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Timeout occurring quite regularly

I have a small network with a local database server (PostgreSQL) and 6 clients. It's all Windows.

Several times per day a timeout occurs when one of the clients sends SQL Queries to the server.

E.g. the client reports a 2022-04-01 15:24:06,959. tshark is running constantly on both ends and I looked for packets at about 15:23:51 (timeout is 15 seconds).

In the trace I see that the corresponding SQL query is not sent out to the server at 15:23:51.

The clients ip address is fe80::d9db:dcdf:58d3:2d03, the server's ip address is fe80::804b:e959:60b:e90b.

Client trace: https://1drv.ms/u/s!AphLkdj_V9pOqAE6W...

Server trace: https://1drv.ms/u/s!AphLkdj_V9pOqAB-R...

My question is: does the trace show any hints why the server's response does not reach the client? Is it maybe a DNS issue?

EDIT: The traces are anonymized using TraceWrangler. Therefore no PGSQL queries are included in the trace and the DNS queries are cut down to UDP packets.

sugar76's avatar
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sugar76
asked 2022-03-29 14:27:52 +0000, updated 2022-04-03 07:22:03 +0000
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You could use TraceWrangler to mask or remove sensitive data.

André's avatar André (2022-03-30 15:54:24 +0000) edit
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I anonymized traces using TraceWrangler and updated the issue.

sugar76's avatar sugar76 (2022-04-03 07:20:29 +0000) edit
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2 Answers

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The client capture contains ICMPv6 "Packet Too Big" frames (filter: icmpv6.type == 2) containing "MTU: 1492". The server side does not.

So the maximum MTU in the network path is 1492 bytes. The value 1492 suggests that there is a VPN or tunnel in between. PPPoE maybe?

The TCP-SYN from the client shows a MSS of 1440. Thus MTU = 1440 + 40 (IPv6) + 20 (TCP) = 1500 bytes.

So a transmission fails when full packets are send (no fragmentation for IPv6 possible), like a large query. Small queries will pass.

Quick fix: lower the MTU of the client. Better fix the MTU in the network path.

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André
answered 2022-04-07 18:57:15 +0000
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In general cases such as this, you would need to capture at both ends of the link and possibly at intermediate points to determine where packets go missing.

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grahamb
answered 2022-03-29 19:53:28 +0000
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I attached traces from both ends now.

sugar76's avatar sugar76 (2022-04-03 07:20:53 +0000) edit
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