---
title: "Fail to send data on some networks"
url: "https://maker.wiznet.io/forum/13690"
markdown_url: "https://maker.wiznet.io/forum/13690/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "johanan"
created: "2020-02-09T07:06:01+09:00"
last_activity: "2020-02-11T00:23:05+09:00"
language: "en"
views: 500
replies: 4
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# Fail to send data on some networks

## Question

Asked by johanan on 2020-02-09 in Ethernet Chips.

Got a device using W5100S, which works well with some networks providers, but fails with others.
When it send small packets, all is OK. When it starts to send more data, then it fails after some packets has been sent. This happens on some network providers. Same device connected to another network provider works well, without any problems. All devices send/rcv data from same server.
What can I do to debug such problem? does it have anything to do with Max Segment Size Register?
Thanks
Johanan

## Replies

### Reply 1 by irina, 2020-02-10

We not support the fragmentation of TCP.
I think didn’t you use fragmentation of TCP?
You can see same network packet via Wireshark program.Analyzing the packet will show you when it doesn’t work.

### Reply 2 by johanan, 2020-02-10 (in reply to reply 1)

Hello,
it is the device using the W5100S that send a lot of information to the server.

It is using VNC protocol to send a LCD screen, and if the screen has a lot of graphics, it fails at some point.

The packets are sent one by one - each abot 1000 bytes, with the send() Wiznet lib function, blocking mode. So I don’t understand how it can be fragmented.

Please explain more.

Thanks.

### Reply 3 by johanan, 2020-02-10

Thanks,
i will check buffer available size.

Problem is more complex, as in my lab (and some other users) all is working OK and very smoothly.

It is with some customers, and some of them are 3500 km away.

I have been able to output some logs via the device RS232 port and I have one user which allows me to login to his PC and check that logs, but so far I could not find the reason.

This customer can update the device firmware, and is cooperative, but even so, it is very difficult to understand what is the problem.

As I said, its a VNC protocol, In order to “solve” the problem, the device does not show any complex graphics (leaving only text and simple shapes on screen) when communicating by VNC, and this works without any issues, as the amount of data sent per screen refresh is much smaller. Whoever this is just for lowering the flames, and not a solution.

I tried disable delays flag (SF_TCP_NODELAY) but this does not make any noticeable difference.

I must assume that it is something which I don’t understand about under the hood of networks, as in some networks all is working very well, and some not. Very frustrating…

Johanan

### Reply 4 by johanan, 2020-02-10

Hi,
Can I check any register value to see if there is lost or bad acknowledge? then I can resend the packet by device software, before I sent the next packet…

Am I making any sense?

Thanks

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Source: https://maker.wiznet.io/forum/13690