NTP Request Packet

udp request ntp cortex-m3 lwip
Sharpless512 picture Sharpless512 · Jan 5, 2013 · Viewed 26.9k times · Source

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: enter image description here

I hope you guys can help me.

Answer

Tomuo picture Tomuo · May 23, 2014

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)

Related questions

Can two applications listen to the same port?

Can two applications on the same machine bind to the same port and IP address? Taking it a step further, can one app listen to requests coming from a certain IP and the other to another remote IP? I know …

Read More
What is the largest Safe UDP Packet Size on the Internet

I've read a number of articles about UDP packet sizes but have been unable to come to a conclusion on whats correct. A number of services restrict the largest UDP packet to 512 bytes (like dns) Given the minimum MTU on …

Read More
What are examples of TCP and UDP in real life?

I know the difference between the two on a technical level. But in real life, can anyone provide examples (the more the better) of applications (uses) of TCP and UDP to demonstrate the difference?

Read More