W5100/Ethernet cannot use socket as UDP after using it as TCP

Here is the serial monitor for that:

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3287)
Socket#2:0x0 80 D:192.168.1.254(3286)
Socket#3:0x0 80 D:192.168.1.254(3290)
Unix time = 1366989984
Client request: GET /?t=&r= HTTP/1.1

POST data:
t=0
r=0
Sending response
done
Client request: GET /favicon.ico HTTP/1.1

POST data:
Sending response
done

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3291)
Socket#2:0x0 80 D:192.168.1.254(3292)
Socket#3:0x0 80 D:192.168.1.254(3290)
Unix time = 1366989994

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3291)
Socket#2:0x0 80 D:192.168.1.254(3292)
Socket#3:0x0 80 D:192.168.1.254(3290)
Unix time = 1366990004

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3291)
Socket#2:0x0 80 D:192.168.1.254(3292)
Socket#3:0x0 80 D:192.168.1.254(3290)
Unix time = 1366990014

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3291)
Socket#2:0x0 80 D:192.168.1.254(3292)
Socket#3:0x0 80 D:192.168.1.254(3290)
Unix time = 1366990024

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3291)
Socket#2:0x0 80 D:192.168.1.254(3292)
Socket#3:0x0 80 D:192.168.1.254(3290)
Unix time = 1366990034

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3291)
Socket#2:0x0 80 D:192.168.1.254(3292)
Socket#3:0x0 80 D:192.168.1.254(3290)
Unix time = 1366990044

Dude. Thanks a ton. I appreciate immensely what you're doing here. Unfortunately that's not exactly the same use case. You only refreshed the browser once. Notice that in my example after the first time I refreshed I still got responses back from the NTP, but after the second time I refreshed, the NTP stopped. Seems odd, but here's my explanation:

It seems that it's actually related to transition from UDP to a listening socket->connected transition, to being used for UDP again. When you refresh the first time, you're using socket 0 (which has never been used for UDP) to do the TCP, and the listening port goes to socket 1. The UDP time server then happens on socket 0 which hasn't been used for UDP before TCP. If you wait about 10 seconds and then refresh, it'll use socket 1 to do the TCP conn, and then listen back on socket 0. At that point the UDP will take place on socket 1 (which has done the UDP to TCP transition) and it will fail.

Ugh. this is too complicated a scenario here. Sorry sorry sorry. I'm wondering if you can do 1 more test:

  1. Start the code.
  2. wait about 15 seconds.
  3. refresh your browser.
  4. wait about 15 seconds.
  5. refresh your browser again.

Incidentally, I tried to make a simpler scenario by writing some code that connects to itself, but apparently the W5100 won't connect to itself.

Like this?

Start!
Socket#0:0x22 8888 D:0.0.0.0(0)
Socket#1:0x14 80 D:0.0.0.0(0)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
Client request: GET / HTTP/1.1

POST data:
Sending response
done

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x0 80 D:192.168.1.254(3719)
Socket#2:0x17 80 D:192.168.1.254(3720)
Socket#3:0x14 80 D:0.0.0.0(0)
Unix time = 1366996102
Client request: GET /?t=4&r=3 HTTP/1.1

POST data:
t=4
r=3
Sending response
done

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x0 80 D:192.168.1.254(3719)
Socket#2:0x0 80 D:192.168.1.254(3720)
Socket#3:0x14 80 D:0.0.0.0(0)
Unix time = 1366996112
Client request: GET /?t=5&r=1 HTTP/1.1

POST data:
t=5
r=1
Sending response
done

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x14 80 D:192.168.1.254(3719)
Socket#2:0x0 80 D:192.168.1.254(3720)
Socket#3:0x0 80 D:192.168.1.254(3724)
Unix time = 1366996122
Client request: GET /?t=5&r=1 HTTP/1.1

POST data:
t=5
r=1
Sending response
done

Socket#0:0x22 8888 D:198.55.111.50(123)
Socket#1:0x0 80 D:192.168.1.254(3725)
Socket#2:0x17 80 D:192.168.1.254(3726)
Socket#3:0x14 80 D:192.168.1.254(3724)
Unix time = 1366996132
Client request: GET /favicon.ico HTTP/1.1

POST data:
Sending response
done

edit: Removed my NTP server ip.

I guess I don't understand how your UDP connection got setup on socket 0 and the TCP listening socket is on socket 1? In the code it calls server.begin() before we've done anything with UDP, so that should use socket 0, and the first showSockStatus() gets called before we've ever done anything with UDP, but your first message shows a UDP socket listening on port 8888 as socket 0. My first message looks like this:

Start!
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 0 D:0.0.0.0(0)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)

Here is yours:

void setup() {
	Serial.begin(9600);
	Ethernet.begin(mac, ip, INADDR_NONE, gateway, subnet);
	delay(500);

	Serial.println("Start!");
// here is where the first socket is assigned:
	server.begin();
	ShowSockStatus();
	timer = millis();
}

Here is mine:

void setup() {
	Serial.begin(9600);

        pinMode(4, OUTPUT);
        digitalWrite(4, HIGH);
        
	Ethernet.begin(mac, ip, INADDR_NONE, gateway, subnet);
	delay(500);

// here is socket #0
	Udp.begin(8888);

	Serial.println("Start!");

// here is socket #1
	server.begin();
	ShowSockStatus();
	timer = millis();
}

OK. I see now why yours is working. You dedicate a socket to the UDP connection that never is freed. So you never reuse a socket that was previously used as a UDP socket as a TCP socket and vice versa. Thus you avoid the issue. In my version the call to CheckTime will create the UDP socket and then tear it down, freeing the socket for use by other clients.

What you've got is a workaround for the problem I'm having, but the problem I'm having is still a bug. The W5100 only has 4 sockets available, and dedicating one of them full time to an NTP client when you only need to use that socket once every few days, and you only need it for about 500ms can be wasteful. Creating the UDP socket and tearing it down should be a reasonable scenario, but it doesn't work.

You have 4 sockets. Use them wisely. I like the full time UDP socket, and would like to reserve a socket for client stuff when a server is running. Here is my opinion:

Note my sketch uses all the sockets. One (socket#0) for UDP, and the rest (#1-3) for the server.

It sounds like this guy is having exactly the same problem as I am, but he doesn't know it. He thinks he's running out of sockets, but in actuality the issue is a bug somewhere that won't allow you to reuse a socket that was previously allocated as a TCP socket as a UDP socket.

Your workaround is find for many scenarios, but not mine. I'm trying to build a simple web server. One that syncs time via UDP periodically and also includes a TFTP server so you can upload content to the SD card. TFTP uses 1 UDP socket for the server, and one for the client if it connects. So, to avoid the bug that I've identified here, I'd have to dedicate one socket to the NTP client, one socket to the TFTP server, one socket to the TFTP client, and then I'd have one remaining socket to use as a my listening socket for HTTP. Of course that means I don't have a socket to use if someone actually connected to my HTTP server.

In other words, I'm sunk unless I can figure out how to resolve this bug. (or upgrade to the 5200, which seems like a waste)

Incidentally, another workaround is to actually reinitialize the W5100 after every TCP client disconnects. (i.e. call Ethernet.begin() again) Of course that means you will also have to go reattach your HTTP server and UDP servers. Super kludgy, but it may be the only way around this bug.

That one UDP socket will work for every UDP function, like dhcp, ntp, dns, etc...all use that socket. One socket, all that stuff is covered. :slight_smile:

I know when I am having a problem, but this isn't it. If this was an Apache server, I would expect more.

I will give you the benefit of the doubt, and try to begin and stop a UDP socket and see how it affects the server and the UDP sockets. I'm not sure why I would do that, and I don't really expect it to work. You are assuming that you will need all the sockets for the server simultaneously at times, which would guarantee a NTP fail in that time.

You actually bring up a fantastic point with your DNS example. When you call EthernetClient::connect(char * host, uint8_t port), it does exactly what I do with NTP. It dynamically creates a UDP socket for DNS, connects to a server, and then releases that socket. That should work just fine, except that this bug exists. I'm certain that I can create a scenario with a http server that has a few connects() in it that will start failing on DNS resolution because of the bug. There would be no solution either, because you can't manually specify a socket for the DNS server to use. It just dynamically allocates one from the unused pool.

Sometimes I amaze myself! 8)

Remove the Udp.begin(8888) from setup, and use this checkTime function. How does it do? Works ok here.

void checkTime()
{
	if(!Udp.begin(8888)) {
          Serial.println("UDP fail");
          return;
        }

  	sendNTPpacket(Udp, timeServer); // send an NTP packet to a time server
	Serial.println("");
	ShowSockStatus();
	// wait to see if a reply is available
	delay(1000);  
	if ( Udp.parsePacket() ) {  
		// We've received a packet, read the data from it
		Udp.read(packetBuffer,NTP_PACKET_SIZE);  // read the packet into the buffer

		//the timestamp starts at byte 40 of the received packet and is four bytes,
		// or two words, long. First, esxtract the two words:

		unsigned long highWord = word(packetBuffer[40], packetBuffer[41]);
		unsigned long lowWord = word(packetBuffer[42], packetBuffer[43]);  
		// combine the four bytes (two words) into a long integer
		// this is NTP time (seconds since Jan 1 1900):
		unsigned long secsSince1900 = highWord << 16 | lowWord;  

		// now convert NTP time into everyday time:
		Serial.print("Unix time = ");
		// Unix time starts on Jan 1 1970. In seconds, that's 2208988800:
		const unsigned long seventyYears = 2208988800UL;     
		// subtract seventy years:
		unsigned long epoch = secsSince1900 - seventyYears;  
		// print Unix time:
		Serial.println(epoch);  
	}  
	else
		Serial.println("UDP FAIL!");
	Udp.stop();

}

Here's a much simpler example using the built in DNS library:

#include <SPI.h>
#include <Ethernet.h>
#include <EthernetUdp.h>
#include <utility/w5100.h>
#include "Dns.h"

void ShowSockStatus()
{
	for (int i = 0; i < MAX_SOCK_NUM; i++) {
		Serial.print("Socket#");
		Serial.print(i);
		uint8_t s = W5100.readSnSR(i);
		Serial.print(":0x");
		Serial.print(s,16);
		Serial.print(" ");
		Serial.print(W5100.readSnPORT(i));
		Serial.print(" D:");
		uint8_t dip[4];
		W5100.readSnDIPR(i, dip);
		for (int j=0; j<4; j++) {
			Serial.print(dip[j],10);
			if (j<3) Serial.print(".");
		}
		Serial.print("(");
		Serial.print(W5100.readSnDPORT(i));
		Serial.println(")");
	}
}

byte mac[] = {  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

EthernetServer server(80);
IPAddress dnsserver(192,168,10,1);
IPAddress ip(192,168,10,20);
IPAddress gateway(192,168,10, 2);
IPAddress subnet(255, 255, 255, 0);

long timer;

void setup() {
	Serial.begin(9600);
	Ethernet.begin(mac, ip, dnsserver, gateway, subnet);
	delay(500);

	Serial.println("Start!");
	server.begin();
	ShowSockStatus();
	timer = millis();
}

void loop()
{ 
	//Check if a web client has attached.
	EthernetClient client = server.available();
	if (client) {
		Serial.println("new Client");
		ShowSockStatus();
		boolean currentLineIsBlank = true;
		while (client.connected()) {
			if (client.available()) {
				char c = client.read();
				Serial.print(c);
				if (c == '\n' && currentLineIsBlank) {
					client.stop();
				}
				if (c == '\n') {
					// you're starting a new line
					currentLineIsBlank = true;
				} 
				else if (c != '\r') {
					// you've gotten a character on the current line
					currentLineIsBlank = false;
				}
			}
		}
		// give the web browser time to receive the data
		delay(1);
		// close the connection:
		client.stop();
		Serial.println("client disconnected");
	}

	if ((millis() - timer) > 5000) {
		DNSClient dns;
		IPAddress remote_addr;
		dns.begin(dnsserver);
		if (dns.getHostByName("www.yahoo.com", remote_addr) == 1)
		{
			for (int i=0; i<4; i++) {
				Serial.print(remote_addr[i]);
				if (i!=3)
					Serial.print('.');
				else
					Serial.println("");
			}
		}
		else
			Serial.println("DNS FAIL!");
		timer = millis();
		ShowSockStatus();
	}
}

And the output

Start!
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 0 D:0.0.0.0(0)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
98.139.183.24
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1030 D:192.168.10.1(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
98.138.253.109
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1024 D:192.168.10.1(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
98.138.253.109
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1024 D:192.168.10.1(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x17 80 D:192.168.10.213(11298)
Socket#1:0x17 80 D:192.168.10.213(11299)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.10.20
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
98.139.183.24
Socket#0:0x0 1035 D:192.168.10.1(53)
Socket#1:0x17 80 D:192.168.10.213(11299)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
98.138.253.109
Socket#0:0x0 1029 D:192.168.10.1(53)
Socket#1:0x17 80 D:192.168.10.213(11299)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
98.139.183.24
Socket#0:0x0 1039 D:192.168.10.1(53)
Socket#1:0x0 80 D:192.168.10.213(11299)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x17 80 D:192.168.10.213(11312)
Socket#1:0x14 80 D:192.168.10.213(11299)
Socket#2:0x17 80 D:192.168.10.213(11311)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.10.20
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
98.138.253.109
Socket#0:0x17 80 D:192.168.10.213(11312)
Socket#1:0x14 80 D:192.168.10.213(11299)
Socket#2:0x0 1033 D:192.168.10.1(53)
Socket#3:0x0 0 D:0.0.0.0(0)
98.139.183.24
Socket#0:0x17 80 D:192.168.10.213(11312)
Socket#1:0x14 80 D:192.168.10.213(11299)
Socket#2:0x0 1028 D:192.168.10.1(53)
Socket#3:0x0 0 D:0.0.0.0(0)
98.138.253.109
Socket#0:0x17 80 D:192.168.10.213(11312)
Socket#1:0x14 80 D:192.168.10.213(11299)
Socket#2:0x0 1039 D:192.168.10.1(53)
Socket#3:0x0 0 D:0.0.0.0(0)
DNS FAIL!
Socket#0:0x0 1034 D:192.168.10.1(53)
Socket#1:0x14 80 D:192.168.10.213(11299)
Socket#2:0x0 1039 D:192.168.10.1(53)
Socket#3:0x0 0 D:0.0.0.0(0)
DNS FAIL!
Socket#0:0x0 1033 D:192.168.10.1(53)
Socket#1:0x14 80 D:192.168.10.213(11299)
Socket#2:0x0 1039 D:192.168.10.1(53)
Socket#3:0x0 0 D:0.0.0.0(0)

Notice how it works right up till it tries to reuse socket #0 to do the DNS (which has previously been used as UDP/DNS and then TCP). Then it fails...

If I use a reliable dns server, I cannot replicate your error with my code. Both NTP and DNS work. Is 192.168.10.1 a good dns server? My NTP and DNS changes sockets (they both use the same socket) and has no problem. ??

Are you using the code I put in the message? There shouldn't be any NTP packets going across because there's nothing NTP related in that code.

It's a reliable DNS server. If I never browse to the web port on the Arduino then it'll sit there and poll forever with no issues. In fact it's been up polling for the past 3 hours and has not had a single failure. As soon as I browse to we web port though, it fails (well, I actually have to browse twice for it to happen).

Are you using the code I put in the message?

I am testing the code from your reply #16 now. I changed the network settings and dns server to mine, and have it resolving my local domain name instead of yahoo. It has not failed yet, despite the requests to the server and the "No data received" message.

When resolving yahoo, I got errors if I try to resolve the name too often, no matter if I loaded the webpage or not.

edit: This is after I removed my SD card from the shield slot. NTP, dhcp and dns have a problem if there is a card in the slot and not disabled or initialized correctly.

It has been running about an hour with several dozen requests to the server, and not one fail of either service.

Thanks Tim. I should have mentioned the SD card thing. I didn't include any initialization code to turn of the SD card.

I guess I'd love to see your output if you have it available...

I'm beginning to think that your setup is just not exhibiting the same things that mine is. Maybe there's a batch of 5100s that have bugs? Maybe it's my version of the Ethernet shield. (I've got a couple that I bought off eBay). I don't know. I've used two different shields with three different boards (2 megas and 1 Uno). I've got no idea why it seems so repeatable on my setup with very varied scenarios, but we can't seem to get yours to exhibit the same thing.

Give me a bit to setup the test again. I am testing the ethernet shield's dhcp client function. I think my router has a bug in the dhcp server service. I will replace my public ip with a phoney. I don't need any DoS attacks. I get enough, thanks.

Check back in a bit... :slight_smile:

Does seem odd that it would fail for a DNS request that only came once every 5 seconds. That's pretty far from a DoS. Anyway, I changed my code to use 4.2.2.1 public DNS. I posted it on the other thread...

I am concerned about DoS when publishing my public ip, and not because of this code. Here is my serial monitor display for a short run. It will go on for hours like this.

Start!
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 0 D:0.0.0.0(0)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1024 D:xx.xx.58.115(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1036 D:xx.xx.58.115(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1031 D:xx.xx.58.115(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x17 80 D:192.168.1.254(3691)
Socket#1:0x17 80 D:192.168.1.254(3692)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x0 1026 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3692)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3692)
Socket#2:0x17 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x0 1038 D:xx.xx.58.115(53)
Socket#2:0x17 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x0 1034 D:xx.xx.58.115(53)
Socket#2:0x0 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x17 80 D:192.168.1.254(3694)
Socket#1:0x17 80 D:192.168.1.254(3695)
Socket#2:0x14 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x0 1029 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3695)
Socket#2:0x14 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3695)
Socket#2:0x17 80 D:192.168.1.254(3698)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x0 1024 D:xx.xx.58.115(53)
Socket#2:0x17 80 D:192.168.1.254(3698)
Socket#3:0x0 0 D:0.0.0.0(0)
a
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1024 D:xx.xx.58.115(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1036 D:xx.xx.58.115(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:0.0.0.0(0)
Socket#1:0x0 1031 D:xx.xx.58.115(53)
Socket#2:0x0 0 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x17 80 D:192.168.1.254(3691)
Socket#1:0x17 80 D:192.168.1.254(3692)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x0 1026 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3692)
Socket#2:0x14 80 D:0.0.0.0(0)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3692)
Socket#2:0x17 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x0 1038 D:xx.xx.58.115(53)
Socket#2:0x17 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
xx.xx.58.116
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x0 1034 D:xx.xx.58.115(53)
Socket#2:0x0 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x17 80 D:192.168.1.254(3694)
Socket#1:0x17 80 D:192.168.1.254(3695)
Socket#2:0x14 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x0 1029 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3695)
Socket#2:0x14 80 D:192.168.1.254(3693)
Socket#3:0x0 0 D:0.0.0.0(0)
new Client
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x17 80 D:192.168.1.254(3695)
Socket#2:0x17 80 D:192.168.1.254(3698)
Socket#3:0x0 0 D:0.0.0.0(0)
GET / HTTP/1.1
Host: 192.168.2.2
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 5.1) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.64 Safari/537.31
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3

client disconnected
xx.xx.58.116
Socket#0:0x14 80 D:xx.xx.58.115(53)
Socket#1:0x0 1024 D:xx.xx.58.115(53)
Socket#2:0x17 80 D:192.168.1.254(3698)
Socket#3:0x0 0 D:0.0.0.0(0)

my 2 cents;-

UDP is a lightweight protocol that by design doesn't handle things like packet sequencing. It does not guarantee that your packets will arrive in order (It does not even guarantee that your packets will arrive at all.) .
Say send command set voltage=1 v, then set voltage=4 v, You might get 0v or 1v or 4v if default is 0v. TCP is a better choice if you want robust packet delivery and sequencing.
If you're limited to using UDP you would have to develop a method of identifying the out of sequence packets and resequencing them or request to resend, but for Arduino is tough task.

a lot people implement UDP with Arduino, I mean neither OP nor this thread please hold fire.

I guess either they drink too much Italian coffee or I drink too much, might be both. :stuck_out_tongue: