---
title: "UDP server multiple clients"
url: "https://maker.wiznet.io/forum/11882"
markdown_url: "https://maker.wiznet.io/forum/11882/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "rmszal"
created: "2016-05-21T14:53:07+09:00"
last_activity: "2017-05-04T08:38:55+09:00"
language: "en"
views: 906
replies: 2
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# UDP server multiple clients

## Question

Asked by rmszal on 2016-05-21 in Ethernet Chips.

Hello,

I am using W5500 socket in UDP mode to act as a server and communicate with smartphone app. It works great when I have one client “connected”[sending packets] to device, but when I try to make multiple clients connections, I have problems with long response time. It sometimes gets up to 600 ms to execute sendto() function and as i figured out most of this time is spend waiting for Sn_IR_SENDOK flag. I am using ioLibrary socket API version 1.0.2.

Are there any issues with W5500 that I should be aware when I try to deal with multiple clients connections?

I will really appreciate any advice.

## Replies

### Reply 1 by rmszal, 2016-05-23

It turns out that this problem is connected with sending ARP requests before packets, when destination IP is changing, just like if W5500 holds only one ARP record for socket[FORCE_ARP option is disabled]. Is it how this chip works?

I’ve been thinking about workaround this issue by creating ARP table on microcontroller side. Thus I have a question: if I read a Sn_DHAR register right after sending UDP packet, will that be a destination MAC address, retrieved through ARP request?

### Reply 2 by rmszal, 2016-05-24

FYI: Solution that I’ve mentioned above is working correctly.

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Source: https://maker.wiznet.io/forum/11882