NTP Request Packet

Question

I'm trying to figure out what I need to send (client) in the NTP request package to retrieve a NTP package from the server. I'm working with the LWIP on Cortex M3, Stellaris LM3S6965

I understand that I will recieve a UDP header and then the NTP protocol with the different timestamps the remove the latency. I probable need to make an UDP header but what do I need to add as data?

wireshark image:

I hope you guys can help me.

Operatic

Answer 1

The client request packet is the same as the server reply packet - just set the MODE bits in the first word to 3 (Client) to be sure.

Send the whole 48 byte packet to the server, it will reply with the same.

The simplest packet would be 0x1B followed by 47 zeroes. (Version = 3, mode = 3)

Laccolith

Correct answer, after sending such format to NTP server, I received some reply (previously the "TimeOutException" was thrown). Perhaps you could also tell how to extract time from the received packet? That is, from the 48 bytes received, how to print time as "YYYY:MM:DD:HH:MM:SS" (or as some other format)? Which bits correspond to what? – Reinhold

Answer 2

This is for starters: http://www.eecis.udel.edu/~mills/ntp/html/warp.html

Check this out in case you haven't yet: https://www.rfc-editor.org/rfc/rfc5905

Then look at this: http://wiki.wireshark.org/NTP and check out the sample pcap files that they have uploaded.

I am not sure if this helped, but I hope so.

Busyness

Yeah Already checked those links. But I can only see the recieved files from ntp not the sent files. – Operatic

Answer 3

I have coded an Arduino to connect to an NTP server using this code here,

http://www.instructables.com/id/Arduino-Internet-Time-Client/step2/Code/

Look at the method called getTimeAndDate, and sendNTPpacket.

That is the packet that is sent. This is setting up a buffer and shows binary (0b) and hex (0x) being set up in the 48 character buffer. The address is the NTP time server,

memset(packetBuffer, 0, NTP_PACKET_SIZE);
packetBuffer[0] = 0b11100011;
packetBuffer[1] = 0;
packetBuffer[2] = 6;
packetBuffer[3] = 0xEC;
packetBuffer[12]  = 49;
packetBuffer[13]  = 0x4E;
packetBuffer[14]  = 49;
packetBuffer[15]  = 52;                 
Udp.beginPacket(address, 123);
Udp.write(packetBuffer,NTP_PACKET_SIZE);
Udp.endPacket();

Here is what happens to the received packet,

Udp.read(packetBuffer,NTP_PACKET_SIZE);  // read the packet into the buffer
unsigned long highWord, lowWord, epoch;
highWord = word(packetBuffer[40], packetBuffer[41]);
lowWord = word(packetBuffer[42], packetBuffer[43]); 
epoch = highWord << 16 | lowWord;
epoch = epoch - 2208988800 + timeZoneOffset;
flag=1;
setTime(epoch);

setTime is part of the arduino time library, so the epoch should be the number of seconds since Jan 1, 1900 as suggested here (search for epoch),

https://en.wikipedia.org/wiki/Network_Time_Protocol

But in case you want a C# version, I found this here, compiled the code under the excepted answer and it works. This will likely make more sense to you, and does show the use of epoch 1/1/1900.

How to Query an NTP Server using C#?

Can easily see the similarity.

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