Re: PVAccess UDP/TCP connections failing - no more data in UDP packet

[Michael Davidsaver via Tech-talk <tech-talk@aps.anl.gov>]

On 10/28/25 4:24 PM, Kacper Kłys DMCS via Tech-talk wrote:

Hi guys,

I have a problem with the PVA connection with my IOC hosted on the ARM buildroot-based system. Even though the IOC claims that the PVA server is up and running, I cannot use pvget to read any PVs. I tried using pvget from the same host and another server from the same subnet. The PVs are available over CA, and there are no problems with reading them using CA. 

Thank you for putting together so much information for an initial report!

"no more data in UDP packet" indicates an apparent packet truncation.  In this case, of a CMD_SEARCH_RESPONSE (0x04)  Unfortunately, the mini packet dump being logged does not include the reported offset 77.

I think the most direct way to show if actual truncation occurs would be a packet capture with wireshark (see the bottom of the README.md for an example with the tshark CLI).  Use of the cashark dissector is option, but should flag some errors right away.

https://github.com/mdavidsaver/cashark

Since you report CA searches working, I suspect no actual truncation is occurring.  Still, we should confirm this.  And doing so may give some hint at what is actually going wrong.

I'm using EPICS 7.0.7, PVA configuration on the IOC side:

pvAccess Server v7.1.6
Active configuration (w/ defaults)
EPICS_PVAS_INTF_ADDR_LIST = 10.1.6.3:5075
EPICS_PVAS_BEACON_ADDR_LIST =
EPICS_PVAS_AUTO_BEACON_ADDR_LIST = YES
EPICS_PVAS_BEACON_PERIOD = 15
EPICS_PVAS_BROADCAST_PORT = 5076
EPICS_PVAS_SERVER_PORT = 5075
EPICS_PVAS_PROVIDER_NAMES = QSRV

As far as I understand logs, the pvget is able to find PV via UDP, but later, there is no TCP connection for data transmission. I tested PVA-related ports - they are open. In the system, there is no running firewall. To eliminate the influence of any network filtration, switches, etc, I connected my pc to the server with IOC via Ethernet cable - still could not read any PV via PVA.

The client and IOC console are full of messages like:

Error on UDP RX 10.1.6.3:43426 -> 0.0.0.0:54341 at 77 : no more data in UDP packet : 77:77 for 4
0x00 ca024004 2d000000 c9490169 00000000  ..@. -... .I.i ....
0x10 47435038 04000000 00000000 00000000  GCP8 .... .... ....
0x20 0000ffff 0a010603 d3130374 63700100  .... .... ...t cp..
0x30 41302000 00                          A0 . .
2025-10-28T23:55:18.920 Sending 72 bytes 0.0.0.0:54341 -> 10.1.6.253:5076.
2025-10-28T23:55:18.920 Sending 72 bytes 0.0.0.0:54341 -> 10.1.6.3:5076.
2025-10-28T23:55:18.932 UDP Client Rx (53) 0.0.0.0:54341 <- 10.1.6.3:43426

I'm attaching full logs from the IOC console and pvget debug mode.

I'd be grateful for any tips on how to fix this.

Cheers

Kacper