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Drop Tx without a reason

GiovaBx
Occasional Visitor

Drop Tx without a reason

Hi to all,

I'm facing a strange problem that I can't explain by myself. I have to admin that I'm really a novice in this kind this, so I hope that one of you can help me to understand what it is wrong.

Please be kind, if you think that this is a dump question with an obvious explanation.

I have a really simple network topology

- 2 devices (high resolution cameras) are connected to the switch with a 1Gbps cable.

- 1 Gbps cable connects the switch to a Gigabit port in the PC

Both the PC and the switch are configured to use Jumbo packets (the camera sends Jumbo UDP packets).

Each camera sends every 100 ms an image (that correspond to about 260 packets of 8192 bytes). Please note that the cameras are synchronized and then they send the data at the same time (differences of 20-30 us).

Just to give some more numbers. If I active just one camera from the PC side I can measure:

- 10 fps

- about 2600 pkt/s

- about 18 MB/s

If I increase the frame rate I can easily reach the maximum bandwidth of the gigabit without a packet loss, so the connection between each camera and the switch and between the switch and the PC works without problems.

In the above situation, all is working correctly since just one camera sends its data. Once I activate also the second one,  on PC side I detect a lot of packets loss (in each packets that the camera sends there is a packet counter).

The measured incoming bandwidth instead of being 32 MB/s (2 times 18) is just 25 MB/s. Then it seems that the packets never reach the PC (or at least I can't read them).

I made varius test with different PC, network cards and HP switches (1410, 1820 and 3800 and even with a dumb Netgear), without any significant difference.

Using a Procurve 3800 I can understand a better what happen by looking at the statistics of the port connected to the PC: as soon as the second camera starts to sends its frames on that port the Drop Tx counts!

So for some reasons the switch drops the packet and never forward them to the final destination.

Can someone explain me why this should happen? I can't believe that a "monster" like a procurve 3800 can't handle "burst" of more or less 500 packets every 10ms!

Thanks in advance.

Giovanni

3 REPLIES
Vince-Whirlwind
Honored Contributor

Re: Drop Tx without a reason

It's not likely the switch can't send the packets, more likely it's the PC that's not accepting them. Probably something happening at the application that is meant to be receiving the packets.

GiovaBx
Occasional Visitor

Re: Drop Tx without a reason

Is there any pratical way to understand why these packets are dropped?

The logs doesn't show anything.

Thanks

Vince-Whirlwind
Honored Contributor

Re: Drop Tx without a reason

You would need to look at the logs on the device that is not accepting them. If the switch can't deliver them, they time out and are dropped.