---
title: "TCP loopback server on W5500 connects but does not receive data on the socket."
url: "https://maker.wiznet.io/forum/14345"
markdown_url: "https://maker.wiznet.io/forum/14345/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "gtrevi"
created: "2021-09-08T01:14:11+09:00"
last_activity: "2021-09-08T01:14:11+09:00"
language: "en"
views: 602
replies: 0
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# TCP loopback server on W5500 connects but does not receive data on the socket.

## Question

Asked by gtrevi on 2021-09-08 in Ethernet Chips.

I’m using the latest Wiz driver and running the standard `loopback_tcps()` server (192.168.0.123:5000) on a WIZ850io module (with a W5500), and attempting to communicate with a tcp client (192.168.0.100) running on a PC on the same private (home) network.

The client is successfully connecting to the server (listening on port 5000), but the first `send()` from the client does not trigger any receive on the WizChip (the `loopback_tcps()` continues to poll `getSn_RX_RSR()` but the returned size is always zero):

[![image](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/f/f1238269e8d72b3847b10e4edd9ca9f14565a9ce.png) image874×235 12.4 KB](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/f/f1238269e8d72b3847b10e4edd9ca9f14565a9ce.png)

Here is a Wireshark capture showing the successful connection, but no subsequent bytes received despite the PC client having sent 1024 bytes with the posix `send()`

[![image](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/f/f93224c89f2c5abc3f080831cc16426027b3a1e6.png) image1200×404 47.1 KB](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/f/f93224c89f2c5abc3f080831cc16426027b3a1e6.png)

Looking up the `TCP ZeroWindowProbe` error, it seems to mean that the receiver (the W5500 server) has reduced its receive buffer (a.k.a. window) to zero, basically telling the sender (the PC client) to stop sending (usually for performance reasons).

Any suggestions much appreciated!

Thank you!

---

Source: https://maker.wiznet.io/forum/14345