

Module

You can display the module that issues a SNA message in the message by setting the MSGMOD start option to YES. See Adding the originating module to the message text for more information about the MSGMOD start option.

Routing code

2

Descriptor code

4

Automation

Not applicable.

Example

IST2245I XMIT STALL DETECTED FOR RTP CNR0002F TO NETA.SSCP1A
Parent topic: IST messages for VTAM network operators IST2000I – IST2417I